The TEM is equipped with high-performance additional packages in order to be not just a 'high resolution electron microscope', but a complete and unique characterization platform able to be a fundamental support for excellence researches in all the different fields of interest for Sapienza.
Considering the large number of possible users, the TEM platform has been configured to pursue the ultimate capability to acquire the data required by the user in the shortest possible time, in the more friendly way through an operative interface providing continuous user support, and with control systems that have to ensure precise execution according to the user requirements.
The main characteristics of the TEM column would have to fulfill the technical specifications detailed in the following, considering the above requirements and context of use, taking into the due account the wide-spreaded foreseen applications.
====TEM column technical specifications====
- Accelerating voltage: 20-200 kV.
- Electron source: field emission gun (FEG), high stability and brightness Schottky type, with a reduced energy spread guaranteeing an improved atomic resolution imaging as well as strongly enhanced STEM performance (point resolution better than 0,19 nm).
- auto-recalling of accelerating voltage alignment.
- Brightness: 2x10^8 A/cm^2 (at 200 kV).
- Probe current (at 200kV): 2.4 nA at 1.0 nm.
- Specimen tilt: up to +/- 35 deg on all grid (with double tilt holder for 3 mm diameter grid), up to +/- 80 deg (with tomography holder for 3 mm diameter grid).
- Magnification: from 20X to 2,000,000X.
- Independent electromagnetic deflector for each illumination mode (TEM, EDS, NBD, CBD).
- Convergent Beam Electron Diffraction: convergence angle from 1.5 to 80 mrad with acceptance angle of +/- 10°
- Camera length: from 15 to 2.000 mm.
- Anticontaminating trap.
- Side-entry specimen stage with microactive eucentric goniometer with piezo movement.
- Utilities for the functionality of the TEM, including auto-diagnostic and safety functions.
- CMOS Camera System ensuring reduced noise and high image quality.
The TEM platform is equipped with a suitable software to record series of images from an object at different tilt angles with respect to the electron beam in order to obtain that object's 3D dimensional structure using a back projection technique.
Different modalities of imaging have to be guaranteed, depending primarily on the object. For example, structures that show strong diffraction effects, very common in Materials Science, tend to be studied most effectively in STEM mode using images acquired on a High Angle Annular Dark Field (HAADF) detector. Conversely, specimens in Life Sciences, characterized by an abundance of low Z elements and thus fairly devoid of diffraction effects, tend to be studied mostly using TEM bright field. The TEM platform will have to provide a solution for every aspect of microscopy that targets 3D structures.
The TEM platform includes the following analyses thanks to the installation of additional ancillary tools enahncing and estending the performances and capabilities of characterization.
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I) PRECESSION ELECTRON DIFFRACTION (PED) analysis - Precession device for controlling the TEM beam resulting in the acquisition of PED patterns. The system have to utilize the standard beam tilt coils, and have to reduce the spot size, by a minimum factor of 5, at large precession angles (> 1°). Precession angle can be continuously varied from 0 to 4°. Precession frequency have to be adjustable from 0.5 to 1500 Hz. PED is a specialized method to collect electron diffraction patterns in a transmission electron microscope (TEM). By rotating (precessing) a tilted incident electron beam around the central axis of the microscope, a PED pattern is formed by integration over a collection of diffraction conditions. This produces a quasi-kinematical diffraction pattern that is more suitable as input into direct methods algorithms to determine the crystal structure of the sample. The main applications of PED can be summarized as in the following:
A) CRYSTALLOGRAPHY. The advantages of precession electron diffraction make it one of the preferred methods (maybe the most powerful) of electron crystallography that is to determine the three dimensional arrangement of atoms in a crystalline material. In particular, PED can be applied for: Symmetry determination; the so-called Direct methods; Ab Initio structure determination; Automated diffraction tomography
B) ORIENTATION MAPPING: the relative orientation of crystalline grains (up to crystallites with sizes of very few nam) and/or phases helps understand material texture at the micro and nano scales
C) STRAIN MAPPING and METROLOGY: PED technique represents a straightforward method to obtain strain mapping analysis with a later resolutiomn bettere than 4 nm resolution and sensitivity better than 0.02%
D) e-PDF (electron Pair Distribution Function): this approach, analogous to that one based on the use of X-rays, allows the investigation at local scale of electron diffraction (ED) patterns from nanocrystalline or amorphous materials. e-PDF technique analyze the interatomic distances, bonding and possible short/large scale order of nanocrystalline /amorphous materials at nm scale , enabling to monitor in situ solid state reactions, structure of glassy materials, layered thin films quality and amorphous/ recrystallization studies in semiconductor devices.
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II) MICROANALYSIS. The system is equipped with an Energy Dispersive X-Ray Spectrometer (EDS) with the following main features: Active area: 100 mm^2; Solid angle: 1,0 sr; Resolution: better than 133 eV; Range of detectable elements: from Be to U; Software for qualitative and quantitative analysis, point, linescan, mapping analysis, auto-identification of the peaks; Auto driftcorrection of the specimen during the analysis; System for minimizing minimize spurious peaks during EDS analysis; Double tilt beryllium holder.
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III) CRYO package. It allows to perform Cryo analyses and includes: A CryoTransfer System for loading frozen sections or suspensions into TEM holder and transferring frost-free at temperatures </=165°C; it will include a liquid N cooled specimen holder and tools for the frost-free transfer up to 4 grids; N-cooled holders are cryopomped by an activated charcoal trap and have a built-in heater to regenerate the trap; A Cold Stage Controller that will allow temperature to be measured and controlled between the minimum temperature and RT; A Bench top Dry Pumping Station to produce a clean, dry vacuum in the 10^-6 Torr range; with at least two pumping ports to facilitate cry-holder evacuation.
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IV) DIGITAL STEM package. The digitally controlled STEM system with acquisition software integrated in the TEM GUI includes: A Bright Field (BF) detector; A High Angle Annular Dark Field (HAADF) detector; A digital acquisition module. The STEM system guarantees: Magnification in the range 100x-150.000.000x; STEM-DF resolution: better than 0,19 nm (Schottky source) and 0,26 nm (Cold source).