Chris Guhrenz

Cold Flow as Versatile Approach for Stable and Highly Luminescent Quantum Dot-Salt Composites.

Since the beginning of the 1980s, colloidally synthesized quantum dots (QDs) have been in the focus of interest due to their possible implementation for color conversion, luminescent light concentrators, and lasing. For all these applications, the QDs benefit from being embedded into a host matrix to ensure stability and usability. Many different host materials used for this purpose still have their individual shortcomings. Here, we present a universal, fast, and flexible approach for the direct incorporation of a wide range of QDs into inorganic ionic crystals using cold flow. The QD solution is mixed with a finely milled salt, followed by the removal of the solvent under vacuum. Under high pressure (GPa), the salt powder loaded with QDs transforms into transparent pellets. This effect is well-known for many inorganic salts (e.g., KCl, KBr, KI, NaCl, CsI, AgCl) from, e.g., sample preparation for IR spectroscopy. With this approach, we are able to obtain strongly luminescent QD-salt composites, have precise control over the loading, and provide a chemically robust matrix ensuring long-term stability of the embedded QDs. Furthermore, we show the photo-, chemical, and thermal stability of the composite materials and their use as color conversion layers for a white light-emitting diode (w-LED). The method presented can potentially be used for all kinds of nanoparticles synthesized in organic as well as in aqueous media.

Hybrid N-Butylamine-Based Ligands for Switching the Colloidal Solubility and Regimentation of Inorganic-Capped Nanocrystals

We report on a simple and effective technique of tuning the colloidal solubility of inorganic-capped CdSe and CdSe/CdS core/shell nanocrystals (NCs) from highly polar to nonpolar media using n-butylamine molecules. The introduction of the short and volatile organic amine mainly results in a modification of the labile diffusion region of the inorganic-capped NCs, enabling a significant extension of their dispersibility and improving the ability to form long-range assemblies. Moreover, the hybrid n-butylamine/inorganic capping can be thermally decomposed under mild heat treatment, making this approach of surface functionalization well-compatible with a low-temperature, solution-processed device fabrication. Particularly, a field-effect transistor-based on n-butylamine/Ga-I-complex-capped 4.5 nm CdSe NC solids shows excellent transport characteristics with electron mobilities up to 2 cm2/(V·s) and a high current modulation value (>104) at a low operation voltage (<2 V).

Transfer of Inorganic-Capped Nanocrystals into Aqueous Media

We report on a novel and simple approach to surface ligand design of CdSe-based nanocrystals (NCs) with biocompatible, heterobifunctional polyethylene glycol (PEG) molecules. This method provides high transfer yields of the NCs into aqueous media with preservation of the narrow and symmetric emission bands of the initial organic-capped NCs regardless of their interior crystal structure and surface chemistry. The PEG-functionalized NCs show small sizes, high photoluminescence quantum yields of up to 75%, as well as impressive optical and colloidal stability. This universal approach is applied to different fluorescent nanomaterials (CdSe/CdS, CdSe/CdSCdxZn1-xS, and CdSe/CdS/ZnS), extending the great potential of organic-capped NCs for biological applications.

Versatile Tri(pyrazolyl)phosphanes - Application as phosphorus precursors for the synthesis of highly emitting InP/ZnS quantum dots

Tri(pyrazolyl)phosphanes (5R1,R2 ) are utilized as an alternative, cheap and low-toxic phosphorus source for the convenient synthesis of InP/ZnS quantum dots (QDs). From these precursors, remarkably long-term stable stock solutions (>6 months) of P(OLA)3 (OLAH=oleylamine) are generated from which the respective pyrazoles are conveniently recovered. P(OLA)3 acts simultaneously as phosphorus source and reducing agent in the synthesis of highly emitting InP/ZnS core/shell QDs. These QDs are characterized by a spectral range between 530-620 nm and photoluminescence quantum yields (PL QYs) between 51-62 %. A proof-of-concept white light-emitting diode (LED) applying the InP/ZnS QDs as a color-conversion layer was built to demonstrate their applicability and processibility.

Tetrazole-Stabilized Gold Nanoparticles for Catalytic Applications

Abstract Herein, we report on a proof-of-concept application of tetrazole-stabilized Au nanoparticles (NPs) for CO oxidation. After impregnation of the support material TiO2 with the tetrazole-stabilized Au NPs (diameter<5 nm), a thermal heat treatment under oxygen is used to remove the tetrazole from the NP surface. The resulting surfactant-free NPs are used in the CO oxidation and show enhanced catalytic activity in comparison to the untreated samples demonstrating the potential of tetrazole-stabilized NPs for various catalytic applications.

Quenching of quantum dots luminescence under light irradiation and its influence on the biological application

Applications of quantum dots (QDs) can be limited by their stability under intensive irradiation. We have studied the stability of core/shell QD photoluminescence under the irradiation typical for the laboratory or industrial environment. Our results show that QD photoluminescence stability is determined by the structure and thickness of their shell; the latter factor has the strongest effect on the QD resistance to irradiation. It has been shown that the observed photodegradation of QDs is basically caused by the transfer of excited charge carriers outside the QD. An increase in the carrier tunneling length has been shown to be an efficient way to prevent quenching of QD photoluminescence. Storage of chloroform QDs solution under the light should be avoided due to the decrease of the biological applicability.