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What is Mxene?

Mxene is a class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides that exhibit unique properties such as high electrical conductivity, excellent mechanical strength, and high surface areas. Mxenes were first discovered in 2011 by researchers at Drexel University and have since gained significant attention in the scientific community due to their potential applications in various fields such as energy storage, catalysis, and sensing.

There are several different types of mxenes that have been synthesized, with the most common being titanium carbide (Ti3C2), which is typically prepared by selectively etching aluminum atoms from layered ternary carbides known as MAX phases. Other types of mxenes include vanadium carbide (V2C), niobium carbide (Nb2C), and tantalum carbide (Ta4C3), among others.

The preparation of mxenes typically involves the following steps:

1. Synthesis of MAX phase: The first step in preparing mxenes is to synthesize the parent MAX phase material, which is a layered ternary compound consisting of a transition metal (M), a group A element (A), and carbon or nitrogen (X). Common MAX phases include Ti3AlC2, V2AlC, and Nb4AlC3.

2. Selective etching: The next step involves selectively etching the A element (usually aluminum) from the MAX phase using strong acids or other etchants. This process leaves behind a layered structure of transition metal carbides, nitrides, or carbonitrides, which are the mxene precursors.

3. Intercalation: In some cases, additional intercalation steps may be performed to introduce other elements or molecules between the layers of mxene to modify its properties.

4. Delamination: The final step in preparing mxenes involves delaminating the layered structure to obtain single or few-layered sheets of mxene. This can be achieved through mechanical exfoliation, sonication, or other methods.

Once prepared, mxenes can be further functionalized or integrated into various devices and applications. Their unique combination of properties makes them promising candidates for use in energy storage devices such as batteries and supercapacitors, as well as in catalysis, electromagnetic shielding, and water purification.

Therefore, mxenes represent a new class of 2D materials with exciting potential for a wide range of applications. Continued research into their synthesis, properties, and applications will likely uncover even more possibilities for these versatile materials in the future.

Raman spectroscopy for characterization of Mxene

Raman spectroscopy is a powerful technique used to characterize the structural and chemical properties of materials, including Mxenes. Mxenes, a class of two-dimensional transition metal carbides, nitrides, and carbonitrides, have gained significant attention in the scientific community due to their unique properties and potential applications in various fields. In this article, we will explore how Raman spectroscopy can be utilized to study and analyze Mxene materials.

Raman spectroscopy is a non-destructive analytical technique that provides information about the vibrational modes of a material. When a material is irradiated with monochromatic light, some of the incident photons are scattered at different energies due to interactions with the material’s molecular vibrations. These energy shifts, known as Raman shifts, provide valuable insights into the material’s chemical composition, crystal structure, and bonding characteristics.

For Mxenes, Raman spectroscopy offers several advantages in characterizing their properties. One key advantage is the ability to identify the presence of different functional groups and chemical bonds within the Mxene structure. The Raman spectrum of Mxenes typically exhibits characteristic peaks corresponding to the stretching and bending vibrations of metal-carbon or metal-nitrogen bonds, as well as other functional groups present in the material.

Additionally, Raman spectroscopy can be used to determine the crystallinity and layer thickness of Mxene samples. The intensity and position of Raman peaks can provide information about the stacking order and interlayer interactions within the Mxene structure. By analyzing the Raman spectra of Mxenes obtained from different synthesis methods or processing conditions, researchers can gain valuable insights into the structural properties of these materials.

Furthermore, Raman spectroscopy can be employed to study the electronic properties of Mxenes. By analyzing the Raman spectra at different excitation wavelengths or under different environmental conditions, researchers can probe the charge carrier dynamics, doping effects, and electronic band structure of Mxene materials. This information is crucial for understanding the electrical conductivity and optoelectronic properties of Mxenes, which are important for their applications in energy storage and electronic devices.

In conclusion, Raman spectroscopy is a versatile tool for characterizing Mxene materials and gaining insights into their structural, chemical, and electronic properties. By utilizing Raman spectroscopy in conjunction with other analytical techniques, researchers can further elucidate the fundamental properties of Mxenes and optimize their performance for various applications. Continued research in this area will undoubtedly contribute to unlocking the full potential of Mxene materials in the field of materials science and beyond.

XRD technique for characterization of Mxene

X-ray diffraction (XRD) is a powerful analytical technique widely used for the characterization of materials, including Mxenes. Mxenes, a class of two-dimensional transition metal carbides, nitrides, and carbonitrides, have garnered significant interest in the scientific community due to their unique properties and potential applications in various fields. In this article, we will explore how XRD can be utilized to study and analyze the structural properties of Mxene materials.

X-ray diffraction is based on the principle of Bragg’s law, which states that when X-rays are incident on a crystalline material, they will be diffracted at specific angles depending on the crystal structure and interatomic spacing of the material. By measuring the intensity and angle of the diffracted X-rays, researchers can obtain valuable information about the crystal structure, phase composition, crystallite size, and lattice parameters of a material.

For Mxenes, X-ray diffraction is a valuable tool for determining their crystal structure and phase composition. The XRD pattern of Mxene materials typically exhibits sharp diffraction peaks corresponding to the ordered atomic arrangement within the crystal lattice. By analyzing the positions and intensities of these peaks, researchers can identify the crystallographic phases present in the Mxene sample and determine the crystal symmetry and unit cell parameters.

Moreover, XRD can be used to study the layer stacking and interlayer spacing of Mxene materials. The interlayer distance between adjacent Mxene layers can be calculated from the position of the diffraction peaks in the XRD pattern. By analyzing the changes in interlayer spacing under different synthesis conditions or processing methods, researchers can gain insights into the structural properties and stability of Mxenes.

Additionally, X-ray diffraction can provide information about the crystallite size and degree of crystallinity of Mxene samples. The broadening of XRD peaks is often used to estimate the average crystallite size of the material, with smaller peak widths indicating smaller crystallite sizes. By quantifying the crystallite size distribution in Mxene samples, researchers can assess the degree of structural ordering and defects present in the material.

Furthermore, X-ray diffraction can be employed to investigate the thermal stability and phase transformations of Mxene materials. By performing in situ XRD measurements at different temperatures or under controlled atmospheres, researchers can monitor changes in the crystal structure and phase composition of Mxenes as a function of temperature or environmental conditions. This information is crucial for understanding the thermal behavior and performance of Mxene materials in high-temperature applications.

In conclusion, X-ray diffraction is a versatile technique for characterizing the structural properties of Mxene materials and gaining insights into their crystallographic features, interlayer spacing, crystallite size, and phase composition. By combining XRD with other analytical techniques, researchers can further elucidate the fundamental properties of Mxenes and optimize their performance for various applications. Continued research in this area will undoubtedly contribute to advancing our understanding of Mxene materials and harnessing their full potential in materials science and technology.

FT-IR spectroscopy for characterization of Mxene

Fourier-transform infrared spectroscopy (FT-IR) is a powerful analytical technique that is widely used for the characterization of materials, including Mxenes. Mxenes, a class of two-dimensional transition metal carbides, nitrides, and carbonitrides, have garnered significant interest in the scientific community due to their unique properties and potential applications in various fields. In this article, we will explore how FT-IR can be utilized to study and analyze the structural and chemical properties of Mxene materials.

FT-IR spectroscopy is based on the principle that molecules absorb infrared radiation at specific frequencies that are characteristic of their chemical bonds and functional groups. When infrared light is passed through a sample, certain wavelengths are absorbed by the sample, resulting in the excitation of molecular vibrations. By measuring the intensity of the absorbed infrared radiation as a function of wavelength, researchers can obtain valuable information about the chemical composition, bonding environment, and structural properties of a material.

For Mxenes, FT-IR spectroscopy is a valuable tool for identifying the functional groups present in the material and probing the bonding interactions between the transition metal atoms, carbon or nitrogen atoms, and other constituents. The FT-IR spectrum of Mxene materials typically exhibits characteristic absorption bands corresponding to the vibrational modes of different chemical groups, such as C-C, C-H, C=O, and M-X bonds (where M represents the transition metal and X represents carbon or nitrogen).

By analyzing the positions and intensities of these absorption bands in the FT-IR spectrum, researchers can identify the functional groups present in the Mxene sample and gain insights into the chemical structure and composition of the material. For example, the presence of specific absorption bands can indicate the presence of carbide or nitride groups in the Mxene structure, while shifts in peak positions can provide information about the coordination environment of the transition metal atoms.

Moreover, FT-IR spectroscopy can be used to study the surface chemistry and functionalization of Mxene materials. By analyzing changes in the FT-IR spectrum before and after surface modification or functionalization reactions, researchers can monitor the introduction of new chemical groups or functional moieties onto the Mxene surface. This information is crucial for tailoring the surface properties and reactivity of Mxenes for specific applications, such as catalysis, sensing, or energy storage.

Additionally, FT-IR spectroscopy can provide insights into the thermal stability and decomposition behavior of Mxene materials. By performing in situ FT-IR measurements at different temperatures or under controlled atmospheres, researchers can monitor changes in the infrared absorption bands associated with thermal degradation processes. This information is essential for understanding the thermal behavior and stability of Mxene materials under different environmental conditions.

In conclusion, Fourier-transform infrared spectroscopy is a versatile technique for characterizing the structural and chemical properties of Mxene materials and gaining insights into their functional groups, bonding interactions, surface chemistry, and thermal behavior. By combining FT-IR with other analytical techniques, researchers can further elucidate the fundamental properties of Mxenes and optimize their performance for various applications. Continued research in this area will undoubtedly contribute to advancing our understanding of Mxene materials and unlocking their full potential in materials science and technology.

XPS for characterization of Mxene

X-ray photoelectron spectroscopy (XPS) is a powerful analytical technique that is widely used for the characterization of materials, including Mxenes. Mxenes, a class of two-dimensional transition metal carbides, nitrides, and carbonitrides, have garnered significant interest in the scientific community due to their unique properties and potential applications in various fields. In this article, we will explore how XPS can be utilized to study and analyze the surface chemistry, elemental composition, and electronic structure of Mxene materials.

X-ray photoelectron spectroscopy is based on the principle that when a material is irradiated with X-rays, electrons from the inner shells of atoms are ejected, resulting in the emission of photoelectrons. By measuring the kinetic energy and intensity of these emitted electrons, researchers can obtain valuable information about the elemental composition, chemical bonding, oxidation states, and surface properties of a material.

For Mxenes, XPS spectroscopy is a valuable tool for probing the surface chemistry and elemental composition of the material. The XPS spectrum of Mxene materials typically exhibits characteristic peaks corresponding to the core levels of different elements present in the sample, such as transition metals (M), carbon (C), nitrogen (N), and oxygen (O). By analyzing the positions and intensities of these peaks, researchers can identify the elemental composition of the Mxene sample and gain insights into the bonding environment and oxidation states of the constituent elements.

Moreover, XPS can provide information about the electronic structure and valence band properties of Mxene materials. By analyzing the valence band spectrum obtained from XPS measurements, researchers can study the energy distribution of valence electrons in the material and investigate the electronic interactions between different atomic species. This information is crucial for understanding the electronic properties and charge transfer mechanisms in Mxene materials, which are important for their performance in various applications, such as energy storage, catalysis, and sensing.

Additionally, XPS spectroscopy can be used to study the surface functionalization and chemical modifications of Mxene materials. By performing XPS measurements before and after surface treatments or functionalization reactions, researchers can monitor changes in the elemental composition, chemical states, and surface functionalities of the Mxene sample. This information is essential for tailoring the surface properties and reactivity of Mxenes for specific applications and optimizing their performance in various technological applications.

Furthermore, XPS can provide insights into the stability and degradation behavior of Mxene materials under different environmental conditions. By performing in situ XPS measurements at elevated temperatures or under controlled atmospheres, researchers can monitor changes in the chemical states and oxidation states of the Mxene sample during thermal treatments or exposure to reactive gases. This information is crucial for understanding the thermal stability and reactivity of Mxene materials and optimizing their performance for high-temperature applications.

In conclusion, X-ray photoelectron spectroscopy is a versatile technique for characterizing the surface chemistry, elemental composition, electronic structure, and stability of Mxene materials. By combining XPS with other analytical techniques, researchers can gain comprehensive insights into the fundamental properties of Mxenes and tailor their surface properties for specific applications. Continued research in this area will undoubtedly contribute to advancing our understanding of Mxene materials and unlocking their full potential in materials science and technology.

UV-Vis spectroscopy for characterization of Mxene

Ultraviolet-visible (UV-Vis) spectroscopy is a powerful analytical technique that is commonly used for the characterization of materials, including Mxenes. Mxenes, a class of two-dimensional transition metal carbides, nitrides, and carbonitrides, have garnered significant interest in the scientific community due to their unique properties and potential applications in various fields. In this article, we will explore how UV-Vis spectroscopy can be utilized to study and analyze the optical properties, electronic transitions, and bandgap of Mxene materials.

UV-Vis spectroscopy is based on the principle that when a material is irradiated with ultraviolet or visible light, electrons in the material can be excited from the ground state to higher energy states. By measuring the absorption or transmission of light at different wavelengths, researchers can obtain valuable information about the electronic transitions, band structure, and optical properties of the material.

For Mxenes, UV-Vis spectroscopy is a valuable tool for probing the electronic structure and optical properties of the material. The UV-Vis spectrum of Mxene materials typically exhibits characteristic absorption peaks corresponding to electronic transitions between different energy levels in the material. These absorption peaks can provide insights into the bandgap energy, electronic band structure, and optical transitions in Mxene materials.

The bandgap energy of a material is a critical parameter that determines its electronic and optical properties. By analyzing the absorption spectrum obtained from UV-Vis measurements, researchers can estimate the bandgap energy of Mxene materials and gain insights into their electronic band structure. The bandgap energy of Mxenes can be influenced by various factors, such as the composition, structure, and surface functionalization of the material, making UV-Vis spectroscopy an essential tool for studying and optimizing the optical properties of Mxenes for specific applications.

Moreover, UV-Vis spectroscopy can provide information about the electronic transitions and excitonic effects in Mxene materials. Excitonic effects arise from the interaction between photo-excited electrons and holes in a material, leading to the formation of excitons with distinct optical properties. By analyzing the absorption spectrum and peak shapes in UV-Vis measurements, researchers can study the excitonic effects in Mxene materials and investigate their impact on the optical properties and charge carrier dynamics of the material.

Additionally, UV-Vis spectroscopy can be used to study the surface plasmon resonance (SPR) properties of Mxene materials. SPR is a phenomenon that occurs when free electrons in a material collectively oscillate in response to incident light, leading to enhanced light absorption and scattering at specific wavelengths. By performing UV-Vis measurements at different angles or polarizations, researchers can investigate the SPR properties of Mxene materials and tailor their optical properties for applications such as sensors, photodetectors, and plasmonic devices.

Furthermore, UV-Vis spectroscopy can be employed to study the stability and degradation behaviour of Mxene materials under different environmental conditions. By performing in situ UV-Vis measurements under controlled temperatures or atmospheres, researchers can monitor changes in the optical properties and electronic transitions of the material during thermal treatments or exposure to reactive gases. This information is crucial for understanding the stability and reactivity of Mxene materials and optimizing their performance for applications requiring high temperatures or harsh environments.

In conclusion, UV-Vis spectroscopy is a versatile technique for characterizing the optical properties, electronic transitions, and bandgap of Mxene materials. By combining UV-Vis spectroscopy with other analytical techniques, researchers can gain comprehensive insights into the fundamental properties of Mxenes and tailor their optical properties for specific applications. Continued research in this area will undoubtedly contribute to advancing our understanding of Mxene materials and unlocking their full potential in materials science and technology.

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In the realm of chemistry, the concept of allotropy unveils the mesmerizing ability of an element to exist in multiple forms, known as allotropes, each exhibiting distinct physical and chemical properties. Among the myriad elements that showcase this intriguing phenomenon, carbon stands out as a versatile and captivating element with a plethora of allotropes. Understanding the diverse carbon allotropes, their mechanical and chemical properties, as well as the characterization methods used to unveil their secrets, is essential for unlocking their potential in various scientific and technological applications.

Carbon Allotropes: A Kaleidoscope of Structures and Properties

Carbon, with its ability to form strong covalent bonds and diverse molecular structures, manifests in several allotropes, each with unique properties and applications. Here are some of the prominent carbon allotropes:

1. Diamond: The epitome of elegance and durability, diamond features a three-dimensional network of carbon atoms arranged in a tetrahedral structure. Renowned for its exceptional hardness, thermal conductivity, and optical properties, diamond finds applications in jewellery, cutting tools, and industrial abrasives.

2. Graphite: In contrast to diamond’s rigid structure, graphite embodies layers of carbon atoms arranged in hexagonal rings, imparting lubricating properties. Graphite is commonly used in pencil leads, lubricants, and electrodes due to its soft and slippery nature.

3. Graphene: A single layer of graphite arranged in a two-dimensional hexagonal lattice structure, graphene boasts remarkable mechanical strength, electrical conductivity, and thermal properties. This wonder material holds promise for applications in electronics, energy storage, and sensors.

4. Carbon Nanotubes: These cylindrical structures composed of rolled-up graphene sheets exhibit exceptional mechanical strength, electrical conductivity, and thermal properties. Carbon nanotubes find applications in nanotechnology, composites, and electronics due to their unique structural characteristics.

5. Fullerenes: Hollow carbon molecules with cage-like structures, fullerenes like Buckminsterfullerene (C60) possess intriguing properties such as high electron affinity and reactivity. Fullerenes are utilized in diverse fields ranging from drug delivery to superconductors.

Mechanical and Chemical Properties of Carbon Allotropes

Each carbon allotrope showcases a distinctive set of mechanical and chemical properties based on its unique structure and bonding arrangement:

– Diamond: Exceptional hardness, transparency, high thermal conductivity.
– Graphite: Softness, lubricating properties, opaque nature.
– Graphene: High electrical conductivity, mechanical strength, thermal conductivity.
– Carbon Nanotubes: Exceptional mechanical strength, electrical conductivity, and thermal properties.
– Fullerenes: High electron affinity, reactivity, unique cage-like structures.

Characterization Methods for Carbon Allotropes

To unravel the mysteries of carbon allotropes and understand their properties at a molecular level, various sophisticated characterization techniques are employed:

1. Fourier Transform Infrared Spectroscopy (FTIR)

Fourier-transform infrared (FTIR) spectroscopy is another powerful analytical technique that can aid in the characterization of different allotropies of carbon by providing information about their chemical bonding, functional groups, and structural properties. Here’s how FTIR analysis can be utilized to study various carbon allotropes:

a. Functional Group Identification: FTIR spectroscopy can be used to identify specific functional groups present in different carbon allotropes based on the characteristic absorption bands observed in their infrared spectra. For example, the presence of sp2 and sp3 hybridized carbon bonds in graphene, carbon nanotubes, and diamond can be distinguished by analyzing the peaks corresponding to C=C and C-H stretching vibrations, respectively. Additionally, functional groups such as hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), and epoxy (-O-) groups can be detected in carbon materials through their distinctive IR absorption bands, allowing researchers to assess the surface chemistry and reactivity of the allotropes.

b. Structural Analysis: FTIR spectroscopy can provide insights into the structural characteristics of carbon allotropes by probing the vibrational modes of carbon-carbon bonds and other chemical interactions within the materials. The presence of sp2 and sp3 hybridized carbon atoms, aromatic rings, double bonds, and functional groups can be inferred from the intensity, position, and shape of the absorption bands in the FTIR spectrum. By correlating the vibrational frequencies of carbon allotropes with their structural features, researchers can elucidate the bonding configurations, lattice arrangements, and crystallographic orientations of the materials.

c. Surface Modification and Functionalization: FTIR spectroscopy is a valuable tool for studying surface modifications, functionalization reactions, and chemical interactions on the surface of carbon allotropes. By comparing the FTIR spectra of pristine and modified carbon samples, researchers can identify changes in the absorption bands associated with functional groups introduced during surface treatments, chemical derivatization, or doping processes. This enables the characterization of surface functionalization strategies, quantification of surface coverage, and evaluation of chemical stability in functionalized carbon materials.

d. Quantitative Analysis: FTIR spectroscopy can be utilized for quantitative analysis of functional groups, impurities, and contaminants in carbon allotropes by measuring the absorbance intensities at specific wavenumbers corresponding to characteristic vibrational modes. By establishing calibration curves or using peak area integration methods, researchers can quantify the relative concentrations of different functional groups or impurities in a carbon sample, providing valuable information about its chemical composition, purity, and quality.

e. Stability and Degradation Studies: FTIR spectroscopy can be employed to investigate the stability, degradation mechanisms, and chemical reactivity of carbon allotropes under various environmental conditions. By monitoring changes in the FTIR spectra over time or upon exposure to external factors (e.g., temperature, humidity, oxidation), researchers can assess the material’s resistance to degradation, identify degradation products or by-products, and elucidate the underlying chemical processes that influence its performance and longevity.

2. Raman Spectroscopy

By studying the vibrational modes of carbon materials, Raman spectroscopy offers valuable information about their structural properties and defects. Raman spectroscopy is a powerful analytical technique that can provide valuable insights into the structural and vibrational properties of different carbon allotropes. Here’s how Raman spectroscopy can help characterize various carbon allotropes:

a. Structural Analysis: Raman spectroscopy can distinguish between different carbon allotropes based on their unique structural characteristics. Each allotrope exhibits specific Raman-active vibrational modes, allowing researchers to identify and differentiate between diamond, graphite, graphene, carbon nanotubes, and fullerenes.

b. Defect Detection: Carbon allotropes may contain defects or impurities that can influence their properties. Raman spectroscopy can detect and characterize these defects by analyzing changes in the Raman spectra, such as shifts in peak positions or intensity variations. This information is crucial for understanding the quality and purity of carbon materials.

c. Quantitative Analysis: Raman spectroscopy can be used for quantitative analysis of carbon allotropes, providing information about the relative abundance of different phases or structures within a sample. By correlating Raman spectral features with specific carbon allotropes, researchers can quantitatively assess the composition and distribution of various forms of carbon in a sample.

d. Chemical Functionalization: Raman spectroscopy is sensitive to chemical modifications and functional groups present on the surface of carbon allotropes. By analyzing changes in Raman spectra upon functionalization or chemical treatment, researchers can characterize the interaction between carbon materials and other substances, enabling the design of tailored functionalized carbon materials for specific applications.

3. X-ray Photoelectron Spectroscopy (XPS)

XPS is another valuable technique that can aid in the characterization of different allotropies of carbon. Here’s how XPS analysis can provide insights into the structural and chemical properties of various carbon allotropes:

a. Elemental Composition: XPS analysis can determine the elemental composition of carbon allotropes by measuring the binding energies of core-level electrons, such as the carbon 1s peak. Different carbon allotropes exhibit distinct binding energy values for their core-level electrons due to variations in the local chemical environment and bonding configurations. By comparing the XPS spectra of carbon allotropes with reference data, researchers can identify the presence of specific elements and quantify their relative concentrations.

b. Chemical State Analysis: XPS analysis can reveal information about the chemical state and bonding characteristics of carbon allotropes. The peak shapes, positions, and intensities in the XPS spectra provide insights into the oxidation state, functional groups, and bonding configurations present in a carbon sample. For example, XPS can differentiate between sp2 and sp3 hybridized carbon atoms in graphene and diamond, respectively, based on their distinct chemical environments and electronic structures.

c. Surface Sensitivity: XPS analysis is a surface-sensitive technique that probes the top few nanometers of a material, making it well-suited for characterizing the surface chemistry of carbon allotropes. By analyzing the elemental composition and chemical states at the surface of a carbon sample, researchers can gain valuable information about surface contaminants, functionalization, and modifications that may influence the material’s properties and reactivity.

d. Dopant Identification: XPS analysis can help identify dopants or impurities incorporated into carbon allotropes to modify their electronic, optical, or catalytic properties. By analyzing the XPS spectra of doped carbon materials, researchers can detect changes in the core-level binding energies and chemical states of the dopant atoms, providing insights into their distribution, concentration, and interaction with the host carbon lattice.

e. Depth Profiling: XPS analysis can also be combined with depth profiling techniques to investigate the chemical composition and structure of carbon allotropes as a function of depth below the surface. Depth profiling methods, such as angle-resolved XPS or sputter depth profiling, allow researchers to study the layer-by-layer composition, doping profiles, and interface properties of carbon materials, enabling a comprehensive understanding of their structure-property relationships.

4. Ultraviolet-Visible Spectroscopy (UV-Vis)

UV-Vis spectroscopy aids in studying the optical properties of carbon allotropes, including absorption and emission spectra.

UV-Vis spectroscopy is another valuable technique that can aid in the characterization of different allotropies of carbon by providing insights into their electronic and optical properties. Here’s how UV-Vis analysis can be utilized to study various carbon allotropes:

a. Bandgap Determination: UV-Vis spectroscopy can be used to determine the bandgap energy of carbon allotropes, which is a crucial parameter that influences their electronic and optical properties. By measuring the absorption spectrum of a carbon sample in the UV and visible regions, researchers can identify the onset of absorption (i.e., the bandgap energy) and characterize the material’s semiconducting or insulating behavior. Different carbon allotropes, such as graphene, carbon nanotubes, and diamond, exhibit distinct bandgap energies due to variations in their electronic structure and bonding configurations.

b. Optical Absorption Features: UV-Vis spectroscopy can reveal information about the optical absorption features of carbon allotropes, such as excitonic transitions, interband transitions, and localized electronic states. The absorption spectrum of a carbon sample can exhibit characteristic peaks, shoulders, or broad absorption bands corresponding to specific electronic transitions within the material. By analyzing the shape, intensity, and position of these absorption features, researchers can gain insights into the electronic structure, energy levels, and optical properties of different carbon allotropes.

c. Defects and Functional Groups: UV-Vis spectroscopy can be used to detect defects, functional groups, and chemical modifications in carbon allotropes that affect their electronic and optical properties. Defect-induced states, surface functionalization, and doping can introduce additional absorption features or modify the intensity of existing peaks in the UV-Vis spectrum of a carbon sample. By comparing the UV-Vis spectra of pristine and modified carbon materials, researchers can identify changes in the electronic structure, bandgap energy, and optical response resulting from defects or functionalization.

d. Quantitative Analysis: UV-Vis spectroscopy can also be employed for quantitative analysis of carbon allotropes by correlating the absorption intensity with the concentration of specific components or impurities in a sample. By measuring the absorbance at characteristic wavelengths and establishing calibration curves for different carbon species or dopants, researchers can quantify the relative abundance of components in a complex mixture or determine the doping level in doped carbon materials.

e. Stability and Degradation Studies: UV-Vis spectroscopy can provide valuable information about the stability, degradation, and photochemical behavior of carbon allotropes under various environmental conditions. By monitoring changes in the UV-Vis absorption spectrum over time or under different exposure conditions (e.g., light irradiation, temperature variations), researchers can assess the material’s photochemical stability, degradation mechanisms, and resistance to environmental factors that may impact its performance and longevity.

5. X-ray Diffraction (XRD)

X-ray diffraction (XRD) analysis is another powerful technique that can provide valuable insights into the structural properties of different carbon allotropes. Here’s how XRD analysis can help characterize various allotropies of carbon:

a. Crystal Structure Determination: XRD analysis can be used to determine the crystal structure of carbon allotropes by analyzing the diffraction patterns generated when X-rays interact with the periodic arrangement of atoms in a material. Different carbon allotropes have distinct crystal structures, such as the hexagonal lattice of graphite, the cubic structure of diamond, and the helical structure of carbon nanotubes. By comparing experimental XRD patterns with reference data, researchers can identify and confirm the crystal structure of a carbon allotrope.

b. Phase Identification: XRD analysis can help identify and distinguish between different phases or polymorphs of carbon allotropes present in a sample. By analyzing the positions and intensities of diffraction peaks in the XRD pattern, researchers can determine the presence of specific allotropes, such as graphite, diamond, graphene, carbon nanotubes, and fullerenes. This information is essential for characterizing the composition and phase distribution within a carbon sample.

c. Crystallite Size and Orientation: XRD analysis can provide information about the crystallite size and orientation of carbon allotropes. By analyzing the broadening of XRD peaks, researchers can estimate the average crystallite size of a material, which is crucial for understanding its structural properties. Additionally, XRD can reveal information about the preferred orientation or texture of crystallites within a sample, offering insights into the growth and alignment of carbon allotropes.

d. Strain Analysis: XRD analysis can also be used to investigate the presence of strain or defects in carbon allotropes. Changes in the peak positions and peak shapes in the XRD pattern can indicate the presence of lattice strain, dislocations, or defects in the crystal structure of a material. By quantifying these structural imperfections, researchers can assess the mechanical stability and performance of carbon allotropes.

e. Thermal Stability and Phase Transitions: XRD analysis can be employed to study the thermal stability and phase transitions of carbon allotropes under varying temperature and pressure conditions. By monitoring changes in the XRD patterns as a function of temperature or pressure, researchers can identify phase transformations, melting points, and structural changes in carbon materials, providing crucial information for understanding their behaviour under different environmental conditions.

Conclusion

In conclusion, the captivating world of carbon allotropes unveils a treasure trove of possibilities for scientific exploration and technological innovation. By delving into the diverse structures and properties of carbon allotropes and employing advanced characterization methods, researchers can unlock the full potential of these fascinating materials across a wide range of applications. The allure of carbon allotropes continues to inspire groundbreaking discoveries and advancements in materials science and beyond.

Raman and Fourier Transform Infrared (FTIR) spectroscopy are two of the most widely used analytical techniques in the field of chemistry. Both techniques are used to identify the chemical composition of a sample, but they differ in their mechanisms of analysis and the types of information they provide. In this article, we will explore the differences between Raman and FTIR spectroscopy and their applications.

Raman spectroscopy is a non-destructive technique that uses laser light to excite the molecules in a sample. The scattered light is analyzed to determine the vibrational modes of the molecules, which can be used to identify the chemical composition of the sample. The Raman effect was first discovered by C.V. Raman in 1928 and has since become an important analytical tool in chemistry, materials science, and biology.

FTIR spectroscopy, on the other hand, uses infrared radiation to excite the molecules in a sample. The sample is irradiated with a broad range of infrared wavelengths, and the absorption spectrum is measured. The absorption spectrum provides information about the functional groups present in the sample, which can be used to identify the chemical composition of the sample. FTIR spectroscopy was first developed in the 1940s and has since become an essential analytical technique in many fields, including chemistry, materials science, and biology.

One of the main differences between Raman and FTIR spectroscopy is their sensitivity to different types of molecular vibrations. Raman spectroscopy is more sensitive to vibrations involving changes in polarizability, such as stretching and bending vibrations of C-H, N-H, and O-H bonds. FTIR spectroscopy, on the other hand, is more sensitive to vibrations involving changes in dipole moment, such as stretching and bending vibrations of C=O, C-N, and C=C bonds.

Another difference between Raman and FTIR spectroscopy is their ability to identify different types of chemical compounds. Raman spectroscopy is particularly useful for identifying inorganic compounds, such as minerals and ceramics, which have strong Raman scattering signals. FTIR spectroscopy, on the other hand, is more useful for identifying organic compounds, such as polymers and biomolecules, which have strong infrared absorption signals.

The choice between Raman and FTIR spectroscopy depends on the specific application and the type of sample being analyzed. Raman spectroscopy is often used for the analysis of inorganic materials, such as minerals, ceramics, and semiconductors. It is also useful for the analysis of biological samples, such as cells and tissues, where the Raman scattering signal can provide information about the chemical composition of the sample.

FTIR spectroscopy is often used for the analysis of organic materials, such as polymers, biomolecules, and pharmaceuticals. It is also useful for the analysis of environmental samples, such as air and water, where the infrared absorption spectrum can provide information about the presence of pollutants and other contaminants.

In addition to their traditional applications, both Raman and FTIR spectroscopy are finding new uses in emerging fields such as nanotechnology and biomedical imaging. Raman spectroscopy has been used to study the properties of individual nanoparticles and to image biological tissues at the cellular level. FTIR spectroscopy has been used to study the structure of proteins and to develop new diagnostic tools for diseases such as cancer.

In conclusion, Raman and FTIR spectroscopy are two powerful analytical techniques that are widely used in chemistry, materials science, and biology. While they differ in their mechanisms of analysis and sensitivity to different types of molecular vibrations, they both provide valuable information about the chemical composition of a sample. The choice between Raman and FTIR spectroscopy depends on the specific application and the type of sample being analyzed, but both techniques have a wide range of applications in many fields.

Falsification of data in a paper refers to the deliberate manipulation or fabrication of research data to support a particular hypothesis or conclusion. This can include altering or omitting data, selectively reporting results, or creating false data altogether.

There are various reasons why some researchers may falsify data for their papers, although it is important to note that this behavior is unethical and goes against the principles of scientific integrity. Some of the reasons may include:

1. Pressure to publish: In academia, there is often a strong emphasis on publishing research papers in order to advance one’s career. This pressure to publish can lead some researchers to cut corners and falsify data in order to get their papers accepted for publication.

2. Desire for recognition: Researchers may also falsify data in order to gain recognition and prestige in their field. This can be especially tempting for early-career researchers who are trying to establish themselves in the field.

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We present here results on a Raman spectroscopic study of the deposited defected graphene on Si substrates by chemical vapor deposition (thermal decomposition of acetone). The graphene films are not deposited on the (001) Si substrate directly but on two types of interlayers of mixed phases unintentionally deposited on the substrates: а diamond-like carbon (designated here as DLC) and amorphous carbon (designated here as αC) are dominated ones.

The performed thorough Raman spectroscopic study of as-deposited as well as exfoliated specimens by two different techniques using different excitation wavelengths (488, 514, and 613 nm) as well as polarized Raman spectroscopy establishes that the composition of the designated DLC layers varies with depth: the initial layers on the Si substrate consist of DLC, nanodiamond species, and C₇₀fullerenes while the upper ones are dominated by DLC with an occasional presence of C₇₀ fullerenes. The αC interlayer is dominated by turbostratic graphite and contains a larger quantity of C₇₀ than the DLC-designated interlayers. The results of polarized and unpolarized Raman spectroscopic studies of as-grown and exfoliated graphene films tend to assume that single- to three-layered defected graphene is deposited on the interlayers. It can be concluded that the observed slight upshift of the 2D band as well as the broadening of 2D band should be related to the strain and doping.

1. Introduction

Graphene is a one-atom-thick layered material that consists of completely sp²-bonded carbon atoms tightly packed into a honeycomb lattice. It has a lot of unique properties promising a huge number of possible applications (see, e.g., [1]). A lot of different ways of synthesizing graphene were experimentally tested during the last decade; however, only thermally and plasma-assisted chemical vapor deposition (CVD/PECVD) on metal substrates (copper, nickel, etc.) [2, 3] as well as epitaxial growth on SiC substrates and so on [4, 5] were developed for industrial application. The latter method is based on C (or Si) termination of the (0001)_C (or (0001)_Si) SiC surface and requires high temperature and expensive SiC substrates. The CVD method is based on the plasma-enhanced thermal decomposition of a carbon-containing precursor on a catalytic metal surface. This method provides high reliability and relatively high quality of graphene films, and now, there are a lot of suppliers of reactors for PECVD of graphene. The most preferred precursor is methane (CH₄) as the chemical bond in CH₄ is relatively strong and prevents fast decomposition of the reagent at temperatures below 1000°C (see, e.g., [6]). However, production for microelectronic applications requires transfer of the graphene layers on an insulating surface and, consequently, a large number of additional defects affecting the properties of graphene can be introduced. Therefore, the problem with the deposition of graphene on silicon (or surfaces compatible with silicon technology such as SiO₂) still remains unsolved. We demonstrated the possible application of acetone as a precursor in a thermally assisted CVD and showed that few-layered defected graphene/folded graphene can be deposited on commercially available metal foils—Ni, (Cu_0.5Ni_0.5), μ-metal, and stainless steel SS 304 in a recently published work [7]. Further, we established (see [8]) by Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and grazing incidence X-ray diffraction (GIXRD) as well as by X-ray photoelectron spectroscopy (XPS) the presence of single- to few-layered defected graphene on two different types of interlayers deposited on (100) Si surface: (i) a diamond-like carbon (DLC) layer with some SiC contents (in the range below 5w%) and some residual quantities of SiO₂, and (ii) a complex amorphous carbon layer consisting of a mixture of sp²– and sp³-hybridized carbon as well as very small amount of fullerenes, SiO, and so on.

Here, we focus our experimental study on the Raman spectroscopic characterization of defected as-deposited graphene layers (including polarized spectroscopy) as well as graphene flakes exfoliated from similar specimens by two different ways using 488, 514, and 633 nm excitation laser wavelengths aiming at unambiguous confirmation of the graphene deposition of as well as the identification of the exact composition of the interlayers between the Si substrate and graphene layer/s.

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2. Experimental

2.1. CVD Process

We use 2 inches in diameter (001) Si substrates and a horizontal tube quartz CVD reactor with an internal diameter of approximately 70 mm. The experimental setup also consists of a gas-supply system (inlet and outlet parts), a thermostat with acetone evaporation alert/indication system, a quartz substrate holder, and a resistive heating furnace. The CVD process is based on thermal decomposition of acetone in Ar main gas flow. The deposition temperature was in the range 1150–1160°C. The temperature of the thermostat was kept at 12°C. In order to prevent the supersaturation in the high-temperature zone of the reactor, we used a “pulsed” regime in experiments by alternating the flow of the gas mixture of Ar + C₃H₆O) for 3 min on top of the main flow of pure Ar of about 150–180 cm³/min for 1.5 min for each pulse. The optimal results (predominantly single-layered graphene) were obtained after two deposition pulses.

2.2. Exfoliation

We exfoliated the carbon films deposited on (001) Si substrates by the following two different techniques:(i)The Scotch tape method (see, e.g., [9]): we put tightly the adhesive Scotch tape on the multilayered graphene side of the specimens. After peeling the tape off the specimen, a single- to few-layered graphene remains on the tape’s surface and the interlayer between the upper few layers of graphene and the substrate becomes accessible for spectroscopic examination. Then, we put tightly the Scotch tape with graphene flakes either on 320 nm SiO₂/Si or on glass substrate. About 30–50% of the graphene flakes remain adhered to the SiO₂ or glass substrate after removing the tape due to the Van der Waals force.(ii)We also adhered the multilayered graphene side of the specimens to epoxy resin. After careful cleavage, the most part of the graphene layer/s remains on the surface of the resin. Then, the adhered to the resin graphene film becomes accessible for spectroscopic examination. The Raman spectrum of the epoxy resin does not contain strong peaks around the 2D band of graphene (the area around 2630–2670 cm⁻¹). We established that the 2D band of graphene is clearly distinguishable for graphene regions lying on gas bubbles close to the surface of the resin; otherwise, the 2D band of graphene is weak.

2.3. Characterization

The Raman measurements were carried out in backscattering geometry at a micro-Raman HORIBA Jobin Yvon Labram HR 800 visible spectrometer equipped with a Peltier-cooled CCD detector with a He-Ne (633 nm wavelength and 0.5 mW) laser excitation. The 514 nm (about 23 mW) as well as 488 nm (about 24 mW) lines of an external Ar laser were also used. The laser beam was focused on a spot of about 1 μm in diameter, the spectral resolution being 0.5, 0.7, and 1 cm⁻¹, respectively, or better.

The Raman spectrum of graphene is a clearly established fingerprint of this 2D material (see [10]). The main first-order features in the Raman spectra of graphene and defect-infested graphene excited at 633 nm wavelength are the following:(i)G band (~1582 cm⁻¹) is the only band in graphene allowed by selection rules for first-order Raman effect; it is ascribed to optical (iTO and LO) doubly degenerate phonons of E_2g symmetry at the Γ point (initially described by Tuinstra and Koenig [11]).(ii)D band (~1330 cm⁻¹) is due to breathing-like bands of C hexagonal rings (corresponding to transverse optical phonons near the K point) and requires a defect for its activation via an intervalley double-resonance Raman process (see [12]).(iii)D’ band (at about 1615 cm⁻¹; defect induced similarly to the D band) occurs via an intravalley double-resonance process (see, e.g., [13]).(iv)D” band (at about 1145 cm⁻¹) is resulting from double-resonance intervalley scattering of LA phonons on defects (see [14]). The intensity of this band should be about 100 times lower than that of the D band.

Overtones and combination bands:(i)2D band (historically known from graphite and carbon nanotube-related literature as G’- peak) appears at about 2648–2665 cm⁻¹. It is clearly shown [15–20] that the shape and width of the 2D band can be used for the identification of the mono-, bi-, and three-layered graphene.(ii)The overtone of the D’- peak (2D’) and combination G (phonons), as well as (D+D’) bands, occur around 3230, 2450, and 2930 cm⁻¹, respectively (see [21]).

3. Results and Discussion

Two areas with different surface morphologies are observed by optical microscopy (Figure 1(a)): a clear relief (ridge-like formations) lying along <001> directions covers the first area denoted as DLC while the second area (denoted as αC) is covered by an inhomogeneous film with a constant depth. It should be also mentioned that optical inhomogeneities are observed on the DLC as well as αC-marked areas.
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(e)Figure 1Optical microscopy image of the surface morphology of (a) as-deposited graphene and graphene-related phases on diamond-like carbon (DLC) and amorphous carbon (αC) interlayers. The arrows remarked [100] and [010] directions of the Si substrate. The marker represents 20 μm. (b) The exfoliated and transferred graphene flakes on 320 nm SiO₂. The Raman spectra are taken from “+”-marked positions. The marker represents 30 μm. (c) The layers remaining on the surface of the substrate after exfoliation by Scotch tape. The Raman spectra are taken from the “+”-marked positions near points 1, 2, and 3. The marker represents 30 μm. (d) The exfoliated and transferred graphene flakes on glass substrate. The Raman spectra are taken from the “+”-marked positions near points 1 and 2. The marker represents 20 μm. (e) A graphene flake on air bubble near the epoxy resin surface. The Raman spectra are taken from the square-marked position. The marker represents 20 μm.

It should be recalled that the Raman spectrum (excited at 633 nm laser wavelength) taken from αC- and DLC-denoted areas (see [8]) contains all features typical for graphene: symmetric and clearly pronounced 2D band with full width at a half maximum (FWHM) of 40–56 cm⁻¹, I_2D/I_G ratio between 2 and 3.5, and I_2D/I_D ratio between 2 and 4. However, the 2D band appears at about 2660–2668 cm⁻¹ (for single- and bilayered graphene, respectively), that is, it is blueshifted by about 20 cm⁻¹ relative to the results presented in [15, 22, 23].

Due to the double-resonance origin of most of the monitored spectral features, we perform a Raman spectroscopy examination of as-deposited defected graphene at 488, 514, and 633 nm excitation wavelengths and the results are presented in Figure 2. The 2D bands are blueshifted by about 20 cm⁻¹ and can be typically deconvoluted into (a) a single Lorentzian with FWHM of about 40-41 cm⁻¹ (see Figure 3(a)); (b) four Lorentzians (FWHM of 22 (±1) cm⁻¹) for 2D band with total width of 45–56 cm⁻¹ (see Figure 3(b)); and (c) six Lorentzians (Figure 3(c)) for 2D band with total width larger than 56 cm⁻¹. The results of the deconvolution indicate the presence of single-, bi-, and three-layered defected graphene, respectively (see [15–20]). We did not establish a clear difference between the graphene layers deposited on αC and DLC interlayers; however, bi- and three-layered areas were more frequently observed on DLC interlayers. The results for predominantly single-layered (SL) and bilayered (BL) defected graphene (according to the deconvolution of 2D bands) are summarized in Table 1.

Figure 2Raman spectra taken from as-grown films excited at 633 nm (red trace), 514 nm (blue trace), and 488 nm (green trace) laser wavelengths.
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(c)Figure 3Deconvolution of 2D band identified as coming from single-layered (a), bilayered (b), and three-layered defected graphene deposited on αC. The spectrum is excited at 633 nm laser wavelength.Table 1Summarized results of Raman spectroscopy examination of as-deposited defected graphene films.

In order to access the interlayers as well as graphene flakes for Raman examination, the so-called Scotch tape method was initially used for exfoliation. The Raman spectra of the graphene flakes exfoliated in this way with some occasional amorphous (αC) interlayers transferred to Si/SiO₂ (300 nm) or glass substrate are shown in Figures 4 and 5, respectively.

Figure 4The Raman spectrum of defected 1-2-layered graphene transferred on 320 nm SiO₂. The 2D band is symmetric and appears at 2658-2659 cm⁻¹ with FWHM of 38–40 cm⁻¹ (measured in point 2) and 40–42 cm⁻¹ (measured in point 1).

Figure 5The Raman spectrum of the interlayer (point 1, Figure 1(c)) of αC after exfoliation by Scotch tape. The features observed at about 1450 and 1530 cm⁻¹ are typical for C₇₀ fullerenes.

A lot of flakes (of the order of 10²) were transferred on Si/SiO₂ and examined by Raman spectroscopy. The Raman spectra are enhanced due to interference effects caused by the SiO₂ 300 nm layer over the Si substrate, and I_2D/I_G varies in the range 3.5-6.0. However, it was impossible to isolate single-layered graphene flake (or to obtain clear Raman response of single-layered graphene) in this way. The exfoliated flakes were never transparent (see point 1 in Figure 1(b)). The best spectra were recorded from the points in a darker contrast (point 2 in Figure 1(b)), but the FWHM of 2D Raman band remains >35 cm⁻¹. Moreover, the D” band slightly overlaps with the second order of Si substrate when the spectrum is excited at 514 as well as 488 nm laser wavelengths.

After peeling the tape off the specimen, the interlayer between the upper flake and the substrate is accessed. The remaining interlayers have different optical contrasts (see Figure 1(c)) and Raman spectra: the spectrum of typically retained interlayer (point 1 in Figure 1(c)) in Figure 5 is very similar to that of turbostratic graphite (see [24]), but weak peaks of C₇₀ fullerenes (the features observed at about 1450 and 1530 cm⁻¹ (see [25, 26])) are also clearly distinguished (Figure 5). The strong modes of fullerenes C₇₀ at about 1180 and 1568 cm⁻¹ are merged with D” and G bands.

The Raman spectra (Figure 6) taken from points 2 and 3 (Figure 1(c)) are similar as they contain the most prominent modes of C₇₀ peaks at 1160, 1220, 1454, 1526, and 1565 cm⁻¹ [25, 26], nanodiamond (Nd) peaks at 1330 and 1620 cm⁻¹ (see [27]), and turbostratic graphite. The D, G, and D’ bands are found at 1335, 1590, and 1612 cm⁻¹, respectively, but in a different proportion: the spectrum from point 3 is dominated by the peaks of C₇₀ and Nd while the spectrum from point 2 is dominated by turbostratic graphite (see Figure 6). It should be also remarked that features of C₆₀ fullerenes (see, e.g., [25, 26]) were not observed.

Figure 6Raman spectra of the interlayer that remains on the substrate after exfoliation by the Scotch tape method taken from points 2 and 3 (Figure 1(c)).

As it was mentioned above, the D” band overlaps with the second-order band of Si substrate especially when the spectrum is excited at 488 and 514 nm laser wavelengths. In order to distinguish the dispersion of the D” band of several Scotch tape methods, exfoliated flakes were transferred on glass substrates. The flakes have very similar surface morphology to those transferred on SiO₂/Si substrates (Figure 1(d)). The Raman spectrum of such flakes is not significantly different from that of the as-deposited layers (Figure 7); however, the D” band appears at 1096 (for 488 nm excitation) and at 1135 cm⁻¹ (for 633 nm excitation), respectively, that is, they coincide with the data of Herziger et al. [14].

Figure 7The Raman spectra of as-grown graphene on αC excited at 488 nm (green trace) and 633 nm (black trace) wavelengths. The similar spectra of exfoliated graphene transferred on a glass substrate excited at 488 nm (blue trace) and 633 nm (red trace). The inset: magnified part of the region 900–1200 cm⁻¹.

According to the above results, we conclude that the exfoliation by the Scotch tape method does not enable splitting up between the defected graphene and the interlayers (especially the αC-designated one). Another way for exfoliation was probed (by exfoliation on epoxy resin), and the optical micrograph image of the area of the edge of a resin bubble and the Raman spectrum taken from this area (excited at 633 nm) are shown in Figures 1(e) and 8, respectively. The Raman spectrum of epoxy resin does not contain any features in the 2D region of graphene (upper trace in Figure 8); hence, 2D bands of a single- and bilayered graphene were identified at the edge of a lot of bubbles on the surface of the resin (Figure 1(e)). It should be clearly remarked that the measured FWHM of the 2D band of such single-layered graphene is about 27–29 cm⁻¹, but it is situated at 2654–2656 cm⁻¹, that is, it remains upshifted with about 10–15 cm⁻¹.

Figure 8Raman spectra of graphene films situated on air bubbles/cavities. The 2D band is situated at 2655 cm⁻¹ and has FWHM ~28 cm⁻¹ (i.e., it corresponds to single-layered graphene—blue trace).

Recently, Li et al. [28] established that the intensity of 2D band varies as a cosine to the fourth power when the laser propagation direction is parallel to the graphene layer and the polarization is rotated around it. They also derived the orientation distribution function of monolayered graphene as well as that of graphene paper and highly oriented pyrolytic graphite. We perform similar measurements in X(Y_φY_φ)X geometry, φ being the angle between the incident laser beam polarization and the graphene layer plane; Z is the axis perpendicular to the graphene plane, and the laser beam propagates transversely to the graphene layer along the X direction (see Figure 9(a)). The excitation laser beam was focused in a manner to comprise no more than 30% of the edge of the Si substrate and graphene film (Figure 9(a)). The parallel scattering geometry was used. The measurements were performed starting from φ = 0° (corresponding to X(YY)X in Porto notations) and finished at φ = 180°. The preliminary results of these rotational angle-dependent Raman measurements of as-deposited specimen are presented in Figure 9. The signal significantly drops upon changing the angle from 0° to 90° and increases again in the interval between 90 and 180° which resembles indeed the cos⁴ law. At 90° (corresponding to X(ZZ)X in Porto notations), the Raman signal is very weak but still observable (Figure 9), and the rotational angle-independent features of C₇₀ fullerenes and nanodiamond (Nd) dominate the spectrum. The residual features in the Raman spectra taken at φ = 90° point out that the measured polarized Raman spectra are taken from graphene deposited on DLC interlayer. The measurements in this scattering geometry (X(YY)X in Porto notations) access measurements of the interlayer/s without exfoliation. On the other hand, the polarized Raman study confirms the deposition of graphene because the intensities of the most prominent Raman features of graphite (D, G, and 2D bands) show similar behavior in similar conditions as those of graphene. However, the intensity of the Raman features of graphene decreases significantly slower than those of graphene as it is shown in [28].
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(b)Figure 9(a) Optical photography of the specimen in X(YY)X geometry (in Porto notations). The inset: optical photography of the specimen in Z(YY)Z geometry (in Porto notations). The arrow-remarked laser spots are eye guide showing the real area of the laser spot during measurements. The marker represents 10 μm. (b) Spatially resolved Raman spectra of as-deposited defected graphene at 633 nm excitation.

It is worth noting that the 2D band from the single-layered graphene regions is symmetric and strong, but it is somewhat broadened with FWHM of about 40–42 cm⁻¹and is blueshifted by 15–20 cm⁻¹ in as-grown specimens. It is well known that such behavior is usually related to strain (see [29–32]) and doping [33]. Moreover, Lee et al. [34] and Bouhafs et al. [35] experimentally studied the influence of these parameters on the position and FWHM of G and 2D bands in single- and bi-/multilayered graphene, respectively. The deduced simple plot of the 2D versus G band positions enables distinguishing the influence of doping and strain on the positions of G and 2D bands. In our single-layered specimens, the G band is slightly uphifted by 1-2 cm⁻¹ while the 2D band is more significantly blueshifted and broadened by 10–20 cm⁻¹. Therefore it can be assumed that the 2D band blueshift and broadening are due to the lattice strain predominantly as well as to the doping. It can be suggested that the lattice strain is due to the bonding between graphene and the interlayers while the doping should be related to charge transfer from the interlayers/interfaces to graphene as well as to different intrinsic (grain boundaries, etc.) and extrinsic (trapped nitrogen, oxygen, and impurities during the deposition) defects, that is, it can be related to the influence of the interlayers/substrates as well as of the deposition process.

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4. Conclusions

We extended the analysis of defected graphene deposited by CVD as well as the two types of interlayers between the defected graphene layer/s and Si substrates by both unpolarized and polarized Raman spectroscopy. The performed Raman spectroscopy examination of as-deposited defected graphene at 488, 514, and 633 nm excitation wavelengths enables the most of the monitored spectral features of double-resonance origin (D, D”, and 2D bands). The Raman studies of exfoliation by the so-called Scotch tape method revealed that (a) the composition of the designated DLC interlayers varies with depth: the initial layers on the Si substrate consist of a mixed phase of turbostratic graphite, nanodiamond/diamond-like carbon, and C₇₀ fullerenes while the upper ones are dominated by diamond-like carbon and some C₇₀ fullerenes and (b) the amorphous carbon interlayer is dominated by turbostratic graphite and contains a larger quantity of C₇₀ than the DLC-designated interlayers. Single- and bilayered defected graphene flakes were exfoliated on epoxy resin. The preliminary results of polarized Raman experiments show that the intensity of the 2D band varies as a cosine to the fourth power when the laser propagation direction is parallel to the graphene layer and the polarization is rotated around it which is an additional indication of the deposition of single-layered graphene. The results of Raman spectroscopic studies of as-grown and exfoliated graphene films tend to assume that the observed slight upshift of the 2D band as well as the broadening of 2D band is due to the strain and can be related to the bonding between the graphene and the interlayers, that is, it could be regarded as an influence of the interlayers between the defected graphene and the Si substrates.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

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Authors: T. I. Milenov,¹ E. Valcheva,² and V. N. Popov²

In OriginPro, the Peak Analyzer is capable of creating and subtracting baseline. There are various ways for baseline creation. You can generate baseline anchor points automatically or manually and then connect them with interpolation or fit them with a function.

Steps

Create baseline with 2nd Derivative method

1. Start a New Workbook and import the <Origin EXE Folder>\Samples\Spectroscopy\Baseline.dat. Highlight the second column. In the main menu, select Analysis: Peaks and Baseline: Peak Analyzerto open the dialog of the Peak Analyzer.
2. In the first page (the Goal page) of the Peak Analyzer, select Create Baseline as Goal. Click Next to go to the Baseline Mode page.
3. Next we are going to create an user defined baseline for this spectrum by defining anchor points. On the Baseline Mode page, select User defined from the Baseline Mode dropdown list. Check on Snap to Spectrum to make sure when you add or move a baseline anchor point, it will be pulled back onto the spectrum.
4. The first two methods in Anchor Points Finding are the most commonly used, and they can find anchor points automatically based on the derivative of the spectrum. If the baseline is approximately constant, 1st Derivative and 2nd Derivative is more powerful, otherwise, we should use 2nd Derivative. In this example, the baseline is more curly, so we use 2nd Derivative(zeros) method for locating anchor points.For other methods in Anchor Points Finding and related smoothing parameters, you can refer to the link: Baseline Mode Page.
5. Now click the Find button in the Baseline Anchor Points group. You can preview the anchor points in preview window.
6. Click Next to go to the Create Baseline page, select Interpolation in the Connect by drop-down list. In Interpolation method group, select Spline from the drop-down list. You can preview the spectrum in preview window, and then click Finish to get the baseline data.

Create baseline with ALS Method (Pro)

1. Start from the Baseline Mode page, and select the Asymmetric Least Squares Smoothing Baseline (ALS) as the baseline method. The ALS baseline can be tuned easily with a few parameters without pre-selecting any anchor points.
2. Click Next go to Asymmetric Least Squares Smoothing Baseline page, adjust the parameters to make the baseline optimal, then click Finish button to output the results table and graph.

Subtract Baseline from a Spectrum

1. If you want to subtract baseline, select Subtract Baseline as the Goal at start page.
2. After you created a baseline, click Next button to go to Subtract Baseline page.
3. Click Subtract button for previewing the subtracted data. The baseline data and subtracted spectrum will be outputted after clicking Finish button. The figures below displayed the Subtract Baselinepage and the preview of the subtracted spectrum.

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In this course the general introduction to Raman spectroscopy and microscopy will be provided and practical tips as well as examples will be given. The capability of Raman spectroscopy for the analysis of real-life samples (paint components, clays, coating materials, etc.) taken from historical and archaeological objects will be discussed.

1. Principles of Raman spectroscopy

Raman spectroscopy is widely used in the investigation of cultural heritage materials due to its high spatial resolution (typically in the range of 1 to 10 µm), large amount of obtainable information, non-destructivity and ability to perform in-situ analysis.^1,2 With Raman spectroscopy it’s possible to analyse various materials: minerals, inorganic and organic pigments, binding media, varnishes, ceramics, plastics, textile fibres etc.²

The following video explains the principles and instrumentation of Raman spectroscopy.https://www.uttv.ee/embed?id=29055

Similarly to infrared spectroscopy, Raman spectroscopy is classified as vibrational spectroscopy.³ Raman spectroscopy is based on Raman scattering (or Raman effect) that reveals the vibrational, rotational and other low frequency modes of molecules⁴. In this technique, the sample is exposed to an intense beam of monochromatic light (typically a laser beam) in the frequency range of visible, near-infrared or near-ultraviolet region.⁵ The electromagnetic radiation, interacting with a substance, can be transmitted, absorbed, or scattered⁶. When the monochromatic radiation is scattered by molecules, the majority of the radiation undergoes the common Rayleigh scattering (radiation’s  frequency/wavelength is unchanged). However, a small fraction of the scattered radiation is observed to have a slightly different frequency from that of the incident radiation. This is known as the Raman effect⁷. The Raman lines show up pairwise. The dominant Stokes lines have a lower frequency (longer wavelength) than the initial radiation, whereas the weaker (often nondetectable) anti-Stokes lines have a higher frequency (shorter wavelength).^4,5 The frequency shifts are virtually independent of the excitation wavelength and are characteristic of the particular substance/molecule. Usually one only records the relatively strong Stokes lines, which therefore are attributed a positive frequency shift. Such spectral coordinate is called the Raman shift and measured in wavenumbers (in cm^-1).⁴ See scheme in Figure 1.

Figure 1. Scheme of Raman scattering.

In Raman spectroscopy, as it is a scattering technique, samples are simply placed in the laser beam and the scattered radiation is collected and analysed⁸. Raman spectrometer measures the wavelength-dependent intensity of the inelastically scattered light.

The obtained Raman spectra are essentially vibrational spectra. Hence, if presented in the Raman shift scale, they are directly comparable to corresponding infrared absorption spectra (see Figure 2). However, Raman spectrum arises in a different manner and the rules, which vibrations are Raman-active (and thus produce signals in the spectrum), are different. It turns out that a vibration is Raman-active (i.e. revealed as a spectral line in the Raman spectrum), if the polarizability of the molecule changes during the vibration.⁷ It often happens that vibrations that are active (or give high-intensity signals) in Raman scattering are inactive (or give low-intensity signals) in the infrared, and vice versa.⁷Therefore, Raman spectra often provide complementary information to IR spectra.

Figure 2. Raman (laser 514.5 nm) and IR spectra of benzene.

1.1. Instrumentation

There are two types of Raman spectrometers: dispersive spectrometers (based on the use of diffraction grating) and interferometer containing Fourier-transform Raman spectrometers (FT-Raman)⁹.

In general the main components of Raman spectrometers are presented on the following scheme:

In Raman spectroscopy, the choice of excitation wavelength and intensity is very important. Different wavelengths are suitable for the analysis of different types of material. The wavelength will affect the Raman intensity, spatial resolution, background fluorescence, and potential damage to the sample. Almost exclusively lasers are used as excitation sources, because they are highly monochromatic, give high-intensity radiation and can be efficiently focused due to their high coherence. Only continuous wave (CW) lasers are used, as pulsed lasers easily damage the sample. Some popular CW lasers are presented in Table 1. Traditionally, laser wavelengths up to 830 nm have been used in dispersive instruments while the 1064 nm laser line has been employed in FT-Raman setups. With the availability of sensitive InGaAs array detectors, it has become meaningful to use also the 1064 nm lasers with dispersive Raman instruments.

Table 1. Laser sources for Raman spectroscopy.

Raman scattering efficiency decreases with increasing excitation wavelength as λ⁻⁴. However, short-wavelength lasers more easily induce fluorescence, absorb in the sample or cause other undesirable effects due to their high photon energy. Hence, most common laser wavelengths in Raman spectroscopy are in the visible and NIR region (such as 633 or 785 nm) which offer low fluorescence whilst retaining relatively high Raman intensity. For samples which exhibit fluorescence even under red excitation (for example dyes), the 1064 nm laser may be needed. While near-infrared lasers have a smaller photon energy, compared to visible lasers, they are usually more powerful, in order to compensate for the reduced Raman scattering efficiency. Therefore, they may still damage the sample. It is especially important for strongly absorbing (black) samples, in which case the UV/visible lasers (operating at lower intensities) may yield a stronger Raman signal.

Dispersive Raman spectrometers

A dispersive spectrometer utilizes a diffraction grating to angularly disperse the light. As a result, at the detector plane, different wavelengths become spatially separated. Nevertheless, prior to entering the spectrometer, the incoming light should go through a special edge or notch filter to suppress the primary (Raman-scattered) light and thereby reduce the scattering inside the spectrometer. A matrix detector is used to record the dispersed spectrum. Typically, a silicon-based cooled CCD is used, which is very sensitive in the visible and NIR region (up to 1100 nm).

FT-Raman spectrometers

Commercial FT-Raman spectrometers were introduced in the late 1980s¹⁰. Their operating principle is similar to that of FTIR spectrometers and is based on an interferometer. As the Raman-scattered light enters the instrument, the interferometer selectively modulates the individual spectral components by systematically changing an optical path length difference. The resulting beam of light is recorded by a point detector. FT-Raman is superior to a dispersive instrument in the near-IR region beyond 1000 nm. Commonly, the 1064 nm laser excitation along with germanium or indium gallium arsenide (InGaAs) detector is used. They also offer excellent wavelength accuracy and can potentially combine IR absorption and Raman measurement capacity in single instrument. However, FT-Raman frequently needs to use high laser intensities due to the reduced Raman scattering efficiency at longer wavelengths, which may damage the sample.

Different types of Raman Spectroscopy

A variety of Raman instruments and special techniques are used for the analysis of cultural heritage materials. The choice of instrument determines the sensitivity, spectral range and resolution, spatial resolution, availability of different excitation sources, and convenience of operation. 

• Micro-Raman spectrometer (or Raman microscope) is the most common bench-top Raman instrument. A high-resolution spectrometer (either dispersive or FT) and one or several laser sources are coupled through an optical microscope. The excitation beam is focused and the secondary emission is collected simultaneously by the microscope objective in backscattering geometry. A high-numerical aperture (NA) objective yields both a high spatial resolution and a high collection efficiency.
• Surface-enhanced Raman spectroscopy (SERS) involves inelastic light scattering by molecules placed close to nanometal surfaces, which amplify the scattering by plasmonic resonance. One approach is to study molecules adsorbed onto corrugated metal surfaces such as silver or gold nanoparticles¹¹. Another approach is to stimulate the molecules by a sharp metal tip. Such tip-enhanced Raman spectroscopy is typically implemented by combining a confocal microscope and a scanning probe microscope. 
• In Resonance Raman spectroscopy (RRS) the incident photon energy is close in energy to an electronic transition of a compound or material under examination. 
• In a portable Raman spectrometer, a miniature dispersive spectrometer and a small laser source are integrated into a portable, hand-held device. Hence, the instrument can be used to perform in situ analysis in museums, archives, also outdoors on archaeological sites for the analysis of mural or cave paintings. Such portable devices frequently employ a fiber-optic probes. 

1.2. Problems with Raman spectroscopy

Compared to IR absorption, the primary disadvantage of Raman spectroscopy is the fluorescent background (see Figure 3). As Raman scattering is inherently weak, one has to use an intense laser beam for excitation, and for many materials, this results in a strong fluorescence – either due to the material itself of impurities. Sometimes even trace impurities – if they are strongly fluorescent – can lead to disturbing fluorescence background. Fortunately, Raman lines are spectrally close to the laser beam whereas fluorescence has typically a large Stokes shift. 

Figure 3. Example of the fluorescence in the Raman spectrum of red lead containing paint.

Relative to the Raman signal, the fluorescent background can be highly intense and even the tail of the fluorescence band may obscure the Raman spectrum. Although the problem can be partially resolved by careful sample preparation, time resolved spectroscopy or coherent anti-Stokes Raman spectroscopy (CARS), there will always be experiments that remain difficult to perform.⁷

In addition to fluorescence, intense focused laser irradiation can cause heating and degradation of the sample. The problems are typical for organic, soft, photosensitive or dark/coloured materials whereas transparent inorganic materials have usually quite high damage threshold.

2. Analysis with Raman spectroscopy

In the following video Senior Research Fellow Dr. Valter Kiisk demonstrates and explains how to perform measurements with a typical micro-Raman spectrometer.https://www.uttv.ee/embed?id=29396

Identification of the composition of the studied material is often based on the comparison of its Raman spectrum with a spectral library of reference materials.¹² Different papers and books have been published from where Raman spectra or information about excitation wavelengths and list of wavenumbers in the Raman spectra are available ^5,13,14. Also a very valuable on-line database is made available by the Infrared & Raman Users Group (IRUG)– http://irug.org/ – from where different Raman (and also IR) spectra of cultural heritage materials can be obtained free of charge.

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Raman spectroscopy is a powerful vibrational technique used widely in chemistry, materials science, geology, biology, and industrial laboratories. To make the most of this analytical tool, proper software is essential for spectrum acquisition, visualization, processing, and interpretation. While many commercial solutions exist, there are also free and open tools that are capable, flexible, and ideal for researchers, students, and laboratories on a budget.

1. Spectragryph – Free Optical Spectroscopy Software

Spectragryph Official Site (free for academic and private use)

Spectragryph is a versatile and widely-used optical spectroscopy package supporting Raman, FTIR, UV-VIS, fluorescence, LIBS, and XRF formats. It allows users to import multiple vendor file formats, plot and edit spectra, perform baseline correction, smoothing, peak labeling, and automated batch processing. The software is free for academic and private use, and offers features such as spectral database search, multi-spectrum display, and hardware control for live data acquisition. Spectroscopy

Key Advantages:

• Multi-format support and drag-and-drop data handling
• Batch export, undo/redo, and interactive visualization
• Ability to integrate free spectral libraries like RRUFF mineral spectra Spectroscopy

⚠️ Note: Distribution and licensing for commercial use require a paid license; free academic licenses may need re-verification over time. Spectroscopy

2. RRUFF – Free Raman Database & Identification Tools

RRUFF Raman & Mineral Database (with download tools)

The RRUFF Project provides a comprehensive open database of Raman spectra, X-ray diffraction patterns, and chemical data for thousands of minerals. Although the original “CrystalSleuth” software (used to search and compare spectra) is bundled in legacy downloads, the RRUFF data itself can be imported into many newer tools and databases for free identification and research purposes. RRUFF

Why It’s Useful:

• Large library of reference Raman spectra for minerals
• Free access for educational and research use
• Can be used with software such as Spectragryph or third-party viewers RRUFF

3. Raman Tool Set – Free Basic Raman Processor

While not listed in your original links, Raman Tool Set is a notable free program dedicated to Raman data analysis. It supports baseline correction, normalization, smoothing, and chemometric functions like PCA and cluster analysis — all valuable for simple spectral processing without cost. Wikipedia

Commercial Software You May Compare

Though not free, these commercial solutions represent the professional standard in Raman data handling. Including them in your article gives context about what users gain by paying — and helps highlight the value of free alternatives.

Bruker OPUS

Bruker’s OPUS software is a professional spectroscopy suite supporting IR, NIR, and Raman data acquisition and evaluation. It offers advanced visualization, database tools, and compliance-ready features for regulated environments. Bruker

Thermo Scientific GRAMS/AI

GRAMS/AI is a robust spectroscopy platform often bundled with Thermo instruments. It provides extensive processing and analysis functions for Raman as well as other spectroscopies, widely used in research and industry (Thermo Scientific documentation). Invalid URL

Renishaw Raman Software

Renishaw offers dedicated software for its Raman instruments, focused on spectral acquisition, processing, and material identification. This platform integrates features for instrument control and data analysis tailored to Renishaw hardware. Invalid URL

HORIBA LabSpec 6

LabSpec 6 is a comprehensive spectroscopy suite used with HORIBA Raman systems. It includes visualization, hyperspectral mapping, baseline correction, multivariate analysis, and reporting tools. This software illustrates the advanced feature sets available in commercial packages. Spectroscopy Online

Raman-Analytik Software

Some spectrometer vendors (e.g., Raman-Analytik) provide their own analysis tools with fast fingerprinting, background removal, and database search capabilities — often bundled with hardware or available for download. Raman Analytik

Tips for Using Free Raman Software

• Combine tools with databases: Pair free analysis software (like Spectragryph) with open spectral libraries such as RRUFF for improved identification.
• Watch format compatibility: Free tools vary in supported file types — converting proprietary formats (e.g., Bruker or Renishaw raw files) may require intermediate export or converters.
• Consider workflow needs: If advanced imaging, 3D mapping, or multivariate quantification is required, commercial packages may offer higher performance — but for basic peak analysis, free tools suffice.

Introduction to Raman spectroscopy

When an electromagnetic radiation passes through a transparent medium, existing species scatter part of the beam in all directions. In 1928, C. V. Raman discovered that the wavelength corresponding to a small fraction of the radiation scattered by certain molecules was different from the wavelength of the original radiation (ie, inelastic scattering occurs). Wavelengths vary depending on the molecular structure of the compounds. Raman spectroscopy is based on the analysis of these differences to determine the molecular structure of different compounds [1].

Scattering is a physical process in which a type of radiation such as light, sound, or even a beam of moving particles (such as ions, electrons, etc.) collides with particles or different surfaces in a direct path in which He is moving and deviates and is forced to move in one or more other directions (Figure 1). Scattering usually occurs in all directions [2].
Due to the collision of light with matter, we will have two types of scattering according to the wavelength of the scattered radiation:

1. Rayleigh scattering is caused by particles that are much smaller than the wavelength of the radiation. Due to this type of scattering, the radiation wavelength does not change and is also classified as elastic scattering. The most obvious example of this type of scattering is the blue color of the sky, which occurs due to the scattering of shorter wavelengths in the visible spectrum.
2. Raman scattering, in which the initial wavelength changes due to the transfer of energy between the photons and the matter molecules, and the wavelength increases due to the loss of energy, or the wavelength decreases due to the capture of energy. Finds. The magnitude of these energy changes (whether decreasing or increasing) is proportional to the frequency of the molecular vibrations of the light scattering species. Raman scattering will be divided into two general categories. The first group, which has a longer wavelength (less energy) than the original radiation, is called Stokes, and the second group, which has a shorter wavelength (more energy) than the original radiation, is known as anti-Stokes. 2].
3. Spectrum Raman
   Figure 2 shows a part of a Raman spectrum for CCl4 species in which the sample is irradiated with a laser source with a wavelength of 488 nm. In a horizontal axis Raman spectrum, generally in terms of the scattered radiation wave number (ῡ) or, as shown below, in terms of the changes made in the scattered beam wave number (ῡ2) relative to the source radiation wave number (ῡ1), ie in terms of the wave number changes ( 2) (which in practice indicates the scatter created in a specific wave number). While the vertical axis shows the intensity of the peaks in relative terms. Note that the relationship between the wave number of a radiation and its wavelength (λ), frequency (υ) and energy (E) is as follows and has a unit of cm-1:
   ῡ = 1 / λ and ῡ = υ / c
   E = hυ = hcῡ

As can be seen in the figure below, the Stokes lines are more intense, which is justified by their higher probability of occurrence, as photons are more likely to lose energy due to contact with the material environment than to receive them. Is energy. Another thing to keep in mind is that the amount of Raman Shifts (written numerically above the peaks) is independent of the laser wavelength used to excite the sample. It should also be noted that Riley scattering is located exactly at the wavelength equal to the source wavelength, its displacement rate is zero and its intensity is much higher than the Stokes and anti-Stokes lines [2].

Before continuing the discussion, it is necessary to point out that due to the continuity of the material, in order to better understand the following sections, it is better to first read the article on infrared spectroscopy. Below, due to the great similarity and complementarity of infrared and Raman spectroscopy techniques, a comparison is made on the differences.

3. Investigation of differences between Raman technique and infrared spectroscopy
   Studies have shown that shifts in the wavelength (wave number) of the source due to Raman scattering are in the infrared spectral range. In simpler terms, the difference between the energy of the source radiation and the scattered radiation is equal to the energy of the waves in the middle infrared range (see the article Infrared Spectroscopy). As mentioned in the article on infrared spectroscopy, this amount of energy is sufficient only for transitions between molecular vibrational levels of molecules (Molecular Vibrational Levels), and in this respect two methods are similar to each other. The Raman scattering spectrum and the infrared spectrum for a particular species are often very similar. There are many similarities between the two methods, but it should be noted that despite these similarities, the two techniques are different in principle and theory in that they are usually used as a complement to each other. In the paper introducing the infrared spectroscopy method, it is mentioned that one of the necessary conditions for a particular bond to be active in infrared spectroscopy is to cause a net change in dipole moment due to the absorption of radiation (Refer to the main article).

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It is the shift in wavelength of the inelastically scattered radiation that provides the chemical and structural information. Raman shifted photons can be of either higher or lower energy, depending upon the vibrational state of the molecule under study. A simplified energy diagram that illustrates these concepts is shown below.

Stokes radiation occurs at lower energy (longer wavelength) than the Rayleigh radiation, and anti-Stokes radiation has greater energy. The energy increase or decrease is related to the vibrational energy levels in the ground electronic state of the molecule, and as such, the observed Raman shift of the Stokes and anti-Stokes features are a direct measure of the vibrational energies of the molecule. A schematic Raman spectrum may appear as shown below.

The energy of the scattered radiation is less than the incident radiation for the Stokes line and the energy of the scattered radiation is more than the incident radiation for the anti-Stokes line. The energy increase or decrease from the excitation is related to the vibrational energy spacing in the ground electronic state of the molecule and therefore the wavenumber of the Stokes and anti-Stokes lines are a direct measure of the vibrational energies of the molecule.

In the example spectrum, notice that the Stokes and anti-Stokes lines are equally displaced from the Rayleigh line. This occurs because in either case one vibrational quantum of energy is gained or lost. Also, note that the anti-Stokes line is much less intense than the Stokes line. This occurs because only molecules that are vibrationally excited prior to irradiation can give rise to the anti-Stokes line. Hence, in Raman spectroscopy, only the more intense Stokes line is normally measured – Raman scattering is a relatively weak process. The number of photons Raman scattered is quite small. However, there are several processes which can be used to enhance the sensitivity of a Raman measurement.

Simplified energy diagram

If the wavelength of the exciting laser coincides with an electronic absorption of a molecule, the intensity of Raman-active vibrations associated with the absorbing chromophore are enhanced by a factor of 102 to 104. This resonance enhancement or resonance Raman effect can be extremely useful, not just in significantly lowering the detection limits, but also in introducing electronic selectivety. Thus the resonance Raman technique is used for providing both structural and electronic insight into species of interest.

Metalloporphyrins, carotenoids and several other classes of biologically important molecules have strongly allowed electronic transitions in the visible, making them ideal candidates for resonance Raman spectroscopy. Resonance selectivity has a further practical use, in that spectrum of the chromophoric moiety is resonance enhanced and that of the surrounding environment is not. For biological chromophores, this means that absorbing active centres can be specifically probed by visible excitation wavelengths, and not the surrounding protein matrix (which would require UV lasers to bring into resonance).

Resonance Raman spectroscopy is also an important probe of the chemistry of metal centred complexes, fullerenes, polydiacetylenes and other “exotic” molecules which strongly absorb in the visible. Although many more molecules absorb in the ultraviolet, the high cost of lasers and optics for this spectral region have limited ultraviolet (UV) resonance Raman spectroscopy to a small number of specialist groups.

Schematic Raman spectrum

Vibrations which are resonantly enhanced fall into two or three general mechanistic classes. The most common case is Franck-Condon enhancement, in which a component of the normal coordinate of the vibration occurs in a direction in which the molecule expands during an electronic excitation. The more the molecule expands along this axis when it absorbs light, the larger the enhancement factor. The easily visualized ring breathing (in-plane expansion) modes of porphyrins fall into this class. Vibrations which couple two electronic excited states are also resonantly enhanced, through a mechanism called vibronic enhancement. In both cases, enhancement factors roughly follow the intensities of the absorption spectrum. The fuller theory of resonance enhancement is beyond the scope of this section.

Resonance enhancement does not begin at a sharply defined wavelength. In fact, enhancement of 5x to 10x is observed if the exciting laser is within even a few 100 wavenumbers below the electronic transition of a molecule. This “pre-resonance” enhancement can be experimentally useful.

The Raman scattering from a compound (or ion) adsorbed on or even within a few Angstroms of a structured metal surface can be 103 to 106x greater than in solution. This surface-enhanced Raman scattering is strongest on silver, but is observable on gold and copper as well. At practical excitation wavelengths, enhancement on other metals is unimportant.

SERS arises from two mechanisms:

1. The first is an enhanced electromagnetic field produced at the surface of the metal. When the wavelength of the incident light is close to the plasma wavelength of the metal, conduction electrons in the metal surface are excited into an extended surface electronic excited state called a surface plasmon resonance. Molecules adsorbed or in close proximity to the surface experience an exceptionally large electromagnetic field. Vibrational modes normal to the surface are most strongly enhanced.
2. The second mode of enhancement is by the formation of a charge-transfer complex between the surface and analyte molecule. The electronic transitions of many charge transfer complexes are in the visible, so that resonance enhancement occurs. Molecules with lone pair electrons or pi clouds show the strongest SERS. The effect was first discovered with pyridine.

Other aromatic nitrogen or oxygen containing compounds, such as aromatic amines or phenols, are strongly SERS active. The effect can also be seen with other electron-rich functionalities such as carboxylic acids. The intensity of the surface plasmon resonance is dependent on many factors including the wavelength of the incident light and the morphology of the metal surface. The wavelength should match the plasma wavelength of the metal. This is about 382 nm for a 5μm silver particle, but can be as high as 600nm for larger ellipsoidal silver particles. The plasma wavelength is to the red of 650nm for copper and gold, the other two metals which show SERS at wavelengths in the 350-1000 nm region. The best morphology for surface plasmon resonance excitation is a small (<100nm) particle or an atomically rough surface. SERS is commonly employed to study monolayers of materials adsorbed on metals, including electrodes.

Other popular surfaces include colloids, metal films on dielectric substrates and, recently, arrays of metal particles bound to metal or dielectric colloids through short linkages. Although SERS allows easy observation of Raman spectra from solution concentrations in the micromolar (10x-6) range,non-reproducability of quantitative measurements has in the past marred its utility for analytical purposes. However, standardization in production of SERS active media is steadily improving its potential in this area also.

UVRRS is a powerful tool in the molecular analysis of complex biological systems. Most biological systems absorb UV radiation and hence have the ability to offer resonance with UV Raman excitation. This results in the highly selective resonance Raman effect enabling enhancement of important biological targets such as protein or DNA. For example, excitation around 200nm enhances the Raman peaks from vibrations of amide groups; excitation around 220nm enhances peaks from certain aromatic residues. The Raman scatter from water is weak, allowing for analysis of very weak aqueous systems.

Fiber optic UVRRS configuration

Due to the selective nature of UVRRS, a tunable laser is typically required as the excitation source. Since truly tunable continuous-wave lasers are not yet available, a Nd:YAG-pumped dye laser with frequency-doubled output is one suitable UVRRS system. Depending on the dyes used, this laser setup can give almost any required UV wavelength. Intensified CCDs (ICCDs) with UV photocathodes, back-illuminated CCDs or CCDs with UV enhancing (BASF lumogen)coatings can be used as detectors for UVRRS. These detectors are used on account of their high detection efficiency and multichannel capabilities. The primary obstacle to the merging of the worlds of UVRRS and fiber-optic spectroscopy is solarization, the process by which UV radiation causes opacity of fiber-optics (even quite pure silica fibers). This opacity impairs transmission, rendering standard fiber-optics useless for UVRRS.

Species of Interest

Pulsed lasers are typically utilized in the study of short-lived species. A laser pulse can be supplied to a molecular system with enough energy to redistribute the electrons in a molecule causing the formation of an excited state as illustrated on the right. The Raman spectrum of this excited state molecule can be studied either using the same laser pulse or a different pulse from a second laser (single color and two-color pulsed Raman). Excited states of interest can have lifetimes, from picoseconds to milliseconds, but the majority can be studied using gating in the order of 5ns. As the majority of excited states are generated using UV and visible lasers, photocathodes with high UV and visible Quantum Efficiencies (QEs) are typically suitable.

Schematic of pump-probe (two color) Raman

The simplest pulsed laser experiments are so-called single-color experiments where high irradiance laser pulses are used both to initiate the photoreaction, and then to Raman probe the transient species created within the pulse width. By opening the intensifier tube as shown on the right, only the Raman spectrum of the excited state will be recorded. This pulse/ICCD gate combination will be repeated and accumulated hundreds to thousands of times in order to achieve a good overall signal-to-noise ratio with high dynamic range.

In Time Resolved Resonance Raman (TR3) spectroscopy, pairs of laser pulses of different wavelength are used to photolyse (optically “pump”) and then to Raman probe the transient species of interest. The spectral window of the spectrograph/detector is chosen so that it corresponds to the frequency range of the Raman scattering from the probe laser.

Pulsed two color Raman layout with delays under the control of a delay generator

In Time Resolved Resonance Raman (TR3) spectroscopy, pairs of laser pulses of different wavelength are used to photolyse (optically “pump”) and then to Raman probe the transient species of interest. The spectral window of the spectrograph/detector is chosen so that it corresponds to the frequency range of the Raman scattering from the probe laser.

The time evolution of the transient signal is monitored by recording a series of spectra at different delays after the photolysis event, i.e. at a series of time delays between the excitation and probe pulses. The ICCD camera or either of the lasers can supply the trigger. A delay generator is used to control the delays.

In Raman microscopy, a research grade optical microscope is coupled to the excitation laser and the spectrometer, thus producing a platform capable of obtaining both conventional images and in addition generating Raman Spectra from sample areas approaching the diffraction limit (~1 micron). Imaging and spectroscopy can be combined to generate “Raman cubes”, 3- dimensional data sets, yielding spectral information at every pixel of the 2D image.

A motorized xyz microscope stage can be used to automatically record spectral files, which will constitute the basis of Raman images, Raman maps or a set of Raman spectra recorded from preselected points. Specific software routines will allow the quick and easy reconstruction of these maps. The possibility of generating two-dimensional and three-dimensional images of a sample, using various special features, is an evident advantage over either traditional spectroscopy or microscopy.

Time delay sequences

The first ever Raman “instrument” was constructed in 1928. This instrument used monochromatized sunlight as a light source and a human eye as a detector. Raman instrumentation was developed (based around arc lamps and photographic plates) and soon became very popular up until the 1950s. Since these early days, Raman instrumentation has evolved markedly. Modern instrumentation typically consists of a laser, Rayleigh filter, a few lenses, a spectrograph and a detector (typically a CCD or ICCD).

Typical Continuous Wave (CW) Raman layout

One of the major advantages of dispersive Raman is that it offers the possibility to select the optimal laser excitation wavelength to permit the recording of the best Raman information. For example, wavelengths can be selected to offer the best resonance with the sample under investigation.

One might also need to tune wavelength to avoid fluorescence and thermal emission backgrounds. Nowadays, it is possible to use laser lines from UV, (down to 200nm) up to the infrared, (1.06μm Nd:YAG laser line), from microWatts up to several Watts.

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Light interacts with matter in different ways, transmitting through some materials, while reflecting or scattering off others. Both the material and the colour (wavelength) of the light affect this interaction. We call the study of this light ‘spectroscopy’. Which parts of the visible spectrum enter our eyes determines which colours we perceive.

A substance might appear blue, for example, if it absorbs the red parts of the spectrum of light falling upon it, only reflecting (or scattering) the blue parts into our eyes.

Raman spectroscopy looks at the scattered light

If you were to shine blue light—from just one part of the spectrum—onto the material, you might expect to just see blue light reflected from it, or no light at all if it is completely absorbed (i.e. a black material).

However, by using a Raman spectrometer, you can see that often a very tiny fraction of the scattered light has a different colour. It has changed frequency because, during the scattering process, its energy changed by interacting with molecular vibrations. This is the Raman scattering process, named after its discoverer, the famous Indian physicist C.V. Raman. He was awarded the 1930 physics Nobel Prize for this great discovery.

By studying the vibration of the atoms we can discover the chemical composition and other useful information about the material.

The Raman effect is very weak; only about 1 part in 10 million of the scattered light has a shifted colour. This is too weak to see with the naked eye, so we analyse the light with a highly sensitive spectrometer.

Raman spectrometers

These systems consist of:

• one or more single coloured light sources (lasers)
• lenses (both to focus the light onto the sample and to collect the scattered light)
• filters (to purify the reflected and scattered light so that only the Raman light is collected)
• a means of splitting the light into its constituent colours (normally a diffraction grating or prism)
• a very sensitive detector (to detect the weak light)
• a device such as a computer to control the whole system, display the spectrum and enable this information to be analysed

Raman scattering offers significant advantages for the investigation of materials over other analytical techniques, such as x-raying them or seeing how they absorb light (e.g. infrared absorption or ultraviolet absorption).

aman spectroscopy reveals the chemical and structural composition of samples. Generally, all materials produce Raman spectra, with the exception of pure metals.

Raman scattering

Raman scattering occurs when light interacts with molecular vibrations. This is similar to the more widely known infrared absorption spectroscopy, but different rules apply. A change in molecular polarisability is required during the vibration for the Raman effect to occur.

You will see some vibrations in the Raman spectrum that are not visible in the infrared spectrum, and vice-versa, because of the different selection rules. For example, Raman spectroscopy is superb for studying the carbon atoms that make up the structure of diamond, unlike infrared absorption spectroscopy.

Scattered light

The first step in producing a Raman spectrum is to illuminate your sample with a monochromatic light source, such as a laser.

Most of the light that scatters off is unchanged in energy (‘Rayleigh scattered’). A minute fraction—perhaps 1 part in 10 million—has lost or gained energy (‘Raman scattered’). This Raman shift occurs because photons (particles of light) exchange part of their energy with molecular vibrations in the material.

Where energy is lost the Raman scattering is designated as ‘Stokes’; where energy is gained the Raman scattering is designated as ‘anti-Stokes’. We rarely use anti-Stokes Raman light as it is less intense than the Stokes, however it does represent equivalent vibrational information of the molecule.

Vibrating atoms

The change in energy depends on the frequency of vibration of the molecule. If it is very fast (high frequency)—light atoms held together with strong bonds—the energy change is significant. If it is very slow (low frequency)—heavy atoms held together with weak bonds—the energy change is small.

Raman spectrometers

Renishaw inVia systems consist of:

• single or multiple lasers, from UV (244 nm) to IR (1064 nm) – switch with a single click
• high quality objective lenses, from high confocal 100× to long working distance and immersion options
• custom designed motorised spectrometer lenses­ – automatically align for each configuration
• laser-line-specific Rayleigh filters with a dual filter arrangement to optimise sensitivity
• highest quality master diffraction gratings provide exceptional dispersion and longevity
• thermoelectrically cooled (- 70 ºC) CCD detector – stable and sensitive
• high specification multi-core PC for data collection and analysis

Raman spectra

We graphically depict the results of our measurements as Raman spectra. We plot the intensity of the scattered light (y-axis) for each energy (frequency) of light (x-axis). The frequency is traditionally measured in a unit called the wavenumber (number of waves per cm, cm^-1).

We plot the x-axis frequencies relative to that of the laser as it is the shift in energy of the light that is of particular interest.

How do I get the information I want from my spectrum?

You can tell a great deal about a material from its Raman spectrum, with different features relating to different aspects of the material.

The key features are:

The Raman shifts and relative intensities of all of the Raman bands of the material
With this, we can identify the material.

Individual band changes
A band may shift, narrow or broaden, or vary in intensity. These changes can reveal information about stresses in the sample, variations in crystallinity, and the amount of material respectively.

Variations in spectra with position on the sample
This will reveal changes in the uniformity (homogeneity) of the material. You can analyse at several arbitrary points, or systematically measure an array of points (enabling the production of images of composition, stress, crystallinity, etc.)

What do the Raman bands represent?

It is easy to understand the Raman spectrum of crystals with a regular array of identical atoms, all in the same configuration (such as the carbon atoms in diamond). In these cases, you often see just one dominant Raman band (because there is just one molecular environment of the crystal).

The Raman spectrum of polystyrene, however, is much more complex because the molecule is less symmetric and has hydrogen atoms in addition to carbon atoms. There are also different bond types connecting the atoms.

Vibration frequencies

The frequencies of vibration depend on the masses of the atoms involved and the strength of the bonds between them. Heavy atoms and weak bonds have low Raman shifts. Light atoms and strong bonds have high Raman shifts.

We see the high frequency carbon-hydrogen (C-H) vibrations in the polystyrene spectrum at about 3000 cm^-1. The low frequency carbon-carbon (C-C) vibrations are at around 800 cm^-1. The C-H vibrations have a higher frequency than the C-C vibrations because hydrogen is lighter than carbon.

We see the vibrations of two carbon atoms linked by strong double bonds (C=C) at around 1600 cm^-1. This is at a higher frequency than two carbon atoms lined by a weaker single bond (C-C, 800 cm^-1).

You can use these simple rules to explain many of the features of Raman spectra.

Vibrations in detail

You can see more subtle effects if you inspect spectra closely. The strength of bonds also affects their vibration rates. For example, the C-H vibrations of polystyrene appear in two bands, at approximately 2900 cm^-1 and 3050 cm^-1. The carbons in the former are part of carbon chains (‘aliphatic’), whereas the carbons in the latter form part of carbon rings (‘aromatic’).

You can view the vibrations of a complex molecule as partly consisting of many simple diatomic vibrations. However the full richness of the Raman spectrum can only be understood by considering the vibrations of larger groups of atoms (such as the expanding/contracting ‘breathing mode’ of the aromatic carbon ring that appears at 1000 cm^-1 in polystyrene).

Low frequency vibrations

You can also study Raman bands with low Raman shifts, below 100 cm^-1. These originate from very heavy atoms or very large-scale vibrations, such as the whole crystal lattice vibrating. Renishaw’s Raman instruments enable you to study these modes and explore a wide range of materials and crystals, and distinguish between different crystalline forms (polymorphs).

The big picture

A Raman spectrum therefore consists of a range of features, each associated with a vibrational mode. The spectrum is unique to the material and enables you to identify it. It is important to note that, although a full understanding of the vibrational modes is of interest, you rarely need this as you can use a reference database for identification.

When a sample is illuminated by a laser, both Raman scattering and photoluminescence (PL) can occur. The latter can be many times stronger than the former and can prevent successful Raman analysis.

PL comprises both fluorescence and phosphorescence processes and originates from an absorption/emission process between different electronic energy levels in the material. The amount and type of PL depends on which material you are studying and which laser wavelength you are using. Unwanted fluorescence interference can normally be avoided by choosing an appropriate laser wavelength.

• Energy diagram showing absorption of light and the processes involved in the emission of light as fluorescence and phosphorescence.

What PL can tell us

In many cases photoluminescence carries useful information that can facilitate sample analysis and augment the Raman data. inVia confocal Raman microscopes are suited to the analysis of both Raman scattering and PL.

Fluorescence imaging (a type of PL) is often employed in the biological sciences, where fluorescent tags are used to reveal the presence and distribution of molecular species. However, this approach is more invasive than Raman analysis, which is typically tag-free. Renishaw’s inVia confocal Raman microscope can be used to generate images of fluorescent tags, but more commonly provides valuable tag-free chemical information.

You can also use PL to study crystal defects, such as atomic vacancies and substitutions. This is of particular importance for materials such as diamond and silicon carbide (SiC). Not only can you identify the defect, but you can also tell if the crystal has internal stresses.

• Stress image generated from the ruby R2 PL band position

How to avoid PL backgrounds

Occasionally PL bands are strong and broad, masking Raman information. You can counter this by using a different laser wavelength. This can move the Raman bands away from the peak emission of the PL band and may even avoid generation of the PL entirely.

Ideally, a Raman instrument should be able to switch rapidly and easily between different laser wavelengths, so that you can select or avoid PL features, depending on your requirements.

Raman images (sometimes referred to as maps) depict a variation in spectral information from different points on, or in your sample. They can take the form of one-dimensional profiles, two-dimensional images, or three-dimensional rendered volumes. With them, you can rapidly see how a Raman parameter alters with position.

The parameter could be as simple as the intensity of a particular Raman band, or you could derive it from a more complicated analysis of the whole Raman spectrum.

The two main methods of collecting the spectral data to generate these images are Raman mapping and Raman imaging.

• White light and Raman images of washing powder

Raman mapping

Raman mapping collects a spectral hypercube (a Raman spectrum from each position on the sample in a single file), rather than a simple intensity image. The hypercube is analysed to produce Raman images.

There are several Raman mapping methods, such as:

• Point-by-point mapping
  The laser is focused to a spot. A motorised stage moves the sample under the laser. Spectra are sequentially acquired from an array of sample points spanning the defined region of interest. Fast versions of this are Renishaw’s StreamHR™ and StreamHR Rapide.
• Line focus mapping
  This is similar to point-by-point mapping, but the laser illuminates a line on the sample, rather than a spot. This enables you to simultaneously collect spectra from multiple positions on the sample, saving time. With this method you can use higher laser powers without damaging the sample (reducing exposure times). Renishaw’s StreamLine™ is a sophisticated modern implementation of this concept.

It is important to consider the potentially undesirable effects of undersampling when mapping. This is most clearly illustrated when point-by-point mapping: parts of the sample will be ‘missed’ if the laser spot is smaller than the spacing between acquisition points. Renishaw has solved this problem through the use of the StreamLine™ Slalom mode.

Generating Raman images from map data

Once all the Raman spectra are collected from the mapping experiment, they can be analysed to produce profiles, images or rendered volumes. Analysis options in Renishaw’s WiRE software include:

• Intensity at one frequency in the spectrum
  This produces an equivalent image to that from Raman imaging. These are quick to generate but may be misleading because it is not possible to differentiate between intensities arising from a Raman band of interest and those associated with a broad background fluorescence.
• Curve fit parameters
  All the spectra in the set have a theoretical curve fitted to one of the Raman bands. Images are then made based on the theoretical curve parameters for each spectrum. Images are often made using the centre frequency of the curve (band), or the full width at half maximum (FWHM), as this is sensitive to stresses and crystallinity within the sample respectively.
• Multivariate parameters
  Images can be generated using chemometric tools, such as generic principal component analysis (PCA), or Renishaw’s Empty Modelling™, which is optimised for Raman data. The Empty Modelling method reveals systematic variations between the Raman spectra, and highlights the distribution of these variations across the sample as an image. This is achieved without the need for prior knowledge of what is present within the sample, which greatly simplifies the analysis process. Multivariate analysis is very powerful because it uses information from the entire spectrum, not just one part of it (intensity at one frequency) or one curve-fitted band. This typically results in higher quality Raman images.

Raman imaging

Raman imaging is analogous to taking a photograph; spectral intensity values are collected simultaneously from the entire area of interest. The laser illuminates a square or circular region on the sample. The light is filtered so that the intensity of just one narrow part of the spectrum is recorded on the detector.

The single image collected contains limited information, just the intensity of the light at that frequency. However, these images can be acquired rapidly. This is especially true if you have a high power laser; because the light is spread over an area, you can use all the power without damaging your samples, with correspondingly short exposure times.

Two-dimensional images are typically produced using this method. Renishaw’s True Raman Imaging is an example of Raman imaging.

Note that it is possible to collect intensity values covering multiple points of the spectrum by using multiple and/or tuneable filters.

Spatial resolution

Point-by-point Raman mapping

Spatial resolution is determined by a combination of the laser spot size and the spacing between acquisition points on the sample.

• Laser spot size
  This is a function of the objective magnification and the laser wavelength (higher magnification and shorter wavelengths produce smaller spot sizes)
• Spacing between acquisition points on the sample (sampling)
  This is a function of the sample stage (ideally stages should have a large travel range while still enabling a step size down to 100 nm, smaller than the smallest spot size)

Raman imaging

Spatial resolution is determined by the magnification of the optics in the system and the size of the elements in the detector. Ultimately this is limited, by the inherent wavelike nature of light, to a little under a micrometre.