Decanethiol Self-assembled Monolayers Research Articles

Inelastic Electron Tunneling Spectroscopy on Decanethiol at Elevated Temperatures

Inelastic electron tunneling spectroscopy (IETS) measurements using scanning tunneling microscopy (STM) on a decanethiol self-assembled monolayer on Au(111) are presented. The results presented are obtained in the temperature range 77−300 K, which is a much higher temperature than what is commonly used for STM-IETS measurements. Two peaks at ±34 meV (probably Au−S or S−C stretch mode) and ±156 meV (C−C stretch mode or a CH2 mode) are resolved.

Electrochemical Oxidative Formation and Reductive Desorption of a Self-Assembled Monolayer of Decanethiol on a Au(111) Surface in KOH Ethanol Solution

Electrochemical oxidative adsorption and reductive desorption of a self-assembled monolayer (SAM) of decanethiol on a Au(111) single crystal electrode were examined in 0.1 M KOH ethanol solution containing various concentrations of decanethiol ranging from 1 muM to 1 mM. Anodic and cathodic current peaks corresponding to the adsorption and desorption of decanethiol, respectively, were observed in cyclic voltammograms of a Au(111) single crystal electrode obtained in 0.1 M KOH ethanol solution containing more than 10 muM of decanethiol. Positions of both peaks depended on the concentration of decanethiol, and they shifted negatively by ca. 0.057 V/decade with increase in decanethiol concentration. This result confirms that the adsorption and desorption of decanethiol is a one-electron process. The reductive charge, which consists of desorption charge and capacitive charge, increased when the sweep rate was decreased and the decanethiol concentration was increased and reached the saturated value of 103 (+/-5%) muC cm-2, which corresponds to the reductive charge of thiol SAM of full coverage with a ( radical3 x radical3)R30 degrees structure. Potentiostatic SAM formation was also investigated by holding the potential at +0.1 V. The reductive charge, i.e., the coverage of the SAM, increased with time and reached the saturated value of 103 (+/-5%) muC cm-2, corresponding to full coverage, after holding the potential at +0.1 V for a certain period of time. The time when the amount of adsorbed thiolate reached full coverage depended on the concentration of decanethiol. The higher the concentration was, the faster full coverage was reached. The desorption peak shifted negatively as the holding time at +0.1 V was increased even after the adsorbed amount had reached full coverage. These results suggest that the ordering of decanethiol SAMs requires a much longer time than the time required for full coverage adsorption. The position of the reductive desorption peak was independent of the thiol concentration if the electrode was kept at +0.1 V for long enough so that a highly ordered SAM was formed. The cathodic peak shifted negatively as the sweep rate was increased, showing that reductive desorption of the SAM was rather slow. The rate constant for the reductive desorption was determined from the potential dependent peak shift to be 0.24 s-1, which is in good agreement with the value obtained for a SAM prepared without potential control, indicating that the quality of the electrochemically prepared SAM is as good as that of the SAM prepared nonelectrochemically.

Voltammetric Determination of Prochlorperazine and Ethopropazine Using a Gold Electrode Modified with Decanethiol SAM

The voltammetric behavior of prochlorperazine and ethopropazine at a decanethiol (DEC) self-assembled monolayer (SAM) modified gold electrode (DEC/Au) has been studied. It was observed that prochlorperazine exhibited an anodic peak at about 0.60 V (vs SCE), while ethopropazine exhibited two anodic peaks at about 0.49 V and 0.58 V on DEC/Au in pH 10 sodium carbonate buffer. This was due to their different electrochemical oxidation mechanisms. In this case, the oxidation of prochlorperazine and ethopropazine included one 2e step and two 1e steps, respectively. In the presence of some reductants such as ascorbic acid, the oxidation products of them can catalyze the oxidation of the reductants and thus make the peaks grow. In addition, it was found that the SAM structure became not so compact when prochlorperazine and ethopropazine were present, resulting from their permeating in the SAM. Various conditions were optimized for their determination. Under the selected conditions (i.e. 0.080 M pH 10 sodium carbonate buffer; scan rate: 100 mV/s; accumulation potential: –0.4 V or 0 V; accumulation time: 60 s), the peak currents were linear to prochlorperazine concentration in the ranges of 0.1~2.0 μM and 5.0~50 μM, and linear with ethopropazine in the ranges of 10 nM~0.1 μM and 0.5~20 μM. The RSD was 4.28% for 8 successive measurements of 1.0 μM prochlorperazine. The influence of some coexistents was examined.

Study of the interaction potential between He and a self-assembled monolayer of decanethiol

Self-assembled monolayers of decanethiol on Au(111) have been investigated by means of low energy He beam scattering. In particular the specular reflectivity has been measured and features associated with resonances due to bound states of He atom interacting with the CH3 groups of the standing-up organic molecules have been observed. The analysis of the data has allowed the determination of four bound state levels and the He–surface interaction potential averaged over the unit cell has been estimated.

Electrochemical study and detection of perphenazine using a gold electrode modified with decanethiol SAM

A novel method for the determination of perphenazine has been developed. The method is based on the accumulation of perphenazine at a gold electrode modified with decanethiol (DEC) self-assembled monolayer (SAM) and its oxidation at about 0.6 V (vs. saturated calomel electrode (SCE)). Because some coexistent electroactives were blocked and perphenazine was selectively accumulated by the SAM, the electrode exhibited good selectivity and sensitivity. Various conditions were optimized for practical application. Under the selected conditions (i.e. 0.05 M pH 10 sodium borate buffer, accumulation time: 120 s, accumulation potential: −0.4 V, scan rate: 100 mV s −1), the anodic stripping peak current was linear to perphenazine concentration in the ranges of 6×10 −9–5×l0 −7 and 5×10 −7–5×10 −6 M with correlation coefficients of 0.998 and 0.995, respectively. For a 1.0×10 −6 M perphenazine solution, the relative standard deviation of peak height was 2.3% ( n=8). This method was applied to the determination of perphenazine in some drugs and the recovery was 92–101%. In addition, it was found that in the presence of perphenazine, the SAM structure changed a little and more needle holes appeared. However, the SAM could recover the original form when perphenazine and its redox product were removed from the monolayer by repeatedly cycling the electrode in a blank solution for a minute. The modified electrode was characterized by alternating current impedance and electrochemical probe.

Structures of Annealed Decanethiol Self-Assembled Monolayers on Au(111): an Ultrahigh Vacuum Scanning Tunneling Microscopy Study

While the structures of self-assembled monolayers (SAMs) of alkanethiols on Au(111) are extensively studied and well-known, new structures and complex phase behavior have been progressively discovered when coverage of these layers falls below saturation. Structures and phase transitions of annealed decanethiol monolayers on Au(111) surfaces were systematically investigated using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Rich structures were revealed as a result of annealing in UHV. At temperatures below 345 K, no significant changes in coverage were observed, although the size of two-dimensional crystalline c(4√3 × 2√3)R30° domains increases as annealing progresses. A two-dimensional melting occurs at 345 ± 5 K and was captured in situ from time-dependent STM studies. Above 400 K, significant desorption takes place. In the temperature range of 345−400 K, within which desorption progresses to gradually decrease the surface coverage, a variety of striped phases have been observed, each having distinct molecular-level packing and unit cells. Well-known striped phases have been confirmed: (p × √3), with p values (integer or half-integer multiples of the Au(111) periodicity) of 7.5, 9, and 11. In addition, new structures such as mixed striped phases and mesh-like structures are revealed, which are often found to coexist with the regions of pure striped phases. The systematic investigations of the structural and phase evolution shed light on the SAM desorption process at the molecular level.

Electronic Tunneling through Self-Assembled Monolayers of Phenylene Oligomers on Au(111)

We investigated the electrical properties of self-assembled monolayers (SAMs), which are composed of a single component of benzylic derivatives of oligophenylene thiols, HSCH2(C6H4)m-1C6H5 (m=1, 2, 3) on an Au(111) substrate, using conducting atomic force microscopy (AFM). The use of an Au-coated AFM tip with a decanethiol SAM resulted in a bilayer junction at the contact area between the Au-coated tip with the decanethiol SAM and the Au(111) substrate covered with one of the phenylene oligomer SAMs, i.e., Au-decanethiol SAM/oligophenylene SAM-Au(111). The electrical properties of the phenylene oligomers were studied in terms of the dependence of the observed tunneling current on the length of the oligomers at the same constant sample bias voltage. An exponential decrease in the tunneling current was observed with increasing length of the oligomers. The decay constant, β, of the tunneling current through the SAMs is related to the electronic structure of the molecules. The present study gave a β value of 5.5 nm-1 for the phenylene oligomers by measuring the current–voltage (I–V) curves of the bilayer junctions and plotting the resulting current (at +1.0 V) as a function of the length of the oligomers. The I–V curves were symmetrical around V=0. The value of β did not depend significantly on the applied voltage and was close to reported ones of 6.6 nm-1 for electron transfer through thin organic films of a series of corresponding Hg-hexadecanethiol SAM/oligophenylene SAM-Ag junctions and of 4.0 nm-1 for photoinduced electron transfer in bis-porphyrin donor-acceptor compounds with polyphenylene spacers. The slight difference among these three values was attributed to the difference in barrier height not for electron transfer but for hole transfer through the phenylene oligomers.

Chemistry on surface-confined molecules: an approach to anchor isolated functional units to surfaces.

The synthesis of surface-confined, nanometer-sized dendrimers and Au nanoparticles was performed starting from single Pd(II) pincer adsorbate molecules (10) embedded as isolated species into 11-mercapto-1-undecanol and decanethiol self-assembled monolayers (SAMs) on gold. The coordination of monolayer-protected Au nanoclusters (MPCs) bearing phosphine moieties at the periphery (13), or dendritic wedges (8) having a phosphine group at the focal point, to SAMs containing individual Pd(II) pincer molecules was monitored by tapping mode atomic force microscopy (TM AFM). The individual Pd(II) pincer molecules embedded in the decanethiol SAM were visualized by their coordination to phosphine MPCs 13; isolated objects with a height of 3.5 +/- 0.7 nm were observed by TM AFM. Reaction of these embedded Pd(II) pincer molecules with the dendritic wedge 8 yielded individual molecules with a height of 4.3 +/- 0.2 nm.

Microcontact Printing with Heavyweight Inks

Thioether derivatives with 1–4 thioether moieties were used as inks in microcontact printing on gold for the reproduction of patterns as they combine good monolayer quality with synthetic versatility, and high molecular weights. Self-assembled monolayers (SAMs) on gold of compounds 1 and 2 had a quality comparable to SAMs of decanethiol, both regarding monolayer order and etch resistance. Etch resistances of SAMs of 3 and 4 were lower. Resulting structures after etching of patterned SAMs using 1, 2, and 3 were of good quality for patterns with feature sizes on the same stamp ranging from 1–200 μm. Patterns of 4 were not reproduced. Overall, compounds 1 and 2 are good candidates for low-diffusion inks.

Microcontact Printing with Heavyweight Inks

Thioether derivatives with 1–4 thioether moieties were used as inks in microcontact printing on gold for the reproduction of patterns as they combine good monolayer quality with synthetic versatility, and high molecular weights. Self-assembled monolayers (SAMs) on gold of compounds 1 and 2 had a quality comparable to SAMs of decanethiol, both regarding monolayer order and etch resistance. Etch resistances of SAMs of 3 and 4 were lower. Resulting structures after etching of patterned SAMs using 1, 2, and 3 were of good quality for patterns with feature sizes on the same stamp ranging from 1–200 μm. Patterns of 4 were not reproduced. Overall, compounds 1 and 2 are good candidates for low-diffusion inks.

Organic semiconducting thin film growth on an organic substrate:3,4,9,10-perylenetetracarboxylic dianhydride on a monolayer of decanethiol self-assembled on Au(111)

We use surface x-ray diffraction to study the structure of organic-organic heterojunctions grown by organic molecular-beam deposition. In particular, we study films of $3,4,9,10$-perylenetetracarboxylic dianhydride (PTCDA) grown on a decanethiol self-assembled monolayer (SAM) on a Au(111) surface. The deposition of several $(\ensuremath{\approx}16)$ monolayers of PTCDA results in unstrained crystalline films whose (012) lattice planes are rotated $21.6\ifmmode^\circ\else\textdegree\fi{}$ with respect to the $〈112\ifmmode\bar\else\textasciimacron\fi{}〉$ Au azimuthal direction. This alignment, which is different from that of PTCDA on the bare Au(111) surface, is most likely caused by the corrugation of the SAM surface [with the $c(4\ifmmode\times\else\texttimes\fi{}2)$ superlattice of the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\mathrm{R}30\ifmmode^\circ\else\textdegree\fi{}$ unit cell]. The SAM structure was found to be unaltered by the presence of the PTCDA overlayer. In addition, the heterogeneous PTCDA/SAM/Au structure, acting as an x-ray interferometer with the SAM as a spacer, allows for the precise determination of the SAM thickness.

Self-assembled monolayers of decanethiol on Au(111)/mica

We report here a preparation for thin gold films on mica substrates. We have investigated the influence of the substrate temperature and the evaporation rate on the morphology of the films. After careful outgasing of the substrate, 100 nm of Au is evaporated onto the mica surface maintained at high temperature. After slow cooling, ex situ characterizations are performed using AFM and STM. For our purposes, the best compromise between roughness and grain size is found to occur for an evaporation rate of 2 ˚ A s −1 onto a mica substrate maintained at 460 • C. We have used these substrates for STM and AFM study of decanethiol self-assembled monolayers (SAMs). We present results for gold samples immersed for a few seconds in decanethiol solutions, revealing an incomplete organization of the films. The organization process is discussed through comparison between AFM and STM data recorded on the SAMs. Then we present molecular resolution STM pictures of ordered SAMs for longer immersion times.

Annealed decanethiol monolayers on Au(111) – intermediate phases between structures with high and low molecular surface density

Self-assembled monolayers of decanethiol on Au(111) have been investigated using high resolution scanning tunneling microscopy (STM). After the annealing of the as-adsorbed densely packed c(2 3 ×4 3 )R30° structure and before the formation of a previously described low density (11.5× 3 ) stripe phase three different phases with intermediate molecular surface density have been observed. We present highly resolved and distortion-corrected STM images for both the (7.5× 3 ) stripe phase and the meshlike structure. We describe a mixed (7.5/11.5× 3 ) stripe phase that shows a switching of the stripe arrangement.

Scanning tunneling microscope investigations of organic heterostructures prepared by a combination of self-assembly and molecular beam epitaxy

We report the realization of organic–organic heteroepitaxy by combining liquid-phase self-assembly with ultrahigh vacuum (UHV) gas-phase molecular beam epitaxy. As a model system, we have used self-assembled monolayers (SAMs) prepared by exposing an Au(111)–mica substrate to a dilute solution of decanethiol in ethanol, with subsequent evaporation of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) dye molecules. The well-known (3×3)R30° superstructure of almost upright standing molecules after chemisorption is replaced by the 11.5×3 (33.2Å) pin-stripe phase with flat lying molecules when the samples are annealed in UHV and coverage decreases. The deposition of PTCDA induces reordering and displacement in the decanethiol SAM. Additional to the 33.2Å periodicity, the previously reported 22Å thiol stripe phase can be observed by scanning tunneling microscopy. Several PTCDA structures are observed: single and double rows of PTCDA that grow along decanethiol stripes on top of the flat-lying alkane chain groups, as well as densely packed PTCDA monolayer domains embedded into the thiol layer. These exhibit the well-known herringbone structure or a novel square lattice structure. We have also investigated thicker PTCDA islands and observed molecular resolution for a thickness of several monolayers. The structure of the three-dimensional islands can be identified as the α bulk modification.

Electrochemical Study of a Polypyrrole Film/Decanethiol Self-Assembled Monolayer on a Gold Electrode

Cyclic voltammetry and chronoamperometry were used to study the deposition of polypyrrole on a decanethiol self-assembled monolayer modified gold electrode (PPY/SAM/Au). The voltammetric behavior of the PPY/SAM film was investigated in the presence of several different electrolytes. It is found that the SAM shows great influence on the nucleation and growth of the PPY film. The reaction of the SAM and the anions causes the different voltammetric behavior of the polymerization of pyrrole on the modified electrode. Chronoamperometry shows the nucleation and growth of the PPY is initially inhibited but followed by a rapid increase. The SAM also influences electrochemical behavior of PPY film. Experiments show that the SAM can greatly depress the diffusion of anions in the PPY film, and minimize the background capacitance current.

Electrochemical Study of a Polypyrrole Film/Decanethiol Self-Assembled Monolayer on a Gold Electrode

Cyclic voltammetry and chronoamperometry were used to study the deposition of polypyrrole on a decanethiol self-assembled monolayer modified gold electrode (PPY/SAM/Au). The voltammetric behavior of the PPY/SAM film was investigated in the presence of several different electrolytes. It is found that the SAM shows great influence on the nucleation and growth of the PPY film. The reaction of the SAM and the anions causes the different voltammetric behavior of the polymerization of pyrrole on the modified electrode. Chronoamperometry shows the nucleation and growth of the PPY is initially inhibited but followed by a rapid increase. The SAM also influences electrochemical behavior of PPY film. Experiments show that the SAM can greatly depress the diffusion of anions in the PPY film, and minimize the background capacitance current.

Exchange reaction of a decanethiol self-assembled monolayer with porphyrin disulfides observed by surface plasmon enhanced fluorescence spectroscopy

The treatment of a decanethiol SAM with a solution of a porphyrin disulfide gives a SAM inlaid with the porphyrin chromophores adopting J-aggregate-like partially stacked and well oriented structures by site-selective exchange reaction in the SAM.

Flat Lying Pin-Stripe Phase of Decanethiol Self-Assembled Monolayers on Au(111)

Self-assembled monolayers with low molecular surface density of decanethiol on Au(111) have been prepared by annealing of the densely packed (√3 × √3)R30° structure. High-resolution scanning tunneling microscope (STM) imaging reveals the formation of a 11.5 × √3 stripe structure where the molecules are aligned along the substrate surface in double lamellae. We can deduce from the images that the molecules bind to specific sites at the Au(111) surface rather than adsorb as didecane disulfides. For the first time, a lattice structure for an alkanethiol stripe phase can be derived from the obtained real-space data.

Evidence for Nondestructive Adsorption of Dialkyl Sulfides on Gold

The nondestructive adsorption of long chain dialkyl sulfides on gold is reported. Self-assembled monolayers of decanethiol, didecyl disulfide, and didecyl sulfide were prepared and characterized by XPS, grazing-angle FT-IR, wettability studies, and TOF-SIMS. The first three techniques provide evidence that well-packed monolayers are formed. The positive SIMS spectrum of didecyl sulfide showed fragments that originate from intact dialkyl sulfide attached to gold without C−S bond cleavage. Our results underline the generally accepted nondestructive adsorption process of dialkyl sulfides and contradict the recently reported cleavage of one of the alkyl sulfur bonds of sulfides upon adsorption.