The structure and phase diagram of Langmuir films of alcohols on mercury

The coverage-dependent phase behavior of molecular films of alcohols (CH3(CH2)n-2CH2OH, denoted as CnOH) on mercury was studied for chain lengths 8 < or = n < or = 28, using surface tensiometry and surface specific X-ray methods. Phases with surface-normal-oriented molecules are found at high coverage, showing the CS, S, and LS phases found also on water. Phases comprising surface parallel molecules, which do not exist on water, are found here at low coverage. For the lowest coverage a two-dimensional gas phase is found, followed, upon increasing the coverage, by an n-dependent sequence of condensed phases of up to four layers of surface-parallel molecules before converting to the surface-normal phases. In contrast with the surface-normal phases, all of the surface-parallel phases are found to lack long-range order in the surface-parallel direction. Adsorption energies are derived from the phase diagram for the alkyl chain and the alcohol headgroup.     

Alkyl-thiol Langmuir films on the surface of liquid mercury

The coverage dependent phase behavior of monolayers of alkyl thiols (CH3(CH2)(n-1)SH, denoted as CnSH) on mercury was studied for chain lengths 9 相似文献   

Formation of alkanethiol and alkanedithiol monolayers on GaAs(001)

The formation of alkanethiol (H-(CH2)n-SH, n = 8-18) and 1,8-octanedithiol (HS-(CH2)8-SH) monolayer films on n-type GaAs(001) has been systematically studied. We observed a nonlinear dependence of the film thickness on molecular length, which is drastically different from monolayer films of the same molecules on metals. For 8 < or = n < or = 14, the films are only 3-4.5 A thick, significantly smaller than the corresponding molecular length. For n = 16 and 18, the measured film thicknesses were 9 and 11 A, respectively, consistent with molecules orienting with a tilt angle of approximately 60 degrees from the surface normal. Unlike the alkanethiols, the thickness of the 1,8-octanedithiol monolayer is almost the same as its molecular length, indicating that dithiol molecules orient vertically with only one thiol end group bound to the GaAs surface. Additional support for this conclusion comes from the fact that X-ray photoelectron spectroscopy of the 1,8-octanedithiol monolayer clearly resolves two types of S atoms in the monolayer: those bound to the GaAs surface and those existing as free thiols. A suggestion was made on the mechanisms for alkanethiol and alkanedithiol monolayer formation.     

Temperature dependence of the structure of Langmuir films of normal-alkanes on liquid mercury

The temperature dependent phase behavior of Langmuir films of n-alkanes [CH3(CH2)(n-2)CH3, denote Cn] on mercury was studied for chain lengths 19< or =n< or =22 and temperatures 15< or =T< or =44 degrees C, using surface tensiometry and surface x-ray diffraction methods. In contrast with Langmuir films on water, where molecules invariably orient roughly surface normal, alkanes on mercury are always oriented surface parallel and show no long-range in-plane order at any surface pressure. A gas and several condensed phases of single, double, and triple layers of lying-down molecules are found, depending on n and T. At high coverages, the alkanes studied here show transitions from a triple to a double to a single layer with increasing temperature. The transition temperature from a double to a single layer is found to be approximately 5 degrees C, lower than the bulk rotator-to-liquid melting temperature, while the transition from a triple to a double layer is about as much below the double-to-single layer transition. Both monolayer and bulk transition temperatures show a linear increase with n with identical slopes of approximately 4.5 degrees C/CH2 within the range of n values addressed here. It is suggested that the film and bulk transitions are both driven by a common cause: the proliferation of gauche defects in the chain with increasing temperature.     

Comparison of bulk and thin film structures of the liquid crystal 28OBC: measurements and simulations

Freely-suspended liquid crystalline films of ethyl 4'-n-octyloxybiphenyl-4-carboxylate (28OBC) were prepared and transferred onto different substrates which enable detailed structural characterization. The structures of these thin film assemblies, which are only accessible in this way, were determined and compared with the crystal and molecular structure of 28OBC as formed by crystallization from toluene solution. The compound crystallizes in the monoclinic space group P21/c, a = 11.168(1) A, b = 7.595(2) A, c = 49.106(1) A, beta = 94.01(1) degree, Z = 8. The two symmetrically independent molecules of the asymmetric unit have been used as starting geometries for semi-empirical MO calculations. The difference between the experimentally observed and the optimized molecular structures is interpreted as the influence of the crystal field. The structures of the crystalline and E film phases have been investigated by SAXR and TED and the former has been found to be different from the bulk structure. The structural relationships between the different phases are discussed and a suggestion for the crystalline film structure is given as deduced from simulations of electron diffraction patterns.     

Self-assembly and odd-even effects of cis-unsaturated carboxylic acids on highly oriented pyrolytic graphite

The self-assembly of several cis-unsaturated carboxylic acids of the structure cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH on highly oriented pyrolytic graphite (HOPG) was studied. The impact of the interior cis-CH=CH group and the molecular chain length on their self-assembled structures was considered. Due to the cis conformation of the -HC=CH- group in the interior of these molecules, they display self-assembled structures significantly different from saturated acids with all-trans configurations. As an example of the class of molecules cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) (p=8, n=7), cis-CH3(CH2)7CH=CH(CH2)13COOH self-assembles into two kinds of enantiomer domains with opposite 2-D chirality. Due to the steric restriction of the interior cis-HC=CH group, all chains with acid groups are packed at the same side of a lamella, a head-to-head arrangement which is different from the head-to-tail packing of saturated all-trans acids. However, cis-CH3(CH2)7CH=CH(CH2)8COOH, considered as one example of the group cis-CH3(CH2)p-1CH=CH(CH2)2n-2COOH (p not equal 2n-1) (p=8, n=5), does not form any stable self-assembled domain, consistent with the molecular arrangement model. This difference in self-assembly behavior between cis-CH3(CH2)p-1CH=CH(CH2)2n-1COOH (p not equal 2n) and cis-CH3(CH2)p-1CH=CHC2n-2COOH (p not equal 2n-1) shows an odd-even chain-length effect of cis-CH3(CH2)p-1CH=CH(CH2)m-1COOH (p not equal m, m=2n or 2n-1). For another category of molecules, cis-unsaturated acids with equal numbers of all-trans carbon atoms on both sides of the cis-CH=CH group, cis-CH3(CH2)m-1CH=CH(CH2)m-1COOH (m=2n or 2n-1), display another odd-even effect. cis-CH3(CH2)7CH=CH(CH2)7COOH, one example of cis-CH3(CH2)2n-1-CH=CH(CH2)2n-1COOH (n=4), is predicted to form both an enantiomer and a nonchiral racemic structure, which is in accordance with the experimental observation of its self-assembled monolayer. However, cis-CH3(CH2)2n-2CH=CH(CH2)2n-2COOH does not form a stable self-assembled domain due to the same steric repulsion as that seen in the cis-CH3(CH2)7CH=CH(CH2)8COOH structure. These odd-even effects demonstrate that molecular self-assembly can be significantly tailored by slightly changing the molecular chain length.     

Effect of load on structural and frictional properties of alkanethiol self-assembled monolayers on gold: some odd-even effects

We have conducted molecular dynamics simulations to study the frictional properties of alkanethiols CH(3)(CH(2))(n-1)SH (Cn, 12 ≤ n ≤ 15) self-assembled monolayers (SAMs) on Au(111) surfaces, under various loading and shearing conditions. For the examined alkanethiols, we found some evidence of the friction coefficient being dependent on the number of carbon atoms in the molecule being odd or even. Alkanethiols with n = odd show consistently higher friction coefficients than those with n = even. Such odd-even effect seems to be independent of the sliding velocity. However, the effect is significant only at lower loads (<700 MPa). The structural origin of this odd-even effect has been discussed. The effect of loading on the structure is also studied. For dodecanethiol (n = 12) we find the film responds to increased loading initially by increasing the tilt and then by deformation of individual molecules. SAM-Au contacts under shear show periodic storage and release of energy and a clear stick-slip pattern in the shear stress, film thickness, and the tilt and tilt orientation angles.     

Theoretical investigation on the replacement of CH groups by N atoms in caged structure (CH)8

A molecular design was performed for the caged molecule (CH)8: the replacement of CH groups by N atoms to increase the content of N as well as reduce the content of H. A series of caged molecules were obtained: (CH)xN(8-x) (0 < or = n < or = 8). The studied aspects are as follows: (i) molecular geometries and electronic structures, (ii) the analysis of the electronic structure using natural bond orbital (NBO) and atoms in molecules (AIM), and (iii) some physicochemical properties of studied molecules, such as the dipole moments, IR vibrational spectra, NMR chemical shifts, heats of formation, and relative specific impulses, were provided. Our studies show that these molecules should be a kind of potential and novel energetic material. Our work provides some useful information for the experimental study of these molecules. The effect of the substitution of N atoms for CH groups on the properties of this kind of caged molecule is presented.     

Complexity in the self-assembly of bifunctional molecules on HOPG: the influence of solvent functionality on self-assembled structures

Self-assembled monolayers of bifunctional molecules HOOC(CH2)nCOOH (n = 20, 18, 16, 14, 12, 10), HOOC(CH2)nCH2OH (n = 13, 14), and HOCH2(CH2)14CH2OH dissolved in octanoic acid were investigated using scanning tunneling microscopy, to understand the self-assembly of bifunctional molecules and the influence of a carboxylic acid solvent on the formation of self-assembled structures on HOPG. In the series of di-acids (HOOC(CH2)nCOOH), only HOOC(CH2)20COOH forms stable coadsorption structures with the solvent octanoic acid. The remaining di-acids form stable single-component monolayers and do not coadsorb with solvent octanoic acid. Coadsorption structures involving mixtures of di-acids were observed. This result suggests that coadsorption with acid solvent or with other di-acids occurs to maximize hydrogen-bond density in the overlayer. A quantitative model based on this concept is proposed. For hetero-bifunctional molecules HOOC(CH2)nCH2OH (n = 13, 14), the coadsorption of HOOC(CH2)14CH2OH and octanoic acid at the molecular level produces a microscopic mesh made of homogeneously arranged openings with a dimension of approximately 12.5 A x approximately 5.0 A x approximately 1.8 A. For the hetero-bifunctional molecule HOOC(CH2)13CH2OH, hydroxyl groups of two adjacent lamellae assemble to form a herringbone geometry, and the two carboxylic acid groups assemble with a straight head-to-head configuration. In addition, a new mixed hydrogen-bonding network of COOH...O-H was observed in another self-assembled structure of this molecule. The bifunctional molecule HOCH2(CH2)14CH2OH exhibits multiple packing patterns on HOPG via different hydrogen-bonding networks. HOCH2(CH2)14CH2OH self-assembles using the H-O...O-H network typical of the n-alcohol herringbone structure, forming an asymmetric adsorbate on HOPG. It also forms domains with another hydrogen-bonding network, in which molecules in adjacent lamellae are parallel to each other. This investigation demonstrates the complexity and diversity of self-assembled structures formed from bifunctional molecules on solid surfaces. It also indicates that a solvent with the same functional group as the solute can significantly impact the formation of the self-assembled structures of these bifunctional molecules.     

Inverse opals of molecularly imprinted hydrogels for the detection of bisphenol A and pH sensing

Inverse opal films of molecularly imprinted polymers (MIP) were elaborated using the colloidal crystal template method. The colloidal crystals of silica particles were built by the Langmuir-Blodgett technique, allowing a perfect control of the film thickness. Polymerization in the interspaces of the colloidal crystal in the presence of bisphenol A (BPA) and removal of the used template provides 3D-ordered macroporous methacrylic acid-based hydrogel films in which nanocavities derived from bisphenol A are distributed within the thin walls of the inverse opal hydrogel. The equilibrium swelling properties of the nonimprinted (NIPs) and molecularly imprinted polymers (MIPs) were studied as a function of pH and bisphenol A concentration, while the molecular structures of the bulk hydrogels were analyzed using a cross-linked network structure theory. This study showed an increase in nanopore (mesh) size in the MIPs after BPA extraction as compared to NIPs, in agreement with the presence of nanocavities left by the molecular imprints of the template molecule. The resulting inverse opals were found to display large responses to external stimuli (pH or BPA) with Bragg diffraction peak shifts depending upon the hydrogel film thickness. The film thickness was therefore shown to be a critical parameter for improving the sensing capacities of inverse opal hydrogel films deposited on a substrate.     

Interaction of methanol with amorphous solid water

The interaction of methanol (MeOH) with amorphous solid water (ASW) composed of D2O molecules, prepared at 125 K on a polycrystalline Ag substrate, was studied with metastable-impact-electron spectroscopy, reflection-absorption infrared spectroscopy, and temperature-programmed desorption mass spectroscopy. In connection with the experiments, classical molecular dynamics (MD) simulations have been performed on a single CH3OH molecule adsorbed at the ice surface (T=190 K), providing further insights into the binding and adsorption properties of the molecule at the ice surface. Consistently with the experimental deductions and previous studies, MeOH is found to adsorb with the hydroxyl group pointing toward dangling bonds of the ice surface, the CH3 group being oriented upwards, slightly tilted with respect to the surface normal. It forms the toplayer up to the onset of the simultaneous desorption of D2O and MeOH. At low coverage the adsorption is dominated by the formation of two strong hydrogen bonds as evidenced by the MD results. During the buildup of the first methanol layer on top of an ASW film the MeOH-MeOH interaction via hydrogen-bond formation becomes of importance as well. The interaction of D2O with solid methanol films and the codeposition of MeOH and D2O were also investigated experimentally; these experiments showed that D2O molecules supplied to a solid methanol film become embedded into the film.     

Molecular orientation of evaporated pentacene films on gold: alignment effect of self-assembled monolayer

Pentacene films deposited on self-assembled monolayers (SAMs) bearing different terminal functional groups have been studied by reflection-absorption IR, grazing angle XRD, NEXAFS, AFM, and SEM analyses. A film with pentacene molecules nearly perpendicularly oriented was observed on Au surfaces covered with an SAM of alkanethiol derivative of X-(CH2)(n)-SH, with X = -CH(3), -COOH, -OH, -CN, -NH(2), C(60), or an aromatic thiol p-terphenylmethanethiol. On the other hand, a film with the pentacene molecular plane nearly parallel to the substrate surface was found on bare Au surface. A similar molecular orientation was found in thinner ( approximately 5 nm) and thicker (100 nm) deposited films. Films deposited on different surfaces exhibit distinct morphologies: with apparently smaller and rod-shaped grains on clean bare Au surface but larger and islandlike crystals on SAM-modified surfaces. X-ray photoemission electron microscopy (X-PEEM) was used to analyze the orientation of pentacene molecules deposited on a SAM-patterned Au surface. With the micro-NEXAFS spectra and PEEM image analysis, the microarea-selective orientation control on Au was characterized. The ability to control the packing orientation in organic molecular crystals is of great interest in fabricating organic field effect transistors because of the anisotropic nature of charge transport in organic semiconducting materials.     

Structure of mercaptobiphenyl monolayers on mercury

The molecular-scale structure and phase behavior of single-component Langmuir films of 4'-methyl-4-mercaptobiphenyl (MMB) and 4'-perfluoromethyl-4-mercaptobiphenyl (FMMB) on mercury were studied using surface tensiometry, grazing incidence X-ray diffraction, and X-ray reflectivity. At low coverages, a condensed but in-plane disordered single layer of surface-parallel molecules is found for both compounds. At high coverages, both compounds exhibit in-plane-ordered phases of standing-up molecules. For MMB, the biphenyl core dominates the structure, yielding a centered-rectangular unit cell with an area A(x) of 21.8 A(2)/molecule, with molecules tilted by approximately 14 degrees from the surface normal in the nearest-neighbor direction, and a coherence length xi of >1000 A for the crystalline domains. For FMMB, the perfluoromethyl group dominates the structure, yielding a hexagonal unit cell with untilted molecules, an area A(x) of 24.2 A(2)/molecule, and a much smaller xi of approximately 110 A. The structure is discussed in comparison with self-assembled monolayers of MMB on crystalline Au(111) and similar-length alkanethiolate SAMs on Au(111) and on mercury. The differences in the structure are discussed and traced to the differences in the substrate's surface structure, and in the molecular cross section and rigidity.     

High Molecular Orientation in Mono- and Trilayer Polydiacetylene Films Imaged by Atomic Force Microscopy

Atomically flat monolayer and trilayer films of polydiacetylenes have been prepared on mica and silicon using a horizontal deposition technique from a pure water subphase. Langmuir films of 10,12-pentacosadiynoic acid (I) and N-(2-ethanol)-10,12-pentacosadiynamide (II) were compressed to 20 mN/m and subsequently polymerized by UV irradiation at the air-water interface. Blue and red forms of the films were prepared by varying exposure times and incident power. Polymerization to the blue-phase films produced slight contractions of 2 and 5% for the films of II and I, respectively. Longer UV exposures yielded red-phase films with dramatic film contraction of 15 and 32% for II and I, respectively. The horizontal deposition technique provided transfer ratios of unity with minimal film stress or structure modification. Atomic force microscopy images revealed nearly complete coverage of the substrate with atomically flat films. Crystalline domains of up to 100 micrometers of highly oriented polydiacetylene molecules were observed. The results reported herein provide insight into the roles of molecular packing and chain orientations in converting the monomeric film to the polymerized blue and red phases. Copyright 2000 Academic Press.     

Effect of molecule-substrate interaction on thin-film structures and molecular orientation of pentacene on silver and gold

Evaporated pentacene thin films with thicknesses from several nm to 150 nm on gold and silver substrates have been studied by ultraviolet photoelectron spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). It was found that pentacene thin-film structures, particularly their molecular orientations, are strongly influenced by the metal substrates. UPS measurements revealed a distinct change in the valence band structures of pentacene on Au compared to those on Ag, which is attributed to the different packing between adjacent molecules. Using NEXAFS, we observed 74+/-5 degrees and 46+/-5 degrees molecular tilt angles on Ag and Au, respectively, for all measured thicknesses. We propose that pentacene molecules stand up on the surface and form the "thin-film phase" structure on Ag. On Au, pentacene films grow in domains with molecules either lying flat or standing up on the substrate. Such a mixture of two crystalline phases leads to an average tilt angle of 46 degrees for the whole film and the change in valence band structures. STM and distance-voltage (z-V) spectroscopy studies confirm the existence of two crystalline phases on Au with different conducting properties. z-V spectra on the low conducting phase clearly indicate its nature as "thin-film phase".     

Organophosphine/phosphite stabilized disilver(I) methanedisulphonates: synthesis, solid state structures and their potential use as MOCVD precursors

New disilver(I) methanedisulphonate complexes [CH(2)(SO(3))(2)Ag(2)·L(n)] (L = PPh(3); n=2, 2a; n=3, 2b; n=4, 2c; n=5, 2d; n=6, 2e; L=P(OEt)(3); n=2, 2f; n=4, 2g; n=6, 2h) were prepared by the reaction of [CH(2)(SO(3))(2)Ag(2)], which could be synthesized from methanedisulphonic acid and Ag(2)CO(3) in water, with triphenylphosphine or triethylphosphite in dichloromethane under a nitrogen atmosphere. The solid state structures of three complexes 2c, 2d and 2f were determined by single X-ray structure analysis. Hot-wall metal organic chemical vapor deposition (MOCVD) experiments were carried out at 395 °C, 420 °C and 450 °C using 2g as precursor for the deposition of silver films, respectively. The silver film with high purity obtained at 420 °C is dense and homogeneous, which is composed of many well isolated, granular particulates spreading all over the substrate surface.     

AFM characterization of nanoscale ribbons grown from a series of oligo(p-phenyleneethynylene) derivatives

The key to optimizing the properties of molecular scale wires lies in understanding and controlling the solid-state morphologies. This paper examines the influence of oligomer chain length, solvent, and concentration on the formation of nanoscale ribbons on mica substrates from solutions of oligo(p-phenyleneethynylene)s (OPEs) with hexyloxy side chains and thioacetyl end groups. The OPEs are of different molecular chain lengths, in which the numbers ofp-dihexyloxyphenyleneethynylene repeat units, n, are 1, 3, 5, and 7, respectively, with their two ends capped with 4-thioacetylphenyl alligator groups. The atomic force microscope (AFM) is employed to investigate the thin film morphology and study the self-assembled organizations. Solvent and concentration are found to exert a strong influence on thin film morphology. Under suitable conditions, OPEs with 7 p-dihexyloxyphenyleneethynylene repeat units are driven to form micrometer-long nanoribbons, oriented preferably along the 3-fold symmetry axes of the mica substrate. The cross section of the nanoribbons is composed of 7 molecules as evaluated by AFM characterization. On the other hand, oligomers with shorter chain lengths (n = 1, 3, and 5) produce thin films featuring globular nanoaggregates, chains consisting of elongated grains, and rods, respectively. Plausible reasons for the variation in thin film morphology are discussed, based on the results obtained from investigation of oligomer chain length, solvent, and concentration effects. A subtle balance among molecular size and physicochemical properties of solute molecules, solvent molecules, and substrate is crucial for the formation of desired structures. Among them, oligomer chain length plays a key role in thin film morphology, and the critical number of repeat units in OPE/poly(p-phenyleneethynylene) molecules for the formation of nanoribbon structures with a molecular cross section is supposed to be 8 or 9.     

Demonstrating the feasibility of monitoring the molecular-level structures of moving polymer/silane interfaces during silane diffusion using SFG

In this paper, the feasibility of monitoring molecular structures at a moving polymer/liquid interface by sum frequency generation (SFG) vibrational spectroscopy has been demonstrated. N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane (AATM, NH2(CH2)2NH(CH2)3Si(OCH3)3) has been brought into contact with a deuterated poly(methyl methacrylate) (d-PMMA) film, and the interfacial silane structure has been monitored using SFG. Upon initial contact, the SFG spectra can be detected, but as time progresses, the spectral intensity changes and finally disappears. Additional experiments indicate that these silane molecules can diffuse into the polymer film and the detected SFG signals are actually from the moving polymer/silane interface. Our results show that the molecular order of the polymer/silane interface exists during the entire diffusion process and is lost when the silane molecules traverse through the thickness of the d-PMMA film. The loss of the SFG signal is due to the formation of a new disordered substrate/silane interface, which contributes no detectable SFG signal. The kinetics of the diffusion of the silane into the polymer have been deduced from the time-dependent SFG signals detected from the AATM molecules as they diffuse through polymer films of different thickness.     

Infrared spectroscopic study of C(2)F(6) monolayers and bilayers on graphite

We report an experimental study of adsorbed films of C(2)F(6) on graphite by using infrared reflection absorption spectroscopy supplemented by ellipsometry. The vibrational C-F stretch modes nu(5) (parallel to the molecular axis) and nu(7) (perpendicular) in the film are strongly blueshifted by dynamic dipole coupling, and these shifts are sensitive to lattice spacing and molecular tilt. The relative strength of the absorption peaks mainly depends on the tilt angle relative to the surface normal. We use the strength data to estimate the tilt angle across the known monolayer phases, information that is difficult to obtain by other techniques. Although only the surface-normal component of the induced dipole moment appreciably couples to the external infrared field, surface-parallel components contribute to the intralayer coupling and hence to the frequency shifts for tilted molecules. Comparison to model calculations for a range of herringbone tilt configurations allows us to draw conclusions regarding the pattern of tilt azimuths. On this basis, we offer a revised interpretation of the origin of the Ising-type ordering transition found by Arndt et al. [Phys. Rev. Lett. 80, 1686 (1998)] in heat capacity measurements. Our phase boundaries for monolayer phases above 80 K are in good agreement with earlier results of the Saarbrucken group. We identify three distinct bilayer phases near saturation in isothermal pressure scans from ellipsometric steps and spectroscopic signatures. In temperature scans, we find evidence for several monolayer phases more dense than the well-established 2 x 2 commensurate phase and for a stable trilayer phase below about 60 K.     

Electron transport through thin organic films in metal--insulator--metal junctions based on self-assembled monolayers.

This paper describes an experimentally simple system for measuring rates of electron transport across organic thin films having a range of molecular structures. The system uses a metal--insulator--metal junction based on self-assembled monolayers (SAMs); it is particularly easy to assemble. The junction consists of a SAM supported on a silver film (Ag-SAM(1)) in contact with a second SAM supported on the surface of a drop of mercury (Hg-SAM(2))--that is, a Ag-SAM(1)SAM(2)-Hg junction. SAM(1) and SAM(2) can be derived from the same or different thiols. The current that flowed across junctions with SAMs of aliphatic thiols or aromatic thiols on Ag and a SAM of hexadecane thiol on Hg depended both on the molecular structure and on the thickness of the SAM on Ag: the current density at a bias of 0.5 V ranged from 2 x 10(-10) A/cm(2) for HS(CH(2))(15)CH(3) on Ag to 1 x 10(-6) A/cm(2) for HS(CH(2))(7)CH(3) on Ag, and from 3 x 10(-6) A/cm(2) for HS(Ph)(3)H (Ph = 1,4-C(6)H(4)) on Ag to 7 x 10(-4) A/cm(2) for HSPhH on Ag. The current density increased roughly linearly with the area of contact between SAM(1) and SAM(2), and it was not different between Ag films that were 100 or 200 nm thick. The current--voltage curves were symmetrical around V = 0. The current density decreased with increasing distance between the electrodes according to the relation I = I(0)e(-beta d(Ag,Hg)), where d(Ag,Hg) is the distance between the electrodes, and beta is the structure-dependent attenuation factor for the molecules making up SAM(1). At an applied potential of 0.5 V, beta was 0.87 +/- 0.1 A(-1) for alkanethiols, 0.61 +/- 0.1 A(-1) for oligophenylene thiols, and 0.67 +/- 0.1 A(-1) for benzylic derivatives of oligophenylene thiols. The values of beta did not depend significantly on applied potential over the range of 0.1 to 1 V. These junctions provide a test bed with which to screen the intrinsic electrical properties of SAMs made up of molecules with different structures; information obtained using these junctions will be useful in correlating molecular structure and rates of electron transport.