Analysis of defect luminescence in Ga-doped ZnO nanoparticles

Colloidal Approaches to Zinc Oxide Nanocrystals

Zinc oxide is an extensively studied semiconductor with a wide band gap in the near-UV. Its many interesting properties have found use in optics, electronics, catalysis, sensing, as well as biomedicine and microbiology. In the nanoscale regime the functional properties of ZnO can be precisely tuned by manipulating its size, shape, chemical composition (doping), and surface states. In this review, we focus on the colloidal synthesis of ZnO nanocrystals (NCs) and provide a critical analysis of the synthetic methods currently available for preparing ZnO colloids. First, we outline key thermodynamic considerations for the nucleation and growth of colloidal nanoparticles, including an analysis of different reaction methodologies and of the role of dopant ions on nanoparticle formation. We then comprehensively review and discuss the literature on ZnO NC systems, including reactions in polar solvents that traditionally occur at low temperatures upon addition of a base, and high temperature reactions in organic, nonpolar solvents. A specific section is dedicated to doped NCs, highlighting both synthetic aspects and structure-property relationships. The versatility of these methods to achieve morphological and compositional control in ZnO is explicated. We then showcase some of the key applications of ZnO NCs, both as suspended colloids and as deposited coatings on supporting substrates. Finally, a critical analysis of the current state of the art for ZnO colloidal NCs is presented along with existing challenges and future directions for the field.

Effects of Ambience on Thermal-Diffusion Type Ga-doping Process for ZnO Nanoparticles

Various annealing atmospheres were employed during our unique thermal-diffusion type Ga-doping process to investigate the surface, structural, optical, and electrical properties of Ga-doped zinc oxide (ZnO) nanoparticle (NP) layers. ZnO NPs were synthesized using an arc-discharge-mediated gas evaporation method, followed by Ga-doping under open-air, N2, O2, wet, and dry air atmospheric conditions at 800 °C to obtain the low resistive spray-coated NP layers. The I–V results revealed that the Ga-doped ZnO NP layer successfully reduced the sheet resistance in the open air (8.0 × 102 Ω/sq) and wet air atmosphere (8.8 × 102 Ω/sq) compared with un-doped ZnO (4.6 × 106 Ω/sq). Humidity plays a key role in the successful improvement of sheet resistance during Ga-doping. X-ray diffraction patterns demonstrated hexagonal wurtzite structures with increased crystallite sizes of 103 nm and 88 nm after doping in open air and wet air atmospheres, respectively. The red-shift of UV intensity indicates successful Ga-doping, and the atmospheric effects were confirmed through the analysis of the defect spectrum. Improved electrical conductivity was also confirmed using the thin-film-transistor-based structure. The current controllability by applying the gate electric-field was also confirmed, indicating the possibility of transistor channel application using the obtained ZnO NP layers.

Cytotoxicity of ZnO Nanoparticle Under Dark via Oxygen Vacancy Dependent Reactive Oxygen Species Generation

Antimicrobial and cytotoxic effect of zinc oxide nanomaterials are popularly thought due to zinc ion leaching, but the exact mechanism of cytotoxicity is controversial and not fully understood. Recent works...

Defect engineering, microstructural examination and improvement of ultrafast third harmonic generation in GaZnO nanostructures: a study of e-beam irradiation

Electron beam induced effects on defect engineering and structural, morphological and optical properties of Ga doped ZnO (GaZnO) nanostructures for improved ultrafast nonlinear optical properties are presented. A microstructural analysis was carried out based on the Scherrer, Williamson-Hall, and size-strain models. All three models reveal a peak broadening effect upon electron beam irradiation (EBI) and the crystallite size of the films shows a decrease of 30% compared to unirradiated nanostructures. The decrease in intensity, variation in the peak position and broadening of the Raman E2H mode confirm that the EBI treatment introduces disorder into the nanostructures. The interband gap emissions observed in photoluminescence spectra are primarily due to defect-related emissions originating from intrinsic defects such as Zni, Oi, VZn, VO, VZn+, VO+ and OZn. The O1s core-level spectra show that the peak related to oxygen vacancy defects is suppressed upon EBI. Surface morphology studies reveal that the nucleation barriers of GaZnO nanostructures are reduced upon irradiation treatment resulting in a coalescence mechanism. Third harmonic generation studies show that higher electron-beam doses lead to the occurrence of enhanced THG signals due to a drastic change in the occupation of localized defect levels. Thermally induced nonlinear optical studies depict an improved χ(3) of 1.71 × 10-3 esu upon irradiation due to enhanced FCA induced TPA mechanism and non-radiative transitions which indicates the credibility of the grown films in photonic devices.

Mapping the structural, electrical, and optical properties of hydrothermally grown phosphorus-doped ZnO nanorods for optoelectronic device applications

The phosphorus-doped ZnO nanorods were prepared using hydrothermal process, whose structural modifications as a function of doping concentration were investigated using X-ray diffraction. The dopant concentration-dependent enhancement in length and diameter of the nanorods had established the phosphorus doping in ZnO nanorods. The gradual transformation in the type of conductivity as observed from the variation of carrier concentration and Hall coefficient had further confirmed the phosphorus doping. The modification of carrier concentration in the ZnO nanorods due to phosphorus doping was understood on the basis of the amphoteric nature of the phosphorus. The ZnO nanorods in the absence of phosphorus showed the photoluminescence (PL) in the range of the ultraviolet (UV) and visible regimes. The UV emission, i.e. near-band-edge emission of ZnO, was found to be red-shifted after the doping of phosphorus, which was attributed to donor-acceptor pair formation. The observed emissions in the visible regime were due to the deep level emissions that were aroused from various defects in ZnO. The Al-doped ZnO seed layer was found to be responsible for the observed near-infrared (NIR) emission. The PL emission in UV and visible regimes can cover a wide range of applications from biological to optoelectronic devices.

Defect-related multicolour emissions in ZnO smoke: from violet, over green to yellow

The nature of defects in ZnO smoke was studied at different stages of the material's history by combining photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopy. In contrast to studies previously reported on ZnO nanopowders, high vacuum conditions (P < 10-5 mbar) have been applied during sample storage, handling and spectroscopic investigations. Two pairs of violet-PL/EPR signals (2.88 eV/ g = 1.956 and 2.80 eV/ g = 1.960) were observed in the as-synthesized ZnO powder and attributed to surface (dominant) and bulk zinc interstitials (Zni+). Upon annealing in O2-poor conditions, green-PL emission (2.41 eV) and EPR signal at g = 2.002 develop along with EPR signals specific of superoxide radicals (O2-). In the absence of any external O2 supply, the oxygen necessary for the creation of a notable amount of O2- is provided by the lattice of ZnO smoke, so that the green emission and its EPR counterpart are unambiguously assigned to singly charged oxygen vacancies (VO+). Annealing at high PO2 results in a broad PL emission (∼2.07 eV) without an EPR counterpart. This yellow emission was assigned to peroxide-like surface species (O22-). Overall, this study shows that the visible emissions in ZnO smoke nanopowders can range from violet, over green to yellow as a function of sample history and that the corresponding PL/EPR fingerprints can serve as guidelines for the recognition of defects in other ZnO types.

Investigation of Dual-Ion Beam Sputter-Instigated Plasmon Generation in TCOs: A Case Study of GZO

The use of the high free-electron concentration in heavily doped semiconductor enables the realization of plasmons. We report a novel approach to generate plasmons in Ga:ZnO (GZO) thin films in the wide spectral range of ∼1.87-10.04 eV. In the grown GZO thin films, dual-ion beam sputtering (DIBS) instigated plasmon is observed because of the formation of different metallic nanoclusters are reported. Moreover, formation of the nanoclusters and generation of plasmons are verified by field emission scanning electron microscope, electron energy loss spectra obtained by ultraviolet photoelectron spectroscopy, and spectroscopic ellipsometry analysis. Moreover, the calculation of valence bulk, valence surface, and particle plasmon resonance energies are performed, and indexing of each plasmon peaks with corresponding plasmon energy peak of the different nanoclusters is carried out. Further, the use of DIBS-instigated plasmon-enhanced GZO can be a novel mean to improve the performance of photovoltaic, photodetector, and sensing devices.

Degenerately Doped Metal Oxide Nanocrystals as Plasmonic and Chemoresistive Gas Sensors

Highly doped wide band gap metal oxide nanocrystals have recently been proposed as building blocks for applications as transparent electrodes, electrochromics, plasmonics, and optoelectronics in general. Here we demonstrate the application of gallium-doped zinc oxide (GZO) nanocrystals as novel plasmonic and chemiresistive sensors for the detection of hazardous gases including hydrogen (H₂) and nitrogen dioxide (NO₂). GZO nanocrystals with a tunable surface plasmon resonance in the near-infrared are obtained using a colloidal heat-up synthesis. Thanks to the strong sensitivity of the plasmon resonances to chemical and electrical changes occurring at the surface of the nanocrystals, such optical features can be used to detect the presence of toxic gases. By monitoring the changes in the dopant-induced plasmon resonance in the near-infrared, we demonstrate that GZO thin films prepared depositing an assembly of highly doped GZO colloids are able to optically detect both oxidizing and reducing gases at mild (<100 °C) operating temperatures. Combined optical and electrical measurements show that trivalent dopants within ZnO nanocrystals enhance the gas sensing response compared to undoped ZnO. Moreover, improved sub-ppm of NO₂ gas sensitivity is achieved by activating the sensors response through combined purple-blue (λ = 430 nm) light irradiation and mild heating at 75 °C. In addition, these thin films based on degenerately doped semiconductors are highly transparent in the visible range, enabling the fabrication of "invisible" gas sensors. The use of highly doped semiconductive nanocrystals for both IR plasmonic and chemiresistive sensors represent a marked advancement toward the development of highly sensitive and selective devices.