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Monday, 21 January 2013

Single-atom doping of metal phthalocyanines

Publishing in Nature Materials, a team led by ICREA Prof Pietro Gambardella reports on single-atom doping of copper and nickel phthalocyanines with lithium using STM.

Dr Cornelius Krull, Tenure Track Group Leader Dr Aitor Mugarza and ICREA Professor Dr Pietro Gambardella of ICN’s Atomic Manipulation and Spectroscopy Group, together with Dr Roberto Robles of CIN2 (CSIC-ICN), have just published a study in Nature Materials on the doping of organic (i.e. carbon-based) compounds with lithium atoms, entitled “Site and orbital-dependent charge donation and spin manipulation in electron-doped metal phthalocyanines”. Their findings should prove very utile for the development of novel materials for new classes of electronic devices.

Doping, the deliberate addition of specific impurities to a material in order to selectively modify its electronic or magnetic behaviour, is a fundamental process in the fabrication of semiconductor devices. Although doping of inorganic materials—such as traditional semiconductors—has been extensively studied, doping of organic materials is less understood. This has limited the development of new classes of materials for future organic electronics, such as organic superconductors, which promise improved performance and unique functionalities.

In their study, Dr Krull and co-workers used a class of metal-organic compounds known as metal phthalocyanines as a model system. These compounds have an elegant, symmetric structure comprising eight rings arranged in a floral-motif that surround a metal (in this case, nickel or copper) atom core, offering diverse sites where dopant atoms theoretically could be inserted. They deposited the metal phthalocyanines onto a silver substrate, and then used Scanning Tunnelling Microscopy (STM) to selectively dope the molecules with individual lithium atoms, which they moved around and “inserted” into the phthalocyanines molecules using the microscope tip.

The team identified, with the help of ab-initio electronic structure calculations, the doping sites in the phthalocyanines molecules that were favoured by the lithium atoms and their corresponding charge and spin states. They demonstrated that, depending on the adsorbing site of the lithium dopant, the donated charge would go either to the metal ion or the ligand part. Taking into account the various positions of the dopant, they found that a single dopant atom could provide a molecular charge (Q) of 1 or 2 electrons, and a molecular spin (S) of 0 or ½, and determined the maximum number of electrons that could be added per molecule (two) by doping it with multiple lithium atoms. They also investigated the role of the silver substrate on the doping procedure. For instance, the team determined that the charge transfer in the phthalocyanine molecules was narrowly defined by their interaction with the silver substrate, which acted as both an electron donor (for undoped molecules) and electron acceptor (for doped ones).

Lastly, they discovered that the dopant atoms exhibit a “long-range”, substrate-induced interaction with the phthalocyanines molecules—namely, that lithium atoms as far away as 3 nm from the centre of the molecules could affect their conductance gap (analogous to the band gap in semiconductors).

To access the article, “Site and orbital-dependent charge donation and spin manipulation in electron-doped metal phthalocyanines”, click here.