Ultralow-Frequency Collective Compression Mode and Strong Interlayer Coupling in Multilayer Black Phosphorus.

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Black phosphorus (BP) is a 2D layered material with significant interest due to its unique properties.
• Understanding the structural characteristics of BP is crucial for elucidating its outstanding properties.
• Existing methods for determining BP thickness have limitations.

Purpose of the Study:

• To investigate the underlying structural characters of black phosphorus (BP).
• To report the discovery of a novel ultralow-frequency collective compression mode (CCM) in BP.
• To establish a new, accurate method for determining the thickness of BP layers.

Main Methods:

• Raman spectroscopy measurements were performed on BP.
• Phonon frequency analysis was conducted.
• First-principles calculations were employed to support experimental findings.

Main Results:

• An unprecedented ultralow-frequency collective compression mode (CCM) was observed in BP.
• The CCM indicates unusually strong interlayer coupling in BP.
• Raman modes, including the CCM, showed sensitive shifts with the number of layers, enabling accurate thickness determination up to tens of atomic layers.

Conclusions:

• The discovery of the CCM provides fundamental insights into the strong interlayer coupling in BP.
• The observed Raman mode shifts offer a practical and accurate tool for determining BP thickness, surpassing previous methods.
• These findings enhance the exploration of BP as a promising 2D semiconductor material.