Chemical Vapor Deposited Monolayer MoS2 Research Articles

Mobility Enhancement in CVD-Grown Monolayer MoS2 Via Patterned Substrate-Induced Nonuniform Straining.

The extraordinary mechanical properties of two-dimensional transition-metal dichalcogenides make them ideal candidates for investigating strain-induced control of various physical properties. Here we explore the role of nonuniform strain in modulating optical, electronic, and transport properties of semiconducting, chemical vapor deposited monolayer MoS2, on periodically nanostructured substrates. A combination of spatially resolved spectroscopic and electronic properties explore and quantify the differential strain distribution and carrier density on a monolayer, as it conformally drapes over the periodic nanostructures. The observed accumulation in electron density at the strained regions is supported by theoretical calculations which form the likely basis for the ensuing ×60 increase in field effect mobility in strained samples. Though spatially nonuniform, the pattern-induced strain is shown to be readily controlled by changing the periodicity of the nanostructures thus providing a robust yet useful macroscopic control on strain and mobility in these systems.

Scanning Near-Field Optical Microscopy of Ultrathin Gold Films.

Ultrathin metal films are an essential platform for two-dimensional (2D) material compatible and flexible optoelectronics. Characterization of thin and ultrathin film-based devices requires a thorough consideration of the crystalline structure and local optical and electrical properties of the metal-2D material interface since they could be dramatically different from the bulk material. Recently, it was demonstrated that the growth of gold on the chemical vapor deposited monolayer MoS2 leads to a continuous metal film that preserves plasmonic optical response and conductivity even at thicknesses below 10 nm. Here, we examined the optical response and morphology of ultrathin gold films deposited on exfoliated MoS2 crystal flakes on the SiO2/Si substrate via scattering-type scanning near-field optical microscopy (s-SNOM). We demonstrate a direct relationship between the ability of thin film to support guided surface plasmon polaritons (SPP) and the s-SNOM signal intensity with a very high spatial resolution. Using this relationship, we observed the evolution of the structure of gold films grown on SiO2 and MoS2 with an increase in thickness. The continuous morphology and superior ability with respect to supporting SPPs of the ultrathin (≤10 nm) gold on MoS2 is further confirmed with scanning electron microscopy and direct observation of SPP fringes via s-SNOM. Our results establish s-SNOM as a tool for testing plasmonic films and motivate further theoretical research on the impact of the interplay between the guided modes and the local optical properties on the s-SNOM signal.

Exciton-plasmon coupling and giant photoluminescence enhancement in monolayer MoS2 through hierarchically designed TiO2/Au/MoS2 ternary core−shell heterostructure

Enhancing the light coupling efficiency of large-area monolayer molybdenum disulfide (1L-MoS2) is one of the major challenges for its successful applications in optoelectronics and photonics. Herein, we demonstrate a dramatically enhanced photoluminescence (PL) emission from direct chemical vapor deposited monolayer MoS2 on a fluorine-doped TiO2/Au nanoparticle plasmonic substrate, where the PL intensity is enhanced by nearly three orders of magnitude, highest among the reported values. The formation of TiO2/Au/1L-MoS2 ternary core–shell heterojunction is evidenced by the high-resolution transmission electron microscopy and Raman analyses. Localized surface plasmon resonance induced enhanced absorption and improved light coupling in the system was revealed from the UV–vis absorption and Raman spectroscopy analyzes. Our studies reveal that the observed giant PL enhancement in 1L-MoS2 results from two major aspects: firstly, the heavy p-doping of the MoS2 lattice is caused by the transfer of the excess electrons from the MoS2 to TiO2 at the interface, which enhances the neutral exciton emissions and restrains the trion formation. Secondly, the localized surface plasmon in Au NPs underneath the 1L-MoS2 film initiates exciton-plasmon coupling between excitons of the 1L-MoS2 and surface plasmons of the Au NPs at the MoS2/Au interface. The PL and Raman analyses further confirm the p-doping effect. We isolate the contributions of plasmon enhancement from the theoretical calculation of the field enhancement factor using the effective medium approximation of plasmonic heterostructure, which is in excellent agreement with the experimental data. This work paves a way for the rational design of the plasmonic heterostructure for the effective improvement in the light emission efficiency of 1L-MoS2, and may enable engineering the different contributions to enhance the optoelectronic performance of 2D heterostructures.

Dopants and grain boundary effects in monolayer MoS2: a first-principles study.

The structural, electronic and magnetic properties of large area chemical vapor deposited monolayer MoS2 rely significantly on the presence of grain boundaries (GBs) and defects. In this study, first-principles calculations were performed to investigate the electronic and magnetic properties of transition metal doped MoS2 GBs. The experimentally observed 60° tilt GBs were demonstrated with four different atomic configurations and the nonmagnetic 4|8ud GB has the lowest formation energy among the considered models. Further calculations of 4|8ud GBs doped with TMs, such as V, Cr, Mn, Fe, Co and Ni, indicate that dopants can significantly lower the formation energies of the doped GBs compared to the perfect monolayer MoS2 by occupying the GB region instead of within the grains. Magnetism can be achieved in doped GB systems by careful defect engineering. CoMo, MnMo and Niint in 4|8ud GBs are predicted to be magnetic and simultaneously energetically favorable. The electron coupling between the doped TM and surrounding GB atoms is expected to induce magnetism and high electron mobilities into the systems. This study may pave the way for optimal design of MoS2-based electronic and spintronic devices.

Rectifying effect in a MoS2 monolayer crossed with an electro-spun PEDOT-PSS nano-ribbon

A chemical vapor-deposited monolayer MoS2 crystal was crossed with an electro-spun PEDOT-PSS nano-ribbon under ambient conditions. The current–voltage (I–V) curve measured across the hetero-junction was nonlinear and asymmetric, similar to a diode. Under thermal equilibrium, electrons flowed from the MoS2 conduction band into the PEDOT-PSS LUMO level. This occurred via band bending that established a constant Fermi level across the interface. Consequently, a potential barrier was formed that restricted the current. Under normal operation in air, the diode turn-on voltage was 0.1 V and the rectification ratio at ± 1 V was 20. The thermionic emission Schottky junction model was employed for data analysis. The ideality factor was 1.9, and height of the barrier was 0.18 eV. The easy fabrication, low turn-on voltage and high rectification ratio could make this diode useful in cheap, low-power-consumption signal rectifiers.

Controllable Tunneling Triboelectrification of Two-Dimensional Chemical Vapor Deposited MoS2

Tunneling triboelectrification of chemical vapor deposited monolayer MoS2 has been characterized at nanoscale with contact-mode atomic force microscopy (AFM) and Kelvin force microscopy (KFM). Although charges can be trapped on insulators like SiO2 by conventional triboelectrification, triboelectric charges tunneling through MoS2 and localized at the underlying substrate exhibit more than two orders of magnitude longer lifetime. Their polarity and density can be modified by triboelectric process with various bias voltages applied to Pt-coated AFM tips, and the saturated density is almost 30 times higher than the reported result of SiO2. Thus, the controllable tunneling triboelectric properties of MoS2 on insulating substrates can provide guidance to build a new class of two-dimensional (2D) MoS2-based nanoelectronic devices.

Interface passivation and trap reduction via hydrogen fluoride for molybdenum disulfide on silicon oxide back-gate transistors

Layered semiconductor molybdenum disulfide (MoS2) has recently emerged as a promising material for flexible electronic and optoelectronic devices because of its finite bandgap and high degree of gate control. Here, we report a hydrogen fluoride (HF) passivation technique for improving the carrier mobility and interface quality of chemical vapor deposited monolayer MoS2 on a SiO2/Si substrate. After passivation, the fabricated MoS2 back-gate transistors demonstrate a more than double improvement in average electron mobility, a reduced gate hysteresis gap of 3 V, and a low interface trapped charge density of ∼5.8 × 1011 cm−2. The improvements are attributed to the satisfied interface dangling bonds, thus a reduction of interface trap states and trapped charges. Surface x-ray photoelectron spectroscopy analysis and first-principles simulation were performed to verify the HF passivation effect. The results here highlight the necessity of a MoS2/dielectric passivation strategy and provides a viable route for enhancing the performance of MoS2 nano-electronic devices.

Chemical vapor deposited monolayer MoS2 top-gate MOSFET with atomic-layer-deposited ZrO2 as gate dielectric

For the first time, ZrO2 dielectric deposition on pristine monolayer MoS2 by atomic layer deposition (ALD) is demonstrated and ZrO2/MoS2 top-gate MOSFETs have been fabricated. ALD ZrO2 overcoat, like other high-k oxides such as HfO2 and Al2O3, was shown to enhance the MoS2 channel mobility. As a result, an on/off current ratio of over 107, a subthreshold slope of 276 mV dec−1, and a field-effect electron mobility of 12.1 cm2 V−1 s−1 have been achieved. The maximum drain current of the MOSFET with a top-gate length of 4 μm and a source/drain spacing of 9 μm is measured to be 1.4 μA μm−1 at VDS = 5 V. The gate leakage current is below 10−2 A cm−2 under a gate bias of 10 V. A high dielectric breakdown field of 4.9 MV cm−1 is obtained. Gate hysteresis and frequency-dependent capacitance–voltage measurements were also performed to characterize the ZrO2/MoS2 interface quality, which yielded an interface state density of ∼3 × 1012 cm−2 eV−1.

Role of sulphur atoms on stress relaxation and crack propagation in monolayer MoS2

We present in-situ transmission electron microscopy of crack propagation in a freestanding monolayer MoS2 and molecular dynamic analysis of the underlying mechanisms. Chemical vapor deposited monolayer MoS2 was transferred from sapphire substrate using interfacial etching for defect and contamination minimization. Atomic resolution imaging shows crack tip atoms sustaining 14.5% strain before bond breaking, while the stress field decays at unprecedented rate of 2.15 GPa Å−1. Crack propagation is seen mostly in the zig-zag direction in both model and experiment, suggesting that the mechanics of fracture is not brittle. Our computational model captures the mechanics of the experimental observations on crack propagation in MoS2. While molybdenum atoms carry most of the mechanical load, we show that the sliding motion of weakly bonded sulphur atoms mediate crack tip stress relaxation, which helps the tip sustain very high, localized stress levels.

Elastic properties of chemical-vapor-deposited monolayer MoS2, WS2, and their bilayer heterostructures.

Elastic properties of materials are an important factor in their integration in applications. Chemical vapor deposited (CVD) monolayer semiconductors are proposed as key components in industrial-scale flexible devices and building blocks of two-dimensional (2D) van der Waals heterostructures. However, their mechanical and elastic properties have not been fully characterized. Here we report high 2D elastic moduli of CVD monolayer MoS2 and WS2 (∼170 N/m), which is very close to the value of exfoliated MoS2 monolayers and almost half the value of the strongest material, graphene. The 2D moduli of their bilayer heterostructures are lower than the sum of 2D modulus of each layer but comparable to the corresponding bilayer homostructure, implying similar interactions between the hetero monolayers as between homo monolayers. These results not only provide deep insight into understanding interlayer interactions in 2D van der Waals structures but also potentially allow engineering of their elastic properties as desired.