  Spatial positioning of nanocrystal building blocks on a solid surface is a
prerequisite for assembling individual nanoparticles into functional devices.
Here, we report on the graphoepitaxial liquid-solid growth of nanowires of the
photovoltaic compound CH$_3$NH$_3$PbI$_3$ in open nanofluidic channels. The
guided growth, visualized in real-time with a simple optical microscope,
undergoes through a metastable solvatomorph formation in polar aprotic
solvents. The presently discovered crystallization leads to the fabrication of
mm2-sized surfaces composed of perovskite nanowires having controlled sizes,
cross-sectional shapes, aspect ratios and orientation which have not been
achieved thus far by other deposition methods. The automation of this general
strategy paves the way towards fabrication of wafer-scale perovskite nanowire
thin films well-suited for various optoelectronic devices, e.g. solar cells,
lasers, light-emitting diodes and photodetectors.
