Controlled Synthesis of Ultrathin PtSe2 Nanosheets with Thickness-Tunable Electrical and Magnetoelectrical Properties

Janus Doping of Sulfur into Platinum Diselenide Ribbons

2D platinum diselenide (PtSe2), a novel member of the transition metal dichalcogenides (TMDCs) family, possesses many excellent properties, including a layer-dependent bandgap, high carrier mobility, and broadband response, making it promise for applications in technologies like field-effect transistors and room-temperature photodetectors. Doping represents an effective method to modify the electrical properties of 2D TMDCs and to bestow upon them additional functions. However, to date, little research has been conducted on the successful doping of 2D PtSe2 for modification. In this study, sulfur (S) powder is utilized during the chemical vapor deposition growth process of 2D PtSe2 ribbons and successfully integrated into the PtSe2 lattice through substitutional doping. The Au substrate significantly decreases the substitution energy of Se atoms in the lower layer of PtSe2, resulting in the formation of the Janus PtSSe structure. S-doped PtSe2 ribbons demonstrate significant symmetry breaking and enhanced electrical properties, showcasing a strong nonlinear optical response and certain synaptic plasticity, further simulating some neuromorphological processes. This study not only demonstrates a viable method for controllable doping and modification of 2D PtSe2 but also establishes a platform for exploring the characteristics of Janus TMDCs.

Wafer-Scale Synthesis of Highly Oriented 2D Topological Semimetal PtTe2 via Tellurization

Platinum ditelluride (1T-PtTe2) is a two-dimensional (2D) topological semimetal with a distinctive band structure and flexibility of van der Waals integration as a promising candidate for future electronics and spintronics. Although the synthesis of large-scale, uniform, and highly crystalline films of 2D semimetals system is a prerequisite for device application, the synthetic methods meeting these criteria are still lacking. Here, we introduce an approach to synthesize highly oriented 2D topological semimetal PtTe2 using a thermally assisted conversion called tellurization, which is a cost-efficient method compared to the other epitaxial deposition methods. We demonstrate that achieving highly crystalline 1T-PtTe2 using tellurization is not dependent on epitaxy but rather relies on two critical factors: (i) the crystallinity of the predeposited platinum (Pt) film and (ii) the surface coverage ratio of the Pt film considering lateral lattice expansion during transformation. By optimizing the surface coverage ratio of the epitaxial Pt film, we successfully obtained 2 in. wafer-scale uniformity without in-plane misalignment between antiparallelly oriented domains. The electronic band structure of 2D topological PtTe2 is clearly resolved in momentum space, and we observed an interesting 6-fold gapped Dirac cone at the Fermi surface. Furthermore, ultrahigh electrical conductivity down to ∼3.8 nm, which is consistent with that of single crystal PtTe2, was observed, proving its ultralow defect density.

Large-Substrate-Terrace Confined Growth of Arrayed Ultrathin PtSe2 Ribbons on Step-Bunched Vicinal Au(001) Facets Toward Electrocatalytic Applications

Ultrathin PtSe2 ribbons can host spin-polarized edge states and distinct edge electrocatalytic activity, emerging as a promising candidate for versatile applications in various fields. However, the direct synthesis is still challenging and the growth mechanism is still unclear. Herein, the arrayed growth of ultrathin PtSe2 ribbons on bunched vicinal Au(001) facets, via a facile chemical vapor deposition (CVD) route is reported. The ultrathin PtSe2 flakes can transform from traditional irregular shapes to desired ribbon shapes by increasing the height of bunched and unidirectionally oriented Au steps (with step height hstep) is found. This crossover, occurring at hstep ≈ 3.0 nm, defines the tailored growth from step-flow to single-terrace-confined modes, as validated by density functional theory calculations of the different system energies. On the millimeter-scale single-crystal Au(001) films with aligned steps, the arrayed ultrathin PtSe2 ribbons with tunable width of ≈20-1000 nm, which are then served as prototype electrocatalysts for hydrogen evolution reaction (HER) is achieved. This work should represent a huge leap in the direct synthesis and the mechanism exploration of arrayed ultrathin transition-metal dichalcogenides (TMDCs) ribbons, which should stimulate further explorations of the edge-related physical properties and practical applications.

Synthesis of 2D PtSe2 Nanolayers on Glass Substrates and Their Integration in Near-Infrared Light Shutters

PtSe2 has asserted its key role among the emerging 2D transition metal dichalcogenides, however, its simplified growth process with controlled number of layers, high crystalline quality, and on inexpensive substrates is still challenging. Here, we report the synthesis details of PtSe2 layers on soda lime glass substrates by selenization of predeposited Pt layers using the thermally assisted conversion method at atmospheric pressure. PtSe2 syntheses are confirmed by X-ray photoelectron spectroscopy and Raman analysis. The layers were further investigated with transmission electron microscopy and optical ellipsometry, revealing the thickness and its dependence on the metal precursor sputtering time. Finally, the integration of PtSe2 as transparent conductive layers in polymer-dispersed liquid crystal structures operating as near-infrared light shutters is demonstrated and device performance is discussed. The proposed simple and inexpensive synthesis approach opens up new directions toward PtSe2 potential technological applications, including ITO-free optoelectronics.

Sm and Gd Contacts in 2D Semiconductors for High-Performance Electronics and Spintronics

Two-dimensional (2D) semiconductors have attracted great attention due to their rich electronic properties and even been considered to have the potential to extend Moore's Law. However, the Schottky barrier between the metal and 2D semiconductor is formed due to the metal-induced gap states (MIGS), which greatly hinder the development of 2D semiconductor transistors in large-scale integrated circuits. Meanwhile, most air-stable 2D semiconductors are nonmagnetic, limiting the possibility of spintronic application. Here, we report a new strategy to suppress the MIGS and reduce the Schottky barrier height on 2D semiconductors (MoS2, WS2, and WSe2) by using lanthanide metal (Sm and Gd) contacts. It was found the lanthanide contacts exhibit a good Ohmic property with a near-zero Schottky barrier. As a result, the carrier mobility of MoS2 transistors reaches 118 cm2/(V s). Furthermore, Gd-contact MoS2 transistors show the typical magnetic property where the magnetoresistance reaches 2.7% at 5 K. By studying its spin valve effect, it was demonstrated that the nonlocal magnetoresistance is 4.1% and spin polarization is 3.25%. This study provides a promising pathway for high-performance 2D electronic and spintronics, which may open a new strategy for future computing-in-memory architecture.

Reversible Transition of Semiconducting PtSe2 and Metallic PtTe2 for Scalable All-2D Edge-Contacted FETs

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers are highly promising as field-effect transistor (FET) channels in the atomic-scale limit. However, accomplishing this superiority in scaled-up FETs remains challenging due to their van der Waals (vdW) bonding nature with respect to conventional metal electrodes. Herein, we report a scalable approach to fabricate centimeter-scale all-2D FET arrays of platinum diselenide (PtSe2) with in-plane platinum ditelluride (PtTe2) edge contacts, mitigating the aforementioned challenges. We realized a reversible transition between semiconducting PtSe2 and metallic PtTe2 via a low-temperature anion exchange reaction compatible with the back-end-of-line (BEOL) processes. All-2D PtSe2 FETs seamlessly edge-contacted with transited metallic PtTe2 exhibited significant performance improvements compared to those with surface-contacted gold electrodes, e.g., an increase of carrier mobility and on/off ratio by over an order of magnitude, achieving a maximum hole mobility of ∼50.30 cm2 V-1 s-1 at room temperature. This study opens up new opportunities toward atomically thin 2D-TMD-based circuitries with extraordinary functionalities.

Influence of PEGylation on WS2 Nanosheets and Its Application in Photothermal Therapy

Photothermal therapy (PTT) is an alternative cancer therapy with minimal side effects and higher efficiency and selectivity. In this study, WS2 nanosheets were developed by ultrasonic exfoliation with different ratios of polyethylene glycol (PEG), and their effects on physicochemical properties were studied. The utilization of PEG during sonication significantly influenced the size and thickness of the resulting WS2 nanosheet layers, which was confirmed through scanning electron microscopy, atomic force microscopy, and dynamic light scattering analyses. PEG functionalization also improved the dispersibility of WS2 in aqueous solution by making its surface hydrophilic, which resulted in better biocompatibility. The intrinsic near-infrared absorbance of the nanosheets positions them as valuable agents for PTT. The study further explores the efficacy of these nanosheets as photothermal agents in the ablation of MDAMB-231 breast cancer cells. Although the use of PEG to demonstrate exfoliation and biocompatibility for WS2 has been reported previously, the effect of PEGylation on various physicochemical properties has not been studied in-depth until now. This study paves the way for the use of highly versatile PEG across a range of 2D material systems.

Probing Carrier Dynamics in Large-Scale MBE-Grown PtSe2 Films by Terahertz Spectroscopy

Atomically thin platinum diselenide (PtSe2) films are promising for applications in the fields of electronics, spintronics, and photodetectors owing to their tunable electronic structure and high carrier mobility. Using terahertz (THz) spectroscopy techniques, we investigated the layer-dependent semiconducting-to-metallic phase transition and associated intrinsic carrier dynamics in large-scale PtSe2 films grown by molecular beam epitaxy. The uniformity of large-scale PtSe2 films was characterized by spatially and frequency-resolved THz-based sheet conductivity mapping. Furthermore, we use an optical-pump-THz-probe technique to study the transport dynamics of photoexcited carriers and explore light-induced intergrain carrier transport in PtSe2 films. We demonstrate large-scale THz-based mapping of the electrical properties of transition metal dichalcogenide films and show that the two noncontact THz-based approaches provide insight in the spatial and temporal properties of PtSe2 films.

Mechanistic understanding of the interfacial properties of metal-PtSe2 contacts

With the advantages of a moderate band gap, high carrier mobility and good environmental stability, two-dimensional (2D) semiconductors show promising applications in next-generation electronics. However, the accustomed metal-2D semiconductor contact may lead to a strong Fermi level pinning (FLP) effect, which severely limits the practical performance of 2D electronics. Herein, the interfacial properties of the contacts between a promising 2D semiconductor, PtSe2, and a sequence of metal electrodes are systematically investigated. The strong interfacial interactions formed in all metal-PtSe2 contacts lead to chemical bonds and a significant interfacial dipole, resulting in a vertical Schottky barrier for Ag, Au, Pd and Pt-based systems and a lateral Schottky barrier for Al, Cu, Sc and Ti-based systems, with a strong FLP effect. Remarkably, the tunneling probability for most metal-PtSe2 is significantly high and the tunneling-specific resistivity is two orders of magnitude lower than that of the state-of-the-art contacts, demonstrating the high efficiency for electron injection from metals to PtSe2. Moreover, the introduction of h-BN as a buffer layer leads to a weakened FLP effect (S = 0.50) and the transformation into p-type Schottky contact for Pt-PtSe2 contacts. These results reveal the underlying mechanism of the interfacial properties of metal-PtSe2 contacts, which is useful for designing advanced 2D semiconductor-based electronics.

Controlled Synthesis of Sub-Millimeter Nonlayered WO2 Nanoplates via a WSe2 -Assisted Method

2D metal oxides (2DMOs) have stimulated tremendous attention due to their distinct electronic structures and abundant surface chemistry. However, it remains a standing challenge for the synthesis of 2DMOs because of their intrinsic 3D lattice structure and ultrahigh synthesis temperature. Here, a reliable WSe₂ -assisted chemical vapor deposition (CVD) strategy to grow nonlayered WO₂ nanoplates with tunable thickness and lateral dimension is reported. Optical microscopy and scanning electron microscopy studies demonstrate that the WO₂ nanoplates exhibit a well-faceted rhombic geometry with a lateral dimension up to the sub-millimeter level (≈135 µm), which is the largest size of 2DMO single crystals obtained by CVD to date. Scanning transmission electron microscopy studies reveal that the nanoplates are high-quality single crystals. Electrical measurements show the nanoplates exhibit metallic behavior with strong anisotropic resistance, outstanding conductivity of 1.1 × 10⁶  S m^-1 , and breakdown current density of 7.1 × 10⁷  A cm^-2 . More interestingly, low-temperature magnetotransport studies demonstrate that the nanoplates show a quantum-interference-induced weak-localization effect. The developed WSe₂ -assisted strategy for the growth of WO₂ nanoplates can enrich the library of 2DMO materials and provide a material platform for other property explorations based on 2D WO₂ .

Energy Dissipation and Electrical Breakdown in Multilayer PtSe2 Electronics

Investigating the energy dissipation in micro- and nanoscale is fundamental to improve the performance and reliability of two-dimensional (2D) electronics. Recently, 2D platinum selenide (PtSe₂) has drawn extensive attention in developing next-generation functional devices due to its distinctive fusion of versatile properties. Toward practical applications of PtSe₂ devices, it is essential to understand the interfacial thermal properties between PtSe₂ and its substrate. Among them, the thermal boundary conductance (TBC) has played a critical role for out-of-plane heat dissipation of PtSe₂ devices. Here, we identify the energy dissipation behavior of multilayer PtSe₂ devices and extract the actual TBC value of the PtSe₂/SiO₂ interface by Raman thermometry with electrical bias. The obtained TBC value is about 8.6 MW m^-2 K^-1, and it belongs to the low end of as-known solid-solid interfaces, suggesting possible applications regarding thermoelectric devices or others reliant on a large temperature gradient. Furthermore, the maximum current density of the PtSe₂ device determines its threshold power, which is crucial for improving device design and guiding future applications. Therefore, we explore the electrical breakdown profile of the multilayer PtSe₂ device, revealing the breakdown current density of 17.7 MA cm^-2 and threshold power density of 0.2 MW cm^-2, which are larger than typical values for commonly used aluminum and copper. These results provide key insights into the energy dissipation of PtSe₂ devices and make PtSe₂ an excellent candidate for thermal confinement applications and nanometer-thin interconnects, which will benefit the development of energy-efficient functional 2D devices.

Abnormal linear dichroism transition in two-dimensional PdPS

The linear dichroism (LD) conversion shows promising applications for polarized detectors, optical transition and light propagation. However, polarity reversal always occurs at a certain wavelength in LD materials, which can only distinguish two wavelength bands as wavelength-selective photodetectors. In this study, the multi-degree-of-freedom of optical anisotropy based on 2D PdPS flakes is carefully described, in which four critical switching wavelengths are observed. Remarkably, the quadruple LD conversion shows a significant wavelength-dependent behavior, allowing us to pinpoint five wavelength bands, 200-239 nm, 239-259 nm, 259-469 nm, 469-546 nm, and 546-700 nm, for a wavelength-selective approach to photodetectors. In addition, the polarized photoresponse under 532 nm was realized with an anisotropy factor of ∼1.51 and further illustrated the in-plane anisotropy. Raman spectroscopy of PdPS flakes also shows strong phonon anisotropy. The unique wavelength-selective property shows great potential for the miniaturization and integration of photodetectors.

Fabrication of Large-Area Short-Wave Infrared Array Photodetectors under High Operating Temperature by High Quality PtS2 Continuous Films

A narrow bandgap of a few layers of platinic disulfide (PtS2) has shown great advantages in large-area array photodetectors for wide spectra photodetection, which is necessary for infrared imaging and infrared sensing under extreme conditions. The photodetection performance of two dimensional materials is highly dependent on the crystalline quality of the film, especially under high operating temperatures. Herein, we developed large area uniform array photodetectors using a chemical vapor deposition grown on PtS2 films for short-wave infrared photodetection at high operating temperature. Due to the high uniformity and crystalline quality of as-grown large area PtS2 films, as-fabricated PtS2 field effect transistors have shown a broadband photo-response from 532 to 2200 nm with a wide working temperature from room temperature to 373 K. The photo-responsivity (R) and specific detectivity (D*) of room temperature and 373 K are about 3.20 A/W and 1.24 × 107 Jones, and 839 mA/W and 6.1 × 106 Jones, at 1550 nm, respectively. Our studies pave the way to create an effective strategy for fabricating large-area short-wave infrared (SWIR) array photodetectors with high operating temperatures using chemical vapor deposition (CVD) grown PtS2 films.