Session D27: Fullerenes, Nano-membranes, and Quasicrystals

The equilibrium and electronic structure of large icosahedral fullerenes using an all-electron fully analytic density functional theory

We have recently developed a fast, variational and fully analytic density functional theory (ADFT). Instead of numerical integration it employs analytic integration using Gaussian basis sets and the calculus of variations to express the molecular orbitals as well as the Kohn-Sham potential in linear-combination of atomic orbital form. We first parametrize the ADFT to provide the experimental geometry of C$_{60}$ fullerene. Using this parametrization, the triple zeta 6-311G(d,p) orbital basis, and density fitting with exchange-correlation bases that include up to $f$ functions, the geometries of C$_{240}$, C$_{540}$, C$_{960}$, C$_{1500}$, and C$_{2160}$ fullerenes are optimized. The equilibrium structures of these fullerenes are polyhedral in nature, confirming the previous predictions by tight-binding methods. Bond distances are converging towards those of graphene. The evolution of electron removal energies, electron affinities, and singlet excitation energies from C$_{60}$ to C$_{2160}$ is studied using the $\Delta \, SCF$ and transition-state methods.   

Fullerene in a Metal-Organic Matrix: Design of the Electronic Structure

By combining C$_{60}$ fullerenes and a metal-organic framework, a novel material has been designed with enhanced electronic properties aimed at improving the superconducting transition temperature. Combining these materials within the same structure gives new possibilities to tailor the electronic properties. Higher superconducting transition temperatures have previously been achieved in the fullerene solids by intercalating with ever-larger alkali atoms into the C$_{60}$ crystal. The current study demonstrates by means of state-of-the-art calculations that MOF can be used as a placeholder to set the distance between C$_{60}$ fullerenes introduced inside their pores giving another means to increase the distance between them and further tailor their electronic properties.   

The Smoluchowski Effect and Step-Edge Behavior of Nanocars and Azofullerenes

The nanocar molecule - four fullerene wheels connected by rotating alkyne axles to a central chassis - was the first molecule designed and fabricated specifically for nanoscale manipulation. We have investigated the imaging and manipulation of the nanocar molecule on Au(111) by variable-temperature STM, with specific focus on their unique dynamic step-crossing and -straddling abilities. Our static analysis of the molecules adsorbed at step edges under the influence of the Smoluchowski effect has begun to explain the complex interactions of their behavior in these regions, with an eye towards surface manipulation in three dimensions. Further manipulation studies also attempt to elucidate the fullerene-substrate interactions that make rolling manipulation possible, with special attention paid to the azofullerene dimer - one of the first specifically designed and tested molecules incorporating two simple mechanical functions -- actuation of the ``azo'' unit and rolling / rotation of the wheel-like fullerenes.   

Cage-Core Interactions in Fullerenes Enclosing Metal Clusters with Multiple Scandium and Yttrium Atoms.

Pronounced stability has been reported for metallofullerenes of the form NSc3@CN (N = 68, 78) /1/. In response of these and related findings, Density Functional Theory studies have been performed on the relation between cage-core interactions and the geometry as well as stability of endofullerenes with metal impurities containing Sc and Y. Substantial electron transfer from the metal core to the fullerene cage combines with electron backdonation, involving the interaction between the occupied orbitals of the negatively charged cage and the unoccupied d orbitals of the positively charged core. The Hueckel 4n+2 rule, well established in organic chemistry, is shown to provide a valuable heuristic tool for understanding the intramolecular electron transfer and the related stability gain /1/. The usefulness of the aromaticity concept for explaining and predicting the architecture of metallofullerenes is further exemplified by the units Sc2@C84 and Y2@C84 which were analyzed in spin triplet and singlet conditions. The Sc2 core turns out to be realized by two separated ions, while Y2 forms a bound subunit. These findings are in agreement with conclusions based on the 4n + 2 rule, assisted by Nucleus Independent Chemical Shift (NICS) calculations. /1/ Stevenson, S.; Fowler, P.W.; Heine, T.; Duchamp, J.C.; Rice, G.; Glass, T.; Harich, K.; Hadju, F.; Bible, R.; Dorn, H.C. Nature, 2000, 408, 427, /2/ S. S. Park, D. Liu, F. Hagelberg, J. Phys. Chem. A 109, 8865 (2005).   

Hydrogen Storage in Novel Carbon-based Nanostructured Materials

One of the biggest challenges facing a future hydrogen economy is that of onboard vehicular hydrogen storage, for which novel carbon-based nanostructured materials have emerged as potential candidates. Towards this end, we present the synthesis and characterization of ``bucky dumbbell,'' a new organometallic compound comprised of two buckyballs complexed to a central iron atom. This new compound has been characterized using both $^{13}$C solid-state NMR and Raman spectroscopy, and electron spin paramagnetic resonance spectroscopy reveals the presence of Fe$^{3+}$. Temperature-programmed desorption has revealed a new hydrogen binding site via the appearance of a peak centered at approximately -50 \r{ }C, indicating the hydrogen is stabilized at a temperature significantly above that expected for physisorption but still lower than that of C-H bond formation. Comparison with C$_{60}$ under the same hydrogen exposure and heating conditions shows almost no hydrogen adsorption, and the exact binding energy (or desorption activation energy, E$_{d})$ for the bucky dumbbell shows an enhanced value of $\sim $6.2 kJ/mol. Initial volumetric analyses conducted at 77K and 3 bar show a storage capacity of $\sim $0.4 wt{\%}. The synthesis and analysis of other novel fullerene-based organometallic hydrogen complexes will also be discussed.   

Microscopic ESR study of N@C$_{60}$ using a Magnetic Resonance Force Microscope

We report electron spin resonance studies of the endohedral fullerene N@C$_{60}$ using the novel technique of magnetic resonance force microscopy (MRFM). These studies are performed at temperatures down to 1 K on both thin films of N@C$_{60}$ and in samples where the endohedral fullerene is incorporated into a bulk crystalline matrix\footnote{B. Naydenov, C. Spudat, W. Harneit, H. I. Suss, J. Hullinger, J. Nuss, M. Jansen, Chem. Phys. Lett., 424, 327 (2006)}. Utilizing the large magnetic field gradients ($\sim$ 10$^5$ Tesla/meter) in the vicinity of our micromagnetic probe tip, we are able to selectively probe the electron spins in sub--micron volumes. Further, our schemes for spin manipulation allow us to measure the spin--lattice relaxation rate (T$_1^{-1}$) with a spatial resolution in one dimension of approximately 20 nanometers. We will also discuss our efforts to improve the sensitivity of our microscope for detecting {\em individual} electronic spins.   

Dimensional evolution of the electronic and structural properties of K$_{x}$C$_{60}$ multilayers studied by Scanning Tunneling Microscopy

We investigate the effect of dimensionality on the properties of potassium doped C$_{60}$ (K$_{x}$C$_{60})$ by studying thin films with precisely controlled doping levels and layer structures using scanning tunneling microscopy and spectroscopy. We observe systematic variation in spatial and electronic structure as the films change from the 2D to the quasi-3D regime. In metallic K$_{3}$C$_{60}$, the large electronic density of states at the Fermi level (E$_{F})$ is seen to split, with a small gap opening at E$_{F}$. In the Jahn-Teller-induced K$_{4}$C$_{60}$ insulator, the energy gap around E$_{F}$ increases monotonically with increased film thickness. In K$_{5}$C$_{60}$, the spectra change from a re-entrant metal in the first layer to an insulator in the third layer. These trends can be explained by considering the increase of Coulomb repulsion in multilayers as screening from the metal substrate is reduced. These results highlight the role of strong electron correlation and dimensionality in determining the properties of doped fullerides.   

Crystal structure of Rb$_{4}$C$_{60}$ under pressure

We show that Rb$_{4}$C$_{60}$ transforms from its orientationally disordered tetragonal structure at ambient pressure to an orthorhombic phase in the neighborhood of 0.4 GPa. Lattice parameters, interfullerene distances, and closest Rb-C distances evolve continuously up to 2.2 GPa. Rietveld refinements establish that the high pressure phase is isostructural to Cs$_{4}$C$_{60}$. The previously observed conducting phase at 0.8 GPa is therefore structurally distinct from the ambient pressure insulator.   

Structural characterization and molecular dynamics of fullerene or fullerene-derivative nanowhiskers

Recently, a new type of fibrous fullerene crystals called fullerene nanowhisker has been reported by a liquid-liquid interfacial precipitation method using saturated $m$-xylene solution of fullerene and isopropyl alcohol. Considerable interests have been generated in the structure and properties of fullerene or fullerene-derivative nanowhiskers. In this study, we present the results of structural characterization and molecular dynamics of C$_{60}$, C$_{70}$ and C$_{61}$H$_{2}$ --nanowhiskers(NWs) by x-ray diffraction and solid state NMR. The XRD pattern of as-grown C$_{60}$-NWs have a hexagonal structure with lattice constants of $a$=23.732 and $c$=10.126. Both solid-state $^{13}$C-CP/MAS and wideline $^{1}$H-NMR measurement clearly shows that $m$-xylene molecules are included in NWs. Both lineshape and spin-lattice relaxation time of wideline $^{13}$C-NMR measurements clearly show that C$_{60}$-NWs$ exhibited the phase transition at 250 K. Detailed results on the molecular dynamics and the other properties for C$_{60}$-, C$_{70}$- or C$_{61}$H$_{2}$-NWs will be presented.   

Formation of SiC Clusters with Bucky Diamond Structures

SiC clusters with bucky diamond structures have been found in a quantum-mechanical molecular dynamics study based on our recently developed self-consistent and environment dependent Hamiltonian in the framework of a linear combination of atomic orbitals [1]. Starting from a spherically truncated bulk diamond structure, stable structures of SiC clusters containing 147 atoms were studied for various compositions of Si and C atoms. In particular, the following initial configurations were considered: (i) C-rich configuration with Si-core, (ii) Si-rich configuration with C-core, and (iii) an almost equal admixture Si and C atoms. It is found that in the first case Si atoms are dragged to the exterior and a cage-like structure formed, while in the second case some C atoms remain in the interior region and some move to the exterior region forming distorted tetrahedral structures with Si atoms. Finally, in the third case, the bucky-diamond structure is obtained, where the interior has a diamond-like structure and the exterior a fullerene-like structure. The reason why (SiC)$_{147}$ clusters form different stable structures can be understood based on hybridization characteristics of Si (\textit{sp}$^{3 })$ and C atoms (\textit{sp}$^{1}$, \textit{sp}$^{2}$, and \textit{sp}$^{3 })$, respectively. [1] Leahy \textit{et al}. Phys. Rev. B74, 155408 (2006).   

In-situ microscopic investigations of the nucleation and growth of C$_{60}$ films on Bi(0001)/Si(111)

Growth of epitaxial C$_{60}$ films on Si is of particular interest for technological reasons. However, strong interaction between the C$_{60}$ molecules and the clean Si induces film growth in the Stransky-Krastanov mode with only local ordering in the first monolayer. Passivation of the Si dangling bonds -- for example with hydrogen -- leads to van der Waals bonding of adsorbates and thus higher degree of crystallinity in C$_{60}$ film, but the true relation between surface properties, and the crystallinity of the fullerene film is not yet fully understood. In this work, C$_{60}$ thin films were grown by UHV deposition on Si(111) substrate covered with thin Bi(0001) passivation layer. Real-time, dark-field low-energy electron microscope (LEEM) investigation of the growth revealed that C$_{60}$ film nucleates in fcc(111) phase, having an epitaxial relation with the Bi(0001) surface. At a growth temperature of $\sim $400K, preferential nucleation of C$_{60}$ at Bi twin boundaries has been detected. Low-energy electron diffraction (LEED) confirmed that film had a single orientation and an excellent crystallinity. The in-plane lattice parameter in the C$_{60}$ films with thickness up to 3ML has been measured to be 10.04 $\pm $ 0.02 A, which is very close to the bulk value of 10.01 A.   

Elastic strain-sharing as a means of fabricating strained-Si(110) nanomembranes

Hole mobility is higher in Si(110) than it is in Si(001), and straining Si(110) produces further improvements, making strained-Si(110) desirable for p-MOS devices. We describe elastic strain sharing in Si:SiGe:Si(110) heterostructure membranes, which generates flexible, transferable, and dislocation-free strained-Si(110) nanomembranes. Membranes are grown by chemical vapor deposition on the Si template layer of (110) silicon-on-insulator (SOI) substrates. Selective etching of the buried oxide layer `releases' the epitaxial tri-layer system. X-Ray diffraction measurements show that the heterostructure elastically relaxes by transferring strain from compression in the alloy layer, into tensile strain in the Si layers, and we will discuss the achieved mobility values. The XRD line scans exhibit narrow peak widths and thickness fringes, which are both signatures of high-quality (negligible dislocation density) single-crystal strained-Si.   

Dislocation-free, uniformly strained Si fabricated by Si nano-membrane (SiNM) technology

It is known that the interface of a thin film with the substrate on which it is grown plays an important role in dislocation nucleation and kinetic critical thickness. A crystalline-amorphous interface reduces the line energy of dislocations and makes strained structures on SiO$_{2}$ [e.g., strained-Si-on-insulator (sSOI) or a strained SiGe film grown on SOI], susceptible to dislocation formation. We describe fabrication of elastic strain-sharing Si nanomembranes and demonstrate that these strained structures are more thermally stable than strained structures on noncompliant substrates. Our studies with low-energy electron microscopy (LEEM) and x-ray absorption spectroscopy (XAS) show that the structures have a more uniform strain than the strained Si fabricated by conventional SmartCut{\textregistered} sSOI technology.   

Mechanism of quasicrystal nucleation and growth

On cooling, liquids ordinarily solidify into glasses or into crystalline phases with long- range periodic ordering. However, it is also possible to form quasicrystals, ordered solids with long-range aperiodicity. Although quasicrystals have been observed in many materials, their formation is poorly understood. We present the results of a molecular simulation study to elucidate the process by which quasicrystals form from supercooled liquids. We show that, as has been speculated in previous theoretical and experimental works, icosahedral clusters play a significant role in quasicrystal formation. Specifically, icosahedral clusters facilitate the formation of the so-called quasicrystal ``critical'' nucleus, and, together with phasons, facilitate the complicated mechanism that allows quasicrystals to grow aperiodic structures via local interactions. Our findings suggest that direct correlations between liquid ordering and solid structure may be a requisite property for quaiscrystal-forming systems, and is consistent with the class of systems that are known to form quasicrystals experimentally.   

Specific heat of rhombohedral C60 polymer in the temperature range of 2-300K

Under high temperature of 700 K and high pressure of 6 GPa, we have prepared a batch of C60 polymer. XRD data confirmed it is rhombohedral phase and solid 13C NMR showed a formation of sp3 bond between two neighbor C60 in (111) plane. We have measured the specific heat of C60 polymer and pristine C60 by PPMS in the range from 2 to 300 K. The experimental result of pristine C60 agreed well with previous report. For C60 polymer, above T=80 K it is found that temperature dependence of the specific heat is similar to that of pristine C60 besides an anomaly from order-disorder phase transition at 260K, but in range from 2 to 80K the specific heat is much less than that of pristine C60. Assuming three- (3D) and two-dimensional (2D) Debye phonon modes to contribute respectively to the specific heat in different temperature zone, the calculated values of specific heat have got a good agreement with the experimental data in the whole temperature range. These results show the 2D planar modes but not 3D modes are a dominator to the specific heat of C60 polymer, and the low-frequency intermolecular modes of C60 lattice are restrained in the case of C60 polymer by sp3 bonds from 2+2 cycloaddition reaction.