Applied Physics Letters

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American Institute of Physics / AIP
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Wideband reduction of in-duct noise using acoustic metamaterial with serially connected resonators made with MPP and cavities
Volume 116, Issue 25, June 2020. For the design of duct silencers, one should satisfy the essential constraints on the sound attenuation band, additional volume, and backpressure. For wideband sound attenuation, various acoustic metamaterials (AMM) using multiple resonators have been proposed. However, they often do not satisfy the spatial constraint, and the blocking of the conduit makes them impractical. This study proposes a compact silencing AMM unit for wideband sound reduction without deteriorating the mechanical performance. Previous works on the stacked micro-perforated panels (MPP) with different backing air gaps provide the basic idea of this work, which reveals the benefit of multiple resonators in adjusting the bandwidths to attain a wideband attenuation characteristic. The resistive element is also exploited in the MPP for suppressing the acoustic transparency of the detuned resonators. The formulated theoretical design method is tested by using a resonant unit cell configured with a serial connection of quadruple MPP layers, each air gap with a length of 30 mm and a uniform sectional area of 8 × 8 mm2. For minimizing the occupied volume, each cell surrounds the outer periphery of the main duct by folding, and the cell entry is flush-mounted on the duct wall. The test is conducted with the main duct of 30 × 30 mm2, and the attached 50 cells are arranged periodically with a 10-mm interval. The additional width of the duct is less than 1% of the wavelength. The measured power transmission coefficient is less than 0.2 for the range of 0.4–4.05 kHz, which agrees well with the prediction.
Interface chemistry of pristine TiN/La:[math] capacitors
Volume 116, Issue 25, June 2020. We present a hard and soft x-ray photoelectron spectroscopy study of the interface chemistry in pristine TiN/La-doped [math]/TiN capacitors. An oxynitride phase (∼1.3 nm) is formed at the top interface, while a [math] phase was detected near the bottom interface. The oxygen vacancy ([math]) concentration is higher at the top interface than in the film due to oxygen scavenging by the top electrode. The [math] concentration was also found to increase from ∼1.5 to 1.9 × [math] when increasing La doping from 1.7 to 2.7 mol. %. Two La dopants are compensated by the formation of one positively charged [math].
Light-induced degradation and self-healing inside CH3NH3PbI3-based solar cells
Volume 116, Issue 25, June 2020. CH3NH3PbI3 (MAPbI3)-based perovskite solar cells (PSCs) with special hole and electron transport layers (HTL and ETL) were prepared to study their light-induced degradation. Obvious degradation was observed under initial light exposure not only at the device level but also at the film morphology and electronic structure level. Device performance parameters, such as short-circuit current (JSC), power conversion efficiency, fill factor, and hysteresis effect, were aggravated with an initial light exposure of less than ∼8 h at 1 sun intensity. Meanwhile, the deteriorated crystallinity and electronic structure of the MAPbI3 film were also detected with x-ray diffraction, ultraviolet photoelectron spectroscopy, and UV-Visible absorption spectroscopy. The observed degradation is rationally related to the light-induced decomposition of MAPbI3. However, the degradation can be partly recovered with the following light exposure resulting in self-healing of the devices and MAPbI3 films. The self-healing behavior should be ascribed to the conversion of decomposition products back to MAPbI3, because the intermediates are wrapped tightly in the photoactive layer by the compact coverlayers of HTLs and ETLs and some reversible reactions occur consequently. The mechanism of self-healing is discussed by introducing the trapped states derived from ion migration. The PSCs prepared here imply a good optical stability and thus a good performance facilitated by tight wrapping of the active MAPbI3.
Does sunlight always accelerate water droplet evaporation'
Volume 116, Issue 25, June 2020. We investigate droplet evaporation, which is a natural phenomenon but the mechanism is not well understood. We are surprised to find that sunlight irradiation does not always enhance droplet evaporation, which is against the common sense that “Sun accelerates water evaporation.” This is true at least for short-time evaporation. A whole droplet lifetime consists of two regimes of evaporation: a light induced deterioration regime and an acceleration regime. The deterioration regime is explained by the decreased temperature difference from the droplet bottom to apex, weakening Marangoni flow to hinder conduction heat transfer from the substrate to the droplet. The enhanced regime is explained by the reduced light energy reflection via the droplet surface. The substrate conduction heat transfer and radiation heat transfer of light are coupled to dominate evaporation. The two mechanisms create opposite contributions, resulting in a constant evaporation rate for sunlight irradiation on a droplet. However, natural light decreases the evaporation rate vs time. Hence, evaporation rates with and without sunlight irradiation cross at a specific time. Our work enhances the fundamental understanding of droplet evaporation and provides a useful guideline for efficient solar energy utilization.
Adjustable super-resolution microscopy with diffractive spot array illumination
Volume 116, Issue 25, June 2020. Diffractive super-resolution spot arrays offer the possibility of adjustable super-resolution microscopic imaging. By inserting a phase-only diffractive optical element (DOE) into the illumination system of a standard microscope, super-resolution information of the sample can be obtained by spots. Here, we report an adjustable super-resolution microscopy (ASM) that the imaging resolution and the number of spot arrays can be adjusted by DOEs. The results of 3 × 3 and 5 × 5 spot arrays with 50% and 70% of the Airy spot size are, respectively, realized to support the flexibility of the ASM. The resolution test target was used as a sample to show the ASM can achieve about double-resolution experimentally, illuminated by a uniform 3 × 3 spot array with 50% of the Airy spot size under a small numerical aperture objective. Moreover, imaging of cellular mitochondria was performed, substantially realized resolution beyond the diffraction limit. The ability to adjust the super-resolution of microscopy using DOEs is of great importance for further nanoscale imaging.
The growth and characteristics of In2Se3/(Bi1−xInx)2Se3 superlattices with asymmetric graded interfaces by molecular beam epitaxy
Volume 116, Issue 25, June 2020. The growth of In2Se3/Bi2Se3 superlattices (SLs) by molecular beam epitaxy at an elevated temperature is explored. The crystalline phase structure of In2Se3 layers in the as-grown SLs is determined to be α-In2Se3. The diffusion of In from In2Se3 to Bi2Se3 is significantly promoted, while Bi diffusion into In2Se3 layers is insignificant as manifested by the in situ lattice evolution analysis, so that the achieved SL structure is of graded (Bi1−xInx)2Se3 solid-solution layers periodically separated by α-In2Se3 layers. The lattice vibration characteristics due to phonon confinement in the achieved SLs are also exhibited.
WO3/ZnO nanowire heterojunction as hole transport channel for building up persistent holographic fringes
Volume 116, Issue 25, June 2020. Transition metal oxides exhibit an excellent photochromic property, which can be applied in optical information storage. However, the decolorization reaction hinders the formation of optical bi-stable states of the oxides. Here, we deposit WO3 nanoparticles into ZnO nanowire arrays to form a multi-site heterojunction. The heterojunction micro-interface acts as a spatially dispersed hole transport channel to suppress the WO3 carrier recombination and to prolong the relaxation time more than 104 s. The photochromic response range covers the whole visible-NIR regions, and the absorption modulation amplitude is improved significantly. Persistent holographic fringes with alternated arrangement of WO3 and HxWO3 show a diffraction efficiency reaching 1.1% even at a very low writing power. This work is expected to be one of the most pursued application strategies for the integration of storage and the display of high-density information.
Thermal and radiation response of 4H–SiC Schottky diodes with direct-write electrical contacts
Volume 116, Issue 25, June 2020. A high-sensitivity 4H–SiC temperature sensor and an alpha detector have been fabricated using additively printed metal contacts. The surface morphology and electrical conductivity of the printed electrodes were established prior to Schottky diode development. 4H–SiC Schottky diodes with direct-write printed silver contacts on the 5 μm-thick epilayer on 4H–SiC were characterized electrically in terms of the forward and reverse current–voltage and high-frequency capacitance–voltage characteristics. The turn-on voltage of the Schottky diodes, as established from the forward current–voltage characteristics measured up to a temperature of 400 °C, showed a linear temperature dependence. Schottky diodes with direct-write printed Ag electrodes were able to measure alpha particles emitted from Americium-241. The high temperature and radiation response of the Schottky diodes show their suitability for multi-modal sensor fusion on the 4H–SiC platform for harsh environment applications.
Bifunctional superlens for simultaneous flexural and acoustic wave superfocusing
Volume 116, Issue 25, June 2020. Superfocusing of acoustic and elastic waves is generally achieved by the combination of negative refraction and the enhancement of the evanescent waves. Here, we numerically and experimentally demonstrate the bifunctionality of a superlens that can simultaneously focus acoustic and flexural waves beyond the diffraction limit. The designed structure is composed of a two-dimensional arrangement of pillars that act as rigid scatterers for the sound waves and as resonant scatterers for the flexural waves. The band structure presents modes with negative dispersion bands allowing negative refraction for both types of waves within the frequency range of 6.9–7.4 kHz, which is induced by the Bragg scattering effect. Edge modes that enhance the evanescent waves through resonant coupling appear around 7.2 kHz for the flexural and sound wave. The simultaneous superlensing is then observed at this frequency. Our finding will enlighten multiphysical and multifunctional wave manipulations and could have pragmatic applications involving multiwave devices.
Electrostatic control of dewetting dynamics
Volume 116, Issue 25, June 2020. The stability of liquid films on surfaces is critically important in microscale patterning and the semiconductor industry. If the film is sufficiently thin, it may spontaneously dewet from the surface. The timescale and rate of dewetting depend on the film repellency of the surface and the properties of the liquid. Therefore, control over the repellency requires modifying surface chemistry and liquid properties to obtain the desired rate of film retraction. Here, we report how the dynamics of a receding thin liquid stripe to a spherical cap droplet can be controlled by programming surface repellency through a non-contact electrostatic method. We observe excellent agreement between the expected scaling of the dynamics for a wide range of voltage-selected final contact angles. Our results provide a method of controlling the dynamics of dewetting with high precision and locality relevant to printing and directed templating.
Microwave applications of photonic topological insulators
Volume 116, Issue 25, June 2020. This Perspective examines the emerging applications of photonic topological insulators (PTIs) in the microwave domain. The introduction of topological protection of light has revolutionized the traditional perspective of wave propagation through the demonstration of backscatter-free waveguides in the presence of sharp bending and strong structural defects. The pseudospin degree of freedom of light enables the invention of unprecedented topological photonic devices with useful functionalities. Our aim is to present a brief introduction of recent developments in microwave PTI demonstrations. We give a clear comparison of different PTI realizations, summarize the key features giving rise to topological protection, and present a discussion of the advantages and disadvantages of PTI technology compared to existing microwave device technology. We conclude with forward-looking perspectives of how the advantages of this technology can best be exploited.
Electrically controlled rapid adiabatic passage in a single quantum dot
Volume 116, Issue 25, June 2020. We demonstrate electrically controlled robust state preparation of an exciton qubit by rapid adiabatic passage with Fourier-limited laser pulses. In our approach, resonant ps laser pulses are applied to generate excitonic population in a quantum dot, whereas synchronously applied ps electric transients provide a controlled sweep of the exciton transition energy. The ps electric transients applied to the quantum dot in a diode structure result in ultrafast Stark shifts of the exciton energy on time scales below the decoherence time of the exciton. We experimentally demonstrate that the tailored electric chirp of the exciton energy leads to a controlled rapid adiabatic passage, which results in a robust state preparation of the exciton. Our experimental results are confirmed by a theoretical analysis of the chirped coherent manipulation of the exciton two level system. Our approach toward optoelectronic quantum control paves the way for broader applications that require a scalable control of functional coherent systems.
Revealing the importance of light extraction efficiency in InGaN/GaN microLEDs via chemical treatment and dielectric passivation
Volume 116, Issue 25, June 2020. Chemical etching and Al2O3 dielectric passivation were used to minimize nonradiative sidewall defects in InGaN/GaN microLEDs (mesa diameter = 2–100 μm), resulting in an increase in external quantum efficiency (EQE) as the LED size was decreased. Peak EQEs increased from 8%–10% to 12%–13.5% for mesa diameters from 100 μm to 2 μm, respectively, and no measurable leakage currents were seen in current density–voltage (J–V) characteristics. The position and shape of EQE curves for all devices were essentially identical, indicating size-independent ABC model (Shockley–Read–Hall, radiative, and Auger recombination) coefficients-behavior that is not typical of microLEDs as the size decreases. These trends can be explained by enhancement in light extraction efficiency (LEE), which is only observable when sidewall defects are minimized, for the smallest LED sizes. Detailed ray-tracing simulations substantiate the LEE enhancements.
Graphene as reusable substrate for bialkali photocathodes
Volume 116, Issue 25, June 2020. Bialkali photocathodes, such as cesium potassium antimonide (CsK2Sb), can generate a high-brightness electron beam using a high-power green laser. These photocathode materials have potential applications in advanced accelerators and electron microscopes. It is known that the quantum efficiency (QE) of these photocathodes is affected severely by their substrates; however, reusability of the substrates is not well known. Here, we use graphene, silicon (Si), and molybdenum (Mo) substrates to evaluate the effects of substrates on the QE of redeposited CsK2Sb photocathodes after thermal cleanings. We found that the QE of CsK2Sb photocathodes redeposited on a graphene substrate after thermal cleaning at 500 °C remained largely unchanged. On the other hand, the QE of redeposited photocathodes on Si and Mo substrates after thermal cleaning at the same temperature decreased drastically. We used x-ray photoelectron spectroscopy to quantitatively evaluate the residues of photocathodes after thermal cleaning at 400 °C and 500 °C. We found that Sb, K, and Cs are removed by thermal cleaning at 500 °C for the graphene substrate, but all or the majority of these elements remained on the Si and Mo substrates. The results were consistent with our density functional theory calculations for the case of Si, which we investigated. Furthermore, our angle-resolved photoemission spectroscopy on graphene indicated that its intrinsic electronic structure is preserved after photocathode deposition and thermal cleaning at 500 °C. Hence, we attributed the difference in the amount of photocathode residue to the unique dangling-bond-free surface of inert graphene. Our results provide a foundation for graphene-based reusable substrates for high-QE semiconductor photocathodes.
Ultrabroadband and independent polarization of optical amplification with InGaAs-based indium-rich cluster quantum-confined structure
Volume 116, Issue 25, June 2020. This Letter reports polarization-independent optical amplification over an ultrabroad spectral range by semiconductor optical amplifiers. The technique uses an InGaAs-based indium-rich cluster (IRC) quantum-confined structure as the active medium and obtains comparable optical gain for both transverse electric (TE) and transverse magnetic (TM) polarization modes in the spectral ranges of 905–1005 and 905–970 nm, respectively. The device thus provides independent optical amplification for TE and TM polarizations over a common bandwidth of 65 nm. The difference between the amplified intensities of TE and TM modes is
Influence of quantum dot morphology on the optical properties of GaSb/GaAs multilayers
Volume 116, Issue 25, June 2020. We examine the influence of quantum dot (QD) morphology on the optical properties of two-dimensional (2D) GaSb/GaAs multilayers, with and without three-dimensional nanostructures. Using nanostructure sizes from scanning transmission electron microscopy and local Sb compositions from local-electrode atom-probe tomography as input into self-consistent Schrödinger–Poisson simulations based on 8 × 8 k·p theory, we compute confinement energies for QDs, circular arrangements of smaller QDs, termed QD-rings, and 2D layers on GaAs substrates. The computed confinement energies and the measured photoluminescence emission energies increase from QDs to QD-rings to 2D layers, enabling direct association of nanostructure morphologies with the optical properties of the GaSb/GaAs multilayers. This work opens up opportunities for tailoring near to far infrared optoelectronic devices by varying the QD morphology.
Enhancement of acoustic spin pumping by acoustic distributed Bragg reflector cavity
Volume 116, Issue 25, June 2020. Surface acoustic waves (SAWs) in the GHz frequency range can inject spin currents dynamically into adjacent non-magnetic layers via the spin pumping effect associated with ferromagnetic resonance. Here, we demonstrate an enhancement of acoustic ferromagnetic resonance and spin current generation by a pair of SAW reflector gratings, which form an acoustic analog of the distributed Bragg reflector cavity. In the experiment, we confirmed 2.04 ± 0.02 times larger SAW power absorption in a device with cavity than in the case of no acoustic cavity. We confirmed up to 2.96 ± 0.02 times larger spin current generation by measuring electric voltages generated by the inverse Edelstein effect at the interface between Cu and Bi2O3. The results suggest that acoustic cavities would be useful to enhance the conversion efficiency in SAW driven coupled magnon–phonon dynamics.
High performance high power textured piezoceramics
Volume 116, Issue 25, June 2020. Crystallographic grain-oriented ceramics (also referred to as textured ceramics) are known to exhibit a high soft piezoelectric response. However, the role of texturing in hard piezoelectric materials is not well understood and it has been difficult to obtain a balance of hard and soft properties in the same material. Here, we investigate the hard and soft piezoelectric behavior of [001]PC-textured 0.05Pb(Mn1/3Sb2/3)O3-0.95[0.4Pb(Mg1/3Nb2/3)O3-0.25PbZrO3-0.35PbTiO3] (PMnS-PMN-PZT) ceramics to illustrate the influence of texturing degree. The results demonstrate that textured PMnS-PMN-PZT ceramics exhibit a 170% higher longitudinal mode piezoelectric coefficient (d33) with only 16% reduction in the mechanical quality factor (Qm). Random PMnS-PMN-PZT ceramics were found to exhibit a d33 of 259 pC/N and a Qm of 982, while textured ceramics sintered at the same temperature demonstrated a d33 of 445 and a Qm of 824. Electric field dependent x-ray diffraction is utilized to confirm the existence of internal bias generated from defect dipoles, providing the signature for hard behavior. Temperature dependent measurement of d33 and Qm for textured PMnS-PMN-PZT ceramics indicated high stability up to 120 °C.
Structural investigation of ferroelectric BiFeO3–BaTiO3 solid solutions near the rhombohedral–pseudocubic phase boundary
Volume 116, Issue 25, June 2020. The structural properties of single crystals of BiFeO3–BaTiO3 (BF–BT) ferroelectric oxide solid solution with a slightly BF-rich composition (72BF–28BT; toward the rhombohedral-phase side) from the rhombohedral–pseudocubic phase boundary were investigated by transmission electron microscopy (TEM) and reciprocal space mapping (RSM), including three-dimensional (3D) RSM. According to the TEM results, the 72BF–28BT specimen had a domain structure similar to that of a rhombohedral crystal and the polarization direction appeared to be approximately ⟨111⟩, corresponding to a ferroelectric crystal with rhombohedral symmetry, although some distorted behavior suggested that the crystal symmetry may not have been strictly rhombohedral. Fine and complex domain structures were also observed inside the rectangular domains. The RSM results also indicated that the crystal had rhombohedral-like symmetry, although several discrepancies were observed, such as unexpectedly split diffraction peaks and distortion-related diffraction distribution in the reciprocal space. The structural behavior observed in the TEM images and 2D and 3D RSM images suggested that the symmetry of the 72BF–28BT specimen was lower than rhombohedral and may have been monoclinic. Structural disorder was detected along the distorted direction from rhombohedral symmetry, which could interfere with obtaining structural information via powder diffraction experiments and lead to inaccurate identification of the crystal structure as a higher-symmetry one.
Atomic-scale imaging of interfacial polarization in cuprate-titanate heterostructures
Volume 116, Issue 25, June 2020. The interfaces in oxide heterostructures that bring novel physical phenomena and functionalities have attracted great attention in fundamental research and device applications. For uncovering structure–property relationships of oxide heterostructures, direct evidence of the atomic-scale structure of heterointerfaces is highly desired. Here, we report on studying the structure of interfaces between YBa2Cu3O7-δ thin films and SrTiO3 substrates by means of aberration-corrected ultrahigh-resolution electron microscopy. Employing advanced imaging and spectroscopic techniques, shifts of atoms at the interface away from the regular lattice sites are measured, leading to the interfacial polarity. The local polarization induced by the atomic shifts directs toward the cuprate films and is estimated to be about 36.1 μC/cm2. The observed interfacial polar layer is understood by the special atomic configuration across the interface, which could modulate the electrical properties in superconducting devices that are based on the ferroelectric/superconductor heterosystems.
Droplet motion on contrasting striated surfaces
Volume 116, Issue 25, June 2020. Liquid droplets move readily under the influence of surface tension gradients on their substrates. Substrates decorated with parallel microgrooves, or striations, presenting the advantage of homogeneous chemical properties yet varying the topological characteristics on either side of a straight-line boundary, are considered in this study. The basic type of geometry consists of hydrophobic micro-striations/rails perpendicular to the boundary, with the systematic variation of the width to spacing ratio, thus changing the solid–liquid contact fraction and inducing a well-defined wettability contrast across the boundary. Droplets in the Cassie–Baxter state, straddling the boundary, move along the wettability contrast in order to reduce the overall surface free energy. The results show the importance of the average solid fraction and contrasting fraction in a wide range of given geometries across the boundary on droplet motion. A unified criterion for contrasting striated surfaces, which describes the displacement and the velocity of the droplets, is suggested, providing guidelines for droplet manipulation on micro-striated/railed surfaces.
The resistance of Cu nanowire–nanowire junctions and electro-optical modeling of Cu nanowire networks
Volume 116, Issue 25, June 2020. Flexible transparent conductors made from networks of metallic nanowires are a potential replacement for conventional, non-flexible, and transparent conducting materials such as indium tin oxide. Cu nanowires are particularly interesting as cost-effective alternatives to Ag nanowires—the most investigated metallic nanowire to date. To optimize the conductivity of Cu nanowire networks, the resistance contributions from the material and nanowire junctions must be independently known. In this paper, we report the resistivity values (ρ) of individual solution-grown Cu nanowires ⟨ρ⟩ = 20.1 ± 1.3 nΩ m and the junction resistance (Rjxn) between two overlapping Cu nanowires ⟨Rjxn⟩ = 205.7 ± 57.7 Ω. These electrical data are incorporated into an electro-optical model that generates analogs for Cu nanowire networks, which accurately predict without the use of fitting factors the optical transmittance and sheet resistance of the transparent electrode. The model's predictions are validated using experimental data from the literature of Cu nanowire networks composed of a wide range of aspect ratios (nanowire length/diameter). The separation of the material resistance and the junction resistance allows the effectiveness of post-deposition processing methods to be evaluated, aiding research and industry groups in adopting a materials-by-design approach.
Gated Hall and field-effect transport characterization of e-mode ZnO TFTs
Volume 116, Issue 25, June 2020. Two methods of measuring the electronic transport properties of a material are transistor DC-voltage and the Hall effect. Hall mobility measurements of normally off semiconductors can be done by electrostatic doping to lower resistance in the channel. We show that by measuring both, we can compare any value (raw measured as well as calculated data) directly to any other value along an index of FET gate and drain voltage across the entire safe operating area of the device. Our gated Hall technique intrinsic calculations of Hall mobility, typically possible only for bulk or doped materials, for thin-film transistor materials stack up with thickness scaled to practical values.
Diamagnetic coupling for magnetic tuning in nano-thin films
Volume 116, Issue 25, June 2020. Combining nanoscale thin films of magnetic and non-magnetic phases in various hetero-structures has generated a rich variety of new magnetic and magneto-transport phenomena and applications. Here, we propose a coupling between ferromagnetic and diamagnetic layers. We used this diamagnetic coupling to improve the exchange bias field of a diamagnet/ferromagnet/anti-ferromagnet hetero-structure by up to 212%, as evidenced in the experiments presented here. Since diamagnets have very special properties, including temperature independent negative magnetic susceptibilities, this coupling could be a powerful tool in future synthesis of solid state nanostructures such as exchange bias systems, spintronic devices, magnetic random access memories, sensors, and multiferroics.
Optoelectronic domain-wall motion for logic computing
Volume 116, Issue 25, June 2020. Logic computing in magnetic domain walls is investigated using the interplay of all-optical helicity-dependent switching and current-induced spin–orbit torque switching. By simultaneously controlling current and laser pulses, logic functions of AND, OR, NAND, and NOR are experimentally demonstrated through the anomalous Hall effect and verified by micromagnetic simulations. The optoelectronic domain-wall motion is energy-efficient compared to the traditional all-current approach and provides another degree of freedom for the realization of logic applications.
Plasmonic effects of copper nanoparticles in polymer photovoltaic devices for outdoor and indoor applications
Volume 116, Issue 25, June 2020. The use of metal nanoparticles (NPs) that can trigger localized surface plasmon resonance (LSPR) is an effective method for improving the performance of organic photovoltaics (OPVs). Currently, most plasmonic NPs are based on noble metals, including gold and silver; their high cost limits their commercial applications in the cost-effective OPVs. Herein, copper (Cu) NPs, which are more abundant and cheaper, are adopted to fabricate OPVs. To avoid oxidation of Cu NPs, they are positioned at the cathode interface, so that their fabrication could be implemented in an inert environment. The resulting OPVs exhibited improved power conversion efficiencies (PCEs) under illumination at 1 sun, and the device enhancement could be attributed to the LSPR effects of Cu NPs. Further, their potential to enhance the performance of OPVs under indoor lighting conditions is evaluated. The enhancement factor of PCEs was higher, while the light source had a lower color temperature. It could be due to the fact that the main plasmonic band of the Cu NPs is localized in the red spectral range. The results reveal the consideration of matching between the LSPR spectral range and the emission spectra of the artificial light sources is very critical for indoor applications.
Effects of airborne hydrocarbon adsorption on pool boiling heat transfer
Volume 116, Issue 25, June 2020. During pool boiling, a significantly high heat flux leads to the transition from nucleate boiling to film boiling, where a vapor film forms over the boiling surface, drastically increasing thermal resistance. This transition at the critical heat flux (CHF) results in an abrupt increase in surface temperature and can lead to catastrophic failure of the boiler. However, reported CHF values vary greatly, even for smooth surfaces of the same material; for example, the CHF values on flat silicon and silicon dioxide surfaces vary across studies by up to 49% and 84%, respectively. Here, we address this discrepancy by accounting for hydrocarbon adsorption on boiling surface. Hydrocarbon adsorption on smooth boiling surfaces decreases surface wettability, hindering the ability to maintain liquid contact with the surface and, thus, lowering the pool boiling CHF. To investigate hydrocarbon adsorption kinetics under ambient conditions and the subsequent effect on CHF, we cleaned flat silicon dioxide samples with argon plasma to remove hydrocarbon contaminants and then exposed them to laboratory air for different periods of time before conducting pool boiling experiments. Pool boiling results along with x-ray photoelectron spectroscopy data showed that the amount of adsorbed hydrocarbon increased with exposure time in air, which resulted in a decrease in wettability and, accordingly, a decrease in CHF. This work has important implications for understanding the spread in CHF values reported in the literature and may serve as a guideline for the preparation of boiling surfaces to achieve consistent experimental results.
Squeezed light induced two-photon absorption fluorescence of fluorescein biomarkers
Volume 116, Issue 25, June 2020. Two-photon absorption (TPA) fluorescence of biomarkers has been decisive in advancing the fields of biosensing and deep-tissue in vivo imaging of live specimens. However, due to the extremely small TPA cross section and the quadratic dependence on the input photon flux, extremely high peak-intensity pulsed lasers are imperative, which can result in significant photo- and thermal damage. Previous works on entangled TPA with spontaneous parametric downconversion light sources found a linear dependence on the input photon-pair flux, but are limited by low optical powers, along with a very broad spectrum. We report that by using a high-flux squeezed light source for TPA, a fluorescence enhancement of [math] is achieved in fluorescein biomarkers as compared to classical TPA. Moreover, a polynomial behavior of the TPA rate is observed in the the laser dye 4-dicyanomethylene-2-methyl-6-(p(dimethylamino)styryl)-4H-pyran in dimethyl sulphoxide.
Single-shot multi-planar wave-front measurement with multi-focal Fibonacci sieves
Volume 116, Issue 25, June 2020. Wave-front measurement based on coherent diffraction imaging (CDI) is a promising method for measuring wave-front aberrations, which has wide applications ranging from optical testing to adaptive optics. This study proposes a single-shot multi-planar wave-front measurement with multi-focal Fibonacci sieves to reconstruct the wave-front distribution of small transmissive objects. A Fibonacci sieve was designed to simultaneously capture multi-planar diffraction patterns at a single recording plane; thus, a multi-planar CDI algorithm can be used to reconstruct the test wave-front by a set of extracted sub-graphs. Its feasibility was proved in the optical region experimentally. Since diffractive optical elements used in the experiment are amplitude-only elements, the proposed wave-front measurement method opens up the possibility of practical real-time and on-line wave-front measurement ranging from x rays to terahertz.
Acoustic levitation in mid-air: Recent advances, challenges, and future perspectives
Volume 116, Issue 25, June 2020. Mid-air acoustic levitation is becoming a powerful tool to suspend and manipulate millimetric objects. Because of its unique characteristics, acoustic levitation is suitable to trap a wide variety of materials such as liquids, solids, soap bubbles, and even living creatures. Acoustic levitation can also be combined with noncontact measurement systems, allowing contactless analysis and characterization of levitating samples. In this article, we review some of the advances that have been made over the last decade. We also present the technical challenges that must be overcome in order to extend the capability of current acoustic levitation devices and, finally, we point out future directions for acoustic levitation.
Switching between spectral broadening and narrowing of the exciton absorption band of a CH3NH3PbI3 film on altering the polarity of an applied electric field
Volume 116, Issue 25, June 2020. Electric field-induced broadening and narrowing of an exciton absorption band have been observed to depend on the polarity of the applied electric field for methylammonium lead triiodide perovskite (MAPbI3) as a thin film sandwiched between a conducting film of fluorine-doped tin oxide (FTO) and an insulating film of poly(methyl methacrylate) (PMMA). The width of the absorption band increased or decreased when the direction of the applied field was toward FTO or PMMA, respectively, indicating that the spectral broadening and narrowing become switchable on altering the direction of the applied electric field. When titanium oxide (TiO2) in a compact layer was introduced between the MAPbI3 and FTO layers, the linear electric field effect that depended on the polarity was either not observed or decreased significantly although the quadratic field effects on the exciton absorption band were similarly observed with or without the TiO2 layer.
Terahertz faraday rotation of magneto-optical films enhanced by helical metasurface
Volume 116, Issue 25, June 2020. The Faraday rotation effect of both the La: yttrium iron garnet (YIG) film and the YIG metasurface were experimentally and numerically investigated in the terahertz (THz) region. A THz magneto-optic polarization measurement system was used to observe the transmission, resonance, and magneto-optical effect of the La:YIG film and YIG metasurface. The THz artificial chirality and resonance localization of the helical metasurface generate the superchiral THz field, which enhances the THz magneto-optical effect of the YIG film. The results show that the Faraday effect of the YIG metasurface is about three times that of the pure YIG film, whose differential rotation angle increases from 8° to over 24°. This work achieves more sensitively active polarization control of THz waves, which is of great significance for THz polarization conversion, sensing, and non-reciprocal transmission.
The formation mechanism of voids in physical vapor deposited AlN epilayer during high temperature annealing
Volume 116, Issue 25, June 2020. The voids will be formed in the physical vapor deposited (PVD)-AlN epilayer after high temperature annealing. In this work, the formation mechanism of voids and its effect on crystal quality are investigated. Based on microstructural analysis and first principles calculations, it is confirmed that (1) the dislocations mainly gather around the voids and the strain status around the voids is similar to other regions in the same PVD-AlN epilayer, (2) the paired dislocations with opposite signs prefer to move closer and react with each other during high temperature annealing, thus contributing to the formation of voids, (3) the voids provide the inner surface for dislocations to terminate, decreasing the density of the threading dislocation propagating to the surface, and (4) the emergence of dislocations is energetically favorable and the energy dropped by 5.93 eV after the two isolated dislocation lines fused into a void by overcoming a barrier of 1.34 eV. The present work is of great significance for improving the quality and performance of AlN materials and devices.
Termination-dependence of Fermi level pinning at rare-earth arsenide/GaAs interfaces
Volume 116, Issue 25, June 2020. The properties of metal/semiconductor interfaces are generally described by the metal-induced gap states (MIGS) model. However, rare-earth (RE) arsenide interfaces are found not to follow the MIGS model in having very different Schottky barrier heights (SBHs) for the Ga- or As-terminations of polar (100) or (111) RE-As/GaAs interfaces. Density function supercell calculations find this effect is due to localized defect interface states located on the mis-coordinated atoms of these interfaces that pin their SBHs at very different energies for each termination as determined by the anion sublattice bonding. Band offsets of semiconducting ScN/GaN interfaces also depend on their termination as determined by the same defect interface states. This pinning mechanism dominates any MIGS mechanism when it arises. Nonpolar (110) interfaces have little change in bonding, so they have no defect interface states, and we find their SBH is pinned by MIGS at the charge neutrality level. Hence, traditional MIGS models should be extended to include such interface states in a more general description.
Elastocaloric effect in vanadium (IV) oxide
Volume 116, Issue 25, June 2020. Elastocaloric cooling utilizes the latent heat associated with stress-induced reversible phase transformations to achieve cooling. Currently, the key barrier to this technology is its prohibitive cost due to the high elastocaloric material cost and the large stress required to drive the cooling cycle. Vanadium (IV) oxide (VO2) is a good candidate, and it is relatively cheap. Our calorimetry study shows it exhibits a reversible phase transformation with a large latent heat of 31.5 J/g as well as excellent functional stability. Its transformation temperature and latent heat are tunable via heat treatment. We demonstrate that VO2 powders can be cyclically compressed in a steel tube using a steel plunger to drive the elastocaloric effect. The application of relatively low stress of 300 MPa is sufficient to result in a reversible temperature change of 0.5 °C on the powder compact. Further improvement of reversible temperature change to 1.6 °C under 300 MPa is achieved by adding conductive copper powders. Future efforts should focus on improving material properties such as heat capacity and thermal conductivity for candidate ceramic oxides to maximize elastocaloric effects.
Carbon related hillock formation and its impact on the optoelectronic properties of GaN/AlGaN heterostructures grown on Si(111)
Volume 116, Issue 25, June 2020. The integration of GaN on Si as large scale substrates still faces many hurdles. Besides the large difference in the lattice constant and the high thermal mismatch existing between GaN and Si, spiral hillock growth phenomena are common problems in the development of thick GaN layers. In this work, hexagonal hillocks were observed on GaN/AlGaN high-electron-mobility transistor heterostructures grown on Si(111) by metal-organic chemical vapor deposition. The presence of these morphological and structural defects is attributed to the presence of localized contamination at the AlN/Si interface. These carbon-based defects cause highly defective regions in the AlN seed layer, which propagate through all the AlGaN buffer layers inducing the formation of V-shaped pits at the AlGaN interfaces. In hillock regions of the wafers, Raman spectroscopy indicates disturbed two-dimensional electron gas characteristics resulting from GaN/AlGaN interface roughness and a decreased amount of free carriers in the potential well. Energy-dispersive x-ray spectroscopy reveals Ga accumulation inside the V-pits and in nanopipes below, which is responsible for defective areas and the increased GaN growth rate resulting in hillock formation. Photoluminescence measurements confirm the presence of Ga-rich material reducing the inherent gallium vacancy concentration. Here, the reduced amount of Ga-vacancies acting as a shallow acceptor suppresses the ultraviolet luminescence band from donor–acceptor pair transition.
Different types of band alignment at MoS2/(Al, Ga, In)N heterointerfaces
Volume 116, Issue 25, June 2020. Heterojunction band offset parameters are critical for designing and fabricating junction-based devices as these parameters play a crucial role in determining the optical and electronic properties of a device. Herein, we report the band discontinuities at the MoS2/III-nitride (InN, GaN, and AlN) heterointerfaces. Few-layer MoS2 thin films are deposited by pulsed laser deposition on III-nitrides/c-sapphire substrates. Band offsets [valence band offset (VBO) and conduction band offset (CBO)] at the heterojunctions are determined by high-resolution x-ray photoelectron spectroscopy. The estimated band alignments are found to be type-I (VBO: 2.34 eV, CBO: 2.59 eV), type-II (VBO: 2.38 eV, CBO: 0.32 eV), and type-III (VBO: 2.23 eV, CBO: 2.87 eV) for MoS2/AlN, MoS2/GaN, and MoS2/InN, respectively. Such determination of the band offsets of 2D/3D heterojunctions paves a way to understand and design the futuristic photonic and electronic devices using these material systems.
Chlorine termination of selenium dangling bonds decreases tellurium-diffusion-induced acceptor states in hexagonal selenium
Volume 116, Issue 25, June 2020. In this study, we examined the effects of doping chlorine (Cl) into crystalline selenium (c-Se) on diminishing the tellurium-diffusion-induced shallow acceptor states associated with the dangling bonds that appear at the ends of Se chains by theoretical analysis with the first-principles calculations. In this model, the negatively charged dangling bonds can be neutralized by inserting monovalent Cl atoms. Furthermore, the defect termination by doping Cl into fabricated Se thin films was experimentally demonstrated. Low-temperature cathodoluminescence measurements were implemented to show that acceptor-related emissions from the c-Se films were significantly decreased by Cl termination of the dangling bonds, experimentally confirming the results of the simulations.
Electrical and optical properties of degenerate and semi-insulating ZnGa2O4: Electron/phonon scattering elucidated by quantum magnetoconductivity
Volume 116, Issue 25, June 2020. We study the electrical and optical properties of degenerate ZnGa2O4 films grown by metalorganic chemical vapor deposition (MOCVD) on sapphire and semi-insulating films grown by pulsed laser deposition (PLD) on fused silica. After a forming-gas anneal at 700 °C, the MOCVD film is highly conducting, with a room-temperature carrier concentration of 2 × 1020 cm−3, a mobility of 20 cm2/V s, and direct bandgap absorptions at 3.65 eV and 4.60 eV. Under the same annealing conditions, the PLD film is semi-insulating, with a direct bandgap absorption at 5.25 eV. The phonon structure, important for electrical and thermal conduction as well as superconductivity and other quantum phenomena, is very complicated due to the large number of atoms (and, thus, phonon branches) in the unit cell. However, we show that the phonon contributions to electron mobility (μph) can be directly measured by quantum-based magnetoconductivity over the temperature span T = 10–200 K. From an approximate analytical formula, μph = function (Tph, T), we calculate an effective phonon energy kTph(T) that takes account of all phonon contributions at temperature T. For T = 10–200 K, the value of kTph ranges from about 10 to 90 meV, consistent with the energy range of the ZnGa2O4 phonon density of states (at 0 K) calculated by density functional theory. The total measured mobility can then be modeled by μtot−1 = μii−1 + μph−1, where μii is the mobility due to ionized-impurity scattering. With a high bandgap, controllable conductivity, high breakdown voltage, and bulk-growth capability, ZnGa2O4 offers opportunities for high-power electronics and UV detectors.
Electronic structures and magnetization reversal in [math]
Volume 116, Issue 25, June 2020. By employing temperature (T)-dependent soft x-ray magnetic circular dichroism (XMCD) in the Fe and Cr 2p absorption edges, the electronic structures of [math]O4 spinel ferrite, which exhibits magnetization compensation, have been investigated. This work provides evidence that (i) both Fe and Cr ions are trivalent, (ii) most of [math] ions occupy the A (Td) sites, while [math] ions occupy the B (Oh) sites, (iii) the magnetic moments of Fe and Cr ions are coupled antiferromagnetically, and (iv) they are reversed at [math] K. The sum-rule analysis of Fe and Cr 2p XMCD spectra reveals that the orbital magnetic moments of Fe and Cr ions in [math]O4 are much larger than those of metallic Fe and Cr, implying the large spin–orbit coupling and the non-fully occupied [math] orbitals of [math] and [math] ions. Based on these findings, we have provided a comprehensive model for the electronic and spin configurations of Fe and Cr ions in (Fe)A[[math]]BO4.
Vanadium gate-controlled Josephson half-wave nanorectifier
Volume 116, Issue 25, June 2020. Recently, the possibility to tune the critical current of conventional metallic superconductors via electrostatic gating was shown in wires, Josephson weak-links, and superconductor-normal metal–superconductor junctions. Here, we exploit such a technique to demonstrate a gate-controlled vanadium-based Dayem nano-bridge operated as a half-wave rectifier at 3 K. Our devices exploit the gate-driven modulation of the critical current of the Josephson junction and the resulting steep variation of its normal-state resistance, to convert an AC signal applied to the gate electrode into a DC one across the junction. All-metallic superconducting gated rectifiers could provide the enabling technology to realize tunable photon detectors and diodes useful for superconducting electronics circuitry.
Exfoliated hexagonal BN as gate dielectric for InSb nanowire quantum dots with improved gate hysteresis and charge noise
Volume 116, Issue 25, June 2020. We characterize InSb quantum dots induced by bottom finger gates within a nanowire that is grown via the vapor–liquid–solid process. The gates are separated from the nanowire by an exfoliated 35 nm thin hexagonal BN flake. We probe the Coulomb diamonds of the gate-induced quantum dot exhibiting a charging energy of [math] meV and orbital excitation energies up to 0.3 meV. The gate hysteresis for sweeps covering 5 Coulomb diamonds reveals an energy hysteresis of only 60 μeV between upward and downward sweeps. Charge noise is studied via long-term measurements at the slope of a Coulomb peak revealing a potential fluctuation of ∼1 μeV/[math] at 1 Hz. This makes h-BN a dielectric with the currently lowest gate hysteresis and lowest low-frequency potential fluctuations reported for low-gap III–V nanowires. The extracted values are similar to state-of-the-art quantum dots within Si/SiGe and Si/SiO2 systems.
Reversible transition between bipolar resistive switching and threshold switching in 2D layered III–VI semiconductor GaSe
Volume 116, Issue 25, June 2020. Recently, two-dimensional (2D) layered materials have emerged as promising candidates for resistive switching (RS) devices. However, challenges in controllable conversion of RS types in such 2D materials still remain. Here, we report the experimental realization of reversible transition between non-volatile bipolar resistive switching (BRS) and volatile threshold switching (TS) in 2D layered III–VI semiconductor gallium selenide (GaSe) nanosheets through appropriately setting the compliance current (Icc). Under a relatively high Icc value of 1 mA, the device shows non-volatile BRS performance with a high ON/OFF ratio of nearly 104, a long retention time of 12 000 s, and a high endurance of 1200 switching cycles. Furthermore, under a relatively low Icc (lower than 10 μA), the volatile TS behaviors can be observed. For the former, the large Icc can generate stable conductive filaments (CFs) of Ga vacancy. Thus, the breakage of the stable CFs needs a high reverse voltage to re-align the Ga vacancy. For the latter, the low Icc generated unstable CFs can be broken by the current induced Joule heat. This study establishes the feasibility of integrating different RS types in 2D layered semiconductor nanosheets and understanding the underlying physical mechanism of different RS types in the 2D platform.
All-organic flexible logical computing system based on electrical polarization of ferroelectric polymers
Volume 116, Issue 25, June 2020. Artificial intelligence refers to the ability of a machine to study and think like a human. In the human brain, these functions rely on changes and transmission of electrical polarization among neurons. Ferroelectric polymer membranes exhibit electrical polarization that is tunable under the action of external stimuli, such as mechanical stresses and electric fields, and they, thus, exhibit signal transmission characteristics similar to those of neurons. In this paper, we describe the fabrication of a flexible computing device based on ferroelectric polymers that are capable of performing Boolean logic operations and have data retention capabilities and high state discrimination down to the nanoscale, thereby enabling dense packaging and low-power logic state switching. It is also able to adhere to human skin. This demonstration of a logical computing system based on polarization will help in the further exploration and development of artificial intelligence.
Resistance drift in Ge2Sb2Te5 phase change memory line cells at low temperatures and its response to photoexcitation
Volume 116, Issue 25, June 2020. Resistance drift in amorphous Ge2Sb2Te5 is experimentally characterized in melt-quenched line cells in the range of 300 K to 125 K and is observed to follow the previously reported power-law behavior with drift coefficients in the range of 0.07 to 0.11 in the dark, linearly decreasing with 1/kT. While these drift coefficients measured in the dark are similar to commonly observed drift coefficients (∼0.1) at and above room temperature, measurements under light show a significantly lower drift coefficient (0.05 under illumination vs 0.09 in the dark at 150 K). Periodic on/off switching of light shows a sudden decrease/increase in resistance, attributed to photo-excited carriers, followed by a very slow response (∼30 min at 150 K) attributed to contribution of electron traps and slow trap-to-trap charge exchanges. A device-level electronic model is used to relate these experimental findings to gradual charging of electron traps in amorphous Ge2Sb2Te5, which gives rise to growth of a potential barrier for holes in time and, hence, resistance drift.
Solute strongly impacts freezing under confinement
Volume 116, Issue 25, June 2020. The presence of liquid water in frozen media impacts the strength of soils, the growth of frost heave, plant life and microbial activities, or the durability of infrastructures in cold regions. If the effect of confinement alone on freezing is well known, water is never pure and solutes depressing the freezing point are naturally found. However, the combination of confinement and solute is poorly understood. Here, we study in situ the freezing of water in a model porous medium made of densely packed particles with various salt (KCl) concentrations. We demonstrate a synergistic effect of solute with confinement: the freezing front, initially heterogeneous due to confinement, drives solute enrichment in the remaining liquid, further depressing its freezing point. This increases the local freezing point depression and results in much larger mushy layers where ice and liquid water coexist. We compare our experimental freezing profile with theory and estimate the local solute concentration to increase by more than one order of magnitude through the freezing process. These results imply that even low solute concentrations may have important effects on the distribution of water in frozen porous media and should help explain the variety of freezing patterns observed experimentally. This may be critical for cryo-tolerance of construction materials and organisms and will help understanding solute precipitation and redistribution in soils.
Large valley degeneracy and high thermoelectric performance in p-type Ba8Cu6Ge40-based clathrates
Volume 116, Issue 25, June 2020. We demonstrate both theoretically and experimentally the high thermoelectric performance of p-type Ba8Cu6Ge40. Density functional theory calculations for Ba8Cu6Ge40 find that the valence band maximum consists of 12-fold degenerated valleys with light band effective masses, indicative of excellent electronic properties. It is also indicated that changing the Cu/Ge ratio is effective in controlling the carrier type. Motivated by these calculation results, a series of Ba8Cu6−xGe40+x samples with different Ge substitution amounts x are fabricated and the transport properties are characterized. The carrier type is effectively controlled with x, and the p-type Ba8Cu5.7Ge40.3 sample shows a high power factor of ∼1 mW/mK2, much higher than the previously reported values of n-type samples. These results emphasize the importance of the descriptor-based investigation into the electronic structures of clathrate thermoelectric materials.
An analysis of carrier dynamics in methylammonium lead triiodide perovskite solar cells using cross correlation noise spectroscopy
Volume 116, Issue 25, June 2020. Using cross correlation current noise spectroscopy, we have investigated carrier dynamics in methylammonium lead triiodide solar cells. This method provides space selectivity for devices with a planar multi-layered structure, effectively amplifying current noise contributions coming from the most resistive element of the stack. In the studied solar cells, we observe near full-scale shot noise, indicating the dominance of noise generation by a single source, likely the interface between the perovskite and the spiro-organic 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene hole-transport layer. We argue that the strong 1/f noise term has contributions from both the perovskite layer and interfaces. It displays a non-ideal dependence on photocurrent, [math] (instead of usual [math]), which is likely due to current-induced halide migration. Finally, we observe generation–recombination noise. We argue that this contribution is due to bimolecular recombination in the perovskite bulk absorption layer. Extrapolating our results, we estimate that at standard 1 sun illumination, the electron–hole recombination time is 5 μs.
Measurement of the dissociation rates of ion clusters in ionic liquid ion sources
Volume 116, Issue 25, June 2020. Ionic liquid ion sources utilize electric fields to evaporate and accelerate ions and ion clusters to ∼1 keV energies. Ion clusters may dissociate after evaporation, which is not a well-characterized phenomenon and has relevant consequences in many applications. We measure the dissociation rate-constants of ion clusters for several ionic liquids. It is found that ion cluster dissociation occurs on timescales of the order of 1–5 μs and follows a constant-rate equation in the region outside the ion source. Using the measured rate-constants, we estimate the post-emission ion cluster temperatures. We also qualify the way the electric field enhances the rate-constants. Finally, our work supports the hypothesis that ion clusters with many degrees of freedom have lower dissociation rates.
Publisher's Note: “Band engineering in epitaxial monolayer transition metal dichalcogenides alloy MoxW1−xSe2 thin films” [App. Phys. Lett. 116, 193101 (2020)]
Volume 116, Issue 25, June 2020.
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