Recently, Luo Peng and his team, who are engaged in postdoctoral research at the University of Pennsylvania in the United States, have successfully created an organic semiconductor glass film.

This organic semiconductor glass film combines high density and high stability, which helps to improve luminous efficiency and extend the service life of devices.

It is expected to serve as the core material for various OLED (Organic Light-Emitting Diode) displays.

When the organic semiconductor glass film is deposited onto a flexible substrate, it is expected to be directly applicable to flexible displays.As long as the cooling rate is fast enough, any material can form a glass.

Glass materials, also known as amorphous materials, refer to a category of materials with a disordered microstructure.

In addition to the commonly recognized glass such as windows, wine bottles, and optical fibers whose main component is silicon dioxide, plastics, rubbers, organic semiconductor light-emitting films of displays, amorphous alloys, and so on, all belong to glass materials.

Glass materials permeate every aspect of our daily lives and are also one of the materials with the longest history of human use. The Atlantic has written that glass is the most important material for mankind [1].

Glass is usually formed by the cooling of liquids. As long as the cooling rate is fast enough, any material can form a glass.Glass materials, due to their microstructure resembling that of a liquid and being in a non-equilibrium metastable state thermodynamically, always undergo slow internal molecular movements tending towards an equilibrium state, leading to aging phenomena in the material, such as the aging and cracking of plastic water pipes.

In 1986, the U.S. Space Shuttle Challenger exploded 74 seconds after liftoff, resulting in the loss of all seven astronauts on board.

The cause of this disaster was simply due to the aging and failure of a rubber ring, which developed cracks, leading to poor sealing and fuel leakage.

Therefore, improving the stability of glass materials and overcoming aging is one of the most important topics in the field of glass.

In addition to liquid cooling, glass materials can also be prepared by physical vapor deposition. Physical vapor deposition is like the game Tetris, where individual vaporized molecules fall layer by layer onto the substrate, accumulating to form thin film materials.In 2007, a team from the University of Wisconsin-Madison reported in the journal Science[2] that by adjusting the substrate temperature of physical vapor deposition, it is possible to obtain glass materials with extremely high density and stability.

People refer to such materials as "super-stable glass" because ordinary glass materials prepared by the same composition through liquid cooling methods, if they want to achieve such high stability, would require annealing treatment for hundreds of millions of years.

For example, a piece of amber that has existed in nature for billions of years, if it is heated to a liquid state and then cooled down, that is, to return it to its initial state of formation.

At this time, it will be found that over the billions of years, its density has increased by 1~2% under the influence of temperature. The preparation method of super-stable glass can achieve such a density for the glass within 2 hours.

The formation mechanism of super-stable glass involves another important topic in the field of glass: surface dynamics. Simply put, the molecular motion on the surface of the glass is at least several orders of magnitude higher than that of the internal molecules.During the process of vapor deposition, the molecules that are deposited onto the substrate can rearrange their stacking manner (structural rearrangement) in an extremely short time before being covered by subsequently deposited molecules, thereby achieving a state of very high density and stability.

The special formation method of ultra-stable glass takes advantage of the characteristic that surface molecules have higher mobility than internal molecules. This allows the molecules to reach a stable state within about 1 second, which would take hundreds of millions of years to achieve inside the glass.

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, optoelectronic properties, and so on of glass [3].

According to the introduction, the structure and properties of ultra-stable glass can be adjusted by controlling the rate of vapor deposition and the substrate temperature.The slower the deposition rate, the longer the molecules that have been deposited onto the surface will have to adjust themselves before being covered and fully solidified by subsequent molecules, thus achieving a more stable structural arrangement.

The higher the substrate temperature, the greater the mobility of the surface molecules, and the thickness of the surface layer with sufficient mobility is also greater. However, at the same time, the thermodynamic driving force towards equilibrium is lower.

Therefore, there is an optimal temperature range for the formation of ultra-stable glass. Under conventional deposition rates, the substrate temperature at which the glass can achieve the best stability generally occurs around 0.8 to 0.85 times the glass transition temperature.

In the past decade or so of research on ultra-stable glass, hard substrates such as silicon or metal materials have been used almost exclusively.

In this case, the formation process of ultra-stable glass: is essentially a "self-assembly" process determined by the mobility of the surface molecules at the corresponding temperature.However, apart from using the sedimentation rate to adjust the size of the time window for surface molecules to "lose mobility before being covered," people basically have no additional control over the formation process of ultra-stable glasses.

 

 

 

"Condense 3000 years into 2 hours"

 

Unlike previous studies, in this research, Luo Peng and others used a soft silicone material with an elastic modulus only a few ten-thousandths of silicon material, which has extremely high elasticity, as the substrate.

 

At the beginning of the study, he mainly wanted to understand whether the formation of ultra-stable glasses might be due to internal stress caused by hard substrates, and whether using a soft substrate would destroy the stability of the glass.So, he first prepared an organic silicon film a few nanometers thick on a silicon wafer by spin coating method, to be used as a soft substrate.

 

Ensuring the uniformity and surface smoothness of the soft substrate film is very important for the subsequent accurate and reliable measurement of the thickness and optical properties of the deposited molecular glass.

 

Subsequently, he used physical vapor deposition to prepare a molecular glass film on the substrate. The deposited glass film was characterized in terms of structure and performance using ellipsometry and synchrotron wide-angle X-ray scattering.

 

With the continuous accumulation of experimental data, he found that deposition on the soft substrate could not only prepare ultra-stable glass, but also its density and stability were significantly higher than the glass film deposited directly on the silicon wafer.

 

In other words, Luo Peng used a very soft substrate, but obtained a harder glass material. This seems to violate common sense and is completely beyond his own expectations.Later, he designed numerous control experiments to repeatedly verify the results, ultimately confirming the conclusions of this study.

In general, he and his colleagues found that compared to rigid substrates, using soft substrates can significantly improve the density and stability of vapor-deposited organic semiconductor glass thin films, as well as alter their molecular packing and orientation structure.

To achieve such a high density on a silicon substrate, the deposition rate would need to be reduced by at least ten million times.

That is to say, by using soft substrates, he has shortened the slow deposition process that would take 3000 years on a rigid substrate to just 2 hours.

Moreover, further adjusting the substrate's elasticity can even more extensively control the structure and stability of the glass.The research findings indicate that a soft substrate can significantly enhance the mobility of molecules on the surface of glass, accelerating the rearrangement process towards a stable equilibrium structure, thereby improving the stability of the glass.

This implies that the substrate's elasticity provides a completely new window for controlling the dynamics of the glass surface, and consequently, the structure and properties of the glass.

In other words, this method can elevate the stability of glass to an unprecedented level. The influence of the soft substrate can extend at least 170 nanometers away from the interface, which is a very large impact range relative to the size of the deposited molecules (1 nanometer).

Regarding the related paper, the reviewers commented that this discovery represents a significant advancement in material development through physical vapor deposition (PVD), and it has established our fundamental understanding of how soft substrates can change the local dynamics of glass materials.

This marks a paradigm shift in understanding the glass formation process in vapor deposition. Up to now, it has been assumed that the properties of the substrate have an influence range on the structure of vapor-deposited glass of no more than 10 nanometers.This work, by performing vapor deposition on a soft substrate, clearly refutes the aforementioned conclusion. That is, deposition on a soft substrate can yield glass materials much closer to the equilibrium state than in previous studies, thus bringing us a step closer to the elusive so-called "ideal glass" state under a reasonable deposition rate.

Recently, the related paper was published in Nature Materials[4] with the title "High-density stable glasses formed on soft substrates."

Luo Peng is the first author, and Professor Zahra Fakhraai from the University of Pennsylvania is the corresponding author.

Luo Peng stated that the soft substrate can affect the mobility of glass surface molecules 170 nanometers away from the interface, and there is currently no reasonable theoretical explanation for this phenomenon.

Therefore, he is preparing to design more experiments to study this phenomenon, to further understand the mechanism of action of the soft substrate and how this effect is transmitted over such a large range.Additionally, they are preparing to apply the current experimental method to other different soft substrates and glass systems.

 

He stated: "The ultimate goal of materials science research is to be able to precisely control the formation process of materials at the molecular level, thereby regulating the structure and properties of materials according to demand."

 

An important insight brought by this achievement is that the mobility of surface molecules, as well as the process of surface molecules tending to equilibrium during the vapor deposition process, can be effectively adjusted by other external means.

 

Therefore, he also intends to find more direct means that can regulate the self-assembly process of surface molecules during the vapor deposition process, in order to further improve the stability of glass films."Midway Through the Experiment, a Child Was Born"

Additionally, Luo Peng recalled: "During this research period, I also experienced the birth of a child."

It was a day in July 2023 when Luo Peng conducted part of the experiment in the lab in the morning and accompanied his wife for a prenatal checkup at noon. There was a minor issue during the checkup, and for safety reasons, the doctor required the hospitalization for childbirth after the assessment.

However, Luo Peng's experiment was not yet completed, and the lunchbox he had for lunch was still in the office refrigerator. Although there was only one week left until the expected delivery date, he was not mentally fully prepared.

After the doctor arranged for the hospitalization, Luo Peng hurried back home to get the things he had prepared in advance, feeling both excited and a bit uneasy throughout the process. Fortunately, the child was born smoothly.After being discharged and returning home, he took advantage of the child's sleep to go to the laboratory to continue the unfinished experiments.

"Over the past few days, it feels quite magical, because the Penn Hospital is right across from my laboratory, so it feels like I was doing experiments halfway and then went to give birth to a child, and then came back to continue the experiments," said Luo Peng.

Of course, his full commitment to the laboratory work is inseparable from his wife's understanding and support.

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 and a doctorate respectively.

Subsequently, he successively carried out postdoctoral research and visiting research at the University of Illinois at Urbana-Champaign and the National Institute of Standards and Technology in the United States.Since July 2021, he has been a postdoctoral research fellow at the University of Pennsylvania, focusing primarily on the dynamics of glass surfaces and ultra-stable glasses.

Soon after, he will join the Institute of Physics, Chinese Academy of Sciences, as a distinguished research fellow, to continue his research in glass materials and physics.