Mastering Capillary Forces to Move Silicon Nanowires from Nano to Macro

Silicon nanowires (SiNWs) have been a milestone in the exploitation of nanotechnology as a tool to develop efficient thermoelectric materials. By breaking the unfortunate tie between thermal and electric conductivity, the making of quasi-1D nanostructures let improve Si figure of merit (ZT) at room temperature from a meager 0.01 up to ≈ 1 [1, 2]. Nonetheless, usability of SiNWs in real-world devices has met the hurdles commonly encountered by nanostructures. As noted [3], for most applications thermoelectric devices need bulk materials, exchanging heat over large areas and across macroscopic distances. For SiNWs obtained by extreme lithography [2] this can be hardly obtained at acceptable costs. Instead, bottom-up approaches such as metal-assisted chemical etching (MACE) disclose the possibility of preparing wires with arbitrary lengths over large surfaces, enabling in principle full scalability from the nanoscale to macroscopic systems. However, on the MACE route three issues must be cleared, namely (1) the possibility of obtaining crystalline highly doped SiNWs; (2) the control of capillary forces bundling wires; and (3) the series resistance of the substrate. In this communication we report about the optimization of one-pot MACE to prepare SiNWs with lengths in excess to 0.1 mm [4]. Single-crystalline Si (100) wafers, both p and n-type with doping levels between 10¹⁵ and 10¹⁹ cm^-3, were etched in HF/AgNO₃ solutions at temperatures between 15 and 25 °C. We show that, contrary to previous reports, highly doped SiNWs are crystalline, with an encapsulating amorphized layer of only a few nanometers [4] that may be successfully removed (when needed) by wet etching. Quite independently of the doping, as-prepared SiNWs display bundling because of the capillary forces due to the solvent-wire interaction, ultimately causing SiNWs to bind to each other at their free ends. When supported SiNWs are supposed to act as thermoelectric legs in a device, bundling degrades system performances, as it increases the electrical resistance per area unit. An easy procedure to minimize capillary forces and the subsequent folding of SiNWs will be presented, making a combined use of chemical etching to control SiNW hydrophilicity and of solvent of graded polarity to control solvent-surface interactions. Non-bundled SiNWs also display enhanced (lower) electrical and thermal contact resistances, enabling the fabrication of thermoelectric devices with the conventional Π-type geometry [5]. However, capillarity is not always a negative feature. We will report for the first time about the making of unsupported, substrate-free Si ‘nanofelts’, where SiNWs are kept together by random bundling at their opposite ends. Lack of the series thermal and electric substrate resistance along with possibility of obtaining intertwined thick SiNW assemblies discloses exciting novel opportunities to scale thermoelectric nanosystems to the macroscopic scale.<br/><br/>S.M. acknowledges the support received by the Università Italo-Francese through the 2016 Vinci program, grant C3-132.<br/> <br/>[1] A. I. Hochbaum, R. Chen, R.D. Delgado, W. Liang, E.C. Garnett, M. Najarian, A. Majumdar, P. Yang, Enhanced Thermoelectric Performance of Rough Silicon Nanowires. <i>Nature</i>, 451 (2008) 163.<br/>[2] A.I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J.K. Yu, W.A. Goddard, J.R. Heath, Silicon Nanowires as Efficient Thermoelectric Materials, <i>Nature,</i> 451 (2008) 168.<br/>[3] J.P. Heremans, M.S. Dresselhaus, L.E. Bell, D.T. Morelli, When Thermoelectrics Reached the Nanoscale. <i>Nat. Nanotechnol.</i>, 8 (2013) 471.<br/>[4] S. Magagna, D. Narducci, C. Alfonso, E. Dimaggio, G. Pennelli, A. Charaï, On the mechanism ruling the morphology of silicon nanowires obtained by one-pot metal-assisted chemical etching, <i>Nanotechnology</i>, 31 (2020) 404002.<br/>[5] S. Elyamny, E. Dimaggio, S. Magagna, D. Narducci, G. Pennelli, High Power Thermoelectric Generator Based on Vertical Silicon Nanowires, <i>Nano Lett.</i>, 20 (2020) 4748.