Liquid-Phase Exfoliation of Bismuth Telluride Iodide (BiTeI): Structural and Optical Properties of Single-/Few-Layer Flakes

Abstract

Bismuth telluride halides (BiTeX) are Rashba-type crystals with several potential applications ranging from spintronics and nonlinear optics to energy. Their layered structures and low cleavage energies allow their production in a twodimensional form, opening the path to miniaturized device concepts. The possibility to exfoliate bulk BiTeX crystals in the liquid represents a useful tool to formulate a large variety of functional inks for large-scale and cost-effective device manufacturing. Nevertheless, the exfoliation of BiTeI by means of mechanical and electrochemical exfoliation proved to be challenging. In this work, we report the first ultrasonicationassisted liquid-phase exfoliation (LPE) of BiTeI crystals. By screening solvents with different surface tension and Hildebrandt parameters, we maximize the exfoliation efficiency by minimizing the Gibbs free energy of the mixture solvent/BiTeI crystal. The most effective solvents for the BiTeI exfoliation have a surface tension close to 28 mN m −1 and a Hildebrandt parameter between 19 and 25 MPa 0.5 . The morphological, structural, and chemical properties of the LPE-produced single-/few-layer BiTeI flakes (average thickness of ∼3 nm) are evaluated through microscopic and optical characterizations, confirming their crystallinity. Second-harmonic generation measurements confirm the non-centrosymmetric structure of both bulk and exfoliated materials, revealing a large nonlinear optical response of BiTeI flakes due to the presence of strong quantum confinement effects and the absence of typical phase-matching requirements encountered in bulk nonlinear crystals. We estimated a second-order nonlinearity at 0.8 eV of |χ (2) | ∼ 1 nm V −1 , which is 10 times larger than in bulk BiTeI crystals and is of the same order of magnitude as in other semiconducting monolayers (e.g., MoS 2 ).

Two-dimensional BiTeI as a novel perovskite additive for printable perovskite solar cells

“…Such parameters' value/ range minimizes the Gibbs free energy of the mixture solvent/ BiTeI crystals, maximizing the exfoliation yield and stabilizing the exfoliated material dispersion. 63,78,79 A solvent exchange treatment of the as-produced BiTeI dispersion was then carried out to redisperse the exfoliated materials in an anhydrous DMF : DMSO (4 : 1 vol./vol.) mixture, which was used to dissolve the quadrupole cation perovskite precursors.…”

“…61 Other examples of applications of BiTeI are thermoelectric devices, 48 spintronics, 45 catalysis (e.g., nitrogen reduction reaction 62 ) and non-linear optics. 63 By taking advantage of the natural cleavage planes offered at the interface between Te and I, the exfoliation of BiTeI in 2D forms is a turning point to fully exploit its surface properties, 60,63 as well as its integration as an additive in thin lms of functional materials, including metal halide perovskites of PSCs. The predicted cleavage energy of BiTeI to obtain its monolayer is as low as ∼90 meV per atom, 63 which is of the same order as those reported for other layered materials, for example, metal chalcogenides [64][65][66] and graphite.…”

“…63 By taking advantage of the natural cleavage planes offered at the interface between Te and I, the exfoliation of BiTeI in 2D forms is a turning point to fully exploit its surface properties, 60,63 as well as its integration as an additive in thin lms of functional materials, including metal halide perovskites of PSCs. The predicted cleavage energy of BiTeI to obtain its monolayer is as low as ∼90 meV per atom, 63 which is of the same order as those reported for other layered materials, for example, metal chalcogenides [64][65][66] and graphite. 67 Single-/few-layer BiTeI akes have been recently obtained through both mechanical 68 and liquid-phase exfoliation (LPE) methods.…”

“…The limitations of the previous exfoliation techniques have been recently solved by prototypical ultrasonication-assisted LPE, which preserved the structural characteristics of bulk BiTeI crystals (except for the layer number reduction). 63 In particular, alcoholic solvents can minimize the Gibbs free energy of the solvent/BiTeI mixture, the primary condition to attain an efficient LPE of layered materials, 69,70 leading to scalable production of BiTeI inks for large-scale, high-throughput manufacturing of devices, including solar cells. [71][72][73] Owing to the metallic character of BiTeI, and by rationalizing the main characteristics of 2D materials previously used as additives for perovskites, we rst evaluated the impact of LPEexfoliated BiTeI akes as additives for perovskite absorbers in inverted PSCs.…”

Janus zirconium halide ZrXY (X, Y = Br, Cl and F) monolayers with high lattice thermal conductivity and strong visible-light absorption

“…22 The presence of giant Rashba splitting in the non-centrosymmetric BiTeI monolayer opens the way toward spintronics and nonlinear optics. 22,23 Janus In 2 SSe has been predicted with broken out of plane symmetry, having indirect band gap and suitable for optoelectronic nanodevice applications. 24 Bui et al explored the structural, electronic and optical properties of Janus Ga 2 SSe, Ga 2 STe and Ga 2 SeTe monolayers and proposed these monolayers for ultraviolet detectors and photovoltaic absorber applications.…”