Anisotropic Electron–Phonon Interactions in Angle-Resolved Raman Study of Strained Black Phosphorus

Abstract

Few-layer black phosphorus (BP) with an in-plane puckered crystalline structure has attracted intense interest for strain engineering due to both its significant anisotropy in mechanical and electrical properties and its high intrinsic strain limit. Here, we investigated the phonon response of few layer BP under uniaxial tensile strain (∼7%) with in situ polarized Raman spectroscopy. Together with the first-principles density functional theory (DFT) analysis, the anisotropic Poisson’s ratio in few-layer BP was verified as one of the primary factors that caused the large discrepancy in the trend of reported Raman frequency shift for strained BP, armchair (AC) direction in particular. By carefully including and excluding the anisotropic Poisson’s ratio in the DFT emulations, we rebuilt both trends reported for Raman mode shifts. Furthermore, the angle-resolved Raman spectroscopy was conducted in situ under tensile strain for systematic investigation of the in-plane anisotropy of BP phonon response. The experimentally observed thickness and crystallographic orientation dependence is elaborated using DFT theory as having a strong correlation between the strain-perturbated electronic-band structure and the phonon vibration modes. This study provides insight, both experimentally and theoretically, for the complex electron–phonon interaction behavior in strained BP, which enables diverse possibilities for the strain engineering of electrical and optical properties in BP and similar two-dimensional nanomaterials.

Thermal Transport in 2D Semiconductors—Considerations for Device Applications

“…This inconsistent phonon response to strain possibly explains the recent experimental observation that Raman sensitive peak A g 1 , B 2g , and A g 2 modes could have opposite responses for tensile strain ZZ-and AC-directions. [81] By using a polymeric low-k dielectric to stretch the substrate, a high tensile strain of more than 7% was achieved in ref. [81], which nonetheless is still short of the theoretical predication of the high strain limit for ZZ and AC directions of 27% and 30%, [82] respectively.…”

“…[81] By using a polymeric low-k dielectric to stretch the substrate, a high tensile strain of more than 7% was achieved in ref. [81], which nonetheless is still short of the theoretical predication of the high strain limit for ZZ and AC directions of 27% and 30%, [82] respectively. Interestingly, the in-plane modes, B 2g and A g 2 , show a red/blue shift when a strain is introduced along the ZZ and AC directions, respectively, due to the special in-plane puckered structure of BP.…”

Anisotropic Phonon Response of Few‐Layer PdSe2 under Uniaxial Strain

“…It has been widely used to determine the crystallographic orientations, [ 2,5,27–29 ] layer numbers, [ 30–39 ] and strains of various 2D materials. [ 40–48 ] In particular, the phonon response under strain was found to be anisotropic in several 2D materials such as black phosphorus and ReSe 2 . [ 45,49,50 ] Recently, Oyedele and co‐workers reported the Raman spectra of layered PdSe 2 , [ 21 ] and the observed low frequency (LF) phonon modes were used to identify the numbers of layers.…”

“…[ 40–48 ] In particular, the phonon response under strain was found to be anisotropic in several 2D materials such as black phosphorus and ReSe 2 . [ 45,49,50 ] Recently, Oyedele and co‐workers reported the Raman spectra of layered PdSe 2 , [ 21 ] and the observed low frequency (LF) phonon modes were used to identify the numbers of layers. [ 51 ] Nevertheless, the experimental investigations on the phonon response of strained PdSe 2 are still lacking, which, however, is crucial for the integration of few‐layer PdSe 2 into flexible electronic devices.…”

“…We found near‐zero Poisson’s ratios under the uniaxial tensile strain, suggesting that the tensile strain applied in PdSe 2 is effectively uniaxial. In contrast, for many other anisotropic 2D materials such as black phosphorus, the Poisson’s ratios are notably larger, [ 45 ] and hence a compressive strain is automatically induced perpendicular to the applied tensile strain direction, leading to an effective biaxial strain in the system. The near‐zero Poisson’s ratio in few‐layer PdSe 2 is a major factor contributing to the observed anisotropic phonon response.…”