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Alkanethiolates (ATs) forming self-assembled monolayers (SAMs) on coinage metal and semiconductor substrates have been used successfully for decades for tailoring the properties of these surfaces. Here, we provide a detailed analysis of a highly promising class of AT-based systems, which are modified by one or more dipolar carboxylic acid ester groups embedded into the alkyl backbone. To obtain comprehensive insight, we study nine different embedded-dipole monolayers and five reference nonsubstituted SAMs. We systematically varied lengths of the alkyl segments, ester group orientations, and number of ester groups contained in the chain. To understand the structural and electronic properties of the SAMs, we employ a variety of complementary experimental techniques, namely, infrared reflection absorption spectroscopy (IRS), high-resolution X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), atomic force microscopy (AFM), and Kelvin probe (KP) AFM. These experiments are comp...

In this Article, we experimentally measure the adsorption kinetics of human serum albumin (HSA) on a hydrophobic hexadecanethiolated gold surface. We use micromachined quartz crystal resonators with fundamental frequency of 83 MHz to accomplish these measurements in real time. In this work, we focus on two key results: (i) asymptotic behavior of the sensor responses upon HSA adsorption and (ii) the jamming limit of adsorbed layer formed by both single-injection and multi-injection experiments with the same value of final concentration. We develop a new interface-depletion modified random sequential adsorption (RSA) model to elucidate the adsorption kinetics and the transport properties of the protein molecules. Analysis of the experimentally measured data shows that the results can be explained on the basis of the exponentially depleting interfacial layer RSA model. To better understand the origin of the formation of the interfacial depletion region where the supply of protein molecules is dramatically reduced, we performed a series of molecular dynamics (MD) simulations using the ReaxFF method. These simulations predict that the resulting adsorption of the protein molecules on the thiolated surface results in a specific orientation at the interface and the diffusion constant of the protein molecules in this layer is significantly reduced. This interplay between the surface adsorption rate and the reduced diffusion coefficient leads to the depletion of the protein molecules in the interfacial layer where the concentration of the protein molecules is much less than the bulk concentration and explains the observed slowdown of the HSA adsorption characteristics on a hydrophobic surface.

Cyanide monolayers on Au{111} restructure from a hexagonal close-packed lattice to a mixed-orientation "ribbon" structure through thermal annealing. The new surface structure loses most of the observed surface features characterizing the initial as-adsorbed system with "ribbon" domain boundaries isolating rotationally offset surface regions where the orientation is guided by the underlying gold lattice. A blue shift to higher frequencies of the CN vibration to 2235cm-1 with respect to the as-adsorbed CN/Au{111} vibration at 2146 cm-1 is observed. In addition, a new low-frequency mode is observed at 145 cm-1, suggesting a chemical environment change similar to gold-cyanide crystallization. We discuss this new structure with respect to a mixed cyanide/isocyanide monolayer and propose a bonding scheme consisting of Au-CN and Au-NC bound molecules that are oriented normal to the Au{111} surface.

This article focuses on the use of molecular films as building blocks for nanolithography. More specifically, it reviews efforts aimed at utilizing organic molecular assemblies in overcoming the limitations of lithography, including self-patterning and directed patterning. It considers the methods of patterning self-assembled organic monolayer films through soft-lithographic methods such as microcontact printing and nanoimprint lithography, through direct ‘write’ or ‘machine’ processes with a nanometer-sized tip and through exposure to electron or photon beams. It also discusses efforts to pattern the organic assemblies via the physicochemical self-assembling interactions, including patterning via phase separation of chemically different molecules and insertion of guest adsorbates into host matrices. Furthermore, it examines the efforts that have been made to couple patterned molecular assemblies with inorganic thin-film growth methods to form spatially constrained, three-dimensional thin films. Finally, it describes a hybrid self-assembly/conventional lithography (i.e. molecular rulers) approach to forming nanostructures.

Static charge transport (CT) properties of nitrile-substituted oligophenylenes and oligo(phenylene ethynylene)s (NC-OPh and NC-OPE, respectively) assembled via the thiolate anchor on gold substrates were measured by the mercury drop junction technique. The derived attenuation factors (β), viz. 0.53 ± 0.1 and 0.30 ± 0.08 A–1 for the NC-OPh and NC-OPE monolayers, respectively, correlate well with the literature values for the analogous nonsubstituted systems, suggesting that the attachment of the nitrile group to the OPh or OPE backbone does not significantly affect their transport properties. This finding provides a basis for the use of the nitrile moiety as a resonantly addressable group and as specific charge injection site in the measurements of dynamic CT by resonant Auger electron spectroscopy. The comparison between the static and dynamic β values for the NC-OPh monolayers implies that the static value corresponds to nonresonant injection conditions. This suggests, potentially, that the static CT can...

A vibronic resonance between Au{111} surface states and adsorbed CN vibrations has been predicted, which we target for study. We have formed stable monolayers of cyanide on Au{111} and observe a hexagonal close-packed lattice with a nearest neighbor distance of 3.8 ± 0.5 A. Cyanide orients normal to the surface attached via a Au–C bond. We show that the substrate–molecule coupling is particularly strong, leading to ultrafast electron transfer from the cyanide molecules to the Au{111} substrate as measured by resonant Auger spectroscopy using the core–hole clock method. The CN/Au{111} system is a simple example of a strongly interacting adsorbate–substrate system and will be the subject of a number of further studies, as discussed.

Microbolometer-grade vanadium oxide (VOx) thin films with 1.3 < x < 2.0 were prepared by pulsed direct-current (DC) sputtering using substrate bias in a controlled oxygen and argon environment. These films were systematically alloyed with Ti, Nb, Mo, and Zr using a second gun and radiofrequency (RF) reactive co-sputtering to probe the effects of the transition metals on the film charge transport characteristics. The results reveal that the temperature coefficient of resistance (TCR) and resistivity are unexpectedly similar for alloyed and unalloyed films up to alloy compositions in the ∼20 at.% range. Analysis of the film structures for the case of the 17% Nb-alloyed film by glancing-angle x-ray diffraction and transmission electron microscopy shows that the microstructure remains even with the addition of high concentrations of alloy metal, demonstrating the robust character of the VOx films to maintain favorable electrical transport properties for bolometer applications. Postdeposition thermal annealing of the alloyed VOx films further reveals improvement of electrical properties compared with unalloyed films, indicating a direction for further improvements in the materials.

Femtosecond charge-transfer dynamics in self-assembled monolayers of cyano-terminated ethane-thiolate on gold substrates was investigated with the core hole clock method. By exploiting symmetry selection rules rather than energetic selection, electrons from the nitrogen K-shell are state-selectively excited into the two symmetry-split π* orbitals of the cyano end group with X-ray photons of well-defined polarization. The charge-transfer times from these temporarily occupied orbitals to the metal substrate differ significantly. Theoretical calculations show that these two π* orbitals extend differently onto the alkane backbone and the anchoring sulfur atom, thus causing the observed dependence of the electron-transfer dynamics on the symmetry of the orbital.

Self-assembled monolayers (SAMs) of nitrile-substituted oligo(phenylene ethynylene) thiols (NC-OPEn) with a variable chain length n (n ranging from one to three structural units) on Au(111) were studied by synchrotron-based high-resolution X-ray photoelectron spectroscopy and near-edge absorption fine-structure spectroscopy. The experimental data suggest that the NC-OPEn molecules form well-defined SAMs on Au(111), with all the molecules bound to the substrate through the gold–thiolate anchor and the nitrile tail groups located at the SAM–ambient interface. The packing density in these SAMs was found to be close to that of alkanethiolate monolayers on Au(111), independent of the chain length. Similar behavior was found for the molecular inclination, with an average tilt angle of ~33–36° for all the target systems. In contrast, the average twist of the OPEn backbone (planar conformation) was found to depend on the molecular length, being close to 45° for the films comprising the short OPE chains and ~53.5° for the long chains. Analysis of the data suggests that the attachment of the nitrile moiety, which served as a spectroscopic marker group, to the OPEn backbone did not significantly affect the molecular orientation in the SAMs.

In this paper, we use micromachined, high-frequency, quartz bulk acoustic wave resonators to systematically study the physical and viscoelastic properties of spontaneously adsorbed globular protein films with molecular weights (MWs) spanning three decades on hydrophobic surfaces. Specifically, changes in the frequency and the Q -factor of the micromachined resonator array were studied as a function of concentration for five proteins, namely human serum albumin (HSA), immunoglobulin G (IgG), human fibrinogen (Fib), alpha-2-macroglobulin (AMG), and immunoglobulin M (IgM) at the fundamental and third resonance modes. The results obtained were interpreted using equivalent electrical impedance models for the multilayer stack on the quartz crystal microbalances surface. Discrete changes in the protein adsorption and the viscoelastic behavior with solution concentration were observed for all the five protein films. The spherical core-shell protein model is used to provide a simple explanation of the results. The work presents the first systematic and quantitative evaluation of the density, thickness, viscosity, and elastic modulus of adsorbed globular protein films and demonstrates the advantages of using micromachined high-frequency bulk acoustic wave resonators for obtaining these types of data.

Immersion of oxidized aluminum substrates in ethanol solutions of poly(acrylic acid) (PAA), followed by extensive solvent immersion, results in tenaciously chemisorbed, nanometer scale, controllable thickness films for a wide range of solution concentrations and molecular weights. Atomic force microscope images reveal isolated polymer globules from adsorption in low-concentration solutions with crossover to conformal, highly uniform, nanometer-thickness films at higher concentrations, an indication that the chemisorbing chains start to overlap and trap underlying segments to form planar chemisorbed films only two or three chains in thickness. Quantitative IR reflection spectroscopy in combination with chemical derivitization on a standard set of 1.0(±0.2) nm thick films reveals a film structure with 5.5(±1) chemisorbed -CO(-)(2) groups/nm(2) and 6.3 unattached -CO(2)H groups/nm(2), with up to ∼3.6/nm(2) available for chemical derivitization, a comparable number to typical self-assembled monolayer coverages of ∼4-5 molecules/nm(2). Thermal treatment of the ∼1 nm chemisorbed films, at even extreme temperatures of ∼150 °C, results in almost no anhydride formation via adjacent -CO(2)H condensation, in strong contrast to bulk PAA, a clear indication that the films have a frozen glass structure with effectively no segment and side group mobility. Overall, these results demonstrate that these limiting thickness nanometer films provide a model surface for understanding the behavior of strongly bound polymer chains at substrates and show potential as a path to creating highly stable, chemically functionalized inorganic substrates with highly variable surface properties.

The maximum capacity of a hydrophobic adsorbent is interpreted in terms of square or hexagonal (cubic and face-centered-cubic, FCC) interfacial packing models of adsorbed blood proteins in a way that accommodates experimental measurements by the solution-depletion method and quartz-crystal-microbalance (QCM) for the human proteins serum albumin (HSA, 66 kDa), immunoglobulin G (IgG, 160 kDa), fibrinogen (Fib, 341 kDa), and immunoglobulin M (IgM, 1000 kDa). A simple analysis shows that adsorbent capacity is capped by a fixed mass/volume (e.g. mg/mL) surface-region (interphase) concentration and not molar concentration. Nearly analytical agreement between the packing models and experiment suggests that, at surface saturation, above-mentioned proteins assemble within the interphase in a manner that approximates a well-ordered array. HSA saturates a hydrophobic adsorbent with the equivalent of a single square-or-hexagonally-packed layer of hydrated molecules whereas the larger proteins occupy two-or-more layers, depending on the specific protein under consideration and analytical method used to measure adsorbate mass (solution depletion or QCM). Square-or-hexagonal (cubic and FCC) packing models cannot be clearly distinguished by comparison to experimental data. QCM measurement of adsorbent capacity is shown to be significantly different than that measured by solution depletion for similar hydrophobic adsorbents. The underlying reason is traced to the fact that QCM measures contribution of both core protein, water of hydration, and interphase water whereas solution depletion measures only the contribution of core protein. It is further shown that thickness of the interphase directly measured by QCM systematically exceeds that inferred from solution-depletion measurements, presumably because the static model used to interpret solution depletion does not accurately capture the complexities of the viscoelastic interfacial environment probed by QCM.

We have designed and fabricated 60 MHz bulk acoustic wave transducers and demonstrated the efficacy of these devices for probing the gravimetric and viscoelastic properties of layers in intimate contact with one of the electrodes. By integrating a 100 nm thick nanoporous film as the top electrode of the quartz resonator, we have been able to obtain enhanced sensitivity to mass loading and investigate the physical properties of liquids confined within 25–35 nm pores. By experimentally measuring the change in resonance frequency and Q- factor at the first and third resonance modes and using a detailed acoustic model in which the trapped liquid is treated as viscoelastic layer under a viscous liquid, the properties of trapped water-glycerol mixture confined in nanopores has been deduced.

Unambiguous identification of analyte biomolecules with high quantitative sensitivity and specificity continues to be a biosensing challenge. In this work, we demonstrate a micromachined quartz resonator array with nanoporous gold electrode as a highly sensitive substrate for biosensing applications. Furthermore, using surface enhanced Raman spectroscopy (SERS) the nanoporous gold substrate can be used to obtain label free detection of biomolecules with very high selectivity. Enhanced gravimetric sensitivity to 24-mer oligonucleotide binding reaction was achieved due to the increased electrode surface area. This dual mode sensing of biomaterials allows for unique identification of the attached biomolecules on the sensor surface via SERS whereas the quartz resonator provides a very accurate quantitative measure of the analyte. This paper presents the first results on the application of nanoporous gold (np- Au) electroded micromachined quartz resonators for detection of DNA hybridization reaction using simultaneous gravimetric and SERS measurements.

The dynamics of the charge transfer (CT) in alkanethiolate self-assembled monolayers is addressed by resonant Auger spectroscopy using the core hole clock method. The CT pathway was unambiguously defined by resonant excitation of the nitrile tailgroup attached to the alkyl backbone. The length of this backbone was varied to monitor the respective dependence of the CT time. It was found that, similar to the static conductance, this dependence can be coarsely described by an exponential function with an attenuation factor of 0.93 per methylene unit (0.72 A−1; a tunneling along the chain was assumed). As a result, the CT time is quite long even for a relatively short alkyl chain; in particular, it is ca. 100 fs for the chain consisting of only four CH2 units. In contrast, the CT time associated with the thiolate headgroup anchor was found to be quite short, viz. 2.3 fs, which suggests an efficient interfacial electronic coupling between the aliphatic backbone of the CnCN molecules and the substrate over the ...

In this paper we use micromachined, high-frequency, quartz bulk acoustic wave resonator arrays to evaluate the physical and viscoelastic properties of adsorbed globular protein films with molecular weights spanning two orders of magnitude. Specifically, changes in the frequency and the Q-factor of the micromachined resonator array were studied as a function of concentration for three proteins, namely Human Serum Albumin (HSA), Immunoglobulin G (IgG) and Human Fibrinogen (Fib) at the fundamental and third resonance modes. The results obtained are interpreted using continuum mechanics approach for multilayer stack on the QCM surface. Discrete changes in the protein adsorption rate constant and the viscoelastic behavior was observed for all the three protein films. The thickness, density, elastic modulus and viscosity of protein layer were successfully obtained through this analysis.

Micromachined quartz crystal resonator arrays operating in the 66–69 MHz fundamental mode range were tested for their ability to provide high sensitivity to mass loading and viscoelastic properties of biomolecules and biomolecular assemblies. Calibrations using viscous water–glycerol mixtures give the expected linear dependence on the square root of the density–viscosity product. Sequential adsorption of avidin layers interspersed with dithiobis(sulfosuccinimidylpropionate) (DTSSP) or biotynilated bovine albumin (BBA) cross-linker layers provided thick and planar viscoelastic layers for testing. The data reveal a high Q- factor sensitivity and the observed complex impedance changes could be accounted for using a layer dependent, variable viscosity model treated with continuum mechanics approach. The best interpretation of the frequency and Q- factor changes indicates that the layer material properties are most likely dominated by the frictional effects arising from the inter-linking molecules between the layers. These results show the ability of these high frequency micro resonators as an incisive tool for analyzing both static loading and dynamic viscoelastic properties of bimolecular adsorbates.

A new class of nonlithographically prepared surface enhanced Raman spectroscopy (SERS) substrates based on metalized, nanostructured poly(p-xylylene) films has been developed and optimized for surface plasmon response with a view to applications of SERS detection of microbial pathogens, specifically, bacteria and viruses. The main emphasis has been on achieving high spot to spot, sample to sample reproducibility of the SERS signals while maintaining useful enhancement factors. The use of these surfaces, metalized with either Ag or Au, provides a noninvasive and nondestructive method for spectral fingerprint analyses of both bacteria and viruses. Examples are given for the detection of bacteria (E. coli and B. cereus) and viruses (respiratory syncytial virus and Coxsackievirus). Our method is able to distinguish Gram positive from Gram negative bacterial strains as well as enveloped and nonenveloped viruses. The results demonstrate the development of a new class of SERS substrates which can provide rapid, selective identification of infectious agents without amplification of cultures.

An 8-pixel micromachined quartz crystal resonator array with a fundamental resonance frequency of 66 MHz has been designed, fabricated, and tested. A compact impedance-spectrum-analyzer electronic interface has been developed and combined with the quartz resonator array to form the biosensing system. The sensor array was calibrated using water-glycerol solutions, and the performance was found to be exactly as expected. Measurement of the crosstalk between the sensor pixels showed an isolation of ~ 30 dB. Selective functionalization of the pixels was achieved through the use of aqueous 3, 3'-Dithiobis (sulfosuccinimidylpropionate) (DTSSP) molecules. The adsorption of avidin on DTSSP gave a frequency signal of 60 kHz in comparison to unfunctionalized pixels. The specific adsorption of avidin on functionalized pixels was confirmed through fluorescence microscopy. Comparing the performance of the micromachined quartz crystal microbalance (QCM) with a commercial 5-MHz device, we found that the micromachined QCM has a 4.25 times higher signal-to-noise ratio. Based on the measurement of the noise and using three times the frequency noise as the limit for the detection of avidin molecules, we expect to resolve a minimum of ~ 1/960 of a monolayer of avidin corresponding to an aerial mass density resolution of 0.7 ng/cm2.

Surface-enhanced Raman spectroscopy (SERS) as the basis of rapid and reliable biosensing techniques has the significant advantages of minimal sample preparation and ease of operation, when compared with other methods. The SERS technique has received much attention over the years owing to its ability to reach single-molecule detection and for applications such as rapid DNA sequencing. Its use for biomedical analysis, however, has lagged behind because of the difficulties in finding effective substrates for accommodating biological entities that also provide high sensitivity and highly reproducible SERS responses from sample to sample. If these challenges are met, SERS could potentially be extremely useful in the field of medical diagnostics to detect infectious agents. Here, we report the development of SERS substrates that provide a highly uniform and reproducible bioanalysis surface and demonstrate examples of ‘‘fingerprint’’ signals from different types of bacteria, in which the signals differ based on variations in the cell membranes. Our method is unique in controlling surface morphology and topology prior to SERS-active metal deposition. As a result, we created a novel substrate that has a uniform and reproducible SERS signal over a large (>1mm 2 ) surface. The advantage of using these new types of substrates is that no template or lithography is involved, thus providing a simple, inexpensive, and quick method to achieve highly sensitive and spatially uniform SERS signals for biomedical applications.

Self-assembled monolayers (SAMs) on Au{111} prepared from the midchain ester functionalized thiols, HS(CH2)mCO2(CH2)n-1CH3 with m,n = 10,5; 10,10; 15,5; and 5,10, along with deuterated analogs, were characterized by wetting, ellipsometry, near edge X-ray absorption fine structure spectroscopy, quantitative infrared spectroscopy, high resolution X-ray photoelectron spectroscopy (HRXPS), and density functional theory calculations. The SAMs can be viewed as layered structures in which the bottom −(CH2)− alkyl segment (between the ester group and the substrate) exhibits typical conformational ordering and orientation analogous to long chain alkanethiolate SAMs, while the upper segment (ambient side) is significantly more conformationally disordered. The presence of the ester moiety leads to the formation of a strong electric dipole layer with a component of 1.05(±0.09) Debye normal to the surface. This dipole layer exhibits a strong electrostatic effect on the XPS spectra in which the C 1s photoelectron kinet...

We have designed and fabricated 25-microm-thick quartz resonators operating at a fundamental resonance frequency of approximately 62 MHz. The results show a substantial increase in the mass sensitivity compared to single monolithic commercial resonators operating at lower frequencies in the approximately 5-10-MHz range. The overall performance of the micromachined resonators is demonstrated for the example of human serum albumin protein adsorption from aqueous buffer solutions onto gold electrodes functionalized with self-assembled monolayers. The results show a saturation adsorption frequency change of 6.8 kHz as opposed to 40 Hz for a commercial approximately 5-MHz sensor under identical loading conditions. From the analysis of the adsorption isotherm, the equilibrium adsorption constant of the adsorption of the protein layer was found to be K = 8.03 x 10(6) M(-1), which is in agreement with the values reported in the literature. The high sensitivity of the miniaturized QCM devices can be a significant advantage in both vapor and solution adsorption analyses.

Whereas static conductance of molecules arranged in self-assembled monolayers has been extensively studied, little is known about the dynamics of the charge transfer in these systems. By the example of alkanethiolate films, we show that the latter phenomena can be successfully addressed by resonant Auger spectroscopy, using the core hole clock method. The charge transfer pathway was unambiguously defined by resonant excitation of the nitrile tailgroup attached to the alkyl backbone. Extension of this approach to other functional groups is possible, leading to new information valuable for molecular electronics and other applications.

Structural trends for a homologous series of n-alkanethiolate self-assembled monolayers (SAMs), C(n)H(2n+1)S- with 12or = nor = 19, on GaAs(001), studied by a combination of grazing incidence X-ray diffraction and infrared spectroscopy, along with ancillary probes, show an overall decay in organization with decreasing n, with the largest changes occurring below n = 15-16. The long-chain monolayers form a mosaic structure withor =10 nm domains of molecules organized in an incommensurate pseudo-hcp arrangement with nearest neighbor distances of 4.70 and 5.02 A, a 21.2 A(2) area per chain, two chains per subcell in a herringbone packing with a chain tilt angle of 14 degrees , and preferential domain alignment along the substrate [110]([110]) step edge direction. In contrast, for n14 no evidence of translational ordering is seen and the alkyl chains exhibit a loss of conformational ordering and coverage relative to the n16 cases. A 4'-methyl-biphenyl-4-thiolate companion SAM shows evidence for ordered structures but with lattice parameters close to those expected for a structure commensurate with the intrinsic GaAs(001) square lattice. These trends are explained on the basis of competitions between lattice, interfacial, and intermolecular forces controlling the nanoscale structures of the SAMs. Overall these results provide an important aspect of understanding the effects of SAM formation on surface properties such as electronic and chemical passivation.

High-resolution X-ray photoelectron spectroscopy (HRXPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and Raman scattering have been used to characterize the bonding and electronic properties of the interfaces formed by ambient-temperature, solution self-assembly of octadecanethiol and dodecanethiol under oxygen-free conditions on GaAs(001) surfaces. The combination of HRXPS and ToF-SIMS shows that both monolayers form direct S−[GaAs] attachment to the bare substrate with dominant As−S bonding and some fraction of Ga−S bonding, but the densely packed octadecanethiolate self-assembled monolayers (SAMs) are significantly more effective in protecting the interface from oxide regrowth compared to the less dense dodecanethiolate SAMs. Raman scattering measurements of the GaAs LO phonon modes indicate that the formation of these direct, oxide-free S−[GaAs] interfaces does not induce any significant changes in the GaAs surface electronic states, whereas control experiments with inorganic sulfide t...

Implementation of quantum information processing based on spatially localized electronic spins in stable molecular radicals is discussed. The necessary operating conditions for such molecules are formulated in self-assembled monolayer (SAM) systems. As a model system we start with 1,3-diketone types of neutral radicals. Using first principles quantum chemical calculations we prove that these molecules have stable localized electron spin, which may represent a qubit in quantum information processing.

Thin nanoporous gold (np-Au) films, ranging in thickness from approximately 40 to 1600 nm, have been prepared by selective chemical etching of Ag from Ag/Au alloy films supported on planar substrates. A combination of scanning electron microscopy (SEM) imaging, synchrotron grazing incidence small angle X-ray scattering, and N2 adsorption surface area measurements shows the films to exhibit a porous structure with intertwined gold fibrils exhibiting a spectrum of feature sizes and spacings ranging from several to hundreds of nanometers. Spectroscopic ellipsometry measurements (300-800 nm) reveal the onset of surface plasmon types of features with increase of film thicknesses into the approximately 200 nm film thickness range. Raman scattering measurements for films functionalized with a self-assembled monolayer formed from 4-fluorobenzenethiol show significant enhancements which vary sharply with film thickness and etching times. The maximum enhancement factors reach approximately 10(4) for 632.8 nm excitation, peak sharply in the approximately 200 nm thickness range for films prepared at optimum etching times, and show high spot to spot reproducibility with approximately 1 microm laser spot sizes, an indication that these films could be useful as durable, highly reproducible surface-enhanced Raman substrates.

Adsorption isotherms of nine globular proteins with molecular weight (MW) spanning 10–1000 kDa confirm that interfacial energetics of protein adsorption to a hydrophobic solid/aqueous-buffer (solid–liquid, SL) interface are not fundamentally different than adsorption to the water–air (liquid–vapour, LV) interface. Adsorption dynamics dampen to a steady-state (equilibrium) within a 1 h observation time and protein adsorption appears to be reversible, following expectations of Gibbs' adsorption isotherm. Adsorption isotherms constructed from concentration-dependent advancing contact angles θ a of buffered-protein solutions on methyl-terminated, self-assembled monolayer surfaces show that maximum advancing spreading pressure, , falls within a relatively narrow band characteristic of all proteins studied, mirroring results obtained at the LV surface. Furthermore, Π a isotherms exhibited a ‘Traube-rule-like’ progression in MW similar to the ordering observed at the LV surface wherein molar concentrations required to reach a specified spreading pressure Π a decreased with increasing MW. Finally, neither Gibbs' surface excess quantities [ Γ sl − Γ sv ] nor Γ lv varied significantly with protein MW. The ratio {[ Γ sl − Γ sv ]/ Γ lv }∼1, implying both that Γ sv ∼0 and chemical activity of protein at SL and LV surfaces was identical. These results are collectively interpreted to mean that water controls protein adsorption to hydrophobic surfaces and that the mechanism of protein adsorption can be understood from this perspective for a diverse set of proteins with very different composition.

The electronic properties of alkanethiolate [CH3(CH2)nS-, n = 9 and 11] and alkaneselenolate [CH3(CH2)nSe-, n = 9 and 11] self-assembled monolayers on Au{111} have been quantitatively compared. Simultaneously acquired apparent tunneling barrier height (ATBH) and scanning tunneling microscopy (STM) images reveal that alkanethiolate molecules have a lower barrier to tunneling, and therefore a higher conductance than alkaneselenolates of the same alkyl chain length. Molecular and contact conductance differences were elucidated by using observed STM topographic tunneling height differences between the analogous species. This apparent topographic difference combined with comparative ATBH data indicate that the observed decrease in conductance for alkaneselenolates compared to alkanethiolates originates exclusively from the Au-chalcogenide physical, chemical, and electronic contact.

A systematic comparison of Wilhelmy-balance tensiometry (WBT), tilting-plate goniometry (TPG) and captive-drop goniometry (CDG), using a variety of silane-treated glass surfaces of variable wetting characteristics is reported. WBT was assumed to be the benchmark of comparison (gold standard). Advancing angles measured by TPG and CDG were in statistical agreement with corresponding WBT measurements. In contrast, receding angles from both goniometric techniques were systematically higher than WBT.

In situ time-of-flight secondary ion mass spectrometry, infrared spectroscopy, and X-ray photoelectron spectroscopy measurements have been used to characterize the interfacial chemistry that occurs upon physical vapor deposition of Ti and Ca atoms onto a -OCH(3) terminated alkanethiolate self-assembled monolayer (SAM) on Au{111}. While the final result for both metals is near-exhaustive degradation of the methoxy terminal group and partial degradation of the alkyl chains to inorganic products such as carbides, hydrides, and oxides, the reaction mechanisms differ significantly. Titanium reacts in parallel with the -OCH(3) and -CH(2)- units, extensively degrading the latter until a metallic overlayer forms preventing further degradation. At this point, there is a cessation of the Ti-SAM reactions. In contrast, Ca is initially consumed by the -OCH(3) terminal group via a reaction mechanism involving two -OCH(3) groups; subsequent depositions lead to alkyl chain degradation, but at a rate slower than that for Ti deposition. These results demonstrate the subtle differences in chemistry that can arise in the vapor deposition of reactive metals, and have important implications for the behavior of electrical interfaces in organic and molecular devices made with Ti or Ca top contacts.

Vibrationally resonant sum-frequency generation (VR-SFG) and spectroscopic ellipsometry (SE) have been used to characterize self-assembled monolayer films of unsubstituted and mononitro-substituted oligo(phenylene−ethynylene) molecules on vapor-deposited Au substrates. When combined with quantum chemical calculations of the relevant transition moment directions, orientation distributions and electronic excitation spectra are obtained. The orientation distribution from VR-SFG is in good agreement with previous IR reflection studies, indicating both molecules are tilted from the surface normal by ∼30°. The calculated resonant hyperpolarizabilities are in good agreement with experimental spectra. The optical polarizability extracted from SE suggests strong intermolecular interactions, consistent with molecular exciton theory.

Coexisting adsorbate phases in high-coverage decaneselenolate and dodecaneselenolate [CH3(CH2)nSe, n = 9 and 11] self-assembled monolayers on Au{111} have been characterized by scanning tunneling microscopy and consist of two types: a densely packed distorted hexagonal lattice incommensurate to the underlying gold substrate, as revealed by the observation of a moire pattern, and a commensurate linear missing-row structure. Examination of the nearest neighbor distances in the tightly packed lattice reveal two distinct repeat distances of 4.90 and 5.20 A, which complements previous surface X-ray data. The linear missing row structure manifests in several variants of the (√3 × 3√3)R30° unit cell differentiated by whether the molecules bind at 2- or 3-fold substrate sites. While the number of molecules within this unit cell is typically two, in some cases an additional alkaneselenolate molecule is located at a site one Au atom lower than the rest. The structural conclusions are supported by excellent agreeme...

The design of future molecular electronic devices requires a firm understanding of the conduction mechanisms that determine their electrical characteristics. Progress toward this goal has been hindered by complications in controlling the exact configuration and makeup of fabricated molecular junctions, thus limiting the availability of quantitative experimental data for developing cohesive theories to model and predict molecular transport. This article summarizes recent research aimed at developing well-controlled systems for comparing molecular conduction and vibrational spectra using crossed-wire and in-wire metal–molecule–metal junctions. Systematic variations in molecular structure and metal–molecule contacts show strong quantitative agreement in device properties, while spectroscopic data provide evidence that the properties are due to the molecular junction. Further investigations using these and other molecular junction test beds will provide the needed experimental data to advance fundamental understanding of molecular transport and facilitate future molecular electronics applications.

The measured effect of temperature on charge transport through individual chemisorbed 1-nitro-2,5-di(phenylethynyl--mercapto)benzene molecules spanning a gold electrode gap is reported for the range of 10–300 K. Conduction is dominated by coherent superexchange at low temperatures and shifts to an incoherent hopping process above bias-dependent temperature thresholds. The results are consistent with proposed theoretical mechanisms in which bias-dependent heat dissipation from inelastic vibrational scattering within the molecule raises the temperature of the junction above that of the surrounding system, which in turn leads to the shift in the prevailing conduction process with increasing bias.

Nanowire metal−molecule−metal junctions containing dithoilated molecules of dodecane (C12), oligo(phenylene ethynylene) (OPE), and oligo(phenylene vinylene) (OPV) were prepared by replicating the pores of sub-40 nm diameter polycarbonate track etched membranes. Bottom Au−S or Pd−S contacts were made by potential-assisted molecule assembly onto the tips of the first segment of the electrochemically deposited nanowires. Top S−Ag or S−Pd contacts were formed by depositing Ag or Pd nanoparticles, which also served as a thin seed layer for electrodeposition of the second nanowire segment. Room-temperature current−voltage (I−V) measurements of individual nanowires show that the conductance of junctions formed with π-conjugated oligomers are several orders of magnitude larger than the saturated alkanes, with the OPV junctions having the highest conductance. The molecular wire junction conductance was also found to be dependent on the metal contacts with symmetric Pd−Pd junctions yielding the best metal−molecule ...

Self-assembled monolayers (SAMs) formed from nitrile-functionalized alkanethiols (AT), NC(CH2)16SH (NC-C16), on (111) gold and silver substrates were characterized by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and contact angle measurements. The average chain tilt angles in NC-C16/Ag and NC-C16/Au were estimated to be 29.5° ± 5° and 42.5° ± 5° from the surface normal, respectively, while the data suggest lower ordering for NC-C16/Au. The −C⋮N bonds were found to be predominantly oriented in the surface plane with a tilt angle of 65° ± 7° for both NC-C16/Au and NC-C16/Ag. Comparison with previous data on CH3-terminated SAMs reveals that substitution of weakly interacting CH3 groups by the CN entities results in an increase in the average tilt angles of the alkyl chains by ∼7.5° and ∼17.5° in AT/Au and AT/Ag, respectively. A strong electrostatic interaction between the polar nitrile groups is assumed to underlie the structural behavior by controlling a balance ...

Aluminum, copper and silver atoms are found to form a weakly solvated quasi-isotropic layer when vapor-deposited onto methoxy groups exposed at the surface of a hexadecanethiolate self-assembled monolayer on Au {1 1 1}. The nature of the interactions was revealed using SIMS, XPS and IRS, and supported by DFT calculations. This method complements 3D gas-phase cluster experiments by providing an approach for controlling solvation geometry and bonding via the molecular parameters of the monolayer. The results are discussed in terms of their applicability to the design of controlled interfaces, particularly metal contacts in molecular electronic devices.

Films of both methoxy-terminated alkanethiols and a molecular wire candidate on Au{1 1 1} substrates were exposed to a variety of transition and alkali earth metals (Al, Au, Ag, Ca, Cr, Fe, Cu, Mg, Ti). The results show that aggressively reacting metals, such as Ti, destroy the organic monolayer and metals of intermediate reactivity, e.g. Cu, react at the termini and also penetrate the monolayer and react with the substrate. The results of these investigations provide a basis by which future construction of molecular devices based on desired chemical reactivity may proceed.

We tracked over time the conductance switching of single and bundled phenylene ethynylene oligomers isolated in matrices of alkanethiolate monolayers. The persistence times for isolated and bundled molecules in either the ON or OFF switch state range from seconds to tens of hours. When the surrounding matrix is well ordered, the rate at which the inserted molecules switch is low. Conversely, when the surrounding matrix is poorly ordered, the inserted molecules switch more often. We conclude that the switching is a result of conformational changes in the molecules or bundles, rather than electrostatic effects of charge transfer.

Self-assembled monolayer films of a metallotetraazaphthalocyanine were prepared using two different reaction conditions. The films were characterized by ellipsometry, AFM, electronic absorption spe ...

Molecular resolution scanning tunneling microscopy (STM) imaging and high sensitivity infrared reflection spectroscopy (IRS) measurements have been combined to characterize the structures of mixed self-assembled monolayers (SAMs) of fully conjugated, linear thiolate-terminated molecules and short chain (8−12) n-alkanethiolates on Au{111}. Immersion of preformed, ordered alkanethiolate SAMs into dilute solutions of the conjugated molecules results in two-dimensional matrix isolation of conjugated adsorbates in the host SAM. The post-immersion host SAM matrix shows retained alkanethiolate ordering with the guest molecules inserted both singly into boundaries between SAM structural domains and in bundles at substrate step edges. Inserted molecules of lengths in the ∼15 A range adopt surface orientations similar to those of alkanethiolate molecules at all compositions, including the pure conjugated SAMs. In contrast, the configuration of an inserted, long conjugated molecule (∼40 A), varies monotonically with...

The interaction of vapor-deposited Al atoms with self-assembled monolayers of HS(CH2)15CO2H chemisorbed at polycrystalline Au(111) surfaces has been studied using time-of-flight secondary-ion mass spectrometry, X-ray photoelectron spectroscopy, and infrared spectroscopy. The Al deposition was performed incrementally at room temperature. The Al atoms do not penetrate into the organic monolayer, but rather they remain at the vacuum interface where they undergo chemical interactions solely with the CO2H groups. Reaction of the CO2H groups continues until slightly more than one atom per reacting group is deposited, on average; thereafter, no further reaction is observed. However, 20−25% of the CO2H groups remain unreacted, regardless of the Al coverage. These results are explained on the basis of a combination of chemical and steric effects.

The interaction of vapor-deposited Al atoms with self-assembled monolayers (SAMs) of HS(CH2)15- CH3 and HS(CH2)15CO2CH3 chemisorbed at Au{111} surfaces was studied using X-ray photoelectron spectroscopy, infrared spectroscopy, time-of-flight secondary ion mass spectrometry, and spectroscopic ellipsometry. For the CH3-terminated SAM, no reaction with C- Ho r C-C bonds was observed. For total Al doses up to 12 atoms/nm 2 , penetration to the Au-S interface occurs with no disruption of the average chain conformation and tilt, indicating formation of a highly uniform 1:1 Al adlayer on the Au. Subsequently, penetration ceases and a metallic overlayer begins to form at the SAM-vacuum interface. These results are explained in terms of an initial dynamic hopping of the -S headgroups on the Au lattice, which opens transient diffusion channels to the Au-S interface, and the closing of these channels upon completion of the adlayer. In contrast, Al atom interactions with the CO2CH3-terminated SAM are restricted to the vacuum interface, where in the initial stages discrete organometallic products form via reaction with the CO 2CH3 group. First, a 1:1 complex forms with a reduced CdO bond and an intact CH3 moiety. Further exposure leads to the additional reaction of about four Al atoms per ester, after which a metallic overlayer nucleates in the form of clusters. After the growth progresses to 30 A, the clusters coalesce into a uniform metallic film. These results illustrate the extraordinary degree of control that organic substrates can exert during the course of metal film formation.

Alkanethiolate monolayers on polycrystalline gold, formed by typical solution self-assembly conditions with immersion in dilute ethanolic solution of asymmetric dialkyl disulfides CH3(CH2)n-1SS(CH2)15CH3 (n = 4, 8, 12), were found to consist predominately of CH3(CH2)15S- moieties. This observation shows that large deviations from expected 1:1 compositions can occur. The data support an exchange mechanism involving dissociative chemisorption of the S−S moiety and a subsequent replacement of the shorter chains by the longer ones via short chain disulfide or thiolate desorption and readsorption of mixed chain length disulfide. The efficiency of the substitution process increases with increasing length difference of the hydrocarbon chains in the respective disulfides.

Using electrochemical and chemical probes, we demonstrate that thin films of crystalline polymethylene (PM), formed via solution decomposition of diazomethane on gold surfaces, exist in the form of a heterogeneous distribution with microscopically small pores at low average PM coverages. For evaporated gold substrates, as the PM loading increases above ∼0.8 μg·cm-2, a mass density equivalent to an ∼7 nm thick, fully dense planar film, a transition from a micropore to a blocked electrode occurs. This microstructural picture is confirmed by the ability to backfill open areas with alkanethiolates and electropolymerized aniline. Parallel experiments on sputter-deposited gold films show that the blocking threshold occurs at much lower average PM coverages and alkanethiolate chemisorption data suggest that the open pores are approaching the sizes of individual molecules. These results can be rationalized by a more uniform nucleation of PM nanocrystals across sputter-deposited relative to evaporated surfaces. Th...

The deposition of 2 A of Al metal onto a monolayer of methylester-terminated alkanethiolate (HS(CH 2 ) 15 CO 2 CH 3 ) self-assembled on polycrystalline Au(111) was studied using time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS) and infrared reflectance spectroscopy (IRS). The deposited Al was found to be highly reactive with the oxygen atoms in the self-assembled monolayer terminal functional group. No reactivity between Al and the methylene backbone of the monolayer was observed, nor was any Al observed at the monolayer/Au interface. However, the deposition of Al does induce some chain disordering.

Rubbing-induced surface reconstruction of the polyimide poly(biphenyl dianhydride-p-phenylenediamine) (BPDA-PDA), in the form of 5−300 nm thin films supported on silicon substrates, has been studied using high-precision polarized infrared vibrational spectroscopy in conjunction with spectral simulations. Under unidirectional rubbing, a reversible, selective reorientation of domains of initially well-ordered chains is observed, with the amount of reoriented material equivalent to an ∼1−3 nm thick surface layer, independent of the total film thickness. Accompanying this process are extensive perturbations of the imide ring structures, which include bond distortions of the [OC−N−CO] units and out-of-plane rotation of the rings around the chain axis, effectively causing a molecular-scale roughening along the aligned chain axes while maintaining the periodic repeat unit spacing. These effects are proposed to arise from dynamic chain−chain steric interactions that occur as chains align under the applied buffing...