PdSe2 is a promising two dimensional noble metal dichalcogenide with inherent in-plane anisotropy due to its pentagonal structure and finds application in electronic, photonic, and thermoelectric devices. Herein, we present...

Ultrathin Bi2O2Se nanostructures possess ubiquitous structural and optical properties suitable for future optoelectronics. Herein, we have studied the unique structural and optoelectronic functionalities of ultrathin Bi2O2Se nanocrystals and their modulation...

Surface-enhanced Raman spectroscopy (SERS), a very powerful tool for the identification of molecular species, has relied mostly on noble metal-based substrates to obtain a high enhancement factor. In this work, we demonstrate that self-driven intrinsic defects in 2D palladium di-selenide (PdSe2) dendrites grown at low temperature (280 °C) act as hotspots for high SERS enhancement. We grow 2D dendritic PdSe2 with ample intrinsic defects to exploit it for SERS application. X-ray electron spectroscopy (XPS) analysis reveals 9.3% outer layer and 4.7% interior Se vacancies. A detailed examination of atomic-scale defects revealed Se vacancy (VSe) coupled with Se–Pd–Se vacancy (VSe-Pd-Se) in monolayer PdSe2, and an array of line defects (Se vacancies) and nanopores in bilayer PdSe2 dendrites. Interestingly, our studies reveal that Se vacancy-rich PdSe2 gives rise to line defects that act like hotspots for SERS enhancement. Remarkably, the vacancy-rich dendritic PdSe2 shows a SERS enhancement factor >105 and can detect RhB at a concentration down to 10−8  M. We speculate that the topological line defects and the edge construction in PdSe2 dendrites act as metallic wire or edge, which is partly responsible for the high enhancement in the SERS signal. The high SERS sensitivity is explained on the basis of multiple charge transfer processes combined with the predicted metal-like behavior of the defected 2D PdSe2. Our conclusions are fully supported by the density functional theory calculation of the electronic density of states of the defective bilayer (2L) PdSe2, which remarkably exhibits metallic character. Being a defect-enabled SERS substrate, dendritic 2D PdSe2 fills the gap between conventional plasmonic SERS substrate and plasmon-free SERS substrate.

Interlayer charge transfer (CT) based on band alignment plays a vital role in various optoelectronic applications, such as photoluminescence modulation, superior photoconduction, etc. The layer-by-layer integration of atomically thin materials with different band alignments is an efficient approach to witness CT. To study interlayer CT, the stacking of ultrathin van der Waals (vdW) materials has been studied extensively, while the stacking of vdW materials with non-van der Waals (nvdW) materials is least explored. Herein, we present the stacking of an nvdW twodimensional (2D) Bi 2 O 2 Se layer over a vdW 2D MoS 2 (monolayer) and study the interlayer coupling and CT across the 2D interface. Studies through various spectroscopic and microscopic tools and density functional theory calculations reveal that significant interlayer CT occurs across the heterolayers due to the favorable band alignment of type-II across the junction. Interestingly, the CT from the 2D Bi 2 O 2 Se layer to the monolayer MoS 2 results in photoluminescence (PL) quenching in the MoS 2 layer and enhanced photoconduction in the HS. Low-temperature PL studies reveal that the robust interlayer coupling between the heterolayers enhances the CT process. The modified Varshni fit reveals that the electron−phonon coupling constant (Huang−Rhys factor) is higher for trions (1.13) than for neutral excitons (0.66) in the heterostructure. Upon photoexcitation, the trion−phonon coupling is stronger than the neutral exciton−phonon coupling in the heterostructure system. The additional doping caused by photogenerated CT was quantified by solving the coupled rate equations using a four-level model, and the results are fully consistent with the CT estimated from the density functional theory (DFT) calculation. These results are significant for understanding the interaction between vdW and nvdW 2D heterostructures and further exploration of such 2D heterostructures in future optoelectronic applications. KEYWORDS: type-II heterostructure, charge transfer, 2D Bi 2 O 2 Se, monolayer MoS 2 , electron−phonon coupling

The appealing success of non-van der Waals (Non-VdW) two-dimensional (2D) Bismuth oxiselenise (Bi2O2Se) crystals in optoelectronics provides an exciting avenue to investigate its fundamental physical properties. To date, the majority...