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Posted: Nov 11, 2014

Optical transmittance of multilayer graphene films

(Nanowerk Spotlight) The search for novel transparent electrode materials with good stability, high transparency and excellent conductivity is driven by the required trade-off between transparency and conductivity: Metals are very conductive but not transparent; plastics are quite transparent but not conductive. However, many optoelectronic applications ideally require electrodes with both, high transparency and high conductivity.
Researchers are confident that graphene may outperform existing transparent conductive materials and early research results support this (read more: "Lithium-doped graphene approaches the limits of transparency and conductivity").
However, monolayer graphene might not be sufficient for fabricating a highly conductive electrode. The dilemma is that the transmittance of graphene film decreases as the number of layers increases.
"It therefore is of great importance to have a fast and reliable method to determine the number of layers in the fabrication and measurement of multilayer graphene," Shou-En Zhu, a PhD researcher in the Precision and Microsystems Engineering Department at Delft University of Technology, tells Nanowerk. "Having a simple relation to determine how many graphene layers can meet the transmittance requirement and provide good conductivity at the same time would be a very valuable tool."
Zhu is first author of a recent paper in EPL (Europhysics Letters) ("Optical transmittance of multilayer graphene") that studies the optical transmittance of multilayer graphene films up to 65 layers thick.
Optical transmittance of CVD multilayer graphene and simulation results
Optical transmittance of CVD multilayer graphene and simulation results. The red dots and blue dots are the experiment data points from multi-stacking. The gray dashed line indicates the theory curve from nonlinear equation. The magenta hollow circles and the black stars indicate the simulation data from ABC- and ABA-stacked multilayer graphene films, respectively. (Reprinted with permission by IOP Publishing)
"By combing large-scale tight-binding simulation and optical measurement on CVD multilayer graphene, we found the optical transmission through graphene films in the visible region to be solely determined by the number of graphene layers," says Zhu. "We show that the optical transmittance measurement is more reliable in the determination of the number of layers than the commonly used the Raman spectroscopy."
He adds that this optical transmittance measurement can be applied also to other 2D materials with weak van der Waals interlayer interaction as well.
In their experiments, the researchers, which included Zhu's colleague professor Guido Janssen and assistant professor Shengjun Yuan from Radboud University of Nijmegen, show that a monolayer of graphene can absorb 2.6% of light at 550 nm wavelength – which is 13% higher than the value of 2.3% reported previously in the visible light.
As a result of their study, the team provides an equation to determine how many layers of graphene films are required for specific applications in the field of transparent electrode and surface coating.
"Our numerical and experimental studies of the optical transmittance in multilayer graphene films show that the nonlinear equation T = (1+1.13παN/2)-2 gives a good description of the light transmittance through multilayer graphene in the visible light range," Zhu summarizes the findings.
In practice, large-scale graphene films or flakes can be transferred or spun coat onto transparent substrate as conductive and transparent electrodes. Both transmittance and electrical conductivity are crucial for this kind of application. This work provides a quick and direct method for determining the number of graphene layers.
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