Size effects on the band gap of eRCN/DG particles as a function of X_H (black dashed lines); the long-dashed lines indicate the upper and lower validity limits for the eRCN model, X_H = 0.62 and 0.17, respectively. The grey lines with square data points indicate the size-dependent band gap derived using π − π^∗ band constraints (for E_g(bulk) = 0.25, 0.75, 1.25, 1.50, 2.00 eV,  ≡ X_H = 0.06, 0.17, 0.29, 0.35 and 0.47, respectively, from bottom to top) and the solid (blue) line shows the approximate empirical limit, E_g [eV]  =  { (a [nm] )^-1 − 0.2 } , above which no aromatic cluster size effects need to be taken into account (see Sect. B.1.1).