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In semiconductors, photoexcited electrons and holes (carriers) initiallyoccupy high-energy states, but quickly lose energy to phonons and relax to theband edge within a picosecond [1]. Increasing the lifetime of carriers inlight-absorbing materials is necessary to improve open-circuit voltage inphotovoltaics [2], charge separation in organic solar cells [3], and chargetransfer in photodetection de vices [4]. Here we demonstrate long lifetimesover one hundred picoseconds for electron-hole pairs in graphene quantum dots(GQDs) due to large transition energies and weak coupling to excitonic statesbelow the fundamental band gap. This possibility for a large transition energyto bound excitons is due to graphene's poor screening, illustrating a uniquemechanism in this QD to occupy higher-energy states for long timescales. GQDedges can be terminated with either armchair or zigzag carbon patterns, andthis edge structure changes excited state lifetimes by orders of magnitude.These results indicate nanoscale control of carrier lifetimes inoptoelectronics.

Preventing rapid energy loss from electron-hole pairs to phonons in graphene quantum dots