AP 5301/8301Instrumental Methods of Analysis

and Laboratory

Lecture 4

Microscopy (III): Transmission Electron Microscopy (TEM)

Prof YU Kin Man

E-mail: kinmanyu@cityu.edu.hk

Tel: 3442-7813

Office: P6422

1

mailto:kinmanyu@cityu.edu.hk

Lecture 4: Outline Introduction:

Development of transmission electron microscope

Essential parts and functions

Operation principles

TEM specimen preparation

Imaging modes: brief field, dark field and high resolution

TEM diffraction

Diffraction basics

TEM diffraction patterns

Selected area electron diffraction

Convergent beam electron diffraction

Scanning transmission electron microscopy (STEM)

Z-contrast imaging

Electron probe microanalysis

Electron energy loss spectroscopy

Energy dispersive and wavelength dispersive x-ray spectroscopy

2

OM TEM SEM

Magneticlenses

detector

CRTCathode Ray Tube

Light sourceSource of electrons

Condenser

Specimen

Objective

Eyepiece

Projector Specimen

Optical and electron microscopes3

Transmission electron microscope4

TEM: an introduction

E (keV) Wavelength (pm)

50 5.36

80 4.18

100 3.70

200 2.51

300 1.97

5

Electrons at 300 keV have a ~2 and a diffraction limited resolution ~1 pm

In practice TEM resolution is far from these

limits

Imperfections (aberrations) of magnetic lenses

are the limiting factor

A short history:

1897 J. J. Thompson Discovers the electron

1924 Louis de Broglie: identifies the wavelength for electrons as = /

1926 H. Busch: magnetic or electric fields act as lenses for electrons

1929 E. Ruska: Ph.D thesis on magnetic lenses

1931 Knoll & Ruska: built the 1st electron microscope (EM)

1931 Davisson & Calbrick: properties of electrostatic lenses

1934 Driest & Muller: surpass resolution of the Light Microscope

1938 von Borries & Ruska: first practical EM (Siemens) - 10 nm resolution

1940 RCA: commercial EM with 2.4 nm resolution

2000 new developments, cryomicroscopes, primary energies up to 1 MeV

Comparison: SEM and TEM

TEM SEM

Electron beam Broad, static beam Beam focused to fine point and

scan over specimen

Electron path passes through thin specimen. scans over surface of specimen

Specimens Specially prepared thin

specimens supported on TEM

grids.

Sample can be any thickness and is

mounted on an aluminum stub.

Specimen stage Located halfway down column. At the bottom of the column.

Image formation Transmitted electrons collectively

focused by the objective lens and

magnified to create a real image

Beam is scanned along the surface

of the specimen to build up the

image

Image display On fluorescent screen. On TV monitor.

Image nature Image is a two dimensional

projection of the sample.

Image is of the surface of the

sample

Magnification Up to 5,000,000x ~250,000x

Resolution ~0.2 nm ~2-5 nm

6

Advantages

TEMs offer very powerful magnification and resolution.

TEMs have a wide-range of applications and can be utilized in a variety of

different scientific, educational and industrial fields

TEMs provide information on element and compound structure.

Images are high-quality and detailed.

Chemical information with analytical attachments

Disadvantages

TEMs are large and very expensive (USD 300K to >1M)

Laborious sample preparation.

Operation and analysis requires special training.

Samples are limited to small size (mm) and must be electron transparent.

TEMs require special housing and maintenance.

Images are black and white .

TEM: advantages and disadvantages7

Transmission electron microscopy (TEM)8

Two unique features of transmission electron microscopy (TEM) are its high

lateral spatial resolution (better than 0.2 nm) and its capability to provide

both image and diffraction information from a single sample.

Hence TEM can be used to obtain full morphological, crystallographic,

atomic structural and microanalytical such as chemical composition (at

nm scale), bonding (distance and angle), electronic structure,

coordination number data from the sample.

Diffraction

SpectroscopyImaging

TEM: operation principle Primary electrons generated by electron

gun and focused by stages of condenser

lenses into bundles

Electrons illuminate the sample:

at low magnification, a spread beam is used

to illuminate a large area

at high magnification, a strongly condensed

beam is used

The pattern of electrons leaving the object,

reaches the objective lens forms the image.

The image is greatly enlarged by a projector

lens.

The traversing electrons (transmission)

reach the scintillator plate at the base of the

column of the microscope.

The scintillator contains phosphor

compounds that can absorb the energy of

the striking electrons and convert it to light

flashes, forming an image

9

Control brightness,

convergence

Control contrast

A disc of metal

TEM: operation principle10

TEM: essential parts and functions11

Electron Gun

EDS Detector

Condenser

LensSpecimen

HolderObjective Lens

Magnifying Lenses

CM200 (200kV)

SAD Aperture

Fluorescenc

e screen

Cost: $4,000,000

Column

Binocular

LN2

Specimen Holder

a split polepiece objective lens

holder

beam

Heating and strainingTwin specimen holder

Double tilt heating

Rotation, tilting, heating, cooling and straining

TEM: specimen preparationTEM is a microscopy technique whereby a beam of electrons is transmitted through an ultrathin specimen, interacting with the specimen as it passes through it. Materials for TEM must be specially prepared to thicknesses which allow electrons to transmit through the sample (~10-200 nm).

13

In addition to be thin, samples have to be:

Electrically conductive

Stable under vacuum

Free from hydrocarbon contaminants

No artefacts

For nanoparticles or thin foils, e.g. graphene, disperse crystals or powders on

a carbon film on a Cu grid

Thin foil

TEM: specimen preparation

For solid samples, there are different methods:

Mechanical: Mechanical polishing down to electron

transparency

Cleavage

Ultramicrotomy-using a (diamond) knife

blade

Crushing

Mechanical+ionic/chemical Grinding, dimpling, ion milling

Using keV Ar ions focused on the sample to

thin it down

Focused ion beam (FIB)

Electro-chemical polishing

Chemical polishing or etching

14

ion milling

Focused ion beam

(FIB)

TEM: Cross-section specimen preparation15

Cross sectional TEM:

characterization of multilayer materials

layers thickness measurement

layers and interfaces structure analysis

Cross-sectional TEM image of a silicate-

titanate film containing 10 nm gold particles

http://www.nanoanalysis.co.jp/en/busin

ess/case_example_49.html

TEM operation16

TEM offers two methods of specimen observation, diffraction mode and image

mode. The objective lens forms a diffraction pattern in the back focal plane with

electrons scattered by the sample and combines them to generate an image in the

image plane.

Whether the diffraction pattern or the image

appears on the viewing screen depends on

the strength of the intermediate lens.

The diffraction pattern is entirely equivalent

to an X-ray diffraction pattern.

The image mode produces an image of the

illuminated sample area

In image mode, the post-specimen lenses

are set to examine the information in the

transmitted signal at the image plane of the

objective lens.

There are three primary image modes that

are used in conventional TEM work, bright-

field microscopy, dark-field microscopy,

and high-resolution electron microscopy.

Use of apertures Condenser aperture:

Limit the beam divergence (reducing the

diameter of the discs in the convergent

electron diffraction pattern).

Limit the number of electrons hitting the

sample (reducing the intensity)

Objective aperture:

Control the contrast in the image.

Allow certain reflections to contribute to the

image.

Bright field imaging (central beam, 000),

Dark field imaging (one reflection, g),

High resolution Images (several reflections

from a zone axis).

Selected area aperture:

Select diffraction patterns from small (>

1m) areas of the specimen.

Allows only electrons going through an area

on the sample that is limited by the SAD

aperture to contribute to the diffraction

pattern (SAD pattern).

TEM imaging: bright field

Bright field (BF): a small objective aperture is used to block all diffracted beams and to pass only the transmitted(undiffracted) electron beam.

Contrast arises in a bright-field image when thickness or compositional variations or structural anomalies are present.

Regions in which intensity is scattered (defects) appear dark

High-Z material appear darker than the low-Z material

In crystalline materials, dark contrast regions in bright-field usually originate from areas that are aligned for Bragg diffraction

18

In image mode, the post-specimen lenses are set to examine the information in the

transmitted signal at the image plane of the objective lens. The scattered electron

waves finally recombine, forming an image with recognizable details related to the

sample microstructure (or atomic structure). There are three primary image modes:

TEM BF image of

microcrystalline ZrO2.

some crystals appear with

dark contrast since they

are oriented (almost)

parallel to a zone axis

(Bragg contrast).

TEM imaging: dark field Dark field (DF): a small objective aperture is

used to select a diffracted beam and block all other beams.

Undistorted crystal lattice appears dark since little scattered intensity arises from these regions to contribute brightness.

dislocations (defects) appear as brightlines on a dark background

19

In the DF image (right), some

of the microcrystals appear with

bright contrast, namely such

whose diffracted beams partly

pass the objective aperture

(a) Bright-field (BF) micrograph of

multilayer cross-section sample Ni/Co

multilayera; (b) Dark-field (DF) TEM

image.

TEM imaging: high resolution Phase contrast or high resolution (HREM): use

the non-diffracted and at least one diffracted

beams by using a large (or none) objective

aperture and add them back together, phase and

intensity to form an image

When viewed at high-magnification, it is

possible to see contrast in the image in the

form of periodic fringes that represent direct

resolution of the Bragg diffracting planes

The contrast is referred to as phase contrast

20

TDSi

BN

Objective

aperture

Electron diffraction pattern recorded from both BN film on Si substrate.

High resolution TEM

image of a RuO2

nanorod

High Resolution Transmission Electron

Microscope (HRTEM) Image of a Grain

Boundary Film in Strontium-Titinate

TEM diffraction

Electrons like X-rays are scattered by atoms

and can be used to analyze crystal structures

in a similar way.

As in X-ray diffraction (XRD), the scattering

event can be described as a reflection of the

beams at planes of atoms (lattice planes)

There are however fundamental differences:

Electrons have a much shorter wavelength

than the X-rays

X-rays are scattered by the electrons that make

up the bulk of the atom. Electrons are charged

particles and interact with the electrons

surrounding atoms and also the nucleus.

The elastic cross section of the electron is ca.

106 times larger than that of X-rays.

Electron beams can be focused using

electromagnetic lenses

21

TEM: electron diffraction22

= 2 sin Braggs law:

x-ray electrons

= 1.54 (Cu K) = 0.037 (100kV)

A wide range of = 0.26 = 4

For electron diffraction, the incident beam has to

be almost parallel to the planes for diffraction to

occur, so that = 2

=

d =

1

L is the camera length (mm)

r is the distance between T and D spots

1/d is the reciprocal of interplanar distance (1)

Specimen foil

e-

L 2

r

e-beamZone axis

of crystal

sample

= sin 2 2

hkl

Real lattice

[001]

For electrons: =1.5

+1062

Reciprocal latticeReciprocal lattice is another way to view a crystal lattice and is used to

understand diffraction patterns. A dimension of 1/d (-1) is used in

reciprocal lattices.

g reciprocal lattice vector

23

TEM: diffraction intensity

Spot (ring) intensity: 2

24

=

2( + + )

where is the atomic scattering factor, and is dependent on atomic number

, , are the fractional distances within the unit cell

, , is the Miller indices of the plane

Atomic scattering factor:

sin2

where Z is the atomic number of the atom

Structure Factor:

lattice

+

basis Crystal structure

We can consider the BCC structure as a simple

cubic lattice with a two atom basis, with atoms at

[000] and []

=

2( + + )

= 0 + 2(1

2 +

1

2 +

1

2)

= 1 + ( + + )

Hence: = 2 + + 0 + +

For a monatomic BCC crystal diffraction from (111), (003), (201), (221), etc. are missing and these are the forbidden diffractions

TEM: structure factor (example)

(000)

( 1 2 1

2 1

2)

25

(010)

(100)

TEM: diffraction pattern26

Real lattice

reciprocal lattice

2

T D

For a simple cubic structure

=

2 + 2 + 2

T

r

r010

100 110

= /

Diffraction pattern: points with space distance proportional to the reciprocal

of the interplanar spacing (1/d) in the direction of the normal to the plane

Polycrystalline materials

The electron diffraction pattern is a set of rings, with some spots depending on

the crystallite sizes.

Nano to Amorphous materials

As the crystal size get smaller (nm) the

rings get more diffuse and eventually

become halo-like when the material

becomes amorphous

TEM: diffraction pattern

Al single crystalPolycrystalline Pt

silicide (PtSi)

Silicon with epitaxial nickel

silicides ( Si - NiSi - NiSi2)

Polycrystalline nickel mono

silicide (NiSi) on top of

single crystalline silicon

(Si).

Amorphous GaNAsnanocrystalline GaNAs

27

TEM: selected area electron diffraction (SAED)

Combined with sample tilting,

diffraction images of single

crystallites can be obtained in

various orientations.

Single crystals of a few

hundred nm in size can be

examined in this way.

28

Selected Area Electron Diffraction SAED is probably the most commonly used

TEM technique.

A selected area aperture is located underneath the

sample holder and can be adjusted to block parts of

the beam so as to examine just selected areas of the

sample.

SAED aperture

Many grains covered by SAED aperture

TEM: diffraction pattern

Each grain is a single crystal

A single grain Two grainsAnother grain

(different orientation)More grains Many grains

29

TEM: convergent beam electron diffraction

Parallel beam (SAED) Convergent beam (CBED)

disksT D

Convergence angle

Spatial

resolution

beam size

[hkl]

30

http://www.feic.com/support/tem/silicon.htmhttp://www.feic.com/support/tem/silicon.htm

TEM: convergent beam electron diffraction31

Convergent Beam Electron Diffraction (CBED): converging the electrons in a

cone onto the specimen, one can in effect perform a diffraction experiment over

several incident angles simultaneously. This technique can reveal the full three-

dimensional symmetry of the crystal. Each spot in SAED then becomes a

disk within which variations in intensity

can be seen.

CBED patterns contain a wealth of

information about symmetry and

thickness of specimen.

The information is generated from

small regions beyond reach of other

techniques (

TEM: convergent beam electron diffraction32

The convergence semiangle, , can be

adjusted by changing the C2 aperture. The

size of the diffraction disk depends on .

Depending on different patterns are

produced.

Electrons are scattered in all directions in the

convergent conical illumination.

Each point in the disc can be scattered by the

same 2. Therefore the diffracted electrons

also form discs, one for each Bragg reflection.

Weaknesses:

Limited to crystalline specimens

Complicated analysis, normally compared to

computer simulated pattern.

The focused beam gives a very high current

density which causes damage to the sample.

Specimens are typically cooled with LN2

CBED: phase identification in BaAl2Si2O8

200oC 400oC 800oC

Hexagonal Orthorhombic Hexagonal

6mm 2mm 6mm

CBED (top) and SAED (bottom) patterns6 - rotation axis (rotation about axis by 360/6 degrees) m mirror plane

mm

33

Scanning Transmission Electron Microscopy (STEM)34

In a STEM the electron beam is focused into a narrow spot which is scanned

over the sample in a rastering mode.

With STEM we can use many more of these signals in a highly spatially

resolved way than we can with TEM

Z-contrast image

EELS

EDS

CL

SEM

Scanning Transmission Electron Microscopy (STEM)

The rastering of the beam across

the sample makes these

microscopes suitable for analysis

techniques such as mapping by

energy dispersive X-ray (EDX)

spectroscopy

electron energy loss

spectroscopy (EELS)

annular dark-field imaging

(ADF).

By using a high-angle detector

(high angle annular dark-field

HAADF), atomic resolution

images where the contrast is

directly related to the atomic

number (z-contrast image) can

be formed.

SAED =0.26o or ~6.4 mrads

I Z2

35

X-rays

EDX detector

luminescence

STEM: Z-contrast imaging36

Low angle scattering: Coulombic

interaction with the electron cloud

Higher angle scattering: Coulombic

interaction with the nucleusRutherford

scattering with cross section

() 2

Rutherford scattering will dominate when the

scattering angle > screening parameter

=0.1171/3

1/2

,

e.g. Cu for 200 keV e-beam, 25

Z-sensitive electrons can be collected by

using a detector/camera length combination

that gives large collection angles (e.g.

>80100 mrad): high-angle annular dark-

field (HAADF)

STEM: HAADF images37

STEM HAADF micrographs of 2 layers

of Bi absorbed along the general GBs of

a Ni polycrystal quenched from 700oC

http://www.jeol.co.jp/en/products/

detail/JEM-2800.html

HREM-TEM HR Z-contrast STEM http://www.microscopy.ethz.ch/HD-1.htm

Pt pn TiO2 Pt on C foil

(a) HRTEM and (b) HAADF-STEM images of Pt

nanoparticles (diameter 1-2 nm) dispersed on

ceria. Krumeich and Mller

SrTiO3

Electron probe microanalysis (EPMA)38

Electrons lose energy through inner-shell

ionizations are useful for detecting the

elemental components of a material.

In Electron Energy Loss Spectroscopy

(EELS) characteristic spectral signature,

termed the edge profile, is derived from the

excitation of discrete inner shell levels to

empty states above the Fermi level.

By studying the detailed shape of the

spectral profiles measured in EELS, the

electronic structure, chemical bonding, and

average nearest neighbor distances for each

atomic species detected can be derived.

Quantitative elemental concentration

determinations can be obtained for the

elements 3 35 using a standard-less data analysis procedure

Electron energy loss spectroscopy (EELS)

EPMA: electron energy loss process39

Measures the changes in the energy

distribution of an electron beam

transmitted through a thin specimen.

The energy loss process is the primary

interaction event.

All other sources of analytical information

( i.e. X-rays, Auger electrons, etc.) are

secondary products of the initial inelastic

event.

EPMA: EELS spectrum40

Region 1: zero loss peak,

represents electrons that have

passed through the specimen

suffering either negligible or no

energy losses

Region 2 (~1-50 eV): low loss

regime, exhibits a series of broad

spectral features related to inelastic

scattering with the valence electron

structure of the material.

In metallic systems these peaks arise

due to a collective excitation of the

valence electrons, and are termed

plasmon oscillations or peaks

Region 3 (extending to 100-1000 eV): a series of edges resulting from

electrons that have lost energy corresponding to the creation of vacancies in

the deeper core levels of the atom (K, L, M shells).

Edge energies are characteristic for each element and therefore can identify

different elements and their quantity (edge height).

EPMA: EELS elemental mapping41

a) HREM image of a carbon

nanotube.

b) Carbon map at the same

region.

c) EELS spectrum

d) Intensity profile of carbon map

perpendicular to the tube axis.

The intensity profile

corresponds well to the

calculated number

distribution of carbon atom

(solid line) based on the size

and the shape of nanotube.

The intensity dip at center

part corresponds to 20

carbon atoms.

http://eels.kuicr.kyoto-u.ac.jp/eels.en.html

EPMA: EELS spectrum42

The inner shell edge profile in

EELS varies with the edge type

(K, L, M, etc.), the electronic

structure, and the chemical

bonding. The details of the profile

is a measure of the empty local

density of states above the

Fermi level of the elemental

species being studied.

For example, Carbon edge from

graphite, C60 and diamond show

very different fine structures.

Comparing spectra with data

library or computation can reveals

the bonding state and local

electronic structure of the

particular sample.

http://eels.kuicr.kyoto-u.ac.jp/eels.en.html

EELS: examples43

Two-dimensional EELS elemental

mapping of Fe (red) and Pt (green) in a

PtFe nanowire

Zhu et al. JACS, 137 (32 (2015)

a) Ti L2,3-edges elemental map; b) La M4,5-edges

elemental map; c) Sr L2,3-edges at 1940 eV

elemental map; d) Mn L2,3-edges elemental map;

e) colorized map using the color scheme from

Figures 9a-d.

SrTiO3/SrLaMnO3 interface

http://www.gatan.com/atomic-level-eels-mapping-using-high-energy-edges-dualeels-mode

Ti La

Sr Mn

SrLaMnO3SrTiO3

EPMA: Energy dispersive x-ray spectroscopy44

Energy-dispersive X-ray spectroscopy (EDS, EDX, or XEDS), sometimes called

energy dispersive X-ray analysis (EDXA) or energy dispersive X-ray microanalysis

(EDXMA), is an analytical technique used for the elemental analysis or chemical

characterization of a sample. It relies on an interaction of some source of X-ray

excitation and a sample.

A high-energy beam of charged particles such as electrons or protons (PIXE),

or a beam of X-rays (XRF), is focused into the sample.

The incident beam excites an electron in an inner shell, ejecting it from the

shell while creating an electron hole.

An electron from an outer shell fills the

hole, and the difference in energy

between the two shells may be released

in the form of an X-ray.

The emitted x-rays are characteristic to

specific elements and can be measured

by an energy-dispersive spectrometer

giving information on the identity and

amount of the atoms in the sample.

EDS detectors: Si(Li), Ge(Li)45

A ED-spectrometer is p-n junction (or Schottky) of a high purity Si or Ge semiconductor crystal (typically compensated with Li).

A high negative voltage is applied over the crystal (500-1000 V) create a depletion width larger than the x-ray penetration depth (mm).

When x-rays enter the crystal electron-hole pairs are formed and the number is proportional to the energy of the x-ray.

The pairs are swept across the semiconductor creating a current pulse with an amplitude proportional to the energy.

The crystal is cooled (using a LN2 dewar or thermal-electric cooled) to reduce thermal excitation (noise).

Measuring the amplitude and counting produces the ED-spectrum.

Energy resolution ~100-150 eV

EDS: characteristic x-rays46

Characteristic x-ray line energy=

Relative intensities of major x-ray lines

1 = 100 1 = 100 1,2 = 100

2 = 50 2 = 50 = 60

1 = 15 30 1 = 50

2 = 1 10 2 = 250

3 = 6 15 3 = 1 6

4 = 3 5

1 = 1 10

EDS: in SEM/TEM/STEM47

SEM-EDS analysis: example48

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy

(EDS or EDX) microanalysis for calcium oxalate (CaOx) crystals.

Chen et al. Kidney intnl. 80, 369 (2011)

SEM-EDS elemental mapping49

Fe3O4/graphene prepared at a low concentration of Fe2+ ions

Lim et al., in Advanced Topics on Crystal Growth, Chapter 12 (2013) ISBN 978-953-51-1010-1

EDS vs EELS mapping50

Fast joint EELS / EDS color map across a 32 nm

transistor device

http://www.gatan.com/techniques/edsedx

EELS / EDS color map of a SrTiO3 crystal

Wavelength dispersive x-ray spectroscopyWavelength-dispersive X-ray spectroscopy (WDXRF or WDS) analyzes the wavelength (instead of the energy in EDS) of the emitted x-rays.

51

Note that: =

=

12.26/

So we can either measure the energy or

wavelength of an emitted x-ray

Wavelength Dispersive Spectrometers

measure by diffraction from a crystal

utilizing Braggs law:

n = 2 sin = 1,2,3

In WDS the emitted X-rays are diffracted by

a crystal and counted by a detector.

The intensity of the diffracted X-rays is

recorded as a function of the diffraction

angle.

WDS can achieve superb energy resolution

of a few eV.

WDS52

Zr L-line portion of an ED spectrum of zirconia

(ideally, ZrO2) containing Y acquired using 15kV.

Blue: WDS energy scan of the same spectral region

EPMA: WDS vs EDS53

WDS EDS

Spectra acceptance One element/run Entire spectrum in one shot

Collection time > 10 mins Mins

Sensitive elements Better for lighter elements

(Be, B, C, N, O)

Resolution ~few eV ~130 eV

Probe size ~200 nm ~5 nm

Max count rate ~50000 cps

EPMA: EELS vs EDS54

EELS EDS

Energy resolution ~0.1 eV ~130 eV

Energy range 0-3000 eV 1-50 keV

Element range Better for light elements Better for heavy

elements

Ease of use Medium high

Spatial resolution Good beam broadening

Information Elemental, coordination,

bonding

Only elemental

Quantification Easy Easy

Peak overlap No Can be severe

Related techniques: x-ray fluorescence55

X-ray fluorescence (XRF) is the emission of characteristic X-rays from a material that has been excited by bombarding with high-energy X-rays. Characteristic x-rays can be measured either in energy or wavelength dispersive mode.

Hot cathode tube (Coolidge tube) is the most

common x-ray source.

electrons are produced by thermionic effect from a

tungsten filament heated by an electric current.

A high voltage potential is applied between the cathode

and the anode, the electrons are thus accelerated

The anode is usually made out of tungsten or

molybdenum. So the x-ray generated are characteristic

x-rays of the anode materials

High intensity sources: rotating anode, synchrotron

Comparison: XRF and EPMA56

SEM-EDS

(STEM)

ED-XRF

probe Electron X-ray

Sample

applicability

Conductive samples Conductive or

insulating

Vacuum

requirement

Yes (10

Analysis

time

Minutes Minutes