Session L16: Focus Session: van der Waals Bonding in Advanced Materials: Molecular Systems and Interfaces

Effect of Substrate Chemistry on the Adsorption of Olympicene Radical: A vdW inclusive DFT study

We investigate the effects of surface chemistry on the adsorption characteristics of the Olympicene radical on Au and Pt(111) surfaces using vdW inclusive density functional theory (DFT) employing the optimized vdW-DF and vdW-DF2 methods. Adsorption characteristics such as adsorption energy, adsorption geometry, electronic structure, charge transfer, and charge redistribution will be presented. The effect of substrate electronic structure on the nature of bonding will be discussed. Comparison on the nature of bonding will be presented with our previously reported results on the adsorption of Olympicene radical on Cu(111) to provide a more complete picture.   

First Principles Investigation of the C3 Coefficients for Molecular Adsorption on Transition Metal Surfaces

C6 coefficients are used to investigate the strength of the long-range interactions for weakly interacting dimers as a function of separation distance. These coefficients are useful both as a measure for the accuracy of the various van der Waals (vdW) inclusive methods as well as reference for use in large-scale molecular dynamics simulations. In the case of molecule-surface interaction, the C3 coefficient is the counterpart to the C6 coefficient that is used for testing the interaction of dimers. We will present the results of the vdW inclusive density functional theory (DFT) calculations evaluating the C3 coefficients for the adsorption of M/X(110) and X(111), with X: Ag, Au, Cu, Pt, Pd, Ni, Rh and M: Benzene, Thiophene, Sexithiophene, Pentacene and Olympicene, as described by the PBE exchange-correlation functional and the self-consistent vdW-DF, optimized vdW-DFs and vdW-DF2 functionals.   

Phonon dispersion of acene molecular crystals using van der Waals-corrected density functional theory

Acene molecular crystals are interesting testbeds for the study of phenomena relevant to organic optoelectronics, including charge separation and carrier transport. In such processes, scattering from lattice vibrations is an important dissipation mechanism. Despite their central role in dissipation processes, there are few calculations of phonon spectra in acene crystals. Here, we carry out van der Waals-corrected density functional theory calculations of the ground-state structure and phonon band structure of acene molecular crystals, comparing to neutron diffraction data where applicable. We use a finite-differences method, and compare the performance of several approaches -- including standard generalized gradient approximations (GGA) such as PBE, PBE plus pair-wise vdW corrections, and vdW density functionals -- to experiments for solid naphthalene and pentacene.   

Probing Molecule-Molecule Interactions Through Atomic Force Spectroscopy

We investigate the role of molecule-molecule interactions at the single-molecule level using a custom high-resolution atomic force microscope (AFM). We perform break-junction measurements using a gold substrate and gold-coated AFM cantilever on a series of methyl-sulfide terminated alkane chains. We measure, simultaneously, two independent quantities for each junction: force and conductance. This gives us insight into junction elongation and rupture processes. We use conductance as a signature of the junction structure and electronic characteristics and use the measured force to understand its mechanical properties. We find that molecular junctions form with one or two molecules bridging the gap between the cantilever and substrate, with the two-molecule junction having roughly twice the conductance of the one-molecule junction. More importantly, we find that the probability to form a two-molecule junction is higher for alkanes with an odd number of carbon atoms indicating that the van der Waals interactions between the two molecules might be important in forming these junctions. We discuss the implications of these results and compare them to those obtained for conjugated molecules.   

Forces and Dynamics in Aromatic Overlayers on Metal Surfaces

Organic thin films have been the subject of intense research because of their suitability for applications in molecular electronics. The beneficial properties of these films are sensitive to the structure of the film. However, predicting and controlling organic thin-film structures is still a significant challenge. Owing to computational requirements, first-principles calculations cannot probe the link between thin-film deposition conditions and film structure. In this talk, we will discuss a multi-scale approach applied to quantify structures and dynamics of a thin film of benzene on Ag(111). Based on first-principles calculations, we developed a force field to describe the interaction of benzene with Ag(111). We applied this force-field to describe several aspects of this system, including its order-disorder phase transition and its desorption kinetics. Despite the apparent simplicity of this vdW dominated system, it exhibits surprising complexity in binding site preference and in ordering, leading to an interesting interplay between pi-conjugated electrons of benzene and surface-state electrons of Ag(111).   

Quantifying molecule-surface interactions using AFM-based single-molecule manipulation

Scanning probe microscopy plays an important role in the investigation of molecular adsorption. Promising is the possibility to probe the molecule-surface interaction while tuning its strength through AFM tip-induced single-molecule manipulation. Here, we outline a strategy to achieve quantitative understanding of such manipulation experiments [1]. The example of qPlus sensor based PTCDA molecule lifting experiments is used to demonstrate how different aspects of the molecule-surface interaction, namely the short-range adsorption potential [2], the asymptotic van der Waals potential [3], local chemical bonds which are the source of the surface corrugation [4], and molecule-molecule interactions [5] can be measured with SPM and interpreted by the help of force-field simulations.\\[4pt] [1] N. Fournier et al., Phys. Rev. B 84, 035435 (2011)\\[0pt] [2] C. Wagner et al., Phys. Rev. Lett. 109, 076102 (2012)\\[0pt] [3] C. Wagner et al., Nature Communications 2014 in press\\[0pt] [4] C. Wagner et al., Beilstein J. Nanotechnol. 2014, 5, 202\\[0pt] [5] M. F. B. Green et al., Beilstein J. Nanotechnol. 2014, 5, 1926.   

Many-Body Dispersion Interactions in Molecular Materials

In this work, we have developed an efficient method for obtaining an accurate theoretical description of van der Waals (vdW) interactions that includes both long-range Coulomb electrodynamic response screening effects as well as treatment of the many-body vdW energy to infinite order. This method goes beyond the standard $C_6/R^6$ pairwise additive approximation and can easily be coupled to a wide array of theoretical methods, ranging from classical force fields to higher-level quantum chemical calculations. To demonstrate the increasingly important role played by many-body vdW interactions in large, structurally complex molecular systems, we use this method to investigate several pertinent molecular properties, such as binding energies/affinities in gas-phase molecular dimers and supramolecular complexes, relative conformational energetics in small polypeptides, and thermodynamic stabilities among competing molecular crystal polymorphs.   

Predicting Elastic Properties of $\beta$-HMX from First-principles Calculations

We investigate the performance of the van der Waals (vdW) functions in predicting the elastic constants of the $\beta$-polymorph of cyclotetramethylene tetranitramine (HMX) energetic molecular crystal using density functional theory (DFT) calculations. We confirm that the accuracy of the elastic constants is significantly improved using the vdW corrections with environment dependent $C_6$ together with PBE and revised PBE exchange-correlation functionals. The elastic constants obtained using PBE-D3(0) calculations yield the most accurate mechanical response of $\beta$-HMX, with compared to the experimental stress-strain data. The PBEsol without vdW corrections can also predict the elastic constants well. Our results suggest that PBE-D3 calculations are reliable in predicting the elastic constants of this material.   

Quantitative bond energetics in atomic-scale junctions with significant van der Waals character

A direct measurement of the potential energy surface that characterizes individual chemical bonds in complex materials has fundamental significance for many disciplines. Here, we demonstrate that the energy profile for metallic single-atom contacts and single-molecule junctions can be mapped by fitting ambient atomic force microscope measurements carried out in the near-equilibrium regime to a physical, but simple, functional form.[1] In particular we are able to extract bond energies for metal-molecule link bonds in cases where the interaction has significant contribution from nonspecific interactions attributed to van der Waals (vdW) interactions at short length scale in addition to specific donor-acceptor bonds.[2] Our approach significantly expands the quantitative information extracted from these measurements, allowing direct comparisons to density functional theory (DFT) calculations instead of relying on trends in bond rupture forces alone. [1] S.V. Aradhya, A. Nielsen, M.S. Hybertsen, L. Venkataraman, ACS Nano 8, 7522--7530 (2014) [2] S.V. Aradhya, M. Frei, M.S. Hybertsen, L. Venkataraman, Nature Materials, 11, 872-876, (2012)   

Charge transfer from first principles: self-consistent GW applied to donor-acceptor systems

Charge transfer in donor-acceptor systems (DAS) is determined by the relative alignment between the frontier orbitals of the donor and the acceptor. Semi-local approximations to density functional theory (DFT) may give a qualitatively wrong level alignment in DAS, leading to unphysical fractional electron transfer in weakly bound donor-acceptor pairs. $GW$ calculations based on first-order perturbation theory ($G_0W_0$) correct the level alignment, but leave unaffected the electron density. We demonstrate that self-consistent $GW$ (sc$GW$) provides an ideal framework for the description of charge transfer in DAS. Moreover, sc$GW$ seamlessly accounts for many-body correlations and van der Waals interactions. As in $G_0W_0$, the sc$GW$ level alignment is in agreement with experimental reference data. However in sc$GW$, also the electron density is treated at the $GW$ level and, therefore, it is consistent with the level alignment between donor and acceptor leading to a qualitatively correct description of charge-transfer properties.   

Van der Waals Interactions in Aspirin

The ability of molecules to yield multiple solid forms, or polymorphs, has significance for diverse applications ranging from drug design and food chemistry to nonlinear optics and hydrogen storage. In particular, aspirin has been used and studied for over a century, but has only recently been shown to have an additional polymorphic form, known as form II. Since the two observed solid forms of aspirin are degenerate in terms of lattice energy, kinetic effects have been suggested to determine the metastability of the less abundant form II. Here, first-principles calculations provide an alternative explanation based on free-energy differences at room temperature. The explicit consideration of many-body van der Waals interactions in the free energy demonstrates that the stability of the most abundant form of aspirin is due to a subtle coupling between collective electronic fluctuations and quantized lattice vibrations. In addition, a systematic analysis of the elastic properties of the two forms of aspirin rules out mechanical instability of form II as making it metastable [A. M. Reilly and A. Tkatchenko, Phys. Rev. Lett. 113, 055701 (2014).]   

Determination of Surface-Substrate Adsorption Energy using the Exchange-Hole Dipole Moment

Calculated surface-substrate binding energies are usually underestimated because conventional density functionals do not include dispersion, which is necessary to capture the van der Waals interactions that lead to weak physiadsorption. The exchange-hole dipole moment (XDM) model is a non-empirical density-functional approach to model dispersion. Adsorption energies for several aromatic molecules and nuclebases on noble metal surfaces were calculated using B86bPBE-XDM. In this talk, I compare the calculated adsorption energies with experiment and present implications for future applications to modeling surface interactions. \\[4pt] [1] A.\ Otero-de-la-Roza and E.\ R.\ Johnson, \textsl{J.\ Chem.\ Phys.} \textbf{138} 204109 (2013).\\[0pt] [2] A.\ Otero-de-la-Roza and E.\ R.\ Johnson, \textsl{J.\ Chem.\ Phys.} \textbf{137} 054103 (2012).\\[0pt] [3] A.\ Otero-de-la-Roza and E.\ R.\ Johnson, \textsl{J.\ Chem.\ Phys.} \textbf{136} 204109 (2012).   

Van der Waals Dispersion Interactions and Excited States of Oligoacene Molecular Crystals

Molecular crystals are a prototypical class of van der Waals (vdWs)-bound organic materials with novel excited state properties relevant for photovoltaics applications. Predicting the structure and excited state properties of oligoacene crystals presents a challenge for standard density functional theory (DFT), as standard functionals do not have long-range dispersion, and DFT does not yield excited-state properties. In this work, we use a combination of vdW-corrected DFT -- both pair-wise correction methods and correlation functionals -- and many-body perturbation theory to study the geometry and excited states of oligoacene crystals. We find that vdWs methods can predict lattice constants up to 1\% of the experimental measurements. Low lying excited states computed with MBPT compare well with experiments, and are found to be quite sensitive to geometry. Our study reveals the importance of vdWs dispersion interactions to the determination of excited states; moreover, our work suggests routes for predictive calculations, in which both structures and excited states are calculated entirely from first-principles.