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The temperature-dependent hard X-ray excited optical luminescence (XEOL) spectroscopy was used to study the optical properties of O and Zn polarity of a c-plane single crystal ZnO wafer. By analyzing the XEOL and XRD, we found an unprecedented blue shift of the free exciton transition with increasing the excited carrier density as tuning the X-ray energy across the Zn K-edge, and the O-polar face possesses better crystal structure than the Zn-polar one. This spectral blue shift is attributed to the Coulomb screening of the spontaneous polarization by the excited free carriers that result in decreasing the exciton-phonon Fröhlich interaction to reduce exciton binding energy. [ABSTRACT FROM AUTHOR]