Quantum spin liquid states have garnered significant attention as potential precursors for high-temperature superconductors. Researchers are aiming to achieve high-temperature superconductivity through regulation. However, previous studies have indicated that candidate materials with honeycomb structures, such as Na2IrO3 and alpha-Li2IrO3, remain in a magnetically ordered and insulating state. Pressure serves as an effective regulatory tool by adjusting atomic interactions through interatomic spacing manipulation, thereby influencing the band structure near the Fermi surface and consequently tuning quantum-state evolution. In this study, interlayer Li were substituted by Ag atoms in alpha-Li2IrO3 to obtain the Ag3LiIr2O6, and its transitions in structure and physical properties as functions of temperature and pressure were investigated. It has been observed that Ag3LiIr2O6 remains stable between -190 and 300(degrees)C without undergoing any structural phase transitions. High-pressure phase transitions occur at 3.0-7.5 and 12.0-16.1 GPa. The first structural phase transition, as deduced from high-pressure x-ray diffraction and Raman spectroscopy, is associated with Ir-Ir dimerization and IrO6 octahedral distortion. Corresponding resistance measurements indicate a decreasing rate reduction in resistance near 5.2 GPa due to dimerization. Further compression leads to the existence of a minimum room-temperature resistance at similar to 19.5 GPa. A transition from negative to positive magnetoresistance occurs at 12.4 GPa under 2 K. Further analysis suggests that the transition from negative to positive magnetoresistance may be connected to the valence change of Ag from +1 to 0. Although the desired insulator-to-metal transition was not achieved, we have explored the correlation between structural and physical property transitions under high pressure, laying the groundwork for future investigations.

Gray arsenic is a two-dimensional semimetal material that has attracted much attention due to its rich and fascinating properties such as ultrahigh carrier mobility and giant magnetoresistance properties. In this work, the structural and electrical properties of gray arsenic under high pressure have been systematically explored. Two structural phase transitions, A7 phase of gray arsenic to simple cubic phase and simple cubic phase to host -guest phase, were revealed at -28 and -35 GPa, respectively. And the transition to host -guest phase completed at -46 GPa. Superconductivity appeared after -35 GPa and the maximum of Tc, -5 K, was revealed in pure host -guest phase at -54 GPa, which is currently the highest recorded Tc of arsenic. The anisotropic upper critical field of superconductivities revealed the presence of mixed phase and the relative crystal orientations of the high-pressure phases.

Recently exfoliated monolayer and multilayered transition metal dichalcogenides have gathered significant interest based on their tunable bandgap and extremely high carrier mobility. We have investigated the Raman and photoluminescence spectra of monolayer and multilayer WS2 as a function of pressure. The Raman-inactive mode B1u, which is activated by structural disorder, was revealed at 6.7 GPa in monolayers, at 8.0 GPa in bilayers, and at 13.7 GPa in multilayers, respectively. With the enhancement of pressure-induced interlayer interaction, the crystal phase transition due to layer sliding like 2Hc to 2Ha occurs at 14.8 and 18.7 GPa in bilayers and multilayers, as evidenced by the split of E12g and B1u. The electronic phase transition of the monolayer is supposed to be a direct K-K bandgap changing to an indirect Λ-K bandgap at 2.6 GPa. These observations contribute to a better understanding of the impact of interlayer interactions on the modulation of WS2 energy bands and structure, as well as fundamental studies of two-dimensional layered materials, which can inform the development of device applications.

Van der Waals heterostructures have recently attracted great interest of the scientific community due to their rich exotic physical properties and extensive application prospects. Therefore, we conducted pressure-dependent Raman and photoluminescence spectroscopic studies on MoS2-WS2 heterostructures in different twist angles (24.5 and 54 degrees). Thus, it was confirmed that as the interlayer interaction increases under pressure, an electronic phase transition and a structural phase transition due to layer sliding are observed at -1.8 and -3.8 GPa in the HS-24.5 degrees structures, while no phase transition is observed in the HS-54 degrees structures. As a result of a larger tunable interlayer space in HS-24.5 degrees structures, optical properties of HS-24.5 degrees structures are more pressure-sensitive than those of the HS-54 degrees structure. It is expected that this work will help comprehensively establish the correlation between the interlayer interactions and optical properties of vdW HSs at the atomic level. Understanding this correlation is crucial for the development of new excitonic devices.

In this study, we propose an unconventional approach to investigate the pressure-induced changes in crystal structure, electrical transport, and optical bandgap by examining bulk FeCl2 and FeCl2 films with a thickness of 600 nm. Our analysis involved Raman spectroscopy, electrical transport measurements, and Ultra-violet-visible (UV-vis) absorption spectroscopy. Analyzing the Raman spectroscopy data, we discovered subtle distinctions in the sequence of structural phase transitions between bulk FeCl2 and the 600 nm thick film of FeCl2 under varying pressures. Specifically, while bulk FeCl2 underwent an insulating-to-metal transition at 52 GPa, the film of FeCl2 maintained its semiconductor behavior even at pressures as high as 60 GPa. Furthermore, by studying the optical absorption spectra, we observed a consistent decrease in the direct band gap of bulk FeCl2 with increasing pressure. In contrast, the film of FeCl2 exhibited an increase in the optical band gap beyond 34.1 GPa. These remarkable findings offer valuable insights into the manipulation of the mechanical, electrical, and optical properties of layered nanomaterials.

High pressure is an effective and clean method for tuning lattice structures and electronic states. In this study, we report on the crystal growth and experimental study of the pressure effect on the three-dimensional Dirac semimetal BaAgBi. Within the pressure range of 51.8 GPa studied in this work, BaAgBi remained metallic and no superconductivity was observed down to 1.6 K. The resistance is primarily governed by the electron-phonon and s-d electron-electron scattering. High-pressure synchrotron X-ray diffraction experiments indicated the presence of two high-pressure phases below 50 GPa, and the crystal structures were proposed to be Pnma and Pca21, respectively. By refining the high-pressure X-ray data, we plotted the pressure dependence of unit cell volumes and obtained the bulk modulus for all three phases.

Violet phosphorus, as a kind of two-dimensional material, has become an excellent candidate for optoelectronic devices as it makes up for the drawbacks of black phosphorus applications. This study explores the structural and electrical properties of violet phosphorus under high-pressure conditions. Two structure phase transitions are revealed by the spectra and electromagnetic transport measurements. At similar to 8 GPa, a structural transition from monoclinic to A7 phase is observed with the appearance of superconductivity. The transition from the A7 phase to the simple cubic phase starts at similar to 13 GPa and completes after similar to 30 GPa at low temperatures. In addition, the band gap of violet phosphorus decreases linearly with the increase of pressure until it disappears after the transition to the A7 phase. The anisotropic upper magnetic field of superconductivities in A7 and simple cubic phases showed the relative crystal orientations of the high-pressure phases compared with initial violet phosphorus crystal phase.

Our work studied the structural changes of Gd@C 82 under high pressure and successfully synthesized Gd-doped diamond which has the paramagnetic properties using Gd@C 82 precursors under high pressure and high temperature.

Pressure-induced emission enhancement is a new and effectivewayto manipulate the color of organic materials. However, the specificfluorescence enhancement mechanism and the causes of the color changehave not been previously well explored. Herein, two rofecoxib derivatives,MOX2 and Y7, were synthesized with methyl sulfonyl and cyan as theterminal groups on the para-position of the benzenering ② (Ph-②), and their fluorescence behavior was investigatedat 0-20 GPa. The two compounds exhibit opposite pressure-tunedfluorescent behavior in the 0-3.1 GPa range. Theoretical calculationindicates that the -SO2CH3 group of MOX2directs the dihedral angle between the Ph-② and Ph-③ring planes to an optimized steric position, producing a large sterichindrance and neutralizing the influence of pressure on the benzenering, which results in the invariance of the fluorescence peak position.The change in this dihedral angle is mainly caused by the gradualformation of hydrogen bonds between the O atoms on -SO2 and the H atoms of the benzene ring, resulting in a 6.3-foldincrease in the fluorescence intensity. Conversely, the luminescenceintensity of Y7 is monotonously suppressed as the pressure is increaseddue to the smaller size of the -C N substituent. Thisstudy provides a reference for regulating the luminescence propertiesof organic molecules.

Organic solar cells have become an important development direction in solar cell materials because of their low cost, light weight, and good flexibility. However, the size of their bandgap is difficult to continuously regulate, resulting in a low power conversion efficiency. In this work, an organic molecule TPEPA was synthesized, and its luminescence performance and polymerization under high pressure were studied by performing in situ Raman, IR, fluorescence, and UV-vis spectroscopy. The Raman and IR spectroscopic results show that single bonds (C-H, C-Ph) and long chains (C-C[triple bond, length as m-dash]C-C) are more unstable and prone to amorphization under high pressure. At 10 GPa, the TPEPA molecule undergoes a transition of amorphization accompanied by a few polymerizations in the C[triple bond, length as m-dash]C bond structure. After holding pressure at 20 GPa for one day and releasing to ambient pressure, the other peaks almost disappeared, while the new peak of C(sp3)-H from the polymerization of the benzene ring was observed, indicating that the irreversible amorphization and polymerization did occur. UV-vis spectra results show that the bandgap is reduced from 2.9 eV to 1.3 eV, which is just in the maximum conversion efficiency bandgap range (1.3-1.4 eV) of p-n junction solar cell materials. This pressure is within the working pressure range of a large volume press, which is favorable in applications of large-scale synthesis. Our strategy may provide a method for the large-scale synthesis of novel organic solar cell materials.

Relaxor ferroelectric crystals of lead magnesium niobate–lead titanate (PMN-xPT) have attracted great attention due to their extraordinary dielectric, piezoelectric, and electromechanical properties. PMN-xPT shows different relaxor behavior and structural phase transitions in a wide temperature and chemical component range. Here, we studied spectroscopy and ferroelectricity of PMN-0.28PT under high pressure. The appearance of a new Raman band and the sudden redshift of UV-vis absorption edge imply a structural phase transition at about 8 GPa. More importantly, the ferroelectricity of the sample is suppressed above a pressure of 5 GPa, and there is no ferroelectricity under further compression. We suggest that the disappearance of ferroelectricity may be related to the polar nanoregions being suppressed by pressure. Our observations of ferroelectricity disappearance above 5 GPa indicate the B-site cation rearrangement in the several nanometers region.

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 pi-pi 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(sp(2) + sp(3))-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 C=C 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 transstilbene 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.

We have developed a new type of multi-functional mechanochromic material by integrating a phenyl rotator and a donor–acceptor unit into the structure of BPMA.

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.

The pressure-induced phase transition and polymerization of nitrogen-rich molecules are widely focused on due to their extreme importance for the development of green high-energy-density materials. Here, we present a study of the phase-transition behaviour and chemical reaction of 1H-tetrazole up to 100 GPa using in situ Raman, IR, X-ray diffraction, neutron diffraction techniques and theoretical calculations. A phase transition above 2.6 GPa was identified and the high-pressure structure was determined with one molecule in a unit cell instead of two molecules as reported before. The 1H-tetrazole polymerized reversibly below 100 GPa, probably through carbon-nitrogen bonding instead of nitrogen-nitrogen bonding. Our studies update the structure model of the high-pressure phase of 1H-tetrazole, and present the possible intermolecular bonding route for the first time, which gives new insights to understand the phase transition and chemical reaction of nitrogen-rich compounds, and is of benefit for designing new high-energy-density materials.

Black phosphorus (BP) shows great potential in electronic and optoelectronic applications; however, maintaining the stable performance of BP devices over temperature is still challenging. Here, a novel BP field-effect transistor (FET) fabricated on the atomic layer deposited AlN/SiO2/Si substrate is demonstrated. Electrical measurement results show that BP FETs on the AlN substrate possess superior electrical performance compared with those fabricated on the conventional SiO2/Si substrate. It exhibits a large on-off current ratio of 5 × 108, a low subthreshold swing of <0.26 V/dec, and a high normalized field-effect carrier mobility of 1071 cm2 V−1 s−1 in the temperature range from 77 to 400 K. However, these stable electrical performances are not found in the BP FETs on SiO2/Si substrate when the temperature increases up to 400 K; instead, the electrical performance of BP FETs on the SiO2/Si substrate degrades drastically. Furthermore, to gain a physical understanding on the stable performance of BP FETs on the AlN substrate, low-frequency noise analysis was performed, and it revealed that the AlN film plays a significant role in suppressing the lattice scattering and charge trapping effects at high temperatures.

Pressure-induced polymerization (PIP) of metal acetylides is a novel method to synthesize a metal-carbon framework and polycarbide materials with unique structures and properties. However, the pressure required for the PIP of C-2(2-) is too high for large-scale synthesis. In this work, we investigated the PIP of monosodium acetylide (NaC2H) by performing in situ Raman spectroscopy, infrared spectroscopy, X-ray diffraction, and impedance spectroscopy up to 30 GPa and ex situ gas chromatography-mass spectrometry on the recovered sample. NaC2H experiences a phase transition at 7 GPa and polymerizes at 14 GPa, which is the lowest PIP pressure of acetylide to date and already in the working range of a large volume press. At the reaction threshold, the nearest intermolecular C center dot center dot center dot C distance is about 2.9 angstrom, which is almost the same as that of CaC2 and indicates a topochemical initiation. The PIP is mainly a free radical addition process. The termination of the free radicals limits the composition of the produced polycarbide anions CxHyn- within x - 2 <= y + n <= x + 2. Our work discloses the threshold of the intermolecular distance for the PIP of acetylide and proposes the reaction mechanism, which furthers the investigation of its high-pressure chemical reaction.

Two-dimensional (2D) materials have been proposed as promising candidate for spintronic applications due to their atomic crystal structure and physical properties. Here, we introduce exfoliated few-layer tungsten diselenide (WSe2) as spacer in a Py/WSe2/Py vertical spin valve. In this junction, the WSe2 spacer exhibits metallic behavior. We observed negative magnetoresistance (MR) with a ratio of -1.1% at 4 K and -0.21% at 300 K. A general phenomenological analysis of the negative MR property is discussed. Our result is anticipated to be beneficial for future spintronic applications. (C) 2017 Elsevier B.V. All rights reserved.