Two-Photon-Induced Blue Shift of Core and Shell Optical Transitions in Colloidal CdSe/CdS Quasi-Type II Quantum Rods

Abstract

The spectral dependence of the two-photon absorption in CdSe/CdS core/shell nanocrystal heterorods has been studied via two-photon-induced luminescence excitation spectroscopy. We verified that the two-photon absorption in these samples is a purely nonlinear phenomenon, excluding the contribution from multistep linear absorption mediated by defect states. A large absorption cross section was observed for CdSe/CdS core/shell quantum rods, in the range of 10(5) GM (1 GM = 10(-50) cm(4) s phot(-1)), scaling with the total nanocrystal volume and thus independent of the core emission wavelength. In the two-photon luminescence excitation spectra, peaks are strongly blue-shifted with respect to the one-photon absorption peaks, for both core and shell transitions. The experimental results are confirmed by k·p calculations, which attribute the shift to both different parity selection rules that apply to one-photon and two-photon transitions and a low oscillator strength for two-photon transitions close to the ground-state one-photon absorption. In contrast with lead chalcogenide quantum dots, we found no evidence of a breakdown of the optical selection rules, despite the presence of band anisotropy, via the anisotropic hole masses, and the explicitly induced reduction of the electron wave function symmetry via the rod shape of the shell. The anisotropy does lead to an unexpected splitting of the electron P-states in the case of a large CdSe core encapsulated in a thin CdS shell. Hence, tuning of the core and shell dimensions and the concurrent transition from type I to quasi-type II carrier localization enables unprecedented control over the band-edge two-photon absorption.

Tuning Hole and Electron Transfer from Photoexcited CdSe Quantum Dots to Phenol Derivatives: Effect of Electron‐Donating and ‐Withdrawing Moieties

“…[21][22][23] It has been realized that it is very important to dissociatet he excitons before they go for annihilation.E xtraction of charge carriers can take place through electrono rh ole transfer from photoexcited QDs to as uitable molecular adsorbate or another semiconductor,b yw hich ultrafaste xciton-exciton annihilation can be reduced drastically.S everal literature reports are available in whiche xcitond issociation hasb een demonstrated through charget ransfer (CT) in different QD/molecule composite [24][25][26][27][28][29][30] and QD/semiconductor nanoparticle (TiO 2 ) [31][32][33] systems. [42,43] Exciton dissociation in the core-shell structure in the presence of am olecular adsorbate has also been reported by us. [34][35][36][37][38][39][40][41] In addition, an intermediate structure, knowna sq uasi-typeIIe xists, in which, due to low band offset between either valence (VB) or conduction bands(CB) of both core and shell, one of the carrier delocalizes in either core or shell, leadingt oe fficient charge separation.…”

Two Photon Absorption in II–VI Semiconductors: The Influence of Dimensionality and Size

“…A weak contribution of the first excitonic resonance to the TPA spectrum in comparison to energies in the quasi-continuum, where strong TPA is observed, has been reported for CdSe QDs, 32,37 CdS NRs and QDs, 18 as well as for CdSe/CdS dots in rods. 40 For NPLs we see an extreme increase of two-photon absorption from two-photon energies resonant to the first excitonic transition to the continuum absorption region. The increase has a stepwise character because the nanoplatelets' step-like 2D density of states leads to three steps (for the HH, LH, and SO continua), shifted to higher energies with respect to the first HH, LH and SO exciton transitions by the exciton binding energies, estimated to the order of 200 meV.…”

“…35 Blanton et al 36 presented TPIF of single particles and relative TPA spectra of CdSe QDs. 37 More exotic particles such as doped 38 or surfactant-capped 39 QDs and quasi type II CdSe/CdS dots in rods 40 with TPA cross sections up to 10 5 GM also have been investigated.…”

High-Efficiency All-Solution-Processed Light-Emitting Diodes Based on Anisotropic Colloidal Heterostructures with Polar Polymer Injecting Layers

“…Anisotropic structures have been widely explored as emitters in lasers 35,36 , light down-converters 37,38 , and two-photon absorbers 39 , but their application to QD-LEDs is still marginal with respect to the traditional spherical QDs 40 . Nevertheless, CdSe/CdS/ZnSe nanorods have been recently shown to boost both the efficiency and brightness of QD-LEDs with respect to spherical QDs, possibly due to favorable band offset and enhanced light outcoupling 41 , thus making elongated structures promising (diameter = 5.1 nm) overcoated with a 6.8 nm thick and 26.8 nm long CdS rod (see schematic in the top panel in Fig.1d and the transmission electron micrograph in Supporting Fig.S1).…”