Many envisioned carbon nanotube (CNT) applications, such as device interconnections in integrated circuits, require directed growth of aligned CNTs, and low-resistance high-strength CNT junctions with tunable chemistry, stability, and electronic properties. However, forming CNT-CNT junctions on the substrate plane in a scalabe fashion, to enable in-plane device circuitry and interconnections, remains to be realized. This invention is based on the discovery that high current densities can slice, weld, and chemically functionalize multiwalled CNTs and alter their electrical properties. The conductance of thin film assemblies of such CNTs increases by 150%, indicating an increase in the number of low-resistance pathways caused by CNT junction formation. Such welded, high-conductance CNT networks could be useful for device and sensor applications, and may serve as high mechanical toughness mat fillers that are amenable to integration with nanocomposite matrices.