The key challenge for a nanomaterial based sensor is how to synthesize in bulk quantity and fabricate an actual device with insightful understanding of operational mechanisms during performance. I report here effective, controllable methods that exploit the concepts of the "green approach" to synthesize two different one-dimensional nanostructures, including tin oxide nanowires and carbon nanotubes. The syntheses are followed by product characterization and sensing device fabrications as well as sensor performance understanding at the molecular level. Sensor-analyte response and recovery kinetics are also presented. The first part of the thesis describes bulk-scale synthesis and characterization of tin oxide nanowires by the molten salt synthetic method and the nanowire doping with antimony (n-types) and lithium. The work builds on the success of using n-doped SnO2 nanoparticles to selectively detect chlorine gas at room temperature. Replacing n-doped nanoparticles with n-doped nanowires reduces the number of inter-particle electron hops between sensing electrodes. The nanowire based sensors show unprecedented 5 ppb detectability of corrosive Cl2 gas concentration in air. At the higher range, 10 ppm of Cl2 gas leads to a 250 fold increase in the device resistance. During sensor recovery, FT-IR studies show that dichlorine monoxide (Cl2O) and chlorine dioxide (ClO2) are the desorbing species. Long term stability of devices is affected by lattice oxygen vacancies replaced by chlorine atoms. Bulk-scale synthesis of multiwall carbon nanotube (MWCNTs) was achieved by a novel inexpensive synthetic method. The green chemistry method uses the non-toxic and easy to handle solid carbon source naphthalene. The synthesis is carried out by simply heating naphthalene and organometallic precursors as catalysts in a sealed glass tube. Synthesis at 610º C leads to MWCNTs of 50 nm diameter and lengths exceeding well over microns. MWCNT doping is attempted with nitrogen (n-type) and boron (p-type) precursors. Palladium nanoparticles decorated on as-synthesized MWCNTs are employed for specific detection of explosive hydrogen gas with concentrations far below the explosive concentration limits. During performance, the sensor exhibits abnormal response behaviors at hydrogen gas concentrations higher than 1%. A model of charge carrier inversion, brought about by reduction of MWCNT by hydrogen molecules dissociated by Pd nanoparticles is proposed.

The principal aim of this thesis is to study the electrical transport properties of metal nanowires. Specifically, we have focussed on investigating the resistance fluctuations of Ag and Cu nanowires of diameters ranging from 15nm to 200nm and studied the instabilities that set in when the diameter is reduced below a certain range. The nanowires were grown electrochemically inside polycarbonate and alumina templates. X-ray diffraction studies on the samples showed the presence of a HCP 4H phase in the Ag nanowires in addition to the usual FCC phase, which is seen in bulk Ag. The relative ratios of these two phases were a maximum for nanowires of diameter 30nm. The X-ray diffraction studies also showed that the samples were of high chemical purity. TEM studies revealed that the wires are single crystalline in nature. Once the wires are released from the template, the wires of diameter 15nm were seen to break down spontaneously into globules due to Rayleigh instability. Wires of larger diameter tended to neck down to smaller radius but did not break down completely into globules. Both the Ag and Cu nanowire arrays had a fairly linear temperature dependence of resistance down to about 100K and reached a residual resistance below 40-50K. The temperature dependence of resistance could be fitted to a Bloch-Grüneisen formula over the entire temperature range. We found that n = 5 gave the best fit for the wires of all diameters showing that the dominant contribution to the temperature dependence of the resistivity in theses nanowires come from electron-acoustic phonon interactions. The resistivities of the wires were seen to increase as the wire diameter was decreased. This increase in the resistivity of the wires could be attributed to surface scattering of conduction electrons. In nanowires of diameter 15nm of both Ag and Cu, the relative variance of resistance fluctuations <(ΔR)2>/R2 showed a prominent peak at around ~ 220K for the Ag nanowire and ~ 260K for the Cu wire. Ag wires of diameter 20nm showed a much-reduced peak in noise at a somewhat higher temperature while this feature was completely absent in wires of larger diameter as also for the reference Ag film. The noise in wires of diameter larger than 20nm was similar to that of the reference film. For wires of diameter 15nm as we approach T*, the power spectral density showed a severe deviation from 1/f nature. We could establish that the extra fluctuation seen in the nanowires of the narrowest diameters could originate from the Rayleigh instability. The measured resistance fluctuation was found to have a magnitude similar to that estimated from a simple model of a wire showing volume preserving fluctuation. In the temperature range T ≤ 100K we observed very large non-Gaussian resistance fluctuations in a narrow temperature range for Ag and Cu wires of diameter 30nm with the fluctuations becoming much smaller as the diameter of the wires deviated from 30nm. In wires of diameter larger than 50nm the noise was almost independent of temperature in this range. The power spectrum of the resistance fluctuations also developed a large additional low frequency component near TP. We could establish that the appearance of this noise at a certain temperature (~30 – 50K) is due to the onset of martensite strain accommodation in these nanowires. To summarize, we measured the resistance and resistance fluctuations of Ag and Cu nanowires of diameters ranging from 15nm to 200nm in the temperature range 4.2-300K. The temperature dependence of resistance could be fitted to a Bloch-Grüneisen formula over the entire temperature range of measurement (4.2K-300K). The contribution of electron-phonon scattering to the resistivity was found to be similar to that of bulk. The defect free nature of our samples allowed us to identify two novel sources of noise in these nanowires. At high temperatures Rayleigh instability causes the noise levels in wires of diameter around 15nm to increase. At lower temperatures the formation of martensite state leads to an increase in noise in wires of small diameters.