Session G31: Focus Session: Van der Waals Interactions in Complex Materials: Bridging Theory and Experiment III

Non-additivity of molecule-surface van der Waals potentials from force measurements

Van der Waals (vdW) forces act ubiquitously in condensed matter. Their description as an inherently quantum mechanical phenomenon was developed for single atoms and homogeneous macroscopic bodies by London, Casimir, and Lifshitz. For intermediate-sized objects like organic molecules an atomistic description is required, but explicit first principles calculations are very difficult since correlations between many interacting electrons have to be considered. Hence, semi-empirical correction schemes are often used that simplify the vdW interaction to a sum over atom-pair potentials. A similar gap exists between successful measurements of vdW and Casimir forces for single atoms on the one hand and macroscopic bodies on the other, as comparable experiments for molecules are absent. I will present experiments in which long-range vdW potentials between a series of related molecules and a metal surface have been determined experimentally. The experiments rely on the extremely sensitive force detection of an atomic force microscope in combination with its molecular manipulation capabilities. The results allow us to confirm the asymptotic force law and to quantify the non-additive part of the vdW interaction which is particularly challenging for theory. In the present case, cooperative effects account for 10{\%} of the total interaction. This effect is of general validity in molecules and thus relevant at the intersection of chemistry, physics, biology, and materials science.   

Determination of Surface-Substrate Adsorption Energy usin 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).   

Trends in Adsorption Characteristics of Organic Molecules on Transition Metal Surfaces: Role of Surface Chemistry and van der Waals Interactions

The accurate description of interface characteristics between organic molecules and metal surfaces has long been debated in theoretical studies. A well-founded description of interface geometry and adsorption energy is highly desirable for these systems. Using first principles calculations with the inclusion of van der Waals interactions, we examine the adsorption characteristics of a few organic molecules on several transition metal surfaces. Our aim is to obtain insights into the role of vdW interactions in the adsorption characteristics as well as to build an understanding on how these functionals treat the adsorption on varying surface chemistries. Furthermore, the comparisons made between the results obtained using different vdW functionals for each organic molecule type provide the means to assess their performance.   

Adsorption characteristics of Thiophene on Cu and Ni(100): role of van der Waals

We apply density functional theory, with and without the inclusion of self-consistent van der Waals (vdWs) interactions (optB86, optB88, optPBE, revPBE, rPW86), to study the adsorption of thiophene (C$_{\mathrm{4}}$H$_{\mathrm{4}}$S) on Cu(100) and Ni(100). Our calculations reveal that the C$_{\mathrm{4}}$H$_{\mathrm{4}}$S molecule adsorbs, on either substrate, with its molecular plane parallel to the surface with the sulfur close to the bridge site. The inclusion of vdWs interactions results in a significant increase in the binding energy of thiophene on Cu(100) (from 0.12 eV to up to 0.77 eV), while the adsorption height is also modified from 3.2 A down to, at most, 2.38 A, depending on the functional used. The Ni(100) case presents a similar behavior for the binding energy (enhancement from 1.56 eV to up to 2.34 eV), but the adsorption heights increase from 2.12 {\AA} up to 2.32 {\AA}. In addition to adsorption geometry and energetics, we present the results and analysis of the electronic properties (charge transfer, changes in the d-band of the substrate, and change in the work function) of these two systems to complement our understanding of the molecule-substrate bonding. Our results suggest that the adsorption characteristics are dependent on the type of functional used; opt-type functionals (optB86, optB88, optPBE) are found to produce stronger bonding as compared to PBE, revPBE and rPW86.   

The Effect of van der Waals Interactions on the Sexithiophene Adsorption on Ag(110)

We use density functional theory to study the adsorption of Sexithiophene (6T) on Ag(110). Special attention is given for exploring the effects of van der Waals interactions on the adsorption geometry and energy using vdW-DF family functionals. The 6T molecule is found to bind to the Ag(110) surface via two orientations, with the long molecular axis parallel to the [001] and [110] directions. Including van der Waals interactions resulted in a substantial increase in the binding energy (from 0.6 eV to 4 eV), while the binding height is slightly modified (from 3.1 {\AA} to 2.75 {\AA}). Both the binding energies and heights show significant variations depending on the vdW functional used: the opt-type functionals (optB86, optB88, optPBE) further enhance the adsorption energy when compared to those obtained using PBE, revPBE, or rPW86 functionals. Upon adsorption, there is a small, however, noticeable broadening and a shift (towards higher binding energy) in the position of the d-band center of the substrate surface atoms is observed. Note that, the absence of charge transfer, interfacial states, changes in the atomic structure of the molecule or the substrate suggests that the bonding characteristic of the 6T/Ag(110) system can be categorized as weak chemisorption or strong physisorption.   

Van der Waals quantum friction and fluctuation theorems

We use general concepts of statistical mechanics to compute the quantum frictional force on an atom moving at constant velocity above a planar surface. We derive the zero-temperature frictional force using a non-equilibrium fluctuation-dissipation relation, and show that in the large-time, steady-state regime quantum friction scales as the cubic power of the atom's velocity. We also discuss how approaches based on Wigner-Weisskopf and quantum regression approximations fail to predict the correct steady-state zero temperature frictional force, mainly due to the low frequency nature of quantum friction.   

Probe the Corrugation and Friction of Cu(111) toward Ne and Xe: First Principles Studies

The interaction between rare-gas (RG) and metal surface is typically described as the sum of two contributions: van der Waals attraction at large RG-metal distances, and Pauli repulsion at short distances. In the repulsive range, RG atoms can see a corrugated or anticorrugated potential surface, depending on the change of charge density profile of the surface atoms. The probe of the corrugation effects near the attractive part is also important since the corrugated or anticorrugated charge distribution at the surface can significantly change the physical properties of the whole system. In this letter, we show that also near the negative potential well of Ne and Xe monolayers on Cu(111), we can observe different surface corrugations: while the potential surface of Ne on Cu(111) is corrugated, it is anticorrugated for Xe/Cu(111). The analyses of electronic properties reveal that the weak hybridization of RG p- and substrate d-states is critical for the surface anticorrugation. Studies of the activation energies along sliding paths imply that Ne motion is much faster than Xe on Cu(111). Density functional calculations with self-consistent nonlocal van der Waals functional were used throughout our studies.   

Water films on transition metal surfaces: A physical disclosure of adsorption energy

Our work reports novel physical models derived from DFT calculations including van der Waals forces for the adsorption of different water motifs: ice bilayer, $\surd $37 x$\surd $37-R 25.3$^{\circ}$ and rosette on transition metal surfaces. This energy decomposition scheme is obtained by analyzing the two driving energies of adsorption: water-water and water-metal interactions. The former explained by single water adsorption and the latter by ice resonance stabilization. These two magnitudes drive, to different extent, the adsorption of ice bilayer and $\surd $37 whereas rosette motif lacks the resonance contribution. The equations successfully reproduce and predict the experimental results for the wettability and the dissociation of water films on the fcc(111) and hcp(0001) facets of Pd, Pt, Ru Ir, Rh, Au, and Ag. So happens for the temperature of the hydrophobic/hydrophilic water film transition and for the effect of the surface roughness on it. Furthermore, the metastability and the wettability of other water films like $\surd $39 x$\surd $39-R 16.1$^{\circ}$ can be anticipated by the rationalization of their geometry and their dissociation state.   

Probing Adsorption Interactions In Metal-Organic Frameworks Using X-ray Spectroscopy and Density Functional Theory

Metal-organic frameworks (MOFs) are currently among the most promising materials for gas separation applications such as carbon capture. We explore the local electronic signatures of molecular adsorption at coordinatively unsaturated binding sites in the metal-organic framework Mg-MOF-74 using X-ray spectroscopy and first principles calculations. \textit{In situ} measurements at the Mg $K$-edge reveal distinct pre-edge absorption features associated with the unique, open coordination of the Mg sites. These spectral features are suppressed upon adsorption of CO$_{2}$ and $N,N'$-dimethylformamide. Density functional theory shows that these spectral changes arise from modifications of local symmetry around the Mg sites upon gas uptake and are strongly dependent on the metal-adsorbate binding strength. Similar sensitivity to local symmetry is expected for any open metal site, making X-ray spectroscopy an ideal tool for examining adsorption in such MOFs.   

Design of a metal-organic framework with enhanced back bonding for the separation of N2 and CH4

Removing dinitrogen, an omnipresent but noncombustible contaminant, from natural gas or other methane-rich gases is an extraordinarily difficult separation based on physical properties alone, as both gases lack a permanent dipole and have similar polarizabilities, boiling points, and kinetic diameters. In this work, by using dispersion-corrected density functionals and wavefunction approaches, we predict a new metal-organic framework (MOF) of potential utility for the highly selective and efficient separation of dinitrogen from methane, a particularly challenging separation of critical value for utilizing natural gas. Selective back bonding interactions from the vanadium(II) cation centers in V-MOF-74 to the unoccupied $\pi$* orbitals of N2 can be used to separate N2/CH4 mixtures. We compare our calculations with the experimentally characterized Fe-MOF-74.   

First-Principles Prediction of Small Molecule Adsorption in MOF-74 Variants

Using density functional theory (DFT), we predict binding energies of flue gas molecules (CO, CO$_{2}$, H$_{2}$O, H$_{2}$S, N$_{2}$, NH$_{3}$, SO$_{2}$, and H$_{2})$ and small hydrocarbons (CH$_{4}$, C$_{2}$H$_{2}$, C$_{2}$H$_{4}$, C$_{2}$H$_{6}$, C$_{3}$H$_{6}$, and C$_{3}$H$_{8})$ in a variety of ``MOF-74'' variants.~ Using a harmonic approximation to compute quantum zero-point and thermal corrections, we compute binding enthalpies for comparison with experimental heats of adsorption.~ Our study is performed using vdW-DF2, a fully nonlocal dispersion-corrected density functional along with Hubbard U corrections on 3$d$-orbital electrons as appropriate.~ We study MOF-74 variants, ``M-MOF-74'', where ``M'' is chosen to be any divalent third-row metal cation (M$=$ Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn).~ Additionally, we study ``M-MOF-74'' systems with ``meta-dobdc'' as the linker (as compared to the traditional ``para-dobdc'').~ We compare with experiment when available and find reasonable agreement. ~We identify trends, and compare with experiment where available, finding excellent agreement. This work supported by DOE through the EFRC on Gas Separations for Clean Energy Technologies; computational resources provided by NERSC.~   

High-throughput screening of small-molecule adsorption in MOF-74

Using high-throughput screening coupled with state-of-the-art van der Waals density functional theory, we investigate the adsorption properties of four important molecules, H$_2$, CO$_2$, CH$_4$, and H$_2$O in MOF-74-$\mathcal{M}$ with $\mathcal{M} =\;$Be, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Zr, Nb, Ru, Rh, Pd, La, W, Os, Ir, and Pt. We show that high-throughput techniques can aid in speeding up the development and refinement of effective materials for hydrogen storage, carbon capture, and gas separation. The exploration of the configurational adsorption space allows us to extract crucial information concerning, for example, the competition of water with CO$_2$ for the adsorption binding sites. We find that only a few noble metals---Rh, Pd, Os, Ir, and Pt---favor the adsorption of CO$_2$ and hence are potential candidates for effective carbon-capture materials. Our findings further reveal significant differences in the binding characteristics of H$_2$, CO$_2$, CH$_4$, and H$_2$O within the MOF structure, indicating that molecular blends can be successfully separated by these nano-porous materials.   

The DeNO$_x$ process and NO$_2$ adsorption in MOF74

Due to the harmful character of NO$_2$ and its slow decomposition rate, the use of catalytic materials for its removal (DeNO$_x$ process) has attracted a lot of attention. The high porosity and highly reactive uncoordinated metal centers of MOF74 have led us to investigate the use of Mg- and Zn-MOF74 as materials for trapping NO$_2$ with resistance to poisoning by SO$_2$. In this combined theoretical and experimental study, we investigate the interaction between the unsaturated metal centers of the MOF and the NO$_2$ guest molecules. For our theoretical modeling we use ab initio calculations at the DFT level, utilizing vdW-DF to capture the significant van der Waals component of the interaction between NO$_2$ and the MOF. We present detailed first-principle results concerning the adsorption energies and geometries, as well as vibration frequencies of the NO$_2$ molecule adsorbed in the MOF. Our experimental efforts (IR and Raman spectroscopy) have shown a blue shift to 1684 cm$^{-1}$ in the vibration stretching mode of the NO$_2$ upon adsorption and a thermal stability up to 150$^{\circ}$C. Our first-principle calculations and experimental results show a remarkable agreement, allowing us to give a complete picture of the adsorption of NO$_2$ molecules in the MOF74 structure.