Dual emission and optical gain in PbS/CdS nanocrystals: Role of shell volume and of core/shell interface

Abstract

Colloidal semiconductor nanocrystals (NCs) emitting simultaneously at two distinct wavelengths are of great interest for multiple applications ranging from ultrasensitive self-calibrating nanoscale sensing to light-emitting diodes and white-light emission. The physical mechanism governing dual emission in core/shell NCs composed of two materials is still under investigation; in particular, the roles of the volume of the shell and of the core/shell interface are not well understood. Here we compare three different PbS/CdS NCs with identical cores, but shells of different thicknesses and different core/shell interfaces. We study photoluminescence (PL) characteristics and ultrafast exciton dynamics with subpicosecond time resolution. We find that dual emission is possible for both thick and thin quantum-confined shells, and for different core/shell interfaces. We show that very thick shells allow the decoupling of shell excitons from core ones and we fully describe the dynamic exciton barrier, connected to efficient shell PL. We discuss the efficiency of shell PL in relation to the properties of the NCs and show that it can give rise to optical gain. Our results provide a comprehensive understanding of the physical phenomena governing dual-emission mechanisms in NCs.

Photoinduced electron transfer in novel CdSe–Cu2Se type II core–shell quantum dots

“…This type of dual emission has already been reported in similar systems and the mechanism responsible is still under research. [45][46][47][48] With same CdSe core size, CdSe-Cu 2 Se structures with higher shell thickness were also grown, that exhibited more or less the same features but with a nearly vanishing shoulder peak. The absorption and emission features of the bare CdSe and CdSe-Cu 2 Se core-shell QDs with different shell thicknesses are provided in the ESI (Fig.…”

Tailoring the Heterostructure of Colloidal Quantum Dots for Ratiometric Optical Nanothermometry

“…[1,[9][10][11]18,31,35,[42][43][44][45][46][47][48][49] Compared with other types of phosphors (dyes, polymers, carbon dots, and upconverting materials), [11,24,36,42,44,50] semiconductor QDs have demonstrated their potential as optical temperature sensors. [21,31,35,38,43,47,48,[51][52][53][54][55][56][57][58][59][60][61][62][63] QDs have size/shape/composition tunable absorption/emission spectrum from the visible to the near infrared (NIR) range, narrow emission peak width, high absorption coefficient, high QY, excellent photostability, and simple synthetic approach. [64][65][66][67][68][69][70]…”

“…Doped ion emission is caused by the energy transfer of electron-hole pairs from the host to the localized Mn 2+ ion and the subsequent electronic transition of Mn 2+ d-orbitals (i.e., 4 T 1 − 6 A 1 ); [51][52][53][54] iii) two band edge emissions in core/shell/shell QDs (e.g., PbS/CdS/CdSe QD). [46] The interfacial shell serves as energy barrier; iv) two band edge emissions in core/thick-shell QDs (e.g., PbS/CdS QD and CdSe/CdS QD) [33,56,59,61] or core/ tetrapods or rods, [98,99] due to spatial indirect emission; v) two band edge emissions in hybrid nanostructures (e.g., PbS/CdS/ ZnS QDs linked with upconversion nanocrystals). [100] Compared to type ii-iv), in the type case i), usually the trap emission cannot be controlled because the trap-emission of QDs is typically very sensitive to the surrounding chemical environment.…”

“…Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim excellent optical properties. [57,61,62,94,101] Depending on the relative band energy levels of core and shell materials, the band structure in core/shell QDs can be defined as type I, type II, and reverted type I. [102][103][104][105][106][107][108][109][110] In particular, in the type I structure, both electrons and holes are confined in the core region, leading to higher QY and better photostability compared to plain QDs.…”

“…A cation exchange approach was further used to synthesize PbS/CdS QDs, which is widely applied for the production of core/shell structured chalcogenide QDs. [61][62][63] The CdS shell thickness can be controlled by varying the reaction parameters, such as core size, Cd(oleate) 2 concentration, molar ratio of Cd(oleate) 2 /PbS, reaction temperature, and reaction time. Furthermore, SILAR was used to coat the CdSe on the core/shell PbS/CdS QDs.…”