Multilayer phase diagram ofCF4adsorbed on graphite

Abstract

Layer transitions in CF4 adsorbed on graphite are studied by vapor-pressure isotherm measurements in the range one to eight layers and 52 K & T & 104 K, using an ellipsometric technique to measure the coverage on a single surface. A maximum of two layers adsorb below 71 K. Above 71 K we observe for the first time the condensation of a double-layer (third and fourth), which splits into separate layers above a triple point at 75 K. The fifth layer appears at 80 K, the sixth at 87 K, and at least eight discrete layers are seen over several kelvin above the bulk melting point at 89.5 K. The slopes of the layer chemical potentials versus temperature yield estimates of the locations of phase transitions and of entropy differences.In recent years a rich variety of two-dimensional phases and phase transitions has been found in monolayer physisorbed systems. ' There is evidence of a similar richness in multilayer systems.Attention has focused particularly on wetting, that is, the stability of uniform thick films with respect to bulk condensed phases, 5 on melting of wetting films and its relation to bulk melting, and on layering in wetting films and the relation of the layer critical points to roughening of the bulk phase surface. At intermediate thicknesses the topology of the phase diagrams may involve layer appearance transitions at finite temperatures, prewetting, melting of individual layers or of multilayer phases, ' layer critical points, " and layer coalescence or splitting transitions. ' The last has not been observed previously experimentally.If a particular layer is stable over an appreciable temperature range, there may be additional phase transitions involving changes in structure or molecular orientation. 'The system CF4 on graphite exhibits many of the phenomena just mentioned. X-ray' and neutron' diffraction measurements have been reported, but so far only for the first layer. Zhang, Kim, and Chan (ZKC) have made an extensive heat-capacity study covering the monolayer, ' ' bilayer, ' and multilayer ' regimes. Volumetric isotherm measurements have been reported by Dolle, Matecki, and Thorny, ' including two isotherms below the bulk melting temperature at 89.5 K. Gay, Bienfait, and Suzanne' have studied CF4 on graphite below 50 K by reflection highenergy electron diffraction and interpret their results as showing a wetting transition at 37 K. They also report stepwise attenuation of substrate low-energy electron diffraction intensity, which indicates condensation of a third and possibly fourth layer in isotherms between 51 and 58 K.In this paper we report extensive adsorption measurements for CF4 on a single surface of highly oriented pyrolytic graphite.The CF4 coverage on a 0.1x0.2-mm area of graphite is measured by a phase-modulation ellipsometric technique. ' The ellipsometer output I& at the modulation frequency is directly proportional to coverage, in the range of interest. For very thick uniform films, Ii and the output at twice the modulation frequency I2 should vary periodically with film thickness...

Ellipsometric Study of Multilayer Growth and Wetting of C2Cl2F4 Physisorbed on Graphite

“…The rare gases Ar, , Kr, and Xe 8,9 show complete wetting in both the liquid and solid states. For all other (nonspherical) molecules such as CF 4 and C 2 H 4 a first-order wetting transition has been observed which coincides with the triple point of the bulk system. Carbon monoxide also falls into this class even though wetting does not appear at the liquid−solid transition but at the triple point connected with the structural α−β transition of the bulk system .…”

“…This means that Freon-114 belongs to the group of molecules that show triple-point wetting on graphite. [10][11][12]15 However, unlike CF 2 Cl 2 , the molecule C 2 F 4 Cl 2 does not show metastability and irregular fluctuations in the vicinity of the wetting transition.…”

“…We have shown that C 2 F 4 Cl 2 wets highly oriented pyrolytic graphite completely at temperatures above T t = 179.2 K. Below this temperature, only one (at T = 98.65 K) to four layers (at T = 172.31 K) of C 2 Cl 2 F 4 adsorb on graphite. This means that Freon-114 belongs to the group of molecules that show triple-point wetting on graphite. − , However, unlike CF 2 Cl 2 , the molecule C 2 F 4 Cl 2 does not show metastability and irregular fluctuations in the vicinity of the wetting transition.…”

“…Therefore, the interpretation of such isotherms is difficult. Using ellipsometry to monitor adsorbate thickness, − ,, high-quality multilayer adsorption isotherms of rare gases and smaller molecules have been obtained on highly ordered pyrolytic graphite (HOPG). This method analyzes the change in polarization state of light reflected from the adsorbed film.…”

Adsorption of CF4 on graphite preplated with a monolayer of CF3Cl

“…Previous studies have shown that CF 4 on graphite has a complex phase diagram in both the monolayer 5-7 and the multilayer regimes. 8,9 The monolayer phase diagram proposed by Zhang et al 7 is reproduced in Fig. 1.…”