Ultrafast Self‐Limited Growth of Strictly Monolayer WSe2 Crystals

The controllable synthesis of uniform tungsten diselenide (WSe₂) is crucial for its emerging applications due to the high sensitivity of its extraordinary physicochemical properties to its layer numbers. However, undesirable multilayer regions inevitably form during the fabrication of WSe₂ via the traditional chemical vapor deposition process resulted from the lack of significantly energetically favorable competition between layer accumulation and size expansion. This work innovatively introduces Cu to occupy the hexagonal site positioned at the center of the six membered ring of the WSe₂ surface, thus filtrates the undesired reaction path through precisely thermodynamical control and achieves self‐limited growth WSe₂ crystals. The as‐obtained WSe₂ crystals are characterized as strictly single‐layer over the entire wafer. Furthermore, the strictly self‐limited growth behavior can achieve the “win–win” cooperation with the synthesis efficiency. The fastest growth (≈15 times of the growth rate in the previous work) of strictly monolayer WSe₂ crystals thus far is realized due to the high‐efficiency simultaneous selenization process. The as‐proposed ultrafast Cu‐assisted self‐limited growth method opens a new avenue to fabricate strictly monolayer transition metal dichalcogenides crystals and further promotes their practical applications in the future industrial applications.     

GaP/GaNP Heterojunctions for Efficient Solar‐Driven Water Oxidation

The growth and characterization of an n‐GaP/i‐GaNP/p⁺‐GaP thin film heterojunction synthesized using a gas‐source molecular beam epitaxy (MBE) method, and its application for efficient solar‐driven water oxidation is reported. The TiO₂/Ni passivated n‐GaP/i‐GaNP/p⁺‐GaP thin film heterojunction provides much higher photoanodic performance in 1 m KOH solution than the TiO₂/Ni‐coated n‐GaP substrate, leading to much lower onset potential and much higher photocurrent. There is a significant photoanodic potential shift of 764 mV at a photocurrent of 0.34 mA cm^?2, leading to an onset potential of ≈0.4 V versus reversible hydrogen electrode (RHE) at 0.34 mA cm^?2 for the heterojunction. The photocurrent at the water oxidation potential (1.23 V vs RHE) is 1.46 and 7.26 mA cm^?2 for the coated n‐GaP and n‐GaP/i‐GaNP/p⁺‐GaP photoanodes, respectively. The passivated heterojunction offers a maximum applied bias photon‐to‐current efficiency (ABPE) of 1.9% while the ABPE of the coated n‐GaP sample is almost zero. Furthermore, the coated n‐GaP/i‐GaNP/p⁺‐GaP heterojunction photoanode provides a broad absorption spectrum up to ≈620 nm with incident photon‐to‐current efficiencies (IPCEs) of over 40% from ≈400 to ≈560 nm. The high low‐bias performance and broad absorption of the wide‐bandgap GaP/GaNP heterojunctions render them as a promising photoanode material for tandem photoelectrochemical (PEC) cells to carry out overall solar water splitting.     

Lateral Monolayer MoSe2–WSe2 p–n Heterojunctions with Giant Built‐In Potentials

2D transition metal dichalcogenides (TMDs) have exhibited strong application potentials in new emerging electronics because of their atomic thin structure and excellent flexibility, which is out of field of tradition silicon technology. Similar to 3D p–n junctions, 2D p–n heterojunctions by laterally connecting TMDs with different majority charge carriers (electrons and holes), provide ideal platform for current rectifiers, light‐emitting diodes, diode lasers and photovoltaic devices. Here, growth and electrical studies of atomic thin high‐quality p–n heterojunctions between molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂) by one‐step chemical vapor deposition method are reported. These p–n heterojunctions exhibit high built‐in potential (≈0.7 eV), resulting in large current rectification ratio without any gate control for diodes, and fast response time (≈6 ms) for self‐powered photodetectors. The simple one‐step growth and electrical studies of monolayer lateral heterojunctions open up the possibility to use TMD heterojunctions for functional devices.     

Eliminating Overerase Behavior by Designing Energy Band in High‐Speed Charge‐Trap Memory Based on WSe2

Atomic crystal charge trap memory, as a new concept of nonvolatile memory, possesses an atomic level flatness interface, which makes them promising candidates for replacing conventional FLASH memory in the future. Here, a 2D material WSe₂ and a 3D Al₂O₃/HfO₂/Al₂O₃ charge‐trap stack are combined to form a charge‐trap memory device with a separation of control gate and memory stack. In this device, the charges are erased/written by built‐in electric field, which significantly enhances the write speed to 1 µs. More importantly, owing to the elaborate design of the energy band structure, the memory only captures electrons with a large electron memory window over 20 V and trap selectivity about 13, both of them are the state‐of‐the‐art values ever reported in FLASH memory based on 2D materials. Therefore, it is demonstrated that high‐performance charge trap memory based on WSe₂ without the fatal overerase issue in conventional FLASH memory can be realized to practical application.     

Direct Synthesis of a Self‐Assembled WSe2/MoS2 Heterostructure Array and its Optoelectrical Properties

Functional van der Waals heterojunctions of transition metal dichalcogenides are emerging as a potential candidate for the basis of next‐generation logic devices and optoelectronics. However, the complexity of synthesis processes so far has delayed the successful integration of the heterostructure device array within a large scale, which is necessary for practical applications. Here, a direct synthesis method is introduced to fabricate an array of self‐assembled WSe₂/MoS₂ heterostructures through facile solution‐based directional precipitation. By manipulating the internal convection flow (i.e., Marangoni flow) of the solution, the WSe₂ wires are selectively stacked over the MoS₂ wires at a specific angle, which enables the formation of parallel‐ and cross‐aligned heterostructures. The realized WSe₂/MoS₂‐based p–n heterojunction shows not only high rectification (ideality factor: 1.18) but also promising optoelectrical properties with a high responsivity of 5.39 A W^?1 and response speed of 16 µs. As a feasible application, a WSe₂/MoS₂‐based photodiode array (10 × 10) is demonstrated, which proves that the photosensing system can detect the position and intensity of an external light source. The solution‐based growth of hierarchical structures with various alignments could offer a method for the further development of large‐area electronic and optoelectronic applications.     

Vacancy‐Induced Synaptic Behavior in 2D WS2 Nanosheet–Based Memristor for Low‐Power Neuromorphic Computing

Memristors with nonvolatile memory characteristics have been expected to open a new era for neuromorphic computing and digital logic. However, existing memristor devices based on oxygen vacancy or metal‐ion conductive filament mechanisms generally have large operating currents, which are difficult to meet low‐power consumption requirements. Therefore, it is very necessary to develop new materials to realize memristor devices that are different from the mechanisms of oxygen vacancy or metal‐ion conductive filaments to realize low‐power operation. Herein, high‐performance and low‐power consumption memristors based on 2D WS₂ with 2H phase are demonstrated, which show fast ON (OFF) switching times of 13 ns (14 ns), low program current of 1 µA in the ON state, and SET (RESET) energy reaching the level of femtojoules. Moreover, the memristor can mimic basic biological synaptic functions. Importantly, it is proposed that the generation of sulfur and tungsten vacancies and electron hopping between vacancies are dominantly responsible for the resistance switching performance. Density functional theory calculations show that the defect states formed by sulfur and tungsten vacancies are at deep levels, which prevent charge leakage and facilitate the realization of low‐power consumption for neuromorphic computing application.     

Chemically Tuned p‐ and n‐Type WSe2 Monolayers with High Carrier Mobility for Advanced Electronics

Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post‐silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom‐thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p‐ and n‐type semiconductors is essential for various device applications, such as complementary metal‐oxide‐semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe₂ is demonstrated by chemical doping. Two different molecules, 4‐nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe₂ field‐effect transistors (FETs) to p‐ and n‐type, respectively. Moreover, the chemically doped WSe₂ show increased effective carrier mobilities of 82 and 25 cm² V^?1s^?1 for holes and electrons, respectively, which are much higher than those of the pristine WSe₂. The doping effects are studied by photoluminescence, Raman, X‐ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe₂ FETs are integrated into CMOS inverters, exhibiting extremely low power consumption ( ≈ 0.17 nW). Furthermore, a p‐n junction within single WSe₂ grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe₂ will contribute to the development of TMDC‐based advanced electronics.