Paresh Shimpi

Low temperature synthesis and characterization of MgO/ZnO composite nanowire arrays

Large scale dendritic MgO/ZnO composite nanowire arrays have been successfully synthesized on Si substrates using a two-step sequential hydrothermal synthesis at low temperature for the first time. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and x-ray photoelectron spectroscopy (XPS) were systematically carried out to confirm and elaborate the potentially localized Mg surface alloying process into the ZnO nanowire arrays. Both room temperature and low temperature (40 K) photoluminescence results revealed an enhanced and blue-shifted near-band-edge (NBE) ultraviolet (UV) emission for the MgO/ZnO nanowires compared to those of the pure ZnO nanowire arrays, indicating the success of Mg alloying into ZnO nanowires. This enhancement might be due to the 155 degrees C hydrothermal process and the amorphous MgO layer in the MgO/ZnO nanowires. The specific template of densely packed ZnO nanowire arrays was suggested to be instrumental in enabling this type of MgO/ZnO composite nanowire growth.

Structure and magnetic properties of three-dimensional (La,Sr)MnO3 nanofilms on ZnO nanorod arrays

Three-dimensional (3D) cubic perovskite (La,Sr)MnO3 (LSMO) nanofilms have been deposited on ZnO nanorod arrays with controlled dimensionality and crystallinity by radio frequency (rf) magnetron sputtering and post thermal annealing. Compared to the two-dimensional (2D) LSMO nanofilm on flat Si, the structure and magnetic properties of 3D LSMO nanofilms on ZnO nanorod arrays have a strong anisotropic morphology and thickness dependence. Ferromagnetic property has been observed in both 2D and 3D LSMO nanofilms while a ferromagnetic–superparamagnetic transition was revaled in 3D LSMO nanofilms on ZnO nanorod array with decreasing nanofilm thickness, due to a large surface dispersion effect. The LSMO/ZnO nanofilm/nanorod structures could open up new avenues for intriguing magnetic properties studies and applications of nanoscale perovskites.Three-dimensional (3D) cubic perovskite (La,Sr)MnO3 (LSMO) nanofilms have been deposited on ZnO nanorod arrays with controlled dimensionality and crystallinity by radio frequency (rf) magnetron sputtering and post thermal annealing. Compared to the two-dimensional (2D) LSMO nanofilm on flat Si, the structure and magnetic properties of 3D LSMO nanofilms on ZnO nanorod arrays have a strong anisotropic morphology and thickness dependence. Ferromagnetic property has been observed in both 2D and 3D LSMO nanofilms while a ferromagnetic–superparamagnetic transition was revaled in 3D LSMO nanofilms on ZnO nanorod array with decreasing nanofilm thickness, due to a large surface dispersion effect. The LSMO/ZnO nanofilm/nanorod structures could open up new avenues for intriguing magnetic properties studies and applications of nanoscale perovskites.

Annealing induced nanostructure and photoluminescence property evolution in solution-processed Mg-alloyed ZnO nanowires

Solution-processed Mg-alloyed ZnO nanowire arrays have been achieved recently without using high temperature annealing process. By introducing thermal annealing processes in oxygen-rich ambient condition, the UV near-band-edge (NBE) emission was surprisingly mitigated until disappeared with annealing temperature increasing from 400 to 900 °C. As the annealing temperature increased, intensity of UV peak decreased while intensity of visible peak (490–520 nm) increased. The structure evolution upon thermal annealing was revealed to be responsible for these abnormal photoluminescence property variations, where unusual (Zn,Mg)1.7SiO4 epitaxially evolved on ZnMgO nanowires surface and contributed to the quenching of UV NBE emission. The structure evolution induced UV-NBE quenching and nanoscale localized alloying in semiconductor ZnMgO nanowires could bring up opportunities in catalysis, optoelectronics, spintronics, and sensors.

(La,Sr)CoO3/ZnO nanofilm–nanorod diode arrays for photo-responsive moisture and humidity detection

Large scale (La,Sr)CoO3 (LSCO)/ZnO nanofilm–nanorod diode arrays have been successfully fabricated using a combination of hydrothermal synthesis and colloidal deposition. With well-controlled dimensionality, crystallinity, crystal structures and device structures, LSCO/ZnO nanofilm–nanorod diode arrays display an excellent rectifying current–voltage (I–V) characteristic under ±1 V bias with negligible leakage current upon reverse bias. These nanostructured diode arrays have been found to be sensitive to UV illumination and different relative humidities at room temperature upon forward bias. A negative photoconductivity response is revealed upon UV illumination on the diode arrays as a result of the desorption process of nanofilm–nanorod surface moisture. The forward current of LSCO/ZnO nanofilm–nanorod diodes increases significantly with increasing relative humidity. These unique nanostructured diode arrays could be useful as photo-responsive moisture and humidity detectors.

Thermal oxidation of Cu nanofilm on three-dimensional ZnO nanorod arrays

Three-dimensional (3D) ZnO–CuO core-shell nanorod arrays have been synthesized by a three-step process on silicon (100) substrates. A hydrothermal method was used to grow 3D ZnO nanorod arrays, followed by deposition of Cu nanofilm using sputtering, which was oxidized subsequently at 400 °C to form CuO shell surrounding ZnO nanorod core. The control over oxygen flow and pressure during the Cu nanofilm oxidation was found to improve the uniformity and intactness of CuO shell surrounding ZnO nanorod core. With higher oxygen flow, more conformal CuO thin film coating was achieved on the 3D ZnO nanorods array as a result of more sufficient oxygen access at the bottom portion of ZnO nanorods array. Higher pressure during thermal oxidation favored the formation of non-conformal ZnO–CuO core-shell nanorods and impurity Zn2SiO4 possibly at the interface of ZnO and Si substrate. UV-vis absorption spectroscopy revealed higher absorption efficiency in the visible region in ZnO–CuO core-shell nanorods than in pure ZnO nanorods. The fabricated ZnO–CuO core-shell nanorods could be useful nanoscale building blocks in solar cells and light emitting devices.

Oxide-catalyzed growth of Ag2O/Zn2SnO4 hybrid nanowires and their reversible catalytic ambient ethanol/oxygen detection

Large scale hybrid nanowires consisting of Zn2SnO4 periodic nanowires and Ag2O nanoparticles on both nanowire surfaces and tips have been successfully synthesized at 650 °C using a unique one-step silver oxide catalyzed vapor-solid-solid (VSS) growth process. The single crystal Zn2SnO4 nanowires mainly grew along the [13] direction with three major morphologies including straight linear, L-shape, and radiative sparse. Two-dimensional nucleation and periodic ledge growth processes were proposed to be responsible for the Zn2SnO4 periodical nanowire formation. These Ag2O/Zn2SnO4 hybrid nanowires were responsive to ethanol at ∼150 °C upon ∼150 ppm ethanol pulses, with one order of magnitude electrical conductivity decrease. This surprising decrease might be triggered by ethanol pulses associated with Ag2O nanoparticles on nanowire surfaces and electrode-nanowire film interfaces, leading to a catalytic activation of ambient oxygen detection. Ar plasma treatment on the nanowire surface inversely leads to conductivity increase upon ethanol pulses, which suggests a successful plasma removal of catalytic Ag2O and oxygen ions, therefore enabling the detection of ethanol molecules, instead of ambient oxygen. This reversible catalytic ambient ethanol/oxygen detection mechanism enabled by the hybrid nanowire configurations could provide a new path for designing smart gas detection devices compatible with multiple-transient-gas detection.

Carbon-assisted lateral self-assembly of amorphous silica nanowires

Amorphous silica nanowires generally grow in either a metal-catalyzed vapour–liquid–solid (VLS) process or a solid–liquid–solid (SLS) process. So far, ordered alignment of impurity-free SiOx (1 < x < 2) nanowires has not been achieved. In this study, without using metal catalysts, laterally aligned amorphous silica nanowire arrays have been successfully synthesized on a Si(100) substrate surface at ∼700–1100 °C. These amorphous silica nanowires have diameters of ∼100–350 nm and lengths of ∼1–20 μm, which are uniformly decorated with ∼4–11 nm SiO nanocrystals as a result of possible thermal decomposition of amorphous silica at high temperature. A carbon-assisted and lattice strain driven mechanism might be responsible for the lateral alignment of these SiO nanocrystal decorated amorphous silica nanowires.

Hierarchical oxide-based composite nanostructures for energy, environmental, and sensing applications

Self-assembled composite nanostructures integrate various basic nano-elements such as nanoparticles, nanofilms and nanowires toward realizing multifunctional characteristics, which promises an important route with potentially high reward for the fast evolving nanoscience and nanotechnology. A broad array of hierarchical metal oxide based nanostructures have been designed and fabricated in our research group, involving semiconductor metal oxides, ternary functional oxides such as perovskites and spinels and quaternary dielectric hydroxyl metal oxides with diverse applications in efficient energy harvesting/saving/utilization, environmental protection/control, chemical sensing and thus impacting major grand challenges in the area of materials and nanotechnology. Two of our latest research activities have been highlighted specifically in semiconductor oxide alloy nanowires and metal oxide/perovskite composite nanowires, which could impact the application sectors in ultraviolet/blue lighting, visible solar absorption, vehicle and industry emission control, chemical sensing and control for vehicle combustors and power plants.