Synthesis and Band Structure of 2-D Honeycomb Semiconductor Superlattices

In 2-D semiconductor systems, it is possible to superimpose an additional periodicity on the nanoscale that can deeply influence the band structure of the semiconductor. For instance, when a 2-D semiconductor crystal is molded into a honeycomb geometry with a periodicity on the 5 - 50 nm scale, the band gap of the semiconductor remains comparable to that of a quantum well, but the highest valence and lowest conduction bands show a LINEAR (instead of quadratic) relation to the carrier momentum at the K-points¹. As a consequence, such honeycomb semiconductors host massless electrons and holes by doping or optical excitation.

Semiconductor crystals can be molded into a honeycomb geometry by bottom-up self-assembly of nanocrystals^2,3, or by top-down lithography⁴.
 In this lecture, I will show the progress that we made on the self-assembly and lithographic pathways. The Dirac-type band structure and the first spectroscopic results will be discussed as well.

References:

1 Dirac Cones, Topological Edge States, and Nontrivial Flat Bands in Two-Dimensional Semiconductors with a Honeycomb Nanogeometry. Phys. Rev. X 4, 011010, doi:10.1103/PhysRevX.4.011010 (2014).

2 Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices. Science 344, 1377-1380, doi:10.1126/science.1252642 (2014).

3 In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystals. Nat. Mater. 15, 1248-1254, doi:10.1038/nmat4746 (2016).

4. Transport Properties of a two-dimensional PbSe square superstructure in an electrolyte gated transistor, NanoLetters 2017.