Methyl Termination Research Articles

Green Nanoengineered Fabrics: Waste-Derived Polyphenol-Zinc@ Silica Core-Shell Reactive Janus Nanoparticles for Functional Fabrics.

Fabricating Janus nanoparticle-functionalized fabrics with UV protection, strength enhancement, self-cleaning properties, and wash durability, with a biocompatible nature, is crucial in modern functional fabrics engineering. Particularly, tailoring multifunctional nanoparticles capable of exhibiting several distinct properties, utilizing low-cost raw materials, and adhering to green chemistry principles is pivotal. A fabrication strategy for developing multifunctional reactive Janus nanoparticles, utilizing waste-derived natural polyphenol (quercetin-3-glucuronide, myricetin-3-galactoside, gossypin, phlorizin, kaempferol, myricetin-3-arabinoside)-integrated zinc-silica core-shell Janus nanoparticles with UV protection, strength enhancement, and self-cleaning properties, is proposed. Polyphenols were utilized as sustainable precursors for synthesizing zinc-polyphenol complexes, which were then encapsulated within a silica shell to form a core-shell structure. Furthermore, Janus particles were created by introducing a bifunctional layer with half amine/carboxylic acid and half methyl terminals, imparting reactive hydrophilic and hydrophobic properties. Janus-coated textiles and leather exhibited significant attenuation of harmful UV radiation, with water contact angle measurements confirming improved water repellency. The coexistence of natural phenols and bifunctional groups within a material bolstered textile strength, fostering superior adhesion and markedly enhancing wash durability. This eco-friendly approach, utilizing waste-derived materials, presents a promising solution for sustainable textile engineering with enhanced performance in UV protection and water resistance, thereby contributing to the advancement of green nanotechnology in textile applications.

Single Nucleotides Moving through Nanoslits Composed of Self-Assembled Monolayers via Equilibrium and Nonequilibrium Molecular Dynamics.

Nonequilibrium molecular dynamics (MD) simulations were used to study the effect of three chemical surface groups on the separation of DNA mononucleotide velocity (or time-of-flight) distributions as they pass through nanoslits. We used nanoslits functionalize with self-assembled monolayers (SAMs) since they have relatively smooth surfaces. The SAM molecules were terminated with either a methyl, methylformyl, or phenoxy group, and the nucleotides were driven electrophoretically with an electric field intensity of 0.1 V/nm in slits about 3 nm wide. Although these large driving forces are physically difficult to achieve experimentally, the simulations are still of great value as they provide molecular level insight into nucleotide translocation events and allow comparison of different surfaces. Nucleotides adsorbed and desorbed from the slit surface multiple times during the simulations. The required slit length for 99% accuracy in identifying the deoxynucleotide monophosphates (dNMPs), based on the separation of the distributions of time of flight, was used to compare the surfaces with shorter lengths indicating more efficient separation. The lengths were 6.5 μm for phenoxy-terminated SAMs, 270 μm for methylformyl-terminated SAMs, and 2400 μm for methyl-terminated SAMs. Our study showed that a slit with a section with methyl termination and the second section with methylformyl termination lead to a required length of 120 μm, which was significantly lower than for only a methylformyl- or methyl-terminated surface.

Area-Selective Atomic Layer Deposition of TiN Using Trimethoxy(octadecyl)silane as a Passivation Layer.

Area-selective deposition (ASD) offers tremendous advantages when compared with conventional patterning processes, such as the possibility of achieving three-dimensional features in a bottom-up additive fashion. Recently, ASD is gaining more and more attention from IC manufacturers and equipment and material suppliers. Through combination of self-assembled monolayer (SAM) surface passivation of the nongrowth substrate area and atomic layer deposition (ALD) on the growth area, ASD selective to the growth area can be achieved. With the purpose of screening SAM precursors to provide optimal passivation performance on SiO2, various siloxane precursors with different terminal groups and alkyl chains were investigated. Additionally, the surface dependence and growth inhibition of TiN ALD on -NH2, -CF3, and -CH3 terminations is investigated. We demonstrated the methyl termination of the SAM precursor combined with a C18 alkyl chain plays an important role in broadening the ALD selectivity window by suppressing precursor adsorption. Owing to the high surface coverage, excellent thermal stability and longer carbon chain length, an optimized trimethoxy(octadecyl)silane (TMODS) film deposited from liquid phase was able to provide a selectivity higher than 0.99 up to 20 nm ALD film deposited on hydroxyl-terminated Si oxide. The approach followed in this work can allow extending the ASD process window, and it is relevant for a wide variety of applications.

Molecular basis and potential applications of capsaicinoids and capsinoids against the elongation of etiolated wheat (Triticum aestivum L.) coleoptiles in foods

Capsaicinoids and capsinoids from dietary peppers have promising sensory properties and bioactivity, but the molecular basis of their penetration mechanism through cell lipid bilayers and its relationship to their bioavailability as food constituents are still poorly understood. Herein, statistically significant linear and quadratic quantitative structure-activity relationships were constructed to derive the essential structural elements required for their bioactivity against the elongation of etiolated wheat coleoptiles that mainly occurs via penetration. The resultant optimal models had high predictivity and reliability (r2 > 0.825 and r2pred > 0.950), which elucidate the importance of steric structural elements. Besides, their mechanistic hypothesis and rational design strategy were proposed, and the correlation between this bioactivity and their food-sensory properties was supposed. Finally, the bioactivity of newly designed analogs with methyl terminals and/or conjugated CC links was screened. Hopefully, this work would benefit the better understanding of their penetration mechanism and facile identification of bioactive analogs for designing food/drug formulations.

Enhanced Stability and Efficiency for Photoelectrochemical Iodide Oxidation by Methyl Termination and Electrochemical Pt Deposition on n-Type Si Microwire Arrays

Arrays of Si microwires doped n-type (n-Si) and surface-functionalized with methyl groups have been used, with or without deposition of Pt electrocatalysts, to photoelectrochemically oxidize I–(aq) to I3–(aq) in 7.6 M HI(aq). Under conditions of iodide oxidation, methyl-terminated n-Si microwire arrays exhibited stable short-circuit photocurrents over a time scale of days, albeit with low energy-conversion efficiencies. In contrast, electrochemical deposition of Pt onto methyl-terminated n-Si microwire arrays consistently yielded energy-conversion efficiencies of ∼2% for iodide oxidation, with an open-circuit photovoltage of ∼400 mV and a short-circuit photocurrent density of ∼10 mA cm–2 under 100 mW cm–2 of simulated air mass 1.5G solar illumination. Platinized electrodes were stable for >200 h of continuous operation, with no discernible loss of Si or Pt. Pt deposited using electron-beam evaporation also resulted in stable photoanodic operation of the methyl-terminated n-Si microwire arrays but yielded substantially lower photovoltages than when Pt was deposited electrochemically.

ATR FTIR Spectroscopic Study on Insect Body Surface Lipids Rich in Methylene-Interrupted Diene.

To protect themselves, insects cover their bodies with what is called cuticular lipid. The cuticular lipid of an American cockroach has a unique lipid content; the most abundant is a cis-alkadiene, cis, cis-6,9-heptacosadiene, amounting to about 70%, which is followed by a branched alkane 3-methylpentacosane. In order to clarify the structural features of the unique lipid composition below the critical temperature, the cuticular lipid was studied by Fourier transform infrared (FTIR) spectroscopy in combination with an attenuated total reflection (ATR) sampling technique. The infrared spectra measured on an extracted lipid sample at 20 °C suggested that the lipid keeps an appreciable level of conformational and lateral packing regularity, in spite of a high cis-unsaturated lipid content, and also a highly disordered condition around the methyl terminals and cis-olefin groups. The CH2 scissoring and the CH2 rocking regions showed the characteristics of the O⊥ subcell. The same characteristics were observed also by in situ measurements on a forewing of the American cockroach. Combining the spectral features of these bands and the physicochemical properties of each component, it can be inferred that saturated lipids form highly ordered domains within the liquid containing the cis, cis-diene as the main component. For comparison, the cuticular lipid of a male cricket, which consisted of many different hydrocarbons, including 15% of unsaturated hydrocarbons, showed a lower regularity both in the conformation and in the lateral packing of hydrocarbon chains. These results imply that not only the degree of cis-unsaturation but also the chemical structure diversity of hydrocarbons are the important factors to determine the physicochemical properties of cuticular lipid.

A systematic rheological study of alkyl amine surfactants for fluid mobility control in hydrocarbon reservoirs

The basis of this study is to identify the versatility of N,N,N′-trimethyl-N′-tallow-1,3-diaminopropane (DTTM) surfactant in high saline environments. The surfactant was examined with sodium chloride, NaCl, to understand how triggers such as salt, pH, temperature, and surfactant concentration influences the viscoelastic response of the surfactant solution. The DTTM surfactant and salt (NaCl) concentrations used in steady-state shear viscosity analysis range from 0.2 wt% to 2 wt% and 5 wt% to 25 wt%, respectively. Along with DTTM results, three similar chemical structures are investigated to understand how viscosity changes with alterations in tail and head group composition. It was found that DTTM surfactant has the capability of transitioning from a foam-bearing to viscoelastic state at low surfactant concentrations under moderate to high saline conditions. A longer tail length promotes viscoelasticity and shear-thinning behavior. Terminals consisting of hydroxides or ethoxylates have a lower viscosity than that of methyl terminals. A head group consisting of two nitrogen atoms has a higher viscosity than those containing one nitrogen atom. The rheological characterization of DTTM presented in this paper is part of a larger study in determining the capability of this surfactant to foam CO2 for improving mobility control in CO2 enhanced oil recovery in high saline oil formations.

Synthesis of Polyethylenes with Controlled Branching with α-Diimine Nickel Catalysts and Revisiting Formation of Long-Chain Branching

The synthesis of polyethylenes with precise branching, especially long-chain branching (LCB), using ethylene monomer as a single feedstock is of a significant academic and industrial interest. On the basis of the ortho-aryl effect, a series of α-diimine nickel complexes with monoaryl-substituted anilines has been designed and prepared for the synthesis of the polyethylenes with controlled branching. The introduction of the ortho-aryl on aniline moieties enhanced the branching control ability of the α-diimine nickel catalysts. A different mechanistic model was proposed to interpret the presence of methyl and LCB but absence of other short branches in the obtained polyethylenes. LCB was formed by ethylene insertion into the primary Ni-alkyl species originating from nickel migration to methyl terminal of the growing chain because of restricted ethylene insertion into secondary Ni-alkyl species with an α-ethyl or a bulkier alkyl group.

DFT calculations of the surface and the electronic structure of silicon core approximants for sensing

Silicon core approximants terminated with hydrogen, hydroxyl and methyl termination were investigated by density-functional-theory calculations within B3LYP hybrid functional formalism using 6–31G(d) Gaussian type molecular-orbital basis set. Optimized geometric structures including bond lengths and bond angles for all clusters were obtained. The electronic structures of the various clusters are calculated including density of state analysis, natural bond orbital analysis and the HOMO-LUMO orbitals analysis. The influences of the Si surface bond termination on charge transfer and local breakdown of the MO symmetry were discussed. UV–vis spectra of all the clusters mentioned above are also calculated. The g tensor for all structures are studied and discussed. All data show the SiO double bond is found to be the most probable surface structures of the Si quantum dots exposed to the atmosphere of H2O, O2 and CH3CH2OH gases. Of course it still need for further studies and verification of experiments. The investigation will provide a theoretical basis for the sensing of silicon quantum dots.

Fabrication of axial p-n junction silicon nanopillar devices and application in photovoltaics

A photovoltaic device based on plasma-etched silicon nanopillar (SiNP) arrays is demonstrated. Highly ordered, aligned and perpendicular to the substrate pure crystalline silicon nanopillars, that follow the axial p-n junction geometry, are fabricated by polystyrene colloidal particle self-assembly followed by cryogenic silicon plasma etching. The SiNPs surface electrical passivation by both methyl termination and hydrogen termination is examined and the encapsulation of the pillars by PMMA and transparent conductive oxide is demonstrated. The total reflectance (both specular and diffuse) of SiNP arrays of different period and diameter is measured. The total reflectance of silicon is reduced significantly in a wide range of the optical spectrum. Electrical characterization of bundles of nanowires was performed in terms of I-V measurements under light illumination. A short-circuit current density (Jsc) of 18.9mA/cm2, an open-circuit voltage (Voc) of 367mV and a fill factor (FF) of 0.59 were obtained on such axial p-n SiNP array photovoltaic devices.

Controlling noncovalent interactions between a lysine-rich α-helical peptide and self-assembled monolayers of alkanethiols on Au through functional group diversity

Reliably attaching a structured biomolecule to an inorganic substrate would enable the preparation of surfaces that incorporate both biological and inorganic functions and structures. To this end, we have previously developed a procedure using the copper(I)-catalyzed click reaction to tether synthetic α-helical peptides carrying two alkyne groups to well-ordered alkanethiol self-assembled monolayers (SAM) on a Au(111) surface, in which the SAM is composed of a mixture of methyl and azide termination. Proteins, however, are composed of many diverse functional groups, and this composition directly effects protein structure, interactions, and reactivity. Here, we explore the utility of mixed SAMs with alternative terminating functional groups to tune and direct the reactivity of the surface through noncovalent peptide-surface interactions. We study both polar surfaces (OH-terminated) and charged surfaces (COOH- and NH3-terminated, which are negatively and positively charged, respectively, under our reaction conditions). Surfaces were functionalized with a bipolar peptide composed of Lys and Leu residues that could express different interactions through either hydrophilic and/or charge (Lys) or hydrophobic (Leu) influences. X-ray photoelectron spectroscopy (XPS) and surface infrared spectroscopy were used to characterize surfaces at all stages of the peptide functionalization procedure. This strategy resulted in a high density of surface-bound α-helices without aggregation. Mixed SAMs that included a positively charged alkanethiol along with the azide-terminated thiol resulted in a more efficient reaction and better alignment of the peptide with the azide on the surface. Negatively charged surfaces increased physisorption of the peptide, which was then removed during sample rinsing. This work demonstrates that varying easily controlled chemical inputs during the functionalization steps allows the reaction conditions to be balanced for the chemical needs of a particular biomolecule or substrate.

Enhancement of self-assembly and gelation ability of N,N’-didodecanoyl ethylenediamine organogelator by terminal functionalization

Functionalization at the methyl terminals of N,N′-didodecanoyl ethylenediamine was found to provide a much more powerful low molecular-weight gelators. Replacement of the terminal methyl groups of diamides with carboxy groups effectively enhanced the molecular self-assembly. Terminal functionalization enables organogelator to switch to hydrogelator. FTIR spectra and SEM imaging indicated that an intermolecular hydrogen bonding of the central amide groups were not interfered by terminal functional groups.

Atomic Surface Structure of CH3-Ge(111) Characterized by Helium Atom Diffraction and Density Functional Theory

The atomic-scale surface structure of methyl-terminated germanium (111) has been characterized by using a combination of helium atom scattering and density functional theory. High-resolution helium diffraction patterns taken along both the ⟨121⟩ and the ⟨011⟩ azimuthal directions reveal a hexagonal packing arrangement with a 4.00 ± 0.02 A lattice constant, indicating a commensurate (1 × 1) methyl termination of the primitive Ge(111) surface. Taking advantage of Bragg and anti-Bragg diffraction conditions, a step height of 3.28 ± 0.02 A at the surface has been extracted using variable de Broglie wavelength specular scattering; this measurement agrees well with bulk values from CH3-Ge(111) electronic structure calculations reported herein. Density functional theory showed that methyl termination of the Ge(111) surface induces a mild inward relaxation of 1.66% and 0.60% from bulk values for the first and second Ge–Ge bilayer spacings, respectively. The DFT-calculated rotational activation barrier of a sin...

Sum frequency generation image reconstruction: aliphatic membrane under spherical cap geometry.

The article explores an opportunity to approach structural properties of phospholipid membranes using Sum Frequency Generation microscopy. To establish the principles of sum frequency generation image reconstruction in such systems, at first approach, we may adopt an idealistic spherical cap uniform assembly of hydrocarbon molecules. Quantum mechanical studies for decanoic acid (used here as a representative molecular system) provide necessary information on transition dipole moments and Raman tensors of the normal modes specific to methyl terminal - a typical moiety in aliphatic (and phospholipid) membranes. Relative degree of localization and frequencies of the normal modes of methyl terminals make nonlinearities of this moiety to be promising in structural analysis using Sum Frequency Generation imaging. Accordingly, the article describes derivations of relevant macroscopic nonlinearities and suggests a mapping procedure to translate amplitudes of the nonlinearities onto microscopy image plane according to geometry of spherical assembly, local molecular orientation, and optical geometry. Reconstructed images indicate a possibility to extract local curvature of bilayer envelopes of spherical character. This may have practical implications for structural extractions in membrane systems of practical relevance.

What a difference a bond makes: the structural, chemical, and physical properties of methyl-terminated Si(111) surfaces.

The chemical, electronic, and structural properties of surfaces are affected by the chemical termination of the surface. Two-step halogenation/alkylation of silicon provides a scalable, wet-chemical method for grafting molecules onto the silicon surface. Unlike other commonly studied wet-chemical methods of surface modification, such as self-assembly of monolayers on metals or hydrosilylation on silicon, the two-step method enables attachment of small alkyl chains, even methyl groups, to a silicon surface with high surface coverage and homogeneity. The methyl-terminated Si(111) surface, by comparison to hydrogen-terminated Si(111), offers a unique opportunity to study the effects of the first surface bond connecting the overlayer to the surface. This Account describes studies of methyl-terminated Si(111), which have shown that the H-Si(111) and CH3-Si(111) surfaces are structurally nearly identical, yet impart significantly different chemical and electronic properties to the resulting Si surface. The structure of methyl-terminated Si(111) formed by a two-step halogenation/methylation process has been studied by a variety of spectroscopic methods. A covalent Si-C bond is oriented normal to the surface, with the methyl group situated directly atop a surface Si atom. Multiple spectroscopic methods have shown that methyl groups achieve essentially complete coverage of the surface atoms while maintaining the atomically flat, terraced structure of the original H-Si(111) surface. Thus, the H-Si(111) and CH3-Si(111) surface share essentially identical structures aside from the replacement of a Si-H bond with a Si-C bond. Despite their structural similarity, hydrogen and methyl termination exhibit markedly different chemical passivation. Specifically, CH3-Si(111) exhibits significantly greater oxidation resistance than H-Si(111) in air and in aqueous electrolyte under photoanodic current flow. Both surfaces exhibit similar thermal stability in vacuum, and the Si-H and Si-C bond strengths are expected to be very similar, so the results suggest that methyl termination presents a greater kinetic barrier to oxidation of the underlying Si surface. Hydrogen termination of Si(111) provides nearly perfect electronic passivation of surface states (i.e., less than 1 electronic defect per 40 million surface atoms), but this electronic passivation is rapidly degraded by oxidation in air or under electrochemical conditions. In contrast, methyl termination provides excellent electronic passivation that resists degradation due to oxidation. Moreover, alkylation modifies the surface electronic structure by creating a surface dipole that effectively changes the electron affinity of the Si surface, facilitating modification of the charge-transfer kinetics across Si/metal or Si/electrolyte junctions. This Account also briefly describes recent studies of mixed monolayers formed by the halogenation/alkylation of silicon. Mixed monolayers allow attachment of bulkier groups that enable secondary chemistry at the surface (e.g., attachment of molecular catalysts or seeding of atomic layer deposition) to be combined with methyl termination of remaining unreacted surface sites. Thus, secondary chemistry can be enabled while maintaining the chemical and electronic passivation provided by complete termination of surface atoms with Si-C bonds. Such studies of mixed monolayers demonstrate the potential use of a wet-chemical surface modification scheme that combines both chemical and electronic passivation.

Methyl Termination and ATR-FTIR Evaluation of n-Si(111) Electrode towards Photoelectrochemical Cell Fabrication

We confirmed methyl termination on n-Si(111) surface by ATR-FTIR measurement, which was fabricated by a photo chloro-reaction and its methylation. The coverage of the methylation was about 63.7%, and the surface was not re-terminated by hydrogen. Photoelectrochemical properties of the n-Si(111) were measured as an electrode for a photoelectrochemical cell, and an onset potential obtaining photocurrent for the methyl terminated n-Si(111) electrode was observed as negative shift at 70 mV comparing with that of the hydrogen terminated n-Si(111) electrode. Therefore, the negative shift would be expected for improving open circuit voltage towards solar cell.

The role of surface chemistry-induced cell characteristics on nonviral gene delivery to mouse fibroblasts

BackgroundGene delivery approaches serve as a platform to modify gene expression of a cell population with applications including functional genomics, tissue engineering, and gene therapy. The delivery of exogenous genetic material via nonviral vectors has proven to be less toxic and to cause less of an immune response in comparison to viral vectors, but with decreased efficiency of gene transfer. Attempts have been made to improve nonviral gene transfer efficiency by modifying physicochemical properties of gene delivery vectors as well as developing new delivery techniques. In order to further improve and understand nonviral gene delivery, our approach focuses on the cell-material interface, since materials are known to modulate cell behavior, potentially rendering cells more responsive to nonviral gene transfer. In this study, self-assembled monolayers of alkanethiols on gold were employed as model biomaterial interfaces with varying surface chemistries. NIH/3T3 mouse fibroblasts were seeded on the modified surfaces and transfected using either lipid- or polymer- based complexing agents.ResultsTransfection was increased in cells on charged hydrophilic surfaces presenting carboxylic acid terminal functional groups, while cells on uncharged hydrophobic surfaces presenting methyl terminations demonstrated reduced transfection for both complexing agents. Surface–induced cellular characteristics that were hypothesized to affect nonviral gene transfer were subsequently investigated. Cells on charged hydrophilic surfaces presented higher cell densities, more cell spreading, more cells with ellipsoid morphologies, and increased quantities of focal adhesions and cytoskeleton features within cells, in contrast to cell on uncharged hydrophobic surfaces, and these cell behaviors were subsequently correlated to transfection characteristics.ConclusionsExtracellular influences on nonviral gene delivery were investigated by evaluating the upregulation and downregulation of transgene expression as a function of the cell behaviors induced by changes in the cells’ microenvronments. This study demonstrates that simple surface modifications can lead to changes in the efficiency of nonviral gene delivery. In addition, statistically significant differences in various surface-induced cell characteristics were statistically correlated to transfection trends in fibroblasts using both lipid and polymer mediated DNA delivery approaches. The correlations between the evaluated complexing agents and cell behaviors (cell density, spreading, shape, cytoskeleton, focal adhesions, and viability) suggest that polymer-mediated transfection is correlated to cell morphological traits while lipid-mediated transfection correlates to proliferative characteristics.

Simulated Structure and Nonlinear Vibrational Spectra of Water Next to Hydrophobic and Hydrophilic Solid Surfaces

Molecular dynamics simulations have been used to study the structure of water molecules adjacent to solid hydrophobic and hydrophilic surfaces. The hydrophobic surfaces resemble self-assembled monolayers with methyl termination, whereas the hydrophilic surfaces are terminated with hydroxyl groups. The resulting water structure is characterized by its density profile, order parameters, and molecular tilt-twist distribution as a function of distance from the surface. In both cases, results are compared to those obtained in bulk water and also to the vapor–water interface. To make a deeper connection to experimental studies, we have applied a frequency-domain approach to calculate the nonlinear vibrational spectra of the O–H stretching response. We have observed that, despite the sharp atomic discontinuity imposed by the surface, water next to a hydrophobic surface is similar in structure and spectral response to what is observed for the more diffuse vapor–water interface. At the hydrophilic surface, water ordering persists for a greater distance from the surface, and therefore the spectral response accumulates over a greater depth. In the strongly hydrogen bonded side of the spectrum, this is seen as an increased nonlinear susceptibility. However, in the energy region of the uncoupled O–H oscillators we demonstrate that the low experimental signal is likely not due to an absence of those species but instead a net cancellation of the microscopic response due to opposing water orientations over a distance well within the experimental coherence length.

Molecular Junctions of Self-Assembled Monolayers with Conducting Polymer Contacts

We present a method to fabricate individually addressable junctions of self-assembled monolayers (SAMs) that builds on previous studies which have shown that soft conductive polymer top contacts virtually eliminate shorts through the SAMs. We demonstrate devices with nanoscale lateral dimensions, representing an order of magnitude reduction in device area, with high yield and relatively low device-to-device variation, improving several features of previous soft contact devices. The devices are formed in pores in an inorganic dielectric layer with features defined by e-beam lithography and dry etching. We replace the aqueous PEDOT:PSS conductive polymer used in prior devices with Aedotron P, a low-viscosity, amphiphilic polymer, allowing incorporation of self-assembled monolayers with either hydrophobic or hydrophilic termination with the same junction geometry and materials. We demonstrate the adaptability of this new design by presenting transport measurements on SAMs composed of alkanethiols with methyl, thiol, carboxyl, and azide terminations. We establish that the observed room-temperature tunnel barrier is primarily a function of monolayer thickness, independent of the terminal group's hydrophilicity. Finally, we investigate the temperature dependence of transport and show that the low-temperature behavior is based on the energy distribution of sites from which carriers can tunnel between the polymer and gold contacts, as described by a model of variable-range hopping transport in a disordered conductor.

First principles analysis of the initial oxidation of Si(001) and Si(111) surfaces terminated with H and CH3

Transition state analyses have been carried out within a density functional theory setting to explain and quantify the distinctly different ways in which hydrogen and methyl terminations serve to protect silicon surfaces from the earliest onset of oxidation. We find that oxidation occurs via direct dissociative adsorption, without any energy barrier, on Si(111) and reconstructed Si(001) that have been hydrogen terminated; oxidation initiates with a barrier of only 0.05 eV on unreconstructed Si(001). The commonly measured protection afforded by hydrogen is shown to derive from a coverage-dependent dissociation rate combined with barriers to the hopping of adsorbed oxygen atoms. Methyl termination, in contrast, offers an additional level of protection because oxygen must first undergo interactions with these ligands in a three-step process with significant energy barriers: adsorption of O(2) into a C-H bond to form a C-O-O-H intermediate; decomposition of C-O-O-H into C-O-H and C=O intermediates; and, finally, hopping of oxygen atoms from ligands to the substrate.