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

Abstract

The large bulk bandgap (1.35 eV) and Bohr radius (~10 nm) of InP semiconductor nanocrystals provides bandgap tunability over a wide spectral range, providing superior color tuning compared to that of CdSe quantum dots. In this paper, the dependence of the bandgap, photoluminescence emission, and exciton radiative lifetime of core/shell quantum dot heterostructures has been investigated using colloidal InP core nanocrystals with multiple diameters (1.5, 2.5, and 3.7 nm). The shell thickness and composition dependence of the bandgap for type-I and type-II heterostructures was observed by coating the InP core with ZnS, ZnSe, CdS, or CdSe through one to ten iterations of a successive ion layer adsorption and reaction (SILAR)-based shell deposition. The empirical results are compared to bandgap energy predictions made with effective mass modeling. Photoluminescence emission colors have been successfully tuned throughout the visible and into the near infrared (NIR) wavelength ranges for type-I and type-II heterostructures, respectively. Based on sizing data from transmission electron microscopy (TEM), it is observed that at the same particle diameter, average radiative lifetimes can differ as much as 20-fold across different shell compositions due to the relative positions of valence and conduction bands. In this direct comparison of InP/ZnS, InP/ZnSe, InP/CdS, and InP/CdSe core/shell heterostructures, we clearly delineate the impact of core size, shell composition, and shell thickness on the resulting optical properties. Specifically, Zn-based shells yield type-I structures that are color tuned through core size, while the Cd-based shells yield type-II particles that emit in the NIR regardless of the starting core size if several layers of CdS(e) have been successfully deposited. Particles with thicker CdS(e) shells exhibit longer photoluminescence lifetimes, while little shell-thickness dependence is observed for the Zn-based shells. Taken together, these InP-based heterostructures demonstrate the extent to which we are able to precisely tailor the material properties of core/shell particles using core/shell dimensions and composition as variables.

A high-temperature continuous stirred-tank reactor cascade for the multistep synthesis of InP/ZnS quantum dots

“…During ZnS deposition on InP core, the addition of sulfur (TOP : S) at elevated temperatures did not alter significantly PL peak position, meaning that surface etching (PL peak blue shift) was not significant. This is in line with previous studies reporting the synthesis of InP/ZnS with TOP : S. 48 In addition, due to relatively low reactivity of zinc precursors, InP/ZnS QDs synthesized at 270°C and 280°C exhibit low energy tails, which indicate trap emission. 48 Such low energy tails suggest incomplete ZnS shell overcoating and it is in direct correlation with the corresponding emission linewidths and QYs (below 25%) of the samples (Fig.…”

“…However, when the temperature was increased up to 310°C (thicker shells), the emission linewidths and QYs were decreased and remained relatively stable at ∼60 nm and ∼22%, respectively. Similar to a previous study, 48 this effect is supposedly due to the strain effects (higher lattice mismatch of InP/ZnS -7.8% comparing to InP/ZnSe -4.9%) and surface defects after a specific number of monolayers deposited on the surface of InP cores. A more effective strategy for enhanced passivation of InP cores would require a double-shell strategy, first by depositing a ZnSe or ZnSeS shell and then by a subsequent deposition of a thin ZnS shell.…”

Influence of the Core/Shell Structure of Indium Phosphide Based Quantum Dots on Their Photostability and Cytotoxicity

“…ZnSe has a lower mismatch (3.3%) and several works reported the growth of either pure ZnSe or graded ZnSe x S 1−x shells on InP or InZnP (Lim et al, 2011; Tessier et al, 2015; Pietra et al, 2016; Chandrasekaran et al, 2017). However, ZnSe provides a weaker carrier confinement than ZnS due to the smaller band gap and is more sensitive to oxidation than ZnS (Toufanian et al, 2018). Therefore, it has been suggested to add a thicker outer ZnS shell on a graded intermediate ZnSe x S 1−x shell (Lim et al, 2013; Ramasamy et al, 2017; Wang et al, 2017).…”

A versatile and accessible polymer coating for functionalizable zwitterionic quantum dots with high DNA grafting efficiency

“…InP/ZnS and InP/ZnSe/ZnS QDs produced using a previously published SILAR method were used to demonstrate generalizability to a cadmium-free system. 5 A one-pot synthesis of CdSe/CdS/ZnS alloyed shell particles was used to generate the green CdSe-based emitters for the energy transfer study described below. 6 Polymer functionalization.…”