Highly efficient gate-tunable photocurrent generation in vertical heterostructures of layered materials

A self-encapsulated broadband phototransistor based on a hybrid of graphene and black phosphorus nanosheets

A phototransistor based on a hybrid of graphene and BP nanosheets with a facile fabrication method and remarkable performance is presented. Unlike previously reported single BP flake-based devices, this phototransistor employs diverse BP nanosheets with different sizes and layer numbers. The wet transfer process of graphene is exploited to integrate the liquid-exfoliated BP nanosheets into the device smoothly. Due to the diversity of BP nanosheets, the device demonstrates a broadband photo-response in the spectrum from 360 nm to 785 nm. The photo-response mechanism is revealed to be the photogating effect caused by the discrete BP nanosheets adsorbed on graphene. The phototransistor has a responsivity of 7.7 × 103 A W-1 in the near-UV region with a wide conductive channel of 200 μm. Moreover, the simplified wet transfer process of graphene leaves a self-encapsulated layer of PMMA on the as-prepared device, inducing a good atmospheric stability in the device. This report provides a valid, implantable, and facile strategy to apply BP nanosheets in a broadband, high-performing and air-stable photodetector.

General synthesis of two-dimensional van der Waals heterostructure arrays

Two-dimensional van der Waals heterostructures (vdWHs) have attracted considerable interest^1-4. However, most vdWHs reported so far  are created by an arduous micromechanical exfoliation and manual restacking process⁵, which-although versatile for proof-of-concept demonstrations^6-16 and fundamental studies^17-30-is clearly not scalable for practical technologies. Here we report a general synthetic strategy for two-dimensional vdWH arrays between metallic transition-metal dichalcogenides (m-TMDs) and semiconducting TMDs (s-TMDs). By selectively patterning nucleation sites on monolayer or bilayer s-TMDs, we precisely control the nucleation and growth of diverse m-TMDs with designable periodic arrangements and tunable lateral dimensions at the predesignated spatial locations, producing a series of vdWH arrays, including VSe₂/WSe₂, NiTe₂/WSe₂, CoTe₂/WSe₂, NbTe₂/WSe₂, VS₂/WSe₂, VSe₂/MoS₂ and VSe₂/WS₂. Systematic scanning transmission electron microscopy studies reveal nearly ideal vdW interfaces with widely tunable moiré superlattices. With the atomically clean vdW interface, we further show that the m-TMDs function as highly reliable synthetic vdW contacts for the underlying WSe₂ with excellent device performance and yield, delivering a high ON-current density of up to 900 microamperes per micrometre in bilayer WSe₂ transistors. This general synthesis of diverse two-dimensional vdWH arrays provides a versatile material platform for exploring exotic physics and promises a scalable pathway to high-performance devices.

Anisotropy of Excitons in Two-Dimensional Perovskite Crystals

Two-dimensional (2D) perovskites show great potential for optoelectronic applications due to their bandgap tunability, extremely large excition binding energy, and large crystal anisotropy compared with their three-dimensional counterparts. To fully explore exciton-based applications and improve their performance, it is essential to understand the exciton behavior in 2D perovskites. Here, we investigate exciton anisotropy within the crystallographic plane and cross plane of (C₄H₉NH₃)₂PbI₄ 2D perovskite crystals by polarization-resolved photoluminescence, reflection, and photoconductivity studies. We observe a polarization-dependent emission evolution and an enhanced self-trapped exciton emission with an oblique incident excitation from the cross plane. Furthermore, the anisotropy of excitons in (C₄H₉NH₃)₂PbI₄ 2D perovskite crystals is identified by polarization-resolved photoluminescence and photoconductivity measurement, and a completely opposite polarization-dependent behavior was observed for free excitons and self-trapped excitons. We attribute this different anisotropy to the existence of out-of-plane excitons and different optical selection rule for free excitons and self-trapped excitons. Our findings will shed light on designing and improving the performance of exciton-based optoelectronic devices in 2D perovskites.

Tensile Strain-Controlled Photogenerated Carrier Dynamics at the van der Waals Heterostructure Interface

Customizing the photogenerated carrier dynamics would make the two-dimensional (2D) materials highly adaptable to various application scenarios. On the basis of time-domain ab initio nonadiabatic molecular dynamics simulation, we find that 4% tensile strain can suppress the electron transfer at the van der Waals heterostructure MoS₂/WS₂ interface. Our analysis shows that after the electron-hole pair is excited in the K valley in WS₂ direct electron transfer from WS₂@K to MoS₂@K is very difficult because of the weak interlayer coupling in the K valley, and thus, it happens through the T valley as WS₂@K-MoS₂@T-MoS₂@K. When the tensile strain is applied, the energy of WS₂@K is decreased, resulting in the suppression of electron transfer. Our study suggests that tuning of the interlayer charge-transfer dynamics by external strain is possible, which provides valuable insights into the functional design of photonic devices based on 2D materials.

Tailoring Quantum Tunneling in a Vanadium-Doped WSe2/SnSe2 Heterostructure

2D van der Waals layered heterostructures allow for a variety of energy band offsets, which help in developing valuable multifunctional devices. However, p-n diodes, which are typical and versatile, are still limited by the material choice due to the fixed band structures. Here, the vanadium dopant concentration is modulated in monolayer WSe2 via chemical vapor deposition to demonstrate tunable multifunctional quantum tunneling diodes by vertically stacking SnSe2 layers at room temperature. This is implemented by substituting tungsten atoms with vanadium atoms in WSe2 to provoke the p-type doping effect in order to efficiently modulate the Fermi level. The precise control of the vanadium doping concentration is the key to achieving the desired quantum tunneling diode behaviors by tuning the proper band alignment for charge transfer across the heterostructure. By constructing a p-n diode for p-type V-doped WSe2 and heavily degenerate n-type SnSe2, the type-II band alignment at low V-doping concentration is clearly shown, which evolves into the type-III broken-gap alignment at heavy V-doping concentration to reveal a variety of diode behaviors such as forward diode, backward diode, negative differential resistance, and ohmic resistance.

Bridging the van der Waals Interface for Advanced Optoelectronic Devices

Van der Waals (vdW) heterostructures exhibit excellent optoelectronic properties and novel functionalities. However, their applicability is impeded due to the common issue of the tunneling barrier, which arises from the vdW gap; this significantly increases the injection resistance of the photoexcited carriers. Herein, a generic strategy is demonstrated to eliminate the vdW gap in a broad class of heterostructures. It is observed that the vdW gap in the interface is bridged via strong orbital hybridization between the interface dangling bonds of nonlayered chalcogenide semiconductors and the artificially induced vacancies of transition metal chalcogenides (TMDCs). The photoresponse times of bridged PbS/ReS₂ , PbS/MoSe₂ , and PbS/MoS₂ are ≈30, 51, and 43 µs, respectively. The photon-triggered on/off ratio of the bridged PbS/MoS₂ , ZnSe/MoS₂ , and ZnTe/MoS₂ heterostructures exceed 10⁶ , 10⁵ , and 10⁵ , respectively. These are several orders of magnitude higher than common vdW heterostructures. The findings obtained in this study present a versatile strategy for overcoming the performance limitations of vdW heterostructures.

Ferroelectric and dipole control of band alignment in the two dimensional InTe/In2Se3 heterostructure

Two dimensional (2D) ferroelectric materials are gaining growing attention due to their nontrival ferroelectricity, and the 2D ferroelectric heterostructures with tunable electronic, optoelectronic, or even magnetic properties, show many novel properties that do not exist in their constituents. In this work, by using the first-principles calculations, we investigate the ferroelectric and dipole control of electronic structures of the 2D ferroelectric heterostructure InTe/In₂Se₃. It is found that band alignment is closely dependent on the ferroelectric polarization of In₂Se₃. By switching the polarization of In₂Se₃, the band alignment of InTe/In₂Se₃ switches from a staggered (type II) to a straddling type (type I), and the band gap changes from indirect gap 0.76 eV to direct gap 0.15 eV. When the ferroelectric field of In₂Se₃ is reversed, the band alignment of InTe/In₂Se₃ switches from type-I to type-II, and the band gap changes from indirect gap 0.76 eV to direct gap 0.15 eV. In addition, we find that the interlayer dipole can also effectively modulate the band structure and induce the type-I to type-II band alignment transition. Our present results indicate that the 2D ferroelectric heterostructure with the tunable band alignment and band gap can be of great significance in the optoelectronic devices.

Schottky Barrier Variable Graphene/Multilayer-MoS2 Heterojunction Transistor Used to Overcome Short Channel Effects

A single-layer MoS₂ achieves excellent gate controllability within the nanoscale channel length of a field-effect transistor (FET) owing to an ultra-short screening length. However, multilayer MoS₂ (ML-MoS₂) is more vulnerable to short channel effects (SCEs) owing to its thickness and long screening length. We eliminated the SCEs in an ML-MoS₂ FET (thickness of 4-13 nm) at a channel length of sub-30 nm using a Schottky barrier (SB) variable graphene/ML-MoS₂ heterojunction. Although the band modulation in the ML-MoS₂ channel worsens with a decrease in the channel length, which is similar to the SCEs occurring in conventional FETs, the variable Fermi level (E_F) of a graphene electrode along the gate voltage allows control of the SB at the graphene/MoS₂ junction and backs up the current modulation through a variable SB. Electrical measurements and a theoretical band simulation demonstrate the efficient SB modulation of our graphene nanogap (GrNG) ML-MoS₂ FET with three distinct carrier transports along V_gs: a thermionic emission at a low SB, Fowler-Nordheim tunneling at a moderate SB, and direct tunneling at a high SB. Our GrNG FET shows an extremely high on-off current ratio of ∼10⁸, which is approximately three-orders of magnitude better than a previously reported metal nanogap (MeNG) FET and a self-aligned metal/graphene nanogap FET with a similar MoS₂ thickness. Our GrNG FET also exhibits a 100,000-times higher on-off ratio, 100-times lower subthreshold swing, and 10-times lower drain induced barrier.

Ultra-Broadband, High Speed, and High-Quantum-Efficiency Photodetectors Based on Black Phosphorus

Black phosphorus (BP), a narrow band gap semiconductor without out-of-plane dangling bonds, has shown promise for broadband and integrable photodetector applications. Simultaneously exhibiting high speed and high-efficiency operation, however, remains a critical challenge for current BP-based photodetectors. Here, we demonstrate a photodetector based on the BP-based van der Waals heterostructures. The developed photodetector enables broadband responses in the visible to mid-infrared range with external quantum efficiency ranging from 20 to 52% at room temperature. These results together with noise measurements indicate that the photodetector can detect light in the picowatt range. Furthermore, the demonstrated BP detector has ultrafast rise (1.8 ns) and fall (1.68 ns) times, and its photoresponse exhibits reproducible switching behavior even under consecutive and rapid light intensity modulations (2100 cycles, 200 MHz), as indicated by the eye-diagram measurement. By leveraging these features, we show our BP heterostructures can be configured as a point-like detector in a scanning confocal microscopy, useful for mid-infrared imaging applications.

Light-Driven WSe2-ZnO Junction Field-Effect Transistors for High-Performance Photodetection

Assembling nanomaterials into hybrid structures provides a promising and flexible route to reach ultrahigh responsivity by introducing a trap-assisted gain (G) mechanism. However, the high-gain photodetectors benefitting from long carrier lifetime often possess slow response time (t) due to the inherent G-t tradeoff. Here, a light-driven junction field-effect transistor (LJFET), consisting of an n-type ZnO belt as the channel material and a p-type WSe2 nanosheet as a photoactive gate material, to break the G-t tradeoff through decoupling the gain from carrier lifetime is reported. The photoactive gate material WSe2 under illumination enables a conductive path for externally applied voltage, which modulates the depletion region within the ZnO channel efficiently. The gain and response time are separately determined by the field effect modulation and the switching speed of LJFET. As a result, a high responsivity of 4.83 × 103 A W-1 with a gain of ≈104 and a rapid response time of ≈10 µs are obtained simultaneously. The LJFET architecture offers a new approach to realize high-gain and fast-response photodetectors without the G-t tradeoff.

Electronic and photocatalytic properties of two-dimensional boron phosphide/SiC van der Waals heterostructure with direct type-II band alignment: a first principles study

Designing van der Waals (vdW) heterostructures of two-dimensional materials is an efficient way to realize amazing properties as well as opening opportunities for applications in solar energy conversion and nanoelectronic and optoelectronic devices. In this work, we investigate the electronic, optical, and photocatalytic properties of a boron phosphide–SiC (BP–SiC) vdW heterostructure using first-principles calculations. The relaxed configuration is obtained from the binding energies, inter-layer distance, and thermal stability. We show that the BP–SiC vdW heterostructure has a direct band gap with type-II band alignment, which separates the free electrons and holes at the interface. Furthermore, the calculated absorption spectra demonstrate that the optical properties of the BP–SiC heterostructure are enhanced compared with those of the constituent monolayers. The intensity of optical absorption can reach up to about 10⁵ cm⁻¹. The band edges of the BP–SiC heterostructure are located at energetically favourable positions, indicating that the BP–SiC heterostructure is able to split water under working conditions of pH = 0–3. Our theoretical results provide not only a fascinating insight into the essential properties of the BP–SiC vdW heterostructure, but also helpful information for the experimental design of new vdW heterostructures.

We investigate the structural, electronic, optical and photocatalytic properties of boron phosphide and SiC monolayers and their corresponding van der Waals heterostructure by density functional theory.

Enhanced electrical and broad spectral (UV-Vis-NIR) photodetection in a Gr/ReSe2/Gr heterojunction

Excellent electrical and photoelectrical study of vertical integration by layered two-dimensional materials having gate tunable broad spectral (UV-Vis-NIR) light detection response.

Robust edge photocurrent response on layered type II Weyl semimetal WTe2

Photosensing and energy harvesting based on exotic properties of quantum materials and new operation principles have great potential to break the fundamental performance limit of conventional photodetectors and solar cells. Weyl semimetals have demonstrated novel optoelectronic properties that promise potential applications in photodetection and energy harvesting arising from their gapless linear dispersion and Berry field enhanced nonlinear optical effect at the vicinity of Weyl nodes. In this work, we demonstrate robust photocurrent generation at the edge of Td-WTe2, a type-II Weyl semimetal, due to crystalline-symmetry breaking along certain crystal fracture directions and possibly enhanced by robust fermi-arc type surface states. This edge response is highly generic and arises universally in a wide class of quantum materials with similar crystal symmetries. The robust and generic edge current response provides a charge separation mechanism for photosensing and energy harvesting over broad wavelength range.

Gate-Induced Metal-Insulator Transition in 2D van der Waals Layers of Copper Indium Selenide Based Field-Effect Transistors

The existence of an exquisite phenomenon such as a metal-insulator transition (MIT) in two-dimensional (2D) systems, where completely different electronic functionalities in the same system can emerge simply by regulating parameters such as charge carrier density in them, is noteworthy. Such tunability in material properties can lead to several applications where precise tuning of function specific properties are desirable. Here, we report on our observation on the occurrence of MIT in the 2D material system of copper indium selenide (CuIn₇Se₁₁). Clear evidence of the metallic nature of conductivity (σ) under the influence of electrostatic doping via the gate, which crosses over to an insulating phase upon lowering the temperature, was observed by investigating the temperature and gate dependence of σ in CuIn₇Se₁₁ field-effect transistor devices. At higher charge carrier densities (n > 10¹² cm^-1), we found that σ ∼ (n)^α with α ∼ 2, which suggests the presence of bare Coulomb impurity scattering within the studied range of temperature (280 K > T > 20 K). Our analysis of the conductivity data following the principles of percolation theory of transition where σ ∼ (n - n_C)^δ show that the critical percolation exponent δ(T) has average values ∼1.57 ± 0.27 and 1.02 ± 0.35 within the measured temperature range for the two devices and it is close to the 2D percolation exponent value of 1.33. We believe that the 2D MIT seen in our system is due to the charge density inhomogeneity caused by electrostatic doping and unscreened charge impurity scattering that leads to a percolation driven transition. The findings reported here for CuIn₇Se₁₁ system provide a different material platform to investigate MIT in 2D and are crucial in order to understand the fundamental basis of electronic interactions and charge-transport phenomenon in other unexplored 2D electron systems.

Graphene Sandwich Stable Perovskite Quantum-Dot Light-Emissive Ultrasensitive and Ultrafast Broadband Vertical Phototransistors

Dual-functional devices that can simultaneously detect light and emit light have a tremendous appeal for multiple applications, including displays, sensors, defense, and high-speed optical communication. Despite the tremendous efforts of scientists, the progress of integration of a phototransistor, where the built-in electric field separates the photogenerated excitons, and a light-emitting diode, where the radiative recombination can be enhanced by band offset, into a single device remains a challenge. Combining the superior properties of perovskite quantum dots (PQDs) and graphene, here we report a light-emissive, ultrasensitive, ultrafast, and broadband vertical phototransistor that can simultaneously act as an efficient photodetector and light emitter within a single device. The estimated value of the external quantum efficiency of the vertical phototransistor is ∼1.2 × 10¹⁰% with a photoresponsivity of >10⁹ A W^-1 and a response time of <50 μs, which exceed all the presently reported vertical phototransistor devices. We also demonstrate that the modulation of the Dirac point of graphene efficiently tunes both amplitude and polarity of the photocurrent. The device exhibits a green emission having a quantum efficiency of 5.6%. The moisture-insensitive and environmentally stable, light-emissive, ultrafast, and ultrasensitive broadband phototransistor creates a useful route for dual-functional optoelectronic devices.

Multifarious Interfaces, Band Alignments, and Formation Asymmetry of WSe2-MoSe2 Heterojunction Grown by Molecular-Beam Epitaxy

Monolayer (ML) transition-metal dichalcogenides (TMDs) continue to attract research attention, and the heterojunctions formed by vertically stacking or laterally stitching two different TMDs, e.g., MoSe₂ and WSe₂, may have many interesting electronic and optical properties and thus are at the center stage of current research. Experimentally realizing such heterojunctions with desired interface morphologies and electronic properties is of great demand. In this work, we report a diverse interface structure in molecular-beam epitaxial WSe₂-MoSe₂ heterojunction. The corresponding electronic bands show type-II band alignment for both monolayer ML-ML and ML-bilayer lateral junctions irrespective of the presence or absence of step states. Interestingly, a strong anisotropy in lateral heterojunction formation is observed, where sharp interfaces are obtained only when WSe₂ deposition precedes MoSe₂. Reversing the deposition order leads to alloying of the two materials without a notable boundary. This is explained by a step segregation process as suggested by the first-principles total energy calculations.

Emerging Devices Based on Two-Dimensional Monolayer Materials for Energy Harvesting

Two-dimensional (2-D) materials of atomic thickness have attracted considerable interest due to their excellent electrical, optoelectronic, mechanical, and thermal properties, which make them attractive for electronic devices, sensors, and energy systems. Scavenging the otherwise wasted energy from the ambient environment into electrical power holds promise to address the emerging energy needs, in particular for the portable and wearable devices. The versatile properties of 2-D materials together with their atomically thin body create diverse possibilities for the conversion of ambient energy. The present review focuses on the recent key advances in emerging energy-harvesting devices based on monolayer 2-D materials through various mechanisms such as photovoltaic, thermoelectric, piezoelectric, triboelectric, and hydrovoltaic devices, as well as progress for harvesting the osmotic pressure and Wi-Fi wireless energy. The representative achievements regarding the monolayer heterostructures and hybrid devices are also discussed. Finally, we provide a discussion of the challenges and opportunities for 2-D monolayer material-based energy-harvesting devices in the development of self-powered electronics and wearable technologies.

Passivation of Layered Gallium Telluride by Double Encapsulation with Graphene

Layered semiconductor gallium telluride (GaTe) undergoes a rapid structural transition to a degraded phase in ambient conditions, limiting its utility in devices such as optical switches. In this work, we demonstrate that the degradation process in GaTe flakes can be slowed down dramatically via encapsulation with graphene. Through examining Raman signatures of degradation, we show that the choice of substrate significantly impacts the degradation rate and that the process is accelerated by the transfer of GaTe to hydrophilic substrates such as SiO₂/Si. We find that double encapsulation with both top and bottom graphene layers can extend the lifetime of the material for several weeks. The photoresponse of flakes encapsulated in this way is only reduced by 17.6 ± 0.4% after 2 weeks, whereas unencapsulated flakes display no response after this time. Our results demonstrate the potential for alternative, van der Waals material-based passivation strategies in unstable layered materials and highlight the need for careful selection of substrates for 2D electronic devices.

Phase Transition and Superconductivity Enhancement in Se-Substituted MoTe2 Thin Films

Consecutively tailoring few-layer transition metal dichalcogenides MX₂ from 2H to T_d phase may realize the long-sought topological superconductivity in a single material system by incorporating superconductivity and the quantum spin Hall effect together. Here, this study demonstrates that a consecutive structural phase transition from T_d to 1T' to 2H polytype can be realized by increasing the Se concentration in Se-substituted MoTe₂ thin films. More importantly, the Se-substitution is found to dramatically enhance the superconductivity of the MoTe₂ thin film, which is interpreted as the introduction of two-band superconductivity. The chemical-constituent-induced phase transition offers a new strategy to study the s^+- superconductivity and the possible topological superconductivity, as well as to develop phase-sensitive devices based on MX₂ materials.

Atomic mechanism of strong interactions at the graphene/sapphire interface

For atomically thin two-dimensional materials, interfacial effects may dominate the entire response of devices, because most of the atoms are in the interface/surface. Graphene/sapphire has great application in electronic devices and semiconductor thin-film growth, but the nature of this interface is largely unknown. Here we find that the sapphire surface has a strong interaction with some of the carbon atoms in graphene to form a C-O-Al configuration, indicating that the interface interaction is no longer a simple van der Waals interaction. In addition, the structural relaxation of sapphire near the interface is significantly suppressed and very different from that of a bare sapphire surface. Such an interfacial C-O-Al bond is formed during graphene growth at high temperature. Our study provides valuable insights into understanding the electronic structures of graphene on sapphire and remote control of epitaxy growth of thin films by using a graphene–sapphire substrate.

Understanding the atomic arrangement and binding nature of 2D materials with substrates is crucial to understand their properties and utilize their functions. Here, authors report that at high temperature graphene and α-Al₂O₃ substrate form a C-O-Al bond, having strong interactions, while interfacial structural relaxations of sapphire remain suppressed.

Photocurrent Direction Control and Increased Photovoltaic Effects in All-2D Ultrathin Vertical Heterostructures Using Asymmetric h-BN Tunneling Barriers

Two-dimensional (2D) materials are atomically thick and without out-of-plane dangling bonds. As a result, they could break the confinement of lattice matching, and thus can be freely mixed and matched together to construct vertical van der Waals heterostructures. Here, we demonstrated an asymmetrical vertical structure of graphene/hexagonal boron nitride (h-BN)/tungsten disulfide (WS₂)/graphene using all chemical vapor deposition grown 2D materials. Three building blocks are utilized in this construction: conductive graphene as a good alternative for the metal electrode due to its tunable Fermi level and ultrathin nature, semiconducting transition-metal dichalcogenides (TMDs) as an ultrathin photoactive material, and insulating h-BNas a tunneling barrier. Such an asymmetrical vertical structure exhibits a much stronger photovoltaic effect than the symmetrical vertical one without h-BN. By changing the sequence of h-BN in the vertical stack, we could even control the electron flow direction. Also, improvement has been further made by increasing the thickness of h-BN. The photovoltaic effect is attributed to different possibilities of excited electrons on TMDs to migrate to top and bottom graphene electrodes, which is caused by potential differences introduced by an insulating h-BN layer. This study shows that h-BN could be effectively used as a tunneling barrier in the asymmetrical vertical heterostructure to improve photovoltaic effect and control the electron flow direction, which is crucial for the design of other 2D vertical heterostructures to meet various needs of electronic and optoelectronic devices.

Vertically Stacked CVD-Grown 2D Heterostructure for Wafer-Scale Electronics

This paper demonstrates, for the first time, wafer-scale graphene/MoS₂ heterostructures prepared by chemical vapor deposition (CVD) and their application in vertical transistors and logic gates. A CVD-grown bulk MoS₂ layer is utilized as the vertical channel, whereas CVD-grown monolayer graphene is used as the tunable work-function electrode. The short vertical channel of the transistor is formed by sandwiching bulk MoS₂ between the bottom indium tin oxide (ITO, drain electrode) and the top graphene (source electrode). The electron injection barriers at the graphene-MoS₂ junction and ITO-MoS₂ junction are modulated effectively through variation of the Schottky barrier height and its effective barrier width, respectively, because of the work-function tunability of the graphene electrode. The resulting vertical transistor with the CVD-grown MoS₂/graphene heterostructure exhibits a current density exceeding 7 A/cm², a subthreshold swing of 410 mV/dec, and an on-off current ratio exceeding 10³. The large-area synthesis, transfer, and patterning processes of both semiconducting MoS₂ and metallic graphene facilitate construction of a wafer-scale array of transistors and logic gates such as NOT, NAND, and NOR.

Gas Phase Electrodeposition Enabling the Programmable Three-Dimensional Growth of a Multimodal Room Temperature Nanobridge Gas Sensor Array

Parallel three-dimensional (3D) growth of different nanomaterials with submicrometer resolution is a promising approach to overcome some technological and economic limits encountered in planar integrated homogeneous films. The programmable multimaterial gas phase nanoparticle electrodeposition concept enables the fabrication of a 3D multimodal conductometric gas sensor array. The approach requires the deposition of more than one nanomaterial to achieve orthogonal sensing capabilities and multigas sensitivity and selectivity. The demonstrated "electronic nose"-like array contains 1080 self-aligning 3D nanobridge-based electrical connections of platinum, nickel oxide, and gold on a single chip. The nanobridges form through a nearest neighbor Coulombic interaction. Each gas sensitive bridge is a 3D structure composed of a porous but electrically conducting nanoparticle network. It was found that this architecture is unique, because it does not require external heating to operate. A trimodal sensor array will be demonstrated to detect various levels of ammonia (NH₃), carbon monoxide (CO), and hydrogen sulfide (H₂S). A 3 × 3 transfer matrix will be discussed to correlate the recorded signals with the gas composition.

Versatile Electronic Devices Based on WSe2/SnSe2 Vertical van der Waals Heterostructures

Van der Waals heterostructures formed by stacking of various two-dimensional materials are promising in electronic applications. However, the performances of most reported electronic devices based on van der Waals heterostructures are far away from those of existing (Si, Ge, and III-V bulk material based) technologies. Here, we report high-performance heterostructure devices based on vertically stacked tungsten diselenide and tin diselenide. Due to the unique band alignment and the atomic thickness of the material, both charge carrier transport and energy barrier can be effectively modulated by the applied electrical field. As a result, the heterostructure devices show superb characteristics, with a high current on/off ratio of ∼3 × 10⁸, an average subthreshold slope of 126 mV/dec over 5 dec of current change due to band-to-band tunneling, an ultrahigh rectification ratio of ∼3 × 10⁸, and a current density of more than 10⁴ A/cm². Furthermore, a small signal half-wave rectifier circuit based on a majority-carrier-transport-dominated diode is successfully demonstrated, showing great potential in future high-speed electronic applications.

Highly Sensitive, Fast Graphene Photodetector with Responsivity >106 A/W Using a Floating Quantum Well Gate

Graphene, owing to its zero-band-gap electronic structure, is promising as an absorption material for ultra-wideband photodetection applications. However, graphene-absorption-based detectors inherently suffer from poor responsivity because of weak absorption and fast photocarrier recombination, limiting their viability for low-intensity light detection. Here, we use a graphene/WS₂/MoS₂ vertical heterojunction to demonstrate a highly sensitive photodetector, where the graphene layer serves dual purposes, namely, as the light absorption layer and also as the carrier conduction channel, thus maintaining the broadband nature of the photodetector. A fraction of the photoelectrons in graphene encounter ultrafast interlayer transfer to a floating monolayer MoS₂ quantum well, providing a strong quantum-confined photogating effect. The photodetector shows a responsivity of 4.4 × 10⁶ A/W at 30 fW incident power, outperforming photodetectors reported till date where graphene is used as a light absorption material by several orders. In addition, the proposed photodetector exhibits an extremely low noise equivalent power of <4 fW/ Hz and a fast response (∼milliseconds) with zero reminiscent photocurrent. The findings are attractive toward the demonstration of a graphene-based highly sensitive, fast, broadband photodetection technology.

Ultrahigh-Performance Optoelectronics Demonstrated in Ultrathin Perovskite-Based Vertical Semiconductor Heterostructures

Two-dimensional (2D) atomic layered semiconductor (e.g., transition metal dichalcogenides, TMDCs) heterostructures display diverse novel interfacial carrier properties and have potential applications in constructing next generation highly compact electronics and optoelectronics devices. However, the optoelectronic performance of this kind of semiconductor heterostructures has difficulty reaching the expectations of practical applications, due to the intrinsic weak optical absorption of the atomic-thick component layers. Here, combining the extraordinary optoelectronic properties of quantum-confined organic-inorganic hybrid perovskite (PVK), we design an ultrathin PVK/TMDC vertical semiconductor heterostructure configuration and realize the controlled vapor-phase growth of highly crystalline few-nanometer-thick PVK layers on TMDCs monolayers. The achieved ultrathin PVKs show strong thickness-induced quantum confinement effect, and simultaneously form band alignment-engineered heterointerfaces with the underlying TMDCs, resulting in highly efficient interfacial charge separation and transport. Electrical devices constructed with the as-grown ultrathin PVK/WS₂ heterostructures show ambipolar transport originating from p-type PVK and n-type WS₂, and exhibit outstanding optoelectronic characteristics, with the optimized response time and photoresponsivity reaching 64 μs and 11174.2 A/W, respectively, both of which are 4 orders of magnitude better than the heterostructures with a thick PVK layer, and also represent the best among all previously reported 2D layered semiconductor heterostructures. This work provides opportunities for 2D vertical semiconductor heterostructures via incorporating ultrathin PVK layers in high-performance integrated optoelectronics.

Ultrahigh Gauge Factor in Graphene/MoS2 Heterojunction Field Effect Transistor with Variable Schottky Barrier

Piezoelectricity of transition metal dichalcogenides (TMDs) under mechanical strain has been theoretically and experimentally studied. Powerful strain sensors using Schottky barrier variation in TMD/metal junctions as a result of the strain-induced lattice distortion and associated ion-charge polarization were demonstrated. However, the nearly fixed work function of metal electrodes limits the variation range of a Schottky barrier. We demonstrate a highly sensitive strain sensor using a variable Schottky barrier in a MoS₂/graphene heterostructure field effect transistor (FET). The low density of states near the Dirac point in graphene allows large modulation of the graphene Fermi level and corresponding Schottky barrier in a MoS₂/graphene junction by strain-induced polarized charges of MoS₂. Our theoretical simulations and temperature-dependent electrical measurements show that the Schottky barrier change is maximized by placing the Fermi level of the graphene at the charge neutral (Dirac) point by applying gate voltage. As a result, the maximum Schottky barrier change (ΔΦ_SB) and corresponding current change ratio under 0.17% strain reach 118 meV and 978, respectively, resulting in an ultrahigh gauge factor of 575 294, which is approximately 500 times higher than that of metal/TMD junction strain sensors (1160) and 140 times higher than the conventional strain sensors (4036). The ultrahigh sensitivity of graphene/MoS₂ heterostructure FETs can be developed for next-generation electronic and mechanical-electronic devices.

High performance tin diselenide photodetectors dependent on thickness: a vertical graphene sandwiched device and interfacial mechanism

In recent years, with the rapid development of transfer technologies related to graphene and other two-dimensional layered materials (2DLMs), graphene sandwiched 2DLMs have been confirmed to be outstanding tunneling and optoelectronic devices. Here, compared to the planar SnSe2-Au device, the SnSe2 device with different thicknesses (12-256 nm) is incorporated into graphene sandwiched structures for photodetection. The results indicate that the photoresponse properties are dependent on the thickness and gate voltage. In particular, under 532 nm illumination and at a Vg of +80 V, the SnSe2 device with a thickness of 96.5 nm shows an impressively high responsivity of 1.3 × 103 A W-1, an external quantum efficiency of 3 × 105%, and a detectivity of 1.2 × 1012 Jones. Besides, a high response speed (a rise time of 30.2 ms and a decay time of 27.2 ms) and flat photoswitching behavior are achieved without the gate voltage. In addition, the intrinsic mechanisms are further discussed through the relative spatial potential difference and the band alignment diagrams of the graphene-SnSe2-graphene and Au-SnSe2-Au structures. These findings indicate that SnSe2 has great potential for practical applications in next generation high performance optoelectronics.

Efficient Gate Modulation in a Screening-Engineered MoS2/Single-Walled Carbon Nanotube Network Heterojunction Vertical Field-Effect Transistor

In this report, a screening-engineered carbon nanotube (CNT) network/MoS₂/metal heterojunction vertical field effect transistor (CNT-VFET) is fabricated for an efficient gate modulation independent of the drain voltage. The gate field in the CNT-VFET transports through the empty space of the CNT network without any screening layer and directly modulates the MoS₂ semiconductor energy band, while the gate field from the Si back gate is mostly screened by the graphene layer. Consequently, the on/off ratio of CNT-VFET maintained 10³ in overall drain voltages, which is 10 times and 1000 times higher than that of the graphene (Gr) VFET at V_sd = 0.1 (ratio = 81.9) and 1 V (ratio = 3), respectively. An energy band diagram simulation shows that the Schottky barrier modulation of CNT/MoS₂ contact along the sweeping gate bias is independent of the drain voltage. On the other hand, the gate modulation of Gr/MoS₂ is considerably reduced with increased drain voltage because more electrons are drawn into the graphene electrode and screens the gate field by applying a higher drain voltage to the graphene/MoS₂/metal capacitor.

Graphene-WS2 heterostructures by a lithography free method: their electrical properties

We have developed a lithography free technique for the fabrication of two-dimensional (2D) material based heterostructures. We fabricated graphene-WS₂ heterostructured devices using a transmission electron microscope grid as a shadow mask and their electrical transport characteristics were studied by electrical and magneto transport measurements. Graphene was directly deposited on a Si/SiO₂ substrate by radio frequency plasma enhanced chemical vapor deposition. WS₂ was synthesized by first depositing WO₃ followed by sulfurization. The temperature dependence of the resistance and magnetoresistance are measured for graphene, WS₂, and graphene-WS₂ heterostructure. At low temperatures, the transport is found to follow the variable-range hopping (VRH) process, where logarithmic R exhibits a T ^-1/3 temperature dependence, an evidence for the 2D Mott VRH transport. The measured low-field magnetoresistance also exhibits a quadratic magnetic field dependence ∼B ², consistent with the 2D Mott VRH transport.

Gate-Tunable and Programmable n-InGaAs/Black Phosphorus Heterojunction Diodes

Semiconductor heterostructures have enabled numerous applications in diodes, photodetectors, junction field-effect transistors, and memory devices. Two-dimensional (2D) materials and III-V compound semiconductors are two representative materials providing excellent heterojunction platforms for the fabrication of heterostructure devices. The marriage between these semiconductors with completely different crystal structures may enable a new heterojunction with unprecedented physical properties. In this study, we demonstrate a multifunctional heterostructure device based on 2D black phosphorus and n-InGaAs nanomembrane semiconductors that exhibit gate-tunable, photoresponsive, and programmable diode characteristics. The device exhibits clear rectification with a large gate-tunable forward current, which displays rectification and switching with a maximum rectification ratio of 4600 and an on/off ratio exceeding 10⁵, respectively. The device also offers nonvolatile memory properties, including large hysteresis and stable retention of storage charges. By combining the memory and gate-tunable rectifying properties, the rectification ratio of the device can be controlled and memorized from 0.06 to 400. Moreover, the device can generate three different electrical signals by combining a photoresponsivity of 0.704 A/W with the gate-tunable property, offering potential applications, for example, multiple logic operator. This work presents a heterostructure design based on 2D and III-V compound semiconductors, showing unique physical properties for the development of multifunctional heterostructure devices.

Next-Generation Multifunctional Carbon-Metal Nanohybrids for Energy and Environmental Applications

Nanotechnology has unprecedentedly revolutionized human societies over the past decades and will continue to advance our broad societal goals in the coming decades. The research, development, and particularly the application of engineered nanomaterials have shifted the focus from "less efficient" single-component nanomaterials toward "superior-performance", next-generation multifunctional nanohybrids. Carbon nanomaterials (e.g., carbon nanotubes, graphene family nanomaterials, carbon dots, and graphitic carbon nitride) and metal/metal oxide nanoparticles (e.g., Ag, Au, CdS, Cu₂O, MoS₂, TiO₂, and ZnO) combinations are the most commonly pursued nanohybrids (carbon-metal nanohybrids; CMNHs), which exhibit appealing properties and promising multifunctionalities for addressing multiple complex challenges faced by humanity at the critical energy-water-environment (EWE) nexus. In this frontier review, we first highlight the altered and newly emerging properties (e.g., electronic and optical attributes, particle size, shape, morphology, crystallinity, dimensionality, carbon/metal ratio, and hybridization mode) of CMNHs that are distinct from those of their parent component materials. We then illustrate how these important newly emerging properties and functions of CMNHs direct their performances at the EWE nexus including energy harvesting (e.g., H₂O splitting and CO₂ conversion), water treatment (e.g., contaminant removal and membrane technology), and environmental sensing and in situ nanoremediation. This review concludes with identifications of critical knowledge gaps and future research directions for maximizing the benefits of next-generation multifunctional CMNHs at the EWE nexus and beyond.

A Scalable, Solution-Based Approach to Tuning the Solubility and Improving the Photoluminescence of Chemically Exfoliated MoS2

MoS₂ are two-dimensional (2D) materials that exhibit emerging photoluminescence (PL) at the monolayer level and have potential optoelectronic applications. Monolayers of MoS₂ typically achieved by mechanical exfoliation (Me), chemical vapor deposition (CVD), and chemical exfoliation (Ce) via lithium intercalation contain numerous defects that significantly reduce their PL efficiency. Several studies have reported overcoming poor PL in mechanically exfoliated and CVD-grown MoS₂, but such studies for chemically exfoliated MoS₂ (Ce-MoS₂) have not been reported. Here, we report a solution-based method of enhancing the PL of Ce-MoS₂ by reacting with molecules with suitable functional groups at high temperatures. Reaction with dodecanethiol (DDT) generates PL that is more intense than mechanically exfoliated MoS₂ (Me-MoS₂) with high crystallinity and has a significantly broader range of wavelengths. Based on ultraviolet-visible, Fourier transform infrared, X-ray photoemission, and PL spectroscopy as well as transmission electron and PL imaging, we propose that the present method modifies PL properties of Ce-MoS₂ by simultaneously annealing, replacing molybdenum-oxygen with molybdenum-sulfur bonds, inducing strain, and generating a nanopolycrystalline structure. This work points to such defect engineering using molecules as an effective means to modify the properties of Ce-MoS₂ and layered transition-metal dichalcogenides more generally.

Engineering the Dimensional Interface of BiVO4-2D Reduced Graphene Oxide (RGO) Nanocomposite for Enhanced Visible Light Photocatalytic Performance

Graphene as a two-dimensional (2D) nanoplatform is beneficial for assembling a 2D heterojunction photocatalytic system to promote electron transfer in semiconductor composites. Here a BiVO₄ nanosheets/reduced graphene oxide (RGO) based 2D-2D heterojunction photocatalytic system as well as 0D-2D BiVO₄ nanoparticles/RGO and 1D-2D BiVO₄ nanotubes/RGO nanocomposites are fabricated by a feasible solvothermal process. During the synthesis; the growth of BiVO₄ and the intimate interfacial contact between BiVO₄ and RGO occur simultaneously. Compared to 0D-2D and 1D-2D heterojunctions, the resulting 2D-2D BiVO₄ nanosheets/RGO composites yield superior chemical coupling; leading to exhibit higher photocatalytic activity toward the degradation of acetaminophen under visible light irradiation. Photoluminescence (PL) and photocurrent experiments revealed that the apparent electron transfer rate in 2D-2D BiVO₄ nanosheets/RGO composites is faster than that in 0D-2D BiVO₄ nanoparticles/RGO composites. The experimental findings presented here clearly demonstrate that the 2D-2D heterojunction interface can highlight the optoelectronic coupling between nanomaterials and promote the electron–hole separation. This study will motivate new developments in dimensionality factors on designing the heterojunction photocatalysts and promote their photodegradation photocatalytic application in environmental issues.

Nonadiabatic Dynamics Simulations Reveal Distinct Effects of the Thickness of PTB7 on Interfacial Electron and Hole Transfer Dynamics in PTB7@MoS2 Heterostructures

Mixed-dimensional hybrid heterostructures have attracted a lot of experimental attention because they can provide an ideal charge-separated interface for optoelectronic and photonic applications. In this Letter, we have employed first-principles DFT calculations and nonadiabatic dynamics simulations to explore photoinduced interfacial electron and hole transfer processes in two PTB7- nL@MoS₂ models ( n = 1 and 5). The interfacial electron transfer is found to be ultrafast and completes within ca. 10 fs in both PTB7-1L@MoS₂ and PTB7-5L@MoS₂ models, which demonstrates that the electron transfer is not sensitive to the thickness of the PTB7 polymer. Differently, the interfacial hole transfer is sensitive to the thickness of the PTB7 polymer. The transfer time is estimated to be ca. 70 ps in PTB7-1L@MoS₂, while it is significantly accelerated to ca. 1 ps in PTB7-5L@MoS₂. Finally, we have found that the electron transfer is mainly controlled by adiabatic electron evolution, whereas in the hole transfer, nonadiabatic hoppings play a dominant role. These findings are useful for the design of excellent charge-separated interfaces of mixed-dimensional TMD-based heterojunctions for a variety of optoelectronic applications.

Photoinduced Doping To Enable Tunable and High-Performance Anti-Ambipolar MoTe2/MoS2 Heterotransistors

van der Waals (vdW) p-n heterojunctions formed by two-dimensional nanomaterials exhibit many physical properties and deliver functionalities to enable future electronic and optoelectronic devices. In this report, we demonstrate a tunable and high-performance anti-ambipolar transistor based on MoTe₂/MoS₂ heterojunction through in situ photoinduced doping. The device demonstrates a high on/off ratio of 10⁵ with a large on-state current of several micro-amps. The peak position of the drain-source current in the transfer curve can be adjusted through the doping level across a large dynamic range. In addition, we have fabricated a tunable multivalue inverter based on the heterojunction that demonstrates precise control over its output logic states and window of midlogic through source-drain bias adjustment. The heterojunction also exhibits excellent photodetection and photovoltaic performances. Dynamic and precise modulation of the anti-ambipolar transport properties may inspire functional devices and applications of two-dimensional nanomaterials and their heterostructures of various kinds.

Plasmon Excited Ultrahot Carriers and Negative Differential Photoresponse in a Vertical Graphene van der Waals Heterostructure

Photogenerated nonequilibrium hot carriers play a key role in graphene's intriguing optoelectronic properties. Compared to conventional photoexcitation, plasmon excitation can be engineered to enhance and control the generation and dynamics of hot carriers. Here, we report an unusual negative differential photoresponse of plasmon-induced "ultrahot" electrons in a graphene-boron nitride-graphene tunneling junction. We demonstrate nanocrescent gold plasmonic nanostructures that substantially enhance the absorption of long-wavelength photons whose energy is greatly below the tunneling barrier and significantly boost the electron thermalization in graphene. We further analyze the generation and transfer of ultrahot electrons under different bias and power conditions. We find that the competition among thermionic emission, the carrier-cooling effect, and the field effect results in a hitherto unusual negative differential photoresponse in the photocurrent-bias plot. Our results not only exemplify a promising platform for detecting low-energy photons, enhancing the photoresponse, and reducing the dark current but also reveal the critically coupled pathways for harvesting ultrahot carriers.

Coexistence of Different Charge-Transfer Mechanisms in the Hot-Carrier Dynamics of Hybrid Plasmonic Nanomaterials

Hot-carrier dynamics at the interfaces of semiconductors and nanoclusters is of significant importance for photovoltaic and photocatalytic applications. Plasmon-driven charge separation processes are considered to be only dependent on the type of donor-acceptor interactions, that is, the conventional hot-electron-transfer mechanism for van der Waals interactions and the plasmon-induced interfacial charge-transfer transition mechanism for chemical bonds. Here, we demonstrate that the two mechanisms can coexist in a nanoparticle-semiconductor hybrid nanomaterial, both leading to faster transfer than carrier relaxation. The origin of the two mechanisms is attributed to the spatial polarization of the excited hot carriers, where the longitudinal state couples to semiconductors more strongly than the transverse state. Our findings provide a new insight into the photoinduced carrier dynamics, which is relevant for many applications in solar energy conversion, including efficient water splitting, photocatalysis, and photovoltaics.

Oxygen-Induced In Situ Manipulation of the Interlayer Coupling and Exciton Recombination in Bi2Se3/MoS2 2D Heterostructures

Two-dimensional (2D) heterostructures are more than a sum of the parent 2D materials, but are also a product of the interlayer coupling, which can induce new properties. In this paper, we present a method to tune the interlayer coupling in Bi₂Se₃/MoS₂ 2D heterostructures by regulating the oxygen presence in the atmosphere, while applying laser or thermal energy. Our data suggest that the interlayer coupling is tuned through the diffusive intercalation and deintercalation of oxygen molecules. When one layer of Bi₂Se₃ is grown on monolayer MoS₂, an influential interlayer coupling is formed, which quenches the signature photoluminescence (PL) peaks. However, thermally treating in the presence of oxygen disrupts the interlayer coupling, facilitating the emergence of the MoS₂ PL peak. Our density functional theory calculations predict that intercalated oxygen increases the interlayer separation ∼17%, disrupting the interlayer coupling and inducing the layers to behave more electronically independent. The interlayer coupling can then be restored by thermally treating in N₂ or Ar, where the peaks will requench. Hence, this is an interesting oxygen-induced switching between "non-radiative" and "radiative" exciton recombination. This switching can also be accomplished locally, controllably, and reversibly using a low-power focused laser, while changing the environment from pure N₂ to air. This allows for the interlayer coupling to be precisely manipulated with submicron spatial resolution, facilitating site-programmable 2D light-emitting pixels whose emission intensity could be precisely varied by a factor exceeding 200×. Our results show that these atomically thin 2D heterostructures may be excellent candidates for oxygen sensing.

Modulation of terahertz radiation from graphene surface plasmon polaritons via surface acoustic wave

We present a theoretical study of terahertz (THz) radiation induced by surface plasmon polaritons (SPPs) on a graphene layer under modulation by a surface acoustic wave (SAW). In our gedanken experiment, SPPs are excited by an electron beam moving on a graphene layer situated on a piezoelectric MoS₂ flake. Under modulation by the SAW field, charge carriers are periodically distributed over the MoS₂ flake, and this causes periodically distributed permittivity. The periodic permittivity structure of the MoS₂ flake folds the SPP dispersion curve back into the center of the first Brillouin zone, in a manner analogous to a crystal, leading to THz radiation emission with conservation of the wavevectors between the SPPs and the electromagnetic waves. Both the frequency and the intensity of the THz radiation are tuned by adjusting the chemical potential of the graphene layer, the MoS₂ flake doping density, and the wavelength and period of the external SAW field. A maximum energy conversion efficiency as high as ninety percent was obtained from our model calculations. These results indicate an opportunity to develop highly tunable and integratable THz sources based on graphene devices.

Electrical control of spatial resolution in mixed-dimensional heterostructured photodetectors

Low-dimensional nanomaterials, such as one-dimensional (1D) nanomaterials and layered 2D materials, have exhibited significance for their respective unique electronic and optoelectronic properties. Here we show that a mixed-dimensional heterostructure with building blocks from multiple dimensions will present a synergistic effect on photodetection. A carbon nanotube (CNT)-[Formula: see text]-graphene photodetector is representative on this issue. Its spatial resolution can be electrically switched between high-resolution mode (HRM) and low-resolution mode (LRM) revealed by scanning photocurrent microscopy (SPCM). The reconfigurable spatial resolution can be attributed to the asymmetric geometry and the gate-tunable Fermi levels of these low-dimensional materials. Significantly, an interference fringe with 334 nm in period was successfully discriminated by the device working at HRM, confirming the efficient electrical control. Electrical control of spatial resolution in CNT-[Formula: see text]-graphene devices reveals the potential of the mixed-dimensional architectures in future nanoelectronics and nano-optoelectronics.

Conformal hexagonal-boron nitride dielectric interface for tungsten diselenide devices with improved mobility and thermal dissipation

Relatively low mobility and thermal conductance create challenges for application of tungsten diselenide (WSe₂) in high performance devices. Dielectric interface is of extremely importance for improving carrier transport and heat spreading in a semiconductor device. Here, by near-equilibrium plasma-enhanced chemical vapour deposition, we realize catalyst-free growth of poly-crystalline two-dimensional hexagonal-boron nitride (2D-BN) with domains around 20~ 200 nm directly on SiO₂/Si, quartz, sapphire, silicon or SiO₂/Si with three-dimensional patterns at 300 °C. Owing to the atomically-clean van-der-Walls conformal interface and the fact that 2D-BN can better bridge the vibrational spectrum across the interface and protect interfacial heat conduction against substrate roughness, both improved performance and thermal dissipation of WSe₂ field-effect transistor are realized with mobility around 56~ 121 cm² V⁻¹ s⁻¹ and saturated power intensity up to 4.23 × 10³ W cm⁻². Owing to its simplicity, conformal growth on three-dimensional surface, compatibility with microelectronic process, it has potential for application in future two-dimensional electronics.

Plasma-enhanced chemical vapour deposition (PECVD) is an industrially compatible microelectronics technology. Here, the authors use PECVD to obtain low-temperature, catalyst-free growth of poly-crystalline two-dimensional hexagonal-boron nitride, thus enabling superior thermal dissipation in WSe₂ field-effect transistors with mobility up to 121 cm² V⁻¹ s⁻¹.

WSe2 Photovoltaic Device Based on Intramolecular p-n Junction

High quality p-n junctions based on 2D layered materials (2DLMs) are urgent to exploit, because of their unique properties such as flexibility, high absorption, and high tunability which may be utilized in next-generation photovoltaic devices. Based on transfer technology, large amounts of vertical heterojunctions based on 2DLMs are investigated. However, the complicated fabrication process and the inevitable defects at the interfaces greatly limit their application prospects. Here, an in-plane intramolecular WSe₂ p-n junction is realized, in which the n-type region and p-type region are chemically doped by polyethyleneimine and electrically doped by the back-gate, respectively. An ideal factor of 1.66 is achieved, proving the high quality of the p-n junction realized by this method. As a photovoltaic detector, the device possesses a responsivity of 80 mA W^-1 (≈20% external quantum efficiency), a specific detectivity of over 10¹¹ Jones and fast response features (200 µs rising time and 16 µs falling time) at zero bias, simultaneously. Moreover, a large open-circuit voltage of 0.38 V and an external power conversion efficiency of ≈1.4% realized by the device also promises its potential in microcell applications.

Bias-Modulated High Photoelectric Response of Graphene-Nanocrystallite Embedded Carbon Film Coated on n-Silicon

We propose that bias-modulated graphene-nanocrystallites (GNs) grown vertically can enhance the photoelectric property of carbon film coated on n-Si substrate. In this work, GN-embedded carbon (GNEC) films were deposited by the electron cyclotron resonance (ECR) sputtering technique. Under a reverse diode bias which lifts the Dirac point of GNs to a higher value, the GNEC film/n-Si device achieved a high photocurrent responsivity of 0.35 A/W. The bias-modulated position of the Dirac point resulted in a tunable ON/OFF ratio and a variable spectral response peak. Moreover, due to the standing structured GNs keeping the transport channels, a response time of 2.2 μs was achieved. This work sheds light on the bias-control wavelength-sensitive photodetector applications.

Investigating the interlayer electron transport and its influence on the whole electric properties of black phosphorus

Two-dimensional (2D) nanomaterials have attracted great attention in next generation electronic and optoelectronic technologies due to the unique layered structure and excellent physical and chemical properties. However, the mechanism of transmission along the vertical direction of 2D semiconductor materials has not been investigated. Here, we use first-principles calculations to explore the bandgap energies along different directions, and fabricate a vertical, a lateral and a mixture-structured black phosphorus field effect transistor (BPFET) to study the electrical characteristics along different directions under variable temperatures. The variable temperature test indicates that the mixture-structured device performs more like a lateral device, while the conductance along the vertical direction is hard to be tuned by temperature and electrical field. The unchanged conductance under electric field and variable temperatures allows the vertical device to act as a fixed resistor, promising possible application for the prospective electronic and optoelectronic devices.

High Photoresponsivity in Ultrathin 2D Lateral Graphene:WS2:Graphene Photodetectors Using Direct CVD Growth

We show that reducing the degree of van der Waals overlapping in all 2D ultrathin lateral devices composed of graphene:WS₂:graphene leads to significant increase in photodetector responsivity. This is achieved by directly growing WS₂ using chemical vapor deposition (CVD) in prepatterned graphene gaps to create epitaxial interfaces. Direct-CVD-grown graphene:WS₂:graphene lateral photodetecting transistors exhibit high photoresponsivities reaching 121 A/W under 2.7 × 10⁵ mW/cm² 532 nm illumination, which is around 2 orders of magnitude higher than similar devices made by the layer-by-layer transfer method. The photoresponsivity of our direct-CVD-grown device shows negative correlation with illumination power under different gate voltages, which is different from similar devices made by the transfer method. We show that the high photoresponsivity is due to the lowering of effective Schottky barrier height by improving the contact between graphene and WS₂. Furthermore, the direct CVD growth reduces overlapping sections of WS₂:Gr and leads to more uniform lateral systems. This approach provides insights into scalable manufacturing of high-quality 2D lateral electronic and optoelectronic devices.

Acoustic extraordinary transmission manipulation based on proximity effects of heterojunctions

Heterojunctions between two crystalline semiconductor layers or regions can always lead to engineering the electronic energy bands in various devices, including transistors, solar cells, lasers, and organic electronic devices. The performance of these heterojunction devices depends crucially on the band alignments and their bending at the interfaces, which have been investigated for years according to Anderson's rule, Schottky-Mott rule, Lindhard theory, quantum capacitance, and so on. Here, we demonstrate that by engineering two different acoustic waveguides with forbidden bands, one can achieve an acoustic heterojunction with an extraordinary transmission peak arising in the middle of the former gaps. We experimentally reveal that such a transmission is spatially dependent and disappears for a special junction structure. The junction proximity effect has been realized by manipulating the acoustic impedance ratios, which have been proven to be related to the geometrical (Zak) phases of the bulk bands. Acoustic heterojunctions bring the concepts of quantum physics into the classical waves and the macroscopic scale, opening up the investigations of phononic, photonic, and microwave innovation devices.