Q: What made you curious about molybdenum ditelluride (MoTe₂)? Other researchers couldn’t even fabricate it well and you went ahead and developed a new way to make it and then started experimenting with it.

A: Two years ago, I was in a meeting with my research center’s director, Young Hee Lee, and he suggested many materials as candidates for our deep research.
After that I focused on MoTe₂ because it is made from tellurium which has good electrical properties and it is a material that other people haven’t explored. Scientists understand that tellurium is hard to fabricate because it sublimates so easily which leads to many defects. When we focused on this material synthesis, we really tried to avoid the tellurium vacancy which occurs because of the sublimation during the production. This process of using the flux method was published in Nature Physics a few months ago. The flux method isn’t new, but people neglected to use it to make MoTe₂.

Q: Why did the team decide to use the flux method to fabricate MoTe₂?

A: In the Nature Physics article there are several corresponding authors: Director Young Hee Lee, me, and Professor Sung Wng Kim who is one of the material scientists in our department. In a meeting, Sung Wng Kim suggested using the flux method and by adding some extra tellurium we could avoid the deficiency problem.
The flux method is a very common method in material science but I am a physicist, so I wasn’t familiar with the method. In our center we have various backgrounds and once or every two weeks we have an active discussion together.
I suggested that we take a look at MoTe₂ since people had neglected it in the past because they ran into problems with the tellurium vacancy.

Q: As described in the Science paper, you used lasers to change the state of the MoTe₂ from semiconducting 2H to metallic 1T’. How did the team come up with that idea?

A: The question we had was ‘How do we convert the 2H to 1T’ on a 2D scale? Also, how can we make local phase transition? We needed to think about things in the nanoscale. We wanted to make local heating, and thought of several options, but using a laser was the most efficient way to locally heat the area.
It turned out that the laser created a tellurium vacancy because it caused to the tellurium to sublimate and allowed the conversion of 2H to 1T’. Since the tellurium sublimated and created a vacancy, it allowed the phase change at a lower temperature.

Q: How did you start working in this area of physics?

A: I spent 7 years working on my PhD and 2 of those in Paris, France as part of the exchange student system. Paris was really important to me. At Seoul National University (SNU) it was really all about the STM machine and looking at atoms but in Paris it was material study, like MoTe₂.
Usually it is rare to find a researcher who knows both atomic science and also material/device science. For atomic science I studied in SNU, for devices it was Samsung and materials it was in Paris.

Q: Your time in France seems like it had a big impact on you. Do you miss anything from your time there?

A: I miss several things. My research attitude and research style may have been influenced by the French way. Like IBS and many other systems they have a unique culture and style. To me, French people seem talkative and they always discuss things. After lunch they have coffee time and discuss science, they take a long dessert time but discuss and initiate good ideas and good projects. I really miss that relaxed atmosphere for science.
My professor’s friend at the university, Albert Fert, got the Nobel Prize in physics while I was there and Fert did his research in this relaxed French ambience.
Of course I am enjoying my research life, but everything is so busy here, everything is passing too quickly. So I miss this free ambience, but I am still young. I want to actively do my research and this is a big point in some sense.

Q: How did your time interacting with Albert Fert shape your attitude towards doing research?

A: One important thing is that I saw how a Nobel Laureate really does his research. Although he was old, about 70 years old when I first met him, he was comfortable discussing math and logic, even with many undergraduate students. I learned that doing science or physics does not only mean using complicated math or logic. This really gave me a sort of confidence about myself and that I can also do good research. Many Korean scientists admire Nobel Laureates but do not ever really have a chance to see how they work. In this sense, I was really lucky.
Doing science and making simple models to explain phenomena are fun things according to him and this is also my belief about doing science. I like to think, “Fun life with fun physics”. In other words, I am now confident that this attitude can also produce good things like the Nobel Prize.

Q: You are in a unique situation in that you get to work in the same research center as your wife and she is the first author of your most recent paper. Do you think that your working together has any added benefits—or any extra difficulties—that other researchers who aren't married do not encounter?

A: For me, this is a great thing. I can devote myself to my research and my partner understands. As you can imagine, we sometimes discuss common subjects. Personally, I do not have other things than science for now, so I do not see other difficulties as a couple who are scientists. For family things, we are just like other people and I do not see any big differences.

Q: What do you plan to study next?

A: I am still young, so I want to extend my research scope so I voluntarily discuss new ideas and outcomes with other scientists. These days I am working in the energy science department in Sungkyunkwan University and alternative energy is always being emphasized in our department.
Energy generation, like photovoltaic cells are very important and popular so I am trying to meet with scientists who are working with alternative renewable energy, specifically I am interested in hydrogen energy. Lately I have been working with hydrogen production and hydrogen energy.
In the hydrogen economy there are three important parts: production, storage and usage. I am interested in hydrogen production. In middle school students do a simple experiment where they apply some voltage to water and make make oxygen and hydrogen. It is easy to think that it is just like a toy, an experiment only for children. But these days I see that this is how to make hydrogen effectively.
For producing hydrogen, platinum is the best material but it is very expensive and people want to replace it with something else. Actually I found that MoTe₂ is quite promising as an electrode for hydrogen production. I think, as an energy department scientist, that hydrogen production is a good research topic and MoTe₂ is also very good, it isn’t perfect like platinum but we still have some work to do as researchers. I am doing a very primitive experiment with my student now where we attach a battery to MoTe₂ and put it in a beaker and record the voltage to see if it is a viable replacement for platinum…and I am excited about this.

Heejun Yang’s paper Phase patterning for ohmic homojunction contact in MoTe₂ appears in the August issue of Science and Bandgap opening in few-layered monoclinic MoTe₂ appeared in the May issue of Nature Physics

Interviewed by Daniel Kopperud