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