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  1. Abstract

    Two-dimensional (2D) materials such as semiconductors and ferroelectrics are promising for future energy-efficient logic devices because of their extraordinary electronic properties at atomic thickness. In this work, we investigated a van der Waals heterostructure composited of 2D semiconducting MoS2and 2D ferroelectric CuInP2S6(CIPS) and NiPS3. Instead of using 2D ferroelectrics as conventional gate dielectric layers, here we applied CIPS and NiPS3as a ferroelectric capping layer, and investigated a long-distance coupling effect with the gate upon the sandwiched 2D MoS2channels. Our experimental results showed an outstanding enhancement of the electrodynamic gating in 2D MoS2transistors, represented by a significant reduction of subthreshold swing at room temperature. This was due to the coupling-induced polarization of 2D ferroelectrics at 2D semiconductor surface which led to an effective and dynamic magnification of the gate capacitance. Meanwhile, the electrostatic gating was remained steady after adding the ferroelectric capping layer, providing ease and compatibility for further implementation with existing circuit and system design. Our work demonstrates the long-distance coupling effect of 2D ferroelectrics in a capping architecture, reveals its impacts from both electrodynamic and electrostatic perspectives, and expands the potential of 2D ferroelectrics to further improve the performance of energy-efficient nanoelectronics.

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  2. Abstract Two-dimensional (2D) molybdenum disulfide (MoS 2 ) has been recognized as a potential substitution of platinum (Pt) for electrochemical hydrogen evolution reaction (HER). However, the broad adoption of MoS 2 is hindered by its limited number of active sites and low inherent electrical conductivity. In this work, we employed a one-step solvothermal synthesis technique to construct a ternary hybrid structure consisting of dual-phase MoS 2, titanium carbide (Ti 3 C 2 ) MXene, and carbon nanotubes (CNTs), and demonstrated synergistic effects for active site exposure, surface area enlargement, and electrical conductivity improvement of the catalyst. The dual-phase MoS 2 (DP-MoS 2 ) is directly formed on the MXene with CNTs acting as crosslinks between 2D islands. The existence of edge-enriched metallic phase MoS 2 , the conductive backbone of MXene along with the crosslink function of CNTs clearly improves the overall HER performance of the ternary nanocomposite. Moreover, the integration of MoS 2 with MXene not only increases the interlayer distance of the 2D layers but also partially suppresses the MXene oxidation and the 2D layer restacking, leading to good catalytic stability. As a result, an overpotential of 169 mV and a low Tafel slope of 51 mV/dec was successfully achieved. This work paves a way for 2D-based electrocatalyst engineering and sheds light on the development of the next-generation noble metal-free HER electrocatalysts. 
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  3. The development of active catalysts for hydrogen evolution reaction (HER) made from low-cost materials constitutes a crucial challenge in the utilization of hydrogen energy. Earth-abundant molybdenum disulfide (MoS 2 ) has been discovered recently with good activity and stability for HER. In this report, we employ a hydrothermal technique for MoS 2 synthesis which is a cost-effective and environmentally friendly approach and has the potential for future mass production. Machine-learning (ML) techniques are built and subsequently used within a Bayesian Optimization framework to validate the optimal parameter combinations for synthesizing high-quality MoS 2 catalyst within the limited parameter space. Compared with the heavy-labor and time-consuming trial-and-error approach, the ML techniques provide a more efficient toolkit to assist exploration of the most effective HER catalyst in hydrothermal synthesis. To investigate the structure-property relationship, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and various electrochemical characterizations have been conducted to investigate the superiority of the ML validated optimized sample. A strong correlation between the material structure and the HER performance has been observed for the optimized MoS 2 catalyst. 
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  4. null (Ed.)
    High contact resistance is one of the primary concerns for electronic device applications of two-dimensional (2D) layered semiconductors. Here, we explore the enhanced carrier transport through metal–semiconductor interfaces in WS 2 field effect transistors (FETs) by introducing a typical transition metal, Cu, with two different doping strategies: (i) a “generalized” Cu doping by using randomly distributed Cu atoms along the channel and (ii) a “localized” Cu doping by adapting an ultrathin Cu layer at the metal–semiconductor interface. Compared to the pristine WS 2 FETs, both the generalized Cu atomic dopant and localized Cu contact decoration can provide a Schottky-to-Ohmic contact transition owing to the reduced contact resistances by 1–3 orders of magnitude, and consequently elevate electron mobilities by 5–7 times. Our work demonstrates that the introduction of transition metal can be an efficient and reliable technique to enhance the carrier transport and device performance in 2D TMD FETs. 
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  5. Abstract

    Realizing van der Waals (vdW) epitaxy in the 1980s represents a breakthrough that circumvents the stringent lattice matching and processing compatibility requirements in conventional covalent heteroepitaxy. However, due to the weak vdW interactions, there is little control over film qualities by the substrate. Typically, discrete domains with a spread of misorientation angles are formed, limiting the applicability of vdW epitaxy. Here, the epitaxial growth of monocrystalline, covalent Cr5Te82D crystals on monolayer vdW WSe2by chemical vapor deposition is reported, driven by interfacial dative bond formation. The lattice of Cr5Te8, with a lateral dimension of a few tens of micrometers, is fully commensurate with that of WSe2via 3 × 3 (Cr5Te8)/7 × 7 (WSe2) supercell matching, forming a single‐crystalline moiré superlattice. This work establishes a conceptually distinct paradigm of thin‐film epitaxy, termed “dative epitaxy”, which takes full advantage of covalent epitaxy with chemical bonding for fixing the atomic registry and crystal orientation, while circumventing its stringent lattice matching and processing compatibility requirements; conversely, it ensures the full flexibility of vdW epitaxy, while avoiding its poor orientation control. Cr5Te82D crystals grown by dative epitaxy exhibit square magnetic hysteresis, suggesting minimized interfacial defects that can serve as pinning sites.

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  6. Abstract

    2D semiconductors such as monolayer molybdenum disulfide (MoS2) are promising material candidates for next‐generation nanoelectronics. However, there are fundamental challenges related to their metal–semiconductor (MS) contacts, which limit the performance potential for practical device applications. In this work, 2D monolayer hexagonal boron nitride (h‐BN) is exploited as an ultrathin decorating layer to form a metal–insulator–semiconductor (MIS) contact, and an innovative device architecture is designed as a platform to reveal a novel diode‐like selective enhancement of the carrier transport through the MIS contact. The contact resistance is significantly reduced when the electrons are transported from the semiconductor to the metal, but is barely affected when the electrons are transported oppositely. A concept of carrier collection barrier is proposed to interpret this intriguing phenomenon as well as a negative Schottky barrier height obtained from temperature‐dependent measurements, and the critical role of the collection barrier at the drain end is shown for the overall transistor performance.

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