207th ECS Meeting, Abstract #922, copyright ECS
Alkali-doped metal-phthalocyanines: electronic properties and structure
M.F. Craciun(1), S. Rogge(1), Y. Iwasa(2), and A. F.
Morpurgo(1)
(1) Kavli Institute of Nanoscience, Lorentzweg 1, 2628 CJ
Delft, the Netherlands
(2) Tohoku University, IMR, Aoba Ku, Sendai, Miyagi
9808577, Japan
In the solid state, much of the very rich physical behavior of C60 originates from the possibility to introduce charge
carrier by means of intercalation with alkali atoms. Surprisingly, in spite of the fascinating physics that is observed in alkali-doped C60, only rather limited research
has been devoted to study the electronic properties of other alkali-doped molecular systems. In particular, for instance, no metallic behavior of the electrical conductance has been observed for any molecule other than C60. Motivated by this fact we have started
systematic investigations of the electronic and structural properties of alkali-doped metal phthalocyanines films and micro-crystals.
Metal-phthalocyanines (MPc’s) are a large class of isostructural planar molecules consisting of a circular ligand ring made of Carbon, Nitrogen, and Hydrogen, and of a central metal atom. The metal atom determines the electronic properties of the molecules, such as the HOMO and LUMO orbitals and the molecular spin in the ground state. For this reason, MPc’s exhibit a rich variety of electronic properties, which have been subject of investigations in the past. Nevertheless, these investigations have been far from systematic and the possibilities offered by this large class of molecules remain to be explored.
Here we report a systematic comparative study of the electronic properties of five different MPc’s (CuPc, NiPc, CoPc, FePc, and MnPc) doped with Potassium atoms. Specifically, for these five molecules we have studied the electrical transport properties of doped thin-films, as a function of Potassium concentration and of temperature. In all cases, we find that the films –insulating in their pristine state- can be brought into a metallic state upon doping. Upon doping even further the films go back into an insulating state, when the potassium concentration is approximately four atoms per molecule. This demonstrates the occurrence of an insulator-metal-insulator transition in all MPc’s films, which is phenomenologically very similar to what has been so far observed only in C60.
In the case of CuPc films, we have confirmed the occurrence of the insulator-metal-insulator transition by means of local tunnelling spectroscopy, using a scanning tunnelling microscope. In these experiments, we observe a gap in the tunnelling I-V curves of the pristine films. Upon doping, the size of the gap decreases and eventually vanishes. Further doping results in the reappearance of the gap. This behavior is consistent with the observed doping dependence of the electrical conduction.
We have also observed robust experimental differences in the precise behavior of the film conductance as a function of doping. Specifically, CuPc and NiPc, for which electrons transfer from the potassium atoms are supposed to occupy the same ligand orbital, exhibit an identical dependence of the conductance on doping. This
dependence is different for CoPc, FePc, and MnPc, in which orbitals centered on the metal atoms are also involved in the charge transfer process.
In order to understand the precise structure of the doped MPc’s, we have performed additional studies of K-CuPc micro-crystals by means of x-Ray diffraction, Raman spectroscopy, and SQUID magnetization measurements. The results of these investigations clearly demonstrate the presence of at least two different stable phases of potassium intercalated CuPc. One of the two phases can be produced experimentally with a high degree of purity, so that the solution of the crystalline structure is possible (work currently in progress). Both the structural analysis performed so far, as well as the Raman shift observed in this phase, suggest that a very large charge transfer from the potassium atoms to the molecules is occurring. Although the complete analysis of the data will be necessary to reach a firm conclusion, preliminary work indicates that in this phase up to five electrons per molecule may be transferred from the Potassium atoms to the CuPc molecules. This charge transfer has a strong effect on the magnetic properties of the materials, which is observed in the experiments.
So far, the only molecule in which experiments comparable to those reported here have been previously performed is C60, in which a remarkable richness in
physical behavior has been observed (e.g., superconducting and Mott-Hubbard insulating phases). Finding a similar behavior in an entire class of molecules open new possibilities for the investigation of molecular materials at high charge carrier density. For instance, in MPc’s it is possible to tune the molecular spin by appropriately choosing the central metal atom (e.g., MnPc has S= 3/2; for FePc, S= 1; in CoPc and CuPc, S=1/2,; NiPc has S=0) and the exchange energy is much larger than in C60. This may result in new interesting magnetic
phenomena. In addition, in many MPc’s the relevant orbitals are two-fold degenerate, contrary to the case of C60, in which the degeneracy is threefold. As recently
suggested theoretically by Tosatti and coworkers, this difference is a key aspect for the generation of electronic states close to half-filling, which have been predicted to exhibit novel electronic phenomena.
