Electron-doping-enhanced trion formation in monolayer molybdenum disulfide functionalized with cesium carbonate

Quantum tunneling high-speed nano-excitonic modulator

High-speed electrical control of nano-optoelectronic properties in two-dimensional semiconductors is a building block for the development of excitonic devices, allowing the seamless integration of nano-electronics and -photonics. Here, we demonstrate a high-speed electrical modulation of nanoscale exciton behaviors in a MoS2 monolayer at room temperature through a quantum tunneling nanoplasmonic cavity. Electrical control of tunneling electrons between Au tip and MoS2 monolayer facilitates the dynamic switching of neutral exciton- and trion-dominant states at the nanoscale. Through tip-induced spectroscopic analysis, we locally characterize the modified recombination dynamics, resulting in a significant change in the photoluminescence quantum yield. Furthermore, by obtaining a time-resolved second-order correlation function, we demonstrate that this electrically-driven nanoscale exciton-trion interconversion achieves a modulation frequency of up to 8 MHz. Our approach provides a versatile platform for dynamically manipulating nano-optoelectronic properties in the form of transformable excitonic quasiparticles, including valley polarization, recombination, and transport dynamics.

Indirect-To-Direct Bandgap Crossover and Room-Temperature Valley Polarization of Multilayer MoS2 Achieved by Electrochemical Intercalation

Monolayer (1L) group VI transition metal dichalcogenides (TMDs) exhibit broken inversion symmetry and strong spin-orbit coupling, offering promising applications in optoelectronics and valleytronics. Despite their direct bandgap, high absorption coefficient, and spin-valley locking in K or K' valleys, the ultra-short valley lifetime limits their room-temperature applications. In contrast, multilayer TMDs, with more absorptive layers, sacrifice the direct bandgap and valley polarization upon gaining inversion symmetry from the bilayer structure. It is demonstrated that multilayer molybdenum disulfide (MoS2) can maintain 1) a structure with broken inversion symmetry and strong spin-orbit coupling, 2) a direct bandgap with high photoluminescence (PL) intensity, and 3) stable valley polarization up to room temperature. Through the intercalation of organic 1-ethyl-3-methylimidazolium (EMIM+) ions, multilayer MoS2 not only exhibits layer decoupling but also benefits from an electron doping effect. This results in a hundredfold increase in PL intensity and stable valley polarization, achieving 55% and 16% degrees of valley polarization at 3 K and room temperature, respectively. The persistent valley polarization at room temperature, due to interlayer decoupling and trion dominance facilitated by a gate-free method, opens up potential applications in valley-selective optoelectronics and valley transistors.

Enhanced Performance of P-Channel CuIBr Thin-Film Transistor by ITO Surface Charge-Transfer Doping

The process development and optimization of p-type semiconductors and p-channel thin-film transistors (TFTs) are essential for the development of high-performance circuits. In this study, the Br-doped CuI (CuIBr) TFTs are proposed by the solution process to control copper vacancy generation and suppress excess holes formation in p-type CuI films and improve current modulation capabilities for CuI TFTs. The CuIBr films exhibit a uniform surface morphology and good crystalline quality. The on/off current (ION/IOFF) ratio of CuIBr TFTs increased from 103 to 106 with an increase in the Br doping ratio from 0 to 15%. Furthermore, the performance and operational stability of CuIBr TFTs are significantly enhanced by indium tin oxide (ITO) surface charge-transfer doping. The results obtained from the first-principles calculations well explain the electron-doping effect of ITO overlayer in CuIBr TFT. Eventually, the CuIBr TFT with 15% Br content exhibits a high ION/IOFF ratio of 3 × 106 and a high hole field-effect mobility (μFE) of 7.0 cm2 V-1 s-1. The band-like charge transport in CuIBr TFT is confirmed by the temperature-dependent measurement. This study paves the way for the realization of transparent complementary circuits and wearable electronics.

Electronic Devices Based on Heterostructures of 2D Materials and Self-Assembled Monolayers

2D materials (2DMs), known for their atomically ultrathin structure, exhibit remarkable electrical and optical properties. Similarly, molecular self-assembled monolayers (SAMs) with comparable atomic thickness show an abundance of designable structures and properties. The strategy of constructing electronic devices through unique heterostructures formed by van der Waals assembly between 2DMs and molecular SAMs not only enables device miniaturization, but also allows for convenient adjustment of their structures and functions. In this review, the fundamental structures and fabrication methods of three different types of electronic devices dominated by 2DM-SAM heterojunctions with varying architectures are timely elaborated. Based on these heterojunctions, their fundamental functionalities and characteristics, as well as the regulation of their performance by external stimuli, are further discussed.

Chemical passivation of 2D transition metal dichalcogenides: strategies, mechanisms, and prospects for optoelectronic applications

The interest in obtaining high-quality monolayer transition metal dichalcogenides (TMDs) for optoelectronic device applications has been growing dramatically. However, the prevalence of defects and unwanted doping in these materials remain challenges, as they both limit optical properties and device performance. Surface chemical treatments of monolayer TMDs have been effective in improving their photoluminescence yield and charge transport properties. In this scenario, a systematic understanding of the underlying mechanism of chemical treatments will lead to a rational design of passivation strategies in future research, ultimately taking a step toward practical optoelectronic applications. We will therefore describe in this mini-review the strategies, progress, mechanisms, and prospects of chemical treatments to passivate and improve the optoelectronic properties of TMDs.

Layer-by-layer thinning of two-dimensional materials

Etching technology - one of the representative modern semiconductor device makers - serves as a broad descriptor for the process of removing material from the surfaces of various materials, whether partially or entirely. Meanwhile, thinning technology represents a novel and highly specialized approach within the realm of etching technology. It indicates the importance of achieving an exceptionally sophisticated and precise removal of material, layer-by-layer, at the nanoscale. Notably, thinning technology has gained substantial momentum, particularly in top-down strategies aimed at pushing the frontiers of nano-worlds. This rapid development in thinning technology has generated substantial interest among researchers from diverse backgrounds, including those in the fields of chemistry, physics, and engineering. Precisely and expertly controlling the layer numbers of 2D materials through the thinning procedure has been considered as a crucial step. This is because the thinning processes lead to variations in the electrical and optical characteristics. In this comprehensive review, the strategies for top-down thinning of representative 2D materials (e.g., graphene, black phosphorus, MoS2, h-BN, WS2, MoSe2, and WSe2) based on conventional plasma-assisted thinning, integrated cyclic plasma-assisted thinning, laser-assisted thinning, metal-assisted splitting, and layer-resolved splitting are covered in detail, along with their mechanisms and benefits. Additionally, this review further explores the latest advancements in terms of the potential advantages of semiconductor devices achieved by top-down 2D material thinning procedures.

Anomalous layer-dependent photoluminescence spectra of supertwisted spiral WS2

Twisted stacking of two-dimensional materials with broken inversion symmetry, such as spiral MoTe2 nanopyramids and supertwisted spiral WS2, emerge extremely strong second- and third-harmonic generation. Unlike well-studied nonlinear optical effects in these newly synthesized layered materials, photoluminescence (PL) spectra and exciton information involving their optoelectronic applications remain unknown. Here, we report layer- and power-dependent PL spectra of the supertwisted spiral WS2. The anomalous layer-dependent PL evolutions that PL intensity almost linearly increases with the rise of layer thickness have been determined. Furthermore, from the power-dependent spectra, we find the power exponents of the supertwisted spiral WS2 are smaller than 1, while those of the conventional multilayer WS2 are bigger than 1. These two abnormal phenomena indicate the enlarged interlayer spacing and the decoupling interlayer interaction in the supertwisted spiral WS2. These observations provide insight into PL features in the supertwisted spiral materials and may pave the way for further optoelectronic devices based on the twisted stacking materials.

Graphene-like emerging 2D materials: recent progress, challenges and future outlook

Owing to the unique physical and chemical properties of 2D materials and the great success of graphene in various applications, the scientific community has been influenced to explore a new class of graphene-like 2D materials for next-generation technological applications. Consequently, many alternative layered and non-layered 2D materials, including h-BN, TMDs, and MXenes, have been synthesized recently for applications related to the 4th industrial revolution. In this review, recent progress in state-of-the-art research on 2D materials, including their synthesis routes, characterization and application-oriented properties, has been highlighted. The evolving applications of 2D materials in the areas of electronics, optoelectronics, spintronic devices, sensors, high-performance and transparent electrodes, energy conversion and storage, electromagnetic interference shielding, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nanocomposites are discussed. In particular, the state-of-the-art applications, challenges, and outlook of every class of 2D material are also presented as concluding remarks to guide this fast-progressing class of 2D materials beyond graphene for scientific research into next-generation materials.

p-Type Two-Dimensional Semiconductors: From Materials Preparation to Electronic Applications

Two-dimensional (2D) materials are regarded as promising candidates in many applications, including electronics and optoelectronics, because of their superior properties, including atomic-level thickness, tunable bandgaps, large specific surface area, and high carrier mobility. In order to bring 2D materials from the laboratory to industrialized applications, materials preparation is the first prerequisite. Compared to the n-type analogs, the family of p-type 2D semiconductors is relatively small, which limits the broad integration of 2D semiconductors in practical applications such as complementary logic circuits. So far, many efforts have been made in the preparation of p-type 2D semiconductors. In this review, we overview recent progresses achieved in the preparation of p-type 2D semiconductors and highlight some promising methods to realize their controllable preparation by following both the top-down and bottom-up strategies. Then, we summarize some significant application of p-type 2D semiconductors in electronic and optoelectronic devices and their superiorities. In end, we conclude the challenges existed in this field and propose the potential opportunities in aspects from the discovery of novel p-type 2D semiconductors, their controlled mass preparation, compatible engineering with silicon production line, high-κ dielectric materials, to integration and applications of p-type 2D semiconductors and their heterostructures in electronic and optoelectronic devices. Overall, we believe that this review will guide the design of preparation systems to fulfill the controllable growth of p-type 2D semiconductors with high quality and thus lay the foundations for their potential application in electronics and optoelectronics.

All-optical control of high-purity trions in nanoscale waveguide

The generation of high-purity localized trions, dynamic exciton-trion interconversion, and their spatial modulation in two-dimensional (2D) semiconductors are building blocks for the realization of trion-based optoelectronic devices. Here, we present a method for the all-optical control of the exciton-to-trion conversion process and its spatial distributions in a MoS₂ monolayer. We induce a nanoscale strain gradient in a 2D crystal transferred on a lateral metal-insulator-metal (MIM) waveguide and exploit propagating surface plasmon polaritons (SPPs) to localize hot electrons. These significantly increase the electrons and efficiently funnel excitons in the lateral MIM waveguide, facilitating complete exciton-to-trion conversion even at ambient conditions. Additionally, we modulate the SPP mode using adaptive wavefront shaping, enabling all-optical control of the exciton-to-trion conversion rate and trion distribution in a reversible manner. Our work provides a platform for harnessing excitonic quasiparticles efficiently in the form of trions at ambient conditions, enabling high-efficiency photoconversion.

Hysteresis-Free Contact Doping for High-Performance Two-Dimensional Electronics

Contact doping is considered crucial for reducing the contact resistance of two-dimensional (2D) transistors. However, a process for achieving robust contact doping for 2D electronics is lacking. Here, we developed a two-step doping method for effectively doping 2D materials through a defect-repairing process. The method achieves strong and hysteresis-free doping and is suitable for use with the most widely used transition-metal dichalcogenides. Through our method, we achieved a record-high sheet conductance (0.16 mS·sq^-1 without gating) of monolayer MoS₂ and a high mobility and carrier concentration (4.1 × 10¹³ cm^-2). We employed our robust method for the successful contact doping of a monolayer MoS₂ Au-contact device, obtaining a contact resistance as low as 1.2 kΩ·μm. Our method represents an effective means of fabricating high-performance 2D transistors.

MoS2/WSe2 vdW Heterostructures Decorated with PbS Quantum Dots for the Development of High-Performance Photovoltaic and Broadband Photodiodes

van der Waals heterostructures (vdWHs) overcoming the lattice and processing limitations of conventional heterostructures provide an opportunity to develop high-performance 2D vdWH solar cells and photodiodes. However, it is challenging to improve the sensitivity and response speed of 2D vdWH photovoltaic devices due to the low light absorption efficiency and electron/hole traps in heterointerfaces. Here, we design a PbS/MoS₂/WSe₂ heterostructure photodiode in which a light-sensitive PbS quantum dot (QD) layer combined with a MoS₂/WSe₂ heterostructure significantly enhances the photovoltaic response. The electron current in the heterostructure is increased by the effective collection of photogenerated electrons induced by PbS QDs. The device exhibits a broadband photovoltaic response from 405 to 1064 nm with a maximum responsivity of 0.76 A/W and a specific detectivity of 5.15 × 10¹¹ Jones. In particular, the response speed is not limited by multiple electron traps in the PbS QDs/2D material heterointerface, and a fast rising/decaying time of 43/48 μs and a -3 dB cutoff frequency of over 10 kHz are achieved. The negative differential capacitance and frequency dependence of capacitance demonstrate the presence of interface states in the MoS₂/WSe₂ heterointerface that hamper the improvement of the response speed. The scheme to enhance photovoltaic performance without sacrificing response speed provides opportunities for the development of high-performance 2D vdWH optoelectronic devices.

Reconfigurable carrier type and photodetection of MoTe2 of various thicknesses by deep ultraviolet light illumination

Tuning of the Fermi level in transition metal dichalcogenides (TMDCs) leads to devices with excellent electrical and optical properties. In this study, we controlled the Fermi level of MoTe₂ by deep ultraviolet (DUV) light illumination in different gaseous environments. Specifically, we investigated the reconfigurable carrier type of an intrinsic p-MoTe₂ flake that gradually transformed into n-MoTe₂ after illumination with DUV light for 30, 60, 90, 120, 160, 250, 500, 900, and 1200 s in a nitrogen (N₂) gas environment. Subsequently, we illuminated this n-MoTe₂ sample with DUV light in oxygen (O₂) gas and reversed its carrier polarity toward p-MoTe₂. However, using this doping scheme to reveal the effect of DUV light on various layers (3-30 nm) of MoTe₂ is challenging. The DUV + N₂ treatment significantly altered the polarity of MoTe₂ of different thicknesses from p-type to n-type under the DUV + N₂ treatment, but the DUV + O₂ treatment did not completely alter the polarity of thicker n-MoTe₂ flakes to p-type. In addition, we investigated the photoresponse of MoTe₂ after DUV light treatment in N₂ and O₂ gas environments. From the time-resolved photoresponsivity at different polarity states of MoTe₂, we have shown that the response time of the DUV + O₂ treated p-MoTe₂ is faster than that of the pristine and doped n-MoTe₂ films. These carrier polarity modulations and photoresponse paves the way for wider applications of MoTe₂ in optoelectronic devices.

Excitons and Trions in MoS2 Quantum Dots: The Influence of the Dispersing Medium

Single-layer MoS₂ has been reported to exhibit strong excitonic and trionic signatures in its photoluminescence (PL) spectra. Here, we report that the emission spectra of MoS₂ QDs strongly depend on the dielectric constant of the solvent and the relative difference in the electronegativity between the solvent and QDs. Due to the difference in electronegativity, electrons are either added to the QD or withdrawn from it. Consequently, depending upon the dielectric permittivity and the electronegativity of the surrounding medium, the signature peaks of excitons and trions exhibit a significant change in the PL spectra of MoS₂ QDs. Our findings are helpful to understand the effect of the surrounding environment on the optical properties of QDs and the importance of the selection of solvent since MoS₂ QDs are potential candidates for valleytronics applications.

Damage-Free Charge Transfer Doping of 2D Transition Metal Dichalcogenide Channels by van der Waals Stamping of MoO3 and LiF

To dope 2D semiconductor channels, charge-transfer doping has generally been done by thermal deposition of inorganic or organic thin-film layers on top of the 2D channel in bottom-gate field-effect transistors (FETs). The doping effects are reproducible in most cases. However, such thermal deposition will damage the surface of 2D channels due to the kinetic energy of depositing atoms, causing hysteresis or certain degradation. Here, a more desirable charge-transfer doping process is suggested. A damage-free charge-transfer doping is conducted for 2D MoTe₂ (or MoS₂ ) channels using a polydimethylsiloxane stamp. MoO₃ or LiF is initially deposited on the stamp as a doping medium. Hysteresis-minimized transfer characteristics are achieved from stamp-doped FETs, while other devices with direct thermal deposition-doped channels show large hysteresis. The stamping method seems to induce a van der Waals-like damage-free interface between the channel and doping media. The stamp-induced doping is also well applied for a MoTe₂ -based complementary inverter because MoO₃ - and LiF-doping by separate stamps effectively modifies two ambipolar MoTe₂ channels to p- and n-type, respectively.

Halide Perovskite Nanocrystal-Enabled Stabilization of Transition Metal Dichalcogenide Nanosheets

Transition metal dichalcogenide (TMD) nanosheets exfoliated in the liquid phase are of significant interest owing to their potential for scalable and flexible photoelectronic applications. Although various dispersants such as surfactants, oligomers, and polymers are used to obtain highly exfoliated TMD nanosheets, most of them are electrically insulating and need to be removed; otherwise, the photoelectric properties of the TMD nanosheets degrade. Here, inorganic halide perovskite nanocrystals (NCs) of CsPbX₃  (X = Cl, Br, or I) are presented as non-destructive dispersants capable of dispersing TMD nanosheets in the liquid phase and enhancing the photodetection properties of the nanosheets, thus eliminating the need to remove the dispersant. MoSe₂ nanosheets dispersed in the liquid phase are adsorbed with CsPbCl₃  NCs. The CsPbCl₃ nanocrystals on MoSe₂ efficiently withdraw electrons from the nanosheets, and suppress the dark current of the MoSe₂ nanosheets, leading to flexible near-infrared MoSe₂  photodetectors with a high ON/OFF photocurrent ratio and detectivity. Moreover, lanthanide ion-doped CsPbCl₃  NCs enhance the ON/OFF current ratio to >10⁶ . Meanwhile, the dispersion stability of the MoSe₂  nanosheets exfoliated with the perovskite NCs is sufficiently high.

Direct Detection of Inhomogeneity in CVD-Grown 2D TMD Materials via K-Means Clustering Raman Analysis

It is known that complex growth environments often induce inhomogeneity in two-dimensional (2D) materials and significantly restrict their applications. In this paper, we proposed an efficient method to analyze the inhomogeneity of 2D materials by combination of Raman spectroscopy and unsupervised k-means clustering analysis. Taking advantage of k-means analysis, it can provide not only the characteristic Raman spectrum for each cluster but also the cluster spatial maps. It has been demonstrated that inhomogeneities and their spatial distributions are simultaneously revealed in all CVD-grown MoS₂, WS₂ and WSe₂ samples. Uniform p-type doping and varied tensile strain were found in polycrystalline monolayer MoS₂ from the grain boundary and edges to the grain center (single crystal). The bilayer MoS₂ with AA and AB stacking are shown to have relatively uniform p-doping but a gradual increase of compressive strain from center to the periphery. Irregular distribution of 2LA(M)/E2g1 mode in WS₂ and E2g1 mode in WSe₂ is revealed due to defect and strain, respectively. All the inhomogeneity could be directly characterized in color-coded Raman imaging with correlated characteristic spectra. Moreover, the influence of strain and doping in the MoS₂ can be well decoupled and be spatially verified by correlating with the clustered maps. Our k-means clustering Raman analysis can dramatically simplify the inhomogeneity analysis for large Raman data in 2D materials, paving the way towards direct evaluation for high quality 2D materials.

Metal oxide charge transfer complex for effective energy band tailoring in multilayer optoelectronics

Metal oxides are intensively used for multilayered optoelectronic devices such as organic light-emitting diodes (OLEDs). Many approaches have been explored to improve device performance by engineering electrical properties. However, conventional methods cannot enable both energy level manipulation and conductivity enhancement for achieving optimum energy band configurations. Here, we introduce a metal oxide charge transfer complex (NiO:MoO₃-complex), which is composed of few-nm-size MoO₃ domains embedded in NiO matrices, as a highly tunable carrier injection material. Charge transfer at the finely dispersed interfaces of NiO and MoO₃ throughout the entire film enables effective energy level modulation over a wide work function range of 4.47 – 6.34 eV along with enhanced electrical conductivity. The high performance of NiO:MoO₃-complex is confirmed by achieving 189% improved current efficiency compared to that of MoO₃-based green OLEDs and also an external quantum efficiency of 17% when applied to blue OLEDs, which is superior to 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile-based conventional devices.

One pathway for improving the performance of optoelectronics is the tailoring energy bands of the charge transport layer. Here, Kim et al present a charge transfer complex composed out of nanodomains of MoO₃ embedded within an NiO matrix, significantly improving green and blue OLED performance.

Spontaneous n-Doping in Growing Monolayer MoS2 by Alkali Metal Compound-Promoted CVD

Monolayer MoS₂ has emerged as one of the most promising candidate materials for future semiconductor devices because of its fascinating physical properties and optoelectronic performance. Recently, the utilization of alkali metal compounds as promoters in CVD growth has been demonstrated to be a facile strategy for growing monolayer MoS₂ and other 2D TMDs with large domain sizes. In this work, we systematically investigated the residues derived from alkali metal compounds and the spontaneous n-doping effect on monolayer MoS₂ in alkali metal compound-promoted CVD growth. When using NaOH and other alkali metal compounds as promoters, it is found that the Raman peak of the A_1g mode red shifted with a broadening width and the PL intensity of the A peak decreased with a red shift, which was attributed to the spontaneous n-doping effect during growth. Moreover, the growth using varying amounts of NaOH promoter suggests that the n-doping level could be controlled by the amount of promoter. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) showed the existence of cation-derived residues in the form of a Na-O cluster physiosorbed on top of monolayer MoS₂, which was also confirmed by the transfer experiment. The NaOH treatment experiment and density functional theory (DFT) calculations demonstrate that sodium hydroxide clusters, which could be converted from a combination of Na-O clusters and water vapor, could produce an n-doping effect on monolayer MoS₂. This study provides a facile route to controllably grow monolayer 2D materials with a desired doping level without further treatment.

Controllable Doping in 2D Layered Materials

For each generation of semiconductors, the issue of doping techniques is always placed at the top of the priority list since it determines whether a material can be used in the electronic and optoelectronic industry or not. When it comes to 2D materials, significant challenges have been found in controllably doping 2D semiconductors into p- or n-type, let alone developing a continuous control of this process. Here, a unique self-modulated doping characteristic in 2D layered materials such as PtSSe, PtS_0.8 Se_1.2 , PdSe₂ , and WSe₂ is reported. The varying number of vertically stacked-monolayers is the critical factor for controllably tuning the same material from p-type to intrinsic, and to n-type doping. Importantly, it is found that the thickness-induced lattice deformation makes defects in PtSSe transit from Pt vacancies to anion vacancies based on dynamic and thermodynamic analyses, which leads to p- and n-type conductance, respectively. By thickness-modulated doping, WSe₂ diode exhibits a high rectification ratio of 4400 and a large open-circuit voltage of 0.38 V. Meanwhile, the PtSSe detector overcomes the shortcoming of large dark-current in narrow-bandgap optoelectronic devices. All these findings provide a brand-new perspective for fundamental scientific studies and applications.

Trion dynamics and charge photogeneration in MoS2 nanosheets prepared by liquid phase exfoliation

Since excitonic quasiparticles, including excitons, trions and charges, have a great influence on the photoelectric characteristics of two-dimensional (2D) transition metal dichalcogenides (TMDs), systematic explorations of the trion dynamics and charge photogeneration in 2D TMDs are important for their future optoelectronic applications. Here, broadband femtosecond transient absorption spectroscopic experiments are performed first to investigate the peak shifting and broadening kinetics in MoS₂ nanosheets in solution prepared by liquid phase exfoliation (LPE-MoS₂, ∼9 layers, 9L), which reveal that the binding energies for the A-, B-, and C-exciton states are ∼77 meV, ∼76 meV, and -70 meV (the energy difference between free charges and excitons; the negative sign for C-excitons means a spontaneous dissociation nature in band-nesting regions), respectively. Then, the trion dynamics and charge photogeneration in LPE-MoS₂ nanosheets have been studied in detail, demonstrating that they are comparable to those in chemical vapor deposition grown MoS₂ films (1L-, 3L- and 7L-MoS₂). These experimental results suggest that LPE-TMD nanosheets also have the potential for use in charge-related optoelectronic devices based on 2D TMDs.

Charge-Transfer Effect and Enhanced Photoresponsivity of WS2- and MoSe2-Based Field Effect Transistors with π-Conjugated Polyelectrolyte

The characteristics of field effect transistors (FETs) fabricated using two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be modulated by surface treatment of the active layers. In this study, an ionic π-conjugated polyelectrolyte, poly(9,9-bis(4'-sulfonatobutyl)fluorene-alt-1,4-phenylene) potassium (FPS-K), was used for the surface treatment of MoSe₂ and WS₂ FETs. The photoluminescence (PL) intensities of monolayer (1L)-MoSe₂ and 1L-WS₂ clearly decreased, and the PL peaks were red-shifted after FPS-K treatment, suggesting a charge-transfer effect. In addition, the n-channel current of both the MoSe₂ and WS₂ FETs increased and the threshold voltage (V_th) shifted negatively after FPS-K treatment owing to the charge-transfer effect. The photoresponsivity of the MoSe₂ FET under light irradiation (λ_ex = 455 nm) increased considerably, from 5300 A W^-1 to approximately 10 000 A W^-1, after FPS-K treatment, and similar behavior was observed in the WS₂ FET. The results can be explained in terms of the increase in electron concentration due to photogating. The external quantum efficiency and photodetectivity of both FETs were also enhanced by the charge-transfer effect resulting from surface treatment with FPS-K containing mobile cations (K⁺) and fixed anions (SO₃^-), as well as by the photogating effect. The variation in charge-carrier density due to the photogating and charge-transfer effects is estimated to be approximately 2 × 10¹² cm^-2. The results suggest that π-conjugated polyelectrolytes such as FPS-K can be a promising candidate for the passivation of TMDC-based FETs and obtaining enhanced photoresponsivity.

A Roadmap for Disruptive Applications and Heterogeneous Integration Using Two-Dimensional Materials: State-of-the-Art and Technological Challenges

This Mini Review attempts to establish a roadmap for two-dimensional (2D) material-based microelectronic technologies for future/disruptive applications with a vision for the semiconductor industry to enable a universal technology platform for heterogeneous integration. The heterogeneous integration would involve integrating orthogonal capabilities, such as different forms of computing (classical, neuromorphic, and quantum), all forms of sensing, digital and analog memories, energy harvesting, and so forth, all in a single chip using a universal technology platform. We have reviewed the state-of-the-art 2D materials such as graphene, transition metal dichalcogenides, phosphorene and hexagonal boron nitride, and so forth, and how they offer unique possibilities for a range of futuristic/disruptive applications. Besides, we have discussed the technological and fundamental challenges in enabling such a universal technology platform, where the world stands today, and what gaps are required to be filled.

Visualization of band offsets at few-layer MoS2/Ge heterojunction

The visualization of band alignment for designing heterostructures between transition metal dichalcogenides and germanium plays a vital role in a deeper understanding of carrier dynamics at the heterointerface. Here, to study the band alignment across the MoS₂/Ge heterojunction, we have deposited a wafer-scale highly crystalline few atomic layers MoS₂film via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The Raman and XRD spectra of as-fabricated MoS₂/Ge heterojunction expose the presence of highly crystalline few atomic layer MoS₂on top of Ge substrate. Interestingly, we found a type-II band alignment at the MoS₂/Ge heterointerface having valence band, and conduction band offset values of 0.88 and 0.21 eV, which can provide very efficient recombination through spatially confining charge carriers. The calculation of band offset parameters offers a promising way for device engineering across the MoS₂/Ge heterojunction interface. Moreover, to demonstrate the practicability of the fabricated heterostructure, we explored the suitability of our device for broadband photodetection applications.

Enhancement of Photodetective Properties on Multilayered MoS2 Thin Film Transistors via Self-Assembled Poly-L-Lysine Treatment and Their Potential Application in Optical Sensors

Photodetectors and display backplane transistors based on molybdenum disulfide (MoS2) have been regarded as promising topics. However, most studies have focused on the improvement in the performances of the MoS2 photodetector itself or emerging applications. In this study, to suggest a better insight into the photodetector performances of MoS2 thin film transistors (TFTs), as photosensors for possible integrated system, we performed a comparative study on the photoresponse of MoS2 and hydrogenated amorphous silicon (a-Si:H) TFTs. As a result, in the various wavelengths and optical power ranges, MoS2 TFTs exhibit 2~4 orders larger photo responsivities and detectivities. The overall quantitative comparison of photoresponse in single device and inverters confirms a much better performance by the MoS2 photodetectors. Furthermore, as a strategy to improve the field effect mobility and photoresponse of the MoS2 TFTs, molecular doping via poly-L-lysine (PLL) treatment was applied to the MoS2 TFTs. Transfer and output characteristics of the MoS2 TFTs clearly show improved photocurrent generation under a wide range of illuminations (740~365 nm). These results provide useful insights for considering MoS2 as a next-generation photodetector in flat panel displays and makes it more attractive due to the fact of its potential as a high-performance photodetector enabled by a novel doping technique.

Modulation of MoTe2/MoS2 van der Waals heterojunctions for multifunctional devices using N2O plasma with an opposite doping effect

van der Waals layered heterojunctions have a variety of band offsets that open up possibilities for a wide range of novel and multifunctional devices. However, due to their poor pristine carrier concentrations and limited band modulation methods, multifunctional p-n heterojunctions are very difficult to achieve. In this report, we developed a highly effective N2O plasma process to treat MoTe2/MoS2 heterojunctions. This allowed us to adjust the hole and electron concentrations in the two materials independently and simultaneously. More importantly, for the first time, we were able to create opposite doping on the two sides of the junction through a single-step treatment. With a very wide doping range from pristine to degenerate levels, a MoTe2/MoS2 heterojunction can be modulated to behave as a forward rectifying diode with enhanced rectifying ratio and as a tunneling transistor with negative differential resistance at room temperature. The new approach provides an effective and generic doping scheme for heterojunctions to construct versatile and multifunctional electronic devices.

Constructing Surface Plasmon Resonance on Bi2WO6 to Boost High-Selective CO2 Reduction for Methane

Plasmonic Bi₂WO₆ with strong localized surface plasmon resonance (LSPR) around the 500-1400 region is successfully constructed by electron doping. Oxygen vacancies on W-O-W (V1) and Bi-O-Bi (V2) sites are precisely controlled to obtain Bi₂WO₆-V₁ with LSPR and Bi₂WO₆-V₂ with defect absorption. Density functional theory (DFT) calculation demonstrates that the V1-induced energy state facilitates photoelectron collection for a long lifetime, resulting in LSPR of Bi₂WO₆. Photoelectron trapping on V1 sites is demonstrated by a single-particle photoluminescence (PL) study, and 93% PL quenching efficiency is observed. With strong LSPR, plasmonic Bi₂WO₆-V₁ exhibits highly selective methane generation with a rate of 9.95 μmol g^-1 h^-1 during the CO₂ reduction reaction (CO₂-RR), which is 26-fold higher than 0.37 μmol g^-1 h^-1 of BiWO₃-V₂ under UV-visible light irradiation. LSPR-dependent methane generation is confirmed by various photocatalytic results of plasmonic Bi₂WO₆ with tunable LSPR and different light excitations. Furthermore, the DFT-simulated pathway of CO₂-RR and in situ Fourier transform infrared spectra on the surface of Bi₂WO₆ prove that V1 sites facilitate CH₄ generation. Our work provides a strategy to obtain nonmetallic plasmonic materials by electron doping.

Metallic surface doping of metal halide perovskites

Intentional doping is the core of semiconductor technologies to tune electrical and optical properties of semiconductors for electronic devices, however, it has shown to be a grand challenge for halide perovskites. Here, we show that some metal ions, such as silver, strontium, cerium ions, which exist in the precursors of halide perovskites as impurities, can n-dope the surface of perovskites from being intrinsic to metallic. The low solubility of these ions in halide perovskite crystals excludes the metal impurities to perovskite surfaces, leaving the interior of perovskite crystals intrinsic. Computation shows these metal ions introduce many electronic states close to the conduction band minimum of perovskites and induce n-doping, which is in striking contrast to passivating ions such as potassium and rubidium ion. The discovery of metallic surface doping of perovskites enables new device and material designs that combine the intrinsic interior and heavily doped surface of perovskites.

Intentional doping is important in semiconductors in order to tune the material property, yet the mechanism in metal halide perovskite is not well-understood. Here, the authors use silver, strontium, and cerium ions to showcase n-type doping on perovskite surface even up to metallic state.

Mechanisms and Applications of Steady-State Photoluminescence Spectroscopy in Two-Dimensional Transition-Metal Dichalcogenides

Two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors exhibit many important structural and optoelectronic properties, such as strong light-matter interactions, direct bandgaps tunable from visible to near-infrared regions, flexibility and atomic thickness, quantum-confinement effects, valley polarization possibilities, and so on. Therefore, they are regarded as a very promising class of materials for next-generation state-of-the-art nano/micro optoelectronic devices. To explore different applications and device structures based on 2D TMDs, intrinsic material properties, their relationships, and evolutions with fabrication parameters need to be deeply understood, very often through a combination of various characterization techniques. Among them, steady-state photoluminescence (PL) spectroscopy has been extensively employed. This class of techniques is fast, contactless, and nondestructive and can provide very high spatial resolution. Therefore, it can be used to obtain optoelectronic properties from samples of various sizes (from microns to centimeters) during the fabrication process without complex sample preparation. In this article, the mechanism and applications of steady-state PL spectroscopy in 2D TMDs are reviewed. The first part of this review details the physics of PL phenomena in semiconductors and common techniques to acquire and analyze PL spectra. The second part introduces various applications of PL spectroscopy in 2D TMDs. Finally, a broader perspective is discussed to highlight some limitations and untapped opportunities of PL spectroscopy in characterizing 2D TMDs.

In Situ Study of Molecular Doping of Chlorine on MoS2 Field Effect Transistor Device in Ultrahigh Vacuum Conditions

We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS₂ channel when the channel part is exposed to Cl₂ gas. The gas exposure and the electrical measurement of the MoS₂ FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS₂ FET can provide and understand the surface properties. With the Cl₂ gas exposure to the channel, the plot of the drain current versus the gate voltage (I _d-V _g curve) shifts monotonically toward the positive direction of V _g, suggesting that the adsorbate acts as an electron acceptor. The I _d-V _g shifts are numerically estimated by measuring the onset of I _d (threshold voltage, V _th) and the mobility as a function of the dosing amounts of the Cl₂ gas. The behaviors of both the V _th shift and the mobility with the Cl₂ dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS₂ surface up to the monolayer coverage. The charge transfer makes the V _th shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS₂ channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl₂ molecule. The monotonic shift of V _th with the coverage suggests that the MoS₂ device sensor has a good sensitivity to detect 10^-3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.

Energy-Efficient Tunneling Field-Effect Transistors for Low-Power Device Applications: Challenges and Opportunities

Conventional field-effect transistors (FETs) have long been considered a fundamental electronic component for a diverse range of devices. However, nanoelectronic circuits based on FETs are not energy efficient because they require a large supply voltage for switching applications. To reduce the supply voltage in standard FETs, which is hampered by the 60 mV/decade limit established by the subthreshold swing (SS), a new class of FETs have been designed, tunnel FETs (TFETs). A TFET utilizes charge-carrier transportation in device channels using quantum mechanical based band-to-band tunneling despite of conventional thermal injection. The TFETs fabricated with thin semiconducting film or nanowires can attain a 100-fold power drop compared to complementary metal-oxide-semiconductor (CMOS) transistors. As a result, the use of TFETs and CMOS technology together could ameliorate integrated circuits for low-power devices. The discovery of two-dimensional (2D) materials with a diverse range of electronic properties has also opened new gateways for condensed matter physics, nanotechnology, and material science, thus potentially improving TFET-based devices in terms of device design and performance. In this review, state-of-art TFET devices exhibiting different semiconducting channels and geometries are comprehensively reviewed followed by a brief discussion of the challenges that remain for the development of high-performance devices. Lastly, future prospects are presented for the improvement of device design and the working efficiency of TFETs.

Combinatorial Large-Area MoS2/Anatase-TiO2 Interface: A Pathway to Emergent Optical and Optoelectronic Functionalities

The interface of transition-metal dichalcogenides (TMDCs) and high-k dielectric transition-metal oxides (TMOs) had triggered umpteen discourses because of the indubitable impact of TMOs in reducing the contact resistances and restraining the Fermi-level pinning for the metal-TMDC contacts. In the present work, we focus on the unresolved tumults of large-area TMDC/TMO interfaces, grown by adopting different techniques. Here, on a pulsed laser-deposited MoS₂ thin film, a layer of TiO₂ is grown by atomic layer deposition (ALD) and pulsed laser deposition (PLD). These two different techniques emanate the layer of TiO₂ with different crystallinities, thicknesses, and interfacial morphologies, subsequently influencing the electronic and optical properties of the interfaces. Contrasting the earlier reports of n-type doping at the exfoliated MoS₂/TiO₂ interfaces, the large-area MoS₂/anatase-TiO₂ films had realized a p-type doping of the underneath MoS₂, manifesting a boost in the extent of p-type doping with increasing thickness of TiO₂, as emerged from the X-ray photoelectron spectra. Density functional analysis of the MoS₂/anatase-TiO₂ interfaces, with pristine and interfacial defect configurations, could correlate the interdependence of doping and the terminating atomic surface of TiO₂ on MoS₂. The optical properties of the interface, encompassing photoluminescence, transient absorption and z-scan two-photon absorption, indicate the presence of defect-induced localized midgap levels in MoS₂/TiO₂ (PLD) and a relatively defect-free interface in MoS₂/TiO₂ (ALD), corroborating nicely with the corresponding theoretical analysis. From the investigation of optical properties, we indicate that the MoS₂/TiO₂ (PLD) interface may act as a promising saturable absorber, having a significant nonlinear response for the sub-band-gap excitations. Moreover, the MoS₂/TiO₂ (PLD) interface had exemplified better phototransport properties. A potential application of MoS₂/TiO₂ (PLD) is demonstrated by the fabrication of a p-type phototransistor with the ionic-gel top gate. This endeavor to analyze and perceive the MoS₂/TiO₂ interface establishes the prospectives of large-area interfaces in the field of optics and optoelectronics.

TFT Channel Materials for Display Applications: From Amorphous Silicon to Transition Metal Dichalcogenides

As the need for super-high-resolution displays with various form factors has increased, it has become necessary to produce high-performance thin-film transistors (TFTs) that enable faster switching and higher current driving of each pixel in the display. Over the past few decades, hydrogenated amorphous silicon (a-Si:H) has been widely utilized as a TFT channel material. More recently, to meet the requirement of new types of displays such as organic light-emitting diode displays, and also to overcome the performance and reliability issues of a-Si:H, low-temperature polycrystalline silicon and amorphous oxide semiconductors have partly replaced a-Si:H channel materials. Basic material properties and device structures of TFTs in commercial displays are explored, and then the potential of atomically thin layered transition metal dichalcogenides as next-generation channel materials is discussed.

Distinctive Field-Effect Transistors and Ternary Inverters Using Cross-Type WSe2/MoS2 Heterojunctions Treated with Polymer Acid

The electrical and optical characteristics of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be improved by surface modification. In this study, distinctive field-effect transistors (FETs) were realized by forming cross-type 2D WSe₂/MoS₂ p-n heterojunctions through surface treatment using poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-PMAA). The FETs were applied to new ternary inverters as multivalued logic circuits (MVLCs). Laser confocal microscope photoluminescence spectroscopy indicated the generation of trions in the WSe₂ and MoS₂ layers, and the intensity decreased after PMMA-co-PMAA treatment. For the cross-type WSe₂/MoS₂ p-n heterojunction FETs subjected to PMMA-co-PMAA treatment, the channel current and the region of anti-ambipolar transistor characteristics increased considerably, and ternary inverter characteristics with three stable logic states, "1", "1/2", and "0", were realized. Interestingly, the intermediate logic state 1/2, which results from the negative differential transconductance characteristics, was realized by the turn-on of all component FETs, as the current of the FETs increased after PMMA-co-PMAA treatment. The electron-rich carboxyl acid moieties in PMMA-co-PMAA can undergo coordination with the metal Mo or W atoms present in the Se or S vacancies, respectively, resulting in the modulation of charge density. These features yielded distinctive FETs and ternary inverters for MVLCs using cross-type WSe₂/MoS₂ heterojunctions.

Interface engineering of two-dimensional transition metal dichalcogenides towards next-generation electronic devices: recent advances and challenges

Over the past decade, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted tremendous research interest for future electronics owing to their atomically thin thickness, compelling properties and various potential applications. However, interface engineering including contact optimization and channel modulations for 2D TMDCs represents fundamental challenges in ultimate performance of ultrathin electronics. This article provides a comprehensive overview of the basic understanding of contacts and channel engineering of 2D TMDCs and emerging electronics benefiting from these varying approaches. In particular, we elucidate multifarious contact engineering approaches such as edge contact, phase engineering and metal transfer to suppress the Fermi level pinning effect at the metal/TMDC interface, various channel treatment avenues such as van der Waals heterostructures, surface charge transfer doping to modulate the device properties, and as well the novel electronics constructed by interface engineering such as diodes, circuits and memories. Finally, we conclude this review by addressing the current challenges facing 2D TMDCs towards next-generation electronics and offering our insights into future directions of this field.

Enhanced Carrier-Exciton Interactions in Monolayer MoS2 under Applied Voltages

Carrier-exciton interactions in two-dimensional transition metal dichalcogenides (TMDs) is one of the crucial elements for limiting the performance of their optoelectronic devices. Here, we have experimentally studied the carrier-exciton interactions in a monolayer MoS₂-based two-terminal device. Such two-terminal device without a gate electrode is generally considered as invalid to modulate the carrier concentration in active materials, while the photoluminescence peak exhibits a red shift and decay with increasing applied voltages. Time-resolved photoluminescence spectroscopy and photoluminescence multipeak fittings verify that such changes of photoluminescence peaks result from enhanced carrier-exciton interactions with increasing electron concentration induce the charged exciton increasing. To characterize the level of the carrier-exciton interactions, a quantitative relationship between the Raman shift of out-of-plane mode and changes in electron concentration has been established using the mass action model. This work provides an appropriate supplement for understanding the carrier-exciton interactions in TMD-based two-terminal optoelectronic devices.

Surface group-modified MXene nano-flake doping of monolayer tungsten disulfides

Exciton/trion-involved optoelectronic properties have attracted exponential amount of attention for various applications ranging from optoelectronics, valleytronics to electronics. Herein, we report a new chemical (MXene) doping strategy to modulate the negative trion and neutral exciton for achieving high photoluminescence yield of atomically thin transition metal dichalcogenides, enabled by the regulation of carrier densities to promote electron-bound trion-to-exciton transition via charge transfer from TMDCs to MXene. As a proof of concept, the MXene nano-flake-doped tungsten disulfide is demonstrated to obtain an enhanced PL efficiency of up to ∼five folds, which obviously exceeds the reported efficiency upon electrical and/or plasma doping strategies. The PL enhancement degree can also be modulated by tuning the corresponding surface functional groups of MXene nano-flakes, reflecting that the electron-withdrawing functional groups play a vital role in this charge transfer process. These findings offer promising clues to control the optoelectronic properties of TMDCs and expand the scope of the application of MXene nano-flakes, suggesting a possibility to construct a new heterostructure junction based on MXenes and TMDCs.

Vapor Phase Selective Growth of Two-Dimensional Perovskite/WS2 Heterostructures for Optoelectronic Applications

Organic-inorganic hybrid perovskites have attracted increased interest owing to their exceptional optoelectronic properties and promising applications. Monolayers of transition metal dichalcogenides (TMDCs), such as tungsten disulfide (WS₂), are also intriguing because of their unique optoelectronic properties and their atomically thin and flexible structures. Therefore, the combination of these different types of materials is very attractive in terms of fundamental science of interface interaction, as well as for the realization of ultrathin optoelectronic devices with high performance. Here, we demonstrate the controlled synthesis of two-dimensional (2D) perovskite/WS₂ heterostructures by an all vapor-phase growth approach. This involves the chemical vapor deposition (CVD) growth of monolayer WS₂, followed by the vapor-phase selective deposition of 2D PbI₂ onto the WS₂ with the successive conversion of PbI₂ to organic-inorganic perovskite (CH₃NH₃PbI₃). Moreover, the selective growth of the perovskite on prepatterned WS₂ enables the direct synthesis of patterned heterostructures, avoiding any damage to the perovskite. The photodetectors utilizing the perovskite/WS₂ heterostructure show increased responsivities compared with isolated thin perovskite obtained by conventional solution methods. The integration of 2D perovskite with TMDCs opens a new avenue to fabricate advanced devices by combining their unique properties and overcoming current processing difficulties of perovskites.

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^-1 s^-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.

A high-performance hydrogen sensor based on a reverse-biased MoS2/GaN heterojunction

We report a MoS₂/GaN heterojunction-based gas sensor by depositing MoS₂ over a GaN substrate via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The microscopic and spectroscopic measurements expose the presence of highly crystalline and homogenous few atomic layer MoS₂ on top of molecular beam epitaxially grown GaN film. Upon hydrogen exposure, the molecular adsorption tuned the barrier height at the MoS₂/GaN interface under the reverse biased condition, thus resulting in high sensitivity. Our results reveal that temperature strongly affects the sensitivity of the device and it increases from 21% to 157% for 1% hydrogen with an increase in temperature (25-150 °C). For a deeper understanding of carrier dynamics at the heterointerface, we visualized the band alignment across the MoS₂/GaN heterojunction having valence band and conduction band offset values of 1.75 and 0.28 eV. The sensing mechanism was demonstrated based on an energy band diagram at the MoS₂/GaN interface in the presence and absence of hydrogen exposure. The proposed methodology can be readily applied to other combinations of heterostructures for sensing different gas analytes.

Convection-Flow-Assisted Preparation of a Strong Electron Dopant, Benzyl Viologen, for Surface-Charge Transfer Doping of Molybdenum Disulfide

Transition metal dichalcogenides (TMDCs) have received attention as atomically thin post-silicon semiconducting materials. Tuning the carrier concentrations of the TMDCs is important, but their thin structure requires a non-destructive modulation method. Recently, a surface-charge transfer doping method was developed based on contacting molecules on TMDCs, and the method succeeded in achieving a large modulation of the electronic structures. The successful dopant is a neutral benzyl viologen (BV0); however, the problem remains of how to effectively prepare the BV0 molecules. A reduction process with NaBH4 in water has been proposed as a preparation method, but the NaBH4 simultaneously reacts vigorously with the water. Here, a simple method is developed, in which the reaction vial is placed on a hotplate and a fragment of air-stable metal is used instead of NaBH4 to prepare the BV0 dopant molecules. The prepared BV0 molecules show a strong doping ability in terms of achieving a degenerate situation of a TMDC, MoS2. A key finding in this preparation method is that a convection flow in the vial effectively transports the produced BV0 to a collection solvent. This method is simple and safe and facilitates the tuning of the optoelectronic properties of nanomaterials by the easily-handled dopant molecules.

MoS2 Field-Effect Transistor-Amyloid-β1-42 Hybrid Device for Signal Amplified Detection of MMP-9

The detection of circulating protein (CP) is very important for the diagnosis and therapeutics of cancer. Conventional techniques based on a specific antibody-antigen interaction are still lacking because of a shortage of cost effectiveness, complicated sandwich structure and tagging process, and inconsistent detection of CP due to the inherent instability of antibodies. Herein, we demonstrate a hybrid device consisting of two-dimensional (2D) nanoscale molybdenum disulfide (MoS₂) field-effect transistor (FET) with an amyloid-β_1-42 (Aβ_1-42) functionalized surface, which amplifies electric signals of the FET in order to detect matrix metalloproteinase-9 (MMP-9), which is a certain type of CP that degrades Aβ_1-42. With the hybrid device, we detected the concentrations of MMP-9 in the range from 1 pM to 10 nM. Moreover, using tapping-mode atomic force microscopy and Kelvin probe force microscopy, we verified that the signal amplification corresponding to the MMP-9 concentrations was caused by the reduced length and the decreased surface potential of degraded Aβ_1-42 due to MMP-9. The hybrid device studied in this paper can be very useful for monitoring MMP-9 activity, as well as serving as a sensing platform for the electrical signal amplification of 2D MoS₂ FET-biosensors.

High Optical Response of Niobium-Doped WSe₂-Layered Crystals

The optical properties of WSe₂-layered crystals doped with 0.5% niobium (Nb) grown by the chemical vapor transport method were characterized by piezoreflectance (PzR), photoconductivity (PC) spectroscopy, frequency-dependent photocurrent, and time-resolved photoresponse. With the incorporation of 0.5% Nb, the WSe₂ crystal showed slight blue shifts in the near band edge excitonic transitions and exhibited strongly enhanced photoresponsivity. Frequency-dependent photocurrent and time-resolved photoresponse were measured to explore the kinetic decay processes of carriers. Our results show the potential application of layered crystals for photodetection devices based on Nb-doped WSe₂-layered crystals.

Enhanced Performance of a CVD MoS2 Photodetector by Chemical in Situ n-Type Doping

Transition metal dichalcogenides (TMDs) are a category of promising two-dimensional (2D) materials for the optoelectronic devices, and their unique characteristics include tunable band gap, nondangling bonds as well as compatibility to large-scale fabrication, for instance, chemical vapor deposition (CVD). MoS₂ is one of the first TMDs that is well studied in the photodetection area widely. However, the low photoresponse restricts its applications in photodetectors unless the device is applied with ultrahigh source-drain voltage ( V_DS) and gate voltage ( V_GS). In this work, the photoresponse of a MoS₂ photodetector was improved by a chemical in situ doping method using gold chloride hydrate. The responsivity and specific detectivity were increased to 99.9 A/W and 9.4 × 10¹² Jones under low V_DS (0.1 V) and V_GS (0 V), which are 14.6 times and 4.8 times higher than those of a pristine photodetector, respectively. The photoresponse enhancement results from chlorine n-type doping in CVD MoS₂ which reduces the trapping of photoinduced electrons and promotes the photogating effect. This novel doping strategy leads to great applications of high-performance MoS₂ photodetectors potentially and opens a new avenue to enhance photoresponse for other 2D materials.

Doping engineering and functionalization of two-dimensional metal chalcogenides

Two-dimensional (2D) layered metal chalcogenides (MXs) have significant potential for use in flexible transistors, optoelectronics, sensing and memory devices beyond the state-of-the-art technology. To pursue ultimate performance, precisely controlled doping engineering of 2D MXs is desired for tailoring their physical and chemical properties in functional devices. In this review, we highlight the recent progress in the doping engineering of 2D MXs, covering that enabled by substitution, exterior charge transfer, intercalation and the electrostatic doping mechanism. A variety of novel doping engineering examples leading to Janus structures, defect curing effects, zero-valent intercalation and deliberately devised floating gate modulation will be discussed together with their intriguing application prospects. The choice of doping strategies and sources for functionalizing MXs will be provided to facilitate ongoing research in this field toward multifunctional applications.

Nonvolatile and Programmable Photodoping in MoTe2 for Photoresist-Free Complementary Electronic Devices

2D transition-metal dichalcogenide (TMD)-based electronic devices have been extensively explored toward the post-Moore era. Huge efforts have been devoted to modulating the doping profile of TMDs to achieve 2D p-n junctions and inverters, the fundamental units in logic circuits. Here, photoinduced nonvolatile and programmable electron doping in MoTe₂ based on a heterostructure of MoTe₂ and hexagonal boron nitride (BN) is reported. The electron transport property in the MoTe₂ device can be precisely controlled by modulating the magnitude of the photodoping gate exerted on BN. Through tuning the polarity of the photodoping gate exerted on BN under illumination, such a doping effect in MoTe₂ can be programmed with excellent repeatability and is retained for over 14 d in the absence of an external perturbation. By spatially controlling the photodoping region in MoTe₂ , a photoresist-free p-n junction and inverter in the MoTe₂ homostructure are achieved. The MoTe₂ diode exhibits a near-unity ideality factor of ≈1.13 with a rectification ratio of ≈1.7 × 10⁴ . Moreover, the gain of the MoTe₂ inverter reaches ≈98, which is among the highest values for 2D-material-based homoinverters. These findings promise photodoping as an effective method to achieve 2D-TMDs-based nonvolatile and programmable complementary electronic devices.

Two-dimensional black phosphorus: its fabrication, functionalization and applications

Phosphorus, one of the most abundant elements in the Earth (∼0.1%), has attracted much attention in the last five years since the rediscovery of two-dimensional (2D) black phosphorus (BP) in 2014. The successful scaling down of BP endows this 'old material' with new vitality, resulting from the intriguing semiconducting properties in the atomic scale limit, i.e. layer-dependent bandgap that covers from the visible light to mid-infrared light spectrum as well as hole-dominated ambipolar transport characteristics. Intensive research effort has been devoted to the fabrication, characterization, functionalization and application of BP and other phosphorus allotropes. In this review article, we summarize the fundamental properties and fabrication techniques of BP, with particular emphasis on the recent progress in molecular beam epitaxy growth of 2D phosphorus. Subsequently, we highlight recent progress in BP (opto)electronic device applications achieved via customized manipulation methods, such as interface, defect and bandgap engineering as well as forming Lego-like stacked heterostructures.