Deep-level Emission Intensity Research Articles

In English

Zinc oxide nanostructures (NS) were grown on thin discontinuous films of noble metals of silver and gold in order to study their structure, optical properties as well as photocatalytic and antiviral activity. The paper presents the results of X-ray diffraction study, scanning electron microscopy study, photoluminescence and Raman measurements. X-ray diffraction experiments demonstrate similar patterns for all grown ZnO nanostructures. The SEM images of ZnO NS grown on Ag/Si and Au/Si wafers demonstrate more dense surface microstructure compared to ZnO NS grown on bare Si wafers. The most intensive ultraviolet and deep level emissions are observed for ZnO NS grown on Ag/Si wafers. Increase in thicknesses of Ag island film from 5 nm to 10 nm gives significant increase in intensity of ultraviolet and deep level emissions. Photocatalysis of grown ZnO nanostructures was studied by methyl orange dye degradation. Superior photocatalytic results are demonstrated by ZnO nanostructures grown on Ag/Si wafers, for which constants of dye degradation were twice higher than for ZnO nanostructures grown on Si and Au/Si substrates. The photocatalytic results correlates with photoluminescence spectra: more intensive photoluminescence in ultraviolet and visible ranges of optical spectrum leads to better photocatalytic performance. The cytotoxic effect of ZnO nanostructures was studied without photoactivation by the help of cell cultures MDCK and Hep-2 while the virucidal effect of ZnO nanostructures was studied by the help of Influenza A virus (H1N1) (strain FM / 1/47) and human adenovirus serotype 2 (HAdV2). ZnO nanostructures in a 1:10 dilution were not toxic to Hep-2 and MDCK cells. Most of the tested ZnO nanostructures exhibited no virucidal activity against human adenovirus serotype 2 (HAdV2) and influenza A virus (H1N1) (strain FM / 1/47) in the absence of photoexcitation.

N-ion-implanted ZnO microtubes for highly-efficient UV detection

Here we report a N-ion-implanted ZnO microtube fabricated by optical vapor supersaturated precipitation (OVSP) and ion implantation techniques. The N ions with concentration of ∼2.4 × 1023 cm−3 were mainly located below 100 nm of the surface in the acceptor-rich ZnO (A-ZnO) microtubes. Due to the presence of N in ZnO lattice as NO defects, an ∼15-fold enhancement of near-band edge (NBE)/deep-level emission (DLE) intensity ratio was achieved. The yellow luminescence and the anomalous negative thermal quenching (NTQ) effect further confirmed the NO defect level of 1.3 eV above the valence band maximum in A-ZnO:N microtubes. The implantation of N ions can expand the depletion layer between A-ZnO:N detector and atmospheric species, by which fast photoresponse down to 0.1 s was realized by a metal-semiconductor-metal (MSM) UV detector. Compared with the A-ZnO UV detector, the photoresponsivity of the MSM UV detector was enhanced by ∼10 folds. This work opens up new opportunities to design effective UV detectors for a variety of practical applications.

In Situ Growth of PbS Nanoparticles without Organic Linker on ZnO Nanostructures via Successive Ionic Layer Adsorption and Reaction (SILAR)

The process of effective solar energy harvesting and conversion requires efficient photon absorption, followed by charge generation and separation, then electron transfer. Nanostructured materials have been considered as potential building blocks for the development of future generations of solar cells. Much attention has been given to wide-bandgap semiconductor nanowires, combined and sensitized with low-bandgap semiconductors effectively attached to the nanowires for low-cost and highly efficient solar cells. Here, the in situ growth of lead sulfide (PbS) nanoparticles on the surface of zinc oxide (ZnO) nanowires grown by the Successive Ionic Layer Adsorption and Reaction (SILAR) technique is presented for different numbers of cycles. The morphology and structure of PbS nanoparticles are confirmed by Scanning Electron Microscopy (SEM), revealing the decoration of the nanowires with the PbS nanoparticles, Transmission Electron Microscopy (TEM) and HR-TEM, showing the tight attachment of PbS nanoparticles on the surface of the ZnO nanowires. The Selected Area Electron Diffraction (SAED) confirms the crystallization of the PbS. Photoluminescence spectra show a broad and more intense deep-level emission band.

Structural and optical properties of ZnO nanorods: The effect of concentration and pH of the growth solution

We prepared ZnO nanorods using a hydrothermal method in the c-axis orientation and perpendicular on an ITO substrate. The structural and optical properties were examined for samples prepared under various concentrations and pHs. The XRD results showed that the Bragg peak (002) is more intense than other peaks. This result reveals the tendency of the uniaxial growth of nanorods. The FE-SEM images showed that increasing the concentration increases the diameter and length of the nanorods, while the orientation remains almost constant. On the other hand, with increasing the pH, the diameter and length of nanorods decrease, so that at pH = 11, there is even no trace of nanorods formation. The photoluminescence spectra show that the intensity of near band edge emission (NBE) peaks was higher than the intensity of deep-level emission (DLE) peaks, which indicates the high crystalline quality of the samples. These results show the distinct effect of concentration and pH and provide a possibility to find an optimal condition to yield a sample with selected structural and optical properties.

Deeper insights into the photoluminescence properties and (photo)chemical reactivity of cadmium red (CdS1\u2212xSex) paints in renowned twentieth century paintings by state-of-the-art investigations at multiple length scales

Cadmium red is the name used for denoting a class of twentieth century artists’ pigments described by the general formula CdS1−xSex. For their vibrant hues and excellent covering power, a number of renowned modern and contemporary painters, including Jackson Pollock, often used cadmium reds. As direct band gap semiconductors, CdS1−xSex compounds undergo direct radiative recombination (with emissions from the green to orange region) and radiative deactivation from intragap trapping states due to crystal defects, which give rise to two peculiar red-NIR emissions, known as deep level emissions (DLEs). The positions of the DLEs mainly depend on the Se content of CdS1−xSex; thus, photoluminescence and diffuse reflectance vis–NIR spectroscopy have been profitably used for the non-invasive identification of different cadmium red varieties in artworks over the last decade. Systematic knowledge is however currently lacking on what are the parameters related to intrinsic crystal defects of CdS1−xSex and environmental factors influencing the spectral properties of DLEs as well as on the overall (photo)chemical reactivity of cadmium reds in paint matrixes. Here, we present the application of a novel multi-length scale and multi-method approach to deepen insights into the photoluminescence properties and (photo)chemical reactivity of cadmium reds in oil paintings by combining both well established and new non-invasive/non-destructive analytical techniques, including macro-scale vis–NIR and vibrational spectroscopies and micro-/nano-scale advanced electron microscopy mapping and X-ray methods employing synchrotron radiation and conventional sources. Macro-scale vis–NIR spectroscopy data obtained from the in situ non-invasive analysis of nine masterpieces by Gerardo Dottori, Jackson Pollock and Nicolas de Staël allowed classifying the CdS1−xSex-paints in three groups, according to the relative intensity of the two DLE bands. These outcomes, combined with results from micro-/nano-scale electron microscopy mapping and X-ray analysis of a set of CdS1−xSex powders and artificially aged paint mock-ups, indicated that the relative intensity of DLEs is not affected by the morphology, microstructure and local atomic environment of the pigment particles but it is influenced by the presence of moisture. Furthermore, the extensive study of artificially aged oil paint mock-ups permitted us to provide first evidence of the tendency of cadmium reds toward photo-degradation and to establish that the conversion of CdS1−xSex to CdSO4 and/or oxalates is triggered by the oil binding medium and moisture level and depends on the Se content. Based on these findings, we could interpret the localized presence of CdSO4 and cadmium oxalate as alteration products of the original cadmium red paints in two paintings by Pollock.Graphical abstract

Enhancement of organic solar cell efficiency by altering the zinc oxide photoanode nanostructure morphology

The current paper examines the effects of zinc oxide nanostructure configurations, as photo-anode formations of organic solar cells, on the performance of power conversion. To this end, some experiments were conducted during which a near band edge emission red shift of ~ 0.11 eV from nanoparticles to vertically oriented nano-rods was observed. This bandgap narrowing promotes transferring of photo-excited electrons towards the conduction band of photo-anode. A ~ 48% decrease in the deep level emission intensity revealed a smaller non-radiative waves emission due to lower level of crystal disorder. Using vertically oriented zinc oxide nanorods as photo-anodes, the photovoltaic efficiency of the organic solar cell improved considerably. The nano-rod-structured photo-anodes showed a 0.22 V rise in the open-circuit voltage, from 0.76 to 0.98 V, and a 2.08 times increment in the overall conversion performance, compared to the zinc oxide nanoparticle-structured photo-anodes. This superior performance is attributed to a greater chance of charge recombination and light-trapping in the cells, more efficient light absorption, and high level of crystallinity that grants easier electron mobility for vertically oriented zinc oxide nanorods. Moreover, a lower charge-transfer resistance (0.85 Ω) was achieved due to better electro-catalytic action for oxygen reduction for vertical nanorods compared to the other two zinc oxide configurations (1.62 Ω and 4.06 Ω). This boosted the cell performance by increasing the short-circuit current density (JSC). The fabricated solar cell may contribute to sustainable and environmentally friendly electricity generation process through reducing the consumption of non-renewable energy sources.

Photo generated charge transport studies of defects-induced shuttlecock-shaped ZnO/Ag hybrid nanostructures

We report the stimulating effects of interfacial charge transfer process between spherical Ag nanoparticles and shuttlecock-shaped ZnO nanostructures observed by UV−visible spectroscopy and x-ray absorption spectroscopy. In specific, ZnO nanorods and shuttlecock-shaped ZnO/Ag nanostructures were developed using a simple chemical colloidal method and characterized for structural variations using XRD. The observed red shift in plasmonic peak and the increase in Urbach energy signify interfacial interactions and increased randomness in the hybrid ZnO/Ag nanostructures. Simultaneously, the enhanced intensity of deep-level emission in the ZnO/Ag hybrid suggests the increased recombination rate of electron-hole pairs. The red and blue emissions evolving with temperature subsequently suggests the presence of oxygen vacancies or zinc interstitials in the system. The decrease in intensities and emerging features in O K-edge and Zn L-edge indicates the charge transfer from Ag to ZnO at the interface of ZnO/Ag hybrids. Moreover, the differences in absorption edges with alternating light on/off conditions were analyzed for the exploitation of this ZnO-based system in various applications.

Effect of Electrolyte Concentration on Epitaxial Growth of ZnO on Cu Substrates through Electrochemical Deposition

The roles of zinc and nitrate ions in the electrochemical growth of ZnO were examined in this study. ZnO was deposited on a polycrystalline Cu substrate from electrolyte containing 0.0025–0.08 M zinc nitrate and 0.1–2.0 M potassium nitrate. The morphology, epitaxial orientation relationship, interfacial microstructure, and photoluminescence (PL) properties of the ZnO deposits were characterized. The ZnO morphology was found to be controlled mostly by the zinc ion concentration in the electrolyte. Here, 50–400 nm wide hexagonal pillars were deposited from electrolyte with potassium nitrate concentration as high as 0.4 M. All the deposited films had strong 〈0001〉//ND fibre texture. Most of the ZnO crystallites were grown epitaxially on Cu grains regardless of the electrolyte concentration. The major epitaxial orientation relationship was identified as (0001)ZnO//(103)Cu and [11 0]ZnO//[010]Cu, and three other epitaxial orientation relationships were also identified. Samples consisting of nanopillars or plates exhibited excellent PL characteristics with near bandgap–deep level emission intensity ratios of 2.4–4.3 and full width at half maximum of the NBE peak of 122–131 meV.

On the interpretation of cathodoluminescence intensity maps of wide band gap nanowires

It is commonly assumed in the spatially resolved cathodoluminescence (CL) studies of wide band gap (WBG) nanowires (NWs) that the CL intensity maps of deep-level (DL) and near band edge (NBE) emission reflect the spatial distribution of defects in these structures. On this basis, crucial conclusions about the technological growth conditions of NWs are drawn. However, here we showed using three-dimensional finite element analysis that in the case of WBG NWs, which exhibit surface band bending, the CL intensity maps of DL and NBE emission do not reflect the distribution of defects but, instead, the electric field strength in NWs. In particular, we found that independently of the defect concentration distribution, the DL emission intensity is always the highest in the areas where the electric field is the strongest and the lowest where the electric field is absent, while the NBE emission intensity exhibits the opposite trends. We explained this finding by the strong influence of the electric field on the spatial distribution of radiative recombination rates. Overall, our results indicate that (i) the frequently observed spatially inhomogeneous CL intensity distribution in WBG NWs can result from the presence of electric fields but not, as widely accepted, non-uniform defect distribution and (ii) spatially resolved CL spectroscopy measurements on the WBG NWs in most cases can not provide any quantitative information about the DL defect distribution.

Influence of growth temperature and duration on different properties of ultra-long ZnO nanorods grown by modified chemical bath deposition method

In this work, ultra-long ZnO nanorods (NRs) were grown on ZnO buffered Si substrates by using direct heat substrate-modified chemical bath deposition (DHS-MCBD) method. Initially, the growth temperature was varied in the range 140 °C to 250 °C while keeping the time fixed at 4h. In the second stage, the growth temperature was kept fixed at 220 °C and the time was changed from 4h to 8 h. The morphological, structural and optical properties of ZnO NRs at different growth temperatures were investigated by using field emission scanning electron microscope (FESEM), x-ray diffraction (XRD) and photoluminescence (PL) spectroscopy techniques. The surface morphology results revealed the formation of hexagonal-shaped, ultra-long and vertically-aligned ZnO NRs. The average length of the NRs was increased from 5.7 μm to 24.5 μm by increasing the growth temperature from 140 °C to 250 °C. The XRD results specified the formation of c-axis oriented diffraction peak corresponding to hexagonal ZnO NRs in all the samples. The intensity of this peak was increased by increasing the growth temperature up to 220 °C, however, on further increasing the temperature to 250 °C, the c-axis peak intensity was decreased. The PL analysis indicated higher UV to deep level emission intensity ratio (IUV/IDLE) of the NRs grown at 220 °C as compared to the others. Similarly, by changing the growth time from 4h to 8h, the length of ZnO NRs was also increased. The results obtained were correlated and the chemical reactions for ZnO NRs growth were discussed in this paper.

Enhancement of the deep-level emission and its chemical origin in hexagonal boron nitride

Defect-related deep-level emissions at ∼4 eV from hexagonal boron nitride (h-BN) have been extensively investigated during the past decades. Although the emission has often been associated with deep level impurities such as carbon and oxygen, the structural and chemical origins of the emission center have not yet been identified. Here, we perform systematic photoluminescence measurements and quantitative trace impurity analysis of variously heat-treated h-BN samples with different deep-level emission intensities. In contrast to the common belief, no positive correlation between the impurity (carbon and oxygen) concentration and the deep-level emission intensity was found. We also demonstrate that the intensity of the deep-level emission is significantly enhanced by high-temperature heating (∼1900 °C) under an N2 environment and subsequent post-annealing at ∼900 °C. These results suggest that intrinsic defects created under N2-rich conditions are responsible for the deep-level emission. Furthermore, we found that besides the well-recognized zero-phonon line (ZPL) at ∼4.1 eV, additional ZPLs are induced in the near ultraviolet (NUV) region by appropriate annealing of the N2-treated h-BN sample. The results of temperature-dependent and time-resolved PL measurements on these NUV emission bands are given and discussed.

Influence of crystallographic polarity on the opto-electrical properties of polycrystalline ZnO thin films deposited by magnetron sputtering

Herein, the influence of crystallographic polarity on the properties of ZnO polycrystalline thin films deposited by magnetron sputtering, has been studied. The electrical and optical properties of undoped ZnO and heavily Ga-doped ZnO thin films showed significant differences between Zn polarity and O polarity. The Hall mobility and carrier concentration of the undoped ZnO thin films with Zn polarity were higher than those with O polarity. The ZnO film with Zn polarity showed a dominant near-band-edge emission, while that with O polarity showed relatively intense deep-level emission in the visible wavelength region. It was suggested that acceptor-type defects tend to be formed in the ZnO films with O polarity. The Ga-doped ZnO films deposited on the Zn-polar ZnO layer showed a higher carrier concentration and Hall mobility as compared to deposition on O-polar ZnO. Crystallographic polarity is thus a very important factor for controlling the properties of polycrystalline ZnO-based thin films deposited on glass substrates.

Electrodeposited ZnO morphology transformations under the influence of SeO2 additive: Rods, disks, nanosheets network

In this study various morphologies of ZnO including rods, disks, and nanosheets network were synthesised by electrodeposition method. The influence of the SeO2 additive concentration on morphological, structural, and optical properties of ZnO was studied by using scanning electron microscopy, X-ray diffractometry (XRD), energy-dispersive X-ray analysis (EDX), optical (UV–Vis) and photoluminescence (PL) spectroscopy methods. The formation of the disk-like ZnO structure with a diameter of ca. 300–500nm and thickness ca. 80–100nm was observed when 0.03mmol/L of SeO2 was added to the solution. Further increase of dopant concentration up to 0.2mmol/L in the solution yielded a highly-porous network structure composed of thin nanosheets with a wall thickness of ca. 20nm. According to the XRD results, all structures were composed of ZnO, rod-like crystals were strongly c-axis oriented, whereas disks and nanosheets network structures were a-axis oriented. EDX analysis revealed that Se was present in the disks and nanosheets structures. PL study of the rod sample showed UV emission at ~370nm, whereas the disks and nanosheets network showed intense deep-level emission band centred at ~540nm. The disks and nanosheets exhibited total transmittance of ∼80–90% in the region of 380–780nm, whereas transmittance of the rods was increasing from 5% to 50% in this region. According to haze factor calculations, samples possessed scattering ability of 20% for disks, 45% for sheets and ca. 50% for rods. The probable mechanism for the formation of ZnO structures was discussed in detail.

Synthesis of ZnO nanorods by one step microwave-assisted hydrothermal route for electronic device applications

In the present work, zinc oxide (ZnO) nanorods (NRs) were prepared by a simple Microwave-assisted hydrothermal synthesis route by using a 100 ml Teflon container. The structural, crystalline nature and purity of the synthesized material was studied by powder X-ray diffraction studies. The optical absorption wavelength observed at 368 nm has been shifted to 377 nm during hydrothermal synthesis of ZnO NRs. The shifting of wavelength reduces the energy band gap from bulk state of 3.52–3.32 eV during the Microwave-assisted hydrothermal process. There is no shift in peak observed in photoluminescence spectrum between ZnO NRs and ZnO nanoparticles (NPs) but increase in temperature increases the Deep Level Emission (DLE) intensity due to the loss of oxygen during the hydrothermal process. The FTIR analysis confirms the presence of various functional groups in the ZnO NRs and NPs. The elemental, shape and surface morphology of the ZnO NRs and NPs were determined by using EDAX and FE–SEM analysis. The surface properties of ZnO NPs and ZnO NRs were studied by BET analysis. Thermal stability of the material was increased from 180 to 277 °C after the hydrothermal synthesis. The zeta potential analysis reveals the stability of the synthesized ZnO nanostructures. The variation of dielectric constant, dielectric loss and ac conductivity with frequency for the various temperatures was measured and discussed the suitability of the material for the electronic device fabrication.

Band gap engineering, enhanced morphology and photoluminescence of un-doped, Ga and/or Al-doped ZnO nanoparticles by reflux precipitation method

In this study, un-doped ZnO, Ga/Al-doped and co-doped ZnO nanoparticles (NPs) at 2mol% Ga doping (GZO), 2mol% Al doping and combined Ga and Al doping concentrations summing up to 2mol% (ranging from 1:1 to 1.9:0.1mol% Ga/Al) were synthesized using the reflux precipitation method. The effects of increasing the mole ratios of Ga in the ZnO host material or reducing Al mol%, on the structural, luminescence and optical properties of impurity-doped ZnO were investigated along with the properties of ZnO NPs. The X-ray diffraction patterns for all samples exhibit highly crystalline NPs with a hexagonal wurtzite structure. The crystallite size and average relative intensity of the diffraction peaks of the samples increased with increasing Ga doping indicating improved crystallinity with an increase in the content of gallium in the ratio. Photoluminescence spectra also displayed an increase in exciton peak emission with increasing Ga doping levels and reached a maximum at a doping concentration of 2.0mol% Ga, as well as a corresponding decrease in the deep level emission intensity, indicative that the best optical quality was obtained at the 2mol% Ga level. It was observed a blue shift in the PL exciton emission peak position and the band edge emission of the reflectance spectra with increasing concentration of Ga in the sample. The optical band gap calculated from the reflectance data also increased with increasing Ga concentration. These GZO NPs with improved crystallinity, minimum lattice stress, and enhanced luminescence intensity and optical properties would be suitable for deposition on a glass substrate to form the seed layer to produce a transparent conducting oxide for a dye- sensitized solar cell based thin films.

Effect of solution concentration on surface morphology, optical properties and solar light response of ZnO thin films

The effects of zinc nitrate solution concentrations on the crystal structures, surface morphologies and optical properties of ZnO thin films were examined. X-ray diffraction patterns and scanning electron microscopy images indicated that all the samples exhibited the hexagonal structure and the morphologies of the thin films changed from 1D nanorods to 2D nanosheets with the increasing of solution concentration. The thin film grown in the 0.075 M solution presents the 3D network nanostructure, which was composed of many very thin 2D ZnO nanosheets, and possesses the largest first LO phonon mode, optical band gap, ratio for the intensity of deep level emission to that of UV emission. The results indicated that the optical properties of the ZnO thin films were affected by the solution concentration. Finally, the as-deposited ZnO thin film was used to generate a photocurrent and as a photocatalyst. Observations showed that the thin film grown in the 0.075 M solution exhibits the largest photocurrent density and degradation rate constant per unit area. The formation mechanism of the enhanced photoresponse and photocatalytic performance for the ZnO thin film has been investigated in detail.

Synergistic effects of Mo and F doping on the quality factor of ZnO thin films prepared by a fully automated home-made nebulizer spray technique

Transparent conducting oxide films of undoped, Mo doped, Mo+F co-doped ZnO were deposited using a facile homemade nebulizer spray pyrolysis technique. The effects of Mo and F doping on the structural, optical, electrical and surface morphological properties were investigated using XRD, UV-vis-NIR spectroscopy, I–V and Hall probe techniques, FESEM and AFM, and XPS, respectively. The XRD analysis confirms that all the films are well crystallized with hexagonal wurtzite structure. All the synthesized samples exhibit high transmittance (above 85%) in the visible region. The current-voltage (I–V) characteristics show the ohmic conduction nature of the films. The Hall probe measurements show that the synergistic effects of Mo and F doping cause desirable improvements in the quality factor of the ZnO films. A minimum resistivity of 5.12×10−3Ωcm with remarkably higher values of mobility and carrier concentration is achieved for Mo (2 at.%)+F (15 at.%) co-doped ZnO films. A considerable variation in the intensity of deep level emission caused by Mo and F doping is observed in the photoluminescence (PL) studies. The presence of the constituent elements in the samples is confirmed by XPS analysis.

Effects of MgO buffer annealing on optical and electrical quality of P-MBE grown ZnO films on c-sapphire

Zinc oxide (ZnO) has been attracting much attention because of its potential applications in photonic and optoelectronic devices. In this present study, we investigated the effect of MgO buffer annealing on the optical and electrical quality of P-MBE grown ZnO films on c-sapphire with MgO buffer layer. The optical quality was observed by low-temperature PL (photoluminescence) measurement in the near band edge emission region measured at 10K and at 77K. The emission line located at 3.368eV dominates the spectrum in both samples (ZnO with and without MgO buffer annealing) at 10K and 77K. This emission can be divided into two peaks, 3.367eV and 3.363eV and assigned as I2 (ionized donor bound excitons emission) and I4 (Hydrogen donor related emission), respectively. The relative intensity of these donor bound exactions to free exaction emission of the sample without MgO buffer annealing is greater than that of the sample with MgO buffer annealing. Comparison of the PL spectra of ZnO with and without annealing revealed that the intensity of free exciton emission from the sample with MgO buffer annealing is twice of that from the sample without annealing. We also found that the intensity of deep-level broad emission is reduced by about 1/3 by MgO-buffer annealing. Hence, the decrease of deep level emission intensity and the increase of free exciton emission intensity by annealing of MgO buffer corresponds to the reduction of defects of the ZnO film. The PL properties also suggest that there are fewer nonradiative recombination centers in ZnO layers with MgO buffer annealing than those in ZnO layers grown without MgO buffer annealing. The electrical quality was measured by room temperature Hall measurements. We found that the samples have a background n-type carrier concentration. The ZnO samples with MgO buffer annealing has a carrier concentration of 1.17×1017 cm-3 and Hall mobility of 120 cm2/V.s, while the ZnO sample without MgO buffer annealing has a carrier concentration of 2.63 × 1016 cm-3 and Hall mobility of 105 cm2/V.s. The improvement of electron mobility of ZnO films by MgO buffer annealing is due to a decrease in dislocation density. We conclude that annealing of MgO buffer layer increases the optical and electrical quality of the ZnO films. These results agree with the structural quality as observed by HRXRD.

Wet Chemically Synthesized CuO Bipods and their Optical Properties.

Metal oxide nanostructures are being investigated widely due to their strong optical absorption, efficient photoluminescence, abundant of availability and structural stability which lead them as a possible substitute of Si based solar cells and many optoelectronic and sensor devices. Due to their non-toxic nature they are supposed to be promising materials for drug delivery and medical research. Among several metal oxides, cupric oxide (CuO) is being investigated nowadays due to their high transparency and visible luminescence. It is a p-type semiconductor of band gap varying from 1.3 eV to 2.1 eV depending of the structure and process of fabrication. This low band gap of CuO nanostructures leads its application in photoconductive and photothermal applications. This leads rigorous investigations on CuO nanostructures. A simple wet chemical methods has been used to synthesize CuO bipods. Predetermined amount of Copper sulphate penta-hydrate (CuSO4, 5H2O) was dissolved in double distilled de-ionized water to prepare 0.5 M solution. Predetermined amount of Lithium hydroxide (LiOH) was dissolved in de-ionized water to prepare 1 M solution. Under rigorous magnetic stirring of the LiOH solution CuSO4 solution was added drop by drop for five minutes and the stirring was maintained for 2 hours at room temperature (30°C). After the reaction a white precipitate was observed at the bottom of the flask. This solution was then aged for 24 hours. The color of the precipitate solution was turned into grey. The precipitate was filtered and subsequently washed with distilled de-ionized water thrice for the removal of unreacted salts if any. The precipitate was then dried in an ordinary furnace at 150°C for further characterization. X-ray diffraction data confirmed the formation of well crystalline CuO having monoclinic unit cell structure. The crystallite size was ~ 12 nm as calculated from the XRD pattern. Transmission electron microscope images revealed that the bipod-like structure is composed of several crystallites. The lengths of the bipods are ~ 300 nm with a waist size ~ 100 nm. The UV-visible spectroscopic data revealed a strong absorption at ~ 376 nm and the band gap was calculated to be 2.51 eV. FTIR spectrum revealed the existence of only Cu=O bonds. In the Raman shift data two strong peaks were found at 273 cm-1, and 608 cm-1 that owe to the Ag mode and Bg mode respectively. Room temperature photoluminescence spectroscopy revealed that the CuO nanoparticles exhibit broad strong emission at ~ 472 nm. After decomposing we observed three peaks at 436, 469 and 495 nm. The emission peak at 469 nm arises due to near band edge transition. The emission peak at 495 nm arises due to shallow level defect states related transitions. In conclusion, we have fabricated CuO nano-bipods using a simple wet chemical method. XRD pattern indicates the formation of well crystalline and pure CuO without any impurity. The nano-bipods exhibit sharp absorption peak at ~ 376 nm with optical band gap energy of 2.51 eV. FTIR spectrum and Raman shift data confirms the formation of CuO with the presence of Ag mode (273 cm-1) and Bg mode (608 cm-1). The fabricated CuO nano-bipods exhibit strong PL emission at 469 nm owing to the recombination of free excitons at the absorption band edge in the UV region associated with other low intense deep level emission. Few relevant patents to the topic have been reviewed and cited.

Synthesis and optical study of heat-treated ZnO nanopowder for optoelectronic applications

UV emitting ZnO nanopowder was chemically synthesized and subsequently subjected to heat treatment in oxygen atmosphere for potential optoelectronic properties. Characterization including Raman spectroscopy, photoluminescence, SEM, FT-IR and XRD were performed to see the effect of high temperature heat treatment and subsequently oxygen defects on the physical properties of ZnO powder. Chemically prepared product was highly pure polycrystalline w-ZnO with random crystallites orientation. Study showed a magnificent absorption of oxygen by the product as manifested by the decreased intensity of deep-level green emission and E1 (LO) phonon mode. The phonon modes appeared at 276 and 970 cm−1 and which have been assigned to ZnO by the previous researchers under relaxed Raman selection rule were no longer found with heat treatment. UV emission was enhanced and the ratio of UV to green emission (I UV /I green) was correlated with the crystal structure and oxygen vacancies before and after heat treatment. FT-IR study established strong Zn-O bending and stretching bands at 356 and 498 cm−1. SEM analysis demonstrated fine crystallites distribution in ZnO nanopowder with almost spherical morphologies. Reasonably, a more spherical and ordered morphologies with large grains were found with heat treatment. The investigated findings manifested improved structural and optical properties for various optoelectronic and biomedical applications of technological importance.