Two-Dimensional Melting of an Anisotropic Crystal Observed at the Molecular Level

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Abstract
A distinctive two-dimensional (2D) melting transition occurring at nearly 100 degrees Celsius (°C) has been observed in Langmuir-Blodgett films by in situ atomic force microscopy (AFM). A 2D orthorhombic crystal phase melted to a 2D smectic phase at about 91°C. The smectic phase was characterized by 1D molecular periodicity with short-range correlations (about 40 angstroms). At 95°C, the smectic order melted to form a hexatic phase. Infrared spectroscopy measurements were consistent with the AFM observations. These observations support the dislocation-mediated melting scenario for an anisotropic 2D crystal predicted by Ostlund and Halperin. A longer wavelength height modulation was also observed in the smectic and hexatic phases.