Size-Dependent Biexciton Quantum Yields and Carrier Dynamics of Quasi-Two-Dimensional Core/Shell Nanoplatelets

Abstract

Quasi-two-dimensional nanoplatelets (NPLs) possess fundamentally different excitonic properties from zero-dimensional quantum dots. We study lateral size-dependent photon emission statistics and carrier dynamics of individual NPLs using second-order photon correlation (g(τ)) spectroscopy and photoluminescence (PL) intensity-dependent lifetime analysis. Room-temperature radiative lifetimes of NPLs can be derived from maximum PL intensity periods in PL time traces. It first decreases with NPL lateral size and then stays constant, deviating from the electric dipole approximation. Analysis of the PL time traces further reveals that the single exciton quantum yield in NPLs decreases with NPL lateral size and increases with protecting shell thickness, indicating the importance of surface passivation on NPL emission quality. Second-order photon correlation (g(τ)) studies of single NPLs show that the biexciton quantum yield is strongly dependent on the lateral size and single exciton quantum yield of the NPLs. In large NPLs with unity single exciton quantum yield, the corresponding biexciton quantum yield can reach unity. These findings reveal that by careful growth control and core-shell material engineering, NPLs can be of great potential for light amplification and integrated quantum photonic applications.

Low‐Threshold Amplified Spontaneous Emission and Lasing from Thick‐Shell CdSe/CdS Core/Shell Nanoplatelets Enabled by High‐Temperature Growth

“…Among these, CdSe/CdS core/shell NPLs have been increasingly attractive due to their excellent shell‐thickness‐dependent optical performance . CdSe/CdS NPLs are synthesized with higher quantum yields (QYs) and stability as compared to CdSe core‐only NPLs, because of the effective passivation of surface trap states by the epitaxial growth of a wider bandgap shells . In addition to the passivated surface defects, the absorption cross section of CdSe/CdS NPLs was boosted owing to the intrinsically increased optical absorption of the CdS shells .…”

“…In addition to the passivated surface defects, the absorption cross section of CdSe/CdS NPLs was boosted owing to the intrinsically increased optical absorption of the CdS shells . At the same time, the inorganic shells could help to suppress the rapid nonradiative Auger recombination (AR) of excitons with lower Auger rates and much reduced emission intermittency at single particle level, thereby enabling to realize low‐threshold (≈6 µJ cm −2 ) amplified spontaneous emission (ASE) and obtain high threshold for gain saturation . Generally, the epitaxial growth of semiconductor shells in core/shell hetero‐nanoplatelets is performed via the colloidal atomic layer deposition (c‐ALD) method .…”

Higher-Order Photon Correlation as a Tool To Study Exciton Dynamics in Quasi-2D Nanoplatelets

“…[35] Figure S5, presenting the PL intensity for NPLs versus excitation power, demonstrates that saturation does not occur even at the highest pulse energy of respectively. [8] , [36], [37] While the average population for the larger samples approaches saturation, the biexciton population is still substantially smaller than that of the single excitons.…”

“…[7] In contrast, due to the mean value of the lateral separation of excitons in a NPL, Auger rates are substantially lower than in QDs and the biexciton QY (BXQY) can approach that of the single exciton state. [8] Previous studies concluded that since the electron-hole binding energy in NPLs is much higher than the lateral confinement energy, Auger recombination thus occurs through exciton-exciton collisions. [9] Using ensemble experimental approaches, such as time-resolved photoluminescence (PL) and transient absorption, Li et al concluded that the rate of Auger relaxation decreases linearly with increasing the NPL lateral area and is inversely proportional to d 7 , where d is the thickness.…”

Pure Bromide‐Based Perovskite Nanoplatelets for Blue Light‐Emitting Diodes

“…Particularly, Ma et al demonstrated the giant oscillator effect existed in CdSe NPLs . Unlike the 0D nanocrystals, the exciton oscillator strength in NPLs is not only related to the degree of spatial overlap between the electron–hole wavefunctions, but also increased with the area of the exciton coherent motion, which is equivalent to the NPL's lateral size.…”