Tuning Electronic States of a CdSe/ZnS Quantum Dot by Only One Functional Dye Molecule

Abstract

Self-assembly of only one functionalized porphyrin dye molecule with one CdSe/ZnS quantum dot (QD) not only modifies the photoluminescence (PL) intensity but also creates a few energetically clearly distinguishable electronic states, opening additional effective relaxation pathways. The related energy modifications are in the range of 10-30 meV and show a pronounced sensitivity to the specific nature of the respective dye. We assign the emerging energies to surface states. Time-resolved PL spectroscopy in combination with spectral deconvolution reveals that surface properties of QDs are a complex interplay of the nature of the dye molecule and the topography of the ligand layer across a temperature range from 77 to 290 K. This includes a kind of phase transition of trioctylphosphine oxide ligands, switching the nature of surface states observed below and above the phase transition temperature. Most importantly, our findings can be closely related to recent calculations of ligand-induced modifications of surface states of QDs. The identification of the optical properties emerged from a combination of spectroscopy on single QDs and QDs in an ensemble.

Deactivation of Excited States in Nanostructures Containing Cu-Porphyrin Subunit

“…In this case, like for multiporphyrin complexes (described above), a controllable formation of "QD-Porphyrin" nanoassemblies have been realized as a surface passivation of CdSe/ZnS ODs by tetrameso-pyridyl substituted porphyrins (free base and/or Cu-complex) in titration experiments. [29,[90][91][92][93][94][95][96][97][98][99] It is well-known from chemical background that the 3d transition metal Zn 2+ ion (of ZnS shell) has empty 3d 10 orbital while heteroatom N-pyr of the porphyrin mesopyridyl ring is a very good e-donor having an unshared electron pair. Thus, in this case a "key-lock" principle is realized via oneor two-fold non-covalent coordination Zn….N-pyr.…”

“…Temperature variation and related changes in emission intensities for pure QD (curve 1, Figure 10B) reveal sharp changes of TOPO ligand shell structure in a narrow temperature range (T crit ≈220 K) for this surfactant (the so-called "phase transition"). [96,99,123] It should be noted that T crit is significantly higher than the temperature of the glass transition for the solvent mixture (T glass =151.6 K). It is seen from Figure 10B also that at the same conditions, PL intensity for QD in "QD-CuP" and "QD-H 2 P" nanoassemblies shows again a "kink" (curves 2 and 3), which, however, is now much more pronounced for CuP (curve 3) compared with both pure QD and QD-H 2 P (curves 1 and 2).…”

“…[29,[90][91][92][93][94][95][96][97][98][99]123] Structures of QDs and porphyrin molecules as well as necessary details explaining the formation of nanoassemblies "QD-Porphyrin" are shown in Figure 9. The results described below concern to the analysis of the interaction between semiconductor CdSe/ZnS quantum dots and surface attached porphyrin molecules (free base and Cu-complex) in order to evaluate the influence of tetrapyrrolic macrocycle on pathways and mechanisms of exciton relaxation in "QD -Cu-porphyrin" nanoassemblies.…”

“…Interestingly, for this tuning the PL properties of QDs, it does not need a full exchange of the ligand shell, but it works already upon attachment of very few dye molecules. [99] Below it will be comparatively shown it for "QD-CuP" and "QD-H 2 P" nanoassemblies.…”

“…The temperature lowering down to 77 K leads to the strong shortening of mean <t> values down to ~8 ns. [99] At least 3 exponential functions (with decay times τ 1 ≤1 ns, τ 2 ≈4 -7 ns and τ 3 ≈10 -18 ns) are needed within our time resolution of 0.2 ns to approximate the PL decay for ensemble experiments ( Figure 11A). Evaluation of single QD shows also a multi-exponential PL decay at each identified PL intensity ( Figure 11B).…”