Browsing by Author "Fedin, Igor"
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Item Auger-Limited Carrier Recombination and Relaxation in CdSe Colloidal Quantum Wells(ACS, 2015-03-02) Baghani, Erfan; O’Leary, Stephen K.; Fedin, Igor; Talapin, Dmitri V.; Pelton, MatthewUsing time-resolved photoluminescence spectroscopy, we show that two-exciton Auger recombination dominates carrier recombination and cooling dynamics in CdSe nanoplatelets, or colloidal quantum wells. The electron–hole recombination rate depends only on the number of electron–hole pairs present in each nanoplatelet, and is consistent with a two-exciton recombination process over a wide range of exciton densities. The carrier relaxation rate within the conduction and valence bands also depends only on the number of electron–hole pairs present, apart from an initial rapid decay, and is consistent with the cooling rate being limited by reheating due to Auger recombination processes. These Auger-limited recombination and relaxation dynamics are qualitatively different from the carrier dynamics in either colloidal quantum dots or epitaxial quantum wells.Item Hot-Carrier Relaxation in CdSe/CdS Core/Shell Nanoplatelets(ACS Publications, 2019-12-11) Pelton, Matthew; Wang, Yana; Fedin, Igor; Talapin, Dmitri V.; O’Leary, Stephen K.We present time-resolved photoluminescence (PL) spectroscopy of a series of colloidal CdSe/CdS core/shell nanoplatelets with different core and shell thicknesses. Exciton numbers are determined from the integrated PL intensities, and carrier temperatures are determined from the high-energy exponential tail of the PL spectra. For times between 10 and 1000 ps, the measured carrier relaxation dynamics are well described by a simple model of Auger reheating: biexcitonic Auger recombination continually increases the average energy of the carriers (while decreasing their number), and this reheating sets a bottleneck to cooling through electron–phonon coupling. For times between 1 and 10 ps, the relaxation dynamics are consistent with electron–phonon coupling, where the bottleneck is now the decay of the longitudinal optical phonon population. Comparison of relaxation dynamics to recombination dynamics reveals changes in the carrier–phonon coupling for shell thicknesses greater than 4 monolayers.Item Low-Threshold Stimulated Emission Using Colloidal Quantum Wells(ACS, 2014-04-28) She, Chunxing; Fedin, Igor; Dolzhnikov, Dmitriy S.; Demortière, Arnaud; Schaller, Richard D.; Pelton, Matthew; Talapin, Dmitri V.The use of colloidal semiconductor nanocrystals for optical amplification and lasing has been limited by the need for high input power densities. Here we show that colloidal nanoplatelets produce amplified spontaneous emission with thresholds as low as 6 μJ/cm² and gain as high as 600 cm⁻¹, both a significant improvement over colloidal nanocrystals; in addition, gain saturation occurs at pump fluences 2 orders of magnitude higher than the threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and narrow ensemble emission line widths.Item Nonmonotonic Dependence of Auger Recombination Rate on Shell Thickness for CdSe/CdS Core/Shell Nanoplatelets(ACS, 2017-10-10) Pelton, Matthew; Andrews, Jordan J.; Fedin, Igor; Talapin, Dmitri V.; Leng, Haixu; O’Leary, Stephen K.Nonradiative Auger recombination limits the efficiency with which colloidal semiconductor nanocrystals can emit light when they are subjected to strong excitation, with important implications for the application of the nanocrystals in light-emitting diodes and lasers. This has motivated attempts to engineer the structure of the nanocrystals to minimize Auger rates. Here, we study Auger recombination rates in CdSe/CdS core/shell nanoplatelets, or colloidal quantum wells. Using time-resolved photoluminescence measurements, we show that the rate of biexcitonic Auger recombination has a nonmonotonic dependence on the shell thickness, initially decreasing, reaching a minimum for shells with thickness of 2–4 monolayers, and then increasing with further increases in the shell thickness. This nonmonotonic behavior has not been observed previously for biexcitonic recombination in quantum dots, most likely due to inhomogeneous broadening that is not present for the nanoplatelets.Item A room temperature continuous-wave nanolaser using colloidal quantum wells(Nature, 2017-07-26) Yang, Zhili; Pelton, Matthew; Fedin, Igor; Talapin, Dmitri V.; Waks, EdoColloidal semiconductor nanocrystals have emerged as promising active materials for solution-processable optoelectronic and light-emitting devices. In particular, the development of nanocrystal lasers is currently experiencing rapid progress. However, these lasers require large pump powers, and realizing an efficient low-power nanocrystal laser has remained a difficult challenge. Here, we demonstrate a nanolaser using colloidal nanocrystals that exhibits a threshold input power of less than 1 μW, a very low threshold for any laser using colloidal emitters. We use CdSe/CdS core-shell nanoplatelets, which are efficient nanocrystal emitters with the electronic structure of quantum wells, coupled to a photonic-crystal nanobeam cavity that attains high coupling efficiencies. The device achieves stable continuous-wave lasing at room temperature, which is essential for many photonic and optoelectronic applications. Our results show that colloidal nanocrystals are suitable for compact and efficient optoelectronic devices based on versatile and inexpensive solution-processable materials.