Bottom-up Fabrication of Graphene on Silicon/Silica Substrate via a Facile Soft-hard Template Approach

In this work, a novel soft-hard template method towards the direct fabrication of graphene films on silicon/silica substrate is developed via a tri-constituent self-assembly route. Using cetyl trimethyl ammonium bromide (CTAB) as a soft template, silica (SiO2) from tetramethoxysilane as a hard template, and pyrene as a carbon source, the self-assembly process allows the formation of a sandwich-like SiO2/CTAB/pyrene composite, which can be further converted to high quantity graphene films with a thickness of ~1 nm and a size of over 5 μm by thermal treatment. The morphology and thickness of the graphene films can be effectively controlled through the adjustment of the ratio of pyrene to CTAB. Furthermore, a high nonlinear refractive index n2 of ~10−12 m2 W−1 is measured from graphene/silica hybrid film, which is six orders of magnitude larger than that of silicon and comparable to the graphene from chemical vapor deposition process.


Experimental
All the chemicals are purchased from Sinopharm chemical reagent Co., Ltd and used without further purification.
To obtain graphene without silica, the silica frameworks were removed using aqueous HF (10 %), and then dialyzed thoroughly with Millipore water.
For the preparation of graphene/silica hybrid films on silicon and quartz, the above obtained sol (0.1 mL) was spin-coated on the substrate with the size of 1.5 cm x 1.5 cm for 1 minute with a coating spin speed of 3000 rpm. A heat process (100 °C, 24 hours) and then thermal treatment of the resulting films in a nitrogen flow at 900 °C for 2 hours led to the hybrid film on corresponding substrates.

Characterization:
Small-angle X-ray diffraction (XRD) patterns were obtained by a X-Ray Polycrystaline The thickness of graphene was identified by atomic force microscopy (AFM, Multimode Nanoscope III, Bruker, Germany). The samples for AFM measurement were prepared by spotted onto freshly cleaved mica surface and dried in air (~20 minutes). The samples were imaged in air by tapping mode under ambient conditions. Dispersive Raman microscope (Senterra R200-L, Bruker Optics, Germany), ultraviolet-visible (UV/Vis) spectrophotometer (J-4100, Hitachi, Japan) and Z-scan S3 (School of Physical Science and Technology, Soochow University) measurements were used to investigate the properties of graphene/silica hybrid film on quarz. The excitation wavelength of Raman spectra is 532 nm at room temperature. The Z-scan measurement setup has been used as previously reported S1 .
The sample is subjected to pulses emitted form a Nd:YAG picoseconds laser with center wavelength 532 nm, pulse duration 21 ps (full width at half-maximum, FWHM), and repetition rate 10 Hz. The two regimes: open aperture and closed aperture are performed in our experiment. The sensitivity to nonlinear refraction is entirely due to the aperture and removal of the aperture completely eliminates the effect S2 . Therefore, nonlinear absorption coefficients can be extracted from open-aperture experiments at first. Then closed-aperture measurement was carried out. Upon dividing by the open-aperture curve, the nonlinear refractive index was obtained. The up-down curve suggests that a small positive on-axis phase shift ∆Φ (i.e. self-focusing occurs). The magnitude of the phase shift can be determined from the change in normalized transmittance between peak and valley. In the limit of small nonlinear phase change and the far-field condition d >> z0, the curve can be fitted by equation (1): where x=z/z0 is the normalized distance from the focus and z0 is the Rayleigh length.
The nonlinear refractive index n2 can be calculated using the equation (2): where k0 = 2π/λ , L is the sample thickness and I is the peak intensity of the laser. S3 S4 Figure S1. FT-IR spectra of (a) PY-2,(b) GS-2, and (c) the graphene obtained from GS-2 after removal silica component.