Sub-bandgap photoluminescence properties of multilayer h-BN-on-sapphire.

Two-dimensional hexagonal boron nitride (h-BN) materials have garnered increasing attention due to its ability of hosting intrinsic quantum point defects. This paper presents a photoluminescence (PL) mapping study related to sub-bandgap-level emission in bulk-like multilayer h-BN films. Spatial PL intensity distributions were carefully analyzed with 500 nm spatial resolution in terms of zero phonon line (ZPL) and phonon sideband (PSB) emission-peaks and their linewidths, thereby identifying the potential quantum point defects within the films. Two types of ZPL and PSB emissions were confirmed from the point defects located at the non-edge and edge of the films. Our statistical PL data from the non-edge- and edge-areas of the sample consistently reveal broad and narrow emissions, respectively. The measured optical properties of these defects and the associated ZPL peak shift and line broadening as a function of temperature between 77° and 300° K are qualitatively and quantitatively explained. Moreover, an enhancement of the photostable PL emission by at least a factor of ×3 is observed when our pristine h-BN was irradiated with a 532 nm laser.

Generation of Concentric Space-Variant Linear Polarized Light by Dielectric Metalens.

Miniaturized flat and ultrathin optical components with spatial and polarization degrees of freedom are important for optical communications. Here, we use nanostructures that act as tiny phase plates on a dielectric metalens to generate a concentric polarization beam with different orientations along the radial direction. The important discoveries are that (1) the circularly polarized light can be converted into linearly polarized states with a different orientation at near field and that (2) this orientation is strongly correlated to the rotation of the nanostructures on the metalens. Stokes parameters are utilized to investigate the comprehensive polarization states embedded in the optical intensity along the propagation direction. The variation of the spatial polarization states transformed by the dielectric metalens can be properly mapped onto the Poincaré sphere. We believe that the variety of spatial polarizations within a miniaturized configuration provides a new degree of freedom for diverse applications in the future.

Responsivity and detectivity enhancements by graphene overlay on normal-incident multicolor quantum grid infrared photodetectors.

An efficient and effective method to achieve high responsivity and specific detectivity, particularly for normal-incident quantum well infrared photodetectors (QWIPs), is proposed in this study. By combining superlattice (SL) structure, grating structures, and graphene monolayer onto traditional QWIP designs, a graphene-covered multicolor quantum grid infrared photodetector (QGIP) with improved optoelectrical properties is developed. The enhancements of the device's responsivity and specific detectivity are about 7-fold and 20-fold, respectively, which resulted from an increase in the charge depletion region and the generation of extra photoelectrons due to graphene-semiconductor heterojunction. This method provides a potential candidate for future high-performance photodetectors.

Nonvolatile molecular memory with the multilevel states based on MoS2 nanochannel field effect transistor through tuning gate voltage to control molecular configurations.

A new flexible memory element is crucial for mobile and wearable electronics. A new concept for memory operation and innovative device structure with new materials is certainly required to address the bottleneck of memory applications now and in the future. We report a new nonvolatile molecular memory with a new operating mechanism based on two-dimensional (2D) material nanochannel field-effect transistors (FETs). The smallest channel length for our 2D material nanochannel FETs was approximately 30 nm. The modified molecular configuration for charge induced in the nanochannel of the MoS2 FET can be tuned by applying an up-gate voltage pulse, which can vary the channel conductance to exhibit memory states. Through controlling the amounts of triggered molecules through either different gate voltage pulses or gate duration time, multilevel states were obtained in the molecular memory. These new molecular memory transistors exhibited an erase/program ratio of more than three orders of current magnitude and high sensitivity, of a few picoamperes, at the current level. Reproducible operation and four-level states with stable retention and endurance were achieved. We believe this prototype device has potential for use in future memory devices.

Force and light tuning vertical tunneling current in the atomic layered MoS2.

In this work, the vertical electrical transport behavior of bilayer MoS2 under the coupling of force and light was explored by the use of conductive atomic force microscopy. We found that the current-voltage behavior across the tip-MoS2-Pt junction is a tunneling current that can be well fitted by a Simmons approximation. The transport behavior is direct tunneling at low bias and Fowler-Nordheim tunneling at high bias, and the transition voltage and tunnel barrier height are extracted. The effect of force and light on the effective band gap of the junction is investigated. Furthermore, the source-drain current drops surprisingly when we continually increase the force, and the dropping point is altered by the provided light. This mechanism is responsible for the tuning of tunneling barrier height and width by force and light. These results provide a new way to design devices that take advantage of ultrathin two-dimensional materials. Ultrashort channel length electronic components that possess tunneling current are important for establishing high-efficiency electronic and optoelectronic systems.

Strong Rashba-Edelstein Effect-Induced Spin-Orbit Torques in Monolayer Transition Metal Dichalcogenide/Ferromagnet Bilayers.

The electronic and optoelectronic properties of two-dimensional materials have been extensively explored in graphene and layered transition metal dichalcogenides (TMDs). Spintronics in these two-dimensional materials could provide novel opportunities for future electronics, for example, efficient generation of spin current, which should enable the efficient manipulation of magnetic elements. So far, the quantitative determination of charge current-induced spin current and spin-orbit torques (SOTs) on the magnetic layer adjacent to two-dimensional materials is still lacking. Here, we report a large SOT generated by current-induced spin accumulation through the Rashba-Edelstein effect in the composites of monolayer TMD (MoS2 or WSe2)/CoFeB bilayer. The effective spin conductivity corresponding to the SOT turns out to be almost temperature-independent. Our results suggest that the charge-spin conversion in the chemical vapor deposition-grown large-scale monolayer TMDs could potentially lead to high energy efficiency for magnetization reversal and convenient device integration for future spintronics based on two-dimensional materials.

Atomic-Monolayer MoS2 Band-to-Band Tunneling Field-Effect Transistor.

The experimental observation of band-to-band tunneling in novel tunneling field-effect transistors utilizing a monolayer of MoS2 as the conducting channel is demonstrated. Our results indicate that the strong gate-coupling efficiency enabled by two-dimensional materials, such as monolayer MoS2 , results in the direct manifestation of a band-to-band tunneling current and an ambipolar transport.

High-Current Gain Two-Dimensional MoS2-Base Hot-Electron Transistors.

The vertical transport of nonequilibrium charge carriers through semiconductor heterostructures has led to milestones in electronics with the development of the hot-electron transistor. Recently, significant advances have been made with atomically sharp heterostructures implementing various two-dimensional materials. Although graphene-base hot-electron transistors show great promise for electronic switching at high frequencies, they are limited by their low current gain. Here we show that, by choosing MoS2 and HfO2 for the filter barrier interface and using a noncrystalline semiconductor such as ITO for the collector, we can achieve an unprecedentedly high-current gain (α ∼ 0.95) in our hot-electron transistors operating at room temperature. Furthermore, the current gain can be tuned over 2 orders of magnitude with the collector-base voltage albeit this feature currently presents a drawback in the transistor performance metrics such as poor output resistance and poor intrinsic voltage gain. We anticipate our transistors will pave the way toward the realization of novel flexible 2D material-based high-density, low-energy, and high-frequency hot-carrier electronic applications.

Effect of focused ion beam deposition induced contamination on the transport properties of nano devices.

Focused ion beam (FIB) deposition produces unwanted particle contamination beyond the deposition point. This is due to the FIB having a Gaussian distribution. This work investigates the spatial extent of this contamination and its influence on the electrical properties of nano-electronic devices. A correlation study is performed on carbon-nanotube (CNT) devices manufactured using FIB deposition. The devices are observed using transmission electron microscopy (TEM) and these images are correlated with device electrical characteristics. To discover how far Pt-nanoparticle contamination occurs along a CNT after FIB electrical contact deposition careful TEM inspections are performed. The results show FIB deposition efficiently improves electrical contact; however, the practice is accompanied by serious particle contamination near deposition points. These contaminants include metal particles and amorphous elements originating from precursor gases and residual water molecules in the vacuum chamber. Pt-contamination extends for approximately 2 µm from the point of FIB contact deposition. These contaminants cause current fluctuations and alter the transport characteristics of devices. It is recommended that nano-device fabrication occurs at a distance greater than 2 µm from the FIB deposition of an electrical contact.

Resonant tunneling through discrete quantum states in stacked atomic-layered MoS2.

Two-dimensional crystals can be assembled into three-dimensional stacks with atomic layer precision, which have already shown plenty of fascinating physical phenomena and been used for prototype vertical-field-effect-transistors.1,2 In this work, interlayer electron tunneling in stacked high-quality crystalline MoS2 films were investigated. A trilayered MoS2 film was sandwiched between top and bottom electrodes with an adjacent bottom gate, and the discrete energy levels in each layer could be tuned by bias and gate voltages. When the discrete energy levels aligned, a resonant tunneling peak appeared in the current-voltage characteristics. The peak position shifts linearly with perpendicular magnetic field, indicating formation of Landau levels. From this linear dependence, the effective mass and Fermi velocity are determined and are confirmed by electronic structure calculations. These fundamental parameters are useful for exploitation of its unique properties.

Polymer-free patterning of graphene at sub-10-nm scale by low-energy repetitive electron beam.

A polymer-free technique for generating nanopatterns on both synthesized and exfoliated graphene sheets is proposed and demonstrated. A low-energy (5-30 keV) scanning electron beam with variable repetition rates is used to etch suspended and unsuspended graphene sheets on designed locations. The patterning mechanisms involve a defect-induced knockout process in the initial etching stage and a heat-induced curling process in a later stage. Rough pattern edges appear due to inevitable stochastic knockout of carbon atoms or graphene structure imperfection and can be smoothed by thermal annealing. By using this technique, the minimum feature sizes achieved are about 5 nm for suspended and 7 nm for unsuspended graphene. This study demonstrates a polymer-free direct nanopatterning approach for graphene.

Submicron Memtransistors Made from Monocrystalline Molybdenum Disulfide.

Multiterminal memtransistors made from two-dimensional (2D) materials have garnered increasing attention in the pursuit of low-power heterosynaptic neuromorphic circuits. However, existing 2D memtransistors tend to necessitate high set voltages (>1 V) or feature defective channels, posing concerns regarding material integrity and intrinsic properties. Herein, we present a monocrystalline monolayer MoS2 memtransistor designed for operation within submicron regimes. Under reverse drain bias sweeps, our experiments reveal memristive behavior within the device, further controllable through modulation of the gate terminal. This controllability facilitates the consistent manifestation of multistate memory effects. Notably, the memtransistor behavior becomes more significant as the channel length diminishes, particularly with channel lengths below 1.6 µm, showcasing an increase in the switching ratio alongside a decrease in the set voltage with the decreasing channel length. Our optimized memtransistor demonstrates the ability to exhibit individual resistance states spanning 5 orders of magnitude, with switching drain voltages of approximately 0.05 V. To elucidate these findings, we investigate hot carrier effects and their interplay with oxide traps within the HfO2 dielectric. This work highlights the importance of memtransisor behavior in highly scaled 2D transistors, particularly those featuring low contact resistances. This understanding holds the potential to tailor memory characteristics essential for the development of energy-efficient neuromorphic 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.

Self-powered broadband photodetection enabled by facile CVD-grown MoS2/GaN heterostructures.

Achieving self-powered photodetection without biasing is a notable challenge for photodetectors. In this work, we demonstrate the successful fabrication of large-scale van der Waals epitaxial molybdenum disulfide (MoS2) on a p-GaN/sapphire substrate using a straightforward chemical vapor deposition (CVD) technique. Our research primarily centers on the characterization of these photodetectors produced through this method. The MoS2/GaN heterojunction photodetector showcases a broad and extensive photoresponse spanning from ultraviolet A (UVA) to near-infrared (NIR). When illuminated by a 532 nm laser, its self-powered photoresponse is characterized by a rise time (τr) of ∼18.5 ms and a decay time (τd) of ∼123.2 ms. The photodetector achieves a responsivity (R) of ∼0.13 A W-1 and a specific detectivity (D*) of ∼3.8 × 1010 Jones at zero bias. Additionally, while utilizing a 404 nm laser, the photodetector reaches a maximum R and D* of ∼1.7 × 104 A/W and ∼1.6 × 1013 Jones, respectively, at Vb = 5 V. The operational mechanism of the device can be explained by the diode characteristics involving a tunneling current in the presence of reverse bias. The exceptional performance of these photodetectors can be attributed to the pristine interface between the CVD-grown MoS2 and GaN, providing an impeccably clean tunneling surface. Additionally, our investigation has unveiled that MoS2/GaN heterostructure photodetectors, featuring MoS2 coverage percentages spanning from 20% to 50%, exhibit improved responsivity capabilities at an external bias voltage. As a result, this facile CVD growth technique for MoS2 photodetectors holds significant potential for large-scale production in the manufacturing industry.

Photo-response of a nanopore device with a single embedded ZnO nanoparticle.

The photo-response of a ZnO nanoparticle embedded in a nanopore made on a silicon nitride membrane is investigated. The ZnO nanoparticle is manipulated onto the nanopore and sandwiched between aluminum contact electrodes from both the top and bottom. The asymmetric device structure facilitates current-voltage rectification that enables photovoltaic capacity. Under illumination, the device shows open-circuit voltage as well as short-circuit current. The fill factor is found to increase at low temperatures and reaches 48.6% at 100 K. The nanopore structure and the manipulation technique provide a solid platform for exploring the electrical properties of single nanoparticles.

Control of trion-to-exciton conversion in monolayer WS2 by orbital angular momentum of light.

Controlling the density of exciton and trion quasiparticles in monolayer two-dimensional (2D) materials at room temperature by nondestructive techniques is highly desired for the development of future optoelectronic devices. Here, the effects of different orbital angular momentum (OAM) lights on monolayer tungsten disulfide at both room temperature and low temperatures are investigated, which reveal simultaneously enhanced exciton intensity and suppressed trion intensity in the photoluminescence spectra with increasing topological charge of the OAM light. In addition, the trion-to-exciton conversion efficiency is found to increase rapidly with the OAM light at low laser power and decrease with increasing power. Moreover, the trion binding energy and the concentration of unbound electrons are estimated, which shed light on how these quantities depend on OAM. A phenomenological model is proposed to account for the experimental data. These findings pave a way toward manipulating the exciton emission in 2D materials with OAM light for optoelectronic applications.

Twisted Light-Induced Photocurrent in a Silicon Nanowire Field-Effect Transistor.

Light can possess orbital angular momentum (OAM), in addition to spin angular momentum (SAM), which offers nearly infinite possible values of momentum states, allowing a wider degree of freedom for information processing and communications. The OAM of light induces a selection rule that obeys the law of conservation of angular momentum as it interacts with a material, affecting the material's optical and electrical properties. In this work, silicon nanowire field-effect transistors are subjected to light with OAM, also known as twisted light. Electrical measurements on the devices consequently reveal photocurrent enhancements after incrementing the OAM of the incident light from 0ℏ (fundamental mode) to 5ℏ. Such a phenomenon is attributed to the enhancements of the photogating and the photoconductive effects under the influence of the OAM of light, the underlying mechanism of which is proposed and discussed using energy band diagrams. With these observations, a strategy for controlling photocurrent has been introduced, which can be a realization of the application in the field of optoelectronics technology.

Defect-engineered room temperature negative differential resistance in monolayer MoS2 transistors.

The negative differential resistance (NDR) effect has been widely investigated for the development of various electronic devices. Apart from traditional semiconductor-based devices, two-dimensional (2D) transition metal dichalcogenide (TMD)-based field-effect transistors (FETs) have also recently exhibited NDR behavior in several of their heterostructures. However, to observe NDR in the form of monolayer MoS2, theoretical prediction has revealed that the material should be more profoundly affected by sulfur (S) vacancy defects. In this work, monolayer MoS2 FETs with a specific amount of S-vacancy defects are fabricated using three approaches, namely chemical treatment (KOH solution), physical treatment (electron beam bombardment), and as-grown MoS2. Based on systematic studies on the correlation of the S-vacancies with both the device's electron transport characteristics and spectroscopic analysis, the NDR has been clearly observed in the defect-engineered monolayer MoS2 FETs with an S-vacancy (VS) amount of ∼5 ± 0.5%. Consequently, stable NDR behavior can be observed at room temperature, and its peak-to-valley ratio can also be effectively modulated via the gate electric field and light intensity. Through these results, it is envisioned that more electronic applications based on defect-engineered layered TMDs will emerge in the near future.

Twisted Light-Enhanced Photovoltaic Effect.

Twisted light carries a defined orbital angular momentum (OAM) that can enhance forbidden transitions in atoms and even semiconductors. Such attributes can possibly lead to enhancements of the material's photogenerated carriers through improved absorption of incident light photons. The interaction of twisted light and photovoltaic material is, thus, worth studying as more efficient photovoltaic cells are essential these days due to the need for reliable and sustainable energy sources. Two-dimensional (2D) MoS2, with its favorable optoelectronic properties, is a good platform to investigate the effects of twisted light on the photon absorption efficiency of the interacting material. This work, therefore, used twisted light as the exciting light source onto a MoS2 photovoltaic device. We observed that while incrementing the incident light's quantized OAM at fixed optical power, there are apparent improvements in the device's open-circuit voltage (VOC) and short-circuit current (ISC), implying enhancements of the photoresponse. We attribute these enhancements to the OAM of light that has facilitated improved optical absorption efficiency in MoS2. This study proposes a way of unlocking the potentials of 2D-MoS2 and envisions the employment of light's OAM for future energy device applications.

High-Frequency Graphene Base Hot-Electron Transistor.

The integration of graphene and other two-dimensional (2D) materials with existing silicon semiconductor technology is highly desirable. This is due to the diverse advantages and potential applications brought about by the consequent miniaturization of the resulting electronic devices. Nevertheless, such devices that can operate at very high frequencies for high-speed applications are eminently preferred. In this work, we demonstrate a vertical graphene base hot-electron transistor that performs in the radio frequency regime. Our device exhibits a relatively high current density (∼200 A/cm2), high common base current gain (α* ∼ 99.2%), and moderate common emitter current gain (ß* ∼ 2.7) at room temperature with an intrinsic current gain cutoff frequency of around 65 GHz. Furthermore, cutoff frequency can be tuned from 54 to 65 GHz by varying the collector-base bias. We anticipate that this proposed transistor design, built by the integrated 2D material and silicon semiconductor technology, can be a potential candidate to realize extra fast radio frequency tunneling hot-carrier electronics.