Deep-level Emission Band Research Articles

Ruthenium-doped ZnO nanostructures: Growth, characterization, and applications in waveguides and light-emitting devices

ZnO based materials are gaining importance due to its potential applications. In many of these applications, the size, shape and dopability of the structures grown play a determinant role. In the present work, ZnO nano- and microstructures doped with Ru have been grown using the Vapour-Solid (VS) method. Among the different growth parameters, gas flux and initial dopant content seem to play the major role to determine both shape and size of the structures. The main morphologies obtained are self-arranged triangular grids and elongated structures. Based on microscopic observations a simplified growth model is proposed for the grids. An exhaustive characterization has been performed. X-Ray diffraction (XRD) investigations demonstrate good crystallinity and texturing related to preferential growth direction changes due to doping. Luminescence characterization through photo- and cathodoluminescence (PL and CL) techniques reveals typical ZnO near band edge and deep level emission bands at 3.2 and 2.3 eV, respectively. The potential use of elongated structures as waveguides and optical cavities, Fabry-Pèrot resonators, is explored.

Synthesis and Properties of Magnetic-Luminescent Fe3O4@ZnO/C Nanocomposites

A Fe3O4@ZnO/C nanocomposite with a core-shell structure was synthesized using the co-precipitation method. To prevent the aggregation of the Fe3O4 magnetic particles, polyethylene glycol (PEG) was added. The X-ray diffractometer (XRD) results confirmed the formation of Fe3O4 and ZnO phases, with Fe3O4 having a cubic crystal system and ZnO having a hexagonal crystal system. Carbon in Fe3O4@ZnO/C had no effect on the crystal structure of Fe3O4@ZnO. Images from transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that the nanocomposite formed a core-shell structure. The Fourier transform infrared (FTIR) spectra verified the presence of bonds among ZnO, Fe3O4, and carbon. The appearance of the stretching vibration of the C≡C bond on the Fe3O4@ZnO/C sample revealed the nanocomposites’ carbon coupling. Photoluminescence (PL) spectroscopy was used to characterize the optical properties of the nanocomposites. Based on the results of the PL, the sample absorption of visible light was in the wavelength range of 400–700 nm. The photoluminescence of Fe3O4@ZnO differed from that of the Fe3O4@ZnO/C, especially in the deep-level emission (DLE) band. There was a phenomenon of broadening and shift of the band at a shorter wavelength, namely, in the blue wavelength region. Magnetic properties were characterized by vibrating-sample magnetometry (VSM). Based on the VSM results, the sample coupled with carbon exhibited a decrease in magnetic saturation. The presence of carbon changed photon energy into thermal energy. So, this material, apart from being a bioimaging material, can also be developed as a photothermal therapy material.

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.

Optimization of the luminescence and structural properties of Er-doped ZnO nanostructures: effect of dopant concentration and excitation wavelength

The present study reports the effect of Er doping on the structural, morphological, and optical properties of ZnO nanostructures. ZnO: Er nanorods with different doping concentrations were successfully synthesized via a hydrothermal method. X-ray diffraction and Energy-dispersive X-ray spectroscopy analyses suggest the successful incorporation of Er into the wurtzite structure of ZnO; A secondary phase of Er2O3 appears when the Er concentration exceeds 2.5 wt%. The optical band-gap of ZnO was determined from the analysis of ultraviolet–visible spectra, and a decrease of the band gap is emphasized with increasing Er concentration. Photoluminescence spectra display a strong and broad deep-level emission band, which intensified with increasing Er concentration. The current work provides more details about the excitation wavelength effect on the luminescence of pure and Er-doped ZnO materials, finding that the visible photoluminescence emission intensity could be controlled by adjusting the Er concentration as well as the excitation wavelength; better luminescence was obtained with high Er concentration and high excitation wavelength. Moreover, the DFT calculations support the experimental results and confirm that the presence of oxygen vacancies has an effect on the structural and electronic properties of ZnO. Hence, the research findings of this work could provide an experimental and theoretical reference that may motivate future research on the study of ZnO-based optoelectronic applications.

The effect of iron content on properties of ZnO nanoparticles prepared by microwave hydrothermal method

Iron doped (in the range from 1 to 10 mol %) ZnO nanoparticles (NPs) were obtained by microwave hydrothermal method. The effects of iron content on morphology, crystal structure, optical and magnetic properties of ZnO nanoparticles were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), cathodoluminescence (CL), photoluminescence (PL) and magnetic measurements. XRD studies revealed presence of ZnFe2O4 foreign phase in NPs for iron doping concentrations above 5% placing the solubility limit of Fe in ZnO NPs near this value. The photoluminescence and cathodoluminescence spectra show an increase of the intensity ratio between near band edge (NBE) and deep level emission band (DLE) of ZnO, which is tentatively related by us with intrinsic cracking of ZnO crystals. The blue-shift of NBE band maxima was observed as a concentration of Fe dopant increases. The results of magnetic measurements indicate superparamagnetic behavior of nanoparticles.

Effect of Al3+/Si4+ codoping on the structural, optoelectronic and UV sensing properties of ZnO

Abstract

Photoluminescence of ZnO Nanowires: A Review.

One-dimensional ZnO nanostructures (nanowires/nanorods) are attractive materials for applications such as gas sensors, biosensors, solar cells, and photocatalysts. This is due to the relatively easy production process of these kinds of nanostructures with excellent charge carrier transport properties and high crystalline quality. In this work, we review the photoluminescence (PL) properties of single and collective ZnO nanowires and nanorods. As different growth techniques were obtained for the presented samples, a brief review of two popular growth methods, vapor-liquid-solid (VLS) and hydrothermal, is shown. Then, a discussion of the emission process and characteristics of the near-band edge excitonic emission (NBE) and deep-level emission (DLE) bands is presented. Their respective contribution to the total emission of the nanostructure is discussed using the spatial information distribution obtained by scanning transmission electron microscopy−cathodoluminescence (STEM-CL) measurements. Also, the influence of surface effects on the photoluminescence of ZnO nanowires, as well as the temperature dependence, is briefly discussed for both ultraviolet and visible emissions. Finally, we present a discussion of the size reduction effects of the two main photoluminescent bands of ZnO. For a wide emission (near ultra-violet and visible), which has sometimes been attributed to different origins, we present a summary of the different native point defects or trap centers in ZnO as a cause for the different deep-level emission bands.

Effects of annealing on photoluminescence and defect interplay in ZnO bombarded by heavy ions: Crucial role of the ion dose

Bombardment of ZnO with heavy ions generating dense collision cascades is of particular interest because of the formation of nontrivial damage distribution involving a defected layer located between the surface and the bulk damage regions, as seen by Rutherford backscattering spectroscopy in the channeling mode. By correlating photoluminescence and channeling data, we demonstrate that the thermal evolution of defects in wurtzite ZnO single crystals implanted with Cd ions strongly depends on the implanted dose. Specifically, the ion dose has a profound effect on the optical response in the spectral range between the near-band-edge emission and deep-level emission bands. The interplay between interstitial and vacancy type defects during annealing is discussed in relation to the evolution of the multipeak damage distribution.

Effect of Mn doping on defect-related photoluminescence and nanostructure of dense 3-D nano-root network of ZnO

In this paper, we investigate the effect of Mn doping on an interesting morphology of ZnO thin films prepared by sol-gel spin coating technique. The tunability of morphology, structural and optical properties by doping has been reported. X-ray diffraction (XRD) patterns of the samples annealed at 400 °C confirm the formation of hexagonal wurtzite ZnO without any Mn related phases. On introduction of Mn in ZnO lattice, the crystallite size decreases. Field emission scanning electron microscope (FESEM) studies of the samples confirmed the formation of 3-D nanostructured thin films with morphology similar to a dense network of roots. The nano-root network comprises nanoparticles and as the Mn content is increased, the nanoparticles agglomerate. Energy-dispersive X-ray spectroscopy-based color mapping technique was employed to observe the distribution of Zn, Mn, and O elements. On increasing the doping concentration of Mn, there is a blue-shift in the absorption edge in the absorption spectra recorded by UV–Vis spectroscopy. The optical bandgap increases linearly from 3.26 eV to 3.32 eV on increasing the Mn content from 0 at% to 5 at%. Photoluminescence spectroscopy was used to characterize the deep level emissions. A broad deep level emission band arising due to defects was observed which ranges from violet to red region in the undoped ZnO and is significantly suppressed after Mn-doping. Owing to its dense 3-D intertwined nanoroot network, the proposed study is expected to find potential application in gas sensing, photodetection and catalysis.

Blueshift of absorption edge and photoluminescence in Al doped ZnO thin films

ABSTRACTPure and Al doped ZnO thin films are fabricated on quartz substrates by sol–gel method, and then analyzed by X-ray diffraction (XRD), transmittance spectra, and photoluminescence (PL) measurements respectively. XRD results reveal that all the thin films have a preferential c-axis orientation. With the increase of Al doping, however, the peak position of the (002) plane is shifted to a low 2θ value. On the other hand, the data of spectrometer transmittance are obtained, with which the band gap energies of Al doped ZnO films are calculated by a linear fitting method. The band gap is found to be broadening, and the absorption edge has an obvious blueshift to the shorter wavelength with increasing dopant concentration. PL measurement is also conducted, and deep-level (DL) emission and near band edge (NBE) emission are observed in pure ZnO thin films. But DL emissions are depressed when Al is doped into thin films. And the peak of NBE emission has a blueshift to the region of higher photon energy as the Al concentration increases, a performance which tallies with observations through the optical transmittance data. The study demonstrates that the blueshift of optical properties in the ZnO:Al films, can nevertheless be easily manipulated and managed by controlling dopant concentration, a big plus to the said films in their applications in broadband UV photodetectors with highly tunable wavelength resolution.

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.

Size dependent photoluminescence properties of CdZnS nanostructures

Photoluminescence response of cadmium zinc sulfide (CdZnS) nanostructures has been studied through low-temperature and time-resolved photoluminescence measurements. The as synthesize nanostructures are single crystalline having growth along [102] direction. The CdZnS nanostructures exhibited near band edge and deep level emission bands at 2.485eV and 2.09eV, displaying bi-exponential excitonic decay with an average lifetime of 2.56ns and 4.11ns, respectively. Furthermore, single CdZnS nanostructures have been investigated for their size dependent photoluminescence properties. For isolated nanostructures, emission band systematically shifted towards the higher-energy, and defect-related low energy emission peak disappeared with a decrease in nanostructure width. Results revealed that emission originated from the radiative recombination process of the donor and acceptor bound excitons. Detrimental effect of low temperature Photoluminescence measurements have been observed in individual nanostructures.

Dense Plasma Focus Device Based High Growth Rate Room Temperature Synthesis of Nanostructured Zinc Oxide Thin Films

High-energy-density plasmas in plasma focus device are now routinely being used as nonconventional plasma nanotechnology tool with novel processing and deposition parameters which are not available in conventional lowtemperature plasma facilities. This paper reports the synthesis of multilayered polycrystalline intrinsic defects-free nanostructured zinc oxide (ZnO) thin film onto room-temperature silicon substrates at high growth rates using pulsed high-energy-density dense plasma focus device. The device was operated in pure oxygen environment and the ablation target used was a pure zinc rod fitted onto the anode top. Thin films of ZnO were deposited at room temperature using different numbers of dense plasma focus shots and at different distances from the anode top. The as-deposited ZnO showed the formation of polycrystalline hexagonal phase ZnO thin films with strong c-axis orientation. The scanning electron microscopy results showed either uniformly laid nanoparticles or cauliflower aggregates of nanoparticles on the surface of the thin-film samples with average nanoparticle size varying between 23.3 ± 6.1 and 69.7 ± 16.5 nm. The average thicknesses of the multilayered thin films were measured to be in a range of 1.06 ± 0.14 to about 2.82 ± 0.32 μm. The complete disappearance of deep level emission band in photoluminescence spectrum of ZnO thin-film sample deposited using 30 focus shots at deposition distance of 15 cm showed successful synthesis of high crystalline quality intrinsic defects-free as-deposited ZnO thin film. One of the key features of the ZnO thin-film synthesis using nonconventional plasma focus device is the very high average growth rate of about 0.1 μm/shot. At one plasma focus shot per minute operation, the average growth rate comes out to be 0.1 μm/min, which is either higher or comparable with commonly used ZnO deposition methods; but at 10 Hz operation extreme growth rates of about 60 μm/min could be achieved.

Morphological and Photoluminescence analysis of Zinc Oxide thin films deposited by RF sputtering at different substrate temperatures

Zinc oxide (ZnO) thin films were prepared using reactive RF magnetron sputtering of a pure metallic zinc target onto n-type (100) silicon substrates. The evolution of the surface morphology and the optical properties of the films were studied as a function of the substrate temperature, which was varied from ambient to 300°C. X-ray diffraction pattern of ZnO thin film shows the appearance of c-axis oriented (002) peak for all samples shows varying degrees of crystallinity of films. Photoluminescence studies were also carried out (350-700 nm) to study the crystallinity and optically active defects in the films. PL spectra of the film shows UV emission peak depicts good crystallinity of ZnO film where as the intensity of deep level emission band decreases with increase in substrate temperature due to the formation of stoichiometric ZnO film which causes decrease in defect.

Spectroscopy and Topography of Deep-Level Luminescence in Photovoltaic Silicon

The aim of this paper is to identify the origin of a deep-level emission band with a peak at about 0.8 eV observed in photoluminescence from defective areas in multicrystalline Si crystals at room temperature. We compare the band with that in a plastically deformed float-zone and annealed Czochralski-grown Si crystals investigated in detail previously for microelectronic applications and point out the similarities in spectroscopic characteristics and in spatial distributions around dislocations. This prompted us to suggest that the 0.8-eV band consists of two dislocation-related components termed D a1 and D a2 at about 0.79 and 0.94 eV, respectively, and an oxygen-precipitation-related component Db at approximately 0.87 eV. Spatial variations of the three components around dislocation clusters forming small-angle grain boundaries reflect secondary defects or impurities trapped by the strain field around the dislocations, the intrinsic nature of the dislocations, and preferential oxygen precipitation on the dislocations, respectively. The presence of oxygen precipitates in a region emitting the strong Db component was confirmed by highly spatially resolved and highly sensitive secondary ion mass spectroscopy and by mapping of oxygen by luminescence activation using electron irradiation. Anisotropic properties expected for the dislocation-related D a1 component were detected by polarized luminescence imaging.

Polarized photoluminescence imaging analysis around small-angle grain boundaries in multicrystalline silicon wafers for solar cells

We have shown the effectiveness of polarized photoluminescence imaging for analyzing the structural and spectroscopic properties of small-angle grain boundaries (SA-GBs) in multicrystalline Si. The dislocation-related deep-level emission band at approximately 0.79 eV at room temperature was found to be polarized, whereas the band-edge emission did not show the polarization effect. The anisotropy of the 0.79 eV band was classified into two groups depending on the tilt and twist characteristics of SA-GBs determined by the electron backscatter diffraction measurement.

Deep-level emission in ZnO nanowires and bulk crystals: Excitation-intensity dependence versus crystalline quality

The excitation-intensity dependence of the excitonic near-band-edge emission (NBE) and deep-level related emission (DLE) bands in ZnO nanowires and bulk crystals is studied, which show distinctly different power laws. The behavior can be well explained with a rate-equation model taking into account deep donor and acceptor levels with certain capture cross sections for electrons from the conduction band and different radiative lifetimes. In addition, a further crucial ingredient of this model is the background n-type doping concentration inherent in almost all ZnO single crystals. The interplay of the deep defects and the background free-electron concentration in the conduction band at room temperature reproduces the experimental results well over a wide range of excitation intensities (almost five orders of magnitude). The results demonstrate that for many ZnO bulk samples and nanostructures, the relative intensity R = INBE/IDLE can be adjusted over a wide range by varying the excitation intensity, thus, showing that R should not be taken as an indicator for the crystalline quality of ZnO samples unless absolute photoluminescence intensities under calibrated excitation conditions are compared. On the other hand, the results establish an all-optical technique to determine the relative doping levels in different ZnO samples by measuring the excitation-intensity dependence of the UV and visible luminescence bands.

Photoluminescence in MgO-ZnO Nanorods Enhanced by Hydrogen Plasma Treatment

MgO nanorods were fabricated by the thermal evaporation of . The influence of ZnO sheathing and hydrogen plasma exposure on the photoluminescence (PL) of the MgO nanorods was studied. PL measurements of the ZnO-sheathed MgO nanorods showed two main emission bands: the near band edge emission band centered at ~380 nm and the deep level emission band centered at ~590 nm both of which are characteristic of ZnO. The near band edge emission from the ZnO-sheathed MgO nanorods was enhanced with increasing the ZnO shell layer thickness. The near band edge emission from the ZnO-sheathed MgO nanorods appeared to be enhanced further by hydrogen plasma irradiation. The underlying mechanisms for the enhancement of the NBE emission from the MgO nanorods by ZnO sheathing and hydrogen plasma exposure are discussed.

Tuning of undoped ZnO thin film via plasma enhanced atomic layer deposition and its application for an inverted polymer solar cell

We studied the tuning of structural and optical properties of ZnO thin film and its correlation to the efficiency of inverted solar cell using plasma-enhanced atomic layer deposition (PEALD). The sequential injection of DEZn and O2 plasma was employed for the plasma-enhanced atomic layer deposition of ZnO thin film. As the growth temperature of ZnO film was increased from 100 °C to 300 °C, the crystallinity of ZnO film was improved from amorphous to highly ordered (002) direction ploy-crystal due to self crystallization. Increasing oxygen plasma time in PEALD process also introduces growing of hexagonal wurtzite phase of ZnO nanocrystal. Excess of oxygen plasma time induces enhanced deep level emission band (500 ∼ 700 nm) in photoluminescence due to Zn vacancies and other defects. The evolution of structural and optical properties of PEALD ZnO films also involves in change of electrical conductivity by 3 orders of magnitude. The highly tunable PEALD ZnO thin films were employed as the electron conductive layers in inverted polymer solar cells. Our study indicates that both structural and optical properties rather than electrical conductivities of ZnO films play more important role for the effective charge collection in photovoltaic device operation. The ability to tune the materials properties of undoped ZnO films via PEALD should extend their functionality over the wide range of advanced electronic applications.

Antibacterial and Blue shift investigations in sol–gel synthesized CrxZn1−xO Nanostructures

In this paper, a systematic study of structural, morphological, phononic, vibrational and optical properties of Zn1−xCrxO nanostructures has been reported. These particles are synthesized through simple sol–gel chemical technique at low temperature using simply available laboratory chemicals i,e zinc acetate and sodium hydroxide. The detailed structural studies carried out by X-ray diffraction method (XRD) and transmission electron microscopy (TEM) reveals that the as-synthesized nanoparticles are well-crystalline and possessing hexagonal wurtzite structure in 54–57nm range. Scanning electron microscopy (SEM) shows the synthesized nanostrucures were grown in high density with less agglomeration and their different morphologies observed after doping. The presences of functional groups were characterized by employing Fourier transform infrared spectroscopy (FTIR). Compared with the photoluminescence (PL) spectrum of the undoped ZnO, the peak position of ultraviolet (UV) emission of the Cr-doped ZnO nanostructures exhibits remarkable blue shift, also confirmed by Raman studies, and the deep level emission band was sharply suppressed. Band gap of synthesized samples has been evaluated using DRS (UV–Vis mode). It is found that the band gap increases as Cr doping took place which is attributed to the combined effect of size reduction and doping. The synthesized CrxZn1−xO nanostructures showed potential applications in destruction of bacteria.