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Scanning transmission electron microscopy

Scanning transmission electron microscopy (STEM) is a powerful electron microscopy technique that forms images by raster-scanning a finely focused beam of high-energy electrons across an ultrathin specimen and collecting the transmitted electrons with specialized detectors to reveal structural and compositional details at the atomic scale. This method combines the high-resolution imaging capabilities of transmission electron microscopy with the point-by-point scanning approach of scanning electron microscopy, enabling both two-dimensional imaging and three-dimensional tomography while providing simultaneous spectroscopic analysis. The foundational concepts of STEM trace back to the 1930s, when Manfred von Ardenne developed early prototypes of scanning transmission electron microscopes, though limited by technology at the time.^[1] Practical high-resolution STEM emerged in the late 1960s through the work of Albert V. Crewe and colleagues at the University of Chicago, who in 1970 achieved the first visualization of individual heavy atoms (such as thorium) on a thin carbon substrate, demonstrating atomic resolution using annular dark-field detection.^[2] Major breakthroughs occurred in the 1990s and 2000s with the invention and implementation of aberration correctors, which compensated for spherical and chromatic aberrations in the probe-forming lens, reducing the probe size to below 0.5 Å and enabling routine sub-angstrom imaging.^[3] These corrections, pioneered by teams including those at Cambridge University and Oak Ridge National Laboratory, transformed STEM into a versatile tool for quantitative materials characterization.^[3] At its core, STEM operates by accelerating electrons to energies typically between 80 and 300 keV, condensing them into a coherent probe via electromagnetic lenses, and scanning the probe across the specimen in a controlled pattern. Transmitted electrons interact with the sample through elastic and inelastic scattering; low-angle or unscattered electrons are captured by bright-field detectors for phase-contrast imaging, while high-angle scattered electrons are detected by annular dark-field (ADF) detectors to produce incoherent images. In particular, high-angle annular dark-field (HAADF) imaging yields Z-contrast, where signal intensity scales approximately with the square of the atomic number (Z²), providing direct interpretability for compositional mapping without the phase contrast issues of conventional TEM.^[4] Integrated with techniques like electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS), STEM delivers atomic-resolution chemical bonding and elemental distribution data. Recent enhancements, such as monochromation and pixelated detectors for four-dimensional STEM (4D-STEM), further support diffraction-based analysis and dynamic in situ observations. STEM's applications span materials science, nanotechnology, and biology, where it excels in analyzing atomic structures, defects, interfaces, and dynamics in semiconductors, catalysts, batteries, two-dimensional materials, and biomolecules. For instance, it has been instrumental in visualizing single-atom catalysts and quantifying strain in quantum dots, driving advancements in energy storage and quantum technologies. Its multimodal capabilities, supported by open-source software for data processing, make it indispensable for correlative studies combining imaging, spectroscopy, and tomography.

Fundamentals

Basic principles

Scanning transmission electron microscopy (STEM) is a microscopy technique that employs a finely focused electron probe, raster-scanned across a thin specimen, to form images based on the transmitted electrons.^[5] In contrast to conventional transmission electron microscopy (TEM), which uses parallel illumination over a broad area, STEM relies on sequential scanning to build the image pixel by pixel, enabling high-resolution imaging and localized analysis.^[5] The electron probe in STEM is formed by accelerating electrons to high energies and focusing them using a series of condenser lenses, with scanning coils deflecting the beam in a controlled raster pattern across the specimen.^[6] The convergence semi-angle (α), typically ranging from 10 to 30 milliradians, determines the probe's angular spread; a larger α increases the probe intensity and collection efficiency but can broaden the probe size due to aberrations, affecting the achievable resolution.^[5] The probe current density J, which influences signal strength and potential specimen damage, is proportional to the beam current I divided by the square of the probe diameter d, expressed as J \propto \frac{I}{d^2}. Transmitted electrons in STEM arise from interactions within the specimen, primarily elastic scattering (no energy loss, used for structural imaging) and inelastic scattering (energy loss, enabling spectroscopic techniques).^[5] Key operational parameters include the acceleration voltage, commonly 60–300 kV to balance penetration depth and resolution; dwell time per pixel, which affects signal-to-noise ratio; and scan speed, determining the overall acquisition rate.^[7] These parameters involve trade-offs, as higher resolution requires a smaller probe and higher current density, increasing the risk of radiation damage to beam-sensitive specimens.^[8]

Instrumentation and setup

Scanning transmission electron microscopy (STEM) instrumentation consists of a modified transmission electron microscope (TEM) column equipped with specialized components to generate, focus, and scan a finely converged electron probe across a thin specimen. The setup emphasizes high coherence and stability to achieve sub-angstrom spatial resolution, typically operating at accelerating voltages between 20 and 300 kV.^[9] Unlike conventional TEM, which employs plane-wave illumination for parallel imaging, STEM integrates scanning electronics and post-specimen detectors to collect transmitted signals pixel by pixel, enabling versatile high-resolution analysis.^[9] The electron source is a critical component for producing a bright, coherent beam, with modern STEM systems predominantly using cold field emission guns (CFEGs) that emit electrons via quantum tunneling from a sharpened tungsten tip under high vacuum and strong electric fields. These sources provide superior brightness (up to 10^8 A/cm² sr) and reduced energy spread (around 0.3-0.7 eV) compared to thermionic emitters, facilitating sub-angstrom probe formation and high signal-to-noise ratios in imaging and spectroscopy.^[9]^[10] The electrons are accelerated through the anode to energies suitable for penetrating thin samples while minimizing beam damage. The condenser lens system, typically comprising two or three magnetic lenses, focuses the electron beam into a convergent probe with semi-angles adjustable from 0.1 to approximately 60 mrad, enabling probe diameters below 0.1 nm when combined with aberration correctors. These correctors, often hexapole or octupole designs inserted in the condenser path, compensate for spherical aberration and other lens imperfections to achieve the necessary probe sizes for atomic-scale resolution.^[9] The convergence angle plays a key role in balancing probe current, depth of field, and scattering geometry during setup. Raster scanning of the probe across the specimen is accomplished using pairs of electromagnetic deflection coils positioned above and below the objective lens in a double-deflection configuration, which linearizes the scan field and minimizes distortions. Modern systems employ digital control for precise pixelation, allowing dwell times from microseconds to milliseconds per pixel and typical image arrays of 512 × 512 or 1024 × 1024 pixels, with step sizes ranging from 0.001 nm to ~100 nm depending on the field of view.^[9]^[11] Analog control, while simpler, is less common in contemporary setups due to limitations in speed and accuracy for high-resolution data acquisition.^[11] The specimen stage provides precise translation in x, y, and z directions (down to nanometer resolution) and tilt capabilities up to ±70° about a single axis to accommodate sample orientation needs while maintaining stability against vibrations. Samples must be prepared as thin sections, typically less than 100 nm thick, to reduce multiple scattering events that degrade probe coherence and image contrast.^[12]^[13] A high-vacuum environment, maintained at pressures around 10^{-7} to 10^{-10} Pa by turbomolecular and ion pumps, is essential to prevent electron beam scattering by residual gases and hydrocarbon contamination of the sample. Environmental controls, including temperature stabilization to within 0.1 K and electromagnetic shielding, ensure minimal drift during long acquisitions.^[9] Alignment procedures involve optimizing probe current (typically 10 pA to 1 nA) via source extraction voltage adjustments and correcting astigmatism using stigmator coils, often visualized through Ronchigram patterns that reveal wavefront aberrations. These steps, performed prior to imaging, ensure the probe remains focused and symmetric, with aberration correctors tuned to minimize higher-order errors.^[9]^[14]

Historical Development

Early innovations

The concept of scanning transmission electron microscopy (STEM) originated in 1938 when Manfred von Ardenne proposed it as a scanning analog to the scanning electron microscope (SEM), aimed at imaging thicker samples by transmitting a focused electron beam through the specimen to form contrast based on transmitted electrons.^[15] However, the technology remained impractical due to limitations in electron source brightness and probe-forming optics, preventing widespread adoption until the 1970s. During the 1960s, initial prototypes emerged, particularly through efforts at the University of Chicago, where Albert Crewe and colleagues developed early STEM instruments incorporating field emission guns to achieve brighter probes.^[16] A pivotal milestone came in 1970 when Crewe's team demonstrated atomic-resolution imaging of heavy atoms, such as uranium and thorium, using a field emission source in a dedicated STEM setup, marking the first visualization of individual atoms in a transmission mode without intense electric fields.^[17] This breakthrough highlighted STEM's potential for high-contrast imaging of atomic structure, surpassing conventional transmission electron microscopes in resolution for certain applications. In the 1980s, commercialization accelerated with companies like Vacuum Generators (VG Microscopes, later acquired by Thermo Fisher Scientific) producing dedicated STEM instruments, such as the HB5 series, which integrated scanning capabilities with transmission electron microscopy for enhanced analytical functions like energy-dispersive X-ray spectroscopy.^[18] Early STEM systems faced significant challenges from probe aberrations, which limited spatial resolution to approximately 0.5 nm, restricting applications primarily to materials science for imaging defects and interfaces in crystalline samples.^[19] These innovations established STEM as a complementary technique to conventional TEM, emphasizing its scanning mode for targeted, high-resolution probing.

Aberration correction advancements

In scanning transmission electron microscopy (STEM), the primary limitations to achieving atomic-scale resolution in the electron probe arise from spherical and chromatic aberrations in the probe-forming condenser lens system. Spherical aberration, characterized by the coefficient C_s, causes rays at different convergence angles to focus at varying distances, resulting in a blurred probe profile. The extent of this blur is approximated by \delta_s \approx 0.5 C_s \alpha^3, where \alpha is the semi-convergence angle of the probe; for uncorrected lenses at 200 kV, typical C_s values of 1–1.5 mm limit the resolution to around 1 Å when balancing aberration and diffraction effects. Chromatic aberration, quantified by the coefficient C_c, further degrades the probe due to energy spread in the electron source, with \delta_c \approx C_c (\Delta E / E) \alpha, where \Delta E / E is the relative energy spread (typically 0.5–1 eV at 200 kV).^[20]^[21] Breakthroughs in aberration correction began with the design of multipole correctors using quadrupole-octopole configurations, pioneered by theorist Harald Rose and engineer Maximilian Haider from 1998 to 2003. Rose's theoretical framework for aplanatic correction using symmetric multipole arrangements addressed third-order spherical aberration while minimizing higher-order terms, enabling C_s to be tuned to near zero. Haider's team at CEOS implemented the first practical quadrupole-octopole (QO) corrector in a transmission electron microscope (TEM) by 2000, demonstrating resolution improvements to 0.13 nm at 200 kV. These designs built on earlier hexapole concepts but offered superior stability and off-axis correction for scanning probes.^[22]^[23] Early dedicated implementations of such correctors in STEM were achieved by Ondrej Krivanek and colleagues at Nion starting in the late 1990s, with the first operational STEM corrector in 1997.^[24] A pivotal milestone occurred in 2003, when Krivanek's group captured the first aberration-corrected STEM images resolving individual heavy atoms, such as gold, with sub-Ångstrom precision, surpassing prior uncorrected limits.^[20] One example of later integration was a custom QO system into a VG HB603 STEM around 2007, which produced a corrected probe with a full-width at half-maximum of approximately 0.6 Å at 200 kV, as evidenced by high-angle annular dark-field (HAADF) imaging of crystal lattices.^[25] Post-correction probe profiles shifted from aberration-dominated Gaussian-like tails to near-diffraction-limited Airy discs, described by \delta_d \approx 0.61 \lambda / \alpha, where \lambda is the electron wavelength (about 0.025 Å at 200 kV), allowing \alpha up to 40–50 mrad without significant blur.^[20]^[25] By the 2010s, aberration correction enabled routine sub-Ångstrom resolutions, such as 0.5 Å probes at 200 kV, facilitating direct imaging of light elements like carbon and oxygen in materials due to enhanced signal-to-noise from brighter, focused beams. Commercial adoption accelerated with integration into production instruments: JEOL's JEM-ARM200F in 2008–2010 and Hitachi's HD-2700C STEM by 2009, both incorporating QO correctors for seamless operation. These advancements increased usable probe currents by up to 100-fold compared to uncorrected systems, as larger \alpha collected more electrons from the source while maintaining resolution, profoundly impacting applications in atomic-scale structural analysis. However, ongoing challenges include maintaining temporal stability against mechanical vibrations and electrical noise, which can introduce residual aberrations exceeding 0.1 Å over extended imaging sessions.^[26]^[27]^[28]

Imaging Modalities

Annular dark-field imaging

Annular dark-field (ADF) imaging in scanning transmission electron microscopy (STEM) employs an annular detector positioned to collect electrons scattered at high angles, typically greater than 50-100 milliradians (mrad), from the specimen. This configuration forms an incoherent imaging mode by integrating signals from an annular ring in the far-field plane, excluding low-angle transmitted and elastically scattered electrons that pass through the central hole of the detector. The resulting images provide a dark background in regions of vacuum, with brightness corresponding to scattered intensity from the sample, enabling direct visualization without the phase contrast issues common in coherent modes.^[4] The primary contrast mechanism in ADF imaging arises from Rutherford scattering, where the scattering cross-section is proportional to the square of the atomic number (Z²), yielding a "Z-contrast" effect that highlights heavier elements against lighter backgrounds. This makes ADF particularly effective for delineating atomic columns containing heavy atoms in complex structures. The intensity in ADF images can be approximated by the relation I_{\text{ADF}} \propto Z^2 \cdot t \cdot N, where t is the specimen thickness and N is the atomic density, reflecting the cumulative contribution from thermal diffuse scattering at high angles that dominates the signal.^[4] Key advantages of ADF imaging include its incoherent nature, which minimizes interference artifacts and renders the mode largely insensitive to defocus or astigmatism variations, allowing robust imaging of beam-sensitive materials. With typical collection angles featuring an inner radius of 50-100 mrad and an outer radius exceeding 200 mrad, ADF achieves atomic resolution down to 0.1 nm or better for nanostructures, while suppressing diffraction contrast effects that plague low-angle techniques. Aberration correction further enhances this resolution by enabling finer probes.^[4] Despite these strengths, ADF images can exhibit artifacts such as thickness fringes arising from intra-column interference in wedge-shaped specimens, which modulate intensity along atomic columns. In crystalline samples, channeling effects occur due to the probe beam's propagation influenced by the lattice potential, leading to variations in signal intensity that depend on orientation and depth, potentially mimicking compositional changes.^[29] ADF imaging finds widespread applications in characterizing semiconductors, such as mapping dopant distributions and interfaces in silicon-based devices, where Z-contrast reveals heavy impurities like phosphorus or arsenic. In catalysis research, it excels at identifying single heavy atoms or nanoparticles on supports, aiding studies of active sites in materials like platinum on carbon for fuel cells. Unlike scanning electron microscopy (SEM), which provides topographic contrast from thick, surface-sensitive backscattered electrons, ADF operates in transmission mode on thin specimens, delivering subsurface atomic-scale detail with compositional sensitivity.

Bright-field and differential phase contrast

Bright-field imaging in scanning transmission electron microscopy (STEM) utilizes a central detector to capture the unscattered beam and low-angle scattered electrons, yielding primarily mass-thickness contrast where regions of greater thickness or atomic density appear darker due to enhanced electron absorption and scattering. This mode is particularly sensitive to diffraction effects in crystalline specimens, which can modulate contrast through constructive and destructive interference, though it typically provides lower spatial resolution than high-angle annular dark-field imaging owing to the inclusion of diffracted beams in the detection. BF-STEM is effective for visualizing overall sample morphology and light elements but offers limited sensitivity to subtle phase variations from electromagnetic fields.^[30]^[31] Differential phase contrast (DPC) extends low-angle coherent imaging by employing segmented detectors, such as four-quadrant configurations, to quantify beam deflections arising from local electric and magnetic fields that induce phase shifts in the transmitted electrons. The DPC signal measures the displacement of the beam's center-of-mass position, which corresponds to the spatial gradient of the phase shift, enabling direct mapping of projected electromagnetic fields with high sensitivity to weakly scattering features. This technique facilitates atomic-scale visualization of phenomena like strain distributions and magnetic domains by reconstructing the phase gradient into full phase maps via integration.^[32]^[33]^[34] The underlying phase shift \phi in DPC-STEM is described by \phi = \frac{2\pi}{\lambda} \int V \, dz, where \lambda is the electron wavelength, V is the local electrostatic potential, and the integral is along the beam path through the specimen thickness; this shift causes a deflection proportional to \nabla \phi, which the segmented detector resolves as intensity imbalances across quadrants. Originally introduced in the 1970s by Dekkers and de Lang for enhancing phase contrast in STEM, DPC saw limited adoption due to probe aberrations but experienced a revival in the early 2010s with aberration-corrected instruments, achieving sub-nanometer sensitivity for field mapping in materials like ferroelectrics.^[33]^[32]^[35] Despite its strengths, DPC-STEM faces limitations including elevated noise levels when probing weak fields, where shot noise from low electron counts dominates the signal, and the requirement for reference images or background subtraction to calibrate deflections accurately. These challenges are exacerbated in thicker samples due to multiple scattering, which distorts the linear deflection model, necessitating thin specimens (typically <50 nm) for reliable quantitative results. Precise detector alignment, complementary to general STEM instrumentation, is essential to minimize artifacts from beam tilt or astigmatism.^[36]^[37]^[38]

Universal and pixelated detectors

Universal detectors in scanning transmission electron microscopy (STEM) refer to versatile hybrid pixel detectors that facilitate rapid switching between multiple imaging modes, such as bright-field (BF), annular dark-field (ADF), and differential phase contrast (DPC), while also providing energy sensitivity for partial spectroscopic analysis.^[39] These detectors, exemplified by the Medipix and Timepix series developed at CERN, feature pixel arrays (e.g., 256 × 256 pixels for Timepix3) that allow configurable thresholds and counting modes to adapt to different signal types without mechanical adjustments.^[40] For instance, Timepix3 enables event-driven acquisition with timestamps, supporting BF imaging via central pixel summation, ADF through outer ring integration, and DPC by quadrant asymmetry analysis, all within microseconds.^[40] This multifunctionality evolved from earlier single-mode detectors, addressing limitations in traditional setups by enabling simultaneous data collection across modes.^[39] A key application of these pixelated detectors is in four-dimensional STEM (4D-STEM), where an area detector records the full two-dimensional diffraction pattern at each probe position in a two-dimensional scan raster, producing a 4D dataset parameterized by real-space coordinates (x, y) and reciprocal-space coordinates (e.g., scattering angle or momentum k, and sometimes energy E). Typical pixelated detectors, such as those with 100 × 100 or 256 × 256 arrays, capture this information with high spatial resolution, enabling post-acquisition virtual imaging and advanced analyses like orientation mapping of crystal domains or phase retrieval techniques that reduce the need for extensive scanning.^[39] For example, pixelated detection enhances DPC by allowing center-of-mass calculations across the diffraction pattern for precise electric and magnetic field mapping.^[39] The data volume generated in 4D-STEM scans is substantial, often reaching hundreds of gigabytes for moderate-sized datasets (e.g., ~2 GB for a 128 × 128 scan with a 256 × 256 detector) and up to terabytes for high-resolution atomic-scale acquisitions, necessitating efficient storage and processing pipelines.^[39] Post-2015 advancements have focused on direct electron detectors with readout rates exceeding 10^5 electrons per second per pixel, improving signal-to-noise ratios and enabling integration with aberration-corrected STEM for atomic-resolution 4D datasets.^[39] These detectors achieve frame rates up to 1000 frames per second or higher in event-driven modes, supporting dwell times as low as 100 ns for low-dose imaging.^[40] A notable example is the Electron Microscope Pixel Array Detector (EMPAD), introduced in 2016 and refined by 2018, featuring a 128 × 128 pixel array with a 500 μm silicon absorber, offering a dynamic range of over 10^6:1 and linear response to electron fluxes from single electrons to beam currents. The EMPAD supports 4D-STEM at up to 1100 frames per second, facilitating applications like strain mapping at sub-angstrom resolution.^[41] More recent developments as of 2024 include the MerlinEM detector, a 256 × 256 hybrid pixel direct electron detector with a 55 μm pixel pitch, capable of frame rates up to 21,000 fps in 1-bit mode and radiation tolerance from 30 to 300 keV, enhancing 4D-STEM and dynamic in situ imaging.^[42] Despite these advances, challenges persist, including radiation damage to detector sensors from high-energy electrons, which can degrade performance over time, and immense computational demands for handling terabyte-scale datasets, requiring specialized software for real-time processing and analysis.^[39]

Spectroscopic Analysis

Electron energy loss spectroscopy

Electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) exploits the inelastic scattering of electrons to probe the chemical and electronic structure of materials at the atomic scale. When a focused electron beam interacts with a specimen, a fraction of the electrons loses energy through interactions with the sample's atoms, typically in the range of 0.1 to 3 keV, corresponding to excitations of inner-shell electrons or collective oscillations. The transmitted electrons are dispersed by energy using a post-column spectrometer, such as an omega filter or magnetic prism, to generate an energy-loss spectrum that reveals information about the sample's composition and bonding. Core-loss EELS, which involves energy losses above approximately 100 eV from inner-shell ionizations, provides detailed insights into the fine structure of electronic orbitals and chemical bonding. For instance, the near-edge structure at L-edges of transition metals encodes information on oxidation states and coordination environments through characteristic peak shapes and positions. With aberration-corrected STEM, spatial resolutions better than 1 nm can be achieved for these core-loss spectra, enabling nanoscale mapping of elemental distributions and valence states. The inelastic scattering cross-section in EELS is described by the relation \sigma \propto \int \text{Im}\left[-\frac{1}{\varepsilon(q,\omega)}\right] dq, where \varepsilon(q,\omega) is the complex dielectric function of the material, q is the momentum transfer, and \omega is the energy loss. This formalism links the observed spectrum to the material's electronic response, with low-loss EELS (below ~50 eV) particularly sensitive to volume plasmons, interband transitions, and band gap energies. In practice, EELS data acquisition in STEM often employs spectrum imaging mode, where an energy-loss spectrum is recorded at each raster-scanned pixel to produce a multidimensional dataset of spatial and spectral information. To minimize beam-induced damage in sensitive samples, rapid scanning techniques and cryogenic cooling are utilized, allowing for high-fidelity mapping without significant alteration of the specimen. Modern monochromators integrated into the electron source can achieve energy resolutions as fine as ~5 meV (0.005 eV), limited primarily by the inherent width of core-level excitations.^[43] Applications of EELS in STEM include the mapping of dopant distributions in semiconductors, such as boron in silicon, where sub-nanometer resolution reveals segregation at interfaces, and the determination of valence states in battery materials like lithium iron phosphate, aiding in the understanding of electrochemical performance. These capabilities stem from the technique's sensitivity to light elements and electronic structure, though overall resolution is constrained by the energy resolution of the system.

Energy-dispersive X-ray spectroscopy

In scanning transmission electron microscopy (STEM), energy-dispersive X-ray (EDX) spectroscopy enables elemental mapping by detecting characteristic X-rays emitted from the sample when the focused electron probe ionizes inner-shell electrons, leading to atomic relaxation and X-ray emission at specific energies unique to each element.^[44] This process also generates a continuous bremsstrahlung background from decelerating electrons interacting with the sample's atomic nuclei, which must be subtracted to isolate characteristic peaks for analysis.^[44] EDX complements techniques like electron energy loss spectroscopy by providing robust detection of heavier elements (Z > 10) through X-rays that escape the sample more readily than transmitted electrons.^[45] Modern EDX systems in STEM typically employ silicon drift detectors (SDDs), which offer high energy resolution (around 120-130 eV at Mn Kα) and fast readout capabilities, allowing spectrum acquisition during beam scanning.^[46] These detectors are positioned to collect X-rays emitted at shallow angles from the thin sample, with large active areas (up to 150 mm² per detector) and multi-detector configurations achieving solid angles exceeding 1 steradian for high-throughput mapping.^[47] The spatial resolution of EDX in STEM reaches 1-5 nm, primarily limited by the electron probe size and the interaction volume where X-rays originate, though aberration-corrected instruments can approach sub-nanometer scales for select elements.^[48] Quantification of elemental concentrations from EDX spectra relies on the Cliff-Lorimer method, which assumes thin specimens where X-ray absorption and fluorescence effects are minimal.^[49] The method uses experimentally determined k-factors to relate measured X-ray intensities to atomic concentrations via the ratio:

\frac{I_A}{I_B} = k_{AB} \frac{N_A t}{N_B t}

where I_A and I_B are the intensities of characteristic X-rays from elements A and B, N_A and N_B are their atomic densities, t is the specimen thickness (which cancels in uniform-thickness regions), and k_{AB} is the sensitivity factor calibrated against standards.^[49] For thicker or varying samples, absorption corrections may be applied, but the thin-sample approximation holds well in STEM for nanoscale analysis.^[50] EDX mapping in STEM operates in spectrum-imaging mode, where a full X-ray spectrum is recorded at each pixel of a scanned area to generate hyperspectral datasets for elemental distribution maps.^[45] Common artifacts include peak overlaps (e.g., Ti Kβ with Ba Lα) requiring deconvolution and X-ray absorption in the sample that preferentially attenuates low-energy lines from light elements.^[50] Post-2010 advancements, such as windowless SDDs, have enhanced sensitivity to light elements (down to boron) by eliminating the beryllium window's absorption of soft X-rays, enabling atomic-scale EDX mapping integrated with high-resolution STEM imaging.^[51]

Diffraction Techniques

Convergent-beam electron diffraction

Convergent-beam electron diffraction (CBED) employs a highly convergent electron beam with a semi-convergence angle typically exceeding 1 mrad to illuminate a small area of the sample, producing overlapping disks in the back-focal plane of the objective lens that form the diffraction pattern. This convergence leads to a central bright disk for the zero-order Laue zone (ZOLZ) surrounded by higher-order Laue zones (HOLZ), where the disk overlaps provide dynamical diffraction effects essential for structural analysis.^[52] HOLZ lines, appearing as dark or bright lines within the disks due to intersections of the Ewald sphere with reciprocal lattice planes from adjacent zones, are highly sensitive to lattice parameters and enable precise measurement of interplanar spacings. The lattice spacing d is determined using d = \lambda / (2 \sin \theta), where \theta is the Bragg angle for first-order reflection and \lambda is the relativistic de Broglie wavelength of the electrons, approximated as \lambda = h / \sqrt{2 m e V \left(1 + \frac{e V}{2 m c^2}\right)}, with h as Planck's constant, m the electron rest mass, e the electron charge, V the accelerating voltage, and c the speed of light.^[52]^[53] Sample thickness is quantified from the spacing of Kikuchi bands, which arise from inelastic scattering and exhibit intensity oscillations dependent on the crystal thickness.^[52]^[54] CBED pattern analysis reveals crystal symmetry through the ZOLZ, where the arrangement of diffraction disks displays point group symmetries such as rotational axes or mirror planes, facilitating space group determination. Strain mapping is performed by measuring shifts in HOLZ line positions or rocking curves of the central disk, achieving sensitivities down to $7 \times 10^{-5} strain.^[52] In scanning transmission electron microscopy (STEM), CBED leverages nanoscale probe sizes below 10 nm, formed by the condenser lens system, to acquire local crystallographic data for orientation mapping, phase identification in multiphase materials, and characterization of defects like dislocations. Universal detectors enhance CBED in STEM by enabling efficient capture of full two-dimensional diffraction patterns.^[52]^[55] Despite its strengths, CBED requires thin crystalline samples, typically under 100 nm, to minimize multiple scattering that complicates pattern interpretation, and relies on computational indexing algorithms for accurate analysis of complex structures.^[52]

4D-STEM and ptychography

Four-dimensional scanning transmission electron microscopy (4D-STEM) acquires a complete two-dimensional diffraction pattern at each position of a focused electron probe as it scans across the sample, generating a four-dimensional dataset parameterized by real-space probe coordinates (x, y) and reciprocal-space diffraction coordinates (k_x, k_y). This approach, enabled by fast pixelated detectors, captures comprehensive structural information including local orientation, strain, and phase contrast in a single acquisition.^[56] By extending convergent-beam electron diffraction to a scanned probe, 4D-STEM allows for center-of-mass analysis of diffraction patterns to map lattice strain and crystal orientation with nanometer-scale precision.^[58]^[59] Electron ptychography leverages the 4D-STEM dataset from overlapping probe positions to reconstruct quantitative images of the sample's phase and amplitude through iterative phase retrieval algorithms. In this method, the scanned probe illuminates overlapping regions of the specimen, and the resulting diffraction patterns are processed to solve the phase problem inherent in electron scattering. A key algorithm, such as the difference map, iteratively updates estimates of the probe function and specimen transmission function by minimizing inconsistencies between measured and simulated diffraction intensities across overlaps. This yields high-fidelity reconstructions that surpass conventional imaging by incorporating weak scattering signals.^[60]^[61] Ptychographic reconstruction follows principles analogous to Fourier ptychography, where the specimen exit wave is iteratively refined in the Fourier domain to extend the effective resolution. Such methods have achieved resolutions exceeding twice that of direct STEM imaging, with demonstrations reaching approximately 50 pm in the 2020s for atomic-scale structure determination.^[62] Since 2015, advancements in 4D-STEM and ptychography have included GPU-accelerated processing to handle the computational demands of large datasets, enabling real-time analysis and broader adoption. As of 2025, integrations with deep learning have accelerated reconstructions, and applications have expanded to in situ observations and highly beam-sensitive materials such as metal-organic frameworks.^[63]^[64]^[65] These techniques have found applications in beam-sensitive materials, such as organic frameworks, where low-dose acquisitions minimize damage while mapping defects and metrology at the atomic level.^[66]^[60] Despite these progresses, challenges persist, including probe instability that introduces artifacts in phase retrieval and the management of massive datasets, often tens to hundreds of gigabytes for a full scan, which strain storage and processing resources.^[67]^[60]

Quantitative and Computational Methods

Quantitative STEM modeling

Quantitative STEM modeling employs physics-based simulations to predict and interpret scanning transmission electron microscopy (STEM) images with high fidelity, enabling the extraction of quantitative information such as atomic positions, strain fields, and chemical compositions from experimental data. These models primarily rely on the multislice algorithm or Bloch wave methods to simulate electron probe propagation through crystalline specimens, accounting for multiple scattering events. The multislice approach divides the sample into thin slices perpendicular to the beam direction, iteratively computing the electron wavefunction as it interacts with the projected atomic potential in each slice. Input parameters include precise atomic coordinates, accelerating voltage, convergence angle, and thermal effects modeled via Debye-Waller factors, which describe atomic vibrations and ensure accurate intensity predictions for annular dark-field (ADF) and differential phase contrast (DPC) imaging.^[68]^[69] The core of the multislice method is encapsulated in the iterative propagation equation for the electron wavefunction:

\psi(z + \Delta z) = P(\Delta z) \, t(\Delta z) \, \psi(z)

where \psi(z) is the wavefunction at depth z, t(\Delta z) is the transmission function incorporating the slice's projected potential, and P(\Delta z) is the free-space propagator accounting for diffraction between slices. This formulation, originally developed for dynamical electron diffraction, allows simulation of probe-sample interactions under realistic conditions, including aberration coefficients for corrected instruments. Bloch wave methods, alternatively, diagonalize the scattering matrix for periodic structures, offering efficiency for perfect crystals but less flexibility for defects compared to multislice. In practice, these simulations facilitate quantitative analysis by fitting computed images to experimental ADF or DPC data, often minimizing a chi-squared metric to refine parameters like lattice strain or elemental occupancy while quantifying uncertainties through error propagation. For instance, strain mapping in semiconductors or nanoparticles involves optimizing simulated Z-contrast against observed intensities to achieve sub-picometer precision, validated against aberration-corrected STEM experiments on materials like SrTiO₃. Composition profiling similarly uses intensity ratios from multislice outputs to discern atomic species, as demonstrated in alloy interfaces. Dedicated software packages support these workflows: QSTEM implements multislice for arbitrary structures with emphasis on quantitative accuracy; Dr. Probe employs a beamlet partitioning scheme for high-resolution ADF simulations, enabling parallel computation; and PRISM accelerates calculations via interpolated scattering matrices, achieving over 1000-fold speedups with minimal loss in fidelity for large supercells.^[69] Despite their power, quantitative STEM models face limitations, notably high computational demands for simulating thick or disordered supercells, often requiring GPU acceleration or hybrid algorithms to manage terabyte-scale datasets. Thermal effects are typically approximated using the frozen lattice method, which generates multiple static atomic configurations and averages results to mimic vibrations, but this neglects dynamic phonon interactions and can introduce artifacts in low-temperature or beam-sensitive samples. Validation against experiments confirms accuracy for thicknesses up to 20-30 nm, beyond which multiple inelastic scattering degrades predictability.

Data processing and reconstruction

Data processing in scanning transmission electron microscopy (STEM) begins with pre-processing steps to correct instrumental artifacts and enhance image quality. Flat-field correction normalizes pixel-to-pixel variations in detector sensitivity, ensuring uniform illumination across the image field by dividing raw data by a reference flat-field image acquired under uniform conditions. Scan distortion removal addresses non-linear deformations caused by scan coil instabilities or electromagnetic interference, often using orthogonal raster scan pairs to map and rectify positional errors in the probe trajectory. Denoising techniques are essential for low-dose acquisitions to preserve atomic-scale details; principal component analysis (PCA) decomposes spectrum images into orthogonal components, retaining low-variance noise-free modes to achieve significant noise reduction in STEM XEDS data.^[70] Post-2020, deep learning methods like unsupervised denoising autoencoders have emerged, leveraging convolutional neural networks to map noisy STEM series to clean atomic images without paired training data, improving signal fidelity in 4D-STEM datasets. Reconstruction pipelines handle multi-frame or 4D-STEM data through alignment and iterative algorithms to recover phase or structural information. Alignment algorithms, such as non-rigid registration via Smart Align, fuse multiple frames by estimating sub-pixel shifts and distortions, enhancing spatial fidelity and signal-to-noise ratio (SNR) in annular dark-field (ADF) images.^[71] For ptychography, iterative methods like the extended ptychographic iterative engine solve phase retrieval by propagating probe and object estimates across overlapping scan positions, converging to high-resolution reconstructions; recent integrations with neural networks accelerate this process, reducing computation time while maintaining accuracy.^[72] In tomography, similar iterative schemes align tilt series and reconstruct volumes, often referencing simulations from quantitative STEM modeling for probe function refinement.^[73] Software tools facilitate these workflows, with open-source options like HyperSpy enabling interactive loading, visualization, and analysis of multidimensional STEM data through Python-based scripting for tasks like PCA decomposition and spectrum fitting. Proprietary platforms such as Gatan DigitalMicrograph support real-time scripting for 4D-STEM processing, including orientation mapping and spectrum imaging synchronization.^[74] Machine learning integration, exemplified by U-Net architectures, automates atom segmentation by classifying pixels in denoised STEM images, achieving precise localization of atomic positions even in low-SNR conditions.^[75] Performance is quantified by SNR, defined as the mean signal intensity divided by the standard deviation of noise (SNR = μ / σ), where multi-frame averaging can boost SNR by factors of 2–5 without resolution loss.^[76] Handling terabyte-scale 4D-STEM datasets requires GPU-accelerated parallel computing to manage memory and computation demands during reconstruction.^[77] Recent trends from 2020–2025 emphasize AI-driven tools, such as neural networks for automated aberration estimation from through-focal series, enabling post-acquisition correction of defocus and astigmatism to sub-angstrom precision.^[77] Real-time processing pipelines, incorporating deep convolutional networks for on-the-fly decision-making, now support adaptive scanning and immediate feedback in smart STEM setups, reducing acquisition times for dynamic experiments.

Specialized Applications

Tomography and 3D imaging

Scanning transmission electron microscopy (STEM) tomography extends two-dimensional imaging to three-dimensional volumetric reconstruction by acquiring a tilt series of projections, typically using annular dark-field (ADF) or bright-field (BF) detectors. The sample is tilted over a range of angles, commonly from -70° to +70°, to capture images at incremental steps (e.g., 1-2° intervals), enabling the mapping of internal structures in materials such as nanoparticles or crystalline defects.^[78] This approach is particularly suited to STEM due to its high signal-to-noise ratio in ADF mode, which provides Z-contrast for distinguishing atomic number variations across the volume.^[79] Reconstruction of the 3D volume from the tilt series relies on algorithms that invert the projection data, often modeled via the Radon transform, which integrates the object density along lines perpendicular to the projection direction:

Rf(\theta, s) = \int_{-\infty}^{\infty} \int_{-\infty}^{\infty} f(x, y) \, \delta(s - x \cos \theta - y \sin \theta) \, dx \, dy

where f(x, y) is the 2D cross-section, \theta is the tilt angle, s is the radial distance, and \delta is the Dirac delta function; the inverse Radon transform yields the density function for back-projection. Common methods include filtered back-projection (FBP), which applies a ramp filter to correct for blurring, and simultaneous iterative reconstruction technique (SIRT), an iterative approach that minimizes discrepancies between projections and the model for improved accuracy in noisy data.^[79] These techniques achieve isotropic resolutions around 1 nm, sufficient for resolving nanoscale features like void distributions or lattice distortions.^[80] A key limitation is the "missing wedge" artifact, arising from the incomplete angular coverage (typically <180° due to sample holder constraints), which elongates features along the tilt axis and reduces axial resolution; this is mitigated by model-based iterative methods that incorporate prior knowledge of the sample's sparsity or symmetry to inpaint missing projections.^[81] Applications include characterizing nanoparticle morphology, such as the core-shell structure in Au@Ag systems, where tomography reveals asymmetric growth and interface roughness, and mapping dislocation networks in semiconductors like PbSe nanocrystals, quantifying loop densities and strain fields.^[82]^[79] Recent advancements integrate 4D-STEM with tomography, capturing diffraction patterns at each tilt angle to enable orientation-resolved 3D reconstructions, enhancing strain and phase mapping in complex materials.^[83] This builds on traditional tilt-series alignment, using cross-correlation for drift correction during data processing. Challenges persist, including beam-induced sample drift, which distorts projections and necessitates rapid acquisition (e.g., <5 seconds per series), and the limited tilt range exacerbating the missing wedge; open-source software like TomoJ facilitates alignment, reconstruction via FBP or SIRT, and artifact correction through fiducial-free methods.^[80]^[84]

Cryogenic and in situ STEM

Cryogenic scanning transmission electron microscopy (cryo-STEM) enables imaging of beam-sensitive organic materials by cooling samples to approximately 100 K using liquid nitrogen or helium-based holders, preserving their native hydrated state and minimizing radiation damage. Specialized side-entry holders incorporate anti-contaminators, such as retractable liquid-nitrogen-cooled blades positioned near the specimen, to reduce frost buildup and maintain image clarity during extended observations.^[85] This approach has achieved near-atomic resolutions around 3.5 Å for protein structures, as demonstrated in single-particle imaging of biological macromolecules using integrated differential phase contrast (iDPC) modes, such as for the tobacco mosaic virus.^[86] Advancements in 4D-STEM ptychography have enabled sub-nanometer resolution 3D reconstructions of frozen-hydrated proteins.^[87] In situ STEM facilitates real-time observation of dynamic processes by integrating gas cells, heating stages up to 1000°C, and biasing capabilities into the microscope column, allowing studies of reactions such as catalysis under controlled atmospheres up to 1 mbar.^[88] Differential pumping systems, featuring multiple apertures and vacuum stages, isolate the sample environment from the high-vacuum column to prevent beam instability while enabling gas introduction without compromising instrument performance.^[89] These setups often employ environmental transmission electron microscopy (ETEM) configurations adapted for scanning modes, supporting atomic-scale visualization of evolving structures during operando conditions.^[90] Key applications of cryo- and in situ STEM include hydrated biomolecule imaging, where cryo-STEM reveals native conformations of proteins and nucleic acids, and battery material analysis, where in situ STEM tracks electrode evolution during electrochemical cycling.^[91] For instance, early cryo-STEM demonstrations in the 2010s captured DNA nanostructures in vitreous ice, highlighting beam-induced conformational changes at near-atomic scales.^[86] In catalysis, in situ STEM monitors nanoparticle sintering and facet formation under reactive gases, providing insights into active site dynamics.^[89] Challenges in these techniques encompass contamination from residual water vapor in cryo-STEM, necessitating rigorous sample preparation and holder preconditioning, and mechanical drift in in situ setups, which requires post-acquisition correction algorithms for accurate temporal analysis. Drift correction is particularly critical for video-rate imaging, where sub-pixel alignment ensures reliable tracking of fast-evolving features like lithium dendrite growth in batteries.^[92] Advancements from 2015 to 2025 have integrated micro-electro-mechanical systems (MEMS)-based chips into in situ STEM, enabling nanogap chambers below 1 nm for precise control of liquid or gas environments in electrochemical experiments.^[93] Fast direct electron detectors have enhanced temporal resolution to the millisecond regime, supporting high-frame-rate capture of transient events such as catalytic turnover or phase transformations.^[94] These developments, including atomic-resolution cryo-STEM at liquid helium temperatures, expand applications to quantum materials and biological dynamics while integrating with tomography for 3D environmental reconstructions.^[95]

Low-voltage and environmental variants

Low-voltage scanning transmission electron microscopy (STEM) operates at accelerating voltages typically below 100 kV, often in the range of 20–60 kV, to reduce beam-induced damage and enhance contrast for beam-sensitive materials such as organic compounds and light-element structures.^[96] This approach minimizes knock-on damage and radiolysis effects that are more pronounced at higher voltages, allowing for the imaging of delicate samples like biological tissues or 2D materials without significant structural alteration.^[97] Aberration correction is essential in low-voltage STEM to maintain sub-ångström resolution, enabling the visualization of individual atoms in materials like graphene or metal oxides. For instance, at 60 kV, STEM has been used to quantify defect dynamics in 2D WS₂, revealing beam-induced sulfur vacancy formation rates that are lower than at 200 kV, thus preserving sample integrity during extended observation. A key advantage of low-voltage STEM is improved signal-to-noise ratio for electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDS) on light elements, such as oxygen or carbon, due to reduced background scattering and enhanced scattering cross-sections at lower energies.^[98] This has facilitated quantitative mapping of lithium distribution in battery materials at 30 kV, where traditional high-voltage methods suffer from delocalization and damage.^[96] However, challenges include the need for ultra-thin samples (typically <20 nm) to avoid multiple scattering, which limits applicability to bulk materials, and requires advanced preparation techniques like focused ion beam milling.^[97] Seminal advancements in this area stem from the integration of chromatic aberration correctors, as demonstrated in early 60 kV STEM systems that achieved 0.1 nm resolution for light-element imaging. Environmental variants of STEM, often termed environmental STEM (ESTEM), extend imaging capabilities to non-vacuum conditions by incorporating differential pumping systems that maintain pressures up to several millibars of gas (e.g., O₂, H₂, or CO) around the sample while preserving high vacuum in the electron column.^[99] This setup uses specialized holders with microelectromechanical systems (MEMS) to introduce controlled gaseous environments and temperatures up to 1000°C, enabling real-time observation of dynamic processes like catalysis or oxidation at atomic resolution via high-angle annular dark-field (HAADF) imaging.^[100] For example, ESTEM has visualized single-atom migration in Pt/C catalysts during redox cycles, showing reversible encapsulation and exposure of platinum atoms under alternating H₂ and O₂ atmospheres at 1 mbar and 300°C.^[99] The primary benefits of ESTEM include the ability to study realistic operating conditions for nanomaterials, such as gas-solid interactions in heterogeneous catalysts, where structural changes like sintering or faceting occur in situ.^[99] Analytical techniques like EELS and EDS remain viable, providing chemical insights into reaction intermediates, as seen in the oxidation of iron nanoparticles where oxide shell growth was tracked at 0.5 mbar O₂.^[101] Challenges involve managing increased electron scattering from gas molecules, which reduces signal intensity and necessitates low-dose imaging protocols to avoid beam-stimulated reactions, and the limitation to relatively low pressures compared to industrial conditions (typically <10 mbar).^[99] Pioneering work by Boyes and Gai in the 1990s established the foundational ESTEM framework using dedicated gas-cell columns, paving the way for aberration-corrected systems that achieve sub-0.1 nm resolution in reactive environments.^[99]

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https://doi.org/10.1146/annurev-matsci-080921-092646
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Environmental STEM Study of the Oxidation Mechanism for Iron and ...
The oxidation of solution-synthesized iron (Fe) and iron carbide (Fe2C) nanoparticles was studied in an environmental scanning transmission electron ...