Structural and optical properties of Mn-doped ZnO nanocrystalline thin films with the different dopant concentrations

The transparent nanocrystalline thin films of undoped zinc oxide and Mn-doped (Zn_1−xMn_xO) have been deposited on glass substrates via the sol–gel technique using zinc acetate dehydrate and manganese chloride as precursor. The as-deposited films with the different manganese compositions in the range of 2.5–20 at% were pre-heated at 100 °C for 1 h and 200 °C for 2 h, respectively, and then crystallized in air at 560 °C for 2 h. The structural properties and morphologies of the undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/Vis spectroscopy. The analyzed results indicates that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn related phases. Room temperature photoluminescence is observed for the ZnO and Mn-doped ZnO thin films.     

Synthesis of nanocrystalline ZnO nanobelts via pyrolytic decomposition of zinc acetate nanobelts and their gas sensing behavior

The pyrolytic decomposition of layered basic zinc acetate (LBZA) nanobelts (NBs) into nanocrystalline ZnO NBs is investigated using scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). We also report on the gas sensing response of the resulting ZnO nanomaterial to CO. The LBZA NBs are grown at 65 °C in an aqueous solution of zinc acetate dihydrate. AFM and SEM results show as-grown products possess the characteristic layered structure of the LBZA crystals. XRD and XPS results show that annealing as-grown products at 210 °C in air causes a transformation from zinc acetate to nanocrystalline ZnO NBs via thermal decomposition. The ZnO crystalline domain size increases with temperature from 9.2 nm at 200 °C to 94 nm at 1000 °C, as measured from XRD. SEM shows evidence of sintering at 600 °C. The thickness of the NBs, determined via AFM, ranges from 10 to 50 nm and remains approximately constant with annealing temperature. XPS confirmed the chemical transformation from zinc acetate to ZnO and showed a significant remaining zinc hydroxide component for the ZnO NBs consistent with published results. PL measurements at room temperature show a blue shift in peak emission as the nanobelts change from LBZA to ZnO at 200 °C. Above this transition temperature, the ZnO nanobelts possess strong band edge emission at 390 nm and little broad band emission in the visible region. The AFM and SEM images reveal that the crystallites within the nanobelts orientate in rows along the long axis during annealing. This structure provides a high surface area to volume ratio of aligned nanoparticles which is beneficial for gas sensing applications. Gas sensors fabricated from 400 °C annealed nanobelts showed a response of 1.62 when exposed to 200 ppm of CO in dry air at 400 °C, as defined by the ratio of resistance before and during exposure. This indicates that ZnO nanostructures obtained by thermal decomposition of LBZA NBs could provide a cost effective route to high sensitivity gas sensors.     

High c-axis preferred orientation ZnO thin films prepared by oxidation of metallic zinc

Thin films of zinc oxide were grown on glass substrates by thermal oxidation. The metallic zinc films were thermally oxidized at different temperatures ranging from 300 to 600 °C to yield ZnO thin films. The structural property of the thin films was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The X-ray diffraction measurements showed that the films oxidized at 300 °C were not oxidized entirely, and the films deposited at 600 °C had better crystalline quality than the rest. When the oxidation temperature increased above 400 °C, the films exhibited preferred orientation along (002) and high transmittance ranging from 85% to 98% in vis–near-infrared band. Meanwhile, the films showed a UV emission at about 377 nm and green emission. With the increasing of oxidation temperature, the intensity of green emission peak was enhanced, and then decreased, disappearing at 600 °C, and the case of UV emission increased. Furthermore, a strong green emission was observed in the film sintered in pure oxygen atmosphere.     

Spin coating-pyrolysis derived highly c-axis-oriented ZnO layers pre-fired at various temperatures

Transparent ZnO layers were prepared on silica glass substrates by the spin coating-pyrolysis process. As-deposited films were pre-fired at 250 °C for 60 min, at 350 °C for 30 min, and at 500 °C for 10 min, followed by heat treatment at 900 °C for 30 min in air. The ZnO films were analyzed by high resolution X-ray diffraction, field emission-scanning electron microscopy, scanning probe microscopy, and ultraviolet–visible–near infrared spectrophotometry. (0 0 2)-oriented ZnO films were obtained by pre-firing at 350 °C and at 500 °C. All the ZnO films exhibited a high transmittance, above 80%, in the visible region, and showed a sharp fundamental absorption edge at 0.38–0.40 μm. The most highly c-axis-oriented ZnO with a homogeneous surface was observed at a pyrolysis temperature of 350 °C.     

Comparative study on structural and optical properties of ZnO thin films prepared by PLD using ZnO powder target and ceramic target

Zinc oxide thin films have been obtained in O₂ ambient at a pressure of 1.3 Pa by pulsed laser deposition (PLD) using ZnO powder target and ceramic target. The effect of temperature on structural and optical properties of ZnO thin films was investigated systematically by XRD, SEM, FTIR and PL spectra. The results show that the best structural and optical properties can be achieved for ZnO thin film fabricated at 700 °C using powder target and at 400 °C using ceramic target, respectively. The PL spectrum reveals that the efficiency of UV emission of ZnO thin film fabricated by using powder target is low, and the defect emission of ZnO thin film derived from Zn_i and O_i is high.     

Optical properties and structural characteristics of ZnO thin films grown on a-plane sapphire substrates by plasma-assisted molecular beam epitaxy

We investigated structural and optical properties of ZnO thin films grown on (112?0) a-plane sapphire substrates using plasma-assisted molecular beam epitaxy. Negligible biaxial stress in ZnO thin films is due to the use of (112?0) a-plane sapphire substrates and slow substrate cooling. The 14 K photoluminescence spectrum shows a blueshift of energy positions compared with ZnO single crystal. A donor with binding energy of 43 meV and an acceptor with binding energy of ～170 meV are identified by well-resolved photoluminescence spectra. A characteristic emission band at 3.320 eV (so-called A-line) is studied. Based on analysis from photoluminescence spectra, the origin of the A-line, it seems, is more likely an (e, A°) transition, in which defect behaves as an acceptor. The room-temperature photoluminescence is dominated by the FX at 3.307 eV, which is an indication of strongly reduced defect density in ZnO thin films.     

Effect of annealing temperature on the characteristics of ZnO thin films

Effect of annealing temperature on characteristics of sol–gel driven ZnO thin film spin-coated on Si substrate was studied. The UV–visible transmittance of the sol decreased with the increase of the aging time and drastically reduced after 20 days aging time. Granular shape of ZnO crystallites was observed on the surface of the films annealed at 550, 650, and 750 °C, and the crystallite size increased with the increase of the annealing temperature. Consequently nodular shape of crystallites was formed upon increasing the annealing temperature to 850 °C and above. The current–voltage characteristics of the Schottky diodes fabricated with ZnO thin films with various annealing temperatures were measured and analyzed. It is found that, ZnO films showed the Schottky characteristics up to 750 °C annealing temperature. The Schottky diode characteristics were diminished upon increasing the annealing temperature above 850 °C. XPS analysis suggested that the absence of oxygen atoms in its oxidized state in stoichiometric surrounding, might be responsible for the diminished forward current of the Schottky diode when annealed above 850 °C.     

Physical properties of ZnSe thin films: Air and vacuum annealing evolution to buffer layer applications

In order to develop high efficiency solar cell device by replacing conventional hazardous CdS window layer by environmental friendly Zn-based buffer layer, ZnSe thin films of thickness 100 nm were grown on glass and ITO substrates employing electron beam evaporation technique followed by air and vacuum annealing at temperature 100 °C, 200 °C and 300 °C. As-grown and annealed films were subjected to characterization tools like XRD, UV-Vis spectrophotometer, SEM, EDS and source meter. Structural results reveal the amorphous phase, SEM images indicate uniform deposition without pin holes and EDS patterns confirm the deposition. Transmittance is observed to be high in visible region and band gap is found to change with temperature of the treatment and I-V measurements demonstrate ohmic nature. On the basis of optimized results, the films annealed at 200 °C in vacuum may be used as buffer layer to develop high efficiency Cd-based and CIGS thin film solar cells.     

Strong temperature and substrate effect on ZnO nanorod flower structures in modified chemical vapor condensation growth

We have reported low temperature growth (300 °C) of ZnO nanorod flower structures by depositing zinc acetate vapor on Ge (100) substrate in the form of a jet using chemical vapor condensation technique. The flowers were comprised of hierarchical arrangement of highly crystalline ZnO nanorods oriented isotropically around a common nucleus. The temperature window for stability of these structures was found to be very narrow and the formation of the flowers was highly depended on the type of the substrates used. The flower morphology changed to a different hemispherical shape when the growth temperature was increased by only 50 °C while decreasing the growth temperature of the same degrees resulted in an amorphous deposition of ZnO. The temperature and substrate effect has been explained on the basis of adatom kinetics during growth. X-ray diffraction and TEM study revealed wurtzite ZnO nanorods with lattice constants a and c of 3.2 and 5.19 Å, respectively. The flower structures showed strong room temperature photoluminescence having pure excitonic transition at around 3.298 eV.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Photoluminescence study of Ce-doped silica films

Ce-doped silica films with different Ce concentrations were prepared by ion-beam sputtering and ion implantation. The films containing 1.2 at% Ce were annealed at temperatures from 500 °C to 1200 °C in air ambient, and were annealed in different ambient at 1100 °C. Ce-related photoluminescence was observed in films sensitive to the Ce concentrations, annealing temperatures and the annealing ambient. The peak intensity of the photoluminescence band is approximately linear with Ce concentrations. Also, the photoluminescence intensity is dependent on the annealing temperatures and reaches its highest value after annealing at 700 °C. In addition, the experimental results show that compared with the annealing in an air ambient, the photoluminescence intensity can be enhanced with nitrogen gas. There would be no obvious change for the photoluminescence position or shape.     

Effect of phosphorus irradiation on the structural,electrical, and optical characteristics of ZnO thin films

Phosphorus irradiation at a low energy (50 keV) and at a dosage of 8×10¹⁴ ions/cm² was carried out on 〈002〉 ZnO films grown by using a pulsed laser deposition technique (Sample A). Subsequent rapid thermal annealing at 650 °C and 750 °C was performed to remove defects resulting from the irradiation (samples B and C, respectively). Atomic force microscopy was used to determine the root mean square roughness, which was 10.07, 8.66, and 9.31 nm for samples A, B, and C, respectively. Low-temperature photoluminescence measurements revealed increased deep-level defect peaks following irradiation; however, the subsequent annealing minimized the defects. Although the dominant donor-bound exciton peak verifies the n-type conductivity of the films, the free–electron–to–acceptor and donor-to-acceptor pair peaks in the irradiated samples confirm an increase in acceptor concentration.     

Fabrication of PbS thin films composed of highly (200)-oriented nano-/micro-rods in deep eutectic solvent

PbS thin films composed of highly (200)-oriented shuttle-like nano-/micro-rods were successfully fabricated on glass substrates by the environment friendly ionothermal method at 140 °C in deep eutectic solvent (DES). The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), ultraviolet-visible (UV–vis) and photoluminescence (PL) spectra, respectively. The possible mechanism of the oriented growth of PbS nano-/micro-rods was discussed. The PbS thin films composed of shuttle-like nano-/micro-rods exhibited a large absorbance property in the wavelength range of 350–1100 nm, and moreover, the PL spectrum had a broad emission band centered at 490 nm. The shuttle-like PbS nano-/micro-rods-based thin films might have potential application in solar cells.     

The study of structure and optoelectronic properties of ZnS and ZnO films on porous silicon substrates

ZnS and ZnO films were prepared on porous silicon (PS) substrates with the same porosity by pulsed laser deposition (PLD), and the structural, optical and electrical properties of ZnS and ZnO films on PS were investigated at room temperature by X-ray diffraction (XRD), scanning electron microscope (SEM), optical absorption measurement, photoluminescence (PL) and I–V characteristic studies. The prepared ZnS was obtained in the cubic phase along β-ZnS (1 1 1) orientation which showed a perfect match with the earlier report while ZnO films were obtained in c-axis orientation. There appeared some cracks in the surface of ZnS and ZnO films due to the roughness of PS substrates. Luminescence studies of ZnS/PS and ZnO/PS composites indicated room temperature emission in a broad, intense, visible photoluminescence band, which cover the blue emission to red emission, exhibiting intensively white light emission. Based on the I–V characteristic, ZnS/PS heterojunction exhibited the rectifying junction behavior, while the I–V characteristic of ZnO/PS heterostructure was different from that of the common diode, whose reverse current was not saturated.     

Effect of the Si excess on the structure and the optical properties of Nd-doped Si-rich silicon oxide

Nd-doped Si-rich silicon oxide thin films were produced by radio frequency magnetron co-sputtering of three confocal cathodes: Si, SiO₂, and Nd₂O₃, in pure argon plasma at 500 °C. The microstructure and optical properties of the films were investigated versus silicon excess and post-deposition annealing treatment by means of ellipsometry and Fourier transform infrared spectrometry as well as by the photoluminescence method. A notable emission from Nd³⁺ ions was obtained for the as-deposited sample, while the films annealed at 900 °C showed the highest peak intensity. The maximum emission was observed for the films with 4.7 at% of Si excess.     

PPV/PVA/ZnO nanocomposite prepared by complex precursor method and its photovoltaic application

In this article, we describe a new method to prepare a ZnO and conjugated polymer nanocomposite and its application in bulk-heterojunction solar cells. The composite was composed of zinc oxide (ZnO) and poly(phenylene vinylene)/poly(vinyl alcohol) (PPV/PVA). For the preparation, the composite was prepared first through the complex reaction between Zn²⁺ ion and –OH of the PVA–PPV precursor by simply mixing zinc salts and a PVA–PPV precursor aqueous solution at 70 °C. By addition of a concentrated aqueous ammonia into the system, highly regular Zn(OH)₂ nanodots were formed and dispersed in the PVA/PPV precusor mixed solution. The PVA/PPV precursor can well bind Zn²⁺ ion through complex interaction, so act as a template to direct the distribution of ZnO in the process. The nanocomposite films were finally obtained by solution casting and subsequently treated by heating samples at 160 °C for 6 h. TEM observations showed that ZnO nanodots uniformly dispersed in PVA–PPV mixtures. The resulting nanocomposite films possess a large interfacial area between the electron donor and acceptor of the bulk-heterojunction. Improved charge seperation and collection are evidenced by the large photoluminescence intensity difference between pure PPV and composites films, which result in the increase in both open circuit voltage and short circuit current of the hybrid solar cells.     

White-light emission from annealed ZnO:Si nanocomposite thin films

As grown ZnO:Si nanocomposites of different compositional ratios were fabricated by thermal evaporation techniques. These films were subjected to post-deposition annealing under high vacuum at a temperature of 250 °C for 90 min. The photoluminescence (PL) spectra of annealed samples have shown marked improvements both in terms of intensity and broadening. Structural and Raman analyses show formation of a Zn–Si–O shell around ZnO nanoclusters wherein on heating Zn₂SiO₄ compound forms resulting in huge UV, orange and red peaks at 310, 570 and 640 nm in PL. The new emissions due to Zn₂SiO₄ completes white light spectrum. The study not only suggests that 1:2 ratio is the best suited for material manipulation but also shows process at the interface of ZnO nanoclusters and silicon matrix leads to new PL emissions.     

Low-temperature synthesis of Zn2SiO4:Mn green photoluminescence phosphor

Zn₂SiO₄:Mn green phosphor having comparable photoluminescence (PL) efficiency with commercial phosphor has been synthesized at 1000 °C using solid state reactions involving ZnO, silicic acid and manganese acetate. The water of crystallization attached to SiO₂ in silicic acid whose dissociation at 1000 °C seem to promote the sintering efficiency of Zn₂SiO₄:Mn. Incremental ZnO addition and re-firing at 1000 °C promote the diffusion rate of ZnO and SiO₂. The formation of a single crystalline phase of willemite structure in the samples was confirmed by powder XRD measurements. The phosphor exhibit an intense excitation band centered around 275 nm and a relatively weak excitation centered around 380 nm while the broad band green emission peaks at 524 nm. Other parameters studied include PL spectra, grain morphology, ZnO/SiO₂ molar ratio, Mn concentration, co-dopant/flux and the effect of chemical forms of Mn dopant as well as silica on the PL efficiency.     

ESR spectra and thermal diffusivity of Zn–Al layered double hydroxide

Zn–Al–NO₃–LDH was synthesized using the co-precipitation method at pH 7±0.1 and ratio Zn/Al=4. The heat treatment of LDH was studied by X-ray diffraction (XRD) and thermogravimetric analysis (TGA/DTG) to investigate the stability of the LDH structure. The in situ electron spin resonance (ESR) spectra of fresh LDH from room temperature up to 190 °C were obtained, which are due to the presence of nitrate radicals in LDH interlayer. ESR spectra of sintered LDH below 200 °C (ex situ ESR spectra) were investigated, which are also due to the nitrate radicals. However, at 200 °C and above, spectra were due to the oxygen vacancies of ZnO, which was formed during the thermal treatment of LDH. Thermal diffusivity of LDH as a function of in situ temperatures results in a nonlinear relation, which is due to the changing water content of LDH when temperature increases. However, thermal diffusivity of LDH as a function of sintered temperatures showed a linear relation and the slope of these data demonstrated the dependency between thermal diffusivity and water content of LDH below 200 °C. For temperature above 180 °C, the thermal diffusivity behavior was mainly due to the ZnO phase in LDH.     

Structural evolution,growth mechanism and photoluminescence properties of CuWO4 nanocrystals

Copper tungstate (CuWO₄) crystals were synthesized by the sonochemistry (SC) method, and then, heat treated in a conventional furnace at different temperatures for 1 h. The structural evolution, growth mechanism and photoluminescence (PL) properties of these crystals were thoroughly investigated. X-ray diffraction patterns, micro-Raman spectra and Fourier transformed infrared spectra indicated that crystals heat treated and 100 °C and 200 °C have water molecules in their lattice (copper tungstate dihydrate (CuWO₄·2H₂O) with monoclinic structure), when the crystals are calcinated at 300 °C have the presence of two phase (CuWO₄·2H₂O and CuWO₄), while the others heat treated at 400 °C and 500 °C have a single CuWO₄ triclinic structure. Field emission scanning electron microscopy revealed a change in the morphological features of these crystals with the increase of the heat treatment temperature. Transmission electron microscopy (TEM), high resolution-TEM images and selected area electron diffraction were employed to examine the shape, size and structure of these crystals. Ultraviolet–Visible spectra evidenced a decrease of band gap values with the increase of the temperature, which were correlated with the reduction of intermediary energy levels within the band gap. The intense photoluminescence (PL) emission was detected for the sample heat treat at 300 °C for 1 h, which have a mixture of CuWO₄·2H₂O and CuWO₄ phases. Therefore, there is a synergic effect between the intermediary energy levels arising from these two phases during the electronic transitions responsible for PL emissions.