Photophysical characterization of several species of fluorescent silver nanoclusters, encapsulated in oligonucleotide scaffolds, was achieved at the bulk and single molecule level. These studies reveal the presence of a short-lived microsecond blinking component which leads to higher emission rates than exhibited by common organic dyes. This dark state was found to be photo-accessible with a very efficient depopulation transition leading to repopulation of the emissive state. Secondary excitation on resonance with this transition significantly shortens the residence time in the dark state giving rise to as much as 5-fold fluorescence enhancement. Manipulation of the secondary laser can be used to impose a regularly modulated waveform onto the fluorescent signal. Signal processing techniques can be employed to extract the modulated signal from large backgrounds, leading to drastically improved sensitivity. This new imaging concept can be extended, beyond Ag nanoclusters, to common organic fluorophores that demonstrate large dark state quantum yields. These fluorophores simultaneously illustrate the utility of this technique and help to define a general set of parameters for engineering ideal dyes for modulated signal extraction. Ideally suited for fluorescence enhancement, FRET pairs can be used to engineer a wide range of modulatable systems, based on detecting donor emission in the presence of a laser directly exciting the acceptor. The utility of Ag nanoclusters, organic dyes, and FRET systems for improved sensitivity are investigated in this work.

The surface and interface of the metal nanoclusters and semiconducting nanomaterials play a key role in determining the electronic structure and overall photophysical properties. A single strand DNA stabilizes the metal nanoclusters, but it also influences the structural change, solvation free energy, and photophysical properties. On the other hand, surface and interface states in Pb and Cd chalcogenide nanomaterials affect the phonon mediated hot carrier relaxation. We applied DFT and DFT based non-adiabatic dynamics methods to study the surface and interface?s effects on the photoexcited processes. In the first part, we have studied the Ag nanoclusters' photophysical properties that are affected by the structural isomers, redox potential, nucleobase passivation, and cluster size. Ag nanoclusters are shown alternative reduction potential, which makes nanoclusters of singlet spin multiplicity thermodynamically favorable. Besides, the optically bright transition in the range of 2.5-3.5 eV is shown metal to ligand charge transfer. It is modulated by the s+p+d orbital mixing in the hole and electron states. We also simulate the charge transfer from the photoexcited PbS QD to organic dye (PDI) attached to the QD surface. Depending on the linker group and the dipole moment of neighboring passivating ligands, the PDI-QD conformations are varies. In response to structural change, the total dipole moment is modulated, changing its electronic structure and hence the photoexcited electron transfer rate from the PbS QD to PDI. We also investigate the inorganic-inorganic interactions in the PbCl2 bridged PbSe NPL and PbSe|CdSe Janus heterostructure. The energy dissipation rate of hot electrons is slower in NPL than the hot hole, while hot e-h relaxed to the band-edge by ?1.0ps in the QD. The slower relaxation rate is rationalized by a large average intraband energy difference and smaller coupling term. Besides, the hot carriers in the NPL are spatially separated by ?1.00 ps, which is a favorable condition for the carrier multiplication process. In Janus QD, (100) interfacial layer creates a structural mismatch in the CdSe part. Besides, the energy offset between the valance localized on PbSe and CdSe part is minimum in the PbSe Janus QD of an interface of (111) facet.