ABSTRACT
In this chapter, we put on a rigorous basis the concept of electronic
band structure E (k) in semiconductor nanocrystals. We show that, down to a certain limit, this concept has still reasonable
meaning, even if the band structure becomes gradually “fuzzy” and
minigaps appear within the allowed energy bands with decreasing
nanocrystal size. We introduce a general computational method
which allows the reconstruction of a fuzzy electronic band structure
of nanocrystals from ordinary real-space molecular orbitals. The
nanocrystals basically remember the basic features of their “parent”
bulk material. In particular, we demonstrate that hydrogen-capped
silicon nanocrystals, fully relaxed geometrically and electronically,
retain the indirect-bandgap structure down to≤2 nm. Moreover, we reveal, both computationally and experimentally, that mechanical
(tensile) strain applied to the Si nanocrystals via proper surface
capping makes these nanocrystals a direct-bandgap material,
putting themon a parwith standard direct-bandgap semiconductors
like GaAs.
