TRANSMISSION ELECTRON MICROSCOPY

Transmission Electron Microscope (TEM) is a distinct tool in materials characterization to determine the crystal structures and microstructures of materials by use of simultaneous diffraction and imaging techniques. In other words, TEM is the only tool to get both crystallographic and morphological information of materials from a region of few nanometers at magnifications up to million times.

Despite the advantages of TEM such as improved resolution, obtaining crystallographic and morphological information of materials simultaneously, there are some limitations: TEM presents two dimensional images of elecron-material interactions of three dimensional specimens so interpretation of the images is critical. Beam damage effect is particularly important in polymers and biological specimens. Specimen preparation is one of the most critical limitation in TEM. Specimen needs to be thin enough for electron transparency. A solid specimen for TEM examination has strict dimensions of 3 mm diameter and less than 100 nm thickness, which is dictated by design of the microscope goniometer. It is not possible to obtain any information from the specimen unless it is prepared for electron transparency by thinning down to few nanometers.

TEM is based on the principle of sending a coherent electron beam to the thin region at the specimen center and gathering it under the specimen either as the transmitted beam, which passes directly from the specimen, or as the diffracted beams that are scattered from certain crystal planes of the material at Bragg’s angles. Bright field and dark field images so formed are mostly used to identify the submicron features in the sample and to distinguish second phases. Lattice imaging is a more advanced technique to observe projection of lattice planes of materials. Selected Area Diffraction is a technique to determine crystal structures of materials and Convergent Beam Electron Diffraction is a more sophisticated technique to determine complete crystallographic analysis of materials, i.e. determination of space and point groups. Scaning Transmission Electron Microscopy (STEM) mode equipped with High Angle Annular Dark Field (HAADF) detector is used to obtain High Angle Annular Bright Field and High Angle Annular Dark Field images. Since 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, it takes several seconds or even minutes to build up the STEM image but the defects in the imaging lenses do not affect the image resolution, which is controlled by the scanning beam only.

Electron Energy Loss Spectroscopy (EELS) and EDX are indispensable tools of modern quantitative microscopy for elemental analysis of nanometric regions of the material using the energy distribution of electrons inelastically scattered from the specimen and the characteristic X-ray released when the electron beam interacts with the specimen, respectively. Energy Filtering TEM (EFTEM) employs the energy distribution of inelastically scattered electrons to enhance image contrast.

METU CENTRAL LABORATORY TRANSMISSION ELECTRON MICROSCOPES LABORATORIES

METU Central Lab. R&D Department TEM Laboratory have been equipped with two transmission electron microscopes and a specimen preparation laboratory:

• High Contrast TEM, (CTEM):

• FEI Tecnai G2 Spirit BioTwin, 20-120kV

• High Resolution TEM, (RTEM):

• Jeol JEM 2100F HRTEM, 200kV

• TEM Specimen preparation laboratory:

• Ultramicrotome (Leica EM UC6)

• Electropolisher (Struers Tenupol 5 Twin-Jet Electropolisher)

• PIPS (Gatan Precision Ion Polishing System)

• Diamond Saw (Struers Accutom 50 Precision Cutting Machine)

• Dimple Grinder, Disc Punch, Ultrasonic Disc Cutter, Cross Section Kit, Ultrasonic Cleaner (Bandelin RK-210H, 10 lt capacity), Stereomicroscope (Olympus model SZ-61, light source)

entral Laboratory has been equipped with JEOL JEM 2100F Field Emission Gun TEM operated at 80-200kV. In addition to conventional Bright Field, Dark Field, Selected Area Electron Diffraction, Lattice imaging modes, microscope is accessorized with High Angle Annular Dark Field detector in Scanning TEM (STEM) Convergent Beam Electron Diffraction (CBED), Nano Beam Diffraction (NBD) modes for advanced applications as well as Electron Energy Loss Spectroscopy (EELS), (Energy Filtering TEM (EFTEM)) and Electron Dispersive Spectrometer (EDS) for analytical microscopy.

ftsr Reconstruction software program supplied by HREM Research Inc. is available for analysis and simulation of high resolution crystal images.

Technical Specifications of JEOL JEM 2100F HRTEM

A-ILLUMINATION SYSTEM

Accelerating voltage: 80-200 (80,100, 120, 160,200kV) kV in 5 steps

Electron Source: Schottky type Field Emission gun

Voltage stability: better than 2 ppm/min

Beam size: TEM mode: 2-5 nm, EDS/NBD/CBED modes: 0,14-2,4 nm

Condensor aperture: Motorized, 4 stage (10, 40, 100, 200 m)

B-IMAGING SYSTEM

Line resolution: 0.1 nm

Point to point resolution: 0.19 nm

Specimen tilting:

URP: ±25-27°b (edge-center)

HRP: ±35-37° (edge-center)

Magnification: 50X-1 500 000X

Camera Length: 80-2000 mm

Rotation free imaging is standard

Lens current stability better than 1 ppm/min

STEM HAADF:

Resolution: 0.14 nm

Magnification: 2000-150 000 000X

C-SPECIMEN CHAMBER

Side-entry, five axis, motorized goniometer

Specimen drift: maximum 1 nm/min

Recalling the position and tilting amount of the specimen

Specimen movement: URP, x&y-axis:± 2mm z-axis:± 0.1mm

Specimen size: 3 mm diameter

D-ANALYTICAL INSTRUMENTS

GATAN TRIDIEM GIF: PEELS + Energy Filter

EDS: 0.13 srad, X-ray Mapping (long time mapping available due to reduced specimen drift (<1nm/min)), Detection of light elements, down to B

SOFTWARE for Image Reconstruction: FTSR (HREM Research Inc.)

Abbreviations:

HRP: High Resolution Polepiece

URP: Ultra High Resolution Polepiece

STEM: Scanning Transmission Electron Microscope

PEELS: Parallel Electron Energy Loss Spectrometry

GIF: Gatan Image Filter

HAADF: High Angle Annular Dark Field Detector