Few-Layer PdSe2 Sheets: Promising Thermoelectric Materials Driven by High Valley Convergence

A type-I van der Waals heterostructure formed by monolayer WS2 and trilayer PdSe2

Two-dimensional (2D) heterostructures, formed by stacking 2D semiconductors through the van der Waals force, have been extensively studied recently. However, the majority of the heterostructures discovered so far possess type-II interfaces that facilitate interlayer charge separation. Type-I interfaces, on the other hand, confine both electrons and holes in one layer, which is beneficial for optical applications that utilize electron-hole radiative recombination. So far, only a few type-I 2D heterostructures have been achieved, which has limited the construction of multilayer heterostructures with sophisticated band landscapes. Here, we report experimental evidence of a type-I interface between monolayer WS2 and trilayer PdSe2. Two-dimensional PdSe2 has emerged as a promising material for infrared optoelectronic and other applications. We fabricated the heterostructure by stacking an exfoliated monolayer WS2 flake on top of a trilayer PdSe2 film, synthesized by chemical vapor deposition. Photoluminescence spectroscopy measurements revealed that the WS2 exciton peak is significantly quenched in the heterostructure, confirming efficient excitation transfer from WS2 to PdSe2. Femtosecond transient absorption measurements with various pump/probe configurations showed that both electrons and holes photoexcited in the WS2 layer of the heterostructure can efficiently transfer to PdSe2, while neither type of carriers excited in PdSe2 can transfer to WS2. These experimental findings establish a type-I band alignment between monolayer WS2 and trilayer PdSe2. Our results further highlight PdSe2 as an important 2D material for constructing van der Waals heterostructures with emergent electronic and optoelectronic properties.

Beyond metals: theoretical discovery of semiconducting MAX phases and their potential application in thermoelectrics

MAX phase is a family of ceramic compounds, typically known for their metallic properties. However, we show here that some of them may be narrow bandgap semiconductors. Using a series of first-principles calculations, we have investigated the electronic structures of 861 dynamically stable MAX phases. Notably, Sc2SC, Y2SC, Y2SeC, Sc3AuC2, and Y3AuC2 have been identified as semiconductors with band gaps ranging from 0.2 to 0.5 eV. Furthermore, we have assessed the thermodynamic stability of these systems by generating ternary phase diagrams utilizing evolutionary algorithm techniques. Their dynamic stabilities are confirmed by phonon calculations. Additionally, we have explored the potential thermoelectric efficiencies of these materials by combining Boltzmann transport theory with first-principles calculations. The relaxation times are estimated using scattering theory. The zT coefficients for the aforementioned systems fall within the range of 0.5 to 2.5 at temperatures spanning from 300 to 700 K, indicating their suitability for high-temperature thermoelectric applications.

Stable Pentagonal Layered Palladium Diselenide Enables Rapid Electrosynthesis of Hydrogen Peroxide

Electrosynthesis of hydrogen peroxide (H2O2) via the two-electron oxygen reduction reaction (2e- ORR) is promising for various practical applications, such as wastewater treatment. However, few electrocatalysts are active and selective for 2e- ORR yet are also resistant to catalyst leaching under realistic operating conditions. Here, a joint experimental and computational study reveals active and stable 2e- ORR catalysis in neutral media over layered PdSe2 with a unique pentagonal puckered ring structure type. Computations predict active and selective 2e- ORR on the basal plane and edge of PdSe2, but with distinct kinetic behaviors. Electrochemical measurements of hydrothermally synthesized PdSe2 nanoplates show a higher 2e- ORR activity than other Pd-Se compounds (Pd4Se and Pd17Se15). PdSe2 on a gas diffusion electrode can rapidly accumulate H2O2 in buffered neutral solution under a high current density. The electrochemical stability of PdSe2 is further confirmed by long device operational stability, elemental analysis of the catalyst and electrolyte, and synchrotron X-ray absorption spectroscopy. This work establishes a new efficient and stable 2e- ORR catalyst at practical current densities and opens catalyst designs utilizing the unique layered pentagonal structure motif.

Two-dimensional Janus pentagonal MSeTe (M = Ni, Pd, Pt): promising water-splitting photocatalysts and optoelectronic materials

Inspired by the interesting and novel properties exhibited by Janus transition metal dichalcogenides (TMDs) and two-dimensional pentagonal structures, we here investigated the structural stability, mechanical, electronic, photocatalytic, and optical properties for a class of two-dimensional (2D) pentagonal Janus TMDs, namely penta-MSeTe (M = Ni, Pd, Pt) monolayers, by using density functional theory (DFT) combined with Hubbard's correction (U). Our results showed that these monolayers exhibit good structural stability, appropriate band structures for photocatalysts, high visible light absorption, and good photocatalytic applicability. The calculated electronic properties reveal that the penta-MSeTe are semiconductors with a bandgap range of 2.06-2.39 eV, and their band edge positions meet the requirements for water-splitting photocatalysts in various environments (pH = 0-13). We used stress engineering to seek higher solar-to-hydrogen (STH) efficiency in acidic (pH = 0), neutral (pH = 7) and alkaline (pH = 13) environments for penta-MSeTe from 0% to +8% biaxial and uniaxial strains. Our results showed that penta-PdSeTe stretched 8% along the y direction and demonstrates an STH efficiency of up to 29.71% when pH = 0, which breaks the theoretical limit of the conventional photocatalytic model. We also calculated the optical properties and found that they exhibit high absorption (13.11%) in the visible light range and possess a diverse range of hyperbolic regions. Hence, it is anticipated that penta-MSeTe materials hold great promise for applications in photocatalytic water splitting and optoelectronic devices.

Layer-dependent electronic structures and optical properties of two-dimensional PdSSe

Two-dimensional (2D) layered palladium dichalcogenides PdX₂ (X = S and Se) have attracted increasing interest due to their tunable electronic structure and abundant physicochemical properties. Recently, as the sister material of PdX₂, PdSSe has received increasing attention and shows great promise for technological applications and fundamental research. In the present study, we focus on the layer-dependent geometry, electronic structure, and optical properties of PdSSe using first-principles calculations. The lattice shrinkage effect present in the 2D structure is suppressed with increasing number of layers. Attributed to the strong interlayer coupling interactions, the band gap decreases from 2.30 to 0.83 eV with increased thickness. Particularly, the dispersion of the band edges on the high symmetry path changes considerably from the monolayer to bilayer PdSSe, resulting in shifts of the conduction band minimum and valence band maximum. The multilayer PdSSe shows band convergence feature with multi-valley for the conduction band, which are maintained with reduced effective mass. Furthermore, the increasing number of layers drives a wider absorption range in the visible light region, and the light absorption capability increases from ∼10% to ∼30%. Meanwhile, the band edge positions of the multilayer PdSSe are more appropriate for photocatalytic water splitting. Our theoretical study reveals the enhanced valley convergence, conductivity and optical absorption performance of the few-layer PdSSe, which suggests its promising application in thermoelectric conversion, solar harvesting and photocatalysis.

Porous carbon-based metal-free monolayers towards highly stable and flexible wearable thermoelectrics and microelectronics

In the search for high mechanical strength and flexibility, ultrahigh semiconducting speed is crucial for the next generation of microelectronic and wearable electronics. Herein, we propose two 2D graphene-like macrocyclic complex carbon-based monolayers, namely g-MC-A and g-MC-B. Both monolayers are dynamically stable according to phonon dispersion and ab initio molecular dynamics simulations. The yield stress of these two layers reaches half that of graphene, revealing remarkably high mechanical strength. Besides, both monolayers are semiconductors. The electron mobility of g-MC-A is high: up to 10⁴ cm² V^-1 s^-1, comparable to black phosphorene. Furthermore, these two monolayers exhibit excellent inherent conductivity with anisotropic characteristics. Interestingly, an extra valley is observed near the conduction band edge for both layers, further simulation predicted both metal-free monolayers will exhibit ZT > 1, implying high thermoelectric performance. Therefore, these two C-based metal-free layers have promising applications in mechanical enhancement, microelectronics, wearable electronics and thermoelectric devices.

Polarization-resolved and helicity-resolved Raman spectra of monolayer XP3 (X = Ge and In)

Monolayer XP₃ (X = Ge, In) is a theoretically predicted two-dimensional (2D) material with fascinating adsorption efficiency, foreshadowing its potential applications in the photovoltaic and optoelectronic communities. To achieve a comprehensive understanding of its optical properties and to further boost quickly identifying its specific applications, in this paper we systematically investigated the polarization-resolved and helicity-resolved Raman spectra excited by two commonly used laser lines (532 nm and 633 nm) through density functional theory. The dynamical stability of monolayer XP₃ is demonstrated by phonon dispersion. Monolayer GeP₃ and InP₃ are found to exhibit significantly different point group symmetries and thereby Raman properties due to the big difference in atomic size and electronic configurations between the Ge atom and In atom. Raman anisotropy of monolayer XP₃ has been found when the wave vector of linear polarized incident light is parallel to the monolayer, and all the anisotropic Raman active phonons are categorized in terms of the locations of two (four) maxima in polarization angle dependent Raman intensities of the parallel (perpendicular) configuration. The polarization direction averaged Raman spectra have been further discussed according to the characteristics of light absorbance. The calculations of helicity-resolved Raman spectra indicate a stronger helicity selection rule under helical excitation with the wave vector normal to the monolayer. The present work paves the way for the suitable design, characterization and exploitation of the proposed 2D material with controllable surface properties for applications in electronics and optoelectronics.

Theoretical analysis of the thermoelectric properties of penta-PdX2 (X = Se, Te) monolayer

Based on the successful fabrication of PdSe2 monolayers, the electronic and thermoelectric properties of pentagonal PdX2 (X = Se, Te) monolayers were investigated via first-principles calculations and the Boltzmann transport theory. The results showed that the PdX2 monolayer exhibits an indirect bandgap at the Perdew–Burke–Ernzerhof level, as well as electronic and thermoelectric anisotropy in the transmission directions. In the PdTe2 monolayer, P-doping owing to weak electron–phonon coupling is the main reason for the excellent electronic properties of the material. The low phonon velocity and short phonon lifetime decreased the thermal conductivity (κl) of penta-PdTe2. In particular, the thermal conductivity of PdTe2 along the x and y transmission directions was 0.41 and 0.83 Wm−1K−1, respectively. Owing to the anisotropy of κl and electronic structures along the transmission direction of PdX2, an anisotropic thermoelectric quality factor ZT appeared in PdX2. The excellent electronic properties and low lattice thermal conductivity (κl) achieved a high ZT of the penta-PdTe2 monolayer, whereas the maximum ZT of the p- and n-type PdTe2 reached 6.6 and 4.4, respectively. Thus, the results indicate PdTe2 as a promising thermoelectric candidate.

Tunneling transport of 2D anisotropic XC (X = P, As, Sb, Bi) with a direct band gap and high mobility: a DFT coupled with NEGF study

Direct bandgap and significant anisotropic properties are crucial and beneficial for nanoelectronic applications. In this work, through first-principles calculations, we investigate novel two-dimensional (2D) α-XC (X = P, As, Sb, Bi) materials, which possess a direct bandgap of 0.73 to 1.40 eV with remarkable anisotropic electronic properties. Intriguingly, 2D α-XC presents the highest electron mobility near 8 × 10³ cm² V^-1 s^-1 along the Γ-X direction. Moreover, the transfer characteristics of the 2D α-XC TFETs are thoroughly assessed through NEGF methods. AsC TFETs demonstrate an on-state current larger than 2.2 × 10³ μA μm^-1, which can satisfy the International Technology Roadmap for Semiconductors (ITRS) for high-performance requirements. In particular, the minimum value of subthreshold swing of devices is as low as 15 mV dec^-1, indicating excellent device switching characteristics. The relevant calculation results show that 2D α-XC monolayers could be a promising candidate in next-generation high-performance device applications.

Stabilized Synthesis of 2D Verbeekite: Monoclinic PdSe2 Crystals with High Mobility and In-Plane Optical and Electrical Anisotropy

PdSe₂ has a layered structure with an unusual, puckered Cairo pentagonal tiling. Its atomic bond configuration features planar 4-fold-coordinated Pd atoms and intralayer Se-Se bonds that enable polymorphic phases with distinct electronic and quantum properties, especially when atomically thin. PdSe₂ is conventionally orthorhombic, and direct synthesis of its metastable polymorphic phases is still a challenge. Here, we report an ambient-pressure chemical vapor deposition approach to synthesize metastable monoclinic PdSe₂. Monoclinic PdSe₂ is shown to be synthesized selectively under Se-deficient conditions that induce Se vacancies. These defects are shown by first-principles density functional theory calculations to reduce the free energy of the metastable monoclinic phase, thereby stabilizing it during synthesis. The structure and composition of the monoclinic PdSe₂ crystals are identified and characterized by scanning transmission electron microscopy imaging, convergent beam electron diffraction, and electron energy loss spectroscopy. Polarized Raman spectroscopy of the monoclinic PdSe₂ flakes reveals their strong in-plane optical anisotropy. Electrical transport measurements show that the monoclinic PdSe₂ exhibits n-type charge carrier conduction with electron mobilities up to ∼298 cm² V^-1 s^-1 and a strong in-plane electron mobility anisotropy of ∼1.9. The defect-mediated growth pathway identified in this work is promising for phase-selective direct synthesis of other 2D transition metal dichalcogenides.

Electronic, Magnetic, and Optical Properties of Metal Adsorbed g-ZnO Systems

2D ZnO is one of the most attractive materials for potential applications in photocatalysis, gas and light detection, ultraviolet light-emitting diodes, resistive memory, and pressure-sensitive devices. The electronic structures, magnetic properties, and optical properties of M (Li, Na, Mg, Ca, or Ga) and TM (Cr, Co, Cu, Ag, or Au) adsorbed g-ZnO were investigated with density functional theory (DFT). It is found that the band structure, charge density difference, electron spin density, work function, and absorption spectrum of g-ZnO can be tuned by adsorbing M or TM atoms. More specifically, the specific charge transfer occurs between g-ZnO and adsorbed atom, indicating the formation of a covalent bond. The work functions of M adsorbed g-ZnO systems are obviously smaller than that of intrinsic g-ZnO, implying great potential in high-efficiency field emission devices. The Li, Na, Mg, Ca, Ga, Ag, or Au adsorbed g-ZnO systems, the Cr adsorbed g-ZnO system, and the Co or Cu adsorbed g-ZnO systems exhibit non-magnetic semiconductor proprieties, magnetic semiconductor proprieties, and magnetic metal proprieties, respectively. In addition, the magnetic moments of Cr, Co, or Cu adsorbed g-ZnO systems are 4 μB, 3 μB, or 1 μB, respectively, which are mainly derived from adsorbed atoms, suggesting potential applications in nano-scale spintronics devices. Compared with the TM absorbed g-ZnO systems, the M adsorbed g-ZnO systems have more obvious absorption peaks for visible light, particularly for Mg or Ca adsorbed g-ZnO systems. Their absorption peaks appear in the near-infrared region, suggesting great potential in solar photocatalysis. Our work contributes to the design and fabrication of high-efficiency field emission devices, nano-scale spintronics devices, and visible-light responsive photocatalytic 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.

Strain-induced enhancement in the electronic and thermal transport properties of the tin sulphide bilayer

The enhancement in the thermoelectric figure of merit (ZT) of a material is limited by the interplay between the electronic transport coefficients. Here we report the greatly enhanced thermoelectric performance of the SnS bilayer with the application of isotropic strain, due to the simultaneous increase in the Seebeck coefficient and low lattice thermal conductivities. Based on first-principles calculations combined with Boltzmann transport theory, we predict that the band structure of the SnS bilayer can be effectively tuned using the strain, and the Seebeck coefficient is significantly improved for the tensile strain. The lattice thermal conductivities for the bilayer under the tensile strain are quite low (0.21-1.89 W m^-1 K^-1 at 300 K) due to the smaller frequencies of the acoustic phonon modes. Along the zigzag (armchair) direction, the room temperature peak ZT value of 4.96 (2.40) is obtained at a strain of 2% (4%), which is 5.3 (2.03) times higher than the peak ZT of the unstrained bilayer along the zigzag (armchair) direction. Thus the strain-tuned SnS bilayer is a good thermoelectric material with low lattice thermal conductivities and promising ZT values at room temperature.

Theoretical assessment of Raman spectra on MXene Ti2C: from monolayer to bilayer

The structural, vibrational and Raman spectra properties of monolayer and bilayer Ti₂C excited by two commonly used laser lines (532 nm and 633 nm) are investigated by first principles calculations to establish a correlation among layer stacks and optical features for two-dimensional MXenes. The stability of the monolayer and the energetically preferable stacking configuration are demonstrated by phonon dispersion. The monolayer and bilayer Ti₂C systems are found to exhibit different point group symmetries and thereby the Raman properties due to the symmetry breaking of the bilayer structure caused by interlayer van der Waals interactions. We listed all Raman-active modes for monolayer (bilayer) Ti₂C, i.e., one (five) out-of-plane A_1g (A₁) and one (five) pair (pairs) of degenerate in-plane E_g (E) vibration modes. Polarization angle dependent Raman intensity has been discussed in terms of the locations of two (four) maxima in the parallel (perpendicular) configuration, which might be applied in experimentally identifying monolayer and bilayer Ti₂C. The difference in the polarization direction averaged Raman spectra between monolayer and bilayer Ti₂C can be explained by the characteristics of light absorbance.

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.

Pentagonal B2N3-based 3D metallic boron nitride with high energy density

Different from conventional insulating or semiconducting boron nitride,metallicBN has received increasing attention in recent years as its intrinsic metallicity grants it great potential for broad applications. In this study, by assembling the experimentally synthesized pentagonal B₂N₃units, we have proposed the first pentagon-based three-dimensional (3D) metallic boron nitride, labeled penta-B₄N_7.First-principles calculations together with molecular dynamics simulations and convex hull diagram show that penta-B₄N₇is not only thermally, dynamically and mechanically stable, but also three dimensionally metallic. A detailed analysis of its electronic structure reveals that the intrinsic metallicity comes from the delocalized electrons in the partially occupied antibonding N-Nπorbitals. Equally important, the energy density of penta-B₄N₇is found to be 4.07 kJ g^-1, which is the highest among that of all the 3D boron nitrides reported so far.

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.

Unique topological nodal line states and associated exceptional thermoelectric power factor platform in Nb3GeTe6 monolayer and bulk

To date, ideal topological nodal line semimetal (TNLS) candidates in high dynamically stable and high thermally stable two-dimensional (2D) materials are still extremely scarce. Herein, by performing first-principles calculations, on the one hand, we found that three-dimensional Nb₃GeTe₆ bulk possesses a single closed TNL in the k_x = 0 plane and a fourfold TNL in the S-R direction without considering spin-orbit coupling (SOC). Under the SOC effect, a new topological signature, i.e., hourglass-like Dirac nodal line, occurs in Nb₃GeTe₆ bulk. On the other hand, we found that the 2D Nb₃GeTe₆ monolayer features a doubly degenerate TNL along surface X-S paths. Importantly, this monolayer enjoys the following advantages: (i) it has high thermal stability at room temperature and above; (ii) its TNL is nearly flat in energy and is very close to the Fermi level (E_F), which provides a fantastic maximum value platform of the thermoelectric power factor around the E_F; and (iii) no extraneous bands are close to the TNL, near the Fermi level. Moreover, we explore the entanglement between the topological states and thermolectric properties for the 2D Nb₃GeTe₆ monolayer. Our work not only reports the discovery of a novel TNL material, but also builds the link between the TNL and thermoelectric properties.

Two-Dimensional Noble-Metal Chalcogenides and Phosphochalcogenides

Noble-metal chalcogenides, dichalcogenides, and phosphochalcogenides are an emerging class of two-dimensional materials. Quantum confinement (number of layers) and defect engineering enables their properties to be tuned over a broad range, including metal-to-semiconductor transitions, magnetic ordering, and topological surface states. They possess various polytypes, often of similar formation energy, which can be accessed by selective synthesis approaches. They excel in mechanical, optical, and chemical sensing applications, and feature long-term air and moisture stability. In this Minireview, we summarize the recent progress in the field of noble-metal chalcogenides and phosphochalcogenides and highlight the structural complexity and its impact on applications.

Ultra-low lattice thermal conductivity and giant phonon-electric field coupling in hafnium dichalcogenide monolayers

Phonons in crystalline solids are of utmost importance in governing its lattice thermal conductivity (k L). In this work, k L in hafnium (Hf) dichalcogenide monolayers has been investigated based on ab initio DFT coupled to linearized Boltzmann transport equation together with single-mode relaxation-time approximation. Ultra-low k L found in HfS2 (2.19 W m-1 K-1), HfSe2 (1.23 W m-1 K-1) and HfSSe (1.78 W m-1 K-1) monolayers at 300 K, is comparable to that of the state-of-art bulk thermoelectric materials, such as, Bi2Te3 (1.6 W m-1 K-1), PbTe (2.2 W m-1 K-1) and SnSe (2.6 W m-1 K-1). Gigantic longitudinal-transverse optical (LO-TO) splitting of up to 147.7 cm-1 is noticed at the Brillouin zone-centre (Γ-point), which is much higher than that in MoS2 single layer (∼2 cm-1). It is driven by the colossal phonon-electric field coupling arising from the domination of ionic character in the interatomic bonds and Born effective or dynamical charges as high as 7.4e on the Hf ions, which is seven times that on Mo in MoS2 single layer. Enhancement in k L occurs in HfS2 (2.19 to 4.1 W m-1 K-1), HfSe2 (1.23 to 1.7 W m-1 K-1) and HfSSe (1.78 to 2.2 W m-1 K-1) upon the incorporation of the non-analytic correction term. Furthermore, the mode Grüneisen parameter is calculated to be as high as ∼2.0, at room temperature, indicating a strong anharmonicity. Moreover, the contribution of optical phonons to k L is found to be ∼12%, which is significantly high than that in single-layer MoS2. Large atomic mass of Hf (178.5 u), small phonon group velocities (4-5 km s-1), low Debye temperature (∼166 K), low bond and elastic stiffness (Young's modulus ∼75 N m-1), small phonon lifetimes (∼6 ps), low specific heat capacity (∼17 J K-1 mol-1) and strong anharmonicity are collectively found to be the factors responsible for such a low k L. These findings would be immensely helpful in designing thermoelectric interconnects at the nanoscale and 2D material-based energy harvesters.

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.

Two-dimensional MgX2Se4 (X = Al, Ga) monolayers with tunable electronic properties for optoelectronic and photocatalytic applications

Two new two-dimensional (2D) layered materials, namely, MgX₂Se₄ (X = Al, Ga) monolayers, are predicted to possess novel electronic properties. Ab initio electronic structure calculations show that both MgAl₂Se₄ and MgGa₂Se₄ monolayers are direct-gap semiconductors with bandgaps of 3.14 eV and 2.34 eV, respectively. The bandgap of both 2D materials is very sensitive to the in-plane biaxial strain, while the strain induced bandgap changes allow the tuning of optical absorption from the violet to green-light region. Also importantly, the in-plane electron mobility of both 2D materials is predicted to be as high as ∼0.7-1.0 × 10³ cm² V^-1 s^-1, notably higher than that of the MoS₂ sheet (∼200 cm² V^-1 s^-1), while it is comparable to that of black phosphorene (∼1000 cm² V^-1 s^-1), suggesting their potential application in n-type field-effect transistors. Moreover, suitable bandgap and band-edge alignment make the monolayer MgX₂Se₄ a potential photocatalyst for water splitting. Lastly, we show that MgX₂Se₄ possesses a lower monolayer cleavage energy than that of graphite, indicating easy exfoliation of MgX₂Se₄ layers from their bulk.

Strain engineering on the electronic states of two-dimensional GaN/graphene heterostructure

Combining two different layered structures to form a van der Waals (vdW) heterostructure has recently emerged as an intriguing way of designing electronic and optoelectronic devices. Effects of the strain on the electronic properties of GaN/graphene heterostructure are investigated by using first-principles calculation. In the GaN/graphene heterostructure, the strain can control not only the Schottky barrier, but also contact types at the interface. Moreover, when the uniaxial strain is above -1% or the biaxial strain is above 0%, the contact type transforms to ohmic contact. These results provide a detailed understanding of the interfacial properties of GaN/graphene and help to predict the performance of the GaN/graphene heterostructure on nanoelectronics and nanocomposites.

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.

From Two- to Three-Dimensional van der Waals Layered Structures of Boron Crystals: An Ab Initio Study

A remarkable recent advancement has been the successful synthesis of two-dimensional boron monolayers on metal substrates. However, although up to 16 possible bulk allotropes of boron have been reported, none of them possess van der Waals (vdW) layered structures. In this work, starting from the experimentally synthesized monolayer boron sheet (β12 borophene), we explored the possibility for forming vdW layered bulk boron. We found that two β12 borophene sheets cannot form a stable vdW bilayer structure, as covalent-like B-B bonds are formed between them because of the peculiar bonding. Interestingly, when the covalently bonded bilayer borophene sheets are stacked on top of each other, three-dimensional (3D) layered structures are constructed via vdW interlayer interactions, rather than covalent. The 3D vdW layered structures were found to be dynamically stable. The interlayer binding energy is about 20 meV/Å2, which is close to the weakly bound graphene layers in graphite (∼16 meV/Å2). Furthermore, the density functional theory predicted electronic band structure testifies that these vdW bulk boron crystals can behave as good conductors. The insights obtained from this work suggest an opportunity to discover new vdW layered structures of bulk boron, which is expected to be crucial to numerous applications ranging from microelectronic devices to energy storage devices.

Strain-enhanced properties of van der Waals heterostructure based on blue phosphorus and g-GaN as a visible-light-driven photocatalyst for water splitting

Many strategies have been developed to overcome the critical obstacles of fast recombination of photogenerated charges and the limited ability of semiconductor photocatalysts to absorb visible light. Considering all the novel properties of monolayered g-GaN and blue phosphorus (BlueP) which were revealed in recent studies, first-principles calculations were used to systematically investigate the structural stability, electronic energy, band alignment, band bending, and charge difference in the heterostructure formed by these two layered materials. The g-GaN/BlueP heterostructure is constructed by van der Waals (vdW) forces, and it possess a staggered band structure which induces electron transformation because of the different Fermi levels of the two layered materials. By aligning the Fermi levels, an interfacial electric field is built and it causes band bending, which can promote effective separation of photoexcited holes and electrons; the band-bending phenomenon was also calculated according to density functional theory (DFT). Moreover, effects of in-plane strain on the tuned bandgap, energy, and band edge were investigated, and the results show that the optical-absorption performance in the visible-light range can be improved. The findings reported in this paper are expected to provide theoretical support for the use of the g-GaN/BlueP vdW heterostructure as a photocatalyst for water splitting.

Photoluminescence of PdS2 and PdSe2 quantum dots

PdS₂ and PdSe₂ QDs are fabricated via liquid exfoliation using NMP solvent. The PL behaviors of these QD solutions are studied. The obtained results suggest promising optoelectronic applications with group-10 TMD QDs in the future.

Metal and ligand effects on the stability and electronic properties of crystalline two-dimensional metal-benzenehexathiolate coordination compounds

The cohesive energy, phonon spectrum and quantum molecular-dynamic simulation have been used successively to determine whether the crystalline two-dimensional (2D) metal-benzenehexathiolate (M-BHT) coordination compounds are stable or not. The electronic structures of stable M-BHTs and the corresponding inorganic semiconducting materials have been compared. From the point of view of satisfying stoichiometric ratios and saturation of chemical bonds, we designed possible planar molecular structures and demonstrated that there may be two different 2D M-BHTs, i.e. group II-[Formula: see text] and group IV-[Formula: see text]. However, the cohesive energy calculation indicates that the group IV-[Formula: see text] coordination compound cannot be obtained by thermodynamic equilibrium growth. In contrast, [Formula: see text] and [Formula: see text] from the group II-[Formula: see text] have not only thermodynamic stability, but also dynamic stability due to their phonon spectrum with no imaginary frequency. Moreover, they are still the two most stable ones when the bridge atom S of ligand BHT is replaced by the other chalcogens of O, Se and Te. Further studies indicated that [Formula: see text] and [Formula: see text] both have room temperature dynamic stability and exhibit semiconducting. The exceptional stability and relatively narrow band gap make them advantageous over their inorganic counterparts. Our findings open opportunities to search for new 2D planar conducting coordination compound for organic electronic applications.

Novel electronic structures and enhanced optical properties of boron phosphide/blue phosphorene and F4TCNQ/blue phosphorene heterostructures: a DFT + NEGF study

Blue phosphorene (Blue-p), an allotrope of black phosphorene, has attracted extensive interest due to its hexagonal crystal with a flat arranged layer of phosphorus atoms.