Structural and photoluminescent properties of Ni doped ZnO nanorod arrays prepared by hydrothermal method

Self-assembled Ni-doped zinc oxide (Zn_1−xNi_xO, x = 0.05, 0.10, 0.15, i.e., ZnNiO, nominal composition) nanorod arrays vertically grown on the ZnO seed layer covered glass along [0 0 1] direction were synthesized by hydrothermal method. Their images and structures have been characterized by scan electron microscope (SEM), X-ray diffraction (XRD) and Raman spectra, showing that Ni doping is beneficial to the formation of ZnO nanorods with hexagonal cross section and the enhancement of ZnO crystal quality. X-ray photoemission spectroscopy (XPS) study further demonstrated that Ni atoms were successfully doped into ZnO lattices. The photoluminescence (PL) spectra of ZnNiO samples show near bandedge emission (NBE) peaks at about 380 nm at a low excitation power and the NBE peak position redshifts while its intensity continuously increases with the increase of Ni doping concentration. With the excitation power increasing, the NBE peak redshifts from 380 nm to about 400 nm for ZnNiO nanorod arrays. The NBE mechanisms for ZnNiO nanorod arrays have been discussed, which is helpful for understanding their room temperature ferromagnetisms.     

Nonlinear optical properties of Au/ZnO nanoparticle arrays

The triangular-shaped Au/ZnO nanoparticle arrays were fabricated on fused quartz substrate using nanosphere lithography. The structural characterization of the Au/ZnO nanoparticle arrays was investigated by atomic force microscopy. The absorption peak due to the surface plasmon resonance of Au particles at the wavelength of about 570 nm was observed. The nonlinear optical properties of the nanoparticle arrays were measured using the z-scan method at a wavelength of 532 nm with pulse duration of 10 ns. The real and imaginary part of third-order nonlinear optical susceptibility, Re χ⁽³⁾ and Im χ⁽³⁾, were determined to be 1.15 × 10⁻⁶ and −5.36 × 10⁻⁷ esu, respectively. The results show that the Au/ZnO nanoparticle arrays have great potential for future optical devices.     

Nonlinear optical properties of periodic gold nanoparticle arrays

Periodic Au nanoparticle arrays were fabricated on silica substrates using nanosphere lithography. The identical single-layer masks were prepared by self-assembly of polystyrene nanospheres with radius R = 350 nm. The structural characterization of nanosphere masks and periodic particle arrays was investigated by atomic force microscopy. The nonlinear optical properties of the Au nanoparticle arrays were determined using a single beam z-scan method at a wavelength of 532 nm with laser duration of 55 ps. The results show that periodic Au nanoparticle arrays exhibit a fast third-order nonlinear optical response with the nonlinear refractive index and nonlinear absorption coefficient being n₂ = 6.09 × 10⁻⁶ cm²/kW and β = −1.87 × 10⁻⁶ m/W, respectively.     

Enhanced field emission from ZnO nanopencils by using pyramidal Si(1 0 0) substrates

Zinc oxide nanopencil arrays were synthesized on pyramidal Si(1 0 0) substrates via a simple thermal evaporation method. Their field emission properties have been investigated: the turn-on electric field (at the current density of 10 μA/cm²) was about 3.8 V/μm, and the threshold electric field (at the current density of 1 mA/cm²) was 5.8 V/μm. Compared with similar structures grown on flat Si substrates, which were made as references, the pyramidal Si-based ZnO nanopencil arrays appeared to be superior in field emission performance, thus the importance of the non-flat substrates has been accentuated. The pyramidal Si substrates could not only suppress the field screening effect but also improve the field enhancement effect during the field emission process. These findings indicated that using non-flat substrates is an efficient strategy to improve the field emission properties.     

Luminescent characteristics of CaTiO3:Pr thin films prepared by pulsed laser deposition method with various substrates

CaTiO₃:Pr³⁺ films were deposited on different substrates such as Al₂O₃ (0 0 0 1), Si (1 0 0), MgO (1 0 0), and fused silica using pulsed laser deposition method. The crystallinity and surface morphology of these films were investigated by XRD and SEM measurements. The films grown on the different substrates have different crystallinity and morphology. The FWHM of (2 0 0) peak are 0.18, 0.25, 0.28, and 0.30 for Al₂O₃ (0 0 0 1), Si (1 0 0), MgO (1 0 0), and fused silica, respectively. The grain sizes of phosphors grown on different substrates were estimated by using Scherrer's formula and the maximum crystallite size observed for the thin film grown on Al₂O₃ (0 0 0 1). The room temperature PL spectra exhibit only the red emission peak at 613 nm radiated from the transition of (¹D₂ → ³H₄) and the maximum PL intensity for the films grown on the Al₂O₃ (0 0 0 1) is 1.1, 1.4, and 3.7 times higher than that of the CaTiO₃:Pr³⁺ films grown on MgO (1 0 0), Si (1 0 0), and fused Sillica substrates, respectively. The crystallinity, surface morphology and luminescence spectra of thin-film phosphors were highly dependent on substrates.     

Controlled growth and field emission properties of zinc oxide nanopyramid arrays

Single-crystalline, pyramidal zinc oxide nanorods have been synthesized in a large quantity on p-Si substrate via catalyst-free thermal chemical vapor deposition at low temperature. SEM investigations showed that the nanorods were vertically aligned on the substrate, with diameters ranging from 60 to 80 nm and lengths about 1.5 μm. A self-catalysis VLS growth mechanism was proposed for the formation of the ZnO nanorods. The field emission properties of the ZnO nanopyramid arrays were investigated. A turn-on field about 3.8 V/μm was obtained at a current density of 10 μA/cm², and the field emission data was analyzed by applying the Fowler-Nordheim theory. The stability of emission current density under a high voltage was also tested, indicating that the ZnO nanostructures are promising for an application such as field emission sources.     

Effect of calcination temperature on the morphology and surface properties of TiO2 nanotube arrays

Well-ordered TiO₂ nanotube arrays were prepared by electrochemical anodization of titanium in aqueous electrolyte solution of H₃PO₄ + NH₄F at a constant voltage of 20 V for 3 h, followed by calcined at various temperatures. X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL) were used to characterize the samples. The results showed that the as-prepared nanotube arrays before being calcined were amorphous and could transform to anatase phase at a heat treatment temperature higher than 400 °C. As the calcination temperatures increased, crystallization of anatase phase enhanced and rutile phase appeared at 600 °C. However, further increasing the calcination temperature would cause the collapse of nanotube arrays. PL intensity of the nanotube arrays annealed at 500 °C was the lowest, which was probably ascribed to better crystallization together with fewer surface defects of the nanotube arrays.     

Infrared superbroadband emission of Bi ion doped germanium-aluminum-sodium glass

The 96GeO-(3-χ)Al₂O₃-χNa₂O-1NaBiO₃ (χ = 0, 0.5, 1.5 molar percent designated as A1, A2 and A3) and 96GeO-(3.5-ψ)Al₂O₃-ψNa₂O-0.5Bi₂O₃ (ψ = 0.5, 1, 2 molar percent designated as B1, B2 and B3) glasses were prepared by conventional melting method with the measurement of their DTA curve, fluorescence decay curve, transmission, absorption and emission spectra. The near infrared superbroadband emission characteristics of the A1, A2, B1 and B2 glasses peak at ∼1220 nm were observed when pumped by an 800 nm laser diode. The stimulated emission cross section (σ_p) was obtained from the emission spectra. The result indicated that the introduction of Bi⁵⁺ in NaBiO₃ into raw materials could increase the emission intensity of the obtained glasses by 5.6 times than that of Bi³⁺ in Bi₂O₃, and the FWHM (Δλ) and emission lifetime (τ) at 1220 nm increased from 195 nm to 275 nm, and 280 μs to 434 μs. Meanwhile, it was found that the absorption edges were blue-shifted from 486 to 447 nm by comparing those of A1 and B1. The absorption edges were considered to be ascribed to the charge transfer from Bi³⁺ 6s² to Bi⁵⁺ 6s⁰. Therefore we could conclude that the content of Bi⁵⁺ ions in A1 was more than that in B1 glasses. It could be deduced from the emission and absorption spectra that the stronger emission intensity and wider FWHM were due to the higher concentration of Bi⁵⁺ ion in glass. In particular, the increase of Na₂O content was in proportion to the thermal stability and the value of σ_p × τ and σ_p × Δλ of glasses.     

High-pressure and optical properties of lanthanum magnesium hexa-aluminate doped with Mn (LMA:Mn) laser material

The effect of hydrostatic pressure on the emission spectra and fluorescence lifetime (τ) of Mn²⁺ in LaMgAl₁₁O₁₉ (LMA) crystals up to 101 kbar has been studied at room temperature. From the position of the peak (⁴T₁ → ⁶A₁ transition) in the emission spectra, we estimated that the pressure induced red-shift. A variation, slowly decreasing, in the fluorescence lifetime (τ) values for ⁴T₁ → ⁶A₁ transition was observed. The pressure-induced red-shift and lifetime variation could be described by simple models. In the considered pressure range (0-101 kbar), a good agreement between the experimental values and theoretically predicted values was obtained.     

Fine structure on the excitonic emission in AgI nanoparticles embedded in silica glass

Stable photoluminescence (PL) from AgI nanoparticles embedded in silica glass was investigated at room temperature. The Z_1,2 excitonic emission of AgI exhibits fine structure with spacing of ∼0.20 eV (1610 cm⁻¹), which is assigned to the frequency of vibration in interfacial water species. The PL excitation spectrum displays two newly observed bands at 3.45 and 4.35 eV associated with AgI-silica interaction. We suggest that the excitons in AgI are localized in the AgI/SiO₂ interface region before radiative recombination.     

Carbon nanodot arrays grown as replicas of specially widened anodic aluminum oxide pore arrays

Via a specially widened anodic aluminum oxide (AAO) pore arrays, carbon nanodot arrays with uniform size and high density were obtained through filtered cathodic arc plasma (FCAP) technique. The AAO template was prepared in oxalic acid by multi-steps to get a specially enlarged opening which plays an important role in the deposition of nanodots. The morphology of the nanodots was studied by a field emission scanning electron microscopy (FESEM). The diameter of the as-prepared nanodot demonstrated here is about 100 nm at the bottom and less than 40 nm at the top, and the density was estimated to 10¹⁰ cm⁻². Field emission properties of the nanodot arrays were investigated and a low threshold field of 5.1 V/μm at 10 mA/cm² was obtained. In this paper, the carbon nanodot arrays grown as replicas of the specially widened AAO template may support a strategy to realize the fabrication of nanodot arrays with various materials.     

Enhanced optical and field emission properties of CTAB-assisted hydrothermal grown ZnO nanorods

We have developed a simple N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB)-assisted hydrothermal route for the production of ZnO one-dimensional (1D) nanostructures on zinc foil at reaction temperature of 160 °C. With the increase of CTAB concentration, the one-dimensional structures change from microrod to a mixture of nano- and microrod and finally to nanorods. X-ray diffraction studies confirmed the proper phase formation of the grown nanostructures. The room temperature photoluminescence spectra showed that ZnO nanostructures prepared with increased CTAB concentration exhibited enhanced band edge UV emission and also blue shift of the emission peak. All the samples show no defect related green emission. Field emission property of the 1D structures has been investigated in detail. By tuning the CTAB concentration, the field emission property was optimized. The nanorods synthesized with high CTAB showed turn-on and threshold fields of 3.2 and 5 V/μm, respectively, which are comparable to the values for vapour phase synthesized high field emitting ZnO nanostructures.     

A novel blue magnesium strontium aluminate-based phosphor for PDP application

A novel blue light-emitting phosphor, Eu²⁺-doped magnesium strontium aluminate (MgSrAl₁₀O₁₇:Eu²⁺), for plasma display panel (PDP) application was developed. X-ray diffraction (XRD) patterns disclosed that the phosphor annealed at 1500 °C for 5 h was a pure MgSrAl₁₀O₁₇ phase. Field emission scanning electron microscopy (FE-SEM) images showed the particle size of the phosphor was less than 3 μm. The phosphor shows strong and broad blue emission under vacuum ultraviolet (VUV) light excitation. After baking at 400-600 °C and irradiation with VUV light for 300 h, the phosphor still keep excellent VUV luminescence properties exhibiting good stability against high temperature baking and VUV irradiation. The decay time was short as 1.09 μs and the quantum yield was high to 0.77±0.02. All the characteristics indicated that MgSrAl₁₀O₁₇:Eu²⁺ would be a promising blue phosphor for PDP application.     

Characterization of soft-X-ray detectors fabricated with high-quality CVD diamond thin films

We have characterized the performance of soft-X-ray detectors fabricated with undoped and B-doped homoepitaxial diamond layers of high quality which were grown on a commercially available type Ib (1 0 0) substrate by means of a high-power microwave-plasma chemical-vapor-deposition (CVD) method. The signal currents of the diamond-based detectors with thin TiN electrodes formed vertically (along the homoepitaxial growth direction) were measured at room temperature as a function of the applied voltage, V_a, for irradiations of 500-1200 eV soft-X-ray beams ranging from ≈6 × 10⁹ to ≈1 × 10¹¹ photons/s. The deduced apparent quantum efficiencies increased with the increasing V_a and reached to 2.5 × 10³ at V_a = 60 V. As expected from the device structure, the detector performance depended only very slightly on the applied magnetic field up to 10 T. The excellently high sensitivities attained for soft-X-ray photons are discussed in relation to carrier amplification mechanisms which invested the above diamond detectors.     

Enhanced field emission and patterned emitter device fabrication of metal-tetracyanoquinodimethane nanowires array

Ag(TCNQ) and Cu(TCNQ) nanowires were synthesized via vapor-transport reaction method at a low temperature of 100 °C. Field emission properties of the as-obtained nanowires on ITO glass substrates were studied. The turn-on electric fields of Ag(TCNQ) and Cu(TCNQ) nanowires were 9.7 and 7.6 V/μm (with emission current of 10 μA/cm²), respectively. The turn-on electric fields of Ag(TCNQ) and Cu(TCNQ) nanowires decreased to 6 and 2.2 V/μm, and the emission current densities increased by two orders at a field of 8 V/μm with a homogeneous-like metal (e.g. Cu for Cu(TCNQ)) buffer layer to the substrate. The improved field emission is due to the better conduct in the nanowires/substrate interface and higher internal conductance of the nanowires. The patterned field emission cathode was then fabricated by localized growing M-TCNQ nanowires onto mask-deposited metal film buffer layer. The emission luminance was measured to be 810 cd/m² at a field of 8.5 V/μm.     

Effect of growth temperature on the morphology and bonded states of SnO2 nanobaskets

The nanobaskets of SnO₂ were grown on in-house fabricated anodized aluminum oxide pores of 80 nm diameter using plasma enhanced chemical vapor deposition at an RF power of 60 W. Hydrated stannic chloride was used as a precursor and O₂ (20 sccm) as a reactant gas. The deposition was carried out from 350 to 500 °C at a pressure of 0.2 Torr for 15 min each. Deposition at 450 °C results in highly crystalline film with basket like (nanosized) structure. Further increase in the growth temperature (500 °C) results in the deterioration of the basket like structure and collapse of the alumina pores. The grown film is of tetragonal rutile structure grown along the [1 1 0] direction. The change in the film composition and bonded states with growth temperature was evident by the changes in the photoelectron peak intensities of the various constituents. In case of the film grown at 450 °C, Sn 3d_5/2 is found built up of Sn⁴⁺ and O-Sn⁴⁺ and the peaks corresponding to Sn²⁺ and O-Sn²⁺ were not detected.     

Luminescence and energy transfer processes in Lu2SiO5:Ce scintillator

The energy transfer processes in Lu₂SiO₅:Ce³⁺ luminescence was investigated through the temperature dependent luminescence under excitation with VUV-UV. Ce1 center emission peaking at 393 and 422 nm and Ce2 center emission peaking at 462 nm were observed. Ce2 center emission is enhanced with the temperature, which can be explained by the rate of energy transfer from Ce1 center increases when the temperature rises. The Ce1 emission shows the thermal quenching effect under the direct excitation of Ce³⁺ at 262 nm. However, under the interband excitation of 183 nm, the Ce1 center emission exhibits undulating temperature dependence. This is because the emission is governed by thermal quenching and possible thermal enhancement of the transport of free carriers with the rising temperature.     

Magnetic,structural and electrical properties of ordered and disordered Co50Fe50 films

Co₅₀Fe₅₀ films with thickness varying from 100 to 500 Å were deposited on a glass substrate by sputtering process, respectively. Two kinds of CoFe films were studied: one was the as-deposited film, and the other the annealed film. The annealing procedure was to keep the films at 400 °C for 5 h in a vacuum of 5×10⁻⁶ mbar. From the X-ray study, we find that the as-deposited film prefers the CoFe(1 1 0) orientation. Moreover, the body-centered cubic (bcc) CoFe(1 1 0) line is split into two peaks: one corresponding to the ordered body-centered tetragonal (bct) phase, and the other, the disordered bcc phase. After annealing, the peak intensity of the ordered bct phase becomes much stronger, while that of the disordered bcc phase disappears. The annealing has also caused the ordered CoFe(2 0 0) line to appear. When the amount of the ordered bct phase in Co₅₀Fe₅₀ is increased, the saturation magnetization (M_s) and coercivity (H_c) become larger, but the electrical resistivity (ρ) decreases. From the temperature coefficient of resistance (TCR) measurement, we learn that the bct grains in the CoFe film start to grow at temperature 82 °C.     

Growth of carbon nanofibers on aligned zinc oxide nanorods and their field emission properties

Carbon nanofibers were grown by electrodeposition technique onto aligned zinc oxide (ZnO) nanorods deposited by hybrid wet chemical route on glass substrates. X-ray diffraction traces indicated very strong peak for reflections from (0 0 2) planes of ZnO. The Raman spectra were dominated by the presence of G band at about 1597 cm⁻¹ corresponding to the E_2g tangential stretching mode of an ordered graphitic structure with sp² hybridization and a D band at about 1350 cm⁻¹ originating from disordered carbon. Fourier transformed infrared studies indicated the presence of a distinct characteristic absorption peak at ∼511 cm⁻¹ for Zn-O stretching mode. Photoluminescence spectra indicated band edge luminescence of ZnO at ∼3.146 eV along with a low intensity peak at ∼0.877 eV arising out of carbon nanofibers. Field emission properties of these films and their dependence on the CNF coverage on ZnO nanorods are reported here. The average field enhancement factor as determined from the slope of the FN plot was found to vary between 1 × 10³ and 3 × 10³. Both the values of turn-on field and threshold field for CNF/ZnO were lower than pure ZnO nanorods.     

Growth of ultra-thin cerium oxide layers on Cu(1 1 1)

The reactive vacuum deposition of CeO₂ on Cu(1 1 1) surface in oxygen atmosphere provides high quality epitaxial ceria overlayers. We report the growth characteristics of Ce oxide, the structures, and the temperature stability of the oxide phases as investigated by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy. We find that Ce oxide on the Cu(1 1 1) grows initially in the form of islands giving sharp hexagonal LEED pattern of the CeO₂(1 1 1) structure corresponding to the (1.5 × 1.5) structure. The CeO₂-Cu(1 1 1) films exhibited mixed valence states and temperature dependent CeO₂-Ce₂O₃ transition above 900 K due to the vacuum annealing. The transition progressed more rapidly at the surface, probably by formation of oxygen vacancies.