Optical Characterizations for Broadening Emissions from Doping of the Two-Dimensional Tin–Lead Perovskite Films

In this work, we present the optical properties of two-dimensional (2D) perovskite films of BA2SnxPb(1–x)I4 with x = 0–1. The excitonic absorption peaks of the fabricated perovskite samples were shown to shift to lower energies with increase of the Sn proportions in the films, whereas their emission spectra were shown to display dramatic Stokes shifts and spectral broadening from 600 to 1100 nm when compared with their pristine analogues. Power-dependent PL studies indicate that the emissions in the fabricated 2D perovskite samples arise from the self-trapping of excitons, while the spectral broadening originates from the production of multiple defect states upon doping of small amounts of Sn in the Pb-rich samples. The bandwidths of the emission spectra of the Pb–Sn mixed 2D perovskite samples were larger when the Pb proportion was higher than the Sn proportion, a key indicator for increasing the bandwidth of the white-light emission. TA spectral studies reveal that the nature of trap states plays a prominent role in the spectral broadening of the emission of the Pb-rich perovskite samples

Selective Hydrogen Etching Leads to 2D Bi(111) Bilayers on Bi₂Se₃: Large Rashba Splitting in Topological Insulator Heterostructure

Ultrathin bilayers (BLs) of bismuth have been predicated to be a two-dimensional (2D) topological insulator. Here we report on a new route to manufacture the high-quality Bi bilayers from a 3D topological insulator, a top-down approach to prepare large-area and well-ordered Bi(111) BL with deliberate hydrogen etching on epitaxial Bi₂Se₃ films. With scanning tunneling microscopy (STM) and X-ray photoelectron spectra (XPS) <i>in situ</i>, we confirm that the removal of Se from the top of a quintuple layer (QL) is the key factor, leading to a uniform formation of Bi(111) BL in the van der Waals gap between the first and second QL of Bi₂Se₃. The angle resolved photoemission spectroscopy (ARPES) <i>in situ</i> and complementary density functional theory (DFT) calculations show a giant Rashba splitting with a coupling constant of 4.5 eV Å in the Bi(111) BL on Bi₂Se₃. Moreover, the thickness of Bi BLs can be tuned by the amount of hydrogen exposure. Our ARPES and DFT study indicated that the Bi hole-like bands increase with increasing the Bi BL thickness. The selective hydrogen etching is a promising route to produce a uniform ultrathin 2D topological insulator (TI) that is useful for fundamental investigations and applications in spintronics and valleytronics