Z.G. Hu

Vapomechanically Responsive Motion of Microchannel-Programmed Actuators

Materials that respond rapidly and reversibly to external stimuli currently stand among the top choices as actuators for real-world applications. Here, a series of programmable actuators fabricated as single- or bilayer elements is described that can reversibly respond to minute concentrations of acetone vapors. By using templates, microchannel structures are replicated onto the surface of two highly elastic polymers, polyvinylidene fluoride (PVDF) and polyvinyl alcohol, to induce chiral coiling upon exposure to acetone vapors. The vapomechanical coiling is reversible and can be conducted repeatedly over 100 times without apparent fatigue. If they are immersed in liquid acetone, the actuators are saturated with the solvent and temporarily lose their motility but regain their shape and activity within seconds after the solvent evaporates. The desorption of acetone from the PVDF layer is four times faster than its adsorption, and the actuator composed of a single PVDF layer maintains its ability to move over an acetone-soaked filter paper even after several days. The controllable and reproducible sensing capability of this smart material can be utilized for actuating dynamic elements in soft robotics.

Probing and Manipulating the Interfacial Defects of InGaAs Dual‐Layer Metal Oxides at the Atomic Scale

The interface between III-V and metal-oxide-semiconductor materials plays a central role in the operation of high-speed electronic devices, such as transistors and light-emitting diodes. The design of high-performance devices requires a detailed understanding of the electronic structure at the interface. However, the relation between the interface state charges to the electrical failure, such as breakdown of the oxide in the transistor remains unknown. Herein, the defect-driven interfacial electron structure of the Ti/ZrO2/Al2O3/InGaAs system are probed and manipulated using a specifically designed in situ transmission electron microscopy experimental method. The interfacial defects induced by oxygen-atom missing is found the main reason for the device failure. This study unearths the fundamental defect-driven interfacial electric structure of III-V semiconductor materials and paves the way to future high-speed and high-reliability devices.

Structural, optical and electrical properties of delafossite CuGaO2 films grown by sol-gel method

Transparent p-type conductive CuGaO2 films have been fabricated on sapphire substrates by sol–gel method. The stable sol solution for CuGaO2 were formed by copper(II) acetate monohydrate and gallium(III) nitrate hydrate, and the c-axis orientation of CuGaO2 films were strengthened with increasing annealing temperature. The pure phase CuGaO2 film was obtained at 900°C for 30 min in N2 atmosphere, and its microstructure, compositions, optical and electrical properties were analyzed. It was found that the sol-gel derived CuGaO2 films show a high optical transparency (60-80%) in the visible region, the direct and indirect band gaps were approximately 3.56 and 3.24 eV, respectively. It shows a crossover from the thermal activation behavior to that of three-dimensional variable range hopping from the temperature-dependent electrical conductivity at about 160 K.

Synthesis and optical properties of the (La, Mn)-codoped BiFeO3 films on n+-Si(100) substrates

La-doped BiFe0.92Mn0.08O3 films with the composition from 0 to 20% (BLFMx) have been deposited on Si(100) substrates by a sol-gel route. X-ray diffraction analysis shows that the films are polycrystalline and exhibit the pure perovskite phase structure (R3c). The La dopant effects on the surface morphology, dielectric function, and optical transition of the films have been investigated by atomic force microscopy and spectroscopic ellipsometry at room temperature. The dielectric functions of the films have been uniquely extracted by fitting the measured ellipsometric spectra with a three phase layered model (air/film/Si) and the Tauc-Lorentz dielectric function model in the photon energy range of 0.5-3.5 eV. It is found that the optical transitions decrease with increasing La composition, which is crucial for future photoelectric device.

Ellipsometric study of ferroelectric Ba0.4Sr0.6-xMnxTiO3 ceramics from 0.7 to 4.7 eV

Spectroscopic ellipsometry was used to extract the optical properties of Ba0.4Sr0.6-xMnxTiO3 (BSMT) (x from 1% to 20%) ceramics in the 0.7-4.7 eV (260-1700 nm) photon energy range at room temperature. X-ray diffraction analysis showed that BSMT ceramics are polycrystalline and lattice constants with different Mn composition present a slight variation. By reproducing the experimental ellipsometric spectra (Ψ and Δ), the optical constants and optical band gap energy have been obtained. It was found that the refractive index n increases first and then decreases as the photon energy increases from 0.7 to 4.7 eV for all the samples. The extinction coefficient k increases with increasing photon energy. On the other hand, both n and k decrease with increasing doping level of Mn (x ≤ 5%). Direct optical band gap energy is estimated to be 3.45-3.71 eV owing to different Mn doping. The difference of the optical properties can be ascribed to structure distortion with different Mn composition. The present results could be useful for future application of (Ba,Sr)TiO3-based optoelectronic devices.

Temperature dependence of photoluminescence, Raman scattering, and transmittance spectra of anatase Ti1-xFexO2 nanocrystalline films

Anatase Ti1-xFexO2 (x=0, 1%, 2%) nanocrystalline films were prepared on quartz substrates by a facile nonhydrolytic sol-gel route. The structure and optical properties have been studied by X-ray diffraction (XRD), Raman scattering, transmittance spectra and temperature dependent photoluminescence (PL). The B1g, Eg and (A1g+B1g) modes of anatase phase TiO2 can be observed in Raman spectra. Dielectric functions have been extracted by fitting the transmittance spectra in the photon energy range of 0.5-6.5 eV with Adachis model. The pure TiO2 film displays a strong broadening visible luminescence band; however, Fe-doped samples exhibit a very weak luminescence due to the increase of oxygen vacancy concentration in TiO2. With the temperature increases, the PL intensity decreases monotonously and there are five emission peaks for the pure sample in low temperatures, which could be attributed to oxygen vacancies, surface states and F+ center.