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

Abstract

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 α405 nm ≈ 6.5 ± 1.1 × 10(4) cm(-1), α520 nm ≈ 2.1 ± 0.4 × 10(4) cm(-1), and α640 nm ≈ 1.1 ± 0.3 × 10(4) cm(-1) while corresponding rGO α-values are α405 nm ≈ 21.6 ± 0.6 × 10(4) cm(-1), α520 nm ≈ 16.9 ± 0.4 × 10(4) cm(-1), and α640 nm ≈ 14.5 ± 0.4 × 10(4) 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.

Spectroscopy and Microscopy of Graphene Oxide and Reduced Graphene Oxide

“…As an illustration, the last two panels in Fig. 2.2b show that GO's absorption dramatically increases across the visible upon the removal of these functionalities and approaches graphene's 2.3 %A limit [21,26].…”

“…By contrast, GO's optical absorption is nearly an order of magnitude smaller (~0.3 %A, second panel, Fig. 2.2b) [21,22]. Furthermore, GO's absorption spectrum is wavelength dependent; it peaks in the UV/blue edge of the visible spectrum and falls towards the near infrared (NIR).…”

“…To investigate the optical properties of individual GO and rGO sheets as well as their intrasheet optical properties, we have recently developed a unique toolset to probe both the spatially resolved absorption and emission of individual single-layer GO/rGO specimens [21,33]. When these measurements are carried out before, during, and after photolytic GO reduction, they provide an unprecedented look at how GO's optical properties evolve throughout its reduction process.…”

“…This allows the transmittance (T = I/I 0 ) or % absorption [%A= (1 − T) × 100] to be quantified for every pixel (an approximate 250 nm × 250 nm region). The sample's absorption coefficient (α; cm −1 ) is then obtained using [21] a cm…”

Direct mechanochemical cleavage of functional groups from graphene

“…While this is difficult to achieve in threedimensional (3D) crystalline materials, the 2D carbon lattice found in graphene is ideal for mechanochemistry since it is atomically flat, can be produced with exceptional crystalline quality, can be functionalized with a wide range of chemical groups and is easily characterized by conventional surface science techniques 13,14 . Chemically modifying graphene markedly alters its optical 15 , electronic 16 and lubricating 17,18 properties. It should be noted that while prior work has shown that scanning probes can remove functional groups by locally applying heat 16,19 or electronic potential 20 ; mechanochemical cleavage by a scanning probe has not been addressed.…”