Self-assembled Monolayer Surface Research Articles

Attoampere Nanoelectrochemistry: Attoampere Nanoelectrochemistry (Small 29/2021)

High frequency (GHz) signals are sent to a scanning tunneling microscope probe down to the surface of an alkylferrocene self-assembled monolayer in liquid and allow to measure locally electrochemical charge transfer corresponding to currents on the order of attoampere. This technique, presented in article number 2101253 by Nicolas Clément, Georg Gramse, and co-workers, opens the way to unprecedented precision for local measurements in biology, electrochemistry and materials science.

KAT Ligation for Rapid and Facile Covalent Attachment of Biomolecules to Surfaces.

The efficient and bioorthogonal chemical ligation reaction between potassium acyltrifluoroborates (KATs) and hydroxylamines (HAs) was used for the surface functionalization of a self-assembled monolayer (SAM) with biomolecules. An alkane thioether molecule with one terminal KAT group (S-KAT) was synthesized and adsorbed onto a gold surface, placing a KAT group on the top of the monolayer (KAT-SAM). As an initial test case, an aqueous solution of a hydroxylamine (HA) derivative of poly(ethylene glycol) (PEG) (HA-PEG) was added to this KAT-SAM at room temperature to perform the surface KAT ligation. Quartz crystal microbalance with dissipation (QCM-D) monitoring confirmed the rapid attachment of the PEG moiety onto the SAM. By surface characterization methods such as contact angle and ellipsometry, the attachment of PEG layer was confirmed, and covalent amide-bond formation was established by X-ray photoelectron spectroscopy (XPS). In a proof-of-concept study, the applicability of this surface KAT ligation for the attachment of biomolecules to surfaces was tested using a model protein, green fluorescent protein (GFP). A GFP was chemically modified with an HA linker to synthesize HA-GFP and added to the KAT-SAM under aqueous dilute conditions. A rapid attachment of the GFP on the surface was observed in real time by QCM-D. Despite the fact that such biomolecules have a variety of unprotected functional groups within their structures, the surface KAT ligation proceeded rapidly in a chemoselective manner. Our results demonstrate the versatility of the KAT ligation for the covalent attachment of a variety of water-soluble molecules onto SAM surfaces under dilute and biocompatible conditions to form stable, natural amide bonds.

Grain size control of perovskite films based on β-alanine self-assembled monolayers surface treatment

The grain size of the perovskite layer can be tuned by the surface treatment of Ultraviolet ozone (UVO) with β-alanine self-assembled molecular films (SAMs) on the Nickel oxide (NiOx) substrate. The UVO treatment had been used to control the hydroxy (-OH) end-group on NiOx substrate and the condensation reaction of β-alanine SAMs, in which the end-group of the amino group (-NH2) correlates with the nucleation of the perovskite layer. The difference of surface end-group with/without the UVO treatment on the NiOx substrate was revealed by the shift of the oxidation peak of the SAMs layer in the cyclic voltammetry and the electrochemical impedance spectroscopy. It is found that the wettability of NiOx substrate was enhanced by the SAMs treatment time due to the increment of -NH2 end-group and the UVO treatment. Larger grain size of the perovskite layer was achieved on the NiOx substrate with SAMs 15 minutes treatment. Moreover, the optical absorbance of perovskite films in the short wavelength range was enhanced by the assembling time of SAMs. This study provides a method to control the surface morphology of the perovskite active layer in its solar cell device applications.

Molecular Mechanics Study of Flow and Surface Influence in Ligand-Protein Association.

Ligand–protein association is the first and critical step for many biological and chemical processes. This study investigated the molecular association processes under different environments. In biology, cells have different compartments where ligand–protein binding may occur on a membrane. In experiments involving ligand–protein binding, such as the surface plasmon resonance and continuous flow biosynthesis, a substrate flow and surface are required in experimental settings. As compared with a simple binding condition, which includes only the ligand, protein, and solvent, the association rate and processes may be affected by additional ligand transporting forces and other intermolecular interactions between the ligand and environmental objects. We evaluated these environmental factors by using a ligand xk263 binding to HIV protease (HIVp) with atomistic details. Using Brownian dynamics simulations, we modeled xk263 and HIVp association time and probability when a system has xk263 diffusion flux and a non-polar self-assembled monolayer surface. We also examined different protein orientations and accessible surfaces for xk263. To allow xk263 to access to the dimer interface of immobilized HIVp, we simulated the system by placing the protein 20Å above the surface because immobilizing HIVp on a surface prevented xk263 from contacting with the interface. The non-specific interactions increased the binding probability while the association time remained unchanged. When the xk263 diffusion flux increased, the effective xk263 concentration around HIVp, xk263–HIVp association time and binding probability decreased non-linearly regardless of interacting with the self-assembled monolayer surface or not. The work sheds light on the effects of the solvent flow and surface environment on ligand–protein associations and provides a perspective on experimental design.

Cuprous or cupric? How substrate polarity can select for different phases of copper sulfide films in chemical bath deposition.

Copper sulfides have many applications from thermoelectrics to biotechnology. While the properties of different copper sulfide phases are well understood, controlling the deposited copper sulfide stoichiometry remains a significant challenge, especially in solution-phase synthesis techniques. In this work, we investigate the chemical bath deposition of CuxS on functionalized self-assembled monolayers (SAMs). Time-of-flight mass spectrometry, Raman spectroscopy, and x-ray photoelectron spectroscopy are employed to analyze the deposited films. We show that the use of thiourea as a sulfur source leads to the deposition of different copper sulfide phases and is controlled by the interaction of sulfur-containing ions in solution with the functionalized SAMs. For -COOH terminated SAMs, copper sulfide deposition is controlled by the surface polarity of the substrate. At the bath pH used in these experiments, the -COOH terminal groups are deprotonated. The resulting -COO- terminated SAM surface repels negatively charged sulfur-containing ions, leading to the deposition of Cu2S. For -CH3 terminated SAMs, which are non-polar, there is no specific interaction between the SAM terminal group and sulfur-containing ions and CuS is deposited. For -OH terminated SAMs, which have a polar terminal group, there are two competing effects: the repulsion of S-containing ions by the small negative charge of the terminal -OH group and the increase in the concentration of sulfur-containing ions in solution as the bath pH increases. This competition leads to the deposit stoichiometry changing from Cu2S at pH 9 to CuS at pH 12.

Biosensor for electrochemical detection of biomarkers of osteoarthritis

Biosensor for electrochemical detection of biomarkers of osteoarthritis

Molecular Dynamics Simulations of a Catalytic Multivalent Peptide-Nanoparticle Complex.

Molecular modeling of a supramolecular catalytic system is conducted resulting from the assembling between a small peptide and the surface of cationic self-assembled monolayers on gold nanoparticles, through a multiscale iterative approach including atomistic force field development, flexible docking with Brownian Dynamics and µs-long Molecular Dynamics simulations. Self-assembly is a prerequisite for the catalysis, since the catalytic peptides do not display any activity in the absence of the gold nanocluster. Atomistic simulations reveal details of the association dynamics as regulated by defined conformational changes of the peptide due to peptide length and sequence. Our results show the importance of a rational design of the peptide to enhance the catalytic activity of peptide–nanoparticle conjugates and present a viable computational approach toward the design of enzyme mimics having a complex structure–function relationship, for technological and nanomedical applications.

Effect of carbamylation on protein structure and adsorption to self-assembled monolayer surfaces

Protein adsorption research has primarily focused upon the effects of surface chemistry, with almost no emphasis on how changes to proteins that occur in various disease states may influence their adsorption. One such situation occurs with chronic kidney disease where, despite hemodialysis treatment, the retention of urea within the blood compartment leads to protein carbamylation. Protein carbamylation has been shown to alter the function and structure of proteins. This work is focused on understanding how different degrees of carbamylation affect the physicochemical properties (structure, charge, water interactions) of single proteins (α-lactalbumin, albumin, and fibrinogen) and their adsorption to self-assembled monolayers. It was found that, unlike its secondary structure, the protein’s tertiary structure was significantly altered upon carbamylation. Also, compared to native proteins, an increase in carbamylation lead to an increase in the negative surface charge of the protein and a weaker hydration state of the protein. In order to study the effects of different types of neutral surfaces, of different surface-water properties, on protein adsorption both bare and alkanethiol modified (−CH3 or −OH end-groups) Au surfaces with were used as model surfaces. A significant decrease in adsorbed amounts of carbamylated fibrinogen and carbamylated α-lactalbumin, but not for carbamylated albumin, relative to native proteins was observed for both surfaces; suggesting that the increase in negative surface charge is more influential on adsorption than the change in hydration that occurs throughout the protein upon carbamylation. This data suggests that protein alterations that occur due to disease states have a significant effect on the overall protein structure and these changes affect their adsorption to surfaces.

Electrochemical Immunosensor for the Early Detection of Rheumatoid Arthritis Biomarker: Anti-Cyclic Citrullinated Peptide Antibody in Human Serum Based on Avidin-Biotin System.

Rheumatoid arthritis (RA) is a chronic autoimmune disease that produces a progressive inflammatory response that leads to severe pain, swelling, and stiffness in the joints of hands and feet, followed by irreversible damage of the joints. The authors developed a miniaturized, label-free electrochemical impedimetric immunosensor for the sensitive and direct detection of arthritis Anti-CCP-ab biomarker. An interdigitated-chain-shaped microelectrode array (ICE) was fabricated by taking the advantage of microelectromechanical systems. The fabricated ICE was modified with a self-assembled monolayer (SAM) of Mercaptohexanoic acid (MHA) for immobilization of the synthetic peptide bio-receptor (B-CCP). The B-CCP was attached onto the surface of SAM modified ICE through a strong avidin-biotin bio-recognition system. The modified ICE surface with the SAM and bio-molecules (Avidin, B-CCP, Anti-CCP-ab and BSA) was morphologically and electrochemically characterized. The change in the sensor signal upon analyte binding on the electrode surface was probed through the electrochemical impedance spectroscopy (EIS) property of charge-transfer resistance (Rct) of the modified electrodes. EIS measurements were target specific and the sensor response was linearly increased with step wise increase in target analyte (Anti-CCP-ab) concentrations. The developed sensor showed a linear range for the addition of Anti-CCP-ab between 1 IU mL−1 → 800 IU mL−1 in phosphate buffered saline (PBS) and Human serum (HS), respectively. The sensor showed a limit of detection of 0.60 IU mL−1 and 0.82 IU mL−1 in the PBS and HS, respectively. The develop bio-electrode showed a good reproducibility (relative standard deviation (RSD), 1.52%), selectivity and stability (1.5% lost at the end of 20th day) with an acceptable recovery rate (98.0% → 101.18%) and % RSD’s for the detection of Anti-CCP-ab in spiked HS samples.

Area-Selective Atomic Layer Deposition of TiN Using Trimethoxy(octadecyl)silane as a Passivation Layer.

Area-selective deposition (ASD) offers tremendous advantages when compared with conventional patterning processes, such as the possibility of achieving three-dimensional features in a bottom-up additive fashion. Recently, ASD is gaining more and more attention from IC manufacturers and equipment and material suppliers. Through combination of self-assembled monolayer (SAM) surface passivation of the nongrowth substrate area and atomic layer deposition (ALD) on the growth area, ASD selective to the growth area can be achieved. With the purpose of screening SAM precursors to provide optimal passivation performance on SiO2, various siloxane precursors with different terminal groups and alkyl chains were investigated. Additionally, the surface dependence and growth inhibition of TiN ALD on -NH2, -CF3, and -CH3 terminations is investigated. We demonstrated the methyl termination of the SAM precursor combined with a C18 alkyl chain plays an important role in broadening the ALD selectivity window by suppressing precursor adsorption. Owing to the high surface coverage, excellent thermal stability and longer carbon chain length, an optimized trimethoxy(octadecyl)silane (TMODS) film deposited from liquid phase was able to provide a selectivity higher than 0.99 up to 20 nm ALD film deposited on hydroxyl-terminated Si oxide. The approach followed in this work can allow extending the ASD process window, and it is relevant for a wide variety of applications.

Probing fibronectin adsorption on chemically defined surfaces by means of single molecule force microscopy

Atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) and a quartz crystal microbalance (QCM) were respectively employed to probe interfacial characteristics of fibronectin fragment FNIII8–14 and full-length fibronectin (FN) on CH3–, OH–, COOH–, and NH2-terminated alkane-thiol self-assembled monolayers (SAMs). Force-distance curves acquired between hexahistidine-tagged FNIII8–14 immobilised on trisNTA-Ni2+ functionalized AFM cantilevers and the OH and COOH SAM surfaces were predominantly ‘loop-like’ (76% and 94% respectively), suggesting domain unfolding and preference for ‘end-on’ oriented binding, while those generated with NH2 and CH3 SAMs were largely ‘mixed type’ (81% and 86%, respectively) commensurate with unravelling and desorption, and ‘side-on’ binding. Time-dependent binding of FN to SAM-coated QCM crystals occurred in at least two phases: initial rapid coverage over the first 5 min; and variably diminishing adsorption thereafter (5–70 min). Loading profiles and the final hydrated surface concentrations reached (~ 950, ~ 1200, ~ 1400, ~ 1500 ng cm−2 for CH3, OH, COOH and NH2 SAMs) were consistent with: space-filling ‘side-on’ orientation and unfolding on CH3 SAM; greater numbers of FN molecules arranged ‘end-on’ on OH and especially COOH SAMs; and initial ‘side-on’ contact, followed by either (1) gradual tilting to a space-saving ‘end-on’ configuration, or (2) bi-/multi-layer adsorption on NH2 SAM.

Relationship between the surface structure of the gate insulator and the performance of organic thin-film transistors

To investigate the impact on the device performance of different surface structures of the self-assembled monolayer (SAM) used to form the gate insulator of organic field effect transistors (OFETs), sum frequency generation spectroscopy and contact angle measurements were performed on SAM insulators with different surface structures. Furthermore, the surface structure of the SAM was varied with changing its dipping or spin coating formation method. Moreover, the electrical properties of OFETs were examined when only the surface structure of the gate insulator had changed. The results showed that compared with the dipping method, the disorder of alkyl chains (gauche defects) increased on the octadecylphosphonic acid (ODPA) SAM surface, deposited through the spin coating method. Contact angle measurements revealed that the surface free energy of ODPA-SAM increased in tandem with an increase in gauche defects on the SAM surface. Contrary to expectations, the device performance of OFETs with the SAM insulator with gauche defects improved, which is most probably due to the smaller surface free energy difference between ODPA-SAM and poly(3-hexylthiophene) (P3HT), which improved the adhesion at the ODPA-SAM/P3HT interface.

Electrochemical investigation of the intermolecular hydrogen bond on the surface of self-assembled monolayer

Intermolecular hydrogen bond, as a kind of weak interaction, plays a fundamental and important role in the molecular recognition process on cell membrane surface. Based on the complexity of the cell membrane and its microenvironment, the self-assembled monolayer with clear structure is used to simulate the cell membrane to construct the simulation system. The intermolecular hydrogen bond on the surface of the self-assembled is explored at the molecular level, which provided a new perspective for the study of molecular recognition on the cell membrane surface. In this paper, combined with the advantages of scanning electrochemical microscopy, the progress on the intermolecular hydrogen bond on the surface of self-assembled monolayer is introduced from the electrochemical viewpoint.

Density Functional Theory Study of Pd Aggregation on a Pyridine-Terminated Self-Assembled Monolayer.

By using density functional theory calculations, the initial steps towards Pd metal cluster formation on a pyridine‐terminated self‐assembled monolayer (SAM) consisting of 3‐(4‐(pyridine‐4‐yl)phenyl)propane‐1‐thiol on an Au(1 1 1) surface are investigated. Theoretical modelling allows the investigation of structural details of the SAM surface and the metal/SAM interface at the atomic level, which is essential for elucidating the nature of Pd–SAM and Pd–Pd interactions at the liquid/solid interface and gaining insight into the mechanism of metal nucleation in the initial stage of electrodeposition. The structural flexibility of SAM molecules was studied first and the most stable conformation was identified, planar molecules in a herringbone packing, as the model for Pd adsorption. Two binding sites are found for Pd atoms on the pyridine end group of the SAM. The strong interaction between Pd atoms and pyridines illustrates the importance of SAM functionalisation in the metal nucleation process. Consistent with an energetic driving force of approximately −0.3 eV per Pd atom towards Pd aggregation suggested by static calculations, a spontaneous Pd dimerisation is observed in ab initio molecular dynamic studies of the system. Nudged elastic band calculations suggest a potential route with a low energy barrier of 0.10 eV for the Pd atom diffusion and then dimerisation on top of the SAM layer.

Nanoporous Thin Films Formed from Photocleavable Diblock Copolymers on Gold Substrates Modified with Thiolate Self-Assembled Monolayers.

Nanoporous thin films formed on electrodes are considered functional elements of electrochemical sensing systems, thus motivating methods for their development. We report a preparative strategy detailing the effects of surface modification of gold substrates with thiolate self-assembled monolayers (SAMs) on the properties of nanoporous thin films derived from polystyrene-block-poly(ethylene oxide) having a photocleavable o-nitrobenzyl ester junction (PS-hν-PEO). Two PS-hν-PEO having similar PEO volume fractions (≈0.2) but different molecular weights (10 and 23 kg/mol) were used to prepare films (30-100 nm thick) spin-cast on gold substrates unmodified and modified with cysteamine, thioctic acid, and 6-hydroxy-1-hexanethiol SAMs. Solvent vapor annealing followed by PEO removal led to the formation of nanopores with average diameters of 12 and 19 nm from the smaller and larger PS-hν-PEO, respectively. Cyclic voltammograms of 1,1'-ferrocenedimethanol showed that nanoporous films on cysteamine SAMs afforded nanopores reaching the underlying substrates at higher density than those on the other substrates. This result was attributed to balanced affinity of the cysteamine SAM surface with PS and PEO, which enhanced the vertical orientation of PEO microdomains. The generation of carboxyl groups associated with the photocleavage reaction was revealed by pH-dependent changes in the voltammogram of Fe(CN)63- that reflected electrostatic effects regulated by the protonation state of the carboxyl groups. The SAMs underneath the nanoporous films could be replaced by treatment with a thiol solution, as verified by voltammograms of l-ascorbic acid. These results suggest that thiolate SAM modification provides a simple means to control the interfacial orientation of PEO microdomains in thin PS-hν-PEO films.

Molecular Origin of the Odd-Even Effect of Macroscopic Properties of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?

Elucidating the influence of the monolayer interface versus bulk on the macroscopic properties (e.g., surface hydrophobicity, charge transport, and electron transfer) of organic self-assembled monolayers (SAMs) chemically anchored to metal surfaces is a challenge. This article reports the characterization of prototypical SAMs of n-alkanethiolates on gold (CH3(CH2)nSAu, n = 6-19) at the macroscopic scale by electrochemical impedance spectroscopy and contact angle goniometry, and at the molecular level, by infrared reflection absorption spectroscopy. The SAM capacitance, dielectric constant, and surface hydrophobicity exhibit dependencies on both the length (n) and parity (nodd or neven) of the polymethylene chain. The peak positions of the CH2 stretching modes indicate a progressive increase in the chain conformational order with increasing n between n = 6 and 16. SAMs of nodd have a greater degree of structural gauche defects than SAMs of neven. The peak intensities and positions of the CH3 stretching modes are chain length independent but show an odd-even alternation of the spatial orientation of the terminal CH3. The correlations between the different data trends establish that the chain length dependencies of the dielectric constant and surface hydrophobicity originate from changes in the polymethylene chain conformation (bulk), while the odd-even variation arises primarily from a difference in the chemical composition of the interface related to the terminal group orientation. These findings provide new physical insights into the structure-property relation of SAMs for the design of ultrathin film dielectrics as well as the understanding of stereostructural effects on the electrical characteristics of tunnel junctions.

Fast fragmentation during surface-induced dissociation: An examination of peptide size and structure

We present the results of direct dynamics simulations of surface-induced dissociation for protonated versions of AnK, KAn (n = 1, 3, and 5), AcA7K, and AcKA7 for collisions with a fluorinated self-assembled monolayer surface. We focus on elucidating fast fragmentation events, which takes place in coincidence with the collision event. Such events generate a large number of products, and hence, are not easily understood through chemical intuition. Our simulations show distinct differences between the AnK/AcA7K and KAn/AcKA7 series of peptides, with the former being more reactive, and the latter more selective. Backbone rearrangements and sidechain fragmentation are also seen.

Amyloid-β (Aβ42) Peptide Aggregation Rate and Mechanism on Surfaces with Widely Varied Properties: Insights from Brownian Dynamics Simulations.

Amyloid-β (Aβ) plaques, which form by aggregation of harmless Aβ peptide monomers into larger fibrils, are characteristic of neurodegenerative disorders such as Alzheimer's disease. Efforts to treat Alzheimer's disease focus on stopping or reversing the aggregation process that leads to fibril formation. However, effective treatments are elusive due to certain unknown aspects of the process. Many hypotheses point to disruption of cell membranes by adsorbed Aβ monomers or oligomers, but how Aβ behaves and aggregates on surfaces of widely varying properties, such as those present in a cell, is unclear. Elucidating the effects of various surfaces on the dynamics of Aβ and the kinetics of the aggregation process from bulk solution to a surface-adsorbed multimer can help identify what drives aggregation, leading to new methods of intervention by inhibitory drugs or other means. In this work, we used all-atom Brownian dynamics simulations to study the association of two distinct Aβ42 monomer conformations with a surface-adsorbed or free-floating Aβ42 dimer. We calculated the association time, surface interaction energy, surface diffusion coefficient, surface residence time, and the mechanism of association on four different surfaces and two different bulk solution scenarios. In the presence of a surface, the majority of monomers underwent a two-dimensional surface-mediated association that depended primarily on an Aβ42 electrostatic interaction with the self-assembled monolayer (SAM) surfaces. Moreover, aggregation could be inhibited greatly by surfaces with high affinity for Aβ42 and heterogeneous charge distribution. Our results can be used to identify new opportunities for disrupting or reversing the Aβ42 aggregation process.

Molecular Dynamics Simulations of Water Condensation on Surfaces with Tunable Wettability.

Water condensation plays a major role in a wide range of industrial applications. Over the past few years, many studies have shown interest in designing surfaces with enhanced water condensation and removal properties. It is well known that heterogeneous nucleation outperforms homogeneous nucleation in the condensation process. Because heterogeneous nucleation initiates on a surface at a small scale, it is highly desirable to characterize water-surface interactions at the molecular level. Molecular dynamics (MD) simulations can provide direct insight into heterogeneous nucleation and advance surface designs. Existing MD simulations of water condensation on surfaces were conducted by tuning the solid-water van der Waals interaction energy as a substitute for modeling surfaces with different wettabilities. However, this approach cannot reflect the real intermolecular interactions between the surface and water molecules. Here, we report MD simulations of water condensation on realistic surfaces of alkanethiol self-assembled monolayers with different head group chemistries. We show that decreasing surface hydrophobicity significantly increases the electrostatic forces between water molecules and the surface, thus increasing the water condensation rate. We observe a strong correlation between our rate of condensation results and the results from other surface characterization metrics, such as the interfacial thermal conductance, contact angle, and the molecular-scale wettability metric of Garde and co-workers. This work provides insight into the water condensation process at the molecular scale on surfaces with tunable wettability.

Influence of an alkyl spacer on the formation and structure of 4-Fluorobenzenethiol and 4-Fluorobenzenemethanethiol self-assembled monolayers on Au(111)

Self-assembled monolayers (SAMs) prepared by adsorption of 4-Fluorobenzenethiol (p-FBT) and 4-Fluorobenzenemethanethiol (p-FBMT) on Au(111) substrates with different immersion times (ITs) at room temperature have been characterized using scanning tunneling microscopy (STM). The resulting p-FBT and p-FBMT monolayers showed substantial differences concerning their adlayer SAM structure and surface morphology. The structural quality of p-FBT-SAMs was found to depend significantly on the preparation conditions such as the IT. At short IT of ~3 h, the p-FBT-SAMs were found to mainly include disordered phase (α-phase) between the surrounding areas of the gold islands. With increasing IT, a gradual phase transition from the disordered phase (α) into ordered one (β-phase) was observed. Consequently, increasing the IT to ~ 24 h resulted in the formation of well-defined and highly ordered β-phase that is characterized by the commensurate (83×23)R30∘superstructure. In contrast to the p-FBT-SAMs, a full-coverage and close packed monolayers were formed in p-FBMT-SAMs only after very short IT of ~1 min. The upright-standing p-FBMT molecules were found to form a (3×3)R30∘structure. Despite the absence of a periodic variation in the height of the molecules within the molecular domains, we proposed a larger (23×3)R30∘unit cell containing two inequivalent p-FBMT molecules arranged in a herringbone-type structure instead of the smaller (3×3)R30∘unit cell. The considerable difference in the behavior of these closely related molecules which differ only in their structure (an additional methyl group between benzene and S group in p-FBMT), clarifies the delicate balance of the factors which define the energy and the structure of thiol SAMs.