Two-dimensional (2D) PdSe₂ has garnered sustained interest due to its unique puckered pentagonal lattice, which endows it with nonlinear optical activity, strong in-plane anisotropy, sizable bandgap, and environmental stability. However, the advancement of this system toward optoelectronic device applications has been limited by the lack of reliable direct-growth routes for high-quality 2D PdSe₂ crystals with regulated thickness. In this paper, we report the bottom-up growth of few-layer and multilayer 2D PdSe₂ nanosheets on sapphire substrates via a growth-parameter-regulated chemical vapor transport reaction. The high quality of the as-grown PdSe₂ nanosheets is confirmed by aberration-corrected scanning transmission electron microscopy images and low-frequency Raman spectroscopy. We further investigate the strong optical anisotropy of PdSe₂ nanosheets using the second-harmonic generation response and angle-resolved polarized Raman spectroscopy. Moreover, electrical transport measurements reveal large anisotropic ratios of 4.9 for electrical conductivity and 6.3 for carrier mobility. This work opens a new route for the synthesis of variable-thickness 2D pentagonal materials and establishes PdSe₂ as an excellent candidate for polarization-sensitive optoelectronic devices.

Over the last few years, great advances have been achieved in exploration of high-mobility two-dimensional (2D) semiconductors such as metal chalcogenide InSe and noble-transition-metal dichalcogenide PdSe₂. These materials are competitive candidates for constructing next-generation optoelectronic devices owing to their unique crystalline and electronic structures. Moreover, the optical and electronic properties of 2D materials can be efficiently modified via precisely engineering their band structures, which is critical for widening specific applications ranging from high-performance optoelectronics to catalysis and energy harvesting. In this review, we focus on the progress in bandgaps engineering of newly emerging high-mobility 2D semiconductors and their applications in optoelectronic devices, incorporating our recent study in the InSe and PdSe₂ systems. First of all, we discuss the structure-property relationship of typical high-mobility 2D semiconductors (InSe and PdSe₂). Next, we analyze several viable strategies for bandgap engineering, including thickness, strain or pressure, alloying, heterostructure, surface modification, intercalation, and so on. Furthermore, we summarize the optoelectronic devices fabricated with such high-mobility 2D semiconductors. The conclusion and outlook in this topic are finally presented. This review aims to provide valuable insights in bandgap engineering of newly emerging 2D semiconductors and explore their potential in future optoelectronic applications.

Exposure to oxygen alters the physical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). In particular, oxygen in the ambient may influence the device stability of 2D TMDs over time. Engineering the doping of 2D TMDs, especially hole doping is highly desirable towards their device function. Herein, controllable oxygen-induced p-type doping in a range of hexagonal (MoTe₂, WSe₂, MoSe₂ and PtSe₂) and pentagonal (PdSe₂) 2D TMDs are demonstrated. Scanning tunneling microscopy, electrical transport and X-ray photoelectron spectroscopy are used to probe the origin of oxygen-derived hole doping. Three mechanisms are postulated that contribute to the hole doping in 2D TMDs, namely charge transfer from absorbed oxygen molecules, surface oxides, and chalcogen atom substitution. This work provides insights into the doping effects of oxygen, enabling the engineering of 2D TMDs properties for nanoelectronic applications.