Alkyl Monolayers Research Articles

Formation of n-Alkyl Monolayers by Organomercury Deposition on Gold

n-Alkyl self-assembled monolayers can be directly attached to gold through C–Au bonds by the deposition of organomercury salts on gold substrates, as shown here using n-butylmercury and n-octadecylmercury tosylate derivatives. The Hg atoms, which are codeposited during this process, are removed by thermal annealing at 95 °C, resulting in alkyl monolayers having a significantly enhanced thermal stability compared with alkanethiol monolayers, however, a lower degree of conformational order. The monolayer properties are elucidated by X-ray photoemission and IR spectroscopy, STM, ellipsometry, and contact-angle goniometry.

Effect of Doping Density on the Charge Rearrangement and Interface Dipole at the Molecule–Silicon Interface

The interface level alignment of alkyl and alkenyl monolayers, covalently bound to oxide-free Si substrates of various doping levels, is studied using X-ray photoelectron spectroscopy. Using shifts in the C 1s and Si 2p photoelectron peaks as a sensitive probe, we find that charge distribution around the covalent Si–C bond dipole changes according to the initial position of the Fermi level within the Si substrate. This shows that the interface dipole is not fixed but rather changes with the doping level. These results set limits to the applicability of simple models to describe level alignment at interfaces and show that the interface bond and dipole may change according to the electrostatic potential at the interface.

Grafting of Poly(ethylene glycol) on Click Chemistry Modified Si(100) Surfaces

Poly(ethylene glycol) (PEG) is one of the most extensively studied antifouling coatings due to its ability to reduce protein adsorption and improve biocompatibility. Although the use of PEG for antifouling coatings is well established, the stability and density of PEG layers are often inadequate to provide optimum antifouling properties. To improve on these shortcomings, we employed the stepwise construction of PEG layers onto a silicon surface. Acetylene-terminated alkyl monolayers were attached to nonoxidized crystalline silicon surfaces via a one-step hydrosilylation procedure with 1,8-nonadiyne. The acetylene-terminated surfaces were functionalized via a copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction of the surface-bound alkynes with an azide to produce an amine terminated layer. The amine terminated layer was then further conjugated with PEG to produce an antifouling surface. The antifouling surface properties were investigated by testing adsorption of human serum albumin (HSA) and lysozyme (Lys) onto PEG layers from phosphate buffer solutions. Detailed characterization of protein fouling was carried out with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with principal component analysis (PCA). The results revealed no fouling of albumin onto PEG coatings whereas the smaller protein lysozyme adsorbed to a very low extent.

Photodimerization and Micropatterning of Anthracene-Appended Receptors Covalently Bound to Silicon Surfaces: En Route to Write–Read–Erase Molecular Print Board

Anthracene-appended receptor monolayers covalently bound to hydrogen-terminated Si(111) surfaces have been prepared from attachment of an aminoalkane-substituted receptor to a preassembled acid-terminated alkyl monolayer using carbodiimide coupling. Subsequent irradiation at 350 nm of the receptor-modified surface yielded the open → closed conversion of the grafted anthracene-appended receptor, as clearly supported by IR spectroscopy data. Grafting of the open receptor and its subsequent closing did not change significantly the surface topography, and atomic force microscopy (AFM) images showed formation of perfectly dense and homogeneous molecular films without adventitious contamination or aggregated islands. Scanning electrochemical microscopy (SECM) measurements in feedback mode using an appropriate redox mediator (i.e., 9,10-diethoxyanthracene DEA) provided clear evidence for an efficient charge transport process between grafted receptors. However, the kinetics of the mediator regeneration was faster when the receptors were under their closed form. Interestingly, light-activated switching of the anthracene-appended receptor offered the unique opportunity to pattern the surface using UV photolithography through an optical mask. After the lithographic step, the SECM image clearly showed the expected current regeneration contrast between the open and produced closed receptor micropatterns.

Creation and control of defined protein patterns on glass: alkyl chain length controls stability

Here we present a method for the production of defined protein patterns on glass using trimethoxysilane-terminated self-assembled monolayer (SAM) chemistry. The surfaces were characterized by infrared spectroscopy to determine the relationship between alkyl chain length and monolayer order and, in turn, the critical alkyl chain length required to produce a tightly packed SAM. Protein resistance studies, which incorporated a glycol region into the monomer, further highlighted the importance of alkane chain length within the monolayer, as increased chain length was necessary to generate protein resistant regions that persisted for more than a few days. The long-chain trimethoxysilane-terminated monomers that we have developed are stable, can be purified using standard silicon gel chromatography, and can be easily patterned using methods developed for other SAMs. As a result, these monomers and the resulting monolayers produced from them provide a method for the functionalization of a wide variety of oxide substrates, including ordinary glass, which has enormous advantages over previous glycol-terminated trichlorosilane monomers.

Kelvin probe studies of alkyl monolayers on silicon (111) for surface passivation

A chlorination–alkylation procedure has been investigated with a view to improving the surface passivation properties on silicon. We have found that increasing surface coverage of the alkyl monolayer raises the measured SPV and recombination lifetime. A logarithmic trend is observed between the SPV and recombination, showing that monolayer preparation is an important factor in determining the lifetimes observed. The high SPVs measured by Kelvin probe suggest the presence of a silicon surface with a lower concentration of electrons, reducing surface recombination.

Site-Selective Electroless Metallization on Porous Organosilica Films by Multisurface Modification of Alkyl Monolayer and Vacuum Plasma

Taking plasma-enhanced chemical vapor deposited porous SiOCH (p-SiOCH) and octadecyltrichlorosilane (OTS) as model cases, this study elucidates the chemical reaction pathways for alkyl-based self-assembled monolayers (SAMs) on porous carbon-doped organosilica films under N(2)-H(2) vacuum plasma illumination. In contrast to previous findings that carboxylic groups are found in alkyl-based SAMs only by exposure to oxygen-based plasma, this study discovers that, upon exposure to reductive nitrogen-based vacuum plasma, surface carboxylic functional groups can be instantly formed on OTS-coated p-SiOCH films. Particular attention is given to developing a multisurface modification process, starting with the modification of p-SiOCH films by N(2)-H(2) plasma and continuing with SAM deposition and plasma patterning; this ultimately leads to site-selective seeding for the spatially controlled fabrication of Cu-wire metallization by electroless deposition. Plasma diagnosis and X-ray near-edge absorption and Fourier transform infrared spectroscopies show that, by adequately controlling the plasma parameters, the bulk of the p-SiOCH films are free from plasma damage (in terms of degradation in bonding structures and electrical properties); the formation of the seed-trapping carboxylic functional groups on the surface, the key factor for the validity of this new seeding process, is due to a water-mediated chemical oxygenation route.

Advancing and receding angles – Dynamic contact angle measurements on mixed alkyl monolayers

Mixtures of 1-decene and methyl-10-undecenoate with different mole fractions were used to prepared mixed monolayers on silicon (100) surfaces. The formed layers were characterized by dynamic contact angle measurements of two different liquids using the dynamic sessile drop method. The advancing and receding contact angles as well the calculated molar wetting free energies decreased with increasing mole fraction of methyl-10-undecenoate. Otherwise the molar dewetting free energies as well as the contact angle hysteresis stay nearly constant. A difference in the wetting and dewetting properties on mixed monolayers is observed, whereby mainly dispersive interactions are responsible for wetting. Furthermore we show that the contact angle hysteresis is independent of surface roughness, surface composition and surface free energy, which indicates that neither surface roughness nor heterogeneities are responsible for this hysteresis. We attribute the hysteresis to interactions between the liquid and the silicon substrate.

Reduction of Gold Penetration through Phenyl-Terminated Alkyl Monolayers on Silicon

By preparing phenyl-terminated monolayers on hydrogen-terminated silicon (111), we show that their higher surface densities in comparison with n-alkyl monolayers improves their electrical properties (lower reverse-bias currents, higher effective barrier heights, and closer-to-unity ideality factors) when contacted using either mercury drop or thermally deposited gold electrodes. Consistent with these macroscopic results, the ballistic electron emission microscopy characterization shows a significant decrease in ballistic current and higher local barrier height for the phenyl-terminated monolayers, when compared with gold | n-alkyl monolayer | silicon junctions. We propose that increased intermolecular interaction through π–π stacking of the phenyl head-groups stabilizes the monolayer structure at the buried interface and inhibits the penetration of thermally deposited gold atoms.

Controlling Space Charge of Oxide-Free Si by in Situ Modification of Dipolar Alkyl Monolayers

Good passivation of Si, both electrically and chemically, is achieved by monolayers of 1,9-decadiene, directly bound to an oxide-free Si surface. The terminal C═C bond of the decadiene serves for further in situ reaction, without harming the surface passivation, to −OH- or −Br-terminated monolayers that have different dipole moments. Such a two-step procedure meets the conflicting requirements of binding mutually repelling dipolar groups to a surface, while chemically blocking all surface reactive sites. We demonstrate a change of 0.15 eV in the Si surface potential, which translates into a 0.4 eV variation in the Schottky barrier height of a Hg junction to those molecularly modified n-Si surfaces. Charge transport across such junctions is controlled both by tunneling across the molecular monolayer and by the Si space charge. For reliable insight into transport details, we resorted to detailed numerical simulations, which reveal that the Si space charge and the molecular tunneling barriers are coupled. As a result, attenuation due to the molecular tunneling is much weaker than in metal/molecule/metal molecular junctions. Simulation shows also that some interface states are present but that they have a negligible effect on Fermi level pinning. These states are efficiently decoupled from the metal (Hg) and interact mostly with the Si.

Vibrational properties of alkyl monolayers on Si(111) surfaces: Predictions from ab-initio calculations

Vibrational properties of Si(111) surfaces terminated by different functional groups have been investigated using density functional theory (DFT). The variations in methyl-related frequencies in different chemical environments, e.g., in methane, methylsilane and ethylsilane, and the methyl- and ethyl-terminated Si(111) surfaces are well predicted by DFT within the local density approximation. In particular, DFT calculations provide useful information on trends and mode assignments in cases where the surface coverage and morphology are not well established experimentally, e.g., in the case of the ethyl-terminated Si(111) surface. Influences of DFT exchange-correlation functionals and anharmonic effects on computed vibrational frequencies are discussed.

Structure Matters: Correlating temperature dependent electrical transport through alkyl monolayers with vibrational and photoelectron spectroscopies

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 ∼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.

Nanoseeding via Dual Surface Modification of Alkyl Monolayer for Site-Controlled Electroless Metallization

In this work, an attempt to fabricate nanostructured metallization patterns on SiO(2) dielectric layers is made by using plasma-patterned self-assembled monolayers (SAMs), in conjunction with a novel aqueous seeding and electroless process. Taking octadecyltrichlorosilane (OTS) as a test material, the authors demonstrate that optimizing the N(2)-H(2) plasma conditions leads to the successive conversion of the topmost aliphatic chains of alkyl SAMs to carboxyl (COOH) and hydroxyl (C-OH) functional groups, which was previously found in alkyl SAMs only by exposure to "oxygen-based" plasma. Further modifying the plasma-exposed (either COOH or C-OH terminated) regions with an aqueous solution (SC-1) creates surface functionalities that are viable for site-controlled metallic seeding (e.g., Co or Ni) with an adsorption selectivity of greater than 1000:1. Neither the combination of costly PdCl(2) and complex additives nor the demerits of the associated aqueous chemistry (e.g., seed agglomeration and seed sparseness) are involved. Therefore, the seed particles are only 3 nm in size. Simultaneously, there are sufficient particle densities previously unattainable for electroless deposition to trigger highly resolved Cu metallization patterns with a film thickness of less than 10 nm. The formation of the seed-adsorbing sites is discussed, based on a plasma-dissociated, water-mediated chemical oxidation route.

Anthracene and Anthracene:C60 Adduct-Terminated Monolayers Covalently Bound to Hydrogen-Terminated Silicon Surfaces

Anthracene monolayers covalently bound to hydrogen-terminated n-type Si(100) surfaces have been prepared from the attachment of an amine-substituted anthracene derivative to a preassembled acid-terminated alkyl monolayer using carbodiimide coupling. The anthracene headgroups were then used as anchoring sites for C60 following [4 + 2] Diels–Alder cycloaddition. After cycloaddition of C60 on the anthracene layer, the surface roughness determined by atomic force microscopy increased from 3.0 ± 0.7 to 5.0 ± 1.0 Å and the morphology showed uniformly distributed globular features a few nanometers high. Cyclic voltammograms of the anthracene-modified monolayer in the dark were characterized by an ill-defined reversible system at E°′ = −2.05 V vs saturated calomel electrode (SCE), which compares well with the value determined for the anthracene derivative in solution on a platinum electrode. Furthermore, the surface coverage of attached anthracene units was estimated to be (4.6 ± 0.3) × 10–10 mol cm–2, which is consistent with a densely packed monolayer. In contrast, the voltammogram of the C60-modified monolayer did not show multiple reversible one-electron transfers characteristic of the anthracene:C60 adduct. Instead, one irreversible cathodic peak at −1.50 V followed by a reversible system at −2.15 V was observed. These electrochemical differences between surface-confined and dissolved species are assigned to reduced charge transfer kinetics between the underlying semiconductor and bound C60 within a certain potential range. This hypothesis is consistent with the flat band potential Efb value of −0.80 ± 0.05 V vs SCE, determined from capacitance measurements. Moreover, scanning electrochemical microscopy (SECM) measurements in feedback mode provided clear evidence for the electroactive properties of bound C60. The SECM approach curves suggest that both the anthracene and anthracene:C60 layers displayed good conductivity, presumably by electron hopping between adjacent redox sites.

Modulation of flat-band voltage on H-terminated silicon-on-insulator pseudo–metal–oxide–semiconductor field effect transistors by adsorption and reaction events

Accumulation mode pseudo-MOSFETs formed on hydrogen terminated silicon-on-insulator (SOI-H) were used to probe molecular adsorption and reaction events. Current-voltage characteristics of such n-channel devices are found to be sensitive to the environment, with the accumulation threshold voltage, or flat-band voltage, exhibiting large reversible changes upon cycling between ambient atmosphere, high vacuum (<10−5 Torr), and exposure to water and pyridine vapor at pressures in the Torr range. The field-effect mobility is found to be comparatively less affected through these transitions. Oxidation of the H-terminated surface in ambient conditions leads to irreversible shifts in both the flat-band voltage and the field-effect mobility. A photochemical gas phase reaction with decene is used to form a decyl monolayer on the SOI(100)-H surface. Formation of this monolayer is found to result in a relatively small shift of the threshold voltage and only a slight degradation of the field effect mobility, suggesting that alkyl monolayer dielectrics formed in this way could function as good passivating dielectrics in field effect sensing applications.

Conductive AFM patterning on oligo(ethylene glycol)-terminated alkyl monolayers on silicon substrates: proposed mechanism and fabrication of avidin patterns.

Micro- and nanopatterns of biomolecules on inert, ultrathin platforms on nonoxidized silicon are ideal interfaces between silicon-based microelectronics and biological systems. We report here the local oxidation nanolithography with conductive atomic force microscopy (cAFM) on highly protein-resistant, oligo(ethylene glycol) (OEG)-terminated alkyl monolayers on nonoxidized silicon substrates. We propose a mechanism for this process, suggesting that it is possible to oxidize only the top ethylene glycol units to generate carboxylic acid and aldehyde groups on the film surface. We show that avidin molecules can be attached selectively to the oxidized pattern and the density can be varied by altering the bias voltage during cAFM patterning. Biotinylated molecules and nanoparticles are selectively immobilized on the resultant avidin patterns. Since one of the most established methods for immobilization of biomolecules is based on avidin-biotin binding and a wide variety of biotinylated biomolecules are available, this approach represents a versatile means for prototyping any nanostructures presenting these biomolecules on silicon substrates.

(Invited) Functionalization of Germanium Nanowires

Halogen-termination and organic functionalization of germanium (Ge) nanowires is described. X-ray photoelectron spectroscopy (XPS), X-ray photoemission electron spectroscopy (XPEEM), infrared spectroscopy (IR) and transmission electron spectroscopy (TEM) were used to characterize the modified nanowire surfaces. The stability of alkyl and alkanethiol monolayers formed on Cl, Br and I-terminated surfaces are compared. The direct covalent attachment of aryl ligands onto H-Ge nanowires can be achieved by the decomposition of arenediazonium salts in acetonitrile solutions. The influence of the ring substituent on the thickness and uniformity of the functionalization layer was investigated.

(Invited) Functionalization and Behavior in Aqueous Media of Silicon Surfaces for Improved Biochemical Sensing

The silicon surface may be functionalized with organic groups through direct Si-C bonding, which provides the obtained surfaces with improved chemical stability and allows for the immobilization of probe molecules for the elaboration of sensors. We here review recent work done in our group, using carboxyl-terminated alkyl monolayers on model Si(111) surfaces as a benchmark to study in details further surface functionalization, then extending the processes to amorphous hydrogenated silicon and amorphous hydrogenated silicon/carbon alloys, which can be deposited in thin-film form on any substrate. By designing appropriate structures, fluorescence biochips were then obtained, with strongly improved performance in terms of sensitivity, stability and reliability.

Patterning NHS-Terminated SAMs on Germanium

Here we report a simple, robust approach to patterning functional SAMs on germanium. The protocol relies on catalytic soft-lithographic pattern transfer from an elastomeric stamp bearing pendant immobilized sulfonic acid moieties to an NHS-functionalized bilayer molecular system comprising a primary ordered alkyl monolayer and a reactive ester secondary overlayer. The catalytic polyurethane-acrylate stamp was used to form micrometer-scale features of chemically distinct SAMs on germanium. The methodology represents the first example of patterned SAMs on germanium, a semiconductor material.

Ultrafast vibrational dynamics and spectroscopy of a siloxane self-assembled monolayer

Time and frequency domain sum-frequency generation (SFG) were combined to study the dynamics and structure of self-assembled monolayers (SAMs) on a fused silica surface. SFG-free induction decay (SFG-FID) of octadecylsilane SAM in the CH stretching region shows a relatively long time scale oscillation that reveals that six vibrational modes are involved in the response of the system. Five of the modes have commonly been used for the fitting of SFG spectra in the CH stretching region, namely the symmetric stretch and Fermi resonance of the methyl group, the antisymmetric stretch of the methyl, as well as the symmetric and antisymmetric stretches of the methylene group. The assignment of the sixth mode to the terminal CH(2) group was confirmed by performing a density function theory calculation. The SFG-FID measures the vibrational dephasing time (T(2)) of each of the modes, including a specific CH(2) group within the SAM, the terminal CH(2), which had never been measured before. The relatively long (∼1.3 ps) dephasing of the terminal CH(2) suggests that alkyl monolayer structure is close to that of the liquid condensed phase of Langmuir Blodgett films.