Controlled Layer Thinning and p‐Type Doping of WSe2 by Vapor XeF2

This report presents a simple and efficient method of layer thinning and p‐type doping of WSe₂ with vapor XeF₂. With this approach, the surface roughness of thinned WSe₂ can be controlled to below 0.7 nm at an etched depth of 100 nm. By selecting appropriate vapor XeF₂ exposure times, 23‐layer and 109‐layer WSe₂ can be thinned down to monolayer and bilayer, respectively. In addition, the etching rate of WSe₂ exhibits a significant dependence on vapor XeF₂ exposure pressure and thus can be tuned easily for thinning or patterning applications. From Raman, photoluminescence, X‐ray photoelectron spectroscopy (XPS), and electrical characterization, a p‐doping effect of WSe₂ induced by vapor XeF₂ treatment is evident. Based on the surface composition analysis with XPS, the causes of the p‐doping effect can be attributed to the presence of substoichiometric WO_x (x < 3) overlayer, trapped reaction product of WF₆, and nonstoichiometric WSe_x (x > 2). Furthermore, the p‐doping level can be controlled by varying XeF₂ exposure time. The thinning and p‐doping of WSe₂ with vapor XeF₂ have the advantages of easy scale‐up, high etching selectivity, excellent controllability, and compatibility with conventional complementary metal‐oxide‐semiconductor fabrication processes, which is promising for applications of building WSe₂ devices with versatile functionalities.     

Highly Uniform Trilayer Molybdenum Disulfide for Wafer‐Scale Device Fabrication

Molybdenum disulfide (MoS₂) is a layered semiconducting material with a tunable bandgap that is promising for the next generation nanoelectronics as a substitute for graphene or silicon. Despite recent progress, the synthesis of high‐quality and highly uniform MoS₂ on a large scale is still a challenge. In this work, a temperature‐dependent synthesis study of large‐area MoS₂ by direct sulfurization of evaporated Mo thin films on SiO₂ is presented. A variety of physical characterization techniques is employed to investigate the structural quality of the material. The film quality is shown to be similar to geological MoS₂, if synthesized at sufficiently high temperatures (1050 °C). In addition, a highly uniform growth of trilayer MoS₂ with an unprecedented uniformity of ±0.07 nm over a large area (> 10 cm²) is achieved. These films are used to fabricate field‐effect transistors following a straightforward wafer‐scale UV lithography process. The intrinsic field‐effect mobility is estimated to be about cm² V^–1 s^–1 and compared to previous studies. These results represent a significant step towards application of MoS₂ in nanoelectronics and sensing.     

van der Waals Epitaxial Growth of Atomically Thin 2D Metals on Dangling‐Bond‐Free WSe2 and WS2

2D metals have attracted considerable recent attention for their special physical properties, such as charge density waves, magnetism, and superconductivity. However, despite some recent efforts, the synthesis of ultrathin 2D metals nanosheets down to monolayer thickness remains a significant challenge. Herein, by using atomically flat 2D WSe₂ or WS₂ as the growth substrate, the synthesis of atomically thin 2D metallic MTe₂ (M = V, Nb, Ta) single crystals with the thickness down to the monolayer regime and the creation of atomically thin MTe₂/WSe₂ (WS₂) vertical heterojunctions is reported. Comparison with the growth on the SiO₂/Si substrate under the same conditions reveals that the utilization of the dangling‐bond‐free WSe₂ or WS₂ as the van der Waals epitaxy substrates is crucial for the successful realization of atomically thin MTe₂ (M = V, Nb, Ta) nanosheets. It is further shown that the epitaxial grown 2D metals can function as van der Waals contacts for 2D semiconductors with little interface damage and improved electronic performance. This study defines a robust van der Waals epitaxy pathway to ultrathin 2D metals, which is essential for fundamental studies and potential technological applications of this new class of materials at the 2D limit.     

Oriented Growth of Gold Nanowires on MoS2

Layered 2D materials serve as a new class of substrates for templated synthesis of various nanomaterials even with highly dissimilar crystal structures; thus overcoming the lattice constraints of conventional epitaxial processes. Here, molybdenum disulfide (MoS₂) is used as a prototypical model substrate for oriented growth of in‐plane Au nanowires (NWs) despite the nearly 8% lattice mismatch between MoS₂ and Au. Au NWs on the MoS₂ surface are oriented along three symmetrically equivalent directions within the substrate arising from the strong Au–S binding that templates the oriented growth. The kinetics of the growth process are explored through experiments and modeling. Strong charge transfer is observed between Au NWs and MoS₂, resulting in degenerate p‐doping of MoS₂.     

Coexistence of Negative and Positive Photoconductivity in Few-Layer PtSe2 Field-Effect Transistors

Platinum diselenide (PtSe₂) field-effect transistors with ultrathin channel regions exhibit p-type electrical conductivity that is sensitive to temperature and environmental pressure. Exposure to a supercontinuum white light source reveals that positive and negative photoconductivity coexists in the same device. The dominance of one type of photoconductivity over the other is controlled by environmental pressure. Indeed, positive photoconductivity observed in high vacuum converts to negative photoconductivity when the pressure is raised. Density functional theory calculations confirm that physisorbed oxygen molecules on the PtSe₂ surface act as acceptors. The desorption of oxygen molecules from the surface, caused by light irradiation, leads to decreased carrier concentration in the channel conductivity. The understanding of the charge transfer occurring between the physisorbed oxygen molecules and the PtSe₂ film provides an effective route for modulating the density of carriers and the optical properties of the material.     

Sub‐Millimeter‐Scale Monolayer p‐Type H‐Phase VS2

2D H‐phase vanadium disulfide (VS₂) is expected to exhibit tunable semiconductor properties as compared with its metallic T‐phase structure, and thus is of promise for future electronic applications. However, to date such 2D H‐phase VS₂ nanostructures have not been realized in experiment likely due to the polymorphs of vanadium sulfides and thermodynamic instability of H‐phase VS₂. Preparation of H‐phase VS₂ monolayer with lateral size up to 250 µm, as a new member in the 2D transition metal dichalcogenides (TMDs) family, is reported. A unique growth environment is built by introducing the molten salt‐mediated precursor system as well as the epitaxial mica growth platform, which successfully overcomes the aforementioned growth challenges and enables the evolution of 2D H‐phase structure of VS₂. The honeycomb‐like structure of H‐phase VS₂ with broken inversion symmetry is confirmed by spherical aberration‐corrected scanning transmission electron microscopy and second harmonic generation characterization. The phase structure is found to be ultra‐stable up to 500 K. The field‐effect device study further demonstrates the p‐type semiconducting nature of the 2D H‐phase VS₂. The study introduces a new phase‐stable 2D TMDs materials with potential features for future electronic devices.     

Continuous Control and Enhancement of Excitonic Valley Polarization in Monolayer WSe2 by Electrostatic Doping

Enhancement and continuous control of the excitonic valley polarization in electrostatically doped monolayer WSe₂ are demonstrated. Under excitation with circularly polarized light, 20% valley polarization of excitons around the charge neutrality condition at 70 K is increased to 40% by modulating the electron/hole density up to 2 × 10¹² cm^?2. This increase originates from slow valley relaxation for neutral exciton between the K and ?K valleys owing to screening of long‐range e–h exchange interactions by doped carriers. The gate‐dependences of the exciton valley polarization at various temperatures are reproduced by theoretical calculations, which holds potential for next‐generation valleytronic devices continuously controlled by an applied bias voltage.     

Layer‐Controlled and Wafer‐Scale Synthesis of Uniform and High‐Quality Graphene Films on a Polycrystalline Nickel Catalyst

Chemical vapor deposition (CVD) provides a synthesis route for large‐area and high‐quality graphene films. However, layer‐controlled synthesis remains a great challenge on polycrystalline metallic films. Here, a facile and viable synthesis of layer‐controlled and high‐quality graphene films on wafer‐scale Ni surface by the sequentially separated steps of gas carburization, hydrogen exposure, and segregation is developed. The layer numbers of graphene films with large domain sizes are controlled precisely at ambient pressure by modulating the simplified CVD process conditions and hydrogen exposure. The hydrogen exposure assisted with a Ni catalyst plays a critical role in promoting the preferential segregation through removing the carbon layers on the Ni surface and reducing carbon content in the Ni. Excellent electrical and transparent conductive performance, with a room‐temperature mobility of ≈3000 cm² V^?1 s^?1 and a sheet resistance as low as ≈100 Ω per square at ≈90% transmittance, of the twisted few‐layer grapheme films grown on the Ni catalyst is demonstrated.     

Systematic Study of Widely Applicable N‐Doping Strategy for High‐Performance Solution‐Processed Field‐Effect Transistors

A specific design for solution‐processed doping of active semiconducting materials would be a powerful strategy in order to improve device performance in flexible and/or printed electronics. Tetrabutylammonium fluoride and tetrabutylammonium hydroxide contain Lewis base anions, F^? and OH^?, respectively, which are considered as organic dopants for efficient and cost‐effective n‐doping processes both in n‐type organic and nanocarbon‐based semiconductors, such as poly[[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)] (P(NDI2OD‐T2)) and selectively dispersed semiconducting single‐walled carbon nanotubes by π‐conjugated polymers. The dramatic enhancement of electron transport properties in field‐effect transistors is confirmed by the effective electron transfer from the dopants to the semiconductors as well as controllable onset and threshold voltages, convertible charge‐transport polarity, and simultaneously showing excellent device stabilities under ambient air and bias stress conditions. This simple solution‐processed chemical doping approach could facilitate the understanding of both intrinsic and extrinsic charge transport characteristics in organic semiconductors and nanocarbon‐based materials, and is thus widely applicable for developing high‐performance organic and printed electronics and optoelectronics devices.     

Effect of Doping Concentration on Microstructure of Conjugated Polymers and Characteristics in N‐Type Polymer Field‐Effect Transistors

Despite extensive progress in organic field‐effect transistors, there are still far fewer reliable, high‐mobility n‐type polymers than p‐type polymers. It is demonstrated that by using dopants at a critical doping molar ratio (MR), performance of n‐type polymer poly[[N,N9‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,59‐(2,29‐bithiophene)] (P(NDI2DO‐T2)) field‐effect transistors (FETs) can be significantly improved and simultaneously optimized in mobility, on–off ratio, crystallinity, injection, and reliability. In particular, when using the organic dopant bis(cyclopentadienyl)–cobalt(II) (cobaltocene, CoCp₂) at a low concentration (0.05 wt%), the FET mobility is increased from 0.34 to 0.72 cm² V^–1 s^–1, and the threshold voltage was decreased from 32.7 to 8.8 V. The relationship between the MR of dopants and electrical characteristics as well as the evolution in polymer crystallinity revealed by synchrotron X‐ray diffractions are systematically investigated. Deviating from previous discoveries, it is found that mobility increases first and then decreases drastically beyond a critical value of MR. Meanwhile, the intensity and width of the main peak of in‐plane X‐ray diffraction start to decrease at the same critical MR. Thus, the mobility decrease is correlated with the disturbed in‐plane crystallinity of the conjugated polymer, for both organic and inorganic dopants. The method provides a simple and efficient approach to employing dopants to optimize the electrical performance and microstructure of P(NDI2DO‐T2).     

Mercury‐Mediated Organic Semiconductor Surface Doping Monitored by Electrolyte‐Gated Field‐Effect Transistors

Surface doping allows tuning the electronic structure of semiconductors at near‐surface regime and is normally accomplished through the deposition of an ultrathin layer on top or below the host material. Surface doping is particularly appealing in organic field‐effect transistors (OFETs) where charge transport takes place at the first monolayers close to the dielectric surface. However, due to fabrication restrictions that OFET architecture imparts, this is extremely challenging. Here, it is demonstrated that mercury cations, Hg²⁺, can be exploited to control doping levels at the top surface of a thin film of a p‐type organic semiconductor blended with polystyrene. Electrolyte‐ or water‐gated field‐effect transistors, which have its conductive channel at the top surface of the organic thin film, turn out to be a powerful tool for monitoring the process. A positive shift of the threshold voltage is observed in the devices upon Hg²⁺ exposure. Remarkably, this interaction has been proved to be specific to Hg²⁺ with respect to other divalent cations and sensitive down to nanomolar concentrations. Hence, this work also opens new perspectives for employing organic electronic transducers in portable sensors for the detection of an extremely harmful water pollutant without the need of using specific receptors.     

Chloride and Indium‐Chloride‐Complex Inorganic Ligands for Efficient Stabilization of Nanocrystals in Solution and Doping of Nanocrystal Solids

Here, the surface functionalization of CdSe and CdSe/CdS core/shell nanocrystals (NCs) with compact chloride and indium‐chloride‐complex ligands is reported. The ligands provide not only short interparticle distances but additionally control doping and passivation of surface trap states, leading to enhanced electronic coupling in NC‐based arrays. The solids based on these NCs show an excellent electronic transport behavior after heat treatment at the relatively low temperature of 190 °C. Indeed, the indium‐chlorido‐capped 4.5 nm CdSe NC based thin‐film field‐effect transistor reaches a saturation mobility of μ = 4.1 cm² (V s)^?1 accompanied by a low hysteresis, while retaining the typical features of strongly quantum confined semiconductor NCs. The capping with chloride ions preserves the high photoluminescence quantum yield ( ≈ 66%) of CdSe/CdS core/shell NCs even when the CdS shell is relatively thin (six monolayers). The simplicity of the chemical incorporation of chlorine and indium species via solution ligand exchange, the efficient electronic passivation of the NC surface, as well as their high stability as dispersions make these materials especially attractive for wide‐area solution‐processable fabrication of NC‐based devices.     

Systematic Doping Control of CVD Graphene Transistors with Functionalized Aromatic Self‐Assembled Monolayers

Recent reports have shown that self‐assembled monolayers (SAMs) can induce doping effects in graphene transistors. However, a lack of understanding persists surrounding the quantitative relationship between SAM molecular design and its effects on graphene. In order to facilitate the fabrication of next‐generation graphene‐based devices it is important to reliably and predictably control the properties of graphene without negatively impacting its intrinsic high performance. In this study, SAMs with varying dipole magnitudes/directions are utilized and these values are directly correlated to changes in performance seen in graphene transistors. It is found that, by knowing the z‐component of the SAM dipole, one can reliably predict the shift in graphene charge neutrality point after taking into account the influence of the metal electrodes (which also play a role in doping graphene). This relationship is verified through density functional theory and comprehensive device studies utilizing atomic force microscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy, and electrical characterization of graphene transistors. It is shown that properties of graphene transistors can be predictably controlled with SAMs when considering the total doping environment. Additionally, it is found that methylthio‐terminated SAMs strongly interact with graphene allowing for a cleaner graphene transfer and enhanced charge mobility.     

Transition Metal Dichalcogenide‐Based Transistor Circuits for Gray Scale Organic Light‐Emitting Displays

Two types of transition metal dichalcogenide (TMD) transistors are applied to demonstrate their possibility as switching/driving elements for the pixel of organic light‐emitting diode (OLED) display. Such TMD materials are 6 nm thin WSe₂ and MoS₂ as a p‐type and n‐type channel, respectively, and the pixel is thus composed of external green OLED and nanoscale thin channel field effect transistors (FETs) for switching and driving. The maximum mobility of WSe₂‐FETs either as switch or as driver is ≈30 cm² V^?1 s^?1, in linear regime of the gate voltage sweep range. Digital (ON/OFF‐switching) and gray‐scale analogue operations of OLED pixel are nicely demonstrated. MoS₂ nanosheet FET‐based pixel is also demonstrated, although limited to alternating gray scale operation of OLED. Device stability issue is still remaining for future study but TMD channel FETs are very promising and novel for their applications to OLED pixel because of their high mobility and I _D ON/OFF ratio.     

Layered Platinum Dichalcogenides (PtS2, PtSe2, and PtTe2) Electrocatalysis: Monotonic Dependence on the Chalcogen Size

Presently, research in layered transition metal dichalcogenides (TMDs) for numerous electrochemical applications have largely focused on Group 6 TMDs, especially MoS₂ and WS₂, whereas TMDs belonging to other groups are relatively unexplored. This work unravels the electrochemistry of Group 10 TMDs: specifically PtS₂, PtSe₂, and PtTe₂. Here, the inherent electroactivities of these Pt dichalcogenides and the effectiveness of electrochemical activation on their charge transfer and electrocatalytic properties are thoroughly examined. By performing density functional theory (DFT) calculations, the electrochemical and electrocatalytic behaviors of the Pt dichalcogenides are elucidated. The charge transfer and electrocatalytic attributes of the Pt dichalcogenides are strongly associated with their electronic structures. In terms of charge transfer, electrochemical activation has been successful for all Pt dichalcogenides as evident in the faster heterogeneous electron transfer (HET) rates observed in electrochemically reduced Pt dichalcogenides. Interestingly, the hydrogen evolution reaction (HER) performance of the Pt dichalcogenides adheres to a trend of PtTe₂ > PtSe₂ > PtS₂ whereby the HER catalytic property increases down the chalcogen group. Importantly, the DFT study shows this correlation to their electronic property in which PtS₂ is semiconducting, PtSe₂is semimetallic, and PtTe₂ is metallic. Furthermore, Pt dichalcogenides are effectively activated for HER. Distinct electronic structures of Pt dichalcogenides account for their different responses to electrochemical activation. Among all activated Pt dichalcogenides, PtS₂ shows most accentuated improvement as a HER electrocatalyst with an exceptional 50% decline in HER overpotential. Knowledge on Pt dichalcogenides provides valuable insights in the field of TMD electrochemistry, in particular, for the currently underrepresented Group 10 TMDs.     

Organic Junction Field‐Effect Transistor

The realization and performance of a novel organic field‐effect transistor—the organic junction field‐effect transistor (JFET)—is discussed. The transistors are based on the modulation of the thickness of a depletion layer in an organic pin junction with varying gate potential. Based on numerical modeling, suitable layer thicknesses and doping concentrations are identified. Experimentally, organic JFETs are realized and it is shown that the devices clearly exhibit amplification. Changes in the electrical characteristics due to a variation of the intrinsic and the p‐doped layer thickness are rationalized by the numerical model, giving further proof to the proposed operational mechanism.     

Enhanced Quality of Wafer‐Scale MoS2 Films by a Capping Layer Annealing Process

Wafer‐scale, single‐crystalline 2D semiconductors without grain boundaries and defects are needed for developing reliable next‐generation integrated 2D electronics. Unfortunately, few literature reports exist on the growth of 2D semiconductors with single‐crystalline structure at the wafer scale. It is shown that direct sulfurization of as‐deposited epitaxial MoO₂ films (especially, with thicknesses more than ≈5 nm) produces textured MoS₂ films. This texture is inherited from the high density of defects present in the as‐prepared epitaxial MoO₂ film. In order to eliminate the texture of the converted MoS₂ films, a new capping layer annealing process (CLAP) is introduced to improve the crystalline quality of as‐deposited MoO₂ films and minimize its defects. It is demonstrated that sulfurization of the CLAP‐treated MoO₂ films leads to the formation of single‐crystalline MoS₂ films, instead of textured films. It is shown that the single‐crystalline MoS₂ films exhibit field‐effect mobility of 6.3 cm² V^?1 s^?1, which is 15 times higher than that of textured MoS₂. These results can be attributed to the smaller concentration of defects in the single‐crystalline films.     

1T‐Phase WS2 Protein‐Based Biosensor

Metallic 1T‐phase transition metal dichalcogenides have been recognized for their desirable properties like high surface‐to‐volume ratio, high conductivity, and capacitive behavior, making them outstanding for catalytic and sensing applications. Herein, a hydrogen peroxide (H₂O₂) biosensor is constructed by the immobilization of hemoglobin (Hb) on 1T‐phase WS₂ (1T‐WS₂) sheets, and entrapment by glutaraldehyde. 1T‐WS₂ not only displays electrocatalytic activity toward the reduction of H₂O₂ but also provides a high surface‐to‐volume ratio and conductive platform for the immobilization of Hb and facilitation of its electron transfer to the electrode surface. The advantageous role of 1T‐phase WS₂ is further demonstrated for the construction of a heme‐based H₂O₂ biosensor compared to its 1T‐phase MoS_2, MoSe₂, and WSe₂ counterparts. Synergistic interactions between 1T‐WS₂ and Hb result in a H₂O₂ biosensor with high analytical performance in terms of wide range, sensitivity, selectivity, reproducibility, repeatability, and stability. These findings have profound impact in the research fields of electrochemical sensing and biodiagnostics.     

Enhancement of n‐Type Organic Field‐Effect Transistor Performances through Surface Doping with Aminosilanes

Dopants, i.e., electronically active impurities, are added to organic semiconductor materials to control the material's Fermi level and conductivity, to improve injection at the device contacts, or to fill trap states in the active device layers and interfaces. In contrast to bulk doping as achieved by blending or co‐deposition of dopant and semiconductor, surface doping has a lower propensity to introduce additional traps or scattering centers or to even alter the layer morphology relative to the undoped active material layers. In this study, the electrical effects of a very simple, post‐device‐fabrication surface doping process involving various amine group–containing alkoxysilanes on the performance of organic field‐effect transistors (OFETs) made from the well‐known n‐type materials PTCDI‐C8 and N2200 are researched. It is demonstrated that OFETs doped in such a way generally show enhanced characteristics (up to 10 times mobility increase and a significant reduction in threshold voltage) without any adverse effects on the devices' on/off ratio. It is also shown that the efficiency of the doping process is linked to the number of amine groups.