Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Hydrolytic and Thermal Stability of Organic Monolayers on Various Inorganic Substrates
    Bhairamadgi, N.S. ; Pujari, S.P. ; Trovela, F.G. ; Debrassi, A. ; Khamis, A.A.M. ; Alonso Carnicero, J.M. ; Zahrani, A.A. Al; Wennekes, T. ; Al-Turaif, H.A. ; Rijn, C.J.M. van; Alhamed, Y.A. ; Zuilhof, H. - \ 2014
    Langmuir 30 (2014)20. - ISSN 0743-7463 - p. 5829 - 5839.
    self-assembled monolayers - hydrogen-terminated silicon - oxidized si(100) surface - alkyl monolayers - nitride surfaces - aluminum-oxide - gold - phosphonate - films - attachment
    A comparative study is presented of the hydrolytic and thermal stability of 24 different kinds of monolayers on Si(111), Si(100), SiC, SiN, SiO2, CrN, ITO, PAO, Au, and stainless steel surfaces. These surfaces were modified utilizing appropriate organic compounds having a constant alkyl chain length (C18), but with different surface-reactive groups, such as 1-octadecene, 1-octadecyne, 1-octadecyltrichlorosilane, 1-octadecanethiol, 1-octadecylamine and 1-octadecylphosphonic acid. The hydrolytic stability of obtained monolayers was systematically investigated in triplicate in constantly flowing aqueous media at room temperature in acidic (pH 3), basic (pH 11), phosphate buffer saline (PBS) and deionized water (neutral conditions), for a period of 1 day, 7 days, and 30 days, yielding 1152 data points for the hydrolytic stability. The hydrolytic stability was monitored by static contact angle measurements and X-ray photoelectron spectroscopy (XPS). The covalently bound alkyne monolayers on Si(111), Si(100), and SiC were shown to be among the most stable monolayers under acidic and neutral conditions. Additionally, the thermal stability of 14 different monolayers was studied in vacuum using XPS at elevated temperatures (25–600 °C). Similar to the hydrolytic stability, the covalently bound both alkyne and alkene monolayers on Si(111), Si(100) and SiC started to degrade from temperatures above 260 °C, whereas on oxide surfaces (e.g., PAO) phosphonate monolayers even displayed thermal stability up to ~500 °C.
    Light-Activated Electroactive Molecule-Based Microcells Confined on a Silicon Surface
    Fabre, B. ; Li, F. ; Scheres, L.M.W. ; Pujari, S.P. ; Zuilhof, H. - \ 2013
    Angewandte Chemie-International Edition 52 (2013)46. - ISSN 1433-7851 - p. 12024 - 12027.
    hydrogen-terminated silicon - oxide-free silicon - information-storage - alkyl monolayers - charge-storage - logic gates - ferrocene - vinylferrocene - communication - capacitance
    Ultrahigh-capacity molecular AND gates provide the potential for the next-generation dynamic random access memory. The ferrocene-terminated monolayer on oxide-free silicon system allows a highly stable and independent switching with both light and potential, yielding precisely such an AND gate.
    Hybrids of Organic Molecules and Flat, Oxide-Free Silicon: High-Density Monolayers, Electronic Properties, and Functionalization
    Li, Y. ; Calder, S.A. ; Yaffe, O. ; Cahen, D. ; Haick, H. ; Kronik, L. ; Zuilhof, H. - \ 2012
    Langmuir 28 (2012)26. - ISSN 0743-7463 - p. 9920 - 9929.
    hydrogen-terminated si(111) - mixed methyl/allyl monolayers - alkyl monolayers - porous silicon - si surfaces - carrier lifetime - label-free - interfaces - hydrosilylation - passivation
    Since the first report of Si–C bound organic monolayers on oxide-free Si almost two decades ago, a substantial amount of research has focused on studying the fundamental mechanical and electronic properties of these Si/molecule surfaces and interfaces. This feature article covers three closely related topics, including recent advances in achieving high-density organic monolayers (i.e., atomic coverage >55%) on oxide-free Si(111) substrates, an overview of progress in the fundamental understanding of the energetics and electronic properties of hybrid Si/molecule systems, and a brief summary of recent examples of subsequent functionalization on these high-density monolayers, which can significantly expand the range of applicability. Taken together, these topics provide an overview of the present status of this active area of research.
    Hexadecadienyl Monolayers on Hydrogen-Terminated Si(III): Faster Monolayer Formation and Improved Surface Coverage Using the Enyne Moiety
    Rijksen, B.M.G. ; Pujari, S.P. ; Scheres, L.M.W. ; Rijn, C.J.M. van; Baio, J.E. ; Weidner, T. ; Zuilhof, H. - \ 2012
    Langmuir 28 (2012)16. - ISSN 0743-7463 - p. 6577 - 6588.
    self-assembled monolayers - silicon surfaces - alkyl monolayers - organic monolayers - x-ray - molecular simulation - visible-light - si - spectroscopy - attachment
    To further improve the coverage of organic monolayers on hydrogen-terminated silicon (H–Si) surfaces with respect to the hitherto best agents (1-alkynes), it was hypothesized that enynes (H–C=C–HC-CH–R) would be even better reagents for dense monolayer formation. To investigate whether the increased delocalization of ß-carbon radicals by the enyne functionality indeed lowers the activation barrier, the kinetics of monolayer formation by hexadec-3-en-1-yne and 1-hexadecyne on H–Si(111) were followed by studying partially incomplete monolayers. Ellipsometry and static contact angle measurements indeed showed a faster increase of layer thickness and hydrophobicity for the hexadec-3-en-1-yne-derived monolayers. This more rapid monolayer formation was supported by IRRAS and XPS measurements that for the enyne show a faster increase of the CH2 stretching bands and the amount of carbon at the surface (C/Si ratio), respectively. Monolayer formation at room temperature yielded plateau values for hexadec-3-en-1-yne and 1-hexadecyne after 8 and 16 h, respectively. Additional experiments were performed for 16 h at 80° to ensure full completion of the layers, which allows comparison of the quality of both layers. Ellipsometry thicknesses (2.0 nm) and contact angles (111–112°) indicated a high quality of both layers. XPS, in combination with DFT calculations, revealed terminal attachment of hexadec-3-en-1-yne to the H–Si surface, leading to dienyl monolayers. Moreover, analysis of the Si2p region showed no surface oxidation. Quantitative XPS measurements, obtained via rotating Si samples, showed a higher surface coverage for C16 dienyl layers than for C16 alkenyl layers (63% vs 59%). The dense packing of the layers was confirmed by IRRAS and NEXAFS results. Molecular mechanics simulations were undertaken to understand the differences in reactivity and surface coverage. Alkenyl layers show more favorable packing energies for surface coverages up to 50–55%. At higher coverages, this packing energy rises quickly, and there the dienyl packing becomes more favorable. When the binding energies are included the difference becomes more pronounced, and dense packing of dienyl layers becomes more favorable by 2–3 kcal/mol. These combined data show that enynes provide the highest-quality organic monolayers reported on H–Si up to now.
    Mimicking the silicon surface: reactivity of silyl radical cations toward nucleophiles
    Rijksen, B.M.G. ; Lagen, B. van; Zuilhof, H. - \ 2011
    Journal of the American Chemical Society 133 (2011)13. - ISSN 0002-7863 - p. 4998 - 5008.
    hydrogen-terminated silicon - organic monolayers - alkyl monolayers - si(111) surfaces - porous silicon - si - functionalization - oligosilanes - attachment - 1-alkenes
    Radical cations of selected low molecular-weight silicon model compounds were obtained by photoinduced electron transfer. These radical cations react readily with a variety of nucleophiles, regularly used in monolayer fabrication onto hydrogen-terminated silicon. From time-resolved kinetics, it was concluded that the reactions proceed via a bimolecular nucleophilic attack to the radical cation. A secondary kinetic isotope effect indicated that the central Si-H bond is not cleaved in the rate-determining step. Apart from substitution products, also hydrosilylation products were identified in the product mixtures. Observation of the substitution products, combined with the kinetic data, point to an bimolecular reaction mechanism involving Si-Si bond cleavage. The products of this nucleophilic substitution can initiate radical chain reactions leading to hydrosilylation products, which can independently also be initiated by dissociation of the radical cations. Application of these data to the attachment of organic monolayers onto hydrogen-terminated Si surfaces via hydrosilylation leads to the conclusion that the delocalized Si radical cation (a surface-localized hole) can initiate the hydrosilylation chain reaction at the Si surface. Comparison to monolayer experiments shows that this reaction only plays a significant role in the initiation, and not in the propagation steps of Si-C bond making monolayer formation
    Photothermal Micro- and Nanopatterning of Organic/Silicon Interfaces
    Klingebiel, B. ; Scheres, L.M.W. ; Franzka, S. ; Zuilhof, H. ; Hartmann, N. - \ 2010
    Langmuir 26 (2010)9. - ISSN 0743-7463 - p. 6826 - 6831.
    self-assembled monolayers - hydrogen-terminated silicon - atomic-force microscope - scanned probe oxidation - organic monolayers - alkylsiloxane monolayers - alkyl monolayers - visible-light - surfaces - attachment
    Photothermal laser processing of organic monolayers on oxide-free silicon substrates under ambient conditions is investigated. Organic monolayers on Si(100) and Si(111) substrates are prepared via hydrosilylation of H-terminated silicon samples in neat 1-hexadecene and 1-hexadecyne, respectively. Laser processing at ¿ = 514 nm and a 1/e2 spot diameter of 2.6 µm results in local decomposition of the monolayers and oxidation of the exposed substrate. In agreement with the high thermal and chemical stability of these monolayers, a thermokinetic analysis of the data from experiments at distinct laser powers and pulse lengths points to a highly activated process. As a result, processing is strongly nonlinear and allows for subwavelength patterning, with line widths between 0.4 and 1.4 µm. Most remarkably, upon fabrication of dense line patterns, narrow organic monolayer stripes with sharp edges and lateral dimensions of 80 nm are formed. This opens up new perspectives in photothermal engineering of organic/silicon interfaces, e.g., for hybrid microelectronic and sensor applications
    Self-Assembly of Organic Monolayers onto Hydrogen-Terminated Silicon: 1-Alkynes Are Better Than 1-Alkenes
    Scheres, L.M.W. ; Giesbers, M. ; Zuilhof, H. - \ 2010
    Langmuir 26 (2010)13. - ISSN 0743-7463 - p. 10924 - 10929.
    covalently attached monolayers - metal-semiconductor diodes - extremely mild attachment - alkyl monolayers - si(111) surface - visible-light - molecular electronics - thermal-reactions - si - alkenes
    Recently, a new method for the preparation of high-quality organic monolayers with 1-alkynes at room temperature in the dark (i.e., without any external activation) was reported. To pinpoint the precise origin of this self-assembly process and to compare the reactivity of 1-alkenes and 1-alkynes toward hydrogen-terminated Si(111) [H-Si(111)], we followed the gradual formation of both monolayers at room temperature by static water contact angle measurements. Subsequently, attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) were used to obtain detailed information about the structure and quality of the resulting monolayers. Our data clearly demonstrate that 1-alkynes are considerably more reactive toward H-Si(111) than 1-alkenes. 1-Alkynes are able to self-assemble into densely packed hydrophobic monolayers without any external activation (i.e., at room temperature under ambient light and even in the dark) whereas for 1-alkenes under the same conditions hardly any reactivity toward H-Si(111) was observed. The self-assembly of 1-alkynes on H-Si(111) at room temperature is explained by three factors: the higher nucleophilicity of 1-alkynes, which results in a facile attack at the electron-hole pairs at the H-Si surface and easy Si-C bond formation, the stabilization of the ß radical by delocalization over the double bond, and the lower-energy barrier encountered for H abstractions
    Light-enhanced microcontact printing of 1-alkynes onto hydrogen-terminated silicon
    Maat, J. ter; Yang, M. ; Scheres, L.M.W. ; Kuypers, S. ; Zuilhof, H. - \ 2010
    Chemical Communications 46 (2010)42. - ISSN 1359-7345 - p. 8005 - 8007.
    self-assembled monolayers - scanning-electron-microscopy - organic monolayers - alkyl monolayers - visible-light - surfaces - 1-alkenes - alkanethiols - attachment - chemistry
    method for the direct patterning of 1-alkynes onto hydrogen-terminated silicon is presented. It combines microcontact printing with illumination through the stamp, and results in the formation of an alkenyl monolayer. The formation of heterogeneous monolayers is demonstrated by subsequent backfilling
    Micro- and Nanopatterning of Functional Organic Monolayers on Oxide-Free Silicon by Laser-Induced Photothermal Desorption
    Scheres, L. ; Klingebiel, B. ; Maat, J. ter; Giesbers, M. ; Jong, J.H.S.G.M. de; Hartmann, N. ; Zuilhof, H. - \ 2010
    Small 6 (2010)17. - ISSN 1613-6810 - p. 1918 - 1926.
    self-assembled monolayers - hydrogen-terminated silicon - supported phospholipid multilayers - atomic-force microscope - scanned probe oxidation - alkylsiloxane monolayers - alkyl monolayers - surfaces - chemistry - dna
    The photothermal laser patterning of functional organic monolayers, prepared on oxide-free hydrogen-terminated silicon, and subsequent backfilling of the laser-written lines with a second organic monolayer that differs in its terminal functionality, is described. Since the thermal monolayer decomposition process is highly nonlinear in the applied laser power density, subwavelength patterning of the organic monolayers is feasible. After photothermal laser patterning of hexadecenyl monolayers, the lines freed up by the laser are backfilled with functional acid fluoride monolayers. Coupling of cysteamine to the acid fluoride groups and subsequent attachment of Au nanoparticles allows easy characterization of the functional lines by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Depending on the laser power and writing speed, functional lines with widths between 1.1 µm and 250 nm can be created. In addition, trifluoroethyl-terminated (TFE) monolayers are also patterned. Subsequently, the decomposed lines are backfilled with a nonfunctional hexadecenyl monolayer, the TFE stripes are converted into thiol stripes, and then finally covered with Au nanoparticles. By reducing the lateral distance between the laser lines, Au-nanoparticle stripes with widths close to 100 nm are obtained. Finally, in view of the great potential of this type of monolayer in the field of biosensing, the ease of fabricating biofunctional patterns is demonstrated by covalent binding of fluorescently labeled oligo-DNA to acid-fluoride-backfilled laser lines, which—as shown by fluorescence microscopy—is accessible for hybridization
    Microcontact Printing onto Oxide-Free Silicon via Highly Reactive Acid Fluoride-Functionalized Monolayers
    Scheres, L.M.W. ; Maat, J. ter; Giesbers, M. ; Zuilhof, H. - \ 2010
    Small 6 (2010)5. - ISSN 1613-6810 - p. 642 - 650.
    self-assembled monolayers - hydrogen-terminated silicon - covalently attached monolayers - metal-semiconductor diodes - linked organic monolayers - extremely mild attachment - alkyl monolayers - porous silicon - si(111) surfaces - click chemistry
    This work describes a new route for patterning organic monolayers on oxide-free silicon by microcontact printing (µCP) on a preformed, reactive, acid-fluoride-terminated monolayer. This indirect printing approach is fast and easily preserves the oxide-free and well-defined monolayer-silicon interface, which is the most important property for potential applications in biosensing and molecular electronics. Water-contact-angle measurements, ellipsometry, attenuated total reflection infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS) demonstrate the formation of the initial acid-fluoride-terminated monolayers without upside-down attachment. Subsequent printing for twenty seconds with an N-hexadecylamine-inked poly(dimethylsiloxane) stamp results in well-defined 5-µm N-hexadecylamide dots, as evidenced by atomic force microscopy and scanning electron microscopy. Printing with a flat stamp allows investigation of the efficiency of amide formation by µCP and water-contact-angle measurements, ellipsometry, and XPS reveal the quantitative conversion of the acid fluoride groups to the corresponding amide within twenty seconds. The absence of silicon oxide, even after immersion in water for 16 h, demonstrates that the oxide-free monolayer-silicon interface is easily preserved by this patterning route. Finally, it is shown by fluorescence microscopy that complex biomolecules, like functionalized oligo-DNA, can also be immobilized on the oxide-free silicon surface via µCP
    Covalent Attachment of Bent-Core Mesogens to Silicon Surfaces
    Scheres, L. ; Achten, R. ; Giesbers, M. ; Smet, L. de; Arafat, A. ; Sudhölter, E.J.R. ; Marcelis, A.T.M. ; Zuilhof, H. - \ 2009
    Langmuir 25 (2009)3. - ISSN 0743-7463 - p. 1529 - 1533.
    hydrogen-terminated silicon - extremely mild attachment - organic monolayers - alkyl monolayers - liquid-crystals - visible-light - si(100) surfaces - shaped molecules - porous silicon - functionalization
    Two vinyl-terminated bent core-shaped liquid crystalline molecules that exhibit thermotropic antiferroelectric SmCPA phases have been covalently attached onto a hydrogen-terminated silicon(111) surface. The surface attachment was achieved via a mild procedure from a mesitylene solution, using visible tight at room temperature. AFM measurements indicate that a smooth monolayer has been formed. The thickness of the monolayer was evaluated with ellipsometry and X-ray reflectivity. Although the molecules differ in length by four carbon atoms, the thickness of the resulting monolayers was the same. The measured thicknesses correspond quite well with the smectic layer thickness in the bulk liquid crystalline material, suggesting a similar self-organization within the monolayer. From attenuated total reflectance infrared (ATR-IR), which clearly shows the C-H and C=O vibrations, a tilt angle of the mesogens is deduced that also corresponds well with the tilt angle in the liquid crystalline state. X-ray photoelectron spectroscopy (XPS) measurements confirm the high quality of the monolayers, with only marginal silicon oxide formation. The elemental composition and amounts of different O and C atoms deduced from the high-resolution XPS correspond very well with the calculated compositions.
    Site-Specific Immobilization of DNA in Glass Microchannels via Photolithography
    Vong, T. ; Maat, J. ter; Beek, T.A. van; Lagen, B. van; Giesbers, M. ; Hest, J.C.M. van; Zuilhof, H. - \ 2009
    Langmuir 25 (2009)24. - ISSN 0743-7463 - p. 13952 - 13958.
    organic monolayers - covalent attachment - silicon surfaces - micrometer-scale - alkyl monolayers - electrochemistry - lithography - microarrays - strategies - h-si(111)
    For the first time, it microchannel was photochemically patterned with it functional linker. This simple method was developed for the site-specific attachment of DNA via this linker onto silicon oxide surfaces (e.g., fused silica and borosilicate glass), both onto a flat surface and onto the inside of a fused silica microchannel. Sharp boundaries in the micrometer range between modified and unmodified zones were demonstrated by the attachment Of fluorescently labeled DNA oligomers. Studies of repeated hybridization-deliybridization cycles revealed selective and reversible binding of cDNA strands at the explicit locations. On average, similar to 7 x 10(11) fluorescently labeled DNA molecules were hybridized per square centimeter. The modified surfaces were characterized with X-ray photoelectron spectroscopy, infrared microscopy, static contact angle measurements, confocal laser scanning microscopy, and fluorescence detection (to quantify the attachment of the fluorescently labeled DNA).
    Synthesis and cytotoxicity of silicon nanoparticles with covalently attached organic monolayers
    Ruizendaal, L. ; Bhattacharjee, S. ; Pournazari, K. ; Rosso-Vasic, M. ; Haan, L.H.J. de; Alink, G.M. ; Marcelis, A.T.M. ; Zuilhof, H. - \ 2009
    Nanotoxicology 3 (2009)4. - ISSN 1743-5390 - p. 339 - 347.
    quantum dots - alkyl monolayers - in-vivo - terminated silicon - surface - toxicity - nanocrystals - functionalization - cells - assay
    A series of highly monodisperse silicon nanoparticles (Si NPs) with either positively (amine), neutral (azide) or negatively (carboxylic acid) charged covalently attached organic monolayers were synthesized and investigated for their cytotoxicity. Infrared data confirmed the presence of these covalently attached surface groups. The Si NPs were characterized by absorption and fluorescence spectroscopy. The cytotoxicity was investigated in Caco-2 cells by determining the cell viability and proliferation. The IC50 values for the Si NPs ranged from 20 µg/l for the amine-terminated Si NPs, via 550–850 µg/l for the azide-terminated Si NPs to non-toxic (no measureable IC50) for the carboxylic acid-terminated Si NPs. These results indicate a trend in cytotoxicity, depending on surface charge, i.e., that positively charged Si NPs are more cytotoxic than negatively charged Si NPs. Interestingly, it appeared that the cytotoxicity of the Si NP-NH2 depends strongly on the presence of fetal calf serum in the medium.
    Local Probe Oxidation of Self-Assembled Monolayers on Hydrogen-Terminated Silicon
    Yang, M. ; Wouters, D. ; Giesbers, M. ; Schubert, U.S. ; Zuilhof, H. - \ 2009
    ACS Nano 3 (2009)10. - ISSN 1936-0851 - p. 2887 - 2900.
    atomic-force microscopy - covalently attached monolayers - linked organic monolayers - extremely mild attachment - field-induced oxidation - alkyl monolayers - constructive nanolithography - anodization lithography - chemical-modification - molecular simulation
    Local probe oxidation experiments by conductive AFM have been performed on a hexadecyl monolayer and a N-hydroxysuccinimide (NHS)-ester-functionalized undecyl (NHS-UA) monolayer assembled on hydrogen-terminated (i.e., unoxidized) silicon. The oxidation conditions for the mild oxidation of the top terminal groups of monolayers and the deep oxidation of the underlying silicon into silicon oxide were investigated. The results show that the bias threshold for the AFM tip-induced oxidation of the top groups of monolayers on oxide-free silicon can be reduced by 2 V for the methyl-terminated hexadecyl monolayer and even by 3.5 V for the active NHS-ester-terminated undecyl monolayer, in comparison to a methyl-terminated octadecyl trichlorosilane (OTS) monolayer on oxidized silicon. Upon such local mild oxidation, the active NHS ester group of the NHS-UA monolayer is selectively cleaved off to generate carboxyl-containing monolayer nanopatterns, opening further possibilities for subsequent patterned multifunctionalization
    Molecular Electronics at Metal/Semiconductor Junctions. Si Inversion by Sub-Nanometer Molecular Films
    Yaffe, O. ; Scheres, L.M.W. ; Reddy Puniredd, S. ; Stein, N. ; Biller, A. ; Har Lavan, R. ; Shpaisman, H. ; Zuilhof, H. ; Haick, H. ; Cahen, D. ; Vilan, A. - \ 2009
    Nano Letters 9 (2009)6. - ISSN 1530-6984 - p. 2390 - 2394.
    crystalline silicon surfaces - terminated si(111) surfaces - voltaic energy-conversion - linked organic monolayers - mis tunnel-diodes - alkyl monolayers - transport - deposition - quality - barrier
    Electronic transport across n-Si-alkyl monolayer/Hg junctions is, at reverse and low forward bias, independent of alkyl chain length from 18 down to 1 or 2 carbons! This and further recent results indicate that electron transport is minority, rather than majority carrier dominated, occurs via generation and recombination, rather than (the earlier assumed) thermionic emission, and, as such, is rather insensitive to interface properties. The (m)ethyl results show that binding organic molecules directly to semiconductors provides semiconductor/metal interface control options, not accessible otherwise.
    Covalently attached organic monolayers on SiC and SixN4 surfaces: Formation using UV light at room temperature
    Rosso, M. ; Giesbers, M. ; Arafat, A. ; Schroën, C.G.P.H. ; Zuilhof, H. - \ 2009
    Langmuir 25 (2009)4. - ISSN 0743-7463 - p. 2172 - 2180.
    self-assembled monolayers - silicon-nitride surfaces - hydrogen-terminated silicon - chemical-vapor-deposition - extremely mild attachment - diamond surfaces - alkyl monolayers - visible-light - photochemical functionalization - photoelectron-spectroscopy
    We describe the formation of alkyl monolayers on silicon carbide (SiC) and silicon-rich silicon nitride (SixN4) surfaces, using UV irradiation in the presence of alkenes. Both the surface preparation and the monolayer attachment were carried out under ambient conditions. The stable coatings obtained in this way were studied by water contact angle measurements, infrared reflection absorption spectroscopy, X-ray reflectivity, and X-ray photoelectron spectroscopy. Besides unfunctionalized 1-alkenes, methyl undec-10-enoate, and 2,2,2-trifluoroethyl undec-10-enoate were also grafted onto both substrates. The resulting ester-terminated surfaces could then be further reacted after hydrolysis using amide chemistry to easily allow the attachment of amine-containing compounds.
    Covalent attachment of organic monolayers to silicon carbide surfaces
    Rosso, M. ; Arafat, A. ; Schroën, C.G.P.H. ; Giesbers, M. ; Roper, C.S. ; Maboudian, R. ; Zuilhof, H. - \ 2008
    Langmuir 24 (2008)8. - ISSN 0743-7463 - p. 4007 - 4012.
    hydrogen-terminated silicon - self-assembled monolayers - extremely mild attachment - sic surfaces - diamond surfaces - alkyl monolayers - visible-light - amorphous-carbon - nitride surfaces - films
    This work presents the first alkyl monolayers covalently bound on HF-treated silicon carbide surfaces (SiC) through thermal reaction with 1-alkenes. Treatment of SiC with diluted aqueous HF solutions removes the native oxide layer (SiO2) and provides a reactive hydroxyl-covered surface. Very hydrophobic methyl-terminated surfaces (water contact angle = 107) are obtained on flat SiC, whereas attachment of -functionalized 1-alkenes also yields well-defined functionalized surfaces. Infrared reflection absorption spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy measurements are used to characterize the monolayers and show their covalent attachment. The resulting surfaces are shown to be extremely stable under harsh acidic conditions (e.g., no change in after 4 h in 2 M HCl at 90 C), while their stability in alkaline conditions (pH = 11, 60 C) also supersedes that of analogous monolayers such as those on Au, Si, and SiO2. These results are very promising for applications involving functionalized silicon carbide.
    Self-Assembly of High-Quality Covalently Bound Organic Monolayers onto Silicon
    Scheres, L.M.W. ; Arafat, A. ; Zuilhof, H. - \ 2007
    Langmuir 23 (2007). - ISSN 0743-7463 - p. 8343 - 8346.
    hydrogen-terminated silicon - acid mediated hydrosilylation - extremely mild attachment - alkyl monolayers - porous silicon - si(111) surface - visible-light - si - functionalization - 1-alkenes
    A very mild method has been developed to obtain covalently attached alkyl monolayers from the attachment of 1-alkynes onto hydrogen-terminated silicon surfaces at room temperature in the dark. Apart from being the mildest method reported so far for the preparation of such monolayers, their quality, as indicated by water contact angles, XPS, and infrared spectroscopy, equals within experimental error that of the best reported alkyl monolayers on silicon.
    pH Sensitivity of Si-C Linked Organic Monolayers on Crystalline Silicon Surfaces: Titration Experiments, Mott Schottky Analysis and Site-Binding Modeling
    Faber, E.J. ; Sparreboom, W. ; Groeneveld, W. ; Smet, L.C.P.M. de; Bomer, J. ; Olthuis, W. ; Zuilhof, H. ; Sudhölter, E.J.R. ; Bergveld, P. ; Berg, A. van den - \ 2007
    ChemPhysChem 8 (2007)1. - ISSN 1439-4235 - p. 101 - 112.
    self-assembled monolayers - hydrogen-terminated silicon - metal-semiconductor diodes - alkyl monolayers - electron-transfer - native-oxide - electrochemical properties - recombination velocity - n-si(111) electrodes - germanium surfaces
    The electrochemical behavior of SiC linked organic monolayers is studied in electrolyte-insulator-Si devices, under conditions normally encountered in potentiometric biosensors, to gain fundamental knowledge on the behavior of such Si electrodes under practical conditions. This is done via titration experiments, Mott-Schottky data analysis, and data fitting using a site-binding model. The results are compared with those of native SiO2 layers and native SiO2 layers modified with hexamethyldisilazane. All samples display pH sensitivity. The number of SiOH groups on the alkylated samples is calculated to be less than 0.7 % of that of a pure SiO2 insulator, which still causes a pH sensitivity of 25 mV per pH unit in the pH range: 4-7. The alkylated samples hardly suffer from response changes during up- and down-going titrations, which indicates that very little oxide is additionally formed during the measurements. The pKa values of all samples with monolayers (4.0-4.4) are lower than that of native SiO2 (6.0). The long-term drift (of approximately 1 mV h-1) is moderate. The results indicate that biosensors composed of alkylated Si substrates are feasible if a cross-sensitivity towards pH in the sensor signal is taken into account.
    Covalent biofunctionalization of silicon nitride surfaces
    Arafat, A. ; Giesbers, M. ; Rosso, M. ; Sudhölter, E.J.R. ; Schroën, C.G.P.H. ; White, R.G. ; Li Yang, ; Linford, M.R. ; Zuilhof, H. - \ 2007
    Langmuir 23 (2007). - ISSN 0743-7463 - p. 6233 - 6244.
    hydrogen-terminated silicon - self-assembled monolayers - ion mass-spectrometry - laser interference lithography - tof-sims spectra - alkyl monolayers - multivariate-analysis - organic monolayers - porous silicon - scribed silicon
    Covalently attached organic monolayers on etched silicon nitride (SixN4; x 3) surfaces were prepared by reaction of SixN4-coated wafers with neat or solutions of 1-alkenes and 1-alkynes in refluxing mesitylene. The surface modification was monitored by measurement of the static water contact angle, XPS, IRRAS, AFM, and ToF-SIMS, and evidence for the formation of Si-C bonds is presented. The etching can be achieved by dilute HF solutions and yields both Si-H and N-H moieties. The resulting etched SixN4 surfaces are functionalized by terminal carboxylic acid groups in either of two ways: (a) via attachment of a 10-undecenoic acid 2,2,2-trifluoroethyl ester (trifluoro ethanol ester) and subsequent thermal acid hydrolysis; (b) through attachment of a photocleavable ester, and subsequent photochemical cleavage, as this would allow photopatterned functionalized SixN4. The carboxylic acids are successfully used for the attachment of oligopeptides (aspartame) and complete proteins using EDC/NHS chemistry. Finally, an amino-terminated organic monolayer can be formed by reaction of HF-treated SixN4 surfaces with a N-(-undecylenyl)phthalimide, which yields an amino-terminated surface upon deprotection with hydrazine.
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