# Borophene Is a Promising 2D Allotropic Material for Biomedical Devices
## 2 Main Properties of Borophene  
The naive 3D structure of boron makes it neither a metal nor a non-metal: it can be considered a metalloid, commonly used in the production of semiconductors. On the other hand, boron starts to show interesting metallic properties when it holds a two-dimensional structure, similar to other allotropic materials such as graphene. Nevertheless, borophene is made with a different technique; in fact, it is obtained through the physical vapor deposition (PVD) technique, which works by vaporizing boron on a thin film of silver [2] (Figure 1). Borophene surfaces are very different from those observables on graphene; in fact, borophene has ridges with a morphology that strongly depends on how deep the boron atoms bind together. These structural features make borophene a polymorphous and anisotropic material [6]. Some studies carried out using the density functional theory (DFT) have permitted the investigation of three different polymorphic structures of borophene: _2-Pmmm_, \(\beta_{12}\), and \(\chi_{3}\). All these structures can show different behaviors under similar compression and torsion forces with deformation energies lower than 100 meV/atom, a peculiarity that could be suitable for the designing of nanometric devices (Figure 2) [7]. Figure 1: Atomic geometry of borophene on Ag(111), in front (**top**) and side (**bottom**) views. In materials science, thermal conductivity plays an important role in defining the performances and lifecycle of a material. Thermal conductivity of borophene has been studied by several research groups [8], and recently thermal conductivity has been investigated by means of the transmission of phonons along the surface, thus allowing verification that thermal conductivity is in fact linked to anisotropy and frequency of phonons. The properties of borophene have aroused great interest among the scientific community [9; 10], and structural and physical experiments have been performed over the years to find the mechanical coefficients of the different forms of borophene (_2-Pmmn_, \(\beta_{12}\), \(\chi_{3}\)) [10]. Chemical reactions could modify some mechanical values of borophene; for instance, electrochemical fluorination is able to decrease the coefficients on the different forms of borophene [10]. In recent studies on fluorinated boron, particular attention has been paid to the structure of _2-Pmmn_ borophene after a fluorination reaction. In fact, two highly stable anisotropic structures have been obtained during such studies: B4F and B2F [11]. Both these structures were found to be excellent candidates for the production of semiconductors in electronic devices, mainly thanks to their excellent mechanical, thermal, and electrical conductivity properties. Optical coefficients have also been linked to the anisotropy of the structure of borophene; we can obtain different data based on the different direction of the stimuli on the material's surface [5]. It should be noted that the B2F structure has excellent optical conductivity in the visible region, and this peculiarity makes it perfect for optical devices (Figure 3). Figure 3: Optical coefficients linked to the anisotropy of the structure of borophene make it ideal for optical circuits. Figure 2: Crystal structures of \(\beta_{12}\), \(\chi_{3}\) and _2-Pmmn_. Numerous 2D boron nanostructures have been created by research groups over time. However, monoatomic-thin boron layers have energy that is higher than 3D bulk-state boron, and this fact means that the 2D structure of boron is thermodynamically unstable. In this landscape, a metallic substrate is necessary to counteract the 3D nucleation barrier in order to attract the boron atoms into the 2D monoatomic-thin Boron layer form. Borophene that has been successfully synthesized on the surface of Ag(111) by deposition has also been also defined as an \"atomic-thin two-dimensional boron sheet\" [12]. The stability of the different forms of borophene is still debated in the scientific community.