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Abnormal phase transition between twodimensional high-density liquid crystal and lowdensity crystalline solid phases

    The research team of Hui Li professor publishes an article on Nature COMMUNICATIONS recently. Here the group report an entropy-driven phase transition between a high-density liquid crystal and low-density crystalline solid, directly observed by scanning tunneling microscope in carbon monoxide adsorbed on Cu(111).

    Carbon monoxide (CO) adsorption on Cu(111) is an intensively studied system, mainly because copper-based catalysts are widely employed in many important chemical reactions, such as CO oxidation and methanol synthesis. To date, most studies on CO adsorption have focused on low-coverage regime, where the interactions among CO molecules are negligible. However, when the coverage of co increase to high-coverage, the dipole moments of CO molecules can create large repulsive forces, and bring in lateral pressure. And real industrial environments usually correspond to high-density adsorption under high CO pressure. It is thus of interest to investigate the thermodynamic phenomena in CO/Cu(111) adsorption in the high-density regime. Here, we report direct experimental observation of an entropy-driven phase transition from 2D high-density liquid crystal (HDLC) to low-density crystalline solid (LDC) in a CO monolayer adsorbed on a Cu(111) surface, in the temperature range 5~77K. Combined with first-principles calculations, the group of Hui Li find that repulsive dipole-dipole interactions among CO molecules leads to the unconventional thermodynamics of 2D CO. This anomalous 2D liquid crystal-to-solid transition may provide a platform that can be investigated with atomic detail for exploring the unusual thermodynamics of 2D matter. In addition, their results could provide a theoretical basis for design and development of more efficient copper-based catalysis.