EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Assembling nanostructured building blocks into network materials unlocks macroscopic properties inaccessible with monolithic solids. Recently we were able to show that silicon nanowires (SiNWs) can be produced via floating catalyst chemical vapor deposition (FCCVD), a method that enables large-scale and continuous synthesis of SiNWs suspended in the gas phase exploiting metal-catalized vapor-liquid-solid (VLS) growth. [1] The combined high aspect ratio and large concentration of SiNWs (in the gas-phase) enable formation of macroscopic solids solely comprised of percolated SiNWs, such as free-standing sheets and continuous metre-long SiNW tapes.
In an attempt to explore new FCCVD growth conditions, a large range of parameters were tested with emphasis on different precursor (SiH4) to carrier gas ratios (N2 & H2). These resulted in various SiNW morphologies entailing different precursor conversion rates and changing grades of nanowire contamination, stemming from non-catalyzed precursor decomposition. We found that the latter can be effectively characterized using Raman spectroscopy. This enabled us to introduce a novel observable ("crystallinity") that is of high value within the characterization spectrum of nanowire samples. Thus, amongst other, we could identify an unambiguous conversion rate boundary of 0.3% at 90% nanowire crystallinity. We further discuss the crucial role of adding hydrogen to the FCCVD gas mixture for successful nanowire synthesis. Finally, our most effective growth parameters are compared to other known FCCVD processes, such as CNT fiber spinning. [2] Hence, we identify common traits in metal-catalyzed gas phase growth processes which are likely to be applicable to virtually any one-dimensional inorganic nanomaterial known to be able to undergo VLS growth.

[1] Schäufele, Richard S., Miguel Vazquez-Pufleau, and Juan J. Vilatela; Materials Horizons 7.11 (2020): 2978-2984.

[2] Reguero, Víctor, et al; Chemistry of Materials 26.11 (2014): 3550-3557.