Pressure-Induced Phase Transitions in Germanium Telluride: Raman Signatures of Anharmonicity and Oxidation

Raman Spectroscopy on Free-Base Meso-tetra(4-pyridyl) Porphyrin under Conditions of Low Temperature and High Hydrostatic Pressure

We present a Raman spectroscopy study of the vibrational properties of free-base meso-tetra(4-pyridyl) porphyrin polycrystals under various temperature and hydrostatic pressure conditions. The combination of experimental results and Density Functional Theory (DFT) calculations allows us to assign most of the observed Raman bands. The modifications in the Raman spectra when excited with 488 nm and 532 nm laser lights indicate that a resonance effect in the Qy band is taking place. The pressure-dependent results show that the resonance conditions change with increasing pressure, probably due to the shift of the electronic transitions. The temperature-dependent results show that the relative intensities of the Raman modes change at low temperatures, while no frequency shifts are observed. The experimental and theoretical analysis presented here suggest that these molecules are well represented by the C2v point symmetry group.

Gate and Temperature Driven Phase Transitions in Few-Layer MoTe2

MoTe₂ has a stable hexagonal semiconducting phase (2H) as well as two semimetallic phases with monoclinic (1T') and orthorhombic (T_d) structures. A structural change can thus be accompanied by a significant change in electronic transport properties. The two semimetallic phases are connected by a temperature driven transition and could exhibit topological properties. Here we make extensive Raman measurements as a function of layer thickness, temperature, and electrostatic doping on few layer 2H-MoTe₂ and also on 1T'-MoTe₂ and T_d-WTe₂. Recent work in MoTe₂ has raised the possibility of a 2H-1T' transition through technology compatible pathways. It has been claimed that such a transition, of promise for device applications, is activated by electrostatic gating. We investigate this claim and find that few-layer tellurides are characterized by high mobility of Te ions, even in ambient conditions and especially through the variation of external parameters like electric field or temperature. These can generate Te clusters, vacancies at crystalline sites, and facilitate structural transitions. We however find that the purported 2H-1T' transition in MoTe₂ cannot be obtained by a pure electrostatic field.

High figure-of-merit and power generation in high-entropy GeTe-based thermoelectrics

The high-entropy concept provides extended, optimized space of a composition, resulting in unusual transport phenomena and excellent thermoelectric performance. By tuning electron and phonon localization, we enhanced the figure-of-merit value to 2.7 at 750 kelvin in germanium telluride–based high-entropy materials and realized a high experimental conversion efficiency of 13.3% at a temperature difference of 506 kelvin with the fabricated segmented module. By increasing the entropy, the increased crystal symmetry delocalized the distribution of electrons in the distorted rhombohedral structure, resulting in band convergence and improved electrical properties. By contrast, the localized phonons from the entropy-induced disorder dampened the propagation of transverse phonons, which was the origin of the increased anharmonicity and largely depressed lattice thermal conductivity. We provide a paradigm for tuning electron and phonon localization by entropy manipulation, but we have also demonstrated a route for improving the performance of high-entropy thermoelectric materials.

Dynamics of ultrafast phase transitions in MgF2 triggered by laser-induced THz coherent phonons

The advent of free-electron lasers opens new routes for experimental high-pressure physics, which allows studying dynamics of condensed matter with femtosecond resolution. A rapid compression, that can be caused by laser-induced shock impact, leads to the cascade of high-pressure phase transitions. Despite many decades of study, a complete understanding of the lattice response to such a compression remains elusive. Moreover, in the dynamical case (in contrast to quasi-static loading) the thresholds of phase transitions can change significantly. Using the third harmonic pump–probe technique combined with molecular dynamics to simulate the terahertz (THz) spectrum, we revealed the dynamics of ultrafast laser-induced phase transitions in MgF₂ in all-optical experiment. Tight focusing of femtosecond laser pulse into the transparent medium leads to the generation of sub-TPa shock waves and THz coherent phonons. The laser-induced shock wave propagation drastically displaces atoms in the lattice, which leads to phase transitions. We registered a cascade of ultrafast laser-induced phase transitions (P42/mnm ⇒ Pa-3  ⇒ Pnam) in magnesium fluoride as a change in the spectrum of coherent phonons. The phase transition has the characteristic time of 5–10 ps, and the lifetime of each phase is on the order of 40–60 ps. In addition, phonon density of states, simulated by molecular dynamics, together with third-harmonic time-resolved spectra prove that laser-excited phonons in a bulk of dielectrics are generated by displacive excitation (DECP) mechanism in plasma mediated conditions.

Sub-Picosecond Non-Equilibrium States in the Amorphous Phase of GeTe Phase-Change Material Thin Films

The sub-picosecond response of amorphous germanium telluride thin film to a femtosecond laser excitation is investigated using frequency-domain interferometry and ab initio molecular dynamics. The time-resolved measurement of the surface dynamics reveals a shrinkage of the film with a dielectric properties' response faster than 300 fs. The systematic ab initio molecular dynamics simulations in non-equilibrium conditions allow the atomic configurations to be retrieved for ionic temperature from 300 to 1100 K and width of the electron distribution from 0.001 to 1.0 eV. Local order of the structures is characterized by in-depth analysis of the angle distribution, phonon modes, and pair distribution function, which evidence a transition toward a new amorphous electronic excited state close in bonding/structure to the liquid state. The results shed a new light on the optically highly excited states in chalcogenide materials involved in both important processes: phase-change materials in memory devices and ovonic threshold switching phenomenon induced by a static field.

Superconducting Phase Induced by a Local Structure Transition in Amorphous Sb_{2}Se_{3} under High Pressure

Superconductivity and Anderson localization represent two extreme cases of electronic behavior in solids. Surprisingly, these two competing scenarios can occur in the same quantum system, e.g., in an amorphous superconductor. Although the disorder-driven quantum phase transition has attracted much attention, its structural origins remain elusive. Here, we discovered an unambiguous correlation between superconductivity and density in amorphous Sb_{2}Se_{3} at high pressure. Superconductivity first emerges in the high-density amorphous (HDA) phase at about 24 GPa, where the density of glass unexpectedly exceeds its crystalline counterpart, and then shows an enhanced critical temperature when pressure induces crystallization at 51 GPa. Ab initio simulations reveal that the bcc-like local geometry motifs form in the HDA phase, arising from distinct "metavalent bonds." Our results demonstrate that HDA phase is critical for the incipient superconductive behavior.

Pressure-induced structural transitions between successional superconducting phases in GeTe

Being a best-known prototype of phase-change materials, GeTe was reported to possess many high-pressure phases, whose structural evolution and superconductivity remain under debate for decades. Herein, we systematically investigated the pressure dependence of electrical transport and the structural evolution of the GeTe viain situangle-dispersive synchrotron x-ray diffraction and resistance measurements up to 55 GPa. At room temperature, the structural phase transitions from the initial rhombohedral phase to theFm3̄mphase, and then to an orthorhombicPnmaphase, were observed at pressures of about 4 and 13.4 GPa, respectively. Furthermore, the metallization occurred at around 11 GPa, where the superconductivity could also be observed. With increasing pressure, the superconducting transition temperature increases monotonically from 5.7 to 6.4 K and then is independent of pressure above 23 GPa in the purePnmaphase. These results provide insights into the pressure-dependent evolution of the structure and superconductivity in GeTe and have implications for the understanding of other IV-VI semiconductors at high pressure.

Raman spectrum of layered tilkerodeite (Pd2HgSe3) topological insulator: the palladium analogue of jacutingaite (Pt2HgSe3)

The layered mineral tilkerodeite (Pd₂HgSe₃), the palladium analogue of jacutingaite (Pt₂HgSe₃), is a promising quantum spin hall insulator for low-power nanospintronics. In this context, a fast and reliable assessment of its structure is key for exploring fundamental properties and architecture of new Pd₂HgSe₃-based devices. Here, we investigate the first-order Raman spectrum in high-quality, single-crystal bulk tilkerodeite, and analyze the wavenumber relation to its isostructural jacutingaite analogue. By using polarized Raman spectroscopy, symmetry analysis, and first-principles calculations, we assigned all the Raman-active phonons in tilkerodeite, unveiling their wavenumbers, atomic displacement patterns, and symmetries. Our calculations used several exchange-correlation functionals within the density functional perturbation theory framework, reproducing both structure and Raman-active phonon wavenumbers in excellent agreement with experiments. Also, it was found that the influence of the spin-orbit coupling can be neglected in the study of these properties. Finally, we compared the wavenumber and atomic displacement patterns of corresponding Raman-active modes in tilkerodeite and jacutingaite, and found that the effect of the Pd and Pt masses can be neglected on reasoning their wavenumber differences. From this analysis, tilkerodeite is found to be mechanically weaker than jacutingaite against the atomic displacement patterns of these modes. Our findings advance the understanding of the structural properties of a recently discovered layered topological insulator, fundamental to further exploring its electronic, optical, thermal, and mechanical properties, and for device fabrication.

Flux periodic oscillations and phase-coherent transport in GeTe nanowire-based devices

Despite the fact that GeTe is known to be a very interesting material for applications in thermoelectrics and for phase-change memories, the knowledge on its low-temperature transport properties is only limited. We report on phase-coherent phenomena in the magnetotransport of GeTe nanowires. From universal conductance fluctuations measured on GeTe nanowires with Au contacts, a phase-coherence length of about 280 nm at 0.5 K is determined. The distinct phase-coherence is confirmed by the observation of Aharonov-Bohm type oscillations for parallel magnetic fields. We interpret the occurrence of these magnetic flux-periodic oscillations by the formation of a tubular hole accumulation layer. For Nb/GeTe-nanowire/Nb Josephson junctions we obtained a critical current of 0.2 μA at 0.4 K. By applying a perpendicular magnetic field the critical current decreases monotonously with increasing field, whereas in a parallel field the critical current oscillates with a period of the magnetic flux quantum confirming the presence of a tubular hole channel.

Pressure-Dependent Behavior of Defect-Modulated Band Structure in Boron Arsenide

The recent observation of unusually high thermal conductivity exceeding 1000 W m^-1 K^-1 in single-crystal boron arsenide (BAs) has led to interest in the potential application of this semiconductor for thermal management. Although both the electron/hole high mobilities have been calculated for BAs, there is a lack of experimental investigation of its electronic properties. Here, a photoluminescence (PL) measurement of single-crystal BAs at different temperatures and pressures is reported. The measurements reveal an indirect bandgap and two donor-acceptor pair (DAP) recombination transitions. Based on first-principles calculations and time-of-flight secondary-ion mass spectrometry results, the two DAP transitions are confirmed to originate from Si and C impurities occupying shallow energy levels in the bandgap. High-pressure PL spectra show that the donor level with respect to the conduction band minimum shrinks with increasing pressure, which affects the release of free carriers from defect states. These findings suggest the possibility of strain engineering of the transport properties of BAs for application in electronic devices.

Functional Monochalcogenides: Raman Evidence Linking Properties, Structure, and Metavalent Bonding

Pressure- and temperature-dependent Raman scattering in GeSe, SnSe, and GeTe for pressures beyond 50 GPa and for temperatures ranging from 78 to 800 K allow us to identify structural and electronic phase transitions, similarities between GeSe and SnSe, and differences with GeTe. Calculations help to deduce the propensity of GeTe for defect formation and the doping that results from it, which gives rise to strong Raman damping beyond anomalous anharmonicity. These properties are related to the underlying chemical bonding and consistent with a recent classification of bonding in several chalcogenide materials that puts GeTe in a separate class of "incipient" metals.

New Pressure Stabilization Structure in Two-Dimensional PtSe2

The frequency shifts and lattice dynamics to unveil the vibrational properties of platinum diselenide (PtSe₂) are investigated using pressure-dependent polarized Raman scattering at room temperature up to 25 GPa. The two phonon modes E_g and A_1g display similar hardening trends; both the Raman peak positions and full widths at half-maximum have distinct mutation phenomena under high pressure. Especially, the split E_g mode at 4.3 GPa confirms the change of the lattice symmetry. With the aid of the first-principles calculations, a new pressure stabilization structure C2/m of PtSe₂ has been found to be in good agreement with experiments. The band structures calculations reveal that the new phase is a novel type-I Dirac semimetal. The results demonstrate that the pressure-dependent Raman spectra combined with theoretical predictions may open a new window for searching and controlling the phase structure and Dirac cones of two-dimensional materials.

Emerging pnictogen-based 2D semiconductors: sensing and electronic devices

Pnictogens are an intensively studied group of monoelemental two-dimensional materials. This group of elements consists of phosphorus, arsenic, antimony, and bismuth. In this group, the elements adopt two different layered structural allotropes, orthorhombic structure with true van der Waals layered interactions and rhombohedral structure, where covalent interactions between layers are also present. The orthorhombic structure is well known for phosphorus and arsenic, and the rhombohedral structure is the most thermodynamically stable allotropic modification of arsenic, antimony, and bismuth. Due to the electronic structure of pnictogen layers and their semiconducting character, these materials have huge application potential for electronic devices such as transistors and sensors including photosensitive devices as well as gas and electrochemical sensors. While photodetection and gas sensing applications are often related to lithography processed materials, chemical sensing proceeds in a liquid environment (either aqueous or non-aqueous) and can be influenced by surface oxidation of these materials. In this review, we explore the current state of pnictogen applications in sensing and electronic devices including transistors, photodetectors, gas sensors, and chemical/electrochemical sensors.

Probing Spatial Phonon Correlation Length in Post-Transition Metal Monochalcogenide GaS Using Tip-Enhanced Raman Spectroscopy

The knowledge of the phonon coherence length is of great importance for two-dimensional-based materials since phonons can limit the lifetime of charge carriers and heat dissipation. Here we use tip-enhanced Raman spectroscopy (TERS) to measure the spatial correlation length L_c of the A_1g¹ and A_1g² phonons of monolayer and few-layer gallium sulfide (GaS). The differences in L_c values are responsible for different enhancements of the A_1g modes, with A_1g¹ always enhancing more than the A_1g², independently of the number of GaS layers. For five layers, the results show an L_c of 64 and 47 nm for A_1g¹ and A_1g², respectively, and the coherence lengths decrease when decreasing the number of layers, indicating that scattering with the surface roughness plays an important role.