Colloidal CdSe nanocrystals are being investigated for use in transistor, light-emitting diodes, and photovoltaic solar cells. Much of the interest arises from their novel optical and electronic properties, such as their size-tunable band gap. They are also promising materials, because they can be processed from solution. The solubility of a colloidal nanocrystal is determined by its surface chemistry. Researchers have found that exchanging the native surface ligands (present from synthesis) with shorter, conjugated molecules can increase the efficiency of nanocrystal-based devices, leading multiple groups to investigate nanocrystal-ligand interactions.Recently, Weiss et al. reported that exchanging the native ligands with dithiocarbamate molecules causes a red-shift in the absorbance spectra of CdS, CdSe, and PbS nanocrystals. The authors concluded that there is sufficient resonance between the HOMO energy level of the nanocrystal and ligand to allow for delocalization of the 'hole' into the ligand shell, causing the band gap energy shift. They speculated that this class of molecules might improve charge transport in nanocrystal-based photovoltaics.We investigate the effects of dithiocarbamate ligands on the absorbance spectra of colloidal CdSe nanospheres and nanobelts. We confirm that the addition of dithiocarbamate ligands to nanosphere samples causes the absorbance spectra of the nanospheres to red-shift as reported by Weiss et al. However, the addition of dithiocarbamate ligands to CdSe nanobelt samples does not  correlate with a change in the band gap of the nanobelts. To date, most studies of nanocrystal-ligand interactions have focused on nanospheres, although elongated nanocrystal shapes are becoming more common in nanocrystal-based devices. We examine the different effects of dithiocarbamate ligands on these two colloidal nanocrystal shapes in terms of the HOMO & LUMO energies of each nanocrystal shape and addressing differences in ligand binding affinity to each nanocrystal shape.

Comparison of how dithiocarbamate ligands affect the band gap energies of colloidal CdSe nanospheres and nanobelts

247th National Meeting (2014)

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