Polarization-Resolved Raman Study of Bulk-like and Davydov-Induced Vibrational Modes of Exfoliated Black Phosphorus

Abstract

Owing to its crystallographic structure, black phosphorus is one of the few 2D materials expressing strongly anisotropic optical, transport, and mechanical properties. We report on the anisotropy of electron-phonon interactions through a polarization-resolved Raman study of the four vibrational modes of atomically thin black phosphorus (2D phosphane): the three bulk-like modes A, B, and A and the Davydov-induced mode labeled A(B). The complex Raman tensor elements reveal that the relative variation in permittivity of all A modes is irrespective of the atomic motion involved lowest along the zigzag direction, the basal anisotropy of these variations is most pronounced for A and A(B), and interlayer interactions in multilayer samples lead to reduced anisotropy. The bulk-forbidden A(B) mode appears for n ≥ 2 and quickly subsides in thicker layers. It is assigned to a Davydov-induced IR to Raman conversion of the bulk IR mode B and exhibits characteristics similar to A. Although this mode is expected to be weak, an electronic resonance significantly enhances its Raman efficiency such that it becomes a dominant mode in the spectrum of bilayer 2D phosphane.

Production and Patterning of Liquid Phase–Exfoliated 2D Sheets for Applications in Optoelectronics

“…Besides the information on layer thickness of TMDs,[38c] Raman spectroscopy can provide details about atomic coordination and crystal lattice, e.g., trigonal prismatic (2H) versus octahedral (1T) . Figure c also presents the Raman spectra of BP as a function of the number of atomic layers, from the SL to an optically thick bulk sample . From bulk to atomically thin samples, slight frequency variations and the appearance of two new Raman active vibrational modes located above A 1 g and A 2 g were observed.…”

Optical and Optoelectronic Properties of Black Phosphorus and Recent Photonic and Optoelectronic Applications

“…Theoretically, this polarization‐dependent absorption is robust over a wide energy range spanning from 0.33 eV (MIR) up to 3.0 eV (UV) . The significant anisotropic optical absorption of BP leads to strong anisotropy manifested in other related optical properties such as the optical contrast, Raman spectroscopy, PL spectroscopy, photocurrent spectroscopy, and photocarrier dynamics …”

“…Theoretically, this polarization-dependent absorption is robust over a wide energy range spanning from 0.33 eV (MIR) up to 3.0 eV (UV). [93] The significant anisotropic optical absorption of BP leads to strong anisotropy manifested in other related optical properties such as the optical contrast, [95] Raman spectroscopy, [96][97][98][99][100][101] PL spectroscopy, [86,102] photocurrent spectroscopy, [93,103] and photocarrier dynamics. [104][105][106] The anisotropic PL spectra of mono-and few-layer BP are closely related to the anisotropic absorption spectra and share the same symmetry origin.…”

Spatial-Temporal Imaging of Anisotropic Photocarrier Dynamics in Black Phosphorus

“…The crystal orientation is determined specifically from the intensity of the A g 1 peak, which reaches the minimal intensity when the laser polarization is along armchair (x-) direction. 32 SUEM contrast images of the BP flake shown in Figure 1b are presented in Figure 2. A low-pass Gaussian filter is used to suppress the noise of the images for presentation, while raw images are used for quantitative analysis shown later.…”