nanotubes assisted by thionine as a probe. J Phys Chem C 2010;114(45):19234–8. [6] Liu HP, Nishide D, Tanaka T, Kataura H. Large-scale single- chirality separation of single-wall carbon nanotubes by simple gel chromatography. Nat Commun 2011;2:30911–8. [7] Ghosh S, Bachilo SM, Weisman RB. Advanced sorting of single-walled carbon nanotubes by nonlinear density- gradient ultracentrifugation. Nat Nanotechnol 2010;5(6):443–50. [8] Tu XM, Manohar S, Jagota A, Zheng M. DNA sequence motifs for structure-specific recognition and separation of carbon nanotubes. Nature 2009;460(7252):250–3. [9] Duque JG, Densmore CG, Doorn SK. Saturation of Surfactant Structure at the Single-Walled Carbon Nanotube Surface. J Am Chem Soc 2010;132:16165–75. Probing solid state N-doping in graphene by X-ray absorption near-edge structure spectroscopy Jun Zhong a , Jiu-Jun Deng a , Bao-Hua Mao a , Tian Xie a , Xu-Hui Sun a , Zhi-Gang Mou b , Cai-Hao Hong c , Ping Yang b , Sui-Dong Wang a, * a Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials Laboratory (FUNSOM), Soochow University, Suzhou 215123, China b College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China c Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ARTICLE INFO Article history: Received 25 May 2011 Accepted 22 August 2011 Available online 29 July 2011 ABSTRACT The evolution of solid state N-doping in graphene has been probed using X-ray absorption near-edge structure (XANES) spectroscopy. The XANES spectra show that the modification of graphene with N species can be achieved by urea attachment at annealing temperatures lower than 300 °C. A transition from urea to amino species is observed at 400 °C. At higher temperatures, pyridinic and graphitic type doping are achieved. The results indicate that the electronic structure of graphene can be controlled by solid state treatment, involving different N species depending on the annealing process. Ó 2011 Elsevier Ltd. All rights reserved. Graphene has attracted great interests due to its unique physical and chemical properties, which could be used for numerous potential applications [1,2]. To improve the graph- ene functionality, controllable doping represents an effective way to modulate the electronic properties [3–6]. Nitrogen (N) doping has been frequently used to produce the n-type behavior and to improve the conductivity of graphene [5–7]. The synthesis of N-doped graphene had been reported by adding ammonia (NH 3 ) gas during the chemical vapor deposi- tion (CVD) growth of graphene [5]. Large quantity of N-doped reduced graphene oxide (GO) sheets had also been obtained through the thermal annealing of GO in NH 3 [6]. Considering different N-doping processes, it is significant to identify the doping species, which have a great influence on the electronic structure of graphene. Recently, it has been shown that X-ray absorption near- edge structure (XANES) spectroscopy is an useful tool to char- acterize the electronic structure of graphene [8,10]. XANES is an element-specific spectroscopic technique involving the excitation of electrons from a core level to the empty states, and it is particularly powerful to characterize complex sys- tems and get structural information of different species [11]. The electronic structures of pure graphene and GO have been widely investigated using XANES at the carbon (C) and oxygen (O) K-edges [8,9,12–14]. However, the XANES study on the N- doping in graphene is still limited. Zhang et al. identified the gas-phase N-doping in graphene by N K-edge XANES [10], but there is no information on the reaction at low temperatures such as 200 and 300 °C. Moreover, the XANES results at C and O K-edges have not been sufficiently dis- cussed. X-ray photoelectron spectroscopy (XPS) has also been frequently used for characterizing N-doped graphene [5,6,10], but the XPS spectra are usually composed of overlapped peaks, making the identification of different species quite Available online 30 August 2011 0008-6223/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.08.046 * Corresponding author: Fax: +86 512 65882846. E-mail addresses: pyang@suda.edu.cn (P. Yang), wangsd@suda.edu.cn (S.-D. Wang). CARBON 50 (2012) 321 – 341 335