130 research outputs found
On the Nature of Trapped-Hole States in CdS Nanocrystals and the Mechanism of their Diffusion
Recent transient absorption experiments on CdS nanorods suggest that
photoexcited holes rapidly trap to the surface of these particles and then
undergo diffusion along the rod surface. In this paper, we present a
semiperiodic DFT model for the CdS nanocrystal surface, analyze it, and comment
on the nature of both the hole-trap states and the mechanism by which the holes
diffuse. Hole states near the top of the valence band form an energetic near
continuum with the bulk, and localize to the non-bonding sp orbitals on
surface sulfur atoms. After localization, the holes form nonadiabatic small
polarons that move between the sulfur orbitals on the surface of the particle
in a series of uncorrelated, incoherent, thermally-activated hops at room
temperature. The surface-trapped holes are deeply in the weak-electronic
coupling limit and, as a result, undergo slow diffusion.Comment: 4 figure
Molecular Chemistry to the Fore: New Insights into the Fascinating World of Photoactive Colloidal Semiconductor Nanocrystals
Colloidal semiconductor nanocrystals possess unique properties that are unmatched by other chromophores such as organic dyes or transition-metal complexes. These versatile building blocks have generated much scientific interest and found applications in bioimaging, tracking, lighting, lasing, photovoltaics, photocatalysis, thermoelectrics, and spintronics. Despite these advances, important challenges remain, notably how to produce semiconductor nanostructures with predetermined architecture, how to produce metastable semiconductor nanostructures that are hard to isolate by conventional syntheses, and how to control the degree of surface loading or valence per nanocrystal. Molecular chemists are very familiar with these issues and can use their expertise to help solve these challenges. In this Perspective, we present our group\u27s recent work on bottom-up molecular control of nanoscale composition and morphology, low-temperature photochemical routes to semiconductor heterostructures and metastable phases, solar-to-chemical energy conversion with semiconductor-based photocatalysts, and controlled surface modification of colloidal semiconductors that bypasses ligand exchange
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