Construction of conductive multilayer films of biogenic triangular gold nanoparticles and their application in chemical vapour sensing

Metal nanoparticles are interesting building blocks for realizing films for a number of applications that include bio- and chemical sensing. To date, spherical metal nanoparticles have been used to generate functional electrical coatings. In this paper we demonstrate the synthesis of electrically conductive coatings using biologically prepared gold nanotriangles as the building blocks. The gold nanotriangles are prepared by the reduction of aqueous chloroaurate ions using an extract of the lemongrass plant (Cymbopogon flexuosus) which are thereafter assembled onto a variety of substrates by simple solution casting. The conductivity of the film shows a drastic fall upon mild heat treatment, leading to the formation of electrically conductive thin films of nanoparticles. We have also investigated the possibility of using the gold nanotriangle films in vapour sensing. A large fall in film resistance is observed upon exposure to polar molecules such as methanol, while little change occurs upon exposure to weakly polar molecules such as chloroform

Scanning tunneling microscopy/spectroscopy on Au nanoparticles assembled using lauryl amine (LAM) and octadecane thiol (ODT)

In this report, we demonstrate scanning tunneling microscopy and spectroscopy on thin films of lauryl amine (LAM) and octadecane thiol (ODT) protected gold nanoparticles. We show that the zero current in the I-V curves (measure of Coulomb blockade (CB) of the nanoparticles) depends on the properties of the spacer molecule. In both the cases the gap voltage and the tunneling current at which the images are obtained are quite different which is further confirmed from the fitting performed based on the orthodox theory. The values for the capacitance and charging energy obtained from the fitting for ODT capped particles are comparable to the values obtained using spherical capacitor model. In contrast, values of these parameters were found to differ for LAM capped nanoparticles. While imaging, ODT capped nanoparticles were observed to drag along the scan direction leading to ordering of particles. Images of LAM capped gold nanoparticles show local ordering in self-assembly of particles although no evidence of large scale ordering in spatial Fourier transform was seen. These observations suggest that nanoparticles with larger CB would be imaged nonevasively in contrast to small CB systems for which tip induced effects will be dominant. In both the systems the current was found to rise faster than theoretical curves based on the orthodox theory suggesting that mechanism of charge transfer in this case may involve field emission rather than tunneling through a rectangular barrier. An attempt has been made to explain charge transfer based on Fowler-Nordheim (F-N) plots of the I-V curves

Effect of bath temperature on optical and morphology properties of CdS thin films grown by chemical bath deposition

Synthesis of CdS thin films were carried out onto glass substrates by chemical bath deposition (CBD) method using CdCl2asCd2+ and thiourea as S2-ion source with ammonia as a complexing agent. Influence of bath temperature on structural,morphology and optical properties has been systematically and carefully investigated. XRD analysis revealed that the synthesizedCdS films are nanocrystalline having hexagonal structure with (002) preferential orientation. Estimated crystallite size was foundin the range 16-32 nm. The UV-Visible spectroscopy analysis showed that the films have high transmission (> 70%) in visibleand NIR region of solar spectrum. Optical band gap was found > 2.3 eV over the entire range of bath temperature studied. TheRaman spectra for the CdS films deposited at various bath temperatures shows a continuous shift of 1 LO phonon peak towardshigher frequency which suggest the improvement of structural order with increase in bath temperature. The increase in theintensity ratio, 2LO/1LO with bath temperature indicates enhancement of crystallinity of CdS films with increase in bathtemperature. The SEM analysis showed that CdS films deposited at various bath temperatures are smooth, homogeneous, andnearly uniform with randomly oriented spherical nanocrystallites