Characterization of the Interfacial Structures of Core/Shell CdSe/ZnS QDs

Abstract

Core/shell quantum dots (QDs) have been extensively studied, yet their optical properties widely vary among studies. Such variation may arise from the variation in interfacial structures induced by the subtle difference in each synthetic procedure. Here, we studied the interfacial structures of CdSe/ZnS QDs using the time-of-flight medium energy ion-scattering spectroscopy (TOF-MEIS), which offers the radial elemental distributions as well as the overall elemental compositions of QDs. The TOF-MEIS spectra provided strong evidence for the existence of an alloyed layer at the interface between CdSe and ZnS in typical CdSe/ZnS QDs. On the basis of the emission and absorption spectra of QDs sampled during the synthesis, we conclude that such interfacial alloying is caused by the dissolution of CdSe seeds during the synthesis steps. Such a dissolution mechanism is further corroborated by the observation that the ligand environment of solvent (X or L type) leads to different shapes of interfaces.

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.…”

“…1,2,5,11,12 These synthetic strategies usually rely on the fine control of the solvent system by choosing specific precursors with suitable reactivity and their relative amounts while controlling the temperature 13 and reaction time among other parameters. 2,5, 11,12,14,15 Previously, CdSe/ZnS QDs have been extensively studied for both the size-and alloying-dependent optical properties. The ZnS also acts as an effective surface passivating agent for several groups II-VI and III-V QDs 2−4, 8,12,16 due to its superior photochemical stability and wide band gap (∼3.7 eV).…”

“…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.…”

“…Semiconductor quantum dots (QDs), owing to a unique size-dependent photoluminescence (PL) property, − have emerged as highly performant and versatile nanotools for potential use in biolabeling, biosensing, selective targeting, multiplexed bioimaging, theranostics, and diagnostics. − However, the optoelectronic properties, being dependent on the particle size, sometimes hamper their use in nanoelectronics and biological applications . Recent advances have led to the development of emission color-tunable QDs without changing the size by manipulating their composition , via gradient alloying or ion exchange (cationic/anionic)-induced alloying. ,,,, These synthetic strategies usually rely on the fine control of the solvent system by choosing specific precursors with suitable reactivity and their relative amounts while controlling the temperature and reaction time among other parameters. ,,,,, …”

“…However, the larger lattice mismatch (ca. 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, , affecting the optoelectronic properties of QDs. ,,, …”