Synthesis and studies on effect of indium doping on physical properties of electrodeposited CdSe thin films

Semiconducting CdSe and indium doped CdSe (In: CdSe) thin films have been synthesized on stainless steel and fluorine doped tin oxide coated glass substrates in an aqueous medium using a potentiostatic mode of electrodeposition. The doping concentration of indium has been optimized to 0.15 vol% using the reliable photoelectrochemical technique. To study the effect of indium doping these films are characterized using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, elemental mapping, Raman spectroscopy, contact angle measurement and UV–visible spectrophotometry techniques. CdSe and In: CdSe thin films are low crystalline with a cubic crystal structure. The valence states of CdSe and In: CdSe thin films are analyzed by means of XPS. Undoped CdSe thin film shows fiberlike morphology, which transforms into a beautiful web of nanofibers upon doping. The Elemental composition of both films analyzed by means of energy dispersive X-ray spectroscopy. Raman studies show transverse optical and longitudinal optical modes of phonon. Indium doping improves the hydrophilic nature of CdSe photoanode. The optical band gap (direct) found to be decreased from 2.02 to 1.67 eV upon indium doping. Both films are photoactive in nature.     

Investigation of size dependent structural and optical properties of thin films of CdSe quantum dots

Cadmium selenide (CdSe) quantum dots were grown on indium tin oxide substrate using wet chemical technique for possible application as light emitting devices. The structural, morphological and luminescence properties of the as deposited thin films of CdSe Q-dot have been investigated, using X-ray diffraction, transmission electron microscopy, atomic force microscopy and optical and luminescence spectroscopy. The quantum dots have been shown to deposit in an organized array on ITO/glass substrate. The as grown Q-dots exhibited size dependent blue shift in the absorption edge. The effect of quantum confinement also manifested as a blue shift of photoluminescence emission. It is shown that the nanocrystalline CdSe exhibits intense photoluminescence as compared to the large grained polycrystalline CdSe films.     

ESCA study on the mechanism of adherence of metal to silica glass

Indium oxide films formed on the surface of silica glass samples by selective oxidation have been shown to greatly improve the adherence of gold-indium alloy to the silica glass. In order to clarify the role that the oxide films play in the reaction, thin indium films have been evaporated onto silica glass and heated at temperatures of between 973 and 1473 K, both in air and in hydrogen gas. Electron spectroscopy for chemical analysis (ESCA) measurements have then been made to investigate the chemical environments around oxygen and indium atoms at the reacting interface between the oxide and glass. Measured 01s and In3d _5/2 spectra reveal the formation of non-bridging oxygen atoms at the interface, in addition to the original bridging oxygen atoms in silica glass. Introduction of the non-bridging oxygen atoms and indium ions into the silica glass is concluded to be an essential factor in promoting good adherence between the gold alloy and silica glass.     

Structural and optical properties of nanocrystalline CdSe and Al:CdSe thin films for photoelectrochemical application

Nanocrystalline CdSe and Al:CdSe semiconductor thin films have been successfully synthesized onto amorphous and FTO glass substrates by spray pyrolysis technique. Aqueous solutions containing precursors of Cd and Se have been used to obtain good quality films. The optimized films have been characterized for their structural, morphological, wettability and optical properties. X-ray diffraction (XRD) studies show that the films are polycrystalline in nature with hexagonal crystal structure. Scanning electron microscopy (SEM) studies show that the film surface is smooth, uniform and compact in nature. Water wettability study reveals that the films are hydrophilic behavior. The formation of CdSe and Al:CdSe thin film were confirmed with the help of FTIR spectroscopy. UV–vis spectrophotometric measurement showed a direct allowed band gap lying in the range 1.673–1.87 eV. Output characteristics were studied by using cell configuration n- CdSe/Al:CdSe |1 M (NaOH + Na₂ + S)|C. An efficient solar cell having a power conversion efficiency of 0.38% at illumination 25 mW cm⁻² was fabricated.     

Annealing effects on the photoresponse properties of CdSe nanocrystal thin films

The photoresponse properties of the as-prepared and annealed close-packed CdSe nanocrystal (NC) films were investigated under laser illumination by Kelvin probe force microscopy. The annealing process improved the photoresponse speed of the CdSe NC films. The work function of the annealed CdSe NC films changed more rapidly than that of the non-annealed film in air at room temperature. Combined with X-ray photoelectron spectroscopy measurements and thermogravimetric analysis, the observed phenomena can be interpreted that annealing process removed the organic capping agents completely and eliminated oxide on the CdSe surface, which formed the tunnel barrier between NCs in the CdSe NC films. Consequently, it improved the separation rate of photoelectric charges and thus provided high speed photoresponse.     

Large-scale growth of Cu2ZnSnSe4 and Cu2ZnSnSe4/Cu2ZnSnS4 core/shell nanowires

We present a fast and simple protocol for large-scale preparation of quaternary Cu(2)ZnSnSe(4) (CZTSe), as well as CZTSe/Cu(2)ZnSnS(4) (CZTS) core/shell nanowires using CuSe nanowire bundles as self-sacrificial templates. CuSe nanowire bundles were synthesized by reacting Cu(2 - x)Se nanowire bundles with sodium citrate solution. CZTSe nanowires were prepared by reacting CuSe nanowire bundles with Zn(CH(3)COO)(2) and SnCl(2) in triethylene glycol. X-ray diffraction (XRD) and selected area electron diffraction studies show that stannite CZTSe is formed. The formed CZTSe nanowire bundles have diameters of 200-400 nm and lengths of up to hundreds of micrometers. CZTSe/CZTS nanocable bundles with similar morphologies were grown by the addition of some elemental sulfur to the reaction system for growth of CZTSe bundles. The stannite CZTSe/kesterite CZTS core/shell structure of the grown nanocables was confirmed by XRD and high-resolution transmission electron microscope investigation. The influence of S/Se molar ratio in the reaction system on the crystallographic structures and optical properties of CZTSe/CZTS nanocables was studied. The obtained CZTSe/CZTS core/shell nanocable bundles show broad and enhanced optical absorption over the visible and near-infrared region, which is promising for use in photovoltaic applications.     

Photoelectrochemical conversion using sprayed CdSe

CdSe films on titanium have been prepared by spray pyrolysis and characterized by X-ray diffraction and X-ray photon spectroscopy. Photoelectrochemical solar cells with 4·84% conversion efficiency have been fabricated using these electrodes. Spectral response studies indicate absorption at sub-band gap energies. The band gap of CdSe as determined from photo-current studies was 1·66 eV. The flat band potential measured gives a value of −1·54 V with reference to SCE for electrodes prepared using cadmium chloride and selenourea in the 1:1 ratio.     

Structural, optical and photoelectrochemical properties of electrodeposited CdSe thin films

Cadmium Selenide thin films have been electrodeposited from an acidic bath using CdSO₄ as a cadmium source and SeO₂ as a selenium source at pH=3 on to stainless steel and fluorine-tin oxide coated glass substrates. The CdSe films have been characterized by X-ray diffraction, scanning electron microscopy and optical absorption. X-ray diffraction spectra showed that CdSe is polycrystalline with single hexagonal phase. The intensity of the (0 0 2) peak increases remarkably by annealing in nitrogen atmosphere. A microstructural study revealed that the films were uniform and well covered the substrate. Optical absorption studies showed that the bandgap of the CdSe is 1.70 eV. It is observed that the conductivity of the CdSe films increases by annealing in nitrogen atmosphere. The photoelectrochemical activities of CdSe films deposited on stainless steel and fluorine-tin oxide coated glass have been studied by using CdSe/ 1 M NaOH-1 M Na₂S-1 MS / C cell configuration and it is found that films deposited on stainless steel give better performance, photoelectrochemical (PEC) studies also reveal that the CdSe has n-type conductivity.     

Layer-by-layer growth of CdSe-based nanocrystal light-emitting diodes

The partial exchange of surface-passivating trioctylphosphine oxide (TOPO) on CdSe and ZnS-clad CdSe (CdSe/ZnS) nanocrystals with primary amines was utilized to grow ultra-thin films of these nanocrystals under nonaqueous conditions. This growth was achieved using 1,12-diaminododecane in a layer-by-layer assembly format, where one of the amino groups binds with the nanocrystal surface and the other regenerates the interface for further binding of nanocrystals. The nature of the growth is dependent on the relative surface affinity between the TOPO and the primary amine toward the zinc or cadmium sites on the nanocrystals. Using this technique, high-quality luminescent films of these nanocrystals can be built with well-defined thicknesses. Electroluminescent devices have been fabricated using this methodology.     

Interface synthesis, assembly and characterization of close-packed monolayer of prussian blue/polypyrrole nanocomposites

The self-assembly of nanomaterials into ordered two- or three-dimensional structures has been extensively explored over the past years due to the great application potential. This paper reports a simple one-step methodology for the synthesis and assembly of Prussian blue/polypyrrole (PBPPy) nanocomposites into close-packed monolayer at a toluene-water interface where the formation of PBPPy and evaporation of organic phase happen simultaneously. The formed films could be easily transferred onto the glass carbon surface by layer-by-layer technique. The obtained PBPPy nanocomposites and their assembled film were characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, UV-vis spectroscopy and cyclic voltammetry. The PBPPy nanocomposite film on the electrode surface exhibits excellent electron transfer and high electrocatalysis toward the reduction of H2O2. Herein, PPy component in PBPPy nanocomposites plays a great role in this great electrochemical performance. On the one hand, the presence of PPy is helpful of the formation of close-packed monolayer of PBPPy nanocomposites, and the adhesion of the ordered monolayer or multilayer onto the electrode surface due to the molecular interaction of PPy or the surface group of electrode. On the other hand, the conductive PPy facilitates the electron transfer between PB and PB or PB and electrode. The suggested method can be extended to a wide range of nanomaterials assembly and devise development.     

Growth of poly-crystalline silicon-germanium on silicon by aluminum-induced crystallization

The formation of poly-crystalline silicon-germanium films on single-crystalline silicon substrates by the method of aluminum-induced crystallization was investigated. The aluminum and germanium films were evaporated onto the single-crystalline silicon substrate to form an amorphous-germanium/aluminum/single-crystalline silicon structure that was annealed at 450 °C-550 °C for 0-3 h. The structural properties of the films were examined using x-ray diffraction, Raman spectroscopy and Auger electron spectroscopy. The x-ray diffraction patterns confirmed that the initial transition from an amorphous to a poly-crystalline structure occurs after 20 min of aluminum-induced crystallization annealing process at 450 °C. The micro-Raman spectral analysis showed that the aluminum-induced crystallization process yields a better poly-crystalline SiGe film when the film is annealed at 450 °C for 40 min. The growth mechanism of the poly-crystalline silicon-germanium by aluminum-induced crystallization was also studied and is discussed.     

Resonant infrared matrix-assisted pulsed laser evaporation of CdSe colloidal quantum dot/poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-(1-cyano vinylene)phenylene] hybrid nanocomposite thin films

CdSe colloidal quantum dot / poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-(1-cyano vinylene)phenylene] hybrid nanocomposite thin films were deposited using resonant infrared matrix-assisted pulsed laser evaporation. The distributions of CdSe colloidal quantum dots within the polymer matrices of as-grown films were evaluated using transmission electron microscopy, and the optical properties of these films were determined by photoluminescence spectroscopy. These measurements demonstrate that: i) depending upon the deposition parameters used, the CdSe colloidal quantum dot distribution can be tuned between two morphology extremes, i.e. clustering or homogenous dispersion; and ii) the constituent materials of the nanocomposite are not damaged in any way that affects structural or optical properties by the deposition process. The demonstrated ability to control nanoparticle distribution within organic films has not been achieved by other deposition techniques and could enhance the performance of optoelectronic devices based on these materials.     

On the chemistry at the Si,Ti-doped a-C:H/TiC interface

Silicon-titanium-doped a-C:H, deposited via plasma assisted chemical vapor deposition and its chemistry at the titanium carbide interface has been studied via X-ray photoelectron spectroscopy. In the a-C:H film, as well as at the interface, the carbide species TiSiC is formed. Thermal treatment of Si,Ti-a-C:H films on TiC causes an increase in TiSiC at the interface leading to a better adhesion performance.     

Alloy formation at the tetrapod core/arm interface

The nature of the interfacial structure between the core and the arms of a tetrapod quantum dot (QD) formed during the heteroepitaxial growth of a ZnS arm onto a CdSe core is not well understood but can be analyzed through the use of high-frequency electron paramagnetic resonance (HF-EPR) spectroscopy. The spectroscopic resolution at high frequency allows the presence of unique crystal fields reflecting interfacial alloying to be analyzed by incorporating Mn(II) ions as a dopant into the QD to act as an intentional EPR active spectroscopic probe. In addition, the HF-EPR can spectroscopically observe the presence of ion vacancies that are anticipated to form at the heteroepitaxial interface to accommodate structural mismatch. The HF-EPR spectra for Mn(II) are extremely sensitive to perturbations of the microenvironment due to changes in the crystal field. The HF-EPR spectra of Mn(II) in a CdSe (core)/ZnS (arm) tetrapod exhibiting wurtzite symmetry for both core and interface of the tetrapod provide clear evidence of heteroalloying at the core-arm interface and formation of intrinsic dislocations at grain boundaries. The formation of the interfacial alloy and grain boundaries reflects short-range ion migration at the heteroepitaxial layer to reduce strain energy due to the 12% lattice mismatch between the wurtzite lattices of CdSe and ZnS.     

Preparation of nanocrystalline Mg doped CdSe thin films and their optical,photoluminescence, electrical and structural characterization

Phosphors used are mostly rare earth doped complex structures. A simple and unique material system of CdSe:Mg nanocrystalline thin films, which efficiently absorb UV (235 nm) and emit broad spectrum of green-yellow region has been prepared by chemical bath deposition method with average particle size of 52.3 nm, measured using AFM images. The optical absorption studies found that CdSe thin film has direct optical band gap, \({E_g}\) of 2.62 eV that shows a blue shift of 0.88 eV compared to the bulk \({E_g}\) value. Optical, electrical, structural and morphological properties were studied by UV–Vis–NIR spectrophotometer, photoluminescence (PL) emission spectra, dc two-probe method, X-ray diffraction (XRD), and atomic force microscope (AFM). Measured electrical resistivity decreased with increase of doping concentration. Activation energy was also calculated. The results confirm that the CdSe:Mg thin films are in the pure cubic phase. The magnesium concentrations also affect the nanocrystalline nature of the CdSe thin films. The optical band gap and surface roughness of CdSe thin films mostly decrease with 5% doping of Mg. The effect of Mg doping on refractive index, extinction coefficient and other optical parameters was also investigated.     

Electron-microscopic investigation of a SiC/Si(111) structure obtained by solid phase epitaxy

First results of the electron-microscopic investigation of thin silicon carbide (SiC) layers grown on silicon using a new method of solid phase epitaxy are presented. It is shown that, at the initial stage of epitaxial growth, a transition layer is formed which consists of various SiC polytypes. This layer occurs at the interface between the substrate and a single-crystalline SiC layer possessing predominantly a 3C polytype structure. It is established that pores with dimensions ranging from a fraction of micron to several dozen nanometers are formed in a near-surface layer of the silicon substrate, which favor the growth of epitaxial, weakly strained single-crystalline SiC layers.     

Achieve 2-inch-diameter homogeneous GaN films on sapphire substrates by pulsed laser deposition

The 2-inch-diameter homogeneous GaN films have been epitaxially grown on sapphire substrates by pulsed laser deposition (PLD) technique with optimized laser rastering and PLD growth conditions. The as-grown GaN films are characterized by in situ reflection high-energy electron diffraction, white-light interferometry, scanning electron microscopy, atomic force microscopy (AFM), grazing incidence angle X-ray reflectivity, reciprocal space mappings, and micro-Raman spectroscopy for surface morphologies and structural properties. The as-grown 2-inch-diameter single-crystalline GaN films exhibit excellent thickness uniformity with a root-mean-square (RMS) inhomogeneity less than 3.4 % and very smooth surface with a RMS roughness less than 1.3 nm measured by AFM. There is a maximum of 1.2 nm thick interfacial layer existing between the as-grown GaN films and sapphire substrates, and the as-grown 310 nm thick GaN films are almost fully relaxed only with an in-plane compressive strain of 0.044 %. This work demonstrates a possibility for achieving high-quality large-scale GaN films with uniform thickness and atomically abrupt interface by PLD, and is of great interest for the commercial development of GaN-based optoelectronic devices.     

Physico-chemical characterization of thin films obtained by fluorination of GaAs under 5 bar of fluorine

Fluoride-GaAs(100) structures have been prepared by fluorination under 5 bar of fluorine for different temperatures and times. This paper deals with the preparation method and with the physico-chemical characterization of the samples obtained. The bulk of the fluoride layers has been investigated by Rutherford backscattering, IR absorption and Auger electron spectroscopy measurements. It has been demonstrated that for fluorine at this pressure, the best oxidation temperature is close to 200°C. The bulk composition of the films formed at the surface of GaAs substrates is GaF₃. The semiconductor-fluoride interface has been studied using the X-ray photoelectron spectroscopy technique, and arsenic has been found to be bound to fluorine.     

Magnetic force microscopy on nanocrystalline Co films

Pioneer works in ultrathin magnetic films have shown perpendicular magnetic domains in the demagnetized state. The source of this perpendicular anisotropy is the interface anisotropy developed at the interface. Similar domains could be observed in tetragonally distorted ultrathin films due to the magnetoelastic anisotropy. On the other hand, single-crystalline hexagonal close packed (hcp) Co films when grown epitaxially with the c-axis oriented perpendicular to the film plane may show perpendicular stripe magnetic domains even up to a thickness of about 500 nm. In that case the source of perpendicular anisotropy was the magnetocrystalline anisotropy of bulk Co, which favors the c-axis. In this work, we have grown by radio frequency magnetron sputtering Co films in the thickness range 15-4500 nm. We have used various substrates, such as Corning glass, silicon and Al-foil. The substrate temperature was about 350 K. The films have been found by X-ray diffraction experiments to present various structures and textures depending on the preparation conditions, mainly the Ar-pressure and deposition rate. Stripe- and labyrinth-like domain configurations are observed in films textured along the c-axis, and in films with a mixture of hcp and fcc grains, repectively. Films which show mainly fcc or amorphous structure do not form perpendicular domains. The results are discussed with respect to magnetization loops.     

Epitaxial film growth of chromium dioxide by low pressure chemical vapor deposition using chromium carbonyl

Epitaxial chromium dioxide (CrO₂) thin films have been deposited by low pressure chemical vapor deposition (LPCVD) on (100) TiO₂ substrates using the precursor chromium hexacarbonyl (Cr(CO)₆) within a narrow temperature window of 380-400 °C. Normal θ-2θ Bragg x-ray diffraction results show that the predominant phase is CrO₂ with only a small amount of Cr₂O₃ present, mostly at the film surface. The LPCVD films have a reasonably smooth surface morphology with a root mean square roughness of 4 nm on a scale of 5 μm. Raman spectroscopy confirms the existence of rutile CrO₂ in the deposited films, while transmission electron microscopy confirms the single-crystalline nature of the films. The LPCVD films showing a dominant CrO₂ phase exhibit clear uniaxial magnetic anisotropy with the easy axis oriented along the c direction.