We measured the optical response of CuGeO3 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 cm −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 transfer edge hardening with decreasing size. These findings advance

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... e understanding of size effects in low-dimensional copper oxides.