The spontaneous formation of complex interfacial patterns from thermally deposited Sb-4 clusters on HOPG is controlled by the deposition conditions (i.e., coverage and deposition rate) at constant temperature (300 K) under ultrahigh vacuum conditions. Two main driving forces for the pattern formation in this system have been identified. Initially, the crystallization of compact nanoparticles with spherical shape at a maximum diameter of 120 nm drives the system toward irregular, fingerlike shapes. Second, the ramification of these fingerlike nanoparticles is governed by the deposition rate, as the increase of the deposition rate allows the nanoparticle shape to be continuously tuned from fingerlike to further ramified and eventually fractal forms. On the basis of electron microscopy and atomic force microscopy measurements, these phenomena are quantified with a focus on fractal dimension, particle perimeter, and size of the side branches (tip width).