Heavy metal iodides such as mercuric iodide and bismuth tri-iodide are among the materials with the best properties for room temperature semiconductor detection. The use of nanostructures of these compounds - recently synthesized - as precursors for nucleation of oriented layers, opens new perspectives for layer growth and, through them, the possibility of a new generation of detectors with enhanced imaging performance. The mentioned heavy metal iodides were synthesized in this work in 1-octadecene as solvent, using octadecanethiol (ODT) or trioctylphosphine (TOP) as capping agents. This is the first report of morphology and size control of mercuric iodide and bismuth tri-iodide nanostructures using capping agents. We obtained promising results for mercuric iodide with ODT and for bismuth tri-iodide with TOP, which will start a new trend in nucleation of heavy metal iodides. (C) 2014 Elsevier B.V. All rights reserved.