Large Frequency Change with Thickness in Interlayer Breathing Mode - Significant Interlayer Interactions in Few Layer Black Phosphorus

TL;DR

This study combines theoretical and experimental methods to reveal that few-layer black phosphorus exhibits a large frequency shift in its interlayer breathing mode with thickness, indicating strong interlayer interactions beyond van der Waals forces.

Contribution

It provides the first detailed analysis of interlayer vibrational modes in few-layer BP, highlighting the unusually large interlayer force constant and covalent interactions.

Findings

Interlayer breathing mode frequency shifts significantly with thickness.

Interlayer force constant in BP is about 50% larger than in graphene and MoS2.

Strong covalent interlayer interactions influence BP's physical properties.

Abstract

Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics. A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes. We show that the interlayer breathing mode A3g shows a large redshift with increasing thickness; the experimental and theoretical results agreeing well. This thickness dependence is two times larger than that in the chalcogenide materials such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that in graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers, and that interlayer interactions are not merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP, and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP.