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Download This Article PDF Format RSC Advances PAPER View Article Online View Journal | View Issue Pressure-induced metallization in MoSe2 under different pressure conditions† Cite this: RSC Adv.,2019,9,5794 Linfei Yang,ab Lidong Dai, *a Heping Li,a Haiying Hu,a Kaixiang Liu,ab Chang Pu,ab Meiling Hongab and Pengfei Liuc In this study, the vibrational and electrical transport properties of molybdenum diselenide were investigated under both non-hydrostatic and hydrostatic conditions up to 40.2 GPa using the diamond anvil cell in conjunction with Raman spectroscopy, electrical conductivity, high-resolution transmission electron microscopy, atomic force microscopy, and first-principles theoretical calculations. The results obtained indicated that the semiconductor-to-metal electronic phase transition of MoSe2 can be extrapolated by some characteristic parameters including abrupt changes in the full width at half maximum of Raman modes, electrical conductivity and calculated bandgap. Under the non-hydrostatic condition, metallization occurred at 26.1 GPa and it was irreversible. However, reversible metallization occurred at 29.4 GPa under the hydrostatic condition. In addition, the pressure-induced metallization reversibility Creative Commons Attribution 3.0 Unported Licence. Received 16th November 2018 of MoSe can be revealed by high-resolution transmission electron and atomic force microscopy of the Accepted 4th February 2019 2 recovered samples under different hydrostatic conditions. This discrepancy in the metallization DOI: 10.1039/c8ra09441a phenomenon of MoSe2 in different hydrostatic environments was attributed to the mitigated interlayer rsc.li/rsc-advances van der Waals coupling and shear stress caused by the insertion of pressure medium into the layers. Introduction understanding the crystalline structure evolution and electrical properties of AB2-type layered materials, and also promotes its ¼ ¼ The AB2-type (A Mo, W; B S, Se, Te) transition-metal industrial exploitation in electronic devices. This article is licensed under a dichalcogenides (TMDs) have gained much interest owing to The high-pressure vibrational, structural and electrical their unique mechanical, optical and electrical properties, and properties for MoSe2 have been previously investigated by are widely used as solid-state lubricants, photodetectors, and Raman and infrared spectroscopy, electrical resistivity and rst- 1–3 Open Access Article. Published on 15 February 2019. Downloaded 9/28/2021 5:41:52 AM. transistors. Under ambient conditions, most of the principles theoretical calculations. All of these results indicated transition-metal dichalcogenides possess a well-dened layered that MoSe2 underwent a pressure-induced electronic transition structure, in which the molecules are connected by the intra- from being a semiconductor to a metal. However, with regard to layer covalent bond and the interlayer van der Waals (vdW) the metallization pressure point, there are many controversial forces. As an essential parameter, pressure can induce some viewpoints reported in previous studies. Riikov´a et al. con- changes in structural and electrical properties such as phase ducted the rst-principles theoretical calculations up to transition, amorphization and metallization in TMDs, and 130 GPa to study the effect of high pressure on the evolution of hence, pressure is of utmost importance for application in the crystal structure and electronic properties of MoSe2, and electromechanical devices, energy-variable opto-electronics and predicted that it should undergo metallization at a pressure of 4–8 10 photovoltaics. Molybdenum diselenide (MoSe2) is one of the at least 28 GPa. Caramazza et al. investigated the high- typical AB2-type TMDs with an indirect bandgap of 1.10 eV, pressure optical properties of MoSe2 up to 30 GPa by means crystallizing into a stable hexagonal 2Hc structure (space group: of Raman spectroscopy, and reported a pressure of 18 GPa for 9 11 P63/mmc) under ambient conditions. A systematic work on the metallization with the pressure medium of NaCl. Dybala et al. high-pressure behaviour of MoSe2 can be helpful in measured the photore ectance (PR) spectra of MoSe2 to explore its electronic band structure under hydrostatic pressure up to 20 GPa, and found that the metallization of MoSe2 occurred at aKey Laboratory of High-Temperature and High-Pressure Study of the Earth's Interior, 18 GPa with the pressure medium of liquid Daphne 7474.12 Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, Guizhou 550081, China. E-mail: [email protected] Zhao et al. investigated the high-pressure electrical transport bUniversity of Chinese Academy of Sciences, Beijing 100049, China behaviour of MoSe2 up to 60 GPa through the temperature- cState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the dependent electrical resistivity measurements, and their study Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China results indicated that the metallization process was complete at † Electronic supplementary information (ESI) available. See DOI: 40 GPa using the pressure medium of cubic BN.7 10.1039/c8ra09441a 5794 | RSC Adv.,2019,9,5794–5803 This journal is © The Royal Society of Chemistry 2019 View Article Online Paper RSC Advances À In addition, as an important inuential factor, the pressure a spectral resolution of 1.0 cm 1, and the collection time for environments can result in many structural and electronic each spectrum was 120 s. As for electrical conductivity property variations in the binary layered TMDs (e.g. WS2, MoS2 measurements at room temperature and high pressure condi- and ReS2). In the case of WS2, Duwal et al. con rmed that the tions, a T-301 stainless steel was employed as the gasket and isostructural phase transition from 2Hc to 2Ha occurred at pre-indented to a thickness of 50 mm beforehand, and then 37 GPa under the non-hydrostatic condition, and however, a 180 mm hole was drilled in the centre of the gasket by the laser under the hydrostatic condition, the phase structure was always drilling technique. The mixture of boron nitride (BN) and epoxy stable with He being used as the pressure medium.13 Zhuang was compressed into the hole as the insulator, and another hole et al. observed a novel phenomenon that metallization of MoS2 of 100 mm was then drilled to provide an insulated sample was irreversible under the non-hydrostatic condition, while it chamber. The electrical conductivity measurements were was reversible under the hydrostatic condition.14 Zhuang et al. carried out by two-electrode method using a Solartron-1260 also found that metallization of ReS2 under the hydrostatic impedance/gain phase analyzer in the frequency range from À condition occurred at a much higher pressure point than that 10 1 to 107 Hz. The temperature-dependent electrical conduc- under the non-hydrostatic condition.15 As a similar layered tivity measurements were performed in the temperature range structural diselenide, viz. MoSe2, it is possible that its high- from 120 K to 300 K. Similar to the experimental technique by pressure vibrational and electrical properties will be affected Rahman et al., the in situ high-pressure electrical resistivity by different hydrostatic environments. However, till date, there experiment is valid up to 55 GPa in the diamond anvil cell.17 are no relative reports on high-pressure structural and elec- More detailed descriptions on the experimental procedures and ff 18–20 tronic properties in MoSe2 under di erent hydrostatic envi- measurement methods have been reported previously. ronments. Thus, further systematic investigation on MoSe2 is Some microscopical observations on the recovered samples indispensable to explore the physical properties under different were performed at room temperature and high pressure pressure environments. conditions by virtue of the cross-sectional selected-area high- Creative Commons Attribution 3.0 Unported Licence. In the present study, we report a semiconductor-to-metal resolution transmission electron and atomic force micros- transition of MoSe2 under non-hydrostatic and hydrostatic copy. All of the acquired data via atomic force microscopy (AFM) conditions of up to 40.2 GPa by virtue of a series of research and high-resolution transmission electron microscopy methods including Raman spectroscopy, electrical conductivity, (HRTEM) images were collected using a Multimode 8 mass atomic force microscopy (AFM), high-resolution transmission spectrometer (Bruker) and Tecnai G2 F20 S-TWIN TMP, electron microscopy (HRTEM) and rst-principles calculations. respectively. The rst-principles calculations were conducted on Additionally, it was found that the metallization of MoSe2 was the basis of the density functional theory and pseudopotential irreversible under the non-hydrostatic condition, but reversible methods, which were performed using the CASTEP code in the under the hydrostatic condition. Furthermore, the reason for Material Studio package. The generalized gradient approxima- This article is licensed under a the diverse electrical properties displayed by MoSe2 under tions (GGA) in the Perdew–Burke–Ernzerhof (PBE) scheme were different pressure environments has been discussed in detail. applied for the description of the exchange and correlation terms. Structural optimizations were implemented using the Broyden–Fletcher–Goldfarb–Shanno (BFGS) minimization Open Access Article. Published on 15 February 2019. Downloaded 9/28/2021 5:41:52 AM. Experimental algorithm. In order
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