Beautiful work of 2D material stacks for future electronics - layered stacks of molybdenum disulfide (MoS2) encapsulated in boron nitride (BN), with graphene overlapping the edge of the MoS2 to act as electrical contacts as Published by : Holly Evarts,
"Two-Dimensional Semiconductor Comes Clean",
Apr. 27, 2015 and in Nature Nanotechnology below.
In 2013
James Hone, Wang
Fong-Jen Professor of Mechanical Engineering at Columbia Engineering,
and colleagues at Columbia demonstrated that they could dramatically
improve the performance of graphene—highly conducting two-dimensional
(2D) carbon—by encapsulating it in boron nitride (BN), an insulating
material with a similar layered structure. In work published this week
in the Advance Online Publication on
Nature Nanotechnology’s
website, researchers at Columbia Engineering, Harvard, Cornell,
University of Minnesota, Yonsei University in Korea, Danish Technical
University, and the Japanese National Institute of Materials Science
have shown that the performance of another 2D material—molybdenum
disulfide (MoS2)—can be similarly improved by BN-encapsulation.
Molybdenum disulfide encapsulated between layers of boron nitride (Image courtesy of Gwan-Hyoung Lee/Yonsei University).
Schematic cross-section
view of atomic layer of molybdenum disulfide contacted by graphene, and
encapsulated between layers of insulating hexagonal boron nitride.
Credit: Gwan-Hyoung Lee/Columbia Engineering
“These findings provide a demonstration of how to study all 2D
materials,” says Hone, leader of this new study and director of
Columbia’s NSF-funded Materials Research Science and Engineering Center.
“Our combination of BN and graphene electrodes is like a ‘socket’ into
which we can place many other materials and study them in an extremely
clean environment to understand their true properties and potential.
This holds great promise for a broad range of applications including
high-performance electronics, detection and emission of light, and
chemical/bio-sensing.”
Two-dimensional (2D) materials created by “peeling’” atomically thin
layers from bulk crystals are extremely stretchable, optically
transparent, and can be combined with each other and with conventional
electronics in entirely new ways. But these materials—in which all atoms
are at the surface—are by their nature extremely sensitive to their
environment, and their performance often falls far short of theoretical
limits due to contamination and trapped charges in surrounding
insulating layers. The BN-encapsulated graphene that Hone’s group
produced last year has 50× improved electronic mobility—an important
measure of electronic performance—and lower disorder that enables the
study of rich new phenomena at low temperature and high magnetic fields.
“We wanted to see what we could do with MoS2—it’s the best-studied 2D
semiconductor, and, unlike graphene, it can form a transistor that can
be switched fully ‘off’, a property crucial for digital circuits,” notes
Gwan-Hyoung Lee, co-lead author on the paper and assistant professor of
materials science at Yonsei. In the past, MoS2 devices made on common
insulating substrates such as silicon dioxide have shown mobility that
falls below theoretical predictions, varies from sample to sample, and
remains low upon cooling to low temperatures, all indications of a
disordered material. Researchers have not known whether the disorder was
due to the substrate, as in the case of graphene, or due to
imperfections in the material itself.
In the new work, Hone’s team created heterostructures, or layered
stacks, of MoS2 encapsulated in BN, with small flakes of graphene
overlapping the edge of the MoS2 to act as electrical contacts. They
found that the room-temperature mobility was improved by a factor of
about 2, approaching the intrinsic limit. Upon cooling to low
temperature, the mobility increased dramatically, reaching values 5-50×
that those measured previously (depending on the number of atomic
layers). As a further sign of low disorder, these high-mobility samples
also showed strong oscillations in resistance with magnetic field, which
had not been previously seen in any 2D semiconductor.
“This new device structure enables us to study quantum transport
behavior in this material at low temperature for the first time,” added
Columbia Engineering PhD student Xu Cui, the first author of the paper.
By analyzing the low-temperature resistance and quantum oscillations,
the team was able to conclude that the main source of disorder remains
contamination at the interfaces, indicating that further improvements
are possible.
“This work motivates us to further improve our device assembly
techniques, since we have not yet reached the intrinsic limit for this
material,” Hone says. “With further progress, we hope to establish 2D
semiconductors as a new family of electronic materials that rival the
performance of conventional semiconductor heterostructures—but are
created using scotch tape on a lab-bench instead of expensive
high-vacuum systems.”
Multi-terminal transport measurements of MoS2 using a van der Waals heterostructure device platform
Xu Cui, Gwan-Hyoung Lee, Young Duck Kim, Ghidewon Arefe, Pinshane Y.
Huang, Chul-Ho Lee, Daniel A. Chenet, Xian Zhang, Lei Wang, Fan Ye,
Filippo Pizzocchero, Bjarke S. Jessen, Kenji Watanabe, Takashi
Taniguchi, David A. Muller, Tony Low, Philip Kim & James Hone
Nature Nanotechnology(2015)doi:10.1038/nnano.2015.70
Figure 1c: Cross-sectional STEM image of the fabricated device. The zoom-in false-colour image clearly shows the ultra-sharp interfaces between different layers (graphene, 5L; MoS2, 3L;top hBN, 8nm; bottom hBN, 19 nm) [Figure and Abstract used with permission from Nature Publishing Group under License Number 3621820766388]
Atomically thin two-dimensional semiconductors such as MoS2
hold great promise for electrical, optical and mechanical devices and
display novel physical phenomena. However, the electron mobility of
mono- and few-layer MoS2 has so far been substantially below
theoretically predicted limits, which has hampered efforts to observe
its intrinsic quantum transport behaviours. Potential sources of
disorder and scattering include defects such as sulphur vacancies in the
MoS2 itself as well as extrinsic sources such as charged
impurities and remote optical phonons from oxide dielectrics. To reduce
extrinsic scattering, we have developed here a van der Waals
heterostructure device platform where MoS2 layers are fully
encapsulated within hexagonal boron nitride and electrically contacted
in a multi-terminal geometry using gate-tunable graphene electrodes.
Magneto-transport measurements show dramatic improvements in
performance, including a record-high Hall mobility reaching 34,000 cm2 V–1 s–1 for six-layer MoS2
at low temperature, confirming that low-temperature performance in
previous studies was limited by extrinsic interfacial impurities rather
than bulk defects in the MoS2. We also observed Shubnikov–de Haas oscillations in high-mobility monolayer and few-layer MoS2.
Modelling of potential scattering sources and quantum lifetime analysis
indicate that a combination of short-range and long-range interfacial
scattering limits the low-temperature mobility of MoS2.
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