Abnormal Phase Transition Between Two-Dimensional High-Density Liquid Crystal and Low-Density Crystal Phases of Carbon Monoxide on Cu(111)*

A few matters in nature show anomalous density change, i.e., their density decreases upon liquid-to-solid transition. Such an anomalous density change can be either attributed to directional intermolecular hydrogen bonds, e.g., in case of liquid water, or to the directional covalent bonds, e.g., in cases of liquid phosphorus, gallium, and silicon. For two-dimensional (2D) matters, e.g., liquid water confined to a nano-slit, a similar anomalous first-order transition between the high density liquid (HDL) and low density amorphous (LDA) ice was predicted based on atomistic molecular simulation. Herein, we report the first direct experimental observation of an entropy-driven phase transition from the 2D high density liquid crystal (HDLC) to low density crystal (LDC) for the carbon monoxide (CO) monolayer adsorbed on Cu(111) surface, in the temperature range of 5~77 K. This anomalous 2D liquid crystal-to-solid transition provides a novel platform for exploring unusual thermodynamics of 2D matters, in addition to the application driven research of copper-based catalysis such as CO oxidation.