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Showing papers in "Applied Physics Letters in 2015"


Journal ArticleDOI
TL;DR: In this paper, the authors predict anisotropic piezoelectric effects in intrinsic monolayer group IV monochalcogenides (MX, M=Sn or Ge, X=Se or S), including SnSe, SnS, GeSe, and GeS.
Abstract: We predict enormous, anisotropic piezoelectric effects in intrinsic monolayer group IV monochalcogenides (MX, M=Sn or Ge, X=Se or S), including SnSe, SnS, GeSe, and GeS. Using first-principle simulations based on the modern theory of polarization, we find that their piezoelectric coefficients are about one to two orders of magnitude larger than those of other 2D materials, such as MoS2 and GaSe, and bulk quartz and AlN which are widely used in industry. This enhancement is a result of the unique “puckered” C2v symmetry and electronic structure of monolayer group IV monochalcogenides. Given the achieved experimental advances in the fabrication of monolayers, their flexible character, and ability to withstand enormous strain, these 2D structures with giant piezoelectric effects may be promising for a broad range of applications such as nano-sized sensors, piezotronics, and energy harvesting in portable electronic devices.

571 citations


Journal ArticleDOI
TL;DR: In this article, a monolithic perovskite/silicon multi-junction solar cell with a VOC as high as 1.65 V was proposed. But the performance of this cell was not evaluated.
Abstract: With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm2 2-terminal monolithic perovskite/silicon multijunction solar cell with a VOC as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.

496 citations


Journal ArticleDOI
TL;DR: In this article, the role of light trapping and angular restriction in improving the maximal output power of thin-film perovskite photovoltaics has been clarified and the influence of trap-assisted nonradiative recombination on the device efficiency is investigated.
Abstract: With the consideration of photon recycling effect, the efficiency limit of methylammonium lead iodide (CH3NH3PbI3) perovskite solar cells is predicted by a detailed balance model. To obtain convincing predictions, both AM 1.5 spectrum of Sun and experimentally measured complex refractive index of perovskite material are employed in the detailed balance model. The roles of light trapping and angular restriction in improving the maximal output power of thin-film perovskite solar cells are also clarified. The efficiency limit of perovskite cells (without the angular restriction) is about 31%, which approaches to Shockley-Queisser limit (33%) achievable by gallium arsenide (GaAs) cells. Moreover, the Shockley-Queisser limit could be reached with a 200 nm-thick perovskite solar cell, through integrating a wavelength-dependent angular-restriction design with a textured light-trapping structure. Additionally, the influence of the trap-assisted nonradiative recombination on the device efficiency is investigated. The work is fundamentally important to high-performance perovskite photovoltaics.

444 citations


Journal ArticleDOI
TL;DR: In this article, a structural study on the origin of ferroelectricity in Gd doped HfO2 thin films is presented, which provides unambiguous evidence for the existence of a non-centrosymmetric orthorhombic phase that can support spontaneous polarization.
Abstract: Here, we present a structural study on the origin of ferroelectricity in Gd doped HfO2 thin films. We apply aberration corrected high-angle annular dark-field scanning transmission electron microscopy to directly determine the underlying lattice type using projected atom positions and measured lattice parameters. Furthermore, we apply nanoscale electron diffraction methods to visualize the crystal symmetry elements. Combined, the experimental results provide unambiguous evidence for the existence of a non-centrosymmetric orthorhombic phase that can support spontaneous polarization, resolving the origin of ferroelectricity in HfO2 thin films.

430 citations


Journal ArticleDOI
TL;DR: In this article, the authors demonstrate a metamaterial absorber based ultra-sensitive sensing scheme at the terahertz frequencies with significantly enhanced sensitivity and an order of magnitude higher figure of merit compared to planar metasurfaces.
Abstract: Planar metasurfaces and plasmonic resonators have shown great promise for sensing applications across the electromagnetic domain ranging from the microwaves to the optical frequencies. However, these sensors suffer from lower figure of merit and sensitivity due to the radiative and the non-radiative loss channels in the plasmonic metamaterial systems. We demonstrate a metamaterial absorber based ultrasensitive sensing scheme at the terahertz frequencies with significantly enhanced sensitivity and an order of magnitude higher figure of merit compared to planar metasurfaces. Magnetic and electric resonant field enhancement in the impedance matched absorber cavity enables stronger interaction with the dielectric analyte. This finding opens up opportunities for perfect metamaterial absorbers to be applied as efficient sensors in the finger print region of the electromagnetic spectrum with several organic, explosive, and bio-molecules that have unique spectral signature at the terahertz frequencies.

417 citations


Journal ArticleDOI
TL;DR: In this article, the authors demonstrate a silicon heterojunction solar cell with molybdenum oxide hole collector, featuring a fill factor value higher than 80% and certified energy conversion efficiency of 22.5%.
Abstract: Substituting the doped amorphous silicon films at the front of silicon heterojunction solar cells with wide-bandgap transition metal oxides can mitigate parasitic light absorption losses. This was recently proven by replacing p-type amorphous silicon with molybdenum oxide films. In this article, we evidence that annealing above 130 °C—often needed for the curing of printed metal contacts—detrimentally impacts hole collection of such devices. We circumvent this issue by using electrodeposited copper front metallization and demonstrate a silicon heterojunction solar cell with molybdenum oxide hole collector, featuring a fill factor value higher than 80% and certified energy conversion efficiency of 22.5%.

368 citations


Journal ArticleDOI
TL;DR: In this article, the effective minority carrier lifetime (τe) of a large area (151.9 cm2) HJ c-Si solar cell with amorphous Si (a-Si) passivation layer was investigated from the point of view of effective minority lifetime and the impact of τe on fill factor.
Abstract: We have achieved a certified 25.1% conversion efficiency in a large area (151.9 cm2) heterojunction (HJ) crystalline Si (c-Si) solar cell with amorphous Si (a-Si) passivation layer. This efficiency is a world record in a both-side-contacted c-Si solar cell. Our high efficiency HJ c-Si solar cells are investigated from the standpoint of the effective minority carrier lifetime (τe), and the impact of τe on fill factor (FF) is discussed. The τe measurements of our high efficiency HJ c-Si solar cells reveal that τe at an injection level corresponding to an operation point of maximum power is dominated by the carrier recombination at the a-Si/c-Si interface. By optimization of the process conditions, the carrier recombination at the a-Si/c-Si interface is reduced, which leads to an improvement of the FF by an absolute value of 2.7%, and a conversion efficiency of 25.1% has been achieved. These results indicate that the reduction of carrier recombination centers at the a-Si/c-Si interface should be one of the m...

350 citations


Journal ArticleDOI
TL;DR: In this paper, the thermal conductivities of β-Ga2O3 single crystals along four different crystal directions were measured in the temperature range of 80 −495 K using the time domain thermoreflectance method.
Abstract: The thermal conductivities of β-Ga2O3 single crystals along four different crystal directions were measured in the temperature range of 80–495 K using the time domain thermoreflectance method. A large anisotropy was found. At room temperature, the [010] direction has the highest thermal conductivity of 27.0 ± 2.0 W/mK, while that along the [100] direction has the lowest value of 10.9 ± 1.0 W/mK. At high temperatures, the thermal conductivity follows a ∼1/T relationship characteristic of Umklapp phonon scattering, indicating phonon-dominated heat transport in the β-Ga2O3 crystal. The measured experimental thermal conductivity is supported by first-principles calculations, which suggest that the anisotropy in thermal conductivity is due to the differences of the speed of sound along different crystal directions.

350 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reported the observation of ferroelectric characteristics in undoped hafnium oxide thin films in a thickness range of 4-20nm, which were fabricated using atomic layer deposition and embedded into titanium nitride based metal-insulator-metal (MIM) capacitors for electrical evaluation.
Abstract: We report the observation of ferroelectric characteristics in undoped hafnium oxide thin films in a thickness range of 4–20 nm. The undoped films were fabricated using atomic layer deposition (ALD) and embedded into titanium nitride based metal-insulator-metal (MIM) capacitors for electrical evaluation. Structural as well as electrical evidence for the appearance of a ferroelectric phase in pure hafnium oxide was collected with respect to film thickness and thermal budget applied during titanium nitride electrode formation. Using grazing incidence X-Ray diffraction (GIXRD) analysis, we observed an enhanced suppression of the monoclinic phase fraction in favor of an orthorhombic, potentially, ferroelectric phase with decreasing thickness/grain size and for a titanium nitride electrode formation below crystallization temperature. The electrical presence of ferroelectricity was confirmed using polarization measurements. A remanent polarization Pr of up to 10 μC cm−2 as well as a read/write endurance of 1.6 ×...

315 citations


Journal ArticleDOI
TL;DR: In this article, a broadband terahertz (THz) metamaterial absorber was constructed by stacking 12 metallic bars of varying lengths on three polyimide layers with equal spacing, and a broadband absorption spectrum was formed through merging multiple successive resonance peaks.
Abstract: We present the simulation, implementation, and measurement of a broadband terahertz (THz) metamaterial absorber. By stacking 12 metallic bars of varying lengths on three polyimide layers with equal spacing, a broadband absorption spectrum is formed through merging multiple successive resonance peaks. The measured total absorption exceeds 95% from 0.81 to 1.32 THz at the normal incidence and the full width at half maximum is 64% (from 0.76 to 1.48 THz). The absorption decreases with fluctuations as the incident angle increases but remains above 62% even at the incident angle of 40°. The physical explanation to the absorption mechanism is presented and verified by a 9-bar example, which exhibits narrower absorption bandwidth. It is also experimentally demonstrated that the proposed structure is robust against misalignment of each metallic layer.

305 citations


Journal ArticleDOI
TL;DR: In this article, the authors demonstrate that black phosphorus can serve as a broadband saturable absorber and can be used for ultrashort optical pulse generation with performances similar or even better than currently used graphene or carbon nanotubes.
Abstract: Low-dimensional materials, due to their unique and versatile properties, are very interesting for numerous applications in electronics and optoelectronics. Recently rediscovered black phosphorus, with a graphite-like layered structure, can be effectively exfoliated up to the single atomic layer called phosphorene. Contrary to graphene, it possesses a direct band gap controllable by the number of stacked atomic layers. For those reasons, black phosphorus is now intensively investigated and can complement or replace graphene in various photonics and electronics applications. Here, we demonstrate that black phosphorus can serve as a broadband saturable absorber and can be used for ultrashort optical pulse generation. The mechanically exfoliated ∼300 nm thick layers of black phosphorus were transferred onto the fiber core, and under pulsed excitation at 1560 nm wavelength, its transmission increases by 4.6%. We have demonstrated that the saturable absorption of black phosphorus is polarization sensitive. The fabricated device was used to mode-lock an Er-doped fiber laser. The generated optical solitons with the 10.2 nm bandwidth and 272 fs duration were centered at 1550 nm. The obtained results unambiguously show that black phosphorus can be effectively used for ultrashort pulse generation with performances similar or even better than currently used graphene or carbon nanotubes. This application of black phosphorus proves its great potential to future practical use in photonics.

Journal ArticleDOI
TL;DR: In this article, the authors proposed to use magnetic-Cr modulation doping into topological insulators (Bi,Sb)2Te3 films to increase the observable temperature of QAHE.
Abstract: Quantum anomalous Hall effect (QAHE), which generates dissipation-less edge current without external magnetic field, is observed in magnetic-ion doped topological insulators (TIs) such as Cr- and V-doped (Bi,Sb)2Te3. The QAHE emerges when the Fermi level is inside the magnetically induced gap around the original Dirac point of the TI surface state. Although the size of gap is reported to be about 50 meV, the observable temperature of QAHE has been limited below 300 mK. We attempt magnetic-Cr modulation doping into topological insulator (Bi,Sb)2Te3 films to increase the observable temperature of QAHE. By introducing the rich-Cr-doped thin (1 nm) layers at the vicinity of both the surfaces based on non-Cr-doped (Bi,Sb)2Te3 films, we have succeeded in observing the QAHE up to 2 K. The improvement in the observable temperature achieved by this modulation-doping appears to be originating from the suppression of the disorder in the surface state interacting with the rich magnetic moments. Such a superlattice de...

Journal ArticleDOI
TL;DR: In this paper, the authors studied the energy relaxation times of transmon qubits in 3D cavities as a function of dielectric participation ratios of material surfaces and found an approximately proportional relation between the transmon relaxation rates and surface participation ratios.
Abstract: We study the energy relaxation times (T1) of superconducting transmon qubits in 3D cavities as a function of dielectric participation ratios of material surfaces. This surface participation ratio, representing the fraction of electric field energy stored in a dissipative surface layer, is computed by a two-step finite-element simulation and experimentally varied by qubit geometry. With a clean electromagnetic environment and suppressed non-equilibrium quasiparticle density, we find an approximately proportional relation between the transmon relaxation rates and surface participation ratios. These results suggest dielectric dissipation arising from material interfaces is the major limiting factor for the T1 of transmons in 3D circuit quantum electrodynamics architecture. Our analysis also supports the notion of spatial discreteness of surface dielectric dissipation.

Journal ArticleDOI
TL;DR: In this paper, a broadband, ultrafast optical power limiting in reduced graphene oxide (rGO), measured by a single open aperture Z-scan using a white light continuum (WLC) source, is presented.
Abstract: Wavelength dispersion of optical power limiting is an important factor to be considered while designing potential optical limiters for laser safety applications We report the observation of broadband, ultrafast optical limiting in reduced graphene oxide (rGO), measured by a single open aperture Z-scan using a white light continuum (WLC) source WLC Z-scan is fast when the nonlinearity is to be measured over broad wavelength ranges, and it obviates the need for an ultrafast tunable laser making it cost-economic compared to conventional Z-scan The nonlinearity arises from nondegenerate two-photon absorption, owing mostly to the crystallinity and extended π conjugation of rGO

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate an experimental approach to stabilize a room temperature skyrmion ground state in chiral magnetic films via exchange coupling across non-magnetic spacer layers.
Abstract: Possible magnetic skyrmion device applications motivate the search for structures that extend the stability of skyrmion spin textures to ambient temperature. Here, we demonstrate an experimental approach to stabilize a room temperature skyrmion ground state in chiral magnetic films via exchange coupling across non-magnetic spacer layers. Using spin polarized low-energy electron microscopy to measure all three Cartesian components of the magnetization vector, we image the spin textures in Fe/Ni films. We show how tuning the thickness of a copper spacer layer between chiral Fe/Ni films and perpendicularly magnetized Ni layers permits stabilization of a chiral stripe phase, a skyrmion phase, and a single domain phase. This strategy to stabilize skyrmion ground states can be extended to other magnetic thin film systems and may be useful for designing skyrmion based spintronics devices.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate that parasitic leakage currents dominate the current voltage characteristics of organic solar cells measured under illumination intensities less than one sun when the device shunt resistance is too low.
Abstract: In this report, we demonstrate that parasitic leakage currents dominate the current voltage characteristics of organic solar cells measured under illumination intensities less than one sun when the device shunt resistance is too low (<106 Ω cm2). The implications of such effects on common interpretations of the light intensity dependence of the solar cell open circuit voltage, fill factor, short circuit current, and power conversion efficiency are discussed in detail.

Journal ArticleDOI
TL;DR: In this paper, the dielectric properties and electrical hysteresis behaviors of Pb0.97La0.02(Zr0.58Sn0.335Ti0.085)O3 antiferroelectric (AFE) ceramics were investigated with an emphasis on energy storage properties.
Abstract: The dielectric properties and electrical hysteresis behaviors of Pb0.97La0.02(Zr0.58Sn0.335Ti0.085)O3 antiferroelectric (AFE) ceramics were investigated in this work with an emphasis on energy storage properties. Three phase transition points can be detected as temperature increases. AFE and paraelectric phases are found to coexist from 100 °C to 170 °C. The room temperature recoverable energy density is 1.37 J/cm3 at 8.6 kV/mm. With increasing temperature (from 20 °C to 100 °C) and frequency (from 0.01 to 100 Hz) under 8.6 kV/mm, the variation of recoverable energy density was less than 15%, all higher than 1.2 J/cm3. All the corresponding energy efficiencies were no less than 75%. The high energy density, high energy efficiency, and their weak dependence on temperature and frequency during a wide scope indicate that these antiferroelectric ceramics are quite promising to be used for pulse power capacitors applications.

Journal ArticleDOI
TL;DR: In this paper, the impact of the trapped charges on the carrier transport of MoS2-based metal-oxide-semiconductor FETs is evaluated. And the trapped charge density and time constant at different temperatures are extracted.
Abstract: The field effect transistors (FETs) based on thin layer MoS2 often have large hysteresis and unstable threshold voltage in their transfer curves, mainly due to the charge trapping at the oxide-semiconductor interface. In this paper, the charge trapping and de-trapping processes at the SiO2-MoS2 interface are studied. The trapping charge density and time constant at different temperatures are extracted. Making use of the trapped charges, the threshold voltage of the MoS2 based metal-oxide-semiconductor FETs is adjusted from 4 V to −45 V. Furthermore, the impact of the trapped charges on the carrier transport is evaluated. The trapped charges are suggested to give rise to the unscreened Coulomb scattering and/or the variable range hopping in the carrier transport of the MoS2 sheet.

Journal ArticleDOI
TL;DR: In this article, a perovskite solar cell with thin dense Mg-doped TiO2 as hole blocking layers (HBLs) was reported, which outperformed cells using TiO 2 HBLs in several ways: higher open-circuit voltage (Voc) (1.08 V), power conversion efficiency (12.28%), shortcircuit current, and fill factor.
Abstract: In this letter, we report perovskite solar cells with thin dense Mg-doped TiO2 as hole-blocking layers (HBLs), which outperform cells using TiO2 HBLs in several ways: higher open-circuit voltage (Voc) (1.08 V), power conversion efficiency (12.28%), short-circuit current, and fill factor. These properties improvements are attributed to the better properties of Mg-modulated TiO2 as compared to TiO2 such as better optical transmission properties, upshifted conduction band minimum (CBM) and downshifted valence band maximum (VBM), better hole-blocking effect, and higher electron life time. The higher-lying CBM due to the modulation with wider band gap MgO and the formation of magnesium oxide and magnesium hydroxides together resulted in an increment of Voc. In addition, the Mg-modulated TiO2 with lower VBM played a better role in the hole-blocking. The HBL with modulated band position provided better electron transport and hole blocking effects within the device.

Journal ArticleDOI
TL;DR: In this article, a template-assisted selective epitaxy (TASE) was used to construct 3D stacked nanowires and multiple gate field effect transistors (MuG-FETs) co-planar to the SOI layer.
Abstract: III–V nanoscale devices were monolithically integrated on silicon-on-insulator (SOI) substrates by template-assisted selective epitaxy (TASE) using metal organic chemical vapor deposition. Single crystal III–V (InAs, InGaAs, GaAs) nanostructures, such as nanowires, nanostructures containing constrictions, and cross junctions, as well as 3D stacked nanowires were directly obtained by epitaxial filling of lithographically defined oxide templates. The benefit of TASE is exemplified by the straightforward fabrication of nanoscale Hall structures as well as multiple gate field effect transistors (MuG-FETs) grown co-planar to the SOI layer. Hall measurements on InAs nanowire cross junctions revealed an electron mobility of 5400 cm2/V s, while the alongside fabricated InAs MuG-FETs with ten 55 nm wide, 23 nm thick, and 390 nm long channels exhibit an on current of 660 μA/μm and a peak transconductance of 1.0 mS/μm at VDS = 0.5 V. These results demonstrate TASE as a promising fabrication approach for heterogeneou...

Journal ArticleDOI
TL;DR: In this article, a plasmonic perfect absorber was proposed to enhance the optical absorbance ratio of single and three atomic layer graphene, which showed polarization dependence and tolerates a wide range of incident angles, and the peak position and bandwidth of graphene absorption spectra are tunable in a wide wavelength range through a specific structural configuration.
Abstract: Enhancement and manipulation of light absorption in graphene is a significant issue for applications of graphene-based optoelectronic devices. In order to achieve this purpose in the visible region, we demonstrate a design of a graphene optical absorber inspired by metal-dielectric-metal metamaterial for perfect absorption of electromagnetic waves. The optical absorbance ratios of single and three atomic layer graphene are enhanced up to 37.5% and 64.8%, respectively. The graphene absorber shows polarization-dependence and tolerates a wide range of incident angles. Furthermore, the peak position and bandwidth of graphene absorption spectra are tunable in a wide wavelength range through a specific structural configuration. These results imply that graphene in combination with plasmonic perfect absorbers have a promising potential for developing advanced nanophotonic devices.

Journal ArticleDOI
TL;DR: In this article, the authors present microwave-frequency NbTiN resonators on silicon, systematically achieving internal quality factors above 1 1/m in the quantum regime, and they use two techniques to reduce losses associated with two-level systems: an additional substrate surface treatment prior to Nb TiN deposition to optimize the metal-substrate interface and deep reactive-ion etching of the substrate to displace the substrate-vacuum interfaces away from high electric fields.
Abstract: We present microwave-frequency NbTiN resonators on silicon, systematically achieving internal quality factors above 1 M in the quantum regime. We use two techniques to reduce losses associated with two-level systems: an additional substrate surface treatment prior to NbTiN deposition to optimize the metal-substrate interface and deep reactive-ion etching of the substrate to displace the substrate-vacuum interfaces away from high electric fields. The temperature and power dependence of resonator behavior indicate that two-level systems still contribute significantly to energy dissipation, suggesting that more interface optimization could further improve performance.

Journal ArticleDOI
TL;DR: In this paper, a metamaterial medium for electromagnetic energy harvesting based on the full absorption concept is proposed, where the power absorption is mostly dissipated across a resistive load instead of the dielectric substrate.
Abstract: We present the design of a metamaterial medium for electromagnetic energy harvesting based on the full absorption concept. A metamaterial slab was designed comprising 13 × 13 electrically small cells, each loaded with an 82 Ω resistor which mimics the input impedance of a rectification circuitry. Unlike earlier designs of metamaterial absorbers, here the power absorption is mostly dissipated across a resistive load instead of the dielectric substrate. This implies that effective electromagnetic energy harvesting can be achieved. The power is channeled through a via connected to each cell. For a design optimized at 3 GHz, simulation and experimental results show power absorption efficiency of 97% and 93%, respectively.

Journal ArticleDOI
TL;DR: In this paper, a flip-chip architecture for violet-emitting III-nitride (III-N) lightemitting diodes (LEDs) was proposed for high current density and high temperature.
Abstract: We report on violet-emitting III-nitride light-emitting diodes (LEDs) grown on bulk GaN substrates employing a flip-chip architecture. Device performance is optimized for operation at high current density and high temperature, by specific design consideration for the epitaxial layers, extraction efficiency, and electrical injection. The power conversion efficiency reaches a peak value of 84% at 85 °C and remains high at high current density, owing to low current-induced droop and low series resistance.

Journal ArticleDOI
TL;DR: In this article, the authors present measurements of conductance hysteresis on CH3NH3PbI3 perovskite thin films, performed using the double-wave method, in order to investigate the possibility of a ferroelectric response.
Abstract: We present measurements of conductance hysteresis on CH3NH3PbI3 perovskite thin films, performed using the double-wave method, in order to investigate the possibility of a ferroelectric response. A strong frequency dependence of the hysteresis is observed in the range of 0.1 Hz to 150 Hz, with a hysteretic charge density in excess of 1000 μC cm−2 at frequencies below 0.4 Hz—a behaviour uncharacteristic of a ferroelectric response. We show that the observed hysteretic conductance, as well as the presence of a double arc in the impedance spectroscopy, can be fully explained by the migration of mobile ions under bias on a timescale of seconds. Our measurements place an upper limit of ≈1 μC cm−2 on any intrinsic frequency-independent polarisation, ruling out ferroelectricity as the main cause of current-voltage hysteresis and providing further evidence of the importance of ionic migration in modifying the efficiency of CH3NH3PbI3 devices.

Journal ArticleDOI
TL;DR: In this article, the authors reported the experimental realization of perfect sound absorption by sub-wavelength monopole and dipole resonators that exhibit degenerate resonant frequencies through the destructive interference of two resonators' transmission responses, while the matching of their averaged impedances to that of air implies no backscattering, thereby leading to total absorption.
Abstract: We report the experimental realization of perfect sound absorption by sub-wavelength monopole and dipole resonators that exhibit degenerate resonant frequencies. This is achieved through the destructive interference of two resonators' transmission responses, while the matching of their averaged impedances to that of air implies no backscattering, thereby leading to total absorption. Two examples, both using decorated membrane resonators (DMRs) as the basic units, are presented. The first is a flat panel comprising a DMR and a pair of coupled DMRs, while the second one is a ventilated short tube containing a DMR in conjunction with a sidewall DMR backed by a cavity. In both examples, near perfect absorption, up to 99.7%, has been observed with the airborne wavelength up to 1.2 m, which is at least an order of magnitude larger than the composite absorber. Excellent agreement between theory and experiment is obtained.

Journal ArticleDOI
TL;DR: In this article, a honeycomb acoustic metamaterial with a remarkably small mass per unit area at 1.3 kg/m2 was designed, theoretically proven, and then experimentally verified.
Abstract: In this letter, a class of honeycomb acoustic metamaterial possessing lightweight and yet sound-proof properties is designed, theoretically proven, and then experimentally verified. It is here reported that the proposed metamaterial having a remarkably small mass per unit area at 1.3 kg/m2 can achieve low frequency (<500 Hz) sound transmission loss (STL) consistently greater than 45 dB. Furthermore, the sandwich panel which incorporates the honeycomb metamaterial as the core material yields a STL that is consistently greater than 50 dB at low frequencies. The proposed metamaterial is promising for constructing structures that are simultaneously strong, lightweight, and sound-proof.

Journal ArticleDOI
TL;DR: In this article, the optical properties of amorphous and polycrystalline Sb2Se3 thin films prepared by thermal evaporation were investigated using temperature dependent transmission spectrum and temperature dependent photoluminescence.
Abstract: Sb2Se3 is a very promising photovoltaic material because of its attractive material, optical and electrical properties. Very recently, we reported a superstrate CdS/Sb2Se3 solar cell with 5.6% certified efficiency. In this letter, we focused on the optical properties of amorphous and polycrystalline Sb2Se3 thin films prepared by thermal evaporation. Using temperature dependent transmission spectrum and temperature dependent photoluminescence, the indirect optical transition nature and bandgap values as functions of temperature were acquired. Using ellipsometry measurements and Swanepoel's envelope method, the refractive indices as well as the dielectric constant in a wide wavelength range of 193–2615 nm were obtained. These works would lay the foundation for the further development of Sb2Se3 thin film solar cells.

Journal ArticleDOI
TL;DR: In this article, photoluminescence from 2 to 9nm MoS2 quantum dots (QDs) is excluded from the solvent and the absorption and PL spectra are shown to be consistent with the size distribution.
Abstract: The low quantum yield (∼10−5) has restricted practical use of photoluminescence (PL) from MoS2 composed of a few layers, but the quantum confinement effects across two-dimensional planes are believed to be able to boost the PL intensity In this work, PL from 2 to 9 nm MoS2 quantum dots (QDs) is excluded from the solvent and the absorption and PL spectra are shown to be consistent with the size distribution PL from MoS2 QDs is also found to be sensitive to aggregation due to the size effect

Journal ArticleDOI
TL;DR: In this article, a nanometer scale strain mapping technique by means of scanning nano-beam electron diffraction has been proposed, with a high precision of 0.1% at a lateral resolution of 1 nm for a large field of view reaching up to 1 μm.
Abstract: We report on the development of a nanometer scale strain mapping technique by means of scanning nano-beam electron diffraction. Only recently possible due to fast acquisition with a direct electron detector, this technique allows for strain mapping with a high precision of 0.1% at a lateral resolution of 1 nm for a large field of view reaching up to 1 μm. We demonstrate its application to a technologically relevant strain-engineered GaAs/GaAsP hetero-structure and show that the method can even be applied to highly defected regions with substantial changes in local crystal orientation. Strain maps derived from atomically resolved scanning transmission electron microscopy images were used to validate the accuracy, precision and resolution of this versatile technique.