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Zhao, Jiong,Deng, Qingming,Avdoshenko, Stanislav M.,Fu, Lei,Eckert, Jü,rgen,Rü,ü,mmeli, Mark H. National Academy of Sciences 2014 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.111 No.44
<P><B>Significance</B></P><P>The single metal atom has been proposed to be a catalyst during the growth of carbon nanotubes; however, this hypothesis is still not confirmed. Our direct in situ transmission EM observation of the restructuring of the graphene edges interacting with an Fe atom directly revealed the intermediate states: pentagon and hexagon structures. In particular, our experiments and simulations show that the single Fe atom behaves differently on the graphene zigzag and armchair edges, giving insights to the growth mechanisms of various sp<SUP>2</SUP> carbon structures.</P><P>Single-atom catalysts are of great interest because of their high efficiency. In the case of chemically deposited sp<SUP>2</SUP> 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.</P>
Organic Zener Diodes: Tunneling across the Gap in Organic Semiconductor Materials
Kleemann, Hans,Gutierrez, Rafael,Lindner, Frank,Avdoshenko, Stanislav,Manrique, Pedro D.,Lü,ssem, Bjö,rn,Cuniberti, Gianaurelio,Leo, Karl American Chemical Society 2010 Nano letters Vol.10 No.12
<P>Organic Zener diodes with a precisely adjustable reverse breakdown from −3 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 highest occupied molecular orbital−lowest unoccupied molecular orbital gap of neighboring 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.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/nalefd/2010/nalefd.2010.10.issue-12/nl102916n/production/images/medium/nl-2010-02916n_0005.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/nl102916n'>ACS Electronic Supporting Info</A></P>
Popov, Alexey A.,Schiemenz, Sandra,Avdoshenko, Stanislav M.,Yang, Shangfeng,Cuniberti, Gianaurelio,Dunsch, Lothar American Chemical Society 2011 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.115 No.31
<P>While the role of asymmetric nitride clusters on the cage size and symmetry in fullerene-based structures is already well-known, the role of the asymmetric arrangement of metals in nitride clusters on the nitrogen is studied in detail in this work. It is discovered that asymmetric mixed-metal nitride clusters give sufficiently narrow <SUP>14</SUP>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<SUB><I>x</I></SUB>Sc<SUB>3–<I>x</I></SUB>N@C<SUB>80</SUB> and Lu<SUB><I>x</I></SUB>Y<SUB>3–<I>x</I></SUB>N@C<SUB>80</SUB> (<I>x</I> = 0–3) the δ(<SUP>14</SUP>N) values are found to be linear functions of <I>x</I> showing that <SUP>14</SUP>N chemical shifts are additive values with specific increment for each kind of metal atoms. Density functional theory calculations are performed to interpret the experimentally measured spectra. To reveal the main factors affecting <SUP>14</SUP>N chemical shifts in nitride clusterfullerenes, shielding tensor components are analyzed in terms of Ramsey theory both in localized and canonical molecular orbitals. <SUP>14</SUP>N chemical shifts in M<SUB>3</SUB>N@C<SUB>80</SUB> and related systems are shown to be determined solely by nitrogen-localized orbitals and in particular by the p<SUB><I>x,y,z</I></SUB> 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.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2011/jpccck.2011.115.issue-31/jp204290f/production/images/medium/jp-2011-04290f_0004.gif'></P>
Liu, Fupin,Spree, Lukas,Krylov, Denis S.,Velkos, Georgios,Avdoshenko, Stanislav M.,Popov, Alexey A. American Chemical Society 2019 Accounts of chemical research Vol.52 No.10
<P><B>Conspectus</B></P><P>A characteristic phenomenon of lanthanide-fullerene interactions is the transfer of metal valence electrons to the carbon cage. With early lanthanides such as La, a complete transfer of six valence electrons takes place for the metal dimers encapsulated in the fullerene cage. However, the low energy of the σ-type Ln-Ln bonding orbital in the second half of the lanthanide row limits the Ln<SUB>2</SUB> → fullerene transfer to only five electrons. One electron remains in the Ln-Ln bonding orbital, whereas the fullerene cage with a formal charge of −5 is left electron-deficient. Such Ln<SUB>2</SUB>@C<SUB>80</SUB> molecules are unstable in the neutral form but can be stabilized by substitution of one carbon atom by nitrogen to give azafullerenes Ln<SUB>2</SUB>@C<SUB>79</SUB>N or by quenching the unpaired electron on the fullerene cage by reacting it with a chemical such as benzyl bromide, transforming one sp<SUP>2</SUP> carbon into an sp<SUP>3</SUP> carbon and yielding the monoadduct Ln<SUB>2</SUB>@C<SUB>80</SUB>(CH<SUB>2</SUB>Ph). Because of the presence of the Ln-Ln bonding molecular orbital with one electron, the Ln<SUB>2</SUB>@C<SUB>79</SUB>N and Ln<SUB>2</SUB>@C<SUB>80</SUB>(R) molecules feature a unique single-electron Ln-Ln bond and an unconventional +2.5 oxidation state of the lanthanides.</P><P>In this Account, which brings together metallofullerenes, molecular magnets, and lanthanides in unconventional valence states, we review the progress in the studies of dimetallofullerenes with single-electron Ln-Ln bonds and highlight the consequences of the unpaired electron residing in the Ln-Ln bonding orbital for the magnetic interactions between Ln ions. Usually, Ln···Ln exchange coupling in polynuclear lanthanide compounds is weak because of the core nature of 4f electrons. However, when interactions between Ln centers are mediated by a radical bridge, stronger coupling may be achieved because of the diffuse nature of radical-based orbitals. Ultimately, when the role of a radical bridge is played by a single unpaired electron in the Ln-Ln bonding orbital, the strength of the exchange coupling is increased dramatically. Giant exchange coupling in endohedral Ln<SUB>2</SUB> dimers is combined with a rather strong axial ligand field exerted on the lanthanide ions by the fullerene cage and the excess electron density localized between two Ln ions. As a result, Ln<SUB>2</SUB>@C<SUB>79</SUB>N and Ln<SUB>2</SUB>@C<SUB>80</SUB>(CH<SUB>2</SUB>Ph) compounds exhibit slow relaxation of magnetization and exceptionally high blocking temperatures for Ln = Dy and Tb. At low temperatures, the [Ln<SUP>3+</SUP>-e-Ln<SUP>3+</SUP>] fragment behaves as a single giant spin. Furthermore, the Ln-Ln bonding orbital in dimetallofullerenes is redox-active, which allows its population to be changed by electrochemical reactions, thus changing the magnetic properties because the change in the number of electrons residing in the Ln-Ln orbital affects the magnetic structure of the molecule.</P> [FIG OMISSION]</BR>
Velkos, Georgios,Krylov, Denis S.,Kirkpatrick, Kyle,Spree, Lukas,Dubrovin, Vasilii,Bü,chner, Bernd,Avdoshenko, Stanislav M.,Bezmelnitsyn, Valeriy,Davis, Sean,Faust, Paul,Duchamp, James,Dorn, Harry John Wiley and Sons Inc. 2019 Angewandte Chemie. international edition Vol.58 No.18
<P><B>Abstract</B></P><P>The azafullerene Tb<SUB>2</SUB>@C<SUB>79</SUB>N is found to be a single‐molecule magnet with a high 100‐s blocking temperature of magnetization of 24 K and large coercivity. Tb magnetic moments with an easy‐axis single‐ion magnetic anisotropy are strongly coupled by the unpaired spin of the single‐electron Tb−Tb bond. Relaxation of magnetization in Tb<SUB>2</SUB>@C<SUB>79</SUB>N below 15 K proceeds via quantum tunneling of magnetization with the characteristic time <I>τ</I><SUB>QTM</SUB>=16 462±1230 s. At higher temperature, relaxation follows the Orbach mechanism with a barrier of 757±4 K, corresponding to the excited states, in which one of the Tb spins is flipped.</P>