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Peering into interfacial water
《自然-材料》发表北大量子材料中心江颖等在水科学领域的重要研究进展

The interaction of water with the surfaces of solid materials is ubiquitous. Many remarkable physical and chemical properties of water/solid interfaces are governed by H-bonding interaction between water molecules. As a result, the accurate description of H-bonding configuration and directionality is one of the most important fundamental issues in water science. Ideally, attacking this problem requires the access to the internal degrees of freedom of water molecules, i.e. the O-H directionality. However, resolving the internal structure of water has not been possible so far despite massive efforts in the last decades due to the light mass and small size of hydrogen.

Recently, Ying Jiang and colleagues of InternationalCenter for Quantum Materials (ICQM) of PekingUniversity succeeded to achieve submolecular-resolution imaging of individual water monomers and tetramers adsorbed on a Au-supported NaCl(001) film at 5 K, using a cryogenic scanning tunneling microscope (STM). They first decoupled electronically the water molecule from the metal substrate by inserting an insulating NaCl layer and then employed the STM tip as a top gate to tune controllably the molecular density of states of water around the Fermi level. These key steps enabled them to image the frontier molecular orbitals which are spatially locked together with the geometric structures of water molecules. Notably, they were able to discriminate in real space the orientation of water monomers and the H-bonding directionality of water tetramers based on the submolecular-resolution orbital images.

This work opens up the possibility of determining the detailed topology of H-bonded networks at water/solid interfaces with atomic precision, which is only possible through theoretical simulations in the past. The ability to resolve the O-H directionality of water provides further opportunities for probing the dynamics of H-bonded networks at atomic scale such as H-atom transfer and bond rearrangement. In addition, the novel orbital-imaging technique developed in this work reveals new understanding of STM experiments and may be applicable to a broad range of molecular systems and materials. 

This work was published online in Nature Materials on Jan. 5, 2014 [Nature Materials DOI: 10.1038/nmat3848, http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat3848.html]. This work received supports from Ministry of Science and Technology of China, National Natural Science Foundation of China, Ministry of Education of China, and PekingUniversity.

(a) 3D STM topographic image of water monomers and tetramers adsorbed on the NaCl(001) surface. (b) and (c) HOMO and LUMO images of a water monomer, respectively. (d) and (e) HOMO images of two water tetramers with different H-bonding chirality. (f)-(i) Calculated isosurfaces of HOMO and LUMO orbitals, corresponding to (b)-(e). 

    最近,北京大学量子材料中心、量子物质科学协同创新中心的江颖课题组和王恩哥课题组合作,在水科学领域取得重大突破,在国际上首次实现了水分子的亚分子级分辨成像,使得在实空间中直接解析水的氢键网络构型成为可能。相关研究成果于15日以Article的形式在线发表在《自然-材料》[Nature Materials DOI: 10.1038/nmat3848]。江颖和王恩哥是文章的共同通讯作者,博士研究生郭静、孟祥志和陈基是文章的共同第一作者,物理学院的李新征研究员和量子材料中心的施均仁教授在理论方面提供了重要的支持和帮助。这项工作得到了国家基金委、科技部、教育部和北京大学的资助。

水的各种奇特物理和化学性质与水分子之间的氢键相互作用紧密相关,如何在分子水平上确定水的氢键网络构型是水科学领域的关键科学问题之一。由于氢键的形成主要源于氢原子和氧原子之间的静电作用力(O-H…O),要精确描述水的氢键构型,不仅需要判定氧原子的位置,还必须能识别氢原子的位置,也就是要求能在亚分子级水平上探测水分子在空间中的取向。然而,由于氢原子的质量和尺寸都非常小,对水分子进行亚分子级分辨成像极具挑战性。

过去三年,江颖课题组主要致力于超高分辨的扫描探针显微镜系统的研制和开发,深入到单分子的内部展开亚分子级分辨成像和操控研究,并取得了一系列研究进展:在亚纳米尺度对二维自旋晶格的近藤效应进行了实空间成像 [Science 333,324(2011)];探测到了单个萘酞菁分子内部不同的振动模式的空间分布[J. Chem. Phys. 135,014705(2011)];对单个功能化分子内部的化学键实现了选择性操纵[Nature Chemistry 5,36(2013)]

在此基础上,江颖课题组与王恩哥课题组紧密配合,通过仔细的论证和不懈的探索,成功的把亚分子级分辨成像和操控技术应用到水科学领域,开创性的把扫描隧道显微镜的针尖作为顶栅极(top gate),以皮米(10-12m)的精度控制针尖与水分子的距离和耦合强度,调控水分子的轨道态密度在费米能级附近的分布,从而在NaCl(001)薄膜表面上获得了单个水分子和水团簇迄今为止最高分辨的轨道图像。这使得研究人员可以在实验中直接识别水分子的空间取向和水团簇的氢键方向性。结合第一性原理计算,研究人员发现以往报道的盐表面的水分子团簇都不是最稳定的构型,并提出了一种全新的四聚体吸附结构。

该工作不仅为水-盐相互作用的微观机制提供了新的物理图像,而且为分子间氢键相互作用的研究开辟了新的途径。另外,该工作所发展的实验技术还可进一步应用于原子尺度上的氢键动力学研究,比如质子传输、氢键的形成和断裂、振动弛豫等。

aNaCl(001)薄膜表面吸附的单个水分子和水分子四聚体的三维STM图像。图b和图c分别是单个水分子的HOMOLUMO轨道STM图像。图d和图e是两种具有不同氢键手性的水分子四聚体的HOMO轨道STM图像。图f-i:由第一性原理计算得到的与图b-e相对应的轨道图像。