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Synthesis of mesoscale ordered 2D π-conjugated polymers with semiconducting properties

Two-dimensional materials can exhibit intriguing electronic properties that stem from their geometry. The best-known example is graphene’s Dirac cone that gives rise to massless electrons, which originates from the all-carbon hexagonal lattice. Two-dimensional conjugated polymers (2DCPs) can be considered analogs of graphene, but offering greater potential to design geometry and properties by carefully selecting their building blocks. Strikingly, 2DCPs on a kagome lattice (i.e. a trihexagonal tiling) can show both Dirac cones and flat bands, with highly-massive charge carriers.

Though studied for more than a decade the electronic properties of 2DCP remained elusive and were considered to be a scientific niche reserved to theorists, since the poor crystallinity of synthesized polymeric network did not allow for the use of averaging techniques such as angle-resolved photoelectron spectroscopy (ARPES) to probe the desired bands. The production of long-range ordered 2D polymeric networks was therefore a necessary milestone to reach before opening the door to the study of the band structures of topological networks, ultimately realized by a collaboration between the “Istituto di Struttura della Materia” of CNR and a number of Canadian universities (INRS-EMT, McGill and Lakehead). 
The long-range mesoscale-flake crystallinity of the 2DCP was obtained by using a rigid precursor (TBTANG and TBTANGO, see Figure 1a), on a heated Au(111) to enhance the diffusion and avoid defects of the network. The rigid precursors were necessary since they have higher enthalpy of formation for irregular unwanted structures (i.e. non-hexagonal in this case). The quality of the structure was confirmed by STM (Figure 1b), LEED (Figure 1c) and high-resolution XPS (Figure 1d), for two precursors with different functional groups, showing single domains in the excess of 100 × 100 nm2.
The long-range crystallinity allowed the study of the material band structure, a feat achieved at the VUV beamline of the Elettra synchrotron facility, finding the presence of a Dirac cone at the polymer k point (Figure 1e) and flat bands below the Fermi level, as expected from theoretical calculation of freestanding P2TANG polymer made from TBTANG precursor (Figure 1f). A linear fit of the Dirac cone yielded an experimental band velocity of vF= (0.5 ± 0.1)∙10m/s close to the band velocity for graphene on the same Au(111) surface (vF= 0.8∙10m/s).
By exploring the band structure of a topological polymer this work opens for the exploration of a wide range of 2DCP with various lattices, with important consequences both in condensed matter physics, for the study of anomalous Hall effects, surface superconductivity or superfluid transport, and in the implementation of such materials for applications in photonics and electronics. 

Figure 1.    (a)  TBTANG and TBTANGO precursors. (b) P2TANGO obtained by dosing on Au(111) kept at 200 °C. (c) P2TANG LEED at 72 eV. d) P2TANG C 1s spectra with spectral deconvolution of the different components. (e) second derivative of the ARPES map for P2TANG on Au(111) along the  ΓKM direction, where it is possible to observe the Dirac cone feature, with the two peaks converging around 0.55 eV; the theoretical calculated band structure is superimposed. /f) Calculated band structure and projected density of states for freestanding P2TANG.

 

This research was conducted by the following research team:

G. Galeotti,1,2 F. De Marchi,1E. Hamzehpoor,O. MacLean,1M. Rajeswara Rao,3Y. Chen,L. V. Besteiro,1,4D. Dettmann,1,2L. Ferrari,2F. Frezza,2,5P. M. Sheverdyaeva,6R. Liu,7A. K. Kundu,6P. Moras,6M. Ebrahimi,1M.C. Gallagher,7F. Rosei,1D.F. Perepichka,3G. Contini2,5
 
1 Centre Energie, Matériaux et Télécommunications, Institut National de la Recherche Scientifique, Varennes, Québec, Canada 
2 Istituto di Struttura della Materia, CNR, Rome, Italy
Department of Chemistry, McGill University, Québec, Canada 
4 Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu, China
Department of Physics, University of Tor Vergata, Rome, Italy
6 Istituto di Struttura della Materia, CNR, Trieste, Italy
7 Department of Physics, Lakehead University, Thunder Bay, Ontario, Canada


Contact persons:

Giorgio Contini, email: giorgio.contini@cnr.it
 

Reference

G. Galeotti, F. De Marchi, E. Hamzehpoor,O. MacLean, M. R. Rao, Y. Chen,L. V. Besteiro, D. Dettmann, L. Ferrari, F. Frezza, P. M. Sheverdyaeva, R. Liu, A. K. Kundu, P. Moras, M. Ebrahimi, M.C. Gallagher, F. Rosei, D.F. Perepichka and G. Contini, “Synthesis of mesoscale ordered 2D π-conjugated polymers with semiconducting properties”, Nature Materials (2020), doi: 10.1038/s41563-020-0682-z

This work was partially supported by a project Grande Rilevanza Italy-Quebec of the Italian Ministero degli Affari Esteri e della Cooperazione Internazionale (MAECI), Direzione Generale per la Promozione del Sistema Paese.




Last Updated on Friday, 22 May 2020 11:26