Identification of transitions into Rydberg states in the X-ray absorption spectra of condensed long-chain alkanes

Fluorine-Rich Zinc Oxoclusters as Extreme Ultraviolet Photoresists: Chemical Reactions and Lithography Performance

The absorption of extreme ultraviolet (EUV) radiation by a photoresist strongly depends on its atomic composition. Consequently, elements with a high EUV absorption cross section can assist in meeting the demand for higher photon absorbance by the photoresist to improve the sensitivity and reduce the photon shot noise induced roughness. In this work, we enhanced the EUV absorption of the methacrylic acid ligands of Zn oxoclusters by introducing fluorine atoms. We evaluated the lithography performance of this fluorine-rich material as a negative tone EUV photoresist along with extensive spectroscopic and microscopic studies, providing deep insights into the underlying mechanism. UV-vis spectroscopy studies demonstrate that the presence of fluorine in the oxocluster enhances its stability in the thin films to the ambient atmosphere. However, the EUV photoresist sensitivity (D ₅₀) of the fluorine-rich oxocluster is decreased compared to its previously studied methacrylic acid analogue. Scanning transmission X-ray microscopy and in situ X-ray photoelectron spectroscopy in combination with FTIR and UV-vis spectroscopy were used to gain insights into the chemical changes in the material responsible for the solubility switch. The results support decarboxylation of the ligands and subsequent radical-induced polymerization reactions in the thin film upon EUV irradiation. The rupture of carbon-fluorine bonds via dissociative electron attachment offers a parallel way of generating radicals. The mechanistic insights obtained here will be applicable to other hybrid materials and potentially pave the way for the development of EUV materials with better performance.

Additive Lithography-Organic Monolayer Patterning Coupled with an Area-Selective Deposition

The combination of area-selective deposition (ASD) with a patternable organic monolayer provides a versatile additive lithography platform, enabling the generation of a variety of nanoscale feature geometries. Stearate hydroxamic acid self-assembled monolayers (SAMs) were patterned with extreme ultraviolet (λ = 13.5 nm) or electron beam irradiation and developed with ASD to achieve line space patterns as small as 50 nm. Density functional theory was employed to aid in the synthesis of hydroxamic acid derivatives with optimized packing density to enhance the imaging contrast and improve dose sensitivity. Near-edge X-ray absorption fine structure spectroscopy and infrared spectroscopy reveal that the imaging mechanism is based on improved deposition inhibition provided by the cross-linking of the SAM to produce a more effective barrier during a subsequent deposition step. With patterned substrates composed of coplanar copper lines and silicon spacers, hydroxamic acids selectively formed monolayers on the metal portions and could undergo a pattern-wise exposure followed by ASD in the first combination of a patternable monolayer with ASD. This material system presents an additional capability compared to traditional ASD approaches that generally reflect a starting patterned surface. Furthermore, this bottoms-up additive approach to lithography may be a viable alternative to subtractive nanoscale feature generation.

Spectroscopic imaging of surfaces-Sum frequency generation microscopy (SFGM) combined with compressive sensing (CS) technique

Surface chemistry is notoriously difficult to study, in part, due to the decreased number of molecules that contribute to the properties compared to the bulk phase but often has significant effects on the chemical activity of the material. This is especially true in topics such as corrosion, catalysis, wetting, and many others in nature and industry. Sum frequency generation (SFG) spectroscopy was developed for interface studies due to its high molecular selectivity and surface sensitivity, which is quite useful to study the effects of structural inhomogeneity in microscopy. Compressive sensing (CS) combined with SFG spectroscopy minimizes the imaging time while still producing quality images. Selected systems are presented here to demonstrate the capability of CS-SFG microscopy. CS-SFG microscopy successfully distinguished the static monolayer molecular mixtures, the orientations and adsorption of adsorbed molecules by the dip-coating technique, and the localized CO behaviors on polycrystalline Pt electrodes. Further discussion includes dynamic imaging as a future direction in CS-SFG microscopy. As materials and surfaces become more complex, imaging with chemical contrast becomes indispensable to understanding their performance and CS-SFG microscopy seems highly beneficial in this respect.

A dithiocarbamate anchoring group as a flexible platform for interface engineering

The molecular organization and electronic properties of dithiocarbamate (DTC) anchored self-assembled monolayers (SAMs) linked to Au(111) substrates are studied by a combination of X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and state-of-the-art density functional theory calculations. For that, several piperidine/piperazine precursors with different architecture and substitution patterns are selected. The presented data show that the DTC anchor provides a useful building block for monomolecular self-assembly on coinage metals with both sulfur atoms bonded to the substrate in a way similar to what is usually observed for the more commonly applied thiolate docking group. The combination of the DTC group with the quite flexible piperidine/piperazine cyclic linkers results in a dense molecular packing with an upright orientation of the terminal moieties. The latter comprise phenyl rings bearing various substituents, which enables tuning the interfacial dipole over a wide range. Simulations on two prototypical DTC-docked SAMs help to better understand the experimental observations and provide insight into the local origin of the SAM-induced shifts in the electrostatic energy. In particular, a comparison of measured and simulated XP spectra reveals the significant contribution of the DTC group to the interfacial dipole.

Triptycene-terminated thiolate and selenolate monolayers on Au(111)

To study the implications of highly space-demanding organic moieties on the properties of self-assembled monolayers (SAMs), triptycyl thiolates and selenolates with and without methylene spacers on Au(111) surfaces were comprehensively studied using ultra-high vacuum infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy and thermal desorption spectroscopy. Due to packing effects, the molecules in all monolayers are substantially tilted. In the presence of a methylene spacer the tilt is slightly less pronounced. The selenolate monolayers exhibit smaller defect densities and therefore are more densely packed than their thiolate analogues. The Se-Au binding energy in the investigated SAMs was found to be higher than the S-Au binding energy.

Scalable low-cost fabrication of disposable paper sensors for DNA detection

Controlled integration of features that enhance the analytical performance of a sensor chip is a challenging task in the development of paper sensors. A critical issue in the fabrication of low-cost biosensor chips is the activation of the device surface in a reliable and controllable manner compatible with large-scale production. Here, we report stable, well-adherent, and repeatable site-selective deposition of bioreactive amine functionalities and biorepellant polyethylene glycol-like (PEG) functionalities on paper sensors by aerosol-assisted, atmospheric-pressure, plasma-enhanced chemical vapor deposition. This approach requires only 20 s of deposition time, compared to previous reports on cellulose functionalization, which takes hours. A detailed analysis of the near-edge X-ray absorption fine structure (NEXAFS) and its sensitivity to the local electronic structure of the carbon and nitrogen functionalities. σ*, π*, and Rydberg transitions in C and N K-edges are presented. Application of the plasma-processed paper sensors in DNA detection is also demonstrated.

Helical versus all-trans conformations of oligo(ethylene glycol)-terminated alkanethiol self-assembled monolayers

The complex mixture of conformational states exhibited by oligo(ethylene glycol)-terminated alkanethiols on Ag and Au surfaces is explored by polarization-dependent X-ray absorption spectroscopy. Three self-assembled monolayers (SAMs) with known helical or all-trans conformations are used as references to characterize a SAM with unknown conformations. This case study is used as a prototype for developing a systematic framework to extract the conformations of SAMs from the polarization dependence of several orbitals. In the case at hand, these are associated with the C-H/Rydberg bonds of the alkane, the C-H/Rydberg bonds of ethylene glycol, and the C-C bonds of the backbone. The C-H/Rydberg orbitals of the alkane and ethylene glycol are distinguished via the chemical shift of the corresponding C 1s core levels.

Electrical and physical characterization of bilayer carboxylic acid-functionalized molecular layers

We have used flip chip lamination (FCL) to form monolayer and bilayer molecular junctions of carboxylic acid-containing molecules with Cu atom incorporation. Carboxylic acid-terminated monolayers are self-assembled onto ultrasmooth Au by using thiol chemistry and grafted onto n-type Si. Prior to junction formation, monolayers are physically characterized by using polarized infrared absorption spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy, confirming the molecular quality and functional group termination. FCL was used to form monolayer junctions onto H-terminated Si or bilayer junctions of carboxylic acid monolayers on Au and Si. From the electrical measurements, we find that the current through the junction is attenuated as the effective molecular length within the junction increases, indicating that molecules are electrically active within the junction. We find that the electronic transport through the bilayer junction saturates at very thick effective distances possibly because of another electron-transport mechanism that is not nonresonant tunneling as a result of trapped defects or sequential tunneling. In addition, bilayer junctions are fabricated with and without Cu atoms, and we find that the electron transport is not distinguishably different when Cu atoms are within the bilayer.

Effects of self-assembled monolayer structural order, surface homogeneity and surface energy on pentacene morphology and thin film transistor device performance

A systematic study of six phosphonic acid (PA) self-assembled monolayers (SAMs) with tailored molecular structures is performed to evaluate their effectiveness as dielectric modifying layers in organic field-effect transistors (OFETs) and determine the relationship between SAM structural order, surface homogeneity, and surface energy in dictating device performance. SAM structures and surface properties are examined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, contact angle goniometry, and atomic force microscopy (AFM). Top-contact pentacene OFET devices are fabricated on SAM modified Si with a thermally grown oxide layer as a dielectric. For less ordered methyl- and phenyl-terminated alkyl ~(CH₂)₁₂ PA SAMs of varying surface energies, pentacene OFETs show high charge carrier mobilities up to 4.1 cm² V^-1 s^-1. It is hypothesized that for these SAMs, mitigation of molecular scale roughness and subsequent control of surface homogeneity allow for large pentacene grain growth leading to high performance pentacene OFET devices. PA SAMs that contain bulky terminal groups or are highly crystalline in nature do not allow for a homogenous surface at a molecular level and result in charge carrier mobilities of 1.3 cm² V^-1 s^-1 or less. For all molecules used in this study, no causal relationship between SAM surface energy and charge carrier mobility in pentacene FET devices is observed.

Mixed self-assembled monolayers of azobenzene photoswitches with trifluoromethyl and cyano end groups

Mixed self-assembled monolayers (SAMs) of alkanethiolates carrying azobenzene chromophores with either a trifluoromethyl or a cyano substituent have been studied. High-resolution x-ray photoelectron spectroscopy proves that the ratio of adsorbed molecules can be arbitrarily adjusted via the molar fractions in solution. As a function of these molar fractions core level shifts are observed which are attributed to local work-function changes. By simulating the electric dipole field distribution, the continuous core level shifts are ascribed to a homogeneous mixture of molecules with different end groups adsorbed on adjacent lattice sites. Near-edge x-ray absorption fine structure measurements reveal formation of well-ordered SAMs. Despite the difference in dipole moment of the end groups, the molecular tilt and twist angles are identical for both single-component SAMs and a 1:1 mixed SAM.

Two-dimensional superstructures and softened C-H stretching vibrations of cyclohexane on Rh(111): effects of preadsorbed hydrogen

Adsorption structures and interaction of cyclohexane molecules on the clean and hydrogen-preadsorbed Rh(111) surfaces were investigated using scanning tunneling microscopy, spot-profile-analysis low-energy electron diffraction, temperature-programmed desorption, and infrared reflection absorption spectroscopy (IRAS). Various ordered structures of adsorbed cyclohexane were observed as a function of hydrogen and cyclohexane coverages. When the fractional coverage (θ(H)) of preadsorbed hydrogen was below 0.8, four different commensurate or higher-order commensurate superstructures were found as a function of θ(H); whereas more densely packed incommensurate overlayers became dominant at higher θ(H). IRAS measurements showed sharp softened C-H vibrational peaks at 20 K, which originate from the electronic interaction between adsorbed cyclohexane and the Rh surface. The multiple softened C-H stretching peaks in each phase are due to the variation in the adsorption distance from the substrate. At high hydrogen coverages they became attenuated in intensity and eventually diminished at θ(H) = 1. The gradual disappearance of the soft mode correlates well with the structural phase transition from commensurate structures to incommensurate structures with increasing hydrogen coverage. The superstructure of adsorbed cyclohexane is controlled by the delicate balance between adsorbate-adsorbate and adsorbate-substrate interactions which are affected by preadsorbed hydrogen.

Electronic structure of genomic DNA: a photoemission and X-ray absorption study

The electronic structure of genomic DNA has been comprehensively characterized by synchrotron-based X-ray absorption and X-ray photoelectron spectroscopy. Both unoccupied and occupied states close to the Fermi level have been unveiled and attributed to particular sites within the DNA structure. A semiconductor-like electronic structure with a band gap of approximately 2.6 eV has been found at which the pi and pi* orbitals of the nucleobase stack make major contributions to the highest occupied and lowest unoccupied molecular orbitals, respectively, in agreement with previous theoretical predictions.

Molecular self-assembly at bare semiconductor surfaces: cooperative substrate-molecule effects in octadecanethiolate monolayer assemblies on GaAs(111), (110), and (100)

The structures of self-assembled monolayers formed by chemisorption of octadecanethiol onto the surfaces of GaAs(001), (110), (111-A)-Ga, and (111-B)-As have been characterized in detail by a combination of X-ray photoelectron, near-edge X-ray absorption fine structure, and infrared spectroscopies and grazing incidence X-ray diffraction. In all cases, the molecular lattices are ordered with hexagonal symmetry, even for the square and rectangular intrinsic substrate (001) and (110) lattices, and the adsorbate lattice spacings are all incommensurate with their respective intrinsic substrate lattices. These results definitively show that the monolayer organization is driven by intermolecular packing forces to assemble in a hexagonal motif, such as would occur in the approach to a limit for an energetically featureless surface. The accompanying introduction of strain into the soft substrate surface lattice via strong S substrate bonds forces the soft substrate lattice to compliantly respond, introducing quasi-2D strain. A notably poorer organization for the (111-A)-Ga case compared to the (111-B)-As and other faces indicates that that the Ga-terminated surface lattice is more resistant to adsorbate packing-induced stress. Overall, the results show that surface molecular self-assembly must be considered as a strongly cooperative process between the substrate surface and the adsorbate and that inorganic substrate surfaces should not be considered as necessarily rigid when strong intermolecular adsorbate packing forces are operative.

Chemistry at air/water interface versus reaction in a flask: tuning molecular conformation in thin films

Atomic force microscopy and X-ray reflectivity studies of cobalt stearate Langmuir-Blodgett (LB) films (CoStp) deposited from a preformed bulk sample on quartz substrates showed formation of a Volmer-Weber type monolayer but no multilayers as compared to the excellent multilayers of cobalt stearate films (CoStn) deposited at the air/water interface by the usual LB technique, in spite of both showing bidentate bridging type coordination of cobalt ions with the carboxylate group. The difference is attributed to existence of different headgroup conformers, observed from Fourier transform infrared (FTIR) studies. The CoStp films had a higher energy 'boat' conformation with linear O-Co-O linkage, whereas CoStn formed a low energy conformer with a bent O-Co-O configuration (bond angle of 105 degrees). Present results support the necessity of bidentate bridging coordination in multilayer deposition, but rejects its sufficiency by bringing out the crucial role played by air/water interface. Differences in surface pressure-molecular area isotherms and hydrocarbon tail-tail interactions (evident from FTIR spectra) of the films support the above statement. Methyl-methyl interactions observed in CoStn samples suggest hierarchy of supramolecular chemistry at the air/water interface in tuning the C-O-Co bond angle essential to satisfy the wetting condition with the substrate and subsequently form LB multilayers.

Correlation between the molecular structure and photoresponse in aliphatic self-assembled monolayers with azobenzene tailgroups

We have compared the structural and photoisomerization properties of self-assembled monolayers (SAMs) comprising either the trans or cis isomers of azobenzene terminated dithiolane with in-chain amide unit, viz., 4-(phenyldiazenyl)phenyl-4-(1,2-dithiolane-3-yl)-butylcarboxamide ( 1). These films were prepared on Au(111) from solutions of both isomers. Structure and composition of the SAMs were studied by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The photoresponse of the films was monitored in real time by ellipsometry. SAMs fabricated from the trans isomer were found to be densely packed and highly ordered. These films did not show any discernible photoresponse upon irradiation with UV light, which, under favorable conditions, triggers the trans- cis isomerization. In contrast, films prepared from solutions containing predominantly the cis isomer were loosely packed and mostly disordered but exhibited reversible photoreactivity. The results confirm that steric effects, i.e., available free volume, play a dominant role for the photoresponse of aliphatic SAMs bearing the photoactive azobenzene group. The crystal structure of 1 ( trans isomer) exhibits a row-like aggregation of neighboring molecules by weak hydrogen bonds and can be taken as a model for the arrangement of 1 in the monolayer films. Further, in addition to the surface coordination behavior, we have also mimicked the chemisorption of the 1,2-dithiolane moiety onto the gold substrate in molecular coordination chemistry in oxidative addition reactions with the zero-valent platinum complex [Pt(PPh 3) 4].

Effect of the phase states of self-assembled monolayers on pentacene growth and thin-film transistor characteristics

To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of organic thin-film transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and 90 degrees C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temperature. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.

On the importance of purity for the formation of self-assembled monolayers from thiocyanates

Assembly of dodecyl thiocyanate (C12SCN) from ethanol solution onto Au(111)/mica substrates was investigated by scanning tunneling microscopy (STM), near edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray photoelectron spectroscopy (XPS), and infrared reflection-absorption spectroscopy (IRRAS). Contrary to previous reports, thiolate monolayers formed by cleavage of the S-CN bond can be obtained whose quality is at least as good as that of self-assembled monolayers (SAMs) formed directly from the thiol analogue of C12SCN, dodecanethiol (C12SH). However, the achievable quality is strikingly dependent on the purity of the thiocyanate with even low levels of contamination impeding the formation of structurally well-defined monolayers.

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.

Characterization of monolayer formation on aluminum-doped zinc oxide thin films

The optical and electronic properties of aluminum-doped zinc oxide (AZO) thin films on a glass substrate are investigated experimentally and theoretically. Optical studies with coupling in the Kretschmann configuration reveal an angle-dependent plasma frequency in the mid-IR for p-polarized radiation, suggestive of the detection of a Drude plasma frequency. These studies are complemented by oxygen depletion density functional theory studies for the calculation of the charge carrier concentration and plasma frequency for bulk AZO. In addition, we report on the optical and physical properties of thin film adlayers of n-hexadecanethiol (HDT) and n-octadecanethiol (ODT) self-assembled monolayers (SAMs) on AZO surfaces using reflectance FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS), contact angle, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Our characterization of the SAM deposition onto the AZO thin film reveals a range of possible applications for this conducting metal oxide.

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.

Monomolecular films of phthalocyanines: formation, characterization, and expelling by alkanethiols

Adsorption of aluminum-2,3-naphthalocyanine (Al-PC) onto gold (111) substrate from the pure and mixed (with alkanethiols) solutions of the target molecules in ethanol was studied. The resulting films were characterized by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The adsorption from the pure solution resulted in formation of an Al-PC monolayer composed of the strongly inclined molecules. However, a presence of only one molar percent of a thiolated compound (alkanethiol) in the primary solution led to the complete expelling of the Al-PC molecules from the substrate and formation of a one-component alkylthiolate monolayer. The results indicate that an equilibrium formation of mixed monolayers of rodlike thiolated molecules and relatively large planar-geometry molecules, whose interaction with the metal surface is mainly provided by the pi-electron system, is difficult to achieve or is in most cases even impossible under equilibrium conditions and requires an introduction of additional anchor moieties (e.g., thiols) into these molecules, as has been demonstrated by successful coadsorption of hexadecanethiol and thiolated tetraphenylporphyrin.

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).

Radiation Damage to Alkyl Chain Monolayers on Semiconductor Substrates Investigated by Electron Spectroscopy

Monolayers of alkyl chains, attached through direct Si-C bonds to Si(111), via phosphonates to GaAs(100) surfaces, or deposited as alkyl-silane monolayers on SiO2, are investigated by ultraviolet and inverse photoemission spectroscopy and X-ray absorption spectroscopy. Exposure to ultraviolet radiation from a He discharge lamp, or to a beam of energetic electrons, leads to significant damage, presumably associated with radiation- or electron-induced H-abstraction leading to carbon-carbon double-bond formation in the alkyl monolayer. The damage results in an overall distortion of the valence spectrum, in the appearance of (occupied) states above the highest occupied molecular orbital of the alkyl molecule, and in a characteristic (unoccupied state) pi resonance at the edge of the carbon absorption peak. These distortions present a serious challenge for the interpretation of the electronic structure of the monolayer system. We show that extrapolation to zero damage at short exposure times eliminates extrinsic features and allows a meaningful extraction of the density of state of the pristine monolayer from spectroscopy measurements.

Controlled One-Dimensional Nanostructures in Poly(3-hexylthiophene) Thin Film for High-Performance Organic Field-Effect Transistors

With the aim of improving the field-effect mobilities in poly(3-hexylthiophene) (P3HT) thin film transistors, we controlled the nanostructures of P3HT thin film by changing the solvent vapor pressure in a spin-coating chamber during solidification. The transistors with P3HT thin films spin-coated under a high solvent vapor pressure (56.5 KPa), showing the one-dimensional nanowire morphologies, resulted in the relatively high field-effect mobilities (0.02 cm2/(V.s)) that are typically more than 1 order of magnitude higher than those prepared under ambient conditions, showing the featureless morphologies. This can be attributed to the higher solvent vapor pressure during film formation, providing the solvent is allowed to evaporate slowly and the degree of ordering within the P3HT crystalline domains is dramatically improved.

Linear dichroism in the X-ray absorption spectra of linear n-alkanes

The nature of the linear dichroism in the near-edge X-ray absorption fine structure (NEXAFS) spectra of linear n-alkanes is a matter of long-standing controversy. Linear dichroism in the carbon 1s --> sigma*(C-C) transition has been interpreted within a building block model and a molecular orbital model, leading to two different descriptions for the angular dependence of this feature. When used for measurement of molecular orientation, the application of these two different models will lead to different results. We have explored the linear dichroism in the carbon 1s NEXAFS spectra of single crystals of the linear n-alkane hexacontane (n-C60H122). An analysis of the angular dependence in this spectrum shows that the transition dipole moment associated with the carbon 1s --> sigma*(C-C) transition is oriented along the macromolecular chain axis, contradicting the predictions of the building block model. However, other transitions are observed in the sigma*(C-H) and the sigma*(C-C) bands that are orthogonal to the dominant transitions for each band. We also observe that radiation damage can be manifest in the form of molecular reorientation in highly ordered organic thin films.

Matrix effects in the carbon 1s near edge x-ray absorption fine structure spectra of condensed alkanes

The carbon 1s near edge x-ray absorption fine structure (NEXAFS) spectra of simple gaseous alkane molecules differ from the spectra of the same alkane molecules in the condensed phase. The origin of these large, systematic differences is poorly understood. The NEXAFS spectra of gaseous alkanes are interpreted as a progression of core-->Rydberg transitions with distinctive vibronic structure. The interpretation of the NEXAFS spectra of condensed phase alkanes is varied. Specifically, the degree of Rydberg character in the pre-edge core excited states of condensed alkanes is controversial. We determined the character of core excited states in condensed alkanes with a combination of experiment and computational study. From this, we have determined the nature of matrix effects for these species. The high-resolution carbon 1s NEXAFS spectrum of gaseous neopentane is dramatically different from its condensed phase spectrum, a striking illustration of the dramatic spectroscopic changes that occur upon condensation. High quality ab initio calculations of a cluster designed to model the solid phase environment provide definitive evidence for the reduction of Rydberg character and support the assignment of sigma*C-H) valence character in the pre-edge features in the NEXAFS spectra of condensed alkanes.

Surface Potential Switching by Metal Ion Complexation/Decomplexation Using Bipyridinethiolate Monolayers on Gold

Surface potential switching on gold(111) surfaces is induced by complexation/decomplexation reactions of a bipyridine (BP) derivative and palladium(II) chloride, as observed by Kelvin probe force microscopy (KFM). On the basis of the theoretical predictions, a 4-(5-phenylethynyl-2,2'-bipyridine-5'-yl-ethynyl)benzenethiol (PhBP) derivative was synthesized and used as an active monolayer to catch transition metal ions. By using the microcontact printing (CP) technique, micron-size patterned PhBP monolayers, which act as effective hosts to coordinate palladium(II) chloride, were prepared on gold(111) surfaces. The KFM signal decreases by complexation of the Pd(II) chloride in PhBP monolayers and is recovered by removal of Pd ions using an ethylenediamine solution, as confirmed by X-ray photoelectron spectroscopy. This process is reversible, indicating that the surface potential switching is realized by complexation/decomplexation of Pd(II). A CP PhBP monolayer, when it detects the target palladium ion, shows sensitivity for the picomolar level detection judged from surface potential changes in KFM measurements. The dipole moment estimated by the surface potentials is much smaller than the calculated value, indicating that mechanisms for the reduction of the surface dipole moment exist in real monolayers prepared by the CP method.

Molecular Self-Assembly at Bare Semiconductor Surfaces: Preparation and Characterization of Highly Organized Octadecanethiolate Monolayers on GaAs(001)

Through rigorous control of preparation conditions, organized monolayers with a highly reproducible structure can be formed by solution self-assembly of octadecanethiol on GaAs (001) at ambient temperature. A combination of characterization probes reveal a structure with conformationally ordered alkyl chains tilted on average at 14 +/- 1 degrees from the surface normal with a 43 +/- 5 degrees twist, a highly oleophobic and hydrophobic ambient surface, and direct S-GaAs attachment. Analysis of the tilt angle and film thickness data shows a significant mismatch of the average adsorbate molecule spacings with the spacings of an intrinsic GaAs(001) surface lattice. The monolayers are stable up to approximately 100 degrees C and exhibit an overall thermal stability which is lower than that of the same monolayers on Au[111] surfaces. A two-step solution assembly process is observed: rapid adsorption of molecules over the first several hours to form disordered structures with molecules lying close to the substrate surface, followed by a slow densification and asymptotic approach to final ordering. This process, while similar to the assembly of alkanethiols on Au[111], is nearly 2 orders of magnitude slower. Finally, despite differences in assembly rates and the thermal stability, exchange experiments with isotopically tagged molecules show that the octadecanethiol on GaAs(001) monolayers undergo exchange with solute thiol molecules at roughly the same rate as the corresponding exchanges of the same monolayers on Au[111].

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.

Adsorption states of dialkyl ditelluride autooxidized monolayers on Au(111)

Organic ditellurides (R2Te2 where R = n-butyl (C4), n-octyl (C8), and n-cetyl (C16)) were synthesized, and their adsorption states after oxidation on Au(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), theoretical analyses, near-edge X-ray absorption fine structure (NEXAFS) measurements, contact angle measurements, and atomic force microscopy (AFM). The obtained results show that dialkyl ditellurides form autooxidized monolayers (AMs) on the surfaces under ambient conditions and that the oxidation process is accelerated by ambient light. XPS, UPS, and theoretical analyses suggest that the autooxidized ditelluride species consist of polymers or oligomers with Te-O-Te-O network structures stabilized by oxygen bridges between tellurium molecules following the cleavage of tellurium-gold bonds. NEXAFS and contact angle measurements indicate that the average tilt angles of the alkyl chains from the surface normal are smaller for the AMs of dialkyl ditellurides having longer alkyl chains. AFM measurements show defects and roughness features around a few angstroms in height on the surfaces of the dialkyl ditelluride AMs.

Surface-induced conformational changes in poly(3-hexylthiophene) monolayer films

With the aim of elucidating the surface-induced molecular ordering in regioregular poly(3-hexylthiophene) (P3HT) monolayer films, we have controlled the intermolecular interactions at the interface between P3HT and the insulator substrate by using self-assembled monolayers (SAMs) functionalized with two kinds of groups (-NH2 and -CH3). We have found that, depending on the surface properties of such modified insulator substrates, the P3HT chains in the monolayer films can adopt two different conformations (edge-on and face-on). This surprising variation in chain conformation arises because of the specific interactions of the P3HT chains with the modified insulator substrates, which can be explained in terms of the following factors: the unshared electron pairs of the SAM end groups (in the -NH2 system), the pi-H interactions between the thienyl backbone bearing pi systems and the H (hydrogen) atoms of the SAM end groups, and interdigitation between the alkyl chains of P3HT and the alkyl chains of the SAMs (in the -NH2 system).

X-ray Absorption Spectromicroscopy Studies for the Development of Lithography with a Monomolecular Resist

Soft X-ray absorption microscopy was applied to image and characterize molecular patterns produced by electron irradiation of aliphatic and aromatic thiol-derived self-assembled monolayers (SAMs) on Au substrates. The measurements were performed at all relevant absorption edges. The fabricated patterns could be clearly imaged with a lateral resolution better than 150 nm, which, for example, allowed us to distinguish a fine structure of 1 microm features. The X-ray absorption microspot spectra derived from different areas of the SAM patterns provided specific chemical information on pristine and irradiated areas and unexpected features in these patterns. The quality of the microspot spectra is comparable with that of the analogous X-ray absorption spectra acquired with standard equipment from homogeneous SAMs. In particular, a chemical transformation of the functional tail groups within the irradiated areas of the patterned aromatic SAMs could be directly monitored.

Rydberg−Valence Mixing in the Carbon 1s Near-Edge X-ray Absorption Fine Structure Spectra of Gaseous Alkanes

We have acquired high-resolution carbon 1s near-edge X-ray absorption fine structure (NEXAFS) spectra of methane, ethane, propane, isobutane, and neopentane. These experimental measurements are complemented by high-quality ab initio calculations, performed with the improved virtual orbital approximation. The degree and character of Rydberg-valence mixing in the preedge of the NEXAFS spectra of these species is explored. Significant Rydberg-valence mixing only occurs when there are excited states of valence sigma(C-H) character that have the appropriate symmetry to interact with excited states of Rydberg character. Our results show that this mixing is only present when there are C-H bonds to the core excited carbon atom.

Self-Assembled Monolayers of Alkaneselenolates on (111) Gold and Silver

Self-assembled monolayers (SAMs) formed from didodecyl diselenide (C12SeSeC12) and didodecyl selenide (C12SeC12) on (111) Au and Ag substrates were extensively characterized by several complementary techniques. C12SeSeC12 was found to form contamination-free, densely packed, and well-ordered C12Se SAMs on both substrates, whereas the adsorption of C12SeC12 occurred only on Au and resulted in the formation of a SAM-like C12SeC12 film with a low packing density and a conformational disorder. The properties of the C12Se SAMs were compared with those of dodecanethiolate (C12S) SAMs. The packing density, orientational order, and molecular inclination in C12Se/Au and C12S/Au were found to be very similar. In contrast, C12Se/Ag exhibited significantly lower packing density, a lower degree of the conformational and orientational order, and a larger molecular inclination than C12S/Ag. The results suggest a sp3 bonding configuration for the selenium atom on Au and Ag and indicate a larger corrugation of the headgroup-substrate binding energy surface in C12Se/Ag than in C12S/Ag.