Inter-cluster separation induced change in charge transport mechanism in Ni40Pd60 nanoclusters

S G Praveen,D Jaiswal Nagar,C Bansal

doi:10.1038/s41598-019-43581-0

Abstract

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 Ni40Pd60 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.

Highlights

Transport of a degenerate electron gas in a disordered environment has been a subject of intense study[1,2,3,4,5,6]
From Figs 4(e,j) and 5(c), it is clear that R varies as T−1/3, suggesting that the main mechanism of charge transport in the assembly E is that of Mott-variable range hopping rather than tunnelling
We have demonstrated that nanoclusters offer a novel playground for studying the important problem of charge transport in a physical system as the system is slowly built up by making a nano-cluster comprising some number of atoms whose inter-cluster separation is varied

Summary

Change of Transport Mechanism and Dimension

Since the inter-cluster separation for the nano-cluster assembled films A to D varies from 14 nm to 2 nm, which is above the tunnelling transport limit as described above, the change of resistance by three orders of magnitude from assemblies A to D, implies differing levels of disorders, and differing transport mechanism in the different assemblies. From the intercept of the fits, the zero temperature resistance R0 was obtained for the assemblies corresponding to different inter-cluster separation values. Since the assemblies A and B are shown to follow Efros-Shklovskii mechanism of charge transport very well where the charge carriers are known to be strongly localised at a defect site, it is reasonable to assume a small value of the localization length ξ as ~1 nm[21] Using this value, we get the dielectric constant as 162 and 195 for nanocluster assembly A and B respectively. From Figs 4(e,j) and 5(c), it is clear that R varies as T−1/3, suggesting that the main mechanism of charge transport in the assembly E is that of Mott-variable range hopping rather than tunnelling

Conclusions and Outlook

Materials and Methods

Additional Information