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Towards non-invasive imaging of interfacial water by AFM
江颖、王恩哥等实现表面水的非侵扰式成像

Resolving the hydrogen-bonding structure of interfacial water is crucial for understanding many extraordinary physical and chemical properties of water/solid interfaces. To date, scanning probe microscopy (SPM) has been an ideal tool to visualizethe microscopic structureand dynamicsof water at solid surfaces due to high spatial resolution. However, an intrinsic problem of SPM is that all the probes inevitably induce perturbation to the fragile water structure, due to the excitation of the tunneling electrons and the tip-water interaction forces, especially under the close-imaging condition applied in order to achieve ultrahigh spatial resolution. This limitation makes SPM fall short compared with non-invasive spectroscopic methods.

Now, the teams led by Prof. Ying Jiang and Prof. Enge Wang of International Center for Quantum Materials (ICQM) of Peking Universityfind a method to overcome this intractable problem. As published in Nature Communications on 9th Jan. 2018 (doi:10.1038/s41467-017-02635-5), the researchers report the submolecular-resolution imaging of metastable water nanoclusters on a Au-supported NaCl(001) film by probing the high-order electrostatic force using a qPlus-based noncontact atomic force microscopy (AFM) (Fig. 1), which is almost non-invasive to the weakly bonded water clusters.

“At large tip heights where only the long-range van der Waals and electrostatic forces are detectable, the resolutionof AFMis usually quite poorfor weakly polarized molecules.” says Jiang, “However, the situation could be quite different for the strong polar molecules such as water.”

The AFM images of the water tetramers taken with a CO-terminated tip at large tip-water distance show prominent internalfeatures, which resemble the electrostatic potential distribution, whereas, the STM images are featureless. Comparison with the theoretical simulations by the group of Dr. Pavel JelínekfromInstitute of Physics, the Czech Academy of Sciencesreveals that such a high resolution originates from the high-orderelectrostatic force acting between the quadrupole-like CO-tip and the strongly polar water molecules(Fig. 1).

'The magic of the multipole electrostatic forcelies in that on one hand it has short-range character to yield superb submolecular resolution; on the other hand such a weak force signal is still detectable at relatively large tip-water distance, thus avoiding the disturbance of the tip on the water molecules.' adds Jiang. The minimum perturbation of the probes during AFM imaging could be as small as 40~50meV.This technique was found to be very precise for the structural determination of the weakly bonded water clusters and even their metastable states, which have been very difficult to image before with conventional SPM (Fig. 2).

The weakly perturbative imaging achieved in this work defeats the longstanding limitation in the SPM studies of water at surfaces, and may open up a new era of studying the intrinsic or 'hidden' structures of ice/water on surfaces, ion hydration and biological water with atomic precision.The submolecular-resolution AFM images of water obtained by CO-tip not only provide the spatial information of electrostatics, but also allow us to determine the detailed H-bonding structure including the position of the H atoms, which is crucialfor the understanding of H-bonding interaction and dynamics of water.

This work received supports from Ministry of Science and Technology of China, National Natural Science Foundation of China, National Science Fund for Distinguished Young Scholars, Cheung Kong Young Scholar Program, Praemium Academie of the Czech Academy of Sciences, and the Ministry of Education of the Czech Republic.

Article link: Jinbo Peng, Jing Guo, Prokop Hapala, Duanyun Cao, Runze Ma, Bowei Cheng, Limei Xu, Martin Ondráček, Pavel Jelínek*, Enge Wang*, and Ying Jiang*, 'Weakly perturbative imaging of interfacial water with submolecular resolution by atomic force microscopy', Nature CommunicationsDOI: 10.1038/s41467-017-02635-5 (2018).

 

Figure 1. (a) Experimental setup of a qPlus AFM sensor. (b) Schematic showing the high-orderelectrostatic force acting between the quadrupole-like CO-tip and the strongly polar water molecules.

 

Figure 2. Non-invasive AFM imaging of metastable water dimers (a-d) and trimers (e-h) with a CO-tip. The crooked depressions in the AFM images arise from the electrostatic potential distribution of positively charged H, reflecting the position of the H atoms.

水的氢键构型对于理解水很多奇特的物理化学性质十分关键,但是水的氢键网络非常脆弱,很容易被外界所干扰,实验中如何实现水的非侵扰式探测是水科学领域的一个难题。最近,北京大学量子材料科学中心的江颖课题组、王恩哥课题组和捷克科学院物理研究所Pavel Jelínek课题组合作,实现了表面水分子团簇的非侵扰式原子力显微镜成像,并揭示了一系列弱键合的亚稳态结构。相关研究工作于1月9日在线发表在《自然-通讯》【Nature Communications DOI: 10.1038/s41467-017-02635-5】.

江颖课题组长期致力于超高分辨的扫描探针显微镜系统的研制和开发,近年来在表面水的结构和动力学研究中取得了国际领先的研究成果。然而,由于隧道电子的激发以及针尖-水分子之间的相互作用力,扫描探针不可避免地会对弱键合的水分子结构产生扰动。在这个工作中,江颖等发展了新一代基于qPlus的非接触原子力显微镜(nc-AFM)技术,自行制作了高性能qPlus型原子力传感器(图1),解决了AFM和STM双模式扫描的关键技术问题,实现了飞安级电流和皮牛级力信号的同时探测,将针尖-样品之间相互作用力的探测灵敏度推向了极限。

在此基础上,江颖等进一步利用一氧化碳分子对针尖进行化学修饰,调控针尖的电荷分布,通过探测电四极矩针尖与强极性水分子之间的微弱高阶静电力,获得了弱键合的水分子团簇甚至亚稳结构的亚分子级分辨成像,并在原子尺度上确定了其氢键构型和氢原子的位置(图2)。王恩哥、Pavel Jelínek等通过理论模拟,提出了AFM高分辨成像的新机制,揭示了针尖尖端的电荷分布在对极性分子成像中的关键角色。此工作中发展的非侵扰式成像技术突破了长期以来扫描探针显微镜在界面水研究中的瓶颈,打开了研究弱键合水体系(如:疏水界面、离子水合物、生物水等)的大门,具有非常广泛的应用前景。

该工作得到了国家杰出青年科学基金、国家自然科学基金委重点项目、国家重点研发计划项目的支持。江颖、王恩哥以及Pavel Jelínek为文章的共同通讯作者,彭金波、郭静和Prokop Hapala为文章的共同第一作者。

 

图1:(a) qPlus型原子力传感器的实验装置图;(b) 具有电四极矩电荷分布的一氧化碳针尖与强极性水分子之间的高阶静电力。

 

图2:各种亚稳态的水二聚体(a-d)和三聚体(e-h)的亚分子级分辨成像。AFM图中弯曲的暗环来源于水分子氢原子的静电势分布,据此可以确定氢原子的取向。

 

论文链接:Jinbo Peng, Jing Guo, Prokop Hapala, Duanyun Cao, Runze Ma, Bowei Cheng, Limei Xu, Martin Ondrá?ek, Pavel Jelínek*, Enge Wang*, and Ying Jiang*, 'Weakly perturbative imaging of interfacial water with submolecular resolution by atomic force microscopy', Nature CommunicationsDOI: 10.1038/s41467-017-02635-5 (2018).