Researchers
have shown great interest in two-dimensional crystals
recently, because of their thickness-dependent electronic and optical
properties. We have investigated the Raman and photoluminescence spectra
of free-standing monolayer and bilayer MoS2, as a function
of pressure. As the enforcement of layer interaction, an electronic
and a crystal phase transition were revealed at ∼6 GPa and
∼16 GPa, respectively, in bilayer MoS2, while no
phase transition in the monolayer is observed. The electronic phase
transition at ∼6 GPa is supposed to be a direct interband changing
to an indirect Λ-K interband transition, and the new structure
shown at ∼16 GPa is not metallized and supposed to be a transformation
from stacking faults due to layer sliding like 2Hc to 2Ha. The different pressure-induced features of monolayer MoS2, compared with bilayer MoS2, can help to get a
better understanding about the importance of interlayer interaction
on modifying the optical properties of MoS2 and other fundamental
understanding of 2D materials.
HgTe film is widely used for quantum Hall well studies and devices, as it has unique properties, like band gap inversion, carrier‐type switch, and topological evolution depending on the film thickness modulation near the so‐called critical thickness (63.5 Å), while its counterpart bulk materials do not hold these nontrivial properties at ambient pressure. Here, much richer transport properties emerging in bulk HgTe crystal through pressure‐tuning are reported. Not only the above‐mentioned abnormal properties can be realized in a 400 nm thick bulk HgTe single crystal, but superconductivity is also discovered in a series of high‐pressure phases. Combining crystal structure, electrical transport, and Hall coefficient measurements, a p‐n carrier type switching is observed in the first high‐pressure cinnabar phase. Superconductivity emerges after the semiconductor‐to‐metal transition at 3.9 GPa and persists up to 54 GPa, crossing four high‐pressure phases with an increased upper critical field. Density functional theory calculations confirm that a surface‐dominated topologic band structure contributes these exotic properties under high pressure. This discovery presents broad and efficient tuning effects by pressure on the lattice structure and electronic modulations compared to the thickness‐dependent critical properties in 2D and 3D topologic insulators and semimetals.
Pressure evolution of local structure and vibrational dynamics of the perovskite-type relaxor ferroelectric single crystal of 0.935(Na0.5Bi0.5)TiO3-0.065BaTiO3 (NBT-6.5BT) is systematically investigated via in situ Raman spectroscopy. The pressure dependence of phonon modes up to 30 GPa reveals two characteristic pressures: one is at around 4.6 GPa which corresponds to the rhombohedral-to-tetragonal phase transition, showing that the pressure strongly suppresses the coupling between the off-centered A- and B-site cations; the other structural transition involving the oxygen octahedral tilt and vibration occurs at pressure ∼13–15 GPa with certain degree of order-disorder transition, evidenced by the abnormal changes of intensity and FWHM in Raman spectrum.
The comparison of different stereoisomeric organic compounds
under
high pressure has been less investigated. Here, we chose different
stereochemical configurations of cis/trans-stilbene to study the luminescence
properties, polymerization reaction, and structural changes at 0–20
GPa by spectroscopy and XRD. No fluorescence enhancement occurred
in cis-stilbene due to π–π stacking. At 16 GPa,
the IR, UV–vis, and sample color changes show that it undergoes
an irreversible polymerization, that C(sp2)–H changes
to C(sp2 + sp3)–H. However, trans-stilbene
undergoes fluorescence enhancement at 0–4 GPa due to the reduction
of the torsion angle of the benzene ring and the CC bond leading
to the formation of rigid planar molecules, which is further confirmed
by the IR and XRD results. At 8 GPa, the new peaks in UV–vis
and XRD results show the formation of new substances by structural
change. However, the structure of trans-stilbene is more stable, which
leads to the return to the raw state after releasing the pressure,
and a reversible transformation occurs at high pressure. The cis-trans
isomerization under high pressure was also briefly investigated by
combining heating and laser irradiation. The cis → trans-stilbene
transition can only happen under a fixed-range light irradiation,
and the trans → cis-stilbene transition could not happen even
under irradiation with a 360 nm laser, which may provide a new idea
for synthesizing trans isomers with a higher purity.
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