Constitutional Dialogue and Human Dignity: States and Transnational Constitutional Discourse

Freestanding and well-ordered two-dimensional (2D) silica monolayers with tetrahedral (<i>T</i>-silica) and octahedral (<i>O</i>-silica) building blocks are found to be stable by first-principles calculations. <i>T</i>-silica is formed by corner-sharing SiO₄ tetrahedrons in a rectangular network, and <i>O</i>-silica consists of edge-sharing SiO₆ octahedrons. Moreover, the insulating <i>O</i>-silica is the strongest silica monolayer, and can therefore act as a supporting substrate for nanostructures in sensing and catalytic applications. Nanoribbons of <i>T</i>-silica are metallic, while those of <i>O</i>-silica have band gaps regardless of the chirality and width. We find the interaction of <i>O</i>-silica with graphene to be weak, suggesting the possibility of its use as a monolayer dielectric material for graphene-based devices. Considering that the sixfold-coordinated silica exists at high pressure in the bulk phase, the prediction of a small energy difference of <i>O</i>-silica with the synthesized silica bilayer, together with the thermal stability at 1000 K, suggests that synthesis of <i>O</i>-silica can be achieved in experiments

Amino Acid Analogue-Conjugated BN Nanomaterials in a Solvated Phase: First Principles Study of Topology-Dependent Interactions with a Monolayer and a (5,0) Nanotube

Using density functional theory and an implicit solvation model, the relationship between the topology of boron nitride (BN) nanomaterials and the protonated/deprotonated states of amino acid analogues is investigated. In the solvated phase, the calculated results show distinct “physisorbed versus chemisorbed” conditions for the analogues of arginine (Arg)- and aspartic acid (Asp)-conjugated BN nanomaterials, including a monolayer (ML) and a small-diameter zigzag nanotube (NT). Such a distinction does not depend on the functional groups of amino acids but rather depends on the curvature-induced interactions associated with the tubular configuration. Arg and Asp interact with the BNML to form physisorbed complexes irrespective of the state of the amino acids in the solvated phase. For the NT, Arg and Asp form chemisorbed complexes, and the distinct nature of bonds between the donor electron moieties of N_(Arg) and O_(Asp) and the boron of the tubular surface is revealed by the natural bond orbital analysis; stronger s-type bonds for the deprotonated conjugated complexes and slightly weaker p-type dominated bonds for the protonated conjugated complexes. The interaction of neutral Trp with BN nanomaterials results in physisorbed configurations through π-stacking interactions with the indole ring of the Trp and BN nanomaterials. The calculated results form the basis for a theoretical study of more complex protein macromolecules interacting with nanomaterials under physiological conditions

Controlling the Performance of a Three-Terminal Molecular Transistor: Conformational versus Conventional Gating

The effect of conformational changes in the gate arm of a three-terminal device is investigated. In the ground state, the gate (triphenyl) arm is nonplanar, where the middle phenyl ring is approximately 30° out-of-plane relative to other two rings. At this geometry, the calculated tunnel current (<i>I</i>_d) as a function of external bias (<i>V</i>_ds) across the two D–A substituted arms exhibits a typical insulator-semiconductor behavior. Similar <i>I</i>_d–<i>V</i>_ds characteristics is calculated when planarity of the triphenyl arm is restored. However, a significant increase, by more than an order of magnitude, and a distinct variation in the current are predicted in its operational mode (<i>V</i>_ds > 1.5 V) when additional nonplanarity is induced in the triphenyl chain. Analysis of the results suggest that, unlike in “voltage” gating, neither the HOMO–LUMO gap nor the dipole moment of the system undergo significant changes due to pure conformational gating, as observed in this study. Instead, the observed conformational gating affects the current via localization/delocalization of the electronic wave function in the conduction channel. Furthermore, the tunneling current corresponding to conformational gating in two different directions appears to exhibit oscillatory nature with a phase factor of π/2 in the presence of the gate field. The current modulation is found to reach its maximum only under exclusive effect of voltage or conformational gating and diminishes when both of them are present

Amino-Acid-Conjugated Gold Clusters: Interaction of Alanine and Tryptophan with Au₈ and Au₂₀

The stability and electronic properties of gold (Au) clusters interacting with the amino acids alanine (Ala) and tryptophan (Trp) in their canonical and zwitterionic configurations were investigated using first-principles density functional theory (DFT). We found that the geometrical structures of the Au clusters and the polarities of the amino acids determine the nature of the interactions in the gas and solvent phases. In the gas phase, the Au₈ (<i>D</i>_4<i>h</i>) and Au₂₀ (<i>T</i>_<i>d</i>) clusters prefer single-site interactions through the amine group for the canonical amino acids, whereas in the solvent phase, the carboxylic site is preferred for the zwitterionic amino acids. The limited screening of the intermolecular interactions introduced by the solvent medium for the canonical forms of Ala and Trp conjugated with the Au_<i>n</i> complexes suggests that the bonding is primarily covalent in nature. The screening is significantly more pronounced for the zwitterionic complexes for which the electrostatic interactions dominate. The cluster sizes and configurations define the extent of the interactions and the overall stability of the complexes. The structures of the Au_<i>n</i> clusters govern the charge distribution and electrostatic potential, directing the selectivity toward the preferential binding sites with the Ala and Trp amino acids

Carbon-Doped Boron Nitride Nanomesh: Stability and Electronic Properties of Adsorbed Hydrogen and Oxygen

Atomic or molecular preferential adsorption on a surface template provides a facile and feasible means of fabricating ordered low-dimensional nanostructures with tailored functionality for novel applications. In this study, we demonstrate that functionality of C-doped BN nanomesh can be tailored by an external electric field which modifies the strength of the adsorbate binding to the nanomesh. Specifically, selective binding of H, O, H₂, and O₂ at various sites of the C-doped nanomeshwithin the pore, on the wire, and at an intermediate siteis investigated with density functional theory. The calculated results find that atomic species are bound, but the molecular species are not bound to the nanomesh. We have shown that it is possible to modify the adsorbate binding energy with the application of an external field, such that the molecular H₂ can be bound at the pore region of the nanomesh. Interestingly, the work function of the nanomesh has a close correlation with the adsorbate binding energy with the BN nanomesh

Hierarchical Self-Assembly of Noncanonical Guanine Nucleobases on Graphene

Self-assembly characterizes the fundamental basis toward realizing the formation of highly ordered hierarchical heterostructures. A systematic approach toward the supramolecular self-assembly of free-standing guanine nucleobases and the role of graphene as a substrate in directing the monolayer assembly are investigated using the molecular dynamics simulation. We find that the free-standing bases in gas phase aggregate into clusters dominated by intermolecular H-bonds, whereas in solvent, substantial screening of intermolecular interactions results in π-stacked configurations. Interestingly, graphene facilitates the monolayer assembly of the bases mediated through the base–substrate π–π stacking. The bases assemble in a highly compact network in gas phase, whereas in solvent, a high degree of immobilization is attributed to the disruption of intermolecular interactions. Graphene-induced stabilization/aggregation of free-standing guanine bases appears as one of the prerequisites governing molecular ordering and assembly at the solid/liquid interface. The results demonstrate an interplay between intermolecular and π-stacking interactions, central to the molecular recognition, aggregation dynamics, and patterned growth of functional molecules on two-dimensional nanomaterials

Degradation of Alkali-Based Photocathodes from Exposure to Residual Gases: A First-Principles Study

Photocathodes are a key component in the production of electron beams in systems such as X-ray free-electron lasers and X-ray energy-recovery linacs. Alkali-based materials display high quantum efficiency (QE), however, their QE undergoes degradation faster than metal photocathodes even in the high vacuum conditions where they operate. The high reactivity of alkali-based surfaces points to surface reactions with residual gases as one of the most important factors for the degradation of QE. To advance the understanding on the degradation of the QE, we investigated the surface reactivity of common residual gas molecules (e.g., O₂, CO₂, CO, H₂O, N₂, and H₂) on one of the best-known alkali-based photocathode materials, cesium antimonide (Cs₃Sb), using first-principles calculations based on density functional theory. The reaction sites, adsorption energy, and effect in the local electronic structure upon reaction of these molecules on (001), (110), and (111) surfaces of Cs₃Sb were computed and analyzed. The adsorption energy of these molecules on Cs₃Sb follows the trend of O₂ (−4.5 eV) > CO₂ (−1.9 eV) > H₂O (−1.0 eV) > CO (−0.8 eV) > N₂ (−0.3 eV) ≈ H₂ (−0.2 eV), which agrees with experimental data on the effect of these gases on the degradation of QE. The interaction strength is determined by the charge transfer from the surfaces to the molecules. The adsorption and dissociation of O containing molecules modify the surface chemistry such as the composition, structure, charge distribution, surface dipole, and work function of Cs₃Sb, resulting in the degradation of QE with exposure to O₂, CO₂, H₂O, and CO

Can Single-Atom Change Affect Electron Transport Properties of Molecular Nanostructures such as C₆₀ Fullerene?

At the nanoscale, even a single atom change in the structure can noticeably alter the properties, and therefore, the application space of materials. We examine this critical behavior of nanomaterials using fullerene as a model structure by a first-principles density functional theory method coupled with nonequilibrium Green’s function formalism. Two different configurations, namely, (i) endohedral (B@C₆₀ and N@C₆₀), in which the doping atom is encapsulated inside the fullerene cage, and (ii) substitutional (BC₅₉ and NC₅₉), in which the doping atom replaces a C atom on the fullerene cage, are considered. The calculated results reveal that the conductivity for the doped fullerene is higher than that of the pristine fullerene. In the low-bias regime, the current (I) voltage (V) characteristic of the endohedral as well as the substitutional configurations are very similar. However, as the external bias increases beyond 1.0 V, the substitutional BC₅₉ fullerene exhibits a considerably higher magnitude of current than all other species considered, thus suggesting that it can be an effective semiconductor in <i>p</i>-type devices

MoS₂ Quantum Dot: Effects of Passivation, Additional Layer, and <i>h</i>‑BN Substrate on Its Stability and Electronic Properties

The inherent problem of a zero-band gap in graphene has provided motivation to search for the next-generation electronic materials including transition metal dichalcogenides, such as MoS₂. In this study, a triangular MoS₂ quantum dot (QD) is investigated to see the effects of passivation, additional layer, and the <i>h</i>-BN substrate on its geometry, energetics, and electronic properties. The results of density functional theory calculations show that the monolayer QD is metallic in nature, mainly due to the coordinatively unsaturated Mo atoms at the edges. This is reaffirmed by the passivation of the S edge atoms, which does not significantly modify its metallic character. Analysis of the chemical topology finds that the Mo–S bonds associated with the edge atoms are predominantly covalent despite the presence of metallic states. A bilayer QD is more stable than its monolayer counterpart, mainly due to stabilization of the dangling bonds of the edge atoms. The degree of the metallic character is also considerably reduced as demonstrated by the <i>I</i>–<i>V</i> characteristics of a bilayer QD. The binding strength of a monolayer QD to the <i>h-</i>BN substrate is predicted to be weak. The substrate-induced modifications in the electronic structure of the quantum dot are therefore not discernible. We find that the metallic character of the QD deposited on the insulating substrate can therefore be exploited to extend the functionality of MoS₂-based nanostructures in catalysis and electronics applications at the nanoscale level

Nature of Interaction between Semiconducting Nanostructures and Biomolecules: Chalcogenide QDs and BNNT with DNA Molecules

Interactions of DNA oligomers with two categories of semiconducting nanostructureschalcogenide quantum dots (QDs) and boron nitride nanotubes (BNNTs)owing to their widespread presence in bio-inspired processes are investigated using the first-principles density functional theory and continuum solvent model. The chalcogenide QDs interact strongly at their metal centers featuring electrostatic interaction with DNA oligomers at oxygen or nitrogen site, while BNNTs form covalent bonds with DNA oligomers at multiple surface sites. It is found that the different bonding nature leads to distinctly different response to the aqueous environment; the presence of solvent drastically reduces the binding strength of nucleobases with the QDs due to the strong electrostatic screening. This is not the case with BNNTs for which the covalent bonding is barely affected by the solvent. This study thus clearly shows how a solvent medium influences chemical interactions providing guidance for technological applications of bioconjugated systems