Semiconductive and Photoconductive Properties of the Single Molecule Magnets Mn12-Acetate and Fe8Br8

TL;DR

This study investigates the semiconductive and photoconductive behaviors of Mn12-Acetate and Fe8Br8 single crystals, revealing their thermally activated resistivity and photon energy-dependent photoconductivity, with implications for their electronic structure.

Contribution

It provides new resistivity and photoconductivity measurements for Mn12-Acetate and Fe8Br8, linking their electronic properties to crystal structure and existing band structure calculations.

Findings

Both materials exhibit semiconductor-like behavior with specific activation energies.

Photoconductivity increases with photon energy, especially in Mn12-Acetate.

Resistivity is unaffected by X-ray irradiation despite increased resistivity.

Abstract

Resistivity measurements are reported for single crystals of Mn12-Acetate and Fe8Br8. Both materials exhibit a semiconductor-like, thermally activated behavior over the 200-300 K range. The activation energy, Ea, obtained for Mn12-Acetate was 0.37 +/- 0.05 eV, which is to be contrasted with the value of 0.55 eV deduced from the earlier reported absorption edge measurements and the range of 0.3-1 eV from intramolecular density of states calculations, assuming 2Ea = Eg, the optical band gap. For Fe8Br8, Ea was measured as 0.73 +/- 0.1 eV, and is discussed in light of the available approximate band structure calculations. Some plausible pathways are indicated based on the crystal structures of both lattices. For Mn12-Acetate, we also measured photoconductivity in the visible range; the conductivity increased by a factor of about eight on increasing the photon energy from 632.8 nm (red) to 488 nm (blue). X-ray irradiation increased the resistivity, but Ea was insensitive to exposure.