Effects of Size and Localized States in Charge Carrier Dynamics and Performance of Solution-Processed Graphene Quantum Dots/Silicon Heterojunction Near-UV Photodetectors

The influence of size and localized defect states on photogenerated carrier recombination dynamics, which affects the performance of graphene quantum dots (GQDs)-based Si-compatible near-UV heterojunction photodetectors, is reported. GQDs of varying size from ∼3.0 to ∼8.0 nm have been prepared by a top-down method of oxidative cutting of graphene oxide followed by hydrothermal reduction and gradient centrifugation at different speeds. Structural, compositional, and photophysical characteristics as well as photocarrier dynamics of different sized samples have been studied. Spectroscopic features and carrier dynamics of GQDs are effectively controlled by their size and localized surface states, which also determine the average recombination lifetime of photogenerated carriers. Two-terminal vertical heterojunction photodetector devices fabricated by using solution-processed quantum dots exhibit superior performance over a broad spectrum with a peak response in the near-UV (380 nm) region. The device fabricated by using ∼6.0 nm diameter GQDs displayed the highest peak responsivity of 3.5 A/W, showing an interesting correlation with carrier dynamics. To our best knowledge, this is the only report in graphene quantum dots or carbon nanostructure genre showing the direct correlation between size of the quantum dots and localized surface states on photocarrier dynamics and consequential performance of photodetector devices.