The many outstanding properties of graphene have impressed and intrigued scientists for the last few decades. Its transparency to light of all wavelengths combined with a low sheet resistance makes it a promising electrode material for novel optoelectronics. So far, no one has utilized graphene as both the substrate and transparent electrode of a functional optoelectronic device. Here, we demonstrate the use of double-layer graphene as a growth substrate and transparent conductive electrode for

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... an ultraviolet light-emitting diode in a flip-chip configuration, where GaN/AlGaN nanocolumns are grown as the light emitting structure using plasma-assisted molecular beam epitaxy. Although the sheet resistance is increased after nanocolumn growth compared with pristine double-layer graphene, our experiments show that the double-layer graphene functions adequately as an electrode. The GaN/AlGaN nanocolumns are found to exhibit a high crystal quality with no observable defects or stacking faults. Room temperature electroluminescence measurements show a GaN related near bandgap emission peak at 365 nm and no defect-related yellow emission. 2 With the emergence of new semiconductor nanomaterials and heterostructures, new possibilities for optoelectronics arise. The semiconductor materials most commonly used for optoelectronics today, like Si 1 , GaAs, InAs 2 , ZnO 3 and GaN with its alloys 2 , exhibit structural imperfections when grown as heteroepitaxial thin-films, for instance twinning defects, threading dislocations and stacking faults. This is due to a large lattice mismatch between these materials as well as with conventional substrates. By nanostructuring the semiconductor materials in the forms of columns or pyramids, new combinations of materials can be explored, and new substrates can be employed for the epitaxial growth. An intriguing potential substrate for epitaxial growth of semiconductors is graphene, the singlelayer form of carbon, as it can not only act as an atomically thin crystalline growth template, but also has outstanding functional properties when it comes to strength, flexibility, electron and thermal conductivity 4,5 . Hybrid systems based on the growth of semiconductor nanocolumns on different graphitic substrates have been intensively studied in the last decade, with the aim of developing new functionalities and higher efficiency optoelectronic devices as for example solar cells, photodetectors, light emitting diodes (LEDs) and lasers. Such hybrid systems have been demonstrated for GaAs 6 , InAs 7-9 , InAsSb 10 , In(Ga)As 11,12 , ZnO 13,14 and GaN. With regards to the growth of GaN nanocolumns, different graphitic forms have been used as growth substrate, for instance graphite 15 , transferred CVD graphene (single-and multilayer) 16-25 and epitaxial graphene 26 . However, these studies mostly focused on the growth of the nanocolumns, without further demonstration of a hybrid device realization. In addition to the attributes already mentioned, graphene has the attractive property of being transparent in all parts of the electromagnetic spectrum and has been demonstrated as a top-emitting transparent conductive electrode (TCE) for GaN 27,28 and InGaN LEDs 29-31 . In contrast to the traditional TCE in optoelectronic devices, indium tin oxide, graphene is transparent in the whole UV region of the electromagnetic spectrum (100 to 350 nm) 32 , offering a potential solution for devices operating in this region. Recently, our group showed GaN/AlGaN nanocolumn growth with a single-layer graphene substrate as the bottom electrode 25 . However, so far there has been no realization of utilizing graphene simultaneously as the growth substrate and the TCE of a semiconductor device. Here, we