S.S. Talwar

Orange-red luminescence from Cu doped CdS nanophosphor prepared using mixed Langmuir–Blodgett multilayers

Cu doped CdS nanophosphors were fabricated through Langmuir-Blodgett route for the first time. Precursors mixed Langmuir-Blodgett multilayers of cadmium arachidate-copper arachidate were used to grow doped sulfide nanoparticles within the organic matrix through postdeposition treatment with H(2)S gas. Changes in composition and layered structure of precursor multilayers were studied using Fourier transform infrared and x-ray reflection. Uptake of Cu in the multilayers was analyzed by inductively coupled plasma atomic emission spectroscopy measurements. Unannealed H(2)S exposed multilayers containing CdS nanoparticles show strong surface state emission centered at approximately 570 nm, whereas Cu doped CdS nanoparticles show orange-red luminescence. Photoluminescence (PL) spectra of annealed-Cu doped CdS nanoparticles show distinct Cu-related emission compared to annealed-undoped CdS nanoparticles. Power dependent PL measurements of annealed samples show that an efficient carrier recombination takes place at T(2) level of Cu(++). The carrier relaxation from the excitonic states to T(2) level results in the strong orange-red luminescence.

Study of simultaneous reduction and nitrogen doping of graphene oxide Langmuir?Blodgett monolayer sheets by ammonia plasma treatment

Graphene oxide (GO) monolayer sheets, transferred onto Si by the Langmuir-Blodgett technique, were subjected to ammonia plasma treatment at room temperature with the objective of simultaneous reduction and doping. Scanning electron microscopy and atomic force microscopy studies show that plasma treatment at a relatively low power (∼10 W) for up to 15 min does not affect the morphological stability and monolayer character of GO sheets. X-ray photoelectron spectroscopy has been used to study de-oxygenation of GO monolayers and the incorporation of nitrogen in graphitic-N, pyrrolic-N and pyridinic-N forms due to the plasma treatment. The corresponding changes in the valence band electronic structure, density of states at the Fermi level and work function have been investigated by ultraviolet photoelectron spectroscopy. These studies, supported by Raman spectroscopy and electrical conductivity measurements, have shown that a short duration plasma treatment of up to 5 min results in an increase of sp²-C content along with a substantial incorporation of the graphitic-N form, leading to the formation of n-type reduced GO. Prolonged plasma treatment for longer durations results in a decrease of electrical conductivity, which is accompanied by a substantial decrease of sp²-C and an increase in defects and disorder, primarily attributed to the increase in pyridinic-N content.

Variation of monolayer behaviour and molecular packing in zinc arachidate LB films with subphase pH

Abstract Monolayer characteristics and multilayer LB films of zinc arachidate (ZnA) have been studied as a function of subphase pH in the range 4.9–8.0. The monolayer stability was found to be strongly dependent on the subphase pH. Measurement of bilayer spacings in transferred multilayers using X-ray diffraction revealed that depending on subphase pH, three types of molecular packing arrangements corresponding to alkyl chain tilt angles of 0°, 19° and 31° are possible in ZnA multilayers. In certain ranges of subphase pH, more than one type of packing arrangements were found to coexist. These results suggest that the packing arrangement in ZnA multilayers is significantly affected by the properties of the monolayer at the air–water interface and may not be simply related to metal ion electronegativity.

Structure of CdS nanoparticles containing cadmium arachidate LB films

Y-type cadmium arachidate LB films were exposed to H2S gas for different durations. UV-VIS absorption spectra reveal an absorption edge at 455–460 nm in a 30 min H2S exposed film which does not change appreciably on further exposure. The blue shifted absorption edge is attributed to the formation of 30 A CdS nanoparticles. The H2S exposed films exhibit two types of layered structures, that is, the original bilayer spacing of 55.2A as well as an additional bilayer spacing of 44A. The XRD peak intensities corresponding to the new bilayer spacing (44A) increase up to 6h of H2S exposure accompanied by an increase in bandgap absorption. These results suggest that CdS nanoparticles are formed within domains having tilted arachidate molecules distributed throughout the film without completely destroying the original layered structure of the LB film.

Preparation and characterization of mixed LB films of polyaniline and cadmium arachidate

Abstract Langmuir isotherms of mixed monolayers of polyaniline and cadmium arachidate have been studied, and mixed monolayers have been transferred uniformly onto substrates as Langmuir-Blodgett (LB) multilayers. The LB films were characterized by XRD, UV-Vis and FTIR spectroscopy, dc conductivity and cyclic voltammetric measurements. The FTIR results suggest that the films contain arachidate salt, and the XRD results indicate the presence of a layered structure in the mixed LB films.

Structure of CdS–arachidic acid composite LB multilayers

Abstract Langmuir–Blodgett (LB) multilayers of cadmium arachidate were used as precursors to grow semiconducting CdS nanoclusters. The formation of CdS in the multilayers was determined by Fourier transform-infrared (FT-IR), ultraviolet–visible (UV–vis) and Raman spectroscopy. The structural changes occurring as a consequence of CdS formation have been characterized using X-ray reflection (XR) and grazing incidence X-ray diffraction (GIXD) techniques. The CdS containing composite multilayers exhibit the presence of two types of molecular domains, one with close packed herringbone arrangement and the other with tilted molecular chains with no in-plane order. The structural and spectroscopic evidences together suggest that the CdS nanoclusters formed within the arachidic acid LB matrix are quasi two-dimensional in nature with lateral dimension ∼5–10 nm and thickness ∼1.1 nm.

Molecular packing in cadmium and zinc arachidate LB multilayers

Abstract A combination of grazing incidence X-ray reflection/diffraction (GIXR/GIXD) and atomic force microscopy have been used to study the 3D structure in cadmium arachidate (CdA) and single phase zinc arachidate (ZnA) LB multilayers. The CdA multilayers have an ideal close packed herringbone structure. The molecules have a non-centred in-plane arrangement with specific orientational relation between the central and the corner molecules. In contrast, the ZnA multilayers have a hexagonal layer plane packing with tilted molecules. The molecules are loosely packed with rotational freedom about chain axis as observed in ‘rotator’ phases.

Molecular packing in CdS containing conducting polymer composite LB multilayers

Abstract Langmuir–Blodgett (LB) technique has been used to deposit composite multilayers of poly (3-octylthiophene)-cadmium arachidate (POT-CdA) and polyaniline-cadmium arachidate (PANI-CdA). These were used as precursors to develop semiconducting CdS nanoclusters within the conducting polymer based multilayers. The presence of CdS in the multilayers was determined by Fourier transform infrared spectroscopy (FTIR), UV–Vis and Raman spectroscopy. The structural changes occurring as a consequence of CdS formation have been characterized using X-ray reflection and grazing incidence X-ray diffraction (GIXD) techniques. As-deposited POT-CdA multilayers exhibit good vertical as well as in-plane structure similar to that of CdA. In contrast, PANI-CdA multilayers have poor structural order. The in-plane molecular packing in CdA and PANI-CdA multilayers after H2S exposure has mixed domains of rectangular (herringbone) and hexagonal arrangements. In the case of POT-CdA multilayers, however, the original rectangular packing is retained.

Growth of CdS nanocrystallites on graphene oxide Langmuir–Blodgett monolayers

Large area GO-Cd composite Langmuir-Blodgett monolayers were transferred onto Si substrate by introducing Cd(2+) ions into the subphase. The changes in the behaviour of the Langmuir monolayer isotherm in the presence of Cd(2+) ions are attributed to changes in the microstructure and density of the GO sheets on the subphase surface. The uptake of Cd onto the GO monolayers and the effect of subsequent sulphidation were investigated by AFM, FTIR, Raman, XPS and HRTEM techniques. The incorporation of Cd into the GO monolayers causes some overlapping of sheets and extensive formation of wrinkles. Sulphidation of the GO-Cd sheets results in the formation of uniformly distributed CdS nanocrystallites on the entire basal plane of the GO monolayers. The de-bonding of Cd with oxygen functional groups results in a reduction of the wrinkles. The GO sheets function primarily as a platform for the interaction of metal ions with oxygen functionalities and their structure and characteristic features are not affected by either uptake of Cd or formation of CdS.

Structure and polymerisation behaviour of cadmium 10,12-pentacosadiynoate Langmuir-Blodgett films

Two different types of Y-films (a Y-film from Z-type deposition and a Y-film from Y-type deposition) have been observed during the Langmuir-Blodgett (LB) film deposition of metal diynoates, but their structural characterisation has not received much attention. In the present work, a study of LB film deposition of cadmium 10,12-pentacosadiynoate, for evaluating the effects of monolayer aging and concentration of spreading solution, has been carried out and three different types of Y-films of cadmium 10,12-pentacosadiynoate have been obtained. Structural characterisation and study of the topochemical polymerisation of different types of films show remarkably large differences in their bilayer spacing and diacetylene polymerisation behaviour. This study shows that Y-films obtained by overturning in the Z-type build-up process are structurally quite different from the Y-films obtained from the Y-type build-up process.