Optical Materials 107 (2020) 110165 Available online 2 July 2020 0925-3467/© 2020 Elsevier B.V. All rights reserved. Variable angle spectroscopic ellipsometry characterization of turbostratic CVD-grown bilayer and trilayer graphene Grazia Giuseppina Politano a, * , Carlo Vena a , Giovanni Desiderio b , Carlo Versace a, b a Dipartimento di Fisica, Universit� a della Calabria, 87036, Rende, CS, Italy b Licryl CNR/Nanotec c/o Dipartimento di Fisica, Universit� a della Calabria, 87036, Rende, CS, Italy A R T I C L E INFO Keywords: Ellipsometry Bilayer graphene Trilayer graphene Silicon PET Flexible electronics ABSTRACT We report a Variable Angle Spectroscopic Ellipsometry (VASE) characterization of the surface of CVD-grown bilayer and trilayer graphene produced by multiple transfer on SiO 2 /Si and polyethylene terephthalate (PET) substrates. The graphene layers are randomly stacked. The study of the optical properties of single- and few-layer graphene on PET by means of VASE, which has not been published yet, could be useful in the light of novel graphene-based fexible and stretchable electronics applications. The Lorentz models proposed for the optical response of bilayer and trilayer graphene samples ft very well the experimental data. Some interesting properties have been observed. A never-before-reported absorption peak at e3 eV on bilayer and trilayer graphene on SiO 2 /Si is discussed. The absorption peak due to resonant excitons has been found at e 4.4 eV on bilayer graphene on SiO 2 /Si and its value is red-shifted from e4.6 eV in monolayer graphene toe 4.4 eV in bilayer graphene. This peak shift has not been observed on bilayer graphene on PET substrates. 1. Introduction Few-layer graphene has recently attracted much attention due to a broad spectrum of unusual electronic properties and numerous possi- bilities for applications [1]. Bilayer and trilayer graphene are interesting materials especially because of their electrically tunable band gap in presence of a perpen- dicular electric feld [2,3]. Moreover, the crystallographic stacking order in few-layer graphene plays an important role in determining its electronic structure and layer-dependent carrier density [4,5]. Several methods have been used to prepare graphene, including graphite micromechanical exfoliation [6], graphene oxide reduction [7], and Chemical Vapor Deposition (CVD) [8] with the purpose to obtain large areas and more production. Specifcally, CVD is one of the most promising processes to grow high quality mono- and few-layer graphene. Large-scale CVD-grown graphene flms are usually transferred and stacked by a wet process using a polymer as a support material [9]. The resulting stacked graphene is almost entirely turbostratic, i.e. each layer is rotated in a random direction [10]. Even though research on the op- tical properties of few-layer graphene flms with a well-ordered struc- ture have been widely studied [11–14], only few works have investigated the optical properties of turbostratic bilayer and trilayer graphene [15]. One method to study the optical properties of a material is Spec- troscopic Ellipsometry (SE) [16], which is an optical technique with high accuracy and sensitivity. Several research groups have derived the complex refractive index of monolayer graphene by SE [15,17,18]. Matkovi� c et al. [19] have used SE in the visible range to derive the optical constants of graphene using a phenomenological Fano model. Kravets et al. [20] have treated gra- phene as an anisotropic material with arbitrary xy response and a Cauchy response for the z component of the dielectric tensor. Magnozzi et al. [21] have modeled the graphene as the sum of two functions: a Lorentz oscillator and a parametrized function. In addition, previous works [22] have shown that the optical properties of graphene are dependent on the substrate. In our previous work [23], we have studied the optical properties of a CVD-grown monolayer graphene, transferred from a copper substrate onto SiO 2 /Si, using Variable Angle Spectroscopic Ellipsometry (VASE). Moreover, we have carried out VASE studies of graphene-related ma- terials, such as graphene oxide and graphene nanoplatelets thin flms [24,25]. It is worth noticing that only few works have investigated the optical properties of bilayer and trilayer graphene using VASE [15,26]. * Corresponding author. E-mail address: grazia.politano@unical.it (G.G. Politano). Contents lists available at ScienceDirect Optical Materials journal homepage: http://www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2020.110165 Received 21 March 2020; Received in revised form 13 June 2020; Accepted 23 June 2020