Solvent-dependent tuning of blue–green emission of chemically synthesized ZnO nanomaterials with high colour purity and electroluminescence efficiency

A series of nanomaterials of ZnO have been synthesized via chemical co-precipitation method with fixed proportions of precursors and varied solvents. X-ray diffraction confirms wurtzite crystalline structure with nanometric crystallites (< 23 nm). Texture coefficient of crystallographic orientations show remarkable change for switching the solvents from water to alcohol. Morphological study reveals nanomaterials resembling prolate, sphere and oblate shaped structures for the solvents water, methanol and ethanol, respectively, with increasing particle size. All the nanomaterials show a similar absorption band in the UV region; though, more absorption covering a wider region in visible range is observed for nanomaterials prepared in alcoholic solutions. Red shifting in band gap of nanomaterials is correlated with band-tail effect. Variation in Urbach energy indicates that the nature of solvent plays a vital role in creating defects in ZnO, justifying enhanced absorptions in visible region. Photoluminescence (PL) spectra show various emission bands consisting of blue, green and yellow emissions corresponding to different intrinsic defects in nanomaterials. PL displays a tuning trend for blue–green emission by changing solvent from water to alcohol. However, overall enhanced PL intensity and particularly intense blue emission have been achieved by replacing water with alcohol. Tunability in emission colours and high colour purity is observed in the CIE chromaticity analysis. Theoretically, estimated electroluminescence of ZnO prepared in alcohol shows superiority compared to ZnO prepared in water. The mechanism of solvent-mediated defect creation and emission in ZnO will be beneficial for future QD LED applications.

     

Comparative study of structural and optical properties of ZnO nanostructures prepared by three different aqueous solution methods

In the present work, zinc oxide (ZnO) nanoparticles with different morphologies and sizes were successfully synthesized via three different aqueous solution routes named proteic sol–gel, PVA-assisted sol–gel and microwave-assisted hydrothermal method. Sol–gel samples were crystallized into hexagonal structure after calcination at 350 °C, presenting uniform growth and predominantly spherical particles. On the other hand, the sample produced via hydrothermal method assumed nanorod morphology, probably due to the adsorption of ammonium on the surface of ZnO nuclei, which affect the growth orientation of the crystals. All the samples exhibited a sharp UV emission peak, related to excitonic recombination, and a broad emission band in the visible region, attributed to internal transitions in color centers. Sol–gel samples calcined at the lowest temperatures presented an UV emission intensity that was 44 and 89 times higher than the visible emission, which can be related to the passivation of the defects by hydrogen ions. As-prepared hydrothermal sample presented a broad emission band centered at approximately 596 nm, which is possibly related to OH groups adsorbed on the particle surfaces. Nevertheless, the emission band of samples calcined above 800 °C was shifted to 540 nm, which is probably related to oxygen vacancy according to the results from chemical analysis.     

Enhanced free exciton and direct band-edge emissions at room temperature in ultrathin ZnO films grown on Si nanopillars by atomic layer deposition

Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films grown on silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200 °C, an intensive UV emission corresponding to free-exciton recombination (~3.31 eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25 °C, albeit the appearance of the defect-associated visible emission, the UV emission peak was observed to shift by ~60 meV to near the direct band edge (3.37 eV) recombination emission. The high-resolution transmission electron microscopy (HRTEM) showed that the ZnO films obtained at 25 °C were consisting of ZnO nanocrystals with a mean radius of 2 nm embedded in a largely amorphous matrix. Because the Bohr radius of free-exictons in bulk ZnO is ~2.3 nm, the size confinement effect may have occurred and resulted in the observed direct band edge electron-hole recombination. Additionally, the results also demonstrate order of magnitude enhancement in emission efficiency for the ZnO/Si-NP structure, as compared to that of ZnO directly deposited on Si substrate under the same conditions.     

Optical and structural characteristics of ZnO thin films grown by rf magnetron sputtering

Nanostructured ZnO thin films on Pyrex glass substrates were deposited by rf magnetron sputtering at different substrate temperatures. Structural features and surface morphology were studied by X-ray diffraction and atomic force microscopy analyses. Films were found to be transparent in the visible range above 400 nm, having transparency above 90%. Sharp ultraviolet absorption edges around 370 nm were used to extract the optical band gap for samples of different particle sizes. Optical band gap energy for the films varied from 3.24 to 3.32 eV and the electronic transition was of the direct in nature. A correlation of the band gap of nanocrystalline ZnO films with particle size and strain was discussed. Photoluminescence emission in UV range, which is due to near band edge emission is more intense in comparison with the green band emission (due to defect state) was observed in all samples, indicating a good optical quality of the deposited films.     

Template-free solvothermal synthesis of ZnO nanoparticles with controllable size and their size-dependent optical properties

ZnO nanoparticles (NPs) have been synthesized via a facile and template-free solvothermal method. The size of ZnO NPs could be tailored by adjusting the ratio of ethanol to ethylene glycol (EG). Their structure and morphology have been investigated. The as-prepared samples are monodispersed ZnO NPs with controllable sizes of about 24.2, 18.9 and 14.7 nm. The cathodoluminescence (CL) spectra of the samples show that the relative intensity ratio of the visible emission peak at 500-650 nm to the band-edge UV emission peak at 380 nm increases as the particle size decreases. Sample with smaller crystallites would have larger surface area and more oxygen vacancy defects, thus it exhibits higher visible emission peak. The UV-vis absorption spectrum indicates the band gap variation of the ZnO NPs with their size. Moreover, the size-dependent blue shifts of both the CL emission and the UV-vis absorption spectra reveal the effect of quantum confinement.     

Sol–gel derived N-doped ZnO thin films

N-doped ZnO (NZO) thin films have been prepared by a sol–gel method and their electrical and optical properties have been investigated. The prepared NZO films were p-type, and had excellent electrical properties. They had an optical transparency above 85% in the visible range. The UV absorption edge was red-shifted with increasing N-doping concentration. Two emission bands were observed in the photoluminescence (PL) spectra, with one band located in the UV range and the other band consisting of green luminescence. Both UV and green emissions were enhanced with increasing N-doping concentration.     

Morphology and optical properties of ZnO particles modified by diblock copolymer

Biprism, rugby cone and triangle-like shapes of ZnO particles were synthesized directly from an aqueous zinc acetate dihydrate solution in the presence of poly(ethylene oxide)-b-poly(propylene oxide) copolymer and sodium hydroxide at a pH of 8, 10 and 12, respectively. The particle sizes of their ZnO particles decreased with an increase of pH values. Furthermore, it had been found that the estimated band gap value and the emission peak in the UV region showed a blue shift as a dependence on their particle sizes.     

A general combustion approach to multipod ZnO and its characterization

By a general approach of combustion oxidation at high temperature, multipod ZnO was synthesized without any catalysts or additives. The morphology and optical properties of the multipod ZnO were studied in detail. The growth mechanism was discussed preliminarily. An ultraviolet (UV) emission peak at 374 nm and a broad green emission peak centered at 502 nm are observed in photoluminescence spectrum of the multipod ZnO. The multipod structure exhibits significant enhancement of UV emission intensity and green light emission intensity compared with the tetrapod structure, which are attributed to less structural defects and increased surface area respectively. Furthermore, compared with nano-particle and micro-particle ZnO, UV emission peak of multipod ZnO appears a slight blue shift. Due to slim tips of the legs, quantum size effect cause a slight blue shift of UV emission peak. We believe that these optical properties of the multipod structure have extensive applications in nanoscale optical devices.     

Galvanic deposition of ZnO using mixed electrolyte and their photoluminescence properties

The effects of Zn(OAc)₂ concentrations and chemical nature of supporting electrolytes on the galvanic deposition of ZnO have been investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive X-ray (EDX) microanalysis. The results show that the taper-like ZnO crystals are apt to be produced at lower Zn(OAc)₂ concentrations, while the rod-like ZnO crystals tend to be grown at higher Zn(OAc)₂ concentrations. The photoluminescence of as-prepared ZnO nanorods shows that there exist a strong UV emission band, a broad blue band at 468 nm, and a very weak green band at 550 nm. The blue-shift of UV emission is attributed to the Cl doping of ZnO in chloride electrolyte.     

Characteristics of nanostructured ZnO layers deposited in spray plasma device

ZnO and Al doped ZnO thin film have been deposited on glass substrate by "spray plasma" process using an aqueous solution of Zn(NO3)2. XRD patterns revealed polycrystalline character with the typical hexagonal würtzite structure. The preferential c-axis orientation of crystallites depends highly on the operating conditions. Willamson-Hall method and AFM photographs showed a size of crystallites ranging between 20 and 80 nm and a roughness between 6 and 50 nm. Films exhibit a transmittance between 65 and 90% in the visible region. XPS revealed that the stoichiometry of deposited ZnO is Zn/O = 0.93. Fluorescence spectroscopy showed the presence of two bands at 360 nm (UV) and at 410 nm (Blue). The UV band can be attributed to exiton emission. Another important result concerns the non-existence of a "green" band at 500 nm.     

Thermal annealing induced structural and optical properties of Ca doped ZnO nanoparticles

In this letter, the effects of annealing on structural and optical properties of Ca doped ZnO nanoparticles have been investigated. X-ray diffraction analysis reveals that the prepared particles are in hexagonal wurtzite structure and formation of secondary phase related to the Calcite was found after thermal annealing. UV–Vis measurements show free exciton absorption band appeared at 372 nm and increase of band gap with annealing of samples. Room temperature photoluminescence (PL) spectrum of the prepared Ca doped ZnO nanoparticles shows bands which belong to the near band edge emission at 377 nm and green emission at 556 nm. Annealed samples exhibit enhancement in the blue emission band. Raman spectra show the increment in the electron–phonon coupling value with annealing.     

ZrO2和ZrO2:Dy3+纳米晶发光特性

采用液相沉淀法制备了不同掺杂浓度和尺寸的ZrO2和ZrO2:Dy纳米晶,研究了其发光特性.结果表明在纳米ZrO₂中,存在着宽带激发和发射,起源为电子在价带和导带之间的跃迁.在Dy³⁺掺杂的样品中,随着颗粒尺寸的长大,其发光增强.并且共掺杂Li⁺的样品发光强度被极大地提高.随着Dy³⁺浓度的变化,黄发射和蓝发射的强度比(Y/B)发生改变,且浓度猝灭是通过近邻激活剂间的交换作用进行的.     

Augmentation of band gap and photoemission in ZnO by Li doping

The monovalent impurity Lithium is chosen to dope with Zinc oxide (ZnO) in four concentrations by auto-combustion route. The influence of Li on the structural and optical properties of ZnO are discussed. The Li incorporation happens both as substitution and interstitial doping with an increase of grain size and the optical band gap of ZnO. The optical phonon modes are identified from Raman spectra that also gives information about the stress in the samples. The UV and visible emission characteristics of the samples are found from the fluorescence spectra. The origin of the visible emission is explained by defect chemistry. When Li lodges Zn site new acceptor levels of Li are created that causes the yellow emission that is absent in undoped ZnO. Li interstitial creates Zn interstitials that are responsible for blue emission. The green emission is explained as the outcome of the transition between Zn_i and oxygen vacancies.     

Optical characterization of ZnO

The green (2.19 eV) and yellow (2.00 eV) luminescence bands in ZnO polycrystalline samples were studied by photoluminescence, excitation luminescence, time-resolved spectroscopy and lifetime measurements. A shift towards higher energies of the green emission band is observed for temperatures above 35 K indicating that at least two excited levels 74 meV apart are involved in the recombination process.     

Synthesis,structural and optical characterization of Ni-doped ZnO nanoparticles

Nanocrystalline Zn_1−x Ni _x O (x = 0.00, 0.02, 0.04, 0.06, 0.08) powders were synthesized by a simple sol–gel autocombustion method using metal nitrates of zinc, nickel and glycine. Structural and optical properties of the Ni-doped ZnO samples annealed at 800 °C are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis using X-rays (EDAX), UV–visible spectroscopy and photoluminescence (PL). X-ray diffraction analysis reveals that the Ni-doped ZnO crystallizes in a hexagonal wurtzite structure and secondary phase (NiO) was observed with the sensitivity of XRD measurement with the increasing nickel concentration (x ≥ 0.04). The lattice constants of Ni-doped ZnO nanoparticles increase slightly when Ni²⁺ is doped into ZnO lattice. The optical absorption band edge of the nickel doped samples was observed above 387 nm (3.20 eV) along with well-defined absorbance peaks at around 439 (2.82 eV), 615(2.01 eV) and 655 nm (1.89 eV). PL measurements of Ni-doped samples illustrated the strong UV emission band at ~3.02 eV, weak blue emission bands at 2.82 and 2.75 eV, and a strong green emission band at 2.26 eV. The observed red shift in the band gap from UV–visible analysis and near band edge UV emission with Ni doping may be considered to be related to the incorporation of Ni ions into the Zn site of the ZnO lattice.     

Optoelectronics behaviour of ZnO nanorods for UV detection

In this study ZnO nanorods have been synthesized by a chemical precipitation method. The room temperature UV–Vis absorption spectra of the ZnO nanorods indicated two absorption peaks in the UV region, one in the near UV region and the other attributed to the band gap of ZnO. The Photoluminescence spectra of ZnO nanorods show two emission bands, one ultraviolet emission band at 378 nm and the other in the defect related yellow emission band near 550 nm. The stimulated yellow luminescence of ZnO nanorods were affected by the synthesis time and annealing temperature. The same ZnO nanorods were deposited onto the ITO substrate to form a UV photoconductive detector. The ratio of the UV photogenerated current to dark current was as high as nine times under 3 V bias. Hence, these nanorods can be promising materials in the use of UV radiation detection.     

Vertical growth and resonator properties of hexagonally shaped zinc oxide nanonails

We synthesized vertically aligned nail-shaped ZnO nanocrystal arrays on silicon substrates via a combination of a carbothermal reduction method and textured ZnO seeding layers that were precoated on silicon substrates by thermally decomposing zinc acetate, and studied their optical properties using cathodoluminescence (CL) and photoluminescence techniques. The ZnO nanonails show a sharp band-gap edge UV emission and a defect-related broad green emission. Monochromatic CL images of an individual ZnO nanonail show variations in spatial distributions of respective CL bands that had different origins. We attribute the spatial variation of CL images to an uneven distribution of luminescent defects and/or a structure-related light out-coupling from hexagonal ZnO nanostructures. The most distinct CL feature from the hexagonal head of an individual ZnO nanonail was the occurrence of a series of distinct resonant peaks within the visible wavelength range. It appeared that the head of a nanonail played the role of a hexagonal cavity so that polarization-dependent whispering gallery modes were stimulated by electron beam excitation.     

Structural and optoelectronic characterization of prepared and Sb doped ZnO nanoparticles

This paper briefly reports the structural and optoelectronics properties of prepared pure and Sb doped ZnO nanoparticles. Doping with suitable elements offers an efficient method to control and enhance the optical properties of ZnO nanoparticles, which is essential for various optoelectronics applications. Sb doped ZnO nanoparticles have significant concern due to their unique and unusual electrical and optical properties. In the present work, we report the synthesis of Sb doped ZnO successfully with average particle size range from 26 to 29 nm via direct precipitation method. The phase purity and crystallite size of synthesized ZnO and Sb doped nano-sized particles were characterized and examined via X-ray diffraction (XRD) and scanning electron microscopy (SEM). The elemental analyses of undoped and doped ZnO nanoparticles were examined by using energy-dispersive X-ray spectroscopy (EDAX).We investigated and measured the optoelectronics properties of synthesized ZnO and Sb doped ZnO nanoparticles by employing photoluminescence and UV–Visible spectroscopy. The influence of Sb doping on photoluminescence (PL) spectra of ZnO nanoparticles, which consists of UV emission and broad visible emission band, is found to be strongly dependent upon the Sb concentration for all the Sb doped ZnO nanoparticles samples under investigation. The UV–Visible absorption study shows an increase in band gap energy as Sb is incorporated on the ZnO nanoparticles.     

纳米氧化锌的制备及其光致发光性能研究

采用低温蒸发和气相输运方法,研制出ZnO的纳米棒－纳米钉、纳米钉类球状聚集体和具有钉状结构的纳米梳。利用透射电子显微镜和X射线衍射仪对其形貌与结构进行了表征,阐明了ZnO纳米结构的生长机理。用荧光光谱仪测量了退火前后纳米棒－纳米钉结构的光致发光谱,结果显示,在380nm处两者均存在近紫外峰,在500nm处出现绿色宽谱峰,而退火2h后近紫外发光峰窄而强,且发生了少许蓝移,此现象可归因于退火后的ZnO纳米结构具有较低的点缺陷浓度。     

Tunable Spectrum Selectivity for Multiphoton Absorption with Enhanced Visible Light Trapping in ZnO Nanorods

Observation of visible light trapping in zinc oxide (ZnO) nanorods (NRs) correlated to the optical and photoelectrochemical properties is reported. In this study, ZnO NR diameter and c‐axis length respond primarily at two different regions, UV and visible light, respectively. ZnO NR diameter exhibits UV absorption where large ZnO NR diameter area increases light absorption ability leading to high efficient electron–hole pair separation. On the other hand, ZnO NR c‐axis length has a dominant effect in visible light resulting from a multiphoton absorption mechanism due to light reflection and trapping behavior in the free space between adjacent ZnO NRs. Furthermore, oxygen vacancies and defects in ZnO NRs are associated with the broad visible emission band of different energy levels also highlighting the possibility of the multiphoton absorption mechanism. It is demonstrated that the minimum average of ZnO NR c‐axis length must satisfy the linear regression model of Z _p,min = 6.31d to initiate the multiphoton absorption mechanism under visible light. This work indicates the broadening of absorption spectrum from UV to visible light region by incorporating a controllable diameter and c‐axis length on vertically aligned ZnO NRs, which is important in optimizing the design and functionality of electronic devices based on light absorption mechanism.