Continuous Electrical Conductivity Variation in M₃(Hexaiminotriphenylene)₂ (M = Co, Ni, Cu) MOF Alloys

Abstract

We report on the continuous fine-scale tuning of band gaps over 0.4 eV and of the electrical conductivity of over 4 orders of magnitude in a series of highly crystalline binary alloys of two-dimensional electrically conducting metal-organic frameworks M₃(HITP)₂ (M = Co, Ni, Cu; HITP = 2,3,6,7,10,11-hexaiminotriphenylene). The isostructurality in the M₃(HITP)₂ series permits the direct synthesis of binary alloys (M_<i>x</i>M'_3-<i>x</i>)(HITP)₂ (MM' = CuNi, CoNi, and CoCu) with metal compositions precisely controlled by precursor ratios. We attribute the continuous tuning of both band gaps and electrical conductivity to changes in free-carrier concentrations and to subtle differences in the interlayer displacement or spacing, both of which are defined by metal substitution. The activation energy of (Co_<i>x</i>Ni_3-<i>x</i>)(HITP)₂ alloys scales inversely with an increasing Ni percentage, confirming thermally activated bulk transport.