Metal to Insulator Transition, Colossal Seebeck Coefficient and Large Violation of Wiedemann Franz law in Nanoscale Granular Nickel

TL;DR

This study investigates nickel nanoparticles, revealing a size-dependent metal-insulator transition, colossal Seebeck coefficients, and significant violations of the Wiedemann-Franz law, highlighting unconventional charge and heat transport mechanisms.

Contribution

It demonstrates size-controlled tuning of electronic and thermal transport properties, including a transition from metallic to insulating behavior and large Lorenz number deviations.

Findings

Size-dependent metal-insulator transition observed.

Colossal Seebeck coefficient achieved at small sizes.

Large violation of Wiedemann-Franz law in nanoparticles.

Abstract

We report on the electrical and thermal transport properties of nickel nanoparticles with crystallite size from 23.1 to 1.3 nm. These nanoparticles show a systematic metal to insulator transition with the change in the conduction type from n to p type, colossal Seebeck coefficient, and ultralow thermal conductivity at 300 K as the crystallite size drops. The electrical resistivity analysis reveals a dramatic change in the electronic excitation spectrum indicating the opening of an energy gap, and cotunneling and Coulomb blockade of the charge carriers. Seebeck coefficient shows transport energy degradation of charge carriers as transport level moves away from the Fermi level with decrease in crystallite size. The Lorenz number rising to about four orders of magnitude in the metallic regimes with decrease in crystallite size, showing a large violation of the Wiedemann Franz law in these compacted nickel nanoparticles. Such an observation provides the compelling confirmation for unconventional quasiparticle dynamics where the transport of charge and heat is independent of each other. Therefore, such nanoparticles provide an intriguing platform to tune the charge and heat transport, which may be useful for thermoelectrics and heat dissipation in nanocrystal array-based electronics.