Vibronic coupling and band gap trends in CuGeO 3 nanorods

We measured the optical response of ${\mathrm{CuGeO}}_{3}$ nanorods in order to reveal size effects on the electronic properties. The vibronically activated $d$-to-$d$ color band excitations are activated by the 131 and 478 ${\mathrm{cm}}^{\ensuremath{-}1}$ phonons, with the relative contribution of the lower frequency O-Cu-O bending mode increasing with decreasing size until it dominates the process. We also uncover trends in the direct band gap, with the charge transfer edge hardening with decreasing size. These findings advance the understanding of size effects in low-dimensional copper oxides.