Alkanethiol Molecules Research Articles

Evaluation on corrosively dissolved gold induced by alkanethiol monolayer with atomic absorption spectroscopy

We have monitored a gold corrosive dissolution behavior accompanied in n-alkanethiol like n-dodecanethiol assembled process with in situ quartz crystal microbalance (QCM), and then observed it with atomic force microscopy (AFM) which showed an evident image of corrosive defects or holes produced on gold substrate, corresponding to gold dissolution induced by the alkanethiol molecules in the presence of oxygen. For detection of the dissolved gold defects during alkanethiol assembled process, an atomic absorption spectroscopy (AAS) has been carried out in this paper, and the detection limit for the dissolved gold could be evaluated to be 15.4 ng/mL. The amount of dissolved gold from the substrates of gold plates as functions of immersion time, acid media, solvents and thiol concentration has been examined in the oxygen saturated solutions. In comparison with in situ QCM method, the kinetics behavior of the long-term gold corrosion on the gold plates in 1.0 mmol/L of n-dodecanethiol solution determined with AAS method was a slow process, and its corrosion rate on gold dissolution could be evaluated to be about 4.4 × 10 − 5 ng·cm − 2 ·s − 1 , corresponding to 1.3 × 10 8 Au atoms·cm − 2 ·s − 1 , that was much smaller than that of initial rate monitored with in situ QCM. Both kinetics equations obtained with QCM and AAS showed a consistent corrosion behavior on gold surfaces.

Adhesion forces between Staphylococcus epidermidis and surfaces bearing self-assembled monolayers in the presence of model proteins

Self-assembled monolayers (SAMs) are being developed into coatings to reduce microbial biofilm formation on biomaterials. To test anti-adhesion properties, SAMs can be easily constructed on gold, and used to represent a coated biomaterial. However, coatings that prevent bacterial adhesion must also resist protein adsorption. We explored the competitive effects of bacteria and protein for adsorption to SAMs, choosing fetal bovine serum (FBS) to represent protein non-specific binding, and fibronectin (FN) to evaluate ligand/receptor binding. Staphylococcus epidermidis were immobilized on an atomic force microscope (AFM) tip and used as a force probe to detect the interaction forces between bacteria and gold-coated SAMs. The SAMs tested were alkanethiol molecules terminating in isophthalic acid (IPA) or isophthalic acid with silver (IAG). While S. epidermidis showed weak interactions with FBS, the bacteria showed strong adhesion with FN, due to ligand/receptor binding. Bacterial retention and viability experiments were correlated with the force measurements. S. epidermidis interacting with IAG SAMs showed a loss of viability, due to the mobility of silver ions. For most substrata, there was a link between high adhesion forces with bacteria and a high percentage of dead cells being retained on that substratum (even in the absence of a specific biocidal effect, such as silver). This may suggest that high adhesion forces can cause stress to the bacteria which contributed to their death. The relationship between highly adhesive SAMs and bacterial inactivation may be useful in future biomaterial design. When evaluating coatings for biomaterials, it is important to consider the interplay between bacteria, proteins, and the coating material.

Gas-Phase Ion-Mobility Characterization of SAM-Functionalized Au Nanoparticles

We present results of a systematic examination of functionalized gold nanoparticles (Au-NPs) by electrospray-differential mobility analysis (ES-DMA). Commercially available, citrate-stabilized Au colloid solutions (10-60 nm) were sized using ES-DMA, from which changes in particle size of less than 0.3 nm were readily discerned. It was found that the formation of salt particles and the coating of Au-NPs by salt during the electrospray process can interfere with the mobility analysis, which required the development of sample preparation and data correction protocols to extract correct values for the Au-NP size. Formation of self-assembled monolayers (SAMs) of alkanethiol molecules on the Au-NP surface was detected from a change in particle mobility, which could be modeled to extract the surface packing density of SAMs. A gas-phase temperature-programmed desorption (TPD) kinetic study of SAMs on Au-NPs found the data to be consistent with a second-order Arrhenius-based rate law, yielding an Arrhenius factor of 1.0 x 10 (11) s (-1) and an activation energy approximately 105 kJ/mol. For the size range of SAM-modified Au-NP we considered, the effect of surface curvature on the energetics of binding of carboxylic acid terminated SAMs is evidently negligible, with binding energies determined by TPD agreeing with those reported for the same SAMs on planar surfaces. This study suggests that the ES-DMA can be added to the tool set of characterization methods used to study the structure and properties of coated nanoparticles.

Alkanethiol/Au(111) Self-Assembled Monolayers Contain Gold Adatoms: Scanning Tunneling Microscopy before and after Reaction with Atomic Hydrogen

Alkanethiol self-assembled monolayers on Au(111) are widely studied, yet the exact nature of the sulfur-gold bond is still debated. Recent studies suggest that Au(111) is significantly reconstructed, with alkanethiol molecules binding to gold adatoms on the surface. These adatoms are observed using scanning tunneling microscopy before and after removing the organic monolayer with an atomic hydrogen beam. Upon monolayer removal, changes in the gold substrate are seen in the formation of bright, triangularly shaped islands, decreasing size of surface vacancy islands, and faceting of terrace edges. A 0.143 +/- 0.033 increase in gold coverage after monolayer removal shows that there is one additional gold adatom for every two octanethiol molecules on the surface.

Measuring Relative Barrier Heights in Molecular Electronic Junctions with Transition Voltage Spectroscopy

Though molecular devices exhibiting potentially useful electrical behavior have been demonstrated, a deep understanding of the factors that influence charge transport in molecular electronic junctions has yet to be fully realized. Recent work has shown that a mechanistic transition occurs from direct tunneling to field emission in molecular electronic devices. The magnitude of the voltage required to enact this transition is molecule-specific, and thus measurement of the transition voltage constitutes a form of spectroscopy. Here we determine that the transition voltage for a series of alkanethiol molecules is invariant with molecular length, while the transition voltage of a conjugated molecule depends directly on the manner in which the conjugation pathway has been extended. Finally, by examining the transition voltage as a function of contact metal, we show that this technique can be used to determine the dominant charge carrier for a given molecular junction.

Lithography on GaP(1 0 0) surfaces

Two types of lithographic methods were used to modify GaP(1 0 0) surfaces with commercially available alkanethiol molecules: microcontact printing (μCP) and “dip-pen” nanolithography (DPN). The patterned surfaces were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). The characterization was done in order to understand the quality of each type of pattern, its chemical composition, and the organization of the molecules on the surface. Differences between the two lithographic methods used to do lithography on the GaP(1 0 0) in this study were dependent on the chosen molecular “ink”.

Electron Transfer Mediating Properties of Hydrocarbons as a Function of Chain Length: A Differential Scanning Conductive Tip Atomic Force Microscopy Investigation

The development of novel molecular and biomolecular devices relies on the understanding of charge transport across molecule-substrate interfaces. However, different strategies adopted so far for fabricating and studying transport through metal-molecule-metal junctions yield values for the transport coefficients that differ by up to orders of magnitude even for the same junction. Conductive tip atomic force microscopy (CT-AFM) allows for the simultaneous measurement of transport and morphological properties of molecular assemblies, but absolute transport measurements depend on the nature of the AFM tip-molecule contact. In this work we present a differential approach to the study of metal-molecule-metal junctions based on the combination of AFM-driven nanolithography and CT-AFM. We nanograft patches of alkanethiol molecules in a self-assembled monolayer of alkanethiol molecules of different chain length and measure by CT-AFM the morphology and the transport properties of the nanopatches and of the reference layer. The method allows for the determination of the differential resistance between the two molecular layers and is thus independent of environmental factors. The validity of this approach is demonstrated by measuring the tunneling decay constant of alkanethiols as a function of their length.

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.

An Efficient Monte Carlo Algorithm for the Fast Equilibration and Atomistic Simulation of Alkanethiol Self-Assembled Monolayers on a Au(111) Substrate

A new Monte Carlo algorithm is presented for the simulation of atomistically detailed alkanethiol self-assembled monolayers (R-SH) on a Au(111) surface. Built on a set of simpler but also more complex (sometimes nonphysical) moves, the new algorithm is capable of efficiently driving all alkanethiol molecules to the Au(111) surface, thereby leading to full surface coverage, irrespective of the initial setup of the system. This circumvents a significant limitation of previous methods in which the simulations typically started from optimally packed structures on the substrate close to thermal equilibrium. Further, by considering an extended ensemble of configurations each one of which corresponds to a different value of the sulfur-sulfur repulsive core potential, sigmass, and by allowing for configurations to swap between systems characterized by different sigmass values, the new algorithm can adequately simulate model R-SH/Au(111) systems for values of sigmass ranging from 4.25 A corresponding to the Hautman-Klein molecular model (J. Chem. Phys. 1989, 91, 4994; 1990, 93, 7483) to 4.97 A corresponding to the Siepmann-McDonald model (Langmuir 1993, 9, 2351), and practically any chain length. Detailed results are presented quantifying the efficiency and robustness of the new method. Representative simulation data for the dependence of the structural and conformational properties of the formed monolayer on the details of the employed molecular model are reported and discussed; an investigation of the variation of molecular organization and ordering on the Au(111) substrate for three CH3-(CH2)n-SH/Au(111) systems with n=9, 15, and 21 is also included.

Adsorption Processes of Thiolated Cyclodextrins on Au(111) Substrates: A Way to Organize Ordered Structures of "Bulky" Molecules on Solid Substrates

Adsorption processes of thiolated cyclodextrins onto the self-assembeled monolayers of octanethiol formed on Au(111) substrates were investigated with a scanning tunneling microscope (STM). Our results revealed that the exchange of alkanethiolate molecules with the thiolated cyclodextrins occurred preferentially at the boundaries of the domains of a c(4×2) structure. Moreover, the adsorbed thiolated cyclodextrins were found to form one-dimensional chain structures. Our approach presented here provides a way to organize ordered structures of bulky molecules on solid substrates.

Inelastic Electron Tunneling Spectroscopy of Alkane Monolayers with Dissimilar Attachment Chemistry to Gold

We compare the low-temperature electron transport properties of alkyl monolayers which utilize different attachment strategies to gold. Inelastic electron tunneling spectroscopy (IETS) and current-voltage analysis were performed on molecular junctions incorporating alkyl-dithiocarbamate and alkanethiolate self-assembled monolayers of similar length. Alkyl-dithiocarbamate monolayers were formed by the condensation of dioctylamine or didecylamine with carbon disulfide in anhydrous ethanol and compared to alkanethiolate SAMs of 1-decanethiol and 1-dodecanethiol, respectively. The electron transport properties of each monolayer were examined using magnetically assembled microsphere junctions under high-vacuum conditions at low temperature. IETS was employed to differentiate the films on the basis of vibrational modes which are characteristic of each method of attachment. We use quantum chemical simulations of model compounds to calculate frequency and intensity of predicted signals arising from molecular vibrations to aid in the accurate assignment of the spectra. A qualitative comparison of our devices also reveals an increase in current density when utilizing dithiocarbamate attachment to gold compared to alkanethiolate molecules of similar length.

1-Adamantanethiolate Monolayer Displacement Kinetics Follow a Universal Form

Alkanethiol molecules in solution displace 1-adamantanethiolate self-assembled monolayers on Au{111}, ultimately leading to complete molecular exchange. Specifically, here, fast insertion of n-dodecanethiolate at defects in the original 1-adamantanethiolate monolayer nucleates an island growth phase, which is followed by slow ordering of the n-dodecanethiolate domains into a denser and more crystalline form. Langmuir-based kinetics, which describe alkanethiolate adsorption on bare Au{111}, fail to model this displacement reaction. Instead, a Johnson-Mehl-Avrami-Kolmogorov model of perimeter-dependent island growth yields good agreement with kinetic data obtained by Fourier transform infrared spectrometry over 100-fold variation in n-dodecanethiol concentration. Rescaling the growth rate at each concentration collapses all the data onto a single universal curve, suggesting that displacement is a scale-free process. The rate of displacement varies as the square-root of the n-dodecanethiol concentration across the 0.01-1.0 mM range studied.

Degradation of octanethiol self-assembled monolayers from hydrogen-atom exposure: A molecular-scale study using scanning tunneling microscopy

Octanethiol self-assembled monolayers were exposed to gas-phase hydrogen atoms, and the resulting changes in the order and chemical structure of the surface were monitored using scanning tunneling microscopy (STM). Extensive damage to the monolayer was observed in the form of both dark and bright features in STM images. These changes began along domain boundaries and moved into close-packed regions of the monolayer as hydrogen-atom exposure time increased. Increasing exposure also results in an accelerated rate of observed surface changes, indicating that the reactivity of the surface increases as a result of initial gas-surface reactions. Complex restructuring of the alkanethiol monolayer is observed, including defect formation and the disordering of the alkanethiol monolayer. However, in some cases the monolayer demonstrates the capability of self-healing, with local annealing and reordering of close-packed domains. This annealing and reordering likely results from increased mobility of surface-bound alkanethiolates in the vicinity of monolayer defects, or from diffusion and readsorption of transiently formed alkanethiol molecules.

Gas Phase Formation of Dense Alkanethiol Layers on GaAs(110)

We present a study of the growth and thermal stability of hexanethiol (C6) films on GaAs(110) by direct recoil spectroscopy with time-of-flight analysis. We compare our results with the better known case of C6 adsorption on Au(111). In contrast to the two-step adsorption kinetics observed for Au surfaces after lengthy exposures, data for C6 adsorption on the GaAs(110) surface are consistent with the formation of a single dense phase of C6 molecules at lower exposures. On the contrary, in solution preparation, dense phases can only be obtained on GaAs for long alkanethiols and after lengthy immersions. The C6 layer has a first desorption peak at 325 K, where partial desorption of the alkanethiol molecules takes place. Fits to the desorption curves result in a 1 eV adsorption energy, in agreement with a chemisorption process. Increasing the temperature to 500 K results in the S-C bond scission with only S remaining on the GaAs(110) surface. The possibility of forming dense, short-alkanethiol layers on semiconductor surfaces from the vapor phase could have a strong impact for a wide range of self-assembled monolayer applications, with only minimal care not to surpass room temperature once the layer has been formed in order to avoid molecular desorption.

Phase transition of silver nanoparticles from aqueous solution to chloroform with the help of inclusioncomplexes of p-sulfonated calix[4]arene and alkanethiol molecules and its application in the size sorting ofnanoparticles

Silver nanoparticles were synthesized in aqueous solution andcapped with an inclusion complex of octadecanethiol (ODT) andp-sulfonated calix[4]arene (pSC4). By vigorous shaking of a biphasic mixture of thepSC4-threaded ODT-stabilized silver hydrosol and chloroform, the phase transitionof the silver nanoparticles could be observed in the course of the transition ofcolour from the aqueous phase to the organic phase. The procedure of phasetransition was independently verified by UV–vis spectroscopy, transmissionelectron microscopy (TEM), Fourier transform infrared (FTIR) spectra, and1H nuclear magnetic resonance (NMR). In shaking the biphasic mixture, it was believed thatpSC4 molecules were desorbed from the inclusion complex and the protective layer of silvernanoparticles shifted from hydrophilic to hydrophobic, which drove the phase transition ofsilver nanoparticles from aqueous solution into chloroform. More interestingly, the efficiencyin the phase transition of aqueous silver nanoparticles rigidly depended on their sizes,which maybe used in the size sorting of nanoparticles.

Negative resist behavior of neutral sodium atoms deposited on self-assembled monolayers

The authors report their initial studies of the negative resist behavior of neutral sodium atoms deposited on alkanethiol molecules during neutral atom lithographic processing. Their results show that neutral sodium atoms incident upon octadecanethiol, the longest molecule of the various alkanethiols among alkanethiol self-assembled monolayers (SAMs), formed the most robust negative resist during the patterning process and made high quality patterning profile. In order to interpret the nature of the surface interaction between neutral sodium atoms and SAMs, they examine the surface components using x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and rinse tests. Their results indicate that sodium neutral atoms do not chemically react with or physically damage the SAMs but rather accumulate on and possibly diffuse into the SAMs.

The effect of terminal group on the electronic transport property of alkanethiol self-assembled monolayer

The electronic transport properties of the alkanethiol self-assembled monolayer (SAM) with various terminal groups on Au (111) surfaces have been investigated by the conducting atomic force microscopy (CAFM). The results show that the terminal group has obvious effect on the conductance of the SAMs. Combining the X-ray photoelectron spectrum (XPS) analysis and CAFM, we have further studied the relationship between the conductance of SAM and the C1s binding energy of the terminal group of the molecule. It is found that the higher the binding energy, the lower the conductance of the SAM. We attribute this behavior to that the high binding energy would induce strong electronic localization in the terminal group of alkanethiol molecule, which results in raising the effective barrier of the electron injecting into the molecular chains and reducing the transport ability of the electrons in the molecule. In addition, we also find that different group terminated SAMs have different surface potentials, which can shift the zero point of I-V curve of SAMs.

Collision-induced annealing of octanethiol self-assembled monolayers by high-kinetic-energy xenon atoms

Collisions with high-energy xenon atoms (1.3 eV) induce structural changes in octanethiol self-assembled monolayers on Au(111). These changes are characterized at the molecular scale using an in situ scanning tunneling microscope. Gas-surface collisions induce three types of structural transformations: domain boundary annealing, vacancy island diffusion, and phase changes. Collision-induced changes that occur tend to increase order and create more stable structures on the surface. We propose a mechanism where monolayer transformations are driven by large amounts of vibrational energy localized in the alkanethiol molecules. Because we monitor incremental changes over small regions of the surface, we can obtain structural information about octanethiol monolayers that cannot be observed directly in scanning tunneling microscopy images.

X-ray photoelectron spectroscopy study of self-assembled monolayers of alkanethiols on (001) GaAs

Self-assembled monolayers of several alkanethiol molecules, with varying chain length and terminal groups, were investigated using x-ray photoelectron spectroscopy (XPS). The chain lengths varied from 10 to 15 methylene units, and the terminal group was either hydrophilic carboxylic acid and hydroxyl or hydrophobic methyl. The alkanethiol molecules were deposited on GaAs surfaces from liquid solutions. The impact of atmospheric exposure was examined by investigating one set of samples stored under atmospheric conditions and a second set that was stored in a nitrogen atmosphere prior to analysis. Carbon, oxygen, gallium, and arsenic core level XPS spectra were obtained on all surfaces. The intensity of the gallium and arsenic core levels indicates a considerable difference in the Ga∕As ratio dependent on the terminal group of the alkanethiol. Additionally, the carbon and oxygen spectra indicate varying chemical bonding on the surface with the alkanethiol’s having a carboxylic acid terminal group showing a more complex carbon and oxygen bonding.

Study on nanoscale abrasive interaction between nanoprobe and self-assembled molecular surface for probe-based nanolithography process

Mechano-chemical scanning probe lithography (MC-SPL) is a nanofabrication technique that is based on the nanoscale abrasive interaction between a sharp nanoprobe and an ultrathin organic resist-coated surface. This work reviews the fundamental aspects of the MC-SPL process. The abrasive interaction at the tip-workpiece contact interface that occurs in the MC-SPL process was investigated by experimentally as well as by molecular dynamics simulation. Experimental results showed that severe wear of the tip that can generate just after coming into contact with the sample and the defects on the organic resists lead to the pattern broadening. In addition, the results from the simulation using the tips modeled with various shapes disclosed that the scribed pattern width was largely affected more by the shape of the tip rather than by the tip-surface contact area and thus the severe pattern broadening and large displacement of alkanethiol molecules around the tip could occur in the case of using the tip with large cone angle. It was found that these phenomena were due to the strong van der Waals interactions between the alkanethiol molecules.