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Top 20 Most Read Articles
May 2013
The 20 articles with the most full-text downloads during the month, in descending order.
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Transient photoreflectance of AlInN/GaN heterostructures AIP Advances 2, 042148 (2012); http://dx.doi.org/10.1063/1.4768670 (7 pages) Online Publication Date: 20 November 2012
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Time-resolved photoreflectance (PR) in AlInN/GaN heterostructures was applied to study carrier dynamics at energies extending from the uniform AlInN alloy band gap to the band gap of GaN. PR at the AlInN band gap has been found to have subpicosecond decay. Such ultrafast carrier relaxation from the extended to the sub-band edge states implies that the localization sites are small and dense, most probably originating from the In-rich clusters. At energies below the AlInN band gap, a complicated energy dependence of the PR signal is attributed to the properties of the localized states and to the modulation of the interface electric field by photoexcitation.
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Dynamic nonlinear thermal optical effects in coupled ring resonators AIP Advances 2, 032131 (2012); http://dx.doi.org/10.1063/1.4743962 (8 pages) Online Publication Date: 2 August 2012
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We investigate the dynamic nonlinear thermal optical effects in a photonic system of two coupled ring resonators. A bus waveguide is used to couple light in and out of one of the coupled resonators. Based on the coupling from the bus to the resonator, the coupling between the resonators and the intrinsic loss of each individual resonator, the system transmission spectrum can be classified by three different categories: coupled-resonator-induced absorption, coupled-resonator-induced transparency and over coupled resonance splitting. Dynamic thermal optical effects due to linear absorption have been analyzed for each category as a function of the input power. The heat power in each resonator determines the thermal dynamics in this coupled resonator system. Multiple “shark fins” and power competition between resonators can be foreseen. Also, the nonlinear absorption induced thermal effects have been discussed.
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Vibrational stability of graphene AIP Advances 3, 052101 (2013); http://dx.doi.org/10.1063/1.4804244 (10 pages) Online Publication Date: 2 May 2013
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The mechanical stability of graphene as temperature rises is analyzed based on three different self-consistent phonon (SCP) models. Compared with three-dimensional (3-D) materials, the critical temperature Ti at which instability occurs for graphene is much closer to its melting temperature Tm obtained from Monte Carlo simulation (Ti ≃ 2Tm, K. V. Zakharchenko, A. Fasolino, J. H. Los, and M. I. Katsnelson, J. Phys. Condens. Matter 23, 202202). This suggests that thermal vibration plays a significant role in melting of graphene while melting for 3-D materials is often dominated by topologic defects. This peculiar property of graphene derives from its high structural anisotropy, which is characterized by the vibrational anisotropic coefficient (VAC), defined upon its Lindermann ratios in different directions. For any carbon based material with a graphene-like structure, the VAC value must be smaller than 5.4 to maintain its stability. It is also found that the high VAC value of graphene is responsible for its negative thermal expansion coefficient at low temperature range. We believe that the VAC can be regarded as a new criterion concerning the vibrational stability of any low-dimensional (low-D) materials. |
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Electrostatic properties of few-layer MoS2 films AIP Advances 3, 042125 (2013); http://dx.doi.org/10.1063/1.4802921 (6 pages) Online Publication Date: 18 April 2013
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Two-dimensional MoS2-based materials are considered to be one of the most attractive materials for next-generation nanoelectronics. The electrostatic properties are important in designing and understanding the performance of MoS2-based devices. By using Kelvin probe force microscopy, we show that few-layer MoS2 sheets exhibit uniform surface potential and charge distributions on their surfaces but have relatively lower surface potentials on the edges, folded areas as well as defect grain boundaries. |
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Non-aqueous energy storage devices using graphene nanosheets synthesized by green route AIP Advances 3, 042112 (2013); http://dx.doi.org/10.1063/1.4802243 (9 pages) Online Publication Date: 12 April 2013
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In this paper we report the use of triethylene glycol reduced graphene oxide (TRGO) as an electrode material for non-aqueous energy storage devices such as supercapacitors and Li-ion batteries. TRGO based non–aqueous symmetric supercapacitor is constructed and shown to deliver maximum energy and power densities of 60.4 Wh kg–1 and 0.15 kW kg–1, respectively. More importantly, symmetric supercapacitor shows an extraordinary cycleability (5000 cycles) with over 80% of capacitance retention. In addition, Li-storage properties of TRGO are also evaluated in half-cell configuration (Li/TRGO) and shown to deliver a reversible capacity of ∼705 mAh g–1 with good cycleability at constant current density of 37 mA g–1. This result clearly suggests that green-synthesized graphene can be effectively used as a prospective electrode material for non-aqueous energy storage systems such as Li-ion batteries and supercapacitors. |
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A Raman spectroscopic investigation of graphite oxide derived graphene AIP Advances 2, 032183 (2012); http://dx.doi.org/10.1063/1.4756995 (13 pages) Online Publication Date: 25 September 2012
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Graphene sheets that are now routinely obtained by the exfoliation/reduction of graphite oxide exhibit Raman spectra unlike traditional graphene systems. The general attributes of the Raman spectra of these ‘wrinkled graphene’ are first reaffirmed by evaluating the spectra of samples prepared by seven different exfoliation-reduction methods. These graphene sheets exhibit highly broadened D and G Raman bands and in addition, have a modulated bump in place of the conventional 2D (G′) band. It is shown that the high wavenumber ‘bump’ can be resolved into the conventional 2D band and several defect activated peaks such as G*, D+D′ and 2D′. The broad G band could also be deconvoluted into the actual G band and the D′ band, thereby attributing the broadening in the G band to the presence of this defect activated band. Two additional modes, named as D* at 1190 cm-1 and D** at ∼1500 cm-1 could be identified. These peculiar features in the Raman spectrum of ‘graphene’ are attributed to the highly disordered and wrinkled (defective) morphology of the sheets. The affect of defects are further augmented due to the finite crystallite size of these graphene sheets. The dispersion in the band positions and peak intensities with respect to the laser energy are also demonstrated.
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Strain induced phase transitions in silicene bilayers: a first principles and tight-binding study AIP Advances 3, 052102 (2013); http://dx.doi.org/10.1063/1.4804246 (10 pages) Online Publication Date: 2 May 2013
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Using first principles and tight-binding calculations, we have investigated the structures of silicene bilayers under the isotropic tensile strain. We find that (i) the strain induce several barrierless phase transitions. (ii) After the phase transitions, the bilayer structures become planar, similar with the AA-stacking graphene bilayers, but combined with the strong covalent interlayer bonds. The tight-binding results demonstrate that this silicene bilayer is characterized by intralayer sp2 hybridization and the interlayer sp1 hybridization. (iii) The electronic properties of the silicene bilayers change from semiconducting to metallic with the increase of strain.
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Ultraviolet-visible spectroscopy of graphene oxides AIP Advances 2, 032146 (2012); http://dx.doi.org/10.1063/1.4747817 (5 pages) Online Publication Date: 17 August 2012
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Dispersions of few-layer (1-3 layers), multi-layer (4-10 layers) and thick-layer (>10 layers) graphene oxide (GO) were prepared by a modified Hummers method with different mass ratios of KMnO4 to graphite. Ultraviolet-visible (UV-vis) spectroscopic data show that few-layer GO dispersions can be distinguished from multi- and thick-layer dispersions by a more intense peak at 230 nm. Atomic force microscopy (AFM) images of few-layer GO contain a single peak, those of multi-layer GO exhibit a shoulder and those of thick-layer GO do not contain a peak or shoulder. These findings allow qualitative analysis of GO dispersions. X-ray photoelectron spectra (XPS) show that the change of UV-vis absorption intensity of GO is caused by a conjugative effect related to chromophore aggregation that influences the π-π* plasmon peak.
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Reconfigurable resistive switching devices based on individual tungsten trioxide nanowires AIP Advances 3, 042137 (2013); http://dx.doi.org/10.1063/1.4804067 (7 pages) Online Publication Date: 30 April 2013
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In the two-terminal Au/WO3 nanowire/Au electronic device with two Schottky barriers, drifting of oxygen vacancies under strong electric field induced by the bias voltage applied at short distance will result in the effective width of the reverse biased Schottky barrier decreasing, and then result in the memristive effect or resistive switching phenomenon. By unidirectional bias voltage sweeping, the Au/WO3 Schottky contact can be turned gradually and reversibly into Ohmic contact, and then the two-terminal Au/WO3 nanowire/Au resistive switching device can be reconfigured gradually and reversibly from non-rectifying state to either a forward or reverse rectifying state. |
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Control of two-dimensional growth of AlN and high Al-content AlGaN-based MQWs for deep-UV LEDs AIP Advances 3, 052103 (2013); http://dx.doi.org/10.1063/1.4804247 (6 pages) Online Publication Date: 2 May 2013
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Dense and atomically flat AlN film with root-mean-square roughness value of 0.32 nm was grown on sapphire substrate at a relatively lower temperature by using a three-step epitaxy technique. On the basis of this AlN template, AlGaN-based multiple quantum wells (MQWs) with atomically flat hetero-interfaces were epitaxially grown to suppress nonradiative recombination by introducing In as a surfactant during simultaneous source supply. As a result, single intense- and narrow-peaked photoluminescence was obtained from the MQWs. Finally, the deep ultraviolet light emitting diodes with well-behaved I-V characteristic and strong electroluminescence in the range of 256–312 nm were fabricated successfully. |
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Thermal transport in nanostructures AIP Advances 2, 041410 (2012); http://dx.doi.org/10.1063/1.4773462 (24 pages) Online Publication Date: 28 December 2012
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This review summarizes recent studies of thermal transport in nanoscaled semiconductors. Different from bulk materials, new physics and novel thermal properties arise in low dimensional nanostructures, such as the abnormal heat conduction, the size dependence of thermal conductivity, phonon boundary/edge scatterings. It is also demonstrated that phonons transport super-diffusively in low dimensional structures, in other words, Fourier's law is not applicable. Based on manipulating phonons, we also discuss envisioned applications of nanostructures in a broad area, ranging from thermoelectrics, heat dissipation to phononic devices.
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AIP Advances 2, 010901 (2012); http://dx.doi.org/10.1063/1.3699622 (3 pages) Online Publication Date: 29 March 2012
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Physics of cancer propagation: A game theory perspective AIP Advances 2, 011202 (2012); http://dx.doi.org/10.1063/1.3699043 (10 pages) Online Publication Date: 22 March 2012
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This is a theoretical paper which examines at a game theoretical perspective the dynamics of cooperators and cheater cells under metabolic stress conditions and high spatial heterogeneity. Although the ultimate aim of this work is to understand the dynamics of cancer tumor evolution under stress, we use a simple bacterial model to gain fundamental insights into the progression of resistance to drugs under high competition and stress conditions.
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AIP Advances 3, 032137 (2013); http://dx.doi.org/10.1063/1.4798305 (6 pages) Online Publication Date: 20 March 2013
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Transparent bottom-gate amorphous Indium-Gallium-Zinc Oxide (a-IGZO) thin-film transistors (TFTs) had been successfully fabricated at relative low temperature. The influence of reaction gas ratio of N2O and SiH4 during the growth of etching stop layer (SiOx) on the characteristics of a-IGZO TFTs was investigated. The transfer characteristics of the TFTs were changed markedly because active layer of a-IGZO films was modified by plasma in the growth process of SiOx. By optimizing the deposition parameters of etching stop layer process, a-IGZO TFTs were manufactured and exhibited good performance with a field-effect mobility of 8.5 cm2V-1s-1, a threshold voltage of 1.3 V, and good stability under gate bias stress of 20 V for 10000 s.
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Atomic scale study of thermal reduction of nano goethite coexisting with magnetite AIP Advances 3, 022101 (2013); http://dx.doi.org/10.1063/1.4790614 (15 pages) Online Publication Date: 1 February 2013
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Evolution of the local structure and magnetic properties of nano particles of goethite having magnetite as a composite due to controlled annealing treatments in vacuum has been studied using Mossbauer spectroscopy. Importance of size, defect associated with structural OH- for the observed structural and magnetic properties of goethite has been emphasized in this study. Present Mossbauer results show that thermal annealing at low temperatures (420-550 K) lead to a partial conversion / reduction of orthorhombic goethite to cubic spinel oxides such as maghemite and off-stochiometric magnetite. This study further establishes that annealing treatments beyond 650 K predominantly results in topotactic conversion of goethite to haematite. Underlying physics of the transitions of goethite to iron oxides and the important role of desorbed hydrogen for the orthorhombic to cubic structural transitions has been elucidated in this study.
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First-principles calculations of mechanical and electronic properties of silicene under strain AIP Advances 2, 022159 (2012); http://dx.doi.org/10.1063/1.4732134 (6 pages) Online Publication Date: 25 June 2012
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We perform first-principles calculations of mechanical and electronic properties of silicene under strains. The in-plane stiffness of silicene is much smaller than that of graphene. The yielding strain of silicene under uniform expansion in the ideal conditions is about 20%. The homogeneous strain can introduce a semimetal-metal transition. The semimetal state of silicene, in which the Dirac cone locates at the Fermi level, can only persist up to tensile strain of 7% with nearly invariant Fermi velocity. For larger strains, silicene changes into a conventional metal. The work function is found to change significantly under biaxial strain. Our calculations show that strain tuning is important for applications of silicene in nanoelectronics.
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Review Article: Structures of phthalocyanine molecules on surfaces studied by STM AIP Advances 2, 041402 (2012); http://dx.doi.org/10.1063/1.4773458 (10 pages) Online Publication Date: 28 December 2012
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This review mainly focuses on progress recently achieved in the growth of phthalocyanine molecules on single-crystal surfaces of sub-monolayer up to few-monolayer thin films studied by scanning tunneling microscopy in our groups. On metallic surfaces such as Au(111), Ag(111) and Cu(111), molecular superstructures are determined by combining directional intermolecular interactions caused by symmetry reduction, molecule-substrate interactions and indirect long-range interactions due to quantum interference of surface state electrons. On semiconducting TiO2 surface, molecular assembling structures are dictated by the strong molecule-substrate interaction. However, on insulating NaCl film, molecule-molecule interaction dominates over the molecule-NaCl coupling, leading to molecular growth behavior. Knowledge obtained from these studies would help people better understand the physicochemical properties of the phthalocyanine molecules at surfaces so that their new applications could be further explored and uncovered in the future.
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Triboelectric charging of insulating polymers–some new perspectives AIP Advances 2, 010701 (2012); http://dx.doi.org/10.1063/1.3687233 (9 pages) Online Publication Date: 8 February 2012
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Tribolectric charging results from contact between surfaces, but precisely what is meant by each is not defined or understood, as they relate to charging. The recent microscopic evidence that contact charging can result from material transfer provides incentive to examine how contact charging is affected by these two factors. It is suggested that vigorous rubbing or pressing of two polymers results in transfer of deeper layers than would result from light contacts. Different layers can have substantially different compositions because polymers are typically not homogeneous as a function of depth, so contact and surface are related in this way. This could account for charge transfer between identical polymers, especially in asymmetric contacts in which the frictional force on one polymer differs from that on the other, so that material from different depths is transferred. This review outlines the roles of physics, chemistry and surface analysis in sufficient detail to focus on these subjects. It also makes suggestions how these concepts could be applied to some of the current leading edge research in this area.
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Role of defect states in magnetic and electrical properties of ZnO nanowires AIP Advances 3, 042110 (2013); http://dx.doi.org/10.1063/1.4801937 (6 pages) Online Publication Date: 9 April 2013
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The perfect crystalline nature along with a defect ridden surface controls the electrical and magnetic properties of ZnO nanowires. Herein, a soft chemical approach is presented to grow ZnO nanowires in powder as well as highly oriented nanowire film form. Photoluminescence measurements reveal high surface defects in as-grown nanowire and post growth annealing treatment in argon and oxygen atmosphere reduces intensity of defect emissions. Magnetic measurements illustrate the ferromagnetic nature of submicron sized zinc oxide (ZnO) nanorods arising due to singly charged oxygen vacancies. Nanowires show diamagnetic behavior when annealed at higher temperature in oxygen while argon annealing does not affect the magnetic behavior. In an analogous manner, we also investigated the effect of surface defects on electrical properties and correlated electrical conductivity with a responsible defect state. |
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Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament AIP Advances 2, 012151 (2012); http://dx.doi.org/10.1063/1.3690961 (13 pages) Online Publication Date: 17 February 2012
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In the perspective of the laser lightning rod, the ability of femtosecond filaments to trigger and to guide large scale discharges has been studied for several years. The present paper reports recent experimental results showing for the first time that filaments are able not only to trigger and guide but also to divert an electric discharge from its normal path. Laser filaments are also able to divert the spark without contact between laser and electrodes at large distance from the laser. A comparison between negative and positive discharge polarities also reveals important discrepancies in the guiding mechanism.
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