High-yield
epitaxial bottom-up growth of silicon nanowires is still
challenging but desirable for various applications such as antireflective
coatings, solar cells, and high-aspect-ratio scanning probes. Hence,
pristine single-crystalline silicon surfaces are, in principle, required
as a growth substrate, but reoxidation occurring prior to nanowire
growth obstructs epitaxial growth significantly. Here, we present
an approach that relies on Al/Au alloy catalysts for gas-phase silicon
nanowire synthesis, allowing intrinsically an in situ removal of a
native silicon-oxide layer during the initial growth stages. This
approach yields reliable and superior epitaxial growth of silicon
nanowires on single-crystalline silicon substrates.
Bottom-up grown single-crystalline silicon nanowires (SiNWs) are highly intriguing to build nanoscale probes, for instance for atomic force microscopy (AFM), due to their mechanical robustness and high aspect ratio geometry. Several strategies to build such nanowire-equipped probes were explored but their fabrication is still elaborate, time-consuming and relies partly on single-crystalline substrates. Here, we explore a new strategy to fabricate AFM probes that are equipped with single-SiNW scanning tips. The conceptual evaluation begins with a discussion on the overall design and softness of such probes based on finite-element-method simulations. For the experimental realization, SiNWs were grown by the well-established gold-catalyzed vapor–liquid–solid method employing gaseous monosilane. As-grown SiNWs were subsequently transferred onto flexible membranes and even freestanding AFM microcantilever beams via mechanical nanowire contact printing. Elongation of the deposited nanowires by so-called regrowth was triggered by reusing the original gold catalyst to yield the prospective AFM scanning tip. SiNW-equipped scanning probes were created in this manner and were successfully employed for topography imaging. Although a multitude of challenges remains, the created probes showed an overall convincing performance and a superior durability.
Aligned large-scale deposition of nanowires grown in a bottom-up manner with high yield is a persisting challenge but required to assemble single-nanowire devices effectively. Contact printing is a powerful strategy in this regard but requires so far adequate adjustment of the tribological surface interactions between nanowires and target substrate, e.g. by microtechnological surface patterning, chemical modifications or lift-off strategies. To expand the technological possibilities, we explored two-directional pressure-controlled contact printing as an alternative approach to efficiently transfer nanowires with controlled density and alignment angle onto target substrates through vertical-force control. To better understand this technology and the mechanical behavior of nanowires during the contact printing process, the dynamic bending behavior of nanowires under varying printing conditions is modeled by using the finite element method. We show that the density and angular orientation of transferred nanowires can be controlled using this three-axis printing approach, which thus enables potentially a controlled nanowire device fabrication on a large scale.
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