Production of graphene sheets by direct dispersion with aromatic healing agents

Materials: Synthetic graphite powder (<20 μm), 1-pyrenemethylamine (Py-NH₂) hydrochloride from Sigma-Aldrich, and 1,3,6,8- pyrenetetrasulfonic acid (Py-SO₃) tetrasodium salt hydrate from Acros Organic were purchased and used as received. All solutions were prepared using deionized water (18.2MV) (Nanopure water, Barnstead), which was also used to rinse and clean the samples. Dispersion of graphene with pyrene molecules: Stock solutions of Py-NH₂ and Py-SO₃ with a concentration 0.4 mg mL-1 were prepared in deionized water by vigorous stirring for 1 h. Graphite powder was added into the resulted solutions, in which the weight ratio between the pyrene derivatives to the graphite powder is 4:1. Direct exfoliation of graphite to graphene sheets was performed by sonication of the obtained mixture solution with Sonics VX-130 (130W, 45%) in an ice bath. The exfoliation process was monitored by recording the fluorescence spectra of the suspension at different exfoliation period. All fluorescence measurements were performed using a Cary-Eclipse fluorescence spectrophotometer (Varian, Inc, Palo Alto, CA). The obtained grey dispersion was then centrifuged at 4000 rpm for 20min to remove unexfoliated graphite using a Beckman J2-21 centrifuge (usually a very small amount). The supernatant containing graphene sheets was dialyzed three times with an Amicon YM-50 centrifugal filter unit (Millipore) to remove most of the free pyrene molecules. The removal of free pyrene was monitored by measuring UV-vis and emission spectra of the solution after each dialysis. The yield of graphene sheets was estimated to be 50%. The resulted solution was directly used to prepare graphene films with a vacuum filtration method. Atomic force microscopy: The Py-NH₂ and Py-SO₃ exfoliated graphene samples (after being extensively dialyzed, normally 25 times for Py-NH ₂ and 10 times for Py-SO₃) were imaged with a tapping mode Nanoscope IIIa AFM instrument (Veeco instrument, Santa Barbara, CA, USA) in air. In order to image the graphene sheets, 2 μL of the prepared solutions were deposited on freshly cleaved mica. After a 3-5 min of incubation, the mica surface was rinsed with 1 drops of DI water and dried in a fume hood for 20- 30 min. During imaging, a 125-μm-long rectangular silicon cantilever/tip assembly (Model: MPP-12100, Veeco) was used with a resonance frequency of approximately 127-170 kHz, a spring constant of approximately 5 Nm^-1, and a tip radius of less than 10 nm. The applied frequency was set on the lower side of the resonance frequency and scan rate was ∼1.0 Hz. Height differences were obtained from section analysis of the topographic images. In the figures variations in height are indicated by color coding. X-Ray Photoelectron Spectroscopy: XPS spectra were obtained with a Perkin-Elmer hemispherical analyzer with a non-monochromatic Mg Ka X-ray source (hn=253.6 eV). At 17.9 eV pass energy, the full width at half maximum (FWHM) of the Cu 2p 3/2 core level is 1.2 eV. All core-level photoemission peaks were referenced to the Au 4f 7/2 peak with a binding energy of 83.7 eV. Raman spectroscopy: Raman spectra were acquired with a micro-Raman spectroscope (Renishaw 1000) assembled with a confocal imaging microscope, with an excitation energy of 1.96 eV (632.8 nm) and a power around 0.1W∼0.3W. Spectra are acquired using a 30 s exposure time and two accumulations. Optical and electrical properties of the dispersed graphene sheets: UV-vis-NIR absorption spectroscopy was used to characterize the electronic states of the exfoliated graphene sheets. All spectra were obtained using a Cary 500 UV-vis-NIR spectrophotometer in double-beam mode. Preparation of graphene films: Graphene films with different thickness were prepared from the corresponding suspension by vacuum filtration using Anodisc 47 inorganic membranes with 200-nm pores (Whatman Ltd.). After filtration, the thin films were dried in air for 15-20 min. The sheet resistance of the films was determined by a 302 manual four-point resistivity probe (Lucas Laboratories). To study the optical properties, these films were transferred from the anodisc filter membranes onto PDMS sheets and the sheet transmittance was measured using a Cary 500 UV- vis-NIR spectrophotometer in double-beam mode in the wavelength range of 400-800 nm. The transmittance reported here was corrected by subtracting the absorption of the same thickness PDMS sheet at each wavelength from the measured absorption curves. To make a transparent and highly conductive film, graphene films on quartz were prepared by drop coating. The films were annealed at different temperatures with a Lindberg Blue oven in high-purity Ar. Electrical and optical properties of the annealed films were measured after being cooled to room temperatures.