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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    Dynamics of substituted Alkyl monolayers covalently bonded to silicon: A broadband admittance spectroscopy study
    Godet, C. ; Fadjie-Djomkam, A.B. ; Ababou-Girard, S. ; Tricot, S. ; Turban, P. ; Li, Y. ; Pujari, S.P. ; Scheres, L.M.W. ; Zuilhof, H. ; Fabre, B. - \ 2014
    The Journal of Physical Chemistry Part C: Nanomaterials and Interfaces 118 (2014)13. - ISSN 1932-7447 - p. 6773 - 6787.
    self-assembled monolayers - hydrogen-terminated silicon - atomic-force microscopy - oxide-free silicon - molecular-dynamics - organic monolayers - si(111) surfaces - dielectric-relaxation - electrical-properties - electronic-properties
    The relaxation dynamics of surface-bound n-alkyl chains was studied by broadband admittance spectroscopy (10 mHz–10 MHz) measured at low temperature (130–300 K) in the reverse bias regime of rectifying Hg//organic monolayer (OML)–n-doped Si tunnel junctions. To obtain molecular-level information on the structure and dynamics of grafted monolayers, carboxyl or amide dipolar moieties were located either at the top free surface with variable acid concentration (0%, 5%, and 100%) or at the inner position in the alkyl backbone (100% amide units). Two classes of dipolar relaxation mechanisms are found with different thermally activated behavior. At low T, only peak A is observed (f ˜ 102–105 Hz) with very small activation energy (EA = 20–40 meV) and pre-exponential factor (f 0A ˜ 103–106 Hz). With increasing T, peak B also appears, with higher values of activation energy, EB = 0.25–0.40 eV, and pre-exponential factor (f 0B ˜ 108–1010 Hz). The bias-independent relaxation mechanism A, with very low activation energy typical of dipole–dipole interaction, is attributed to extrinsic relaxation of adventitious H2O molecules in hydrogen-bond clusters. Mechanism B is attributed to intrinsic relaxation of the alkyl chain assembly. In the acid series, the relative intensity of peak B is consistent with the acid group coverage given by XPS, in contrast with peak A, and its activation energy reveals increased motional constraints in the acid-substituted OML. The shape of dipolar relaxation peaks, discussed in the framework of Dissado-Hill/Jonscher theories for many-body interactions, is useful to discriminate near-substrate and molecular tail relaxations through order/disorder effects.
    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.
    Mono-Fluorinated Alkyne-Derived SAMs on Oxide-Free Si(III) Surfaces: Preparation, Characterization and Tuning of the Si Workfunction
    Pujari, S.P. ; Andel, E. van; Yaffe, O. ; Cahen, D. ; Weidner, T. ; Rijn, C.J.M. van; Zuilhof, H. - \ 2013
    Langmuir 29 (2013)2. - ISSN 0743-7463 - p. 570 - 580.
    self-assembled monolayers - ray photoelectron diffraction - hydrogen-terminated silicon - indium tin oxide - organic monolayers - molecular electronics - gold electrodes - work function - transport - junctions
    Organic monolayers derived from ¿-fluoro-1-alkynes of varying carbon chain lengths (C(10)-C(18)) were prepared on Si(111) surfaces, resulting in changes of the physical and electronic properties of the surface. Analysis of the monolayers using XPS, Infrared Reflection Absorption Spectroscopy, ellipsometry and static water contact angle measurements provided information regarding the monolayer thickness, the tilt angle, and the surface coverage. Additionally, PCFF molecular mechanics studies were used to obtain information on the optimal packing density and the layer thickness, which were compared to the experimentally found data. From the results, it can be concluded that the monolayers derived from longer chain lengths are more ordered, possess a lower tilt angle, and have a higher surface coverage than monolayers derived from shorter chains. We also demonstrate that by substitution of an H by F atom in the terminal group, it is possible to controllably modify the surface potential and energy barrier for charge transport in a full metal/monolayer-semiconductor (MOMS) junction.
    Structure Matters: Correlating Temperature Dependent Electrical Transport through Alkyl Monolayers with Vibrational and Photoelectron Spectroscopies
    Shpaisman, H. ; Seitz, H. ; Yaffe, O. ; Roodenko, K. ; Scheres, L.M.W. ; Zuilhof, H. ; Chabal, Y.J. ; Sueyoshi, T. ; Kera, S. ; Ueno, N. ; Vilan, A. ; Cahen, D. - \ 2012
    Chemical Science 3 (2012). - ISSN 2041-6520 - p. 851 - 862.
    self-assembled monolayers - hydrogen-terminated silicon - h stretching modes - molecular junctions - charge-transport - through-bond - infrared-spectroscopy - electronic transport - organic monolayers - thiol monolayers
    Freezing out of molecular motion and increased molecular tilt enhance the efficiency of electron transport through alkyl chain monolayers that are directly chemically bound to oxide-free Si. As a result, the current across such monolayers increases as the temperature decreases from room temperature to [similar]80 K, i.e., opposite to thermally activated transport such as hopping or semiconductor transport. The 30-fold change for transport through an 18-carbon long alkyl monolayer is several times the resistance change for actual metals over this range. FTIR vibrational spectroscopic measurements indicate that cooling increases the packing density and reduces the motional freedom of the alkyl chains by first stretching the chains and then gradually tilting the adsorbed molecules away from the surface normal. Ultraviolet photoelectron spectroscopy shows drastic sharpening of the valence band structure as the temperature decreases, which we ascribe to decreased electron–phonon coupling. Although conformational changes are typical in soft molecular systems, in molecular electronics they are rarely observed experimentally or considered theoretically. Our findings, though, indicate that the molecular conformational changes are a prominent feature, which imply behavior that differs qualitatively from that described by models of electronic transport through inorganic mesoscopic solids
    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
    Organic Monolayers onto Oxide-Free Silicon with Improved Surface Coverage: Alkynes versus Alkenes
    Scheres, L.M.W. ; Maat, J. ter; Giesbers, M. ; Zuilhof, H. - \ 2010
    Langmuir 26 (2010)7. - ISSN 0743-7463 - p. 4790 - 4795.
    covalently attached monolayers - acid mediated hydrosilylation - hydrogen-terminated silicon - metal-semiconductor diodes - extremely mild attachment - si(111) surface - porous silicon - visible-light - si - quality
    On H-Si(111), monolayer assembly with 1-alkenes results in alkyl monolayers with a Si-C-C linkage to the silicon substrate, while 1-alkynes yield alkenyl monolayers with a Si-C-C linkage. To investigate the influence of the different linkage groups on the final monolayer structure, organic monolayers were prepared from 1-alkenes and 1-alkynes with chain lengths from C12 to C18, and the final monolayer structures were studied in detail by static water contact angles measurements, ellipsometry, attenuated total reflectance infrared (ATR-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The thicknesses, tilt angles, and packing densities of the alkyl monolayers are in good agreement with literature values, whereas increased thicknesses, reduced tilt angles, and improved packing densities were observed for the alkenyl monolayers. Finally, the surface coverages for alkyl monolayers were determined to be 50-55% (in line with literature values), while those for the alkenyl monolayers increased with the chain length from 55% for C12 to as high as 65% for C18! The latter value is very close to the theoretical maximum of 69% obtainable on H-Si(111). Such enhanced monolayer quality and increased surface coverage of the alkenyl monolayers, in combination with the oxidation-inhibiting nature of the Si-C-C linkage, significantly increases the chance of successful implementation of organic monolayers on oxide-free silicon in molecular electronic and biosensor devices, especially in view of the importance of a defect-free monolayer structure and the corresponding stability of the monolayer-silicon interface
    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
    Functional monolayers on oxide-free silicon surfaces via thiol-ene click chemistry
    Caipa Campos, M.A. ; Paulusse, J.M.J. ; Zuilhof, H. - \ 2010
    Chemical Communications 46 (2010)30. - ISSN 1359-7345 - p. 5512 - 5514.
    hydrogen-terminated silicon - self-assembled monolayers - organic monolayers - visible-light - attachment - peptide
    Thiol–ene click chemistry was used for the attachment of a variety of functional molecules onto oxide-free Si(111) surfaces using very mild conditions; the efficient nature of this coupling strategy allowed for successful light-induced micropatterning and thus provides a novel route towards biofunctional electronics
    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
    Controlled Oxidation, Biofunctionalization, and Patterning of Alkyl Monolayers on Silicon and Silicon Nitride Surfaces using Plasma Treatment
    Rosso, M. ; Giesbers, M. ; Schroën, C.G.P.H. ; Zuilhof, H. - \ 2010
    Langmuir 26 (2010)2. - ISSN 0743-7463 - p. 866 - 872.
    self-assembled monolayers - covalently attached monolayers - hydrogen-terminated silicon - linked organic monolayers - extremely mild attachment - low-temperature plasma - ultrafiltration membranes - photochemical attachment - molecular electronics - visible-light
    A new method is presented for the fast and reproducible functionalization of silicon and silicon nitride surfaces coated with covalently attached alkyl monolayers. After formation of a methyl-terminated 1-hexadecyl monolayer on H-terminated Si(100) and Si(111) surfaces, short plasma treatments (1-3 s) are sufficient to create oxidized functionalities without damaging the underlying oxide-free silicon. The new functional groups can, e.g., be derivatized using the reaction of surface aldehyde groups with primary amines to form imine bonds. In this way, plasma-treated monolayers on silicon or silicon nitride surfaces were successfully coated with nanoparticles, or proteins such as avidin. In addition, we demonstrate the possibility of micropatterning, using a soft contact mask during the plasma treatment. Using water contact angle measurements, ellipsometry, XPS, IRRAS, AFM, and reflectometry, proof of principle is demonstrated of a yet unexplored way to form patterned alkyl monolayers on oxide-free silicon surfaces
    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.
    Efficient Energy Transfer between Silicon Nanoparticles and a Ru-Polypyridine Complex
    Rosso-Vasic, M. ; Cola, L. de; Zuilhof, H. - \ 2009
    The Journal of Physical Chemistry Part C: Nanomaterials and Interfaces 113 (2009)6. - ISSN 1932-7447 - p. 2235 - 2240.
    hydrogen-terminated silicon - covalently attached monolayers - extremely mild attachment - nanocrystal quantum dots - porous silicon - visible-light - organic monolayers - surface functionalization - charge-transfer - hydrosilylation
    Blue-emitting amine-terminated Si nanoparticles (NPs; size, 1.57 ± 0.24 nm) are functionalized with a Ru(bpy)2(spb)2+ [bpy = 2,2'-bipyridine; spb = 4-(p-N-succinimidylcarboxyphenyl)-2,2'-bipyridine] complex. The distance between the dye and the Si core is controlled by different alkyl chain lengths (-C3H6, -C6H12, and -C11H22), and the thus formed Si NPs are two-chromophore systems that exhibit dual-emission in two separate regions: blue (450 nm, from Si core) and red (630 nm, from Ru dye). By measuring the Si/Ru ratio, the extinction coefficient of amino-terminated Si NPs was experimentally determined for the first time (2.6 × 105 M-1 cm-1). Energy transfer from Si NPs to acceptor molecules [Ru(bpy)2(spb)2+] is observed by steady-state and time-resolved fluorescence, and its distance-dependent efficiency is shown to be up to 55% in the case of a short alkyl spacer (-C3H6). Energy transfer rates are for all examined cases in the (0.2-2.2) × 109 s-1 range
    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.
    Alkyl-Functionalized Oxide-Free Silicon Nanoparticles: Synthesis and Optical Properties
    Rosso-Vasic, M. ; Spruijt, E. ; Lagen, B. van; Cola, L. de; Zuilhof, H. - \ 2008
    Small 4 (2008)10. - ISSN 1613-6810 - p. 1835 - 1841.
    hydrogen-terminated silicon - porous silicon - surface functionalization - nanocrystals - hydrosilylation - monolayers - luminescence - spectroscopy - photoluminescence - nanoclusters
    Highly monodisperse silicon nanoparticles (1.57 ± 0.21 nm) are synthesized with a covalently attached alkyl monolayer on a gram scale. Infrared spectroscopy shows that these silicon nanoparticles contain only a few oxygen atoms per nanoparticle. XPS spectra clearly show the presence of unoxidized Si and attached alkyl chains. Owing to the relatively efficient synthesis (yields 100-fold higher than of those previously reported) the molar extinction coefficient can be measured: max = 1.7 × 10-4 M-1cm-1, only a factor of 4 lower than that of CdS and CdSe nanoparticles of that size. The quantum yield of emission ranges from 0.12 (C10H21-capping) to 0.23 (C16H33-capping). UV/Vis absorption and emission spectroscopy show clear vibrational progressions (974 ± 14 cm-1; up to five vibrational bands visible at room temperature), resembling bulk SiC phonons, which support the monodispersity observed by TEM. This was also confirmed by time-resolved fluorescence anisotropy measurements, which display a strictly monoexponential decay that can only be indicative of monodisperse, ball-shaped nanoparticles
    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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