(color online) Thermoelectric properties of NWs. Thermoelectric figure of merit as a function of the number of atomic layers in the shell, N_S for (a) clean Si/Ge and N S ′ for (b) clean Ge/Si NWs with different core sizes N_C and N C ′ . Electronic (c) and phononic (d) transmission functions through wires with different surface roughness disorder. In panel (c) the transmission of the pristine wires are shown by dashed lines. Panels adapted with permission from (a) and (b) Ref. [100], ©2015 Springer; (c) and (d) Ref. [122], ©2012 ACS.