Study of nitrogen doping of graphene via in-situ transport measurements Rong Zhao a , Tareq Afaneh a , Ruchira Dharmasena a , Jacek Jasinski b , Gamini Sumanasekera a,b,n , Victor Henner c,d a Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292, USA b Conn Center for Renewable Energy, University of Louisville, Louisville, KY 40292, USA c Department of Theoretical Physics, Perm State University, Perm 614990, Russia d Department of Mathematics, Perm Technical University, Perm 614990, Russia article info Article history: Received 6 July 2015 Received in revised form 21 December 2015 Accepted 3 March 2016 Available online 4 March 2016 Keywords: Graphene Nitrogen doping Thermopower Weak localization Magnetoresistance abstract Here we report in-situ monitoring of electrical transport properties of graphene subjected to sequential and controlled nitrogen plasma doping. The nitrogen is presumed to be incorporated in to the carbon lattice of graphene by making covalent bonding as observed by the swinging of the sign of the ther- mopower from (initial) positive to (eventual) negative. Electrical transport properties for nitrogen-doped graphene are believed to be governed by the enhanced scattering due to nitrogen dopants and presence of localized states in the conduction band induced by doping. Our results are well supported by Raman and XPS results. Crown Copyright & 2016 Published by Elsevier B.V. All rights reserved. 1. Introduction As a novel nanomaterial with a single sheet of carbon atoms arranged in a honeycomb lattice, graphene has attracted strong scientific and technological interests [1] since its discovery in 2004 [2]. The fascinating properties of graphene, such as extremely large carrier mobility [3], high thermal conductivity [3], tunable band gap [4] and quantum Hall effect [5], have led to its great applica- tion potentials in the devices such as transistors [6], rechargeable lithium ion batteries [7], ultra-sensitive sensors [8] and super- capacitors [9]. We have previously reported in-situ electrical properties of graphene during covalent functionalization with oxygen [10], hydrogen [11] and fluorine [12]. However, substitu- tional doping of carbon in graphene with appropriate elements can change the physical and chemical properties of graphene in a more dramatic way leading to important device manufacturing. Among all these different doping, nitrogen doping of graphene is expected to introduce additional n-type carriers in graphene sys- tems which is crucial for applications in high frequency semi- conductor devices [13] and enhanced catalysis for energy conversion and storage [14–17]. Numerous approaches have been proposed to synthesize ni- trogen-doped graphene, including nitrogen-containing precursors in chemical vapor deposition (CVD) [18,19], thermal annealing of graphene oxide in ammonia [20] and N 2 /NH 3 plasma treatment [21–23]. In this work we present in-situ electrical transport properties of graphene during plasma assisted nitrogen doping. 2. Experimental Graphene was synthesized by chemical vapor deposition on copper foils using CH 4 gas and transferred to Si/SiO 2 or glass substrates as described in [24,25]. Electrical transport studies were performed on graphene transferred onto glass substrates. Two Chromel (KP/Au  7 at% Fe (Au:Fe) thermocouples and a platinum resistive heater were utilized for thermopower measurements. Two additional copper wires were used for simultaneous 4-probe resistance measurements as described in Ref. [26]. A custom-de- signed split ring capacitively coupled RF plasma system (13.56 MHz, max. power 600 W) was used at room temperature to generate nitrogen plasma. Plasma exposure time was established by in situ monitoring of the change in resistance and thermopower of the sample. For low-temperature measurements, a chip carrier Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B http://dx.doi.org/10.1016/j.physb.2016.03.007 0921-4526/Crown Copyright & 2016 Published by Elsevier B.V. All rights reserved. n Corresponding author at: Department of Physics and Astronomy, University of Louisville, Louisville, KY 40292, USA. E-mail address: gusuma01@louisville.edu (G. Sumanasekera). Physica B 490 (2016) 21–24