(Invited) Colloidal Semiconductor Nanocrystals Under Intense Optical Excitations Towards Efficient Lasing and Hot-Carrier Harvesting (2020)

Colloidal semiconductor nanocrystals appeal to broad range of applications including bio-imaging and displays where essential optical properties such as photoluminescence quantum yields are typically characterized under weak optical excitation conditions producing less than one exciton per particle on average. On the other hand, applications such as lasing and high-brightness lighting require intense excitation conditions producing more than one exciton per nanocrystal on average. Unfortunately, prior reports showed that multiple excitons in these nanocrystals are annihilated at extremely high rates due to nonradiative Auger recombination. Therefore, energy of the multiple excitons is lost to heating before it can be used and converted into electrical or light energy. In the first of the talk, I will present a colloidal nanocrystal system, which provides remarkable performance in lasing applications. Recently, we showed that atomically-flat colloidal CdSe nanoplatelets exhibit record-high optical gain coefficients under intense optical excitation conditions [1]. In contrast, optical gain coefficients in conventional CdSe nanocrystals (e.g., core/shell quantum dots) saturate at very low excitation fluences; typically at a fluence that is two times the gain threshold. In the case of the CdSe nanoplatelets, optical gain saturates at an exceptionally high level which is an order of magnitude larger than those of prototypical nanocrystals. As a result, CdSe nanoplatelets attain giant modal gain coefficients which we directly measure to be on the order of 6000 cm -1 with the intrinsic material gain coefficients exceeding 15,000 cm -1 . The measured modal gain in the nanoplatelets is the highest among any lasing media measured at room temperature to date. This indicates that nanoplatelets sustain optically excited multiple excitons and enable their strong contribution to the gain before the nonradiative Auger recombination kicks in. We hypothesize that multiexcitons in the nanoplatelets form highly stable biexcitonic species as claimed in epitaxial quantum wells at low temperatures before [2]. In the second part of the talk, I will present a new technique to monitor how colloidal nanocrystals heat up at the nanoscale following an intense optical excitation, which is important for applications requiring intense excitation conditions. Previously, a phonon bottleneck effect between excited state electrons and optical phonons have been predicted but has never been observed experimentally [3]. This implied that nanocrystals should heat up rather quickly (sub-picosecond to picosecond timescale) by transferring the energy stored in the electronic system into the lattice via electron – phonon coupling. However, timescales of the heating up nanocrystals, which involve both electron – phonon and phonon – phonon couplings, could not be straightforwardly characterized with the existing time-resolved techniques. Here, we utilize ultrafast electron diffraction measurements, where we probe the lattice of the nanocrystals through following an optical pump. With this method, we resolve the intrinsic timescales of the heat transfer within the colloidal nanocrystals [4]. Distinct from previous optical studies, we observe highly slow heating up of the nanocrystals which strikingly depend on the excitations whether generated in the core or the shell of the nanocrystals. [1] B. Guzelturk et al. Nano Letters 19, 277 (2019) [2] D. A. B. Miller, “Optical physics of Quantum Wells” [3] R. Schaller et al. Phys. Rev. Lett. 95, 196401 (2005) [4] B. Guzelturk et al. (in-preparation)