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Single-atom catalysts are of great interest because of their high efficiency. In the case of chemically deposited sp(2) carbon, the implementation of a single transition metal atom for growth can provide crucial insight into the formation mechanisms of graphene and carbon nanotubes. This knowledge is particularly important if we are to overcome fabrication difficulties in these materials and fully take advantage of their distinct band structures and physical properties. In this work, we present atomically resolved transmission EM in situ investigations of single Fe atoms at graphene edges. Our in situ observations show individual iron atoms diffusing along an edge either removing or adding carbon atoms (viz., catalytic action). The experimental observations of the catalytic behavior of a single Fe atom are in excellent agreement with supporting theoretical studies. In addition, the kinetics of Fe atoms at graphene edges are shown to exhibit anomalous diffusion, which again, is in agreement with our theoretical investigations.

Hexasubstituted fullerenes with the skew pentagonal pyramid (SPP) addition pattern are predomi- nantly formed in many types of reac- tions and represent important and ver- satile building blocks for supramolecu- lar chemistry, biomedical and optoelec- tronic applications. Regioselective syn- thesis and characterization of the new SPP derivative, C60A3)4(CN)H, in this work led to the experimental identifi- cation of the new family of "superhalo- gen fullerene radicals", species with the gas-phase electron affinity higher than that of the most electronegative halogens, F and Cl. Low-temperature photoelectron spectroscopy and DFT studies of different C60X5 radicals reveal a profound effect of X groups on their electron affinities (EA), which vary from 2.76 eV (X = CH3 )t o 4.47 eV (X = CN). The measured gas- phase EA of the newly synthesized C60- A3)4CN equals 4.28 (1) eV, which is about 1 eV higher than the EA of Cl atom. An observed remarkable stabili- ty of C60A3)4CN in solution under ambient conditions opens new venues for design of air-stable molecular com- plexes and salts for supramolecular structures of electroactive functional materials.

A method for the selective synthesis of sulfide clusterfullerenes Dy2S@C2n is developed. Addition of methane to the reactive atmosphere reduces the formation of empty fullerenes in the arc-discharge synthesis, whereas the use of Dy2S3 as a source of metal and sulfur affords sulfide clusterfullerenes as the main fullerene products along with smaller amounts of carbide clusterfullerenes. Two isomers of Dy2S@C82 with Cs(6) and C3v(8) cage symmetry, Dy2S@C72-Cs(10528), and a carbide clusterfullerene Dy2C2@C82-Cs(6) were isolated. The molecular structure of both Dy2S@C82 isomers was elucidated by single-crystal X-ray diffraction. SQUID magnetometry demonstrates that all of these clusterfullerenes exhibit hysteresis of magnetization, with Dy2S@C82-C3v(8) being the strongest single molecule magnet in the series. DC- and AC-susceptibility measurements were used to determine magnetization relaxation times in the temperature range from 1.6 K to 70 K. Unprecedented magnetization relaxation dynamics with three consequent Orbach processes and energy barriers of 10.5, 48, and 1232 K are determined for Dy2S@C82-C3v(8). Dy2S@C82-Cs(6) exhibits faster relaxation of magnetization with two barriers of 15.2 and 523 K. Ab initio calculations were used to interpret experimental data and compare the Dy-sulfide clusterfullerenes to other Dy-clusterfullerenes. The smallest and largest barriers are ascribed to the exchange/dipolar barrier and relaxation via crystal-field states, respectively, whereas an intermediate energy barrier of 48 K in Dy2S@C82-C3v(8) is assigned to the local phonon mode, corresponding to the librational motion of the Dy2S cluster inside the carbon cage.

Increasing the temperature at which molecules behave as single-molecule magnets is a serious challenge in molecular magnetism. One of the ways to address this problem is to create the molecules with strongly coupled lanthanide ions. In this work, endohedral metallofullerenes Y2@C80 and Dy2@C80 are obtained in the form of air-stable benzyl monoadducts. Both feature an unpaired electron trapped between metal ions, thus forming a single-electron metal-metal bond. Giant exchange interactions between lanthanide ions and the unpaired electron result in single-molecule magnetism of Dy2@C80(CH2Ph) with a record-high 100 s blocking temperature of 18 K. All magnetic moments in Dy2@C80(CH2Ph) are parallel and couple ferromagnetically to form a single spin unit of 21 μB with a dysprosium-electron exchange constant of 32 cm−1. The barrier of the magnetization reversal of 613 K is assigned to the state in which the spin of one Dy centre is flipped., Single molecule magnets have demonstrated promise for information storage, molecular spintronics and quantum computing, but are limited by their low operational temperatures. Here, Popov and coworkers prepare a SMM with a high blocking temperature of 18 K by trapping two lanthanide ions with a single-electron bond inside a fullerene.

A remarkably large magnetic anisotropy energy of 305 K is computed by quantum chemistry methods for divalent Fe$^{2+}$ $d^6$ substitutes at Li-ion sites with $D_{6h}$ point-group symmetry within the solid-state matrix of Li$_3$N. This is similar to values calculated by the same approach and confirmed experimentally for linearly coordinated monovalent Fe$^{1+}$ $d^7$ species, among the largest so far in the research area of single-molecule magnets. Our ab initio results therefore mark a new exciting exploration path in the search for superior single-molecule magnets, rooted in the $d_{xy}^{1.5} d_{x^2-y^2}^{1.5} d_{z^2}^1 d_{yz}^1 d_{zx}^1$ configuration of $d^6$ transition-metal ions with linear or quasilinear nearest-neighbor coordination. This $d^6$ axial anisotropy may be kept robust even for symmetries lower than $D_{6h}$, provided the ligand and farther-neighbor environment is engineered such that the $d_{xy}^{1.5} d_{x^2-y^2}^{1.5} d_{z^2}^1 d_{yz}^1 d_{zx}^1$ - $d_{xy}^{1} d_{x^2-y^2}^{1} d_{z^2}^2 d_{yz}^1 d_{zx}^1$ splitting remains large enough., Comment: 5 pages, 1 figure

Charge carrier transport under reverse voltage conditions is of major relevance in devices like organic photo-detectors, organic solar cells (tandem cells), organic light emitting diodes (generation contacts), and organic Zener diodes. We present organic pin-diodes comprising molecular doped layers of pentacene and C60 with an adjustable and reversible reverse breakdown behavior. We discuss the electric field and temperature dependence of the breakdown mechanism and propose a coherent charge transport scenario to describe the experimental findings. Within this model a field assisted tunneling of charge carriers over a rather large distance from valence to conductance states (and vice versa) governs the breakdown behavior. This is in accordance to experimental observations where charge carriers can overcome a layer thickness of 110 nm in the breakdown regime.

Graphene layers have been targeted in the last years as excellent host materials for sensing a remarkable variety of gases and molecules. Such sensing abilities can also benefit other important scientific fields such as medicine and biology. This has automatically led scientists to probe graphene as a potential platform for sequencing DNA strands. In this work, we use robust numerical tools to model the dynamic and electronic properties of molecular sensor devices composed of a graphene nanopore through which DNA molecules are driven by external electric fields. We performed molecular dynamic simulations to determine the relation between the intensity of the electric field and the translocation time spent by the DNA to pass through the pore. Our results reveal that one can have extra control on the DNA passage when four additional graphene layers are deposited on the top of the main graphene platform containing the pore in a 2 × 2 grid arrangement. In addition to the dynamic analysis, we carried electronic transport calculations on realistic pore structures with diameters reaching nanometer scales. The transmission obtained along the graphene sensor at the Fermi level is affected by the presence of the DNA. However, it is rather hard to distinguish the respective nucleobases. This scenario can be significantly altered when the transport is conducted away from the Fermi level of the graphene platform. Under an energy shift, we observed that the graphene pore manifests selectiveness toward DNA nucleobases.

We report the first example of a donor–acceptor corannulene‐containing hybrid material with rapid ligand‐to‐ligand energy transfer (ET). Additionally, we provide the first time‐resolved photoluminescence (PL) data for any corannulene‐based compounds in the solid state. Comprehensive analysis of PL data in combination with theoretical calculations of donor–acceptor exciton coupling was employed to estimate ET rate and efficiency in the prepared material. The ligand‐to‐ligand ET rate calculated using two models is comparable with that observed in fullerene‐containing materials, which are generally considered for molecular electronics development. Thus, the presented studies not only demonstrate the possibility of merging the intrinsic properties of π‐bowls, specifically corannulene derivatives, with the versatility of crystalline hybrid scaffolds, but could also foreshadow the engineering of a novel class of hierarchical corannulene‐based hybrid materials for optoelectronic devices.

Herein, we report the first example of a crystalline metal-donor-fullerene framework, in which control of the donor-fullerene mutual orientation was achieved through chemical bond formation, in particular, by metal coordination. The (13) C cross-polarization magic-angle spinning NMR spectroscopy, X-ray diffraction, and time-resolved fluorescence spectroscopy were performed for comprehensive structural analysis and energy-transfer (ET) studies of the fulleretic donor-acceptor scaffold. Furthermore, in combination with photoluminescence measurements, the theoretical calculations of the spectral overlap function, Förster radius, excitation energies, and band structure were employed to elucidate the photophysical and ET processes in the prepared fulleretic material. We envision that the well-defined fulleretic donor-acceptor materials could contribute not only to the basic science of fullerene chemistry but would also be used towards effective development of organic photovoltaics and molecular electronics.

Charge migration is a ubiquitous phenomenon with profound implications throughout many areas of chemistry, physics, biology and materials science. The long-term vision of designing functional materials with tailored molecular scale properties has triggered an increasing quest to identify prototypical systems where truly molecular conduction pathways play a fundamental role. Such pathways can be formed due to the molecular organization of various organic materials and are widely used to discuss electronic properties at the nanometer scale. Here, we present a computational methodology to study charge propagation in organic molecular stacks at nano and sub-nanoscales and exploit this methodology to demonstrate that moving charge carriers strongly affect the values of the physical quantities controlling their motion. The approach is also expected to find broad application in the field of charge migration in soft matter systems.

Graphene|metal|ligand systems open a new realm in surface magnetochemistry. We show that by trapping metal atoms in the two-dimensional potential lattice of a graphene-ligand interface it is possible to build a chemical analogue of an optical lattice, a key setup in quantum information and strongly correlated systems. Employing sophisticated first-principles calculations, we studied electronic and dynamic properties of graphene|metal|ligand assemblies and showed that there is a general principle--spin-charge separation in π-d systems--that underlies the possibility of synthesizing and controlling such systems. We find that ligands can work as a local gate to control the properties of trapped metal atoms and can impose bosonic or fermionic character on such atomic nets, depending on the ligand's nature. Remarkably, the magnetization energy in such systems reaches record-high values of ca. 400 meV, which makes the respective magnetic phenomena utilizable at room temperature. Accompanied by spin polarization of the graphene π-conjugated system it leads to spin-valve materials and brings the realization of quantum computing one step closer.

In this work, the multibox (M-box) simulation scheme is introduced, which can be considered as a generalization of the QM/MM scheme for multifragment (molecular) systems. This scheme exploits the natural locality of multifragment molecular-based systems by mapping the system into force-coupled block subspaces. Where defined in this way, the entire system can be fully modeled under a quantum mechanical force field. This allows the description of each subspace explicitly by means of a robust electronic structure theory without the requirement for large computational resources. An adequate block-to-block coupling by means of shared subsystem fragments is applied to preserve the long-distance structural correlation in the system during a molecular dynamic (MD) simulation. Since electronic structure descriptions play a central role in the formulation of several parametric models for charge or energy transport, we expect that this space-time correlated scheme can become a reliable computational tool for charge/energy transport/transfer applications. The efficiency of the method is demonstrated by performing statistical and time-resolved analysis using both the multifragment box and full ab initio approaches. We illustrate the method using as examples the melting process of a one-dimensional benzene chain (weak interaction situation) and NVE dynamics for the CnHn polymeric chain (strong interaction situation). We also have extended the threshold of applicability of our model, demonstrating how it can cope with MD simulation with more complex systems and processes.

While the role of asymmetric nitride clusters on the cage size and symmetry in fullerene-based structures is known, the role of the asymmetric arrangement of metals in nitride clusters on the nitrogen is not studied now. It is discovered that asymmetric mixed-metal nitride clusters give sufficiently narrow 14 N NMR signals to make NMR the method of choice to characterize the endohedral cluster from the inside. In the series of mixed-metal nitride clusterfullerenes Lu x Sc 3-x N@C 80 and Lu x Y 3-x N@C 80 (x = 0-3) the δ( 14 N) values are found to be linear functions of x showing that 14 N chemical shifts are additive values with specific increment for each kind of metal atoms. DFT calculations are performed to interpret the experimentally measured spectra. To reveal the main factors affecting 14 N chemical shifts in nitride clusterfullerenes, shielding tensor components are analyzed in terms of Ramsey theory both in localized and canonical molecular orbitals. 14 N chemical shifts in M 3 N@C 80 and related systems are shown to be determined solely by nitrogen-localized orbitals, and in particular by the p x,y,z atomic orbitals of nitrogen. As a result, the peculiarities of the nitrogen shielding in nitride clusterfullerenes can be interpreted by the simple analysis of the nitrogen-projected density of states and its variation in different chemical environments.

In contrast with empty fullerenes, nitride clusterfullerenes usually exhibit irreversible reduction steps at moderate electrochemical scan rates. However, these reduction steps are chemically reversible, indicating that reversible follow-up reaction takes place. To explain this phenomenon, we analyze in this work if anion-radicals of nitride clusterfullerenes are more prone to dimerization than anion-radicals of empty fullerenes. Extensive DFT computations are performed to find the most stable dianionic dimeric structures of Sc3N@C68, Sc3N@C80, Sc3N@C80(CF3)2, [5,6] and [6,6] pyrrolidine adducts of Sc3N@C80 and Y3N@C80, a series of Y3N@C2n (2n = 78, 80, 84, 86, 88), as well as those of empty fullerenes C60, C70, and C84. Dimerization energies of the most stable isomers are computed in the gas phase, with the use of van der Waals corrections, and in solution. It is found that dianionic dimers of nonderivatized nitride clusterfullerenes are substantially more stable than those of empty fullerenes, which can...

We examine at the DFT level of theory the topology of side wall functionalization of the (5,5)metallic single-wall carbon nanotube (CNT) with carbenes (>CX(2)) and its effect on electronic properties of the system. It is demonstrated that specific substructures/topology known to stabilize functionalized fullerene molecules can play the same role in CNTs, as well. Upon deepening of functionalization, "uniformity" of addition motives and related continuous changes in properties transform into regular addition patterns with isolated aromatic islands on the nanotube backbone that give rise to jump-like changes in electronic structure.

One isomer of C 60 ( i -C 3 F 7 ) 8 , three isomers of C 60 ( i -C 3 F 7 ) 6 , and the first mixed perfluoroalkylated fullerene, C 60 (CF 3 ) 2 ( i -C 3 F 7 ) 2 , have been isolated by HPLC from a mixture prepared by reaction of C 60 with heptafluoroisopropyl iodide in a glass ampoule at 260–290 °C. The molecular structures of the four new compounds have been determined by means of X-ray single crystal diffraction partially also by use of synchrotron radiation. Theoretical calculations at the DFT level of theory have been employed to rationalize the energetics of isomers and of C 60 -R f binding.

Electron impact study of isomer S 6 -C 60 (CF 3 ) 12 resulted in the ionization efficiency curves (IEC) of singly and doubly charged positive ions and electron capture resonances for negative ones. The appearance energy (AE) values of the fragment C 60 (CF 3 ) n ions appear to correlate linearly with n down to n = 0 (C 60 ion) and its value for each additional act of CF 3 detachment rises by 2–3 eV. Observations of metastable ions reveal several cases of quasi-simultaneous detachment of two CF 3 -groups. Energy shift between ionization energy of S 6 -C 60 (CF 3 ) 12 and AE of the first fragment C 60 (CF 3 ) 11 ion was observed as 8.8 eV for singly and 8.3 eV for doubly charged ions. To rationalize this energy shift a DFT calculation of the orbital energies for the C 60 (CF 3 ) 12 + cation have been carried out and energy gap between HOMO and LUMO orbitals was estimated. High probability of occupation of the localized σ* orbital and CF 3 -elimination require four electron excitation and can be effected via a multielectron transition from the multitude of the densely distributed occupied π orbitals. The analogies with the fragmentation of C 60 with C 2 -elimination are mentioned.

The Bingel reaction of poly(trifluoromethyl)fullerene p7mp-C70(CF3)10 with diethyl malonate and CBr4 in the presence of bases yields the C70(CF3)10[C(CO2Et)2] cycloadduct as a major product, along with two C70(CF3)10[CH(CO2Et)] isomers. An XRD study of the main compound demonstrates that a [2 + 1] cycloaddition occurs at the unoccupied pole of the p7mp-C70(CF3)10 molecule. The observed regiochemical selectivity of the [2 + 1] cycloaddition is shown to be favored from both energetic and orbital reactivity viewpoints.

We report the first detection of carbon nanotubes in the gas phase in a matrix-assisted laser desorption/ionization (MALDI) mass spectrometric experiment. These observations open the possibility of studying isolated nanotubes in the gas phase by means of various spectroscopic methods and a possible way to separate them.

A number of C60(CF3)n compounds with n = 2–10 have been synthesized by the reaction of C60 with silver trifluoroacetate and successfully isolated by means of HPLC. This resulted in the first crystal structure determination of six lower trifluoromethyl derivatives with n = 2 (single isomer), 4 (two isomers), and 6 (three isomers). A kinetic model of sequential trifluoromethylation based on the Bell–Evans–Polanyi principle has been used to explain the experimentally observed isomeric distribution in the mixtures of C60(CF3)n compounds up ton = 6. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Three isomers of C60(CF3)16 and one isomer of C60(CF3)18 have been isolated by HPLC from a mixture prepared by trifluoromethylation of C60 with CF3I in a glass ampoule at 380–400 °C. The molecular structures of the four new compounds have been determined by means of X-ray single crystal diffraction and discussed in terms of mechanistic pathways of their formation and relative stability according to the DFT calculations.

Pentafluoroethyl derivatives of [60]fullerene C60(C2F5)n (n = 6, 8, and 10) were synthesized by the reaction of C60 with C2F5I in glass ampoules at 380–440 °C. Isomers of composition C60(C2F5)6 (one isomer), C60(C2F5)8 (five isomers), and C60(C2F5)10 (two isomers) were isolated by chromatographic separation. Their molecular structures were established by X-ray diffraction. The relative stabilities of isomers were compared by density functional theory calculations.

In covalent mechanochemistry, precise application of mechanical stress to molecules of interest (“mechanophores”) is used to induce to promote desired reaction pathways. Computational prediction of such phenomena and rational mechanophore design involves the computationally costly task of finding relevant transition-state saddles on force-deformed molecular potential energy surfaces (PESs). Finding a transition state often requires an initial guess about the pathway by which the reaction will proceed. Unfortunately, chemical intuition often fails when predicting likely consequences of mechanical stress applied to molecular systems. Here, we describe a fully deterministic method for finding mechanochemically relevant transition states and reaction pathways. The method is based on the observation that application of a sufficiently high mechanical force will eventually destabilize any molecular structure. Mathematically, such destabilization proceeds via a “catastrophe” occurring at a critical force where the energy minimum corresponding to the stable molecular structure coalesces with a transition state. Catastrophe theory predicts the force-deformed PES to have universal behavior in the vicinity of the critical force, allowing us to deduce the molecular structure of the transition state just below the critical force analytically. We then use the previously developed method of tracking transition-state evolution with the force to map out the entire reaction path and to predict the complete force dependence of the reaction barrier. Beyond its applications in mechanochemistry, this approach may be useful as a general method of finding transition states using fictitious forces to target specific reaction mechanisms.

The volatile fluorofullerene products of high-temperature reactions of C60 with the ternary manganese(III, IV) fluorides KMnF4, KMnF5, A2MnF6 (A+ = Li+, K+, Cs+), and K3MnF6 were monitored as a function of reaction temperature, reaction time, and stoichiometric ratio by in situ Knudsen-cell mass spectrometry. When combined with fluorofullerene product ratios from larger-scale (bulk) screening reactions with the same reagents, an optimized set of conditions was found that yielded the greatest amount of C60F8 (KMnF4/C60 mol ratio 28–30, 470 °C, 4–5 h). Two isomers of C60F8 were purified by HPLC, one of which has not been previously reported. Quantum chemical calculations at the DFT level combined with 1D and 2D 19F NMR, FTIR, and FT-Raman spectroscopy indicate that the C60F8 isomer previously reported to be 1,2,3,8,9,12,15,16-C60F8 is actually 1,2,3,6,9,12,15,18-C60F8, making it the first high-temperature fluorofullerene with non-contiguous fluorine atoms. The new isomer, which was found to be 1,2,7,8,9,12,13,14-C60F8, is predicted to be 5.5 kJ mol−1 more stable than 1,2,3,6,9,12,15,18-C60F8 at the DFT level. In addition, new DFT calculations and spectroscopic data indicate that the compound previously isolated from the high-temperature reaction of C60 and K2PtF6 and reported to be 16-CF3-1,2,3,8,9,12,15-C60F7 is actually 18-CF3-1,2,3,6,8,12,15-C60F7.

A novel isomer of C70(CF3)6 has been isolated by HPLC from a mixture prepared by trifluoromethylation of C70 with CF3COOAg. The X-ray structure revealed an unprecedented arrangement of CF3 groups forming a p3mp ribbon. This result provides additional evidence of the preferable formation of trifluoromethylated fullerene molecules comprising a single continuous ribbon of edge-sharing para- and meta-C6(CF3)2 hexagons.

Five C70(CF3)14 isomers have been isolated chromatographically from the mixture produced in the ampoule reaction between C70 and CF3I at 390 °C. Molecular structures of four isomers have been determined in a single-crystal X-ray diffraction study. A quantum chemical survey of the theoretically possible isomers demonstrated that the structures obtained are energetically favorable but that there is probably no full thermodynamic control in the trifluoromethylation process.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Electron ionization, thermal surface ionization, matrix-assisted laser desorption/ionization, electrospray ionization, and electron capture mass spectrometric techniques have been applied to the analysis of mixtures of trifluoromethylated C 60 derivatives. For the first time ionization energy and electron affinity values for some of the molecules under investigation have been reported. These ionization characteristics have allowed to explain the differences in the results obtained by means of different mass spectral techniques.

Singly and doubly charged anions of fluorofullerenes were produced by means of electrospray ionization from acetonitrile/dichlorobenzene solutions of C 60 F 36 and C 60 F 48 doped with organic electron donors. These anions can be formally viewed as products of mono- and bis-substitution of fluorine atoms by electrons. Photoelectron spectra of C 60 F 34 2− and C 60 F 46 2− allow estimating the second electron affinity of the corresponding neutral species to be 2.4 (1) and 3.2 (1) eV, respectively. Quantum chemical calculations at the DFT level of theory suggest that the observed dianions form due to fluorine rearrangements on the carbon shell, which is governed by their high relative stability.

We discuss how to define and to compute internal forces in a molecule subjected to mechanical stress. Because of the inherently many-body character of intramolecular interactions, internal forces cannot be uniquely defined without specifying a set of internal coordinates used to describe the molecular structure. When such a set is comprised of 3N − 6 interactomic distances (N being the number of atoms) and includes the bond lengths of interest, we show that the associated forces, while satisfying the equation F = ∂V/∂R (where R is the bond length, F is the internal force in this bond, and V is the potential energy of the molecule), can be determined from the molecular geometry alone. We illustrate these ideas using several toy models ranging from small molecules to a graphene sheet and show that the magnitude of the internal force in a bond is not necessarily a good predictor of its strength in response to mechanical loading. At the same time, analysis of internal forces reveals interesting phenomena such as the force multiplication effect, where weak external forces may, e.g., be used to break strong bonds, and offers insight into the catch-bond phenomenon where chemical reactivity is suppressed through application of a force.

We propose a method for computing the activation barrier for chemical reactions involving molecules subjected to mechanical stress. The method avoids reactant and transition-state saddle optimizations at every force by, instead, solving the differential equations governing the force dependence of the critical points (i.e., minima and saddles) on the system's potential energy surface (PES). As a result, only zero-force geometry optimization (or, more generally, optimization performed at a single force value) is required by the method. In many cases, minima and transition-state saddles only exist within a range of forces and disappear beyond a certain critical point. Our method identifies such force-induced instabilities as points at which one of the Hessian eigenvalues vanishes. We elucidate the nature of those instabilities as fold and cusp catastrophes, where two or three critical points on the force-modified PES coalesce, and provide a classification of various physically distinct instability scenarios, each illustrated with a concrete chemical example.

Non-equilibrium molecular dynamics simulations reveal that the thermal conductance of Si nanowires with periodic corrugations is lower than that of smooth wires with cross-sections equivalent to the constricted portions. This reduction in conductance is up to 30% and tends to plateau with increasing corrugation height. Spatially resolved temperature and heat current maps provide a microscopic understanding of this effect; we find that 80% of the heat current is carried through the constricted area even for high-amplitude corrugations. More importantly, we show that temperature gradient inversion and heat current vortices at the ridge peaks establish fundamental limits on maximum conductance reduction. (C) 2013 AIP Publishing LLC.

Quantum interference (QI) effects in molecular devices have drawn increasing attention over the past years due to their unique features observed in the conductance spectrum. For the further development of single molecular devices exploiting QI effects, it is of great theoretical and practical interest to develop simple methods controlling the emergence and the positions of QI effects like anti-resonances or Fano line shapes in conductance spectra. In this work, starting from a well-known generic molecular junction with a side group (T-shaped molecule), we propose a simple graphical method to visualize the conditions for the appearance of quantum interference, Fano resonances or anti-resonances, in the conductance spectrum. By introducing a simple graphical representation (parabolic diagram), we can easily visualize the relation between the electronic parameters and the positions of normal resonant peaks and anti-resonant peaks induced by quantum interference in the conductance spectrum. This parabolic model not only can predict the emergence and energetic position of quantum interference from a few electronic parameters but also can enable one to know the coupling between the side group and the main conduction channel from measurements in the case of orthogonal basis. The results obtained within the parabolic model are validated using density-functional based quantum transport calculations in realistic T-shaped molecular junctions.

Photoinduced charge generation (PCG) dynamics are notoriously difficult to correlate with specific molecular properties in device relevant polymer:fullerene organic photovoltaic blend films due to the highly complex nature of the solid state blend morphology. Here, this study uses six judiciously selected trifluoromethylfullerenes blended with the prototypical polymer poly(3-hexylthiophene) and measure the PCG dynamics in 50 fs–500 ns time scales with time-resolved microwave conductivity and femtosecond transient absorption spectroscopy. The isomeric purity and thorough chemical characterization of the fullerenes used in this study allow for a detailed correlation between molecular properties, driving force, local intermolecular electronic coupling and, ultimately, the efficiency of PCG yield. The findings show that the molecular design of the fullerene not only determines inter-fullerene electronic coupling, but also influences the decay dynamics of free holes in the donor phase even when the polymer microstructure remains unchanged.

Endohedral metallofullerenes (EMFs) are able to encapsulate up to four metal atoms. In EMFs, metal atoms are positively charged because of the electron transfer from the endohedral metal atoms to the carbon cage. It results in the strong Coulomb repulsion between the positively charged ions trapped in the confined inner space of the fullerene. At the same time, in many EMFs, such as Lu2@C76, Y2@C79N, M2@C82 (M = Sc, Y, Lu, etc.), Y3@C80, or Sc4O2@C80, metals do not adopt their highest oxidation states, thus yielding a possibility of the covalent metal–metal bonding. In some other EMFs (e.g., La2@C80), metal–metal bonding evolves as the result of the electrochemical or chemical reduction, which leads to the population of the metal-based LUMO with pronounced metal–metal bonding character. This article highlights different aspects of the metal–metal bonding in EMFs. It is concluded that the valence state of the metal atoms in dimetallofullerenes is not dependent on their third ionization potential, but is determined by their ns2(n − 1)d1 → ns1(n − 1)d2 excitation energies. Peculiarities of the metal–metal bonding in EMFs are described in terms of molecular orbital analysis as well as topological approaches such as Quantum Theory of Atoms in Molecules and Electron Localization Function. Interplay of Coulomb repulsion and covalent bonding is analyzed in the framework of the Interacting Quantum Atom approach.

We shed light on the catalyst-free growth of graphene from amorphous carbon (a-C) by current-induced annealing by witnessing the mechanism both with in-situ transmission electron microscopy and with molecular dynamics simulations. Both in experiment and in simulation, we observe that small a-C clusters on top of a graphene substrate rearrange and crystallize into graphene patches. The process is aided by the high temperatures involved and by the van der Waals interactions with the substrate. Furthermore, in the presence of a-C, graphene can grow from the borders of holes and form a seamless graphene sheet, a novel finding that has not been reported before and that is reproduced by the simulations as well. These findings open up new avenues for bottom-up engineering of graphene-based devices.

Simply super! The perfluoroalkylfullerene C(60)(C(2)F(5))(5) H is the first structurally characterized perfluoroalkylated fullerene-based acid and is also predicted to be the first gas-phase fullerene-based superacid.

Charge transport characteristics of short double-strand (ds) DNA including mismatches are stud- ied within a methodology combining molecular dynamics (MD) simulations and electronic structure calculations based on a fragment orbital approach. Electronic parameters and transmission prob- abilities are computed along the MD trajectory. We find that in the course of the MD simulation the energetic position of frontier orbitals may be interchanged. As a result, the highest occupied molecular orbital (HOMO) can temporarily have a large weight on the backbones as a function of time. This shows that care must be taken when projecting the electronic structure onto effective low-dimensional model Hamiltonians to calculate transport properties. Further, the transport cal- culations indicate a suppression of the charge migration efficiency when introducing a single GT or AC mismatch in the DNA sequence.

Organic Zener diodes with a precisely adjustable reverse breakdown from -3 V to -15 V without any influence on the forward current-voltage curve are realized. This is accomplished by controlling the width of the charge depletion zone in a pin-diode with an accuracy of one nanometer independently of the doping concentration and the thickness of the intrinsic layer. The breakdown effect with its exponential current voltage behavior and a weak temperature dependence is explained by a tunneling mechanism across the HOMO-LUMO gap of neigh- boring molecules. The experimental data are confirmed by a minimal Hamiltonian model approach, including coherent tunneling and incoherent hopping processes as possible charge transport pathways through the effective device region., to appear in Nano Letters Article ASAP (2010)

In situ low-voltage aberration corrected transmission electron microscopy (TEM) observations of the dynamic entrapment of a C60 molecule in the saddle of a bent double-walled carbon nanotube is presented. The fullerene interaction is non-covalent, suggesting that enhanced π-π interactions (van der Waals forces) are responsible. Classical molecular dynamics calculations confirm that the increased interaction area associated with a buckle is sufficient to trap a fullerene. Moreover, they show hopping behavior in agreement with our experimental observations. Our findings further our understanding of carbon nanostructure interactions, which are important in the rapidly developing field of low-voltage aberration corrected TEM and nano-carbon device fabrication. © The Author(s) 2010.

New isomers of C60(CF3)12 and C60(CF3)14 have been isolated from mixtures obtained via reaction of C60 or S6-C60(CF3)12 with CF3I; they were characterized by single crystal XRD study and investigated theoretically by means of DFT calculations.

A number of C60(CF3)n compounds with n = 2–10 have been synthesized by the reaction of C60 with silver trifluoroacetate and successfully isolated by means of HPLC. This resulted in the first crystal structure determination of six lower trifluoromethyl derivatives with n = 2 (single isomer), 4 (two isomers), and 6 (three isomers). A kinetic model of sequential trifluoromethylation based on the Bell–Evans–Polanyi principle has been used to explain the experimentally observed isomeric distribution in the mixtures of C60(CF3)n compounds up ton = 6. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

New isomers of C(60)(CF(3))(12) and C(60)(CF(3))(14) have been isolated from mixtures obtained via reaction of C(60) or S(6)-C(60)(CF(3))(12) with CF(3)I; they were characterized by single crystal XRD study and investigated theoretically by means of DFT calculations.

X-ray diffraction study of the substance originally believed to be C(60)F(16) reveals a double-caged structure, (C(60)F(16))(C(60)); MALDI mass spectra, 19F NMR spectral data and reasons for stability are discussed.

Refluxing of the o-DCB solution of C60 with CF2ClCOONa and 18-crown-6 leads to formation of C60(CF2)n (n = 1–3); the monoadduct C60(CF2) has been found to consist of the main [6,6]- and minor [5,6]-isomers, both having an open structure.

Two isomers of C70(CF3)12 have been isolated from a mixture obtained by trifluoromethylation of C70 with CF3I; their molecular structures determined by X-ray crystallography are in good agreement with the results of theoretical DFT calculations for the most stable C70(CF3)12 isomers.