CHEMICAL SCIENCE

Chinese scientists have made important achievements in visualizing atomic manufacturing


Recently, the team of Professor Sun Litao of Southeast University, together with the team of Professor Zheng Haimei of lawrence Berkeley National Laboratory in the United States and professor Haiping Fang of East China University of Science and Technology, combined with experiments and molecular simulations, revealed for the first time the mechanism of the complete solid-liquid-gas three-phase reaction in the etching process from the atomic scale.

On May 26, 2022, the study was published in Nature Materials under the title “Solid–liquid–gas reaction accelerated by gas molecule tunnelling-like effect” The research team also used this document to celebrate the 120th anniversary of Southeast University.

The corresponding authors of the work are Professor Sun Litao of Southeast University, Professor Zheng Haimei of Lawrence Berkeley National Laboratory, and Professor Fang Haiping of East China University of Science and Technology. Dr. Wang Wen (currently working at Zhengzhou University), Associate Researcher Xu Tao, and Associate Researcher Chen Jige are the co-first authors.

Based on the in situ electron microscopy system, the research team observed the whole process of accelerated (~20-fold) wet etching of nanobubbles in real time, and for the first time revealed the complete solid-liquid-gas three-phase reaction mechanism in the etching process from the atomic scale, providing a new means of realization and manufacturing principle for the development of efficient and high-precision manufacturing processes and methods.

Wet etching is widely used in important fields such as semiconductor manufacturing, but the direction of wet etching is limited in selectivity, and it is difficult to obtain micro-nano structures with precise and controllable dimensions. The solid-liquid-gas reaction at the micro-nano scale is the basic physico-chemical process in the manufacture of integrated circuits, and also involves key processes such as cleaning and polishing in transistor processing. Current advanced transistor devices such as 7nm and 5nm have strict sub-nanometer accuracy requirements for the geometric dimensions of internal metals, semiconductors, and dielectric layers. Limited by the means of characterization, the above process development can only rely on offline detection methods to characterize. The results of this study play a fundamental supporting role in establishing process parameter-structure size models and accelerating process research and development.

The solid-liquid-gas three-opposite involved in the study should be widely present in nature and industry, in addition to wet etching, there are also atmospheric corrosion, biological aerobic respiration, photocatalysis, fuel cells and so on. Due to the difficulty of tracking the evolution of individual particles and the three-phase interface at the nanoscale, there has been a lack of quantitative analysis of reaction kinetics and an accurate understanding of gas transport mechanisms at three-phase interfaces. Professor Sun Litao’s team used electron beam spokes to lysate water to generate oxygen bubbles, and constructed and observed in real time the solid-liquid-gas three-phase reaction of oxygen bubble etching gold nanorods in hydrogen bromide aqueous solution (as shown in Figure 1).

Figure 1: Schematic diagram of a solid-liquid-gas reaction established in a liquid pool.

It was observed that when there were no nanobubbles around the gold nanorods, the nanorods were gradually oxidized and etched into a smooth ellipsoid on the surface and eventually disappeared; but when there were nanobubbles around the gold nanorods, the nanorods near the location of the nanobubbles were accelerated etched and evolved into a locally concave structure. It is worth noting that when local depression occurs, the nanorods and nanobubbles are not in direct contact, and there is an ultra-thin liquid film between the two (as shown in Figure 2). Quantitative analysis of a large number of experimental results shows that the etch rate is significantly increased (above an order of magnitude) only when the distance between the nanobubble and the solid is less than the critical size (~1 nm); otherwise, the etch rate is almost unchanged. The discovery that there is a critical distance between nanobubbles involved in etch reactions subverts the traditional belief that “the closer the bubbles are to the solid reactants, the faster they react.”

Figure 2: Etching process of gold nanorods in the presence of oxygen nanobubbles.

Figure 3: Etching process when there are oxygen nanobubbles on top of nanorods.

Professor Fang Haiping of East China University of Science and Technology and Associate Researcher Chen Jige of the Shanghai Institute for Advanced Study of the Chinese Academy of Sciences, etc., using classical molecular dynamics and first-principle molecular dynamics simulations, pointed out that the presence of nanobubbles did not affect the adsorption position of bromine ions on the surface of gold nanorods, and the adsorption of oxygen molecules released from nanobubbles on the surface of gold nanorods is the key to accelerating the reaction. When the thickness of the liquid layer between the surface of the nanobubble and the gold nanorod is greater than ~1 nm, the oxygen molecules released by the nanobubbles pass through the liquid layer through the concentration gradient-led diffusion to the surface of the gold nanorod, which is slower. However, when the thickness of the liquid layer between the nanobubble and the surface of the gold nanorod is reduced to ~1 nm, the transport process of oxygen molecules has a “like-like penetration” effect, and the oxygen molecules pass through the liquid layer at a very high speed and are adsorbed to the surface of the gold nanorod, thereby greatly accelerating the etching reaction.

The study reveals for the first time a complete solid-liquid-gas reaction path at the atomic scale: (1) when the liquid layer thickness is greater than the critical value, oxygen molecules undergo concentration gradient-led diffusion in the liquid layer; (2) when the liquid layer thickness is less than the critical value, oxygen molecules are rapidly adsorbed on the solid surface under the action of van der Waals force; (3) oxygen molecules participate in chemical reactions on the solid surface (as shown in Figure 4). This achievement makes it possible for the wet etching technology to greatly improve the controllability of the etching direction and size, and it is also very likely to develop into a new technology in the field of micro-nano machining in the future. In addition, the researchers proposed several methods suitable for improving the three-phase reaction in different scenarios, which is of great significance for the future regulation of micro-nano processing and heterogeneous catalysis involving solid-liquid-gas three-phase processes.

Figure 4: Solid-liquid-gas etching mechanism of gold nanorods.

In order to verify the universality of the mechanism, Professor Sun Litao’s team also studied the etching of oxygen bubbles on palladium nanocubes in aqueous hydrogen bromide solution, and reached a consistent conclusion.

This work has been supported by the National Outstanding Youth Fund Project, the National Major Scientific Research Instrument and Equipment Development Project, the National Natural Science Foundation of China International Cooperation Project, the National Natural Science Foundation of China, the Shanghai Municipal Natural Science Foundation and other projects. (Source: Science Network)

Related paper information:https://doi.org/10.1038/s41563-022-01261-x



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