Unraveling High Thermal Conductivity with In-Plane Anisotropy Observed in Suspended SiP₂

The anisotropic thermal transport properties of low-symmetry two-dimensional materials play an important role in understanding heat dissipation and optimizing thermal management in integrated devices. Examples of efficient energy dissipation and enhanced power sustainability have been demonstrated in nanodevices based on materials with anisotropic thermal transport properties. However, the exploration of materials with high thermal conductivity and strong in-plane anisotropy remains challenging. Herein, we demonstrate the observation of anisotropic in-plane thermal conductivities of few-layer SiP₂ based on the micro-Raman thermometry method. For suspended SiP₂ nanoflake, the thermal conductivity parallel to P-P chain direction (κ_∥) can reach 131 W m^-1 K^-1 and perpendicular to P-P chain direction (κ_⊥) is 89 W m^-1 K^-1 at room temperature, resulting in a significant anisotropic ratio (κ_∥/κ_⊥) of 1.47. Note that such a large anisotropic ratio mainly results from the higher phonon group velocity along the P-P chain direction. We also found that the thermal conductivity can be effectively modulated by increasing the SiP₂ thickness, reaching a value as high as 202 W m^-1 K^-1 (120 W m^-1 K^-1) for κ_∥ (κ_⊥) at 111 nm thickness, which is the highest among layered anisotropic phosphide materials. Notably, the anisotropic ratio always remains at a high level between 1.47 and 1.68, regardless of the variation of SiP₂ thickness. Our observation provides a new platform to verify the fundamental theory of thermal transport and a crucial guidance for designing efficient thermal management schemes of anisotropic electronic devices.