Formation Of Organic Monolayers Research Articles

Molecularly Modified Electrodes for Efficient Triboelectric Nanogenerators

The integration of organic molecules into monolayers on triboelectric layers and electrodes has significantly improved the performance of triboelectric nanogenerators (TENGs) in recent years. By modifying surfaces at the molecular level, one can enhance durability, power density, and cost‐efficiency, leading to flexible, lightweight, and more efficient devices. A simple chemistry of organic monolayer formation allows a precise control over orientation, coverage, consistency, and functionality. These monolayers boost surface charge density and output voltage while influencing surface polarization and dipole interactions. This review focuses on recent advances in chemical modification of electrodes for controlling surface charge density and altering surface dipoles, emphasizing the significance of organic monolayers. A new concept of Schottky‐based TENGs is also introduced that explores chemically modified sliding surfaces. Furthermore, the importance of flexoelectricity and its contribution to triboelectricity is discussed. By addressing current challenges and outlining future directions, this review underscores the crucial role of surface chemistry in advancing TENGs.

Formation of Organic Monolayers on KF-Etched Si Surfaces

Silicon is the most commonly used material in the microelectronics industry, due to its inherent advantages of high natural abundance, low cost, and high purity, coupled with the chemical and electrical stability at the interface with its oxide. For molecular electronics applications, oxide-free Si surfaces are widely used because of the relative ease of removing the oxide (SiOx) by chemical means, yielding a surface which forms strong covalent bonds with a wide range of chemical functional groups; another advantage is that these surfaces remain oxide-free in the absence of oxidising agents. Standard procedures require the use of either HF, NH4F, or a mixture of both as the etching solution; however, these two chemicals are highly corrosive and toxic, posing a significant risk to the experimentalist. Here, we report that for silicon wafers etched by using potassium fluoride, a less toxic chemical, the resulting surface is free of oxides and can be functionalized by self-assembled monolayers of 1,8-nonadiyne. To demonstrate this, Si/SiOx wafers were etched by using either KF or NH4F, followed by hydrosilylation with 1,8-nonadiyne and a click reaction of the terminal alkyne with azidomethylferrocene. The surface coverages and electron transfer kinetics of the ferrocene-terminated KF-etched surfaces are comparable to those formed by acidic fluoride etching procedures. This is the first study comparing the differences between surfaces functionalized by self-assembled monolayers of 1,8-nonadiyne which were etched by KF and NH4F. KF could be used as a replacement chemical for etching silicon wafers when a less corrosive and toxic chemical is required.

Study of three new halogenated oxoquinolinecarbohydrazide N-phosphonate derivatives as corrosion inhibitor for mild steel in acid environment

Three new and original halogenated oxoquinolinecarbohydrazide N-phosphonate derivatives (DHOP-F, DHOP-Cl and DHOP-Br) were prepared from the corresponding 4-oxo-1,4-dihydroquinoline-3-carboxylates (81, 71 and 65% yields, respectively), characterized by IR, 1H, 13C and 31P NMR spectroscopy and HRMS and evaluated as corrosion inhibitors for mild steel AISI 1020 in hydrochloric acid media. Electrochemical Impedance Spectroscopy, Linear Polarization Resistance and Weight Loss Study revealed an anticorrosive efficiency of 93.0, 84.2 and 66.8%, respectively, for DHOP-Br, DHOP-Cl and DHOP-F, at a very low concentration (0.48 mmol L−1). Physical chemical calculations showed that the adsorption process is spontaneous, and all molecules obey the Langmuir theory. Data crossing between electrochemical and gravimetric measurements reveals a protection mechanism of adsorption by organic monolayer formation on the metal surface. Polarization curves proves that the organic molecules are all mixed-type corrosion inhibitors and Atomic Force Microscopy images suggest the formation of a protective layer on the metal surface. Ab initio calculations are in great agreement with the experimental results and, therefore, used to rationalize the reason why the bromide derivative is the best inhibitor.

Redox-Active Ferrocene grafted on H-Terminated Si(111): Electrochemical Characterization of the Charge Transport Mechanism and Dynamics

Electroactive self-assembled monolayers (SAMs) bearing a ferrocene (Fc) redox couple were chemically assembled on H-terminated semiconducting degenerate-doped n-type Si(111) substrate. This allows to create a Si(111)|organic-spacer|Fc hybrid interface, where the ferrocene moiety is covalently immobilized on the silicon, via two alkyl molecular spacers of different length. Organic monolayer formation was probed by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) and X-ray photoelectron spectroscopy (XPS) measurements, which were also used to estimate thickness and surface assembled monolayer (SAM) surface coverage. Atomic force microscopy (AFM) measurements allowed to ascertain surface morphology and roughness. The single electron transfer process, between the ferrocene redox probe and the Si electrode surface, was probed by cyclic voltammetry (CV) measurements. CVs recorded at different scan rates, in the 10 to 500 mV s−1 range, allowed to determine peak-to-peak separation, half-wave potential, and charge-transfer rate constant (KET). The experimental findings suggest that the electron transfer is a one electron quasi-reversible process. The present demonstration of surface engineering of functional redox-active organometallic molecule can be efficient in the field of molecular electronics, surface-base redox chemistry, opto-electronic applications.

Surface chemistry of a hydrogenated mesoporous p-type silicon

The finality of this work is devoted to the grafting of organic molecules on hydrogen passivated mesoporous silicon surfaces. The study would aid in the development for the formation of organic monolayers on silicon surface to be exploited for different applications such as the realisation of biosensors and medical devices. The basic material is silicon which has been first investigated by FTIR at atomistic plane during the anodic forward and backward polarization (i.e. “go” and “return”). For this study, we applied a numerical program based on least squares method to infrared absorbance spectra obtained by an in situ attenuated total reflection on p-type silicon in diluted HF electrolyte. Our numerical treatment is based on the fitting of the different bands of IR absorbance into Gaussians corresponding to the different modes of vibration of molecular groups such as siloxanes and hydrides. An adjustment of these absorbance bands is done systematically. The areas under the fitted bands permit one to follow the intensity of the different modes of vibration that exist during the anodic forward and backward polarization in order to compare the reversibility of the phenomenon of the anodic dissolution of silicon. It permits also to follow the evolution between the hydrogen silicon termination at forward and backward scanning applied potential. Finally a comparison between the states of the initial and final surface was carried out. We confirm the presence of clearly four and three distinct vibration modes of siloxanes/hydroxide, SiOx, and hydrides, SiHx, respectively. The results show clearly that the adsorbed species found in the final stage after an electrochemical treatment consist of surface hydrogen and they show also that the surface morphology is different compared to the starting one, which is considered as reference.It is clear that the H-terminated of porous silicon surface is hydrophobic in nature. The hydrophobic character of this surface makes difficult the grafting of a probe which serves to get other molecules; where from the necessity of the presence of a hydrophilic surface on the porous silicon surface. This will facilitate the penetration and the grafting of the molecules probes.So to improve the penetration and the grafting of the organic molecules or the immobilization of the probes on the hydrogenated mesoporous silicon surfaces under wet chemical conditions an intermediate step is required. In this second study, we use the following procedure. After the porosification of the silicon by electrochemical anodisation, the porous SiH layer returns a hydrophobic surface. This stage is a starting point of multistep scheme for the surface modification. The next step is the thermal hydrosilylation in order to have an acidic surface. The acidic surface was then modified by the chains of Poly(ethylene glycol) (PEG) which is a highly investigated polymer for the covalent modification of biological macromolecules and surfaces. The grafting of PEG returns a hydrophilic surface confirmed by the IR results.

Microwave-assisted formation of organic monolayers from 1-alkenes on silicon carbide.

The rate of formation of covalently linked organic monolayers on HF-etched silicon carbide (SiC) is greatly increased by microwave irradiation. Upon microwave treatment for 60 min at 100 °C (60 W), 1-alkenes yield densely packed, covalently attached monolayers on flat SiC surfaces, a process that typically takes 16 h at 130 °C under thermal conditions. This approach was extended to SiC microparticles. The monolayers were characterized by X-ray photoelectron spectroscopy and static water contact angle measurements. The microwave-assisted reaction is compatible with terminal functionalities such as alkenes that enable subsequent versatile "click" chemistry reactions, further broadening the range and applicability of chemically modified SiC surfaces.

Spectroelectrochemical Behaviour of p-Aminobenzenethiol (p-ABT) on Platinum Group Metals

Researchers have particular interest in amino-terminated monolayers because they have been used for several applications. Among them, SAMs based on aromatic thiols have been vastly studied. The formation of organic monolayers by self-assembly is directed by a specific interaction between a terminal functional group and the surface, in this case the bond formed between the thiol sulphur and the metal. Aminobenzenethiols (ABTs) are frequently studied as SERS probe molecules and particularly the p-ABT is commonly used as molecular junction and building block in SAMs. Several contributions have been reported about the p-ABT adsorbed on Ag and Au due to the same specific behaviour under certain experimental circumstances. SERS from the self-assembled monolayer of p-ABT on Ag and Au is significantly different from the normal Raman spectrum of the molecule in solid state. In a normal Raman spectrum only bands that correspond to completely symmetric a1 vibrations are observed, but in its SERS spectrum, non-a1-type bands are also present (1143, 1390 and 1432 cm-1). However, the SERS from benzenethiol on these metals are substantially the same as the normal Raman spectrum. It was first speculated [1, 2] that the alteration of the spectrum of p-ABT could be due to the CT mechanism contribution, as a consequence of the alteration of the electronic states of benzenethiol by the introduction of an -NH2 group on the benzene ring. Alternatively, it was shown, based on the spectral similarity of p-ABT and 4,4’-DMAB, that the non-a1-type bands correspond to N=N stretching vibrations of 4,4’-DMAB produced from p-ABT via a catalytic coupling reaction [3]. Recently, the formation of the azo-derivative, observed in the cases of gold and silver surfaces, was not detected on copper surfaces [4] pointing out that the metal could play an important role in the azobenzene derivative formation. In our knowledge no studies have been done with Pt-group metals, which are generally accepted to be weak electromagnetic enhancing metals under conditions of visible wavelength excitation. Consequently, the CT mechanism could play an important role. We report in this work a spectroelectrochemical study with different Pt-group metals, for which a correlation between their voltammetric behaviour and the SERS spectra is established.Figure. (A) SERS spectra and (B) Cyclic voltammetry of p-ABT on Pt nanoparticles depostited on glassy carbon. Test solution: 0.1 M NaClO4 + 0.001 M p-ABT. Sweep rate: 50 mVs-1 [1] Osawa, M., Matsuda, N., Yoshii, K., Uchida, I., J. Phys. Chem. 98 (1994) 12702[2] Kim, K., Kim, K.L., Shin, K.S., J. Phys. Chem. C 117 (2013) 5975[3]Huang, Y.F. Zhu, H.P., Liu, G.K., Wu, D.Y., Ren, B. Tian, Z.Q. J. Am. Chem. Soc. 132 (2010) 9244[4] Dendisova, M., Havranek, L., Oncak, M. Matejka, P. J. Phys. Chem. C 117 (2013) 21245

Formation of Organic Monolayers on Silicon via Gas-Phase Photochemical Reactions

A new method for the formation of molecular monolayers on silicon surfaces utilizing gas-phase photochemical reactions is reported. Hydrogen-terminated Si(111) surfaces were exposed to various gas-phase molecules (hexene, benzaldehyde, and allylamine) and irradiated with ultraviolet light from a mercury lamp. The surfaces were studied with in situ Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and scanning tunneling microscopy. The generation of gas-phase radicals was found to be the initiator for organic monolayer formation via the abstraction of hydrogen from the H/Si(111) surface. Monolayer growth can occur through either a radical chain reaction mechanism or through direct radical attachment to the silicon dangling bonds.

A Kinetic Model of the Formation of Organic Monolayers on Hydrogen-Terminated Silicon by Hydrosilation of Alkenes

We have analyzed a kinetic model for the formation of organic monolayers based on a previously suggested free radical chain mechanism for the reaction of unsaturated molecules with hydrogen-terminated silicon surfaces (Linford, M. R.; Fenter, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc 1995, 117, 3145). A direct consequence of this mechanism is the nonexponential growth of the monolayer, and this has been observed spectroscopically. In the model, the initiation of silyl radicals on the surface is pseudo first order with rate constant, ki, and the rate of propagation is determined by the concentration of radicals and unreacted Si-H nearest neighbor sites with a rate constant, kp. This propagation step determines the rate at which the monolayer forms by addition of alkene molecules to form a track of molecules that constitute a self-avoiding random walk on the surface. The initiation step describes how frequently new random walks commence. A termination step by which the radicals are destroyed is also included. The solution of the kinetic equations yields the fraction of alkylated surface sites and the mean length of the random walks as a function of time. In mean-field approximation we show that (1) the average length of the random walk is proportional to (kp/ki)1/2, (2) the monolayer surface coverage grows exponentially only after an induction period, (3) the effective first-order rate constant describing the growth of the monolayer and the induction period (kt) is k = (2ki kp)1/2, (4) at long times the effective first-order rate constant drops to ki, and (5) the overall activation energy for the growth kinetics is the mean of the activation energies for the initiation and propagation steps. Monte Carlo simulations of the mechanism produce qualitatively similar kinetic plots, but the mean random walk length (and effective rate constant) is overestimated by the mean field approximation and when kp >> ki, we find k approximately ki0.7kp0.3 and Ea = (0.7Ei+ 0.3Ep). However the most striking prediction of the Monte Carlo simulations is that at long times, t >> 1/k, the effective first-order rate constant decreases to ki even in the absence of a chemical termination step. Experimental kinetic data for the reaction of undec-1-ene with hydrogen-terminated porous silicon under thermal reflux in toluene and ethylbenzene gave a value of k = 0.06 min(-1) and an activation energy of 107 kJ mol(-1). The activation energy is in reasonable agreement with density functional calculations of the transition state energies for the initiation and propagation steps.

Electrochemically driven organic monolayer formation on silicon surfaces using alkylammonium and alkylphosphonium reagents

The functionalization of silicon surfaces with organic monolayers, bound through Si–C bonds, is an area of wide interest due to the technological promise of organosilicon hybrid devices, but also to investigate fundamental surface reactivity. In this paper, the use of alkylammonium and alkylphosphonium cations as sources of organic moieties to bind to hydrogen-terminated flat and porous silicon is demonstrated. Tetraalkylammonium, tetraalkyl/arylphosphonium reagents, and alkyl pyridinium salts can be utilized, but trialkylammonium salts cannot as they yield substantial surface oxidation. Under electrochemical conditions, either potentiostatic or galvanostatic modes, alkyl groups derived from the ammonium or phosphonium salts are grafted to the silicon surface and are bound through Si–C bonds. Covalent attachment of the organic monolayers to the surface was demonstrated by XPS, AFM scribing, and FTIR. The mechanism may proceed via reduction of the ammonium salt yielding alkyl radicals, R , which may be reduced to R − and attack surface Si–Si bonds, leading to Si–C bonds, or the formation of silyl anions (≡Si −) under the cathodic conditions followed by nucleophilic attack on the trialkylammonium cation.

Monolayer Pattern Evolution via Substrate Strain-Mediated Spinodal Decomposition

Investigations of octylsilane (C8H17SiH3) monolayer pattern formation on Au(111) are reported. Scanning tunneling microscopy data display the evolution of a approximately 6 nm scale pattern of interwoven features concomitant with ejection of surface Au atoms and relaxation of the Au(111) 23xsqrt[3] surface reconstruction. Numerical simulations suggest the surface dynamics are governed by a substrate strain-mediated spinodal decomposition mechanism, novel to organic monolayer formation. Collectively, the experimental and theoretical data indicate strain-inducing Si-Au bond interactions drive the pattern formation and the alkyl chains play a negligible role.

Dynamic in Situ Characterization of Organic Monolayer Formation via a Novel Substrate-Mediated Mechanism

Ultrahigh vacuum scanning tunneling microscopy data investigating octylsilane (C8H17SiH3) monolayer pattern formation on Au(111) are presented. The irregular monolayer pattern exhibits a 60 A length scale. Formation of the octylsilane monolayer relaxes the Au(111) 23 x square root3 surface reconstruction and ejects surface Au atoms. Au adatom diffusion epitaxially extends the Au(111) crystal lattice via step edge growth and island formation. The chemisorbed monolayer covers the entire Au surface at saturation exposure. Theoretical and experimental data suggest the presence of two octylsilane molecular adsorption phases: an atop site yielding a pentacoordinate Si atom and a surface vacancy site yielding a tetracoordinate Si atom. Theoretical simulations investigating two-phase monolayer self-assembly dynamics on a solid surface suggest pattern formation results from strain-induced spinodal decomposition of the two adsorption phases. Collectively, the theoretical and experimental data indicate octylsilane monolayer pattern formation is a result of interfacial Au-Si interactions and the alkyl chains play a negligible role in the monolayer pattern formation mechanism.

Formation of Organic Monolayer on a Hydrogen Terminated Si(111) Surface via Silicon-Carbon Bond Monitored by ATR FT-IR and SFG Spectroscopy: Effect of Orientational Order on the Reaction Rate

Abstract The formation of an octadecyl monolayer on a hydrogen terminated Si(111) surface in neat octadecene at 200 °C was followed by attenuated total reflectance Fourier transform infrared (ATR FT-IR) and sum frequency generation (SFG) spectroscopy and ellipsometry. Until ca. 60% of the monolayer was formed, the monolayer formation proceeded with simple first-order reaction kinetics and the monolayer was disordered. As the coverage steadily increased, the order of the monolayer became very high and the rate of the monolayer formation became significantly slower.

Two-Step Approach to the Formation of Organic Monolayers on the Silicon Oxide Surface

A new synthetic strategy has been developed for the formation of densely packed monolayers on oxide surfaces. This technique involves a simple two-step procedure in which the oxide surface is initially chlorinated and then exposed to alcohol molecules for monolayer formation via Si−O linkages. We demonstrate this approach with the formation of octadecanol (C18) monolayers on the oxide-terminated Si(111) surface. Characterization of the resulting C18H37O− monolayer by a combination of techniques reveals a densely packed film with partial disorder. At the highest surface coverage, the thickness of the film is determined to be 24 A. Kinetics of monolayer growth is characterized by a fast initial step at low coverage; the rate constant decreases by 3 orders of magnitude when surface coverage approaches saturation.

Effects of Organic Monolayer Formation on Electrochemiluminescence Behavior of Porous Silicon

The effects of various organic monolayers on the surface of porous silicon on the electrochemiluminescence (ECL, also referred to as electroluminescence in the literature) characteristics were investigated. Surfaces were terminated with alkyne, alkene, and alkyl functionalities through cathodic electrografting (CEG), Lewis acid-mediated hydrosilylation (LA), and anodic electrografting (AEG), respectively. ECL was induced through the known formic acid/sodium formate electron injection system. Alkyl-terminated surfaces produced through AEG yielded the brightest emission of any of the functionalized surfaces although ECL emission was about half as intense as that from the underivatized Si−H-terminated surface. The lifetime, however, was extended by a factor of 2, and these surfaces demonstrate an unprecedented recharging phenomenon. When ECL ceases, a brief 10 s application of a cathodic bias restores most of the ECL emission intensity. This process can be cycled about 10 times, and results in a substantially greater light output than that from any of the surfaces examined here, including the Si−H-terminated surface. Dodecenyl-terminated surfaces, produced through Lewis acid-mediated hydrosilylation of 1-dodecyne, show the greatest lifetimes, an order of magnitude longer than that of an Si−H-terminated surface, but their emission intensity is unfortunately very low. The induction times for light emission to occur decreased for all the functionalized surfaces, as compared to the native Si−H surface. Explanations for the effects of organic monolayer formation on ECL observed here are described.

Organic monolayers on Si(111) by electrochemical method

This work details the formation of organic monolayers on Si(111) by electrochemical methods. We show that grafting of phenyl groups is possible by reduction of +N 2–Ar–X cations where the substituent X may be Br, NO 2, COOH, CN, C n H 2 n+1 ( n=1, 4, 12). Characterizations show that the electrochemical process is self stopped after completion of the first monolayer whose structure is (2×1) close packed in the case X=Br and CH 3 as observed by STM. The stability and passivating properties of films are also investigated.

Synthesis of Novel n-Alkatrienyltrichlorosilane - Versatile Reagent and Precursor for Organic Monolayer Formation

Preparation of (Z,Z,Z)-9,12,15-octadecatrienyltrichlorosilane (2) by the reaction of the corresponding Grignard reagent with SiCl4 is described. The novel compound of the type RSiX3, useful for various organometallic applications and with the ability to form close-packed ordered monolayers chemically bonded to the surface, was thoroughly characterized by spectral analysis.

A convenient route to thiol terminated peptides for conjugation and surface functionalization strategies.

The derivatization of poly(p-(chloromethyl)styrene-co-divinylbenzene) (Merrifield resin) with N-(tert-butoxycarbonyl)-2-aminoethanethiol is presented as a convenient route for the generation of thiol terminated peptides using a solid phase methodology. Maximum resin substitution reached 92% (773 mumol/g) after 24 h. However, at 30 min, yields exceeded 400 mumol/g, above which the resin is suitable for solid phase peptide synthesis. Thiol terminated peptides are well-suited for subsequent chemical conjugation reactions or for the formation of organic monolayers on metal substrates.

Detection of submonolayer 18O on a gold surface by nuclear reaction analysis

A gold substrate is the preferred solid surface for formation of an organic self-assembled monolayer (SAM). Device fabrication process may require the gold film to be exposed to photolithographic processing and plasma treatment prior to molecular assembly. It has been observed that oxygen plasma treatment prevents the formation of SAMs: however, subsequent treatment with an argon plasma allows assembly of the organic monolayers. To understand the mechanisms involved, a plasma containing 98% 18O was used and the film surface was analysed using the 18O(p, α)15N nuclear reaction. A proton beam energy of 830 keV was chosen because a broad resonance in the excitation curve maximises the oxygen detection sensitivity. A silicon surface barrier detector was employed to detect α particles. The analysis showed that after oxygen plasma treatment the gold layer was covered with about 0.8 × 1015 oxygen atoms cm−2. The presence of this oxygen submonolayer prevents the sulfur-gold chemical interaction taking place as an essential precursor to the organic monolayer formation.

Two-dimensional phase formation by heterocoalescence of a surface active droplet at the aqueous/mercury interface

We propose a novel experimental technique for the characterization of the initial steps of organic monolayer formation at a fluid interface: mercury/aqueous dispersion of insoluble surfactants. The electrochemical methodology, chronoamperometry at a dropping mercury electrode, is based on suppression of the polargoraphic maximum of 10 −3 M Hg(II) in artificial seawater. Individual droplets (diameter ⩾ 1μm) of neutral and electroactive molecules (1-heptadecene, methyl oleate, squalene) transform into compact molecular layers at the mercury electrode/seawater interface. As a result, the mercury electrode surface is composed of two coexisting and immiscible domains: metallic and organically coated. On the organically coated fraction of the surface of the mercury electrode, Hg(II) reduction proceeds with a measurable nucleation and growth step forming a compact monolayer of elemental mercury on top of the organic film. The attachment of the organic film to the electrode substrate through the collision and spreading of insoluble surfactant droplets provides a convenient in situ technique for the preparation and investigation of organic or organometallic films.