The main goal of the project is to develop and apply smart strategies, which are dedicated to characterize beam-sensitive nanostructures using quantitative scanning transmission electron microscopy STEM imaging. This will allow one to use a minimum electron dose to detect single atoms, to determine their atom types and to precisely measure positions of atoms. In this manner, beam damage will be drastically reduced or will even be ruled out completely. The project will focus on the development of novel theoretical methods to optimize the shape of the incoming electron probe and the probe scanning strategy, to experimentally realize the predicted designs and to apply the methods to challenging and relevant beam-sensitive nanostructures.
Electrons are usually generated in an electron microscope by a process known as thermionic emission from a filament, usually tungsten, in the same manner as a light bulbor alternatively by field electron emission.
The transmitted beam contains information about electron density, phase and periodicity ; this beam is used to form an image. Layout of optical components in a basic TEM Hairpin style tungsten filament Single crystal LaB6 filament From the top down, the TEM consists of an emission source, which may be a tungsten filament or needle, or a lanthanum hexaboride LaB6 single crystal source.
The electron source is typically mounted in a Wehnelt cylinder to provide preliminary focus of the emitted electrons into a beam. The upper lenses of the TEM then further focus the electron beam to the desired size and location.
The interaction of electrons with a magnetic field will cause electrons to move according to the left hand rulethus allowing for electromagnets to manipulate the electron beam.
The use of magnetic fields allows for the formation of a magnetic lens of variable focusing power, the lens shape originating due to the distribution of magnetic flux. Additionally, electrostatic fields can cause the electrons to be deflected through a constant angle.
Coupling of two deflections in opposing directions with a small intermediate gap allows for the formation of a shift in the beam path, allowing for beam shifting in TEM, which is important for STEM.
From these two effects, as well as the use of an electron imaging system, sufficient control over the beam path is possible for TEM operation[ citation needed ]. The optical configuration of a TEM can be Transmission electron microscopy phd thesis changed, unlike that for an optical microscope, as lenses in the beam path can be enabled, have their strength changed, or be disabled entirely simply via rapid electrical switching, the speed of which is limited by effects such as the magnetic hysteresis of the lenses.
Optics[ edit ] The lenses of a TEM allow for beam convergence, with the angle of convergence as a variable parameter, giving the TEM the ability to change magnification simply by modifying the amount of current that flows through the coil, quadrupole or hexapole lenses.
The quadrupole lens is an arrangement of electromagnetic coils at the vertices of the square, enabling the generation of a lensing magnetic fields, the hexapole configuration simply enhances the lens symmetry by using six, rather than four coils.
Typically a TEM consists of three stages of lensing. The stages are the condenser lenses, the objective lenses, and the projector lenses. The condenser lenses are responsible for primary beam formation, while the objective lenses focus the beam that comes through the sample itself in STEM scanning mode, there are also objective lenses above the sample to make the incident electron beam convergent.
The projector lenses are used to expand the beam onto the phosphor screen or other imaging device, such as film. The magnification of the TEM is due to the ratio of the distances between the specimen and the objective lens' image plane.
It is noted that TEM optical configurations differ significantly with implementation, with manufacturers using custom lens configurations, such as in spherical aberration corrected instruments,  or TEMs using energy filtering to correct electron chromatic aberration.
Components[ edit ] The electron source of the TEM is at the top, where the lensing system 4,7 and 8 focuses the beam on the specimen and then projects it onto the viewing screen The beam control is on the right 13 and 14 A TEM is composed of several components, which include a vacuum system in which the electrons travel, an electron emission source for generation of the electron stream, a series of electromagnetic lenses, as well as electrostatic plates.
The latter two allow the operator to guide and manipulate the beam as required. Also required is a device to allow the insertion into, motion within, and removal of specimens from the beam path. Imaging devices are subsequently used to create an image from the electrons that exit the system.
TEM components such as specimen holders and film cartridges must be routinely inserted or replaced requiring a system with the ability to re-evacuate on a regular basis.
As such, TEMs are equipped with multiple pumping systems and airlocks and are not permanently vacuum sealed. The vacuum system for evacuating a TEM to an operating pressure level consists of several stages.
Initially, a low or roughing vacuum is achieved with either a rotary vane pump or diaphragm pumps setting a sufficiently low pressure to allow the operation of a turbo-molecular or diffusion pump establishing high vacuum level necessary for operations.
To allow for the low vacuum pump to not require continuous operation, while continually operating the turbo-molecular pumps, the vacuum side of a low-pressure pump may be connected to chambers which accommodate the exhaust gases from the turbo-molecular pump.
For these very low pressures, either an ion pump or a getter material is used.
Poor vacuum in a TEM can cause several problems ranging from the deposition of gas inside the TEM onto the specimen while viewed in a process known as electron beam induced deposition to more severe cathode damages caused by electrical discharge.
The specimen holders hold a standard size of sample grid or self-supporting specimen. Standard TEM grid sizes are 3. The sample is placed onto the meshed area having a diameter of approximately 2.
Usual grid materials are copper, molybdenum, gold or platinum. This grid is placed into the sample holder, which is paired with the specimen stage.Thesis submitted for the degree of Doctor of Philosophy not only helped and guided me throughout the course of my PhD, but has inspired me with her enthusiasm for science and teaching of TEM.
Grain Boundary domain dynamics investigated by Transmission Electron Microscopy in modified BaTiO 3 displaying PTCR effect K. M. Holsgrove, A. M. sion electron microscopy (TEM) and x-ray diﬀraction. InAs NWs can be used in a broad range of applications, including detectors, high speed electronics and low temperature.
(RBS), Auger electron spectroscopy (AES), cross-sectional transmission electron microscopy (X-TEM), scanning electron microscopy (SEM), and electrical resistivity (ER) measurements during temperature cycling have been used for a fine structural characterization.
Transmission Electron Microscopy A textbook for Materials Science Press JJ Thompson - Discovery of the Electron H. Bush Magnetic/Electric Fields as Lenses E.
Ruska PhD Thesis Magnetic lenses Knoll and Ruska 1st EM built benjaminpohle.com Chicago - Scanning Transmission Electron Microscope ~ nm resolution probe. Transmission electron microscopy and atomic force microscopy (AFM) are used to characterize the microstructures in the SiO 2 film.
The limitations of these techniques for the study of the nanostructures are addressed in this paper and it is suggested that AFM combined with etching can yield a structural spectroscopy with very good sensitivity.
Structural and Chemical Investigations of Nanotribology Using In Situ Transmission Electron Microscopy and Defect Based Analytical Modeling A DISSERTATION SUBMITTED TO THE GRADUATE SCHOOL PhD, but I take everything you say very seriously and I could not ask for a .