Phase transitions of ethylene monolayers on graphite

Abstract

Low-energy electron diffraction (LEED) measurements have detected seven solid phases for monolayer ethylene on graphite. Electron-beam induced effects (which presumably include decomposition and/or polymerization of the adsorbed ethylene) were minimized by exposing the adsorbed layer to the nanoamp beam for <100 s per crystal; data were obtained from four different crystals. We concentrate here on the phase transitions from the molecular-axisorientationally disordered low density (DLD) phase to the newly discovered commensurate molecular-axis-disordered low density (CDLD) phase and on the melting of the CDLD phase. The change in the principle q vector in the DLD-CDLD phase transition is presented for several different coverages. These data indicate an interesting incommensurate–commensurate (IC–C) transition which has not been suspected from previous studies. This transition preempts the melting transition: as a consequence we can rule out the possibility of a continuous melting transition as suggested by earlier heat capacity and neutron scattering experiments. In addition, LEED images of the fluid phase produced by melting of the CDLD phase are presented which show a modulated-ring-type structure.

Ordered phases of ethylene adsorbed on charged fullerenes and their aggregates

“…One was observed when the low-density phase undergoes a transition from orientationally disordered incommensurate to orientationally disordered commensurate, although not to the commensurate phase because the molecules are too large when they lie flat. Contrary to conclusions drawn in earlier work the authors argued that this phase melts discontinuously [16,17] . Two additional commensurate, orientationally disordered phases were identified by Eden and Fain at high density, with the molecules being in the T-shaped configuration [16,17] .…”

“…Contrary to conclusions drawn in earlier work the authors argued that this phase melts discontinuously [16,17] . Two additional commensurate, orientationally disordered phases were identified by Eden and Fain at high density, with the molecules being in the T-shaped configuration [16,17] . One of them, the commensurate phase, formed at temperatures above 58 K and densities ≈0.9 ⩽ x ⩽ ≈1.03.…”

“…Above 35 K the molecules rotate freely around an axis normal to the surface thus destroying the herringbone pattern. According to an electron diffraction study by Eden and Fain [16,17] , the commensurate phase appears at temperatures above 58 K and densities ≈0.9 ⩽ x ⩽ ≈1.03; the phase melts below 70 K.…”

“…Molecular dynamics studies supported the idea that the corrugation of the graphite surface has a negligible effect on the structure of the ethylene adsorbate [7,13,15] . However, in 1991 Eden and Fain published low-energy electron diffraction data which revealed the existence of three additional solid phases at high temperatures [16,17] . One was observed when the low-density phase undergoes a transition from orientationally disordered incommensurate to orientationally disordered commensurate, although not to the commensurate phase because the molecules are too large when they lie flat.…”

Occurrence of two-dimensional smectic liquid crystal inn-propanol adsorbed on graphite

“…My, 61.30.Eb, 64.70.Dv, 68.35.Rh The prediction of dislocation-mediated melting theory [1-3] that the melting of a two-dimensional (2D) solid could be fundamentally diflFerent from that observed in ordinary three-dimensional (3D) systems has caused a hot controversy among both theorists and experimentalists in the past decade [4][5][6][7][8][9][10][11][12]. Most of these studies, however, are concerned with isotropic monolayers and the melting of an anisotropic monolayer seems to have received little attention to date [7,8,10,13].…”