Phase transitions in monolayer hydrogen and deuterium on graphite

Abstract

Submonolayer phase transitions of parahydrogen and orthodeuterium on graphite have been investigated with positronium annihilation. It is shown that in contrast with conventional specific-heat measurements, quantitative assessments of gas in the mixed commensurate solid plus gas phases are possible, which will lead to a redrawing of the phase diagrams.The study of physisorbed monolayers on smooth graphite surfaces is now a mature field. The adsorption of a range of gases, from hydrogen to xenon, has been well investigated with a variety of methods, including isotherms, specific heats, neutron scattering, low-energy electron diffraction, x-ray diffraction, and others. The nature of the twodimensional ͑2D͒ phase transitions and the role of the underlying carbon substrate have been especially interesting. 1 H 2 and D 2 monolayers on graphite allow one to study ordering phenomena and phase transitions in quantum systems. In both cases at low coverages, a commensurate ()ϫ)) R30°phase forms due to the corrugation of the carbon surface potential, and at coverages near monolayer completion an incommensurate triangular 2D solid phase is created. Specific-heat measurements on submonolayers have allowed phase diagrams to be drawn for H 2 ͑Refs. 2 and 3͒ and D 2 ͑Ref. 4͒ that describe the transitions from 2D gas to commensurate solid plus gas. These measurements, however, based on broad peaks, are not very informative. They do indicate differences between H 2 and D 2 , which have been attributed to differing zero point motions of the quantum particles, but they cannot give quantitative descriptions of the mixed gas/solid phases.In this paper we report on measurements with a new method employing positronium ͑Ps͒ formation, which offers the prospect of detailed examination of these phase transitions.Positronium offers a unique means of studying the behavior of two-dimensional systems. It is well known that positrons injected into metals quickly thermalize by electronic and phononic excitations and may then diffuse to a surface to cause the emission of Ps by capturing a surface electron. 5 In the special case of graphite, Sferlazzo et al. 6 with a positron beam experiment observed that under normal conditions at room temperature little emission occurs. However, their 2-␥ angular correlation study revealed that large amounts of Ps were emitted at high temperatures. The lack at low temperatures was attributed to the band structure of graphite, 7 which showed that parallel momentum conservation would forbid surface electrons at the top of filled bands participating in Ps formation, whereas at high temperatures the abundance could be explained with a mechanism in which momentum conservation was satisfied by the emission and absorption of phonons. 6 The basal plane of graphite offers an ideally smooth substrate for the thermodynamic study of condensed gas mono-layers: their creation and their phase transitions. 8 The exfoliated form of graphite, grafoil, is a leaflike structure ͑of typical dimension, 100 nm͒ 8 with basal plane surfa...

Solvation of carbonaceous molecules by para-H2 and ortho-D2 clusters. I. Polycyclic aromatic hydrocarbons

“…The presence of 2D gas in the commensurate solid phase has also been evidenced by positronium annihilation measurements. 27 In graphene, and similarly to 4 He, the stable phase changes between the √ 3 × √ 3 commensurate solid and the liquid depending on the pressure or surface density. 28,29 For both quantum gases, the reconciling view of supersolidity has also been suggested to occur in adsorbed clusters.…”

Solvation of carbonaceous molecules by para-H2 and ortho-D2 clusters. II. Fullerenes

“…[5][6][7][8][9][10][11][12]16 Besides decreasing temperature, one way of crystallizing assemblies of hydrogen molecules consists of adsorbing them on carbonaceous substrates such as graphite. [17][18][19][20][21] For instance, the submonolayer at 1/3 coverage forms the √ 3 × √ 3 commensurate structure [20][21][22] and finite para-H 2 clusters can be solidlike and also display supersolidity. 23 Freezing on graphitic substrates is largely driven by the stronger binding of the adsorbed molecules, which tends to pin them on particular sites and usually suppress bosonic exchange, [20][21][22]24,25 as also known in the case of helium.…”

“…[17][18][19][20][21] For instance, the submonolayer at 1/3 coverage forms the √ 3 × √ 3 commensurate structure [20][21][22] and finite para-H 2 clusters can be solidlike and also display supersolidity. 23 Freezing on graphitic substrates is largely driven by the stronger binding of the adsorbed molecules, which tends to pin them on particular sites and usually suppress bosonic exchange, [20][21][22]24,25 as also known in the case of helium. 26,27 To a large extent, similar phenomenology has been found to occur a) Electronic mail: florent.calvo@univ-grenoble-alpes.fr for helium around polycyclic aromatic hydrocarbons (PAHs), which can be considered as flat finite-size graphene units.…”

Hydrogen as an Unstable Shallow Donor in Oxides

“…The nature of the two-dimensional phase transitions and the role of the underlying hexagonal carbon substrate have been found to be especially interesting (Bruch et al 1997). We have ourselves examined phase transitions in monolayer hydrogen and deuterium on graphite (Morton et al 2000).…”

“…the temperatures and shapes-depend on the adsorption energies and sometimes the two-dimensional phase changes. For example the curve for hydrogen at the lowest temperatures shows first a rise corresponding to increasing monolayer gas coverage, the plateau due to there being insufficient gas to increase the coverage further, followed by a sharp drop in the Ps formed due to the monolayer suffering a phase change to a mixed phase of commensurate solid plus gas (Morton et al 2000). One can also observe the differences in the proportions of para-Ps and ortho-Ps by comparing the relative heights of S and R in the two figures.…”