Structure, microstructure and physical properties of ZnO based materials in various forms: bulk, thin film and nano (original) (raw)
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Journal of Physics D-applied Physics, 2007
ZnO is a unique material that offers about a dozen different application possibilities. In spite of the fact that the ZnO lattice is amenable to metal ion doping (3d and 4f), the physics of doping in ZnO is not completely understood. This paper presents a review of previous research works on ZnO and also highlights results of our research activities on ZnO. The review pertains to the work on Al and Mg doping for conductivity and band gap tuning in ZnO followed by a report on transition metal (TM) ion doped ZnO. This review also highlights the work on the transport and optical studies of TM ion doped ZnO, nanostructured growth (ZnO polycrystalline and thin films) by different methods and the formation of unique nano-and microstructures obtained by pulsed laser deposition and chemical methods. This is followed by results on ZnO encapsulated Fe 3 O 4 nanoparticles that show promising trends suitable for various applications. We have also reviewed the non-linear characteristic studies of ZnO based heterostructures followed by an analysis on the work carried out on ZnO based phosphors, which include mainly the nanocrystalline ZnO encapsulated SiO 2 , a new class of phosphor that is suitable for white light emission.
Optik, 2019
Doping of group II elements in ZnO is an efficient way to enhance the optical properties of ZnO nanoparticles. For this purpose, Zn 1-x Mg x O nanoparticles (x = 0, 0.02, 0.04, 0.06, 0.08 and 0.1) are synthesized using co-precipitation technique. Effect of 'Mg' doping on structural, morphological, optical and thermal properties of ZnO nanoparticles is investigated. XRD results verify that synthesized nanoparticles are poly-crystalline with typical hexagonal wurtzite structure and possess no other impurity or dopant phases. Crystallite size is increased with the increase in 'Mg' content. SEM analysis reveals that polyhedral grains are aggregated with the increase in doping concentration of Mg. For the highest dopant concentration, surface morphology is entirely changed with the formation of nanowires. Optical band gap energy of Mg-doped ZnO nanoparticles is greater than that of pristine ZnO nanoparticles. The blue shift in band gap is observed with Mg content x ≤ 0.04, followed by red shift for higher Mg content. The decrease in phase transition and increase in decomposition temperature for Mg-doped ZnO nanoparticles suggest their thermal stability. Tailoring of band gap makes ZnO nanoparticles a promising material for photocatalysis, optoelectronic and display devices. 1. Introduction Zinc oxide (ZnO) is considered as one of the front runners among different metal oxide semiconductors due to its fascinating physical and chemical properties. It has wide direct band gap energy (˜3.2-3.37 eV) and high exciton binding energy (60 meV). These properties make ZnO an efficient material for short wavelength optoelectronic and nanoelectronic devices [1,2]. Its superior piezoelectric property and stability against photo-corrosion make it attractive for surface acoustic wave devices, gas sensors, solar cells and transparent electrodes [3-5]. Its non-toxicity, biocompatibility, chemical stability and photochemical properties make it suitable for drug delivery, antibacterial and photo-catalytic applications [6-8]. ZnO, at nanoscale, exhibits unique structural, optical, electronic and chemical properties, entirely different from its bulk counterpart. The properties of ZnO nanostructures depend on crystalline structure and morphology, particle size and their shape, which in turn are dependent on synthesis methods of ZnO nanostructures. In general, physical, chemical and green methods are employed to fabricate ZnO nanomaterials [9-12]. Among these, chemical method is a simple and cost-effective technique. It can be performed at room or low temperature, using a wide range of precursors and synthesis conditions such as concentration of reactants, time, temperature etc. Control of these parameters results into ZnO nanostructures with different geometries and sizes. These
Journal of Luminescence, 2014
In this work 0%, 3%, 5%, 7% and 10% Fe doped ZnO nanoparticles (NPs) have been successfully synthesized by the sol-gel method. The compositional, structural and optical studies have been investigated by XRD, FESEM equipped with EDS, TEM with SAED, UV-visible and Photoluminescence spectroscopy. XRD, SEM and TEM results confirmed the formation of nanoparticles with polycrystalline single phase nature with hexagonal wurtzite structure. The crystallite size has been found to vary between 10 and 20 nm with changes in the doping concentration of Fe. The stretching bonds in ZnO have been observed in FTIR spectra. The UV-visible and PL spectra show a red shift with increase of dopant concentration. The band gaps for all samples were calculated by the Tauc relation and narrowing of band gap from 3.33 eV to 3.26 eV with increasing Fe dopant concentration up to 10% was found. The band gap is narrowing because of the s-d and p-d exchange interactions which introduce a negative and a positive correction to the conduction and the valence-band edges respectively.
Comparative first principles study of ZnO doped with group III elements
Journal of Alloys and Compounds, 2016
In this paper, we present a comparative study of structural, electronic, optical and electrical properties of ZnO doped with aluminum, gallium and indium. The calculated structural parameters were obtained using the Generalized Gradient Approximation. The influence of doping ZnO on growth directions was investigated. The electronic structure, absorption coefficient, reflectivity, refraction index, and transparency were determined using the modified Becke-Johnson potential implemented in the Wien2K code. The electrical conductivity was calculated for all compounds using the Boltzmann transport equation. We have found that these elements present a good optical and electrical conductivity with a significant improvement for the ZnO doped Al compared to the ZnO doped Ga or In. The results for pure and doped ZnO are in a good agreement with experimental and other theoretical studies and confirm their usability in photovoltaic devices.
Influence of d-d transition bands on electrical resistivity in Ni doped polycrystalline ZnO
Applied Physics Letters, 2006
We report on the transport and optical properties of Ni doped ZnO polycrystalline samples. Ni doping in ZnO could be achieved to a small concentration ͑2 mol % ͒. Diffuse reflectance spectroscopy of doped ZnO showed the existence of d-d transition bands at 430, 580, and 655 nm which are characteristic of Ni ͑II͒ with tetrahedral symmetry. Resistivity was found to be activated. The value of activation energy of undoped ZnO was about 90 meV. It was found to decrease to 60 meV for Zn 0.99 Ni 0.01 O and to 10 meV for Zn 0.98 Ni 0.02 O. The decrease in resistivity was found to be in accordance with the impurity d-band splitting model.
Influence of Cu doping on the structural, electrical and optical properties of ZnO
Pure and Cu-doped zinc oxide (ZnO) nanoparticles were prepared using a chemical method. The dopant concentration (Cu/Zn in atomic percentage (wt%)) is varied from 0 to 3 wt%. Structural characterization of the samples performed using X-ray diffraction (XRD) confirmed that all the nanoparticles of zinc oxide are having polycrystalline nature. Morphological studies were conducted using field emission scanning electron microscopy (FESEM) to confirm the grain size and texture. Electrical measurements showed that the AC conductivity initially decreases and then rises with increasing Cu concentration. The UV–Vis studies showed absorbance peaks in the 200– 800 nm region. It is found that the absorbance does not significantly change with doping. This fact is further confirmed from the band-gap calculations using the reflectance graphs. When analysed in terms of Burstein–Moss shift, an increase of band gap from 3.42 to 3.54 eV with increasing Cu concentration is observed. In the photoluminescence (PL) studies a red-shift is observed with increasing dopant concentration.
Metal doped ZnO (MZO, metal = Ag, Cd, Cu) with different metal ion doped concentrations were synthesized by sol-gel method. The structural and optical properties were characterized by UV–Vis spectroscopy and Fourier transform infrared (FTIR). With metal ion doping content increase, a red shift in band gap is observed. The red shift in band edge absorption peak in UV-Vis absorbance spectrum with increasing metal content also confirm the doping of metal in ZnO nanostructure. The band gap of ZnO resistivity was also changed according to the metal doping amounts and a kind of dopant. The results showed that each metal ion that has closer ionic radius to Zn2+ could change optical band gap more than other.
Fundamentals of zinc oxide as a semiconductor
Reports on Progress in Physics, 2009
In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.
Optical Properties of ZnO and Related Compounds Transparent Conductive Zinc Oxide
2008
Upon alloying with MgO or CdO, the fundamental band gap of ZnO can be shifted to higher or lower energies, respectively (Tab. 3.1) . Furthermore, the electrical n-type conductivity of ZnO can be controlled over many orders of magnitude by doping with Al or Ga . On the other hand, reproducible p-type conductivity in ZnO is still a challenge. Doping with group-I elements (Li, Na, K, etc.), which are supposed to substitute the Zn-atoms, or doping with group-V elements (N, P, As, Sb, etc.), which are supposed to substitute the O-atoms, are promising pathways towards p-type conductivity . Upon alloying with Mn or other transition metals, ZnO can reveal ferromagnetic properties with a Curie temperature above . Essential for the performance of the above addressed materials is the knowledge of fundamental properties.