Weibing Chen

Metallic 1T phase source/drain electrodes for field effect transistors from chemical vapor deposited MoS2

Two dimensional transition metal dichalcogenides (2D TMDs) offer promise as opto-electronic materials due to their direct band gap and reasonably good mobility values. However, most metals form high resistance contacts on semiconducting TMDs such as MoS2. The large contact resistance limits the performance of devices. Unlike bulk materials, low contact resistance cannot be stably achieved in 2D materials by doping. Here we build on our previous work in which we demonstrated that it is possible to achieve low contact resistance electrodes by phase transformation. We show that similar to the previously demonstrated mechanically exfoliated samples, it is possible to decrease the contact resistance and enhance the FET performance by locally inducing and patterning the metallic 1T phase of MoS2 on chemically vapor deposited material. The device properties are substantially improved with 1T phase source/drain electrodes.

Surface functionalization of two-dimensional metal chalcogenides by Lewis acid–base chemistry

Precise control of the electronic surface states of two-dimensional (2D) materials could improve their versatility and widen their applicability in electronics and sensing. To this end, chemical surface functionalization has been used to adjust the electronic properties of 2D materials. So far, however, chemical functionalization has relied on lattice defects and physisorption methods that inevitably modify the topological characteristics of the atomic layers. Here we make use of the lone pair electrons found in most of 2D metal chalcogenides and report a functionalization method via a Lewis acid-base reaction that does not alter the host structure. Atomic layers of n-type InSe react with Ti(4+) to form planar p-type [Ti(4+)n(InSe)] coordination complexes. Using this strategy, we fabricate planar p-n junctions on 2D InSe with improved rectification and photovoltaic properties, without requiring heterostructure growth procedures or device fabrication processes. We also show that this functionalization approach works with other Lewis acids (such as B(3+), Al(3+) and Sn(4+)) and can be applied to other 2D materials (for example MoS2, MoSe2). Finally, we show that it is possible to use Lewis acid-base chemistry as a bridge to connect molecules to 2D atomic layers and fabricate a proof-of-principle dye-sensitized photosensing device.

Janus Monolayer Transition-Metal Dichalcogenides

The crystal configuration of sandwiched S-Mo-Se structure (Janus SMoSe) at the monolayer limit has been synthesized and carefully characterized in this work. By controlled sulfurization of monolayer MoSe2, the top layer of selenium atoms is substituted by sulfur atoms, while the bottom selenium layer remains intact. The structure of this material is systematically investigated by Raman, photoluminescence, transmission electron microscopy, and X-ray photoelectron spectroscopy and confirmed by time-of-flight secondary ion mass spectrometry. Density functional theory (DFT) calculations are performed to better understand the Raman vibration modes and electronic structures of the Janus SMoSe monolayer, which are found to correlate well with corresponding experimental results. Finally, high basal plane hydrogen evolution reaction activity is discovered for the Janus monolayer, and DFT calculation implies that the activity originates from the synergistic effect of the intrinsic defects and structural strain inherent in the Janus structure.

Brittle Fracture of 2D MoSe2

An in situ quantitative tensile testing platform is developed to enable the uniform in-plane loading of a freestanding membrane of 2D materials inside a scanning electron microscope. The in situ tensile testing reveals the brittle fracture of large-area MoSe2 crystals and measures their fracture strength for the first time.

Synthesis of High-Quality Graphene and Hexagonal Boron Nitride Monolayer In-Plane Heterostructure on Cu–Ni Alloy

Graphene/hexagonal boron nitride (h‐BN) monolayer in‐plane heterostructure offers a novel material platform for both fundamental research and device applications. To obtain such a heterostructure in high quality via controllable synthetic approaches is still challenging. In this work, in‐plane epitaxy of graphene/h‐BN heterostructure is demonstrated on Cu–Ni substrates. The introduction of nickel to copper substrate not only enhances the capability of decomposing polyaminoborane residues but also promotes graphene growth via isothermal segregation. On the alloy surface partially covered by h‐BN, graphene is found to nucleate at the corners of the as‐formed h‐BN grains, and the high growth rate for graphene minimizes the damage of graphene‐growth process on h‐BN lattice. As a result, high‐quality graphene/h‐BN in‐plane heterostructure with epitaxial relationship can be formed, which is supported by extensive characterizations. Photodetector device applications are demonstrated based on the in‐plane heterostructure. The success will have important impact on future research and applications based on this unique material platform.

Unveil the Size-Dependent Mechanical Behaviors of Individual CNT/SiC Composite Nanofibers by In Situ Tensile Tests in SEM

In situ quantitative tensile tests of individual carbon nanotube (CNT)/SiC core-shell nanofibers are carried out in both a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The incorporation of CNTs into a SiC matrix led to improved elastic modulus and fracture strength of the CNT/SiC nanofibers as compared to SiC alone.

Direct Assessment of the Toxicity of Molybdenum Disulfide Atomically Thin Film and Microparticles via Cytotoxicity and Patch Testing

The low toxicity of molybdenum disulfide (MoS2 ) atomically thin film and microparticles is confirmed via cytotoxicity and patch testing in this report. The toxicity of MoS2 thin film and microparticles is extensively studied but is still inconclusive due to potential organic contamination in the preparations of samples. Such contamination is avoided here through preparing MoS2 atomically thin film via direct sulfurization of molybdenum thin film on quartz plate, which permits a direct assessment of its toxicity without any contamination. Six different types of cells, including normal, cancer, and immortal cells, are cultured in the media containing MoS2 thin film on quartz plates or dispersed MoS2 microparticles and their viability is evaluated with respect to the concentrations of samples. Detached thin films from the quartz plates are also investigated to estimate the toxicity of dispersed MoS2 in biological media. Allergy testing on skin of guinea pigs is also conducted to understand their effect on animal skins. By avoiding possible organic contamination, the low toxicity of MoS2 atomically thin film and microparticles to cells and animal skins paves the way for its applications in flexible biosensing/bioimaging devices and biocompatible coatings.

Synergetic photoluminescence enhancement of monolayer MoS2 via surface plasmon resonance and defect repair

The weak light-absorption and low quantum yield (QY) in monolayer MoS2 are great challenges for the applications of this material in practical optoelectronic devices. Here, we report on a synergistic strategy to obtain highly enhanced photoluminescence (PL) of monolayer MoS2 by simultaneously improving the intensity of the electromagnetic field around MoS2 and the QY of MoS2. Self-assembled sub-monolayer Au nanoparticles underneath the monolayer MoS2 and bis(trifluoromethane)sulfonimide (TFSI) treatment to the MoS2 surface are used to boost the excitation field and the QY, respectively. An enhancement factor of the PL intensity as high as 280 is achieved. The enhancement mechanisms are analyzed by inspecting the contribution of the PL spectra from A excitons and A− trions under different conditions. Our study takes a further step to developing high-performance optoelectronic devices based on monolayer MoS2.

Photoluminescence quenching in hybrid gold/MoSe 2 nanosheets

Transition Metal Dichalcogenide (TMD) materials have increasingly gained attention, due to their unique optical, spintronic, and electronic properties [1]. These properties at the monolayer limit are by part a result of the ultimate confinement imposed on their excitonic transitions that originate a direct band-gap and a lack of inversion symmetry in their crystallographic structure [2]. Many promising recent efforts in control of excitonic transitions in these materials have been devoted to study of their coupling with plasmonic nanoresonators. Plasmonic nanoresonators are known for their ability to control and modify the optical response of materials in their proximity [3]. These findings have motivated the study of emergent phenomena associated with the plasmon exciton interaction in these hybrid systems [4-5], which include enhancement [6] and quenching of the TMDs photoluminescence [7].