Trion Emission Dominates the Low-Temperature Photoluminescence of CdSe Nanoplatelets

Abstract

Colloidal nanoplatelets (NPLs) are atomically flat, quasi-two-dimensional particles of a semiconductor. Despite intense interest in their optical properties, several observations concerning the emission of CdSe NPLs remain puzzling. While their ensemble photoluminescence spectrum consists of a single narrow peak at room temperature, two distinct emission features appear at temperatures below ~160 K. Several competing explanations for the origin of this two-color emission have been proposed. Here, we present temperature-and time-dependent experiments demonstrating that the two emission colors are due to two subpopulations of uncharged and charged NPLs. We study dilute films of isolated NPLs, thus excluding any explanation relying on collective effects due to NPL stacking. Temperature-dependent measurements explain that trion emission from charged NPLs is bright at cryogenic temperatures, while temperature activation of nonradiative Auger recombination quenches the trion emission above 160 K. Our findings clarify many of the questions surrounding the photoluminescence of CdSe NPLs.

Manipulation of the Optical Properties of Colloidal 2D CdSe Nanoplatelets

“…Previous low‐temperature PL studies have shown that a single CdSe NPL only has one band‐edge exciton emission, whereas CdSe NPL stacks upon ethanol addition exhibit an additional PL emission peak in the low‐energy side at a cryogenic temperature. Although several hypotheses [ 154–156 ] have been proposed to interpret the origin of this additional PL peak, phonon line emission, as one of the possible mechanisms, was tentatively proposed to account for the collective effects in the NPL stacks films. [ 157 ] Transient electric birefringence study of CdSe NPLs’ helical structure demonstrated the presence of a dominant in‐plane permanent dipole along with a small amount of out‐of‐plane dipole.…”

Nature and Control of Shakeup Processes in Colloidal Nanoplatelets

“…We analyze the emission spectra of trions in core-only and core/shell NPLs. Negative trions are studied unless otherwise noted, as it is the most frequently reported species in these structures, 23,24,33,34 but the conclusions do not depend on the sign of the charged exciton (see Fig. S2 in the Supporting Information, SI).…”

“…In core-only CdSe NPLs, the low temperature photoluminescence is thought to arise from subpopulations of excitons and negative trions. 24,33,46 Very recently, Antolinez and co-workers have reported that the X − emission shows a distinct peak or a shoulder (depending on the film thickness) redshifted from the trion band edge transition. The redshift is ∼ 19 meV and the relative height 15 − 25% that of the main peak.…”

“…5c,d), where only one SU peak has been mea-sured. 24 It is then not surprising that, despite investigating different charge locations (Figs.…”

“…12 A similar feature was soon after reported in core-only CdSe NPLs, although in this case only two peaks were measured. 24 Having ruled out more conventional mechanisms, such as exciton-phonon interaction or spectral diffusion, the nature of the additional peaks was tentatively ascribed to SU processes of negative trions (X − ). These are partly radiative Auger processes, whereby an electronhole pair recombines radiatively but transfers part of its energy to the remaining electron by exciting it into a higher single-electron level (in-plane excitation).…”