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This review is focused upon unique optical properties of rare earth (RE) doped ZnO nanostructures derived by a cost-effective sol-gel method. Doping of RE elements improves the properties of ZnO due to their special electronic structure, small crystal field splitting and partially filled 4f energy level, surrounded by full 5s and 5p orbitals. RE-element-induced defects regulate energy transfer dynamics, which can add superior features in ZnO. The biocompatibility of ZnO along with its special calibre of sustained tuneable features by RE doping makes it an excellent candidate for a highly demanding field, i.e. bio-imaging. Here, five RE elements, Tb, Ce, Gd, Y and Eu, are chosen to comprehensively review alterations in the optical parameters of ZnO by their doping. Band gap tuning and luminous properties of RE doped ZnO nanostructures, as well as their features and applications in diverse sectors, are majorly highlighted. [ABSTRACT FROM AUTHOR]

Undoped and Ca doped ZnO Wurtzite nanostructures were prepared via sol-gel co-precipitation. Heavy Ca doping causes structural deterioration due to larger ionic radius of Ca2+. Strain-induced deformation has been analysed by W-H method. Rietveld analysis has revealed changes in bond length due to doping. Optical transparency and band gap have been determined by UV-Vis and Tauc's plot, respectively. Band gap is seen to increase with doping. PL study with three different excitation wavelengths, before and after heat treatment of the samples, recommends that 300 nm is the most suitable for obtaining emission in the blue region. SEM has revealed that the low amount of doping causes clustering of particles, but heavy doping reduces it. [ABSTRACT FROM AUTHOR]

Highly transparent thin films of aluminium doped ZnO along with specific silver co-doping are deposited by sol-gel spin coating, which show preferred single orientation along (100) plane. Presence of Al dopant reduces, whereas co-doping with Ag improves crystallinity. Films show sharp absorption in near UV region and band gap observes minor shifts. UV and blue emission peaks are seen in the photoluminescence spectra. UV and blue emission peaks, respectively, demonstrate a fairly large i.e. 43% and 77% rise in the normalised intensity. Presence of Ag causes rise in UV peak because of enhanced excitonic recombination probability. Rise of blue emission intensity is ascribed to creation of Ag-Al interstitials. Surface roughness increases to 210.0 nm with Al increment into ZnO and drops drastically to 81.1 nm by Ag co-doping. Boundaries of nanostructures club together with rise in Al amount and thereafter Ag co-doping is seen to reduce the particle size. [ABSTRACT FROM AUTHOR]

MgZnO/ZnO bilayers (Mg concentration of ∼30%) have been grown and subsequently annealed at different temperatures in the range of 600-900 °C with the specific interest of studying the effect of inter-diffusion of Mg on the photoluminescence (PL) properties of the bilayers. The influence of Mg diffusion and material homogenization is evaluated through absorption, PL, and secondary ion mass spectrometry (SIMS) measurements. No appreciable change in the spectral positions is seen either in PL or absorption up to an annealing temperature of 700 °C, which is also supported by SIMS. However at higher annealing temperatures, diffusion of Mg into the ZnO layer is clearly evident in SIMS profile, which results in the red-shift (blue-shift) of spectral positions of MgZnO (ZnO) layer, respectively. Finally, for the sample annealed at 900 °C, the two layers are completely merged providing a single peak at ∼3.60 eV in PL/absorption corresponding to a completely homogenized MgZnO layer. Spectroscopic results are corroborated by the numerical simulations based on a simple theoretical model, which correlates the observed PL spectra of the heterostructures with the experimental Mg diffusion profiles across the heterointerface, as measured by SIMS. [ABSTRACT FROM AUTHOR]

Pulsed laser deposition technique was used for growing thin films of ZnO on Si (100) substrate held at different temperatures (Ts). All the as-deposited films have shown a preferential c-axis orientation associated with varying grain size as a function of Ts ranging from 100 to 600 °C. Current-voltage (I-V) characteristics of these films show Ohmic behavior over the entire Ts range studied. These films were subjected to annealing at 800 °C in air ambient for 4 h. The grain size was observed to increase after the annealing process for all the films deposited at different Ts. Interestingly, these annealed films show nonlinear variation of current with applied voltage, very similar to the one observed in doped ZnO varistors. The nonlinear parameters such as the asymmetric behavior of change in current on the polarity reversal of voltage, the plateau region, and the break down voltage are observed to depend on Ts. This nonlinear behavior can perhaps be explained on the basis of electronic conduction model proposed for the bulk, doped ZnO varistors. The role of insulating intergranular layers between the disoriented microcrystallites is expected to be similar to that played by insulating intergranular layers in the doped ZnO varistors. [ABSTRACT FROM AUTHOR]

Electron interference effects and transition from weakly localized to a strongly localized transport regime is observed in Cu doped ZnO thin films grown by pulsed laser deposition on c-sapphire substrates. The doping concentration of Cu was varied from 0% to 10%. Up to the doping concentration of ~ 0.5%, the films showed weakly localized behavior where quantum corrections to conductivity due to electron interference was active. At these doping concentrations, a transition from 3D to 2D weak localization was also observed as the measurement temperature was decreased. But at Cu concentrations of 5% and beyond, the films were found to show behavior of strong localization where the transport at low temperature was dominated by hopping mechanism. [ABSTRACT FROM AUTHOR]

n -ZnO/p-Si heterojunctions were grown by atomic layer deposition (ALD) on (100) p-Si substrates at different growth temperatures in the range of ~100–250 °C. The current-voltage characterization of all the heterojunctions showed typical rectifying behavior, a true signature of a p-n junction diode. The diode grown at 100 °C were having significantly lower reverse saturation current (~21 nA) and high rectification factor (~120) compared to those grown at relatively higher temperatures such as 200 or 250 °C. From capacitance-voltage measurements, it was found that the depletion width in the ZnO side of n -ZnO/p-Si diode was maximum (~60 nm) for the diode grown at 100 °C and decreased gradually to ~3 nm for the diodes grown at high temperatures of 250 °C. The electron concentration in ZnO films was found to increase significantly on increasing the growth temperature from ~100 to 250 °C. The junction capacitance also showed an increasing trend with increase in the growth temperature. The variation of diode parameters with growth temperature has been discussed in terms of carrier concentration in ZnO films and associated growth mechanisms of the ALD. Such low temperature grown n -ZnO/p-Si diodes with lower reverse saturation current and large depletion width may be suitable for photo detection applications. [ABSTRACT FROM AUTHOR]

We report the effects of electron interference on temperature dependent transport properties of two dimensional electron gas (2DEG) confined at the interface in polycrystalline MgZnO/ZnO heterostructures grown by pulsed laser deposition on c-alumina substrates. On increasing Mg concentration in the MgZnO layer, the sheet electron concentration was found to increase and the sheet resistance was found to decrease. In addition, the electron concentration and mobility were almost temperature independent in the range from 4.2 to 300 K, indicating the formation of 2DEG at the interface. The temperature dependent resistivity measurements showed a negative temperature coefficient of resistivity at low temperatures together with negative magnetoresistance. These were found to be caused by electron interference effects, and the experimental data could be explained using the models of quantum corrections to conductivity. [ABSTRACT FROM AUTHOR]

Synchrotron based photoemission spectroscopy study shows a type-II band alignment at pulsed laser deposited epitaxial ZnO layer and GaP substrate. Additionally, the ZnO/GaP hetero-junction is comprised of an interfacial GaOx layer. Valence band onsets corresponding to ZnO, GaOx, and GaP materials enabled us to determine the valence band offset (DEV) values of 2.81 eV, 0.35 eV, and 2.46 eV at ZnO/GaP, ZnO/GaOx, and GaP/GaOx hetero-junctions, respectively. ΔEV values determined directly from the difference of valence band onsets are in corroboration with that obtained by Kraut's method. The band alignment and interface study of ZnO/GaP hetero-junction can be used to design optoelectronic devices. [ABSTRACT FROM AUTHOR]

Spread of antibiotic resistant bacteria through water pose severe health risks to society. In this context, a systematic study is reported for photocatalytic removal of multidrug resistant (MDR) Escherichia coli and related genes from water by using Alumina/ZnO heterostructures under visible light. 500 mg/L of the photocatalyst could disinfect ≈ 106 CFU/mL of target bacteria in water within 240 min of visible light irradiation. Lipid peroxidation, DNA and protein leakage studies have suggested the compromisation of bacterial cell membrane which was further corroborated from electron microscopy images. MDR Escherichia coli has registered a loss of resistance towards fifteen antibiotics with a down-regulation in the antibiotic resistance genes after exposure to photocatalytic process. Proposed process was also validated for disinfection of MDR Staphylococcus haemolyticus and antibiotic susceptible Escherichia coli. Successful disinfection of fecal coliform was achieved in water samples collected from three major rivers and a waste water treatment plant. The in vivo toxicity study of the treated water on mice model has not revealed any remarkable impact on gut health. With high effectiveness in disinfecting bacteria coupled with biocompatibility of the treated water towards animal model may proffer the present technique as a propitious off-grid water disinfection technique for real-world applications. A systematic study is reported for Photocatalytic removal of Extended-spectrum beta-lactamase producing Escherichia coli and related genes from water using Alumina/ZnO heterostructures under visible light. [Display omitted] • Photocatalytic disinfection of ESBL-producing E. coli using Alumina/ZnO under visible light is reported. • Lipid peroxidation, DNA and protein leakage studies have suggested the compromisation of bacterial cell membrane. • After photocatalysis, E. coli registered a loss of antibiotic resistance and down-regulation in the resistance genes. • Process was validated for disinfection of antibiotic resistant S. haemolyticus and antibiotic susceptible E. coli. • Successful disinfection of fecal coliform was achieved in real water. [ABSTRACT FROM AUTHOR]

Ensembles of alumina capped ZnO quantum dots (ZQDs) were grown using pulsed laser deposition. The ZQDs of mean radii comparable to and smaller than the pertinent excitonic Bohr radius (∼2.34 nm), called ultrasmall quantum dots, show size dependent optical absorption edges, which follow the strong confinement model. In this model the confinement energy and Coulombic interaction energy of the localized electron-hole pairs are significantly higher than their correlation energy and the optical transitions are perceived to be nonexcitonic in nature. In photoluminescence (PL) spectra of such ZQDs of mean radius of ∼2.3 nm at temperatures of 6 K and above, the primary recombinations are found to be due to the surface bound and Al donor bound electron-hole pairs. The band-edge recombination peak of the PL spectra appeared at about 70 K and above, which was found to be about 166 meV Stoke and/or thermally redshifted with respect to the experimentally observed absorption edge. Almost all of the PL spectra at different temperatures conspicuously showed the LO and 2LO phonon replicas of the primary transitions, suggesting strong coupling between the recombining charge carriers and the LO phonon, which is rather unusual for nonexcitonic recombinations. The temperature dependent PL peak positions followed the well known Varshni's relation with fitting parameters close to that of the bulk ZnO. The peak intensity of the observed PL transitions followed the normal law of thermal quenching which could be fitted with the Arrhenius equation having activation energy of about 10 meV. Temperature dependence of full width at half maximum of the PL peaks when fitted with the Hellmann and O'Neill models did not result in a close match. However, from this fit one could estimate a value of the carrier-LO phonon coupling coefficient of ∼980 meV, which is higher than that reported earlier for the ZQDs. These observations are hitherto unfamiliar and expected to provide further insight into the basic optical processes in alumina capped ultrasmall ZQDs. [ABSTRACT FROM AUTHOR]

Understanding the behavior of engineered nanoparticles in the environment and within organisms is perhaps the biggest obstacle to the safe development of nanotechnologies. Reliable tracing is a particular issue for nanoparticles such as ZnO, because Zn is an essential element and a common pollutant thus present at elevated background concentrations. We synthesized isotopically enriched (89.6%) with a rare isotope of Zn (67Zn) ZnO nanoparticles and measured the uptake of 67Zn by L. stagnalis exposed to diatoms amended with the particles. Stable isotope technique is sufficiently sensitive to determine the uptake of Zn at an exposure equivalent to lower concentration range (<15 μg g−1). Without a tracer, detection of newly accumulated Zn is significant at Zn exposure concentration only above 5000 μg g−1 which represents some of the most contaminated Zn conditions. Only by using a tracer we can study Zn uptake at a range of environmentally realistic exposure conditions. [Copyright &y& Elsevier]

Abstract: The objective of the study is to describe a new approach of combining quantum dots technology with anti-cancer drug therapy. In this regard, we communicate the preliminary research on the synthesis of blue-light emitting ZnO quantum dots (QDs) combined with biodegradable chitosan (N-acetylglucosamine) for tumor-targeted drug delivery. The results presented here indicate that the proposed new generation of QDs loaded with anti-cancer agents and encapsulated with biocompatible polymer represent a potential platform to deliver tumor-targeted drugs and document the delivery process, if desired. Non-toxic water-dispersed ZnO QDs with long-term fluorescence stability were synthesized by a chemical hydrolysis method, encapsulated with chitosan and loaded with anti-cancer drug. Chitosan enhanced the stability of the QDs because of the hydrophilicity and cationic charge of chitosan. The study points toward the application of water-dispersed ZnO QDs with long-term fluorescence stability for design of new drug release carrier. [Copyright &y& Elsevier]

Abstract: Temperature dependence of the photoluminescence (PL) transitions in the range of 10–300K was studied for ZnO thin films grown on sapphire by pulsed laser deposition. The low temperature PL spectra were dominated by recombination of donor bound excitons (B X) and their phonon replicas. With increasing temperature, free exciton (F X) PL and the associated LO phonon replicas increased in intensity at the expense of their bound counterparts. The B X peak with line width of ∼6meV at 10K exhibited thermal activation energy of ∼17meV, consistent with the exciton-defect binding energy. The separation between the F X and B X peak positions was found to reduce with increasing temperature, which was attributed to the transformation of B X into the shallower donor bound exciton complexes at consecutive lower energy states with increasing temperature, which are possible in ZnO. The energy separation between F X peak and its corresponding 1-LO phonon replica showed stronger dependence on temperature than that of 2-LO phonon replica. However, their bound counterparts did not exhibit this behavior. The observed temperature dependence of the energy separation between the free exciton and it is LO phonon replicas are explained by considering the kinetic energy of free exciton. The observed PL transitions and their temperature dependence are consistent with observations made with bulk ZnO crystals implying high crystalline and optical quality of the grown films. [Copyright &y& Elsevier]

Removal of organic dye-based impurities from water has been achieved traditionally by the use of photocatalysts. ZnO is a commonly used photocatalyst that works when irradiated by UV irradiation. Since the UV fraction in a solar spectrum is very small, there is a scope of utilizing the near-infrared and visible spectrum for the process of dye degradation. We synthesized a novel tri-dopant upconversion nanoparticle (NaYF 4 :Yb,Tm,Nd) system using a simplified microwave-assisted technique and combined it with ZnO particles for methylene blue dye degradation studies. We examined the role of neodymium (Nd3+) dopant on the efficiency of dye degradation when illuminated under an inexpensive tungsten halogen lamp source. X-ray diffraction studies revealed that the presence of Nd3+ in the crystal lattice leads to a reduction in the particle size due to shrinkage in the crystallite size. The presence of Nd3+ in upconversion nanoparticles revealed several characteristic absorption bands from 520 nm to 870 nm along with the characteristic 980 nm corresponding to (ytterbium) Yb3+. The power-dependent study at 980 nm confirmed the multi-photon process of upconversion. The degradation efficiency revealed up to 92% degradation for 5 mg L−1 of dye. The degradation efficiency of upconversion nanoparticles and ZnO system in the presence of Nd3+ was 72% and in the absence of Nd3+ was 58%, for a dye concentration of 10 mg L−1. The dye degradation rate constant was higher for NaYF 4 :Yb,Tm,Nd and ZnO system (0.0041 min−1) compared to NaYF 4 :Yb,Tm, and ZnO set (0.0029 min−1). Thus, the presence of Nd3+ widens the absorption spectra of UCNP in both the visible and near-infrared (NIR) range. [Display omitted] • Successful synthesis of Nd, Yb, Tm doped NaYF 4 upconversion nanoparticles • Successful dye degradation utilising the visible and NIR spectrum and triggering the photocatalysis process of ZnO • Presence of Nd dopant increases the dye degradation • Efficient dye removal of upto (72%) under illumination in simple halogen lamp [ABSTRACT FROM AUTHOR]

Abstract: Pulsed Laser Deposition of high crystalline quality thin ZnO films on sapphire substrates necessitates a growth temperature of about 750 °C or higher while sharp interfaces in a multilayer structure suitable for optoelectronic devices are obtained at comparatively low growth temperatures. To meet these contradictory requirements a growth scheme has been evolved in which a ZnO buffer layer deposited at about 750 °C was found to facilitate high crystalline quality growth of a ZnO over layer even at 400 °C. The buffer layer grown at 750 °C was found to have its in plane lattice 30° rotated compared to that of the sapphire substrate and so was the case with the ZnO over layer grown at 400 °C on the buffer. The full width at half maximum of the high resolution X-ray diffraction rocking curves of (00.2) and (10.4) peaks of ZnO films grown at low temperature on the buffer were found to be ∼0.18° and ∼0.28°, respectively, which are comparable to 0.11° and 0.23° as observed in the case of films deposited directly at 750 °C and much better than the values of 0.46° and 0.48° seen in case of the films grown at 400 °C without any buffer layer. The buffer assisted low temperature ZnO growth also decreased the r.m.s. value of the surface roughness of the film to about 8 Å from about 40 Å of the one grown at high temperature without any buffer. [Copyright &y& Elsevier]

The composition of nitrogen laser (wavelength 337 nm, pulse width 3 ns) induced ablation plumes from ZnSe and ZnO targets was studied at different laser fluences using a time-of-flight mass spectrometer. In the case of ZnSe, abundant ion signals corresponding to Se and ZnSe clusters, some of which were Se-rich, were detected with fluence-dependent distributions. At a laser fluence of 1250 J m[sup -2], clusters with elevated intensity were observed at sizes of 6, 13, 19, 23 and 33 ZnSe molecules (‘magic numbers’), which match quite well with the earlier observation by others of magic numbers of chemically similar ZnS produced by a conventional vaporization and quenching scheme. In the case of ZnO, we detected the presence of atomic Zn and molecular species of ZnO, as well as a series of (ZnO)[sub n]-type clusters with fluence-dependent distributions. Unlike the case of ZnSe, no magic numbers were observed for ZnO. [ABSTRACT FROM AUTHOR]

Efficient room temperature (RT) photoluminescence (PL) is achieved on ZnO multiple quantum wells (MQWs) grown on sapphire by pulsed laser deposition using a buffer assisted growth scheme. Absorption spectra of these MQWs at RT showed the excitonic features entwined with the band edges, which pointed to the excitonic nature of the PL transitions. At RT the band edge of these MQWs shifted from ∼3.36 to 3.78 eV on decreasing the well layer thickness from ∼4 to 1 nm. In the range from 10 K to RT, the PL spectral linewidth increased and the peak shifted monotonically towards red with increasing temperature. [ABSTRACT FROM AUTHOR]

Studies on band-offset and band-alignment of heterojunction of highly c-axis oriented ZnO thin films grown on n-Ge (1 1 1) by pulsed laser deposition show a type-II band alignment with the valence band offset (ΔEV) of 3.1 ± 0.2 eV. The valence band spectra of this heterojunction show band onsets corresponding to Ge, interfacial GeOx, and ZnO layers. This observation also enabled us to determine ΔEV of ZnO/GeOx heterojunction to be 1.4 ± 0.2 eV. These studies provide further insight into the band alignment of ZnO/GeOx/Ge system wherein the observed large value of ΔEV of ZnO/Ge can be used for heterojunction based optoelectronic devices. [ABSTRACT FROM AUTHOR]

ZnO band-gap was engineered for and by growing its quantum wells and nanoparticle matrices using pulsed laser deposition. Alloy thin films of MgxZn1-xO grown with high crystalline quality by variable oxygen ambient pressure methodology developed by us could continuously increase the band gap up to about 3.8 eV. Increasing Cd concentration in the alloy thin films of CdyZn1-yO grown with good crystalline quality monotonically decreased the band gap up to about 2.9 eV. Multilayer matrices of alumina capped ZnO nanoparticles with mean radii in the range of 1.8–3.6 nm were grown to obtain size-dependent band gap up to about 4.5 eV. These nanoparticles are understood to grow without the formation of a wetting layer on the substrate. [Copyright &y& Elsevier]

We have observed weak localization induced by disorder in thin films of Gd doped ZnO (Gd:ZnO) deposited by pulsed laser deposition (PLD). Disorder parameter (K f l) of all the Gd:ZnO thin films was found to be greater than 1, which indicates that thin films of Gd:ZnO in this study are in metallic side of metal-insulator transition. However, temperature dependent resistivity studies of all the Gd:ZnO thin films exhibits negative temperature coefficient of resistance (TCR) showing semiconductor like nature at low temperatures. This anomaly in temperature dependent resistivity behaviour has been attributed to the dominant presence of weak localization. Magnetoresistance measurements confirms the dominant influence of weak localization in the conducting mechanism of Gd:ZnO films. Phase coherent length was found to be varied with the concentration of Gd and reports the highest value at ~360 nm for Gd:ZnO thin film with 1 at.% of Gd concentration.. • Disorder induced weak localization was observed in thin films of Gd doped ZnO deposited by pulsed laser deposition. • The observation of negative magnetoresistance confirms the dominant role of weak localization in the Gd doped ZnO films. • Phase coherent length was found to be varied with the concentration of Gd and reports the highest value at ~ 360 nm. • Electron electron interaction acts as a dominant phase breaking mechanism of weak localization in Gd:ZnO thin films. [ABSTRACT FROM AUTHOR]