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The effects of a negative direct-current (dc) voltage on the striated structures and the electrical parameters have been studied by multi-fold experimental diagnostics and particle-in-cell/Monte Carlo collision (PIC/MCC) simulations in a dc superposed rf (radio-frequency, 8 MHz) capacitive discharge in CF₄. The measured electron density, the electron-impact excitation rate and the electrical parameters are compared with the corresponding simulation results. It was found that the plasma bulk is compressed and the electron density decreases slightly at low magnitude of the dc voltage, |V_dc|, while the plasma bulk broadens at high |V_dc| when the ionization induced by secondary electrons adjacent to the powered electrode is enhanced. By increasing |V_dc|, the region of the strong ionization/excitation shrinks towards the edge of the collapsing sheath adjacent to the grounded electrode. And the ionization adjacent to the powered electrode is significantly suppressed during the collapsing phase of the sheath. This is mainly attributed to the fact that the dc component of the bulk electric field and the spatial gradient of the electron density are simultaneously enhanced during the transition from the ‘striation’ mode to ‘striation-γ’ hybrid mode when increasing |V_dc|. Besides, we found that the rf power deposition is somewhat suppressed at low |V_dc|, and is enhanced at high |V_dc|. The magnitude of the dc current exhibits a complex dependence on |V_dc|, due to nonlinear change of the plasma dc resistance.