Graphene
has recently become a popular material when discussing ultrathin
two-dimensional nanosheets, but layered transition metal dichalcogenides (TMDs)
have shown promise for use in applications in which graphene is not a
possibility. Because of their unique and
diverse chemical properties, TMDs have been looked at as an attractive for use
in applications as diverse as chemically active semiconductors, supercapacitors
and quantum
devices. A South Korean team
working at IBS has created a method for the production of two different
homogenous TMD crystal configurations made of molybdenum telluride (MoTe2)
which exhibit a reversible structural phase transition.
TMDs are generally used in the form of
atomically thin, mono- or few-layered sheets which can be produced by
mechanically slicing off thin layers from a large piece of bulk material or
creating them via a synthesis using their constituent elements. They are a
seemingly straightforward combination of a transition metal sandwiched between
2 chalcogenides. However, what is
especially of interest are the Periodic Table Group 6 TMDs which are classified
by their MX2 configuration with M representing molybdenum (Mo) or
tungsten (W) and X representing 2 chalcogenides of either, sulfur (S), selenium
(Se) or tellurium (Te).
The team used a novel production method for
the creation of the MoTe2 crystal, which they were able to
synthesize in two forms: a single trigonal cell (1T’-MoTe2) and a
two layer hexagonal cell (2H-MoTe2).
In previous attempts, other researchers produced MoTe2 and
discovered that there was a tellurium deficiency in their crystals. Tellurium has a relatively low (~400° C) sublimation temperature which made fabrication of homogenous
crystals quite difficult. To solve this
problem, the Korean team used the flux method and combined finely ground Mo and
Te powders to liquid sodium chloride (NaCl) in two glass tubes. They was placed
in an alumina cylinder in a 10-5 torr vacuum and heated to 1,100° C for 12 hours. Both samples
were slowly cooled to 900° C and then one
sample was quenched
in water and immediately cooled to room temperature which resulted in 1T’-MoTe2.
The other sample was continuously slow-cooled from 900° C to room
temperature which yielded 2H-MoTe2.
After obtaining both types of the newly-formed,
homogeneous MoTe2, the researchers began testing the samples. Using a Fourier Transform Infrared
Spectrometer (FTIR) on a sample of 2H-MoTe2, a bandgap of 0.9 eV was
observed, which is consistent with numerous previously collected data. However, when the FTIR was used to analyze a
few-layered (1~10 layers) 1T’-MoTe2 sample, it was discovered that
it had an absorption band edge near 60 meV.
This was the first ever observation of a bandgap in this structure type
(monoclinic) TMD. Also of note was that
the bandgap that is evident in a few-layered sample closes as the number of
layers increases from few-layered to bulk as the 1T’-MoTe2 properties
change from semiconducting to metallic.
The bulk 1T’-MoTe2 crystals
exhibit a maximum carrier mobility of 4,000 cm2 V-1 s-1
and a giant magnetoresistance of 16,000% in a magnetic field of 14T at 1.8 K.
The unique properties of the different MoTe2
configurations discovered by the IBS team create the possibility for novel
applications that have been limited by materials that do not possess the
necessary properties for their creation.
According to the team, “This class of semiconducting MoTe2
unlocks the possibility of topological quantum devices”.
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Notes
for editors
References
*Title of Paper: Bandgap opening in
few-layered monoclinic MoTe2, NATURE PHYSICS, DOI: 10.1038/NPHYS3314
*Authors: Dong Hoon Keum, Suyeon Cho, Jung Ho
Kim, Duk-Hyun Choe, Ha-June Sung, Min Kan, Haeyong Kang, Jae-Yeol Hwang,
SungWng Kim, Heejun Yang, K. J. Chang and Young Hee Lee
For further information or to request media
assistance, please contact: Mr. Shi Bo Shim, Head of Department of
Communications, Institute for Basic Science (+82-42-878-8189; sibo@ibs.re.kr)
or Mr. Daniel Kopperud, Department of Communications, Institute for Basic
Science (+82-42-878-8275; dpkopperud@ibs.re.kr)
About Institute for Basic Science (IBS)
The IBS was founded in 2011 by the government of the Republic of Korea. With
the sole purpose of driving forward the development of basic science in Korea,
IBS will be comprised of a total of 50 research centers in all fields of basic
science, including mathematics, physics, chemistry, life science, earth science
and interdisciplinary science. IBS has launched 24 research centers as of
January 2015. There are one mathematics, eight physics, six chemistry, seven
life science, and two interdisciplinary research centers.
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