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Electron Microscopy

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Advanced Characterization Techniques for Energy Storage

By Tianchan Jiang

ScatteringDiffraction

Imaging

SpectroscopyElectron

Microscopy

• Microscopy: obtaining magnified images to study the morphology, structure, and shape of various features, including grains, phases, embedded phases, embedded particles, etc.

• Spectroscopy: investigation of chemical composition and chemistry of the solid.

• Within spectroscopy, bulk techniques such as infrared, Raman, and Rutherford backscattering require minimal sample preparation and are not touched upon.

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Microscopy vs. Spectroscopy

SpectroscopyElectron

Microscopy

• Optical microscopy (OM): SurfaceMaximum magnification~1000×.- Reflection- Transmission- Phase contrast- Polarized light

• Electron microscopy (EM): Surface & internal- Scanning electron microscopy (SEM)

Surface and internal morphology with 1000Å - Transmission electron microscope (TEM)

Phase determination capability. Crystallographic information from~4000 Å2 area.

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Microscopy: Optical vs. Electron

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Limits of resolution

light vs. electronsRayleigh’s criterion (resolution limitation) :

sin61.0

2 n

d

Green light Electrons: 200kV

Compare electron to light

=400nm =0.0025nm

sin1 sin0.1

n ~1.7 (oil) n ~1.0 (vacuum)

=150nm =0.02nm

1/10th the size of an atom!why electrons

d

5


Specimen (10 -100 nm thick)

All these electrons carry on the information of the structure and chemistry of the specimen due to the electron-specimen interactions

Incident electron

beamBackscattered electronsBSE

Auger electrons

AES

SEMSecondary electrons

X-rays

Composition analysis (EDS, EDX, etc)

Direct beam

Scattered electrons

Elastic scattering: TEM, HR-TEM, STEM, SAED, CBED)Inelastic scattering: EELS 6

Electron Scattering

Electron Microscopy

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Electron Microscopy

• TEM, Bright Field/Dark Field (BF/DF)

• High Resolution Transmission Electron Microscopy (HR-TEM)

• High Resolution Scanning Transmission Electron Microscopy (HR-STEM)

• Energy-Filtered Transmission Electron Microscopy (EFTEM)

• In-situ Transmission Electron Microscopy (In-

situ TEM)

Diffraction8

Electron Microscopy

SEM: scanning electron microscopy(HR)TEM: (high-resolution) transmission electron microscopy

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Working Ranges of Various Types of Microscopes

Incident electron beam

Specimen

Imaging/diffraction/spectroscopy

TEM deals mainly with internal structure

Direct beam

Diffracted beam

Diffracted beam

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Transmission Electron Microscope (TEM)

Object plane

Backfocal plane

Image plane

• To see the diffraction pattern you have to adjust the imaging system lenses so that the backfocal plane of the objective lens acts as the object plane for the immediate lens. Then the diffraction pattern is projected onto the viewing screen.

• To see an image you need to readjust the intermediate lens so that its object plane is the image plane of the objective lens. Then an image is projected onto the viewing screen.

f

Forming diffraction patterns and images

(form diffraction pattern)

(form TEM image)

Object lens

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TEM Imaging System

• There are two basic modes of TEM operation, namely the bright-field mode, where the (000) transmitted beam contributes to the image, and the dark-field imaging mode, in which the (000) beam is excluded.

• In the bright field (BF) mode of the TEM, an aperture is placed in the back focal plane of the objective lens which allows only the direct beam to pass.

• In this case, the image results from a weakening of the direct beam by its interaction with the sample. Therefore, mass-thickness and diffraction contrast contribute to image formation: thick areas, areas in which heavy atoms are enriched, and crystalline areas appear with dark contrast.

• In dark field (DF) images, the direct beam is blocked by the aperture while one or more diffracted beams are allowed to pass the objective aperture. Since diffracted beams have strongly interacted with the specimen, very useful information is present in DF images, e.g., about planar defects, stacking faults or particle size.

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Bright-Field and Dark-Field Imaging


• Diffraction contrast imaging One beam selected for imaging

Transmitted beam selected – “bright field” imaging

Diffracted beam selected – “dark field” imaging

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Bright field TEM image Dark field TEM image

Bright-field and dark field images of silicon nanostrucure

Diffraction contrast imaging

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In order to translate the electron scatter into interpretable amplitude contrast we select either the direct beam or some of the diffracted beams in the selected area diffraction (SAD) pattern to form bright field (BF) and dark field (DF) images, respectively.

Usually we tilt the incident beam such that the scattered electrons remain on axis, creating a centered dark-field (CDF) image.

To TEM image system (BF) To TEM image system (DF)

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This mode makes use of the specific Bragg diffracted electrons to image the region from which they originated.

It allows to link diffraction information with specific regions in the sample.

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Dark-Field Imaging

The selected area diffraction (SAD) contains a bright central spot which contains the direct electrons and some scattered electrons. We form a bright-field image using the central spot:

The procedure is:

1) Inserting an objective aperture into the backfocal plane of the objective lens.

2) Using the external drives to move the aperture so that only the direct beam is selected to go thorough the aperture.

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• High resolution electron microscopy can resolve object details smaller than 1 nm (10-9 m).

• It can be used to image the interior structure of the specimen (comparing to atomic resolution scanning tunneling microscopy, only at the surface).

• Comparing to atomic resolution provided by X-ray diffraction (averaging information), HREM can provide information on the local structure.

• Direct imaging of atom arrangements, in particular the structural defects, interface, dislocations.

Chem. Commun., 2008, 6271

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High Resolution Transmission Electron Microscopy (HR-TEM)

High-resolution imaging: Interference of transmitted and diffracted electron beams

HR-TEM is based on phase contrast, i.e., the phase difference of scattered electron waves after transmitting the TEM specimen

specimen

1),,(0i

Aezyx

2),,( i

DAezyx

• The phase of the incident electron wave is changed by the specimen.

• The change in the phase (from 1 to 2) of the electron waves carries on the atomic structure information of the specimen

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• HR-TEM is an imaging mode of the transmission electron microscope (TEM) that allows imaging the crystallographic structure of a sample at an atomic scale.

• Because of its high resolution, it is an invaluable tool to study nanoscale properties of crystalline material such as semiconductors, metals and ceramics.

• At present, the highest resolution realized is 0.8 Å. Ongoing research and development will soon push the resolution of HR-TEM to 0.5 Å. At these small scales, individual atoms and crystalline defects can be imaged. a Ag particle supported on ZnO

Pratsinis group, Particle Technology Laboratory


Gun crossover

C1 lens

C1 crossover

C2 lens-focused

C2 diaphragm

Focused beam

Specimen

Optic axis

JEOL JEM2100F Scanning Transmission Electron Microscopy 21


• High resolution image contrast depends strongly on defocus, astigmatism, beam tilt, crystal tilt, etc. All of these cause the lattice fringes to become stronger or weaker or move.

• A structure image is when the lattice image is a good representation of the crystal structure, with heavy atoms as dark spots.

• To get a structure image:

- The specimen must be thin- Work in flat and clean regions of the specimen- The crystal must be on axis- The beam must be on axis- No astigmatism- The microscope must be at the correct focus (Scherzer focus)- The lattice spacing must be within the resolution limit of the microscope

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Experimental Considerations of HR-TEM

• For microscope:

- Illumination has a high degree of coherence (small source, small spread of the wavelength).

- Field emission gun (FEG) microscope provide better resolution than LaB6

- Mechanics and electronics should be sufficiently stable

- Electron lenses should have small aberrations.- Perform current and voltage centering of the objective lens routinely and

frequently at high magnification.- Remove objective aperture (or use a large one).

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Experimental Considerations of HR-TEM

• Mode: TEM BF (microprobe)• C1 aperture: 2 mm (largest)• C2 aperture: 100/50 um (second/third largest)• Objective aperture: 100 um (largest, slightly larger than the

point resolution)

• Emission: <100 mA• Spot size: 1• Magnification: 500 kx to 750 kx> 750kx for quantitative analysis of CCD-recorded images.

• Exposure time: 3 - 5 seconds with emulsion setting 5.6 < 1.5 seconds to minimize the effect of specimen drift.

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HR-TEM Recommended Conditions

• The spatial coherence is maximized while minimizing the energy spread in the beam.

• A beam is bright enough to allow a reasonable exposure time at high magnification.

• C2 aperture:

Smaller aperture ----- smaller beam size, higher coherencyEffective when the specimen charges. (The total beam current on the specimen is reduced.)

Larger aperture ----- larger beam size, lower coherencyEffective when the specimen contaminates. (The edge of the beam can be far from the area of interest.)

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HR-TEM Recommended Conditions

• When we use convergent-beam mode, the convergence destroys the coherency and the image contrast, so to see an image we have to scan the beam; this mode of operation of the illumination system is standard for STEM.

Gun crossover

C1 lens

C1 crossover

C2 lens-focused

C2 diaphragm

Focused beam

Specimen

Optic axis

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The Illumination System of HR-TEM

• Microscope alignment objective lens astigmatism, beam tilt

• Thin Foil SpecimenThin area with minimal ion-thinning damage

• Zone axisAlignment for the area of interest

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Important Parameters for HR-TEM

• Coarse preparation of samples:– Small objects (mounted on grids):

• Strew• Spray• Cleave• Crush

– Disc cutter (optionally mounted on grids)– Grinding device

• Intermediate preparation:– Dimple grinder

• Fine preparation:– Chemical polisher– Electropolisher– Ion thinning mill

• PIMS: precision milling (using SEM on very small areas (1 X 1 μm2)

• PIPS: precision ion polishing (at 4° angle) removes surface roughness with minimum surface damage

• Beam blockers may be needed to mask epoxy or easily etched areas

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Technology of Specimen Preparation: Bulk Materials

Forming diffraction patterns and images: the STEM imaging system

• The beam has to scan parallel to the optic axis at all times so that it mimics the parallel beam in a TEM even though it is scanning.

• One potentially very big advantage of forming images this way is that we don’t use lenses, as in an SEM. The defects in the imaging lenses do not affect image resolution, which can limit TEM images, is absent in STEM images, so it is advantageous when dealing with a thick specimen using STEM.

• However, due to some drawbacks, STEM images are not widely used particularly for crystalline specimens.

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High Resolution Scanning Transmission Electron Microscopy(HR-STEM)

SE detector

EDX detectorBS detector

Specimenholder

Image deflection coils

Magnification system

Differentialpumping aperture

TVPEELS/Imaging filter

STEM BF/DF

Signals and Detectors:

• In TEM (Transmission electron microscopy)- Energy Filter- TV/CCD camera- Plate camera

• In STEM (Scanning TEM)- BF/DF- HAADF- BS & SE (SEM)

• In STEM and TEM- EDX & PEELS

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TEM vs. STEM

• The STEM signal generated at any point on the specimen is detected, amplified, and a proportional signal is displayed at an equivalent point on the CRT. The image builds up over several seconds or even minutes.

• The variable signal which merges from the BF detector depends on the intensity in the direct beam from that point on the specimen.

Front focal plane of objective lens

Pivot point of scanning system

Upper polepiece of objective lens

Convergent scanning beam

Specimen

Lower polepiece of objective lens

Back focal plane of objective lens

Diffraction beam in stationary DP

Direct beam in stationary DP

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Bright-Field STEM Images

DP: diffraction pattern

• Select any of the scattered electrons

• Unlike TEM, in which we tilt the incident beam so the scattered electrons that we want to form the image travel down the optic axis and are selected by the objective aperture, in STEM if we want a specific beam of scattered electrons to fall on the BF detector, we can simply shift the stationary diffraction pattern so that the beam is on the optic axis.

• It’s simple to do this with the diffraction pattern centering controls, or displacing the C2 aperture.

Front focal plane of objective lens

Pivot point of scanning system

Upper polepiece of objective lens

Convergent scanning beamSpecimen

Lower polepiece of objective lens

Back focal plane of objective lens

Diffraction beam in stationary DP

Direct beam in stationary DP

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Dark-Field STEM Images

• Rather than using the BF detector for DF imaging, we usually use an annular detector, which surrounds the BF detector, and then all the scattered electrons fall onto that detector. We call this annular dark-field (ADF) imaging.

• It has certain advantages, depending on the contrast mechanism operating in the specimen.

• The ADF detector is centered on the optic axis and has a hole in the middle, within which the BF detector sits. The resultant ADF image in this example is complementary to the BF image.

• High angle annular dark field (HAADF) imaging: Collects incoherent scattering, yields atomic resolution

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Annual Dark-Field STEM Images

• Any of the STEM images that we have just described appear on the CRT screen at a magnification that is controlled by the scan dimensions on the specimen, not the lenses of the TEM.

• This is a fundamental difference between scanning and static image formation. Scanning images are not magnified by lenses (and are thus not affected by aberrations in the imaging lenses).

• If the scanned area on the specimen is 1 cm × 1 cm, and the resultant image is displayed on a CRT with an area 10 cm × 10 cm (which is the standard size of the record CRT, though rarely the size of the viewing CRT screen) then the magnification is 10×. If the scanning dimension is reduced to 1 mm, the magnification is 100×, and so on, up to magnifications in excess of 106 ×, which are common in dedicated STEMs.

• As with the TEM, we have to calibrate the STEM magnification and the camera length of the diffraction pattern we use to create the images.

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Magnification in STEM

Sample Rod

JEOL JEM2100F Scanning Transmission Electron Microscopy 35

STEM Facility

• Electrons collected on axis give the Bright Field image. Most of the electrons are unscattered or inelasticallyscattered.

• Electron collected off-axis are the dark field signal. They are either elastically scattered or inelasticallyscattered electrons.

• The Annular Dark Field (ADF) signal from the annular detector, is sensitive to the atomic number of the atoms -- the greater the atomic number, the greater is the scattering intensity. In some circumstances, this can lead to single atom sensitivity.

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STEM Imaging ModesZ contrast

Dark field imaging High resolution imagingBright field imaging

Objective lens

Specimen

Objective aperture

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Comparison of BF, DF, and High Resolution Imaging Mode

• The most obvious distinction between phase-contrast imaging and other forms of TEM imaging is the number of beams collected by the objective aperture or an electron detector.

• Bright/dark field images require that we select a single beam using the objective aperture. A phase-contrast image requires the selection of more than one beam. In general, the more beams collected, the higher the resolution.

• HR-TEM image is an interference pattern between the forward-scattered and diffracted electron waves from the specimen. Interference patterns require close attention to the phase of the electron waves.

High resolution imaging38

Comparison of BF, DF, and High Resolution Imaging Mode

• It is a contrast-enhancement technique. It improves contrast in images and diffraction patterns by removing inelastically scattered electrons that produce heavy background.

• It is a mapping technique. It creates elemental (chemical) maps by forming images with inelastically scattered electrons.

• It is an analytical technique. It records electron energy-loss spectra (and maps) to provide precise chemical analysis of the samples.

Energy-filtered transmission electron microscopy (EFTEM), also known as electron spectroscopic imaging (ESI), couples an electron energy loss imaging spectrometer to a conventional transmission electron microscope to enable the direct quantitative imaging of elements within the specimen.

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Energy-Filtered Transmission Electron Microscopy (EFTEM)(Electron Spectroscopic Imaging (ESI))

Mostly, the so-called three-window method is applied for mappings:

• An image is taken after a suitable ionization edge of the corresponding element (post-edge image, ΔE3).

• Two additional images (pre-edge image 1, ΔE1, and pre-edge image 2, ΔE2) are recorded at energy losses smaller than the ionization edge.

• Only the electrons passing through the selected energy slit contribute to these images. The pre-edge images are used for an approximate determination of the unspecific background which is then subtracted from the post-edge image leading to an elemental map with enhanced contrast. Alternatively, the post-edge image can be divided by a pre-edge image (ratio method).

The energy filter allows to record electron energy loss spectra (EELS) and element specific images (elemental maps) by means of the electron spectroscopic imaging technique.

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K edge of carbon (C_k)

The background intensity in post-the edge image is determined from the pre-edge image 1 and 2, and which is then subtracted from the post-edge image leading to an elemental map with enhanced contrast

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Energy filtering can also be used to improve diffraction patterns eliminating scattered, but not diffracted, electrons from the image. Like transmitted electrons diffracted electrons have no energy loss.

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In-situ Transmission Electron Microscopy (In-situ TEM)

• Dynamic experiments:‒ In situ experiments remove the doubts that exist whenever you observe

materials after heat treatment or after deformation and then try to infer what actually happened at temperature or during deformation.

‒ It is implicit in most TEM investigations that cooling a heat-treated specimen to room temperature or removing the applied stress does not change the microstructure.

‒ Having this assumption, we draw conclusions about what happened during our experiment. However, this assumption is clearly not valid for many situations.

‒ Nevertheless, we generally view our specimen at ambient temperatures and not under load.

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• Limitations:‒ Difficult to perform on thin specimens: when the surface properties dominate the

bulk, as is often the case in thin specimens, TEM images and analyses can be misleading.

‒ Surface diffusion is much more rapid than bulk diffusion and defects are subject to different stress states.

‒ Preparation difficulties.

• To overcome the limitations:

‒ Use much thicker specimens.

‒ Higher voltages are required.

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• Applications:

‒ Developments in electron optics, stage design, and recording media since the 1960s mean that combined HR-TEM and in situ experimentation is now possible at intermediate voltages. This combination is a powerful tool, permitting the observation of reactions at the atomic level, such as the motion of individual ledges at interfaces, although lower-resolution images are no less impressive.

‒ However, the experiments performed in-situ are taking place under conditions that do not approach the bulk conditions experienced by many engineering materials.

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• Convergent Beam Electron Diffraction (CBED)

• Coherent Nanodiffraction

Spectroscopy46

Electron Microscopy

• We have to be very cautious in interpreting selected area diffraction (SAD) pattern from small areas:

The diameter of the smallest area you can select by SAD is about ~0.5 m with an error of similar dimensions.

This size is large compared to the dimension of many crystalline features that interest us in materials science.

For example, many crystal defects and second-phase precipitation which influence the properties of materials are much smaller than 0.5 m.

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Why Convergent Beam Electron Diffraction (CBED)?

• The technique of CBED overcomes both of these limitations and also generates much new diffraction information.

• SAD patterns contain only rather imprecise two-dimensional crystallographic information because the Bragg conditions are relaxed for a thin specimen and small grains within the specimen.

CBED patterns consist of discs of intensity (rather than spots) which are rich in detail and can be exploited to reveal various aspects of specimen microstructure.

CBED is a micro-analytical technique that uses a convergent or focused beam of electrons to obtain diffraction patterns from small specimen regions.

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• Spatial resolution is determined by the focused incident probe size (very small probe size, we call nano-probe).

• Typical application of CBED include:

Specimen thickness

Unit cell and precise lattice parameter

Crystal system and 3D crystal symmetry

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Focus the electron beam on a small area of your specimen to form a convergent-beam electron diffraction (CBED) pattern Condenser II

Upper objective

Specimen

Lower objective

Back Focal Plane

Convergent angle

• Conventional electron diffraction techniques use a parallel incident bema. These techniques are called selected area diffraction (SAD).

When the SAD patterns from area less than ~ 0.5 m, the SAD cannot be used to precisely describe the material structure. The minimum size of SAD aperture usually is 0.5 m.

• In contrast, convergent beam electron diffraction (CBED) uses a convergent beam of electrons to limit the area of the specimen which contributes to the diffraction pattern.

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Convergent Beam Electron Diffraction (CBED)

Parallel incident beam

Sample

Objective lens

Direct beam Direct beam

Screen

Selected area electron diffraction

Condenser II

Upper objective

Specimen

Lower objective

Back Focal Plane

Convergent angle

Convergent beam electron diffraction

Aperture

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Convergent Beam Vs. Parallel Beam

• Each spot by CBED then becomes a disc within which variations in intensity can usually be seen.

• Such patterns initially seem more difficult to interpret but they contain a wealth of information about the symmetry and thickness of the crystal and are widely used in TEM.

• The big advantage of CBED over SAD techniques is that most of the information is generated from small regions beyond the reach of other diffraction techniques. The CBED technique is also called micro-diffraction technique.

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• Local contamination (CBED requires clean specimen, ultrahigh vacuum in TEM)

• The convergent beam may heat or damage the region of the specimen as you examine it.

Potential drawbacks in using CBED

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Factors affecting CBED pattern

• Specimen height h• Selected-area aperture • Probe size S• C2 aperture• Specimen thickness

There are four microscope variables you need to control when forming a CBED pattern:

The beam convergent semi-angle .

The camera length (L) (i.e., the magnification)

The focus of the pattern

The size of the beam

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Experimental Approach to Obtaining CBED Patterns

Select C2 aperture You can adjust the convergence semiangle

by changing the C2 aperture.

The size of the diffraction disks depends on .

To get a pattern of non-overlaping disks, you must select a C2 aperture such that 2 < 2B for the particular specimen and orientation. Typically, the Bragg angle is a few milliradians, and C2 apertures in the 10-50 m will usually ensure that you can get a pattern of non-overlaping disks.

Optic axisGun crossover

C1 lens

C1 crossover

C2 lens

Reduced convergence angle

Specimen

C2 aperture

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Experimental Approach to Obtaining CBED Patterns

Medium2

Small 2

The pattern of non-overlaping disks

Large2

The pattern of substantial overlaping disks

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CBED Patterns with Different Convergent Angle

Disadvantages

Weak reflections harder to see Does not show diffuse scatter. For example, from disordered materials Not good for powder patterns – ring patterns.

Advantages of CBED

Pattern from small region of sample Pattern from well defined area Better Kikuchi lines More accurate orientation Easy to track tilting

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Coherent Nanodiffraction

• Analyze single particle with very high spatial resolution (~25-50nm).

• Understand reaction mechanisms and structural change within the single particle.

• Observe strain caused by mechanical as well as chemical phenomena.

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Specimen

nBack Focal Pla

ICondenser I

UpperObjective

LowerObjective

c) Convergent Beam Electron Diffraction

• Both a) selected area electron diffraction (SAED) and b) nano-area electron diffraction (NED) use parallel illumination.

• SAED limits the sample volume contributing to electron diffraction by using an aperture in the image plane of the image forming lens (objective).

• NED achieves a very small probe by imaging the condenser aperture on the sample using a third condenser lens (eg M= 1/20).

• Convergent beam electron diffraction (CBED) uses a focused probe.

SA Aperture

VirtualAperture

Specimen

Lower ObjectiveLens Back Focal Plan

a) Selected Area Electron Diffraction

PP’

Upper Objective

Lower ObjectiveSpecimen

Condenser Lens III

C2 Aperture10µm

Back Focal Plane

b) Nano-area Electron Diffraction

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Three Modes of Electron Diffraction

Coherent Nanodiffraction Images

FePO4Color-bar represents diffraction intensityParticle is approximately 1um in longest dimension

FePO4Color-bar represents diffraction intensity

Low Angle Diffraction IntensityOverall Diffraction Over Angle Range

60by APS Sector 26, Prof. Jordi Cabana’s group in UIC

Coherent Nanodiffraction Images

FePO4Color-bar represents diffraction intensity

High Angle Diffraction Intensity

FePO4Color-bar represents pixel on detector(2D detector is 1024x1024 pixels)

2θ Peak Position


D-Spacing Map Using Coherent Nanodiffraction

FePO4

Color-bar represents d-spacing (Å)FePO4 d=2.987Å LiFePO4 d=3.001Å


• Electron Energy Loss Spectroscopy (EELS), Valence Electron Energy Loss Spectrometry (VEELS)

• Energy Dispersive Spectroscopy/ Wavelength-

Dispersive Spectroscopy (EDS/WDS)

Scanning

63

Electron Microscopy

• Measure the amount of energy lost by the incident electrons.

• Similar spatial resolution, energy resolution of ~ 1 eV.

- Characteristic features (ionization edges) in the EEL spectrum offer an alternative means of identifying and quantifying the elements present.

- Provide information about the chemical and crystallographic structure of the specimen, down to an atomic level.

EELS spectrum from CaCO3

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Electron Energy Loss Spectroscopy (EELS)

• Energy-loss instrumentation is based on the magnetic-prism spectrometer. Electrostatic designs have been tried but they provide either inadequate resolution or nonlinear dispersion, and they become impractical for electron energies above 100 keV.

• In the simple straight-edged prism, a magnetic field B (typically 0.01 T, produced between triangular-shaped polepieces of an electromagnet) bends the incident beam into a circle of radius R and deflects it through a large angle (typically 90°).

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White(polychromatic

light)

Glassprism

Dispersedspectrum

Projector lens crossover

Entranceaperture

Magnetic prism(Magnetically isolated drift-tube)

Dispersion plane

Ray paths through a magnetic prism spectrometer showing dispersion and focusing of the electrons in the plane of the spectrometer;

Compare the nonfocusing action of a glass prism on visible light

66


The advantages of the plasmon-based method over the core loss analysis are numerous:

• Acquisition times for VEELS spectra are at least a factor 100 faster than those of core losses.

• Processing of the spectra is straightforward, independent of the expertise of the analyst.

• In composite electrodes, when alloys are covered by polymers or carbon, the extraction of edge counts in core loss spectra becomes extremely difficult.

J. Danet, T. Brousse, K. Rasim, D. Guyomard and P. Moreau, Physical Chemistry Chemical Physics, 12, 220 (2010).67

Valence Electron Energy Loss Spectroscopy (VEELS)

EDS spectrum from Si

• Detection of characteristic X-rays excited by incident electrons

• Spatial resolution on the order of probe size (can be as high as 2-3 Å)

Element Wt% At%CK 51.87 67.88OK 12.15 11.94

AlK 02.08 01.21SiK 33.90 18.97

Matrix Correction ZAF

Kv 20.0 Mag 20000 Tilt 0.0 MicronsPerPixY 0.025

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Energy Dispersive X-ray Spectroscopy (EDX or EDS)

• Scanning Electron Microscope (SEM)

• Backscattered Electron Microscope (BSE)

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Electron Microscopy

AdditionalTechniques

• A small electron beam spot (compared to interaction volume ≈ 1 μm in diameter) is scanned repeatedly over the surface area of the sample.

• Slight variation in surface topography produce marked variations in the strength of the beam of the secondary electrons (SEs) ejected from the sample surface.

• The SE signal is displayed on a television screen in a scanning pattern synchronized with the electron beam scan of the sample surface.

• Magnification possible with the SEM is limited by the beam spot size (better than optical microscope, but less than TEM).

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Scanning Electron Microscopy (SEM)

• SEM image looks like a visual image of a large scale piece.

• Depth of field of the SEM allows a irregular surface to be inspected (compare to optical microscope require flat, polished surface).

• Incident beam generates characteristic wavelength of X-rays that identify the elemental composition of the material.

SEM image of carbon nanotubes(CNT)

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Scanning Electron Microscopy (SEM)

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How SEM works

Electron column (gun and lenses) produces a small spot

Deflection system controls image magnification

Scanned image is formed by point by point

Electron detectors collect the signal

Viewing and record monitors display image as beam scans

• Everhart-Thornley (E-T) detector: collects both backscattered electrons (BSE) & secondary electrons (SE)

- Collector screen positively biased: SE, BSE- Collector screen negatively biased: only BSE

• Solid-State BSE detector: collects BSE, compositional image• EBSD detector (phosphor screen, CCD): Electron backscattering diffraction,

Orientation imaging map• STEM detector• EDS (Energy-Dispersive X-ray Spectrometry) : collects x-rays• WDS (Wavelength-Dispersive Spectrometry) : collects x-rays used in

analytical SEM (EPMA)

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Electron Detectors

• TEM offers much higher spatial resolution than SEM

• TEM offers additional facilities for electron beam microanalysis not available on SEM.

SEM, which mainly offers experimental tools for study of surfaces and near surface regions

TEM, which enables exploration of internal structure of thin specimen.

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Compare TEM with SEM

• TEM uses very fast electrons (typically 100-300 keV) to transmit them through a thin foil specimen;

• SEM uses slower electrons (typically 1-20 KeV) which are reflected from the sample surface to form SEM images

• When passing through the specimen (i.e., TEM), fast electrons can suffer the following most significant fates:

1) Transmission without interaction – undeflected;

2) Elastic scattering – deflected without loss of energy;

3) Inelastic scattering – deflected with loss of significant energy, leading to secondary emission of X-rays or other excitations of specimen.

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Compare TEM with SEM

Specimen (10 -100 nm thick)

Incident electron

beamBackscattered electronsBSE

Auger electrons

AES

SEMSecondary electrons

X-rays

Composition analysis (EDS, EDX, etc)

Direct beam

Scattered electrons

Elastic scattering: TEM, HR-TEM, STEM, SAED, CBED)Inelastic scattering: EELS 76

Where is Backscattered Electron Scattering?

• Undergo elastic scattering events- Direction changes (0 – 180o)- Electron energy does not change (ΔE ~ 0)- Columbic interaction with nuclei

• Enough deviation from the incident beam path to return to the surface (> 90o)

• Backscatter coefficient, η

η =𝑛𝐵𝑆𝐸

𝑛𝐵= 𝑖𝐵𝑆𝐸

𝑖𝐵

• Characteristics of BSE- Effect of atomic number Z

Region I & II: BSERegion III: SE

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Backscattered Electron Microscope (BSE)

Electron Beam

Secondary Electrons (SE)BackscatteredElectrons (BSE)

Collector

Scintillator

Light Pipe

PMT AMP

CRTIntensity

Collector Voltage:+300 V for SEs and BSEs-100 V for BSEs only

+12 V

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Operator Controls

• Accelerating voltage and emission current• Condenser lens control

- beam current- beam size

• Objective lens control- focus the beam

• Signal control- contrast- brightnessline scan waveform on an oscilloscope

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Atomic Number Contrast

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Backscattered Electron Microscope (BSE) Images

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Additional Techniques

Magnetic Analysis

Nuclear Magnetic Resonance Spectroscopy (NMR)

Thermal Analysis

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Magnetic Analysis


Thermal Analysis

Introduction of Magnetism

Magnetic Analysis

• When there are unpaired electrons in the system, we observe magnetic behavior that is related to the number and orbital arrangement of the unpaired electrons.

• The magnetic behavior is determined by measuring the magnetic polarization of a substance by a magnetic field.

Permanent magnet Magnetic flux or magnetic lines of force

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Introduction of Magnetism

Magnetic Analysis

• It is convenient to define a quantity called magnetic induction, B, in order to describe the behavior of substances in a field.

𝑩 = 𝑯𝟎 + 𝟒𝝅𝑴

• 𝑯𝟎: the applied field strength, 𝑴: magnetization, i.e. the intensity of magnetization per unit volume.

Magnetic field lines of flux for a paramagnetic substance in a field

Magnetic field lines of flux for a diamagnetic substance in a field

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Types of Magnetic Behavior

Magnetic Analysis

Type Sign Magnitude Field Dependence of χ

Origin

Diamagnetism - 10-6 emu units Independent Field induced, paired electron circulations

Paramagnetism + 0 to 10-4 emu units Independent Angular momentum of the electron

Ferromagnetism + 10-4 to 10-2 emu units Dependent Spin alignment from dipole-dipole interaction of

moments on adjacent atoms, ↿↿

Antiferromagnetism + 0 to 10-4 emu units Dependent Spin paring, ↿⇂ , from dipole-dipole

interactions 85

Temperature Dependence

Magnetic Analysis

Neel temperature: T maximum occurs in antiferromagnetic plot.Curie temperature: T break occurs in ferromagnetic plot.Currie-Weiss behavior: 𝜒 = 𝐶

𝑇−𝜃

Molar susceptibility: 𝜒 =𝑀

𝐻

Effective magnetic moment: 𝜇𝑒𝑓𝑓 = 2.828 𝜒𝑇 Τ1 2 𝐵𝑀

↓Units of Bohr magneton

𝜇𝑒𝑓𝑓 𝑠𝑝𝑖𝑛 − 𝑜𝑛𝑙𝑦 = 𝑔 𝑆 𝑆 + 1 ൗ1 2 𝐵𝑀

UnpairedElectrons

𝑆 𝜇𝑒𝑓𝑓 𝑠𝑝𝑖𝑛 − 𝑜𝑛𝑙𝑦

𝐵𝑀

1 ½ 1.73

2 1 2.83

3 3/2 3.87

4 2 4.90

5 5/2 5.92

6 3 6.93

7 7/2 7.94

S: Spin angular momentum operator86

Applications of Susceptibility Measurements

Magnetic Analysis

Spin-Orbit Coupling𝜇𝑒𝑓𝑓 𝑠𝑝𝑖𝑛 − 𝑜𝑛𝑙𝑦 = 𝑔 𝐽 𝐽 + 1 ൗ1 2 𝐵𝑀

J: quantum number

The magnetic moments of transition metal ion complexes are often quite characteristic of the electronic ground state and structure of the complex.

Intramolecular Effects

S. Han, H. Kim, J. Kim, Y. Jung, Modulating the magnetic behavior of Fe(II)-MOF-74 by the high electron affinity of the guest molecule. Physical Chemistry Chemical Physics 17, 16977-16982 (2015).

The contribution of dyz orbital that has partial unpaired electrontransferred from O2, to the super-exchange interaction between Fe atoms along the 1D chain.

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Measurements of Magnetic Susceptibilities

Magnetic Analysis

• The Gouy Methodo Employ a long, uniform glass tube packed with the solid materials or solution, which is

suspended in a homogeneous magnetic field.o The sample is weighed in and out of the field, and the weight difference is related to the

susceptibility and field strength. If a standard of known susceptibility is used, the field strength need not be known.

• The Faraday Methodo Use a small amount of sample that is suspended in an inhomogeneous field such that 𝐻 Τ𝜕𝐻

𝜕𝑋 is a constant over the entire volume of the sample.

o The method is very sensitive, so small samples can be used and studies can be made on solutions.

• Susceptibility of Single Crystalso The anisotropy in the susceptibility can be determined.o This information has several important applications, as we shall see in the study of nuclear

magnetic resonance (NMR) and electron paramagnetic resonance (EPR) of transition metal ions.

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Magnetic Analysis


Thermal Analysis

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Principle

The individual moments in the sample add vectorially to give a net magnetization, 𝑀.𝑀 = σ𝑖 𝜇𝑖

In an ensemble of spins in a field, those orientations aligned with the field will be lower in energy and preferred. However, thermal energies oppose total alignment; experimentally, only a small net magnetization is observed. The equation for the motion of precession of 𝑀 is similar to that for Ԧ𝜇, i.e.,

ሶ𝑀 = - 𝛾𝐻0 ×𝑀

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The Bloch EquationsIn order to understand many of the applications of NMR, it is necessary to appreciate the change in magnetization of the system with time as the H1 field is applied. This result is applied be the Bloch equation.

ሶ𝑀 = - 𝛾𝐻𝑒𝑓𝑓 ×𝑀 −1

𝑇2𝑀𝑢 Ԧ𝑒𝑢 +𝑀𝑣 Ԧ𝑒𝑣 − 𝑀𝑧 −𝑀0 Ԧ𝑒𝑧

𝐻𝑒𝑓𝑓 = 𝐻0 −𝜔1

𝛾Ԧ𝑒𝑧 + 𝐻1 Ԧ𝑒𝑢

Relaxation effectsTorque from the magnetic field

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The Bloch Equations

ሶ𝑀 = - 𝛾𝐻𝑒𝑓𝑓 ×𝑀 −1

𝑇2𝑀𝑢 Ԧ𝑒𝑢 +𝑀𝑣 Ԧ𝑒𝑣 − 𝑀𝑧 −𝑀0 Ԧ𝑒𝑧

ሶ𝑀𝑢 =𝑑𝑀𝑢

𝑑𝑡= − 𝜔0 − 𝜔1 𝑀𝑣 −

𝑀𝑢

𝑇2

ሶ𝑀𝑣 =𝑑𝑀𝑣

𝑑𝑡= 𝜔0 − 𝜔1 𝑀𝑢 −

𝑀𝑣

𝑇2+ 𝛾𝐻1𝑀𝑧

ሶ𝑀𝑧 =𝑑𝑀𝑧

𝑑𝑡= 𝛾𝐻1𝑀𝑣 +

𝑀0 −𝑀𝑧

𝑇1

Larmor frequency 𝜔0 = 𝛾𝐻0The u, v reference frame is rotating at angular velocity 𝜔1

The NMR Experiment

• In this method, one applies a strong homogenous magnetic field, causing the nuclei to process. Radiation of energy comparable to ∆E is then imposed with a radio frequency transmitter, producing H1.

• When the applied frequency from the radio transmitter is equal to the Larmorfrequency, the two are said to be in resonance, and a u,v-component is induced which can be detected.

• This is the condition in 𝐻𝑒𝑓𝑓 = 𝐻0 −𝜔1

𝛾Ԧ𝑒𝑧 +𝐻1 Ԧ𝑒𝑢 when 𝐻0 ≈

𝜔1

𝛾.

• ∆𝐸 = ℎ𝑣, 𝜔 = 2𝜋𝑣 ⟾ ∆𝐸~𝜔. Energy will be extracted from the r.f. source only when this resonance condition (𝜔 = 2𝜋𝑣) is fulfilled.

• With an electronic detector, one can observe the frequency at which a u,v-component is induced or at which the loss of energy from the transmitter occurs, allowing the resonance frequency to be measured. 93


The NMR Experiment

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Magnet Magnet

Sample tube R.F. Amplifier Detector

Radio transmitter

Sample holder

NMR Applications in batteries:

Nuclear Magnetic Resonance Spectroscopy(NMR)

• Static 11B (9.4 T) NMR spectra of pieces of TiC-CDC film soaked with different volumes of NEt4BF4/ACN electrolyte.

• Spectra are shown for (a) TiC-CDC-800 and (b) TiC-CDC-600. Each spectrum is the result of co-adding 1024 transients, separated by a recycle interval of 10 s. (c) TiC-CDC pores sizes for carbons considered in this work as well as solvated and desolvated anion size are shown.

• dn is the pore size where n% of the pore volume is below that size such that d50 is the average pore size (d85 is referred to as the maximum pore size).

A. C. Forse et al., Nuclear magnetic resonance study of ion adsorption on microporous carbide-derived carbon. Physical Chemistry Chemical Physics 15, 7722-7730 (2013).

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Applications to circumvent technology issue in batteries:


• To study structural changes in the materials

• Some detailed applications:

o Assign 7Li and 27Al NMR spectra of materials by DFT calculations for structural optimization. o Detect dynamic Jahn-Teller distortion.o Find the preferential sites of a specific element.o Monitor structural changes as a function of charge by DFT calculated shifts.

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Magnetic Analysis


Thermal Analysis

Simultaneous Thermal Analysis (STA)

Thermal Analysis

• Simultaneous Thermal Analysis (STA) generally refers to the simultaneous application of thermogravimtry analysis (TGA) and differential scanning calorimetry (DSC) to one and the same sample in a single instrument.

• The test conditions are perfectly identical for the TGA and DSC signals (same atmosphere, gas flow rate, vapor pressure of the sample, heating rate, thermal contact to the sample crucible and sensor, radiation effect, etc.).

• The information gathered can even be enhanced by coupling the STA instrument to an Evolved Gas Analyzer (EGA) like Fourier transform infrared spectroscopy (FTIR) or mass spectrometry (MS).

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Thermogravimtry Analysis (TGA)

The schematic of TGA measurement

• The sample is heated under nitrogen or synthetic air with constant heat rate while the difference of the mass during this process is measured.

• A mass loss indicates that a degradation of the measured substance takes place. The reaction with oxygen from the synthetic air, for example, could lead to an increase of mass.

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Differential Scanning Calorimetry (DSC)

DSC measurement

• The DSC can be used to obtain the thermal critical points like melting point, enthalpy specific heat or glass transition temperature of substances.

• The sample and an empty reference crucible is heated at constant heat flow.

• A difference of the temperature of both crucibles is caused by the thermal critical points of the sample and can be detected.

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Applications of Thermal Analysis in Batteries

DSC heating traces

Electrolyte Study

(1-x) PY14TFSI-(x) PY13FSI-0.3 M LiPF6- mixtures (5 °C min-1, N2 atmosphere)

Variable-temperature TGA heating tracesQ. Zhou, W. A. Henderson, G. B. Appetecchi, S. Passerini, Phase Behavior and Thermal Properties of Ternary Ionic Liquid-Lithium Salt (IL-IL-LiX) Electrolytes. Journal of Physical Chemistry C 114, 6201-6204 (2010). 101

Technique List

• Electron SpectroscopyImaging

TEM, Bright Field/Dark Field (BF/DF)High Resolution Transmission Electron

Microscopy (HR-TEM)High Resolution Scanning Transmission

Electron Microscopy (HR-STEM)Energy-Filtered Transmission Electron

Microscopy (EFTEM) In-situ Transmission Electron Microscopy (In-

situ TEM)Diffraction

Convergent Beam Electron Diffraction (CBED)Coherent Nanodiffraction

SpectroscopyElectron Energy Loss Spectroscopy (EELS),

Valence Electron Energy Loss Spectrometry (VEELS)

Energy Dispersive Spectroscopy/ Wavelength-Dispersive Spectroscopy (EDS/WDS)Scanning

Scanning Electron Microscope (SEM)Backscattered Electron Microscope (BSE)

• Additional TechniquesMagnetic Analysis Nuclear Magnetic Resonance Spectroscopy

(NMR) Thermal Analysis

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Electron Microscopy – Imaging Applications

Imaging BF & DF Imaging Nanoscale morphological changesHR-TEM Structural changes at atomic scale or light elements

HR-STEM) HAADF: Structural changes/chemical sensitivity at the atomic scale (high Z)ABF: Imaging lithium & other light elements study

EFTEM/ESI Changes in elemental distribution in light elements like Li, C, O, F In-situ TEM Dynamic studies; reaction pathways; formation/dissolution

Diffraction CBED Phase identification; local strainCoherent

NanodiffractionNanoscale structural changes

Spectroscopy EELS Valence change in low Z /Li, C, O, F; local coordination change VEELS Local changes in band gap; alloy characterization

Determine multiple solid-state properties of materials at nanoscaleEDX / EDS Determine local chemistry High Z (best); mapping

Scanning SEM Surface topographyBSE General visualization of local chemistry (high Z)

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Electron Microscopy Applications

Technology issues that could be applied to:

- Electrochemical performance - New electrode materials study and understanding materials synthesis process- Solid electrode-electrolyte interface (SEI)- Degradation mechanism during cycling & storage- Morphology and phase distribution during cycling- Stability (thermal decomposition)

References/Further Readings

• Transmission Electron Microscopy, D.B. Williams and C. Barry Carter (1996) (Plenum Press, New York) ISBN 0-306-45324.

• X. H. Liu and J. Y. Huang, Energy & Environmental Science, 4, 3844 (2011).

• R. Huang and Y. Ikuhara, Current Opinion in Solid State & Materials Science, 16, 31 (2012).

• X. H. Liu, Y. Liu, A. Kushima, S. L. Zhang, T. Zhu, J. Li and J. Y. Huang, Advanced Energy Materials, 2, 722 (2012).

• C. M. Wang, Journal of Materials Research, 30, 326 (2015).

• J. N. Weker and M. F. Toney, Advanced Functional Materials, 25, 1622 (2015).

• J. Danet, T. Brousse, K. Rasim, D. Guyomard and P. Moreau, Physical Chemistry Chemical Physics, 12, 220 (2010).

• http://www.electrical4u.com/what-is-magnetic-field/

• https://en.wikipedia.org/wiki/Magnetochemistry

• Wikimedia Commons, the free media repository

• Bestech Sensor and Teaching equipment105

http://www.electrical4u.com/what-is-magnetic-field/

https://en.wikipedia.org/wiki/Magnetochemistry

Acknowledgement

• Dr. M Stanley Whittingham, Binghamton University (SUNY)• Dr. Guangwen Zhou, Binghamton University (SUNY)• Dr. Wayne Jones, Binghamton University (SUNY)• Dr. Jordi Cabana, University of Illinois at Chicago• Qiyue Yin, Binghamton University (SUNY) ; Brookhaven National Laboratory

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