AP 5301/8301 Instrumental Methods of Analysis and Laboratory Lecture 4 Microscopy (III): Transmission Electron Microscopy (TEM)

Transcription

1 1 AP 5301/8301 Instrumental Methods of Analysis and Laboratory Lecture 4 Microscopy (III): Transmission Electron Microscopy (TEM) Prof YU Kin Man kinmanyu@cityu.edu.hk Tel: Office: P6422

2 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

3 Optical and electron microscopes 3 Light source Condenser Specimen Source of electrons Magnetic lenses Objective Eyepiece Projector Specimen CRT Cathode Ray Tube detector OM TEM SEM

4 Transmission electron microscope 4

5 TEM: an introduction A short history: 1897 J. J. Thompson Discovers the electron 1924 Louis de Broglie: identifies the wavelength for electrons as λ = h/mv 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 5 Electrons at 300 kev have a λ~2 pm 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 E (kev) Wavelength (pm)

6 Comparison: SEM and TEM 6 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 Image display On fluorescent screen. On TV monitor. Image nature Image is a two dimensional projection of the sample. Magnification Up to 5,000,000x ~250,000x Resolution ~0.2 nm ~2-5 nm Beam is scanned along the surface of the specimen to build up the image Image is of the surface of the sample

7 TEM: advantages and disadvantages 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 7 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.

8 Transmission electron microscopy (TEM) 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. 8 Diffraction Imaging Spectroscopy

9 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

10 TEM: operation principle 10 A disc of metal Control brightness, convergence Control contrast

11 TEM: essential parts and functions Column CM200 (200kV) Electron Gun EDS Detector 11 Condenser Lens Objective Lens SAD Aperture Magnifying Lenses Binocular Fluorescenc e screen LN 2 Specimen Holder Cost: $4,000,000

12 Specimen Holder beam Rotation, tilting, heating, cooling and straining holder a split polepiece objective lens Double tilt heating Twin specimen holder Heating and straining

13 TEM: specimen preparation TEM 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 (~ nm). In addition to be thin, samples have to be: Electrically conductive Stable under vacuum Free from hydrocarbon contaminants No artefacts 13 For nanoparticles or thin foils, e.g. graphene, disperse crystals or powders on a carbon film on a Cu grid Thin foil

14 TEM: specimen preparation 14 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 ion milling Focused ion beam (FIB)

15 TEM: Cross-section specimen preparation 15 Cross sectional TEM: characterization of multilayer materials layers thickness measurement layers and interfaces structure analysis Cross-sectional TEM image of a silicatetitanate film containing 10 nm gold particles ess/case_example_49.html

16 TEM operation 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, brightfield microscopy, dark-field microscopy, and high-resolution electron microscopy. 16

17 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 (> 1µm) 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).

18 TEM imaging: bright field 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: 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 TEM BF image of microcrystalline ZrO2. some crystals appear with dark contrast since they are oriented (almost) parallel to a zone axis (Bragg contrast). 18

19 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 bright lines 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.

20 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 Si D T BN Objective aperture High resolution TEM image of a RuO2 nanorod High Resolution Transmission Electron Microscope (HRTEM) Image of a Grain Boundary Film in Strontium-Titinate Electron diffraction pattern recorded from both BN film on Si substrate.

21 TEM diffraction 21 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 times larger than that of X-rays. Electron beams can be focused using electromagnetic lenses

22 TEM: electron diffraction e - Bragg s law: λ = 2d hkl sin θ hkl 22 d hkl e-beam Zone axis of crystal L 2 Specimen foil x-ray λ = 1.54A (Cu K) A wide range of θ hkl electrons λ = 0.037A (100kV) θ = 0.26 o for d = 4A [001] Real lattice sample r L r = sin 2θ 2θ hkl For electrons: λ nm = For electron diffraction, the incident beam has to be almost parallel to the planes for diffraction to occur, so that λ = 2d hkl θ hkl r L = λ d r = λl 1 d L is the camera length (mm) r is the distance between T and D spots 1/d is the reciprocal of interplanar distance (A 1 ) 1.5 V+10 6 V 2

23 Reciprocal lattice Reciprocal lattice is another way to view a crystal lattice and is used to understand diffraction patterns. A dimension of 1/d (Å -1 ) is used in reciprocal lattices. g reciprocal lattice vector 23

24 TEM: diffraction intensity 24 + lattice basis Spot (ring) intensity: I hkl F hkl 2 Crystal structure Structure Factor: F hkl = basis f j exp 2πi(hu j + kv j + lw j ) where f j is the atomic scattering factor, and is dependent on atomic number u j,v j, w j are the fractional distances within the unit cell h, k, l is the Miller indices of the plane Atomic scattering factor: λ f θ sin θ Z 2 where Z is the atomic number of the atom

25 TEM: structure factor (example) 25 We can consider the BCC structure as a simple cubic lattice with a two atom basis, with atoms at [000] and [½½½] F hkl = basis f j exp 2πi(hu j + kv j + lw j ) F hkl = fexp i0 + fexp 2πi( 1 2 h k l) ( ) F hkl = f 1 + exp iπ(h + k + l) (000) Hence: F hkl = 2f if h + k + l is even 0 if h + k + l is odd For a monatomic BCC crystal diffraction from (111), (003), (201), (221), etc. are missing and these are the forbidden diffractions

26 TEM: diffraction pattern 26 (010) (100) For a simple cubic structure a d hkl = h 2 + k 2 + l θ T r 010 Real lattice T r D reciprocal lattice r hkl = Lλ/d hkl 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

27 TEM: diffraction pattern Polycrystalline materials The electron diffraction pattern is a set of rings, with some spots depending on the crystallite sizes. 27 Al single crystal Polycrystalline Pt silicide (PtSi) Silicon with epitaxial nickel silicides ( Si - NiSi - NiSi 2 ) Polycrystalline nickel mono silicide (NiSi) on top of single crystalline silicon (Si). 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 nanocrystalline GaNAs Amorphous GaNAs

28 TEM: selected area electron diffraction (SAED) 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. 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

29 TEM: diffraction pattern Each grain is a single crystal 29 A single grain Another grain (different orientation) Two grains More grains Many grains SAED aperture Many grains covered by SAED aperture

30 TEM: convergent beam electron diffraction 30 Parallel beam (SAED) Convergent beam (CBED) Convergence angle Spatial resolution beam size T D disks [hkl]

31 TEM: convergent beam electron diffraction 31 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 threedimensional symmetry of the crystal. Each spot in SAED then becomes a disk within which variations in intensity can be seen. CBED patterns of a Cu2ZnSnS4 crystal in the [010] orientation disc and line patterns packed with information 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 (<1 nm) Applications: Phase identification Symmetry determination Phase fingerprinting Thickness measurement Strain and lattice parameter Structure factor

32 TEM: convergent beam electron diffraction 32 The convergence semiangle, α, can be adjusted by changing the C2 aperture. The size of the diffraction disk depends on α. Depending on α 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 2θ. 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 LN 2

33 33 CBED: phase identification in BaAl 2 Si 2 O 8 Hexagonal Orthorhombic Hexagonal m m 6mm 2mm 6mm 200 o C 400 o C 800 o C <0001> CBED (top) and SAED (bottom) patterns 6 - rotation axis (rotation about axis by 360/6 degrees) m mirror plane

34 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 SEM EDS EELS CL Z-contrast image

35 Scanning Transmission Electron Microscopy (STEM) 35 EDX detector X-rays luminescence I Z 2 SAED =0.26 o or ~6.4 mrads 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.

36 STEM: Z-contrast imaging 36 Low angle scattering: Coulombic interaction with the electron cloud Higher angle scattering: Coulombic interaction with the nucleus Rutherford scattering with cross section σ R σ R (θ) Z 2 Rutherford scattering will dominate when the scattering angle > screening parameter θ o θ o = 0.117Z1/3 E o 1/2, E o in kev e.g. Cu for 200 kev e-beam, θ o 25 mrad Z-sensitive electrons can be collected by using a detector/camera length combination that gives large collection angles (e.g. β > mrad): high-angle annular darkfield (HAADF)

37 STEM: HAADF images 37 STEM HAADF micrographs of 2 layers of Bi absorbed along the general GBs of a Ni polycrystal quenched from 700 o C SrTiO 3 detail/jem-2800.html (a) HRTEM and (b) HAADF-STEM images of Pt nanoparticles (diameter 1-2 nm) dispersed on ceria. Krumeich and Müller HREM-TEM HR Z-contrast STEM Pt pn TiO 2 Pt on C foil

38 Electron probe microanalysis (EPMA) 38 Electron energy loss spectroscopy (EELS) 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 Z 35 using a standard-less data analysis procedure

39 EPMA: electron energy loss process 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. 39

40 EPMA: EELS spectrum 40 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 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).

41 EPMA: EELS elemental mapping 41 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.

42 EPMA: EELS spectrum 42 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.

43 EELS: examples 43 SrTiO3/SrLaMnO3 interface Ti La Sr Mn Two-dimensional EELS elemental mapping of Fe (red) and Pt (green) in a PtFe nanowire Zhu et al. JACS, 137 (32 (2015) SrTiO 3 SrLaMnO 3 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.

44 EPMA: Energy dispersive x-ray spectroscopy 44 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.

45 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 ( 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 e h 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 ~ ev

46 EDS: characteristic x-rays 46 Characteristic x-ray line energy= E final E initial Relative intensities of major x-ray lines K α1 = 100 L α1 = 100 M α1,2 = 100 K α2 = 50 L α2 = 50 M β = 60 K β1 = L β1 = 50 K β2 = 1 10 L β2 = 250 K β3 = 6 15 L β3 = 1 6 L β4 = 3 5 L γ1 = 1 10

47 EDS: in SEM/TEM/STEM 47

48 SEM-EDS analysis: example 48 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)

49 SEM-EDS elemental mapping 49 Fe 3 O 4 /graphene prepared at a low concentration of Fe 2+ ions Lim et al., in Advanced Topics on Crystal Growth, Chapter 12 (2013) ISBN

50 EDS vs EELS mapping 50 EELS / EDS color map of a SrTiO3 crystal Fast joint EELS / EDS color map across a 32 nm transistor device

51 Wavelength dispersive x-ray spectroscopy Wavelength-dispersive X-ray spectroscopy (WDXRF or WDS) analyzes the wavelength (instead of the energy in EDS) of the emitted x-rays. 51 Note that: E ev = hc λ 12.26/E kev nm or λ A = 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: nλ = 2d sin θ where n = 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.

52 WDS 52 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

53 EPMA: WDS vs EDS 53 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 <2000 cps Detection limits 100 ppm ppm Spectral artifacts rare Peak overlap

54 EPMA: EELS vs EDS 54 EELS Energy resolution ~0.1 ev ~130 ev Energy range ev 1-50 kev EDS Element range Better for light elements Better for heavy elements Ease of use Medium high Spatial resolution Good beam broadening Information Elemental, coordination, bonding Quantification Easy Easy Only elemental Peak overlap No Can be severe

55 Related techniques: x-ray fluorescence 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. 55 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

56 Comparison: XRF and EPMA 56 SEM-EDS (STEM) probe Electron X-ray Sample applicability Vacuum requirement Sample depth Conductive samples Yes (<10-5 Torr) Surface (few to 100 nm) ED-XRF Conductive or insulating Can be done in air mm to mm Probe size Down to nm Down to typically mm Cost of equipment Detection limit Medium to high similar Low to medium Elements Down to B or N Typically Z>10 Analysis time Minutes Minutes