Organic Monolayers Research Articles

Directed Assembly of Au-Tipped 1D Inorganic Nanostructures via Nanolithographic Docking.

Controlled assembly of nanostructures is a key challenge in nanotechnology. In this work, we introduce an approach for the controlled assembly of 1D nanodumbbells-Au-tipped semiconductor nanorods-into arbitrary 2D higher architectures, by their chemical docking to nanopatterned functionalities. We realized the docking functionalities via nanoimprinted metallic nanodots functionalized with an organic monolayer, whose terminal thiol groups chemically bind the nanodumbbell tips. We demonstrated that the functional nanopattern encodes the nanodumbbell assembly and can be designed to deterministically position nanodumbbells in two possible modes. In the single-docking mode, the nanodot arrays are designed with a spacing that exceeds the nanodumbbell length, restricting each nanodumbbell to dock with one edge and physically connect with its free edge to one of the neighboring nanodumbbells. Alternatively, in the double-docking mode, the nanodots are spaced to exactly fit the nanodumbbell length, allowing nanodumbbell docking with both edges. We found that the docking kinetics can be described by a random attachment model, and verified that for the used docking chemistry, nanodumbbells that are docked to the same dot do not interact with each other. Our work demonstrates the possibility for massively parallel positioning of sub-100 nm 1D semiconductor nanostructures, and can potentially enable their future integration into functional nanodevices and nanosystems.

Interfacial Electron Beam Lithography: Chemical Monolayer Nanopatterning via Electron-Beam-Induced Interfacial Solid-Phase Oxidation.

Chemical nanopatterning-the deliberate nanoscale modification of the chemical nature of a solid surface-is conveniently realized using organic monolayer coatings to impart well-defined chemical functionalities to selected surface regions of the coated solid. Most monolayer patterning methods, however, exploit destructive processes that introduce topographic as well as other undesired structural and chemical transformations along with the desired surface chemical modification. In particular in electron beam lithography (EBL), organic monolayers have been used mainly as ultrathin resists capable of improving the resolution of patterning via local deposition or removal of material. On the basis of the recent discovery of a class of radiation-induced interfacial chemical transformations confined to the contact surface between two solids, we have advanced a direct, nondestructive EBL approach to chemical nanopatterning-interfacial electron beam lithography (IEBL)-demonstrated here by the e-beam-induced local oxidation of the -CH3 surface moieties of a highly ordered self-assembled n-alkylsilane monolayer to -COOH while fully preserving the monolayer structural integrity and molecular organization. In this conceptually different EBL process, the traditional resist is replaced by a thin film coating that acts as a site-activated reagent/catalyst in the chemical modification of the coated surface, here the top surface of the to-be-patterned monolayer. Structural and chemical transformations induced in the thin film coating and the underlying monolayer upon exposure to the electron beam were elucidated using a semiquantitative surface characterization methodology that combines multimode AFM imaging with postpatterning surface chemical modifications and quantitative micro-FTIR measurements. IEBL offers attractive opportunities in chemical nanopatterning, for example, by enabling the application of the advanced EBL technology to the straightforward nanoscale functionalization of the simplest commonly used organosilane monolayers.

Cover Feature: Interaction of Single Cells with 2D Organic Monolayers: A Scanning Electrochemical Microscopy Study (ChemElectroChem 20/2018)

Cover Feature: Interaction of Single Cells with 2D Organic Monolayers: A Scanning Electrochemical Microscopy Study (ChemElectroChem 20/2018)

A Computational Comparison of Soft Landing of α-Helical vs Globular Peptides.

The effect of secondary structure on the soft landing process is investigated through direct dynamics simulations of AcA7K and AcKA7 colliding with a fluorinated, organic self-assembled monolayer (FSAM) surface. The α-helical (AcA7K) and globular (AcKA7) peptides each exhibited a similar probability of soft landing with normal incidence at all collision energies considered. Rapid conformational changes were quantified through the calculation of the time dependent, conformational entropy production that took place during the collision events, which is consistent with the prior structural measurements made by Laskin and co-workers on these systems. AcA7K produces more entropy during the collisions than AcKA7.

Interaction of Single Cells with 2D Organic Monolayers: A Scanning Electrochemical Microscopy Study

AbstractCell shape is a valuable indicator of cell health status and interactions with the extracellular materials. With the aim to investigate how individual cells respond to chemical microenvironments, we employed 2D monomolecular layers of small organic molecules, characterized by very low roughness. MCF10A cells were grown on these surfaces and characterized by using scanning electrochemical microscopy. The single cell morphologies were analyzed to define a “figure of merit” for the different cell–surface interactions. Such “figure of merit” quantifies the interaction with the microenvironment. This method is applicable to any cell state (metastatic, physiological or pathological) and any surface.

A facile method for the fabrication of hierarchical nanosized metal catalysts

Fabrication of well-defined metal nanoparticles with homogeneous distribution is of great importance to enhance the catalysts’ effectiveness and to minimize their cost. Here we report the electrodeposition of palladium on modified carbon substrates with an aryl-based organic monolayer and its electrocatalytic performance towards two model reactions with high involvement in energy storage and production systems, i.e. the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER) in alkaline medium. Main results indicate that the organic layer dramatically affects the growth, size and distribution of palladium nanoparticles on the conductive substrate, which provokes a great enhancement of the catalytic performance for both studied reactions. These results demonstrate the potential of synthetic methodology for cost-effective production of metallic catalysts.

Multifrequency Kelvin probe force microscopy on self assembled molecular layers and quantitative assessment of images’ quality

The application of single-pass multifrequency Kelvin probe force microscopy (KPFM) for topography and contact potential difference (CPD) measurements of organic self-assembled monolayers (SAM) is demonstrated. Four modes of mechanical and electrical cantilever excitation were tested in order to obtain the best possible resolution in the CPD measurements. The algorithm using maximum capacity of information channel for quantitative image quality assessment was proposed to compare and assess the quality of the recorded images and imaging modes. The improvement of the quality of CPD imaging in multiresonance operation was confirmed.

Mechanistic insights into CTAB assisted TiO2 crystal growth with largely exposed high energy crystal facets

Pristine anatase titanium dioxide (TiO2) photocatalysts were prepared by a modified hydrothermal method using cetyltrimethylammonium bromide (CTAB) as the capping agent. The so-formed TiO2 crystals showed superhydrophilic behavior in a short irradiation time span, without adopting any modifications to TiO2 like doping or hybridizing with organic monolayers. At an optimum pH of 3, the CTAB molecules were effectively adhered to the high energy facets of TiO2 favoring a crystal growth elongation in [001] direction leading to the exposure of high energy {101}/{100} and {001} facets. Variations in crystal morphology, size and exposing facets are investigated in detail using XRD, Raman and TEM, whereas the optical properties are studied using UV–vis diffuse reflectance spectroscopy, PL and TSCPC techniques. Acetic acid acting as a bridging ligand attaching the Ti centres in a twisted conformation effectively led to the anatase phase of TiO2. The shape and size of the CTAB micelles were greatly dependent on the pH of the reaction system, which in turn decided the exposure percentage of different facets. Anatase TiO2 prepared at optimum pH 3 were ellipsoidal and showed 83% photocatalytic degradation (k = 9.23 × 10−3 min-1) of Methylene Blue as the model dye, and the water contact angle of 17° under UV irradiation. When synthesis is repeated with double the quantity of water, TiO2 shared ellipsoidal and cuboidal morphologies and were superhydrophilic showing contact angle of 9° within 5 min UV irradiation, along with a superior photodegradation of 93% (k = 12.9 × 10−3 min-1).

Substrate-induced shifts and screening in the fluorescence spectra of supramolecular adsorbed organic monolayers.

We have investigated the influence of the substrate on the fluorescence of adsorbed organic molecules. Monolayer films of perylene-3,4,9,10-tetracarboxylic-3,4,9,10-diimide (PTCDI), a supramolecular network formed from PTCDI and melamine, and perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride have been deposited on hexagonal boron nitride (hBN). The principal peaks in the fluorescence spectra of these films were red-shifted by up to 0.37 eV relative to published measurements for molecules in helium droplets. Smaller shifts (∼0.03 eV) arising from interactions between neighbouring molecules are investigated by comparing the fluorescence of distinct arrangements of PTCDI, which are templated by supramolecular self-assembly and determined with molecular resolution using atomic force microscopy under ambient conditions. We compare our experimental results with red-shifts calculated using a combination of a perturbative model and density functional theory which account for, respectively, resonant and non-resonant effects of a dielectric hBN substrate. We show that the substrate gives rise to a red-shift in the fluorescence of an adsorbed molecule and also screens the interactions between neighbouring transition dipole moments; both these effects depend on the refractive index of the substrate.

Real-space imaging with pattern recognition of a ligand-protected Ag374 nanocluster at sub-molecular resolution

High-resolution real-space imaging of nanoparticle surfaces is desirable for better understanding of surface composition and morphology, molecular interactions at the surface, and nanoparticle chemical functionality in its environment. However, achieving molecular or sub-molecular resolution has proven to be very challenging, due to highly curved nanoparticle surfaces and often insufficient knowledge of the monolayer composition. Here, we demonstrate sub-molecular resolution in scanning tunneling microscopy imaging of thiol monolayer of a 5 nm nanoparticle Ag374 protected by tert-butyl benzene thiol. The experimental data is confirmed by comparisons through a pattern recognition algorithm to simulated topography images from density functional theory using the known total structure of the Ag374 nanocluster. Our work demonstrates a working methodology for investigations of structure and composition of organic monolayers on curved nanoparticle surfaces, which helps designing functionalities for nanoparticle-based applications.

Cytosolic Diffusion and Peptide-Assisted Nuclear Shuttling of Peptide-Substituted Circa 102 Gold Atom Nanoclusters in Living Cells

For biological and medical applications, a definable nanomaterial and knowledge on its fate after administration are highly recommended if not mandatory. Here, we synthesized a water-soluble gold nanocluster by sodium borohydride reduction of chloroauric acid in the presence of 5,5′-dithio-bis(2-nitrobenzoic acid). The resulting gold nanocluster displayed the physical characteristics of a cluster containing an inner core of about 100 gold atoms that was surrounded by an organic monolayer made of about 30 thioaminobenzoic acids (TAB) and 14 anionic thionitrobenzoic acids (TNB). The mixed TAB-,TNB-protected gold nanocluster reacted well in water with thiolated peptides containing a nuclear localization signal (NLS) or a nuclear export signal (NES) mainly by exchange of the TNB, providing gold nanoclusters equipped with 8–9 intracellular active peptides and a remaining ligand coverage consisting mostly of the zwitterion TAB. The behavior of these peptide–gold nanoclusters inside the cytosol and nucleus of ce...

Metallization of self-assembled organic monolayer surfaces by Pd nanocluster deposition

A condensation-cell-type cluster aggregation source was used to deposit Pd clusters on the self-assembled monolayers of four different types of organic molecules. Mica slides covered by Au (111) were coated with self-assembled monolayer films of n-dodecanethiol, 4-mercaptopyridine, dimethyldithiocarbamate, and diethyldithiocarbamate, and the behaviour of Pd clusters on these surfaces, as well as on the bare Au (111) surface, was characterized using scanning tunnelling microscopy and X-ray photoelectron spectroscopy. The aim of this study was to present an alternative means for the metallization of organic layers, through the use of preformed clusters that, unlike single adatoms, are less likely to penetrate the organic layer and cause unwanted interfacial layers or short circuits, and deduce its suitability for the various types of organic self-assembled monolayers. Our experimental results indicate that all of the studied self-assembled monolayers are impenetrable for Pd clusters deposited at thermal energies. Contrary to most methods of metallization, Pd cluster deposition therefore provides an efficient means of growing metal overlayers on organic self-assembled monolayers.

Reactive Monolayers in Directed Additive Manufacturing - Area Selective Atomic Layer Deposition

Additive manufacturing generally describes a process of producing macro scale 3D-objects by the addition (or formation) of one patterned layer, of the overall object, onto a subsequent layer. In this fashion the target structure is produced in a layer-by-layer process. Selective area atomic layer deposition (SA-ALD) is, fundamentally, an additive manufacturing process where the layer-by-layer process occurs at the atomic level. A patterned surface is generated by exploiting disparate surface chemistries where a metal oxide is deposited in a targeted area. One method of enabling this process is through the use of organic monolayers that can form from a collection of well organized small molecules onto one surface and not another. These self-assembled monolayers (SAMs) can act as effective barriers to block the deposition of up to hundreds of thermal ALD cycles or can be used in combination with repair strategies to extend selective deposition capabilities. Recent efforts have focused on increasing the durability of these monolayer blocking layers. To this end we have designed SAM components that contain an aligning hydrogen bonding component that aid in directing the formation of well aligned monolayers as well as acting as a supramolecular cross-link. Furthermore, the components contain a photoactive diyne moiety that can be subsequently polymerized after irradiation to produce a robust ALD barrier. On structured surfaces this photoactive SAM achieved better deactivation properties than commercially available materials providing access to materials that produce a robust barrier in shorter formation times and withstand a great number of ALD cycles.

Sorting of Molecular Building Blocks from Solution to Surface.

We demonstrate that molecular gradients on an organic monolayer is formed by preferential binding of ruthenium complexes from solutions also containing equimolar amounts of isostructural osmium complexes. The monolayer consists of a nanometer-thick assembly of 1,3,5-tris(4-pyridylethenyl)benzene (TPEB) covalently attached to a silicon or metal-oxide surface. The molecular gradient of ruthenium and osmium complexes is orthogonal to the surface plane. This gradient propagates throughout the molecular assembly with thicknesses over 30 nm. Using other monolayers consisting of closely related organic molecules or metal complexes results in the formation of molecular assemblies having an homogeneous and equimolar distribution of ruthenium and osmium complexes. Spectroscopic and computational studies revealed that the geometry of the complexes and the electronic properties of their ligands are nearly identical. These subtle differences cause the isostructural osmium and ruthenium complexes to pack differently on modified surfaces as also demonstrated in crystals grown from solution. The different packing behavior, combined with the organic monolayer significantly contributes to the observed differences in chemical composition on the surface.

Magnetic Levitation Tribometer: A Point-Contact Friction

We developed a new magnetic levitation tribometer (MLT) to study point-contact friction via pendulum method. The device allows us to carry out fast and accurate point-contact measurement of friction between a pair of materials. The MTL parameters were found with the help of test experiments with the steel ball / glass plate pair. The friction coefficient values are coherent with the results known in the literature (Serway and Beichner in Physics for scientists and engineers, 5th edn, Saunders College Publishing, Orlando, 2000). The proposed MLT technique at loads lower than $$10^{-3}$$ N is very promising for non-destructive investigation of frictional properties of organic monolayers.

Mechanism of Phosphorus Transport Through Silicon Oxide During Phosphonic Acid Monolayer Doping

Monolayer doping (MLD) is a relatively new method to incorporate shallow dopants from an adsorbed organic monolayer. To prevent evaporation of the dopant-containing organic layer during thermal pro...

Monolayered Graphene Oxide as a Low Contact Resistance Protection Layer in Alkanethiol Solid-State Devices

Vapor deposition of metals has long been the primary method for making contact with organic molecules in electronic devices in a fast and scalable manner. However, direct metal evaporation has proven to be the primary cause of device failure in solid-state molecular devices due to the degradation of the self-assembled molecular monolayers. The introduction of a protective interlayer between the molecular monolayer and the evaporated top electrode greatly improves the yield of working devices but at the cost of an increased internal contact resistance that depends on the nature of the interlayer and its interface with both organic molecules and metal top electrode. In the present work, we investigate the performance of a single layered graphene oxide as an atomically thin interlayer in solid-state molecular devices. We show that a single layered graphene oxide sheet is sufficient to protect an organic monolayer of alkane thiols from metal-induced degradation and short-circuiting. Remarkably, and despite gr...

Charge Transport through Single Molecules Connected to Semiconductor Electrodes

Single molecule electrical measurements have revolutionised our understanding of how charge flows through individual molecules and biomolecules in a wide variety of environments and are important for applications ranging from energy technology, DNA sequencing and interfacial electrochemistry. Such measurements have been typically made with metallic contacts but lately we have managed to extend this to the semiconductor gallium arsenide. This advance is important since future applications of molecular electronics will rely on integrating molecules alongside conventional semiconductor electronic technologies. In a recent publication in Nano Letters [1] we have shown that it is possible to make measurements of single molecules connected at one end to gallium arsenide and at the other end to a gold scanning tunnelling microscope tip. Using this methodology we can record current-voltage response of semiconductor – molecule – metal devices and measure the electrical conductance of single molecules in such junctions. For these measurements of charge transfer at the at the semiconductor-molecule interface the gallium arsenide surfaces are modified with organic self-assembled monolayers. Semiconductor / molecule(s) / metal junctions are formed by bringing a gold STM tip into contact with the molecular monolayer. Such junctions behave as a Schottky diode with the rectification ratio of the fabricated device being dependent on both the nature of the organic backbone and the electronic properties of the semiconductor. As well as recording I-V characteristics with the STM tip contacting the molecular monolayer, by using a time-dependent STM technique we were also able to detect the contribution of a single molecule to charge transport across the molecular junction. As well as showing that it is possible to form single molecule devices contacted to the semiconductor gallium arsenide we have also recently demonstrated that such single molecule devices show a strong photocurrent response.[2] Gallium arsenide is able to efficiently absorb visible light as it is a III-V direct bandgap (1.42 eV at room temperature) semiconductor and illumination of the molecular junction led to large changes in the current-voltage response. The junction configuration is schematically illustrated in the figure from reference [2] which shows a molecule bridging between the GaAs surface and the gold STM tip. The hemispheres in the GaAs substrate in this illustration depict the space charge layer (SCL) from which the charge carriers are collected. The resulting photo-generated charge carriers are transported to the metal electrode (STM tip) through the bridging organic dithiol giving the photo-current. Importantly, the photo-current response in these hybrid junctions can be controlled through the choice of molecular bridge as well as the doping density of the semiconductor amd the light intensity and wavelength.[2] In the dark high current rectification (> 103) is achieved with low-doping GaAs and saturated alkyl bridges and appreciable photocurrent is achieved upon illumination of these junctions. The response of the device is greatly influenced by molecular orbital alignment and this can be controlled by switching from saturated molecular bridges to conjugated ones with lower HOMO-LUMO gaps. Good photodiode functionality can be achieved with a space charge layer of only a few nanometres. Since the response of the molecular junctions is controlled by the combined properties of the semiconductor and the molecular wire this significantly extend of the single-molecule electronics “tool-box” as well as providing new methodologies for studying charge transfer at semiconductor-molecule interfaces. The presentation concludes with discussion of the perspective of single molecule photo-electrochemistry.

Injection of Spin-Polarized Electrons into a AlGaN/GaN Device from an Electrochemical Cell: Evidence for an Extremely Long Spin Lifetime

Spin-polarized electrons are injected from an electrochemical cell through a chiral self-assembled organic monolayer into a AlGaN/GaN device in which a shallow two-dimensional electron gas (2DEG) layer is formed. The injection is monitored by a microwave signal that indicates a coherent spin lifetime that exceeds 10 ms at room temperature. The signal was found to be magnetic field independent; however, it depends on the current of the injected electrons, on the length of the chiral molecules, and on the existence of 2DEG.

Indirect photopatterning of functionalized organic monolayers via copper-catalyzed “click chemistry”

Solution-based lithographic surface modification of an organic monolayer on a solid substrate is attained based on selective area photo-reduction of copper (II) to copper (I) to catalyze the azide-alkyne dipolar cycloaddition “click” reaction. X-ray photoelectron spectroscopy is used to confirm patterning, and spectroscopic results are analyzed and supplemented with computational models to confirm the surface chemistry. It is determined that this surface modification approach requires irradiation of the solid substrate with all necessary components present in solution. This method requires only minutes of irradiation to result in spatial and temporal control of the covalent surface functionalization of a monolayer and offers the potential for wavelength tunability that may be desirable in many applications utilizing organic monolayers.