L. Kou - Academia.edu (original) (raw)

Papers by L. Kou

$Research paper thumbnail of Refractive index and hygroscopic stability of AlxGa1−xAs native oxides$

Applied Physics Letters, 1999

We present prism coupling measurements on AIXGal.XAs native oxides showing the dependent~~c E!vE'... more We present prism coupling measurements on AIXGal.XAs native oxides showing the dependent~~c E!vE'D refractive index on composition (0.3 SXS0.97), oxidation temperature (400q<500), and carrier g purity. Index values range from n=l .490 (x=O.9, 400 "C) to 1.707 (x=O.3, 500 "C). The oxides are *)Electronic mail: dhall(j?nd.edu ' '. .

Atmospheric Environment, 1999

ABSTRACT

$Research paper thumbnail of Refractive indices of water and ice in the 065- to 25-μm spectral range$

Applied Optics, 1993

0C are reported. The k spectrum for water in the spectral region 0.65-2.5 pLm is found to be in e... more 0C are reported. The k spectrum for water in the spectral region 0.65-2.5 pLm is found to be in excellent agreement with those of previous studies. The k values for polycrystalline ice in the 1.44-2.50-ptm region eliminate the large uncertainties existing among previously published conflicting sets of data. The imaginary part of refractive index of supercooled water shows a systematic shift of absorption peaks toward the longer wavelengths compared with that of water at warmer temperatures.

The journal of physical chemistry letters, Jan 7, 2015

Two-dimensional materials tend to become crumpled according to the Mermin-Wagner theorem, and the... more Two-dimensional materials tend to become crumpled according to the Mermin-Wagner theorem, and the resulting ripple deformation may significantly influence electronic properties as observed in graphene and MoS2. Here, we unveil by first-principles calculations a new, highly anisotropic ripple pattern in phosphorene, a monolayer black phosphorus, where compression-induced ripple deformation occurs only along the zigzag direction in the strain range up to 10%, but not the armchair direction. This direction-selective ripple deformation mode in phosphorene stems from its puckered structure with coupled hinge-like bonding configurations and the resulting anisotropic Poisson ratio. We also construct an analytical model using classical elasticity theory for ripple deformation in phosphorene under arbitrary strain. The present results offer new insights into the mechanisms governing the structural and electronic properties of phosphorene crucial to its device applications.

2008 Symposium on Piezoelectricity, Acoustic Waves, and Device Applications, 2008

The wurtzite ZnO material exhibits excellent piezoelectric property along the [0001]-direction be... more The wurtzite ZnO material exhibits excellent piezoelectric property along the [0001]-direction because of the noncentrosymmetric structure. As a typical II-VI wide band gap compound, it has been long studied as a piezoelectric material. Here we review the previous theoretical and experimental researches on the piezoelectric properties of ZnO and its nanostructures. Some practical applications in nanodevices are also exhibited. The present review could serve as a good reference for future investigations in the relative fields, and also indicates potential applications in nanoscale devices. Our contributions on this review topic are focused on the theoretical investigations of piezoelectricity of ZnO nanostructures, including nanowires and nanofilms, by using first-principles calculations. For the nanowires, size-dependent axial piezoelectricity in [0001]-oriented ZnO nanowires with diameters ranging from 0.4 to 3.0 nm is investigated. It is shown that the effective piezoelectric constant e33 of the nanowires increases with increasing diameter, and is approximately one order smaller than the bulk value due to the structural change on the boundary of the nanowires. The absolute value of the axial piezoelectricity induced by the radial strain e31 is around half of the effective e33, which is similar to the bulk case. For the ZnO nanofilms, we find that the effective piezoelectric constant e33 of ZnO nanofilms is also size dependent, and increases with increasing thickness in the nanoscale simulated in our work. When the film thickness is larger than 2.4 nm, the corresponding piezoelectric coefficient becomes higher than that of bulk ZnO. The enhancement over the bulk value reaches 11% when the film thickness is 2.9 nm.

Chemistry of Materials, 2014

ABSTRACT Macroscopic assembled, self-standing graphene and graphene oxide (GO) films have been de... more ABSTRACT Macroscopic assembled, self-standing graphene and graphene oxide (GO) films have been demonstrated as promising materials in many emerging fields, such as Li ion battery electrodes, supercapacitors, heat spreaders, gas separation, and water desalination. Such films were mainly available on centimeter-scale via the time- and energy-consuming vacuum-filtration method, which seriously impedes their progress and large-scale applications. Due to the incompatibility between large-scale and ordered assembly structures, it remains a big challenge to access large-area assembled graphene thick films. Here, we report for the first time a fast wet-spinning assembly strategy to produce continuous GO and graphene thick films. A 20 m long, 5 cm wide, well-defined GO film was readily achieved at a speed of 1 m min–1. The continuous, strong GO films were easily woven into bamboo-mat-like fabrics and scrolled into highly flexible continuous fibers. The reduced graphene films with high thermal and moderate electrical conductivities were directly used as fast-response deicing electrothermal mats. The fast yet controllable wet-spinning assembly approach paves the way for industrial-scale utilization of graphene.

Nano Letters, 2015

One of the major obstacles to a wide application range of the quantum spin Hall (QSH) effect is t... more One of the major obstacles to a wide application range of the quantum spin Hall (QSH) effect is the lack of suitable QSH insulators with a large bulk gap. By means of first-principles calculations including relativistic effects, we predict that methyl-functionalized bismuth, antimony, and lead bilayers (Me-Bi, Me-Sb, and Me-Pb) are 2D topological insulators (TIs) with protected Dirac type topological helical edge states, and thus suitable QSH systems. In addition to the explicitly obtained topological edge states, the nontrivial topological characteristic of these systems is confirmed by the calculated nontrivial Z2 topological invariant. The TI characteristics are intrinsic to the studied materials and are not subject to lateral quantum confinement at edges, as confirmed by explicit simulation of the corresponding nanoribbons. It is worthwhile to point out that the large nontrivial bulk gaps of 0.934 eV (Me-Bi), 0.386 eV (Me-Sb), and 0.964 eV (Me-Pb) are derived from the strong spin-orbit coupling within the px and py orbitals and would be large enough for room-temperature application. Moreover, we show that the topological properties in these three systems are robust against mechanical deformation. These novel 2D TIs with such giant topological energy gaps are promising platforms for topological phenomena and possible applications at high temperature.

The Journal of Physical Chemistry Letters, 2014

ABSTRACT Recent reports on the fabrication of phosphorene, i.e., mono- or few-layer black phospho... more ABSTRACT Recent reports on the fabrication of phosphorene, i.e., mono- or few-layer black phosphorus, have raised exciting prospects of an outstanding two-dimensional (2D) material that exhibits excellent properties for nanodevice applications. Here we study by first-principles calculations the adsorption of CO, CO2, NH3, NO and NO2 gas molecules on a mono-layer phosphorene. Our results predict superior sensing performance of phosphorene that rivals or even surpasses other 2D materials such as graphene and MoS2. We determine the optimal adsorption positions of these molecules on the phosphorene and identify molecular doping, i.e., charge transfer between the molecules and phosphorene, as the driving mechanism for the high adsorption strength. We further calculated the current-voltage (I-V) relation using a non-equilibrium Greens function (NEGF) formalism. The transport features show large (one to two orders of magnitude) anisotropy along different (armchair or zigzag) directions, which is consistent with the anisotropic electronic band structure of phosphorene. Remarkably, the I-V relation exhibits distinct responses with a marked change of the I-V relation along either the armchair or the zigzag directions depending on the type of molecules. Such selectivity and sensitivity to adsorption makes phosphorene a superior gas sensor that promises wide-ranging applications.

Nanotechnology, 2014

We show that the hexagonal structure of single-layer molybdenum disulphide (MoS2), under uniaxial... more We show that the hexagonal structure of single-layer molybdenum disulphide (MoS2), under uniaxial tension along a zigzag direction for large deformations, can transfer to a new quadrilateral structure by molecular dynamics (MD) simulations when the temperature is below 40 K. The new phase remains stable after unloading, even at room temperature. The Young&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s modulus of the new phase along the zigzag direction is about 2.5 times higher than that of normal MoS2. Checking against density functional theory calculations shows that the new phase is preserved and displays excellent electrical conductivity. Our results provide physical insights into the origins of the new phase transition of MoS2 at low temperatures.

Nanotechnology, Jan 13, 2015

Ripple is a common deformation in two-dimensional materials due to localized strain, which is exp... more Ripple is a common deformation in two-dimensional materials due to localized strain, which is expected to greatly influence the physical properties. The effects of the ripple deformation in the MoS2 layer on their physics, however, are rarely addressed experimentally. We here grow atomically thin MoS2 nanostructures by employing a vapor phase deposition method without any catalyst and observed the ripples in MoS2 nanostructures. The MoS2 ripples exhibit quasi-periodical ripple structures in the MoS2 surface. The heights of the ripples vary from several angstroms to tens of nanometers and the wavelength is in the range of several hundred nanometers. The growth mechanism of rippled MoS2 nanostructures is elucidated. We have also simultaneously investigated the electrostatic properties of MoS2 ripples by using Kelvin probe force microscopy, which shows inhomogeneous surface potential and charge distributions for MoS2 ripple nanostructures with different local strains.

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In the framework of first-principles calculations, we investigate the structural and electronic p... more In the framework of first-principles calculations, we investigate the structural and electronic properties of graphene in contact with as well as sandwiched between WS 2 and WSe 2 monolayers.

The search for inversion asymmetric topological insulators (IATIs) persists as an effect for real... more The search for inversion asymmetric topological insulators (IATIs) persists as an effect for realizing new topological phenomena. However, so for only a few IATIs have been discovered and there is no IATI exhibiting a large band gap exceeding 0.6 eV. Using first-principles calculations, we predict a series of new IATIs in saturated Group III-Bi bilayers. We show that all these IATIs preserve extraordinary large bulk band gaps which are well above room-temperature, allowing for viable applications in room-temperature spintronic devices. More importantly, most of these systems display large bulk band gaps that far exceed 0.6 eV and, part of them even are up to ~1 eV, which are larger than any IATIs ever reported. The nontrivial topological situation in these systems is confirmed by the identified band inversion of the band structures and an explicit demonstration of the topological edge states. Interestingly, the nontrivial band order characteristics are intrinsic to most of these materials and are not subject to spin-orbit coupling. Owning to their asymmetric structures, remarkable Rashba spin splitting is produced in both the valence and conduction bands of these systems. These predictions strongly revive these new systems as excellent candidates for IATI-based novel applications.

Carbon, 2015

Graphene is the first model system of two-dimensional topological insulator (TI), also known as q... more Graphene is the first model system of two-dimensional topological insulator (TI), also known as quantum spin Hall (QSH) insulator. The QSH effect in graphene, however, has eluded direct experimental detection because of its extremely small energy gap due to the weak spin-orbit coupling. Here we predict by ab initio calculations a giant (three orders of magnitude) proximity induced enhancement of the TI energy gap in the graphene layer that is sandwiched between thin slabs of Bi 2 Te 3 (or Sb 2 Te 3 , MoTe 2 ). This gap (∼ 1 meV) is accessible by existing experimental techniques, and it can be further enhanced by tuning the interlayer distance via compression. We reveal by a tight-binding study that the QSH state in graphene is driven by the Kane-Mele interaction in competition with Kekulé deformation and symmetry breaking. The present work identifies a new family of graphene-based TIs with an observable and controllable bulk energy gap in the graphene layer, thus opening a new avenue for direct verification and exploration of the long-sought QSH effect in graphene.

ACS Nano, 2014

We predict a family of robust two-dimensional (2D) topological insulators in van der Waals hetero... more We predict a family of robust two-dimensional (2D) topological insulators in van der Waals heterostructures comprising graphene and chalcogenides BiTeX (X = Cl, Br, and I).

Nanoscale, 2015

Supercapacitors with porous electrodes of graphene macroscopic assembly are supposed to have high... more Supercapacitors with porous electrodes of graphene macroscopic assembly are supposed to have high energy storage capacity. However, a great number of &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;close pores&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; in porous graphene electrodes are invalid because electrolyte ions cannot infiltrate. A quick method to prepare porous graphene electrodes with reduced &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;close pores&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; is essential for higher energy storage. Here we propose a wet-spinning assembly approach based on the liquid crystal behavior of graphene oxide to continuously spin orientational graphene hydrogel films with &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;open pores&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;, which are used directly as binder-free supercapacitor electrodes. The resulting supercapacitor electrodes show better electrochemical performance than those with disordered graphene sheets. Furthermore, three reduction methods including hydrothermal treatment, hydrazine and hydroiodic acid reduction are used to evaluate the specific capacitances of the graphene hydrogel film. Hydrazine-reduced graphene hydrogel film shows the highest capacitance of 203 F g(-1) at 1 A g(-1) and maintains 67.1% specific capacitance (140 F g(-1)) at 50 A g(-1). The combination of scalable wet-spinning technology and orientational structure makes graphene hydrogel films an ideal electrode material for supercapacitors.

International journal of pharmaceutics, Jan 10, 2012

To compare the dissolution and bioavailability for nimodipine microcrystals and nanocrystals, and... more To compare the dissolution and bioavailability for nimodipine microcrystals and nanocrystals, and to determine the critical size range in improving the oral absorption of nimodipine. Nimodipine microcrystals and nanocrystals were prepared using a microprecipitation method. The particle size was determined with a laser diffraction method. X-ray powder diffraction was applied to inspect the potential crystal form transition. The aqueous solubility was determined by shaking flasks, and the dissolution behavior was evaluated using the paddle method. The pharmacokinetics was performed in beagle dogs in a crossover experimental design. Three nimodipine colloidal dispersions (16296.7, 4060.0 and 833.3 nm) were prepared, respectively. Nimodipine had undergone crystal form transition during microprecipitation process, but experienced no conversion under the high-pressure homogenization. The colloidal dispersions did not show any difference in aqueous equilibrium solubility. Additionally, the...

The Journal of Physical Chemistry C, 2010

We report on the electric-field-and H chemical-absorption-induced band manipulations of armchair ... more We report on the electric-field-and H chemical-absorption-induced band manipulations of armchair ZnO nanoribbons using first-principles calculations. It is shown that the band gap of a semiconducting armchair nanoribbon can be reduced monotonically with increasing transverse field strength, demonstrating a giant Stark effect. The critical field strength to completely close the band gap decreases with increasing ribbon width, while it is almost independent of the stacking thickness. On the other hand, the nanoribbon with the edges fully passivated shows an enhanced gap but a slightly weaker Stark effect. We also observe hydrogentermination-induced metallization of the ribbons when only the edge O atoms are passivated, which results from an n-type doping effect. These findings suggest potential ways of band engineering in armchair ZnO

The Journal of Physical Chemistry C, 2011

By use of first-principles calculations, we examine the effects of uniaxial strain and radial def... more By use of first-principles calculations, we examine the effects of uniaxial strain and radial deformation on electronic properties of zigzag ZnO nanotubes. Our results show that local strain or deformation can cause significant reduction of the band gap owing to quantum-confined Stark effect induced by the built-in electric polarization. Driven by this polarization field, the charge carriers are separated with hole and electron states localized on the opposite ends of the tube. In sharp contrast, uniform tensile strain tends to widen the band gap while compressive strain and radial deformation have negligible effects on the band gap, although they can produce considerable shifts in edge-state energies. The present results reveal the key role of local strain as an effective tool in tuning the properties of zigzag ZnO nanotubes. Such local-strain-induced electronic structure modulation suggests an effective approach to design and implementation of ZnO nanotubes in nanoscale devices.