(159d) Multiple Energy “Exciton-Shelves” in Quantum-Dot-DNA Nanobioelectronic Materials
Quantum dots (QDs) are semiconductor nanocrystallites with size-dependent multiple quantum-confined states which are being explored for utilizing broadband radiation. While DNA has been used for the self-assembly of nanocrystals, it has not been investigated for the formation of simultaneous conduction pathways for transporting multiple energy charges or excitons. These exciton shelves can be formed by coupling the conduction band, valence band, and hot-carriers states in QDs with different HOMO-LUMO levels of DNA nucleobases, resulting from varying degrees of conjugation in the nucleobases. Here, we present studies on the electronic density of states (DOS) in naturally occurring nucleobases (guanine, thymine, cytosine, and adenine), which energetically couple with quantized states in semiconductor QDs. Using scanning tunneling spectroscopy of single nanoparticle-DNA constructs, we demonstrate composite DOS of chemically-coupled DNA oligonucleotides and cadmium chalcogenide QDs (CdS, CdSe, CdTe). While perfectly aligned QD-DNA states lead to exciton-shelves for multiple energy charge transport and energy transfer, mismatched energy levels in QD-DNA hybrids introduce intra-bandgap states which can lead to charge trapping and recombination. Using single nanoparticle and ensemble device measurements we also show successful extraction and transport of both bandedge and high-energy charge carriers, and energy transport using excitons. These results can have important implications for the development of a new class of nano-bio electronics and biological transducers.