Session J6: Focus Session: van der Waals Bonding in Advanced Materials - Layered Structures & Mechanical Properties

Improved Description of Soft Layered Materials with van der Waals Density Functional Theory

The accurate description of dispersion forces with approaches based on density functional theory has long been a coveted goal and is currently one of the most active areas of research in computational physics/chemistry. We have tested two new functionals, the optimized Becke88 van der Waals (optB88-vdW) and optimized PBE van der Waals (optPBE-vdW) [1, 2] to describe materials where van~der~Waals interactions dominate. Structural (bond length and interlayer distance) and energetic parameters (atomization and interlayer binding energies) of graphite and \textit{hexagonal}-boron nitride have been calculated using these functionals and we show that our calculations are in very good agreement with experiments and higher level theoretical calculations. From these calculations it is possible to conclude that optB88- vdW and optPBE-vdW are promising functionals for the accurate description of systems held together mainly by dispersion forces. \\[4pt] [1] Klime\v{s} \textit{et al.}, J. Phys.: Condens. Matter \textbf{22}, 022201 (2010). \\[0pt] [2] Klime\v{s} \textit{et al.}, Phys. Rev. B \textbf{83}, 195131 (2011).   

Moir\'e pattern of a single layer MoS$_2$ grown on Cu(111)

We present results of first principles calculations of the geometric and electronic structures of a single layer of Molybdenum disulfide (MoS$_2$) on Cu(111) utilizing the van der Waals density functional [1]. The lowest energy Moir\'e structure consists of $(4\times 4)$ MoS$_2$ on $(5\times 5)$ Cu(111), in agreement with experimental observation [2]. Examination of the local density of electronic states and charge redistribution shows that the layer is not purely physisorbed on the surface, rather there exists a chemical interaction between it and the Cu surface atoms. Interestingly the MoS$_2$ film is found to be not appreciably buckled, while the atoms in the top Cu layer gets reorganized and vertically disordered. The sizes of Moir\'e patterns for a single layer of MoS$_2$ adsorbed on several other close packed metal surfaces are estimated by minimizing the lattice mismatch between the film and the substrate. \\[4pt] [1] M. Dion \textit{et al}, Phys. Rev. Lett. \textbf{92}, 246401 (2004) \\[0pt] [2] D. Kim \textit{et al}, Langmuir \textbf{27}, 11650 (2011)   

Observation of tightly bound charged excitons in monolayer MoS$_{2}$

Recent advances in the development of atomically thin layers of materials have opened up many new research opportunities. In particular, the transition metal-dichalcogenide molybdenum disulfide (MoS$_{2})$ has been shown to cross over from a dark indirect semiconductor to a highly luminescent direct gap material in the limit of monolayer thickness.\footnote{Mak et al. \textit{Phys. Rev. Lett.} \textbf{105}, 136805 (2010); Splendiani et al. \textit{Nano Letters} \textbf{10}, 1271-1275, (2010).} Here we report results of studies of the optical absorption and photoluminescence of a monolayer MoS$_{2}$ field-effect transistor (FET) at 10 K. In the limit of very low doping, the optical properties are dominated by an excitonic feature at $\sim $ 1.9 eV. As the doping density is increased, a new resonance emerges on the low-energy side of the exciton. This feature has been identified as a trion, the bound state of an exciton and an additional electron (or hole). The absorbance and photoluminescence of both the trion and exciton can be tuned by electrostatic doping. A large trion binding energy, exceeding room temperature, is inferred. Our observation can be understood in terms of the dynamical many-body response of a 2D electron gas to the optically created hole and reflects the unusually strong many-body interactions in this 2D system.   

Beyond graphene: atomic scale structure of quasi-2D van der Waals materials

Graphene, a single atomic layer of graphite, has attracted much attention for its unique electronic and mechanical properties. But what role does reduced dimensionality play in other 2D atomic crystals? In this talk we present scanning tunneling microscopy (STM) measurements of the atomic scale structure and spectroscopy of related quasi-2D materials including the transition metal dichalcogenides (TMDs), and their heterostructures with graphene.   

Here to stay: Stability and structure of (6$\sqrt{3}\times$6$\sqrt{3}$)-R30$^\circ$ graphene on SiC(111) by all-electron DFT including van der Waals effects

SiC is a favorite growth substrate for mono- and few-layer graphene by Si sublimation. On the Si side, large ordered graphene areas can be obtained as commensurate (6$\sqrt{3}\times$6$\sqrt{3}$)-R30$^\circ$ (``6$\sqrt{3}$'') periodic films [1]. From a thermodynamic perspective, graphene formation competes with several other phases, including graphite on one side, and several Si-rich reconstructions on the other. The question whether 6$\sqrt{3}$ graphene on SiC(111) is thermodynamically or just kinetically stabilized could be answered by density functional theory (DFT) calculations but for two challenges: (1) The needed surface slab systems are extremely large (up to 2000 atoms in this work), and (2) most standard DFT functionals do not include van der Waals effects. We here show by all-electron DFT including van der Waals effects (PBE+vdW [2]) that the 6$\sqrt{3}$ graphene-like phases on SiC(111) are thermodynamically stable compared to competing surface phases, and obtain the full structure including the substrate-induced graphene corrugation. The impact of strain in smaller-cell approximants and that of possible graphene defects is discussed. [1] K. Emtsev \emph{et al.}, Nature Materials \textbf{8}, 203 (2009). [2] A. Tkatchenko, M. Scheffler, PRL \textbf{102}, 073005 (2009).   

Edge-Edge interactions in stacked graphene

Graphene is often considered as the ultimate material for nanoscale electronics due to its unique structure-dependent properties. It has recently been shown that edge reconstruction and reshaping using combined electron irradiation and Joule heating could be used to control graphene properties towards nanodevice design. For instance, HRTEM experiments on few-layered graphene have revealed the presence of small graphene patches and platelets over larger graphene domains. While these platelets usually move freely over the larger graphene surface, they sometimes get locked in positions close to the edges of the larger sheet, thereby modifying the local electronic environment. We modeled this behavior with extensive density functional calculations using the van der Waals functional of Dion \emph{et al.} Local interactions at the edges are found to be sufficiently strong to explain the presence of stacking configurations (e.g. AA) that are known to be energetically unfavorable in 2D graphene. Our results explain the observed dynamics of these stacked platelets and provide a deeper understanding of graphene edge reconstruction.   

Influence of water on the electronic structure of metal supported graphene: Insight from van der Waals density functional theory

We investigate the interaction between water and metal supported graphene through van der Waals density functional theory calculations. Our results show a systematic increase in the adsorption energy of water on graphene in the presence of underlying metal substrates. In addition, we find that the electronic nature of the graphene-metal contacts behave differently upon water adsorption: in the case of a weak, physical graphene-metal contact, the charge carrier doping level of graphene is tuned by water, resulting in a Fermi level shift on the order of 100 meV. In the case of a strong chemical graphene-metal contact, the ? and ?* bands of graphene are hardly perturbed by water adsorption. These results illustrate the correlated nature of the interactions between water, graphene, and metal substrates, and show that the electronic structure and the doping level of graphene can be controlled by water deposition.   

Alkanes adsorbed on graphene: a vdW-DF study

Studies of small chains of molecules adsorbed on graphene are important for application of graphene in possible devices, not least its use as a gas sensor for the detection of single molecules. In this work we present a density functional theory study of the first ten n-alkanes adsorbed on graphene using the vdW-DF functional. We compare our adsorption energies to temperature programmed desorption measurements finding a similar linear scaling with the number of carbon atoms in the chain. The presence of an offset when extrapolating to the case of no molecules on the surface is also confirmed.   

To wet or not to wet? Dispersion forces tip the balance for water ice on metals

For almost 30 years now, density functional theory (DFT) has been used to explore the molecular level details of water-metal interfaces. However, since the typical generalized gradient approximation exchange-correlation functionals used in these studies do not account for van der Waals (vdW) dispersion forces, the role dispersion plays in water adsorption remains unclear. Here, we tackle this issue head on applying a newly developed non-local functional [J. Klime\v{s} {\it et al}., J. Phys.: Condens. Matter {\bf 22}, 022201 (2010)] to two of the most widely studied water-ice adsorption systems, namely water on Cu(110) and Ru(0001). We show that non-local correlations contribute substantially to the water-metal bond and that this is an important factor in governing the relative stabilities of wetting layers and 3D bulk ice [J. Carrasco {\it et al}., Phys. Rev. Lett. {\bf 106}, 026101 (2011)]. Due to the greater polarizability of the substrate metal atoms, non-local correlations between water and the metal exceed those between water within ice. This sheds light on a long-standing problem, wherein common DFT exchange-correlation functionals incorrectly predict that none of the low temperature experimentally characterized ice-like wetting layers are thermodynamically stable.   

Understanding the magnetic properties present at hybrid organic-ferromagnetic interfaces and the role of the van der Waals interaction

The design of nanoscale spintronic elements in multifunctional devices relies on a clear theoretical understanding of the physics at the electrode-organic system interfaces and in particular, the functionality of specific molecules in a given organic-metal surface environment. The density functional theory provides a framework where a realistic understanding of these systems with predictive power can be expected. However, only very recent functionals describe the exchange correlation of the organic molecule-metal interface reliably including the van der Waals interaction. We show that this has a great influence in particular on specific flat absorbed $\pi$-conjugated electron systems. Our first-principles calculations performed for several organic molecules containing $\pi(p_z)$ electrons adsorbed onto a magnetic substrate show that the magnetic properties such as the local spin polarization, molecular magnetic moments and their spatial orientation can be specifically tuned by using substituents with different electronegativities. References: [1] N. Atodiresei et al., PRL 102, 136809 (2009); [2] J. Brede et al., PRL 105, 047204 (2010); [3] N. Atodiresei PRL 105, 066601 (2010); [4] C. Busse et al., PRL 107, 036101 (2011). [5] N. Atodiresei et al., PRB 84, 172402 (2011).   

Simultaneous conductance and mechanical measurements on single molecule junctions reveal enhanced binding due to van der Waals interactions

Quantitative measurement of Van der Waals (vdW) interactions at the single molecule level remains challenging in experiments and its accurate inclusion in first principles theory is also complex. Here we report simultaneous measurement of force and electrical conductance across Au-molecule-Au junctions using a conducting atomic force microscope (AFM) for 4,4'-bipyridine (BP) and 1,2-bis(4-pyridyl)ethylene (BPE) molecules. For each of these molecules two distinct molecular junction structures are observed with characteristic conductances, consistent with previous studies utilizing scanning tunneling microscopy (STM). These two structures are found to have very different mechanical properties. Specifically, we find that the higher conductance junctions have a significantly larger rupture force and stiffness than those that show the lower conductance. They also have a larger rupture force than Au point contacts, suggesting multiple points of contact. Density functional theory (DFT) calculations suggest that the rupture force for the low conductance structure is well characterized as arising from N-Au donor acceptor interaction. However, the large rupture force and stiffness of the high conductance structure is most naturally explained as being due to the vdW contributions.   

Mechanical properties of van der Waals interactions at compositionally graded interfaces between metals and organic semiconductors

Interfacial bonding plays an important role in energy transfer (e.g., electrical conduction, thermal transport, or acoustic coupling) in composite materials as well as electronic and optoelectronic devices. In this work we use an ultrafast laser to excite vibrational modes in a thin aluminum film that is in contact with a small molecular organic semiconductor (copper phthalocyanine, CuPc). From the measured acoustic dynamics, we derive the fundamental mechanical properties of the van der Waals bonding at the Al/CuPc interface, and further study how these mechanical properties change in a series of samples as the interface is compositionally graded. The implications of these results are discussed in the contexts of interfacial thermal resistance, organic optoelectronic devices, and thermoelectric energy conversion.   

Benchmark data base for accurate van der Waals interaction in inorganic fragments

A range of inorganic materials, such as Sb, As, P, S, Se are built from van der Waals (vdW) interacting units forming the crystals, which neither the standard DFT GGA description as well as cheap quantum chemistry methods, such as MP2, do not describe correctly. We use this data base, for which have performed ultra accurate CCSD(T) calculations in complete basis set limit, to test the alternative approximate theories, such as Grimme [1], Langreth-Lundqvist [2], and Tkachenko-Scheffler [3]. While none of these theories gives entirely correct description, Grimme consistently provides more accurate results than Langreth-Lundqvist, which tend to overestimate the distances and underestimate the interaction energies for this set of systems. Contrary Tkachenko-Scheffler appear to yield surprisingly accurate and computationally cheap and convenient description applicable also for systems with appreciable charge transfer. \\[4pt] [1] S. Grimme, J. Comp. Chem. \textbf{27}, 1787 (2006) \\[0pt] [2] K. Lee, \textit{et al.}, Phys. Rev. B 82 081101 (R) (2010) \\[0pt] [3] Tkachenko and M. Scheffler Phys. Rev. Lett. \textbf{102} 073005 (2009).   

Interlayer binding energy of graphite: A mesoscopic determination from deformation

Despite the interlayer binding energy (BE) being one of the most important material properties of graphite, direct experimental determination is yet to be reported. In this talk, we present a novel experimental method to directly measure the interlayer BE of HOPG. By employing the self-retraction motion of a graphite flake in a graphite island (Phys. Rev. Lett. 100, 067205 (2008)), we assembled a graphite top flake spanning a graphite step, yielding a contact area ($\sim\mu$m$^2$) with a graphite platform. STM scan showed that the interface was atomically smooth. The deformation of the top flake should be determined by the BE with the graphite platform. Thus using a finite element model to simulate the top-flake height profiles measured by AFM, we determine the graphite BE as 0.19($\pm $0.01)J/m$^{2}$, which can serve as a benchmark for other theoretical and experimental works. Our proposed method can be easily extended to measure the BEs between graphite/graphene and other types of substrates. It can also be used in other systems, particularly lamellar materials and thin films.   

Helium atom diffraction from a monolayer solid with a beam that penetrates to the substrate

Diffraction of a thermal energy helium atomic beam is evaluated in the situation that the target monolayer lattice is so dilated that the atomic beam penetrates to the region between the monolayer and the substrate. Parameters are chosen to be representative of a hypothetical $p(1 \times 1)$ commensurate monolayer solid of H$_2$/KCl(001). Depending on the spacing between the monolayer and the substrate and on the angle of incidence, there are cases where part of the incident beam is trapped in the interlayer region for times exceeding 50 ps. The methodology is a direct extension of the wave-packet propagation used for the scattering of helium from the quantum monolayer solid H$_2$/NaCl(001).\footnote{L. W. Bruch, F. Y. Hansen and F. Traeger, J. Chem. Phys. {\bf 134}, 194308 (2011)}