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Atomically resolved single-molecule triplet quenching
The nonequilibrium triplet state of molecules plays an important role in photocatalysis, organic photovoltaics, and photodynamic therapy. We report the direct measurement of the triplet lifetime of an individual pentacene molecule on an insulating surface with atomic resolution by introducing an electronic pump-probe method in atomic force microscopy. Strong quenching of the triplet lifetime is observed if oxygen molecules are coadsorbed in close proximity. By means of single-molecule manipulation techniques, different arrangements with oxygen molecules were created and characterized with atomic precision, allowing for the direct correlation of molecular arrangements with the lifetime of the quenched triplet. Such electrical addressing of long-lived triplets of single molecules, combined with atomic-scale manipulation, offers previously unexplored routes to control and study local spin-spin interactions.
Publication: Science 373, 452 (2021). (Featured in “Perspective” of Science 373, 392 (2021)). PDF Download
Report: Science, Phy.org, Chemistry World
中文报道/介绍：知社学术圈
Magic hydration-number effect on ion transport
Ion hydration and transport at interfaces are relevant to a wide range of applied fields and natural processes. Interfacial effects are particularly profound in confined geometries such as nanometre-sized channels, where the mechanisms of ion transport in bulk solutions may not apply. To correlate atomic structure with the transport properties of hydrated ions, both the interfacial inhomogeneity and the complex competing interactions among ions, water and surfaces require detailed molecular-level characterization. Here we constructed individual sodium ion (Na+) hydrates on a NaCl(001) surface by progressively attaching single water molecules (one to five) to the Na+ ion using a combined scanning tunnelling microscopy and noncontact atomic force microscopy system. We found that the Na+ ion hydrated with three water molecules diffuses orders of magnitude more quickly than other ion hydrates. Ab initio calculations revealed that such high ion mobility arises from the existence of a metastable state, in which the three water molecules around the Na+ ion can rotate collectively with a rather small energy barrier. This scenario would apply even at room temperature according to our classical molecular dynamics simulations. Our work suggests that anomalously high diffusion rates for specific hydration numbers of ions are generally determined by the degree of symmetry match between the hydrates and the surface lattice.
Publication: Nature 557, 701-705 (2018) PDF Download
Report: PKU news (english edition), Nature Review Chemisty, Nature Research Chemistry Community
中文报道/介绍: 北京大学新闻网, 新浪新闻, 新华网, CCTV（视频）, 人民日报, 中国科学网, 光明日报
H-sensitive imaging
Scanning probe microscopy has been extensively applied to probe interfacial water in many interdisciplinary fields but the disturbance of the probes on the hydrogen-bonding structure of water has remained an intractable problem. Here, we report submolecular-resolution imaging of the water clusters on a NaCl(001) surface within the nearly noninvasive region by a qPlus-based noncontact atomic force microscopy. Comparison with theoretical simulations reveals that the key lies in probing the weak high-order electrostatic force between the quadrupole-like CO-terminated tip and the polar water molecules at large tip–water distances. This interaction allows the imaging and structural determination of the weakly bonded water clusters and even of their metastable states with negligible disturbance. This work may open an avenue for studying the intrinsic structure and dynamics of ice or water on surfaces, ion hydration, and biological water with atomic precision.
Publication: Nature Communications 9,122 (2018) PDF Download
Report: PKU news (english edition)
Direct visualization of concerted proton tunnelling
Proton transfer through hydrogen bonds plays a fundamental role in many physical, chemical and biological processes. Proton dynamics is susceptible to quantum tunnelling, which typically involves many hydrogen bonds simultaneously, leading to correlated many-body tunnelling. In contrast to the well-studied incoherent single-particle tunnelling, our understanding of many-body tunnelling is still in its infancy. Here we report the real-space observation of concerted proton tunnelling in a cyclic water tetramer using a cryogenic scanning tunnelling microscope. This is achieved by monitoring the reversible interconversion of the hydrogen-bonding chirality of the water tetramer with a chlorine-terminated scanning tunnelling microscope tip. We found that the presence of the Cl anion at the tip apex may either enhance or suppress the concerted tunnelling process, depending on the details of the coupling symmetry between the Cl ion and the protons. Our work opens up the possibility of controlling the quantum states of protons with atomic-scale precision.
Publication: Nature Physics 11, 235-239 (2015). (Featured in “News and Views” of Nature Physics 11, 216 (2015).) PDF Download
Report: Nature Physics, AsianScientist, Phys.org
中文报道/介绍: 北京大学新闻
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