Volume 231, Issue 1 - Special Issue: Colloidal nanostructures of various dimensionality and hierarchy / Guest Editor: Christian Klinke

The size-controlled synthesis of cationic particles by differential reduction of HAuCl 4 precursor in the presence of NaBH 4 and 1-aminoundecane-12-thiol (AUT) is reported. The number of seed particles is determined by the fraction of the initially Au precursor reduced by NaBH 4 present in the reaction mixture, which are then grown larger by the AUT, acting as both weak reducing agent and stabilizing surfactant. By this methodology, size controlled synthesis is achieved in a two-step one-pot synthesis at room temperature.

We studied the aggregation of AuNP induced by small aromatic molecules under different conditions. In water, the aggregation was found to be difficult to control. Phase transfer of the particles into toluene by using oleylamine as a ligand allows for a more controlled and reliable synthesis. Using nonane-1,9-dithiol as a control, our experiments demonstrate that the molecular structure of the linker has a decisive influence on the aggregation. Aromatic dithiols yielded spherical aggregates in the range of 100 nm, whereas the aliphatic linker produced large aggregates in the µm range. The length of the aromatic linker (2 vs. 3 phenylene units) strongly affected aggregation kinetics and the structure of the produced aggregates. With UV/Vis and DLS based experiments it was possible to distinguish the process of ligand layer formation and aggregation. Our results will help to develop syntheses of defined spherical aggregates and possibly more complex structures.

Herein, we study how optical properties of colloidal dispersions of noble metal nanoparticles (Au and Ag) are affected by processes such as aggregation and oxidative dissolution. The optical contributions of these processes to the extinction spectra in the UV-vis region are often overlapped, making difficult its interpretation. In this regard, modeling the UV-vis spectra (in particular absorbance curve, peaks position, intensity and full width at half maximum -FWHM) of each process separately offers a powerful tool to identify the transformation of NPs under relevant and complex scenarios, such as in biological media. The proper identification of these transformations is crucial to understand the biological effects of the NPs.

Herein, we report on a proof-of-concept application of tetrazole-stabilized Au nanoparticles (NPs) for CO oxidation. After impregnation of the support material TiO 2 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.

The catalytic properties of cryogelated noble metal aerogel monoliths out of aqueous colloids are investigated using the oxidation of carbon monoxide (CO) as a model reaction, in order to evaluate their potential for catalytic applications. Aerogels built of self-supporting platinum (Pt) and palladium (Pd) nanocrystals (NCs) have a directly accessible catalyst surface and show catalytic performance similar to state of the art catalysts while being support-free and therefore ultralight materials. In addition, these materials provide properties like room temperature CO conversion and spontaneous catalytic reactions. However, full material aerogel catalysts come with the side effect of limited thermal stability, which will have to be overcome in future.

The current state-of-the-art of the fabrication and photophysics of graded shells in quantum dots is reviewed. Graded shells, i.e. partially alloyed interfaces between core and shell or between two shells of semiconductor nanoheterostructures, have been demonstrated to improve fluorescence properties and suppress non-radiative pathways of exciton dynamics. By simply looking at linear optics on the level of single excitons this is reflected in increased photoluminescence quantum yields. However, it is shown that graded shells have further beneficial implications for band structure engineering and multiexciton dynamics such as optical gain and charge carrier multiplication.

In the present manuscript we develop a method to determine all characteristic dimensions of CdSe seeded CdS nanorods solely via their extinction spectra without the need for electron microscopical investigations. In detail, the core diameter as well as the overall diameter and length and the molar extinction coefficient can all be derived from characteristic points in the absorption spectra. We carefully investigate in which size regime our assumptions are valid and give an estimation of the expected error, making it possible for the reader to decide whether this method is sufficiently accurate for their respective system. Our method displays a comfortable and fast route to analyze these nowadays often used nanorods.

Two-dimensional (2D) semiconductors hold high potential for the implementation of efficient ultrathin electronics (e.g. field-effect transistors, light emitting diodes and solar cell devices). In recent years, colloidal methods to synthesize ultrathin 2D materials have been developed that offer alternatives (like the production of non-layered 2D materials and upscaling) to mechanical exfoliation methods. By focusing on optoelectronic applications, it is important to characterize the nature and dynamics of photoexcited states in these materials. In this paper, we use ultrafast transient absorption (TA) and terahertz (THz) spectroscopy as optimal tools for such a characterization. We choose recently synthesized ultrathin colloidal 2D InSe nanosheets (inorganic layer thickness 0.8–1.7 nm; ≤5 nm including ligands) for discussing TA and THz spectroscopic studies and elucidate their charge carrier dynamics under photoexcitation with TA. THz spectroscopy is then used to extract contactless AC mobilities as high as 20±2 cm 2 /Vs in single InSe layers. The obtained results underpin the general applicability of TA and THz spectroscopy for characterizing photoexcited states in 2D semiconductors.

In this work, we investigate the electrical and dielectric response of lead sulfide (PbS) nanoparticle (NP) films with impedance spectroscopy. In particular, the influence of the ligand passivation on the surface trap state density of PbS NPs is demonstrated by comparing two different types of ligands: ethane-1,2-dithiol (EDT) and 3-sulfanylpropanoic acid (MPA). We observe that the MPA treatment passivates the PbS surface more efficiently than EDT. By analyzing the dielectric loss spectra, we are able to visualize shallow trap states in the bulk of PbS-EDT films and correlate this with the dispersive response observed in the impedance spectra. Evidence of deep trap states is revealed for both PbS-EDT and PbS-MPA diodes. Under illumination, the PbS-MPA and PbS-EDT films demonstrate almost identical trap profiles, showing solely the deep trap state densities. We conclude that the deep traps are related to the stoichiometry of the PbS NPs.

We use the photochromic organic semiconductor 1,2-Bis(5′-carboxy-2′-methylthien-3′-yl)-cyclopentene (DTCP) to cross-link PbS quantum dots assembled into thin films. The ligand exchange is monitored by means of vibrational spectroscopy (FT-IR) and core-level X-ray photoemission spectroscopy (XPS). Transport measurements in a field-effect transistor (FET) set-up reveal ambipolar behavior with hole and electron mobilities on the order of 10 −4 cm 2 /Vs and 10 −5 cm 2 /Vs, respectively. Exposure to UV light from a 4 W UV lamp does not significantly change the transport properties, indicating that switching of DTCP is hindered in the hybrid film. We find a pronounced photo-conductance with rapid and reversible photo-response on the order of few seconds, which we attribute to (de-)filling of QD trap states. Our results indicate that hybrid, nanostructured networks of PbS QDs cross-linked with DTCP can be obtained by the presented procedure but that switching of the QD-bound DTCP appears to be hindered compared to the pure, unbound molecular species. We discuss future means to address this problem.