Charge Carrier Cooling Bottleneck Opens Up Nonexcitonic Gain Mechanisms in Colloidal CdSe Quantum Wells

Abstract

Ultrathin two-dimensional (2D) materials have received much attention in the past years for a wide variety of photonic applications because of their pronounced roomtemperature excitonic features, leading to unique properties in terms of light−matter interaction. However, only a few studies focus on light amplification and the complex photophysics at high excitation density. The beneficial nature of strong excitonic effects on optical gain remain hence unquantified, and despite the increased binding energies of the excitonic species, it remains unclear what the involvement of 2D excitons would be in optical gain. Here, we use colloidal CdSe nanoplatelets as a model system for colloidal 2D materials and show, using a quantitative and combinatory approach to ultrafast spectroscopy, that several excitation density-dependent optical gain regimes exist. At low density, optical gain originates from excitonic molecules delivering large material gains up to 20 000 cm −1 with an Auger limited lifetime of a few hundred picoseconds. At increasing pair density, we observe a persistence of this excitonic gain regime and the unexpected coexistence of blue-shifted and significantly enhanced optical gain up to 10 5 cm −1. We show that this peculiar situation originates from a carrier cooling bottleneck at high density that limits further exciton formation from unbound charge carriers. The insulating (multi-)exciton gas is found to coexist with the conductive phase, indicating the absence of a full Mott transition. Our results shed a new light on the photophysics of excitons in strongly excited 2D materials and pave the way for the development of more efficient (broadband) optical gain media and/or high exciton density applications.

Explaining observed stability of excitons in highly excited CdSe nanoplatelets

“…In order to stay close to experiments, our results are obtained for the CdSe nanoplatelets of 4.5 monolayers studied by Ref. [51], which sets m e = 0.27 m 0 and m h = 0.45 m 0 as the effective electron and hole masses, with m 0 the bare electron mass, corresponding to the n = 4 case in Ref. [54].…”

“…The white horizontal solid line corresponds to the region experimentally explored in Ref. [51], at the constant temperature T = 294 K. The white dashed line marks the points for which the density of excitons and biexcitons is equal. The circles ( , top right) and squares ( , from bottom to top right) dotted lines represent the photoexcitation density above which excitons and biexcitons become quantum degenerate, respectively.…”

“…Even though excitons and biexcitons have been thoroughly experimented with, it was not until recently that an unexpected stability of excitons in CdSe nanoplatelets at high densities was observed, which was in turn explained by the rather unimportant screening effects of the free charges [51,52]. Armed with this newfound understanding, it follows that the formation of more complex species is very likely as well.…”

Tuning exciton diffusion, mobility and emission line width in CdSe nanoplatelets via lateral size

“…28, we investigate in this paper the exciton (quasiparticle) diffusion constant, exciton mobility and scattering rate using temperature dependent photoluminescence (PL) spectroscopy and theoretical modeling for CdSe nanoplatelets, a model system for 2D semiconductors of finite lateral size. Unlike charge carrier mobility determination by classical transport measurements with contacts or contact-less THz and microwave conductivity measurements, 29,30 we determine the diffusion coefficient and mobility of neutral excitons by connecting the temperature dependent emission line width of the considered colloidal CdSe quantum wells to the microscopic scattering rates and subsequently to the (exciton) diffusivity. We demonstrate that our method shows good agreement with theoretical modeling and other methods, e.g.…”

A bright future for colloidal quantum dot lasers

“…Even so, optical gain in nanoplatelets is often discussed in similar terms as QDs, where the word exciton is interchanged. However, excitons in 2D materials differ vastly from electron-hole pairs in the 0D counterparts, and it remains unclear what the exact gain mechanism is in 2D nanoplatelets 33 . Gain in quantum wells, for example, is typically mediated at room temperature by free charge carriers and at cryogenic temperatures by excitonic molecules, i.e., bound quasi-particles consisting of two excitons; the latter give similarly disruptive gain coefficients up to 10 5 cm −1 , comparable to nanoplatelets at room temperature 34,35 .…”