Heterogeneous Charge Transfer of Colloidal Nanocrystals in Ionic Liquids

Abstract

[a] Colloidal group II B -VI semiconductor nanocrystals (so-called quantum dots or QDs) have attracted increasing interest among the research community because of their mesoscopic, size-dependent electro-optical properties. These properties make these novel materials highly desirable for potential applications, such as labeling of biomolecules, [1][2][3] catalysis, [4] phosphors, [5] light-emitting diodes (LEDs) [6][7][8][9][10][11][12][13] or photovoltaic cells. [14][15][16] Especially the latter applications require a fundamental knowledge of the charge-transfer kinetics of these particles. So far, it was not possible to measure these parameters in colloidal nanocrystals, because the appropriate electrochemical techniques require a supporting electrolyte to be added to the colloid, which causes such particles to precipitate. Therefore, only nanocrystal films (aggregates) and "water-soluble" colloidal nanocrystals were studied under this perspective. [17][18][19] Ionic liquids (ILs) have recently emerged as neoteric solvents for all kinds of chemical applications.[20] Among many other fields, they have been used for organometallic catalysis [21] and process engineering, as well as for electrochemical applications, [22] such as electrodeposition of semiconductors. [23] ILs are liquid materials that-similar to salts-consist completely of ions, but as opposed to commonly known "molten salts", which are high-melting and highly corrosive, ILs have a relatively low melting point (below 100 8C by a common definition; [24] many are liquid at room temperature and below), are often of relatively low viscosity and noncorrosive. Thus, they can be used as solvents for chemical applications.ILs are excellent electrolytes for electrochemical measurements because they are very flexible regarding their anions, cations and substituents (ILs are frequently called "designer solvents" [25] ). The realization of a nanocrystal colloid in ILs not only combines two intriguing fields, but also enables electrokinetic measurements of colloidal nanocrystals with hydrophobic ligands to be performed for the first time. Herein, we report the transfer of colloidal nanocrystals into an appropriate IL and the first electrokinetic measurements with these new colloids. We were not only able to observe the quantum-size effect (QSE) with differently sized nanocrystals, but also to estimate charge-transfer rates on ionization.For measuring the charge transfer of colloidal nanocrystals two requirements must be met: First, the electrolyte (namely, the IL) has not to evince oxidation and reduction processes in the region of the ionization potential and electron affinity of the nanocrystals. Secondly, the nanocrystals have to be colloidally stable within this IL. "As synthesized" nanocrystals are mostly sterically stabilized with nonpolar ligands. Therefore, we prepared different ILs with increased nonpolar character. The ILs are based on two cations: 1-methyl-3-alkyl-imidazolium (C x mim), which is by far the most widely used IL cation in the chemica...

Interactions between Redox Complexes and Semiconductor Quantum Dots Coupled via a Peptide Bridge

“…A comparison between our data and previous results, reported by Nann and co-workers, suggests that the QD oxidation state corresponds to surface defect states (which can be constituted by atomic vacancies or oxygen adsorption)31. Positions of the lowest (highest) energy level in the QD conduction (valence) band, extracted from previously reported measurements on CdSe nanocrystals emitting at ~590 nm are displayed in comparison (Figure 2)31, 32. We should also emphasize that our experiments were carried out in buffer solutions where electrolytic conduction is rather high compared to organic electrolytic solutions.…”

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Investigating the Control by Quantum Confinement and Surface Ligand Coating of Photocatalytic Efficiency in Chalcopyrite Copper Indium Diselenide Nanocrystals

“…56,58 Secondly, a large oxidation peak current was observed at higher potential (after the band-gap). It is know that OLA could undergo dynamic exchange and be present unbound in solution 71 and thus we believe large anodic peak current could be due to oxidation of free OLA.…”

Light triggered detection of aminophenyl phosphate with a quantum dot based enzyme electrode

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