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 two-dimensional 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 ultrasonication-assisted 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 MPa0.5. 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., MoS2).

BeDimensional S p A via Lungotorrente Secca 30R 16163 Genova Italy

Department of Inorganic Chemistry University of Chemistry and Technology Prague Technická 5 16628 Prague 6 Czech Republic

Department of Physics University of Calabria Via P Bucci cubo 31 C Rende Cosenza 87036 Italy

Dipartimento di Chimica e Chimica Industriale Università degli Studi di Genova via Dodecaneso 31 16146 Genoa Italy

Dipartimento di Fisica Politecnico di Milano Piazza Leonardo da Vinci 32 20133 Milano Italy

Functional Nanosystems Istituto Italiano di Tecnologia via Morego 30 16163 Genova Italy

Graphene Labs Istituto Italiano di Tecnologia via Morego 30 16163 Genova Italy

Nanochemistry Department Istituto Italiano di Tecnologia via Morego 30 Genova 16163 Italy

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