Ultra-small (r<2 nm), stable (>1 year), mixed valence copper oxide quantum dots with anomalous band gap

TL;DR

This paper reports a green synthesis method for ultra-small, stable, monodisperse copper oxide quantum dots with anomalously high band gap, highlighting the role of mixed valence states and off-stoichiometry.

Contribution

It introduces a digestive ripening process for copper oxide QDs, achieving stable, monodisperse, ultra-small particles with anomalous band gap behavior, a novel finding for transition metal oxides.

Findings

Quantum dots are stable for over a year.

Band gap observed at 5.3 eV, higher than expected.

Copper exists in mixed valence states in the material.

Abstract

Ultra-small (r<2 nm) semiconductor quantum dots (QDs) have attracted attention for applications ranging from dye sensitized solar cells to sensing due to its tunable electronic structure and band gap, and large specific surface area. However obtaining monodisperse QDs and stabilization in this size regime remains a challenge. A recent report on digestive ripening of an oxide system showed substantial promise in addressing these requirements of QDs. In this work, we report a green solution, soft chemical (chimie douce) approach for synthesis of quasi-spherical, ultra-small, stable, and monodispersed copper oxide QDs (r<2 nm) based on digestive ripening (DR). It may be noted this is only the second transition metal oxide system in which DR is reported so far. DR involves the refluxing of polydispersed colloidal nanoparticles in the presence of surface active agents (e.g. triethanolamine (TEA)) that leads to fairly monodispersed nanoparticles. It has been noticed that capping with TEA results in reduction in the average particle diameter from 6 +/- 4 nm to 2.4 +/- 0.5 nm and an increase of zeta potential ({\xi}) from +12 +/- 2 mV to +31 +/- 2 mV. These copper oxide QDs are monodispersed (size ~ 2.4 +/- 0.5 nm), and stable (>1 year). In addition, these quantum dots show an anomalous increase in band gap (5.3 eV), inexplicable using Brus' equation. XPS indicates that Cu exists in mixed valence state in this material. Based on our observations, we suggest that off-stoichiometry in copper oxide, which is seemingly substantial at these length scales, is responsible for the observed anomaly in band gap.