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Low concentration hydrogen gas detection is of paramount importance both in space applications as well as medical applications. It is also critically important for safe handling of hydrogen below the explosive limit. Here, we report a novel hybrid Pd metal-polymer chemiresistive sensor that can sense 0.5% hydrogen ($H_2$) gas in ambient conditions of temperature and pressure with the highest reported sensitivity($\sim$30%) obtained earlier by a physical deposition technique, making it an extremely good sensor for real life low concentration hydrogen gas detection. The sensor is easy to fabricate and is also extremely cost-effective for commercial applications. The obtained hybrid chemiresistive sensor comprises palladium (Pd) nanocrystals bound by oxygen and nitrogen atoms of a stabilizer Polyvinylepyrollidone (PVP), grown on top of a selfassembled monolayer. The exceptional rise time-constant is proposed to arise from hydrogen loading at the (111) surface of the palladium nanocrystal which is a very fast process and subsequent fast diffusion of the H atoms from the surface into the bulk. An effort to increase the number of available sites by UV-ozone cleaning, resulted in a degradation of the sensing device due to the poisoning of the available sites by oxygen.

This work reports a systematic study of the evolution of charge transport mechanism in granular ultra-thin films of palladium of thickness varying between 6nm and 2nm. While the films with thickness > 4nm exhibit metallic behaviour, that at 3nm thickness undergoes a metal-insulator transition at 19.5K. In contrast, the 2nm thick film remained insulating at all temperatures. with transport following Mott's variable range hopping. At room temperature, while the thicker film exhibit resistance decrease on H$_2$ exposure. the insulating film showed an anomalous initial resistance increase before switching to a subsequent decrease. The nanostructure dependent transport and the ensuing H$_2$ response is modeled on a percolation model, which also explores the relevance of film thickness as a macroscopic control parameter to engineer the desired system response in granular metal films.