Browsing by Author "Shi, Chuqiao"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    Advancing Nanobeam Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM) for Strain Analysis
    (2024-04-03) Shi, Chuqiao; Han, Yimo
    This dissertation delves into the realm of nanobeam four-dimensional scanning transmission electron microscopy (4D-STEM), and its application for characterizing the intricate microstructures of two-dimensional (2D) materials and nano-catalysts. It leverages recent technological breakthroughs in pixelated, fast direct electron detectors that enable the comprehensive collection of momentum-space data at every scan in STEM. This integration of spatial and momentum dimensions enables the generation of rich 4D datasets. The wealth of information captured by these advanced detectors, though vast, poses interpretative challenges due to its complexity. Consequently, this thesis is dedicated to the development and application of novel analytical methodologies for the extraction of crystallographic information from such voluminous 4D-STEM datasets, addressing key problems in materials science. In Chapter 2, the study applies 4D-STEM to investigate the broad structural characteristics of van der Waals 2D ferroelectric SnSe. The research uncovers significant in-plane lattice distortions and out-of-plane stacking variations. This has led to the discovery of considerable lattice strains and distinctive ferroelectric-to-antiferroelectric domain walls which hold implications for the material's physical properties and potential device applications. Chapter 3 shifts the focus to the surface strain of core-shell nano-catalysts' structure. Through meticulous analysis via 4D-STEM, it is demonstrated that cube-shaped Au@Pd particles with sharp-tipped cores exhibit a coherent, dislocation-free heteroepitaxial interface even when the shell thickness is considerably greater than that of comparable nanocatalysts with rounded cores. This finding suggests a route to enhancing the strain stability of such structures, which is paramount in their application as catalysts. Chapter 4 ventures into the machine leaning methods to process extensive 4D-STEM datasets autonomously. This innovative, data-driven approach effectively discerns various material deformations, including strain, lattice distortions, and bending contours. Such detailed comprehension of lattice alterations is crucial for the advancement of material characterization techniques and the ensuing implications for materials science. Overall, the thesis advances the understanding of complex material systems through innovative 4D-STEM analysis and machine learning, potentially impacting the design and application of nanoscale materials and advancing technological frontiers across various disciplines.
  • Thumbnail Image
    Item
    Area-selective atomic layer deposition on 2D monolayer lateral superlattices
    (Springer Nature, 2024) Park, Jeongwon; Kwak, Seung Jae; Kang, Sumin; Oh, Saeyoung; Shin, Bongki; Noh, Gichang; Kim, Tae Soo; Kim, Changhwan; Park, Hyeonbin; Oh, Seung Hoon; Kang, Woojin; Hur, Namwook; Chai, Hyun-Jun; Kang, Minsoo; Kwon, Seongdae; Lee, Jaehyun; Lee, Yongjoon; Moon, Eoram; Shi, Chuqiao; Lou, Jun; Lee, Won Bo; Kwak, Joon Young; Yang, Heejun; Chung, Taek-Mo; Eom, Taeyong; Suh, Joonki; Han, Yimo; Jeong, Hu Young; Kim, YongJoo; Kang, Kibum
    The advanced patterning process is the basis of integration technology to realize the development of next-generation high-speed, low-power consumption devices. Recently, area-selective atomic layer deposition (AS-ALD), which allows the direct deposition of target materials on the desired area using a deposition barrier, has emerged as an alternative patterning process. However, the AS-ALD process remains challenging to use for the improvement of patterning resolution and selectivity. In this study, we report a superlattice-based AS-ALD (SAS-ALD) process using a two-dimensional (2D) MoS2-MoSe2 lateral superlattice as a pre-defining template. We achieved a minimum half pitch size of a sub-10 nm scale for the resulting AS-ALD on the 2D superlattice template by controlling the duration time of chemical vapor deposition (CVD) precursors. SAS-ALD introduces a mechanism that enables selectivity through the adsorption and diffusion processes of ALD precursors, distinctly different from conventional AS-ALD method. This technique facilitates selective deposition even on small pattern sizes and is compatible with the use of highly reactive precursors like trimethyl aluminum. Moreover, it allows for the selective deposition of a variety of materials, including Al2O3, HfO2, Ru, Te, and Sb2Se3.
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Preserving surface strain in nanocatalysts via morphology control
    (AAAS, 2024) Shi, Chuqiao; Cheng, Zhihua; Leonardi, Alberto; Yang, Yao; Engel, Michael; Jones, Matthew R.; Han, Yimo
    Engineering strain critically affects the properties of materials and has extensive applications in semiconductors and quantum systems. However, the deployment of strain-engineered nanocatalysts faces challenges, in particular in maintaining highly strained nanocrystals under reaction conditions. Here, we introduce a morphology-dependent effect that stabilizes surface strain even under harsh reaction conditions. Using four-dimensional scanning transmission electron microscopy (4D-STEM), we found that cube-shaped core-shell Au@Pd nanoparticles with sharp-edged morphologies sustain coherent heteroepitaxial interfaces with larger critical thicknesses than morphologies with rounded edges. This configuration inhibits dislocation nucleation due to reduced shear stress at corners, as indicated by molecular dynamics simulations. A Suzuki-type cross-coupling reaction shows that our approach achieves a fourfold increase in activity over conventional nanocatalysts, owing to the enhanced stability of surface strain. These findings contribute to advancing the development of advanced nanocatalysts and indicate broader applications for strain engineering in various fields.
  • Thumbnail Image
    Item
    Reversible non-volatile electronic switching in a near-room-temperature van der Waals ferromagnet
    (Springer Nature, 2024) Wu, Han; Chen, Lei; Malinowski, Paul; Jang, Bo Gyu; Deng, Qinwen; Scott, Kirsty; Huang, Jianwei; Ruff, Jacob P. C.; He, Yu; Chen, Xiang; Hu, Chaowei; Yue, Ziqin; Oh, Ji Seop; Teng, Xiaokun; Guo, Yucheng; Klemm, Mason; Shi, Chuqiao; Shi, Yue; Setty, Chandan; Werner, Tyler; Hashimoto, Makoto; Lu, Donghui; Yilmaz, Turgut; Vescovo, Elio; Mo, Sung-Kwan; Fedorov, Alexei; Denlinger, Jonathan D.; Xie, Yaofeng; Gao, Bin; Kono, Junichiro; Dai, Pengcheng; Han, Yimo; Xu, Xiaodong; Birgeneau, Robert J.; Zhu, Jian-Xin; da Silva Neto, Eduardo H.; Wu, Liang; Chu, Jiun-Haw; Si, Qimiao; Yi, Ming; Rice Center for Quantum Materials
    Non-volatile phase-change memory devices utilize local heating to toggle between crystalline and amorphous states with distinct electrical properties. Expanding on this kind of switching to two topologically distinct phases requires controlled non-volatile switching between two crystalline phases with distinct symmetries. Here, we report the observation of reversible and non-volatile switching between two stable and closely related crystal structures, with remarkably distinct electronic structures, in the near-room-temperature van der Waals ferromagnet Fe5−δGeTe2. We show that the switching is enabled by the ordering and disordering of Fe site vacancies that results in distinct crystalline symmetries of the two phases, which can be controlled by a thermal annealing and quenching method. The two phases are distinguished by the presence of topological nodal lines due to the preserved global inversion symmetry in the site-disordered phase, flat bands resulting from quantum destructive interference on a bipartite lattice, and broken inversion symmetry in the site-ordered phase.