1. Could you explain the significance of your article to the non-specialist?

Molecules, supra-molecular structures and semiconductor quantum dots are increasingly used as the active components in prototype opto-electrical devices with miniaturized dimensions and novel functions. This means that we need to be able to measure the electronic properties of such single, individual nano-objects.

In this paper, we explore the conditions under which scanning tunnelling microscopy and spectroscopy can be used to reach this goal and yield quantitative information on the energy levels and Coulomb interactions of semiconductor quantum dots.

 

2. What has motivated you to conduct this work?

Nanometre-sized semiconductor particles, so-called quantum dots or artificial atoms, have exciting size-dependent opto-electronic properties. Scanning tunnelling microscopy provides a means to do electronic spectroscopy with sub-nanometre spatial resolution which allows us to understand their electronic properties in detail on a single nanocrystal level. This is crucial for their use in novel applications, such as solar cells, LEDs or lasers.

 

3. Where do you see this work developing in the future?

These colloidal quantum dots can be used as building blocks for more complex architectures where new, collective properties are expected to emerge. The properties of the assemblies depend on the individual building blocks as well as the electronic interactions between them. We are currently investigating quantum mechanical coupling in two-dimensional arrays of PbSe nanocrystals, where the high spatial resolution of STM is essential in getting information on how local order is reflected in the electronic properties.

 

4. Are there any particular challenges facing future research in this area?

It is possible to map the spatial shape of the wave-functions with STM by performing spectroscopy at each point over the quantum dot. The amplitude of the tunnelling resonances is related to the square of the wave-function at the position of the STM tip. This is an extremely challenging experiment and has been demonstrated only for a very limited number of systems (carbon nanotubes by the group of Dekker, InAs quantum dots by the groups of Wiesendanger and Millo). In coupled quantum dots, it would be fascinating to map the orbitals in real space.