N2 Monolayer Research Articles

(B3CB3)N2 Monolayer Consisting of CO2-Type Superatomic Molecules with Sandwich Hexacoordinate Carbons.

Searching for a useful way to build stable hypercoordinate carbon species is a great way to enrich carbon-based chemical rules. Here, we extend the super valence bond model into superatom-atom superbonding to theoretically predict a two-dimensional (B3CB3)N2 monolayer, which is the first design of a stable nonplanar hexacoordinate carbon containing material, featuring a sandwich B3CB3 configuration. Chemical bonding analyses indicate that its stability originates from the unique super CO2 structures, where each tricoordinate B3 unit has six delocalized electrons acting as a super oxygen (S2P4), and the B3CB3 unit mimics the bonding pattern of the CO2 molecule via the superatom-atom super double bond. Thus, the (B3CB3)N2 monolayer can be visualized as an assembly of super CO2 units linked by N atoms. The calculated moderate direct band gap (∼1.71 eV) and high light absorption coefficient of the (B3CB3)N2 monolayer make it a promising candidate for further applications in electronics and optoelectronics. This work provides a possible pathway to design hypercoordinate carbon materials via superatom-atom cluster assembly, which is expected to be applied across diverse fields.

Simulation studies of secondary electron yield with electron transport from Cu (110) surfaces containing C2, N2, CO2, or NO2 adsorbates

Secondary electron yields of (110) copper surfaces, covered with either carbon, nitrogen, or their dioxides, have been studied by employing combined first principles methods for the material properties and Monte Carlo simulations for electron transport. Furthermore, by studying electron transport inside the Cu system and modeling the power loss taking account of the inelastic electron scattering within the material, changes in the thermal energy of the system have been modeled. The physical reasons behind the increase and decrease of the yield for each system from an electronic perspective are discussed. In agreement with results observed in studies of secondary electron emission, it is shown that the formation of C2 and N2 monolayers reduce the secondary electron yields, while CO2 and NO2 increase the yield significantly. It is demonstrated that in the case of C2 and N2 formation, changes in the surface electronic barrier reduce the probability of electron escape from the Cu surface, resulting in lower secondary electron emission. Formation of CO2 and NO2, on the other hand, reduce the electronic barrier effects. In addition, due to weak bonding of the CO2 layer with the Cu host, the surface provides an additional source of secondary electrons resulting in higher electronic emission yield. Moreover, the NO2 adsorbate creates a surface electric field that changes the surface electron energy and increases the electron escape probability. Additionally, it is verified that thermal change in the system is negligible and so during secondary electron emission measurements, negligible (if any) surface adsorption or desorption could occur.

Assessment of Mineralogical, Textural, and Water Chemistry Changes During Long-Distance Tailings-Slurry Transport

An exploratory study demonstrated that subtle changes in solids and process water were caused by long-distance turbulent transport of tailings from the concentrator to the impoundment of a Cu (Mo) porphyry copper deposit. Slurry water chemical analysis was complemented by modelling potential phase precipitation or dissolution and speciation of dissolved metals. It was found that transport did not affect major insoluble mineral phases. However, the degrees of liberation of several phyllosilicates, as determined by automated mineralogy, were reduced close to the impoundment, which points to separation of clay particles from tectosilicate surfaces by the turbulence; this also was observed by SEM examination of micro-aggregate specimens. Reduction of maximum particle sizes, increased N2 monolayer adsorption, and resultant specific surface areas indicate that transport modified the micro-aggregates. Major element water chemistry is controlled by the presence of soluble mineral phases, such as gypsum, and reagents, such as lime, in the flotation process. Changes in the dissolved concentrations of some elements could potentially affect tailings deportment in the impoundment. Increased concentrations of Al may affect the clay settling behaviour, while Mo and As levels will require treatment prior to the discharge of water from the tailings impoundment. This study demonstrated that systematic scrutiny of tailings slurries leaving the concentrator and before entering the impoundment can be potentially useful, though care will have to be exercised to replicate sample handling and analytical conditions, during any long-term monitoring.

Structures and thermal properties of the N2 monolayer on Pb(1 1 1)

The physisorption of N2 molecules has long been a model system of molecular adsorption. We present a low-energy electron diffraction (LEED) study of the adsorption structures and thermodynamics of monolayer N2 on Pb(1 1 1). The results indicate that the monolayer structure has a triangular incommensurate center-of-mass lattice, and that the N2-substrate interaction is weaker than that observed on other metal surfaces. The N2 monolayer undergoes a phase transition between an orientationally ordered phase (low-temperature) and an orientationally disordered phase at a temperature of 20 K. Potential energy and quasiharmonic calculations indicate that the weak N2-Pb(1 1 1) interaction is the main contributing factor for the difference in orientational order of incommensurate N2 monolayers on Pb(1 1 1) and other similar metal surfaces.

Thermolubricity of gas monolayers on graphene.

Nanofriction of Xe, Kr and N₂ monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. It was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N₂ confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates.

Orientational ordering in physisorbed molecular layers induced by surface geometry and confinement

AbstractMonte Carlo calculations are performed to study the orientational ordering of a model two‐dimensional molecular admonolayer confined between two monatomic steps of a square substrate. Aside from the orientational influence of the substrate on the molecular axes, we show strong competition between the lateral interactions and the confinement potential due to the steps, which is responsible for more or less abrupt orientational transitions depending on the strength of the various interactions, the width of the confinement, and the temperature. An application to the adsorption of N2 monolayer adsorbed on Cu/CuO stripe substrate is discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

Adsorption and structure of N2 on Pt(111)

The adsorption and structure of monolayer N2 films on Pt(111) have been investigated in a combined experimental and theoretical study. Using specular helium scattering, the heat of adsorption for the unconstrained N2 monolayer was found to be q1=158±5meV per molecule, and the 2D heat of condensation was found to be q2D=21±3meV. At low temperature (20K), the nitrogen molecules arrange in a (3×3) high-order commensurate phase. This structure irreversibly transforms into a (4×4) structure after annealing at 40K or if N2 is directly adsorbed at this higher temperature. Increasing the coverage beyond completion of the unconstrained monolayer, the (3×3) phase experiences a Novaco–McTague rotation of±4.2° out of the [11̄0] high-symmetry direction and, in some cases, a secondary rotation by ∼19.8°. Based on realistic interaction potentials, the structure of the N2 monolayer is investigated by molecular dynamics simulations. The (3×3) and (4×4) structures are obtained as respective equilibrium structures for slightly different coverages with the same total binding energy per molecule in both cases. With increasing temperature, the (4×4) structure becomes the most stable phase. In the (3×3) phase, each unit cell contains four molecules, and the adlayer is aligned with the close packed direction of the Pt(111) substrate. The unit cell of the (4×4) phase contains seven molecules, and the close packed direction of the adlayer is rotated by 19.1° with respect to the substrate [11̄0] direction. Within the two structures, the molecules exhibit a different in-plane herringbone orientational order. Furthermore, the molecular dynamics simulations provide evidence for an orientational transition within the (4×4) phase at about 30K.

Substrate induced crystallization of amorphous solid water at low temperatures

We show that N2 monolayer desorption from ice surfaces is a quantitative, highly sensitive method for following the surface crystallization kinetics at low temperatures. Vapor deposited water films on a crystalline ice substrate exhibit amorphous growth at temperatures below ∼110 K. The rate of crystallization for these amorphous films is dramatically accelerated compared to the rate of crystallization observed for the amorphous films deposited directly on Pt(111). We find that the crystalline ice substrate acts as a two-dimensional nucleus for the growth of the crystalline phase, thereby accelerating the crystallization kinetics.

Diatomic molecules, rotations, and path-integral Monte Carlo simulations: N2 and H2 on graphite

The rotational motion of homonuclear diatomic molecules confined to two dimensions at finite temperatures is discussed within the framework of path-integral Monte Carlo (PIMC) techniques. For single rotators the symmetry restriction on the total wave function coupling nuclear spin and rotations of these diatomic molecules is carried over to PIMC for fermionic and bosonic diatomic molecules. Three experimentally relevant quantum statistical averages are formulated, and quantum effects due to discrete level spacing and exchange are separated with the help of these averages. The method is applied to single N2 and H2 rotators adsorbed on graphite in the frozen-in crystal field which is due to the commensurate (√3×√3)R30° ‘‘2-in’’ herringbone phase. Contrary to H2, exchange effects are negligible for N2 in the relevant temperature range. The resulting sign problem for certain combinations of molecule and averaging procedure is discussed. PIMC simulations of the phase transition from the translationally √3-ordered and orientationally disordered phase to the herringbone phase were carried out for complete N2 monolayers without a symmetry restriction on the wave function. Due to dispersive quantum fluctuations, transition temperature and ground-state order parameter are depressed by roughly 10% as compared to classical MC simulations of the same realistic model. In addition, the PIMC results are compared to quasiharmonic and quasiclassical approximations. The quasiharmonic treatment yields the correct order parameter suppression, the quasiclassical simulation the lowering of the transition temperature, but only the full quantum PIMC simulations describe the entire temperature range of interest correctly.

Features of the Histogram Monte Carlo Method: Application to N2 Monolayer Melting on Graphite

Previously developed methods that determine properties of a system over a range of thermodynamic points, using information accumulated at a single point, are summarized, as is an extension called the multiple histogram method. These strategies are applied to Monte Carlo calculations of melting for N2 adlayers on graphite. The results are used to show the utility of the multiple histogram method and the inadequacy of the single point method for this application to a physical system.

Calculated melting behavior of partial and complete N2 monolayers deposited on graphite.

The melting transition of ${\mathrm{N}}_{2}$ on graphite is examined using a Monte Carlo procedure with several different boundary conditions. On the basis of a simple interpretation of the experimental environment, we are able to explain and accurately reproduce the large change in melting temperature between partial and monolayer coverages.

Predicted properties of the uniaxially incommensurate phase of N2 monolayers on graphite.

A Monte Carlo calculation and an analytic model calculation are used to determine the properties of ${\mathrm{N}}_{2}$ monolayers on graphite at surface densities \ensuremath{\rho} just above that of the 2 \ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 commensurate ground state, with \ensuremath{\rho}=1. At densities 1.006 75\ensuremath{\le}\ensuremath{\rho}\ensuremath{\lesssim}1.0563 it is established that a nonuniform, striped, uniaxial incommensurate, in-plane herringbone phase is thermodynamically stable, and there is evidence that this state may continuously evolve toward the commensurate phase as \ensuremath{\rho} approaches unity, encumbered only by the physical size of the monolayer domains. The width of the uniaxial incommensurate stripes appears to be independent of density and the lattice parameters and molecular orientations in the stripes are consistent with broadly determined experimental values. At densities \ensuremath{\rho}g1.05 the stripes become poorly defined and a modulation around commensurate lattice points of an otherwise uniform uniaxial incommensurate structure occurs. It is speculated that mass-density waves may be supported by this system.