Effect Of Self-assembled Monolayers Research Articles

Lateral dipole moments induced by all-cis-pentafluorocyclohexyl groups cause unanticipated effects in self-assembled monolayers

All-cis-hexafluoro- and all-cis-pentafluoro-cyclohexane (PFCH) derivatives are new kinds of materials, the structures and properties of which are dominated by the highly dipolar Janus-face motif. Here, we report on the effects of integrating the PFCH groups into self-assembled monolayers (SAMs) of alkanethiolates on Au(111). Monolayers with an odd (eleven) and even (twelve) number of methylene groups were characterized in detail by several complementary experimental tools, supported by theoretical calculations. Surprisingly, all the data show a high similarity of both kinds of monolayers, nearly lacking the typically observed odd-even effects. These new monolayers have a packing density about 1/3 lower than that of non-substituted alkanethiolate monolayers, caused by the bulkiness of the PFCH moieties. The orientations of the PFCH groups and the alkyl chains could be determined independently, suggesting a conformation similar to the one found in the solid state structure of an analogous compound. Although in the SAMs the PFCH groups are slightly tilted away from the surface normal with the axial fluorine atoms pointing downwards, most of the dipole moments of the group remain oriented parallel to the surface, which is a unique feature for a SAM system. The consequences are much lower water contact angles compared to other partly fluorinated SAMs as well as rather moderate work function values. The interaction between the terminal PFCH moieties results in an enhanced stability of the PFCH-decorated SAMs toward exchange reaction with potential molecular substituents in spite of the lower packing density of these films.

Substrate Roughness and Tilt Angle Dependence of Sum-Frequency Generation Odd–Even Effects in Self-Assembled Monolayers

Substrate Roughness and Tilt Angle Dependence of Sum-Frequency Generation Odd–Even Effects in Self-Assembled Monolayers

Effect of Self-Assembled Monolayers (SAMs) as Surface Passivation on the Flexible a-InSnZnO Thin-Film Transistors

To overcome the environment susceptibility of flexible amorphous InSnZnO (a-ITZO) thin-film transistors (TFTs), a surface passivation method utilizing n -octyltriethoxysilane (OTES) self-assembled monolayers (SAMs) is developed. The electrical characteristics of the developed transistors indicate that device performance can be enhanced upon OTES passivation, exhibiting high mobility ( -.19.4 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ), a steep subthreshold slope (-.90 mV/dec), near-zero threshold voltage (-.0.6 V), and high ON- OFF current ratio (-.7.9 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> ). The passivation layer can effectively improve the stability of flexible ITZO TFTs even under positive bias stress (PBS) and negative bias stress (NBS), and only 0.8/1.3 V threshold voltage shifts are shown after PBS/NBS. In addition, the OTES-passivated transistors exhibit good mechanical reliability and maintain its electrical characteristics during the 10-k bending period, without drastic decline. These results demonstrate that the SAM passivation method is suitable for the fabrication of flexible high-performance metal-oxide electronic devices.

Solution processable diketopyrrolopyrrole semiconductor: towards bio-electronic applications

In this paper, the possibility to use diketopyrrolopyrrole (DPP) for the construction of electrical devices designed to interact with animal cells was studied. For this purpose, the biocompatibility and electrical properties of the selected DPP derivative (3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethyl-hexyl)pyrrolo[3,4-c]pyrrole-1,4-dione) [referred as DPP(TBFu)2] were researched. The electrical properties were studied using model organic field-effect transistors. Mainly investigated was under what conditions maximum charge carrier mobility can be achieved. Using the cumulative effect of self-assembled monolayers on dielectrics and electrodes and detailed thermal analysis of the DPP, a higher charge carrier mobility was achieved than has been previously reported (5.5 × 10−3 cm2 V−1 s−1). The biocompatibility was studied based on a culture of 3T3 fibroblasts. This research revealed that DPP(TBFu)2 can be used in applications involving direct contact with living animal cells. The conclusions found with these model devices can be applied to components suitable for biosensing applications, e.g., water- or electrolyte-gated organic field-effect transistors.

Supramolecular effects in self-assembled monolayers: general discussion.

Supramolecular effects in self-assembled monolayers: general discussion.

Spectroscopic evidence for the origin of odd-even effects in self-assembled monolayers and effects of substrate roughness.

This paper reports the effects of substrate roughness on the odd-even effect in n-alkanethiolate self-assembled monolayers (SAMs) probed by vibrational sum frequency generation (SFG) spectroscopy. By fabricating SAMs on surfaces across the so-called odd-even limit, we demonstrate that differentiation of the vibrational frequencies of CH3 from SAMs derived from alkyl thiols with either odd (SAMO) or even (SAME) numbers of carbons depends on the roughness of the substrate on which they are formed. Odd-even oscillation in SFG susceptibility amplitudes was observed for spectra derived from SAME and SAMO fabricated on flat surfaces (RMS roughness = 0.4 nm) but not on rougher surfaces (RMS roughness = 2.38 nm). In addition, we discovered that local chemical environments for the terminal CH3 group have a chain-length dependence. There seems to be a transition at around C13, beyond which SAMs become "solid-like".

Effect of Self-Assembled Monolayers on the Locally Electrodeposited Silver Thin Layers

The localized electrodeposition of silver on gold coated with self-assembled monolayers (SAMs) by scanning electrochemical microscopy (SECM) is reported. The SAMs were ω-functionalized alkanethiols of the form X–(CH2)2SH, X = OH, NH2, CO2H, SO3H, as well as 4-mercaptobenzoic acid. The SAMs were characterized by X-ray photoelectron spectroscopy (XPS) and cycling voltammetry (CV). The anodic dissolution of a Ag microelectrode, which was held within a few microns from the Au surface, formed a well-controlled flux of Ag+. Deposition of silver nanostructures was driven by the electrochemical reduction of the Ag+ on the Au surface. The effect of the functional group on the Ag local deposition was studied and compared with bulk deposition on the same SAMs. For bulk deposition, we found that the interaction between Ag+ ions and the functional group of the alkanethiols slowed the kinetics of Ag deposition, shifting the deposition to potential that is more negative and caused the formation of large, well-faceted Ag...

Effects of Immersion Solvent on Photovoltaic and Photophysical Properties of Porphyrin-Sensitized Solar Cells.

Memory effects in self-assembled monolayers (SAMs) of zinc porphyrin carboxylic acid on TiO2 electrodes have been demonstrated for the first time by evaluating the photovoltaic and electron transfer properties of porphyrin-sensitized solar cells prepared by using different immersion solvents sequentially. The structure of the SAM of the porphyrin on the TiO2 was maintained even after treating the porphyrin monolayer with different neat immersion solvents (memory effect), whereas it was altered by treatment with solutions containing different porphyrins (inverse memory effect). Infrared spectroscopy shows that the porphyrins in the SAM on the TiO2 could be exchanged with the same or analogous porphyrin, leading to a change in the structure of the porphyrin SAM. The memory and inverse memory effects are well correlated with a change in porphyrin geometry, mainly the tilt angle of the porphyrin along the long molecular axis from the surface normal on the TiO2, as well as with kinetics of electron transfer between the porphyrin and TiO2. Such a new structure-function relationship for DSSCs will be very useful for the rational design and optimization of photoelectrochemical and photovoltaic properties of molecular assemblies on semiconductor surfaces.

Interfaces analysis by impedance spectroscopy and transient current spectroscopy on semiconducting polymers based metal–insulator–semiconductor capacitors

Impedance and transient current measurements on metal–insulator–semiconductor (MIS) capacitors are used as tools to thoroughly investigate the bulk and interface electronic transport properties of semiconducting polymers, i.e. poly(3-hexylthiophene) (P3HT). Distinct features were observed at both interfaces, i.e. metal–semiconductor and semiconductor–insulator. The results revealed a dispersive transport in the bulk due to the band tail of the localized states, presence of interface states at the interface between the insulator and the semiconductor and formation of a less conductive small layer at the interface semiconductor–metal contact due to intrusions of sputtered Au particles. Effects of self-assembled monolayers (SAMs) treatments of the gate insulating dielectric were investigated showing that treating the gate dielectric with either ozone or hexamethyldisilazane (HMDS) or octyltrichlorosilane (OTS) alter not only the interface semiconductor–insulator but the bulk properties as well. An exponential density of states with a width parameter of 38–58meV depending on the surface treatment was found to be representative of the band tail of P3HT. Though both OTS and HMDS treatments slightly increase the density of interface states, only OTS treated samples showed a decrease in disorder parameter of the bulk. The latter fact can be attributed to an increase of the grain size due to a favored π-π stacking film growth. An outcome explaining the already reported increase of the lateral mobility and decrease of the vertical mobility observed upon OTS treatment of the gate insulating dielectric in poly(3-hexylthiophene) based devices.

Effects of Self-Assembled Monolayers with Fluorinated Alkyl Chain

The electrical characteristics of fluorinated self-assembled monolayers (FSAMs) in organic light-emitting diodes (OLEDs) were investigated according to the change in the lengths of their alkyl chains. As the number of fluorine atom increases, the device with long alkyl chain length allows the flow of a large amount of current because of the improved barrier height of the hole injection. However, the current efficiency under long alkyl chain length was decreased by the switched carrier balance. The device with the shortest FSAM alkyl chain length demonstrated an approximately 68% higher current efficiency than that with the longest alkyl chain length.

Effects of Self-Assembled Monolayers with Different Chemical Groups on Ovarian Cancer Cell Line BehaviorIn Vitro

In addition to serving as a physical support, the extracellular matrix (ECM) actively influences cell behavior. However, the definitive effects of different chemical structures present in the ECM on cell behavior remain obscure. The current study aimed to investigate the effects of different chemical structures present in the ECM on cellular physiology using the ovarian cancer cell line SKOV-3 as a model. Self-assembled monolayers (SAMs) with different chemical modifications, including methyl (-CH3), hydroxyl (-OH), amino (-NH2), carboxyl (-COOH), and mercapto (-SH) groups, were used as microenvironmental models to explore the effects of different structures on SKOV-3 cells. The cell morphology, cell adhesion, cytotoxicity, and functional alterations in cancer cells cultured on different SAMs were analyzed. The results showed that SKOV-3 cells cultured on -NH2surfaces exhibited the largest contact area, whereas those on -CH3surfaces exhibited the smallest contact area and mostly rounded morphologies. Additionally, -NH2and -COOH promoted cell proliferation and adhesion, whereas CH3inhibited adhesion, leading to G1 arrest during the cell cycle and resulting in cell apoptosis. This study may provide useful information for reconstruction of the ECM and for controlling cell behavior in related areas of study.

Structural ordering versus energy band alignment: Effects of self-assembled monolayers on the metal/semiconductor interfaces of small molecule organic thin-film transistors

A model describing the influence of self-assembled monolayers on the contact resistance of bottom-contact organic thin-film transistors is presented. The model takes the contact geometry, the energy level alignment and the structural order of the organic films into consideration when describing the contact effects of organic transistors. The treatment of the metal source/drain electrodes of the transistors by self-assembled monolayers allows for tuning the work function of the metal contact and an improved ordering of the organic molecules on top of the source/drain contacts. The results reveal that the contact resistance is mainly determined by the molecular ordering, rather than the tuning of the work function. The model is compared to experimentally measured contact resistances for different self-assembled monolayers.

Effects of self-assembled monolayers on amperometric glucose biosensors based on an organic–inorganic hybrid system

Ultrathin-form amperometric glucose biosensors have been fabricated on gold substrates by using self-assembled monolayers (SAMs) of carboxylic acid groups (COOH). The glucose oxidase (GOx) was covalently immobilized on the SAMs and covered with Langmuir–Blodgett films including nanometer-sized clusters of Prussian Blue (PB) playing a role of mediator for glucose detection, which makes the present sensor, Au/SAM/GOx/PB, work at very low potentials of approximately 0.0V vs. Ag/AgCl. An amperometric biosensor comprising a SAM of 4-mercaptobenzoic acid (MBA) exhibited a fast response current of 3s, a detection limit of 12.5μM, a high sensitivity of 50nA/(cm2mM), and a long-term stable linearity ranging from12.5μM to 70mM. The sensitivity is significantly affected by the conductivity of the SAM layer. The analysis of the electron transfer confirmed a high conductive nature of MBA among carboxylic acid groups. The obtained linearity in a high-concentration region is attributed to both the ultrathin nature and high electron transfer of the MBA SAM.

Effect of Self-Assembled Monolayer (SAM) on the Oxide Semiconductor Thin Film Transistor

In this paper, we proposed the self-assembled monolayer (SAM) as a protection layer against plasma and chemically induced damages to the back interface of an oxide semiconductor during the deposition of the passivation layer. When a thin-film transistor (TFT) is passivated with plasma-enhanced chemical-vapor deposition (PECVD) SiOx and solution-based materials, the back interface of the oxide semiconductor could be exposed to plasma and chemically induced damages, respectively. We employed SAMs on the back surface of the oxide semiconductor prior to the passivation process to suppress such damage. The hydrophobic Cl-SAM (3-chloropropyltriethoxysilane) suppressed the degradation in mobility and subthreshold slope (SS) due to ion bombardment during plasma treatment. The hydrophobic CH3-SAM (octyltriethoxysilane) successfully blocked chemically induced damage due to solution-based passivation.

Improved Charge Separation Properties of Organic Hetero-Junction Solar Cells by Self-Assembled Monolayers Anchored Ag Nanoparticles

We investigate the effect of self-assembled monolayers and localized surface plasmons of silver nano-particles on an organic solar cell consisting of zinc phthalocyanine as an active layer. The device was fabricated by covalent attachment of silver nanoparticles on n-type silicon substrates using self-assembled monolayer of 4-mercaptophenol. Power conversion efficiency is increased up to 8 times as compared to a reference device with merely 0.13% photo-conversion efficiency containing no self-assembled monolayers and silver nano-particles. We believe that improved conductivity at the interface due to the aromatic self-assembled monolayer and the increased local electric field experienced by the active layer in presence of silver nano-particles act in synergy towards the higher population of excitons and dissipation of charge.

Effects of Self-Assembled Monolayers on Selective Crystallization of Tolbutamide

Selective crystallization of tolbutamide on SAMs with the similar molecule structure of tolbutamide and the functional groups of methyl, trifluoromethyl, and phenyl respectively was studied through characterizations of XRD and crystal morphologies. It is indicated that at low supersaturation tolbutamide (TB) crystallized into form II on the methyl-terminated SAMs and trifluoromethyl-terminated SAMs, whereas the highest bioavailable TB crystals of form IV were obtained on phenyl-terminated SAMs, differing from the precipitated crystals from the solution including TB forms I, II, and III together. The preferential growth orders of the crystallographically important faces of TB forms I–IV were assessed on different functionalized SAMs through molecular modeling based on the prediction of equilibrium morphologies of crystals, which is in accordance with the experimental results. Further time-resolved Raman spectra of TB crystals grown on phenyl-terminated SAMs illustrate that the surface functional groups are...

Effect of Self-Assembled Monolayers on Charge Injection and Transport in Poly(3-hexylthiophene)-Based Field-Effect Transistors at Different Channel Length Scales

Charge injection and transport in bottom-contact regioregular-poly(3-hexylthiophene) (rr-P3HT) based field-effect transistors (FETs), wherein the Au source and drain contacts are modified by self-assembled monolayers (SAMs), is reported at different channel length scales. Ultraviolet photoelectron spectroscopy is used to measure the change in metal work function upon treatment with four SAMs consisting of thiol-adsorbates of different chemical composition. Treatment of FETs with electron-poor (electron-rich) SAMs resulted in an increase (decrease) in contact metal work function because of the electron-withdrawing (-donating) tendency of the polar molecules. The change in metal work function affects charge injection and is reflected in the form of the modulation of the contact resistance, R(C). For example, R(C) decreased to 0.18 MΩ in the case of the (electron-poor) 3,5-bis-trifluoromethylbenzenethiol treated contacts from the value of 0.61 MΩ measured in the case of clean Au-contacts, whereas it increased to 0.97 MΩ in the case of the (electron-rich) 3-thiomethylthiophene treated contacts. Field-effect mobility values are observed to be affected in short-channel devices (<20 μm) but not in long-channel devices. This channel-length-dependent behavior of mobility is attributed to grain-boundary limited charge transport at longer channel lengths in these devices.

Effect of Self-Assembled Monolayers on the Performance of Organic Photovoltaic Cells

The improvement of the performance of organic photovoltaic cells (OPVCs) and the photogeneration process in these devices may occur via multiple mechanisms depending on their structure and/or architecture. For this purpose we investigate how self-assembled monolayers of thiol molecules (C12H25SH and 3T(CH2)6SH) and benzoic acid molecules (ABA and NBA) affect the efficiency and the photogeneration of free carriers in a sexithiophene based photovoltaic cells. Firstly, we provide the results of absorption spectra for samples with SAM of thiol that show there effect on orientation of 6T molecules on these structures and the organization degree of the thiol molecules on ITO substrate. Afterward, we describe from current vs. applied voltage after illumination, the enhancement of the performance of these cells. In the second, we study the effect of SAM of benzoic acids molecules on the photovoltaic behavior. A theoretical model is used for quantitative description of the open circuit voltage as a function of carrier’s generation rates at the electrodes. The results of I-V characterization under illumination show that open circuit voltage as well as short circuit current is dramatically affected by the dipolar layer. The orientation and the magnitude of dipole moment of benzoic acid molecules are the crucial factors that affect the organic photovoltaic parameters.

Vertical alignment of liquid crystals by ordering effect of selfassembled monolayers on the Ion‐beam‐irradiated anisotropic surface

In this paper, vertically aligned (VA) liquid crystal (LC) modes were investigated using the alkyl chain ordering effect of self‐assembled monolayers (SAMs) prepared on the anisotropic inorganic surface. On the anisotropic surface prepared through oblique ion beam irradiations, the SAM molecules are adsorbed, producing macroscopic alkyl chain ordering, which can determine the pretilt direction of the vertically aligned LC molecules through the intermolecular interactions on the surface.

Effects of Self-Assembled Monolayer and PFPE Lubricant on Wear Characteristics of Flat Silicon Tips

The effects of self-assembled monolayer (SAM) and perfluoropolyether (PFPE) lubricant on the wear characteristics of flat silicon tips were investigated. The wear test consisted of sliding the silicon tips fabricated on a flat silicon specimen against SAM and PFPE (Z-tetraol) coated silicon (100) wafer. The tips were slid at a low speed for about 15 km under an applied load of 39.2 μN. The wear volume of the tip was obtained by measuring the tip profile using an Atomic Force Microscope (AFM). It was found that the coatings were effective in reducing the wear of the tips by an order of magnitude from 10−6 to 10−7.