Exploration of new ferromagnetic, semiconducting and biocompatible NbX (X = Cl, Br or I) monolayers with considerable visible and infrared light absorption.

Ferromagnetic character and biocompatible properties have become key factors for developing next-generation spintronic devices and show potential in biomedical applications. Unfortunately, the Mn-containing monolayer is not biocompatible though it has been extensively studied, and the Cr-containing monolayer is not environmental friendly, although these monolayers are ferromagnetic. Herein, we systematically investigated new types of 2D ferromagnetic monolayers NbX (X = Cl, Br or I) by means of first principles calculations together with mean field approximation based on the classical Heisenberg model. The small cleavage energy and high in-plane stiffness have been calculated to evaluate the feasibility of exfoliating the monolayers from their layered bulk phase. Spin-polarized calculations together with self-consistently determined Hubbard U were utilized to assess a strong correlation energy, which demonstrated that NbX (X = Cl, Br or I) monolayers are ferromagnetic. The calculated Curie temperatures for NbCl, NbBr and NbI were 31, 56 and 87 K, respectively, which may be increased by external strain, or electron or hole doping. Moreover, the NbX (X = Cl, Br or I) monolayers exhibited strong visible and infrared light absorption. The biocompatibility, ferromagnetism and considerable visible and infrared light absorption render the NbX (X = Cl, Br or I) monolayers with great potential application in next-generation biocompatible spintronic and optoelectronic devices.