Nano-materials: Silicon Quantum Dots

Quantum Dots (QDs) display enhanced optical properties compared to bulk counterparts, due to the process of quantum confinement. Those made of silicon are particularly advantageous in that they are bioinert and non-toxic, a significant advantage for potential bio-medical applications, and in addition can be produced relatively inexpensively.

However, in contrast to QDs made from other materials, for example II-VI compounds, those made of silicon are particularly vulnerable to oxidation. This is an acute problem for the use of these systems, especially in bio-medical terms, because oxidation can change the nature and yield of optical responses. For example it has been experimentally observed that exposure to atmosphere or aqeuous solution can result in both red- and blue-shifted photoluminescence (PL) peaks (associated with the HOMO-LUMO gap) as well as the creation of additional peaks at lower temperatures.

An understanding of the mechanism behind such optical effects is required so that control can be implemented upon the yield and energy of these systems for effective use in industry. As a result there has been a surge of interest in accurate ab-initio modelling of such systems, leading to the broad conclusions that:

  1. red shifts are due to the introduction of surface, oxygen related, centers that introduce exciton traps into the HOMO-LUMO gap of the QD,
  2. blue shifts are due to the formation of a capping layer of SiO2 leading to an increased quantum confinement.


Highest occupied molecular orbital of a partially oxidized 1 nm Si-QD

However the nature of the surface centers, both the structure and stability, remains in doubt. The silanone (Si=O) functional group has been proposed often as an ideal candidate, electronically, for the production of red-shifted PL spectra. However early studies on its stability have shown it to be very unstable in the presence of a water molecule. We have extended this work examining the effect of stabilization due to back-bonded silicon and the effect of charging, recently published. Of particular interest is the latter effect, which can yield both stabilization and optically active centers. In addition, the mechanisms for formation and propensity for aggregation of the silicon dioxide capping layers also requires clarification. We have shown that the energetics of oxygen diffusion within and into QDs is very different to the values obtained for the bulk. Both areas of research are ongoing.


Highest occupied molecular orbital of Silanone
Lowest unoccupied molecular orbital of Silanone