2D Quantum Materials Group

Nanoscience and nanotechnology of low-dimensional quantum materials

Interests

Electrons and phonons confined in low-dimensional nanostructures respond differently to external stimuli, leading to phenomena that are distinctly different from those in bulk materials. Low-dimensional nanostructures, therefore, provide a fascinating platform for exploring novel phenomena. We work on various one- or two-dimensional materials, including carbon nanotubes, graphene, transition metal dichalcogenides and their heterostructures. Our goal is to open up the next generation of nanoscience with open-minded thinking that is not limited by conventional frameworks such as physics and chemistry.

Design of novel nanostructures

Given the desired physical properties, we ask, "What kind of structure should be designed?" and "How can we realize these structures?" It is essential to create novel nanostructures by going back to the fundamentals and developing the appropriate processes. By combining (1) advanced crystal growth methods, such as molecular beam epitaxy and metal-organic chemical vapor deposition, with (2) manipulation techniques that precisely stack various nanostructures with micrometer precision, we are able to design a variety of nanostructures with atomic precision. Our goal is to produce functional crystalline solids on demand for the next generation of nanoscience.

Exploring Physical Properties

Our objective is to explore the unique characteristics of low-dimensional quantum materials by analyzing their optical and electronic properties in depth. To accomplish this, we create nanodevices using semiconductor microfabrication techniques, conduct advanced spectroscopic and imaging measurements, and assess electrical conductivity across various external fields. In addition to understanding these fundamental physical properties, we also aim to make advancements in quantum technology. We are particularly enthusiastic about the potential of low-dimensional quantum materials, such as two-dimensional materials, to contribute to the second quantum revolution. Our goal is to develop quantum light sources and quantum sensing devices that can facilitate this revolution by applying the knowledge acquired from our fundamental research.

Papers published

In the following, brief introductions of some of the recent papers are given.
You can see a list of all publications in the following sites.

Google Scholar

Researchers ID

Research Gate

Recent papers (selected)

Gate-modulated spectroscopy

M. Xue et al., Appl. Phys. Lett (2023)
The excited states of excitons were observed with high sensitivity by lock-in detection of reflected light modulated by applying gate voltage. We detected the 2s state, whose signal is usually a weak, not only for excitons but also for charged excitons. In the future, we will selectively detect excitons from local states such as Moire excitons.
Photovoltaic effect in heterostructures

S. Zhang et al., Nanoscale (2023)
We report that photocurrent flows in van der Waals heterostructures even without a PN junction; the IV characteristic is linear, and a photocurrent of 28 nA flows under light irradiation even with zero bias. Photocurrent mapping revealed that this photocurrent originates from the stacking structure.
Post doping towards 2D crystals

Y. Murai et al., ACS Nano (2021)
We have developed a post doping method for two-dimensional crystals. We can dope various elements substitutionally into frameworks of 2D crystals by this method, which will lead to various functional devices and new 2D crystals.
Optical properties of ultraflat MoSe2

T. Hotta et al., ACS Nano (2021) and arXiv submitted on 4 Jun. (2020)
We have established a new method for the preparation of ultraflat 2D crystals. An ultraflat monolayer MoSe2 prepared shows extremely sharp optical transitions and enhanced exciton-exciton collisions, providing an ideal playground for investigating many-body effects in 2D systems.
Exciton diffusion in a 2D crystal

T. Hotta et al., Phys. Rev. B. (2020) and arXiv submitted on 28 Apr. (2019)
We have obesrved exciton diffusion in a 2D crystal, hBN/MoSe2/hBN. We found that the number of scattering sources was small in a high-quality 2D heterostructure, leading to high exciton mobility at low temperatures.
The moire potential in a DWCNT

S. Zhao et al., Phys. Rev. Lett. (2020) and arXiv submitted on 22 Jun. (2019)
We have successfully observed a drastic change in the electronic bandstructure of a DWCNT through Rayleigh scattering spectroscopy on individual DWCNTs. The change in electronic bandstructure arises from long-period moire potential. This paper was selected as an Editor's suggestion in PRL!
Electronic structure of hBN/MoS₂/hBN

Uchiyama, Y. et al., npj 2D materials & applications (2019)
We found that a momentum-forbidden dark state is more stable than the bright exciton in this system due to bandstructure modulation arising from orbital hybridization between MoS2 and hBN. In hBN/MoS2/hBN heterostructures, the most stable exciton is not the bright exciton anymore!
Interlayer excitons in WS₂/MoS₂

Okada, M. et al., ACS Nano 12, 3, 2498 (2018)
The origin of low-energy PL peaks found in PL spectra of WS₂/MoS₂ was addressed. We found that, in a multi-valley system, momentum-forbidden interlayer excitons plays an important role.
Biexcitons in WS2/hBN

Okada, M. et al., Scientific Reports, 7, 322 (2017)
We have observed the efficient formation of biexcitons in WS2/hBN. Enhanced exciton diffusion plays a crucial role in the formation of biexcitons.
Electronic transport in GNRs

Wang, Q. et al., ACS Nano, 10, 1475 (2016)
We have performed in-situ measurements of electronic transport of graphene nanoribbons (GNRs) in a TEM. In this work, we found that the thinning of a graphene nanoribbon down to ca. 2 nm has led to the opening bandgap of ~ several hundreds of milli-electron volts.
MBE growth of monolayer NbSe2

Hotta, T. et al., Appl. Phys. Lett., 109, 133101 (2016)
We have succesfully grown monolayer NbSe2 by molecular beam epitaxy method. This paper was selected as a cover picture of Appl. Phys. Lett.!
WS2/hBN heterostructures

Okada, M. et al., ACS Nano, 8, 8273 (2014)
We have realized WS2/hBN heterostructures with clean interface by the multi-furnace chemical vapor deposition method. We found that ultraflat hBN can strongly suppress inhomogeneous broadening in PL spectra of monolayer WS2.