A. Dinia

Growth and characterization of electrodeposited Cu2O thin films

This work demonstrates the electrodeposition of cuprous oxide (Cu2O) thin films onto a fluorine-doped tin oxide (FTO)-coated conducting glass substrates from Cu(II) sulfate solution with C6H8O7 chelating agent. During cyclic voltammetry experiences, the potential interval where the electrodeposition of Cu2O is carried out was established. The thin films were obtained potentiostatically and were characterized through different techniques. From the Mott–Schottky measurements, the flat-band potential and the acceptor density for the Cu2O thin films are determined. All the films showed a p-type semiconductor character with a carrier density varying between 2.41 × 1018 cm−3 and 5.38 × 1018 cm−3. This little difference is attributed to the increase of the stoichiometric defects in the films with the deposition potential. Atomic force microscopy analysis showed that the Cu2O thin films obtained at high potential are more homogenous in appearance and present lower crystallites size. X-ray diffraction measurements indicate a cubic structure with good crystallization state and the deposition potential was found to have an influence on the size of the crystallites. The optical measurements show a direct band gap between 2.07–2.49 eV depending on the applied potential.

Optical and structural properties of Nd doped SnO2 powder fabricated by the sol–gel method

We report on the structural and optical properties of undoped and neodymium doped SnO2 powders (0, 1, 3, and 5 at% of Nd) synthesized by the sol–gel method. SEM and TEM microscopy techniques reveal a nanometric scale of the powders. We show that the tetragonal rutile phase is achieved after annealing at 700 °C. The crystallite size of the doped SnO2 is found to decrease gradually with the increase of Nd content without changing the SnO2 structure. A strong decrease in the intensity of the Raman peaks is noted for doped powders, which can be attributed to the location of Nd3+ ions at the Sn sites indicating Nd incorporation into the host matrix. For the first time the optical properties were studied by UV-Vis-NIR spectroscopy and revealed Nd related absorption bands in the SnO2 matrix. The investigation of the photoluminescence properties shows broad emission centred around 550–650 nm originating from defects present in the SnO2 host matrix. Under 325 nm laser excitation, a strong photoluminescence of trivalent Nd is observed in the infrared region and shows Nd related emission peaks at 885, 1065, and 1336 nm. Such a strong PL signal under laser excitation indicates that Nd3+ is optically active. The excitation dependent PL (PLE) recorded in the 450–700 nm range confirms the presence of active Nd3+ successfully inserted into the SnO2 host matrix.

Effect of Al concentrations on the electrodeposition and properties of transparent Al-doped ZnO thin films

Al-doped zinc oxide (AZO) thin films are prepared on polycrystalline fluorine-doped tin oxide-coated conducting glass substrates from nitrates baths by the electrodeposition process at 70xa0°C. The electrochemical, morphological, structural and optical properties of the AZO thin films were investigated in terms of different Al concentration in the starting solution. It was found that the carrier density of AZO thin films varied between −3.11 and −5.56xa0×xa01020xa0cm−3 when the Al concentration was between 0 and 5 at.%. Atomic force microscopy images reveal that the concentration of Al has a very significant influence on the surface morphology and roughness of thin AZO. X-ray diffraction spectra demonstrate preferential (002) crystallographic orientation having c-axis perpendicular to the surface of the substrate and average crystallites size of the films was about 33–54xa0nm. With increasing Al doping, AZO films have a strong improved crystalline quality. As compared to pure ZnO, Al-doped ZnO exhibited lower crystallinity and there is a shift in the (002) diffraction peak to higher angles. Due to the doping of Al of any concentration, the films were found to be showing >80xa0% transparency. As Al concentration increased the optical band gap was also found to be increase from 3.22 to 3.47xa0eV. The room-temperature photoluminescence spectra indicated that the introduction of Al can improve the intensity of ultraviolet (UV) emission, thus suggesting its greater prospects in UV optoelectronic devices. A detailed comparison and apprehension of electrochemical, optical and structural properties of ZnO and ZnO:Al thin films is done for the determination of optimum concentration of Al doping.

Photoluminescence properties of rare earth (Nd, Yb, Sm, Pr)-doped CeO2 pellets prepared by solid-state reaction

Several structural and optical properties of ceria (band gap, refractive index and lattice parameter) make this material very promising for applications in optoelectronics and photovoltaics. In this paper, we show that CeO2 can be efficiently functionalized by doping with trivalent rare earth ions to give rise to photon management properties. The trivalent ions can be successfully inserted by solid-state reaction of the elementary oxide powders. By combining the information obtained from the absorbance spectra with that of the PL excitation spectra, we demonstrate the presence of the trivalent ions in CeO2 and provide insight in the electronic level structure and transfer mechanism. In particular, we prove that both the complex absorption spectra and the energy transfer mechanisms cannot be fully explained without considering the presence of isolated Ce3+ ions in CeO2.

Annealing treatment for restoring and controlling the interface morphology of organic photovoltaic cells with interfacial sputtered ZnO films on P3HT:PCBM active layers

In this paper, we report on the photovoltaic properties of conventional organic photovoltaic solar cells integrating a sputtered ZnO interfacial film deposited on the absorber P3HT:PCBM layer. An emphasis has been put on the influence of the annealing temperature and time for restoring and controlling the P3HT:PCBM/ZnO interface morphology, which can be damaged by the sputtering process. We show a significant improvement in the current–voltage (J–V) characteristics upon annealing up to 160 °C. This is evidenced by the reduction of the S-shape of these curves systematically observed for the cells integrating thick (100 nm) sputtered ZnO films. This approach was also highlighted on cells containing thinner (20 nm) ZnO films using a longer annealing process at 140 °C, which led to a significant improvement of the power conversion efficiency compared with the value recorded in as-prepared cells or in cells with no interfacial ZnO layer. These photovoltaic performances have been related to the change of the morphology of the absorber layer and to the vertical phase segregation of P3HT:PCBM at the interface with ZnO. Optical microscopy, scanning electron microscopy and atomic force microscopy have been performed in order to confirm this approach.

Effect of nitrate concentration on the electrochemical growth and properties of ZnO nanostructures

Zinc oxide (ZnO) nanostructures were deposited under potentiostatic control on indium tin oxide coated glass substrate from an aqueous solution containing zinc nitrates. Voltammograms were recorded to determine the optimal potential region for the deposition of ZnO. The deposition was carried out at various concentrations of Zn+2 and constant bath temperature (65xa0°C). The nucleation and growth kinetics at the initial stages of ZnO studied by current transients indicated a 3D island growth (Volmer–Weber). It is characterized by an instantaneous nucleation mechanism followed by diffusion-limited growth. The Mott–Schottky measurements, the flat band potential and the donor density for the ZnO nanostructures were determined. The morphological, structural, and optical properties of the nanostructures have been investigated. Scanning electron microscopy images showed different sizes and morphologies of the nanostructures which depends on the concentrations of Zn+2. X-ray diffraction study confirms the wurtzite phase of the ZnO nanostructures with high crystallinity. UV–visible spectra showed a significant optical transmission (up to 90xa0%), which decreased with Zn2+ concentrations.xa0The energy band gap values have been estimated to be in the range 3.36–3.54xa0eV.

The influence of pH electrolyte on the electrochemical deposition and properties of nickel thin films

Ni thin films were electrodeposited on gold substrate from chloride solution with different pH at room temperature. The effect of electrolyte pH on Ni coatings was studied by using the cyclic voltammetry, the scanning electron microscopy (SEM), x-ray diffraction, and alternating gradient force magnetometer measurements. From electrochemical measurements, the onset potential for reduction of Ni was gradually shifted towards more cathodic scan with increase in pH; this is due to the protons in the case of low pH values and to the hydroxide ions in the case of higher pH values. The SEM study showed that a granular and compact structure of the electrodeposited Ni layers and the variation of film morphology with bath pH are established. The x-ray diffraction spectra revealed the formation of fcc structure Ni thin films with a preferential orientation along the Ni(111). The size of the deposited crystals in both the cases has been found to be in the range of 49–153xa0nm. Magnetic properties such as coercivity and saturation magnetization showed strong dependence on the electrolyte solution pH and consequently the crystallite size. Coercivity higher than 130–160xa0Oe was achieved for a pH value of 4 to 5. The differences observed in the magnetic properties were attributed to the structural changes caused by the electrolyte pH.

Effect of the thickness of the ZnO buffer layer on the properties of electrodeposited p-Cu2O/n-ZnO/n-AZO heterojunctions

Transparent conducting Cu2O/non-doped ZnO/Al-doped ZnO/FTO heterojunction solar cells were fabricated by a three-step electrodeposition by inserting a thin non-doped ZnO film as a buffer layer between a n-AZO thin film and a p-Cu2O nanostructure. The effect of the thickness of the buffer layer on the properties of the heterojunction was investigated by means of a number of techniques. Mott–Schottky electrochemical impedance analysis showed a p-type conductivity for the Cu2O layers and an n-type conductivity for the doped and undoped ZnO films. Analysis also showed that the flat band and carrier concentration of the ZnO thin films varied with the thickness of the layer of ZnO. From field emission scanning electron microscopy (FE-SEM) observation, when the thickness of ZnO was increased, the grains size and the morphology of Cu2O was affected; in addition, the cubic structure of Cu2O was damaged. This was confirmed by the atomic force microscopy (AFM) images, which showed that the surface morphology transformed from a pyramid shape to a granular form when the thickness of ZnO increased. The X-ray diffraction (XRD) analysis indicated that with Cu2O, the undoped and the doped ZnO nanostructures have a polycrystalline nature and a cubic and hexagonal wurtzite structure with (111) and (101) preferential orientations, respectively. We also noted a high transmittance of 65% from the UV-Vis spectra and a band gap energy as large as 2.4 eV was found. The current–voltage (I–V) characteristics of p-Cu2O/n-ZnO/n-AZO heterojunctions with different ZnO buffer layer thicknesses were investigated. The results showed that p-Cu2O/n-ZnO/n-AZO heterojunctions have a well-defined rectifying behavior.

Effect of strontium deficiency on the structural, magnetic and magnetocaloric properties of La0.65Eu0.05Sr0.3−xMnO3 (0 ≤ x ≤ 0.15) perovskites

We have investigated the effect of nominal strontium deficiency on the structure, magnetic and magnetocaloric properties of La0.65Eu0.05Sr0.3−xMnO3 (x = 0, 0.10 and 0.15) perovskites. The nanocrystalline samples were prepared by the sol–gel-based Pechini method. Rietveld refinement of the X-ray diffraction patterns, shows the formation of single-phase compositions with rhombohedral symmetry (space group Rc, no. 167). Raman spectra at room temperature reveal a gradual change in phonon modes with increasing nominal strontium deficiency. All the samples undergo paramagnetic–ferromagnetic (PM–FM) transition. The Curie temperature decreases linearly with increasing x and changes from 355 K for x = 0 to 280 K for x = 0.15. Arrott plot analyses and a universal curve method were applied for studying the order of the magnetic transition in this system, found to be of second order. As strontium deficiency content increased further, peak entropy values were seen to decline. However, a simultaneous broadening of the ΔSmaxM peaks led to enhanced relative cooling power (RCP) in the Sr-deficiency samples of up to 22% over that of La0.65Eu0.05Sr0.3MnO3. La0.65Eu0.05Sr0.15MnO3 exhibits the largest RCP value of 283 J kg−1 at 280 K among the compounds investigated up to 5 T applied field. Through these results, La0.65Eu0.05Sr0.3−xMnO3 materials are strongly suggested for use as active refrigerants for magnetic refrigeration technology near and above room temperature.

Efficient energy transfer from ZnO to Nd3+ ions in Nd-doped ZnO films deposited by magnetron reactive sputtering

In this paper, a detailed study of the luminescent properties of Nd3+ ions in sputtered ZnO thin films is reported for the first time. Experimental evidence is provided showing that Nd is inserted and optically active in the ZnO matrix. Despite the small amount (<2%) of rare earth in these thin ZnO films, intense luminescence signals have been collected, indicating efficient infrared emission of Nd3+ in ZnO. Direct excitation of Nd3+ ions in the ZnO matrix was possible, suggesting that most of the Nd atoms are in the 3+ form at all deposition temperatures. Moreover, intense Nd3+ emission has been recorded also when the host was excited, indicating that an efficient energy transfer occurs from ZnO to Nd ions. Both the transfer efficiency and the Nd3+ concentration seem to depend on the deposition temperature. In particular, indirect excitation of the sample deposited at 400 °C generates a richer emission pattern compared to lower temperatures. The careful analysis of the luminescence data indicated that the new pattern comes from Nd sites that cannot be efficiently directly excited, but that are characterized by intense emission under indirect excitation of the host. The possible transfer mechanisms leading to this behavior will be outlined.