arXiv subjects

Yongqing Cai

Publications and source records attributed to Yongqing Cai.

At least 91 records · Page 5Linked to original sources

arXiv2018

A First-Principles Study on the Adsorption of Small Molecules on Antimonene: Oxidation Tendency and Stability

Antimonene, a new group-VA 2D semiconducting material beyond phosphorene, was recently synthesized through various approaches and was shown to exhibit a good structural integrity in ambient conditions and various interesting properties. In this work, we perform systematical first-principles investigations on the interactions of antimonene with the small molecules CO, NO, NO2, H2O, O2, NH3 and H2. It is found that NO, NO2, H2O, O2, and NH3 serve as charge acceptors, while CO shows a negligible charge transfer. H2 acts as a charge donor to antimonene with the amount of charge transfer being ten times that of H2 on phosphorene. The interaction of the O2 molecule with antimonene is much stronger than that with phosphorene. Surprisingly, the kinetic barrier for the splitting of the O2 molecule on antimonene is low (~0.40 eV), suggesting that pristine antimonene may undergo oxidation in ambient conditions, especially at elevated temperatures. Fortunately, the acceptor role of H2O on antimonene, opposite to a donor role in phosphorene, helps to suppress further structural degradation of the oxidized antimonene by preventing the proton transfer between water molecules and oxygen species to form acids.

cond-mat.mtrl-sci↗

arXiv2017

Exploring the Charge Localization and Band Gap Opening of Borophene: A First-Principles Study

Recently synthesized two-dimensional (2D) boron, borophene, exhibits a novel metallic behavior rooted in the s-p orbital hybridization, distinctively different from other 2D materials such as sulfides/selenides and semi-metallic graphene. This unique feature of borophene implies new routes for charge delocalization and band gap opening. Herein, using first-principles calculations, we explore the routes to localize the carriers and open the band gap of borophene via chemical functionalization, ribbon construction, and defect engineering. The metallicity of borophene is found to be remarkably robust against H- and F-functionalization and the presence of vacancies. Interestingly, a strong odd-even oscillation of the electronic structure with width is revealed for H-functionalized borophene nanoribbons, while an ultra-high work function (~ 7.83 eV) is found for the F-functionalized borophene due to its strong charge transfer to the atomic adsorbates.

cond-mat.mtrl-sci↗

arXiv2017

Atomic-scale mechanisms of defect- and light-induced oxidation and degradation of InSe

Layered indium selenide (InSe), a new two-dimensional (2D) material with a hexagonal structure and semiconducting characteristic, is gaining increasing attention owing to its intriguing electronic properties. Here, by using first-principles calculations, we reveal that perfect InSe possesses a high chemical stability against oxidation, superior to MoS2. However, the presence of intrinsic Se vacancy (VSe) and light illumination can markedly affect the surface activity. In particular, the excess electrons associated with the exposed In atoms at the VSe site under illumination are able to remarkably reduce the dissociation barrier of O2 to ~0.2 eV. Moreover, at ambient conditions, the splitting of O2 enables the formation of substitutional (apical) oxygen atomic species, which further cause the trapping and subsequent rapid splitting of H2O molecules and ultimately the formation of hydroxyl groups. Our findings uncover the causes and underlying mechanisms of InSe surface degradation via the defect-photo promoted oxidations. Such results will be beneficial in developing strategies for the storage of InSe material and its applications for surface passivation with boron nitride, graphene or In-based oxide layers.

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

Strain and Water Effects on the Electronic Structure and Chemical Activity of In-Plane Graphene/Silicene Heterostructure

By using first-principles calculations, the electronic structure of planar and strained in-plane graphene/silicene heterostructure is studied. The heterostructure is found to be metallic in a strain range from -7% (compression) to +7% (tension). The effect of compressive/tensile strain on the chemical activity of the in-plane graphene/silicene heterostructure is examined by studying its interaction with the H2O molecule. It shows that compressive/tensile strain is able to increase the binding energy of H2O compared with the adsorption on a planar surface, and the charge transfer between the water molecule and the graphene/silicene sheet can be modulated by strain. Moreover, the presence of the boron-nitride (BN)-substrate significantly influences the chemical activity of the graphene/silicene heterostructure upon its interaction with the H2O molecule and may cause an increase/decrease of the charge transfer between the H2O molecule and the heterostructure. These findings provide insights into the modulation of electronic properties of the in-plane free-standing/substrate-supported graphene/silicene heterostructure, and render possible ways to control its electronic structure, carrier density and redox characteristics, which may be useful for its potential applications in nanoelectronics and gas sensors.

cond-mat.mtrl-sci↗

arXiv2016

Large Electronic Anisotropy and Enhanced Chemical Activity of Highly Rippled Phosphorene

We investigate the electronic structure and chemical activity of rippled phosphorene induced by large compressive strains via first-principles calculation. It is found that phosphorene is extraordinarily bendable, enabling the accommodation of ripples with large curvatures. Such highly rippled phosphorene shows a strong anisotropy in electronic properties. For ripples along the armchair direction, the band gap changes from 0.84 to 0.51 eV for the compressive strain up to -20% and further compression shows no significant effect, for ripples along the zigzag direction, semiconductor to metal transition occurs. Within the rippled phosphorene, the local electronic properties, such as the modulated band gap and the alignments of frontier orbitals, are found to be highly spatially dependent, which may be used for modulating the injection and confinement of carriers for optical and photovoltaic applications. The examination of the interaction of a physisorbed NO molecule with the rippled phosphorene under different compressive strains shows that the chemical activities of the phosphorene are significantly enhanced at the top and bottom peaks of the ripples, indicated by the enhanced adsorption and charge transfer between them. All these features can be ascribed to the effect of curvatures, which modifies the orbital coupling between atoms at the ripple peaks.

cond-mat.mtrl-sci↗

arXiv2016

The Role of H2O and O2 Molecules and Phosphorus Vacancies in the Structure Instability of Phosphorene

The poor structural stability of phosphorene in air was commonly ascribed to humidity and oxygen molecules. Recent exfoliation of phosphorene in deoxygenated water promotes the need to re-examine the role of H2O and O2 molecules. Considering the presence of high population of vacancies in phosphorene, we investigate the interaction of H2O and O2 molecules with vacancy-contained phosphorene using first-principles calculations. In contrast to the common notion that physisorbed molecules tend to have a stronger adsorption at vacancy sites, we show that H2O has nearly the same adsorption energy at the vacancy site as that at the perfect one. Charge transfer analysis shows that O2 is a strong electron scavenger, which transfers the lone-pair electrons of the phosphorus atoms to the 2π* antibonding orbital of O2. As a result, the barrier for the O-O bond splitting to form O-P bonds is reduced from 0.81 eV at the perfect site to 0.59 eV at the defect site, leading to an about 5000 faster oxidizing rate at the defect site than at the perfect site at room temperature. Hence, our work reveals that the vacancy in phosphorene shows a stronger oxygen affinity than the perfect phosphorene lattice site. Structural degradation of phosphorene due to oxidization may occur rapidly at edges and grain boundaries where vacancies tend to agglomerate.

cond-mat.mtrl-sci↗

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

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↗

arXiv2015

Exciton-dominated Dielectric Function of Atomically Thin MoS2 Films

We systematically measure the dielectric function of atomically thin MoS2 films with different layer numbers and demonstrate that excitonic effects play a dominant role in the dielectric function when the films are less than 5-7 layers thick. The dielectric function shows an anomalous dependence on the layer number. It decreases with the layer number increasing when the films are less than 5-7 layers thick but turns to increase with the layer number for thicker films. We show that this is because the excitonic effect is very strong in the thin MoS2 films and its contribution to the dielectric function may dominate over the contribution of the band structure. We also extract the value of layer-dependent exciton binding energy and Bohr radius in the films by fitting the experimental results with an intuitive model. The dominance of excitonic effects is in stark contrast with what reported at conventional materials whose dielectric functions are usually dictated by band structures. The knowledge of the dielectric function may enable capabilities to engineer the light-matter interactions of atomically thin MoS2 films for the development of novel photonic devices, such as metamaterials, waveguides, light absorbers, and light emitters.

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↗

arXiv2015

Direction dependent thermal conductivity of monolayer phosphorene: parameterization of Stillinger-Weber potential and molecular dynamics study

A Stillinger-Weber interatomic potential is parameterized for phosphorene. It well reproduces the crystal structure, cohesive energy and phonon dispersion predicted by first-principles calculations. The thermal conductivity of phosphorene is further explored by equilibrium molecular dynamics simulations adopting the optimal set of potential parameters. At room temperature, the intrinsic thermal conductivities along zigzag and armchair directions are about 152.7 and 33.0 W/mK, respectively, with a large anisotropy ratio of five. The remarkably directional dependence of thermal conductivity in phosphorene, consistent with previous reports, is mainly due to the strong anisotropy of phonon group velocities, and weak anisotropy of phonon lifetimes as revealed by lattice dynamics calculations. Moreover, the effective phonon mean free paths at zigzag and armchair directions are about 141.4 and 43.4nm, respectively.

cond-mat.mes-hall↗

arXiv2015

Giant Phononic Anisotropy and Unusual Anharmonicity of Phosphorene: Interlayer Coupling and Strain Engineering

Phosphorene, an emerging elemental two-dimensional (2D) direct band gap semiconductor with fascinating structural and electronic properties distinctively different from other 2D materials such as graphene and MoS2, is promising for novel nanoelectronic and optoelectronic applications. Phonons, as one of the most important collective excitations, are at the heart for the device performance, as their interactions with electrons and photons govern the carrier mobility and light-emitting efficiency of the material. Here, through a detailed first-principles study, it is demonstrated that monolayer phosphorene exhibits a giant phononic anisotropy, and remarkably, this anisotropy is squarely opposite to its electronic counterpart and can be tuned effectively by strain engineering. By sampling the whole Brillouin zone for the mono-layer phosphorene, several "hidden" directions are found, along which small-momentum phonons are "frozen" with strain and possess the smallest degree of anharmonicity. Unexpectedly, these "hidden" directions are intrinsically different from those usually studied armchair and zigzag directions. Light is also shed on the anisotropy of interlayer coupling of few-layer phosphorene by examining the rigid-layer vibrations. These highly anisotropic and strain-tunable characteristics of phosphorene offer new possibilities for its applications in thermal management, thermoelectronics, nanoelectronics and phononics.

cond-mat.mtrl-sci↗

arXiv2015

Energetics, Charge Transfer and Magnetism of Small Molecules Physisorbed on Phosphorene

First-principles calculations are performed to investigate the interaction of physisorbed small molecules, including CO, H2, H2O, NH3, NO, NO2, and O2, with phosphorene, and their energetics, charge transfer, and magnetic moment are evaluated on the basis of dispersion corrected density functional theory. Our calculations reveal that CO, H2, H2O and NH3 molecules act as a weak donor, whereas O2 and NO2 act as a strong acceptor. While NO molecule donates electrons to graphene, it receives electrons from phosphorene. Among all the investigated molecules, NO2 has the strongest interaction through hybridizing its frontier orbitals with the 3p orbital of phosphorene. The nontrivial and distinct charge transfer occurring between phosphorene and these physisorbed molecules not only renders phosphorene promising for application as a gas sensor, but also provides an effective route to modulating the polarity and density of carriers in phosphorene. In addition, the binding energy of H2 on phosphorene is found to be 0.13 eV/H2, indicating that phosphorene is suitable for both stable room-temperature hydrogen storage and its subsequent facile release.

cond-mat.mtrl-sci↗

arXiv2014

Microstructural Evolution of Charged Defects in the Fatigue Process of Polycrystalline BiFeO3 Thin Films

Fatigue failure in ferroelectrics has been intensively investigated in the past few decades. Most of the mechanisms discussed for ferroelectric fatigue have been built on the "hypothesis of variation in charged defects", which however are rarely evidenced by experimental observation. Here, using a combination of complex impedance spectra techniques, piezoresponse force microscopy and first-principles theory, we examine the microscopic evolution and redistribution of charged defects during the electrical cycling in BiFeO3 thin films. The dynamic formation and melting behaviors of oxygen vacancy (VO) order are identified during the fatigue process. It reveals that the isolated VO tend to self-order along grain boundaries to form a planar-aligned structure, which blocks the domain reversals. Upon further electrical cycling, migration of VO within vacancy clusters is accommodated with a lower energy barrier (~0.2 eV) and facilitates the formation of nearby-electrode layer incorporated with highly concentrated VO. The interplay between the macroscopic fatigue and microscopic evolution of charged defects clearly demonstrates the role of ordered VO cluster in the fatigue failure of BiFeO3 thin films.

cond-mat.mtrl-sci↗

arXiv2014

Layer-dependent Band Alignment and Work Function of Few-Layer Phosphorene

Using first-principles calculations, we study the electronic properties of few-layer phosphorene focusing on layer-dependent behavior of band gap, work function and band alignment and carrier effective mass. It is found that few-layer phosphorene shows a robust direct band gap character, and its band gap decreases with the number of layers following a power law. The work function decreases rapidly from monolayer (5.16 eV) to trilayer (4.56 eV), and then slowly upon further increasing the layer number. Compared to monolayer phosphorene, there is a drastic decrease of hole effective mass along the ridge (zigzag) direction for bilayer phosphorene, indicating a strong interlayer coupling and screening effect. Our study suggests that 1). Few-layer phosphorene with a layer-dependent band gap and a robust direct band gap character is promising for efficient solar energy harvest. 2). Few-layer phosphorene outperforms monolayer counterpart in terms of a lighter carrier effective mass, a higher carrier density and a weaker scattering due to enhanced screening. 3). The layer-dependent band edges and work functions of few-layer phosphorene allow for modification of Schottky barrier with enhanced carrier injection efficiency. It is expected that few-layer phosphorene will present abundant opportunities for a plethora of new electronic applications.

cond-mat.mtrl-sci↗