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

Sulfur isotope engineering in heterostructures of transition metal dichalcogenides

Heterostructuring of two-dimensional materials offers a robust platform to precisely tune optoelectronic properties through interlayer interactions. Here we achieved a strong interlayer coupling in a double-layered heterostructure of sulfur isotope-modified adjacent MoS2 monolayers via two-step chemical vapor deposition growth. The strong interlayer coupling in the MoS2(34S)/MoS2(32S) was affirmed by low-frequency shear and breathing modes in the Raman spectra. The photoluminescence emission spectra showed that isotope-induced changes in the electronic structure and strong interlayer coupling led to the suppression of intralayer excitons, resulting in dominant emission from the MoS2(32S) layer. Time-resolved photoluminescence experiments indicated faster lifetimes in the MoS2(34S)/MoS2(32S) heterostructure compared to the conventional bilayers with the natural isotopic abundance, highlighting nuanced interlayer exciton dynamics due to the isotopic modification. This study underscores the great potential of isotope engineering in van der Waals heterostructures, as it enables tailoring the band structure and exciton dynamics at the nuclear level without the need of chemical modification.

Thermodynamic Origin-Based In Situ Electrochemical Construction of Reversible p-n Heterojunctions for Optimal Stability in Potassium Ion Storage

Heterojunctions in electrode materials offer diverse improvements during the cycling process of energy storage devices, such as volume change buffering, accelerated ion/electron transfer, and better electrode structure integrity, however, obtaining optimal heterostructures with nanoscale domains remains challenging within constrained materials. A novel in situ electrochemical method is introduced to develop a reversible CuSe/PSe p-n heterojunction (CPS-h) from Cu3PSe4 as starting material, targeting maximum stability in potassium ion storage. The CPS-h formation is thermodynamically favorable, characterized by its superior reversibility, minimized diffusion barriers, and enhanced conversion post K+ interaction. Within CPS-h, the synergy of the intrinsic electric field and P-Se bonds enhance electrode stability, effectively countering the Se shuttling phenomenon. The specific orientation between CuSe and PSe leads to a 35° lattice mismatch generates large space at the interface, promoting efficient K ion migration. The Mott-Schottky analysis validates the consistent reversibility of CPS-h, underlining its electrochemical reliability. Notably, CPS-h demonstrates a negligible 0.005% capacity reduction over 10,000 half-cell cycles and remains stable through 2,000 and 4,000 cycles in full cells and hybrid capacitors, respectively. This study emphasizes the pivotal role of electrochemical dynamics in formulating highly stable p-n heterojunctions, representing a significant advancement in potassium-ion battery (PIB) electrode engineering.

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.

Multiplexed Complementary Signal Transmission for a Self-Regulating Artificial Nervous System

Neuromorphic engineering has emerged as a promising research field that can enable efficient and sophisticated signal transmission by mimicking the biological nervous system. This paper presents an artificial nervous system capable of facile self-regulation via multiplexed complementary signals. Based on the tunable nature of the Schottky barrier of a complementary signal integration circuit, a pair of complementary signals is successfully integrated to realize efficient signal transmission. As a proof of concept, a feedback-based blood glucose level control system is constructed by incorporating a glucose/insulin sensor, a complementary signal integration circuit, an artificial synapse, and an artificial neuron circuit. Certain amounts of glucose and insulin in the initial state are detected by each sensor and reflected as positive and negative amplitudes of the multiplexed presynaptic pulses, respectively. Subsequently, the pulses are converted to postsynaptic current, which triggered the injection of glucose or insulin in a way that confined the glucose level to a desirable range. The proposed artificial nervous system demonstrates the notable potential of practical advances in complementary control engineering.

Heterostructure Engineering of 2D Superlattice Materials for Electrocatalysis

Exploring low-cost and high-efficient electrocatalyst is an exigent task in developing novel sustainable energy conversion systems, such as fuel cells and electrocatalytic fuel generations. 2D materials, specifically 2D superlattice materials focused here, featured highly accessible active areas, high density of active sites, and high compatibility with property-complementary materials to form heterostructures with desired synergetic effects, have demonstrated to be promising electrocatalysts for boosting the performance of sustainable energy conversion and storage devices. Nevertheless, the reaction kinetics, and in particular, the functional mechanisms of the 2D superlattice-based catalysts yet remain ambiguous. In this review, based on the recent progress of 2D superlattice materials in electrocatalysis applications, the rational design and fabrication of 2D superlattices are first summarized and the application of 2D superlattices in electrocatalysis is then specifically discussed. Finally, perspectives on the current challenges and the strategies for the future design of 2D superlattice materials are outlined. This review attempts to establish an intrinsic correlation between the 2D superlattice heterostructures and the catalytic properties, so as to provide some insights into developing high-performance electrocatalysts for next-generation sustainable energy conversion and storage.

Direct fabrication of high-quality vertical graphene nanowalls on arbitrary substrates without catalysts for tidal power generation

The non-catalytic preparation of high-quality vertical graphene nanowalls (VGN) and graphene-based high output power hydrovoltaic effect power generation devices has always been difficult to achieve. In this work, we successfully prepared VGN with defect density, few layers and submicron domain size on a variety of substrates without catalysts through reasonable adjustment of growth conditions by the hot-wire chemical vapor deposition (HWCVD) method. The Raman test of the VGN prepared under optimal conditions showed that its I_D/I_G value was less than 1, and I_2D/I_G was more than 2.8. The deposition pressure was a key factor affecting the crystallization quality of the VGN. A suitable deposition pressure of 500 Pa could screen the active carbon clusters involved in the growth of nanowalls. The VGN prepared had excellent electrical properties and output of dropping-ion-droplet nano-power generation devices. Because of the larger crystal domain area and smaller contact angle of the VGN, the maximum output power exhibited at 100 Pa was 15.7 μW, which exceeded the value produced by other reported hydrovoltaic energy harvesting devices. All of them confirmed that VGN with improved quality had high application prospects in nano-energy devices.

Constructing van der Waals heterostructures by dry-transfer assembly for novel optoelectronic device

Since the first successful exfoliation of graphene, the superior physical and chemical properties of two-dimensional (2D) materials, such as atomic thickness, strong in-plane bonding energy and weak inter-layer van der Waals (vdW) force have attracted wide attention. Meanwhile, there is a surge of interest in novel physics which is absent in bulk materials. Thus, vertical stacking of 2D materials could be critical to discover such physics and develop novel optoelectronic applications. Although vdW heterostructures have been grown by chemical vapor deposition, the available choices of materials for stacking is limited and the device yield is yet to be improved. Another approach to build vdW heterostructure relies on wet/dry transfer techniques like stacking Lego bricks. Although previous reviews have surveyed various wet transfer techniques, novel dry transfer techniques have been recently been demonstrated, featuring clean and sharp interfaces, which also gets rid of contamination, wrinkles, bubbles formed during wet transfer. This review summarizes the optimized dry transfer methods, which paves the way towards high-quality 2D material heterostructures with optimized interfaces. Such transfer techniques also lead to new physical phenomena while enable novel optoelectronic applications on artificial vdW heterostructures, which are discussed in the last part of this review.

A Review of Phototransistors Using Metal Oxide Semiconductors: Research Progress and Future Directions

Metal oxide thin-film transistors have been continuously researched and mass-produced in the display industry. However, their phototransistors are still in their infancy. In particular, utilizing metal oxide semiconductors as phototransistors is difficult because of the limited light absorption wavelength range and persistent photocurrent (PPC) phenomenon. Numerous studies have attempted to improve the detectable light wavelength range and the PPC phenomenon. Here, recent studies on metal oxide phototransistors are reviewed, which have improved the range of light wavelengths and the PPC phenomenon by introducing an absorption layer of oxide or non-oxide hybrid structure. The materials of the absorption layer applied to absorb long-wavelength light are classified into oxides, chalcogenides, organic materials, perovskites, and nanodots. Finally, next-generation convergence studies combined with other research fields are introduced and future research directions are detailed.

Monolithic Tandem Multicolor Image Sensor Based on Electrochromic Color-Radix Demultiplexing

Optical data acquisition has been set as one of the milestones to testify the developments aimed at harnessing the full potential of the spatial and temporal data processing capabilities of the advanced semiconductor technology. A highly promising approach to drive the level of acquisition beyond the current technological node is the vertical integration of multiple photodetectors. However, vertical integration so far requires the same level of circuit complexity as lateral integration from the incapability of monolithic integration. Here, an electrochromic device architecture is introduced that enables realization of a monolithic tandem multicolor photodetector. The device, composed of vertically stacked p-type and n-type graphene barristors, is demonstrated to be capable of regulating the balanced charge transport under any desired illumination wavelengths. It exhibits variable anti-ambipolar charge transport behavior, which yields sensitive voltage-controlled photoconductive gain spectra. These electrical behaviors are utilized to fabricate an optoelectronic logic sensor that can demultiplex the desired color coordinate or wavelength in the constituent array with high color accuracy.

Synaptic transistors and neuromorphic systems based on carbon nano-materials

Carbon-based materials possessing a nanometer size and unique electrical properties perfectly address the two critical issues of transistors, the low power consumption and scalability, and are considered as a promising material in next-generation synaptic devices. In this review, carbon-based synaptic transistors were systematically summarized. In the carbon nanotube section, the synthesis of carbon nanotubes, purification of carbon nanotubes, the effect of architecture on the device performance and related carbon nanotube-based devices for neuromorphic computing were discussed. In the graphene section, the synthesis of graphene and its derivative, as well as graphene-based devices for neuromorphic computing, was systematically studied. Finally, the current challenges for carbon-based synaptic transistors were discussed.

Substitutional Fluorine Doping of Large-Area Molybdenum Disulfide Monolayer Films for Flexible Inverter Device Arrays

Reliable and controllable doping of transition metal dichalcogenides (TMDCs) is a mandatory requirement for practical large-scale electronic applications. However, most of the literature on the doping methodologies of TMDCs has focused on n-type doping and multilayer TMDC rather than a monolayer one enabling large-scale growth. Herein, we report substitutional fluorine doping of a chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS₂) monolayer film using a delicate SF₆ plasma treatment. Our SF₆-treated MoS₂ monolayer shows a p-type doping effect with fluorine substitution. The doping concentration is controlled by the plasma treatment time (2-4.9 atom %) while maintaining the structural integrity of the MoS₂ monolayer. Such reliable and tunable substitutional doping is attributed to preventing direct ion bombardment to the MoS₂ monolayer by our gentle plasma treatment system. Finally, we fabricated MoS₂ homojunction flexible inverter device arrays based on the pristine and SF₆-treated MoS₂ monolayer. A clear switching behavior is observed, and the voltage gain is approximately 8 at an applied V_DD of 2 V, which is comparable to that of CVD-grown MoS₂-based inverter devices reported previously. Obtained voltage gain is also stable even after 500 bending cycles at an applied strain of 0.5%.

Ion Gel Capacitively Coupled Tribotronic Gating for Multiparameter Distance Sensing

Developing sophisticated device architectures is of great significance to go beyond Moore's law with versatility toward human-machine interaction and artificial intelligence. Tribotronics/tribo-iontronics offer a direct way to controlling the transport properties of semiconductor devices by mechanical actions, which fundamentally relies on how to enhance the tribotronic gating effect through device engineering. Here, we propose a universal method to enhance the tribotronic properties through electric double layer (EDL) capacitive coupling. By preparing an ion gel layer on top of tribotronic graphene transistor, we demonstrate a dual-mode field effect transistor (i.e., a tribotronic transistor with capacitively coupled ion gel and an ion-gel-gated graphene transistor with a second tribotronic gate). The resulted tribotronic gating performances are greatly improved by twice for the on-state current and four times for the on/off ratio (the first mode). It can also be utilized as a multiparameter distance sensor with drain current increased by ∼600 μA and threshold voltage shifted by ∼0.8 V under a mechanical displacement of 0.25 mm (the second mode). The proposed methodology of EDL capacitive coupling offers a facile and efficient way to designing more sophisticated tribotronic devices with superior performance and multifunctional sensations.

Lowering the Schottky barrier height of G/WSSe van der Waals heterostructures by changing the interlayer coupling and applying external biaxial strain

Graphene-based van der Waals (vdW) heterostructures composed of two-dimensional transition metal dichalcogenides (TMDs) and graphene show great potential in the design and manufacture of field effect transistors.

Synthesis of NbSe2 single-crystalline nanosheet arrays for UV photodetectors

Single-crystalline NbSe₂ nanosheet arrays were synthesized via a CVD method. The NbSe₂ nanosheet arrays based photodetectors show very high responsivity and external quantum efficiency to UV light.