ZnO Nanocrystal Networks Near the Insulator-Metal Transition: Tuning Contact Radius and Electron Density with Intense Pulsed Light

TL;DR

This study demonstrates tuning of ZnO nanocrystal networks near the insulator-metal transition by independently adjusting contact radius and electron density using intense pulsed light, revealing critical behavior of the transition.

Contribution

It introduces a method to independently control contact radius and electron density in ZnO nanocrystal networks using IPL, advancing understanding of the insulator-metal transition in nanomaterials.

Findings

Conductivity scaling indicates proximity to the quantum phase transition.

Critical behavior of dielectric constant and localization length was observed.

Samples remain insulating, suggesting a higher critical n ho^3 value than predicted.

Abstract

Networks of ligand-free semiconductor nanocrystals (NCs) offer a valuable combination of high carrier mobility and optoelectronic properties tunable via quantum confinement. In principle, maximizing carrier mobility entails crossing the insulator-metal transition (IMT), where carriers become delocalized. A recent theoretical study predicted that this transition occurs at n\rho^3 ~ 0.3, where n is the carrier density and \rho is the interparticle contact radius. In this work, we satisfy this criterion in networks of plasma-synthesized ZnO NCs by using intense pulsed light (IPL) annealing to tune n and \rho independently. IPL applied to as-deposited NCs increases \rho by inducing sintering, and IPL applied after the NCs are coated with Al2O3 by atomic layer deposition increases n by removing electron-trapping surface hydroxyls. This procedure does not substantially alter NC size or composition and is potentially applicable to a wide variety of nanomaterials. As we increase n\rho^3 to at least twice the predicted critical value, we observe conductivity scaling consistent with arrival at the critical region of a continuous quantum phase transition. This allows us to determine the critical behavior of the dielectric constant and electron localization length at the IMT. However, our samples remain on the insulating side of the critical region, which suggests that the critical value of n\rho^3 may in fact be significantly higher than 0.3.