Direct Observation of Single Layer Graphene Oxide Reduction through Spatially Resolved, Single Sheet Absorption/Emission Microscopy

Laser reduction of graphene oxide (GO) offers unique opportunities for the rapid, nonchemical production of graphene. By tuning relevant reduction parameters, the band gap and conductivity of reduced GO can be precisely controlled. In situ monitoring of single layer GO reduction is therefore essential. In this report, we show the direct observation of laser-induced, single layer GO reduction through correlated changes to its absorption and emission. Absorption/emission movies illustrate the initial stages of single layer GO reduction, its transition to reduced-GO (rGO) as well as its subsequent decomposition upon prolonged laser illumination. These studies reveal GO’s photoreduction life cycle and through it native GO/rGO absorption coefficients, their intrasheet distributions as well as their spatial heterogeneities. Extracted absorption coefficients for unreduced GO are α_{405 nm} ≈ 6.5 ± 1.1 × 10⁴ cm^–1, α_{520 nm} ≈ 2.1 ± 0.4 × 10⁴ cm^–1, and α_{640 nm} ≈ 1.1 ± 0.3 × 10⁴ cm^–1 while corresponding rGO α-values are α_{405 nm} ≈ 21.6 ± 0.6 × 10⁴ cm^–1, α_{520 nm} ≈ 16.9 ± 0.4 × 10⁴ cm^–1, and α_{640 nm} ≈ 14.5 ± 0.4 × 10⁴ cm^–1. More importantly, the correlated absorption/emission imaging provides us with unprecedented insight into GO’s underlying photoreduction mechanism, given our ability to spatially resolve its kinetics and to connect local rate constants to activation energies. On a broader level, the developed absorption imaging is general and can be applied toward investigating the optical properties of other two-dimensional materials, especially those that are nonemissive and are invisible to current single molecule optical techniques