Quantum Size Effects of Agₙ Clusters on Carbon Nanotubes

The electronic structures of different Ag clusters adsorbed on metallic and semiconducting carbon nanotubes (CNTs) were studied using a first-principles density functional theory method. More precisely, we have considered Agₙ with n = 4, 13, 55, and 147 atoms to describe the quantum size effects associated with the Agₙ–CNT interfaces. Although we observed a sharp transition from a molecular to a bulky behavior from Ag₄ to Ag₅₅, the electronic structure properties of Ag₅₅ and Ag₁₄₇ were found to be very similar to the extended slab Ag(111) surface. When in contact with the CNTs, Agₙ is chemisorbed when n = 4 and 13, while it is physisorbed for larger systems (n > 55) similarly to the Ag(111) slab surface. As a result, the adsorption energy of Agₙ to CNTs decreases from around 300–100 meV/atom when n changes from 4 to ∞. This variation in cluster size is consistent with the calculated fluctuation of electron charge from Agₙ to the CNTs that sharply decreases with the Ag cluster size. Following an adsorption of CO on different sites of Agₙ–CNT, Agₙ became systematically more weakly bound to the CNT surface. This change is discussed in terms of variations of charge transfer, dipole moment, and charge reorganization within the Ag–CNT system. Our overall calculations suggest that small Ag clusters would be a much better choice for developing a CNT-based sensor for CO in terms of both sensitivity and stability.