Structural stability and vibrational analysis of beryllium sulfide BeS from the bulk to the (n,0) nanotubes. An ab initio description

Abstract Beryllium sulfide (BeS) in different forms from molecule, bulk, monolayer ( h -BeS), to the single-walled nanotube obtained by wrapping the h -BeS monolayer along the ( n ,0) hexagonal lattice vector for n varying from n = 6 to 64, has been examined. Density functional theory (DFT) with an all electron basis set and the hybrid DFT/B3LYP level have been applied to compute energetic and geometrical, electronic and vibrational, elastic and piezoelectric properties, where the trend towards the hexagonal monolayer ( h -BeS) in the limit of large tube radius is obtained. The vibrational properties including Raman spectra and polarizabilities are evaluated via the Coupled Perturbed Hartree–Fock and Kohn–Sham (CPHF/KS) computational schemes. For the first time, the IR vibrational modes at higher tube diameter and their convergence to the h -BeS monolayer limit are reported where the vibrational and electronic contributions to both perpendicular and parallel components of polarizability tensor are additionally discussed. For the (n,0) BeS nanotube family, three ranges of IR active phonon modes are determined. The first one located in the 0–300 cm −1 frequency domain, goes regularly to zero when the tube diameter increases. Both the second (300–400 cm −1 ) and the third (700–800 cm −1 ) ones tend smoothly with different slope, towards the two optical vibrational modes of the h -BeS layer. These theoretical models can be extended to investigate further issues, such as the effect caused by the addition of a dopant, the influence of the substitutional fraction of nanotube atoms and the interaction of molecules outside and/or inside of the nanotube.