Sub-Nanosecond Radiative Decay Time of Excitons in Large Size Nanocrystals and Nanoplatelets

Sub-nanosecond radiative decay time was first observed for weakly bound excitons in bulk semiconductors and was understood in terms of the phenomenon known as giant oscillator strength (GOS).¹ GOS is a quantum phenomenon connected with coherent excitation of excitons over entire volume of exciton localization and is counterintuitive because the radiative decay time is inversely proportional to this volume. Consequently, it was predicted theoretically² that exciton weakly confined in a spherical nanocrystal (NC), which radius <i>a</i> is much larger than the exciton radius <i>a</i>_ex is characterized by GOS which strength, <i>f </i>_NC=<i>f</i>₀(<i>a</i>/<i>a</i>_ex)³ &gt;&gt;<i> f</i>₀ , where <i>f</i>₀is the exciton oscillator strength. Indeed ~ 100 ps radiative decay times was observed in CuCl³and large size CsPbX₃ (X=Cl,Br,I) perovskite NCs.⁴ The sub-nanosecond radiative decay time connected with GOS was also observed in quantum wells and nanopletelets.⁵ In all cases the superfast radiative decay time observed generally at liquid helium temperatures increases with temperature. In my talk I will discuss the origin of the GOS of excitons and their sub-picosecond decay time, the effect of the temperature and line broadening in in nanoplatelets. <br/>¹E. I Rashba and G. E. Gurgenishvili, Edge absorption theory in semiconductors. <i>Sov. Phys. Solid State,</i> <b>4</b>, 759-760 (1962).<br/>² Al. L Efros and A. L. Efros, Interband absorption of light in a semiconductor sphere, <i>Sov. Phys. Semicond, </i> <b>16</b>, 772-775 (1982).<br/>³T. Itoh and T. Kirihara, Excitons in CuCl Microcrystals Embedded in NaCl, <i>J. Lumin</i>. <b>31−32</b>, 120−122 (1984).<br/>⁴M. A. Becker<i>, et al.</i> “Bright triplet excitons in caesium lead halide perovskites,” <i>Nature</i>, <b>553</b>, 189-193 (2018).<br/> ⁵S. Ithurria, M. D. Tessier, B. Mahler, R. P. S. Lobo, B. Dubertret, Al. L. Efros, Colloidal Nanoplatelets with Two- Dimensional Electronic Structure. <i>Nat. Mater</i>. <b>10,</b> 936−941 (2011).