Odd-even effects in photoemission from terphenyl-substituted alkanethiolate self-assembled monolayers

Self-assembled monolayers (SAMs) formed from 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)2-(CH2)nSH (TPn, n = 1-6) on polycrystalline (111) gold and silver substrates have been characterized by synchrotron-based high-resolution X-ray photoelectron spectroscopy. The intensities, binding energy positions, and width of most photoemission lines exhibited pronounced odd-even effects, i.e., systematic and periodic variation, depending on either odd or even number of the methylene units in the aliphatic linker of the TPn molecules. The detailed analysis of these effects provides important information on the bonding and arrangement of the chemisorbed sulfur headgroups in the TPn films and balance of the structural forces in alkanethiolate SAMs.     

Odd-even effects on the structure, stability, and phase transition of alkanethiol self-assembled monolayers

Molecular dynamics simulations were conducted to predict the structural properties and phase transition temperatures of n-alkanethiols CH(3)(CH(2))(n-1)SH (Cn, 4 ≤ n ≤ 22) self-assembled monolayers (SAMs) on Au (111) surfaces. We studied the effects of chain length on the structural properties, including tilt and orientation angles, and on phase transition temperature. We found clear dependence of the structural properties, on both the number of carbon atoms, n; and on n being odd or even. Alkanethiols with n ≤ 7 show liquid-like behavior and large rotational mobility, whereas those with n ≥ 12 are well-ordered and stable. For 12 ≤ n ≤ 15, odd-even effects are observed, where for n = odd, larger tilt angles, oriented in the direction of their next next nearest neighbor (NNNN), and for n = even, lower tilt angles, mostly tilted toward next nearest neighbor (NNN), were observed. For 15 ≤ n ≤ 19, we find tilt angle and orientation to be independent of n. For all alkanethiols, a gradual decrease of the tilt angle occurred by increasing the temperature from 300 to 420 K. Order-disorder phase transitions occurred at a certain temperature. This was signified by abrupt instabilities in the tilt orientation angle. This transition temperature showed an enhancement of ～67-100 °C over the melting point of the corresponding n-alkane bulk system. This enhancement depended on n, and was larger for n = odd. Overall, we found that odd alkanethiols show better structural and thermal stability, and smaller gauche defects.     

Odd-even effects in self-assembled monolayers of omega-(biphenyl-4-yl)alkanethiols: a first-principles study

Conjugated molecules with a saturated alkyl linker between a thiol docking group and the pi-conjugated core have been shown to form self-assembled monolayers (SAMs) with a high degree of long-range order and uniformity. Additionally, pronounced odd-even effects have been observed in a number of properties characterizing these SAMs. We focus on omega-(biphenyl-4-yl)alkanethiols with n = 0-6 -(CH2)n- units deposited on Au(111) and investigate the microscopic origin of these odd-even effects in terms of the local sulfur-gold bonding geometry by employing first-principles calculations. An additional structural parameter, the torsion angle between the two phenyl rings in the biphenyl moiety, is identified and its relation to the experimentally observed odd-even effects is discussed. More importantly, we address relevant quantities for the application of these SAMs in molecular electronic devices, in particular, the modification of the work function of the underlying metal substrate and the energetic alignment of the molecular orbitals in the SAM with the Fermi level. While no clear trend emerges for the former, we find pronounced odd-even effects for the latter. Furthermore, the insertion of a single methylene unit between the biphenyl core and the thiol appears to largely decouple the valence electronic systems of the pi-conjugated segment and the gold substrate. Our results thus provide a solid theoretical basis for the interface energetics in this important class of systems.     

Odd-even effect and metal induced structural convergence in self-assembled monolayers of bipyridine derivatives

Scanning tunneling microscopy (STM) observations reveal that bipyridine derivatives which exhibit various two-dimensional structures due to the odd-even chain length effect are converged into a lamellar structure upon metal coordination.     

Organization-induced charge redistribution in self-assembled organic monolayers on gold

The charge redistribution that occurs within dipolar molecules as they self-assemble into organized organic monolayer films has been studied. The extent of charge transfer is probed by work function measurements, using low-energy photoelectron spectroscopy (LEPS), contact potential difference (CPD), and X-ray photoelectron spectroscopy (XPS), with the latter providing fine details about the internal charge distribution along the molecule. In addition, two-photon photoelectron spectroscopy is applied to investigate the electronic structure of the adsorbed layers. We show that charge transfer acts to reduce the dipole-dipole interaction between the molecules but may either decrease or increase the molecule-to-surface dipole moment.     

Multiscale charge injection and transport properties in self-assembled monolayers of biphenyl thiols with varying torsion angles

This article describes the molecular structure-function relationship for a series of biphenylthiol derivatives with varying torsional degree of freedom in their molecular backbone when self-assembled on gold electrodes. These biphenylthiol molecules chemisorbed on Au exhibit different tilt angles with respect to the surface normal and different packing densities. The charge transport through the biphenylthiol self-assembled monolayers (SAMs) showed a characteristic decay trend with the effective monolayer thickness. Based on parallel pathways model the tunneling decay factor β was estimated to be 0.27??(-1) . The hole mobility of poly(3-hexylthiophene)-based thin-film transistors incorporating a biphenylthiol SAM coating the Au source and drain electrodes revealed a dependence on the injection barrier with the highest occupied molecular orbital (HOMO) level of the semiconductor. The possible role of the resistivity of the SAMs on transistor electrodes on the threshold voltage shift is discussed. The control over the chemical structure, electronic properties, and packing order of the SAMs provides a versatile platform to regulate the charge injection in organic electronic devices.     

Molecular chirality and charge transfer through self-assembled scaffold monolayers

The effect of molecular chirality on electron transmission is explored by photoelectrochemistry. Thiol-terminated chiral scaffold molecules containing a porphyrin chromophore were self-assembled on gold surfaces to form a monolayer. Incorporation of the SAM-coated gold into an electrochemical cell and illumination with visible light generated a cathodic photocurrent. When using circularly polarized light, the photocurrent displayed an asymmetry (different magnitude of photocurrent for right versus left polarization) that changed with the molecular chirality (left- or right-handedness of the scaffold). A symmetry constraint on the electronic coupling between the porphyrin and the organic scaffold is proposed as a possible mechanism for the photocurrent asymmetry.     

Surface-assisted transient displacement charge technique. I. Photoinduced charge transfer in self-assembled monolayers

Surface-assisted photoinduced transient displacement charge (SPTDC) technique was developed in order to study light-induced charge transfer in surface-bound molecules and applied to investigation of self-assembled monolayers of 7-diethylaminocoumarin and 2,4-dinitrophenylamine. The dipole moment change measured by SPTDC correlates reasonably well with that measured in solution by standard PTDC technique and with semiempirical calculations. Shortening of the excited-state lifetime of surface-immobilized coumarin due to stimulated emission was observed in both fluorescence and dipole measurements. The dipole signal decline in low-polarity solvents indicates the importance of dipole-dipole interaction that causes reorientation of molecules upon photoexcitation.     

Effective anion sensing based on the ability of copper to affect electron transport across self-assembled monolayers

The ability of copper ions to affect the charge-transfer resistance of self-assembled monolayers (SAMs) of a tris-(2-pyridylmethyl)amine-based ligand on to gold electrodes is used to create a novel, sensitive and selective electrochemical cyanide sensor.     

Odd-even chain length-dependent order in pH-switchable self-assembled layers

The reversible self-assembly of a series of bipolar amphiphiles, alpha,omega-bis(3- or 4-amidinophenoxy)alkanes (chain length n = 5-12), on mercaptoalkanoic acid-functionalized gold surfaces (chain length n = 10, 11, 14, 15) has been studied by in-situ ellipsometry, IR reflection absorption spectroscopy (IRAS), and atomic force microscopy (AFM). The layer order, amphiphile orientation, and tendency to form bilayers depends on the position of the amidine substituent, the alkyl chain length of both the amidine amphiphile and the underlying acid self-assembled monolayer (SAM), and whether the amidine alkyl chain contained an even or odd number of methylene groups. Thus, para-substituted bisbenzamidines containing more than six methylene groups (n>6) and with an odd number (n = 7, 9, 11) tended to form bilayered structures, whereas those containing an even number formed monolayers when adsorbed on SAMs of the long-chain acids (n = 14, 15). This behavior also correlated with the average tilt angle of the benzene moieties of the amphiphiles, as estimated by IRAS. The odd-numbered chains gave lower tilt angles than the even-numbered ones, and a possible model that accounts for these results is proposed. IRAS also revealed a higher order of the odd-numbered chains and an increasing hydrogen-bonding contribution with increasing chain length. Additional evidence for the proposed bilayered assemblies and their reversibility was obtained by AFM. Images obtained from the assembly of decamidine on a SAM of mercaptohexadecanoic acid in a pH 9 borate buffer revealed domains of similar size to that of the underlying acid SAM (20-30 nm), but less densely packed. By acidifying the solution, the second layer was destabilized and a very smooth layer with few defects appeared. Further acidification to pH 3 also destabilized the first layer.     

Charge transport through polyene self-assembled monolayers from multiscale computer simulations

We combine first-principles density-functional theory with matrix Green's function calculations to predict the structures and charge transport characteristics of self-assembled monolayers (SAMs) of four classes of systems in contact with Au(111) electrodes: conjugated polyene chains (n = 4, 8, 12, 16, and 30) thiolated at one or both ends and saturated alkane chains (n = 4, 8, 12, and 16) thiolated at one or both ends. For the polyene SAMs, we find no decay in the current as a function of chain length and conclude that these 1-3 nm long polyene SAMs act as metallic wires. We also find that the polyene-monothiolate leads to a contact resistance only 2.8 times higher than that for the polyene-dithiolate chains, indicating that the device conductance is dominated by the properties of the molecular connector with less importance in having a second molecule-electrode contact. For the alkane SAMs, we observe the normal exponential decay in the current as a function of the chain length with a decay constant of beta(n) = 0.82 for the alkane-monothiolate and 0.88 for the alkane-dithiolate. We find that the contact resistance for the alkane-monothiolate is 12.5 times higher than that for the alkane-dithiolate chains, reflecting the extra resistance due to the weak contact on the nonthiolated end. These contrasting charge transport characteristics of alkane and polyene SAMs and their contact dependence are explained in terms of the atomic projected density of states.     

Structure-dependent charge transport and storage in self-assembled monolayers of compounds of interest in molecular electronics: effects of tip material, headgroup, and surface concentration

The electrical properties of self-assembled monolayers (SAMs) on a gold surface have been explored to address the relation between the conductance of a molecule and its electronic structure. We probe interfacial electron transfer processes, particularly those involving electroactive groups, of SAMs of thiolates on Au by using shear force-based scanning probe microscopy (SPM) combined with current-voltage (i-V) and current-distance (i-d) measurements. Peak-shaped i-V curves were obtained for the nitro- and amino-based SAMs studied here. Peak-shaped cathodic i-V curves for nitro-based SAMs were observed at negative potentials in both forward and reverse scans and were used to define the threshold tip bias, V(TH), for electric conduction. For a SAM of 2',5'-dinitro-4,4'-bis(phenylethynyl)-1-benzenethiolate, VII, V(TH) was nearly independent of the tip material [Ir, Pt, Ir-Pt (20-80%), Pd, Ni, Au, Ag, In]. For all of the SAMs studied, the current decreased exponentially with increasing distance, d, between tip and substrate. The exponential attenuation factors (beta values) were lower for the nitro-based SAMs studied here, as compared with alkylthiol-based SAMs. Both V(TH) and beta of the nitro-based SAMs also depended strongly on the molecular headgroup on the end benzene ring addressed by the tip. Finally, we confirmed the "memory" effect observed for nitro-based SAMs. For mixed SAMs of VII and hexadecanethiol, I, the fraction of the charge collected in the negative tip bias region that can be read out at a positive tip bias on reverse scan (up to 38%) depended on the film composition and decreased with an increasing fraction of I, suggesting that lateral electron hopping among molecules of VII occurs in the vicinity of the tip.     

Maleimido-terminated self-assembled monolayers

Four approaches have been explored for the preparation of maleimido-functionalized self-assembled monolayers (SAMs) on silicon. SAMs prepared by self-assembly of maleimido-functionalized alkyltrichlorosilanes (11-maleimido-undecyl-trichlorosilane) on oxide-covered silicon yield higher signals from maleimido functionalities in ATR-IR (attenuated total reflection IR) spectroscopy and XPS (X-ray photoelectron spectroscopy) than the other three methods. The surface composition of maleimido groups was tailored further by the formation of mixed monolayers with nonfunctionalized alkyltrichlorosilanes (decyltrichlorosilane). The order of the alkyl chains within the monolayers only slightly depends on the composition of the mixed monolayers. We utilized the maleimido-terminated SAMs to bind various nucleophilic compounds, alkylamines, alkylthiols, and thiol-tagged DNA oligonucleotides by means of conjugate addition.     

Charge transport through self-assembled monolayers of compounds of interest in molecular electronics

The electrical properties of self-assembled monolayers (SAMs) on metal surfaces have been explored for a series of molecules to address the relation between the behavior of a molecule and its structure. We probed interfacial electron transfer processes, particularly those involving unoccupied states, of SAMs of thiolates or arylates on Au by using shear force-based scanning probe microscopy (SPM) combined with current-voltage (i-V) and current-distance (i-d) measurements. The i-V curves of hexadecanethiol in the low bias regime were symmetric around 0 V and the current increased exponentially with V at high bias voltage. Different than hexadecanethiol, reversible peak-shaped i-V characteristics were obtained for most of the nitro-based oligo(phenylene ethynylene) SAMs studied here, indicating that part of the conduction mechanism of these junctions involved resonance tunneling. These reversible peaked i-V curves, often described as a negative differential resistance (NDR) effect of the junction, can be used to define a threshold tip bias, V(TH), for resonant conduction. We also found that for all of the SAMs studied here, the current decreased with increasing distance, d, between tip and substrate. The attenuation factor beta of hexadecanethiol was high, ranging from 1.3 to 1.4 A(-1), and was nearly independent of the tip bias. The beta-values for nitro-based molecules were low and depended strongly on the tip bias, ranging from 0.15 A(-1) for tetranitro oligo(phenylene ethynylene) thiol, VII, to 0.50 A(-1) for dinitro oligo(phenylene) thiol, VI, at a -3.0 V tip bias. Both the V(TH) and beta values of these nitro-based SAMs were also strongly dependent on the structures of the molecules, e.g. the number of electroactive substituent groups on the central benzene, the molecular wire backbone, the anchoring linkage, and the headgroup. We also observed charge storage on nitro-based molecules. For a SAM of the dintro compound, V, approximately 25% of charge collected in the negative scan is stored in the molecules and can be collected at positive voltages. A possible mechanism involving lateral electron hopping is proposed to explain this phenomenon.     

Surface-assisted transient displacement charge technique. II. Effect of gases on photoinduced charge transfer in self-assembled monolayers

Surface-assisted photoinduced transient displacement charge (SPTDC) technique was used to study charge transfer in self-assembled monolayers of 7-diethylaminocoumarin covalently linked to an oxide surface in the atmosphere of different gases. The dipole signal was found to be opposite to that in solution and dependent on the nature of the gas and its pressure. The results were explained by collision-induced relaxation that impedes uninhibited tilting of molecules onto the surface. Collisions with paramagnetic oxygen induce intersystem crossing to long-lived triplet dipolar states of coumarin with the rate close to half of that for the collision rate.     

Functionalized fullerenes in self-assembled monolayers

Anisotropy of intermolecular and molecule-substrate interactions holds the key to controlling the arrangement of fullerenes into 2D self-assembled monolayers (SAMs). The chemical reactivity of fullerenes allows functionalization of the carbon cages with sulfur-containing groups, thiols and thioethers, which facilitates the reliable adsorption of these molecules on gold substrates. A series of structurally related molecules, eight of which are new fullerene compounds, allows systematic investigation of the structural and functional parameters defining the geometry of fullerene SAMs. Scanning tunnelling microscopy (STM) measurements reveal that the chemical nature of the anchoring group appears to be crucial for the long-range order in fullerenes: the assembly of thiol-functionalized fullerenes is governed by strong molecule-surface interactions, which prohibit formation of ordered molecular arrays, while thioether-functionalized fullerenes, which have a weaker interaction with the surface than the thiols, form a variety of ordered 2D molecular arrays owing to noncovalent intermolecular interactions. A linear row of fullerene molecules is a recurring structural feature of the ordered SAMs, but the relative alignment and the spacing between the fullerene rows is strongly dependent on the size and shape of the spacer group linking the fullerene cage and the anchoring group. Careful control of the chemical functionality on the carbon cages enables positioning of fullerenes into at least four different packing arrangements, none of which have been observed before. Our new strategy for the controlled arrangement of fullerenes on surfaces at the molecular level will advance the development of practical applications for these nanomaterials.     

In situ electrodeposited nanoparticles for facilitating electron transfer across self-assembled monolayers in biosensor design

Gold nanoparticles (AuNPs) were synthesized in situ and electrodeposited onto Au substrate. The AuNPs modified interface facilitates electron transfer across self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA). After activation of surface carboxyl groups with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide, the interface displayed good stability for immobilization of biomolecules. These modification processes were characterized by contact angle measurement, cyclic voltammetry and electrochemical impedance spectra. The immobilized acetylcholinesterase (AChE), as a model, showed excellent activity to its substrate, leading to a stable AChE biosensor. Under the optimal experimental conditions, the inhibition of malathion on AChE biosensor was proportional to its concentration in two ranges, from 0.001 to 0.1 μg mL⁻¹ and from 0.1 to 25 μg mL⁻¹, with detection limit of 0.001 μg mL⁻¹. The simple method showed good reproducibility and acceptable stability, which had potential application in biosensor design.     

Mechanical and charge transport properties of alkanethiol self-assembled monolayers on a au(111) surface: the role of molecular tilt

The relationship between charge transport and mechanical properties of alkanethiol self-assembled monolayers (SAMs) on Au(111) films has been investigated using an atomic force microscope with a conductive tip. Molecular tilts induced by the pressure applied by the tip cause stepwise increases in film conductivity. A decay constant beta = 0.57 +/- 0.03 A-1 was found for the current passing through the film as a function of tip-substrate separation due to this molecular tilt. This is significantly smaller than the value of approximately 1 A-1 found when the separation is varied by changing the length of the alkanethiol molecules. Calculations indicate that, for isolated dithiol molecules S-bonded to hollow sites, the junction conductance does not vary significantly as a function of molecular tilt. The impact of S-Au bonding on SAM conductance is discussed.