Scanning transmission electron microscopy: A review of high angle annular dark field and annular bright field imaging and applications in lithium-ion batteries

Scanning transmission electron microscopy（STEM） has been shown as powerful tools for material characterization,especially after the appearance of aberration-corrector which greatly enhances the resolution of STEM. High angle annular dark field（HAADF） and annular bright field（ABF） imaging of the aberration-corrected STEM are widely used due to their high-resolution capabilities and easily interpretable image contrasts. However, HAADF mode of the STEM is still limited in detecting light elements due to the weak electron-scattering power. ABF mode of the STEM could detect light and heavy elements simultaneously, providing unprecedented opportunities for probing unknown structures of materials. Atomiclevel structure investigation of materials has been achieved by means of these imaging modes, which is invaluable in many fields for either improving properties of materials or developing new materials. This paper aims to provide a introduction of HAADF and ABF imaging techniques and reviews their applications in characterization of cathode materials, study of electrochemical reaction mechanisms, and exploring the effective design of lithium-ion batteries（LIBs）. The future prospects of the STEM are also discussed.

A machine perspective of atomic defects in scanning transmission electron microscopy

Enabled by the advances in aberration-corrected scanning transmission electron microscopy(STEM),atomic-resolution real space imaging of materials has allowed a direct structure-property investigation.Traditional ways of quantitative data analysis suffer from low yield and poor accuracy.New ideas in the field of computer vision and machine learning have provided more momentum to harness the wealth of big data and sophisticated information in STEM data analytics,which has transformed STEM from a localized characterization technique to a macroscopic tool with intelligence.In this review article,we discuss the prime significance of defect topology and density in two-dimensional(2D)materials,which have proved to be a powerful means to tune a wide range of properties.Subsequently,we systematically review advanced data analysis methods that have demonstrated promising prospects in analyzing STEM data,particularly for identifying structural defects,with high throughput and veracity.A unified framework for atomic structure identification is also summarized.

Improved Cohen-Sutherland algorithm for TGS transmission imaging

Tomographic gamma scanner(TGS),an advancedγ-ray nondestructive analysis technique,can locate and analyze nuclides in radioactive nuclear waste,and TGS can be categorized into two processes:e.g.,transmission measurement and emission measurement.Specifically,transmission measurements provide the basis for accurate measurement of nonuniform radionuclide content in TGS scanning.The scan data were obtained using the Monte Carlo tool Geant4 simulation,and 25 voxels were divided into five lengths and five widths in a square barrel.In this study,an encoding cropping algorithm based on draped foot vector judgment was adopted to rapidly calculate the voxel trace matrix within a square bucket of nuclear waste,and the transmission images were reconstructed using ordered subset expectation maximization.The results indicated that the cropping speed of the improved coding algorithm was significantly higher than that of the original algorithm,and the relative mean deviation and root-mean-square error between the reconstructed attenuation coefficient and the reference standard value tended to decrease with an increase in the cropped line segments in the voxel;the Pearson correlation coefficient tended to converge to 1.0.The image quality evaluation parameters of the high media-density materials were better than those of the low media-density materials in the above three indexes.The reconstruction effect was relatively poor for more complex filling materials.When there were more than 10 cropped line segments in the voxel,the reconstruction data generally tended to be stable.The graphical trimming algorithm can rapidly calculate the trace matrix of the scanned voxels;it exhibits the advantages of speed and efficiency and can serve as a novel method to solve the trace matrix of TGS nuclear waste transmission scans.

Multiphase cooperation for multilevel strain accommodation in a single-crystalline BiFeO_(3) thin film

The functionalities and diverse metastable phases of multiferroic BiFeO_(3)(BFO)thin films depend on the misfit strain.Although mixed phase-induced strain relaxation in multiphase BFO thin films is well known,it is unclear whether a singlecrystalline BFO thin film can accommodate misfit strain without the involvement of its polymorphs.Thus,understanding the strain relaxation behavior is key to elucidating the lattice strain–property relationship.In this study,a correlative strain analysis based on dark-field inline electron holography(DIH)and quantitative scanning transmission electron microscopy(STEM)was performed to reveal the structural mechanism for strain accommodation of a single-crystalline BFO thin film.The nanoscale DIH strain analysis results indicated a random combination of multiple strain states that acted as a primary strain relief,forming irregularly strained nanodomains.The STEM-based bond length measurement of the corresponding strained nanodomains revealed a unique strain accommodation behavior achieved by a statistical combination of multiple modes of distorted structures on the unit-cell scale.The globally integrated strain for each nanodomain was estimated to be close to1.5%,irrespective of the nanoscale strain states,which was consistent with the fully strained BFO film on the SrTiO_(3) substrate.Density functional theory calculations suggested that strain accommodation by the combination of metastable phases was energetically favored compared to single-phase-mediated relaxation.This discovery allows a comprehensive understanding of strain accommodation behavior in ferroelectric oxide films,such as BFO,with various low-symmetry polymorphs.

Atomically self-healing of structural defects in monolayer WSe_(2)

Minimizing disorder and defects is crucial for realizing the full potential of two-dimensional transition metal dichalcogenides(TMDs) materials and improving device performance to desired properties. However, the methods in defect controlcurrently face challenges with overly large operational areas and a lack of precision in targeting specific defects. Therefore,we propose a new method for the precise and universal defect healing of TMD materials, integrating real-time imaging withscanning transmission electron microscopy (STEM). This method employs electron beam irradiation to stimulate the diffusionmigration of surface-adsorbed adatoms on TMD materials grown by low-temperature molecular beam epitaxy (MBE),and heal defects within the diffusion range. This approach covers defect repairs ranging from zero-dimensional vacancydefects to two-dimensional grain orientation alignment, demonstrating its universality in terms of the types of samples anddefects. These findings offer insights into the use of atomic-level focused electron beams at appropriate voltages in STEMfor defect healing, providing valuable experience for achieving atomic-level precise fabrication of TMD materials.

Visualizing extended defects at the atomic level in a Bi_(2)Sr_(2)CaCu_(2)O8_(+σ) superconducting wire

The microstructure significantly influences the superconducting properties.Herein,the defect structures and atomic arrangements in high-temperature Bi_(2)Sr_(2)CaCu_(2)O8_(+σ) superconducting wire are directly characterized via stateof-the-art scanning transmission electron microscopy.Interstitial oxygen atoms are observed in both the charge reservoir layers and grain boundaries in the doped superconductor.Inclusion phases with varied numbers of CuO_(2) layers are found,and twist interfaces with different angles are identified.This study provides insights into the structures of Bi-2212 wire and lays the groundwork for guiding the design of microstructures and optimizing the production methods to enhance superconducting performance.

Probing nickelate superconductors at atomic scale:A STEM review

The discovery of nickelate superconductors,including doped infinite-layer(IL)nickelates RNiO2(R=La,Pr,Nd),layered square-planar nickelate Nd6Ni5O12,and the Ruddlesden–Popper(RP)phase La3Ni2O7,has spurred immense interest in fundamental research and potential applications.Scanning transmission electron microscopy(STEM)has proven crucial for understanding structure–property correlations in these diverse nickelate superconducting systems.In this review,we summarize the key findings from various modes of STEM,elucidating the mechanism of different nickelate superconductors.We also discuss future perspectives on emerging STEM techniques for unraveling the pairing mechanism in the“nickel age”of superconductivity.

Nanoscale cathodoluminescence spectroscopy probing the nitride quantum wells in an electron microscope

To gain further understanding of the luminescence properties of multiquantum wells and the factors affecting them on a microscopic level,cathodoluminescence combined with scanning transmission electron microscopy and spectroscopy was used to measure the luminescence of In_(0.15)Ga_(0.85)N five-period multiquantum wells.The lattice-composition-energy relationship was established with the help of energy-dispersive x-ray spectroscopy,and the bandgaps of In_(0.15)Ga_(0.85)N and GaN in multiple quantum wells were extracted by electron energy loss spectroscopy to understand the features of cathodoluminescence spectra.The luminescence differences between different periods of multiquantum wells and the effects of defects such as composition fluctuation and dislocations on the luminescence of multiple quantum wells were revealed.Our study establishing the direct relationship between the atomic structure of In_(x)Ga_(1-x)N multiquantum wells and photoelectric properties provides useful information for nitride applications.

Symmetry quantification and segmentation in STEM imaging through Zernike moments

We present a method using Zernike moments for quantifying rotational and reflectional symmetries in scanning transmission electron microscopy(STEM)images,aimed at improving structural analysis of materials at the atomic scale.This technique is effective against common imaging noises and is potentially suited for low-dose imaging and identifying quantum defects.We showcase its utility in the unsupervised segmentation of polytypes in a twisted bilayer TaS_(2),enabling accurate differentiation of structural phases and monitoring transitions caused by electron beam effects.This approach enhances the analysis of structural variations in crystalline materials,marking a notable advancement in the characterization of structures in materials science.

Enhanced Electrical Properties of Bi_(2−x)Sb_(x)Te_(3) Nanoflake Thin Films Through Interface Engineering

The structure–property relationship at interfaces is difficult to probe for thermoelectric materials with a complex interfacial microstructure.Designing thermoelectric materials with a simple,structurally-uniform interface provides a facile way to understand how these interfaces influence the transport properties.Here,we synthesized Bi_(2−x)Sb_(x)Te_(3)(x=0,0.1,0.2,0.4)nanoflakes using a hydrothermal method,and prepared Bi_(2−x)Sb_(x)Te_(3) thin films with predominantly(0001)interfaces by stacking the nanoflakes through spin coating.The influence of the annealing temperature and Sb content on the(0001)interface structure was systematically investigated at atomic scale using aberration-corrected scanning transmission electron microscopy.Annealing and Sb doping facilitate atom diffusion and migration between adjacent nanoflakes along the(0001)interface.As such it enhances interfacial connectivity and improves the electrical transport properties.Interfac reactions create new interfaces that increase the scattering and the Seebeck coefficient.Due to the simultaneous optimization of electrical conductivity and Seebeck coefficient,the maximum power factor of the Bi_(1.8)Sb_(0.2)Te_(3) nanoflake films reaches 1.72 mW m^(−1)K^(−2),which is 43%higher than that of a pure Bi_(2)Te_(3) thin film.

Multidimensional images and aberrations in STEM

Recent advances in scanning transmission electron microscopy(STEM)have led to increased development of multidimensional STEM imaging modalities and novel image reconstruction methods.This interest arises because the main electron lens in a modern transmission electron microscope usually has a diffraction-space information limit that is significantly better than the real-space resolution of the same lens.This state-of-affairs is sometimes shared by other scattering methods in modern physics and contributes to a broader excitement surrounding multidimensional techniques that scan a probe while recording diffraction-space images,such as ptychography and scanning nano-beam diffraction.However,the contrasting resolution in the two spaces raises the question as to what is limiting their effective performance.Here,we examine this paradox by considering the effects of aberrations in both image and diffraction planes,and likewise separate the contributions of pre-and post-sample aberrations.This consideration provides insight into aberration-measurement techniques and might also indicate improvements for super-resolution techniques.

Atomic-scale insights of indium segregation and its suppression by GaAs insertion layer in InGaAs/AlGaAs multiple quantum wells

The In segregation and its suppression in InGaAs/AlGaAs quantum well are investigated by using high-resolution x-ray diffraction(XRD)and photoluminescence(PL),combined with the state-of-the-art aberration corrected scanning transmission electron microscopy(Cs-STEM)techniques.To facility our study,we grow two multiple quantum wells(MQWs)samples,which are almost identical except that in sample B a thin GaAs layer is inserted in each of the InGaAs well and AlGaAs barrier layer comparing to pristine InGaAs/AlGaAs MQWs(sample A).Our study indeed shows the direct evidences that In segregation occurs in the InGaAs/AlGaAs interface,and the effect of the Ga As insertion layer on suppressing the segregation of In atoms is also demonstrated on the atomic-scale.Therefore,the atomic-scale insights are provided to understand the segregation behavior of In atoms and to unravel the underlying mechanism of the effect of GaAs insertion layer on the improvement of crystallinity,interface roughness,and further an enhanced optical performance of InGaAs/AlGaAs QWs.

Lithium mapping in a Mg-9Li-4Al-1Zn alloy using electron energy-loss spectroscopy

Magnesium-lithium alloys with high lithium content have been attracting significant attention because of their low density,high formability and corrosion resistance.These properties are dependent on the distribution of lithium,which is difficult to map in the presence of magnesium.In this work,a ratio spectrum-imaging method with electron energy-loss spectroscopy(EELS)is demonstrated,which enables the mapping of lithium.In application to LAZ941(Mg-9Li-4Al-1Zn in wt.%),this technique revealed that a key precipitate in the microstructure,previously thought by some to be Mg_(17)Al_(12),is in fact rich in lithium.This result was corroborated with a structural investigation by high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM),showing this phase to be Al_(1-x)Zn_(x)Li,with x<<1.This work indicates the potential offered by this technique for mapping lithium in materials.

Development of advanced electron tomography in materials science based on TEM and STEM

The recent developments of electron tomography（ET） based on transmission electron microscopy（TEM） and scanning transmission electron microscopy（STEM） in the field of materials science were introduced. The various types of ET based on TEM as well as STEM were described in detail, which included bright-field（BF）-TEM tomography, dark-field（DF）-TEM tomography, weak-beam dark-field（WBDF）-TEM tomography, annular dark-field（ADF）-TEM tomography, energy-filtered transmission electron microscopy（EFTEM） tomography, high-angle annular dark-field（HAADF）-STEM tomography, ADF-STEM tomography, incoherent bright field（IBF）-STEM tomography, electron energy loss spectroscopy（EELS）-STEM tomography and X-ray energy dispersive spectrometry（XEDS）-STEM tomography, and so on. The optimized tilt series such as dual-axis tilt tomography, on-axis tilt tomography, conical tilt tomography and equally-sloped tomography（EST） were reported. The advanced reconstruction algorithms, such as discrete algebraic reconstruction technique（DART）, compressed sensing（CS） algorithm and EST were overviewed. At last, the development tendency of ET in materials science was presented.

Synthesis and Characterization of GaN Nanorods by Ammoniating Ga_2O_3 /Co Films Deposited on Si(111) Substrates

GaN nanorods are successfully synthesized on Si（111） substrates with magnetron sputtering through ammoniating Ga2O3/Co films at 950℃. X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy,and Fourier-transform infrared spectroscopy are used to characterize the samples. The results demonstrate that the nanorods are single-crystal GaN with a hexagonal wurtzite structure and possess relatively smooth surfaces. The growth mechanism of GaN nanorods is also discussed.

Direct View of Cr Atoms Doped in Anatase TiO2（001） Thin Film

Imaging the doping elements is doped TiO2 thin film. But it is critical for understanding the photocatalytic activity of still a challenge to characterize the interactions between the dopants and the TiO2 lattice at the atomic level. Here, we use high angle annular dark- field/annular bright-field scanning transmission electron microscope （HAADF/ABF-STEM） combined with electron energy loss spectroscopy （EELS） to directly image the individual Cr atoms doped in anatase TiO2（001） thin film from [100] direction. The Cr dopants, which are clearly imaged through the atomic-resolution EELS mappings while can not be seen by HADDF/ABF-STEM, occupy both the substitutional sites of Ti atoms and the interstitial sites of TiO2 matrix. Most of them preferentially locate at the substitutional sites of Ti atoms. These results provide the direct evidence for the doping structure of Cr-doped A- TiO2 thin film at the atomic level and also prove the EELS mapping is an excellent technique for characterizing the doped materials.

Dilute long period stacking/order(LPSO)-variant phases along the composition gradient in a Mg-Ho-Cu alloy

We have systematically investigated the microstructures of as-cast Mg_(97.49)Ho_(1.99)Cu_(0.43)Zr_(0.09)alloy by atomic resolution high-angle annular dark field scanning transmission electron microscopy(HAADF-STEM), revealing the coexistence of 18R, 14H and 24R long period stacking/order(LPSO) phases with fully coherent interfaces along step-like composition gradient in a blocky intermetallic compound distributed at grain boundary. The short-range order(SRO) L1_(2)-type Cu_(6)Ho_(8)clusters embedded across AB’C’A-stacking fault layers are directly revealed at atomic scale. Importantly, the order degree of SRO clusters in the present dilute alloy is significant lower than previous 6M and 7M in-plane order reported in ternary Mg-TM(transition metal)-RE(rare earth) alloys, which can be well matched by 9M in-plane order. This directly demonstrates that SRO in-plane L1_(2)-type clusters can be expanded into more dilute composition regions bounded along the definite TM/RE ratio of 3/4. In addition, the estimated chemical compositions of solute enriched stacking fault(SESF) in all LPSO variants are almost identical with the ideal SESF composition of 9M in-plane order, regardless of the type of LPSO phases. The results further support the viewpoint that robust L1_(2)-type TM_(6)RE_(8)clusters play an important role in governing LPSO phase formation.

Study on corrosion resistance of artificially aged 7075 aluminium alloy by using Cs-corrected STEM

This research studied the mechanism of the corrosion resistance enhancement of artificially aged 7075 aluminium alloy using advanced Cs-corrected scanning transmission electron microscopy(STEM).The corrosion behaviors of artificially aged 7075 aluminium alloys in a 3.5 wt.%NaCl solution were investigated by impedance spectra,equivalent circuit analyses,polarization measurements and immersion tests.The results show that a longer aging treatment leads to better corrosion resistance,which can be attributed to the following microstructural features,as revealed by STEM.The Cu segregation at grain boundaries under over-aged conditions helps retard intergranular corrosion.The Mg(Zn,Cu)_(2) precipitates formed on the surfaces of Al_(18)Mg_(3)(Cr,Mn)_(2) dispersoids effectively insulate the dispersoids as cathodes in corrosion,from the Al matrix.This study demonstrates a potential strategy to design corrosion-resistant alloys achieved by proper alloying and subsequent aging.

Synthesis of large-scale GaN nanowires by ammoniating Ga_2O_3 films on Co layer deposited on Si(111) substrates

A mass of GaN nanowires has been successfully synthesized on Si（111） substrates by magnetron sputtering through ammoniating Ga2O3/Co films at 950℃. X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscope and Fourier transformed infrared spectra are used to characterize the samples. The results demonstrate that the nanowires are of single-crystal GaN with a hexagonal wurtzite structure and possess relatively smooth surfaces. The growth mechanism of GaN nanowires is also discussed.

High-throughput combinatorial approach expeditesthe synthesis of a lead-free relaxor ferroelectric system

Developing novel lead-free ferroelectric materials is crucial for next-generationmicroelectronic technologies that are energy efficient and environmentfriendly.However,materials discovery and property optimization are typicallytime-consuming due to the limited throughput of traditional synthesismethods.In this work,we use a high-throughput combinatorial synthesisapproach to fabricate lead-free ferroelectric superlattices and solid solutions of(Ba_(0.7)Ca_(0.3))TiO_(3)(BCT)and Ba(Zr_(0.2)Ti_(0.8))O_(3)(BZT)phases with continuous variationof composition and layer thickness.High-resolution x-ray diffraction(XRD)and analytical scanning transmission electron microscopy(STEM)demonstratehigh film quality and well-controlled compositional gradients.Ferroelectricand dielectric property measurements identify the“optimal propertypoint”achieved at the composition of 48BZT–52BCT.Displacement vectormaps reveal that ferroelectric domain sizes are tunable by varying{BCT–BZT}Nsuperlattice geometry.This high-throughput synthesis approach can be appliedto many other material systems to expedite new materials discovery and properties optimization,allowing for the exploration of a large area of phasespace within a single growth.