Supplement: Transient absorption and photocurrent microscopy show hot electron supercollisions describe the rate-limiting relaxation step in graphene

Devices with suspended graphene stripes are fabricated by transferring pre-patterned CVD-grown graphene on to prefabricated device structures with patterned microtrenches. The CVD graphene is grown on Alfa Aesar 0.025mm, 99.8% pure copper foils. The as-grown graphene covered copper piece is then spin-coated with photoresist (MICROPOSIT S1813). Arrays of stripes with different widths are then patterned onto the copper piece using photolithography. The graphene in the exposed area is then removed with oxygen plasma. After stripping the remaining photoresist, the copper piece with the patterned graphene stripes is coated with a thin layer (~50 nm) of PMMA and put into copper etch (CE-200). After all the cupper substrate is dissolved, the remaining PMMA/graphene floats are transferred into 6 baths of DI water to clean the graphene. The floats are then transferred onto the pre-fabricated substrates with the graphene stripes roughly aligned perpendicular to the microtrenches. After the PMMA piece completely dries and conforms to the substrate surface, the sample is placed in anisole followed by acetone to dissolve the PMMA layer. Finally, the devices are dried using a critical point dryer to preserve the suspended graphene stripes across the microtrenches. Images of a typical resulting device are shown in Fig. 5b (inset) of the main text.