Self-assembled Film of Cholesterol Molecules on the Au (111): An STM Study (original) (raw)
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Langmuir, 2007
We report a study of acid-terminated self-assembled monolayers of alkanethiols of different length, 11mercaptoundecanoic acid (11-MUA) and 16-mercaptohexadecanoic acid (16-MHDA), on Au(111). Scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and contact angle techniques were used for characterization, and the results were compared with those obtained from n-alkanethiols of similar chain length, providing a detailed description of the two-dimensional crystalline structure. Molecular resolution STM images show that 11-MUA forms a dense-packed monolayer arranged in a ( 3 × 3)R30°structure with a c(2 × 4) superlattice, where the simple hexagonal phase, the c(2 × 4) superlattice, and nonordered areas coexist. 16-MHDA assembles in a uniform monolayer with similar morphology to that of 11-MUA, but molecular resolution could not be reached in STM due to both the hydrophilicity of the acid groups and the poor conductivity of the thick monolayer. Nevertheless, the monolayer thicknesses estimated by XPS and electrochemistry and the highly blocking character of the film observed by electrochemistry as well as the low water contact angle are consistent with 16-MHDA molecules forming a compact monolayer on the Au(111) substrate with fully extended alkyl chains and acid groups pointing away from the surface. The results obtained for 16-MHDA were reproducible under different preparation conditions such as the addition or omission of acetic acid to the ethanolic solution. Contrary to other reports, we demonstrate that ordered acid-terminated self-assembled monolayers are obtained with the same preparation conditions as those of the methyl-terminated ones, without any additional treatment.
Structure of dioctadecyl l-glutamide-derived lipid self-assembled monolayers on Au(111) surface
Applied Surface Science, 2006
An L-glutamic acid-derived lipid with a terminal thiol has been synthesized and its corresponding self-assembled structure on Au(1 1 1) surfaces described. The surface morphology of the lipid self-assembled monolayer (SAM) exhibits nano-order patterning, where the height of the monolayer (approximately 1.5 nm) could be interpreted as due to the monolayer structure estimated from molecular models. The molecular orientation in the monolayer is almost perpendicular to the Au(1 1 1) surface depending on the three-point hydrogen-bonding sites in the molecule. #
Nanotechnology
Self-Assembled Monolayers (SAM) on Au(111) are able to control the functionality of a gold surface. We use scanning tunnelling microscopy (STM) in air and contact angle measurements to compare the morphology and the chemistry of three alkylthiol SAMs differing by their tail groups: 1,9-nonanedithiol (NDT) and 1,4-butanedithiol (BDT) and 11mercaptoundecanol (MUOH). STM reveals very different morphologies: hexagonal lattice for MUOH and parallel rows for NDT and BDT. In the case of NDT, we find that the thiol tail groups may form disulfide bridges for long immersion times. The availability of-SH group for chemical reactions is checked by attaching gold nanoparticles (AuNPs). When the thiol tail group is available, AuNPs readily attach as shown with atomic force microscopy (AFM). When disulfide bridges are formed the gold surface is not able to bind nanoparticles.
Thiocholesterol on gold: A nanoporous molecular assembly
Langmuir, 1996
The formation of thiocholesterol (TC) monolayers on gold has been studied by ellipsometry, contact angle measurements, infrared spectroscopy, and cyclic voltammetry. Subsequent treatment of the TC assembly with 11-mercaptodeuterioundecanoic acid (MDUA) shows that the average surface coverage is about 65% of that of a self-assembled alkanethiolate monolayer and that it has a large number of molecular defects. These defects exist because of a mismatch between the size and shape of the TC molecule and the pinning distance at the Au(111) crystal lattice. Potential uses of these defect-rich structures are microelectrode arrays for electroanalytical and biosensor applications.
Self-assembled monolayers of alkanethiols on Au(111): surface structures, defects and dynamics
Physical Chemistry Chemical Physics, 2005
The surface structures, defects and dynamics of self-assembled monolayers (SAMs) on Au(111) are reviewed. In the case of the well-known c(4 Â 2) and O3 Â O3 R301 surface structures, the present discussion is centered on the determination of the adsorption sites. A more complex scenario emerges for the striped phases, where a variety of surface structures that depends on surface coverage are described. Recently reported surface structures at non-saturation coverage show the richness of the self-assembly process. The study of surface dynamics sheds light on the relative stability of some of these surface structures. Typical defects at the alkanethiol monolayer are shown and discussed in relation to SAMs applications.
Langmuir, 2008
The solution self-assembly of R,ω-alkanedithiols onto Au(111) was investigated using atomic force microscopy (AFM). A heterogeneous surface morphology is apparent for 1,8-octanedithiol and for 1,9-nonanedithiol self-assembled monolayers (SAMs) prepared by solution immersion as compared to methyl-terminated n-alkanethiols. Local views from AFM images reveal a layer of mixed molecular orientations for R,ω-alkanedithiols, which evidence surface structures with heights corresponding to both lying-down and standing-up orientations. For dithiol SAMs prepared by solution self-assembly, the majority of R,ω-alkanedithiol molecules chemisorb with both thiol end groups bound to the Au(111) surface with the backbone of the alkane chain aligned parallel to the surface. However, AFM images disclose that there are also islands of standing molecules scattered throughout the surface. To measure the thickness of R,ω-alkanedithiol SAMs with angstrom sensitivity, methyl-terminated n-alkanethiols with known dimensions were used as molecular rulers. Under conditions of spatially constrained self-assembly, nanopatterns of R,ω-alkanedithiols written by nanografting formed monolayers with heights corresponding to an upright configuration.
Surface structure and interface dynamics of alkanethiol self-assembled monolayers on Au(111)
The journal of physical chemistry. B, 2006
Scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS) were used to examine the structural transitions and interface dynamics of octanethiol (OT) self-assembled monolayers (SAMs) caused by long-term storage or annealing at an elevated temperature. We found that the structural transitions of OT SAMs from the c(4 x 2) superlattice to the (6 x square root 3) superlattice resulting from long-term storage were caused by both the dynamic movement of the adsorbed sulfur atoms on several adsorption sites of the Au(111) surface and the change of molecular orientation in the ordered layer. Moreover, it was found that the chemical structure of the sulfur headgroups does not change from monomer to dimer by the temporal change of SAMs at room temperature. Contrary to the results of the long-term-stored SAMs, it was found that the annealing process did not modify either the interfacial or chemical structures of the sulfur headgroups or the two-dimension...
The use of organic thin layers as active component in materials science and electronic devices is presently considered a potential alternative to conventional semiconductor based nano scale electronics since it directly provides precise well-defined nano-scale components for electronic devices which eventually allows for simple processing and device fabrication. In the area of interface and surface science covalently bound Self-Assembled Monolayers (SAMs) became of significant importance. In particular aromatic based materials became of paramount interest, since they exhibit strong intermolecular interactions, chemical stability and charge transport properties across metal-organic interface.
Journal of Physical Chemistry B, 1998
Computational studies including geometry optimizations and molecular dynamics (MD) simulations are carried out for self-assembled monolayers of n-alkanethiols (RSH, R) C 16 H 33 , C 17 H 35) and 4′-alkoxybiphenyl-4thiols (ROC 12 H 8 SH, R) C 16 H 33 , C 17 H 35) on the (111) surface of gold with a full atomic representation force field. In this work, we combine the information derived from scanning tunneling microscopy (STM), surface reflection infrared spectra (IR), and computational studies to uncover the origins of different odd-even effects observed by IR for long chain n-alkanethiols and 4′-alkoxybiphenyl-4-thiols and then to establish a relationship between chemical structures of the headgroups and packing structures of thiols on Au(111). Computationally, the odd-even effect is monitored by the relative magnitude of z-components (the direction normal to the Au surface) of the methyl group. Although both n-alkanethiols and 4′-alkoxybiphenyl-4-thiols occupy the same spacing on Au(111), according to our simulation results, their favored packing structures are different. Because the headgroup of 4′-alkoxybiphenyl-4-thiol (-SC 12 H 8 O-) is more rigid than that of n-alkanethiol (-S-), 4′-alkoxybiphenyl-4-thiol prefers more structured packing arrangements and thus its odd-even effect is stronger than that of n-alkanethiol. In other words, the flexibility of the headgroup greatly influences the variety of possible packing structures. Finally, with this new relationship, we are able to rationalize the strong oddeven effect of a new SAM molecule, n-alkyldithioic acid, on Au(111).
Langmuir, 2010
A characterization of the 1,8-octanedithiol (ODT) self-assembled monolayer (SAM) formed from a Triton X-100 lyotropic medium has been conducted by electrochemical techniques. It is found that an ODT layer of standing-up molecules is obtained at short modification time without removing oxygen from the medium. The electrochemical study shows that the ODT layer formed after 15 min of modification time has similar electron-transfer blocking properties to the layers formed from organic solvents at much longer modification times. On the basis of XPS data, it is demonstrated that the inability to bind gold nanoparticles (AuNPs) is due to the presence of extra ODT molecules either interdigited or on top of the layer. Treatment consisting of an acid washing step following the formation of the ODT-Au(111) SAM produces a layer that is able to attach AuNPs as demonstrated by electrochemical techniques and atomic force microscopy (AFM) images.