Janus layers and electronic structure of 1T-(TiSeS)2

TiS2-TiSe2 is one of the most studied titanium based solid solution systems. However, so far, all research on it has only focused on its disordered phase. Here, we systematically investigate its ordered phases. Using a structure search method based on the particle swarm optimization (CALYPSO) algorithm, we identify TiSeS-156 and discover a new structure (1T-(TiSeS)2). Based on first principles theory, their phonon spectra, formation energy, mechanical, electronic, thermal, and optical properties, as well as chemical bond analysis and synthetic pathways, have been investigated. The primitive cell of TiSeS-156 has three atoms and has a space group of P3m1 (no. 156). 1T-(TiSeS)2 has six atoms and has P3̄m1 symmetry (no. 164). TiSeS-156 and 1T-(TiSeS)2 are constructed by stacking the S-Ti-Se Janus layer materials. TiSeS-156 and 1T-(TiSeS)2 are narrow-gap semiconductors. The localized nature of the Ti(3d) states of TiSeS-156 and 1T-(TiSeS)2 leads to their semiconductor properties. 1T-(TiSeS)2 and TiSeS-156 have very similar mechanical, electronic, thermal, and optical properties of 1T-TiS2 and 1T-TiSe2, and are members of the 2D hexagonal lattice transition metal dichalcogenide layered material family. However, compared with 1T-TiS2 and 1T-TiSe2, TiSeS-156 and 1T-(TiSeS)2 have a wider range of potential applications, such as photovoltaic devices and photocatalysis, due to their S-Ti-Se Janus layer structure. They also provide a pathway for the preparation of Janus TiSeS monolayer and multi-layer materials. Moreover, our findings provide crucial insights for understanding the rich and complex crystal structures of the TiS2-TiSe2 system, which have broad implications for further exploration of this class of promising materials.

Tensile strain and finite size modulation of low lattice thermal conductivity in monolayer TMDCs (HfSe2 and ZrS2) from first-principles: a comparative study

With excellent physical and chemical properties, 2D TMDC materials have been widely used in engineering applications, but they inevitably suffer from the dual effects of strain and device size. As typical 2D TMDCs, HfSe₂ and ZrS₂ are reported to have excellent thermoelectric properties. Thermal transport properties have great significance for exerting the performance of materials, ensuring device lifetime and stable operation, but current research is not detailed enough. Here, first-principles combined with the phonon Boltzmann transport equation are used to study the phonon transport inside monolayer HfSe₂ and ZrS₂ under tensile strain and finite size, and explore the band structure properties. Our research shows that they have similar phonon dispersion curve structures, and the band gap of HfSe₂ increases monotonically with the increase of tensile strain, while the bandgap of ZrS₂ increases and then decreases with the increase of tensile strain. Thermal conductivity has obvious strain dependence: with the increase of tensile strain, the thermal conductivity of HfSe₂ gradually decreases, while that of ZrS₂ increases slightly, and then gradually decreases. Reducing the system size can limit the contribution of phonons with a long mean free path, significantly decreasing thermal conductivity through the controlling effect of tensile strain. The mode contribution of thermal conductivity is systematically investigated, and anharmonic properties including mode and frequency-level scattering rates, group velocity and Grüneisen parameters are used to explain the associated mechanism. Phonon scattering processes and channels in various cases are discussed in detail. Our research provides a detailed understanding of the phonon transport and electronic structural properties of low thermal conductivity monolayers of HfSe₂ and ZrS₂, and further completes the study of thermal transport of the two materials under strain and size tuning, which will provide a foundation for further popularization and application.

Four-phonon and electron-phonon scattering effects on thermal properties in two-dimensional 2H-TaS2

Thermal transport characteristics of monolayer trigonal prismatic tantalum disulfide (2H-TaS₂) are investigated using first-principles calculations combined with the Boltzmann transport equation. Due to a large acoustic-optical phonon gap of 1.85 THz, the four-phonon (4ph) scattering significantly reduces the room-temperature phononic thermal conductivity (κ_ph). With the further inclusion of phonon-electron scattering, κ_ph reduces to 1.78 W mK^-1. Nevertheless, the total thermal conductivity (κ_total) of 7.82 W mK^-1 is dominated by the electronic thermal conductivity (κ_e) of 6.04 W mK^-1. Due to the electron-phonon coupling, κ_e differs from the typical estimation based on the Wiedemann-Franz law with a constant Sommerfeld value. This work provides new insights into the physical mechanisms for thermal transport in metallic 2D systems with strong anharmonic and electron-phonon coupling effects. The phonon scattering beyond three-phonon (3ph) scattering and even κ_e are typically overlooked in computations, and the constant Sommerfeld value is widely used for separating κ_e and κ_ph from the experimental thermal conductivity. These conclusions have implications for both the computational and experimental measurements of the thermal properties of transition metal dichalcogenides.

Photothermal and Enhanced Photocatalytic Therapies Conduce to Synergistic Anticancer Phototherapy with Biodegradable Titanium Diselenide Nanosheets

Nanomaterial-based photothermal and photocatalytic therapies are effective against various types of cancers. However, combining two or more materials is considered necessary to achieve the synergistic anticancer effects of photothermal and photocatalytic therapy, which made the preparation process complicated. Herein, the authors describe simple 2D titanium diselenide (TiSe₂ ) nanosheets (NSs) that can couple photothermal therapy with photocatalytic therapy. The TiSe₂ NSs are prepared using a liquid exfoliation method. They show a layered structure and possess high photothermal conversion efficiency (65.58%) and good biocompatibility. Notably, upon near-infrared irradiation, these NSs exhibit good photocatalytic properties with enhanced reactive oxygen species generation and H₂ O₂ decomposition in vitro. They can also achieve high temperatures, with heat improving their catalytic ability to further amplify oxidative stress and glutathione depletion in cancer cells. Furthermore, molecular mechanism studies reveal that the synergistic effects of photothermal and enhanced photocatalytic therapy can simultaneously lead to apoptosis and necrosis in cancer cells via the HSP90/JAK3/NF-κB/IKB-α/Caspase-3 pathway. Systemic exploration reveals that the TiSe₂ NSs has an appreciable degradation rate and accumulates passively in tumor tissue, where they facilitate photothermal and photocatalytic effects without obvious toxicity. Their study thus indicates the high potential of biodegradable TiSe₂ NSs in synergistic phototherapy for cancer treatment.

Novel thermoelectric performance of 2D 1T- Se2Te and SeTe2with ultralow lattice thermal conductivity but high carrier mobility

The design and search for efficient thermoelectric materials that can directly convert waste heat into electricity have been of great interest in recent years since they have practical applications in overcoming the challenges of global warming and the energy crisis. In this work, two new two-dimensional 1T-phase group-VI binary compounds Se₂Te and SeTe₂with outstanding thermoelectric performances are predicted using first-principles calculations combined with Boltzmann transport theory. The dynamic stability is confirmed based on phonon dispersion. It is found that the spin-orbit coupling effect has a significant impact on the band structure of SeTe₂, and induces a transformation from indirect to direct band gap. The electronic and phononic transport properties of the Se₂Te and SeTe₂monolayer are calculated and discussed. High carrier mobility (up to 3744.321 and 2295.413 cm²V^-1S^-1for electron and hole, respectively) is exhibited, suggesting great applications in nanoelectronic devices. Furthermore, the maximum thermoelectric figure of meritzTof SeTe₂for n-type and p-type is 2.88, 1.99 and 5.94, 3.60 at 300 K and 600 K, respectively, which is larger than that of most reported 2D thermoelectric materials. The surprising thermoelectric properties arise from the ultralow lattice thermal conductivityk_l(0.25 and 1.89 W m^-1K^-1for SeTe₂and Se₂Te at 300 K), and the origin of ultralow lattice thermal conductivity is revealed. The present results suggest that 1T-phase Se₂Te and SeTe₂monolayer are promising candidates for thermoelectric applications.