High-κ perovskite membranes as insulators for two-dimensional transistors.

The scaling of silicon metal-oxide-semiconductor field-effect transistors has followed Moore's law for decades, but the physical thinning of silicon at sub-ten-nanometre technology nodes introduces issues such as leakage currents1. Two-dimensional (2D) layered semiconductors, with an atomic thickness that allows superior gate-field penetration, are of interest as channel materials for future transistors2,3. However, the integration of high-dielectric-constant (κ) materials with 2D materials, while scaling their capacitance equivalent thickness (CET), has proved challenging. Here we explore transferrable ultrahigh-κ single-crystalline perovskite strontium-titanium-oxide membranes as a gate dielectric for 2D field-effect transistors. Our perovskite membranes exhibit a desirable sub-one-nanometre CET with a low leakage current (less than 10-2 amperes per square centimetre at 2.5 megavolts per centimetre). We find that the van der Waals gap between strontium-titanium-oxide dielectrics and 2D semiconductors mitigates the unfavourable fringing-induced barrier-lowering effect resulting from the use of ultrahigh-κ dielectrics4. Typical short-channel transistors made of scalable molybdenum-disulfide films by chemical vapour deposition and strontium-titanium-oxide dielectrics exhibit steep subthreshold swings down to about 70 millivolts per decade and on/off current ratios up to 107, which matches the low-power specifications suggested by the latest International Roadmap for Devices and Systems5.

Ultralow contact resistance between semimetal and monolayer semiconductors.

Advanced beyond-silicon electronic technology requires both channel materials and also ultralow-resistance contacts to be discovered1,2. Atomically thin two-dimensional semiconductors have great potential for realizing high-performance electronic devices1,3. However, owing to metal-induced gap states (MIGS)4-7, energy barriers at the metal-semiconductor interface-which fundamentally lead to high contact resistance and poor current-delivery capability-have constrained the improvement of two-dimensional semiconductor transistors so far2,8,9. Here we report ohmic contact between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where the MIGS are sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a contact resistance of 123 ohm micrometres and an on-state current density of 1,135 microamps per micrometre on monolayer MoS2; these two values are, to the best of our knowledge, the lowest and highest yet recorded, respectively. We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS2, WS2 and WSe2. Our reported contact resistances are a substantial improvement for two-dimensional semiconductors, and approach the quantum limit. This technology unveils the potential of high-performance monolayer transistors that are on par with state-of-the-art three-dimensional semiconductors, enabling further device downscaling and extending Moore's law.

Fast water transport and molecular sieving through ultrathin ordered conjugated-polymer-framework membranes.

The development of membranes that block solutes while allowing rapid water transport is of great importance. The microstructure of the membrane needs to be rationally designed at the molecular level to achieve precise molecular sieving and high water flux simultaneously. We report the design and fabrication of ultrathin, ordered conjugated-polymer-framework (CPF) films with thicknesses down to 1 nm via chemical vapour deposition and their performance as separation membranes. Our CPF membranes inherently have regular rhombic sub-nanometre (10.3 × 3.7 Å) channels, unlike membranes made of carbon nanotubes or graphene, whose separation performance depends on the alignment or stacking of materials. The optimized membrane exhibited a high water/NaCl selectivity of ∼6,900 and water permeance of ∼112 mol m-2 h-1 bar-1, and salt rejection >99.5% in high-salinity mixed-ion separations driven by osmotic pressure. Molecular dynamics simulations revealed that water molecules quickly and collectively pass through the membrane by forming a continuous three-dimensional network within the hydrophobic channels. The advent of ordered CPF provides a route towards developing carbon-based membranes for precise molecular separation.

Unveiling defect-mediated carrier dynamics in monolayer semiconductors by spatiotemporal microwave imaging.

The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly extracted from the spatial profile and temporal relaxation of microwave signals, respectively. Time-resolved experiments indicate that the critical process for photoexcited carriers is the escape of holes from trap states, which prolongs the apparent lifetime of mobile electrons in the conduction band. As a result, counterintuitively, the long-lived photoconductivity signal is higher in chemical-vapor deposited (CVD) samples than exfoliated monolayers due to the presence of traps that inhibits recombination. Our work reveals the intrinsic time and length scales of electrical response to photoexcitation in van der Waals materials, which is essential for their applications in optoelectronic devices.

Wafer-scale single-orientation 2D layers by atomic edge-guided epitaxial growth.

Two-dimensional (2D) layered materials hold tremendous promise for post-Si nanoelectronics due to their unique optical and electrical properties. Significant advances have been achieved in device fabrication and synthesis routes for 2D nanoelectronics over the past decade; however, one major bottleneck preventing their immediate applications has been the lack of a reproducible approach for growing wafer-scale single-crystal films despite tremendous progress in recent experimental demonstrations. In this tutorial review, we provide a systematic summary of the critical factors-including crystal/substrate symmetry and energy consideration-necessary for synthesizing single-orientation 2D layers. In particular, we focus on the discussions of the atomic edge-guided epitaxial growth, which assists in unidirectional nucleation for the wafer-scale growth of single-crystal 2D layers.

Conducting Polyaniline for Antifouling Ultrafiltration Membranes: Solutions and Challenges.

Conjugated polyaniline can impact the field of water filtration membranes due to its hydrophilic and antibacterial nature, facile and inexpensive synthesis procedure, heat and acid tolerance, and unique doping/dedoping chemistry. However, the gelation effect, its rigid backbone, and the limited hydrophilicity of polyaniline severely restrict the adaptability to membranes and their antifouling performance. This Mini Review summarizes important works of polyaniline-related ultrafiltration membranes, highlighting solutions to conquer engineering obstacles in processing and challenges in enhancing surface hydrophilicity with an emphasis on chemistry. As a pH-responsive polymer convertible to a conductive salt, this classic material should continue to bring unconventional advances into the realm of water filtration membranes.

Ledge-directed epitaxy of continuously self-aligned single-crystalline nanoribbons of transition metal dichalcogenides.

Two-dimensional transition metal dichalcogenide nanoribbons are touted as the future extreme device downscaling for advanced logic and memory devices but remain a formidable synthetic challenge. Here, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer and single-crystalline MoS2 nanoribbons on ß-gallium (III) oxide (ß-Ga2O3) (100) substrates. LDE MoS2 nanoribbons have spatial uniformity over a long range and transport characteristics on par with those seen in exfoliated benchmarks. Prototype MoS2-nanoribbon-based field-effect transistors exhibit high on/off ratios of 108 and an averaged room temperature electron mobility of 65 cm2 V-1 s-1. The MoS2 nanoribbons can be readily transferred to arbitrary substrates while the underlying ß-Ga2O3 can be reused after mechanical exfoliation. We further demonstrate LDE as a versatile epitaxy platform for the growth of p-type WSe2 nanoribbons and lateral heterostructures made of p-WSe2 and n-MoS2 nanoribbons for futuristic electronics applications.

Low temperature excitonic spectroscopy and dynamics as a probe of quality in hybrid perovskite thin films.

We have developed a framework for using temperature dependent static and dynamic photoluminescence (PL) of hybrid organic-inorganic perovskites (PVSKs) to characterize lattice defects in thin films, based on the presence of nanodomains at low temperature. Our high-stability PVSK films are fabricated using a novel continuous liquid interface propagation technique, and in the tetragonal phase (T > 120 K), they exhibit bi-exponential recombination from free charge carriers with an average PL lifetime of ∼200 ns. Below 120 K, the emergence of the orthorhombic phase is accompanied by a reduction in lifetimes by an order of magnitude, which we establish to be the result of a crossover from free carrier to exciton-dominated radiative recombination. Analysis of the PL as a function of excitation power at different temperatures provides direct evidence that the exciton binding energy is different in the two phases, and using these results, we present a theoretical approach to estimate this variable binding energy. Our findings explain this anomalous low temperature behavior for the first time, attributing it to an inherent fundamental property of the hybrid PVSKs that can be used as an effective probe of thin film quality.

Transfer of 2D Films: From Imperfection to Perfection.

Atomically thin 2D films and their van der Waals heterostructures have demonstrated immense potential for breakthroughs and innovations in science and technology. Integrating 2D films into electronics and optoelectronics devices and their applications in electronics and optoelectronics can lead to improve device efficiencies and tunability. Consequently, there has been steady progress in large-area 2D films for both front- and back-end technologies, with a keen interest in optimizing different growth and synthetic techniques. Parallelly, a significant amount of attention has been directed toward efficient transfer techniques of 2D films on different substrates. Current methods for synthesizing 2D films often involve high-temperature synthesis, precursors, and growth stimulants with highly chemical reactivity. This limitation hinders the widespread applications of 2D films. As a result, reports concerning transfer strategies of 2D films from bare substrates to target substrates have proliferated, showcasing varying degrees of cleanliness, surface damage, and material uniformity. This review aims to evaluate, discuss, and provide an overview of the most advanced transfer methods to date, encompassing wet, dry, and quasi-dry transfer methods. The processes, mechanisms, and pros and cons of each transfer method are critically summarized. Furthermore, we discuss the feasibility of these 2D film transfer methods, concerning their applications in devices and various technology platforms.

Enhanced Photogating Gain in Scalable MoS2 Plasmonic Photodetectors via Resonant Plasmonic Metasurfaces.

Absorption of photons in atomically thin materials has become a challenge in the realization of ultrathin, high-performance optoelectronics. While numerous schemes have been used to enhance absorption in 2D semiconductors, such enhanced device performance in scalable monolayer photodetectors remains unattained. Here, we demonstrate wafer-scale integration of monolayer single-crystal MoS2 photodetectors with a nitride-based resonant plasmonic metasurface to achieve a high detectivity of 2.58 × 1012 Jones with a record-low dark current of 8 pA and long-term stability over 40 days. Upon comparison with control devices, we observe an overall enhancement factor of >100; this can be attributed to the local strong EM field enhanced photogating effect by the resonant plasmonic metasurface. Considering the compatibility of 2D semiconductors and hafnium nitride with the Si CMOS process and their scalability across wafer sizes, our results facilitate the smooth incorporation of 2D semiconductor-based photodetectors into the fields of imaging, sensing, and optical communication applications.

Versatile solution for growing thin films of conducting polymers.

The method employed for depositing nanostructures of conducting polymers dictates potential uses in a variety of applications such as organic solar cells, light-emitting diodes, electrochromics, and sensors. A simple and scalable film fabrication technique that allows reproducible control of thickness, and morphological homogeneity at the nanoscale, is an attractive option for industrial applications. Here we demonstrate that under the proper conditions of volume, doping, and polymer concentration, films consisting of monolayers of conducting polymer nanofibers such as polyaniline, polythiophene, and poly(3-hexylthiophene) can be produced in a matter of seconds. A thermodynamically driven solution-based process leads to the growth of transparent thin films of interfacially adsorbed nanofibers. High quality transparent thin films are deposited at ambient conditions on virtually any substrate. This inexpensive process uses solutions that are recyclable and affords a new technique in the field of conducting polymers for coating large substrate areas.

Electrically switchable anisotropic polariton propagation in a ferroelectric van der Waals semiconductor.

Tailoring of the propagation dynamics of exciton-polaritons in two-dimensional quantum materials has shown extraordinary promise to enable nanoscale control of electromagnetic fields. Varying permittivities along crystal directions within layers of material systems, can lead to an in-plane anisotropic dispersion of polaritons. Exploiting this physics as a control strategy for manipulating the directional propagation of the polaritons is desired and remains elusive. Here we explore the in-plane anisotropic exciton-polariton propagation in SnSe, a group-IV monochalcogenide semiconductor that forms ferroelectric domains and shows room-temperature excitonic behaviour. Exciton-polaritons are launched in SnSe multilayer plates, and their propagation dynamics and dispersion are studied. This propagation of exciton-polaritons allows for nanoscale imaging of the in-plane ferroelectric domains. Finally, we demonstrate the electric switching of the exciton-polaritons in the ferroelectric domains of this complex van der Waals system. The study suggests that systems such as group-IV monochalcogenides could serve as excellent ferroic platforms for actively reconfigurable polaritonic optical devices.

Interfacial Reconstructed Layer Controls the Orientation of Monolayer Transition-Metal Dichalcogenides.

Growing continuous monolayer films of transition-metal dichalcogenides (TMDs) without the disruption of grain boundaries is essential to realize the full potential of these materials for future electronics and optoelectronics, but it remains a formidable challenge. It is generally believed that controlling the TMDs orientations on epitaxial substrates stems from matching the atomic registry, symmetry, and penetrable van der Waals forces. Interfacial reconstruction within the exceedingly narrow substrate-epilayer gap has been anticipated. However, its role in the growth mechanism has not been intensively investigated. Here, we report the experimental conformation of an interfacial reconstructed (IR) layer within the substrate-epilayer gap. Such an IR layer profoundly impacts the orientations of nucleating TMDs domains and, thus, affects the materials' properties. These findings provide deeper insights into the buried interface that could have profound implications for the development of TMD-based electronics and optoelectronics.

Giant Nonlinear Optical Response via Coherent Stacking of In-Plane Ferroelectric Layers.

Thin ferroelectric materials hold great promise for compact nonvolatile memory and nonlinear optical and optoelectronic devices. Herein, an ultrathin in-plane ferroelectric material that exhibits a giant nonlinear optical effect, group-IV monochalcogenide SnSe, is reported. Nanometer-scale ferroelectric domains with ≈90°/270° twin boundaries or ≈180° domain walls are revealed in physical-vapor-deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second-harmonic fields and the as-resulted second-harmonic generation was observed to be 100 times more intense than monolayer WS2 . This work demonstrates in-plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on-chip nonlinear optical components and nonvolatile memory devices.

Oriented lateral growth of two-dimensional materials on c-plane sapphire.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) represent the ultimate thickness for scaling down channel materials. They provide a tantalizing solution to push the limit of semiconductor technology nodes in the sub-1 nm range. One key challenge with 2D semiconducting TMD channel materials is to achieve large-scale batch growth on insulating substrates of single crystals with spatial homogeneity and compelling electrical properties. Recent studies have claimed the epitaxy growth of wafer-scale, single-crystal 2D TMDs on a c-plane sapphire substrate with deliberately engineered off-cut angles. It has been postulated that exposed step edges break the energy degeneracy of nucleation and thus drive the seamless stitching of mono-oriented flakes. Here we show that a more dominant factor should be considered: in particular, the interaction of 2D TMD grains with the exposed oxygen-aluminium atomic plane establishes an energy-minimized 2D TMD-sapphire configuration. Reconstructing the surfaces of c-plane sapphire substrates to only a single type of atomic plane (plane symmetry) already guarantees the single-crystal epitaxy of monolayer TMDs without the aid of step edges. Electrical results evidence the structural uniformity of the monolayers. Our findings elucidate a long-standing question that curbs the wafer-scale batch epitaxy of 2D TMD single crystals-an important step towards using 2D materials for future electronics. Experiments extended to perovskite materials also support the argument that the interaction with sapphire atomic surfaces is more dominant than step-edge docking.

Scalable CMOS back-end-of-line-compatible AlScN/two-dimensional channel ferroelectric field-effect transistors.

Three-dimensional monolithic integration of memory devices with logic transistors is a frontier challenge in computer hardware. This integration is essential for augmenting computational power concurrent with enhanced energy efficiency in big data applications such as artificial intelligence. Despite decades of efforts, there remains an urgent need for reliable, compact, fast, energy-efficient and scalable memory devices. Ferroelectric field-effect transistors (FE-FETs) are a promising candidate, but requisite scalability and performance in a back-end-of-line process have proven challenging. Here we present back-end-of-line-compatible FE-FETs using two-dimensional MoS2 channels and AlScN ferroelectric materials, all grown via wafer-scalable processes. A large array of FE-FETs with memory windows larger than 7.8 V, ON/OFF ratios greater than 107 and ON-current density greater than 250 µA um-1, all at ~80 nm channel length are demonstrated. The FE-FETs show stable retention up to 10 years by extension, and endurance greater than 104 cycles in addition to 4-bit pulse-programmable memory features, thereby opening a path towards the three-dimensional heterointegration of a two-dimensional semiconductor memory with silicon complementary metal-oxide-semiconductor logic.

Two-Dimensional Cs2AgBiBr6/WS2 Heterostructure-Based Photodetector with Boosted Detectivity via Interfacial Engineering.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) monolayers have been widely used for optoelectronic devices because of their ultrasensitivity to light detection acquired from their direct gap properties. However, the small cross-section of photon absorption in the atomically thin layer thickness significantly limits the generation of photocarriers, restricting their performance. Here, we integrate monolayer WS2 with 2D perovskites Cs2AgBiBr6, which serve as the light absorption layer, to greatly enhance the photosensitivity of WS2. The efficient charge transfer at the Cs2AgBiBr6/WS2 heterojunction is evidenced by the shortened photoluminescence (PL) decay time of Cs2AgBiBr6. Scanning photocurrent microscopy of Cs2AgBiBr6/WS2/graphene reveals that improved charge extraction from graphene leads to an enhanced photoresponse. The 2D Cs2AgBiBr6/WS2/graphene vertical heterostructure photodetector exhibits a high detectivity (D*) of 1.5 × 1013 Jones with a fast response time of 52.3 µs/53.6 µs and an on/off ratio of 1.02 × 104. It is worth noting that this 2D heterostructure photodetector can realize self-powered light detection behavior with an open-circuit voltage of ∼0.75 V. The results suggest that the 2D perovskites can effectively improve the TMDC layer-based photodetectors for low-power consumption photoelectrical applications.

Unusual Activity of Rationally Designed Cobalt Phosphide/Oxide Heterostructure Composite for Hydrogen Production in Alkaline Medium.

Design and development of an efficient, nonprecious catalyst with structural features and functionality necessary for driving the hydrogen evolution reaction (HER) in an alkaline medium remain a formidable challenge. At the root of the functional limitation is the inability to tune the active catalytic sites while overcoming the poor reaction kinetics observed under basic conditions. Herein, we report a facile approach to enable the selective design of an electrochemically efficient cobalt phosphide oxide composite catalyst on carbon cloth (CoP-CoxOy/CC), with good activity and durability toward HER in alkaline medium (η10 = -43 mV). Theoretical studies revealed that the redistribution of electrons at laterally dispersed Co phosphide/oxide interfaces gives rise to a synergistic effect in the heterostructured composite, by which various Co oxide phases initiate the dissociation of the alkaline water molecule. Meanwhile, the highly active CoP further facilitates the adsorption-desorption process of water electrolysis, leading to extremely high HER activity.

Unraveling the Correlation between Raman and Photoluminescence in Monolayer MoS2 through Machine-Learning Models.

2D transition metal dichalcogenides (TMDCs) with intense and tunable photoluminescence (PL) have opened up new opportunities for optoelectronic and photonic applications such as light-emitting diodes, photodetectors, and single-photon emitters. Among the standard characterization tools for 2D materials, Raman spectroscopy stands out as a fast and non-destructive technique capable of probing material's crystallinity and perturbations such as doping and strain. However, a comprehensive understanding of the correlation between photoluminescence and Raman spectra in monolayer MoS2 remains elusive due to its highly nonlinear nature. Here, the connections between PL signatures and Raman modes are systematically explored, providing comprehensive insights into the physical mechanisms correlating PL and Raman features. This study's analysis further disentangles the strain and doping contributions from the Raman spectra through machine-learning models. First, a dense convolutional network (DenseNet) to predict PL maps by spatial Raman maps is deployed. Moreover, a gradient boosted trees model (XGBoost) with Shapley additive explanation (SHAP) to bridge the impact of individual Raman features in PL features is applied. Last, a support vector machine (SVM) to project PL features on Raman frequencies is adopted. This work may serve as a methodology for applying machine learning to characterizations of 2D materials.

Bismuth-based mixed-anion compounds for anode materials in rechargeable batteries.

A facile solvothermal synthesis approach for chemical composition control in ternary Bi-S-I systems is reported by simply controlling the sulfide concentration. We demonstrate the application of these bismuth-based ternary mixed-anion compounds as high capacity anode materials in rechargeable batteries. Cells utilising Bi13S18I2 achieved an initial capacity value of 807 mA h g-1, while those with BiSI/Bi13S18I2 a value of 1087 mA h g-1 in lithium-ion battery systems.