The growth direction of nanowires on a substrate is a complex phenomenon influenced by many factors, such as lattice matching between the nanowire material and substrate, surface energy, epitaxial strain, growth conditions, and more. It's challenging to provide a definitive explanation without specific details about the growing conditions and methods, but I can provide some general insights that might help.

In your case, with iron (Fe) nanowires growing on a strontium titanate (SrTiO3) substrate, it's plausible that the nanowires preferentially grow perpendicularly due to a combination of factors, which could indeed include interfacial energy.

In your specific case, you might need to consider the detailed experimental conditions and materials' properties to get more insight into why the Fe nanowires are growing perpendicularly. Collaborating with a materials scientist or physicist specializing in nanowire growth could be valuable to understand the exact reasons behind the observed growth direction.