Author Archives: carvalho

Silicon nanocrystals with diameters between 1 and 3 nm and surfaces passivated by chlorine or a mixture of chlorine and hydrogen were modeled using density functional theory, and their properties compared with those of fully hydrogenated nanocrystals. It is found that fully and partially chlorinated nanocrystals are stable, and have higher electron affinity, higher ionization energy and lower optical absorption energy threshold. As the hydrogenated silicon nanocrystals, chlorinated silicon nanocrystals doped with phosphorus or boron require a high activation energy to transfer an electron or hole, respectively, to undoped silicon nanocrystals. The electronic levels of surface dangling bonds are similar for both types of surface passivation, although in the chlorinated silicon nanocrystals some fall outside the narrower energy gap.

F4-TCNQ doped Si-NC

We use first-principles models to demonstrate how an organic oxidizing agent F4-TCNQ (7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane) modifies the electronic structure of silicon nanocrystals, suggesting it may enhance p-type carrier density and mobility. The proximity of the lowest unoccupied level of F4-TCNQ to the highest occupied level of the Si nanocrystals leads to the formation of an empty hybrid state overlapping both the nanocrystal and molecule, reducing the excitation energy to ∼0.8–1 eV in vacuum.
Hence, it is suggested that F4-TCNQ can serve both as a surface oxidant and as a mediator for hole hopping between adjacent nanocrystals in p-type doped silicon nanocrystal networks.

Phys. Rev. B 84, 125437 (2011)