Abstract
We review recent experimental work to utilize the size dependence of the luminescence quenching of colloidal semiconductor quantum dots induced by functionalized porphyrin molecules attached to the surface to describe a photoluminescence (PL) quenching process which is different from usual models of charge transfer (CT) or Foerster resonant energy transfer (FRET). Steady-state and picosecond time-resolved measurements were carried out for nanocomposites based on colloidal CdSe/ZnS and CdSe quantum dots (QDs) of various sizes and surfacely attached tetra-mesopyridyl-substituted porphyrin molecules (“Quantum Dot-Porphyrin” nanocomposites), in toluene at 295 K. It was found that the major part of the observed strong quenching of QD PL in “QD-Porphyrin” nanocomposites can neither be assigned to FRET nor to photoinduced charge transfer between the QD and the chromophore. This PL quenching depends on QD size and shell and is stronger for smaller quantum dots: QD PL quenching rate constants scale inversely with the QD diameter. Based on the comparison of experimental data and quantum mechanical calculations, it has been concluded that QD PL quenching in “QD-Porphyrin” nanocomposites can be understood in terms of a tunneling of the electron (of the excited electron-hole pair) followed by a (self-) localization of the electron or formation of trap states. The major contribution to PL quenching is found to be proportional to the calculated quantum-confined exciton wave function at the QD surface. Our findings highlight that single functionalized molecules can be considered as one of the probes for the complex interface physics and dynamics of colloidal semiconductor QD.
Highlights
Semiconductor quantum dots (QD), known as “nanocrystals,” are structures with electronic and optical properties that can be engineered through the size of the structure, not just the composition
We review recent experimental work to utilize the size dependence of the luminescence quenching of colloidal semiconductor quantum dots induced by functionalized porphyrin molecules attached to the surface to describe a photoluminescence (PL) quenching process which is different from usual models of charge transfer (CT) or Foerster resonant energy transfer (FRET)
It follows from experimental Stern-Volmer PL quenching plots I0/I(x) and quantum mechanical calculations for the electron wave functions that the specificity of the exciton nonradiative decay in “QD-porphyrin” nanocomposites is due to the charge tunneling through ZnS barrier in quantum confinement conditions
Summary
Semiconductor quantum dots (QD), known as “nanocrystals,” are structures with electronic and optical properties that can be engineered through the size of the structure, not just the composition. While quantum confinement is basically understood, the anchoring of functional organic molecules to tunable wide-gap semiconductor colloidal QDs using various approaches is still of considerable scientific and practical interest [9, 11, 15, 28,29,30], as the particular chemical composition of the surfactant shell decisively affects the photophysical properties of the assembly, especially the PL quantum yield. Colloidal QDs are bright emitters and characterized by a large absorption cross-section [2,3,4] Their photoluminescence (PL) quantum efficiency has shown to be sensitive to a number of influences that originate either from the ligand shell [22,23,24] or directly from the QD core [31], the QD surface [32,33,34], and the surrounding matrix [35]
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