Centimeter-Scale Few-Layer PdS2: Fabrication and Physical Properties

Theoretical Investigation of Stacked Two-Dimensional Transition-Metal Dichalcogenide Materials: The Role of Chemical Species and Number of Monolayers

We report a theoretical investigation, based on density functional theory calculations, of the role of chalcogen species and the number of monolayers in the physical-chemical properties of multilayer two-dimensional transition-metal dichalcogenides (TMDs, MQ2), where M belongs to groups 8 and 10 of the periodic table, Q = S, Se, or Te, and the multilayer is composed of 1 to 6 layers. From the analysis of structural energetic, and electronic properties, we found significant changes in lattice parameters and exfoliation energies as a function of the number of layers, particularly affected by the chalcogen Q species. The TMDs in group 8 exhibit similar lattice parameters for the same choice of chalcogens, making them suitable for constructing commensurate heterostructures, while the crystal phase and the lattice parameter of the TMDs in group 10 strongly depend on the choice of the transition-metal species. Furthermore, the decreasing trend of electronegativity from S to Te results in stronger exfoliation energies due to lower surface charges, thus governing the structural and electronic characteristics of few-layer TMDs. We find unexpected electronic characteristics, such as band gap increases driven by spin-orbit coupling for certain compositions, the emergence of polarization electric fields due to point inversion symmetry breaking, and semiconductor-to-metal transitions with minimal layer additions to the monolayer. The presence of sulfur improves the sensitivity of the surface properties, enabling precise tuning of band edge positions with the layer number.

Direct Syntheses of 2D Noble Transition Metal Dichalcogenides Toward Electronics, Optoelectronics, and Electrocatalysis-Related Applications

2D noble transition metal dichalcogenides (nTMDCs, PdX2 and PtX2, where X═S, Se, Te) have emerged as a new class of 2D materials, owing to their unique puckered pentagonal structure in 2D PdS2 and PdSe2, largely tunable band structures or band gaps with decreasing the layer thickness at the 2D limit, strong interlayer interactions, superior optoelectronic properties, high edge catalytic properties, etc. Directly synthesizing 2D nTMDCs domains or thin films with large-area uniformity, tunable thickness, and high crystalline quality is the premise for exploring these salient properties and developing a wide range of applications. Hereby, this review summarizes recent progress in the direct syntheses and characterizations of 2D nTMDCs, mainly focusing on the thermally assisted conversion (TAC) and chemical vapor deposition (CVD) methods, by using various metal and chalcogen-contained precursors. Meanwhile, the applications of directly synthesized 2D nTMDCs in various fields, such as high-performance field effect transistors (FETs), broadband photodetectors, superior catalysts in hydrogen evolution reactions, and ultra-sensitive piezo resistance sensors, are also discussed. Finally, challenges and prospects regarding the direct syntheses of high-quality 2D nTMDCs and their applications in next-generation electronic and optoelectronic devices, as well as novel catalysts beyond noble metals are overviewed.

Stabilization and Surface Functionalization of Palladium Disulfide Nanoparticles with Acetylene Derivatives

Metal chalcogenide nanoparticles have been attracting extensive attention in diverse fields. Traditionally these nanoparticles are stabilized by organic ligands such as thiols and amines involving nonconjugated core-ligand interfacial interactions. In the present study, a facile wet-chemistry method is described for the synthesis of palladium disulfide (PdS2) nanoparticles capped with acetylene derivatives. Spectroscopic and electrochemical measurements suggest that conjugated Pd-C≡ linkages are formed at the core-ligand interface and facilitate electronic coupling and hence manipulation of the nanoparticle optical and electronic properties. The unique interfacial linkages also allow further functionalization of the nanoparticles by metathesis reaction with olefin derivatives, as manifested in the reaction with vinylferrocene. This research opens new avenues for the structural engineering and functionalization of metal chalcogenide nanoparticles.

In-Plane Sliding Ferroelectricity Realized in Penta-PdSe2/Penta-PtSe2 van der Waals Heterostructure

Different from conventional 2D sliding ferroelectrics with polarization switchable in the out-of-plane via interlayer sliding, we show the existence of in-plane sliding ferroelectricity in a bilayer of a pentagon-based van der Waals heterostructure formed by vertically stacking an experimentally synthesized penta-PdSe2 sheet and a crystal lattice well-matched penta-PtSe2 sheet. From the 128 sliding patterns, four stable configurations are found that exhibit in-plane sliding ferroelectricity with an ultralow polarization switching barrier of 1.91 meV/atom and a high ferroelectric polarization of ±17.11 × 10-10 C m-1. Following the ferroelectric transition among the stable sliding configurations, significant changes in carrier mobility, electrical conductivity, and second harmonic generation are identified. In particular, the ferroelectric stacking configurations are found to possess a negative Poisson's ratio, facilitating the experimental characterization of the sliding ferroelectric effect. This study demonstrates that pentagonal sheets can be used to realize 2D in-plane sliding ferroelectrics going beyond the existing ones.

Layer-dependent ultrafast carrier dynamics of PdSe2 investigated by photoemission electron microscopy

For atomically thin two-dimensional materials, variations in layer thickness can result in significant changes in the electronic energy band structure and physicochemical properties, thereby influencing the carrier dynamics and device performance. In this work, we employ time- and energy-resolved photoemission electron microscopy to reveal the ultrafast carrier dynamics of PdSe2 with different layer thicknesses. We find that for few-layer PdSe2 with a semiconductor phase, an ultrafast hot carrier cooling on a timescale of approximately 0.3 ps and an ultrafast defect trapping on a timescale of approximately 1.3 ps are unveiled, followed by a slower decay of approximately tens of picoseconds. However, for bulk PdSe2 with a semimetal phase, only an ultrafast hot carrier cooling and a slower decay of approximately tens of picoseconds are observed, while the contribution of defect trapping is suppressed with the increase of layer number. Theoretical calculations of the electronic energy band structure further confirm the transition from a semiconductor to a semimetal. Our work demonstrates that TR- and ER-PEEM with ultrahigh spatiotemporal resolution and wide-field imaging capability has great advantages in revealing the intricate details of ultrafast carrier dynamics of nanomaterials.

The growth mechanism of PtS2 single crystal

PtS2, a member of the group 10 transition metal dichalcogenides (TMDs), has received extensive attention because of its excellent electrical properties and air stability. However, there are few reports on the preparation of single-crystal PtS2 in the literature, and the growth mechanism of single crystal PtS2 is not well elucidated. In this work, we proposed a method of preparation that combines magnetron sputtering and chemical vapor transport to obtain monocrystalline PtS2 on a SiO2/Si substrate. By controlling the growth temperature and time, we have synthesized a single crystalline PtS2 of hexagonal shape and size of 1-2 μm on a silicon substrate. Combining the molecular dynamics simulation, the growth mechanism of single crystal PtS2 was investigated both experimentally and theoretically. The synthesis method has a short production cycle and low cost, which opens the door for the fabrication of other TMDs single crystals.

Large-area single-crystal TMD growth modulated by sapphire substrates

Transition metal dichalcogenides (TMDs) have recently attracted extensive attention due to their unique physical and chemical properties; however, the preparation of large-area TMD single crystals is still a great challenge. Chemical vapor deposition (CVD) is an effective method to synthesize large-area and high-quality TMD films, in which sapphires as suitable substrates play a crucial role in anchoring the source material, promoting nucleation and modulating epitaxial growth. In this review, we provide an insightful overview of different epitaxial mechanisms and growth behaviors associated with the atomic structure of sapphire surfaces and the growth parameters. First, we summarize three epitaxial growth mechanisms of TMDs on sapphire substrates, namely, van der Waals epitaxy, step-guided epitaxy, and dual-coupling-guided epitaxy. Second, we introduce the effects of polishing, cutting, and annealing processing of the sapphire surface on the TMD growth. Finally, we discuss the influence of other growth parameters, such as temperature, pressure, carrier gas, and substrate position, on the growth kinetics of TMDs. This review might provide deep insights into the controllable growth of large-area single-crystal TMDs on sapphires, which will propel their practical applications in high-performance nanoelectronics and optoelectronics.

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.

Penta-SiCN monolayer as a well-balanced performance anode material for Li-ion batteries

Lithium-ion batteries (LIBs) remain irreplaceable for clean energy storage applications. The intrinsic metallic nature of penta-SiCN ensures its promising application in the electrodes of LIBs. Using first-principles calculations, we evaluate the performance of the intrinsic metallic penta-SiCN monolayer as the anode material for LIBs. Penta-SiCN exhibits a low diffusion energy barrier (0.107 eV) for Li atom migration on Si18C18N18, while the diffusion energy barrier for vacancy migration on Li17Si18C18N18 is only 0.006 eV. Additionally, penta-SiCN possesses a high theoretical capacity of 1485.98 mA h g-1, average open-circuit voltage of 0.97 V, and small volume expansion of 1%. Remarkably, penta-SiCN exhibits robust wettability towards the electrolytes (solvent molecules and metal salts) widely used in commercial LIBs, indicating the excellent compatibility in electrode applications. These intriguing theoretical findings make penta-SiCN a high performance anode material for LIBs.

Type-1 Pentagonal Tiling Realized in 2D Penta-SrP2 Sheet

According to the systematic classification of pentagon-based two-dimensional (2D) materials [ Phys. Rep. 2022, 964, 1], only type-2 and type-4 out of the 15 pentagonal tiling patterns have been realized in 2D materials so far. Here, we propose the first stable pentagon-based 2D material characterized by the type-1 pentagonal tiling pattern named penta-SrP2. We find that penta-SrP2 is not only thermally and mechanically stable but also dynamically stable when the temperature is above 200 K derived from the calculations by taking both phonon renormalization and thermal expansion into consideration. Moreover, the penta-SrP2 sheet is semiconducting with an indirect band gap of 0.96 eV. These findings expand the family of pentagon-based 2D materials in morphology and provide a new perspective to explore the dynamical stability of high-temperature phases.

Two-dimensional β-noble-transition-metal chalcogenide: novel highly stable semiconductors with manifold outstanding optoelectronic properties and strong in-plane anisotropy

In this work, five two-dimensional (2D) noble-transition-metal chalcogenide (NTMC) semiconductors, namely β-NX (N = Au, Ag; X = S, Se, Te), were designed and predicted by first-principles simulations. Structurally, the monolayer β-NX materials have good energetic, mechanical, dynamical, and thermal stability. They contain two inequivalent noble-transition-metal atoms in the unit cell, and the N-X bond comprises a partial ionic bond and a partial covalent bond. Regarding the electronic properties, the β-NX materials are indirect-band-gap semiconductors with appropriate band-gap values. They have tiny electron effective masses. The hole effective masses exhibit significant differences in different directions, indicating strongly anisotropic hole mobility. In addition, the coexistence of linear and square-planar channels means that the diffusion and transport of carriers should be anisotropic. In terms of optical properties, the β-NX materials show high absorption coefficients. The absorption and reflection characteristics reveal strong anisotropy in different directions. Therefore, the β-NX materials are indirect-band-gap semiconductors with good stability, high absorption coefficients, and strong mechanical, electronic, transport, and optical anisotropy. In the future, they could have great potential as 2D semiconductors in nano-electronics and nano-optoelectronics.

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.

Centimeter-Scale PdS2 Ultrathin Films with High Mobility and Broadband Photoresponse

2D materials with mixed crystal phase will lead to the nonuniformity of performance and go against the practical application. Therefore, it is of great significance to develop a valid method to synthesize 2D materials with typical stoichiometry. Here, 2D palladium sulfides with centimeter scale and uniform stoichiometric ratio are synthesized via controlling the sulfurization temperature of palladium thin films. The relationship between sulfurization temperature and products is investigated in depth. Besides, the high-quality 2D PdS₂ films are synthesized via sulfurization at the temperature of 450-550 °C, which would be compatible with back-end-of-line processes in semiconductor industry with considering of process temperature. The PdS₂ films show an n-type semiconducting behavior with high mobility of 10.4 cm² V^-1 s^-1 . The PdS₂ photodetector presents a broadband photoresponse from 450 to 1550 nm. These findings provide a reliable way to synthesizing high-quality and large-area 2D materials with uniform crystal phase. The result suggests that 2D PdS₂ has significant potential in future nanoelectronics and optoelectronic applications.

First-Principles Prediction of New 2D p-SiPN: A Wide Bandgap Semiconductor

Pentagonal two-dimensional ternary sheets are an emerging class of materials because of their novel characteristic and wide range of applications. In this work, we use first-principles density functional theory (DFT) calculations to identify a new pentagonal SiPN, p-SiPN, which is geometrically, thermodynamically, dynamically, and mechanically stable, and has promising experimental potential. The new p-SiPN shows an indirect bandgap semiconducting behavior that is highly tunable with applied equ-biaxial strain. It is mechanically isotropic, along the x-y in-plane direction, and is a soft material possessing high elasticity and ultimate strain. In addition, its exceptional anisotropic optical response with strong UV light absorbance, and small reflectivity and electron energy loss make it a potential material for optoelectronics and nanomechanics.

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...