Optical and Electrical Properties of Spin Coated Bilayer Graphene

Abstract

Graphene is a 2D material with high transparency, high carrier mobility, and high electrical conductivity [1-4]. In this work, a thin graphene layer was coated on Si through spin coating, and dip coating technique.Graphene ink was prepared by expanding the graphite flakes through microwave irradiation followed by ultrasonic liquid phase exfoliation. 5 mg of the resulting graphene was dispersed in 1 ml of IPA through a probe sonication technique to obtain the graphene suspension/ink. To form the graphene/Si junction, pieces of 3 x 3 cm2 of n-type Si were used. In addition, same size of fused silica were used to measure the reflectance of deposited graphene. The graphene layer in the first sample was formed by spin coating 40 µl of graphene ink on the Si and the fused silica at a speed of 200 rpm for 40 s. The second sample was prepared by dip coating technique, where 13 ml of the graphene ink was added to 47 ml of IPA, followed by dipping Si and the fused silica pieces with a speed of 20 mm/min till they immersed completely and were kept for 1 min and were moved up at the same speed and dried at room temperature. This dip-coating process was repeated 10 times for each sample.The morphology of graphene-coated area was analyzed using the SEM technique. The SEM images of the spin-coated shows that the surface is fully covered with graphene flakes of different sizes stacked on each other with varied orientations, this shows the ability to obtain a continuous coating of graphene by spin coating. Also, Hall effect measurements were performed, and it showed that the conductivity of the Si substrate was 1.95x10-2 ohm-1cm-1, and it increased to 2.74 ohm-1cm-1 after adding the graphene. Finally, the reflectance of the graphene on fused silica was measured using a UV-VIS spectrophotometer, and it showed that the reflectance decreased from 8% to 4% in the range from 360 to 860 nm after adding the graphene.For dip-coated sample, the SEM image shows some voids, and empty areas linked with by graphene structure with a width of few microns, this shows that the dip-coating process needs to be optimized to get a full coverage of the surface. The electrical measurement of this sample showed that the conductivity increased to 2.66 ohm-1cm-1 after the dip coating of graphene. For the reflectance measurements, it was observed that the graphene has decreased the reflectance to 6-7% in the range from 360 to 860 nm.Finally, spin coating at higher speed, and less amount of graphene can be tested, to obtain a thinner graphene layer. For the dip coating process, higher graphene concentration in IPA, and longer immersion time will be investigated to get a better coverage.