Linear Tunability of the Band Gap and Two-Dimensional (2D) to Three-Dimensional (3D) Isostructural Transition in WSe₂ under High Pressure

Abstract

Transition metal dichalcogenides (TMDs) have recently gained tremendous interest for use in electronic and optoelectronic applications. Unfortunately, the electronic structure or band gap of most TMDs shows noncontinuously tunable characteristics, which limits their application to energy-variable optoelectronics. Thus, layered materials with better tunability in their electronic structures and band gaps are desired. Herein, we experimentally demonstrated that layered WSe2 possessed highly tunable transport properties under various pressures, with a linearly decreasing band gap that culminates in metallization. Pressure tuned the band gap of WSe2 linearly, at a rate of 25 meV/GPa. The high tunability of WSe2 was attributed to the larger electron orbitals of W2+ and Se2– in WSe2 compared to the Mo2+ and S2– in MoS2. WSe2 underwent an isostructural phase transition from a 2D layered structure to a 3D structure at approximately 51.7 GPa, where a conversion from van der Waals (vdW) to covalent-like bonding was observed in the valence electron localization function (ELF). Our results present an important advance toward controlling the band structure of layered materials and suggest significant implications for energy-variable optoelectronic devices via pressure engineering.

Thermoelectric Properties of Compressed Titanium and Zirconium Trichalcogenides

“…6,19,20,29,51,52 Previous investigations on 2D transition-metal dichalcogenides, such as TX 2 (T = Mo and W and X = S, Se, and Te) having fundamental band gap values of about 1.1−1.4 eV, showed that they turn to metals when subjected to high-pressure application above 40−60 GPa in WSe 2 , 53−55 36−37 GPa in WS 2 , 56,57 30−40 GPa in MoSe 2 , 58,59 19 GPa in MoTe 2 , 58 and above 20−40 GPa in MoS 2 . 60−65 These semiconductor−metal transitions either were accompanied by isostructural transformations [54][55][56]60,63,64 or happened without structural changes. 58,59 High-pressure structural investigations on TiS 3 demonstrated that its original ZrSe 3 -type lattice persists under compression up to 22 GPa, and beyond this point, it suffers to an isosymmetric transition.…”

2D Materials and Heterostructures at Extreme Pressure

“…High-pressure not only enables an in-depth understanding of the structure-property relations for 2D materials and corresponding heterostructures but also modifies their optoelectronic properties. [14,17,20,48,167,[201][202][203] These high-pressure phased properties through vdW interaction engineering extend the potential applications of 2D materials in optical, electronic, and optoelectronic fields. [16] Although some optimized properties might not be retained after releasing the pressure, they are instructive for the synthesis of new 2D materials as well as designs of novel functional devices under high pressure.…”

“…It shows a slow-down drop of resistance and a tendency toward a low-temperature curve with the increase of magnetic fields, proving the emergence of the superconducting phase. [180] Chi et al also investigated the www.advancedsciencenews.com www.advancedscience.com [39] (CH 3 NH 3 )PbI 3 Above 60 (RT) [42] Cs 3 Bi 2 I 9 28 (338 K) [ 200] 2D TMDs ZrTe 2 T-ZrTe 2 ≈ 2 (H-Zr-Te 2 ≈ 6) [ 201] 1T-TiSe 2 2-4 (1.8 K) [ 191] WTe 2 10.5 (2.8 K); [ 183] 4-5 (300 K) [ 184] WS 2 22 (280 K) [17] WSe 2 51.7 (RT) [ 202] ZrS 2 5.6-25 [ 186] 5.6-25 (1.1-1.9 K) or 25-100 (0.3-0.1 K) [ 186] MoTe 2 14.9 [ 203] MoS 2 30 (270 K); [ 167] 9 (RT) [19] 90 (3 K) [ 167] Intermediate state: 10-19; semiconducting state: 0-10 [19] CdI 2 62 (240 K) [ 174] 35 (270 K) [ 174] semiconducting phase FeCL 2 47 (300 K) [ 204] NI 2 19 (310 K) [ 205] MgC 2 Ambient pressure (15 K) [ 206] BP 1.7, 10 [ 207] Above 5 (6-13 K); [ 181] above 10 (5-10 K); [ 182] 11-30 (4-10.7 K), 15 (6 K) [ 207] MoSe 2 40.7-60 [20] 0-40.7 semiconducting phase CaC 2 43 (7.9 K) and 95 (9.8 K) [ 185] CaC 6 7.5 (15.1 K) [ 208] superconductivity in pristine 2H a -MoS 2 and this is explained by the occurrence of a new flat Fermi pocket at ultrahigh pressure. [167] Using the external magnetic field, superconductivity was again verified.…”

“…Development of carbon-based electronic devices such as transistors or strain sensors 2D TMD Highly tunable transport properties including the decreased resistivity or enhanced electrical conductivity; [ 17,20,167,202,203,231] enhanced onset of the critical temperature for superconductivity; [ 167] enhanced mobility and electron concentrations as well as ionization of impurity levels; [ 231] suppression of magnetoresistance, reconstruction of Fermi surface (the decrease of hole and increase of electron ones) [ 183] Bandgap narrowing [ 17,20,167,[201][202][203] Electronic structure and bandgap engineering; energy variable optoelectronic and photovoltaic design; alternative routing of high-temperature superconductivity; [ 167] optoelectronic gain modulation [19] vdW heterostructures Enhanced doping level: 0.4 × 10 13 -3.2 × 10 13 cm −2 ; [37] enhanced charging effects in alcohol mixture PTM-based experiments [46] Tuning of electronic and band structures…”

Pressure-Induced Enhancement of Broad-Band White Light Emission in Butylammonium Lead Bromide

“…Because the Raman frequencies are very sensitive to strain, in situ Raman spectra were measured upon compression to probe subtle local changes of lattice vibrations. 39 The lowfrequency and high-frequency modes are associated with the inorganic PbBr 6 octahedral network and organic CH 3 (CH 2 ) 3 NH 3 cations, respectively. 40 Previous work suggests that the modes of PbBr 6 rotations and distortions dominate the low-frequency Raman spectra.…”