Functionalization of CdSe semiconductor nanocrystals with organic charge-transporting ligands

Abstract

[1,1′-Biphenyl]-4,4′-diamine-N,N′-bis(3-methylphenyl)-N,N′-diphenyl (TPD), a well-studied hole transporting material, has been sulfonated using acetyl sulfate and subsequently used as a passivating ligand in the synthesis of CdSe quantum dots (QDs). It is further demonstrated that QDs synthesized through this approach are able to serve as efficient photosensitizers in photoconductive (PC) inorganic–organic hybrid composites. Fourier-transform infrared spectroscopy confirms that the sulfonate group has been bonded to the TPD molecule. UV/visible absorption and photoluminescence (PL) spectroscopy of the sulfonated TPD (STPD) indicate that sulfonation does not significantly alter the electronic properties of TPD. Furthermore, mass spectrometry shows STPD is primarily mono-sulfonated. The STPD capped CdSe QDs (STPD-QCdSe) clearly exhibit the effect of quantum confinement in their UV/visible absorption spectra. The PL spectra of STPD-QCdSe suggest the STPD is attached to the surface of QCdSe. The morphology of STPD-QCdSe has been studied using transmission electron microscopy. The results indicate the STPD-QCdSe are approximately spherical with diameters of [3.1, 3.9] nm and highly crystalline. To demonstrate the enhancement in charge-transfer efficiency associated with STPD-QCdSe, two types of PC composites were fabricated. The first was photosensitized through the inclusion of STPD-QCdSe, and the other included CdSe QDs capped with trioctylphosphine (TOPO-QCdSe), with molecular TPD serving as the charge transport matrix in both cases. The PC was measured as a function of the external electric field, E, with the PC of the composite sensitized with STPD-QCdSe exceeding that of the composite sensitized with TOPO-QCdSe by a factor of ∼15 with E = 10 V μm⁻¹.