Studies on growth and characterization of ternary CdS1−xSex alloy thin films deposited by chemical bath deposition technique

Ternary alloyed CdS_1−xSe_x thin films of variable composition ‘x’ were grown by the simple and economical chemical bath deposition technique. The as-grown thin films were characterized for structural, compositional, surface morphological, optical and electrical studies. The X-ray diffraction (XRD) patterns of the sample indicated that all the samples were polycrystalline in nature with hexagonal structure. Scanning electron microscopy (SEM) micrographs showed uniform morphology with spherical shaped grains distributed over entire glass substrate. EDAX studies confirmed that the CdS_1−xSe_x films were having approximately same stoichiometry initially as well as finally. Room temperature optical measurements showed that band gap engineering could be realized in CdS_1−xSe_x thin films via modulation in composition ‘x’. Electrical resistivity of CdS_1−xSe_x thin films for various compositions was found to be low. The broad and fine tunable band gap properties of ternary CdS_1−xSe_x thin films have potential applications in opto-electronic devices.     

Origin of photoluminescence in nanocrystalline Si:H films

We have studied the origin of photoluminescence (PL) from hydrogenated nanocrystalline silicon (nc-Si:H) films produced by a plasma-enhanced chemical vapor deposition technique using SiF₄/SiH₄/H₂ gas mixtures. The nc-Si:H films were characterized using X-ray diffraction, infrared, Raman spectroscopy, optical absorption and stress, and were examined for PL by varying the deposition temperature (T_d) under two different hydrogen flow rate ([H₂]) conditions. The PL exhibited two peaks at around 1.7-1.75 and 2.2-2.3 eV. The peak energy, E_PL, of the 1.7-1.75-eV PL band was found to shift as T_d or [H₂] changes. It was found that the decrease in T_d acts to decrease the average grain size, 〈δ〉, and to increase both the optical band gap, , and the E_PL values. By contrast, the increase in [H₂] decreased the 〈δ〉 value, while increased the values of and E_PL. Thus, as either T_d decreases or [H₂] increases, it is found that a decrease in 〈δ〉 corresponds well with increases in and E_PL. As a consequence, it was suggested that an increase in E_PL of the 1.7-1.75-eV PL band can be connected with an increase in , through a decrease in 〈δ〉. However, the PL process cannot be connected with the transition between both the bands, related to formation of nanocrystals. Based on these results, it was proposed that the use of both low T_d and high [H₂] conditions would allow to grow nc-Si:H films with small grains.     

Characterization of semiconductor core-shell nanoparticles by resonant Raman scattering and photoluminescence spectroscopy

Colloidal CdSe nanoparticles (NPs), passivated with CdS and ZnS, were characterized by resonant Raman scattering and photoluminescence (PL). The effect of the passivating shell, its volume and formation procedure on optical and vibrational spectra is discussed. Analyzing the Raman peaks due to optical phonons inside the core and those related to the core-shell interface allows some understanding of the relation between the core-shell structure and its PL properties to be achieved. In particular, a compositional intermixing at the core/shell interface of the NPs was deduced from the Raman spectra, which can noticeably affect their PL intensity.     

Optical properties of selenium-tellurium nanostructured thin film grown by thermal evaporation

Se-Te nanostructured thin films were deposited on glass substrates in the presence of oxygen and argon by thermal evaporation. The properties of Se-Te thin films strongly depend on the deposition method. During the process used, the substrate is cooled to a temperature of 77 K employing liquid nitrogen. The nanostructured thin films of Se_100−xTe_x (where x=4, 8 and 16) are deposited on glass substrate. The surface morphology of the deposited films was investigated through Scanning Electron Microscopy (SEM). The typical size of these nanostructures is in the range 40-100 nm and the length is of the order of several micrometers. The optical parameters i.e. optical gap (E_g), absorption coefficient (α), and extinction coefficient (k) are calculated in the wavelength range 190-1100 nm. It was found that the optical band gap decreased from 3.4 to 2.9 eV when Te concentration was increased in the Se_100−xTe_x nanostructured thin films. The large bandgap may be attributed to the decrease in particle size which clearly exhibits a quantum size effect. XRD analysis was performed to confirm glassy nature of the nanostructured thin films.     

Preparation and optical properties of Cu2O hollow microsphere film and hollow nanosphere powder via a simple liquid reduction approach

In this work, we report a simple liquid reduction approach to prepare Cu₂O hollow microsphere film and hollow nanosphere powder with Cu(OH)₂ nanorods as precursor and ascorbic acid as the reductant at 60 °C. When Cu(OH)₂ nanorod array film grown on a copper foil is used as the precursor, Cu₂O thin film made up of hollow microspheres with average diameter of 1.2 μm is successfully prepared. When the Cu(OH)₂ nanorods are scraped from the copper foil and then used as the precursor, Cu₂O hollow nanosphere powder with the average diameter of 270 nm is obtained. The samples are characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and ultraviolet-vis light (UV-vis) absorption spectra. A possible formation mechanism of Cu₂O hollow spheres is discussed.     

Structural and optical properties of a-Si1−xCx:H films synthesized by dc magnetron sputtering technique

Hydrogenated amorphous SiC films (a-Si_1−xC_x:H) were prepared by dc magnetron sputtering technique on p-type Si(1 0 0) and corning 9075 substrates at low temperature, by using 32 sprigs of silicon carbide (6H-SiC). The deposited a-Si_1−xC_x:H film was realized under a mixture of argon and hydrogen gases. The a-Si_1−xC_x:H films have been investigated by scanning electronic microscopy equipped with an EDS system (SEM-EDS), X-ray diffraction (XRD), secondary ions mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR), UV-vis-IR spectrophotometry, and photoluminescence (PL). XRD results showed that the deposited film was amorphous with a structure as a-Si_0.80C_0.20:H corresponding to 20 at.% carbon. The photoluminescence response of the samples was observed in the visible range at room temperature with two peaks centred at 463 nm (2.68 eV) and 542 nm (2.29 eV). In addition, the dependence of photoluminescence behaviour on film thickness for a certain carbon composition in hydrogenated amorphous SiC films (a-Si_1−xC_x:H) has been investigated.     

Structural studies of annealed neodymia-silica composite synthesized by solgel technique

Nd₂O₃-SiO₂ binary oxide was prepared by solgel technique using tetra-ethoxysilane and neodymium nitrate as precursor materials and HCl as a catalyst. The prepared samples were subjected to heat treatment in the temperature range from 600 to 1100 °C for different time duration. Characterization of heat treated samples was carried out by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The effect of sintering temperature and time on structural changes of Nd-doped silica has been discussed. The sample sintered at 1100 °C for 12 h shows the formation of monoclinic Nd₂O₃ nanocrystallites in silica matrix with average grain size ∼18 nm.     

Blue luminescent silicon nanocrystals prepared by short pulsed laser ablation in liquid media

The pulsed laser processing in liquid media is an attractive alternative to produce room temperature luminescent silicon nanocrystals (Si-ncs). We report on a blue luminescent Si-ncs preparation by using nanosecond pulsed laser (Nd:YAG, KrF excimer) processing in transparent polymer and water. The Si-ncs fabrication is assured by ablation of crystalline silicon target immersed in liquids. During the processing and following aging in liquids, oxide based liquid media, induce shell formation around fresh nanocrystals that provides a natural and stable form of surface passivation. The stable room temperature blue-photoluminescent Si-ncs are prepared with maxima located around ∼440 nm with corresponding optical band gap around ∼2.8 eV (∼430 nm). Due to the reduction of surface defects, the Si-ncs preparation in water, leads to a narrowing of full-width-half-maxima of the photoluminescence spectra.     

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.     

Decay dynamics of ultraviolet photoluminescence in ZnO nanocrystals

Time resolved photoluminescence (PL) measurements at low temperature are performed on colloidal ZnO nanocrystals dispersed in t-butanol. Considering the particle size dependence of the decay times we conclude that the luminescence is composed of two trap related emissions one of which undergoes lifetime shortening due to a non-radiative process. Initial fast shift of the spectrum within 30 ps is observed and it is interpreted as a fast hole cooling just after the excitation.     

Cr-doping induced ferromagnetic behavior in antiferromagnetic EuTiO3 nanoparticles

We have synthesized a series of high quality EuTi_1−xCr_xO₃ (x = 0.0, 0.02, and 0.04) nanoparticles by simple sol-gel technique. The averaged grain size of these obtained nanoparticles displays no obvious change with Cr-doping and is about 100 nm. The structural and magnetic properties of EuTi_1−xCr_xO₃ (x = 0.0, 0.02, and 0.04) samples were detailedly investigated. It is found that the G-antiferromagnetic (G-AFM) ordering of pure EuTiO₃ can be significantly modified with slight Cr-doping, and finally the ferromagnetic behavior is enhanced for EuTi_1−xCr_xO₃ system with Cr-doping.     

Preparation and luminescent properties of europium-doped yttria fibers by electrospinning

Sub-micrometer-sized fibers of europium-doped yttria (Y₂O₃:Eu³⁺) were prepared by electrospinning followed by high-temperature calcinations for the first time. The fibers were with diameters of 200-400 nm and lengths of several 10 μm and cubic in phase. The spectral properties of the Y₂O₃:Eu³⁺ fibers were studied, in contrast with those of bulk powders. The results indicated that in the present Y₂O₃:Eu³⁺ fibers the excited charge transfer band had slightly blue shift in comparison with that in the bulk due to weaker covalence of Eu-O bonds. In addition, both of the lifetimes of the ⁵D₁ and ⁵D₀ states in the fibers became shorter than that in the bulk due to improved nonradiative transition rates.     

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 activity of mesoporous Nb2O5 for photocatalytic hydrogen production

The mesoporous Nb₂O₅ photocatalysts were synthesized via an evaporation-induced self-assembly (EISA) method. The mesoporous structure of the as-made samples was studied by small-angle X-ray diffraction, N₂ adsorption-desorption isothermal and transmission electron microscopy. The increase of the calcination temperature during the synthesis resulted in enhanced crystallization, but decreased mesoporosity of the samples. The later was found to have a crucial influence on the photocatalytic activity by bringing on decreased BET surface area and especially increscent pore wall thickness. The advantage of the mesoporous Nb₂O₅ was also proved by performing 20 times higher photocatalytic activity than a bulk Nb₂O₅ without any porosity. A model was given to describe the effect of mesoporosity on the transportation and recombination of carriers.     

Size-dependent photo-induced shift of the first exciton band in CdTe quantum dots in glass prepared by a two-stage heat-treatment process

CdTe nanocrystals were grown from commercially available RG850 Schott filter glass by two-step heat-treatment process which almost doubles the particle to matrix volume fraction. A calculation shows that a quantized-state effective mass model in the strong confinement regime might be used to deduce the average radius for the nanocrystals larger than 2 nm in radius from the energetic position of the first exciton peak in optical absorption spectrum. Size-induced shift of ∼360 meV in the first exciton peak position was observed. The steady state photoluminescence spectra exhibit a broad band red shifted relative to the first exciton band, which indicates the existence of shallow trap states. The non-linear optical properties of CdTe nanocrystals were studied by room temperature resonant photoabsorption spectroscopy. The differential absorption spectra had three-lobed structure whose size-dependent evolution was explained by bleaching of the absorption, red shift and broadening in the Gaussian absorption band used to fit the first exciton peak. A maximum red shift of 2.32 meV for the average nanocrystal radius of 4.65 nm was estimated by fitting the photomodulation spectra with a combination of first and second derivative Gaussian absorption bands. We presume that the red shift is induced by the electric field of trapped charges in surface states. Internal electric field strengths of 23 and 65 kV/cm were predicted for the average nanocrystal radii of 3.95 and 4.65 nm, respectively, with the help of second-order perturbation theory in the strong confinement limit.     

Electromagnetic field distribution on a rough gold thin film: An experimental study as a function of the gold thickness

The electromagnetic field distribution on an illuminated rough gold surface has been investigated by apertureless scanning near-field optical microscopy. The sample consists of an alumina substrate with a variable gold coverage ranging from 0 to 30 monolayers (ML). For such small thicknesses, the metal layer is not continuous but exhibits a certain roughness. We have studied the influence of this thickness on the electromagnetic field localization on the surface. For a gold coverage smaller than 10 equivalent monolayers, the electromagnetic field is almost uniform on the surface. For 10 and 14 ML, the field becomes inhomogeneous and isolated, localized peaks start to be visible. The width of the peaks is smaller than 50 nm. Above 14 ML, strong variations are apparent everywhere on the sample. Their amplitude tends to saturate beyond 24 ML. A complete statistical study of the sample (standard deviation, Fourier analysis) is performed.     

Spin-deposited nanocrystalline lithium ferrite thin films: Fabrication and characterization

Thin films of lithium ferrite (with general composition Li_0.5Fe_2.5O₄) were fabricated at low temperatures (up to 650 °C) by citrate-route using spin-deposition technique. Deposited films consisted of nanometer-sized grains as evidenced by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. XRD patterns for annealed films showed broad peaks exhibiting a spinel phase. Size of nanocrystallites is estimated to be 3-7 nm using Scherrer's equation. Average grain size ∼8.5 nm is observed from TEM images of films annealed at 650 °C. Scanning electron micrographs show the formation of spherical aggregates of around 130 nm in diameter. The AFM analysis clearly evidenced the development of nanograins even at low (∼500 °C) annealing temperatures. Significant decrease in complex dielectric permittivity (∈′ − j∈″) with frequency is observed in the low frequency (100 Hz-1 MHz) as well as in X-band microwave frequency (8-12 GHz) region. ∈′ is found to be in the range of 15.7-33.9 in low frequency region, whereas in X-band microwave frequency region, it is found to lie between 3.9 and 4.9. Similarly, ∈″ is found to be 0.16-5.9 in the low frequency region, and 0.002-0.024 in the X-band microwave frequency region. Room temperature dc resistivity of these films is estimated to lie in the range of 10⁶-10⁸ Ω cm. These results strongly suggest that citrate-route processed nanocrystalline lithium ferrite thin films are promising candidates for monolithic microwave integrated circuits (MMICs).     

Raman spectroscopic and photoluminescence study of single-crystalline SnO2 nanowires

Single-crystalline SnO₂ nanowires with sizes of 4-14 nm in diameter and 100-500 nm in length were produced in a molten salt approach by using hydrothermal synthesized precursor. Structural characters of the nanowires were examined by X-ray diffraction and high-resolution electron transmission microscopy. Raman, photoluminescence and X-ray photoelectron spectra of the samples were examined under heat treatments. Three new Raman modes at 691, 514 and 358 cm⁻¹ were recorded and assigned. The former two are attributed to activation of original Raman-forbidden A_2uLO mode and the third is attributed to defects in small-sized nanowires. A strong photoluminescence is observed at about 600 nm, the temperature effects is examined and the origin of the PL process is discussed via X-ray photoelectron spectra.     

Optical properties of silylene-biphenylene copolymer films

We have measured absorption, photoluminescence, and photoluminescence excitation spectra, and the photoluminescence time response for films of silylene-biphenylene copolymer, ((C₆H₄)₂(Si(CH₃)₂)_m)_n with m=1,2,4, and 6. The excitation spectra clearly reveal that the lowest absorption band in each copolymer consists of two bands, i.e., a band at 4.7 eV and a band of which energy depends on m. Since the latter band is absent in the copolymer with m=1, the former band is attributed to the lowest π−π* transition in biphenylene subunits. The latter band is attributed to the lowest σ−σ* transition in the silylene subunits, considering its dependence on m. In contrast to the result for solution, the peak energy of photoluminescence band is independent of m. The band has a Stokes shift of more than 1 eV and a large band width of 0.5 eV. The time responses of photoluminescence intensity consist of more than two decay components and the intensity decays more slowly at smaller energy. The large Stokes shift is explained as due to excimer formation between biphenylene subunits. In order to explain the energy dependence of time responses, energy migration is discussed.     

Photoluminescence scanning on InAs/InGaAs quantum dot structures

The photoluminescence spectra of InAs quantum dots (QDs) embedded into four types of In_xGa_1−xAs/GaAs (x = 0.10, 0.15, 0.20 and 0.25) multi quantum well MBE structures have been investigated at 300 K in dependence on the QD position on the wafer. PL mapping was performed with 325 nm HeCd laser (35 mW) focused down to 200 μm (110 W/cm²) as the excitation source. The structures with x = 0.15 In/Ga composition in the In_xGa_1−xAs capping layer exhibited the maximum photoluminescence intensity. Strong inhomogeneity of the PL intensity is observed by mapping samples with the In/Ga composition of x ≥ 0.20-0.25. The reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300 K as follows from the quantum dot PL mapping. The mechanism of this effect has been analyzed. PL peak shifts versus capping layer composition are discussed as well.