Ferromagnetic Graphene Nanoribbons: Edge Termination with Organic Radicals

Cho, Daeheum,Ko, Kyoung Chul,Park, Heesoo,Lee, Jin Yong American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.18

<P>The intramolecular magnetic exchange coupling of edge terminated zigzag graphene nanoribbon (ZGNR) was studied with density functional theory calculations. In order to examine the applicability of the spin alternation rule and a classification scheme for radicals and couplers on functionalized graphene nanoribbons, we investigated the magnetic behaviors of pristine zigzag graphene nanoribbon with eight zigzag chains (8-ZGNR) and 8-ZGNRs terminated with trimethylenemethane (TMM) and 6-oxoverdazyl (OVER) radicals,that is, TMM-ZGNR-TMM (TZT), OVER-ZGNR-OVER (OZO), and TMM-ZGNR-OVER (TZO). As expected, only ZGNR terminated with different group radicals on each edge (TZO) had a ferromagnetic (high-spin) ground state with an energy gap of 39 meV/supercell (321.57 cm<SUP>–1</SUP>) relative to the low-spin state. This strongly supports the validity of the spin alternation rule and the classification scheme for radicals and couplers on extensively conjugated large graphene nanoribbons. TZT and OZO were found to have an antiferromagnetic (low-spin) ground state with magnetic coupling weaker than that of interedge antiferromagnetic superexchange of pristine 8-ZGNR. Based on the spin distribution pattern on magnetic ground states, GNR prefers to have each edge in antiferromagnetic order, which satisfies Lieb’s theorem on the Hubbard model and spin alternation rule. All of the terminated ZGNRs exhibited semiconducting properties with an energy gap of 0.06–0.21 eV.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2015/jpccck.2015.119.issue-18/acs.jpcc.5b01288/production/images/medium/jp-2015-01288f_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp5b01288'>ACS Electronic Supporting Info</A></P>

Organic magnetic diradicals (radical-coupler-radical): standardization of couplers for strong ferromagnetism.

Cho, Daeheum,Ko, Kyoung Chul,Lee, Jin Yong American Chemical Society 2014 The Journal of physical chemistry A Vol.118 No.27

<P>The intramolecular magnetic coupling constant (J) values of sets of diradicals linked to bis-DTDA, OVER, and NN radicals (DTDA, OVER, and NN groups) through an aromatic coupler were studied by unrestricted density functional theory calculations (UB3LYP/6-311++G(d,p)). Among 15 aromatic couplers, 9 compounds with an odd number of carbon atoms along its spin coupling path were found to interact ferromagnetically upon coupling with bisradicals while the other 6 couplers with an even number of carbon atoms along its spin coupling path give rise to antiferromagnetic coupling. The overall trends in the strength of magnetic interactions of aromatic couplers were preserved for DTDA, OVER, and NN groups so that the trend can be utilized as an index for the magnetic strength of a given coupler. It was found that the differences in the nucleus-independent chemical shift (NICS), bond order of connecting bonds, and Mulliken atomic spin density at connected atoms between triplet and BS states are closely related to the intramolecular magnetic behavior. 2,4- and 2,5-phosphole couplers exhibit the strongest intramolecular ferromagnetic and antiferromagnetic interactions among 15 aromatic couplers when linked to diverse bisradicals.</P>

Effect of Hartree-Fock exact exchange on intramolecular magnetic coupling constants of organic diradicals

Cho, Daeheum,Ko, Kyoung Chul,Ikabata, Yasuhiro,Wakayama, Kazufumi,Yoshikawa, Takeshi,Nakai, Hiromi,Lee, Jin Yong American Institute of Physics 2015 The Journal of chemical physics Vol.142 No.2

<P>The intramolecular magnetic coupling constant (J) of diradical systems linked with five- or six-membered aromatic rings was calculated to obtain the scaling factor (experimental J/calculated J ratio) for various density functional theory (DFT) functionals. Scaling factors of group A (PBE, TPSSh, B3LYP, B97-1, X3LYP, PBE0, and BH&HLYP) and B (M06-L, M06, M06-2X, and M06-HF) were shown to decrease as the amount of Hartree-Fock exact exchange (HFx) increases, in other words, overestimation of calculated J becomes more severe as the HFx increases. We further investigated the effect of HFx fraction of DFT functional on J value, spin contamination, and spin density distributions by comparing the B3LYP analogues containing different amount of HFx. It was revealed that spin contamination and spin densities at each atom increases as the HFx increases. Above all, newly developed BLYP-5 functional, which has 5% of HFx, was found to have the scaling factor of 1.029, indicating that calculated J values are very close to that of experimental values without scaling. BLYP-5 has potential to be utilized for accurate evaluation of intramolecular magnetic coupling constant (J) of diradicals linked by five- or six-membered aromatic ring couplers.</P>

Theoretical Study of Extremely Long yet Stable Carbon-Carbon Bonds: Effect of Attractive C···H Interactions and Small Radical Stabilization of Diamondoids

Cho, Daeheum,Ikabata, Yasuhiro,Yoshikawa, Takeshi,Lee, Jin Yong,Nakai, Hiromi Chemical Society of Japan 2015 Bulletin of the Chemical Society of Japan Vol. No.

Effects of size and interparticle interaction of silica nanoparticles on dispersion and electrical conductivity of silver/epoxy nanocomposites

Woo Cho, Hyun,Nam, Seungwoong,Lim, Soonho,Kim, Daeheum,Kim, Heesuk,June Sung, Bong American Institute of Physics 2014 Journal of Applied Physics Vol.115 No.15

<P>The agglomeration of nanoparticles (NPs) occurs due to attractive interaction between NPs and worsens the physical properties of materials such as electrical conductivity. When the attractive interaction is sufficiently strong, the agglomerates of NPs may be arrested dynamically in non-equilibrium state with a large relaxation time. We show that when conductive silver NPs form agglomerates in epoxy matrices, one can tune the effective interaction between silver NPs in epoxy matrices by introducing auxiliary non-conductive silica NPs and may prevent the agglomeration easily. More interestingly, as the size of the auxiliary silica NPs decreases, the silver NPs disperse better, thus increasing the electrical conductivity by orders of magnitude. We also perform Monte Carlo simulations and show that the auxiliary silica NPs influence the morphology of silver NPs not entropically but energetically. (C) 2014 AIP Publishing LLC.</P>

Doping Effect on Edge-Terminated Ferromagnetic Graphene Nanoribbons

Nam, Yeonsig,Cho, Daeheum,Lee, Jin Yong American Chemical Society 2016 The Journal of Physical Chemistry Part C Vol.120 No.20

<P>The doping effect on intramolecular magnetic exchange coupling of an edge-terminated zigzag graphene nanoribbon (ZGNR) with organic radicals was studied with density functional theory calculation. We investigated magnetic behaviors of boron (B)- and nitrogen (N)-doped ZGNRs, terminated with trimethylenemethane (TMM) and 6-oxoverdazyl (OVER) radicals, that is, TMM-ZGNR-TMM, OVER-ZGNR-OVER, and TMM-ZGNR-OVER. A doping with B or N on the spin-coupling pathway of radical-ZGNR-radical changed the spin distribution pattern of each system and hence its magnetic ground configuration, magnetic coupling strength, and magnetic moment. The first doping switched the magnetic ground configuration of a system from antiferromagnetic (AFM) to ferromagnetic (FM) and vice versa. An additional doping switched it back to its original magnetic ground configuration. Moreover, N doping on a radical-terminated edge increased the magnetic coupling strength as compared with the undoped system, while B doping decreased it. Furthermore, B or N doping on a TMM-terminated edge increased the magnetic moment of the system, while the same doping on an OVER-terminated edge decreased it. Our results demonstrate a possibility of reversible spin control of organic magnetic materials from AFM to FM and vice versa by chemical doping and the enhancement of the magnetic coupling strength of edge-terminated ZGNRs.</P>

Enhancement of Electrical and Thermomechanical Properties of Silver Nanowire Composites by the Introduction of Nonconductive Nanoparticles: Experiment and Simulation

Nam, Seungwoong,Cho, Hyun W.,Lim, Soonho,Kim, Daeheum,Kim, Heesuk,Sung, Bong J. American Chemical Society 2013 ACS NANO Vol.7 No.1

<P>Electrically conductive polymer nanocomposites have been applied extensively in many fields to develop the next generation of devices. Large amounts of conductive nanofillers in polymer matrices are, however, often required for a sufficiently high electrical conductivity, which in turn deteriorates the desired thermomechanical properties. We illustrate a novel but facile strategy to improve the electrical conductivity and the thermomechanical property of silver nanowire/polymer nanocomposites. We find that one may increase the electrical conductivity of silver nanowire/polymer nanocomposites by up to about 8 orders of magnitude by introducing silica nanoparticles with nanocomposites. The electrical percolation threshold volume fraction of silver nanowires decreases from 0.12 to 0.02. Thermomechanical properties also improve as silica nanoparticles are introduced. We carry out extensive Monte Carlo simulations to elucidate the effects of silica nanoparticles at a molecular level and find that van der Waals attractive interaction between silica nanoparticles and silver nanowires dominates over the depletion-induced interaction between silver nanowires, thus improving the dispersion of silver nanowires. Without silica nanoparticles, silver nanowires tend to aggregate, which is why additional silver nanowires are required for a desired electrical conductivity. On the other hand, with silica nanoparticles mixed, the electrical percolating network is likely to form at a smaller volume fraction of silver nanowires.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/ancac3/2013/ancac3.2013.7.issue-1/nn305439t/production/images/medium/nn-2012-05439t_0006.gif'></P>

Periodicity of band gaps of chiral α-graphyne nanotubes

Kang, Baotao,Cho, Daeheum,Lee, Jin Yong The Royal Society of Chemistry 2017 Physical chemistry chemical physics Vol.19 No.11

<P>Electronic structures of zigzag (n,0), armchair (n,n), and chiral (n,m) alpha-graphyne nanotubes (alpha GNTs) with n = 2- 7 were investigated using density functional tight binding calculations. Oscillatory behavior of the band gaps with a period of every ( n - m) = 3 was found for each tube. According to the periodicity, aGNTs could be classified into three families, and their band gaps were in the increasing order of ( n - m) = 3a < 3a + 1 < 3a + 2. Among the three families, aGNTs with ( n - m) = 3a became effectively semimetallic when the tube size was larger than approximately 2 nm, while the other families remained semiconducting.</P>

Electronic and Nuclear Contributions to Vibrational Stark Shifts of Hydroxyl Stretching Frequencies of Water Clusters

Lim, Jong Hyeon,Cho, Daeheum,Kang, Heon,Lee, Jin Yong American Chemical Society 2018 The Journal of Physical Chemistry Part C Vol.122 No.24

<P>In spite of the importance of vibrational Stark effect (VSE) and many attempts, origin of VSE is still unclear in molecular level. Here, we studied on origin of VSE of hydroxyl stretching vibration in small water clusters (monomer, dimer, and tetramer) assuming that VSE can be separated by nuclear and electronic contribution. We calculated total Stark tuning rate (Δμ<SUB>tot</SUB>) and its nuclear contribution (Δμ<SUB>geom</SUB>) using the ab initio method, then the electronic contribution (Δμ<SUB>elec</SUB>) was simply obtained by the difference, Δμ<SUB>tot</SUB> - Δμ<SUB>geom</SUB>. In all cases, the nuclear contribution has dominant contribution to VSE. The hydroxyl stretching mode with neighboring hydrogen acceptor showed larger Δμ<SUB>geom</SUB> than that of dangling bonds. Furthermore, the calculated Δμ<SUB>geom</SUB> became larger in larger cluster due to the hydrogen bond network. The comparison between Stark tuning rates including and excluding anharmonicity supports the importance of potential anharmonicity in VSE, as previously reported. Interestingly, a good linear relationship is observed between the hydroxyl stretch frequency (ν<SUB>geom</SUB>) and hydroxyl bond length and also between the Stark tuning rate (Δμ<SUB>geom</SUB>) and the change of hydroxyl bond length. Similarly, the electronic contribution of calculated frequencies and Stark tuning rate (Δμ<SUB>elec</SUB>) showed a good linear relationship with atomic charge derived by electronic perturbation (Δq<SUB>elec</SUB>) and change of that (Δ(Δ<I>q</I><SUB>elec</SUB>)), respectively.</P> [FIG OMISSION]</BR>

Systematic Approach To Design Organic Magnetic Molecules: Strongly Coupled Diradicals with Ethylene Coupler

Ko, Kyoung Chul,Cho, Daeheum,Lee, Jin Yong American Chemical Society 2012 The journal of physical chemistry. A, Molecules, s Vol.116 No.25

<P>The intramolecular magnetic coupling constant (<I>J</I>) values of diradical systems linked with two monoradicals through a coupler (para-substituted phenyl acetylene (Model I), meta-substituted phenyl acetylene (Model II), ethylene (Model III)) were investigated by unrestricted density functional theory calculations. We divided eight monoradicals into α-group and β-group according to Mulliken spin density values of the connected atoms. The overall trends in the strength of magnetic interactions of diradicals were found to be identical in three different model systems. The diradicals with para-substituted phenyl acetylene coupler resulted in almost twice stronger intramolecular magnetic coupling interactions of the corresponding diradicals as compared to the meta-substituted one with opposite magnetism. <B>NN</B>-Ethylene-<B>PO</B> (nitronyl nitroxide radical coupled to phenoxyl radical via ethylene coupler) was calculated to have the strongest magnetic coupling constant with ferromagnetism, and to be even stronger (more than twice) than <B>NN</B>-ethylene-<B>NN</B> (nitronyl nitroxide diradical with ethylene coupler), which was reported to have strong antiferromagnetic interactions in a previous experiment. It was found that the spin density values of the connected atoms are closely related to the determination of magnetic interactions and <I>J</I> values. The spin states of the ground state in diradical systems were explained by means of the spin alternation rule.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpcafh/2012/jpcafh.2012.116.issue-25/jp211225j/production/images/medium/jp-2011-11225j_0002.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp211225j'>ACS Electronic Supporting Info</A></P>