Optical response of carbon nanotubes functionalized with (free-base, Zn) porphyrins, and phthalocyanines: A DFT study

We use density-functional theory calculations to study the stability, electronic, and optical properties of free-base and Zn porphyrins and phthalocyanines (H${}_2$P, H${}_2$Pc, ZnP, and ZnPc) noncovalently attached onto a semiconducting carbon nanotube (CNT). The macrocycle physisorption is described by van der Waals density functional while optical response is obtained through the imaginary part of the dielectric function. Our results show a rather strong macrocycle binding energy, ranging from 1.0 to 1.5 eV, whereas the CNT geometry and electronic properties are weakly affected by the adsorbates. The optical spectrum shows that CNT-porphyrins and CNT-phthalocyanines assemblies would absorb at different energies of the visible solar radiation spectrum, which would increase the conversion energy efficiency in a photovoltaic device including both macrocycles.

Figure 1

Equilibrium geometries of the free-base porphyrin (a) and the free-base phthalocyanine (b) in the adsorption position II. Green and red balls represent C atoms of the CNT and the macrocycles, respectively. Blue and gray balls represent N and H atoms.

Figure 2

Electronic bands structure of free-base and Zn macrocycles adsorbed on the CNT for a wave vector along the tube axis ($\Gamma$Y). (a) Pristine CNT represented with four unit cell, (b) CNT-H${}_2$P, (c) CNT-ZnP, (d) Pristine CNT represented with six unit cell, (e) CNT-H${}_2$Pc, and (f) CNT-ZnPc.

Figure 3

Charge density difference after the macrocycle adsorption on the CNT (0.0002 $e$/Å${}^3$). (a) CNT-H${}_2$Pc and (b) CNT-ZnPc. Blue and red isosurfaces represent charge depletion and accumulation, respectively.

Figure 4

Schematic representation of energy levels and relevant optical transitions at the $\Gamma$ point, according to the Gouterman model. The black (red) horizontal lines represent CNT (macrocycles) energy levels.

Figure 5

Imaginary part of the dielectric function of the CNT-H${}_2$P [(a)–(c)] and CNT-ZnP [(d)–(f)] complexes as compared with the pristine CNT. Different polarization for the incident light are considered: (a) and (d) Incident light polarized parallel to the CNT axis. (b) and (e) Incident light polarized perpendicular to the CNT axis ($x$ direction). (c) and (f) Incident light polarized perpendicular to the CNT axis ($y$ direction). The vertical lines represent the position of the peaks associated to the $Q$ bands.

Figure 6

Imaginary part of the dielectric function for the CNT-H${}_2$Pc [(a)–(c)] and CNT-ZnPc [(d)–(f)] complexes as compared with the pristine CNT. Different polarization of the incident light are considered: (a) and (d) Incident light polarized parallel to the CNT axis. (b) and (e) Incident light polarized perpendicular to the CNT axis ($x$ direction). (c) and (f) Incident light polarized perpendicular to the CNT axis ($y$ direction). The vertical lines represent the position of the peaks associated to the $Q$ bands.