WSe 2 2D p-type semiconductor-based electronic devices for information technology: Design, preparation, and applications

The pioneering exfoliation of monolayer tungsten diselenide has greatly inspired researchers toward semiconducting applications. WSe 2 belongs to a family of transition-metal dichalcogenides. Similar to graphene, WSe 2 and analogous dichalcogenides have layered structures with weak van der Waals interactions between two adjacent layers. First, the readers are presented with the fundamentals of WSe 2, such as types, morphologies, and properties. Here, we report the characterization principles and practices such as microscopy, spectroscopy, and diffraction. Second, the methods for obtaining high-quality WSe 2 , such as exfoliation, hydrothermal and chemical vapor deposition, are briefly listed. With advantages of light weight, flexibility, and high quantum efficiency, 2D materials may have a niche in optoelectronics as building blocks in p-n junctions. Therefore, we introduce a state-of-the-art demonstration of heterostructure devices employing the p-type WSe 2 semiconductor. The device architectures include field-effect transistors, photodetectors, gas sensors, and photovoltaic solar cells. Due to its unique electronic, optical, and energy band properties, WSe 2 has been increasingly investigated due to the conductivity of the p-type charge carrier upon palladium contact. Eventually, the dynamic research on WSe 2 and van der Waals heterostructures is summarized to arouse the passion of the 2D research community.


| Bulk 3D materials
Early in 1991, a single crystal of WSe 2 bulk was synthesized using the chemical vapor transport method. 15It has a layered structure analogous to that of graphite (Figure 1D).Bulk WSe 2 is a semiconductor processing an indirect bandgap (1.25 eV). 44,453][54][55] Bulk WSe 2 was used previously in photovoltaic cells 56,57 and for photoelectrochemical (PEC) hydrogen production 58 because of its suitable bandgap and long-term stability.
In 2004, the first isolation of graphene 59 triggered great interest in obtaining thin layered TMDC and monolayer WSe 2 .Indeed, in 2005, monolayer MoS 2 and NbSe 2 were first exfoliated 60 with the well-known Scotch-tapeassisted microcleavage approach. 61
The typical micrographs of monolayer WSe 2 are shown in Figure 1B,C.For monolayers, the 2H orientation is preserved for consistency, although the orientation H is also used.The change in the number of electrons from d 2 to d 3 results in structural destabilization from 2H to 1T, as shown in Figure 1E.

| Why is WSe 2 important?
From a viewpoint of conventional semiconductor physics, the type of conductive charges determines the p-type or ntype transistor, which composes the basic unit of microelectronics.The fundamental building blocks of photoelectronics are p-n junctions, [102][103][104] which are a heterostructure of two types of materials.][112][113][114] Most TMDC materials such as MoSe 2 [115][116][117] and WS 2 [118][119][120][121][122][123][124] are n-type semiconductors, 125,126 but WSe 2 features a p-type charge carrier, 127,128 that is, hole conducting. 11,129Indeed, WSe 2 is one of the most frequently used p-type 2D semiconducting materials upon contact with palladium 130 or chemical doping of NO. 131 Thereby, WSe 2 has showed great interests for electronic engineering for device design.
After considering the quantum size limitation, the manybody electrostatic attraction [132][133][134] generates the exciton in WSe 2 and determines the yield of free charge carriers.The dissociation of exciton, 135 typically demanding a built-in electric field 136,137 in WSe 2 becomes of vital importance because an e-h pair bonded together with Coulombic interaction 138 cannot be separated with kinetic energy at room temperature.Here screening effect 139 plays a role in bandgap renormalization. 140ndeed WSe 2 materials have attracted intensive efforts for fundamental electronic processes such as the formation of exciton, 141 bi-exciton, [142][143][144] and trion. 145Then the dynamics of exciton related energy states 146,147 including their generation, 148 diffusion, 149,150 annihilation, 151 and lifetime 152,153 should be intensively investigated with ultrafast optical spectroscopy 154,155 excited with large power narrow linewidth light, that is, supercontinuum white lasers.
In addition, the dissociation of exciton demands great efforts to understand the behind mechanism for the liberation of an electron-hole pair.Also, interesting phenomena emerge for the study of bandgap renormalization, 156 direct 157,158 or indirect exciton, 159,160 out of plane polarization, dark exciton states, [161][162][163] exciton lake, 164 ferroelectric regulation of exciton, 165 and Stark shift. 166,167In parallel to the quantum physics, proof of concept devices have emerged for demonstration of quantum information devices. 168More contributions are required for fully understanding the potential of WSe 2 in quantum techniques.
8][179][180][181][182] Here, these new understanding of WSe 2 may have been attributed to the enlargement of its size 183 and the incorporation of low-temperature physics [184][185][186] as well as ultrafast optical pump-probe spectroscopy [187][188][189] and timeresolved terahertz spectroscopy. 190o sum up, WSe 2 remains less studied and unknown for the fundamental electronic process such as exciton based dynamics [191][192][193] while n-type TMDCs such as MoS 2 have been intensively explored. 194,195To employ the advantages of optoelectronic devices of 2D TMDCs, we need to

| PROPERTIES
We now come to the properties of WSe 2 to understand its structure, transport, electric conductivity, and optical features.

| Atomic structures
WSe 2 has crystal structures composed of weakly bonded sandwich-layered structures Se-W-Se, where a W atomic layer is embedded inside two Se layers.The Brillouin zone is shown in Figure 2A.These three atomic layers form monolayer WSe 2 , and they are hexagonally packed from the top, which is the typical 2H phase.In addition, adjacent layers reside on top of the pristine monolayer via a weak van der Waals force, which leads to the formation of a bilayer.
Single-crystal WSe 2 is a perfect crystal structure when synthesized using the chemical vapor transport method.When using post-treatment such as plasma and chemical etching, defects may be introduced.
The WSe 2 monolayer has a high defect density and randomly distributed defects.These defects are classified into four types including Se atom vacancy, W atom vacancy, double atom vacancy (V Se-Se , V Se2 , and V W2 ), and large vacancies (V WSe3 , V WSe6 ). 197,198When different elements are doped in WSe 2 , the change in the atomic structure results in multiple functions.Recently, oxygen substitutional defects (O Se ) were observed due to the presence of WO 3 and low dissociation energy of O 2 after passivating Se vacancies. 199Here, oxygen interstitials (O ins ) were also determined with STM and STEM as well as a theoretical calculation based on DFT.

| Electronic energy band structure
The electronic properties of WSe 2 determine its device performance in electronics and optoelectronics by means of band structure and density of states.First, first-principle calculations were used to generate the band structure for bulk WSe 2 and thin layered WSe 2 (Figure 2B).
With decreasing layer numbers, WSe 2 experiences a transition from an indirect bandgap in its bulk state to a direct bandgap for its individual monolayer.Additionally, the bandgap changes with the tuning of the layer number.The bandgap determines the emission wavelength of the photoluminescence spectra, and hence, PL peaks have different positions (Figure 2C,D).
Then, the stacking of two neighboring layers affects the bandgap of WSe 2 .For instance, the Bernal stacking of bilayer WSe 2 results in triple-band degeneracy, as evidenced by photoluminescence tests. 196dditionally, external strain leads to a large change in the bandgap of WSe 2 .For example, its bandgap is inversely proportional to the strain. 200The stress regulation of 2D TMDC properties can expand their application in the fields of wearable photoelectric devices, catalysis, energy storage, and sensors. 201Monolayer WSe 2 is fully suitable for photovoltaic applications 202 because efficient light absorption stems from a direct optical gap and also inhibits the radiative recombination of indirect excitons. 203

| Electronic transport properties
Monolayer WSe 2 has an electron mobility of approximately 142 cm 2 ÁV −1 Ás −1 with indium contact and hole mobility of 140 cm 2 ÁV −1 Ás −1 with NO 2 doping. 204][207] However, the charge carrier mobility of WSe 2 samples is still limited due to the scattering 208 caused by structural defects, 209 grain boundaries, 172,210 and charged impurities.Further improvement in the charge carrier mobility can enhance the performance of electronic devices such as transistors.

| Optical properties
With the development of visual characterization methods, such as optical microscopy, WSe 2 can be easily observed by distinguishing the color difference from that of its supporting SiO 2 substrate, typically 300 nm thick, over a Si chip.
With tuning the thickness of WSe 2 , the color of the thin layers showed a transition from deep red, deep purple, and lavender to magenta as observed by an optical microscope.The difference in the layer number of WSe 2 can be distinguished with various characterization tools, such as photoluminescence spectroscopy, [211][212][213] Raman spectroscopy, 7,196,214 scanning electron microscopy, 215,216 transmission electron microscopy, 217 electron diffraction, [218][219][220][221][222] scanning tunneling microscopy, [223][224][225] and atomic force microscopy. 58onlinear transmission of WSe 2 may lead to the laser application as a saturable absorber material for optics. 226dditionally, the defects of WSe 2 can be resolved by transmission electron microscopy. 227High-resolution transmission electron microscopy reveals that WSe 2 nanocrystals possess hexagonal lattice fringes under bright-field mode and have a 0.27 nm interlayer spacing, which is different from that of MoSe 2 , that is, 0.282 nm. 228,229imilarly, the point defects are easily recognized after single-photo emission measurements. 199The narrow line emitting centers (NLEs) around WSe 2 flakes act as singlephoton emitters. 230The NLEs survive at a temperature range and have negligible changes under vacuum compared with open air. 231

| PREPARATION OF 2D WSE 2 NANOSHEETS BY EXFOLIATION METHODS
Two strategies are employed to synthesize WSe 2 monolayer crystals.One is the top-down method, including mechanical exfoliation, electrochemical exfoliation, chemical exfoliation, lithiation, and delamination.The other is the bottom-up strategy, which refers to molecular beam epitaxy, metal precursor plus post selenization, and chemical vapor deposition (CVD).In this section, we first discuss the exfoliation methods.

| Mechanical exfoliation
Mechanical exfoliation describes the simple process for isolating the WSe 2 individual layer out of the bulk WSe 2 material residing on a substrate, such as Si/SiO 2 .
For a mechanical exfoliation process, first, WSe 2 bulk is pressed firmly onto scotch tape; then, the tape is gently peeled away, and the part of the tape with WSe 2 sheets is refolded with a clean adhesive until the remaining piece becomes light grey.Eventually, one can press the tape with the WSe 2 nanosheets on Si substrates with 300 nm SiO 2 and peel off the tape to obtain monolayer and multilayer WSe 2 nanosheets.To clean the sample surface, the WSe 2 sample can be rinsed in acetone, water, and propanol to remove organic residues. 58owever, the low yield restricts the industrialization of this process despite its high quality.

| Chemical exfoliation by organic solvents
Chemical exfoliation, with the assistance of organic molecules to expand the interlayer spacing, facilitates the delamination of 2D materials using weak forces such as sonication, which is analogous to the exfoliation of graphene. 232TMDCs such as WS 2 , MoS 2 , MoSe 2 , WSe 2 , and MoTe 2 have been successfully isolated with different organic solvents.The chemical structure of the different solvents and their dispersibility has a direct relationship with the exfoliation efficiency.
Additionally, the tuning of the dispersed concentration 228,233 affects exfoliation with sonication in various solvents (Figure 3).
Such chemical exfoliation-derived WSe 2 nanosheets can be coated on an fluorine-doped tin oxide (FTO) substrate in an electrodeposition process (Figure 3).The resultant thin film of nanocrystalline WSe 2 has shown a great photoresponse in a heterojunction photodetector.
Liquid exfoliation is a facile and wet method for TMDC nanosheet production.The intercalation of organic molecules provides sufficient energy to expand the layer spacing and yield exfoliated nanosheets.These nanosheets can be obtained in a colloidal suspension and are a few layers thick in the supernatant in Eppendorf tubes after sonication and liquid separation. 234 typical protocol is depicted as follows.First, 5 mg of bulk WSe 2 is added to 5 mL of pure and mixed solvents and water with surfactant.Then, sonication in an ice bath is performed with a probe tip for the cyclic program.Eventually, the exfoliated material is isolated from the supernatant and precipitated by centrifugation. 234 improve the yield, a modified liquid-phase ultrasonication method was developed with improved synthesis efficiency. 235

| Electrochemical exfoliation
The solvent electrochemical exfoliation is an efficient strategy for the preparation and functionalization of 2D nanosheets.
There are two different routes of electrochemical exfoliation.TMDCs are mixed with alkali metals in a tertbutyllithium phase and then immersed into the water so that isolation occurs upon the intercalated alkali metals reacting with water.Subsequently, the gaseous byproducts promote the continuous isolation of TMDC nanosheets. 236ere, the resultant nanosheets have lateral sizes of several microns.The other method, so-called bipolar electrochemical exfoliation, further decreases the size of the nanosheets to nanoparticles of 2D TMDC via a generated electrical field to promote electrochemical reactions. 237he method has the advantage of a high production rate in the aqueous solution phase but possesses the disadvantage of producing nanosheet dispersions with low concentrations.Hydrothermal methods have the advantages of simplicity and low energy consumption.][240][241][242] WSe 2 films are synthesized with N,N-dimethylformamide as the solvent for selenium powder, which cooperates with a reducing agent, that is, sodium borohydride and sodium tungstate dehydrate.The reactions occur over a quartz substrate at high pressure maintained by a stainless-steel autoclave. 229ow we bring the readers the big picture of CVD of WSe 2 .

| SYNTHESIS OF 2D WSE 2 MATERIALS WITH CVD METHODS
The growth of the atomic thickness can be controlled via physical vapor deposition (PVD) and CVD.Compared with PVD-grown films, the CVD-grown samples usually show a greater domain size.Furthermore, in CVD samples, the antisite defects are theoretically less detrimental for optoelectronic applications than those in PVD samples. 243rior to growth, the substrates are cleaned with sophisticated procedures including soaking in semiconductor cleaning agents, rinsing in organic solvents, and sonicating in deionized water.
CVD is one of the most efficient, widely used, and popular synthesis routes to prepare WSe 2 .There are various kinds of CVD processes, including low-pressure 244 and ambientpressure CVD, 243,245 metal-organic CVD, 246,247 thermal CVD, 248,249 plasma CVD or so-called plasma-assisted CVD, 250,251 plasma-enhanced CVD (PECVD), 252,253 and pulsed-laser deposition. 254With analysis technology being developed, precise control of WSe 2 synthesis, that is, layer number and grain size, becomes possible.

| Low-pressure and ambientpressure CVD
WSe 2 triangular domains and membranes are both grown through atmospheric pressure CVD.Ambient-pressure CVD refers to heating the working chamber at a pressure of 1 atm. 255ow-pressure CVD works in a pumped chamber with a gas pressure of approximately 1000 Pa (ca.0.01 atm or 10 mbar). 256Under low-pressure CVD conditions, gas molecules have a large mean free path and a greater collision probability with the substrate; hence, the grain size can be enhanced with reduced nuclei density, which eventually can improve the film quality. 243

| Types of precursors
The CVD parameters that significantly affect the resultant WSe 2 are the tungsten feedstock, salt incorporation, selenium supply, hydrogen concentration, and argon flow rate.

| The choice of tungsten-containing precursors
WSe 2 can be synthesized using precursors such as WO 2.9 , WO 3 , WOCl 4 , W(CO) 6 , and WCl 6 . 257Instead of elemental W sources, transitional metal oxides, WO 3 , are often selected as transitional metal sources.WSe 2 forms through the reaction of WO 3 species, that is, the volatile suboxides species of WO 3-x , with hydrogen and expands in size on the substrate. 258

| Salt assistance
Salt-assisted strategies have shown great advantages in CVD growth of TMDC. 205In other words, the incorporation of salt promotes the growth dynamics of WSe 2 , that is, selenium and WO 3 react in a rapid fashion in the presence of halide (Cl − , Br − , and I − ) ions.The alkali halides, that is, NaCl, KCl, KBr, and KI, which include halide ions, can react with tungsten oxides (high melting point) to form intermediate tungsten halides (low melting point) or tungsten dihalide dioxide, for example, WO 2 Cl 2 .Such an intermediate tungsten-containing species can lower the reaction melting points and increase the rate of the reactants.Prior to the CVD reaction, the alkali halides can be mixed together with tungsten precursors, [259][260][261] mounted next to tungsten oxides, [262][263][264] and deposited onto a substrate. 265,266Very recently, NaCl was directly used as a substrate to grow MoS 2 nanoribbons, 267 which may suggest the direct synthesis of WSe 2 over an alkali halide. 268

| Se crack
Early in 1991, the influence of selenium on WSe 2 growth was investigated, that is, the formation of a micrometer thin film.According to the fundamentals of physical chemistry, growth dynamics are determined by three factors, that is, the concentration of the reactants, the temperature, and the catalyst, of which the concentration has the most influence.First, the growth and orientation of WSe 2 films are affected by the selenium vapor pressure, which is controlled by the temperature.Upon vaporization, the selenium vapor is mainly composed of Se 8 molecules.
When further increasing the temperature, Se 8 decomposed into more reactive radicals of Se 8-x , 269 such as Se 4 or Se 2 .
Here, the chemistry behind the increased reaction rate is the increased concentration of Se radicals, for example, Se 2 or Se monomer at elevated temperature.Note that the minimum temperature is located in the range of 548 ± 10 K. Second, different temperatures of the selenium source lead to different orientations.A WSe 2 film with a stoichiometric composition can be achieved by soft selenization. 269hermal cracking of selenium influences the process of selenization.In the selenization reaction of copper, indium, and gallium alloys, the highly cracked-Se atmosphere lowers the reaction threshold of CuGaSe 2 with increasing temperature and promotes the reaction forming a Cu(In, Ga)Se 2 solid. 270n addition, the selenization process can be enhanced by nickel catalysis, 271 plasma assistance, 272 and laser irradiation. 273

| H 2 incorporation
Hydrogen is frequently employed in pretreating catalyst support for 2D material synthesis.For example, during graphene or related sp 2 carbon growth, hydrogen plays a dual role in decomposing the hydrocarbon to boost the concentrations of reactive carbon radicals and in etching the surrounding amorphous carbon as a cleaning agent. 274,275As for TMDC synthesis, hydrogen, on the one hand, functions as a cracking agent to decompose the molecular ring of S 8 or Se 8 . 276,277The produced S or Se radicals possess high reactivity that can improve the growth rate.This also affects the morphology (planar 278 or pyramid 279 ) and lateral grain size of WSe 2 .
On the other hand, hydrogen etches the growth frontier of WSe 2 by liberating the Se atoms in the form of gaseous molecules, such as hydrogen selenide.Here, desorption of selenium species, resulting in etching of WSe 2 , has competition with chemisorption of Se species, which leads to selenization on surface sites.When the hydrogen supply is negligible, the pure Se radicals have relatively low reactivity with transition metal atoms, and WSe 2 formation is limited.When gradually increasing the hydrogen content, the WSe 2 synthesis rate can be enhanced.However, a further increase in the hydrogen flow rate, that is, an overly sufficient supply of hydrogen, results in inhibition of selenization.With a low hydrogen partial pressure, triangular crystals are the predominant form.With increasing hydrogen flow rate, hexagonal WSe 2 crystals are predominantly deposited on the substrates (Figure 4). 280

| Gas flow rate
The gas flow rate influences the concentration of reactants and affects growth dynamics.As the flow rate decreases, adatoms and nucleation sites over the surfaces of the substrate are reduced, which influences the average nuclei density and may thereby leave more blank space to boost the average domain size. 281With increasing flow rate, the growth rate is promoted.However, a high gas flow rate leaves little time for the decomposition and reaction of gaseous species. 282he gas flow parameter also affects the thermodynamics of 2D material synthesis.When the gas valve is switched on, gas molecules are introduced into the chamber.These freshly introduced molecules typically have an average temperature of 25 C when coming out of the cylinder and possess low internal energy.When heating up in the pre-heater ahead of the reaction zone, which occurs in a three-zone horizontal furnace, the energy level of these precursors is elevated close to the maximal energy level, which allows for the synthetic reaction to occur.When the flow rate of the carrier gas increases, an inert gas such as argon is added (to dilute the reactive gases due to safety concerns).

| Metal-organic CVD
In the metal-organic chemical vapor deposition (MOCVD) technique, tungsten-containing organic compounds such as W(CO) 6 and selenium-containing gas such as (CH 3 ) 2 Se are evaporated and deposited on a substrate at elevated temperatures.An early report of large domain TMDC monolayer control via MOCVD was published in 2015. 282he process features fine control of TMDC growth, that is, domain size, nucleation density, and shape.Indeed, the WSe 2 size is proportional to the process temperature and pressure.
An MOCVD system is fulfilled with an atmosphere of tungsten hexacarbonyl, W(CO) 6 , and dimethyl selenium, (CH 3 ) 2 Se, as shown in Figure 5.The energy of the tungsten and selenium precursors along the dissociative reaction coordinates can be evaluated by W(CO) 6 and (CH 3 ) 2 Se dissociation, respectively.MOCVD processes with other W-, Se-, and Te-containing precursors have corresponding dissociation energies. 283

| Thermal CVD
The thermal CVD of tungsten diselenides refers to the selenization of tungsten oxides.Typically, oxide precursors such as WO 3 powder are placed in a high-temperature furnace, and Se powder is placed upstream and kept at a lower temperature than WO 3 .The substrates are located above the WO 3 downstream where the Se and WO 3 vapors reach.A mixed gas of H 2 and Ar carries the Se vapor into the substrate zone and initializes the selenization reaction.Typically, the synthetic reaction lasts 15 min when the zones with WO 3 and the substrate reach 890 C-900 C and the low-temperature zone with Se powder is heated to 250 C and 260 C. Finally, the synthesis is terminated and cooling occurs, and the sample is removed for further characterization. 283

| Plasma chemical vapor deposition
PECVD is a well-developed industrial technology, particularly in thin-film production.Plasma CVD requires a reduced temperature for operation compared with thermally driven CVD.A plasma is generated by igniting and maintaining a high-frequency voltage to a gas with low partial pressure.In PECVD, the plasmas are typically composed of electrons, ions, and neutral species in both excited and ground states.

| Others
Other strategies have been employed to boost growth.For instance, large triangular domains were synthesized in the presence of Cu vapor, 14 which plays a role as a catalyst to decrease the activation energy of WSe 2 formation.
Cold wall CVD has also shown success in WSe 2 synthesis.Compared with the volatilizing of solid selenium sources and tungsten oxides, the incorporation of gaseous sources such as W(CO) 6 and H 2 Se has the advantages of easy control in inductively heated cold-wall CVD.The results demonstrate the reproducible epitaxial growth of a large area monolayer film of WSe 2 . 284ost recently, pulsed-laser deposition 285 and atomic layer deposition 223 have become generally available for WSe 2 film formation. 229fter knowing all kinds of synthesis approaches, comprehensive understanding of growth mechanism is still missing and yet is achieved the synthesis protocol for full film monolayer WSe 2 single crystal over a large area.Currently, the limitations and challenges of WSe 2 synthesis research include three aspects.First, the individual domain of monolayer WSe 2 can be synthesized through CVD with an average grain size of 100 μm. 205owever, the monolayer WSe 2 grains cannot coalesce into strictly monolayer films; viz., the substrate is deposited with isolated small grains. 77Second, precise layer control seems difficult, although full coverage of WSe 2 can be achieved by elongating the growth time.Hence, homogeneity control for uniform thin film fabrication of strictly monolayer WSe 2 remains a challenge.Third, a continuous WSe 2 thin film of tens of nanometers can be derived from the selenization of W thin films.However, these thin films are composed of nanocrystalline grains that show limited electric transport performances. 286To sum up, these issues from aspects of material synthesis require continuous and productive contributions from process optimization related scientists and engineers.
Indeed, the thermodynamics and kinetics of WSe 2 growth have yet been reported.One may borrow idea from analogous 2D materials in the means of temperaturedependent growth, [287][288][289] catalyst assistance, 290 vapor phase synthesis, [291][292][293][294][295] selection of feedstock, [296][297][298] in situ characterization tools, [299][300][301] and external energy input 302 other than thermal sublimation.Then the chemical parameters such as the activation energy for regulating the nuclei formation 303 and grain expansion 304 could be derived from the Arrhenius plots. 288Next, defects 305,306 play an important role in regulating the electric transport performances.In addition, machine learning 300,301 could be employed for the design of novel types of 2D materials or alloyed phases.

| APPLICATIONS
Graphene has diverse fundamental and technological applications.Its features of chemical inertness and adsorption of certain molecules offer opportunities for application with 2D TMDCs, for example, sensing, electronic devices, and energy storage. 307TMDC heterostructures also provide many applications.ZnO/WSe 2 heterostructures can be employed as a photocatalyst for water splitting toward hydrogen production. 308

| Electronics
The combination of WSe 2 with many other materials has been explored in electronic devices.After multilayer WSe 2 is sandwiched between top and bottom graphene layers, one can observe quantum interference and Coulomb drag at room temperature. 309Te 2 (M = V, Nb, or Ta) nanosheets can be deposited on a WSe 2 or WS 2 substrate in an epitaxial fashion.Epitaxially grown two-dimensional metallic MTe 2 has been used as the contact for semiconductors with improved electronic performances. 310

| Field effect transistors
In an field-effect transistor (FET) with few-layer WSe 2 , a charge carrier mobility up to 70.1 cm 2 ÁV −1 Ás −1 and an outpaced 10 6 ON/OFF current ratio was achieved with an Al 2 O 3 top-gate dielectric. 311he source and drain electrodes are typically composed of Ti/Au (Au is sometimes replaced with Pt or Pd). 312,313Se 2 FETs have a bipolar characteristic that experiences significant suppression after employing the electron-withdrawing function of graphene oxide. 314lso, FET device performances degrade in the presence of a residue, such as polymethylmethacrylate (PMMA), which decreases carrier mobility. 313For example, backgated MoS 2 and WSe 2 FETs become p-type semiconductors with PMMA residue. 313 WSe 2 device was integrated with a VO 2 stripe as a dielectric for a drain electrode to complete a sensing and switching device (Figure 6).
The device is fabricated with Ti as the electrode for the source and h-BN as the insulating layer for the gate electrode.The fabrication process is shown in Figure 6.In such switching devices, phase-change materials such as VO 2 can enrich the sensitivity, that is, decrease the threshold of the gate voltage to switch on the transistor. 315dvanced FET configurations have been verified with the WSe 2 homojunction for tunnel FET application.(Figure 7) Such incorporation of large gate dielectric, as well as gate spacing, promotes high subthreshold swing in the band to band tunneling. 316 addition, a p-i-n junction of WSe 2 homojunction has been achieved with selective ionic liquid doping under a bias voltage. 317The accumulation of holes and electrons in the channel simultaneously causes static electricity to be generated in the p-i-n junction.When the device is cooled, the carriers induced by electricdouble-layer transistors are electrostatically fixed, and a p-i-n junction is formed. 317Here, the carrier density of the WSe 2 electric-double-layer transistor is 10 13 cm −2 , and the Portable electronics 318 may require the embedding of energy generators for self-powering, [319][320][321] selfsupporting, 322,323 and energy storage components [324][325][326][327] and useful life estimation, 328,329 which can be integrated for electronic systems for and internet of things. 319Also, tactile sensors [330][331][332] targeting for electronic skin usage may require the signal amplification with electronic components such as synergistic sensors, 333-335 transistors [336][337][338][339] and memory devices. 340

| Gas sensors
2][343][344] Their large specific surface area ratio enhances the sensitivity of the sensors. 345 WS 2x Se 2-2x alloy gas sensor was engineered by thermal evaporation technology. 345Of four different x ratios, the large-area 3L WS 0.96 Se 1.04 alloy showed the optimal response upon NO 2 exposure in the gas sensors (Figure 8A-C).Therelationship between device performance and structure has been thoroughly analyzed with the characteristics shown by the linear I-V plot (Figure 8D).The energy band alignment  changes with the S composition (x) (Figure 8E), WSe 2 and WS 2x Se 2-2x alloy energy band (Figure 8F).Through cycles of NO 2 , the gas sensors have similar current changes (Figure 8G,H).As the whole concentration increases, the alloyed 2D materials boost the gas sensor current.

| Photoelectronics: Photodetectors
Thin-film solar cells may stem from the large volume of light absorber materials, 346,347 therefore the commercial photovoltaic panels [348][349][350][351] favor the strategy of micrometer thick device.For low-dimensional materials, one may explore the high detectivity with high light conversion efficiency, [352][353][354][355][356] low threshold for sensing, 357,358 plasmonic enhancement, 359,360 and the flexibility over a plastic substrate. 361,362Moreover these advantages could be applied in the photocatalysis 363 and electrocatalysis. 364he extraordinary electrical properties of WSe 2 satisfy its use in photodetectors.Furthermore, the p-n heterojunction layer formed by stacking WSe 2 and other TMDCs contributes to improving the photoelectric performance of these devices.
The responsivity of the WSe 2 photodetector is demonstrated by the configuration with a constant drain source, V ds , under direct monochromatic illumination (Figure 9). 365The current-voltage (I-V) plots under visible light and time-resolved photocurrent (I pc -t) 285 show that the bias voltage is proportional to the separation efficiency of the electron-hole pairs and inversely proportional to the carrier transport time.The transient photocurrent shows that the process current initially increases and then slowly decreases after the light turns on.In contrast, the device achieves fast current relaxation in darkness.Obviously, a positive correlation between the incident power density and photocurrent is confirmed.
A summary of the reported WSe 2 -based photodetectors has been listed in Table 1.

| Piezotronics
Piezoelectricity is the electric charge generated in solidstate noncentrosymmetric materials when applying mechanical strain, [414][415][416][417][418] that is, electricity produced from pressure. 419In the early stages of research, conventional wurtzite crystals such as ZnO [420][421][422] showed great piezoelectricity, 423,424 that is, a coefficient of 14.3 pm V −1 , 420 but these crystals are fragile and inhomogeneous (after production) and cannot be employed in large-area, flexible 425 and stretchable electronics. 426Therefore, novel alternative materials for boosting piezoelectric performance are needed.Indeed, 2D chalcogenide materials possess both great piezoelectric properties 427,428 and excellent flexibility. 429,430mong TMDCs, WSe 2 is one of the best piezoelectric materials with a low piezoelectric coefficient of 2.64 pm V −1 . 431oreover, bilayer, twisted WSe 2 shows advantages over monolayer 432,433 and Bernal bilayer WSe 2 434,435 owing to the low flexibility of the monolayer and loss of noncentosymmetry in Bernal bilayer WSe 2 .Multiple WSe 2 piezoelectric devices have been integrated into a nanogenerator array for self-powering an LCD display (Figure 10).

| Piezo-phototronics
][439][440] In these devices, the piezoelectric-polarization charge upon strain can be used for modulating a photoelectronic process. 441Indeed, such piezophototronic effects 442 have been demonstrated in GaN [443][444][445][446] and wurtzite ZnObased devices, [447][448][449] such as solar cells, LEDs, and photodetectors. 450,451][454] Thus, the strain-induced polarization charge 428,455,456 can play a role in tuning transport behaviors, that is, it can avoid the use of gate electrodes and voltages and simplify the entire device configuration. 457ndeed, the demonstration of piezophototronic devices with WSe 2 /MoS 2 p-n junctions has been achieved over a flexible substrate of polyethylene terephthalate (Figure 11).
When applying strain ranging from −0.85% to 0.53%, the polarization charges in WSe 2 can realign the band diagram, which causes the acceleration or deceleration of hole transport, that is, the manipulation of electric transport. 457Eventually, the optimal photoresponsivity was realized to be 1.86 times that in the dark state by introducing a −0.62% strain.Such piezophototronic effects may shed light on novel applications of ultrathin photoelectronics.
Beside the inorganic heterostructures, the incorporation of organic semiconductor has emerged and become an important research trend.][463][464][465] The investigation of organic/WSe 2 interface has led to a milestone for comprehensive understanding 466 of the physics, light-matter interaction, and device behaviors.Typical organic semiconductors have been chosen among the species such as metal phthalocyanines, 223,467 fluorinated fullerenes, 468,469 perylene-tetracarboxylic, 470 rubrene 471 and pentacene 472 as well as their related derivatives. 473Indeed, the organic/TMDC heterojunction showed promising rectifying ratio of 10 5 , which is comparable to 474 and may exceed some inorganic p-n junctions. 104

| Memory based on complementary metal-oxide semiconductor techniques
The complementary metal-oxide semiconductor (CMOS) is a fabrication process that uses symmetrical and complementary n-type and p-type MOS FETs for logic gate applications. 475he CMOS inverter composed of p-WSe 2 and the n-MoS 2 FET showed negligible hysteresis with static voltage transfer because of the encapsulation of h-BN. 476he CMOS inverters of p-WSe 2 and n-MoS 2 display drain saturation currents of 3.1 and 18.0 μA and on/off current ratios of 4.5 × 10 6 and 1 × 10 8 , respectively, and a voltage gain (up to 3) at a supply voltage of V DD = 1 V (Figure 12).
WSe 2 as a channel material has been successfully applied in CMOS static random-access memory (SRAM).It features low power consumption through direct current analysis. 477The tri-gate n-FET 45 is compatible with read/write technology, and the device realizes stable operation of SRAM with a VDD of 0.8 V.The p-type MOSFET 478,479 is induced through doping by an airstable oxygen plasma.1][482] Due to the ultrathin nature of WSe 2 , its application in flexible electronics can be foreseen.Writing and erasing characteristics of a WSe 2 -based memory are shown in Figure 13.

| Logic inverter
Logic gates are important components for processing 0 and 1 signals in digital computing.Gate circuits include AND, NOT (inverter), and OR and their combinations Copyright (2015) American Chemical Society such as NOR, NAND, XNOR (equivalence gate), and XOR (exclusive-OR gate).Among them, the inverter is the most fundamental gate.
A complementary logic inverter has been fabricated with n-type and p-type FETs of WSe 2 .The Pt metal contact leads to p-type FET formation, and doping with potassium ions results in n-type FET performance.The current switching ON/OFF ratio of both FET devices exceeds 10 4 .Eventually, the DC voltage gain of such an inverter exceeds 12. 478 Indeed, a WSe 2 CMOS inverter with a full logic swing, high voltage gain, and an adaptable noise margin has been demonstrated. 484igh-performance air-stable WSe 2 CMOS technology 485 exhibits the advantages of a high noise margin, full logic swing, and good voltage transfer characteristics, which allow the inverter to possess a low static power consumption and a large voltage gain (Figure 14).
Adding tetrafluoro-tetracyanoquinodimethane (F 4 TCNQ) to the WSe 2 channel can increase the tunneling current. 485lectron charge transfer occurs from the WSe 2 layers to F 4 TCNQ-PMMA to achieve localized p doping.
The WSe 2 nanosheet and ZnO nanowire FETs form a hybrid complementary logic inverter (Figure 15).
The first WSe 2 channel acts as a p-type conductor.Second, ZnO functions as an n-type channel.Then, Ni/ZnO acts as a Schottky junction and is applied a forward bias voltage with the Ni contact connecting to the source electrode of the WSe 2 device.When the Ni electrode is connected with the ground, Ni/ZnO is set at a reversed bias voltage. 486

| Rectifiers
A rectifier refers to an electrical device that converts an alternating current to a direct current and features a one-directional flow.Such a process is well recognized as rectification because it regulates the current direction.Quite often, a rectifier is composed of a metal and semiconductor contact, which is termed the Schottky junction.Metal/WSe 2 can form a rectifier, for example, Cr and Ir. 487Additionally, the ionic liquid-gated WSe 2 transistor has Schottky contact behavior.In 2D materials, graphene acts as a semimetal, and TMDC or phosphorene produces a semiconductor. 489herefore, a graphene/TMDC contact exhibits typical rectifier performance. 490,491ndeed, a rectifier with graphene/WSe 2 architecture exhibits an ideality factor of 1.83 as well as a rectification ratio of 10 4 .Such rectification performances can be further regulated with the frequency and duration of the pulsed gate voltage. 492Note that an interlayer of h-BN is often inserted between graphene and WSe 2 to form a metal-insulator-semiconductor architecture for better charge modulation (Figure 16). 493or example, WSe 2 and black phosphorus (BP) heterojunctions can form rectifiers. Based on gate voltage tunable ambipolar charge carriers in WSe 2 and BP, heterojunctions have shown rectification properties. 102Here, isotype heterostructures of BP-WSe 2 can be modulated to form p-p and n-n junctions, which feature reification performances. 102Moreover, such a heterojunction can be tuned into a p-n diode, which has shown significant photovoltaic performance with 1.70% power conversion efficiency, a V OC of 0.35 V, and an external quantum efficiency of approximately 23% at a gate voltage of −10 V. 102 Rectifiers also include boost rectifiers, 494 three-phase active rectifiers, 495 and pulse width modulation rectifiers. 496,497ote that In/WSe 2 204 and Pd/WSe 2 498 can form ohmic contacts for smooth charge transfer and function as an amplifier.Additionally, oxides with high work functions lead to ohmic contact, that is, the MoO x /WSe 2 interface. 499Here, ohmic contact provides low resistance contact to enhance the charge transport properties, such as the carrier mobility and on/off ratio.

| Light-emitting diodes
A light-emitting diode is a light source that emits light when current flows through a semiconductor.
The electrons recombine with holes in the semiconductor and liberate the energy as photons.The color of light is determined by the band gap of the semiconductor.Monolayer WSe 2 possesses a direct bandgap of 1.6 eV, which corresponds to near infrared light (775 nm).The external quantum efficiency of a WSe 2 device, that is, a luminescence quantum well, reaches 5% 500 and increases with temperature.Indeed, this performance is over 200 times higher than that of MoS 2 and MoSe 2 .The positive bias and negative bias of the device are asymmetrical (Figure 17).The temperature-dependent difference results from the inversion of spin-orbit splitting over states of the conduction band in WSe 2 .This leads to dark excitation.
The WSe 2 ambipolar transistor forms electrostatic p-in junctions with a semiconductor channel, thereby emitting circularly polarized electroluminescence.This phenomenon can be understood as a planar electric field controlling the overlap of electron holes. 501Here, a valley-optoelectronics technology could be developed by employing the valley degree of freedom.
A light source that emits a single photon is a key drive for quantum technology.A single photon can be emitted at the site of a single defect in the WSe 2 channel.A monolayer WSe 2 p-i-n device with h-BN 502 as the insulator can electrically drive the emission of a single photon by employing both lateral and vertical van der Waals heterostructures. 503The electroluminescence spectral lines are narrow, which matches well with excitons from optically excited defects.Here, the intensity contour of the spatially resolved emission mapping presents its maximum in the confined region, that is, the defect sites of the WSe 2 sample.The features of linearly polarized selection rules as well as the doublet of exchange splitting confirm the existence of single photon emission.
The resistance of a WSe 2 /h-BN/few-layer graphene device is determined by the vertical tunneling resistance. 485epending on the bias conditions, 504 the device exhibits electroluminescence of positrons and negative electrons (Figure 18).
Recently, a transient electroluminescent device 505 has been demonstrated for 2D TMDC material.Also, monolayer WSe 2 displayed a photoluminescence quantum yield of unity under defect passivation treatment. 506ransient operation reduces the dependence of the device electroluminescence 506 on the polarity or height of the Schottky barrier.A brief summarization of the structure and property has been illustrated in Figure 19.
Electroluminescence is achieved by adding an alternating voltage between the gate and the semiconductor to achieve an undoped transparent millimeter-scale single-layer semiconductor WSe 2 device. 506ndeed, the WSe 2 /MoS 2 heterojunction suppresses the radiation recombination rate through h-BN and injects carriers between the layers to establish interlayer excitons [507][508][509][510] to achieve tunable electroluminescence.Selective injection of electrons and holes can be attained through the graphene electrode.Some of the Auger carriers are recovered by relaxation and then converted into electroluminescence. 511onolayer WSe 2 can be optimized with halide growth and substrate decoupling, and the photoluminescence quantum ratio obtained is superior to that of WSe 2 produced through mechanical exfoliation. 245

| SUMMARY AND OUTLOOK
In this work, a broad comprehensive picture of research on two-dimensional WSe 2 has been presented.First, we introduced the fundamentals of WSe 2 , including its crystal structure, electronic structure, bandgap, electronic properties, and optical properties.More importantly, monolayer WSe 2 -based electric devices were discussed, such as sensors, field effect transistors, and photodetectors.Eventually, complicated device architectures have been demonstrated as proof of concept such as piezotronics, piezophototronics, memory, logic inverter, and light emitting diodes.
Currently, opportunities remain for studies in four areas.First, the synthesis strategies of WSe 2 should be continuously optimized for better understanding the vapor-liquid-solid mode 267 and self-alignment mechanism of (ie, previously shown in growth of graphene 512,513 and h-BN 514,515 over surfaces of liquid metal) of individual crystalline domains.Subsequently, large crystalline WSe 2 islands 516 are sewed up as a seamless single crystalline film, viz., the fabrication of wafer-scale, homogeneous WSe 2 thin films 517 with precise layer control 14,256 is still needed.In a word, it is particularly important to prepare TMDC materials as a single-layer continuous film and ideally a single crystal domain over a wafer scale. 518urthermore, the transfer 519,520 or low temperature direct growth 517 of WSe 2 over flexible [521][522][523] and stretchable substrates [524][525][526] could lead to progress in wearable electronics.3][534][535][536][537] With the incorporation of polymeric support, WSe 2 devices can be integrated into wearable electronics [538][539][540][541][542][543][544] and the internet of things. 545,546econd, the tuning of charge transport behaviors may be enriched with the doping strategy 547 or defect engineering. 548Here, doping 13 with foreign atoms, 549,550  F I G U R E 1 9 The scheme of structure, properties, and applications of WSe 2 .Basic devices are introduced, including photodetectors, gas sensors, and field effect transistors.Eventually, readers are presented with novel information devices, such as lightemitting diodes, rectifiers, logic inverters, CMOS circuits, piezotronics, and piezophototronics molecules, [551][552][553][554] or defects 555,556 induced with vacancies may lead to the elevated concentration of charge carrier and the regulation of energy levels of acceptor and donor.Also, selection of surface contact could bring new electrical conductivity, that is, interface engineering 557,558 with polymeric modification 559 and quantum dots 560 based surface decoration (also with metallic nanoparticles). 561hird, device arrays [562][563][564] and system integration 565 out of high-quality WSe 2 materials should be considered.For example, the recognition of simple or complicated shapes could be optimized, viz., the 2D mapping of the photocurrent of each pixel throughout the full area of the WSe 2 continuous film.Here, the difference of current at different region of WSe 2 could be achieved with the selective light illumination, that is, the projection of a light beam generated from a patterned hard mask.
In addition, individual electronic device based on WSe 2 has been fabricated with e-beam lithography 566 for direct writing of a targeted pattern over a WSe 2 sample. 209he responsiveness of individual WSe 2 photodetector could be experimentally realized with approaching its theoretical limit in the means of optimizing materials and device fabrication process.Here, the device array could be manufactured with channel length of 5 μm, and the yield of working devices could be improved with good homogeneity over a centimeter scale, that is, with a big single crystalline domain of WSe 2 over a sub-centimeter scale.Here, the equipment of electron beam lithography is necessary to achieve sub-10 μm line width but increases the total fabrication cost.However, device cost could be largely reduced with employing UV light lithography process (for the fabrication of ca.20 μm line width). 255,313Therefore, pixel recognition with low resolution seems worthy for exploration in the aim of cutting cost.
Last but not the least, the fundamental properties of WSe 2 , such as exciton, spin transport, and valley transport 567 have emerged as hot spot topics with the input of tools for low temperature physics and photophysics, 568,569 that is, scanning tunneling microscopy, current sensing atomic force microscope, 570 femtosecond time-resolved pump-probe spectroscopy, 571,572 absorption and photoluminescence spectroscopy, 140,[573][574][575] electric probe station with strong magnetic field (hall measurement), 576 magneto spectroscopy, 577 and in situ transmission electron microscopy with applying bias voltage.1][582] A small twisted angle between van der Waals heterostructure may cause formation of an interlayer moiré exciton. 1,583Similar exciton physics may exist for electronhole liquid 584 or so called droplets. 585With assistance of magnet and spin, the device out of spin transfer and torque may result in the size decrease and upscaling of logic gate circuits for information processing.Also, the valleytronics of WSe 2 and its planar heterostructures with other 2D semiconductor 586 may lead to the high density and low voltage data storage compared to conventional magnetic disk storage as well as NAND flash memory.Indeed valley polarization could be achieved upon applying strain 587 over 2D material as a waveguide and its depolarization may occur upon phonon absorption. 588Also, exciton diffusion and polarization and luminescent emission 589 provide a platform of excitonic circuits for testifying the photonics and quantum communication. 590verall, lots of opportunities remain for WSe 2 research and hence we call for research community from physics, chemistry, and engineering to continuously contribute to the understanding of such an important material.The synergetics of physical chemistry (including thermodynamics and kinetics), materials science, computer science, and other cross-field disciplines may lead to successful synthesis of large area-controlled growth of tungsten diselenides and fruitful device applications.

F I G U R E 1
Crystal structure of a transition metal dichalcogenide, MX 2 .(A) Atomic configuration of a representative MX 2 structure, demonstrated with yellow balls (halogen atoms, X) and green balls (metal atoms, M). (B) Optical microscopy graph and (C) atomic force microscopy micrograph of a flake of monolayer WSe 2 .(D) Photograph of WSe 2 bulk material.(E) Three structural phases of WSe 2 , including 2H, 3R, and 1T polytypes.WSe 2 has a lattice constant a ranging from 0.31 to 0.37 nm, as well as an interlayer spacing of 0.65 nm.(A) Reprinted (adapted) with permission from Ref. 13.Copyright (2016) American Chemical Society.(B, C) Reprinted (adapted) with permission from Ref. 14.Copyright (2016) Wiley-VCH Verlag.(D) Reprinted (adapted) with permission from Ref. 15.Copyright (1993) Elsevier.(E) Reprinted (adapted) with permission from Ref. 16.Copyright (2013) American Chemical Society investigate and review the most recent advances with WSe 2 in aspects of both synthesis and devices.

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I G U R E 3 Schematic diagram of WSe 2 device preparation.(A) Direct vapor transport technique for the synthesis of bulk materials.(B) Liquid phase exfoliation technique.(C) Photograph of colloidal nanocrystalline suspensions of WSe 2 and MoSe 2 nanosheets.(D) Schematics of the electrophoretic deposition method for nanosheet coating over transparent conducting FTO substrates.(E) Photograph of MoSe 2 /WSe 2 heterojunction thin films over FTO substrates.(F) Time-dependent photoresponse of MoSe 2 , WSe 2 , and their heterostructures.Reprinted (adapted) with permission from Ref. 228.Copyright (2019) American Physical Society.FTO, fluorine-doped tin oxide 3.Thermal deposition methods (hydrothermal)

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I G U R E 4 Synthesis of single-crystal WSe 2 .(A) Scheme of a CVD furnace with a protocol for fast heating of Se sources.(B, C) Optical images of triangular and hexagonal WSe 2 crystals on Si/SiO 2 .(D) Size demonstration of one individual WSe 2 crystal.(E, F) Optical images of WSe 2 crystals on Si /SiO 2 /Si 3 N 4 .(G) Magnified optical graph of a large-size WSe 2 crystal showing the edge.Scale bar 50 μm.(H) Histogram of the number counts of crystal size for triangular and hexagonal crystals.AFM height (I) and phase (J) images.Reprinted (adapted) with permission from Ref. 280.Copyright (2015) American Scientific Publishers.AFM, atomic force microscope

F I G U R E 5
WSe 2 synthesis in a metal-organic chemical vapor deposition furnace.(A) Schematic of the reaction of precursors to form WSe 2 over the substrate.(B) Fake-colored AFM micrograph of WSe 2 single crystal.(C) Raman spectra of WSe 2 grown over different substrates.Reprinted (adapted) with permission from Ref. 282.Copyright (2015) American Chemical Society Transistor performances of a WSe 2 channel with VO 2 as the drain dielectric.(A) Scheme of the WSe 2 field-effect transistor.Device fabrication process: (B) VO 2 film growth on Al 2 O 3 with pulsed laser deposition.(C) Patterning of VO 2 with reactive ion etching.(D) WSe 2 deposition onto a VO 2 -coated substrate through dry transfer.(E) Coating of Ti/Au electrodes.(F) The h-BN transfer for covering the WSe 2 channel.(G) Fabrication of metal electrodes.(H) Resistance vs temperature of VO 2 .(I) Current-voltage curves of the WSe 2 /VO 2 heterostructures at different temperatures.Qualitative band diagrams of Ti, WSe 2 , and VO 2 (inset).Reprinted (adapted) with permission from Ref. 315.Copyright (2019) American Chemical Society

F I G U R E 7
Structural representation and electrical performance curves of the WSe 2 p-n homojunction field-effect transistor.(A) Scheme of the WSe 2 device with electric doping through two top gate electrodes.(B) Transfer characteristic curves under different drain voltage.Output I-V characteristics of forward-bias voltage (C) and transfer curves at reverse bias (D).(E) Contour map of diode current vs double top gate voltage.BTBT, band-to-band tunneling; TGD, top gate above drain; TGS, top gate over source.Reprinted (adapted) with permission from Ref. 316.Copyright (2019) Wiley-VCH Verlag carrier mobility exceeds 10 2 cm 2 ÁV −1 Ás −1 .The current of the drain electrodes exhibits a reverse response at a temperature of 433 C.

F I G U R E 8
Gas sensor of WSe 2 -based alloyed materials.(A) Atomic configuration of WS 2x Se 2-2x alloy residing over a substrate.(B) Scheme of the gas sensor.(C) Photographs of the finger electrode over the WSe 2 alloy sample and bare substrate.(D) Current vs voltage curves of gas sensors of different 2D materials.(E) Energy band alignment at the Pd/TMDC interface.(F) Energy band diagram of the Pd/TMDC/Pd device architecture.(G) Gas-sensing performance of the device with exposure to a small amount of NO 2 .Inset: Energy band diagram of the device.(H) Response of each gas sensor with different TMDCs corresponding to the concentrations of NO 2 gas.Reprinted (adapted) with permission from Ref. 345.Copyright (2018) American Physical Society

F I G U R E 9
Electronic and photoelectronic response characteristics of the WSe 2 photodetector.Scheme of the WSe 2 photodetector from perspective view with (A) asymmetric contact and (B) symmetric contact.(C) Photocurrent response of a device illuminated with different power.(D) The responsivity and photocurrent of a WSe 2 photodetector under different incident power.(E) The relationship between responsivity and light power.(F) Open-circuit voltage vs incident power at metal/WSe 2 Schottky junction.(G) The photocurrent response upon light irradiation.(F) The time-dependent photoresponse of photodetectors of WSe 2 and WSe 2 /quantum dots hybrids.(A-F) Reprinted (adapted) with permission from Ref. 365.Copyright (2018) Wiley-VCH Verlag.(G-H) Reprinted (adapted) with permission from Ref. 366.Copyright (2018) American Chemical Society T A B L E 1 Photodetector performances of TMDCs WSe 2 piezoelectric nanogenerator for a self-powered LCD device.(A) Demonstration of one individual WSe 2 piezoelectric nanogenerator.(B) Optical micrograph of the WSe 2 device with Cr/Au electrodes.Embedded is a flexible device at a bending status consisting of WSe 2 over a PET substrate.(C) Optical graph of a PENG array of five twisted bilayer WSe 2 devices.(D) Photograph of 20 integrated twisted bilayer WSe 2 PENG devices in a flat table (left) as well as a strain platform (right).(E) Scheme of the shunt connection of 20 PENG devices for multiplying the output current.(F) Output current of an integrated WSe 2 PENG array with a shunt connecting different numbers of individual devices.(G) Output current-voltage of such an integrated PENG array.(H) Four snapshots captured from a movie of an LCD display powered by integrated WSe 2 PENG arrays in a strain cycle.Reprinted (adapted) with permission from Ref. 436.Copyright (2017) Wiley-VCH Verlag

1 1
Photoelectronic performances of a piezophototronic device of WSe 2 .WSe 2 /MoS 2 heterojunction overlapping characterization including an AFM micrograph (A) and optical graph of the photodiode devices (B).(C) Schematic of the test platform for the operating piezophototronic device.(D) I-t curve with illumination at different power densities.The energy band of the WSe 2 /MoS 2 photodiode without strain and at zero bias and (E) with moderate positive voltage polarization (F).Reprinted (adapted) with permission from Ref. 457.Copyright (2018) Wiley-VCH Verlag Schematic and electrical performance of a CMOS-fabricated logic gate with p-WSe 2 and n-MoS 2 FETs.(A) Schematic illustration.(B) Circuit diagram of the logic gate, such as the inverter.(C) I DS and V DS curves measured at different V GS for FETs of p-WSe 2 (left: negative voltage) and n-MoS 2 (right: positive voltage).(D) Transfer characteristics dependent on the gate voltage: MoS 2 (left) and WSe 2 (right).(E) Voltage gain curves (right) and static voltage transfer characteristics curve (left).(F) I-V curve of the CMOS inverter gate under V DD = 1 V. Reprinted (adapted) with permission from Ref. 476.Copyright (2018) Wiley-VCH Verlag

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I G U R E 1 3 Read/write operation of memory based on the WSe 2 CMOS technique.(A) The 1 state with I DS beyond 10 −12 A written with 5 V pulse and long maintained.(B) The memory state being written to 1 (beyond 10 −11 A) and erased to 0 (beyond 10 −12 A) with different pulsed voltage.Reprinted (adapted) with permission from Ref. 483.Copyright (2019) Wiley-VCH Verlag F I G U R E 1 4 Structure and band alignment of the WSe 2 device fabricated by the CMOS technique.(A) Schematic of the device architecture.(B) Optical micrograph of a WSe 2 CMOS device with AlO x -encapsulated n-MOS and F 4 TCNQ-PMMA-doped p-MOS.(C) Strategy of p-type doping.(D) Band diagram of the aforementioned substances.Reprinted (adapted) with permission from Ref. 485.

F I G U R E 1 6
Characteristic curve and schematic diagram of WSe 2 rectification.(A) Optical microscopic image of gated graphene/WSe 2 Schottky junction.(B) Side view of the rectifying device.(C) Diagram of the flat energy band of such a junction biased at a positive gate voltage.(D) Output current vs voltage curve of a Schottky junction at different gate voltage.(E) Diode ideality and leakage current depending on gate voltage.(F).Reprinted (adapted) with permission from Ref. 493.Copyright (2019) Wiley-VCH Verlag

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I G U R E 1 7 Electro-and photo-luminescence performances of WSe 2 devices with h-BN encapsulation.Contour maps of electroluminescence with bias voltages: (A) Negative.(C) Positive.(B) Contour maps of the photoluminescence of the WSe 2 device.(D, F) Electroluminescence spectrum.(E) Photoluminescence spectrum.Reprinted (adapted) with permission from Ref. 500.Copyright (2015) American Chemical Society

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I G U R E 1 8 Schematic diagram of a metal-insulatorsemiconductor consisting of WSe 2 /h-BN/few-layer graphene (A).Electroluminescence image of triangle WSe 2 under positive (B) and negative (C) voltages.Reprinted (adapted) with permission from Ref. 504.Copyright (2019) American Chemical Society