S. Kassavetis

Evidence for graphite-like hexagonal AlN nanosheets epitaxially grown on single crystal Ag(111)

Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced energy bandgap as expected for hexagonal AlN.

Optical Properties and Plasmonic Performance of Titanium Nitride

Titanium nitride (TiN) is one of the most well-established engineering materials nowadays. TiN can overcome most of the drawbacks of palsmonic metals due to its high electron conductivity and mobility, high melting point and due to the compatibility of its growth with Complementary Metal Oxide Semiconductor (CMOS) technology. In this work, we review the dielectric function spectra of TiN and we evaluate the plasmonic performance of TiN by calculating (i) the Surface Plasmon Polariton (SPP) dispersion relations and (ii) the Localized Surface Plasmon Resonance (LSPR) band of TiN nanoparticles, and we demonstrate a significant plasmonic performance of TiN.

Oxygen-plasma-modified biomimetic nanofibrous scaffolds for enhanced compatibility of cardiovascular implants

Summary Electrospun nanofibrous scaffolds have been extensively used in several biomedical applications for tissue engineering due to their morphological resemblance to the extracellular matrix (ECM). Especially, there is a need for the cardiovascular implants to exhibit a nanostructured surface that mimics the native endothelium in order to promote endothelialization and to reduce the complications of thrombosis and implant failure. Thus, we herein fabricated poly-ε-caprolactone (PCL) electrospun nanofibrous scaffolds, to serve as coatings for cardiovascular implants and guide tissue regeneration. Oxygen plasma treatment was applied in order to modify the surface chemistry of the scaffold and its effect on cell attachment and growth was evaluated. The conditions of the surface modification were properly adjusted in order to define those conditions of the treatment that result in surfaces favorable for cell growth, while maintaining morphological integrity and mechanical behavior. Goniometry (contact angle measurements), scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) measurements were used to evaluate the morphological and chemical changes induced by the plasma treatment. Moreover, depth-sensing nanoindentation was performed to study the resistance of the plasma-treated scaffolds to plastic deformation. Lastly, the cell studies indicated that all scaffolds were cytocompatible, with the plasma-treated ones expressing a more pronounced cell viability and adhesion. All the above findings demonstrate the great potential of these biomimetic tissue-engineering constructs as efficient coatings for enhanced compatibility of cardiovascular implants.

Self-assembled plasmonic templates produced by microwave annealing: applications to surface-enhanced Raman scattering

Perhaps the simplest method for creating metal nanoparticles on a substrate is by driving their self-assembly with the thermal annealing of a thin metal film. By properly tuning the annealing parameters one hopes to discover a recipe that allows the pre-determined design of the NP arrangement. However, thermal treatment is known for detrimental effects and is not really the manufacturers route of choice when it comes to large-scale applications. An alternative method is the use of microwave annealing, a method that has never been applied for metal processing, due to the high reflectance of microwave radiation at the surface of a metal. However, in this work we challenge the widely used nanostructuring methods by proving the microwaves annealing ability to produce plasmonic templates, out of extremely thin metal films, by simply using a domestic microwave oven apparatus. We show that this process is generic and independent of the deposition method used for the metal and we further quantify the suitability of these plasmonic templates for use in surface-enhanced Raman scattering applications.

Plasmonic spectral tunability of conductive ternary nitrides

Conductive binary transition metal nitrides, such as TiN and ZrN, have emerged as a category of promising alternative plasmonic materials. In this work, we show that ternary transition metal nitrides such as TixTa1−xN, TixZr1−xN, TixAl1−xN, and ZrxTa1−xN share the important plasmonic features with their binary counterparts, while having the additional asset of the exceptional spectral tunability in the entire visible (400–700 nm) and UVA (315–400 nm) spectral ranges depending on their net valence electrons. In particular, we demonstrate that such ternary nitrides can exhibit maximum field enhancement factors comparable with gold in the aforementioned broadband range. We also critically evaluate the structural features that affect the quality factor of the plasmon resonance and we provide rules of thumb for the selection and growth of materials for nitride plasmonics.

The WebLabs of the University of Cambridge: A study of securing remote instrumentation

Safe deployment of web interfaces for remote instrumentation requires that the laboratory system be protected from harmful manipulation by end users or attacks from malicious software over the internet. Industrial control systems, although highly relevant to contemporary engineering education and an essential component of many remote experiments, are typically only designed to run in a secured local area network and cannot safely be exposed to the internet because they lack a sufficiently robust security infrastructure. They also typically require the installation of proprietary software on the end user system which is an obstacle for deployment in learning scenarios at universities. Facing these challenges when bringing the Chemical Engineering WebLabs at the University of Cambridge online, the Computing Center of the University of Stuttgart and the University of Cambridge developed a framework to allow the secure deployment of industrial controller software in remote learning applications; this framework is generic, has a low-barrier for students as it only requires an internet browser and Java™ installation, and it satisfies the high security demands of most university infrastructure providers. Furthermore, the framework has the potential to be applied to almost any remote laboratory setup and is compatible with all commonly-used operating systems at the user end.

Photoluminescence enhancement of ZnO via coupling with surface plasmons on Al thin films

We present that the ultra-violet emission of ZnO can be enhanced, as much as six-times its integral intensity, using an Al thin interlayer film between the Si substrate and ZnO thin film and a post-fabrication laser annealing process. The laser annealing is a cold process that preserves the chemical state and integrity of the underlying aluminum layer, while it is essential for the improvement of the ZnO performance as a light emitter and leads to enhanced emission in the visible and in the ultraviolet spectral ranges. In all cases, the metal interlayer enhances the intensity of the emitted light, either through coupling of the surface plasmon that is excited at the Al/ZnO interface, in the case of light-emitting ZnO in the ultraviolet region, or by the increased back reflection from the Al layer, in the case of the visible emission. In order to evaluate the process and develop a solid understanding of the relevant physical phenomena, we investigated the effects of various metals as interlayers (Al, Ag, a...

Nanomechanical and Nanotribological Properties of Silicon Oxide Thin Films on Polymeric Membranes

The present work reports on the nanomechanical properties of SiOx films, 60 and 160 nm thick, (with x~1.7) deposited by Electron Beam Evaporation onto poly(ethylene terephthalate) (PET) membranes. The nanomechanical properties of both SiOx films and PET were examined in light of depth-sensing nanoindentation experiments, under conditions of maximum contact load in the range of 0.03-1 mN. The continuous stiffness measurement technique was used to monitor the hardness (H) and the elastic modulus (E) as a function of the penetration depth. The SiOx/PET system exhibit a Η of 3 GPa, increased by a factor of 9 compared to the hardness of the uncoated PET, while Ε was found to increase by a factor of 5 and provide sufficient adhesion to the PET substrate. Low load scratch tests were performed in the load range from 0.6 to 7.5mN to explore the deformation mechanisms and friction behavior of a SiOx/PET system and SiOx adhesion to the PET substrate. The low friction coefficients (0.02-0.2) obtained at low (<3mN) normal (contact) loads indicated an almost full elastic response for SiOx/PET. In the normal load range 3-7.5 mN, the coefficient of friction increased with normal load (0.2-0.3), suggesting that plastic deformation becomes an important mode of deformation. The dependence of the elastic/plastic deformation on the normal load was confirmed by nanoindentation testing.

Structural factors determining the nanomechanical performance of transition metal nitride films

Chromium nitride (CrN) and Titanium nitride (TiN) thin films were deposited employing unbalanced magnetron sputtering (UBMS) for various values of substrate bias voltage (V b ). The structural characterization of the films in terms of phase identification and the density determination was achieved utilizing X-Ray techniques (XRD and XRR respectively), while the internal stresses were calculated by the change of the substrates curvature using Stoneys equation. The nanomecahnical properties of the films (hardness and elastic modulus) were investigated using the Nanoindentation (NI) technique in the Continuous Stiffness Measurement (CSM) configuration. According to the NI results the hardness of the films ranges between between 15–25 GPa while the elastic modulus between 180–250 GPa. The analysis revealed that the hardness of the films is maximum when their orientation is pure (either [111] or [100]), while it is minimized under mixed orientation regime. Furthermore, the hardness of the films increases as the internal compressive stresses and the mass density increase. The latter can be validated through the comparison of the results concerning the above films to reported results for TiN films prepared by balanced magnetron sputtering (BMS)

Theoretical Considerations and a Mathematical Model for the Analysis of the Biomechanical Response of Human Keratinized Oral Mucosa.

Removable complete and partial dentures are supported by the residual alveolar ridges consisting of mucosa, submucosa, periosteum, and bone. An understanding of the biomechanical behavior of the oral mucosa is essential in order to improve the denture-bearing foundations for complete and partially edentulous patients. The purpose of this paper was to examine the biomechanical behavior of the soft tissues supporting a removable denture and develop a model for that reason. Keratinized oral mucosa blocks with their underlying bone were harvested from the maxillary palatal area adjacent to the edentulous ridges of a cadaver. The compressive response of the oral mucosa was tested by using atomic force microscopy. The specimens were first scanned in order their topography to be obtained. The mechanical properties of the specimens were tested using a single crystal silicon pyramidal tip, which traversed toward the keratinized oral mucosa specimens. Loading-unloading cycles were registered and four mathematical models were tested using MATLAB to note which one approximates the force-displacement curve as close as possible: a. spherical, b. conical, c. third order polynomial, d. Murphy (fourth order polynomial, non-linear Hertzian based). The third order polynomial model showed the best accuracy in representing the force-displacement data of the tested specimens. A model was developed in order to analyze the biomechanical behavior of the human oral keratinized mucosa and obtain information about its mechanical properties.