Article pubs.acs.org/JPCC
Interaction between Coronene and Graphite from TemperatureProgrammed Desorption and DFT-vdW Calculations: Importance of Entropic Effects and Insights into Graphite Interlayer Binding John D. Thrower,* Emil E. Friis, Anders L. Skov, Louis Nilsson, Mie Andersen, Lara Ferrighi, Bjarke Jørgensen, Saoud Baouche,† Richard Balog, Bjørk Hammer, and Liv Hornekær Department of Physics and Astronomy and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 Aarhus C, Denmark ABSTRACT: The adsorption of polycyclic aromatic hydrocarbon (PAH) molecules on graphitic surfaces provides a model system with which to investigate weak van der Waals (vdW) interactions. There are few experimental investigations of either the interaction between large PAH molecules and graphite or the binding between graphite layers. Determining the adsorption energy in these molecular systems provides a valuable benchmark for validating theoretical methods for implementing van der Waals interactions and, hence, also a means to investigate the interlayer binding in graphite. Here, we investigate the interaction between the coronene molecule and highly oriented pyrolytic graphite by using temperatureprogrammed desorption. We show how entropic effects play an important role in governing the desorption kinetics for large molecules such as coronene and must be taken into account in order to derive a realistic binding energy. DFT calculations demonstrate that the optB88-vdW functional is able to reproduce the experimentally derived binding energy. We use our experimental value to estimate the interlayer binding energy in graphite, considering the effect of intermolecular interactions found in the molecular system. The resulting value is again well reproduced by the optB88-vdW functional, indicating that the optB88-vdW functional is well-suited to describe the interaction between systems dominated by graphitic vdW interactions. significantly,7 since they do not properly account for vdW interactions. The M06-L fitted functional,8 despite being semilocal, has been proposed to give a binding of 41 meV/C for graphite.9 The early version of vdW-DF was found to underestimate the interlayer binding with a value of 24 meV/ C.10 Recently, however, the development of vdW density functionals has made great improvements. The newly developed vdW functionals such as the optB88-vdW and optPBE-vdW11 have been shown to reproduce the high-level results well, giving values for the binding in the range of 60−65 meV/C.12 Still, there are limited experimental investigations with which to benchmark these theoretical studies. Girifalco et al. obtained a value of 43 ± 5 meV/C atom from analyzing heat of wetting data,13 while Benedict et al. found a value of 35 ± 15 10 meV/C atom by considering the collapse of carbon nanotubes.14 More recently, Lui et al. examined the deformation of a graphene flake sitting across a graphite step with AFM to obtain an interlayer binding energy of 31 ± 2 meV.15 It is desirable to obtain a more direct measure by investigating, for example the binding between a smaller graphene flake and the graph-
1. INTRODUCTION Noncovalent bonding is important in the binding of many systems and governs molecular interactions in fields as diverse as biochemistry1 and molecular electronics.2 Aromatic molecules often play important roles in such systems, and an understanding of the interactions between these molecules is necessary. Noncovalent bonding between large aromatic systems at short intermolecular distances involves π−π interactions in which dispersion forces dominate.3 Graphite exemplifies such weak interactions, being a highly anisotropic layered structure with strong covalent sp2 bonds between carbon atoms within each plane and weak van der Waals (vdW) interactions between the planes. Over the past decade there has been renewed interest in graphitic materials, largely driven by the isolation of graphene, a single graphitic layer, with its remarkable mechanical and electronic properties.4 The theoretical determination of the strength of vdW interactions is frequently challenging, with calculations yielding a large spread in energies for the interlayer binding of graphite, depending on the level of theory chosen. High-level calculations such as the random phase approximation and quantum Monte Carlo methods give results in the range of 48− 56 meV/C atom.5,6 Standard DFT functionals such as the local density approximation and the generalized gradient approximation functionals underestimate the interlayer binding © XXXX American Chemical Society
Received: April 29, 2013 Revised: June 7, 2013
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dx.doi.org/10.1021/jp404240h | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
ments were also used to monitor the quality of the HOPG substrate, of which there was no noticeable decrease following multiple coronene exposures. Coronene (C24H12; SigmaAldrich, sublimed 99%) was deposited on the HOPG surface using a home-built Knudsen cell type evaporation source held at 165 °C. The coronene was degassed thoroughly prior to the measurements and the purity assessed by directly dosing into the quadrupole mass spectrometer (QMS). No contaminants, with the exception of expected fragments associated with 13C species, were detected. The evaporator was moved close to the substrate during film deposition in order to reduce the amount of coronene deposited on the sample holder. The efficiency of the water cooling resulted in any small coronene desorption peak from the mount being at a significantly higher temperature than that from HOPG and could therefore be removed easily during the baselining procedure. In all thermal desorption experiments a temperature ramp of 1 K s−1 was used, and desorbing species were detected using a differentially pumped QMS (Extrel CMS LLC) with a cross-beam ionization source. All thermal desorption measurements reported here present ion signals for the coronene parent ion (C24H12+) with a mass-tocharge ratio (m/z) of 300. Scanning tunneling microscopy (STM) measurements were performed in a second UHV chamber equipped with the Aarhus STM.19 The HOPG was mounted in a similar way to that used for thermal desorption measurements, and coronene was dosed using a second evaporation source. Coronene was dosed onto the sample prior to transfer to the STM where images were obtained with the sample held at 120 K. STM images were postprocessed using the WSxM software package.20 2.2. Theoretical Methods. The density functional theory calculations were performed using the optB88-vdW functional11 for the description of exchange-correlation effects. The projector-augmented wave (PAW) method, as implemented in the real space grid-based method, GPAW,21 was used for the handling of the Kohn−Sham wave functions. The graphite surface was modeled by two layers of graphene in AB stacking. The addition of one more layer of graphene has been tested, and it does not affect the binding energies of the coronene molecule. For calculating the binding energy of the isolated molecule a (7 × 7) graphite cell with (2 × 2) k-points was used, whereas for the dimer complexes a (8 × 11) cell was used with (2 × 1) k-points. For the monolayer structures (2 × 2) k-points were used both for the (5 × 5) cell and for the slightly smaller rotated cells. 2D periodic boundary conditions were employed parallel to the surface, and a vacuum region of 6 Å separated the slab from the cell boundaries perpendicular to the surface. The grid spacing was 0.18 Å. The structures were relaxed until the maximum force on every atom was below 0.02 eV/Å. The optB88-vdW optimized graphene lattice constant of 2.465 Å was used throughout the study, except for the calculations comparing different exchange-correlation functionals where the individually optimized lattice constant was used for each functional. This value compares well to the experimentally determined graphite lattice constant of 2.46 Å.22 For the calculation of binding energies zero-point energies were not taken into account since a previous study on a similar system, namely the benzene crystal, found that this correction amounts to only 10−35 meV.23
ite(0001) surface. For this purpose polycyclic aromatic hydrocarbons (PAHs), as hydrogenated graphene flakes, provide a suitable model system. The interaction between PAHs and graphene is also of fundamental interest in terms of modifying the properties of graphene. DFT calculations have shown that the adsorption of PAH derivatives can lead to n- or p-doping of graphene while O- and N-substituted PAHs can lead to the opening of a band gap through the mixing of adsorbate and graphene orbitals.16 By studying their thermal desorption from highly oriented pyrolytic graphite (HOPG), Zacharia et al. determined the binding energy of several PAH molecules from which they derived a graphite interlayer binding of 52 ± 5 meV/C atom.17 In that study, for larger PAHs they were unable to clearly distinguish between desorption from multilayers and the monolayer adsorbed directly on the graphite surface. In addition, the analysis of thermal desorption kinetics necessary for extracting the binding energy is complicated for large molecules as a result of the increasing difficulty of estimating the pre-exponential factor. This arises as a result of the increase in entropy associated with the increasing number of molecular degrees of freedom. Thus, extracting the desorption energy requires careful consideration of the entropy of the system. In this article we revisit the interaction between PAHs and graphite by investigating the adsorption of coronene (C24H12) on HOPG and its thermal desorption through temperatureprogrammed desorption (TPD). In contrast to the previous study,17 we observe a distinct desorption peak for desorption of the monolayer, confirmed through scanning tunneling microscopy (STM) measurements. The monolayer exhibits desorption kinetics that are consistent with the transition from a highly mobile 2D gas at low coverage to a more hindered state at saturation coverage. By considering transition state theory and a recent empirical approach for determining preexponential factors, we determine a coronene/HOPG binding energy of 1.8 ± 0.05 eV, significantly higher than that found previously. We also perform DFT calculations using the optB88-vdW functional which yields a coronene binding energy of 1.9 eV, in good agreement with the experimentally determined desorption energy. Our experimental values allow us to estimate the graphite interlayer binding to be 57 ± 4 meV/C atom, a value that is well reproduced by the optB88vdW functional.
2. METHODS 2.1. Experimental Methods. Thermal desorption measurements were conducted in an ultrahigh vacuum (UHV) chamber in which a base pressure of
