Nanocrystallization has been an important approach for reducing thermal conductivity in thermoelectric materials due to limits on phonon mean-free path imposed by the characteristic structural size. We report on thermal conductivity as low as 0.3 Wm⁻¹K⁻¹ of nanocrystalline silicon thin films prepared by plasma-enhanced chemical-vapor deposition as grain size is reduced to 2.8 nm by controlling hydrogen dilution of silane gas during growth. A multilayered film composed by alternating growth conditions, with layer thicknesses of 3.6 nm, is measured to have a thermal conductivity 30% and 15% lower than its two constituents. Our quantitative analysis attributes the strong reduction of thermal conductivity with decreasing grain size to the magnifying effect of porosity which occurs concomitantly due to increased mass density fluctuations. Our results demonstrate that ultrasmall grain sizes, multilayering, and porosity, all at a similar nanometer-size scale, may be a promising way to engineer thermoelectric materials.

由于特征结构尺寸对声子平均自由路径的限制，纳米晶化一直是降低热电材料热导率的重要方法。我们报告的热导率低至 0.3 Wm ^-1 K ^-1通过控制生长过程中硅烷气体的氢稀释，通过等离子体增强化学气相沉积制备的纳米晶硅薄膜的晶粒尺寸减小到 2.8 nm。由交替生长条件组成的多层薄膜，层厚为 3.6 nm，经测量具有比其两种成分低 30% 和 15% 的热导率。我们的定量分析将热导率随着晶粒尺寸的减小而大大降低归因于孔隙率的放大效应，而孔隙率的放大效应是由于质量密度波动的增加而导致的。我们的结果表明，超小晶粒尺寸、多层结构和孔隙率，都在类似的纳米尺寸尺度上，可能是一种很有前途的热电材料设计方法。