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

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.

Direct Observation of Photon Induced Giant Band Renormalization in 2D PdSe2 Dichalcogenide by Transient Absorption Spectroscopy

Insight into fundamental light-matter interaction as well as underlying photo-physical processes is crucial for the development of novel optoelectronic devices. Palladium diselenide (PdSe2 ), an important representative of emerging 2D noble metal dichalcogenides, has gain considerable attention owing to its unique optical, physical, and chemical properties. In this study, 2D PdSe2 nanosheets (NSs) are prepared using the liquid-phase exfoliation method. A broadband carrier relaxation dynamics from visible to near-infrared bands are revealed using a time-resolved transient absorption spectrometer, giving results that indicate band filling and bandgap renormalization (BGR) effects in the 2D PdSe2 NSs. The observed blue-shift of the transient absorption spectra at the primary stage and the subsequent red-shift can be ascribed to this BGR effect. These findings reveal the many-body character of the 2D TMDs material and may hold keys for applications in the field of optoelectronics and ultrafast photonics.

Synthesis of Two-Dimensional MoO2 Nanoplates with Large Linear Magnetoresistance and Nonlinear Hall Effect

Two-dimensional (2D) materials with large linear magnetoresistance (LMR) are very interesting owing to their potential application in magnetic storage or sensor devices. Here, we report the synthesis of 2D MoO₂ nanoplates grown by a chemical vapor deposition (CVD) method and observe large LMR and nonlinear Hall behavior in MoO₂ nanoplates. As-obtained MoO₂ nanoplates exhibit rhombic shapes and high crystallinity. Electrical studies indicate that MoO₂ nanoplates feature a metallic nature with an excellent conductivity of up to 3.7 × 10⁷ S m^-1 at 2.5 K. MoO₂ nanoplates display a large LMR of up to 455% at 3 K and -9 T. A thickness-dependent LMR analysis suggests that LMR values increase upon increasing the thickness of nanoplates. Besides, nonlinearity has been found in the magnetic-field-dependent Hall resistance, which decreases with increasing temperatures. Our studies highlight that MoO₂ nanoplates are promising materials for fundamental studies and potential applications in magnetic storage devices.

Graphene Bridge Heterostructure Devices for Negative Differential Transconductance Circuit Applications

Two-dimensional van der Waals (2D vdW) material-based heterostructure devices have been widely studied for high-end electronic applications owing to their heterojunction properties. In this study, we demonstrate graphene (Gr)-bridge heterostructure devices consisting of laterally series-connected ambipolar semiconductor/Gr-bridge/n-type molybdenum disulfide as a channel material for field-effect transistors (FET). Unlike conventional FET operation, our Gr-bridge devices exhibit non-classical transfer characteristics (humped transfer curve), thus possessing a negative differential transconductance. These phenomena are interpreted as the operating behavior in two series-connected FETs, and they result from the gate-tunable contact capacity of the Gr-bridge layer. Multi-value logic inverters and frequency tripler circuits are successfully demonstrated using ambipolar semiconductors with narrow- and wide-bandgap materials as more advanced circuit applications based on non-classical transfer characteristics. Thus, we believe that our innovative and straightforward device structure engineering will be a promising technique for future multi-functional circuit applications of 2D nanoelectronics.

Waveguide-Integrated PdSe2 Photodetector over a Broad Infrared Wavelength Range

Hybrid integration of van der Waals materials on a photonic platform enables diverse exploration of novel active functions and significant improvement in device performance for next-generation integrated photonic circuits, but developing waveguide-integrated photodetectors based on conventionally investigated transition metal dichalcogenide materials at the full optical telecommunication bands and mid-infrared range is still a challenge. Here, we integrate PdSe₂ with silicon waveguide for on-chip photodetection with a high responsivity from 1260 to 1565 nm, a low noise-equivalent power of 4.0 pW·Hz^-0.5, a 3-dB bandwidth of 1.5 GHz, and a measured data rate of 2.5 Gbit·s^-1. The achieved PdSe₂ photodetectors provide new insights to explore the integration of novel van der Waals materials with integrated photonic platforms and exhibit great potential for diverse applications over a broad infrared range of wavelengths, such as on-chip sensing and spectroscopy.

MBE-grown ultrathin PtTe2 films and their layer-dependent electronic structures

2D platinum ditelluride (PtTe₂) has received significant attention for 2D photodetector applications due to its novel physical properties. One of the critical factors that affect device performance is the film quality. Here, using molecular beam epitaxy, we investigate the role of growth temperature in determining the film quality of PtTe₂ on highly oriented pyrolytic graphite, and unveil its layer-dependent electronic properties by X-ray photoelectron spectroscopy, Raman spectroscopy, and scanning tunneling microscopy/spectroscopy (STM/STS), as well as density functional theory (DFT) calculations. At low growth temperature (≤250 °C), the PtTe₂ film prefers a stack of the monolayer and bilayer, while at ≈300 °C large-area continuous bilayer films are formed. In contrast, high growth temperature (>300 °C) leads to the formation of thick films with high Te deficiency and poor crystallinity. Theoretical calculations confirm the higher thermal stability of bilayer PtTe₂ over other layer numbers above a critical crystal size of ≈100 nm². STS shows that PtTe₂ is a semiconductor in the monolayer with a bandgap of 0.80 ± 0.05 eV, and changes to a semimetal from the bilayer. DFT calculations support our experimental results and suggest an indirect bandgap structure of the monolayer. This work provides a systematic study of the layer-dependent electronic structure of 2D PtTe₂, and demonstrates that with appropriate substrate and growth temperature choices, high-quality ultrathin PtTe₂ films can be obtained, important for device applications.

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...

Thermodynamic Perspective on the Oxidation of Layered Materials and Surface Oxide Amelioration in 2D Devices

Surface oxidation is an unneglectable problem for 2D semiconductors because it hinders the practical application of 2D material-based devices. In this research, the oxidation of layered materials is investigated by a thermodynamic approach to verify their oxidation tendency. It was found that almost all 2D materials are thermodynamically unstable in the presence of oxygen at room temperature. Two potential solutions for surface oxidation are proposed in this work: (i) the conversion of the surface oxides to functional oxides through the deposition of active metals and (ii) the recovery of original 2D materials from the surface oxides by 2D material heterostructure formation with the same chalcogen group. Supported by thermodynamic calculations, both approaches are feasible to ameliorate the surface oxides of 2D materials by the appropriate selection of metals for deposition or 2D materials for heterostructure formation. Thermodynamic data of 64 elements and 75 2D materials are included and compared in this research, which can improve gate insulator or electrode contact material selection in 2D devices to solve the surface oxidation issue. For instance, yttrium and titanium are good candidates for surface oxide conversion, while zirconium and hafnium chalcogenide can trigger the recovery of original 2D materials from their surface oxides. The systematic diagrams presented in this work can serve as a guideline for considering surface oxidation in future device fabrication from various 2D materials.

Fabrication of 2D PdSe2/3D CdTe Mixed-Dimensional van der Waals Heterojunction for Broadband Infrared Detection

Room-temperature infrared photodetectors are in great demand because of their vitally important applications in the military and civilian fields, which has inspired intensive studies in recent decades. Here, we present the fabrication of a large-size PdSe₂/CdTe mixed-dimensional van der Waals (vdW) heterojunction for room-temperature infrared detection. Taking advantage of the wide light absorption of the multilayer PdSe₂, high-quality vdW interface, and unique mixed-dimensional device geometry, the present device is capable of detecting an ultrawide light up to long-wave infrared (LWIR) of 10.6 μm at room temperature. In addition, our photodetector exhibits a good capability to follow short pulse infrared signals with a quick response rate of 70 ns, a large responsivity of 324.7 mA/W, and a reasonable specific detectivity of 3.3 × 10¹² Jones. Significantly, the assembled photodetector is highly sensitive to a polarized infrared light signal with a decent polarization sensitivity of 4.4. More importantly, an outstanding LWIR imaging capability based on the PdSe₂/CdTe vdW heterojunction at nonrefrigeration condition is demonstrated. Our work paves a novel route for the design of highly polarization-sensitive, room-temperature infrared photodetectors.

Observation of Anisotropic Magnetoresistance in Layered Nonmagnetic Semiconducting PdSe2

Anisotropy in crystals usually has remarkable consequences in two-dimensional (2D) materials, for example, black phosphorus, PdSe₂, and SnS, arising from different lattice periodicities along different crystallographic directions. Electrical anisotropy has been successfully demonstrated in 2D materials, but anisotropic magnetoresistance in 2D materials is rarely studied. Herein, we report anisotropic magnetoresistance in layered nonmagnetic semiconducting PdSe₂ flakes. Anisotropic magnetoresistance along the two crystalline axes under a perpendicular magnetic field is demonstrated, and the magnetoresistance along the a-axis is apparently different from the magnetoresistance along the b-axis. The magnetoresistance can also be flexibly tuned by applying a gate voltage, leveraging the semiconductor properties of PdSe₂. The computed anisotropic electronic density of states and electronic mobility with ab initio density functional calculations support the anisotropic and measured magnetoresistance. Our findings advance the understanding of magnetoresistance in anisotropic transition-metal dichalcogenides and pave the way for potential applications in anisotropic spintronic devices.

Electronic and structural characterisation of polycrystalline platinum disulfide thin films

We employ a combination of scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) to investigate the properties of layered PtS₂, synthesised via thermally assisted conversion (TAC) of a metallic Pt thin film. STM measurements reveal the 1T crystal structure of PtS₂, and the lattice constant is determined to be 3.58 ± 0.03 Å. STS allowed the electronic structure of individual PtS₂ crystallites to be directly probed and a bandgap of ∼1.03 eV was determined for a 3.8 nm thick flake at liquid nitrogen temperature. These findings substantially expand understanding of the atomic and electronic structure of PtS₂ and indicate that STM is a powerful tool capable of locally probing non-uniform polycrystalline films, such as those produced by TAC. Prior to STM/STS measurements the quality of synthesised TAC PtS₂ was analysed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. These results are of relevance to applications-focussed studies centred on PtS₂ and may inform future efforts to optimise the synthesis conditions for thin film PtS₂.

Semiconducting thin-film polycrystalline PtS₂ is characterised by atomically resolved scanning tunnelling microscopy and spectroscopy.