Phonon properties of biphenylene monolayer by first-principles calculations

Recently, the synthesis of biphenylene inspires the substantial attention on the two-dimensional allotrope of carbon. Although elastic, thermal, and electronic properties of biphenylene have been reported, phonon modes and the origin of anisotropy in biphenylene are still unclear. In this work, combining the first-principles calculations and theoretical analysis, we investigate the properties of optical and acoustic phonons in monolayer biphenylene. There are nine Raman-active and five infrared-active modes which can be excited by the Raman or infrared laser. Interestingly, a Raman-active single phonon mode (Ag3) is observed, and its frequency is up to 49.67 THz at the Brillouin zone-center point. This provides promising potential for biphenylene monolayer in the application of phonon lasers, quantum nonlinear elements, and quantum mechanical resonators. Meantime, the Grüneisen constant of an Ag3 mode is up to 2.07 at the zone-center point, suggesting that its Raman spectroscopy can be used to identify the lattice strain and temperature of biphenylene. To explore the origin of anisotropy in biphenylene, we calculate the covalency and cophonicity and find that the inconsistent speed of motion and different intensities of hybridization between these inequivalent carbon atoms should take responsibility for the direction dependent thermal and elastic properties in biphenylene.