Ping Sheng

The Hong Kong University of Science and Technology, Chiu-lung, Kowloon City, Hong Kong

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Publications (351)1415.72 Total impact

  • Maijia Liao · Li Wan · Shixin Xu · Chun Liu · Ping Sheng
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    ABSTRACT: The Poisson Boltzmann equation is known for its success in describing the Debye layer that arises from the charge separation phenomenon at the silica/water interface. However, by treating only the mobile ionic charges in the liquid, the Poisson Boltzmann equation accounts for only half of the electrical double layer, with the other half, the surface charge layer, being beyond its computational domain. In this work, we take a holistic approach to the charge separation phenomenon at the silica/water interface by treating, within a single computational domain, the electrical double layer that comprises both the mobile ions in the liquid and the surface charge density. The Poisson Nernst Planck equations are used as the rigorous basis for our methodology. The holistic approach has the advantage of being able to predict surface charge variations that arise either from the addition of salt and acid to the liquid, or from the decrease of the liquid channel width to below twice the Debye length. As the electrical double layer must be overall neutral, we use this constraint to derive both the form of the static limit of the Poisson Nernst Planck equations, as well as a global chemical potential that replaces the classical zeta potential as the boundary value for the PB equation, which can be re-derived from our formalism. We present several predictions of our theory that are beyond the framework of the PB equation alone, e.g., the surface capacitance and the so-called pK and pL values, the isoelectronic point at which the surface charge layer is neutralized, and the appearance of a Donnan potential that arises from the formation of an electrical double layer at the inlet regions of a nano-channel connected to the bulk reservoir. All theory predictions are shown to be in good agreement with the experimental observations.
    No preview · Article · Jan 2016
  • Shuyu Chen · Han Wang · Tiezheng Qian · Ping Sheng
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    ABSTRACT: The lack of a first-principles derivation has made the hydrodynamic boundary condition a classical issue for the past century. The fact that the fluid can have interfacial structures adds additional complications and ambiguities to the problem. Here we report the use of molecular dynamics to identify from equilibrium thermal fluctuations the hydrodynamic modes in a fluid confined by solid walls, thereby extending the application of the fluctuation-dissipation theorem to yield not only the accurate location of the hydrodynamic boundary at the molecular scale, but also the relevant parameter value(s) for the description of the macroscopic boundary condition. We present molecular dynamics results on two examples to illustrate the application of this approach - one on the hydrophilic case and one on the hydrophobic case. It is shown that the use of the orthogonality condition of the modes can uniquely locate the hydrodynamic boundary to be inside the fluid in both cases, separated from the molecular solid-liquid interface by a small distance Δ that is a few molecules in size. The eigenvalue equation of the hydrodynamic modes directly yields the slip length, which is about equal to Δ in the hydrophilic case but is larger than Δ in the hydrophobic case. From the decay time we also obtain the bulk viscosity which is in good agreement with the value obtained from dynamic simulations. To complete the picture, we derive the Green-Kubo relation for a finite fluid system and show that the boundary fluctuations decouple from the bulk only in the infinite-fluid-channel limit; and in that limit we recover the interfacial fluctuation-dissipation theorem first presented by Bocquet and Barrat. The coupling between the bulk and the boundary fluctuations provides both the justification and the reason for the effectiveness of the present approach, which promises broad utility for probing the hydrodynamic boundary conditions relevant to structured or elastic interfaces, as well as two-phase immiscible flows. © 2015 American Physical Society.
    No preview · Article · Oct 2015 · Physical Review E
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    Min Yang · Chong Meng · Caixing Fu · Yong Li · Zhiyu Yang · Ping Sheng
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    ABSTRACT: We report the experimental realization of perfect sound absorption by sub-wavelength monopole and dipole resonators that exhibit degenerate resonant frequencies. This is achieved through the destructive interference of two resonators' transmission responses, while the matching of their averaged impedances to that of air implies no backscattering, thereby leading to total absorption. Two examples, both using decorated membrane resonators (DMRs) as the basic units, are presented. The first is a flat panel comprising a DMR and a pair of coupled DMRs, while the second one is a ventilated short tube containing a DMR in conjunction with a sidewall DMR backed by a cavity. In both examples, near perfect absorption, up to 99.7%, has been observed with the airborne wavelength up to 1.2 m, which is at least an order of magnitude larger than the composite absorber. Excellent agreement between theory and experiment is obtained.
    Full-text · Article · Sep 2015 · Applied Physics Letters
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    ABSTRACT: We derive and numerically demonstrate that perfect absorption of elastic waves can be achieved in two types of ultra-thin elastic meta-films: one requires a large value of almost pure imaginary effective mass density and a free space boundary, while the other requires a small value of almost pure imaginary effective modulus and a hard wall boundary. When the pure imaginary density or modulus exhibits certain frequency dispersions, the perfect absorption effect becomes broadband, even in the low frequency regime. Through a model analysis, we find that such almost pure imaginary effective mass density with required dispersion for perfect absorption can be achieved by elastic metamaterials with large damping. Our work provides a feasible approach to realize broadband perfect absorption of elastic waves in ultra-thin films.
    Full-text · Article · Jul 2015 · Scientific Reports
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    Songwen Xiao · Guancong Ma · Yong Li · Zhiyu Yang · Ping Sheng
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    ABSTRACT: By employing a metal-coated central platelet and a rigid mesh electrode which is transparent to acoustic wave, we show that the membrane-type acoustic metamaterials (MAMs) can be easily tuned by applying an external voltage. With static voltage, the MAM's eigenfrequencies and therefore the phase of the transmitted wave are tunable up to 70 Hz. The MAM's vibration can be significantly suppressed or enhanced by using phase-matched AC voltage. Functionalities such as phase modulation and acoustic switch with on/off ratio up to 21.3 dB are demonstrated.
    Full-text · Article · Mar 2015 · Applied Physics Letters
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    ABSTRACT: Bilayer graphene, unlike monolayer graphene, provides a tunable electronic energy gap when an electrical field is applied across the layers, therefore holds great potential in semiconductor industry. Here, we demonstrate a facile technique to obtain bilayer graphene structures on the growth substrate by controlling oxidation to remove monolayers, while retaining the bilayer electronic properties. We found that this precise oxidation process selectively destructs monolayers while preserves the qualities of bilayers, evidenced by the expected quantum hall effect and exceptional room temperature carrier mobilities of ~3500 cm2 v−1 s−1 obtained from electrical transport measurement. In addition, visualization of bilayers, which serves as nuclei for graphene growth, opens the door for the understanding the actual mechanism of graphene growth process which eventually can lead to the optimized synthesis.
    Full-text · Article · Mar 2015 · RSC Advances
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    Min Yang · Yong Li · Chong Meng · Caixing Fu · Jun Mei · Zhiyu Yang · Ping Sheng
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    ABSTRACT: Decorated membrane, comprising a thin layer of elastic film with small rigid platelets fixed on top, has been found to be an efficient absorber of low frequency sound. In this work we consider the problem of sound absorption from a perspective aimed at deriving upper bounds under different scenarios, i.e., whether the sound is incident from one side only or from both sides, and whether there is a reflecting surface on the back side of the membrane. By considering the negligible thickness of the membrane, usually on the order of a fraction of one millimeter, we derive a relation showing that the sum of the incoming sound waves' (complex) pressure amplitudes, averaged over the area of the membrane, must be equal to that of the outgoing waves. By using this relation, and without going to any details of the wave solutions, it is shown that the maximum absorption achievable from one-side incident is 50%, while the maximum absorption with a back reflecting surface can reach 100%. The latter was attained by the hybridized resonances. All the results are shown to be in excellent agreement with the experiments. This generalized perspective, when used together with the Green function formalism, can be useful in gaining insights and delineating the constraints on what are achievable in scatterings and absorption by thin film structures.
    Full-text · Article · Feb 2015 · Comptes Rendus Mecanique
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    Meng Xiao · Guancong Ma · Zhiyu Yang · Ping Sheng · Z Q Zhang · C T Chan
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    ABSTRACT: The geometric phase that specifically characterizes the topological property of bulk bands in one-dimensional periodic systems is known as the Zak phase. Recently, it has been found that topological notions can also characterize the topological phase of mechanical isostatic lattices. Here, we present a theoretical framework and two experimental methods to determine the Zak phase in a periodic acoustic system. We constructed a phononic crystal with a topological transition point in the acoustic band structure where the band inverts and the Zak phase in the bulk band changes following a shift in system parameters. As a consequence, the topological characteristics of the bandgap change and interface states form at the boundary separating two phononic crystals having diierent bandgap topological characteristics. Such acoustic interface states with large sound intensity enhancement are observed at the phononic crystal interfaces. We use a simple photonic crystal system to demonstrate geometric phase effects and the existence of topological transition points in acoustic systems.
    Full-text · Article · Feb 2015 · Nature Physics
  • Shuo Guo · Chun Huen Lee · Ping Sheng · Penger Tong
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    ABSTRACT: We report a direct measurement of the friction coefficient ξ_{c} of two fluctuating contact lines formed on a fiber surface when a long glass fiber intersects the two water-air interfaces of a thin soap film. The glass fiber of diameter d in the range of 0.4-4 μm and length 100-300 μm is glued onto the front end of a rectangular cantilever used for atomic force microscopy. As a sensitive mechanical resonator, the hanging fiber probe can accurately measure a minute change of its viscous damping caused by the soap film. By measuring the broadening of the resonant peak of the hanging fiber probe with varying viscosity η of the soap film and different surface treatments of the glass fiber, we confirm that the contact line dissipation obeys a universal scaling law, ξ_{c}=απdη, where the coefficient α=1.1±0.3 is insensitive to the change of liquid-solid contact angle. The experimental result is in good agreement with the numerical result based on the phase field model under the generalized Navier boundary conditions.
    No preview · Article · Jan 2015 · Physical Review E
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    ABSTRACT: The metal-insulator transition is one of the remarkable electrical properties of atomically thin molybdenum disulphide. Although the theory of electron-electron interactions has been used in modelling the metal-insulator transition in molybdenum disulphide, the underlying mechanism and detailed transition process still remain largely unexplored. Here we demonstrate that the vertical metal-insulator-semiconductor heterostructures built from atomically thin molybdenum disulphide are ideal capacitor structures for probing the electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of molybdenum disulphide over a wide excitation frequency and temperature range. By combining capacitance and transport measurements, we have observed a percolation-type metal-insulator transition, driven by density inhomogeneities of electron states, in monolayer and multilayer molybdenum disulphide. In addition, the valence band of thin molybdenum disulphide layers and their intrinsic properties are accessed.
    Full-text · Article · Jan 2015 · Nature Communications
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    ABSTRACT: Bilayer graphene (BLG) can provide a tunable band gap when exposed to a vertical electric field. We report here an approach to the synthesis of large single-crystal BLG structures with diameters up to 0.54 mm. We found that both absorption-diffusion and gas-phase penetration mechanisms contributed to the growth of the lower second layer and that the absorption-diffusion mechanism favors faster BLG growth. Our strategy was to suppress nucleation in the growth of the first layer using an established surface oxidation method to maintain a low coverage of graphene on Cu foil. We subsequently maximized the growth of the second layer by increasing the duration of absorption-diffusion. The chemical treatment used to polish the Cu surface to reduce the nucleation of growth in the monolayer increased the nucleation density during the growth of the second layer. Electron transport measurements on dual-gated field-effect transistors showed that the BLG fabricated was of high quality with a sizeable tunable band gap. Our approach may have broad applications for the controlled synthesis of bilayers in materials chemistry.
    Full-text · Article · Jan 2015 · Nanoscale
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    Min Yang · Guancong Ma · Zhiyu Yang · Ping Sheng

    Preview · Article · Jan 2015
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    ABSTRACT: We report new developments on the chemical vapor deposition growth of 0.4 nm single-walled carbon nanotubes (SWCNTs) inside the linear channels of the aluminophosphate zeolite, AlPO4-5 (AFI), single crystals (0.4 nm-SWCNT@AFI). Ethylene (C2H4) and carbon monoxide (CO) were used as the feedstock. Polarized Raman spectroscopy was used to analyze the structure and quality of SWCNTs, both the radial breathing mode and G-band are much clearer and stronger than the samples grown by the old process which used template tripropylamine molecules for growing SWCNT@AFI. From the Raman spectra, it is clearly seen that the RBM is composed of two peaks at 535 and 551 cm−1. By using the pseudopotential module in Material Studio to calculate the Raman lines, the 535 cm−1 peak is attributed to the (5,0) SWCNTs and the 551 cm−1 peak to the (3,3) SWCNTs. The abundance of (4,2) is relatively small. Thermal gravity analysis showed that while the samples grown by CO display less than 1 wt% of carbon, for the samples heated in C2H4 atmosphere the weight percentage of SWCNTs is around 10%, which implies ∼30% of the AFI channels are occupied with SWCNTs, a significant increase compared with the previous samples.
    No preview · Article · Sep 2014 · Carbon
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    ABSTRACT: By carrying out simultaneous longitudinal and Hall measurements in graphene, we find that the 1/f noise for the charge carrier density is negatively correlated to that of mobility, with a governing behavior that differs significantly from the relation between their mean values. The correlation in the noise data can be quantitatively explained by a single-parameter theory whose underlying physics is the trapping and detrapping of the fluctuating charge carriers by the oppositely charged Coulomb scattering centers. This can alter the effective density of long-range scattering centers in a transient manner, with the consequent fluctuating effect on the mobility.
    No preview · Article · Aug 2014 · Physical Review B
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    Guancong Ma · Min Yang · Songwen Xiao · Zhiyu Yang · Ping Sheng
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    ABSTRACT: An impedance-matched surface has the property that an incident wave generates no reflection. Here we demonstrate that by using a simple construction, an acoustically reflecting surface can acquire hybrid resonances and becomes impedance-matched to airborne sound at tunable frequencies, such that no reflection is generated. Each resonant cell of the metasurface is deep-subwavelength in all its spatial dimensions, with its thickness less than the peak absorption wavelength by two orders of magnitude. As there can be no transmission, the impedance-matched acoustic wave is hence either completely absorbed at one or multiple frequencies, or converted into other form(s) of energy, such as an electrical current. A high acoustic-electrical energy conversion efficiency of 23% is achieved.
    Full-text · Article · Jun 2014 · Nature Materials
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    Yong Jian Wang · Zuli Xu · Ping Sheng · Penger Tong
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    ABSTRACT: A systematic study of the electric-field-induced forces between a solid glass sphere and a flat gold-plated substrate filled with an insulating liquid has been carried out. Using atomic force microscopy, we measure the electrostatic force f(s, V) between the sphere and substrate as a function of the surface separation s and applied voltage V. The measured f(s, V) is found to be well described by an equation for a conducting sphere. Further force measurements for the "wet" porous glass spheres filled with an aqueous solution of urea and the dried porous glass spheres filled with (dry) air suggest that there is a water layer of a few nanometers in thickness adsorbed on the hydrophilic glass surface under ambient conditions. This adsorbed water layer is more conductive than the dielectric core of the glass sphere, making the sphere surface to be at a potential close to that of the cantilever electrode. As a result, the electric field is strongly concentrated in the gap region between the glass sphere and gold-plate substrate and thus their electrostatic attraction is enhanced. This surface conductivity effect is further supported by the thermal gravimetric analysis (TGA) and force response measurements to a time-dependent electric field. The experiment clearly demonstrates that the adsorption of a conductive water layer on a hydrophilic surface plays a dominant role in determining the electrostatic interaction between the dielectric sphere and substrate.
    Full-text · Article · Jun 2014 · The European Physical Journal Applied Physics
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    Dataset: 1D goes 2D

    Full-text · Dataset · May 2014
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    ABSTRACT: In this paper, we report a first-principles study of the lattice dynamics of small graphene nanoribbon with zigzag edges. Our investigation is based on spin polarized density functional calculations (DFT). Nesting properties in the electronic band structure are very different for nanoribbons with unpolarized, ferromagnetic, and antiferromagnetic configurations. As a result, the phonon spectrum and nesting related softening in phonon frequencies differ in these cases. The unpolarized and ferromagnetic structures show nesting related phonon softening and considerable electron phonon linewidth, while for the antiferromagnetic structure, a band gap at the Fermi energy eliminates these effects. Saturating the nanoribbon edge with hydrogen has negligible effect on the phonon spectra for the magnetic structures while for the unpolarized configuration all structures without hydrogen are unstable due to soft phonon modes. The electron-phonon coupling coefficients have also been calculated and implications for Peierls transition and superconductivity are discussed.
    No preview · Article · May 2014 · Physical Review B
  • Li Wan · Shixin Xu · Maijia Liao · Chun Liu · Ping Sheng
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    ABSTRACT: In this work we treat the Poisson-Nernst-Planck (PNP) equations as the basis for a consistent framework of the electrokinetic (EK) effects. The static limit of the PNP equations is shown to be the charge-conserving Poisson-Boltzmann (CCPB) equation, with guaranteed charge neutrality within the computational domain. We propose a surface potential trap model that attributes an energy cost to the interfacial charge dissociation. In conjunction with the CCPB, the surface potential trap can effect a surface-specific adsorbed charge layer . By defining a chemical potential that arises from the charge neutrality constraint, a re-formulated CCPB can be reduced to the form of the Poisson-Boltzmann (PB) equation, whose prediction of the Debye screening layer profile is in excellent agreement with that of the PB equation when the channel width is much larger than the Debye length. However, important differences emerge when the channel width is small so that the Debye screening layers from the opposite sides of the channel overlap with each other. In particular, the theory automatically yields a variation of that is generally known as the “charge regulation” behavior, attendant with predictions of force variation as a function of nanoscale separation between two charged surfaces that is in good agreement with the experiments, with no adjustable or additional parameters. We give a generalized definition of the potential that reflects the strength of the EK effect; its variations with the concentration of surface specific and non-specific salt ions are shown to be in good agreement with the experiments. To delineate the behavior of the electro-osmotic (EO) effect, the coupled PNP and Navier-Stokes (NS) equations are solved numerically under an applied electric field tangential to the fluid-solid interface. The EO effect is shown to exhibit intrinsic time dependence that is non-inertial in its origin. Under a step function applied electric field, a pulse of fluid flow is followed by relaxation to a new ion distribution, owing to the diffusive counter current. We have numerically evaluated the Onsager coefficients associated with the EO effect, L21, and its reverse SP effect, L12, and show L12=L21 in accordance with the Onsager relation. We conclude by noting some of the challenges ahead.
    No preview · Article · Mar 2014 · Physical Review X
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    Min Yang · Guancong Ma · Ying Wu · Zhiyu Yang · Ping Sheng
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    ABSTRACT: We present a homogenization scheme for acoustic metamaterials that is based on reproducing the lowest orders of scattering amplitudes from a finite volume of metamaterials. This approach is noted to differ significantly from that of coherent potential approximation, which is based on adjusting the effective-medium parameters to minimize scatterings in the long-wavelength limit. With the aid of metamaterials' eigenstates, the effective parameters, such as mass density and elastic modulus can be obtained by matching the surface responses of a metamaterial's structural unit cell with a piece of homogenized material. From the Green's theorem applied to the exterior domain problem, matching the surface responses is noted to be the same as reproducing the scattering amplitudes. We verify our scheme by applying it to three different examples: a layered lattice, a two-dimensional hexagonal lattice, and a decorated-membrane system. It is shown that the predicted characteristics and wave fields agree almost exactly with numerical simulations and experiments and the scheme's validity is constrained by the number of dominant surface multipoles instead of the usual long-wavelength assumption. In particular, the validity extends to the full band in one dimension and to regimes near the boundaries of the Brillouin zone in two dimensions.
    Full-text · Article · Feb 2014 · Physical Review B

Publication Stats

11k Citations
1,415.72 Total Impact Points

Institutions

  • 1970-2015
    • The Hong Kong University of Science and Technology
      • • Department of Physics
      • • Institute for Advanced Study (IAS)
      • • Department of Mathematics
      Chiu-lung, Kowloon City, Hong Kong
  • 2014
    • Pennsylvania State University
      • Department of Mathematics
      University Park, Maryland, United States
  • 2001-2012
    • Nano Science and Technology Institute
      Austin, Texas, United States
    • Tsinghua University
      • Center for Advanced Study
      Beijing, Beijing Shi, China
  • 2002
    • Wuhan University
      • Department of Physics
      Wu-han-shih, Hubei, China
  • 1997
    • The Chinese University of Hong Kong
      • Department of Physics
      Hong Kong, Hong Kong