Pressure induced metallization without structural transition in layered MoSe2 - Dr. Qiaoshi Zeng
Shanghai, June 19, 2015 — The electronic structure transition (insulator to metal or semiconductor to metal transition) is generally accompanied or followed by a first-order structural transition. However, using multiple experimental techniques and ab-initio calculatios, new research from a group including Qiaoshi Zeng from HPSTAR find that the metallization process does not involve any crystal structure change in MoSe2, which allows its energycontinuous tunability for potential opto-electronic or photovoltaic applications. Their work is published on recent Nature Communications (DOI: 10.1038/ncomms8312).
Transition metal dichalcogenides (TMDs) have attracted intense scientific and engineering interest since the discovering of graphene in 2004. In the past decade, people mainly use three experimental methods: applying electrical field, utilizing quantum confinement with samples thinning down into monolayers, or employing stress or strain to vary the electronic structure of TMDs. In recent years, pressure is found to be a powerful tool for tuning mechanical and optical properties in TMDs, eg. MoS2.
At ambient conditions, MoS2 and MoSe2 are iso-structural in crystal structures and have highly similar electronic structures, so it’s nature to assume asimilar structure transition would also occur in MoSe2 just like that happened in MoS2. However, in this new study, both high-pressure x-ray diffraction data and Raman spectroscopy indicated no crystal structure change on MoSe2 up to some 600, 000 atmospheric pressure (60 gigapasals). To explore the observed stability of MoSe2, the team carry out ab-initio simulation, find that the energy barrier for layer sliding in MoSe2 is much higher than that in MoS2, and the high-pressure phase of MoSe2 bear higher energies than the initial structures, which possibly make the crystal-structure transition unfavorable. Additionally, somewhat more surprising is the metallization of MoSe2 at about 400, 000 times Earth’s atmosphere (40 GPa) from high-pressure IR spectroscopy and temperature-dependent resistivity measurements. These results demonstrate that pressure doesn’t result in crystal structure change but the electronic structure transition, which is rarely seen in TMDs.
Caption: (A) Schematic of the high pressure DAC set up; (B) Temperature-pressure-resistivity contour map; (C) Normalized cell parameters a/a0 and c/c0 versus pressure; (D) Evolution of band gap under pressure.
Through high-pressure IR spectroscopy, electrical resistivity measurement combined with ab-initio calculations, the team find that the band-gap of MoSe2 (inthe range ofvisible to IRregion) as well as its electronic structure both exhibit strong dependence on pressure. “These may allow MoSe2, one representative TMD, to be applied in energy-variable opto-electronics and photovoltaics, although the limitation of sample size (0.05 to 0.1 mm) must be taken into account in future investigation”, said Zhao Zhao from Stanford University, the lead author of this research.
“Compared with other methods, pressure is a powerful method to change structure and properties of energy materials just like it works on the metalization of MoSe2 , and crystalliztion in metallic glasses” Zeng said, a co-author of this work. “This highlights pressure’s unique role in tuning the electronic, optical, and mechanical properties of such materials for more and betterapplications.”