Dot-in-Rod Nanostructures

our experts: Mareike Dittmar (synthesis), Vincent Mittag (synthesis), Jannik Rebmann (synthesis), 
Sebastian Hentschel (photocatalysis), Florian Johst (spectroscopy), Hans Werners (spectroscopy).
Please also have a look on the graduate school "Nanohybrid"

Synthesis of Dot-in-Rod Nanostructures and Similar Nanostructures

Wet-chemical fabrication methods using Schlenk technique allows for the synthesis of nanostructures with well-defined dimensions. The choice of precursors, solvents, ligands, reaction conditions, and reaction duration influences the growth of the nanocrystals. Dot-in-Rod nanostructures are typically obtained in a two-step synthesis as depicted in the following scheme (path I): (i) synthesis of spherical nanocrystals upon supersaturation of the desired material followed by a (ii) growth phase. The second step represents an anisotropic growth of the shell material by hot-injection method.

Different types of dot-in-rod structures can be obtained, depending on the combined materials. CdSe/CdS (core/shell) dot-in-rods are well investigated. ZnSe/CdS dot-in-rods, on the other hand, feature a relatively new material combination and exhibits unique optical properties. Furthermore, ZnSe and CdSe are miscible materials. Accordingly, cation exchange/alloy formation occurs during the synthesis. By TEM, optical spectroscopy, and different methods using X-rays, we extensively study the ZnSe/CdS dot-in-rods and their properties.

CdSe/CdS dot-in-rods as well as ZnSe/CdS dot-in-rods can also be treated by post-synthesis modifications. By selective growth, as depicted in path II, hybrid semiconductor-metal nanocrystals can be fabricated. Thus, different metals can be precipitated at one tip of the elongated shell material. These hybrid semiconductor-metal nanostructures are suitable for photocatalytic applications. The underlying processes are the photoexcitation of electron-hole pairs inside the semiconductor and the subsequent spatial separation of electrons and holes across the metal-semiconductor interface before the charge carriers contribute to the catalyzed chemical reaction.

Moreover, tips of CdSe/CdS dot-in-rods can be transformed to PbS (see path III). This transformation results in CdSe/CdS/PbS double quantum dots and is achieved by two different approaches. The first approach utilizes a direct cation-exchange reaction from CdS to PbS. For the cation-exchange, lead halides in oleylamine are used. The used halide influences the properties of the obtained CdSe/CdS/PbS double quantum dots. In the second approach is a heat-up synthesis using a single-source precursor to grow PbS quantum dots exclusively on one of the tips of each dot-in-rod.

Photocatalysis by Hybrid Semiconductor-Metal Nanostructures

Hybrid semiconductor-metal nanostructures (e.g. Pt-tipped  CdSe/CdS or ZnSe/CdS dot-in-rods) are suitable for photocatalytic hydrogen production from aqueous solutions. We are investigating these hybrid structures with respect to their ability for the involved processes. For this, we use a flow reactor with water cooling and a mass spectrometer specifically designed for gases. To get the nanoparticles dispersed in aqueous phases, it requires ligand exchange from the nonpolar phase to the aqueous polar phase. Hence, different types of ligands are investigated with respect to their stabilization of the nanoparticles on the one hand and their influence on hydrogen production on the other. In addition, cyclic voltammetry is used to investigate the band-edge energies of the semiconductor materials of the hybrid structures.

Spectroscopy

Exciting our semiconductor nanostructures with light generates a bound state consisting of a negative electron and a positive hole, called exciton. The properties of this quasi-particle depend on the size, the shape, and the materials of the corresponding nanostructures. This inherently affects the emission energy and kinetic of the exciton recombination.

We focus on type-I (CdSe/CdS) and type-II (ZnSe/CdS) dot-in-rod nanostructures that show fluorescence in the energy range of visible light. In the type-I system, both charge carriers are localized in the same material, which is CdSe in our case. The CdS keeps the exciton away from surface energy states that would quench the fluorescence. In the other case, which is a type-II system, both charge carriers are separated in one of each material. The type-II systems thereby allow us to easily separate the charge carriers. The charge-carrier distribution is reflected in the recombination kinetics as well as in the emission spectra.

For an in-depth characterization, we analyze single particles to directly correlate the relation between structure and (optical/electronic) properties. The emission linewidth can be reduced by performing the spectroscopic experiments at cryogenic conditions (< 10 K) thereby revealing further optical phenomena. (Examples for these measurements can be found in ACS Nano 2017, 11, 12, 12185–12192 and Nano Lett. 2014, 14, 11, 6655–6659.)

Publications

• A.Hinsch, S.-H. Lohmann, C. Strelow, T. Kipp, C. Würth, D. Geißler, A. Kornowski, C. Wolter, H. Weller, U. Resch-Genger, A. Mews, Fluorescence Quantum Yield and Single-Particle Emission of CdSe Dot/CdS Rod Nanocrystals, The Journal of Physical Chemistry C 2019 123 (39), 24338-24346.
• S.-H. Lohmann, P. Harder, F. Bourier, C. Strelow, A. Mews, T. Kipp, Influence of Interface-Driven Strain on the Spectral Diffusion Properties of Core/Shell CdSe/CdS Dot/Rod Nanoparticles, The Journal of Physical Chemistry C 2019 123 (8), 5099-5109.
• S.-H. Lohmann, C. Strelow, A. Mews, T. Kipp, Surface Charges on CdSe-Dot/CdS-Rod Nanocrystals: Measuring and Modeling the Diffusion of Exciton-Fluorescence Rates and Energies, ACS Nano 2017 11 (12), 12185-12192.
• A. Mews, J. Zhao,  A Bright Outlook for Quantum Dots, Nature Photon 2007, 1, 683–684.