Zhixin Hu,1,2 Xianghua Kong,1,2, Jingsi Qiao,1,2, Bruce Normand1,2 and Wei Ji1,2,
1Department of Physics, Renmin University of China, Beijing 100872, China
2Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-Nano Devices, Renmin University of China, Beijing 100872, China
DOI:10.1039/C5NR06293D Publication Date:
Few-layer black phosphorus (FLBP) is a material with high potential for nano- and optoelectronics applications. We perform a theoretical investigation of its vibrational properties by discussing six Raman-observable phonons, including three optical, one breathing and two shear modes. With increasing sample thickness, we find anomalous redshifts of the frequencies for each optical mode but a blueshift for the armchair shear mode. Our calculations also show splitting of the phonon branches, due to anomalous surface effects, and strong phonon-phonon coupling. By computing uniaxial stress effects, inter-atomic force constants and electron densities, we provide a compelling demonstration that these properties are the consequence of strong and highly directional interlayer interactions arising from electronic hybridization of the lone electron-pairs of FLBP. This exceptional interlayer coupling mechanism governs the stacking stability of BP layers and thus opens a new avenue beyond van der Waals epitaxy for understanding the design of two-dimensional heterostructures.