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Electrical computers have revolutionized society over the past several decades, but questions have remained about their ability to perform in extreme environments, such as their stability at high-temperature conditions. This has motivated the recent surge of interest in developing mechanical computing platforms at all length scales, including the nanoscale, in which traditional electrical computers are augmented with mechanical ones. However, the most proposed nanomechanical bits are volatile memory bits based on the dynamic response of nanomechanical resonators, and as such, there is a need to develop robust and reprogrammable entirely non-volatile nanomechanical bits. Here, we exploit the multiple quasi-stable configurations of the graphene/fullerene/graphene (GFG) van der Waals heterostructure to work as a novel nonvolatile nanomechanical bit. The GFG heterostructure is unique and robust in that it can return to its initial state without further mechanical input, it can be used for logic functions at relevant operating temperatures through simple application of uniaxial strain, it can exhibit reprogrammability between five basic logic gates (NOT, AND, NAND, OR, NOR) by varying the magnitude or direction of applied strain, and it can be used to represent combinatorial logic through full and half adders. These findings provide a new opportunity to develop mechanical computers based on the large class of nanomaterials. [ABSTRACT FROM AUTHOR]

Two-dimensional lateral heterostructures exhibit novel electronic and optical properties that are induced by their in-plane interface for which the mechanical properties of the interface are important for the stability of the lateral heterostructure. Therefore, we performed molecular dynamics simulations and developed a convolutional neural network-based machine learning model to study the fracture properties of the interface in a graphene/hexagonal boron nitride lateral heterostructure. The molecular dynamics (MD) simulations show that the shape of the interface can cause an 80% difference in the fracture stress and the fracture strain for the interface. By using 11 500 training samples obtained with help of high-cost MD simulation, the machine learning model is able to search out the strongest interfaces with the largest fracture strain and fracture stress in a large sample space with over 150 000 structures. By analyzing the atomic configuration of these strongest interfaces, we disclose two major factors dominating the interface strength, including the interface roughness and the strength of the chemical bond across the interface. We also explore the correlation between the fracture properties and the thermal conductivity for these lateral heterostructures by examining the bond type and the shape of the graphene/hexagonal boron nitride interface. We find that interfaces comprised of stronger bonds and smoother zigzag interfaces can relieve the abrupt change of the acoustic velocity, leading to the enhancement of the interface thermal conductivity. These findings will be valuable for the application of the two-dimensional lateral heterostructure in electronic devices. [ABSTRACT FROM AUTHOR]

The field of topological mechanics has recently emerged due to the interest in robustly transporting various types of energy in a flaw and defect-insensitive fashion. While there have been a significant number of studies based on discovering and proposing topological materials and structures, very few have focused on tuning the resulting topological bandgaps, which is critical because the bandgap frequency is fixed once the structure has been fabricated. Here, we perform both lattice dynamical calculations and molecular dynamical simulations to investigate strain effects on the phononic topological bandgaps in two-dimensional monolayer hexagonal boron nitride. Our studies demonstrate that while the topologically protected phononic bandgaps are not closed even for severely deformed hexagonal boron nitride and are relatively insensitive to uniaxial tension and shear strains, the position of the frequency gap can be efficiently tuned in a wide range through the application of biaxial strains. Overall, this work thus demonstrates that topological phonons are robust against the effects of mechanical strain engineering and shed light on the tunability of the topological bandgaps in nanomaterials. [ABSTRACT FROM AUTHOR]

Nanomotors serve as nanoscale engines by converting various energies into mechanical energy. In addition to the huge number of existing nanomotors, we propose a simple nanomotor based on the hollow carbon nanosphere (i.e., fullerene) that is full of gas. We investigate the acceleration of the nanosphere by leakage of gas through a nanopore by molecular dynamics simulations. The nanosphere can be driven to a high speed of 100 m/s under proper simulation conditions, which can be further tuned by temperature, gas density, and pore diameter. We observe rotation of the pore direction during the acceleration process for a nanosphere of different pore diameters. The acceleration process can be well described by the Meshchersky theory. We also simulate the deceleration process of the nanosphere due to the damping force of the gas, which can be analyzed in terms of the kinetic motion of gas molecules. The nanomotor proposed in this work shall be realizable in experiments and may be useful in driving the mechanic motion of fullerenes. [ABSTRACT FROM AUTHOR]

We propose a tubular van der Waals heterostructure by rolling up graphene and MoS2 atomic layers into a tubular form. We show that the interlayer space of this heterostructure can be varied in a specific range. This specific range is related to the interlayer van der Waals force, the ultrahigh in-plane stiffness and small bending modulus of graphene, and the brittle characteristic properties of MoS2. This variability of the interlayer space can be utilized to efficiently tune the mechanical properties of the tubular van der Waals heterostructure. More specifically, we demonstrate that the Poisson's ratio of the tubular van der Waals heterostructure can be manipulated by a factor of two by varying the interlayer space in the range 1.44–4.44 Å. This work provides a basis for applications of a new member of the van der Waals heterostructure family with a tunable Poisson's ratio. [ABSTRACT FROM AUTHOR]

A key parameter for the performance of ceramic topcoats in a thermal barrier coating is the porosity, which is sensitive to many growing conditions. In the present work, molecular dynamics simulations are performed to investigate the microscopic mechanism for the dependence of the porosity on several experimental conditions in the atmospheric plasma spraying deposition process, including temperature, substrate roughness, and the thickness-to-radius ratio of the yttrium-stabilized zirconia shell. These factors can significantly modify the porosity through changing the buckling mode of the yttrium-stabilized zirconia shell. Our findings provide some microscopic mechanisms for improving the deposition quality of the atmospheric plasma spraying thermal barrier coatings. [ABSTRACT FROM AUTHOR]

We investigate the strain engineering and the edge effect for mechanical properties in graphene nanoribbons. The free edges of the graphene nanoribbons are warped due to compressive edge stresses. There is a structural transformation for the free edges from the three-dimensional warping configuration to the two-dimensional planar structure at the critical strain ϵc = 0.7%, at which the applied mechanical stress is equal to the intrinsic compressive edge stress. This structural transformation leads to step-like changes in several mechanical properties studied in the present work, including the Young's modulus, the Poisson's ratio, the quality factor of nanomechanical resonators, and the phonon edge mode. [ABSTRACT FROM AUTHOR]

The Lennard-Jones potential is widely used to describe the interlayer interactions within layered materials like graphene. However, it is also widely known that this potential strongly underestimates the frictional properties for layered materials. Here, we propose to supplement the Lennard-Jones potential by a Gaussian-type potential, which enables more accurate calculations of the frictional properties of two-dimensional layered materials. Furthermore, the Gaussian potential is computationally simple as it introduces only one additional potential parameter that is determined by the interlayer shear mode in the layered structure. The resulting Lennard-Jones-Gaussian potential is applied to compute the interlayer cohesive energy and frictional energy for graphene, MoS2, black phosphorus, and their heterostructures. [ABSTRACT FROM AUTHOR]

We perform molecular dynamics simulations to study the registry effect on the thermal conductivity of few-layer graphene. The interlayer interaction is described by either the Lennard-Jones potential or the registry-dependent potential. Our calculations show that the thermal conductivity in few-layer graphene from both potentials are close to each other, i.e., the registry effect is essentially not important. It is because the thermal transport in few-layer graphene is mainly limited by the interlayer breathing mode, which is insensitive to the registry. [ABSTRACT FROM AUTHOR]

We present a parameterization of the Stillinger-Weber potential to describe the interatomic interactions within single-layer MoS2 (SLMoS2). The potential parameters are fitted to an experimentally obtained phonon spectrum, and the resulting empirical potential provides a good description for the energy gap and the crossover in the phonon spectrum. Using this potential, we perform classical molecular dynamics simulations to study chirality, size, and strain effects on the Young's modulus and the thermal conductivity of SLMoS2. We demonstrate the importance of the free edges on the mechanical and thermal properties of SLMoS2 nanoribbons. Specifically, while edge effects are found to reduce the Young's modulus of SLMoS2 nanoribbons, the free edges also reduce the thermal stability of SLMoS2 nanoribbons, which may induce melting well below the bulk melt temperature. Finally, uniaxial strain is found to efficiently manipulate the thermal conductivity of infinite, periodic SLMoS2. [ABSTRACT FROM AUTHOR]

The interlayer energy of the twisting bilayer graphene is investigated by the molecular mechanics method using both the registry-dependent potential and the Lennard-Jones potential. Both potentials show that the interlayer energy is independent of the twisting angle θ, except in the two boundary regions θ≈0° or 60°, where the interlayer energy is proportional to the square of the twisting arc length. The calculation results are successfully interpreted by a single atom model. An important information from our findings is that, from the energy point of view, there is no preference for the twisting angle in the experimental bilayer graphene samples, which actually explains the diverse twisting angles in the experiment. [ABSTRACT FROM AUTHOR]

We apply united-atom (UA) nonequilibrium molecular dynamics simulations to predict the thermal conductivity for amorphous polyethylene at room temperature with chain length (CL) increasing from 4 to 1260. The thermal conductivity is determined by the competition of the two mechanisms: (1) The collision among the UA beads and (2) the phonon vibration. The collision mechanism mainly dominates CL≤7, while the phonon vibration mechanism mainly determines CL ≥ 140. The competition between the two mechanisms determines 12 ≤ CL ≤ 140. In particular, the thermal conductivity for 8 ≤ CL ≤ 11 cannot be obtained accurately due to the aggregation of the beads. [ABSTRACT FROM AUTHOR]

We show that the two molecular mechanics models, the stick-spiral and the beam models, predict considerably different mechanical properties of materials based on energy equivalence. The difference between the two models is independent of the materials since all parameters of the beam model are obtained from the harmonic potentials. We demonstrate this difference for finite width graphene nanoribbons and a single polyethylene chain comparing results of the molecular dynamics (MD) simulations with harmonic potentials and the finite element method with the beam model. We also find that the difference strongly depends on the loading modes, chirality and width of the graphene nanoribbons, and it increases with decreasing width of the nanoribbons under pure bending condition. The maximum difference of the predicted mechanical properties using the two models can exceed 300% in different loading modes. Comparing the two models with the MD results of AIREBO potential, we find that the stick-spiral model overestimates and the beam model underestimates the mechanical properties in narrow armchair graphene nanoribbons under pure bending condition. [ABSTRACT FROM AUTHOR]

The upper limit of the thermal conductivity and the mechanical strength are predicted for the polyethylene chain, by performing the ab initio calculation and applying the quantum mechanical non-equilibrium Green's function approach. Specially, there are two main findings from our calculation: (1) the thermal conductivity can reach a high value of 310 Wm-1 K-1 in a 100 nm polyethylene chain at room temperature and the thermal conductivity increases with the length of the chain; (2) the Young's modulus in the polyethylene chain is as high as 374.5 GPa, and the polyethylene chain can sustain 32.85%±0.05% (ultimate) strain before undergoing structural phase transition into gaseous ethylene. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations are performed to investigate edge effects on the quality factor of graphene nanoresonators with different edge configurations and of various sizes. If the periodic boundary condition is applied, very high quality factors (3 × 105) are obtained for all kinds of graphene nanoresonators. However, if the free boundary condition is applied, quality factors will be greatly reduced by two effects resulting from free edges: the imaginary edge vibration effect and the artificial effect. Imaginary edge vibrations will flip between a pair of doubly degenerate warping states during the mechanical oscillation of nanoresonators. The flipping process breaks the coherence of the mechanical oscillation of the nanoresonator, which is the dominant mechanism for extremely low quality factors. There is an artificial effect if the mechanical oscillation of the graphene nanoresonator is actuated according to an artificial vibration (non-natural vibration of the system), which slightly reduces the quality factor. The artificial effect can be eliminated by actuating the mechanical oscillation according to a natural vibration of the nanoresonator. Our simulations provide an explanation for the recent experiment, where the measured quality factor is low and varies between identical samples with free edges. [ABSTRACT FROM AUTHOR]

The electron-phonon interaction (EPI) effect in single-walled carbon nanotube is investigated by the nonequilibrium Green's function approach within the Born approximation. Special attention is paid to the EPI induced Joule heating phenomenon and the thermoelectric properties in both metallic armchair (10, 10) tube and semiconductor zigzag (10, 0) tube. For Joule heat in the metallic (10, 10) tube, the theoretical results for the breakdown bias voltage is quite comparable with the experimental value. It is found that the Joule heat can be greatly enhanced by increasing the chemical potential, while the role of the temperature is not so important for Joule heat. In the zigzag (10, 0) tube, the Joule heat is smaller than the armchair tube, resulting from nonzero bandgap in the electron band structure. For the electronic conductance Ge and electron thermal conductance σel, the EPI has important effect at higher temperature or higher chemical potential. Compared with ballistic transport, there is an opposite tendency for Ge to decrease with increasing temperature after EPI is considered. This is due to the dominant effect of the electron phonon scattering mechanism in the electron transport in this situation. There is an interesting 'electron-drag' phenomenon for the phonon thermal conductance in case of low temperature and high chemical potential, where phonons are dragged by electrons from low temperature region into high temperature region through EPI effect. [ABSTRACT FROM AUTHOR]

The phonon and electron transport in single-walled carbon nanotubes (SWCNT) are investigated using the nonequilibrium Green's function approach. In zigzag SWCNT (n,0) with mod(n,3)≠0, the thermal conductance is mainly attributed to the phonon transport, while the electron only has few percentage contribution. The maximum value of the figure of merit (ZT) is about 0.2 in this type of SWCNT. The ZT is considerably larger in narrower SWCNT because of enhanced Seebeck coefficient. ZT is smaller in the armchair SWCNT, where Seebeck coefficient is small due to zero band gap. It is found that the cluster isotopic doping can reduce the phonon thermal conductance obviously and enhance the value of ZT. The uniaxial elongation and compress strain depresses phonons in whole frequency region, leading to the reduction in the phonon thermal conductance in whole temperature range. Interestingly, the elongation strain can affect the phonon transport more seriously than the compress strain, because the high frequency G mode is completely filtered out under elongation strain ε>0.05. The strain also has important effect on the subband edges of the electron band structure by smoothing the steps in the electron transmission function. The ZT is decreased by strain as the reduction in the electronic conductance overcomes the reduction in the thermal conductance. [ABSTRACT FROM AUTHOR]

A universal exponential factor, γc=π/2, is disclosed for the dimensional crossover of few-layer graphene (FLG) from two-dimensional graphene to three-dimensional graphite. γc is found by analyzing available experimental data on different properties of FLG with varying thickness. A theoretical study on the phonon spectrum of the vertical acoustic mode in FLG is carried out to further check this exponential factor γc. Interestingly, the same exponential factor appears in the dimensional crossover of the phonon mode. It turns out that the exponential factor γc is related to the homogeneous Helmholtz-like molal equation in the mass transfer with a first order chemical reaction. The finding should provide valuable information for experimentalists and theorists in the future investigation on thickness dependent properties of FLG. [ABSTRACT FROM AUTHOR]

The topological effect on thermal conductivity is investigated through the comparison among graphene nanoribbons, carbon nanorings, and the Möbius-like graphene strips (MGS) by molecular dynamics simulation. It is found that the thermal conductivity of MGS is less than one half of that of graphene nanoribbons. The underlying mechanism whereby MGS acquire such low thermal conductivity may be attributable to the enhanced phonon-phonon scattering and localization property, which are induced by the nontrivial topology of Möbius strip. Moreover, by counting in the dimensions of MGS, a lower length/width ratio reduces its thermal conductivity, as the phonon-phonon scattering and localization within might be further elevated. [ABSTRACT FROM AUTHOR]

Structure transformation by heat treatment in single-walled carbon nanotubes (SWCNT) is investigated using molecular dynamics simulation. The critical temperature for the collapse of pure SWCNT is as high as 4655 K due to strong covalent carbon-carbon bonding. Above 2000 K, the cross section of SWCNT changes from circle to ellipse. The self-repairing capability is then investigated and two efficient processes are observed for the SWCNT to repair themselves. (1) In the first mechanism, vacancy defects aggregate to form a bigger hole, and a bottleneck junction is constructed nearby. (2) In the second mechanism, a local curvature is generated around the isolate vacancy to smooth the SWCNT. Benefit from the powerful self-repairing capability, defective SWCNT can seek a stable configuration at high temperatures; thus the critical temperature for collapse is insensitive to the vacancy defect density. [ABSTRACT FROM AUTHOR]

Thermal conductivity of graphene nanoribbons (GNR) with length 106 Å and width 4.92 Å after isotopic doping is investigated by molecular dynamics with quantum correction. Two interesting phenomena are found, (1) isotopic doping reduces thermal conductivity effectively in low doping region, and the reduction slows down in high doping region, (2) thermal conductivity increases with increasing temperature in both pure and doped GNR, but the increasing behavior is much more slow in the doped GNR than that in pure ones. Further studies reveal that the physics of these two phenomena is related to the localized phonon modes, whose number increases quickly (slowly) with increasing isotopic doping in low (high) isotopic doping region. [ABSTRACT FROM AUTHOR]

The ultrashort baseline system is widely used in ships and underwater navigation and positioning. The misalignment and level arm with the inertial measurement unit are the two sources of positioning inaccuracy. The accuracy of calibration is usually affected by measurement noise and linearization of the observation equation. In order to improve the calibration accuracy, the variational Bayesian unscented Kalman filter (VBUKF) method is proposed for the calibration of the ultrashort baseline installation error in the paper. The detailed derivation of VBUKF for the calibration is presented in the paper. Simulation experiments and field experiments were carried out, respectively, to verify the algorithm. The simulation results show that the proposed method can calibrate the installation error of the sensors in real time on line. The field experiment verified that the algorithm improves the calibration accuracy of the installation error under the large misalignment. The positioning accuracy is also improved compared with the traditional method. [ABSTRACT FROM AUTHOR]

Auxetic materials (materials with negative Poisson's ratio) and nanomaterials have independently been, for many years, two of the most active research fields in material science. Recently, these formerly independent fields have begun to intersect in new and interesting ways due to the recent discovery of auxeticity in nanomaterials like graphene, metal nanoplates, black phosphorus, and others. Here, we review the research emerging at the intersection of auxeticity and nanomaterials. We first survey the atomistic mechanisms, both intrinsic and extrinsic, that have been found, primarily through atomistic simulations, to cause auxeticity in nanomaterials. We then outline the available experimental evidence for auxetic nanomaterials. In order to lay the groundwork for future work in this exciting area, we close by discussing several future prospects as well as the current challenges in this field. [ABSTRACT FROM AUTHOR]

We derive analytical expressions for the Young's modulus and the Poisson's ratio of the graphene nanoribbon, in which free edges are warped by the compressive edge stress. Our analytical formulas explicitly illustrate the reduction of the Young's modulus by the warped free edges, leading to the obvious width dependence for the Young's modulus of the graphene nanoribbon. The Poisson's ratio is also reduced by the warped free edges, and negative Poisson's ratio can be achieved in the graphene nanoribbon with an ultra-narrow width. These results are comparable with previous theoretical works. [ABSTRACT FROM AUTHOR]

Nanotechnology has opened a new area in science and engineering, leading to the development of novel nano-electromechanical systems such as nanoresonators with ultra-high resonant frequencies. The ultra-high-frequency resonators facilitate wide-ranging applications such as ultra-high sensitive sensing, molecular transportation, molecular separation, high-frequency signal processing, and biological imaging. This paper reviews recent studies on dynamic characteristics of nanoresonators. A variety of theoretical approaches, i.e., continuum modeling, molecular simulations, and multiscale methods, in modeling of nanoresonators are reviewed. The potential application of nanoresonators in design of sensor devices and molecular transportation systems is introduced. The essence of nanoresonator sensors for detection of atoms and molecules with vibration and wave propagation analyses is outlined. The sensitivity of the resonator sensors and their feasibility in detecting different atoms and molecules are particularly discussed. Furthermore, the applicability of molecular transportation using the propagation of mechanical waves in nanoresonators is presented. An extended application of the transportation methods for building nanofiltering systems with ultra-high selectivity is surveyed. The article aims to provide an up-to-date review on the mechanical properties and applications of nanoresonators, and inspire additional potential of the resonators. [ABSTRACT FROM AUTHOR]

The topic of this review is the effects of electron-phonon interaction (EPI) on the transport properties of molecular nano-conductors. A nano-conductor connects to two electron leads and two phonon leads, possibly at different temperatures or chemical potentials. The EPI appears only in the nano-conductor. We focus on its effects on charge and energy transport. We introduce three approaches. For weak EPI, we use the nonequilibrium Green's function method to treat it perturbatively. We derive the expressions for the charge and heat currents. For weak system-lead couplings, we use the quantum master equation approach. In both cases, we use a simple single level model to study the effects of EPI on the system's thermoelectric transport properties. It is also interesting to look at the effect of currents on the dynamics of the phonon system. For this, we derive a semi-classical generalized Langevin equation to describe the nano-conductor's atomic dynamics, taking the nonequilibrium electron system, as well as the rest of the atomic degrees of freedom as effective baths. We show simple applications of this approach to the problem of energy transfer between electrons and phonons. [ABSTRACT FROM AUTHOR]

We perform classic molecular dynamics simulations to comparatively investigate the mechanical properties of single-layer MoS2 and a graphene/MoS2/graphene heterostructure under uniaxial tension. We show that the lattice mismatch between MoS2 and graphene will lead to an spontaneous strain energy in the interface. The Young's modulus of the heterostructure is much larger than that of MoS2. While the stiffness is enhanced, the yield strain of the heterostructure is considerably smaller than the MoS2 due to lateral buckling of the outer graphene layers owning to the applied mechanical tension. [ABSTRACT FROM AUTHOR]

The temperature-dependent mechanical properties of single-layer molybdenum disulphide (MoS2) are obtained using molecular dynamics (MD) nanoindentation simulations. The Young's moduli, maximum load stress, and maximum loading strain decrease with increasing temperature from 4.2 K to 500 K. The obtained Young's moduli are in good agreement with those using our MD uniaxial tension simulations and the available experimental results. The tendency of maximum loading strain with different temperature is opposite with that of metal materials due to the short range Stillinger-Weber potentials in MoS2. Furthermore, the indenter tip radius and fitting strain effect on the mechanical properties are also discussed. [ABSTRACT FROM AUTHOR]

The extreme reduction of the thermal conductivity by defects and folds in carbon nanocoils (CNCs) are first demonstrated using nonequilibrium molecular dynamics simulations. The thermal conductivity for two different defect types with five different folds in the CNCs is extensively studied and the maximum reduction can be up to 70% at both room temperature and 600 K by comparison of the corresponding straight single-walled carbon nanotubes. We reveal that the phonon scattering by coupled defects and folds can govern the reduction of the thermal conductivity by calculating phonon polarization vectors. [ABSTRACT FROM AUTHOR]

We perform classical molecular dynamics simulations to demonstrate the application of kinked silicon nanowires (KSiNWs) as nanoscale springs. The spring-like oscillation in gigahertz frequency range is successfully actuated using a similar procedure as the actuation of a classical mass spring oscillator. We detect the spring-like mechanical oscillation and some other low-frequency oscillations by the energy spectrum analysis, where a dimensional crossover phenomenon is observed for the transverse mode in KSiNWs with decreasing aspect ratio. Our findings shed light on the elastic properties of the KSiNW and open a way for its application in nanomechanical devices. [ABSTRACT FROM AUTHOR]

We study thermal transport in folded graphene nanoribbons using molecular dynamics simulations and the non-equilibrium Green's function method. It is found that the thermal conductivity of flat graphene nanoribbons can be modulated by folding and changing interlayer couplings. The analysis of transmission reveals that the reduction of thermal conductivity is due to scattering of low frequency phonons by the folds. Our results suggest that folding can be utilized in the modulation of thermal transport properties in graphene and other two dimensional materials. [ABSTRACT FROM AUTHOR]

Presents a generalized Sagdeev equation governing solitary kinetic Alfven waves (SKAW) and the criterion for the existence of SKAW which are valid for different ranges of plasma pressure parameter. Possible application of results to electromagnetic spikes observed by the Freja scientific satellite; Expression showing the behavior of dispersion.

Presents an analytical solution of finite-amplitude solitary kinetic Alfven waves. Basic equations and dispersion relation; Comparison of the solution with the solution of the Korteweg-de Vries equation in the small amplitude limit; Relations between the width and amplitude of the waves.

The minimum thermal conductance versus supercell size (ds) is revealed in graphene and boron nitride superlattice with ds far below the phonon mean free path. The minimum value is reached at a constant ratio of ds/L ≈ 5%, where L is the thickness of the superlattice; thus, the minimum point of ds depends on L. The phenomenon is attributed to the localization property and the number of confined modes in the superlattice. With the increase of ds, the localization of the confined mode is enhanced while the number of confined modes decreases, which directly results in the minimum thermal conductance. [ABSTRACT FROM AUTHOR]

The thermal conductance in graphene nanoribbon with a vacancy or silicon point defect is investigated by nonequilibrium Green's function (NEGF) formalism combined with first-principles calculations of density-functional theory with local density approximation. The thermal conductance is very sensitive to the position of the vacancy defect, while insensitive to the position of silicon defect. A vacancy defect situated at the center of the nanoribbon generates a saddlelike surface, which greatly reduces the thermal conductance by strong scattering to all phonon modes; while an edge vacancy defect only results in a further reconstruction of the edge and slightly reduces the thermal conductance. [ABSTRACT FROM AUTHOR]

The presence of GaN nanowires grown primarily normal to the surface of a sapphire substrate has a dramatic influence on the polarization dependence of laser light reflectivity at λ = 1550-nm wavelength. Even at 12% substrate surface coverage, there is a factor of 2 enhancement in polarization dependence of reflectivity relative to bulk sapphire at values of incident angle greater than φ = 72°. [ABSTRACT FROM AUTHOR]