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The Scanning Electron Microscope (SEM) produces images by probing the specimen with a focused electron beam that is scanned across a rectangular area of the specimen (raster scanning).

There are two families of electron guns:

• Conventional thermionic emitters such as Tungsten (W) or Lanthanum hexaboride (LaB6) tipped filaments.
• Tungsten field emission gun (FEG) , warm or Cold FEG. A pointed emitter is held at several kilovolts (2000-7000 V) so that there is sufficient potential at the emitter surface to cause field electron emission.

Field emission gun (FEG) is used to produce an electron beam that is smaller in diameter, more coherent and up to three orders of magnitude greater current density or brightness.

scrollable

Energy of electrons is depending of Voltage: 1 Kev to 50KeV

Current (A): Number of electrons /unit of time

1 amp = 1 coulomb/sec 1 coulomb ~ 6 x10¹⁸ electrons

Example if the current measured at sample is around 10^-9A to 10^-12 A then the number of electrons is around 6X10⁶ to 6X10⁹ electrons/sec.

Environmental Scanning Electron Microscope (ESEM)

ESEM is a variety of SEM called environmental scanning electron microscope. It can produce images of sufficient quality and resolution with the samples being wet or contained in low vacuum or gas. This greatly facilitates imaging biological samples that are unstable in the high vacuum of conventional electron microscopes. The major disadvantage of transmission electron microscope is the need for extremely thin sections of the specimens, typically about 100 nanometers. Biological specimens are typically required to be chemically fixed, dehydrated and embedded in a polymer resin to stabilize them sufficiently to allow ultrathin sectioning. Sections of biological specimens, organic polymers and similar materials may require special treatment with heavy atom labels in order to achieve the required image contrast.

ESEM is especially useful for non-metallic, uncoated and biological materials. The presence of gas, mainly Argon, around a sample permits to work with pressure greater than 500 Pa compared to conventional SEM requirements samples under vacuum about 10-3 to 10-4 Pa. This vacuum level creates the possibility to operate on non-conductive samples without any preparation or hydrated specimens without charging.

Transmission Electron Microscope (TEM)

In a Transmission Electron Microscope (TEM), the electron beam is accelerated by an anode typically at +100 keV (40 to 400 keV) with respect to the cathode, focused by electrostatic and electromagnetic lenses, and transmitted through the specimen that is in part transparent to electrons and in part scatters them out of the beam. When it emerges from the specimen, the electron beam carries information about the structure of the specimen that is magnified by the objective lens system of the microscope.

The spatial variation in this information (the “image”) may be viewed by projecting the magnified electron image onto a fluorescent viewing screen coated with a phosphor or scintillator material such as zinc sulfide. Alternatively, the image can be photographically recorded by exposing a photographic film or plate directly to the electron beam, or a high-resolution phosphor may be coupled by means of a lens optical system or a fiber optic light-guide to the sensor of a digital camera. The image detected by the digital camera may be displayed on a monitor or computer.

A transmission electron microscope can achieve better than 50 pm resolution and magnifications of up to about 10,000,000x whereas most light microscopes are limited by diffraction to about 200 nm resolution and useful magnifications below 2000x. Generally, the image resolution of an SEM is at least an order of magnitude poorer than that of a TEM. However, because the SEM image relies on surface processes rather than transmission, it is able to image bulk samples up to many centimeters in size and (depending on instrument design and settings) has a great depth of field, and so can produce images that are good representations of the three dimensional shape of the sample.

The Scanning Transmission Electron Microscope (STEM) rasters a focused incident probe across a specimen that (as with the TEM) has been thinned to facilitate detection of electrons scattered through the specimen. The high resolution of the TEM is thus possible in STEM. The focusing action (and aberrations) occurs before the electrons hit the specimen in the STEM, but afterward in the TEM.

Focused ion beam (FIB)

Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. A FIB setup is a scientific instrument that resembles a scanning electron microscope (SEM). However, while the SEM uses a focused beam of electrons to image the sample in the chamber, a FIB setup uses a focused beam of ions instead. Unlike an electron microscope, FIB is inherently destructive to the specimen.

When the high-energy gallium ions strike the sample, they will sputter atoms from the surface. Gallium atoms will also be implanted into the top few nanometers of the surface, and the surface will be made amorphous. A FIB-SEM consists in a system with both electron and ion beam columns, allowing the same feature to be investigated using either of the beams. A FIB-SEM system uses a beam of Ga+ ion to mill into the surface to locate a feature or defect of interest. The integrated SEM then uses a focused beam of electrons to image the sample in the chamber.

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1. XPST

XPST is a program package for the analysis of X-ray Photoelectron Spectroscopy (XPS) data. It includes various graphical interfaces as well as commandline functions to facilitate the workup of XPS data.

When a XPST fit project is started, a corresponding subfolder with all required data is generated and saved within the Igor experiment. You can generate fit templates and you can export entire fit projects to share them with your co-workers. As a special feature, a flexible multiplet function was implemented to facilitate a convenient analysis of complex spectra. XPST can handle any number of peaks.

There is also a youtube channel with tutorials.

Several changes in the newest version of XPST were made according to this book about programming Igor.

Installation

XPST was initially developed with Igor 5, but a major revision was made with Igor 7. XPST works also nicely with Igor 8. The Igor 7/8 version is not compatible with Igor 6. If you still run Igor 6, you have to download a previous release.

• Unpack the .zip file
• Copy/Move the folder ‘XPST’ to the folder ‘Igor Procedures’ in Igor’s main folder
• Copy/Move the folder ‘XPSTHelp’ to the folder ‘Igor Help Files’ in Igor’s main folder
• Restart Igor

Before you upgrade to a newer version of XPST, please remove all files associated with your old version from Igor’s folders.
 

Stay in touch …

If you want to stay informed about updates or other issues, just send a blank mail to: xpst_update@freenet.de
This is not a support contact – it only serves to keep you informed about changes.
You can sign off anytime you want using the same address.
 

More functions …

Besides the graphical interfaces, XPST comes with several commandline functions. For example:

• WhereIs() …. returns the absolute path of a selected wave (so it can be easily found)
• WaveOverlap() …. computes the overlap of two selected waves and saves it to a new wave
• CursorCut() …. cuts out regions from a selected wave

Limitations

• Data with ‘kinetic energy’ as x-axis (in waveform format or not) can not be analyzed with the Fit Assistant. Only a positive ‘binding energy’ scale works – however, this should be the most common case. If there is a strong demand for kinetic energies, it could be implemented in future versions.

See: https://www.wavemetrics.com/project/XPStools

2. CasaXPS

CasaXPS processing software offers powerful analysis techniques for both spectral and imaging data. The system originally designed for XPS and Auger data now offers features covering a wide range of analytical techniques including ToF SIMS, dynamic SIMS and many more.

Features in CasaXPS include:

·         Full quantification including transmission correction.

·         Configurable quantification reports.

·         Background type ranging from the standard Linear, Shirley and Tougaard to user-defined cubic-spline approximations.

·         Asymmetric and symmetric line-shapes: Doniach-Sunjic, Voigt, Gaussian-Lorentzian and line-shapes defined from data.

·         Easy-to-use propagation of processing, annotation and peak models to other data.

·         Batch processing for repetitive tasks, including configurable processing, display layout with automatic printing and report generation.

·         State-of-the-art image processing for XPS spectromicroscopy offering quantified chemical-state XPS images.

See: http://www.casaxps.com/

3. Originlab

Origin provides powerful and versatile tools such as Peak Analyzer, Quick Peaks Gadget, Integration Gadget, etc. for baseline correction, peak detection, peak integration and peak fitting.

Origin provides baseline detection and subtraction. Key features include:

• Create baseline with many baseline modes
  • Commonly used baseline types such as Constant, Straight line, Use Existing Dataset, End Points Weighted, Min&Max
  • Automatically find anchor points for baseline, modify the points, create baseline by interpolation or fitting
  • Create baseline for XPS using Shirley or Tougaard method and adjust corresponding parameters to optimize
  • Create baseline using Asymmetric Least Squares (ALS) Smoothing method and adjust corresponding parameters to optimize
• Subtract baseline

Origin allows you to search for peaks including hidden (“convoluted”) peaks and filter out unwanted peaks or manually add or remove peaks.

• Savitzky-Golay smoothing on the spectrum before peak finding

In Origin, you can integrate data with multiple peaks, to obtain peak areas, FWHM and other peak characteristics. Baseline subtraction is supported before peak integration. 

Available options include: 

• Directly select desired data range on graph
• Instantly view results of peak area and FWHM
• Subtract baseline from peak data
• Auto detect peak positions
• Auto determine peak widths for overlapped peaks
• Individually set peak widths
• Fit peaks and obtain fitted peak areas

Origin provides many tools to perform peak fitting: 

• Quick Peaks Gadget  : Visually correct baseline, find and fit peaks
• Multiple Peak Fit: Manually pick peak positions and fit peaks with same function. No baseline correction
• Peak Analyzer: Correct baseline, find peaks and fit by Peak Analyzer wizard
• Nonlinear Curve Fit Dialog: Fit multiple peaks with replicas in the nonlinear curve fit dialog

Available options for peak fitting include:

• Fit peaks with built-in or user-defined functions
• Fit multiple peaks with different functions
• Control fitting process using bounds and constraints
• Fix or share peak parameters
• Vary baseline parameters along with peak fitting

See: https://www.originlab.com/index.aspx?go=Products/Origin/DataAnalysis/PeakAnalysis#Peak_Fitting_PRO

4. XPSPEAK

Free, fully featured, software for the analysis of XPS spectra written by Raymund Kwok.XPSPeak is a XPS Peak Fitting Program.The portable app creates a sandbox folder in its current location, where it stores all its settings and temporary files. Can be downloaded from the US, UK or Hong Kong.

See: https://xpspeak.software.informer.com/4.1/

5. Unifit

See: https://home.uni-leipzig.de/unifit/downloads.htm

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What is XPS?

XPS is a surface-sensitive technique based on the photoelectric effect, which occurs when atoms or molecules are irradiated by photons of suitable energy, resulting in the ejection of electrons. The kinetic energy of the ejected electrons depends upon the elemental core level from which they originated.

Using this information, XPS data can be used to determine the elemental composition of the surface and, in most cases, the bonding environment of the elements.

The schematic figure shown to the right, illustrates the XPS procedure, where x-rays are used to excite and eject photoelectrons from a sodium chloride molecule on a substrate.

XPS detects and quantifies the ejected photoelectrons, which are proportional to the amount present in the uppermost layers of the surface.

To understand more about these uppermost layers, further details on the XPS penetration depth and attenuation length of the X-ray photons are required.

Normal XPS can provide information from the top 10nm (approximately) layer of the surface.

Schematic diagram of photoelectron emission from a sodium atom under x-ray exposure, and (right) example of XPS survey scan spectra of sodium chloride with the Na1s electron XPS peak highlighted in the spectra.

Image provided courtesy of Thermo Fisher Scientific.

Theoretically, XPS should be able to detect all elements. However, helium (He) does not readily form solid compounds and its 1s orbital has a tiny cross-section for photoemission.

Hydrogen (H) also has a tiny cross-section and suffers from having to share its only electron in forming compounds, which then resides in a valence-like orbital, the energy of which varies from compound to compound.

Features of the Thermo Fisher ESCALAB 250Xi XPS instrument

The Thermo Fisher Scientific ESCALAB 250Xi is the most recent advancement in the ESCALAB series.

The instrument is an optimised multi-method platform that is expandable and comes with excellent flexibility and configurability. Its cutting edge technology is driven by smart software and hardware.

Equipped with a micro-focusing X-ray monochromator designed to deliver optimum XPS performance, the instrument ensures maximum sample throughput.

The multi-technique capability and availability of a range of preparation chambers and devices provides the solution to any surface analytical problem.

ESCALAB 250Xi XPS instrumentESCALAB standard sample loading chamber

Some notable features of the instrument include:

• High sensitivity spectroscopy
• Small area XPS
• Depth profiling capability with MAGCIS (Monatomic & Gas Cluster Ion Source)
• Ion scattering spectroscopy (ISS)
• Reflected electron energy loss spectroscopy (REELS)
• Micro-focused X-ray spot
• Efficient Charge Neutralisation
• Angle Resolved Spectroscopy
• Ultra-high sensitivity and energy resolution

XPS analysis sample requirement

XPS is a surface analytical technique that can be used to study the surface properties of a range of sample types. This includes both inorganic and organic (ex-situ and samples suitable to be placed under ultra-high vacuum) materials, polymers, semiconductors, metals, composite materials, geological and archaeological samples, ceramics, and glasses amongst others.

• The ideal dimension for an XPS analysis sample is 1cm X 1cm, with a maximum of 0.5cm thickness. Samples with lengths up to 5cm and widths up to 2.2cm can also be analysed. Samples below 0.5cm x 0.5cm dimension are difficult to mount on the sample stage. XPS analysis depends on the spot size. Ideally we can measure surface areas (spots) greater than 300µm X 300µm, as the smallest X-ray spot/exposure area is 200µm X 200µm for general analysis in our instrument. Please let us know your sample size and type in advance.
• Powder samples can also be measured using XPS. However, please discuss this with us in advance.
• Sample handling is crucial for XPS analysis. As XPS measures the top few atomic layers on the surface (different for depth profiling), it is very easy to contaminate samples with fingerprints. Ideally, XPS samples should be shipped and transferred inside a Wafer Carrier Box used in the semiconductor industry. However, any sealed container that doesn’t alter surface chemistry should be sufficient. Samples stored in zip-lock polymer bags are not ideal.

XPS Instrument Analysis Chamber view.

Image provided courtesy of Thermo Fisher Scientific.

Applications of XPS

XPS surface analysis can provide answers to a wide range of research problems. The following are examples of research questions addressed using XPS by researchers at the University of Brighton and elsewhere.

What material did we make / purchase?

XPS can provide data about the elemental distribution on the surface of a sample.

The limit of detection is 0.1 atomic per cent or better. In the figure (right), two XPS survey scans of gold coated QCM crystals are shown – the inset is a zoomed-in version of the Ti2p area of the spectra.

The two crystals are from two separate purchased batches. As can be seen, batch one (red) contains around 23 atomic per cent of titanium, making the crystal not fit for purpose, while batch two (green) is ‘pure’ as claimed by the manufacturer.

Survey scan spectra of two different batch QCM crystals. (S. Ray, unpublished data)

How thick is the coating/contamination on a surface?

XPS can measure precisely the thickness (below 10nm) of the surface adsorbed layer. This is useful in understanding the nature of surface contamination, and also in studying biomolecular adsorption on implants.

The spectra (right) is a comparison of XPS and Ellipsometric Thickness measurements of three different proteins adsorbed on hydrophobic surfaces, showing extremely close agreement.

XPS and Ellipsometric Thickness measurement comparison of three different proteins adsorbed on hydrophobic surfaces.

What chemical states are present on my surface?

XPS can identify and quantify the nature of the chemical states on a surface, and can help in visualising the surface functionalisation, essential for applications including bio-chemical sensors and chemical conjugations.

In the example (below left image), the experimental verification of graphene oxide reduction is shown, while the right image illustrates the success of a pluoronics (triblock copolymer) coating on reduced graphene oxide.

XPS C1s narrow scan spectra of (left) reduced graphene oxide and (right) pluronics coated reduced graphene oxide (S. Ray, unpublished data)

How can we analyse and quantify contamination or doping in our sample?

XPS can reveal surface or just below-surface contamination, and can successfully quantify any organic and inorganic contaminants and/or doping.

One example of doping quantification would be analysing the nitrogen doping effect on the properties of graphene. From the XPS spectra (right), the amount and bonding nature of nitrogen with graphene could be quantified.

Image on left: Nitrogen (N1s) narrow scan spectra and peak deconvolution to find the nature of nitrogen bonded to graphene (S. Ray, unpublished data).

Can XPS measure the thickness of coatings on nanoparticles?

In his 2017 publication, Professor David Castner (University of Washington) states, “Single particle information from electron microscopy combined with XPS sensitivity in determining composition make a powerful combination for nanoparticle analysis” ( Powell et al., 2017, J. Physical. Chemistry C).

XPS can measure precisely the thickness of single layer or multiple layers of coatings on nano-micro particles. Currently there are numerous situations where nanoparticles are used (e.g. in targeted drug delivery, sunscreens, and antimicrobial socks).

To functionalise these particles according to their target use, a proper understanding of the coating on the nanoparticles is required. In the case of multifunction nanoparticle use for targeted drug delivery, quantification of the single, double or triple layers is necessary.

XPS can help in understanding many other questions, including:

• What is the effect of heat, aging, chemical treatment, or real world use on my samples?
• What coating is on the surface?
• What is wrong inside my thick film?
• Are the thicknesses of film layers correct?
• Is the surface chemistry of a sample uniform?

Accessing the Surface Analysis Laboratory

If you have queries about how XPS can help in your research and/or industrial project, please do not hesitate to get in touch by telephone or email. We are happy to discuss your research requirements and provide quotations as necessary.

We particularly welcome proposals for joint research funding applications.

We also offer an analysis-only service, but can also provide full interpretation of results on request. XPS analysis costs can be charged per sample, per day/half-day or according to your needs.

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Introduction

X-Ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA), is one of the most widely used surface techniques in materials science and chemistry. It allows the determination of atomic composition of the sample in a non-destructive manner, as well as other chemical information, such as binding constants, oxidation states and speciation. The sample under study is subjected to irradiation by a high energy X-ray source. The X-rays penetrate only 5 – 20 Å into the sample, allowing for surface specific, rather than bulk chemical, analysis. As an atom absorbs the X-rays, the energy of the X-ray will cause a K-shell electron to be ejected, as illustrated by Figure 1.13.11.13.1. The K-shell is the lowest energy shell of the atom. The ejected electron has a kinetic energy (KE) that is related to the energy of the incident beam (hν), the electron binding energy (BE), and the work function of the spectrometer (φ) (1.13.11.13.1). Thus, the binding energy of the electron can be calculated.

BE = hν − KE − ψs(1.13.1)(1.13.1)BE = hν − KE − ψs

Table 1.13.11.13.1 shows the binding energy of the ejected electron, and the orbital from which the electron is ejected, which is characteristic of each element. The number of electrons detected with a specific binding energy is proportional to the number of corresponding atoms in the sample. This then provides the percent of each atom in the sample.

The chemical environment and oxidation state of the atom can be determined through the shifts of the peaks within the range expected (Table 1.13.21.13.2). If the electrons are shielded then it is easier, or requires less energy, to remove them from the atom, i.e., the binding energy is low. The corresponding peaks will shift to a lower energy in the expected range. If the core electrons are not shielded as much, such as the atom being in a high oxidation state, then just the opposite occurs. Similar effects occur with electronegative or electropositive elements in the chemical environment of the atom in question. By synthesizing compounds with known structures, patterns can be formed by using XPS and structures of unknown compounds can be determined.

Sample preparation is important for XPS. Although the technique was originally developed for use with thin, flat films, XPS can be used with powders. In order to use XPS with powders, a different method of sample preparation is required. One of the more common methods is to press the powder into a high purity indium foil. A different approach is to dissolve the powder in a quickly evaporating solvent, if possible, which can then be drop-casted onto a substrate. Using sticky carbon tape to adhere the powder to a disc or pressing the sample into a tablet are an option as well. Each of these sample preparations are designed to make the powder compact, as powder not attached to the substrate will contaminate the vacuum chamber. The sample also needs to be completely dry. If it is not, solvent present in the sample can destroy the necessary high vacuum and contaminate the machine, affecting the data of the current and future samples.

Analyzing Functionalized Surfaces

Depth Profiling

When analyzing a sample (Figure 1.13.21.13.2 a) by XPS, questions often arise that deal with layers of the sample. For example, is the sample homogenous, with a consistent composition throughout, or layered, with certain elements or components residing in specific places in the sample? (Figure 1.13.21.13.2 b,c). A simple way to determine the answer to this question is to perform a depth analysis. By sputtering away the sample, data can be collected at different depths within the sample. It should be noted that sputtering is a destructive process. Within the XPS instrument, the sample is subjected to an Ar⁺ ion beam that etches the surface. This creates a hole in the surface, allowing the X-rays to hit layers that would not have otherwise been analyzed. However, it should be realized that different surfaces and layers may be etched at different rates, meaning the same amount of etching does not occur during the same amount of time, depending on the element or compound currently being sputtered.

It is important to note that hydrocarbons sputter very easily and can contaminate the high vacuum of the XPS instrument and thus later samples. They can also migrate to a recently sputtered (and hence unfunctionalized) surface after a short amount of time, so it is imperative to sputter and take a measurement quickly, otherwise the sputtering may appear to have had no effect.

Functionalized Films

When running XPS, it is important that the sample is prepared correctly. If it is not, there is a high chance of ruining not only data acquisition, but the instrument as well. With organic functionalization, it is very important to ensure the surface functional group (or as is the case with many functionalized nanoparticles, the surfactant) is immobile on the surface of the substrate. If it is removed easily in the vacuum chamber, it not only will give erroneous data, but it will contaminate the machine, which may then contaminate future samples. This is particularly important when studying thiol functionalization of gold samples, as thiol groups bond strongly with the gold. If there is any loose thiol group contaminating the machine, the thiol will attach itself to any gold sample subsequently placed in the instrument, providing erroneous data. Fortunately, with the above exception, preparing samples that have been functionalized is not much different than standard preparation procedures. However, methods for analysis may have to be modified in order to obtain good, consistent data.

A common method for the analysis of surface modified material is angle resolved X-ray photoelectron spectroscopy (ARXPS). ARXPS is a non-destructive alternative to sputtering, as it relies upon using a series of small angles to analyze the top layer of the sample, giving a better picture of the surface than standard XPS. ARXPS allows for the analysis of the topmost layer of atoms to be analyzed, as opposed to standard XPS, which will analyze a few layers of atoms into the sample, as illustrated in Figure 1.13.31.13.3. ARXPS is often used to analyze surface contaminations, such as oxidation, and surface modification or passivation. Though the methodology and limitations are beyond the scope of this module, it is important to remember that, like normal XPS, ARXPS assumes homogeneous layers are present in samples, which can give erroneous data, should the layers be heterogeneous.

Limitations of XPS

There are many limitations to XPS that are not based on the samples or preparation, but on the machine itself. One such limitation is that XPS cannot detect hydrogen or helium. This, of course, leads to a ratio of elements in the sample that is not entirely accurate, as there is always some amount of hydrogen. It is a common fallacy to assume the percent of atoms obtained from XPS data are completely accurate due to this presence of undetected hydrogen (Table 1.13.11.13.1).

It is possible to indirectly measure the amount of hydrogen in a sample using XPS, but it is not very accurate and has to be done in a roundabout, often time consuming manner. If the sample contains hydrogen with a partial positive charge (i.e. OH), the sample can be washed in sodium naphthalenide (C₁₀H₈Na). This replaces this hydrogen with sodium, which can then be measured. The sodium to oxygen ratio that is obtained infers the hydrogen to oxygen ratio, assuming that all the hydrogen atoms have reacted.

XPS can only give an average measurement, as the electrons lower down in the sample will lose more energy as they pass other atoms while the electrons on the surface retain their original kinetic energy. The electrons from lower layers can also undergo inelastic or elastic scattering, seen in Figure 1.13.41.13.4. This scattering may have a significant impact on data at higher angles of emission. The beam itself is also relatively wide, with the smallest width ranging from 10 – 200 μm, lending to the observed average composition inside the beam area. Due to this, XPS cannot differentiate sections of elements if the sections are smaller than the size of the beam.

Sample reaction or degredation are important considerations. Caution should be exercised when analyzing polymers, as they are often chemically active and X-rays will provide energy to start degrading the polymer, altering the properties of the sample. One method found to help overcome this particular limitation is to use angle-resolved X-ray photoelectron spectroscopy (ARXPS). XPS can often reduce certain metal salts, such as Cu²⁺. This reduction will give peaks that indicate a certain set of properties or chemical environments when it could be completely different. It needs to be understood that charges can build up on the surface of the sample due to a number of reasons, specifically due to the loss of electrons during the XPS experiment. The charge on the surface will interact with the electrons escaping from the sample, affecting the data obtained. If the charge collecting is positive, the electrons that have been knocked off will be attracted to the charge, slowing the electrons. The detector will pick up a lower kinetic energy of the electrons, and thus calculate a different binding energy than the one expected, giving peaks which could be labeled with an incorrect oxidation state or chemical environment. To overcome this, the spectra must be charge referenced by one of the following methods: using the naturally occurring graphite peak as a reference, sputtering with gold and using the gold peak as a reference or flooding the sample with the ion gun and waiting until the desired peak stops shifting.

Limitations with Surfactants and Sputtering

While it is known that sputtering is destructive, there are a few other limitations that are not often considered. As mentioned above, the beam of X-rays is relatively large, giving an average composition in the analysis. Sputtering has the same limitation. If the surfactant or layers are not homogeneous, then when the sputtering is finished and detection begins, the analysis will show a homogeneous section, due to the size of both the beam and sputtered area, while it is actually separate sections of elements.

The chemistry of the compounds can be changed with sputtering, as it removes atoms that were bonded, changing the oxidation state of a metal or the hybridization of a non-metal. It can also introduce charges if the sample is non-conducting or supported on a non-conducting surface.

Using XPS to Analyze Metal Nanoparticles

Introduction

X-ray photoelectron spectroscopy (XPS) is a surface technique developed for use with thin films. More recently, however, it has been used to analyze the chemical and elemental composition of nanoparticles. The complication of nanoparticles is that they are neither flat nor larger than the diameter of the beam, creating issues when using the data obtained at face value. Samples of nanoparticles will often be large aggregates of particles. This creates problems with the analysis acquisition, as there can be a variety of cross-sections, as seen in Figure 1.13.51.13.5. This acquisition problem is also compounded by the fact that the surfactant may not be completely covering the particle, as the curvature of the particle creates defects and divots. Even if it is possible to create a monolayer of particles on a support, other issues are still present. The background support will be analyzed with the particle, due to their small size and the size of the beam and the depth at which it can penetrate.

Many other factors can introduce changes in nanoparticles and their properties. There can be probe, environmental, proximity, and sample preparation effects. The dynamics of particles can wildly vary depending on the reactivity of the particle itself. Sputtering can also be a problem. The beam used to sputter will be roughly the same size or larger than the particles. This means that what appears in the data is not a section of particle, but an average composition of several particles.

Each of these issues needs to be taken into account and preventative measures need to be used so the data is the best representation possible.

Sample Preparation

Sample preparation of nanoparticles is very important when using XPS. Certain particles, such as iron oxides without surfactants, will interact readily with oxygen in the air. This causes the particles to gain a layer of oxygen contamination. When the particles are then analyzed, oxygen appears where it should not and the oxidation state of the metal may be changed. As shown by these particles, which call for handling, mounting and analysis without exposure to air, knowing the reactivity of the nanoparticles in the sample is very important even before starting analysis. If the reactivity of the nanoparticle is known, such as the reactivity of oxygen and iron, then preventative steps can be taken in sample preparation in order to obtain the best analysis possible.

When preparing a sample for XPS, a powder form is often used. This preparation, however, will lead to aggregation of nanoparticles. If analysis is performed on such a sample, the data obtained will be an average of composition of each nanoparticle. If composition of a single particle is what is desired, then this average composition will not be sufficient. Fortunately, there are other methods of sample preparation. Samples can be supported on a substrate, which will allow for analysis of single particles. A pictorial representation in Figure 1.13.61.13.6 shows the different types of samples that can occur with nanoparticles.

Analysis Limitations

Nanoparticles are dynamic; their properties can change when exposed to new chemical environments, leading to a new set of applications. It is the dynamics of nanoparticles that makes them so useful and is one of the reasons why scientists strive to understand their properties. However, it is this dynamic ability that makes analysis difficult to do properly. Nanoparticles are easily damaged and can change properties over time or with exposure to air, light or any other environment, chemical or otherwise. Surface analysis is often difficult because of the high rate of contamination. Once the particles are inserted into XPS, even more limitations appear.

Probe Effects

There are often artifacts introduced from the simple mechanism of conducting the analysis. When XPS is used to analyze the relatively large surface of thin films, there is small change in temperature as energy is transferred. The thin films, however, are large enough that this small change in energy has to significant change to its properties. A nanoparticle is much smaller. Even a small amount of energy can drastically change the shape of particles, in turn changing the properties, giving a much different set of data than expected.

The electron beam itself can affect how the particles are supported on a substrate. Theoretically, nanoparticles would be considered separate from each other and any other chemical environments, such as solvents or substrates. This, however, is not possible, as the particles must be suspended in a solution or placed on a substrate when attempting analysis. The chemical environment around the particle will have some amount of interaction with the particle. This interaction will change characteristics of the nanoparticles, such as oxidation states or partial charges, which will then shift the peaks observed. If particles can be separated and suspended on a substrate, the supporting material will also be analyzed due to the fact that the X-ray beam is larger than the size of each individual particle. If the substrate is made of porous materials, it can adsorb gases and those will be detected along with the substrate and the particle, giving erroneous data.

Environmental Effects

Nanoparticles will often react, or at least interact, with their environments. If the particles are highly reactive, there will often be induced charges in the near environment of the particle. Gold nanoparticles have a well-documented ability to undergo plasmon interactions with each other. When XPS is performed on these particles, the charges will change the kinetic energy of the electrons, shifting the apparent binding energy. When working with nanoparticles that are well known for creating charges, it is often best to use an ion gun or a coating of gold. The purpose of the ion gun or gold coating is to try to move peaks back to their appropriate energies. If the peaks do not move, then the chance of there being no induced charge is high and thus the obtained data is fairly reliable.

Proximity Effects

The proximity of the particles to each other will cause interactions between the particles. If there is a charge accumulation near one particle, and that particle is in close proximity with other particles, the charge will become enhanced as it spreads, affecting the signal strength and the binding energies of the electrons. While the knowledge of charge enhancement could be useful to potential applications, it is not beneficial if knowledge of the various properties of individual particles is sought.

Less isolated (i.e., less crowded) particles will have different properties as compared to more isolated particles. A good example of this is the plasmon effect in gold nanoparticles. The closer gold nanoparticles are to each other, the more likely they will induce the plasmon effect. This can change the properties of the particles, such as oxidation states and partial charges. These changes will then shift peaks seen in XPS spectra. These proximity effects are often introduced in the sample preparation. This, of course, shows why it is important to prepare samples correctly to get desired results.

Conclusions

Unfortunately there is no good general procedure for all nanoparticles samples. There are too many variables within each sample to create a basic procedure. A scientist wanting to use XPS to analyze nanoparticles must first understand the drawbacks and limitations of using their sample as well as how to counteract the artifacts that will be introduced in order to properly use XPS.

One must never make the assumption that nanoparticles are flat. This assumption will only lead to a misrepresentation of the particles. Once the curvature and stacking of the particles, as well as their interactions with each other are taken into account, XPS can be run.

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Principle

X-ray Photoelectron Spectroscopy (XPS) or Electron Spectroscopy for Chemical Analysis (ESCA) is a technique which analyzes the elements constituting the sample surface, its composition, and chemical bonding state by irradiating x-rays on the sample surface, and measuring the kinetic energy of the photoelectrons emitted from the sample surface. XPS instrument using Al Kα rays can generally obtain information on elements within a few nms of the sample surface.

Additionally, the change in bond energy (chemical shift) caused by the electron state surrounding the atoms to be analyzed, such as atomic valence charges and interatomic distances, tend to be greater than the chemical shift observed in AES, which makes the relative ease with which the state of chemical bonds can be identified another advantage of XPS.

Charge compensation mechanism (dual beam charge neutralization)

XPS is used for element/chemical state analysis for a wide range of solid samples from conductive to insulating materials. However, with insulating material samples, a positive charge occurs in the x-ray irradiated area due to the generation of photoelectrons. A spectrum measured in a positively charged state shifts to the high bond energy side (low kinetic energy side) compared to its actual position, making it difficult to grasp the correct energy position. Thus, with insulating material samples, charge neutralization is necessary during measurement. The dual beam technique, which irradiates a low energy electron beam and an ion beam simultaneously, is a neutralization method which stabilizes uneven charges on the surface in a self-repairing way, and is capable of stable charge neutralization for a wide range of insulating materials. It is also an essential feature for microscopic area analysis.

X-ray photoelectron spectroscopy (XPS Analysis) also called Electron
Spectroscopy for Chemical Analysis (ESCA) is a chemical surface
analysis method. XPS measures the chemical composition of the outermost
100 Å of a sample. Measurements can be made at greater depths by
ion sputter etching to remove surface layers.

All elements except for H and He can be detected at concentrations above 0.05 to 1.0 atom %, depending on the element. In addition, chemical bonding information can be determined from detailed analysis. Conductive and nonconductive samples can be measured and the technique is well suited for polymeric materials. The sampled area varies from 1 mm down to 30 µm in diameter.

X-ray Photoelectron Spectroscopy Analysis (XPS)

In XPS, also known as Electron Spectroscopy for Chemical Analysis (ESCA), X-rays bombard a sample creating ionized atoms and ejecting free electrons. The energies of these free electrons are related to their binding energies in the original atom. By measuring these characteristic energies, XPS Analysis identifies the chemical elements present in the sample. XPS provides both elemental and, to a certain extent, chemical information in the top 3-30 atomic layers (10-100Å) in solid samples. The sensitivity varies between 0.01-1 atom% dependent upon the element. It can do nondestructive depth profiling to 100 Å and detect all elements except H and He. Ion sputtering combined with XPS is used to accomplish deeper profiling. XPS is especially good for obtaining elemental surface composition of unknown materials, including conductors and insulators.

Critical problem solving with surface analysis is enhanced by reducing the probe area when using XPS Analysis. Small-spot XPS instruments probe for composition, chemistry, and contamination in 0.01 mm2 areas. It also makes XPS sputter depth profiles a reality.

One of the primary reasons for using XPS Surface Analysis to analyze samples is its inherent high surface sensitivity. This results from the fact that nearly all of the electrons which are used for analysis escape from only the outermost four to five atomic layers of the material. This high surface sensitivity permits the easy detection of most surface concentrated elements that would be undetectable by bulk or quasi-bulk techniques, e.g. XRD, XRF, EDS or Electron Microprobe. Remember, chemistry begins at the surface.

Imagine that a sample surface is contaminated by 20% coverage of Si from a silicone lubricant. Using XPS Analysis, the Si atoms represent ~6% of the atoms present in the 4-atom deep sampling volume. However, by using one of the bulk or quasi-bulk techniques, the Si atoms now represent ~0.03% or less of the >1 µm deep sampling volume. Given that surface Si concentrations as low as 0.10.% can be detected, the advantage of XPS over bulk techniques is readily apparent.

One very important reason for using XPS Surface Aanlysis is that it is nondestructive. XPS uses very soft (low energy) x-rays that produce minimum energy input to the sample during analysis. Electron beam analysis techniques concentrate a high amount of energy in a small region and can be very destructive toward organic materials or other thermally sensitive compounds. Bulk analysis techniques often require that the sample be powdered and placed in a matrix material introducing a high probability of altering or entirely losing some surface species.

In addition to providing a detailed elemental surface composition, XPS Analysis provides even more information about the detected elements. Changes in the chemical environment or oxidation state of an atom can cause corresponding changes in the energies of the electrons that are ejected and analyzed. These energy shifts or “chemical shifts” have been well studied and tabulated for many different compounds. By measuring these shifts, it is possible in most cases to accurately assign the chemical environment of a given element.

Another important advantage of XPS over electron beam techniques, i.e. AES, Electron Microprobe, etc., is its ability to analyze insulating specimens with relative ease. Since the analysis beam (x-rays) does not consist of charged particles, the insulating specimen is not required to conduct away any charge buildup due to incidence of the analysis beam itself. The specimen is only required to conduct away enough charge to compensate for the small number of electrons which were ejected from the sample. This small positive charge buildup is easily compensated for by use of a “flood gun”, which directs low energy electrons to the sample surface.

In addition to the inherent advantages of using XPS generally, the small-spot instrument that Rocky Mountain Laboratories’ employs has a number of special features that give an enormous edge over other instruments. The sample transfer and sample chamber configuration allows the analysis of samples a s large as 3.75″ diameter x 0.375″ high. Or, many specimens may be mounted and measured by software automation, if they are of uniform size and shape. The minimum size is limited only by the size of the smallest x-ray beam (50 µm), which has been used to analyze a single 10 µm organic fiber.

The largest x-ray spot (image of the x-ray beam on the sample) is 1-2 mm and is used primarily for rapid data acquisition during survey scans. The smallest x-ray spot is most often used for analysis of small heterogeneous features on a larger sample or simply for analysis of a very small sample. Because the x-ray spot is smaller than in other XPS instruments, remarkably rapid and precise depth profiles are now routine, since both raster size and beam voltage of the ion etching gun can be greatly reduced.

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Interaction of an electron beam with a sample target produces a variety of emissions, including x-rays. An energy-dispersive (EDS) detector is used to separate the characteristic x-rays of different elements into an energy spectrum, and EDS system software is used to analyze the energy spectrum in order to determine the abundance of specific elements. EDS can be used to find the chemical composition of materials down to a spot size of a few microns, and to create element composition maps over a much broader raster area. Together, these capabilities provide fundamental compositional information for a wide variety of materials.

How it Works – EDS

Show caption

EDS systems are typically integrated into either an SEM or EPMA instrument. EDS systems include a sensitive x-ray detector, a liquid nitrogen dewar for cooling, and software to collect and analyze energy spectra. The detector is mounted in the sample chamber of the main instrument at the end of a long arm, which is itself cooled by liquid nitrogen. The most common detectors are made of Si(Li) crystals that operate at low voltages to improve sensitivity, but recent advances in detector technology make availabale so-called “silicon drift detectors” that operate at higher count rates without liquid nitrogen cooling.

An EDS detector contains a crystal that absorbs the energy of incoming x-rays by ionization, yielding free electrons in the crystal that become conductive and produce an electrical charge bias. The x-ray absorption thus converts the energy of individual x-rays into electrical voltages of proportional size; the electrical pulses correspond to the characteristic x-rays of the element.

Strengths

• When used in “spot” mode, a user can acquire a full elemental spectrum in only a few seconds. Supporting software makes it possible to readily identify peaks, which makes EDS a great survey tool to quickly identify unknown phases prior to quantitative analysis.
• EDS can be used in semi-quantitative mode to determine chemical composition by peak-height ratio relative to a standard.

Limitations

• There are energy peak overlaps among different elements, particularly those corresponding to x-rays generated by emission from different energy-level shells (K, L and M) in different elements. For example, there are close overlaps of Mn-K_α and Cr-K_β, or Ti-K_α and various L lines in Ba. Particularly at higher energies, individual peaks may correspond to several different elements; in this case, the user can apply deconvolution methods to try peak separation, or simply consider which elements make “most sense” given the known context of the sample.
• Because the wavelength-dispersive (WDS) method is more precise and capable of detecting lower elemental abundances, EDS is less commonly used for actual chemical analysis although improvements in detector resolution make EDS a reliable and precise alternative.
• EDS cannot detect the lightest elements, typically below the atomic number of Na for detectors equipped with a Be window. Polymer-based thin windows allow for detection of light elements, depending on the instrument and operating conditions.

Results

A typical EDS spectrum is portrayed as a plot of x-ray counts vs. energy (in keV). Energy peaks correspond to the various elements in the sample. Generally they are narrow and readily resolved, but many elements yield multiple peaks. For example, iron commonly shows strong K_α and K_βpeaks. Elements in low abundance will generate x-ray peaks that may not be resolvable from the background radiation.

EDS spectrum of multi-element glass (NIST K309) containing O, Al, Si, Ca, Ba and Fe (Goldstein et al., 2003). Details

EDS spectrum of biotite, containing detectable Mg, Al, Si, K, Ti and Fe (from Goodge, 2003). Details

References

• Severin, Kenneth P., 2004, Energy Dispersive Spectrometry of Common Rock Forming Minerals. Kluwer Academic Publishers, 225 p.–Highly recommended reference book of representative EDS spectra of the rock-forming minerals, as well as practical tips for spectral acquisition and interpretation.
• Goldstein, J. (2003) Scanning electron microscopy and x-ray microanalysis. Kluwer Adacemic/Plenum Pulbishers, 689 p.
• Reimer, L. (1998) Scanning electron microscopy : physics of image formation and microanalysis. Springer, 527 p.
• Egerton, R. F. (2005) Physical principles of electron microscopy : an introduction to TEM, SEM, and AEM. Springer, 202.
• Clarke, A. R. (2002) Microscopy techniques for materials science. CRC Press (electronic resource)

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To satisfy this curiosity, many inventions have been devised. One of them is the optical microscope. The human eye can distinguish objects down to about 0.2 mm. Optical microscopes reveal small objects, which would be otherwise invisible to the human eye, by magnifying them with the help of a combination of glass lenses. If we raise the amplification rate (magnification) of an optical microscope higher and higher, can we see an atom?

Electron microscopes enable clear observation of micro-structures, which is not possible with optical microscopes. Moreover, they also make it possible to analyze substance structures and obtain atomic level information by using an electron beam. The electron microscope is an epoch-making invention used throughout the world to investigate an atomic world that we could hardly imagine.

The difference between Electron Beam and Light

fig1. Ripples caused by the difference in the magnitude of the wave

As explained above, the ability to distinguish a small structure, that is resolution, largely depends on the wavelength of the “wave” used to illuminate the specimen.

The nature of this “wave” may be easily understood by comparing it to the wave pattern arising when a small stone is thrown into a lake. Assume the waves on the water surface come into contact with a rock protruding above the surface. If the rock is larger than the length between the crests of the waves (wavelength), then the wave pattern does not continue behind the rock (Fig,1). This creates a shadow. If the rock is smaller than the wavelength, however, the wave pattern will not be interrupted behind the rock and there is no shadow. In this case, the existence of the rock cannot be detected.

Whereas the wavelength of visible light is 400 to 800 nm (1 nanometer is one 100,000th of 0.1mm), the wavelength of the electron beam, which is used as a light source in the electron microscope, varies depending on the accelerating voltage. The accelerating voltages commonly used are 100 to 200 kV (corresponding to wavelengths of 0.0037nm to 0.0025nm).

This wavelength is far shorter than that of light, and sufficient to distinguish the arrangements of atoms (several nanometers). For the optical microscope the combination of the lens is varied to alter the magnification. In contrast, for the electron microscope, the intensity of the electric current passed to the electromagnets is varied to change the intensity of the magnetic field. This corresponds to the changing the thickness of a convex lens. In fact, by manipulating the electric current, the magnification can be freely controlled.

Another characteristic “electron diffraction”

TEMs employ a high voltage electron beam in order to create an image. An electron gun at the top of a TEM emits electrons that travel through the microscope’s vacuum tube. Rather than having a glass lens focusing the light (as in the case of light microscopes), the TEM employs an electromagnetic lens which focuses the electrons into a very fine beam. This beam then passes through the specimen, which is very thin, and the electrons either scatter or hit a fluorescent screen at the bottom of the microscope. An image of the specimen with its assorted parts shown in different shades according to its density appears on the screen. This image can be then studied directly within the TEM or photographed.  Figure 1 shows a diagram of a TEM and its basic parts. 

Fig. 1 Simplified diagram of a transmission electron microscope.  Drawing by Graham Colm, courtesy of Wikimedia Commons.

What Are the Differences Between a TEM and a Light Microscope?

Although TEMs and light microscopes operate on the same basic principles, there are several differences between the two. The main difference is that TEMs use electrons rather than light in order to magnify images. The power of the light microscope is limited by the wavelength of light and can magnify something up to 2,000 times. Electron microscopes, on the other hand, can produce much more highly magnified images because the beam of electrons has a smaller wavelength which creates images of higher resolution. (Resolution is the degree of sharpness of an image.) Figure 2 compares the magnification of a light microscope to that of a TEM. 

Fig. 2 [left] Cotton stem; area in the circle is the phloem tissue. Light microscope x250. Photo by K. Esau.  [right] Enlarged image of cotton phloem tissue showing a sieve element (top cell) and a companion cell (bottom cell), TEM x8,000. Photo by J. Thorsch.

How Are TEM Specimens Prepared?

Specimens must be very thin so that electrons are able to pass through the tissue. This may be done by cutting very thin slices of a specimen’s tissue using an ultramicrotome.  The tissue must first be put in a chemical solution to preserve the cell structure.  The tissue must also be completely dehydrated (all water removed). 

                                 Fig. 3 Ultramicrotome.  Photo by J. Thorsch.                                                             Fig. 4 Microtome grid.  Image by Laurie Hannah

Once preserved and dehydrated, tissue samples are placed in hard, clean plastic.  The plastic supports the tissue while it is being thinly cut with the ultramicrotome (Fig. 3).

After sections are cut and mounted on grids, (tiny circular disks with openings,) a solution of lead is used to stain the tissue (Fig. 4).  The lead provides contrast to the tissue by staining certain cell parts.  When placed in the electron microscope, the electrons are scattered by the lead.  They do not penetrate the tissue or hit the fluorescent screen, leaving those areas dark. 

Esau’s Work With the TEM

Esau started using the TEM in her research in the early 1960s.  When she moved to UC Santa Barbara in 1963, the campus purchased a Siemens electron microscope for her. She then received a grant from the National Science Foundation in 1969 for another new microscope which she used for the remainder of her career in Santa Barbara. The TEM significantly improved her understanding of the relationship between plants and viruses. Electron microscopy also aided in clarifying the functioning of sieve elements, the food conducting cells in plants. Without the TEM, much of this research would not have been possible. 

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The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. The signals that derive from electron-sample interactions reveal information about the sample including external morphology (texture), chemical composition, and crystalline structure and orientation of materials making up the sample.

In most applications, data are collected over a selected area of the surface of the sample, and a 2-dimensional image is generated that displays spatial variations in these properties. Areas ranging from approximately 1 cm to 5 microns in width can be imaged in a scanning mode using conventional SEM techniques (magnification ranging from 20X to approximately 30,000X, spatial resolution of 50 to 100 nm). The SEM is also capable of performing analyses of selected point locations on the sample; this approach is especially useful in qualitatively or semi-quantitatively determining chemical compositions (using EDS), crystalline structure, and crystal orientations (using EBSD). The design and function of the SEM is very similar to the EPMA and considerable overlap in capabilities exists between the two instruments.

Fundamental Principles of Scanning Electron Microscopy (SEM)

Accelerated electrons in an SEM carry significant amounts of kinetic energy, and this energy is dissipated as a variety of signals produced by electron-sample interactions when the incident electrons are decelerated in the solid sample. These signals include secondary electrons (that produce SEM images), backscattered electrons (BSE), diffracted backscattered electrons (EBSD that are used to determine crystal structures and orientations of minerals), photons (characteristic X-rays that are used for elemental analysis and continuum X-rays), visible light (cathodoluminescence–CL), and heat. Secondary electrons and backscattered electrons are commonly used for imaging samples: secondary electrons are most valuable for showing morphology and topography on samples and backscattered electrons are most valuable for illustrating contrasts in composition in multiphase samples (i.e. for rapid phase discrimination). X-ray generation is produced by inelastic collisions of the incident electrons with electrons in discrete ortitals (shells) of atoms in the sample. As the excited electrons return to lower energy states, they yield X-rays that are of a fixed wavelength (that is related to the difference in energy levels of electrons in different shells for a given element). Thus, characteristic X-rays are produced for each element in a mineral that is “excited” by the electron beam. SEM analysis is considered to be “non-destructive”; that is, x-rays generated by electron interactions do not lead to volume loss of the sample, so it is possible to analyze the same materials repeatedly.

Scanning Electron Microscopy (SEM) Instrumentation – How Does It Work?

Essential components of all SEMs include the following:

• Electron Source (“Gun”)
• Electron Lenses
• Sample Stage
• Detectors for all signals of interest
• Display / Data output devices
• Infrastructure Requirements:
  • Power Supply
  • Vacuum System
  • Cooling system
  • Vibration-free floor
  • Room free of ambient magnetic and electric fields

SEMs always have at least one detector (usually a secondary electron detector), and most have additional detectors. The specific capabilities of a particular instrument are critically dependent on which detectors it accommodates.

Applications

The SEM is routinely used to generate high-resolution images of shapes of objects (SEI) and to show spatial variations in chemical compositions: 1) acquiring elemental maps or spot chemical analyses using EDS, 2)discrimination of phases based on mean atomic number (commonly related to relative density) using BSE, and 3) compositional maps based on differences in trace element “activitors” (typically transition metal and Rare Earth elements) using CL. The SEM is also widely used to identify phases based on qualitative chemical analysis and/or crystalline structure. Precise measurement of very small features and objects down to 50 nm in size is also accomplished using the SEM. Backescattered electron images (BSE) can be used for rapid discrimination of phases in multiphase samples. SEMs equipped with diffracted backscattered electron detectors (EBSD) can be used to examine microfabric and crystallographic orientation in many materials.

Strengths and Limitations of Scanning Electron Microscopy (SEM)?

Strengths

There is arguably no other instrument with the breadth of applications in the study of solid materials that compares with the SEM. The SEM is critical in all fields that require characterization of solid materials. While this contribution is most concerned with geological applications, it is important to note that these applications are a very small subset of the scientific and industrial applications that exist for this instrumentation. Most SEM’s are comparatively easy to operate, with user-friendly “intuitive” interfaces. Many applications require minimal sample preparation. For many applications, data acquisition is rapid (less than 5 minutes/image for SEI, BSE, spot EDS analyses.) Modern SEMs generate data in digital formats, which are highly portable.

Limitations

Samples must be solid and they must fit into the microscope chamber. Maximum size in horizontal dimensions is usually on the order of 10 cm, vertical dimensions are generally much more limited and rarely exceed 40 mm. For most instruments samples must be stable in a vacuum on the order of 10^-5 – 10^-6 torr. Samples likely to outgas at low pressures (rocks saturated with hydrocarbons, “wet” samples such as coal, organic materials or swelling clays, and samples likely to decrepitate at low pressure) are unsuitable for examination in conventional SEM’s. However, “low vacuum” and “environmental” SEMs also exist, and many of these types of samples can be successfully examined in these specialized instruments. EDS detectors on SEM’s cannot detect very light elements (H, He, and Li), and many instruments cannot detect elements with atomic numbers less than 11 (Na). Most SEMs use a solid state x-ray detector (EDS), and while these detectors are very fast and easy to utilize, they have relatively poor energy resolution and sensitivity to elements present in low abundances when compared to wavelength dispersive x-ray detectors (WDS) on most electron probe microanalyzers (EPMA). An electrically conductive coating must be applied to electrically insulating samples for study in conventional SEM’s, unless the instrument is capable of operation in a low vacuum mode.

User’s Guide – Sample Collection and Preparation

Sample preparation can be minimal or elaborate for SEM analysis, depending on the nature of the samples and the data required. Minimal preparation includes acquisition of a sample that will fit into the SEM chamber and some accommodation to prevent charge build-up on electrically insulating samples. Most electrically insulating samples are coated with a thin layer of conducting material, commonly carbon, gold, or some other metal or alloy. The choice of material for conductive coatings depends on the data to be acquired: carbon is most desirable if elemental analysis is a priority, while metal coatings are most effective for high resolution electron imaging applications. Alternatively, an electrically insulating sample can be examined without a conductive coating in an instrument capable of “low vacuum” operation.