Optical study of annealed cobalt-porous silicon nanocomposites

International audienceWe report Raman and photoluminescence studies of cobalt-porous silicon nanocomposites (PS/Co). Cobalt was introduced in porous silicon (PS) by immersion method using CoCl2 aqueous solution. The presence of cobalt in PS matrix was identified by FTIR spectroscopy and EDX analyses. The Raman spectroscopy revealed the presence of Si bonded to cobalt oxide in PS/Co. We discuss also the Raman spectra of PS and PS/Co samples under different annealing temperatures ranging from room temperature (RT) to 600 °C. The optical properties of PS and PS/Co were studied by photoluminescence (PL). The highest PL intensity was observed for an immersion time of 60 min. For long duration, the deposited cobalt quantity acts as energy trap and promotes the non-radiative energy transfer; it is the autoextinction phenomenon. We have studied also the effect of the annealing temperature on the PL of both PS and PS/Co samples. For PS, the annealing process leads to a rapid oxidation of the Si nanocrystallites (nc-Si). In the case of PS/Co sample, two different mechanisms are proposed; one is the desorption of Si-Hx(x=2,3) with the formation of cobalt oxide for annealing temperature less than 450 °C which causes the increasing of PL intensity and the stability of PL energy, the other mechanism is the transformation of the porous silicon to silica at high temperatures View the MathML source(≻450°C) which leads to the decreasing of the PL intensity and the blue shift of the PL curve

Broadband luminescence in defect-engineered electrochemically produced porous Si/ZnO nanostructures

The fabrication, by an all electrochemical process, of porous Si/ZnO nanostructures with engineered structural defects, leading to strong and broadband deep level emission from ZnO, is presented. Such nanostructures are fabricated by a combination of metal-assisted chemical etching of Si and direct current electrodeposition of ZnO. It makes the whole fabrication process low-cost, compatible with Complementary Metal-Oxide Semiconductor technology, scalable and easily industrialised. The photoluminescence spectra of the porous Si/ZnO nanostructures reveal a correlation between the lineshape, as well as the strength of the emission, with the morphology of the underlying porous Si, that control the induced defects in the ZnO. Appropriate fabrication conditions of the porous Si lead to exceptionally bright Gaussian-type emission that covers almost the entire visible spectrum, indicating that porous Si/ZnO nanostructures could be a cornerstone material towards white-light-emitting devices

N 2 + ion bombardment effect on the band gap of anatase TiO2 ultrathin films

International audienc

The effect of tin doping on physical properties of cobalt oxide thin films

International audienceThis work deals with the effect of Sn doping at mole percentages of 1 %, 2 %, 3 % and 4 % on the structural, morphological, and optical properties of Co3O4 thin films. These physical properties were carried out by X-Ray Diffraction (XRD), Raman, Atomic Force Microscopy (AFM), scanning electron microscope (SEM), Ultraviolet-Visible-Near infrared spectroscopy (UV-Vis-NIR) and spectroscopic ellipsometry (SE). Co3O4 thin films were grown on amorphous glass substrates by spray pyrolysis technique. Experimental and modeling rigorous studies using these different techniques particularly SE were achieved in order to determinate the effect of tin doping on physical properties of cobalt oxide thin films. The incorporation of Sn into the Co sites affects drastically the optical transitions values as well as other optical properties such as the dielectric function, the band gap, the refractive index and the extinction coefficient. All doped samples exhibited a relatively higher absorption coefficient compared to the undoped ones, greater than 105 cm−1 over a wide energy range. SE analysis revealed a smaller transition of approximately 0.77 eV considered as fundamental band gap energy that was assigned to a 3d-d type within the Co2+ tetrahedral site

Raman study of Cd1−xZnxTe phonons and phonon–polaritons—Experiment and ab initio calculations

Backward/near-forward Raman scattering and ab initio Raman/phonon calculations are combined, together with x-ray diffraction and ellipsometry measurements to further inform the debate on the compact phonon behavior of the II–VI Cd1−xZnxTe alloy. The compacity favors the coupling of polar optic modes in both the transverse and longitudinal symmetries via the related (EL,T) long-wave electric fields. The EL-coupling achieves maximum in the Zn-dilute limit, which enhances the (upper) ZnTe-like (impurity) mode at the expense of the (lower) CdTe-like (matrix-like) one, leaving the impression of a unique {Cd-Te,Zn−Te}-mixed longitudinal optic (LO) phonon across most of the composition domain. However, the purely mechanical (non-polar) transverse optic (PM-TO) phonons, that hardly couple, reveal an underlying three-mode {1 × (Cd-Te),2 × (Zn-Te)} fine structure that distinguishes between Zn–Te vibrations in Zn- and Cd-like environments up to second neighbors. Further refinement arises by exploring the phonon–polariton (i.e., polar-TO) regime at large Zn content. On reducing the scattering angle, the ET-coupling develops into a sequential softening of phonon–polaritons from ZnTe- down to CdTe-like ones, which transiently unveils a bimodal pattern behind the Cd–Te signal. Altogether, this results in a (rare) canonical four-mode {2 × (Cd-Te),2 × (Zn-Te)} percolation pattern for Cd1−xZnxTe, i.e., a close II–VI replica of the twin III−V In1−xGaxAs one—yet differing by two apparent LO modes and a sensitivity of bond vibrations limited to first-neighbors. Retrospectively, the difference in sensitivity of bond vibrations to the local environment between In1−xGaxAs (limited to first neighbors) and Cd1−xZnxTe (extending up to second neighbors) emerges as a rule throughout common (covalent) III–V and (ionic) II–VI semiconductor alloys