(color online) Thermal conductivity of core–shell and porous NWs. (a) Overall thermal conductivity of the Si–Ge core–shell NW as a function of the number of layers in the Ge shell for different cross-sectional areas of the Si core. (b) Time dependence of normalized heat current autocorrelation function for Si NWs, Si NTs, and Ge/Si core–shell NWs. (c) Thermal conductivity with three levels of porosity corresponding to different etching conditions. (d) Measured thermal conductivity of Samples #1 to #8 versus dose. The inset is the same data plotted on a logarithmic scale. Panels adapted with permission from (a) Ref. [105], ©2011 ACS; (b) Ref. [36], ©2011 AIP; (c) Ref. [38], ©2012 Springer; (d) Ref. [112], ©2017 NPG.