Spectral widths and Stokes shifts in InP-based quantum dots

Abstract

InP-based quantum dots (QDs) have Stokes shifts and photoluminescence (PL) line widths that are larger than in II–VI semiconductor QDs with comparable exciton energies. The mechanisms responsible for these spectral characteristics are investigated in this paper. Upon comparing different semiconductors, we find the Stokes shift decreases in the following order: InP > CdTe > CdSe. We also find that the Stokes shift decreases with core size and decreases upon deposition of a ZnSe shell. We suggest that the Stokes shift is largely due to different absorption and luminescent states in the angular momentum fine structure. The energy difference between the fine structure levels, and hence the Stokes shifts, are controlled by the electron–hole exchange interaction. Luminescence polarization results are reported and are consistent with this assignment. Spectral widths are controlled by the extent of homogeneous and inhomogeneous broadening. We report PL and PL excitation (PLE) spectra that facilitate assessing the roles of homogeneous and different inhomogeneous broadening mechanisms in the spectra of zinc-treated InP and InP/ZnSe/ZnS particles. There are two distinct types of inhomogeneous broadening: size inhomogeneity and core–shell interface inhomogeneity. The latter results in a distribution of core–shell band offsets and is caused by interfacial dipoles associated with In–Se or P–Zn bonding. Quantitative modeling of the spectra shows that the offset inhomogeneity is comparable to but somewhat smaller than the size inhomogeneity. The combination of these two types of inhomogeneity also explains several aspects of reversible hole trapping dynamics involving localized In3+/VZn2− impurity states in the ZnSe shells.

Positive Trions in InP/ZnSe/ZnS Colloidal Nanocrystals

“…Preliminary theoretical estimations predict that the 1 P h 1 S e transition has higher energy than the 1 S h 1 P e transition. Note that the energy separation of about 350 meV is smaller than about 500 meV reported for the 1 P h 1 P e –1 S h 1 S e energy separation …”

“…5−9 These recent advances make InP-based NCs promising as emitters for solidstate lighting, lasing, and bioimaging applications. 7,8,10,11 Emission line of InP-based NC ensembles is broad compared to the Cd-based NCs, 12 due to the size inhomogeneity and core/shell interface inhomogeneity within the ensemble of NCs, 13 which complicates their optical studies. However, several pioneering studies of the optical properties of InPbased NCs have already been published.…”

“…InP-based colloidal nanocrystals (NCs) are an ideal candidate to replace more toxic Cd-based NCs, as InP has a bandgap close to that of CdSe. There has been steady progress in the improvement of the synthesis protocols since the 1990s, − and now, InP-based NCs with high quantum yield and stability can be fabricated. − These recent advances make InP-based NCs promising as emitters for solid-state lighting, lasing, and bioimaging applications. ,,, Emission line of InP-based NC ensembles is broad compared to the Cd-based NCs, due to the size inhomogeneity and core/shell interface inhomogeneity within the ensemble of NCs, which complicates their optical studies. However, several pioneering studies of the optical properties of InP-based NCs have already been published. − …”

Angular Momentum Fine Structure in InP/ZnSe Quantum Dots

“…We note that the reported bright−bright splitting is consistent with the 57 meV bright− bright splitting estimated from the analysis of detection wavelength dependent PLE spectra. 4 An analogous assignment was adopted by Berkinsky et al with a dark-bright splitting reported to be about 4 meV. 5 These results suggest that the darkbright and bright−bright splittings correspond to the energy differences between the ±2 and ±1 L and the ±1 L and the allowed upper ±1 U , 0 U states, respectively, as shown in Figure 1A.…”

“…We will focus on the same red-emitting InP/ZnSe/ZnS particles used in the radiative rate studies, for which very good PLE and PLE anisotropy spectra have been published. 4 The PLE anisotropy results allow determination of the ±1 U /0 U splitting, as discussed below.…”