Ziyuan Lin

Extraordinarily Strong Interlayer Interaction in 2D Layered PtS2

Platinum disulfide (PtS2 ), a new member of the group-10 transition-metal dichalcogenides, is studied experimentally and theoretically. The indirect bandgap of PtS2 can be drastically tuned from 1.6 eV (monolayer) to 0.25 eV (bulk counterpart), and the interlayer mechanical coupling is almost isotropic. It can be explained by strongly interlayer interaction from the pz orbital hybridization of S atoms.

Controllable Growth of Large-Size Crystalline MoS2 and Resist-Free Transfer Assisted with a Cu Thin Film.

Two-dimensional MoS2 is a promising material for future nanoelectronics and optoelectronics. It has remained a great challenge to grow large-size crystalline and high surface coverage monolayer MoS2. In this work, we investigate the controllable growth of monolayer MoS2 evolving from triangular flakes to continuous thin films by optimizing the concentration of gaseous MoS2, which has been shown a both thermodynamic and kinetic growth factor. A single-crystal monolayer MoS2 larger than 300 μm was successfully grown by suppressing the nuclei density and supplying sufficient source. Furthermore, we present a facile process of transferring the centimeter scale MoS2 assisted with a copper thin film. Our results show the absence of observable residues or wrinkles after we transfer MoS2 from the growth substrates onto flat substrates using this technique, which can be further extended to transfer other two-dimensional layered materials.

Direct TEM observations of growth mechanisms of two-dimensional MoS2 flakes.

A microscopic understanding of the growth mechanism of two-dimensional materials is of particular importance for controllable synthesis of functional nanostructures. Because of the lack of direct and insightful observations, how to control the orientation and the size of two-dimensional material grains is still under debate. Here we discern distinct formation stages for MoS2 flakes from the thermolysis of ammonium thiomolybdates using in situ transmission electron microscopy. In the initial stage (400 °C), vertically aligned MoS2 structures grow in a layer-by-layer mode. With the increasing temperature of up to 780 °C, the orientation of MoS2 structures becomes horizontal. When the growth temperature reaches 850 °C, the crystalline size of MoS2 increases by merging adjacent flakes. Our study shows direct observations of MoS2 growth as the temperature evolves, and sheds light on the controllable orientation and grain size of two-dimensional materials.

Infrared light gated MoS 2 field effect transistor

Molybdenum disulfide (MoS₂) as a promising 2D material has attracted extensive attentions due to its unique physical, optical and electrical properties. In this work, we demonstrate an infrared (IR) light gated MoS₂ transistor through a device composed of MoS₂ monolayer and a ferroelectric single crystal Pb(Mg(1/3)Nb(2/3))O₃-PbTiO₃ (PMN-PT). With a monolayer MoS₂ onto the top surface of (111) PMN-PT crystal, the drain current of MoS₂ channel can be modulated with infrared illumination and this modulation process is reversible. Thus, the transistor can work as a new kind of IR photodetector with a high IR responsivity of 114%/Wcm⁻². The IR response of MoS₂ transistor is attributed to the polarization change of PMN-PT single crystal induced by the pyroelectric effect which results in a field effect. Our result promises the application of MoS₂ 2D material in infrared optoelectronic devices. Combining with the intrinsic photocurrent feature of MoS₂ in the visible range, the MoS₂ on ferroelectric single crystal may be sensitive to a broadband wavelength of light.

Improved interfacial H2O supply by surface hydroxyl groups for enhanced alkaline hydrogen evolution

Robust hydrogen evolution reaction (HER) in alkaline solution using a cost-effective catalyst is still a critical challenge for industrial H2O electrolysis. The dramatic depletion of interfacial H2O and hindered OH− diffusion bring about a harsh interfacial environment and thereby lead to sluggish hydrogen evolution. Here we develop heterogeneous catalysts based on defect-rich three-dimensional graphene (M/3D graphene) and achieve improved HER kinetics via surface hydroxyl group modification on the graphene surface. Our theoretical studies suggest that the surface hydroxyl groups derived from the hydroxylation process effectively attract multi-overlayer H2O clusters without any thermodynamic barrier and form a reservoir to continuously supply H2O for catalytic sites. Electrochemical characterizations indicate that the HER kinetics per active electrochemical surface area have been greatly improved after crafting abundant hydroxyl groups on the 3D graphene framework, which can be attributed to sufficient proton donors and an ameliorative interfacial pH environment. Benefiting from the surface hydroxyl group modification, the MoS2/3D graphene structure facilitates the HER at a low onset potential of 60 mV in alkaline medium and exhibits excellent durability after 3000 cycles. This surface modification strategy reveals the importance of interfacial H2O supply in HER kinetics and provides general guidance on designing highly efficient catalysts in alkaline medium.

High thermally conductive and electrically insulating 2D boron nitride nanosheet for efficient heat dissipation of high-power transistors

High-power transistors suffer greatly from inefficient heat dissipation of the hotspots, which elevate the local temperature and significantly degrade the performance and reliability of the high-power devices. Although various thermal management methods at package-level have been demonstrated, the heat dissipation from non-uniform hotspots at micro/nanoscale still persist in the high power transistors. Here, we develop a method for local thermal management using thermally conductive and electrical insulating few-layer hexagonal boron nitride (h-BN) as heat spreaders and thick counterpart as heat sinks. The electrically insulating characteristic of h-BN nanosheet allows it to be intimately contacted with the hotspot region that is located at the gate electrode edge near the drain side of a high-electron-mobility transistor (HEMT). The high thermal conductivity of h-BN nanosheet, which is quantitatively measured by Raman thermography, reduces the temperature of the hotspot by introducing an additional heat transporting pathway. Our DC and radio-frequency characterizations of the HEMT show the improvement of saturation current, cut-off frequency and maximum oscillation frequency. The finite element simulations show a temperature decrease of ~32 °C at the hotspot with the use of h-BN nanosheet. This method can be further extended for the micro/nanoscale thermal management of other high-power devices.

Phase and Facet Control of Molybdenum Carbide Nanosheet Observed by In Situ TEM

Transition metal carbides are of great potential for electrochemical applications. The phase and facet of molybdenum carbides greatly affect the electrochemical performance. Carburization of MoO3 inside a transmission electron microscope to monitor the growth process of molybdenum carbides is performed. Carbon sources with different activities are used and the controllable growth of molybdenum carbides is investigated. The results show that the relatively inert amorphous carbon film produces Mo2 C, where the interstitial sites formed by hexagonal closed packing molybdenum atoms are partially occupied by carbon atoms. In contrast, the carbon decomposed from the sucrose has a high portion of sp3 hybridized and crosslinked carbon atoms with high reactivity, leading to the formation of MoC with full occupation of interstitial sites by carbon atoms. In addition, the MoC growth experiences a (111) to (100) facets change with the increase of temperature. The (111) facet formed at low temperature has Mo-terminated or C-terminated surface with higher surface energy and higher reactivity, while the (100) facet with 1:1 C/Mo ratio on the surface exhibits enhanced stability. The phase and facet control by carbon source and temperature allow us to tune the crystal structures and surface atoms as well as their electrochemical properties.

Improved performance of HEMTs with BN as heat dissipation

High electron mobility transistors (HEMTs) are of great interest in applications requiring high frequency and high power. However, self-heating due to the inefficient heat dissipation limits the performance of the transistors. Here we improve the thermal management of HEMTs utilizing the few-layer boron nitride as heat spreader, which is applicable in the integrated circuits. DC and RF performance of the transistors are both enhanced with the adopted heat dissipation. It is attributed to temperature reduction around the hotspot in the transistors, indicated by the finite element simulation.