Orientation Of Self-assembled Monolayers Research Articles

Tetrapodal Diazatriptycene Enforces Orthogonal Orientation in Self-Assembled Monolayers.

Conformationally rigid multipodal molecules should control the orientation and packing density of functional head groups upon self-assembly on solid supports. Common tripods frequently fail in this regard because of inhomogeneous bonding configuration and stochastic orientation. These issues are circumvented by a suitable tetrapodal diazatriptycene moiety, bearing four thiol-anchoring groups, as demonstrated in the present study. Such molecules form well-defined self-assembled monolayers (SAMs) on Au(111) substrates, whereby the tetrapodal scaffold enforces a nearly upright orientation of the terminal head group with respect to the substrate, with at least three of the four anchoring groups providing thiolate-like covalent attachment to the surface. Functionalization by condensation chemistry allows a large variety of functional head groups to be introduced to the tetrapod, paving the path toward advanced surface engineering and sensor fabrication.

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.

Fourier-transform infrared and photoluminescence spectroscopies of self-assembled monolayers of long-chain thiols on (001) GaAs

Self-assembled monolayers (SAMs) of various thiols have shown the potential to protect freshly fabricated or chemically cleaned GaAs surfaces from oxidization, adsorption of foreign atoms, and∕or surface defect formation. We have employed an attenuated total reflection Fourier-transform infrared spectroscopic technique to investigate the process of formation of long-chain thiols, comprising ten or more methylene chains, on the surface of (001) GaAs. A strong infrared (IR) signal was measured for all the investigated GaAs-thiol interfaces. Varying the type of terminal groups, from hydrophilic to hydrophobic, significantly changes the IR intensity of the methylene stretching vibration, indicating different methylene chain orientation in SAMs. Consequently, these SAMs exhibited different passivation performance to the (001) GaAs surface as judged by the intensity of the GaAs-related photoluminescence signal.

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.

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.

Orientation modulation of a synthetic polypeptide in self-assembled monolayers: a TOF-SIMS study.

Structure and orientation of molecules are key properties of functionalized surfaces. Using time-of-flight secondary ion mass spectrometry (TOF-SIMS), here we investigate how to modulate these parameters upon the immobilization process varying the conditions of self-assembly. The molecule of interest, a template-assembled synthetic protein (TASP), consists of a central peptide ring with orthogonally arranged residues. Thioalkane chains allow the directed self-assembly of the molecule on a gold surface; four serine residues on the opposite side of the ring can be used as anchoring sites for various functional sensing molecules. The TASP conformation and its orientation in self-assembled monolayers (SAMs) play a central role for the accessibility of these serine residues. To study the influence of the self-assembly conditions, two series of samples were prepared. Pure TASP monolayers of different surface densities are compared to mixed TASP/alkanethiol monolayers prepared by sequential adsorption varying sequence and particular incubation times as well as by coadsorption modifying incubation times and TASP/alkanethiol mass ratios. Switching the TASP orientation from a state where the molecules are lying flat on the surface to an upright orientation turned out to be possible by inserting the TASP into a preformed alkanethiol monolayer of an appropriate surface density. This study demonstrates that TOF-SIMS is an excellent tool not only to investigate the surface composition, but also the molecular structure of functionalized surfaces.

Bonding and Orientation in Self-Assembled Monolayers of Oligophenyldithiols on Au Substrates

The structure and orientation within prototype self-assembled monolayers (SAMs) containing aromatic subunits absorbed on Au substrates has been investigated using infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning tunneling microscopy (STM). Several oligophenyl(di)thiols, namely biphenylthiol (HS−C6H4−C6H5, BPT), biphenyldithiol (HS−C6H4−C6H4−SH, BPDT), biphenyldimethyldithiol (HS−CH2−C6H4−C6H4−CH2−SH, BPDMT), and terphenyldimethyldithiol (HS−CH2−C6H4−C6H4−C6H4−CH2−SH, TPDMT) have been studied. At least one of these molecules, BPDT, has recently been discussed in connection with the fabrication of a simple electronic device using the aromatic oligophenylthiolate SAM as an active component. Our results reveal that organodithiols with a short oligophenyl backbone, that is, BPDT and BPDMT, do not form well oriented layers but demonstrate that longer backbones, for example, a terphenyl unit, do indeed lead to the formation of SAMs with a high degree of molecular orientation.

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...

Optical properties and molecular orientation of self-assembled monolayer using surface plasmon resonance spectroscopy

We report evidence of photoisomerization-induced changes in thin films of spiroxazine-alkanethiol self-assembled monolayers (SAM) by surface plasmon resonance (SPR). Geometry optimization by MO calculations reveals that the tilt angle of spiroxazine moiety is around 30°.

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.

Effect of the molecular interaction on molecular packing and orientation in azobenzene-functionalized self-assembled monolayers on gold

The effect of van der Waals interaction between alkyl chains on the molecular packing density and orientation in the self-assembled monolayers (SAMs) of a series of novel functionalized azobenzene on gold has been studied by reflectance infrared spectroscopy and contact angle measurement. The aromatic interaction between the azobenzene groups and hydrogen bonding between the amide groups help to organize the molecules, forming a well-defined structure. Larger van der Waals interactions introduced by alkyl chains with longer length at the front of the azobenzene group lead to a higher packing density and a larger tilting angle of the molecules.

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.