There are a variety of technologies available for detecting and recording the images, diffraction patterns, and electron energy loss spectra produced using transmission electron microscopy (TEM).
Traditionally, TEM images or diffraction patterns could be observed using a fluorescent viewing screen, consisting of powdered ZnS or ZnS/CdS, which is excited by the electron beam via cathodoluminescence. [1] Once the microscopist could see a suitable image on their viewing screen, images could then be recorded using photographic film. For electron microscopes, film typically consisted of a gelatin and silver halide emulsion layer on a plastic support base. [2] The silver halide would be converted to silver upon exposure to the electron beam, and the film could then be chemically developed to form an image, which could be digitized for analysis using a film scanner. [2] In modern TEMs, film has largely been replaced by electronic detectors.
Charge coupled device (CCD) cameras were first applied to transmission electron microscopy in the 1980s and later became widespread. [3] [4] For use in a TEM, CCDs are typically coupled with a scintillator such as single crystal Yttrium aluminium garnet (YAG) in which electrons from the electron beam are converted to photons, which are then transferred to the sensor of the CCD via a fiber optic plate. [1] The main reason for this is that direct exposure to the high energy electron beam risks damaging the sensor CCD. A typical CCD for a TEM will also incorporate a Peltier cooling device to reduce the temperature of the sensor to approximately -30 °C, which reduces dark current and improves signal-to-noise. [1]
Since 2006, scintillator and fiber optic coupled cameras based on complementary metal oxide semiconductor (CMOS) electronics have become commercially available for TEM. [5] CMOS cameras have some advantages for electron microscopy compared to CCD cameras. One advantage is that CMOS cameras are less prone than CCD cameras to blooming, i.e. the spreading of charge from oversaturated pixels into nearby pixels. [6] Another advantage is that CMOS cameras can have faster readout speeds. [7]
The use of scintillators to convert electrons to photons in CCD and CMOS cameras reduces the detective quantum efficiency (DQE) of these devices. Direct electron detectors, which have no scintillator and are directly exposed to the electron beam, typically offer higher DQE than scintillator-coupled cameras. [2] [8] There are two main types of direct electron detectors, both of which were first introduced to electron microscopy in the 2000s. [9] [10]
A hybrid pixel detector, also known as a pixel array detector (PAD) features a sensor chip bonded to a separate electronics chip with each pixel read out in parallel. The pixels are typically wide and thick e.g. 150 x 150 x 500 µm for the electron microscope pixel array detector (EMPAD) described by Tate et al. [11] This large pixel size allows each pixel to fully absorb high-energy electrons, enabling high dynamic range. However, the large pixel size limits the number of pixels that can be incorporated into a sensor. [11]
A monolithic active pixel sensor (MAPS) for TEM is a CMOS-based detector that has been radiation hardened to withstand direct exposure to the electron beam. The sensitive layer of the MAPS is typically very thin, with a thickness as low as 8 μm. [10] This reduces the lateral spread of electrons from the electron beam within the detective layer of the sensor, allowing for smaller pixel sizes e.g. 6.5 x 6.5 µm for a Direct Electron DE-16. [12] Smaller pixel size allows for a large number of pixels to be incorporated into a sensor, although the dynamic range is typically more limited than for a hybrid pixel detector. [12]
In scanning TEM (STEM), a focused probe is rastered over an area of interest, and a signal is recorded at each probe position to form an image. This typically requires different types of detector from conventional TEM imaging, in which a broad area of the specimen is illuminated. Traditional STEM imaging involves detectors, such as the annular dark-field (ADF) detector, which integrate the signal resulting from electrons from within a given range of scattering angles at each position of the raster. Such detectors may typically consist of a scintillator connected to photomultiplier tube. [13]
Segmented STEM detectors, first introduced in 1994 allow differential phase contrast information to be obtained. [14]
4D STEM involves the use of an imaging camera, such as they hybrid pixel or MAPS direct electron detectors described above, to record an entire convergent beam electron diffraction (CBED) pattern at each STEM raster position. [12] The resulting four-dimensional dataset can then be analyzed to reconstruct arbitrary STEM images, or extract other types of information from the specimen, such as strain, or electric and magnetic field maps. [15]
A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the surface topography and composition of the sample. The electron beam is scanned in a raster scan pattern, and the position of the beam is combined with the intensity of the detected signal to produce an image. In the most common SEM mode, secondary electrons emitted by atoms excited by the electron beam are detected using a secondary electron detector. The number of secondary electrons that can be detected, and thus the signal intensity, depends, among other things, on specimen topography. Some SEMs can achieve resolutions better than 1 nanometer.
Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a sensor such as a scintillator attached to a charge-coupled device.
Electron diffraction refers to changes in the direction of electron beams due to interactions with atoms. Close to the atoms the changes are described as Fresnel diffraction; far away they are called Fraunhofer diffraction. The resulting map of the directions of the electrons far from the sample is called a diffraction pattern, see for instance Figure 1. These patterns are similar to x-ray and neutron diffraction patterns, and are used to study the atomic structure of gases, liquids, surfaces and bulk solids. Electron diffraction also plays a major role in the contrast of images in electron microscopes.
Photoemission electron microscopy is a type of electron microscopy that utilizes local variations in electron emission to generate image contrast. The excitation is usually produced by ultraviolet light, synchrotron radiation or X-ray sources. PEEM measures the coefficient indirectly by collecting the emitted secondary electrons generated in the electron cascade that follows the creation of the primary core hole in the absorption process. PEEM is a surface sensitive technique because the emitted electrons originate from a shallow layer. In physics, this technique is referred to as PEEM, which goes together naturally with low-energy electron diffraction (LEED), and low-energy electron microscopy (LEEM). In biology, it is called photoelectron microscopy (PEM), which fits with photoelectron spectroscopy (PES), transmission electron microscopy (TEM), and scanning electron microscopy (SEM).
Electron backscatter diffraction (EBSD) is a scanning electron microscopy (SEM) technique used to study the crystallographic structure of materials. EBSD is carried out in a scanning electron microscope equipped with an EBSD detector comprising at least a phosphorescent screen, a compact lens and a low-light camera. In this configuration, the SEM incident beam hits the tilted sample. As backscattered electrons leave the sample, they interact with the crystal's periodic atomic lattice planes and diffract according to Bragg's law at various scattering angles before reaching the phosphor screen forming Kikuchi patterns (EBSPs). EBSD spatial resolution depends on many factors, including the nature of the material under study and the sample preparation. Thus, EBSPs can be indexed to provide information about the material's grain structure, grain orientation, and phase at the micro-scale. EBSD is applied for impurities and defect studies, plastic deformation, and statistical analysis for average misorientation, grain size, and crystallographic texture. EBSD can also be combined with energy-dispersive X-ray spectroscopy (EDS), cathodoluminescence (CL), and wavelength-dispersive X-ray spectroscopy (WDS) for advanced phase identification and materials discovery.
Electron crystallography is a method to determine the arrangement of atoms in solids using a transmission electron microscope (TEM). It can involve the use of high-resolution transmission electron microscopy images, electron diffraction patterns including convergent-beam electron diffraction or combinations of these. It has been successful in determining some bulk structures, and also surface structures. Two related methods are low-energy electron diffraction which has solved the structure of many surfaces, and reflection high-energy electron diffraction which is used to monitor surfaces often during growth.
A scanning transmission electron microscope (STEM) is a type of transmission electron microscope (TEM). Pronunciation is [stɛm] or [ɛsti:i:ɛm]. As with a conventional transmission electron microscope (CTEM), images are formed by electrons passing through a sufficiently thin specimen. However, unlike CTEM, in STEM the electron beam is focused to a fine spot which is then scanned over the sample in a raster illumination system constructed so that the sample is illuminated at each point with the beam parallel to the optical axis. The rastering of the beam across the sample makes STEM suitable for analytical techniques such as Z-contrast annular dark-field imaging, and spectroscopic mapping by energy dispersive X-ray (EDX) spectroscopy, or electron energy loss spectroscopy (EELS). These signals can be obtained simultaneously, allowing direct correlation of images and spectroscopic data.
Annular dark-field imaging is a method of mapping samples in a scanning transmission electron microscope (STEM). These images are formed by collecting scattered electrons with an annular dark-field detector.
High-resolution transmission electron microscopy is an imaging mode of specialized transmission electron microscopes that allows for direct imaging of the atomic structure of samples. It is a powerful tool to study properties of materials on the atomic scale, such as semiconductors, metals, nanoparticles and sp2-bonded carbon. While this term is often also used to refer to high resolution scanning transmission electron microscopy, mostly in high angle annular dark field mode, this article describes mainly the imaging of an object by recording the two-dimensional spatial wave amplitude distribution in the image plane, similar to a "classic" light microscope. For disambiguation, the technique is also often referred to as phase contrast transmission electron microscopy, although this term is less appropriate. At present, the highest point resolution realised in high resolution transmission electron microscopy is around 0.5 ångströms (0.050 nm). At these small scales, individual atoms of a crystal and defects can be resolved. For 3-dimensional crystals, it is necessary to combine several views, taken from different angles, into a 3D map. This technique is called electron tomography.
Electron holography is holography with electron matter waves. Dennis Gabor invented holography in 1948 when he tried to improve image resolution in electron microscope. The first attempts to perform holography with electron waves were made by Haine and Mulvey in 1952; they recorded holograms of zinc oxide crystals with 60 keV electrons, demonstrating reconstructions with approximately 1 nm resolution. In 1955, G. Möllenstedt and H. Düker invented an electron biprism, thus enabling the recording of electron holograms in off-axis scheme. There are many different possible configurations for electron holography, with more than 20 documented in 1992 by Cowley. Usually, high spatial and temporal coherence of the electron beam are required to perform holographic measurements.
Kikuchi lines are patterns of electrons formed by scattering. They pair up to form bands in electron diffraction from single crystal specimens, there to serve as "roads in orientation-space" for microscopists uncertain of what they are looking at. In transmission electron microscopes, they are easily seen in diffraction from regions of the specimen thick enough for multiple scattering. Unlike diffraction spots, which blink on and off as one tilts the crystal, Kikuchi bands mark orientation space with well-defined intersections as well as paths connecting one intersection to the next.
Ptychography is a computational method of microscopic imaging. It generates images by processing many coherent interference patterns that have been scattered from an object of interest. Its defining characteristic is translational invariance, which means that the interference patterns are generated by one constant function moving laterally by a known amount with respect to another constant function. The interference patterns occur some distance away from these two components, so that the scattered waves spread out and "fold" into one another as shown in the figure.
Low-voltage electron microscope (LVEM) is an electron microscope which operates at accelerating voltages of a few kiloelectronvolts or less. Traditional electron microscopes use accelerating voltages in the range of 10-1000 keV.
Precession electron diffraction (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.
A stigmator is a component of electron microscopes that reduces astigmatism of the beam by imposing a weak electric or magnetic quadrupole field on the electron beam.
Microcrystal electron diffraction, or MicroED, is a CryoEM method that was developed by the Gonen laboratory in late 2013 at the Janelia Research Campus of the Howard Hughes Medical Institute. MicroED is a form of electron crystallography where thin 3D crystals are used for structure determination by electron diffraction. Prior to this demonstration, macromolecular (protein) electron crystallography was only used on 2D crystals, for example.
John Marius Rodenburg is Professor in the Department of Electronic and Electrical Engineering at the University of Sheffield.
Joanne Etheridge is an Australian physicist. She is Director of the Monash Centre for Electron Microscopy and Professor in the Department of Materials Science and Engineering at Monash University.
4D scanning transmission electron microscopy is a subset of scanning transmission electron microscopy (STEM) which utilizes a pixelated electron detector to capture a convergent beam electron diffraction (CBED) pattern at each scan location. This technique captures a 2 dimensional reciprocal space image associated with each scan point as the beam rasters across a 2 dimensional region in real space, hence the name 4D STEM. Its development was enabled by evolution in STEM detectors and improvements computational power. The technique has applications in visual diffraction imaging, phase orientation and strain mapping, phase contrast analysis, among others.
Angus J Wilkinson is a professor of materials science based at University of Oxford. He is a specialist in micromechanics, electron microscopy and crystal plasticity. He assists in overseeing the MicroMechanics group while focusing on the fundamentals of material deformation. He developed the HR-EBSD method for mapping stress and dislocation density at high spatial resolution used at the micron scale in mechanical testing and micro-cantilevers to extract data on mechanical properties that are relevant to materials engineering.
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