Yanping Huang

Pressure-Dependent Light Emission of Charged and Neutral Excitons in Monolayer MoSe2

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.

Structural stability and compressive behavior of ZrH2 under hydrostatic pressure and nonhydrostatic pressure

The important transition metal dihydride ZrH2 has been characterized using in situ synchrotron X-ray diffraction combined with diamond anvil cell techniques and ab initio calculations. The effect of a pressure-transmitting medium on the structural stability and compressive behavior was investigated under both hydrostatic pressure and nonhydrostatic pressure conditions. The ambient I4/mmm structure is stable under both nonhydrostatic and hydrostatic compressions. The supplementary theoretical calculations have proposed that the I4/mmm structure transformed into the P4/nmm structure above 100 GPa confirming the stability of the I4/mmm structure during the experimental runs. The difference in the volume reduction between the two compressions becomes larger with increasing pressure. Up to about 50 GPa, the volume collapse of nonhydrostatic compression is 6% relative to the hydrostatic compression. The present study offers a new approach with nonhydrostatic compression or shear stress for finding higher volumetric density hydrogen structures in metal hydrides.

A Novel High-Density Phase and Amorphization of Nitrogen-Rich 1H-Tetrazole (CH 2 N 4 ) under High Pressure

The high-pressure behaviors of nitrogen-rich 1H-tetrazole (CH2N4) have been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman scattering up to 75u2009GPa. A first crystalline-to-crystalline phase transition is observed and identified above ~3u2009GPa with a large volume collapse (∼18% at 4.4u2009GPa) from phase I to phase II. The new phase II forms a dimer-like structure, belonging to P1 space group. Then, a crystalline-to-amorphous phase transition takes place over a large pressure range of 13.8 to 50u2009GPa, which is accompanied by an interphase region approaching paracrystalline state. When decompression from 75u2009GPa to ambient conditions, the final product keeps an irreversible amorphous state. Our ultraviolet (UV) absorption spectrum suggests the final product exhibits an increase in molecular conjugation.

Experimental verification of the high pressure crystal structures in NH3BH3

A detailed high-pressure study on NH3BH3 has been carried out using in situ synchrotron X-ray diffraction (XRD) and Raman scattering with a diamond anvil cell up to 20 and 33 GPa, respectively. The Rietveld refinement based on the XRD pattern and analysis of Raman data indicate two first-order phase transitions from the ambient pressure I4 mm structure (α-NH3BH3) to a high pressure Cmc21 phase (β-NH3BH3) at 2.14 GPa, and further into a monoclinic P21 (Z = 2) phase (γ-NH3BH3) at 9.67 GPa. Fitting the measured volumetric compression data to the third order Birch-Murnaghan equation of state reveals a bulk modulus of B0 = 9.9 ± 0.5 and 17.0 ± 3.0 GPa (with fixed B0 () = 4) for the β-NH3BH3 below and above 5 GPa, respectively. Still, with the splitting of the NBH rock mode in Raman experiment, it is concluded that a second-order isostructural phase transition occurs at 5 GPa. By analyzing the dihydrogen bonding framework, the origin of the isostructural phase transition is attributed to the number of dihydrogen bondings per molecule in the Cmc21 phase increasing from 12 to 14 at 5 GPa.

Pressure-induced structural transformation of CaC2

The high pressure structural changes of calcium carbide CaC2 have been investigated with Raman spectroscopy and synchrotron X-ray diffraction (XRD) techniques in a diamond anvil cell at room temperature. At ambient conditions, two forms of CaC2 co-exist. Above 4.9 GPa, monoclinic CaC2-ii diminished indicating the structural phase transition from CaC2-ii to CaC2-i. At about 7.0 GPa, both XRD patterns and Raman spectra confirmed that CaC2-i transforms into a metallic Cmcm structure which contains polymeric carbon chains. Along with the phase transition, the isolated C2 dumbbells are polymerized into zigzag chains resulting in a large volume collapse with 22.4%. Above 30.0 GPa, the XRD patterns of CaC2 become featureless and remain featureless upon decompression, suggesting an irreversible amorphization of CaC2.

High pressure Raman spectroscopy investigation on acetonitrile and acetonitrile–water mixture

High-pressure Raman scattering studies on pure acetonitrile and an acetonitrile–water mixture at a molar ratio of (nCH3CNu2006:u2006nH2O) 1u2006:u20067.25 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.

Structural properties of ammonium iodide under high pressure

The high-pressure behavior of ammonium iodide (NH4I) has been investigated by in situ synchrotron X-ray diffraction (XRD) and Raman scattering up to 40 GPa. The first-order phase transition from phase IV to V is confirmed by XRD measurements for the first time. Fitting the measured volumetric compression data to the third order Birch–Murnaghan equation of state reveals bulk moduli of B0 = 14.3 ± 1.3 and 28.5 ± 2.6 GPa (with fixed B′0 = 4) for phases IV and V, respectively. Still, by analyzing the red shift of the N–H symmetric and asymmetric modes and the intramolecular distances, it is concluded that hydrogen bonding is the dominant effect upon compression over the whole pressure range.

In situ synchrotron X-ray diffraction with laser-heated diamond anvil cells study of Pt up to 95 GPa and 3150 K

Platinum (Pt) has been widely studied for pressure calibration in high pressure–temperature ranges. We have for the first time performed in situ synchrotron X-ray diffraction (XRD) with laser-heated diamond anvil cells to study the P–V–T equation of state (EOS) for Pt up to 95 GPa and 3150 K. MgO was used for pressure calibration. A detailed analysis of the room-temperature compression curve was fitted with the third-order Birch–Murnaghan (BM) EOS, which yields ambient volume V0 = 60.3 A3, isothermal bulk modulus K0 = 308 GPa, and its pressure derivative K′0 = 4.1. A least-squares fit of the P–V–T data to a high-temperature (BM) EOS yielded K′0 = 5.5 ± 2, K0 = 274 ± 36 GPa, αKT(V0, T) = 0.003 ± 0.0003 GPa K−1 and (∂KT/∂T)V = 0.03 ± 0.01 GPa K−1 with V0 = 60.3 A3. Within a reasonable range, it is found that the EOS of this study is consistent with the known EOS of Pt. The present technique and results cover the P–T range between the resistive heating and the shock compression experimental data in the literature.

Elastic properties of single crystal hydrogen sulfide: A Brillouin scattering study under high pressure-temperature

We have performed high pressure-temperature Brillouin scattering measurements on single crystal hydrogen sulfide using externally heated diamond anvil cell techniques. The pressure dependences of the acoustic velocities, isothermal elastic constants, and moduli of single crystal hydrogen sulfide have been determined along four isotherms. Both elastic constants and moduli increase monotonously with pressure along each isotherm, while they show a decreased tendency with temperature elevated under the same pressure points. The experimental equation of state of single crystal hydrogen sulfide is obtained by fitting with a third-order Birch-Murnaghan and Tait equation. It is proposed that the effect of hydrogen bonds contributes to the unique tendency of elastic anisotropy in single crystal hydrogen sulfide. Through our work, we have extended the melting curve and phase diagram of hydrogen sulfide up to 12u2009GPa and 580u2009K.We have performed high pressure-temperature Brillouin scattering measurements on single crystal hydrogen sulfide using externally heated diamond anvil cell techniques. The pressure dependences of the acoustic velocities, isothermal elastic constants, and moduli of single crystal hydrogen sulfide have been determined along four isotherms. Both elastic constants and moduli increase monotonously with pressure along each isotherm, while they show a decreased tendency with temperature elevated under the same pressure points. The experimental equation of state of single crystal hydrogen sulfide is obtained by fitting with a third-order Birch-Murnaghan and Tait equation. It is proposed that the effect of hydrogen bonds contributes to the unique tendency of elastic anisotropy in single crystal hydrogen sulfide. Through our work, we have extended the melting curve and phase diagram of hydrogen sulfide up to 12u2009GPa and 580u2009K.

The stability of B6 octahedron in BaB6 under high pressure

We have performed in situ synchrotron X-ray diffraction and first-principles calculations to explore the compression behavior of barium hexaboride (BaB6) under high pressure. No phase transitions in our experiment are observed up to 49.3 GPa at ambient temperature. It is found that the ambient cage structure (Pmm) is still stable with a basic covalent network during the experimental pressure run. The results of our theoretical calculations show that the ambient structure might transform into three dynamically stable structures (Cmmm, Cmcm and I4/mmm) at 78 GPa, 97 GPa and 105 GPa respectively. The energy band calculations indicate that the sample is still a semiconductor with a narrow gap at 50 GPa.