(Invited) Porous Silicon Nanowires for SERS Platforms and Thermo-Electrical Materials
Authors: Cara Eleonora , De Leo Natascia , Ferrarese Lupi Federico , Giovannozzi Andrea , Beckhoff Burkhard , Hönicke Philipp , Lopeandia Aitor , Rodriguez Viejo Javier , Saghi Zineb , Boarino Luca 
Published: December 2024

Since the discovery of Metal Assisted Chemical Etching (MACE) 1, the direct dependence of the resulting silicon nanostructures as extruded by the metal mask has led to several applications on large area, like photovoltaics2, Surface Enhanced Raman Scattering platforms 3, energy harvesting and thermo-electrics4. Coupling MACE with nanosphere lithography5 allowed the fabrication of silicon nanowire arrays with desired dimensions and functionalities over a large area.
In this communication, an overview of some applications of porous silicon nanowires will be given with particular attention to the realization of porous nanowires, losing structural stiffness and gaining high flexibility, used for the development of gold-coated active substrates for surface-enhanced Raman spectroscopy (SERS) and for the development of thermoelectric devices where the thermal transport properties of single nanowires are of interest.
Surface-enhanced Raman spectroscopy, discovered in 19746, is a promising analytical tool for detecting chemical and biological species at single molecule levels in liquid and gas phases. Its specificity and sensitivity have led to applications in electrochemistry, environmental analysis, and bio-sensing. Thanks to the development of electromagnetic and chemical theories to explain SERS, it is now widely accepted that the phenomenon is primarily attributed to electromagnetic field enhancement. The light enhancement is achieved through the excitation of localized surface plasmon resonances (LSPRs) in gaps, crevices, or sharp features of plasmonic materials, typically noble and coinage metals with nanoscale features. This process generates Raman hot spots due to the proximity of metal nanostructures separated by a few nanometers. In our work, we optimized the fabrication of gold-coated flexible porous silicon nanowires understanding the formation of the hot spots at the tip-to-tip sites of nanowires bundles7 and controlling the patterning over large area obtained by nanospheres self-assembly to correlate the nanospheres distribution in a monolayer to the final SERS substrates enhancement performances and homogeneity8. Moreover, we integrated the SERS measurements with the absolute quantification of the number of active molecules contributing to the SERS signal by means of reference-free synchrotron-based X-ray fluorescence measurements9. The progress in this work on SERS will be discussed.
Furthermore, in the field of thermoelectric materials and devices, an accurate evaluation of the thermal and electrical conductivity of single nanowires is mandatory, so our efforts are addressed to the production and the advanced characterization of single porous silicon nanowires. MACE represents the most suitable method to produce long nanowires with desired diameter and aspect ratio (> 1:200) for single-wire electrical and thermal characterization10. Porous silicon nanowires of 100 nm of diameter obtained by MACE from highly doped substrates have been nanomanipulated, bonded and measured on a custom-designed MEMS platform 18, and the thermal conductivity resulted being of 0.8 W/MK, lower than thermal silicon dioxide values and almost two orders of magnitude less than crystalline bulk silicon. The thermal and electrical conductivity of the silicon nanowires can be modified by conformally coating the nanostructures with ALD-based deposition of ZnO. The STEM characterization of the structural properties of the coated single nanowires is discussed together with the thermal and electrical conductivity.

Acknowledgements

Part of this work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101007417, having benefited from the access provided by CEA LETI in Grenoble within the framework of the NFFA-Europe Pilot Transnational Access Activity, proposal [ID310]. Part of this work has been carried out at Nanofacility Piemonte, a laboratory supported by the ‘‘Compagnia di San Paolo’’ Foundation, and at QR Lab - Micro & Nanolaboratories, INRiM.

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