Abstract
The intermolecular lattice vibrations in small‐molecule organic semiconductors have a strong impact on their functional properties. Existing models treat the lattice vibrations within the harmonic approximation. In this work, polarization‐orientation (PO) Raman measurements are used to monitor the temperature‐evolution of the symmetry of lattice vibrations in anthracene and pentacene single crystals. Combined with first‐principles calculations, it is shown that at 10 K, the lattice dynamics of the crystals are indeed harmonic. However, as the temperature is increased, specific lattice modes gradually lose their PO dependence and become more liquid‐like. This finding is indicative of a dynamic symmetry breaking of the crystal structure and shows clear evidence of the strongly anharmonic nature of these vibrations. Pentacene also shows an apparent phase transition between 80 and 150 K, indicated by a change in the vibrational symmetry of one of the lattice modes. These findings lay the groundwork for accurate predictions of the electronic properties of high‐mobility organic semiconductors at room temperature.
| Original language | English |
|---|---|
| Article number | 1908028 |
| Number of pages | 6 |
| Journal | Advanced Materials |
| Volume | 32 |
| Issue number | 10 |
| Early online date | 31 Jan 2020 |
| DOIs | |
| Publication status | Published - 12 Mar 2020 |
Funding
The authors thank Tsachi Livneh for fruitful discussions and Lior Segev for software development. O.Y. acknowledges funding from: BSF (grant No. 2016362), ERC (850041 - ANHARMONIC), Benoziyo EF, Ilse Katz Institute, H. C. Krenter Institute, Soref Fund, C. Stiftung, A. & S. Rochlin Foundation, E. A. Drake and R. Drake, and the Harold Perlman Family. D.A.E. acknowledges funding from: Alexander von Humboldt Foundation within the framework of the Sofja Kovalevskaja Award, the Technical University of Munich - Institute for Advanced Study (grant Agreement No. 291763), and by the Deutsche Forschungsgemeinschaft (EXC 2089/1 - 390776260).