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.