Transition of thermal rectification in silicon nanocones

Current understanding of thermal rectification asserts that the rectification ratio (R), which measures the relative heat flux between two ends of a nanostructure, is determined by its geometric asymmetry. The higher the asymmetry, the higher the R. However, by using nonequilibrium molecular dynamics method we have calculated thermal transport in Si nanocones as an example, the results show that such an understanding may be incorrect and R may not increase monotonically with geometric asymmetry. Rather, R exhibits a sharp reverse when the vertex angle ({\theta}) of the nanocone is approximately 90{\deg}. In other words, when {\theta} > 90{\deg}, R decreases, rather than increasing. We show that this abnormal behavior is originated from a change in the thermal transport mechanism. At small {\theta}s, phonon transport is dominated by localized modes, especially for transport from tip to bottom. At large {\theta}s, however, these localized modes disappear, leading to R decrease.