Showing papers in "Applied Physics Letters in 2010"
TL;DR: Using atomic resolved scanning tunneling microscopy, the experimental evidence of a silicene sheet epitaxially grown on a close-packed silver surface [Ag(111] was presented in this article, which was achieved via direct condensation of a silicon atomic flux onto the single-crystal substrate in ultrahigh vacuum conditions.
Abstract: Using atomic resolved scanning tunneling microscopy, we present here the experimental evidence of a silicene sheet (graphenelike structure) epitaxially grown on a close-packed silver surface [Ag(111)]. This has been achieved via direct condensation of a silicon atomic flux onto the single-crystal substrate in ultrahigh vacuum conditions. A highly ordered silicon structure, arranged within a honeycomb lattice, is synthesized and present two silicon sublattices occupying positions at different heights (0.02 nm) indicating possible sp2-sp3 hybridizations.
1,253 citations
TL;DR: In this article, an ultrathin, wide-angle, subwavelength high performance metamaterial absorber for optical frequencies is presented. But the experimental results show that an absorption peak of 88% is achieved at the wavelength of ∼1.58μm, though theoretical results give near perfect absorption.
Abstract: High absorption efficiency is particularly desirable at present for various microtechnological applications including microbolometers, photodectors, coherent thermal emitters, and solar cells. Here we report the design, characterization, and experimental demonstration of an ultrathin, wide-angle, subwavelength high performance metamaterial absorber for optical frequencies. Experimental results show that an absorption peak of 88% is achieved at the wavelength of ∼1.58 μm, though theoretical results give near perfect absorption.
1,147 citations
TL;DR: In this paper, an elongated ferromagnetic cantilevered beam having a base and an opposing end is used for scavenging vibrational energy from an external excitation source.
Abstract: The present invention relates to the field of energy harvesting. More particularly, embodiments of the invention relate to methods, systems, and devices for scavenging vibration- based energy from an ambient vibration source. Specific embodiments of the present invention include an energy harvesting device comprising: a) an elongated ferromagnetic cantilevered beam having a base and an opposing end; b) a plurality of piezoceramic elements operably connected to the base of the beam; c) a first support member for supporting the beam at its base; and d) two permanent magnets disposed on a second support member; wherein the base end of the beam is operably connected to the support member such that the beam is suspended lengthwise from the support member at its base and the opposing end of the beam is free and is disposed a selected distance above and between the magnets; and wherein the piezoceramic elements are operably connected in parallel to each other, such that during operation the beam is capable of scavenging vibrational energy from an external excitation source and the piezoceramic elements are capable of converting the harmonic or random vibrational energy into electrical energy. The piezo-magneto-elastic generator results in a 200% increase in the open- circuit voltage amplitude (hence promising an 800% increase in the power amplitude). The inventive piezo-magneto-elastic generator can be applied for use in piezoelectric energy harvesting, as well as in electromagnetic, electrostatic and magnetostrictive energy harvesting techniques and their hybrid combinations with similar devices.
958 citations
TL;DR: In this article, the self-aligned silicon nanoribbons on Ag(110) with honeycomb, graphene-like structure were observed by scanning tunneling microscopy (STM) and ab initio calculations based on density functional theory.
Abstract: Scanning tunneling microscopy (STM) and ab initio calculations based on density functional theory (DFT) were used to study the self-aligned silicon nanoribbons on Ag(110) with honeycomb, graphene-like structure. The silicon honeycombs structure on top of the silver substrate is clearly observed by STM, while the DFT calculations confirm that the Si atoms adopt spontaneously this new silicon structure.
915 citations
TL;DR: In this paper, the role of oxygen vacancies and various impurities in the electrical and optical properties of the transparent conducting oxide β-Ga2O3 was investigated using hybrid functionals.
Abstract: Using hybrid functionals we have investigated the role of oxygen vacancies and various impurities in the electrical and optical properties of the transparent conducting oxide β-Ga2O3. We find that oxygen vacancies are deep donors, and thus cannot explain the unintentional n-type conductivity. Instead, we attribute the conductivity to common background impurities such as silicon and hydrogen. Monatomic hydrogen has low formation energies and acts as a shallow donor in both interstitial and substitutional configurations. We also explore other dopants, where substitutional forms of Si, Ge, Sn, F, and Cl are shown to behave as shallow donors.
713 citations
TL;DR: In this paper, a longitudinal spin-Seebeck effect (SSE) is proposed, in which a magnon-induced spin current is injected parallel to a temperature gradient from a ferromagnet into an attached paramagnetic metal.
Abstract: We propose a longitudinal spin-Seebeck effect (SSE), in which a magnon-induced spin current is injected parallel to a temperature gradient from a ferromagnet into an attached paramagnetic metal. The longitudinal SSE is measured in a simple and versatile system composed of a ferrimagnetic insulator Y3Fe5O12 slab and a Pt film by means of the inverse spin-Hall effect. The experimental results highlight the intriguing character of the longitudinal SSE due to its own geometric configuration.
693 citations
TL;DR: In this paper, the authors used an environmental scanning electron microscope to study frost formation and its impact on icephobic properties of super-hydrophobic surfaces, and showed that frost nucleation occurs indiscriminately on super hydrophobic textures without any particular spatial preference.
Abstract: We study frost formation and its impact on icephobic properties of superhydrophobic surfaces. Using an environmental scanning electron microscope, we show that frost nucleation occurs indiscriminately on superhydrophobic textures without any particular spatial preference. Ice adhesion measurements on superhydrophobic surfaces susceptible to frost formation show increased adhesion over smooth surfaces with a strong linear trend with the total surface area. These studies indicate that frost formation significantly compromises the icephobic properties of superhydrophobic surfaces and poses serious limitations to the use of superhydrophobic surfaces as icephobic surface treatments for both on-ground and in-flight applications.
652 citations
TL;DR: In this paper, high efficiency solar cells with up to 6.8% efficiency were obtained with absorber layer thicknesses less than 1μm and annealing times in the minutes.
Abstract: High efficiency Cu2ZnSnS4 solar cells have been fabricated on glass substrates by thermal evaporation of Cu, Zn, Sn, and S. Solar cells with up to 6.8% efficiency were obtained with absorber layer thicknesses less than 1 μm and annealing times in the minutes. Detailed electrical analysis of the devices indicate that the performance of the devices is limited by high series resistance, a “double diode” behavior of the current voltage characteristics, and an open circuit voltage that is limited by a carrier recombination process with an activation energy below the band gap of the material.
588 citations
TL;DR: In this paper, the authors demonstrate over 1×1010 open-loop switching cycles from a simple memristive device stack of Pt/TaOx/Ta, and compare this system to a similar device stack based on titanium oxides to obtain insight into the solid-state thermodynamic and kinetic factors that influence endurance in metal-oxide memristors.
Abstract: We demonstrate over 1×1010 open-loop switching cycles from a simple memristive device stack of Pt/TaOx/Ta. We compare this system to a similar device stack based on titanium oxides to obtain insight into the solid-state thermodynamic and kinetic factors that influence endurance in metal-oxide memristors.
570 citations
TL;DR: In this article, the electronic properties of straight, 1.6 nm wide, silicene nanoribbons on Ag(110), arranged in a one-dimensional grating with a pitch of 2 nm, whose high-resolution scanning tunneling microscopy images reveal a honeycomb geometry.
Abstract: We report on the electronic properties of straight, 1.6 nm wide, silicene nanoribbons on Ag(110), arranged in a one-dimensional grating with a pitch of 2 nm, whose high-resolution scanning tunneling microscopy images reveal a honeycomb geometry. Angle-resolved photoemission shows quantum confined electronic states of one-dimensional character. The silicon band dispersion along the direction of the nanoribbons suggests a behavior analogous to the Dirac cones of graphene on different substrates.
555 citations
TL;DR: In this paper, the effects of carbon on the electrical and optical properties of GaN were investigated using hybrid functional calculations, and it was shown that carbon substituting for N (CN) has an ionization energy of 0.90 eV.
Abstract: Using hybrid functional calculations we investigate the effects of carbon on the electrical and optical properties of GaN. In contrast to the currently accepted view that C substituting for N (CN) is a shallow acceptor, we find that CN has an ionization energy of 0.90 eV. Our calculated absorption and emission lines also indicate that CN is a likely source for the yellow luminescence that is frequently observed in GaN, solving the longstanding puzzle of the nature of the C-related defect involved in yellow emission. Our results suggest that previous experimental data, analyzed under the assumption that CN acts as a shallow acceptor, should be re-examined.
TL;DR: In this article, the authors describe an ultra-sensitive atomic magnetometer based on optically pumped potassium atoms operating in a spin exchange relaxation free regime, and demonstrate magnetic field sensitivity of 160 εaT/Hz1/2 in a gradiometer arrangement with a measurement volume of 0.45 cm3 and energy resolution per unit bandwidth of 44ℏ.
Abstract: We describe an ultrasensitive atomic magnetometer based on optically pumped potassium atoms operating in a spin-exchange relaxation free regime. We demonstrate magnetic field sensitivity of 160 aT/Hz1/2 in a gradiometer arrangement with a measurement volume of 0.45 cm3 and energy resolution per unit bandwidth of 44ℏ. As an example of an application enabled by such a magnetometer, we describe measurements of weak remnant rock magnetization as a function of temperature with a sensitivity on the order of 10−10 emu/cm3/Hz1/2 and temperatures up to 420°C.
TL;DR: In this article, a temperature-composition phase diagram is proposed that exhibits compositionally driven phase transitions with easy paths for both polarization rotation and polarization extension, which is best known at temperature-driven ferroelectric-paraelectric phase transitions.
Abstract: Many ferroelectric solid solutions exhibit enhanced electromechanical properties at the morphotropic boundary separating two phases with different orientations of polarization. The mechanism of properties enhancement is associated with easy paths for polarization rotation in anisotropically flattened free energy profile. Another mechanism of properties enhancement related to free energy flattening is polarization extension. It is best known at temperature-driven ferroelectric-paraelectric phase transitions and may lead to exceedingly large properties. Its disadvantage is temperature instability of the enhancement. In this paper a temperature-composition phase diagram is proposed that exhibits compositionally driven-phase transitions with easy paths for both polarization rotation and polarization extension.
TL;DR: In this paper, the authors proposed that to maximize the solar cell performance, growth of Cu2ZnSnS4 under Cu-poor/Zn-rich conditions will be optimal, if the precipitation of ZnS can be avoided by kinetic barriers.
Abstract: Cu2ZnSnS4 is one of the most promising quaternary absorber materials for thin-film solar cells Examination of the thermodynamic stability of this quaternary compound reveals that the stable chemical potential region for the formation of stoichiometric compound is small Under these conditions, the dominant defect will be p-type CuZn antisite, which has an acceptor level deeper than the Cu vacancy The dominant self-compensated defect pair in this quaternary compound is [CuZn−+ZnCu+]0, which leads to the formation of various polytype structures of Cu2ZnSnS4 We propose that to maximize the solar cell performance, growth of Cu2ZnSnS4 under Cu-poor/Zn-rich conditions will be optimal, if the precipitation of ZnS can be avoided by kinetic barriers
TL;DR: In this paper, a wide range (1570-1600 nm) continuous wavelength tunable dissipative solitons could be formed in an erbium doped fiber laser mode locked with few layer graphene.
Abstract: Atomic layer graphene possesses wavelength-insensitive ultrafast saturable absorption, which can be exploited as a “full-band” mode locker. Taking advantage of the wide band saturable absorption of the graphene, we demonstrate experimentally that wide range (1570–1600 nm) continuous wavelength tunable dissipative solitons could be formed in an erbium doped fiber laser mode locked with few layer graphene.
TL;DR: In this paper, the authors show that gas-phase doping by means of NH3 plasma exposure is a highly flexible and manufacturable process for graphene electronics, and that the amount of charge transfer can be fine tuned by controlling the exposure time and monitored by the systematic shift in the Raman G mode and the Gds−Vg curves.
Abstract: Here we show that gas-phase doping by means of NH3 plasma exposure is a highly flexible and manufacturable process for graphene electronics. The nitrogen-containing radicals can readily form covalent bonds with the carbon lattice and keep stable in the postannealing for damage restoration. The amount of charge transfer can be fine tuned by controlling the exposure time and monitored by the systematic shift in the Raman G mode and the Gds−Vg curves in transport measurements. The maximum doping level can reach 1.5×1013 cm−2.
TL;DR: In this article, the effect of applied electric field EG on thickness dependent magnetic anisotropy of sputtered Co40Fe40B20 sandwiched with MgO and Ta was investigated.
Abstract: We have investigated the effect of applied electric field EG on thickness dependent magnetic anisotropy of sputtered Co40Fe40B20 sandwiched with MgO and Ta. The range of CoFeB thickness explored is 2 nm and below. As the thickness is reduced, the easy axis of magnetization becomes perpendicular from in-plane. We show that perpendicular magnetic anisotropy of in-plane samples and coercivity of perpendicular samples can be modified by applying EG at room temperature. Furthermore, superparamagnetic behavior is observed for CoFeB layers with further reduced thickness below ≈0.9 nm, where electric-field effect is also observed below their blocking temperature.
TL;DR: Karalis et al. as mentioned in this paper explored the effect of adding multiple devices on the tuning and overall efficiency of the power transfer, and demonstrate this scheme experimentally for the case of coupling objects of different sizes.
Abstract: Electromagnetic resonators strongly coupled through their near-fields [A. Karalis, J. D. Joannopoulos, and M. Soljacic, Ann. Phys. 323, 34 (2008); A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M. Soljacic, Science 317, 83 (2007)] are able to achieve efficient wireless power transfer from a source to a device separated by distances multiple times larger than the characteristic sizes of the resonators. This midrange approach is therefore suitable for remotely powering several devices from a single source. We explore the effect of adding multiple devices on the tuning and overall efficiency of the power transfer, and demonstrate this scheme experimentally for the case of coupling objects of different sizes: a large source (1 m2 in area) powering two smaller devices (each ≃0.07 m2 in area).
TL;DR: In this article, the authors exploit the nonlinear behavior of a bistable composite plate with bonded piezoelectric patches for broadband nonlinear energy harvesting, and the response of the structure is experimentally investigated revealing different large amplitude oscillations.
Abstract: Recently, the idea of using nonlinearity to enhance the performance of vibration-based energy harvesters has been investigated. Nonlinear energy harvesting devices have been shown to be capable of operating over wider frequency ranges delivering more power than their linear counterparts, rendering them more suitable for real applications. In this paper, we propose to exploit the rich nonlinear behavior of a bistable composite plate with bonded piezoelectric patches for broadband nonlinear energy harvesting. The response of the structure is experimentally investigated revealing different large amplitude oscillations. Substantially large power is extracted over a wide frequency range achieving broadband nonlinear energy harvesting.
TL;DR: In this paper, the authors show that thin membrane-type acoustic metamaterials can serve as a total reflection nodal surface at certain frequencies, which implies that several membrane panels can be stacked to achieve broad-frequency effectiveness.
Abstract: We show experimentally that thin membrane-type acoustic metamaterials can serve as a total reflection nodal surface at certain frequencies. The small decay length of the evanescent waves at these frequencies implies that several membrane panels can be stacked to achieve broad-frequency effectiveness. We report the realization of acoustic metamaterial panels with thickness ≤15 mm and weight ≤3 kg/m2 demonstrating 19.5 dB of internal sound transmission loss (STL) at around 200 Hz, and stacked panels with thickness ≤60 mm and weight ≤15 kg/m2 demonstrating an average STL of >40 dB over a broad range from 50 to 1000 Hz.
TL;DR: In this article, the authors examined mobility and saturation velocity in graphene on SiO2 above room temperature (300-500 K) and at high fields (∼1V/μm).
Abstract: We examine mobility and saturation velocity in graphene on SiO2 above room temperature (300–500 K) and at high fields (∼1 V/μm). Data are analyzed with practical models including gated carriers, thermal generation, “puddle” charge, and Joule heating. Both mobility and saturation velocity decrease with rising temperature above 300 K, and with rising carrier density above 2×1012 cm−2. Saturation velocity is >3×107 cm/s at low carrier density, and remains greater than in Si up to 1.2×1013 cm−2. Transport appears primarily limited by the SiO2 substrate but results suggest intrinsic graphene saturation velocity could be more than twice that observed here.
TL;DR: In this paper, the authors reported the generation of 174 fs pulses from a graphene-based fiber laser for ultrafast spectroscopy with high temporal resolution, which is an ideal ultrawide-band saturable absorber.
Abstract: Ultrafast fiber lasers with broad bandwidth are in great demand for a variety of applications, such as spectroscopy, biomedical diagnosis, and optical communications. Sub 200 fs pulses are required for ultrafast spectroscopy with high temporal resolution. Graphene is an ideal ultrawide-band saturable absorber. We report the generation of 174 fs pulses from a graphene-based fiber laser.
Abstract: A facile method is reported for the direct (polymer-free) transfer of layer-area graphene from metal growth substrates to selected target substrates. The direct route, by avoiding several wet chemical steps and accompanying mechanical stresses and contamination common to all presently reported layer-area graphene transfer methods, enables fabrication of layer-area graphene devices with unprecedented quality. To demonstrate, we directly transfer layer-area graphene from Cu growth substrates to holey amorphous carbon transmission electron microscopy (TEM) grids, resulting in robust, clean, full-coverage graphene grids ideal for high resolution TEM.
TL;DR: In this paper, the authors demonstrate that smooth and flat surfaces combining hydrophilic and hydrophobic patterns improve pool boiling performance, up to a 65% and 100% increase in critical heat flux and heat transfer coefficients.
Abstract: We demonstrate that smooth and flat surfaces combining hydrophilic and hydrophobic patterns improve pool boiling performance. Compared to a hydrophilic surface with 7° wetting angle, the measured critical heat flux and heat transfer coefficients of the enhanced surfaces are, up to respectively, 65% and 100% higher. Different networks combining hydrophilic and hydrophobic regions are characterized. While all tested networks enhance the heat transfer coefficient, large enhancements of critical heat flux are typically found for hydrophilic networks featuring hydrophobic islands. Hydrophilic networks indeed are shown to prevent the formation of an insulating vapor layer. © 2010 American Institute of Physics. doi:10.1063/1.3485057 Boiling is an efficient process to transfer large amounts of heat at a prescribed temperature because of the large latent heat of vaporization. The term flow boiling describes the boiling of liquids forced to move along hot surfaces, while in pool boiling, the topic handled in this paper, the liquid is stagnant and in contact with a hot solid surface. 1 Besides the common experience of boiling water in an electric kettle, pool boiling has applications in metallurgy, high performance heat exchangers, and immersion cooling of electronics. Pool boiling performance is measured with two parameters, the heat transfer coefficient HTC and the critical heat flux CHF. The CHF is measured by increasing the surface temperature until a transition from high HTC to very low HTC occurs. This signifies the formation of a vapor film insulating the liquid from the heated surface, a phenomenon called dry out. Several characteristics determine the performance of a boiling surface. Nucleation sites in appropriate number and dimensions need to be provided such as cavities, rough areas, or hydrophobic islands. 2 As of today, the performance of boiling surfaces has been increased by using wicking structures to prevent dry out, 3 by increasing the surface area with fins or fluidized bed, 3‐6 and by enhancing the wettability of the surface. 5‐10 The latter strategy is justified by experiments of Wang and Dhir, 11 showing that the CHF was
TL;DR: In this article, a deterministic design of an ultrahigh Q-factor, wavelength-scale photonic crystal nanobeam cavity is proposed and experimentally demonstrated using this approach, cavities with Q>106 and on-resonance transmission T>90% are designed.
Abstract: A deterministic design of an ultrahigh Q-factor, wavelength-scale photonic crystal nanobeam cavity is proposed and experimentally demonstrated. Using this approach, cavities with Q>106 and on-resonance transmission T>90% are designed. The devices, fabricated in silicon and capped with a low refractive index polymer, have experimental Q=80 000 and T=73%. This is, to the best of our knowledge, the highest transmission measured in deterministically designed, wavelength-scale high-Q cavities.
TL;DR: In this paper, the preparation of ultrathin, transparent graphene films for use in supercapacitor applications is described, revealing a very homogeneous surface with intimate contact between graphene sheets.
Abstract: This study reports the preparation of ultrathin, transparent graphene films for use in supercapacitor applications. The surface morphology of the films was investigated by scanning electron microscopy and transmission electron microscopy, revealing a very homogeneous surface with intimate contact between graphene sheets. Electrochemical characterization demonstrated nearly ideal electrical double layer capacitive behavior. The capacitance obtained from charge-discharge analysis is 135 F/g for a film of approximately 25 nm which has a transmittance of 70% at 550 nm and a high power density of 7200 W/kg in 2 M KCl electrolyte.
TL;DR: In this paper, a novel technology for controlling the composition of InGaN quantum wells on the same wafer was developed, which paved the way for the monolithic integration of three primary-color nano-light-emitting diodes.
Abstract: A novel technology for controlling the In composition of InGaN quantum wells on the same wafer was developed, which paved the way for the monolithic integration of three-primary-color nano-light-emitting diodes. In the experiment, InGaN/GaN multiple quantum well nanocolumn arrays with nanocolumn diameters from 137 to 270 nm were prepared on the same substrate with the Ti-mask selective area growth by rf-plasma-assisted molecular beam epitaxy. The emission color changed from blue to red (from 479 to 632 nm in wavelength) with increasing nanocolumn diameter. The emission color change mechanism was clearly explained by the beam shadow effect of the neighboring nanocolumns.
TL;DR: A mechanically exfoliated graphene flake (? 150×380??m2) on a silicon wafer with 98 nm silicon dioxide on top was scanned with a spectroscopic ellipsometer with a focused spot at an angle of 55°.
Abstract: A mechanically exfoliated graphene flake ( ? 150×380??m2) on a silicon wafer with 98 nm silicon dioxide on top was scanned with a spectroscopic ellipsometer with a focused spot ( ? 100×55??m2) at an angle of 55°. The spectroscopic ellipsometric data were analyzed with an optical model in which the optical constants were parameterized by B-splines. This parameterization is the key for the simultaneous accurate determination of the optical constants in the wavelength range 210–1000 nm and the thickness of graphene, which was found to be 3.4 A.
TL;DR: In this article, the authors show that O-vacancy acts as a hole trap and plays a role in negative bias illumination stress instability in amorphous In-Ga-Zn-O thin film transistors.
Abstract: We find that O-vacancy (VO) acts as a hole trap and plays a role in negative bias illumination stress instability in amorphous In–Ga–Zn–O thin film transistors. Photoexcited holes drift toward the channel/dielectric interface due to small potential barriers and can be captured by VO in the dielectrics. While some of VO+2 defects are very stable at room temperature, their original deep states are recovered via electron capture upon annealing. We also find that VO+2 can diffuse in amorphous phase, inducing hole accumulation near the interface under negative gate bias.
TL;DR: In this paper, a bilayered chiral metamaterial is proposed to realize a 90° polarization rotator, whose giant optical activity is due to the transverse magnetic dipole coupling among the metallic wire pairs of enantiomeric patterns.
Abstract: A bilayered chiral metamaterial is proposed to realize a 90° polarization rotator, whose giant optical activity is due to the transverse magnetic dipole coupling among the metallic wire pairs of enantiomeric patterns. By transmission through this thin bilayered structure of less than λ/30 thick, a linearly polarized wave is converted to its cross polarization with a resonant polarization conversion efficiency of over 90%. It is demonstrated that the chirality in the propagation direction makes this efficient cross-polarization conversion possible.