Transmissions Ele lectron Mic icroscopy (T (TEM) Basic principles - PowerPoint PPT Presentation

Transmissions Ele lectron Mic icroscopy (T (TEM) Basic principles Diffraction Imaging Specimen preparation A.E. Gunnæs MENA3100 V18

Electron interaction with the (thin) specimen e - Typical specimen thickness Backscattered electrons ~ 100 nm or less Auger electrons Secondary electrons Cathodoluminescence X-rays Gas Absorbed electrons Heating Specimen EBIC Cooling Inelastically scattered Elastically scattered electrons electrons Transmitted electrons Electrons interacts 100-1000 times stronger with matter than X-rays -need thin samples

Operatin ing modes Convergent beam Parallel beam Can be scanned (STEM mode) Specimen Spectroscopy and mapping (EDS and EELS) Example of EDS mapping in STEM mode. Cu 2 O (sputtering, 600nm) TiO 2 (ALD, 10 nm) AZO (sputtering, ~200 nm) Quartz (1mm) S. Gorantla STEM: Scanning transmission electron microscopy EDS: Energy dispersive spectroscopy HAADF: High angular annular dark field EELS: Electron energy loss spectroscopy

TEM is is based on three possible set of f techniqes Imaging Spectroscopy Diffraction Chemistry and elecronic With spatial resolution From regions down to a few states (EDS and EELS). down to the atomic level nm (CBED). Spatial and energy (HREM and STEM) resolution down to the atomic level and ~0.1 eV. 200 nm SAD pattern BF TEM image CBED: Convergent beam electron diffraction HREM: High resolution electron microscopy SAD: Selected area diffraction BF: Bright field

Imaging and resolution Resolution of the eyes:~ 0.1-0.2 mm Resolution in a visible light microscope: ~200 nm Modern TEMs with Cs correctors have sub Å resolution! A.E. Gunnæs

The in Th interestin ing obje jects for r TE TEM is is lo local l structure and inhomogeneit inh itie ies in in sp specim imens Important for material properties Interfaces CuO Defects ZnO Precipitates HAADF image Local atomic structure and composition, Electronic structure and chemical bonding S. Gorantla Strain analysis around a dislocation core at the CuO-ZnO interface

An example of a TEM study: Identification of an unknown phase in a thin film A.E. Gunnæs

Specimen: thin film of BiFeO 3 + unknown phase Metal organic compound on Pt BiFeO 3 Heat treatment at 350 o C (10 min) to remove organic parts. Pt Process repeated three times before final heat treatment at 500-700 o C (20 min) . TiO 2 (intermetallic phase grown) Lim Glue SiO 2 Bi Bi O O O Bi Si Bi Fe Bi O Fe Fe O O 200 nm O O Fe Fe Bi O A.E. Gunnæs Bi Fe Bi O O BF TEM image of the cross section of the specimen O Bi Bi Bi Fe Fe O Fe O O O O Fe Bi Goal to produce single phase: Fe Bi O Fe Bi BiFeO 3 with space grupe: R3C and celle dimentions: a= 5.588 Å c=13.867 Å Bi Bi

Determination of the Bravais-lattice of an unknown crystalline phase Tilting series around a dens row of reflections in the reciprocal space 0 o 50 nm A. E. Gunnæs 19 o 25 o Positions of the reflections in the reciprocal space 40 o 52 o Courtesy : Dr. Jürgen Thomas, IFW-Dresden, Germany

Electron Diffraction in TEM Elastic scattered electrons Only the direction of v is changing. (Bragg scattering) Elastic scattering is due to Coulomb interaction between the incident electrons and the electric charge of the electron clouds and the nucleus. (Rutherford scattering). The elastic scattering is due to the average position of the atoms in the lattice. Reflections satisfying Braggs law: 2dsin θ =n λ Electrons interacts 100-1000 times stronger with matter than X-rays -can detect weak reflections not observed with XRD technique Courtesy : Dr. Jürgen Thomas, IFW-Dresden, Germany

Bravais-lattice and cell parameters 011 111 001 101 c [011] [100] [101] b 010 110 d = L λ / R 100 a From the tilt series we find that the unknown phase has a primitive orthorhombic Bravias-lattice with cell parameters: 6.04 Å a= 6,04 Å, b= 7.94 Å og c=8.66 Å α = β = γ = 90 o 7.94 Å

Chemical analysis by use of EDS and EELS Ukjent fase BiFeO 3 BiFe 2 O 5 Nr_2_1evprc.PICT 40 14 O - K 35 12 Fe - L2,3 CCD counts x 1000 30 10 25 8 20 6 BiFeO 3 Ukjent fase 4 15 2 10 -0 500 eV forskyvning, 1 eV pr. kanal -0 200 400 600 800 1000 Energy Loss (eV)

Published structure A.G. Tutov og V.N. Markin The x-ray structural analysis of the antiferromagnetic Bi 2 Fe 4 O 9 and the isotypical combinations Bi 2 Ga 4 O 9 and Bi 2 Al 4 O 9 Izvestiya Akademii Nauk SSSR, Neorganicheskie Materialy (1970), 6, 2014-2017. Romgruppe: Pbam nr. 55, celleparametre: 7,94 Å, 8,44 Å, 6.01Å Bi O Bi O x y z O Bi O Bi Bi 4g 0,176 0,175 0 Fe O O Fe Fe 4h 0,349 0,333 0,5 O Fe O Fe O O Fe 4f 0 0,5 0,244 O Fe Fe O O O Fe Fe O 4g 0,14 0,435 0 O O Fe O O 8i 0,385 0,207 0,242 O Fe O O Fe O O 4h 0,133 0,427 0,5 Fe Bi O O 2b 0 0 0,5 Bi O O Bi O Bi P Celle parameters found with electron diffraction (a= 6,04 Å, b= 7.94 Å and c=8.66 Å) fits reasonably well with the previously published data for the Bi 2 Fe 4 O 9 phase. The disagreement in the c-axis may be due to the fact that we have been studying a thin film grown on a crystalline substrate and is not a bulk sample. The conditions for reflections from the space group Pbam is in agreement with observations done with electron diffraction. Conclusion: The unknown phase has been identified as Bi 2 Fe 4 O 9 with space group Pbam with cell parameters a= 6,04 Å, b= 7.94 Å and c=8.66 Å.

The construction of a TEM A.E. Gunnæs

A.E. Gunnæs

Ba Basic ic TE TEM Electron gun Apertures 1. and 2. condenser lenses Sample Objective lens Sample Vacuum in the column holder better than 10 -6 Pa Intermediate lenses Projector lens Fluorescence Recording media screen (Cameras, detectors) Similar components as a transmission light microscope

The ele lectron source • Two types of emission guns: • Thermionic emission • W or LaB 6 • Field emission Cold FEG Schottky FEG W ZrO/W Thermionic emission

Thermionic guns Filament heated to give thermionic emission -Directly (W) or indirectly (LaB 6 ) Filament negative potential to ground Wehnelt produces a small negative bias -Brings electrons to cross over

Field emission gun • The principle: • The strength of an electric field E is considerably increased at sharp points. E=V/r • r W < 0.1 µm, V=1 kV → E = 10 10 V/m • Lowers the work-function barrier so that electrons can tunnel out of the tungsten. • Surface has to be pristine (no contamination or oxide) • Ultra high vacuum condition (Cold FEG) or poorer vacuum if tip is heated (”thermal” FE; ZrO surface tratments → Schottky emitters).

Resolution (JEOL2100F: 0.19 nm) -Spherical - Chromatic The point resolution in a TEM is limited by the aberrations of the lenses. -Astigmatism

Ele lectromagnetic le lenses A charged particle such as an electron, is deflected by a magnetic field. The direction and magnitude of the force F , on the electron is given by the vector equation: F= -e(v x B)

Ba Basic ic TE TEM Electron gun Apertures 1. and 2. condenser lenses Sample Objective lens Sample Vacuum in the column holder better than 10 -6 Pa Intermediate lenses Projector lens Fluorescence Recording media screen (Cameras, detectors) Similar components as a transmission light microscope

Simplified ray diagram Parallel incoming electron beam Si 3,8 Å Sample 1,1 nm Objective lense Diffraction plane (back focal plane) Objective aperture Image plane Selected area aperture

Selected area diffraction Parallel incoming electron beam • Diffraction from a single crystal in a polycrystalline sample if the aperture is small enough/crystal large enough. Specimen with two crystals (red and blue) • Orientation relationships can be determined. • ~2% accuracy of lattice parameters Objective lense – XRD is much more accurate Pattern on the screen Diffraction pattern Image plane Selected area aperture

Electron Diffraction in TEM Amorphous phase Poly crystalline sample Single Crystals Interface between two different phases epitaxially grown The orientation relationship between the phases can be determined with ED. 25

Dif iffr fractio ion with ith lar large SAD aperture, , rin ring and sp spot patterns SAD Similarities to XRD

Why do we observe many reflections in one diffraction pattern? 2dsin θ B = λ Cu K alpha X-ray:  = 150 pm Electrons at 200 kV:  = 2.5 pm Courtesy : Dr. Jürgen Thomas, IFW- Dresden, Germany

Il Illustration with the Ewald Sphere The radius of the Ewald sphere is 1/  (=k) Cu K alpha X-ray:  = 150 pm => small k Electrons at 200 kV:  = 2.5 pm => large k k o k Resiprocal lattice of a crystal

ED and form effects The dimensions of the specimen affects the shape of the resiprocal lattice poins Resiprocal space Real space

The intensity distribution around each reciprocal lattice point is spread out in the form of spikes directed normal to the specimen. 2d sin θ = n λ λ 200kV = 0.00251 nm Θ ~1 o I( k’ - k )I=(2/ λ )sin θ B = g

Zone axis and Laue zones Zone axis [uvw] (hkl) uh+vk+wl= 0