Probing nuclear quantum effects in water with scanning tunneling microscopy and spectroscopy
International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, P. R. China
Quantum behaviors of protons in terms of tunneling and zero-point motion have significant effects on water properties, structure, and dynamics even at room and at higher temperature. In spite of tremendous theoretical and experimental efforts, accurate and quantitative description of the nuclear quantum effects (NQEs) in water is still challenging, due to the difficulty of accessing the internal degrees of freedom of water molecules. Using a low-temperature scanning tunneling microscope (STM), we are able to resolve in real space the internal structure, that is, the O-H directionality, of individual water molecules adsorbed on a solid surface [1,2]. The key steps are decoupling electronically the water from the metal substrate by inserting an insulating NaCl layer and gating the molecular density of states of water around the Fermi level via tip-water coupling. These techniques allow us not only to directly visualize the many-body quantum tunneling of protons within the H-bonded network , but also to quantify the impact of quantum fluctuation on the strength of hydrogen bonds by resonantly enhanced inelastic electron tunneling spectroscopy (IETS) . Our work opens up the possibility of exploring the quantum nature of hydrogen bonds at single-bond limit.
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