Boneberg, Johannes
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Photoswitching Affinity and Mechanism of Multivalent Lectin Ligands
Multivalent receptor-ligand binding is a key principle in a plethora of biological recognition processes. Immense binding affinities can be achieved with the correct spatial orientation of the ligands. Accordingly, the incorporation of photoswitches, that can be used to reversibly change the spatial orientation of molecules, into multivalent ligands is a means to alter the binding affinity and possibly also the binding mode of such ligands. We report a divalent ligand for the model lectin wheat germ agglutinin (WGA) containing an arylazopyrazole photoswitch. This switch, that has been recently introduced as an alternative to the more commonly used azobenzene moiety, is characterized by almost quantitative E / Z photoswitching in both directions, high quantum yields, and high thermal stability of the Z isomer. The ligand was designed in a way that only one of the isomers is able to bridge adjacent binding sites of WGA leading to a chelating binding mode. Photoswitching induces an unprecedentedly high change in lectin binding affinity as determined by isothermal titration calorimetry (ITC). Furthermore, additional dynamic light scattering (DLS) data suggest that the binding mode of the ligand changes from chelating binding of the E isomer to crosslinking binding of the Z isomer.
Performance enhancement in Sb2S3 solar cell processed with direct laser interference patterning
Direct laser interference patterning (DLIP) is used to fabricate large-area, periodic surface patterns on Sb2S3 substrates to enhance the performance of solar cells. Comparing the power conversion efficiencies (PCE) to the reference cell on flat Sb2S3 film, a relative increase of 73% is observed for the DLIP patterned device. Our systematic study reveals that DLIP promotes beneficial crystallization of the Sb2S3 film. Light scattering is increased and recombination is depressed in the textured Sb2S3 film. It is expected that our study can provide a roadmap for the further development of photovoltaic devices (PVs) based on chalcogenide semiconductors.
Ultraviolet Deactivation of Silane-Functionalized Surfaces : A Scalable Approach for Patterned Nanoparticle Assembly
Developing optoelectronic devices, biological or chemical sensors, displays, and other devices based on nanoparticles (NPs) requires designing tailored NP assemblies on solid substrates, and often with a given surface positioning. In our study, we discuss a new soft-lithographic method for patterning an organic layer, which is capable of binding gold nanoparticles (AuNPs) to the surface. AuNPs with a citrate shell were 17 nm in diameter and prepared by the Turkevich protocol. Our method is based on controlling the binding capability of (3-aminopropyl)trimethoxysilane (APTES)-coated surface by deactivating the −NH2 terminal groups of APTES under the action of UV-generated ozone in air. We show that partial and complete deactivation can be achieved depending on the atmosphere and exposure time. Using a shadow mask during irradiation, we furthermore show that our method can be applied for creating micron-scale arrays of NPs on APTES-coated substrates with a spatial resolution down to ∼1.5 μm, currently limited by the properties of the mask.
Position-controlled laser-induced creation of rutile TiO2 nanostructures
For potential applications of nanostructures, control over their position is important. In this report, we introduce two continuous wave laser-based lithography techniques which allow texturing thin TiO2 films to create a fine rutile TiO2 structure on silicon via spatially confined oxidation or a solid-liquid-solid phase transition, for initial layers, we use titanium and anatase TiO2, respectively. A frequency-doubled Nd:YAG laser at a wavelength of 532 nm is employed for the lithography process and the samples are characterized with scanning electron microscopy. The local orientation of the created rutile crystals is determined by the spatial orientation of hydrothermally grown rutile TiO2 nanorods. Depending on the technique, we obtain either randomly aligned or highly ordered nanorod ensembles. An additional chemically inert SiO2 cover layer suppresses the chemical and electronic surface properties of TiO2 and is removed locally with the laser treatment. Hence, the resulting texture provides a specific topography and crystal structure as well as a high contrast of surface properties on a nanoscale, including the position-controlled growth of TiO2 nanorods.
Optical near-field imaging and nanostructuring by means of laser ablation
In this review we consider the development of optical near-field imaging and nanostructuring by means of laser ablation since its early stages around the turn of the century. The interaction of short, intense laser pulses with nanoparticles on a surface leads to laterally tightly confined, strongly enhanced electromagnetic fields below and around the nano-objects, which can easily give rise to nanoablation. This effect can be exploited for structuring substrate surfaces on a length scale well below the diffraction limit, one to two orders smaller than the incident laser wavelength. We report on structure formation by the optical near field of both dielectric and metallic nano-objects, the latter allowing even stronger and more localized enhancement of the electromagnetic field due to the excitation of plasmon modes. Structuring with this method enables one to nanopattern large areas in a one-step parallel process with just one laser pulse irradiation, and in the course of time various improvements have been added to this technique, so that also more complex and even arbitrary structures can be produced by means of nanoablation. The near-field patterns generated on the surface can be read out with high resolution techniques like scanning electron microscopy and atomic force microscopy and provide thus a valuable tool—in conjunction with numerical calculations like finite difference time domain (FDTD) simulations—for a deeper understanding of the optical and plasmonic properties of nanostructures and their applications.
In-situ control of on-chip angstrom gaps, atomic switches, and molecular junctions by light irradiation
Pairs of electrodes with nanometer gap, termed as nano-gapped electrodes, are fundamental building blocks for the fabrication of nanometer-sized devices and are essential for the examination of molecular properties and extreme nano-optics. Although modern fabrication techniques make it feasible to fabricate nanometer gaps, it is still a formidable challenge to fabricate adjustable gaps arrays with angstrom preci sion. Here, we demonstrate that in-situ adjustable nanogaps (arrays) with sub-angstrom precision can be fabricated via laser irradiation on the substrate which supports the electrode pairs. We further demonstrate that atomic-level metal contacts can be switched and the direction of the switching can be selectively controlled by the laser irradiation position. By varying the laser power gradually, the nanogap’s size can be continuously changed, providing a reliable break junction technique to address the properties of single- molecule junctions. The small spatial focus size of the laser beam makes it feasible to realize addressable on-chip molecular junction arrays.
Surface plasmon enhanced switching kinetics of molecular photochromic films on gold nanohole arrays
Diarylethene molecules are discussed as possible optical switches, which can reversibly transition between completely conjugated (closed) and nonconjugated (open) forms with different electrical conductance and optical absorbance, by exposure to UV and visible light. However, the opening reaction exhibits, in general, much lower quantum yield than the closing process, hindering their usage in optoelectronic devices. To enhance the opening process, which is supported by visible light, we employ the plasmonic field enhancement of gold films perforated with nanoholes. We show that gold nanohole arrays reveal strong optical transmission in the visible range (~60%) and pronounced enhancement of field intensities, resulting in around 50% faster switching kinetics of the molecular species in comparison with quartz substrates. The experimental UV-Vis measurements are verified with Finite-Difference Time-Domain simulation that confirm the obtained results. Thus, we propose gold nanohole arrays as transparent and conductive plasmonic material that accelerates visible-light-triggered chemical reactions including molecular switching.
Interaction between plasmonic silver nanorod arrays and nanosecond pulsed laser
Plasmonic metamaterials have recently received increasing attention due to their favorable optical and electrical properties. The physics of the interaction between plasmonic metamaterials and lasers are rich and open opportunities for applications, such as sensing, imaging and photovoltaics. Here, we investigate the mechanism of the interaction between Ag nanorod arrays and nanosecond (ns) pulsed laser. The experimental results show that ns laser pulses significantly improve the crystallinity of the Ag nanorod arrays. Meanwhile, the laser evaporates some of the Ag into the air. This work paves the way for future high-performance plasmonic meta-devices.
Simulation of Chemical Order–Disorder Transitions Induced Thermally at the Nanoscale for Magnetic Recording and Data Storage
In memory nanodevices based on phase changes induced thermally, the process of information recording is a reversible transition between the structurally ordered (crystalline) and disordered (amorphous) phases that can provide a difference in the physical properties of these two states, for example, in optical reflectivity, electrical resistivity, or magnetic permeability. It is of particular interest to explore whether the chemical disorder is erasable, rewritable, and scalable in solid alloys upon their exposure to short heating pulses. Here, we model this process by assuming second-order phase transitions between chemically ordered and disordered states in the atomic lattice. Our simulations reveal that nanosecond laser irradiation concentrated within a nanoscale spot on the sample surface is able to induce reversible chemical-order (B2)-disorder (A2) transformations (CODTs) in intermetallic Fe-rich FexAl1–x alloys that exhibit the disorder-induced ferromagnetism. A realization of this concept would provide an alternative approach to current technologies for magnetic recording and data storage, in which the written bits are represented by regions with not a different polarity but with a different magnitude of magnetization. We envision that the proposed approach can be realized with tools used currently for heat-assisted magnetic recording (HAMR), for example, with a near-field transducer (NFT). A specific design for CODT-based magnetic recording media is proposed.