On July 27, 2022, the team of Professor Zhou Guangwen of the State University of New York at Binghamton, in collaboration with the team of Professor Judith C. Yang of the University of Pittsburgh and Dr. Dmitri N. Zakharov of Brookhaven National Laboratory, published an article in the journal Nature entitled “Dislocation-induced stop-and-go kinetics of interfacial.” transformations” research results.
This result reports that interface mismatch mismatch climbing delays and leads to stop-and-go interface phase transition behavior during semi-common lattice interface phase transition. Experiments and simulations have shown that dislocation climbing requires long-range diffusion of atoms to the interface dislocation nucleus, during which the interface phase transition is paused until the climb is complete. The corresponding author of the paper is Zhou Guangwen; The first author is Sun Xianhu and Wu Dongxiang.
Most engineered materials have heterogeneous microstructures. They are either balanced by controlling the phases or by the superposition of different materials as in the preparation of thin films. In both processes, in order to achieve a state of thermodynamic equilibrium, the microstructure produces structural relaxation through the interaction of internal interfaces. Unlike the dislocations in the block that increase the energy of the system, the introduction of mismatch dislocations at the heterogeneous interface can release lattice mismatch strains and corresponding strain energies, thereby making the material system tend to a more stable equilibrium state. These mismatches are surrounded by compressed and tensile strain sites around the interface and have a significant impact on the function and structure of heterogeneous systems, such as interface adhesion, mass transfer, and interface charge distribution. Therefore, fundamentally understanding the dynamic coupling between mismatch, interface structure and chemistry, and function has been a long-term research topic. However, due to the fact that mismatches are buried inside the interface of materials, coupled with the difficulty of experimental measurements at the atomic scale, directly detecting dynamic mismatches and dynamic interfaces has been a major challenge. Transmission electron microscopy (TEM) is one of the few tools capable of directly observing buried dislocations, elucidating the mismatches, structure, and interface mismatch at the atomic scale, but all of these studies are based on static structures. Obviously, people are more eager to observe and understand the dynamic behavior of mismatch in the interface during the interface phase transition. But this is nearly impossible to achieve with traditional microscopy, as it requires applying heat and chemistry to drive interfacial transitions, while also capturing rapidly evolving interfaces in real time at the atomic scale.
In order to solve this scientific and technical problem, in this work, Zhou Guangwen’s team used ambient in situ transmission electron microscopy (ETEM) to activate the Cu2O/Cu interface reaction through hydrogen atmosphere and high temperature heating, while observing the Cu2O → Cu interface transition at high temporal and spatial resolution. The Cu2O/Cu interface was chosen as a model system because of its prominent role in many technical areas, such as heterogeneous catalysis, gas sensing, solar conversion, and emission control.
During the experiment, Zhou Guangwen’s team captured a stop-and-go (intermittent) interface phase transition behavior regulated by interface misalignment. By combining these in situ atom-scale observations with density functional theory (DFT) simulations, Zhou’s team proposed that interfacial dislocation delayed interfacial phase transitions are due to the need to compensate for missing metal atoms in the interfacial dislocation nucleus, which need to be diffused over a long distance to interfacial dislocation. Because mismatch represents an important and pervasive structural defect at heterogeneous interfaces, the associated interface phase transitions play a key role in the microstructural evolution of multiple material systems, properties, and reactions, such as metallurgy, thin-film material preparation, catalysis, and corrosion. Therefore, the phenomenon reported in this paper has broad relevance and important engineering significance.
Figure 1: Structure characterization of cu2O/Cu semi-colattice interface.
Figure 2: In situ observation of the flow of atomic steps at the intermittent interface.
Figure 3: Flow of intermittent interface atomic steps mismatched by Mismatch adjustment during Cu2O → Cu interface phase transition.
Figure 4: Cu2O→Mechanism study of intermittent phase transition at Cu interface.
The Cu2O/Cu interface formed by the in situ metal oxide copper single crystal film provides an ideal platform for observing the dynamic interaction of mismatch and interface phase transition. Cu and Cu2O themselves have a large lattice mismatch (about 14.5%), so it is thermodynamically impossible to form a stable common lattice interface. The introduction of a series of mismatches can effectively release the strain energy of the interface, which in turn leads to the generation of semi-common lattice interfaces.
H2 easily reacts with lattice O on the surface of Cu2O to form H2O molecules. As the surface of the H2O molecule is desorbed, the oxidized surface is accompanied by the formation of O vacancies. Since lattice O only needs to overcome a small energy barrier to diffuse from the Cu2O/Cu interface to the surface, the surface O vacancies can also be easily migrated to the Cu2O/Cu interface.
After the secondary deletion of lattice O before the interfacial atomic step, adjacent Cu atoms spontaneously relax transversely toward the O vacancy position, resulting in them being perfectly matched to the underlying Cu lattice. The lateral relaxation of the Cu atom toward the O vacancy position leads to the appearance of Cu vacancy groups before the interfacial atomic step. The formation of Cu vacancy groups is due to the small number of Cu atoms on the Cu2O side matched to the Cu side. It is the Cu vacancy mass produced by these structural relaxations that causes the interfacial atomic step flow spikes to be misplaced at the interface nucleus. Only when the copper vacancy mass is fully filled with additional copper atoms will the interfacial atomic step resume flow and the phase transition continues to occur. Therefore, the overall upward climb of the interface misalignment line to the new Cu2O/Cu interface is a necessary condition for the interface atomic step to resume the phase transition.
Experiments and simulations have shown that the atoms required for dislocation climbing need to diffuse from the Cu matrix over a long distance to the interfacial dislocation nucleus, during which the interfacial phase transition is suspended until the climb is complete. In view of the prevalence of the interfacial step flow mechanism in the solid-solid phase transition process, the important role of mismatch in regulating the dynamics of solid-solid phase transformation may be directly applicable to oxidation reaction, nitriding reaction, vulcanization reaction, silicification reaction, phase precipitation reaction, solid phase displacement reaction, and multi-layer structure formed by mutual diffusion, among which the atomic process of controlling interface phase change has many correlations with this work, including interface step, mismatch mismatch and vacancy auxiliary diffusion.
This work was supported by the Division of Materials Science and Engineering in the Office of Basic Energy Sciences of the U.S. Department of Energy (DE-SC0001135). (Source: Science Network)
Related paper information:https://doi.org/10.1038/s41586-022-04880-1