Evidence of phase transition in porous silicon

We have studied the ageing effects on the photoluminescence (PL) of porous silicon (PS) prepared by anodisation in HF solution of low concentration (3% ). It was found that the PL behaviour within oxidation is different from that elaborated in habitual condition. The natural oxidation of PS layers shows two opposite behaviours of the PL before stabilisation. The PL intensity decreases significantly after an initial strong and fast increase. At low temperature, the PL exhibits large band and multi-peaks structures. It was shown by electron paramagnetic resonance (EPR) and Raman measurements that a possible phase transition from crystalline to amorphous phase can occur. We show that each transformation is related to preparation conditions

Photoluminescence properties of porous silicon nanocomposites

Abstract Different porous silicon (PS) layers were impregnated with rhodamine 6G (Rh) solution in order to form Rh/PS nanocomposites. The effect of the porous matrix (fresh, oxidised, type p + ) on the propriety of photoluminescence (PL) has been investigated. It was found that the luminescence of this nanocomposite is provided by an energy transfer from PS nanocristallites to rhodamine and from interaction of dye molecules with the chemical species on the Si surface. An antistokes PL has been observed using a He-Ne laser excitation. The dependence of its intensity via power excitation suggests a process of two-step two-photon absorption as origin of this PL. Nanocomposite formed by PS and ZnO has been also investigated. We show that this transparent oxide does not degrade the skeleton of PS and does not inhibit PL. The PL band shifts to high energy and the intensity becomes stable providing the passivation of the dangling bonds on the Si-surface by ZnO

Origin of the photoluminescence shifts in porous silicon

The origin of the photoluminescence (PL) shifts in Porous Silicon (PS) is discussed according to a quantum confinement – based model, in which we modelize the PS layer as a mixture of quantum dots and wires. It was shown that a PL blueshift or redshift may occur during laser irradiation of PS, depending on preparation conditions. No PL shift was observed for some PS samples, even after a long ageing in air, due to the presence of an amorphous silicon phase detected from Raman spectroscopy measurements. It was found that the presence of the amorphous phase plays an important role in the PL behaviour of oxidised PS

Structure and electrical characterization of ZnO-Ag phosphate glasses

Silver zinc phosphate glasses with a composition (40 − x/2) P2O5 – (40 − x/2) Na2O–20ZnO–x AgNO3 (x = 5, 10 and 15 mol%) were prepared of the glasses via conventional melt-quenching technique. From the Raman spectrum, the structure of the glasses was analyzed. Conduction and relaxation mechanisms in these glasses were studied using impedance spectroscopy in a frequency range from 10 Hz to 13 MHz and a temperature range from 323 K to 623 K. The dependence of electrical data on frequency was analyzed in the framework of the Nyquist's plot and Jonscher’s power law. The semicircles observed in the plots indicate a double relaxation process. The studied materials exhibit a significant contribution of bulk and interfacial effect to electrical conduction and to non-Debye relaxation process. The dc conductivity (σdc) follows Arrhenius behavior with temperature. The ac and dc conductivities of the samples were found to increase with the increase in temperature. The conductivity variation for P2O5-Na2O-ZnO glasses doped with various concentrations of AgNO3 was explained by the presence of ionic contribution.The dielectric characterizations include measurements involving the variation of the dielectric constant as well as the dielectric loss with frequency. The dielectric studies show low values for the dielectric constant and loss at high frequencies. Dependence of the electrical modulus of the glasses on frequency and temperature presented a relaxation phenomenon. Keywords: Glasses, Impedance spectroscopy, Conduction, Dielectri

Transport Mechanisms and Dielectric Features of Mg-Doped ZnO Nanocrystals for Device Applications

Magnesium-doped zinc oxide “ZnO:Mg” nanocrystals (NCs) were fabricated using a sol gel method. The Mg concentration impact on the structural, morphological, electrical, and dielectric characteristics of ZnO:Mg NCs were inspected. X-ray diffraction (XRD) patterns display the hexagonal wurtzite structure without any additional phase. TEM images revealed the nanometric size of the particles with a spherical-like shape. The electrical conductivity of the ZnO NCs, thermally activated, was found to be dependent on the Mg content. The impedance spectra were represented via a corresponding circuit formed by a resistor and constant phase element (CPE). A non-Debye type relaxation was located through the analyses of the complex impedance. The conductivity diminished with the incorporation of the Mg element. The AC conductivity is reduced by raising the temperature. Its plot obeys the Arrhenius law demonstrating a single activation energy during the conduction process. The complex impedance highlighted the existence of a Debye-type dielectric dispersion. The various ZnO:Mg samples demonstrate high values of dielectric constant with small dielectric losses for both medium and high-frequency regions. Interestingly, the Mg doping with 3% content exhibits colossal dielectric constant (more than 2 × 104) over wide temperature and frequency ranges, with Debye-like relaxation. The study of the electrical modulus versus the frequency and at different temperatures confirms the non-Debye relaxation. The obtained results reveal the importance of the ZnO:Mg NCs for device applications. This encourages their application in energy storage

Mixed Ionic and Electronic Conduction in TeO2-ZnO-V2O5 Glasses towards Good Dielectric Features

The melt-quenching technique was used to synthesize tellurite glasses of the chemical composition 80TeO2-(20-x) ZnO-xV2O5. X-ray diffraction (XRD) patterns indicate the amorphous nature of the prepared glasses. Raman and FTIR measurements demonstrate a progressive substitution of the Te-O-Te linkages by the Te-O-V bridges and the formation of VO4 and VO5 units by a change of the vanadium coordination due to the higher number of oxygens incorporated by further addition of V2O5. The AC conductivity was investigated in the frequency range of 40 Hz to 107 Hz between 473 K to 573 K. A good coherence of the AC conductivity was found using a model correlating the barrier hopping (CPH) and the dominant conduction process changes from ionic to polaronic with the addition of V2O5. The dielectric constant exhibits high values in the range of lower and medium frequencies. Both variations of the electric modulus and the dielectric loss parameters with frequency and temperature showed a relaxation character mainly assigned to the vanadate phases. The electric modulus displays a non-Debye dielectric dispersion and a relaxation process. The present results open the door to future zinc-tellurite glasses-doped vanadium exploitation as a potential electrolyte-based material for solid-state batteries

Structural Defect Impact on Changing Optical Response and Raising Unpredicted Ferromagnetic Behaviour in (111) Preferentially Oriented Nanocrystalline NiO Films

NiO thin films deposed on a glass substrate, “NiO/glass”, are successfully prepared using a spray pyrolysis technique (SPT) at 460 °C and characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray, Atomic force microscopy (AFM), spectroscopic ellipsometry (SE), Photoluminescence (PL) and diverse electric and magnetic studies. The structural investigation shows that the synthesized films crystallized in a cubic structure with (111) preferential orientation. The NiO layers exhibit a uniform grain of regular sizes with aggregates randomly distributed across their surface. The optical properties of the NiO thin films evidenced a normal optical dispersion as well as good transparency of the NiO films. An unpredicted ferromagnetic aspect was raised due to the high oxygen presence in the synthetized material. A high thermal dependency of the conductivity, as well as a semiconductor behavior of the grown NiO material, is also demonstrated