Highly Photoluminescent InP Based Core Alloy Shell QDs from Air-Stable Precursors: Excitation Wavelength Dependent Photoluminescence Quantum Yield, Photoluminescence Decay Dynamics, and Single Particle Blinking Dynamics

Abstract

InP based quantum dots (QDs) are coming in a big way as an alternative to toxic Cd, or Pb based QDs. Unlike many literature reports in this work, green-yellow-orange-red emitting highly photoluminescent (PLQY as high as 65%) and photostable InP/ZnSeS core/alloy shell quantum dots (CAS QDs) have been synthesized using a less toxic, air-stable aminophosphine precursor (P­(DMA)3). Unlike literature predictions in this paper, we show that green-yellow-orange-red emitting InP based alloyed QDs can be prepared with InCl3 only. We report here the hitherto unobserved and quite interesting excitation wavelength dependent PLQY for all of these green-yellow-orange-red emitting InP based CAS QDs. PLQY increases monotonically with increasing excitation wavelength. Significant deviation of the PL excitation spectrum from the absorption spectrum has been observed in the shorter wavelength region. This observation is perhaps because the surface mediated nonradiative pathways predominate over radiative charge carrier recombination when excited at shorter wavelength. PL decay for these QDs generally follows a triexponential decay equation with the shortest lifetime of 3–10 ns, the moderate one with a lifetime of 24–30 ns, and the longest one with a lifetime > 60 ns. Moderate and long lifetimes have been shown to be associated with two mutually interdependent excited-state decay channels, and the competition between these two decay channels dictates the PLQY of these CAS QDs. The moderate lifetime has been shown to be associated with an electron–hole recombination process, and the long lifetime is associated with delayed emission from the band edge due to interaction with the manifold of shallow traps. Quite interestingly, amplitude of the moderate lifetime (dynamical property) has been observed to be correlated with the PLQY (spectral property). PL decay for all of these InP based CAS QDs has been observed to be excitation wavelength independent. However, PL decay gets slower with increasing monitoring wavelength. Thus, the presence of shallow trap states is evidenced. Single particle blinking dynamics of InP based CAS QDs has been investigated for the first time. We could achieve the lowest reported magnitude of the m ON exponent for InP based QDs and the value is 1.19, which speaks about the much longer On-times or, in other words, superiority of our InP based CAS QD system in comparison to other reported InP based QDs, for example, InP core only, or InP/ZnS, InP/ZnSe/ZnS, InP/GaP/ZnS core/shell or core/shell/shell QD systems.

A White Light‐Emitting Quantum Dot Complex for Single Particle Level Interaction with Dopamine Leading to Changes in Color and Blinking Profile

“…The single particle nature of WLE QDC (deposited on a glass cover slip) was demonstrated based on the blinking activity, that is, time‐dependent alteration between on state (emitting state) and off state (dark state) using CLSM under superresolution. This is a primary signature, of single luminescent nanocrystals, which distinguishes them from multiple nanocrystals . Figure E shows the superresolution representative image of the white colored particles as obtained from Figure D (iii).…”

“…In our case, the probability distributions of the events (on and off) could be fitted well by a truncated power law through a log–log plot extracted from the obtained data points for 40 single particles (Figure H,I), the details of which are described in the Experimental Section. It is to be mentioned here that the exponent of the power‐law distribution (known as α on and α off ) of the on and off events is an important parameter with regard to the luminescence behavior of a blinking single nanocrystal . Hence, any chemical reaction or physical interaction that alters the emission characteristics of a single nanocrystal may lead to change in the exponents (α on and α off ) and their ratio (α on /α off ) in the blinking statistics .…”

“…It is to be mentioned here that the exponent of the power‐law distribution (known as α on and α off ) of the on and off events is an important parameter with regard to the luminescence behavior of a blinking single nanocrystal . Hence, any chemical reaction or physical interaction that alters the emission characteristics of a single nanocrystal may lead to change in the exponents (α on and α off ) and their ratio (α on /α off ) in the blinking statistics . In other words, the exponents (α on and α off ) and their ratio (α on /α off ) of a blinking single nanocrystal can be considered useful in describing the presence of any chemically reactive analyte on the surface and thus have important consequences in sensing application .…”

“…Hence, any chemical reaction or physical interaction that alters the emission characteristics of a single nanocrystal may lead to change in the exponents (α on and α off ) and their ratio (α on /α off ) in the blinking statistics . In other words, the exponents (α on and α off ) and their ratio (α on /α off ) of a blinking single nanocrystal can be considered useful in describing the presence of any chemically reactive analyte on the surface and thus have important consequences in sensing application . Importantly, the value of the exponent (α on ) of the on‐duration probability distribution ( P on ) was found to be lesser compared to the exponent (α off ) of the off‐duration probability distribution ( P off ) for single WLE QDC (α on = 0.42 ± 0.04 < α off = 1.75 ± 0.03; Figure H,I, Table S4, Supporting Information).…”

“…Chemical methods of suppression of blinking can also be considered to contain hint of the nature of the environment surrounding the emissive Qdot. Notably, the changes in the blinking statistics, especially the exponents of the on and/or off probability distribution, in the presence of an analyte could also be considered a characteristic useful in sensing.…”