Control of polymorphism in solution-processed organic thin film transistors by self-assembled monolayers

Polymorphism of organic semiconductor films is of key importance for the performance of organic thin film transistors(OTFTs).Herein, we demonstrate that the polymorphism of solution-processed organic semiconductors in thin film transistors can be controlled by finely tuning the surface nanostructures of substrates with self-assembled monolayers(SAMs). It is found that the SAMs of 12-cyclohexyldodecylphosphonic acid(CDPA) and 12-phenyldodecylphosphonic acid(Ph DPA) induce different polymorphs in the dip-coated films of 2-dodecyl[1]benzothieno[3,2-b][1]benzothiophene(BTBT-C12). The film of BTBT-C12 on CDPA exhibits field effect mobility as high as 28.1 cm~2 V~(-1) s~(-1) for holes, which is higher than that of BTBT-C12 on Ph DPA by three times. The high mobility of BTBT-C12 on CDPA is attributable to the highly oriented films of BTBT-C12 with a reduced in-plane lattice and high molecular alignment.     

Micropatterning of lanthanum-based oxide thin film on self-assembled monolayers

We studied the direct micropatterning of a lanthanum-based thin film on a template of self-assembled monolayers in an aqueous solution at 80 degrees C. The template composed of silanol and octadecyl areas was prepared by UV-modified octadecyltrichlorosilane SAMs through a photomask. The amorphous La(2)O(CO(3))(2) x H(2)O thin films were selectively deposited in the silanol regions. Crystallized La(2)O(3) was obtained after heating at 800 degrees C in air.     

Electron transport through thin organic films in metal--insulator--metal junctions based on self-assembled monolayers.

This paper describes an experimentally simple system for measuring rates of electron transport across organic thin films having a range of molecular structures. The system uses a metal--insulator--metal junction based on self-assembled monolayers (SAMs); it is particularly easy to assemble. The junction consists of a SAM supported on a silver film (Ag-SAM(1)) in contact with a second SAM supported on the surface of a drop of mercury (Hg-SAM(2))--that is, a Ag-SAM(1)SAM(2)-Hg junction. SAM(1) and SAM(2) can be derived from the same or different thiols. The current that flowed across junctions with SAMs of aliphatic thiols or aromatic thiols on Ag and a SAM of hexadecane thiol on Hg depended both on the molecular structure and on the thickness of the SAM on Ag: the current density at a bias of 0.5 V ranged from 2 x 10(-10) A/cm(2) for HS(CH(2))(15)CH(3) on Ag to 1 x 10(-6) A/cm(2) for HS(CH(2))(7)CH(3) on Ag, and from 3 x 10(-6) A/cm(2) for HS(Ph)(3)H (Ph = 1,4-C(6)H(4)) on Ag to 7 x 10(-4) A/cm(2) for HSPhH on Ag. The current density increased roughly linearly with the area of contact between SAM(1) and SAM(2), and it was not different between Ag films that were 100 or 200 nm thick. The current--voltage curves were symmetrical around V = 0. The current density decreased with increasing distance between the electrodes according to the relation I = I(0)e(-beta d(Ag,Hg)), where d(Ag,Hg) is the distance between the electrodes, and beta is the structure-dependent attenuation factor for the molecules making up SAM(1). At an applied potential of 0.5 V, beta was 0.87 +/- 0.1 A(-1) for alkanethiols, 0.61 +/- 0.1 A(-1) for oligophenylene thiols, and 0.67 +/- 0.1 A(-1) for benzylic derivatives of oligophenylene thiols. The values of beta did not depend significantly on applied potential over the range of 0.1 to 1 V. These junctions provide a test bed with which to screen the intrinsic electrical properties of SAMs made up of molecules with different structures; information obtained using these junctions will be useful in correlating molecular structure and rates of electron transport.     

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.     

Fe(CO)5 thin films adsorbed on Au(111) and on self-assembled organic monolayers: I. Structure

The adsorption of Fe(CO)(5) onto Au(111)/mica and C(4), C(8), C(12), and C(16) SAMs/Au(111)/mica surfaces has been studied using infrared spectroscopy to elucidate the coverage-dependent structures of these films and the intermolecular couplings that determine the form of the spectra. For all substrates, the first layer is composed of molecules physisorbed with one axial and two equatorial carbonyl groups directed toward the substrate; subsequent layers are preferentially oriented with the C(3) molecular axis aligned perpendicular to the substrate (i.e., one axial carbonyl group directed toward the substrate). The axial vibrational band systematically shifts to higher frequencies with increasing surface coverage because of the effects of intermolecular coupling of the quasiparallel transition dipole moments. The strong effects of dipolar coupling are also witnessed by the trends of the band positions when the distance to the image plane is systematically varied using highly organized self-assembled organic substrates; no band shifts are observed when dilute Fe(CO)(5) is embedded in Xe matrixes under identical experimental conditions. The as-deposited films are structurally stable below 125 K on Au(111)/mica surfaces and below 100 K on the organic self-assembled monolayers. The instability of the films above these temperatures demonstrates that the as-adsorbed films do not form thermodynamically well-defined phases but are structurally metastable. The results presented herein and in the companion paper provide a consistent framework to interpret the spectroscopy of these systems that resolves outstanding issues concerning these films and provides a structural model that explains the dynamic properties of these films during exposure to low-energy electron beams.     

Bismuth sulfide thin films with low resistivity on self-assembled monolayers

Using self-assembled monolayers (SAMs), highly crystalline bismuth sulfide thin films with low electrical resistivity have been prepared from aqueous solution at low temperature (40-70 degrees C). The nucleation and growth process of Bi2S3 thin films was investigated in detail by XPS, AES, SEM, XRD, SAED, and HRTEM. Solution conditions have marked effects on the microstructure, growth rate, and mechanism of Bi2S3 films. Increased solution temperature resulted in a higher growth rate and a shorter induction time due to a higher supersaturation degree. In the solution of pH 1.12, homogeneous nucleation and the attachment process dominated the formation of Bi2S3 films. In contrast, at pH 0.47 Bi2S3 thin films were formed via heterogeneous nucleation and growth. The c-axial orientation of bismuthinite films was enhanced with the increase of reaction time. By controlling the solution supersaturation and reaction duration, highly crystalline Bi2S3 films composed of closely packed and coalescent crystallites could be realized, whose dark electrical resistivity could reach as low as 0.014 Omega cm without any post-treatment.     

Stepwise synthesis of Gly-Gly-His on gold surfaces modified with mixed self-assembled monolayers

Peptide-modified electrode surfaces have been shown to have excellent recognition properties for metal ions. An efficient method of screening a potential peptide for its selectivity for a given metal would involve the synthesis of the peptide directly on the electrode surface. This paper outlines a procedure in which the tripeptide Gly-Gly-His was synthesized one amino acid at a time on a gold surface modified with a self-assembled monolayer of the mixed alkanethiolates 3-mercaptopropionic acid (MPA) and 3-mercaptopropane (MP). Electrochemistry and high-resolution mass spectrometry were used to elucidate the structure of the adsorbed species and follow the synthesis. The amino acids can be attached only to MPA, but the presence of a diluting unreactive molecule of MP reduces steric crowding about the reaction center. The maximum coverage of synthesized tripeptide occurs at a ratio of MPA/MP of 1:1.     

Microscopic wetting of self-assembled monolayers with different surfaces: a combined molecular dynamics and quantum mechanics study

Molecular dynamics simulations are used to study the micronature of the organization of water molecules on the flat surface of well-ordered self-assembled monolayers (SAMs) of 18-carbon alkanethiolate chains bound to a silicon (111) substrate. Six different headgroups (-CH(3), -C═C, -OCH(3), -CN, -NH(2), -COOH) are used to tune the character of the surface from hydrophobic to hydrophilic, while the level of hydration is consistent on all six SAM surfaces. Quantum mechanics calculations are employed to optimize each alkyl chain of the different SAMs with one water molecule and to investigate changes in the configuration of each headgroup under hydration. We report the changes of the structure of the six SAMs with different surfaces in the presence of water, and the area of the wetted surface of each SAM, depending on the terminal group. Our results suggest that a corrugated and hydrophobic surface will be formed if the headgroups of SAM surface are not able to form H-bonds either with water molecules or between adjacent groups. In contrast, the formation of hydrogen bonds not only among polar heads but also between polar heads and water may enhance the SAM surface hydrophilicity and corrugation. We explicitly discuss the micromechanisms for the hydration of three hydrophilic SAM (CN-, NH(2)- and COOH-terminated) surfaces, which is helpful to superhydrophilic surface design of SAM in biomimetic materials.     

Direct electrochemistry of cytochrome c on a phosphonic acid terminated self-assembled monolayers

The direct electrochemistry of cytochrome c (cyt c) on a gold electrode modified with 3-mercaptopropylphosphonic acid [HS-(CH₂)₃-PO₃H₂, MPPA] self-assembled monolayers (SAMs) was for the first time investigated. Electrochemical measurements and surface-enhanced infrared absorption spectroscopic reveal that the adsorption kinetics of cyt c on the MPPA-SAMs is very fast (saturation adsorption is completed within 5 s) and the immobilized cyt c molecules retain their native secondary protein structure. The nature of interaction between cyt c and -PO₃H₂ groups is mainly the electrostatic interaction. The direct electrochemistry of the immobilized cyt c on the -PO₃H₂ terminated SAMs with short chain is nearly reversible. Its formal potential (E⁰′ = 18 ± 3 mV vs. SCE) is very close to that of cyt c in an aqueous solution (E⁰′ = 18-22 mV vs. SCE). In addition, the electron transfer rate of cyt c immobilized on -PO₃H₂ terminated SAMs is relatively slow as compared to -SO₃H and -COOH terminated SAMs, indicating excess negative charge density on the SAMs surface will decrease the electron transfer rate of cyt c.     

Interfacial friction of surfaces grafted with one- and two-component self-assembled monolayers

We present a quantitative study of the nanoscale frictional properties of one-component (pure) and two-component (mixed) alkylsilane self-assembled monolayers (SAMs). The load and velocity dependence of the friction force was measured in air and ethanol using lateral force microscopy (LFM). It was observed that for SAMs with well-ordered structure (pure SAMs and mixed SAMs composed of two long chain molecules) friction depends nonlinearly on load, at low loads, both in air and in ethanol. These observations are consistent with the low-load contact area predictions of the Johnson-Kendall-Roberts (JKR) theory, indicating that for well-ordered SAMs friction force is proportional to contact area and that the true contact area is determined by elastic deformation of the SAM by the LFM probe. In ambient air, the magnitude of the friction force measured using mixed SAMs is found to be similar to that obtained using pure SAMs at the same external load. Changing the medium to ethanol, however, leads to dramatically lower friction in the mixed SAMs. An analysis of the friction data using a thermally activated Eyring model that takes into account the monolayer viscoelasticity suggests that the better friction properties of the mixed SAMs are a consequence of greater disorder and higher molecular mobility in the outer layer/canopy. These findings indicate that multi-tiered SAM coatings comprising a highly ordered underlayer and a disordered, mobile canopy can provide the basis for low-friction coatings for small mechanical systems.     

In situ growth of CuS thin films on functionalized self-assembled monolayers using chemical bath deposition

Nano-structured CuS thin films were deposited on the functionalized -NH(2)-terminated self-assembled monolayers (SAMs) surface by chemical bath deposition (CBD). The deposition mechanism of CuS on the -NH(2)-terminated group was systematically investigated using field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscope (XPS), UV-vis absorption. The optical, electrical and photoelectrochemical performance of CuS thin films incorporating with the X-ray diffraction (XRD) analysis confirmed the nanocrystalline nature of CuS with hexagonal crystal structure and also revealed that CuS thin film is a p-type semiconductor with high electrical conductivity (12.3Ω/□). The functionalized SAMs terminal group plays a key role in the deposition of CuS thin films. The growth of CuS on the varying SAMs surface shows different deposition mechanisms. On -NH(2)-terminated surfaces, a combination of ion-by-ion growth and cluster-by-cluster deposition can interpret the observed behavior. On -OH- and -CH(3)-terminated surfaces, the dominant growth mechanism on the surface is cluster-by-cluster deposition in the solution. According to this principle, the patterned CuS microarrays with different feature sizes were successfully deposited on -NH(2)-terminated SAMs regions of -NH(2)/-CH(3) patterned SAMs surface.     

Evaporation of microdroplets of ethanol-water mixtures on gold surfaces modified with self-assembled monolayers

The wetting property and evaporation behavior of ethanol-water mixtures of various concentrations on gold surfaces modified with 1-decanethiolate self-assembled monolayers (SAMs) were studied by digital contact angle analysis. It has been shown that the initial contact angle decreases monotonically with increased concentration of ethanol in the mixture. Evaporation studies revealed a general trend with a preliminary increase in contact angle accompanied with a decrease in contact area, then a constant contact angle accompanied with a slower, linear decrease in contact area. At the very beginning of the evaporation process, the contact angles showed a rapid decrease for the microdroplets of a binary mixture with equal volume fractions (i.e., 50% ethanol). Three distinct stages of the evaporation profile for the ethanol-water mixtures were observed, which differ from the inclusive "pinning" and "shrinking" behavior observed for the pure liquid case. Ultimately, the study makes possible the use of an evaporation profile to monitor the change in concentration of a binary system and allows a better understanding of the interactions between liquid microdroplets with solid substrates.     

Electric field induced switching of poly(ethylene glycol) terminated self-assembled monolayers: a parallel molecular dynamics simulation

Effects of electric field on the structure of poly(ethylene glycol) (PEG) terminated alkanethiol self-assembled monolayer (SAM) on gold have been studied using parallel molecular dynamics method. An applied electric field triggers a conformational transition from all-trans to a mostly gauche conformation. The polarity of the electric field has a significant effect on the surface structure of PEG leading to a profound effect on the hydrophilicity of the surface. The electric field applied antiparallel to the surface normal causes a reversible transition to an ordered state in which the oxygen atoms are exposed. On the other hand, an electric field applied in a direction parallel to the surface normal introduces considerable disorder in the system and the oxygen atoms are buried inside. The parallel field affects the overall tilt structure of SAMs more adversely than the antiparallel field.     

Multiscale modeling of self-assembled monolayers of thiophenes on electronic material surfaces

Computer simulation programs, spanning different time and length scales, are used to describe the fundamentals of thin film growth morphology in organic self-assembled monolayers using thiophenes on gold as representative systems. Ab initio calculations created a catalog of the energetics between two N-[4-(thien-2ylethynyl)phenyl] hydroxyl ("1P" molecules) in vacuum and interactions in three orthogonal orientations (parallel, perpendicular, and gamma-phase) to a Au (111) surface. This energetic dataset was supplied as the input for kinetic Monte Carlo simulations of dimer and trimer representations of small organic molecules to describe both sub-monolayer and multilayer growth on a series of hypothetical model substrates. On strongly binding metallic-like substrates, sub-monolayers of the model organic molecules formed ordered phases in the x and y directions at high temperatures and a disordered polycrystalline structure at low temperatures with the molecules lying down. Only at high temperatures was a "phase inversion" observed from a completely flat to an upright structure, suggesting the upright phase to be kinetically limited. Results for multilayer deposition of 1P molecules on three substrates which differ in their binding energy to the molecule (from non-interacting to strongly binding substrates) provided a rich view of the polymorphism that can result from differing choices of temperature and flux conditions. Irrespective of the binding energy of the molecule to the substrate, on highly corrugated surfaces we always observed 3D-island growth of multiple layers of the thiophenes, in contrast to Stranski-Krastanov or Frank-van der Merwe growth on more uniform substrates. The qualitative picture we obtained agrees with the growth habits of other small organic molecule systems like the acene series. Finally, molecular dynamics studies were used to understand the packing structures of stable polymorphs of thiophene SAMs. Different deposition conditions and substrate-molecule binding captured different regimes of growth morphology, some of which have already been observed experimentally.     

Preparation of high quality electrical insulator self-assembled monolayers on gold. Experimental investigation of the conduction mechanism through organic thin films

Self-assembled monolayers (SAMs) form highly ordered, stable dielectrics on conductive surfaces. Being able to attach larger-area contacts in a MIM (metal-insulator-metal) diode, their electrical properties can be determined. In this paper, the electrical conduction through thiolate SAMs of different alkyl chain lengths formed on gold surfaces were studied and discussed. The influence of the headgroup with respect to the surface quality and prevention of short circuits is investigated. Phenoxy terminated alkanethiols were found to form high quality SAMs with perfect insulating properties. Synthesis of the required terminally substituted long chain thiols have been developed. The I(V) characteristics of MIM structures formed with these SAMs are measured and simulated according to theoretical tunneling models for electrical conductivity through thin organic layers. SAM based electronic devices will become especially important for future nanoscale applications, where they can serve as insulators, gate dielectric of FETs, resistors, and capacitor structures.     

The effect of the surface coverage of the phosphonic acid terminated self-assembled monolayers on the electrochemical behavior of dopamine

The surface coverage of 3-mercaptopropylphosphonic acid (HS-CH₂CH₂CH₂-PO₃H₂, MPPA) self-assembled monolayers (SAMs) on gold surface can be controlled by the dissociation degree of phosphonic acid groups (-PO₃H₂) in the bulk solution and adsorption time of MPPA molecules under the basic condition. Electrochemical measurements show that the low-density MPPA-SAMs modified gold electrode enhances significantly the kinetics of electron transfer of dopamine (DA), and improves the antifouling capability of modified electrode towards DA oxidation. The present results offer crucial information for design and optimization of the electrochemical sensors for DA determination.     

SECM characterization of immobilised enzymes by self-assembled monolayers on titanium dioxide surfaces

SECM in generator-collector mode was used to detect the presence of immobilised enzymes on titanium dioxide layers which were chemically or electrochemically generated with possible application as chemical sensors and biosensors. Glucose oxidase (GOx) and horseradish peroxidase (HRP) were immobilised by SAM generation using aminopropyltriethoxysilane (APTES) and ascorbic acid. The enzymes were successfully immobilised on two different TiO(2) surfaces. A simple test of durability of the system was made and a model of SAM organisation is presented.     

Reversible covalent patterning of self-assembled monolayers on gold and silicon oxide surfaces

This paper describes the generation of reversible patterns of self-assembled monolayers (SAMs) on gold and silicon oxide surfaces via the formation of reversible covalent bonds. The reactions of (patterned) SAMs of 11-amino-1-undecanethiol (11-AUT) with propanal, pentanal, decanal, or terephthaldialdehyde result in dense imine monolayers. The regeneration of these imine monolayers to the 11-AUT monolayer is obtained by hydrolysis at pH 3. The (patterned) monolayers were characterized by Fourier transform infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, contact angle and electrochemical measurements, and atomic force microscopy. Imines can also be formed by microcontact printing of amines on terephthaldialdehyde-terminated substrates. Lucifer Yellow ethylenediamine was employed as a fluorescent amine-containing marker to visualize the reversible covalent patterning on a terephthaldialdehyde-terminated glass surface by confocal microscopy. These experiments demonstrate that with reversible covalent chemistry it is possible to print and erase chemical patterns on surfaces repeatedly.     

Adsorption of sulfite oxidase on self-assembled monolayers from molecular dynamics simulations

Sulfite oxidase (SO) is an enzyme catalyzing the terminal step of the metabolism of sulfur-containing amino acids that is essential for almost all living organisms. The catalytic activity of SO in vertebrates strongly depends on the efficiency of the intramolecular electron transfer (IET) between the catalytic Moco domain and the cytochrome b5 (cyt b5) domain. The IET process is assumed to be mediated by large domain motions of the cyt b5 domains within the enzyme. Thus, the interaction of SO with charged surfaces may affect the mobility of the cyt b5 domain required for IET and consequently hinder SO activation. In this study, we present a molecular dynamics approach to investigating the ionic strength dependence of the initial surface adsorption of SO in two different conformations-the crystallographic structure and the model structure for an activated SO-onto mixed amino- and hydroxyl-terminated SAMs. The results show for both conformations at low ionic strengths a strong adsorption of the cyt b5 units onto the SAM, which inhibits the domain motion event required for IET. Under higher ion concentrations, however, the interaction with the surface is weakened by the negatively charged ions acting as a buffer and competing in adsorption with the cathodic cyt b5 domains. This competition prevents the immobilization of the cytochrome b5 units onto the surface, allowing the intramolecular domain motions favoring IET. Our predictions support the interpretation of recent experimental spectroelectrochemical studies on SO.