Adsorption of lanthanide double-decker phthalocyanines on single-walled carbon nanotubes: structural changes and electronic properties as studied by density functional theory

Abstract

Context Molecular modeling of carbon nanotubes and lanthanide double-decker phthalocyanines hybrids is challenging due to the presence of 4f-electrons. In this paper, we analyzed the trends in structural changes and electronic properties when a lanthanide (La, Gd, and Lu) bisphthalocyanine molecule is adsorbed on the surface of two single-walled carbon nanotubes (SWCNTs) models: armchair and zigzag. The density functional theory (DFT) computations showed that the height of bisphthalocyanines complexes (LnPc2) when adsorbed on a nanotube (LnPc2+SWCNT) is the structural feature which is most affected by the nanotube model. The formation energy of the LnPc2+SWCNT hybrid depends on the metal atom and the nanotube chirality. LaPc2 and LuPc2 bind stronger to the zigzag nanotube, while for GdPc2, bonding to the armchair nanotube is the stronger one. The HOMO-LUMO gap energy (Egap) shows a correlation between the nature of lanthanide and the nanotube chirality. In the case of adsorption on armchair nanotube, Egap tends to match the gap of isolated LnPc2, whereas for adsorption on the zigzag nanotube, it is closer to the value for the isolated nanotube model. The spin density is localized on the phthalocyanines ligands (plus on Gd in the case of GdPc2), when the bisphthalocyanine is adsorbed on the surface of the armchair nanotube. For bonding to zigzag nanotube (ZNT), it extends over both components, except for LaPc2+ZNT, where spin density is found on the nanotube only. Method All DFT calculations were carried out using the DMol3 module of Material Studio 8.0 software package from Accelrys Inc. The computational technique chosen was the general gradient approximation functional PBE in combination with a long-range dispersion correction developed by Grimme (PBE-D2), the double numerical basis set DN, and the DFT semi-core pseudopotentials.

Adsorption of Gadolinium Bisphthalocyanine on Atomically Flat Surfaces: Comparison of Graphene and Hexagonal Boron Nitride from DFT Calculations

“…A theoretical analysis of bonding strength, geometries, and electronic parameters of noncovalent complexes of GdPc 2 with graphene and hBN cluster models of variable size was performed by employing the numerical-based DFT module DMol 3 of the Materials Studio suite [25][26][27][28]. As in all previous related works mentioned above [17][18][19][20][21][22][23][24], the PBE (by Perdew-Burke-Ernzerhof [29]) general gradient approximation function was used in conjunction with the empirical dispersion correction introduced by Grimme [30], that is, the PBE-D2 combination. (One should note that for noncovalent complexes of tetraazaannulenes with carbon nanoclusters, PBE-D2 yields more realistic geometries than, for example, the widely used hybrid functional B3LYP [31].…”

“…As already mentioned in the introduction, the size of the double-numerical basis set (DN in [23,24]) was increased by adding a polarization d-function to all non-H atoms, that is, to DND (which is equivalent to the 6-31G(d) Pople-type basis set). The settings employed for the full geometry optimization and the calculation of the electronic parameters included the use of DFT semi-core pseudopotentials (DSPPs) and a real space (or orbital) cutoff of 5.0 Å, as dictated by the presence of gadolinium atoms.…”

“…In principle, it was found that the use of a very low value of 0.0001 Ha (equivalent temperature of 31.6 K) yields stable and consistent results, which are essentially identical to those afforded by applying Fermi occupancy (that is, zero smearing). Consequently, we usually set 0.0001 Ha as the target value to elaborate on final publishable results [17][18][19][20][21][22][23][24]. While for the derivatives of Tb and some other lanthanides, this value does not allow to achieve SCF convergence and has to be increased, the Gd-containing systems (like the ones incorporating La and Lu atoms) often can be successfully treated at the Fermi occupancy.…”

“…Nevertheless, along with the serious (sometimes unresolvable) problems of self-consistent field (SCF) convergence for the Tb-and Dy-containing systems, we found that the case of their closest neighbor Gd might be as simple as the cases of La (with a totally empty 4f shell) and Lu (with a completely filled 4f shell), due to the half-filled 4f shell and zero orbital moment. This property allowed us to successfully perform calculations for GdPc 2 adsorbed on carbon nanotube [23] and graphene (both pristine and defect-containing) models [24].…”

“…In both works [23,24], we focused mostly on the variations in the geometry and interaction strength of GdPc 2 with different carbon nanoclusters. At the same time, perhaps the most interesting (from our point of view) result is related to the spin coupling (or alignment) pattern.…”