Monolayer Of Molecules Research Articles

Role of Metal Centers in Tuning the Electronic Properties of Graphene-Based Conductive Interfaces

A major bottleneck in the fabrication of efficient bio-organic nanoelectronic devices resides in the strong charge recombination that is present at the different interfaces forming the complex system. An efficient way to overcome this bottleneck is to add a self-assembled monolayer (SAM) of molecules between the biological material and the electrode that promotes an efficient direct electron transfer whilst minimising wasteful processes of charge recombination. In this work, the presence of a pyrene-nitrilotriacetic acid layer carrying different metal centers as SAM physisorbed on graphene is fully described by mean of electrochemical analysis, field emission scanning electron microscopy, photoelectrochemical characterisation and theoretical calculations. Our multidisciplinary study reveals that the metal center holds the key role for the efficient electron transfer at the interface. While Ni2+ is responsible for an electron transfer from SAM to graphene, Co2+ and Cu2+ force an opposite transfer, from graphene to SAM. Moreover, since Cu2+ inhibits the electron transfer due to a strong charge recombination, Co2+ seems the transition metal of choice for the efficient electron transfer.

Gas adsorption on the pristine monolayer GeP3: A first-principles calculation

Monolayer GeP3 has promising applications due to a small indirect band gap and high carrier mobility (Nano Lett, 2017, 17, 1833–1838). By employing the first-principles calculation, we investigate the adsorption behavior of five representative gas molecules (CH4, N2, H2, CO, and H2S) on the surface of monolayer GeP3. Our calculations indicate that GeP3 monolayer can directly adsorb these gas molecules. Also, there are impressive adsorption energies and notable charge transfers between GeP3 monolayer and gas molecules. After adsorbing these gas molecules, the electronic property of GeP3 monolayer can be changed dramatically. In addition, the calculated recovery time of monolayer GeP3-based sensor at 300 K is also suitable for CH4, N2 and H2. These results suggest that monolayer GeP3 might have a potential application in molecule sensor for CH4, N2 and H2.

A new alternative to use soybean hulls on the adsorptive removal of aqueous dyestuff

This work aimed to use raw soybean hulls (RSH) as an adsorbent on the removal of BF-4B reactive red dye. Kinetic and equilibrium experiments were performed using the best conditions obtained from preliminary tests. Kinetic data were better represented by the pseudo-second order model, evidencing three well-differentiated mass transfer regions. By a statistical analysis of the fit of 7 equilibrium models and 2 model combinations to experimental data, it was observed that the adsorption occurred in a single molecule monolayer for final dye concentrations lower than ~63 mg L−1. After this concentration, a possible multimolecular monolayer was verified, evidencing the formation of aggregates or micelles, since the Langmuir-BET model was the best fit. The maximum adsorption capacity estimated was about 19 mg g−1. The results have proved that the soybean hulls could be used as an alternative adsorbent in the treatment of textile effluents, without the need for a pretreatment.

Studied on the dynamic adsorption process of Lycium barbarum polysaccharide in the POPC/DPPC monolayers.

In this study, the interaction between Lycium barbarum polysaccharide (LBP) and unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or saturated 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was explored using the Langmuir films technique and atomic force microscopy (AFM). Comparing the pure lipid monolayer with the mixed monolayers, the π-A isotherms of the mixed monolayers shifted to larger molecular areas when LBP was added to the subphase. The compression modulus showed that the compressibility of the monolayer films decreased with the addition of LBP. Adsorption curves revealed that the variation in the surface pressure of LBP with POPC was larger than that with DPPC. This phenomenon was verified by the AFM images and the number of each lipid molecule combining with polysaccharide molecules in the mixed monolayer (Ap value), indicating that hydrophobic interactions between LBP and POPC are stronger than those of DPPC. These findings lay the foundation for exploring the pharmacological mechanism of LBP as an in vivo therapeutic.

Anomalously Large Spectral Shifts near the Quantum Tunnelling Limit in Plasmonic Rulers with Subatomic Resolution.

The resonance wavelength of a coupled plasmonic system is extremely sensitive to the distance between its metallic surfaces, resulting in "plasmon rulers". We explore this behavior in the subnanometer regime using self-assembled monolayers of bis-phthalocyanine molecules in a nanoparticle-on-mirror (NPoM) construct. These allow unprecedented subangstrom control over spacer thickness via choice of metal center, in a gap-size regime at the quantum-mechanical limit of plasmonic enhancement. A dramatic shift in the coupled plasmon resonance is observed as the gap size is varied from 0.39 to 0.41 nm. Existing theoretical models are unable to account for the observed spectral tuning, which requires inclusion of the quantum-classical interface, emphasizing the need for new treatments of light at the subnanoscale.

Alignment of liquid crystals using Langmuir‒Blodgett films of unsymmetrical bent-core liquid crystals

ABSTRACTThe properties of the thin films of liquid crystal (LC) molecules can be governed easily by external fields. The anisotropic structure of the LC molecules has a large impact on the electrical and optical properties of the film. The Langmuir monolayer (LM) of LC molecules at the air–water interface is known to exhibit a variety of surface phases which can be transferred onto a solid substrate using the Langmuir‒Blodgett (LB) technique. Here, we have studied the LM and LB films of asymmetrically substituted bent-core LC molecules. The morphology of LB film of the molecules is found to be a controlling parameter for aligning bulk LC in the nematic phase. It was found that the LB films of the bent-core molecules possessing defects favour the planar orientation of nematic LC, whereas the LB films with fewer defects show homeotropic alignment. The defect in LB films may introduce splay or bend distortions in the nematic near the alignment layer which can govern the planar alignment of the bulk LC. The uniform layer of LB film facilitates the molecules of nematic to anchor vertically due to a strong van der Waals interaction between the aliphatic chains leading to a homeotropic alignment.

Efficient Photodoping of Graphene in Perovskite–Graphene Heterostructure

AbstractGraphene has been shown to be a prospective platform for active photonic devices with exotic optical properties. Employed in these devices, the graphene photodoping mechanism would allow for the remote spatiotemporal doping control by means of illumination, not restricted by the physical gate electrodes. This paper reports on the efficient graphene photodoping in graphene‐CH3NH3PbI3 perovskite heterostructure recently spotlighted for photodetector applications. To maximize the photoresponse, the heterostructure is optimized by systematically introducing additional layers of the self‐assembled monolayer of octadecyltrichlorosilane molecules, MoO3, and poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate for improving interfacial charge transfer properties. The photodoping amount estimated from the photocurrent measurements is 2.7 × 1012 cm−2 at zero gate bias—more than twice higher than previously observed in graphene‐perovskite heterostructures. Without external gate bias, the estimated graphene Fermi energy varies from 0.37 to 0.43 eV due to photodoping, which is sufficient to operate graphene‐based active photonic devices at mid‐infrared frequencies, and similar to that typically achievable with the conventional electrostatic gating. Furthermore, this work highlights the missing yet important aspect of the characterization method for the phototransistors with a 2D channel, resulting from the mismatch between the units of the current and power densities.

Dynamically Switching the Electronic and Electrostatic Properties of Indium–Tin Oxide Electrodes with Photochromic Monolayers: Toward Photoswitchable Optoelectronic Devices

The chemical modification of electrodes with organic materials is a common approach to tuning the electronic and electrostatic landscapes between interlayers in optoelectronic devices, thus facilitating charge injection at the electrode/semiconductor interfaces and improving their performance. The use of photochromic molecules for surface modification allows a dynamic control of the electronic and electrostatic properties of the electrode and thereby enables additional functionalities in such devices. Here, we show that the electronic properties of a transparent indium–tin oxide (ITO) electrode are reversibly and dynamically modified by depositing organic photochromic switches (diarylethenes) in the form of self-assembled monolayers (SAMs). By combining a range of surface characterization and density functional theory calculations, we present a detailed picture of the SAM binding to ITO, the packing density of molecules, their orientation, and work function modification of the ITO surface due to SAM deposition. Upon illumination with UV and green light, we observe a reversible shift of the frontier occupied levels by 0.7 eV and concomitantly a reversible work function change of ca. 60 meV. Our results prove the viability of dynamic switching of the electronic properties of the electrode with external light stimuli upon modification with a monolayer of photochromic molecules, which could be used to fabricate ITO-based photoswitchable optoelectronic devices.

Potential of Borage flower aqueous extract as an environmentally sustainable corrosion inhibitor for acid corrosion of mild steel: Electrochemical and theoretical studies

The presence of numerous active compounds like lactic acid, nicotinic acid, and carotene in the Borage flower extract (BFE) converted it to a potent source of powerful sustainable compounds for mild steel (MS) corrosion mitigation in the solution of 1 M hydrochloric acid. The corrosion inhibition mechanism was assessed by potentiodynamic polarization (PPL) and the impact of inhibitor content and exposure time on the inhibition extent of the BFE was investigated by electrochemical impedance spectroscopy (EIS). Different constituents of Borage flower extract were examined by FT-IR and UV–Vis methods. Surface morphology was studied by AFM and SEM analyses, and the surface hydrophilicity/hydrophobicity was assessed by contact angle test. EIS findings evidenced that with the increase in Borage flower extract content in HCl solution and immersion time the power of corrosion prevention increased in HCl environment and achieved the highest value of 91% at 800 ppm after 5 h. On the basis of polarization curves it was found that the Borage flower extract showed a mixed effect in corrosion inhibition with a minor cathodic prevalence. The SEM and AFM examinations demonstrated that the adsorption of BFE components on the MS surface gave rise to a smooth surface morphology with few damaged areas. The analysis of contact angle confirmed that by the increase of BFE amount the surface hydrophobicity was improved. Also, the surface coverage by green inhibitor molecules was in accordance with Langmuir isotherm, indicating that a monolayer of inhibitor molecules without intermolecular interactions covered the steel surface. Additionally, the molecular simulations demonstrated that the organic molecules of BFE adsorbed on the steel substrate. The outcomes of theoretical investigations suggested the donor-acceptor interactions of green inhibitor species.

Characterising glass transition temperatures and glass dynamics in mesoporous silica-based amorphous drugs.

In this study the glass transition temperatures (Tgα and Tgβ) in mesoporous silica-based amorphous drugs were characterized. For this purpose, mesoporous silica Parteck SLC (MPS) was loaded with the drugs ibuprofen and carvedilol, either below, at, or above the monomolecular drug loading capacities, i.e. the concentration at which the entire MPS surface is covered with a monolayer of drug molecules. The resulting amorphous forms were analysed using X-ray powder diffraction and the thermal behaviour was characterised with differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The drug monolayer did not contribute to the thermal signal in DSC. Using DMA however, it could be shown that the monolayer indeed exhibited a very weak Tgα, and that the temperature range of this transition did not differ from that of the quench cooled amorphous drugs. Theoretical ab initio molecular dynamics simulations revealed that the nature of hydrogen bonding geometry of the functional groups interacting with the MPS surface were similar to that of the respective crystalline drugs, which results in restricted molecular motions for those functional groups. On the other hand, the non-interacting parts of the molecules exhibited molecular motions similar to what is observed in pure amorphous drugs. As a result of the interactions of the monolayer with the MPS surface, the monomolecular drug layer did not reveal a Tgβ. However, a Tgβ was found at any drug-MPS ratios above the monomolecular drug loading capacity as a result of the excess drug which forms a "true" amorphous phase. Overall, this study demonstrated that drug molecules forming an amorphous monolayer on the surfaces of a mesoporous silica particle, even though they are restricted in their mobility, exhibit a Tgα, but lack a Tgβ, whereas any excess drug confined in the MPS pores showed similar properties as the pure amorphous drug. These findings will help to increase the overall understanding of drug loaded MS systems, including their physical stability as well as release properties.

Theoretical Description of Mixed Film Formation at the Air/Water Interface: Carboxylic Acids-Alcohols.

The thermodynamic parameters of formation and clusterization of aliphatic alcohols CnH2n+1OH and carboxylic acids CnH2n+1COOH (n = 6–16) are calculated using the quantum-chemical semiempirical PM3 method. Four types of dimers are constructed in two directions of the spread monolayer comprising the most energetically advantageous monomer structures. The hydrophobic chains of alcohol and carboxylic acid molecules in the regarded dimers are found to be tilted within 12° to the normal of the spread monolayer. The structures of the mixed and pure surfactant dimers are the basis for the mixed alcohol–carboxylic acid monolayers of the following types: two dimensional (2D) film 1 with single distribution of the individual component in the other one, when the molecules of the first component do not interact with each other but are completely surrounded by the molecules of the second component; 2D film 2 with domain structure, when the film consists of “islands” of the individual components. The dependences of the clusterization Gibbs’ energy per one monolayer molecule on the molar fraction of the components for the mixed 2D films 1 formed by surfactants with equal alkyl chain length are found to be limited from top to bottom by the corresponding dependences for pure components. This indicates the absence of synergetic interaction between the hydrophilic head groups of carboxylic acids and alcohols and conforms to the available experimental data. The formation of the described types of mixed films is competitive. The preferential formation of 2D films 1 with single distribution of the first component among the molecules of the second one is possible when the length of the carboxylic acid hydrocarbon chain is longer by Δn = 1–2 methylene units than that of the corresponding alcohol alkyl chain. According to the fractionally linear law, the highest possible content of the carboxylic acids in such 2D films 1 depends on the Δn value and does not exceed 33.3%.

The equivalence of quasi-chemical and statistical description of adsorbate molecule association in a localised adsorption monolayer on a homogeneous solid surface

Abstract The analysis of Berezin and Kiselev’s concept assuming linear association of molecules in a localised adsorption monolayer on the homogeneous surface of a solid adsorbent was carried out. An inconsistency due to the lack of consideration of equilibrium concentration of free adsorption sites in the formulation of the expression for the association constant has been pointed out. It was shown that the correct form of this function leads to the final adsorption equation being identical to the specific case of the Fowler-Guggenheim equation. The obtained result has been generalised to cases of branched adsorbateadsorbate associations. A new adsorption equation limiting the association to at most the dimers has been introduced. Critical conditions for the two-dimensional condensation of the adsorption layer were determined. By applying the formalism of canonical ensemble, full equivalence of the phenomenological and statistical methods was demonstrated in the description of the intermolecular interactions in the localised adsorption monolayer.

Self-assembled nematic colloidal motors powered by light

Biological motors are marvels of nature that inspire creation of their synthetic counterparts with comparable nanoscale dimensions, high efficiency and diverse functions. Molecular motors have been synthesized, but obtaining nanomotors through self-assembly remains challenging. Here we describe a self-assembled colloidal motor with a repetitive light-driven rotation of transparent micro-particles immersed in a liquid crystal and powered by a continuous exposure to unstructured ~1 nW light. A monolayer of azobenzene molecules defines how the liquid crystal’s optical axis mechanically couples to the particle’s surface, as well as how they jointly rotate as the light’s polarization changes. The rotating particle twists the liquid crystal, which changes polarization of traversing light. The resulting feedback mechanism yields a continuous opto-mechanical cycle and drives the unidirectional particle spinning, with handedness and frequency robustly controlled by polarization and intensity of light. Our findings may lead to opto-mechanical devices and colloidal machines compatible with liquid crystal display technology.

Self-Retraction of Surfactant Droplets on a Superhydrophilic Surface.

In the present work, an interesting droplet self-retraction phenomenon of myristyltrimethylammonium bromide (C14TAB) solution on the superhydrophilic mica surface was observed. The self-retraction could only occur within a concentration range from 0.01 to 16 cmc. The maximum variation of the contact angle (from 24.99 to 76.85°) was observed in droplets with a concentration of 0.1 cmc. The self-retraction mechanism was studied based on high-speed photography, surface analyses, surface energy calculation, and model fitting. It was proved that there was a monolayer of C14TAB molecules adsorbed on the mica surface, which formed through the electrostatic interaction between the negative mica and the positively charged headgroups. The formation of this monolayer took only a short time of <67 ms since the droplet touched the surface, and the monolayer was well-distributed and hydrophobic with a surface free energy of 76.041 mJ/m2 (20% of mica's surface free energy). The reduction of surface tension led to an imbalance at the borderline of the droplet, causing self-retraction. A model was established to simulate the self-retraction process and it agreed with the experimental data well. This study has significant implication in understanding the surface modification caused by surfactant molecules.

Abstract 17323: Polyinosinic-Polycytidylic Acid Primes Cardiac Progenitors From Human Induced Pluripotent Stem Cells for Enhanced Cell Therapy and Cardiomyocyte Maturation

Cardiomyocytes derived from human induced pluripotent stem cells (hiPS-CMs) hold promise for disease modeling, drug discovery, and therapy, but the challenge remains to create mature cardiomyocytes like those found in the adult heart. While groups have increased the maturity of hiPS-CMs in extended culture with electrical, metabolic, and mechanical stimulation, we hypothesized that epigenetic modulation during the formation of cardiac progenitors (hiPS-CPCs) could enhance their capacity to form mature CMs. We found that priming with the innate immune agonist polyinosinic-polycytidylic acid (pIC) decreased cardiac lineage-HDAC expression during the formation of hiPS-CPCs in defined small molecule monolayer differentiation. While both untreated and primed day 5 hiPS-CPCs contained equivalent &gt;80% purity of KDR+PDGRFα+ CPC populations, gene expression studies using RNAseq demonstrated that pIC priming enhanced the early cardiogenic and Notch signaling programs. When both groups were differentiated in basal media, primed hiPS-CPCs gave rise to more mature cardiomyocytes based on larger cell size, increased optical action potential upstroke velocity, greater oxidative metabolism, enhanced sarcomere maturation, and upregulated transcriptional markers of CM maturation including cTnI, cardiac actin, and αMHC. These maturation effects of pIC treatment were blocked by the Notch inhibitor DAPT. Most impressively, primed hiPS-CPCs improved survival as well as myocardial systolic/diastolic function in a mouse model of myocardial infarction (Figure). Conclusion: pIC-primed hiPS-CPCs with the capacity to give rise to hiPS-CMs of enhanced maturation hold promise for improved disease modeling, drug screening and cell therapy.

Interface Electrostatics Dictates the Electron Transport via Bioelectronic Junctions.

Different batches of Si wafers with nominally the same specifications were found to respond differently to identical chemical surface treatments aimed at regrowing Si oxide on them. We found that the oxides produced on different batches of wafer differ electrically, thereby affecting solid-state electron transport (ETp) via protein films assembled on them. These results led to the another set of experiments, where we studied this phenomenon using two distinct chemical methods to regrow oxides on the same batch of Si wafers. Wehave characterized the surfaces of the regrown oxides and of monolayers of linker molecules that connect proteins with the oxides and examined ETp via ultrathin layers of the protein bacteriorhodopsin, assembled on them. Our results illustrate the crucial role of (near) surface charges on the substrate in defining the ETp characteristics across the proteins. This is expressed most strikingly in the observed current's temperature dependences, and we propose that these are governed by the electrostatic landscape at the electrode-protein interface rather than by intrinsic protein properties. This study's major finding, relevant to protein bioelectronics, is that protein-electrode coupling in junctions is a decisive factor in ETp across them. Hence,surfaceelectrostaticscan create a barrier that dominates charge transport and controls the transport mode across the junction. Our findings' wider importance lies in their relevance to hybrid junctions of Si with (polyelectrolyte) biomolecules, a likely direction for future bioelectronics. A remarkable corollary of presented results is that once an electron is injected into the protein, transport within the proteins is so efficient that it does not encounter a measurable barrier down to 160 K.

Patterning Nanogaps: Spatial Control of the Distribution of Nanogaps between Gold Nanoparticles and Gold Substrates

Narrow nanogaps formed between nanostructures act as hot spots, where the plasmonic properties are significantly enhanced. Consequently, the ability to create and control nanogaps is highly desirable for many plasmon-based applications. Nanoparticles-on-mirror (NPoM) is an attractive system that allows one to produce nanogaps on two-dimensional surfaces with great flexibility. NPoM is formed by adsorbing gold nanoparticles (AuNPs) on self-assembled monolayers (SAMs) of molecules on Au substrates. The properties of the resulting nanogaps are defined by the SAM molecular spacer and the shape and size of the adsorbed AuNPs. In this paper, we present a method for controlling the spatial distribution of the nanogaps with micrometer resolution. UV irradiation of the SAMs leads to desorption of the thiol molecules from the surface via photooxidation, which hinders the subsequent adsorption of AuNPs on the surface. By applying spatioselective irradiation, spatially controlled NPoM patterns are constructed. Furthe...

Effect of the Molecular Polarizability of SAMs on the Work Function Modification of Gold: Closed‐ versus Open‐Shell Donor–Acceptor SAMs

AbstractCharge injection barriers at metal/organic interfaces can be tuned by modifying the work function of metallic electrodes using self‐assembled monolayers (SAMs) of polar molecules. An interesting example of polar molecules is offered by donor–acceptor (D–A) dyads based on ferrocene (Fc) as electron‐donor unit and either a polychlorotriphenylmethyl radical or a polychlorotriphenylmethane as electron‐acceptor units, connected by a π‐conjugated vinylene bridge. The D–A radical exhibits high chemical and thermal stability and presents different electronic, optical, and magnetic properties with respect to the closed‐shell form. The magnitude of the shift in the charge injection barriers for these two D–A systems is estimated by means of surface potential measurements performed by Kelvin probe force microscopy. The experimental data are compared with density functional theory calculations, which evidence the importance of the molecular dipole moments and polarizabilities to understand the experimental values. In order to achieve high work function shifts of metals upon SAM formation, the molecules forming the SAM have to exhibit both a high permanent dipole moment and a low polarizability along the direction normal to the substrate. In presence of polarizable molecules, the work function shifts can be enhanced by reducing the intermolecular interactions; by using mixed SAMs with active molecules embedded into a passive matrix.

Capillarity-Driven Oil Flow in Nanopores: Darcy Scale Analysis of Lucas–Washburn Imbibition Dynamics

We present gravimetrical and optical imaging experiments on the capillarity-driven imbibition of silicone oils in monolithic silica glasses traversed by 3D networks of pores (mesoporous Vycor glass with 6.5 nm or 10 nm pore diameters). As evidenced by a robust square root of time Lucas–Washburn (L–W) filling kinetics, the capillary rise is governed by a balance of capillarity and viscous drag forces in the absence of inertia and gravitational effects over the entire experimental times studied, ranging from a few seconds up to 10 days. A video on the infiltration process corroborates a collective pore filling as well as pronounced imbibition front broadening resulting from the capillarity and permeability disorder, typical of Vycor glasses. The transport process is analyzed within a Darcy scale description, considering a generalized prefactor of the L–W law, termed Lucas–Washburn–Darcy imbibition ability. It assumes a Hagen–Poiseuille velocity profile in the pores and depends on the porosity, the mean pore diameter, the tortuosity and the velocity slip length and thus on the effective hydraulic pore diameter. For both matrices a reduced imbibition speed and thus reduced imbibition ability, compared to the one assuming the nominal pore diameter, bulk fluidity and bulk capillarity, can be quantitatively traced to an immobile, pore wall adsorbed boundary layer of 1.4 nm thickness. Presumably, it consists of a monolayer of water molecules adsorbed on the hydrophilic pore walls covered by a monolayer of flat-laying silicone oil molecules. Our study highlights the importance of immobile nanoscopic boundary layers on the flow in tight oil reservoirs as well as the validity of the Darcy scale description for transport in mesoporous media.

A novel route to get functionality in nanoelectronics: Controlling the charge transport by the subtle impact of the coverage of self-assembled monolayers on the conformation of floppy molecules adsorbed on metallic electrodes

Molecules at interfaces can exhibit a behavior that is very different from that in bulk phases. In this paper we will analyze one important factor that makes the difference between these two situations. Unlike in bulk situations—which are characterized by a practically constant density and a given type of crystal structure—molecules in a self-assembled monolayer (SAM) adsorbed on a substrate electrode are characterized by a density (coverage) and an ordering that can be widely tuned. The theoretical results presented in this paper indicate that tuning the coverage of SAMs consisting of floppy molecules may have a strong impact on the molecular conformation. Therefore, it may represent a route to achieve interface-driven control over the charge transfer/transport at molecular scale. Specifically, we consider three floppy molecular species—H-(C6H4)2-SH (biphenylmonothiol), HS-(C6H4)2-SH (4,4′-dithiol-1,1′-biphenyl), and NC5H4-C6H4-SH (4-(4-pyridinyl) benzenethiol)—consisting of two benzene or benzene-based rings that can easily rotate relative to each other. Our calculations reveal that, by varying the SAM coverage, the torsional angle can be varied by a factor of two. This reflects itself in a change of conductance over one order of magnitude, a fact that can be exploited in designing molecular switches.