Bismuth telluride iodide monolayer flakes with nonlinear optical response obtained via gold-assisted mechanical exfoliation

Bismuth telluride iodide (BiTeI) is a layered material known as Rashba semiconductor, which is promising for several applications such as spintronics, nonlinear optics, and energy-related devices. Due to its layered structure and low cleavage energy, BiTeI is particularly suitable for producing two-dimensional (2D) BiTeI crystals, allowing for the development of devices at the nanoscale. However, traditional methods of mechanically exfoliating BiTeI have encountered significant challenges. To address this, we used the so-called gold-assisted mechanical exfoliation to successfully obtain 2D BiTeI flakes. Through extensive characterization employing a range of techniques including Raman spectroscopy and second-harmonic generation (SHG) measurements, we examine the morphological, structural, and chemical properties of the gold-assisted mechanically exfoliated BiTeI flakes. We confirm their crystalline nature while also providing information about strain and defects present within the flakes. Moreover, SHG measurements revealed a significant nonlinear optical response, and the non-centrosymmetric structure observed in these flakes can be attributed to quantum confinement effects and the absence of phase-matching requirements typically found in bulk nonlinear crystals.