Columns that reveal the world
- Getting up close and personal with the researchers -

My research topic is inorganic thin films. Many people may not understand what is meant when they are suddenly told about inorganic thin films. Inorganic thin films are very thin coatings of inorganic materials such as metals and ceramics. The thickness of the thin films is about 1 to 100 nanometers. This is only 1/10000 to 1/100000th the thickness of a Japanese hair. In fact, these inorganic thin films are deeply connected to our daily lives, and will play a very important role in future environmental and energy efficiency technologies.

For example, the touch panels used in smartphones and ticket vending machines at stations are made by coating a transparent, electrically conductive thin film on top of an electrically non-conductive polymer film. Also, the tiny computer chips built into various electrical appliances, including personal computers, and even supercomputers, are the culmination of various thin-film technologies. In other words, without inorganic thin-film technology, smartphones and computers would not have been made, and we would not have enjoyed the convenience of today's lifestyle.

In addition, inorganic thin films are also used in the window glass of high-rise buildings, displays, solar cells, and residential building materials. By coating the surface of window glass with a thin film that transmits visible light and reflects infrared rays, heat does not enter the building even when it is exposed to strong sunlight in the summer, leading to energy savings. Research is being conducted on solar cell solar cells to use thin films to generate electricity more efficiently. In addition, as for residential building materials, coating with a thin film of titanium oxide, a photocatalyst, makes it possible to create exterior walls that automatically break down dirt using sunlight. In this way, we are unknowingly exposed to products that use inorganic thin films.

 
Inorganic thin film technology is used in smartphones and a variety of other products.

Inorganic thin films can be divided into several types depending on the method of production and the materials used. Plating, which is used to make accessories and craft parts, is also a type of thin film. In the case of plating, the item to be coated is placed in an electrolyte containing metal ions, and electricity is applied to grow a thin film on the surface. Since the coating is done by putting a substance into the liquid, it is also called liquid phase deposition.

In contrast, the currently mainstream method is vapor phase epitaxy, in which the material used to make the thin film is vaporized and deposited on the surface of a substance. Among vapor phase epitaxy methods, we are focusing on a method called sputtering and are researching technologies for making various thin films. The sputtering method uses an ionized gas called plasma in a vacuum chamber to bombard the surface of a solid such as a metal with accelerated ions, vaporizing the solid at the atomic level and depositing a thin film on the desired substrate.

Thanks to the development of this method, it is now possible to coat uniform thin films on a wide range of objects, from extremely small semiconductor devices known as nano- and micro-devices to building materials several meters in size. The sputtering method has such a wide range of applications that it is said to be applicable to any type of material. Moreover, this method allows for control of crystal structures at the atomic level in a non-equilibrium state, making it possible to create functional thin films with new functions and structures that do not yet exist in the world.

I originally studied the properties of solids in the field of condensed matter physics, and then joined a company and worked at a research institute for nine years. At the institute, I conducted research and development in the field of materials science, focusing on manufacturing, by synthesizing materials and developing devices with special functions. In the process, I also worked on research in the field of inorganic chemistry to develop materials and devices with special functions, but the sputtering method was an interdisciplinary field that required knowledge of both condensed matter physics and inorganic materials chemistry, which I had been working on up until then. I became aware of the fascination with this interdisciplinary field, and began researching the synthesis process of inorganic thin films using the sputtering method.

Synthesizing highly functional inorganic thin films using a sputtering device. Plasma (ionized gas) is generated by discharging electricity in a vacuum chamber, and the resulting ions are used to cause sputtering.

Curiosity is important to me as I conduct my research. There are many fascinating phenomena in the natural world. However, even when we come across interesting phenomena, we often miss them because we don't understand how fascinating they are. One such example is transparent conductive film. There is a deep relationship between the properties and color of a material, and it is generally believed that transparent or white solids, unlike metals, do not conduct electricity. However, transparent conductive film possesses a property that is beyond common sense, being able to conduct electricity despite being a transparent material.

To be amazed or interested in transparent conductive films, you need a certain level of physics knowledge. Some people may think this raises the bar. However, on the other hand, it also means that by studying thoroughly, the world that you find interesting will broaden and deepen.

Metals conduct electricity because they have free electrons that can move around freely inside them. When a voltage is applied to a metal, the free electrons move in the same direction, causing an electric current to flow. These free electrons then reflect and absorb the light that hits their surface, which is why metals appear shiny and glossy to us. However, there are many intriguing questions about what makes a material that is transparent to visible light and conducts electricity, and what the electrons inside it are like.

Currently, thanks to the efforts of many researchers, many types of transparent conductive films have been developed, making it possible to produce touch panels and other devices. We are intrigued by transparent conductive films and have pursued research into them in order to gain a deeper understanding of how they conduct electricity. As a result, we have established a technology for creating transparent conductive films with the desired electrical and thermal conductivity, from films that are close to insulators to those that conduct electricity like metals, by depositing the atoms that make up the thin film on the surface of a material while precisely controlling the deposition of these films. This technology makes it possible to dramatically improve the performance of optical and electronic devices with complex functions.

In addition to transparent conductive films, we are also simultaneously conducting research on thin films with various functions. This is possible because the students in the lab are actively engaged in their research. Many of the lab's students are 4th year undergraduates and many graduate students. When 4th year undergraduates join the lab, they are often unable to move forward without being taught many things, as they are trying to tackle research activities for the first time. However, as they come to understand the appeal of research through discussions with graduate students, they begin to take a more and more proactive approach to their research. In addition, Aoyama Gakuin University places great emphasis on English education, and many graduate students present their research at international conferences held overseas, and actively discuss in English with researchers from around the world who are conducting cutting-edge research. When I see the students' positive attitude, I feel very encouraged and motivated to take on new challenges.

In our laboratory, we are researching synthesis methods for thin films with various properties, such as thermal conductivity, light transmittance, and thermoelectric conversion properties, in addition to electrical conductivity, by controlling the growth conditions of the thin film. With current technology, we can create thin films that exhibit various designed physical properties, but it is even more difficult to give them the ability to reversibly change or switch these physical properties. If we could create thin films whose electrical conductivity, thermal conductivity, light transmittance, etc. could be freely and reversibly controlled while in use, we could realize on-demand functions and greatly contribute to high energy efficiency. If such material technology could be established, it is expected to become a fundamental technology for solving the energy and environmental problems currently facing humanity. I would like to conduct research with students so that such a sustainable, safe and secure society can be realized.

(Published in July 2020)

At the Transparent Conductive Materials Conference (held in Crete, Greece), a graduate student presented his research to researchers from around the world.

Note
College of Science and Engineering has strengthened its English education through the compulsory English subject "English Core" for first and second year students, where students are divided into classes based on proficiency level in the four skills (reading, writing, listening, speaking).
The school offers an "International Program in Science and Engineering" that includes specialized subjects that use a lot of English and has a curriculum that allows students to graduate in four years even if they study abroad for an extended period at a partner or accredited school.
https://www.aoyama.ac.jp/faculty/science/