Self-Supported Silicon Nanowire Assemblies: From Nano to Macro

Single-crystalline silicon is widely recognized as a suboptimal thermoelectric material. When heavily doped, it can achieve high power factors (approximately 10 mW/mK²). However, its high thermal conductivity (approximately 140 W/mK at 300 K) severely diminishes its figure of merit, resulting in a modest 0.01 at room temperature. Nevertheless, the prospect of transforming silicon into an effective thermoelectric material holds significant implications for expanding the utilization of thermoelectrics in both heat harvesting and cooling. As a result, research has zeroed in on leveraging nanotechnology to reduce its thermal conductivity. Boukai et al.1 and Hochbaum et al.2 demonstrated in 2008 that rough, single-crystalline silicon nanowires (NWs) exhibit a thermal conductivity of ≈ 5 W/mK due to inelastic scattering of phonons at wire boundaries, with their figure of merit at 300 K reaching approximately 1. While low dimensionality has proven to be a potent strategy for enhancing thermoelectric efficiency, it also results in reduced heat absorption, as a significant portion of the available heat flux is redirected through the material template when NWs are grown parallel to the substrate. Conversely, when NWs are aligned perpendicular to it, the temperature difference primarily occurs on the substrate. Consequently, higher efficiencies come at the expense of lower converted heat flux, leading to lower electric power densities. To circumvent these limitations, dense, unsupported nanowire arrays are required. During this presentation, recent advancements in the fabrication of Si NWs through metal-assisted chemical etching (MACE)3 will be elucidated. Our exploration began with an analysis of the physical chemistry governing the MACE process. We demonstrated that the competing 2- and 4-electron etching processes are contingent on oxide etching kinetics, accounting for the comparable etching rates observed in p- and n-type silicon. Additionally, we provided evidence for the crystalline nature of NWs even at high doping levels4. While MACE conditions dictate wire length and nanomorphology, NW shapes and aggregation are primarily influenced by pre- and post-processing conditions. Specifically, we established that drying conditions regulate wire bundling, while the pristine termination of the Si wafer (whether oxidized or hydrogen-terminated) dictates their shape. Although bundling is detrimental for thermoelectric generation in supported NWs, it can be harnessed to obtain fully self-supported NW aggregates5. This opens the possibility of utilizing NW assemblies as macroscopic legs in conventional Π-type thermoelectric generators or coolers. However, achieving this goal requires the deposition of metal contacts on top of NWs. We have explored electrochemical methods to obtain ohmic contacts on supported NWs6, although contact resistances could not meet our satisfaction. This, we are currently developing alternate strategies to embed NW aggregates using polymers, which will be presented and discussed. 1 A.I. Boukai et al., Nature 451, 168 (2008). 2 A.I. Hochbaum et al., Nature 451, 163 (2008). 3 Z. Huang et al., Adv. Mater. 23, 285 (2011). 4 S. Magagna et al., Nanotechnology 31, 404002 (2020). 5 F. Giulio et al., ACS Appl. Electron. Mater. (2023) 10.1021/acsaelm.3c01014. 6 S. Elyamny et al., Nano Lett. 20, 4748 (2020).