Recently, Luo Peng and his team, who are engaged in postdoctoral research at the University of Pennsylvania in the United States, have successfully fabricated an organic semiconductor glass thin film.
This organosemiconductor glass film combines high density and high stability to help improve luminous efficiency and extend device lifetime.
It is expected to be used as a core material for various OLED (Organic Light-Emitting Diode) displays.
When organic semiconductor glass films are deposited on flexible substrates, they are expected to be used directly on flexible displays.
图 | 罗鹏（来源：罗鹏）
As long as the cooling rate is fast enough, any material can form glass
Glass materials, also known as amorphous materials, refer to a class of materials with disordered microstructures.
In addition to glass, which is mainly composed of silica, such as windows, wine bottles, and optical fibers, plastics, rubber, organic semiconductor light-emitting films for displays, and amorphous alloys are all glass materials.
Glass is a material that pervades every aspect of our daily lives and is one of the materials that has been used by humans for the longest time. The Atlantic once wrote that glass is the most important material for human beings [1].
Glass is usually formed by liquid cooling. As long as the cooling rate is fast enough, any material can form glass.
Due to the microstructure of glass materials like liquids, and thermodynamically in a non-equilibrium metastable state, its internal molecules always move slowly towards the equilibrium state, resulting in the aging of materials, such as the aging and cracking of plastic water pipes.
In 1986, the U.S. space shuttle Challenger exploded 74 seconds after liftoff, killing all seven astronauts on board.
The cause of this disaster is only due to the aging and failure of one rubber ring, which causes cracks, resulting in poor sealing and fuel leakage.
Therefore, improving the stability of glass materials and overcoming aging is one of the most important issues in the glass field.
In addition to liquid cooling, glass materials can also be prepared by physical vapor deposition. Physical vapor deposition is like a game of Tetris, where individual vaporized molecules fall onto the substrate one by one, stacking up to form a thin film material.
In 2007, a team from the University of Wisconsin-Madison reported in the journal Science [2] that glass materials with extremely high density and stability can be obtained by adjusting the substrate temperature of physical vapor deposition.
This type of material is called "ultra-stable glass" because ordinary glass materials prepared by liquid cooling with the same composition need to be annealed for hundreds of millions of years if they want to achieve such high stability.
For example, if a piece of amber that has existed in nature for billions of years is heated to a liquid state and then cooled down, it is returned to the state in which it was first formed.
At this time, it will be found that in these billions of years, its density has increased by 1~2% under the action of temperature. The preparation method of ultra-stable glass can achieve such a density in less than 2 hours.
The formation mechanism of ultrastable glass involves another important topic in the field of glass: surface dynamics. In simple terms, molecules on the surface of glass are at least orders of magnitude more capable of moving than molecules inside.
In the process of vapor deposition, the molecules deposited onto the substrate can rearrange their stacking (structural rearrangement) in a very short period of time before they are covered by subsequent deposited molecules, thus achieving a very high-density stable state.
This special method of forming ultra-stable glass takes advantage of the fact that the surface molecules have a higher ability to move than the internal molecules, so that the molecules can complete the stable state that would take hundreds of millions of years to reach inside the glass in about 1 second.
It can be said that the discovery of ultra-stable glass has completely changed our understanding of the glass formation process, enabling us to effectively control the microstructure, density, stability, mechanics, and optoelectronic properties of glass [3].
As a result, the discovery of ultrastable glass is widely regarded as one of the most exciting recent advances in the field of glass research.
According to reports, the structure and performance of ultra-stable glass can be controlled by the rate of vapor deposition and the temperature of the substrate.
The slower the deposition rate, the longer the molecules deposited onto the surface will have to adjust themselves to achieve a more stable structural arrangement before being covered and completely frozen by subsequent molecules.
The higher the substrate temperature, the higher the motility of the surface molecules, and the greater the thickness of the surface layer with sufficient motility. But at the same time, the thermodynamic driving force tends to be lower in equilibrium.
Therefore, there is an optimal temperature range for the formation of ultra-stable glass. At conventional deposition rates, the substrate temperature at which the glass is optimally stable typically occurs around 0.8 to 0.85 times the glass transition temperature.
In the past decade or so, hard substrates such as silicon or metal materials have been used in the research of ultra-stable glass.
In this case, the formation of ultra-stable glass is basically a "self-assembly" process determined by the ability of the surface molecules to move themselves at the corresponding temperature.
However, apart from using the deposition rate to adjust the size of the time window before the surface molecules lose their ability to move due to covering, there is little additional control over the formation of ultrastable glass.
"Cutting 3000 years to 2 hours"
Different from previous studies, in this study, Luo Peng et al. used a soft silicone material with an elastic modulus of only 1/10,000 of that of silicon and extremely high elasticity as the substrate.
At the beginning of his research, he mainly wanted to find out whether the formation of ultra-stable glass could be caused by the internal stress caused by the hard substrate, and whether the use of soft substrate would destroy the stability of the glass.
Therefore, he first prepared a layer of silicone film several nanometers thick on silicon wafers by spin coating method to be used as a soft substrate.
Ensuring the uniformity and surface flatness of soft substrate films is important for the subsequent accurate and reliable measurement of the thickness and optical properties of deposited molecular glasses.
Subsequently, molecular glass films were prepared on substrates by physical vapor deposition. Ellipsic polarization techniques and synchrotron radiation wide-angle X-ray scattering were used to characterize the structure and properties of deposited glass films.
With the accumulation of experimental data, he found that deposition on soft substrates can not only produce ultra-stable glass, but also has a significantly higher density and stability than glass films deposited directly on silicon wafers.
（来源：Nature Materials）
In other words, Luo Peng used a very soft substrate and obtained a harder glass material. This may seem counterintuitive and completely beyond your expectations.
Later, he designed a number of control experiments and repeatedly verified the results, and finally determined the conclusion.
Overall, he and his colleagues found that the use of soft substrates can significantly improve the density and stability of vapor-deposition organic semiconductor glass films, as well as change their molecular packing and orientation structure, compared to hard substrates.
（来源：Nature Materials）
If you want to deposit on a silicon substrate and achieve such a high density, you need to reduce the deposition rate by at least 10 million times.
This means that by using a soft substrate, he was able to reduce the slow deposition process that took 3,000 years on a hard substrate to 2 hours.
Moreover, further adjustment of the elasticity of the substrate can also control the structure and stability of the glass to a greater extent.
（来源：Nature Materials）
The results show that the soft substrate can significantly improve the movement ability of the molecules on the surface of the glass, accelerate the rearrangement process of the stable structure in the equilibrium state, and thus improve the stability of the glass.
This means that substrate elasticity provides a whole new window into the dynamics of the glass surface, and thus the structure and properties of the glass.
In other words, this method can improve the stability of glass to an unprecedented level. The influence of soft substrates can extend beyond the interface by at least 170 nm, which is a very large range relative to the size of the deposited molecule (1 nm).
（来源：Nature Materials）
In response to the paper, the reviewers commented that this discovery is a significant advance in material development by physical vapor deposition (PVD) and establishes our basic understanding of how soft substrates alter the local dynamics of glass materials.
This marks a paradigm shift in understanding the process of glass formation in vapor deposition. Until now, it has been thought that the nature of the substrate has no effect on vapor deposition glass structures in the range of up to 10 nanometers.
In this work, the above conclusions are clearly refuted by vapor deposition on soft substrates. That is, the deposition of glass materials on soft substrates can obtain glass materials that are far closer to the equilibrium state than in previous studies, so that they can be one step closer to the elusive so-called "ideal glass" state at a reasonable deposition rate.
日前，相关论文以《在软衬底上形成高密度稳定玻璃》（High-density stable glasses formed on soft substrates）为题发表在 Nature Materials[4]。
Peng Luo is the first author, and Zahra Fakhraai, a professor at the University of Pennsylvania, serves as the corresponding author.
图 | 相关论文（来源：Nature Materials）
According to Luo, soft substrates can affect the ability of glass surface molecules to move up to 170 nanometers away from the interface, and there is currently no reasonable theoretical explanation for this phenomenon.
Therefore, he plans to design more experiments to study this phenomenon and further understand the mechanism of action of soft substrates and how this effect is transmitted to such a large scale.
In addition, it is ready to apply the current experimental methods to different other soft substrates and glass systems.
"The ultimate goal of materials science research is to be able to precisely manipulate the formation process of materials at the molecular level, so that the structure and properties of materials can be controlled according to demand," he said. ”
An important implication of this achievement is that the movement ability of surface molecules and the process of surface molecules tending to equilibrium during vapor deposition can be effectively adjusted by other external means.
Therefore, he also intends to find a more direct means to manipulate the self-assembly process of surface molecules during vapor deposition to further improve the stability of glass films.
"Halfway through the experiment, I had a baby"
In addition, Luo Peng recalled: "During this study, I also experienced the birth of a child. ”
It was one day in July 2023, Luo Peng went to the laboratory in the morning to conduct part of the experiment, and accompanied his wife to go for a prenatal checkup at noon.
However, Luo Peng's experiment was not finished yet, and the lunch boxes for lunch were still in the office refrigerator. Although he was only a week away from his due date, he was still not fully prepared psychologically.
The doctor said that after arranging hospitalization, Luo Peng hurried home to get the things he had prepared before, and he was both excited and a little uneasy throughout the whole process. However, fortunately, the child was born without incident.
After he was discharged from the hospital and went home, he took advantage of the time when the child was asleep to go to the laboratory to continue the unfinished experiment.
"For the whole few days, it felt amazing because Penn Hospital was right across the street from my lab, so it felt like I was halfway through the experiment and had a baby, and then I came back and continued the experiment. Luo Peng said.
Of course, he can devote himself to laboratory work without the understanding and support of his wife.
It is also reported that Luo Peng graduated from South China University of Technology and the Institute of Physics of the Chinese Academy of Sciences with a bachelor's degree and a doctorate degree respectively.
He then conducted postdoctoral and visiting research at the University of Illinois at Urbana-Champaign and the National Institute of Standards and Technology.
Since July 2021, he has been a postdoctoral researcher at the University of Pennsylvania, focusing on glass surface dynamics and ultrastable glass.
Soon after, he will join the Institute of Physics of the Chinese Academy of Sciences as a distinguished researcher to continue his research on glass materials and physics.
Resources:
1.HTTPS://www.theatic.com/technology/archive/2018/04/humankinds-most-important-material/557315//
2.https://www.science.org/doi/full/10.1126/science.1135795
3.https://www.annualreviews.org/doi/abs/10.1146/annurev-physchem-042018-052708
4.Luo, P., Wolf, S.E., Govind, S. et al. High-density stable glasses formed on soft substrates. Nat. Mater. (2024). https://doi.org/10.1038/s41563-024-01828-w
Operation/Typesetting: He Chenlong