The molecular level modification of surfaces: from self-assembled monolayers to complex molecular assemblies

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Surface modification through the formation of a self-assembled monolayer (SAM) can effectively engineer the physicochemical properties of the surface/material. However, the precise design of multifunctional SAMs at the molecular level is still a major challenge. Here, we jointly use N-heterocyclic carbenes (NHCs) and thiols to form multifunctional hetero-SAM systems that demonstrate excellent chemical stability, electrical conductivity, and, in silico, catalytic activity. This synergistic effect is facilitated by the high surface mobility and electron-rich nature of NHCs, combined with the strong binding strength of thiols. Scanning tunneling microscopy, electrical conductivity, and scanning electron microscope measurements, as well as density functional theory calculations, were employed to explore the synergistic interactions in the supramolecular SAMs. The van der Waals integration of ballbot-type NHCs and thiols enables the SAMs to exhibit both superior surface anticorrosion properties (attributing to the shift in the d-band center) and low surface resistance originating from the band alignment. Moreover, we find that the deposition sequence of flat-lying NHCs and thiols results in SAMs with different configurations, which can further tune the mechanistic pathway in silico in the acetylene hydrogenation process. Our results provide essential molecular insights into the local electronic control of the new SAM/metal interface and the high stability of the emergent multifunctionality (NHC/thiol)-SAMs forming self-assembled lamellae structures in the nanometer regime.

Self-assembled monolayers (SAMs) are ultra-thin organic monolayers, which can be used in different ways to assemble functionalized surface structures. This potential is caused by the ability of the SAMs to tie further molecules and components through the terminal groups of the organic layers. Additional applications for microfluidics and micromechanics require micro and nano structuring of the SAMs. In combination with multi-photon lithography (MPL) SAMs are offering advantageous properties as ultra thin layers. Thus, the processing with single pulses is feasible and results in very short processing times without the appearance of bubbles and formation of particles compared to photo resists. For our experiments, we used a non-collinear optical parametric amplifier (NOPA) which has the ability to generate short pulses of sub-30fs in the visible and near-infrared (NIR) range of light. The NOPA can be tuned in the range of 480 nm to 950 nm without spectral gaps.We used thiol based SAMs as ultra thin layers on gold substrates. The selected laser power offers the possibility of ablation of the SAMs without damaging the gold layer. Thereby we investigate the characteristics of thiol-based SAMs as monomolecular resists during etching of gold. We also investigate the wavelength dependencies of the substrate to get an optimal process window for the ablation process. Minimum structure sizes at a 1/e laser spot diameter of about 1.6 µm are close to 1/5 of the spot diameter.Self-assembled monolayers (SAMs) are ultra-thin organic monolayers, which can be used in different ways to assemble functionalized surface structures. This potential is caused by the ability of the SAMs to tie further molecules and components through the terminal groups of the organic layers. Additional applications for microfluidics and micromechanics require micro and nano structuring of the SAMs. In combination with multi-photon lithography (MPL) SAMs are offering advantageous properties as ultra thin layers. Thus, the processing with single pulses is feasible and results in very short processing times without the appearance of bubbles and formation of particles compared to photo resists. For our experiments, we used a non-collinear optical parametric amplifier (NOPA) which has the ability to generate short pulses of sub-30fs in the visible and near-infrared (NIR) range of light. The NOPA can be tuned in the range of 480 nm to 950 nm without spectral gaps.We used thiol based SAMs as ultra thin layers o...

Performance improvement of organic solar cell via incorporation of donor type self-assembled interfacial monolayer

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

A lot of strategies for smart approaches on surfaces were applied such as hydrogel layer, polymer brushes or self-assembly monolayers (SAM). [1] Nowadays switchable zwitterionic materials consisting of molecules with internally balanced charge between positive ammonium and negative carboxy group are promising candidates for this application. [2] They can combine antifouling properties of their zwitterion state and complexation or sticky character in their pre-zwitterionic carboxybetaine ester form. Zwitterionic forms possess antibiofouling properties due to electrostatic interaction between charged moieties, highly hydration capability and overall neutral charge in material as well as biomimetic character because zwitterions are structural similarity to biomembranes. We showed that modifications of surface by zwitterionic based self-assemble monolayer allow enhance detection limit of biosensors down to 10–15 M for analyte, [3,4] or improve electrorheological response. [5]Carboxybetaine esters have cationic character and permit complexation with polyanionic bioabsorbents as well as character of counter ion can adjust wettability and interaction with biomolecules.These studies will present on the utilization of pre-zwiterionic molecules: carboxybetaine based derivates formed from lipoic acid precursor in order to modify surface for construction of impedimetric lectin biosensors and for tuning wettability and interaction with DNA and other charged (bio)molecules.Novel pre-zwitterionic carboxybetaine ester (hydrolysable and photolysable) derivates were synthetized by protocol consists of several synthetic steps and fully characterized. Subsequently, modification of a gold surface was performed by a self-assembled monolayer deposited from a solution containing prezwitterion molecules. Self-assembly monolayer, formed from derivates, was characterized by set instrumentation as atomic force microscopy, quartz crystal microbalance XPS, contact angle etc.Hydrolysable carboxybetaine derivate was able to from complex with polycationic DNA molecules to preconcentrate and release at pH dependent manner. During course of hydrolysis carboxybetaine ester is transferred to carboxybetaine zwitterionic form to promote DNA release due to formation of carboxylate negative charge. Additionally, gradient in wettability can be observed within progress of hydrolysis and present of long perfluorinated or aliphatic types of counter ions. For example switch in wettability can be achieved only by simple and rapid couterion exchange between superhydrophilic (contact angle (CA) below 10° (to very high hydrophobilic (CA over 140°) on rough gold surface. After completed hydrolyses zwitterionic surface can be utilized as a platform for biosensor surface with nonfouling properties. Carboxylic functionality allows immobilizing sensing molecules as lectins for electrochemical impedance spectroscopy by means of EDC/NHS chemistry. This methodology provides opportunity for ultrasensitive detection up to 10–15 M of lectins which may result of a biomarker discovery on several diseases in whole media.Moreover utilization of photolabile ester of carboxybetaine derivates allowing spatially control wettability and pattering with photomask was performed. Photolabile 2-nitrophenyl methyl ester group was introduced to pre-zwitterionic molecule and after irradiation of prepared surface with light at 365 nm was transformed from carboxybetaine ester group to zwitterionic carboxybetaine. Progress of photolysis can be observed by change of surface zeta potential, quartz crystal microbalance and contact angle measurement. This irreversible switch along with different interaction of biological species before and after photolysis will be discussed in this contribution as well.This contribution was made possible by NPRP grant 6-381-1-078 from the Qatar National Research Fund (a member of the Qatar Foundation). The statements contained are entirely the responsibility of the authors.

Silicon nanowire-based metal-oxide-semiconductor field-effect transistors (SiNW MOSFETs) have been demonstrated excellent sensitivity and stability after surface modification and functionalization of nanowires. Chemical molecules have been applied to functionalize the surface of silicon surface. Silane coupling agents are good candidates for forming self-assembled monolayers (SAMs) by chemically interacting with silicon oxide. Those chemically modified SAMs can provide a functional surface to further conjugate biomolecules on SiNW MOSFETs. After functionalization, SiNW MOSFETs with tunably biocompatible surface can sustain a functional biointerface for biological tests. In this work, SiNW MOSFETs were fabricated using the standard I-line stepper of MOS semiconducting process and then visualized by scanning electron microscopy (SEM). The n-type SiNW MOSFETs devices were fabricated after the process of trimming, the scale of nanowire was down to a level of approximate 165 nm. 3-aminopropyl trimethoxysilane (APTMS) and 3-mercaptopropyl trimethoxysilane (MPTMS) SAMs were independently used to modify the surface of SiNW MOSFETs for pH sensing in biological buffer solution. Atomic force microscopy (AFM) and electron spectroscopy for chemical analysis (ESCA) were applied to characterize before and after surface modification. AFM found APTMS and MPTMS were successfully modified on silicon substrates. The average vertical length of APTMS and MPTMS SAMs from our AFM observation was around 2.628 nm and 2.698 nm, respectively. ESCA showed the specifically functional amino (-NH2) groups and mercapto (-SH) groups on each APTMS and MPTMS modified silicon substrates. The specific amine functional group at 399.4 eV occurred after the modification of APTMS on silicon substrate in N1s spectra. S2p spectra showed the specific binding at 163.6 eV (C-SH) and 165.8 eV (-C-S-S-C-) after the modification of MPTMS on silicon substrate. Those disulfide bonds further influenced the organization of MPTMS-SAM on the surface; therefore, the APTMS had better SAM performance on our silicon substrate. On the other hand, electrical measuring system was used for elucidating that the suitable surface modification would have great impact on the sensing response and sensitivity. Varied biological PBS solutions at different pH values showed that unmodified SiNW MOSFETs were sensitive to the H^+ ion change. When the pH level of the solution increased, the drain current of the unmodified SiNW MOSFETs decreased accordingly. In comparison with unmodified nanowires in current measurement, the changes of current of APTMS or MPTMS modified nanowires were enhanced in sensing of different pH solutions. Our results also showed that amino and mercapto groups of APTMS and MPTMS can improve the protonation and deprotonation reactions in different pH solutions. Both APTMS and MPTMS modified SiNW MOSFETs in pH sensings possessed good electrical sensing response and sensitivity in contrast with unmodified one. Moreover, in consequence of lower mercaptal groups of MPTMS on NWs, the relatively minor signal responses to varied pH solutions in MPTMS modified SiNW MOSFETs. The electrical measurement showed that the amino groups of APTMS significantly improve the sensitivity of SiNW MOSFET in different pH sensings. Our results showed that adequate modification could provide a functionable surface for SiNW MOSFETs. We inferred the APTMS modified SiNW MOSFETs could be a real-time sensor for different pH levels detection and further applied in monitoring biological environment in the future.

Molecular electronic control over plasmons offers a promising route for on-chip integrated molecular plasmonic devices for information processing and computing. To move beyond the currently available technologies and to miniaturize plasmonic devices, molecular electronic plasmon sources are required. Here, we report on-chip molecular electronic plasmon sources consisting of tunnel junctions based on self-assembled monolayers sandwiched between two metallic electrodes that excite localized plasmons, and surface plasmon polaritons, with tunnelling electrons. The plasmons originate from single, diffraction-limited spots within the junctions, follow power-law distributed photon statistics, and have well-defined polarization orientations. The structure of the self-assembled monolayer and the applied bias influence the observed polarization. We also show molecular electronic control of the plasmon intensity by changing the chemical structure of the molecules and by bias-selective excitation of plasmons using molecular diodes. Self-assembled molecular monolayers between metal electrodes are demonstrated as an electronic plasmon source.

Self-assembled monolayers (SAMs) have attracted considerable attention because of their wide applications to biosensors, optoelectronic devices, control of wettability and biocompatibility, corrosion resistance, etc. Among various types of SAMs formed on numerous kinds of substrates, the SAMs of alkanethiols on gold have extensively been studied, primarily because it is relatively easy and simple to form well-organized monolayers on gold compared with the methods used for other substrates. In addition, the gold surface enables us to effortlessly use analytical techniques, including surface plasmon resonace spectroscopy, quartz crystal microbalance, reflection absorption infrared spectroscopy and ellisometry, in the mechanistic studies and potential applications of SAMs. Especially, the SAMs of long-chain alkanethiols with ω-functional groups, such as OH, COOH, and NH2, have attracted a great deal of interest, because the use of these alkanethiols would yield more interesting properties of surfaces through post-modification reactions, such as nucleophilic substitution, nucleophilic addition, esterification, and acylation, in addition to their own chemical properties. For example, the surface modifications of OH-, COOH-, and NH2-terminated SAMs have been used for the attachment of biopolymers, such as oligonucleotides and peptides, onto gold surfaces, which is the essential step for the application to biotechnology. Since the post-modifications and applications of SAMs are performed under various conditions of temperature, pH and solvents, studies on the stability of SAMs against potentially destructive conditions of monolayers are important for wider applications of SAMs as well as design of post-modification reactions. The air-stability of alkanethiolbased monolayers on gold was studied with long-chain alkanethiols for one week to several months. Their longterm stability was also investigated in biological media, because biomaterials and biological devices, which use SAMs as a platform, necessitate prolonged exposure to the biological media. Recently, the effect of storage conditions, such as air, N2, ethanol, phosphate buffer and H2O, on the stability of mixed poly(ethylene oxide)-thiol SAMs was studied to ensure the long-term preservation of biosensing property. In addition to these studies on the stability of SAMs, the pH dependency of the stability of SAMs in aqueous solutions is to be established, since the numerous surface modification reactions and applications of SAMs are performed at various pH values. As a related work, the dissociation phenomena of thiolate ligands from the surface of cadmium chalcogenide nanocrystals have been examined by varying the pH values. In this work, we investigated pHdependent stability of alkanethiol SAMs with various functional groups at their terminals, in order to give a basic but crucial information for post-modifications and other SAMbased reactions. The SAMs of long-chain alkanethiols with CH3, OH, COOH, or NH2 group were prepared on gold, and their stability was studied for one week at pH values from 1 to 14 by measuring the changes in the ellipsometric thickness. For the preparation of the SAMs on gold, a clean gold substrate was immersed for 20 h in each alkanethiol solution of HS-(CH2)15-CH3, HS-(CH2)16-OH, HS-(CH2)15-COOH, or HS-(CH2)16-NH2. The SAMs were characterized by FTIR spectroscopy (Figure 1). Figure 1a shows the IR peaks

We developed a self-assembled monolayer (SAM) surface modification of an indium tin oxide (ITO; SnO2–In2O3) by a dry method for ink-jet printing processes and compared this with the SAM surface modifications by wet and dry methods. We focused on an analysis of the work function increase and changes in chemical bonding at the ITO surface interface with the SAM. In addition, we demonstrated that the causes of the work function increase of the ITO were the binding energy changes of the oxygen atoms on the ITO surface and an improvement in the hole injection from the ITO with the SAM to an organic layer in the organic device. The SAM surface modification decreased the threshold voltage and increased the current density in the organic device. These changes were due to the energy barrier caused by the work function difference being removed at the interface between the ITO with a high work function (5.3 to 5.5 eV) and the organic layer. Additionally, it was observed that the current density and luminance characteristics of the device were improved by the SAM surface modification. There was less organic solvent contamination at the interface between the ITO and the SAM by the dry method because no organic solvent was used.

The main distinctive feature of modern surface science is that it deals with single crystal surfaces, which are well-defined from the viewpoint of their structure and composition. However, for the preparation of crystal surfaces and their analysis, the requirement of UHV equipment greatly limits applications. While atomically clean surfaces analyzed under artificial measurement conditions still play an important role for fundamental understanding, it should also be the goal of modern surface science to explore the role of surfaces under realistic conditions. Indeed, in recent years, surface science is moving to more complicated materials and to studies of the solid–liquid interface. The latter has great relevance in biological processes and technological applications, e.g. catalysis, lubrication and corrosion prevention. Also, the modification of surfaces by self-assembled monolayers (SAMs) is an important process taking place at the solid–liquid interface. SAMs are ordered molecular assemblies formed by the adsorption of an active surfactant on a solid surface. Based on the constituent surfactant, a variety of SAMs with different properties and functionalities can easily be prepared with potential applications in various branches of surface technology. Furthermore, SAMs are excellent model systems amenable to investigating competing surfactant–substrate and surfactant–surfactant intermolecular interactions. The main goal of this thesis is to provide insight into the formation of SAMs at the solid–liquid interface by using in situ experimental techniques. Fourier-transform infrared spectroscopy (FT-IR) applied in the attenuated total internal reflection (ATR) mode was used as the prime technique to probe the self-assembly of chiral peptides or derivatives thereof from the liquid phase to gold surfaces. It is demonstrated that ATR is an excellent in situ analysis method that provides rich molecular-level information, such as kinetics of adsorption, structural changes and orientation of molecules within the SAM. The insight was further broadened by using ATR in combination with modulation excitation spectroscopy (MES). The latter benefits from highlighting spectral changes due to the periodic stimulation of the sample by an external parameter. MES was used to further explore the structure of SAMs and it was shown that the latter may undergo significant reversible structural changes by periodically stimulating the molecules within the adsorbate layer. One of the most interesting and valuable properties a chiral SAM or surface can have is its ability to discriminate between enantiomers of a chiral compound, which is crucial for heterogeneous enantioselective catalysis and chiral recognition. It is therefore desirable to develop techniques that can not only quantify enantiodiscrimination but also shed light on its origin. Such techniques should ideally provide molecular-level information and combine (surface-) sensitivity with selectivity for the chiral information, a combination of criteria that is difficult to meet simultaneously. It is demonstrated that ATR in combination with MES is a powerful technique to meet these requirements. For the first time, enantiodiscrimination between chiral SAMs and an analyte molecule was probed and unique molecular-level insight on the origin of chiral recognition was obtained. Additional experimental techniques used for this work were quartz crystal microbalance (QCM) and polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS). QCM is extremely sensitive to mass uptake on the sensor surface and the technique was used to investigate adsorption kinetics and self-assembly of molecules from the liquid phase in situ. Furthermore, computational methods at the density functional theory (DFT) level were routinely used for vibrational analysis of surfactant molecules and calculations helped to interpret experimental spectra.

Surface modification and functionalization through the self-assembled monolayer and graft polymerization

Understanding and controlling charge transport across multiple parallel molecules are fundamental to the creation of innovative functional electronic components, as future molecular devices will likely be multimolecular. The smallest possible molecular ensemble to address this challenge is a dual-molecule junction device, which has potential to unravel the effects of intermolecular crosstalk on electronic transport at the molecular level that cannot be elucidated using either conventional single-molecule or self-assembled monolayer (SAM) techniques. Herein, we demonstrate the fabrication of a scanning tunneling microscopy (STM) dual-molecule junction device, which utilizes noncovalent interactions and allows for direct comparison to the conventional STM single-molecule device. STM-break junction (BJ) measurements reveal a decrease in conductance of 10% per molecule from the dual-molecule to the single-molecule junction device. Quantum transport simulations indicate that this decrease is attributable to intermolecular crosstalk (i.e., intermolecular π-π interactions), with possible contributions from substrate-mediated coupling (i.e., molecule-electrode). This study provides the first experimental evidence to interpret intermolecular crosstalk in electronic transport at the STM-BJ level and translates the experimental observations into meaningful molecular information to enhance our fundamental knowledge of this subject matter. This approach is pertinent to the design and development of future multimolecular electronic components and also to other dual-molecular systems where such crosstalk is mediated by various noncovalent intermolecular interactions (e.g., electrostatic and hydrogen bonding).

Polymer-dispersed liquid crystal (PDLC) films of 4-cyano-4‘-pentylbiphenyl (5CB) were fabricated between two quartz substrates, the surfaces of which had been modified with the self-assembled monolayers (SAMs) of CH3−(CH2)17−Si(OMe)3 (1), HS−(CH2)10−Si(OEt)3 (2), and NC−(CH2)11−Si(OEt)3 (3). The SAM-modification effects on the molecular aggregation of 5CB were investigated by steady-state and time-resolved fluorescence analysis for the PDLC films. Remarkably, it was found that selective excitation of the interface layer with the substrate surface gave both the monomer and excimer emissions of 5CB in relative intensities, depending on the chemical nature of the SAM surfaces. While the monomer and excimer emissions appeared in comparative intensities in the case of the unmodified quartz surface, the surface modification with the SAM of 1 resulted in a dominant contribution of the excimer emission. By contrast, the monomer emission was much stronger than the excimer emission in the case of the surface modified by the SAM of 2. The surface modification with the SAM of 3 gave a fluorescence spectrum very similar to that in the case of the unmodified surface. Fluorescence decay analysis for the PDLC films revealed that the excimer emission consists of two components with shorter (1.3−1.6 ns) and longer (10−12 ns) lifetimes, whose relative contributions depend on the SAM modifications. The molecular pictures of 5CB depicted from the decay dynamics are in good agreement with those derived from the steady-state fluorescence behavior of the PDLC films. Electrooptic devices based on the PDLC films were constructed by using indium−tin oxide transparent electrodes modified with the SAMs, and it was confirmed that the electrooptic responses again significantly depend on the modifications of the substrate surface. The dependency of the fluorescence and electrooptic behavior on the surface modifications for the PDLC films has been discussed in terms of anchoring effects of the substrate surfaces, which effectively work even in heterogeneous materials such as PDLCs.

A rapid surface modification technique for the formation of self-assembled monolayers (SAMs) of alkanethiols on gold thin films using microwave heating in <10 min is reported. In this regard, SAMs of two model alkanethiols, 11-mercaptoundecanoic acid (11-MUDA, to generate a hydrophilic surface) and undecanethiol (UDET, a hydrophobic surface), were successfully formed on gold thin films using selective microwave heating in (1) a semicontinuous fashion and (2) a continuous fashion at room temperature (24 h, control experiment, no microwave heating). The formation of SAMs of 11-MUDA and UDET was confirmed by contact angle measurements, Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The contact angles for water on SAMs formed by the selective microwave heating and conventional room temperature incubation technique (24 h) were measured to be similar for 11-MUDA and UDET. FT-IR spectroscopy results confirmed that the internal structures of SAMs prepared using both microwave heating and room temperature were similar. XPS results revealed that the organic and sulfate contaminants found on bare gold thin films were replaced by SAMs after the surface modification process had been conducted using both microwave heating and room temperature.