Browsing by Author "Yu, Henry"

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    Electronic Properties and Applications of 2D Materials
    (2016-08-02) Yu, Henry; Yakobson, Boris I.
    2D materials has become one of the most exciting areas of research, since the report of graphene in 2005. For graphene, the high mobility ($\sim$ 15,000 cm$^2$V$^{-1}$s$^{-1}$)draws much attention to its potential as high speed electronic devices. Its cone-like electronic dispersion, resembling that of relativistic massless Dirac fermions, entertains many exotic and interesting behaviors, making it an ideal system for the study of relativistic particles. Aside from graphene, many other 2D materials have also been successfully made, including MoS$_2$, h-BN, 2D Boron, etc. Although these newly made materials have already exhibited several good characteristics, they are also distinctively different from traditional 3D materials. This means that a deeper understanding of 2D materials is imperative, to capture and utilize their unique features. The abundant atomistic modeling methods nowadays enable us to investigate the various aspects of 2D materials (or any system in general), with the help of computers. Density Functional Theory (DFT) based methods can give very accurate descriptions of the ground state properties of materials, including their charge density, mechanical moduli or the optimal structure; on top of DFT, many body theory methods also allow for the construction of excitation processes based on the DFT results. In the case of large structures, in which DFT may not be affordable, the tight-binding method can be an excellent alternative with a much lower cost. In this work, I will employ these atomistic methods to the understanding of the distinctive features of 2D materials, especially their electronic, or even electro-mechanical properties. First, I have found that the graphite screw dislocations (GSD), a family of graphene-like structures as nanoribbons, turn out to be superior nano-solenoids, producing magnetic field up to 1T at typical voltage. Second, I have successfully modeled the strain-induced Landau quantization of the graphene band structure in large structures ($N\sim 10^5$), showing the possibility of strain engineering for the design of desired Landau levels in actual devices. Finally, I have discovered several universal features for 2D lateral junctions. 2D junctions, due to the weak electronic screening, turn out to be not merely a miniaturized version of its 3D counterpart. The scaling laws, depletion region and several practical consequences are analyzed and quantified.
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    Electronics in 2D: heterojunction, property modulation, and growth
    (2021-02-25) Yu, Henry; Yakobson, Boris I.
    The crowning achievements of modern electronics stems from decades of research on the fundamental physical, chemical and growth properties of silicon. Ever advancing efficiency in silicon wafer synthesis and our ability to further miniaturize transistors have given us exponential growth in computing speed over the past several decades, a trend famously termed as the Moore’s Law. With the transistor size approaching physical limits of silicon in the near future, the pressure of continuing Moore’s law have made 2D materials a promising candidate for next generation electronics, due to their amazing properties. One prominent feature of 2D materials is their low dimensionality, which causes weak electronic screening compared to 3D systems. This can render traditional device fabrication methods, such as forming contacts or carrier doping, useless, posing a potential challenge for future 2D electronics. Hence, in this thesis I will combine atomistic calculations and continuum models to reveal the new physics of, and propose new ways to build 2D electronic devices, utilizing their low dimensionality. First I will present an analysis on 2D coplanar metal-semiconductor (MS) contacts, which are ubiquitous in electronic devices. Weak electronic screening in low dimensions immediately implies that interface states should have weak effects on 2D MS contacts, in contrast to their 3D counterparts. Using a multi-scale model, I will use Gr|MoS2 as an example to show that the notorious Fermi level pinning is greatly suppressed in 2D MS contacts, due to low dimensionality. Apart from the new physics in 2D heterojunctions, weak screening also makes carrier doping challenging, one of the major means to modulate properties in traditional devices. Fortunately, the layered geometry of 2D materials make strain or defect engineering of properties very easy, through substrate engineering. In this regard, I will present an ab initio based framework to predict the elastic state of 2D crystals on general curved surfaces. Using this framework, I will demonstrate how to transform MoS2 and phosphorene into photonic devices by a nano-pillar surface. Also, as a more specific and exotic example, I will demonstrate how to pattern graphene to make “graphene straintronics”, creating valley current filters or surface tunable superconductors. Finally, for more general surface shapes, the interplay of surface curvature and material growth dynamics causes more drastic changes to the crystal, i.e. formation of defects. Exploiting this phenomenon to achieve defect engineering, I will present a new curved-space phase-field model for 2D material growth on curved surfaces. This model predicts material defect formation either due to the surface topological requirements or the release of elastic energy.
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    Strain tolerance of two-dimensional crystal growth on curved surfaces
    (AAAS, 2019) Wang, Kai; Puretzky, Alexander A.; Hu, Zhili; Srijanto, Bernadeta R.; Li, Xufan; Gupta, Nitant; Yu, Henry; Tian, Mengkun; Mahjouri-Samani, Masoud; Gao, Xiang; Oyedele, Akinola; Rouleau, Christopher M.; Eres, Gyula; Yakobson, Boris I.; Yoon, Mina; Xiao, Kai; Geohegan, David B.
    Two-dimensional (2D) crystal growth over substrate features is fundamentally guided by the Gauss-Bonnet theorem, which mandates that rigid, planar crystals cannot conform to surfaces with nonzero Gaussian curvature. Here, we reveal how topographic curvature of lithographically designed substrate features govern the strain and growth dynamics of triangular WS2 monolayer single crystals. Single crystals grow conformally without strain over deep trenches and other features with zero Gaussian curvature; however, features with nonzero Gaussian curvature can easily impart sufficient strain to initiate grain boundaries and fractured growth in different directions. Within a strain-tolerant regime, however, triangular single crystals can accommodate considerable (<1.1%) localized strain exerted by surface features that shift the bandgap up to 150 meV. Within this regime, the crystal growth accelerates in specific directions, which we describe using a growth model. These results present a previously unexplored strategy to strain-engineer the growth directions and optoelectronic properties of 2D crystals.