[en] Highlights: • ZnO nanorods were grown by DC reactive magnetron sputtering of zinc target. • Substrate temperature critically controls the morphology of ZnO films/nanorods. • Vertically aligned and separated nanorods grow only in the range of 700–750 °C. • Substantial decrease in surface diffusion length is responsible for nanorod growth. • Increase in diameter with deposition flux is attributed to enhanced surface diffusion. - Abstract: DC reactive magnetron sputtering of zinc target in argon-oxygen sputtering atmosphere has been used to grow ZnO thin films/nanorods on Si in a wide substrate temperature range of 300–750 °C and under different sputtering conditions, namely, DC power, sputtering pressure and oxygen percentage in the sputtering atmosphere. Powder X-ray diffraction, Raman spectroscopy and a combination of top-down and cross-sectional scanning electron microscopy studies of ZnO films and nanorods grown under different conditions, have shown that substrate temperature critically controls their growth behavior and morphology, eventually resulting in the growth of vertically c-axis oriented, highly aligned and separated ZnO nanorods at substrate temperatures of 700–750 °C. The strongly substrate temperature dependent growth of nanorods is explained by considering that the growth above 600 °C, takes place in the ‘desorption regime’, in which, the surface diffusion length decreases exponentially with temperature. The diameter of nanorods increases with increase of DC power or decrease of sputtering pressure, which is attributed to the increase of surface diffusion length at higher deposition flux. The morphology of ZnO nanorods is not significantly affected by oxygen percentage in the sputtering atmosphere, since it does not influence the deposition flux.

[en] CdS nanocrystallites formed in ordered fatty acid LB multilayers exhibited strong surface states emission ∝550 nm and weak excitonic emission ∝400 nm. Treatment with aqueous CdCl₂ resulted in the suppression of surface states emission and enhancement of the blue excitonic emission. Subsequent annealing in air at 200 C caused an order of magnitude enhancement of excitonic emission. The growth of nanocrystallites during annealing as seen from the red-shift of excitonic absorption and emission is suppressed by the CdCl₂ treatment. The hindered growth of nanocrystallites, the significant enhancement of excitonic emission from CdS, and the suppression of surface states emission are attributed to surface passivation of CdS nanocrystallites by surface oxide formation. (orig.)

[en] CdA and CdA-ZnA mixed multilayers were prepared by LB process and characterized by XPS. CdS and CdZnS alloy nanoparticles were formed by post-deposition sulphidation and heat treated in temperature range of 150-250 deg C. A blue shift is observed in absorption and PL peaks of alloy nanoparticles w.r.t. the corresponding features in CdS, confirming the alloy formation. PL studies showed a strong enhancement of excitonic emission and suppression of defect states luminescence on heat treatment up to 200 deg C. (author)

[en] CdS nanocrystallites were prepared by sulphidation of cadmium arachidate LB multilayers. The composite multilayers were treated with aqueous CdCl₂ and heat treated in the range of 100-250 deg C. PL studies show that annealing of CdCl₂ treated multilayers results in strong enhancement of excitonic emission due to passivation of surface states. Atomic force microscopy studies indicate that up to 200 deg C, the CdA present in the multilayers assists in passivation of surface defects and restricts the growth of CdS nanocrystallites. (author)

[en] ZnO nanorods (NRs) were grown by chemical bath deposition on sputtered GaN over Si with and without sputtered ZnO seed layers. The effect of ZnO seed layer thickness, precursor concentration and growth temperature on the morphology and photoluminescence (PL) of ZnO-NRs has been studied. Scanning electron microscopy studies at different stages of growth have shown that the thickness of ZnO seed layer is critically important for controlling the growth behavior, morphology and density of ZnO-NRs on GaN surface. ZnO-NRs on bare GaN/Si grow with a large diameter and small aspect ratio of ∼4, displaying the tendency of lateral growth. Introduction of a thin ZnO seed layer (10 nm) under optimized precursor concentration and temperature drastically increases the aspect ratio to ∼16, due to partial coverage of ZnO on GaN surface and a moderate density of nucleation with small critical size. ZnO seed layers of higher thickness (50 nm and 100 nm) result in reduced aspect ratio due to increase in nucleation density and limited availability of reacting species. Increase in precursor concentration results in pronounced lateral growth and the decrease in growth temperature also results in compact nanorods with reduced aspect ratios. Room temperature photoluminescence (PL) studies show that ZnO-NRs on GaN, grown with or without ZnO seed layer under optimized precursor concentration and temperature, display high near-band-edge luminescence and negligible defect emission, compared to the nanorods on a ZnO seed layer over Si, as well as those grown at higher precursor concentration and lower temperatures. The enhanced PL is attributed to the absence of crystalline defects at nanorod interfaces due to lateral coalescence, arising from the moderate density and slight misalignment of the nanorods. - Highlights: • ZnO nanorods grown on sputtered GaN film display strong tendency of lateral growth. • Nanorods grown on 10 nm ZnO/GaN display moderate density and high aspect ratios. • Thicker ZnO seed layers result in high density of nanorods and small aspect ratios. • Nanorods on GaN and ZnO/GaN show strong band edge and negligible defect emissions. • Lateral coalescence of nanorods results in degradation of photoluminescence.

[en] Polycrystalline ZnO films were deposited on quartz substrates by reactive sputtering of zinc target. X-ray powder diffraction, pole figure analysis and high resolution measurements along with transmission electron microscopy, Raman and photoluminescence studies were carried out to study the microstructure, crystallinity and optically active defects in the films. All the films deposited in the substrate temperature range from room temperature to 600 deg. C exhibited strong c-axis preferred orientation. The changes in preferred orientation of crystallites with substrate temperature were attributed to its being determined by preferential nucleation at lower temperatures and surface diffusion at higher temperatures. A detailed microstructural analysis showed that with increase in substrate temperature from 300 deg. C to 600 deg. C, a significant reduction in micro-strain to ∼ 10^-3 takes place, along with a marginal increase in crystallite size. Raman and photoluminescence studies have shown that the films deposited below 300 deg. C possessed poor crystalline quality. The film deposited at 600 deg. C yielded the most intense and narrow (∼ 102 meV) band edge luminescence at room temperature, though it did not exhibit the strongest c-axis orientation of crystallites. This is attributed to its superior crystalline quality and absence of oxygen-deficiency related defects

[en] Polyion complexation in mixed Langmuir and Langmuir Blodgett (LB) films of photochromic amphiphilic azobenzene carboxylic acids, 11-[4-(4-hexylphenyl)azo] phenoxyundecanoic acid, 11-(4-phenylazo)phenoxyundecanoic acid, and diamine grafted poly(methylmethaacrylate) polymers has been studied. Monolayer behaviour of the pure components and mixed films was studied through pressure-area isotherms and LB films were characterized by spectroscopic, X-ray diffraction and Atomic force microscopy techniques. Aggregation (H-type), often observed in LB films of pure amphiphilic azo acids, was partly avoided in the mixed LB films as indicated by absorption spectral studies. Photoisomerization of the polyion complexed LB films was also studied. The results altogether demonstrate that amine grafted polymer enter into a polyion complexation with azo acid carboxylate group. LB films could be obtained by transfer of the composite monolayers and these LB films exhibited different levels of aggregation of the azo acids. Reversible photoisomerization was observed in LB films with unaggregated azo acid

[en] ZnO and Zn_1-xCd_xO nanocrystallites were prepared by oxidation of zinc arachidate-arachidic acid and zinc arachidate-cadmium arachidate-arachidic acid LB multilayers, respectively. The metal content of the multilayers was controlled by manipulation of subphase composition and pH. Precursor multilayers were oxidized in the temperature range of 400 deg. C-700 deg. C. The formation of ZnO and Zn_1-xCd_xO was confirmed by UV-Visible spectroscopy. Uniformly distributed, isolated and nearly mono-dispersed nanocrystallites of ZnO (11 ± 3 nm) and Zn_1-xCd_xO (18 ± 6 nm) were obtained

[en] Polycrystalline GaN films were deposited on quartz and ZnO buffer layers over quartz by reactive sputtering of a GaAs target in 100% nitrogen at 550 deg. C and 700 deg. C. Micro-structural investigations of the films were carried out using high resolution X-ray diffraction, atomic force microscopy and Raman spectroscopy. GaN films deposited on ZnO buffer layers exhibit strongly preferred (0002) orientation of crystallites. In particular, the film deposited at 700 deg. C on ZnO buffer layer over amorphous quartz substrate showed large crystallite size, both along and perpendicular to growth direction, strong and nearly complete c-axis orientation of crystallites with tilt of ∼ 2.5 deg. and low value of micro-strain ∼ 2 x 10^-3. The significant improvement in crystallinity and orientation of crystallites in the GaN film is attributed to the presence of the ZnO buffer layer on quartz substrate and its small lattice mismatch (1.8%) with GaN

[en] Core-shell CdS/ZnS nanoparticles in arachidic acid film were prepared through a novel Langmuir-Blodgett (LB) approach. Post-deposition treatment of the precursor LB multilayers of cadmium arachidate with H₂S gas followed by intercalation of Zn²⁺ ions and further sulfidation result in the formation of CdS/ZnS nanoparticles in the LB film. The formation of these nanoparticles and resulting changes in layered structures were studied by FTIR and X-ray reflection measurements. The optical properties were studied using UV-vis absorption and photoluminescence spectroscopy. A red-shift in the absorption spectrum and enhancement of CdS excitonic emission together with reduction of surface states emission suggest that after the intercalation step, a thin layer of ZnS surrounds the CdS nanoparticles, thus forming a core-shell structure. Subsequent to the second sulfidation, a further red-shift in absorption suggests the formation of a thicker ZnS coating on CdS. Electron diffraction of CdS nanoparticles coated with thicker ZnS showed the diffraction patterns of only ZnS, as expected for core-shell structures