Dambarudhar Mohanta

Defect mediated optical emission of randomly oriented ZnO nanorods and unusual rectifying behavior of Schottky nanojunctions

We report on the interrelation of optical emission of randomly oriented ZnO nanorod system with the carrier transport properties of Ag/ZnO nanorod-based rectifying junctions. The ZnO nanorods, exhibiting a hexagonal wurtzite phase, were fabricated by a cost-effective rapid thermal annealing process and at different annealing temperatures. The photoluminescence spectra of the as grown samples have revealed various Zn and O related native defects (e.g., vacancies, interstitials etc.) located at ∼400, 428, 491, and 535 nm. As evident from the I-V characteristic curves, though all the Ag/ZnO nanojunctions show Schottky behavior, the nanorods grown at a temperature of 550 °C and 650 °C are characterized by very large ideality factors of respective values 35.4 and 33.2, apart from displaying unusually high reverse currents. Whereas, the samples grown at 450 °C and 750 °C show usual rectifying nature having relatively lower ideality factors (18.4 and 12.2), along with low leakage-current under reverse biasing. T...

Irradiation induced grain growth and surface emission enhancement of chemically tailored ZnS : Mn/PVOH nanoparticles by Cl+9 ion impact

Manganese doped zinc sulfide nanoparticles are fabricated on polyvinyl alcohol dielectric matrix. They are bombarded with energetic chlorine ions (100 MeV). The size of the crystallites is found to increase with ion fluence due to melting led grain growth under ion irradiation. The increased size as a result of grain growth has been observed both in the optical absorption spectra in terms of redshift and in electron microscopic images. The photoluminescence (PL) study was carried out by band to band excitation (λex = 220 nm) upon ZnS : Mn, which results into two emission peaks corresponding to surface states and Mn+2 emission, respectively. The ion fluence for irradiation experiment so chosen were 1 × 1011, 5 × 1011, 5 × 1012 and 1013 Cl/cm2.

Optical absorption study of 100-MeV chlorine ion-irradiated hydroxyl-free ZnO semiconductor quantum dots

We report here synthesis and optical absorption study of energetic ion-irradiated hydroxyl-free ZnO semiconductor quantum dots. Quantum dot samples were synthesized by a quenching technique and 100-MeV chlorine ion was selected for the irradiation experiment with doses 1×1011, 5×1011, 5×1012, and 1013 ions/cm2. With an increase in fluence, the optical absorption spectra of irradiated quantum dots reveal redshift of the energy-gap parameter with respect to unirradiated (virgin) quantum dots. The narrowing of the energy gap of nanoparticles indicate particle growth under ion irradiation which is confirmed from transmission electron microscope images. The possible reason for such variations was encountered using an effective-mass approximation model which fits well for small effective mass and high-dielectric constant (e0).

Structural and optoelectronic properties of Eu2+-doped nanoscale barium titanates of pseudo-cubic form

The effect of europium (Eu)-doping on the optoelectronic carrier transition properties of pseudo-cubic barium titanate (BT) nanostructured system is being reported. Referring to x-ray diffractograms, apart from the diffraction peaks related to perovskite BT structure, non-existence of any additional peaks due to byproducts has revealed that Eu has undergone substitutional doping into BT host lattice. We speculate that adequate growth of a cubic overlayer over the tetragonal core has led to suppressed tetragonality (c/a ratio) features. We notice substantial decrease in the carrier transition exponent (n value), from its normal value, when doping level was varied within 0%-14%. While the overall photoluminescence response is improved with Eu-doping, the BT system was expected to experience concentration quenching. The emission peak at ∼455 nm was attributed to Eu2+ mediated 4f65d1→4f7 carrier transitions. Investigating optoelectronic properties of non-ferroelectric perovskite nanostructured system has a di...

Directed growth characteristics and optoelectronic properties of Eu-doped ZnO nanorods and urchins

Inexpensive fabrication of europium (Eu3+)-doped zinc oxide (ZnO) nanoparticles, nanorods, and urchin systems obtained by rapid thermal annealing is being reported. The polyvinyl alcohol films containing a reactant mixture [Zn(CH3COO)2, Eu(CH3COO)3, and NaOH] were casted on Al-foils followed by thermal annealing at 80, 300, and 650 °C. Gradual change-over from spherically symmetric nanoparticles to nanorods along with urchin like structures are obtained under different annealing environment. The nanorods and urchins are expected to have grown as a result of spontaneous decomposition of Zn(OH)2 followed by unidirectional growth. The nanorods are dislodged from the regular urchin structures at a high annealing temperature of 650 °C because of the loss of the crystalline substrate support. The as-received ZnO products possessed hexagonal wurtzite structure for all the annealing cases-as evident from the x-ray diffraction patterns. The photoluminescence study on the samples has revealed dominant defect relate...

ZnO nanorod-based UV photodetection and the role of persistent photoconductivity

We report on the substantial persistent photoconductivity (PPC) response exhibited by the zinc oxide (ZnO) nanorod-based ultraviolet (UV) photodetection system. An increase in photocurrent and, hence, rise in PPC was observed for larger UV exposure times at regular intervals. Triggered by quantum efficiency, the increment in sustained conduction band electrons is proposed as the main reason behind the increased photocurrent response. In contrast, the trap centers located below the conduction band are expected to slow down the recombination rate, which accounts for the rise in PPC. The lowering of PPC upon annealing suggests the surface dependent nature of the PPC. The growth and decay mechanism of PPC has a direct relevance while assessing figure of merit of prototype nanostructure-based optical sensor and UV photodetectors.

Rapid hydrothermal route to synthesize cubic-phase gadolinium oxide nanorods

An inexpensive fabrication route and growth mechanism is being reported for obtaining quality gadolinium oxide (Gd2O3) nanoscale rods. The elongated nanoscale systems, as produced via a hydrothermal process, were characterized by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), optical absorption spectroscopy, photoluminescence (PL) spectroscopy, Raman spectroscopy and magnetic hysteresis measurements. XRD patterns of the nanorods, as-prepared from independent precursors of different pH, depict a cubic crystal phase and an average crystallite size of 5–6.5 nm. As revealed from HRTEM micrographs, diameter of the nanorods prepared at pH = 13.3 (∼ 7 nm) was much smaller than the rods prepared at pH = 10.8 (∼ 19 nm). However, the aspect ratio was more than double in the former case than the latter case. PL response was found to be dominated by defect mediated emissions, whereas Raman spectrum of a given specimen (pH = 10.8) has revealed characteristic Fg + Ag modes of cubic phase of Gd2O3 nanorods, apart from other independent modes. Furthermore, M ∼ H plot of the nanorod system (pH = 10.8) exhibited slight departure from the ideal superparamagnetic behaviour, with low remanence and coercive field values. The exploitation of one-dimensional Gd2O3 nanorods have immense potential in the production of advanced contrast agents, smart drives and also in making novel ferrofluids of technological relevance.

Interfacial charge transfer mechanism in nanostructured TiO2–ZnS coupled network for single electron device applications

We report on the production of nanoporous TiO2 network sensitized by ZnS nanospheres as an idealized scheme to facilitate interfacial charge transfer effects. The nanoporous TiO2 system was fabricated on the 0.1μm thick Al substrate from titanium isopropoxide [Ti(i−OC3H7)] and 1-butanol (C4H9OH) as requisite precursor. The Zn++ ions are internally adsorbed to provide heterogeneous coupled TiO2–ZnS nanosystem. The I-V response shows transistor characteristics which suggests sharp rise in current with forward biasing voltage before attaining saturation. It is expected that with the increase in signal frequency more number of trap carriers being able to follow signal assist higher carrier transfer rate across the interface in the coupled system and hence saturation current (IS) increases. However, in all the cases saturation occurs around finite biasing voltage, i.e., 3.6V. This ensures that the surface states (which normally lie within the forbidden gap and below the conduction bands for electrons) mainly p...

Photochromism and magneto-optic response of ZnO:Mn semiconductor quantum dots fabricated by microemulsion route

ZnO:Mn semiconductor quantum dots were prepared by solution casting led microemulsion route. Quantum dots of average size ∼2 nm were noticed in transmission electron micrographs. The present work highlights colour change phenomena (photochromic effect) of quantum dots while subjected to photon illumination. The magneto-optic measurements e.g. magnetic field (H) vs angle of rotation (θ) show step like behavior and is ascribed to the quantum confinement effect of diluted magnetic ZnO:Mn nanostructures. Further, underlying mechanism responsible for exhibiting photochromism and magneto-optic effects are also discussed.

Influence of ion bombardment on the photoluminescence response of embedded CdS nanoparticles

Semiconductor nanoparticles (CdS) were fabricated by an inexpensive chemical route using polyvinyl alcohol (PVA) as the dielectric host matrix. Nano-CdS in PVA were subjected to ion irradiation (using oxygen, chlorine and gold) in the medium energy range (80–100 MeV) and under fluence variation of 1011–1013 ions/cm2. The nature of light emission was found to be drastically different in each of the three cases. Photoluminescence spectra of oxygen irradiated samples exhibit band edge emission (2.8 eV) as well as trap related emission (1.76 eV) whereas band edge emission is found to be bleached out for chlorine ion irradiated nano-CdS. The intense broad PL peaks, noticeable in the case of gold ion irradiated samples suggest superposition of the two peaks — namely, band edge emission and trap related emission. Furthermore, in the case of gold ion irradiated nano-CdS, energy shift in the PL spectra reveals variation in size distribution caused by the extra pressure effect of heavy gold ion beams. The mechanism of such a difference as a result of ion irradiation-type and ion-fluence is discussed in detail.