Abnormal Thickness-Dependent Thermal Transport in Suspended 2D PdSe₂

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Research on two-dimensional (2D) materials initially concentrated on highly symmetrical structures like graphene and hexagonal boron nitride (h-BN).
• Recently, low-symmetry 2D materials, such as palladium diselenide (PdSe2), have gained significant interest due to their unique properties, including layer-dependent bandgaps, favorable mechanical characteristics, and excellent thermoelectric performance.

Purpose of the Study:

• To investigate the phonon thermal transport properties of PdSe2 with a pentagonal fold structure.
• To explore the relationship between the thickness of PdSe2 flakes and their thermal conductivity.
• To understand the underlying mechanisms responsible for the observed thickness-dependent thermal transport.

Main Methods:

• Experimental measurement of thermal conductivity in PdSe2 flakes using the thermal bridge method.
• Fabrication of PdSe2 flakes with varying thicknesses, ranging from a few nanometers to several tens of nanometers.
• Atomistic modeling to analyze lattice contraction, phonon group velocity, and phonon mode upshifts.

Main Results:

• Thermal conductivity of PdSe2 flakes increased from 5.04 W m^-1 K^-1 for 60 nm thick flakes to 34.51 W m^-1 K^-1 for few-layer flakes.
• Atomistic simulations revealed that decreasing thickness leads to lattice contraction and enhanced phonon group velocity.
• An upshift in optical phonon modes was observed with reduced thickness, contributing to decreased acoustic phonon scattering and increased thermal conductivity.

Conclusions:

• PdSe2 exhibits an anomalous thickness-dependent thermal transport behavior, with thermal conductivity increasing as the material becomes thinner.
• Lattice contraction and altered phonon dynamics are identified as key factors driving this unusual phenomenon.
• The findings provide crucial insights into nanoscale thermal transport in 2D materials and suggest potential applications in thermal management.