In the past two decades doped nanocrystals (NCs) have attracted a lot of attention because of their interesting electronic structure and unique optical properties. Recently, copper (Cu) - doped NCs have been studied extensively for understanding of their broadband Stokes-shifted emission and to utilize them in different optoelectronic applications including luminescent solar concentrators (LSCs). Here we show successful Cu-doping in atomically flat NCs, the colloidal quantum wells (CQWs), resulting strong broadband, largely Stokes-shifted and tunable visible-near-infrared (NIR) emission. [1-2] We have divided this work into four complementary parts. (i) Synthesis and understanding of the detailed photophysics of Cu-doped CdSe CQWs possessing excellent optical properties, which include very high PL quantum efficiency with step-like absorption profiles. (ii) Based on these exceptional properties, these NIR emitting doped CQWs were successfully applied to LSCs and have shown superior performance for solar light harvesting. Using detailed reabsorption studies, we demonstrated our doped CQWs as excellent candidates for next generation LSCs.[1,3] (iii) By utilizing the interaction between the quantum confined host carriers (band edge) and the Cu dopant ions, we observed room-temperature radiative biexciton states in these doped CQWs under continuous-wave excitation at pumping fluence as low as ~10 W/cm². Using the optical gain arising from the biexciton population, doped CQWs film embedded in the vertical cavity surface-emitting laser (VCSEL) structure yields an ultralow lasing threshold under sub-nanosecond pulsed excitation. These findings set our doped-CdSe CQWs as a new member for correlated excitonic studies and biexciton lasing devices. (iv) By utilizing dopant emission from Cu- doped CQWs, we propose a mutual fluorescence resonance energy transfer (FRET) between two distinct CQWs which play the role of donor and acceptor simultaneously. Using detailed time-resolved and photoluminescence excitation spectroscopies we demonstrated that band-edge excitons in Cu-doped CQWs effectively transfer the excitation to excitons in undoped CQWs whose energy is also harvested backwards by the dopants. This unique mutual FRET mechanism allows controllable conversion among three primary colors in the CQW complex. [4] Overall, these efforts allowed to utilize doping concepts in 2D atomically flat CQWs to their full potential, which can be extended into different host cores/dopants and new 2D architectures for future color conversion applications.

References

[1] Sharma, M.; Gungor, K.; Yeltik, A.; Olutas, M.; Guzelturk, B.; Kelestemur, Y.; Erdem, T.; Delikanli, S.; McBride, J. R.; Demir, H. V. Near-Unity Emitting Copper-Doped Colloidal Semiconductor Quantum Wells for Luminescent Solar Concentrators. Adv. Mater. 2017, 29, 1700821

 [2]. Sharma, M.; Olutas, M.; Yeltik, A.; Kelestemur, Y.; Sharma, A.; Delikanli, S.; Guzelturk, B.; Gungor, K.; McBride, J. R.; Demir, H. V. Understanding the Journey of Dopant Copper Ions in Atomically Flat Colloidal Nanocrystals of CdSe Nanoplatelets Using Partial Cation Exchange Reactions. Chem. Mater. 2018, 30, 3265– 3275

[3] Sharma, A; Sharma, M; Gungor, K; Olutas, M; Dede, D; Demir, H.V. Near-Infrared-Emitting Five Monolayer Thick Copper Doped CdSe Nanoplatelets, Adv. Optical Mater, 2019, 1900831 DOI:10.1002/adom.201900831

[4] Yu, J; Sharma, M; Delikanli, S; Birowosuto, M.D; Demir, H.V; Dang, C, Mutual Energy Transfer in a Binary Colloidal Quantum Well Complex, J. Phys. Chem. Lett., 10, 5193-5199 (2019).