Density functional theoretical study of pentacene/noble metal interfaces with van der Waals corrections: Vacuum level shifts and electronic structures

Abstract

In order to clarify factors determining the interface dipole, we have studied the electronic structures of pentacene adsorbed on Cu͑111͒, Ag͑111͒, and Au͑111͒ by using first-principles density functional theoretical calculations. In the structural optimization, a semiempirical van der Waals ͑vdW͒ approach ͓S. Grimme, J. Comput. Chem. 27, 1787 ͑2006͔͒ is employed to include long-range vdW interactions and is shown to reproduce pentacene-metal distances quite accurately. The pentacene-metal distances for Cu, Ag, and Au are evaluated to be 0.24, 0.29, and 0.32 nm, respectively, and work function changes calculated by using the theoretically optimized adsorption geometries are in good agreement with the experimental values, indicating the validity of the present approach in the prediction of the interface dipole at metal/organic interfaces. We examined systematically how the geometric factors, especially the pentacene-substrate distance ͑Z C ͒, and the electronic properties of the metal substrates contribute to the interface dipole. We found that at Z C Ն 0.35 nm, the work function changes ͑⌬'s͒ do not depend on the substrate work function ͑ m ͒, indicating that the interface level alignment is nearly in the Schottky limit, whereas at Z C Յ 0.25 nm, ⌬'s vary nearly linearly with m , and the interface level alignment is in the Bardeen limit. Our results indicate the importance of both the geometric and the electronic factors in predicting the interface dipoles. The calculated electronic structure shows that on Au, the long-range vdW interaction dominates the pentacene-substrate interaction, whereas on Cu and Ag, the chemical hybridization contributes to the interaction.

Charging energy and barrier height of pentacene on Au(111): A local-orbital hybrid-functional density functional theory approach

“…The experimental evidence 19,48,49 indicates that this interface dipole is 0.95 eV, in good agreement with our results. Morikawa et al 20 have calculated an interface dipole of 1.19 eV for a 6 × √ 7 pentacene/Au(111) structure and a pentacene-Au distance of 3.2 Å; the interface dipole and the metal/organic distance are both in good agreement with our calculations.…”

“…In a second step, we calculate the WCI (thin red curve) and add the C 6 /r 6 -like van der Waals energy (thin green curve) to obtain the total energy (thick green curve). Therefore, we obtain an equilibrium distance of around 3.2 Å, similar to the one found in other calculations, 20 and a binding energy of 1.8 eV per molecule. This energy overestimates by ∼ 0.7 eV the experimental value of ∼ 1.1eV, 38 probably due to the approximation used for the factor f D (R).…”

“…As mentioned in that reference, using a more complete basis set changes significantly the molecular levels; however, the shift operator and the hybrid potential (or the scissor operator) used in our calculations allow us to fit the molecule energy gap and its energy position to the experimental values: these corrections make our calculations of the charging energy and the interface dipole reasonably well-converged quantities (this is checked independently by the good agreement found with the calculations of Morikawa et al ). 20 An exception to this conclusion is the calculation of the pillow dipole that we find to depend largely on the atomic basis set; as discussed in Sec. III A, for an extended basis set the pillow potential for a full monolayer is 0.25 eV, a value that is largely compensated by the surface dipole correction.…”

“…; we use this one because at lower coverages pentacene molecules tend to have larger spacings between the rows of molecules such as in the chosen type. 19 Other authors have analyzed theoretically the 6 × √ 7 geometry using the density functional theory (DFT) techniques within generalized gradient approximation (GGA); 20 for the sake of comparison, we have also calculated this geometry [ Fig. 1(b)].…”

The role of the van der Waals interactions in the adsorption of anthracene and pentacene on the Ag(111) surface

“…6, in agreement with experimental observations 10,45,46 . Our results are also in good agreement with previous theoretical studies: from PBE calculations we found d = 3.938Å and E ads = −0.119 eV, which compare well with previous GGA results (d = 3.7-4.12Å and E ads within the range of −0.078 to −0.108 eV) 47,48 ; using the PBE+vdW surf method we found d = 2.910Å and E ads = −2.396 eV, which is in good agreement with previous results obtained by Toyoda et al 47 using the pairwise DFT-D method (d = 2.9Å and E ads = −2.28 eV), but differ from the values obtained by the same authors using the nonlocal vdW-DF method (d = 3.7Å and E ads = −1.62 eV) 47 . It should be pointed out that the vdW-DF method tends to overestimate the adsorption heights, even though it provides reliable adsorption energies 12,22 -recent vdW-DF functionals (e.g., optB86b-vdW and rev-vdW-DF2), however, have been shown to give both accurate adsorption height and energy 49 .…”