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Self-assembled bolaamphiphilic perylene bisimide (PBI) containing an amino acid appended fluorescent semiconducting soft material (hydrogel) has been discovered at physiological pH. This new organic material based on self-assembled perylene bisimide appended amino acid-based bolaamphiphile (PBI-C11-Y) has been well characterized using various techniques including UV-Vis, fluorescence, X-ray diffraction, FT-IR, transmission electron microscopic (TEM) and atomic force microscopic (AFM) studies. Interestingly, the UV-Vis absorption properties of the soft-material are dependent on the pH of the medium. This PBI-conjugated amino acid appended gelator molecule contains a centrally located perylene bisimide moiety as well as an aromatic amino acid l-tyrosine at the side chains, which are extremely useful for interacting with the delocalized large π-surface of GO (graphene oxide) or RGO (reduced graphene oxide) to form a GO/RGO containing hybrid hydrogel. Graphene oxide and reduced graphene oxide have been successfully incorporated into the nanofibrillar network structure of the PBI-C11-Y based gel to make nanohybrid systems. The I-V profile of the semiconducting photo-responsive soft-material of the PBI-C11-Y has been successfully tuned upon the incorporation of GO and RGO within the gel-based soft material. This PBI-C11-Y xerogel based structure shows photo-switching behaviour upon exposure to white light. The ON/OFF ratio of the PBI-C11-Y can be modulated upon the inclusion of GO and RGO within the hydrogel matrix. Furthermore, the OFF state stability of the PBI-C11-Y xerogel material has been increased upon the inclusion of RGO. Regulation of the photo-switching behaviour of the PBI-C11-Y based xerogel holds promise for making PBI-containing amino acid appended biomaterials with interesting properties in future.

SiO2 spacer layers of various thicknesses in between an Au and ZnO nanorod (NR) layer (Au/ZnO) have been introduced and the luminescence properties of the layered ensemble have been investigated. When ZnO NRs are grown directly on a granular Au layer, the ultraviolet (UV) emission of ZnO was enhanced nearly 2 times due to surface plasmon resonance (SPR) mediated charge transfer between Au and ZnO. A significant improvement is observed when the SiO2 spacer layer is introduced in between Au and ZnO. The UV/VIS emission intensity ratio of Au/SiO2/ZnO NRs' becomes ∼6 times higher than that of the pristine ZnO NRs. A SPR mediated electron transfer from Au/SiO2 to ZnO causes such an enhanced UV emission. The spacer diminishes the reverse electron transfer process from the conduction band of ZnO to the Fermi level of Au, a most competitive reason for PL quenching, which also accounts for the emission enhancement.

Understanding the mechanism behind high photocurrent gain is very important to realize a highly functional material for photodetector devices. Herein, we report a very high ultraviolet photocurrent gain of 2.8 × 105 in hydrothermally grown Cu-doped ZnO nanorod arrays which is two orders of magnitude higher as compared to the undoped sample. Trapping of carriers under dark and de-trapping them under illumination by Cu-related defects is responsible for high gain. The trap state at ∼1.65 eV is attributed to the [{CuZn+(3d10)}− − Zni+(4s1)]0 type of defects. A model is shown to explain the dark and photocurrent states in the doped samples.

Reduced graphene oxide (rGO) has been engineered with a hybrid of dispersed Ag and ZnO nanoparticles by a one-step hydrothermal technique using graphene oxide, AgNO3 and Zn(CH3COO)2 precursors without adding any external toxic reagent. Formation of the Ag–ZnO–rGO hybrid is confirmed from X-ray diffractometry, high resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The hybrid shows an enhanced and faster ultraviolet photocatalytic property i.e. 97.2% degradation of methyl orange in 90 minutes as compared to the values of 56%, 84% and 60% by bare ZnO, Ag–ZnO and ZnO–rGO, respectively. The enhanced photocatalytic property is due to an efficient charge transfer process from ZnO to both Ag and rGO. This result would be beneficial for synthesizing efficient ZnO-based ternary photocatalysts with different combinations of metal and graphene, and understanding their photocatalytic performance in photoreaction processes.

A new, stable, semiconducting, photo-responsive, amino acid (L-tyrosine) based perylene bisimide derivative (PBI-Y) was found to form pH-sensitive hydrogels. This gelator molecule (PBI-Y) has undergone hydrogelation at 50 mM phosphate buffer at pH 5.00–9.00. Different minimum gelation concentration values (MGC) were recorded in different pHs at the same phosphate buffer strength (50 mM). The hydrogel was also characterized using different techniques, including X-ray diffraction analysis, rheology and FT-IR study. The fluorescence and cyclic voltammetric responses of the PBI-Y hydrogelator can be tuned by changing the pH. The PBI-Y hydrogelator has shown self-assembled nanofibrillar network structures, as it has been evident from the field emission scanning electron microscopic (FE-SEM), as well as transmission electron microscopic analyses (TEM). This PBI-Y hydrogel has also shown interesting semiconducting behaviour. This PBI-Y hydrogelator-based photoconductor was found to exhibit excellent photo-switching behaviour with a high photo-response value. This holds a future promise for the creation of PBI-conjugated functional soft-materials-based photodetectors and photovoltaics.

We demonstrate the feasibility of fabricating a solution-processed novel core–shell coaxial n–p nanorod (NR) heterojunction consisting of an n-type ZnO NR core and a p-type CuS shell through an ion-exchange route. The field emission scanning electron and high resolution transmission electron microscopy experiments show evidence of the formation of core–shell NR arrays manifesting a clear change in the side walls of the NRs, which become rougher due to CuS shell formation over ZnO NRs. The core–shell heterojunction shows diode behavior with an ideality factor of 5.8. The diode shows a very fast response time (

A strong spin-spin interaction between oxygen vacancy and Mg in ZnMgO nanorods has been manifested by electron paramagnetic resonance (EPR) studies from symmetrical hyperfine structure. Mg probably occupies Zn sites as a result of meta-stable coordination. 25Mg have nuclear spin I = 5/2 that can give rise to spin resonance with the unpaired spin S = 1/2 of VO+ in ZnO having (2n + 1) = 6 number of splitting as observed in the EPR spectra. On heating at 500 °C, the hyperfine structure disappears as Mg moves to the interstitial sites. The results are further supported by x-ray diffraction and photoluminescence results.

ZnO nanowires (NWs) have been decorated with Pd nanoparticles of sizes less than 10 nm (Pd-ZnO NWs) via a chemical solution route. The microstructural characterizations have been done using field emission scanning electron and high-resolution transmission electron microscopes. The effects of attaching Pd nanoparticles to the walls of ZnO NWs have been investigated by studying the ultraviolet (UV) photosensitivity and photoluminescence (PL) properties. The surface-modified NWs show a UV photosensitivity more than double and a response seven times faster compared to the bare NWs. The photocarrier relaxation under the steady UV illumination condition is quite different in Pd-ZnO NWs. The higher and faster photosensitivity has been explained on the basis of photocarrier transfer from the conduction band of ZnO to the Fermi level of Pd and subsequent electron trapping by the adsorbed O(2) molecules on the NWs' surface, which have been presented through a proposed model. The PL spectrum of Pd-ZnO NWs shows that the intensities of the band-edge and defect-related emissions decrease and increase, respectively, due to Pd anchoring, the effect being pronounced as the density of Pd nanoparticles increases.

We have demonstrated a novel and simple low-cost method to grow beaded nanochains of ZnO using an aqueous chemical growth method. Whatman filter paper (40) is used as the template. The filter paper is generally made up of cellulose fibers along which the growth of beaded ZnO nanoparticles (NPs) is initiated. When the filter paper is burnt at 700 degrees C temperature, the NPs appear as a beaded nanochain morphology while those synthesized without the filter paper form lumped nanostructures without any regular shape and size. A model has been proposed to explain the growth mechanism. A sharp and strong green emission has been observed for the template-grown sample in contrast to a broad and less intense hump of the without template-grown sample. The beaded nanochains shows 64% photocatalytic degradation of methyl orange (MO) under UV irradiation, which is much superior to a value of only 22% shown by the lumped sample. Not only can this low-cost simple template-based synthesis be applied to grow other nanostructures of similar morphology but is also promising for enhancing the properties in the multifunctional materials.

A highly enhanced ultraviolet (UV) photodetection property has been achieved in 0.1% Cu doped and 0.1% Cu–2% Li co-doped sol–gel ZnO thin films. The 0.1% Cu doped and 0.1% Cu–2% Li co-doped films show UV photosensitivity values (photo-to-dark current ratio) of around 2000 and 5000, respectively, which is one order of magnitude higher than the undoped film. The co-doped film shows a decrease in the photocurrent growth and decay time constants compared with the Cu doped one. The increased photosensitivity indicates that these low-cost doped and co-doped films are promising for UV photodetection applications.

The anomalous photocurrent decay in aqueous solution grown ZnO nanowires (NWs) under steady ultraviolet light illumination have been investigated. The photocurrent growth-decay measurements using the above-band and subband gap light excitation energies in the as-grown and annealed NWs show that while a VZn-related defect complex is formed by the surface adsorbed H2O molecules, a faster carrier trapping by the surface adsorbed O2 molecules and a slower carrier recombination at the defect, Zni cause the photocurrent decay under steady illumination supported by the results of the photocurrent spectra and photoluminescence measurements. The predicted mechanism has been explained through a model.

A simple route for the synthesis of a novel mesoporous zinc oxide material having wurtzite like nanocrystalline pore walls by using Schiff-base amine as template is reported, which shows very high BET surface area (456 m(2) g(-1)) and remarkably enhanced photoconductivity and photoluminescence at room temperature under visible light irradiation vis-à-vis bulk ZnO material.

We have investigated the carrier relaxation process during photoconduction in quasi-one-dimensional (Q1D) ZnO nanowires (NWs) of diameters 29–36nm on different substrates using photocurrent transient measurements. Ultraviolet (UV) sensitive NWs show around three to four orders of change in the photo-to-dark current ratio. Under steady UV illumination, the photocarrier relaxation occurs through two-electron process—carrier loss due to the trapping by the surface states and recombination at the deep defect states. The results demonstrate that the carrier relaxation during photoconduction in Q1D NWs of diameter comparable to the Debye length is also dominated by the surface states.

Single layer ZnO/polyaniline (PANI) inorganic/organic hybrid structure fabricated on glass substrate by depositing polyaniline film on the sol-gel ZnO thin film shows a rectifying behavior indicating the formation of a diode. The current transport mechanism is modeled through an energy band diagram. The heterojunction is sensitive to UV illumination. The photo-to-dark current ratio of the junction is about 46 for −5V bias. The photoresponse parameters of the diode are found to be better than those of only ZnO film of similar thickness. The results indicate that the ZnO/PANI hybrid structure might be promising for the UV photodetection applications.

MgxZn1−xO (0.0⩽x⩽0.12) thin films have been deposited on glass substrates by sol-gel process. The hexagonal wurtzite structure of ZnO is retained in the Mg-substituted films. From the photoresponse measurements, it is observed that both ZnO and substituted films are UV sensitive and with increasing Mg content, the sensitivity shifts toward the shorter wavelength side with a gradual decrease in magnitude. The UV-to-visible current ratio remains almost constant up to x=0.08 and thereafter decreases. The decay time for 90% photocurrent gradually decreases to less than 4s for x=0.05 followed by an increase for higher values of x. The decay time is faster than the growth time for all the substituted films. The change in the photoconductivity with Mg content (x) is correlated to the microstructural change. A trap level, found to be involved in the photoconductivity, is located at ∼0.8eV below the conduction band. The photo-to-dark current ratio (Iph∕Id) gradually decreases from three orders of magnitude to one ...

MgxZn1−xO (x=0.0–0.20) ternary alloy thin films have been deposited by sol-gel technique and the composition related structural, microstructural, electrical, and optical properties are investigated. All the films have the hexagonal wurtzite structure. In the composition range of 0.05

Dendrite-like GaN nanowires have been grown by direct reaction of gallium metal with ammonia without any catalyst in a simple grinding-mediated two-step process, the second step being annealing of the as-synthesized GaN powder at 800°C. The samples have been characterized by x-ray diffractometry, transmission electron microscopy (TEM), and photoluminescence (PL). TEM results show that many of the synthesized nanowires are of diameters less than the Bohr exciton radius of GaN. A large blue shift of the excitonic peak in the photoluminescence (PL) spectra indicates quantum confinement of the carriers. These results are significant for the growth of dendrite-like GaN nanowires with a small aspect ratio.

The effect of oxygen mixed in nitrogen on p-type activation in Mg-doped GaN epilayers grown by metalorganic chemical vapor deposition (MOCVD) was investigated. The samples annealed in N2/O2 (1%) ambient exhibited the best electrical properties with respect to hole concentration. SIMS data suggested that oxygen reacted with hydrogen present in the Mg-doped GaN samples during the thermal annealing process, thereby enhancing the activation of Mg acceptors.

Reactive ion etching of GaN has been carried out in two different plasma chemistries using 10Cl2/10Ar and 1CH4/9Cl2/10Ar at various applied radio frequency (rf) power. We have addressed the postetch surface characteristics of GaN, and the effect of CH4 addition to chlorine plasma and the variation of rf power are discussed. Surface contamination, roughness, and stoichiometry have been determined from the results of secondary ion mass spectrometry. It is observed that in the case of plasma with 5% CH4, above 125 W, contamination of carbon is minimal and roughness is similar to that of the as-grown sample. At rf power less than 125 W, the Ga/N ratio of the etched surface is almost unity, whereas at rf power more than 125 W, it is more than unity. When the GaN surface is etched with only chlorine plasma, the surface is Ga excess at all rf power applied. The intensity of the main band-to-band photoluminescence peak is found to decrease when 10Cl2/10Ar is used, and the intensity was a minimum at 200 W. When 5%...

Reactive ion etching (RIE) of p-GaN and p-Al x Ga1-x N has been investigated using Cl2/CH4/Ar plasmas in the conventional RIE technique. It has been found that variation of CH4 percentage in gas mixtures leads to changes in the etch rate of both p-GaN and p-Al0.15Ga0.85N. The maximum etch rate for p-GaN has been found to be 257 nm/min in Cl2/Ar plasma containing 2.5% CH4 and the value is 170 nm/min for p-Al0.15Ga0.85N in Cl2/Ar plasma containing 10% CH4. With increasing rf power, the etch rates for p-GaN and p-Al0.15Ga0.85N reach as high as 433 nm/min and 255 nm/min respectively. Under optimum conditions of gas composition, radiofrequency (rf) power, and temperature, an anisotropic and smooth etch profile is obtained. The etched surface exhibits small roughness.

Reactive ion etching (RIE) of GaN and Al x Ga1-x N has been investigated using Cl2, CH4 and Ar gas mixtures. It has been found that a variation in the CH4 percentage in the gas mixture leads to a change in the etch rate of both GaN and Al0.85Ga0.15N with a etch selectivity of 4.2:1 for GaN over Al x Ga1-x N for an optimum value of 2.5% of CH4. This gas composition also yields an anisotropic and smooth etching profile. An etch rate of 257 nm/min of GaN has been obtained which is the highest value ever reported with the RIE technique.

Optical thresholds, that correspond to a level located at 1 eV above the valence band, are observed by photocapacitance techniques in n-type Mg-doped GaN. In undoped GaN, this level has been previously related to the yellow emission detected by photoluminescence. In Mg-doped GaN, this yellow luminescence is only observed for excitation energies below the Mg-related band (2.9 - 3 eV). This result evidences that Mg-doping may reduce but not avoid the formation of the yellow band related defects in n-type and semiinsulating Mg-doped samples. The fact that the yellow luminescence is not observed for excitation energies above the bandgap may be justified by a higher efficiency of the Mg-related recombination path.

Undoped layers of GaN grown by MOVPE on sapphire substrates have been characterized by photoluminescence, photocapacitance and photoinduced current transient spectroscopy (PICTS). Photocapacitance reveals in all samples two specific signatures at photon energies of 1 eV and 2.5 eV. The photocapacitance decrease observed at 1 eV seems to be due to an electron capture process from the valence band, whereas the capacitance increase at 2.5 eV is related to an electron emission process. The fact that the capacitance step at 1 eV is only seen after photoionization at energies above 2.5 eV, and the observed correlation between its amplitude and the photoluminescence intensity of the “yellow band”, lead us to conclude that both transitions are linked to the same trap, which is also suggested to be responsible for the yellow band. The position of this trap, at 2.5 eV below the conduction band, is confirmed by PICTS measurements, that show a hole thermal emission activation energy of 0.9 eV at 350 K.

Optical and transport properties of wurtzite GaN layers, grown by plasma-assisted molecular-beam epitaxy on Si(111) substrates, have been investigated. An emission at 3.455 eV, analyzed by continuous-wave and time-resolved luminescence in undoped and Si-doped GaN layers, is assigned to excitons bound to Si donors with an optical binding energy of 50 meV. A common origin of this peak, for undoped and Si-doped GaN, is backed by secondary-ion-mass spectroscopy that evidences a Si diffusion from the substrate into the GaN layer for growth temperatures above 660 \ifmmode^\circ\else\textdegree\fi{}C. Simultaneously, Ga diffusion into the Si substrate generates a highly $p$-type conductive layer at the GaN/Si interface, leading to unreliable Hall data in undoped and lightly doped layers. Positron annihilation reveals a concomitant vacancy cluster generation at the GaN/Si interface in samples grown above 660 \ifmmode^\circ\else\textdegree\fi{}C. No traces of the ``yellow band'' are detected either in undoped or in Si-doped samples.