Nanocrystal self-assembly: a bottom up approach to 2-D semiconductors with electrical non-trivial properties
W. H. Evers, M. Dijkstra, C. Delerue*, E. Kalesaki* and D. Vanmaekelbergh
Debye Institute for Nanomaterials Science, University of Utrecht
IEMN-ISEN and University of Lille
The emergence of graphene as an unconventional, physically interesting and possibly useful material has raised general interest in the properties of electrons confined in a hexagonal or honeycomb lattice of other materials, whether of atomic or nanoscale nature. In that respect, robust semiconductor lattices that have a honeycomb lattice with a translational period in the nanometer range and low effective carrier mass would be of large interest; first of all these systems keep their semiconductor properties but obtain valence and conduction bands with massless Dirac fermions; second, they open the possibility to combine this with the degrees of freedom from the atomic lattice, e.g. a strong spin-orbit coupling.
In this lecture, it will first be shown that 2-D semiconductor superlattices that are atomically coherent and at the same time have a nanoscale periodicity (square and honeycomb) can be prepared by nanocrystal self-assembly. Second, I will present the results of atomistic tight-binding calculations of the electronic band structure of these materials (see figure). It was found that honeycomb superlattices lead to two separated Dirac-type conduction bands, with in some cases a non-trivial band-opening at the K-point. Third, I will present the first results of scanning tunnelling experiments and transport measurements for CdSe honeycomb semiconductors.
1 Evers, W. H. et al. Low-dimensional semiconductor superlattices formed by geometric control over nanocrystal attachment. NanoLetters 13, 2317-1323 (2013).
2 Kalesaki, E., Delerue, C., Morais Smith, C., Allan, G. & Vanmaekelbergh, D. Dirac cones, topological edge states and non-trivial flatbands in two-dimensional semiconductors with a honeycomb nano-geometry. PRX, under review (2013).
3 Kalesaki, E., Allan, G., Vanmaekelbergh, D. & Delerue, C. Electronic structure of atomically-coherent square semiconductor superlattices with dimensionality below two. PRB, submitted (2013).