Diphenylthiocarbazone (dithizone)-assisted solvothermal synthesis and optical properties of one-dimensional CdS nanostructures

One-dimensional (1D) cadmium sulfide (CdS) nanostructures with various aspect ratios were successfully synthesized by a diphenylthiocarbazone (dithizone)-assisted solvothermal method. The results showed that the dithizone-assisted synthesized samples had larger aspect ratio than that prepared in the absence of dithizone, and CdS nanowires with the highest aspect ratio were obtained with an appropriate dithizone amount (0.03 g/50 ml ethylenediamine in the present system). All the 1D CdS nanostructures were in hexagonal wurtzite phase. The as-synthesized large-scale CdS nanowires were in diameters ranging from 70 to 80 nm, length up to 20 μm, and aspect ratios of 250-285. Further characterization indicated that the CdS nanowires were single crystalline with a preferential growth orientation of [0 0 2], c-axis. Two optical absorption peaks were observed at about 488 nm and 502 nm for the CdS nanowire sample with high aspect ratio in the optical absorption spectroscopy, which could be attributed to the nanometer effect of nanowires. It was found that the additive dithizone was a crucial factor in controlling the morphology and optical properties of the 1D CdS nanostructures. The growth mechanism of 1D CdS nanostructure and the effects of dithizone in the present system were discussed.     

Photoelectrochemical investigations of cadmium sulphide (CdS) thin film electrodes prepared by spray pyrolysis

Polycrystalline cadmium sulphide (CdS) thin films have been prepared by spraying a mixture of an equimolar aqueous solutions of cadmium chloride and thiourea on preheated fluorine doped tin oxide (FTO) coated glass substrates at different substrate temperatures. The cell configurations n-CdS/1 M (NaOH + Na₂S + S)/C were used for studying the capacitance-voltage (C-V) characteristics in dark, current-voltage (I-V) characteristics in dark and under illumination, photovoltaic power output and spectral response characteristics of the as deposited thin films. Photoelectrochemical study shows that as deposited CdS thin films exhibits n-type of conductivity. The spectral response characteristics of the films at room temperature show a prominent sharp peak at 500 nm leading to optical bandgap energy of 2.48 eV. It is found that fill factor and efficiency are maximum for photoelectrode deposited at 300 °C. This is due to low resistance; high flat band potential, maximum open circuit voltage as well as maximum short-circuit current. The measured values of efficiency (η) and fill factor (FF) are found to be 0.17% and 0.38 respectively for film deposited at 300 °C.     

Constitution, microstructure, and battery performance of magnetron sputtered Li-Co-O thin film cathodes for lithium-ion batteries as a function of the working gas pressure

Li-Co-O thin film cathodes have been deposited onto Si and stainless steel substrates by RF magnetron sputtering from a ceramic LiCoO₂ target at various working gas pressures from 0.15 to 25 Pa. Composition, crystal structure and thin film morphology were examined and properties such as intrinsic stress, conductivity and film density were determined. As-deposited films at 0.15 Pa as well as in the range between 5 Pa and 10 Pa working gas pressure showed a nanocrystalline metastable rocksalt structure with disordered cation arrangement and were nearly stoichiometric. To induce a cation ordering the films were annealed in a furnace at temperatures between 100 and 600 °C for 3 h in argon/oxygen atmosphere (Ar:O₂ = 4.5:5) of 10 Pa. This cation ordering process was observed by XRD and Raman spectroscopy. For the films deposited at 10 Pa gas pressure an annealing temperature of 600 °C leads to the formation of the high temperature phase HT-LiCoO₂ with a layered structure. The Raman spectrum of the films deposited at 0.15 Pa and annealed at 400 °C indicates the formation of the low temperature phase LT-LiCoO₂ with a cubic spinel-related structure, which is assumed to be stabilized due to high compressive stress in the film. The electrochemical characterisation of annealed thin film cathodes revealed that the discharge capacity strongly depends on the crystal structure. Thin Li-Co-O films with a perfect layered HT-LiCoO₂ structure showed the highest discharge capacities.     

Structural, morphological, luminescent and electronic properties of sprayed aluminium incorporated iron oxide thin films

Thin films of aluminium incorporated Fe₂O₃, synthesized by simple chemical spray pyrolysis on to glass substrates using aqueous solutions of analytical reagent grade ferric trichloride and aluminium nitrate as precursors. The influence of aluminium doping on to morphological properties, contact angle, X-ray photoelectron spectroscopy, photoluminescence and thermal conductivity properties have been investigated. The preparative parameters have been optimized to obtain good quality thin films which are uniform and well adherent to the substrate. The FE-SEM and AFM micrographs depict the films are compact and homogeneous (spindle-shaped hematite nanostructures) with varying grain sizes (average grain size ~ 20-60 nm). Contact angle measurement show the films are hydrophobic in nature. The chemical composition and valence states of constituent elements in Fe₂O₃ are analyzed by X-ray photoelectron spectroscopy. The excitonic strong violet emission has been observed in photoluminescence. The specific heat and thermal conductivity study shows the phonon conduction behavior is dominant in these polycrystalline films. We studied interparticle interactions like grains, grain boundary effects using complex impedance spectroscopy.     

Thermal stability of electrical properties of ZnO:Al films deposited by room temperature magnetron sputtering

In order to investigate the thermal stability of electrical properties for aluminum doped zinc oxide (ZnO:Al, AZO) films deposited by direct current reactive magnetron sputtering, AZO films deposited from an alloy target (0.8 wt.% Al) on soda-lime glasses were annealed in argon gas at different temperatures. A data capturer was applied to monitor and collect real-time sheet resistance (R_s) of the films throughout the annealing. Results revealed that R_s of the film heated at 100 °C was reduced throughout the annealing, however, conductivity of the films annealed over 100 °C was improved at early stage but then deteriorated all along to the end. Some novel R_s change points which need more penetrations were detected. The experimental results obtained from electron diffraction spectrum, X-ray diffraction pattern, X-ray photoelectron spectrum, and Hall measurement were analyzed to explore the effect of the annealing on the electrical properties of AZO films. It was found that the exotic element, which might influence the film properties, was not observed. It was also suggested that the transformation of the crystalline structure and surface chemical bonding states, which resulted in the decrease of carrier concentration and mobility could be the reason for the conductivity degeneration of the films annealed at higher temperature.     

The effect of the microstructure and the surface topography on the electrical properties of thin Ag films deposited by high power pulsed magnetron sputtering

In this work thin silver (Ag) films are grown employing high power pulsed magnetron sputtering (HPPMS) for various pulse on/off time configurations, as well as by dc magnetron sputtering (dcMS), for reference. It is shown that the increase of the pulse off-time from 450 μs to 3450 μs, while the pulse on-time is kept constant at 50 μs, results in an increase of the peak target current (I_Tp) from 3 A to 22 A. The increase of I_Tp is accompanied by an increase of the ion flux towards the growing film. This is particularly pronounced for I_Tp > 11 A. The microstructure, the surface topography and the electrical properties of Ag films grown at I_Tp = 11 A, I_Tp = 22 A and by dcMS are investigated, as a function of the film thickness d. It is shown that for d > 20 nm the electrical resistivity of films sputtered at I_Tp = 22 A is similar to that of films grown by dcMS. Slightly higher values are measured for films grown at I_Tp = 11 A. It is found that in this thickness range the film conductivity is strongly affected by the vertical grain size and the scattering of the charged carriers at the film interfaces. For d < 15 nm the resistivity of films deposited at I_Tp = 22 A is substantially lower as compared to that of films grown by dcMS. Films deposited at I_Tp = 11 A exhibit also in this case a higher conductivity. In this thickness regime the electronic transport and, thus the conductivity are profoundly determined by the surface topography and the film density.     

High-temperature thermoelectric properties of late rare earth-doped Ca3Co4O9+δ

Misfit-layered oxides Ca_3−xLn_xCo₄O_9+δ with Ln = Dy, Er, Ho, and Lu were synthesized using solid state reactions. The resulting samples were hot-pressed (HP) at 1123 K in air for 2 h under a uniaxial pressure of 60 MPa. Thermoelectric properties of Ca_3−xLn_xCo₄O_9+δ were investigated up to 1200 K. Both the Seebeck coefficient and electrical resistivity increase upon Ln substitution for Ca. Among the Ln-doped samples, the magnitude of Seebeck coefficient tends to increase with decreasing ionic radius of Ln³⁺. The Ln-doped samples exhibit a lower thermal conductivity than the non-doped one due to a decrease of their lattice thermal conductivity. The dimensionless figure of merit, ZT, reaches 0.36 at 1073 K for the Ca_2.8Lu_0.2Co₄O_9+δ sample, which is about 1.6 times larger than that for the non-doped counterpart.     

Annealing driven performance and optical effects in nanocrystalline SnO2 thin films induced by pulsed delivery

Tin dioxide thin films were prepared successfully by pulsed laser deposition techniques on glass substrates. The thin films were then annealed for 30 min from 50 °C to 550 °C at 50 °C intervals. The influence of the annealing temperature on the microstructure and optical properties of SnO₂ thin films was investigated using X-ray diffraction, optical transmittance and reflectance measurements. Various optical parameters, such as optical band gas energy, refractive index and optical conductivity were calculated from the optical transmittance and reflectance data recorded in the wavelength range 300-2500 nm. We found that the SnO₂ thin film annealed at temperatures up to 400 °C is a good window material for solar cell application. Our experimental results indicated that SnO₂ thin films with the high optical quality could be synthesized by pulsed laser deposition techniques.     

Modulation of field emission properties of DLC films with the incorporation of nanocrystalline silver nanoparticles by CVD technique

Composite films containing Ag nanoparticles embedded in diamond-like carbon (DLC) matrix were deposited on glass substrates by using capacitively coupled plasma (CCP) r.f. chemical vapour deposition technique (CVD). Amount of silver incorporation was controlled by the amount of argon in the argon and methane mixture in the plasma. Field emission properties of these films are reported here. Films became sp² rich with increased incorporation of silver nanoparticles in the DLC matrix. Field enhancement factor increased significantly for films with higher silver content in the DLC matrix. Work function (?) values obtained from the Fowler-Nordheim model varied between 0.04 and 0.10 eV while the field factor (β) varied between 336 and 2759. The critical field was found to vary between 10 and 100 V/μm.     

Microstructure and thickness dependent magnetic properties of nanogranular Co-Zn-O thin films for microwave applications

Co-based granular thin films with in-plane anisotropy were deposited on Si substrate by magnetron sputtering. The films have a phase structure of Co nanocrystallites and amorphous Zn-O inter-granular phases. The Co nanograins with uniform size of 8-10 nm are evenly distributed in the amorphous matrix. This structure gives the films relatively high resistivities. The as-deposited films with thickness larger than 100 nm have low coercivity (<10 Oe) along both easy and hard directions. The dynamic properties in the frequency range up 5 GHz for the films with various thicknesses have been investigated. High values of permeability (μ′ up to 560 and μ″ up to 1000) and ferromagnetic resonance frequency (FMR) up to 4.1 GHz have been obtained in these films. The FMR frequency decreases with increasing thickness, because of the increases in real and imaginary permeabilities. The high frequency characteristics have complicated dependences on the resistivity, anisotropy field, and magnetization. The microwave properties of Co-Zn-O films can be adjusted in a relatively wide range by changing film thickness, which makes these films promising for absorber applications.     

Enhanced power factor of Indium co-doped ZnO:Al thin films deposited by RF sputtering for high temperature thermoelectrics

An improvement in the thermoelectric power factor of Al doped ZnO has been achieved by means of co-doping with indium using a dual magnetron sputtering system. The concentration of indium in the film was varied from 0 to 10 atomic % by varying the RF power of the In target, with the ZnO:Al target fixed at 100 W. It has been found that the films with In concentrations at or below 5 at.% have no significant change in microstructure, and yet a marked improvement in thermopower. At higher doping levels, the Seebeck coefficient continues to increase, however poly-crystallinity is induced in the ZnO matrix which results in a considerable decrease in electrical conductivity. This factor ultimately has a negative impact on the materials power factor. Taking into account the films studied, (ZnO)Al_.03In_.02 exhibited the best thermoelectric properties with an electrical conductivity of 5.88 × 10² S/cm and a Seebeck coefficient of −220 μV/K at 975 K, resulting in a power factor is 22.1 × 10⁻⁴ Wm⁻¹ K⁻², which is three times greater than for the film with no In doping. Film microstructure, composition, and thermal stability were investigated using X-ray diffraction, scanning electron microscopy, and Auger electron spectroscopy.     

Nickel-lithium oxide alloy transparent conducting films deposited by spray pyrolysis technique

In this research, nickel oxide (NiO) transparent semiconducting films are prepared by spray pyrolysis technique on glass substrates. The effect of Ni concentration in initial solution and substrate temperature on the structural, electrical, thermoelectrical, optical and photoconductivity properties of NiO thin films are studied. The results of investigations show that optimum Ni concentration and suitable substrate temperature for preparation of basic undoped NiO thin films with p-type conductivity and high optical transparency is 0.1 M and 450 °C, respectively. Then, by using these optimized deposition parameters, nickel-lithium oxide ((Li:Ni)O_x) alloy films are prepared. The XRD structural analysis indicate the formation of the cubic structure of NiO and (Li:Ni)O_x alloy films. Also, in high Li doping levels, Ni₂O₃ and NiCl₂ phases are observed. The electrical measurements show that the resistance of the films decreases with increasing Li level up to 50 at%. For these films, the optical band gap and carrier concentration are obtained to be 3.6 eV and 10¹⁵-10¹⁸ cm⁻³, respectively.     

ZnO nanocoral reef grown on porous silicon substrates without catalyst

Porous silicon (PS) technology is utilized to grow coral reef-like ZnO nanostructures on the surface of Si substrates with rough morphology. Flower-like aligned ZnO nanorods are also fabricated directly onto the silicon substrates through zinc powder evaporation using a simple thermal evaporation method without a catalyst for comparison. The characteristics of these nanostructures are investigated using field-emission scanning electron microscopy, grazing-angle X-ray diffraction (XRD), and photoluminescence (PL) measurements of structures grown on both Si and porous Si substrates. The texture coefficient obtained from the XRD spectra indicates that the coral reef-like nanostructures are highly oriented on the porous silicon substrate with decreasing nanorods length and diameter from 800-900 nm to 3.5-5.5 μm and from 217-229 nm to 0.6-0.7 μm, respectively. The PL spectra show that for ZnO nanocoral reefs the UV emission shifts slightly towards lower frequency and the intensity increase with the improvement of ZnO crystallization. This non-catalyst growth technique on the rough surface of substrates may have potential applications in the fabrication of nanoelectronic and nanooptical devices.     

Realization of stable p-type ZnO thin films using Li-N dual acceptors

Lithium and nitrogen dual acceptors-doped p-type ZnO thin films have been prepared using spray pyrolysis technique. The influence of dual acceptor (Li, N) doping on the structural, electrical, and optical properties of (Li, N):ZnO films are investigated in detail. The (Li, N):ZnO films exhibit good crystallinity with a preferred c-axis orientation. From AFM studies, it is found that the surface roughness of the thin films increases with the increase of doping percentage. The Hall Effect measurements showed p-type conductivity. The Hall measurements have been performed periodically up to seven months and it is observed that the films show p-type conductivity throughout the period of observation. The samples with Li:N ratio of 8:8 mol% showed the lowest resistivity of 35.78 Ω cm, while sample with Li:N ratio of 6:6 mol% showed highest carrier concentration. The PL spectra of (Li, N):ZnO films show a strong UV emission at room temperature. Furthermore, PL spectra show low intensity in deep level transition, indicating a low density of native defects. This indicates that the formation of intrinsic defects is effectively suppressed by dual acceptor (Li, N) doping in ZnO thin films. The chemical bonding states of N and Li in the films were examined by XPS analysis.     

Synthesis and characterization of cuprous oxide dendrites: New simplified green hydrothermal route

Cuprous oxide (Cu₂O) dendrites were prepared by simple hydrothermal route at two different temperatures using starch as reducing and stabilizing agent. Scanning Electron Microscopy (SEM) revealed the alterations in morphology with reaction temperature and time. The spherical nanoparticles obtained at lower reaction temperature self-assembled into distinct dendritic nanostructures at high temperature. The mechanism of formation of dendrite over the polysaccharide template has been discussed. Transmission Electron Microscopy (TEM) revealed that the crystalline size of these dendrites in one dimension is about 50 nm. The nanoparticles were characterized by UV-vis and photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), FT-IR and Thermal Gravimetry Analyzer (TGA). Impedance analysis of the nanostructures showed conductivity to be a function of temperature.     

Sb doping behavior and its effect on crystal structure, conductivity and photoluminescence of ZnO film in depositing and annealing processes

The Sb-doped ZnO (ZnO:Sb) and undoped ZnO films with wurtzite structure and (0 0 2) preferred orientation were deposited on Si(1 0 0) substrate at 550 °C. It is deduced from XRD and XPS measurements that the Sb in the as-grown ZnO:Sb has high oxidation state and dopes in the form of oxygen-rich Sb-O clusters, which results in a large inner stress and a great increase of the c-axis lattice constant. After annealing at 750 °C under vacuum, the c-axis lattice constant of the ZnO:Sb decreases sharply to near the value of ZnO bulk, the electrical properties change from n-type to p-type and the PL intensity ratio of the visible to ultraviolet emission band goes down greatly, as the Sb content increases from 0 to 2.1 at.%. EDS and XRD measurements indicate that some of Sb dopants escape from the ZnO:Sb films and the oxygen-rich Sb-O clusters vanished after the annealing process. The effect of the change in Sb doping behavior on crystal structure, conductivity and PL is discussed in detail.     

Formation of zinc aluminum mixed metal oxide nanostructures

We report a method for synthesizing zinc aluminum layered double hydroxide (ZnAl:LDH) nanostructures at room temperature. The ZnAl:LDH nanoplates could be converted into zinc aluminum mixed metal oxide (MMO) nanostructures by calcination in air. The crystalline nature and morphology of the MMO nanostructures could be tuned by varying the calcination temperature. At low calcination temperatures (450-650 °C), nanostructures were composed of crystalline ZnO regions and amorphous regions. The crystalline orientations of the ZnO crystal grains were almost identical throughout the nanostructure. At calcination temperatures above 750 °C, ZnAl₂O₄ crystal grains appeared and amorphous regions could not be found in MMO nanostructures. As the calcination temperature increased, the crystal grain size and surface roughness of MMO nanostructures increased. Calcination at 950 °C resulted in the formation of MMO nanoparticles. The optical properties of the MMO nanostructures were probed by UV-vis diffuse reflectance spectroscopy. The spectra varied depending on their dimensions and crystalline natures.     

Studies of nanostructured copper/hydrogenated amorphous carbon multilayer films

Nanostructured copper/hydrogenated amorphous carbon (a-C:H) multilayer grown in a low base vacuum (1 × 10⁻³ Torr) system combining plasma-enhanced chemical vapor deposition and sputtering techniques. These nanostructured multilayer were found to exhibit improved electrical, optical, surface and structural properties, compared to that of monolayer a-C:H films. The residual stresses of such multilayer structure were found well below 1 GPa. Scanning electron microscopy and atomic force microscopy results revealed a nanostructured surface morphology and low surface roughnesses values. X-ray photoelectron spectroscopy, secondary ion mass spectroscopy and energy dispersive X-ray analysis confirmed a very small amount of copper in these films. These structures exhibited very high optical transparency in the near infrared region (∼90%) and the optical band gap varied from 1.35 to 1.7 eV. It was noticed that the temperature dependent conductivity improved due to the presence of both copper and the nano-structured morphology.     

New process for synthesis of ZnO thin films: Microstructural, optical and electrical characterization

Nanocrystalline ZnO thin films were prepared on glass substrates by using spin coating technique. The effect of annealing temperature (400-700 °C) on structural, compositional, microstructural, morphological, electrical and optical properties of ZnO thin films were studied by X-ray diffraction (XRD), Energy dispersive Spectroscopy (EDS), Atomic Force Microscopy (AFM), High Resolution Transmission Microscopy (HRTEM), Scanning Electron Microscopy (SEM), Electrical conductivity and UV-visible Spectroscopy (UV-vis). XRD measurements show that all the films are nanocrystallized in the hexagonal wurtzite structure and present a random orientation. The crystallite size increases with increasing annealing temperature. These modifications influence the optical properties. The AFM analysis revealed that the surface morphology is smooth. The HRTEM analysis of ZnO thin film annealed at 700 °C confirms nanocrystalline nature of film. The SEM results shows that a uniform surface morphology and the nanoparticles are fine with an average grain size of about 40-60 nm. The dc room temperature electrical conductivity of ZnO thin films were increased from 10⁻⁶ to 10⁻⁵ (Ω cm)⁻¹ with increase in annealing temperature. The electron carrier concentration (n) and mobility (μ) of ZnO films annealed at 400-700 °C were estimated to be of the order of 4.75-7.10 × 10¹⁹ cm⁻³ and 2.98-5.20 × 10⁻⁵ cm² V⁻¹ S⁻¹.The optical band gap has been determined from the absorption coefficient. We found that the optical band gap energy decreases from 3.32 eV to 3.18 eV with increasing annealing temperature between 400 and 700 °C. This means that the optical quality of ZnO films is improved by annealing.It is observed that the ZnO thin film annealing at 700 °C has a smooth and flat texture suited for different optoelectronic applications.     

Effect of lanthanum doping on linear and nonlinear-optical properties of Bi3TiNbO9 thin films

Lanthanum doped Bi₃TiNbO₉ thin films (LBTN-x, La³⁺ contents x = 5%, 15%, 25% and 35 mol.%) with layered perovskite structure were fabricated on fused silica by pulsed laser deposition method. Their linear and nonlinear optical properties were studied by transmittance measurement and Z-Scan method. All films exhibit good transmittance (>55%) in visible region. For lanthanum doping content are x = 5%, 15% and 25 mol.%, the nonlinear absorption coefficient of LBTN-x thin films increases with the La³⁺ content, then it drops down at x = 35 mol.% when the content of La³⁺ in (Bi₂O₂)²⁺ layers is high enough to aggravate the orthorhombic distortion of the octahedra. We found that, 25 mol.% is the optimal La³⁺ content for LBTN-x thin films to have the largest nonlinear absorption coefficient making the LBTN-x film a promising candidate for absorbing-type optical device applications.