This work reports a systematic study of the evolution of charge transport properties in granular ultra-thin films of palladium of thickness varying between 6 nm and 2 nm. While the...

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The transport properties of films assembled from metal nanoclusters can be significantly different from the metals in their bulk or thin film forms due to quantum confinement effects and several competing energy and length scales. For a film composed of metal nanoclusters as its building blocks, the cluster size and the inter-cluster separation are parameters that can be varied experimentally. Here we show that the electrical conductivity of a film composed of silver nanoclusters can be changed by 9 orders of magnitude as a function of the average inter-cluster separation while keeping the average cluster size same. For inter-cluster separations of 9 nanometres or more the conductivity is insulating type whereas for lesser inter-cluster separations the conductivity behaviour is metallic type with a positive temperature coefficient of resistance. In the intermediate range between the two regions, a very interesting temperature-independent conductivity is seen. Our work provides a new paradigm for design of artificial solid structures composed of nanoclusters. The properties of these nanostructures could be tuned by varying the inter-cluster distances to get the desired properties in the same material.

Nanoclusters offer a fascinating possibility of studying the evolution of properties of a physical system by varying the number, size and inter-cluster separation of a given cluster to go from one limit to another. By systematically varying the inter-cluster separation in a nanocluster assembly of Ni 40 Pd 60 alloy, that is known to be a metal in bulk, we observe an unusual and hitherto unreported, spatial dimension change as well as a change in the transport mechanism. In the nanocluster form, the temperature dependent resistance shows an activated behavior for virtually all inter-cluster separations, contrary to, the bulk metallic behaviour. At large average inter-cluster separation, the transport happens via three dimensional Efros-Shklovskii hopping, due to the opening of a Coulomb gap at the Fermi surface. With a reduction in the inter-cluster separation, the transport mechanism changes from three dimensional Efros-Shklovskii hopping to that of a three dimensional Mott variable range hopping (VRH) due to the closing up of the gap. With a further reduction in average inter-cluster separation, the three dimensional Mott VRH changes to that of a two dimensional Mott VRH with additional signatures of an insulator to a weak metal-like transition in this particular assembly. So, nanoclusters offer a paradigm for studying the important problem of evolution of charge transport in physical systems with the possibility of directly tuning the average inter-cluster separation enabling the system to go from insulating to metallic limit via intermediate changes in the charge transport mechanism.

Low-concentration hydrogen (H 2 ) gas detection is of paramount importance in space applications as well as in medical applications. It is also critically important for safe handling of hydrogen below the explosive limit. Here, we report a hybrid Pd metal−polymer chemiresistive sensor that can sense 0.5% H 2 gas in ambient conditions of temperature and pressure with the highest sensitivity (∼50%) reported until now, making it an extremely good sensor for real-life low-concentration H 2 gas detection. The sensor is easy to fabricate and is also extremely cost-effective for commercial applications. The obtained hybrid chemiresistive sensor comprises palladium nanocrystals bound by oxygen and nitrogen atoms of a stabilizer, polyvinylpyrrolidone, grown on top of a self-assembled monolayer. The 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 degradation of the sensing device due to poisoning of the available sites by oxygen.

This work reports on a novel and simple synthetic route for the growth of metal–organic crystal [Cu(C2O4)(4‐aminopyridine)2(H2O)]n of large size using the technique of liquid–liquid diffusion or layer diffusion. Single‐crystal X‐ray diffraction measurements revealed a very good quality of the grown single crystals with a small value 1.101 of goodness‐of‐fit R. Rietveld refinement done on powder X‐ray diffractogram obtained on few single crystals crushed together revealed a very small value of R as 3.45, indicating very good crystal quality in a batch of crystals. Density functional theory with three different basis sets generated the optimized geometry of a monomeric unit as well as its vibrational spectra. Comparison between experimentally obtained bond lengths, bond angles, IR frequencies, etc. suggests (B3LYP/LanL2DZ, B3LYP/6‐311++ G(d,p) basis set to describe the properties the best. Magnetic susceptibility measurements confirm the metal–organic crystal [Cu(C2O4)(4‐aminopyridine)2(H2O)]n to be a very good representation of a spin 1/2 Heisenberg antiferromagnet.

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