Octanethiol Self-assembled Monolayers Research Articles

D2O Water Interaction with Mixed Alkane Thiol Monolayers of Tuned Hydrophobic and Hydrophilic Character

We have used temperature-programmed desorption (TPD) experiments to characterize the interaction between D2O water and a self-assembled monolayer (SAM) of mixed hydrophobic and hydrophilic thiols on Au(111). Two-component SAM surfaces with tuned hydrophobic and hydrophilic character have been formed by exposing gold samples to solutions of octanethiol and 6-mercaptohexanoic acid in varying millimolar concentrations. Water desorption spectra from the pure hydrophobic surface exhibit first-order desorption at low coverage with the onset of zero-order character at ∼1 Langmuir with narrow peaks at ∼145 K. In contrast, desorption of D2O water from the pure carboxylic acid-terminated surface exhibits broad peaks shifted to higher temperatures. Interestingly, water TPD spectra from a 50% octanethiol/50% 6-mercaptohexanoic acid surface closely resemble desorption from the purely hydrophobic octanethiol SAM. Increasing the surface acid fraction beyond 50% shifts the TPD profiles to higher temperatures with long, high-temperature tails that approach the behavior of water desorbing from the hydrophilic 6-mercaptohexanoic acid surface. We discuss the implications for water interaction with “textured” organic surfaces as well as with atmospherically relevant organic aerosol particles. In particular, the results suggest that significant surface oxidation is necessary to impact the interaction of water with organic aerosol surfaces.

Electrochemical Quartz Crystal Microbalance Study of Azurin Adsorption onto an Alkanethiol Self-Assembled Monolayer on Gold

A quartz crystal microbalance coupled with electrochemistry was used to examine the adsorption of azurin on a gold electrode modified with a self-assembled monolayer of octanethiol. Azurin adsorbed irreversibly to form a densely packed monolayer. The rate of azurin adsorption was related to the bulk concentration of azurin in solution within the concentration range studied. At a high azurin concentration (2.75 muM), adsorption was rapid with a stable adsorption maximum attained in 2-3 min. At a lower azurin solution concentration (0.35 muM), the time to reach a stable adsorption maximum was approximately 30 min. Interestingly, the maximum surface concentration attained for all solution concentrations studied by the QCM method was 25 +/- 1 pmol cm-2, close to that predicted for monolayer coverage. The dissipation was monitored during adsorption, and only small changes were detected, implying a rigid adsorption model, as needed when using the Sauerbrey equation. Cyclic voltammetric data were consistent with a one-electron, surface-confined CuII/CuI azurin process with fast electron-transfer kinetics. The electroactive surface concentration calculated using voltammetry was 7 +/- 1 pmol cm-2. The differences between the QCM and voltammetrically determined surface coverage values reflect, predominantly, the different measurement methods but imply that all surface-confined azurin is not electrochemically active on the time scale of cyclic voltammetry.

Molecular-scale investigation of octanethiol self-assembled monolayers on Au(1 1 1) prepared by solution and vapor deposition at high temperature

Octanethiol (OT) self-assembled monolayers (SAMs) were prepared on Au(111) by solution and ambient-pressure vapor deposition methods at 75°C. Surface structures were investigated with scanning tunneling microscopy (STM) to understand molecular-scale structural features of OT SAMs according to deposition method and temperature. OT SAMs prepared by solution deposition have large-ordered domains corresponding to ∼100nm with a c(4×2) superlattice, but disordered phases and structural defects in the ordered domains also formed from desorption of adsorbed molecules from the surface. However, high-quality OT SAMs were obtained by vapor deposition and showed large well-ordered domains of 20–50nm without formation of disordered phases. The formation of vacancy islands was markedly influenced by deposition method. This study demonstrated that the formation and structure of OT SAMs on Au(111) were influenced by deposition method and temperature.

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.

Collision-induced diffusion and vacancy migration in alkanethiol monolayers on Au(1 1 1)

Scanning tunneling microscopy is used to characterize the collision-induced migration of molecules within well-ordered octanethiol and nonanethiol self-assembled monolayers. A seeded molecular beam is used to create xenon atoms with a kinetic energy of 1.3 eV, and collisions with these atoms cause measurable changes in alkanethiol monolayer surface structure. Migration rates are calculated and compared for molecules in close-packed domains, at domain-boundary defects, and along the perimeter of vacancy-island defects. The number of nearest-neighbor molecules (within the 5 Å lattice distance) is strongly predictive of molecular stability with respect to xenon bombardment, and the overall dependence of stability on nearest neighbors is well fit by a simple exponential curve. The incident direction of the molecular beam is not observed to influence the direction of molecular motion; however, in some cases, migration correlates strongly to surface lattice directions. Finally, there is no evidence that substrate restructuring or gold-atom diffusion accompanies alkanethiol migration under these non-equilibrium conditions.

Chemical Dynamics Simulations of CO2Scattering off a Fluorinated Self-Assembled Monolayer Surface

Chemical dynamics simulations at collision energies of 3.0, 10.6, and 20.0 kcal/mol were performed to study energy transfer in perpendicular (θi = 0°) collisions of carbon dioxide with a perfluorinated octanethiol self-assembled monolayer (F-SAM) surface. An accurate carbon dioxide + F-SAM intermolecular potential was developed from ab initio calculations extrapolated to the complete basis set limit. Three types of collision events are observed in the trajectories: direct scattering, physisorption on the top of the surface, and penetration of the surface. Energy transfer to carbon dioxide rotation and the final translational energy of carbon dioxide are both analyzed for each of these trajectory types. Penetration leads to near complete thermal accommodation with the F-SAM, whereas accommodation is not reached for either the direct or physisorption trajectories. The distributions of the translational energy and rotational angular momentum, P(Ef) and P(J), of the scattered CO2 molecules and for the differ...

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.

Interaction Control Between Endohedral Metallofullerene and Metal Substrate by Introducing Alkanethiol Self-Assembled Monolayer

The interaction control between endohedral metallofullerenes and a metal substrate has been demonstrated by introducing hexanethiol, octanethiol, and decanethiol self-assembled monolayers (SAMs) as the interlayer. We observe the electric properties of terbium endohedral metallofullerenes (Tb@C82) on alkanethiol SAMs with different chain lengths by scanning tunneling microscopy (STM) and spectroscopy (STS). Based on the comparison of the high-resolution STM images of a Tb@C82 molecule on hexanethiol and octanethiol SAMs, the interaction between Tb@C82 and a hexanethiol SAM is found to be larger than that between Tb@C82 and an octanethiol SAM; this is because at 68 K, the rotational states of Tb@C82 terminate only on the hexanethiol SAM. Furthermore, we find that the tunneling current-voltage characteristics of Tb@C82 on the hexanethiol SAM show the rectifying effects that are also caused by the molecular energy level shifts of Tb@C82 molecules due to the large interaction.

Self-Assembled Monolayers on Pt(111): Molecular Packing Structure and Strain Effects Observed by Scanning Tunneling Microscopy

Self-assembled monolayers (SAMs) of octanethiol and benzeneethanethiol were deposited on clean Pt(111) surfaces in ultrahigh vacuum (UHV). Highly resolved images of these SAMs produced by an in situ scanning tunneling microscope (STM) showed that both systems organize into a super-structure mosaic of domains of locally ordered, closely packed molecules. Analysis of the STM images indicated a (square root 3 x square root 3)R30 degrees unit cell for the octanethiol SAMs and a 4(square root 3 x square root 3)R30 degrees periodicity based on 2 x 2 basic molecular packing for the benzeneethanethiol SAMs under the coverage conditions investigated. SAMs on Pt(111) exhibited differences in molecular packing and a lower density of disordered regions than SAMs on Au(111). Electron transport measurements were performed using scanning tunneling spectroscopy. Benzeneethanethiol/Pt(111) junctions exhibited a higher conductance than octanethiol/Pt(111) junctions.

Structural changes of an octanethiol monolayer via hyperthermal rare-gas collisions

In situ scanning tunneling microscopy is used to measure the effect of hyperthermal rare-gas bombardment on octanethiol self-assembled monolayers. Close-packed monolayers remain largely unchanged, even after repeated collisions with 0.4 eV argon and 1.3 eV xenon atoms. In contrast, gas-surface collisions do induce structural changes in the octanethiol film near defects, domain boundaries, and disordered regions, with relatively larger changes observed for xenon-atom bombardment.

Surface Structure and Interface Dynamics of Alkanethiol Self-Assembled Monolayers on Au(111)

Scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS) were used to examine the structural transitions and interface dynamics of octanethiol (OT) self-assembled monolayers (SAMs) caused by long-term storage or annealing at an elevated temperature. We found that the structural transitions of OT SAMs from the c(4 x 2) superlattice to the (6 x square root 3) superlattice resulting from long-term storage were caused by both the dynamic movement of the adsorbed sulfur atoms on several adsorption sites of the Au(111) surface and the change of molecular orientation in the ordered layer. Moreover, it was found that the chemical structure of the sulfur headgroups does not change from monomer to dimer by the temporal change of SAMs at room temperature. Contrary to the results of the long-term-stored SAMs, it was found that the annealing process did not modify either the interfacial or chemical structures of the sulfur headgroups or the two-dimensional c(4 x 2) domain structure. Our results will be very useful for a better understanding of the interface dynamics and stability of sulfur atoms in alkanethiol SAMs on Au(111) surfaces.

Single Molecular Orientation Switching of an Endohedral Metallofullerene

The single molecular orientation switching of the Tb@C82 endohedral metallofullerene has been studied by using low-temperature ultrahigh vacuum (UHV) scanning tunneling microscopy (STM). An octanethiol self-assembled monolayer (SAM) was introduced between Tb@C82 and the Au111 substrate to control the thermal rotational states of Tb@C82. Scanning tunneling spectroscopy (STS) of Tb@C82 on an octanethiol SAM at 13 K demonstrated hysteresis including negative differential conductance (NDC). This observed hysteresis and NDC is interpreted in terms of a switching of the Tb@C82 molecular orientation caused by the interaction between its electric dipole moment and an external electric field.

Replacement of adsorbed alkanethiolate on Au with carboxyl-terminated thiol in solution: effect of alkyl chain length

We have investigated the effect of the alkyl chain length on the replacement of the self-assembled alkanethiolate on Au surface with carboxyl-terminated thiol dissolved in solution. Immersion time-dependent replacement of self-assembled alkanethiolates on Au substrate was monitored by lateral force microscope (LFM), contact angle measurement, and infrared spectroscopy (IRS). Both the self-assembled monolayers (SAM) of 1-octadecanethiol (ODT) and octanethiol (OT) were reacted with carboxyl-terminated 11-mercaptoundecanoic acid (MUA) molecules in ethanol. OT SAM was rapidly replaced by MUA molecules. More than 50% of OT molecules were replaced by MUA within the initial 50 h. In contrast, less than 10% of ODT molecules were replaced by MUA within the same time scale. This can be explained in terms of the relative stability between methyl- and carboxyl-terminated thiolates on Au surface, depending upon the chain length and the lateral hydrogen bonding.

Stable, Nanoscale Glycosphingolipid Films for Use in Sensing Applications

AbstractWe have developed a means of producing thin, oriented lipid monolayers which are stable under repeated washing and which may be useful in biosensing or surface- coating applications. Phosphotidylcholine (PC) and the glycosphingolipid (GSL) GM1 were used as representative lipids for this process. Initially, a mixed self-assembled monolayer of octanethiol and hexadecanethiol was constructed on a clean gold surface. This hydrophobic surface was then brought into contact with a thin lipid layer that had been deposited at the air/liquid interface of a solution by evaporating a mixture of lipid in hexane on top of a layer of water. The lipid layer, now deposited on the gold surface, was then heated to cause intercalation of the fatty acid and alkanethiol chains, and cooled to form a highly stable film which withstood repeated rinsing and solution exposure. Presence and stability of the film was confirmed via ellipsometry, FTIR, and QCM, with an average overall thickness of ∼3.5 nm. These films may be potentially useful in biotoxin detection or as a protein resistant layer for the prevention of biofouling.

Self-Assembled Monolayers Exposed by Metastable Helium forNano-Patterning: Octanethiol and Dodecanethiol

A large-area gold pattern with nanoscale edge on muscovite mica wasfabricated by using a metastable helium beam and octanethiol (OT) anddodecanethiol (DDT) self-assembled monolayers (SAM), in which the widthof edge was typically ~70 nm for the OT SAM and ~90 nm forthe DDT SAM, respectively. The mask was reproduced with high fidelity.Combined the analysis of roughness with grain size, more flat surfaceand sharper edge of patterns were obtained by using the shorter chainmolecules such as the OT. All the information indicated that the OT SAMson atomically flat surfaces can be used as a resist for exposure tometastable atom beams.

Patterning of gold film on muscovite mica by using a helium-metastable atom beam and an octanethiol self-assembled monolayer

Using a helium-metastable atom beam and an octanethiol (OT) self-assembled monolayer (SAM), we carried out atom lithography to the gold film on muscovite mica having a flat surface. A large intact area and a clear pattern with a nanoscale width ∼80–100 nm of the etched step was obtained. The gold surface, as well as the surface of the mica substrate, was flat. From the analysis of the roughness, appreciable gold islands were not detected in the area exposed to the He* atom beam. This finding demonstrates that the OT SAMs on atomically flat surfaces can be used as a resist for exposure to metastable-atom beams.

Tunneling Current-Distance Characteristic of Scanning Vibrating Probe / 1-Alkanethiol Self-Assembled Monolayer (SAM) / Au (111) Structure

ABSTRACTWe report measurements of the tunneling current – distance (I-d) dependence above alkanethiol self-assembled monolayers (SAMs) on Au (111) substrates with high probe voltage. From the semilogarithmic I-d plots of hexanethiol and octanethiol SAMs, a kink in the tunneling current slopes is clearly observed, which shows the point where the STM tip contacts the end of the SAMs. The conductance decay constants of the vacuum layer and the alkanethiol SAMs are estimated from the tunneling current slopes. We also discuss the contact conductance of alkanethiol SAMs.

Scanning tunneling microscopy and X-ray photoelectron spectroscopy of silver deposited octanethiol self-assembled monolayers

Ag deposited self-assembled monolayers (SAMs) of octanethiol (CH3(CH2)7SH) have been studied using scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). At the initial stage of Ag deposition, monatomic height islands, ∼7×1011cm−2 in density, grow at the SAMs/Au(111) interface and become larger as more Ag atoms are deposited up to a full monolayer coverage of Ag. The differences of the nucleation density and the growth property between Ag and Au islands can be attributed to the higher mobility of Ag atoms and the difference of the molecular packing on these islands. XPS analysis of this structure (SAMs/Ag monolayer/Au) shows that the Ag 3d5/2 binding energy is shifted −0.3 eV with respect to bulk Ag, the C 1s binding energy is ∼0.3 eV higher than that before Ag deposition, and the S 2p3/2 binding energy exhibits little shift before and after deposition. The origin of the shift can be explained by the change of the dipole at the interface and the electrical isolation of alkyl chains from the surroundings.

Alkanethiol self-assembled monolayers on Au(111) surfaces investigated by non-contact AFM

Self-assembled monolayers (SAMs) of alkanethiol molecules (CH3–(CH2)n-1–SH, Cn) deposited on Au(111) surfaces were investigated by ultrahigh vacuum non-contact atomic force microscopy (UHV-NC-AFM). Octanethiol SAMs (C8) and hexadecanethiol (C16) SAMs self-assembled from ethanol solutions were used. Molecularly resolved NC-AFM images of longer-chain alkanethiol (C16) SAMs, which are difficult to be observed by STM, are presented for the first time. The results show that the molecules in this film are hexagonally packed, making (√3×√3)R30° structures. Imaging conditions were more stable in the case of SAMs with longer-chain molecules. This is probably because the “rigidity” depends on the chain length of the molecules. In addition, the high-resolution images of the stripe structures in lower-density region of C8 SAMs were obtained by NC-AFM.

Underpotential Deposition of Silver onto Gold Substrates Covered with Self-Assembled Monolayers of Alkanethiols To Induce Intervention of the Silver between the Monolayer and the Gold Substrate

Underpotential deposition (UPD) of Ag on Au(111)/mica electrodes coated with a self-assembled monolayer (SAM) of propanethiol or octanethiol and reductive desorption of the SAM after conducting UPD have been studied using voltammetry, XPS measurement, and scanning tunneling microscopy (STM). The reductive desorption potential of the SAM was changed by UPD of Ag from a characteristic value obtained at Au to that at Ag, indicating that the UPD of Ag took place through the SAM layer in such a way as to intervene between the SAM and the Au electrode. No significant loss of thiol molecules occurred during the Ag deposition. The rate of UPD through the SAM of propanethiol was so fast as to be completed within 10 s, while that for the SAM of octanethiol took ca. 50 min or more to build up the SAM/Ag/Au structure. Voltammetric results indicated that the UPD of Ag proceeded initially at molecular defects in the SAM of octanethiol and that the resulting Ag islands grew laterally to limiting coverage. Ex situ STM ob...