Self-assembled monolayers on Au microelectrodes

Long chain alkanethiols self-assembled monolayers (SAMs) formed on Au microelectrodes showed higher sensitivity towards defects than the same monolayers on macroelectrodes. The analysis of cyclic voltammetry and electrochemical impedance spectroscopy (EIS) experiments performed on covered microelectrodes were consistent with the formation of pinholes of about 10 nm in diameter. Moreover, the EIS data exhibited a specific behavior that was interpreted invoking the short circuiting of the pinholes impedance by the surrounding surface of the microelectrode in the high frequency domain, whereas in the low frequencies, the surface covered by the SAM was assume to act as an insulator.     

Thiol-terminated monolayers on oxide-free Si: assembly of semiconductor-alkyl-S-metal junctions

Self-assembled monolayers formed by thermal hydrosilylation of a trifluoroacetyl-protected alkenylthiol on Si-H surfaces, followed by removal of the protecting groups, yield essentially oxide-free monolayers suitable for the formation of Si-C11H22-S-Hg and Si-C11H22-S-Au junctions in which the alkyl chains are chemically bound to the silicon surface (via Si-C bonds) and the metal electrode (via Hg-S or Au-S bonds). Two barriers to charge transport are present in the system: at low bias the current is temperature activated and hence limited by thermionic emission over the Schottky barrier in the silicon, whereas as at high bias transport is limited by tunneling through the organic monolayer. The thiol-terminated monolayer on oxide-free silicon provides a well-characterized system allowing a careful study of the importance of the interfacial bond to the metal electrode for current transport through saturated molecules.     

DC- and RF-GD-OES measurements of adsorbed organic monolayers on copper

Our direct current (DC)- and radiofrequency glow discharge optical emission spectroscopy (RF-GD-OES) measurements of adsorbed organic monolayers were inspired by the work of Shimizu et al., who presented the first example of depth profile analysis of an adsorbed monolayer by RF-GD-OES in 2004. The great potential of RF-GD-OES for analyses of layers with thicknesses in the subnanometer range was surprising. Shimizu et al. discussed not only the qualitative detection of atoms of the organic monolayer (C, H, N, S), but also the determination of the different orientation of the molecules relative to the surface due to a significant peak sequence. This latter assumption was questioned in the analytical community. We intend to demonstrate the potential of the GD-OES technique for surface analysis in terms of reliability and reproducibility by using an advanced vacuum instrumentation and presputtering with silicon. It will be shown that comparable measurements can be reproduced not only with RF-GD-OES but, above all, also with DC-GD-OES. The experimental steps to adsorb thiourea molecules on a copper substrate are described in detail. Further experiments with other organic molecules, e.g. benzotriazole (BTA) or benzothiazole (BTH), disprove the predicted correlation between the orientation of the molecules relative to the surface and the occurrence of peak separation. Ultimately, a quantification of compounds of the organic monolayer in the case of adsorbed thiourea is achieved.      

Hybrids of organic molecules and flat, oxide-free silicon: high-density monolayers, electronic properties, and functionalization

Since the first report of Si-C bound organic monolayers on oxide-free Si almost two decades ago, a substantial amount of research has focused on studying the fundamental mechanical and electronic properties of these Si/molecule surfaces and interfaces. This feature article covers three closely related topics, including recent advances in achieving high-density organic monolayers (i.e., atomic coverage >55%) on oxide-free Si(111) substrates, an overview of progress in the fundamental understanding of the energetics and electronic properties of hybrid Si/molecule systems, and a brief summary of recent examples of subsequent functionalization on these high-density monolayers, which can significantly expand the range of applicability. Taken together, these topics provide an overview of the present status of this active area of research.     

Self-assembled monolayers of alkylsiloxanes on SrTiO3 substrates

The formation and structure of alkyltrichlorosilanes on several types of SrTiO₃ substrates have been studied. The silanes adsorb spontaneously from a hexadecane solution and form monolayers on all the substrates used. Characterization has been performed by atomic force microscopy, wettability, angle resolved X-ray photoelectron spectroscopy, reflection absorption infrared spectroscopy, and spectroscopic ellipsometry. It was found that highly ordered and densely packed monolayers were formed below a certain temperature.     

Self-assembled monolayers of alkylphosphonic acid on GaN substrates

In this paper we describe the formation and characterization of self-assembled monolayers of octadecylphosphonic acid (ODPA) on epitaxial (0001) GaN films on sapphire. By immersing the substrate in its toluene solution, ODPA strongly adsorbed onto UV/O 3-treated GaN to give a hydrophobic surface. Spectroscopic ellipsometry verified the formation of a well-packed monolayer of ODPA on the GaN substrate. In contrast, adsorption of other primarily substituted hydrocarbons (C n H 2 n+1 X; n = 16-18; X = -COOH, -NH 2, -SH, and -OH) offered less hydrophobic surfaces, reflecting their weaker interaction with the GaN substrate surfaces. A UV/O 3-treated N-polar GaN had a high affinity to the -COOH group in addition to ODPA, possibly reflecting the basic properties of the surface. These observations suggested that the molecular adsorption was primarily based on hydrogen bond interactions between the surface oxide layer on the GaN substrate and the polar functional groups of the molecules. The as-prepared ODPA monolayers were desorbed from the GaN substrates by soaking in an aqueous solution, particularly in a basic solution. However, ODPA monolayers heated at 160 degrees C exhibited suppressed desorption in acidic and neutral aqueous solution maybe due to covalent bond formation between ODPA and the surface. X-ray photoelectron spectroscopy provided insight into the effect of the UV/O 3 treatment on the surface composition of the GaN substrate and also the ODPA monolayer formation. These results demonstrate that the surface of a GaN substrate can be tailored with organic molecules having an alkylphosphonic acid moiety for future sensor and device applications.     

Self-assembled monolayers at electrode metal surfaces

Neutral organic compounds, dissolved in an electrolyte in contact with an electrode, adsorb and form different monolayers which may range from dilute to compact films. In some instances, non-electroactive organic molecules are highly associated and form 2D condensed phases which are characterized by the presence of phase transitions. The occurence of these self-assembled monolayers is discussed on the basis of experimental results obtained at equilibrium as well as under dynamic conditions. Self-assembling depends on the relative magnitude of the interactions involving the surfactant, the solvent and the electrode. Adequate potential-step programmes have been successfully used to trigger the formation of the ordered phase. It is found that the kinetics are controlled by a nucleation and growth mechanism. According to the experimental conditions, a deterministic or stochastic behaviour is observed. The amplitude of the supersaturation, given by the surface free energy gap between the final and metastable states, is independently controlled by the potential, temperature and surfactant concentration. The classical nucleation theory allows the determination of key parameters such as the line tension, the radius and free energy of formation of the critical nucleus. Ion and electron transfer processes through condensed monolayers are also briefly described.     

Self-assembled monolayers of semifluorinated alkaneselenolates on noble metal substrates

Self-assembled monolayers (SAMs) formed from semifluorinated dialkyldiselenol (CF(3)(CF(2))(5)(CH(2))(2)Se-)(2) (F6H2SeSeH2F6) on polycrystalline Au(111) and Ag(111) were characterized by high-resolution X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, near edge X-ray absorption fine structure spectroscopy, scanning tunneling microscopy, and contact angle measurements. The Se-Se linkage of F6H2SeSeH2F6 was found to be cleaved upon the adsorption, followed by the formation of selenolate-metal bond. The resulting F6H2Se SAMs are well-ordered, densely packed, and contamination-free. The packing density of these films is governed by the bulky fluorocarbon part, which exhibits the expected helical conformation. A noncommensurate hexagonal arrangement of the F6H2Se molecules with an average nearest-neighbor spacing of about 5.8 +/- 0.2 A, close to the van der Waals diameter the fluorocarbon chain, was observed on Au(111). The orientation of the fluorocarbon chains in the F6H2Se SAMs does not depend on the substrate-the average tilt angle of these moieties was estimated to be about 21-22 degrees on both Au and Ag.     

Self-assembled monolayers of ferrocene-substituted biphenyl ethynyl thiols on gold

Homogeneous and mixed [with biphenylthiol (BPT)] self-assembled monolayers (SAMs) of ferrocene-substituted biphenyl ethynyl thiols (Fc) were prepared on Au(111) substrates and characterized by several complementary spectroscopic techniques. The mixed films were fabricated either by subsequent immersion of the substrates into the BPT and Fc solutions or by immersion of the substrate into a mixed solution of BPT and Fc. The first procedure resulted in the preparation of high-quality mixed SAMs, in which the Fc molecules were stochastically distributed in the BPT matrix and well-separated from each other. The portion of these molecules in such films could be precisely varied from ca. 7 to 42% by selection of the immersion time in the BPT solution. The films prepared from the mixed solution exhibited a phase separation between the Fc and BPT constituents. These films contained mostly the Fc molecules ( approximately 80-90%), showing, thus, a significant deviation from the relative content of the target molecules in the primary solution (a 1:1 ratio). This finding shows that the Fc molecules, when competing with BPT, preferably adsorb onto Au(111) substrate, suggesting a significant impact of the ferrocene groups onto the structure-building interactions responsible for molecular self-assembly.     

Self-assembled monolayers based on phenanthroline-gold(111) bonding

The self-assembly of long-alkyl-chain substituted phenanthroline derivatives on highly oriented pyrolitic graphite (HOPG) and gold(111) is compared. Whereas the adsorption on HOPG is controlled by the affinity of alkyl chains for the substrate, which leads to flat-lying adsorbed molecules, alignments of upright-oriented molecules are formed on gold(111). This situation is explained by the bonding of chelating species with gold(111) surfaces and by the pi-stacking interaction between conjugated moieties. This intermediate situation between strong thiol-like chemical bonding and the weak n-alkane-like physical adsorption opens the route toward laterally organized functional molecular assemblies.     

Self-assembled monolayers (SAMs) with photo-functionalities

Self-assembled monolayers (SAMs) of alkylthiol on metals, especially on gold, with photo-functionalities, such as photo-induced electron transfer, control of photo-electrochemical properties, control of electron transfer by photoisomerization, luminescence, and photo-patterning, are reviewed.     

Self-assembled monolayers of aromatic selenolates on noble metal substrates

Self-assembled monolayers (SAMs) formed from bis(biphenyl-4-yl) diselenide (BBPDSe) on Au(111) and Ag(111) substrates have been characterized by high-resolution X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, infrared reflection absorption spectroscopy, water contact angle measurements, and scanning tunneling microscopy (STM). BBPDSe was found to form contamination-free, densely packed, and well-ordered biphenyl selenolate (BPSe) SAMs on both Au and Ag. Spectroscopic data suggest very similar packing density, orientational order, and molecular inclination in BPSe/Au and BPSe/Ag. STM data give a similar intermolecular spacing of 5.3 +/- 0.4 A on both Au and Ag but exhibit differences in the exact arrangement of the BPSe molecules on these two substrates, with the (2 square root[3] x square root[3])R30 degrees and (square root[3] x square root[3])R30 degrees unit cells on Au and Ag, respectively. There is strong evidence for adsorbate-mediated substrate restructuring in the case of Au, whereas no clear statement on this issue can be made in the case of Ag. The film quality of the BPSe SAMs is superior to their thiol analogues, which is presumably related to a better ability of the selenolates to adjust the surface lattice of the substrate to the most favorable 2D arrangement of the adsorbate molecules. This suggests that aromatic selenolates represent an attractive alternative to the respective thiols.     

Self-assembled organic monolayers terminated in perfluoroalkyl pentafluoro-lambda(6)-sulfanyl (-SF5) chemistry on gold

Recently synthesized (Winter, R.; Nixon, P. G.; Gard, G. L.; Radford, D. H.; Holcomb, N. R.; Grainger, D. W. J. Fluorine Chem. 2001, 107, 23-30) SF5-terminated perfluoroalkyl thiols (SF5(CF2)nCH2CH2SH, where n = 2, 4, and 6) and a symmetric SF5-terminated dialkyl disulfide ([SF5-CH=CH-(CH2)8-S-]2) were assembled as thin films chemisorbed onto gold surfaces. The adsorbed monolayer films of these SF5-containing molecules on polycrystalline gold were compared using ellipsometry, contact angle, X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and infrared spectroscopy (FTIR) surface analytical methods. The resulting SF5-dialkyl disulfide monolayer film shows moderate angle dependence in depth-dependent XPS analysis, suggesting a preferentially oriented film. The SF5-terminated perfluoroalkyl thiols exhibit angular-dependent XPS compositional variance depending on perfluoroalkyl chain length, consistent with improved film assembly (increasingly hydrophobic, fewer defects, and more vertical chain orientation increasing film thickness) with increasing chain length. Tof-SIMS measurements indicate that both full parent ions for these film-forming molecules and the unique SF5 terminal group are readily detectable from the thin films without substantial contamination from other adsorbates.     

Si-C-bound alkyl chains on oxide-free Si: towards versatile solution preparation of electronic transport quality monolayers

We show that electronic transport quality alkyl chain mono-layers can be prepared from dilute solution, rather than from neat alkanes, and on Si (100) instead of (111) surfaces. High monolayer quality was deduced from XPS and from comparing current-voltage curves of Hg/alkyl/Si junctions with those for junctions with monolayers made from neat alkanes. XPS shows that limited surface oxidation does not harm the integrity of the monolayer. Solution preparation significantly widens the range of molecules that can be used for transport studies.     

Self-assembled monolayers of alkanethiolates on palladium are good etch resists

This paper describes microcontact printing (muCP) of long-chain alkanethiolates on palladium, followed by solution-phase etching with an iron(III)-based etchant, to make patterned structures. The commonly used soft-lithographic procedure for fabricating microstructures-muCP of SAMs on gold-has three shortcomings: a significant surface density of pinhole defects, substantial edge roughness, and incompatibility with processes used in CMOS fabrication. Microcontact printing on palladium gives fewer defects and smaller edge roughness than on gold, and is compatible with CMOS. The mechanism by which etch-resistant patterns are formed is different for palladium and gold. The Pd/S interfacial layer formed by the reaction of palladium films with sulfur-containing compounds provides good resistance to etches independently of the barrier to access the surface provided by the film of (CH2)n groups in the long-chain SAMs. This barrier is the basis of the etch resistance of SAMs on gold, but only supplements the etch resistance of the sulfur-containing interfacial layer on palladium. Characterization of the SAM formed from hexadecanethiol on palladium is described.     

Self-assembled silane monolayers: fabrication with nanoscale uniformity

Illustrating direct connections between surface chemical events and mechanical and topological characteristics of self-assembled monolayers derived from octadecyltrichlorosilane (OTS) adsorption on Si(100), layers prepared in the presence and absence of moisture have been characterized. Uniform and robust self-assembled monolayers are demonstrated provided the Si(100) surface is fully hydroxylated by treatment in piranha solution and the dried surface is exposed to OTS under strict anhydrous conditions. With nanoscale resolution, the uniform mechanical properties are confirmed by interfacial force microscopy while the uniform topological properties are evident in atomic force microscopy images. The monolayer character of the OTS coverage is confirmed by X-ray photoelectron spectroscopy, ellipsometry, and patterning experiments. Analogous surfaces, prepared in the presence of moisture, exhibit nonuniform topological and mechanical properties.     

Self-assembled monolayers (SAMs) for electrochemical sensing

The past, present, and future of the application of self-assembled monolayers (SAMs) in electroanalytical chemistry is reviewed. SAMs for electroanalytical applications have been introduced in the early 1990s and since then have been exploited for the detection of different species ranging from metal ions to biomolecules and microorganisms. This review describes the different types of monolayers, surfaces on which they have been assembled, the various analytes, which were determined, and the various electrochemical techniques employed. The prospective and perspectives of this topic are discussed.