An Extensive Study of the Electrical Conduction in HKUST-1 Single Crystals

Abstract

Metal-Organic Frameworks (MOFs), which is three-dimensional (3-D) nanoporous materials composed of metal ions and organic ligands coordinated together, has recently attracted enormous attention as functional materials for electronic devices owing to their structural and functional flexibility in the design, as well as relatively high thermal stability [1, 2]. Although there are some reports so far that guest molecule doping drastically improves the electrical conduction of MOFs, e.g. the electrical conductivity of HKUST-1 (Cu3(btc)2) increases by infiltration of TCNQ, most of those experiments has been done using pellet or polycrystalline film samples [3, 4]. To extract an intrinsic electrical current conduction, avoiding the influences of grain boundaries and the residual stress introduced into the film due to the substrate mismatching, the use of single crystalline sample is essential. In this work, we investigated the dominant conduction mechanism of TCNQ-infiltrated bulk HKUST-1 single crystals which include much less grain boundaries and residual stress, by measuring the electrical conductivity as functions of a bias voltage, temperature, and pressure. Sub-mm sized HKUST-1 single crystals were fabricated and infiltrated with TCNQ using a saturated TCNQ/CH2Cl2 solution. The temperature dependence of DC current between 30 °C to 110 °C in the ambient shows that a current at 10 V decreases with increasing temperature down to undetectable value of less than 1 x 10-12 A, although the current value at 30 °C of around 1 x 10-6 A is similar to that of polycrystalline films [3]. We also confirmed that the temperature dependence of DC current for un-infiltrated HKUST-1 single crystals almost agrees with those of the TCNQ-infiltrated ones. Therefore, TCNQ infiltration is not effective in improving an electrical conductivity and the relatively high electrical conductivity at 30 °C is attributed to the presence of water absorbed by samples from the ambient, which was confirmed by thermogravimetric analysis. These results indicate that the current we measured was attributed to the ionic conduction probably due to the electrolysis of water inside the nanoporous structure not to the electronic conduction. Therefore, it is implied that the drastic improvement of the conductivity in the TCNQ-infiltrated HKUST-1 reported in ref. [3] is due to the use of film instead of bulk single crystal. It is suggested that the residual stress introduced into the HKUST-1 film by substrate mismatching plays a key role in the improved electronic conduction of the TCNQ-infiltrated HKUST-1 system. We will also discuss the effect of applying high pressure to the single crystals.