Field-Effect Tunneling Transistor Based on Vertical Graphene Heterostructures

Electrically Oriented Antibodies on Transistor for Monitoring Several Copies of Methylated DNA

An antibody transistor is a promising biosensing platform for the diagnosis and monitoring of various diseases. Nevertheless, the low concentration and short half-life of biomarkers require biodetection at the trace-molecule level, which remains a challenge for existing antibody transistors. Herein, we demonstrate a graphene field-effect transistor (gFET) with electrically oriented antibody probes (EOA-gFET) for monitoring several copies of methylated DNA. The electric field confines the orientation of antibody probes on graphene and diminishes the distance between graphene and methylated DNAs captured by antibodies, generating more induced charges on graphene and amplifying the electric signal. EOA-gFET realizes a limit of detection (LoD) of ∼0.12 copy μL-1, reaching the lowest LoD reported before. EOA-gFET shows a distinguishable signal for liver cancer clinical serum samples within ∼6 min, which proves its potential as a powerful tool for disease screening and diagnosis.

Photonics in Multimaterial Lateral Heterostructures Combining Group IV Chalcogenide van der Waals Semiconductors

Lateral heterostructures combining two multilayer group IV chalcogenide van der Waals semiconductors have attracted interest for optoelectronics, twistronics, and valleytronics, owing to their structural anisotropy, bulk-like electronic properties, enhanced optical thickness, and vertical interfaces enabling in-plane charge manipulation/separation, perpendicular to the trajectory of incident light. Group IV monochalcogenides support propagating photonic waveguide modes, but their interference gives rise to complex light emission patterns throughout the visible/near-infrared range both in uniform flakes and single-interface lateral heterostructures. Here, this work demonstrates the judicious integration of pure and alloyed monochalcogenide crystals into multimaterial heterostructures with unique photonic properties, notably the ability to select photonic modes with targeted discrete energies through geometric factors rather than band engineering. SnS-GeS1-xSex-GeSe-GeS1-xSex heterostructures with a GeS1-xSex active layer sandwiched laterally between GeSe and SnS, semiconductors with similar optical constants but smaller bandgaps, were designed and realized via sequential vapor transport synthesis. Raman spectroscopy, electron microscopy/diffraction, and energy-dispersive X-ray spectroscopy confirm a high crystal quality of the laterally stitched components with sharp interfaces. Nanometer-scale cathodoluminescence spectroscopy provides evidence for a facile transfer of electron-hole pairs across the lateral interfaces and demonstrates the selection of photon emission at discrete energies in the laterally embedded active (GeS1- xSex) part of the heterostructure.

Efficient Modulation of Schottky to Ohmic Contact in MoSi2N4/M3C2 (M = Zn, Cd, Hg) van der Waals Heterostructures

A strong Fermi level pinning (FLP) effect can induce a large Schottky barrier in metal/semiconductor contacts; reducing the Schottky barrier height (SBH) to form an Ohmic contact (OhC) is a critical problem in designing high-performance electronic devices. Herein, we report the interfacial electronic features and efficient modulation of the Schottky contact (ShC) to OhC for MoSi2N4/M3C2 (M = Zn, Cd, Hg) van der Waals heterostructures (vdWHs). We find that the MoSi2N4/M3C2 vdWHs can form a p-type ShC with small SBH with the calculated pinning factor S ≈ 0.8 for MoSi2N4/M3C2 contacts. These results indicate that the FLP effect can be effectively suppressed in MoSi2N4 contact with M3C2. Moreover, the interfacial properties and SBH of MoSi2N4/Zn3C2 vdWHs can be effectively modulated by a perpendicular electric field and biaxial strain. In particular, an efficient OhC can be achieved in MoSi2N4/Zn3C2 vdWHs by applying a positive electric field of 0.5 V/Å and strain of ±8%.

Ultrafast Non-Volatile Floating-Gate Memory Based on All-2D Materials

The explosive growth of massive-data storage and the demand for ultrafast data processing require innovative memory devices with exceptional performance. 2D materials and their van der Waal heterostructures with atomically sharp interfaces hold great promise for innovations in memory devices. Here, this work presents non-volatile, floating-gate memory devices with all functional layers made of 2D materials, achieving ultrafast programming/erasing speeds (20 ns), high extinction ratios (up to 108), and multi-bit storage capability. These devices also exhibit long-term data retention exceeding 10 years, facilitated by a high gate-coupling ratio (GCR) and atomically sharp interfaces between functional layers. Additionally, this work demonstrates the realization of an "OR" logic gate on a single-device unit by synergistic electrical and optical operations. The present results provide a solid foundation for next-generation ultrahigh-speed, ultralong lifespan, non-volatile memory devices, with a potential for scale-up manufacturing and flexible electronics applications.

Recent progress in emerging two-dimensional organic-inorganic van der Waals heterojunctions

Two-dimensional (2D) materials have attracted significant attention in recent decades due to their exceptional optoelectronic properties. Among them, to meet the growing demand for multifunctional applications, 2D organic-inorganic van der Waals (vdW) heterojunctions have become increasingly popular in the development of optoelectronic devices. These heterojunctions demonstrate impressive capability to synergistically combine the favourable characteristics of organic and inorganic materials, thereby offering a wide range of advantages. Also, they enable the creation of innovative device structures and introduce novel functionalities in existing 2D materials, avoiding the need for lattice matching in different material systems. Presently, researchers are actively working on improving the performance of devices based on 2D organic-inorganic vdW heterojunctions by focusing on enhancing the quality of 2D materials, precise stacking methods, energy band regulation, and material selection. Therefore, this review presents a thorough examination of the emerging 2D organic-inorganic vdW heterojunctions, including their classification, fabrication, and corresponding devices. Additionally, this review offers profound and comprehensive insight into the challenges in this field to inspire future research directions. It is expected to propel researchers to harness the extraordinary capabilities of 2D organic-inorganic vdW heterojunctions for a wider range of applications by further advancing the understanding of their fundamental properties, expanding the range of available materials, and exploring novel device architectures. The ongoing research and development in this field hold potential to unlock captivating advancements and foster practical applications across diverse industries.

van der Waals Contact for Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for next-generation electronics owing to their atomically thin structures and surfaces devoid of dangling bonds. However, establishing high-quality metal contacts with TMDs presents a critical challenge, primarily attributed to their ultrathin bodies and delicate lattices. These distinctive characteristics render them susceptible to physical damage and chemical reactions when conventional metallization approaches involving "high-energy" processes are implemented. To tackle this challenge, the concept of van der Waals (vdW) contacts has recently been proposed as a "low-energy" alternative. Within the vdW geometry, metal contacts can be physically laminated or gently deposited onto the 2D channel of TMDs, ensuring the formation of atomically clean and electronically sharp contact interfaces while preserving the inherent properties of the 2D TMDs. Consequently, a considerable number of vdW contact devices have been extensively investigated, revealing unprecedented transport physics or exceptional device performance that was previously unachievable. This review presents recent advancements in vdW contacts for TMD transistors, discussing the merits, limitations, and prospects associated with each device geometry. By doing so, our purpose is to offer a comprehensive understanding of the current research landscape and provide insights into future directions within this rapidly evolving field.

Graphyne-based 3D porous structure and its sandwich-type graphene structure for alkali metal ion battery anode materials

In order to develop candidate materials for more metal ion battery anodes, a three-dimensional (3D) porous structure 3D-PGY was designed based on graphyne, and a sandwich structure graphene/PGY/graphene (G/PGY/G) was constructed by adjusting the distance between two layers of graphene with 3D-PGY as the middle layer. Systematic calculations have shown that 3D-PGY is thermally and mechanically stable even at temperatures up to 1000 K. Li can migrate in multiple diffusion directions in two structures because of its smaller radius while Na and K ions can only migrate through the larger pores. The energy barriers of Li, Na and K ions in 3D-PGY are 0.18, 0.43 and 0.27 eV respectively. After forming the sandwich structure with graphene, the minimum energy barriers of Li, Na and K ions are decreased to 0.12, 0.37 and 0.24 eV, respectively. As the anode for Li, Na, and K ion batteries, the theoretical specific capacities of 3D-PGY are about 558 mA h g-1, and the average open circuit voltages of 3D-PGY and G/PGY/G are about 0.48/0.52/0.29 and 1.08/1.04/1.39 V, respectively. Finally, using ab initio molecular dynamics simulations, the diffusion coefficients for 3D-PGY at different temperatures, as well as for G/PGY/G at 400 K were obtained. The Li, Na and K ions in both structures can diffuse rapidly and have good rate capabilities. These excellent performances show that the graphyne-based 3D porous structure and its sandwich-type graphene structure are very promising for the development of new battery materials.

Room Temperature Negative Differential Resistance with High Peak Current in MoS2/WSe2 Heterostructures

Two-dimensional transition metal dichalcogenide (2D TMD) semiconductors allow facile integration of p- and n-type materials without a lattice mismatch. Here, we demonstrate gate-tunable n- and p-type junctions based on vertical heterostructures of MoS2 and WSe2 using van der Waals (vdW) contacts. The p-n junction shows negative differential resistance (NDR) due to Fowler-Nordheim (F-N) tunneling through the triangular barrier formed by applying a global back-gate bias (VGS). We also show that the integration of hexagonal boron nitride (h-BN) as an insulating tunnel barrier between MoS2 and WSe2 leads to the formation of sharp band edges and unintentional inelastic tunnelling current. The devices based on vdW contacts, global VGS, and h-BN tunnel barriers exhibit NDR with a peak current (Ipeak) of 315 μA, suggesting that the approach may be useful for applications.

Crucial role of interfacial interaction in 2D polar SiGe/GeC heterostructures

The planar charge transfer is a distinctive characteristic of the two-dimensional (2D) polar materials. When such 2D polar materials are involved in vertical heterostructures (VHs), in addition to the van der Waals (vdW) interlayer interaction, the interfacial interaction triggered by the in-plane charge transfer will play a crucial role. To deeply understand such mechanism, we conducted a comprehensive theoretical study focusing on the structural stability and electronic properties of 2D polar VHs built by commensurate SiGe/GeC bilayers with four species ordering patterns (classified as a C-group with patterns I and II and a Ge-group with patterns III and IV, respectively). It was found that the commensurate SiGe/GeC VHs are mainly stabilized by interfacial interactions (including the electrostatic interlayer bonding, the vdW force, as well as thesp2/sp3orbital hybridization), with the Ge-group being the most energetically favorable than the C-group. A net charge redistribution occurs between adjacent layers, which is significant (∼0.23-0.25 e cell-1) in patterns II and IV, but slightly small (∼0.05-0.09 e cell-1) in patterns I and III, respectively, forming spontaneousp-nheterojunctions. Such interlayer charge transfer could also lead to a polarization in the interfacial region, with the electron depletion (accumulation) close to the GeC layer and the electron accumulation (depletion) close to the SiGe layer in the C-group (the Ge-group). This type of interface dipoles could induce a built-in electric field and help to promote photogenerated electrons (holes) migration. Furthermore, a semi-metal nature with a tiny direct band gap at the SiGe layer and a semiconducting nature at the GeC layer indicate that the commensurate SiG/GeC VHs possess a type-I band alignment of heterojunction and have a wide spectrum of light absorption capabilities, indicating its promising applications for enhancing light-matter interaction and interfacial engineering.

High current treated-passivated graphene (CTPG) towards stable nanoelectronic and spintronic circuits

Achieving enhanced and stable electrical quality of scalable graphene is crucial for practical graphene device applications. Accordingly, encapsulation has emerged as an approach for improving electrical transport in graphene. In this study, we demonstrate high-current treatment of graphene passivated by AlOx nanofilms as a new means to enhance the electrical quality of graphene for its scalable utilization. Our experiments and electrical measurements on large-scale chemical vapor-deposited (CVD) graphene devices reveal that high-current treatment causes persistent and irreversible de-trapping density in both bare graphene and graphene covered by AlOx. Strikingly, despite possible interfacial defects in graphene covered with AlOx, the high-current treatment enhances its carrier mobility by up to 200% in contrast to bare graphene samples, where mobility decreases. Spatially resolved Raman spectroscopy mapping confirms that surface passivation by AlOx, followed by the current treatment, reduces the number of sp3 defects in graphene. These results suggest that for current treated-passivated graphene (CTPG), the high-current treatment considerably reduces charged impurity and trapped charge densities, thereby reducing Coulomb scattering while mitigating any electromigration of carbon atoms. Our study unveils CTPG as an innovative system for practical utilization in graphene nanoelectronic and spintronic integrated circuits.

Graphene/Hexagonal Boron Nitride Heterostructures for O2 Activation and CO Oxidation: Metal-Free Catalysts by Design

Pristine graphene and h-BN monolayers are chemically inert to oxygen and thus exhibit very limited catalytic activity toward O2 activation. Herein, we show that graphene/h-BN heterostructures exhibit a surprising O2 activation capability. We theoretically designed ten graphene/h-BN heterostructures with three types of interfaces and investigated their catalytic activities toward O2 activation and CO-oxidation. In general, O2 can be molecularly chemisorbed and activated on electron-rich graphene/h-BN heterostructures. Electron-deficient graphene/h-BN heterostructures can lead to dissociative O2 adsorption with relatively low dissociation energy barriers (<0.4 eV). For CO-oxidation, the computed energy barrier can be as low as 0.67 eV. The high catalytic activities toward O2 stem from either electron-deficient heterostructures' accumulated electrons or electron richness and low work function for the electron-rich heterostructures. Although the catalytic activities of graphene/h-BN heterostructures depend strongly on the interface type, they are insensitive to the patterns of BN-substitutes, hence benefiting applicability of a wide range of heterostructures.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

In Situ Self-Fluorescence 3D Imaging of Micro/Nano Damage in Silicone Gel for Understanding Insulation Failure under High-Frequency Electric Fields

Strong electromagnetic and heat flux stresses can induce severe damage to solid insulation materials, leading to faults in power equipment and power electronics devices. However, in the absence of suitable in situ imaging methods for observing the development and morphology of electrical damage within insulation materials, the mechanism of insulation failure under high-frequency electric fields has remained elusive. In this work, a recently discovered fluorescence self-excitation phenomenon in electrical damage channels of polymers is used as the basis for a laser confocal imaging method that is able to realize three-dimensional (3D) in situ imaging of electrical tree channels in silicone gel through nondestructive means. Based on the reconstructed morphology of the damaged area, a spatial equivalent calculation model is proposed for analysis of the 3D geometric features of electrical trees. The insulation failure mechanism of silicone gel under electric fields of different frequencies is analyzed through ReaxFF molecular dynamics simulations of the thermal cracking process. This work provides a new method for in situ nondestructive 3D imaging of micro/nanoscale damage structures within polymers with potential applications to material analysis and defect diagnosis.

Time-resolving state-specific molecular dissociation with XUV broadband absorption spectroscopy

The electronic and nuclear dynamics inside molecules are essential for chemical reactions, where different pathways typically unfold on ultrafast timescales. Extreme ultraviolet (XUV) light pulses generated by free-electron lasers (FELs) allow atomic-site and electronic-state selectivity, triggering specific molecular dynamics while providing femtosecond resolution. Yet, time-resolved experiments are either blind to neutral fragments or limited by the spectral bandwidth of FEL pulses. Here, we combine a broadband XUV probe pulse from high-order harmonic generation with an FEL pump pulse to observe dissociation pathways leading to fragments in different quantum states. We temporally resolve the dissociation of a specific O2+ state into two competing channels by measuring the resonances of ionic and neutral fragments. This scheme can be applied to investigate convoluted dynamics in larger molecules relevant to diverse science fields.

One-atom-thick hexagonal boron nitride co-catalyst for enhanced oxygen evolution reactions

Developing efficient (co-)catalysts with optimized interfacial mass and charge transport properties is essential for enhanced oxygen evolution reaction (OER) via electrochemical water splitting. Here we report one-atom-thick hexagonal boron nitride (hBN) as an attractive co-catalyst with enhanced OER efficiency. Various electrocatalytic electrodes are encapsulated with centimeter-sized hBN films which are dense and impermeable so that only the hBN surfaces are directly exposed to reactive species. For example, hBN covered Ni-Fe (oxy)hydroxide anodes show an ultralow Tafel slope of ~30 mV dec-1 with improved reaction current by about 10 times, reaching ~2000 mA cm-2 (at an overpotential of ~490 mV) for over 150 h. The mass activity of hBN co-catalyst is found exceeding that of commercialized catalysts by up to five orders of magnitude. Using isotope experiments and simulations, we attribute the results to the adsorption of oxygen-containing intermediates at the insulating co-catalyst, where localized electrons facilitate the deprotonation processes at electrodes. Little impedance to electron transfer is observed from hBN film encapsulation due to its ultimate thickness. Therefore, our work also offers insights into mechanisms of interfacial reactions at the very first atomic layer of electrodes.

Metal Phosphorous Chalcogenide: A Promising Material for Advanced Energy Storage Systems

The development of efficient and affordable electrode materials is crucial for clean energy storage systems, which are considered a promising strategy for addressing energy crises and environmental issues. Metal phosphorous chalcogenides (MPX3 ) are a fascinating class of two-dimensional materials with a tunable layered structure and high ion conductivity, making them particularly attractive for energy storage applications. This review article aims to comprehensively summarize the latest research progress on MPX3 materials, with a focus on their preparation methods and modulation strategies. Additionally, the diverse applications of these novel materials in alkali metal ion batteries, metal-air batteries, and all-solid-state batteries are highlighted. Finally, the challenges and opportunities of MPX3 materials are presented to inspire their better potential in energy storage applications. This review provides valuable insights into the promising future of MPX3 materials in clean energy storage systems.

Three-Dimensional MoS2/MXene Heterostructure Aerogel for Chemical Gas Sensors with Superior Sensitivity and Stability

The concept of integrating diverse functional 2D materials into a heterostructure provides platforms for exploring physics that cannot be accessed in a single 2D material. Here, physically mixing two 2D materials, MXene and MoS2, followed by freeze-drying is utilized to successfully fabricate a 3D MoS2/MXene van der Waals heterostructure aerogel. The low-temperature synthetic approach effectively suppresses significant oxidation of the Ti3C2Tx MXene and results in a hierarchical and freestanding 3D heterostructure composed of high-quality MoS2 and MXene nanosheets. Functionalization of MXene with a MoS2 catalytic layer substantially improves sensitivity and long-term stability toward detection of NO2 gas, and computational studies are coupled with experimental results to elucidate that the mechanism behind enhancements in the gas-sensing properties is effective inhibition of HNO2 formation on the MXene surface, due to the presence of MoS2. Overall, this study has a great potential for expansion of applicability to other classes of two-dimensional materials as a general synthesis method, to be applied in future fields of catalysis and electronics.

High-Density Vertical Transistors with Pitch Size Down to 20 nm

Vertical field effect transistors (VFETs) have attracted considerable interest for developing ultra-scaled devices. In particular, individual VFET can be stacked on top of another and does not consume additional chip footprint beyond what is needed for a single device at the bottom, representing another dimension for high-density transistors. However, high-density VFETs with small pitch size are difficult to fabricate and is largely limited by the trade-offs between drain thickness and its conductivity. Here, a simple approach is reported to scale the drain to sub-10 nm. By combining 7 nm thick Au with monolayer graphene, the hybrid drain demonstrates metallic behavior with low sheet resistance of ≈100 Ω sq-1 . By van der Waals laminating the hybrid drain on top of 3 nm thick channel and scaling gate stack, the total VFET pitch size down to 20 nm and demonstrates a higher on-state current of 730 A cm-2 . Furthermore, three individual VFETs together are vertically stacked within a vertical distance of 59 nm, representing the record low pitch size for vertical transistors. The method pushes the scaling limit and pitch size limit of VFET, opening up a new pathway for high-density vertical transistors and integrated circuits.

Probing the Inelastic Electron Tunneling via the Photocurrent in a Vertical Graphene van der Waals Heterostructure

Inelastic electron tunneling (IET), accompanied by energy transfer between the tunneling charge carriers and other elementary excitations, is widely used to investigate the collective modes and quasiparticles in solid-state materials. In general, the inelastic contribution to the tunneling current is small compared to the elastic part and is therefore only prominent in the second derivative of the tunneling current with respect to the bias voltage. Here we demonstrate a direct observation of the IET by measuring the photoresponse in a graphene-based vertical tunnel junction device. Characteristic peaks/valleys are observed in the bias-voltage-dependent tunneling photocurrent at low temperatures, which barely shift with the gate voltage applied to graphene and diminish gradually as the temperature increases. By comparing with the second-order differential conductance spectra, we establish that these features are associated with the phonon-assisted IET. A simple model based on the photoexcited hot-carrier tunneling in graphene qualitatively explains the response. Our study points to a promising means of probing the low-energy elementary excitations utilizing the graphene-based van der Waals (vdW) heterostructures.

Realizing On/Off Ratios over 104 for Sub-2 nm Vertical Transistors

Vertical transistors hold promise for the development of ultrascaled transistors. However, their on/off ratios are limited by a strong source-drain tunneling current in the off state, particularly for vertical devices with a sub-5 nm channel length. Here, we report an approach for suppressing the off-state tunneling current by designing the barrier height via a van der Waals metal contact. Via lamination of the Pt electrode on a MoS2 vertical transistor, a high Schottky barrier is observed due to their large work function difference, thus suppressing direct tunneling currents. Meanwhile, this "low-energy" lamination process ensures an optimized metal/MoS2 interface with minimized interface states and defects. Together, the highest on/off ratios of 5 × 105 and 104 are realized in vertical transistors with 5 and 2 nm channel lengths, respectively. Our work not only pushes the on/off ratio limit of vertical transistors but also provides a general rule for reducing short-channel effects in ultrascaled devices.

Stacking transfer of wafer-scale graphene-based van der Waals superlattices

High-quality graphene-based van der Waals superlattices are crucial for investigating physical properties and developing functional devices. However, achieving homogeneous wafer-scale graphene-based superlattices with controlled twist angles is challenging. Here, we present a flat-to-flat transfer method for fabricating wafer-scale graphene and graphene-based superlattices. The aqueous solution between graphene and substrate is removed by a two-step spinning-assisted dehydration procedure with the optimal wetting angle. Proton-assisted treatment is further used to clean graphene surfaces and interfaces, which also decouples graphene and neutralizes the doping levels. Twist angles between different layers are accurately controlled by adjusting the macroscopic stacking angle through their wafer flats. Transferred films exhibit minimal defects, homogeneous morphology, and uniform electrical properties over wafer scale. Even at room temperature, robust quantum Hall effects are observed in graphene films with centimetre-scale linewidth. Our stacking transfer method can facilitate the fabrication of graphene-based van der Waals superlattices and accelerate functional device applications.

Electrical field and biaxial strain tunable electronic properties of the PtSe2/Hf2CO2 heterostructure

The structure and electronic properties of two-dimensional vertical van der Waals PtSe2/Hf2CO2 heterostructure have been investigated based on first-principles calculations. The results show that the PtSe2 and Hf2CO2 monolayers form a type-I heterostructure with both the conduction band minimum (CBM) and valence band maximum (VBM) located at the Hf2CO2 layer. The electronic properties of PtSe2/Hf2CO2 heterostructure can be effectively adjusted by applying external electric field or biaxial strain. The transition in band alignment from type-I to type-II can be manipulated by controlling the strength and direction of the electric field. Additionally, the transition from type-I to type-II have also taken place under the strains, and the band gap of the PtSe2/Hf2CO2 heterostructure decreases with increasing the compressive or tensible strain. Under a strong strain of -8%, the PtSe2/Hf2CO2 heterostructure can transform from semiconductor to metal. These findings provide a promising method to tune the electronic properties of PtSe2/Hf2CO2 heterostructure and design a new vdW heterostructure in the applications for electronic and optoelectronic devices.

A Light-Programmed Rewritable Lattice-Mediated Multistate Memory for High-Density Data Storage

Mainstream non-volatile memory (NVM) devices based on floating gate structures or phase-change/ferroelectric materials face inherent limitations that compromise their suitability for long-term data storage. To address this challenge, a novel memory device based on light-programmed lattice engineering of thin rhenium disulfide (ReS2 ) flakes is proposed. By inducing sulfur vacancies in the ReS2 channel through light illumination, the device's electrical conductivity is modified accordingly and multiple conductance states for data storage therefore are generated. The device exhibits more than 128 distinct states with linearly increasing conductance, corresponding to a sevenfold increase in storage density. Through further optimization to achieve atomic-level precision in defect creation, it is possible to achieve even higher storage densities. These states are extremely stable in vacuum or inert ambient showing long retention of >10 years, while they can be erased upon exposure to the air. The ReS2 memory device can maintain its stability over multiple program-erase operation cycles and shows superior wavelength discrimination capability for incident light in the range of 405-785 nm. This device represents a significant contribution to NVM technology by offering the ability to store information in multistate memory and enabling filter-free color image recorder applications.

Dual transmission channels at metal-MoS2/WSe2 hetero-bilayer interfaces

van der Waals heterostructures (vdWHs) open the possibility of creating novel semiconductor materials at the atomic scale that demonstrate totally new physics and enable unique functionalities, and have therefore attracted great interest in the fields of advanced electronic and optoelectronic devices. However, the interactions between metals and vdWHs semiconductors require further investigation as they directly affect or limit the advancement of high-performance electronic devices. Here we study the contact behavior of MoS₂/WSe₂ vdWHs in contact with a series of bulk metals using ab initio electronic structure calculations and quantum transport simulations. Our study shows that dual transmission paths for electrons and holes exist at the metal-MoS₂/WSe₂ hetero-bilayer interfaces. In addition, the metal-induced bandgap state (MIGS) of the original monolayer disappears due to the creation of the heterolayer, which weakens the Fermi level pinning (FLP) effect. We also find that the creation of the heterolayer causes a change in the Schottky barrier height (SBH) of the non-ohmic contact systems, whilst this does not occur so easily in the ohmic contact systems. In addition, our results indicate that when Al, Ag and Au are in contact with a MoS₂/WSe₂ hetero-bilayer semiconductor, a low contact barrier exists throughout the whole transmission process causing the charge to tunnel to the MoS₂ layer, irrespective of whether the MoS₂ is in contact with the metals as the nearest layer or as the next-nearest layer. Our work not only offers new insights into electrical contact issues between metals and hetero-bilayer semiconductors, but also provides guidance for the design of high-performance vdWHs semiconductor devices.

Enhanced Layer-Breathing Modes in van der Waals Heterostructures Based on Twisted Bilayer Graphene

The characterization of interlayer coupling in two-dimensional van der Waals heterostructures (vdWHs) is essential to understand their quantum behaviors and structural functionalities. Interlayer shear and layer-breathing (LB) phonons carry rich information on interlayer interaction, but they are usually too weak to be detected via standard Raman spectroscopy due to the weak electron-phonon coupling (EPC). Here, we report a universal strategy to enhance LB modes of vdWHs based on twisted bilayer graphene (tBLG). In both tBLG/hBN and tBLG/MoS₂ vdWHs, the resonantly excited electrons in tBLG can strongly couple to LB phonons extended over the entire layers in the vdWHs, whose resonance condition is tunable by the twist angle of tBLG. In vdWHs containing twisted graphene layers with multiple twisted interfaces, the EPC of LB phonons coming from the collective LB vibrations of entire heterostructure layers can be tuned by resonant excitation of programmable van Hove singularities according to each twisted interface. The universality and tunability of enhanced LB phonons by tBLG make it a promising method to investigate EPC and interlayer interaction in related vdWHs.

Wafer-Scale Single Crystal Hexagonal Boron Nitride Layers Grown by Submicron-Spacing Vapor Deposition

The direct growth of wafer-scale single crystal two-dimensional (2D) hexagonal boron nitride (h-BN) layer with a controllable thickness is highly desirable for 2D-material-based device applications. Here, for the first time, a facile submicron-spacing vapor deposition (SSVD) method is reported to achieve 2-inch single crystal h-BN layers with controllable thickness from monolayer to tens of nanometers on the dielectric sapphire substrates using a boron film as the solid source. In the SSVD growth, the boron film is fully covered by the same-sized sapphire substrate with a submicron spacing, leading to an efficient vapor diffusion transport. The epitaxial h-BN layer exhibits extremely high crystalline quality, as demonstrated by both a sharp Raman E_2g vibration mode (12 cm^-1 ) and a narrow X-ray rocking curve (0.10°). Furthermore, a deep ultraviolet photodetector and a ZrS₂ /h-BN heterostructure fabricated from the h-BN layer demonstrate its fascinating properties and potential applications. This facile method to synthesize wafer-scale single crystal h-BN layers with controllable thickness paves the way to future 2D semiconductor-based electronics and optoelectronics.

Graphene Spin Valves for Spin Logic Devices

An alternative to charge-based electronics identifies the spin degree of freedom for information communication and processing. The long spin-diffusion length in graphene at room temperature demonstrates its ability for highly scalable spintronics. The development of the graphene spin valve (SV) has inspired spin devices in graphene including spin field-effect transistors and spin majority logic gates. A comprehensive picture of spin transport in graphene SVs is required for further development of spin logic. This review examines the advances in graphene SVs and their role in the development of spin logic devices. Different transport and scattering mechanisms in charge and spin are discussed. Furthermore, the on/off switching energy between graphene SVs and charge-based FETs is compared to highlight their prospects for low-power devices. The challenges and perspectives that need to be addressed for the future development of spin logic devices are then outlined.

Spin-gapless van der Waals heterostructure for spin gating through magnetic injection devices

Spin-gapless semiconductors (SGSs) are new magnetic zero-bandgap materials whose band structure is extremely sensitive to external influences (pressure or magnetic fields) and have great potential for high-speed and low-energy spintronics applications. The first-principles method was used to systematically study the heterostructures constructed of an asymmetric surface-functionalized Janus MXene material, Cr₂NOF, and a two-dimensional hexagonal lattice (2DH) semiconductor material and to study the effects of the electronic structure, Curie temperature, magnetism, and the design of unusual band structures and magnetic injection in the bilayer to obtain an SGS structure. Through the design and construction of Cr₂NOF/2DH van der Waals heterojunction spintronic devices, the spin-filtering effect of the devices can reach 100%, especially, realizing spin gating through magnetic injection. We report the transport mechanism of the heterojunction spintronic devices to achieve the goal of a controllable optimization of the device functions, which provides a theoretical basis for the design of MXene van der Waals heterojunctions for high-efficiency and low-power-consumption spintronic devices.

Localised strain and doping of 2D materials

There is a growing interest in 2D materials-based devices as the replacement for established materials, such as silicon and metal oxides in microelectronics and sensing, respectively. However, the atomically thin nature of 2D materials makes them susceptible to slight variations caused by their immediate environment, inducing doping and strain, which can vary between, and even microscopically within, devices. One of the misapprehensions for using 2D materials is the consideration of unanimous intrinsic properties over different support surfaces. The interfacial interaction, intrinsic structural disorder and external strain modulate the properties of 2D materials and govern the device performance. The understanding, measurement and control of these factors are thus one of the significant challenges for the adoption of 2D materials in industrial electronics, sensing, and polymer composites. This topical review provides a comprehensive overview of the effect of strain-induced lattice deformation and its relationship with physical and electronic properties. Using the example of graphene and MoS₂ (as the prototypical 2D semiconductor), we rationalise the importance of scanning probe techniques and Raman spectroscopy to elucidate strain and doping in 2D materials. These effects can be directly and accurately characterised through Raman shifts in a non-destructive manner. A generalised model has been presented that deconvolutes the intertwined relationship between strain and doping in graphene and MoS₂ that could apply to other members of the 2D materials family. The emerging field of straintronics is presented, where the controlled application of strain over 2D materials induces tuneable physical and electronic properties. These perspectives highlight practical considerations for strain engineering and related microelectromechanical applications.

Graphene Strain-Effect Transistor with Colossal ON/OFF Current Ratio Enabled by Reversible Nanocrack Formation in Metal Electrodes on Piezoelectric Substrates

Extraordinarily high carrier mobility in graphene has led to many remarkable discoveries in physics and at the same time invoked great interest in graphene-based electronic devices and sensors. However, the poor ON/OFF current ratio observed in graphene field-effect transistors has stymied its use in many applications. Here, we introduce a graphene strain-effect transistor (GSET) with a colossal ON/OFF current ratio in excess of 10⁷ by exploiting strain-induced reversible nanocrack formation in the source/drain metal contacts with the help of a piezoelectric gate stack. GSETs also exhibit steep switching with a subthreshold swing (SS) < 1 mV/decade averaged over ∼6 orders of magnitude change in the source-to-drain current for both electron and hole branch amidst a finite hysteresis window. We also demonstrate high device yield and strain endurance for GSETs. We believe that GSETs can significantly expand the application space for graphene-based technologies beyond what is currently envisioned.

Electron-phonon interaction toward engineering carrier mobility of periodic edge structured graphene nanoribbons

Graphene nanoribbons have many extraordinary electrical properties and are the candidates for semiconductor industry. In this research, we propose a design of Coved GNRs with periodic structure ranged from 4 to 8 nm or more, of which the size is within practical feature sizes by advanced lithography tools. The carrier transport properties of Coved GNRs with the periodic coved shape are designed to break the localized electronic state and reducing electron-phonon scattering. In this way, the mobility of Coved GNRs can be enhanced by orders compared with the zigzag GNRs in same width. Moreover, in contrast to occasional zero bandgap transition of armchair and zigzag GNRs without precision control in atomic level, the Coved GNRs with periodic edge structures can exclude the zero bandgap conditions, which makes practical the mass production process. The designed Coved-GNRs is fabricated over the Germanium (110) substrate where the graphene can be prepared in the single-crystalline and single-oriented formants and the edge of GNRs is later repaired under "balanced condition growth" and we demonstrate that the propose coved structures are compatible to current fabrication facility.

Formation of Graphene on Gold-Nickel Surface Alloys

Nickel (Ni)-catalyzed growth of a single- or rotated-graphene layer is a well-established process above 800 K. In this report, a Au-catalyzed, low-temperature, and facile route at 500 K for graphene formation is described. The substantially lower temperature is enabled by the presence of a surface alloy of Au atoms embedded within Ni(111), which catalyzes the outward segregation of carbon atoms buried in the Ni bulk at temperatures as low as 400-450 K. The resulting surface-bound carbon in turn coalesces into graphene above 450-500 K. Control experiments on a Ni(111) surface show no evidence of carbon segregation or graphene formation at these temperatures. Graphene is identified by its out-of-plane optical phonon mode at 750 cm^-1 and its longitudinal/transverse optical phonon modes at 1470 cm^-1 while surface carbon is identified by its C-Ni stretch mode at 540 cm^-1, as probed by high-resolution electron energy-loss spectroscopy. Dispersion measurements of the phonon modes confirm the presence of graphene. Graphene formation is observed to be maximum at 0.4 ML Au coverage. The results of these systematic molecular-level investigations open the door to graphene synthesis at the low temperatures required for integration with complementary metal-oxide-semiconductor processes.

Electrical Contacts With 2D Materials: Current Developments and Future Prospects

Current electrical contact models are occasionally insufficient at the nanoscale owing to the wide variations in outcomes between 2D mono and multi-layered and bulk materials that result from their distinctive electrostatics and geometries. Contrarily, devices based on 2D semiconductors present a significant challenge due to the requirement for electrical contact with resistances close to the quantum limit. The next generation of low-power devices is already hindered by the lack of high-quality and low-contact-resistance contacts on 2D materials. The physics and materials science of electrical contact resistance in 2D materials-based nanoelectronics, interface configurations, charge injection mechanisms, and numerical modeling of electrical contacts, as well as the most pressing issues that need to be resolved in the field of research and development, will all be covered in this review.

Hybrid Heterostructures to Generate Long-Lived and Mobile Photocarriers in Graphene

We report the generation of long-lived and highly mobile photocarriers in hybrid van der Waals heterostructures that are formed by monolayer graphene, few-layer transition metal dichalcogenides, and the organic semiconductor F₈ZnPc. Samples are fabricated by dry transfer of mechanically exfoliated MoS₂ or WS₂ few-layer flakes on a graphene film, followed by deposition of F₈ZnPc. Transient absorption microscopy measurements are performed to study the photocarrier dynamics. In heterostructures of F₈ZnPc/few-layer-MoS₂/graphene, electrons excited in F₈ZnPc can transfer to graphene and thus be separated from the holes that reside in F₈ZnPc. By increasing the thickness of MoS₂, these electrons acquire long recombination lifetimes of over 100 ps and a high mobility of 2800 cm² V^-1 s^-1. Graphene doping with mobile holes is also demonstrated with WS₂ as the middle layers. These artificial heterostructures can improve the performance of graphene-based optoelectronic devices.

Dual-mode frequency multiplier in graphene-base hot electron transistor

Since quantum computers have been gradually introduced in countries around the world, the development of the many related quantum components that can operate independently of temperature has become more important for enabling mature products with low power dissipation and high efficiency. As an alternative to studying cryo-CMOSs (complementary metal-oxide-semiconductors) to achieve this goal, quantum tunneling devices based on 2D materials can be examined instead. In this work, a vertical graphene-based quantum tunneling transistor has been used as a frequency modulator. The transistor can operate via different quantum tunneling mechanisms and generates, by applying the appropriate bias, voltage-resistance curves characteristic of variable nonlinear resistance for both base and emitter voltages. We experimentally demonstrate frequency modulation from input signals over the range of 100 kHz to 10 MHz, enabling a tunable frequency doubler or tripler in just a single transistor. This frequency multiplication with a tunneling mechanism makes the graphene-based tunneling device a promising option for frequency electronics in the emerging field of quantum technologies.

Graphene barristors for de novo optoelectronics

Graphene-based vertical Schottky-barrier transistors (SBTs), renowned as graphene barristors, have emerged as a feasible candidate to fundamentally expand the horizon of conventional transistor technology. The remote tunability of graphene's electronic properties could endorse multi-stimuli responsive functionalities for a broad range of electronic and optoelectronic applications of transistors, with the capability of incorporating nanochannel architecture with dramatically reduced footprints from the vertical integrations. In this Feature Article, we provide a comprehensive overview of the progress made in the field of SBTs over the last 10 years, starting from the operating principles, materials evolution, and processing developments. Depending on the types of stimuli such as electrical, optical, and mechanical stresses, various fields of applications from conventional digital logic circuits to sensory technologies are highlighted. Finally, more advanced applications toward beyond-Moore electronics are discussed, featuring recent advancements in neuromorphic devices based on SBTs.

One-pot liquid-phase synthesis of MoS2-WS2van der waals heterostructures for broadband photodetection

Two dimensional (2D) van der Waals heterostructures (vdWHs) have unique potential in facilitating the stacking of layers of different 2D materials for optoelectronic devices with superior characteristics. However, the fabrication of large area all-2D heterostructures is still challenging towards realizing practical devices at a reduced cost. In the present work, we have demonstrated a rapid yet simple, impurity-free and efficient sonication-assisted chemical exfoliation approach to synthesize hybrid vdWHs based on 2D molybdenum disulphide (MoS₂) and tungsten disulphide (WS₂), with high yield. Microscopic and spectroscopic studies have confirmed the successful exfoliation of layered 2D materials and formation of their hybrid heterostructures. The co-existence of 2D MoS₂and WS₂in the vdWH hybrids is established by optical absorption and Raman shift measurements along with their chemical stiochiometry determined by x-ray photoelectron spectroscopy. The spectral response of the vdWH/Si (2D/3D) heterojunction photodetector fabricated using the as-synthesized material is found to exhibit broadband photoresponse compared to that of the individual 2D MoS₂and WS₂devices. The peak responsivity and detectivity are found to be as high as ∼2.15 A W^-1and 2.05 × 10¹¹Jones, respectively for an applied bias of -5 V. The ease of fabrication with appreciable performance of the chemically synthesized vdWH-based devices have revealed their potential use for large area optoelectronic applications on Si-compatible CMOS platforms.

Engineered Two-Dimensional Nanostructures as SERS Substrates for Biomolecule Sensing: A Review

Two-dimensional nanostructures (2DNS) attract tremendous interest and have emerged as potential materials for a variety of applications, including biomolecule sensing, due to their high surface-to-volume ratio, tuneable optical and electronic properties. Advancements in the engineering of 2DNS and associated technologies have opened up new opportunities. Surface-enhanced Raman scattering (SERS) is a rapid, highly sensitive, non-destructive analytical technique with exceptional signal amplification potential. Several structurally and chemically engineered 2DNS with added advantages (e.g., π-π* interaction), over plasmonic SERS substrates, have been developed specifically towards biomolecule sensing in a complex matrix, such as biological fluids. This review focuses on the recent developments of 2DNS-SERS substrates for biomolecule sensor applications. The recent advancements in engineered 2DNS, particularly for SERS substrates, have been systematically surveyed. In SERS substrates, 2DNS are used as either a standalone signal enhancer or as support for the dispersion of plasmonic nanostructures. The current challenges and future opportunities in this synergetic combination have also been discussed. Given the prospects in the design and preparation of newer 2DNS, this review can give a critical view on the current status, challenges and opportunities to extrapolate their applications in biomolecule detection.

Hexagonal Boron Nitride for Next-Generation Photonics and Electronics

Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it is explored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of structural and optical properties of h-BN have expanded potential applications into emerging electronics and photonics fields. h-BN shows a very efficient deep-ultraviolet band-edge emission despite its indirect-bandgap nature, as well as stable room-temperature single-photon emission over a wide wavelength range, showing a great potential for next-generation photonics. In addition, h-BN is extensively being adopted as active media for low-energy electronics, including nonvolatile resistive switching memory, radio-frequency devices, and low-dielectric-constant materials for next-generation electronics.

Stick-Slip Dynamics of Moiré Superstructures in Polycrystalline 2D Material Interfaces

A new frictional mechanism, based on collective stick-slip motion of moiré superstructures across polycrystalline two-dimensional material interfaces, is predicted. The dissipative stick-slip behavior originates from an energetic bistability between low- and high-commensurability configurations of large-scale moiré superstructures. When the grain boundary separates between grains of small and large interfacial twist angle, the corresponding moiré periods are significantly different, resulting in forbidden grain boundary crossing of the moiré superstructures during shear induced motion. For small twist angle grains, where the moiré periods are much larger than the lattice constant, this results in multiple reflections of collective surface waves between the surrounding grain boundaries. In combination with the individual grain boundary dislocation snap-through buckling mechanism dominating at the low normal load regime, the friction exhibits nonmonotonic behavior with the normal load. While the discovered phenomenon is demonstrated for h-BN/graphene polycrystalline junctions, it is expected to be of general nature and occur in many other large-scale layered material interfaces.

Band Gap Opening in Borophene/GaN and Borophene/ZnO Van der Waals Heterostructures Using Axial Deformation: First-Principles Study

One of the topical problems of materials science is the production of van der Waals heterostructures with the desired properties. Borophene is considered to be among the promising 2D materials for the design of van der Waals heterostructures and their application in electronic nanodevices. In this paper, we considered new atomic configurations of van der Waals heterostructures for a potential application in nano- and optoelectronics: (1) a configuration based on buckled triangular borophene and gallium nitride (GaN) 2D monolayers; and (2) a configuration based on buckled triangular borophene and zinc oxide (ZnO) 2D monolayers. The influence of mechanical deformations on the electronic structure of borophene/GaN and borophene/ZnO van der Waals heterostructures are studied using the first-principles calculations based on density functional theory (DFT) within a double zeta plus polarization (DZP) basis set. Four types of deformation are considered: uniaxial (along the Y axis)/biaxial (along the X and Y axes) stretching and uniaxial (along the Y axis)/biaxial (along the X and Y axes) compression. The main objective of this study is to identify the most effective types of deformation from the standpoint of tuning the electronic properties of the material, namely the possibility of opening the energy gap in the band structure. For each case of deformation, the band structure and density of the electronic states (DOS) are calculated. It is found that the borophene/GaN heterostructure is more sensitive to axial compression while the borophene/ZnO heterostructure is more sensitive to axial stretching. The energy gap appears in the band structure of borophene/GaN heterostructure at uniaxial compression by 14% (gap size of 0.028 eV) and at biaxial compression by 4% (gap size of 0.018 eV). The energy gap appears in the band structure of a borophene/ZnO heterostructure at uniaxial stretching by 10% (gap size 0.063 eV) and at biaxial compression by 6% (0.012 eV). It is predicted that similar heterostructures with an emerging energy gap can be used for various nano- and optoelectronic applications, including Schottky barrier photodetectors.

Chemical Vapor Deposition of Quaternary 2D BiCuSeO p-Type Semiconductor with Intrinsic Degeneracy

2D BiCuSeO is an intrinsic p-type degenerate semiconductor due to its self-doping effect, which possesses great potential to fabricate high-performance 2D-2D tunnel field-effect transistors (TFETs). However, the controllable synthesis of multinary 2D materials by chemical vapor deposition (CVD) is still a challenge due to the restriction of thermodynamics. Here, the CVD synthesis of quaternary 2D BiCuSeO nanosheets is realized. As-grown BiCuSeO nanosheets with thickness down to ≈6.1 nm (≈7 layers) and domain size of ≈277 µm show excellent ambient stability. Intrinsic p-type degeneracy of BiCuSeO, capable of maintaining even in a few layers, is comprehensively unveiled. By varying the thicknesses and temperatures, the carrier concentration of BiCuSeO nanosheets can be adjusted in the range of 10¹⁹ to 10²¹ cm^-3 , and the Hall mobility of BiCuSeO is ≈191 cm² V^-1 s^-1 (at 2 K). Furthermore, taking advantage of the p-type degeneracy of BiCuSeO, a prototypical BiCuSeO/MoS₂ TFET is fabricated. The emergence of the negative differential resistance trend and multifunctional diodes by modulating the gate voltage and temperature reveal the great practical implementation potential of BiCuSeO nanosheets. These results pave way for the CVD synthesis of multinary 2D materials and rational design of high-performance tunnel devices.

Strain-Tuneable Magnetism and Spintronics of Distorted Monovacancies in Graphene

The electronic and spintronic properties of the monovacancies in freestanding and isotopically compressed graphene are investigated using hybrid exchange density functional perturbation theory. When the effects of electronic self-interaction are taken into account, an integer magnetic moment of 2 μ_B is identified for a Jahn-Teller reconstructed V₁(5-9) monovacancy in freestanding graphene. For graphene with stable ripples induced by a compressive strain of 5%, a bond reconstruction produces a V₁(55-66) structure for the monovacancy, which is localized at the saddle points of the ripple. The sizeable local distortion induced by reconstruction modifies both the geometric and electronic properties of rippled graphene and quenches the magnetic moment of the vacancy due to the sp³ hybridization of the central atom. The nonmagnetic V₁(55-66) structure is found to be stable on rippled structures, with the formation energy ∼2.3 eV lower than that of the metastable distorted V₁(5-9) structures localized at sites other than the saddle points. The electronic ground state of distorted V₁(5-9) corresponds to a wide range of fractional magnetic moments (0.50-1.25 μ_B). The computed relative stabilities and the electronic and magnetic properties of the V₁(5-9) structures are found to be closely related to their local distortions. This analysis of the fundamental properties of defective graphene under compression suggests a number of strategies for generating regular defect patterns with tuneable magnetic and electronic properties and may, therefore, be used as a novel technique to achieve more precise control of graphene electronic structure for various application scenarios such as transistors, strain sensors, and directed chemisorption.

Electronic Properties of Hexagonal Graphene Quantum Rings from TAO-DFT

The reliable prediction of electronic properties associated with graphene nanosystems can be challenging for conventional electronic structure methods, such as Kohn-Sham (KS) density functional theory (DFT), due to the presence of strong static correlation effects in these systems. To address this challenge, TAO (thermally assisted occupation) DFT has been recently proposed. In the present study, we employ TAO-DFT to predict the electronic properties of n-HGQRs (i.e., the hexagonal graphene quantum rings consisting of n aromatic rings fused together at each side). From TAO-DFT, the ground states of n-HGQRs are singlets for all the cases investigated (n = 3-15). As the system size increases, there should be a transition from the nonradical to polyradical nature of ground-state n-HGQR. The latter should be intimately related to the localization of active TAO-orbitals at the inner and outer edges of n-HGQR, which increases with increasing system size.

High-Temperature Quantum Hall Effect in Graphite-Gated Graphene Heterostructure Devices with High Carrier Mobility

Since the discovery of the quantum Hall effect in 1980, it has attracted intense interest in condensed matter physics and has led to a new type of metrological standard by utilizing the resistance quantum. Graphene, a true two-dimensional electron gas material, has demonstrated the half-integer quantum Hall effect and composite-fermion fractional quantum Hall effect due to its unique massless Dirac fermions and ultra-high carrier mobility. Here, we use a monolayer graphene encapsulated with hexagonal boron nitride and few-layer graphite to fabricate micrometer-scale graphene Hall devices. The application of a graphite gate electrode significantly screens the phonon scattering from a conventional SiO₂/Si substrate, and thus enhances the carrier mobility of graphene. At a low temperature, the carrier mobility of graphene devices can reach 3 × 10⁵ cm²/V·s, and at room temperature, the carrier mobility can still exceed 1 × 10⁵ cm²/V·s, which is very helpful for the development of high-temperature quantum Hall effects under moderate magnetic fields. At a low temperature of 1.6 K, a series of half-integer quantum Hall plateaus are well-observed in graphene with a magnetic field of 1 T. More importantly, the ν = ±2 quantum Hall plateau clearly persists up to 150 K with only a few-tesla magnetic field. These findings show that graphite-gated high-mobility graphene devices hold great potential for high-sensitivity Hall sensors and resistance metrology standards for the new Système International d'unités.

Miniaturized spectrometers with a tunable van der Waals junction

Miniaturized computational spectrometers, which can obtain incident spectra using a combination of device spectral responses and reconstruction algorithms, are essential for on-chip and implantable applications. Highly sensitive spectral measurement using a single detector allows the footprints of such spectrometers to be scaled down while achieving spectral resolution approaching that of benchtop systems. We report a high-performance computational spectrometer based on a single van der Waals junction with an electrically tunable transport-mediated spectral response. We achieve high peak wavelength accuracy (∼0.36 nanometers), high spectral resolution (∼3 nanometers), broad operation bandwidth (from ∼405 to 845 nanometers), and proof-of-concept spectral imaging. Our approach provides a route toward ultraminiaturization and offers unprecedented performance in accuracy, resolution, and operation bandwidth for single-detector computational spectrometers.

Hot Carrier Transfer in PtSe2/Graphene Enabled by the Hot Phonon Bottleneck

The charge transfer (CT) process of two-dimensional (2D) graphene/transition metal dichalcogenides (TMDs) heterostructures makes the photoelectric conversion ability of TMDs into a wider spectral range for the light harvester and photoelectric detector applications. However, the direct in situ investigation of the hot carrier transport in graphene/TMDs heterostructures has been rarely reported. Herein, using the optical pump and a terahertz (THz) probe (OPTP) spectroscopy, the CT process from graphene to five-layer PtSe₂ in the PtSe₂/graphene (P/G) heterostructure is demonstrated to be related to the pump fluence, which is enabled by the hot phonon bottleneck (HPB) effect in graphene. Furthermore, the frequency dispersion conductivity and the THz emission spectroscopy of the P/G heterostructure confirmed the existence of interlayer CT and its pump fluence-dependent behavior. Our results provide in-depth physical insights into the CT mechanism at the P/G van der Waals interface, which is crucial for further exploration of optoelectronic devices based on P/G heterostructures.