Probing vibrational modes in an organic semiconductor using neutrons
10 Feb 2021
- Rosie de Laune



ISIS scientists have used inelastic neutron scattering to study the low energy vibrational modes of the organic semiconductor TCNQ.


​TCNQ and its vibrations, A. Berlie and H. Cavaye, Phys. Chem. Chem. Phys., 2021, 23, 2899 - Reproduced by permission of The Royal Society of Chemistry​


TCNQ (7,7,8,8-tetracyanoquinodimethane) is an organic compound that can conduct electricity, thought to be due to its ability to form a structure made up of layers of two-dimensional molecules. To develop ways that its unusual properties could be applied, it's important for scientists to understand its structure.

An important part of this structural characterisation is the study of the different ways the molecule can vibrate; each different motion is known as a vibrational mode. Within organic materials, these often involve the hydrogen atoms in the structure and, since neutrons interact strongly with hydrogen, this makes inelastic neutron scattering (INS) an ideal tool for investigating these modes.

In addition, unlike other techniques that require certain selection rules to be met, such as Raman scattering, all vibrational modes are visible in neutron spectroscopy. ISIS scientists Adam Berlie and Hamish Cavaye used the TOSCA instrument at ISIS, and the PELICAN instrument at the Bragg Institute, ANSTO, to study the vibrational modes of TCNQ.

Their study, published in Phys. Chem. Chem. Phys., measured, for the first time, the temperature-dependence of these vibrational modes. Neutrons are also sensitive to the different isotopes of hydrogen and so, by substituting hydrogen for deuterium in some samples, they were able to see the impact of this change on the molecular vibrations.

Focussing on the low energy region, they found that some of the modes were sensitive to the change from hydrogen to deuterium, especially at temperatures below 180 K. Therefore, at low temperatures, deuteration may prove to be an effective way to tune the properties of the material. 

Further information

The full paper can be found at DOI: 10.1039/D0CP06253G 

Contact: de Laune, Rosie (STFC,RAL,ISIS)