Quasi-Type II Carrier Distribution in CdSe/CdS Core/Shell Quantum Dots with Type I Band Alignment

Abstract

Band alignments are essential for understanding the optical properties and carrier transfer of core/shell QDs. As CdSe/CdS core/shell QDs with increasing shell thickness represent red-shifted absorption and luminescence spectra, weakened oscillator strength of the lowest electronic transition, and elongated luminescence lifetime, they are assigned to quasi-type II band alignment. However, femtosecond transient absorption spectroscopy with state-selective excitation revealed a type I band alignment of the CdSe/CdS QDs with a thin CdS shell, in which the excited electron is localized in the CdSe core with core excitation while delocalized in the whole QDs with shell excitation, even though a quasi-type II carrier distribution was observed with steady-state spectroscopy. In the type I core/shell QDs, the CdS shell acts as an energy barrier in surface electron and hole-trapping processes. The time constant of the hole-trapping process of the CdSe core (∼10 ps) was elongated 10 times owing to a tunnel effect through the high energy barrier of the CdS shell, which was estimated from the decay related to the biexcitonic induced spectral shift. The biexcitonic spectral shift induced by a ∼100 ps hole-trapping process was also observed at the 1S­(e)–2S3/2(h) transition. Our results from transient absorption spectroscopy with state-selective excitation are useful to clarify band alignment and carrier distribution of hetero-nanostructures, which could help to objectively extract charge carriers in photovoltaic applications.

A comparative study demonstrates strong size tunability of carrier–phonon coupling in CdSe-based 2D and 0D nanocrystals

“…For CdSe-CdS dots type I band alignment is also suggested, 54 but is still under discussion. 55 The performed renormalization by the surface area is also substantiated by the results of Takagahara et al 56 for spherical CdSe core-only quantum dots. They found a 1/r 2 ∝ 1/A dependence for coupling in dots, where r is the crystallite radius.…”

Bandgap Engineering of Indium Phosphide-Based Core/Shell Heterostructures Through Shell Composition and Thickness

“…The latter approach renders QDs emissive through passivation of surface trap states at the core/shell interface, thus inhibiting non-radiative recombination pathways while protecting the core nanocrystal against photo-oxidation (Battaglia et al, 2003 ). The bandgap of the shell material, as well as the position of its conduction and valence bands relative to those of the core NC, dictates the bandgap structure of the resulting heterostructure and, thereby, the resulting optoelectronic properties (Li and Wang, 2004 ; Wang et al, 2018 ).…”

Core/Shell Quantum Dots Solar Cells

“…This particular case of core/shell structured QDs is called quasi‐type‐II (Figure 1d). 47,63–65 For example, CdSe/CdS core/shell QDs (either small core or thick shell) exhibits a quasi‐type‐II structure due to the lower CB offset.…”

“…In “giant” core/shell QDs, the thick shell provides complete isolation of core from the surrounding environment, which enhances the thermal, photophysical and chemical stability 53,100–103. The most importantly by tailoring the shell composition as well as thickness, electronic band alignment of “giant” core/shell QDs changes and leads to the formation of a quasi‐type‐II core/shell structure in which holes are confined in the core region and electrons are delocalized in shell region 63,97,104,105. This decrease in the spatial overlap between the electron and hole wavefunctions leads to prolonged electron lifetime and significantly suppressed the nonradiative auger recombination 95.…”