Aspects of semiconductivity in soft, porous metal-organic framework crystals

Abstract

Metal-organic frameworks (MOFs) are known for their vast design space of possible structures, covering a wide range of often porous crystal structures and physical properties. Electrical conductivity, though, was—until very recently—not a feature usually associated with MOFs. On the other hand, well defined porous media such as MOFs, showing some measure of conductivity, could find uses in a huge number of fields ranging from electrochemistry to electronics and sensing. In this work, we therefore investigate the different aspects contributing to the bad conductivity in MOFs. Using Bardeen–Shockley deformation potential theory, we devise an approach that allows us to gauge all factors influencing the conductivity, including the availability of free charge carriers and their mobility. The latter itself is determined by the effective masses of the charge carriers, the material’s elastic constants, and the deformation potential constants, which measure an effective electron-phonon coupling. Based on these parameters, we study charge carrier mobility in metal (1,2,3)-triazolate MOF crystals, M(ta)2, where the metal is either iron, zinc, or ruthenium. Thereby, Zn(ta)2 was experimentally shown to have little to no conductivity, while Fe(ta)2 is one of the best currently known MOF semiconductors. Disregarding the fact that all three investigated MOFs show near-zero carrier densities due to their large bandgaps, our calculations reproduce the trends between Zn(ta)2 and Fe(ta)2. In contrast to that we find the Ru(ta)2 MOF, which to date has not been synthesized experimentally, to yield even better performance than iron triazolate. In summary, assuming, fox example, light doping to counter the large bandgap, our analysis of the factors influencing conductivity in MOFs allows us not only to confirm experimental trends but also to predict new, as yet unknown semiconducting MOF crystals.

Ultrathin porphyrin and tetra-indole covalent organic frameworks for organic electronics applications

“…where e is the elementary charge, ̵ h is the reduced Planck constant, k B is the Boltzmann constant, T is the temperature of the system, m * is the effective mass of the carrier, C 2D is the in-plane stiffness, and E 1 is the Bardeen-Shockley deformation potential (DP). 40,44,[63][64][65] Hole and electron effective masses are calculated via quadratic fitting of the valence band (VB) and conduction band (CB) extrema, respectively. Although the utilization of the PBE XC functional is known to underestimate the electronic bandgap, 66 the morphology of the band structure is consistent with results from hybrid XC functionals.…”

Charge Separation and Charge Carrier Mobility in Photocatalytic Metal‐Organic Frameworks

“…[30,31] The outstanding performance of these systems originates in the dispersion of their electronic band structure, [32] which in turn can be directly related to the effective mass of the charge carriers. [33] Elucidating the charge separation and charge carrier mobility from electronic structure calculations will not only provide insights into the fundamental properties of MOFs materials but also aid in the discovery of novel systems with improved characteristics for photocatalysis. In this context, high-throughput (HT) screening techniques become particularly useful to encompass the almost unlimited number of hypothetical structures that can be synthesized from distinct metal-node and organiclinker combinations.…”

“…[ 30,31 ] The outstanding performance of these systems originates in the dispersion of their electronic band structure, [ 32 ] which in turn can be directly related to the effective mass of the charge carriers. [ 33 ]…”

Ligand engineering in Cu(ii) paddle wheel metal–organic frameworks for enhanced semiconductivity

“…5 Despite these advances and continuous growing interest in these systems, their electronic transport properties are only poorly understood. 6 A better understanding of the underlying principles of charge transport in MOFs is imperative, since electrically conductive MOFs could improve important technologies such as supercapacitors, 7 batteries 8 and fuel cells. 9 The number of known intrinsically conductive MOFs is small, although it has increased substantially during the last decade.…”