Interfacial Monolayer Research Articles

Interfacial Flat-Lying Molecular Monolayers for Performance Enhancement in Organic Field-Effect Transistors.

Organic field-effect transistors (OFETs) are the most fundamental device units in organic electronics. Interface engineering at the semiconductor/dielectric interface is an effective approach for improving device performance, particularly for enhancing charge transport in conducting channels. Here, we report flat-lying molecular monolayers that exhibit good uniformity and high crystallinity at the semiconductor/dielectric interface, deposited through slow thermal evaporation. Transistor devices achieve high carrier mobility up to 2.80 cm2 V-1 s-1, which represents a remarkably improvement in device performance compared with devices that are completely based on fast-evaporated films. Interfacial flat-lying monolayers benefit charge transport by suppressing the polarization of dipoles and narrowing the broadening of trap density of states. Our work provides a promising strategy for enhancing the performance of OFETs by using interfacial flat-lying molecular monolayers.

Tuning Slonczewski-like torque and Dzyaloshinskii–Moriya interaction by inserting a Pt spacer layer in Ta/CoFeB/MgO structures

Slonczewski-like torque and the Dzyaloshinskii–Moriya interaction (DMI) are important factors in current-induced magnetization switching and domain-wall motion seen in ferromagnetic metal (FM)/heavy metal (HM) structures. We demonstrate the tuning of both factors by inserting a thin Pt layer between Ta and CoFeB in the Ta/CoFeB/MgO structures. The results suggest that the Slonczewski-like torque and DMI decreases with increasing Pt thickness (tPt) in the range 0–1 nm. In consequence, the critical switching current density from the induced spin-orbit torque (SOT) increases whereas the required in-plane field for deterministic switching decreases. The sign of the DMI reverses around tPt = 1 nm, confirming that D has the opposite sign at the Ta/CoFeB and Pt/CoFeB interfaces; but its intensity saturates at tPt = 3 nm, suggesting that several interface monolayers may contribute to the DMI. Our results verifies that a thin HM interlayer may be a suitable route to tailor the SOTs and DMI at the HM/FM interface, as well as the current-induced magnetization switching in these structures.

Electric-field-controlled interface dipole modulation for Si-based memory devices

Various nonvolatile memory devices have been investigated to replace Si-based flash memories or emulate synaptic plasticity for next-generation neuromorphic computing. A crucial criterion to achieve low-cost high-density memory chips is material compatibility with conventional Si technologies. In this paper, we propose and demonstrate a new memory concept, interface dipole modulation (IDM) memory. IDM can be integrated as a Si field-effect transistor (FET) based memory device. The first demonstration of this concept employed a HfO2/Si MOS capacitor where the interface monolayer (ML) TiO2 functions as a dipole modulator. However, this configuration is unsuitable for Si-FET-based devices due to its large interface state density (Dit). Consequently, we propose, a multi-stacked amorphous HfO2/1-ML TiO2/SiO2 IDM structure to realize a low Dit and a wide memory window. Herein we describe the quasi-static and pulse response characteristics of multi-stacked IDM MOS capacitors and demonstrate flash-type and analog memory operations of an IDM FET device.

The Lγ Phase of Pulmonary Surfactant.

To determine how different components affect the structure of pulmonary surfactant, we measured X-ray scattering by samples derived from calf surfactant. The surfactant phospholipids demonstrated the essential characteristics of the Lγ phase: a unit cell with a lattice constant appropriate for two bilayers, and crystalline chains detected by wide-angle X-ray scattering (WAXS). The electron density profile, obtained from scattering by oriented films at different relative humidities (70-97%), showed that the two bilayers, arranged as mirror images, each contain two distinct leaflets with different thicknesses and profiles. The detailed structures suggest one ordered leaflet that would contain crystalline chains and one disordered monolayer likely to contain the anionic compounds, which constitute ∼10% of the surfactant phospholipids. The spacing and temperature dependence detected by WAXS fit with an ordered leaflet composed of dipalmitoyl phosphatidylcholine. Physiological levels of cholesterol had no effect on this structure. Removing the anionic phospholipids prevented formation of the Lγ phase. The cationic surfactant proteins inhibited Lγ structures, but at levels unlikely related to charge. Because the Lγ phase, if arranged properly, could produce a self-assembled ordered interfacial monolayer, the structure could have important functional consequences. Physiological levels of the proteins, however, inhibit formation of the Lγ structures at high relative humidities, making their physiological significance uncertain.

Band alignment of 2D WS2/HfO2 interfaces from x-ray photoelectron spectroscopy and first-principles calculations

We report on the growth of two-dimensional (2D) WS2 on high-k HfO2/Si substrates by reactive sputtering deposition. Raman, x-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy characterizations indicate that the 2D WS2 layers exhibit high-quality crystallinity and exact stoichiometry. Through high-resolution XPS valence spectra, we find a type I alignment at the interface of monolayer WS2/HfO2 with a valence band offset (VBO) of 1.95 eV and a conduction band offset (CBO) of 1.57 eV. The VBO and CBO are also found to increase up to 2.24 eV and 2.09 eV, respectively, with increasing WS2 layers. This is consistent with the results obtained from our first-principles calculations. Our theoretical calculations reveal that the remarkable splitting and shift of the W 5dz2 orbital originating from interlayer orbital coupling in thicker WS2 films induce a reduction of its bandgap, leading to an increase in both the VBO and CBO. This observation can be attributed to the asymmetric splitting at different high symmetric k-points caused by the interlayer orbital coupling.

Molecular Dynamics Simulation of 4-N-Hexyl-4’-Cyanobiphenyl Adsorbed at the Air-Water Interface

The interfacial behavior of 4-n-hexyl-4’-cyanobiphenyl (6CB) molecules at the air-water interface is investigated by full atomistic molecular dynamics simulations. To understand the morphology and the structure of adsorbed 6CB molecules in detail, the snapshots and mass density profiles of the simulation system are generated. The average tilt angles between the interface normal and various vectors defined in the rigid and alkyl parts of 6CB are in good agreement with the experimental data available. The interfacial thickness and monolayer width are obtained from the mass density profiles of water and 6CB phase, respectively. The second and fourth rank orientational order parameters of cyanobiphenyl core are found to be larger than those of an elastic alkyl chain. Bond order parameters for 6CB are also calculated. The calculated oxygen-oxygen radial distribution function and hydrogen bonding statistics for bulk water are compared with those for the interfacial region. The surface tensions of the systems are calculated. All simulation results are compared with the available literature data.

A facile approach to fabricating ultrathin layers of reduced graphene oxide on planar solids

Fabrication of graphene-based surface coatings through self-assembly may provide an affordable alternative to chemical vacuum deposition. Herein, we exploited the self-assembly of graphene oxide at the oil/water interfaces to form monolayers of 2D carbons on solid surfaces with different surface energy. We showed that interfacial monolayers with controlled packing density of graphene oxide can be deposited on the hydrophilic surfaces such as silicon wafers and quartz glass as well as on the hydrophobic surface of Teflon. Graphene oxide attained flat arrangements in the monolayers on hydrophilic surfaces and yielded the films of partially scrolled particles on the surface of Teflon. The as-formed graphene oxide surface coatings underwent rapid reduction under microwave irradiation at 1000 W. The efficiency of reduction was dependent on the ability of the supporting material to absorb microwaves: silicon wafer > quartz glass > Teflon. The single layers of graphene oxide reduced on the surface of silicon wafers showed extraordinary low sheet resistance 1.2 kΩ·sq−1, whereas those on Teflon exhibited low electrical properties (3.0 × 105 kΩ·sq−1). The results suggest that this facile and scalable soft-matter method for producing surface films of graphene oxide can be extended to other practically relevant combinations of graphene-based colloids and supporting materials.

Molecular dynamics study of 5CB at the air-water interface: From gas to beyond the monolayer collapse

We extend our earlier fully atomistic molecular dynamics (MD) study [Gürbulak, O., and Cebe, E. (2017)] to the analysis of the interfacial behaviour of 4-n-pentyl-4′-cyanobiphenyl (5CB) at the air/water interface with surface coverages ranging from 0.07 to 5.01 molecules/nm2. The density profiles of interfacial species and simulated tilt angles lead to the alignment variation of molecules from planar to tilted with increasing surface density of 5CB that is in good accord with experiments. Order parameters of various ranks and CH bond order parameters relevant for deuterium nuclear magnetic resonance spectra were computed. The orientational order of mesogen molecules is considerably enhanced with respect to that for pure nematic 5CB, and increases also with surface density. The calculated interfacial thickness, monolayer width, and radial distribution functions were compared with previously reported results.

Natural Deposition Strategy for Interfacial, Self-Assembled, Large-Scale, Densely Packed, Monolayer Film with Ligand-Exchanged Gold Nanorods for In Situ Surface-Enhanced Raman Scattering Drug Detection.

Liquid interfacial self-assembly of metal nanoparticles holds great promise for its various applications, such as in tunable optical devices, plasmonics, sensors, and catalysis. However, the construction of large-area, ordered, anisotropic, nanoparticle monolayers and the acquisition of self-assembled interface films are still significant challenges. Herein, a rapid, validated method to fabricate large-scale, close-packed nanomaterials at the cyclohexane/water interface, in which hydrophilic cetyltrimethylammonium bromide coated nanoparticles and gold nanorods (AuNRs) self-assemble into densely packed 2D arrays by regulating the surface ligand and suitable inducer, is reported. Decorating AuNRs with polyvinylpyrrolidone not only extensively decreases the charge of AuNRs, but also diminishes repulsive forces. More importantly, a general, facile, novel technique to transfer an interfacial monolayer through a designed in situ reaction cell linked to a microfluidic chip is revealed. The self-assembled nanofilm can then automatically settle on the substrate and be directly detected in the reaction cell in situ by means of a portable Raman spectrometer. Moreover, a close-packed monolayer of self-assembled AuNRs provides massive, efficient hotspots to create great surface-enhanced Raman scattering (SERS) enhancement, which provides high sensitivity and reproducibility as the SERS-active substrate. Furthermore, this strategy was exploited to detect drug molecules in human urine for cyclohexane-extracted targets acting as the oil phase to form an oil/water interface. A portable Raman spectrometer was employed to detect methamphetamine down to 100 ppb levels in human urine, exhibiting excellent practicability. As a universal platform, handy tool, and fast pretreatment method with a good capability for drug detection in biological systems, this technique shows great promise for rapid, credible, and on-spot drug detection.

Evidence of Tailoring the Interfacial Chemical Composition in Normal Structure Hybrid Organohalide Perovskites by a Self-Assembled Monolayer.

Current-voltage hysteresis is a major issue for normal architecture organo-halide perovskite solar cells. In this manuscript we reveal a several-angstrom thick methylammonium iodide-rich interface between the perovskite and the metal oxide. Surface functionalization via self-assembled monolayers allowed us to control the composition of the interface monolayer from Pb poor to Pb rich, which, in parallel, suppresses hysteresis in perovskite solar cells. The bulk of the perovskite films is not affected by the interface engineering and remains highly crystalline in the surface-normal direction over the whole film thickness. The subnanometer structural modifications of the buried interface were revealed by X-ray reflectivity, which is most sensitive to monitor changes in the mass density of only several-angstrom thin interfacial layers as a function of substrate functionalization. From Kelvin probe force microscopy study on a solar cell cross section, we further demonstrate local variations of the potential on different electron-transporting layers within a solar cell. On the basis of these findings, we present a unifying model explaining hysteresis in perovskite solar cells, giving an insight into one crucial aspect of hysteresis for the first time and paving way for new strategies in the field of perovskite-based opto-electronic devices.

Determining electrospun morphology from the properties of protein-polymer solutions.

Integrating natural macromolecules, e.g. proteins, is a progressive trend in the fabrication of biocompatible sub-micrometer fibers with tunable diameters using the electrospinning technique. The correlation between solution properties and electrospun morphology is critical; it is quite clear for synthetic linear polymer solutions but remains uncertain for solutions with protein. Here, we report the determination of electrospun morphology in protein-polymer solutions of poly(ethylene oxide) (PEO) and zein, a storage protein from corn. The viscosity of the zein/PEO mixed solutions can be well described using the Lederer-Roegiers equation and decreases with the increase of the fraction of zein. The surface tension sharply decreases above a critical concentration at the saturation of the interfacial monolayer. Correspondingly, the different electrospun morphologies-from bead, coexisting bead and fiber, to fiber and ribbon-were mapped onto a ternary phase diagram and a viscosity contour plot. Such coupling provides a clear way to determine the electrospun morphology from solution properties. The occurrence of electrospun fibers partially follows two empirical rules, while the critical point revealed from surface tension has the best approximation. The diameters of electrospun fibers were found to have a scaling relationship against concentration, zero-shear viscosity and surface tension of solutions. These scaling exponents were compared with those from typical polymer solutions. The analysis suggests that aqueous ethanol gives different solvent qualities to zein and PEO solutions, resulting in the irregular shape in the phase diagram that correlates solution properties and electrospun morphologies.

Directing the hetero-growth of lattice-mismatched surface-mounted metal–organic frameworks by functionalizing the interface

A new strategy was developed to grow hetero-structured surface-mounted metal–organic frameworks by engineering the mono-layer interface between two lattice-mismatched components.

Molecular dynamics simulation of four typical surfactants at oil/water interface

ABSTRACTIn the present study, we have performed molecular dynamics simulations to describe the microscopic behaviors of the anionic, nonionic, zwitterion, and gemini surfactants at oil/water interface. The abilities of reducing the interfacial tension and forming the stable interfacial film of the four surfactants have been investigated through evaluating interfacial thickness, interface formation energy and radial distribution function. The results show that the four kinds of surfactants can form in stable oil/water interface of monolayer, and the gemini surfactant can form the more stable monolayer. The results of the above three parameters demonstrate that the gemini surfactant has the best simulation effect in the four surfactants. From the calculated interfacial tension values, the gemini surfactant possess the more powerful ability of reducing the interfacial tension than others, so it is more suitable to be used as the surfactant for flooding. In addition, the effects of different electric field intensities on surfactants were calculated, through the radial distribution function of the hydrophilic group in the surfactant and the oxygen atom in the water. We have found that the adding of the periodic electric field can significantly affect the diffusion behavior of the molecules, and nonionic surfactant has stronger demulsification capability than others.

Interfacial monolayer graphene growth on arbitrary substrate by nickel-assisted ion implantation

Direct synthesis of monolayer graphene on arbitrary substrate (such as SiO2, Al2O3, glass, and Si3N4) is demonstrated through a universal and controllable approach, i.e., carbon ion implantation technique. By tuning the implantation energy to precisely implant carbon ions into the thin Ni film, which is pre-deposited on the objective substrate, followed by post-annealing and fast-cooling processes, monolayer graphene films are directly synthesized on the arbitrary objective substrate. Micro-Raman spectroscopy, STM, and TEM are cooperatively utilized to verify that the synthesized graphene is monolayer with high quality. Moreover, field-effect transistors are fabricated with the directly synthesized monolayer graphene on SiO2/Si substrate to reveal the corresponding electrical properties. This study provides an avenue for direct growth of graphene on arbitrary substrate, which offers more flexibility in the experimental conditions, especially the experimental atmosphere. In addition, involving the ion implantation technique may pave the way for wafer-scale graphene synthesis, thus benefitting the application of graphene in micro-/nano-electronic field.

Unoccupied Interface and Molecular States in Thiol and Dithiol Monolayers.

The electronic structure of self-assembled monolayers (SAMs) formed by thiols of different lengths and dithiol molecules bound to Au(111) has been characterized. Inverse photoemission spectroscopy (IPES) and density functional theory have been used to describe the molecule/Au substrate system. All molecular layers display a clear signal in the IPES data at the edge of the lowest unoccupied system orbital (LUSO), roughly 3 eV above the Fermi level. There is also evidence, in both the experimental data and the calculation, of a finite density of states just below the LUSO edge, which has been recognized as localized at the Au-substrate interface. Regardless of the molecular lengths and in addition to this induced density of interface states, an apparent antibonding Au-S state has been identified in the IPES data for both molecular systems. The main difference between the electronic structures of thiol and dithiol SAMs is a shift in the energy of the antibonding state.

Asymmetrical two-dimensional electron gas in superlattices consisting of insulating GdTiO3 and BaTiO3

Two-dimensional electron gas due to semiconductor interfaces can have high mobility and exhibits superconductivity, magnetism, and other exotic properties that are unexpected in constituent bulk materials. We study crystal structures, electronic states, and magnetism of short-period (BTO)m/(GTO)2 (m=2 and 4) superlattices consisting of insulating BaTiO3 (BTO) and GdTiO3 (GTO) by first principles calculations. Our investigation shows that the middle Ti-O monolayer in the GTO layer becomes metallic. Although both GTO and BTO share a common constituent, the Ti-O atomic layer, the M-O (M=Ba, Ti, and Gd) displacements along the c axis in each monolayer reflect the asymmetry of two interfacial Ti-O monolayers and the large distortion of the TiO6 octahedra in such superlattices, against electronic reconstruction at the interfaces and thus differentially changing the d energy levels of the three Ti-O monolayers related with the GTO layer. Such superlattices are interesting for potential spintronics applications because of their unique asymmetrical two-dimensional electron-gas properties and possible useful spin-orbit effects.

Interaction of a model apolipoprotein, apoLp-III, with an oil-phospholipid interface

Lipid droplets are “small” organelles that play an important role in de novo synthesis of new membrane, and steroid hormones, as well as in energy storage. The way proteins interact specifically with the oil-(phospho-)lipid monolayer interface of lipid droplets is a relatively unexplored but crucial question. Here, we use our home built liquid droplet tensiometer to mimic intracellular lipid droplets and study protein-lipid interactions at this interface. As model neutral lipid binding protein, we use apoLp-III, an amphipathic α-helix bundle protein. This domain is also found in proteins from the perilipin family and in apoE. Protein binding to the monolayer is studied by the decrease in the oil/water surface tension. Previous work used POPC (one of the major lipids found on lipid droplets) to form the phospholipid monolayer on the triolein surface. Here we expand this work by incorporating other lipids with different physico-chemical properties to study the effect of charge and lipid head-group size. This study sheds light on the affinity of this important protein domain to interact with lipids.

π‐Conjugated Organosilica Semiconductors: Toward Robust Organic Electronics

The use of novel organosilica materials embedding π‐conjugated moieties as semiconductor into field‐effect transistors is demonstrated. The chosen π‐conjugated core is a [1]benzothieno[3,2‐b][1]benzothiophene that is modified with hydrolyzable and crosslinkable triethoxysilyl moieties. After polycondensation, this compound forms a hybrid material composed of charge transport pathways as well as insulating sublayers made of silicon oxide (SiOx). The hybrid material behaves as a semiconductor and is subsequently integrated as active layer into field‐effect transistors. These precursors show J‐type aggregation that evolves toward H‐type aggregates during the sol–gel process, which improve charge transport. Taking advantage of the sol–gel chemistry involved here, hybrid field‐effect transistors that are fully crosslinked with covalent bonds are built. Molecules are crosslinked to each other, covalently bonded to the silicon oxide dielectric, and also covalently bonded to the gold electrodes, thanks to the use of an appropriate additional interfacial monolayer. This is the first report of fully covalent transistors. Those devices with modest mobilities show impressive resilience against polar, aliphatic, and aromatics solvents even under sonication. This study opens the route toward a new class of hybrid materials to create highly robust electronic applications.

Nanoscale Chemical Imaging of Interfacial Monolayers by Tip‐Enhanced Raman Spectroscopy

AbstractWe report an investigation of interfacial fluorinated hydrocarbon (carboxylic‐fantrip) monolayers by nanoscale imaging using tip‐enhanced Raman spectroscopy (TERS) and density functional theory (DFT) calculations. By comparing TERS images of a sub‐monolayer prepared by spin‐coating and a π–π‐stacked monolayer on Au(111) in which the molecular orientation is confined, specific Raman peaks shift and line widths narrow in the transferred LB monolayer. Based on DFT calculations that take into account dispersion corrections and surface selection rules, these specific effects are proposed to originate from π–π stacking and molecular orientation restriction. TERS shows the possibility to distinguish between a random and locked orientation with a spatial resolution of less than 10 nm. This work combines experimental TERS imaging with theoretical DFT calculations and opens up the possibility of studying molecular orientations and intermolecular interaction at the nanoscale and molecular level.

Nanoscale Chemical Imaging of Interfacial Monolayers by Tip-Enhanced Raman Spectroscopy.

We report an investigation of interfacial fluorinated hydrocarbon (carboxylic-fantrip) monolayers by nanoscale imaging using tip-enhanced Raman spectroscopy (TERS) and density functional theory (DFT) calculations. By comparing TERS images of a sub-monolayer prepared by spin-coating and a π-π-stacked monolayer on Au(111) in which the molecular orientation is confined, specific Raman peaks shift and line widths narrow in the transferred LB monolayer. Based on DFT calculations that take into account dispersion corrections and surface selection rules, these specific effects are proposed to originate from π-π stacking and molecular orientation restriction. TERS shows the possibility to distinguish between a random and locked orientation with a spatial resolution of less than 10 nm. This work combines experimental TERS imaging with theoretical DFT calculations and opens up the possibility of studying molecular orientations and intermolecular interaction at the nanoscale and molecular level.