It becomes clear that the chemical bonds of the SMI itself and the SMI-surface bonds can be attacked by the precursor or the co-reactant. For this system, the Si-Osurf bond is the most prone to be broken by the precursor, initiating unwanted ALD growth. Again, a dependence of the reactivity on the SMI layer density was found. While for an isolated SMI molecule (Fig. .a) at the surface, no difference between TMA and TEA was found in terms of the thermodynamic and kinetic signatures of the reaction (both show moderate barriers and strong thermodynamic driving force), at a realistic density of 2.5 SMIs per nm 2 the picture changes completely (Fig. .b). The precursors are more strongly bonded to the SMI-surface bond although the layer is denser. This can be explained by an increase in dispersion attraction. However, the activation energy more than doubles (ΔE ‡ = 217 kJ•mol -1 for TMA and 190 kJ•mol -1 for TEA) which essentially prohibits this reaction at typical ALD conditions. Thus, although the precursors adsorb, they cannot react. This is the essence of the concept of blocking by reactivity reduction (Fig. .d). When the SMI density is increased further to 3.4 SMI per nm 2 , both precursors are blocked via adsorption prevention (Fig. .c). In Fig. .b, there is an additional reaction channel for TMA with a much lower barrier (ΔE ‡ = 92 kJ•mol -1 ). Thus, it is proposed that the reason for the low selectivity of TMA is not the penetration of the layer but the ability to still react via ligand-exchange mechanisms in a sterically crowded environment. That the bonding mode of the inhibitor can play a crucial role for the decomposition reaction has been shown for Hacac. Here, the monodentate bonding configuration gets attacked by the precursor while the bidentate structure is inert. Fig. . Activation and reaction energies for the disintegration of the SiSMI -Osurf bond with trimethylaluminum (TMA) and triethylaluminum (TEA) for (a) a single small molecule inhibitor (SMI) and (b) an inhibitor layer density of 2.5 SMIs/nm 2 . Energies (in brackets dispersion contribution) are given with respect to the free reactants in kJ•mol -1 . Reprint with permission from reference [214] .