Defect-Mediated Phase Transformation in Anisotropic Two-Dimensional PdSe2 Crystals for Seamless Electrical Contacts

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.

Spatially Controlled Phase Transition in MoTe2 Driven by Focused Ion Beam Irradiations

Phase transitions play an important role in tuning the physical properties of two-dimensional (2D) materials as well as developing their high-performance device applications. Here, we reported the observation of a phase transition in few-layered MoTe2 flakes by the irradiation of gallium (Ga+) ions using a focused ion beam (FIB) system. The semiconducting 2H phase of MoTe2 can be controllably converted to the metallic 1T'-like phase via Te defect engineering during irradiations. By taking advantage of the nanometer-sized Ga+ ion probe proved by FIB, in-plane 1T'-2H homojunctions of MoTe2 at submicrometer scale can be fabricated. Furthermore, we demonstrate the improvement of device performance (on-state current over 2 orders of magnitude higher) in MoTe2 transistors using the patterned 1T'-like phase regions as contact electrodes. Our study provides a new strategy to drive the phase transitions in MoTe2, tune their properties, and develop high-performance devices, which also extends the applications of FIB technology in 2D materials and their devices.

Strain-Assisted Phase Transformation in Two-Dimensional Transition-Metal Dichalcogenides

Two-dimensional polymorphic transition-metal dichalcogenides have drawn attention for their diverse applications. This work explores the complex interplay between strain-induced phase transformation and crack growth behavior in annealed nanocrystalline MoS2. Employing molecular dynamics (MD) simulations, this research focuses on the effect of grain size, misorientation, and annealing on phase evolution and their effects on the mechanical behavior of MoS2. First, examining phase transformation in monocrystalline MoS2 under various stress states reveals distinct behaviors depending on the initial phase (1T or 2H) and crystallographic orientation with respect to loading directions. Notably, transformation from a layered hexagonal to a body-centered tetragonal structure is more noticeable when strain in a zigzag direction is applied to the 1T sample. As such, single crystalline MoS2 with a 1T phase exhibits a 16% lower fracture stress in the armchair direction compared to that with a 2H phase. On the other hand, the 1T phase shows a 5% higher phonon lifetime compared to the 2H phase with similar phonon group velocities. Next, the influence of thermal energy and mechanical stress on the phase transformation of nanocrystalline MoS2 is investigated through annealing and quenching cycles, uncovering 60 and 44% irreversibility of phase transformation for an average grain size of 3 and 11 nm, respectively. Besides, the evolution of nanocrystalline samples with different initial phases and grain sizes is studied under uniaxial and biaxial stress. This study shows an inverse pseudo-Hall-Petch effect with exponents of 0.11 and 0.09 for 2H and 1T, respectively. The study reveals that phase transformation can occur concurrently with crack initiation and propagation with the 1T phase exhibiting a 19% lower grain size sensitivity of fracture stress compared to the 2H phase.

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.

Multi-Layer Palladium Diselenide as a Contact Material for Two-Dimensional Tungsten Diselenide Field-Effect Transistors

Tungsten diselenide (WSe2) has emerged as a promising ambipolar semiconductor material for field-effect transistors (FETs) due to its unique electronic properties, including a sizeable band gap, high carrier mobility, and remarkable on-off ratio. However, engineering the contacts to WSe2 remains an issue, and high contact barriers prevent the utilization of the full performance in electronic applications. Furthermore, it could be possible to tune the contacts to WSe2 for effective electron or hole injection and consequently pin the threshold voltage to either conduction or valence band. This would be the way to achieve complementary metal-oxide-semiconductor devices without doping of the channel material.This study investigates the behaviour of two-dimensional WSe2 field-effect transistors with multi-layer palladium diselenide (PdSe2) as a contact material. We demonstrate that PdSe2 contacts favour hole injection while preserving the ambipolar nature of the channel material. This consequently yields high-performance p-type WSe2 devices with PdSe2 van der Waals contacts. Further, we explore the tunability of the contact interface by selective laser alteration of the WSe2 under the contacts, enabling pinning of the threshold voltage to the valence band of WSe2, yielding pure p-type operation of the devices.

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.

Quantum octets in high mobility pentagonal two-dimensional PdSe2

Two-dimensional (2D) materials have drawn immense interests in scientific and technological communities, owing to their extraordinary properties and their tunability by gating, proximity, strain and external fields. For electronic applications, an ideal 2D material would have high mobility, air stability, sizable band gap, and be compatible with large scale synthesis. Here we demonstrate air stable field effect transistors using atomically thin few-layer PdSe2 sheets that are sandwiched between hexagonal BN (hBN), with large saturation current > 350 μA/μm, and high field effect mobilities of ~ 700 and 10,000 cm2/Vs at 300 K and 2 K, respectively. At low temperatures, magnetotransport studies reveal unique octets in quantum oscillations that persist at all densities, arising from 2-fold spin and 4-fold valley degeneracies, which can be broken by in-plane and out-of-plane magnetic fields toward quantum Hall spin and orbital ferromagnetism.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

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.

Probing Intrinsic Defect-Induced Trap States and Hopping Transport in Two-Dimensional PdSe2 Semiconductor Devices

Palladium diselenide (PdSe₂), as an emerging two-dimensional (2D) layered material, is gaining growing attention in nanoelectronics and optoelectronics due to its thickness-dependent band gap, high carrier mobility, and good air stability. However, its asymmetric pentagon structure is inclined to breed defects. Herein, the intrinsic Se vacancy-induced trap states and their influence on the hopping transport in PdSe₂ are systematically investigated. We provide direct evidence that Se vacancies exist in the fresh PdSe₂ samples, which results in the localized trapping states inside the band gap. For the few-layer PdSe₂, at 77 K, the trap density (D_it) near the midgap is about 2.2 × 10¹³ cm^-2 eV^-1, whereas at 295 K, the D_it value increases to ∼7.1 × 10¹³ cm^-2 eV^-1. By comparison, the multilayer PdSe₂ shows nonobvious temperature-dependent trap behaviors with almost unchanged D_it values of ∼8.1 × 10¹² cm^-2 eV^-1 at midgap in the temperature range between 77 and 295 K. Thus, trap states in the few-layer PdSe₂ are more vulnerable to temperature effect. Transport measurements demonstrated that both few-layer and multilayer PdSe₂ field-effect transistor (FET) devices show n-type dominant ambipolar behaviors. The electron mobility in the multilayer PdSe₂ FET is nearly 15-fold higher than that in the few-layer PdSe₂ FET at 315 K, probably owing to the decreased effective mass and suppression of charge impurity scattering in the thicker channel material. However, both FET devices exhibit variable-range hopping over a temperature range from 77 to 240 K and thermally activated hopping at temperatures above 240 K. The hopping transport mechanism is strongly associated with the Se vacancy-induced localized states with poor screening and strong potential fluctuations. This study reveals the important role of structural defects in tailoring and improving the charge transport properties of PdSe₂.

Formation of In-Plane Semiconductor-Metal Contacts in 2D Platinum Telluride by Converting PtTe2 to Pt2Te2

Monolayer PtTe₂ is a narrow gap semiconductor while Pt₂Te₂ is a metal. Here we show that the former can be transformed into the latter by reaction with vapor-deposited Pt atoms. The transformation occurs by nucleating the Pt₂Te₂ phase within PtTe₂ islands, so that a metal-semiconductor junction is formed. A flat band structure is found with the Fermi level of the metal aligning with that of the intrinsically p-doped PtTe₂. This is achieved by an interface dipole that accommodates the ∼0.2 eV shift in the work functions of the two materials. First-principles calculations indicate that the origin of the interface dipole is the atomic scale charge redistributions at the heterojunction. The demonstrated compositional phase transformation of a 2D semiconductor into a 2D metal is a promising approach for making in-plane metal contacts that are required for efficient charge injection and is of particular interest for semiconductors with large spin-orbit coupling, like PtTe₂.

Excitation-Dependent Anisotropic Raman Response of Atomically Thin Pentagonal PdSe2

The group-10 noble-metal dichalcogenides have recently emerged as a promising group of two-dimensional materials due to their unique crystal structures and fascinating physical properties. In this work, the resonance enhancement of the interlayer breathing mode (B1) and intralayer A_g ¹ and A_g ³ modes in atomically thin pentagonal PdSe₂ were studied using angle-resolved polarized Raman spectroscopy with 13 excitation wavelengths. Under the excitation energies of 2.33, 2.38, and 2.41 eV, the Raman intensities of both the low-frequency breathing mode B1 and high-frequency mode A_g ¹ of all the thicknesses are the strongest when the incident polarization is parallel to the a axis of PdSe₂, serving as a fast identification of the crystal orientation of few-layer PdSe₂. We demonstrated that the intensities of B1, A_g ¹, and A_g ³ modes are the strongest with the excitation energies between 2.18 and 2.38 eV when the incident polarization is parallel to PdSe₂ a axis, which arises from the resonance enhancement caused by the absorption. Our investigation reveals the underlying interplay of the anisotropic electron-phonon and electron-photon interactions in the Raman scattering process of atomically thin PdSe₂. It paves the way for future applications on PdSe₂-based optoelectronics.

Recent Progress in Contact Engineering of Field-Effect Transistor Based on Two-Dimensional Materials

Two-dimensional (2D) semiconductors have been considered as promising candidates to fabricate ultimately scaled field-effect transistors (FETs), due to the atomically thin thickness and high carrier mobility. However, the performance of FETs based on 2D semiconductors has been limited by extrinsic factors, including high contact resistance, strong interfacial scattering, and unintentional doping. Among these challenges, contact resistance is a dominant issue, and important progress has been made in recent years. In this review, the Schottky-Mott model is introduced to show the ideal Schottky barrier, and we further discuss the contribution of the Fermi-level pinning effect to the high contact resistance in 2D semiconductor devices. In 2D FETs, Fermi-level pinning is attributed to the high-energy metal deposition process, which would damage the lattice of atomically thin 2D semiconductors and induce the pinning of the metal Fermi level. Then, two contact structures and the strategies to fabricate low-contact-resistance short-channel 2D FETs are introduced. Finally, our review provides practical guidelines for the realization of high-performance 2D-semiconductors-based FETs with low contact resistance and discusses the outlook of this field.

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.

Recent Progress in 1D Contacts for 2D-Material-Based Devices

Recent studies have intensively examined 2D materials (2DMs) as promising materials for use in future quantum devices due to their atomic thinness. However, a major limitation occurs when 2DMs are in contact with metals: a van der Waals (vdW) gap is generated at the 2DM-metal interfaces, which induces metal-induced gap states that are responsible for an uncontrollable Schottky barrier (SB), Fermi-level pinning (FLP), and high contact resistance (R_C ), thereby substantially lowering the electronic mobility of 2DM-based devices. Here, vdW-gap-free 1D edge contact is reviewed for use in 2D devices with substantially suppressed carrier scattering of 2DMs with hexagonal boron nitride (hBN) encapsulation. The 1D contact further enables uniform carrier transport across multilayered 2DM channels, high-density transistor integration independent of scaling, and the fabrication of double-gate transistors suitable for demonstrating unique quantum phenomena of 2DMs. The existing 1D contact methods are reviewed first. As a promising technology toward the large-scale production of 2D devices, seamless lateral contacts are reviewed in detail. The electronic, optoelectronic, and quantum devices developed via 1D contacts are subsequently discussed. Finally, the challenges regarding the reliability of 1D contacts are addressed, followed by an outlook of 1D contact methods.

Atomic and structural modifications of two-dimensional transition metal dichalcogenides for various advanced applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs) and their heterostructures have attracted significant interest in both academia and industry because of their unusual physical and chemical properties. They offer numerous applications, such as electronic, optoelectronic, and spintronic devices, in addition to energy storage and conversion. Atomic and structural modifications of van der Waals layered materials are required to achieve unique and versatile properties for advanced applications. This review presents a discussion on the atomic-scale and structural modifications of 2D TMDs and their heterostructures via post-treatment. Atomic-scale modifications such as vacancy generation, substitutional doping, functionalization and repair of 2D TMDs and structural modifications including phase transitions and construction of heterostructures are discussed. Such modifications on the physical and chemical properties of 2D TMDs enable the development of various advanced applications including electronic and optoelectronic devices, sensing, catalysis, nanogenerators, and memory and neuromorphic devices. Finally, the challenges and prospects of various post-treatment techniques and related future advanced applications are addressed.

High output mode-locked laser empowered by defect regulation in 2D Bi2O2Se saturable absorber

Atomically thin Bi2O2Se has emerged as a novel two-dimensional (2D) material with an ultrabroadband nonlinear optical response, high carrier mobility and excellent air stability, showing great potential for the realization of optical modulators. Here, we demonstrate a femtosecond solid-state laser at 1.0 µm with Bi2O2Se nanoplates as a saturable absorber (SA). Upon further defect regulation in 2D Bi2O2Se, the average power of the mode-locked laser is improved from 421 mW to 665 mW, while the pulse width is decreased from 587 fs to 266 fs. Moderate Ar+ plasma treatments are employed to precisely regulate the O and Se defect states in Bi2O2Se nanoplates. Nondegenerate pump-probe measurements show that defect engineering effectively accelerates the trapping rate and defect-assisted Auger recombination rate of photocarriers. The saturation intensity is improved from 3.6 ± 0.2 to 12.8 ± 0.6 MW cm−2 after the optimized defect regulation. The enhanced saturable absorption and ultrafast carrier lifetime endow the high-performance mode-locked laser with both large output power and short pulse duration.

Interfacial Reaction and Diffusion at the One-Dimensional Interface of Two-Dimensional PtSe2

Two-dimensional (2D) PtSe₂ has potential applications in near-infrared optoelectronics because its band gap can be tuned by varying the layer thickness. There are several different platinum-selenide phases with different stoichiometries that result from high-temperature processing. In this report, we use in situ scanning/transmission electron microscopy (STEM) to investigate high-temperature phase transitions in 2D PtSe₂ and observe interfacial reactions as well as the Kirkendall effect. The 2D nature of PtSe₂ plays a key role in the unique one-dimensional interfaces that result during the formation of Se-poor phases (PtSe and PtSe_1-x) at the edges of the PtSe₂ crystals. The activation energy extracted for this formation suggests that the process is mediated by Se vacancies, as evidenced by the large strain variations in the material made via 4D STEM measurements. The observation of the Kirkendall effect in a 2D material suggests routes to engineer 1D edge chemistry for contact engineering in device applications.

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.

Understanding Heterogeneities in Quantum Materials

Quantum materials are usually heterogeneous, with structural defects, impurities, surfaces, edges, interfaces, and disorder. These heterogeneities are sometimes viewed as liabilities within conventional systems; however, their electronic and magnetic structures often define and affect the quantum phenomena such as coherence, interaction, entanglement, and topological effects in the host system. Therefore, a critical need is to understand the roles of heterogeneities in order to endow materials with new quantum functions for energy and quantum information science applications. In this article, several representative examples are reviewed on the recent progress in connecting the heterogeneities to the quantum behaviors of real materials. Specifically, three intertwined topic areas are assessed: i) Reveal the structural, electronic, magnetic, vibrational, and optical degrees of freedom of heterogeneities. ii) Understand the effect of heterogeneities on the behaviors of quantum states in host material systems. iii) Control heterogeneities for new quantum functions. This progress is achieved by establishing the atomistic-level structure-property relationships associated with heterogeneities in quantum materials. The understanding of the interactions between electronic, magnetic, photonic, and vibrational states of heterogeneities enables the design of new quantum materials, including topological matter and quantum light emitters based on heterogenous 2D materials.

Pd17Se15 alloy on Se sphere with high anti-poisoning ability for alcohol fuel electrooxidation

Electron-deficient effect of Pd in the Pd17Se15 catalyst effectively weakens the adsorption of CO poisoning species and enhances the electrocatalytic performance of alcohol electrooxidation in an alkaline medium.

Selective Antisite Defect Formation in WS2 Monolayers via Reactive Growth on Dilute W-Au Alloy Substrates

Defects are ubiquitous in 2D materials and can affect the structure and properties of the materials and also introduce new functionalities. Methods to adjust the structure and density of defects during bottom-up synthesis are required to control the growth of 2D materials with tailored optical and electronic properties. Here, the authors present an Au-assisted chemical vapor deposition approach to selectively form S_W and S2_W antisite defects, whereby one or two sulfur atoms substitute for a tungsten atom in WS₂ monolayers. Guided by first-principles calculations, they describe a new method that can maintain tungsten-poor growth conditions relative to sulfur via the low solubility of W atoms in a gold/W alloy, thereby significantly reducing the formation energy of the antisite defects during the growth of WS₂ . The atomic structure and composition of the antisite defects are unambiguously identified by Z-contrast scanning transmission electron microscopy and electron energy-loss spectroscopy, and their total concentration is statistically determined, with levels up to ≈5.0%. Scanning tunneling microscopy/spectroscopy measurements and first-principles calculations further verified these antisite defects and revealed the localized defect states in the bandgap of WS₂ monolayers. This bottom-up synthesis method to form antisite defects should apply in the synthesis of other 2D materials.

Topological bands in PdSe2 pentagonal monolayer

The electronic structure of monolayer pentagonal palladium diselenide (PdSe2) is analyzed from the topological band theory perspective. Employing first-principles calculations, effective models and symmetry indicators we find that the low-lying...

Highly Efficient, Ultrabroad PdSe2 Phototransistors from Visible to Terahertz Driven by Mutiphysical Mechanism

The noble transition metal dichalcogenide palladium diselenide (PdSe₂) is an ideal candidate material for broad-spectrum photodetection owing to the large bandgap tunability, high mobility, low thermal conductivity, and large Seebeck coefficient. In this study, self-powered ultrabroadband PdSe₂ photodetectors from the visible-infrared to terahertz (THz) region driven by a mutiphysical mechanism are reported. In the visible-infrared region, the photogenerated electron-hole pairs in the PdSe₂ body are quickly separated by the built-in electric field at the metal-semiconductor interface and achieve a photoresponsivity of 28 A·W^-1 at 405 nm and 0.4 A·W^-1 at 1850 nm. In the THz region, PdSe₂ photodetectors display a room-temperature responsivity of 20 mA·W^-1 at 0.10 THz and 5 mA·W^-1 at 0.24 THz based on efficient production of hot carriers in an antenna-assisted structure. Owing to the fast response speed of ∼7.5 μs and low noise equivalent power of ∼900 pW·Hz^-1/2, high-resolution transmission THz imaging is demonstrated under an ambient environment at room temperature. Our research validates the great potential of PdSe₂ for broadband photodetection and provides a possibility for future optoelectronic applications.

Penta-PdPSe: A New 2D Pentagonal Material with Highly In-Plane Optical, Electronic, and Optoelectronic Anisotropy

Due to their low-symmetry lattice characteristics and intrinsic in-plane anisotropy, 2D pentagonal materials, a new class of 2D materials composed entirely of pentagonal atomic rings, are attracting increasing research attention. However, the existence of these 2D materials has not been proven experimentally until the recent discovery of PdSe₂ . Herein, penta-PdPSe, a new 2D pentagonal material with a novel low-symmetry puckered pentagonal structure, is introduced to the 2D family. Interestingly, a peculiar polyanion of [SePPSe]^4- is discovered in this material, which is the biggest polyanion in 2D materials yet discovered. Strong intrinsic in-plane anisotropic behavior endows penta-PdPSe with highly anisotropic optical, electronic, and optoelectronic properties. Impressively, few-layer penta-PdPSe-based phototransistor not only achieves excellent electronic performances, a moderate electron mobility of 21.37 cm² V^-1 s^-1 and a high on/off ratio of up to 10⁸ , but it also has a high photoresponsivity of ≈5.07 × 10³ A W^-1 at 635 nm, which is ascribed to the photogating effect. More importantly, penta-PdPSe also exhibits a large anisotropic conductance (σ_max /σ_max  = 3.85) and responsivity (R_max /R_min  = 6.17 at 808 nm), superior to most 2D anisotropic materials. These findings make penta-PdPSe an ideal material for the design of next-generation anisotropic devices.

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.

Flexible ferroelasticity in monolayer PdS2: a DFT study

As an unusual mechanical response, the ferroelastic phenomenon in two-dimensional materials has been reported both experimentally and theoretically in recent years. Here, we present the theoretical findings of ferroelastic switching in monolayer PdS2. We demonstrate four types of PdS2 allotropes, showing excellent ferroelasticity with low ferroelastic barriers and strong switching signals. The ferroelastic transitions in monolayer PdS2 include the lattice rotation in penta-α PdS2, the transformation between penta-α PdS2 and penta-β PdS2, the transformation between penta-α PdS2 and penta-γ PdS2, the transformation between penta-β PdS2 and penta-γ PdS2, the transformation between penta-α PdS2 and δ PdS2, and the lattice rotation in δ PdS2. The ferroelastic transitions between these four allotropes have revealed the flexible ferroelasticity in monolayer PdS2. Specifically, the flexible switching in PdS2 allotropes may efficiently control the anisotropic transport of electrons. Thus, the presence of these outstanding mechanical properties endows PdS2 with great potential for applications in next-generation shape memory devices.

Two-dimensional Weyl points and nodal lines in pentagonal materials and their optical response

Two-dimensional pentagonal structures based on the Cairo tiling are the basis of a family of layered materials with appealing physical properties. In this work we present a theoretical study of the symmetry-based electronic and optical properties of these pentagonal materials. We provide a complete classification of the space groups that support pentagonal structures for binary and ternary systems. By means of first-principles calculations, the electronic band structures and the local spin textures in momentum space are analyzed for four examples of these materials, namely, PdSeTe, PdSeS, InP₅ and GeBi₂, all of which are dynamically stable. Our results show that pentagonal structures can be realized in chiral and achiral lattices with Weyl nodes pinned at high-symmetry points and nodal lines along the Brillouin zone boundary; these degeneracies are protected by the combined action of crystalline and time-reversal symmetries. Additionally, we computed the linear and nonlinear optical features of the proposed pentagonal materials and discuss some particular features such as the shift current, which shows an enhancement due to the presence of nodal lines and points, and their possible applications.

Defect Engineering of Two-Dimensional Transition-Metal Dichalcogenides: Applications, Challenges, and Opportunities

Atomic defects, being the most prevalent zero-dimensional topological defects, are ubiquitous in a wide range of 2D transition-metal dichalcogenides (TMDs). They could be intrinsic, formed during the initial sample growth, or created by postprocessing. Despite the majority of TMDs being largely unaffected after losing chalcogen atoms in the outermost layer, a spectrum of properties, including optical, electrical, and chemical properties, can be significantly modulated, and potentially invoke applicable functionalities utilized in many applications. Hence, controlling chalcogen atomic defects provides an alternative avenue for engineering a wide range of physical and chemical properties of 2D TMDs. In this article, we review recent progress on the role of chalcogen atomic defects in engineering 2D TMDs, with a particular focus on device performance improvements. Various approaches for creating chalcogen atomic defects including nonstoichiometric synthesis and postgrowth treatment, together with their characterization and interpretation are systematically overviewed. The tailoring of optical, electrical, and magnetic properties, along with the device performance enhancement in electronic, optoelectronic, chemical sensing, biomedical, and catalytic activity are discussed in detail. Postformation dynamic evolution and repair of chalcogen atomic defects are also introduced. Finally, we offer our perspective on the challenges and opportunities in this field.

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.

Giant Thickness-Tunable Bandgap and Robust Air Stability of 2D Palladium Diselenide

Uncovering the thickness-dependent electronic property and environmental stability for 2D materials are crucial issues for promoting their applications in high-performance electronic and optoelectronic devices. Herein, the extrahigh air stability and giant tunable electronic bandgap of chemical vapor deposition (CVD)-derived few-layer PdSe₂ on Au foils, by using scanning tunneling microscope/spectroscopy (STM/STS), are reported. The robust stability of 2D PdSe₂ is uncovered by the observation of nearly defect/adsorption-free atomic lattices on long-time air-exposed samples. A one-to-one correspondence between the electronic bandgap (from ≈1.15 to ≈0 eV) and thickness of PdSe₂ /Au (from bilayer to bulk) is established. It is also revealed that few-layer semiconducting PdSe₂ flakes present zero-gap edges, induced by hybridization of Pd 4d and Se 4p orbitals. This work hereby provides straightforward evidence for the thickness-tunable electronic property and air stability of 2D semiconductors, thus shedding light on their applications in next-generation electronic devices.

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.

Semiconducting few-layer PdSe2 and Pd2Se3: native point defects and contacts with native metallic Pd17Se15

PdSe2 is a unique layered two-dimensional (2D) material with pentagonal structural motif and anisotropic properties. In addition, its strong interlayer interaction leads to new 2D form of the exfoliated monolayer, that is, Pd2Se3. Despite the increasing interest in these emerging 2D materials, the landscape of the native point defects, as a fundamental materials property, has not been revealed. In this work, we systematically investigate different types of defects in mono- and bi-layer PdSe2 and monolayer Pd2Se3. In contrast to the common expectation, Se vacancy is not the readily formed defect. Instead, Se-excess defects, such as SePd antisite and Se interstitial, are more likely to form over a majority of the allowed range of the atomic chemical potentials. Se-deficiency defect, Pd interstitial, is able to form under the Se-poor condition in bilayer PdSe2. The defect-mediated interlayer fusion model in the formation of monolayer Pd2Se3 from bilayer PdSe2 is reformulated. These dominant defects are found to stay in the neutral charge state, partly explaining the ambipolar behavior of the PdSe2 transistors. Finally, the stacked and lateral contacts between these few-layer semiconductors and the native Pd17Se15 metal are also studied. All these interfaces show p-type contact properties. This work reveals the important materials properties of few-layer PdSe2 and Pd2Se3 for the better development of new functional devices.

Thermoelectric Penta-Silicene with a High Room-Temperature Figure of Merit

Silicon is one of the most frequently used chemical elements of the periodic table in nanotechnology (Goodilin et al., ACS Nano 2019, 13, 10879-10886). Two-dimensional silicene, a silicon analogue of graphene, has been readily obtained to make field-effect transistors since 2015 (Tao et al., Nat. Nanotechnol. 2015, 10, 227; Tsai et al., Nat. Commun. 2013, 4, 1500). Recently, as new members of the silicene family, penta-silicene and its nanoribbon have been experimentally grown on a Ag(110) surface with exotic electronic properties (Cerdá et al., Nat. Commun. 2016, 7, 13076; Sheng et al., Nano Lett. 2018, 18, 2937-2942). However, the thermoelectric performance of penta-silicene has not been so far studied, which would hinder its potential applications of electric generation from waste heat and solid-state Peltier coolers. Based on the Boltzmann transport theory and ab initio calculations, we find that penta-silicene shows remarkable room-temperature figures of merit ZT of 3.4 and 3.0 at the reachable hole and electron concentrations, respectively. We attribute this high ZT to the superior "pudding-mold" electronic band structure and ultralow lattice thermal conductivity. The discovery provides new insight into the transport property of pentagonal nanostructures and highlights the potential applications of thermoelectric materials at room temperature.

Atomically Precise PdSe2 Pentagonal Nanoribbons

We report atomically precise pentagonal PdSe₂ nanoribbons (PNRs) fabricated on a pristine PdSe₂ substrate with a hybrid method of top-down and bottom-up processes. The PNRs form a uniform array of dimer structure with a width of 2.4 nm and length of more than 200 nm. In situ four-probe scanning tunneling microscopy (STM) reveals metallic behavior of PNRs with ballistic transport for at least 20 nm in length. Density functional theory calculations produce a semiconducting density of states of isolated PNRs and find that the band gap narrows and disappears quickly once considering coupling between PNR stacking layers or interaction with the PdSe₂ substrate. The coupling of PNRs is further corroborated by Raman spectroscopy and field-effect transistor measurements. The facile method of fabricating atomically precise PNRs offers an air-stable functional material for dimensional control.

Direct Laser Patterning and Phase Transformation of 2D PdSe2 Films for On-Demand Device Fabrication

Heterophase homojunction formation in atomically thin 2D layers is of great importance for next-generation nanoelectronics and optoelectronics applications. Technologically challenging, controllable transformation between the semiconducting and metallic phases of transition metal chalcogenides is of particular importance. Here, we demonstrate that controlled laser irradiation can be used to directly ablate PdSe₂ thin films using high power or trigger the local transformation of PdSe₂ into a metallic phase PdSe_2-x using lower laser power. Such transformations are possible due to the low decomposition temperature of PdSe₂ and a variety of stable phases compared to other 2D transition metal dichalcogenides. Scanning transmission electron microscopy is used to reveal the laser-induced Se-deficient phases of PdSe₂ material. The process sensitivity to the laser power allows patterning flexibility for resist-free device fabrication. The laser-patterned devices demonstrate that a laser-induced metallic phase PdSe_2-x is stable with increased conductivity by a factor of about 20 compared to PdSe₂. These findings contribute to the development of nanoscale devices with homojunctions and scalable methods to achieve structural transformations in 2D materials.

Striated 2D Lattice with Sub-nm 1D Etch Channels by Controlled Thermally Induced Phase Transformations of PdSe2

2D crystals are typically uniform and periodic in-plane with stacked sheet-like structure in the out-of-plane direction. Breaking the in-plane 2D symmetry by creating unique lattice structures offers anisotropic electronic and optical responses that have potential in nanoelectronics. However, creating nanoscale-modulated anisotropic 2D lattices is challenging and is mostly done using top-down lithographic methods with ≈10 nm resolution. A phase transformation mechanism for creating 2D striated lattice systems is revealed, where controlled thermal annealing induces Se loss in few-layered PdSe₂ and leads to 1D sub-nm etched channels in Pd₂ Se₃ bilayers. These striated 2D crystals cannot be described by a typical unit cells of 1-2 Å for crystals, but rather long range nanoscale periodicity in each three directions. The 1D channels give rise to localized conduction states, which have no bulk layered counterpart or monolayer form. These results show how the known family of 2D crystals can be extended beyond those that exist as bulk layered van der Waals crystals by exploiting phase transformations by elemental depletion in binary systems.