Tuning the Work Function of Graphene-on-Quartz with a High Weight Molecular Acceptor C. Christodoulou, †,§ A. Giannakopoulos, ‡,§ M. V. Nardi, † G. Ligorio, † M. Oehzelt, †,∥ L. Chen, ‡ L. Pasquali, ⊥,#,∇ M. Timpel, † A. Giglia, # S. Nannarone, ⊥,# P. Norman, ○ M. Linares, ○ K. Parvez, ◆ K. Mü llen, ◆ D. Beljonne,* ,‡ and N. Koch* ,†,∥ † Institut fü r Physik, Humboldt-Universitä t zu Berlin, Brook-Taylor-Straße 6, 12489 Berlin, Germany ‡ Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, 7000 Mons, Belgium ∥ Helmholtz-Zentrum Berlin fü r Materialien und Energie GmbH, Albert-Einstein-Straße 16, 12489 Berlin, Germany ⊥ Engineering Department “E. Ferrari”, University of Modena e Reggio Emilia, Via Vignolese 905, 41125 Modena, Italy # IOM-CNR, Area Science Park, SS. 14 Km. 163.5, 34149 Basovizza, Trieste, Italy ∇ Department of Physics, University of Johannesburg, P. O. Box 524, Auckland Park 2006, South Africa ○ Department of Physics, Chemistry and Biology, Linkö ping University, SE-58183 Linkö ping, Sweden ◆ Max Planck Institute fü r Polymerforschung, Ackermannweg 10, 55128 Mainz, Germany ABSTRACT: Ultraviolet and X-ray photoelectron spectroscopies in combi- nation with density functional theory (DFT) calculations were used to study the change in the work function (Φ) of graphene, supported by quartz, as induced by adsorption of hexaazatriphenylene−hexacarbonitrile (HATCN). Near edge X-ray absorption fine structure spectroscopy (NEXAFS) and DFT modeling show that a molecular-density-dependent reorientation of HATCN from a planar to a vertically inclined adsorption geometry occurs upon increasing surface coverage. This, in conjunction with the orientation- dependent magnitude of the interface dipole, allows one to explain the evolution of graphene Φ from 4.5 eV up to 5.7 eV, rendering the molecularly modified graphene-on-quartz a highly suitable hole injection electrode. ■ INTRODUCTION Graphene is the two-dimensional hexagonal arrangement of sp 2 - hybridized carbon atoms that attracted unparalleled scientific interest after it was established to be stable in its free-standing form and that its charge carriers are mass-less Dirac fermions. 1−3 Its unique electronic 4 and mechanical properties 5 make this material attractive for electronics, e.g., as transparent electrode. Graphene grown via chemical vapor deposition on copper foil 6 has been proven to provide large-area, highly crystalline sheets that can be easily transferred to any substrate of interest. 7 In the present work we study graphene supported by an insulating transparent substrate, namely quartz, which is an arrangement already used to epitaxially grow perfluoropentacene. 8 Further- more, it is a good candidate to replace the commonly used but low-abundant indium tin oxide (ITO) (on glass or quartz), which, in addition, has the disadvantage of indium and tin diffusion into organic semiconductor layers. 9 To improve the performance of the graphene as an electrode, modification of its work function is required in order to better match the energy levels of common charge transport materials and to achieve low charge injection barriers. One way to increase the work function (Φ) of common metal electrodes to further hole injection is to modify their surface by depositing a molecular acceptor monolayer. 10 This has also been demonstrated to work for graphene, e.g., with the acceptor tetrafluorotetracyanoquinodi- methane (F4TCNQ) 11,12 However, F4TCNQ is not suitable for practical applications due to its low molecular weight (276.15 g/ mol) and thus high volatility even at room temperature (sublimation starts at ∼85 °C). 13 We circumvent the problem of volatility by using instead hexaazatriphenylene−hexacarboni- trile (HATCN), a molecular acceptor with comparably high molecular weight (384 g/mol) and sublimation temperature in vacuum at ∼220 °C, 14 already used as a mediation layer for improving the power efficiency of tandem OLEDs 15 by decreasing the charge injection barrier. First-principles computational techniques provide significant insights into the electronic properties of molecular adsorbates on electrode materials 16,17 and are essential in rationalizing the outcomes of the experiments conducted. Recent first-principles calculations on HATCN/graphene have already indicated a substantial Φ modification and the persistence of large charge carrier mobilities in the resulting doped graphene. 18 We now signi ficantly extend the theoretical modeling efforts to consistently help in understanding the structural and electronic Received: December 14, 2013 Revised: February 11, 2014 Published: February 15, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 4784 dx.doi.org/10.1021/jp4122408 | J. Phys. Chem. C 2014, 118, 4784−4790