Temperature and Magnetic-Field Dependence of Radiative Decay in Colloidal Germanium Quantum Dots

Abstract

We conduct spectroscopic and theoretical studies of photoluminescence (PL) from Ge quantum dots (QDs) fabricated via colloidal synthesis. The dynamics of late-time PL exhibit a pronounced dependence on temperature and applied magnetic field, which can be explained by radiative decay involving two closely spaced, slowly emitting exciton states. In 3.5 nm QDs, these states are separated by ∼1 meV and are characterized by ∼82 μs and ∼18 μs lifetimes. By using a four-band formalism, we calculate the fine structure of the indirect band-edge exciton arising from the electron-hole exchange interaction and the Coulomb interaction of the Γ-point hole with the anisotropic charge density of the L-point electron. The calculations suggest that the observed PL dynamics can be explained by phonon-assisted recombination of excitons thermally distributed between the lower-energy "dark" state with the momentum projection J = ± 2 and a higher energy "bright" state with J = ± 1. A fairly small difference between lifetimes of these states is due to their mixing induced by the exchange term unique to crystals with a highly symmetric cubic lattice such as Ge.

Direct Evaluation of the Quantum Confinement Effect in Single Isolated Ge Nanocrystals

“…Comparing the PL and STS data may provide then the first experimental estimate for this interaction and for the effective masses involved in these two manifestations of the QC effect in Ge NCs. With that respect it is interesting to note that while the tight-binding model fits well our STS data over the whole range of NC sizes (3 to 10.5 nm), Robel et al show 14 that it accounts well for their optical data measured 11 on small (<4 nm) NCs. On the contrary, the behavior of larger NCs (>5 nm), measured by other groups, 12,13 is well described by the effective mass approximation.…”

Germanium–Tin/Cadmium Sulfide Core/Shell Nanocrystals with Enhanced Near-Infrared Photoluminescence

“…53 Recently, Robel and coworkers monitored the combined effect of temperature and high magnetic field on the PL lifetimes of Ge nanocrystals, which showed splitting between closely-spaced states as well as mixing between dark and bright states all contribute toward the indirect PL. 67 The intricacies of these widely varying optical properties are not clear: surface states very likely play a role for the luminescence in the visible region, as this extent of a blue shift from the bulk band gap is not explained by confinement alone.…”

Investigation of the photophysical properties of energy-relevant inorganic nanocrystals

“…96 Recently, Robel and coworkers monitored the combined effect of temperature and high magnetic field on the PL lifetimes of Ge nanocrystals, which showed splitting between closely-spaced states as well as mixing between dark and bright states all contribute toward the indirect PL. 109 The intricacies of these widely varying optical properties are not clear: surface states very likely play a role for the luminescence in the visible region, as this extent of a blue shift from the bulk band gap is not explained by confinement alone.…”