Symmetry Engineering of Two-dimensional Materials and Their Transport and Optoelectronic Properties

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主論文の要旨
論 文 題 目 Symmetry Engineering of Two-dimensional Materials and
Their Transport and Optoelectronic Properties (対称性を制御した二次
元物質の光電子物性）
氏

名 張 紹春 (ZHANG Shaochun)

論 文 内 容 の 要 旨
Since the discovery of graphene, research on two-dimensional (2D) materials has undergone
rapid development owing to its unique properties, thus widening the landscape of fundamental
research and technological advances. A rich variety of marvelous physical phenomena have been
discovered in 2D materials, especially in transition metal dichalcogenides (TMDs) and their
heterostructures, such as piezoelectricity, superconductivity and Hall effect, etc. Symmetry in
TMDs crystals, including rotational symmetry, mirror symmetry, and inversion symmetry, is
essential to determinate their properties. Symmetry breaking in 2D materials play a crucial role
in modulating the physical properties.
Meanwhile, the photovoltaic effect (PVE), which emerges in noncentrosymmetric materials,
has ignited the search for new PVE materials. Typically, PVE appears in inhomogeneous
semiconductors with p–n junctions, where the spontaneously formed built-in potential generates
a photovoltage in response to light irradiation. In contrast, the bulk photovoltaic effect (BPVE),
which appears in noncentrosymmetric materials, does not require a built-in potential and is free
from the Shockley-Queisser (SQ) limit. TMDs, with low dimensionality, smaller band gap and
flexible crystal structure, are a promising class of materials to realize BPVE. In this thesis, we
focus on the PVE in TMDs with breaking symmetry.
Chapter 2 reports the symmetry breaking via assembling WS 2/MoS2 van der Waal
heterostructures. Although WS2 and MoS2 have C3 symmetry and sets of mirror planes, the
symmetry can probably be altered at WS2 and MoS2 interface due to interlayer interaction.
Photocurrent mapping measurements have demonstrated that photocurrent up to 28 nA appears

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without applying bias voltages under 996 µW light excitation, while is almost zero when
excitation laser sport is placed outside the stacked region. This work suggests that simply
stacking two different layered structures can lead to observing PVE, togethering with the
current-voltage (I-V) characteristics and power dependence of photocurrent, which is
consistent with BPVE.
In chapter 3, we report lifting the inversion symmetry in TMDs by strain. We
systematically study the electronic polarization induced by strain, including in-plane and
out-of-plane directions. Photovoltaic response in the out-of-plane direction is much stronger
than that in the in-plane direction. For observing the out-of-plane photocurrent arising
from the out-of-plane polarization, we used a graphene/WS2/graphene (G/WS2/G) van der
Waals heterostructure, where the top and bottom graphene work as transparent electrodes.
In addition, we put a micro-width bar underneath the heterostructure to bend WS 2. Using
this structure, ~ 0.3 A/cm2 photocurrent density at an excitation power of ~ 10 W/cm 2
(λ=633 nm) at room temperature was observed, which is very close to the highest
photoresponse in low-dimensional system based on BPVE (WS2 nanotubes).
Chapter 4 reports removing symmetry in MoS 2 by patterning 2D flakes into
nanoribbons. MoS2 nanoribbons along neither the armchair nor zigzag direction were
prepared to break the C 3 rational and mirror symmetries simultaneously, and significant
photocurrent density can be detected along whole nanoribbons. This study suggests that
removing the rational and mirror symmetries by reducing the effective dimensionality from
2D to 1D materials through a simple design can lead to a non-zero PVE. Meanwhile, it
provides a simple approach to obtaining materials that exhibit PVE and offers a deeper
understanding of symmetry engineering.