Browsing by Author "Avdoshenko, Stanislav M."
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- ItemCarbon cage isomers and magnetic Dy⋯Dy interactions in Dy2O@C88 and Dy2C2@C88 metallofullerenes(Cambridge : RSC, 2022) Yang, Wei; Velkos, Georgios; Sudarkova, Svetlana; Büchner, Bernd; Avdoshenko, Stanislav M.; Liu, Fupin; Popov, Alexey A.; Chen, NingThree isomers of Dy2O@C88 and two isomers of Dy2C2@C88 were synthesized and structurally characterized by single-crystal X-ray diffraction, vibrational spectroscopy, and DFT calculations. Both types of clusterfullerenes feature 4-fold electron transfer to the carbon cage, thus resulting in the same carbon cage isomers identified as C1(26), Cs(32), and D2(35). The studies of Dy⋯Dy superexchange interactions in Dy2O and Dy2C2 clusters revealed that the O2− bridge favors antiferromagnetic coupling whereas the acetylide group C22− supports ferromagnetic coupling of Dy magnetic moments. The strength of the coupling showed a considerable variability in different cage isomers. All metallofullerenes exhibited slow relaxation of magnetization and magnetic hysteresis. In Dy2O@C88 isomers the hysteresis remained open up to 7-9 K, while in Dy2C2@C88 the hysteresis loops were closed already at 2.5 K. This study demonstrated that both the endohedral bridge between metal atoms and the fullerene cage play an important role in magnetic interactions and relaxation of magnetization.
- ItemEndohedral metal-nitride cluster ordering in metallofullerene-NiII(OEP) complexes and crystals : A theoretical study(Cambridge : RSC Publ., 2019) Dubrovin, Vasilii; Gan, Li-Hua; Büchner, Bernd; Popov, Alexey A.; Avdoshenko, Stanislav M.The ordering of endohedral clusterfullerenes Sc3N@C80 and YSc2N@C80 co-crystallized with Ni(OEP) and isolated complexes with Ni(OEP) have been investigated theoretically. Having used multiple orientations of M3N clusters inside the cages with Fibonacci sampling, we describe the effect of intermolecular interactions on the orientation of the endohedral cluster. © the Owner Societies.
- ItemFulleretic well-defined scaffolds: Donor–fullerene alignment through metal coordination and its effect on photophysics(Hoboken, NJ : Wiley, 2016) Williams, Derek E.; Dolgopolova, Ekaterina A.; Godfrey, Danielle C.; Ermolaeva, Evgeniya D.; Pellechia, Perry J.; Greytak, Andrew B.; Smith, Mark D.; Avdoshenko, Stanislav M.; Popov, Alexey A.; Shustova, Natalia B.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 13C 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.
- ItemHierarchical Corannulene‐Based Materials: Energy Transfer and Solid‐State Photophysics(Weinheim : Wiley-VCH, 2017-3-23) Rice, Allison M.; Fellows, W. Brett; Dolgopolova, Ekaterina A.; Greytak, Andrew B.; Vannucci, Aaron K.; Smith, Mark D.; Karakalos, Stavros G.; Krause, Jeanette A.; Avdoshenko, Stanislav M.; Popov, Alexey A.; Shustova, Natalia B.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.
- ItemHigh Blocking Temperature of Magnetization and Giant Coercivity in the Azafullerene Tb 2 @C 79 N with a Single-Electron Terbium–Terbium Bond(Weinheim : Wiley-VCH, 2019) Velkos, Georgios; Krylov, Denis S.; Kirkpatrick, Kyle; Spree, Lukas; Dubrovin, Vasilii; Büchner, Bernd; Avdoshenko, Stanislav M.; Bezmelnitsyn, Valeriy; Davis, Sean; Faust, Paul; Duchamp, James; Dorn, Harry C.; Popov, Alexey A.The azafullerene Tb 2 @C 79 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 2 @C 79 N below 15 K proceeds via quantum tunneling of magnetization with the characteristic time τ QTM =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. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemMagnetic hysteresis and strong ferromagnetic coupling of sulfur-bridged Dy ions in clusterfullerene Dy2S@C82(Cambridge : RSC, 2020) Krylov, Denis; Velkos, Georgios; Chen, Chia-Hsiang; Büchner, Bernd; Kostanyan, Aram; Greber, Thomas; Avdoshenko, Stanislav M.; Popov, Alexey A.Two isomers of metallofullerene Dy2S@C82 with sulfur-bridged Dy ions exhibit broad magnetic hysteresis with sharp steps at sub-Kelvin temperature. Analysis of the level crossing events for different orientations of a magnetic field showed that even in powder samples, the hysteresis steps caused by quantum tunneling of magnetization can provide precise information on the strength of intramolecular Dy⋯Dy interactions. A comparison of different methods to determine the energy difference between ferromagnetic and antiferromagnetic states showed that sub-Kelvin hysteresis gives the most robust and reliable values. The ground state in Dy2S@C82 has ferromagnetic coupling of Dy magnetic moments, whereas the state with antiferromagnetic coupling in Cs and C3v cage isomers is 10.7 and 5.1 cm-1 higher, respectively. The value for the Cs isomer is among the highest found in metallofullerenes and is considerably larger than that reported in non-fullerene dinuclear molecular magnets. Magnetization relaxation times measured in zero magnetic field at sub-Kelvin temperatures tend to level off near 900 and 3200 s in Cs and C3v isomers. These times correspond to the quantum tunneling relaxation mechanism, in which the whole magnetic moment of the Dy2S@C82 molecule flips at once as a single entity. © the Partner Organisations.
- ItemMagnetic Hysteresis at 10 K in Single Molecule Magnet Self‐Assembled on Gold(Weinheim : Wiley-VCH, 2021) Chen, Chia-Hsiang; Spree, Lukas; Koutsouflakis, Emmanouil; Krylov, Denis S.; Liu, Fupin; Brandenburg, Ariane; Velkos, Georgios; Schimmel, Sebastian; Avdoshenko, Stanislav M.; Federov, Alexander; Weschke, Eugen; Choueikani, Fadi; Ohresser, Philippe; Dreiser, Jan; Büchner, Bernd; Popov, Alexey A.Tremendous progress in the development of single molecule magnets (SMMs) raises the question of their device integration. On this route, understanding the properties of low‐dimensional assemblies of SMMs, in particular in contact with electrodes, is a necessary but difficult step. Here, it is shown that fullerene SMM self‐assembled on metal substrate from solution retains magnetic hysteresis up to 10 K. Fullerene‐SMM DySc2N@C80 and Dy2ScN@C80 are derivatized to introduce a thioacetate group, which is used to graft SMMs on gold. Magnetic properties of grafted SMMs are studied by X‐ray magnetic circular dichroism and compared to the films of nonderivatized fullerenes prepared by sublimation. In self‐assembled films, the magnetic moments of the Dy ions are preferentially aligned parallel to the surface, which is different from the disordered orientation of endohedral clusters in nonfunctionalized fullerenes. Whereas chemical derivatization reduces the blocking temperature of magnetization and narrows the hysteresis of Dy2ScN@C80, for DySc2N@C80 equally broad hysteresis is observed as in the fullerene multilayer. Magnetic bistability in the DySc2N@C80 grafted on gold is sustained up to 10 K. This study demonstrates that self‐assembly of fullerene‐SMM derivatives offers a facile solution‐based procedure for the preparation of functional magnetic sub‐monolayers with excellent SMM performance.
- ItemRobust Single Molecule Magnet Monolayers on Graphene and Graphite with Magnetic Hysteresis up to 28 K(Weinheim : Wiley-VCH, 2021) Spree, Lukas; Liu, Fupin; Neu, Volker; Rosenkranz, Marco; Velkos, Georgios; Wang, Yaofeng; Schiemenz, Sandra; Dreiser, Jan; Gargiani, Pierluigi; Valvidares, Manuel; Chen, Chia-Hsiang; Büchner, Bernd; Avdoshenko, Stanislav M.; Popov, Alexey A.The chemical functionalization of fullerene single molecule magnet Tb2@C80(CH2Ph) enables the facile preparation of robust monolayers on graphene and highly oriented pyrolytic graphite from solution without impairing their magnetic properties. Monolayers of endohedral fullerene functionalized with pyrene exhibit magnetic bistability up to a temperature of 28 K. The use of pyrene terminated linker molecules opens the way to devise integration of spin carrying units encapsulated by fullerene cages on graphitic substrates, be it single-molecule magnets or qubit candidates. © 2021 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH
- ItemSingle-Electron Lanthanide-Lanthanide Bonds Inside Fullerenes toward Robust Redox-Active Molecular Magnets(Washington, DC : ACS Publications, 2019) Liu, Fupin; Spree, Lukas; Krylov, Denis S.; Velkos, Georgios; Avdoshenko, Stanislav M.; Popov, Alexey A.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 Ln2 → 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 Ln2@C80 molecules are unstable in the neutral form but can be stabilized by substitution of one carbon atom by nitrogen to give azafullerenes Ln2@C79N or by quenching the unpaired electron on the fullerene cage by reacting it with a chemical such as benzyl bromide, transforming one sp2 carbon into an sp3 carbon and yielding the monoadduct Ln2@C80(CH2Ph). Because of the presence of the Ln-Ln bonding molecular orbital with one electron, the Ln2@C79N and Ln2@C80(R) molecules feature a unique single-electron Ln-Ln bond and an unconventional +2.5 oxidation state of the lanthanides.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 Ln2 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, Ln2@C79N and Ln2@C80(CH2Ph) compounds exhibit slow relaxation of magnetization and exceptionally high blocking temperatures for Ln = Dy and Tb. At low temperatures, the [Ln3+-e-Ln3+] 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. © 2019 American Chemical Society.
- ItemSingle-Molecule Magnets DyM2N@C80 and Dy2MN@C80 (M=Sc, Lu): The Impact of Diamagnetic Metals on Dy3+ Magnetic Anisotropy, Dy⋅⋅⋅Dy Coupling, and Mixing of Molecular and Lattice Vibrations(Weinheim : Wiley-VCH, 2020) Spree, Lukas; Schlesier, Christin; Kostanyan, Aram; Westerström, Rasmus; Greber, Thomas; Büchner, Bernd; Avdoshenko, Stanislav M.; Popov, Alexey A.The substitution of scandium in fullerene single-molecule magnets (SMMs) DySc2N@C80 and Dy2ScN@C80 by lutetium has been studied to explore the influence of the diamagnetic metal on the SMM performance of dysprosium nitride clusterfullerenes. The use of lutetium led to an improved SMM performance of DyLu2N@C80, which shows a higher blocking temperature of magnetization (TB=9.5 K), longer relaxation times, and broader hysteresis than DySc2N@C80 (TB=6.9 K). At the same time, Dy2LuN@C80 was found to have a similar blocking temperature of magnetization to Dy2ScN@C80 (TB=8 K), but substantially different interactions between the magnetic moments of the dysprosium ions in the Dy2MN clusters. Surprisingly, although the intramolecular dipolar interactions in Dy2LuN@C80 and Dy2ScN@C80 are of similar strength, the exchange interactions in Dy2LuN@C80 are close to zero. Analysis of the low-frequency molecular and lattice vibrations showed strong mixing of the lattice modes and endohedral cluster librations in k-space. This mixing simplifies the spin–lattice relaxation by conserving the momentum during the spin flip and helping to distribute the moment and energy further into the lattice. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemStripe-yzmagnetic order in the triangular-lattice antiferromagnet KCeS2(Bristol : IOP Publ., 2021) Kulbakov, Anton A.; Avdoshenko, Stanislav M.; Puente-Orench, Inés; Deeb, Mahmoud; Doerr, Mathias; Schlender, Philipp; Doert, Thomas; Inosov, Dmytro S.Yb- and Ce-based delafossites were recently identified as effective spin-1/2 antiferromagnets on the triangular lattice. Several Yb-based systems, such as NaYbO2, NaYbS2, and NaYbSe2, exhibit no long-range order down to the lowest measured temperatures and therefore serve as putative candidates for the realization of a quantum spin liquid. However, their isostructural Ce-based counterpart KCeS2 exhibits magnetic order below TN = 400 mK, which was so far identified only in thermodynamic measurements. Here we reveal the magnetic structure of this long-range ordered phase using magnetic neutron diffraction. We show that it represents the so-called 'stripe-yz' type of antiferromagnetic order with spins lying approximately in the triangular-lattice planes orthogonal to the nearest-neighbor Ce–Ce bonds. No structural lattice distortions are revealed below TN, indicating that the triangular lattice of Ce3+ ions remains geometrically perfect down to the lowest temperatures. We propose an effective Hamiltonian for KCeS2, based on a fit to the results of ab initio calculations, and demonstrate that its magnetic ground state matches the experimental spin structure.
- ItemSubstrate-Independent Magnetic Bistability in Monolayers of the Single-Molecule Magnet Dy2ScN@C80 on Metals and Insulators(Weinheim : Wiley-VCH, 2020) Krylov, Denis S.; Schimmel, Sebastian; Dubrovin, Vasilii; Liu, Fupin; Nguyen, T.T. Nhung; Spree, Lukas; Chen, Chia-Hsiang; Velkos, Georgios; Bulbucan, Claudiu; Westerström, Rasmus; Studniarek, Michał; Dreiser, Jan; Hess, Christian; Büchner, Bernd; Avdoshenko, Stanislav M.; Popov, Alexey A.Magnetic hysteresis is demonstrated for monolayers of the single-molecule magnet (SMM) Dy2ScN@C80 deposited on Au(111), Ag(100), and MgO|Ag(100) surfaces by vacuum sublimation. The topography and electronic structure of Dy2ScN@C80 adsorbed on Au(111) were studied by STM. X-ray magnetic CD studies show that the Dy2ScN@C80 monolayers exhibit similarly broad magnetic hysteresis independent on the substrate used, but the orientation of the Dy2ScN cluster depends strongly on the surface. DFT calculations show that the extent of the electronic interaction of the fullerene molecules with the surface is increasing dramatically from MgO to Au(111) and Ag(100). However, the charge redistribution at the fullerene-surface interface is fully absorbed by the carbon cage, leaving the state of the endohedral cluster intact. This Faraday cage effect of the fullerene preserves the magnetic bistability of fullerene-SMMs on conducting substrates and facilitates their application in molecular spintronics. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
- ItemUnderstanding the catalyst-free transformation of amorphous carbon into graphene by current-induced annealing(London : Nature Publishing Group, 2013) Barreiro, Amelia; Börrnert, Felix; Avdoshenko, Stanislav M.; Rellinghaus, Bernd; Cuniberti, Gianaurelio; Rümmeli, Mark H.; Vandersypen, Lieven M.K.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.
- ItemUsing internal strain and mass to modulate Dy⋯Dy coupling and relaxation of magnetization in heterobimetallic metallofullerenes DyM2N@C80 and Dy2MN@C80 (M = Sc, Y, La, Lu)(Cambridge : RSC, 2022) Hao, Yajuan; Velkos, Georgios; Schiemenz, Sandra; Rosenkranz, Marco; Wang, Yaofeng; Büchner, Bernd; Avdoshenko, Stanislav M.; Popov, Alexey A.; Liu, FupinEndohedral clusters inside metallofullerenes experience considerable inner strain when the size of the hosting cage is comparably small. This strain can be tuned in mixed-metal metallofullerenes by combining metals of different sizes. Here we demonstrate that the internal strain and mass can be used as variables to control Dy⋯Dy coupling and relaxation of magnetization in Dy-metallofullerenes. Mixed-metal nitride clusterfullerenes DyxY3−xN@Ih-C80 (x = 0-3) and Dy2LaN@Ih-C80 combining Dy with diamagnetic rare-earth elements, Y and La, were synthesized and characterized by single-crystal X-ray diffraction, SQUID magnetometry, ab initio calculations, and spectroscopic techniques. DyxY3−xN clusters showed a planar structure, but the slightly larger size of Dy3+ in comparison with that of Y3+ resulted in increased elongation of the nitrogen thermal ellipsoid, showing enhancement of the out-of-plane vibrational amplitude. When Dy was combined with larger La, the Dy2LaN cluster appeared strongly pyramidal with the distance between two nitrogen sites of 1.15(1) Å, whereas DyLa2N@C80 could not be obtained in a separable yield. Magnetic studies revealed that the relaxation of magnetization and blocking temperature of magnetization in the DyM2N@C80 series (M = Sc, Y, Lu) correlated with the mass of M, with DySc2N@C80 showing the fastest and DyLu2N@C80 the slowest relaxation. Ab initio calculations predicted very similar g-tensors for Dy3+ ground state pseudospin in all studied DyM2N@C80 molecules, suggesting that the variation in relaxation is caused by different vibrational spectra of these compounds. In the Dy2MN@C80 series (M = Sc, Y, La, Lu), the magnetic and hysteretic behavior was found to correlate with Dy⋯Dy coupling, which in turn appears to depend on the size of M3+. Across the Dy2MN@C80 series, the energy difference between ferromagnetic and antiferromagnetic states changes from 5.6 cm−1 in Dy2ScN@C80 to 3.0 cm−1 in Dy2LuN@C80, 1.0 cm−1 in Dy2YN@C80, and −0.8 cm−1 in Dy2LaN@C80. The coupling of Dy ions suppresses the zero-field quantum tunnelling of magnetization but opens new relaxation channels, making the relaxation rate dependent on the coupling strengths. DyY2N@C80 and Dy2YN@C80 were found to be non-luminescent, while the luminescence reported for DyY2N@C80 was caused by traces of Y3N@C80 and Y2ScN@C80