Reconfigurable Micro/Nano-Optical Devices Based on Phase Transitions: From Materials, Mechanisms to Applications

In recent years, numerous efforts have been devoted to exploring innovative micro/nano-optical devices (MNODs) with reconfigurable functionality, which is highly significant because of the progressively increasing requirements for next-generation photonic systems. Fortunately, phase change materials (PCMs) provide an extremely competitive pathway to achieve this goal. The phase transitions induce significant changes to materials in optical, electrical properties or shapes, triggering great research interests in applying PCMs to reconfigurable micro/nano-optical devices (RMNODs). More specifically, the PCMs-based RMNODs can interact with incident light in on-demand or adaptive manners and thus realize unique functions. In this review, RMNODs based on phase transitions are systematically summarized and comprehensively overviewed from materials, phase change mechanisms to applications. The reconfigurable optical devices consisting of three kinds of typical PCMs are emphatically introduced, including chalcogenides, transition metal oxides, and shape memory alloys, highlighting the reversible state switch and dramatic contrast of optical responses along with designated utilities generated by phase transition. Finally, a comprehensive summary of the whole content is given, discussing the challenge and outlooking the potential development of the PCMs-based RMNODs in the future.

Far-Infrared Signatures for a Two-Step Pressure-Driven Metallization in Transition Metal Dichalcogenides

We present a high-pressure investigation of the semiconductor-to-metal transition in MoS₂ and WS₂ carried out by synchrotron-based far-infrared spectroscopy, to reconcile the controversial estimates of the metallization pressure found in the literature and gain new insight into the mechanisms ruling this electronic transition. Two spectral descriptors are found indicative of the onset of metallicity and of the origin of the free carriers in the metallic state: the absorbance spectral weight, whose abrupt increase defines the metallization pressure threshold, and the asymmetric line shape of the E_1u peak, whose pressure evolution, interpreted within the Fano model, suggests the electrons in the metallic state originate from n-type doping levels. Combining our results with those reported in the literature, we hypothesize a two-step mechanism is at work in the metallization process, in which the pressure-induced hybridization between doping and conduction band states drives an early metallic behavior, while the band gap closes at higher pressures.

Recent developments in 2D transition metal dichalcogenides: phase transition and applications of the (quasi-)metallic phases

The advent of two-dimensional transition metal dichalcogenides (2D-TMDs) has led to an extensive amount of interest amongst scientists and engineers alike and an intensive amount of research has brought about major breakthroughs in the electronic and optical properties of 2D materials. This in turn has generated considerable interest in novel device applications. With the polymorphic structural features of 2D-TMDs, this class of materials can exhibit both semiconducting and metallic (quasi-metallic) properties in their respective phases. This polymorphic property further increases the interest in 2D-TMDs both in fundamental research and for their potential utilization in novel high-performance device applications. In this review, we highlight the unique structural properties of few-layer and monolayer TMDs in the metallic 1T- and quasi-metallic 1T'-phases, and how these phases dictate their electronic and optical properties. An overview of the semiconducting-to-(quasi)-metallic phase transition of 2D-TMD systems will be covered along with a discussion on the phase transition mechanisms. The current development in the applications of (quasi)-metallic 2D-TMDs will be presented ranging from high-performance electronic and optoelectronic devices to energy storage, catalysis, piezoelectric and thermoelectric devices, and topological insulator and neuromorphic computing applications. We conclude our review by highlighting the challenges confronting the utilization of TMD-based systems and projecting the future developmental trends with an outlook of the progress needed to propel this exciting field forward.

Wrinkle and near-resonance effects on the vibrational and electronic properties in compressed monolayer MoSe2

Pressure has been considered as an effective technique to modulate the structural, electronic, and optical properties of transition metal dichalcogenide (TMDs) materials. Here, by performing in situ high pressure Raman, photoluminescence (PL) and absorption measurements, we systematically investigated the vibrational and electronic properties evolution of monolayer MoSe₂ grown on a SiO₂/Si substrate under high pressure. When the pressure increased up to 4.84 GPa, an unexpected phonon mode at 367 cm^-1 appeared, which was identified as the Raman-inactive A₂'' mode and was activated under high pressure. Combined with the analysis of absorption spectroscopy, this phenomenon can be attributed to the pressure-induced wrinkle and near-resonance effects in compressed monolayer MoSe₂. Subsequently, A₁' split into two peaks after 7.44 GPa, providing further distinct evidence for the pressure-induced wrinkle effect in compressed monolayer MoSe₂. Moreover, this wrinkle effect can also lead to a rapid quenching of photoluminescence in monolayer MoSe₂. These results suggest that the substrate plays an important role in determining the vibrational and electronic properties of compressed monolayer MoSe₂, and can provide valuable information on the electronic and optoelectronic applications of monolayer MoSe₂ under extreme conditions.

Pressure-induced isostructural electronic topological transitions in 2H-MoTe2: x-ray diffraction and first-principles study

Synchrotron x-ray diffraction measurements on powder 2H-MoTe₂ (P6₃/mmc) up to ∼46 GPa have been performed along with first-principles based density functional theoretical analysis to probe the isostructural transition in low pressure regime and two electronic topological transitions (ETT) of Lifshitz-type in high pressure regime. The low pressure isostructural transition at ∼7 GPa is associated with the lattice parameter ratio c/a anomaly and the change in the compressibility of individual layers. The pressure dependence of the volume by linearizing the Birch-Murnaghan equation of state as a function of Eulerian strain shows a clear change of the bulk modulus at the ETT pressure of ∼20 GPa. The minimum of c/a ratio around 32 GPa is associated with the change in topology of electron pockets marked as second ETT of Lifshitz-type. We do not observe any structural transition up to the maximum applied pressure of ∼46 GPa under quasi-hydrostatic condition.

2D Materials and Heterostructures at Extreme Pressure

2D materials possess wide-tuning properties ranging from semiconducting and metallization to superconducting, etc., which are determined by their structure, empowering them to be appealing in optoelectronic and photovoltaic applications. Pressure is an effective and clean tool that allows modifications of the electronic structure, crystal structure, morphologies, and compositions of 2D materials through van der Waals (vdW) interaction engineering. This enables an insightful understanding of the variable vdW interaction induced structural changes, structure-property relations as well as contributes to the versatile implications of 2D materials. Here, the recent progress of high-pressure research toward 2D materials and heterostructures, involving graphene, boron nitride, transition metal dichalcogenides, 2D perovskites, black phosphorene, MXene, and covalent-organic frameworks, using diamond anvil cell is summarized. A detailed analysis of pressurized structure, phonon dynamics, superconducting, metallization, doping together with optical property is performed. Further, the pressure-induced optimized properties and potential applications as well as the vision of engineering the vdW interactions in heterostructures are highlighted. Finally, conclusions and outlook are presented on the way forward.

Direct synthesis of metastable phases of 2D transition metal dichalcogenides

The different polymorphic phases of transition metal dichalcogenides (TMDs) have attracted enormous interest in the last decade. The metastable metallic and small band gap phases of group VI TMDs displayed leading performance for electrocatalytic hydrogen evolution, high volumetric capacitance and some of them exhibit large gap quantum spin Hall (QSH) insulating behaviour. Metastable 1T(1T') phases require higher formation energy, as compared to the thermodynamically stable 2H phase, thus in standard chemical vapour deposition and vapour transport processes the materials normally grow in the 2H phases. Only destabilization of their 2H phase via external means, such as charge transfer or high electric field, allows the conversion of the crystal structure into the 1T(1T') phase. Bottom-up synthesis of materials in the 1T(1T') phases in measurable quantities would broaden their prospective applications and practical utilization. There is an emerging evidence that some of these 1T(1T') phases can be directly synthesized via bottom-up vapour- and liquid-phase methods. This review will provide an overview of the synthesis strategies which have been designed to achieve the crystal phase control in TMDs, and the chemical mechanisms that can drive the synthesis of metastable phases. We will provide a critical comparison between growth pathways in vapour- and liquid-phase synthesis techniques. Morphological and chemical characteristics of synthesized materials will be described along with their ability to act as electrocatalysts for the hydrogen evolution reaction from water. Phase stability and reversibility will be discussed and new potential applications will be introduced. This review aims at providing insights into the fundamental understanding of the favourable synthetic conditions for the stabilization of metastable TMD crystals and at stimulating future advancements in the field of large-scale synthesis of materials with crystal phase control.

Recent progress of TMD nanomaterials: phase transitions and applications

Transition metal dichalcogenides (TMDs) show wide ranges of electronic properties ranging from semiconducting, semi-metallic to metallic due to their remarkable structural differences. To obtain 2D TMDs with specific properties, it is extremely important to develop particular strategies to obtain specific phase structures. Phase engineering is a traditional method to achieve transformation from one phase to another controllably. Control of such transformations enables the control of properties and access to a range of properties, otherwise inaccessible. Then extraordinary structural, electronic and optical properties lead to a broad range of potential applications. In this review, we introduce the various electronic properties of 2D TMDs and their polymorphs, and strategies and mechanisms for phase transitions, and phase transition kinetics. Moreover, the potential applications of 2D TMDs in energy storage and conversion, including electro/photocatalysts, batteries/supercapacitors and electronic devices, are also discussed. Finally, opportunities and challenges are highlighted. This review may further promote the development of TMD phase engineering and shed light on other two-dimensional materials of fundamental interest and with potential ranges of applications.

A new criterion for the prediction of solid-state phase transition in TMDs

The classical thermodynamic criterion for phase transition predicts whether the phase transition will occur according to whether the nth derivative of the state parameter is discontinuous, and the continuity verification of multi-order derivatives increases the difficulty and complexity of judgment for phase transition to a certain extent. Based on the reverse shifts of the DOS curves near the Fermi level, we propose a new criterion for solid-state phase transition named Conch Criterion, which has been verified in the TMD system. The new criterion can observe the occurrence of phase transition from another perspective besides the thermodynamic properties while mutually confirming the thermodynamic criterion.

Pressure-induced enhancement in the thermoelectric properties of monolayer and bilayer SnSe2

The electronic structures of monolayer and bilayer SnSe₂ under pressure were investigated by using first-principles calculations including van der Waals interactions. For monolayer SnSe₂, the variation of electronic structure under pressure is controlled by pressure-dependent lattice parameters. For bilayer SnSe₂, the changes in electronic structure under pressure are dominated by intralayer and interlayer atomic interactions. The n-type thermoelectric properties of monolayer and bilayer SnSe₂ under pressure were calculated on the basis of the semi-classical Boltzmann transport theory. It was found that the electrical conductivity of monolayer and bilayer SnSe₂ can be enhanced under pressure, and such dependence can be attributed to the pressure-induced changes of the Se-Sn antibonding states in conduction band. Finally, the doping dependence of power factors of n-type monolayer and bilayer SnSe₂ at three different pressures were estimated, and the results unveiled that thermoelectric performance of n-type monolayer and bilayer SnSe₂ can be improved by applying external pressure. This study benefits to understand the nature of the transport properties for monolayer and bilayer SnSe₂ under pressure, and it offers valuable insight for designing high-performance thermoelectric few-layered SnSe₂ through strain engineering induced by external pressure.

Pressure-dependent semiconductor to semimetal and Lifshitz transitions in 2H-MoTe2: Raman and first-principles studies

High pressure Raman spectroscopy of bulk 2H-MoTe₂ up to  ∼29 GPa is shown to reveal two phase transitions (at  ∼6 and 16.5 GPa), which are analyzed using first-principles density functional theoretical calculations. The transition at 6 GPa is marked by changes in the pressure coefficients of A _1g and [Formula: see text] Raman mode frequencies as well as in their relative intensity. Our calculations show that this is an isostructural semiconductor to a semimetal transition. The transition at  ∼16.5 GPa is identified with the changes in linewidths of the Raman modes as well as in the pressure coefficients of their frequencies. Our theoretical analysis clearly shows that the structure remains the same up to 30 GPa. However, the topology of the Fermi-surface evolves as a function of pressure, and abrupt appearance of electron and hole pockets at [Formula: see text] GPa marks a Lifshitz transition.

Tellurization Velocity-Dependent Metallic-Semiconducting-Metallic Phase Evolution in Chemical Vapor Deposition Growth of Large-Area, Few-Layer MoTe2

Phase engineering of two-dimensional (2D) transition metal dichalcogenides (TMDs) such as MoTe₂ offers tremendous opportunities in various device applications. However, most of the existing methods so far only address the small-area local phase change or the growth of certain kinds of phases of MoTe₂ film by laser irradiation, mechanical strain, or procursor type. Obtaining facile, tunable, reversible, and continuous-phase transition and evolution between different phases in direct growth of large-area, few-layer MoTe₂ still remains challenging. Here, we develop a facile method to achieve phase control and transition and report a highly tunable, tellurization velocity-dependent metallic-semiconducting-metallic phase evolution in chemical vapor deposition (CVD) growth of large-area, few-layer MoTe₂. We found four different phase stages, including two different types of coexistence phases of both 2H and 1 T' phases, 100% 2H phase, and 100% 1T' phase, would emerge, relying on the adopted tellurization velocity. Importantly, the tellurization velocity should be extremely controlled to obtain 100% 2H phase MoTe₂, while 100% 1T' phase requires a fast tellurization velocity. We further found that such metallic-semiconducting-metallic phase evolution took place with a homogeneous spatial distribution and differs from previous reports in which obvious phase separations are usually found during the phase transition. The resulting MoTe₂ shows high quality with room-temperature mobility comparable with mechanically exfoliated materials. The results might impact large-scale phase engineering of TMDs and other 2D materials for Weyl semimetal topological physics and potential 2D semiconductor device applications.

Two-dimensional hydrogenated molybdenum and tungsten dinitrides MN2H2 (M = Mo, W) as novel quantum spin hall insulators with high stability

Based on first-principles calculations, we predict the existence of the quantum spin Hall (QSH) effect in hydrogenated transition-metal nitrides MN₂H₂ (M = Mo, W), showing high structural stability. MN₂H₂ monolayers are identified to be intrinsic topological insulators (TIs) with protected Dirac type topological helical edge states, and show robust topological features against the large stretching strain. Besides, sizeable intrinsic nontrivial band gaps (70-124 meV) ensure the QSH effect in MN₂H₂ at room temperature. The pure d-d band inversion was revealed. More interestingly, the topological phase transition between a QSH phase and a topological semimetallic phase can be induced by applying in-plane strain.