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

ARTICLE https://doi.org/10.1038/s41467-020-17032-8 OPEN Sub-single exciton optical gain threshold in colloidal semiconductor quantum wells with gradient alloy shelling 1234567890():,; Nima Taghipour1, Savas Delikanli1,2, Sushant Shendre2, Mustafa Sak1, Mingjie Li3, Furkan Isik1, Ibrahim Tanriover1, Burak Guzelturk 4, Tze Chien Sum 3 & Hilmi Volkan Demir 1,2,3 ✉ 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 solutionprocessable electrically-driven colloidal lasers. 1 Department of Electrical and Electronics Engineering, Department of Physics, UNAM-Institute of Materials Science and Nanotechnology, Bilkent University, Ankara 06800, Turkey. 2 Luminous! Centre of Excellence for Semiconductor Lighting and Displays, School of Electrical and Electronic Engineering, School of Physical and Mathematical Sciences, School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore. 3 School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 639798, Singapore. 4 Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA. ✉email: NATURE COMMUNICATIONS | (2020)11:3305 | https://doi.org/10.1038/s41467-020-17032-8 | www.nature.com/naturecommunications 1 ARTICLE S NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17032-8 olution-processed semiconductor nanocrystals offer a versatile and promising gain medium for light-amplification applications1–4 owing to their broad spectral tunability1,5,6, low-cost production and flexibility of using them in a broad range of matrices1,4. As a consequence, the utilization of colloidal semiconductor quantum dots (CQDs) as a gain medium has been experiencing a tremendous growth in the last few decades. In addition, alternative nanoemitters, e.g., semiconductor nanorods7 and perovskite nanocrystals8,9, have recently emerged as an auspicious gain material for stimulated emission and lasing. Yet another promising candidate, colloidal semiconductor quantum wells (CQWs), the so-called nanoplatelets (NPLs)10–12, have been shown to be excellent in optical gain applications thanks to their giant oscillator strength13 and ultralarge modal gain coefficient14. Using different heterostructures of NPLs allows for ultralowamplified spontaneous emission (ASE) thresholds10,12. However, their gain performance still suffers from nonradiative Auger recombination, wherein the released energy from recombination of the electron-hole is transferred to a third carrier. In common II–VI semiconductor nanocrystals, because of the non-unity degeneracy of the electron and hole states involved in emission, light amplification requires an ensemble-averaged number of excitons per nanocrystal greater than one ((N) > 1), thereupon multi-excitonic Auger recombination strongly affects the dynamics of the carriers. Previously, various efforts have been reported to tackle the issue of the fast Auger decay in nanocrystals including the smoothing of their confinement potential15 and introducing the optical gain in single-exciton regime2,7,16. These methods have been shown to effectively enhance the optical gain performance of CQDs either by reducing influence or inactivating of the Auger process. On the other hand, in atomically flat CQWs, significant reduction in ASE and lasing threshold have previously been obtained in core/shell12 and core/crown10,11 heterostructures. Nonetheless, the demonstrated approaches have not addressed the fundamental issue of the optical gain in CQWs where the multi-exciton ((N) > 1) nature of the light amplification results in fast decay of the carriers by a few hundred of picoseconds timescale17,18 as a result of the Auger process. One way of inactivating of Auger recombination in CQWs is to employ the concept of single-exciton gain mechanism for optical amplification as formerly demonstrated in various heterostructures of the CQDs2,7,16. Particularly, a finite Stokes shift of the stimulated emission with respect to absorption peak can be employed to achieve optical gain in sub-single exciton regime where the Auger process is mostly inactivated2,19. As presented in our previous work20, the Stokes shift is increased with the increasing Zn content in the shell. This shift is finally maximized in CdSe/ CdS@ZnS core/crown@shell heterostructure. As a result, the Zn concentration in our CdSe/CdS/Cd1-xZnxS core/crown@alloyedshell CQWs can be modified to tune the Stokes shift to engineer the optical gain in these CQW heterostructures. In the current study, we have exploited the finite Stokes shift as a tool for achieving optical gain threshold in sub-single exciton regime ((N) < 1) in quasi-type-II CdSe/CdS@Cd1-xZnxS core/ crown@gradient-alloyed shell (C/C@GS) CQWs, which presents an important step towards the evolution of semiconductor CQW lasers. In addition, these core/crown@gradient-alloyed shell CQWs offer a promising solution for suppression of the Auger process with their smooth confinement potential. We demonstrated an exceptionally low stimulated emission threshold of ~820 nJ cm−2, corresponding to an average number of e–h pairs of 0.84 per NPL, which is also fully supported by nonlinear absorption measurements through ultrafast transient absorption spectroscopy. Sub-single exciton optical gain regime is also confirmed by linear dependence of the normalized absorption 2 changes. The extremely large absorption cross-section (5.06 × 10−13 cm2) of our engineered NPLs, accompanied by an extremely large net modal gain coefficient of ~1960 cm−1, and a long net optical gain lifetime of ~830 ps result in such ultralow optical gain thresholds and lead to record-long stable ASE. Employing specifically engineered core/crown@gradient-alloyed shell heterostructures, we present a linearly polarized single-mode lasing from a vertical-cavity surface-emitting laser enabling a record low lasing threshold of ~7.46 μJ cm−2. Results For this study, we synthesized CdSe/CdS@Cd1 (...truncated)