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Electron microscope

An electron microscope is a type of microscope that uses a beam of accelerated electrons as a source of illumination to produce highly magnified images of specimens, achieving resolutions as fine as 0.1 nanometers, far surpassing the 200-nanometer limit of traditional light microscopes due to the much shorter wavelength of electrons compared to visible light.^[1]^[2] The instrument operates in a vacuum, where electrons are generated from a tungsten filament, accelerated by high voltages typically ranging from 20 to 120 kilovolts, and focused using electromagnetic lenses to form an image either by transmission through ultrathin samples or by scanning surfaces.^[1] There are two main variants: the transmission electron microscope (TEM), which passes electrons through a specimen less than 0.5 micrometers thick to reveal internal atomic structures at magnifications exceeding 1 million times, and the scanning electron microscope (SEM), which raster-scans a focused electron beam over a sample's surface to detect secondary or backscattered electrons, providing three-dimensional topographic details at resolutions around 1 nanometer and magnifications up to 400,000 times.^[1]^[3] These principles enable detailed analysis of material composition, crystal structures, and biological ultrastructures that are impossible to resolve with optical methods.^[2] The invention of the electron microscope traces back to 1931, when Ernst Ruska and Max Knoll at the Technical University of Berlin constructed the first prototype TEM, building on Louis de Broglie's 1924 wave-particle duality theory for electrons; Ruska later refined it into a practical instrument by 1933 and received the Nobel Prize in Physics in 1986 for this breakthrough.^[1]^[2] SEM development followed, with initial concepts proposed by Knoll in 1935 and a functional prototype achieved by Manfred von Ardenne in 1937.^[1]^[3] The first commercial TEM, the Siemens EM100, was introduced in 1939, marking the transition from laboratory curiosity to essential tool in scientific research.^[1] Since its inception, electron microscopy has transformed numerous disciplines by providing unparalleled insights into nanoscale phenomena, including viral structures in biology, defect analysis in semiconductors for materials science, and atomic arrangements in nanomaterials for nanotechnology applications.^[2] Modern advancements, such as aberration-corrected lenses and field-emission electron sources, have pushed resolutions below 0.5 angstroms, enabling even finer chemical and elemental mapping through techniques like energy-dispersive X-ray spectroscopy.^[3] Despite challenges like sample preparation requirements and vacuum operation, its superior resolving power continues to drive discoveries across the physical and life sciences.^[1]

Principles of operation

Electron optics and wave nature

The wave nature of electrons, first proposed by Louis de Broglie in 1924, enables their use in microscopy by associating a wavelength with particles based on their momentum. The de Broglie wavelength \lambda is given by the formula \lambda = h / p, where h is Planck's constant and p is the electron's momentum.^[4] For accelerated electrons, this wavelength is inversely proportional to their velocity, allowing high-energy electrons (typically 100–300 keV in microscopes) to achieve de Broglie wavelengths on the order of picometers, such as approximately 3.88 pm at 100 keV.^[5] This is orders of magnitude shorter than visible light wavelengths (around 400–700 nm), which limits light microscopy resolution to about 200 nm, whereas electrons enable sub-angstrom (less than 0.1 nm) imaging potential.^[5]^[6] In electron microscopy, the interaction of the electron beam with matter primarily involves scattering events that form the basis for image contrast. Elastic scattering occurs when an electron is deflected without significant energy loss, preserving its kinetic energy while changing direction, often through Coulomb interactions with atomic nuclei; high-angle elastic events are described by Rutherford scattering, where the differential cross-section is proportional to $1/\sin^4(\theta/2) and \theta is the scattering angle.^[7] In contrast, inelastic scattering involves energy transfer to the sample, such as exciting inner-shell electrons or phonons, leading to energy losses of a few electron volts to thousands, which can generate signals like X-rays or secondary electrons but also broadens the beam.^[8] These interactions, dominated by elastic scattering for imaging due to its role in phase shifts and diffraction, allow electrons to probe atomic-scale structures far beyond light-based methods. Electron optics relies on magnetic fields to manipulate the beam, analogous to glass lenses in light microscopy but exploiting the charged nature of electrons. The focusing principle stems from the Lorentz force, \mathbf{F} = -e (\mathbf{v} \times \mathbf{B}), where e is the electron charge, \mathbf{v} is its velocity, and \mathbf{B} is the magnetic field; in a solenoid or magnetic lens, the azimuthal field component causes off-axis electrons to spiral inward, converging the beam at a focal point. Electromagnetic lenses, combining static magnetic fields with variable currents, enable adjustable focal lengths and beam control, forming images through successive focusing stages. However, imperfections in these fields introduce aberrations that degrade image quality. Spherical aberration in electron optics arises from the lens's inability to focus rays parallel to the optic axis at the same point, with peripheral rays focusing closer than paraxial ones due to stronger field gradients at larger radii, resulting in a blurred disk of confusion proportional to the third power of the aperture angle.^[9] Chromatic aberration occurs because electrons with slightly different energies (from beam energy spread or scattering) experience varying refractive indices in the magnetic field, causing lower-energy electrons to focus shorter than higher-energy ones and further limiting resolution.^[9] These aberrations, inherent to static round lenses as proven by Scherzer's theorem in 1936, restrict practical resolutions despite the short de Broglie wavelength, though they can be mitigated in principle by optimizing lens design without altering the fundamental wave properties.^[10]

Resolution limits and magnification

The resolution in transmission electron microscopy (TEM) is fundamentally governed by the Abbe diffraction limit, adapted for electron optics as d = 0.61 \lambda / \sin \alpha, where d is the minimum resolvable distance, \lambda is the de Broglie wavelength of the electrons, and \alpha is the semi-angle of the objective aperture.^[11] This formula highlights how shorter electron wavelengths—on the order of picometers at typical accelerating voltages—combined with optimized aperture angles, enable resolutions far superior to light microscopy. However, practical implementation requires careful control of optical parameters to approach this theoretical bound. Magnification in electron microscopes is defined as the ratio of the image size to the object size, M = image size / object size, and serves to enlarge features for visualization. In TEM, practical magnification limits reach up to 50 million times, allowing detailed examination of atomic-scale structures, though higher values are often limited by specimen stability and detector capabilities rather than optical constraints.^[12] Several factors impose practical limits on resolution beyond the diffraction barrier, including lens aberrations such as spherical and chromatic effects, which distort the electron beam focus; beam divergence, which reduces coherence; and stability requirements for voltage, current, and mechanical vibrations that prevent drift during imaging.^[6] These elements collectively degrade image contrast and sharpness, necessitating precise instrumentation to mitigate their impact. A key distinction exists between point resolution and the information limit in electron microscopy. Point resolution refers to the smallest distance at which two points can be distinguished under ideal coherent illumination, primarily limited by spherical aberration in uncorrected systems. In contrast, the information limit accounts for partial coherence effects from chromatic aberration and energy spread in the electron source, which dampen high-frequency information transfer through an envelope function, effectively reducing resolvable details. Modern aberration-corrected instruments achieve effective resolutions around 0.05 nm by pushing this information limit, enabling atomic-scale imaging in materials science applications.^[13]^[14] Across electron microscope types, resolution varies significantly: TEM and scanning transmission electron microscopy (STEM) routinely attain sub-nanometer levels, often below 0.1 nm with corrections, while scanning electron microscopy (SEM) is typically limited to approximately 1 nm due to its surface-scanning nature and larger probe sizes.^[15]

History

Early development and invention

The discovery of the electron by J. J. Thomson in 1897 marked a pivotal moment in physics, as his experiments with cathode rays demonstrated that these rays consisted of negatively charged particles far smaller and lighter than atoms, which he termed "corpuscles" (later renamed electrons). This identification of the electron as a discrete particle provided the essential building block for subsequent technologies involving electron beams.^[16] Building on this, Louis de Broglie proposed in 1924 that all matter, including electrons, exhibits wave-particle duality, hypothesizing that particles possess an associated wavelength inversely proportional to their momentum.^[17] This revolutionary idea extended quantum concepts from light to matter, suggesting that electrons could be manipulated like waves for imaging purposes. The wave nature of electrons was experimentally verified in 1927 through the Davisson-Germer experiment, conducted at Bell Laboratories, where electrons diffracted off a nickel crystal lattice produced interference patterns consistent with de Broglie's predicted wavelength of approximately 0.165 nm for 54 eV electrons. This confirmation established electrons as suitable for high-resolution optics, far surpassing the wavelength limitations of visible light (around 500 nm).^[18] These foundational concepts—electrons as particles with wave properties—directly inspired the development of electron microscopy in the late 1920s and early 1930s. In 1931, Ernst Ruska and Max Knoll at the Technical University of Berlin constructed the first prototype transmission electron microscope, using magnetic coils as lenses to focus an electron beam and achieve a magnification of 400× on a test grid, exceeding the resolution of contemporary light microscopes. This device demonstrated the feasibility of electron-optical imaging but suffered from beam instability and low contrast. Independently, in May 1931, Reinhold Rüdenberg at Siemens-Schuckertwerke filed a patent for an electron microscope design using electrostatic lenses (German patent application, leading to US Patent 2,058,914 in 1936).^[19] By 1933, Ruska refined the prototype with improved magnetic lenses, attaining a magnification of 12,000× and enabling clearer imaging of fine structures, such as etched platinum sheets.^[20] A key innovation came in 1932 when Ruska, in collaboration with Bodo von Borries, patented a short-focal-length magnetic polepiece lens (German Patent No. 680284), which minimized aberrations and allowed precise electron beam control essential for practical microscopy. This design became the basis for all subsequent magnetic electron microscopes.^[20] These advancements culminated in 1939 with Siemens & Halske producing the first commercial transmission electron microscope, the Super Microscope, capable of 20,000× magnification and resolutions down to 10 nm, marking the transition from laboratory prototypes to industrial tools.^[21] Parallel efforts in the United States during the 1930s, including early electron optics research at institutions like MIT, yielded prototypes such as the 1938 transmission electron microscope built at the University of Toronto (the first successful one in North America), but these initial designs achieved only limited resolution and stability compared to German innovations.^[22]

Key advancements and commercialization

During World War II, the United States emerged as a leader in electron microscopy development, driven by the need for advanced materials analysis in military applications. Vladimir Zworykin, leading a team at the Radio Corporation of America (RCA), oversaw the creation of improved transmission electron microscopes (TEMs) in the early 1940s, including a more affordable model demonstrated in 1942 that enabled broader wartime deployment for studying alloys and other materials.^[23]^[24] The invention of the scanning electron microscope (SEM) built on early prototypes from the 1930s. In 1935, Max Knoll constructed the first SEM prototype, demonstrating basic scanning principles for surface imaging. Two years later, in 1937, Manfred von Ardenne advanced the design with a scanning transmission system using a finely focused electron probe, laying foundational concepts for high-resolution surface examination. Commercialization followed postwar, with practical prototypes developed in the early 1950s by Charles Oatley and his team at the University of Cambridge, culminating in the first commercial SEM, the Stereoscan, released by the Cambridge Scientific Instrument Company in 1965, which facilitated industrial adoption for materials characterization.^[25]^[26] Further innovations in the 1960s expanded electron microscopy capabilities. In 1966, Albert Crewe at the University of Chicago developed the scanning transmission electron microscope (STEM) with an annular dark-field detector, enabling atomic-scale imaging by collecting scattered electrons from a focused probe, which improved contrast for heavy atoms in biological and materials samples.^[27]^[28] Commercialization accelerated in the mid-20th century, particularly through Japanese manufacturers. Hitachi began producing electron microscopes in the 1940s and entered the international market in the 1950s with reliable TEM models, while Japan Electron Optics Laboratory (JEOL), founded in 1949, started manufacturing magnetic TEMs in 1950 and expanded globally by the mid-1950s, making high-vacuum instruments more accessible for research and industry. By the 1970s, advancements in lens design and electron sources pushed TEM resolution to 0.2 nm, as exemplified by JEOL's JEM-100B model introduced in 1968, which supported detailed crystallographic studies.^[29]^[30]^[31] The field's impact was recognized in 1986 when Ernst Ruska received the Nobel Prize in Physics for his foundational work in electron optics and the design of the first electron microscope, highlighting its transformative role in scientific imaging up to that point.^[32]

Types of electron microscopes

Transmission electron microscope (TEM)

The transmission electron microscope (TEM) is configured such that a beam of accelerated electrons passes through an ultra-thin specimen, generally less than 100 nm in thickness, to produce an image from the electrons transmitted through the sample.^[33] This electron-transparent preparation allows for the visualization of internal structures at the nanoscale, with the beam interacting via scattering and absorption to generate contrast.^[34] The setup relies on electromagnetic lenses to focus the beam, similar to general electron optics principles.^[33] In TEM imaging, two primary modes are employed: bright-field and dark-field. Bright-field imaging collects the direct, unscattered transmitted electrons, resulting in areas of higher mass-thickness appearing darker due to increased scattering.^[34] Conversely, dark-field imaging utilizes scattered electrons, often those diffracted at specific angles, to highlight crystalline features and defects by blocking the direct beam.^[33] TEM finds extensive applications in revealing biological ultrastructures, such as the morphology of viruses including polyomaviruses and coronaviruses, enabling direct visualization and diagnosis in clinical samples.^[35] In materials science, it is used to examine defects like dislocations in metallic nanostructures and carbon-based materials, providing insights into phase evolution and atomic arrangements.^[36] These capabilities support fields from virology to nanomaterial engineering.^[37] TEM instruments achieve magnifications ranging from 10× to 1,000,000×, with resolutions approaching 0.1 nm in aberration-corrected systems, allowing atomic-scale imaging.^[34] Historically, the first practical applications of TEM in metallurgy occurred in the 1940s, when Robert Heidenreich at Bell Labs used electrolytic thinning to study metal crystal textures in 1949.^[28]

Scanning electron microscope (SEM)

The scanning electron microscope (SEM) operates by directing a finely focused beam of electrons across the surface of a sample in a raster-scanning pattern, point by point, to generate images based on the emitted signals from electron-sample interactions.^[38] This sequential scanning allows for the construction of detailed topographic maps of the specimen's surface, with the intensity of detected signals determining the brightness of each pixel in the resulting image.^[39] SEM accommodates bulk samples up to several centimeters in scale, unlike transmission-based methods that require ultrathin sections, enabling the examination of three-dimensional objects without extensive dissection.^[40] Additionally, environmental SEM variants operate at lower vacuum levels, facilitating imaging of hydrated or uncoated specimens such as biological materials that might otherwise degrade under high vacuum.^[41] The SEM's depth of field is approximately 300 times greater than that of optical microscopes, owing to the small interaction volume of the electron beam with the sample surface, which allows sharp focus across a wide range of heights in the specimen.^[42] Typical SEM resolution ranges from 0.5 to 10 nm, depending on beam energy, probe current, and sample preparation, while magnification can reach up to 1,000,000× for high-detail surface features.^[39]^[43] These capabilities arise from the short de Broglie wavelength of electrons and precise beam control, enabling visualization of nanoscale surface structures.^[39] Emitted signals, such as secondary electrons for topography and backscattered electrons for compositional contrast, are detected to form the image.^[38] SEM finds widespread applications in analyzing fracture surfaces to identify failure mechanisms in materials, such as crack propagation paths in metals or composites.^[44] In biology, it reveals intricate surface details on specimens like insect exoskeletons or cellular membranes after appropriate coating.^[45] Forensic investigations employ SEM to examine trace evidence, including gunshot residue particles or fiber morphologies for evidentiary matching.^[46]

Scanning transmission electron microscope (STEM)

The scanning transmission electron microscope (STEM) is a versatile instrument that integrates the scanning probe approach with transmission electron microscopy principles, enabling high-resolution imaging of thin specimens at the atomic scale. In STEM, a finely focused electron beam is raster-scanned across the sample, allowing for the collection of transmitted electrons to form images that reveal internal structure and composition with exceptional detail. This hybrid design facilitates simultaneous acquisition of multiple signal types, making it particularly suited for analytical studies in materials science.^[47] The basic setup of a STEM involves a high-brightness electron source, such as a field emission gun, which generates a probe beam converged to a sub-angstrom diameter and scanned over the specimen using deflection coils. The thin sample, typically less than 100 nm thick, is positioned such that electrons transmit through it, interacting via elastic and inelastic scattering processes. Multiple detectors are arranged below the sample to capture these transmitted electrons at various angles: a bright-field detector collects unscattered or low-angle scattered beams for phase-contrast information, while segmented or annular detectors record higher-angle scattering for amplitude-contrast details. This configuration allows for real-time imaging and simultaneous data collection from diverse signals, enhancing the efficiency of nanoscale characterization.^[47]^[48] The development of STEM traces back to the pioneering efforts of Albert Crewe and his team at the University of Chicago in the late 1960s and 1970s, who constructed the first practical instrument capable of atomic-resolution imaging using a field-emission source and annular detectors. Crewe's innovations, including the demonstration of single-atom visibility in 1970, laid the foundation for modern STEM by emphasizing Z-contrast imaging over conventional bright-field modes. By the 1980s, commercial STEM systems emerged, and with the advent of aberration correction in the 2000s, STEM became a standard tool in analytical electron microscopy for its ability to combine high spatial resolution with spectroscopic capabilities. Today, STEM instruments are integral to advanced laboratories worldwide, supporting multidisciplinary research from condensed matter physics to nanotechnology.^[49]^[50] A key imaging mode in STEM is annular dark-field (ADF) imaging, which employs a ring-shaped detector to collect high-angle scattered electrons, producing Z-contrast images where intensity scales with the square of the atomic number (Z²) due to incoherent Rutherford scattering. This sensitivity to atomic number enables clear visualization of heavy elements against lighter backgrounds without the phase contrast artifacts common in bright-field TEM, making ADF-STEM ideal for locating impurities or mapping elemental distributions in complex alloys. The contrast arises primarily from thermal diffuse scattering at elevated angles, minimizing contributions from crystalline diffraction and providing chemically intuitive images.^[51]^[52] STEM finds extensive applications in atomic-scale materials analysis, particularly for studying catalyst nanoparticles where it reveals active site distributions and structural dynamics under operational conditions. For instance, in situ STEM observations of platinum-based catalysts during reactions provide insights into particle sintering and facet evolution, aiding the design of more efficient heterogeneous catalysts. Another prominent application is 4D-STEM, which records a full diffraction pattern at each scan position to enable strain mapping, quantifying lattice distortions in semiconductors or 2D materials with nanometer precision. This technique has been instrumental in analyzing strain fields around defects in graphene heterostructures, informing device performance optimization.^[47]^[53]^[54] With aberration correction, STEM achieves sub-0.05 nm resolution, allowing direct imaging of atomic columns and even light elements like oxygen in oxides. This breakthrough, realized in instruments like the TEAM 0.5 project microscope in 2007, extends the probe size limit from ~0.1 nm to below 50 pm, enabling quantitative analysis of beam-sensitive materials without significant damage. Such resolutions have revolutionized fields like quantum materials research by permitting the observation of subtle bonding arrangements and electronic inhomogeneities.^[55]

Instrumentation and components

Electron sources

Electron sources in electron microscopes generate beams of electrons that are accelerated and focused to illuminate specimens, with the source type determining key beam properties such as brightness, coherence, and stability.^[57] The primary types include thermionic emission guns and field emission guns (FEGs), each suited to different imaging demands based on their emission mechanisms and performance characteristics.^[58] Thermionic guns, the most common for standard applications, rely on heating a cathode filament to emit electrons via thermal excitation. Typically, these use a tungsten hairpin filament heated to approximately 2700 K, producing a beam with moderate brightness of about 10^5 A/cm² sr.^[58] This design is robust and cost-effective, making it widely adopted in conventional transmission electron microscopes (TEMs) and scanning electron microscopes (SEMs) where ultra-high resolution is not required.^[59] However, the larger source size (crossover diameter around 50 μm) limits beam coherence compared to advanced sources.^[60] Field emission guns (FEGs) offer superior performance for high-resolution imaging by extracting electrons through quantum tunneling under a strong electric field applied to a sharply pointed cathode. These include cold field emission guns, operating at room temperature, and Schottky (thermal-assisted) FEGs, which heat the emitter slightly to enhance stability.^[61] FEGs achieve much higher brightness, typically 10^8 to 10^9 A/cm² sr, enabling smaller probe sizes and better signal-to-noise ratios in demanding applications.^[62] Their use is prevalent in advanced TEMs and SEMs for atomic-scale imaging.^[57] A fundamental limitation of all electron sources is shot noise, arising from the Poisson statistics of electron emission, where the variance in electron count equals the mean number of electrons, leading to fluctuations that degrade image quality.^[63] Additionally, the energy spread (ΔE) of the emitted electrons—typically 1–3 eV for thermionic guns and 0.3–0.7 eV for FEGs—contributes to chromatic aberration downstream, as electrons of varying energies focus differently.^[64] This spread is narrower in FEGs due to the absence of thermal broadening, improving resolution in aberration-sensitive systems.^[65] Electrons from the source are accelerated by voltages ranging from 1 kV in low-energy SEMs to 400 kV in high-voltage TEMs, with 100–300 kV often providing an optimal balance between achieving short de Broglie wavelengths for high resolution and minimizing beam-induced sample damage in sensitive materials.^[66] Higher voltages reduce the relative impact of energy spread on aberrations but increase penetration and potential structural alteration in specimens.^[67]

Lenses and beam control

In electron microscopes, electromagnetic lenses serve as the primary optical elements for focusing and directing the electron beam, replacing glass lenses used in light microscopy due to the inability of materials to refract electrons effectively. These lenses operate on the principle of the Lorentz force, where a magnetic field generated by current-carrying coils deflects charged electrons toward the optical axis. Solenoid designs consist of helical windings around a soft iron core to produce a uniform axial magnetic field, while aperture designs incorporate pole pieces and a central aperture to shape the field more precisely, enhancing focusing power and reducing aberrations. The focal length f of such a magnetic lens is approximated by

f \approx \frac{V}{e B^2 r^2 / (8 m)},

where V is the accelerating voltage, e is the electron charge, B is the magnetic field strength, r is the bore radius, and m is the electron rest mass; this relation highlights the lens's sensitivity to field strength and beam energy.^[68] Condenser lenses, typically one or two in series above the sample, control beam convergence and illumination intensity by adjusting the current to vary their focal length, allowing the electron beam—emanating from the source—to be demagnified into a fine probe or diverged for parallel illumination in diffraction experiments. The objective lens, positioned closest to the sample, collects and focuses transmitted or scattered electrons to form the initial image, with its short focal length (often 1–5 mm) enabling high magnification up to 1 million times while determining key resolution limits through beam-sample interactions. Together, these lenses form a compound optical system that achieves sub-angstrom precision in modern instruments.^[69] In scanning modes, such as those employed in scanning electron microscopes (SEM) and scanning transmission electron microscopes (STEM), scanning coils provide beam deflection via orthogonal magnetic fields, rastering the focused probe across the sample in a pixel-by-pixel pattern to build images sequentially. These coils enable scan speeds up to $10^6 points per second, balancing acquisition time with signal quality for dynamic or large-area imaging. Alignment systems, including stigmators and deflectors, ensure beam symmetry; stigmators apply differential currents to correct astigmatism arising from lens imperfections, while deflectors fine-tune the beam path to center it on the optical axis, preventing off-axis aberrations and maintaining resolution.^[70]

Detectors and vacuum systems

In electron microscopes, detectors are crucial for capturing the various signals generated by electron-sample interactions, such as transmitted electrons in transmission electron microscopy (TEM) and secondary electrons in scanning electron microscopy (SEM). For transmitted electrons, which pass through thin samples to form images based on beam intensity, detectors typically consist of a scintillator that converts electron energy into light, coupled to a photodiode or photomultiplier tube for signal amplification and detection.^[71] This setup enables high-sensitivity imaging of internal sample structures by quantifying the transmitted beam's variations. In contrast, the Everhart-Thornley detector, widely used for secondary electrons—low-energy electrons emitted from the sample surface providing topographic information—employs a scintillator biased to attract and accelerate these electrons, followed by a photomultiplier tube for efficient signal collection. Electron microscopes require high vacuum environments to minimize electron scattering by residual gas molecules, ensuring a mean free path sufficient for the electron beam to reach the sample unaltered. Operating pressures typically range from ~1 Pa (or higher, up to ~1000 Pa) in low-vacuum or environmental scanning modes to 10^{-10} Pa in ultra-high vacuum transmission systems, with conventional high-vacuum SEM and TEM operating around 10^{-5} to 10^{-7} Pa.^[72] To achieve these levels, turbomolecular pumps, which use high-speed rotating blades to transfer momentum to gas molecules, are employed for initial high-vacuum evacuation, often backed by rotary vane roughing pumps.^[72] Ion pumps, utilizing strong electric fields to ionize and bury gas molecules into a titanium cathode, maintain ultra-high vacuums without moving parts, complementing turbomolecular systems in critical regions like the electron gun. Differential pumping is essential in electron microscopes to accommodate varying pressure requirements across components, preventing contamination while allowing sample introduction. This involves separate vacuum chambers connected by small apertures: the electron gun chamber is maintained at approximately 10^{-7} Pa to protect the source from outgassing, while the sample chamber in SEMs operates at around 10^{-5} Pa for standard imaging.^[73] Turbomolecular and ion pumps are strategically placed in each stage to sustain these gradients, with the aperture limiting gas flow between compartments. For imaging hydrated or beam-sensitive samples, variable pressure SEM (VP-SEM) or environmental SEM (ESEM) adaptations introduce controlled gas pressures up to several hundred Pa in the sample chamber, using differential pumping to isolate the high-vacuum column.^[74] This allows observation of wet biological specimens without dehydration, as water vapor neutralizes charging and maintains sample integrity during secondary electron imaging.^[75]

Imaging and analytical modes

Surface imaging modes

Surface imaging modes in scanning electron microscopy (SEM) primarily rely on electrons emitted from the sample surface following interaction with the incident electron beam. These modes utilize secondary electrons (SE) and backscattered electrons (BSE) to generate images that reveal surface topography and compositional variations, respectively. By scanning the focused beam across the sample and detecting these emitted signals, high-resolution surface details can be visualized without requiring transmission through the specimen.^[76] Secondary electrons are low-energy electrons, typically with energies less than 50 eV, generated through inelastic scattering events where the incident beam excites and ejects electrons from the sample's outer atomic shells.^[77] These SEs originate from a very shallow depth, with an escape depth of approximately 1-5 nm, making them highly sensitive to surface topography and providing excellent contrast for features like edges, roughness, and texture.^[78] In SE imaging, the yield of detected electrons varies with local surface orientation relative to the detector, enhancing the three-dimensional appearance of the image.^[76] Backscattered electrons, in contrast, are high-energy electrons (generally >50 eV, approaching the incident beam energy) produced by elastic scattering within the sample, where primary electrons are redirected back toward the surface.^[79] BSEs emerge from a larger interaction volume, on the order of micrometers in depth and lateral extent, and their yield depends strongly on the atomic number (Z) of the elements present, resulting in Z-contrast imaging where heavier elements appear brighter due to higher backscattering efficiency.^[80] This mode is particularly useful for distinguishing phases or inclusions in materials based on composition, though it offers less topographic detail than SE imaging.^[76] The efficiency of SE and BSE emission is characterized by yield curves, which plot the secondary electron yield δ(E₀) and backscattered electron yield η(E₀) as functions of the incident beam energy E₀. The SE yield δ(E₀) typically rises from near zero at low E₀, peaks at a maximum (often around 200-1000 eV depending on the material), and then decreases asymptotically, reflecting the balance between electron generation and escape probability.^[81] Similarly, the BSE yield η(E₀) increases monotonically with E₀ and atomic number, approaching values up to 0.5-0.6 for high-Z materials at keV energies, which informs optimal beam conditions for imaging.^[80] These curves, derived from experimental measurements and Monte Carlo simulations, guide the selection of accelerating voltage to maximize signal while minimizing beam damage or charging.^[82] For non-conductive samples, which can accumulate charge under the electron beam and distort images, variable pressure (or environmental) SEM modes introduce a low-pressure gas (typically 0.1-10 Torr of water vapor or nitrogen) into the specimen chamber to facilitate charge neutralization.^[83] The incident electrons ionize the gas molecules, producing positive ions that are attracted to negatively charged regions on the sample surface, while free electrons from ionization can also contribute to balancing the charge; this gas-mediated process allows direct imaging of insulators like biological tissues or polymers without conductive coatings.^[84] The presence of gas slightly scatters the primary beam and amplifies secondary signals through cascades, but it enables hydrated or wet sample observation under near-native conditions.^[85]

Transmission and diffraction modes

In transmission electron microscopy (TEM), transmitted electrons are utilized to reveal internal structures and crystallographic details of thin specimens. These modes exploit the interaction of the electron beam with the sample, where unscattered or minimally scattered electrons pass through, while diffracted electrons provide information on atomic arrangements. Key techniques include bright-field imaging, dark-field imaging, electron diffraction patterns, and phase contrast methods, each offering distinct contrast mechanisms for analyzing materials from metals to biomolecules.^[86] Bright-field TEM imaging employs the direct, unscattered electron beam to form the image, with an objective aperture blocking scattered electrons to enhance contrast. In this mode, regions of the sample that transmit more electrons appear brighter, while areas with higher mass density or thickness scatter electrons more effectively, resulting in darker features due to mass-thickness contrast. This contrast arises because heavier atoms or thicker sections increase electron scattering probability, reducing the intensity of transmitted electrons reaching the detector. Bright-field mode is particularly effective for visualizing overall morphology and identifying dense inclusions in materials like alloys or biological tissues stained for enhanced scattering.^[86]^[87] Dark-field TEM imaging, in contrast, selects diffracted electrons by centering a specific diffraction spot on the optic axis while blocking the direct beam, producing a dark background with bright features from scattering sites. This mode highlights crystal defects such as dislocations or stacking faults, as these regions bend the lattice and direct diffracted electrons into the aperture, increasing local intensity. It is also valuable for distinguishing crystallographic phases in polycrystalline samples by selectively imaging electrons from particular orientations. By emphasizing scattered electrons, dark-field reveals structural heterogeneities that may be obscured in bright-field views.^[86]^[88] Electron diffraction modes provide crystallographic information by analyzing the angular distribution of transmitted electrons. In selected area electron diffraction (SAD), a small aperture selects a region (typically 0.5–5 μm) of the sample, and a parallel electron beam produces a spot pattern on the detector, where spot positions correspond to reciprocal lattice vectors. These patterns obey Bragg's law, given by

n\lambda = 2d \sin\theta

where n is an integer, \lambda is the electron wavelength, d is the interplanar spacing, and \theta is the diffraction angle; measuring \theta allows determination of d for phase identification and orientation mapping. Convergent beam electron diffraction (CBED) improves resolution by using a focused, convergent beam (probe size <50 nm) to generate disk-like patterns with internal symmetry, enabling point group determination and thickness measurement from higher-order Laue zones. CBED offers higher spatial precision than SAD for local structure analysis in nanomaterials.^[88]^[89]^[90] Phase contrast modes enhance visibility of weakly scattering objects, such as unstained biomolecules, by converting phase shifts in the electron wave to amplitude differences. The defocus method achieves this by slightly underfocusing the objective lens, transferring low-spatial-frequency phase information into contrast, though it limits resolution due to spherical aberration at larger defocus values. The Zernike phase plate method inserts a thin-film plate (e.g., amorphous carbon, ~30 nm thick with a central hole) in the back focal plane to shift the phase of unscattered electrons by \pi/2 relative to scattered ones, providing uniform contrast at focus without resolution loss. This approach has enabled high-resolution imaging of proteins like GroEL at ~1.2 nm and viruses, improving signal-to-noise for cryo-TEM applications.^[91]^[92]

Spectroscopic analysis modes

Spectroscopic analysis modes in electron microscopy utilize the interactions between the electron beam and sample to extract chemical composition, electronic structure, and bonding information. These techniques rely on detecting emitted X-rays or energy losses from inelastic scattering events, enabling elemental identification and quantification at high spatial resolutions. In transmission electron microscopes (TEM) and scanning transmission electron microscopes (STEM), such modes complement imaging by providing analytical data from regions as small as a few nanometers. Energy-dispersive X-ray spectroscopy (EDS), also known as EDX, detects X-rays generated when incident electrons interact with sample atoms, producing both a continuous bremsstrahlung spectrum from decelerated electrons and characteristic X-rays unique to each element's atomic structure. The bremsstrahlung background provides information on beam energy, while characteristic peaks allow elemental identification from beryllium to uranium, with detection limits around 0.1 wt% for most elements. Quantification in thin specimens, typically less than 100 nm thick, employs the Cliff-Lorimer method, which relates the ratio of characteristic X-ray intensities from two elements to their concentration ratio using experimentally determined k-factors, assuming negligible absorption and fluorescence effects. This approach, developed for STEM applications, achieves accuracies of 1-5% relative error for major elements in alloys and minerals.^[93] Electron energy-loss spectroscopy (EELS) measures the energy lost by transmitted electrons due to inelastic scattering, typically in the range of 10-1000 eV, revealing details about electronic excitations and core-level transitions. Low-loss EELS (<50 eV) probes plasmons and valence electrons, while core-loss EELS (>50 eV) identifies elements via ionization edges analogous to X-ray absorption. The fine structure near these edges, known as energy-loss near-edge structure (ELNES), encodes bonding and coordination information by reflecting the density of unoccupied states; for example, shifts in the silicon L-edge ELNES indicate Si-O versus Si-Si bonding environments. EELS offers superior energy resolution (0.1-1 eV) compared to EDS, enabling analysis of light elements like lithium and hydrogen, though it requires thinner samples (<50 nm) to minimize multiple scattering. Wavelength-dispersive spectroscopy (WDS) provides higher energy resolution (about 5-20 eV) than EDS (typically 130 eV), making it particularly effective for resolving overlapping peaks and quantifying light elements such as boron, carbon, nitrogen, and oxygen in electron microprobes and TEM attachments. By diffracting X-rays with a curved crystal analyzer tuned to specific wavelengths via Bragg's law, WDS achieves lower detection limits (down to 10 ppm) and better peak-to-background ratios, though it requires sequential scanning and longer acquisition times. This mode is valuable for precise compositional analysis in geological and materials samples where EDS sensitivity to light elements is limited. In STEM, spectrum imaging combines raster scanning with EDS or EELS acquisition at each pixel to generate nanoscale elemental maps, achieving spatial resolutions of 1-5 nm for heavy elements and 5-10 nm for lighter ones. This technique produces hyperspectral datasets where elemental distributions are extracted via background subtraction and peak fitting, enabling visualization of segregation, precipitates, or diffusion profiles; for instance, in bimetallic nanoparticles, it reveals core-shell structures with atomic percent precision. Such mapping supports correlative studies of structure-property relationships in nanomaterials.^[94]

Advanced techniques and instruments

Aberration correction

Aberration correction in electron microscopes addresses the inherent imperfections of electromagnetic lenses, primarily spherical and chromatic aberrations, which limit spatial resolution by blurring the electron probe. Spherical aberration, characterized by the coefficient C_s, causes electrons at different radial distances from the optical axis to focus at varying points, degrading image sharpness; this can be corrected using multipole lens systems, such as quadrupole-octupole configurations that dynamically adjust the electron trajectories to compensate for these deviations. The quadrupole-octupole corrector, developed by Krivanek and colleagues, was first demonstrated in 1999 on a scanning transmission electron microscope (STEM), enabling sub-angstrom probe sizes by expanding the usable angular aperture without introducing additional aberrations.^[95] Chromatic aberration arises from variations in electron energy, leading to defocusing of off-energy electrons; correction methods include energy filtering to select a narrow energy bandwidth or Wien filters, which employ crossed electric and magnetic fields to counteract energy-dependent deflections without dispersing the beam spatially. Wien filters, as implemented in modern correctors, provide precise chromatic correction (C_c) by operating in a velocity-mapping mode that aligns trajectories for electrons of different energies. These systems, often integrated with spherical aberration correctors, have been refined since the early 2000s to handle both axial and off-axial aberrations comprehensively.^[96] The implementation of aberration correction has dramatically enhanced resolution in STEM, reducing the information limit from approximately 0.2 nm to below 0.05 nm at typical accelerating voltages of 100–300 kV, as achieved in early corrected instruments like the TEAM 0.5 microscope. This improvement facilitates atomic-resolution annular dark-field (ADF) STEM imaging, where high-angle scattering provides Z-contrast sensitive to atomic number, allowing visualization of light elements such as oxygen and carbon alongside heavier atoms in complex materials. By minimizing probe tails and enabling brighter, smaller probes, correction suppresses background noise, improving signal-to-noise ratios for quantitative analysis of atomic structures and defects.^[55]^[97] Commercial adoption began in the early 2000s with systems from Nion, Inc., featuring proprietary quadrupole-octupole correctors for STEM, and CEOS GmbH, offering hexapole-based correctors adaptable to transmission electron microscopes (TEM) and STEM platforms. These technologies, initially prototypes, became standard in high-end instruments by the 2010s, with over 500 corrected microscopes installed globally, enabling routine sub-0.1 nm imaging in materials science and nanotechnology research. Ongoing advancements, such as integrated fifth-order corrections, continue to push resolution limits while maintaining stability for long-duration experiments.^[98]

Cryo-electron microscopy

Cryo-electron microscopy (cryo-EM) is a specialized imaging technique that enables the visualization of biological specimens in their near-native, frozen-hydrated state, preserving delicate structures without the artifacts introduced by chemical fixation or staining. By rapidly freezing samples to form vitreous ice, cryo-EM minimizes dehydration and structural distortion, allowing high-resolution imaging of macromolecules and cellular components at cryogenic temperatures. This method builds on transmission electron microscopy (TEM) principles but incorporates low-temperature environments to reduce beam-induced damage and maintain sample integrity.^[99] The vitrification process is central to cryo-EM, involving the ultra-rapid freezing of aqueous biological samples to prevent ice crystal formation, which would otherwise damage cellular structures. Samples are typically applied to a grid, blotted to thin the suspension, and plunge-frozen into liquid ethane cooled by liquid nitrogen, achieving cooling rates exceeding 10^5 K/s to form amorphous (vitreous) ice. This non-crystalline state preserves the hydrated conformation of proteins and complexes, avoiding the disruptive effects of hexagonal or cubic ice crystals. Automated systems like the Vitrobot facilitate reproducible vitrification, ensuring uniform ice thickness for optimal imaging.^[100] In cryo-TEM and cryo-scanning TEM (cryo-STEM), specimens are imaged using specialized stages maintained at approximately 90-100 K to keep the vitreous ice stable and suppress thermal motion. The electron beam is operated at low doses, typically 10-50 electrons/Å², to minimize radiation damage, which can cause bond breakage and conformational changes in sensitive biomolecules. Direct electron detectors and phase plates enhance signal-to-noise ratios under these conditions, enabling the capture of high-fidelity projections. Cryo-STEM complements cryo-TEM by providing annular dark-field imaging for thicker samples, further reducing dose requirements through focused probes.^[101]^[102]^[103] Single-particle analysis in cryo-EM involves collecting thousands to hundreds of thousands of two-dimensional projections of purified macromolecules in random orientations, followed by computational alignment and 3D reconstruction to achieve resolutions often better than 3 Å, typically in the range of 1.5-4 Å. This approach, pioneered by Joachim Frank, uses algorithms like RELION or cryoSPARC to classify particles, correct for heterogeneity, and generate density maps that reveal atomic details without averaging over crystalline arrays. For flexible proteins, advanced methods such as 3D variability analysis capture conformational dynamics from the dataset.^[104]^[105] Key advances in cryo-EM were recognized by the 2017 Nobel Prize in Chemistry, awarded to Jacques Dubochet for developing vitrification, Joachim Frank for single-particle reconstruction, and Richard Henderson for demonstrating atomic-resolution potential. Complementing these, cryo-electron tomography (cryo-ET) extends the technique to in situ cellular imaging, acquiring tilt series from frozen sections to reconstruct 3D tomograms of macromolecular arrangements within their native context, often at 3-10 Å resolution. By 2025, cryo-EM has become routine for protein structure determination, with over 30,000 entries in the Protein Data Bank, including landmark structures like the SARS-CoV-2 spike protein that informed vaccine design.^[106]^[107]

Sample preparation

Methods for biological samples

Biological samples for electron microscopy require careful preparation to preserve their delicate, hydrated structures while ensuring compatibility with the instrument's high vacuum environment. Fixation is the initial step, stabilizing cellular components against subsequent processing. Chemical fixation commonly employs glutaraldehyde, a cross-linking agent that penetrates tissues and reacts with proteins to form stable bonds, typically at concentrations of 2-4% in a buffered solution such as 0.1 M sodium cacodylate at pH 7.4 for 1-2 hours or overnight at 4°C.^[108]^[109] This is often followed by secondary fixation with osmium tetroxide (1-2%) to enhance membrane preservation by reacting with lipids, applied for 30-60 minutes at room temperature.^[38] Cryo-fixation alternatives, such as plunge-freezing into liquid ethane cooled by liquid nitrogen, rapidly vitrify samples (up to 10-20 μm thick) without ice crystal formation, preserving native hydration and ultrastructure in milliseconds.^[110] High-pressure freezing (HPF) extends this to thicker specimens (up to 200 μm) by applying 2,000 bar pressure during freezing to suppress ice nucleation, commonly using devices like the Leica EM HPM100.^[111] After fixation, samples are dehydrated using a graded series of ethanol (30-100%) to remove water, which is incompatible with epoxy resin embedding, followed by infiltration with resin such as Epon or LR White over 24-48 hours. Sectioning via ultramicrotomy produces thin slices suitable for transmission electron microscopy (TEM), typically 50-100 nm thick to minimize beam scattering while allowing electron transmission. This process uses an ultramicrotome equipped with a diamond knife, where the embedded block advances incrementally (e.g., 70 nm per section) against the knife edge at speeds of 0.5-2 mm/s, collecting ribbons of sections on copper grids floated on water in the knife boat.^[112]^[113] Diamond knives, with edge radii below 10 nm, provide superior durability and sharpness for biological resins compared to glass knives, reducing compression artifacts during cutting at room temperature or cryotemperatures (-90 to -110°C for frozen-hydrated samples).^[114] Contrast enhancement through staining is essential for visualizing cellular details in TEM, as biological tissues inherently offer low electron density. Heavy metal stains like uranyl acetate (2-4% aqueous or methanolic) bind to proteins, nucleic acids, and lipids, providing negative staining by scattering electrons around structures; sections are immersed for 5-10 minutes followed by rinsing.^[115]^[116] Lead citrate (Reynolds' formulation) is then applied for 2-5 minutes to further stain proteins and ribonucleoprotein granules, often yielding double-contrast images where membranes appear as dark lines.^[117] For cryo-prepared samples, freeze-substitution replaces ice with organic solvents (e.g., acetone containing uranyl acetate and osmium tetroxide) at -90°C over 48-72 hours, gradually warming to room temperature before resin embedding, to maintain structural fidelity without recrystallization.^[118] Despite these methods, challenges persist in preserving biological integrity. Dehydration during chemical processing can induce shrinkage artifacts, with tissues contracting up to 20-30% due to loss of hydration and lipid extraction, distorting relative dimensions and creating voids.^[119]^[120] Beam-induced damage in the microscope further exacerbates shrinkage, as hydrated or resin-embedded samples lose mass under electron irradiation, leading to bubbling or cracking; this is mitigated by low-dose imaging but remains a limitation for beam-sensitive organics.^[121] Cryo-methods reduce such artifacts by avoiding dehydration, though they require specialized equipment and may introduce devitrification if not handled below -130°C.^[111]

Methods for inorganic materials

Sample preparation for inorganic materials such as metals, ceramics, and semiconductors in electron microscopy focuses on achieving a clean, thin, and structurally representative surface or section to reveal microstructure without introducing artifacts. These rigid samples typically require mechanical and milling techniques to thin them to electron transparency (often <100 nm for TEM) or conductivity (for SEM), contrasting with softer biological specimens. Key methods include polishing, milling, coating, and etching, selected based on material hardness and analysis goals.^[122] Mechanical polishing is a foundational step for preparing bulk inorganic samples, particularly hard and brittle ones like ceramics and metals, to create a flat surface suitable for further thinning. The process begins with grinding using silicon carbide papers (from 400 to 1200 grit) to remove excess material, followed by sequential polishing with diamond abrasives on cloths or films, starting at 9 μm and progressing to <1 μm particle size for a mirror-like finish. This yields a scratch-free surface that minimizes topographic artifacts in SEM imaging or serves as a precursor for TEM foil preparation. For metals, the polished disc (typically 3 mm diameter) is then often electropolished to achieve electron transparency; in twin-jet electropolishing, the sample acts as the anode in an electrolyte bath (e.g., perchloric acid in acetic anhydride for aluminum), with controlled voltage (10-30 V) and temperature to dissolve material preferentially at high-current-density regions, producing foils ~50-100 nm thick at the perforation center. This method is widely used for metals like steel and aluminum alloys, ensuring damage-free thinning for high-resolution TEM.^[123]^[124]^[125] Focused ion beam (FIB) milling enables site-specific preparation of inorganic samples, ideal for semiconductors and layered ceramics where precise cross-sections are needed. A gallium (Ga+) ion beam, accelerated at 30 kV, is raster-scanned to sputter material, creating lamellae ~100 nm thick and 10-20 μm wide by protective platinum deposition followed by milling from both sides. This technique preserves local microstructure, such as interfaces in silicon wafers or nanoparticle composites, and is essential for 3D tomography in FIB-SEM workflows. Unlike broad mechanical methods, FIB allows targeted extraction from bulk samples without extensive handling.^[122]^[126] For non-conductive inorganic samples like ceramics or oxides in SEM, coating via sputtering enhances imaging by preventing charge buildup under the electron beam. A thin layer (~5-10 nm) of conductive material is deposited in a vacuum chamber using a magnetron sputter coater; gold (Au) provides high conductivity and fine grains (~2 nm) for topographic detail, while carbon (C) is preferred for energy-dispersive X-ray spectroscopy (EDS) to avoid spectral overlap. The process involves plasma ionization of argon gas to eject target atoms onto the sample, typically at 10-50 mA for 30-60 seconds, ensuring uniform coverage without altering the underlying structure.^[127]^[128] Etching techniques reveal fine microstructural features like grain boundaries in polished inorganic samples by selectively removing material. Chemical etching involves immersion in reagents such as alcoholic nitric acid for steels or oxalic acid for high-alloy metals, which preferentially attacks boundaries due to higher reactivity, creating topographic contrast visible in SEM (e.g., boundaries appear as dark valleys in secondary electron mode). Ion etching, using argon ions at 4-6 kV in a broad-beam miller, provides damage-free thinning and boundary delineation for ceramics, often at a 15-18° incidence angle to avoid redeposition. These methods enhance phase contrast without excessive material removal, typically etching for 10-30 seconds to 1-2 μm depth.^[129]^[130] These preparation methods support critical applications in characterizing inorganic microstructures, such as assessing nanoparticle dispersion in metal matrices via drop-casting diluted suspensions onto TEM grids followed by FIB lift-out for 3D analysis, or identifying phases in ceramics through etched SEM images combined with EDS mapping. For instance, electropolished foils enable diffraction patterns for phase confirmation in alloy precipitates, while coated and etched samples reveal grain boundary segregation in semiconductors, informing material performance in electronics and catalysis.^[36]^[122]

Limitations and challenges

Technical and operational drawbacks

Electron microscopes are notoriously expensive, with transmission electron microscopes (TEMs) typically costing from around $80,000 for entry-level models to exceeding $3 million for advanced systems as of 2025,^[131] while scanning electron microscopes (SEMs) range from $60,000 to $150,000 for benchtop models to over $1 million for high-resolution systems depending on features.^[132] Maintenance is equally intensive, involving annual service contracts that can exceed $100,000 for high-resolution systems as of 2025 due to the need for specialized parts, regular calibration, and vacuum system upkeep.^[133] A primary operational limitation stems from the high-vacuum environment required for electron beam propagation, which scatters electrons in gaseous atmospheres and prevents imaging of live or hydrated samples without specialized enclosures; though low-vacuum or environmental SEMs allow imaging of hydrated specimens in variable pressure modes.^[134] This vacuum constraint necessitates extensive sample preparation, often taking hours to days, including dehydration, fixation, and sectioning to ensure compatibility.^[135] Throughput in electron microscopy is inherently low compared to light microscopy, as imaging occurs serially—scanning point-by-point or through thin sections—rather than capturing parallel fields of view, resulting in longer acquisition times for large areas or volumes.^[136]^[137] Radiation damage further complicates operations, with knock-on displacement prevalent in inorganic materials under high-energy beams, causing atomic ejection above threshold energies around 200-300 keV, while radiolysis dominates in organic specimens, leading to bond breakage and structural degradation at electron doses exceeding 20-50 electrons per square angstrom.^[138]^[139] Effective use of electron microscopes demands significant operator expertise, including precise alignment of electromagnetic lenses, optimization of beam parameters, and skilled interpretation of high-contrast images, often requiring years of specialized training equivalent to graduate-level proficiency in materials or biological applications.^[140]^[141]

Artifacts and sample damage

In electron microscopy, charging artifacts arise when non-conductive samples accumulate electric charge from the incident electron beam, leading to deflection of the beam and resulting image distortions such as bright or dark spots and geometric aberrations.^[142] This effect is particularly pronounced in scanning electron microscopy (SEM) of insulating materials, where the lack of conductivity prevents charge dissipation, causing the sample surface to act as a dynamic lens that shifts image focus.^[143] To mitigate charging, samples are often coated with a thin conductive layer, such as carbon or gold, which allows electrons to dissipate without significantly altering surface topography.^[144] Drift artifacts manifest as blurring or distortion in images due to unintended sample movement relative to the beam, often caused by thermal vibrations or mechanical instabilities in the microscope stage.^[145] Contamination, meanwhile, occurs from the buildup of hydrocarbons or other residues on the sample surface under the beam, forming a carbonaceous layer that further blurs features and reduces contrast over time. These issues are exacerbated in high-vacuum environments, where residual gases can polymerize under electron irradiation, leading to gradual image degradation during prolonged exposure. Beam damage encompasses several mechanisms induced by electron bombardment, including radiolytic bond breaking, knock-on displacement, and thermal effects. Heating results from inelastic scattering that converts beam energy into phonon excitations, with the temperature rise proportional to the beam current and inversely proportional to the sample's thermal conductivity and the size of the irradiated area; this can cause structural changes or melting in sensitive materials.^[146] Sputtering involves the ejection of atoms from the sample surface due to momentum transfer from high-energy electrons, leading to erosion and topographic alterations, while bubbling in polymers arises from radiolysis-induced gas evolution that forms voids and disrupts morphology.^[147] These damage types limit the achievable resolution, particularly for beam-sensitive specimens like biological tissues or nanomaterials.^[148] Contrast artifacts in transmission electron microscopy (TEM) include overfocus halos, which appear as bright Fresnel fringes around features in phase contrast imaging due to defocus enhancing edge visibility at the expense of accuracy.^[149] Thickness fringes, or interference patterns from electron wave interactions in wedge-shaped samples, can mimic structural details and mislead interpretation of material density variations.^[150] Mitigation strategies for these artifacts and damage include low-dose imaging techniques, which minimize electron fluence to below damage thresholds—typically 10-20 electrons per Å²—while employing computational averaging to recover signal from noisy data.^[151] Cryo-cooling preserves sample integrity by reducing molecular motion and damage rates by several-fold compared to room temperature, enabling imaging of hydrated, native-state structures with minimal alteration.^[152]^[153]

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A sample is scanned by a focused electron beam in scanning electron microscopy, which produces secondary or backscattered electrons as it contacts the material.
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FEI Titan G2 80-200 ChemiSTEM | er-c
Technical Specifications ; X-FEG brightness @ 200 kV, 1.8×109 A/cm2/sr ; Symmetrical analytical S-TWIN objective lens, ~ 5 mm ; Information limit (TEM) @ 200 kV ...
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Beating the shot-noise limit | Nature Physics
Oct 14, 2012 · The electron-beam shot-noise expression (1) is a direct consequence of the Poisson statistics of random particle number distribution, and ...
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Electron Source - an overview | ScienceDirect Topics
Electrons are extracted from the cathode by either thermionic emission from heating it to ≈2000–2800 K, or by cold field emission (CFE) in an electrostatic ...
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Intensity basics - EDS - MyScope
In the SEM, accelerating voltages are typically 5-30 keV but in TEMs much higher accelerating voltages, 100-400 keV or more, are used.
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Choice of operating voltage for a transmission electron microscope
Mar 12, 2014 · An accelerating voltage of 100-300kV remains a good choice for the majority of TEM or STEM specimens, avoiding the expense of high-voltage microscopy.Missing: range SEM 1-400
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None
### Summary of Electromagnetic Lenses for Electron Microscopes
[68]
A Technical Introduction to Transmission Electron Microscopy for ...
Mar 4, 2020 · ... electron optics, and for the design of the first electron microscope. ... transmission electron microscopy (HR-TEM). To achieve this, a ...
[69]
Experimental Methods in Chemical Engineering: SEM & XuM
Mar 14, 2022 · LaB6 crystals are brighter (1 × 106 A cm−2 sr−1) last longer (200 ... The scanning coils are a deflection system that moves the probe in a point ...
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[PDF] Random Access Direct Parallel Detection of Electron Energy Loss ...
Commercial semiconductor arrays like charge- coupled devices (CCDs) or photodiode arrays (PDAs) are used as parallel detectors for EELS. In the direct mode, ...
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Vacuum systems for analytical instruments
Various vacuum pump technologies are used in electron microscopy; most commonly these are: rotary vane pumps,; diaphragm pumps,; scroll pumps,; turbomolecular ...
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[PDF] Development of an Ultra High Resolution Scanning Electron ...
turbo molecular pump. The pressure ultimately becomes 1 X 10-7 Pa in the gun chamber and is in the low 10-5 Pa range in the specimen chamber. Applications.
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Environmental Scanning Electron Microscope and Focused Ion ...
Variable humid environments and variable pressures can be applied to specimen chambers, making it possible to study hydrated and uncoated specimens in their ...
[74]
Variable pressure and environmental scanning electron microscopy
Specific operating conditions of elevated pressures combined with sample cooling (usually restricted to the environmental SEM range) can allow hydrated samples ...
[75]
Scanning Electron Microscopy | Electrons in SEM - US
Secondary electrons, however, originate from the atoms of the sample. They are a result of inelastic interactions between the electron beam and the sample. BSE ...
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[PDF] Scanning Electron Microscopy (SEM)
In the scanning electron microscope a stream of primary electrons is focused onto the sample surface resulting in a number of different particles or waves ...
[77]
secondary electron, SE | Glossary | JEOL Ltd.
That is, the escape depth of secondary electrons from the specimen is as small as 5 to 10 nm for most metals. Thus, the secondary electrons are emitted more at ...
[78]
backscattered electron yield, backscattered electron coefficient ...
The ratio of the number of backscattered electrons emitted from a specimen to the number of incident electrons (primary electrons) onto the specimen.
[79]
Backscattered Electron - an overview | ScienceDirect Topics
The backscattered electrons have the same energy as incident electrons and display intensity or contrast variations that depend on atomic number (Z). Since the ...
[80]
Origin of secondary‐electron‐emission yield‐curve parameters
Aug 1, 1975 · From an analysis of the one‐dimensional constant‐loss theory of secondary electron emission, maximum yield (δ m ), primary electron energy at maximum yield (E ...
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Measuring the backscattering coefficient and secondary electron ...
Measuring the backscattering coefficient and secondary electron yield inside a scanning electron microscope - Reimer - 1980 - Scanning - Wiley Online Library.
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low-vacuum SEM, Natural SEM, Wet SEM, variable pressure ... - JEOL
Fig. (b) Feature of charge neutralization. The residual gas molecules in the specimen chamber become cations by colliding with the incident electrons and the ...Missing: mediated | Show results with:mediated
[83]
Charge neutralisation of insulating surfaces in the SEM by gas ...
Aug 9, 2025 · The ionized nitrogen gas molecules neutralize the accumulated charge on the sample surface[35, 63, 64] (Fig 3C). The amount of nitrogen gas and ...Missing: mediated | Show results with:mediated
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Variable Pressure Scanning Electron Microscopy
Aug 14, 2014 · As an alternative method, variable pressure SEM, which is also sometimes called environmental SEM, can be used. Here a controlled amount of gas, ...
[85]
TEM: Bright field versus dark field - Chemistry LibreTexts
Aug 21, 2022 · In the bright field image the unscattered (transmitted) electron beam is selected with the aperture, and the scattered electrons are blocked.Missing: direct | Show results with:direct
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[PDF] 7. Diffraction Contrast in TEM Images
“Contrast” is the appearance of a feature in an image. Contrast in bright- field (BF) and dark-field (DF) TEM images is usually “diffraction contrast,” or ...<|separator|>
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Selected Area Diffraction - an overview | ScienceDirect Topics
Selected Area Electron Diffraction (SAED) is defined as a microscopic technique used to study crystal structures by observing diffraction patterns produced when ...
[88]
[PDF] Convergent beam electron diffraction - RRUFF Project
In convergent-beam electron diffraction (CBED) a highly convergent electron beam is focussed on to a small (~< 50 nm) area of the sample.
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Convergent Beam Electron Diffraction - ScienceDirect.com
The technique of convergent-beam electron diffraction (CBED) utilizes a convergent focused beam to obtain diffraction patterns from small specimen regions. CBED ...
[90]
Phase contrast electron microscopy: development of thin-film phase ...
Mar 13, 2008 · The first technique uses a Zernike phase plate, which is made of a uniform amorphous carbon film that completely covers the aperture of an ...
[91]
Improved Zernike-type phase contrast for transmission electron ...
Apr 9, 2015 · This imaging mode can be used to image typical phase objects such as unstained biological molecules or cryosections of biological tissue.
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(PDF) Energy-dispersive X-ray spectroscopy - ResearchGate
Energy dispersive X-ray spectroscopy (EDX—sometimes also called EDS or EDXS). is a robust and commonly used technique for chemical characterisation and. imaging ...
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Energy Dispersive X-ray Tomography for 3D Elemental Mapping of ...
Jul 5, 2016 · Energy dispersive X-ray spectroscopy within the scanning transmission electron microscope (STEM) provides accurate elemental analysis with high spatial ...Protocol · 1. Nanoparticle Synthesis · 2. Tem Sample Preparation
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Towards sub-Å electron beams - ScienceDirect.com
Aberration correction permits increasing the useful angular range. It is a subject with a 60 year history [1] dating back to the fundamental work of Scherzer [2] ...
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Cc-Corrector | Corrected Electron Optical Systems - CEOS GmbH
In order to compensate for chromatic aberration the operation principle of a Wien Filter could be applied. Such a filter is an electron optical device with a ...
[96]
TEAM 0.5 - Molecular Foundry
The imaging aberration corrector fully corrects for coherent axial aberrations up to 3rd order and partially compensates for 4th and 5th order aberrations. TEAM ...Missing: sub- | Show results with:sub-
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Nion: The company that transformed microscopy - 2022
Nov 21, 2022 · From aberration correction to single atom vibrational spectroscopy, Nion has taken electron microscopy to the nanoscale and isn't stopping yet.
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The Nobel Prize in Chemistry 2017 - NobelPrize.org
The Nobel Prize in Chemistry 2017 was awarded jointly to Jacques Dubochet, Joachim Frank and Richard Henderson for developing cryo-electron microscopy.
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Automated vitrification of cryo-EM samples with controllable ... - Nature
May 27, 2022 · Cryo fixation into vitreous water (amorphous ice) by fast freezing of biological samples can deliver nearly perfect structural preservation ...
[100]
Cryogenic electron ptychographic single particle analysis with wide ...
May 25, 2023 · Radiation damage relative to transmission electron microscopy of biological specimens at low temperature: a review. ... Low-dose cryo ...
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Low-dose cryo electron ptychography via non-convex Bayesian ...
Aug 29, 2017 · Scanning with small spots of several 10 nm in size over a vitrified sample has shown to reduce beam induced specimen movement in real-space ...
[102]
Low-dose cryo-electron ptychography of proteins at sub-nanometer ...
Sep 14, 2024 · Conventional TEM (CTEM), using dose-fractionated cryo-EM on a Titan Krios G4 equipped with a 300 kV cold-FEG electron source and a Falcon4i ...Results · Ptychography Of Cryo-Em... · 4d-Stem Ptychography...
[103]
3DFlex: determining structure and motion of flexible proteins from ...
May 11, 2023 · Single-particle cryo-EM collects thousands of static two-dimensional (2D) particle images that, in aggregate, may span the target protein's ...
[104]
Isotropic reconstruction for electron tomography with deep learning
Oct 29, 2022 · The advent of single-particle cryoEM has made it routine to determine structures of isolated macromolecular complexes at 2–4 Å resolution by ...
[105]
Press release: The 2017 Nobel Prize in Chemistry - NobelPrize.org
Oct 4, 2017 · The Nobel Prize in Chemistry 2017 is awarded to Jacques Dubochet, Joachim Frank and Richard Henderson for the development of cryo-electron microscopy.
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TEM Fixation - Protocols - Microscopy
For immersion fixation, use 2.5% glutaraldehyde (must be EM grade) in 0.1M buffer. The time of fixation is dependent upon the dimensions of the sample to be ...
[107]
Fixation of Biological Samples
Glutaraldehyde is primarily used to fix samples for electron microscopy (EM). It is often diluted to 2-4% in sodium cacodylate buffer just before use. Fixation ...
[108]
Freezing methods - TEM - MyScope
Cryo-fixation methods include plunge freezing, metal mirror freezing, high pressure freezing, and self-pressurized freezing. Plunge freezing uses ethane or ...
[109]
[PDF] EM Sample Preparation High Pressure Freezing
There are currently two common methods employed; plunge freezing and high pressure freezing. Cryo-fixation has two distinct advantages over chemical fixation.
[110]
Essential Guide to Ultramicrotomy - Leica Microsystems
The sectioning is performed by a vertical movement of the specimen over the extremely sharp blade of a fixed glass or diamond knife. Removing the sections ...
[111]
Ultramicrotomy for Electron Microscopy - Bitesize Bio
Nov 1, 2024 · A diamond or glass knife can also be used for trimming by first exposing the surface of the sample and then trimming resin away from the left ...
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DiATOME Diamond Blades & Knives | Homepage | DiATOME
Diamond blades and diamond knives are essential microtomy tools utilized for semithin and ultrathin sectioning, sample preparation, and other application ...
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Brief Introduction to Contrasting for EM Sample Preparation
Oct 2, 2013 · The uranyl acetate (UA), which enhances the contrast by interaction with lipids and proteins, forms a yellow, needle-like crystal precipitate if ...Stains · Uranyl Acetate · Manual Contrasting
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TEM sample preparation techniques | University of Gothenburg
Dec 8, 2023 · Uranyl acetate is a heavy metal salt which binds to proteins, lipids and nucleic acids, providing additional contrast. Some authors believe it ...
[115]
Staining sectioned biological specimens for transmission electron ...
The most common post-staining of sections is done on grids by aqueous uranyl acetate followed by lead citrate. When it is apparent that simple, aqueous uranium ...
[116]
An introduction to sample preparation and imaging by cryo-electron ...
Freeze substitution of high-pressure frozen samples: the visibility of biological membranes is improved when the substitution medium contains water. J ...
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Artifacts - TEM - MyScope
Dehydration and resin infiltration can result in artifacts. Removing water too quickly can result in shrinkage artifacts. Poor resin infiltration and ...
[118]
Towards native-state imaging in biological context in the electron ...
Most resins are not miscible with water so the sample first needs to be dehydrated using solvents, which can cause artifacts due to shrinkage.<|control11|><|separator|>
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(PDF) Electron Microscopy Preparation of biological samples ...
Sep 27, 2025 · cellular lipid extraction and sudden shrinkage of the cell. To preserve the lipid content during dehydration: 1. Proper fixation of ...
[120]
Method of Inorganic Sample Preparation for Transmission Electron ...
May 23, 2025 · This paper provides an overview of the most commonly used inorganic sample preparation methods for TEM analysis, including mechanical polishing, ...
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TEM sample preparation method of mechanical polishing + ion milling
vi) Polish the sample with a series of diamond abrasive films with decreasing grain sizes as listed in Table 2805a. Each polishing step removes the scratches ...
[122]
Twin‐jet electropolishing for damage‐free transmission electron ...
Twin-jet electropolishing (an extension of electropolishing) is a fast, damage-free and cheap method for preparing TEM specimens from electrically conductive ...Missing: inorganic | Show results with:inorganic
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Electro-polishing - TEM - MyScope
This technique is only relevant to metal samples. It uses a temperature controlled bath and a flow of current. The bath acts as an electrolyte.
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Focused Ion Beam (FIB) Sample Milling for High Performance ...
Using focused ion beam (FIB) systems provide better quality imaging over conventional sample preparation for transmission electron microcscopy (TEM).
[125]
Sputter coating for SEM: how this sample preparation technique ...
This extra step involves coating your sample with an additional thin layer (~10 nm) of a conductive material, such as gold, silver, platinum or chromium etc.
[126]
Sample Coating for SEM | JEOL Resources
Sputter coaters are a great option for imaging applications. Adding a metal coating such as gold or platinum allows the user to image a nonconductive sample ...
[127]
Metallographic etching insight | Struers.com
Dissolution etching enables specific attacks at grain boundaries, surfaces, or phases. During precipitation etching (also known as color etching), a thin layer ...
[128]
[PDF] Sample Preparation Techniques for Transmission Electron ...
These very thin sample preparations are called thin foils after the early metal- lurgical samples that were prepared by beating gold into a very thin foil. The ...
[129]
Electron Microscope Price, including Cost of 50 Different Models
Feb 22, 2022 · Transmission Electron Microscopes (TEM) cost $100,000 to $10,000,000 for new and $125,000 to $900,000 for used instruments. Dual Beam or SEM/FIB ...
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How Much Does a Scanning Electron Microscope (SEM) Cost?
Typically, new SEMs cost anywhere from $50,000 to over $1,000,000. The cost of scanning electron microscopes (SEMs) can vary significantly depending on several ...
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How Much Does a Transmission Electron Microscope Cost? - Excedr
May 27, 2025 · Maintenance and service contracts. High-resolution TEMs require consistent upkeep: Annual service contracts: Range from $30,000 to over $100,000 ...<|control11|><|separator|>
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Why can electron microscopes magnify only dead organisms?
Jun 4, 2024 · This vacuum environment is unsuitable for living organisms, as it removes moisture and gases necessary for life. Therefore, the requirement for ...
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Preparing samples for the electron microscope
Feb 29, 2012 · Samples must be fixed to stabilize them, dried to withstand the vacuum, and for TEM, cut into thin sections. For SEM, they are coated with ...
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The Difference Between SEM and Optical Microscopes
SEMs are superior in terms of resolving power and depth of focus. However, optical microscopes are generally easier and quicker to use.
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Fast SEM Imaging - Delmic
The increased throughput of faster electron microscopes is highly beneficial for volume EM, as larger volumes can be imaged within a smaller amount of time.
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Mechanisms of radiation damage in beam‐sensitive specimens, for ...
Jul 17, 2012 · Knock-on damage is predominant in conductors, where radiolysis is suppressed because of the high electron density. In such specimens, an ...Abstract · INTRODUCTION · INORGANIC SPECIMENS
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Energy-dependent knock-on damage of organic–inorganic hybrid ...
Sep 20, 2021 · As we can see, the knock-on mechanism is effective only if the incident energy is higher than 2.3 keV, which is the threshold incident energy ...Missing: dose | Show results with:dose
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[PDF] SA572 - Electron Microscopy Specialist
Experience. Minimum one to three years of electron microscopy experience is required. Image analysis experience, preferred. Knowledge, Skills and Abilities.
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MSA | CEMT Certification Program - Microscopy Society of America
Provides the only certification of technologists in biological transmission electron microscopy available in the Americas.<|separator|>
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Utilizing the Charging Effect in Scanning Electron Microscopy - PMC
The charging effect of an insulating specimen from electron beam (e-beam) irradiation may be utilized to facilitate imaging in the scanning electron microscope ...
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Reduction of SEM charging artefacts in native cryogenic biological ...
Jun 4, 2025 · However, with secondary electron SEM imaging, samples can be subject to electrical charging during scanning, leading to the presence of ...
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[PDF] Charging and its Mitigation Presented at Proceedings of SPIE
Scanning electron microscopes are used extensively in research and advanced manufacturing for materials characteriza- tion, metrology and process control.
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Correction of Scanning Electron Microscope Imaging Artifacts in a ...
Mar 12, 2019 · The drift distortion artifact is a consequence of undesired motion of the specimen relative to the electron beam while the scanning process is ...
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Analysis of complex, beam-sensitive materials by transmission ... - NIH
The mechanisms by which a material is damaged by the electron beam can be categorized by the different types of electron scattering experienced: either, (i) ...
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[PDF] Cryo-electron tomography related radiation-damage parameters for ...
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Negative‐Stain Transmission Electron Microscopy of Molecular ...
An overfocus particle will have a halo (Fresnel fringe) of white signal, while an underfocus particle (the desired defocus offset) will have a halo of black ...
[147]
[PDF] Size analysis of single fullerene molecules by electron microscopy
10 molecules were tethered by chemical bonding to carbon black particles to facilitate HRTEM imaging and sizing of known fullerenes.
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Cryo-Electron Tomography for Structural Characterization of ...
Low dose imaging is used to minimize the exposure of the sample to the beam to prevent damaging it before or while recording a tilt series. It allows the ...Missing: mitigation | Show results with:mitigation
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Cryo-electron microscopy: A primer for the non-microscopist - PMC
Imaging at liquid nitrogen temperatures reduces the extent of radiation damage by as much as 6-fold compared to ambient temperatures [28]. This means that, for ...