We report on the major improvement in UV photosensitivity and faster photoresponse from vertically aligned ZnO nanowires (NWs) by means of rapid thermal annealing (RTA). The ZnO NWs were grown by vapor-liquid-solid method and subsequently RTA treated at 700°C and 800°C for 120 s. The UV photosensitivity (photo-to-dark current ratio) is 4.5 × 103 for the as-grown NWs and after RTA treatment it is enhanced by a factor of five. The photocurrent (PC) spectra of the as-grown and RTA-treated NWs show a strong peak in the UV region and two other relatively weak peaks in the visible region. The photoresponse measurement shows a bi-exponential growth and bi-exponential decay of the PC from as-grown as well as RTA-treated ZnO NWs. The growth and decay time constants are reduced after the RTA treatment indicating a faster photoresponse. The dark current-voltage characteristics clearly show the presence of surface defects-related trap centers on the as-grown ZnO NWs and after RTA treatment it is significantly reduced. The RTA processing diminishes the surface defect-related trap centers and modifies the surface of the ZnO NWs, resulting in enhanced PC and faster photoresponse. These results demonstrated the effectiveness of RTA processing for achieving improved photosensitivity of ZnO NWs.

We demonstrate graphene-assisted controlled fabrication of various ZnO 1D nanostructures on the SiO2/graphene substrate at a low temperature (540 °C) and elucidate the growth mechanism. Monolayer and a few layer graphene prepared by chemical vapor deposition (CVD) and subsequently coated with a thin Au layer followed by rapid thermal annealing is shown to result in highly aligned wurtzite ZnO nanorods (NRs) with clear hexagonal facets. On the other hand, direct growth on CVD graphene without a Au catalyst layer resulted in a randomly oriented growth of dense ZnO nanoribbons (NRBs). The role of in-plane defects and preferential clustering of Au atoms on the defect sites of graphene on the growth of highly aligned ZnO NRs/nanowires (NWs) on graphene was established from micro-Raman and high-resolution transmission electron microscopy analyses. Further, we demonstrate strong UV and visible photoluminescence (PL) from the as-grown and post-growth annealed ZnO NRs, NWs, and NRBs, and the origin of the PL emission is correlated well with the X-ray photoelectron spectroscopy analysis. Our results hint toward an epitaxial growth of aligned ZnO NRs on graphene by a vapor-liquid-solid mechanism and establish the importance of defect engineering in graphene for controlled fabrication of graphene-semiconductor NW hybrids with improved optoelectronic functionalities.

We have investigated the structural, optical and ferromagnetic properties of undoped and Fe-doped TiO 2 nanoribbons (NRbs) grown by a solvothermal method. A strong room temperature ferromagnetism (RTFM) is observed in both undoped and Fe-doped TiO 2 NRbs. Fe-doped TiO 2 NRbs exhibited a ∼4.8-fold enhancement in RTFM as compared to the undoped NRbs grown under similar conditions. However, the RTFM decreases at higher Fe concentration, possibly due to antiferromagnetic ordering between nearby Fe 3+ ions caused by a super exchange interaction. X-ray diffraction patterns reveal the pure TiO 2 (B) phase, the TiO 2 (B)-anatase mixed phase and the anatase-rutile mixed phase of the TiO 2 structure. Field emission scanning electron microscopy and transmission electron microscopy observations reveal NRbs with uniform pore distribution and nanopits formed on the surface for both undoped and Fe-doped NRbs. These samples exhibit strong visible photoluminescence associated with oxygen vacancies and the ferromagnetic hysteresis loop, both of which are strongly enhanced after vacuum annealing. Optical absorption, electron spin resonance and x-ray photoelectron spectroscopic analyses are performed to elucidate the origin of RTFM. The observed RTFM in undoped and Fe-doped TiO 2 NRbs is qualitatively explained through a model involving bound magnetic polarons, which include an electron locally trapped by an oxygen vacancy with the trapped electron occupying an orbital overlapping with the unpaired electron (3d 1) of a Ti 3+ ion and/or the unpaired electron (3d 5) of a Fe 3+ ion. The development of TiO 2 NRbs with tunable optical and magnetic properties constitutes an important step towards realizing improved magneto-optical and spintronic devices from novel TiO 2 nanostructures.

Surface effects in a semiconductor photonic nanowire and spectral stability of an embedded single quantum dot Appl. Phys. Lett. 99, 233106 (2011) Strong modification of the reflection from birefringent layers of semiconductor nanowires by nanoshells Appl. Phys. Lett. 99, 201108 (2011) Optical and electrical properties of gold nanowires synthesized by electrochemical deposition J. Appl. Phys. 110, 094301 (2011) Broad spectral response in composition-graded CdSSe single nanowires via waveguiding excitation Appl. Phys. Lett. 99, 181111 (2011) Additional information on J. Appl. Phys.We have investigated the mechanism of photoinduced charge transport and origin of enhanced PC and PL from Au and Ti NPs decorated vertically aligned ZnO NWs arrays. Uniform decoration of metal NPs on the surface of the ZnO NWs was confirmed by high resolution electron microscopy imaging. Absorption spectra also indicate the presence of metal NPs layer with different thicknesses. At a fixed bias, the dark current of the Au/ZnO heterostructures decreases with the increase of Au coverage, while the Ti/ZnO heterostructures show very high dark current. The photocurrent (PC) spectra show a strong peak at the UV region due to the band-edge absorption followed by generation of the photocarriers and two other peaks in the visible region. For the Au/ ZnO heterostructures, the PC increases with increase of Au coverage up to a certain thickness and then decreased. On the other hand, the PC in Ti/ZnO heterostructures increases with the increase of the Ti coverage. The PL spectra for both the system are consistent with the respective PC spectra, which show significant improvement in the band-edge related UV emission and quenching of green emission. The Ti/ZnO heterostructures shows much faster photoresponse compared to the Au/ZnO heterostructures. The changes in the PC and PL spectra with the size of the metal NPs are studied systematically and explained. A model based on energy band alignment is proposed to explain the results.

Over the last decades, semiconductor nanowires have been extensively studied for their applications in several disciplines. Many of these applications require heterostructures, which can be defined as the combination of two or more materials within the same nanowire structure. Recent research on the ZnO nanowires (NWs) heterostructures shows the possibility to overcome the existing limitation of the bare ZnO NWs based optoelectronic devices. Prospects for the commercialization of ZnO NWs based devices using the heterostructure approach are intensively pursued at present. This review article summarizes the current status of research worldwide and the findings from our group on various aspects of the ZnO NW heterostructures: fabrication methodologies, structural characterization, photophysical properties and related emerging applications. The fascinating properties of the ZnO NW heterostructures and its applications in different devices, mainly UV photodetectors, light-emitting-diodes, dye sensitized or quantum dots sensitized solar cells and photoelectrochemical cells are discussed. Focus of the article is on the impact of heterostructure approach on the selective properties and performance of the ZnO NWs based devices. The improvements in the key parameters of these devices are also discussed to highlight the effectiveness of the heterostructure approach and a future outlook of the field is presented at the end.