  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.
