Organic charge-transfer phase formation in thin films of the BEDT-TTF/TCNQ donor-acceptor system

TL;DR

This study investigates the formation of charge transfer phases in thin films of the BEDT-TTF/TCNQ system using vapor deposition, revealing phase formation during annealing and methods for layer preparation.

Contribution

It demonstrates the formation of a monoclinic BEDT-TTF/TCNQ charge transfer phase via vapor deposition and annealing, and introduces new layer preparation techniques.

Findings

Charge transfer phase forms during annealing.

Monoclinic BEDT-TTF/TCNQ identified by X-ray diffraction.

Layer orientation achieved at 100°C using vapor phase transport.

Abstract

We have performed charge transfer phase formation studies on the donor/acceptor system bis(ethylendithio)tetrathiafulvalene,(BEDT-TTF)/tetracyanoquinodimethane,(TCNQ) by means of physical vapor deposition. We prepared donor/acceptor bilayer structures on glass and Si(100)/SiO_2 substrates held at room temperature and analyzed the layer structures by optical microscopy, X-ray diffraction and focused ion beam cross sectioning before and after annealing. We found clear evidence for the formation of a charge transfer phase during the annealing procedure. For the as-grown samples we could not detect the occurrence of a charge transfer phase. X-ray diffraction indicated that the monoclinic variant of the (BEDT-TTF)-TCNQ was formed. This was further corroborated by single-source evaporation experiments from pre-reacted (BEDT-TTF)-TCNQ obtained from solution growth, and in particular from co-evaporation experiments of (BEDT-TTF)-TCNQ and TCNQ. In the course of these experiments we found that (0,l,l)-oriented BEDT-TTF layers can be prepared on alpha-Al_2O_3 (1,1,-2,0) substrates at about 100 C using (BEDT-TTF)-TCNQ as source material. We speculate that due to its high vapor pressure the TCNQ component serves as a carrier gas for BEDT-TTF vapor phase transport.