The interlayer interaction is vitally important for the layer-dependent properties, such as band structure, carrier mobility, and thermal conductivity. PtSe₂, one of typical group-10 transition metal dichalcogenides (TMDCs), undergoes a dramatical bandgap shrinking from an indirect bandgap semiconductor for monolayer to a semimetal for bulk. The sharp decline of the bandgap with increasing layer thickness in PtSe₂ implies that the interaction between adjacent layers might be very large, which is desirable for future device application like high-frequency (terahertz) micro-mechanical resonators.

Recently, a study led by Prof. WANG Jun from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, reported the real-time observation of the interlayer lattice vibrations in bilayer and few-layer PtSe₂ in time domain by means of the coherent phonon method. Their study has been published in Photonics Research.

In their experiment, PtSe₂ films with different thicknesses ranging from 2 to 22 layers were prepared by chemical vapor deposition technology. Under the excitation of 380 femtosecond laser pulses at 1040 nm with a repetition rate of 100 KHz, the lattice vibrations occurred in the PtSe₂ films, then the coherent phonons and the periodic bandgap renormalization were generated, and consequently the transmission of the laser changed periodically.

They used the pump-probe system to observe and analyze the transmission change, and obtained two kinds of interlayer interactions, the layer-breathing mode and the standing wave mode. By fitting the former with different layers, the interlayer breathing force constant standing for perpendicular coupling (per effective atom) was derived as 7.5 N/m, much larger than that of graphene. And from the latter, the out-of-plane sound velocity of PtSe₂ was determined to be 1720 m/s, which was firstly measured experimentally.

The study clearly demonstrates the large interlayer interaction in PtSe₂ films, which provides the theoretical and experimental guidance for studying interactions in two-dimensional materials and indicates its promising potential in nanophotonic devices.

This work was supported by the Chinese National Natural Science Foundation, the Strategic Priority Research Program of CAS, the Key Research Program of Frontier Science of CAS, and the Program of Shanghai Academic Research Leader.

The layer-dependent interlayer interaction modes in PtSe₂ films.(Image by SIOM)

Diagram of vibrational displacements of the layer-dependent interlayer interaction modes in PtSe₂.(Image by SIOM)

Article website:
https://www.osapublishing.org/prj/abstract.cfm?uri=prj-7-12-1416

Contact:
Mr. CAO Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: caoyong@siom.ac.cn