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 Cu_2n−xSe_nL_m-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)∝ω^α, 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 Cu₁₄₆Se₇₃L_m 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 Cu₁₄₄Se₇₃L_m where the lowest electronic excitation is found near 1.7 eV in comparison with the optical gap of bulk Cu₂Se of 1 eV. For Cu₁₂Se₆L_m 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.