[show abstract][hide abstract] ABSTRACT: Nickel and other metal nanoparticles are known to be active as catalysts in the synthesis of carbon nanofibers. In this paper we investigate how dewetting and break-up of nickel thin films depends on film thickness, film–substrate interaction and pretreatment conditions. This is evaluated for films evaporated on oxidized silicon and fused silica substrates with or without tantalum coating, which were subsequently exposed to different pretreatment atmospheres (vacuum, nitrogen, air and hydrogen; 1 h, 650 °C). Atomic force microscopy, scanning electron microscopy and energy dispersive X-ray analysis were used to characterize the films. Pretreated Ni films were subjected to a thermal catalytic chemical vapor deposition procedure with brief ethylene exposures (0.5–3 min, 635 °C). It was found that only on the spherical nanoparticles originating from a hydrogen pretreatment of a Ni film with Ta adhesion layer, homogeneously distributed, randomly-oriented, well-attached, and semi-crystalline carbon nanofibers be synthesized.
Thin Solid Films 05/2013; 534:341-347. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: Surfaces that repel both water and oil effectively (contact angles > 150°) are rare. Here we detail the microfabrication method of silicon surfaces with such properties. The method is based on careful tuning of the process conditions in a reactive etching protocol. We investigate the influence of SF6, O2 and CHF3 gases during the etching process using the same pitch of a photolithographic mask. Varying the loading conditions during etching, we optimized the conditions to fabricate homogeneous pedestal-like structures. The roughness of the microstructures could also effectively be controlled by tuning the dry plasma etching conditions. The wetting behavior of the resulting microstructures was evaluated in terms of the water and oil contact angles. Excitingly, the surfaces can be engineered from superhydrophobic to omniphobic by variation of the aforementioned predefined parameters.
Journal of Micromechanics and Microengineering 12/2012; 23(2):025004. · 1.79 Impact Factor
[show abstract][hide abstract] ABSTRACT: h i g h l i g h t s " Carbon nano fibers (CNFs) were grown on Si based Ni thin films. " Adhesion of the CNFs varies with oxidation state and composition of the substrate. " We characterized the wettability (water contact angle) on this surfaces. " The wettability varies between superhydrophobic and slightly hydrophilic. g r a p h i c a l a b s t r a c t 80 120 160 θ a b s t r a c t This paper describes how layers of carbon nanofibers (CNFs) with a controllable wettability can be synthe-sized by means of thermal catalytic chemical vapor deposition on nickel-based thin films on oxidized sil-icon supports. In order to achieve well-adhesive CNF-layers with a uniform surface coverage and tunable wettability without the necessity of post-synthesis treatments, a series of synthesis parameters is investi-gated: the pretreatment atmosphere (hydrogen or oxygen; 2 h, 500 °C), the use of ethylene (C 2 H 4) or an eth-ylene/hydrogen (C 2 H 4 /H 2) mixture as hydrocarbon source, and the growth time (in the range 5–60 min). Fast and uniform CNF-growth is found on reduced Ni-based thin films using C 2 H 4 /H 2 at a synthesis tem-perature of 635 °C. The CNF-layers on Ni are superhydrophobic or highly hydrophobic for all growth times, but their adhesion to the support is poor for growth times >30 min. In contrast, the adhesion of CNF-layers on Ni/Ta is excellent. Moreover, the wettability of these as-synthesized CNF-layers can be controlled by var-iation of the growth time: from superhydrophobic (610 min) to hydrophilic (P50 min). CNF-layers with such tunable wettability can be easily integrated in flow channels of silicon-based microfluidic systems, thereby offering numerous applications. Ó 2012 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 11/2012; · 3.47 Impact Factor
[show abstract][hide abstract] ABSTRACT: Evaporation-driven particle self-assembly can be used to generate three-dimensional microstructures. We present a unique method to create colloidal microstructures in which we can control the amount of particles and their packing fraction. To this end, we evaporate colloidal dispersion droplets on a special type of superhydrophobic microstructured surface, on which the droplet remains in Cassie-Baxter state during the entire evaporative process. The remainders of the droplet consist of a massive spherical cluster of the microspheres, with diameters ranging from a few tens up to several hundreds of microns. We present scaling arguments to show how the final particle packing fraction of these balls depends on the dynamics of the droplet evaporation, particle size, and number of particles in the system.
Proceedings of the National Academy of Sciences 09/2012; 109(41):16455-8. · 9.74 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper investigates the self-assembly of electrically charged, millimeter-sized polymer objects, agitated mechanically inside planar bounded containers. The objects charge through contact electrification and self-assemble into rosettes, chains, or macroscopic Coulombic crystals, depending on the experimental conditions used. Surfaces that exhibit contact angles close to 180 for both polar and non-polar solvents are rare. Here we report the fabrication of such ''omniphobic'' surfaces by photolithography. We investigate their stability against a so-called wetting transition during evaporation of millimetric water droplets by systematically varying the shape and surface roughness of the micropillars on the surface. We show that a low edge curvature of the top of the micropillars strongly delays the transition, while it completely disappears when the surface roughness is increased. We compare these experimental findings with existing models that describe the Cassie–Baxter to Wenzel transition and conclude that new models are needed which include the hurdle of an energy barrier for the wetting transition. Our results reveal that by increasing the roughness of the micropillars we do not affect the apparent equilibrium contact angle of the droplets. The dynamic robustness of the surface is, however, dramatically enhanced by an increase of the surface roughness.
[show abstract][hide abstract] ABSTRACT: The local deposition of catalysts is desired in a wide range of catalytic microsystems (microreactors and sensors). In this study, we investigate technologies enabling deposition and patterning of catalyst thin films in a manner compatible with standard micromachining processes. We evaluate and compare deposition techniques based on a combination of a self-assembly, soft-lithography and conventional micromachining. Platinum (Pt) and palladium (Pd) were used as model catalysts, both as a sputtered thin film and as nanoparticles supported on γ-alumina. The thin films were characterized and tested in terms of their catalytic activity based on CO chemisorption measurements, stability and reproducibility.
Journal of Micromechanics and Microengineering 03/2012; 22(4):045023. · 1.79 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nanopatterns on titanium may enhance endosseous implant biofunctionality. To enable biological studies to prove this hypothesis, we developed a scalable method of fabricating nanogrooved titanium substrates. We defined nanogrooves by nanoimprint lithography (NIL) and a subsequent pattern transfer to the surface of ASTM grade 2 bulk titanium applying a soft-mask for chlorine-based reactive ion etching (RIE). With respect to direct write lithographic techniques the method introduced here is fast and capable of delivering uniformly patterned areas of at least 4 cm(2). A dedicated silicon nanostamp process has been designed to generate the required thickness of the soft-mask for the NIL-RIE pattern transfer. Stamps with pitch sizes from 1000 nm down to 300 nm were fabricated using laser interference lithography (LIL) and deep cryogenic silicon RIE. Although silicon nanomachining was proven to produce smaller pitch sizes of 200 nm and 150 nm respectively, successful pattern transfer to titanium was only possible down to a pitch of 300 nm. Hence, the smallest nanogrooves have a width of 140 nm. An x-ray photoelectron spectroscopy study showed that only very few contaminations arise from the fabrication process and a cytotoxicity assay on the nanopatterned surfaces confirmed that the obtained nanogrooved titanium specimens are suitable for in vivo studies in implantology research.
[show abstract][hide abstract] ABSTRACT: In this work the applicability of X-ray fluorescence spectroscopy (XRF) for fast, accurate and non-destructive determination of the thickness of a variety of single-layer and multi-layer metal thin films deposited on glass and silicon is investigated. Data obtained with XRF is compared with information from profilometry and images from scanning electron microscopy (SEM). Whereas thickness determinations based on profilometry and cross-sectional SEM-imaging have restrictions with respect to thickness of metal stacks or hardness of the metals, XRF has no such limitations. Moreover, XRF can discriminate between sublayers in a multi-layer film, and can also be utilized for compositional analysis and density estimations. Good agreement between thickness data obtained with XRF, profilometry and SEM-images is found, under the justifiable assumption that the density of sputter-deposited and evaporated thin films is ca. 5% below that of bulk metals. Similar XRF-results are found for non-patterned areas (64 mm2 metal) as well as lithographically patterned areas containing a series of small metal lines (total metal surface ca. 8 mm2). As a consequence, it is concluded that XRF is a versatile technique for analysis, verification, control or evaluation of the thickness, density or (elemental) composition of thin metal film line-patterns, during their fabrication as well as prior or post to applications.
Thin Solid Films 01/2012; 520(6):1740–1744. · 1.60 Impact Factor
[show abstract][hide abstract] ABSTRACT: Organised nanotopography mimicking the natural extracellular matrix can be used to control morphology, cell motility, and differentiation. However, it is still unknown how specific cell types react with specific patterns. Both initial adhesion and preferential cell migration may be important to initiate and increase cell locomotion and coverage with cells, and thus achieve an enhanced wound healing response around an implantable material. Therefore, the aim of this study was to evaluate how MC3T3-E1 osteoblast initial adhesion and directional migration are influenced by nanogrooves with pitches ranging from 150 nm up to 1000 nm. In this study, we used a multi-patterned substrate with five different groove patterns and a smooth area with either a concentric or radial orientation. Initial cell adhesion measurements after 10 s were performed using atomic force spectroscopy-assisted single-cell force spectroscopy, and demonstrated that nascent cell adhesion was highly induced by a 600 nm pitch and reduced by a 150 nm pitch. Addition of RGD peptide significantly reduced adhesion, indicating that integrins and cell adhesive proteins (e.g. fibronectin or vitronectin) are key factors in specific cell adhesion on nanogrooved substrates. Also, cell migration was highly dependent on the groove pitch; the highest directional migration parallel to the grooves was observed on a 600 nm pitch, whereas a 150 nm pitch restrained directional cell migration. From this study, we conclude that grooves with a pitch of 600 nm may be favourable to enhance fast wound closure, thereby promoting tissue regeneration.
[show abstract][hide abstract] ABSTRACT: In this work, an experimental investigation of the single- and multiphase flows of two sets of fluids, CO2–ethanol and CO2–methanol, in a non-adiabatic microfluidic T-junction is presented. The operating conditions ranged from 7 to 18 MPa, and from 294 to 474 K. The feed mass fraction of CO2 in the mixtures was 0.95 and 0.87, respectively. Under these operating conditions, CO2 was either in liquid, gas or supercritical state; and the mixtures experienced a miscible single phase or a vapour–liquid equilibrium (VLE), with two separated phases. Taylor, annular and wavy were the two-phase flow regimes obtained in the VLE region. In the single phase region, the observed flows were classified into standard single-phase flows, “pseudo” two-phase flows and local phenomena in the T-junction. Flow regime maps were generated, based on temperature and pressure conditions. Two-phase flow void fractions and several parameters of Taylor flow were analysed. They showed a clear dependency on temperature, but were mostly insensitive to pressure. A continuous accumulation of liquid, either in the CO2 channel or at the CO2-side wall after the T-junction, disturbed most of the experiments in VLE conditions by randomly generating liquid plugs. This phenomenon is analysed, and capillary and wetting effects due to local Marangoni stresses are suggested as possible causes.
Microfluidics and Nanofluidics 01/2012; 12(6). · 3.22 Impact Factor
[show abstract][hide abstract] ABSTRACT: We explore the functionality of a complete chemoemitter platform mimicking the biosynthetic pathways and release of female sex pheromones of the Spodoptera littoralis moth. This artificial gland system consists of a microreactor in which the pheromone biosynthesis takes place, and a micromachined evaporator that releases ratiometrically-encoded blend into the environment. The artificial gland was implemented for the investigation and detection of patterns by highly sensitive olfaction system of male moths in electroantennographic and behavioral assays. Thus, a new information emission scheme by utilizing semiochemicals as a data carrier was demonstrated.
[show abstract][hide abstract] ABSTRACT: The observation of charge injection from carbon nanofibers (CNFs) into liquid hexane under ambient conditions is reported. A CNF-coated electrode and a counter electrode are brought into micrometer proximity in a quasi-parallel geometry using a strain-gauge-based proximity sensor. Controlled charge injection is obtained at interelectrode distances of 4, 6, 9, and 15 μm. The resulting emission current shows an onset of about 3 V/μm , and it follows the Fowler-Nordheim behavior. The work reported here opens new applications for free electron chemistry in liquids and novel liquid field emitter devices.
IEEE Transactions on Electron Devices 11/2011; · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this work we present a low-cost, low-power, small sample volume microcalorimetric sensor for the measurement of reaction heats. The polysilicon-based microcalorimetric sensor combines several advantages: (i) complementary metal oxide semiconductor technology (CMOS) for future integration; (ii) elements of silicon micromachining (MEMS) to control thermal performance; (iii) heterogeneous catalysts for selective detection and analysis of individual gas compounds; and (iv) microfluidics for optimized control over the reaction conditions.
Sensors and Actuators A Physical 10/2011; 169(2):308-316. · 1.84 Impact Factor
[show abstract][hide abstract] ABSTRACT: Thin films of titanium silicide (TiSi2) formed on heavily boron-doped polycrystalline silicon (poly-Si/B+) were applied for the first time for resistive temperature sensing. The temperature sensors exhibited a high-temperature coefficient of resistance of 3.8 × 10−3 °C−1, a linear dependence of resistance on temperature and an excellent thermal and electrical stability up to 800 °C. This work discusses the fabrication method and the morphological and electrical characterization of the TiSi2/poly-Si thin film resistors throughout the stages of its formation.
Journal of Micromechanics and Microengineering 09/2011; 21(10):105022. · 1.79 Impact Factor
[show abstract][hide abstract] ABSTRACT: A supramolecular platform based on self-assembled monolayers (SAMs) has been implemented in a microfluidic device. The system has been applied for the sensing of two different analyte types: biologically relevant phosphate anions and aromatic carboxylic acids, which are important for anthrax detection. A Eu(III)-EDTA complex was bound to β-cyclodextrin monolayers via orthogonal supramolecular host-guest interactions. The self-assembly of the Eu(III)-EDTA conjugate and naphthalene β-diketone as an antenna resulted in the formation of a highly luminescent lanthanide complex on the microchannel surface. Detection of different phosphate anions and aromatic carboxylic acids was demonstrated by monitoring the decrease in red emission following displacement of the antenna by the analyte. Among these analytes, adenosine triphosphate (ATP) and pyrophosphate, as well as dipicolinic acid (DPA) which is a biomarker for anthrax, showed a strong response. Parallel fabrication of five sensing SAMs in a single multichannel chip was performed, as a first demonstration of phosphate and carboxylic acid screening in a multiplexed format that allows a general detection platform for both analyte systems in a single test run with μM and nM detection sensitivity for ATP and DPA, respectively.
International Journal of Molecular Sciences 01/2011; 12(11):7335-51. · 2.46 Impact Factor