In daily life, the warning sign of "Be Careful and Slippery" has a very high appearance rate. This makes us wonder, "Why is the ice so slippery?" Is there any connection between the skating of the ice and water? Is the reason why the ice surface is so slippery because there is a thin layer of water on the surface?
Weiming Lake in Beida, which is also ice and water. (Picture provided by the research group)
Recently, Professor Jiang Ying, Professor Xu Limei, Field Distinguished Researcher, Academician Wang Enge, etc., from the Quantum Materials Science Center of the School of Physics of Peking University and the light element quantum materials cross platform of the Beijing Huairou Comprehensive National Science Center, have worked closely with each other, using the home-made qPlus scanning probe microscope that has been independently developed and commercialized, The atomic resolution image of hexagonal ice surface, which is the most common in nature, was obtained for the first time. This achievement paper was published in the journal Nature on May 22.
The research team found that the ice surface will begin to melt at minus 153 degrees Celsius. Combined with theoretical calculations, the team revealed the microscopic mechanism of this process, ending the 170 year long debate on ice surface pre melting. The research results were featured in the research bulletin of Nature.
Mysterious Ice Surface
Water is the source of life, and ice, as an important solid form of water, widely exists in nature.
In addition, in interstellar space, ice covered dust particles are the key carriers for the generation of complex organic molecules. Therefore, the study of ice surface is of great significance for exploring the origin of life and material sources. However, due to the lack of atomic scale experimental characterization means, our understanding of the ice surface is still at a very preliminary stage, and even a basic question - what is the surface structure of ice, has not yet been clarified.
In addition, the ice surface often begins to melt at a temperature lower than its melting point (0 ℃), which is called ice pre melting. The pre melting phenomenon is very important for understanding the lubrication phenomenon of ice surface, the formation and life of clouds, and the melting process of glaciers.
The atomic resolution imaging of the bulk ice surface and the pre melting process in real space is the key to understand the pre melting layer and has always been the goal of people.
For many years, scientists generally believed that ice would not appear disorder and melt until its temperature was about 70 degrees below zero (about 200 K). The study of ice surface characteristics has always relied on spectroscopic techniques such as spectral analysis and electron diffraction.
In the micro world, the scanning probe microscope is one of the most reliable "eyes" for us to study the surface structure. The probe of the scanning probe microscope is like a hand with the size of a single atom. If you touch the surface with this "hand", you can get the surface atomic level morphological characteristics, so as to detect the surface structure. Among the numerous members of the scanning probe microscope family, the "qPlus scanning probe microscope" is a leader in atomic level surface structure detection.
Jiang Ying's team has successfully independently developed the domestic prototype of qPlus scanning microscope, and has performed excellently in reducing the amplitude noise of the atomic force sensor and improving the quality factor. A general carbon monoxide molecule modified tip technology has been developed, which can achieve stable atomic resolution imaging on various insulator surfaces. This technology has minimal disturbance to the detection object, which is crucial for the study of water molecules, because the hydrogen bond network of water molecules is very fragile and vulnerable to external interference. This technology has opened up a new direction of "atomic scale water science".
These domestic high-end scientific research equipment and technological innovation provide a new perspective and powerful means for exploring the micro mysteries of ice and other materials, which is expected to reveal the complex structure and unique properties hidden on the surface of ice.
For more than 170 years, the problem of ice surface structure and pre melting layer has remained unsolved. What kind of world is hidden under the mysterious veil of the ice surface? Now, with this domestic "microscopic hand", the mysterious world of ice is no longer out of reach.
Uncover the secrets of domestic "microscopic hand"
Jiang Ying's research group has been committed to the independent research, development and application of high-resolution scanning probe microscope for a long time, innovatively developed a set of qPlus scanning probe technology based on high-order electrostatic force, and took the lead in achieving hydrogen nuclear imaging in the world. In 2022, the research team completed the localization prototype of the qPlus scanning probe microscope, and then transferred the relevant core patents to Beijing Huairou Zhongke Aikomi Co., Ltd. Through joint research of schools and enterprises, the localization of the whole system was achieved.
In this work, the research team further broke through the limitation that the insulator surface cannot be modified in situ with a needle tip, and developed a universal carbon monoxide molecule modified needle tip technology, which can achieve stable atomic level resolution imaging on various insulator surfaces. It is worth mentioning that the domestic scanning probe microscope has obtained higher quality data than the imported equipment, which provides a key support for the structural analysis of the ice surface. Based on the home-made equipment, researchers obtained the atomic level resolution image of ice Ih, the most common surface in nature, for the first time, and realized the accurate recognition of the surface hydrogen bond network and the accurate positioning of the distribution of hydrogen nuclei.
This study found that there are two stacking modes of hexagonal ice (Ih) and cubic ice (Ic) at the base plane of hexagonal ice, which is different from the ideal ice surface that was generally believed to have only one stacking mode of Ih. The Ih and Ic crystal domains are connected by the five - and eight membered ring defects of water molecules to achieve seamless stacking in layers on a nanoscale. By precisely controlling the temperature and pressure of ice growth, researchers found a long-range ordered periodic superstructure on the ice surface, in which Ic and Ih nanocrystalline domains with regular size are alternately arranged.
By analyzing the distribution of hydrogen nuclei on the super structure surface and combining with the first principle calculation, the researchers found that this unique hydrogen bond network structure can significantly reduce the electrostatic repulsion energy between suspended hydrogen nuclei on the ice surface, thus making it more stable than the ideal ice surface. This breakthrough discovery refreshed people's traditional understanding of the ice surface and ended the long-term debate on the ice surface structure and hydrogen order.
In order to further explore the pre melting process of the ice surface, the researchers carried out systematic variable temperature growth experiments and found that the ice surface began to melt at minus 153 ℃ (120 K). At the initial stage of melting, domains of different sizes begin to appear locally in the originally long-range ordered superstructures. With the further increase of growth temperature, the super structure order on the ice surface disappears completely.
At the same time, a large area of surface disorder occurs near the domain boundaries, in which a localized planar cluster structure can often be observed. The theoretical calculation shows that the structure is metastable, and its formation process involves the adjustment of hydrogen bond network in the surface water molecular layer and the fracture of hydrogen bond between layers, resulting in a large area of surface disorder. This structure plays a key role in the initial pre melting process of the ice surface.
Profound influence on many disciplines
This work overturned the traditional understanding of ice surface structure and pre melting mechanism for a long time. The high density distribution of domain boundaries introduced by the ice surface reconstruction promotes the occurrence of pre melting, which makes the ice surface become disordered at a very low temperature (120 K). The temperature generated by this phenomenon is far lower than the pre melting initial temperature (more than 200 K) generally considered in previous studies. Considering that the temperature at the beginning of pre melting is equivalent to the lowest temperature of the earth in the atmosphere, this indicates that in the natural environment, most ice surfaces have been in the disordered state of pre melting or quasi liquid state.
Therefore, to understand various physical and chemical properties related to ice on the earth, surface defects and metastable effects formed in the process of pre melting need to be considered. These discoveries have opened a new chapter in ice science research and will have a profound impact on many disciplines such as materials science, tribology, biology, atmospheric science, interstellar chemistry, etc.
This work was highly recognized and appreciated by three reviewers of Nature. They said that this is one of the most impressive and complete papers that they have read over the years, affirming the innovative application and unprecedented resolution of the qPlus atomic force microscope in ice surface research. Using the qPlus cryogenic atomic force microscope technology to image the ice surface at the atomic level is a major technological innovation, The obtained resolution is unprecedented in ice surface imaging. At the same time, it is pointed out that it is of broad significance to explore the microstructure and pre melting process of ice surface.
They said that these findings may have a profound impact on atmospheric science, material science and other fields.
Relevant paper information: https://doi.org/10.1038/s41586-024-07427-8
