Functional Self-assembled Monolayers Research Articles

(Invited) Integration of Functional Nanostructures and Nanoparticles into Micro- and Nanoelectronic Components and Systems

In recent decades, there has been a significant evolution of microelectronic and semiconductor technologies towards the nanoscale. Both bottom-up and top-down integration concepts of functional nanostructures and nanoparticles into micro- and nanofabricated electronic components have become critical issues that decisively affect the performance of smart electronic systems in various application areas such as environmental sensing and energy harvesting.Functional nanostructures and nanoparticles of different shapes, sizes, morphologies, and materials are now used in various technical and industrial applications, medicine, pharmacy, biology, electronics, power engineering, ecology, and many other fields. However, the complex engineering challenge here is the integration of functional nanostructures, such as molecular components (e.g., functional self-assembled molecular monolayers or entities of desoxyribonucleic acid, DNA), organic or inorganic nanoparticles (metal, semiconductor, colloidal materials), or similar building blocks into nanolithographically fabricated environments.Manufacturing paradigms for this integration go beyond current CMOS- and MEMS-fabrication schemes. Scale-bridging and hybrid manufacturing schemes that combine established top-down manufacturing techniques for microelectronic devices, including aspects of nanotechnology, with bottom-up nanoassembly of molecules on unstructured and pre-structured substrates are required. Microsystems technology based on the combination of conventional semiconductor processing with biotechnological approaches has thus become an emerging field with manifold applications, especially in the field of sensor technology.This contribution describes two illustrative and innovative research questions in nanopatterning and system integration at the micro-nano-interface to motivate applications with new and unprecedented functionalities. The first research question focuses on network-based biocomputing (NBC) using biological agents driven by biomolecular motor proteins in a lithographically fabricated nanofluidic channel system. This approach opens up opportunities for energy-efficient solutions to complex mathematical problems as they make their way through the NBC network. Methods are required to create switchable and rewritable nano-networks to address different mathematical problems using a single NBC-chip. Therefore, stimuli-responsive materials must be selectively and locally deposited into the nano-network to control the transport of the filaments through the network.The second research question focuses on nanophotonic devices based on DNA origami. The high-precision arrangement of nanoscopic substructures using specific binding sites (capture ends) of the DNA origami enables the precise arrangement of DNA origami functionalized with individualized metal nanoparticles. This arrangement could be used to exploit individual spectral response mediated by plasmonic effects.From a technological perspective, top-down lithographic nanostructuring technologies and bottom-up nanostructuring technologies are the basis for new device concepts. The adaptability of nanostructures to the world of biological molecules and other nanoscopic building blocks such as quantum dots and nanoparticles enables novel, even personalized, devices and engineering solutions.

Electrochemical Characterization of the Self-Assembled 4,4’-Bipyridine Layers at the Electrode | Ionic Liquid Interface

Self-assembled monolayers (SAMs) have significant importance in many modern applications1-2. The properties of SAMs depend on many aspects, such as the characteristics of individual molecules, forming the monolayer; by the interactions between these molecules; and SAM interactions with the electrolyte; the crystalline surface of the substrate; etc3. Therefore, the fundamental studies of these systems are crucial in determining the function of SAMs in various applications4-5.The aim of the current work was to study the adsorption characteristics of 4,4’- bipyridine from ionic liquid media at different electrodes6.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted to characterize the electrochemical behavior of the self-assembled 4,4’-bipyridine (4,4’-BP) layers at the Sb(111) and Cd(0001) | 4,4’-BP+1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) interface. All the experiments were carried out in a three-electrode electrochemical cell in a glove-box, i.e., a nitrogen-controlled atmosphere.CV method was used to examine the electrochemical characteristics of the system under investigation within the potential region −0.9 V < E < 0.0 V for Sb(111) and −2.1 V < E < -0.8 V for Cd(0001) vs. Ag|AgCl. The current values increase rapidly at the more negative end of the studied potential region, indicating that reduction processes started at the Sb(111) and Cd(0001) | 4,4’-BP+EMImBF4 interface. In contrast with the data measured in EMImBF4, where the system shows ideal polarisable behaviour in the whole potential range. Interfacial capacitance (C s) values were calculated using the relation Cs=−(Z′′2\U0001d70bf)−1 . Capacitance values calculated depend on the composition of the electrolyte (i.e., on the concentration of 4,4’-BP), electrode material as well as on applied potential. Cs values are lower in the mixture than those measured in pure EMMImBF4 at a certain potential region in the case of both electrode materials. This effect is caused by the adsorption of 4,4’-BP molecules at the electrode surface. The shape of the phase angle vs. frequency (f) curves shows that mixed kinetic processes occur in the whole f range measured (i.e. diffusion and charge transfer steps in addition to adsorption).Acknowledgments:This work was supported by the Estonian Research Council grant PSG249, and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-0011).

Electrochemical Characterization of the Self-Assembled 4,4’-Bipyridine Layers at the Sb(111) | Ionic Liquid Interface

Self-assembled monolayers (SAMs) have significant importance in many modern applications1-2. The characteristics of SAMs depend on many aspects, like the characteristics of individual molecules, forming the monolayer; by the interactions between these molecules; and by SAM interactions with the electrolyte; the crystalline surface of the substrate; etc3. Therefore, the fundamental studies of these systems are crucial to determine the function of SAMs in various applications4-5.The aim of the current work was to study the adsorption characteristics of 4,4’- bipyridine from ionic liquid media and compare the results with previous data measured in aqueous electrolyte solutions.Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted to characterize the electrochemical behavior of the self-assembled 4,4’-bipyridine (4,4’-BP) layers at the Sb(111) | 4,4’-BP+1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) interface. All the experiments were carried out in a three-electrode electrochemical cell in a glove-box, i.e., nitrogen-controlled atmosphere.CV method was used to determine and to evaluate the electrochemical characteristics of the system under investigation within potential region, −0.9 V < E < 0.0 V vs. Ag|AgCl. The current values increase rapidly at E < −0.8 V, indicating that a reduction process started at the Sb(111) | 4,4’-BP+EMImBF4 interface. Thus, unlikely to the data measured in EMImBF4, where the system shows ideal polarisable behaviour in the whole potential range. Interfacial capacitance (C s) values were calculated using relation C s =−(Z′′2\U0001d70bf) −1 . Capacitance values calculated depend on the composition of the electrolyte (i.e., on the concentration of 4,4’-BP) as well as on the potential applied. C s values are lower in the mixture than those measured in pure EMImBF4 at E > – 0.7 V. This effect is caused by the adsorption and formation of the compact layer of 4,4’-BP molecules at the electrode surface. The C s vs. E dependences of the mixture increase significantly at E < −0.8 V (similarly to CV curves), which is not visible in the case of pure EMImBF4. This is also confirmed by small negative phase angle values at E < −0.8 V, where the shape of the phase angle vs. frequency (f) curves shows that mixed kinetic processes occurring in the whole f range measured (i.e. diffusion and charge transfer steps in addition to adsorption). At E > −0.8 V, phase angle values are close to − 80° for Sb(111)|4,4’-BP + EMImBF4 in a wide range of f. Only at low frequency values the mixed kinetic processes (adsorption, diffusion and partial charge transfer) were observed.The formation of ordered 4,4’-BP arrangements was most pronounced in the potential range −0.7 V < E < 0.0 V due to the reductive desorption of 4,4’BP at E < −0.8 V.Acknowledgments:This work was supported by the Estonian Research Council grant PSG249, and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-0011).

Co-assembled Monolayers as Hole-Selective Contact for High-Performance Inverted Perovskite Solar Cells with Optimized Recombination Loss and Long-Term Stability.

Self-assembled monolayers (SAMs) have been widely employed as an effective way to modify interfaces of electronic/optoelectronic devices. To achieve a good control of the growth and molecular functionality of SAMs, we develop a co-assembled monolayer (co-SAM) for obtaining efficient hole selection and suppressed recombination at the hole-selective interface in inverted perovskite solar cells (PSCs). By engineering the position of methoxy substituents, an aligned energy level and favorable dipole moment can be obtained in our newly synthesized SAM, ((2,7-dimethoxy-9H-carbazol-9-yl) methyl) phosphonic acid (DC-PA). An alkyl ammonium containing SAM is co-assembled to further optimize the surface functionalization and interaction with perovskite layer on top. A champion device with an excellent power conversion efficiency (PCE) of 23.59 % and improved device stability are achieved. This work demonstrates the advantage of using co-SAM in improving performance and stability of PSCs.

Chemical-recognition-driven selectivity of SnO2-nanowire-based gas sensors

The sensing capabilities of semiconducting metal oxide (SMO)-based gas sensors, which are a promising type of sensors, were improved in the present study using a novel method that enhanced gas selectivity toward various gas molecules. A simple and effective post-modification with functional self-assembled monolayer (SAM) molecules enhanced the selective sensing properties. The chemical-affinity-driven interaction between SAM and gas molecules enabled selective gas sensing, and the small size of SnO2 nanowires (NWs; diameter = ~50 nm) provided a large sensing area. Moreover, simple alteration of the chemical moiety in SAM molecules facilitated the tuning of SAM-induced selectivity over a broad range of collections. The binding of SAMs on the NWs was analyzed by infrared spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy, with the selective gas sensing being investigated under various sensing conditions. The results from the molecular dynamics simulations supported the proposed selective sensing mechanism. The combination of passivation and selective gas sensing resulted in a simple method for the selective gas sensing of SnO2 NWs, and the concept could be generally expanded to other types of SMO-based sensing platforms.

Epitaxial Electrodeposition of Cu(111) onto an l-Cysteine Self-Assembled Monolayer on Au(111) and Epitaxial Lift-Off of Single-Crystal-like Cu Foils for Flexible Electronics

Functional self-assembled monolayers (SAMs) of thiols on single-crystal metals provide two-dimensional (2D) soft templates for the highly ordered growth of crystalline materials. An epitaxial Cu(11...

Influence of Terminal Carboxyl Groups on the Structure and Reactivity of Functionalized m-Carboranethiolate Self-Assembled Monolayers

The structure and function of self-assembled monolayers (SAMs) at the nanoscale are determined by the steric and electronic effects of their building blocks. Carboranethiol molecules form pristine ...

Enhancing the Sensitivity of Biotinylated Surfaces by Tailoring the Design of the Mixed Self-Assembled Monolayer Synthesis.

Thiolated self-assembled monolayers (SAMs) are typically used to anchor on a gold surface biomolecules serving as recognition elements for biosensor applications. Here, the design and synthesis of N-(2-hydroxyethyl)-3-mercaptopropanamide (NMPA) in biotinylated mixed SAMs is proposed as an alternative strategy with respect to on-site multistep functionalization of SAMs prepared from solutions of commercially available thiols. In this study, the mixed SAM deposited from a 10:1 solution of 3-mercaptopropionic acid (3MPA) and 11-mercaptoundecanoic acid (11MUA) is compared to that resulting from a 10:1 solution of NMPA:11MUA. To this end, surface plasmon resonance (SPR) and attenuated total reflectance infrared (ATR-IR) experiments have been carried out on both mixed SAMs after biotinylation. The study demonstrated how the fine tuning of the SAM features impacts directly on both the biofunctionalization steps, i.e., the biotin anchoring, and the biorecognition properties evaluated upon exposure to streptavidin analyte. Higher affinity for the target analyte with reduced nonspecific binding and lower detection limit has been demonstrated when NMPA is chosen as the more abundant starting thiol. Molecular dynamics simulations complemented the experimental findings providing a molecular rationale behind the performance of the biotinylated mixed SAMs. The present study confirms the importance of the functionalization design for the development of a highly performing biosensor.

Oriented Crystallization on Organic Monolayers to Control Concomitant Polymorphism

Nucleation events and crystal growth can be guided by molecular recognition at interfaces through intermolecular interactions. The short-acting antimicrobial sulfa drug sulfathiazole is known for its concomitant crystallization, which has five known polymorphs, due to conformational flexibility and hydrogen-bond synthon variation. In its development stage of a drug the issue of concomitant crystallization needs to be addressed with respect to patent litigation, including legal actions to protect patents against infringement. A functional self-assembled monolayer (SAM) of organic thiol on a gold surface has been employed as an efficient approach to control concomitant nucleation of such flexible drugs. The crystallization on a SAM surface is mostly kinetically driven and often leads to the nucleation of novel metastable forms. Spectroscopic, thermal analysis and X-ray diffraction studies reveal that a previously unknown, sixth form of the drug nucleates on the designed SAM surface.

Homogeneous functional self-assembled monolayers: Faradaic impedance baseline signal drift suppression for high-sensitivity immunosensing of C-reactive protein

The limit of detection (LOD) of affinity sensors based on alkanethiol self-assembled monolayers (SAMs) system can be improved by either signal amplification and/or noise reduction. The latter includes baseline signal drift arising, in part, from, monolayer imperfections (and variations in this through repeats) as well as electrical noise of both the sensor and transducer. In reagentless “label free” assays signal drift carries with it the possibility of assay false-positive results (if signal drift is positive) or false-negatives (if specific signal is swamped by downward drift). Faradaic electrochemical impedance spectroscopy (FEIS) sensors based on SAM interfaces have been shown to exhibit considerable baseline signal drift, which consequently affects their assaying capabilities. This study reports on the development of a simple two-step pre-treatment method for generating functional SAMs of 11-mercaptoundecanoic acid (MUA) on Au with a highly suppressed baseline signal drift. For electrochemical characterisation of the SAMs, the charge-transfer resistance (Rct), defect presence (pinhole radius and separation), capacitance, and dielectric constant were evaluated. The interface pre-treatment generates films of substantially improved homogeneity that are subsequently functionalised with anti-human C-reactive protein. CRP, an acute-phase protein, is detectable down to femtomolar levels without any amplification; this is a 2–3 order of magnitude lower detection limit than that typically accessible for analyses of this type. The developed protocols thus present a convenient and general route to suppress baseline signal drifts and improve the limits of detection of affinity sensors based on Faradaic impedance.

First principles investigation on long alkyl chain-based surface anchoring for self-assembled bilayer

Bilayer assembly is important for energy conversion devices such as the photoelectrochemical water-splitting system, in which a second layer such as the catalyst is required to deposit onto a functional self-assembled monolayer. In this manuscript, we employ first principles calculations to reveal the nanoscopic structures and properties of a long alkyl chain stabilized bilayer assembly on metal oxide substrate. The calculation confirms the viability of employing the long alkyl chain as an effective anchoring method to stabilize the self-assembled bilayer. The calculation constructs the potential energy surface and identifies a charge transfer character induced by the long alkyl chain interactions. We expect that the long alkyl chains could facilitate the preparation of more advanced surface structures for energy conversion devices which rely on self-assembled multilayers. This study helps the fundamental understanding of photoelectrochemical photoelectrodes and optoelectronic devices with the multi-layer molecular assembly.

1 H-1,2,3-Triazol-5-ylidenes: Readily Available Mesoionic Carbenes.

Classical carbenes are usually described as neutral compounds featuring a divalent carbon with only six electrons in their valence shell. It was only in 1988 that our group prepared the first isolable example, in which the carbene center was stabilized by a push-pull effect, using a phosphino and a silyl substituent. In the last 30 years, a myriad of acyclic and cyclic push-pull and push-push carbenes, bearing different heteroatom substituents, have been isolated. Among them, the so-called N-heterocyclic carbenes (NHCs), which include cyclic (alkyl)(amino)carbenes (CAACs), are arguably the most popular. They have found a vast number of applications ranging from catalysis to material science, and even in medicine. In this Account, we focus on the synthesis, structure, electronic properties, coordination, and applications of a different class of stable cyclic carbenes, namely, 1 H-1,2,3-triazol-5-ylidenes. In contrast with NHCs and CAACs, these compounds have no reasonable canonical resonance forms that can be drawn showing a carbene without additional charges. According to the IUPAC, they belong to the family of mesoionic compounds and thus they are named mesoionic carbenes (MICs). In 2010, we prepared the first stable 1,2,3-triazol-5-ylidene, via a CuAAC reaction, followed by alkylation of the resulting 1,2,3-triazole, and deprotonation. Later, we synthesized more robust N3-arylated counterparts from 1,3-diarylated-1 H-1,2,3-triazolium salts. Both synthetic routes can be carried out in multigram scales, making these MICS readily available. Importantly, MICs do not dimerize which contrasts with NHCs that can give the corresponding Wanzlick-type olefin. This property leads to relaxed steric requirements for their isolation; even C-unsubstituted MICs can be stored for months in the solid state at room temperature. The practicality and easily scalable syntheses of MICs allow for the preparation of polycarbenes, such as bis(1,2,3-triazol-5-ylidenes) (i-bitz), the analogues of the well-known 2,2'-bipyridines (bpy). MIC-transition metal complexes are excellent precatalysts for variety of chemical transformations, which include hydrohydrazination of alkynes, olefin metathesis, reductive formylation of amines with carbon dioxide and diphenylsilane, hydrogenation and dehydrogenation of N-heteroarenes in water, cycloisomerization of enynes, asymmetric Suzuki-Miyaura cross-coupling, and water oxidation (WO) reactions. Besides their catalytic applications, MIC-transition metal complexes have found applications in material sciences as exemplified by the preparation of the first iron(III) complex that is luminescent at room temperature. The peculiar properties of mesoionic triazolylidenes, combined with their enhanced stability, position them as excellent candidates to address some current challenges such as access to high-oxidation-state 3d metal complexes, the stabilization of highly reactive main group elements, the stabilization of nanoparticles, the preparation of efficient catalysts and photosensitizers based on earth-abundant transition metals, and the functionalization of self-assembled monolayers (SAMs) on gold.

Functional Biointerfaces Based on Mixed Zwitterionic Self-Assembled Monolayers for Biosensing Applications.

Surface modification for biosensors has focused attention for improvement of their sensitivity and specificity, particularly for the detection in complex medium. In this work, we have synthesized zwitterionic carboxybetaine-thiols (CB-thiols) and sulfobetaine-thiols (SB-thiols) for modification of gold substrates to form a functional self-assembled monolayer (SAM) for the immunoassay in a surface plasmon resonance (SPR) biosensor. X-ray photoelectron spectroscopy (XPS), contact angle goniometer, and cyclic voltammetry were applied for characterizations of elemental composition, surface wettability, and packing density, respectively. The antifouling properties of the SAMs were accessed by quantitative analysis of protein and bacterial adsorption. The results from the SAMs with a single component indicated that the SB-thiol SAM provides better surface hydrophilicity, fouling resistance, and packing density as compared to the CB-thiol SAM, likely due to the ionic association of CB moieties. However, the CB-thiol with the functional carboxylate group plays a critical role in postmodification of biomolecules via commercially available amine coupling chemistry. Thus, the mixed SAMs were prepared to integrate the unique characteristics from CB- and SB-thiols to control compositions and surface properties. The immunoassay was performed in the SPR biosensor, showing that the zwitterionic mixed SAM enables immobilization of biorecognition elements (BREs), and improved sensitivity and specificity. Consequently, the work reveals excellent and attractive versatility, antifouling, and functionalizable properties of zwitterionic mixed SAMs comprising CB- and SB-thiols for biosensing applications. This surface chemistry is expected to be applicable to monitor specific molecular recognition events.

Robust Surface Plasmon Resonance Chips for Repetitive and Accurate Analysis of Lignin-Peptide Interactions.

We have developed novel surface plasmon resonance (SPR) sensor chips whose surfaces bear newly synthesized functional self-assembled monolayer (SAM) anchoring lignin through covalent chemical bonds. The SPR sensor chips are remarkably robust and suitable for repetitive and accurate measurement of noncovalent lignin–peptide interactions, which is of significant interest in the chemical or biochemical conversion of renewable woody biomass to valuable chemical feedstocks. The lignin-anchored SAMs were prepared for the first time by click chemistry based on an azide–alkyne Huisgen cycloaddition: mixed SAMs are fabricated on gold thin film using a mixture of alkynyl and methyl thioalkyloligo(ethylene oxide) disulfides and then reacted with azidated milled wood lignins to furnish the functional SAMs anchoring lignins covalently. The resulting SAMs were characterized using infrared reflection–absorption, Raman, and X-ray photoelectron spectroscopies to confirm covalent immobilization of the lignins to the SAMs via triazole linkages and also to reveal that the SAM formation induces a helical conformation of the ethylene oxide chains. Further, SPR measurements of the noncovalent lignin–peptide interactions using lignin-binding peptides have demonstrated high reproducibility and durability of the prepared lignin-anchored sensor chips.

Electrochemistry of Metalloproteins Attached through Functional Self-Assembled Monolayers on Gold and Ferromagnetic Electrodes.

We report the experimental results of a study of the electron-transfer processes of redox-active metalloproteins bound to mixed self-assembled monolayers (SAMs) on magnetic (nickel or ultrathin gold-coated nickel) or nonmagnetic (gold) electrodes. Metalloproteins, such as hemoglobin (Hb), Cytochrome C (Cyt C), and Cyt C oxidase, are attached through electrostatic interactions to the free carboxylate or imidazole groups present in the mixed SAMs. The formation of both mixed SAMs and SAM/metalloprotein heterostructures were confirmed by using advanced surface analysis techniques, such as polarization modulation infrared reflection absorption spectroscopy and aqueous contact angle measurements. Electrochemical measurements indicated a stronger electronic coupling between Hb and Cyt C oxidase and the mixed-SAM-coated gold or gold-coated-nickel electrodes, whereas a weaker coupling was found between the protein and the pure nickel electrode. Surface coverage and the electron-transfer rate constant were estimated from the cyclic voltammetry data.

Adjustment of the Work Function of Pyridine and Pyrimidine Substituted Aromatic Self-Assembled Monolayers by Electron Irradiation

Self-assembled monolayers (SAMs) are frequently used to manipulate injection barriers in organic electronics by introduction of a specific dipole moment at the interfaces between the electrodes and adjacent organic layers. This is usually achieved by the selection of a proper dipolar terminal tail group comprising the SAM–ambient interface, which was recently complemented by embedding such a group into the molecular backbone. Here we show that the work function of SAMs can also be adjusted by electron irradiation in a quite broad range and in controlled fashion as far as these films contain pyridine or pyrimidine group. This effect is demonstrated by the example of several representative aromatic SAMs with either terminal pyridine group or embedded pyrimidine group. The observed behavior is presumably related to specific chemical transformations involving the nitrogen atoms in these moieties. The SAMs with the embedded pyrimidine group are then especially attractive because this moiety is decoupled from t...

Hole Injection in N-Type Organic Semiconductors by Tuning Metal Work Function with Functional Self-Assembled Monolayers

Hole Injection in <i>N</i>-Type Organic Semiconductors by Tuning Metal Work Function with Functional Self-Assembled Monolayers

Thiols make for better catalysts: Au nanoparticles supported on functional SBA-15 for catalysis of Ullmann-type homocouplings.

A strategy for arraying small gold nanoparticles on a mesoporous support modified with single-component or mixed self-assembled monolayers is described. The use of mixed surface modifiers allows easy access to a range of surface chemistries and modalities of interaction between nanoparticles and supports. A combination of thiol groups and linear semifluorinated chains effectively stabilized the nanoparticles against aggregation, while preserving their catalytic activity. The thiol-fluorous-supported catalyst was found active in Ullmann-type homocoupling of aryl halides and showed exceptional selectivity in this reaction.

Etching of Crystalline ZnO Surfaces upon Phosphonic Acid Adsorption: Guidelines for the Realization of Well-Engineered Functional Self-Assembled Monolayers.

Functionalization of metal oxides by means of covalently bound self-assembled monolayers (SAMs) offers a tailoring of surface electronic properties such as their work function and, in combination with its large charge carrier mobility, renders ZnO a promising conductive oxide for use as transparent electrode material in optoelectronic devices. In this study, we show that the formation of phosphonic acid-anchored SAMs on ZnO competes with an unwanted chemical side reaction, leading to the formation of surface precipitates and severe surface damage at prolonged immersion times of several days. Combining atomic force microscopy (AFM), X-ray diffraction (XRD), and thermal desorption spectroscopy (TDS), the stability and structure of the aggregates formed upon immersion of ZnO single crystal surfaces of different orientations [(0001̅), (0001), and (101̅0)] in phenylphosphonic acid (PPA) solution were studied. By intentionally increasing the immersion time to more than 1 week, large crystalline precipitates are formed, which are identified as zinc phosphonate. Moreover, the energetics and the reaction pathway of this transformation have been evaluated using density functional theory (DFT), showing that zinc phosphonate is thermodynamically more favorable than phosphonic acid SAMs on ZnO. Precipitation is also found for phosphonic acids with fluorinated aromatic backbones, while less precipitation occurs upon formation of SAMs with phenylphosphinic anchoring units. By contrast, no precipitates are formed when PPA monolayer films are prepared by sublimation under vacuum conditions, yielding smooth surfaces without noticeable etching.

SERS of the Positive Charge Bearing Pyridinium Ring Terminated Self-Assembled Monolayers: Structure and Bonding Spectral Markers

Bifunctional self-assembled monolayers (SAMs) are widely used for construction of surfaces with desirable properties. To predict and control the function of SAMs, molecular level understanding of monolayer architecture is required. In this work, structure and bonding of positive charge bearing SAM of N-(6-mercapto)hexylpyridinium (MHP) on Au and Ag electrodes was probed by SERS, first-principles calculations, isotopic substitution, and reductive desorption voltammetry. Based on analysis of immersion time- and temperature-dependent SERS spectra as well as DFT calculations, the marker bands for MHP structure, bonding, and orientation have been established. The presence of a band around 1083 cm–1 was found to be a reliable SERS marker for the all-trans conformation of hydrocarbon chain. Soft C–H stretching mode near 2832 cm–1 was recognized as a marker for the probing of direct interaction of alkyl chain with the metal. Based on solvent-dependent Raman spectra the pyridinium ring ═C–H band (ν2) was proposed ...