Laser Direct Writing of Copper-Graphene Heterostructures for Flexible Devices

Abstract

Copper conductors, essential building blocks for modern electronics, degrade in performance as devices become thinner and flexible, due to oxidation, grain boundary scattering, poor substrate adhesion, and mechanical fatigue. The hybridization of graphene with metals and metal oxides such as copper-graphene (Cu-Gr) holds promise for enhancing flexible devices by overcoming these challenges. However, current fabrication techniques involve complex, multi-step mixing processes and lack a clear understanding of how graphene integrates with copper. Additionally, the effects of these fabrication methods on copper nucleation, the graphene-copper interface, and microstructure remain unclear, as do their impacts on electrical, thermal, environmental, and mechanical properties. This thesis investigates these topics through three detailed studies using laser direct writing (LDW). First, to simplify the fabrication process, a minimalist LDW technique was developed to create graphene-metal heterostructures and flexible devices by layered fabrication of laser-reduced graphene oxide (LrGO) followed by reduction of CuOx, ZnOx, and FeOx nanomaterials from metal-ion precursors. Supplied laser energy during fabrication, controlled through laser processing parameters, tuned the oxygen functional groups on the LrGO surface and determined the metal oxide composition, which enabled the process to program sensor and junction responses. The sensors showed ranged tunability in: normalized current gains from −2.7 to 3.5, response times of 0.02 and 15 s, and recovery times of 0.04 and 6 s. Additionally, LDW produced LrGO/CuOₓ PN junctions and bipolar transistors with rectification ratios up to 160 and common-emitter current gains of 35.5–38.2. Second, to overcome the weak bonding and voids characteristic of planar interfaces in layer by layer assembled composites, LDW was adapted for simultaneous fabrication of graphene and copper. By tailoring plasma plume physics through a confinement mechanism, the local energy input was controlled to toggle between keyhole and conduction irradiation modes, which respectively governed graphene formation and copper reduction. Modified LDW to initiate keyhole and conduction simultaneously produced distinct Cu-Gr nanocomposite structures, including copper-coated graphene and copper nanoparticles embedded within graphene. The resulting interconnects reached a resistivity of 9.37 × 10⁻⁸ Ω·m and breakdown current density of 1.61 × 10⁸ A·cm⁻², approaching annealed copper. Third, graphene flakes were dispersed in the copper precursor prior to laser irradiation, facilitating the in situ growth of copper on graphene. This growth pathway promotes intimate interfacial contact and uniform distribution while reducing the gaps, contamination, and agglomeration commonly associated with layered assembly of composites, or post-synthesis mixing. Graphene flakes, which substantially changed copper growth mechanisms under laser irradiation, acted as preferential nucleation sites that lowered the minimum laser energy for copper nucleation from approximately 1.5 to 0.4 J mm⁻³, producing a higher density of copper nanoparticles that sintered into a continuous network encapsulating the graphene. The resulting dense composite reached approximately 98% relative density, thermal conductivity up to 1095 W m⁻¹ K⁻¹, and sheet resistance as low as 0.15 Ω sq⁻¹. In summary, this thesis establishes LDW as a process with precise control over graphene and copper formation, progressively increasing graphene integration in Cu-Gr nanocomposites for flexible conductors, sensors, and thermally conductive films.