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The effect of molecular steric properties on the melting of quasi-two-dimensional solids is investigated by comparing results of molecular dynamics simulations of the melting of butane and hexane monolayers adsorbed on the basal-plane surface of graphite. These molecules differ only in their length, being members of the n-alkane series [CH3(CH2)n_2CH3] where n=4 for butane and n=6 for hexane. The simulations employ a skeletal model, which does not include the hydrogen atoms explicitly, to represent the intermolecular and molecule-substrate interactions. Nearest-neighbor intramolecular bonds are fixed in length, but the molecular flexibility is preserved by allowing the bend and dihedral torsion angles to vary. The simulations show a qualitatively different melting behavior for the butane and hexane monolayers consistent with neutron and x-ray scattering experiments. The melting of the low-temperature herringbone (HB) phase of the butane monolayer is abrupt and characterized by a simultaneous breakdown of translational order and the orientational order of the molecules about the surface normal. In contrast, the hexane monolayer exhibits polymorphism in that the solid HB phase transforms to a rectangular-centered structure with a short coherence length in coexistence with a fluid phase. A significant result of the simulations is that they demonstrate the importance of molecular flexibility on the nature of the melting transition. The formation of gauche molecules is essential for the melting process in the hexane monolayer but unimportant for butane. The effect of molecular length on the qualitative nature of the melting process is discussed for both monolayers. J. INTRODUCTIONFor some time we have been interested in the question of how the melting of quasi-two-dimensional (2D) physisorbed monolayers is influenced by the steric properties of the constituent molecules. 1 It seems plausible that the melting behavior of 2D solids should depend more strongly on molecular shape than in the bulk phase. For example, if one considers a rigid rod-shaped molecule adsorbed with its long axis parallel to a surface, the potential energy barrier to rotation about the surface normal could be quite large compared to the steric hindrance encountered in the bulk phase. This is because molecules in the bulk do not remain confined to or near a single plane. In general, orientational and translational disorder can be achieved more easily than in a monolayer due to exchange of molecules between neighboring layers.Our approach to studying the effect of molecular steric properties on monolayer melting has been to consider a series of isostructural rod-shaped molecules, the n-alkanes [CH3(CH2)n_2CH3], physisorbed on a graphite basalplane surface. We have investigated the dependence of their monolayer melting behavior on the length of the molecule (n) using both neutron scattering and x-ray diffraction techniques. crease in the monolayer melting point as the length of the molecule increases in the series ethane (n = 2 ), butane (n =4), and...

A neutron scattering study of the structure of 4He films adsorbed on graphite is reported. Diffraction from helium monolayers at a temperature of 1.2 K shows the formation of an incommensurate, triangular-lattice solid of high density. As the coverage is increased above two layers, the diffraction pattern changes indicating solidification of a second layer. The observed two-layer patterns can be indexed with either a pair of incommensurate, triangular-lattice solid layers of different densities or a close-packed bilayer; the experimental information available is not sufficient to make a more precise identification. A measurement of the height of the first helium layer above the graphite basal plane was also made. This was done by determining the coverage-dependent shift in the position of the graphite (002) diffraction peak (assumed to arise from interference between film and substrate scattering) and fitting it to a simple structural model. Values for the monolayer height above the graphite plane and for the lattice constants of the possible bilayer structures are given.

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Using high-resolution ellipsometry and stray light intensity measurements, we have investigated during successive heating-cooling cycles the optical thickness and surface roughness of thin dotriacontane (n-C 32 H 66 ) films adsorbed from a heptane (n-C 7 H 16 ) solution onto SiO 2 -coated Si͑100͒ single-crystal substrates. Our results suggest a model of a solid dotriacontane film that has a phase closest to the SiO 2 surface in which the long-axis of the molecules is oriented parallel to the interface. Above this ''parallel film'' phase, a solid monolayer adsorbs in which the molecules are oriented perpendicular to the interface. At still higher coverages and at temperatures below the bulk melting point at T b ϭ341 K, solid bulk particles coexist on top of the ''perpendicular film.'' For higher temperatures in the range T b ϽTϽT s where T s ϭ345 K is the wetting temperature of the bulk phase, the coexisting bulk particles melt into droplets; and for TϾT s , a uniformly thick fluid film wets to the parallel film phase. This structure of the alkane/SiO 2 interfacial region differs qualitatively from that which occurs in the surface freezing effect at the bulk alkane fluid/vapor interface. In that case, there is again a perpendicular film phase adjacent to the air interface but no parallel film phase intervenes between it and the bulk alkane fluid. Similarities and differences between our model of the alkane/SiO 2 interface and one proposed recently will be discussed. Our ellipsometric measurements also show evidence of a crystalline-to-plastic transition in the perpendicular film phase similar to that occurring in the solid bulk particles present at higher coverages. In addition, we have performed high-resolution ellipsometry and stray-light measurements on dotriacontane films deposited from solution onto highly oriented pyrolytic graphite substrates. After film deposition, these substrates proved to be less stable in air than SiO 2 .

Quasielastic neutron scattering experiments and molecular dynamics ͑MD͒ simulations have been used to investigate molecular diffusive motion near the melting transition of monolayers of flexible rod-shaped molecules. The experiments were conducted on butane and hexane monolayers adsorbed on an exfoliated graphite substrate. For butane, quasielastic scattering broader than the experimental energy resolution width of 70 eV appears abruptly at the monolayer melting point of T m ϭ116 K, whereas, for the hexane monolayer, it appears 20 K below the melting transition (T m ϭ170 K). To facilitate comparison with experiment, quasielastic spectra calculated from the MD simulations were analyzed using the same models and fitting algorithms as for the neutron spectra. This combination of techniques gives a microscopic picture of the melting process in these two monolayers which is consistent with earlier neutron diffraction experiments. Butane melts abruptly to a liquid phase where the molecules in the trans conformation translationally diffuse while rotating about their center of mass. In the case of the hexane monolayer, the MD simulations show that the appearance of quasielastic scattering below T m coincides with transformation of some molecules from trans to gauche conformations. Furthermore, if gauche molecules are prevented from forming in the simulation, the calculated incoherent scattering function contains no quasielastic component below T m . Modeling of both the neutron and simulated hexane monolayer spectra below T m favors a plastic phase in which there is nearly isotropic rotational diffusion of the gauche molecules about their center of mass, but no translational diffusion. The elastic scattering observed above T m is consistent with the coexistence of solid monolayer clusters with a fluid phase, as predicted by the simulations. For T/T m у1.3, the elastic scattering vanishes from the neutron spectra where the simulation indicates the presence of a fluid phase alone. The qualitative similarities between the observed and simulated quasielastic spectra lend support to a previously proposed ''footprint reduction'' mechanism of melting in monolayers of flexible, rod-shaped molecules. © 1997 American Institute of Physics. ͓S0021-9606͑97͒50237-6͔