Improved method for the preparation of carboxylic acid and amine terminated self-assembled monolayers of alkanethiolates

We present an improved method to prepare carboxylic acid (COOH) and amine (NH2) terminated self-assembled monolayers (SAMs) of alkanethiolates. In this method, a small amount of CF3COOH (for COOH-SAM) or N(CH2CH3)3 (for NH2-SAM) is added into the ethanolic solution of alkanethiols during SAM formation. The freshly formed COOH- and NH2-SAMs are then rinsed with an ethanolic solution of NH4OH or CH3COOH, respectively. Both SAMs prepared with the improved method show better quality in terms of surface chemical composition, roughness, and wettability as measured by X-ray photoelectron spectroscopy, atomic force microscopy, and contact angle, respectively. The formation of better SAMs can be attributed to the disruption of interplane hydrogen bonds.     

Self-assembled carbon nanotubes on gold: polarization-modulated infrared reflection-absorption spectroscopy, high-resolution X-ray photoemission spectroscopy, and near-edge X-ray absorption fine structure spectroscopy study

Recently we reported noncovalent functionalization of nanotubes in an aqueous medium with ionic liquid-based surfactants, 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), resulting in positively charged single-wall carbon nanotube (SWNT)-1,2 composites. Thiolation of SWNTs with 2 provides their self-assembly on gold as well as templating gold nanoparticles on SWNT sidewalls via a covalent -S-Au bond. In this investigation, we studied the electronic structure, intermolecular interactions, and packing within noncovalently thiolated SWNTs and also nanotube alignment in the bulk of SWNT-2 dried droplets and self-assembled submonolayers (SAMs) on gold by high-resolution X-ray photoemission spectroscopy (HRXPS), C K-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). HRXPS data confirmed the noncovalent nature of interactions within the nanocomposite of thiolated nanotubes. In PM-IRRAS spectra of SWNT SAMs on gold, the IR-active vibrational SWNT modes have been observed and identified. According to PM-IRRAS data, the hydrocarbon chains of 2 are oriented with less tilt angle to the bare gold normal in a SAM deposited from an SWNT-2 dispersion than those of 1 deposited from an SWNT-1 dispersion on the mercaptoethanesulfonic acid-primed gold. For both the dried SWNT-2 bulk and the SWNT-2 SAM on gold, the C K-edge NEXAFS spectra revealed the presence of CH-pi interactions between hydrocarbon chains of 2 and the pi electronic nanotube structure due to the highly resolved vibronic fine structure of carbon 1s --> R*/sigma*C-H series of states in the alkyl chain of 2. For the SWNT-2 bulk, the observed splitting and upshift of the SWNT pi* orbitals in the NEXAFS spectrum indicated the presence of pi-pi interactions. In the NEXAFS spectrum of the SWNT-2 SAM on gold, the upshifted values of the photon energy for R*/sigma*C-H transitions indicated close contact of 2 with nanotubes and with a gold surface. The angle-dependent NEXAFS for the SWNT-2 bulk showed that most of the molecules of 2 are aligned along the nanotubes, which are self-organized with orientation parallel to the substrate plane, whereas the NEXAFS for the SWNT-2 SAM revealed a more normal orientation of functionality 2 on gold compared with that in the SWNT-2 bulk.     

Metallization of a thiol-terminated organic surface using chemical vapor deposition

The deposition and the subsequent decomposition of an organometallic precursor, (eta (3)-allyl)(eta (5)-cyclopentadienyl)palladium [Cp(allyl)Pd], on an organic surface exposed by self-assembled monolayers (SAM) was studied using X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRRAS). The interfacial chemical reactions of the vapor-deposited metal precursor with the pendant thiol group of the SAMs made from oligophenyldithiols, which are either prepared directly (terphenyldimethyldithiol, TPDMT) or by a deprotection route from SAMs formed by a monoacylated derivative of biphenyldimethyldithiol (dep. BPDMAc-1) have been studied in detail. When the TPDMT-SAMs were exposed to Cp(allyl)Pd vapor, a Pd (2+)/allyl-terminated SAM surface was obtained (to a lower extent this was also the case for dep. BPDMAc-1 SAMs), which was stable against exposure to H 2 gas. Reduction to Pd (0) by H 2 was only observed when small amounts of Pd (0) were already present, for example, after prolonged exposure to the precursor. The catalytic activity of the small Pd (0) particles also caused a decomposition of the SAMs upon exposure to air.     

Chemistry on surface-confined molecules: an approach to anchor isolated functional units to surfaces.

The synthesis of surface-confined, nanometer-sized dendrimers and Au nanoparticles was performed starting from single Pd(II) pincer adsorbate molecules (10) embedded as isolated species into 11-mercapto-1-undecanol and decanethiol self-assembled monolayers (SAMs) on gold. The coordination of monolayer-protected Au nanoclusters (MPCs) bearing phosphine moieties at the periphery (13), or dendritic wedges (8) having a phosphine group at the focal point, to SAMs containing individual Pd(II) pincer molecules was monitored by tapping mode atomic force microscopy (TM AFM). The individual Pd(II) pincer molecules embedded in the decanethiol SAM were visualized by their coordination to phosphine MPCs 13; isolated objects with a height of 3.5 +/- 0.7 nm were observed by TM AFM. Reaction of these embedded Pd(II) pincer molecules with the dendritic wedge 8 yielded individual molecules with a height of 4.3 +/- 0.2 nm.     

Structural development of mercaptophenol self-assembled monolayers and the overlying mineral phase during templated CaCO3 crystallization from a transient amorphous film

Formation of biomineral structures is increasingly attributed to directed growth of a mineral phase from an amorphous precursor on an organic matrix. While many in vitro studies have used calcite formation on organothiol self-assembled monolayers (SAMs) as a model system to investigate this process, they have generally focused on the stability of amorphous calcium carbonate (ACC) or maximizing control over the order of the final mineral phase. Little is known about the early stages of mineral formation, particularly the structural evolution of the SAM and mineral. Here we use near-edge X-ray absorption spectroscopy (NEXAFS), photoemission spectroscopy (PES), X-ray diffraction (XRD), and scanning electron microscopy (SEM) to address this gap in knowledge by examining the changes in order and bonding of mercaptophenol (MP) SAMs on Au(111) during the initial stages of mineral formation as well as the mechanism of ACC to calcite transformation during template-directed crystallization. We demonstrate that formation of ACC on the MP SAMs brings about a profound change in the morphology of the monolayers: although the as-prepared MP SAMs are composed of monomers with well-defined orientations, precipitation of the amorphous mineral phase results in substantial structural disorder within the monolayers. Significantly, a preferential face of nucleation is observed for crystallization of calcite from ACC on the SAM surfaces despite this static disorder.     

Tripodal Binding Units for Self-Assembled Monolayers on Gold: A Comparison of Thiol and Thioether Headgroups

Whereas thiols and thioethers are frequently used as binding units of oligodentate precursor molecules to fabricate self-assembled monolayers (SAMs) on coinage metal and semiconductor surfaces, their use for tridentate bonding configuration is still questionable. Against this background, novel tridentate thiol ligands, PhSi(CH(2)SH)(3) (PTT) and p-Ph-C(6)H(4)Si(CH(2)SH)(3) (BPTT), were synthesized and used as tripodal adsorbate molecules for the fabrication of SAMs on Au(111). These SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The PTT and BPTT films were compared with the analogous systems comprised of same tripodal ligands with thioether instead of thiol binding units (anchors). XPS and NEXAFS data suggest that the binding uniformity, packing density, and molecular alignment of the thiol-based ligands in the respective SAMs is superior to their thioether counterparts. In addition, the thiol-based films showed significantly lower levels of contamination. Significantly, the quality of the PTT SAMs on Au(111) was found to be even higher than that of the films formed from the respective monodentate counterpart, benzenethiol. The results obtained allow for making some general conclusions on the specific character of molecular self-assembly in the case of tridentate ligands.     

Investigation of the mechanism of nickel electroless deposition on functionalized self-assembled monolayers

We have investigated the seedless electroless deposition (ELD) of Ni on functionalized self-assembled monolayers (SAMs) using scanning electron and optical microscopies, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. For all SAMs studied, the Ni deposition rate is dependent on the bath pH, deposition temperature, and complexing agent. In contrast to the physical vapor deposition of Ni on SAMs, electrolessly deposited Ni does not penetrate through the SAM. This behavior indicates that ELD is a suitable technique for the deposition of low-to-moderate reactivity on organic thin films. We demonstrate that Ni can be selectively deposited on SAMs using two different methods. First, selectivity can be imparted by the formation of Ni(II)-surface complexes. As a demonstration, we selectively deposited Ni on the -COOH terminated SAM areas of patterned -COOH/-CH(3) or -COOH/-OH terminated SAMs. Here, Ni(2+) ions form Ni(2+)-carboxylate complexes with the -COOH terminal group, which comprise the nucleation sites for subsequent metal deposition. Second, we demonstrate that nickel is selectively deposited on the -CH(3) terminated SAM areas of a patterned -OH/-CH(3) terminated SAM. In this case, the Ni(2+) ion does not specifically interact with the -CH(3) terminal group. Rather, selectivity is imparted by the interaction of the reductant, dimethylamine borane (DMAB), with the -OH and -CH(3) terminal groups.     

Thermal and mechanical aging of self-assembled monolayers as studied by near edge X-ray absorption fine structure

Self-assembled monolayers (SAMs) enable significant changes in the surface energy and/or specific interactions of surfaces, which are desirable for microelectromechanical systems (MEMS), superhydrophobic coatings, sensors, and other applications. However, SAMs often exhibit poor durability and rapid degradation upon mechanical, thermal, or moisture exposure. The chemical and orientational changes in SAMs due to mechanical and thermal degradation were investigated using near-edge X-ray absorption fine structure (NEXAFS) and the water contact angle. SAMs were based on unfluorinated or fluorinated linear hydrocarbons that form highly oriented and densely packed structures on silicon substrates. Complex chemical and orientational changes were observed via NEXAFS following degradation. Under heating in a dry, oxygen-rich environment, unfluorinated SAMs tended to cleave at C-C bonds on the main chain; below 250 °C, CH(3) groups were sequentially cleaved toward the surface, whereas above 250 °C, remaining hydrocarbon groups were converted to a graphitic coating dominated by C═C bonds. Under similar conditions, fluorinated SAMs began their chemical degradation at 350 °C and above, although the orientation decreased steadily from 150 to 300 °C; at and above 350 °C, the preferential removal of F occurred and the SAM was slowly converted to a graphitic layer. By contrast, under vacuum the fluorinated molecules were very thermally stable, showing good stability up to 550 °C; when degradation occurred, entire molecules were removed. Mechanical degradation followed two routes; both unfluorinated and fluorinated SAMs that were mechanically rubbed with smooth surfaces exhibited severe chemical degradation of the molecules, leading to an amorphous and poorly defined layer with C═C, C-C, C-H, and C-F bonds. Unfluorinated and fluorinated surfaces that were mechanically rubbed in the presence of free silicon particulates showed the rapid and complete destruction of both the molecular orientation and the protective SAM layer, even for short exposure periods. The resulting NEXAFS spectra were very similar to those produced by heating to 550 °C, suggesting that the friction created by granular particles may lead to extreme local heating.     

Immobilization of the nanoparticle monolayer onto self-assembled monolayers by combined sterically enhanced hydrophobic and electrophoretic forces

The immobilization of surface-derivatized gold nanoparticles onto methyl-terminated self-assembled monolayers (SAMs) on gold surface was achieved by the cooperation of hydrophobic and electrophoretic forces. Electrochemical and scanning probe microscopy techniques were utilized to explore the influence of the SAM's structure and properties of the nanoparticle/SAM/gold system. SAMs prepared from 1-decanethiol (DT) and 2-mercapto-3-n-octylthiophene (MOT) were used as hydrophobic substrates. The DT SAM is a closely packed and organized monolayer, which can effectively block the underlying gold and inhibit a variety of solution species including organic and inorganic molecules from penetrating, whereas the MOT monolayer is poorly packed or disorganized (because of a large difference in dimension between the thiophene head and the alkylchain tail) and permeable to many organic probes in aqueous solution but not to inorganic probes. Thus, the MOT monolayer provides a more energetically favorable hydrophobic surface for the penetration and adsorption of organic species than the DT monolayer. This hypothesis is supported by experiments in which the density of hydrophobically immobilized nanoparticles on the MOT SAM is much larger than that on the DT SAM. The results also suggest new approaches for modification of macroscopic surfaces with nanoscopic particles.     

Self-assembly, characterization, and chemical stability of isocyanide-bound molecular wire monolayers on gold and palladium surfaces

Self-assembled monolayers (SAMs) of the isocyano derivative of 4,4'-di(phenylene-ethynylene)benzene (1), a member of the "OPE" family of "molecular wires" of current interest in molecular electronics, have been prepared on smooth, {111} textured films of Au and Pd. For assembly in oxygen-free environments with freshly deposited metal surfaces, infrared reflection spectroscopy (IRS) indicates the molecules assume a tilted structure with average tilt angles of 18-24 degrees from the surface normal. The combination of IRS, X-ray photoelectron spectroscopy, and density functional theory calculations all support a single sigma-type bond of the -NC group to the Au surface and a sigma/pi-type of bond to the Pd surface. Both SAMs show significant chemical instability when exposed to typical ambient conditions. In the case of the Au SAM, even a few hours storage in air results in significant oxidation of the -NC moieties to -NCO (isocyanate) with an accompanying decrease in surface chemical bonding, as evidenced by a significant increase in instability toward dissolution in solvent. In the case of the Pd SAM, similar air exposure does not result in incorporation of oxygen or loss of solvent resistance but rather results in a chemically altered interface which is attributed to polymerization of the -NC moieties to quasi-2D poly(imine) structures. Conductance probe atomic force microscope measurements show the conductance of the degraded Pd SAMs can diminish by approximately 2 orders of magnitude, an indication that the SAM-Pd electrical contact has severely degraded. These results underscore the importance of careful control of the assembly procedures for aromatic isocyanide SAMs, particularly for applications in molecular electronics where the molecule-electrode junction is critical to the operational characteristics of the device.     

Adsorption of oxygenates on alkanethiol-functionalized Pd(111) surfaces: mechanistic insights into the role of self-assembled monolayers on catalysis

Recent work shows that coating a supported palladium catalyst with a self-assembled monolayer (SAM) of alkanethiols can dramatically improve selectivity in the hydrogenation of 1-epoxy-3-butene (EpB) to 1-epoxybutane. Here, we present the results of surface-level investigations of the adsorption of EpB and related molecules on SAM-coated Pd(111), with an aim of identifying mechanistic explanations for the observed catalytic behavior. Alkanethiol SAM-covered Pd(111) surfaces were prepared by conventional techniques and transferred to ultrahigh vacuum, where they were characterized using Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) of EpB and other probe molecules. Whereas previous studies have shown that EpB undergoes rapid decomposition via epoxide ring opening on uncoated Pd(111), TPD studies show that EpB does not undergo substantial ring opening on SAM-covered surfaces but rather desorbs intact at temperatures less than 300 K. Systematic comparisons of EpB desorption spectra to spectra for other C(4) oxygenates suggest that the SAM creates a kinetic barrier to epoxide ring-opening reactions that does not exist on the uncoated surface. The EpB desorption spectra as a function of exposure show behavior similar to the desorption of olefins from Pd(111), indicating that the binding of the olefin functionality, in contrast to that of the epoxide ring, is not significantly perturbed. EpB desorption spectra from surfaces with less well-ordered SAMs show the presence of weakly bound states not observed on well-ordered SAM surfaces. The lower activity observed on catalysts covered with less well-ordered SAMs is hypothesized to occur due to partial confinement of adsorbates into these weakly bound, less active states.     

In situ hydrolysis of imine derivatives on Au(111) for the formation of aromatic mixed self-assembled monolayers: multitechnique analysis of this tunable surface modification

This paper presents a novel method for preparing aromatic, mixed self-assembled monolayers (SAMs) with a dilute surface fraction coverage of protonated amine via in situ hydrolysis of C═N double bond on gold surface. Two imine compounds, (4'-(4-(trifluoromethyl)benzylideneamino)biphenyl-4-yl)methanethiol (CF(3)-C(6)H(4)-CH═N-C(6)H(4)-C(6)H(4)-CH(2)-SH, TFBABPMT) and (4'-(4-cyanobenzylideneamino)biphenyl-4-yl)methanethiol (CN-C(6)H(4)-CH═N-C(6)H(4)-C(6)H(4)-CH(2)-SH, CBABPMT), self-assembled on Au(111) to form highly ordered monolayers, which was demonstrated by infrared reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). A nearly upright molecular orientation for CF(3)- and CN-terminated SAM was detected by near edge X-ray absorption fine structure (NEXAFS) measurements. Afterward, the acidic catalyzed hydrolysis was carried out in chloroform or an aqueous solution of acetic acid (pH = 3). Systematic studies of this hydrolysis process for CN-terminated SAM in acetic acid at 25 °C were performed by NEXAFS measurements. It was found that about 30% of the imine double bonds gradually cleaved in the first 40 min. Subsequently, a larger hydrolysis rate was observed due to the freer penetration of acetic acid in the SAM and resultant more open molecular packing. Furthermore, the molecular orientation in mixed SAMs did not change during the whole hydrolysis process. This partially hydrolyzed surface contains a controlled amount of free amines/ammonium ions which can be used for further chemical modifications.     

Effect of ring substitution position on the structural conformation of mercaptobenzoic acid self-assembled monolayers on Au(111)

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, photoemission spectroscopy (PES), and contact angle measurements have been used to examine the structure and bonding of self-assembled monolayers (SAMs) prepared on Au(111) from the positional isomers of mercaptobenzoic acid (MBA). The isomer of MBA and solvent chosen in SAM preparation has considerable bearing upon film morphology. Carbon K-edge NEXAFS measurements indicate that the monomers of 2-, 3-, and 4-MBA have well-defined orientations within their respective SAMs. Monomers of 3- and 4-MBA assume an upright orientation on the Au substrates in monolayers prepared using an acetic acid in ethanol solvent. The aryl ring and carboxyl group of these molecules are tilted from the surface normal by a colatitudal angle of approximately 30 degrees . Preparation of 4-MBA SAMs using pure ethanol solvent, a more traditional means of synthesis, had no appreciable effect upon the monomer orientation. Nonetheless, S(2p) PES measurements illustrate that it results in extensive bilayer formation via carboxyl group hydrogen-bonding between 4-MBA monomers. In 2-MBA monolayers prepared using acetic acid/ethanol solvent, the monomers adopt a more prostrate orientation on the Au substrates, in which the aryl ring and carboxyl group of the molecules are tilted approximately 50 degrees from the surface normal. This configuration is consistent with an interaction between both the mercaptan sulfur and carboxyl group of 2-MBA with the underlying substrate. S(2p) and C(1s) PES experiments provide supporting evidence for a bidentate interaction between 2-MBA and Au(111).     

Grafting of monocarboxylic substituted polychlorotriphenylmethyl radicals onto a COOH-functionalized self-assembled monolayer through copper (II) metal ions

A monocarboxylic substituted polychlorotriphenylmethyl radical (PTMCOOH) has been grafted onto a COOH-functionalized SAM (mercaptohexadecanoic acid, MHDA SAM), using copper (II) metal ions as linkers between the carboxyl groups of the SAM and the ligand. The metal-radical adlayer has been characterized thoroughly using different surface analysis techniques, such as contact angle, IRRAS, XPS, SPR, ToF-SIMS, SFM, and NEXAFS. The magnetic character was confirmed by EPR. The density of unoccupied states was investigated using X-ray absorption spectroscopy. A low-energy peak in the NEXAFS spectrum directly revealed the presence of partially occupied electronic levels, thus proving the open-shell character of the grafted ligands. SEM measurements on a laterally patterned sample prepared by muCP of MHDA in a matrix of hexadecane thiolate (a CH 3-terminated SAM) was performed to demonstrate that the metal-assisted anchoring of the open-shell ligand occurs selectively on the COOH terminated SAM. These results represent an easy and new approach to anchor organic radicals on surfaces and constitute a first step toward the growth of magnetic metal-organic radical-based frameworks on solid substrates.     

Comprehensive investigation of self-assembled monolayer formation on ferromagnetic thin film surfaces

We report a simple, universal method for forming high surface coverage SAMs on ferromagnetic thin (< or =100 nm) films of Ni, Co, and Fe. Unlike previous reports, our technique is broadly applicable to different types of SAMs and surface types. Our data constitutes the first comprehensive examination of SAM formation on three different ferromagnetic surface types using two different surface-binding chemistries (thiol and isocyanide) under three different preparation conditions: (1) SAM formation on electroreduced films using a newly developed electroreduction approach, (2) SAM formation on freshly evaporated surfaces in the glovebox, and (3) SAM formation on films exposed to atmospheric conditions beforehand. The extent of SAM formation for all three conditions was probed by cyclic voltammetry for surfaces functionalized with either (11-thiolundecyl)ferrocene (Fc-(CH2) 11-SH) or (11-isocyanoundecyl)ferrocene (Fc-(CH2) 11-NC). SAM formation was also probed for straight-chain molecules, hexadecanethiol and hexadecaneisocyanide, with contact angle measurements, X-ray photoelectron spectroscopy, and reflection-absorption infrared spectroscopy (RAIRS). The results show that high surface coverage SAMs with low surface-oxide content can be achieved for thin, evaporated Ni and Co films using our electroreduction process with thiols. The extent of SAM formation on electroreduced films is comparable to what has been observed for SAMs/Au and to what we observe for SAMs/Ni, Co, and Fe samples prepared in the glovebox.     

Room-temperature isolation of V(benzene)2 sandwich clusters via soft-landing into n-alkanethiol self-assembled monolayers

The adsorption state and thermal stability of V(benzene)2 sandwich clusters soft-landed onto a self-assembled monolayer of different chain-length n-alkanethiols (Cn-SAM, n = 8, 12, 16, 18, and 22) were studied by means of infrared reflection absorption spectroscopy (IRAS) and temperature-programmed desorption (TPD). The IRAS measurement confirmed that V(benzene)2 clusters are molecularly adsorbed and maintain a sandwich structure on all of the SAM substrates. In addition, the clusters supported on the SAM substrates are oriented with their molecular axes tilted 70-80 degrees off the surface normal. An Arrhenius analysis of the TPD spectra reveals that the activation energy for the desorption of the supported clusters increases linearly with the chain length of the SAMs. For the longest chain C22-SAM, the activation energy reaches approximately 150 kJ/mol, and the thermal desorption of the supported clusters can be considerably suppressed near room temperature. The clear chain-length-dependent thermal stability of the supported clusters observed here can be explained well in terms of the cluster penetration into the SAM matrixes.     

Characterizing the molecular order of phosphonic acid self-assembled monolayers on indium tin oxide surfaces

Self-assembled monolayers (SAMs) of alkanephosphonic acids with chain lengths between 8 and 18 carbon units were formed on thin films of indium tin oxide (ITO) sputter-deposited on silicon substrates with 400 nm thermally grown SiO(2). The silicon substrates, while not intended for use in near-IR or visible optics applications, do provide smooth surfaces that permit systematic engineering of grain size and surface roughness as a function of the sputter pressure. Argon sputter pressures from 4 to 20 mTorr show systematic changes in surface morphology ranging from smooth, micrometer-sized grain structures to <50 nm grains with 3× higher surface roughness. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments are conducted for alkanephosphonic acids deposited on these wide range of ITO surfaces to evaluate the effects of these morphological features on monolayer ordering. Results indicate that long-chain SAMs are more highly ordered, and have a smaller tilt angle, than short-chain SAMs. Surprisingly, the 1-octadecyl phosphonic acids maintain their order as the lateral grain dimensions of the ITO surface shrink to ～50 nm. It is only when the ITO surface roughness becomes greater than the SAM chain length (～15 ?) that SAMs are observed to become relatively disordered.     

Tailoring the Cu(100) work function by substituted benzenethiolate self-assembled monolayers

The structure and electronic interface properties of five differently substituted benzenethiol based self-assembled monolayers (SAMs) on Cu(100) have been studied by means of low energy electron diffraction, thermal desorption spectroscopy, X-ray absorption spectroscopy (NEXAFS), and UV photoelectron spectroscopy. Because highly ordered SAMs are formed of which lateral density had been precisely determined, effective molecular dipole moments were derived from the measured work function shifts. These values are compared with gas phase dipole moments computed by quantum chemical calculations for the individual thiol molecules considering the molecular orientation determined from NEXAFS data. Furthermore, this comparison yields clear evidence for a coverage dependent depolarization effect of the adsorbed molecules within the SAMs.     

Chemical grafting of biphenyl self-assembled monolayers on ultrananocrystalline diamond

We have investigated the formation of self-assembled monolayers (SAMs) of 4'-nitro-1,1-biphenyl-4-diazonium tetrafluoroborate (NBD) onto ultrananocrystalline diamond (UNCD) thin films. In contrast to the common approach to modify diamond and diamond-like substrates by electrografting, the SAM was formed from the saturated solution of NBD in acetonitrile by pure chemical grafting. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and near edge X-ray absorption fine structure spectroscopy (NEXAFS) have been used to verify the direct covalent attachment of the 4'-nitro-1,1-biphenyl (NB) SAM on the diamond substrate via stable C-C bonds and to estimate the monolayer packing density. The results confirm the presence of a very stable, homogeneous and dense monolayer. Additionally, the terminal nitro group of the NB SAM can be readily converted into an amino group by X-ray irradiation as well as electrochemistry. This opens the possibility of in situ electrochemical modification as well as the creation of chemical patterns (chemical lithography) in the SAM on UNCD substrates and enables a variety of consecutive chemical functionalization for sensing and molecular electronics applications.