The invention relates to a sub-20 nm nanofiller for use in a matrix material, to the matrix resulting therefrom, to a process for preparing said matrix, and to the use of said nanofiller, the nanofiller comprising functionalized polyhedral oligomeric silicon-oxygen cluster units of the formula [(RaXbSiO1.5)m(RcXdSiO)n(ReXfSi2.05)o(RgXhSi2O2)p]with: a, b, c=0-1; d=1-2; e, g, f=0-3; h=1-4; m·b+n·d+o·f+p·h≦4; m+n+o+p≧4; a+b=1; c+d=2; e+f=3 and g+h=4; R=hydrogen atom, alkyl, cycloalkyl, alkenyl, cycloalkenyl, alkynyl, cycloalkynyl, aryl, heteroaryl group or polymer unit, which are in each case substituted or unsubstituted, or further functionalized polyhedral oligomeric silicon-oxygen cluster units, which are attached by way of a polymer unit or a bridging unit, X=oxy, hydroxyl, alkoxy, carboxyl, silyl, alkylsilyl, alkoxysilyl, siloxy, alkylsiloxy, alkoxysiloxy, silylalkyl, alkoxysilylalkyl, alkylsilylalkyl, halogen, epoxy, ester, fluoroalkyl, isocyanate, blocked isocyanate, acrylate, methacrylate, nitrile, amino, phosphine group or substituents of the type R containing at least one such group of the type X, the substituents of the type R being identical or different and the substituents of the type X being identical or different, with the proviso that there are not more than four substituents of the type X per cluster unit.