Semiconductive and Photoconductive Properties of the Single Molecule Magnets Mn$_{12}$-Acetate and Fe$_8$Br$_8$

TL;DR

This study investigates the semiconductive and photoconductive behaviors of single molecule magnets Mn$_{12}$-Acetate and Fe$_8$Br$_8$, revealing their activation energies, optical properties, and effects of irradiation, contributing to understanding their electronic structures.

Contribution

It provides the first detailed resistivity and photoconductivity measurements for these single molecule magnets, linking their electronic properties to crystal structure and optical excitation.

Findings

Mn$_{12}$-Acetate has an activation energy of 0.37 eV.

Fe$_8$Br$_8$ exhibits an activation energy of 0.73 eV.

Photoconductivity increases with photon energy, especially in the visible range.

Abstract

Resistivity measurements are reported for single crystals of Mn$_{12}$-Acetate and Fe$_8$Br$_8$. Both materials exhibit a semiconductor-like, thermally activated behavior over the 200-300 K range. The activation energy, $E_a$, obtained for Mn$_{12}$-Acetate was 0.37 $\pm$ 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 $2E_a$= $E_g$, the optical band gap. For Fe$_8$Br$_8$, $E_a$ was measured as 0.73 $\pm$ 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 Mn$_{12}$-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 $E_a$ was insensitive to exposure.