Organic Monolayers 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

Creating a suprazyme: integrating a molecular enzyme mimic with a nanozyme for enhanced catalysis.

Enzyme mimics, due to their limited complexity, traditionally display low catalytic efficiency. Herein we present a strategy that enables the transformation of a slow-acting catalyst into a highly active one by creating a non-covalent suprastructure, termed "suprazyme". We show that cucurbit[7]uril macrocycles, rudimentary molecular enzyme mimics, embedded within an anionic monolayer on the surface of gold nanoparticles, outperform individual cucurbit[7]urils as well as nanoparticles, which also exhibit catalytic enzyme-like activity and thus act as nanozymes, by over 50 times, showcasing a 1044-fold acceleration in a model oxime formation reaction. The superior performance of such a suprazyme is attributed to a synergistic interplay between the organic monolayer and macrocycles, which is accompanied by a decreased local polarity and pH that favors the acid-catalyzed condensation process. The proposed approach holds promise for developing diverse suprazymes, contingent upon achieving a complementary structure and mechanism of action between the molecular catalyst and nanoparticles.

Ozonolysis–oxidation-driven top-down strategy for the target preparation of ultrathin 2D metal–organic framework monolayers

A novel chemical exfoliation method is developed for the target preparation of 2D MOF monolayers from the 3D pillar-layered MOFs.

Spin polarized current in chiral organic radical monolayers.

The chirality-induced spin selectivity (CISS) effect is the capability of chiral molecules to act as spin filters, i.e. to selectively sort flowing electrons based on their spin states. The application of this captivating phenomenon holds great promise in the realm of molecular spintronics, where the primary focus lies in advancing technologies based on chiral molecules to regulate the injection and coherence of spin-polarized currents. In this context, we conducted a study to explore the spin filtering capabilities of a monolayer of the thia-bridged triarylamine hetero[4]helicene radical cation chemisorbed on a metallic surface. Magnetic-conductive atomic force microscopy revealed efficient electron spin filtering at exceptionally low potentials. Furthermore, we constructed a spintronic device by incorporating a monolayer of these molecules in between two electrodes, obtaining an asymmetric magnetoresistance trend with signal inversion in accordance with the handedness of the enantiomer involved, indicative of the presence of the CISS effect. Our findings underscore the significance of thia[4]azahelicene organic radicals as promising candidates for the development of quantum information operations based on the CISS effect as a tool to control the molecular spin states.

Galinstan Liquid Metal Electrical Contacts for Monolayer-Modified Silicon Surfaces.

Galinstan is the brand name for a low-melting gallium-based alloy, which is a promising nontoxic alternative to mercury, the only elemental metal found in the liquid state at room temperature. Liquid alloys such as Galinstan have found applications as electromechanical actuators, sensors, and soft contacts for molecular electronics. In this work, we validate the scope of Galinstan top contacts to probe the electrical characteristics of Schottky junctions made on Si(111) and Si(211) crystals modified with Si-C-bound organic monolayers. We show that the surface-to-volume ratio of the Galinstan drop used as a macroscopic contact defines the junction stability. Further, we explore chemical strategies to increase Galinstan surface tension to obtain control over the junction area, hence improving the repeatability and reproducibility of current-voltage (I-V) measurements. We explore Galinstan top contacts as a means to monitor changes in rectification ratios caused by surface reactions and use these data, most notably the static junction leakage, toward making qualitative predictions on the DC outputs recorded when these semiconductor systems are incorporated in Schottky-based triboelectric nanogenerators. We found that the introduction of iron particles leads to poor data repeatability for capacitance-voltage (C-V) measurements but has only a small negative impact in a dynamic current measurement (I-V).

Surface-Modified Pd/CeO2 Single-Atom Catalyst Shows Increased Activity for Suzuki Cross-Coupling.

Single-atom catalysts (SACs) comprise catalytically active atoms dispersed on supports; they combine the high activity and site uniformity of homogeneous catalysts with the ease of separability of heterogeneous catalysts. However, SACs lack fine control over the active site, provided by ligands in homogeneous catalysts. In this work, we demonstrate that modification of the support with an organic monolayer is a viable approach to improving the catalytic performance. The addition of catechol-type monolayers to a Pd/CeO2 SAC increases its catalytic activity for Suzuki cross-coupling, a central reaction in the synthesis of fine chemicals and pharmaceuticals. Kinetic trials reveal that the coating reduces the activation energy from 49 ± 9 to 22 ± 5 kJ/mol and produces a 4-fold rate enhancement at 25 °C, an effect we attribute to π-π interactions between the reactant and the catechol coating. Further development of this approach could vastly increase the utility of SACs in organic synthesis.

Antireflection Substrates for Determining the Number of Layers of Few-Layer Hexagonal Boron Nitride Films and for Visualizing Organic Monolayers

Antireflection Substrates for Determining the Number of Layers of Few-Layer Hexagonal Boron Nitride Films and for Visualizing Organic Monolayers

Adsorption of fatty acid potassium salts on diamond-like carbon: Effects of alkyl chain lengths and adsorptive conditions

Organic monolayers on solid substrates have received considerable attention due to their excellent versatilities in many fields. In this paper, molecular dynamics (MD) simulations were performed to study the adsorption of fatty acid potassium salts (FAPS) on diamond-like carbon (DLC) substrates. The results indicated that adsorbed molecules tend to aggregate on DLC surfaces due to the intense attractions between FAPS molecules. In order to obtain evenly distributed molecular films, the external energy input is needed to facilitate molecular adsorption and overcome the potential barriers. Also, the effects of alkyl chain lengths on molecular adsorption were investigated. A positive correlation was observed between the alkyl length and the adsorptive energy exerted by DLC substrates. Furthermore, the Coulomb interaction between adsorbed molecules is the most powerful interaction within the MD system. Affected by the Coulomb interactions, adsorbed molecules would undergo severe deformation. And a net-like structure composed of polar ends is formed in molecular films. The hollow area within the network is full of alkyl chains of molecules. It can be concluded that the powerful Coulomb interactions between adsorbed molecules play a dominant role in determining the adsorption behavior and structural properties of potassium salts on DLC substrates.

Aqueous Radical Initiated Oxidation of an Organic Monolayer at the Air-Water Interface as a Proxy for Thin Films on Atmospheric Aerosol Studied with Neutron Reflectometry.

Neutron reflectometry has been used to study the radical initiated oxidation of a monolayer of the lipid 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) at the air-solution interface by aqueous-phase hydroxyl, sulfate, and nitrate radicals. The oxidation of organic films at the surface of atmospheric aqueous aerosols can influence the optical properties of the aerosol and consequently can impact Earth's radiative balance and contribute to modern climate change. The amount of material at the air-solution interface was found to decrease on exposure to aqueous-phase radicals which was consistent with a multistep degradation mechanism, i.e., the products of reaction of the DSPC film with aqueous radicals were also surface active. The multistep degradation mechanism suggests that lipid molecules in the thin film degrade to form progressively shorter chain surface active products and several reactive steps are required to remove the film from the air-solution interface. Bimolecular rate constants for oxidation via the aqueous phase OH radical cluster around 1010 dm3 mol-1 s-1. Calculations to determine the film lifetime indicate that it will take ∼4-5 days for the film to degrade to 50% of its initial amount in the atmosphere, and therefore attack by aqueous radicals on organic films could be atmospherically important relative to typical atmospheric aerosol lifetimes.

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.

Excited-State Odd-Even Effect in Through-Space Interactions.

The odd-even effect is a fantastic phenomenon in nature, which has been applied in diverse fields such as organic self-assembled monolayers and liquid crystals. Currently, the origin of each odd-even effect remains elusive, and all of the reported odd-even effects are related to the ground-state properties. Here, we discover an excited-state odd-even effect in the through-space interaction (TSI) of nonconjugated tetraphenylalkanes (TPAs). The TPAs with an even number of alkyl carbon atoms (C2-TPA, C4-TPA, and C6-TPA) show strong TSI, long-wavelength emission, and high QY. However, the odd ones (C1-TPA, C3-TPA, C5-TPA, and C7-TPA) are almost nonexistent with negligible QY. Systematically experimental and theoretical results reveal that the excited-state odd-even effect is synthetically determined by three factors: alkyl geometry, molecular movability, and intermolecular packing. Moreover, these flexible luminescent TPAs possess tremendous advantages in fluorescent information encryptions. This work extends the odd-even effect to photophysics, demonstrating its substantial importance and universality in nature.

Selective Deposition of N-Heterocyclic Carbene Monolayers on Designated Au Microelectrodes within an Electrode Array.

The incorporation of organic self-assembled monolayers (SAMs) in microelectronic devices requires precise spatial control over the self-assembly process. In this work, selective deposition of N-heterocyclic carbenes (NHCs) on specific electrodes within a two-microelectrode array is achieved by using pulsed electrodeposition. Spectroscopic analysis of the NHC-coated electrode arrays reveals that each electrode is selectively coated with a designated NHC. The impact of NHC monolayers on the electrodes' work function is quantified using Kelvin probe force microscopy. These measurements demonstrate that the work function values of each electrode can be independently tuned by the adsorption of a specific NHC. The presented deposition method enables to selectively coat designated microelectrodes in an electrode array with chosen NHC monolayers for tuning their chemical and electronic functionality.

Collective States in Molecular Monolayers on 2D Materials.

Collective excited states form in organic two-dimensional layers through Coulomb coupling of the molecular transition dipole moments. They manifest as characteristic strong and narrow peaks in the excitation and emission spectra that are shifted to lower energies compared with the monomer transition. We study experimentally and theoretically how robust the collective states are against homogeneous and inhomogeneous broadening, as well as spatial disorder that occurs in real molecular monolayers. Using a microscopic model for a two-dimensional dipole lattice in real space, we calculate the properties of collective states and their extinction spectra. We find that the collective states persist even for 1-10% random variation in the molecular position and in the transition frequency, with a peak position and integrated intensity similar to those for the perfectly ordered system. We measured the optical response of a monolayer of the perylene derivative MePTCDI on two-dimensional materials. On the wide-band-gap insulator hexagonal boron nitride, it shows strong emission from the collective state with a line width that is dominated by the inhomogeneous broadening of the molecular state. When the semimetal graphene is used as a substrate, however, the luminescence is completely quenched. By combining optical absorption, luminescence, and multiwavelength Raman scattering, we verify that the MePTCDI molecules form very similar collective monolayer states on hexagonal boron nitride and graphene substrates, but on graphene the line width is dominated by nonradiative excitation transfer from the molecules to the substrate. Our study highlights the transition from the localized molecular state of the monomer to a delocalized collective state in the two-dimensional molecular lattice that is entirely based on Coulomb coupling between optically active excitations of the electrons and molecular vibrations. The excellent properties of organic monolayers make them promising candidates for components of soft-matter optoelectronic devices.

Anomalous Mechanical and Electrical Interplay in a Covalent Organic Framework Monolayer Membrane.

Ion transport through nanoconfinement, driven by both electrical and mechanical forces, has drawn ever-increasing attention, due to its high similarity to stress-sensitive ion channels in biological systems. Previous studies have reported only pressure-induced enhancement in ion conductance in low-permeable systems such as nanotubes, nanoslits, or single nanopores. This enhancement is generally explained by the ion accumulation caused by the capacitive effect in low-permeable systems. Here, we fabricate a highly permeable COF monolayer membrane to investigate ion transport behavior driven by both electrical and mechanical forces. Our results show an anomalous conductance reduction activated by external mechanical force, which is contrary to the capacitive effect-dominated conductance enhancement observed in low-permeable nanopores or channels. Through simulations, we uncovered a distinct electrical-mechanical interplay mechanism that depends on the relative rate between the ion diffusion from the boundary layer to the membrane surface and the ion transport through the membrane. The high pore density of the COF monolayer membrane reduces the charge accumulation caused by the capacitive effect, resulting in fewer accumulated ions near the membrane surface. Additionally, the high membrane permeability greatly accelerates the dissipation of the accumulated ions under mechanical pressure, weakening the effect of the capacitive layer on the streaming current. As a result, the ions accumulated on the electrodes, rather than in the capacitive layer, dominating the streaming current and giving rise to a distinct electrical-mechanical interplay mechanism compared to that in low-permeable nanopores or channels. Our study provides new insights into the interplay between electrical and mechanical forces in ultra-permeable systems.

Suppressing Interfacial Side Reactions of Anode-Free Lithium Batteries by an Organic Salt Monolayer.

Anode-free lithium (Li) batteries are attractive owing to their high energy density. However, Li loss by forming solid-electrolyte interphase (SEI) during cell activation leads to a ≈25% capacity decrease, and the capacity constantly fades upon cycling due to the side reactions on the copper (Cu) current collector. This paper reports high-initial-efficiency, long-cycle-life, and long-calendar-life anode-free Li batteries by using an organic Li salt monolayer bonded on Cu. The functional salt, namely lithium ((4-carbamoylphenyl)sulfonyl)(fluorosulfonyl)imide, electrochemically decomposes and passivates the Cu surface, which reduces Li sacrifice by SEI formation and suppresses galvanic Li corrosion and Li-electrolyte reactions during cycling. This work records a LiF-rich interphase on Cu and guided Li nucleation and growth. A 93.6% initial Li deposition efficiency is realized in a regular carbonate electrolyte, and the galvanic current is decreased to ≈40 nA cm-2 , merely one-tenth of bare Cu. After cell activation, 95.2% capacity is retained for a Cu|LiNi0.8 Mn0.1 Co0.1 pouch cell with a theoretical capacity of 200 mAh, and the cell is operated over 600 cycles. Calendar aging showed no damage to cell performance.

Monolayer organic thin films as particle-contamination–resistant coatings

Three organic monolayers coatings were developed and tested for their effectiveness to increase cleaning efficiency of attached microscale particles by air flows. The experiments were performed using silica substrates coated with these organic thin films and subsequently exposed to stainless-steel and silica microparticles as a model of contamination. Laser-induced–damage tests confirmed that the coatings do not affect the laser-induced–damage threshold values. The particle exposure results suggest that although the accumulation of particles is not significantly affected under the experimental conditions used in this work, the coated substrates exhibit significantly improved cleaning efficiency with a gas flow. A size-distribution analysis was conducted to study the adsorption and cleaning efficiency of particles of different sizes. It was observed that larger size (> 5-μm) particles can be removed from coated substrates with almost 100% efficiency. It was also determined that the coatings improve the cleaning efficiency of the smaller particles (≤ 5 μm) by 17% to 30% for the stainless steel metal and 19% to 38% for the silica particles.

DFT Study of Cl− Ingress into Organic Self-Assembled Monolayers on Aluminum

We address the mechanism by which organic layers on aluminum substrate hinder the penetration of Cl− toward the metal substrate. Localized corrosion by chlorides on Al and its alloys is a major problem, and organic molecules that form self-assembled monolayers on metal substrates may provide efficient corrosion protection. In one of our previous works, we established experimentally that long-chain n-alkyl carboxylic acids form protective layers against Cl− corrosion on Al substrates. In a different work, we identified, using implicit models of the organic layer and metal substrate, two essential effects by which organic layers hinder the penetration of Cl− ions toward the metal substrate. The first effect is due to the inferior solvation of ions in the organic layer compared to that in an aqueous solvent. The second effect is due to the electric field at the electrochemical interface, and the extent to which it affects the penetration of Cl− depends on the electrode potential and the thickness of the organic layer. Both effects are related to a low dielectric constant of the self-assembled monolayer. In the present study, we continue our investigation and explicitly model the organic monolayer and Al substrate using density-functional-theory calculations. To this end, we consider organic monolayers consisting of either dodecanoic- or hexanoic-acid molecules. Current calculations confirm the findings of the simplified implicit models, i.e. the energy barrier for the Cl− penetration increases with the thickness of the organic monolayer and with Cl− concentration in the monolayer. Furthermore, we propose a new mechanism by which Cl− penetrates the organic monolayer. Due to the considerably inferior solvation of Cl− in the organic layer compared to that in water, calculations suggest that it is energetically easier to locally “open” the organic monolayer by creating a hole large enough to accommodate water molecules and Cl−. The presence of water molecules ensures a stronger Cl− solvation and a better electrostatic screening between anions. While the energy barrier for the Cl− penetration via the local “opening” mechanism is suggested to be smaller than for the penetration of Cl− into dense homogeneous organic monolayer, it is still significant enough to pose a considerable kinetic barrier for the penetration of Cl− from the aqueous solution into the organic monolayer at room temperature.

Electrochemical grafting assisted modification of graphite surface by 3,4,5-trimethoxybenzenediazonium based monolayer

A conventional route to create such nanoscale electronic devices using the autonomous ordering and assembly of organic molecules on atomically well-defined surfaces has been proposed. However, these thin films are unstable in realistic environments due to weak interaction between the organic molecules and crystal surfaces. Therefore, enhancing the interfacial interaction between them is assigned as the key approach to extend the scope of application of these promising monolayer thin films. In this work, we demonstrate the formation of an organic monolayer, namely 3,4,5-trimethoxy diazonium (3,4,5-TMD), on a Highly Ordered Pyrolytic Graphite (HOPG) electrode by using the power of electrochemical method. The structural properties at the nano scale as well as the bonding nature between 3,4,5-TMD adlayer and HOPG electrode at the interface were investigated by a combination of cyclic voltammetry (CV), atomic force spectroscopy (AFM) and Raman spectroscopy. As a result, the 3,4,5-TMD molecules covalently bond to carbon atoms located at the HOPG interface for forming a monolayer with its thickness of 0.92 ± 0.02 nm. This finding opens a partway to apply the electrochemical grafting for covalent functionalization of 2D materials on HOPG and other metallic surfaces.

Direct-current output of silicon–organic monolayer–platinum Schottky TENGs: Elusive friction-output relationship

Triboelectric nanogenerators (TENGs) are an emerging energy harvesting technology able to convert ubiquitous mechanical energy into electricity. Friction, static charging and flexoelectricity are all involved in the mechanism underpinning TENG operation, but their relative contribution has remained elusive. Here we used dynamic and static conductive atomic force microscopy (C-AFM) measurements on monolayer-modified silicon crystals to detect evidence of a relationship between friction and zero-bias current, and between pressure and the direction of the putative flexovoltage. We demonstrate that a static electricity-related tribovoltage is probably responsible for a friction excess, and that surprisingly this friction excess is found to be dependent on the doping level and type of the silicon substrate. Such friction excess is however no longer measurable once current is allowed to flow across the junction. This observation points to an electrostatic origin of friction in silicon-based Schottky TENGs, and suggests that the zero external bias DC current is at least in part an electronic flow to neutralize static charges. Further, the sign of the zero-bias current, but not its magnitude, is independent of the semiconductor doping type, which is again suggestive of surface statics being a main contributor to the zero-bias output rather than exclusively a space-charge effect. We also reveal the presence of a junction flexovoltage under pressures common in AFM experiments (GPa), even for negligible lateral friction. In a static Pt–monolayer–n-type Si junction the flexovoltage carries the same sign as the tribovoltage, and can reach such magnitude to overwrite external voltages as high as 2 V. The immediate implication is that the flexovoltage is likely to have i) a strong contribution to the zero-bias output of a n-Si Schottky TENG, ii) a negative effect on the output of a p-Si TENG, and iii) its detection can be straightforward, as we discovered that flexoelectricity manifests as an “inverted diode”: a n-type Si–platinum diode with negligible current even when the n-type material is negatively biased as long as the “static” diode remains under a large normal pressure.

Perovskite Solar Cells with an SbX3 (X = Cl, I) Interface Dipole Layer

Forming an interface dipole moment pointing from the absorber to the oxide substrate can improve the open-circuit voltage (VOC) of perovskite solar cells (PSCs) without affecting photocurrent. In past studies, however, interface dipole layers were generally formed using organic self-assembled monolayers or polyelectrolytes, which raises concerns about intrinsic stability. Meanwhile, perovskites’ substrate-dependent electronic properties question the apparent correlation between the oxide’s work function and the built-in voltage. In this work, we propose an interface dipole layer based on solution-processable metal halides, which increases the built-in voltage of the n–i–p diode by simultaneously reducing the electron transport layer’s work function and strengthening the n-type characteristics of the perovskite absorber. The n–i–p-structured PSCs with the SbX3 dipole layer achieve substantially enhanced VOC (1.171 V) and power conversion efficiency (23.11%). Further experiments and calculations confirm that the proposed dipole model can explain the VOC enhancement qualitatively and quantitatively.