An international team of scientists, involving researchers from the Regional Centre of Advanced Technologies and Materials (RCPTM), the Faculty of Science, Palacký University Olomouc, and the Institute of Physics of the Czech Academy of Sciences (FZU), has designed and experimentally verified the possibility of preparing single-dimensional, carbon-based conductive polymers. As carbon is one of the most abundant elements, new polymer conductors have potentially lower production costs than normal metal conductors, along with greater stability and the opportunity to control their material properties. The joint work of Czech, Spanish and Swiss scientists, which was published this week in the journal Nature Nanotechnology 1, introduces a novel approach to designing non-metallic conductors, which could be used in solar energy applications, optical technologies or nanoelectronics. The work is so important that the editors of Nature Nanotechnology included a special commentary on the article in their News & Views section (https://www.nature.com/articles/s41565-020-0667-8).
“The advantage of the new polymers is the possibility of controlling their electronic and optical properties along with expected higher stability compared to current conductive polymers, which are enriched by foreign elements. Their synthesis is simple and easily reproducible. The possibility of constructing stable carbon conductive polymers paves the way for miniaturizing and enhancing the performance of a number of electronic components,” says Pavel Jelínek, who leads the Czech team.
Metal conductors, which are currently an integral part of most commercial electrical and electronic devices, pass an electrical current by means of free electrons in their structures. In most cases, organic carbon and hydrogen-based molecules do not contain free electrons and therefore act as insulators. However, organic conductors, so-called conductive polymers, are known to carry electric current thanks to enrichment by other elements. These elements supply or extract electrons from the structure of carbon polymers, creating the essential free electrical charge responsible for high electrical conductivity. The discoverers of these polymers were awarded the Nobel Prize in Chemistry in 2000. Advantages of polymer conductors over conventional metal conductors include low-cost production, easy processing using conventional technologies, better mechanical properties and the potential for manipulating their electrical and optical characteristics. Some have found applications in organic LEDs, solar cells, transistors or different types of sensors. However, the main disadvantage of existing conductive polymers is their low chemical and thermal stability, which is linked to the presence of foreign elements in their structure. A number of laboratories around the world are, therefore, trying to prepare new types of conductive polymers that do not contain such elements. It was the Czech-Spanish-Swiss team that was the first to succeed in this challenge.
“In our work, we studied so-called p-conjugated polymers, which are characterized by alternating simple and double bonds between carbon atoms. Nevertheless, on the basis of the polymer's internal stress compensation and its electron structure, an appropriate choice of the polymer's basic construction units can be made to prepare a one-dimensional system that is located near the phase transition. It was the use of the right starting molecules that produced the highly conductive polymer with free electrons without the necessary presence of foreign elements. This approach to the synthesis of 1D conductive polymers may lead to the development of a new generation of organic conductors for molecular electronics,” says Jelínek.
The synthesis of 1D polymer chains took place on a gold surface. The chemical structure and electrical properties were examined by scientists using a scanning microscope with a chemically modified tip that enabled imaging of individual molecules (Fig. 1.) “Conductive polymers were prepared by applying appropriate molecules, developed by Spanish colleagues, to the gold surface. Their subsequent heat treatment led to the formation of long 1D chains without any structural disturbances. The basic building units of the polymers were interconnected carbon bridges. In addition, the electrical properties of 1D polymers can be tuned using just the right choice of basic construction units, moving towards the development of 1D organic semiconductors, for example. Such polymers could find applications not only in the development of molecular electronics, but also in new optoelectronic devices or organic solar cells,” says Bruno de la Torre from both FZU and RCPTM.
Following the collaboration of the Spanish and Czech teams, the results of the study have recently led to the development of chemical protocols for polymer synthesis, the preparation of which is not possible using normal processes.2 “The work in Nature Nanotechnology shows unique possibilities for surface chemistry, where different chemical rules are applied compared to reactions taking place in liquid or gaseous environments. This enables the preparation of completely unique materials such as 1D molecular conductors, with their conductivity resulting directly from their structure. These findings could help address a number of other scientific challenges and prepare a new generation of low dimensional structures with completely new optical, magnetic and electrical properties,” adds Radek Zbořil from RCPTM, Olomouc.
1 B. Cirera, A. Sánchez-Grande, B. de la Torre, J. Santos, Sh. Edalatmanesh, E. Rodríguez-Sánchez, K. Lauwaet, B. Mallada, R. Zbořil, R. Miranda, O. Gröning, P. Jelínek, N. Martín, D. Ecija,Tailoring topological order and π-conjugation to engineer quasi-metallic polymers, Nature Nanotech. (2020) DOI: 10.1038/s41565-020-0668-7.
2 A. Sánchez-Grande, B. de la Torre, J. Santos, B. Cirera, K. Lauwaet, T. Chutora, S. Edalatmanesh, P. Mutombo, J. Rosen, R. Zbořil, R. Miranda, J. Björk, P. Jelínek, N. Martín and D. Écija," ANGEWANDTE CHEMIE INTERNATIONAL EDITION vol. 58, iss. 20, pp. 6559-6563, 2019. DOI: 10.1002/anie.201814154.
Fig. 1 Structure of a conductive 1D polymer imaged using atomic force microscopy (top). An image from a scanning tunnelling microscope showing so-called free electron radicals on the polymer's ends (bottom). (Image source: B. de la Torre)
Fig. 2 A schematic representation of the structure of the 1D conductive polymer on the gold surface with the tip of the scanning microscope used to characterise it. (Image source: M. Pykal)