Atomic-scale Characterization Research Articles

Sub-Angstrom Characterization of the Structural Origin for High In-Plane Anisotropy in 2D GeS2.

Materials with layered crystal structures and high in-plane anisotropy, such as black phosphorus, present unique properties and thus promise for applications in electronic and photonic devices. Recently, the layered structures of GeS2 and GeSe2 were utilized for high-performance polarization-sensitive photodetection in the short wavelength region due to their high in-plane optical anisotropy and wide band gap. The highly complex, low-symmetric (monoclinic) crystal structures are at the origin of the high in-plane optical anisotropy, but the structural nature of the corresponding nanostructures remains to be fully understood. Here, we present an atomic-scale characterization of monoclinic GeS2 nanostructures and quantify the in-plane structural anisotropy at the sub-angstrom level in real space by Cs-corrected scanning transmission electron microscopy. We elucidate the origin of this high in-plane anisotropy in terms of ordered and disordered arrangement of [GeS4] tetrahedra in GeS2 monolayers, through density functional theory (DFT) calculations and orbital-based bonding analyses. We also demonstrate high in-plane mechanical, electronic, and optical anisotropies in monolayer GeS2 and envision phase transitions under uniaxial strain that could potentially be exploited for nonvolatile memory applications.

Functional gas sensing nanomaterials: A panoramic view

The alarming rise of indoor pollution and the need to combat the associated negative effects have promoted increasing attention in modernizing the chemical sensing technologies by newly designed materials with rich and tunable functionalities at atomic or molecular levels. With the appealing physical, chemical, optical, and electronic properties for various potential applications, the state-of-art gas-sensing nanomaterials and their future perspectives are well-documented and summarized in this paper. Specifically, the key performance attributes are addressed in detail such as the sensitivity, selectivity, reversibility, operating temperature, response time, and detection limit. As such, this review provides both critical insights in exploring and understanding various gas sensing nanomaterials and points out limitations and opportunities for further developments, such as morphology control, doping and surface alteration, atomic-scale characterization, and applications in different fields. Finally, the challenges and outlooks are discussed on the basis of the current developments.

Atomic electron tomography in three and four dimensions

Abstract

Concentration-Dependent High-Temperature Deformation Mechanism of Hexagonal Close-Packed Ti-Al Alloys

The dependence of Al alloying on the high-temperature deformation physics of Ti-Al alloys was investigated in the present study. The mechanisms of planar dislocation slip, enhanced 〈c + a〉 activity and stacking faults were operative in low-alloyed Ti-Al model materials upon plastic deformation at 600 °C. Increasing Al content up to 10.2 at. pct triggered significant pyramidal slip and $$ \left\{ {10\bar{1}2} \right\} $$ twins, highlighting the role of Al alloying. Atomic-scale characterization revealed the nature of the topologically discontinuous “stepped” twin boundary, consisting of alternately stacked basal/prismatic interface couples without a traditional rigorous twin invariant plane. This is the first time significant twinning has been observed in severely alloyed polycrystalline hcp metallic alloys, especially plastically deforming at 600 °C with a low strain rate of 1 × 10−4 s−1. A lattice-reorientation-based twinning mechanism, as well as its influence on mechanical properties, was proposed and discussed.

Atomic-scale insights into the interfacial instability of superlubricity in hydrogenated amorphous carbon films.

The origin of instability or even disappearance of the superlubricity state in hydrogenated amorphous carbon (a-C:H) film in the presence of oxygen or water molecules is still controversial. Here, we address this puzzle regarding the tribochemical activities of sliding interfaces at the nanoscale. The results reveal that gaseous oxygen molecules disable the antifriction capacity of a-C:H by surface dehydrogenation of tribo-affected hydrocarbon bonds. In comparison, oxygen incorporation into the hydrocarbon matrix induces the formation of a low-density surface shear band, owing to which the friction state depends on the oxygen content. High friction of a-C:H film in humid environment originates from the "tumor-like" heterogeneous structures as formed in the highly oxidized tribolayer. Notably, an appropriate doping of silicon can completely shield the moisture effect by forming a silica-like tribolayer. These outcomes shed substantial lights upon the roadmap for achieving robust superlubricity of carbon films in a wide range of environments.

Grinding strain induced antiferroelectric-ferroelectric-antiferroelectric sandwich structure in bulk ceramics

The grinding strain induced ferroelectric (FE) to antiferroelectric (AFE) phase transition with associated hysteresis loops tuning were found in bulk Pb0.99(Zr0.97Ti0.03)0.98Nb0.02O3 ceramics. The induced AFE phases can keep stable unless applying heat treatment or electric field. The atomic-scale characterizations reveal that the perfect coherent FE/AFE interface appears either diffuse or sharp depending on its relative orientation to polarization. The induced surface AFE layers on FE bulk can tune hysteresis loops from symmetric to asymmetric and this effect increased with the decrease of thickness of samples. These findings provide new pathway for manipulate phase state and discover novel properties in ferroics.

Revealing multiple classes of stable quantum emitters in hexagonal boron nitride with correlated optical and electron microscopy.

Defects in hexagonal boron nitride (hBN) exhibit high-brightness, room-temperature quantum emission, but their large spectral variability and unknown local structure challenge their technological utility. Here, we directly correlate hBN quantum emission with local strain using a combination of photoluminescence (PL), cathodoluminescence (CL) and nanobeam electron diffraction. Across 40 emitters, we observe zero phonon lines (ZPLs) in PL and CL ranging from 540 to 720 nm. CL mapping reveals that multiple defects and distinct defect species located within an optically diffraction-limited region can each contribute to the observed PL spectra. Local strain maps indicate that strain is not required to activate the emitters and is not solely responsible for the observed ZPL spectral range. Instead, at least four distinct defect classes are responsible for the observed emission range, and all four classes are stable upon both optical and electron illumination. Our results provide a foundation for future atomic-scale optical characterization of colour centres.

Atomic-scale structural evolution of electrode materials in Li-ion batteries: a review

Owing to the high spatial resolution at the atomic scale, the transmission electron microscopy (TEM) or scanning transmission electron microscopy is demonstrated as a promising characterization method to unveil the charge storage mechanism of electrode materials in Li-ion batteries. The structural evolution of electrode materials during charge/discharge process can be directly observed by using TEM. The detailed analysis establishes a relationship between the structure of electrode material and battery performance. Herein, we present a brief review of the atomic-scale characterization in Li-ion batteries, including Li (de)insertion mechanism (both cations and anions charge-compensation mechanism), migration of transition metal ions, and surface phase transition. The in-depth microscopic analysis reveals the detailed structural characteristics, which influence the properties of LIBs, establish the structure–function relationship, and facilitate the development of Li-ion batteries.

Interfacial Nanostructure of 2D Ti3C2/Graphene Quantum Dots Hybrid Multicoating for Ultralow Wear

The 2D Ti3C2/graphene quantum dots (GQDs) composite coating contains numerous merits, which offers underlying solutions for lubrication issues in common conditions. Herein, specific efforts are devoted to explore and develop the antiwear interfacial performances of 2D Ti3C2/GQDs hybrid multicoating in common environment. In addition, combination with nanodiamond would impose significant effects on improving lubricity of binary composite coating to obtain long‐durability ternary composite coating. Meanwhile, macroscale and atomicscale characterizations are utilized to detect the lubrication behavior of binary and ternary composite coating to illuminate the influence of compositions on the establishment of ultralow‐wear interfaces on slip process. These testing results highlight an outspread lubrication mechanism for 2D Ti3C2/GQDs hybrid multicoating. They suggest that 2D Ti3C2/GQDs and 2D Ti3C2/nanodiamond/GQDs composite coating exhibit ultralow wear with rubbing against polytetrafluoroethylene ball. A newly nanostructured tribofilm is formed on the sliding contact. Also the 2D Ti3C2/nanodiamond/GQDs ternary coating has long‐durability and ultralow‐wear during sliding for a long time.

The effects of vanadium substitution on one-dimensional tunnel structures of cryptomelane: Combined TEM and DFT study

Rational design of energy storage materials requires mechanistic understanding which relies on atomic scale characterization of structure and defects. Here we utilize vanadium (V)-substituted one-dimensional cryptomelane as a model material to study structural features critical to electrochemical reversibility of the cathode in lithium-ion batteries. We found the substitution of manganese (Mn) atoms in the tunnel wall with V enhances structural uniformity in the parent KMn8O16 material by inhibiting formation of irregular (2 × 3 and 2 × 1) manganese oxide tunnel structures. Upon lithiation, both transmission electron microscopy (TEM) and density functional theory (DFT) reveal significant structural evolution, eventually resulting in phase separation and formation of highly reduced manganese oxide. During this process, Mn4+ accepts electrons donated by intercalated Li, while the V4+ substituent is not electrochemically reduced but acts to stabilize the tunnel framework structure. As a result, the V substituted material shows higher operating voltage and improved electrochemical reversibility. Our results highlight the benefits of V-substitution in cryptomelane, and the significance of integrating characterization with simulation for understanding and developing design principles for lithium ion battery materials.

Quality control of on-surface-synthesised seven-atom wide armchair graphene nanoribbons.

On-surface synthesis is a powerful method for fabricating atomically precise graphene nanoribbons (GNRs), but the products always include defective structures. In this study, scanning tunnelling microscopy and atomic force microscopy were used to determine the length distribution of armchair-edge GNRs with a width of seven carbon atoms (7-AGNRs) synthesised on Au(111) and to characterise defective structures. The product quality was improved by increasing the precursor deposition amount because of a preference for intermolecular polymerisation over intramolecular cyclodehydrogenation at a high coverage. However, the annealing rate had a complex effect on the quality, with a low rate elongating 7-AGNRs but degenerating the length uniformity. These insights advance the understanding of the critical parameters for obtaining high-quality products in high yield by on-surface synthesis.

Solid wetting-layers in inorganic nano-reactors: the water in imogolite nanotube case.

By combined use of wide-angle X-ray scattering, thermo-gravimetric analysis, inelastic neutron scattering, density functional theory and density functional theory molecular dynamics simulations, we investigate the structure, dynamics and stability of the water wetting-layer in single-walled aluminogermanate imogolite nanotubes (SW Ge-INTs): an archetypal system for synthetically controllable and monodisperse nano-reactors. We demonstrate that the water wetting-layer is strongly bound and solid-like up to 300 K under atmospheric pressure, with dynamics markedly different from that of bulk water. Atomic-scale characterisation of the wetting-layer reveals organisation of the H2O molecules in a curved triangular sublattice stabilised by the formation of three H-bonds to the nanotube's inner surface, with covalent interactions sufficiently strong to promote energetically favourable decoupling of the H2O molecules in the adlayer. The evidenced changes in the local composition, structure, electrostatics and dynamics of the Ge-INT's inner surface upon the formation of the solid wetting-layer demonstrate solvent-mediated functionalisation of the nanotube's cavity at room temperature and pressure, suggesting new strategies for the design of nano-rectors towards potential control of chemical reactivity in nano-confined volumes.

Suppressing Cation Migration and Reducing Particle Cracks in a Layered Fe-Based Cathode for Advanced Sodium-Ion Batteries.

Sodium-ion batteries have huge potential in large-scale energy storage applications. Layered Fe-based oxides are one of the desirable cathode materials due to abundance in the earth crust and high activity in electrochemical processes. However, Fe-ion migration to Na layers is one of the major hurdles leading to irreversible structural degradation. Herein, it is revealed that distinct Fe-ion migration in cycling NaFeO2 (NFO) should be mainly responsible for the strong local lattice strain and resulting particle cracks, all of which results in the deterioration of electrochemical performance. More importantly, a strategy of Ru doping could effectively suppress the Fe-ion migration and then reduce the local lattice strain and the particle cracks, finally to greatly enhance the sodium storage performance. Atomic-scale characterization shows that NFO electrode after cycling presents the intense lattice strain locally, accompanied by the remarkable particle cracks. Whereas, Ru-doped NFO electrode maintains the well-ordered layered structure by inhibiting the Fe-O distortion, so as to eliminate the resulting side effect. As a result, Ru-doped NFO could greatly improve the comprehensive electrochemical performance by delivering a reversible capacity of 120 mA h g-1 , about 80% capacity retention after 100 cycles. The findings provide new insights for designing high-performance electrodes for sodium-ion batteries.

Scalable BEOL compatible 2D tungsten diselenide

Low-temperature synthesis of two-dimensional (2D) transition metal dichalcogenides (TMDs) is a key challenge for their integration with complementary metal-oxide-semiconductor (CMOS) technology at ‘back-end-of-line (BEOL)’. Most low-temperature synthesis utilizes alkali salts, oxide-based metals, and methyl-group based chalcogen precursors which do not meet current BEOL requirements for contaminant-free manufacturing and process scalability. In this study, we benchmark a carbon and alkali salt-free synthesis of fully coalesced, stoichiometric 2D WSe2 films on amorphous SiO2/Si substrates at BEOL- compatible temperatures (475 °C) via gas-source metal-organic chemical deposition. This work highlights the necessity of a Se-rich environment in a kinetically limited growth regime for successful integration of low-temperature 2D WSe2. Atomic-scale characterization reveals that BEOL WSe2 is polycrystalline with domain size of ~200 nm and band gap of 1.8 eV. Back-gated and electrolyte double layer gated field-effect transistors (FETs) exhibit increased ON currents as high as 4 µA µm−1 and ON/OFF ratios of ~106, demonstrating a 100× improvement compared to previously reported BEOL compatible TMDs.

Atomic-scale characterization of the site occupancies in the B2 FeAl phase: Effects of Ni addition, temperature and lattice misfit

A microscopic phase-field model is constructed to study the influences of Ni addition, temperature and lattice misfit on the site occupancies in complex B2 FeAl–Ni intermetallic phase. The tendencies of six site occupancies are explored through tracing the atomic site occupation probability. The results reveal that six site occupancies are monotone functions of Ni concentration, temperature and the lattice misfit, respectively. For the antisite behavior of Fe (AlFe), Al (FeAl) and the substitution behavior of Ni (AlNi, FeNi), the ranks of four defect occupancies are identical under three factors. The transformation rate and ordering degree of the complex FeAl–Ni are dependent of three factors. The mechanism of configuration of six site occupancies is revealed from the energy analysis. The results are in accordance with experimental and theoretical data and provide theoretical support to the questioned validity of the lattice misfit effect on the ordering behavior in intermetallics.

Phase Segmentation in Atom-Probe Tomography Using Deep Learning-Based Edge Detection

Atom-probe tomography (APT) facilitates nano- and atomic-scale characterization and analysis of microstructural features. Specifically, APT is well suited to study the interfacial properties of granular or heterophase systems. Traditionally, the identification of the interface between, for precipitate and matrix phases, in APT data has been obtained either by extracting iso-concentration surfaces based on a user-supplied concentration value or by manually perturbing the concentration value until the iso-concentration surface qualitatively matches the interface. These approaches are subjective, not scalable, and may lead to inconsistencies due to local composition inhomogeneities. We introduce a digital image segmentation approach based on deep neural networks that transfer learned knowledge from natural images to automatically segment the data obtained from APT into different phases. This approach not only provides an efficient way to segment the data and extract interfacial properties but does so without the need for expensive interface labeling for training the segmentation model. We consider here a system with a precipitate phase in a matrix and with three different interface modalities—layered, isolated, and interconnected—that are obtained for different relative geometries of the precipitate phase. We demonstrate the accuracy of our segmentation approach through qualitative visualization of the interfaces, as well as through quantitative comparisons with proximity histograms obtained by using more traditional approaches.

Electronic properties of chemically doped graphene

The controlled modification of graphene's electronic structure through chemical doping is a promising way to expand its possible applications. It is also a subject of interest for fundamental research as the effects of the dopants on the electronic structure of a two-dimensional (2D) material differ vastly from chemical doping of conventional 3D semiconductors such as silicon. In this research update, the authors describe the main results obtained so far on the modification of the electronic properties of graphene upon chemical doping. This includes the atomic scale characterization of the dopant configuration and charge distribution as probed by scanning tunneling microscopy and spectroscopy, momentum-resolved band structure modification as probed by angle-resolved photoemission spectroscopy, and detailed scattering mechanisms information obtained from magnetoresistance measurements. They give an exhaustive and critical account of the above-mentioned subjects, describe open questions, and suggest future research directions.

Cold Microscopy Solves Hot Problems in Energy Research

Many material systems and phenomena elude nano- and atomic-scale characterization through electron microscopy due to the impact of the high energy beam that is employed. Cryogenic microscopy methods pioneered in structural biology offer a way to overcome this limitation by enabling the formation of images with an extremely low dose of electrons using automated data acquisition and advanced detectors. These methods are just beginning to be applied to non-biological materials, and their utility is demonstrated in a pair of recent papers by Cui and colleagues.

Elucidating the composition of PtAg surface alloys with atomic-scale imaging and spectroscopy.

Silver-based heterogeneous catalysts, modified with a range of elements, have found industrial application in several reactions in which selectivity is a challenge. Alloying small amounts of Pt into Ag has the potential to greatly enhance the somewhat low reactivity of Ag while maintaining high selectivity and resilience to poisoning. This single-atom alloy approach has had many successes for other alloy combinations but has yet to be investigated for PtAg. Using scanning tunneling microscopy (STM) and STM-based spectroscopy, we characterized the atomic-scale surface structure of a range of submonolayer amounts of Pt deposited on and in Ag(111) as a function of temperature. Near room temperature, intermixing of PtAg results in multiple metastable structures on the surface. Increasing the alloying temperature results in a higher concentration of isolated Pt atoms in the regions near Ag step edges as well as direct exchange of Pt atoms into Ag terraces. Furthermore, STM-based work function measurements allow us to identify Pt rich areas of the samples. We use CO temperature programmed desorption to confirm our STM assignments and quantify CO binding strengths that are compared with theory. Importantly, we find that CO, a common catalyst poison, binds more weakly to Pt atoms in the Ag surface than extended Pt ensembles. Taken together, this atomic-scale characterization of model PtAg surface alloys provides a starting point to investigate how the size and structure of Pt ensembles affect reaction pathways on the alloy and can inform the design of alloy catalysts with improved catalytic properties and resilience to poisoning.

Surface Nucleation of the Paraelectric Phase in Ferroelectric BaTiO3: Atomic Scale Mapping

In ferroelectricity, atomic-scale dipole moments interact collectively to produce strong electromechanical coupling and switchable macroscopic polarization. Hence, the functionality of ferroelectrics emerges at a solid–solid phase transformation that is accompanied by a sudden disappearance of an inversion symmetry. Much effort has been made to understand the ferroelectric transition at the polarization length scale. Nevertheless, the dipole-moment origin of ferroelectricity has remained elusive. Here, we used variable-temperature high-resolution transmission electron microscopy to reveal the dipole-moment dynamics during the ferroelectric-to-paraelectric transition. We show that the transition occurs when paraelectric nuclei of the size of a couple of unit cells emerge near the surface. Upon heating, the cubic phase grows sidewalk toward the bulk. We quantified the nucleation barrier and show dominancy of mechanical interactions over electrical interaction, helping us demonstrate similarities to predictions of domain nucleation during electric-field switching. Our work motivates dynamic atomic-scale characterizations of solid–solid transitions in other materials.