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The orthorhombic O and O′ structure phases of ABO3-type perovskites possess the same symmetry and quite different physical properties. In this work, a pressure induced isostructural O′–O phase transition with volume expansion and less distortion in DyFeO3 orthoferrite was experimentally observed and studied by high-pressure x-ray diffraction and Raman spectroscopy measurements. Up to 35.2 GPa, DyFeO3 crystallizes in an orthorhombic Pbnm symmetry structure. With no change in the symmetry group, distinct O′-phase and O-phase were verified below 13.1 GPa and above 15.1 GPa, respectively. Meanwhile, the lattice becomes less distorted and metrically pseudo-cubic on the high-pressure side. The cell volume expands by a step in the intermediate pressure window accompanied by a decrease in spontaneous strain. It is suggested that FeO6 octahedra are more compressible than DyO12 sites under pressure. This work claimed the physical mechanism underlying phonon dynamics and variation of local structure symmetry of DyFeO3, which can be considered as the basic view for RFeO3 perovskites. [ABSTRACT FROM AUTHOR]

Beta gallium oxide (β-Ga2O3) is an emerging ultrawide bandgap (∼4.8 eV) semiconductor with attractive properties for future power and radio frequency (RF) electronics, optoelectronics, and sensors for detecting gases and solar-blind ultraviolet radiation. Beyond such promises, β-Ga2O3 crystal possesses excellent mechanical properties, making it pertinent as a material for micro/nanoelectromechanical systems (M/NEMS). Here, we present an overview and perspective on the emerging β-Ga2O3 M/NEMS and their roles in supplementing Ga2O3 power and RF electronics. We review the development of β-Ga2O3 micro/nanomechanical devices and precise extraction of mechanical properties from these devices. We evaluate the design for frequency scaling up to over 4 GHz by tuning the device geometry and dimensions. Toward technological applications, β-Ga2O3 M/NEMS are analyzed in two aspects: β-Ga2O3 vibrating channel transistors for potential integration with β-Ga2O3 power and RF electronics with operating frequency beyond 1 GHz, and β-Ga2O3 resonant transducers for photon radiation detection with scaling of responsivity and response time. With analytical prediction, we envision challenges and propose strategies and schemes in efficient electromechanical transduction engineering, frequency scaling, and design and fabrication for future development of β-Ga2O3 M/NEMS. [ABSTRACT FROM AUTHOR]

We report on the nondestructive measurement of Young's modulus of thin-film single crystal beta gallium oxide (β-Ga2O3) out of its nanoscale mechanical structures by measuring their fundamental mode resonance frequencies. From the measurements, we extract the Young's modulus in the (100) plane, EY,(100) = 261.4 ± 20.6 GPa, for β-Ga2O3 nanoflakes synthesized by low-pressure chemical vapor deposition (LPCVD), and in the [010] direction, EY,[010] = 245.8 ± 9.2 GPa, for β-Ga2O3 nanobelts mechanically cleaved from bulk β-Ga2O3 crystal grown by the edge-defined film-fed growth (EFG) method. The Young's moduli extracted directly on nanomechanical resonant device platforms are comparable to theoretical values from first-principle calculations and experimentally extracted values from bulk crystal. This study yields important quantitative nanomechanical properties of β-Ga2O3 crystals and helps pave the way for further engineering of β-Ga2O3 micro/nanoelectromechanical systems (M/NEMS) and transducers. [ABSTRACT FROM AUTHOR]

We report on device physics modeling, assisted by experimental measurement results, for all-electronic transduction of resonant electromechanical motion of nanoscale β-Ga2O3 vibrating channel transistors (VCTs), including both direct readout of high frequency transmission current (without frequency down-conversion) and frequency modulation (FM) down-conversion techniques. The β-Ga2O3 VCTs under consideration have fundamental mode resonance frequencies at ∼25 MHz–1 GHz, with quality factors (Qs) of ∼100–5000 and transconductance gm at ∼0.2 nS–5 μS. In analysis of signal transduction with varying device parameters, the transistor's gate trench depth z0 and transconductance gm play key roles in improving the electromechanical coupling and device performance. Reduction of trench depth z0 can engender a major enhancement in readout current. Reducing channel thickness h and increasing channel length L can improve the readout current while downshifting the resonance frequency at the same time. We design β-Ga2O3 VCT with a suspended modulation doped field effect transistor structure for efficient operation in the GHz range. This study paves the way for future engineering of all-electronic transduction and integration of high frequency β-Ga2O3 resonators on chip with β-Ga2O3 electronics and optoelectronics. [ABSTRACT FROM AUTHOR]

Among transition-metal nitrides, the mononitride γ-MoN with a rock-salt structure has drawn particular attention because it has been predicted to possess excellent mechanical and electronic properties, especially the high superconducting temperature around 30 K. However, synthesis of bulk γ-MoNx with the nitrogen concentration, x, more than 0.5 is still challenging, leading to contradictions on its phase stability and properties. In this work, we formulated a high-pressure synthesis reaction for the formation of single-crystal γ-MoNx with a remarkably high nitrogen concentration value of x ≈ 0.67. This nitride possesses a high asymptotic hardness of ∼24 GPa, which is so far the second hardest among metal nitrides. Impressively, the expected superconductivity is absent in the as-synthesized product. We further performed density functional theory calculations to clarify the structural stability and the absence of superconductivity in stoichiometric γ-MoN. We find that the ground state of γ-MoN is theoretically explored to be a Mott insulator with an antiferromagnetic phase, while a paramagnetic configuration is adopted at the ambient conditions. Such magnetic properties would explain the structural stability and the absence of superconductivity in the as-synthesized γ-MoNx with a high nitrogen concentration. [ABSTRACT FROM AUTHOR]

Mechanoluminescence (ML) refers to the photon emission from materials subjected to mechanical stimuli. A general concept concerning ML materials is that they cannot directly record a stress history (mechanical record; MR). Here, we report an approach that retrieves a quantitative recording of stress history through the afterglow characteristics analysis of a specific multi-piezo ML material Li0.12Na0.88NbO3:Pr (LNNO), which has been designated to exist near the structural phase boundary of rhombohedral R3c and orthorhombic P21ma. Contrary to the general expectation that the afterglow characteristics are irrelevant to mechanical loading history, LNNO was strongly correlated with the stress distribution after UV excitation. This enabled us to read the recorded stress precisely from the afterglow image of the material. Furthermore, the spatial distribution of the MR was also realized, showing the capability of the readout of stress history in both time and space. This unusual MR function is attributed to eliminating shallow trap sites, which affected the afterglow characteristics upon applying mechanical load. [ABSTRACT FROM AUTHOR]

Photoelectron spectroscopy was used to investigate poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and their blends on various conductive substrates. The study shows a P3HT-rich layer at the top of the P3HT:PCBM blend films. The energy level alignment of the top P3HT changes with the work function of the substrate and the PCBM concentration at the bottom surface of the blend film. The results can be explained using the integer charge transfer model. [ABSTRACT FROM AUTHOR]

In this work, fabrication of organic thin film transistors (OTFTs) using a phenyltrimethoxysilane (PhTMS) modified SiO2 insulator greatly improves the device electrical properties over those with plain or octadecyltrichlorosilane (OTS) modified SiO2, particularly improves the carrier mobility, the subthreshold slope, and channel resistance resulted from reduced density of charge trapping states at the semiconductor/insulator interface. The pentacene OTFTs with modification from PhTMS (3.5‰ v/v) achieves carrier mobility of 1.03 cm2/V s, on/off current ratio of 1.98×105, and subthreshold slope of 0.20 V/decade. This work renders a new, simple approach to significantly improve the OTFT performance. [ABSTRACT FROM AUTHOR]

We report the realization of the dynamic image of stress distribution by developing a remarkably strong mechanoluminescence (ML) material of Sr[sub 0.975]Al[sub 2]O[sub 3.985]:Eu[sub 0.01], which can emit four orders of magnitude larger intensity than that of the reported strong ML material of quartz crystal. This ML material can be mixed in the target composite or coated on the surface to sense stress by emitting visible light. This method is applicable to the dynamic visualization of stress distribution in a solid not only in the atmosphere but also in an aqueous environment. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Electron-surface interaction plays a fundamental role in surface science, which could evolve into an electronic avalanche under a high electrical field, resulting in devastating device failures. In the present study, using a thin polymer film approach, the effective surface layer, whose thickness is estimated to be about 200 nm, where the electron-surface interaction occurs is directly probed. The morphological evolution of thin polymer films with thicknesses from nanometers to micrometers is investigated with a focus on its influence on the electron avalanche process (or flashover) under a dc electric field in vacuum. It is found that the film thickness dependence of flashover is divided into three parts, i.e., fast increase, slow increase, and saturation, each of which has a dominant microscopic mechanism. The results indicate that the secondary electron emission (SEE) yield decreases significantly even when a discontinuous polymer layer is deposited, which varies little afterward. In contrast, the shallow surface traps develop into deep ones with the film thickness. The density ratio of deep traps increases exponentially after a continuous film is formed. The clear transition from SEE dominated to surface charge trap dominated flashover and their unique dependence on film morphology provide a deeper insight into the electron-surface interaction, which can be used for theoretical modeling, surface modification, and advanced functional devices. [ABSTRACT FROM AUTHOR]

Highly efficient fluorescence organic light-emitting diodes (OLEDs) based on the mixed 4,4',4''- tris[3-methylphenyl(phenyl)amino]triphenylamine:tris-[3-(3-pyridyl)mesityl]borane (1:1) system are reported. The electroluminescence due to the exciplex emission is red shifted when the thickness of the electron-transporting layer increases. The prepared OLEDs achieve a low turn-on voltage of 2.1V, a high current efficiency of 36.79 cd/A, and a very high luminescence of 17 100 cd/m², as well as a low efficiency roll-off. The current efficiency of the optimized OLED is maintained at more than 28.33 cd/A up to 10 000 cd m-2. The detailed recombination mechanism of the prepared OLEDs is investigated by the transient electroluminescence method. It is concluded that there are no contributions from trapped charges and annihilations of triplet-triplet excitons to the detected electroluminescence. [ABSTRACT FROM AUTHOR]

We demonstrate a highly efficient inverted bulk heterojunction polymer solar cell based on regioregular poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester with a low temperature annealed interfacial buffer layer, cesium carbonate (Cs2CO3). This approach improves the power conversion efficiency of the inverted cell from 2.3% to 4.2%, with short-circuit current of 11.17 mA/cm2, open-circuit voltage of 0.59 V, and fill factor of 63% under AM1.5G 100 mW/cm2 irradiation. This result is comparable to the previous regular structure device on the same system. Ultraviolet photoelectron spectroscopy shows that the work function of annealed Cs2CO3 layer decreases from 3.45 to 3.06 eV. Further x-ray photoelectron spectroscopy results reveal that Cs2CO3 can decompose into low work function, doped cesium oxide Cs2O upon annealing, which is accountable for the work-function reduction and device efficiency improvement. [ABSTRACT FROM AUTHOR]

An acoustic metasurface with a sub-wavelength thickness can manipulate acoustic wavefronts freely by the introduction of abrupt phase variation. However, the existence of a narrow bandwidth and a low transmittance limits further applications. Here, we present a broadband and highly transparent acoustic metasurface based on a frequency-independent generalized acoustic Snell's law and pentamode metamaterials. The proposal employs a gradient velocity to redirect refracted waves and pentamode metamaterials to improve impedance matching between the metasurface and the background medium. Excellent wavefront manipulation based on the metasurface is further demonstrated by anomalous refraction, generation of non-diffracting Bessel beam, and sub-wavelength flat focusing. [ABSTRACT FROM AUTHOR]

UV-blue light was obtained from a thin-film electroluminescence device using Gd[sub 3]Ga[sub 5]O[sub 12]:Ag as a light-emitting layer, which was deposited by using electron-beam evaporation. The crystal composition and structure of Gd[sub 3]Ga[sub 5]O[sub 12]:Ag were studied by x-ray powder diffraction. The Gd[sub 3]Ga[sub 5]O[sub 12]:Ag has a photoluminescence emission which peaked at around 397 and 467 nm, which were attributed to the oxide vacancies and Ag[sup +], respectively. The brightness of 32 cd/m2 was obtained when an alternating voltage of 130 V at 1 kHz was applied. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]