Oscar E. Vilches

Phase Transitions of Adsorbed Atoms on the Surface of a Carbon Nanotube

Phase Transitions on Carbon Nanotubes The nature of phase transitions changes with system dimensionality. Many aspects of two-dimensional systems have been explored by adsorbing rare gases on graphite surfaces. Wang et al. (p. 552) reduce the dimensionality further by examining phase transitions of argon and krypton on single-walled carbon nanotubes, following the extent of surface coverage and mapping out phase transitions by using the nanotube as a resonator. Changing the conductance and thus the density of surface electrons also allowed exploration of the effect of adsorbate-surface interactions. The adsorption behavior of rare gases is followed through changes in resonance frequency of a single-walled carbon nanotube. Phase transitions of adsorbed atoms and molecules on two-dimensional substrates are well explored, but similar transitions in the one-dimensional limit have been more difficult to study experimentally. Suspended carbon nanotubes can act as nanoscale resonators with remarkable electromechanical properties and the ability to detect adsorption at the level of single atoms. We used single-walled carbon nanotube resonators to study the phase behavior of adsorbed argon and krypton atoms as well as their coupling to the substrate electrons. By monitoring the resonance frequency in the presence of gases, we observed the formation of monolayers on the cylindrical surface, phase transitions within them, and simultaneous modification of the electrical conductance.

Adsorption of H2 and D2 on Carbon Nanotube Bundles

Adsorption isotherms of H2 and D2 deposited on single wall, closed end carbon nanotube bundles have been measured. Two temperature ranges were covered, a) T>77 K to study adsorption on high energy binding sites, and b) T<45 K to characterize adsorption on the graphite-like (graphene) outside surface of the bundles. For reference, N2 and Ar isotherms were measured at T>77 K. We observe two well defined steps in the isotherms that correspond to adsorption on at least two distinct locations, grooves/interstitials and the graphene surface. The calculated isosteric heats of adsorption, Qst, in the grooves/interstitials are approximately twice the value on the graphene.

Apparent new phase of monolayer3He and4He films adsorbed on Grafoil as determined from heat capacity measurements

Heat capacity measurements in monolayer3He and4He films adsorbed on Grafoil at densities higher than the one corresponding to the substrate lattice registered phase show a series of sharp, narrow peaks at 1 K for densities between 0.072 and 0.077Å−2. The exact nature of the transition cannot be determined from this measurement alone, but several possibilities are discussed. It has been determined that the melting line of two-dimensional solid films starts atn=0.078Å−2 for both isotopes. Extensive heat capacity measurements at and above this density are presented for3He, and some new measurements for4He are shown to complement measurements reported elsewhere. The solid3He measurements are compared to predictions of recent models for melting in two dimensions.

Adsorption of on carbon nanotube bundles

Abstract We have measured adsorption isotherms of helium on bundles of single walled carbon nanotubes (SWCNs) in the temperature range from 2 to 14 K . The isotherms and resulting isosteric heat suggest the bundles have sites with differing binding energy for adsorption on the first layer. The results are consistent with the standard interpretation that the bundles have high binding energy sites in either the interstitial channels or the grooves where two tubes meet on the external surface, and a lower energy, graphene like binding to the rest of the external surface. We present a comparison with results of experimental and theoretical studies on the same system.

Specific heat of monolayer films of3He-4He mixtures adsorbed on Grafoil between 0.04 and 4.2 K

The specific heat of mixtures of 3He and 4He adsorbed on Grafoil has been measured at four partial monolayer coverages corresponding to areal densities of 0.0279, 0.0419, 0.0485, and 0.066 »−2 for 0.04 < T < 4.2 K. The 3He concentration in the mixtures was x3 = 0, 0.050, 0.091, 0.201, 0.485, 0.650, 0.908, and 0.999. A feature in the specific heat not seen in measurements with the pure isotopes is found for the two intermediate coverages at about 0.4 K. The measurements are analyzed at the higher temperatures using a theoretical virial expansion for two-dimensional gaseous mixtures. The entropy of mixing calculated by numerical integration of the specific heat curves shows that at 4 K the two isotopes are mixed. While mixing effects develop at temperatures below 1 K, the evidence for phase separation as in bulk mixtures is inconclusive.

Phases of helium monolayers: search and discovery

Abstract In this article we describe from a historical perspective the experimental and theoretical interplay amongst the three authors, their students, and many colleagues which led to the initial discoveries of several of the phases of 4 He and 3 He adsorbed on graphite.

Search for low temperature liquid H2 in 2-D films

Thermodynamic, scattering, and nmr measurements on films of H2, HD, D2, and mixtures of these molecules adsorbed on MgO and graphite have found phase diagrams for individual monolayers with solid-liquid-vapor triple points ranging from 7.2K (for the 2nd layer of H2 on MgO) to 5.74K (for a monolayer of H2 on top of a solid layer of D2 on graphite). We review the experimental evidence for the proposed phase diagrams.

Heat capacity of H2 adsorbed on Ne and on D2 preplated graphite

We have performed heat capacity measurement on H2 adsorbed on graphite preplated with either a monolayer of Ne or a monolayer of D2, to search for low temperature 2D liquid H2. We find evidence indicating that up to 80% of the Ne preplating is displaced by H2, followed by H2 growing on itself. This is in contrast to H2 on D2 preplated graphite in which H2 grows on D2 and possibly 2D liquid H2 exists down to 5.74K.

Heat capacity of monolayer3He adsorbed on H2-plated graphite above 0.2K

The heat capacity of monolayer films of3He adsorbed on 2.15 layers of H2 plating exfoliated graphite has been measured for T>0.2K. Films with densityn<0.03 Å−2 show two-dimensional (2-d) Fermi gas behavior with no condensation, in contrast to similar films of4He which show 2-d condensation below 0.85 K. Forn>0.064 Å−2 the3He films show condensation into apparent solid phases as reported by other research groups. Then-T phase boundaries for these phases has been carefully mapped.

Adsorption isotherms and heat capacity of 4He adsorbed on MgO smoke

Adsorption isotherms at temperatures of 4.23, 3.97, and 3.76 K, and heat capacity measurements for three coverages in the range between 1.0<T<4.2 K are reported for 4He adsorbed on MgO. The isotherms were measured on a substrate consisting of pure MgO smoke in a quartz container. The heat capacity measurements were made on a sample consisting of MgO smoke deposited on quartz fibers, the fibers then enclosed in a quartz calorimeter cell. The isotherms show no special features, being taken considerably above the temperature of any expected fluid-vapor coexistence. The heat capacity measurements show solid-like behavior for the two highest coverages, while the lower coverage run (at approximately 0.029 A−2) shows two-dimensional gas-like behavior. The heat of adsorption determined from the isotherms is qst ≅ 62 K, considerably smaller than for 4He/graphite. Desorption effects are apparent in the heat capacity measurements above about 3 K.