We investigate the structural and electrical properties of carbon-ion-implanted ultrananocrystalline diamond(UNCD)films.Impedance spectroscopy measurements show that the impedance of diamond grains is relatively stable,while that of grain boundaries(GBs)(Rb)significantly increases after the C+ implantation,and decreases with the increase in the annealing temperature(Ta)from 650 ℃ to 1000 ℃.This implies that the C+ implantation has a more significant impact on the conductivity of GBs.Conductive atomic force microscopy demonstrates that the number of conductive sites increases in GB regions at Ta above 900 ℃,owing to the formation of a nanographitic phase confirmed by high-resolution transmission electronic microscopy.Visible-light Raman spectra show that resistive trans-polyacetylene oligomers desorb from GBs at Ta above 900 ℃,which leads to lower Rb of samples annealed at 900 and 1000 ℃.With the increase in Ta to 1000 ℃,diamond grains become smaller with longer GBs modified by a more ordered nanographitic phase,supplying more conductive sites and leading to a lower Rb.