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Vertical Aligned (VA) Nanowire (NW) arrays have a straightforward application in photonics because the nanowire alignment is one of the most desirable features for the realization of real photonic devices. As an example, VA NWs can be used as natural resonance cavities in lasers without any fabricated mirror [1] while, dealing with LEDs, VA NWs lead to wave-guided emission [2]. GaAs is a direct bangap semiconductor, hence GaAs NWs are easy candidates to nanophotonics. Vertical alignment is thought to depend on the crystallographic orientation of the substrate as well as on the NW growth conditions on that specific substrate. We report here results on GaAs NWs, grown vertically aligned but subsequently driven to self-assembly. The self-assembly phenomenon has been rarely reported on GaAs NWs [3] as well as on ZnO NWs and nanorods [4, 5]. The few papers on GaAs NWs’ self-assembly explain this phenomenon by the electrostatic interaction between NWs. For the first time to authors’ knowledge, we propose that the GaAs NWs’ bundling should be explained by a different theoretical approach, based on adhesion forces that would act on the NWs. We exploit a theory until now devised to study the physics and mechanics of contact and adhesion of fibrillar interfaces [6]. According to this theory, the lateral collapse among adjacent micro-beams, that is their bundling, occurs if the beam height is larger than a critical length Lc. This critical length depends on the material mechanical properties and the array geometry. In this contribution we report the experimental proofs that the post-growth vertical misalignment of the NWs intervenes when the phenomenon of lateral collapse between neighboring NWs occurs. We, therefore, demonstrate for the first time that the NW bundling is driven by adhesion forces. Consequently, the existence of a critical length for the NW collapse should be taken into account, when designing NW arrays for their implementation in nanoelectronic devices. REFERENCES [1] Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R. & Yang, P. (2001). Room-temperature ultraviolet nanowire nanolasers. Science, Vol. 292, No. 5523, 1897-1899 (Jun 2001) [2] Könenkamp, R.; Word, R. C. & Schlegel, C., Vertical nanowire light-emitting diode. Applied Physics Letters, Vol. 85, No. 24, 6004-6006 (Dec 2004) [3] Dai X., et al., Tailoring the Vapor_Liquid_Solid Growth toward the Self-Assembly of GaAs Nanowire Junctions, Nano. Lett. 2011, 11, 4947–4952 [4] X. Wang et al., Self-attraction among aligned Au-ZnO nanorods under electron beam, Applied Physics Letters 86, 013111 (2005) [5] Liu et al., Bending and bundling of metal-free vertical free ZnO NWs due to electrostatic interaction, Nanotechnology 19, 185607 (2008) [6] N. J. Glassmaker et al., Design of biomimetic fibrillar interfaces: 1. Making contact, J. R. Soc. Interface, 1, 23–33 (2004) |
