Properties of copper (fluoro-)phthalocyanine layers deposited on epitaxial graphene

Jun Ren,Efthimios Kaxiras,Xu-Cun Ma,Sheng Meng,Qi-Kun Xue,Yi-Lin Wang

doi:10.1063/1.3590277

Abstract

We investigate the atomic structure and electronic properties of monolayers of copper phthalocyanines (CuPc) deposited on epitaxial graphene substrate. We focus in particular on hexadecafluorophthalocyanine (F(16)CuPc), using both theoretical and experimental (scanning tunneling microscopy - STM) studies. For the individual CuPc and F(16)CuPc molecules, we calculated the electronic and optical properties using density functional theory (DFT) and time-dependent DFT and found a red-shift in the absorption peaks of F(16)CuPc relative to those of CuPc. In F(16)CuPc, the electronic wavefunctions are more polarized toward the electronegative fluorine atoms and away from the Cu atom at the center of the molecule. When adsorbed on graphene, the molecules lie flat and form closely packed patterns: F(16)CuPc forms a hexagonal pattern with two well-ordered alternating α and β stripes while CuPc arranges into a square lattice. The competition between molecule-substrate and intermolecular van der Waals interactions plays a crucial role in establishing the molecular patterns leading to tunable electron transfer from graphene to the molecules. This transfer is controlled by the layer thickness of, or the applied voltage on, epitaxial graphene resulting in selective F(16)CuPc adsorption, as observed in STM experiments. In addition, phthalocyanine adsorption modifies the electronic structure of the underlying graphene substrate introducing intensity smoothing in the range of 2-3 eV below the Dirac point (E(D)) and a small peak in the density of states at ∼0.4 eV above E(D).

Highlights

Phthalocyanines (Pc’s) and their derivatives, a class of aromatic compounds and a major component in various types of organic solar cells, have received much attention over the past decade
Copper phthalocyanine (CuPc) is often used as an electron-donor material in contact with materials that have high electron affinity such as the fullerene C60.14,15 A sizable charge transfer occurs from metal substrates such as Al, to CuPc at the metal-organic interface,16,17 while little charge transfer was observed at the interface between CuPc and highly oriented pyrolytic graphite (HOPG)
We considered the two typical semiconducting phthalocyanines: CuPc (Fig. 1, top) for ptype and F16CuPc (Fig. 1, bottom) for n-type semiconductors adopted in organic solar cells

Summary

Introduction

Phthalocyanines (Pc’s) and their derivatives, a class of aromatic compounds and a major component in various types of organic solar cells, have received much attention over the past decade. Copper phthalocyanine (CuPc) is often used as an electron-donor material in contact with materials that have high electron affinity such as the fullerene C60.14,15 A sizable charge transfer occurs from metal substrates such as Al, to CuPc at the metal-organic interface, while little charge transfer was observed at the interface between CuPc and highly oriented pyrolytic graphite (HOPG).. A thin film of copper hexadecafluorophthalocyanine, F16CuPc, is a promising n-type π -conjugated organic semiconductor employed as an electron acceptor. With the rapid development of graphene technology in the past few years, high-quality large-scale graphene films can be produced and the carrier type and concentration in graphene An atomically flat two-dimensional singlelayer of C atoms arranged in a honeycomb lattice, has emerged as a promising material for generation electronic devices due to its interesting physical properties. With the rapid development of graphene technology in the past few years, high-quality large-scale graphene films can be produced and the carrier type and concentration in graphene

Methods

Results

Conclusion