Browsing by Author "Zhang, Tianyi"

Now showing 1 - 5 of 5
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    Domain-dependent strain and stacking in two-dimensional van der Waals ferroelectrics
    (Springer Nature, 2023) Shi, Chuqiao; Mao, Nannan; Zhang, Kena; Zhang, Tianyi; Chiu, Ming-Hui; Ashen, Kenna; Wang, Bo; Tang, Xiuyu; Guo, Galio; Lei, Shiming; Chen, Longqing; Cao, Ye; Qian, Xiaofeng; Kong, Jing; Han, Yimo
    Van der Waals (vdW) ferroelectrics have attracted significant attention for their potential in next-generation nano-electronics. Two-dimensional (2D) group-IV monochalcogenides have emerged as a promising candidate due to their strong room temperature in-plane polarization down to a monolayer limit. However, their polarization is strongly coupled with the lattice strain and stacking orders, which impact their electronic properties. Here, we utilize four-dimensional scanning transmission electron microscopy (4D-STEM) to simultaneously probe the in-plane strain and out-of-plane stacking in vdW SnSe. Specifically, we observe large lattice strain up to 4% with a gradient across ~50 nm to compensate lattice mismatch at domain walls, mitigating defects initiation. Additionally, we discover the unusual ferroelectric-to-antiferroelectric domain walls stabilized by vdW force and may lead to anisotropic nonlinear optical responses. Our findings provide a comprehensive understanding of in-plane and out-of-plane structures affecting domain properties in vdW SnSe, laying the foundation for domain wall engineering in vdW ferroelectrics.
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    First Arrival Differential LiDAR
    (2022-06-14) Zhang, Tianyi; Veeraraghavan, Ashok
    Single-photon avalanche diode (SPAD) based LiDAR is becoming the de-facto choice for 3D imaging in demanding applications such as autonomous vehicles due to their improved depth resolution, sensitivity, and long ranges of operation. However, they suffer from three significant limitations: (a) the additional time-of-arrival dimension results in a data throughput bottleneck, (b) limited spatial resolution due to either low fill-factor (flash LiDAR) or scanning time (scanning-based LiDAR), and (c) course depth resolution due to quantization of photon timing by existing timing circuitries. We propose a novel, in-pixel computing architecture that we term first arrival differential (FAD) LiDAR, where instead of recording quantized time-of-arrival information at individual pixels, we record a temporal differential measurement between pairs of pixels. The differential measurement is dependent on the relative order of photon arrivals at the two pixels (within a cycle) and creates a one-to-one mapping between this differential measurement and depth differences between the two pixels. We perform detailed system analysis and characterization using both analytical derivations, and experimental emulation using a scanning-based single-photon avalanche diode. We demonstrate several advantages of this design such as 10- 100x lower data throughput, and a greater than 10x reduction in required in-pixel chip footprint area, all the while maintaining (or in some cases improving) depth resolution and depth range.
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    Growth of Large-Sized 2D Ultrathin SnSe Crystals with In-Plane Ferroelectricity
    (Wiley, 2023) Chiu, Ming-Hui; Ji, Xiang; Zhang, Tianyi; Mao, Nannan; Luo, Yue; Shi, Chuqiao; Zheng, Xudong; Liu, Hongwei; Han, Yimo; Wilson, William L.; Luo, Zhengtang; Tung, Vincent; Kong, Jing
    Tin (II) selenide (SnSe) is an emerging 2D material with many intriguing properties, such as record-high thermoelectric figure of merit (ZT), purely in-plane ferroelectricity, and excellent nonlinear optical properties. To explore these functional properties and related applications, a crucial step is to develop controllable routes to synthesize large-area, ultrathin, and high-quality SnSe crystals. Physical vapor deposition (PVD) constitutes a reliable method to synthesize 2D SnSe, however, effects of various growth parameters have not yet been systematically investigated, and current PVD-synthesized flakes are often thick (>10 nm) with small lateral sizes (<10 µm). In this work, high-quality 2D SnSe crystals are synthesized via low-pressure PVD, which display in-plane ferroelectric domains observed by piezoresponse force microscopy and polarization-dependent reflection spectroscopy. Detailed studies regarding the roles of various parameters are further carried out, including substrate pre-annealing, growth duration, temperature, and pressure, which enable to rationally optimize the growth and obtain 2D SnSe crystals with lateral sizes up to ≈23.0 µm and thicknesses down to ≈2.0 nm (3–4 layers). This work paves the way for the controlled growth of large-area 2D SnSe, facilitating the future exploration of many interesting multiferroic properties and applications with atomic thickness.
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    Sulfur Vacancy Related Optical Transitions in Graded Alloys of MoxW1-xS2 Monolayers
    (Wiley, 2024) Ghafariasl, Mahdi; Zhang, Tianyi; Ward, Zachary D.; Zhou, Da; Sanchez, David; Swaminathan, Venkataraman; Terrones, Humberto; Terrones, Mauricio; Abate, Yohannes
    Engineering electronic bandgaps is crucial for applications in information technology, sensing, and renewable energy. Transition metal dichalcogenides (TMDCs) offer a versatile platform for bandgap modulation through alloying, doping, and heterostructure formation. Here, the synthesis of a 2D MoxW1-xS2 graded alloy is reported, featuring a Mo-rich center that transitions to W-rich edges, achieving a tunable bandgap of 1.85 to 1.95 eV when moving from the center to the edge of the flake. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy showed the presence of sulfur monovacancy, VS, whose concentration varied across the graded MoxW1-xS2 layer as a function of Mo content with the highest value in the Mo-rich center region. Optical spectroscopy measurements supported by ab initio calculations reveal a doublet electronic state of VS, which is split due to the spin-orbit interaction, with energy levels close to the conduction band or deep in the bandgap depending on whether the vacancy is surrounded by W atoms or Mo atoms. This unique electronic configuration of VS in the alloy gave rise to four spin-allowed optical transitions between the VS levels and the valence bands. The study demonstrates the potential of defect and optical engineering in 2D monolayers for advanced device applications.
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    Thermoelectrics
    (Rice University, 5/4/2017) Hwang, Alex; Morera, Alfonso; Phuathavornskul, Fasai; Zhang, Tianyi; Department of Electrical and Computer Engineering