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

摘要

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.