Monolayer PdSe2: A promising two-dimensional thermoelectric material

Thermal Transport Properties of Two-Dimensional Janus MoXSiN2 (X = S, Se, and Te)

The design of Janus materials offers an effective means of regulating both their physical and chemical properties, leading to their application in various fields. However, the underlying mechanism governing the modulation of the thermal transport characteristics through the construction of Janus materials remains unclear. In this work, we introduce VI-group elements into the MoSi2N4 structure, yielding two-dimensional Janus MoXSiN2 (X = S, Se, and Te) and systematically investigate their thermal transport properties based on first-principles calculation methods. Our findings reveal that the lattice thermal conductivities (κl) of MoSSiN2, MoSeSiN2, and MoTeSiN2 are 47.2, 24.3, and 40.6 W/mK at 300 K, respectively, significantly lower than that of MoSi2N4 (224 W/mK). Such low κl values mainly come from the introduction of X atoms, which enhances phonon scattering and reduces phonon vibration frequencies. In addition, MoTeSiN2 exhibits a higher κl compared to MoSeSiN2, contrary to the trend observed in most materials containing VI-group elements, where κl decreases gradually from S to Te. This anomalous behavior can be attributed to the competitive result between its lower phonon vibrational frequency and weaker phonon anharmonicity of MoTeSiN2. This work elucidates the inherent mechanism governing the modulation of thermal transport properties in Janus materials, thereby enhancing the potential application of Janus MoXSiN2 in engineering thermal management.

Two-dimensional Mo1-xB2 with ordered metal vacancies obtained for advanced thermoelectric applications based on first-principles calculations

The study and development of high thermoelectric properties is crucial for the next generation of microelectronic and wearable electronics. Derived from the recent experimental realization of layers of transition metal molybdenum and boride, we report the theoretical realization of advanced thermoelectric properties in two-dimensional (2D) transition metal boride Mo1-xB2 (x = 0, 0.05, 0.10, 0.125, 0.15)-based defect sheets. The introduction of metal vacancies results in stronger d-p exchange interactions and hybridization between the Mo-d and B-p atoms. Meanwhile, the ordered metal vacancies enabled transition metal borides (n-type Mo0.9B2) to widen the d-bandwidth and raise the d-band center, leading to a relatively high carrier mobility of 3262 cm2 V-1 s-1 and conductivity twice that of a bug-free n-type MoB2 layer, which indicates that it presents good electronic and thermal transport properties. Furthermore, investigations of the thermoelectric performance exhibit a maximum ZT of up to 3.29, which is superior to those of currently reported 2D materials. Modulation by defect engineering suggests that 2D transition metal boride sheets with ordered metal vacancies have promising applications in microelectronics, wearable electronics and thermoelectric devices.

Extraordinary Polarization and Thickness Dependences of Photocarrier Dynamics in PdSe2 Ribbons

Pentagonal palladium diselenide (PdSe2) stands out for its exceptional optoelectronic properties, including high carrier mobility, tunable bandgap, and anisotropic electronic and optical responses. Herein, we systematically investigate photocarrier dynamics in PdSe2 ribbons using polarization-resolved optical pump-probe spectroscopy. In thin PdSe2 ribbons with a semiconductor phase, the photocarrier dynamics are found to be dominated by intraband hot-carrier cooling, interband recombination, and the exciton effect, showing weak crystalline orientation dependences. Conversely, in thick semimetal-phase PdSe2 ribbons, the photocarrier relaxations governed by the electron-optical/acoustic phonon scattering strongly depend on the sample orientation, albeit with a degradation in in-plane anisotropy following hot-carrier cooling. Furthermore, we analyze the correlations between photocarrier dynamics and anisotropic energy dispersions of electronic structures across a wide range in k space, as well as the contributions from the anisotropic electron-phonon couplings. Our study provides crucial insights for developing polarization-sensitive photoelectronic devices based on PdSe2.

Room-Temperature High-Performance Photodetector and Phototransistor Based on PdSe2/ZnIn2S4 Alloy Heterojunctions

Various semiconductor devices have been developed based on 2D heterojunction materials owing to their distinctive optoelectronic properties. However, to achieve efficient charge transfer at their interface remains a major challenge. Herein, an alloy heterojunction concept is proposed. The sulfur vacancies in ZnIn2S4 are filled with selenium atoms of PdSe2. This chemically bonded heterojunction can significantly enhance the separation of photocarriers, providing notable advantages in the field of photoelectric conversion. As a demonstration, a two-terminal photodetector based on the PdSe2/ZnIn2S4 heterojunction materials is fabricated. The photodetector exhibits stable operation in ambient conditions, showcasing superior performance in terms of large photocurrent, high responsivity (48.8 mA W-1) and detectivity (1.98 × 1011 Jones). To further validate the excellent optoelectronic performance of the heterojunction, a tri-terminal phototransistor is also fabricated. Benefiting from gate voltage modulation, the photocurrent is amplified to milliampere level, and the responsivity is increased to 229.14 mA W-1. These findings collectively demonstrate the significant potential of the chemically bonded PdSe2/ZnIn2S4 alloy heterojunction for future optoelectronic applications.

Abnormal Thickness-Dependent Thermal Transport in Suspended 2D PdSe2

Research on 2D materials originally focused on the highly symmetrical materials like graphene, h-BN. Recently, 2D materials with low-symmetry lattice such as PdSe2 have drawn extensive attention, due to the interesting layer-dependent bandgap, promising mechanical properties and excellent thermoelectric performance, etc. In this work, the phonon thermal transport is studied in PdSe2 with a pentagonal fold structure. The thermal conductivity of PdSe2 flakes with different thicknesses ranging from few nanometers to several tens of nanometers is measured through the thermal bridge method, where the thermal conductivity increases from 5.04 W mk-1 for 60 nm PdSe2 to 34.51 W mk-1 for the few-layer one. The atomistic modelings uncover that with the thickness thinning down, the lattice of PdSe2 becomes contracted and the phonon group velocity is enhanced, leading to the abnormal increase in the thermal conductivity. And the upshift of the optical phonon modes contributes to the increase of the thermal conductivity as well by creating less acoustic phonon scattering as the thickness reduces. This study probes the interesting abnormal thickness-dependent thermal transport in 2D materials, which promotes the potential thermal management at nanoscale.

Ultrafast transient absorption measurements of photocarrier dynamics in PdSe2

We investigate the photocarrier dynamics in bulk PdSe2, a layered transition metal dichalcogenide with a novel pentagonal structure and unique electronic and optical properties. Using femtosecond transient absorption microscopy, we study the behavior of photocarriers in mechanically exfoliated bulk PdSe2 flakes at room temperature. By employing a 400 nm ultrafast laser pulse, electron-hole pairs are generated, and their dynamics are probed using an 800 nm detection pulse. Our findings reveal that the lifetime of photocarriers in bulk PdSe2 is approximately 210 ps. Furthermore, by spatially resolving the differential reflection signal, we determine a photocarrier diffusion coefficient of about 7.3 cm2 s-1. Based on these results, we estimate a diffusion length of around 400 nm and a photocarrier mobility of approximately 300 cm2 V-1 s-1. These results shed light on the ultrafast optoelectronic properties of PdSe2, offer valuable insights into photocarriers in this emerging material, and enable design of high-performance optoelectronic devices based on PdSe2.

Thermoelectric Properties of the Corbino Disk in Graphene

Thermopower and the Lorentz number for an edge-free (Corbino) graphene disk in the quantum Hall regime is calculated within the Landauer-Büttiker formalism. By varying the electrochemical potential, we find that amplitude of the Seebeck coefficient follows a modified Goldsmid-Sharp relation, with the energy gap defined by the interval between the zero and the first Landau levels in bulk graphene. An analogous relation for the Lorentz number is also determined. Thus, these thermoelectric properties are solely defined by the magnetic field, the temperature, the Fermi velocity in graphene, and fundamental constants including the electron charge, the Planck and Boltzmann constants, being independent of the geometric dimensions of the system. This suggests that the Corbino disk in graphene may operate as a thermoelectric thermometer, allowing to measure small temperature differences between two reservoirs, if the mean temperature magnetic field are known.

Insights into Structural, Electronic, and Transport Properties of Pentagonal PdSe2 Nanotubes Using First-Principles Calculations

One-dimensional (1D) novel pentagonal materials have gained significant attention as a new class of materials with unique properties that could influence future technologies. In this report, we studied the structural, electronic, and transport properties of 1D pentagonal PdSe₂ nanotubes (p-PdSe₂ NTs). The stability and electronic properties of p-PdSe₂ NTs with different tube sizes and under uniaxial strain were investigated using density functional theory (DFT). The studied structures showed an indirect-to-direct bandgap transition with slight variation in the bandgap as the tube diameter increased. Specifically, (5 × 5) p-PdSe₂ NT, (6 × 6) p-PdSe₂ NT, (7 × 7) p-PdSe₂ NT, and (8 × 8) p-PdSe₂ NT are indirect bandgap semiconductors, while (9 × 9) p-PdSe₂ NT exhibits a direct bandgap. In addition, under low uniaxial strain, the surveyed structures were stable and maintained the pentagonal ring structure. The structures were fragmented under tensile strain of 24%, and compression of -18% for sample (5 × 5) and -20% for sample (9 × 9). The electronic band structure and bandgap were strongly affected by uniaxial strain. The evolution of the bandgap vs. the strain was linear. The bandgap of p-PdSe₂ NT experienced an indirect-direct-indirect or a direct-indirect-direct transition when axial strain was applied. A deformability effect in the current modulation was observed when the bias voltage ranged from about 1.4 to 2.0 V or from -1.2 to -2.0 V. Calculation of the field effect I-V characteristic showed that the on/off ratio was large with bias potentials from 1.5 to 2.0 V. This ratio increased when the inside of the nanotube contained a dielectric. The results of this investigation provide a better understanding of p-PdSe₂ NTs, and open up potential applications in next-generation electronic devices and electromechanical sensors.

Excellent Thermoelectric Performance of 2D CuMN2 (M = Sb, Bi; N = S, Se) at Room Temperature

2D copper-based semiconductors generally possess low lattice thermal conductivity due to their strong anharmonic scattering and quantum confinement effect, making them promising candidate materials in the field of high-performance thermoelectric devices. In this work, we proposed four 2D copper-based materials, namely CuSbS₂, CuSbSe₂, CuBiS₂, and CuBiSe₂. Based on the framework of density functional theory and Boltzmann transport equation, we revealed that the monolayers possess high stability and narrow band gaps of 0.57~1.10 eV. Moreover, the high carrier mobilities (10²~10³ cm²·V^-1·s^-1) of these monolayers lead to high conductivities (10⁶~10⁷ Ω^-1·m^-1) and high-power factors (18.04~47.34 mW/mK²). Besides, as the strong phonon-phonon anharmonic scattering, the monolayers also show ultra-low lattice thermal conductivities of 0.23~3.30 W/mK at 300 K. As results show, all the monolayers for both p-type and n-type simultaneously show high thermoelectric figure of merit (ZT) of about 0.91~1.53 at room temperature.

Computational analysis of the enhancement of photoelectrolysis using transition metal dichalcogenide heterostructures

Finding a material with all the desired properties for a photocatalytic water splitter is a challenge yet to be overcome, requiring both a surface with ideal energetics for all steps in the hydrogen and oxygen evolution reactions (HER and OER) and a bulk band gap large enough to mediate said steps. We have instead examined separating these challenges by investigating the energetic properties of two-dimensional transition metal dichalcogenides (TMDCs) that could be used as a surface coating to a material with a large enough bulk band gap. First we investigated the energetics of monolayer MoS₂and PdSe₂using density functional theory and then investigated how these energetics changed when they were combined into a heterostructure. Our results show that the surface properties were practically (<0.2 eV) unchanged when combined and the MoS₂layer aligns well with the OER and HER. This work highlights the potential of TMDC monolayers as surface coatings for bulk materials that have sufficient band gaps for photocatalytic applications.

Mid-Infrared Optoelectronic Devices Based on Two-Dimensional Materials beyond Graphene: Status and Trends

Since atomically thin two-dimensional (2D) graphene was successfully synthesized in 2004, it has garnered considerable interest due to its advanced properties. However, the weak optical absorption and zero bandgap strictly limit its further development in optoelectronic applications. In this regard, other 2D materials, including black phosphorus (BP), transition metal dichalcogenides (TMDCs), 2D Te nanoflakes, and so forth, possess advantage properties, such as tunable bandgap, high carrier mobility, ultra-broadband optical absorption, and response, enable 2D materials to hold great potential for next-generation optoelectronic devices, in particular, mid-infrared (MIR) band, which has attracted much attention due to its intensive applications, such as target acquisition, remote sensing, optical communication, and night vision. Motivated by this, this article will focus on the recent progress of semiconducting 2D materials in MIR optoelectronic devices that present a suitable category of 2D materials for light emission devices, modulators, and photodetectors in the MIR band. The challenges encountered and prospects are summarized at the end. We believe that milestone investigations of 2D materials beyond graphene-based MIR optoelectronic devices will emerge soon, and their positive contribution to the nano device commercialization is highly expected.

Puckered Penta-like PdPX (X = O, S, Te) Semiconducting Nanosheets: First-Principles Study of the Mechanical, Electro-Optical, and Photocatalytic Properties

The atomic, electronic, optical, and mechanical properties of penta-like two-dimensional PdPX (X = O, S, Te) nanosheets have been systematically investigated using density functional theory calculations. All three PdPX nanosheets exhibit dynamic and mechanical stability on the basis of an analysis of phonon dispersions and the Born criteria, respectively. The PdPX monolayers are found to be brittle structures. Our calculations demonstrate that the PdPX nanosheets exhibit semiconducting characteristics with indirect band gaps of 0.93 (1.99), 1.34 (2.11), and 0.74 (1.51) eV for X = O, S, Te, respectively, using the PBE (HSE06) functional, where PdPTe is the best material for visible-light photocatalytic water splitting. Our findings give important basic characteristics of penta-like two-dimensional PdPX materials and should motivate further theoretical and experimental investigations of these interesting materials.

Strain modulating electronic band gaps and SQ efficiencies of semiconductor 2D PdQ2 (Q = S, Se) monolayer

We studied the physical, electronic transport and optical properties of a unique pentagonal PdQ₂ (Q = S, Se) monolayers. The dynamic stability of 2Dwrinkle like-PdQ₂ is proven by positive phonon frequencies in the phonon dispersion curve. The optimized structural parameters of wrinkled pentagonal PdQ₂ are in good agreement with the available experimental results. The ultimate tensile strength (UTHS) was calculated and found that, penta-PdS₂ monolayer can withstand up to 16% (18%) strain along x (y) direction with 3.44 GPa (3.43 GPa). While, penta-PdSe₂ monolayer can withstand up to 17% (19%) strain along x (y) dirrection with 3.46 GPa (3.40 GPa). It is found that, the penta-PdQ₂ monolayers has the semiconducting behavior with indirect band gap of 0.94 and 1.26 eV for 2D-PdS₂ and 2D-PdSe₂, respectively. More interestingly, at room temperacture, the hole mobilty (electron mobility) obtained for 2D-PdS₂ and PdSe₂ are 67.43 (258.06) cm² V⁻¹ s⁻¹ and 1518.81 (442.49) cm² V⁻¹ s⁻¹, respectively. In addition, I-V characteristics of PdSe₂ monolayer show strong negative differential conductance (NDC) region near the 3.57 V. The Shockly-Queisser (SQ) effeciency prameters of PdQ₂ monolayers are also explored and the highest SQ efficeinciy obtained for PdS₂ is 33.93% at −5% strain and for PdSe₂ is 33.94% at −2% strain. The penta-PdQ₂ exhibits high optical absorption intensity in the UV region, up to 4.04 × 10⁵ (for PdS₂) and 5.28 × 10⁵ (for PdSe₂), which is suitable for applications in optoelectronic devices. Thus, the ultrathin PdQ₂ monolayers could be potential material for next-generation solar-cell applications and high performance nanodevices.

High-Performance, Polarization-Sensitive, Long-Wave Infrared Photodetection via Photothermoelectric Effect with Asymmetric van der Waals Contacts

Long-wavelength infrared (LWIR) photodetection is important for heat-seeking technologies, such as thermal imaging, all-weather surveillance, and missile guidance. Among various detection techniques, photothermoelectric (PTE) detectors are promising in that they can realize ultra-broadband photodetection at room temperature without an external power supply. However, their performance in terms of speed, responsivity, and noise level in the LWIR regime still needs further improvement. Here, we demonstrated a high-performance PTE photodetector based on low-symmetry palladium selenide (PdSe₂) with asymmetric van der Waals contacts. The temperature gradient induced by asymmetric van der Waals contacts even under global illumination drives carrier diffusion to produce a photovoltage via the PTE effect. A responsivity of over 13 V/W, a response time of ∼50 μs, and a noise equivalent power of less than 7 nW/Hz^1/2 are obtained in the 4.6-10.5 μm regime at room temperature. Furthermore, due to the anisotropic absorption of PdSe₂, the detector exhibits a linear polarization angle sensitive response with an anisotropy ratio of 2.06 at 4.6 μm and 1.21 at 10.5 μm, respectively. Our proposed device architecture provides an alternative strategy to design high-performance photodetectors in the LWIR regime by utilizing van der Waals layered materials.

Atomically Thin 2D van der Waals Magnetic Materials: Fabrications, Structure, Magnetic Properties and Applications

Two-dimensional (2D) van der Waals (vdW) magnetic materials are considered to be ideal candidates for the fabrication of spintronic devices because of their low dimensionality, allowing the quantization of electronic states and more degrees of freedom for device modulation. With the discovery of few-layer Cr2Ge2Te6 and monolayer CrI3 ferromagnets, the magnetism of 2D vdW materials is becoming a research focus in the fields of material science and physics. In theory, taking the Heisenberg model with finite-range exchange interactions as an example, low dimensionality and ferromagnetism are in competition. In other words, it is difficult for 2D materials to maintain their magnetism. However, the introduction of anisotropy in 2D magnetic materials enables the realization of long-range ferromagnetic order in atomically layered materials, which may offer new effective means for the design of 2D ferromagnets with high Curie temperature. Herein, current advances in the field of 2D vdW magnetic crystals, as well as intrinsic and induced ferromagnetism or antiferromagnetism, physical properties, device fabrication, and potential applications, are briefly summarized and discussed.

Low-cost pentagonal NiX2 (X=S, Se, and Te) monolayers with strong anisotropy as potential thermoelectric materials

Pentagonal compounds, as a new family of 2D materials, have recently been extensively studied in the fields of electrocatalysis, photovoltaics, and thermoelectrics. Encouraged by the successful synthesis of pentagonal PdSe2,...

Prediction of stable silver selenide-based energy materials sustained by rubidium selenide alloying

Silver selenide (Ag2Se) is a ductile material with a low lattice thermal conductivity that can be a valuable substitute for both PbSe and Bi2Se3 for Pb toxicity free and Bi scarcity.

Thermoelectric, spin-dependent optical and quantum transport properties of 2D half-metallic Co2Se3

In this paper, the thermoelectric, spin-dependent optical and quantum transport properties of two-dimensional(2D) Co2Se3 monolayer are investigated using first principle calculations. The stability of Co2Se3 monolayer is confirmed by energy-cohesive...

Large In-Plane Anisotropic Terahertz Emission Induced by Asymmetric Polarization in Low-Symmetric PdSe2

Palladium diselenide (PdSe₂) exhibits air stability, low symmetry, and high carrier mobility, resulting in unique in-plane anisotropy for polarized optoelectronic devices. However, the relationship of the symmetry and the terahertz (THz) radiation remains elusive yet significant for both the THz source in technology and nonlinear optical physics in science. Herein, we observed large in-plane anisotropic THz radiation from multilayer PdSe₂ under femtosecond laser excitation. The THz emission demonstrates 2α dependence on the optical polarization angle from the resonant optical rectification combined with a background from the photocarrier acceleration under the surface depletion field. Interestingly, the in-plane THz emission along and perpendicular to the puckered direction demonstrates large anisotropy. Furthermore, the THz time-domain signals exhibit reversed polarities along the positive and negative puckered directions. This asymmetric polarization could relate to the bonding of Pd-Se, resulting in the unidirectional photon-induced current. Our results bridge the gap between the low-symmetry two-dimensional materials and the THz technology, which could promote the development of THz-polarized devices based on low-symmetry layered materials.

Enhanced thermoelectric properties in Sb/Ge core/shell nanowires through vacancy modulation

In the present work, we have modified the physical and electronic structure of Sb/Ge core/shell nanowires via vacancy creation and doping with foreign atoms with the aim to improve their thermoelectric energy conversion efficiency. Sb/Ge-NWs having a diameter of 1.5 Å show metallicity with 2G_o quantum conductance. The stability of the nanowires is assessed through the calculation of their formation energy. The formation of one vacancy at either the Sb- and Ge-site modifies substantially the electronic properties. From the comparison of the thermoelectric properties of the nanowires with and without the vacancy, we have found that the figure of merit for the Sb/Ge NW with one Sb vacancy increases of 0.18 compared to the pristine NW. The NW doping with different transition metals: Fe, Co, Ni and Cu have been found to also enhance the conversion efficiency. Thus, our calculations show that the thermoelectric performance of metal–semiconductor core–shell NWs can be in principle improved as much as 80% by vacancy formation and doping.

Thickness-dependent anisotropic transport of phonons and charges in few-layered PdSe2

So far, layered PdSe₂ has attracted much attention due to its completely tunable band-gap with varying layer numbers, yet the thickness-dependent transporting properties have been rarely studied. We have systematically studied the electronic structures, phonon and charge transport properties, and thermoelectric properties of few-layered (from 1L to 4L) and bulk PdSe₂ by first-principles calculations and Boltzmann transport theory. As the thickness increases, the energy levels of band edges relative to 4s of selenium move oppositely due to their different bonding states, leading to the power-law decrease of the band-gap. Meanwhile, the electron effective mass decreases rapidly while the hole effective mass increases significantly compared with those unperturbed. Calculations on elastic constants reveal that both bulk and few-layered PdSe₂ are mechanically stable, and the bulk is ductile with a Poisson's ratio of 0.27. The shifts of Raman active modes with respect to the thickness as well as their Gruneisen parameters are analyzed and the underlying physics is discussed. At room temperature, the thermal conductivities of the bulk are 7.7, 10.1 and 0.9 W m^-1 K^-1 along the a, b and c axes, respectively. It is found that the low-frequency modes (<2.0 THz) contribute about 80% of in-plane thermal conductivities. Due to the enhanced contribution from the ZA mode, the thermal conductivity of few-layered PdSe₂ is much larger than that of the bulk. The ZA mode is mainly scattered by itself and the Umklapp scattering dominates in the process as the thickness increases. Calculations on charge transport reveal that the electron mobility increases from 2.5-13.2 (1L) to 121.9-167.8 (4L) cm² V^-1 s^-1 with the decreasing anisotropy μ_b/μ_a, while the hole mobility remains to be ∼20 cm² V^-1 s^-1, which is in good agreement with the experimental results. Calculations on the thermoelectric properties reveal that the ZT value as well as the power factor increases largely as the thickness increases and it gets to be optimum for the triple layer. Interestingly, the transport of electrons and phonons is decoupled along the out-of-plane direction, which makes bulk PdSe₂ exhibit good thermoelectric performance along the c axis.

Realization of an Ideal Cairo Tessellation in Nickel Diazenide NiN2: High-Pressure Route to Pentagonal 2D Materials

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nanoelectronics. In this work we report the synthesis of nickel diazenide NiN₂ which possesses atomic-thick layers comprised of Ni₂N₃ pentagons forming Cairo-type tessellation. The layers of NiN₂ are weakly bonded with the calculated exfoliation energy of 0.72 J/m², which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN₂ is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN₂ to be a precursor of a pentagonal 2D material with a tunable direct band gap.

Applications of 2D-Layered Palladium Diselenide and Its van der Waals Heterostructures in Electronics and Optoelectronics

The rapid development of two-dimensional (2D) transition-metal dichalcogenides has been possible owing to their special structures and remarkable properties. In particular, palladium diselenide (PdSe₂) with a novel pentagonal structure and unique physical characteristics have recently attracted extensive research interest. Consequently, tremendous research progress has been achieved regarding the physics, chemistry, and electronics of PdSe₂. Accordingly, in this review, we recapitulate and summarize the most recent research on PdSe₂, including its structure, properties, synthesis, and applications. First, a mechanical exfoliation method to obtain PdSe₂ nanosheets is introduced, and large-area synthesis strategies are explained with respect to chemical vapor deposition and metal selenization. Next, the electronic and optoelectronic properties of PdSe₂ and related heterostructures, such as field-effect transistors, photodetectors, sensors, and thermoelectric devices, are discussed. Subsequently, the integration of systems into infrared image sensors on the basis of PdSe₂ van der Waals heterostructures is explored. Finally, future opportunities are highlighted to serve as a general guide for physicists, chemists, materials scientists, and engineers. Therefore, this comprehensive review may shed light on the research conducted by the 2D material community.

Highly Sensitive, Ultrafast, and Broadband Photo-Detecting Field-Effect Transistor with Transition-Metal Dichalcogenide van der Waals Heterostructures of MoTe2 and PdSe2

Recently, van der Waals heterostructures (vdWHs) based on transition-metal dichalcogenides (TMDs) have attracted significant attention owing to their superior capabilities and multiple functionalities. Herein, a novel vdWH field-effect transistor (FET) composed of molybdenum ditelluride (MoTe2 ) and palladium diselenide (PdSe2 ) is studied for highly sensitive photodetection performance in the broad visible and near-infrared (VNIR) region. A high rectification ratio of 6.3 × 105 is obtained, stemming from the sharp interface and low Schottky barriers of the MoTe2 /PdSe2 vdWHs. It is also successfully demonstrated that the vdWH FET exhibits highly sensitive photo-detecting abilities, such as noticeably high photoresponsivity (1.24 × 105 A W-1 ), specific detectivity (2.42 × 1014 Jones), and good external quantum efficiency (3.5 × 106 ), not only due to the intra-TMD band-to-band transition but also due to the inter-TMD charge transfer (CT) transition. Further, rapid rise (16.1 µs) and decay (31.1 µs) times are obtained under incident light with a wavelength of 2000 nm due to the CT transition, representing an outcome one order of magnitude faster than values currently in the literature. Such TMD-based vdWH FETs would improve the photo-gating characteristics and provide a platform for the realization of a highly sensitive photodetector in the broad VNIR region.

Thermoelectric transports in pristine and functionalized boron phosphide monolayers

Recently, a new monolayer Group III-V material, two-dimensional boron phosphide (BP), has shown great potential for energy storage and energy conversion applications. We study the thermoelectric properties of BP monolayer as well as the effect of functionalization by first-principles calculation and Boltzmann transport theory. Combined with a moderate bandgap of 0.90 eV and ultra-high carrier mobility, a large ZT value of 0.255 at 300 K is predicted for two-dimensional BP. While the drastically reduced thermal conductivity in hydrogenated and fluorinated BP is favored for thermoelectric conversion, the decreased carrier mobility has limited the improvement of thermoelectric figure of merit.

Self-powered and high-performance all-fiber integrated photodetector based on graphene/palladium diselenide heterostructures

An all-fiber integrated photodetector is proposed and demonstrated by assembling a graphene/palladium diselenide (PdSe₂) Van der Waals heterostructure onto the endface of a standard optical fiber. A gold film is covered on the heterostructure working as an electrode and a mirror, which reflects back the unabsorbed residual light for further reusage. Owing to the low bandgap of PdSe₂, the all-fiber photodetector shows a broadband photoresponse from 650 to 1550 nm with a high photoresponsivity of 6.68×10⁴ AW^-1, enabling a low light detection of 42.5 pW. And the fastest temporal response is about 660 µs. Taking advantage of heterostructures, the photodetector can work in self-powered mode with the on/off ratio about 82. These findings provide new strategies for integrating two-dimensional materials into optical fibers to realize integrated all-fiber devices with multi-function applications.

Enhanced thermoelectric efficiency of monolayer InP3under strain: a first-principles study

We study the thermoelectric properties of monolayer indium triphosphide (InP₃) under uniaxial compressive and tensile strains using density functional theory in conjunction with Boltzmann transport formalism. InP₃is a recently predicted two-dimensional (2D) material with a host of interesting multi-functional properties. Though InP₃is a low lattice thermal conductivity material, its thermoelectric figure of merit,ZTis found to be low. We thoroughly examined how its thermoelectric transport properties evolve under external strain. We find that the tensile (t) and compressive (c) strains have contrasting effects on the transport coefficients, both leading to the same effect of enhancing theZTvalue strongly. Whilet-strain enhances the power factor dramatically,c-strain gives rise to an ultra-low lattice thermal conductivity. Both these effects lead to an enhancement ofZTvalue at high temperatures by an order of magnitude compared to the corresponding value for free InP₃. The maximumZTvalue of InP₃at 800 K is found to be ∼0.4 undert-strain and ∼0.32 underc-strain, values which are comparable to those observed for some of the leading 2D thermoelectric materials. Another finding relevant to optoelectronic properties is that underc-strain the material shows a transition from an indirect to a direct band gap semiconductor with an accompanying increase in the valley degeneracy. The structural, electronic, and thermal properties of the material are thoroughly analyzed and discussed.

In-Plane Anisotropic Thermal Conductivity of Low-Symmetry PdSe2

Low-symmetry two-dimensional (2D) materials have exhibited novel anisotropic properties in optics, electronics, and mechanics. Such characteristics have opened up new avenues for fundamental research on nano-electronic devices. In-plane thermal conductivity plays a pivotal role in the electronic performance of devices. This article reports a systematic study of the in-plane anisotropic thermal conductivity of PdSe2 with a pentagonal, low-symmetry structure. An in-plane anisotropic ratio up to 1.42 was observed by the micro-Raman thermometry method. In the Raman scattering spectrum, we extracted a frequency shift from the Ag3 mode with the most sensitivity to temperature. The anisotropic thermal conductivity was deduced by analyzing the heat diffusion equations of suspended PdSe2 films. With the increase in thickness, the anisotropy ratio decreased gradually because the thermal conductivity in the x-direction increased faster than in the y-direction. The anisotropic thermal conductivity provides thermal management strategies for the next generation of nano-electronic devices based on PdSe2.

Electronic and topological properties of group-10 transition metal dichalcogenides

The group 10 transition metal dichalcogenides (TMDs) (MX ₂: M = Ni, Pd, Pt; X = S, Se, Te) have attracted much attention in the last few decades because of observation of exotic phases and phenomena such as superconductivity (SC), topological surface states (TSSs), type II Dirac fermions, helical spin texture, Rashba effect, 3D Dirac plasmons, metal-insulator transitions, charge density waves (CDW) etc. In this review, we cover the experimental and theoretical progress on the physical phenomena influenced by the strong electron-electron correlation of the group-10 TMDs from the past to the present. We have especially emphasized on the SC and topological phases in the bulk as well as in atomically thin materials.

Phase Variations and Layer Epitaxy of 2D PdSe2 Grown on 2D Monolayers by Direct Selenization of Molecular Pd Precursors

Two-dimensional (2D) materials and van der Waals heterostructures with atomic-scale thickness provide enormous potential for advanced science and technology. However, insufficient knowledge of compatible synthesis impedes wafer-scale production. PdSe₂ and Pd₂Se₃ are two of the noble transition-metal chalcogenides with excellent physical properties that have recently emerged as promising materials for electronics, optoelectronics, catalyst, and sensors. This research presents a feasible approach to synthesize PdSe₂ and Pd₂Se₃ with inherently asymmetric structure on honeycomb lattice 2D monolayer substrates of graphene and MoS₂. We directly deposit a molecular transition-metal precursor complex on the surface of the 2D substrates, followed by low-temperature selenization by chemical vapor flow. Parameter control leads to tuning of the material from monolayer nanocrystals with Pd₂Se₃ phase, to continuous few-layer PdSe₂ films. Annular dark-field scanning transmission electron microscopy (ADF-STEM) reveals the structure, phase variations, and heteroepitaxy at the atomic level. PdSe₂ with unconventional interlayer stacking shifts appeared as the kinetic product, whereas the bilayer PdSe₂ and monolayer Pd₂Se₃ are the thermodynamic product. The epitaxial alignment of interlayer rotation and translation between the PdSe₂ and underlying 2D substrate was also revealed by ADF-STEM. These results offer both nanoscale and atomic-level insights into direct growth of van der Waals heterostructures, as well as an innovative method for 2D synthesis by predetermined nucleation.

Electron irradiation of multilayer [Formula: see text] field effect transistors

Palladium diselenide ([Formula: see text]) is a recently isolated layered material that has attracted a lot of interest for its pentagonal structure, air stability and electrical properties that are largely tunable by the number of layers. In this work, multilayer [Formula: see text] is used as the channel of back-gate field-effect transistors, which are studied under repeated electron irradiations. Source-drain [Formula: see text] electrodes enable contacts with resistance below [Formula: see text]. The transistors exhibit a prevailing n-type conduction in high vacuum, which reversibly turns into ambipolar electric transport at atmospheric pressure. Irradiation by [Formula: see text] electrons suppresses the channel conductance and promptly transforms the device from n-type to p-type. An electron fluence as low as [Formula: see text] dramatically changes the transistor behavior, demonstrating a high sensitivity of [Formula: see text] to electron irradiation. The sensitivity is lost after a few exposures, with a saturation condition being reached for fluence higher than [Formula: see text]. The damage induced by high electron fluence is irreversible as the device persists in the radiation-modified state for several hours, if kept in vacuum and at room temperature. With the support of numerical simulation, we explain such a behavior by electron-induced Se atom vacancy formation and charge trapping in slow trap states at the [Formula: see text] interface.

Data-driven discovery of 3D and 2D thermoelectric materials

In this work, we first perform a systematic search for high-efficiency three-dimensional (3D) and two-dimensional (2D) thermoelectric materials by combining semiclassical transport techniques with density functional theory (DFT) calculations and then train machine-learning models on the thermoelectric data. Out of 36 000 three-dimensional and 900 two-dimensional materials currently in the publicly available JARVIS-DFT database, we identify 2932 3D and 148 2D promising thermoelectric materials using a multi-steps screening procedure, where specific thresholds are chosen for key quantities like bandgaps, Seebeck coefficients and power factors. We compute the Seebeck coefficients for all the materials currently in the database and validate our calculations by comparing our results, for a subset of materials, to experimental and existing computational datasets. We also investigate the effect of chemical, structural, crystallographic and dimensionality trends on thermoelectric performance. We predict several classes of efficient 3D and 2D materials such as Ba(MgX)2(X = P, As, Bi), X2YZ6(X = K, Rb, Y=Pd, Pt, Z = Cl, Br), K2PtX2(X = S, Se), NbCu3X4(X = S, Se, Te), Sr2XYO6(X = Ta, Zn, Y=Ga, Mo), TaCu3X4(X = S, Se, Te), and XYN (X = Ti, Zr, Y=Cl, Br). Finally, as high-throughput DFT is computationally expensive, we train machine learning models using gradient boosting decision trees and classical force-field inspired descriptors for n-and p-type Seebeck coefficients and power factors, to quickly pre-screen materials for guiding the next set of DFT calculations. The dataset and tools are made publicly available at the websites:https://www.ctcms.nist.gov/~knc6/JVASP.html,https://www.ctcms.nist.gov/jarvisml/andhttps://jarvis.nist.gov/.

Anomalous in-plane lattice thermal conductivity in an atomically thin two-dimensional α-GeTe layer

Low lattice thermal conductivity is one of the most important physical quantities for phononic device applications. Thus, we investigated the in-plane lattice thermal conductivity of mono- and bi-layer α-GeTe systems. The lattice thermal conductivity of the monolayer system along the zigzag direction was 0.43 (W mK-1) while it was 0.21 (W mK-1) along the armchair direction at 300 K, and the lattice thermal conductivity mostly originated from the out-of-plane acoustic mode. In the bilayer system, it was significantly suppressed to 0.044 (W mK-1) and 0.047 (W mK-1) along the zigzag and armchair directions, respectively, at 300 K. Particularly, the out-of-plane acoustic mode in the bilayer had a tremendous Grüneisen parameter and this led to an ultralow in-plane lattice thermal conductivity in the bilayer, and the optical mode dominated the contribution to the lattice thermal conductivity. Our findings may raise intriguing issues regarding the thermoelectric effect, heat insulators, and phononic device applications, and stimulate further experimental studies to verify our theoretical predictions.

Two-Dimensional Palladium Diselenide with Strong In-Plane Optical Anisotropy and High Mobility Grown by Chemical Vapor Deposition

Two-dimensional (2D) palladium diselenide (PdSe₂ ) has strong interlayer coupling and a puckered pentagonal structure, leading to remarkable layer-dependent electronic structures and highly anisotropic in-plane optical and electronic properties. However, the lack of high-quality, 2D PdSe₂ crystals grown by bottom-up approaches limits the study of their exotic properties and practical applications. In this work, chemical vapor deposition growth of highly crystalline few-layer (≥2 layers) PdSe₂ crystals on various substrates is reported. The high quality of the PdSe₂ crystals is confirmed by low-frequency Raman spectroscopy, scanning transmission electron microscopy, and electrical characterization. In addition, strong in-plane optical anisotropy is demonstrated via polarized Raman spectroscopy and second-harmonic generation maps of the PdSe₂ flakes. A theoretical model based on kinetic Wulff construction theory and density functional theory calculations is developed and described the observed evolution of "square-like" shaped PdSe₂ crystals into rhombus due to the higher nucleation barriers for stable attachment on the (1,1) and (1,-1) edges, which results in their slower growth rates. Few-layer PdSe₂ field-effect transistors reveal tunable ambipolar charge carrier conduction with an electron mobility up to ≈294 cm² V^-1 s^-1 , which is comparable to that of exfoliated PdSe₂ , indicating the promise of this anisotropic 2D material for electronics.

High-Performance p-BP/n-PdSe2 Near-Infrared Photodiodes with a Fast and Gate-Tunable Photoresponse

Van der Waals heterostructures composed of transition-metal dichalcogenide (TMD) materials have become a remarkable compact system that could offer an innovative architecture for advanced engineering in high-performance energy-harvesting and optoelectronic devices. Here, we report a novel van der Waals (vdW) TMD heterojunction photodiode composed of black phosphorus (p-BP) and palladium diselenide (n-PdSe₂), which establish a high and tunable rectification and photoresponsivity. A high rectification up to ≈7.1 × 10⁵ is achieved, which is successfully tuned by employing the back-gate voltage to the heterostructure devices. Besides, the device significantly shows the high and gate-controlled photoresponsivity of R = 9.6 × 10⁵, 4.53 × 10⁵ and 1.63 × 10⁵ A W^-1 under the influence of light of different wavelengths (λ = 532, 1064, and 1310 nm) in visible and near-infrared regions, respectively, because of interlayer optical transition and low Schottky. The device also demonstrates extraordinary values of detectivity (D = 5.8 × 10¹³ Jones) and external quantum efficiency (EQE ≈ 9.4 × 10⁶), which are an order of magnitude higher than the currently reported values. The effective enhancement of photovoltaic characteristics in visible and infrared regions of this TMD heterostructure-based system has a huge potential in the field of optoelectronics to realize high-performance infrared photodetectors.

Layer-Dependent and In-Plane Anisotropic Properties of Low-Temperature Synthesized Few-Layer PdSe2 Single Crystals

Palladium diselenide (PdSe₂), a peculiar noble metal dichalcogenide, has emerged as a new two-dimensional material with high predicted carrier mobility and a widely tunable band gap for device applications. The inherent in-plane anisotropy endowed by the pentagonal structure further renders PdSe₂ promising for novel electronic, photonic, and thermoelectric applications. However, the direct synthesis of few-layer PdSe₂ is still challenging and rarely reported. Here, we demonstrate that few-layer, single-crystal PdSe₂ flakes can be synthesized at a relatively low growth temperature (300 °C) on sapphire substrates using low-pressure chemical vapor deposition (CVD). The well-defined rectangular domain shape and precisely determined layer number of the CVD-grown PdSe₂ enable us to investigate their layer-dependent and in-plane anisotropic properties. The experimentally determined layer-dependent band gap shrinkage combined with first-principle calculations suggest that the interlayer interaction is weaker in few-layer PdSe₂ in comparison with that in bulk crystals. Field-effect transistors based on the CVD-grown PdSe₂ also show performances comparable to those based on exfoliated samples. The low-temperature synthesis method reported here provides a feasible approach to fabricate high-quality few-layer PdSe₂ for device applications.

Pressure and electric field tuning of Schottky contacts in PdSe2/ZT-MoSe2 van der Waals heterostructure

A two-dimensional van der Waals (vdW) heterostructure (PdSe₂/ZT-MoSe₂) has been investigated through vdW corrected density functional theory. ZT-MoSe₂ acts as a Dirac material with an anisotropic Dirac cone and variable Fermi velocity (0.52-1.91 × 10⁵ ms^-1). The intrinsic Schottky barrier height can be effectively tuned by applying external pressure and an electric field to the heterostructure. The p-type Schottky barrier transforms into a p-type ohmic contact at pressure P ≈ 16 GPa. A positive electric field induces p-type ohmic contact while a negative electric field results in the transition from p-type Schottky contact to n-type Schottky contact, and finally to n-type ohmic contact at the higher values of the field. Moreover, the external positive (negative) electric field induces n-type (p-type) doping of ZT-MoSe₂ in the heterostructure and remarkably controls the charge carrier concentration. Our results demonstrate that controlling the external pressure and electric field in a PdSe₂/ZT-MoSe₂ heterostructure can result in an unprecedented opportunity for the design of high-performance nanodevices.

Tuning electronic and optical properties of monolayer PdSe2 by introducing defects: first-principles calculations

Based on the density functional theory, the electronic and optical properties of pristine monolayer PdSe2 with Pd or Se vacancy-defect are investigated. Our results show that the Se defect is energetically more favorable than that of Pd defect. The band gap reduces, and some new midgap states appear after the Pd or Se defects are introduced. In terms of the optical properties, the prominent anisotropic characters are remained. The obvious new peaks of the dielectric constant appear after introducing defects. The light absorption in the visible energy range expands based on the appearance of the midgap states induced by the Pd or Se defects. The changes of the refractive index and reflectivity are similar with those of the dielectric constants and the light absorption. The energy loss spectrum of the PdSe2 with Pd or Se defects is obviously different, which can be used to identify different defects in PdSe2. These findings provide effective strategies to tune electronic and optical properties of monolayer PdSe2 by introducing defects.

Direct Observation of the Linear Dichroism Transition in Two-Dimensional Palladium Diselenide

The linear dichroism (LD) transition within anisotropic photonic materials displays promising prospects for applications in polarization-wavelength-selective detectors, optical switching, and optical communication. In conventional two-dimensional (2D) anisotropic materials, the LD is predominantly uniaxial over a broad spectrum of wavelengths and arises principally from the reduced symmetry of the materials. However, the LD transition behavior in crystalline 2D materials remains elusive. Here, we demonstrate the observation of a unique LD conversion phenomenon at a wavelength of 472 nm in palladium diselenide (PdSe₂) using polarization-resolved absorption spectroscopy. This material exhibits prominent anisotropic responses and a high absorption ratio of α_y/α_x ≈ 1.11 at 364 nm, 1.15 at 532 nm, and 0.84 at 633 nm. We propose that this abnormal LD conversion behavior originates from the forceful localization rules at different parallel energy bands that exist within this material. Furthermore, the robust periodicity of A_g and B_1g modes in polarization-resolved Raman spectroscopy is in good agreement with the theoretical structure symmetry analysis. This indicates the strong intrinsic LD effect in the anisotropic nature of PdSe₂, which offers a macrolevel determination of crystal orientations. Such unique LD conversion features, in combination with strong LD effects, enable the air-stable PdSe₂ to be a potential candidate for technological innovations in multispectral imaging, sensing, and polarization-sensitive and wavelength-controllable photoelectronic applications.

Ultrathin nanowire PdX2 (X = P, As): stability, electronic transport and thermoelectric properties

Nano-structuring PdX₂ (X = P, As) material for thermoelectric applications for the conversion of waste heat into electricity.

Electronic properties and low lattice thermal conductivity (κl) of mono-layer (ML) MoS2: FP-LAPW incorporated with spin–orbit coupling (SOC)

This paper focuses on the electronic and thermoelectric properties of monolayer MoS₂.

Thermoelectric properties of 1 T monolayer pristine and Janus Pd dichalcogenides

In this paper, we investigate the stability and thermoelectric properties of 1 T PdSSe, PdSTe and PdSeTe Janus structures using density functional theory (DFT). All three systems are narrow gap semiconductors with indirect bandgaps of 0.94 eV, 0.33 eV and 0.34 eV respectively. Compared to transition metal dichalcogenide (TMD) monolayers, PdS₂ and PdSe₂ are semiconductors with wider indirect bandgaps of 1.29 eV and 0.69 eV respectively. Phonon dispersion calculations demonstrate that all pristine and Janus structures are mechanically stable despite the presence of negligible negative frequencies around the [Formula: see text] point in PdSTe and PdSeTe. Inspection of the lattice thermal conductivity ([Formula: see text]) shows that these structures are slightly anisotropic in the x and y  directions except for PdSe₂ which shows a higher degree of anisotropy. Influenced by the values of [Formula: see text], the thermal electronic conductivity ([Formula: see text]), the electronic conductivity ([Formula: see text]) and the Seebeck effect (S), the figure of merit along the x (ZT ^xx )and y  (ZT ^yy ) directions register the largest values in the case of electron doping for the PdSe₂ and PdSeTe 2D crystals. Interestingly, the figures of merit of the Janus structures are larger than their corresponding pristine PdX₂ (X  =  S, Se) structures. Once synthesized, such information is crucial for the implementation of the PdXY (Y  =  Se, Te) structures in industrial applications.

High Thermoelectric Performance in Hexagonal 2D PdTe2 Monolayer at Room Temperature

Motivated by the recent fabrication of the hexagonal PdTe₂ monolayer, we investigated the thermoelectric properties of the hexagonal and pentagonal PdTe₂ structures using two approaches. The pentagonal monolayer has not been synthesized yet. The hexagonal layer had an indirect band gap of 0.17 eV while the pentagonal structure had an indirect band gap of 1.18 eV. By applying the semiempirical Wiedemann-Franz law to calculate the electronic thermal conductivity, we found that both hexagonal and pentagonal structures had a very high ZT, more than 3. However, the Wiedemann-Franz law underestimated the electronic thermal conductivity, and this resulted in a high ZT. Thus, we employed the Boltzmann transport equation for the electronic thermal conductivity. At high temperature (>500 K), the pentagonal PdTe₂ structure showed a better thermoelectric performance than the hexagonal structure. However, both structures displayed the same ZT of 0.8 at 300 K. We propose that the hexagonal PdTe₂ can be a potential high performance thermoelectric material at room temperature.

Thermoelectric properties of gapped bilayer graphene

Unlike in conventional semiconductors, both the chemical potential and the band gap in bilayer graphene (BLG) can be tuned via application of external electric field. Among numerous device implications, this property also designates BLG as a candidate for high-performance thermoelectric material. In this theoretical study we have calculated the Seebeck coefficients for abrupt interface separating weakly- and heavily-doped areas in BLG, and for a more realistic rectangular sample of mesoscopic size, contacted by two electrodes. For a given band gap ([Formula: see text]) and temperature (T) the maximal Seebeck coefficient is close to the Goldsmid-Sharp value [Formula: see text], the deviations can be approximated by the asymptotic expression [Formula: see text], with the electron charge  -e, the Boltzmann constant [Formula: see text], and [Formula: see text]. Surprisingly, the effects of trigonal warping term in the BLG low-energy Hamiltonian are clearly visible at few-Kelvin temperatures, for all accessible values of [Formula: see text] meV. We also show that thermoelectric figure of merit is noticeably enhanced (ZT  >  3) when a rigid substrate suppresses out-of-plane vibrations, reducing the contribution from ZA phonons to the thermal conductivity.

Near-infrared optical transitions in PdSe2 phototransistors

We investigate electronic and optoelectronic properties of few-layer palladium diselenide (PdSe2) phototransistors through spatially-resolved photocurrent measurements. A strong photocurrent resonance peak is observed at 1060 nm (1.17 eV), likely attributed to indirect optical transitions in few-layer PdSe2. More interestingly, when the thickness of PdSe2 flakes increases, more and more photocurrent resonance peaks appear in the near-infrared region, suggesting strong interlayer interactions in few-layer PdSe2 help open up more optical transitions between the conduction and valence bands of PdSe2. Moreover, gate-dependent measurements indicate that remarkable photocurrent responses at the junctions between PdSe2 and metal electrodes primarily result from the photovoltaic effect when a PdSe2 phototransistor is in the off-state and are partially attributed to the photothermoelectric effect when the device turns on. We also demonstrate PdSe2 devices with a Seebeck coefficient as high as 74 μV K-1 at room temperature, which is comparable with recent theoretical predications. Additionally, we find that the rise and decay time constants of PdSe2 phototransistors are ∼156 μs and ∼163 μs, respectively, which are more than three orders of magnitude faster than previous PdSe2 work and two orders of magnitude over other noble metal dichalcogenide phototransistors, offering new avenues for engineering future optoelectronics.

Ferroelastic lattice rotation and band-gap engineering in quasi 2D layered-structure PdSe2 under uniaxial stress

Transition metal dichalcogenides (TMDCs) have attracted extensive attention in recent years for their novel physical and chemical properties as well as promising applications in the future. In the present paper, based on first-principles simulations, we focused on the bulk of the TMDC material PdSe₂ and provided new insights into its unique structural properties and electronic structures under uniaxial stress. For the first time, we revealed that this orthorhombic PdSe₂ is an intrinsic ferroelastic material with stress-driven 90° lattice rotation in the layer stacking direction. Strikingly, the ferroelastic phase transition originated from the bond reconstructions in the unusual square-planar (PdSe₄)^2- structural units. Specifically, low switching barriers and strong ferroelastic signals rendered room-temperature shape memory accessible. Moreover, the ferroelastic phase transition was accompanied with semiconductor-to-metal-to-semiconductor transitions under uniaxial compressive stress, which could be applied in electronic switching devices. In addition, the band gap was closely associated to the interlayer spacing, which could be engineered by the uniaxial tensile stress. These outstanding stress-engineered properties suggest that orthorhombic PdSe₂ is a promising material for potential applications in microelectromechanical and nanoelectronic devices.

Flexible Organic Thermoelectric Materials and Devices for Wearable Green Energy Harvesting

In the past few decades, organic thermoelectric materials/devices, which can exhibit remarkable potential in green energy conversion, have drawn great attention and interest due to their easy processing, light weight, intrinsically low thermal conductivity, and mechanical flexibility. Compared to traditional batteries, thermoelectric materials have high prospects as alternative power generators for harvesting green energy. Although crystalline inorganic semiconductors have dominated the fields of thermoelectric materials up to now, their practical applications are limited by their intrinsic fragility and high toxicity. The integration of organic polymers with inorganic nanoparticles has been widely employed to tailor the thermoelectric performance of polymers, which not only can combine the advantages of both components but also display interesting transport phenomena between organic polymers and inorganic nanoparticles. In this review, parameters affecting the thermoelectric properties of materials were briefly introduced. Some recently developed n-type and p-type thermoelectric films and related devices were illustrated along with their thermoelectric performance, methods of preparation, and future applications. This review will help beginners to quickly understand and master basic knowledge of thermoelectric materials, thus inspiring them to design and develop more efficient thermoelectric devices.