Tailoring the excitonic properties in two-dimensional monolayer transition metal dichalcogenides (TMDs) through strain engineering is an effective means to explore their potential applications in optoelectronics and nanoelectronics. Here we report pressure-tuned photon emission of trions and excitons in monolayer MoSe2 via a diamond anvil cell (DAC) through photoluminescence measurements and theoretical calculations. Under quasi-hydrostatic compressive strain, our results show neutral (X0) and charged (X–) exciton emission of monolayer MoSe2 can be effectively tuned by alcohol mixture vs inert argon pressure transmitting media (PTM). During this process, X– emission undergoes a continuous blue shift until reaching saturation, while X0 emission turns up splitting. The pressure-dependent charging effect observed in alcohol mixture PTM results in the increase of the X– exciton component and facilitates the pressure-tuned emission of X– excitons. This substantial tunability of X– and X0 excitons in MoSe2 can be extended to other 2D TMDs, which holds potential for developing strained and optical sensing devices.

Transition-metal dichalcogenides (TMDs) are promising materials for optoelectronic devices. Their lattice and electronic structural evolutions under high strain conditions and their relations remain open questions. We exert pressure on WS2 monolayers on different substrates, namely, Si/SiO2 substrate and diamond anvil surface up to ∼25 GPa. Structural distortions in various degree are disclosed based on the emergence of Raman-inactive B mode. Splits of out-of-plane B and A1′ modes are only observed on Si/SiO2 substrate due to extra strain imported from volume decrease in Si and corrugation of SiO2 surface, and its photoluminescence (PL) quenches quickly because of decreased K–K transition by conspicuous distortion of Brillouin zone. While diamond anvil surface provides better hydrostatic environment, combined analysis of PL and absorption proves that pressure effectively tunes PL emission energy and enhances Coulomb interactions. Knowledge of these distinct pressure tunable characteristics of monolayer WS2 improves further understanding of structural and optical properties of TMDs.

The mechanical properties of special engineering plastic have been intensively studied in recent years. However, there are few studies on the mechanical variation under extreme conditions. In this study, we chose poly(ether ether ketone) (PEEK) to study its mechanical behavior under high temperature and high pressure using Brillouin scattering coupled with an electrical resistance heating technique and diamond anvil cell device. The isothermal compressibility and pressure dependence of the mechanical moduli of PEEK film were examined and determined. A typical negative thermal expansion phenomenon was observed under high temperature and high pressure, which has been explained and discussed by virtue of free volume theory. Comprehensive investigation of elastic properties as a function of pressure not only provides an effective way to understand the mechanical behavior of PEEK under high temperature and high pressure, but also supplies an available theoretical direction for their practical application under extreme conditions.

Interlayer coupling plays critical roles in determining the lattice vibrations of two-dimensional transition-metal dichalcogenides. When compressed, the effects of interlayer coupling remain ambiguous. Pressure-dependent vibrational properties of trilayer and quadlayer MoS2 up to 12.7 GPa were investigated through in situ high pressure Raman spectroscopy measurement. The Raman spectrum reveals different responses to pressure in trilayer and quadlayer MoS2 due to their thickness-dependent interlayer coupling interaction. Combining this data with the first-principles calculations, we demonstrate that the quadlayer MoS2 transforms into an AB′ stacking configuration above 8.6 GPa, where all Mo atoms sit exactly over the Mo atoms in their neighboring layer and all S atoms sit over the centers of the hexagons, while the trilayer MoS2 possesses a distorted and wrinkled 2H structure within our studied pressure range. Our study demonstrates that high pressure Raman spectroscopy measurement is an effective method to explore the structural transformation of ultrathin MoS2 at extreme conditions as well as to explore their complicated interlayer coupling interaction. It should also be of great benefit for the development of nanotechnology, especially for the design and fabrication of different stacking nanometer devices with tailored properties for specific applications.

High-pressure Raman scattering studies on pure acetonitrile and an acetonitrile–water mixture at a molar ratio of (nCH3CN:nH2O) 1:7.25 were performed in a diamond anvil cell at room temperature. The structural transitions of pure acetonitrile from liquid to α phase, α to β phase and β to γ phase were detected from Raman spectra variations at 0.2 GPa, 0.8 GPa and 4.95 GPa, respectively. The acetonitrile–water mixture presented a much higher solidification pressure of 1.25 GPa and the β phase is found to be sustained up to 7 GPa. Through Raman analysis on the solid acetonitrile–water mixture, the acetonitrile clusters were found to exist as separated domains and surrounded by ice crystal domains due to large amounts of water in the mixture. Meanwhile the fluorescence was obviously depressed in the mixture and the Raman peaks can be detected up to 29.88 GPa, while the Raman peaks in pure acetonitrile are undetectable at 21 GPa due to its increased fluorescence.

Brillouin scattering spectra of three silicone oils with different viscosity, including two polydimethylsiloxanes (PDMS) and one polyphenylmethylsiloxane (PPMS), have been studied under high pressure. The acoustic velocity changes of the three silicone oils were measured under pressure and the velocity of all the samples increases with increasing pressure in a similar way. The morphology of the PDMS changed upon compression, possibly accompanied with structural rearrangement or relaxation, but no morphology variation was observed in the PPMS. Based on velocity measurements, the refractive index, density and bulk modulus of the three silicone oils were obtained and the corresponding pressure dependences were compared with some other micro molecule liquids. Obvious differences were observed and discussed. The results indicate that more complicated interactions between the long-chain molecules in silicone oils might result in the larger bulk modulus when compared with the micro molecular liquids.

A novel polyurea containing oligoaniline pendants (PU-p-OA) was synthesized by one-step synthetic route. The molecular structure of PU-p-OA was confirmed by Fourier transform infrared spectra (FTIR), nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC), and its thermal stability was probed via thermal gravimetric analysis (TGA). Due to the introduction of the oligoaniline segments, PU-p-OA showed expected spectroscopic properties and reversible electroactivity. The electrochromic behavior of a PU-p-OA/ITO electrode was investigated by spectrochronoamperometry in detail, exhibiting good electrochromic properties with high contrast value. Moreover, the inhibition effect of the electroactive PU-p-OA coatings on the cold rolled steel (CRS) in 5 wt% sodium chloride solution was investigated by Tafel plot analysis and electrochemical impedance spectroscopy. Furthermore, comparative anticorrosive study among PU-p-OA, normal polyurea (NPU), parent aniline tetramer (PAT), and NPU/PAT blend was accomplished, which indicated that the obtained PU-p-OA was a good corrosion inhibitor for CRS in brine medium.

High-pressure Raman scattering and X-ray diffraction studies on manganese niobate (MnNb2O6) have been carried out in a diamond anvil cell at room temperature up to 41 GPa. A pressure-induced phase transition was observed at 12 GPa accompanied by the softening of two internal vibration modes ν10(B2g) and ν8(Ag). Disappearing of most Raman vibration peaks under high pressure reveals that the NbO6 octahedra distort heavily through the transition; this is evidenced by the X-ray diffraction results, but the octahedral units still exist, as the internal Nb–O strengthening vibration is observable even at the highest pressure in this study. The equation of state for MnNb2O6 in low pressure phase I was obtained with third-order Birch–Murnaghan method, yielding a zero-pressure bulk modulus B0 and its pressure derivative B0′ of 154.4 ± 3.5 GPa and 4.1, respectively. The volume dependencies of the optical lattice mode frequencies and their respective Grüneisen parameters were extracted; it is suggested that the instability of the columbite orthorhombic structure at high pressure is related to the strong deformation of the NbO6 octahedra. Decompression measurements suggest that the pressure-induced transformation is partly reversible.

High pressure X-ray diffraction and Raman scattering studies have been carried out on magnesium niobate (MgNb2O6) in a diamond anvil cell (DAC) at room temperature up to 40 GPa. A pressure-induced phase transition was observed at pressures above 10 GPa accompanied by a softening of the internal ν9 (B3g) modes. Raman scattering results reveal that the distortion of the NbO6 octahedra decreases under high pressure. According to the X-ray diffraction data, the high pressure phase can be indexed with a monoclinic unit cell. The initial orthorhombic phase changes to a monoclinic phase possibly due to the rotation of the NbO6 octahedra, similar to some perovskite ABO3 structures.

To develop a new practical method of purifying and recycling ionic liquids, we performed direct microscopic observations and in situ crystallization of low-melting ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]), in detail by high pressure Raman spectroscopy. Compression of [BMIM][PF6] was measured under pressures up to about 2.0 GPa at temperatures 293−353 K by using a high pressure diamond anvil cell (DAC). At room temperature, with pressure increasing, the characteristic bands of [BMIM][PF6] displayed nonmonotonic pressure-induced frequency shifts, and [BMIM][PF6] experienced the liquid−solid phase transition at about 0.50 GPa. In separate experiments, in situ crystallization of low-melting ionic liquid [BMIM][PF6] were also measured at various P−T regions, in order to improve the understanding of its stability limits. Finally, the T versus P phase diagram of [BMIM][PF6] was constructed, and it showed that the melting point was an increase function of pressure. It was also indicated that the structure changes in the crystalline and liquid states under high pressure might also be associated with conformational changes in the butyl chain. Pressure-released Raman spectra also showed that the phase transition of [BMIM][PF6] was reversible.