Quantum Confined Noble Metal Monolayer Protected Clusters Investigated Using Linear and Nonlinear Microscopy

Abstract

Elemental quantum confined nanocluster systems were previously demonstrated to have unusual optical, electronic, catalytic and magnetic properties suggesting to classify them as a new form of matter. Optical investigations in solution phase ensembles using monolayer protected nanoclusters (MPCs) allowed the community to experimentally confirm that the metal-to-insulator transition in gold occurs at ~300 gold atoms. However, investigations of single nanoclusters using optical microscopy and spectroscopy to determine effects of quantum confinement in MPCs were not reported until now. In my dissertation work, I interrogated isolated single quantum confined Au25 MPCs on a solid substrate. My observations made on isolated and aggregated MPCs on solid using two-photon excited fluorescence (TPEF) near-field scanning optical microscopy (NSOM) revealed that their native quantum confinement effects manifest primarily when they are isolated from aggregates and solution ensembles. This is consistent with the picture of narrowing of the density of states (DOS) when the quantum clusters are removed from aggregates and studied in isolation on solid. Also, it agrees with the enhancement expected for volume-normalized oscillator strengths (f12/V) of electronic transitions in the presence of quantum size effects. In order to obtain isolated single nanoclusters on solid, I devised a procedure where I synthesized MPCs, isolated them in solution phase and then deposited isolated single nanoclusters on solid substrate with ~160 nm average inter-nanocluster distances. Scanning transmission electron microscopy (STEM) confirmed the isolation of single nanoclusters on solid. The investigations of isolated MPCs on solid using aperture-based TPEF NSOM elicit ~30 nm point resolution which is ~5-fold better than the typical confocal point resolution. Also, my findings on possible local field enhancement for MPCs suggest the potential to use isolated MPCs, MPC arrays, meshes or lattices to obtain significantly enhanced TPEF properties that can be used in molecular computing, bioimaging,sensing, and data storage applications.

On a separate investigation, I explored materials that can increase the theoretical efficiency limit of organic photovoltaics (OPV) via ntramolecular singlet exciton fission (iSEF). I interrogated a quinoidal bithiophene molecule in solution that revealed highly efficient ultrafast iSEF with ~180% singlet-to-triplet conversion efficiency. Our finding of iSEF in a small molecule invigorates theoretical and experimental investigations of small molecule iSEF materials to make highly efficient solar cells.