Electronic and spintronic devices using two-dimensional materials

An international committee including leading researchers in the field was selected to assess the research project:

• Dr. Enrique Diez Fernández (Universidad de Salamanca)
• Dr. Daniel Neumaier (AMO GmbH)
• Dr. Rafael Morales Arboleya (Ikerbasque)

The defense consisted of a presentation by the candidate on the main aspects of the Ph.D. thesis project followed by an extended discussion based upon the questions that each one of the members of the committee raised in relation to the research work that has been carried out by Mário Ribeiro during his PhD studies. After its final deliberation, the committee decided to award the candidate the Doctor Degree with the highest mention existing at UPV/EHU (cum laude).

After the defense, we asked Dr. Mário Ribeiro to explain us a bit more about his project:

Which was the subject of your thesis?

In my thesis, I dealt with the transport properties of devices that had two-dimensional layered materials in their architecture. I worked with transition-metal dichalcogenides such as molybdenum disulfide and tungsten diselenide, graphene (exfoliated and grown by chemical vapor deposition), and boron nitride. I mainly used transistors and Hall bars as devices templates to study their applicability. Alongside this, I also worked on other projects, both inside and outside the Nanodevices team. In one case, we used organic molecules to spin dope metals, and in another case, in collaboration with an external institute, we developed the large-scale fabrication of lateral metal/insulator/graphene diodes. The latter even lead to a patent.

Why did you choose this subject?

I joined Nanogune’s Nanodevices team in 2014. By then, the field of two-dimensional materials was already ten years into the “graphene revolution”. But even then it was still a rich playground for further fundamental and applied research. There was a huge fuss about these materials because they allowed research teams to easily get a hold on stable atomically thin layers. When you confine electrons to this thinnest possible thickness, new properties emerge without equivalent in bulk systems. Researching in a fast developing emerging field and in a world-class research center was an easy decision for me.

Which methodology/techniques did you use?

In my case, I had to conceive and plan my experiments, design, fabricate, and characterize my devices. Working with 2D materials requires both high-end facilities and some handcraftsmanship. To use pristine 2D materials you need to exfoliate small ultra-thin pieces from bulky millimeter-sized crystals. You do that by using scotch-tape and optical microscopes. It sounds easy, but requires some effort. On the other hand, fabricating the devices after you isolate the desired flakes requires the use of cleanroom facilities. I did this part of the fabrication in Nanogune’s cleanroom. I mainly used electron-beam lithography and photolithography (for chemical vapor deposition graphene) to pattern the nano-to-micrometer sized devices, and I used deposition chambers to evaporate metals, and dry-etching techniques, as reactive ion etching, to shape the flakes/films. I characterized the transport properties of the devices using variable temperature probe stations, or physical property measurement systems, where you have a good control over the sample temperature (down to cryogenic temperatures of 2 K), of the external magnetic-field (up to 9 T), and of the orientation of the sample (for angular dependences). 

Which have been the main conclusions?

I learned many things! That in molybdenum disulfide field-effect transistors there is a crossover between different low-frequency noise regimes via photodoping. That boron nitride working as a gate dielectric improves the performance of ambipolar tungsten diselenide transistors. That graphene is an excellent contact electrode for n-type solution-processed polymers, and due to its low density of states you enable vertical transistors with excellent figures of merit. I also learned that lateral metal/insulator/graphene diodes benefit from the one-dimensional nature of the contact to enable large current densities, strong rectification, with low associated capacitance. And that quantum effects play a strong role in micrometer to millimeter-sized devices of polycrystalline graphene, with large nonlocal signals emerging from the dissipative spin-filtered edge transport under the application of an external magnetic field.

What could be the contribution of your research for present or future nanotechnologies?

Transistors are a key component of modern information technology. Any improvement or further understanding of the limitations to the performance of transistors composed of new materials will certainly help researchers to surpass the current state-of-the-art devices. Regarding spintronics, my work with graphene introduces an unexpected mechanism to take in account when interpreting nonlocal signals as spin signals. Since graphene has been at the core of many studies towards the generation and detection of spin currents without using magnetic elements, I would expect it to be quite relevant to the community.

How do you feel now that you have finished the thesis?

I was very happy that at the end we managed to get some good results. The PhD experience is always challenging, and trying to catch up with a fast moving field is really demanding. I felt relieved. It was also interesting to see that the most impactful publications came unforeseen from side studies. From my experience, it really pays off to keep a good collaboration with your co-workers. By getting involved in different projects you learn more, you diversify your expertise, your networking improves, and you get more ideas. It also puts you under more strain, but the PhD experience is all about you standing up to the challenge (with the help of some luck!).