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The negative thermal expansion (NTE) behavior provides us an opportunity to design materials with controllable coefficient of thermal expansion (CTE). In this letter, we report a tunable isotropic thermal expansion in the cubic (Sc1-xZrx)F3+8 over a wide temperature and CTE range (al¼=-4.0 to+16.8*10-6 K-1, 298-648 K). The thermal expansion can be well adjusted from strong negative to zero, and finally to large positive. Intriguingly, isotropic zero thermal expansion (al¼2.6*10-7 K-1, 298-648 K) has been observed in the composition of (Sc0.8Zr0.2)F3+8. The controllable thermal expansion in (Sc1-xZrx)F3+8 is correlated to the local structural distortion. Interestingly, the ordered magnetic behavior has been found in the zero thermal expansion compound of (Sc0.8Zr0.2)F3+8 at room temperature, which presumably correlates with the unpaired electron of the lower chemical valence of Zr cation. The present study provides a useful reference to control the thermal expansion and explore the multi-functionalization for NTE materials. [ABSTRACT FROM AUTHOR]

Acoustic energy transfer (AET) has been widely used for contactless energy delivery to implantable devices. However, most of the energy harvesters (ultrasonic receivers) for AET are macro-scale transducers with large volume and limited operation bandwidth. Here, we propose and investigate two microelectromechanical systems diaphragm based piezoelectric ultrasonic energy harvesters (PUEHs) as an alternative for AET. The proposed PUEHs consist of micro-scale diaphragm array with different geometric parameter design. Diaphragms in PUEH-1 have large length to width ratio to achieve broadband property, while its energy harvesting performance is compromised. Diaphragms in PUEH- 2 have smaller length to width ratio and thinner thickness to achieve both broadband property and good energy harvesting performance. Both PUEHs have miniaturized size and wide operation bandwidth that are ideally suitable to be integrated as power source for implantable biomedical devices. PUEH-1 has a merged -6 dB bandwidth of 74.5% with a central frequency of 350 kHz. PUEH-2 has two separate -6 dB bandwidth of 73.7%/30.8% with central frequencies of 285 kHz/650 kHz. They can adapt to various ultrasonic sources with different working frequency spectrum. Maximum output power is 34.3 nW and 84.3 nW for PUEH-1 and PUEH-2 at 1mW/cm2 ultrasound intensity input, respectively. The associated power density is 0.734 μW/cm2 and 4.1 μW/cm2, respectively. Better energy harvesting performance is achieved for PUEH-2 because of the optimized length to width ratio and thickness design. Both PUEHs offer more alignment flexibility with more than 40% power when they are in the range of the ultrasound transmitter. [ABSTRACT FROM AUTHOR]

Aluminum Nitride (AlN) based surface acoustic wave (SAW) pressure sensors for harsh environment applications are of great interest in recent years. Such sensor employs a thick diaphragm (~50 μm) to endure the high pressure, but this seriously limits the sensitivity of these devices. Understanding of the working mechanism and the effect of geometrical parameters will yield the design principles to achieve improved sensitivity. In this letter, the effect of diaphragm on the performance of SAW pressure sensors is studied. AlN based SAW resonators on (100) wafer with different diaphragm shapes are fabricated, packaged, and characterized. Pressure coefficient of frequency (PCF) of pressure sensors with circular diaphragm, rectangular diaphragm (small aspect ratio) and rectangular diaphragm (large aspect ratio) is found to be 0.071 ppm/psi, 0.038 ppm/psi, and 0.171 ppm/psi, respectively. The longitudinal and lateral strains along the SAW propagation direction (h100i direction) have the opposite effects on the frequency change, i.e., longitudinal strain increases the resonant frequency while lateral strain decreases the resonant frequency. Hence, the measured PCF is a combined effect of the two strains, whereby increase in lateral strain results in lower PCF. The ratio of longitudinal/lateral strain is determined by the diaphragm shape. The rectangular diaphragm (large aspect ratio) with only lateral strain thus shows negative PCF. Additionally, by changing the wafer plane, SAW propagation direction and ratio of longitudinal/lateral strains may help to enhance the primary strain effect and minimize the reverse strain effect. This approach could further improve the sensitivity of pressure sensor without sacrificing its high pressure sensing range for harsh environment applications. [ABSTRACT FROM AUTHOR]

A number of technologies have been developed to enhance boiling heat transfer (BHT). The enhancements of BHT depend on the size and geometry of the micro/mini structures and it seems difficult to design a structure that is optimum for all heat transfer conditions. This letter reports a study on adaptive control and enhancement of BHT by shape-memory alloy (SMA) structures. The experimental results of BHT on structured porous surfaces show that the SMA surface with recoverable structures has advantages for heat transfer both in the improvement of heat transfer coefficient and in the extending of operating range. The potential applications of such enhancement structures in diverse heat transfer devices are perhaps the most exciting. [ABSTRACT FROM AUTHOR]

An ultrasonic transducer with a wide frequency bandwidth is always preferred for diagnostic ultrasound imaging, because a wide frequency bandwidth can reduce the duration of an ultrasonic pulse and enhance the axial imaging resolution. However, the frequency bandwidth of both conventional ultrasonic transducer and normal piezoelectric micromachined ultrasonic transducer (pMUT) is quite limited. To overcome this limitation, the mode-merging pMUT is presented in this letter. By using the rectangular membrane with large length/width aspect ratio, several resonant modes are excited within a narrow frequency range. When this pMUT works in a largely damped medium, excited modes are merged together and result in an ultra-wide bandwidth. A-6dB bandwidth of 95% is measured in water for the proposed pMUT without matching layer, which is much broader than that of conventional pMUTs. Benefited from such ultra-wide frequency bandwidth, the pulse duration of 1µs is achieved at a central frequency of 1.24 MHz. If this ultra-wide bandwidth pMUT is utilized to replace the conventional transducer for diagnostic ultrasound imaging, the axial resolution can be significantly enhanced without compromising imaging depth. [ABSTRACT FROM AUTHOR]

An ultrasonic transducer with a wide frequency bandwidth is always preferred for diagnostic ultrasound imaging, because a wide frequency bandwidth can reduce the duration of an ultrasonic pulse and enhance the axial imaging resolution. However, the frequency bandwidth of both conventional ultrasonic transducer and normal piezoelectric micromachined ultrasonic transducer (pMUT) is quite limited. To overcome this limitation, the mode-merging pMUT is presented in this letter. By using the rectangular membrane with large length/width aspect ratio, several resonant modes are excited within a narrow frequency range. When this pMUT works in a largely damped medium, excited modes are merged together and result in an ultra-wide bandwidth. A--6 dB bandwidth of 95% is measured in water for the proposed pMUT without matching layer, which is much broader than that of conventional pMUTs. Benefited from such ultra-wide frequency bandwidth, the pulse duration of 1 is is achieved at a central frequency of 1.24 MHz. If this ultra-wide bandwidth pMUT is utilized to replace the conventional transducer for diagnostic ultrasound imaging, the axial resolution can be significantly enhanced without compromising imaging depth. [ABSTRACT FROM AUTHOR]

Magnetic fringe fields of NiFe films were measured by giant magnetoimpedance (GMI) sensor, in this work. We have observed an interesting GMI phenomenon: the impedance enhanced first and then declined due to the presence of NiFe films, it is suggested that magnetic fringe fields have strengthened the longitudinal external magnetic field. Results indicated that the GMI sensor was able to quantify the magnetized NiFe films from 1 × 5 to 5 × 5 mm². Anyway, this study has extended the application of GMI sensor to film detection, which makes it promising in detecting the defects of soft magnetic devices. [ABSTRACT FROM AUTHOR]

The geometry, stability, and electronic properties of B@Co12@Bn and Co13@Bn clusters with a wide range n=55 up to 92 are studied by ab initio calculations. We find that B@Co12@B80 and Co13@B80 with closed B80 shell are two stable magic clusters with nearly perfect icosahedral symmetry, and B@Co12@B80 is more stable than Co13@B80 energetically. The strong core-shell bonding yields a very large energy gain of ∼30 eV. This high stability is attributed to the favorable closed-shell atomic and electronic structures. The B@Co12@B80 exhibits a large highest occupied and lowest unoccupied energy gap (0.96 eV) that is close to the value for isolated B80 fullerene. [ABSTRACT FROM AUTHOR]

Based on ab initio Monte Carlo approach, finite-temperature magnetism of tetragonal Fe is studied. It is shown that the ground state magnetic structure changes from ferromagnetic to antiferromagnetic as the structure transforms from bcc to fcc. The Curie temperature (TC) or Néel temperatures (TN) are very sensitive to the lattice distortion, which decreases from bcc to fcc and then increases over the fcc range. There are a maximum of TC at the bcc and a minimum of TN around the fcc limit due to the change of the coupling distance and symmetry consideration. [ABSTRACT FROM AUTHOR]

The development of cost-effective gas sensors with improved sensing properties and minimum power consumption for room temperature hydrogen leakage monitoring is in increasing demand. In this context, this report focus on the facile fabrication of ordered mesoporous TiO2 via evaporation-induced self-assembly route. With the controlled doping threshold (3%Co-TiO2), the output resistance change to 1000 ppm H2 is ~4.1×103 with the response time of 66 s. The sensor response exhibits power law dependence with an increase in the hydrogen concentration, where the power law coefficient was found not only specific to the kind of target gas but also related to temperature. Further, the effect of structure integrity with doping level and humidity on sensing characteristics is interpreted in terms of variation in surface potential eVS and depletion region w caused by the adsorption of molecular oxygen O2- 2 . [ABSTRACT FROM AUTHOR]

InGaN/GaN multiple quantum wells (MQWs) have been studied by using cathodoluminescence hyperspectral imaging with high spatial resolution. Variations in peak emission energies and intensities across trenchlike features and V-pits on the surface of the MQWs are investigated. The MQW emission from the region inside trenchlike features is redshifted by approximately 45 meV and more intense than the surrounding planar regions of the sample, whereas emission from the V-pits is blueshifted by about 20 meV and relatively weaker. By employing this technique to the studied nanostructures it is possible to investigate energy and intensity shifts on a 10 nm length scale. [ABSTRACT FROM AUTHOR]

The Fe-doped lead-free 0.95(Na1/2Bi1/2)TiO3-0.05BaTiO3 single crystal, reported in the previous work, which exhibited giant piezoelectric strain response (normalized strain constant up to ~1300 pm/V) and a large quasi-static piezoelectric constant d33 of ~600 pC/N, was investigated here with the emphasis on the domain structure so as to understand the nanoscale mechanism responsible for the giant strain response. Based on in-plane and out-of-plane piezoresponse force microscopy, it was found that Fe ion doping induced domain evolution from macrodomains of several micrometers to nanodomains of tens of nanometers. High-resolution transmission electron microscopy was adopted to give an insight into the fine domain structure after Fe doping. The domain evolution under an in situ electric field was further studied, and the results indicated that the giant strain response originated not only from the polarization rotation around the morphotropic phase boundary but also from the domain contribution. [ABSTRACT FROM AUTHOR]

We report a high frequency ultrasonic transducer up to 50MHz for vessel imaging based on a leadfree (K0.44Na0.56)NbO3-0.5 mol. % Mn (Mn-KNN) single crystal, which has a high electromechanical coupling coefficient kt of 0.64 and a large thickness frequency constant Nt of 3210 kHz ⋅ mm. The Krimholtz, Leedom, and Mattaei (KLM) equivalent circuit model was utilized to simulate and optimize the pulse-echo response combined with PiezoCAD software. Theoretically, a high -6 dB bandwidth of 74.94% was obtained at a center frequency of 50.47MHz and optimized matching conditions. The experimental results showed a center frequency of 51.8MHz with a -6 dB bandwidth of 70.2%. The excellent global performance makes this lead-free single-crystal transducer quite potential in an environmentally friendly vessel imaging transducer. [ABSTRACT FROM AUTHOR]

Organometal halide perovskite is a promising material to fabricate light-emitting diodes (LEDs) via solution processing due to its exceptional optoelectronic properties. However, incomplete precursor conversion and various defect states in the perovskite light-emitting layer lead to low luminance and external quantum efficiency of perovskite LEDs. We show here the addition of an optimum amount of sodium bromide in the methylammonium lead bromide (MAPbBr3) precursor during a one-step perovskite solution casting process can effectively improve the film coverage, enhance the crystallinity, and passivate ionic defects on the surface of MAPbBr3 crystal grains, resulting in LEDs with a reduced turn-on voltage from 2.8 to 2.3V and an enhanced maximum luminance from 1059 to 6942 Cd/m² when comparing with the pristine perovskite-based device. [ABSTRACT FROM AUTHOR]

Bouncing dynamics of impact droplets on solid surfaces intensively appeal to researchers due to the importance in many industrial fields. Here, we found that droplets impacting onto dome convex superhydrophobic surfaces could rapidly bounce off with a 28.5% reduction in the contact time, compared with that on flat superhydrophobic surfaces. This is mainly determined by the retracting process of impact droplets. Under the action of dome convexity, the impact droplet gradually evolves into an annulus shape with a special hydrodynamic distribution. As a consequence, both the inner and external rims of the annulus shape droplet possess a higher retracting velocity under the actions of the inertia force and the surface energy change, respectively. Also, the numerical simulation provides a quantitative evidence to further verify the interpretation on the regimes behind the rapidly detached phenomenon of impact droplets. [ABSTRACT FROM AUTHOR]

We demonstrate that the magneto-optic-Kerr effect with normal light incidence can be used to obtain quantitative optical measurements of both components of spin-orbit-induced torque (both the antidamping and effective-field components) in heavy-metal/ferromagnet bilayers. This is achieved by analyzing the quadratic Kerr effect as well as the polar Kerr effect. The two effects can be distinguished by properly selecting the polarization of the incident light. We use this alloptical technique to determine the spin-orbit torques generated by a series of Pt/Permalloy samples, finding values in excellent agreement with spin-torque ferromagnetic resonance measurements. [ABSTRACT FROM AUTHOR]

We report a 23.4% improvement of conversion efficiency in solar cells based on InGaN/GaN multiple quantum wells by using a patterned sapphire substrate in the fabrication process. The efficiency enhancement is due to the improvement of the crystalline quality, as proven by the reduction of the threading dislocation density. More importantly, the better crystalline quality leads to a positive photovoltaic efficiency temperature coefficient up to 423 K, which shows the property and advantage of wide gap semiconductors like InGaN, signifying the potential of III-nitride based solar cells for high temperature and concentrating solar power applications. [ABSTRACT FROM AUTHOR]

The epitaxial growth of FeSi film on (001) 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-0.32PT) was fabricated by sputtering and confirmed by high-resolution transmission electron microscopy. A fourfold symmetric angular remanent magnetization curve of as-deposited FeSi thin film is well fitted theoretically by considering the cubic magnetocrystalline anisotropy. We found that the fourfold anisotropy decreases slightly when an electric field (E) is applied on the Pt/PMN-0.32PT/FeSi/Ta heterostructures with Pt layer as the positive electrode. However, a magnetic anisotropy transition from fourfold anisotropy to twofold anisotropy occurs under negative E. The strain-electric field curve suggests that the observed different variation trend of magnetic anisotropy results from the asymmetric strain response on the polarity of E. Moreover, once the transition happens, it was irreversible unless the heterostructures are heated above the phase transition temperature of PMN-0.32PT. [ABSTRACT FROM AUTHOR]

Planar anisotropy Nd2Co17 flakes fractured along c crystal plane were fabricated by surfactantassisted high-energy ball milling technique. The magnetic flakes have a diameter range of 5-20 µm and a typical thickness of approximately 120 nm. The frequency dependence of complex permeability of Nd2Co17 epoxy resin composite has been investigated in the frequency range of 0.1-18 GHz. The measurement results show that the natural resonance frequency reaches 12.5 GHz while the initial permeability survives up to 2.26. The superior high frequency properties come from the large out-of-plane anisotropy field and the flake structure fractured along the c crystal plane of Nd2Co17. The planar anisotropic Nd2Co17 flakes have significant potential applications in the high-frequency devices working in the frequency beyond 10 GHz. [ABSTRACT FROM AUTHOR]

The epitaxial FeSi thin films on (001) 0.7[Pb(Mg1/3Nb2/3)O3]-0.3[PbTi0.3O3] (PMN-0.3PT) substrates were fabricated by radio frequency magnetron sputtering. The observed asymmetric strain-electric field curve suggests a tensile strain switching between two perpendicular directions in PMN-0.3PT with the variation of polarity of electric fields. A simple theoretical simulation of the free energy landscape shows that the stable magnetization easy axis (MEA) of FeSi with the inherent cubic magnetocrystalline anisotropy depends on the strength and direction of an extra strain induced uniaxial anisotropy. A reversible and non-volatile 90° switching of MEA by ±6 kV/cm pulses was confirmed by the angular dependence of remanent magnetization and Kerr hysteresis loops in Ta/FeSi/PMN-0.3PT/Pt heterostructures. [ABSTRACT FROM AUTHOR]

Forming good metal/graphene contact is of significance in making graphene devices, while tuning the graphene work function is a valid approach to decrease the contact barrier and then achieve electrodes with low contact resistance. A strain device has been fabricated to apply uniaxial strain to graphene grown by chemical vapor deposition method, and Kelvin probe force microscopy was used to measure the work function of the graphene under strain. The work function of the graphene is found to increase as strain increases. By applying a uniaxial strain of 7%, the work function can be adjusted as large as 0.161 eV. Such a result can be explained by strain induced increase of the density of states in graphene. [ABSTRACT FROM AUTHOR]

The effects of sub-bandgap illumination on electrical properties of CdZnTe:In crystals and spectroscopic performances of the fabricated detectors were discussed. The excitation process of charge carriers through thermal and optical transitions at the deep trap could be described by the modified Shockley-Read-Hall model. The ionization probability of the deep donor shows an increase under illumination, which should be responsible for the variation of electrical properties within CdZnTe bulk materials with infrared (IR) irradiation. By applying Ohm's law, diffusion model and interfacial layer-thermionic-diffusion theory, we obtain the decrease of bulk resistivity and the increase of space charge density in the illuminated crystals. Moreover, the illumination induced ionization will further contribute to improving carrier transport property and charge collection efficiency. Consequently, the application of IR irradiation in the standard working environment is of great significance to improve the spectroscopic characteristics of CdZnTe radiation detectors. [ABSTRACT FROM AUTHOR]

We demonstrate tunable mechanical properties of bone-inspired bionanocomposite scaffolds while maintaining the required viscoelasticity. Mechanical properties such as hardness and elastic modulus of the bionanocomposite scaffolds were controlled by varying mineral concentrations of the bioscaffold. In particular, higher calcium and oxygen contents in the bioscaffold resulted in a significant enhancement in hardness and modulus of the bionanocomposite. Moreover, the phosphorous content appeared to be a determining factor in the hardness and mechanical properties of the bionanocomposites. These results open up the possibility of designing new engineered biocompatible nanoscaffolds with desired and tunable biomimetic functions and biomechanical properties with significant potential for advanced bone tissue engineering platforms and bone substitutes. [ABSTRACT FROM AUTHOR]

We report design and synthesis of polymer-based large-area superhydrophobic carbon nanofiber (CNF) composite coatings for tunable electromagnetic interference shielding and attenuation in the terahertz (THz) frequency regime. Such coatings with different CNF/polymer weight ratios are characterized by a frequency domain THz spectroscopy system. A maximum THz shielding effectiveness of ∼32 dB was measured in the examined frequency range of 570-630 GHz. Coating attenuation level varied with CNF loading. Two-dimensional distributions of power attenuation at 600 GHz showed good spatial uniformity. The present composite coatings, in addition to their self-cleaning property, have high potential for advanced technology high-frequency applications. [ABSTRACT FROM AUTHOR]

We report the direct measurement of a magnetoelectric (ME) exchange between magnetostrictive CoFe2O4 nanopillars in a piezoelectric BiFeO3 matrix for single-layer nanocomposite epitaxial thin films grown on (001) SrTiO3 substrates with SrRuO3 bottom electrodes. The ME coefficient was measured by a magnetic cantilever method and had a maximum value of ∼20 mV/cm Oe. The films possessed saturation polarization (60 μC/cm2) and magnetization (410 emu/cc) properties equivalent to bulk values, with typical hysteresis loops. [ABSTRACT FROM AUTHOR]

Low roll-off power efficiency (ηp) organic light-emitting diodes at high current density were fabricated using a nondoped ultrathin bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2′] iridium (acetylacetonate) phosphorescent layer and thin N,N′-di(naphthalene-2-yl)-N,N′-diphenyl-benzidine separate layer. A maximum ηp of 8.48 lm/W and luminance efficiency of 20.26 cd/A at 2014 cd/m2 were obtained from the device. The ηp roll-off percentage of ultrathin phosphor layer device was reduced to 20% at a current density of 20 mA/cm2, which is about 40% for conventional phosphorescent device. The low roll off of efficiency was attributed to the direct carrier trapping and confining emission zone function of the ultrathin phosphor layer. [ABSTRACT FROM AUTHOR]

We have investigated adsorption and dissociation of water and HfCl4 on Ge/Si(100)-(2×1) surface with density functional theory. The Si–Ge heterodimer and Ge–Ge homodimer are employed to represent the Si1-xGex surface. The activation energy for adsorption of water on Ge–Ge homodimer is much higher than that on Si–Ge heterodimer. No net activation barrier exists during the adsorption of HfCl4 on both SiGe surface dimers. The differences in the potential energy surface between reactions on Si–Ge and Ge–Ge dimers are due to different bond strengths. It should also be noticed that the activation energy for HfCl4 is quite flat, thus HfCl4 adsorbs and dissociates on Ge/Si(100)-(2×1) easily. [ABSTRACT FROM AUTHOR]