Browsing by Author "Wang, Luqing"
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Item Defining shapes of two-dimensional crystals with undefinable edge energies(Springer Nature, 2022) Wang, Luqing; Shirodkar, Sharmila N.; Zhang, Zhuhua; Yakobson, Boris I.The equilibrium shape of crystals is a fundamental property of both aesthetic appeal and practical importance: the shape and its facets control the catalytic, light-emitting, sensing, magnetic and plasmonic behaviors. It is also a visible macro-manifestation of the underlying atomic-scale forces and chemical makeup, most conspicuous in two-dimensional (2D) materials of keen current interest. If the crystal surface/edge energy is known for different directions, its shape can be obtained by the geometric Wulff construction, a tenet of crystal physics; however, if symmetry is lacking, the crystal edge energy cannot be defined or calculated and thus its shape becomes elusive, presenting an insurmountable problem for theory. Here we show how one can proceed with auxiliary edge energies towards a constructive prediction, through well-planned computations, of a unique crystal shape. We demonstrate it for challenging materials such as SnSe, which is of C2v symmetry, and even AgNO2 of C1, which has no symmetry at all.Item First-Principles Study of Properties, Morphology, Synthesis, and Applications of Low-Dimensional Materials(2020-08-13) Wang, Luqing; Yakobson, Boris I.The non-trivial properties of low-dimensional (LD) materials due to quantum confinement effect miraculously advance our understanding of the fundamental physics principles of nature. This thesis explores the electronic and mechanical properties, morphologies, synthesis mechanism, and electro-catalytic applications of LD materials through employing state-of-art computational method of first principles calculations, especially density functional theory (DFT). Regarding electronic and mechanical properties, band gap transitions of MoS2 and WS2, anisotropy of phosphorene and low contact barrier of 2H/1T’ MoTe2 junction were analyzed. About morphologies, strategies for asymmetric two-dimensional (2D) materials were proposed. With respect to synthesis, growth mechanism of borophene on Au (111) and line defects of that on Ag (111) are discussed. For electro-catalytic applications, hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in 2D materials are illustrated. These theoretical investigations agree well with experimental measurements, and provide insights for the understanding of the mechanisms of LD materials.Item Geometric imaging of borophene polymorphs with functionalized probes(Springer Nature, 2019) Liu, Xiaolong; Wang, Luqing; Li, Shaowei; Rahn, Matthew S.; Yakobson, Boris I.; Hersam, Mark C.A common characteristic of borophene polymorphs is the presence of hollow hexagons (HHs) in an otherwise triangular lattice. The vast number of possible HH arrangements underlies the polymorphic nature of borophene, and necessitates direct HH imaging to definitively identify its atomic structure. While borophene has been imaged with scanning tunneling microscopy using conventional metal probes, the convolution of topographic and electronic features hinders unambiguous identification of the atomic lattice. Here, we overcome these limitations by employing CO-functionalized atomic force microscopy to visualize structures corresponding to boron-boron covalent bonds. Additionally, we show that CO-functionalized scanning tunneling microscopy is an equivalent and more accessible technique for HH imaging, confirming the v1/5 and v1/6 borophene models as unifying structures for all observed phases. Using this methodology, a borophene phase diagram is assembled, including a transition from rotationally commensurate to incommensurate phases at high growth temperatures, thus corroborating the chemically discrete nature of borophene.Item Geometric imaging of borophene polymorphs with functionalized probes(Springer Nature, 2019) Liu, Xiaolong; Wang, Luqing; Li, Shaowei; Rahn, Matthew S.; Yakobson, Boris I.; Hersam, Mark C.A common characteristic of borophene polymorphs is the presence of hollow hexagons (HHs) in an otherwise triangular lattice. The vast number of possible HH arrangements underlies the polymorphic nature of borophene, and necessitates direct HH imaging to definitively identify its atomic structure. While borophene has been imaged with scanning tunneling microscopy using conventional metal probes, the convolution of topographic and electronic features hinders unambiguous identification of the atomic lattice. Here, we overcome these limitations by employing CO-functionalized atomic force microscopy to visualize structures corresponding to boron-boron covalent bonds. Additionally, we show that CO-functionalized scanning tunneling microscopy is an equivalent and more accessible technique for HH imaging, confirming the v1/5 and v1/6 borophene models as unifying structures for all observed phases. Using this methodology, a borophene phase diagram is assembled, including a transition from rotationally commensurate to incommensurate phases at high growth temperatures, thus corroborating the chemically discrete nature of borophene.Item Manganese deception on graphene and implications in catalysis(Elsevier, 2018) Ye, Ruquan; Dong, Juncai; Wang, Luqing; Mendoza-Cruz, Rubén; Li, Yilun; An, Peng-Fei; Yacamán, Miguel José; Yakobson, Boris I.; Chen, Dongliang; Tour, James M.Heteroatom-doped metal-free graphene has been widely studied as the catalyst for the oxygen reduction reaction (ORR). Depending on the preparation method and the dopants, the ORR activity varies ranging from a two-electron to a four-electron pathway. The different literature reports are difficult to correlate due to the large variances. However, due to the potential metal contamination, the origin of the ORR activity from “metal-free” graphene remains confusing and inconclusive. Here we decipher the ORR catalytic activities of diverse architectures on graphene derived from reduced graphene oxide. High angle annular dark field scanning transmission electron microscopy, X-ray absorption near edge structure, extended X-ray absorption fine structure, and trace elemental analysis methods are employed. The mechanistic origin of ORR activity is associated with the trace manganese content and reaches its highest performance at an onset potential of 0.94 V when manganese exists as a mononuclear-centered structure within defective graphene. This study exposes the deceptive role of trace metal in formerly thought to be metal-free graphene materials. It also provides insight into the design of better-performing catalyst for ORR by underscoring the coordination chemistry possible for future single-atom catalyst materials.Item Nanoscale Probing of Image-Potential States and Electron Transfer Doping in Borophene Polymorphs(American Chemical Society, 2021) Liu, Xiaolong; Wang, Luqing; Yakobson, Boris I.; Hersam, Mark C.Because synthetic 2D materials are generally stabilized by interfacial coupling to growth substrates, direct probing of interfacial phenomena is critical for understanding their nanoscale structure and properties. Using field-emission resonance spectroscopy with an ultrahigh vacuum scanning tunneling microscope, we reveal Stark-shifted image-potential states of the v1/6 and v1/5 borophene polymorphs on Ag(111) with long lifetimes, suggesting high borophene lattice and interface quality. These image-potential states allow the local work function and interfacial charge transfer of borophene to be probed at the nanoscale and test the widely employed self-doping model of borophene. Supported by apparent barrier height measurements and density functional theory calculations, electron transfer doping occurs for both borophene phases from the Ag(111) substrate. In contradiction with the self-doping model, a higher electron transfer doping level occurs for denser v1/6 borophene compared to v1/5 borophene, thus revealing the importance of substrate effects on borophene electron transfer.