XPS was also utilized as an approach to investigate the thickness of the PEDOT-graphene composite films. Figure shows XPS peaks arising from the presence of SiO2. The XPS peak resulting from Na in Figure shows the SiO2 is a result of the underlying glass rather than environmental contamination. Because the XPS penetration depth is 10 nm or less, the thickness of the composite film must be less than 10 nm. Based on this, the maximum thickness of 10 nm was determined. This value fit within the range predicted by the previous UVvis studies. Using the conservative value of 10 nm for the composite film's thickness, the conductivity of our PEDOT-graphene film was calculated to be, at minimum, 1070 S/cm. Table compares this result to results in the literature.