Thickness-dependent charge transport in few-layer MoS₂ field-effect transistors

Understanding how junction resistances impact the conduction mechanism in nano-networks

Networks of nanowires, nanotubes, and nanosheets are important for many applications in printed electronics. However, the network conductivity and mobility are usually limited by the resistance between the particles, often referred to as the junction resistance. Minimising the junction resistance has proven to be challenging, partly because it is difficult to measure. Here, we develop a simple model for electrical conduction in networks of 1D or 2D nanomaterials that allows us to extract junction and nanoparticle resistances from particle-size-dependent DC network resistivity data. We find junction resistances in porous networks to scale with nanoparticle resistivity and vary from 5 Ω for silver nanosheets to 24 GΩ for WS2 nanosheets. Moreover, our model allows junction and nanoparticle resistances to be obtained simultaneously from AC impedance spectra of semiconducting nanosheet networks. Through our model, we use the impedance data to directly link the high mobility of aligned networks of electrochemically exfoliated MoS2 nanosheets (≈ 7 cm2 V-1 s-1) to low junction resistances of ∼2.3 MΩ. Temperature-dependent impedance measurements also allow us to comprehensively investigate transport mechanisms within the network and quantitatively differentiate intra-nanosheet phonon-limited bandlike transport from inter-nanosheet hopping.

Quaternary, layered, 2D chalcogenide, Mo1-xWxSSe: thickness dependent transport properties

Highly oriented, single crystalline, quaternary alloy chalcogenide crystal, MoxW1-xS2ySe2(1-y), is synthesized using a high temperature chemical vapor transport technique and its transport properties studied over a wide temperature range. Field effect transistors (FET) with bottom gated configuration are fabricated using Mo0.5W0.5SSe flakes of different thicknesses, from a single layer to bulk. The FET characteristics are thickness tunable, with thin flakes (1-4 layers) exhibiting n-type transport behaviour while ambipolar transfer characteristics are observed for thicker flakes (>90 layers). Ambipolar behavior with the dominance of n-type over p-type transport is noted for devices fabricated with layers between 9 and 90. The devices with flake thickness ∼9 layers exhibit a maximum electron mobility 63 ± 4 cm2V-1s-1and anION/IOFFratio >108. A maximum hole mobility 10.3 ± 0.4 cm2V-1s-1is observed for the devices with flake thickness ∼94 layers withION/IOFFratio >102-103observed for the hole conduction. A maximumION/IOFFfor hole conduction, 104is obtained for the devices fabricated with flakes of thickness ∼7-19 layers. The electron Schottky barrier height values are determined to be ∼23.3 meV and ∼74 meV for 2 layer and 94 layers flakes respectively, as measured using low temperature measurements. This indicates that an increase in hole current with thickness is likely to be due to lowering of the band gap as a function of thickness. Furthermore, the contact resistance (Rct) is evaluated using transmission line model (TLM) and is found to be 14 kohm.μm. These results suggest that quaternary alloys of Mo0.5W0.5SSe are potential candidates for various electronic/optoelectronic devices where properties and performance can be tuned within a single composition.

Probing the Electronic and Opto-Electronic Properties of Multilayer MoS2 Field-Effect Transistors at Low Temperatures

Transition metal dichalcogenides (TMDs)-based field-effect transistors (FETs) are being investigated vigorously for their promising applications in optoelectronics. Despite the high optical response reported in the literature, most of them are studied at room temperature. To extend the application of these materials in a photodetector, particularly at a low temperature, detailed understanding of the photo response behavior of these materials at low temperatures is crucial. Here we present a systematic investigation of temperature-dependent electronic and optoelectronic properties of few-layers MoS2 FETs, synthesized using the mechanical exfoliation of bulk MoS2 crystal, on the Si/SiO2 substrate. Our MoS2 FET show a room-temperature field-effect mobility μFE ~40 cm2·V-1·s-1, which increases with decreasing temperature, stabilizing at 80 cm2·V-1·s-1 below 100 K. The temperature-dependent (50 K < T < 300 K) photoconductivity measurements were investigated using a continuous laser source λ = 658 nm (E = 1.88 eV) over a broad range of effective illuminating laser intensity, Peff (0.02 μW < Peff < 0.6 μW). Photoconductivity measurements indicate a fractional power dependence of the steady-state photocurrent. The room-temperature photoresponsivity (R) obtained in these samples was found to be ~2 AW-1, and it increases as a function of decreasing temperature, reaching a maximum at T = 75 K. The optoelectronic properties of MoS2 at a low temperature give an insight into photocurrent generation mechanisms, which will help in altering/improving the performance of TMD-based devices for various applications.

Multilayer In-Plane Heterostructures Based on Transition Metal Dichalcogenides for Advanced Electronics

In-plane heterostructures of transition metal dichalcogenides (TMDCs) have attracted much attention for high-performance electronic and optoelectronic devices. To date, mainly monolayer-based in-plane heterostructures have been prepared by chemical vapor deposition (CVD), and their optical and electrical properties have been investigated. However, the low dielectric properties of monolayers prevent the generation of high concentrations of thermally excited carriers from doped impurities. To solve this issue, multilayer TMDCs are a promising component for various electronic devices due to the availability of degenerate semiconductors. Here, we report the fabrication and transport properties of multilayer TMDC-based in-plane heterostructures. The multilayer in-plane heterostructures are formed through CVD growth of multilayer MoS₂ from the edges of mechanically exfoliated multilayer flakes of WSe₂ or Nb_xMo_1-xS₂. In addition to the in-plane heterostructures, we also confirmed the vertical growth of MoS₂ on the exfoliated flakes. For the WSe₂/MoS₂ sample, an abrupt composition change is confirmed by cross-sectional high-angle annular dark-field scanning transmission electron microscopy. Electrical transport measurements reveal that a tunneling current flows at the Nb_xMo_1-xS₂/MoS₂ in-plane heterointerface, and the band alignment is changed from a staggered gap to a broken gap by electrostatic electron doping of MoS₂. The formation of a staggered gap band alignment of Nb_xMo_1-xS₂/MoS₂ is also supported by first-principles calculations.

Stabilized Synthesis of 2D Verbeekite: Monoclinic PdSe2 Crystals with High Mobility and In-Plane Optical and Electrical Anisotropy

PdSe₂ has a layered structure with an unusual, puckered Cairo pentagonal tiling. Its atomic bond configuration features planar 4-fold-coordinated Pd atoms and intralayer Se-Se bonds that enable polymorphic phases with distinct electronic and quantum properties, especially when atomically thin. PdSe₂ is conventionally orthorhombic, and direct synthesis of its metastable polymorphic phases is still a challenge. Here, we report an ambient-pressure chemical vapor deposition approach to synthesize metastable monoclinic PdSe₂. Monoclinic PdSe₂ is shown to be synthesized selectively under Se-deficient conditions that induce Se vacancies. These defects are shown by first-principles density functional theory calculations to reduce the free energy of the metastable monoclinic phase, thereby stabilizing it during synthesis. The structure and composition of the monoclinic PdSe₂ crystals are identified and characterized by scanning transmission electron microscopy imaging, convergent beam electron diffraction, and electron energy loss spectroscopy. Polarized Raman spectroscopy of the monoclinic PdSe₂ flakes reveals their strong in-plane optical anisotropy. Electrical transport measurements show that the monoclinic PdSe₂ exhibits n-type charge carrier conduction with electron mobilities up to ∼298 cm² V^-1 s^-1 and a strong in-plane electron mobility anisotropy of ∼1.9. The defect-mediated growth pathway identified in this work is promising for phase-selective direct synthesis of other 2D transition metal dichalcogenides.

Abnormally weak intervalley electron scattering in MoS2 monolayer: insights from the matching between electron and phonon bands

It is known that carrier mobility in layered semiconductors generally increases from two-dimensions (2D) to three-dimensions due to fewer scattering channels resulting from decreased densities of electron and phonon states. In this work, we find an abnormal decrease of electron mobility from monolayer to bulk MoS₂. By carefully analyzing the scattering mechanisms, we can attribute such abnormality to the stronger intravalley scattering in the monolayer but weaker intervalley scattering caused by few intervalley scattering channels and weaker corresponding electron-phonon couplings compared to the bulk case. We show that it is the matching between the electronic band structure and phonon spectrum rather than their densities of electronic and phonon states that determines scattering channels. We propose, for the first time, the phonon-energy-resolved matching function to identify the intra- and inter-valley scattering channels. Furthermore, we show that multiple valleys do not necessarily lead to strong intervalley scattering if: (1) the scattering channels, which can be explicitly captured by the distribution of the matching function, are few due to the small matching between the corresponding electron and phonon bands; and/or (2) the multiple valleys are far apart in the reciprocal space and composed of out-of-plane orbitals so that the corresponding electron-phonon coupling strengths are weak. Consequently, the searching scope of high-mobility 2D materials can be reasonably enlarged using the matching function as useful guidance with the help of band edge orbital analysis.

Electrostatically Controllable Channel Thickness and Tunable Low-Frequency Noise Characteristics of Double-Gated Multilayer MoS2 Field-Effect Transistors with h-BN Dielectric

Two-dimensional transition-metal dichalcogenide (TMD) materials have attracted increasing attention in efforts to overcome fundamental issues faced by the complementary metal-oxide-semiconductor industry. Multilayer TMD materials such as MoS₂ can be used for high-performance transistor-based applications; the drive currents are high and the materials handle low-frequency (LF) noise well. We fabricated double-gated multilayer MoS₂ transistors using the h-BN dielectric for the top gate and silicon dioxide for the bottom gate. We systemically investigated the bottom gate voltage (V_b)-controlled electrical characteristics and the top/bottom interface-coupling effects. The effective thickness of the MoS₂ channel (t_{MoS2_eff}) was well modulated by V_b, and t_{MoS2_eff} reduction by negative V_b dramatically improved the I_on/I_off ratio. Numerical simulation and analytical modeling with a variation of the depletion depth under different bias conditions verified the experimental results. We were also the first to observe V_b-tuned LF noise characteristics. Here, we discuss the V_b-affected series resistance and carrier mobility in detail. Our findings greatly enhance the understanding of how double-gated multilayer MoS₂ transistors operate and will facilitate performance optimization in the real world.

Temperature-Driven α-β Phase Transformation and Enhanced Electronic Property of 2H α-In2Se3

In recent years, thin layered indium selenide (In2Se3) has attracted rapidly increasing attention due to its fascinating properties and promising applications. Here, we report the temperature-driven α-β phase transformation and the enhanced electronic property of 2H α-In2Se3. We find that 2H α-In2Se3 transforms to β-In2Se3 when it is heated to a high temperature, and the transformation temperature increases from 550 to 650 K with the thickness decreasing from 67 to 17 nm. Additionally, annealing the sample below the phase transformation temperature can effectively improve the electronic property of a 2H α-In2Se3 field-effect transistor, including increasing the on-state current, decreasing the off-state current, and improving the subthreshold swing. After annealing, not only the contact resistance decreases significantly but also the mobility at 300 K increases more than 2 times to 45.83 cm2 V-1 s-1, which is the highest among the reported values. Our results provide an effective method to improve the electrical property and the stability of the In2Se3 nanodevices.

Analysis of Schottky barrier heights and reduced Fermi-level pinning in monolayer CVD-grown MoS2field-effect-transistors

Chemical vapor deposition (CVD)-grown monolayer (ML) molybdenum disulfide (MoS₂) is a promising material for next-generation integrated electronic systems due to its capability of high-throughput synthesis and compatibility with wafer-scale fabrication. Several studies have described the importance of Schottky barriers in analyzing the transport properties and electrical characteristics of MoS₂field-effect-transistors (FETs) with metal contacts. However, the analysis is typically limited to single devices constructed from exfoliated flakes and should be verified for large-area fabrication methods. In this paper, CVD-grown ML MoS₂was utilized to fabricate large-area (1 cm × 1 cm) FET arrays. Two different types of metal contacts (i.e. Cr/Au and Ti/Au) were used to analyze the temperature-dependent electrical characteristics of ML MoS₂FETs and their corresponding Schottky barrier characteristics. Statistical analysis provides new insight about the properties of metal contacts on CVD-grown MoS₂compared to exfoliated samples. Reduced Schottky barrier heights (SBH) are obtained compared to exfoliated flakes, attributed to a defect-induced enhancement in metallization of CVD-grown samples. Moreover, the dependence of SBH on metal work function indicates a reduction in Fermi level pinning compared to exfoliated flakes, moving towards the Schottky-Mott limit. Optical characterization reveals higher defect concentrations in CVD-grown samples supporting a defect-induced metallization enhancement effect consistent with the electrical SBH experiments.

One order enhancement of charge carrier relaxation rate by tuning structural and optical properties in annealed cobalt doped MoS2 nanosheets

The effective manipulation of excitons is crucial for the realization of exciton-based devices and circuits, and doping is considered a good strategy to achieve this.

Theoretical Study of Anisotropic Carrier Mobility for Two-Dimensional Nb2Se9 Material

Finding new materials with satisfying all the desired criteria for nanodevices is an extremely difficult work. Here, we introduce a novel Nb2Se9 material as a promising candidate, capable of overcoming some physical limitations, such as a suitable band gap, high carrier mobility, and chemical stability. Unlike graphene, it has a noticeable band gap and no dangling bonds at surfaces that deteriorate transport properties, owing to its molecular chain structure. Using density functional theory (DFT) calculations with deformation potential (DP) theory, we find that the electron mobility of 2D Nb2Se9 across the axis direction reaches up to 2.56 × 103 cm2 V-1 s-1 and is approximately 2.5-6 times higher than the mobility of other 2D materials, such as MoS2, black phosphorous, and InSe, at room temperature. Moreover, the mobility of 2D Nb2Se9 is highly anisotropic (μ a /μ c ≈ 6.5). We demonstrate the potential of 2D Nb2Se9 for applications in nanoscale electronic devices and, possibly, mid-infrared photodetectors.

Preparation and Research of Monolayer WS2 FETs Encapsulated by h-BN Material

Functional devices that use vertical van der Waals (vdWs) heterostructure material can effectively combine the properties of single component materials, and the strong interlayer coupling effect can change their electronic and optical properties. According to our research, WS₂/h-BN vertical vdWs heterostructure material can be synthesized by chemical vapor deposition (CVD) and wet transfer methods. Monolayer WS₂ material and WS₂/h-BN vertical vdWs heterostructure material can be tested and characterized using XPS, SEM, EDS, AFM and Raman spectroscopy, which can prove the existence of corresponding materials. When the thickness of the material decreases, the Coulomb scattering amongst two-dimensional (2D) layered materials increases. This is because both the shielding effect and the distance between the channel and the interface layer decrease. FET devices are then fabricated on WS₂/h-BN vdWs heterostructure material by the electron beam lithography and evaporation processes. The effects of vdWs epitaxy on electrical transmission when WS₂/h-BN vdWs heterostructure material is formed are explored. Finally, the related electrical performance of FET devices is tested and analyzed. Our experimental research provides guidance for the use of electronic devices with vdWs heterostructure material.

Low-Power OR Logic Ferroelectric In-Situ Transistor Based on a CuInP2S6/MoS2 Van Der Waals Heterojunction

Due to the limitations of thermodynamics, the Boltzmann distribution of electrons hinders the further reduction of the power consumption of field-effect transistors. However, with the emergence of ferroelectric materials, this problem is expected to be solved. Herein, we demonstrate an OR logic ferroelectric in-situ transistor based on a CIPS/MoS2 Van der Waals heterojunction. Utilizing the electric field amplification of ferroelectric materials, the CIPS/MoS2 vdW ferroelectric transistor offers an average subthreshold swing (SS) of 52 mV/dec over three orders of magnitude, and a minimum SS of 40 mV/dec, which breaks the Boltzmann limit at room temperature. The dual-gated ferroelectric in-situ transistor exhibits excellent OR logic operation with a supply voltage of less than 1 V. The results indicate that the CIPS/MoS2 vdW ferroelectric transistor has great potential in ultra-low-power applications due to its in-situ construction, steep-slope subthreshold swing and low supply voltage.

Gate-Tunable Negative Differential Resistance Behaviors in a hBN-Encapsulated BP-MoS2 Heterojunction

Two-dimensional (2D) heterostructures show great potential in achieving negative differential resistance (NDR) effects by Esaki diodes and or resonant tunneling diodes. However, most of the reported Esaki diode-based NDR devices realized by bulk 2D films lack sufficient gate tunability, and the tuning of NDR behavior from appearing to vanishing remains elusive. Here, a gate-tunable NDR device is reported based on a vertically stacked black phosphorus (BP) and molybdenum disulfide (MoS₂) thin 2D heterojunction. At room temperature, a rectifying ratio of ∼6 orders of magnitude from a reverse rectifying diode to a forward rectifying diode by gate modulation is obtained. Through analyzing the temperature-dependent electrical properties, the tunneling mechanism at a certain gate voltage range is revealed. Moreover, the switchable and continuously gate-tunable NDR behavior is realized with a maximum peak-to-valley ratio of 1.23 at 77 K, as shown in the I_DS mappings by sweeping V_DS under different V_GS. In addition, a compact model for gate-tunable NDR behavior in 2D heterostructures is proposed. To our best knowledge, this is the first demonstration of NDR behavior in BP-MoS₂ heterostructures. Consequently, this work sheds light on the gate-tunable NDR devices and reconfigurable logic devices for realizing ternary and reconfigurable logic systems.

High mobility field-effect transistors based on MoS2crystals grown by the flux method

Two-dimensional (2D) molybdenum disulphide (MoS₂) transition metal dichalcogenides (TMDs) have great potential for use in optical and electronic device applications; however, the performance of MoS₂is limited by its crystal quality, which serves as a measure of the defects and grain boundaries in the grown material. Therefore, the high-quality growth of MoS₂crystals continues to be a critical issue. In this context, we propose the formation of high-quality MoS₂crystals via the flux method. The resulting electrical properties demonstrate the significant impact of crystal morphology on the performance of MoS₂field-effect transistors. MoS₂made with a relatively higher concentration of sulphur (a molar ratio of 2.2) and at a cooling rate of 2.5 °C h^-1yielded good quality and optimally sized crystals. The room-temperature and low-temperature (77 K) electrical transport properties of MoS₂field-effect transistors (FETs) were studied in detail, with and without the use of a hexagonal boron nitride (h-BN) dielectric to address the mobility degradation issue due to scattering at the SiO₂/2D material interface. A maximum field-effect mobility of 113 cm²V^-1s^-1was achieved at 77 K for the MoS₂/h-BN FET following high-quality crystal formation by the flux method. Our results confirm the achievement of large-scale high-quality crystal growth with reduced defect density using the flux method and are key to achieving higher mobility in MoS₂FET devices in parallel with commercially accessible MoS₂crystals.

Investigation of charge trapping mechanism in MoS2field effect transistor by incorporating Al into host La2O3as gate dielectric

The charge trapping effect plays a key role in multi-bit memory devices and brain-like neuron devices. Herein, MoS₂field effect transistors are fabricated, incorporating Al into host La₂O₃as the gate dielectric, which exhibit excellent electrical properties with an on-off ratio in the memory window of ∼10⁶and a memory window ratio of ∼40%. Furthermore, the charge trapping and de-trapping processes were systematically studied, and the time constants are obtained from time-domain characteristics. Making use of the charge trapping effect, the threshold voltage of the device can be continuously adjusted. The oxide layer trap density and the interface state trap density are extracted using the charge separation method. These theoretical studies provide a deeper understanding of ways to control the charge trapping process, benefitting the commercialization of two-dimensional electronic devices and the development of new charge trapping devices.

Few-Layer MoS2 Photodetector Arrays for Ultrasensitive On-Chip Enzymatic Colorimetric Analysis

Enzymatic colorimetric analysis of metabolites provides signatures of energy conversion and biosynthesis associated with disease onsets and progressions. Miniaturized photodetectors based on emerging two-dimensional transition metal dichalcogenides (TMDCs) promise to advance point-of-care diagnosis employing highly sensitive enzymatic colorimetric detection. Reducing diagnosis costs requires a batched multisample assay. The construction of few-layer TMDC photodetector arrays with consistent performance is imperative to realize optical signal detection for a miniature batched multisample enzymatic colorimetric assay. However, few studies have promoted an optical reader with TMDC photodetector arrays for on-chip operation. Here, we constructed 4 × 4 pixel arrays of miniaturized molybdenum disulfide (MoS₂) photodetectors and integrated them with microfluidic enzyme reaction chambers to create an optoelectronic biosensor chip device. The fabricated device allowed us to achieve arrayed on-chip enzymatic colorimetric detection of d-lactate, a blood biomarker signifying the bacterial translocation from the intestine, with a limit of detection that is 1000-fold smaller than the clinical baseline, a 10 min assay time, high selectivity, and reasonably small variability across the entire arrays. The enzyme (Ez)/MoS₂ optoelectronic biosensor unit consistently detected d-lactate in clinically important biofluids, such as saliva, urine, plasma, and serum of swine and humans with a wide detection range (10^-3-10³ μg/mL). Furthermore, the biosensor enabled us to show that high serum d-lactate levels are associated with the symptoms of systemic infection and inflammation. The lensless, optical waveguide-free device architecture should readily facilitate development of a monolithically integrated hand-held module for timely, cost-effective diagnosis of metabolic disorders in near-patient settings.

Low-Temperature Synthesis of Wafer-Scale MoS2-WS2 Vertical Heterostructures by Single-Step Penetrative Plasma Sulfurization

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention owing to their synergetic effects with other 2D materials, such as graphene and hexagonal boron nitride, in TMD-based heterostructures. Therefore, it is important to understand the physical properties of TMD-TMD vertical heterostructures for their applications in next-generation electronic devices. However, the conventional synthesis process of TMD-TMD heterostructures has some critical limitations, such as nonreproducibility and low yield. In this paper, we synthesize wafer-scale MoS₂-WS₂ vertical heterostructures (MWVHs) using plasma-enhanced chemical vapor deposition (PE-CVD) via penetrative single-step sulfurization discovered by time-dependent analysis. This method is available for fabricating uniform large-area vertical heterostructures (4 in.) at a low temperature (300 °C). MWVHs were characterized using various spectroscopic and microscopic techniques, which revealed their uniform nanoscale polycrystallinity and the presence of vertical layers of MoS₂ and WS₂. In addition, wafer-scale MWVHs diodes were fabricated and demonstrated uniform performance by current mapping. Furthermore, mode I fracture tests were performed using large double cantilever beam specimens to confirm the separation of the MWVHs from the SiO₂/Si substrate. Therefore, this study proposes a synthesis mechanism for TMD-TMD heterostructures and provides a fundamental understanding of the interfacial properties of TMD-TMD vertical heterostructures.

Staircase-like transfer characteristics in multilayer MoS2 field-effect transistors

Layered semiconductors, such as MoS2, have attracted interest as channel materials for post-silicon and beyond-CMOS electronics. Much attention has been devoted to the monolayer limit, but the monolayer channel is not necessarily advantageous in terms of the performance of field-effect transistors (FETs). Therefore, it is important to investigate the characteristics of FETs that have multilayer channels. Here, we report the staircase-like transfer characteristics of FETs with exfoliated multilayer MoS2 flakes. Atomic force microscope characterizations reveal that the presence of thinner terraces at the edges of the flakes accompanies the staircase-like characteristics. The anomalous staircase-like characteristics are ascribable to a difference in threshold-voltage shift by charge transfer from surface adsorbates between the channel center and the thinner terrace at the edge. This study reveals the importance of the uniformity of channel thickness.

High-Performance Field-Effect Transistor and Logic Gates Based on GaS-MoS2 van der Waals Heterostructure

This work demonstrates a high-performance and hysteresis-free field-effect transistor based on two-dimensional (2D) semiconductors featuring a van der Waals heterostructure, MoS₂ channel, and GaS gate insulator. The transistor exhibits a subthreshold swing of 63 mV/dec, an on/off ratio over 10⁶ within a gate voltage of 0.4 V, and peak mobility of 83 cm²/(V s) at room temperature. The low-frequency noise characteristics were investigated and described by the Hooge mobility fluctuation model. The results suggest that the van der Waals heterostructure of 2D semiconductors can produce a high-performing interface without dangling bonds and defects caused by lattice mismatch. Furthermore, a logic inverter and a NAND gate are demonstrated, with an inverter voltage gain of 14.5, which is higher than previously reported by MoS₂-based transistors with oxide dielectrics. Therefore, this transistor based on van der Waals heterostructure exhibits considerable potential in digital logic applications with low-power integrated circuits.

Improved Sensitivity in Schottky Contacted Two-Dimensional MoS2 Gas Sensor

Two-dimensional (2D) transition-metal dichalcogenides have attracted significant attention as gas-sensing materials owing to their superior responsivity at room temperature and their possible application as flexible electronic devices. Especially, reliable responsivity and selectivity for various environmentally harmful gases are the main requirements for the future chemiresistive-type gas sensor applications. In this study, we demonstrate improved sensitivity of a 2D MoS₂-based gas sensor by controlling the Schottky barrier height. Chemical vapor deposition process was performed at low temperature to obtain layer-controlled 2D MoS₂, and the NO₂ gas responsivity was confirmed by the fabricated gas sensor. Then, the number of MoS₂ layers was fixed and the types of electrode materials were varied for controlling the Schottky barrier height. As the Schottky barrier height increased, the NO₂ responsivity increased, and it was found to be effective for CO and CO₂ gases, which had little reactivity in 2D MoS₂-based gas sensors.

Raman and electrical transport properties of few-layered arsenic-doped black phosphorus

Black phosphorus (b-P) is an allotrope of phosphorus whose properties have attracted great attention. In contrast to other 2D compounds, or pristine b-P, the properties of b-P alloys have yet to be explored. In this report, we present a detailed study on the Raman spectra and on the temperature dependence of the electrical transport properties of As-doped black phosphorus (b-AsP) for an As fraction x = 0.25. The observed complex Raman spectra were interpreted with the support of Density Functional Theory (DFT) calculations since each original mode splits in three due to P-P, P-As, and As-As bonds. Field-effect transistors (FET) fabricated from few-layered b-AsP exfoliated onto Si/SiO2 substrates exhibit hole-doped like conduction with a room temperature ON/OFF current ratio of ∼103 and an intrinsic field-effect mobility approaching ∼300 cm2 V-1 s-1 at 300 K which increases up to 600 cm2 V-1 s-1 at 100 K when measured via a 4-terminal method. Remarkably, these values are comparable to, or higher, than those initially reported for pristine b-P, indicating that this level of As doping is not detrimental to its transport properties. The ON to OFF current ratio is observed to increase up to 105 at 4 K. At high gate voltages b-AsP displays metallic behavior with the resistivity decreasing with decreasing temperature and saturating below T ∼100 K, indicating a gate-induced insulator to metal transition. Similarly to pristine b-P, its transport properties reveal a high anisotropy between armchair (AC) and zig-zag (ZZ) directions. Electronic band structure computed through periodic dispersion-corrected hybrid Density Functional Theory (DFT) indicate close proximity between the Fermi level and the top of the valence band(s) thus explaining its hole doped character. Our study shows that b-AsP has potential for optoelectronics applications that benefit from its anisotropic character and the ability to tune its band gap as a function of the number of layers and As content.

Probing the Field-Effect Transistor with Monolayer MoS2 Prepared by APCVD

The two-dimensional materials can be used as the channel material of transistor, which can further decrease the size of transistor. In this paper, the molybdenum disulfide (MoS₂) is grown on the SiO₂/Si substrate by atmospheric pressure chemical vapor deposition (APCVD), and the MoS₂ is systematically characterized by the high-resolution optical microscopy, Raman spectroscopy, photoluminescence spectroscopy, and the field emission scanning electron microscopy, which can confirm that the MoS₂ is a monolayer. Then, the monolayer MoS₂ is selected as the channel material to complete the fabrication process of the back-gate field effect transistor (FET). Finally, the electrical characteristics of the monolayer MoS₂-based FET are tested to obtain the electrical performance. The switching ratio is 10³, the field effect mobility is about 0.86 cm²/Vs, the saturation current is 2.75 × 10^-7 A/μm, and the lowest gate leakage current is 10^-12 A. Besides, the monolayer MoS₂ can form the ohmic contact with the Ti/Au metal electrode. Therefore, the electrical performances of monolayer MoS₂-based FET are relatively poor, which requires the further optimization of the monolayer MoS₂ growth process. Meanwhile, it can provide the guidance for the application of monolayer MoS₂-based FETs in the future low-power optoelectronic integrated circuits.

Layer-Dependent Photoinduced Electron Transfer in 0D-2D Lead Sulfide/Cadmium Sulfide-Layered Molybdenum Disulfide Hybrids

We demonstrate layer-dependent electron transfer between core/shell PbS/CdS quantum dots (QDs) and layered MoS₂ via energy band gap engineering of both the donor (QDs) and the acceptor (MoS₂) components. We do this by (i) changing the size of the QD or (ii) by changing the number of layers of MoS₂, and each of these approaches alters the band gap and/or the donor-acceptor separation distance, thus providing a means of tuning the charge-transfer rate. We find the charge-transfer rate to be maximal for QDs of smallest size and for QDs combined with a 5-layer MoS₂ or thicker. We model this layer-dependent charge-transfer rate with a theoretical model derived from Marcus theory previously applied to nonadiabatic electron transfer in weakly coupled systems by considering the QD transferring photogenerated electrons to noninteracting monolayers within a few layers of MoS₂.

Paper-Cut Flexible Multifunctional Electronics Using MoS2 Nanosheet

Art and science represent human creativity and rational thinking, respectively. When the two seemingly opposite fields are intertwined, there is always a life-changing spark. In particular, the integration of ancient traditional Chinese art into the latest electronic devices is always been an unexcavated topic. Fabricating two-dimensional material with a tensile strain less than 3% with an ultimate global stretch has been an important problem that plagues the current flexible electronics field. The current research is limited to material in small scale, and it is always necessary to develop and extend large-sized flexible electronic systems. Here, inspired by the traditional Chinese paper-cut structure, we present a highly deformable multifunctional electronic system based on the MoS2 nanosheet. In this work, we first demonstrate how the traditional paper-cut structure can open the view of flexible electronics. In order to obtain a large area of MoS2 with excellent performance, we use a metal-assisted exfoliation method to transfer MoS2, followed by fabricating a field effect transistor to characterize its excellent electrical properties. Two photodetectors and a temperature sensor are produced with good performance. The mechanical simulation proves that the structure has more advantages in stretchability than other typical paper-cut structures. From the experimental and mechanical point of view, it is proved that the device can work stably under high deformation. We finally show that the device has broad application prospects in highly deformed organs, tissues, and joints. These findings set a good example of traditional Chinese culture to guide innovation in the field of electronic devices.

Electric Double Layer Field-Effect Transistors Using Two-Dimensional (2D) Layers of Copper Indium Selenide (CuIn7Se11)

Innovations in the design of field-effect transistor (FET) devices will be the key to future application development related to ultrathin and low-power device technologies. In order to boost the current semiconductor device industry, new device architectures based on novel materials and system need to be envisioned. Here we report the fabrication of electric double layer field-effect transistors (EDL-FET) with two-dimensional (2D) layers of copper indium selenide (CuIn7Se11) as the channel material and an ionic liquid electrolyte (1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)) as the gate terminal. We found one order of magnitude improvement in the on-off ratio, a five- to six-times increase in the field-effect mobility, and two orders of magnitude in the improvement in the subthreshold swing for ionic liquid gated devices as compared to silicon dioxide (SiO2) back gates. We also show that the performance of EDL-FETs can be enhanced by operating them under dual (top and back) gate conditions. Our investigations suggest that the performance of CuIn7Se11 FETs can be significantly improved when BMIM-PF6 is used as a top gate material (in both single and dual gate geometry) instead of the conventional dielectric layer of the SiO2 gate. These investigations show the potential of 2D material-based EDL-FETs in developing active components of future electronics needed for low-power applications.

Ultrafast Carrier Dynamics of the Exciton and Trion in MoS2 Monolayers Followed by Dissociation Dynamics in Au@MoS2 2D Heterointerfaces

Many-body states like excitons, biexcitons, and trions play an important role in optoelectronic and photovoltaic applications in 2D materials. Herein, we studied carrier dynamics of excitons and trions in monolayer MoS₂ deposited on a SiO₂/Si substrate, before and after Au NP deposition, using femtosecond transient absorption spectroscopy. Luminescence measurements confirm the presence of both an exciton and trion in MoS₂, which are drastically quenched after deposition of Au NPs, indicating electron transfer from photoexcited MoS₂ to Au. Ultrafast study reveals that photogenerated free carriers form excitons with a time scale of ∼500 fs and eventually turn into trions within ∼1.2 ps. Dissociation of excitons and trions has been observed in the presence of Au, with time scales of ∼600 fs and ∼3.7 ps, respectively. Understanding the formation and dissociation dynamics of the exciton and trion in monolayer MoS₂ is going to help immensely to design and develop many new 2D devices.

Fabrication of Stacked MoS2 Bilayer with Weak Interlayer Coupling by Reduced Graphene Oxide Spacer

We fabricated the stacked bilayer molybdenum disulfide (MoS₂) by using reduced graphene oxide (rGO) as a spacer for increasing the optoelectronic properties of MoS₂. The rGO can decrease the interlayer coupling between the stacked bilayer MoS₂ and retain the direct band gap property of MoS₂. We observed a twofold enhancement of the photoluminescence intensity of the stacked MoS₂ bilayer. In the Raman scattering, we observed that the E¹_2g and A_1g modes of the stacked bilayer MoS₂ with rGO were further shifted compared to monolayer MoS₂, which is due to the van der Waals (vdW) interaction and the strain effect between the MoS₂ and rGO layers. The findings of this study will expand the applicability of monolayer MoS₂ for high-performance optoelectronic devices by enhancing the optical properties using a vdW spacer.

Flexible Molybdenum Disulfide (MoS2) Atomic Layers for Wearable Electronics and Optoelectronics

Flexible, stretchable, and bendable materials, including inorganic semiconductors, organic polymers, graphene, and transition metal dichalcogenides (TMDs), are attracting great attention in such areas as wearable electronics, biomedical technologies, foldable displays, and wearable point-of-care biosensors for healthcare. Among a broad range of layered TMDs, atomically thin layered molybdenum disulfide (MoS₂) has been of particular interest, due to its exceptional electronic properties, including tunable bandgap and charge carrier mobility. MoS₂ atomic layers can be used as a channel or a gate dielectric for fabricating atomically thin field-effect transistors (FETs) for electronic and optoelectronic devices. This review briefly introduces the processing and spectroscopic characterization of large-area MoS₂ atomically thin layers. The review summarizes the different strategies in enhancing the charge carrier mobility and switching speed of MoS₂ FETs by integrating high-κ dielectrics, encapsulating layers, and other 2D van der Waals layered materials into flexible MoS₂ device structures. The photoluminescence (PL) of MoS₂ atomic layers has, after chemical treatment, been dramatically improved to near-unity quantum yield. Ultraflexible and wearable active-matrix organic light-emitting diode (AM-OLED) displays and wafer-scale flexible resistive random-access memory (RRAM) arrays have been assembled using flexible MoS₂ transistors. The review discusses the overall recent progress made in developing MoS₂ based flexible FETs, OLED displays, nonvolatile memory (NVM) devices, piezoelectric nanogenerators (PNGs), and sensors for wearable electronic and optoelectronic devices. Finally, it outlines the perspectives and tremendous opportunities offered by a large family of atomically thin-layered TMDs.

Physics of excitons and their transport in two dimensional transition metal dichalcogenide semiconductors

Two-dimensional (2D) group-VI transition metal dichalcogenide (TMD) semiconductors, such as MoS₂, MoSe₂, WS₂ and others manifest strong light matter coupling and exhibit direct band gaps which lie in the visible and infrared spectral regimes. These properties make them potentially interesting candidates for applications in optics and optoelectronics. The excitons found in these materials are tightly bound and dominate the optical response, even at room temperatures. Large binding energies and unique exciton fine structure make these materials an ideal platform to study exciton behaviors in two-dimensional systems. This review article mainly focuses on studies of mechanisms that control dynamics of excitons in 2D systems – an area where there remains a lack of consensus in spite of extensive research. Firstly, we focus on the kinetics of dark and bright excitons based on a rate equation model and discuss on the role of previous ‘unsuspected’ dark excitons in controlling valley polarization. Intrinsically, dark and bright exciton energy splitting plays a key role in modulating the dynamics. In the second part, we review the excitation energy-dependent possible characteristic relaxation pathways of photoexcited carriers in monolayer and bilayer systems. In the third part, we review the extrinsic factors, in particular the defects that are so prevalent in single layer TMDs, affecting exciton dynamics, transport and non-radiative recombination such as exciton–exciton annihilation. Lastly, the optical response due to pump-induced changes in TMD monolayers have been reviewed using femtosecond pump–probe spectroscopy which facilitates the analysis of underlying physical process just after the excitation.

Two-dimensional (2D) group-VI transition metal dichalcogenide (TMD) semiconductors, such as MoS₂, MoSe₂, WS₂ and others manifest strong light matter coupling and exhibit direct band gaps which lie in the visible and infrared spectral regimes.

Experimental Identification of Critical Condition for Drastically Enhancing Thermoelectric Power Factor of Two-Dimensional Layered Materials

Nanostructuring is an extremely promising path to high-performance thermoelectrics. Favorable improvements in thermal conductivity are attainable in many material systems, and theoretical work points to large improvements in electronic properties. However, realization of the electronic benefits in practical materials has been elusive experimentally. A key challenge is that experimental identification of the quantum confinement length, below which the thermoelectric power factor is significantly enhanced, remains elusive due to lack of simultaneous control of size and carrier density. Here we investigate gate-tunable and temperature-dependent thermoelectric transport in γ-phase indium selenide (γ-InSe, n-type semiconductor) samples with thickness varying from 7 to 29 nm. This allows us to properly map out dimension and doping space. Combining theoretical and experimental studies, we reveal that the sharper pre-edge of the conduction-band density of states arising from quantum confinement gives rise to an enhancement of the Seebeck coefficient and the power factor in the thinner InSe samples. Most importantly, we experimentally identify the role of the competition between quantum confinement length and thermal de Broglie wavelength in the enhancement of power factor. Our results provide an important and general experimental guideline for optimizing the power factor and improving the thermoelectric performance of two-dimensional layered semiconductors.

Tunable Ultrafast Nonlinear Optical Properties of Graphene/MoS2 van der Waals Heterostructures and Their Application in Solid-State Bulk Lasers

For van der Waals (vdW) heterostructures, optical and electrical properties ( e.g., saturable absorption and carrier dynamics) are strongly modulated by interlayer coupling, which may be due to effective charge transfer and band structure recombination. General theoretical studies have shown that the complementary properties of graphene and MoS₂ enable the graphene/MoS₂ (G/MoS₂) heterostructure to be used as an important building block for various optoelectronic devices. Here, density functional theory was used to calculate the work function values of G/MoS₂ with different thicknesses of MoS₂, and its relaxation dynamic mechanism was illustrated. The results reveal that the G/MoS₂ heterostructure interlayer coupling can be tuned by changing the thickness of MoS₂, furthering the understanding of the fundamental charge-transfer mechanism in few-layer G/MoS₂ heterostructures. The tunable carrier dynamics and saturable absorption were investigated by pump-probe spectroscopy and open-aperture Z-scan technique, respectively. In the experiments, we compared the performances of Q-switched lasers based on G/MoS₂ heterostructures with different MoS₂ layers. Taking advantage of ultrafast recovery time and good saturable absorption properties, a femtosecond solid-state laser at 1.0 μm with G/MoS₂ heterostructure saturable absorber was successfully achieved. This study on interlayer coupling in G/MoS₂ may allow various vdW heterostructures with controllable stacking to be fabricated and shows the promising applications of vdW heterostructures for ultrafast photonic devices.

High-Performance Wafer-Scale MoS2 Transistors toward Practical Application

Atomic thin transition-metal dichalcogenides (TMDs) are considered as an emerging platform to build next-generation semiconductor devices. However, to date most devices are still based on exfoliated TMD sheets on a micrometer scale. Here, a novel chemical vapor deposition synthesis strategy by introducing multilayer (ML) MoS₂ islands to improve device performance is proposed. A four-probe method is applied to confirm that the contact resistance decreases by one order of magnitude, which can be attributed to a conformal contact by the extra amount of exposed edges from the ML-MoS₂ islands. Based on such continuous MoS₂ films synthesized on a 2 in. insulating substrate, a top-gated field effect transistor (FET) array is fabricated to explore key metrics such as threshold voltage (V _T ) and field effect mobility (μ_FE ) for hundreds of MoS₂ FETs. The statistical results exhibit a surprisingly low variability of these parameters. An average effective μ_FE of 70 cm² V^-1 s^-1 and subthreshold swing of about 150 mV dec^-1 are extracted from these MoS₂ FETs, which are comparable to the best top-gated MoS₂ FETs achieved by mechanical exfoliation. The result is a key step toward scaling 2D-TMDs into functional systems and paves the way for the future development of 2D-TMDs integrated circuits.

Optimized single-layer MoS2 field-effect transistors by non-covalent functionalisation

Field-effect transistors (FETs) with non-covalently functionalised molybdenum disulfide (MoS2) channels grown by chemical vapour deposition (CVD) on SiO2 are reported. The dangling-bond-free surface of MoS2 was functionalised with a perylene bisimide derivative to allow for the deposition of Al2O3 dielectric. This allowed the fabrication of top-gated, fully encapsulated MoS2 FETs. Furthermore, by the definition of vertical contacts on MoS2, devices, in which the channel area was never exposed to polymers, were fabricated. The MoS2 FETs showed some of the highest mobilities for transistors fabricated on SiO2 with Al2O3 as the top-gate dielectric reported so far. Thus, gate-stack engineering using innovative chemistry is a promising approach for the fabrication of reliable electronic devices based on 2D materials.

Photovoltaic Field-Effect Transistors Using a MoS2 and Organic Rubrene van der Waals Hybrid

A several-layer n-type MoS₂ was partially hybridized with an organic crystalline p-type rubrene nanosheet through van der Waals interactions to fabricate a two-dimensional (2-D) lateral-type n-p heterojunction optoelectronic device. The field-effect transistors (FETs) using lateral-type MoS₂/rubrene hybrids exhibited both gate-tunable diode and anti-ambipolar transistor characteristics. The FET devices show the coexistence of n-type states, p-type states, and off-states controlled by the gate bias. From the photocurrent mapping experiments, the gate-bias-dependent photovoltaic effect was observed from the heterojunction regions of the MoS₂/rubrene FETs. Furthermore, the photovoltaic FETs were successfully operated by light irradiation without applying source-drain bias and controlled using gate bias. These devices represent new solar-energy-driven 2-D multifunctional electronic devices.

Atomic-scale etching of hexagonal boron nitride for device integration based on two-dimensional materials

Hexagonal boron nitride (h-BN) is considered an ideal template for electronics based on two-dimensional (2D) materials, owing to its unique properties as a dielectric film. Most studies involving h-BN and its application to electronics have focused on its synthesis using techniques such as chemical vapor deposition, the electrical analysis of its surface state, and the evaluation of its performance. Meanwhile, processing techniques including etching methods have not been widely studied despite their necessity for device fabrication processes. In this study, we propose the atomic-scale etching of h-BN for integration into devices based on 2D materials, using Ar plasma at room temperature. A controllable etching rate, less than 1 nm min-1, was achieved and the low reactivity of the Ar plasma enabled the atomic-scale etching of h-BN down to a monolayer in this top-down approach. Based on the h-BN etching technique for achieving electrical contact with the underlying molybdenum disulfide (MoS2) layer of an h-BN/MoS2 heterostructure, a top-gate MoS2 field-effect transistor (FET) with h-BN gate dielectric was fabricated and characterized by high electrical performance based on the on/off current ratio and carrier mobility.

Progress in Contact, Doping and Mobility Engineering of MoS2: An Atomically Thin 2D Semiconductor

Atomically thin molybdenum disulfide (MoS2), a member of the transition metal dichalcogenide (TMDC) family, has emerged as the prototypical two-dimensional (2D) semiconductor with a multitude of interesting properties and promising device applications spanning all realms of electronics and optoelectronics. While possessing inherent advantages over conventional bulk semiconducting materials (such as Si, Ge and III-Vs) in terms of enabling ultra-short channel and, thus, energy efficient field-effect transistors (FETs), the mechanically flexible and transparent nature of MoS2 makes it even more attractive for use in ubiquitous flexible and transparent electronic systems. However, before the fascinating properties of MoS2 can be effectively harnessed and put to good use in practical and commercial applications, several important technological roadblocks pertaining to its contact, doping and mobility (µ) engineering must be overcome. This paper reviews the important technologically relevant properties of semiconducting 2D TMDCs followed by a discussion of the performance projections of, and the major engineering challenges that confront, 2D MoS2-based devices. Finally, this review provides a comprehensive overview of the various engineering solutions employed, thus far, to address the all-important issues of contact resistance (RC), controllable and area-selective doping, and charge carrier mobility enhancement in these devices. Several key experimental and theoretical results are cited to supplement the discussions and provide further insight.

MoS2 Nanosheets Vertically Grown on Carbonized Corn Stalks as Lithium-Ion Battery Anode

In this study, MoS₂ nanosheets are vertically grown on the inside and outside surfaces of the carbonized corn stalks (CCS) by a simple hydrothermal reaction. The vertically grown structure can not only improve the transmission rate of Li⁺ and electrons but also avoid the agglomeration of the nanosheets. Meanwhile, a new approach of biomass source application is presented. We use CCS instead of graphite powders, which can not only avoid the exploitation of graphite resources, but also be used as a matrix for MoS₂ growth to prevent the electrode from being further decomposed during long cycles and at high current densities. Meanwhile, lithium-ion batteries show remarkable electrochemical performance. They demonstrate a high specific capacity of 1409.5 mA g^-1 at 100 mA g^-1 in the initial cycle. After 250 cycles, the discharge capacity is still as high as 1230.9 mAh g^-1. Even at 4000 mA g^-1, they show a high specific capacity of 777.7 mAh g^-1. Furthermore, the MoS₂/CCS electrodes show long cycle life, and the specific capacity is still up to ∼500 mAh g^-1 at 5000 mA g^-1 after 1000 cycles.

Selection Role of Metal Oxides into Transition Metal Dichalcogenide Monolayers by a Direct Selenization Process

Direct reduction of metal oxides into a few transition metal dichalcogenide (TMDCs) monolayers has been recently explored as an alternative method for large area and uniform deposition. However, not many studies have addressed the characteristics and requirement of the metal oxides into TMDCs by the selenization/sulfurization processes, yielding a wide range of outstanding properties to poor electrical characteristics with nonuniform films. The large difference implies that the process is yet not fully understood. In particular, the selenization/sulfurization at low temperature leads to poor crystallinity films with poor electrical performance, hindering its practical development. A common approach to improve the quality of the selenized/sulfurized films is by further increasing the process temperature, thus requiring additional transfer in order to explore the electrical properties. Here, we show that by finely tuning the quality of the predeposited oxide the selenization/sulfurization temperature can be largely decreased, avoiding major substrate damage and allowing direct device fabrication. The direct relationship between the role of selecting different metal oxides prepared by e-beam evaporation and reactive sputtering and their oxygen deficiency/vacancy leading to quality influence of TMDCs was investigated in detail. Because of its outstanding physical properties, the formation of tungsten diselenide (WSe₂) from the reduction of tungsten oxide (WO _x) was chosen as a model for proof of concept. By optimizing the process parameters and the selection of metal oxides, layered WSe₂ films with controlled atomic thickness can be demonstrated. Interestingly, the domain size and electrical properties of the layered WSe₂ films are highly affected by the quality of the metal oxides, for which the layered WSe₂ film with small domains exhibits a metallic behavior and the layered WSe₂ films with larger domains provides clear semiconducting behavior. Finally, an 8'' wafer scale-layered WSe₂ film was demonstrated, giving a step forward in the development of 2D TMDC electronics in the industry.

Virtual Out-of-Plane Piezoelectric Response in MoS2 Layers Controlled by Ferroelectric Polarization

The MoS₂ carrier distribution can be controlled with the use of a dielectric environment substrate. Ferroelectric thin films are used to investigate the electrical responses at the MoS₂ layer. The MoS₂/(111)-PbTiO₃ vertical heterostructure is investigated, and the electrical responses, including piezoelectricity, are obtained using piezoresponse force microscopy. The piezoelectric response modifications obtained at the MoS₂ layer on the ferroelectric thin films are a result of the depolarizing effect. In particular, the piezoelectricity enhancement is observed at the 19-layer MoS₂ because of an induced dipole effect. By considering the polarization effects of ferroelectric thin films, the electrical responses at the MoS₂ layers can be controlled, and the interfacial carrier distribution at the interface results in different electrical performances at the MoS₂.

MoS2 thin films from a (N t Bu)2(NMe2)2Mo and 1-propanethiol atomic layer deposition process

Potential commercial applications for transition metal dichalcogenide (TMD) semiconductors such as MoS2 rely on unique material properties that are only accessible at monolayer to few-layer thickness regimes. Therefore, production methods that lend themselves to scalable and controllable formation of TMD films on surfaces are desirable for high volume manufacturing of devices based on these materials. We have developed a new thermal atomic layer deposition (ALD) process using bis(tert-butylimido)-bis(dimethylamido)molybdenum and 1-propanethiol to produce MoS2-containing amorphous films. We observe self-limiting reaction behavior with respect to both the Mo and S precursors at a substrate temperature of 350 °C. Film thickness scales linearly with precursor cycling, with growth per cycle values of ≈0.1 nm/cycle. As-deposited films are smooth and contain nitrogen and carbon impurities attributed to poor ligand elimination from the Mo source. Upon high-temperature annealing, a large portion of the impurities are removed, and we obtain few-layer crystalline 2H-MoS2 films.

Very fast hot carrier diffusion in unconstrained MoS2on a glass substrate: discovered by picosecond ET-Raman

The currently reported optical-phonon-scattering-limited carrier mobility of MoS₂ is up to 417 cm² V⁻¹ s⁻¹ with two-side dielectric screening: one normal-κ side and one high-κ side. Herein, using picosecond energy transport state-resolved Raman (ET-Raman), we demonstrated very fast hot carrier diffusion in μm-scale (lateral) unconstrained MoS₂ (1.8–18 nm thick) on a glass substrate; this method enables only one-side normal-κ dielectric screening. The ET-Raman method directly probes the diffusion of the hot carrier and its contribution to phonon transfer without contact and additional sample preparation and provides unprecedented insight into the intrinsic D of MoS₂. The measured D values span from 0.76 to 9.7 cm² s⁻¹. A nonmonotonic thickness-dependent D trend is discovered, and it peaks at 3.0 nm thickness. This is explained by the competition between two physical phenomena: with an increase in sample thickness, the increased screening of the substrate results in higher mobility; moreover, thicker samples are subject to more surface contamination, loose substrate contact and weaker substrate dielectric screening. The corresponding carrier mobility varies from 31.0 to 388.5 cm² V⁻¹ s⁻¹. This mobility is surprisingly high considering the normal-κ and single side dielectric screening by the glass substrate. This is a direct result of the less-damaged structure of MoS₂ that is superior to those of MoS₂ samples reported in literature studies that are subjected to various post-processing techniques to facilitate measurement. The very high hot carrier mobility reduces the local carrier concentration and enhances the Raman signal, which is further confirmed by our Raman signal studies and comparison with theoretical studies.

Very high nonmonotonic thickness-dependent hot carrier diffusivity of MoS₂ in a normal-κ dielectric screening environment was discovered by ET-Raman technique.

Electron-Phonon Interaction in Organic/2D-Transition Metal Dichalcogenide Heterojunctions: A Temperature-Dependent Raman Spectroscopic Study

The heterojunctions of organic/two-dimensional transition metal dichalcogenides (TMDs) have the potential to be used in the next-generation optoelectronic and photonic devices. Herein, we have systemically investigated the temperature-dependent Raman spectroscopy to elucidate the phonon shift and thermal properties of the semiconducting TMD nanosheets grafted by a conjugated polymer (PG-MoS₂ and PG-MoSe₂) forming heterojunctions. Our results reveal that softening of Raman modes of PG-TMDs as temperature increases from 77 to 300 K is due to the negative temperature coefficient (TC) and anharmonicity. The TCs of E¹ _2g and A_1g modes of PG-MoS₂ nanosheets and A_1g mode of PG-MoSe₂ were found to be -0.015, -0.010, and -0.010 cm^-1 K^-1, respectively. The origin of negative TCs is explained on the basis of a double resonance process, which is more active in single- and few-layer MoS₂ and MoSe₂. Interestingly, the temperature-dependent behavior of the phonon modes of PG-MoS₂ and PG-MoSe₂ is similar to that of pristine nanosheets. Grafting by conjugated polymer does not affect the electron-phonon (e-p) interaction in the semiconducting (2H-phase) TMDs, hinting the application potential of such materials in field-effect electronic devices.

High-Mobility InSe Transistors: The Role of Surface Oxides

In search of high-performance field-effect transistors (FETs) made of atomic thin semiconductors, indium selenide (InSe) has held great promise because of its high intrinsic mobility and moderate electronic band gap (1.26 eV). Yet the performance of InSe FETs is decisively determined by the surface oxidation of InSe taking place spontaneously in ambient conditions, setting up a mobility ceiling and causing an uncontrollable current hysteresis. Encapsulation by hexagonal boron nitride (h-BN) has been currently used to cope with this deterioration. Here, we provide insights into the role of surface oxides played in device performance and introduce a dry-oxidation process that forms a dense capping layer on top, where InSe FETs exhibit a record-high two-probe mobility of 423 cm²/V·s at room temperature and 1006 cm²/V·s at liquid nitrogen temperature without the use of h-BN encapsulation or high-κ dielectric screening. Ultrahigh on/off current ratio of >10⁸ and current density of 365 μA/μm can be readily achieved without elaborate engineering of drain/source contacts or gating technique. Thickness-dependent device properties are also studied, with optimized performance shown in FETs comprising of 13 nm thick InSe. The high performance of InSe FETs with ultrathin dry oxide is attributed to the effective unpinning of the Fermi level at the metal contacts, resulting in a low Schottky barrier height of 40 meV in an optimized channel thickness.

Exciton Dynamics, Transport, and Annihilation in Atomically Thin Two-Dimensional Semiconductors

Large binding energy and unique exciton fine structure make the transition metal dichalcogenides (TMDCs) an ideal platform to study exciton behaviors in two-dimensional (2D) systems. While excitons in these systems have been extensively researched, there currently lacks a consensus on mechanisms that control dynamics. In this Perspective, we discuss extrinsic and intrinsic factors in exciton dynamics, transport, and annihilation in 2D TMDCs. Intrinsically, dark and bright exciton energy splitting is likely to play a key role in modulating the dynamics. Extrinsically, defect scattering is prevalent in single-layer TMDCs, which leads to rapid picosecond decay and limits exciton transport. The exciton-exciton annihilation process in single-layer TMDCs is highly efficient, playing an important role in the nonradiative recombination rate in the high exciton density regime. Future challenges and opportunities to control exciton dynamics are discussed.

The hot carrier diffusion coefficient of sub-10 nm virgin MoS2: uncovered by non-contact optical probing

We report a novel approach for non-contact simultaneous determination of the hot carrier diffusion coefficient (D) and interface thermal resistance (R) of sub-10 nm virgin mechanically exfoliated MoS₂ nanosheets on c-Si. The effect of hot carrier diffusion in heat conduction by photon excitation, diffusion, and recombination is identified by varying the heating spot size from 0.294 μm to 1.14 μm (radius) and probing the local temperature rise using Raman spectroscopy. R is determined as 4.46-7.66 × 10^-8 K m² W^-1, indicating excellent contact between MoS₂ and c-Si. D is determined to be 1.18, 1.07, 1.20 and 1.62 cm² s^-1 for 3.6 nm, 5.4 nm, 8.4 nm, and 9.0 nm thick MoS₂ samples, showing little dependence on the thickness. The hot carrier diffusion length (L_D) can be determined without knowledge of the hot carrier's life-time. The four samples L_D is determined as 0.344 (3.6 nm), 0.327 (5.4 nm), 0.346 (8.4 nm), and 0.402 μm (9.0 nm). Unlike previous methods that are implemented by making electrical contact and applying an electric field for D measurement, our technique has the advantage of being truly non-contact and non-invasive, and is able to characterize the electron diffusion behavior of virgin 2D materials. Also it points out that hot carrier diffusion needs to be taken into serious consideration in Raman-based thermal property characterization of 2D materials, especially under very tightly focused laser heating whose spot size is comparable to the hot carrier diffusion length.

Thickness-dependent Schottky barrier height of MoS2 field-effect transistors

2D semiconductors, including transition metal dichalcogenides (TMDs), have been widely studied recently. However, the device performance is deteriorated due to the significant contact resistance. The contact resistance of MoS₂-metal contacts decreases with the thickness of MoS₂. We obtained a Schottky barrier height as low as about 70 meV when MoS₂ is trilayer-thick. It is important to find the optimal choice of contact metal and layer thickness of MoS₂.

An origin of unintentional doping in transition metal dichalcogenides: the role of hydrogen impurities

We theoretically elucidate the origin of unintentional doping in two-dimensional transition-metal dichalcogenides (TMDs), which has been consistently reported by experiment, but which still remains unclear. Our explanation is based on the charge transfer between TMDs and the underlying SiO₂ in which hydrogen impurities with a negative-U property pin the Fermi level of the SiO₂ as well as adjacent TMD layers. Using first-principles calculations, we obtain the pinning point of the Fermi level from the charge transition level of the hydrogen in the SiO₂, ε(+/-), and align it with respect to the band-edge positions of monolayer TMDs. The computational results show that the Fermi levels of TMDs estimated by ε(+/-) successfully explain the conducting polarity (n- or p-type) and relative doping concentrations of thin TMD films. By enlightening on the microscopic origin of unintentional doping in TMDs, we believe that the present work will contribute to precise control of TMD-based electronic devices.