Session D31: Focus Session: van der Waals Bonding in Advanced Materials: Applications to Advanced and Functional Materials

Rotational and vibrational excitations of van der Waals bonded hydrogen in nanoporous materials: calibrating first-principle calculations with experiments

The adsorption of H$_2$ within a metal-organic framework is studied via van der Waals density-functional calculations and maximally-localized-Wannier- function analysis. The calculated low-lying vibrational and rotational energy states as well as the adsorption sites are consistent with experiments. The induced dipole due to H$_2$ bond stretching and its quantum mechanic matrix element is found to be accurately given by a first-principles driven approximation. The resulting calculations of IR intensity explain the experimentally mysteriously missing primary line for para hydrogen. The strengths and positions of lines in the complex spectra of rotational-vibrational transitions are in reasonable agreement with experiment, and a selection rule is obtained.   

CO2 Binding in Zeolitic Imidazolate Frameworks from First Principles Calculations

Zeolitic Imidazolate Frameworks (ZIFs) are excellent candidate carbon capture materials owing to their high surface area, selectivity, and stability. In this work we use electronic-structure based methods to investigate the binding of CO2 in a set of ZIFs that share the same topology but feature different functionalized linkers [1]. Since a large portion of the CO2 binding comes from van der Waals (vdW) forces, we explore several different schemes for incorporating these contributions into ab-initio density-functional-theory (DFT) including vdW-DFT [2]. The results are combined with those of classical simulation studies to allow comparisons between calculations and experimentally measured values of the heat of adsorption and adsorption isotherms [1]. This research is supported by the Energy Frontier Research Center ``Molecularly Engineered Energy Materials,'' funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001342. \newline [1] W. Morris, B. Leung, H. Furukawa, O. K. Yaghi, N. He, H. Hayashi, Y. Houndonougbo, M. Asta, B. B. Laird, and O. M. Yaghi, J. AM. CHEM. SOC. 132, 11006-11008 (2010) \newline [2] M. Dion, H. Rydberg, E. Schroder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Let. 92, 246401 (2004)   

The Importance of van der Waals Interactions in the Stability of the Phases of Mg(BH$_4$)$_2$

As hydrogen gains attention as a potential replacement for fossil fuels, materials to store hydrogen safely and efficiently are becoming increasingly important. Metal borohydrides are attracting much attention for this role and, in particular, Mg(BH\(_4\))\(_2\) is a promising candidate for hydrogen storage because of its relatively high hydrogen content (over 12 wt\%) and the abundance of its constituent elements. This system has been investigated previously, both experimentally and via density functional theory (DFT) studies. These two approaches give conflicting results, however, regarding the identity of the low-temperature ground-state. In this work, we investigate the impact of van der Waals (vdW) interactions on the stability of various phases of Mg(BH\(_4\))\(_2\). vdW interactions are included both through the fully self-consistent vdW-DF approach as well as the semi-empical PBE-D/PBE-D* approach. Our results settle the longstanding discrepancy between theory and experiment, as we find inclusion of vdW interactions stabilizes the experimentally determined ground-state structure at low temperature, relative to those predicted by previous DFT studies.   

Influence of van der Waals contact forces on the deformation mechanics of thin flexible membranes assembled from metallic or semiconducting single-wall carbon nanotubes

Thin membranes of single-wall carbon nanotubes (SWCNTs) assembled from either metallic or semiconducting SWCNTs are subjected to the compressive strains imposed by a stretched elastic substrate, and the mechanical characteristics of the membranes are inferred from the topography of the wrinkling instability that emerges. By depositing comparable films on quartz, we also use optical (UV-Vis-NIR) absorption spectroscopy to compute the effective London dispersion spectra of the purified materials, and from these we compute the attractive part of the van der Waals potential between nanotubes of identical electronic type as a function of separation and relative orientation. We find significant differences in the strength and shape of the contact potential depending on electronic type, which in turn are evident in the modulus and yield strain measured from the deformation of the films.   

Electrostatic Origin of Meandering C60 Chain Formation at ZnPc Interfaces

We present STM investigations of interface-formation and nanophase separation in binary films of zinc phthalocyanine (ZnPc) and C$_{60}$ on Ag(111) and Au(111) supports. We report ZnPc:C$_{60}$ 1-D and 2-D interfaces with distinctive molecular orientations and unusually low C$_{60}$ packing densities. Meandering C$_{60}$ chains of single-molecular width arise without registration to the underlying ZnPc template, islanding into a disordered chain phase. These structures are reminiscent of dipole fluids (albeit of single molecular widths!) We present detailed measurements and analysis of C$_{60}$ wandering chain formation on ZnPc/Ag (111) and ZnPc/Au (111) substrates. We explore the physical origin of these structures through simulations with a model potential that incorporates short-range C$_{60}$ -- C$_{60}$ attraction and a long-range dipolar repulsion. From simulations of realized structures, we~ estimate the effective dipole needed for chain formation. DFT calculations on the C60/ZnPc/Ag(111) structure support these conclusions and provide more detailed insight on the electrostatic interactions that drive chain formation.   

How van der Waals interactions affect alanine-based polypeptides

van der Waals interactions play a critical role among the intramolecular interactions that stabilize secondary structure folding motifs in polypeptides. In this work, we quantify its influence \textit{ab initio} for the series of helix-forming alanine based polypeptides Ac-Ala$_n$-LysH$^+$ ($n=$ 4-15). We show that: (i) applying a van der Waals (vdW) correction based on the self-consistent electron density [2] to the PBE functional, a clear $\alpha$-helical conformational preference emerges at $n$=8, in agreement with experiment [1], while a mostly 3$_{10}$ helical structure is preferred at plain PBE; (ii) a numeric atom-centered orbital basis enhanced specifically to converge conformational energy differences from explicitly correlated methods (MP2, EX+cRPA and beyond [3]) gives us benchmark capabilities for treatments that include long-range correlations outrightly; (iii) exploring the free energy surface through \textit{ab initio} dynamics for longer helices ($n$=15) we see a dramatic influence of vdW interactions for high temperature stability and surface explored by these molecules. Our results demonstrate that we are now in a position to quantify vdW contributions accurately, and thus unravel their critical qualitative role in comparison to other contributions (strain, H-bonds) in medium-sized biomolecules. [1] Kohtani and Jarrold, JACS 108, 8454 (2004); [2] Tkatchenko and Scheffler, PRL 102, 073055 (2009); [3] http://www.fhi-berlin.mpg.de/aims   

Dispersion Forces and Self-assembly of Styrene Nanowires on H-Si(100) 2$\times$1 Surface

We present our first-principles investigation of the influence of dispersion forces (or van der Waals interactions) on the self-assembly of styrene nanowires on the hydrogenated Si(100) 2$\times$1 surface. Using density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulations we demonstrate that the dispersion forces enhance the binding between styrene molecules thus allowing us to tune the preferential growth of long wires for the fabrication of desired nanopatterns. Furthermore, this approach is a step towards accurate fully first-principles studies of the effects of dispersion forces on the dynamics at interfaces, and therefore will be invaluable to our understanding of chemical processes such as self-assembly and the catalysis of organic chemical reactions.   

Morphological Control in the Synthesis of Silver Nanostructures: Role of Polyvinylpyrrolidone

Solution-phase syntheses are useful for assembling metallic nanostructures with desired morphologies. For example, a wide variety of silver nanostructures have been synthesized in the polyol process [1], including nanowires, nanoplates, cubes, etc. Polyvinylpyrrolidone (PVP) plays a key role in controlling nanostructure morphologies in these fabrication processes. Based on experimental observations, the interaction strength between PVP chains and Ag atoms in different crystallographic facets is expected to vary significantly and this shape selectivity is expected to play a key role in directing the formation of various nanostructures. Using first-principles calculations based on density-functional theory including van der Waals interactions, we compute the interactions of the basic elements of a repeat unit in PVP (2-pyrollidone and ethane) with various crystal faces of Ag. Our results indicate that PVP does exhibit the expected structure sensitivity and that this arises from an interesting balance between van der Waals interactions and direct chemical bonding. We discuss the ramifications of our calculations for the assembly of Ag nanostructures. \\[4pt] [1] B. Wiley et al., Chem. Eur. J. 11, 454 (2005).   

Structure and Formation of Synthetic Hemozoin: Insights from First Principles Calculations

Malaria has reemerged due to parasite resistance to synthetic drugs that act by inhibiting crystallization of the malaria pigment, hemozoin (HZ). Understanding the process of HZ nucleation is therefore vital. The crystal structure of synthetic HZ, $\beta $-hematin ($\beta $H), has recently been determined via x-ray diffraction. We employ van der Waals (vdW) corrected density functional theory to study the $\beta $H crystal and its repeat unit, a heme dimer. We find that vdW interactions play a major role in the binding of the heme dimer and the $\beta $H crystal. Accounting for the $\beta $H periodicity is a must for obtaining the correct geometry of the heme dimer, due to vdW interactions with adjacent dimers. The different isomers of the heme dimer are close in energy, consistent with the observed pseudo-polymorphism. We use these findings to comment on $\beta $H crystallization mechanisms.   

Adsorption of methane on Zn(bdc)(ted)0.5 microporous metal-organic framework

Zn(bdc)(ted)0.5 is metal-organic framework crystallized in a tetragonal space group with a 3D porous structure containing intersecting channels of two different sizes. The larger channels are parallel to the c axis and have a cross section 7.5 $\times $ 7.5{\AA}. The smaller channels are along both the a- and b-axes and have a cross section of 4.8 $\times $ 3.2{\AA}. We measured methane adsorption isotherms at several different temperatures between 82 and 102 K. We calculated the effective specific surface area, isosteric heat and binding energy values. Two distinct substeps were observed in the isotherms corresponding to two different adsorption sites. The origin of the substeps will be discussed.   

Understanding H$_{2}$-H$_{2}$ interactions in Metal Organic Frameworks (MOFs) with unsaturated metal centers

Unsaturated Metal Organic Frameworks (MOFs) are particularly interesting due to their high H$_{2}$ uptakes with relatively large isosteric heats of adsorption (Q$_{st }>$8 kJ/mol). This work explores H$_{2}$-H$_{2}$ interactions between adsorbed H$_{2}$ at the different sites in MOF-74 (M$_{2}$(dhtp),dhtp=2,5-dihydroxyterephthalate) and combines IR spectroscopy with vdW-DFT calculations. The adsorption sites in MOF-74 are from highest to lowest binding energies the metal, oxygen, benzene and pore-center sites. The frequency of adsorbed H$_{2}$ at the metal site suffers an additional $\sim $-30 cm$^{-1}$ red shift (for Mg and Zn) and $\sim $-84 cm$^{-1}$ (for Co) when the neighboring oxygen site is occupied. The dipole moment of adsorbed H$_{2}$ is also affected. These interactions extend to the benzene sites for MOF-74-Co. A decrease in dipole moment of H$_{2}$ adsorbed at the metal site is observed with the partial occupation of the benzene sites. However, the complete occupation of the benzene sites induces an additional $\sim $-10 cm$^{-1}$ red shift.   

First-Principles Calculations of the Role of Dispersive Interactions in CO$_2$ binding in metal-organic frameworks

Metal-organic frameworks (MOFs) have attracted much attention over the past 20 years for their possible applications in gas storage. In this study, we provide computational insight into what makes a MOF structure optimum for CO$_2$ capture. We present a density functional theory-based study of the electronic and structural properties of recently synthesized frameworks M'$_3$[(M$_4$Cl)$_3$(BTT)$_8$]$_2$, with M'=extraframework cation and M=Ca [1]. We study the interactions between CO$_2$ and different binding sites, and predict an unexpected favored binding site at the organic linker. We explore how binding energies are affected by the ordering and type of the extraframework cations. Finally, we address the role of dispersion forces by employing a recent non-local van der Waals functional [2], and compare with a DFT+D approach [3].\\[4pt] [1] M. Dinca et al., {\sl J. Am. Chem. Soc.} 128, 16876 (2006)\\[0pt] [2] M. Dion et al., {\sl Phys. Rev. Lett.} 92, 246401 (2004)\\[0pt] [3] A. Tkatchenko et al., {\sl Phys. Rev Lett.} 102, 073005 (2009)