Anti-Adhesive Organosilane Coating Comprising Visibility on Demand.

There is a wide application field for anti-adhesive and hydrophobic coatings, stretching from self-cleaning surfaces over anti-graffiti and release coatings to demolding aids in the production of polymers. The typical materials for the latter are hard coatings, including TiN, CrN, diamond-like carbon, etc. Alternatively, organosilane coatings based on perfluorinated compounds or molecules with long alkyl side chains can be employed. Although these functional layers are generally required to be invisible, there is a demand for a straightforward approach, which enables the temporary control of successful and homogeneous application as well as abrasion and wear of the coatings during use. For this purpose, a visibility-on-demand property was introduced to an already established anti-adhesive organosilane coating by incorporation of 1,8-naphthalimide-N-propyltriethoxysilane (NIPTES) as a fluorescent marker molecule. While the naphthalimide unit provides blue fluorescence under UV irradiation, the ethoxy groups of NIPTES enable the covalent coupling to the coating as a result of the hydrolysis and condensation reactions. As a consequence, the fluorescent marker molecule NIPTES can simply be added to the coating solution as an additional organosilane component, without the need for changes in the approved deposition procedure. The generated fluorescent anti-adhesive coatings were characterized by contact angle measurements, atomic force microscopy (AFM), as well as by different spectroscopic techniques, including FTIR, UV-Vis, fluorescence and X-ray photoelectron spectroscopy (XPS). In addition, the on-demand control function provided by the introduced fluorescence properties was evaluated along an injection molding process.

Probing the charge transfer and electron-hole asymmetry in graphene-graphene quantum dot heterostructure.

In recent years, graphene-based van der Waals (vdW) heterostructures have come into prominence showcasing interesting charge transfer dynamics which is significant for optoelectronic applications. These novel structures are highly tunable depending on several factors such as the combination of the two-dimensional materials, the number of layers and band alignment exhibiting interfacial charge transfer dynamics. Here, we report on a novel graphene based 0D-2D vdW heterostructure between graphene and amine-functionalized graphene quantum dots (GQD) to investigate the interfacial charge transfer and doping possibilities. Using a combination ofab initiosimulations and Kelvin probe force microscopy (KPFM) measurements, we confirm that the incorporation of functional GQDs leads to a charge transfer induced p-type doping in graphene. A shift of the Dirac point by 0.05 eV with respect to the Fermi level (EF) in the graphene from the heterostructure was deduced from the calculated density of states. KPFM measurements revealed an increment in the surface potential of the GQD in the 0D-2D heterostructure by 29 mV with respect to graphene. Furthermore, we conducted power dependent Raman spectroscopy for both graphene and the heterostructure samples. An optical doping-induced gating effect resulted in a stiffening of theGband for electrons and holes in both samples (graphene and the heterostructure), suggesting a breakdown of the adiabatic Born-Oppenheimer approximation. Moreover, charge imbalance and renormalization of the electron-hole dispersion under the additional influence of the doped functional GQDs is pointing to an asymmetry in conduction and carrier mobility.

Two-dimensional talc as a van der Waals material for solid lubrication at the nanoscale.

Talc is a van der Waals and naturally abundant mineral with the chemical formula Mg3Si4O10(OH)2. Two-dimensional (2D) talc could be an alternative to hBN as van der Waals dielectric in 2D heterostructures. Furthermore, due to its good mechanical and frictional properties, 2D talc could be integrated into various hybrid microelectromechanical systems, or used as a functional filler in polymers. However, properties of talcas one of the main representatives of the phyllosilicate (sheet silicates) group are almost completely unexplored when ultrathin crystalline films and monolayers are considered. We investigate 2D talc flakes down to single layer thickness and reveal their efficiency for solid lubrication at the nanoscale. We demonstrate by atomic force microscopy based methods and contact angle measurements that several nanometer thick talc flakes have all properties necessary for efficient lubrication: a low adhesion, hydrophobic nature, and a low friction coefficient of 0.10 ± 0.02. Compared to the silicon-dioxide substrate, 2D talc flakes reduce friction by more than a factor of five, adhesion by around 20%, and energy dissipation by around 7%. Considering our findings, together with the natural abundance of talc, we put forward that 2D talc can be a cost-effective solid lubricant in micro- and nano-mechanical devices.

Single-step fabrication and work function engineering of Langmuir-Blodgett assembled few-layer graphene films with Li and Au salts.

To implement large-area solution-processed graphene films in low-cost transparent conductor applications, it is necessary to have the control over the work function (WF) of the film. In this study we demonstrate a straightforward single-step chemical approach for modulating the work function of graphene films. In our approach, chemical doping of the film is introduced at the moment of its formation. The films are self-assembled from liquid-phase exfoliated few-layer graphene sheet dispersions by Langmuir-Blodgett technique at the water-air interfaces. To achieve a single-step chemical doping, metal standard solutions are introduced instead of water. Li standard solutions (LiCl, LiNO3, Li2CO3) were used as n-dopant, and gold standard solution, H(AuCl4), as p-dopant. Li based salts decrease the work function, while Au based salts increase the work function of the entire film. The maximal doping in both directions yields a significant range of around 0.7 eV for the work function modulation. In all cases when Li-based salts are introduced, electrical properties of the film deteriorate. Further, lithium nitrate (LiNO3) was selected as the best choice for n-type doping since it provides the largest work function modulation (by 400 meV), and the least influence on the electrical properties of the film.

Synthesis and Assembly of Zinc Oxide Microcrystals by a Low-Temperature Dissolution-Reprecipitation Process: Lessons Learned About Twin Formation in Heterogeneous Reactions.

Cobalt-doped zinc oxide single crystals with the shape of hexagonal platelets were synthesized by thermohydrolysis of zinc acetate, cobalt acetate, and hexamethylenetetramine (HMTA) in mixtures of ethanol and water. The mineralization proceeds by a low-temperature dissolution-reprecipitation process from the liquid phase by the formation of basic cobalt zinc salts as intermediates. The crystal shape as well as twin formation of the resulting oxide phase can be influenced by careful choice of the solvent mixture and the amount of doping. An understanding of the course of the reaction was achieved by comprehensive employment of analytical techniques (i.e., SEM, XRD, IR) including an in-depth HRTEM study of precipitates from various reaction stages. In addition, EPR as well as UV/Vis spectroscopic measurements provide information about the insertion of the cobalt dopant into the zincite lattice. The Langmuir-Blodgett (LB) technique is shown to be suitable for depositing coatings of the platelets on glass substrates functionalized with polyelectrolyte multilayers and hence is applied for the formation of monolayers containing domains with ordered tessellation. No major differences are found between deposits on substrates with anionic or cationic surface modification. The adherence to the substrates is sufficient to determine the absolute orientation of the deposited polar single crystals by piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM) studies.

Molecules on rails: friction anisotropy and preferential sliding directions of organic nanocrystallites on two-dimensional materials.

Two-dimensional (2D) materials are envisaged as ultra-thin solid lubricants for nanomechanical systems. So far, their frictional properties at the nanoscale have been studied by standard friction force microscopy. However, lateral manipulation of nanoparticles is a more suitable method to study the dependence of friction on the crystallography of two contacting surfaces. Still, such experiments are lacking. In this study, we combine atomic force microscopy (AFM) based lateral manipulation and molecular dynamics simulations in order to investigate the movements of organic needle-like nanocrystallites grown by van der Waals epitaxy on graphene and hexagonal boron nitride. We observe that nanoneedle fragments - when pushed by an AFM tip - do not move along the original pushing directions. Instead, they slide on the 2D materials preferentially along the needles' growth directions, which act as invisible rails along commensurate directions. Further, when the nanocrystallites were rotated by applying a torque with the AFM tip across the preferential sliding directions, we find an increase of the torsional signal of the AFM cantilever. We demonstrate in conjunction with simulations that both, the significant friction anisotropy and preferential sliding directions are determined by the complex epitaxial relation and arise from the commensurate and incommensurate states between the organic nanocrystallites and the 2D materials.

Reconstruction of the domain orientation distribution function of polycrystalline PZT ceramics using vector piezoresponse force microscopy.

Lead zirconate titanate (PZT) is one of the prominent materials used in polycrystalline piezoelectric devices. Since the ferroelectric domain orientation is the most important parameter affecting the electromechanical performance, analyzing the domain orientation distribution is of great importance for the development and understanding of improved piezoceramic devices. Here, vector piezoresponse force microscopy (vector-PFM) has been applied in order to reconstruct the ferroelectric domain orientation distribution function of polished sections of device-ready polycrystalline lead zirconate titanate (PZT) material. A measurement procedure and a computer program based on the software Mathematica have been developed to automatically evaluate the vector-PFM data for reconstructing the domain orientation function. The method is tested on differently in-plane and out-of-plane poled PZT samples, and the results reveal the expected domain patterns and allow determination of the polarization orientation distribution function at high accuracy.

Probing charge transfer between molecular semiconductors and graphene.

The unique density of states and exceptionally low electrical noise allow graphene-based field effect devices to be utilized as extremely sensitive potentiometers for probing charge transfer with adsorbed species. On the other hand, molecular level alignment at the interface with electrodes can strongly influence the performance of organic-based devices. For this reason, interfacial band engineering is crucial for potential applications of graphene/organic semiconductor heterostructures. Here, we demonstrate charge transfer between graphene and two molecular semiconductors, parahexaphenyl and buckminsterfullerene C60. Through in-situ measurements, we directly probe the charge transfer as the interfacial dipoles are formed. It is found that the adsorbed molecules do not affect electron scattering rates in graphene, indicating that charge transfer is the main mechanism governing the level alignment. From the amount of transferred charge and the molecular coverage of the grown films, the amount of charge transferred per adsorbed molecule is estimated, indicating very weak interaction.

Inkjet Printing of Soft, Stretchable Optical Waveguides through the Photopolymerization of High-Profile Linear Patterns.

Optical waveguides have been fabricated via photopolymerization of stable, inkjet-printed patterns. In order to obtain high-profile lines, the properties of both the ink and the substrate were adjusted. We prove that suitable patterns, with contact angles close to 90°, can be printed by using not fully cured, "sticky" PDMS as a substrate. In addition, we propose a simple sliding-drop experiment to show the crucial difference in how the ink dewets the "sticky" and the fully cured substrate, which is otherwise difficult to demonstrate. The light attenuation vs strain curve of the obtained waveguides was determined experimentally and was found to be almost linear within the measured strain range.

Epitaxy of highly ordered organic semiconductor crystallite networks supported by hexagonal boron nitride.

This study focuses on hexagonal boron nitride as an ultra-thin van der Waals dielectric substrate for the epitaxial growth of highly ordered crystalline networks of the organic semiconductor parahexaphenyl. Atomic force microscopy based morphology analysis combined with density functional theory simulations reveal their epitaxial relation. As a consequence, needle-like crystallites of parahexaphenyl grow with their long axes oriented five degrees off the hexagonal boron nitride zigzag directions. In addition, by tuning the deposition temperature and the thickness of hexagonal boron nitride, ordered networks of needle-like crystallites as long as several tens of micrometers can be obtained. A deeper understanding of the organic crystallites growth and ordering at ultra-thin van der Waals dielectric substrates will lead to grain boundary-free organic field effect devices, limited only by the intrinsic properties of the organic semiconductors.

Island shape anisotropy in organic thin film growth induced by ion-beam irradiated rippled surfaces.

Controlling thin film growth of conjugated molecules is a key factor in organic electronics. Here, we report on the growth of the organic semiconductor para-hexaphenyl (6P) on ion bombarded, rippled TiO2(110) surfaces. These surfaces represent a one-dimensionally patterned substrate with alternating descending and ascending step trains with typical step distances below 1 nm. A clear island shape anisotropy and a discretization of the island width according to the ripple wavelength have been observed which are addressed to anisotropic detachment of molecules differently bound to the island rim at ascending and descending steps. By changing the average ripple length from ∼11 nm to ∼60 nm, the islands' length-to-width ratio could be tuned between ∼1.5 : 1 and ∼4.5 : 1. Further, strong diffusion anisotropy along and perpendicular to the ripples was found to be responsible for an increasing island density with decreasing ripple length.

Photoresponse from single upright-standing ZnO nanorods explored by photoconductive AFM.

BACKGROUND: ZnO nanostructures are promising candidates for the development of novel electronic devices due to their unique electrical and optical properties. Here, photoconductive atomic force microscopy (PC-AFM) has been applied to investigate transient photoconductivity and photocurrent spectra of upright-standing ZnO nanorods (NRs). With a view to evaluate the electronic properties of the NRs and to get information on recombination kinetics, we have also performed time-resolved photoluminescence measurements macroscopically. RESULTS: Persistent photoconductivity from single ZnO NRs was observed for about 1800 s and was studied with the help of photocurrent spectroscopy, which was recorded locally. The photocurrent spectra recorded from single ZnO NRs revealed that the minimum photon energy sufficient for photocurrent excitation is 3.1 eV. This value is at least 100 meV lower than the band-gap energy determined from the photoluminescence experiments. CONCLUSION: The obtained results suggest that the photoresponse in ZnO NRs under ambient conditions originates preferentially from photoexcitation of charge carriers localized at defect states and dominates over the oxygen photodesorption mechanism. Our findings are in agreement with previous theoretical predictions based on density functional theory calculations as well as with earlier experiments carried out at variable oxygen pressure.

Atomic force microscopy based manipulation of graphene using dynamic plowing lithography.

Tapping mode atomic force microscopy (AFM) is employed for dynamic plowing lithography of exfoliated graphene on silicon dioxide substrates. The shape of the graphene sheet is determined by the movement of the vibrating AFM probe. There are two possibilities for lithography depending on the applied force. At moderate forces, the AFM tip only deforms the graphene and generates local strain of the order of 0.1%. For sufficiently large forces the AFM tip can hook graphene and then pull it, thus cutting the graphene along the direction of the tip motion. Electrical characterization by AFM based electric force microscopy, Kelvin probe force microscopy and conductive AFM allows us to distinguish between the truly separated islands and those still connected to the surrounding graphene.

Adsorption/desorption of H2 and CO on Zn-modified Pd(111).

The adsorption and thermal desorption of H(2) and CO on clean and Zn covered Pd(111) surfaces were studied using temperature programmed desorption (TPD), low energy electron diffraction, and Auger electron spectroscopy. The obtained H(2) and CO-TPD results reveal that thick Zn layers (approximately 10 ML) prepared at low temperature (150 K) block the adsorption of H(2) and CO. However, the ZnPd surface alloy which is formed at temperatures above 300 K shows a different behavior. The amount of hydrogen adsorbed on surface sites is reduced by about 1/2 on the ZnPd surface alloy whereupon the diffusion of hydrogen into the subsurface region is not influenced. The initial sticking coefficient decreases from 0.5 on the clean surface to 0.14 on the ZnPd alloy. The TPD spectra for CO on the ZnPd surface alloy show that the heat of adsorption is shifted to much lower values than on clean Pd, yielding a desorption energy of 71+/-2 kJ mol(-1) at low CO coverages. The saturation coverage equals 0.5 ML which means that each Pd atom of the ZnPd surface alloy is occupied by one CO admolecule. Interestingly, however, the initial sticking coefficient for CO on the ZnPd surface alloy is still unity, as on the clean Pd surface.