Ionically Self-assembled Monolayer Research Articles

A computational model for the ionic self-assembly of nanoparticles under the influence of external electric fields

We present a class of cooperative stochastic models for adsorption and evaporation of monomers on two- and three-dimensional lattices subjected to perpendicular external electric fields. These models are motivated by the fabrication of optical coatings using layer-by-layer self-assembly. We report computational and analytical results for the time-dependent particle density and discuss a particular experimental implementation of an ionic self-assembled monolayer under the influence of external electric fields.

Immobilization of a non-heme diiron complex encapsulated in an ammonium-type ionic liquid layer modified on an Au electrode: reactivity of the electrode for O2 reduction.

An unstable diiron(II,II) complex possessing O2 binding ability at low temperature was encapsulated and stabilized in an ammonium-type ionic liquid layer polymerized on an electrode. The encapsulated complex revealed catalytic reactivity for four-electron reduction of O2 at an ambient temperature in aqueous solution.

Improved performances of inkjet-printed poly(3-hexylthiophene) organic thin-film transistors by inserting an ionic self-assembled monolayer

Improved performances of inkjet-printed poly(3-hexylthiophene) organic thin-film transistors have been demonstrated by inserting an ionic self-assembled monolayer.

Ionic self-assembled monolayer for low contact resistance in inkjet-printed coplanar structure organic thin-film transistors

To reduce the contact resistance in inkjet-printed organic thin-film transistors (OTFTs), the use of a newly synthesized ionic self-assembled monolayer (SAM) consisting of an anchoring group, a linker group, and an ionic functional group, is investigated. According to the gated transmission line method (TLM) measurements of a series of OTFT devices, where one type has no charge injection layer, another type having a pentafluorobenzenethiol (PFBT) injection layer, and a third type containing a (6-mercaptohexyl)trimethylammonium bromide (MTAB) ionic SAM, the latter exhibits the lowest contact resistance value of ∼3.1KΩcm. The OTFTs without charge injection layer and with the PFBT SAM have relatively higher contact resistance values of ∼6.4KΩcm and ∼5.0KΩcm, respectively. The reduced contact resistance in the OTFTs with ionic SAMs is attributed to the large charge carrier density induced by the ionic SAM, which allows sufficient tunneling-assisted injection of the carriers from the metal electrode to the polymer semiconductor. These results suggest that the use of appropriate ionic SAM injection layer is an effective way to reduce the contact resistance, hence improving the charge transport characteristics of inkjet-printed OTFTs.

Construction of Ionic Liquid Self-assembled Monolayer on Gold Electrode for Hemoglobin Modifying and Peroxide Biosensing

Construction of Ionic Liquid Self-assembled Monolayer on Gold Electrode for Hemoglobin Modifying and Peroxide Biosensing

Ionic liquid self‐assembled monolayer and ion exchange for surface force control

Adhesion and lateral force were controlled by the formation of a self‐assembled monolayer combined with the surface ion exchange of ionic liquids on a silicon surface. In this study, the functionalized imidazolium ionic liquids were designed and synthesized with the aim of controlling surface force. N‐[3‐(Trimethoxylsilyl)propyl] ethylenediamine molecules were first self‐assembled onto a surface as an anchor layer and then 1‐propionic acid‐3‐methylimidazolium chloride were successfully grafted onto the amino‐modified surface. The surface force was changed by surface ion exchange in various anionic solutions. The self‐assembly and ion exchange processes were detected by means of attenuated total reflectance‐Fourier transform infrared spectrometry and further confirmed by X‐ray photoelectron spectra. Adhesion and friction behaviors were systematically investigated by atomic force/friction force microscope. The results indicated that anions played a great role in determining surface properties. Furthermore, surface adhesion and friction can be possibly quantitatively determined by the counter‐anions on the surface. Copyright © 2011 John Wiley & Sons, Ltd.

Micro/nano-tribological behaviors of crown-type phosphate ionic liquid ultrathin films on self-assembled monolayer modified silicon

A series of novel imidazolium-based crown-type phosphate ionic liquid ultrathin films were fabricated on silicon substrates modified by a self-assembled monolayer. The formation and surface properties of the films were analyzed by means of ellipsometric thickness measurement, X-ray photoelectron spectroscopy, and atomic force microscope. Meanwhile, the adhesive and nano-tribological behaviors of the films were tested by a homemade colloidal probe, and a ball-on-plate tribometer was used to evaluate their micro-tribological performances in the reciprocating mode. As a result, the dual-layer films show enhanced micro/nano-tribological properties as compared to single-component ionic liquid films coated directly on the silicon surfaces, which is ascribed to synergic effect of the steady self-assembled underlayer as a load-carrying phase and the proper amount of flowable ionic liquid fraction with self-replenishment property. Hopefully, the novel dual-layer ultrathin films based on fluorine-free ionic liquids and self-assembled monolayer produced in the present work might find promising applications in the lubrication of micro-electromechanical systems.

Phase transitions of an ionic liquid self-assembled monolayer on Au

The properties of a surface modified with an ionic liquid self-assembled monolayer (IL-SAM) can be tuned by simply changing the deposition temperature. Mid-IR, SERS, and molecular modelling demonstrated that 1-(12-mercaptododecyl)-3-methylimidazolium bromide (MDMIBr) exhibited a crystalline monolayer for deposition temperatures below 25 °C. Above 25 °C, the aliphatic chain collapsed into a disordered conformation. At 40 °C, another phase transition occurs due to the imidazolium group tilting parallel to the surface. Consequently, the wettability of IL-SAM was tuned over a broad range of contact angle (from 20° to nearly 40°) by varying the deposition temperature. Permeation of redox mediators to a Au electrode coated with MDMIBr strongly depends on the net charge of the redox mediator. Electron transfer was excellent for neutral and negatively charged redox mediators on electrodes coated with IL-SAM regardless of deposition temperature.

Second-harmonic generation in resonant waveguide gratings incorporating ionic self-assembled monolayer polymer films.

Experimental results on resonantly excited second-harmonic generation (SHG) in a periodic ionically self-assembled monolayer (ISAM) film are reported. A double-layer guided-mode resonance filter (GMRF) structure is coated with 40 bilayers of pyrlium-based chi(2) ISAM thin film and excited with the fundamental of a Nd:YAG laser. Enhanced second-harmonic conversion in the ISAM film is achieved because of the local field enhancement associated with the fundamental resonating leaky mode. This method of SHG is particularly promising, as the ISAM films under investigation exhibit anomalous dispersion that may be applied for phase matching to improve nonlinear conversion efficiency.

Control of excited state dynamics in ionically self-assembled monolayers of conjugated molecules

We report the fabrication of thin organic layers and photovoltaic devices made from them. Building thin layers of organic materials via the method of ionically self-assembled monolayers (ISAM) provides control over the layer thickness and composition of multilayer structures on a nanometer scale. This allows to accurately dope a photoluminescent host material with energy or charge accepting guests, changing the emissive character of the pure photoluminescent host film to a predominantly non-emissive, charge generating structure. We show that by varying the concentration of the guest Copper phthalocyanine and C60(OH)24 in poly-(para-phenylene-vinylene) we can measure the energy migration as well as dissociation of the exciton and can determine the lifetime and the diffusion radius of the exciton. Increasing the number of dopands in the host material, the photoluminescence emission spectra shift and decrease in intensity reflecting a decrease in the number of excitons transferring to neighboring chains or conjugation segments. For high dopand concentrations the recombination of excitons only happens on the same chain as the generation. Building a device to achieve the optimal guest/host ratio for optimal exciton dissociation is one important step in the design of high efficiency photovoltaic devices.

Enhanced Photovoltaic Response in Ionically Self-Assembled Monolayer Thin-Film Devices

ABSTRACTWe describe detailed studies of ionically self-assembled monolayer (ISAM) photovoltaic (PV) devices incorporating various electron acceptor materials, such as fullerenes and phthalocyanines. Excitons are generated when the conducting polymer is irradiated, and the electron acceptors aid in dissociating the electron/hole pairs before they can radiatively recombine, thus improving the efficiency of the PV process. The ISAM technique allows the deposition of conducting polymer and electron acceptor materials in alternating layers of nanometer-scale thickness. This ensures that every photoexcited electron-hole pair is in proximity to an electron acceptor, thus minimizing electron-hole recombination and increasing the photocurrent. The individual thickness of each monolayer and the interpenetration of adjacent layers can be precisely controlled through the parameters of the electrolyte solutions. Using the ISAM technique, we have demonstrated that it is possible to create ultrathin films (100 nm) of PV material that have enhanced efficiencies.

Photovoltaic cells based on ionically self-assembled nanostructures

We use the technique of ionically self-assembled monolayers (ISAMs) to produce photovoltaic devices of well-controlled thickness and composition. The ISAM nanostructure fabrication method simply involves the alternate dipping of a charged substrate into aqueous cationic and anionic solutions at room temperature. We have employed several approaches to combine the tetrahydrothiophenium precursor of poly( para-phenylene-vinylene) (PPV) with fullerenes and other organic materials. We apply modulation spectroscopy for the electro-optical characterization of the ISAM-devices. The modulation frequency dependence of the photocurrent can be assigned to the influence of trapped charges taking part in the photovoltaic process.

Thickness dependence of second-harmonic generation in thin films fabricated from ionically self-assembled monolayers

An ionically self-assembled monolayer (ISAM) technique for thin-film deposition has been employed to fabricate materials possessing the noncentrosymmetry that is requisite for a second-order, χ(2), nonlinear optical response. As a result of the ionic attraction between successive layers, the ISAM χ(2) films self-assemble into a noncentrosymmetric structure that has exhibited no measurable decay of χ(2) at room temperature over a period of more than one year. The second-harmonic intensity of the films exhibits the expected quadratic dependence on film thickness up to at least 100 bilayers, corresponding to a film thickness of 120 nm. The polarization dependence of the second-harmonic generation yields a value of 35° for the average tilt angle of the nonlinear optical chromophores away from the surface normal.

Novel Polymer Dyes for Nonlinear Optical Applications Using Ionic Self-Assembled Monolayer Technology

The advantages of the ionic self-assembled monolayer (ISAM) technique are demonstrated for the fabrication of non-centrosymmetric thin films of new nonlinear optical (NLO) polymers. Results are presented, e.g., from UV-vis spectroscopy, fluorescence spectroscopy, and second-harmonic generation experiments, that confirm the second-order NLO behavior of these polydye materials. One of the materials is shown to have not only a stronger ω(2) response—which should not decay over time—but also a higher degree of ordering than commercially available polymers.

Novel Polymer Dyes for Nonlinear Optical Applications Using Ionic Self-Assembled Monolayer Technology

Novel Polymer Dyes for Nonlinear Optical Applications Using Ionic Self-Assembled Monolayer Technology