Conformational order of n-dodecanethiol and n-dodecaneselenol monolayers on polycrystalline copper investigated by PM-IRRAS and SFG spectroscopy

Self-assembled monolayers (SAMs) of n-dodecanethiol (C₁₂H₂₅SH) and n-dodecaneselenol (C₁₂H₂₅SeH) on polycrystalline copper have been elaborated with the purpose of achieving densely packed and crystalline-like assemblies. By combining the surface sensitivity of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and sum-frequency generation spectroscopy (SFG), the effect of the self-assembly time (15 min, 30 min, 1 h, 2 h and 24 h) on the formation of n-dodecanethiol and n-dodecaneselenol monolayers on untreated and electrochemically reduced polycrystalline copper has been investigated. On electrochemically reduced copper, PM-IRRAS spectroscopy shows that both molecules are able to form well organized layers. SFG spectroscopy indicates that the C₁₂H₂₅SeH SAMs are slightly better ordered than those achieved with C₁₂H₂₅SH. On untreated copper, the two molecules lead to different film organizations. Both PM-IRRAS and SFG indicate that C₁₂H₂₅SH SAMs are of the same film quality as those obtained on electrochemically reduced copper. On the contrary, C₁₂H₂₅SeH monolayers are invariably poorly organized at the molecular level.     

Absolute Orientation of Molecules with Competing Hydrophilic Head Groups at the Air/Water Interface Probed with Sum Frequency Generation Vibrational Spectroscopy

和频振动光谱(SFG-VS)研究中由基团在界面上取向所引起的光谱增强或相消的干涉现象为研究分子在界面上的绝对取向提供了一种直接的测量方法. 这一方法比SFG实验中复杂的相位测量方法更为直接和简单可行. 以在空气/水界面取向已知的对羟基苯腈(PCP)分子的氰基(-CN)基团为相位参考来获得3,5-二甲基对羟基苯腈(35DMHBN)和2,6-二甲基对羟基苯腈(26DMHBN)分子在空气/水界面的取向信息. 通过对这三种分子的水溶液和它们两两混合溶液界面上-CN基团和频振动光谱强度的比较，发现在空气/水界面的3     

A detailed analysis of the photoemission spectra of basic thioaromatic monolayers on noble metal substrates

Analysis of photoemission spectra of complex thioaromatic self-assembled monolayers (SAMs) requires the knowledge and understanding of such spectra for the basic systems. Keeping this goal in mind, synchrotron-based high-resolution X-ray photoelectron spectroscopy was used to characterize SAMs formed from the simplest thioaromatic compounds, namely thiophenol, 1,1′-biphenyl-4-thiol and 1,1′;4′,1″-terphenyl-4-thiol on evaporated Au(1 1 1) and Ag(1 1 1) substrates. The acquired S 2p and C 1s spectra were analyzed in terms of fine structure and initial and final state effects in the photoemission process. The assignment of the individual spectral features was considered in detail. Conclusions on quality and chemical and structural homogeneity of the investigated SAMs were derived.     

Characterization and protonation behavior of bipyridine thiol self-assembled monolayer on Au(1 1 1) studied using X-ray photoelectron spectroscopy and scanning tunneling microscopy

The adsorption process, molecular arrangement and protonation behavior of self-assembled monolayers (SAMs) of bipyridine thiol on Au(1 1 1) were investigated using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM), with a view towards constructing a molecular ion sensor. When the bipyridine SAMs were immersed in a strong CF₃SO₃H solution for protonation of the bipyridine group, additional N(1s) XPS peaks were generated at higher binding peak positions where the origin of the peak was considered to be the N-H species. We further investigated the relationship between the immersion time for the SAMs and the probability of protonation. We observed a decrease in the probability of protonation with a longer immersion time for the SAMs. We consider that both the bipyridine molecular arrangements and the molecular density on the Au surface are crucial for controlling the probability of protonation based on the STM and XPS data.     

Silver ions adsorbed to self-assembled monolayers of alkanedithiols on gold surfaces form Ag–dithiol–Au multilayer structures

Double-ended alkanedithiols, 1,9-nonanedithiol and 1,5-pentanedithiol, formed self-assembled monolayers (SAMs) on Au(l11) substrates and were used to adsorb silver ions from an ethanolic solution of silver nitrate and formed Ag–dithiol–Au multilayer structures. Ellipsometry, contact angle measurement and X-ray photoelectron spectroscopy (XPS) confirmed that the alkanedithiol molecules formed SAMs with only one-ended thiol groups attached to the Au substrates, which was supported by molecular mechanics calculation. XPS and X-ray Auger electron spectroscopy (XAES) indicated that silver ions were deposited onto the SAMs from the solution by the chemical reaction of silver nitrate with another-ended thiol groups of the SAMs. Atomic force microscopy (AFM) was used to observe SAMs and multilayer structures. Received: 20 January 2000 / Accepted: 18 April 2000 / Published online: 9 August 2000     

Monolayer vs. multilayer self-assembled alkylphosphonate films: X-ray photoelectron spectroscopy studies

Combining functional organic self-assembled monolayers (SAMs) with conventional semiconductor materials is a key step in the development of integrated electronics-based devices. T-BAG (Tethering by Aggregation and Growth) has been shown to be a simple and reliable method to grow SAMs of alkylphosphonates on oxide surfaces. However, distinguishing SAMs from ultra-thin multilayers is a challenge for most conventional surface characterization techniques.Self-assembled films of octadecylphosphonic acid (ODPA) were deposited on oxide-covered silicon coupons, converted to the corresponding phosphonates, and characterized by high resolution angularly resolved X-ray photoelectron spectroscopy (XPS). It was our goal to distinguish among different bonding configurations for phosphorous in the phosphonate head groups, namely, mono-dentate, bi-dentate or tri-dentate interactions with the oxide surface, as well as to assess quantitatively the near-surface layer composition.We also present an innovative method that allows us to distinguish between monolayer and multilayer films of ODPA on silicon oxide surfaces. This method is based on differential surface charging effects in XPS. It was found that variation in the ODPA film thickness causes differential responses of various spectral characteristics to an electrical bias applied to the sample during XPS measurements. Both positive and negative applied biases were found to affect the carbon core-level (C1s) line-shape and intensity in the case of the multilayer ODPA film, whereas line-shapes and intensities of all XPS lines measured for the monolayer film were unaffected by the application of a dc bias in the ±30 V range.     

Control of surface properties of self-assembled monolayers by tuning the degree of molecular asymmetry

We have studied self-assembled monolayers (SAMs) of asymmetric dialkyldisulfide derivatives of the form CH₃-(CH₂)_11+m-S-S-(CH₂)₁₁-OH with m = −4, −3, 0, +2 and +4 on gold. Sub-nanoscale changes in the length of the CH₃-terminated alkylchain have been used to selectively protrude one particular end group in the resulting film. The alteration of the chain length in only two methylene units already results in changes of surface properties, which have been detected with local (chemical force microscopy) and macroscopic (contact angle) techniques. In particular, advancing contact angles can be adjusted between 40° and 80°. The adhesion between a hydrophobic tip and these SAMs in water is determined by the chemical nature of the protruding end group. Chemical force microscopy, X-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy have shown that these SAMs are composed of mixed, well-packed CH₃- and OH-alkylthiolate branches. The surface composition ratio is close to 1:1 for all investigated SAMs.     

交叉传播构型的和频振动光谱

报道和频振动光谱在交叉传播的实验构型下的理论公式推导和实验结果. 在交叉传播的和频振动光谱实验室中,可见光和红外光通过相互垂直的入射面同时照射在界面上,从而避免了对使用同时能够透过可见和红外激光束的光学元件的要求. 这种交叉实验构型能够直接应用到封闭在真空或者压力腔体中的界面,使得在用远红外直接探测金属氧化物及其它低频界面振动模式实验的窗口材料有更多的选择. 这一交叉实验构型的潜在应用包括表面科学、材料科学、基础催化科学以及低温下的分子科学等方面.     

CO stretching vibrations on Pt(111) and Pt(110) studied by sumfrequency generation

Optical sum-frequency generation (SFG) is used to characterize CO stretching vibrations on Pt(111) and Pt(110) surfaces. Different adsorption sites (terminal, bridge and step sites) are identified in the SFG spectra of CO on Pt(111), in good quantitative agreement with previous infrared reflection-absorption experiments on this system. For CO on Pt(110) we only observe CO molecules on terminal sites. The measured CO stretching vibration frequencies on Pt(110), both for low and high coverages, are at variance with the results of previous infrared studies. Our SFG results for CO on Pt(110) are confirmed by independent EELS measurements which, in addition, also reveal the frustrated rotational mode and the metal-CO vibration. The measured frequency of 2065 cm⁻¹ for low CO coverage on Pt(110)-(1 × 2) is consistent with a previously proposed empirical relation between the frequency of an isolated adsorbed CO molecule and the coordination number of the binding Pt surface atom.     

Patterned self-assembled monolayers of alkanethiols on copper nanomembranes by submerged laser ablation

Self-assembled monolayers (SAMs) of alkanethiols are major building blocks for nanotechnology. SAMs provide a functional interface between electrodes and biomolecules, which makes them attractive for biochip fabrication. Although gold has emerged as a standard, copper has several advantages, such as compatibility with semiconductors. However, as copper is easily oxidized in air, patterning SAMs on copper is a challenging task. In this work we demonstrate that submerged laser ablation (SLAB) is well-suited for this purpose, as thiols are exchanged in-situ, avoiding air exposition. Using different types of ω-substituted alkanethiols we show that alkanethiol SAMs on copper surfaces can be patterned using SLAB. The resulting patterns were analyzed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Both methods indicate that the intense laser beam promotes the exchange of thiols at the copper surface. Furthermore, we present a procedure for the production of free-standing copper nanomembranes, oxidation-protected by alkanethiol SAMs. Incubation of copper-coated mica in alkanethiol solutions leads to SAM formation on both surfaces of the copper film due to intercalation of the organic molecules. Corrosion-protected copper nanomembranes were floated onto water, transferred to electron microscopy grids, and subsequently analyzed by electron energy loss spectroscopy (EELS).