Systematic quantitative characterisation of surface nanostructures by scanning probe microscopy of thin-films

In this article, we present a new methodology to characterise surface nanostructures of thin films. The methodology focuses on isolating nanostructures and extracting quantitative information, such as their shape and size, based on atomic force microscopy (AFM) data. We start by compensating the AFM data for some specific classes of measurement artefacts. After that, the methodology employs two distinct approaches. The first, which we call the overlay approach, proceeds by systematically processing the raster data that constitute the scanning probe image in both vertical and horizontal directions. The second approach, based on fuzzy logic, relies on a fuzzy inference engine to classify the surface points. Once classified, these points are clustered into surface structures. We have applied both approaches to characterise organic semiconductor thin films of pentacene on different substrates. In this article, we present results employing both approaches to pentacene films deposited on mica. Using only the overlay approach, we have compared the pentacene film characteristics grown on different substrates. These results are discussed and compared along with the challenges of the two approaches.     

Dynamic Coordination of Eu–Iminodiacetate to Control Fluorochromic Response of Polymer Hydrogels to Multistimuli

New fluorochromic materials that reversibly change their emission properties in response to their environment are of interest for the development of sensors and light‐emitting materials. A new design of Eu‐containing polymer hydrogels showing fast self‐healing and tunable fluorochromic properties in response to five different stimuli, including pH, temperature, metal ions, sonication, and force, is reported. The polymer hydrogels are fabricated using Eu–iminodiacetate (IDA) coordination in a hydrophilic poly(N,N‐dimethylacrylamide) matrix. Dynamic metal–ligand coordination allows reversible formation and disruption of hydrogel networks under various stimuli which makes hydrogels self‐healable and injectable. Such hydrogels show interesting switchable ON/OFF luminescence along with the sol–gel transition through the reversible formation and dissociation of Eu–IDA complexes upon various stimuli. It is demonstrated that Eu‐containing hydrogels display fast and reversible mechanochromic response as well in hydrogels having interpenetrating polymer network. Those multistimuli responsive fluorochromic hydrogels illustrate a new pathway to make smart optical materials, particularly for biological sensors where multistimuli response is required.     

Bioinspired Catechol‐Terminated Self‐Assembled Monolayers with Enhanced Adhesion Properties

The role of the catechol moiety in the adhesive properties of mussel proteins and related synthetic materials has been extensively studied in the last years but still remains elusive. Here, a simplified model approach is presented based on a self‐assembled monolayer (SAM) of upward‐facing catechols thiol‐bound to epitaxial gold substrates. The orientation of the catechol moieties is confirmed by spectroscopy, which also showed lack of significant amounts of interfering o‐quinones. Local force‐distance curves on the SAM measured by atomic force microscopy (AFM) shows an average adhesion force of 45 nN, stronger than that of a reference polydopamine coating, along with higher reproducibility and less statistical dispersion. This is attributed to the superior chemical and topographical homogeneity of the SAM coating. Catechol‐terminated SAMs are also obtained on high‐roughness gold substrates that show the ability to assemble magnetic nanoparticles, despite their lack of enhanced adhesion at the molecular level. Finally, the influence of the catechol group on the formation and quality of the SAM is explored both theoretically (molecular dynamics simulations) and experimentally using direct‐write AFM lithography.     

Spatial Micropatterning of Growth Factors in 3D Hydrogels for Location‐Specific Regulation of Cellular Behaviors

Growth factors are potent stimuli for regulating cell function in tissue engineering strategies, but spatially patterning their presentation in 3D in a facile manner using a single material is challenging. Micropatterning is an attractive tool to modulate the cellular microenvironment with various biochemical and physical cues and study their effects on stem cell behaviors. Implementing heparin's ability to immobilize growth factors, dual‐crosslinkable alginate hydrogels are micropatterned in 3D with photocrosslinkable heparin substrates with various geometries and micropattern sizes, and their capability to establish 3D micropatterns of growth factors within the hydrogels is confirmed. This 3D micropatterning method could be applied to various heparin binding growth factors, such as fibroblast growth factor‐2, vascular endothelial growth factor, transforming growth factor‐betas and bone morphogenetic proteins while retaining the hydrogel's natural degradability and cytocompability. Stem cells encapsulated within these micropatterned hydrogels have exhibited spatially localized growth and differentiation responses corresponding to various growth factor patterns, demonstrating the versatility of the approach in controlling stem cell behavior for tissue engineering and regenerative medicine applications.     

Multimodal Characterization of Resin Embedded and Sliced Polymer Nanoparticles by Means of Tip‐Enhanced Raman Spectroscopy and Force–Distance Curve Based Atomic Force Microscopy

Understanding the property‐function relation of nanoparticles in various application fields involves determining their physicochemical properties, which is still a remaining challenge to date. While a multitude of different characterization tools can be applied, these methods by themselves can only provide an incomplete picture. Therefore, novel analytical techniques are required, which can address both chemical functionality and provide structural information at the same time with high spatial resolution. This is possible by using tip‐enhanced Raman spectroscopy (TERS), but due to its limited depth information, TERS is usually restricted to investigations of the nanoparticle surface. Here, TERS experiments are established on polystyrene nanoparticles (PS NPs) after resin embedding and microtome slicing. With that, unique access to their internal morphological features is gained, and thus, enables differentiation between information obtained for core‐ and shell‐regions. Complementary information is obtained by means of transmission electron microscopy (TEM) and from force–distance curve based atomic force microscopy (FD‐AFM). This multimodal approach achieves a high degree of discrimination between the resin and the polymers used for nanoparticle formulation. The high potential of TERS combined with advanced AFM spectroscopy tools to probe the mechanical properties is applied for quality control of the resin embedding procedure.     

Biokompatible und bioaktive polymere Beschichtungen

Biocompatible and bioactive polymer surface for most biomedical applications of polymers biocompatible surface properties are highly needed. here we present various methods to immobilize biocompatible and also bioactive hydrogels on polymer surfaces. In one approach, macroinitiator‐based reactive layers are attached to polycondensates which are able to allow the grafting‐from of various hydrogels. on the other hand, it is possible to immobilize hydrophilic polymers on various substrates by plasma and e‐beam treatment. Stability, swelling and biocompatibility of the polymer films could be verified. by incorporating ph and thermo‐responsive groups, it is possible additionally to control the swelling behaviour by external triggers.     

Direct Probing of Polarization Charge at Nanoscale Level

Ferroelectric materials possess spontaneous polarization that can be used for multiple applications. Owing to a long‐term development of reducing the sizes of devices, the preparation of ferroelectric materials and devices is entering the nanometer‐scale regime. Accordingly, to evaluate the ferroelectricity, there is a need to investigate the polarization charge at the nanoscale. Nonetheless, it is generally accepted that the detection of polarization charges using a conventional conductive atomic force microscopy (CAFM) without a top electrode is not feasible because the nanometer‐scale radius of an atomic force microscopy (AFM) tip yields a very low signal‐to‐noise ratio. However, the detection is unrelated to the radius of an AFM tip and, in fact, a matter of the switched area. In this work, the direct probing of the polarization charge at the nanoscale is demonstrated using the positive‐up‐negative‐down method based on the conventional CAFM approach without additional corrections or circuits to reduce the parasitic capacitance. The polarization charge densities of 73.7 and 119.0 µC cm^?2 are successfully probed in ferroelectric nanocapacitors and thin films, respectively. The obtained results show the feasibility of the evaluation of polarization charge at the nanoscale and provide a new guideline for evaluating the ferroelectricity at the nanoscale.     

Fabrication of nanostructure on a polymer film using atomic force microscope

SPM based lithographic techniques have been developed to pattern various substrates such as metals, semiconductors, and organic/polymer films due to its simplicity and high spatial precision nanostructure. Fabrication of nanostructure using polymeric materials is a key technique for the development of nanodevices. Here, we report the fabrication of nanostructures from polyacrylicacid (PAA) and polymethacrylicacid (PMAA) film on a silicon substrate using atomic force microscope (AFM). The formation of the nanopattern from the polymer film was studied using electrostatic nanolithography and the optimization of the conditions for nanopatterning of the polymer film was investigated with respect to the applied potential and translational speed of the AFM tip. The nanostructure of size 28 nm was created using the biased AFM tip on the PMAA film coated on Si(100) substrate and found that this method is a direct and reliable method to produce uniform nanostructures on a polymer film.     

Hydrogels Constructed from Engineered Proteins

Due to their various potential biomedical applications, hydrogels based on engineered proteins have attracted considerable interest. Benefitting from significant progress in recombinant DNA technology and protein engineering/design techniques, the field of protein hydrogels has made amazing progress. The latest progress of hydrogels constructed from engineered recombinant proteins are presented, mainly focused on biorecognition‐driven physical hydrogels as well as chemically crosslinked hydrogels. The various bio‐recognition based physical crosslinking strategies are discussed, as well as chemical crosslinking chemistries used to engineer protein hydrogels, and protein hydrogels' various biomedical applications. The future perspectives of this fast evolving field of biomaterials are also discussed.     

Preparation and tribological characteristics of sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane TiO2 films

Silane coupling reagent (3-mercaptopropyl trimethoxysilane (MPTS)) was prepared on silicon substrate to form two-dimensional self-assembled monolayer (SAM) and the terminal–SH group in the film was in situ oxidised to–SO₃H group to endow the film with good chemisorption ability. Thus, TiO₂ thin films were deposited on the oxidised MPTS-SAM to form composite thin films, making use of the chemisorption ability of the–SO₃H group. Atomic force microscope (AFM), and contact angle measurements were used to characterise TiO₂ films. Adhesive force and friction force of TiO₂ thin films and silicon substrate were measured under various applied normal loads and scanning speed of AFM tip. Results showed that the friction force increased with applied normal loads and scanning speed of AFM tip. In order to study the effect of capillary force, tests were performed in various relative humidity (RH). Results showed that the adhesive force of silicon substrate increases with RH and the adhesive force of TiO₂ thin films only increases slightly with RH. Research showed that surfaces with more hydrophobic property revealed the lower adhesive and friction forces.     

Nano-tribological characteristics of TiO<Subscript>2</Subscript> films on 3-mercaptopropyl trimethoxysilane sulfonated self-assembled monolayer

Silane coupling reagent (3-mercaptopropyl trimethoxysilane (MPTS)) was used to prepare twodimensional self-assembled monolayer (SAM) on silicon substrate. The terminal -SH group was in situ oxidized to −SO₃H group to endow the film with good chemisorption ability. Then TiO₂ thin films were deposited on the oxidized MPTS-SAM to form composite thin films, making use of the chemisorption ability of the −SO₃H group. Atomic force microscope (AFM) and contact angle measurements were used to characterize TiO₂ films. Adhesive force and friction force of TiO₂ thin films and silicon substrate were measured under various applied normal loads and scanning speed of AFM tip. Results showed that the friction force increased with applied normal loads and scanning speed of AFM tip. In order to study the effect of capillary force, tests were performed in various relative humidities. Results showed that the adhesive force of silicon substrate increases with relative humidities and the adhesive force of TiO₂ thin films only increases slightly with relative humidity. Research showed that surfaces with more hydrophobic property revealed the lower adhesive and friction forces.     

Junction formation in YBaCuO thin films by scanning probe technologies

A novel method of Josephson junction formation using atomic force microscopy (AFM) is presented. We have investigated surface modifications of YBaCuO films by using AFM with applied voltages. Ridge structures have been observed at the surface of YBaCuO at applied voltages between 4V and 10V, the narrowest ridge line width fabricated being 150nm. The mechanism of the formation of the ridge structure is discussed. Current-voltage characteristics of AFM modified microbridges in YBaCuO thin films have been measured and analyzed.     

Adaptable Hydrogel Networks with Reversible Linkages for Tissue Engineering

Adaptable hydrogels have recently emerged as a promising platform for three‐dimensional (3D) cell encapsulation and culture. In conventional, covalently crosslinked hydrogels, degradation is typically required to allow complex cellular functions to occur, leading to bulk material degradation. In contrast, adaptable hydrogels are formed by reversible crosslinks. Through breaking and re‐formation of the reversible linkages, adaptable hydrogels can be locally modified to permit complex cellular functions while maintaining their long‐term integrity. In addition, these adaptable materials can have biomimetic viscoelastic properties that make them well suited for several biotechnology and medical applications. In this review, an overview of adaptable‐hydrogel design considerations and linkage selections is presented, with a focus on various cell‐compatible crosslinking mechanisms that can be exploited to form adaptable hydrogels for tissue engineering.     

Single-step electrochemical nanolithography of metal thin films by localized etching with an AFM tip

This work introduces electrochemical nanolithography (ENL), a single-step method in which a metal thin film is locally etched without application of a mask on a 100?nm length scale with an electrochemical atomic force microscope (AFM). The method requires the application of ultra-short voltage pulses on the tip (nanosecond range duration, 2-4?V amplitude), while both the sample and the metalized tip are under independent potentiostatic control for full control of interface reactions in an AFM electrochemical cell. It is demonstrated that Cu films as well as Co and Cu/Co sandwich magnetic films can be patterned if negative voltage pulses are applied to the tip. This method also applies to films deposited on an insulating substrate. Moreover the lateral dimension of lithographed structures is tunable, from a few micrometers down to 150?nm, by appropriate choice of ENL conditions. Simulation of the dissolution process is discussed.     

Integrating an ultramicroelectrode in an AFM cantilever: combined technology for enhanced information

We present a novel approach to develop and process a microelectrode integrated in a standard AFM tip. The presented fabrication process allows the integration of an electroactive area at an exactly defined distance above of the end of a scanning probe tip and the subsequent remodeling and sharpening of the original AFM tip using a focused ion beam (FIB) technique (See ref 1 for patent information). Thus, the functionality of scanning electrochemical microscopy (SECM) can be integrated into any standard atomic force microscope (AFM). With the demonstrated approach, a precisely defined and constant distance between the microelectrode and the sample surface can be obtained, alternatively to the indirect determination of this distance usually applied in SECM experiments. Hence, a complete separation of the topographical information and the electrochemical signal is possible. The presented technique is a significant step toward electrochemical imaging with submicrometer electrodes as demonstrated by the development of the first integrated frame submicroelectrode.     

Electrical Scanning Probe Microscopy on Active Organic Electronic Devices

Polymer‐ and small‐molecule‐based organic electronic devices are being developed for applications including electroluminescent displays, transistors, and solar cells due to the promise of low‐cost manufacturing. It has become clear that these materials exhibit nanoscale heterogeneities in their optical and electrical properties that affect device performance, and that this nanoscale structure varies as a function of film processing and device‐fabrication conditions. Thus, there is a need for high‐resolution measurements that directly correlate both electronic and optical properties with local film structure in organic semiconductor films. In this article, we highlight the use of electrical scanning probe microscopy techniques, such as conductive atomic force microscopy (c‐AFM), electrostatic force microscopy (EFM), scanning Kelvin probe microscopy (SKPM), and similar variants to elucidate charge injection/extraction, transport, trapping, and generation/recombination in organic devices. We discuss the use of these tools to probe device structures ranging from light‐emitting diodes (LEDs) and thin‐film transistors (TFT), to light‐emitting electrochemical cells (LECs) and organic photovoltaics.     

Microwave‐Assisted Regeneration of Single‐Walled Carbon Nanotubes from Carbon Fragments

Direct growth of chirality‐controlled single‐walled carbon nanotubes (SWNTs) with metal catalyst free strategy, like cloning or epitaxial growth, has suffered from the low efficiency. The underlying problem is the activation of seed edge. Here an unexpectedly efficient microwave‐assisted pathway to regenerate SWNTs from carbon fragments on SiO₂/Si substrate is demonstrated via Raman spectroscopy and atomic force microscope (AFM) characterization. In this attempt, microwave irradiation provides fast heating to remove polar groups bonded to carbon nanotubes and reduce the spontaneous closure of tubes’ open ends. The survived SWNT and carbon fragments connected to it after plasma treatment are simply microwaved and then they serve as the template for regeneration. Scanning electron microscope and AFM characterizations indicate that the efficiency of the regeneration can reach 100%. And the regenerated SWNT has been proved without any change in chirality compared to the original SWNT. Electrical measurements on regenerated carbon nanotube films indicate 1 and 2 times increase in on/off ratio and on‐state current respectively than original carbon nanotube films obtained from solution‐phase separation, confirming the improvement of SWNT's quality. The microwave‐assisted regeneration is found to be highly effective and would be applied to improve the cloning efficiency of carbon nanotubes potentially.     

Amino Acids and Peptide‐Based Supramolecular Hydrogels for Three‐Dimensional Cell Culture

Supramolecular hydrogels assembled from amino acids and peptide‐derived hydrogelators have shown great potential as biomimetic three‐dimensional (3D) extracellular matrices because of their merits over conventional polymeric hydrogels, such as non‐covalent or physical interactions, controllable self‐assembly, and biocompatibility. These merits enable hydrogels to be made not only by using external stimuli, but also under physiological conditions by rationally designing gelator structures, as well as in situ encapsulation of cells into hydrogels for 3D culture. This review will assess current progress in the preparation of amino acids and peptide‐based hydrogels under various kinds of external stimuli, and in situ encapsulation of cells into the hydrogels, with a focus on understanding the associations between their structures, properties, and functions during cell culture, and the remaining challenges in this field. The amino acids and peptide‐based hydrogelators with rationally designed structures have promising applications in the fields of regenerative medicine, tissue engineering, and pre‐clinical evaluation.     

Dynamic Hyaluronan Hydrogels with Temporally Modulated High Injectability and Stability Using a Biocompatible Catalyst

Injectable and biocompatible hydrogels have become increasingly important for cell transplantation to provide mechanical protection of cells during injection and a stable scaffold for cell adhesion post‐injection. Injectable hydrogels need to be easily pushed through a syringe needle and quickly recover to a gel state, thus generally requiring noncovalent or dynamic cross‐linking. However, a dilemma exists in the design of dynamic hydrogels: hydrogels with fast exchange of cross‐links are easier to eject using less force, but lack long‐term stability; in contrast, slow exchange of cross‐links improves stability, but compromises injectability and thus the ability to protect cells under flow. A new concept to resolve this dilemma using a biocompatible catalyst to modulate the dynamic properties of hydrogels at different time points of application to have both high injectability and high stability is presented. Hyaluronic acid based hydrogels are formed through dynamic covalent hydrazone cross‐linking in the presence of a biocompatible benzimidazole‐based catalyst. The catalyst accelerates the formation and exchange of hydrazone bonds, enhancing injectability, but rapidly diffuses away from the hydrogel after injection to retard the exchange and improve the long‐term stability for cell culture.     

Batch fabrication of atomic force microscopy probes with recessed integrated ring microelectrodes at a wafer level

A batch fabrication process at the wafer-level integrating ring microelectrodes into atomic force microscopy (AFM) tips is presented. The fabrication process results in bifunctional scanning probes combining atomic force microscopy with scanning electrochemical microscopy (AFM-SECM) with a ring microelectrode integrated at a defined distance above the apex of the AFM tip. Silicon carbide is used as AFM tip material, resulting in reduced mechanical tip wear for extended usage. The presented approach for the probe fabrication is based on batch processing using standard microfabrication techniques, which provides bifunctional scanning probes at a wafer scale and at low cost. Additional benefits of batch fabrication include the high processing reproducibility, uniformity, and tuning of the physical properties of the cantilever for optimized AFM dynamic mode operation. The performance of batch-fabricated bifunctional probes was demonstrated by simultaneous imaging micropatterned platinum structures at a silicon dioxide substrate in intermittent (dynamic) and contact mode, respectively, and feedback mode SECM. In both, intermittent and contact mode, the bifunctional probes provided reliable correlated electrochemical and topographical data. In addition, simulations of the diffusion-limited steady-state currents at the integrated electrode using finite element methods were performed for characterizing the developed probes.