arXiv subjects

Zhenyu Li

Publications and source records attributed to Zhenyu Li.

At least 163 records · Page 9Linked to original sources

arXiv2018

Type-II Dirac photons at the metasurfaces

Topological characteristics of energy bands, such as Dirac/Weyl nodes, have attracted substantial interest in condensed matter systems as well as in classical wave systems. Among these energy bands, the type-II Dirac point is a nodal degeneracy with tilted conical dispersion, leading to a peculiar crossing dispersion in the constant energy plane. Such nodal points have recently been found in electronic materials. The analogous topological feature in photonic systems remains a theoretical curiosity, with experimental realization expected to be challenging. Here, we experimentally realize the type-II Dirac point using a planar metasurface architecture, where the band degeneracy point is protected by the underlying mirror symmetry of the metasurface. Gapless edge modes are found and measured at the boundary between the different domains of the symmetry-broken metasurface. Our work shows that metasurfaces are simple and practical platforms for realizing electromagnetic type-II Dirac points, and their planar structure is a distinct advantage that facilitates applications in two-dimensional topological photonics.

physics.app-ph↗

arXiv2018

Interfacial properties of black phosphorus/transition metal carbide van der Waals heterostructures

Owing to its outstanding electronic properties, black phosphorus (BP) is considered as a promising material for next-generation optoelectronic devices. In this work, devices based on BP/MXene (Zrn+1CnT2, T = O, F, OH, n = 1, 2) van der Waals (vdW) heterostructures are designed via first-principles calculations. Zrn+1CnT2 compositions with appropriate work functions lead to the formation of Ohmic contact with BP in the vertical direction. Low Schottky barriers are found along the lateral direction in BP/Zr2CF2, BP/Zr2CO2H2, BP/Zr3C2F2, and BP/Zr3C2O2H2 bilayers, and BP/Zr3C2O2 even exhibits Ohmic contact behavior. BP/Zr2CO2 is a semiconducting heterostructure with type-II band alignment, which facilitates the separation of electron-hole pairs. The band structure of BP/Zr2CO2 can be effectively tuned via a perpendicular electric field, and BP is predicted to undergo a transition from donor to acceptor at a 0.4 V/Å electric field. The versatile electronic properties of the BP/MXene heterostructures examined in this work highlight their promising potential for applications in electronics.

cond-mat.mtrl-sci↗

arXiv2017

Dominant Kinetic Pathways of Graphene Growth in Chemical Vapor Deposition: The Role of Hydrogen

The most popular way to produce graphene nowadays is chemical vapor deposition, where, surprisingly, H$_2$ gas is routinely supplied even though it is a byproduct itself. In this study, by identifying dominant growing pathways via multiscale simulations, we unambiguously reveal the central role hydrogen played in graphene growth. Hydrogen can saturate the edges of a growing graphene island to some extent, depending on the H$_2$ pressure. Although graphene etching by hydrogen has been observed in experiment, hydrogen saturation actually stabilizes graphene edges by reducing the detachment rates of carbon-contained species. Such a new picture well explains some puzzling experimental observations and is also instrumental in growth protocol optimization for two-dimensional atomic crystal van der Waals epitaxy.

cond-mat.mtrl-sci↗

arXiv2017

Steganalysis of 3D Objects Using Statistics of Local Feature Sets

3D steganalysis aims to identify subtle invisible changes produced in graphical objects through digital watermarking or steganography. Sets of statistical representations of 3D features, extracted from both cover and stego 3D mesh objects, are used as inputs into machine learning classifiers in order to decide whether any information was hidden in the given graphical object. According to previous studies, sets of local geometry features can be used to define the differences between stego and cover-objects. The features proposed in this paper include those representing the local object curvature, vertex normals, the local geometry representation in the spherical coordinate system and are considered in various combinations with others. We also analyze the effectiveness of various 3D feature sets applied for steganalysis based on the Pearson correlation coefficient. The classifiers proposed in this study for discriminating the 3D stego and cover-objects include Support Vector Machine and the Fisher Linear Discriminant ensemble. Three different watermarking and steganographic methods are used for hiding information in the 3D objects used for testing the performance of the proposed steganalysis methodology.

cs.CR↗

arXiv2016

Water on Silicene: Hydrogen Bond Autocatalysis Induced Physisorption-Chemisorption-Dissociation Transition

A single water molecule has nothing special. However, macroscopic water displays many anomalous properties at the interface, such as a high surface tension, hydrophobicity and hydrophillicity. Although the underlying mechanism is still elusive, hydrogen bond is expected to have played an important role. An interesting question is if the few-water molecule clusters will be qualitatively different from a single molecule. Using adsorption behavior as an example, by carefully choosing two-dimensional silicene as the substrate material, we demonstrate that water monomer, dimer and trimer show contrasting properties. The additional water molecules in dimer and trimer induce a transition from physisorption to chemisorption then to dissociation on silicene. Such a hydrogen bond autocatalytic effect is expected to have a broad application potential in silicene-based water molecule sensor and metal-free catalyst for water dissociation.

cond-mat.mtrl-sci↗

arXiv2016

Distinct Reconstruction Patterns and Spin-Resolved Electronic States along the Zigzag Edges of Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides represent an emerging class of materials exhibiting various intriguing properties, and integration of such materials for potential device applications will necessarily encounter creation of different boundaries. Using first-principles approaches, here we investigate the structural, electronic, and magnetic properties along two inequivalent zigzag M and X edges of MX$_{2}$ (M=Mo, W; X=S, Se). Along the M edges, we reveal a previously unrecognized but energetically strongly preferred (2x1) reconstruction pattern, which is universally operative for all the MX$_{2}$, characterized by a self-passivation mechanism through place exchanges of the outmost X and M edge atoms. In contrast, the X edges undergo a more moderate (2x1) or (3x1) reconstruction for MoX$_{2}$ or WX$_{2}$, respectively. We further use the prototypical zigzag MoX$_{2}$ nanoribbons to demonstrate that the M and X edges possess distinctly different electronic and magnetic properties, which are discussed for spintronic and catalytic applications.

cond-mat.mtrl-sci↗

arXiv2014

A Smartphone Controlled Handheld Microfluidic Liquid Handling System

Microfluidics and lab-on-a-chip technologies have made it possible to manipulate small volume liquids with unprecedented resolution, automation and integration. However, most current microfluidic systems still rely on bulky off-chip infrastructures such as compressed pressure sources, syringe pumps and computers to achieve complex liquid manipulation functions. Here, we present a handheld automated microfluidic liquid handling system controlled by a smartphone, which is enabled by combining elastomeric on-chip valves and a compact pneumatic system. As a demonstration, we show that the system can automatically perform all the liquid handling steps of a bead-based sandwich immunoassay on a multi-layer PDMS chip without any human intervention. The footprint of the system is 6 by 10.5 by 16.5cm, and the total weight is 829g including battery. Powered by a 12.8V 1500mAh Li battery, the system consumed 2.2W on average during the immunoassay and lasted for 8.7 hrs. This handheld microfluidic liquid handling platform is generally applicable to many biochemical and cell-based assays requiring complex liquid manipulation and sample preparation steps such as FISH, PCR, flow cytometry and nucleic acid sequencing. In particular, the integration of this technology with read-out biosensors may help enable the realization of the long-sought Tricorder-like handheld in-vitro diagnostic (IVD) systems.

physics.ins-det↗

arXiv2014

Bilayer Graphene Growth via a Penetration Mechanism

From both fundamental and technical points of view, a precise control of the layer number of graphene samples is very important. To reach this goal, atomic scale mechanisms of multilayer graphene growth on metal surfaces should be understood. Although it is a geometrically favorable pathway to transport carbon species to interface and then form a new graphene layer there, penetration of a graphene overlayer is not a chemically straightforward process. In this study, the possibility of different active species to penetrate a graphene overlayer on Cu(111) surface is investigated based on first principles calculations. It is found that carbon atom penetration can be realized via an atom exchange process, which leads to a new graphene growth mechanism. Based on this result, a bilayer graphene growth protocol is proposed to obtain high quality samples. Such a penetration possibility also provides a great flexibility for designed growth of graphene nanostructures.

cond-mat.mtrl-sci↗

arXiv2013

Orientation Sensitive Nonlinear Growth of Graphene: A Geometry-determined Epitaxial Growth Mechanism

Although the corresponding carbon-metal interactions can be very different, a similar nonlinear growth behavior of graphene has been observed for different metal substrates. To understand this interesting experimental observation, a multiscale $\lq\lq$standing-on-the-front" kinetic Monte Carlo study is performed. An extraordinary robust geometry effect is identified, which solely determines the growth kinetics and makes the details of carbon-metal interaction not relevant at all. Based on such a geometry-determined mechanism, epitaxial growth behavior of graphene can be easily predicted in many cases. As an example, an orientation-sensitive growth kinetics of graphene on Ir(111) surface has been studied. Our results demonstrate that lattice mismatch pattern at the atomic level plays an important role for macroscopic epitaxial growth.

cond-mat.mtrl-sci↗

arXiv2013

Assemblathon 2: evaluating de novo methods of genome assembly in three vertebrate species

Background - The process of generating raw genome sequence data continues to become cheaper, faster, and more accurate. However, assembly of such data into high-quality, finished genome sequences remains challenging. Many genome assembly tools are available, but they differ greatly in terms of their performance (speed, scalability, hardware requirements, acceptance of newer read technologies) and in their final output (composition of assembled sequence). More importantly, it remains largely unclear how to best assess the quality of assembled genome sequences. The Assemblathon competitions are intended to assess current state-of-the-art methods in genome assembly. Results - In Assemblathon 2, we provided a variety of sequence data to be assembled for three vertebrate species (a bird, a fish, and snake). This resulted in a total of 43 submitted assemblies from 21 participating teams. We evaluated these assemblies using a combination of optical map data, Fosmid sequences, and several statistical methods. From over 100 different metrics, we chose ten key measures by which to assess the overall quality of the assemblies. Conclusions - Many current genome assemblers produced useful assemblies, containing a significant representation of their genes, regulatory sequences, and overall genome structure. However, the high degree of variability between the entries suggests that there is still much room for improvement in the field of genome assembly and that approaches which work well in assembling the genome of one species may not necessarily work well for another.

q-bio.GN↗

arXiv2012

Hydrogenated Bilayer Wurtzite SiC Nanofilms: A Two-Dimensional Bipolar Magnetic Semiconductor Material

Recently, a new kind of spintronics materials, bipolar magnetic semiconductor (BMS), has been proposed. The spin polarization of BMS can be conveniently controlled by a gate voltage, which makes it very attractive in device engineering. Now, the main challenge is finding more BMS materials. In this article, we propose that hydrogenated wurtzite SiC nanofilm is a two-dimensional BMS material. Its BMS character is very robust under the effect of strain, substrate, or even a strong electric field. The proposed two-dimensional BMS material paves the way to use this promising new material in an integrated circuit.

cond-mat.mtrl-sci↗

arXiv2012

Bipolar Magnetic Semiconductors: A New Class of Spintronics Materials

Electrical control of spin polarization is very desirable in spintronics, since electric field can be easily applied locally in contrast with magnetic field. Here, we propose a new concept of bipolar magnetic semiconductor (BMS) in which completely spin-polarized currents with reversible spin polarization can be created and controlled simply by applying a gate voltage. This is a result of the unique electronic structure of BMS, where the valence and conduction bands possess opposite spin polarization when approaching the Fermi level. Our band structure and spin-polarized electronic transport calculations on semi-hydrogenated single-walled carbon nanotubes confirm the existence of BMS materials and demonstrate the electrical control of spin-polarization in them.

cond-mat.mtrl-sci↗

arXiv2012

A First-principles Prediction of Two-Dimensional Superconductivity in Pristine B2C Single layer

Based on first-principles lattice dynamics and electron-phonon coupling calculations, B2C sheet is predicted to be a two-dimensional (2D) phonon-mediated superconductor with a relatively high transition temperature (Tc). The electron-phonon coupling parameter calculated is 0.92, and it is mainly contributed by low frequency out-of-plane phonon modes and electronic states with a π character. When the Coulomb pseudopotential is set to 0.10, the estimated temperature Tc is 19.2 K. To be best of our knowledge, B2C is the first pristine 2D superconductor with a Tc higher than the boiling point of liquid helium.

cond-mat.mtrl-sci↗

arXiv2012

Ordered Semiconducting Nitrogen-Graphene Alloys

The interaction between substitutional nitrogen atoms in graphene is studied by performing first principles calculations. The nearest neighbor interaction between nitrogen dopants is highly repulsive because of the strong electrostatic repulsion between nitrogen atoms, which prevents the full phase separation in nitrogen doped graphene. Interestingly, there are two relatively stable nitrogen-nitrogen pairs due to the anisotropy charge redistribution induced by nitrogen doping. We reveal two stable semiconducting ordered N doped graphene structures C3N and C12N through the cluster expansion technique and particle swarm optimization method. In particular, C12N has a direct band gap of 0.98 eV. The heterojunctions between C12N and graphene nanoribbons might be promising organic solar cells.

cond-mat.mtrl-sci↗

arXiv2011

Atomistic Mechanisms of Nonlinear Graphene Growth on Ir Surface

As a two-dimensional material, graphene can be naturally obtained via epitaxial growth on a suitable substrate. Growth condition optimization usually requires an atomistic level understanding of the growth mechanism. In this article, we perform a mechanistic study about graphene growth on Ir(111) surface by combining first principles calculations and kinetic Monte Carlo (kMC) simulations. Small carbon clusters on the Ir surface are checked first. On terraces, arching chain configurations are favorable in energy and they are also of relatively high mobilities. At steps, some magic two-dimensional compact structures are identified, which show clear relevance to the nucleation process. Attachment of carbon species to a graphene edge is then studied. Due to the effect of substrate, at some edge sites, atomic carbon attachment becomes thermodynamically unfavorable. Graphene growth at these difficult sites has to proceed via cluster attachment, which is the growth rate determining step. Based on such an inhomogeneous growth picture, kMC simulations are made possible by successfully separating different timescales, and they well reproduce the experimentally observed nonlinear kinetics. Different growth rates and nonlinear behaviors are predicted for different graphene orientations, which is consistent with available experimental results. Importantly, as a phenomenon originated from lattice mismatch, inhomogeneity revealed in this case is expected to be quite universal and it should also make important roles in many other hetero-epitaxial systems.

cond-mat.mtrl-sci↗

arXiv2011

Diamondization of Graphene and Graphene-BN Bilayers: Chemical Functionalization and Electronic Structure Engineering

In this article, based on first-principles calculations, we systematically study functionalization induced diamonization of graphene bilayer and graphene-BN hybrid bilayer. With single-side functionalization, the diamondized structures are magnetic semiconductor. Interestingly, if both sides of the bilayer are functionalized, diamondization becomes spontaneous without a barrier. On the other hand, when the bottom layer of the bilayer graphene is replaced by a single hexagonal BN layer, the diamondized structure becomes nonmagnetic metal. The tunable electronic and magnetic properties pave new avenues to construct graphene-based electronics and spintronics devices.

cond-mat.mtrl-sci↗

arXiv2011

Oxygen molecule dissociation on carbon nanostructures with different types of nitrogen doping

Energy barrier of oxygen molecule dissociation on carbon nanotube or graphene with different types of nitrogen doping is investigated using density functional theory. The results show that the energy barriers can be reduced efficiently by all types of nitrogen doping in both carbon nanotubes and graphene. Graphite-like nitrogen and Stone-Wales defect nitrogen decrease the energy barrier more efficiently than pyridine-like nitrogen, and a dissociation barrier lower than 0.2 eV can be obtained. Higher nitrogen concentration reduces the energy barrier much more efficiently for graphite-like nitrogen. These observations are closely related to partial occupation of π* orbitals and change of work functions. Our results thus provide useful insights into the oxygen reduction reactions.

cond-mat.mtrl-sci↗

arXiv2011

First-Principles Thermodynamics of Graphene Growth on Cu Surface

Chemical vapor deposition (CVD) is an important method to synthesis grapheme on a substract. Recently, Cu becomes the most popular CVD substrate for graphene growth. Here, we combine electronic structure calculation, molecular dynamics simulation, and thermodynamics analysis to study the graphene growth process on Cu surface. As a fundamentally important but previously overlooked fact, we find that carbon atoms are thermodynamically unfavorable on Cu surface under typical experimental conditions. The active species for graphene growth should thus mainly be CHx instead of atomic carbon. Based on this new picture, the nucleation behavior can be understood, which explains many experimental observations and also provides us a guide to improve graphene sample quality.

cond-mat.mtrl-sci↗