Complex Monolayer Research Articles

Electrostatically Designing Materials and Interfaces.

Collective electrostatic effects arise from the superposition of electrostatic potentials of periodically arranged (di)polar entities and are known to crucially impact the electronic structures of hybrid interfaces. Here, it is discussed, how they can be used outside the beaten paths of materials design for realizing systems with advanced and sometimes unprecedented properties. The versatility of the approach is demonstrated by applying electrostatic design not only to metal-organic interfaces and adsorbed (complex) monolayers, but also to inter-layer interfaces in van der Waals heterostructures, to polar metal-organic frameworks (MOFs), and to the cylindrical pores of covalent organic frameworks (COFs). The presented design ideas are straightforward to simulate and especially for metal-organic interfaces also their experimental implementation has been amply demonstrated. For van der Waals heterostructures, the needed building blocks are available, while the required assembly approaches are just being developed. Conversely, for MOFs the necessary growth techniques exist, but more work on advanced linker molecules is required. Finally, COF structures exist that contain pores decorated with polar groups, but the electrostatic impact of these groups has been largely ignored so far. All this suggest that the dawn of the age of electrostatic design is currently experienced with potential breakthroughs lying ahead.

Structured Excitation Energy Transfer: Tracking Exciton Diffusion below Sunlight Intensity.

With the increasing demand for new materials for light-harvesting applications, spatiotemporal microscopy techniques are receiving increasing attention as they allow direct observation of the nanoscale diffusion of excitons. However, the use of pulsed and tightly focused laser beams generates light intensities far above those expected under sunlight illumination, leading to photodamage and nonlinear effects that seriously limit the accuracy and applicability of these techniques, especially in biological or atomically thin materials. In this work, we present a novel spatiotemporal microscopy technique that exploits structured excitation in order to dramatically decrease the excitation intensity, up to 10,000-fold when compared with previously reported spatiotemporal photoluminescence microscopy experiments. We tested our method in two different systems, reporting the first exciton diffusion measurement at illumination conditions below sunlight, both considering average power and peak exciton densities in an organic photovoltaic sample (Y6), where we tracked the excitons for up to five recombination lifetimes. Next, nanometer-scale energy transport was directly observed for the first time in both space and time in a printed monolayer of the light-harvesting complex 2 from purple bacteria.

Photosystem I complexes form remarkably stable self-assembled tunneling junctions.

This paper describes large-area molecular tunneling junctions comprising self-assembled monolayers (SAMs) of light-harvesting protein complexes using eutectic Ga-In (EGaIn) as a top contact. The complexes, which are readily isolable in large quantities from spinach leaves, self-assemble on top of SAMs of [6,6]-phenyl-C61-butyric acid (PCBA) on gold (Au) supported by mica substrates (AuMica), which induces them to adopt a preferred orientation with respect to the electron transport chain that runs across the short axis of each complex, leading to temperature-independent rectification. We compared trimeric protein complexes isolated from thermophilic cyanobacteria to monomeric complexes extracted from spinach leaves by measuring charge-transport at variable temperatures and over the course of at least three months. Transport is independent of temperature in the range of 130 to 310 K for both protein complexes, affirming that the likely mechanism is non-resonant tunneling. The junctions rectified current and were stable for at least three months when stored at room temperature in ambient conditions, with the yield of working junctions falling from 100% to 97% over that time. These results demonstrate a straightforward strategy for forming remarkably robust molecular junctions, avoiding the fragility that is common in molecular electronics.

Solid-Phase Electrosynthesis.

ConspectusThe significance of the new synthetic approach is that it can overcome the limitations of conventional methods and produce previously inaccessible polymer structures and materials. The solid-phase synthesis developed by Merrifield in 1964 is widely employed for the synthesis of biological molecules, such as peptides, nucleic acids, and oligosaccharides. Although the variety of iterative reactions available is theoretically implemented for most organic synthesis protocols, they are usually required to have high efficiency against sluggish reaction kinetics at the solid-liquid interface and process with protection and deprotection steps. Generally, unsatisfied reaction dynamics at the solid-liquid interface cannot statistically permit accurate and uniform polymer synthesis of sophisticated structures and functions within an acceptable time scale. To address this challenge, we propose the concept of solid-phase electrosynthesis, which simultaneously enables rapidly surface-initiated uniform electrosynthesis and unidirectional assembly of metallopolymers via kinetically accelerated and statistically allowed iterative growth. In particular, on a self-assembled monolayer (SAM) of the metal complex with electroactive unit A, the iterative monomer with two electroactive units A and B can be alternatively activated by oxidative and reductive potentials for A-A and B-B covalent couplings with the SAM, respectively. This enables topochemical one-by-one additions of the iterative monomers to end-on-oriented self-assembled molecules through alternative redox reactions. Each iterative step is purified by washing. Repeating the same iterative reaction enables further reaction of the unreactive sites on the SAMs and repairs the morphology defects, thereby ensuring the statistically allowed uniform synthesis and fabrication of polymer monolayers. The resulting monolayers exhibit subnanometer-uniform morphology over centimeter-sized areas with crystalline states and show thicknesses similar to theoretical molecular lengths. This demonstrates the unidirectional formation of polymer assemblies, providing a pathway for obtaining highly ordered formation of noncrystalline polymers. The length-controlled electrosynthesis of metallopolymers can be generalized for many types of organic ligands and metal species, enabling quantitative design of the composition and sequence-controlled metallopolymers with the precise relationships of structures and properties. Solid-phase electrosynthesis offers a unique approach to synthesize polymer structures and monolayers with enhanced functionality and superior physical properties, including physical density, modulus, and conductance. Through the utilization of precise and efficient iterative growth, this predictable electrosynthesis, coupled with optical and electrical monitoring, not only expands the scope of current synthetic chemistry but also paves a potential way for the automated generation of optoelectric molecular monolayers with large-area dimensional consistency and enhanced physical performance.

Amplitude modulation and surface wave generation in a complex plasma monolayer.

The response of a two-dimensional plasma crystal to an externally imposed initial perturbation has been explored using molecular dynamics (MD) simulations. A two-dimensional (2D) monolayer of micron-sized charged particles (dust) is formed in the plasma environment under certain conditions. The particles interacting via Yukawa pair potential are confined in the vertical (z[over ̂]) direction by an external parabolic confinement potential, which mimics the combined effect of gravity and the sheath electric field typically present in laboratory dusty plasma experiments. An external perturbation is introduced in the medium by displacing a small central region of particles in the vertical direction. The displaced particles start to oscillate in the vertical direction, and their dynamics get modulated through a parametric decay process generating beats. It has also been shown that the same motion is excited in the dynamics of unperturbed particles. A simple theoretical model is provided to understand the origin of the beat motions of particles. Additionally, in our simulations, concentric circular wavefronts propagating radially outward are observed on the surface of the monolayer. The physical mechanism and parametric dependence of the observed phenomena are discussed in detail. This research sheds light on the medium's ability to exhibit macroscopic softness, a pivotal characteristic of soft matter, while sustaining surface wave modes. Our findings are also relevant to other strongly coupled systems, such as colloids and classical one-component plasmas.

Probing Energy-Level Alignment in Molecular Multilayers by Frequency-Modulation Electrostatic Force Microscopy under Tapping-Mode-Combined Fowler-Nordheim Tunneling Spectroscopy.

The alignment of molecular electronic levels in a molecular multilayer is of crucial importance to realize desired functions for molecular devices. Amplitude-modulation-feedback frequency-modulation electrostatic force microscopy combined with Fowler-Nordheim tunneling spectroscopy is utilized as a probe for the energy-level alignment in an organic multilayer. Bias-dependent electrostatic force spectra were examined for bilayers including a Ru complex as a benchmark multilayer system. Electrostatic properties in the low-bias region were captured well by a single-capacitor model, which indicates weak coupling at the bilayer interface between the Ru complex and self-assembled monolayer. In contrast, in the high-bias region, significant disagreement with the expected electrostatic force was recognized for the bilayers and evaluated as the loss of electrostatic energy through the Fowler-Nordheim tunneling process. Alignment of the lowest unoccupied molecular orbital (LUMO) level of the Ru complex was determined by Fowler-Nordheim emission through resonant tunneling. These results indicate an effective method to probe level alignment at interfaces inside multilayers and to provide the partition factor β that depicts a divided electric field.

Spatial localization of high-frequency particle oscillations in a complex plasma monolayer

We study the spatial distribution of dynamical properties in a strongly coupled confined experimental dust monolayer with a dense central region and a rarefied periphery. We show that the dust particle oscillations spectrum shifts into the low-frequency range from the center of the structure toward its periphery. Correspondingly, the high-frequency particle oscillations are localized in the central region of the system. This effect is quantitatively described within the framework of the model system of trapped screened Coulomb charges. Such spatial localization has a significant effect on the spatial distribution of dynamical parameters, such as the amplitude of particle oscillations and the coupling parameter. The obtained results might be useful for the theory of crystal formation, instabilities and phase transitions in complex (dusty) plasmas, colloidal suspensions, and Coulomb crystals.

Mechanisms of epithelial growth and development in the zebrafish intestine.

The intestinal epithelium is a complex tissue monolayer composed of regionally and functionally specialized intestinal epithelial cells. Given epithelial exposure to harsh and varied luminal conditions, epithelial cells continuously regenerate to sustain the barrier against environmental factors, including microbial invaders. Multipotent intestinal stem cells are essential to epithelial regenerative capacity, generating a programed mixture of absorptive and secretory cell types. Mechanisms of epithelial growth and differentiation in response to endogenous or external stressors remain under investigation. In this review, we highlight the zebrafish, Danio rerio, as a potent model of intestinal epithelial development and function. We describe epithelial composition and key regulators of epithelial renewal to promote the zebrafish as an investigative tool to study epithelial development and growth. We also highlight areas for discovery, particularly in the context of stress-dependent regulation of epithelial function.

Antenna-coupled infrared nanospectroscopy of intramolecular vibrational interaction

Many photonic and electronic molecular properties, as well as chemical and biochemical reactivities are controlled by fast intramolecular vibrational energy redistribution (IVR). This fundamental ultrafast process limits coherence time in applications from photochemistry to single quantum level control. While time-resolved multidimensional IR-spectroscopy can resolve the underlying vibrational interaction dynamics, as a nonlinear optical technique it has been challenging to extend its sensitivity to probe small molecular ensembles, achieve nanoscale spatial resolution, and control intramolecular dynamics. Here, we demonstrate a concept how mode-selective coupling of vibrational resonances to IR nanoantennas can reveal intramolecular vibrational energy transfer. In time-resolved infrared vibrational nanospectroscopy, we measure the Purcell-enhanced decrease of vibrational lifetimes of molecular vibrations while tuning the IR nanoantenna across coupled vibrations. At the example of a Re-carbonyl complex monolayer, we derive an IVR rate of (25±8) cm-1 corresponding to (450±150) fs, as is typical for the fast initial equilibration between symmetric and antisymmetric carbonyl vibrations. We model the enhancement of the cross-vibrational relaxation based on intrinsic intramolecular coupling and extrinsic antenna-enhanced vibrational energy relaxation. The model further suggests an anti-Purcell effect based on antenna and laser-field-driven vibrational mode interference which can counteract IVR-induced relaxation. Nanooptical spectroscopy of antenna-coupled vibrational dynamics thus provides for an approach to probe intramolecular vibrational dynamics with a perspective for vibrational coherent control of small molecular ensembles.

Structure and morphology of bovine serum albumin–lysozyme (BSA–Lys) complex films at air–water interface

The heteroprotein complex formed between the two globular proteins, bovine serum albumin and lysozyme was studied at the air–water interface by varying the subphase pH condition (pH ≈ 4.0–12.0). The surface pressure–mean molecular area isotherms of bovine serum albumin–lysozyme complex monolayers showed that relatively lower value of area per molecule was obtained at pH ≈ 9.2 in contrast to the other subphase pH conditions. Stability of the complex monolayers were analyzed from the normalized molecular area–time curves, which validated the formation of a relatively more stable film at pH ≈ 9.2. The in – situ study of the complex films were explored from Brewster angle microscopy and the corresponding images showed the evolution of elongated and compact circular domains at pH ≈ 7.0 and 9.2 respectively, at a higher surface pressure of 18 mN/m, while mostly homogenous films were visible at all the other pH conditions. The complex films were transferred from the water subphase on to the silicon substrates at a surface pressure of 18 mN/m for further study. The out–of–plane structure and in–plane morphology were obtained from X–ray reflectivity and atomic force microscopy analysis respectively, which showed the formation of a relatively thicker film deposited at pH ≈ 9.2. This unique pH value, i.e., pH ≈ 9.2, which is located in between the isoelectric point of bovine serum albumin and lysozyme, is the preferred pH condition for complex formation through electrostatic attraction of the two protein molecules. The biofilm of this stable protein complex formed at the air–water interface may have potential applications in thin film technology. • Bovine serum albumin–lysozyme complex films are formed on water surface. • Lower value of area per molecule for initial rising of pressure is obtained at pH ≈ 9.2. • Mostly elongated and compact circular domains are found at pH ≈ 7.0 to 9.2. • Relatively thicker film is formed at pH ≈ 9.2 where complex formation is maximum. • Stability of such complex film is maximum at pH ≈ 9.2.

Development of iron-based biochar for enhancing nitrate adsorption: Effects of specific surface area, electrostatic force, and functional groups

The problem of nitrate contamination in water has attracted widespread attention. Original biochar has a poor adsorption capacity for nitrate adsorption. Iron impregnation and acid protonation (base deprotonation) are common modification methods for biochar. In order to develop iron-mediated biochar containing multi-functional groups for enhancing nitrate adsorption, Fe-BC@H and Fe-BC@OH were prepared using a two-stage development process, including an iron-based carbon pyrolysis followed by acid protonation (or base deprotonation). The pseudo-second-order kinetic and Langmuir models can well describe the adsorption process which is a physicochemical complex monolayer adsorption. The data proved that Fe-BC@H (9.35 mg/g NO3−-N) had a stronger adsorption capacity than Fe-BC@OH (2.95 mg/g NO3−-N). Surface morphologies, functional groups, and mineral compositions of Fe-BC@H and Fe-BC@OH were analyzed through Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Characterization results showed that acid protonation can further improve the specific surface area (SSA), pore volume, and Zeta potential of Fe-based biochar, providing more adsorption sites for nitrate and enhancing the electrostatic force between nitrate and biochar. However, these effects were suppressed through base deprotonation. In addition, acid protonation can significantly increase the type and number of functional groups of biochar to enhance the chemisorption of nitrate. Such results suggested that the acid protonation can further improve the adsorption capacity of Fe-based biochar for nitrate, while base deprotonation had an inhibitory effect on that of Fe-based biochar. Overall, this study reveals that specific surface area, electrostatic force, and functional groups are crucial effects of the nitrate adsorption on acid/base modified biochar.

Tapered Optical Fibers Coated with Rare-Earth Complexes for Quantum Applications.

Crystals and fibers doped with rare-earth (RE) ions provide the basis for most of today’s solid-state optical systems, from lasers and telecom devices to emerging potential quantum applications such as quantum memories and optical to microwave conversion. The two platforms, doped crystals and doped fibers, seem mutually exclusive, each having its own strengths and limitations, the former providing high homogeneity and coherence and the latter offering the advantages of robust optical waveguides. Here we present a hybrid platform that does not rely on doping but rather on coating the waveguide—a tapered silica optical fiber—with a monolayer of complexes, each containing a single RE ion. The complexes offer an identical, tailored environment to each ion, thus minimizing inhomogeneity and allowing tuning of their properties to the desired application. Specifically, we use highly luminescent Yb3+[Zn(II)MC (QXA)] complexes, which isolate the RE ion from the environment and suppress nonradiative decay channels. We demonstrate that the beneficial optical transitions of the Yb3+ are retained after deposition on the tapered fiber and observe an excited-state lifetime of over 0.9 ms, on par with state-of-the-art Yb-doped inorganic crystals.

Atomic-Scale Insights into the Lateral and Vertical Epitaxial Growth in Two-Dimensional Pd2Se3-MoS2 Heterostructures.

Two-dimensional (2D) materials form heterostructures in both the lateral and vertical directions when two different materials are interfaced, but with totally different bonding mechanisms of covalent in-plane to van der Waal's layered interactions. Understanding how the competition between lateral and vertical forces influences the epitaxial growth is important for future materials development of complex mixed layered heterostructures. Here, we use atomic-resolution annular dark-field scanning transmission electron microscopy to study the detailed atomic arrangements at mixed 2D heterostructure interfaces composed of two semiconductors with distinctly different crystal symmetry and elemental composition, Pd2Se3:MoS2, in order to understand the role of different chemical bonds on the resultant epitaxy. Pd2Se3 is grown off the step edge in bilayer MoS2, and the vertical and lateral epitaxial relationships of the Pd2Se3-MoS2 heterostructures are investigated. We find that the similarity of geometry at the interface with one metal (Pd or Mo) atoms bonded with two chalcogens (S or Se) are the crucial factors to make the atomically stitched lateral junction of 2D heterostructures. In addition, the vertical van der Waal interactions that are normally dominant in layered materials can be overcome by in-plane forces if the interfacial atomic stitching is high in quality and low in defect density. This knowledge should help guide the approaches for improving the epitaxy in mixed 2D heterostructures and seamless stitching of in-plane 2D heterostructures with various complex monolayer structures.

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

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

Stability of two-dimensional complex plasma monolayers in asymmetric capacitively coupled radio-frequency discharges.

In this article, the stability of a complex plasma monolayer levitating in the sheath of the powered electrode of an asymmetric capacitively coupled radio-frequency argon discharge is studied. Compared to earlier studies, a better integration of the experimental results and theory is achieved by operating with actual experimental control parameters such as the gas pressure and the discharge power. It is shown that for a given microparticle monolayer at a fixed discharge power there exist two threshold pressures: (i) above a specific pressure p_{cryst}, the monolayer always crystallizes; (ii) below a specific pressure p_{MCI}, the crystalline monolayer undergoes the mode-coupling instability and the two-dimensional complex plasma crystal melts. In between p_{MCI} and p_{cryst}, the microparticle monolayer can be either in the fluid phase or the crystal phase: when increasing the pressure from below p_{MCI}, the monolayer remains in the fluid phase until it reaches p_{cryst} at which it recrystallizes; when decreasing the pressure from above p_{cryst}, the monolayer remains in the crystalline phase until it reaches p_{MCI} at which the mode-coupling instability is triggered and the crystal melts. A simple self-consistent sheath model is used to calculate the rf sheath profile, the microparticle charges, and the microparticle resonance frequency as a function of power and background argon pressure. Combined with calculation of the lattice modes the main trends of p_{MCI} as a function of power and background argon pressure are recovered. The threshold of the mode-coupling instability in the crystalline phase is dominated by the crossing of the longitudinal in-plane lattice mode and the out-of plane lattice mode induced by the change of the sheath profile. Ion wakes are shown to have a significant effect too.

Photoactive silicon surfaces functionalized with high-quality and redox-active platinum diimine complex monolayers

Platinum diimine complexes can covalently be grafted onto oxide-free, hydrogen-terminated silicon(111) surfaces into clean and high-quality monolayers. The so modified surfaces offer great prospects as photocathodes for solar-driven electrocatalysis.

DNA-mediated molecular assembly of a triphenylene–surfactant complex monolayer

Bottom-up DNA-guided molecular assembly is a technique of choice in modern organic device engineering. DNA, being a polyelectrolyte, is capable of mediating electrostatic interactions between different molecular building blocks. Here, we report DNA-mediated molecular assembly of an electrostatically driven triphenylene-surfactant complex monolayer. The monolayer, formed at air-water interface with DNA in the subphase, was studied using surface manometry and Brewster angle microscopy. Unlike that of the pristine system, the monolayer in presence of DNA showed enhanced stability and a remarkably high efficiency of deposition on solid supports. The morphologies of deposited films with controlled number of layers were studied using atomic force microscopy. In addition, UV–visible spectroscopy and grazing incidence X-ray diffraction (GIXRD) technique confirmed the presence of DNA in the deposited films. Since both triphenylene and surfactant are technologically relevant molecules, the DNA-mediated molecular assemblies deposited on solid substrates could find applications as functional materials in organic electronics.

Molecular Packing in a Triphenylene‐Surfactant Complex Monolayer: Unambiguous Determination of Electrostatically Driven Molecular Packing in a Triphenylene–Surfactant Complex Monolayer (Adv. Mater. Interfaces 13/2021)

Self-assembly is a technique of choice for the bottom-up fabrication of molecular devices. In article number 2100187, Snehasis Daschakraborty, Alpana Nayak, and co-workers report intricate details of the self-assembly processes and resultant ordered structures of a technologically useful molecular system comprising of discotic liquid crystal and double chain surfactant.

Unambiguous Determination of Electrostatically Driven Molecular Packing in a Triphenylene–Surfactant Complex Monolayer

AbstractEmergence of ordered structures during molecular self‐assembly at interfaces is influenced by a variety of thermodynamic and kinetic factors that are not well understood. Experiments alone can only inform about the final architecture keeping self‐assembly pathways inaccessible. Here with the help of molecular dynamic simulations, the intricate details of the self‐assembly processes and resultant ordered structures of a triphenylene–surfactant complex monolayer at air–water interface are reported. The monolayer is observed to be stable and reversible using surface manometry and real‐time Brewster angle microscopy. Not only that the simulated isotherm and 2D density profiles are in good agreement with experimental observations, the simulation results bespeak the crucial role of specific electrostatic interactions forming the nanostructures at the interface analyzed via orientational structural profiles, radial distribution, and Z‐density profiles. Moreover, real‐space visualization using atomic force microscopy suggests that the molecular packing is preserved in the monolayer even after transferring onto a silicon substrate. Since the rational designing of organic electronics requires nanoscale control of molecular structures on solid substrates, it is anticipated that these results will be of fundamental importance.

Electronic Characterization of a Charge-Transfer Complex Monolayer on Graphene.

Organic charge-transfer complexes (CTCs) formed by strong electron acceptor and strong electron donor molecules are known to exhibit exotic effects such as superconductivity and charge density waves. We present a low-temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) study of a two-dimensional (2D) monolayer CTC of tetrathiafulvalene (TTF) and fluorinated tetracyanoquinodimethane (F4TCNQ), self-assembled on the surface of oxygen-intercalated epitaxial graphene on Ir(111) (G/O/Ir(111)). We confirm the formation of the charge-transfer complex by dI/dV spectroscopy and direct imaging of the singly occupied molecular orbitals. High-resolution spectroscopy reveals a gap at zero bias, suggesting the formation of a correlated ground state at low temperatures. These results point to the possibility to realize and study correlated ground states in charge-transfer complex monolayers on weakly interacting surfaces.