Scientific publications

A graphene liquid cell for transmission electron microscopy (TEM) uses one or two graphene sheets to separate the liquid from the vacuum in the microscope. In principle, graphene is an excellent material for such an application because it allows the highest possible spatial resolution, provides a flexible covering foil, and effectively protects the liquid from evaporating. Examples in open literature have demonstrated atomic-resolution TEM using small liquid pockets and the coverage of whole biological cells with graphene sheets. A total of three different basic types of liquid cells are discerned: (i) one graphene sheet is used to cover a liquid sample supported by a thin membrane of another material (for example, silicon nitride, SiN), (ii) two graphene sheets pressed together leaving liquid pockets with graphene at both sides, and (iii) a spacer material with liquid pockets covered at both sides by graphene. A total of four different process flows are available for liquid cell assembly, but there is not yet a consensus on the best routes, and a number of variations exist. The key step is the transfer of graphene to a liquid sample, which is complicated by practical issues that arise from imperfections in the graphene sheets, such as cracks. This review provides an overview of these different approaches to assembling graphene liquid cells and discusses the main obstacles and ideas to overcome them with the prospect of developing the nanoscale technology needed for graphene liquid cells so that they become available on a routine basis for electron microscopy in liquid. It also provides guidance in selecting the appropriate type of graphene liquid cell and the best assembly method for a specific experiment.

The utilization of abundantly available elements in key technologies is a promising way to save precious and rare metals. Iron and titanium offer the highest abundance of all transition metals in the earth's crust and their application in catalytic processes is preferable regarding sustainable material development. The photocatalytic decontamination of wastewater using visible light-responsive materials is of high interest due to the demand for clean water and the increasing accumulation of harmful organic compounds resulting from medical or industrial waste. Herein, we report on a novel photocatalyst based on the generation of crystalline Fe2O3 and TiO2 on size-optimized colloidal metal-organic framework crystallites. The reusable photocatalyst permits the efficient oxidative degradation of pharmaceutical compounds and toxic dyes under visible light illumination and without the requirement of additives or noble metals. We observed a higher photocatalytic activity for our Fe2O3/TiO2@MIL-101 material than for commercially available Fe2O3, TiO2, and TiO2 (P25).

Two-dimensional atomically flat sheets with a high mechanical flexibility are very attractive as ultrathin membranes but are also inherently challenging for microscopic investigations. We report on a method using Scanning Tunneling Microscopy (STM) under ultra-high vacuum conditions for non-indenting low-force spectroscopy on micrometer-sized freestanding graphene membranes. The method is based on applying quasi-static voltage ramps with active feedback at low tunneling currents and ultimately relies on the attractive electrostatic force between the tip and the membrane. As a result a bulge-test scenario can be established. The convenience and simplicity of the method relies on the fact that the loading force and the membrane deflection detection are both provided simultaneously by the STM. This permits the continuous measurement of the stress-strain relation. Electrostatic forces applied are typically below 1 nN and the membrane deflection is detected at sub-nanometer resolution. Experiments on single-layer graphene membranes with a strain of 0.1% reveal a two-dimensional elastic modulus E2D = 220 N m-1.

Molecular layering of liquids in nanometer-scale confinement is demonstrated for typical lubricant constituents such as polyalphaolefins (PAO) and an ester by means of atomic force microscopy. Layering is observed in force vs. distance curves for poly-(1-decene) tetramers (PAO6) and undecamers (PAO40) and for a 2-ethylhexyl monoester on graphite, mica, and polished steel surfaces and is compared to the layering of hexadecane and 1-hexadecene. On graphite surfaces, the confined molecules are oriented parallel to the surfaces for all liquids, resulting in layers with a thickness comparable to the diameter of the alkyl chains. On mica, confined hexadecane molecules also lie parallel to the surface, while the molecules in the first layer of 1-hexadecene and PAOs take a more upright orientation. Confinement on the oxidized polished steel surfaces results in a molecular layering which most often resembles the layering on graphite and differs significantly from layering on the ionic oxide mica.

Friction and wear of a commercially available polyetheretherketone were investigated by two different testing approaches, namely the standard pin-on-disc (POD) configuration and an unidirectional pin-on-flat (POF) scratch test, in a wide range of pv-products from 0.001 to 8 MPa m/s under dry sliding condition. It was found that the steady state friction coefficient gained from POD tests slightly decreases with increasing sliding velocity from 0.1 to 1 m/s, further increase in the velocity to 4 m/s results in an obvious raise of the friction coefficient. It is assumed that this increase can be attributed to the high interfacial temperature induced strong adhesion between PEEK surface and steel counterbody. No obvious difference of the friction coefficients between POD and POF tests is noted in the studied range. With respect to the wear rate, the wear rate measured from POD increases with monotonously increasing velocity. Possible reasons for these observations are discussed based on the analysis of the worn surfaces of polymer samples and transfer films formed on the steel counterface as well as the investigations on the thermal characteristics of different tribo-systems.

In this work we report the use of polarization active porous 1D Bragg microcavities (BM) prepared by physical vapor deposition at oblique angles for the optofluidic analysis of liquid solutions. These photonic structures consist of a series of stacked highly porous layers of two materials with different refractive indices and high birefringence. Their operational principle implies filling the pores with the analyzed liquid while monitoring with linearly polarized light the associated changes in optical response as a function of the solution refractive index. The response of both polarization active and inactive BMs as optofluidic sensors for the determination of glucose concentration in water solutions has been systematically compared. Different methods of detection, including monitoring the BM wave retarder behavior, are critically compared for both low and high glucose concentrations. Data are taken in transmission and reflection modes and different options explored to prove the incorporation of these nanostructured transducers into microfluidic systems and/or onto the tip of an optical fiber. This analysis has proven the advantages of the polarization active transducer sensors for the optofluidic analysis of liquids and their robustness even in the presence of light source instabilities or misalignments of the optical system used for detection.

The iminostannylene HNSn was successfully incorporated in a molecular cage of composition (Me2RSi–NSn)3(HNSn) with group R either being a methyl (1) or vinyl (2) substituent. An X-ray structure analysis reveals that 2 consists of a distorted Sn4N4 cube. The Sn–N(H) bond lengths [2.189(2) Å] are in the range for Sn4N4 hetero cubanes. When stored in a toluene solution the clusters 1 and 2 decompose slowly into the symmetric cubanes (Me2RSi–NSn)4 [R = Me (3), CHCH2 (4)] and an amorphous and insoluble powder of composition HNSn. The decomposition follows a first order rate law as established for 2 with a half life time t1/2 = 320 d at 20 °C. The compounds 1 and 2 can thus be regarded as a result of interaction between three entities {Me2RSi–NSn} and one entity {HNSn}. We also isolated the twistane-like Me2Si(NtBu)2Sn2NtBu (5) in a crystalline form. The central structure of this molecule, which has almost C2v symmetry, has a trigonal bipyramid Sn2N3 unit with the nitrogen atoms occupying the equatorial plane. Each nitrogen atom has a tert-butyl ligand and two of the N atoms are further connected by the dimethylsilyl group. There is one nitrogen atom in an almost planar environment (only bonding to tert-butyl and two tin atoms) with a remarkable short Sn–N bond length of 2.048(5) Å. Both tin atoms in cage 5 can bond to Cr(CO)5 to form [Me2Si(NtBu)2Sn2NtBu][Cr(CO)5]2 (6) with an almost linear Cr–Sn···Sn–Cr arrangement and Sn–Cr bond lengths of 2.581(1) Å (X-ray diffraction).

We have prepared the first crystalline and 3D periodically ordered mesoporous quaternary semiconductor photocatalyst in an evaporation-induced self-assembly assisted soft-templating process. Using lab synthesized triblock-terpolymer poly(isoprene-b-styrene-b-ethylene oxide) (ISO) a highly ordered 3D interconnected alternating gyroid morphology was achieved exhibiting near and long-range order, as evidenced by small angle X-ray scattering (SAXS) and electron microscopy (TEM/SEM). Moreover, we reveal the formation process on the phase-pure construction of the material's pore-walls with its high crystallinity, which proceeds along a highly stable W5+ compound, by both in situ and ex situ analyses, including X-ray powder diffraction (XRPD), Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance (EPR). The resulting photocatalyst CsTaWO6 with its optimum balance between surface area and ordered mesoporosity ultimately shows superior hydrogen evolution rates over its non-ordered reference in photocatalytic hydrogen production. This work will help to advance new self-assembly preparation pathways towards multi-element multifunctional compounds for different applications, including improved battery and sensor electrode materials.

Manufactured nanomaterials (MNMs) are regarded as key components of innovations in various fields with high potential impact (e.g., energy generation and storage, electronics, photonics, diagnostics, theranostics, or drug delivery agents). Widespread use of MNMs raises concerns about their safety for humans and the environment, possibly limiting the impact of the nanotechnology-based innovation. The development of safe MNMs and nanoproducts has to result in a safe as well as functional material or product. Its safe use, and disposal at the end of its life cycle must be taken into account too. However, not all MNMs are similarly useful for all applications, some might bear a higher hazard potential than others, and use scenarios could lead to different exposure probabilities. To improve both safety and efficacy of nanotechnology, we think that a new proactive approach is necessary, based on pre-regulatory safety assessment and dialogue between stakeholders. On the basis of the work carried out in different European Union (EU) initiatives, developing and integrating MNMs Safe-by-Design and Trusted Environments (NANoREG, ProSafe, and NanoReg2), we present our point of view here. This concept, when fully developed, will allow for cost effective industrial innovation, and an exchange of key information between regulators and innovators. Regulators are thus informed about incoming innovations in good time, supporting a proactive regulatory action. The final goal is to contribute to the nanotechnology governance, having faster, cheaper, effective, and safer nano-products on the market.