Here we propose to exploit the natural instability of thin solid films, i.e. solid state dewetting, to form regular patterns of monocrystalline atomically smooth Si, Si1-xGex and Ge nanostructures that cannot be realized with conventional methods. Additionally, the solid-state dewetting dynamics is guided by pre-patterning the sample by a combination of electron-beam lithography and reactive-ion etching, obtaining precise control over number, size, shape, and relative position of the final structures. Methods and structures will be optimized towards their exploitation mainly in photonic devices application (e.g. anti-reflection coatings, colour-filters, random lasers, quantum emitters and photonic sensors).