The influence of polynucleotide stabilizers on selected properties of quantum-confined cadmium sulfide semiconductor clusters

Abstract

Since initial studies in the early 1980's, interest in quantum-confined semiconductor (Q-SC) materials have continued to increase from the diverse perspectives of chemistry, physics and materials science. Q-SC clusters with particle diameters comparable to or less than their respective bulk 'exciton' sizes ($<$100A for II-VI semiconductors) are often called 'quantum dots'. These Q-SC clusters exhibit unique photophysical properties when compared to bulk material; such properties arise from their size dependent bandgap, incomplete band structure, and three-dimensional confinement of charge carriers. Q-SC clusters require the presence of a stabilizer during particle synthesis to prevent agglomeration into bulk material; however, the chemical interaction between the cluster surface and stabilizer is not well understood. Cluster surfaces are important to the photophysics of quantum dots due to large surface area to volume ratios and the resultant large percentages of total atom composition residing at the surface. This work focuses on the influence of polynucleotide stabilizers on the photophysics of quantum-confined cadmium sulfide (Q-CdS) semiconductor clusters. With the known use of simple polyphosphates to stabilize Q-SC clusters, calf thymus deoxyribonucleic acid (DNA) with its polyphosphate backbone has been examined as a stabilizer for Q-CdS. In this regard, results regarding the characterization of Q-CdS/DNA by absorption spectroscopy, steady-state and time-resolved photoluminescence (PL) spectroscopy, and high resolution transmission electron microscopy (HREM) are presented. To investigate the role or influence of the nucleotide base on cluster properties, polynucleotides of varying base composition have been employed to stabilize the synthesis of Q-CdS. In particular, the properties of Q-CdS stabilized by single-stranded ribonucleic acid (RNA) homopolymers, single-stranded RNA co-polymers of complementary bases, double-stranded RNA homopolymers hydrogen bonded to their complementary strand(s), and DNA from calf thymus, Escherichia coli, and Clostridium perfringens have been examined. Changes induced in the properties of Q-CdS stabilized by these polynucleotides as a consequence of thermal treatment have been measured, and these results are discussed as well. Experiments probing the intrinsic affinity of polynucleotides for semiconductor cluster surfaces have been performed. The interaction between hydroxide-layered Q-CdS clusters and the polynucleotides, Escherichia coli deoxyribonucleic acid and polyadenylic acid, have been examined through photoluminescence quenching studies. In general, clusters of Q-CdS stabilized by DNA exhibit broad luminescence in the 500-700 nm wavelength region attributed to surface defects induced in part by the nucleotide host. Methods to reduce defect states (resulting in more desirable band edge luminescence, 'BEL') have been devised for Q-CdS/DNA. The conditions required to drive trap PL to BEL are presented here as well as studies designed to probe significant changes in stabilizer structure as a consequence of such conditions. In order for Q-SC clusters to be useful in small device applications, research efforts must move from studies of small clusters in solution to material supported on a solid substrate. One method currently under scrutiny in our laboratories is the preparation of thin films of Q-CdS/DNA on derivatized glass substrates. Initial PL studies on these films have revealed properties consistent with Q-CdS formation.