Electronic excitation spectra and transport of ligand stabilized crystalline Cu2n-xSenLm-cluster compounds: spectroellipsometric and impedance studies

Abstract

We report impedance and spectroellipsometric measurements of ligand stabilized Cu2n−xSenLm-cluster compounds for n=6, 22, 35 and 73 (x=0, 2). The impedance spectra for n=35 and 73 are quantitatively described by two Debye relaxation processes—presumably intra- and intercluster processes—with relaxation times between 10−5 to 10−4 s. The frequency dependence of the real part of the electrical conductivity exhibits a power law dependence, σ(ω)∝ωα, with α=1, indicative of a hopping transport mechanism. However, the temperature dependence of the dc-conductivity gives no clear distinction between a variable range hopping or a thermally activated nearest neighbour tunneling transport mechanism. From the magnitude of the dc-conductivity—σ(0)=4.8±1.7×10−6 Ω−1 cm−1 for Cu146Se73Lm at 295 K—it is concluded that clusters of this size are still far away from Cu2Se semiconductor bulk properties. This is supported by spectroellipsometric absorption spectra of Cu144Se73Lm where the lowest electronic excitation is found near 1.7 eV in comparison with the optical gap of bulk Cu2Se of 1 eV. For Cu12Se6Lm the lowest optical excitation at 3 eV agrees well with ab initio calculations. Further excitations in these spectra at lower energies are interpreted by decomposition products of smaller clusters.