Thermal Stability and Anisotropic Sublimation of Two-Dimensional Colloidal Bi2Te3 and Bi2Se3 Nanocrystals

Nanoelectronics-enabled reservoir computing hardware for real-time robotic controls

Traditional robotic vehicle control algorithms, implemented on digital devices with firmware, result in high power consumption and system complexity. Advanced control systems based on different device physics are essential for the advancement of sophisticated robotic vehicles and miniature mobile robots. Here, we present a nanoelectronics-enabled analog control system mimicking conventional controllers' dynamic responses for real-time robotic controls, substantially reducing training cost, power consumption, and footprint. This system uses a reservoir computing network with interconnected memristive channels made from layered semiconductors. The network's nonlinear switching and short-term memory characteristics effectively map input sensory signals to high-dimensional data spaces, enabling the generation of motor control signals with a simply trained readout layer. This approach minimizes software and analog-to-digital conversions, enhancing energy and resource efficiency. We demonstrate this system with two control tasks: rover target tracking and drone lever balancing, achieving similar performance to traditional controllers with ~10-microwatt power consumption. This work paves the way for ultralow-power edge computing in miniature robotic systems.

Plasmonic Nanoparticles Boost Solar-to-Electricity Generation at Ambient Conditions

Sunlight-to-electricity conversion using solar thermoelectric generators (STEGs) is a proven technology to meet our ever-growing energy demand. However, STEGs are often operated under a vacuum with customized thermoelectric materials to achieve high performance. In this work, the incorporation of plasmonic gold nanoparticle (AuNP) based solar absorbers enabled the efficient operation of STEGs under ambient conditions with commercially available thermoelectric devices. AuNPs enhanced the performance of STEG by ∼9 times, yielding an overall solar-to-electricity conversion efficiency of ∼9.6% under 7.5 W cm-2 solar irradiance at ambient conditions. Plasmonic heat dissipated by AuNPs upon solar irradiation was used as the thermal energy source for STEGs. High light absorptivity, photothermal conversion efficiency (∼95%), and thermal conductivity of AuNPs enabled the efficient generation and transfer of heat to STEGs, with minimal radiative and convective heat losses. The power generated from plasmon-powered STEGs is used to run electrical devices as well as produce green hydrogen via the electrolysis of water.

Two-Dimensional Van Der Waals Thin Film and Device

In the rapidly evolving field of thin-film electronics, the emergence of large-area flexible and wearable devices has been a significant milestone. Although organic semiconductor thin films, which can be manufactured through solution processing, have been identified, their utility is often undermined by their poor stability and low carrier mobility under ambient conditions. However, inorganic nanomaterials can be solution-processed and demonstrate outstanding intrinsic properties and structural stability. In particular, a series of two-dimensional (2D) nanosheet/nanoparticle materials have been shown to form stable colloids in their respective solvents. However, the integration of these 2D nanomaterials into continuous large-area thin with precise control of layer thickness and lattice orientation still remains a significant challenge. This review paper undertakes a detailed analysis of van der Waals thin films, derived from 2D materials, in the advancement of thin-film electronics and optoelectronic devices. The superior intrinsic properties and structural stability of inorganic nanomaterials are highlighted, which can be solution-processed and underscor the importance of solution-based processing, establishing it as a cornerstone strategy for scalable electronic and optoelectronic applications. A comprehensive exploration of the challenges and opportunities associated with the utilization of 2D materials for the next generation of thin-film electronics and optoelectronic devices is presented.

Dynamic behavior of metal nanoparticles in MOF materials: analysis with electron microscopy and deep learning

Electron microscopy is a key characterization technique for nanoscale systems, and electron microscopy images are typically recorded and analyzed in terms of the morphology of the objects under study in static mode. The emerging current trend is to analyze the dynamic behavior at the nanoscale observed during electron microscopy measurements. In this work, the study of the stability of MOF structures with different compositions and topologies under conditions of an electron microscope experiment revealed an unusual dynamic behavior of M NPs formed due to the electron-beam-induced transformation of specific frameworks. The transition to the liquid phase led to spatial movement, rapid sintering, and an increase in the M NPs size within seconds. In the case of copper nanoparticles, instantaneous sublimation was observed. The dynamic behavior of Co NPs was analyzed with a computational framework combining deep learning and classic computer vision techniques. The present study for the first time revealed unique information about the stability of a variety of MOFs under an electron beam and the dynamic behavior of the formed M NPs. The formation of Fe, Ni, Cu, and Co NPs was observed from a molecular framework with a specific subsequent behavior - a stable form for Fe, excessive dynamics for Co, and sublimation/condensation for Cu. Two important outcomes of the present study should be mentioned: (i) electron microscopy investigations of MOF samples should be made with care, as decomposition under an electron beam may lead to incorrect results and the appearance of "phantom" nanoparticles; and (ii) MOFs represent an excellent model for fundamental studies of molecular-to-nano transitions in situ in video mode, including a number of dynamic transformations.

Thermolysis-Driven Growth of Vanadium Oxide Nanostructures Revealed by In Situ Transmission Electron Microscopy: Implications for Battery Applications

Understanding the growth modes of 2D transition-metal oxides through direct observation is of vital importance to tailor these materials to desired structures. Here, we demonstrate thermolysis-driven growth of 2D V₂O₅ nanostructures via in situ transmission electron microscopy (TEM). Various growth stages in the formation of 2D V₂O₅ nanostructures through thermal decomposition of a single solid-state NH₄VO₃ precursor are unveiled during the in situ TEM heating. Growth of orthorhombic V₂O₅ 2D nanosheets and 1D nanobelts is observed in real time. The associated temperature ranges in thermolysis-driven growth of V₂O₅ nanostructures are optimized through in situ and ex situ heating. Also, the phase transformation of V₂O₅ to VO₂ was revealed in real time by in situ TEM heating. The in situ thermolysis results were reproduced using ex situ heating, which offers opportunities for upscaling the growth of vanadium oxide-based materials. Our findings offer effective, general, and simple pathways to produce versatile 2D V₂O₅ nanostructures for a range of battery applications.

Salt-Assisted Low-Temperature Growth of 2D Bi2 O2 Se with Controlled Thickness for Electronics

Bi₂ O₂ Se is the most promising 2D material due to its semiconducting feature and high mobility, making it propitious channel material for high-performance electronics that demands highly crystalline Bi₂ O₂ Se at low-growth temperature. Here, a low-temperature salt-assisted chemical vapor deposition approach for growing single-domain Bi₂ O₂ Se on a millimeter scale with thicknesses of multilayer to monolayer is presented. Because of the advantage of thickness-dependent growth, systematical scrutiny of layer-dependent Raman spectroscopy of Bi₂ O₂ Se from monolayer to bulk is investigated, revealing a redshift of the A_1g mode at 162.4 cm^-1 . Moreover, the long-term environmental stability of ≈2.4 nm thick Bi₂ O₂ Se is confirmed after exposing the sample for 1.5 years to air. The backgated field effect transistor (FET) based on a few-layered Bi₂ O₂ Se flake represents decent carrier mobility (≈287 cm²  V^-1 s^-1 ) and an ON/OFF ratio of up to 10⁷ . This report indicates a technique to grow large-domain thickness controlled Bi₂ O₂ Se single crystals for electronics.

Thermal Stability and Sublimation of Two-Dimensional Co9Se8 Nanosheets for Ultrathin and Flexible Nanoelectronic Devices

An understanding of the structural and compositional stability of nanomaterials is significant from both fundamental and technological points of view. Here, we investigate the thermal stability of half-unit-cell thick two-dimensional (2D) Co₉Se₈ nanosheets that are exceptionally interesting because of their half-metallic ferromagnetic properties. By employing in situ heating in the transmission electron microscope (TEM), we find that the nanosheets show good structural and chemical stability without changes to the cubic crystal structure until sublimation of the nanosheets starts at temperatures between 460 and 520 °C. The real-time observations of the sublimation process show preferential removal at {110} type crystal facets. From an analysis of sublimation rates at various temperatures, we find that the sublimation occurs through noncontinuous and punctuated mass loss at lower temperatures while the sublimation is continuous and uniform at higher temperatures. Our findings provide an understanding of the nanoscale structural and compositional stability of 2D Co₉Se₈ nanosheets, which is of importance for their reliable application and sustained performance as ultrathin and flexible nanoelectronic devices.

Melting, Crystallization, and Alloying Dynamics in Nanoscale Bismuth Telluride

It is critical to understand the transformation mechanisms in layered metal chalcogenides to enable controlled synthesis and processing. Here, we develop an alumina encapsulation layer-based in situ transmission electron microscopy (TEM) setup that enables the investigation of melting, crystallization, and alloying of nanoscale bismuth telluride platelets while limiting sublimation in the high-vacuum TEM environment. Heating alumina-encapsulated platelets to 700 °C in situ resulted in melting that initiated at edge planes and proceeded via the movement of a sharp interface. The encapsulated melt was then cooled to induce solidification, with individual nuclei growing to form single crystals with the same basal plane orientation as the original platelet and nonequilibrium crystal shapes imposed by the encapsulation layer. Finally, heating platelets in the presence of antimony caused alloying and lattice strain, along with heterogeneous phase formation. These findings provide new insight into important transformation processes in layered metal chalcogenide materials.

Going beyond the equilibrium crystal shape: re-tracing the morphological evolution in group 5 tetradymite nanocrystals

Nanocrystals of group 5 tetradymites M₂X₃ (where M = Bi and Sb, X = Se and Te) are of high technological relevance in modern topological nanoelectronics. However, there is a current lack of a systematic understanding to predict the preferred nanocrystal morphology in experiments where commonly-used equilibrium thermodynamic models appear to fail. In this work, using first-principles DFT calculations with a rationally-extended ab initio atomistic thermodynamics approach coupled to implicit solvation models and Gibbs-Wulff shape constructions, we demonstrate that this absence of predictive power stems from the limitation of equilibrium thermodynamics. By re-tracing and carefully addressing with a more realistic chemical potential definition, we illustrate this shortcoming can be overcome and afford a more rational route to size-engineer and shape-design highly-functional group 5 tetradymite nanoparticles for targeted applications.

Exfoliated Bi2Te3-enabled membranes for new concept water desalination: Freshwater production meets new routes

Water scarcity forces the science to find the most environmentally friendly propulsion technology for supplying plentiful freshwater at low energy costs. Membrane Distillation well meets criteria of eco-friendly management of natural resources, but it is not yet competitive on scale. Herein, we use a dichalchogenide compound (Bi₂Te₃) as a conceivable source to accelerate the redesign of advanced membranes technologies such as thermally driven membrane distillation. A procedure based on assisted dispersant liquid phase exfoliation is used to fill PVDF membranes. Key insights are gained in the crucial role of this topological material confined in hydrophobic membranes dedicated to recovery of freshwater from synthetic seawater. Intensified water flux together with reduced energy consumption is obtained into one pot, thereby gathering ultrafast production and thermal efficiency in a single device. Bi₂Te₃-enabled membranes show ability to reduce the resistance to mass transfer while high resistance to heat loss is opposite. Permeate flux is kept stable and salt rejection is higher than 99.99% during 23 h-MD test. Our results confirm the effectiveness of chalcogenides as frontier materials for new-concept water desalination through breakthrough thermally-driven membrane distillation, which is regarded as a new low-energy and sustainable solution to address the growing demand for access to freshwater.

Anisotropic heat transfer properties of two-dimensional materials

The anisotropic heat transfer properties of two-dimensional materials play an important role in controlled heat transfer and intelligent heat management. At present, there are many references on anisotropic heat transfer of two-dimensional materials, but less systematic review of their development status, problems, and future directions. In this paper, intrinsic anisotropic heat transfer of two-dimensional materials, influencing factors and control means are introduced. The preparation methods of thin film with two-dimensional material and the influence factors of macroscopic anisotropic thermal properties are summarized. The technology of two-dimensional material oriented arrangement in matrix and the influence factors of macroscopic anisotropic thermal properties of the composite are outlined.

Fully Spin-Transparent Magnetic Interfaces Enabled by the Insertion of a Thin Paramagnetic NiO Layer

Spin backflow and spin-memory loss have been well established to considerably lower the interfacial spin transmissivity of metallic magnetic interfaces and thus the energy efficiency of spin-orbit torque technologies. Here, we report that spin backflow and spin-memory loss at Pt-based heavy metal-ferromagnet interfaces can be effectively eliminated by inserting an insulating paramagnetic NiO layer of optimum thickness. The latter enables the thermal magnon-mediated essentially unity spin-current transmission at room temperature due to considerably enhanced effective spin-mixing conductance of the interface. As a result, we obtain dampinglike spin-orbit torque efficiency per unit current density of up to 0.8 as detected by the standard technology ferromagnet FeCoB and others, which reaches the expected upper-limit spin Hall ratio of Pt. We establish that Pt/NiO and Pt-Hf/NiO are two energy-efficient, integration-friendly, and high-endurance spin-current generators that provide >100 times greater energy efficiency than sputter-deposited topological insulators BiSb and BiSe. Our finding will benefit spin-orbitronic research and advance spin-torque technologies.

Recent Advances of Spatial Self-Phase Modulation in 2D Materials and Passive Photonic Device Applications

Optical nonlinearity in 2D materials excited by spatial Gaussian laser beam is a novel and peculiar optical phenomenon, which exhibits many novel and interesting applications in optical nonlinear devices. Passive photonic devices, such as optical switches, optical logical gates, photonic diodes, and optical modulators, are the key compositions in the future all-optical signal-processing technologies. Passive photonic devices using 2D materials to achieve the device functionality have attracted widespread concern in the past decade. In this Review, an overview of the spatial self-phase modulation (SSPM) in 2D materials is summarized, including the operating mechanism, optical parameter measurement, and tuning for 2D materials, and applications in photonic devices. Moreover, some current challenges are also proposed to solve, and some possible applications of SSPM method are predicted for the future. Therefore, it is anticipated that this summary can contribute to the application of 2D material-based spatial effect in all-optical signal-processing technologies.

Ultrahigh electron mobility induced by strain engineering in direct semiconductor monolayer Bi2TeSe2

The successful commercial applications as thermoelectric devices and, due to their exotic electronic properties, as topological insulators of bismuth telluride (Bi₂Te₃) and bismuth selenide (Bi₂Se₃) have stimulated research interest on Bi₂Se₃/Bi₂Te₃-based chemical compounds. Based on the first-principles calculations, we investigate the electronic, optical, vibrational and transport properties of new monolayer Bi₂TeSe₂ obtained by transmuting one Se atom into its neighboring Te atom in the same group from Bi₂Se₃. We find that the monolayer Bi₂TeSe₂ maintains a stable hexagonal structure up to 700 K. Monolayer Bi₂TeSe₂ possesses a direct bandgap of 0.29 eV due to the strong spin-orbit coupling effects, and it remains a direct semiconductor for strains in a moderate range. The optical absorption covers a wide range from the green region to the ultraviolet region, which may lead to applications in optoelectronic devices like saturable absorbers. An extremely high electron mobility of 20 678 cm² V^-1 s^-1 along the zigzag direction can be achieved by strain engineering with -6% compressive strain, which is nearly ten times larger than the intrinsic mobility. These indicate that monolayer Bi₂TeSe₂ is a promising candidate for future high-speed (opto)electronic devices.

In Situ Atomic-Scale Observation of Kinetic Pathways of Sublimation in Silver Nanoparticles

Uncovering kinetics of sublimation atomically is critical to understanding both natural phenomena and advanced manufacturing technologies. Here, direct in situ atomic-scale observations to understand the effects of size, surface, and defects in the sublimation process of supported silver nanoparticles upon heating within an aberration-corrected transmission electron microscopy are conducted. Atomic-scale evidence to sublimation and atomic rearrangement in small Ag nanoparticles during heating is provided, and it is demonstrated that the sublimation-induced stable surfaces in the particles with a size smaller than ≈30 nm are {111} and {100} planes. The role of surface energy and defects in the uniform and nonuniform sublimation pathways at the atomic scale is also revealed, and it is found that the nanoparticles with low surface energy tend to undergo a uniform sublimation pathway, while those with high surface energy or five-fold twin grain boundary proceed via a nonuniform sublimation pathway. Further dynamic analysis unravels a critical size of ≈8 nm for the transformation from linear to nonlinear sublimation rates in the two pathways. These findings demonstrate that the size, shape, and defects are of paramount importance for the sublimation dynamics in the first-order phase transformation, helping to advance the general understanding of many technological applications.

In Situ Atomic-Scale Oscillation Sublimation of Magnesium under CO2 Conditions

Understanding the interactive role between Mg and CO₂ is crucial for many technological applications, including CO₂ storage, melting protection, corrosion resistance, and ceramic welding. Here we report observations of rapid oscillation sublimation of Mg at room temperature in the presence of both CO₂ gas and electron irradiation using environmental transmission electron microscopy. The sublimation is mainly related to phase transformation of amorphous MgCO₃. Differing from the direct formation of gas-state MgCO₃, which attributes to the sublimation of pure Mg under a mild electron beam dose, a unique oscillation process is detected during the process of Mg sublimation under a harsh electron beam dose. The main reason stems from the first-order reaction of a reversible decomposition-formation of amorphous MgCO₃. These atomic-level results provide some interesting insights into the interactive role between Mg and CO₂ under electron beam irradiation.

Formation of BixSey Phases Upon Annealing of the Topological Insulator Bi2Se3: Stabilization of In-Depth Bismuth Bilayers

The goal of this work is to study transformations that occur upon heating Bi₂Se₃ to temperatures up to 623 K. X-ray diffraction (XRD) and scanning tunneling microscopy (STM) and spectroscopy (STS) techniques were used in our investigation. XRD was measured following the 00L and 01L truncation rods. These measurements revealed that upon heating there is a coexistence of a major Bi₂Se₃ phase and other ones that present structures of quintuple-layers intercalated with Bismuth bilayers. STM measurements of the surface of this material showed the presence of large hexagonal Bi_xSe_y domains embedded in a Bi₂Se₃ matrix. STS experiments were employed to map the local electronic density of states and characterize the modifications imposed by the presence of the additional phases. Finally, density functional theory (DFT) calculations were performed to support these findings.

Overgrowth of Bi2Te3 nanoislands on Fe-based epitaxial ferromagnetic layers

Bi₂Te₃ is deposited by hot wall epitaxy in an attempt to form nanosheets on epitaxially-grown ferromagnetic layers of Fe, Fe₃Si and Co₂FeSi.

In Situ Transmission Electron Microscopy Characterization and Manipulation of Two-Dimensional Layered Materials beyond Graphene

Two-dimensional (2D) ultra-thin materials beyond graphene with rich physical properties and unique layered structures are promising for applications in electronics, chemistry, energy, and bioscience, etc. The interaction mechanisms among the structures, chemical compositions and physical properties of 2D layered materials are critical for fundamental nanosciences and the practical fabrication of next-generation nanodevices. Transmission electron microscopy (TEM), with its high spatial resolution and versatile external fields, is undoubtedly a powerful tool for the static characterization and dynamic manipulation of nanomaterials and nanodevices at the atomic scale. The rapid development of thin-film and precision microelectromechanical systems (MEMS) techniques allows 2D layered materials and nanodevices to be probed and engineered inside TEM under external stimuli such as thermal, electrical, mechanical, liquid/gas environmental, optical, and magnetic fields at the nanoscale. Such advanced technologies leverage the traditional static TEM characterization into an in situ and interactive manipulation of 2D layered materials without sacrificing the resolution or the high vacuum chamber environment, facilitating exploration of the intrinsic structure-property relationship of 2D layered materials. In this Review, the dynamic properties tailored and observed by the most advanced and unprecedented in situ TEM technology are introduced. The challenges in spatial, time and energy resolution are discussed also.

Real-time atomic scale observation of void formation and anisotropic growth in II-VI semiconducting ribbons

Void formation in semiconductors is generally considered to be deteriorating. However, for some systems, void formation and evolution are beneficial and can be used for the fabrication of novel nanostructures. In either scenario, the understanding of void formation and evolution is of both scientific and technical high importance. Herein, using ZnS ribbons as an example, we report real-time observations of void formation and the kinetics of growth at the nano- and atomic scales upon heating. Direct imaging reveals that voids, created by a focused electron beam in wurtzite (WZ) ribbons, have a rectangular shape elongated along the <0001> direction. The voids are enclosed by low-surface-energy planes including {01-10} and {2-1-10}, with minor contribution from the higher-energy {0001} planes. Driven by thermodynamics to minimize surface energy, the voids grow straight along the [000±1] directions, exhibiting a strong anisotropy. Occasionally, we observe oscillatory kinetics involving periodic void growth and shrinkage, likely due to the fluctuation of the local chemical potential leading to a transitional kinetic state. We also reveal that the morphology and growth kinetics of voids are highly structure-dependent. Real-time observation during void growth through the complex WZ-zinc blende (ZB)-WZ structure shows that the void, with an initial elongated rectangular morphology in the WZ domain, transforms into a different shape, dominated by the {110} surfaces, after migrating to a domain of the ZB structure. However, when the void moves from the ZB to the WZ domain, it transforms back into a rectangular shape followed by fast growth along the [0001] direction. Our experimental results, together with density functional theory (DFT) calculations, provide valuable insights into the mechanistic understanding of void formation and evolution in semiconductors. More importantly, our study may shed light on new pathways for the morphological modulation of nanostructures by utilizing the intrinsic anisotropy of void evolution in WZ semiconductors.

Colloidal Monolayer β-In2Se3 Nanosheets with High Photoresponsivity

We report a low-temperature colloidal synthesis of single-layer, five-atom-thick, β-In₂Se₃ nanosheets with lateral sizes tunable from ∼300 to ∼900 nm, using short aminonitriles (dicyandiamide or cyanamide) as shape controlling agents. The phase and the monolayer nature of the nanosheets were ascertained by analyzing the intensity ratio between two diffraction peaks from two-dimensional slabs of the various phases, determined by diffraction simulations. These findings were further backed-up by comparing and fitting the experimental X-ray diffraction pattern with Debye formula simulated patterns and with side-view high-resolution transmission electron microscopy imaging and simulation. The β-In₂Se₃ nanosheets were found to be indirect band gap semiconductors (E_g = 1.55 eV), and single nanosheet photodetectors demonstrated high photoresponsivity and fast response times.

Morphological evolution of hierarchical Bi2Se3/BiOBr nanostructures and enhanced activity for p-nitrophenol reduction by NaBH4

The emerging Bi₂Se₃/BiOBr composite with a flower like microsphere structure prepared by a mild solvothermal method using BiOBr microspheres as a template and presenting an enhanced catalytic activity in the reduction of p-nitrophenol.