Optical Properties of Silicon Clusters in the Presence of Water:  A First Principles Theoretical Analysis

Abstract

We investigate the impact of water on the optical absorption of prototypical silicon clusters. Our clusters contain 5 silicon atoms, tetrahedrally coordinated and passivated with either hydrogen or oxygen. We approach this complex problem by assessing the contributions of three factors: chemical reactivity, thermal equilibration, and dielectric screening. We find that the silanone (Si=O) functional group is not chemically stable in the presence of water and exclude this as a source of significant red shift in absorption in aqueous environments. We perform first principles molecular dynamics simulations of the solvation of a chemically stable, oxygenated silicon cluster with explicit water molecules at 300 K. We find a systematic 0.7 eV red shift in the absorption gap of this cluster, which we attribute to thermally induced fluctuations in the molecular structure. Surprisingly, we find no observable screening impact of the solvent, in contrast with consistent blue shifts observed for similarly sized organic molecules in polar solvents. The predicted red shift is expected to be significantly smaller for larger Si quantum dots produced experimentally, guaranteeing that their vacuum optical properties are preserved even in aqueous environments.

Electron Emission from Diamondoids: A Diffusion Quantum Monte Carlo Study

“…Within Ziegler's sum model for restricted Hartree-Fock states [25], the error incurred by adopting this mixed-state approach is equal to half the singlet-triplet splitting. This error is typically 0.1-0.2 eV in group-IV nanostructures of this size [26], which is small compared with the optical gaps. Previous QMC calculations of the optical gaps of Si nanoparticles [14] using this approach were shown to be in excellent agreement with GW -Bethe-Salpeter-equation (GW -BSE) calculations of the true singlet excitation energies.…”

Excited state properties of Si quantum dots

“…In contrast to a great deal of theoretical studies 24–34 on the energy gap or absorption energy, studies on the emission gap are rather scarce, largely due to the lack of an efficient excited‐state (ES) geometric optimization method. However, a clear understanding of the excited state properties and its dynamics is of utmost necessity as the light emission properties of silicon nanoparticles is intimately related to it.…”

Equilibrium Geometries and Associated Energetic Properties of Mixed Metal−Silicon Clusters from Global Optimization

“…The metal atom in the Si n cluster stabilizes the pure Si cluster. Beck 11,12,13,14). Further analysis revealed that Si n cluster with endohedral metal atom impurities are characterized by enhanced stability, strong size selectively, and a large energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).…”