Quasi-One-Dimensional Metallicity in Compressed CsSnI3.

Low-dimensional metal halides exhibit strong structural and electronic anisotropies, making them candidates for accessing unusual electronic properties. Here, we demonstrate pressure-induced quasi-one-dimensional (quasi-1D) metallicity in δ-CsSnI3. With the application of pressure up to 40 GPa, the initially insulating δ-CsSnI3 transforms to a metallic state. Synchrotron X-ray diffraction and Raman spectroscopy indicate that the starting 1D chain structure of edge-sharing Sn-I octahedra in δ-CsSnI3 is maintained in the high-pressure metallic phase while the SnI6 octahedral chains are distorted. Our experiments combined with first-principles density functional theory calculations reveal that pressure induces Sn-Sn hybridization and enhances Sn-I coupling within the chain, leading to band gap closure and formation of conductive SnI6 distorted octahedral chains. In contrast, the interchain I...I interactions remain minimal, resulting in a highly anisotropic electronic structure and quasi-1D metallicity. Our study offers a high-pressure approach for achieving diverse electronic platforms in the broad family of low-dimensional metal halides.

Large bandgap of pressurized trilayer graphene.

Graphene-based nanodevices have been developed rapidly and are now considered a strong contender for postsilicon electronics. However, one challenge facing graphene-based transistors is opening a sizable bandgap in graphene. The largest bandgap achieved so far is several hundred meV in bilayer graphene, but this value is still far below the threshold for practical applications. Through in situ electrical measurements, we observed a semiconducting character in compressed trilayer graphene by tuning the interlayer interaction with pressure. The optical absorption measurements demonstrate that an intrinsic bandgap of 2.5 ± 0.3 eV could be achieved in such a semiconducting state, and once opened could be preserved to a few GPa. The realization of wide bandgap in compressed trilayer graphene offers opportunities in carbon-based electronic devices.

Pressure-Induced Emission (PIE) and Phase Transition of a Two-dimensional Halide Double Perovskite (BA)4 AgBiBr8 (BA=CH3 (CH2 )3 NH3 + ).

Two-dimensional (2D) halide perovskites have attracted significant attention due to their compositional flexibility and electronic diversity. Understanding the structure-property relationships in 2D double perovskites is essential for their development for optoelectronic applications. In this work, we observed the emergence of pressure-induced emission (PIE) at 2.5 GPa with a broad emission band and large Stokes shift from initially nonfluorescent (BA)4 AgBiBr8 (BA=CH3 (CH2 )3 NH3 + ). The emission intensity increased significantly upon further compression up to 8.2 GPa. Moreover, the band gap narrowed from the starting 2.61 eV to 2.19 eV at 25.0 GPa accompanied by a color change from light yellow to dark yellow. Analysis of combined in situ high-pressure photoluminescence, absorption, and angle-dispersive X-ray diffraction data indicates that the observed PIE can be attributed to the emission from self-trapped excitons. This coincides with [AgBr6 ]5- and [BiBr6 ]3- inter-octahedral tilting which cause a structural phase transition. High-pressure study on (BA)4 AgBiBr8 sheds light on the relationship between the structure and optical properties that may improve the material's potential applications in the fields of pressure sensing, information storage and trademark security.

Correlation between the structural change and the electrical transport properties of indium nitride under high pressure.

We report on the intriguing structural and electrical transport properties of compressed InN. Pronounced anomalies of the resistivity, Hall coefficient, electron concentration, and mobility are observed at ∼11.5 GPa, accompanied by a wurtzite-rocksalt structural transition confirmed using high-pressure XRD measurements and first-principles calculations. The pressure-tuned electrical properties of wurtzite and rocksalt InN are also studied, respectively. Particularly, compression pressure significantly decreases the electron concentration of rocksalt InN by two orders of magnitude and increases the mobility by ten fold. The obvious variations in electrical parameters can be rationalized by our band structure simulations, which reveal a direct-indirect energy crossover at 10 GPa, followed by the rapidly increasing patterns of the energy gap with a pressure coefficient of 33 meV GPa-1. Moreover the electron effective mass and energy gap are found to well satisfy with the k·p model. Definite correlation between the structural change and the electrical transport properties should shed a new light on building InN-based applications in the future.

Pressure-driven semiconducting-semimetallic transition in SnSe.

In this work, we report the pressure-dependent electrical transport and structural properties of SnSe. In our experiments an electronic transition from a semiconducting to semimetallic state was observed at 12.6 GPa, followed by an orthorhombic to monoclinic structural transition. Hall effect measurements indicate that both the carrier concentration and mobility vary abnormally accompanied by the semimetallic electronic transition. First-principles band structure calculations confirm the semiconducting-semimetallic transition, and reveal that the semimetallic character of SnSe can be attributed to the enhanced coupling of Sn-5s, Sn-5p, and Se-3p orbitals under compression that results in the broadening of energy bands and subsequently the closure of the band gap. The pressure modulated variations of electrical transport and structural properties may provide an approach to improving the thermoelectric properties of SnSe.

Electrical transport properties of AlAs under compression: reversible boundary effect.

Herein, we report on the intriguing electrical transport properties of compressed AlAs. The relative permittivity and the resistances of both the grain and bulk boundaries vary abnormally at ∼10.9 GPa, accompanied by the cubic-hexagonal structural transition of AlAs. With further compression, the boundary effect becomes undistinguished, and subsequently, the electrical transport mechanism converts from boundary- to bulk-dominated, which gives rise to a significant reduction in the total resistance of AlAs. After being quenched to ambient pressure, resistances recover to the initial values followed by the re-emergence of the boundary effect. Eg decreases with pressure and its pressure dependence changes at ∼14.0 GPa, which rationalizes the anomalous variation of the electrical transport properties. The experimental results indicate that the boundary effect can be modulated by compression and increases the resistance difference between the two states. This opens up a new possible basis for optimizing the performance of AlAs-based applications, including multilevel phase-change memories.

Cesium-mediated electron redistribution and electron-electron interaction in high-pressure metallic CsPbI3.

Electron-phonon coupling was believed to govern the carrier transport in halide perovskites and related phases. Here we demonstrate that electron-electron interaction enhanced by Cs-involved electron redistribution plays a direct and prominent role in the low-temperature electrical transport of compressed CsPbI3 and renders Fermi liquid (FL)-like behavior. By compressing δ-CsPbI3 to 80 GPa, an insulator-semimetal-metal transition occurs, concomitant with the completion of a slow structural transition from the one-dimensional Pnma (δ) phase to a three-dimensional Pmn21 (ε) phase. Deviation from FL behavior is observed upon CsPbI3 entering the metallic ε phase, which progressively evolves into a FL-like state at 186 GPa. First-principles density functional theory calculations reveal that the enhanced electron-electron coupling results from the sudden increase of the 5d state occupation in Cs and I atoms. Our study presents a promising strategy of cationic manipulation for tuning the electronic structure and carrier scattering of halide perovskites at high pressure.

Preserving a robust CsPbI3 perovskite phase via pressure-directed octahedral tilt.

Functional CsPbI3 perovskite phases are not stable at ambient conditions and spontaneously convert to a non-perovskite δ phase, limiting their applications as solar cell materials. We demonstrate the preservation of a black CsPbI3 perovskite structure to room temperature by subjecting the δ phase to pressures of 0.1 - 0.6 GPa followed by heating and rapid cooling. Synchrotron X-ray diffraction and Raman spectroscopy indicate that this perovskite phase is consistent with orthorhombic γ-CsPbI3. Once formed, γ-CsPbI3 could be then retained after releasing pressure to ambient conditions and shows substantial stability at 35% relative humidity. First-principles density functional theory calculations indicate that compression directs the out-of-phase and in-phase tilt between the [PbI6]4- octahedra which in turn tune the energy difference between δ- and γ-CsPbI3, leading to the preservation of γ-CsPbI3. Here, we present a high-pressure strategy for manipulating the (meta)stability of halide perovskites for the synthesis of desirable phases with enhanced materials functionality.

Pressure-Engineered Optical and Charge Transport Properties of Mn2+/Cu2+ Codoped CsPbCl3 Perovskite Nanocrystals via Structural Progression.

In this work, compared with the corresponding pure CsPbCl3 nanocrystals (NCs) and Mn2+-doped CsPbCl3 NCs, Mn2+/Cu2+-codoped CsPbCl3 NCs exhibited improved photoluminescence (PL) and photoluminescence quantum yields (PL QYs) (57.6%), prolonged PL lifetimes (1.78 ms), and enhanced thermal endurance (523 K) as a result of efficient Mn2+ doping (3.66%) induced by the addition of CuCl2. Furthermore, we applied pressure on Mn2+/Cu2+-codoped CsPbCl3 NCs to reveal that a red shift of photoluminescence followed by a blue shift was caused by band gap evolution and related to the structural phase transition from cubic to orthorhombic. Moreover, we also found that under the preheating condition of 523 K, such phase transition exhibited obvious morphological invariance, accompanied by significantly enhanced conductivity. The pressure applied to the products treated with high temperature enlarged the electrical difference and easily intensified the interface by closer packaging. Interestingly, defect-triggered mixed ionic and electronic conducting (MIEC) was observed in annealed NCs when the applied pressure was 2.9 GPa. The pressure-dependent ionic conduction was closely related to local nanocrystal amorphization and increased deviatoric stress, as clearly described by in situ impedance spectra. Finally, retrieved products exhibited better conductivity (improved by 5-6 times) and enhanced photoelectric response than those when pressure was not applied. Our findings not only reveal the pressure-tuned optical and electrical properties via structural progression but also open up the promising exploration of more amorphous all-inorganic CsPbX3-based photoelectric applications.

Visible light response, electrical transport, and amorphization in compressed organolead iodine perovskites.

Recent scientific advances on organic-inorganic hybrid perovskites are mainly focused on the improvement of power conversion efficiency. So far, how compression tunes their electronic and structural properties remains less understood. By combining in situ photocurrent, impedance spectroscopy, and X-ray diffraction (XRD) measurements, we have studied the electrical transport and structural properties of compressed CH3NH3PbI3 (MAPbI3) nanorods. The visible light response of MAPbI3 remains robust below 3 GPa while it is suppressed when it becomes amorphous. Pressure-induced electrical transport properties of MAPbI3 including resistance, relaxation frequency, and relative permittivity have been investigated under pressure up to 8.5 GPa by in situ impedance spectroscopy measurements. These results indicate that the discontinuous changes of these physical parameters occur around the structural phase transition pressure. The XRD studies of MAPbI3 under high pressure up to 20.9 GPa show that a phase transformation below 0.7 GPa, could be attributed to the tilting and distortion of PbI6 octahedra. And pressure-induced amorphization is reversible at a low density amorphous state but irreversible at a relatively higher density state. Furthermore, the MAPbI3 nanorods crush into nanopieces around 0.9 GPa which helps us to explain why the mixed phase of tetragonal and orthorhombic was observed at 0.5 GPa. The pressure modulated changes of electrical transport and visible light response properties open up a new approach for exploring CH3NH3PbI3-based photo-electronic applications.