arXiv subjects

Gang Zhang

Publications and source records attributed to Gang Zhang.

At least 127 records · Page 7Linked to original sources

arXiv2017

The Best Features of Diamond Nanothread for Nanofiber Application

Carbon fibers, especially those constructed from carbon nanotubes (CNTs), have attracted intensive interests from both scientific and engineering communities due to their outstanding physical properties. In this workHere we report, we find that the recently synthesized recently synthesized ultrathin diamond nanothread (DNT) not only possesses excellent torsional deformation capability, but also has excellent interfacial load transfer efficiency. Comparing with the (10,10) carbon nanotube bundles, (1) By considering a seven strand fiber, the flattening of nanotubes as observed in (10,10) CNT bundles is not observed in DNTdiamond nanothread bundle, which . This endows the DNT bundle withleads to a high torsional elastic limit that is almost three times higherfour times as obtained from the (10,10) CNT bundle. Pull-out tests reveal that (2) Pull-out tests reveal that tthe DNT diamond nanothread bundle has has an interface transfer load of more than twice that of the CNT carbon naontube bundle, corresponding to an order of magnitude higher in terms of . (3) Tthe interfacial shear strength of the DNT bundle is an order of magnitude higher than that of the CNT bundle. Such high interface load transfer efficiency is attributed to the zigzag morphology of DNT, which introduces a strong mechanical interlocking effect at the interface through the stick-slip motion (totally different from the load transfer mechanism in CNT bundle). The aforementioned three aspects are commonly used to gauge the performance of fibers. Obviously,These intriguing features enable DNT diamond nanothread as exhibits excellent potential candidate for constructing next generation carbon fibers.

cond-mat.mtrl-sci↗

arXiv2017

Superior lattice thermal conductance of single layer borophene

By way of the nonequilibrium Green's function simulations and first principles calculations, we report that borophene, a single layer of boron atoms that was fabricated recently, possesses an extraordinarily high lattice thermal conductance in the ballistic transport regime, which even exceeds graphene. In addition to the obvious reasons of light mass and strong bonding of boron atoms, the superior thermal conductance is mainly rooted in its strong structural anisotropy and unusual phonon transmission. For low-frequency phonons, the phonon transmission within borophene is nearly isotropic, similar to that of graphene. For high frequency phonons, however, the transmission is one dimensional, that is, all the phonons travel in one direction, giving rise to its ultrahigh thermal conductance. The present study suggests that borophene is promising for applications in efficient heat dissipation and thermal management, and also an ideal material for revealing fundamentals of dimensionality effect on phonon transport in ballistic regime.

cond-mat.mtrl-sci↗

arXiv2017

Charge Transfer and Functionalization of Monolayer InSe by Physisorption of Small Molecules for Gas Sensing

First-principles calculations are performed to investigate the effects of the adsorption of gas molecules (CO, NO, NO2, H2S, N2, H2O, O2, NH3 and H2) on the electronic properties of atomically thin indium selenium (InSe). Our study shows that the lone-pair states of Se are located at the top of the valence band of InSe and close to the Fermi energy level, implying its high sensitivity to external adsorbates. Among these gas molecules, H2 and H2S are strong donors, NO, NO2, H2O and NH3 are effective acceptors, while CO and N2 exhibit negligible charge transfer. The O2 molecule has very limited oxidizing ability and a relatively weak interaction with InSe which is comparable to the N2 adsorption. A clear band gap narrowing is found for the H2S, NO2, and NH3 adsorbed systems whereas a Fermi level shifting to the conduction band is observed upon a moderate uptake of H2 molecules. Our analysis suggests several interesting applications of InSe: 1) Due to the different interaction behaviors with these external molecules, InSe can be used for gas sensing applications; 2) By monitoring the adsorption/desorption behavior of these gas molecules, the population of hole states in InSe due to photon stimulation or defect production can be quantitatively estimated; and 3) It is promising for novel electronic and optoelectronic applications since the adsorption-induced in-gap states and strong charge transfer are able to change the content and polarity of charged carriers and lead to different optical properties.

cond-mat.mtrl-sci↗

arXiv2017

MoS2-graphene in-plane contact for high interfacial thermal conduction

Recent studies showed that the in-plane and inter-plane thermal conductivities of two-dimensional (2D) MoS2 are low, posing a significant challenge in heat management in MoS2-based electronic devices. To address this challenge, we design the interfaces between MoS2 and graphene by fully utilizing graphene, a 2D material with an ultra-high thermal conduction. We first perform ab initio atomistic simulations to understand the bonding nature and structure stability of the interfaces. Our results show that the designed interfaces, which are found to be connected together by strong covalent bonds between Mo and C atoms, are energetically stable. We then perform molecular dynamics simulations to investigate the interfacial thermal conductance. It is found surprisingly that the interface thermal conductance is high, comparable to that of graphene-metal covalent-bonded interfaces. Importantly, each interfacial Mo-C bond serves as an independent thermal channel, enabling the modulation of interfacial thermal conductance by controlling Mo vacancy concentration at the interface. The present work provides a viable route for heat management in MoS2 based electronic devices.

cond-mat.mtrl-sci↗

arXiv2017

A scalar photon theory for near-field radiative heat transfer

We study a one-dimensional model of radiative heat transfer for which the effect of the electromag- netic field is only from the scalar potential and thereby ignoring the vector potential contribution. This is a valid assumption when the distances between objects are of the order of nanometers. Using Lorenz gauge, the scalar field is quantized with the canonical quantization scheme, giving rise to scalar photons. In the limit as the speed of light approaches infinity, the theory reduces to a pure Coulomb interaction governed by the Poisson equation. The model describes very well parallel plate capacitor physics, where a new length scale related to its capacitance emerges. Shorter than this length scale we see greater radiative heat transfer. This differs markedly from the usual Rytov fluctuational electrodynamics theory in which the enhancement is due to evanescent modes shorter than the thermal wavelengths. Our theory may explain recent experiments where charge fluctuations instead of current fluctuations play a dominant role in radiative heat transfer. Finally, due to the asymmetric electron-bath couplings, thermal rectification effects are also observed and reported.

cond-mat.mes-hall↗

arXiv2017

Phosphorene Edge Reconstruction by Self-Rolling

Edge atomic configuration often plays an important role in dictating the properties of finite-sized two-dimensional (2D) materials. By performing ab initio calculations, we identify a highly stable zigzag edge of phosphorene, which is the most stable one among all the considered edges. Surprisingly, this highly stable edge exhibits a novel nanotube-like structure, which is topologically distinctively different from any previously reported edge reconstruction. We further show that this new edge type can form easily, with an energy barrier of only 0.234 eV. It may be the dominant edge type at room temperature in vacuum condition or even under low hydrogen gas pressure. The calculated band structure reveals that the reconstructed edge possesses a bandgap of 1.23 eV. It is expected that this newly found edge structure may stimulate more studies in uncovering other novel edge types and further exploring their practical applications.

cond-mat.mtrl-sci↗

arXiv2017

Analyzing the Carrier Mobility in Transition-metal Dichalcogenide MoS2 Field-effect Transistors

Transition-metal dichalcogenides (TMDCs) are important class of two-dimensional (2D) layered materials for electronic and optoelectronic applications, due to their ultimate body thickness, sizable and tunable bandgap, and decent theoretical room-temperature mobility of hundreds to thousands cm2/Vs. So far, however, all TMDCs show much lower mobility experimentally because of the collective effects by foreign impurities, which has become one of the most important limitations for their device applications. Here, taking MoS2 as an example, we review the key factors that bring down the mobility in TMDC transistors, including phonons, charged impurities, defects, and charge traps. We introduce a theoretical model that quantitatively captures the scaling of mobility with temperature, carrier density and thickness. By fitting the available mobility data from literature over the past few years, we are able to obtain the density of impurities and traps for a wide range of transistor structures. We show that interface engineering such as oxide surface passivation, high-k dielectrics and BN encapsulation could effectively reduce the impurities, leading to improved device performances. For few-layer TMDCs, we analytically model the lopsided carrier distribution to elucidate the experimental increase of mobility with the number of layers. From our analysis, it is clear that the charge transport in TMDC samples is a very complex problem that must be handled carefully. We hope that this Review can provide new insights and serve as a starting point for further improving the performance of TMDC transistors.

cond-mat.mtrl-sci↗

arXiv2016

Phonon transport in a one-dimensional harmonic chain with long-range interaction and mass disorder

Atomic mass and interatomic interaction are the two key quantities that significantly affect the heat conduction carried by phonons. Here, we study the effects of long-range (LR) interatomic interaction and mass-disorder on the phonon transport in a one-dimensional harmonic chain withup to 10^5 atoms. We find that while LR interaction reduces the transmission of low frequency phonons, it enhances the transmission of high frequency phonons by suppressing the localization effects caused by mass disorder. Therefore, long-range interaction is able to boost heat conductance in the high temperature regime or in the large size regime, where the high frequency modes are important.

cond-mat.mes-hall↗

arXiv2016

Theory of substrate-directed heat dissipation for single-layer graphene and other two-dimensional crystals

We present a theory of the phononic thermal (Kapitza) resistance at the interface between graphene or another single-layer two-dimensional (2D) crystal (e.g. MoS$_{2}$) and a flat substrate, based on a modified version of the cross-plane heat transfer model by Persson, Volokitin and Ueba [J. Phys.: Condens. Matter 23, 045009 (2011)]. We show how intrinsic flexural phonon damping is necessary for obtaining a finite Kapitza resistance and also generalize the theory to encased single-layer 2D crystals with a superstrate. We illustrate our model by computing the thermal boundary conductance (TBC) for bare and SiO$_{2}$-encased single-layer graphene and MoS$_{2}$ on a SiO$_{2}$ substrate, using input parameters from first-principles calculation. The estimated room temperature TBC for bare (encased) graphene and MoS$_{2}$ on SiO$_{2}$ are 34.6 (105) and 3.10 (5.07) MWK$^{-1}$m$^{-2}$, respectively. The theory predicts the existence of a phonon frequency crossover point, below which the low-frequency flexural phonons in the bare 2D crystal do not dissipate energy efficiently to the substrate. We explain within the framework of our theory how the encasement of graphene with a top SiO$_{2}$ layer introduces new low-frequency transmission channels which significantly reduce the graphene-substrate Kapitza resistance. We emphasize that the distinction between bare and encased 2D crystals must be made in the analysis of cross-plane heat dissipation to the substrate.

cond-mat.mes-hall↗

arXiv2016

The Critical Role of Substrate in Stabilizing Phosphorene Nanoflake: A Theoretical Exploration

Phosphorene, a new two-dimensional (2D) semiconductor, has received much interest due to its robust direct band gap and high charge mobility. Currently, however, phosphorene can only be produced by mechanical or liquid exfoliation, and it is still a significant challenge to directly epitaxially grow phosphorene, which greatly hinders its mass production and, thus, applications. In epitaxial growth, the stability of nanoscale cluster or flake on a substrate is crucial. Here, we perform ab initio energy optimizations and molecular dynamics simulations to explore the critical role of substrate on the stability of a representative phosphorene flake. Our calculations show that the stability of the phosphorene nanoflake is strongly dependent on the interaction strength between the nanoflake and substrate. Specifically, the strong interaction (0.75 eV/P atom) with Cu(111) substrate breaks up the phosphorene nanoflake, while the weak interaction (0.063 eV/P atom) with h-BN substrate fails to stabilize its 2D structure. Remarkably, we find that a substrate with a moderate interaction (about 0.35 eV/P atom) is able to stabilize the 2D characteristics of the nanoflake on a realistic time scale. Our findings here provide useful guidelines for searching suitable substrates for the directly epitaxial growth of phosphorene.

cond-mat.mtrl-sci↗

arXiv2016

Highly Itinerant Atomic Vacancies in Phosphorene

Using detailed first-principles calculations, we investigate the hopping rate of vacancies in phosphorene, an emerging elemental 2D material besides graphene. Our work predicts that a direct observation of these mono-vacancies (MVs), showing a highly mobile and anisotropic motion, is possible only at low temperatures around 70 K or below where the thermal activity is greatly suppressed. At room temperature, the motion of a MV is sixteen orders faster than that in graphene, because of the low diffusion barrier of 0.3 eV. Built-in strain associated with the vacancies extends far along the zigzag direction while attenuating rapidly along the armchair direction. We reveal new features of the motion of di-vacancies (DVs) in phosphorene via multiple dissociation-recombination processes of vacancies owing to a small energy cost of ~ 1.05 eV for the splitting of a DV into two MVs. Furthermore, we find that uniaxial tensile strain along the zigzag direction can promote the motion of MVs, while the tensile strain along the armchair direction has the opposite effect. These itinerant features of vacancies, rooted in the unique puckering structure facilitating bond reorganization, enable phosphorene to be a bright new opportunity to broaden the knowledge of the evolution of vacancies, and a proper control of the exceedingly active and anisotropic movement of the vacancies should be critical for applications based on phosphorene.

cond-mat.mtrl-sci↗

arXiv2016

Quantum thermal transport in stanene

By way of the non-equilibrium Green's function simulations and analytical expressions, the quantum thermal conductance of stanene is studied. We find that, due to the existence of Dirac fermion in stanene, the ratio of electron thermal conductance and electric conductance becomes a chemical-potential-dependent quantity, violating the Wiedemann-Franz law. This finding is applicable to any two-dimensional (2D) materials that possess massless Dirac fermions. In strong contrast to the negligible electronic contribution in graphene, surprisingly, the electrons and phonons in stanene carry a comparable heat current. The unusual behaviours in stanene widen our knowledge of quantum thermal transport in 2D materials.

cond-mat.mtrl-sci↗

arXiv2016

From Brittle to Ductile: A Structure Dependent Ductility of Diamond Nanothread

As a potential building block for the next generation of devices or multifunctional materials that are spreading almost every technology sector, one-dimensional (1D) carbon nanomaterial has received intensive research interests. Recently, a new ultra-thin diamond nanothread (DNT) has joined this palette, which is a 1D structure with poly-benzene sections connected by Stone-Wales (SW) transformation defects. Using large-scale molecular dynamics simulations, we found that this sp3 bonded DNT can transit from a brittle to a ductile characteristic by varying the length of the poly-benzene sections, suggesting that DNT possesses entirely different mechanical responses than other 1D carbon allotropies. Analogously, the SW defects behave like a grain boundary that interrupts the consistency of the poly-benzene sections. For a DNT with a fixed length, the yield strength fluctuates in the vicinity of a certain value and is independent of the "grain size". On the other hand, both yield strength and yield strain show a clear dependence on the total length of DNT, which is due to the fact that the failure of the DNT is dominated by the SW defects. Its highly tunable ductility together with its ultra-light density and high Young's modulus makes diamond nanothread ideal for creation of extremely strong three-dimensional nano-architectures.

cond-mat.mtrl-sci↗

arXiv2015

Tunable interaction of superconducting flux qubits in circuit QED

We propose to implement tunable interaction of superconducting flux qubits with cavity-assisted interaction and strong driving. The qubits have a three-level Lambda configuration, and the decay of the excited state will be greatly suppressed due to the effective large detuning. The implemented interaction is insensitive to the cavity field state and can be controlled by modulating the phase difference of the driving fields of the qubits. In particular, our scheme is based on the typical circuit QED setup and thus will provide a simple method towards the tunable interaction of superconducting qubits. Finally, we consider the generation of two and four qubits entangled states with the constructed interaction under the influence of typical decoherence effects.

quant-ph↗

arXiv2015

Thermal conductivity of a new carbon nanotube analogue: the diamond nanothread

Based on the non-equilibrium molecular dynamics simulations, we have studied the thermal conductivities of a novel ultra-thin one-dimensional carbon nanomaterial - diamond nanothread (DNT). Unlike single-wall carbon nanotube (CNT), the existence of the Stone-Wales transformations in DNT endows it with richer thermal transport characteristics. There is a transition from wave-dominated to particle-dominated transport region, which depends on the length of poly-benzene rings. However, independent of the transport region, strong length dependence in thermal conductivity is observed in DNTs with different lengths of poly-benzene ring. The distinctive SW characteristic in DNT provides more degrees of freedom to tune the thermal conductivity not found in the homogeneous structure of CNT. Therefore, DNT is an ideal platform to investigate various thermal transport mechanisms at the nanoscale. Its high tunability raises the potential to design DNTs for different applications, such as thermal connection and temperature management.

cond-mat.mtrl-sci↗

arXiv2015

Realization of Room-Temperature Phonon-limited Carrier Transport in Monolayer MoS2 by Dielectric and Carrier Screening

We show that by combining high-k dielectric substrate and high density of charge carriers, Coulomb impurity can be effectively screened, leading to an unprecedented room-temperature mobility of ~150cm2/Vs in monolayer MoS2. The high sample quality enables us to quantitatively extract the mobility components limited by Coulomb impurities, intrinsic and surface optical phonons, and study their scaling with temperature, carrier density and dielectric constant. The excellent agreement between our theoretical analysis and experimental data demonstrates unambiguously that room-temperature phonon-limited transport is achieved in monolayer MoS2, which is a necessary factor for electronic device applications.

cond-mat.mtrl-sci↗

arXiv2015

High-Performance Monolayer WS2 Field-effect Transistors on High-k Dielectrics

The combination of high-quality Al2O3 dielectric and thiol chemistry passivation can effectively reduce the density of interface traps and Coulomb impurities of WS2, leading to a significant improvement of the mobility and a transition of the charge transport from the insulating to the metallic regime. A record high mobility of 83 cm2/Vs (337 cm2/Vs) is reached at room temperature (low temperature) for monolayer WS2. A theoretical model for electron transport is also developed.

cond-mat.mtrl-sci↗

arXiv2015

The Electronic Properties of Phosphorene/Graphene and Phosphorene/Hexagonal Boron Nitride Heterostructures

Vertical integration of two-dimensional materials has recently emerged as an exciting method for the design of novel electronic and optoelectronic devices. Using density functional theory, we investigatethe structural and electronic properties of two heterostruc-tures, graphene/phosphorene (G/BP) and hexagonal boron nitride/phosphorene (BN/BP). We found that the interlayer distance, binding energy, and charge transfer in G/BP and BN/BP are similar. Interlayer noncovalentbonding is predicted due to the weak coupling between the pz orbital of BP and the π orbital of graphene and BN. A small amount of electron transfer from graphene and BN, scaling with the vertical strain, renders BP slightly n-doped for both heterostructures. Several attractive characteristics of BP, including direct band gap and linear dichroism, are preserved. However, a large redistribution of electrostatic potential across the interface is observed, which may significantly renormalize the carrier dynamics and affect the excitonic behavior of BP. Our work suggests that graphene and BN can be used not only as an effective capping layer to protect BP from its structural and chemical degradation while still maintain its major electronic characteristics, but also as an active layer to tune the carrier dynamics and optical properties of BP.

cond-mat.mtrl-sci↗