Hierarchical Surface Pattern on Ni-Free Ti-Based Bulk Metallic Glass to Control Cell Interactions.

Ni-free Ti-based bulk metallic glasses (BMGs) are exciting materials for biomedical applications because of their outstanding biocompatibility and advantageous mechanical properties. The glassy nature of BMGs allows them to be shaped and patterned via thermoplastic forming (TPF). This work demonstrates the versatility of the TPF technique to create micro- and nano-patterns and hierarchical structures on Ti40 Zr10 Cu34 Pd14 Sn2 BMG. Particularly, a hierarchical structure fabricated by a two-step TPF process integrates 400 nm hexagonal close-packed protrusions on 2.5 µm square protuberances while preserving the advantageous mechanical properties from the as-cast material state. The correlations between thermal history, structure, and mechanical properties are explored. Regarding biocompatibility, Ti40 Zr10 Cu34 Pd14 Sn2 BMGs with four surface topographies (flat, micro-patterned, nano-patterned, and hierarchical-structured surfaces) are investigated using Saos-2 cell lines. Alamar Blue assay and live/dead analysis show that all tested surfaces have good cell proliferation and viability. Patterned surfaces are observed to promote the formation of longer filopodia on the edge of the cytoskeleton, leading to star-shaped and dendritic cell morphologies compared with the flat surface. In addition to potential implant applications, TPF-patterned Ti-BMGs enable a high level of order and design flexibility on the surface topography, expanding the available toolbox for studying cell behavior on rigid and ordered surfaces.

Pinaceae Pine Resins (Black Pine, Shore Pine, Rosin, and Baltic Amber) as Natural Dielectrics for Low Operating Voltage, Hysteresis-Free, Organic Field Effect Transistors.

Four pinaceae pine resins analyzed in this study: black pine, shore pine, Baltic amber, and rosin demonstrate excellent dielectric properties, outstanding film forming, and ease of processability from ethyl alcohol solutions. Their trap-free nature allows fabrication of virtually hysteresis-free organic field effect transistors operating in a low voltage window with excellent stability under bias stress. Such green constituents represent an excellent choice of materials for applications targeting biocompatibility and biodegradability of electronics and sensors, within the overall effort of sustainable electronics development and environmental friendliness.

Photoinduced edge-specific nanoparticle decoration of two-dimensional tungsten diselenide nanoribbons.

Metallic nanoparticles are widely explored for boosting light-matter coupling, optoelectronic response, and improving photocatalytic performance of two-dimensional (2D) materials. However, the target area is restricted to either top or bottom of the 2D flakes. Here, we introduce an approach for edge-specific nanoparticle decoration via light-assisted reduction of silver ions and merging of silver seeds. We observe arrays of the self-limited in size silver nanoparticles along tungsten diselenide WSe2 nanoribbon edges. The density of nanoparticles is tunable by adjusting the laser fluence. Scanning electron microscopy, atomic force microscopy, and Raman spectroscopy are used to investigate the size, distribution, and photo-response of the deposited plasmonic nanoparticles on the quasi-one-dimensional nanoribbons. We report an on-surface synthesis path for creating mixed-dimensional heterostructures and heterojunctions with potential applications in opto-electronics, plasmonics, and catalysis, offering improved light matter coupling, optoelectronics response, and photocatalytic performance of 2D materials.

Electrical monitoring of organic crystal phase transition using MoS2 field effect transistor.

Hybrid van der Waals heterostructures made of 2D materials and organic molecules exploit the high sensitivity of 2D materials to all interfacial modifications and the inherent versatility of the organic compounds. In this study, we are interested in the quinoidal zwitterion/MoS2 hybrid system in which organic crystals are grown by epitaxy on the MoS2 surface and reorganize in another polymorph after thermal annealing. By means of field-effect transistor measurements recorded in situ all along the process, atomic force microscopy and density functional theory calculations we demonstrate that the charge transfer between quinoidal zwitterions and MoS2 strongly depends on the conformation of the molecular film. Remarkably, both the field effect mobility and the current modulation depth of the transistors remain unchanged which opens up promising prospects for efficient devices based on this hybrid system. We also show that MoS2 transistors enable fast and accurate detection of structural modifications that occur during phases transitions of the organic layer. This work highlights that MoS2 transistors are remarkable tools for on-chip detection of molecular events occurring at the nanoscale, which paves the way for the investigation of other dynamical systems.

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.

Assessing Fire-Damage in Historical Papers and Alleviating Damage with Soft Cellulose Nanofibers.

The conservation of historical paper objects with high cultural value is an important societal task. Papers that have been severely damaged by fire, heat, and extinguishing water, are a particularly challenging case, because of the complexity and severity of damage patterns. In-depth analysis of fire-damaged papers, by means of examples from the catastrophic fire in a 17th-century German library, shows the changes, which proceeded from the margin to the center, to go beyond surface charring and formation of hydrophobic carbon-rich layers. The charred paper exhibits structural changes in the nano- and micro-range, with increased porosity and water sorption. In less charred areas, cellulose is affected by both chain cleavage and cross-linking. Based on these results and conclusions with regard to adhesion of auxiliaries, a stabilization method is developed, which coats the damaged paper with a thin layer of cellulose nanofibers. It enables the reliable preservation of the paper and-most importantly-retrieval of the contained historical information: the nanofibers form a flexible, transparent film on the surface and adhere strongly to the damaged matrix, greatly reducing its fragility, giving it stability, and enabling digitization and further handling.

The transverse and longitudinal elastic constants of pulp fibers in paper sheets.

Cellulose fibers are a major industrial input, but due to their irregular shape and anisotropic material response, accurate material characterization is difficult. Single fiber tensile testing is the most popular way to estimate the material properties of individual fibers. However, such tests can only be performed along the axis of the fiber and are associated with problems of enforcing restraints. Alternative indirect approaches, such as micro-mechanical modeling, can help but yield results that are not fully decoupled from the model assumptions. Here, we compare these methods with nanoindentation as a method to extract elastic material constants of the individual fibers. We show that both the longitudinal and the transverse elastic modulus can be determined, additionally enabling the measurement of fiber properties in-situ inside a sheet of paper such that the entire industrial process history is captured. The obtained longitudinal modulus is comparable to traditional methods for larger indents but with a strongly increased scatter as the size of the indentation is decreased further.

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.

Massive and massless charge carriers in an epitaxially strained alkali metal quantum well on graphene.

We show that Cs intercalated bilayer graphene acts as a substrate for the growth of a strained Cs film hosting quantum well states with high electronic quality. The Cs film grows in an fcc phase with a substantially reduced lattice constant of 4.9 Å corresponding to a compressive strain of 11% compared to bulk Cs. We investigate its electronic structure using angle-resolved photoemission spectroscopy and show the coexistence of massless Dirac and massive Schrödinger charge carriers in two dimensions. Analysis of the electronic self-energy of the massive charge carriers reveals the crystallographic direction in which a two-dimensional Fermi gas is realized. Our work introduces the growth of strained metal quantum wells on intercalated Dirac matter.

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.

Design of Friction, Morphology, Wetting, and Protein Affinity by Cellulose Blend Thin Film Composition.

Cellulose derivate phase separation in thin films was applied to generate patterned films with distinct surface morphology. Patterned polymer thin films are utilized in electronics, optics, and biotechnology but films based on bio-polymers are scarce. Film formation, roughness, wetting, and patterning are often investigated when it comes to characterization of the films. Frictional properties, on the other hand, have not been studied extensively. We extend the fundamental understanding of spin coated complex cellulose blend films via revealing their surface friction using Friction Force Microscopy (FFM). Two cellulose derivatives were transformed into two-phase blend films with one phase comprising trimethyl silyl cellulose (TMSC) regenerated to cellulose with hydroxyl groups exposed to the film surface. Adjusting the volume fraction of the spin coating solution resulted in variation of the surface fraction with the other, hydroxypropylcellulose stearate (HPCE) phase. The film morphology confirmed lateral and vertical separation and was translated into effective surface fraction. Phase separation as well as regeneration contributed to the surface morphology resulting in roughness variation of the blend films from 1.1 to 19.8 nm depending on the film composition. Friction analysis was successfully established, and then revealed that the friction coefficient of the films could be tuned and the blend films exhibited lowered friction force coefficient compared to the single-component films. Protein affinity of the films was investigated with bovine serum albumin (BSA) and depended mainly on the surface free energy (SFE) while no direct correlation with roughness or friction was found. BSA adsorption on film formed with 1:1 spinning solution volume ratio was an outlier and exhibited unexpected minimum in adsorption.

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.

Combining adhesive contact mechanics with a viscoelastic material model to probe local material properties by AFM.

Viscoelastic properties are often measured using probe based techniques such as nanoindentation (NI) and atomic force microscopy (AFM). Rarely, however, are these methods verified. In this article, we present a method that combines contact mechanics with a viscoelastic model (VEM) composed of springs and dashpots. We further show how to use this model to determine viscoelastic properties from creep curves recorded by a probe based technique. We focus on using the standard linear solid model and the generalized Maxwell model of order 2. The method operates in the range of 0.01 Hz to 1 Hz. Our approach is suitable for rough surfaces by providing a defined contact area using plastic pre-deformation of the material. The very same procedure is used to evaluate AFM based measurements as well as NI measurements performed on polymer samples made from poly(methyl methacrylate) and polycarbonate. The results of these measurements are then compared to those obtained by tensile creep tests also performed on the same samples. It is found that the tensile test results differ considerably from the results obtained by AFM and NI methods. The similarity between the AFM results and NI results suggests that the proposed method is capable of yielding results comparable to NI but with the advantage of the imaging possibilities of AFM. Furthermore, all three methods allowed a clear distinction between PC and PMMA by means of their respective viscoelastic properties.

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.

From Permeation to Cluster Arrays: Graphene on Ir(111) Exposed to Carbon Vapor.

Our scanning tunneling microscopy and X-ray photoelectron spectroscopy experiments along with first-principles calculations uncover the rich phenomenology and enable a coherent understanding of carbon vapor interaction with graphene on Ir(111). At high temperatures, carbon vapor not only permeates to the metal surface but also densifies the graphene cover. Thereby, in addition to underlayer graphene growth, upon cool down also severe wrinkling of the densified graphene cover is observed. In contrast, at low temperatures the adsorbed carbon largely remains on top and self-organizes into a regular array of fullerene-like, thermally highly stable clusters that are covalently bonded to the underlying graphene sheet. Thus, a new type of predominantly sp2-hybridized nanostructured and ultrathin carbon material emerges, which may be useful to encage or stably bind metal in finely dispersed form.

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.