Traditional views have long held that the distinction between the solid and liquid states of metals lies in the speed of atomic movement. However, a recent discovery by an international research team is challenging this belief. They found that in extreme temperatures, liquid metal nanodroplets exhibit completely stationary atoms, akin to being “anchored,” which significantly influences the process of metal cooling and solidification.
When metals are in a high-temperature liquid state, internal atoms move rapidly in complex and dense patterns, interacting with each other swiftly, much like the hustle and bustle of people on a crowded street. However, capturing the moment when metal transitions from liquid to solid is extremely challenging, as this phase determines the material’s structure and many of its functional characteristics.
To explore the early stages of metal solidification, scientists from the University of Nottingham in the UK and the University of Ulm in Germany used transmission electron microscopy to observe the process of metal nanodroplets solidifying. They unexpectedly discovered that some atoms remain stationary at extreme temperatures, and the number and positions of these static atoms affect the solidification pathway during cooling.
This new discovery provides fresh insights into mineralization processes, ice formation, and the folding and solidification of protein fibers, while also holding promise for breakthroughs in industries such as pharmaceuticals, aviation, construction, electronics, and catalysis. The research findings were published in the journal “ACS Nano” on December 9th.
The research team had previously produced chemical reaction videos involving single molecules, including the first recording of the breaking and reformation of chemical bonds, allowing the observation of chemical reactions at the atomic level.
In this study, the team utilized high-resolution transmission electron microscopy with spherical aberration and chromatic aberration correction (HRTEM) and developed an innovative method for imaging metal nanograins with atomic-level resolution within a temperature range of 20°C to 800°C.
Initially depositing metal atoms of platinum, gold, and palladium on a graphene substrate, forming nanograins ranging from 3 to 6 nanometers in size, the researchers then heated and observed the melting process into a liquid state.
The experimental results showed that these metal grains melted as expected, with internal atoms beginning rapid motion. However, surprisingly, not all atoms were in rapid motion; some atoms remained stationary as if “pinned” at specific locations on the supporting material.
These positions are known as “point defects,” and the strong metal-carbon bonding makes these stationary atoms exceptionally stable even at high temperatures.
Further investigations revealed that these stationary metal atoms play a crucial role in guiding the solidification of the liquid. By focusing electron beams on graphene, the team found that they could artificially create more of these defects, thereby adjusting the number of anchored atoms in the liquid metal.
Observations indicated that when the number of stationary atoms was relatively low (less than 10) and distributed randomly, metal crystals could smoothly grow from the liquid metal, forming a regular crystal structure until fully solidified. Conversely, a higher number of stationary atoms would create a ring of “atomic corrals” at the edges of the liquid metal, hindering the normal crystallization process.
The encircled liquid platinum metal, even at temperatures as low as 200°C to 300°C (far below platinum’s normal melting point of 1,768°C), remained in a liquid state. Instead of transforming into a regular crystal, it formed an “amorphous solid.” This amorphous metal is highly unstable and can only exist under the continuous restriction of stationary atoms. Once the constraint is released, accumulated stress will be released, reorganizing into crystals.
Researchers stated that previously, nanoparticle confinement techniques were applied to photons and electrons. This achievement is the first to demonstrate that atoms themselves can be confined in a similar manner. In the future, precise control of atomic arrangements on surfaces may lead to the creation of larger and more complex atomic corrals. This precise control could enhance the efficiency of rare metal materials in energy conversion and storage applications.
Professor Andrei Khlobystov, who leads the research team at the University of Nottingham, expressed to the university’s press office, “We typically think of substances having three states: gas, liquid, and solid. The results of this study may herald the emergence of a new type of material, cleverly combining the characteristics of both solid and liquid.”
Professor Ute Kaiser from the University of Ulm’s SALVE Center noted the observation of “wave-particle duality” in electron beams. “We utilize the wave nature of electrons to observe materials and employ their particle nature to deliver pulse energy. These pulses can manipulate atoms, even fixing them at the edge of liquid metal, creating this novel state of material.”
“Wave-particle duality” refers to the fact that electrons exhibit both particle-like characteristics (having definite positions and momentum) and wave-like characteristics (capable of interference and diffraction phenomena).
Dr. Jesum Alves Fernandes, a catalysis expert at the University of Nottingham, added, “Platinum-carbon catalysts are among the most widely used catalysts globally. This phenomenon of confined liquid metal may alter our understanding of how catalysts work and potentially lead to the design of more active and longer-lasting catalysts, thus holding significant practical application potential.”
This work was supported by the German Research Foundation (DFG) EPSRC program (number: EP/V000055/1) and financial backing from the Royal Society of the UK.