Browsing by Author "Han, Yimo"
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Item Advancing Four-Dimensional Scanning Transmission Electron Microscopy for the Strain Analysis of Deformed Thin Films(2024-04-19) Mireles, Adan Joel; Han, YimoThis study presents significant advancements in Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM) for analyzing strain and crystal orientation in thin films by introducing three novel methods. First, we developed an area-selective filtering technique that leverages unsupervised learning to reduce noise in 4D-STEM datasets. This approach achieved up to a 70% noise reduction for WS2-WSe2 superlattice data. Second, we introduce a strain correction method tailored for buckled two-dimensional materials. Guided by kinematical diffraction simulations, this method produces surface morphology maps that enable surface tilt and strain decoupling. Its application to MoSe2-MoS2 heterojunction data successfully reduced compressive strain measurements from an overestimated 6.5% to a more accurate ~1.5%. Lastly, we present a technique for precisely mapping crystal orientation in thin films. This technique was effectively applied to a gold nanoplate using a combination of 4D-STEM data, abTEM multislice simulations, and electron tomography validation. These advancements significantly improve the accuracy of strain measurements and crystallographic analysis, thereby enhancing our understanding of deformed nanofilms and expanding the capabilities of 4D-STEM for future materials science research.Item Advancing Nanobeam Four-Dimensional Scanning Transmission Electron Microscopy (4D-STEM) for Strain Analysis(2024-04-03) Shi, Chuqiao; Han, YimoThis 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.Item Atomically precise nanoclusters predominantly seed gold nanoparticle syntheses(Springer Nature, 2023) Qiao, Liang; Pollard, Nia; Senanayake, Ravithree D.; Yang, Zhi; Kim, Minjung; Ali, Arzeena S.; Hoang, Minh Tam; Yao, Nan; Han, Yimo; Hernandez, Rigoberto; Clayborne, Andre Z.; Jones, Matthew R.Seed-mediated synthesis strategies, in which small gold nanoparticle precursors are added to a growth solution to initiate heterogeneous nucleation, are among the most prevalent, simple, and productive methodologies for generating well-defined colloidal anisotropic nanostructures. However, the size, structure, and chemical properties of the seeds remain poorly understood, which partially explains the lack of mechanistic understanding of many particle growth reactions. Here, we identify the majority component in the seed solution as an atomically precise gold nanocluster, consisting of a 32-atom Au core with 8 halide ligands and 12 neutral ligands constituting a bound ion pair between a halide and the cationic surfactant: Au32X8[AQA+•X-]12 (X = Cl, Br; AQA = alkyl quaternary ammonium). Ligand exchange is dynamic and versatile, occurring on the order of minutes and allowing for the formation of 48 distinct Au32 clusters with AQAX (alkyl quaternary ammonium halide) ligands. Anisotropic nanoparticle syntheses seeded with solutions enriched in Au32X8[AQA+•X-]12 show narrower size distributions and fewer impurity particle shapes, indicating the importance of this cluster as a precursor to the growth of well-defined nanostructures.Item Automatic parameter selection for electron ptychography via Bayesian optimization(Springer Nature, 2022) Cao, Michael C.; Chen, Zhen; Jiang, Yi; Han, YimoElectron ptychography provides new opportunities to resolve atomic structures with deep sub-angstrom spatial resolution and to study electron-beam sensitive materials with high dose efficiency. In practice, obtaining accurate ptychography images requires simultaneously optimizing multiple parameters that are often selected based on trial-and-error, resulting in low-throughput experiments and preventing wider adoption. Here, we develop an automatic parameter selection framework to circumvent this problem using Bayesian optimization with Gaussian processes. With minimal prior knowledge, the workflow efficiently produces ptychographic reconstructions that are superior to those processed by experienced experts. The method also facilitates better experimental designs by exploring optimized experimental parameters from simulated data.Item Battery metal recycling by flash Joule heating(AAAS, 2023) Chen, Weiyin; Chen, Jinhang; Bets, Ksenia V.; Salvatierra, Rodrigo V.; Wyss, Kevin M.; Gao, Guanhui; Choi, Chi Hun; Deng, Bing; Wang, Xin; Li, John Tianci; Kittrell, Carter; La, Nghi; Eddy, Lucas; Scotland, Phelecia; Cheng, Yi; Xu, Shichen; Li, Bowen; Tomson, Mason B.; Han, Yimo; Yakobson, Boris I.; Tour, James M.; Welch Institute for Advanced Materials; NanoCarbon Center; Applied Physics Program; Smalley-Curl InstituteThe staggering accumulation of end-of-life lithium-ion batteries (LIBs) and the growing scarcity of battery metal sources have triggered an urgent call for an effective recycling strategy. However, it is challenging to reclaim these metals with both high efficiency and low environmental footprint. We use here a pulsed dc flash Joule heating (FJH) strategy that heats the black mass, the combined anode and cathode, to >2100 kelvin within seconds, leading to ~1000-fold increase in subsequent leaching kinetics. There are high recovery yields of all the battery metals, regardless of their chemistries, using even diluted acids like 0.01 M HCl, thereby lessening the secondary waste stream. The ultrafast high temperature achieves thermal decomposition of the passivated solid electrolyte interphase and valence state reduction of the hard-to-dissolve metal compounds while mitigating diffusional loss of volatile metals. Life cycle analysis versus present recycling methods shows that FJH significantly reduces the environmental footprint of spent LIB processing while turning it into an economically attractive process.Item Branching phenomena in nanostructure synthesis illuminated by the study of Ni-based nanocomposites(Royal Society of Chemisty, 2023) Qiao, Liang; Fu, Zheng; Zhao, Wenxia; Cui, Yan; Xing, Xin; Xie, Yin; Li, Ji; Gao, Guanhui; Xuan, Zhengxi; Liu, Yang; Lee, Chaeeon; Han, Yimo; Cheng, Yingwen; He, Shengbao; Jones, Matthew R.; Swihart, Mark T.Branching phenomena are ubiquitous in both natural and artificial crystallization processes. The branched nanostructures' emergent properties depend upon their structures, but their structural tunability is limited by an inadequate understanding of their formation mechanisms. Here we developed an ensemble of Nickel-Based nano-Composites (NBCs) to investigate branching phenomena in solution-phase synthesis with precision and in depth. NBCs of 24 morphologies, including dots, core@shell dots, hollow shells, clusters, polyhedra, platelets, dendrites, urchins, and dandelions, were synthesized through systematic adjustment of multiple synthesis parameters. Relationships between the synthesis parameters and the resultant morphologies were analyzed. Classical or non-classical models of nucleation, nascent growth, 1D growth, 2D growth, 3D reconstruction, aggregation, and carburization were defined individually and then integrated to provide a holistic view of the formation mechanism of branched NBCs. Finally, guidelines were extracted and verified to guide the rational solution-phase syntheses of branched nanomaterials with emergent biological, chemical, and physical properties for potential applications in immunology, catalysis, energy storage, and optics. Demonstrating a systematic approach for deconvoluting the formation mechanism and enhancing the synthesis tunability, this work is intended to benefit the conception, development, and improvement of analogous artificial branched nanostructures. Moreover, the progress on this front of synthesis science would, hopefully, deepen our understanding of branching phenomena in nature.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, YimoVan 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.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, JingTin (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.Item Investigation of the structure and elemental composition of 2D Interfaces in Transition Metal Dichalcogenides(2023-03-23) Pieshkov, Tim; Han, YimoThe electronics boom happened with the invention of semiconductor materials with Si becoming the number one in the race. But now, since Si electronics seems to be reaching its limit, the researchers are switching their attention to 2D materials because of their unique electrical, mechanical, or optical properties. Transition metal dichalcogenides (TMDs) are semiconductor materials from this family that show promise to be the next generation of nanoelectronics as well as give possibility to improve energy or informational storage. However, for maintaining high efficiency, the structure of TMDs should have minimum number of defects, misfits, or inclusions of other elements. This indicates a need to understand the structural and elemental composition of the 2D materials to atomic scales, and this is where techniques like (scanning) transmission electron microscopy (S)TEM and energy-dispersive x-ray spectroscopy (EDX) come to aid. In this study, we will discuss sample preparation and analysis techniques for of SnSe, MoS2, WS2, WSe2, and graphene/Al heterostructures on silicon wafer and sapphire substrates, the methods of uncovering the elemental and layer composition of these materials and provide image and signal processing approaches to increase the signal-to-noise ratio of the currently employed techniques. All of these will help us reach a conclusion about atomic structure and elemental composition of the materials and show the possible traps and pitfalls that a microscopist might encounter during similar analysis for either TMDs or other types of 2D materials.Item Vacancy-mediated anomalous phononic and electronic transport in defective half-Heusler ZrNiBi(Springer Nature, 2023) Ren, Wuyang; Xue, Wenhua; Guo, Shuping; He, Ran; Deng, Liangzi; Song, Shaowei; Sotnikov, Andrei; Nielsch, Kornelius; van den Brink, Jeroen; Gao, Guanhui; Chen, Shuo; Han, Yimo; Wu, Jiang; Chu, Ching-Wu; Wang, Zhiming; Wang, Yumei; Ren, ZhifengStudies of vacancy-mediated anomalous transport properties have flourished in diverse fields since these properties endow solid materials with fascinating photoelectric, ferroelectric, and spin-electric behaviors. Although phononic and electronic transport underpin the physical origin of thermoelectrics, vacancy has only played a stereotyped role as a scattering center. Here we reveal the multifunctionality of vacancy in tailoring the transport properties of an emerging thermoelectric material, defective n-type ZrNiBi. The phonon kinetic process is mediated in both propagating velocity and relaxation time: vacancy-induced local soft bonds lower the phonon velocity while acoustic-optical phonon coupling, anisotropic vibrations, and point-defect scattering induced by vacancy shorten the relaxation time. Consequently, defective ZrNiBi exhibits the lowest lattice thermal conductivity among the half-Heusler family. In addition, a vacancy-induced flat band features prominently in its electronic band structure, which is not only desirable for electron-sufficient thermoelectric materials but also interesting for driving other novel physical phenomena. Finally, better thermoelectric performance is established in a ZrNiBi-based compound. Our findings not only demonstrate a promising thermoelectric material but also promote the fascinating vacancy-mediated anomalous transport properties for multidisciplinary explorations.