Low thermal conductivity in nanocrystalline Zn3P2

Abstract

Semiconductors with low lattice thermal conductivity are important in the search for more efficient thermoelectric materials. The thermal conductivity of nanocrystalline (<7 nm) Zn3P2, fabricated in thin film form by pulsed laser deposition, was measured from 80 K to 294 K. The thermal conductivity of the film showed weak temperature dependence in this temperature range and at 294 K had its highest value of 0.49 W/m K. Although Zn3P2 and its family of isomorphic compounds are known to have intrinsically low thermal conductivity, at room temperature the thermal conductivity of this nanocrystalline film is 25% smaller than the calculated minimum thermal conductivity for Zn3P2. Analyzing the thermal conductivity data with the Callaway model revealed that the data could be well fit by considering only boundary scattering and point defect scattering. The boundary scattering length was in good agreement with the film’s average crystallite size of 4.1 nm and the magnitude of the point defect scattering required the formation of VZn-Zni pairs from approximately 23% of the Zn sites. It is believed that a large number of point defects are responsible for the intrinsically low thermal conductivity of bulk Zn3P2 and therefore the exceptionally low thermal conductivity found in the present study results from the nanometer dimensions of the crystallites. As previous studies have reported high Seebeck coefficients and electronic properties that are insensitive to grain boundaries in Zn3P2, the low thermal conductivity observed in the present study suggests that nanocrystalline Zn3P2 should be further explored for use in thermoelectric applications.