Unexpected Optical Blue Shift in Large Colloidal Quantum Dots by Anionic Migration and Exchange

Abstract

Compositional changes taking place during the synthesis of alloyed CdSeZnS nanocrystals (NCs) allow shifting of the optical features to higher energy as the NCs grow. Under certain synthetic conditions, the effect of those changes on the surface/interface chemistry competes with and dominates over the conventional quantum confinement effect in growing NCs. These changes, identified by means of complementary advanced spectroscopic techniques such as XPS (X-ray photoelectron spectroscopy) and XAS (X-ray absorption spectroscopy), are understood in the frame of an ion migration and exchange mechanism taking place during the synthesis. Control over the synthetic routes during NC growth represents an alternative tool to tune the optical properties of colloidal quantum dots, broadening the versatility of the wet chemical methods.

Singlet Molecular Oxygen Generation via Unexpected Emission Color-Tunable CdSe/ZnS Nanocrystals for Applications in Photodynamic Therapy

“…2,5 It is known that thiols/amines may strongly affect the etching and remodeling of QDs, resulting in diminishing the CdSe lattice and cation/anion exchange at the core/shell interface. 2,5 Hence, the heterogenous nucleation kinetics and thermodynamics 21 might lead to the formation of an alloy, 14 consequently shrinking the core. Another possibility is that the thiols can act as the S source; 5 thus, by increasing the proportion of DDT, the additional amount of sulfur in S-OLA/ DDT and S-DDT encouraged the interfacial alloying.…”

“…12%) of CdSe and ZnS induces a high interfacial strain that tends to be released via the formation of misfit dislocations or surface reconstruction and alloying (an energetically favored phenomenon due to the mixing enthalpy of CdSe/ZnS is −20 kJ/mol in the pseudobinary system) at the core/shell interface, 18,19 affecting the optoelectronic properties of QDs. 12,14,20,21 Additionally, semiconductor QDs are known to generate 1 O 2 , a major cytotoxic species implicated in photodynamic therapy (PDT), 22 either directly via the triplet energy transfer (TET) process 4, 23,24 or indirectly by linking to some other molecular acceptors, 25−28 thus acting as photosensitizers/ therapeutic agents in PDT. 25−28 It is known that traditional photosensitizers (PS) are subject to multiple intrinsic limitations, that is, they have poor aqueous solubility and weak photochemical stability and involve singlet−triplet intersystem crossing (ISC) to transfer energy to the triplet ground state of oxygen molecules.…”

Enhanced Multiexciton Emission Property in Gradient Alloy Core/Shell CdZnSeS/ZnS Quantum Dots: Balance between Surface Passivation and Strain-Induced Lattice Defect

“…Both PL and first exciton absorption (1S) peaks are blue-shifted, while the absorption between 300 and 350 nm is increased obviously due to the epitaxial growth of the ZnS shell. The PL/absorption blue-shift may be caused by increasing quantum confinement as the shell grows and in situ alloying of the core with ZnS. , The QY value is increased from 40% of #core to 85% of #3 ML because the high synthesis reaction temperature causes Zn ions to diffuse into the surface of the CdZnSeS core. As a consequence, the growth of the shell forms a continuous lattice parameter change from the core to the shell without the formation of structural defects.…”

Illustrating the Shell Thickness Dependence in Alloyed Core/Shell Quantum-Dot-Based Light-Emitting Diodes by Impedance Spectroscopy

“…For simplicity, the QD samples with an increasing shell thickness of 1.0, 2.1, and 3.3 nm were labeled as QD 1 , QD 2 , and QD 3 , respectively. The normalized UV–vis absorption spectra and PL intensity are shown in Figure d; with increasing shell thickness, a blue-shift can be detected for both the absorption and PL emission spectra, which is mainly due to a different degree of alloying introduced by intradiffusion of zinc and sulfur atoms from the ZnS shell into the core during the shell-coating process. − The combined size and optical properties of QDs are summarized in Table . It is clear that from these optical characterizations, all three samples exhibit similar quantum yields (QY) and full width at half maximum (FWHM), making it hard to determine the most appropriate material structure for the QD-LED fabrication.…”