Sub-single exciton optical gain threshold in colloidal semiconductor quantum wells with gradient alloy shelling

Abstract

Colloidal semiconductor quantum wells have emerged as a promising material platform for use in solution-processable lasers. However, applications relying on their optical gain suffer from nonradiative Auger decay due to multi-excitonic nature of light amplification in II-VI semiconductor nanocrystals. Here, we show sub-single exciton level of optical gain threshold in specially engineered CdSe/CdS@CdZnS core/crown@gradient-alloyed shell quantum wells. This sub-single exciton ensemble-averaged gain threshold of (Ng)≈ 0.84 (per particle) resulting from impeded Auger recombination, along with a large absorption cross-section of quantum wells, enables us to observe the amplified spontaneous emission starting at an ultralow pump fluence of ~ 800 nJ cm−2, at least three-folds better than previously reported values among all colloidal nanocrystals. Finally, using these gradient shelled quantum wells, we demonstrate a vertical cavity surface-emitting laser operating at a low lasing threshold of 7.5 μJ cm−2. These results represent a significant step towards the realization of solution-processable electrically-driven colloidal lasers.

Coreless Fiber‐Based Whispering‐Gallery‐Mode Assisted Lasing from Colloidal Quantum Well Solids

“…Moreover, it has been previously shown that smoothening of the potential barrier effectively suppresses Auger recombination, which was also theoretically predicted in the quantum‐confined structures . Owing to the soft confinement potential of electron wavefunction in our engineered hetero‐CQWs, nonradiative Auger recombination is significantly suppressed . High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) image of the C/C@GAS shell CQWs with 3‐MLs of gradient alloyed shells is given in Figure a,b.…”

“…In this study, we synthesized CdSe/CdS@Cd 1– x Zn x S C/C@GAS via a colloidal atomic layer deposition technique where Cd 1– x Zn x S shells were grown on the seed 4‐monolayer (4‐ML) CdSe/CdS core/crown CQWs . Thanks to this specially engineered heterostructure of our CQWs, the electron wavefunction feels soft confinement potential in the vertical direction of the heterostructure owing to the small offset between the conduction bands of the core and gradually alloyed shell, while the hole wavefunction is mostly localized in the core region due to the large valence band offset . This forms a quasi‐type‐II band structure, and the spatial overlap between the electron‐ and hole‐wavefunctions is reduced in this system compared to the systems with type‐I band alignment as previously demonstrated in CdSe/CdS dot/rod NCs .…”

Optical Gain in Ultrathin Self‐Assembled Bi‐Layers of Colloidal Quantum Wells Enabled by the Mode Confinement in their High‐Index Dielectric Waveguides

“…[9][10][11][12] CQWs exhibit narrow spontaneous emission spectra, suppressed inhomogeneous broadening, giant oscillator strength, and high density of energy states at the band edge, which are highly desired for optical gain and lasing. [13][14][15] Optical gain and lasing using a few hundred nanometers to micrometer thick CQW active regions have thus far been reported. [5,16] However, achieving gain from ultra-thin active CQW film has been a major challenge.…”

“…[9] Although the methods used for material engineering have been successful in reducing loss mechanisms to some extent, the required active film thickness is still typically larger than 100 nm in these giant nanocrystals. [9,15,20] To address the gain performance issue of thin films, we also need to consider the film preparation methods, since the film quality can help reduce losses and improve the gain performance significantly. Spin-coating and drop-casting have been the common techniques for preparing thin films of colloidal nanocrystals.…”

“…[ 9–12 ] CQWs exhibit narrow spontaneous emission spectra, suppressed inhomogeneous broadening, giant oscillator strength, and high density of energy states at the band edge, which are highly desired for optical gain and lasing. [ 13–15 ]…”