Session X20: Van der Waals Bonding in Advanced Materials: Carbon Allotropes and Boron Nitride

Growth systematics for the controlled generation of nanostructured materials

Whilst nanomaterials are well established in the research arena they are yet to be exploited at the industrial scale due to their limited availability. A series of bottlenecks must still be overcome before their outstanding properties can be fully unlocked. This fact applies to nanomaterials across the board including tailored 0D, 1D, 2D structures and combinations thereof. This talk addresses the challenges that need to be overcome related to up-scaling the production of novel nanomaterials, possible solutions and the necessity for the development of dedicated nanomaterials for end- user applications will also be discussed.   

Investigation of intermolecular interactions between single walled nanotubes and conjugated oligomers using the dispersion-corrected DFT methods

The area of carbon nanotubes (CNT)-polymer composites has been progressing rapidly in recent years. Pure CNT and CNT-polymer composites have many useful (industry related) properties: ranging from electronic electrical conductivity to superior strength. However the full potential of using CNTs as reinforcements (in say a polymer matrix) has been severely limited because of complications associated with the dispersion of CNTs. CNTs tend to entangle with each other forming materials that have properties that fall short of the expectations. The goal of this work is to identify the type of conjugated oligomers that are best suited for the dispersion of single walled CNT (SWCNT). For this purpose, various methods of dispersion corrected density functional theory (DFT-D/B97D, /WB97XD, /CAM-B3LYP) have been used to investigate the interaction between the SWCNT and the organic conjugated oligomers with different end groups (aldehyde (ALD) and dithiafulvenyl (DTF)). We investigate the effect of intermolecular interactions on the structure, polarity and energetics of the oligomers and SWCNT combinations. The comparison of results obtained using different DFT approximations is made. Our results show that DFT-endcapped oligomer interact more strongly with CNT than ALD-endcapped oligomer.   

Probing moir\'{e} physics in low angle twisted bilayer graphene

When two sheets of honeycomb structured graphene are stacked together, a moir\'{e} pattern that depend on the twist angle forms. A gapped superlattice band structure is resulted when this angle is small enough so that the energy of moir\'{e} modulated hybridization between wave functions on the two sheets that are separated by only 0.34nm is low enough. Apart from the energy gaps, the superlattice band structure also manifests reduced Fermi velocity which implies comparatively higher density of states, multiple van Hove singularities below and above the gaps, and Hofstadter butterfly physics when magnetic field is applied. We show electronic transport measurements of high-quality low-angle twist bilayer graphene devices fabricated by a novel tear-and-stack technique, at zero, low and high magnetic fields. We also present angle dependence of the electronic structure along with magneto-transport features that possibly imply electron-electron interactions. A brief discussion about the transition between the low-twist and high-twist bilayer graphene, the physics of the latter of which is believed to be essentially based on decoupled monolayer graphene according to our previous work, is included.   

Moire pattern interlayer potentials in van der Waals materials from high level ab~initio calculations

Stacking-dependent interlayer interactions are important for understanding the structural and electronic properties in incommensurable two dimensional material assemblies where long-range moir\'e patterns arise due to small lattice constant mismatch or twist angles. We study the stacking-dependent interlayer coupling energies between graphene (G) and hexagonal boron nitride (BN) single layers for different possible combinations such as G/G, G/BN and BN/BN using high-level EXX+RPA {\it ab initio} calculations. The total energies differ substantially when compared with conventional LDA, but for stacking-dependent total energy differences we find that the dominance of short-range covalent-type binding over the longer-ranged van der Waals tails near equilibrium geometries renders the LDA as a reasonable starting point for ab initio calculation based analyses for the systems we have studied. Our calculations are useful input for study of strains originated by interlayer interactions in incommensurable 2D van der Waals crystals.   

Near-field study in graphene/hBN moir\'{e} superlattices.

Moir\'{e} patterns are periodic superlattice structures that appear when two crystals with a minor lattice mismatch are superimposed. A prominent recent example is that of monolayer graphene placed on a crystal of hexagonal boron nitride (hBN). As a result of the moir\'{e} pattern superlattice created by this stacking, the electronic band structure of graphene is radically altered, acquiring satellite sub-Dirac cones at the superlattice zone boundaries. To probe dynamical response of the moir\'{e} graphene, we use infrared (IR) nano-imaging to explore propagation of surface plasmons, collective oscillations of electrons coupled to IR light. We show that interband transitions associated with the superlattice minibands in concert with free electrons in the Dirac bands produce two additive contributions to composite IR plasmons in graphene moir\'{e} superstructures. This novel form of collective modes is likely to be generic to other forms of moir\'{e}-forming superlattices, including van der Waals heterostructures.   

Optical control of inter-layer distance of hBN: a TDDFT study

In this presentation, we introduce an idea to modify inter-layer distance of hBN by shining IR laser in resonance with the frequency of the optical phonon (A$_{\rm 2u}$ mode). By performing the TDDFT-MD simulation under the IR laser, significant grow in an amplitude of the A$_{\rm 2u}$ phonon mode was observed and inter-layer contraction over 11 \% of the original distance was achieved. The source of the stronger attraction of hBN sheets was attributed with increase of dipole moment of each layer coming from the motions of boron (B) and nitrogen (N) atoms in opposite directions. Since the dipole moments of these layers remain as parallel throughout the A$_{\rm 2u}$ phonon vibration, the increase of attractive force occurs between the two hBN sheets in analogy of the London force. In this talk, we will further discuss proper intensity of IR laser and potential applications of this phenomenon. This work was published in Phys. Rev. Lett {\bf 114}, 116102 (2015).   

Using quantum Monte Carlo for the interaction of water with carbon and BN based substrates and assessing exchange-correlation functionals

The interaction of water with the pure surfaces, graphene and hexagonal boron nitride (h- BN), has received a lot of attention because of interesting phenomena exhibited by these systems and their promising potential applications in clean energy, water purification, hydrogen storage, and bio-sensing. BN doped graphene can also now be made, opening the way to carefully designed hybrid materials. However, much of the fundamental mechanisms regarding the interaction between these surfaces and water is still not well understood. We use quantum Monte Carlo to establish accurate benchmarks for water on a number of carbonaceous and BN based substrates, including 2-dimensional periodic surfaces, for which van der Waals interactions play a key role. The benchmarks are then used to test and understand various exchange-correlation functionals in density functional theory. We find that the physisorption of water is poorly described in terms of the adsorption site and the interaction energy by a range of different classes of exchange- correlation functionals, including some that account for dispersion, and we show where these inadequacies might come from.   

'Guanigma': the revised structure of biogenic anhydrous guanine

Living organisms display a spectrum of colors, produced by pigmentation, structural coloration, or both. A relatively well-studied system, which produces colors via an array of alternating anhydrous guanine crystals and cytoplasm, is responsible for the metallic luster of many fish. The structure of biogenic anhydrous guanine was believed to be the same as that of the synthetic one - a monoclinic polymorph. Here we re-examine the structure of biogenic guanine, using experimental X-ray and electron diffraction (ED) data exposing troublesome inconsistencies - namely, a 'guanigma'. To address this, we sought alternative candidate polymorphs using symmetry and packing considerations, then used first principles calculations to determine whether the selected candidates could be energetically stable. We identified theoretically a different monoclinic polymorph, were able to synthesize it, and to confirm using X-ray diffraction that it is this polymorph that occurs in biogenic samples. However, the ED data were still not consistent with this polymorph, but rather with a theoretically generated orthorhombic polymorph. This apparent inconsistency was resolved by showing how the ED pattern could be affected by crystal structural faults composed of offset molecular layers.   

Sorption Properties of Halogen Containing Graphene Oxide Frameworks.

Physisorption of gases has applications in gas storage (e.g. methane, hydrogen for vehicles) and gas separation (carbon dioxide from flue gas). The van der Waals force in narrow pores is strong enough to condense even supercritical gases to much higher densities. Additionally, differences in the binding energy between different gases and the sorbent surface are sufficient to for gas separations. Beyond adsorption interactions, simple steric (size, shape) effects also play a role in gas separations. One class of materials currently being investigated for numerous gas storage/separation applications is graphene oxide frameworks (GOFs). GOFs consist of layers of graphene/graphene oxide separated by chemical linkers covalently bonded on both sides. This presentation will give results from boronic acid-based GOFs that contain halogen group elements. Effects of different linkers on pore shape will be presented. Physical behavior of the gases investigated (hydrogen, methane, carbon dioxide, nitrogen), including binding energies and steric effects for gas separation will also be presented. The physics mechanism behind pore breathing (expansion and contraction of pore volume) in these materials will be discussed.