Blocking Directional Lithium Diffusion in Solid-State Electrolytes at the Interface: First-Principles Insights into the Impact of the Space Charge Layer.

Understanding the degradation mechanisms in solid-state lithium-ion batteries at interfaces is fundamental for improving battery performance and for designing recycling methodologies for batteries. A key source of battery degradation is the presence of the space charge layer at the solid-state electrolyte-electrode interface and the impact that this layer has on the thermodynamics of the electrolyte structure. Currently, Li10GeP2S12 in its pristine form has one of the highest lithium conductivities and has been used as a template for designing even higher conductivity derived structures. However, being an ionic material with mostly linear diffusion, it is prone to path-blocker defects, which we show here to be especially prevalent in the space charge layer. We analyze the thermodynamic properties of a number of path-blocker defects using density functional theory and their potential crystal decomposition and find that the presence of an electrostatic potential in the space charge layer elevates the likelihood of existence of these defects, which otherwise would not be likely to form in the bulk of the electrolyte away from electrodes. We use ab initio molecular dynamics to assess the impact of these defects on the diffusivity of the crystal and find that they all reduce the lithium diffusivity. While our work focuses on Li10GeP2S12, it is relevant to any solid-state electrolyte with mainly linear diffusion.

Ferroelectric van der Waals heterostructures of CuInP2S6for non-volatile memory device applications.

Two-dimensional (2D) ferroelectric materials are providing promising platforms for creating future nano- and opto-electronics. Here we propose new hybrid van der Waals heterostructures, in which the 2D ferroelectric material CuInP2S6(CIPS) is layered on a 2D semiconductor for near-infrared (NIR) memory device applications. Using density functional theory, we show that the band gap of the hybrid bilayers formed with CIPS can be tuned and that the optical and electronic properties can be successfully modulated via ferroelectric switching. Of the 3712 heterostructures considered, we identified 19 structures that have a type II band alignment and commensurate lattice matches. Of this set, both the CuInP2S6/PbSe and CuInP2S6/Ge2H2heterostructures possess absorption peaks in the NIR region that change position and intensity with switching polarisation, making them suitable for NIR memory devices. The CuInP2S6/ISSb, CuInP2S6/ISbSe, CuInP2S6/ClSbSe and CuInP2S6/ZnI2heterostructures had band gaps which can be switched from direct to indirect with changing the polarisation of CIPS making them suitable for optoelectronics and sensors. The heterostructures formed with CIPS are exciting candidates for stable ferroelectric devices, opening a pathway for tuning the band alignment of van der Waal heterostructures and the creation of modern memory applications that use less energy.

Naturally-meaningful and efficient descriptors: machine learning of material properties based on robust one-shot ab initio descriptors.

Establishing a data-driven pipeline for the discovery of novel materials requires the engineering of material features that can be feasibly calculated and can be applied to predict a material's target properties. Here we propose a new class of descriptors for describing crystal structures, which we term Robust One-Shot Ab initio (ROSA) descriptors. ROSA is computationally cheap and is shown to accurately predict a range of material properties. These simple and intuitive class of descriptors are generated from the energetics of a material at a low level of theory using an incomplete ab initio calculation. We demonstrate how the incorporation of ROSA descriptors in ML-based property prediction leads to accurate predictions over a wide range of crystals, amorphized crystals, metal-organic frameworks and molecules. We believe that the low computational cost and ease of use of these descriptors will significantly improve ML-based predictions.

Improving sensing of formaldehyde using ZnO nanostructures with surface-adsorbed oxygen.

Detection of pollutant gases, such as formaldehyde (HCHO), in our homes and surrounding environment is of high importance for our health and safety. The effect of surface defects and specifically pre-adsorbed oxygen on the gas sensing reaction of HCHO with ZnO nanostructures is largely unknown. Using density functional theory, nonequilibrium Green's function method and ab initio molecular dynamics (AIMD) simulations, we show that the presence of surface oxygen has two key roles in the sensitivity of ZnO towards HCHO: (1) it leads to the presence of charge trap states, which vanish upon the adsorption of HCHO, and (2) it facilitates the dissociative chemisorption of HCHO on the surface. Our ground state and AIMD calculations show that multiple reaction products are produced, which eventually lead to cleaning the surface from the adsorbed species, and hence enhancing the recyclability of the surface. We not only confirm the reaction proposed by experiment, but show that the presence of surface oxygen facilitates other surface reactions as well. Our work provides insights into the gas-surface reaction mechanism of ZnO-nanostructure based gas sensors, not provided before by experiment.

Reactive Oxygen Species Sequestration Induced Synthesis of ß-PbO and Its Polymorphic Transformation to α-PbO at Atomically Thin Regimes.

The emergence of attractive properties in materials at atomically thin regimes has seen an ongoing interest in two-dimensional (2D) materials. An aspect that has lacked focused attention is the effect of 2D material thickness on its crystal structure. As several layered materials naturally exist in mixed metastable phases, it raises an important question of whether a specific polymorph of these mixed-phase materials will be favored at atomically thin limits. This work attempts to address this issue by employing lead monoxide as a model 2D polymorphic system. We propose a reactive oxygen species (ROS) sequestration-mediated liquid-phase exfoliation (LPE) strategy for the facile synthesis of ultrathin PbO. This is followed by a suite of microscopic and spectroscopic analyses of the PbO nanosheets that reveals the polymorphic transformation of orthorhombic (ß) PbO to its tetragonal (α) analogue with reduction in nanosheet thickness. The transformation process reveals an interesting crystal structure of ultrathin 2D PbO where [001]-oriented domains of α-PbO coexist alongside [100]-oriented regions of ß-PbO. Density functional theory (DFT) calculations support our experimental data by revealing a higher thermodynamic stability of the tetragonal phase in PbO monolayers. These findings are likely to instigate interest in carefully evaluating the crystal structures of ultrathin 2D materials while promoting research in understanding the phase transformation across a range of 2D crystals.

Large Area Ultrathin InN and Tin Doped InN Nanosheets Featuring 2D Electron Gases.

Indium nitride (InN) has been of significant interest for creating and studying two-dimensional electron gases (2DEG). Herein we demonstrate the formation of 2DEGs in ultrathin doped and undoped 2D InN nanosheets featuring high carrier mobilities at room temperature. The synthesis is carried out via a two-step liquid metal-based printing method followed by a microwave plasma-enhanced nitridation reaction. Ultrathin InN nanosheets with a thickness of ∼2 ± 0.2 nm were isolated over large areas with lateral dimensions exceeding centimeter scale. Room temperature Hall effect measurements reveal carrier mobilities of ∼216 and ∼148 cm2 V-1 s-1 for undoped and doped InN, respectively. Further analysis suggests the presence of defined quantized states in these ultrathin nitride nanosheets that can be attributed to a 2D electron gas forming due to strong out-of-plane confinement. Overall, the combination of electronic and plasmonic features in undoped and doped ultrathin 2D InN holds promise for creating advanced optoelectronic devices and functional 2D heterostructures.

Tuning the Schottky barrier height in a multiferroic In2Se3/Fe3GeTe2 van der Waals heterojunction.

The ferroelectric material In2Se3 is currently of significant interest due to its built-in polarisation characteristics that can significantly modulate its electronic properties. Here we employ density functional theory to determine the transport characteristics at the metal-semiconductor interface of the two-dimensional multiferroic In2Se3/Fe3GeTe2 heterojunction. We show a significant tuning of the Schottky barrier height as a result of the change in the intrinsic polarisation state of In2Se3: the switching in the electric polarisation of In2Se3 results in the switching of the nature of the Schottky barrier, from being n-type to p-type, and is accompanied by a change in the spin polarisation of the electrons. This switchable Schottky barrier structure can form an essential component in a two-dimensional field effect transistor that can be operated by switching the ferroelectric polarisation, rather than by the application of strain or electric field. The band structure and density of state calculations show that Fe3GeTe2 lends its magnetic and metallic characteristics to the In2Se3 layer, making the In2Se3/Fe3GeTe2 heterojunction a potentially viable multiferroic candidate in nanoelectronic devices like field-effect transistors. Moreover, our findings reveal a transfer of charge carriers from the In2Se3 layer to the Fe3GeTe2 layer, resulting in the formation of an in-built electric field at the metal-semiconductor interface. Our work can substantially broaden the device potential of the In2Se3/Fe3GeTe2 heterojunction in future low-energy electronic devices.

Nicotine Sensors for Wearable Battery-Free Monitoring of Vaping.

Nicotine, an addictive substance in tobacco products and electronic cigarettes (e-cigs), is recognized for increasing the risk of cardiovascular and respiratory disorders. Careful real-time monitoring of nicotine exposure is critical in alleviating the potential health impacts of not just smokers but also those exposed to second-hand and third-hand smoke. Monitoring of nicotine requires suitable sensing material to detect nicotine selectively and testing under free-living conditions in the standard environment. Here, we experimentally demonstrate a vanadium dioxide (VO2)-based nicotine sensor and explain its conductometric mechanisms with compositional analysis and density functional theory (DFT) calculations. For real-time monitoring of nicotine vapor from e-cigarettes in the air, the sensor is integrated with an epidermal near-field communication (NFC) interface that enables battery-free operation and data transmission to smart electronic devices to record and store sensor data. Collectively, the technique of sensor development and integration expands the use of wearable electronics for real-time monitoring of hazardous elements in the environment and biosignals wirelessly.

Surface Functionalization of WS2 Nanosheets with Alkyl Chains for Enhancement of Dispersion Stability and Tribological Properties.

Tungsten disulfide (WS2) exhibits intriguing tribological properties and has been explored as an excellent lubricious material in thin-film and solid lubricants. However, the poor dispersibility of WS2 has been a major challenge for its utilization in liquid lubricant applications. Herein, a top-down integrated approach is presented to synthesize oxygenated WS2 (WS2-O) nanosheets via strong acid-mediated oxidation and ultrasound-assisted exfoliation. The ultrathin sheets of WS2-O, comprising 4-7 molecular lamellae, exhibit oxygen/hydroxyl functionalities. The organosilanes having variable surface-active leaving groups (chloro and ethoxy) are covalently grafted, targeting the hydroxyl/oxygen functionalities on the surface of WS2-O nanosheets. The grafting of organosilanes is governed by the reactivity of chloro and ethoxy leaving groups. The DFT calculations further support the covalent interaction between the WS2-O nanosheets and organosilanes. The alkyl chain-functionalized WS2-O nanosheets displayed excellent dispersibility in mineral lube base oil. A minute dose of chemically functionalized-WS2 (0.2 mg.mL-1) notably enhanced the tribological properties of mineral lube oil by reducing the friction coefficient (52%) and wear volume (79%) for a steel tribopair. Raman analysis of worn surfaces revealed WS2-derived lubricious thin film formation. The improved tribological properties are attributed to ultralow thickness, stable dispersion, and low shear strength of chemically functionalized WS2 nanosheets, along with protective thin film formation over the contact interfaces of a steel tribopair. The present work opens a new avenue toward exploiting low-dimensional nanosheets for minimizing energy losses due to high friction.

Fully Light-Controlled Memory and Neuromorphic Computation in Layered Black Phosphorus.

Imprinting vision as memory is a core attribute of human cognitive learning. Fundamental to artificial intelligence systems are bioinspired neuromorphic vision components for the visible and invisible segments of the electromagnetic spectrum. Realization of a single imaging unit with a combination of in-built memory and signal processing capability is imperative to deploy efficient brain-like vision systems. However, the lack of a platform that can be fully controlled by light without the need to apply alternating polarity electric signals has hampered this technological advance. Here, a neuromorphic imaging element based on a fully light-modulated 2D semiconductor in a simple reconfigurable phototransistor structure is presented. This standalone device exhibits inherent characteristics that enable neuromorphic image pre-processing and recognition. Fundamentally, the unique photoresponse induced by oxidation-related defects in 2D black phosphorus (BP) is exploited to achieve visual memory, wavelength-selective multibit programming, and erasing functions, which allow in-pixel image pre-processing. Furthermore, all-optically driven neuromorphic computation is demonstrated by machine learning to classify numbers and recognize images with an accuracy of over 90%. The devices provide a promising approach toward neurorobotics, human-machine interaction technologies, and scalable bionic systems with visual data storage/buffering and processing.

Liquid-Metal Synthesized Ultrathin SnS Layers for High-Performance Broadband Photodetectors.

Atomically thin materials face an ongoing challenge of scalability, hampering practical deployment despite their fascinating properties. Tin monosulfide (SnS), a low-cost, naturally abundant layered material with a tunable bandgap, displays properties of superior carrier mobility and large absorption coefficient at atomic thicknesses, making it attractive for electronics and optoelectronics. However, the lack of successful synthesis techniques to prepare large-area and stoichiometric atomically thin SnS layers (mainly due to the strong interlayer interactions) has prevented exploration of these properties for versatile applications. Here, SnS layers are printed with thicknesses varying from a single unit cell (0.8 nm) to multiple stacked unit cells (≈1.8 nm) synthesized from metallic liquid tin, with lateral dimensions on the millimeter scale. It is reveal that these large-area SnS layers exhibit a broadband spectral response ranging from deep-ultraviolet (UV) to near-infrared (NIR) wavelengths (i.e., 280-850 nm) with fast photodetection capabilities. For single-unit-cell-thick layered SnS, the photodetectors show upto three orders of magnitude higher responsivity (927 A W-1 ) than commercial photodetectors at a room-temperature operating wavelength of 660 nm. This study opens a new pathway to synthesize reproduceable nanosheets of large lateral sizes for broadband, high-performance photodetectors. It also provides important technological implications for scalable applications in integrated optoelectronic circuits, sensing, and biomedical imaging.

Structural-Defect-Mediated Grafting of Alkylamine on Few-Layer MoS2 and Its Potential for Enhancement of Tribological Properties.

Two-dimensional transition-metal dichalcogenides possess inherent structural characteristics that can be harnessed for enhancement of tribological properties by making them dispersible in lube media. Here, we present a hydrothermal approach to preparing MoS2 nanosheets comprising 4-10 molecular lamellae. A structural-defect-mediated route for grafting of octadecylamine (ODA) on MoS2 nanosheets is outlined. The unsaturated d orbitals of Mo at the sulfur vacancies on the MoS2 surface are coupled with the electron-rich nitrogen center of ODA and yield ODA-functionalized MoS2 (MoS2-ODA). The MoS2-ODA nanosheets exhibit good dispersibility in lube base oil and are used as an additive (optimized dose: 0.1 mg·mL-1) to mineral oil. It is shown that even at low concentration, MoS2-ODA nanosheets significantly reduce the friction (48%) and wear (44%). Microscopy (field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM)) and spectroscopy (Raman and elemental mapping) analyses of worn scars revealed the formation of MoS2-based protective thin films for lowering of friction and wear. This work, therefore, presents a pathway for low-friction lubricants by deploying functionalized low-dimensional material systems.

Electrically Activated UV-A Filters Based on Electrochromic MoO3-x.

Chromism-based optical filters is a niche field of research, due to there being only a handful of electrochromic materials. Typically, electrochromic transition metal oxides such as MoO3 and WO3 are utilized in applications such as smart windows and electrochromic devices (ECD). Herein, we report MoO3-x-based electrically activated ultraviolet (UV) filters. The MoO3-x grown on indium tin oxide (ITO) substrate is mechanically assembled onto an electrically activated proton exchange membrane. Reversible H+ injection/extraction in MoO3-x is employed to switch the optical transmittance, enabling an electrically activated optical filter. The devices exhibit broadband transmission modulation (325-800 nm), with a peak of ∼60% in the UV-A range (350-392 nm). Comparable switching times of 8 s and a coloration efficiency of up to 116 cm2 C-1 are achieved.

Differential Work-Function Enabled Bifunctional Switching in Strontium Titanate Flexible Resistive Memories.

Multifunctional electronic memories capable of demonstrating both analog and digital switching on-demand are extremely attractive for miniaturization of electronics without significant drain on energy consumption. Simultaneously translating functionality onto mechanically conformable platforms will further enhance their suitability. Here, we demonstrate the ability to engineer multifunctionality in strontium titanate (STO)-based resistive random-access memories (ReRAM) on a flexible polyimide platform. By utilizing different bottom electrodes of various work functions while the top electrode is fixed, differential work functions are induced in STO, to induce bipolar or complementary switching behaviors whenever required. This work-function difference-induced bifunctional switching on the flexible platform reveals a streamlined route for achieving flexible artificial neural networks, high density integration, and logic operation using a single ReRAM.

Theoretical insight on the origin of anelasticity in zinc oxide nanowires.

Anelasticity of nanowires has recently attracted attention as an interesting property for high efficiency mechanical damping materials. While the mechanism of anelasticity has so far been analysed using continuum mechanical models based on defect diffusion, the mechanisms behind anelasticity have not yet been determined on an atomic level. Such information is needed in order to be able to design and synthesise new nanomaterials having desired mechanical properties. Here we determine the potential mechanism of anelasticity in narrow zinc oxide nanowires by analyzing the bond stretching and compression within the nanowire structure based on density functional theory. Our approach shows that different local minimum structures are created when different strain patterns are applied which give rise to the anelastic behavior. These findings can be applied for the prediction of potential anelasticity of other nanowire materials.

Electronic transport investigation of redox-switching of azulenequinones/hydroquinones via first-principles studies.

The redox switching of non-alternant azulenequinone/hydroquinone molecules is investigated using density functional theory and the nonequilibrium Green's function. We examined the electronic transport properties of these molecules when subtended between gold electrodes. The results indicated that the reduction of 1,5-azulenequinone and oxidation of 1,7-azulene hydroquinone 2,6-dithiolate lead to a significant enhancement of the current compared to the respective oxidation of 1,5-azulene hydroquinone and reduction of 1,7-azulenequinone, thus "switching on" the transmission. The significance of the position of the functional group on the switching behavior has been analyzed and whether destructive quantum interference exists in the electron transport of the 1,5 position in particular has been addressed. Our work provides theoretical foundations for organic redox switching components in nanoelectronic circuits.

Adsorption of toxic gases on silicene/Ag(111).

Silicene is a two-dimensional nanomaterial, composed of Si atoms arranged into a buckled honeycomb network. It has become of great interest in recent years due to its remarkable properties such as its natural compatibility with current silicon-based technology. Due to its extreme thinness on the nanoscale, and large lateral dimensions, it has potential applications in gas sensing, gas storage and components in modern electronic devices. In this work, density functional theory calculations and ab initio molecular dynamics simulations are used to examine the reaction of SO2, NO2 and H2S on the Si/Ag(111) surface. It was shown that each gas will adsorb on the surface in different orientations and adsorption sites. SO2 and NO2 were found to chemisorb on the surface, whereas H2S was found to physisorb. SO2 and H2S adsorb associatively, whereas NO2 readily dissociates, producing adsorbed oxygen, and gaseous NO. At elevated temperatures, the SO2 and NO2 remain strongly bound to the surface, resulting in poisoning of the silicene, while H2S readily desorbs. Ab initio molecular dynamics also show that NO2 will selectively bind before SO2 when both gases are present in the same environment. This work shows that Si/Ag(111) may provide useful properties for gas sensing and storage applications.

Photochemical Etching of Carbonyl Groups from a Carbon Matrix: The (001) Diamond Surface.

The surface of diamond is reported to undergo nonablative photochemical etching when exposed to ultraviolet (UV) radiation which allows controlled single and partial layer removal of lattice layers. Oxygen termination of surface dangling bonds is known to be crucial for the etching process; however, the exact mechanism of carbon ejection remains unclear. We investigate the interaction of UV laser pulses with oxygen-terminated diamond surfaces using atomic-scale surface characterization combined with first-principles time-dependent density functional theory calculations. We present evidence for laser-induced desorption (LID) from carbonyl functional groups at the diamond {001} surface. The doubly bonded carbonyl group is photoexcited into a triply bonded CO-like state, including scission of the underlying C─C bonds. The carbon removal process in LID is atom by atom; therefore, this mechanism provides a novel "top-down" approach for creating nanostructures on the surface of diamond and other carbon-containing semiconductors.

Electrically Sorted Single-Walled Carbon Nanotubes-Based Electron Transporting Layers for Perovskite Solar Cells.

Incorporation of as prepared single-walled carbon nanotubes (SWCNTs) into the electron transporting layer (ETL) is an effective strategy to enhance the photovoltaic performance of perovskite solar cells (PSCs). However, the fundamental role of the SWCNT electrical types in the PSCs is not well understood. Herein, we prepared semiconducting (s-) and metallic (m-) SWCNT families and integrated them into TiO2 photoelectrodes of the PSCs. Based on experimental and theoretical studies, we found that the electrical type of the nanotubes plays an important role in the devices. In particular, the mixture of s-SWCNTs and m-SWCNTs (2:1 w/w)-based PSCs exhibited a remarkable efficiency of up to 19.35%, which was significantly higher than that of the best control cell (17.04%). In this class of PSCs, semiconducting properties of s-SWCNTs play a critical role in extracting and transporting electrons, whereas m-SWCNTs provide high conductance throughout the electrode.

Tuning the work function of the silicene/4 × 4 Ag(111) surface.

Silicene, the silicon analog of graphene, is an atomically thin two-dimensional material with promising applications in gas sensing, storage and as components in modern electronic devices. Silicene epitaxially grown on the Ag(111) surface can expand the utility of the silver surface by enabling the tuning of its work function through the functionalisation of silicene. Here we examine the electronic and structural properties and the thermodynamic stability of functionalised silicene/4 × 4 Ag(111) using density functional theory calculations coupled with ab initio molecular dynamics (AIMD) simulations. We focus on 11 functional groups, namely phenyl, methyl, hydroxyl, cyano, methoxyl, amino and ethylmethylamine, in addition to 4 halogen atoms. These functional groups are shown to endow the Si/Ag(111) surface with a large variation in the work function. Our AIMD simulations confirm the thermodynamic stability of these 11 functionalised structures. This work shows the possibility of tuning the electronic structure of silicene by functionalisation, which could then be utilized in polymer solar cells and nanoelectronic circuit components.