Contact conductance governs metallicity in conducting metal oxide nanocrystal films

TL;DR

This paper demonstrates that in nanocrystal films, the transition from insulating to metallic behavior is primarily controlled by the conductance at inter-nanocrystal contacts, not just electron concentration.

Contribution

It establishes a phase diagram linking inter-nanocrystal contact conductance to metallicity, highlighting the importance of contact conductance in nanocrystal film transport properties.

Findings

IMT is governed by inter-nanocrystal contact conductance.

Conventional metallic behavior requires contact conductance to match nanocrystal conductance.

A phase diagram for electron transport behavior in nanocrystal films was developed.

Abstract

In bulk semiconductor materials, the insulator-metal transition (IMT) is governed by the concentration of conduction electrons. Meanwhile, even when fabricated from metallic building blocks, nanocrystal films are often insulating with inter-nanocrystal contacts acting as electron transport bottlenecks. Using a library of transparent conducting tin-doped indium oxide nanocrystal films with varied electron concentration, size, and contact area, we test candidate criteria for the IMT and establish a phase diagram for electron transport behavior. From variable temperature conductivity measurements, we learn that both the IMT and a subsequent crossover to conventional metallic behavior near room temperature are governed by the conductance of the inter-nanocrystal contacts. To cross the IMT, inter-nanocrystal coupling must be sufficient to overcome the charging energy of a nanocrystal, while conventional metallic behavior requires contact conductance to reach the conductance of a nanocrystal. This understanding can enable the design and fabrication of metallic conducting materials from nanocrystal building blocks.