S. R. Sainkar

Organization of polymer-capped platinum colloidal particles at the air–water interface

This communication discusses the formation of thin films of platinum colloidal particles by organization at the air–water interface. The platinum clusters were prepared by sodium borohydride reduction of aqueous chloroplatinic acid with the nanoparticles being capped with a polymer, poly(vinylpyrrolidone) (PVP). After separation from the aqueous medium, the capped clusters were dispersed on the surface of water, using the hydrophobic component of the capping agent to organize the clusters at the air–water interface. Pressure-area measurements showed a region of rather low compressibility, characteristic of a reasonably compact, ordered phase of the colloidal particles. The Langmuir monolayer of the platinum clusters was transferred to suitable substrates and analyzed using transmission electron microscopy, quartz crystal microgravimetry and X-ray photoemission spectroscopy. An organized monolayer of platinum clusters was observed indicating that the air–water interface may be used as a medium for obtaining nanoparticle films for device applications.

Structural studies on silver cluster films deposited on softened PVP substrates

Abstract The deposition of metal cluster films on softenable polymer substrates is known to lead to subsurface particulate structure formation for many metal/polymer combinations. The nature of the subsurface film structure is critically dependent on deposition conditions such as polymer substrate temperature, film deposition rate, etc. and the influence of these parameters is now fairly well understood. In this paper, we show that the metal–polymer interaction is a very important parameter which can be used to control the particle size as well as the inter-cluster separation through an investigation of silver cluster films deposited on softened poly(2-vinylpyridine), PVP. The increased wettability of silver in the PVP matrix leads to the formation of relatively small islands separated by small gaps which is in contrast to silver films deposited on polystyrene which yielded highly agglomerated structures with large inter-island separations. The silver/softened PVP film structure has been characterized by scanning electron microscopy (SEM). Further evidence for significant overlap of the electronic properties of the clusters was obtained from electron tunneling measurements as well as by optical absorption spectroscopy. The depth of the silver clusters below the surface was estimated from angle dependent X-ray photoemission spectroscopy (XPS) to be ≈5 nm. The ability to control the particle size as well as to tune the interaction between clusters by varying the metal wettability in the polymer matrix is an aspect of the work which shows promise for further development.

Preparation, characterization and mechanistic features of zirconia films on bare and functionalized gold surfaces

Electrochemical synthesis of microcrystalline ZrO2 at room temperature on bare and functionalized gold surfaces is reported to illustrate the biomimetic conditions of nucleation and growth. ZrO2 films prepared on self-assembled monolayers (SAMs) of pentane-1,5-dithiol (PDT) on polycrystalline gold surfaces reveal a high degree of orientation and a distinct cleavage face, in contrast to monoclinic films, obtained on bare gold surfaces at room temperature. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), impedance measurement, current-transients, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to follow the monolayer formation, Zr-attachment on the SAM-modified surface and subsequent difference in nucleation conditions of ZrO2 formation. The role of SAMs in controlling the size and habit during crystallization of ZrO2 is discussed in terms of a proposed reaction mechanism.

Preparation and characterization of silver particulate structure deposited on softened poly(4-vinylpyridine) substrates

Results of the investigations carried out on particulate silver films, deposited at a rate of 0.4 nm s-1 on poly(4-vinylpyridine) coated substrates held at temperatures in the range 440-490 K in a vacuum of 8 × 10-6 Torr are reported in this paper. A morphological study by scanning electron microscopy and x-ray diffraction was further confirmed by optical absorption studies. The shift in the plasmon resonance towards longer wavelength is ascribed to polymer-metal particle interactions. Electrical resistances immediately after deposition and at room temperature show a regular dependence on the thickness for the films in the temperature range studied. The films deposited at lower substrate temperatures show a lower negative temperature coefficient of resistance when compared to films deposited at higher substrate temperatures. X-ray photoelectron spectroscopy studies indicate the formation of a sub-surface particulate structure at higher substrate temperatures.

Preparation and characterisation of silver particulate films on softened polystyrene substrates

The preparation of silver particulate films on softened polystyrene (PS) substrates and their characterisation using Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS) and optical absorption spectroscopy is reported in this paper. Silver films of 150 nm thickness were vacuum deposited onto PS coated glass substrates held at temperatures in the range 415-475 K at different deposition rates of 4 to 12 A/s. SEM studies indicate that films deposited at 415 K are close to a semicontinuous structure and the structure is discontinuous at higher temperatures. The film morphology is strongly dependent on the deposition rate at any given substrate temperature. The film agglomeration increases with increasing rate of deposition. In the XPS studies, considerable attenuation of the signal corresponding to silver is observed at lower electron take of angles (ETOAs). This indicates that Ag is formed beneath the PS surface. Optical absorption studies showed an interesting red shift of the plasmon resonance wavelength for lower deposition rates again indicating that a sub-surface particulate structure is formed at lower deposition rates. These results are consistent with reported observations.

Attenuation length measurements in cadmium arachidate Langmuir–Blodgett films

Attenuation lengths in cadmium arachidate [(C19H39COO)2Cd] multilayer Langmuir–Blodgett films deposited on thick lead arachidate [(C19H39COO)2Pb] films have been determined in this communication. A discrete layer model appropriate to the ordered structure of these built‐up films is proposed and applied for the first time to the measurements of the x‐ray photoemission intensity variation with electron takeoff angle and attenuation lengths for 950–1350 eV electrons are evaluated. These lengths agree with earlier attenuation length measurements in Langmuir–Blodgett films and are higher than in most inorganic materials.

Production of dextran by Rhizopus sp. immobilized on porous cellulose support

Mycelia from a culture of Rhizopus spp, isolated from soil, were immobilized on cellulose microfibrils support and used for bioconversion of sucrose to dextran. Both batch and continuous modes of operation were investigated. For immobilized mycelia in batch (shake flask) fermentation, dextran yield and specific rate of production were 14 g l−1 and 0.4 g g−1 h as compared to 11 g l−1 and 0.28 g g−1 h for free mycelia. In continuous fermentation, sucrose solution (45 g l−1) flows through the capillaries of the fabric support on which mycelia were pregrown, and is bioconverted at the interface to dextran. The system could be run continuously for a period of 25 days and the emerging broth showed higher yields of dextran presumably as a result of higher interface interactions between substrate and mycelia. The eluate contained 22 g l−1 of dextran, which was higher than expected as a result of evaporative concentration. Lignin in the medium is seen to be beneficial to both the final yield and specific rate of production. The effect of water activity on the bioconversion is described.

Electrostatic entrapment of chloroaurate ions in patterned lipid films and the in situ formation of gold nanoparticles

The formation of gold nanoparticle assemblies in a patterned manner on suitable substrates is described. The protocol for realizing such structures comprises the following steps. In the first step, patterned films of a fatty amine are thermally evaporated onto solid supports using suitable masks (e.g. a TEM grid). Thereafter, the fatty amine film is immersed in chloroauric acid solution and chloroaurate (AuCl4-) ions entrapped in the lipid matrix by electrostatic complexation with the ammonium ions of the fatty amine molecules. The final step involves the reduction of the AuCl4- ions in situ thus leading to the formation of gold nanoparticles within the patterned lipid matrix. The process of metal ion incorporation and reduction may be repeated a number of times to increase the nanoparticle density in the lipid matrix. AuCl4- ion entrapment and formation of gold nanoparticles within the patterned lipid matrix has been followed by quartz crystal microgravimetry, UV-vis spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy and energy dispersive analysis of x-ray measurements. The protocol described shows immense potential for extension to assemblies of nanoparticles in more intricate patterns as well as to the growth of semiconductor quantum dots in such patterns.

Patterned silver nanoparticle films by an ion complexation process in thermally evaporated fatty acid films

The formation of silver nanoparticle films in a patterned manner on suitable substrates is described. The protocol for realising such structures comprises of the following steps. In the first step, patterned films of a fatty acid are thermally evaporated onto solid supports using suitable masks (e.g. a TEM grid). Thereafter, the fatty acid film is immersed in silver nitrate solution and Ag + ions entrapped in the lipid matrix by electrostatic complexation with the carboxylate ions of the fatty acid molecules. The final step involves the reduction of the Ag + ions in situ thus leading to the formation of silver nanoparticles within the patterned lipid matrix. The process of metal ion incorporation and reduction may be repeated a number of times to increase the nanoparticle density in the lipid matrix. The silver nanoparticle density may also be increased by dissolution of the fatty acid molecules in suitable solvents. The process of Ag + ion entrapment and formation of silver nanoparticles within the patterned lipid matrix has been followed by quartz crystal microgravimetry, UV-VIS spectroscopy, FTIR, SEM and EDX. The process described shows immense potential for extension to assemblies of nanoparticles in more intricate patterns as well as to the growth of semiconductor quantum dots in such patterns.

Influence of alcohol on the morphology of BaSO4 crystals grown at the air–water interface

Control over the crystallography and morphology of technologically important minerals is an important goal in the area of crystal engineering. Charged insoluble surfactant monolayers at the air–water interface (Langmuir monolayers) have proven to be popular mineralization templates. In this paper, we investigate the influence of trace quantities of alcohol (ethanol) in the aqueous subphase on the crystallography and morphology of BaSO4 crystals grown in the presence of anionic Langmuir monolayers at two largely different supersaturation ratios. It is observed that the crystals grow in the barite structure with interesting differences in morphology of the crystals grown with and without alcohol in the subphase. Preferential adsorption of ethanol molecules at the air–water interface is expected to influence not only the organization of the surfactant Langmuir monolayer but also to alter the dielectric properties of the interface and, consequently, the electrostatic binding of the ions with the Langmuir monolayer prior to mineralization.