Article pubs.acs.org/JPCC
Production of Nitrogen-Doped Graphene by Low-Energy Nitrogen Implantation W. Zhao,† O. Höfert,† K. Gotterbarm,† J.F. Zhu,‡ C. Papp,*,† and H.-P. Steinrück†,§ †
Lehrstuhl für Physikalische Chemie II, Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, People's Republic of China § Interdisciplinary Center for Interface Controlled Processes and Erlangen Catalysis Resource Center, Universität Erlangen-Nürnberg, Egerlandstr. 3, 91058 Erlangen, Germany ‡
S Supporting Information *
ABSTRACT: Nitrogen doping of graphene is a suitable route to tune the electronic structure of graphene, leading to n-type conductive materials. Herein, we report a simple way to insert nitrogen atoms into graphene by low-energy nitrogen bombardment, forming nitrogen-doped graphene. The formation of nitrogen-doped graphene is investigated with high resolution X-ray photoelectron spectroscopy, allowing to determine the doping level and to identify two different carbon−nitrogen species. By application of different ion implantation energies and times, we demonstrate that a doping level of up to 0.05 monolayers is achievable and that the branching ratio of the two nitrogen species can be varied.
1. INTRODUCTION Graphene is the 2D carbon network with sp2 hybridization showing no band gap and a linear band dispersion near the Fermi edge, with bands crossing at the so-called Dirac point.1,2 This unique band structure leads to massless Dirac fermion behavior and a half-integer Hall effect.2−4 The ultrahigh carrier mobilityeven at room temperatureresults from the ballistic charge carrier transport properties and makes graphene one of the most promising candidates for the postsilicon semiconductor era.5,6 The production of room temperature field effect transistors from graphene has already been reported.7−9 To use graphene in electronic applications imposes further challenges, e.g., the precise tuning of the band gap and the charge carrier concentration. Doping with nitrogen has been considered as an effective approach to fabricate n-type graphene materials.10−12 Lately, the importance of altering the electronic structure of graphene by nitrogen doping was discussed in experimental13 and theoretical investigations.14,15 The theoretical study indicates that the chemical properties of nitrogendoped graphene, such as doping density and the site of the inserted nitrogen atoms, can dramatically change its physical properties, including the electronic and magnetic characteristics.14 Moreover, nitrogen-doped graphene has been shown to be a promising material in the field of lithium batteries,16 fuel cells,17 and supercapacitors.18 Up to now, various chemical methods to fabricate nitrogen-doped graphene have been reported, including chemical vapor deposition, redox reactions, and nitrogen plasma treatment.9,11,13,18−22 In this work, we use a simple way to make nitrogen-doped graphene. First, we start with a highly ordered single-layer graphene on a Ni(111) surface, which was prepared according to literature.23,24 Subsequently, this graphene layer was exposed © 2012 American Chemical Society
to a beam of low energy nitrogen ions, with energies ranging from 25 to 150 eV. Finally, we annealed the nitrogenbombarded graphene layer to 900 K to allow for a regeneration of the perturbed graphene lattice, leading to nitrogen-doped graphene. The different preparation steps are monitored in situ with high resolution X-ray photoelectron spectroscopy. We are able to identify two different doping sites, namely, 2-fold and 3fold bound nitrogen (see schematic drawing in Figure 1), which are denoted as pyridinic and graphitic nitrogen, respectively. Variation of the ion bombardment conditions, such as ion beam energy and bombardment time results in different doping levels and a different branching ratio between pyridinic and graphitic nitrogen.
2. EXPERIMENTAL SECTION The X-ray photoelectron spectroscopy (XPS) experiments were performed in a transportable ultrahigh vacuum (UHV) apparatus with two chambers, namely, a preparation chamber and an analysis chamber. The analysis chamber is equipped with an electron energy analyzer (Omicron EA 125) and a three-stage supersonic molecular beam, used for preparing the graphene layer. The preparation chamber contains low-energy electron diffraction, further dosing facilities, and a sputtering gun. For a more detailed description of the chamber please see a previous work.25 By use of synchrotron radiation from beamline U 49/2 PGM1 at BESSY II, Berlin, the combined resolution of the XP spectra in the C 1s region is ∼200 meV, at a photon energy of 380 eV, and in the N 1s region it is∼230 Received: October 15, 2011 Revised: January 19, 2012 Published: January 26, 2012 5062
dx.doi.org/10.1021/jp209927m | J. Phys. Chem. C 2012, 116, 5062−5066
The Journal of Physical Chemistry C
Article
eV, at a nitrogen pressure of ∼5.5 × 10−6 mbar, leading to a current density of the ion beam of ∼9−13 μA/cm2.
3. RESULTS AND DISCUSSION In the first step a single layer of graphene was prepared on the Ni(111) surface by exposure of the clean surface to propene at 900 K;23 the corresponding C 1s spectrum is shown as black curve in Figure 2a. Thereafter, this layer was exposed to a 50 eV beam of nitrogen ions at low temperature (
