Graphenehas recently become a popular material when discussing ultrathintwo-dimensional nanosheets, but layered transition metal dichalcogenides (TMDs)have shown promise for use in applications in which graphene is not apossibility.  Because of their unique anddiverse chemical properties, TMDs have been looked at as an attractive for usein applications as diverse as chemically active semiconductors, supercapacitorsand quantumdevices.  A South Korean teamworking at IBS has created a method for the production of two differenthomogenous TMD crystal configurations made of molybdenum telluride (MoTe₂)which exhibit a reversible structural phase transition.

TMDs are generally used in the form ofatomically thin, mono- or few-layered sheets which can be produced bymechanically slicing off thin layers from a large piece of bulk material orcreating them via a synthesis using their constituent elements. They are aseemingly straightforward combination of a transition metal sandwiched between2 chalcogenides.  However, what isespecially of interest are the Periodic Table Group 6 TMDs which are classifiedby their MX₂ configuration with M representing molybdenum (Mo) ortungsten (W) and X representing 2 chalcogenides of either, sulfur (S), selenium(Se) or tellurium (Te). 

The team used a novel production method forthe creation of the MoTe₂ crystal, which they were able tosynthesize in two forms: a single trigonal cell (1T’-MoTe₂) and atwo layer hexagonal cell (2H-MoTe₂).In previous attempts, other researchers produced MoTe₂ anddiscovered that there was a tellurium deficiency in their crystals.  Tellurium has a relatively low (~400° C) sublimation temperature which made fabrication of homogenouscrystals quite difficult.  To solve thisproblem, the Korean team used the flux method and combined finely ground Mo andTe powders to liquid sodium chloride (NaCl) in two glass tubes. They was placedin an alumina cylinder in a 10^-5 torr vacuum and heated to 1,100° C for 12 hours.  Both sampleswere slowly cooled to 900° C and then onesample was quenchedin water and immediately cooled to room temperature which resulted in 1T’-MoTe₂.The other sample was continuously slow-cooled from 900° C to roomtemperature which yielded 2H-MoTe₂.

After obtaining both types of the newly-formed,homogeneous MoTe₂, the researchers began testing the samples.   Using a Fourier Transform InfraredSpectrometer (FTIR) on a sample of 2H-MoTe₂, a bandgap of 0.9 eV wasobserved, which is consistent with numerous previously collected data.  However, when the FTIR was used to analyze afew-layered (1~10 layers) 1T’-MoTe₂ sample, it was discovered thatit 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 thatthe bandgap that is evident in a few-layered sample closes as the number oflayers increases from few-layered to bulk as the 1T’-MoTe₂ propertieschange from semiconducting to metallic.The bulk 1T’-MoTe₂  crystalsexhibit a maximum carrier mobility of 4,000 cm2 V^-1 s^-1and a giant magnetoresistance of 16,000% in a magnetic field of 14T at 1.8 K.

The unique properties of the different MoTe₂configurations discovered by the IBS team create the possibility for novelapplications that have been limited by materials that do not possess thenecessary properties for their creation.According to the team, “This class of semiconducting MoTe₂unlocks the possibility of topological quantum devices”. 

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Notesfor editors

References

*Title of Paper: Bandgap opening infew-layered monoclinic MoTe₂, NATURE PHYSICS, DOI: 10.1038/NPHYS3314

*Authors: Dong Hoon Keum, Suyeon Cho, Jung HoKim, 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 mediaassistance, please contact: Mr. Shi Bo Shim, Head of Department ofCommunications, Institute for Basic Science (+82-42-878-8189; sibo@ibs.re.kr)or Mr. Daniel Kopperud, Department of Communications, Institute for BasicScience (+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. Withthe 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 basicscience, including mathematics, physics, chemistry, life science, earth scienceand interdisciplinary science. IBS has launched 24 research centers as ofJanuary 2015. There are one mathematics, eight physics, six chemistry, sevenlife science, and two interdisciplinary research centers.