We measure the porosity and thermal conductivity of electrolessly-etched single-crystalline silicon nanowires by means of an electron-beam heating technique. Upon irradiating the nanowire with a focused electron beam, the power absorbed by the nanowire scales with the cross-sectional area, thus allowing us to ascertain the material porosity. Such porous silicon nanowires exhibit extremely low thermal conductivity (as low as 0.33 W/m-K at 300K for 43% porosity), even lower than that of amorphous silicon.
