Time-thermo-dynamics of anti-Stokes and Stokes scattering and luminescence in 1-nm silicon nanoparticles: Toward an optical nanorefrigerator

The thermodynamics of nanosystems is interesting, as they constitute the transition between the atomistic and solid states. This is empowered by the development of tools to manipulate individual atoms and perform atomistic simulations and fundamental thermos-science, such as microscopic time-symmetry and macroscopic time-asymmetry, the origin of time's arrow, and photo-cryo-refrigeration. We examine here the photo-thermo and time dynamics in 1-nm silicon nanoparticles with tetrahedral-molecular core-shell structure prepared ex situ and suspended in solvents or re-constituted in films. We examined the temperature dependence of the quantum efficiency and time-dynamics of the Stokes luminescence and its energy dependence across the band. With temperature, we get flat lifetimes but with delay in the onset in agreement with a model calculation of above barrier emission. Our atomistic time-dependent density functional theory shows that Stokes heating takes place in the molecular-like shell where the lifetime is in the nanosecond regime, whereas anti-Stokes cooling takes place in the tetrahedral core where the lifetime is in the ms regime. Unlike doped glasses, we observed a 2-order of magnitude increase in the quantum efficiency of the Stokes luminescence at 10° K. The increase in the quantum efficiency at low temperature, the high quantum efficiency of stimulated anti-Stokes scattering and its anti-correlation with the luminescence, and the visible transparency/blindness due to quantum confinement are requirements for solid state photo-cooling, which may afford an all-silicon photo-cryo-refrigeration, with potential full integration into the CMOS silicon industry.