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TEM - Transmission Electron Microscopy
Instrumentation
A brand-new TEM has been delivered on February 2002 and the installation of the main equipment has been completed within July 2002 (funds project CIPE P5AW2). Attachments to expand the capabilities of the TEM has been acquired by using INFM institutional resources:
Energy dispersive spectrometer, analytical low background double tilt specimen holder, scanning transmission electron microscope (STEM) unit with bright field (BF) detector and high angle annular dark field (HAADF) detector
The equipment has been tested and the performances are, in some cases, better with respect the ones guaranteed by the manufacturer. This is mainly due to the mechanical, thermal and magnetic stability of the environment in which the TEM has been installed. In particular the CME at INFM-TASC national laboratory has been the first in Italy to achieve a resolution of 0.14 nm in HAADF imaging.
Jeol JEM 2010F FEG UHR TEM/STEM:
200 kV accelerating voltage
T. A. Field Emission Source ZrO/W[100]
Low Cs UHR pole-piece: (0.47 ±0.01) mm
Scherzer resolution: 0.19 nm
Minimum probe size: 0.12 nm
STEM B. F./HAADF detectors: resolution in z-contrast = 0.12 nm
Oxford Energy Dispersive x-ray Spectrometer (EDS) (Z=5)
Sample preparation laboratory
The TEM specimen preparation laboratory, established during the 2001, has so far realised hundreds of samples in plan view and in cross-section geometry. Equipment for cutting, mechanical thinning and lapping are available. The specimen electron transparency is mainly achieved by ion beam milling with Energy ranging from 2 to 5 KeV by using different suitable kind of gas species (Ar and Xe in particular).
A new machine for a final milling of samples has been recently acquired: the use of very low voltages down to 200eV permits to reduce surface damages and consequently surface roughness; this is of great importance for quantitative HAADF experiments.
Alternatively the use of a Tripod wedge polishing machine is possible that permits in principle ion milling free sample preparation. The possibility of a perfectly wedge sample is also highly beneficial in quantitative analysis.
Plasma cleaner is finally used to remove the hydrocarbon contamination from the TEM specimen and specimen holder prior of inserting in the TEM vacuum. This step is of particular importance when a high intensity electron beam is focused on an area of less than 0.2 nm as during HAADF experiments or during the acquisition of EDS or EELS spectra from a sub-nanometric volume. The specimen preparation laboratory is now committed to find a thinning procedure to achieve better surface roughness with respect the one currently obtained nowadays.
Computation Laboratory
Due to the computational complexity
involved in the HAADF simulation the CME has now a new computation
laboratory. It is in particular of fundamental importance to be able to
perform the multislice simulation for HAADF images in
a reasonable time. This has been possible thanks to the STEM_CELL project.
Stem_cell is a graphical program allowing for a direct user interface.
It permits to handle 3D cells and partial occupation in the most
correct way. Fig 2 shows a snapshot of the program used in this case to simulate the HAADF image of a GaAs nanoparticle
fig2 snapshot of the STEM_CELL program
The core of the program is based on the well known work of E.J. Kirkland but have been improved for a simplier use. The calculations are based on the "frozen phonon approximation" which is based on the fact that the thermal variation is considered to be slower than the elctron trespassing the area of sample close to a single atom. Electron see only a snapshot of the effective atom thermal vibration...
fig 3 schematic representation of the frozen phonon method
The greatest advantage of STEM _CELL is that the multislice routins are implemented with parallel computing thanks to the use of MPI protocol. In this way the computing time can be reduced of more than one order of magnitude
Calculations are performed in collaboration with the computer center of CAMO-ENEA at Casaccia (ROMA) (16 nodes) or locally with a new machine (4 nodes).
STEM_CELL is available for collaborations on specific projects.
SEM
fig 4 The new sem instrument @TASC
For contact email at carlino @ TASC_DOMAIN
TASC_DOMAIN=tasc.infm.it
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