Mild methods to assemble and pattern organic monolayers on hydrogen-terminated Si(111)

Mild methods to assemble well-ordered organic monolayers of olefins on Si(111) using 2,2,6,6-tetramethyl-1-piperidinyloxy and to pattern these monolayers on the micrometer-size scale using soft lithography are reported.     

Molecular simulation of alkyl monolayers on the Si(111)surface

The structure of twelve-carbon monolayers on the H-terminated Si(111) surface is investigated by molecular simulation method. The best substitution percent on Si(111) surface obtained via molecular mechanics calculation is equal to 50%, and the (8×8) simulated cell can be used to depict the structure of alkyl monolayer on Si surface. After two-dimensional cell containing alkyl chains and four-layer Si(111) crystal at the substitution 50% is constructed, the densely packed and well-ordered monolayer on Si(111) surface can be shown through energy minimization in the suitable-size simulation cell. These simulation results are in good agreement with the experiments. These conclusions show that molecular simulation can provide otherwise inaccessible mesoscopic information at the molecular level, and can be considered as an adjunct to experiments.     

Thiol-terminated monolayers on oxide-free Si: assembly of semiconductor-alkyl-S-metal junctions

Self-assembled monolayers formed by thermal hydrosilylation of a trifluoroacetyl-protected alkenylthiol on Si-H surfaces, followed by removal of the protecting groups, yield essentially oxide-free monolayers suitable for the formation of Si-C11H22-S-Hg and Si-C11H22-S-Au junctions in which the alkyl chains are chemically bound to the silicon surface (via Si-C bonds) and the metal electrode (via Hg-S or Au-S bonds). Two barriers to charge transport are present in the system: at low bias the current is temperature activated and hence limited by thermionic emission over the Schottky barrier in the silicon, whereas as at high bias transport is limited by tunneling through the organic monolayer. The thiol-terminated monolayer on oxide-free silicon provides a well-characterized system allowing a careful study of the importance of the interfacial bond to the metal electrode for current transport through saturated molecules.     

Direct photochemical functionalization of Si(111) with undecenol

Direct UV photochemical functionalization of H-terminated Si(111) with bifunctional 10-undecen-1-ol was achieved with selective attachment via its vinyl end, resulting in the formation of a compact monolayer with free terminal alcohol groups. This is due to the faster radical propagation mechanism in hydrosilylation with alkene compared to the nucleophilic attack mechanism of alcohol, which is impeded by intermolecular hydrogen bonding present at room temperature. Evidence from X-ray photoelectron spectroscopy, infrared spectroscopy, and resistance to fluoride etching shows that Si-C is the interfacial bond, and atomic force microscopy shows the presence of a smooth, uniform monolayer conforming to the atomic terraces of the Si(111) surface. The application of such a hydroxyl-terminated monolayer was demonstrated by tethering a bromoinitiator through surface esterification and thereafter subjecting the surface to the surface-initiated atom-transfer radical polymerization of butyl methacrylate. The poly(butyl methacrylate) brushes formed were found to be smooth (R(a) < 0.3 nm) and uniform even for a thin film of 4.0 nm.     

Molecular simulation of alkyl monolayers on the Si(111) surface YUAN

The preparation of monolayers on silicon surface is of growing interest for potential applica-tions in biosensor or semiconductor technology[1—5]. The alkyl modified Si(111) surfaces[6—10] can be obtained using the thermal, catalyzed, or photochemical reaction of hydrogen-terminated sili-con with alkenes, Grignard reagents, and so on. At the same time, the monolayer properties on Si(111) surface have been studied by a variety of experimental methods[8—10] such as X-ray photo-electron spect…     

Molecular modeling of alkyl and alkenyl monolayers on hydrogen-terminated Si(111)

On H-Si(111) surfaces monolayer formation with 1-alkenes results in alkyl monolayers with a Si-C-C linkage, while 1-alkynes yield alkenyl monolayers with a Si-C═C linkage. Recently, considerable structural differences between both types of monolayers were observed, including an increased thickness, improved packing, and higher surface coverage for the alkenyl monolayers. The precise origin thereof could experimentally not be clarified yet. Therefore, octadecyl and octadecenyl monolayers on Si(111) were studied in detail by molecular modeling via PCFF molecular mechanics calculations on periodically repeated slabs of modified surfaces. After energy minimization the packing energies, structural properties, close contacts, and deformations of the Si surfaces of monolayers structures with various substitution percentages and substitution patterns were analyzed. For the octadecyl monolayers all data pointed to a substitution percentage close to 50-55%, which is due the size of the CH(2) groups near the Si surface. This agrees with literature and the experimentally determined coverage of octadecyl monolayers. For the octadecenyl monolayers the minimum in packing energy per chain is calculated around 60% coverage, i.e., close to the experimentally observed value of 65% [Scheres et al. Langmuir 2010, 26, 4790], and this packing energy is less dependent on the substitution percentage than calculated for alkyl layers. Analysis of the chain conformations, close contacts, and Si surface deformation clarifies this, since even at coverages above 60% a relatively low number of close contacts and a negligible deformation of the Si was observed. In order to evaluate the thermodynamic feasibility of the monolayer structures, we estimated the binding energies of 1-alkenes and 1-alkynes to the hydrogen-terminated Si surface at a range of surface coverages by composite high-quality G3 calculations and determined the total energy of monolayer formation by adding the packing energies and the binding energies. It was shown that due to the significantly larger reaction exothermicity of the 1-alkynes, thermodynamically even a substitution percentage as high as 75% is possible for octadecenyl chains. However, because sterically (based on the van der Waals footprint) a coverage of 69% is the maximum for alkyl and alkenyl monolayers, the optimal substitution percentage of octadecenyl monolayers will be presumably close to this latter value, and the experimentally observed 65% is likely close to what is experimentally maximally obtainable with alkenyl monolayers.     

Cross metathesis on olefin-terminated monolayers on Si(111) using the Grubbs' catalyst

This paper reports the functionalization and patterning of olefin-terminated monolayers on Si(111) through cross metathesis. A simple, one-step synthesis of a diolefin--CH2=CH(CH2)9O(CH2)9CH=CH2--was developed from commercially available starting materials. Mixed partially olefin-terminated monolayers of this novel diolefin and 1-octadecene on hydrogen-terminated Si(111) were obtained. The olefins are raised above the rest of the monolayer and thus sterically accessible for further functionalization. Olefin-terminated monolayers were reacted with the Grubbs' first generation catalyst and olefins in solution that were terminated with fluorines, carboxylic acids, alcohols, aldehydes, and alkyl bromides. Characterization of these monolayers using X-ray photoelectron spectroscopy and horizontal attenuated total reflection infrared spectroscopy demonstrated that olefins on the surface had reacted via cross metathesis to expose fluorines, carboxylic acids, aldehydes, alcohols, and bromides. Through calibration experiments, we demonstrated a simple 1:1 correspondence between the ratio of olefins in solution used in the assembly and the final composition of the mixed monolayers. Finally, these monolayers on silicon were patterned on the micrometer-size scale by soft lithography using microfluidic channels patterned into poly(dimethylsiloxane) (PDMS) stamps. Micrometer-wide lines of polymer brushes were synthesized on these monolayers and characterized by scanning electron microscopy. In addition, olefin-terminated monolayers were patterned into micrometer-sized lines exposing carboxylic acids by cross metathesis with olefins in solution. This method of patterning is broadly applicable and can find applications in a variety of fields including the development of biosensors and nanoelectronics.     

X-ray studies of self-assembled organic monolayers grown on hydrogen-terminated Si(111)

The structure of self-assembled monolayers (SAMs) of undecylenic acid methyl ester (SAM-1) and undec-10-enoic acid 2-bromo-ethyl ester (SAM-2) grown on hydrogen-passivated Si(111) were studied by X-ray reflectivity (XRR), X-ray standing waves (XSW), X-ray fluorescence (XRF), atomic force microscopy, and X-ray photoelectron spectroscopy (XPS). The two different SAMs were grown by immersion of H-Si(111) substrates into the two different concentrated esters. UV irradiation during immersion was used to create Si dangling bond sites that act as initiators of the surface free-radical addition process that leads to film growth. The XRR structural analysis reveals that the molecules of SAM-1 and SAM-2 respectively have area densities corresponding to 50% and 57% of the density of Si(111) surface dangling bonds and produce films with less than 4 angstroms root-mean-square roughness that have layer thicknesses of 12.2 and 13.2 angstroms. Considering the molecular lengths, these thicknesses correspond to a 38 degrees and 23 degrees tilt angle for the respective molecules. For SAM-2/Si(111) samples, XRF analysis reveals a 0.58 monolayer (ML) Br total coverage. Single-crystal Bragg diffraction XSW analysis reveals (unexpectedly) that 0.48 ML of these Br atoms are at a Si(111) lattice position height that is identical to the T1 site that was previously found by XSW analysis for Br adsorbed onto Si(111) from a methanol solution and from ultrahigh vacuum. From the combined XPS, XRR, XRF, and XSW evidence, it is concluded that Br abstraction by reactive surface dangling bonds competes with olefin addition to the surface.     

Formation, characterization, and chemistry of undecanoic acid-terminated silicon surfaces: patterning and immobilization of DNA

This paper describes a simple strategy for DNA immobilization on chemically modified and patterned silicon surfaces. The photochemical modification of hydrogen-terminated Si(111) with undecylenic acid leads to the formation of an organic monolayer covalently attached to the surface through Si-C bonds without detectable reaction of the carboxylic acid group, providing indirect support of a free radical mechanism. Chemical activation of the acid function was achieved by a simple chemical route using N-hydroxysuccinimide (NHS) in the presence of N-ethyl-N'-(3-dimethylaminopropyl) carbodiimide hydrochloride. Single strand DNA with a 5'-dodecylamine group was then coupled to the NHS-activated surface by amide bond formation. Using a previously reported chemical patterning approach, we have shown that DNA can be immobilized on silicon surfaces in spatially well-resolved domains. Methoxytetraethyleneglycolamine was used to inhibit nonspecific adsorption. The resulting DNA-modified surfaces have shown good specificity and chemical and thermal stability under hybridization conditions. The sequential reactions on the surface were monitored by ATR-FTIR, X-ray Photoelectron Spectroscopy, and fluorescence spectroscopy.     

Competing mechanisms in the optically activated functionalization of the hydrogen-terminated Si(111) surface

Recent experiments have shown that organic monolayers on silicon surfaces can be formed through the optically activated surface reaction of H-terminated Si surfaces with terminally unsaturated organic molecules (Eves et al. J. Am. Chem. Soc. 2004, 126, 14318; Sun et al. J. Am. Chem. Soc. 2005, 127, 2514). Possible mechanisms for the formation of this monolayer involve the abstraction of a H atom either at the same attachment site of the molecule (Path A) or from a neighboring site (Path B). Using periodic Density Functional Theory calculations together with an efficient method for finding reaction pathways, we examine both optically activated reaction mechanisms for an alkene and an aldehyde reacting with H-Si(111). Our results show that while Path A is energetically more favorable its significant barrier is likely to limit its viability. Path B on the other hand encounters a much lower H atom abstraction barrier and appears to be more viable.     

Mild and efficient functionalization of hydrogen-terminated Si(111) via sonochemical activated hydrosilylation

Efficient chemical functionalization of hydrogen-terminated Si(111) with simple and bifunctional 1-alkenes was achieved via novel sonochemical activated hydrosilylation, utilizing just a simple ultrasonic bath. It is an extremely mild method that allows the specific attachment of unprotected bifunctional alkenes such as undecenol, undecylenic acid, and even a heat/UV-sensitive alkene, bearing an activated leaving group (N-succinimidyl undecylenate), without suffering any degradation.     

Single-component and mixed ferrocene-terminated alkyl monolayers covalently bound to Si(111) surfaces

Self-assembled ferrocene monolayers covalently bound to monocrystalline Si(111) surfaces have been prepared from the attachment of an amine-substituted ferrocene derivative to a pre-assembled acid-terminated alkyl monolayer using carbodiimide coupling. This derivatization strategy yielded nanometer-scale clean, densely packed monolayers, with the ferrocene units being more than 20 A from the semiconductor surface. The amount of immobilized electroactive units could be varied in the range 2 x 10(-11) to approximately 3.5 x 10(-10) mol cm(-2) by diluting the ferrocene-terminated chains by inert n-decyl chains. The highest coverage obtained for the single-component monolayer corresponded to 0.25-0.27 bound ferrocene per surface silicon atom. The electrochemical characteristics of the mixed n-decyl/ferrocene-terminated monolayers were found to not depend significantly on the surface coverage of ferrocene units. The reversible one-electron wave of the ferrocene/ferrocenium couple was observed at E degrees ' = 0.50 +/- 0.01 V vs SCE, and the rate constant of electron transfer kapp was about 50 s(-1).     

Organotin and organogermanium linkers for simple, direct functionalization of polyoxotungstates

Fifteen Keggin-anion-derived polytungstates [TW11O39[MCH2CH2X]]n- (T = Si, Ge, Ga; M = Sn, Ge; X = COOH, COOCH3, CONH2, CN; n = 5, 6) were prepared in aqueous or aqueous-organic solution from the corresponding lacunary polytungstates and trichlorotin and -germanium precursors, and were isolated as caesium salts. The derivatized polytungstates were characterized by elemental analysis, multinuclear NMR spectroscopy, and cyclic voltammetry; they are stable in aqueous solution to pH 6-7. NMR spectroscopy revealed the presence of a second (beta1 or beta3) isomer in the tungstogallate derivatives. Acid hydrolysis of the ester and nitrile derivatives could be achieved without decomposition of the polytungstate moieties, and esterification and amidation of the carboxylate functions was straightforward using standard coupling techniques, e.g. the formation, isolation and characterization of [SiW11O39[Ge(CH2)2CONHCH2COOCH3]]5- from glycine methyl ester. Since the Cl3MCH2CH2X precursors are readily accessible by hydrostannation/germanation reactions with the corresponding alkenes, novel coupled polytungstates, such as [(SiW11O39GeCH2CH2COOCH2)4C]20- from pentaerythritol tetraacrylate, can also be prepared.     

Covalent attachment of acetylene and methylacetylene functionality to Si(111) surfaces: scaffolds for organic surface functionalization while retaining Si-C passivation of Si(111) surface sites

Si(111) surfaces have been functionalized with Si-CC-R species, where R = H or -CH3, using a two-step reaction sequence involving chlorination of H-Si(111) followed by treatment with Na-CC-H or CH3-CC-Na reagents. The resulting surfaces showed no detectable oxidation as evidenced by X-ray photoelectron spectroscopic (XPS) data in the Si 2p region, electrochemical measurements of Si-H oxidation, or infrared spectroscopy. The Si-CC-R-terminated surfaces exhibited a characteristic CC stretch in the infrared at 2179 cm-1, which was strongly polarized perpendicular to the Si(111) surface plane. XPS measurements in the C 1s region showed a low binding energy peak indicative of Si-C bonding, with a coverage that was, within experimental error, identical to that of the CH3-terminated Si(111) surface, which has been shown to fully terminate the Si atop sites on an unreconstructed Si(111) surface. The Si-CC-H-terminated surfaces were further functionalized by exposure to n-C4H9Li followed by exposure to para Br-C6H5-CF3, allowing for introduction of para -C6H5CF3 groups while maintaining the desirable chemical and electrical properties that accompany complete Si-C termination of the atop sites on the Si(111) surface.     

Quantum mechanics calculations of the thermodynamically controlled coverage and structure of alkyl monolayers on Si(111) surfaces

The heat of formation, Delta E, for silicon (111) surfaces terminated with increasing densities of the alkyl groups CH3- (methyl), C2H5- (ethyl), (CH3)2CH- (isopropyl), (CH3)3C- (tert-butyl), CH3(CH2)5- (hexyl), CH3(CH2)7- (octyl), and C6H5- (phenyl) was calculated using quantum mechanics (QM) methods, with unalkylated sites being H-terminated. The free energy, Delta G, for the formation of both Si-C and Si-H bonds from Si-Cl model compounds was also calculated using QM, with four separate Si-H formation mechanisms proposed, to give overall Delta G(S) values for the formation of alkylated Si(111) surfaces through a two step chlorination/alkylation method. The data are in good agreement with measurements of the packing densities for alkylated surfaces formed through this technique, for Si-H free energies of formation, Delta G(H), corresponding to a reaction mechanism including the elimination of two H atoms and the formation of a C=C double bond in either unreacted alkyl Grignard groups or tetrahydrofuran solvent.     

Effect of the electrostatic interaction on the redox reaction of positively charged cytochrome C adsorbed on the negatively charged surfaces of acid-terminated alkanethiol monolayers on a Au(111) electrode

The electrochemical properties of cytochrome c (cyt c) adsorbed on mixed self-assembled monolayers (SAMs) of 2-mercaptoethanesulfonate (MES)/2-mercaptoethanol (MEL) are compared with those on single-component SAMs of MES, MEL, and mercaptopropionic acid (MPA), using cyclic voltammetry and potential-modulated UV-vis reflectance spectroscopy. The rate constant of electron transfer (ET), k(et), of cyt c adsorbed on the SAM of MPA decreases from 1450 +/- 210 s(-1) at pH 7 to 890 +/- 100 s(-1) at pH 9. In contrast, the value of k(et) of cyt c on the SAM of MES is pH-independent at 100 +/- 15 s(-1). Those facts suggest that a large negative charge density on the SAM surface slows down the ET between cyt c and the electrode. The surface charge density of the SAM affects also the amount of electroactive cyt c, Gamma(e), which decreases from 10.0 +/- 1.0 to 5.3 +/- 1.1 pmol cm(-2) with increasing pH from 7 to 9 on the SAM of MPA. Similarly, the k(et) of cyt c adsorbed on the mixed SAMs of MES/MEL sharply decreases from 900 +/- 300 s(-1) to 110 s(-1) as the surface mole fraction of MES increases beyond 0.5, suggesting the presence of a negative surface charge threshold beyond which the rate of ET of cyt c is dramatically lowered. The decrease in the k(et) on the SAMs at high negative charge densities probably results from the confinement of adsorbed cyt c by the strong electrostatic force to an orientation that is not optimal for the ET reaction.     

Characterization of carboxylic acid-terminated self-assembled monolayers by electrochemical impedance spectroscopy and scanning electrochemical microscopy

Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are used to monitor changes in the ionization of monolayers of 11-mercaptoundecanoic acid. When using an anionic redox probe, Fe(CN)6(-4), the charge-transfer resistance of the 11-mercaptoundecanoic acid monolayer-modified interface increases in a sigmoidal fashion as the solution is made basic. The opposite effect is observed when using a cationic redox probe. The inflection points of these two titration curves, however, differ when using the different redox probes. This result is taken as being characteristic of the influence that applied potential has on the ionization of the monolayer. The role of substrate potential on the ionization of the monolayer is further investigated by SECM. The SECM measurement monitors the concentration of Ru(NH3)6(+3) as the potential of the substrate is varied about the potential of zero charge. For monolayers of 11-mercaptoundecanoic acid in solutions buffered near the pKa of the terminal carboxylic acid, potential excursions positive of the PZC cause an increase in the concentration of Ru(NH3)6(+3) local to the interface, and potential excursions negative of the PZC cause a decrease in the local concentration of Ru(NH3)6(+3). Similar experiments conducted with an interface modified with 11-undecanethiol had no impact on the local concentration of Ru(NH3)6(+3). These results are interpreted in terms of the influence that applied potential has on the pH of the solution local to the interface and the impact that this has on the ionization of the monolayer.     

Study on surface acid-base property of carboxylic acid-terminated self-assembled monolayers by cyclic voltammetry and electrochemical impedance spectroscopy

Cyclic voltammetry and electrochemical impedance spec-troscopy were used to study the surface acid-base property of carboxylic acid-terminated self-assembled monolayers(SAMs).A carboxylic acid-terminated thiol,such as thioctic acid(1,2-dithiolane-3-pentanoic acid),was self-assembled on gold electrodes.Electron transfer between the bulk solution and the SAM modified electrode was studied at different pH using Fe(CN)63-as a probe.The surface pka of thioctic acid was determined by cyclic voltammetry and electrochemical impedance spectroscopy to be 5.6 ±0.1 and 5.8±0.1,respectively.The method is compared with other methods of monolayer pKa measurement.