Interfacial Spacing Tunes π-d Charge Transfer to Co-Optimize Conductivity and Strength in Cu-Graphene Composites

Simultaneously enhancing electrical conductivity and tensile strength in copper-graphene composites remains challenging because microstructural strategies that strengthen the metal typically impede carrier transport. Here we establish a spacing-controlled interfacial mechanism: at a Cu-graphene gap of ∼2.5 Å, the interface forms a metallocovalent π-d charge-transfer bond in which graphene's C-2p_z orbital hybridizes with Cu-3d (a "doorway" covalency) while Cu-4s/4p states donate charges into the graphene's π manifold. Density-functional theory combined with nonequilibrium Green's functions (DFT+NEGF) shows that the spacing pins a hybrid interfacial resonance near the Fermi level and maximizes electrode coupling (lead broadenings), producing a peak in the transmission at E_F, T(E_F). The same hybridization stabilizes occupied bonding states, generating a co-peak in the work of separation, Wsep. Projected density of states and Mulliken analysis further reveal a maximum Cu→C charge transfer at ∼2.5 Å, with C-2p gain balanced by Cu-4s/4p depletion. Experiments on pressure-sintered Cu-graphene validate this mechanism, delivering 128% IACS conductivity (vs. ∼100% IACS for identically processed Cu) together with a tensile strength of 589 MPa at 0.4 wt.% graphene addition. These results identify interfacial spacing as a prescriptive knob that simultaneously optimizes transport and adhesion, enabling copper-graphene composites that are both stronger and more conductive.