Controlling the Nature of Etched Si Nanostructures: High- versus Low-Load Metal-Assisted Catalytic Etching (MACE) of Si Powders

Abstract

Metal-assisted catalytic etching (MACE) involving Ag deposited on Si particles has been reported as a facile method for the production of Si nanowires (Si NWs). We show that the structure of Si particles subjected to MACE changes dramatically in response to changing the loading of the Ag catalyst. The

Metal Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

“…The idea that band bending and charge imbalance on the metal nanoparticle polarizes the surrounding Si to engender local electropolishing beneath the metal nanoparticle and remote porosification away from the metal nanoparticle was first proposed and supported by band bending calculations by Kolasinski in 2014 [90], then confirmed through quantitative two-dimensional band bending modelling by Torralba et al in 2016 [35], and confirmed again by Wang et al in 2018 [62]. Tamarov et al [63] expanded these concepts by interpretation of extensive experimental data through theoretical calculations for a full range of heavily, moderately and lightly doped p and n type Si in combination with Cu, Ag, Au, Pd and Pt catalysts.…”

“…When etching is extensive enough to generate an easily weighable mass difference, then reactant depletion must be taken into consideration. Two other factors, discussed further by Tamarov et al [63], that complicate quantitative analysis are that etching and Si mass loss occur during metal deposition and the presence of side reactions of the oxidant. It should also be noted that when pores become long, transport constraints will hinder uniform mixing as is required for simple application of a rate law.…”

“…The introduction of injection enabled quantitative studies of MACE [64] in which it was discovered that the nature of the etching process depends on the loading of the deposited metal. Furthermore, Tamarov et al [63] found that because of the low oxidant concentration, Cu could be used as a catalyst to porosify Si, not just anisotropically etch it.…”

“…2 we see that holes are generated by electron transfer to the oxidant on the surface of the metal nanoparticle catalyst. The nature of band bending at the metal/semiconductor interface [35,36,63,88,89] is determined by the alignment of the metal Fermi level with the semiconductor VBM. Therefore, both the height (and whether the contact is Ohmic or Schottky-like in nature) and width of the barrier that impedes transport of the hole away from the metal/semiconductor interface depends on the composition of the metal and the doping of the semiconductor.…”

“…SiNWs produced by MACE can be either solid-core or porous [60] depending largely on the resistivity of the Si substrate. Low-doped Si regardless of whether it is n type or p type leads to solid core SiNW and highly doped Si succumbs to remote etching and the formation of porous SiNW [36,[61][62][63][64]. Not only can SiNWs be modified with fluoroalkylsilanes to exhibit superhydrophobicity [65], they can also have their rate of biodegradability regulated by controlling their termination [61].…”