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.     

Primary ion fluence dependence in time-of-flight SIMS of self-assembled monolayer of alkyl thiol molecules on Au(1 1 1)—Discussion of static limit

The nondestructive nature of static secondary ion mass spectrometry (SIMS) in the context of studies of self-assembled monolayers (SAMs) of organic molecules has been examined by measuring the primary ion fluence dependence of secondary ion signals with two well-known SAMs, C₁₈H₃₇SH on Au(1 1 1) and C₁₈H₃₇PO₃H₂ on freshly cleaved mica. This SIMS analysis is challenging because the bonding nature is delicate and the areal molecular density is less than 10¹⁵ cm⁻². In SIMS, it is prevalently assumed that if the primary ion fluence is confined to not more than 1 × 10¹² cm⁻², all secondary ion signals should not change by more than 10% and the practically defined static condition is satisfied. Our results from time-of-flight SIMS with the common primary ions of Bi₃⁺, Bi⁺ and Ar⁺, indicate that this prevalent static assumption fails for both model SAMs. The SIMS results from the phosphyl case, which have been recently published, consistently display the evidence of bombardment-induced damage. In comparison, the thiol case presented here shows much more complex primary ion fluence dependence of SIMS signals. It is therefore concluded that practical static analysis should use primary ion fluence not more than 1 × 10¹¹ cm⁻² or should simply record and report the effects of primary ion fluence.     

RA-IR Studies on the Structure of Mercapto-Ended Azobenzene Derivatives in Self-Assembled Monolayers

Self-Assembled Monolayers (SAMs) were prepared from mercapto-ended azobenzene derivatives with the structure of n-C_nH_2n+1, AzoO(CH₂)_mSH (n=4,6,8,10,12; m=3,5). the structure of these SAMs was thoroughly studied with grazing-angle incident reflection absorption FT1R technique and wettability measurement. the results suggested that the plane of Azobenzene system of the assembling molecules in the SAMs lies on its back with an approximate angle of 22° included between the substrate surface normal and the Azobenzene plane. Tail alkyl groups (n-C_nH_2n+1) in these assembling molecules were considered to be in an all-trans conformation, as if they were in a crystalline-like environment. and the C-C-C plane of these all-trans tail alkyl groups, while n≤8, lies also on its back with an angle about 70° between its plane and the substrate surface normal. the conformation of these head groups (-O(CH₂)_mSH) in SAMs are disturbed by many structural factors. While m=3 or 5, the head group chain was proposed to remain in a gauche conformation to fit the upright orientation of the Azobenzene plane. the packing density was investigated by measuring the contact angle of water on these SAMs. the results showed that the self-assembled monolayer films are perfectly packed and the coverage density might be improved with increasing the length of both tail and head alkyl chains.     

Selective resonance population in high resolution electron energy loss spectroscopy of 4-ethylbenzenethiol self-assembled monolayers on Au(1 1 1)

We have investigated the vibrational frequencies and excitation cross-section of self-assembled monolayers (SAMs) of 4-ethylbenzenethiol (CH₃CH₂(C₆H₄)SH) on Au(1 1 1) by high resolution electron energy loss spectroscopy (HREELS). Negative ion resonances were observed in the energy loss intensities as a function of the incident electron energy. Analysis of the C–H stretching modes indicates resonances of different energies are localised in both the ethyl and phenyl functional groups of the SAM molecules, which regulate the observed vibrational lineshape.     

An atomic force microscope study of vanadium-benzene sandwich clusters soft-landed on self-assembled monolayers

Multiple-decker vanadium-benzene sandwich clusters V_n(benzene)_n+1 produced by a laser-vaporization synthesis method were soft-landed onto self-assembled monolayers of alkanethiol (C₁₈H-SAM) and fluorinated alkanethiol (C₁₀F-SAM) at 200 K. Noncontact atomic force microscopy has been used to examine the resulting adsorption states of the clusters landed on the SAMs at room temperature. For each SAM substrate, the aggregates of the deposited clusters were observed at the vacancy islands and near the steps of the SAM surface. The result indicates that, at room temperature, the clusters landed on the SAM substrate thermally diffuse on the surface to form columnar-shape islands around the defect sites of the SAM surface.     

Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold

Self-assembled monolayers (SAMs) of 4-trifluoromethyl-azobenzene-4′-methyleneoxy-alkanethiols (CF₃– C₆H₄–N=N–C₆H₄–O–(CH₂) _n –SH on (111)-oriented poly-crystalline gold films on mica were examined by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The spectra are analyzed with the help of density-functional-theory calculations of the isolated molecule. Only one doublet is detected in the sulphur 2p spectra of the investigated SAMs, consistent with a thiolate bond of the molecule to the gold surface. The C 1s XP spectra and the corresponding XAS π ^* resonance exhibit a rich structure which is assigned to the carbon atoms in the different chemical surroundings. Comparing XPS binding energies of the azobenzene moiety and calculated initial-state shifts reveals comparable screening of all C 1s core holes. While the carbon 1s XPS binding energy lies below the π ^*-resonance excitation-energy, the reversed order is found comparing core ionization and neutral core excitation of the nitrogen 1s core-hole of the azo group. This surprising difference in core-hole binding energies is interpreted as site-dependent polarization screening and charge transfer among the densely packed aromatic moieties. We propose that a quenching of the optical excitation within the molecular layer is thus one major reason for the low trans to cis photo-isomerization rate of azobenzene in aromatic-aliphatic SAMs.     

Selective MOCVD of titanium oxide and zirconium oxide thin films using single molecular precursors on Si(1 0 0) substrates

Titanium oxide (TiO₂) and zirconium oxide (ZrO₂) thin films have been deposited on modified Si(1 0 0) substrates selectively by metal-organic chemical vapor deposition (MOCVD) method using new single molecular precursor of [M(OⁱPr)₂(tbaoac)₂] (M=Ti, Zr; tbaoac=tertiarybutyl-acetoacetate). For changing the characteristic of the Si(1 0 0) surface, micro-contact printing (μCP) method was adapted to make self-assembled monolayers (SAMs) using an octadecyltrichlorosilane (OTS) organic molecule which has -CH₃ terminal group. The single molecular precursors were prepared using metal (Ti, Zr) isopropoxide and tert-butylacetoacetate (tbaoacH) by modifying standard synthetic procedures. Selective depositions of TiO₂ and ZrO₂ were achieved in a home-built horizontal MOCVD reactor in the temperature range of 300-500 °C and deposition pressure of 1×10⁻³-3×10⁻² Torr. N₂ gas (5 sccm) was used as a carrier gas during film depositions. TiO₂ and ZrO₂ thin films were able to deposit on the hydrophilic area selectively. The difference in surface characteristics (hydrophobic/hydrophilic) between the OTS SAMs area and the SiO₂ or Si-OH layer on the Si(1 0 0) substrate led to the site-selectivity of oxide thin film growth.     

Nanoporous structures on ZnO thin films

Porous structures were formed on ZnO thin films which were grown by an electrochemical deposition (ECD) method. The growth processes were carried out in a solution of dimethylsulfoxide (DMSO) zinc perchlorate, Zn(ClO₄)₂, at 120 ^°C on indium tin oxide (ITO) substrates. Optical and structural characterizations of electrochemically grown ZnO thin films have shown that the films possess high (0002) c$c$-axis orientation, high nucleation, high intensity and low FWHM of UV emission at the band edge region and a sharp UV absorption edge. Nanoporous structures were formed via self-assembled monolayers (SAMs) of hexanethiol (C₆SH) and dodecanethiol (C₁₂SH). Scanning electron microscope (SEM) measurements showed that while a nanoporous structure (pore radius 20 nm) is formed on the ZnO thin films by hexanathiol solution, a macroporous structure (pore radius 360 nm) is formed by dodecanethiol solution. No significant variation is observed in X-ray diffraction (XRD) measurements on the ZnO thin films after pore formation. However, photoluminescence (PL) measurements showed that green emission is observed as the dominant emission for the macroporous structures, while no variation is observed for the thin film nanoporous ZnO sample.     

Adsorption of purpald SAMs on silver and gold electrodes: a Raman mapping study

The adsorption modes of 4‐amino‐3‐hydrazino‐5‐mercapto‐1,2,4‐trizole (purpald) self‐assembled monolayers (SAMs) formed on SERS‐active silver and gold electrodes were comparatively studied using surface‐enhanced Raman scattering (SERS), and the self‐assembling procedures were investigated by the Raman mapping technique. Purpald SAMs adopted a titled orientation with S, N2 atoms anchoring to the silver electrode and the  N7H₂ close to the surface, whereas purpald stood up on the gold electrode through S, N5 atoms and with  N8H₂ adjacent to the surface. The density functional theory (DFT) at the level of B3LYP was performed to help explain their different adsorption behaviors on the silver and gold electrodes. Copyright © 2006 John Wiley & Sons, Ltd.     

Investigation on cell biocompatible behaviors of polyaniline film fabricated via electroless surface polymerization

Considering for the potential application in tissue engineering, polyaniline (PANi) film was fabricated via a two-step route: a self-assembled monolayer of C₆H₅NHC₃H₆Si(OMe)₃ was firstly formed on the single-crystal Si substrate; the conducting PANi film was then prepared through electroless surface polymerization of the aniline molecules on the aniline monolayer-bearing silane surface in an acidic aqueous solution. The formation of PANi film on Si surface was confirmed by characterizations of X-ray photoelectron spectroscope (XPS) and specular reflectance Fourier transform infrared (SR-FTIR) spectrum, etc. At last, the proliferation behaviors of PC-12 cells on the PANi film surface were studied by the [3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyl tetrazolium bromide] (MTT) colorimetric assays, acridine orange fluorometric staining, and scanning electron microscope (SEM) observation, etc. The results demonstrate that the as-prepared PANi film provides high ability for cell proliferation, exhibiting promising potentials as surface coating to cultivate neuronal cells for applications in the tissue engineering.     

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.     

Reorientation Response of Magnetic Microspheres Attached to Gold Electrodes Under an Applied Magnetic Field

In this work, we report the mechanical reorientation of thiolated ferromagnetic microspheres bridging a pair of gold electrodes under an external magnetic field. When an external magnetic field (7 kG) is applied during the measurement of the current–voltage characteristics of a carboxyl ferromagnetic microsphere (4 μm diameter) attached to two gold electrodes by self-assembled monolayers (SAMs) of octane dithiol (C₈H₁₈S₂), the current signal is distorted. Rather than due to magnetoresistance, this effect is caused by a mechanical reorientation of the ferromagnetic sphere, which alters the number of SAMs between the sphere and the electrodes and therefore affects conduction. To study the physical reorientation of the ferromagnetic particles, we measure their hysteresis loops while suspended in a liquid solution.     

Self-assembled monolayers on magnesium alloy surfaces from carboxylate ions

Self-assembled monolayers (SAMs) of carboxylate salts (C_n−1H_2n−1COONa, n = 12, 16, 18) were successfully formed on Mg alloy substrate in ethanol solution and characterized by the contact angle measurement, ATR-FTIR, ellipsometry and XPS. The SAMs were regularly and densely anchored to the Mg alloy substrate via monodentate bonding with the tilting angles of about 40°, 33° and 27° for C12, C16 and C18 monolayers, respectively. Electrochemical impedance spectroscopy (EIS) proved the corrosion protection of SAMs for Mg alloy substrate with protective efficiency (PE) of even up to 98.5%.     

Surface XPS characterization of NiTi shape memory alloy after advanced oxidation processes in UV/H2O2 photocatalytic system

Surface structure of NiTi shape memory alloy (SMA) was modified by advanced oxidation processes (AOP) in an ultraviolet (UV)/H₂O₂ photocatalytic system, and then systematically characterized with x-ray photoelectron spectroscopy (XPS). It is found that the AOP in UV/H₂O₂ photocatalytic system leads to formation of titanium oxides film on NiTi substrate. Depth profiles of O, Ni and Ti show such a film possesses a graded interface structure to NiTi substrate and there is no intermediate Ni-rich layer like that produced in conventional high temperature oxidation. Except TiO₂ phase, some titanium suboxides (TiO, Ti₂O₃) may also exist in the titanium oxides film. Oxygen mainly presents in metal oxides and some chemisorbed water and OH⁻ are found in titanium oxides film. Ni nearly reaches zero on the upper surface and relatively depleted in the whole titanium oxides film. The work indicates the AOP in UV/H₂O₂ photocatalytic system is a promising way to favor the widespread application of biomedical NiTi SMA by improving its biocompatibility.     

机械-化学方法在硅(100)表面制备芳香烃重氮盐单层膜

利用机械-化学方法同时实现硅表面的图形化和功能化. 在芳香烃重氮盐(C₆H₅N₂BF₄)中用金刚石刀具刻划单晶硅(100),使单晶硅表面的Si-O键断裂,形成硅的自由基,进而它们与溶液中含有的有机分子共价结合以形成自组装单层膜. 用原子力显微镜对自组装前后的表面形貌进行表征,用飞行时间二次离子质谱和红外光谱对自组装单层膜进行检测和分析,通过确认C₆H₅离子的存在证明自组装单层     

J-aggregates of pseudoisocyanine with long alkyl substituents in thin films: Synthesis, spectral properties, and thermal stability

The formation and properties of J-aggregates in thin solid films of pseudoisocyanines with long N-alkyl groups, obtained by centrifuging from solutions in organic solvents, were studied. It is shown for the first time that nonsymmetric cyanine dyes, containing a C₂H₅ group at one nitrogen atom and a C₁₀H₂₁, C₁₅H₃₁, or C₁₈H₃₇ group at another nitrogen atom, spontaneously form J-aggregates stable at room temperature and pressing a narrow absorption band with a half-width at half maximum of 200 cm^?1. The thermal stability of J-aggregates in thin films of pseudoisocyanines with alkyl substituents decreases in the following order: C₂H₅-C₂H₅> C₂H₅-C₆H₁₃>C₂H₅-C₁₈H₃₇>C₂H₅-C₁₀H₂₁>C₂H₅-C₁₅H₃₁. By introducing 1-octadecyl-2-methylquinolinium iodide in the film, it was found that the J-aggregates studied consist of a small number (2–4) of dye molecules.     

On the interaction of self-assembled C<Subscript>60</Subscript>F<Subscript>18</Subscript> polar molecules with the Ni(100) surface

The current work is dedicated to investigation of the interaction between self-assembled polar molecules of fullerene fluoride C₆₀F₁₈ with the chemically active surface Ni(100) under radiation and heat treatments. X-ray photoelectron spectroscopy is used in combination with quantum-chemical simulation. For the first time, the transformation of an as-deposited dielectric continuous 2D thin film to a 3D island-type assembly with molecular ordering within the islands is shown to take place. The degree of coverage of the Ni surface by C₆₀F₁₈ islands (0.6–0.7) and their height (~6 nm) are estimated. Quantum-chemical simulation shows that the chemisorption energy of the C₆₀F₁₈ molecule on the Ni surface equals ~6.6 eV and fluorine atoms are located at a distance of 1.9 Å above the Ni surface. The results of the investigation provide an opportunity to create nanoscale ordered structures with local changes in the work function.     

Surface modification of gold nanoparticles and their monolayer behavior at the air/water interface

Gold nanoparticles were prepared by two different methods. The first method was chemically grafting the particles with different lengths of alkylthiol (C₆SH, C₁₂SH and C₁₈SH). For the second method, the Au particles were surface modified first by mercaptosuccinic acid (MSA) to render a surface with carboxylic acid groups which play a role to physically adsorb cationic surfactant in chloroform. This method was termed physical/chemical method. In the first method, the effects of alkyl chain length and dispersion solvent on the monolayer behavior of surface modified gold nanoparticles was evaluated. The gold nanoparticles prepared by 1-hexanthiol demonstrated the narrowest size distribution. Most of them showed narrower particle size distributions in chloroform than in hexane. For the physical/chemical method, the particles can spread more uniformly on the water surface which is attributed to the amphiphilic character of the particles at the air/water interface. However, the particles cannot pack closely due to the relatively weak particle-particle interaction. The effect of alkyl chain length was also assessed for the second method.     

Optical properties of natural quantum-well compounds (C6H5-CnH2n-NH3)2PbBr4 (n=1-4)

This paper describes the synthesis and characterization of self-assembled organic-inorganic layered perovskite compounds, (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ (n=1-4). the effect of the number of carbon atoms of the alkyl chain length (n) on optical properties has been studied. (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ films fabricated by spin-coating are microcrystalline form, single phase and oriented with the c-axis. Crystallinity, the maximum PL intensity and the lifetime of exciton emissions varied with the number of carbon atoms. the lowest-energy exciton splits into a few fine-structure levels at low temperatures. Time-resolved photoluminescence spectra reveal that (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ shows both singlet and triplet excitons. with decreasing temperature, triplet exciton emissions become dominant for (C₆H₅-C_nH_2n-NH₃)₂PbBr₄ (n=1-3), while (C₆H₅-C₄H₈-NH₃)₂PbBr₄ shows mainly singlet exciton emissions. The intersystem crossing from excited singlet state to triplet state plays an important role in the relaxation process of excitons.     

Optical surface absorption by surface-adsorbate charge-transfer excitations

Metal-insulator-silver junctions are well suited to measure genuine surface optical excitations, especially electronic transitions to the lowest unoccupied molecular orbitals (LUMO) of adsorbates on silver. This is demonstrated by the increase of internal photoemission current after monolayer coverage with C₂H₄ (which has π^*-LUMO) by a factor of about 2.5, but a missing change in the case of C₂H₆ (which has no π^*-LUMO ). Received: 26 November 1999 / Published online: 8 March 2000