| Research Abstract |
The momolayr of some simple molecules, which is physically adsorbed on a homogeneous surface of solids such as graphite, forms a two-dimensional molecular solid at low temperatures. We investigate the system by calorimetry, diffraction and neutron scattering. Such adsorbed monolayrs often exhibit phase transitions in the solid phase, which is similar to the case of the bulk solid. However, since the balance of the molecular interactions in this system is rather subtle, many phases appear depending upon the coverage and the temperature.
The systems that we investigated were the monolayrs of isotopic methanes, neopentane, adamantane, sulfuryl fluoride, sulfur hexafluoride, camphor, tetrakis (methylthio) methane and others. All the molecules have globular shape and the orientational ordering at low temperatures is particularly interesting. The X-ray diffraction patterns were obtained for the monolayrs of adamantane, sulfur hexafluoride, camphor and tetrakis (methylthio) methane at ambient and low temperatures. The neutron diffraction patterns were also obtained for the monolayrs of fully-deuterated neopentane, fully-deuterated adamantane and sulfur hexafluoride at low temperatures.
While the detailed structural analysis is now under way, the mechanism of the phase transitions seems to be of orientational order-disorder type. The nature of the phase transition of neopentane monolayr was investigated by inelastic and quasielastic neutron scattering. The dynamics in methane monolayrs were investigated by the rotational tunneling spectra observed by inelastic neutron scattering. Investigation of the dielectric properties of the monolayrs, which is our anotherproject, is now in progress by using hexagonal boron nitride as an adsorbent.
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