Development of Growth Theory for Vapor–Liquid–Solid Nanowires: Contact Angle, Truncated Facets, and Crystal Phase

This article tries to circumvent the fundamental uncertainty in the contact angle of droplets catalyzing vapor–liquid–solid nanowires. Based entirely on surface energetic grounds, our model predicts the following behavior. The initial contact angle of the surface droplets is smaller than for developed nanowires. At the beginning, the energetically preferred growth configuration is inward tapered nonwetted side facets such that the contact angle increases and the radius decreases with length. Such isotropic growth continues until a steady state contact angle is reached, which is usually larger than 90° (111° for the plausible parameters of Au-catalyzed GaAs nanowires) but smaller than the Nebol'sin–Shchetinin angle. This value determines the contact angle for stable vertical nanowires at a fixed liquid volume. If the latter keeps increasing for kinetic reasons, the contact angle also increases to the second critical point (128° for our parameters) where vertical interface is transitioned to wetted truncated as in Tersoff's picture. This wetted configuration is stable against the droplet sliding down. The crystal phase is predominantly wurtzite for smaller contact angles and planar growth interfaces according to Glas et al., and pure zincblende for larger contact angles and truncated growth interfaces according to Tersoff et al.