Giant defect emission enhancement from ZnO nanowires through desulfurization process.

Zinc oxide (ZnO) is a stable, direct bandgap semiconductor emitting in the UV with a multitude of technical applications. It is well known that ZnO emission can be shifted into the green for visible light applications through the introduction of defects. However, generating consistent and efficient green emission through this process is challenging, particularly given that the chemical or atomic origin of the green emission in ZnO is still under debate. In this work we present a new method, for which we coin term desulfurization, for creating green emitting ZnO with significantly enhanced quantum efficiency. Solution grown ZnO nanowires are partially converted to ZnS, then desulfurized back to ZnO, resulting in a highly controlled concentration of oxygen defects as determined by X-ray photoelectron spectroscopy and electron paramagnetic resonance. Using this controlled placement of oxygen vacancies we observe a greater than 40-fold enhancement of integrated emission intensity and explore the nature of this enhancement through low temperature photoluminescence experiments

Three Dimensional Nanowire Array Piezo-phototronic and Piezo-photo-magnetotronic Sensors

Piezotronic and piezo-phototronic is a burgeoning field of study which emerges from the coupling of intrinsic materials properties exhibited by non-centrosymmetric semiconductors. In the past decade research efforts were mainly focused on the wurtzite family of 1D nanostructures, with major emphasis on ZnO nanowire nanogenerators, MS piezotronic transistors, LEDs and photodetectors mainly integrated on single nanowires. In view of previously known advantages of charge carrier separation in radial heterojunctions, particularly in type-II core/shell nanowires, it can be anticipated that the performance of photosensing devices can be largely enhanced by piezo-phototronic effect. Moreover, the performance metrics can be further improved in an array of nanowires where geometrical feature enabled multiple reflection can efficiently trap incident illumination. The crux of this dissertation lies in the development of 3D type-II core/shell nanowire array based piezo-phototronic device and also to investigate the effect of magnetic field on ZnO nanowire arrays based piezotronic and piezo-phototronic device for new class of sensors. In this regard, prototype piezo-phototronic broadband photodetectors integrated on two material systems, namely type-II CdSe/ZnTe 3D core/shell nanowire arrays and fully wide band gap type-II ZnO/ZnS 3D core/shell nanowire arrays have been developed where the photodetection performance of each device exhibits high sensitivity, fast response and large responsivity. The application of piezo-phototronic effect further improves the device performance by three to four orders of magnitude change numerically calculated from absolute responsivities at multiple wavelengths. A 3D ZnO nanowire array based new class of piezo-photo-magnetotronic sensor is also developed for detection of pressure, illumination and magnetic field suggesting multiple functionality of a single device where more than one effect can be coupled together to exhibit piezo-magnetotronic or piezo-photo-magnetotronic type of device behavior

Three Dimensional Nanowire Array Piezo-phototronic and Piezo-photo-magnetotronic Sensors

Piezotronic and piezo-phototronic is a burgeoning field of study which emerges from the coupling of intrinsic materials properties exhibited by non-centrosymmetric semiconductors. In the past decade research efforts were mainly focused on the wurtzite family of 1D nanostructures, with major emphasis on ZnO nanowire nanogenerators, MS piezotronic transistors, LEDs and photodetectors mainly integrated on single nanowires. In view of previously known advantages of charge carrier separation in radial heterojunctions, particularly in type-II core/shell nanowires, it can be anticipated that the performance of photosensing devices can be largely enhanced by piezo-phototronic effect. Moreover, the performance metrics can be further improved in an array of nanowires where geometrical feature enabled multiple reflection can efficiently trap incident illumination. The crux of this dissertation lies in the development of 3D type-II core/shell nanowire array based piezo-phototronic device and also to investigate the effect of magnetic field on ZnO nanowire arrays based piezotronic and piezo-phototronic device for new class of sensors. In this regard, prototype piezo-phototronic broadband photodetectors integrated on two material systems, namely type-II CdSe/ZnTe 3D core/shell nanowire arrays and fully wide band gap type-II ZnO/ZnS 3D core/shell nanowire arrays have been developed where the photodetection performance of each device exhibits high sensitivity, fast response and large responsivity. The application of piezo-phototronic effect further improves the device performance by three to four orders of magnitude change numerically calculated from absolute responsivities at multiple wavelengths. A 3D ZnO nanowire array based new class of piezo-photo-magnetotronic sensor is also developed for detection of pressure, illumination and magnetic field suggesting multiple functionality of a single device where more than one effect can be coupled together to exhibit piezo-magnetotronic or piezo-photo-magnetotronic type of device behavior

Synthesis and characterization of aligned ZnO/BeO core/shell nanocable arrays on glass substrate

By sequential hydrothermal growth of ZnO nanowire arrays and thermal evaporation of Be, large-scale vertically aligned ZnO/BeO core/shell nanocable arrays on glass substrate have been successfully synthesized without further heat treatment. Detailed characterizations on the sample morphologies, compositions, and microstructures were systematically carried out, which results disclose the growth behaviors of the ZnO/BeO nanocable. Furthermore, incorporation of BeO shell onto ZnO core resulted in distinct improvement of optical properties of ZnO nanowire, i.e., significant enhancement of near band edge (NBE) emission as well as effective suppression of defects emission in ZnO. In particular, the NBE emission of nanocable sample shows a noticeable blue-shift compared with that of pristine ZnO nanowire, which characteristics most likely originate from Be alloying into ZnO. Consequently, the integration of ZnO and BeO into nanoscale heterostructure could bring up new opportunities in developing ZnO-based device for application in deep ultraviolet region

II-VI Semiconductor Nanowire Array Sensors Based on Piezotronic, Piezo-Phototronic and Piezo-Photo-Magnetotronic Effects

With the rapid progress of nanotechnologies, there are two developing trends for the next generation of sensors: miniaturization and multi-functionality. Device miniaturization requires less power consumption, or even self-powered system. Multi-functional devices are usually based on multi-property coupling effects. Piezoelectric semiconductors have been considered to be potential candidates for self-powered/multi-functional devices due to their piezotronic coupling effect. In this dissertation, ZnO and CdSe nanowire arrays have been synthesized as the piezoelectric semiconductor materials to develop the following self-powered/multi-functional sensors: (1) self-powered gas sensors of ZnO/SnO2, ZnO/In2O3, ZnO/WO3 and CdSe nanowire arrays have been assembled. All these gas sensors are capable of detecting oxidizing gas and reducing gas without any external power supply owing to piezotronic effect which can convert mechanical energies to electrical energy to power the sensors; (2) a self-powered ZnO/ZnSe core/shell nanowire array photodetector has been fabricated. This photodetector is able to detect the entire range of the visible spectrum as well as UV light because of its type II heterostructure. The absolute sensitivity and the percentage change in responsivity of the photodetector were significantly enhanced resulting from the piezo-phototronic effect. The photodetector also exhibited self-powered photodetection behavior; (3) three dimensional nanowire arrays, such as ZnO and ZnO/Co3O4, have been synthesized to investigate piezo-magnetotronic and piezo-photo-magnetotronic effects. Under magnetic field, the magnetic-induced current of ZnO nanowire array decreased as magnetic field increased, and the current difference was magnified by one order of magnitude caused by piezo-magnetotronic effect through applying a stress. In contrast, under UV light illumination, the current response increased with an increment of magnetic field. The current difference was enhanced by at least two orders of magnitude attributed to piezo-photo-magnetotronic effect. Furthermore, ZnO/Co3O4 core/shell structure was employed to further improve the magnetic-induced current difference. This phenomenon projects a potential for multi-functional piezo-magnetotronic and piezo-photo-magnetotronic device development

Silicon nano wires solar cell

Improving the optical absorption capability of solar cells\u27 materials is a crucial factor in increasing their power conversion efficiency. To this end, the absorption can be enhanced by minimizing the reflection and the transmission out from the absorbing layer. While the reflection can be minimized using an antireflection coating, the transmission can be minimized by exploiting a light- trapping mechanism. In this thesis, the Si nanowires have been utilized to enhance the absorption and photocurrent without the need for antireflection coating, and provide high field localization, which in turn enhances the overall efficiency of the solar cell. Vertically orientated single crystalline silicon nanowire (SiNW) arrays with controlled diameters have been fabricated via a metal-assisted chemical etching (MACE) method. The diameter of the fabricated nanowires is controlled by simply varying the etching time in HF/H2O2 solution. The fabricated SiNWs have diameters ranging from 117 to 650 nm and length from 8 to 18 μm. The optical measurements show a significant difference in the reflectance/absorption of the SiNWs with different diameters, where the reflectance increases with increasing the diameter of the SiNWs. The optical absorption also has been measured at different incident light angle to determine the best angle for absorption. The best absorption angle for different diameters was 10o.The SiNWs showed significant photoluminescence (PL) emission spectra with peaks lying between 380 and 670 nm. The PL intensity increases as the diameter increases and shows red shift for peaks at ~ 670 nm. The increase or decrease of reflectivity is coincident with PL intensity at wavelength ~ 660 nm. The x-ray diffraction (XRD) patterns and high-resolution transmission electron microscope (HR-TEM) confirm the high crystallinity of the fabricated SiNWs. In addition, the Raman spectra showed a shift in the first order transverse (1TO) band toward lower frequencies compared to that usually seen for c-Si. The current-voltage characteristics have also been investigated using photoelectrochemical cell. The measurements have been done in two electrolytes; 10% HF 10% and hydrobromic acid (40%) and bromine (3%). The measurements have been done for the fabricated Si nanowires with different diameters under dark and illumination conditions. The resulted photocurrent decreases with increasing the diameter of SiNWs, which has been explained based on the Debye length of SiNWs. Full wave electromagnetic analysis has been performed using finite difference time domain simulations (FDTD) to confirm the effect of change of diameter on the optical properties of the nanowires. The simulation results show good agreement with the experimental findings for the SiNWs of different diameters. Also, the simulation has been done for different incident light angles to investigate the best incident angle that results in the highest absorption and minimum reflection

II-VI Core-Shell Nanowires: Synthesis, Characterizations and Photovoltaic Applications

The emergence of semiconducting nanowires as the new building blocks for photovoltaic (PV) devices has drawn considerable attention because of the great potential of achieving high efficiency and low cost. In special, nanowires with a coaxial structure, namely, core-shell structures have demonstrated significant advantages over other device configurations in terms of radial charge collection and cost reduction. In this dissertation, several core-shell nanowire structures, including ZnO/ZnSe, ZnO/ZnS, and CdSe/ZnTe, have been synthesized and the photovoltaic devices processed from a ZnO/ZnS core-shell nanowire array and a single CdSe/ZnTe core-shell nanowire have been demonstrated. By combining the chemical vapor deposition and pulsed laser deposition (PLD) techniques, type-II heterojunction ZnO/ZnSe and ZnO/ZnS core-shell nanowire array were synthesized on indium-tin-oxide substrates. Their structures and optical properties have been investigated in detail, which revealed that, despite highly mismatched interfaces between the core and shell, both systems exhibited an epitaxial growth relationship. The quenching in photoluminescence but enhancement in photocurrent with faster response upon coating the core with the shell provides the evidence that the charge separation and collection in the type II core-shell nanowire is greatly improved. This demonstration brings much greater flexibility in designing next generation PV devices in terms of material selection and device operation mechanisms for achieving their maximum energy conversion efficiencies at a low cost and in an environmentally friendly manner. In order to achieve a high quality interface in the core-shell nanowire, CdSe and ZnTe, which have close lattice parameters and thermal expansion coefficients, were chosen to fabricate nanowire solar cells. ZnTe and CdSe nanowires were first synthesized by thermal evaporation and the shells were subsequently deposited by PLD. ZnTe/CdSe nanowires represented an inhomogeneous coating while the CdSe/ZnTe core-shell exhibited a conformal coating with obvious ZnTe eptilayer. The final PV device based on an individual CdSe/ZnTe nanowire demonstrated an efficiency of ~1.7%. In addition, a controllable synthesis of CdSe nanowire array on muscovite mica substrate was presented, providing the possibility to harvest hybrid energies in an all-inorganic nanowire array

II-VI Core-Shell Nanowires: Synthesis, Characterizations and Photovoltaic Applications

The emergence of semiconducting nanowires as the new building blocks for photovoltaic (PV) devices has drawn considerable attention because of the great potential of achieving high efficiency and low cost. In special, nanowires with a coaxial structure, namely, core-shell structures have demonstrated significant advantages over other device configurations in terms of radial charge collection and cost reduction. In this dissertation, several core-shell nanowire structures, including ZnO/ZnSe, ZnO/ZnS, and CdSe/ZnTe, have been synthesized and the photovoltaic devices processed from a ZnO/ZnS core-shell nanowire array and a single CdSe/ZnTe core-shell nanowire have been demonstrated. By combining the chemical vapor deposition and pulsed laser deposition (PLD) techniques, type-II heterojunction ZnO/ZnSe and ZnO/ZnS core-shell nanowire array were synthesized on indium-tin-oxide substrates. Their structures and optical properties have been investigated in detail, which revealed that, despite highly mismatched interfaces between the core and shell, both systems exhibited an epitaxial growth relationship. The quenching in photoluminescence but enhancement in photocurrent with faster response upon coating the core with the shell provides the evidence that the charge separation and collection in the type II core-shell nanowire is greatly improved. This demonstration brings much greater flexibility in designing next generation PV devices in terms of material selection and device operation mechanisms for achieving their maximum energy conversion efficiencies at a low cost and in an environmentally friendly manner. In order to achieve a high quality interface in the core-shell nanowire, CdSe and ZnTe, which have close lattice parameters and thermal expansion coefficients, were chosen to fabricate nanowire solar cells. ZnTe and CdSe nanowires were first synthesized by thermal evaporation and the shells were subsequently deposited by PLD. ZnTe/CdSe nanowires represented an inhomogeneous coating while the CdSe/ZnTe core-shell exhibited a conformal coating with obvious ZnTe eptilayer. The final PV device based on an individual CdSe/ZnTe nanowire demonstrated an efficiency of ~1.7%. In addition, a controllable synthesis of CdSe nanowire array on muscovite mica substrate was presented, providing the possibility to harvest hybrid energies in an all-inorganic nanowire array

Water-Mediated Ionic Migration in Memristive Nanowires with a Tunable Resistive Switching Mechanism

partially_open7sìMemristive devices based on electrochemical resistive switching effects have been proposed as promising candidates for in-memory computing and for the realization of artificial neural networks. Despite great efforts toward understanding the nanoionic processes underlying resistive switching phenomena, comprehension of the effect of competing redox processes on device functionalities from the materials perspective still represents a challenge. In this work, we experimentally and theoretically investigate the concurring reactions of silver and moisture and their impact on the electronic properties of a single-crystalline ZnO nanowire (NW). A decrease in electronic conductivity due to surface adsorption of moisture is observed, whereas, at the same time, water molecules reduce the energy barrier for Ag+ ion migration on the NW surface, facilitating the conductive filament formation. By controlling the relative humidity, the ratio of intrinsic electronic conductivity and surface ionic conductivity can be tuned to modulate the device performance. The results achieved on a single-crystalline memristive model system shed new light on the dual nature of the mechanism of how moisture affects resistive switching behavior in memristive devices.openGianluca Milano; Federico Raffone; Michael Luebben; Luca Boarino; Giancarlo Cicero; Ilia Valov; Carlo RicciardiMilano, Gianluca; Raffone, Federico; Luebben, Michael; Boarino, Luca; Cicero, Giancarlo; Valov, Ilia; Ricciardi, Carl