Exploring nanomechanics with high-pressure techniques

Theoretical and Experimental Advances in High-Pressure Behaviors of Nanoparticles

Using compressive mechanical forces, such as pressure, to induce crystallographic phase transitions and mesostructural changes while modulating material properties in nanoparticles (NPs) is a unique way to discover new phase behaviors, create novel nanostructures, and study emerging properties that are difficult to achieve under conventional conditions. In recent decades, NPs of a plethora of chemical compositions, sizes, shapes, surface ligands, and self-assembled mesostructures have been studied under pressure by in-situ scattering and/or spectroscopy techniques. As a result, the fundamental knowledge of pressure-structure-property relationships has been significantly improved, leading to a better understanding of the design guidelines for nanomaterial synthesis. In the present review, we discuss experimental progress in NP high-pressure research conducted primarily over roughly the past four years on semiconductor NPs, metal and metal oxide NPs, and perovskite NPs. We focus on the pressure-induced behaviors of NPs at both the atomic- and mesoscales, inorganic NP property changes upon compression, and the structural and property transitions of perovskite NPs under pressure. We further discuss in depth progress on molecular modeling, including simulations of ligand behavior, phase-change chalcogenides, layered transition metal dichalcogenides, boron nitride, and inorganic and hybrid organic-inorganic perovskites NPs. These models now provide both mechanistic explanations of experimental observations and predictive guidelines for future experimental design. We conclude with a summary and our insights on future directions for exploration of nanomaterial phase transition, coupling, growth, and nanoelectronic and photonic properties.

Squeezing indium arsenide single crystal to ultrafine nanostructured compact bulk

Reciprocating pressure-induced phase transition (RPPT) has been proposed as a new approach to synthesize nanostructured bulk materials with clean grain boundary interfaces for structures that undergo reversible pressure-induced phase transitions. The modulation effects on grain size under different cycle numbers of RPPT for InAs were investigated and the initial single-crystal bulk, with a dimensional size of about 30 μm, was transformed into a nanostructure with an average grain size of 7 nm by the utilization of the in situ high-pressure diamond anvil cell (DAC) technique. To verify the DAC findings, compact nanostructured bulk InAs with grain sizes ranging from 2-20 nm (average = 8 nm) and large dimensions (3.2 mm × 3.2 mm × 0.5 mm) was successfully synthesized from single-crystal InAs using a large volume press (LVP). The smaller work function (3.86 eV) and larger bandgap energy (2.64 eV) of the compact nanostructured bulk InAs phase compared to those of single-crystal InAs demonstrated that the nanostructure affected the macroscopic properties of InAs. The findings confirm the feasibility of synthesizing nanostructured bulk materials via RPPT.

Phase Transitions in Amorphous Germanium under Non-Hydrostatic Compression

As the pioneer semiconductor in transistor, germanium (Ge) has been widely applied in information technology for over half a century. Although many phase transitions in Ge have been reported, the complicated phenomena of the phase structures in amorphous Ge under extreme conditions are still not fully investigated. Here, we report the different routes of phase transition in amorphous Ge under different compression conditions utilizing diamond anvil cell (DAC) combined with synchrotron-based X-ray diffraction (XRD) and Raman spectroscopy techniques. Upon non-hydrostatic compression of amorphous Ge, we observed that shear stress facilitates a reversible pressure-induced phase transformation, in contrast to the pressure-quenchable structure under a hydrostatic compression. These findings afford better understanding of the structural behaviors of Ge under extreme conditions, which contributes to more potential applications in the semiconductor field.

Pressure Tuning of Optical Properties and Structures in All-Inorganic Halide Perovskite Rb7Sb3Cl16

All lead-free inorganic halide perovskites, as efficient solid-state light emission materials, have become ideal green optoelectronic materials to replace lead halide perovskites for diversified lighting and display applications with their excellent stability. Here, we investigated the pressure-derived optical and structural response of a zero-dimensional lead-free perovskite Rb₇Sb₃Cl₁₆ through applying controllable pressure. A pressure-induced blue shift of the broadband emission was achieved, and it was followed by the emission color transformation from yellow to green, which was ascribed to the electron-phonon coupling weakening and the suppression of structural deformation upon lattice contraction. In parallel, the band gap was narrowed by about 0.5 eV as a result of enhanced metal halide orbital overlap under high pressure. This work provides a fundamental understanding for modulating the optical properties of the low-dimensional metal halide perovskites.

Metallization and Superconductivity in the van der Waals Compound CuP2Se through Pressure-Tuning of the Interlayer Coupling

Emergent layered Cu-bearing van der Waals (vdW) compounds have great potentials for use in electrocatalysis, lithium batteries, and electronic and optoelectronic devices. However, many of their alluring properties such as potential superconductivity remain unknown. In this work, using CuP₂Se as a model compound, we explored its electrical transport and structural evolution at pressures up to ∼60 GPa using both experimental determinations and ab initio calculations. We found that CuP₂Se undergoes a semiconductor-to-metal transition at ∼20 GPa at room temperature and a metal-to-superconductor transition at 3.3-5.7 K in the pressure range from 27.0 to 61.4 GPa. At ∼10 and 20 GPa, there are two isostructural changes in the compound, corresponding to, respectively, the emergence of the interlayer coupling and start of interlayer atomic bonding. At a pressure between 35 and 40 GPa, the vdW layers start to slide and then merge, forming a new phase with high coordination numbers. We also found that the Bardeen-Cooper-Schrieffer (BCS) theory describes quite well the pressure dependence of the critical temperature despite occurrence of a possible medium-to-strong electron-phonon coupling, revealing the determinant roles of the enhanced bulk modulus and electron density of states at high pressure. Moreover, nanosizing of CuP₂Se at high pressure further increased the critical temperature even at sizes approaching the Anderson limit. These findings would have important implications for developing novel applications of layered vdW compounds through simple pressure tuning of the interlayer coupling.

Ultra-incompressible High-Entropy Diborides

Transition metal borides are commonly hard and incompressible, offering great opportunities for advanced applications under extreme conditions. Recent studies show that the hardness of high-entropy borides may exceed that of their constituent simple borides due to the "cocktail effect". However, how high-entropy borides deform elastically remains largely unknown. Here, we show that two newly synthesized high-entropy diborides are ultra-incompressible, attaining ∼90% of the incompressibility of single-crystalline diamond and exhibiting a 50-60% enhancement over the density functional theory predictions. This unusual behavior is attributed to a Hall-Petch-like effect resulting from nanosizing under high pressure, which increases the bulk moduli through dynamic dislocation interactions and creation of stacking faults. The exceptionally low compressibility, together with their high phase stabilities, high hardness, and high electric conductance, renders them promising candidates for electromechanics and microelectronic devices that demand strong resistance to environmental impacts, in addition to traditional grinding and abrading.