Vertically-ALIGNED of sub-millimeter ultralong si nanowire arrays and its reduced phonon thermal conductivity

Well-aligned of single crystalline silicon nanowires (SiNWs) arrays are synthesized using Ag-assisted electroless etching processes. By examining a wide range of reaction periods from 1 min up to 12 h, our experimental results show that the lengths of fabricated SiNWs do not maintain the linear relationship with the reaction period but feature three evident transitions instead. We find that the diffusion of HF through Ag dendrites is the rate-limiting step for maintaining the galvanic reaction of etching processes. To overcome these limitations, we report a simple and controllable route employing HNO ₃AgNO₃HF electrolyte solutions, which enables SiNW lengths ranging from several nanometers up to a few hundred micrometers to become linearly dependent on the reaction time. Transmission electron microscopy studies reveal that the SiNWs fabricated by this approach are single crystalline along 100 in axial direction with relatively rough surfaces. In addition, we further measure the thermal conductivities of SiNW arrays with various lengths at 300 K. The resulting value of thermal conductivity in SiNW arrays is only 44 in comparison with bulk Si (100) substrates; that is attributed to the effects of decreased area of phonon transport, as well as increased phonon scattering.