Binary Monolayers Research Articles

Hexagonal group IV-V (IV=C, Si, Ge, V=N, P, As) binary monolayers: First-principles study

Hexagonal group IV-V (IV=C, Si, Ge, V=N, P, As) binary monolayers: First-principles study

Insights into Chemical Bonding Modes and Heat Transport at the Molecular Level.

Despite the demand for nanoscale thermal management technologies of material surfaces and interfaces using organic molecules, heat transport properties at the single molecular level remain elusive due to the experimental difficulty of measuring temperature at the nanoscopic scale. Here we show how chemical bonding modes can affect the heat transport properties of single molecules. We focused on four molecular systems: benzylthiol linked to another phenyl group by either a triple (compound 1), double (3), or amide (4) bond and a common linear alkanethiol (2), all of which are nearly identical in molecular length. We prepared binary self-assembled monolayers (SAMs) using 1 as a common reference in combination with 2-4 and investigated their relative heat transport properties using scanning thermal microscopy (SThM). Two-dimensional temperature mapping of the binary SAMs showed that C≡C and C=C bonds provide more effective pathways for heat transport compared to C-C bonds. Since the amide molecule has resonance structures with C=N double bond character, we expected that its heat transport properties would be comparable to those of the thiols containing triple or double bonds. However, the heat transport properties of this molecule prevailed over the others, most likely due to the formation of additional heat transport pathways caused by intermolecular hydrogen bonding. These findings may provide important guidelines for the design of organic materials for nanoscale thermal management.

Influence of hydrogen-bonded 3-mercaptopropionic acid bilayers on binary self-assembled monolayer formation

The influence of the monolayer order, defect density, and bilayer formation on the formation of binary self-assembled monolayers (SAMs) was investigated via the solution-phase displacement of 3-mercaptopropionic acid (MPA) by 1-decanethiol (DT). The ultrahigh vacuum scanning tunneling microscopy results confirm that well-ordered, pristine MPA SAMs are displaced at slower rates than MPA SAMs with less long-range order and greater defect density. Furthermore, MPA samples containing regions of an MPA bilayer displayed the slowest rates of displacement with DT. For pristine MPA samples and MPA samples with regions of an MPA bilayer, displacement with DT resulted in the formation of the low-density, lying down phase of DT. Our results suggest that the presence of an MPA bilayer, the result of hydrogen bonding between carboxylic acid groups in MPA, significantly lowers the rate of total displacement of MPA by DT compared to moderately defected MPA SAMs. These results highlight the importance of the structure, composition, and intermolecular forces, such as hydrogen bonding, when considering binary SAM formation via solution-phase displacement methods.

Development of a Multiplexed Electrochemical Aptasensor for the Detection of Cyanotoxins.

In this study, we report a multiplexed platform for the simultaneous determination of five marine toxins. The proposed biosensor is based on a disposable electrical printed (DEP) microarray composed of eight individually addressable carbon electrodes. The electrodeposition of gold nanoparticles on the carbon surface offers high conductivity and enlarges the electroactive area. The immobilization of thiolated aptamers on the AuNP-decorated carbon electrodes provides a stable, well-orientated and organized binary self-assembled monolayer for sensitive and accurate detection. A simple electrochemical multiplexed aptasensor based on AuNPs was designed to synchronously detect multiple cyanotoxins, namely, microcystin-LR (MC-LR), Cylindrospermopsin (CYL), anatoxin-α, saxitoxin and okadaic acid (OA). The choice of the five toxins was based on their widespread presence and toxicity to aquatic ecosystems and humans. Taking advantage of the conformational change of the aptamers upon target binding, cyanotoxin detection was achieved by monitoring the resulting electron transfer increase by square-wave voltammetry. Under the optimal conditions, the linear range of the proposed aptasensor was estimated to be from 0.018 nM to 200 nM for all the toxins, except for MC-LR where detection was possible within the range of 0.073 to 150 nM. Excellent sensitivity was achieved with the limits of detection of 0.0033, 0.0045, 0.0034, 0.0053 and 0.0048 nM for MC-LR, CYL, anatoxin-α, saxitoxin and OA, respectively. Selectivity studies were performed to show the absence of cross-reactivity between the five analytes. Finally, the application of the multiplexed aptasensor to tap water samples revealed very good agreement with the calibration curves obtained in buffer. This simple and accurate multiplexed platform could open the window for the simultaneous detection of multiple pollutants in different matrices.

Surface miscibility of Gemini surfactants and DOPE in binary mixed monolayers

AbstractSurface pressure (π)–molecular area (A) isotherms were gathered to characterize the packing of binary mixed Langmuir monolayers of 1,2‐dioleoyl‐sn‐glycero‐3‐phosphoethanolamine (DOPE) and one of two Gemini surfactants (GS), N,N‐bis(dimethyloctadecyl)‐1,7‐nonanediammonium dibromide (18‐7‐18) or 1,9‐bis(octadecyl)‐1,1,9,9‐tetramethyl‐5‐amino‐1,9‐nonanediammonium dibromide (18‐7NH‐18) of varying molar fractions. Information about miscibility behavior was derived from the π–A curves by examining the excess free energy of mixing (ΔGexc) that was calculated through the surface area additivity rule. Surface compressibility modulus (Cs−1) was also used to characterize intermolecular interactions. Mutual interactions between GS and DOPE were analyzed in terms of excess Gibbs energy of mixing and the value of this parameter depended strongly on the composition of the mixed film. GS and DOPE are generally miscible as DOPE reduces intermolecular repulsion between highly charged GS molecules leading to the formation of more densely packed mixed monolayers. However, GS and DOPE are immiscible in equimolar mixtures due to tail group packing mismatch between saturated and unsaturated alkyl chains. The prevalence of attractive synergistic interactions in the monolayers studied differs from a previous finding of antagonistic mixing behavior in GS/DOPE micelles. These results contribute to the understanding of GS‐lipid interactions and packing that are critical to the in vitro and in vivo stability of liposomes composed of these molecules used for non‐viral gene therapy applications.

Impact of the Amphoteric Nature of a Chelating Surfactant on its Interaction with an Anionic Surfactant: A Surface Tension and Neutron Reflectivity Study of Binary Mixed Solutions.

2-Dodecyldiethylenetriaminepentaacetic acid (C12-DTPA) is a chelating, amphoteric surfactant with a bulky headgroup containing eight pH-responsive groups. The hypothesis was that the amphoteric nature of the chelating surfactant would affect the interaction with another surfactant and, consequently, also the composition of mixed surface layers. Binary mixed monolayers of C12-DTPA and the anionic surfactant sodium dodecyl sulfate (SDS) were examined using neutron reflection and surface tension measurements. The experiments were conducted at pH 5, where the C12-DTPA monomers carried a net negative charge. Surface excess calculations at low total surfactant concentration revealed that the chelating surfactant dominated the surface composition. However, as the concentration was raised, the surface composition shifted toward an SDS-dominant state. This phenomenon was attributed to the increased ionic strength at increased concentrations, which altered the balance between competing entropic forces in the system. Interaction parameters for mixed monolayer formation were calculated, following a framework based on regular solution theory. In accordance with the hypothesis, the chelating surfactant's ability to modulate its charge and mitigate repulsive interactions in the surface layer resulted in favorable interactions between the anionic SDS and negatively charged C12-DTPA monomers. These interactions were found to be concentration-dependent, which was consistent with the observed shift in the surface layer composition.

Nanoscale visualization of phase separation in binary supported lipid monolayer using tip-enhanced Raman spectroscopy.

Supported lipid membranes are an important model system to study the phase separation behavior at the nanoscale. However, the conventional nanoanalytical tools often fail to provide reliable chemical characterization of the phase separated domains in a non-destructive and label-free manner. This study demonstrates the application of scanning tunneling microscopy-based tip-enhanced Raman spectroscopy (TERS) to study the nanoscale phase separation in supported d62-DPPC : DOPC lipid monolayers. Hyperspectral TERS imaging successfully revealed a clear segregation of the d62-DPPC-rich and DOPC-rich domains. Interestingly, nanoscale deposits of d62-DPPC were observed inside the DOPC-rich domains and vice versa. High-resolution TERS imaging also revealed the presence of a 40-120 nm wide interfacial region between the d62-DPPC-rich and DOPC-rich domains signifying a smooth transition rather than a sharp boundary between them. The novel insights obtained in this study demonstrate the effectiveness of TERS in studying binary lipid monolayers at the nanoscale.

Spin and electronic property prediction of IV–V binary monolayers using deep knowledge transfer method

The spin–orbit coupling (SOC) correction associated to the electronic and spin properties of IV–V binary monolayers using deep transfer learning model is estimated. The accuracy of more than 99% is obtained for the prediction of electronic properties. In addition, the correlation between the spin-splittings at the K-point for both conduction and valence bands of different compounds is studied by this model. Their corresponding spin properties demonstrate inter-dependent behavior in the conduction band while less correlation is obtained in the valence band. Furthermore, the four most effective band splitting SOC corrections at the valence band are also found using the square of atomic numbers with an error of less than 10−3.

Anti-reflective MX (M = Sc and Y; X = N, P, As, Sb and Bi) monolayers: structural, electronic and optical study

In this paper, the structural, electronic and optical properties of tetragonal binary monolayers of MX (M = Sc, Y; X = As, Bi, N, P, Sb) are investigated using the density functional theory. The optical study demonstrates that ScN and YN compounds are promising anti-reflective materials. All compounds are found to be semiconductors with a bandgap in the range of 0.45–1.8 eV. Among these compounds, ScN and YN have a direct bandgap at Γ-point while the remainings demonstrate an indirect bandgap. It is found that the structural anisotropy controls the anisotropy of the electronic properties. The biaxial strain analysis shows that YBi monolayer has the maximum linear strain bandgap dependency, making it a suitable candidate for pressure sensing applications. The ScN and YN monolayers demonstrate a phase transition from semiconductive to Dirac semi-metallic characteristics at large compressive strains.

Type-II van der Waals heterostructures constructed by V-V binary monolayers for ultra-thin solar cells

The development of solar cells holds great importance in the field of renewable energy and sustainability. Type-II van der Waals heterostructures, constructed from V-V binary monolayers, which exhibit easily-tunable electronic and optical properties, represent promising materials. In this work, we constructed and screened type-II van der Waals heterostructures using V-V binary monolayers for ultra-thin solar cells, and predicted their solar cell performance based on first-principles study. Five stable type-II van der Waals heterostructures were selected from forty-five heterostructures. The results showed that BiP/SbAs and BiP/BiAs heterostructures are promising candidates for solar cells with the power conversion efficiency as high as 17% and well light absorption performance. It will provide insights into the development and refinement of ultra-thin photovoltaic cells utilizing type-II van der Waals heterostructures composed of V-V binary monolayers.

Dynamic Variation of Rectification Observed in Supramolecular Mixed Mercaptoalkanoic Acid.

Functionality in molecular electronics relies on inclusion of molecular orbital energy level within a transmission window. This can be achieved by designing the active molecule with accessible energy levels or by widening the window. While many studies have adopted the first approach, the latter is challenging because defects in the active molecular component cause low breakdown voltages. Here, it is shown that control over the packing structure of monolayer via supramolecular mixing transforms an inert molecule into a highly tunable rectifier. Binary mixed monolayer composed of alkanethiolates with and without carboxylic acid head group as a proof of concept is formed via a surface-exchange reaction. The monolayer withstands high voltages up to |4.5 V| and shows a dynamic rectification-external bias relationship in magnitude and polarity. Sub-highest occupied molecular orbital (HOMO) levels activated by the widened transmission window account for these observations. This work demonstrates that simple supramolecular mixing can imbue new electrical properties in electro-inactive organic molecules.

Electrical and Electronic Properties of Magnesium/Molybdenum Disulfide Heterojunction Field Effect Transistors: A Theoretical Study

The present article designs two-dimensional heterojunction duplex material FETs based on binary monolayer material, Mg and molybdenum disulfide. Despite having a hexagonal crystal structure, the monolayer Mg and molybdenum disulfide have good lattice matching ability, with a mismatch degree of approximately 5%. The electrostatic characteristics of Mg/molybdenum disulfide field effect transistors (FETs) are well suited for compact fabrication. Electronic structure of first-principles investigations, optical, mechanical, and electrochemical properties of MODES field-effect transistors based on density functional theory are mastered in order to master the electrostatic doping associated features of FETs. Based on the Silvaco TCAD platform, this simulation study was performed. There is theoretical value in engineering practice, both in terms of design and application.

Gold-Supported Lipid Membranes Formed by Redox-Triggered Vesicle Fusion on Binary Self-Assembled Monolayers: Ion-Pairing Association and Surface Hydrophilicity.

The assembly of biomimetic, planar supported lipid bilayers (SLBs) by the popular vesicle fusion method, which relies on the spontaneous adsorption and rupture of small unilamellar vesicles from aqueous solution on a solid surface, typically works with a limited range of support materials and lipid systems. We previously reported a conceptual advance in the formation of SLBs from vesicles in the gel or fluid phase using the interfacial ion-pairing association of charged phospholipid headgroups with electrochemically generated cationic ferroceniums bound to a self-assembled monolayer (SAM) chemisorbed to gold. This redox-driven approach lays down a single bilayer membrane on the SAM-modified gold surface at room temperature within minutes and is compatible with both anionic and zwitterionic phospholipids. The present work explores the effects of the surface ferrocene concentration and hydrophobicity/hydrophilicity on the formation of continuous SLBs of dialkyl phosphatidylserine, dialkyl phosphatidylglycerol, and dialkyl phosphatidylcholine using binary SAMs of ferrocenylundecanethiolate (FcC11S) and dodecanethiolate (CH3C11S) or hydroxylundecanethiolate (HOC11S) comprising different surface mole fractions of ferrocene (χFcsurf). An increase in the surface hydrophilicity and surface free energy of the FcC11S/HOC11S SAM mitigates the decrease in the attractive ion-pairing interactions resulting from a reduced χFcsurf. SLBs of ≳80% area coverage form on the FcC11S/HOC11S SAM for all the phospholipid types down to χFcsurf of at least 0.2, composition yielding a water contact angle (θW) of 44 ± 4°. By contrast, a greater number of ion-pairing interactions is required on the hydrophobic FcC11S/CH3C11S surface to drive the vesicle fusion process; bilayers or bilayer patches form at χFcsurf ≳ 0.6 (θW = 97 ± 3°). These findings will aid in tailoring the surface chemistry of redox-active modified surfaces to widen the conditions that yield supported lipid membranes.

Nonadditive Interactions Unlock Small-Particle Mobility in Binary Colloidal Monolayers

We examine the organization and dynamics of binary colloidal monolayers composed of micron-scale silica particles interspersed with smaller-diameter silica particles that serve as minority component impurities. These binary monolayers are prepared at the surface of ionic liquid droplets over a range of size ratios (σ = 0.16-0.66) and are studied with low-dose minimally perturbative scanning electron microscopy (SEM). The high resolution of SEM imaging provides direct tracking of all particle coordinates over time, enabling a complete description of the microscopic state. In these bidisperse size mixtures, particle interactions are nonadditive because interfacial pinning to the droplet surface causes the equators of differently sized particles to lie in separate planes. By varying the size ratio, we control the extent of nonadditivity in order to achieve phase behavior inaccessible to additive 2D systems. Across the range of size ratios, we tune the system from a mobile small-particle phase (σ < 0.24) to an interstitial solid (0.24 < σ < 0.33) and furthermore to a disordered glass (σ > 0.33). These distinct phase regimes are classified through measurements of hexagonal ordering of the large-particle host lattice and the lattice's capacity for small-particle transport. Altogether, we explain these structural and dynamic trends by considering the combined influence of interparticle interactions and the colloidal packing geometry. Our measurements are reproduced in molecular dynamics simulations of 2D nonadditive disks, suggesting an efficient method for describing confined systems with reduced dimensionality representations.

Theoretical design of buckled honeycomb binary ZrX (X=S, Se, Te) monolayers and valley splitting of heterostructures constructed with MoTe2

Based on density functional theory, we design a new family of buckled honeycomb binary ZrX (X = S, Se, Te) monolayers. The feasibility of experimental synthesis can be seen from their negative binding energy and the absence of imaginary frequency in the phonon dispersion. It is found that the buckled honeycomb binary ZrX monolayers show magnetic character with moment range of 1.832–2.004 μB. The ZrTe monolayer is narrow band gap (0.251 eV) semiconductor. Furthermore, we demonstrate that a large valley splitting can be obtained in MoTe2 monolayer by magnetic proximity coupling to a ZrS or ZrSe magnetic monolayer. For the ZrS/MoTe2 (ZrSe/MoTe2) heterostructure, the electronic band of MoTe2 monolayer has achieved a high valley splitting of 66 (80) meV, which is equal to a Zeeman field of 76 (93) T. Our study paves the way to explore promising two-dimensional magnetic materials for practical applications in valleytronic devices.

Ultra-low lattice thermal conductivity induces high-performance thermoelectricity in Janus group-VIA binary monolayers: A comparative investigation

In this paper, the electrical transport, thermal transport, and thermoelectric properties of two new Janus STe2 and SeTe2 monolayers are systematically studied by first-principles calculations, as well as the comparative with available literature’s results using different methods. It is found that the Seebeck coefficient and conductivity have opposite dependence on temperature, and we illustrate this phenomenon in detail. The decrease of the thermoelectric power factor (PF) with temperature originates from the decrease in conductivity. To obtain accurate and convergent lattice thermal conductivity, the root mean square (RMS) is calculated to obtain a reasonable cutoff radius for the calculation of third-order forces. Janus STe2 and SeTe2 monolayers exhibit ultra-low lattice thermal conductivity of 0.2 and 0.133 W/mK at 300 K, which result from the strong coupling effect between the acoustic mode and the low-frequency optical branch, low phonon group velocity, small phonon lifetime, and large anharmonicity. Consequently, ultra-high ZT values of 2.11 (2.09) and 3.28 (4.24) for n-type(p-type) carrier doping of STe2 and SeTe2 are obtained, indicating that they are promising thermoelectric materials.

Hexagonal warping effect in the Janus group-VIA binary monolayers with large Rashba spin splitting and piezoelectricity.

In this paper, the electronic band structure, Rashba effect, hexagonal warping, and piezoelectricity of Janus group-VIA binary monolayers STe2, SeTe2, and Se2Te are investigated based on density functional theory (DFT). Due to the inversion asymmetry and spin-orbit coupling (SOC), the STe2, SeTe2 and Se2Te monolayers exhibit large intrinsic Rashba spin splitting (RSS) at the Γ point with the Rashba parameters 0.19 eV Å, 0.39 eV Å, and 0.34 eV Å, respectively. Interestingly, based on the k·p model via symmetry analysis, the hexagonal warping effect and a nonzero spin projection component Sz arise at a larger constant energy surface due to nonlinear k3 terms. Then, the warping strength λ was obtained by fitting the calculated energy band data. Additionally, in-plane biaxial strain can significantly modulate the band structure and RSS. Furthermore, all these systems exhibit large in-plane and out-of-plane piezoelectricity due to inversion and mirror asymmetry. The calculated piezoelectric coefficients d11 and d31 are about 15-40 pm V-1 and 0.2-0.4 pm V-1, respectively, which are superior to those of most reported Janus monolayers. Because of the large RSS and piezoelectricity, the studied materials have great potential for spintronic and piezoelectric applications.

Mixing Behavior of the Binary Monolayers of Fatty Acids Based on Their Cohesive Energy Differences.

The morphology involving the height difference and the surface roughness of the binary monolayers of saturated fatty acids were evaluated using atomic force microscopy (AFM) to investigate the mixing behavior of their monolayers. AFM observations revealed that the mixed monolayers of (palmitic acid/arachidic acid) and (arachidic acid/lignoceric acid), which had four methylene group differences between fatty acids, were in a molecularly mixed state. Further, the mixed monolayer of (stearic acid/lignoceric acid), which had six methylene group differences, was in a phase-separated state. From the results of the present and previous studies, it became clear that the difference in the cohesive energy between fatty acids, which corresponds to the enthalpy difference, was an important factor in determining whether the molecular aggregation state of a fatty acid mixed monolayer was in a molecularly mixed or phase-separated state. Moreover, the boundary value of cohesive energy difference was approximately 2.5 kJ mol-1 at a subphase temperature of 293 K.

Van der Waals imprinting of black-phosphorus-like binary alloyed monolayers with tunable band gaps and moiré superstructures

Fabrication of isostructural isovalent alloys is a powerful approach to discovering novel materials with emergent properties. This approach has been well established in traditional bulk materials, yet remains to be fully exploited in two-dimensional systems. Here we use black- phosphorus (BP)-like group-VA binary alloyed monolayers on BP-like group IV-VI substrates to demonstrate the enabling power of isomorphism and isovalency in fabricating two-dimensional alloyed monolayers with tunable properties. We achieve the formation of nearly freestanding BP-like ${\mathrm{Sb}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x} (0\ensuremath{\le}x\ensuremath{\le}1)$ monolayers on the GeSe substrate, with a semiconductor-to-metal transition and tunable moir\'e superstructures as $x$ increases. In contrast, BP-like ${\mathrm{Pb}}_{x}{\mathrm{Bi}}_{1\ensuremath{-}x}$ monolayers can be stabilized only for low Pb concentrations, and the contrasting behaviors are attributable to preservation or breakdown of isovalency in the two cases. This study establishes a generic approach to growing alloyed monolayers via van der Waals imprinting, opening a new avenue for materials discoveries with desirable functionalities.

Structure of Amorphous Two-Dimensional Materials: Elemental Monolayer Amorphous Carbon versus Binary Monolayer Amorphous Boron Nitride.

The structure of amorphous materials has been debated since the 1930s as a binary question: amorphous materials are either Zachariasen continuous random networks (Z-CRNs) or Z-CRNs containing crystallites. It was recently demonstrated, however, that amorphous diamond can be synthesized in either form. Here we address the question of the structure of single-atom-thick amorphous monolayers. We reanalyze the results of prior simulations for amorphous graphene and report kinetic Monte Carlo simulations based on alternative algorithms. We find that crystallite-containing Z-CRN is the favored structure of elemental amorphous graphene, as recently fabricated, whereas the most likely structure of binary monolayer amorphous BN is altogether different than either of the two long-debated options: it is a compositionally disordered "pseudo-CRN" comprising a mix of B-N and noncanonical B-B and N-N bonds and containing "pseudocrystallites", namely, honeycomb regions made of noncanonical hexagons. Implications for other nonelemental 2D and bulk amorphous materials are discussed.