Molecular Orientation In Self-assembled Monolayers Research Articles

Comparison of a Fluorinated Aryl Thiol Self-Assembled Monolayer with Its Hydrogenated Counterpart on Polycrystalline Ag Substrates

The effects of perfluorination of aryl thiols on surface coverage, surface electronic properties, and molecular orientation of self-assembled monolayers of thiophenol (TP) and pentafluorothiophenol (F(5)TP) on polycrystalline Ag were evaluated using linear sweep voltammetry, ultraviolet photoelectron spectroscopy (UPS), and surface Raman spectroscopy, respectively. Electrochemical reductive desorption by linear sweep voltammetry indicates a surface coverage for the TP monolayer of (5.07 +/- 1.29) x 10(-10) mol/cm(2), equating to a molecular area of 32.8 +/- 8.3 A(2), and a surface coverage for the F(5)TP monolayer of (1.95 +/- 0.59) x 10(-10) mol/cm(2), equating to an area of 85.2 +/- 25.8 A(2)/molecule. TP-modified Ag exhibits a change in work function (DeltaPhi) of -0.64 eV relative to bare Ag, whereas F(5)TP-modified Ag exhibits a DeltaPhi of +0.54 eV relative to bare Ag. Quantitative analysis of the UPS and reductive desorption results yields molecular pictures of the proposed interfaces with TP molecules tilted <20 degrees from the surface normal in a herringbone pattern spaced 6.4 A apart and F(5)TP molecules in a more disordered arrangement tilted 67 degrees from the surface normal with an intermolecular distance of 10.4 A. Qualitative surface Raman spectroscopic analysis of in-plane and out-of-plane modes for these systems confirms that TP molecules are oriented more vertical than F(5)TP molecules in these monolayers.

Comparative Study of Tetrahydrothiophene and Thiophene Self Assembled Monolayers on Au(111): Structure and Molecular Orientation

Surface structure and molecular orientation of self-assembled monolayers (SAMs) formed by the spontaneous adsorption of tetrahydrothiophene (THT) and thiophene (TP) on Au(111) were investigated by means of scanning tunneling microscopy (STM) and carbon K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. STM imaging revealed that THT SAMs have a commensurate (3 <TEX>${\times}\;2\sqrt[]{3}$</TEX>) structure containing structural defects in ordered domains, whereas TP SAMs are composed of randomly adsorbed domains and paired molecular row domains that can be described as an incommensurate packing structure. The NEXAFS spectroscopy study showed that the average tilt angle of the aliphatic THT ring and <TEX>$\pi$</TEX>-conjugated TP ring in the SAMs were calculated to be about <TEX>$30^o\;and\;40^o$</TEX>, respectively, from the surface normal. It was also observed that the <TEX>$\pi$</TEX>* transition peak in the NEXAFS spectrum of the TP SAMs is very weak, suggesting that a strong interaction between <TEX>$\pi$</TEX>-electrons and the Au surface arises during the self-assembly of TP molecules. In this study, we have clearly demonstrated that the surface structure and adsorption orientation of organic SAMs on Au(111) are strongly influenced by whether the cyclic ring is saturated or unsaturated.

Determining orientational structure of diamondoid thiols attached to silver using near-edge X-ray absorption fine structure spectroscopy

Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) is a powerful tool for determination of molecular orientation in self-assembled monolayers and other surface-attached molecules. A general framework for using NEXAFS to simultaneously determine molecular tilt and twist of rigid molecules attached to surfaces is presented. This framework is applied to self-assembled monolayers of higher diamondoids, hydrocarbon molecules with cubic-diamond-cage structures. Diamondoid monolayers chemisorbed on metal substrates are known to exhibit interesting electronic and surface properties. This work compares molecular orientation in monolayers prepared on silver substrates using two different thiol positional isomers of [121]tetramantane, and thiols derived from two different pentamantane structural isomers, [1212]pentamantane and [1(2,3)4]pentamantane. The observed differences in monolayer structure demonstrate the utility and limitations of NEXAFS spectroscopy and the framework. The results also demonstrate the ability to control diamondoid assembly, in particular the molecular orientational structure, providing a flexible platform for the modification of surface properties with this exciting new class of nanodiamond materials.

Molecule orientation in self-assembled monolayers determined by infrared-visible sum-frequency generation spectroscopy

The molecular orientation in self-assembled monolayers of thiophenol (TP) on Ag(1 1 1) and of para-nitroanilino-dodecane-thiol (NAT) on Au was investigated by infrared-visible sum-frequency generation between 500 and 1500 cm−1. Ab initio calculations were performed to determine the interface nonlinear response. The combined experimental and theoretical results reveal that the S–C bond of TP is tilted 37∘ from the Ag(1 1 1) surface normal, with the aromatic ring plane perpendicular to the surface. On Au, the tilt angle of the NAT molecule stabilizes below 60∘ at the end of film growth.

Characterization of the Molecular Orientation of Self-Assembled Monolayers of Alkanethiols on Obliquely Deposited Gold Films by Using Infrared−Visible Sum-Frequency Spectroscopy

This paper reports the use of infrared (IR)−visible sum-frequency spectroscopy to characterize the surface of gold films deposited by physical vapor deposition at an oblique angle of incidence and the orientation of the methyl end groups of self-assembled monolayers (SAMs) of n-alkanethiols formed on these gold films. The oblique deposition of the gold breaks the macroscopic azimuthal degeneracy that is characteristic of gold films grown without a preferred direction of deposition. By tuning the frequency of the incident IR light to either the symmetric or the asymmetric methyl stretch resonance and by recording the azimuthal dependence of the sum-frequency signal, the orientation of the methyl groups of the n-alkanethiols on the obliquely deposited gold films were shown to be distributed around a preferred in-plane direction coincident with the direction of the gold deposition. The angles between the normal of the sample and the methyl groups were determined to be 85 ± 2° and 44 ± 10° for the SAMs formed from pentadecanethiol and hexadecanethiol, respectively. In contrast, when using gold films that were not obliquely deposited, no preferred in-plane orientation of the methyl groups could be observed.

Orientation of o-, m-, and p-Methylbenzylmercaptans Adsorbed on Au(111) Probed by Broad-Bandwidth Sum Frequency Generation Spectroscopy

The molecular orientation in self-assembled monolayers of ortho, meta, and para isomers of methylbenzylmercaptans (OMBM, MMBM, and PMBM, respectively) on Au(111) has been studied using broad-bandwidth sum frequency generation spectroscopy and cyclic voltammetry. Cyclic voltammograms for the reductive desorption suggest that all methylbenzylmercaptans form closely packed monolayers. Sum frequency vibrational spectra for OMBM, MMBM, and PMBM show the CH3 symmetric stretching vibration mode. The vibrational intensities are greatest in PMBM and smallest in OMBM, implying the difference in the orientation of the methyl group. The vibrational intensities are compared with the hyperpolarizabilities of methylbenzylmercaptans from a calculation based on the density functional theory. OMBM molecules adsorb on Au(111) tilting their benzyl moieties so that the direction of the methyl group is parallel with the surface, whereas MMBM and PMBM molecules adsorb on Au(111) with their benzyl moieties being perpendicular to the surface.

Growth kinetics of self-assembled monolayers of thiophene and terthiophene on Au(111): An infrared spectroscopic study

The growth kinetics of self-assembled monolayers (SAMs) of thiophene compounds on Au(111) surfaces was revealed by Fourier-transform infrared reflection absorption spectroscopy (FT-IR-RAS). Thiophene and terthiophene form well-ordered SAMs on Au(111) surfaces by immersing gold substrates into their ethanol solutions for ca. 15 h. Gibbs free energies for the adsorption processes of thiophene and terthiophene were found to be identical. However, the growth and molecular orientation of SAMs are different between two thiophene compounds. Terthiophene in SAMs orients parallel to the surface. The SAM growth of terthiophene obeys a time-dependent Langmuir scheme. On the other hand, the thiophene SAM undergoes a two-step growth process with unique molecular orientations. In the primary phase, thiophene assumes a parallel orientation on the Au(111) surface. In the second phase, thiophene is oriented close to the normal of the surface. The different growth process between thiophene and terthiophene is attributable to the topology of sulfur positions in the molecules.

Determination of Molecular Orientation in Self-Assembled Monolayers Using IR Absorption Intensities: The Importance of Grinding Effects

The orientation of organic chain molecules grafted to a metal surface is commonly determined using infrared reflection absorption spectroscopy (IRRAS). Whereas the acquisition of the IR spectrum itself is rather straightforward, the determination of orientational parameters (tilt angle, twist angle) is complicated by the requirement to use the bulk reference spectrum of the substance taken to form the self-assembled monolayer (SAM). In the past, mainly two methods have been used to extract orientational information from IR data. The application of the absolute method requires the precise knowledge of the complex refractive index, but the intensities of the IR bands are influenced by Mie scattering of light in the pellet which depends on the particle size and shape and therefore on the grinding time and conditions. On the other hand, the application of the method using relative intensities requires a second IR-active vibration with a differently oriented transition dipole moment (TDM) to eliminate the rela...

Comment on “Molecular orientation determined by second-harmonic generation: Self-assembled monolayers”

A recent paper by Eisert et al. [Phys. Rev. B 58, 10 860 (1998)] describes a second-harmonic generation (SHG) technique for probing molecular orientation in self-assembled monolayers (SAMs). Their results have raised some questions about the frequently used description of the optical properties of thin films. In their paper, the SAM film is considered anisotropic for SHG, but is assumed to be isotropic in terms of its linear optical response. It is likely that these different treatments of the same system have contributed to the unusual perspectives developed in the above paper. The purpose of this communication is to attempt a clarification of these observations. Here, a phenomenological model is outlined that treats linear and nonlinear optics of the interface in a single framework and can be used to analyze SHG from SAMs. It is also shown that, depending on the experimental system, conventional SHG measurements of phase and intensity may not be enough to determine the orientation of SAMs.

Structures of Self-Assembled Monolayers versus Vacuum Deposited Films of Terthiophene

Using X-ray diffraction and Fourier-transform infrared (FT-IR) spectroscopy, we elucidated the molecular orientation in self-assembled monolayers (SAM) and vacuum deposited films of terthiophene. Terthiophene in the SAM aligns parallel to the gold substrate. On the other hand, in the vacuum deposited films, the long axis of terthiophene aligns along the normal to the surface. Although our results contradict the quantum calculations, which predict that thiophene would not interact with gold significantly, we experimentally demonstrated the interaction between terthiophene and gold. A theoretical fitting of FT-IR signals, the out-of-plane –CH and –CH=CH– vibration at 680 cm-1, revealed the SAM formation of terthiophene to be a Langmuir type of adsorption.

Adsorption of amide‐containing alkanethiols on gold

AbstractMonolayers of a new series of amide‐containing alkanethiol compounds, CH3(CH2)n‐2C(O)NHC2H4SH (referred as CnC2SH, n=4, 5, 5, 8), were self‐assembled from ethanol solutions onto gold and their structures have been characterized using the reflectance absorption infrared spectroscopy (RA‐FTIR) and wettability measurement. It has been proved that the CO and N‐H bonds are parallel to the gold surface and the orientations of alkyl chain in these Self‐Assembled Monolayers (SAMs) are perpendicular to the surface. The special orientation of the amide moiety and molecular axis are suggested to be attributed to the hydrogen‐bonding formed between the amide moieties of neighbor molecules. Meanwhile, wettability measurement reveals that the packing degree of the SAMs depends on the length of the alkyl chains in the thiol molecules. These results demonstrate that both the hydrogen‐bonding and the length of the alkyl chain exerts the tremendous influnece on the orientation of amide‐containing alkanethiol molecules. And also, it give us a way to control the molecular orientation in SAMs in molecular level.

Characterization of self-assembled monolayer of thiophenol on gold by Penning ionization electron spectroscopy

The molecular orientation in self-assembled monolayers (SAMs) of thiophenol chemisorbed on polycrystalline gold from aqueous solutions has been investigated by Penning ionization electron spectroscopy (PIES) and ultraviolet photoelectron spectroscopy (UPS). The analysis of the relative band intensity of the Penning spectrum indicates that the phenyl ring in phenyl thiolate stands perpendicular to the substrate plane at room temperature. Upon heating, the phenyl ring becomes tilted in the layer.