Self-assembled Molecular Layers Research Articles

Influence of gamma radiation on self-assembled molecular layers developed on different metals

In this work, self-assembled molecular layers (SAMs), developed from special amphiphilic molecules (undecenyl phosphonic acid) on the top of two types of stainless steels (1.4571 and 1.4841) were in focus. The metal surface was modified by a self-assembled molecular layer with a double bond at the end of the carbon chain. The modification of the structure of the unsaturated hydrophobic chain was done by gamma irradiation to produce a more compact layer. We were interested in the influence of the metal composition on the layer formation. To give answers to these questions SAM layers were deposited on the top of the stainless steels and then the nanolayers were modified by post-treatment (gamma irradiation). The thickness of the surface films was characterized by ellipsometry and the composition of the amphiphilic SAM layers by infrared spectroscopy. Atomic Force Microscope (AFM) visualized the morphological changes due to the layer deposition and the post-treatment; the surfaces were numerically characterized by roughness parameters; contact angle values proved the change in the wettability caused by the metal surface and the gamma irradiation.

Unconventional superconductivity in chiral molecule-TaS2 hybrid superlattices.

Chiral superconductors, a unique class of unconventional superconductors in which the complex superconducting order parameter winds clockwise or anticlockwise in the momentum space1, represent a topologically non-trivial system with intrinsic time-reversal symmetry breaking (TRSB) and direct implications for topological quantum computing2,3. Intrinsic chiral superconductors are extremely rare, with only a few arguable examples, including UTe2, UPt3 and Sr2RuO4 (refs. 4-7). It has been suggested that chiral superconductivity may exist in non-centrosymmetric superconductors8,9, although such non-centrosymmetry is uncommon in typical solid-state superconductors. Alternatively, chiral molecules with neither mirror nor inversion symmetry have been widely investigated. We suggest that an incorporation of chiral molecules into conventional superconductor lattices could introduce non-centrosymmetry and help realize chiral superconductivity10. Here we explore unconventional superconductivity in chiral molecule intercalated TaS2 hybrid superlattices. Our studies reveal an exceptionally large in-plane upper critical field Bc2,|| well beyond the Pauli paramagnetic limit, a robust π-phase shift in Little-Parks measurements and a field-free superconducting diode effect (SDE). These experimental signatures of unconventional superconductivity suggest that the intriguing interplay between crystalline atomic layers and the self-assembled chiral molecular layers may lead to exotic topological materials. Our study highlights that the hybrid superlattices could lay a versatile path to artificial quantum materials by combining a vast library of layered crystals of rich physical properties with the nearly infinite variations of molecules of designable structural motifs and functional groups11.

Chemical Instability-Induced Wettability Patterns on Superhydrophobic Surfaces.

Chemical instability of liquid-repellent surfaces is one of the nontrivial hurdles that hinders their real-world applications. Although much effort has been made to prepare chemically durable liquid-repellent surfaces, little attention has been paid to exploit the instability for versatile use. Herein, we propose to create hydrophilic patterns on a superhydrophobic surface by taking advantage of its chemical instability induced by acid solution treatment. A superhydrophobic Cu(OH)2 nanoneedle-covered Cu plate that shows poor stability towards HCl solution (1.0 M) is taken as an example. The results show that 2.5 min of HCl solution exposure leads to the etching of Cu(OH)2 nanoneedles and the partial removal of the self-assembled fluoroalkyl silane molecular layer, resulting in the wettability transition from superhydrophobocity to hydrophilicity, and the water contact angle decreases from ~160° to ~30°. Hydrophilic dimples with different diameters are then created on the superhydrophobic surfaces by depositing HCl droplets with different volumes. Afterwards, the hydrophilic dimple-patterned superhydrophobic surfaces are used for water droplet manipulations, including controlled transfer, merging, and nanoliter droplet deposition. The results thereby verify the feasibility of creating wettability patterns on superhydrophobic surfaces by using their chemical instability towards corrosive solutions, which broadens the fabrication methods and applications of functional liquid-repellent surfaces.

A reversible self-assembled molecular layer for lithium metal batteries with high energy/power densities at ultra-low temperatures

We have demonstrated a molecular interface engineering strategy to address the design dilemma of low-temperature electrolytes for lithium metal batteries.

Effect of Heat Treatment on the Structure of Self-Assembled Undecenyl Phosphonic Acid Layers Developed on Different Stainless Steel Surfaces

Deterioration caused by corrosion is well known, which can destroy metallic and non-metallic materials alike. Dissolved inhibitors of bionic micro- and nanocoatings can decrease the degree of undesirable corrosion in various ways. In this paper, a self-assembled molecular layer formed from undecenyl phosphonic acid developed on two different steel surfaces was the subject of our experiments. The influence of the metal composition, layer-forming conditions and post-heat treatment was documented by wettability measurements as well as surface roughness parameters; the change in surface morphology caused by the formation of a layer in addition to post-heat treatment was visualized by an atomic force microscope (AFM); and infrared spectroscopy elucidated the bindings of the amphiphilic molecules involved in the self-assembled layer to the metal surface as well as to each other. Over the course of the self-assembling process, the –P(O)(OH)2 head groups can fix the amphiphilic molecule to the solid surface through the metal oxide-hydroxide layer. The hydrophobic alkenyl chains remain together as a result of special forces, namely hydrogen bonds and van der Waals forces, between them. The double bond at the end of the alkenyl chain disturbs how well the layer is ordered. To improve the homogeneity of the molecular layer and increase its level of compactness, the self-assembled molecular (SAM) layer was heat treated to achieve a more compact molecular film that can perfectly cover the metal surface.

Anticorrosion activity of phosphonic acid amphiphile in self-assembled molecular layer

Anticorrosion activity of phosphonic acid amphiphile in self-assembled molecular layer

The effect of media to reduce irradiation dose in crosslinking of self-assembled molecular layers

The present study aims to investigate the possibility of reducing the absorbed dose required to prepare a polymer nanocoating for surface functionalization by modifying the irradiation medium. We have studied the crosslinking of elaidic acid (EA), an unsaturated trans fatty acid, on oxidised copper by gamma irradiation. Three different irradiation media were compared: water purged with N2, water purged with N2O, and air purged with N2. Contact angle measurements were used to determine the hydrophobicity of the surface. The coatings were investigated by electrochemical measurements: The degree of crosslinking was indirectly determined by polarization measurements, while the behaviour of the coatings was studied by electrochemical impedance spectroscopy. The results showed that crosslinking can be achieved in all media studied. The highest dose of 110 kGy is required when irradiation is performed in oxygen-free air, while much lower doses of 40 kGy are required when irradiation is performed in water that has been purged with both gases used. The highest degree of crosslinking is achieved when irradiation is performed in air, while the degree of crosslinking when irradiated in water is highly dependent on the gas used. If N2O is used, the degree of crosslinking is significantly higher. The surface of the coatings is more hydrophobic when polymer coatings are present on the surface, regardless of the irradiation medium.

Cooperative Self-Assembly of Dimer Junctions Driven by π Stacking Leads to Conductance Enhancement.

We demonstrate enhanced electronic transport through dimer molecular junctions, which self-assemble between two gold electrodes in π-π stabilized binding configurations. Single molecule junction conductance measurements show that benzimidazole molecules assemble into dimer junctions with a per-molecule conductance that is higher than that in monomer junctions. Density functional theory calculations reveal that parallel stacking of two benzimidazoles between electrodes is the most energetically favorable due to the large π system. Imidazole is smaller and has greater conformational freedom to access different stacking angles. Transport calculations confirm that the conductance enhancement of benzimidazole dimers results from the changed binding geometry of dimers on gold, which is stabilized and made energetically accessible by intermolecular π stacking. We engineer imidazole derivatives with higher monomer conductance than benzimidazole and large intermolecular interaction that promote cooperative in situ assembly of more transparent dimer junctions and suggest at the potential of molecular devices based on self-assembled molecular layers.

Materials Quest for Advanced Interconnect Metallization in Integrated Circuits.

Integrated circuits (ICs) are challenged to deliver historically anticipated performance improvements while increasing the cost and complexity of the technology with each generation. Front-end-of-line (FEOL) processes have provided various solutions to this predicament, whereas the back-end-of-line (BEOL) processes have taken a step back. With continuous IC scaling, the speed of the entire chip has reached a point where its performance is determined by the performance of the interconnect that bridges billions of transistors and other devices. Consequently, the demand for advanced interconnect metallization rises again, and various aspects must be considered. This review explores the quest for new materials for successfully routing nanoscale interconnects. The challenges in the interconnect structures as physical dimensions shrink are first explored. Then, various problem-solving options are considered based on the properties of materials. New materials are also introduced for barriers, such as 2D materials, self-assembled molecular layers, high-entropy alloys, and conductors, such as Co and Ru, intermetallic compounds, and MAX phases. The comprehensive discussion of each material includes state-of-the-art studies ranging from the characteristics of materials by theoretical calculation to process applications to the current interconnect structures. This review intends to provide a materials-based implementation strategy to bridge the gap between academia and industry.

From enhanced diffusion to ultrafast ballistic motion of hybrid light–matter excitations

Transport of excitons and charge carriers in molecular systems can be enhanced by coherent coupling to photons, giving rise to the formation of hybrid excitations known as polaritons. Such enhancement has far-reaching technological implications; however, the enhancement mechanism and the transport nature of these hybrid excitations remain elusive. Here we map the ultrafast spatiotemporal dynamics of polaritons formed by mixing surface-bound optical waves with Frenkel excitons in a self-assembled molecular layer, resolving polariton dynamics in energy/momentum space. We find that the interplay between the molecular disorder and long-range correlations induced by coherent mixing with light leads to a mobility transition between diffusive and ballistic transport, which can be controlled by varying the light-matter composition of the polaritons. Furthermore, we show that coupling to light enhances the diffusion coefficient of molecular excitons by six orders of magnitude and even leads to ballistic flow at two-thirds the speed of light.

Van der Waals integration of artificial heterostructures and high-order superlattices

<p indent="0mm">The recent blossom in 2D atomic crystals (2DACs) has ignited intense interest in a new type of bond-free van der Waals heterostructures (vdWHs), in which distinct material components are physically brought together within a vdW distance and held together by weak vdW interactions. Without direct chemical bonding between the constituent materials, the vdWHs negate the lattice matching requirements in typical epitaxially bonded heterostructures. Here we briefly summarize the key advances in the construction and fundamental investigation of versatile vdWHs from diverse 2DACs and beyond, and highlight a unique class of vdW superlattices (vdWSLs) consisting of alternating 2D atomic layers and/or self-assembled molecular layers, with tailored structural symmetry, electronic band modulation, interlayer coupling, and chirality. Lastly, we conclude with a brief outlook on the opportunities in exploring such artificial materials to unlock previously inaccessible physical limits and enable new device concepts beyond the reach of the existing materials.

Chiral molecular intercalation superlattices

The discovery of chiral-induced spin selectivity (CISS) opens up the possibility to manipulate spin orientation without external magnetic fields and enables new spintronic device designs1-4. Although many approaches have been explored for introducing CISS into solid-state materials and devices, the resulting systems so far are often plagued by high inhomogeneity, low spin selectivity or limited stability, and have difficulties in forming robust spintronic devices5-8. Here we report a new class of chiral molecular intercalation superlattices (CMIS) as a robust solid-state chiral material platform for exploring CISS. The CMIS were prepared by intercalating layered two-dimensional atomic crystals (2DACs) (such as TaS2 and TiS2) with selected chiral molecules (such as R-α-methylbenzylamine and S-α-methylbenzylamine). The X-ray diffraction and transmission electron microscopy studies demonstrate highly ordered superlattice structures with alternating crystalline atomic layers and self-assembled chiral molecular layers. Circular dichroism studies show clear chirality-dependent signals between right-handed (R-) and left-handed (S-) CMIS. Furthermore, by using the resulting CMIS as the spin-filtering layer, we create spin-selective tunnelling junctions with a distinct chirality-dependent tunnelling current, achieving a tunnelling magnetoresistance ratio of more than300per cent and a spin polarization ratio of more than60per cent. With a large family of 2DACs of widely tunable electronic properties and a vast selection of chiral molecules of designable structural motifs, the CMIS define a rich family of artificial chiral materials for investigating the CISS effect and capturing its potential for new spintronic devices.

Thermodynamics of self-assembled molecular layers of trimesic acid from fields-supported kinetic Monte Carlo simulation.

In this study we present a thermodynamic analysis of several self-assembled molecular layers of trimesic acid (TMA), gas-solid and solid-solid transitions in such layers using the recently proposed Field(s)-supported MultiPhase kinetic Monte Carlo (FsMP/kMC) method. Simulations were performed in an elongated cell comprising a gas-crystal system under an external potential and a damping field imposed on the gas phase and the interphase. The damping field diminishes the intermolecular potentials, which makes it possible to increase the gas phase density by several orders of magnitude. In turn, this provides reliable determination of the chemical potential of the entire system, including the crystal. The effect of temperature and density on entropy and other thermodynamic functions has been thoroughly investigated. At higher temperatures the chicken-wire (CW) structure with the lowest density is the most stable phase. We revealed a phase transition from the close-packed to CW structure at 435 K. Further increase in temperature leads to disassembling CW structure without the appearance of a liquid-like phase. Thermodynamic analysis of the gas phase showed that the critical temperature of the gas is lower than the disassembling temperature. Although the damping field acts like an increase in temperature along the same system, the thermodynamic equilibrium condition is not violated, which is confirmed by the Gibbs-Duhem equation. This makes the FsMP/kMC method a potentially universal tool for the thermodynamic analysis of crystals formed by "heavy" molecules of any complexity.

Ionic Diffusion-Driven Ionovoltaic Transducer for Probing Ion-Molecular Interactions at Solid-Liquid Interface.

Ion–solid surface interactions are one of the fundamental principles in liquid‐interfacing devices ranging from various electrochemical systems to electrolyte‐driven energy conversion devices. The interplays between these two phases, especially containing charge carriers in the solid layer, work as a pivotal role in the operation of these devices, but corresponding details of those effects remain as unrevealed issues in academic fields. Herein, an ion–charge carrier interaction at an electrolyte–semiconductor interface is interrogated with an ion‐dynamics‐induced (ionovoltaic) energy transducer, controlled by interfacial self‐assembled molecules. An electricity generating mechanism from interfacial ionic diffusion is elucidated in terms of the ion–charge carrier interaction, originated from a dipole potential effect of the self‐assembled molecular layer (SAM). In addition, this effect is found to be modulated via chemical functionalization of the interfacial molecular layer and transition metal ion complexation therein. With the aiding of surface analytic techniques and a liquid‐interfacing Hall measurement, electrical behaviors of the device depending on the magnitude of the ion‐ligand complexation are interrogated, thereby demonstrating the ion–charge carrier interplays spanning at electrolyte–SAM‐semiconductor interface. Hence, this system can be applied to study molecular interactions, including chemical and physical influences, occurring at the solid–liquid interfacial region.

Optimised power harvesting by controlling the pressure applied to molecular junctions.

A major potential advantage of creating thermoelectric devices using self-assembled molecular layers is their mechanical flexibility. Previous reports have discussed the advantage of this flexibility from the perspective of facile skin attachment and the ability to avoid mechanical deformation. In this work, we demonstrate that the thermoelectric properties of such molecular devices can be controlled by taking advantage of their mechanical flexibility. The thermoelectric properties of self-assembled monolayers (SAMs) fabricated from thiol terminated molecules were measured with a modified AFM system, and the conformation of the SAMs was controlled by regulating the loading force between the organic thin film and the probe, which changes the tilt angle at the metal-molecule interface. We tracked the thermopower shift vs. the tilt angle of the SAM and showed that changes in both the electrical conductivity and Seebeck coefficient combine to optimize the power factor at a specific angle. This optimization of thermoelectric performance via applied pressure is confirmed through the use of theoretical calculations and is expected to be a general method for optimising the power factor of SAMs.

Thermodynamics, EOS, and heat capacity in molecular modeling of self-assembled molecular layers.

Self-assembled monolayers (SAMs) on solid surfaces represent a rapidly developed class of non-autonomous phases widely used in organic electronics, sensors, catalysis, and other applications. In many cases, the same organic molecules form various stable and metastable polymorphous structures that can transform to each other at certain parameters. A high rigidity of SAMs extremely complicates the evaluation of the chemical potential using standard methods based on thermodynamic integration. This study presents results of molecular modeling of two-dimensional structures of tripod-shaped molecules associated with the trimesic acid (TMA) molecules. A technique used here is based on a recently developed method of external fields imposed on an elongated simulation cell in the framework of a kinetic Monte Carlo algorithm. These fields are the external potential and a damping field that reduces the intermolecular potential and affects the system similar to the increase in temperature. Equations of state (EOS) for several TMA polymorphs have been obtained with the conventional Monte Carlo simulation. It was shown that, in each case, only one constant links the chemical potential obtained with the external field method and the EOS at any temperature and pressure. The heat capacities of SAMs at constant volume and pressure were also determined as functions of temperature and compressibility of the structure at given degrees of freedom. The approach can be used as a general tool for modeling and evaluation of thermodynamic properties of various rigid structures, including SAMs of functional organic molecules.

Formation of Self-Assembled Anticorrosion Films on Different Metals.

The review will first discuss shortly the phenomenon of corrosion and enroll some general possibilities to decrease the rate of this deterioration. The stress will be laid upon the presentation of anticorrosive self-assembled molecular (SAM) layers as well as on the preparation technology that is a simple technique, does not need any special device, and can be applied on different solids (metals or non-metals) alone or in combination. The paper groups the chemicals (mainly amphiphiles) that can develop nanolayers on different pure or oxidized metal surfaces. The history of the self-assembled molecular layer will be discussed from the beginning of the first application up to now. Not only the conditions of the nanofilm preparation as well as their characterization will be discussed, but the methods that can evaluate the corrosion-inhibiting efficiency of the SAM layers under a corrosive environment will be demonstrated as well.

Recent Advances in Barrier Layer of Cu Interconnects.

The barrier layer in Cu technology is essential to prevent Cu from diffusing into the dielectric layer at high temperatures; therefore, it must have a high stability and good adhesion to both Cu and the dielectric layer. In the past three decades, tantalum/tantalum nitride (Ta/TaN) has been widely used as an inter-layer to separate the dielectric layer and the Cu. However, to fulfill the demand for continuous down-scaling of the Cu technology node, traditional materials and technical processes are being challenged. Direct electrochemical deposition of Cu on top of Ta/TaN is not realistic, due to its high resistivity. Therefore, pre-deposition of a Cu seed layer by physical vapor deposition (PVD) or chemical vapor deposition (CVD) is necessary, but the non-uniformity of the Cu seed layer has a devastating effect on the defect-free fill of modern sub-20 or even sub-10 nm Cu technology nodes. New Cu diffusion barrier materials having ultra-thin size, high resistivity and stability are needed for the successful super-fill of trenches at the nanometer scale. In this review, we briefly summarize recent advances in the development of Cu diffusion-proof materials, including metals, metal alloys, self-assembled molecular layers (SAMs), two-dimensional (2D) materials and high-entropy alloys (HEAs). Also, challenges are highlighted and future research directions are suggested.

Large-Area Patterning of Oil-Based Inks on Superhydrophobic TiO2 Nanotubular Layers by Photocatalytic Wettability Conversion

Patterning an oil-based ink on a solid surface based on a wettability difference is of significant importance for the application of offset printing. Herein, we describe a large-area patterning of oil-based ink on a self-organized TiO2 nanotubular layer based on a photocatalytic wettability conversion. The TiO2 nanotubular layer was fabricated by electrochemical anodization, which demonstrated a superhydrophobic wettability after modification with a self-assembled molecular layer. Subsequently, area-selective ultraviolet (UV) irradiation through a pre-designed pattern of water-based UV-resistant ink formed by an ink-jet technique was used to form a wettability difference. After removing the water-based ink, an oil-based ink was capable of depositing selectively on the superhydrophobic area to form the same pattern as the pre-designed pattern of water-based ink. This large-area patterning of an oil-based ink based on the photocatalytic wettability conversion is potentially applicable in offset printing.

Self-transformed parallel structures in strain sensitive Au thin film micropattern embedded on soft elastomer

This work presents a self transformed method of ordering random buckles in thin metal film fabricated on soft elastomeric film for reliable MEMS devices. Detailed modeling of thin metal film buckling and buckle manipulative stresses associated with the self transformed film is investigated. In this study, a thin metal Au film deposited with dc sputtering is used as a strain sensing film on an elastomeric of polydimethylsiloxane (PDMS) film. To increase the adhesion between the metal film and underlying PDMS film, a self-assembled molecular adhesive layer of (3-mercaptopropyl)trimethoxysilane (MPTMS) has been introduced. The random buckles developed in the Au film were successively oriented without using any ridge surface and prestrain PDMS film for the first time. The strain response study of the Au thin-film resistors of different thicknesses has been characterized for determining the normalized variation of resistance for linear operation. The experimental result showed that the Au thin film resistor on PDMS has high strain sensitivity compared to Au film on Si. The microscopic study of the Au thin film resistor confirmed that the observed high resistance change is due to the micro/nanocracks near grain boundaries on the thin Au film. This study is beneficial for ordering random buckles in hybrid thin-film microstructure on soft elastomer material and fabricating high strain sensitive MEMS devices.