Alf Mews

We present a method for preparing NiPt-alloy tips on CdSe-core/CdS-shell dot-in-rod nanoparticles (DRs). The formation of the NiPt tips is separated into a two-step synthesis, where first Ni tips are grown, which then serve as nucleation sites for Pt before an alloying process occurs. Thus, the NiPt tips are formed in a seed-mediated approach. We find that the reactivity of the Ni tips can be controlled by oxygen treatment, while the reactivity of the lateral surface of the DRs can be tuned by ligands. Monitoring the growth dynamics of the NiPt reveals that an oxide layer on the Ni tips delays the nucleation of Pt, but it does not inevitably prevent it if a combination of oleic acid and oleylamine initiates oxide-layer conversion due to oleate formation. Without conversion, the oxide layer can be utilized to inhibit the NiPt formation. The choice of ligands can be exploited to enable or prevent separate Pt particle growth on the lateral semiconductor rod surface. Especially the DRs with NiPt-alloy tips show superior activity toward HER in both electrocatalytic and photocatalytic experiments.

Quantum dots (QDs) offer great potential for various applications, particularly in the infrared spectral range, including telecommunication, security, sensing, photovoltaics and bioimaging. Unlike the well-studied II-VI and IV-VI semiconductor QD systems, the colloidal synthesis of infrared-active, RoHS-compliant III-V semiconductor QDs like indium arsenide (InAs) remains less explored. Growing larger InAs QDs is challenging due to a lack of control over the precursor reactivity because of the high covalent character of In-As bonding, and due to a non-classical growth mechanism. Herein, we report on the first colloidally synthesized ultra-large InAs QDs, achieving record-breaking sizes up to 40.5 nm on average and optical activity in the range even beyond ~ 2600 nm. Besides, we present synthesis routes for InAs clusters, which were used as precursors for two different growth approaches of ultra-large InAs QDs. All nanostructures were prepared from `green` commercially available precursors.

Interfacing artificial devices with the human brain is the central goal of neurotechnology. Yet, our imaginations are often limited by currently available paradigms and technologies. Suggestions for brain–machine interfaces have changed over time, along with the available technology. Mechanical levers and cable winches were used to move parts of the brain during the mechanical age. Sophisticated electronic wiring and remote control have arisen during the electronic age, ultimately leading to plug-and-play computer interfaces. Nonetheless, our brains are so complex that these visions, until recently, largely remained unreachable dreams. The general problem, thus far, is that most of our technology is mechanically and/or electrically engineered, whereas the brain is a living, dynamic entity. As a result, these worlds are difficult to interface with one another. Nanotechnology, which encompasses engineered solid-state objects and integrated circuits, excels at small length scales of single to a few hundred nanometers and, thus, matches the sizes of biomolecules, biomolecular assemblies, and parts of cells. Consequently, we envision nanomaterials and nanotools as opportunities to interface with the brain in alternative ways. Here, we review the existing literature on the use of nanotechnology in brain–machine interfaces and look forward in discussing perspectives and limitations based on the authors’ expertise across a range of complementary disciplinesfrom neuroscience, engineering, physics, and chemistry to biology and medicine, computer science and mathematics, and social science and jurisprudence. We focus on nanotechnology but also include information from related fields when useful and complementary.

Hybrid semiconductor‐metal nanostructures are promising candidates for converting light into chemical energy by photocatalytic hydrogen production. However, many photocatalysts are prone to photocorrosion which limits their long‐term stability and thus their long‐term functionality needed for commercial applications. This study provides an in‐depth insight into the photocorrosion‐induced changes altering the chemical and photophysical properties of platinized CdSe‐seeded CdS nanorods (CdSe/CdS/Pt) during the photocatalytic hydrogen production from aqueous ethanol solutions at very high pH values. Using a combination of mass spectrometry, NMR spectroscopy, electron microscopy, with in situ absorption and fluorescence spectroscopy, it is shown that the ligand shell consisting of 11‐MUA does not directly contribute to the generated hydrogen, although it is gradually degraded by present OH ^• radicals. Instead, the degradation of the ligand shell first leads to higher reactivity over time and finally to aggregation of the photocatalyst once a sweet spot is crossed. The inorganic CdSe/CdS/Pt particles are then reacting with OH ^• radicals to break apart into different pieces to transform into polymeric sulphur and Cd ²⁺ . This shows that the presence of OH ^• radicals can be very beneficial as an effective hole scavenger but can also lead to degradation of the whole colloidal nanostructure at longer reaction times.

We present a novel, straightforward, and reproducible one-pot heating up technique to synthesize core/crown SnSe/SnS nanosheets by a careful adjustment of the sulfur and selenium precursor reactivity. Here, the SnSe nanosheets are prepared via a wet-chemical route using SnCl₂ and selenium phosphines. By adding a highly reactive S-oleylamine complex during the growth of the SnSe sheets at 240 °C, SnS crowns were formed within the SnSe sheets. The number of SnS crowns can be tailored upon repeated addition of S-oleylamine. A combination of transmission electron microscopy, high-resolution transmission electron microscopy, atomic force microscopy, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy proves the formation of highly crystalline core/crown structures.

Exciton-phonon coupling limits the homogeneous emission line width of nanocrystals. Hence, a full understanding of this is crucial. In this work, we statistically investigate exciton-phonon coupling by performing single-particle spectroscopy on colloidal Zn_1-x Cd _x Se/CdS and CdSe/CdS dot-in-rod nanocrystals at cryogenic temperatures (T ≈ 10 K). In situ cation exchange enables us to analyze different band alignments and, thereby, different charge-carrier distributions. We find that the relative intensities of the longitudinal optical S- and Se-type phonon replicas correlate with the charge-carrier distribution. Our experimental findings are complemented with quantum mechanical calculations within the effective mass approximation that hint at the relevance of surface charges.

We present a detailed study on the effects of oxygen on the photoluminescence properties of CdSe/CdS quantum dots (QDs). We investigated the role of oxygen by performing confocal measurements on thin films as well as on single particles while rapidly exchanging the gaseous environment between oxygen and an inert gas atmosphere. We found that the deionization of negatively charged particles by oxygen depends on both the excitation power and the shell thickness of the QDs. For QDs with thin shells, which exhibit strong photoluminescence blinking, we observed that the presence of oxygen affects both band-edge carrier blinking and hot-carrier blinking.

Cation exchange is a versatile method for modifying the material composition and properties of nanostructures. However, control of the degree of exchange and material properties is difficult at the single-particle level. Successive cation exchange from CdSe to Ag₂Se has been utilized here on the same individual nanowires to monitor the change of electronic properties in field-effect transistor devices. The transistors were fabricated by direct synthesis of CdSe nanowires on prepatterned substrates followed by optical lithography. The devices were then subjected to cation exchange by submerging them in an exchange solution containing silver nitrate. By removal of the devices from solution and probing the electrical transport properties at different times, the change in electronic properties of individual nanowires could be monitored throughout the entire exchange reaction from CdSe to Ag₂Se. Transistor characterization revealed that the electrical conductivity can be tuned by up to 8 orders of magnitude and the charge-carrier mobility by 7 orders of magnitude. While analysis of the material composition by energy dispersive X-ray spectroscopy confirmed successful cation exchange from CdSe to Ag₂Se, X-ray fluorescence spectroscopy proved that cation exchange also took place below the contacts. The method presented here demonstrates an efficient way to tune the material composition and access the resulting properties nondestructively at the single-particle level. This approach can be readily applied to many other material systems and can be used to study the electrical properties of nanostructures as a function of material composition or to optimize nanostructure-based devices after fabrication.