Showing papers in "Applied Physics Letters in 2007"
TL;DR: In this article, the authors study the visibility of graphene and show that it depends strongly on both thickness of SiO2 and light wavelength, and they find that ≈100nm is the most suitable wavelength for its visual detection.
Abstract: Microfabrication of graphene devices used in many experimental studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepared on top of Si wafers with a certain thickness of SiO2. The authors study graphene’s visibility and show that it depends strongly on both thickness of SiO2 and light wavelength. They have found that by using monochromatic illumination, graphene can be isolated for any SiO2 thickness, albeit 300nm (the current standard) and, especially, ≈100nm are most suitable for its visual detection. By using a Fresnel-law-based model, they quantitatively describe the experimental data.
1,958 citations
TL;DR: The effect of Ar plasma treatment on amorphous indium gallium zinc oxide (a-IGZO) thin films was investigated in this paper, where the authors attempted to reduce the contact resistance between the Pt∕Ti (source/drain electrode) and a-IZO (channel).
Abstract: The effect of Ar plasma treatment on amorphous indium gallium zinc oxide (a-IGZO) thin films was investigated The net electron carrier concentration (1020–1021cm−3) of the a-IGZO thin films dramatically increased upon their exposure to the Ar plasma compared to that (1014cm−3) of the as-deposited thin film The authors attempted to reduce the contact resistance between the Pt∕Ti (source/drain electrode) and a-IGZO (channel) by using the Ar plasma treatment Without the treatment, the a-IGZO thin film transistors (TFTs) with W∕L=50∕4μm exhibited a moderate field-effect mobility (μFE) of 33cm2∕Vs, subthreshold gate swing (S) of 025V∕decade, and Ion∕off ratio of 4×107 The device performance of the a-IGZO TFTs was significantly improved by the Ar plasma treatment As a result, an excellent S value of 019V∕decade and high Ion∕off ratio of 1×108, as well as a high μFE of 91cm2∕Vs, were achieved for the treated a-IGZO TFTs
1,295 citations
TL;DR: In this paper, the efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes was investigated and it was shown that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MqW region.
Abstract: The efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.
1,252 citations
TL;DR: In this article, an efficiency of 40.7% was achieved for a metamorphic three-junction GaInP∕GaInAs∕Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0W∕cm2, AM1.5D, low aerosol optical depth, 25°C).
Abstract: An efficiency of 40.7% was measured and independently confirmed for a metamorphic three-junction GaInP∕GaInAs∕Ge cell under the standard spectrum for terrestrial concentrator solar cells at 240 suns (24.0W∕cm2, AM1.5D, low aerosol optical depth, 25°C). This is the initial demonstration of a solar cell with over 40% efficiency, and is the highest solar conversion efficiency yet achieved for any type of photovoltaic device. Lattice-matched concentrator cells have now reached 40.1% efficiency. Electron-hole recombination mechanisms are analyzed in metamorphic GaxIn1−xAs and GaxIn1−xP materials, and fundamental power losses are quantified to identify paths to still higher efficiencies.
1,205 citations
TL;DR: In this paper, the Auger recombination coefficient in quasi-bulk InxGa1−xN (x∼9%−15%) layers grown on GaN (0001) is measured by a photoluminescence technique.
Abstract: The Auger recombination coefficient in quasi-bulk InxGa1−xN (x∼9%–15%) layers grown on GaN (0001) is measured by a photoluminescence technique. The samples vary in InN composition, thickness, and threading dislocation density. Throughout this sample set, the measured Auger coefficient ranges from 1.4×10−30to2.0×10−30cm6s−1. The authors argue that an Auger coefficient of this magnitude, combined with the high carrier densities reached in blue and green InGaN∕GaN (0001) quantum well light-emitting diodes (LEDs), is the reason why the maximum external quantum efficiency in these devices is observed at very low current densities. Thus, Auger recombination is the primary nonradiative path for carriers at typical LED operating currents and is the reason behind the drop in efficiency with increasing current even under room-temperature (short-pulsed, low-duty-factor) injection conditions.
1,124 citations
TL;DR: In this paper, the optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells, and a promising current density of ∼1.6mA∕cm2 for 1.8cm2 cells was obtained, with a broad external quantum efficiency of ∼12% at 690nm.
Abstract: Silicon nanowire-based solar cells on metal foil are described. The key benefits of such devices are discussed, followed by optical reflectance, current-voltage, and external quantum efficiency data for a cell design employing a thin amorphous silicon layer deposited on the nanowire array to form the p-n junction. A promising current density of ∼1.6mA∕cm2 for 1.8cm2 cells was obtained, and a broad external quantum efficiency was measured with a maximum value of ∼12% at 690nm. The optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells.
997 citations
TL;DR: In this article, the authors proposed a scheme to achieve two dimensional (2D) acoustic cloaking by mapping the acoustic equations to Maxwell's equations of one polarization in the 2D geometry.
Abstract: A scheme to achieve two dimensional (2D) acoustic cloaking is proposed by Cummer and Schurig [New J. Phys. 9, 45 (2007)] by mapping the acoustic equations to Maxwell’s equations of one polarization in the 2D geometry. We find that the acoustic equation can be mapped to the direct current conductivity equation in three dimensions, which then allows the design of three-dimensional acoustic cloaking using the coordinate transformation scheme. The perfect cloaking effect is confirmed by solving for the scattering problem using the spherical-Bessel function series expansion method.
879 citations
TL;DR: These experiments demonstrate selectivity in the toxic nature of ZnO NP to different bacterial systems and human T lymphocytes and develop selective toxicity to biological systems and controlling it by NP design could lead to biomedical and antibacterial applications.
Abstract: We report on the toxicity of ZnO nanoparticles (NPs) to gram-negative and gram-positive bacterial systems, Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), and primary human immune cells. ZnO NP (~13 nm) showed complete inhibition of E. coli growth at concentrations 3.4 mM, whereas growth of S. aureus was completely inhibited for 1 mM. Parallel experiments using flow cytometry based assays clearly demonstrated that growth inhibitory properties of ZnO NP were accompanied by a corresponding loss of cell viability. Identical ZnO NP had minimal effects on primary human T cell viability at concentrations toxic to both gram-negative and gram-positive bacteria. Collectively, these experiments demonstrate selectivity in the toxic nature of ZnO NP to different bacterial systems and human T lymphocytes. Developing selective toxicity to biological systems and controlling it by NP design could lead to biomedical and antibacterial applications.
858 citations
TL;DR: In this article, strong variations in the Raman spectra for different single-layer graphene samples obtained by micromechanical cleavage were reported, revealing the presence of excess charges even in the absence of intentional doping.
Abstract: We report strong variations in the Raman spectra for different single-layer graphene samples obtained by micromechanical cleavage. This reveals the presence of excess charges, even in the absence of intentional doping. Doping concentrations up to ∼1013cm−2 are estimated from the G peak shift and width and the variation of both position and relative intensity of the second order 2D peak. Asymmetric G peaks indicate charge inhomogeneity on a scale of less than 1μm.
855 citations
TL;DR: In this article, the authors designed AlCoCrFeNiTiTi0.5 alloy by using the strategy of equiatomic ratio and high entropy of mixing, which is composed mainly of body centered cubic solid solution and possesses excellent compressive mechanical properties.
Abstract: Alloys with composition of AlCoCrFeNiTix (x: molar ratio; x=0,0.5,1,1.5) were designed by using the strategy of equiatomic ratio and high entropy of mixing. The alloy system is composed mainly of body centered cubic solid solution and possesses excellent room-temperature compressive mechanical properties. Particularly for AlCoCrFeNiTi0.5 alloy, the yield stress, fracture strength, and plastic strain are as high as 2.26GPa, 3.14GPa, and 23.3%, respectively, which are superior to most of the high-strength alloys such as bulk metallic glasses.
827 citations
TL;DR: In this paper, the authors describe the development of a class of lead-free (0.94−x)Bi0.5Na 0.5TiO3-0.06BaTiO 3−xK0.4NbO3 ceramics.
Abstract: Piezoelectric actuators convert electrical into mechanical energy and are implemented for many large-scale applications such as piezoinjectors and ink jet printers. The performance of these devices is governed by the electric-field-induced strain. Here, the authors describe the development of a class of lead-free (0.94−x)Bi0.5Na0.5TiO3–0.06BaTiO3–xK0.5Na0.5NbO3 ceramics. These can deliver a giant strain (0.45%) under both unipolar and bipolar field loadings, which is even higher than the strain obtained with established ferroelectric Pb(Zr,Ti)O3 ceramics and is comparable to strains obtained in Pb-based antiferroelectrics.
TL;DR: The original version of this article may be found at the Applied Physics Letters website:http://dx.doi.org/10.1063/1.2822891\/\/\/\/\/\/19.28.28
Abstract: The original version of this article may be found at the Applied Physics Letters website:
http://dx.doi.org/10.1063/1.2822891
Copyright (2007) American Institute of Physics
TL;DR: In this paper, a simple yet highly reproducible method to suppress contamination of graphene at low temperature inside the cryostat is presented, which consists of applying a current of several milliamperes through the graphene device, which is here typically a few microns wide.
Abstract: A simple yet highly reproducible method to suppress contamination of graphene at low temperature inside the cryostat is presented. The method consists of applying a current of several milliamperes through the graphene device, which is here typically a few microns wide. This ultrahigh current density is shown to remove contamination adsorbed on the surface. This method is well suited for quantum electron transport studies of undoped graphene devices, and its utility is demonstrated here by measuring the anomalous quantum Hall effect.
TL;DR: Very high-mobility organic transistors are fabricated with purified rubrene single crystals and high-density organosilane self-assembled monolayers in this paper, where the interface with minimized surface levels allows carriers to distribute deep into the crystals by more than a few molecular layers under weak gate electric fields, and the inner channel plays a significant part in the transfer performance.
Abstract: Very high-mobility organic transistors are fabricated with purified rubrene single crystals and high-density organosilane self-assembled monolayers. The interface with minimized surface levels allows carriers to distribute deep into the crystals by more than a few molecular layers under weak gate electric fields, so that the inner channel plays a significant part in the transfer performance. With the in-crystal carriers less affected by scattering mechanisms at the interface, the maximum transistor mobility reaches 18cm2∕Vs and the contact-free intrinsic mobility turned out to be 40cm2∕Vs as the result of four-terminal measurement. These are the highest values ever reported for organic transistors.
TL;DR: In this paper, fundamental results for carrier statistics in two-dimensional sheets and nanoscale ribbons are derived and the quantum capacitance, an important parameter in the electrostatic design of devices, is derived.
Abstract: In this work, fundamental results for carrier statistics in graphene two-dimensional sheets and nanoscale ribbons are derived. Though the behavior of intrinsic carrier densities in two-dimennsional graphene sheets is found to differ drastically from traditional semiconductors, very narrow (sub-10nm) ribbons are found to be similar to traditional narrow-gap semiconductors. The quantum capacitance, an important parameter in the electrostatic design of devices, is derived for both two-dimensional graphene sheets and nanoribbons.
TL;DR: In this article, the authors investigated the tunnel momentum distribution in a magnetic tunnel junction by analyzing the decay of the hot electrons in the Co metal anode after tunneling, using a three-terminal transistor structure.
Abstract: The tunnel momentum distribution in a magnetic tunnel junction is probed by analyzing the decay
of the hot electrons in the Co metal anode after tunneling, using a three-terminal transistor structure
in which the hot-electron attenuation is sensitive to the tunnel momentum distribution. Solid state
amorphous Al2O3 barriers and the vacuum barrier of a scanning tunneling microscope are
compared. For the former the attenuation length in nominally the same Co is strikingly larger factor
of 2, implying a more isotropic tunnel momentum distribution for Al2O3 barriers.
TL;DR: In this article, the effect of nanotube/nanotube contact resistance on the electrical conductivity of carbon nanotubes-based nanocomposites is studied, and the maximum tunneling distance is found to be about 1.8nm.
Abstract: The effect of nanotube/nanotube contact resistance on the electrical conductivity of carbon nanotube–based nanocomposites is studied. The tunneling resistance due to an insulating film of matrix material between crossing nanotubes is calculated by assuming a rectangular potential barrier in the insulating film. Monte Carlo simulations indicate that the tunneling resistance plays a dominant role in the electrical conductivity of composites, and the maximum tunneling distance is found to be about 1.8nm. Electrical conductivities of composites with inplane random distributions of carbon nanotubes follow the scaling law and the critical exponent depends on the level of contact resistance.
TL;DR: In this paper, the degenerate two-photon absorption coefficient β and Kerr nonlinearity n2 were measured for bulk Si at 300K using 200fs pulses with carrier wavelength of 850 <λ <2200nm for which indirect gap transitions occurred.
Abstract: The degenerate two-photon absorption coefficient β and Kerr nonlinearity n2 are measured for bulk Si at 300K using 200fs pulses with carrier wavelength of 850<λ<2200nm for which indirect gap transitions occur. With a broad peak near the indirect gap and maximum value of 2±0.5cm∕GW, the dispersion of β compares favorably with theoretical calculations of Garcia and Kalyanaraman [J. Phys. B 39, 2737 (2006)]. Within our wavelength range, n2 varies by a factor of 4 with a peak value of 1.2×10−13cm2∕W at λ=1800nm.
Journal Article•
TL;DR: In this article, a solution-processed organic thin film transistors (OTFTs) with carrier mobility > 1cm2∕Vs, current on/off ratio greater than 107, and subthreshold slope < 0.3V/decade were fabricated.
Abstract: Using the small molecule organic semiconductor 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene), the authors have fabricated the solution-processed organic thin film transistors (OTFTs) with carrier mobility >1cm2∕Vs, current on/off ratio greater than 107, and subthreshold slope <0.3V/decade. The high mobility TIPS-pentacene solution-processed films are deposited from high boiling point solvents and show strong molecular ordering including molecular terracing. Film ordering varies substantially for different solvents and film deposition techniques and OTFT mobility correlates well with film ordering.
TL;DR: In this article, a femtosecond optical pump pulse from a 10Hz Ti:sapphire laser to match the phonon-polariton phase velocity to the group velocity of the pump pulses in a lithium niobate crystal was achieved.
Abstract: Generation of near single-cycle pulses centered at 0.5THz frequency with up to 10μJ energy, 100μW average power, and 5.0MW peak power was demonstrated by tilting the intensity front of a femtosecond optical pump pulse from a 10Hz Ti:sapphire laser to match the phonon-polariton phase velocity to the group velocity of the pump pulses in a lithium niobate crystal. Terahertz pulse intensity as high as 10MW∕cm2 was achieved. The photon conversion efficiency was 45% and the calculated peak electric field strength at the focus of an off-axis parabolic mirror was 250kV∕cm.
TL;DR: In this article, electric polarization loops are measured at room temperature on highly pure BiFeO3 single crystals synthesized by a flux growth method, and the resulting low leakage currents allow the authors to measure a large spontaneous polarization in excess of 100μCcm−2.
Abstract: Electric polarization loops are measured at room temperature on highly pure BiFeO3 single crystals synthesized by a flux growth method. Because the crystals have a high electrical resistivity, the resulting low leakage currents allow the authors to measure a large spontaneous polarization in excess of 100μCcm−2, a value never reported in the bulk. During electric cycling, the slow degradation of the material leads to an evolution of the hysteresis curves eventually preventing full saturation of the crystals.
TL;DR: In this article, the III-V nitrides were used as a high-performance photovoltaic material with open-circuit voltages up to 2.4V and internal quantum efficiencies as high as 60%.
Abstract: We experimentally demonstrate the III-V nitrides as a high-performance photovoltaic material with open-circuit voltages up to 2.4V and internal quantum efficiencies as high as 60%. GaN and high-band gap InGaN solar cells are designed by modifying PC1D software, grown by standard commercial metal-organic chemical vapor deposition, fabricated into devices of variable sizes and contact configurations, and characterized for material quality and performance. The material is primarily characterized by x-ray diffraction and photoluminescence to understand the implications of crystalline imperfections on photovoltaic performance. Two major challenges facing the III-V nitride photovoltaic technology are phase separation within the material and high-contact resistances.
TL;DR: In this paper, the permittivity and permeability properties of a shell surrounding an enclosed domain are exploited to manipulate electromagnetic waves by a specific transformation of the permitivity of the shell.
Abstract: The authors suggest a way to manipulate electromagnetic waves by introducing a rotation mapping of coordinates that can be realized by a specific transformation of the permittivity and permeability of a shell surrounding an enclosed domain. Inside the enclosed domain, the information from the outside will appear as if it is coming from a different angle. Numerical simulations were performed to illustrate these properties.
TL;DR: In this article, green α-sialon:Yb2+ and red Sr2Si5N8:Eu2+ oxynitride/nitride phosphors have been demonstrated as potential downconversion luminescent materials for white light-emitting diodes.
Abstract: Green α-sialon:Yb2+ and red Sr2Si5N8:Eu2+ oxynitride/nitride phosphors have been demonstrated as potential downconversion luminescent materials for white light-emitting diodes (LEDs). In this letter, the authors attempt to fabricate white LEDs by combining α-sialon:Yb2+ and Sr2Si5N8:Eu2+ with a blue LED die and report their optical properties. These two phosphors lend themselves for use in 2-phosphor-converted white LEDs with promising properties: a wide range of tunable correlated color temperature (2700–6700K), acceptable color rendering index (82–83), and luminous efficacy (17–23lm∕W). These LEDs are acceptable for general lighting.
TL;DR: In this article, the authors determined that Auger recombination is the limiting factor for quantum efficiency for InGaN-GaN (0001) light-emitting diodes (LEDs) at high current density.
Abstract: Auger recombination is determined to be the limiting factor for quantum efficiency for InGaN–GaN (0001) light-emitting diodes (LEDs) at high current density. High-power double-heterostructure (DH) LEDs are grown by metal-organic chemical vapor deposition. By increasing the active layer thickness, DH LEDs can reach a maximum in quantum efficiency at current densities above 200A∕cm2. Encapsulated thin-film flip-chip DH LEDs with peak wavelength of 432nm have an external quantum efficiency of 40% and output power of 2.3W at 2A.
TL;DR: In this article, a dye-sensitized solar cell (DSSC) using a ZnO-nanoflower film photoanode, which was grown by a hydrothermal method at 95°C.
Abstract: In this letter, the authors report a dye-sensitized solar cell (DSSC) using a ZnO-nanoflower film photoanode, which was grown by a hydrothermal method at 95°C. The dye used was cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II) bis-tetrabutylam-monium (N-719). At AM1.5G irradiation with 100mW∕cm2 light intensity, the DSSC based on ZnO-nanoflower film showed an energy conversion efficiency of 1.9%, which is much higher compared to that (1.0%) of the control device constructed using a photoanode of upstanding ZnO-nanorod array fabricated by hydrothermal method as well. The better performance of ZnO-nanoflower DSSC was due to a better dye loading and light harvesting of the ZnO-nanoflower film. The results demonstrate potential application of ZnO-nanoflower array for efficient dye-sensitized solar cells.
TL;DR: In this article, the authors report results of transport studies on high quality, fully epitaxial BiFeO3 thin films grown via pulsed laser deposition on SrRuO3∕DyScO3 (110) substrates.
Abstract: The authors report results of transport studies on high quality, fully epitaxial BiFeO3 thin films grown via pulsed laser deposition on SrRuO3∕DyScO3 (110) substrates. Ferroelectric tests were conducted using symmetric and asymmetric device structures with either SrRuO3 or Pt top electrodes and SrRuO3 bottom electrodes. Comparison between these structures demonstrates the influence of electrode selection on the dominant transport mechanism. Analysis of film electrical response suggests Poole-Frenkel emission as the limiting leakage current mechanism in the symmetric structure. Temperature dependent measurements yield trap ionization energies of ∼0.65–0.8eV. No clear dominant leakage mechanism was observed for the asymmetric structure.
TL;DR: In this article, nonemissive tetraphenylethene (TPE) 1 and diphenylated derivative 2 were induced to emit intensely by aggregate formation and they were turned on at ∼2.9 and ∼5V and emitted blue lights with maximum luminance of ∼1800 and ∼11000cd∕m2, respectively.
Abstract: Nonemissive tetraphenylethene (TPE) 1 and its diphenylated derivative 2 were induced to emit intensely by aggregate formation. Crystalline aggregates of the dyes emitted bluer lights than their amorphous counterparts. The emissions of the TPE dyes could be switched off and on continuously and reversibly by wetting and dewetting with solvent vapors, respectively, manifesting their ability to optically sense volatile organic compounds. The light-emitting diodes fabricated from 1 and 2 were turned on at ∼2.9 and ∼5V and emitted blue lights with maximum luminance of ∼1800 and ∼11000cd∕m2, respectively.
TL;DR: In this article, thin film transistors (TFTs) were fabricated using an amorphous indium gallium zinc oxide (a-IGZO) channel, by rf sputtering at room temperature and for which the channel length and width are patterned by photolithography and dry etching.
Abstract: The authors report on the fabrication of thin film transistors (TFTs), which use an amorphous indium gallium zinc oxide (a-IGZO) channel, by rf sputtering at room temperature and for which the channel length and width are patterned by photolithography and dry etching. To prevent plasma damage to the active channel, a 100-nm-thick SiOx layer deposited by plasma enhanced chemical vapor deposition was adopted as an etch stopper structure. The a-IGZO TFT (W∕L=10μm∕50μm) fabricated on glass exhibited a high field-effect mobility of 35.8cm2∕Vs, a subthreshold gate swing value of 0.59V∕decade, a thrseshold voltage of 5.9V, and an Ion∕off ratio of 4.9×106, which is acceptable for use as the switching transistor of an active-matrix TFT backplane.