Direct atom-by-atom chemical identification of nanostructures and defects of topological insulators

Staggered-layer-boosted flexible Bi2Te3 films with high thermoelectric performance

Room-temperature bismuth telluride (Bi2Te3) thermoelectrics are promising candidates for low-grade heat harvesting. However, the brittleness and inflexibility of Bi2Te3 are far reaching and bring about lifelong drawbacks. Here we demonstrate good pliability over 1,000 bending cycles and high power factors of 4.2 (p type) and 4.6 (n type) mW m-1 K-2 in Bi2Te3-based films that were exfoliated from corresponding single crystals. This unprecedented bendability was ascribed to the in situ observed staggered-layer structure that was spontaneously formed during the fabrication to promote stress propagation whilst maintaining good electrical conductivity. Unexpectedly, the donor-like staggered layer rarely affected the carrier transport of the films, thus maintaining its superior thermoelectric performance. Our flexible generator showed a high normalized power density of 321 W m-2 with a temperature difference of 60 K. These high performances in supple thermoelectric films not only offer useful paradigms for wearable electronics, but also provide key insights into structure-property manipulation in inorganic semiconductors.

Metavalent Bonding in Layered Phase-Change Memory Materials

Metavalent bonding (MVB) is characterized by the competition between electron delocalization as in metallic bonding and electron localization as in covalent or ionic bonding, serving as an essential ingredient in phase-change materials for advanced memory applications. The crystalline phase-change materials exhibits MVB, which stems from the highly aligned p orbitals and results in large dielectric constants. Breaking the alignment of these chemical bonds leads to a drastic reduction in dielectric constants. In this work, it is clarified how MVB develops across the so-called van der Waals-like gaps in layered Sb2 Te3 and Ge-Sb-Te alloys, where coupling of p orbitals is significantly reduced. A type of extended defect involving such gaps in thin films of trigonal Sb2 Te3 is identified by atomic imaging experiments and ab initio simulations. It is shown that this defect has an impact on the structural and optical properties, which is consistent with the presence of non-negligible electron sharing in the gaps. Furthermore, the degree of MVB across the gaps is tailored by applying uniaxial strain, which results in a large variation of dielectric function and reflectivity in the trigonal phase. At last, design strategies are provided for applications utilizing the trigonal phase.

Comprehensive Insight into p-Type Bi2Te3-Based Thermoelectrics near Room Temperature

Bi2Te3 is a well-recognized material for its unique properties in diverse thermoelectric applications near room temperature. The considerable efforts on Bi2Te3-based alloys have been extremely extensive in recent years, and thus the latest breakthroughs in high-performance p-type (Bi, Sb)2Te3 alloys are comprehensively reviewed to further implement applications. Effective strategies to further improve the thermoelectric performance are summarized from the perspective of enhancing the power factor and minimizing the lattice thermal conductivity. Furthermore, the surface states of topological insulators are investigated to provide a possibility of advancing (Bi, Sb)2Te3 thermoelectrics. Finally, future challenges and outlooks are overviewed. This review will provide potential guidance toward designing and developing high-efficient Bi2Te3-based and other thermoelectrics.

Cation Defect Mediated Phase Transition in Potassium Tungsten Bronze

Applying in situ transmission electron microscopy, the phase instability in potassium tungsten bronze (K_xWO₃, 0.18 < x < 0.57) induced by heating was investigated. The atomistic phase transition pathway of monoclinic K_0.20WO₃ → hexagonal K_mWO₃ (0.18 < m < 0.20) → cubic WO₃ induced by cationic defects (K and W vacancies) was directly revealed. Unexpectedly, a K⁺-rich tetragonal K_nWO₃ (0.40 < n < 0.57) phase would nucleate as well, which may result from the blockage of K⁺ diffusion at the grain boundaries. Our results point out the critical role of the cationic defects in mediating the crystal structures in K_xWO₃, which provide reference to rational structural design for extensive high-temperature applications.

Atomic-scale study of nanocatalysts by aberration-corrected electron microscopy

Aberration-corrected electron microscopy (AC-EM) including transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) has become one of the most powerful technologies in the studies of nanocatalysts. With the current spatial resolution of sub-0.5 Å and energy resolution of 10 meV, AC-EM can quantificationally articulate the connection between catalytic properties and atomic configurations of nanocatalysts. However, the restricted irradiation sensitive characteristics of specimens pose an obstacle to solve their intrinsic structure. Low-dose imaging should be applied to overcome this problem. In addition, the choice of appropriate imaging method is also crucial to tackle specific structural problems of nanocatalysts. On the basis of careful management of electron dose and selection of suitable imaging method,in situgas and liquid S/TEM are able to reveal the structure evolution of nanocatalysts in real-time. Further combination with residual gas analysis would deepen the understanding of the catalytic reaction.

Microscopic Mechanism of Carbon-Dopant Manipulating Device Performance in CGeSbTe-Based Phase Change Random Access Memory

Carbon (C)-doped Ge₂Sb₂Te₅ material is a potential candidate in phase change random access memory (PCRAM) because of its superb thermal stability and ultrahigh cycle endurance. Unfortunately, the role and distribution evolution of C-dopant is still not fully understood, especially in practical industrial devices. In this report, with the aid of advanced spherical aberration corrected transmission electron microscopy, the mechanism of microstructure evolution manipulated by C-dopant is clearly defined. The grain-inner C atoms distinctly increase cationic migration energy barriers, which is the fundamental reason for promoting the thermal stability of metastable face-centered-cubic phase and postponing its transition to the hexagonal structure. By current pulses stimulation, the stochastic grain-outer C clusters tend to aggregate in the active area by breaking C-Ge bonding; thus, grain growth and elemental segregation are effectively suppressed to improve device reliability, for example, lower SET resistance, shorter SET time, and enlarged RESET/SET ratio. In short, the visual distribution variations of C-dopant can manipulate the performance of the PCRAM device, having much broader implications for optimizing its microstructure transition and understanding C-doped material system.

Observation of van der Waals reconfiguration in superlattice phase change materials

Phase change memory (PCM) is a leading candidate for nonvolatile memory applications in the big data era. However, the high power consumption, caused by melting GeTe-Sb2Te3-like phase change materials, hinders their applications. A significant step is the proposal to spatially separate GeTe and Sb2Te3 in the form of a superlattice, enabling a higher operating speed and better cyclability at reduced switching energy. However, the physical origin is under intensive debate. Recently, the swapping of the SbTe terminating layers nearest to the van der Waals (vdWs) gap has been claimed to be the mechanism for the superlattice. Here, we reported a direct atomic-scale chemical identification of two kinds of vdWs reconfigurations together with atomic simulations. The vdWs reconfigurations, which occurred at the GeTe and Sb2Te3 boundary, were demonstrated to change the electrical properties and turn this semiconductor into a conductor, leading to the resistance contrast. Besides, strong intermixing of Ge and Sb atoms was directly observed; in the most severe cases, ∼50% of Ge in the GeTe layer diffused into the adjacent Sb2Te3 layer. Our work paves the way for deeper understanding of the phase transition of the GeTe/Sb2Te3 superlattice and the future design of non-volatile memories towards dynamic random access-like memories.

A Review of Germanium-Antimony-Telluride Phase Change Materials for Non-Volatile Memories and Optical Modulators

Chalcogenide phase change materials based on germanium-antimony-tellurides (GST-PCMs) have shown outstanding properties in non-volatile memory (NVM) technologies due to their high write and read speeds, reversible phase transition, high degree of scalability, low power consumption, good data retention, and multi-level storage capability. However, GST-based PCMs have shown recent promise in other domains, such as in spatial light modulation, beam steering, and neuromorphic computing. This paper reviews the progress in GST-based PCMs and methods for improving the performance within the context of new applications that have come to light in recent years.

Mapping the Reactions in a Single Zero-Valent Iron Nanoparticle

Nanoscale zerovalent iron (nZVI) possesses unique functionalities for metal-metalloid removal and sequestration. So far, direct evidence on the heavy metal-nZVI reactions in the solid phase is still limited due to low concentration of heavy metals and small size of nanoparticles. In this work, angstrom-resolution spectral mappings on the reactions of nZVI with chromate, arsenate, nickel, silver, cesium, and zinc ions are presented. This work was achieved with spherical aberration-corrected scanning transmission electron microscopy integrated with high-sensitivity X-ray energy-dispersive spectroscopy-scanning transmission electron microscopy (XEDS-STEM). Results confirm that iron nanoparticles have a core-shell structure. In addition, the removal mechanism significantly depends on the standard potential E⁰ (E⁰ is standard potential w.r.t. standard hydrogen electrode at 25 °C when free ion activity is 1.). For strong oxidizing agents, such as Cr(VI), the removal mechanism is diffusion and encapsulation in the core area of the nZVI particle. For moderate oxidizers, such as As(V) with E⁰ more positive than that of iron, the removal mechanism is adsorption at the surface, followed by diffusion and encapsulation into the particle between the core and the shell. For metal cations with an E⁰ close to or more negative than that of iron, such as Cs(I) and Zn(II), the removal mechanism is sorption or surface-complex formation. For metal cations with E⁰ much more positive than that of iron, such as Ag(I), the removal mechanism is rapid reduction on the surface of nZVI. Meanwhile, metals with E⁰ slightly more positive than that of iron, such as Ni(II), can be immobilized at the nanoparticle surface via sorption and reduction. The synergetic effects of sorption, reduction, and encapsulation mechanisms of nZVI lead to rapid reactions and high efficiency for treatment and immobilization of many toxic heavy metals. Results also demonstrate that the XEDS-STEM technique is a powerful tool for studying reactions in individual nanoparticles and is particularly valuable for mapping trace-level elements in environmental media.

A Review on Disorder-Driven Metal-Insulator Transition in Crystalline Vacancy-Rich GeSbTe Phase-Change Materials

Metal-insulator transition (MIT) is one of the most essential topics in condensed matter physics and materials science. The accompanied drastic change in electrical resistance can be exploited in electronic devices, such as data storage and memory technology. It is generally accepted that the underlying mechanism of most MITs is an interplay of electron correlation effects (Mott type) and disorder effects (Anderson type), and to disentangle the two effects is difficult. Recent progress on the crystalline Ge₁Sb₂Te₄ (GST) compound provides compelling evidence for a disorder-driven MIT. In this work, we discuss the presence of strong disorder in GST, and elucidate its effects on electron localization and transport properties. We also show how the degree of disorder in GST can be reduced via thermal annealing, triggering a disorder-driven metal-insulator transition. The resistance switching by disorder tuning in crystalline GST may enable novel multilevel data storage devices.

Epitaxial Growth of Ternary Topological Insulator Bi2 Te2 Se 2D Crystals on Mica

Nanostructures of ternary topological insulator (TI) Bi₂ Te₂ Se are, in principle, advantageous to the manifestation of topologically nontrivial surface states, due to significantly enhanced surface-to-volume ratio compared with its bulk crystals counterparts. Herein, the synthesis of 2D Bi₂ Te₂ Se crystals on mica via the van der Waals epitaxy method is explored and systematically the growth behaviors during the synthesis process are investigated. Accordingly, 2D Bi₂ Te₂ Se crystals with domain size up to 50 µm large and thickness down to 2 nm are obtained. A pronounced weak antilocalization effect is clearly observed in the 2D Bi₂ Te₂ Se crystals at 2 K. The method for epitaxial growth of 2D ternary Bi₂ Te₂ Se crystals may inspire materials engineering toward enhanced manifestation of the subtle surface states of TIs and thereby facilitate their potential applications in next-generation spintronics.

Visualizing Arsenate Reactions and Encapsulation in a Single Zero-Valent Iron Nanoparticle

A nanostructure-based mechanism is presented on the enrichment, separation, and immobilization of arsenic with nanoscale zero-valent iron (nZVI). The As-Fe reactions are studied with spherical aberration corrected scanning transmission electron microscopy (Cs-STEM). Near-atomic resolution (<1 nm³) electron tomography discovers a thin continuous layer (23 ± 3 Å) of elemental arsenic sandwiched between the iron oxide shell and the zerovalent iron core. This points to a unique mechanism of nanoencapsulation and proves that the outer layer, especially the Fe(0)-oxide interface, is the edge of the As-Fe reactions. Atomic-resolution imaging on the grain boundary provides strong evidence that arsenic atoms diffuse preferably along the nonequilibrium, high-energy, and defective polycrystalline grain boundary of iron oxides. Results also offer direct evidence on the surface sorption or surface complex formation of arsenate on ferric hydroxide (FeOOH). The core-shell structure and unique properties of nZVI clearly underline rapid separation, large capacity, and stability for the treatment of toxic heavy metals such as cadmium, chromium, arsenic, and uranium.

A Second Amorphous Layer Underneath Surface Oxide

Formation of a nanometer-scale oxide surface layer is common when a material is exposed to oxygen-containing environment. Employing aberration-corrected analytical transmission electron microscopy and using single crystal SnSe as an example, we show that for an alloy, a second thin amorphous layer can appear underneath the outmost oxide layer. This inner amorphous layer is not oxide based, but instead originates from solid-state amorphization of the base alloy when its free energy rises to above that of the metastable amorphous state; which is a result of the composition shift due to the preferential depletion of the oxidizing species, in our case, the outgoing Sn reacting with the oxygen atmosphere.

Observation of hidden atomic order at the interface between Fe and topological insulator Bi2Te3

The first compelling evidence of unique atomic order at the ferromagnet Fe/topological insulator Bi₂Te₃ interface.

Extraordinary Off-Stoichiometric Bismuth Telluride for Enhanced n-Type Thermoelectric Power Factor

Thermoelectrics directly converts waste heat into electricity and is considered a promising means of sustainable energy generation. While most of the recent advances in the enhancement of the thermoelectric figure of merit (ZT) resulted from a decrease in lattice thermal conductivity by nanostructuring, there have been very few attempts to enhance electrical transport properties, i.e., the power factor. Here we use nanochemistry to stabilize bulk bismuth telluride (Bi₂Te₃) that violates phase equilibrium, namely, phase-pure n-type K_0.06Bi₂Te_3.18. Incorporated potassium and tellurium in Bi₂Te₃ far exceed their solubility limit, inducing simultaneous increase in the electrical conductivity and the Seebeck coefficient along with decrease in the thermal conductivity. Consequently, a high power factor of ∼43 μW cm^-1 K^-2 and a high ZT > 1.1 at 323 K are achieved. Our current synthetic method can be used to produce a new family of materials with novel physical and chemical characteristics for various applications.

New Insights into Intrinsic Point Defects in V2VI3 Thermoelectric Materials

Defects and defect engineering are at the core of many regimes of material research, including the field of thermoelectric study. The 60-year history of V2VI3 thermoelectric materials is a prime example of how a class of semiconductor material, considered mature several times, can be rejuvenated by better understanding and manipulation of defects. This review aims to provide a systematic account of the underexplored intrinsic point defects in V2VI3 compounds, with regard to (i) their formation and control, and (ii) their interplay with other types of defects towards higher thermoelectric performance. We herein present a convincing case that intrinsic point defects can be actively controlled by extrinsic doping and also via compositional, mechanical, and thermal control at various stages of material synthesis. An up-to-date understanding of intrinsic point defects in V2VI3 compounds is summarized in a (χ, r)-model and applied to elucidating the donor-like effect. These new insights not only enable more innovative defect engineering in other thermoelectric materials but also, in a broad context, contribute to rational defect design in advanced functional materials at large.

Improving Signal-to-Noise Ratio in Scanning Transmission Electron Microscopy Energy-Dispersive X-Ray (STEM-EDX) Spectrum Images Using Single-Atomic-Column Cross-Correlation Averaging

Acquiring an atomic-resolution compositional map of crystalline specimens has become routine practice, thus opening possibilities for extracting subatomic information from such maps. A key challenge for achieving subatomic precision is the improvement of signal-to-noise ratio (SNR) of compositional maps. Here, we report a simple and reliable solution for achieving high-SNR energy-dispersive X-ray (EDX) spectroscopy spectrum images for individual atomic columns. The method is based on standard cross-correlation aided by averaging of single-column EDX maps with modifications in the reference image. It produces EDX maps with minimal specimen drift, beam drift, and scan distortions. Step-by-step procedures to determine a self-consistent reference map with a discussion on the reliability, stability, and limitations of the method are presented here.

Accurate Nanoscale Crystallography in Real-Space Using Scanning Transmission Electron Microscopy

Here, we report reproducible and accurate measurement of crystallographic parameters using scanning transmission electron microscopy. This is made possible by removing drift and residual scan distortion. We demonstrate real-space lattice parameter measurements with <0.1% error for complex-layered chalcogenides Bi2Te3, Bi2Se3, and a Bi2Te2.7Se0.3 nanostructured alloy. Pairing the technique with atomic resolution spectroscopy, we connect local structure with chemistry and bonding. Combining these results with density functional theory, we show that the incorporation of Se into Bi2Te3 causes charge redistribution that anomalously increases the van der Waals gap between building blocks of the layered structure. The results show that atomic resolution imaging with electrons can accurately and robustly quantify crystallography at the nanoscale.

Frenkel-Defect-Mediated Chemical Ordering Transition in a Li-Mn-Ni Spinel Oxide

Using spinel-type Li(Mn(1.5)Ni(0.5) )O4 with two different cations, Mn and Ni, in the oxygen octahedra as a model system, we show that a cation ordering transition takes place through the formation of Frenkel-type point defects. A series of experimental results based on atomic-scale observations and in situ powder diffractions along with ab initio calculations consistently support such defect-mediated transition behavior. In addition to providing a precise suggestion of the intermediate transient states and the resulting kinetic pathway during the transition between two phases, our findings emphasize the significant role of point defects in ordering transformation of complex oxides.

Vertical/Planar Growth and Surface Orientation of Bi2Te3 and Bi2Se3 Topological Insulator Nanoplates

Nanostructures are not only attractive for fundamental research but also offer great promise for bottom-up nanofabrications. In the past, the growth of one-dimensional vertical/planar nanomaterials such as nanowires has made significant progresses. However, works on two-dimensional nanomaterials are still lacking, especially for those grown out of a substrate. We report here a vertical growth of topological insulator, Bi2Se3 and Bi2Te3, nanoplates on mica. In stark contrast to the general belief, these nanoplates are not prisms exposing (100) lateral surfaces, which are expected to minimize the surface area. Instead, they are frustums, enclosed by (01-4), (015), and (001) facets. First-principles calculations, combined with experiments, suggest the importance of surface oxidation in forming these unexpected surfaces.

Surface-orientation-dependent distribution of subsurface cation-exchange defects in olivine-phosphate nanocrystals

Atomic-scale exchange between two different cations of similar size in crystalline oxides is one of the major types of point defects when multiple cations in oxygen interstitials are arrayed in an ordered manner. Although a number of studies have been performed on a variety of Li-intercalation olivine phosphates to determine the distribution of exchange defects in bulk, understanding of the thermodynamic stability of the defects in subsurface regions and its dependency on the crystallographic orientation at the surface has remained elusive. Through a combination of small-angle neutron scattering, atomic-scale direct probing with scanning transmission electron microscopy, and theoretical ab initio calculations, we directly demonstrate that the antisite exchange defects are distributed in a highly anisotropic manner near the surfaces of LiFePO4 crystals. Moreover, a substantial amount of cation exchanges between Li and Fe sites is identified as an energetically favorable configuration in some surface regions, showing excellent agreement with the calculation results of negative defect formation energies. The findings in this study provide insight into developing better ways to avoid degradation of lithium mobility through the surface as well as scientifically notable features regarding the distribution of exchange defects in olivine phosphates.

Scale-invariant quantum anomalous Hall effect in magnetic topological insulators beyond the two-dimensional limit

We investigate the quantum anomalous Hall effect (QAHE) and related chiral transport in the millimeter-size (Cr(0.12)Bi(0.26)Sb(0.62))₂Te₃ films. With high sample quality and robust magnetism at low temperatures, the quantized Hall conductance of e²/h is found to persist even when the film thickness is beyond the two-dimensional (2D) hybridization limit. Meanwhile, the Chern insulator-featured chiral edge conduction is manifested by the nonlocal transport measurements. In contrast to the 2D hybridized thin film, an additional weakly field-dependent longitudinal resistance is observed in the ten-quintuple-layer film, suggesting the influence of the film thickness on the dissipative edge channel in the QAHE regime. The extension of the QAHE into the three-dimensional thickness region addresses the universality of this quantum transport phenomenon and motivates the exploration of new QAHE phases with tunable Chern numbers. In addition, the observation of scale-invariant dissipationless chiral propagation on a macroscopic scale makes a major stride towards ideal low-power interconnect applications.

Proximity induced high-temperature magnetic order in topological insulator--ferrimagnetic insulator heterostructure

Introducing magnetic order in a topological insulator (TI) breaks time-reversal symmetry of the surface states and can thus yield a variety of interesting physics and promises for novel spintronic devices. To date, however, magnetic effects in TIs have been demonstrated only at temperatures far below those needed for practical applications. In this work, we study the magnetic properties of Bi2Se3 surface states (SS) in the proximity of a high Tc ferrimagnetic insulator (FMI), yttrium iron garnet (YIG or Y3Fe5O12). Proximity-induced butterfly and square-shaped magnetoresistance loops are observed by magneto-transport measurements with out-of-plane and in-plane fields, respectively, and can be correlated with the magnetization of the YIG substrate. More importantly, a magnetic signal from the Bi2Se3 up to 130 K is clearly observed by magneto-optical Kerr effect measurements. Our results demonstrate the proximity-induced TI magnetism at higher temperatures, an important step toward room-temperature application of TI-based spintronic devices.