Scanning tunnelling spectroscopy of low-dimensional semiconductor systems

The applicability of tunnelling spectroscopy by scanning tunnelling microscopy (STM) to semiconductor nanoobjects is addressed through two examples: chemically synthesized PbSe nanocrystals and cleaved InAs quantum dots (QDs) embedded in a GaAs matrix. In the case of free-standing nanocrystals on a metallic substrate, measurements of the conductance at low temperature provides the electronic structures of the nanocrystals in the conduction band and a broad transition range is found for the density of states as a function of the nanocrystal dimensions. Use of the scanning ability of the STM also allows the acquisition of spectroscopic images. At room temperature, current images obtained on cleaved InAs QDs for different samples voltages show the formation of standing wave patterns, which are identified as the probability density of the ground and first excited states. Such results open the opportunity to study the electronic structure of more complicated systems based on the assembly of semiconductor nanoobjects such as coupled QDs.