Decanethiol Self-assembled Monolayers Research Articles

Formation and Thermal Stability of Ordered Self-Assembled Monolayers by the Adsorption of Amide-Containing Alkanethiols on Au(111).

We examined the surface structure, binding conditions, electrochemical behavior, and thermal stability of self-assembled monolayers (SAMs) on Au(111) formed by N-(2-mercaptoethyl)heptanamide (MEHA) containing an amide group in an inner alkyl chain using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) to understand the effects of an internal amide group as a function of deposition time. The STM study clearly showed that the structural transitions of MEHA SAMs on Au(111) occurred from the liquid phase to the formation of a closely packed and well-ordered β-phase via a loosely packed α-phase as an intermediate phase, depending on the deposition time. XPS measurements showed that the relative peak intensities of chemisorbed sulfur against Au 4f for MEHA SAMs formed after deposition for 1 min, 10 min, and 1 h were calculated to be 0.0022, 0.0068, and 0.0070, respectively. Based on the STM and XPS results, it is expected that the formation of a well-ordered β-phase is due to an increased adsorption of chemisorbed sulfur and the structural rearrangement of molecular backbones to maximize lateral interactions resulting from a longer deposition period of 1 h. CV measurements showed a significant difference in the electrochemical behavior of MEHA and decanethiol (DT) SAMs as a result of the presence of an internal amide group in the MEHA SAMs. Herein, we report the first high-resolution STM image of well-ordered MEHA SAMs on Au(111) with a (3 × 2√3) superlattice (β-phase). We also found that amide-containing MEHA SAMs were thermally much more stable than DT SAMs due to the formation of internal hydrogen networks in MEHA SAMs. Our molecular-scale STM results provide new insight into the growth process, surface structure, and thermal stability of amide-containing alkanethiols on Au(111).

A Machine-Learning Approach to Correcting Non-Ideal Background Currents in Disordered Self-Assembled Monolayers

In traditional studies of electroactive molecules tethered to metal surfaces through well-ordered self-assembled monolayers (SAMs), cyclic voltammetry is the primary tool used to measure the coverage of the tethered molecules. The charge under the peak in the voltammogram is directly proportional to the coverage of the redox-active molecules. Accurate coverage measurements are possible because well-ordered SAMs exhibit ideal background currents which are easy to subtract from the total current to isolate the SAMs faradaic current. However, for disordered electroactive SAMs with poor surface blocking properties, specific interactions with the metal substrate create non-ideal background currents which are difficult to separate from the SAMs faradaic current. Although analytical approaches exist to account for background currents in other electrochemical systems, they have some fundamental limitations that impede their practical application to electroactive SAMs. In our work, we propose a simple machine-learning approach to empirically correct for background currents in disordered SAMs, and we apply this approach to determine the surface coverage of partially-desorbed ferrocene-terminated monolayers (Fc-SAMs) from gold surfaces. We use LASSO regression and an ensemble of background scans from partially-desorbed unfunctionalized decanethiol SAMs to approximate the Fc-SAM background currents. Using the approximated background, we estimate the coverage of Fc-SAMs after background correction. This technique allows us to explore Fc-SAM coverage as a function of potential when applying potentials near and beyond the onset of the so-called reductive desorption potential of the SAM which induces non-ideal background currents. To verify the accuracy of the coverages determined using our LASSO method, we used two verification methods: 1) backfilling with decanethiol and 2) chemical stripping of the disordered SAM followed by inductively-coupled plasma-mass spectrometry (ICP-MS). The pros and cons of both confirmation methods will be discussed along with a comparison of error rates and limits of detection between traditional background subtraction method and the LASSO method. We propose that this new approach will enable a more quantitative description of disordered SAM films after reductive desorption events.

Nanoscale Work Function Contrast Induced by Decanethiol Self-Assembled Monolayers on Au(111).

In this paper, we obtain maps of the spatial tunnel barrier variations in self-assembled monolayers of organosulfurs on Au(111). Maps down to the sub-nanometer scale are obtained by combining topographic scanning tunneling microscopy images with dI/dz spectroscopy. The square root of the tunnel barrier height is directly proportional to the local work function and the dI/dz signal. We use ratios of the tunnel barriers to study the work function contrast in various decanethiol phases: the lying-down striped β phase, the dense standing-up φ phase, and the oxidized decanesulfonate λ phase. We compare the induced work function variations too: the work function contrast induced by a lying-down striped phase in comparison to the modulation induced by the standing-up φ phase, as well as the oxidized λ phase. By performing these comparisons, we can account for the similarities and differences in the effects of the mechanisms acting on the surface and extract valuable insights into molecular binding to the substrate. The pillow effect, governing the lowering of the work function due to lying-down molecular tails in the striped low density phases, seems to have quite a similar contribution as the surface dipole effect emerging in the dense standing-up decanethiol phases. The dI/dz spectroscopy map of the nonoxidized β phase compared to the map of the oxidized λ phase indicates that the strong binding of molecules to the substrate is no longer present in the latter.

Structural Stability of Physisorbed Air-Oxidized Decanethiols on Au(111)

We have studied the dynamic behavior of decanethiol and air-oxidized decanethiol self-assembled monolayers (SAMs) on Au(111) using time-resolved scanning tunneling microscopy at room temperature. T...

In Situ Vibrational Study of the Reductive Desorption of Alkanethiol Monolayers on Gold by Sum Frequency Generation Spectroscopy

In situ sum frequency generation vibrational spectroscopy (SFG) was used to monitor the reductive desorption of decanethiol (DT) and octadecanethiol (ODT) self-assembled monolayers (SAMs) grown on evaporated gold on silica. At negative potentials, the alkyl chains of both monolayers became disordered as monitored by the appearance of methylene symmetric stretching modes in the collected spectra. The increased tilting of the terminal methyl groups on the chains of the DT monolayer further support this observation. The disappearance of the methyl C–H stretching vibrational modes at the reductive potential suggests that DT molecules diffused away from the surface after reduction. ODT molecules, on the other hand, retained their two-dimensional structure near the gold surface, implied by the strong methyl vibrational modes at the reductive potential. After four reductive cycles, a large portion of the DT SAM was fully desorbed, while the ODT monolayer existed as a combination of physisorbed and chemisorbed mo...

Dynamics of Decanethiol Self-Assembled Monolayers on Au(111) Studied by Time-Resolved Scanning Tunneling Microscopy

We investigated the dynamics of decanethiol self-assembled monolayers on Au(111) surfaces using time-resolved scanning tunneling microscopy at room temperature. The expected ordered phases (β, δ, χ*, and φ) and a disordered phase (ε) were observed. Current-time traces with the feedback loop disabled were recorded at different locations on the surface. The sulfur end group of the decanethiolate molecule exhibits a stochastic two-level switching process when the molecule is adsorbed in a (local) β phase registry. This two-level process is attributed to the diffusion of the Au-thiolate complex between two adjacent adsorption sites. The irregular current jumps in the current-time traces recorded on the tails of decanethiolate molecules in the ordered β, δ, and χ* phases are ascribed to wagging of the alkyl tails. Finally, the disordered phase is characterized by even larger current jumps, which indicates that the tail of the decanethiolate flips up occasionally and makes contact with the tip. Our experiments reveal that the massive dynamics of the self-assembled monolayer is due to diffusion of decanethiol-Au complexes, rather than the diffusion of decanethiolate molecules.

Helium atom diffraction study of low coverage phases of decanethiol self-assembled monolayers prepared by supersonic molecular beam deposition

Thiol self-assembled monolayers (SAMs) on gold surfaces have a wide range of applications including surface patterning and metal contact–organic semiconductor interface modification in organic electronic devices. Here we present our studies about the low coverage phases of decanethiol (CH3(CH2)9SH) SAMs on Au(111) surface, grown in vacuum by supersonic molecular beam deposition. Crystal structures, phase behaviors and desorption energies of these films determined by helium atom diffraction technique are discussed.

A new striped-phase of decanethiol self-assembled monolayers on Au(1 1 1) formed at a high solution temperature

Self-assembled monolayers (SAMs) of decanethiol (C10) on Au(111) prepared at room temperature (RT), 60°C, and 70°C have been investigated using high resolution scanning tunneling microscopy (STM). For SAMs prepared at RT, the c(4×2) superlattice of the basic (3×3)R30° structure was hardly observed in small-scale STM images. Increasing the solution temperature to 60°C results in formation of SAMs having large ordered domains of the c(4×2) superlattice. In contrast to the previous studies, our results indicate the presence of four different topographical heights for the C10 molecules per c(4×2) unit cell. Preparation of C10 SAMs from 70°C of solution temperature leads to the formation of new striped phase with (43×23)R30° unit cell. The area of a single molecule in the (43×23)R30° striped structure amounts to 57.73Å2, which is about 167% larger than that (21.6Å2) for a molecule in the c(4×2) structure.

Coexistence of Closely Packed c(4 × 2) and Striped Phases in Self-Assembled Monolayers of Decylthiocyanates on Au(111)

Decylthiocyanate (DTC) self-assembled monolayers (SAMs) on Au(111) were prepared by solution and vapor phase deposition methods at <TEX>$50^{\circ}C$</TEX> for 24 h. The formation and surface structure of DTC SAMs were examined using scanning tunneling microscopy (STM). STM imaging revealed that DTC SAMs formed in 1 mM ethanol solution at <TEX>$50^{\circ}C$</TEX> were composed of small ordered domains with lateral dimensions of a few nanometers and disordered phases, whereas DTC SAMs formed in the vapor phase at <TEX>$50^{\circ}C$</TEX> contained two ordered phases: a closely packed c(<TEX>$4{\times}2$</TEX>) superlattice and a striped phase with an interstripe spacing of 2.6 - 2.8 nm. It was also found that the ordered domain and vacancy island formation for DTC SAMs on Au(111) differs significantly from that of decanethiol SAMs, suggesting that adsorption mechanism is different from each other. From this study, it was confirmed that DTC SAMs with a high degree of structural order can be obtained by vapor phase deposition.

Atom Probe Microscopy of Self-Assembled Monolayers: Preliminary Results

We have achieved three-dimensional imaging of decanethiol self-assembled monolayers (SAMs) on metal surfaces by atom probe tomography (APT). The present Letter provides preliminary results on Ni [001] and Au [111], shows the analytical potential of APT analysis of SAMs, and details developments in specimen preparation and in data-treatment methodologies. Importantly, the investigation of the mass spectra from analysis of the SAMs revealed no combination of sulfur and hydrogen at the interface between the metal substrates and the organic materials, potentially providing insight about the bonding of the thiols on the substrate.

Effects of concentration and temperature on the formation process of decanethiol self-assembled monolayer on Au(1 1 1) followed by electrochemical reductive desorption

The formation process of self-assembled monolayer (SAM) of decanethiol (C10SH) on Au(1 1 1) single crystal electrode has been investigated for wide range of C10SH concentration (0.1–500 μM) and temperature (253–298 K). The amount and quality of C10SH SAM were determined from area and position, respectively, of reductive desorption peak of the SAM modified Au(1 1 1) electrode measured in 0.5 M KOH aqueous solution. The kinetic analysis indicates that SAM formation proceeded by two steps, diffusion-limited physisorption followed by chemisorption.

Adhesion behaviour of self-assembled alkanethiol monolayers on silver at different stages of growth

We studied the development of interfacial energy γ of alkanethiol self-assembled monolayers on Ag/mica substrates by force-displacement spectroscopy using an atomic force microscope. Self-assembled monolayers of decanethiol [CH3 (CH2)9SH] and octadecanethiol [CH3 (CH2)17SH] were studied. The growth of the monolayer as a function of time was studied by an ellipsometer for three different concentrations of decanethiol and octadecanethiol. The thiols led to well-ordered self-assembled monolayer that showed atomic resolution images by lateral force microscopy mode. The behaviour of γ showed non-monotonic variation in stages of adsorption, which was more prominent for short chain length decanethiol. The γ reached a limiting value of 12 mJ m−2 for decanethiol and 7 mJ m−2 for octadecanethiol.

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.

Comparative study of decyl thiocyanate and decanethiol self-assembled monolayers on gold substrates

In a recent paper Ciszek et al. [J.W. Ciszek, M.P. Stewart, J.M. Tour, J. Am. Chem. Soc. 126 (2004) 13172] showed that organic thiocyanates may be an interesting alternative to the use of thiols for thiolate assemblies. We use scanning tunnelling microscopy (STM), infrared reflection absorption and sum-frequency generation spectroscopies (IRRAS and SFG) in order to study the adsorption properties of decyl thiocyanates (DTCN) and compare them to the decanethiol (DT) ones. Firstly, IRRAS measurements show that DTCN molecules form self-assembled monolayers (SAMs) on gold via a thiolate link with the metallic substrate. Secondly, the DTCN SAM on gold is less ordered than the DT one as highlighted by SFG spectroscopy. Indeed, the intensities of the methyl vibration modes vanish while the methylene ones increase when DTCN molecules are adsorbed on the substrate instead of DT. We explain the differences in SAMs quality on the basis of STM measurements which reveal differences in molecular order and packing.

Time-Dependent Organization and Wettability of Decanethiol Self-Assembled Monolayer on Au(111) Investigated with STM

A detailed study on the time-dependent organization of a decanethiol self-assembled monolayer (SAM) at a designed solution concentration onto a Au(111) surface has been performed with scanning tunneling microscopy (STM). The SAMs were prepared by immersing Au(111) into an ethanol solution containing 1 microM decanethiol with different immersion times. STM images revealed the formation process and adlayer structure of the SAMs. It was found that the molecules self-organized into adlayers from random separation to a well-defined structure. From 10 s, small domains with ordered molecular organization appeared, although random molecules could be observed on Au(111) at the very initial stage. At 30 s, the SAM consisted of uniform short stripes. Each stripe consisted of sets of decanethiol mainly containing eight molecules. With the immersion time increasing, the length of the stripes increased. At 5 min, the alkyl chains overlapped each other between the adjacent stripes, indicating the start of a stacked process. After immersing Au(111) in decanethiol solution for 3 days, a densely packed adlayer with a (radical 3 x radical 3)R30 degrees structure was observed. The formation process and structure of decanethiol SAMs are well related to sample preparation conditions. The wettability of the decanethiolate SAM-modified Au(111) surface was also investigated.

Measuring the Size Dependence of Young's Modulus Using Force Modulation Atomic Force Microscopy

The dependence of the local Young's modulus of organic thin films on the size of the domains at the nanometer scale is systematically investigated. Using atomic force microscopy (AFM) based imaging and lithography, nanostructures with designed size, shape, and functionality are preengineered, e.g., nanostructures of octadecanethiols inlaid in decanethiol self-assembled monolayers (SAMs). These nanostructures are characterized using AFM, followed by force modulation spectroscopy and microscopy measurements. Young's modulus is then extracted from these measurements using a continuum mechanics model. The apparent Young's modulus is found to decrease nonlinearly with the decreasing size of these nanostructures. This systematic study presents conclusive evidence of the size dependence of elasticity in the nanoregime. The approach utilized may be applied to study the size-dependent behavior of various materials and other mechanical properties.

The use of CTAB to control the size of copper nanoparticles and the concentration of alkylthiols on their surfaces

Copper nanoparticles protected by self-assembled monolayers of decanethiol have been prepared by the controlled reduction of aqueous copper salts using the modified Brust synthesis. For comparison, the nanoparticles are synthesized on different concentration of cetyltrimethylammonium bromide (CTAB). TEM, XPS, XRD, IR have been employed to investigate the structure and formation of these nanoparticles. It was found that CTAB plays a critical role in controlling stability and ease to formation 3D SAMs during synthesis.

Single electron tunneling and manipulation of nanoparticles on surfaces at room temperature

This article focuses on surfaces containing nanoparticles and self-assembled monolayers (SAMs). These surfaces provide a simple and reliable platform for measurements of single electron tunneling (SET) properties of metal nanoparticles at room temperature. This approach of interfacial chemistry allows for the elimination of lateral motion of the individual nanoparticles during electronic property studies. The scanning tunneling microscopy (STM) in ultra-high vacuum is used as an accurate and reproducible probe for imaging and I– V characterization of individual or aggregated Au nanoparticles, revealing a large Coulomb gap (1.0 eV) and fine Coulomb staircases (0.2–0.3 eV) at room temperature. The surrounding decanethiol SAM provides an ideal reference for the imaging and I– V measurements of nanoparticles. These measurements provide a quantitative guide for regulating current and voltage, at which individual Au nanoparticles may be detached and manipulated with the STM tip.

Electrochemical behavior and determination of trifluoperazine at decanethiol self-assembled monolayer modified gold electrodes

The electrochemical behavior of trifluoperazine at decanethiol self-assembled monolayer (SAM) modified gold electrodes (i. e. C10 H21 SH/Au) has been studied. Triflu operazine can effectively accumulate on C10 H21 SH Au electrodes and generate a sensitive anodic peak at about 0. 63 V (vs. SCE) in 0.05 mol/L pH 9.1 Na2B1O, buffer solution. Under the selected conditions, the anodic peak current was linear to trifluoperazine concentration in the range of 5.0×107–3.0×1015 mol/L with correlation coefficient of 0.997. the detection limit was 3.0×108 mol/L. This method was applied to the determination of trifluoperazine in drug samples and the recovery was 97.30, 101.03 r. It was found that so dium dodecyl sulfate (SDS) could make the anodic peak current increase. In the presence of SDS, the peak at about 0.63 V turned into two peaks, resulting from the change of the electrochemical mechanism.

Noble Metal Nanoparticles Deposited on Self‐Assembled Monolayers by Pulsed Laser Deposition Show Coulomb Blockade at Room Temperature

Nanometer-sized noble-metal clusters are fabricated on top of alkylthiolate self-assembled monolayers (SAMs) on annealed gold by pulsed laser deposition at elevated pressures. The size distribution of the clusters depends on the metal and on the pressure during the deposition. Scanning tunneling microscopy (STM) and conductive probe atomic force microscopy (CP-AFM) showed that the metal clusters are insulated from the substrate on top of the SAM. Coulomb blockades could be measured at room temperature by STM for palladium clusters on decanethiol SAMs.