Orientational order in nitrogen monolayers adsorbed on graphite at low temperature

Wang, R.; Wang, S. K.; Taub, H.; Newton, James C.; Shechter, H.

doi:10.1103/physrevb.35.5841

Cited by 44 publications

(25 citation statements)

References 21 publications

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“…At higher coverages an almost uniformly compressed triangular incommensurate phase was observed and the superstructure was assigned to a two-out herringbone lattice [6]. Subsequent neutron diffraction experiments corroborated these results [7]. However, a reinvestigation of the diffraction pattern at monolayer completion revealed that a four-sublattice pinwheel structure provided at least as good a fit to the diffraction intensities as the two-out herringbone structure [8].…”

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confidence: 65%

Orientational Ordering on a Corrugated Substrate: Novel Pinwheel Structure forN2Adsorbed on Cu(110)

et al. 1997

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The structure and orientational ordering of N 2 molecules adsorbed on the Cu(110) surface is investigated in a combined experimental and theoretical study. Using He atom diffraction we find that monolayer N 2 forms a high-order commensurate phase with an oblique unit cell. Minimum energy calculations based on realistic potentials show that the unit cell contains seven N 2 molecules arranged in a novel type of pinwheel structure unexpected for linear molecules adsorbed on a smooth surface. Molecular dynamics simulations corroborate this structure and its remarkable thermal stability. [S0031-9007(97)02476-9] PACS numbers: 68.35.Bs, 34.20.Gj, 79.20.Rf In contrast to adsorbed atoms, nonspherical molecules carry additional degrees of freedom related to their relative orientation on the surface. As a result, the corresponding phase diagram can be quite complex. Besides the positional ordering, various orientationally ordered superstructures may exist as well as new classes of order-order and order-disorder phase transitions. The simplest type of adsorbates to study orientational ordering are diatomic homonuclear molecules, such as H 2 or N 2 . In fact, one of the best studied systems is molecular nitrogen adsorbed on the graphite (0001) basal plane [1].Mean-field theory [2,3] has been used to study the ordering of point quadrupoles on a two-dimensional hexagonal lattice under the influence of a homogeneous crystal field (substrate holding potential V c ). A generic phase diagram is obtained in which four distinct orientationally ordered phases can occur depending on temperature and the relative strength of the holding potential and the quadrupole coupling constant G. (i) In the "two-in" herringbone phase, the molecules are lying flat with two possible in-plane orientations. (ii) In the "two-out" phase the molecules also form a two-sublattice herringbone structure, but the molecular axes are tilted by a finite angle with respect to the surface plane. (iii) At smaller jV c ͞Gj a four-sublattice "pinwheel" structure is obtained, in which one pin molecule standing upright is surrounded by six molecules forming the wheel. However, each of the six wheel molecules belongs to two neighboring pinwheels such that the unit cell contains only four molecules. (iv) Finally, for large positive V c (repulsive potential) a ferrorotational "ferro" phase with a single tilted molecule per unit cell is obtained.The large number of experiments on N 2 and CO adsorbed on graphite has, indeed, revealed the occurrence of all three realistic structures (i)-(iii) [1]. Early LEED studies of N 2 on graphite [4,5] have provided the first clear evidence for a commensurate ͑2 p 3 3p 3 ͒R30 ± and a uniaxially compressed in-plane herringbone structure. At higher coverages an almost uniformly compressed triangular incommensurate phase was observed and the superstructure was assigned to a two-out herringbone lattice [6]. Subsequent neutron diffraction experiments corroborated these results [7]. However, a reinvestigation of the diffraction pattern at m...

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confidence: 65%

Orientational Ordering on a Corrugated Substrate: Novel Pinwheel Structure forN2Adsorbed on Cu(110)

et al. 1997

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“…A well-known particularity of this hexagonal substate is the generally small corrugation ͑a few meV, only͒ experienced by small linear admolecules such as N 2 and CO. Three of the four predicted structures were elucidated. 10 LEED experiments 12-14 corroborated by neutron diffraction results 16 showed the occurrence of commensurate in-plane and out-of-plane herringbone geometries, depending on the N 2 coverage. Reinvestigation of the diffraction pattern for this system at monolayer completion revealed 15 that the signals assigned to the compressed out-of-plane herringbone structure could also be interpreted in terms of a four-sublattice pinwheel structure.…”

Section: B Comparison With Other Systemsmentioning

confidence: 84%

“…At low temperature and in the monolayer regime, low-energy electron diffraction ͑LEED͒ ͑Refs. 12-15͒ and neutron diffraction patterns 16 show the occurrence of a commensurate lattice that may be compressed to a uniaxial incommensurate herringbone lattice. Under further compression N 2 may form nonplanar herringbone geometries or pinwheel structures depending on the N 2 coverage and thus on the magnitude of the lateral interactions.…”

Section: Introductionmentioning

confidence: 99%

N2monolayer and bilayer adsorbed on Ag(110) at 15 K: Structure and orientational ordering

et al. 1998

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The growth and the structure of an N 2 monolayer and bilayer physisorbed on Ag͑110͒ at 15 K were investigated by means of thermal desorption spectroscopy and low-energy electron diffraction ͑LEED͒ and compared with structures determined from interaction potential calculations. The N 2 monolayer desorbs at 40 K with a desorption energy of 104 Ϯ 1 meV while the bilayer desorbs at 32 K with an energy of 73 Ϯ 1 meV. The LEED pattern of the monolayer corresponds to a parallelogram structure. For the bilayer a hexagonal structure was found. An energy minimization procedure based on simulated annealing algorithm gives adsorption energy values very consistent with the experimental data for the monolayer and bilayer. It shows, furthermore, that the monolayer geometry corresponds to a herringbone high-order commensurate phase while the second layer displays two isoenergetical structures: a relatively dilute herringbone phase and a dense hexagonal four-sublattice pinwheel structure. This latter structure has the symmetry observed in the LEED pattern.

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“…Nitrogen and Carbon Monoxide on Graphite. Figure shows most of the phase diagram for submonolayer N 2 on the graphite basal plane. − It is remarkably different from that for O 2 . The phases shown are all 2D solids, with the obvious exception of the 2D vapor.…”

Section: Phase Diagrams and Molecule Orientationsmentioning

confidence: 93%

Monolayers of Linear Molecules Adsorbed on the Graphite Basal Plane: Structures and Intermolecular Interactions

Steele

1996

Langmuir

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The monolayer phase diagrams for a number of nonpolar, linear molecules adsorbed on the basal plane of graphite are reviewed. Values for some of the transition temperatures such as the 2D gas-liquid critical points and the melting points of the low-density 2D solids are collected. Upon comparison of these data with each other and with the analogous data for some spherical adsorbate molecules on graphite, it is argued that the ratios of the 2D critical temperatures to the bulk 3D critical temperatures for these linear molecules are essentially independent of molecular shape and quadrupole moment. The melting points are much more sensitive to the details of the molecular size and shape, but two major categories can be discerned: weakly quadrupolar molecules which form 2D lattices with all molecules lying parallel to one another and are often incommensurate, and strongly quadrupolar molecules that form herringbone lattices which are often commensurate with the underlying substrate lattice. When molecular orientation changes greatly upon melting, it is found that melting points are abnormally low compared to the systems where this does not occur.

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Orientational order in nitrogen monolayers adsorbed on graphite at low temperature

Cited by 44 publications

References 21 publications

Orientational Ordering on a Corrugated Substrate: Novel Pinwheel Structure forN2Adsorbed on Cu(110)

Orientational Ordering on a Corrugated Substrate: Novel Pinwheel Structure forN2Adsorbed on Cu(110)

N2monolayer and bilayer adsorbed on Ag(110) at 15 K: Structure and orientational ordering

Monolayers of Linear Molecules Adsorbed on the Graphite Basal Plane: Structures and Intermolecular Interactions

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