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

Abstract

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.

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

“…In addition, it is observed that the I D values increase with temperature in both devices, demonstrating the semiconducting (or insulating) behavior of PdSe 2 . Such insulating behavior was also recently reported by Gu et al in monoclinic PdSe 2 crystals due to the presence of defect-induced localized states . However, their research was conducted on the Si/SiO 2 substrates with a high density of unwanted traps or defects that interfere with the transport behavior of PdSe 2 itself.…”

“…Such insulating behavior was also recently reported by Gu et al in monoclinic PdSe 2 crystals due to the presence of defect-induced localized states. 16 However, their research was conducted on the Si/SiO 2 substrates with a high density of unwanted traps or defects that interfere with the transport behavior of PdSe 2 itself. In contrast, our PdSe 2 FET devices were fabricated on h-BN surfaces for maximum screening out of the defective interference from external factors, including substrates, which allows us to focus on the impact of the intrinsic defects of PdSe 2 on its transport behaviors.…”

“…So far, the experimentally measured mobilities of PdSe 2 field-effect transistors (FETs) are typically lower than the theoretically expected value (>1000 cm 2 V −1 s −1 ). 7,16 Moreover, the small diffusion barrier of Se vacancies contributes to spontaneous thermally induced migration 17 and further aggravates the disorder states in PdSe 2 . Therefore, understanding the temperature-dependent defect behaviors and the corresponding defect-induced evolution of electrical properties in PdSe 2 is very significant.…”

Integration and Applications of Nanomaterials for Ultrafast Photonics

“…Monoclinic‐PdSe 2 was found to have a very high electron mobility anisotropy of 1.9, which is higher than orthorhombic‐ PdSe 2 and BP. [ 311 ] It is suggested that metastable phases of typical 2D materials can hold unique properties. Notably, this method is expected to be used in direct synthesis of metastable phases of other 2D van der Waals materials due to its versatility.…”

“…Recently, an ambient-pressure chemical vapor deposition method was proposed to directly prepare monoclinic-PdSe 2 single crystals on substrates. [311] PdSe 2 has proven to be promising broadband optical modulator and as SAs in ultrafast lasers in the visible and nearinfrared bands. [277,312] However, its metastable phases have scarcely been applied to ultrafast photonic.…”