Qinglin Wang

High-pressure electrical transport properties of KNbO3: Experimental and theoretical approaches

The electrical transport behavior of SnS under high pressure has been investigated by the temperature dependence of electrical resistivity measurement, the in situ Hall-effect measurement, and the first-principle calculation. The experimental results show that SnS undergoes a semiconductor to semimetal transition at ∼10.3 GPa, and this transition is further substantiated by the band-structure calculation. The total and partial density of states predict that the semimetal character of SnS is attributed to the enhanced coupling of Sn-5s, Sn-5p, and S-3p states with application of pressure. In addition, dramatic changes in electrical transport parameters such as the electrical resistivity, the carrier concentration, and the carrier mobility are observed at 12.6 GPa, which are correlated to the pressure-induced Pnma-Cmcm structural phase transition.

Mixed conduction and grain boundary effect in lithium niobate under high pressure

The charge transport behavior of lithium niobate has been investigated by in situ impedance measurement up to 40.6 GPa. The Li+ ionic conduction plays a dominant role in the transport process. The relaxation process is described by the Maxwell-Wagner relaxation arising at the interfaces between grains and grain boundaries. The grain boundary microstructure rearranges after the phase transition, which improves the bulk dielectric performance. The theoretical calculations show that the decrease of bulk permittivity with increasing pressure in the Pnma phase is caused by the pressure-induced enhancement of electron localization around O atoms, which limits the polarization of Nb-O electric dipoles.

Electrical properties and behaviors of cuprous oxide cubes under high pressure.

An accurate in situ electrical resistivity measurement of cuprous oxide cubes has been conducted in a diamond anvil cell at room temperature with pressures up to 25 GPa. The abnormal electrical resistivity variation found at 0.7-2.2 GPa is attributed to the phase transformation from a cubic to a tetragonal structure. Three other discontinuous changes in the electrical resistivity are observed around 8.5, 10.3, and 21.6 GPa, corresponding to the phase transitions from tetragonal to pseudocubic to hexagonal to another hexagonal phase, respectively. The first-principles calculations illustrate that the electrical resistivity decrease of the tetragonal phase is not related to band-gap shrinkage but related to a higher quantity of electrons excited from strain-induced states increasing in band gap with increasing pressure. The results indicate that the Cu(2)O cubes begin to crush at about 15 GPa and completely transform into nanocrystalline at 25 GPa.

Effect of crystallization water on the structural and electrical properties of CuWO4 under high pressure

The effect of crystallization water on the structural and electrical properties of CuWO4 under high pressure has been investigated by in situ X-ray diffraction and alternating current impedance spectra measurements. The crystallization water was found to be a key role in modulating the structural stability of CuWO4 at high pressures. The anhydrous CuWO4 undergoes two pressure-induced structural transitions at 8.8 and 18.5 GPa, respectively, while CuWO4·2H2O keeps its original structure up to 40.5 GPa. Besides, the crystallization water makes the electrical transport behavior of anhydrous CuWO4 and CuWO4·2H2O quite different. The charge carrier transportation is always isotropic in CuWO4·2H2O, but anisotropic in the triclinic and the third phase of anhydrous CuWO4. The grain resistance of CuWO4·2H2O is always larger than that of anhydrous CuWO4 in the entire pressure range. By analyzing the relaxation response, we found that the large number of hydrogen bonds can soften the grain characteristic frequency o...

The determination of ionic transport properties at high pressures in a diamond anvil cell

A two-electrode configuration was adopted in an in situ impedance measurement system to determine the ionic conductivity at high pressures in a diamond anvil cell. In the experimental measurements, Mo thin-films were specifically coated on tops of the diamond anvils to serve as a pair of capacitance-like electrodes for impedance spectrum measurements. In the spectrum analysis, a Warburg impedance element was introduced into the equivalent circuit to reveal the ionic transport property among other physical properties of a material at high pressures. Using this method, we were able to determine the ionic transport character including the ionic conductivity and the diffusion coefficient of a sodium azide solid to 40 GPa.

Enhancement of 1.5 μm emission under 980 nm resonant excitation in Er and Yb co-doped GaN epilayers

The Erbium (Er) doped GaN is a promising gain medium for optical amplifiers and solid-state high energy lasers due to its high thermal conductivity, wide bandgap, mechanical hardness, and ability to emit in the highly useful 1.5 μm window. Finding the mechanisms to enhance the optical absorption efficiency at a resonant pump wavelength and emission efficiency at 1.5 μm is highly desirable. We report here the in-situ synthesis of the Er and Yb co-doped GaN epilayers (Er + Yb:GaN) by metal-organic chemical vapor deposition (MOCVD). It was observed that the 1.5 μm emission intensity of the Er doped GaN (Er:GaN) under 980 nm resonant pump can be boosted by a factor of 7 by co-doping the sample with Yb. The temperature dependent PL emission at 1.5 μm in the Er + Yb:GaN epilayers under an above bandgap excitation revealed a small thermal quenching of 12% from 10 to 300 K. From these results, it can be inferred that the process of energy transfer from Yb3+ to Er3+ ions is highly efficient, and non-radiative reco...

Metallization and Hall-effect of Mg2Ge under high pressure

The electrical transport properties of Mg2Ge under high pressure were studied with the in situ temperature-dependent resistivity and Hall-effect measurements. The theoretically predicted metallization of Mg2Ge was definitely found around 7.4 GPa by the temperature-dependent resistivity measurement. Other two pressure-induced structural phase transitions were also reflected by the measurements. Hall-effect measurement showed that the dominant charge carrier in the metallic Mg2Ge was hole, indicating the “bad metal” nature of Mg2Ge. The Hall mobility and charge carrier concentration results pointed out that the electrical transport behavior in the antifluorite phase was controlled by the increase quantity of drifting electrons under high pressure, but in both anticotunnite and Ni2In-type phases it was governed by the Hall mobility.

High pressure electrical transport behavior in organic semiconductor pentacene

The high pressure electrical transport behavior of pentacene has been investigated by alternating current impedance techniques and direct current resistivity measurement in a diamond anvil cell (DAC). The resistance decreases with increasing pressure below 17.4 GPa, while it increases above 17.4 GPa, which is caused by the transition of pentacene from an ordered state to the disordered state under higher pressure. From the Raman spectra under various pressures, pentacene becomes amorphous above 17.3 GPa, which is consistent with the impedance results. The charge transport operates in the hopping regime with charges jumping between interacting molecules, and the hopping mechanisms are related to the vibration modes. Above 17.4 GPa, the pressure dependence of the relaxation activation energy is 21.7 meV/GPa and pentacene keeps semiconductor characteristics up to 28.3 GPa.

Pressure Dependence of Mixed Conduction and Photo Responsiveness in Organolead Tribromide Perovskites

The electrical transport properties of CH3NH3PbBr3 (MAPbBr3) polycrystals were in situ investigated by alternating-current impedance spectroscopy under high pressures up to 5.6 GPa. It is confirmed that ionic and electronic conductions coexist in MAPbBr3. As pressure below 3.3 GPa ions migration is the predominant process, while above 3.3 GPa electronic conduction becomes the main process. An obvious ionic-electronic transition can be observed. The pressure dependent photo responsiveness of MAPbBr3 was also studied by in situ photocurrent measurements up to 3.8 GPa. The mixed conduction can be clearly seen in photocurrent measurement. Additionally, the photocurrents remain robust below 2.4 GPa, while they are suppressed with pressure-induced partial amorphization. Interestingly, the photoelectric response of MAPbBr3 can be enhanced by high pressure, and the strongest photocurrent value appears in the high-pressure phase II at 0.7 GPa, which is similar to previous results in both MAPbI3 and MASnI3.

Ionic transport and dielectric properties in NaNbO3 under high pressure

The ionic transport and dielectric behaviors in NaNbO3 were studied under pressures up to 29.1 GPa by in situ impedance spectroscopy measurements. The transport process consists of the ionic transfer and the Warburg diffusion process between sample/electrode diffusion layers. A dielectric relaxation with a giant dielectric constant at low frequencies is observed, which is attributed to the “Maxwell-Wagner” interfacial polarization. In the Pbcm phase, the increase in the interaction between the Na+ ions and the NbO6 octahedra results in the enhancement of vibration resonance damping. In the high-pressure phases, the decrease in the relative permittivity with pressure indicates the existence of space charge polarization of the interface layer besides the ionic polarization.