Third-order optical nonlinearity of N-doped graphene oxide nanocomposites at different GO ratios Salimeh Kimiagar a , Fahimeh Abrinaei b, * a Nano Research Lab (NRL), Department of Physics, Central Tehran Branch, Islamic Azad University, Tehran, Iran b Department of Physics, East Tehran Branch, Islamic Azad University, Tehran, Iran article info Article history: Received 17 January 2018 Received in revised form 20 February 2018 Accepted 18 March 2018 Keywords: Nonlinear optical response Nitrogen-doped graphene oxide nanocomposites Z-scan technique abstract In the present work, the influence of GO ratios on the structural, linear and nonlinear optical properties of nitrogen-doped graphene oxide nanocomposites (N-GO NCs) has been studied. N-GO NCs were synthesized by hydrothermal method. The XRD, FTIR, SEM, and TEM results confirmed the reduction of GO by nitrogen doping. The energy band gaps of N-GO NCs calculated from UVeVis analyzed by using Tauc plot. To obtain further insight into potential optical changes in the N-GO NCs by increasing GO contents, Z-scan analysis was performed with nanosecond Nd-YAG laser at 532 nm. The nonlinear ab- sorption coefficient, b, and nonlinear refractive index, n 2 , for N-GO NCs at the laser intensity of 113MW/ cm were measured and an increase was observed in both parameters after addition of nitrogen to GO. The third-order nonlinear optical susceptibilities of N-GO NCs were measured in the order of 10 9 esu. The results showed that N-GO NCs have negative nonlinearity which can be controlled by GO contents to obtain the highest values for nonlinear optical parameters. The nonlinear optical results not only imply that N-GO NCs can serve as an important material in the advancing of optoelectronics but also open new possibilities for the design of new graphene-based materials by variation of N and GO ratios as well as manufacturing conditions. © 2018 Published by Elsevier B.V. 1. Introduction Researchers have shown that doping of graphene with hetero- atoms like nitrogen, boron, and phosphorus can lead to great evolutions in its electronic, chemical, optical, and magnetic prop- erties [1e4]. Among heteroatoms, nitrogen can be more beneficial for two reasons. First, the atomic radii of nitrogen and carbon are close to each other. Second, nitrogen can form the strong valence bonds with carbon atoms due to the presence of 5 valence electrons [5]. Doping of graphene with nitrogen can greatly improve its electrical properties [6]. It can be introduced into the hexagonal carbon network as the graphitic nitrogen (substitution a single C atom by N), the pyridinic nitrogen (substitution by a vacancy as a neighbor, contribution of one p electron to the p system both with sp 2 hybridization) and the pyrrolic nitrogen (contribution of two p electrons to the p system) [7 ,8]. In general, doping of graphene by nitrogen can transform its applications in various fields, such as photocatalyst, lithium battery, molecular sensing and fuel cells [9e12]. Recently, the employment of nitrogen-doped graphene for the nonlinear optical applications has attracted the attention of researchers [13, 14]. This paper presents a glance of the nonlinear optical (NLO) properties of N-GO NCs prepared by using the hydrothermal method. UVeVis absorption spectroscopy is utilized to calculate the optical band gap (E g ) and the linear absorption coefficient (a) of N- GO NCs. X-ray diffraction (XRD) illustrates the formation of the structure for N-GO NCs. Also, the Fourier transform infrared (FTIR) spectra and scanning electron microscopy (SEM) are measured. The NLO properties of N-GO NCs are investigated using a single beam Z- scan technique at a wavelength of 532 nm. The measurements are performed for both open and closed-aperture Z-scan setup. 2. Experimental detail 2.1. Materials synthesis All materials were bought from Merck Company. GO was pre- pared using modified Hummers method which has been described * Corresponding author. E-mail address: f.abrinaei@iauet.ac.ir (F. Abrinaei). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat https://doi.org/10.1016/j.optmat.2018.03.030 0925-3467/© 2018 Published by Elsevier B.V. Optical Materials 79 (2018) 120e128