Carborane functionalized graphene oxide,a precursor for conductive self-assembled monolayers

A new, advanced graphene oxide based material modified with the extremely stable o-carboranoyl 1,2-C₂B₁₀H₁₁ unit was created. The carboranoyl-functionalized graphene oxide was synthesized from Li[1,2-C₂B₁₀H₁₁] and COCl-functionalized graphene-oxide, which was prepared by converting the graphene oxide –COOH groups to COCl with SOCl₂. The carboranoyl-functionalized graphene oxide was characterized using infrared and X-ray photoelectron spectroscopy. The morphology was investigated via transmission, scanning electron, and atomic force microscopies. The electrochemical properties of o-carboranoyl functionalized graphene oxide was studied using cyclic voltammetry, electrochemical impedance spectroscopy and electric force microscopy. This new graphene-based material could in future be used as a precursor for conductive self-assembled monolayers.     

Multiscale patterning of graphene oxide and reduced graphene oxide for flexible supercapacitors

A simple and facile method for multiscale, in-plane patterning of graphene oxide and reduced graphene oxide (GO–rGO) was developed by region-specific reduction of graphene oxide (GO) under a mild irradiation. The UV-induced reduction of graphene oxide was monitored by various spectroscopic techniques, including optical absorption, X-ray photoelectron spectroscopy (XPS), Raman, and X-ray diffraction (XRD), while the resultant GO–rGO patterned film morphology was studied on optical microscope, scanning electron microscope (SEM), and atomic force microscope (AFM). Flexible symmetric and in-plane supercapacitors were fabricated from the GO–rGO patterned polyethylene terephthalate (PET) electrodes to show capacitances up to 141.2 F/g.     

Electrochemical studies of self-assembled monolayers using impedance spectroscopy

Self-assembled monolayers (SAMs) using various organic molecules were constructed on thin thermal silicon dioxide on silicon structures. These structures have potential bio-applications. The monolayers are part of a capacitor sensor based on the electrolyte-insulator-silicon (EIS) structure. The main goal of this work is to investigate the ability of SAMs for acting as an intermediate layer between aqueous solutions and silicon based biosensors. This work presents monolayers based on trimethoxy silanes with various functional groups such as: (a) (3-aminopropyl)-trimethoxysilane, (b) octadecyltrimethoxysilane and (c) trimethoxy(3,3,3-trifluoropropyl)silane. The biosensor is a differential sensor with a built in reference based on the structure of electrolyte-insulator-silicon or in ion-sensitive-field-effect-transistors (ISFETs). While the methyl and fluorine groups can act as the reference part, which is passive to the bio-molecules detection, the capacitor containing the amine groups has the sensing capabilities to the biological molecules. The amine (NH₂) tail groups can be active for those attachments via glutaraldehyde.The film's study and characterization was made by spectroscopic ellipsometry, contact angle method (CA) and FT-IR spectroscopy. The electrical methods such as capacitance-voltage (C-V) and electrochemical impedance spectroscopy (IS) were used to investigate the whole electrolyte/SAM/SiO₂/Si structure. We will show the detection capabilities of these 2D structures, its electrical equivalent model through a simulation fitting and will discuss the application of those structures to bio-sensitive metal/oxide/silicon (MOS) devices.     

Nanofabrication of self-assembled monolayers using scanning probe lithography

This Account focuses on our recent and systematic effort in the development of generic scanning probe lithography (SPL)-based methodologies to produce nanopatterns of self-assembled monolayers (SAMs). The key to achieving high spatial precision is to keep the tip-surface interactions strong and local. The approaches used include two AFM-based methods, nanoshaving and nanografting, which rely on the local force, and two STM-based techniques, electron-induced diffusion and desorption, which use tunneling electrons for fabrication. In this Account we discuss the principle of these procedures and the critical steps in controlling local tip-surface interactions. The advantages of SPL will be illustrated through various examples of production and modification of SAM nanopatterns and their potential applications.     

Hollow graphene spheres self-assembled from graphene oxide sheets by a one-step hydrothermal process

We developed a one-step hydrothermal method to assemble graphene oxide (GO) sheets into hollow graphene spheres (HGSs), using only a GO/H₂SO₄ aqueous suspension as the starting material. Scanning electron microscope, focused ion beam scanning electron microscope and transmission electron microscope images show that the as-prepared HGSs vary from 1 to 3 μm in diameter and have a hollow interior structure. The as-prepared HGSs show a high capacitance of 207 F g⁻¹, as well as good rate capability and cycling stability when used as electrode materials for supercapacitors.     

Formation kinetics and stability of phosphonate self-assembled monolayers on indium–tin oxide

Phosphonate self-assembled monolayers (SAMs) have been widely used for the surface modification of indium–tin oxide (ITO) electrodes; however, their formation kinetics and stability are not well understood. In this paper, we describe our electrochemical studies of the formation kinetics and stability of a series of phosphonate SAMs on ITO electrodes. In particular electrochemical impedance spectroscopic (EIS) and cyclic voltammetric (CV) measurements have been carried out on three carboxy-terminated phosphonate SAMs by using Fe(CN)₆^3−/4− as redox indicators. The dependence of the charge-transfer resistance (obtained from the EIS plots) on the incubation time allows an estimation of the apparent fractional surface coverage of phosphonate SAMs. The apparent formation rate constant (k_obs) was determined by fitting the experimental data to a Langmuir adsorption model. For 3-phosphonopropanoic acid (PPA), the k_obs value increases when the PPA concentration increases in the deposition solution, and is smaller than those of thiolate SAMs on Au. The stability of phosphonate SAMs was investigated in three different media (pure water, phosphate-buffered saline (PBS) solution, and ambient air condition). It has been shown that the phosphonate SAMs are rather stable in either PBS solution or ambient air condition.     

Shaping graphene oxide by electrochemistry: From foams to self-assembled molecular materials

The ability to control the three-dimensional architecture of graphene-based materials following a rational design is essential for technological applications. Here we demonstrate that the electrochemical etching can be used as a surgical tool to tailor the morphology of graphene electrodes and to impart special features, like micrometric channels and controlled mesoporosity (foams). The final materials, thanks to the high surface area, can represent a promising class of carbon-based supercapacitors. Otherwise, new materials can be prepared using a bottom-up strategy that exploits the self-assembly of the graphene oxide quantum dots produced during the electrochemical erosion. The advantages of this second approach reside not only in the possibility to downscale the control over the spatial organization as compared to the use of conventional micrometric graphene sheets, but also in the introduction of the intrinsic luminescent properties of the quantum dots in the final material. As a proof of concept we report the preparation of luminescent nanospheres by exploiting the self-organization of the graphene oxide quantum dots around frozen water nuclei.     

Large area nanopatterning of alkylphosphonate self-assembled monolayers on titanium oxide surfaces by interferometric lithography

We demonstrate that interferometric lithography offers a fast, simple route to nanostructured self-assembled monolayers of alkylphosphonates on the native oxide of titanium. Exposure at 244 nm using a Lloyd's mirror interferometer caused the spatially periodic photocatalytic degradation of the adsorbates, yielding nanopatterns that extended over square centimetre areas. Exposed regions were re-functionalised by a second, contrasting alkylphosphonate, and the resulting patterns were used as templates for the assembly of molecular nanostructures; we demonstrate the fabrication of lines of polymer nanoparticles 46 nm wide. Nanopatterned monolayers were also employed as resists for etching of the metal film. Wires were formed with widths that could be varied between 46 and 126 nm simply by changing the exposure time. Square arrays of Ti dots as small as 35 nm (λ/7) were fabricated using two orthogonal exposures followed by wet etching.     

Potential-assisted deposition of mixed alkanethiol self-assembled monolayers

Preparation of self-assembled monolayers (SAMs) usually involves passive incubation. The recently developed potential-assisted deposition is indeed more selective as well as 100-fold faster than passive adsorption, thereby enhancing the reproducibility of the monolayer deposition.This article aims to identify the electrodeposition conditions necessary to prepare mixed alkanethiol SAMs on gold surface. Parameters such as concentrations in solution, electrode polarization and deposition time were examined for two chain lengths, C₃ (mercaptopropionic acid, MPA) and C₁₈ (octadecanethiol, ODT). The kinetics and composition of the SAMs were systematically characterized by reductive stripping analysis and by surface plasmon enhanced ellipsometry. Control of the surface concentration of the stable binary SAM was achieved by carefully adjusting the MPA fraction at between 70% and 95% in the deposition solution. Three theoretical models were developed. For MPA or ODT SAMs, whatever the adsorbate and the time period, the second order model gives the best fit. For the binary monolayer (MPA-ODT), a co-adsorption model was developed to describe the kinetics before 10 s.     

Electrochemical assessment of electrochemical oxidation stability of self-assembled monolayers on gold and preparation of binary self-assembled monolayers on gold

Binary thiolates self-assembled monolayers (SAMs) on gold have been prepared efficiently by using an electrochemical oxidation combining with replacement reaction method. The electrochemical results show that both the SAMs of mercaptoacetic acid (MAA) and 1-dodecanethiol (DT) on gold surface are oxidized at certain anodic potentials (noting: it is not oxidative redeposition of alkanethiolate monolayers on gold surface), and the degree of electrochemical oxidation of single thiolates-SAMs is function of the polarization potential and polarization time. Therefore, formation of binary MAA-DT thiolates-SAMs can be achieved if MAA or DT is present in solution after the electrochemical oxidation of DT or MAA SAMs on gold electrode. In this case, the ratio of MAA to DT on the binary thiolates-SAMs can be easily controlled by controlling polarization potential and/or polarization time. The present approach enables us to prepare homogeneously binary SAMs with different composition, and assess the electrochemical oxidation stability of single thiolates-SAMs by means of cyclic voltammetry measurements of redox probe.     

Ammonia gas sensors based on chemically reduced graphene oxide sheets self-assembled on Au electrodes

We present a useful ammonia gas sensor based on chemically reduced graphene oxide (rGO) sheets by self-assembly technique to create conductive networks between parallel Au electrodes. Negative graphene oxide (GO) sheets with large sizes (>10 μm) can be easily electrostatically attracted onto positive Au electrodes modified with cysteamine hydrochloride in aqueous solution. The assembled GO sheets on Au electrodes can be directly reduced into rGO sheets by hydrazine or pyrrole vapor and consequently provide the sensing devices based on self-assembled rGO sheets. Preliminary results, which have been presented on the detection of ammonia (NH₃) gas using this facile and scalable fabrication method for practical devices, suggest that pyrrole-vapor-reduced rGO exhibits much better (more than 2.7 times with the concentration of NH₃ at 50 ppm) response to NH₃ than that of rGO reduced from hydrazine vapor. Furthermore, this novel gas sensor based on rGO reduced from pyrrole shows excellent responsive repeatability to NH₃. Overall, the facile electrostatic self-assembly technique in aqueous solution facilitates device fabrication, the resultant self-assembled rGO-based sensing devices, with miniature, low-cost portable characteristics and outstanding sensing performances, which can ensure potential application in gas sensing fields.     

Dipyrromethene-dodecanethiol self-assembled monolayers deposited onto gold electrodes

In the work presented, a dipyrromethene with a thiol group has been applied to gold electrode modification. The blocking behaviour of mixed dipyrromethene-thiol (DPT)/dodecanethiol (DDT) self-assembled monolayers (SAMs) has been studied as a function of pH using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). It has been shown that hydrogen-bonding between adjacent ligands affects the pH properties of SAMs studied. In the modification solution, an optimum ratio dipyrromethene-thiol/dodecanethiol was found to get the strongest pH response of SAM. The dipyrromethene-thiol/dodecanethiol SAMs have been characterized also by FTIR and wettability contact angle measurements.The possible application of the mixed dipyrromethene-thiol/dodecanethiol SAMs presented will be discussed.     

Iron oxide magnetic nanoparticles used as probing agents to study the nanostructure of mixed self-assembled monolayers

Self-assembled monolayers (SAMs) of organic molecules are of exceptional technological importance since they represent a convenient, flexible, and simple system for tuning the chemical and physical properties of surfaces. The fine control of surface properties is directly dependent on the structure of mixed SAMs which is difficult to characterize at the nanoscale with usual techniques such as scanning probe microscopies. In this study, we report on a general method to investigate at the nanoscale the structure of molecular patterns which consist in SAMs of two components. Iron oxide nanoparticles (NPs) have been used as probing agents to study indirectly the structure of mixed SAMs. Mixed SAMs were prepared by the replacement of mercaptododecane (MDD) adsorbed by mercaptoundecanoic acid (MUA) molecules on gold substrates. Therefore, the SAM surface displays both chelating carboxylic terminal groups and non-chelating methylene terminal groups. As NPs have been previously demonstrated to specifically interact with carboxylic acid groups, the increasing density in NPs was correlated with the evolution of the COOH/CH(3) terminal groups ratio. Therefore the structure of mixed SAMs was studied indirectly as well as the kinetic of the replacement reaction and its mechanism. With this aim, we took advantage of the SPR properties of the gold substrate and of the high refractive index of iron oxide nanoparticles to follow their assembling on mixed SAMs as a time resolved study. The high sensitivity and tuning of the SPR signal over a wide range of wavelengths are correlated with the NP density. Furthermore, SEM combined with image analysis has allowed studying the replacement rate of MDD by MUA in SAMs. We took also advantages of the magnetic properties of NPs to evaluate qualitatively the replacement of thiol molecules.     

Selective formation of monodisperse CdSe nanoparticles on functionalized self-assembled monolayers using chemical bath deposition

Using CdSe chemical bath deposition (CBD) we demonstrate the selective growth and deposition of monodisperse nanoparticles on functionalized self-assembled monolayers (SAMs) using time-of-flight secondary ion mass spectrometry and scanning electron microscopy. We show that the deposition mechanism involves both ion-by-ion growth and cluster-by-cluster deposition. On -COOH terminated SAMs strongly adherent CdSe nanoparticles form via a mixed ion-by-ion and cluster-by-cluster mechanism. Initially, Cd²⁺ ions form complexes with the terminal carboxylate groups. The Cd²⁺-carboxylate complexes then act as the nucleation sites for the ion-by-ion growth of CdSe. After a sufficient concentration of Se²⁻ has formed in solution via the hydrolysis of selenosulfate ions, the deposition mechanism switches to cluster-by-cluster deposition. On -OH and -CH₃ terminated SAMs monodisperse CdSe nanoparticles are deposited via cluster-by-cluster deposition and they do not bind strongly to the surface. Finally, under the appropriate experimental conditions we demonstrate the selective deposition of CdSe nanoparticles on patterned -CH₃/-COOH SAMs.     

硅烷自组装单分子层生长条件的优化

对长碳链硅烷在玻璃片表面自组装形成单分子层膜进行研究。以甲苯为溶剂,使辛烷基三乙氧基硅烷(OTES)、十二烷基三甲氧基硅烷(DTMS)、3-胺丙基三乙氧基硅烷(APTES)和十八烷基三氯硅烷(OTS)在玻璃表面进行生长,考察反应时间、反应浓度和可水解基团对自组装单分子层膜的影响。用接触角仪和全反射红外光谱仪(FT-IR)对单分子膜进行表征。结果表明:带有—Cl水解基团的OTS最易生长,而带有乙氧基离去基团的OTES比带有甲氧基的DTMS容易反应。在24℃时,1 mmol/L OTES反应20 min自组装单分子层膜生长很好,并且表面比较规整均一。