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.     

Molecular Beam Epitaxy Scalable Growth of Wafer‐Scale Continuous Semiconducting Monolayer MoTe2 on Inert Amorphous Dielectrics

Monolayer MoTe₂, with the narrowest direct bandgap of ≈1.1 eV among Mo‐ and W‐based transition metal dichalcogenides, has attracted increasing attention as a promising candidate for applications in novel near‐infrared electronics and optoelectronics. Realizing 2D lateral growth is an essential prerequisite for uniform thickness and property control over the large scale, while it is not successful yet. Here, layer‐by‐layer growth of 2 in. wafer‐scale continuous monolayer 2H‐MoTe₂ films on inert SiO₂ dielectrics by molecular beam epitaxy is reported. A single‐step Mo‐flux controlled nucleation and growth process is developed to suppress island growth. Atomically flat 2H‐MoTe₂ with 100% monolayer coverage is successfully grown on inert 2 in. SiO₂/Si wafer, which exhibits highly uniform in‐plane structural continuity and excellent phonon‐limited carrier transport behavior. The dynamics‐controlled growth recipe is also extended to fabricate continuous monolayer 2H‐MoTe₂ on atomic‐layer‐deposited Al₂O₃ dielectric. With the breakthrough in growth of wafer‐scale continuous 2H‐MoTe₂ monolayers on device compatible dielectrics, batch fabrication of high‐mobility monolayer 2H‐MoTe₂ field‐effect transistors and the three‐level integration of vertically stacked monolayer 2H‐MoTe₂ transistor arrays for 3D circuitry are successfully demonstrated. This work provides novel insights into the scalable synthesis of monolayer 2H‐MoTe₂ films on universal substrates and paves the way for the ultimate miniaturization of electronics.     

Tunable Polarity Behavior and Self‐Driven Photoswitching in p‐WSe2/n‐WS2 Heterojunctions

Van der Waals (vdW) p–n heterojunctions consisting of various 2D layer compounds are fascinating new artificial materials that can possess novel physics and functionalities enabling the next‐generation of electronics and optoelectronics devices. Here, it is reported that the WSe₂/WS₂ p–n heterojunctions perform novel electrical transport properties such as distinct rectifying, ambipolar, and hysteresis characteristics. Intriguingly, the novel tunable polarity transition along a route of n‐“anti‐bipolar”–p‐ambipolar is observed in the WSe₂/WS₂ heterojunctions owing to the successive work of conducting channels of junctions, p‐WSe₂ and n‐WS₂ on the electrical transport of the whole systems. The type‐II band alignment obtained from first principle calculations and built‐in potential in this vdW heterojunction can also facilitate the efficient electron–hole separation, thus enabling the significant photovoltaic effect and a much enhanced self‐driven photoswitching response in this system.     

Synthesis of In‐Plane Artificial Lattices of Monolayer Multijunctions

Recently, monolayers of van der Waals materials, including transition metal dichalcogenides (TMDs), are considered ideal building blocks for constructing 2D artificial lattices and heterostructures. Heterostructures with multijunctions of more than two monolayer TMDs are intriguing for exploring new physics and materials properties. Obtaining in‐plane heterojunctions of monolayer TMDs with atomically sharp interfaces is very significant for fundamental research and applications. Currently, multistep synthesis for more than two monolayer TMDs remains a challenge because decomposition or compositional alloying is thermodynamically favored at the high growth temperature. Here, a multistep chemical vapor deposition (CVD) synthesis of the in‐plane multijunctions of monolayer TMDs is presented. A low growth temperature synthesis is developed to avoid compositional fluctuations of as‐grown TMDs, defects formations, and interfacial alloying for high heterointerface quality and thermal stability of monolayer TMDs. With optimized parameters, atomically sharp interfaces are successfully achieved in the synthesis of in‐plane artificial lattices of the WS₂/WSe₂/MoS₂ at reduced growth temperatures. Growth behaviors as well as the heterointerface quality are carefully studied in varying growth parameters. Highly oriented strain patterns are found in the second harmonic generation imaging of the TMD multijunctions, suggesting that the in‐plane heteroepitaxial growth may induce distortion for unique material symmetry.     

A “Phase Separation” Molecular Design Strategy Towards Large‐Area 2D Molecular Crystals

2D molecular crystals (2DMCs) have attracted considerable attention because of their unique optoelectronic properties and potential applications. Taking advantage of the solution processability of organic semiconductors, solution self‐assembly is considered an effective way to grow large‐area 2DMCs. However, this route is largely blocked because a precise molecular design towards 2DMCs is missing and little is known about the relationship between 2D solution self‐assembly and molecular structure. A “phase separation” molecular design strategy towards 2DMCs is proposed and layer‐by‐layer growth of millimeter‐sized monolayer or few‐layer 2DMCs is realized. High‐performance organic phototransistors are constructed based on the 2DMCs with unprecedented photosensitivity (2.58 × 10⁷), high responsivity (1.91 × 10⁴ A W^?1), and high detectivity (4.93 × 10¹⁵ Jones). This “phase separation” molecular design strategy provides a guide for the design and synthesis of novel organic semiconductors that self‐assemble into large‐area 2DMCs for advanced organic (opto)electronics.     

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.     

Mechanical Exfoliation and Characterization of Single‐ and Few‐Layer Nanosheets of WSe2, TaS2, and TaSe2

Single‐ and few‐layer transition‐metal dichalcogenide nanosheets, such as WSe₂, TaS₂, and TaSe₂, are prepared by mechanical exfoliation. A Raman microscope is employed to characterize the single‐layer (1L) to quinary‐layer (5L) WSe₂ nanosheets and WSe₂ single crystals with a laser excitation power ranging from 20 μW to 5.1 mW. Typical first‐order together with some second‐order and combinational Raman modes are observed. A new peak at around 308 cm^?1 is observed in WSe₂ except for the 1L WSe₂, which might arise from interlayer interactions. Red shifting of the A_1g mode and the Raman peak around 308 cm^?1 is observed from 1L to 5L WSe₂. Interestingly, hexagonal‐ and monoclinic‐structured WO₃ thin films are obtained during the local oxidation of thinner (1L–3L) and thicker (4L and 5L) WSe₂ nanosheets, while laser‐burned holes are found during the local oxidation of the WSe₂ single crystal. In addition, the characterization of TaS₂ and TaSe₂ thin layers is also conducted.     

Ultrafast Growth of High‐Quality Monolayer WSe2 on Au

The ultrafast growth of high‐quality uniform monolayer WSe₂ is reported with a growth rate of ≈26 µm s^?1 by chemical vapor deposition on reusable Au substrate, which is ≈2–3 orders of magnitude faster than those of most 2D transition metal dichalcogenides grown on nonmetal substrates. Such ultrafast growth allows for the fabrication of millimeter‐size single‐crystal WSe₂ domains in ≈30 s and large‐area continuous films in ≈60 s. Importantly, the ultrafast grown WSe₂ shows excellent crystal quality and extraordinary electrical performance comparable to those of the mechanically exfoliated samples, with a high mobility up to ≈143 cm² V^?1 s^?1 and ON/OFF ratio up to 9 × 10⁶ at room temperature. Density functional theory calculations reveal that the ultrafast growth of WSe₂ is due to the small energy barriers and exothermic characteristic for the diffusion and attachment of W and Se on the edges of WSe₂ on Au substrate.     

Reliable Piezoelectricity in Bilayer WSe2 for Piezoelectric Nanogenerators

Recently, piezoelectricity has been observed in 2D atomically thin materials, such as hexagonal‐boron nitride, graphene, and transition metal dichalcogenides (TMDs). Specifically, exfoliated monolayer MoS₂ exhibits a high piezoelectricity that is comparable to that of traditional piezoelectric materials. However, monolayer TMD materials are not regarded as suitable for actual piezoelectric devices due to their insufficient mechanical durability for sustained operation while Bernal‐stacked bilayer TMD materials lose noncentrosymmetry and consequently piezoelectricity. Here, it is shown that WSe₂ bilayers fabricated via turbostratic stacking have reliable piezoelectric properties that cannot be obtained from a mechanically exfoliated WSe₂ bilayer with Bernal stacking. Turbostratic stacking refers to the transfer of each chemical vapor deposition (CVD)‐grown WSe₂ monolayer to allow for an increase in degrees of freedom in the bilayer symmetry, leading to noncentrosymmetry in the bilayers. In contrast, CVD‐grown WSe₂ bilayers exhibit very weak piezoelectricity because of the energetics and crystallographic orientation. The flexible piezoelectric WSe₂ bilayers exhibit a prominent mechanical durability of up to 0.95% of strain as well as reliable energy harvesting performance, which is adequate to drive a small liquid crystal display without external energy sources, in contrast to monolayer WSe₂ for which the device performance becomes degraded above a strain of 0.63%.     

3R MoS2 with Broken Inversion Symmetry: A Promising Ultrathin Nonlinear Optical Device

Nonlinear 2D layered crystals provide ideal platforms for applications and fundamental studies in ultrathin nonlinear optical (NLO) devices. However, the NLO frequency conversion efficiency constrained by lattice symmetry is still limited by layer numbers of 2D crystals. In this work, 3R MoS₂ with broken inversion symmetry structure are grown and proved to be excellent NLO 2D crystals from monolayer (0.65 nm) toward bulk‐like (300 nm) dimension. Thickness and wavelength‐dependent second harmonic generation spectra offer the selection rules of appropriate working conditions. A model comprising of bulk nonlinear contribution and interface interaction is proposed to interpret the observed nonlinear behavior. Polarization enhancement with two petals along staggered stacking direction appears in 3R MoS₂ is first observed and the robust polarization of 3R MoS₂ crystal is caused by the retained broken inversion symmetry. The results provide a new arena for realizing ultrathin NLO devices for 2D layered materials.     

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.     

Impact of H‐Doping on n‐Type TMD Channels for Low‐Temperature Band‐Like Transport

Band‐like transport behavior of H‐doped transition metal dichalcogenide (TMD) channels in field effect transistors (FET) is studied by conducting low‐temperature electrical measurements, where MoTe₂, WSe₂, and MoS₂ are chosen for channels. Doped with H atoms through atomic layer deposition, those channels show strong n‐type conduction and their mobility increases without losing on‐state current as the measurement temperature decreases. In contrast, the mobility of unintentionally (naturally) doped TMD FETs always drops at low temperatures whether they are p‐ or n‐type. Density functional theory calculations show that H‐doped MoTe₂, WSe₂, and MoS₂ have Fermi levels above conduction band edge. It is thus concluded that the charge transport behavior in H‐doped TMD channels is metallic showing band‐like transport rather than thermal hopping. These results indicate that H‐doped TMD FETs are practically useful even at low‐temperature ranges.     

Efficient and Layer‐Dependent Exciton Pumping across Atomically Thin Organic–Inorganic Type‐I Heterostructures

The fundamental light–matter interactions in monolayer transition metal dichalcogenides might be significantly engineered by hybridization with their organic counterparts, enabling intriguing optoelectronic applications. Here, atomically thin organic–inorganic (O–I) heterostructures, comprising monolayer MoSe₂ and mono‐/few‐layer single‐crystal pentacene samples, are fabricated. These heterostructures show type‐I band alignments, allowing efficient and layer‐dependent exciton pumping across the O–I interfaces. The interfacial exciton pumping has much higher efficiency (>86 times) than the photoexcitation process in MoSe₂, although the pentacene layer has much lower optical absorption than MoSe₂. This highly enhanced pumping efficiency is attributed to the high quantum yield in pentacene and the ultrafast energy transfer between the O–I interface. Furthermore, those organic counterparts significantly modulate the bindings of charged excitons in monolayer MoSe₂ via their precise dielectric environment engineering. The results open new avenues for exploring fundamental phenomena and novel optoelectronic applications using atomically thin O–I heterostructures.     

Gold-vapor-assisted chemical vapor deposition of aligned monolayer WSe2 with large domain size and fast growth rate

Orientation-controlled growth of two-dimensional (2D) transition metal dichalcogenides (TMDCs) may enable many new electronic and optical applications. However, previous studies reporting aligned growth of WSe₂ usually yielded very small domain sizes. Herein, we introduced gold vapor into the chemical vapor deposition (CVD) process as a catalyst to assist the growth of WSe₂ and successfully achieved highly aligned monolayer WSe₂ triangular flakes grown on c-plane sapphire with large domain sizes (130 µm) and fast growth rate (4.3 µm·s⁻¹). When the aligned WSe₂ domains merged together, a continuous monolayer WSe₂ was formed with good uniformity. After transferring to Si/SiO₂ substrates, field effect transistors were fabricated on the continuous monolayer WSe₂, and an average mobility of 12 cm²·V⁻¹·s⁻¹ was achieved, demonstrating the good quality of the material. This report paves the way to study the effect of catalytic metal vapor in the CVD process of TMDCs and contributes a novel approach to realize the growth of aligned TMDC flakes.

     

Self‐Confined Growth of Ultrathin 2D Nonlayered Wide‐Bandgap Semiconductor CuBr Flakes

2D planar structures of nonlayered wide‐bandgap semiconductors enable distinguished electronic properties, desirable short wavelength emission, and facile construction of 2D heterojunction without lattice match. However, the growth of ultrathin 2D nonlayered materials is limited by their strong covalent bonded nature. Herein, the synthesis of ultrathin 2D nonlayered CuBr nanosheets with a thickness of about 0.91 nm and an edge size of 45 µm via a controllable self‐confined chemical vapor deposition method is described. The enhanced spin‐triplet exciton (Z_f, 2.98 eV) luminescence and polarization‐enhanced second‐harmonic generation based on the 2D CuBr flakes demonstrate the potential of short‐wavelength luminescent applications. Solar‐blind and self‐driven ultraviolet (UV) photodetectors based on the as‐synthesized 2D CuBr flakes exhibit a high photoresponsivity of 3.17 A W^?1, an external quantum efficiency of 1126%, and a detectivity (D*) of 1.4 × 10¹¹ Jones, accompanied by a fast rise time of 32 ms and a decay time of 48 ms. The unique nonlayered structure and novel optical properties of the 2D CuBr flakes, together with their controllable growth, make them a highly promising candidate for future applications in short‐wavelength light‐emitting devices, nonlinear optical devices, and UV photodetectors.     

Gate‐Controlled BP–WSe2 Heterojunction Diode for Logic Rectifiers and Logic Optoelectronics

p–n junctions play an important role in modern semiconductor electronics and optoelectronics, and field‐effect transistors are often used for logic circuits. Here, gate‐controlled logic rectifiers and logic optoelectronic devices based on stacked black phosphorus (BP) and tungsten diselenide (WSe₂) heterojunctions are reported. The gate‐tunable ambipolar charge carriers in BP and WSe₂ enable a flexible, dynamic, and wide modulation on the heterojunctions as isotype (p–p and n–n) and anisotype (p–n) diodes, which exhibit disparate rectifying and photovoltaic properties. Based on such characteristics, it is demonstrated that BP–WSe₂ heterojunction diodes can be developed for high‐performance logic rectifiers and logic optoelectronic devices. Logic optoelectronic devices can convert a light signal to an electric one by applied gate voltages. This work should be helpful to expand the applications of 2D crystals.     

Photoelectrochemical solar cell fabrication with tungsten diselenide single crystals

Recent reports on highly efficient photoelectrochemical solar cells withn-type WSe₂ have prompted us to grown-type single crystals of WSe₂ using a chemical vapour transport method. Different transporting agents have been used. It is seen that SeCl₄ transporter leads to very large single crystals ofp-type WSe₂, whereas the same transporting agent with excess amount of Se leads ton-type single crystals. PEC solar cells fabricated withp-type andn-type crystals thus grown have been reported and the results discussed.     

Single Atomically Sharp Lateral Monolayer p‐n Heterojunction Solar Cells with Extraordinarily High Power Conversion Efficiency

The recent development of 2D monolayer lateral semiconductor has created new paradigm to develop p‐n heterojunctions. Albeit, the growth methods of these heterostructures typically result in alloy structures at the interface, limiting the development for high‐efficiency photovoltaic (PV) devices. Here, the PV properties of sequentially grown alloy‐free 2D monolayer WSe₂‐MoS₂ lateral p‐n heterojunction are explores. The PV devices show an extraordinary power conversion efficiency of 2.56% under AM 1.5G illumination. The large surface active area enables the full exposure of the depletion region, leading to excellent omnidirectional light harvesting characteristic with only 5% reduction of efficiency at incident angles up to 75°. Modeling studies demonstrate the PV devices comply with typical principles, increasing the feasibility for further development. Furthermore, the appropriate electrode‐spacing design can lead to environment‐independent PV properties. These robust PV properties deriving from the atomically sharp lateral p‐n interface can help develop the next‐generation photovoltaics.     

Metal‐Guided Selective Growth of 2D Materials: Demonstration of a Bottom‐Up CMOS Inverter

2D transition metal dichalcogenide (TMD) layered materials are promising for future electronic and optoelectronic applications. The realization of large‐area electronics and circuits strongly relies on wafer‐scale, selective growth of quality 2D TMDs. Here, a scalable method, namely, metal‐guided selective growth (MGSG), is reported. The success of control over the transition‐metal‐precursor vapor pressure, the first concurrent growth of two dissimilar monolayer TMDs, is demonstrated in conjunction with lateral or vertical TMD heterojunctions at precisely desired locations over the entire wafer in a single chemical vapor deposition (VCD) process. Owing to the location selectivity, MGSG allows the growth of p‐ and n‐type TMDs with spatial homogeneity and uniform electrical performance for circuit applications. As a demonstration, the first bottom‐up complementary metal‐oxide‐semiconductor inverter based on p‐type WSe₂ and n‐type MoSe₂ is achieved, which exhibits a high and reproducible voltage gain of 23 with little dependence on position.     

Few‐Layer Silicene Nanosheets with Superior Lithium‐Storage Properties

Silicene, a 2D silicon allotrope with unique low‐buckled structure, has attracted increasing attention in recent years due to its many superior properties. So far, epitaxial growth is one of the very limited ways to obtain high‐quality silicene, which severely impedes the research and application of silicene. Therefore, large‐scale synthesis of silicene is a great challenge, yet urgently desired. Herein, the first scalable preparation of free‐standing high‐quality silicene nanosheets via liquid oxidation and exfoliation of CaSi₂ is reported. This new synthesis strategy successfully induces mild oxidation of the (Si²ⁿ)^2n? layers in CaSi₂ into neutral Si²ⁿ layers without damage of pristine silicene structure and promotes the exfoliation of stacked silicene layers. The obtained silicene sheets are dispersible and ultrathin ones with monolayer or few‐layer thickness and exhibit excellent crystallinity. As a unique 2D layered silicon allotrope, the silicene nanosheets are further explored as new anodes for lithium‐ion batteries and exhibit a nearly theoretical capacity of 721 mAh g^?1 at 0.1 A g^?1 and an extraordinary cycling stability with no capacity decay after 1800 cycles in contrast to previous most silicon anodes showing rapid capacity decay, thus holding great promise for energy storage and beyond.