Organic Monolayer Films Research Articles

Probing of Photocarrier Electrons and Excitons at an Organic Monolayer Film Studied by Two-Photon Photoemission Spectroscopy

Probing of Photocarrier Electrons and Excitons at an Organic Monolayer Film Studied by Two-Photon Photoemission Spectroscopy

Self‐Assembled Nanoporous Metal–Organic Framework Monolayer Film for Osmotic Energy Harvesting

AbstractOsmotic energy represents a promising energy resource because it is sustainable and environmentally benign. Subnanoscale channels are considered as a competitive platform for generating this blue energy due to their highly selective and ultrafast ion transport. However, fabricating functional subnanochannels capable of high energy output remains challenging. Here, a heterogeneous subnanochannel membrane formed by coating a functionalized self‐assembled metal−organic framework (MOF) monolayer (SAMM) film on a porous anodic aluminum oxide membrane, is reported. The SAMM film, with a thickness of ≈160 nm, is fabricated by self‐assembly of poly(methyl methacrylate‐co‐vinylimidazole)‐modified UiO‐66‐NH2 nanoparticles at the water−air interface. In the SAMM, imidazole and NH2 groups provide abundant positive charges, while the angstrom‐scale windows act as ionic filters for selective screening of anions with different hydration diameters. As a result, the heterogeneous membrane exhibits excellent capacity for anion‐selective transport, which contributes to an optimal osmotic power of 6.76 W m−2 under a 100‐fold NaCl gradient, as well as a high Cl−/SO42− selectivity of ≈42.2. Further, the output power is increased to 10.5 W m−2 by methylating imidazole moieties on the MOF surface. This work provides a facile and modular approach to fabricate subnanochannels for enabling highly selective and efficient osmotic energy conversion.

Wet Chemical Cleaning of Organosilane Monolayers

Thin organic self-assembled monolayer films are used to promote adhesion and seal the pores of metal oxides as well as direct the deposition of layers on patterned surfaces. Defects occur as the self-assembled monolayer forms, and the number and type of defects depend on surface preparation, deposition solvent, temperature, time and other parameters. Particles commonly deposit during organosilane self-assembly on metal oxide surfaces. The particles are defects because they are prone to react in subsequent processing, which may not be desirable if the organosilane serves as a pore sealant or passivation layer. Cleaning the organosilane by solvent extraction to remove non-polar agglomerates followed by an aqueous mixture of ammonium hydroxide and hydrogen peroxide, which is Standard Clean 1, a common particle removal step for silicon surfaces, produced monolayers with few agglomerates based on atomic force microscopy without etching the layer. The combined cleaning sequence contained fewer particles than separate cleaning steps, showing that the cleans removed particles with different compositions. The thickness and contact angle of cleaned monolayers was comparable to those made using a costlier solvent.

Pentacene and tetracene molecules and films on H/Si(111): level alignment from hybrid density functional theory

The electronic properties of hybrid organic–inorganic semiconductor interfaces depend strongly on the alignment of the electronic carrier levels in the organic/inorganic components. In the present work, we address this energy level alignment from first principles theory for two paradigmatic organic–inorganic semiconductor interfaces, the singlet fission materials tetracene and pentacene on H/Si(111), using all-electron density functional theory calculations with a hybrid exchange–correlation functional. For isolated tetracene on H/Si(111), a type I-like heterojunction (lowest-energy electron and hole states on Si) is found. For isolated pentacene, the molecular and semiconductor valence band edges are degenerate. For monolayer films, we show how to construct supercell geometries with up to 1192 atoms, which minimize the strain between the inorganic surface and an organic monolayer film. Based on these models, we predict the formation of type II heterojunctions (electron states on Si, hole-like states on the organic species) for both acenes, indicating that charge separation at the interface between the organic and inorganic components is favored. The paper discusses the steps needed to find appropriate low-energy interface geometries for weakly bonded organic molecules and films on inorganic substrates from first principles, a necessary prerequisite for any computational level alignment prediction.

Control of Molecular Bonding Strength on Metal Catalysts with Organic Monolayers for CO2 Reduction.

The development of separate levers for controlling the bonding strength of different reactive species on catalyst surfaces is challenging but essential for the design of highly active and selective catalysts. For example, during CO2 reduction, production of CO often requires balancing a trade-off between the adsorption strength of the reactant and product states: weak binding of CO is desirable from a selectivity perspective, but weak binding of CO2 leads to low activity. Here, we demonstrate a new method of controlling both CO2 adsorption and CO desorption over supported metal catalysts by employing a single self-assembly step where organic monolayer films were deposited on the catalyst support. Binding of phosphonic acid monolayers on supported Pt and Pd catalysts weakened CO binding via a through-support effect. The weakened CO adsorption was generally accompanied by decreased adsorption and reactivity of CO2. However, by the incorporation of basic amine functions at controlled positions in the modifying film, strong CO2 adsorption and hydrogenation reactivity could be restored. Thus, both through-surface and through-space interactions could be manipulated by design of the organic modifiers. After surface modification, the catalysts exhibited significantly improved selectivity (up to ∼99% at conversions near 50%) and activity toward CO production. Moreover, the rate of deactivation was notably reduced due to prevention of CO poisoning.

Coexisting Charge States in a Unary Organic Monolayer Film on a Metal.

The electronic and geometric structures of tetracene films on Ag(110) and Cu(110) have been studied with photoemission tomography and compared to that of pentacene. Despite similar energy level alignment of the two oligoacenes on these surfaces revealed by conventional ultraviolet photoelectron spectroscopy, the momentum-space resolved photoemission tomography reveals a significant difference in both structural and electronic properties of tetracene and pentacene films. Particularly, the saturated monolayer of tetracene on Ag(110) is found to consist of two molecular species that, despite having the same orientation, are electronically very different-while one molecule remains neutral, another is charged because of electron donation from the substrate.

Control of interfacial acid–metal catalysis with organic monolayers

Numerous important reactions consisting of combinations of steps (for example, hydrogenation and dehydration) have been found to require bifunctional catalysts with both a late-transition metal component and an acidic component. Here, we develop a method for preparing and controlling bifunctional sites by employing organic acid-functionalized monolayer films tethered to the support as an alternative to traditional ligand-on-metal strategies. This approach was used to create a reactive interface between the phosphonic acid monolayers and metal particles, where active-site properties such as acid strength were manipulated via tuning of the molecular structure of the organic ligands within the monolayer. After surface modification, the resultant catalysts exhibited markedly improved selectivity and activity towards hydrodeoxygenation of aromatic alcohols and phenolics. Moreover, by tuning the ligand of the acidic modifier, the rate of deactivation was significantly reduced. Bifunctional heterogeneous catalysts are usually prepared by dispersion of a metal on an acidic or basic support. Now a method has been developed to post-functionalize a catalyst and introduce tunable acidity by coating an organic acid layer on the support, resulting in improved performance as showcased for selected hydrodeoxygenation reactions.

Anisotropic contrast optical microscope.

An optical microscope is described that reveals contrast in the Mueller matrix images of a thin, transparent, or semi-transparent specimen located within an anisotropic object plane (anisotropic filter). The specimen changes the anisotropy of the filter and thereby produces contrast within the Mueller matrix images. Here we use an anisotropic filter composed of a semi-transparent, nanostructured thin film with sub-wavelength thickness placed within the object plane. The sample is illuminated as in common optical microscopy but the light is modulated in its polarization using combinations of linear polarizers and phase plate (compensator) to control and analyze the state of polarization. Direct generalized ellipsometry data analysis approaches permit extraction of fundamental Mueller matrix object plane images dispensing with the need of Fourier expansion methods. Generalized ellipsometry model approaches are used for quantitative image analyses. These images are obtained from sets of multiple images obtained under various polarizer, analyzer, and compensator settings. Up to 16 independent Mueller matrix images can be obtained, while our current setup is limited to 11 images normalized by the unpolarized intensity. We demonstrate the anisotropic contrast optical microscope by measuring lithographically defined micro-patterned anisotropic filters, and we quantify the adsorption of an organic self-assembled monolayer film onto the anisotropic filter. Comparison with an isotropic glass slide demonstrates the image enhancement obtained by our method over microscopy without the use of an anisotropic filter. In our current instrument, we estimate the limit of detection for organic volumetric mass within the object plane of ≈49 fg within ≈7 × 7 μm2 object surface area. Compared to a quartz crystal microbalance with dissipation instrumentation, where contemporary limits require a total load of ≈500 pg for detection, the instrumentation demonstrated here improves sensitivity to a total mass required for detection by 4 orders of magnitude. We detail the design and operation principles of the anisotropic contrast optical microscope, and we present further applications to the detection of nanoparticles, to novel approaches for imaging chromatography and to new contrast modalities for observations on living cells.

Tracing the 4000 year history of organic thin films: From monolayers on liquids to multilayers on solids

The recorded history of organic monolayer and multilayer thin films spans approximately 4000 years. Fatty-acid-based monolayers were deposited on water by the ancients for applications ranging from fortune telling in King Hammurabi's time (∼1800 BC, Mesopotamia) to stilling choppy waters for sailors and divers as reported by the Roman philosopher Pliny the Elder in ∼78 AD, and then much later (1774) by the peripatetic American statesman and natural philosopher Benjamin Franklin, to Japanese “floating-ink” art (suminagashi) developed ∼1000 years ago. The modern science of organic monolayers began in the late-1800s/early-1900s with experiments by Lord Rayleigh and the important development by Agnes Pockels, followed two decades later by Irving Langmuir, of the tools and technology to measure the surface tension of liquids, the surface pressure of organic monolayers deposited on water, interfacial properties, molecular conformation of the organic layers, and phase transitions which occur upon compressing the monolayers. In 1935, Katherine Blodgett published a landmark paper showing that multilayers can be synthesized on solid substrates, with controlled thickness and composition, using an apparatus now known as the Langmuir-Blodgett (L-B) trough. A disadvantage of LB films for some applications is that they form weak physisorbed bonds to the substrate. In 1946, Bigelow, Pickett, and Zisman demonstrated, in another seminal paper, the growth of organic self-assembled monolayers (SAMs) via spontaneous adsorption from solution, rather than from the water/air interface, onto SiO2 and metal substrates. SAMs are close-packed two-dimensional organic crystals which exhibit strong covalent bonding to the substrate. The first multicomponent adsorbed monolayers and multilayer SAMs were produced in the early 1980s. Langmuir monolayers, L-B multilayers, and self-assembled mono- and multilayers have found an extraordinarily broad range of applications including controlled wetting, adhesion, electrochemistry, biocompatibility, molecular recognition, biosensing, cell biology, non-linear optics, molecular electronics, solar cells, read/write/erase memory, and magnetism.

Adsorption height alignment at heteromolecular hybrid interfaces

The formation of metalorganic hybrid interfaces is determined by the fine balance between molecule-substrate and molecule-molecule interactions at the interface. Here, we report on a systematic investigation of interfaces between a metal surface and organic monolayer films that consist of two different molecular species, one donor and one acceptor of electronic charge. Our x-ray standing wave data show that in heteromolecular structures, the molecules tend to align themselves to an adsorption height between those observed in the respective homomolecular structures. We attribute this alignment effect to a substrate-mediated charge transfer between the molecules, which causes a mutual enhancement of their respective donor and acceptor characters. We argue that this effect is of general validity for $\ensuremath{\pi}$-conjugated molecules adsorbing on noble metal surfaces.

Multidentate Adsorbates for Self-Assembled Monolayer Films

The spontaneous adsorption of organic molecules on a variety of planar and nonplanar substrates, that is, self assembly, can generate films just one molecule thick. These nanoscale, self-assembled monolayer (SAM) films have been extensively used to engineer surfaces with well-defined properties. Their utility has been demonstrated in a wide range of applications, including wetting, adhesion, lubrication, patterning, and molecular recognition. Many SAM systems have been investigated, but alkanethiols adsorbed on gold are the most successful combination. This pairing offers a variety of advantages, including the ability to tune precisely the interfacial properties of a surface through the well-established organic synthetic methodologies that have been developed for preparing custom ω-terminated alkanethiols. Alkanethiolate monolayers are moderately stable at room temperature; however, these films degrade over time and readily desorb upon moderate heating. This shortcoming limits the use of SAMs in applications involving elevated temperatures or harsh environments. Accordingly, new adsorbates with multiple bonding moieties have been created to enhance the stability and versatility of SAMs. In this Account, we examine a variety of multidentate adsorbate structures that have been used to generate SAMs on planar substrates and on nanoparticles. Each of these chelating adsorbates (bidentates and tridentates) has been designed to generate well-defined organic monolayer films with multiple attachment points to the underlying substrate. This bonding arrangement allows the formation of SAMs with enhanced stability through the entropy-driven "chelate effect". The research examined here demonstrates that multidentate adsorbates provide robust films: they enable the use of SAMs under conditions that are incompatible with SAMs derived from normal alkanethiols. Another advantage offered by multidentate adsorbates is the capacity for new paradigms in thin-film composition. In particular, appropriately designed chelating adsorbates can be engineered to have two or more chemically distinct terminal groups that are covalently linked to the same underlying headgroup, without adding steric bulk that might prove detrimental to the resultant assembly. This strategy allows the generation of homogeneously mixed multicomponent surfaces, overcoming the problem of phase separation or "islanding" that is pervasive when two or more chemically distinct adsorbates are used to form mixed SAMs. Such homogeneously mixed films offer the opportunity to fine-tune the interfacial properties of a substrate and to create unique heterogeneous interfaces that are well defined by the chemical composition of the tailgroups exposed at the surface. The insight derived from these studies opens the door to new uses for SAMs, both in surface engineering applications (such as corrosion resistance and soft lithographic patterning) and in the stabilization and manipulation of nanoparticles.

Stamping oriented molecular monolayers using liquid crystal inks

Thermotropic liquid crystal (LC)-based inks are combined with patterned anchoring stamps to deposit organic monolayer films with simultaneous control over positional and molecular orientational order in a single step.

Synthesis of a New Organic Linker Molecule for Protein Immobilization

Recent biological and medicinal experimental techniques for diagnosis and drug discovery utilize microarrays of proteins immobilized on solid substrates. Such protein “chips” can have proteins on surface attached covalently or non-covalently between proteins and linker molecules frequently by chemisorption of the film components to the substrate from solution. For the well defined systems, organic monolayer films have formed versatile model study. The arrays of proteins attached to a solid surface have a great potential for detecting interactions of protein-ligand, proteinprotein, and antibody-antigen. Among several self-assembling systems, organosulfur compounds including dialkyl sulfides, dialkyl disulfides, and thiols have been investigated intensively on gold. It has been recognized that self-assembled monolayers (SAM) using organosufur compounds on gold are advantageous over other SAM systems because of higher structural order by densely packing long chains. In addition, versatile functionalization of terminal group at the monolayer surface suggests ideal model systems for immobilization and interfacial interaction. Several functional groups including anhydride, succinimidyl ester, maleimide, and phosphinothioester have been applied for the chemoselective binding of proteins. Here, we report a new potential substrate for the covalent and specific immobilization of nucleophilic molecules. The new organic linker, disulfide 1, has 4, 5-difluoro-2-nitroaniline amide moiety on the head and a long carbon chain with a triazole group. We envisioned that the compound would be prepared via 1,3-dipolar cycloaddition of azide 2 and alkyne 3, and hoped that a Meisenheimer complex of 4,5-difluoro-2-nitroaniline amide moiety would serve for a covalent attachment of amine or thiol moieties, and eventually proteins on the surface (Scheme 1). Prior to the synthesis of the molecule, we wanted to evaluate the reactivity of the substrate 1 toward nucleophilic aromatic substitution. Therefore, we first prepared a amide 4 from 4,5-difluoro-2-nitroaniline and 10-undecenoyl chloride and have tried the substitution reaction with various amines and a thiol as a model study (Table 1). The reaction was carried out in THF/water at room temperature. Primary and secondary amines reacted with the nitroaniline in minutes to afford good yields. Cyclic amines provided better than noncyclic secondary amines, and thiol provided the best yield in the shortest reaction time as expected. On the basis of the result, we anticipate the functional groups in proteins, amine or thiol, would show the similar reaction pattern toward the

Infrared spectra of organic monolayer films in a standing wave measured by photon-trap spectroscopy

Photon-trap spectroscopy, a generalized scheme of cavity ringdown spectroscopy, is applied to measure an infrared spectrum of the C-H stretching vibration of alkylsiloxane monolayer films grafted on a silicon substrate. A continuous-wave laser beam is introduced into a high-finesse Fabry-Pérot cavity containing the substrate placed exactly normal to the light beam to minimize optical losses. The lifetime of the light trapped in the cavity is measured to detect optical absorption sensitively. The results show clear dependence of the absorbance on the location of the monolayers with respect to a standing wave formed in the cavity; the absorbance is practically zero when the monolayers on both the surfaces are adjusted at nodes, whereas it is maximized at antinodes. The present experiment is materialized on the basis of the principles established by our previous study [Terasaki et al., J. Opt. Soc. Am. B 22, 675 (2005)].

Covalent molecular functionalization of diamond thin-film transistors

The chemical functionalization and the resulting electrical properties of diamond thin-film transistors (TFTs) modified with organic monolayer films have been explored. Chemical processing steps were developed to produce amine-terminated surfaces in a manner compatible with pre-existing front-surface metal contacts and dielectric protecting layers. The electrical properties of the monolayer-modified TFTs fabricated using different procedures were investigated. Interchanging the order of the functionalization and contact deposition steps changes the devices transconductance by a factor of 100. Electrical measurements of thin-film transistor devices with three different molecular terminating groups show significant changes in threshold voltage that correspond with the expected charges of the functional groups.

Free-electron-like dispersion in an organic monolayer film on a metal substrate

Thin films of molecular organic semiconductors are attracting much interest for use in electronic and optoelectronic applications. The electronic properties of these materials and their interfaces are therefore worth investigating intensively, particularly the degree of electron delocalization that can be achieved. If the delocalization is appreciable, it should be accompanied by an observable electronic band dispersion. But so far only limited experimental data on the intermolecular dispersion of electronic states in molecular materials is available, and the mechanism(s) of electron delocalization in molecular materials are also not well understood. Here we report scanning tunnelling spectroscopy observations of an organic monolayer film on a silver substrate, revealing a completely delocalized two-dimensional band state that is characterized by a metal-like parabolic dispersion with an effective mass of m* = 0.47m(e), where m(e) is the bare electron mass. This dispersion is far stronger than expected for the organic film alone, and arises as a result of strong substrate-mediated coupling between the molecules within the monolayer.

A Reagentless Tyrosinase Biosensor Based on 1,6‐Hexanedithiol and Nano‐Au Self‐Assembled Monolayers

AbstractAn amperometric tyrosinase biosensor was developed via a simple and effective immobilization method using the self‐assembled monolayers (SAMs) technique. The organic monolayer film was first formed by the spontaneous assembly of thiolor sulfur compound (1,6‐hexanedithiol, HDT) from solution onto gold electrode. When these thiol‐rich surfaces were exposed to Au colloid, the sulfurs form strong bonds to gold nanoparticles, anchoring the clusters to the electrode substrate. After the assembly of gold nanoparticles layer, a new nano‐Au surface was obtained. Thus, the tyrosinase could be immobilized onto the electrode. The tyrosinase retained its activity well in such an immobilization matrix. The various experimental variables for the enzyme electrode were optimized. The resulting biosensor can reach 95% of steady‐state current within 10 s, and the trend in the sensitivity of different phenolic compounds was as follows: catechol>phenol>p‐cresol. In addition, the apparent Michaelis–Menten constant (K$\rm{ {_{m}^{app}})}$ and the stability of the enzyme electrode were estimated.

Quantitative analysis of photoelectron angular distribution of single-domain organic monolayer film: NTCDA on GeS(0 0 1)

Angle-resolved photoelectron spectra were measured for a single-domain monolayer film of naphthalene-1,4,5,8-tetracarboxylic dianhydride (NTCDA) prepared on a cleaved GeS(0 0 1) surface using synchrotron radiation. The observed photoelectron angular distributions were analyzed by a calculation using the single-scattering approximation combined with molecular orbital calculation. With the help of the low-energy electron diffraction pattern, the structure of the NTCDA monolayer on GeS(0 0 1) was estimated quantitatively.

Organization-Induced Charge Redistribution in Self-Assembled Organic Monolayers on Gold

The charge redistribution that occurs within dipolar molecules as they self-assemble into organized organic monolayer films has been studied. The extent of charge transfer is probed by work function measurements, using low-energy photoelectron spectroscopy (LEPS), contact potential difference (CPD), and X-ray photoelectron spectroscopy (XPS), with the latter providing fine details about the internal charge distribution along the molecule. In addition, two-photon photoelectron spectroscopy is applied to investigate the electronic structure of the adsorbed layers. We show that charge transfer acts to reduce the dipole-dipole interaction between the molecules but may either decrease or increase the molecule-to-surface dipole moment.

A reusable capacitive immunosensor with a novel immobilization procedure based on 1,6-hexanedithiol and nano-Au self-assembled layers

A novel immobilization procedure of antibody proteins for capacitive immunosensing, based on thiolor sulfur compound (1,6-hexanedithiol, HDT) and colloid Au layers is proposed. The insulating organic monolayer film was first formed by the spontaneous assembly of HDT from solution onto gold. When these thiol-rich surfaces are exposed to Au colloid, the sulfurs form strong bonds to gold nanoparticles, anchoring the clusters to the electrode substrate. After the assembly of gold nanoparticles layer, the original formed organic thiols surface was restored, and a new nano-Au surface was obtained. Thus, the antibody could be immobilized through electrostatic adsorption between nano-Au and the antibody proteins. After use, the formed immunocomplex layer can be rinsed out, via a saline solution with extreme pH. Therefore, the immunosensor can be regenerated repeatedly, highlighting a clear advantage of this new approach with respect to classical immunoassays employing covalent immobilization.