Self-assembled Monolayer Films Research Articles

Synthesis of carbon nanotubes with catalytic iron-containing proteins

We report a novel method for the growth of carbon nanotubes (CNTs) that uses the iron-containing proteins, hemoglobin, myoglobin, or cytochrome c. The proteins were adsorbed onto amine-terminated self-assembled monolayer (SAM) by peptide bonds between the amine groups of the SAMs and the carboxyl groups of the proteins. SAM films allow proteins to be adsorbed onto films with appropriate density that can initiate the growth of well-graphitized multi-walled CNTs with a diameter of approximately 10 nm. These results show that proteins that contain iron atoms in any form have the catalytic ability to initiate the growth of CNTs.

Self-Assembled Monolayer-Immobilized Gold Nanoparticles as Durable, Anti-Stiction Coatings for MEMS

Self-assembled monolayer (SAM) films of p-aminophenyl trimethoxysilane (APhTS) and 3-mercaptopropyl trimethoxysilane (MPTS) were used to immobilize gold nanoparticles (AuNPs) on silicon substrates and silicon-based microdevices, which created robust nanoparticle coatings that reduced microstructure adhesion. The terminal groups of APhTS and MPTS have both been previously shown to strongly interact and/or bind with metals and metallic nanoparticles. Scanning electron microscopy (SEM) analysis indicated that APhTS and MPTS monolayers improved the adhesion of gold nanoparticles deposited on silicon substrates and microstructures. SEM analysis also showed that the gold nanoparticle/organic monolayer (AuNP/APhTS or AuNP/MPTS) films were more robust than non-immobilized AuNP coatings toward both cantilever beam mechanical contact and water erosion testing. The combination of the rough, lower-energy surfaces of AuNP/APhTS and AuNP/MPTS films also effectively reduced the adhesion exhibited between microstructured surfaces by nearly two orders of magnitude as measured by the apparent work of adhesion. Smooth native oxide-coated Si(100) in-plane surfaces typically have an adhesion energy in excess of 30 mJ/m2 while AuNP/APhTS and AuNP/MPTS coatings reduced the adhesion energy to 0.655 and 1.66 mJ/m2, respectively.

Self-Assembled CoPt Nanoparticles Monolayer Film and Its IR Optical Properties

CoPt nanoparticles were prepared by galvanic displacement reaction, followed by a chemical reduction. The CoPt nanoparticles were spherical and the average diameter was about 33 nm obtained from the results of transmission electron microscopy (TEM), high resolution TEM and scanning electron microscopy (SEM). The results of powder X-ray diffration (XRD), energy dispersed X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) demonstrated that the surface of the product was mostly consist of Pt nanoparticles. An ordered monolayer film of CoPt nanoparticles on Si wafer was obtained by a Liquid/Liquid interface technique. In situ FTIR transmission spectral study indicates that the ordered self-assembled monolayer film of the CoPt nanoparticles shows Fano-like infrared effects, while the deposited CoPt nanoparticles exhibit normal enhanced IR adsorption. The results of the present paper demonstrated that the IR optical properties are closely related to the interactions and thickness of the nanomaterials and significant to understand the anomalous IR properties of nanometer materials.

Formation, surface characterization, and electrocatalytic application of self-assembled monolayer films of tetra-substituted manganese, iron, and cobalt benzylthio phthalocyanine complexes

Molecular thin films of manganese (SAM-2), iron (SAM-3), and cobalt (SAM-4) phthalocyanine complexes, non-peripherally tetra-substituted with benzylmercapto, were formed on polycrystalline gold disc electrode by self-assembly technique. Surface characteristics of the films were interrogated by cyclic voltammetry. Significant passivation of voltammetry processes associated with bare gold surface (gold oxidation and underpotential deposition of copper) confirmed formation of the films. Electrocatalytic property of the films was evidenced from better voltammetry responses (less positive oxidation potential and better current signal) of the insecticide, carbofuran, on these films, relative to that on bare gold electrode. In terms of less positive oxidation potential, the FePc derivative (3) gave the best response, while the best current signal was observed on SAM-2-modified gold electrode. The average heterogeneous rate constant, k, for the oxidation of carbofuran was 3.6 × 10−2 cm s−1 on the SAM film with the best current signal (SAM-2).

Role of Self-Assembled Monolayer of 1,2,3-Benzotriazole in Protecting Copper Artifacts Covered with Sulfide Film

Abstract Corrosion inhibition of the self-assembled monolayer (SAM) of 1,2,3-Benzotriazole (BTAH) on virgin and sulfide-covered (tarnished) copper surfaces was studied using impedance, scanning electron microscopy, and Raman spectroscopy. SAM on preexisting sulfide film develops a more improved protection than on the virgin surface. Addition of HS− ions and non-ionic surface active agents (NIS) in SAM-forming solution deteriorate its properties. Raman spectroscopy established formation of polymeric film by adsorption of triazolate ion rather than triazole molecule. A scheme is suggested for improved inhibition of SAM on tarnished copper. Studies indicate a predominant cathodic control of corrosion reaction by the SAM film.

Self-assembled colloidal PbS quantum dots on GaAs substrates

We report the fabrication and analysis of self-assembled monolayer and bilayer films of colloidal PbS quantum dots (QDs) on GaAs (001) substrates. 1,6-hexanedithiol is used as link molecule between QDs and GaAs substrates. Atomic force microscopy (AFM) and photoluminescence (PL) measurements confirm the formation of PbS QD film on GaAs. For the monolayer PbS QD film, the temperature-dependent PL shows a feature typical of close-packed film. For the bilayer PbS QD film fabricated from two different mean-sized PbS QDs, we find that the stacking sequence of QDs with different size affects the quantum yield and emission wavelength of the film.

Kondo Effect in Self-Assembled Manganese Phthalocyanine Monolayer on Pb Islands

The Kondo effect in two-dimensional manganese phthalocyanine (MnPc) self-assembled monolayer films on Pb(111) islands is studied by low-temperature scanning tunneling microscopy. Variation of the Kondo temperature from 50 K to 300 K at different molecule adsorption sites is revealed. It is shown that the variation is mainly due to the change in the width of d orbital, rather than the shift of its energy. The two-dimensional dI/dV mapping reveals the periodic modulation of the Kondo resonance in the self-assembled MnPc monolayer.

Surface properties of self-assembled monolayer films of tetra-substituted cobalt, iron and manganese alkylthio phthalocyanine complexes

Self-assembled monolayer (SAM) films of iron (SAM- 1), cobalt (SAM- 2) and manganese (SAM- 3) phthalocyanine complexes, tetra-substituted with diethylaminoethanethio at the non-peripheral positions, were formed on gold electrode in dimethylformamide (DMF). Electrochemical, impedimentary and surface properties of the SAM films were investigated. Cyclic voltammetry was used to investigate the electrochemical properties of the films. Ability of the films to inhibit common faradaic processes on bare gold surface (gold oxidation, solution redox chemistry of [Fe(H 2O) 6] 3+/[Fe(H 2O) 6] 2+ and underpotential deposition (UDP) of copper) was investigated. Electrochemical impedance spectroscopy (EIS), using [Fe(CN) 6] 3−/4− redox process as a probe, offered insights into the electrical properties of the films/electrode interfaces. Surface properties of the films were probed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The films were employed for the electrocatalytic oxidation of the pesticide, carbofuran. Electrocatalysis was evidenced from enhanced current signal and less positive oxidation potential of the pesticide on each film, relative to that observed on the bare gold electrode. Mechanism of electrocatalytic oxidation of the pesticide was studied using rotating disc electrode voltammetry.

Rapid functionalization of metal nanoparticles by moderator-tunable ligand-exchange process for biosensor designs

A new method for rapid functionalization of metal nanoparticles based on a moderator-tunable 2D nucleation and growth of ligand-exchange domains has been developed for the design of biosensors with nanoparticle-enhanced sensory films and other applications. The proposed functionalization of gold nanoparticles proceeds through the nucleation and avalanche growth of ligand-exchange domains in the self-assembled monolayer film on a gold nanoparticle surface. The nucleation and growth is controllable by a moderator molecule. The experimental system described presents a ligand-exchange process at citrate-capped gold nanoparticles, with homocysteine acting as the incoming ligand and glutathione as the moderator. The kinetics of the nucleation and growth driven ligand-exchange is characterized by a sigmoidal switching function and differs from that of the usually observed Langmuirian pseudo-first-order process corresponding to the random place-exchange. The ultra-fast functionalization process was monitored using resonance elastic light scattering (RELS) spectroscopy and confirmed by UV–vis plasmonic band absorbance measurements.

The effects of humidity and surface free energy on adhesion force between atomic force microscopy tip and a silane self-assembled monolayer film

The relationship between atomic force microscopy probe-sample adhesion force and relative humidity (RH) at five different levels of surface free energy (γs) of an organic self-assembled monolayer (SAM) has been investigated. Different γs levels were achieved by exposing a patterned SiO2/CH3-terminated octyldimethylchlorosilane SAM sample to an ultraviolet (UV)/ozone atmosphere. A model consisting of the Laplace-Kelvin theory for capillary condensation for nanosized probe and probe-sample molecular interaction was derived to describe the adhesion force as a function of RH from 25 to 90% for different SAM γs values. The equations were solved analytically by using an equivalent curvature of the probe tip shape. Experimental results show that the adhesion force increases slightly with RH for nonpolar SAM. However, for polar SAM surfaces, it increases at first, reaches a maximum, and then decreases. Both the rate of increase and the maximum of the adhesion force with humidity are γs-dependent, which is in good agreement with theoretical prediction. The large rise in the adhesion force in this RH range is due to the capillary force.

Solvent Effects on Molecular Packing and Tribological Properties of Octadecyltrichlorosilane Films on Silicon

Self-assembled monolayer films of octadecyltrichlorosilane were prepared on silicon substrates using hexadecane, toluene, chloroform, and dichloromethane to determine the effects of solvent on molecular packing and tribological properties. Topographical atomic force microscopy images were used to evaluate the film quality and determine surface roughness, and tribological measurements, including friction, adhesion, and elasticity, provided additional information on the local nanoscale packing of the films. Our results showed that solvent viscosity and polarity affected the tribological properties of the films, with films prepared using hexadecane exhibiting superior properties. Langmuir-Blodgett experiments indicated that intermolecular interactions were stronger between octadecyltrichlorosilane and hexadecane molecules than for any other solvent in this study. These results demonstrate that solvent properties are an important consideration in monolayer film preparation and optimization for friction controlled surfaces.

Interface dynamics and mechanisms of nanoindented alkanethiol self-assembled monolayers using molecular simulations

The interface and nanoindentation mechanisms of alkanethiol self-assembled monolayers (SAMs) chemisorbed on a gold surface are investigated using molecular dynamics simulation. The mechanisms include the nanoindentation depths, the workpiece temperatures, the numbers of SAM layers, the length of united-atoms per chain, and the shapes of the indenters. The simulation results show that the disorder and the plastic mobility of SAM chains increased with increasing indentation depth. The relaxation force and the plastic energy almost linearly increased with increasing indentation depth. The disorder region after indentation at high temperature is larger than that at low temperature. The adhesive force shows a dependence on temperature during indentation. The potential energy decreases with increasing number of SAM layers. The structural morphologies of the SAMs were not affected at the third layers for SAM film with four layers. The maximum load quickly decreases for film with two SAM layers. The structures of the SAM can be easily tilted and bent when the united-atoms per chain length is long. The SAM atoms become more disorderly and the elastic recovery is smaller when the SAM length of the united-atoms per chain is long after indentation.

Fabrication and characterization of single walled carbon nanotubes-iron phthalocyanine nano-composite: surface properties and electron transport dynamics of its self assembled monolayer film

Nano-composite of single walled carbon nanotubes (SWCNTs) and iron phthalocyanine, peripherally tetra-substituted with diethylaminoethanethiol (complex 2), was fabricated and characterized by infra red (IR) spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM technique gave a convincing image of the nano-composite. Self-assembled monolayer (SAM) film of the composite (SWCNT-2-SAM) was formed on gold electrode. Its surface properties were investigated, relative to that of 2-SAM film, by cyclic voltammetry and scanning electron microscopy (SEM). Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry, using [Fe(CN)6]3− as redox probe, were used to investigate the dynamics of electron transport in the SWCNT-2-SAM film. Lower charge transfer resistance, RCT, of the SWCNT-2-SAM modified Au electrode, relative to that of the 2-SAM modified Au electrode, was an indication of enhanced electron transport in the presence of SWCNT. The SAM film of the nano-composite showed better electrocatalytic behaviour, relative to that of 2-SAM film, towards the oxidation of the insecticide, carbofuran. The electrode (SWCNT-2-SAM) showed good selectivity for carbofuran, in the presence of diazinon (a non-electroactive interferent) and at lower concentration of the electroactive interferents (chlorpyrifos and dichlorfos).

Porphyrin self-assembled monolayers and photodynamic oxidation of tryptophan

The zinc and magnesium metalated derivatives of 5,5′-[12,12′-di(thiododecyloxy)-4,4′-phenyl)]-10,10′,15,15′,20,20′-hexakis(3,3′,4,4′,5,5′-hexakisdecyloxyphenyl)diporphyrin, 1b and 1c, have been synthesised and deposited to form self-assembled monolayer (SAM) films on the surface of gold-coated glass substrates. The SAM films have been characterized by RAIR spectroscopy and fluorescence spectroscopy. The potential for the porphyrin films to catalyze the oxidation of tryptophan within human serum albumin upon irradiation with white light has been demonstrated and attributed to the porphyrins acting as photosensitizers of oxygen to form oxidizing species.

Photolysis Kinetics of 4-(2-(4’-Pyridinyl)ethynyl)benzenic Diazonium Salt and Characterization of Its Self-Assembled Monolayer Film

In this paper, the photolysis kinetics of conjugate 4-(2-(4′-pyridinyl)ethynyl)benzene diazonium salt in acetonitrile and in solid-phase films was investigated by UV-Vis absorption spectroscopy after exposure to ultra-violet light (250 W, 245 nm). Results show that the photolysis kinetics follows first order reaction kinetics and the polar acetonitrile solvent causes the diazonium group to photodecompose more easily. In addition, X-ray photoelectron spectroscopy (XPS) and electrochemical analysis were used to confirm the formation of 4-(2-(4′-pyridinyl)ethynyl) benzene monolayer films which were anchored onto the surface of quartz and indium-tin-oxide (ITO) glass slides by covalent bonds. This method sets the stage for the successful design of organic-metal containing ultrathin films based on a layer-by-layer self-assembly technique.

A Facile Approach for Self-Assembled Gold Nanorods Monolayer Films and Application in Surface-Enhanced Raman Spectroscopy

A facile approach for fabrication of the gold nanorods (GNRs) monolayer films on quartz slides was exploited. The fabrication procedure just involved that the amino-functionalized quartz slides were immersed directly into the GNRs solutions which was prepared by seed-mediated method and treated with poly(vinyl pyrrolidone) (PVP). The intermolecular hydrogen bonds between the carbonyl groups on the PVP and the amino groups on the amino-functionalized quartz slide were responsible for the GNRs immobilization. The optical response in the visible and the near infrared region of resulting GNRs can be easily controlled simply by varying the immersion time of the substrates. GNRs monolayer films with subtle control over the surface coverage and tunable plasmon resonance as surface-enhanced Raman spectroscopy (SERS) substrates were investigated, which exhibited high SERS activity, stability and reproducibility.

Silicon-based Interdigitated Electrode Array for Highly Sensitive Heavy Metal Ion Sensing

Abstract Highly sensitive sensors based on capacitance changes have been developed for the detection of heavy metal ions by utilization of self-assembled monolayer films on silicon oxide surfaces between interdigitated electrodes. The attractive ability of the sensor to efficiently preconcentrate trace amounts of Cu2+ allows a simple and ultrasensitive method for copper ion detection. The wide range linear calibration curve of 1.0×10−13 to 1.0×10−9 M Cu2+ is constructed at 53 Hz. This sensor is highly sensitive and can push the detection limit of copper ion down to 1.0×10−13 M.

Modification of metal/semiconductor junctions by self-assembled monolayer organic films

Two new metal/molecule/semiconductor contacts, Au/n-Si/TDA/Au and Au/p-Si/ODM/Au, were fabricated to understand effect of organic compounds, tridecylamine and octadecylmercaptan self-assembled monolayer (SAM) films, on electrical charge transport properties of the metal/semiconductor junctions. The morphology of the organic monolayers deposited on Si substrates was investigated by atomic force microscopy. The molecular coverage of ODM deposited on p-Si is poorer than that of TDA on n-Si substrate. The ideality factors of the p-Si/ODM and n-Si/TDA diodes were found to be 1.66 and 1.48, respectively. The electrical results show that the tridecylamine monolayer passivated junction has a lower ideality factor. The ideality factor indicates clear dependence on two different type functional groups R-SH (Thiol) and R-NH 2 (Amin) groups and it increases with different functional groups of organic molecule. The barrier height φ b value of the n-Si/TDA diode is smaller than that of p-Si/ODM diode, as a result of chain length of the SAM organic molecules. The interface state density D it values of the diodes were determined using conductance technique. The n-Si/TDA diode has the smaller interface state density according to p-Si/ODM diode. We have evaluated that the organic molecules control the electronic parameters of metal/semiconductor diodes and thus, organic modification helps to get one step closer towards to new organic assisted silicon based microelectronic devices.

Electric Conduction Properties of Self-assembled Monolayer Films of Ru Complexes with Disulfide/Phosphonate Anchors in a Au–(Molecular Ensemble)–(Au Nanoparticle) Junction

Abstract Electric conduction properties of SAM films of Ru complexes with disulfide or phosphonate anchors have been measured by Au-nanoparticle-assisted conducting probe atomic force microscopy. The I–V curves of these films were nonlinear, and the resistivities depend on the headgroups and the layered structure in ways that can be explained by the contact resistance and the insulating effect of the Zn–phosphonate interlayer in metal–molecule–metal junctions. Asymmetry in these films causes a rectification effect on the I–V response.

Low adhesion, non-wetting phosphonate self-assembled monolayer films formed on copper oxide surfaces

Self-assembled monolayer (SAM) films have been formed on oxidized copper (Cu) substrates by reaction with 1H,1H,2H,2H-perfluorodecylphosphonic acid (PFDP), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), and octylphosphonic acid (OP) and then investigated by X-ray photoelectron spectroscopy (XPS), contact angle measurement (CAM), and atomic force microscopy (AFM). The presence of alkyl phosphonate molecules, PFDP, ODP, DP, and OP, on Cu were confirmed by CAM and XPS analysis. No alkyl phosphonate molecules were seen by XPS on unmodified Cu as a control. The PFDP/Cu and ODP/Cu SAMs were found to be very hydrophobic having water sessile drop static contact angles of more than 140°, while DP/Cu and OP/Cu have contact angles of 119° and 76°, respectively. PFDP/Cu, ODP/Cu, DP/Cu, and OP/Cu SAMs were studied by friction force microscopy, a derivative of AFM, to better understand their micro/nanotribological properties. PFDP/Cu, ODP/Cu, and DP/Cu had comparable adhesive force, which is much lower than that for unmodified Cu. ODP/Cu had the lowest friction coefficient followed by PFDP/Cu, DP/Cu, and OP/Cu while unmodified Cu had the highest. XPS data gives some indication that a bidentate bond forms between the alkyl phosphonate molecules and the oxidized Cu surface. Hydrophobic phosphonate SAMs could be useful as corrosion inhibitors in micro/nanoelectronic devices and/or as promoters for anti-wetting, low adhesion surfaces.