Recent Advances in Functional Nanoparticle Assemblies

Assemblies of colloidal nanoparticles (NPs) into various functional superstructures, such as ordered or nonordered, microscopic or macroscopic, and templated or self-supported, have recently attracted a lot of research interest. The continuous development of colloidal nanoparticle synthesis enables a fine-tuning of the structure and properties of such functional superstructures leading to numerous new applications. Herein, it is aimed to summarize a variety of assemblies based on ordered self-assembled NPs, e.g., stacked nanoplatelets, and nonordered self-assembled NPs, e.g., nanoparticle-based aerogels and cryoaerogels, systems ranging from the microscopic to the macroscopic range. Depending on the materials of the nanoparticle building blocks used, e.g., metallic, magnetic, semiconducting, or their combination in hybrid systems, both ordered and nonordered assemblies yield interesting properties for a wide variety of applications, such as catalysis, photocatalysis, or sensing, which are highlighted and discussed

Temperature and Composition Dependent Optical Properties of CdSe/CdS Dot/Rod-Based Aerogel Networks

Employing nanocrystals (NCs) as building blocks of porous aerogel network structures allows the conversion of NC materials into macroscopic solid structures while conserving their unique nanoscopic properties. Understanding the interplay of the network formation and its influence on these properties like size-dependent emission is a key to apply techniques for the fabrication of novel nanocrystal aerogels. In this work, CdSe/CdS dot/rod NCs possessing two different CdSe core sizes were synthesized and converted into porous aerogel network structures. Temperature-dependent steady-state and time-resolved photoluminescence measurements were performed to expand the understanding of the optical and electronic properties of these network structures generated from these two different building blocks and correlate their optical with the structural properties. These investigations reveal the influence of network formation and aerogel production on the network-forming nanocrystals. Based on the two investigated NC building blocks and their aerogel networks, mixed network structures with various ratios of the two building blocks were produced and likewise optically characterized. Since the different building blocks show diverse optical response, this technique presents a straightforward way to color-tune the resulting networks simply by choosing the building block ratio in connection with their quantum yield

Synthesis of InP/ZnS Nanocrystals and Phase Transfer by Hydrolysis of Ester

The synthesis of highly luminescent non-toxic nanocrystals (NCs) and the subsequent phase transfer to aqueous solution by hydrolysis of the crystal-bound ester are presented. Therefore, the synthesis of the spherical semiconductor system InP/ZnS was modified by changing the sulfur precursor in the synthesis from 1-dodecanethiol to dodecyl 3-mercaptopropionate (D3MP). By employing D3MP both as sulfur precursor for the ZnS shell growth and as stabilizing ligand, the phase transfer from organic to aqueous solution can be performed easily. Instead of the usually employed ligand exchange with mercaptopropionic acid, the NCs are only shaken with a sodium borate buffer in order to obtain aqueous soluble NCs by hydrolysis of the ester. In future work, the NCs must be protected against aggregation and the long term stability has to be increased. The optical properties of the samples are investigated by UV/Vis and photoluminescence spectroscopy, and the morphology of the nanoparticles (NPs) before and after phase transfer is determined by transmission electron microscopy

Influencing the coupling between network building blocks in CdSe/CdS dot/rod aerogels by partial cation exchange

The assembly of CdSe/CdS dot/rod nanocrystals (NCs) with variable length of ZnS tips into aerogel networks is presented. To this end, a partial region selective cation exchange procedure is performed replacing Cd by Zn starting at the NC tip. The produced aerogel networks are investigated structurally and optically. The networks of tip-to-tip connected NCs have an intricate band structure with holes confined to the CdSe cores while electrons are delocalized within the CdS also within connected building blocks. However, the ZnS tips act as a barrier of variable length and strength between the NC building blocks partly confining the electrons. This results in NC based aerogel networks with tunable strength of coupling between building blocks

Photoluminescence Lifetime Based Investigations of Linker Mediated Electronic Connectivity Between Substrate and Nanoparticle

The evolution of systems based on nanoparticles as the main component seems to be a self-accelerating process during the last five decades. Hence, an overview across this field gets more and more challenging. It is sometimes rewarding to focus on the fundamental physical phenomenon of the electronic interconnection between the different building blocks of the obtained devices. Therefore, the investigation of charge transport among the utilized particles and their substrate is one of the mandatory steps in the development of semiconductor nanoparticle based devices like e.g., sensors and LEDs. The investigation of the influence of tunneling barriers on the properties of nanoparticle-functionalized surfaces is a challenging task. The different basic influences on the charge transport dynamics are often difficult to separate from each other. Non-invasive and easily viable experiments are still required to resolve the charge distributing mechanisms in the systems. In the presented work, we want to focus on thin and transparent indium tin oxide (ITO) layers covered glass slides since this substrate is frequently utilized in nanoelectronics. CdSe/CdS nanorods (NRs) are applied as an optically addressable probe for the electronic surface states of the conductive glass. The presented experimental design provides the proof of electronic interconnections in ITO coated glass/linker/NR electrodes via easy reproducible functionalization and polishing experiments. UV/Vis absorption and photoluminescence (PL) lifetime measurements revealed changes in the optical properties caused by differences in the charge carrier dynamics between the system. Our work is focused on the modification of charge carrier dynamics due to the application of linker molecules with different functional groups like (3-mercaptopropyl)methoxysilane (MPTMS) and (3-aminopropyl)trimethoxysilane (APTMS). The presented observations are explained with a simple kinetic model

Noble-Metal Nanorod Cryoaerogels with Electrocatalytically Active Surface Sites

Noble-metal-based electrocatalysts usually contain small nanoparticle building blocks to ensure a high specific surface area as the scene for the surface processes. Here, we show that relatively large noble-metal nanorods are also promising candidates to build up functional macrostructures with prominent electrocatalytic activity. After optimizing and upscaling the syntheses of gold nanorods and gold bipyramid-templated silver nanorods, cryoaerogels are fabricated on a conductive substrate via flash freezing and subsequent freeze drying. The versatile cryoaerogelation technique allows the formation of macrostructures with dendritic, open-pore structure facilitating the increase of the accessible nanorod surfaces. It is demonstrated via electrochemical oxidation and stripping test experiments that noble-metal surface sites are electrochemically active in redox reactions. Furthermore, gold nanorod cryoaerogels offer a platform for redox sensing, ethanol oxidation reaction, as well as glucose sensing. Compared to their simply drop-cast and dried counterparts, the noble-metal nanorod cryoaerogels offer enhanced activity due to the open porosity of the fabricated nanostructure while maintaining structural stability

A Versatile Route to Assemble Semiconductor Nanoparticles into Functional Aerogels by Means of Trivalent Cations

3D nanoparticle assemblies offer a unique platform to enhance and extend the functionality and optical/electrical properties of individual nanoparticles. Especially, a self-supported, voluminous, and porous macroscopic material built up from interconnected semiconductor nanoparticles provides new possibilities in the field of sensing, optoelectronics, and photovoltaics. Herein, a method is demonstrated for assembling semiconductor nanoparticle systems containing building blocks possessing different composition, size, shape, and surface ligands. The method is based on the controlled destabilization of the particles triggered by trivalent cations (Y3+, Yb3+, and Al3+). The effect of the cations is investigated via X-ray photoelectron spectroscopy. The macroscopic, self-supported aerogels consist of the hyperbranched network of interconnected CdSe/ CdS dot-in-rods, or CdSe/CdS as well as CdSe/CdTe core-crown nanoplatelets is used to demonstrate the versatility of the procedure. The non-oxidative assembly method takes place at room temperature without thermal activation in several hours and preserves the shape and the fluorescence of the building blocks. The assembled nanoparticle network provides longer exciton lifetimes with retained photoluminescence quantum yields, that make these nanostructured materials a perfect platform for novel multifunctional 3D networks in sensing. Various sets of photoelectrochemical measurements on the interconnected semiconductor nanorod structures also reveal the enhanced charge carrier separation

Luminescence encoding of polymer microbeads with organic dyes and semiconductor quantum dots during polymerization

Luminescence-encoded microbeads are important tools for many applications in the life and material sciences that utilize luminescence detection as well as multiplexing and barcoding strategies. The preparation of such beads often involves the staining of premanufactured beads with molecular luminophores using simple swelling procedures or surface functionalization with layer-by-layer (LbL) techniques. Alternatively, these luminophores are sterically incorporated during the polymerization reaction yielding the polymer beads. The favorable optical properties of semiconductor quantum dots (QDs), which present broadly excitable, size-tunable, narrow emission bands and low photobleaching sensitivity, triggered the preparation of beads stained with QDs. However, the colloidal nature and the surface chemistry of these QDs, which largely controls their luminescence properties, introduce new challenges to bead encoding that have been barely systematically assessed. To establish a straightforward approach for the bead encoding with QDs with minimized loss in luminescence, we systematically assessed the incorporation of oleic acid/oleylamine-stabilized CdSe/CdS-core/shell-QDs into 0.5–2.5 µm-sized polystyrene (PS) microspheres by a simple dispersion polymerization synthesis that was first optimized with the organic dye Nile Red. Parameters addressed for the preparation of luminophore-encoded beads include the use of a polymer-compatible ligand such as benzyldimethyloctadecylammonium chloride (OBDAC) for the QDs, and crosslinking to prevent luminophore leakage. The physico-chemical and optical properties of the resulting beads were investigated with electron microscopy, dynamic light scattering, optical spectroscopy, and fluorescence microscopy. Particle size distribution, fluorescence quantum yield of the encapsulated QDs, and QD leaking stability were used as measures for bead quality. The derived optimized bead encoding procedure enables the reproducible preparation of bright PS microbeads encoded with organic dyes as well as with CdSe/CdS-QDs. Although these beads show a reduced photoluminescence quantum yield compared to the initially very strongly luminescent QDs, with values of about 35%, their photoluminescence quantum yield is nevertheless still moderate

Is freeze-drying an alternative to solvent exchange for the hydration stop of cementitious suspensions?

In order to understand the rheological properties of cementitious suspensions at early stages, among other phases, the formation of ettringite and its time-dependent influence, whether by amount or morphology, has to be examined in detail using a suitable method to stop the hydration process. It is state-of-the-art to exchange water with isopropanol, however, the water initially remains in the system possibly leading to reduced time resolution. Our group raised the question if freeze-drying or the combination of the water-isopropanol exchange with subsequent freeze-drying might be a suitable technique to achieve an almost complete hydration stop at any time. Recently, it was shown under which circumstances low-pressure characterization techniques can be employed without destroying the samples due to loss of crystal bound water. Here, by implementing these recent results, we show under which circumstances freeze-drying indeed can be employed as fast hydration stop method. © 202