Canadian Journal of Chemistry. Spin-dependent electron transport through a Mnphthalocyanine. Draft

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1 Spin-dependent electron transport through a Mnphthalocyanine molecule: an SS-DFT study Journal: Manuscript ID cjc Manuscript Type: Article Date Submitted by the Author: 6-Jun-216 Complete List of Authors: Liu, Shuanglong; NUS Xi, Yongjie; NUS Guo, Na; NUS Yam, Kah Meng; NUS zhang, chun; NUS Keyword: SS-DFT, quantum transport, manganese phthalocyanine

2 Page 1 of 4 1 Spin-dependent electron transport through a Mn-phthalocyanine molecule: an SS-DFT study Shuanglong Liu, Yongjie Xi, Na Guo, Kah Meng Yam, and Chun Zhang 1. Introduction Abstract: We generalize the recently proposed steady-state density functional theory (SS-DFT) to spin dependent cases and theoretically investigate the electronic and transport properties of a Mn-phthalocyanine molecule sandwiched between two graphene nanoribbon leads. The junction filters spin-up (minority spin) electrons while allows spin-down (majority spin) electrons to pass with a filtering efficiency of about 99.5% at low biases. The spin-down electrons are found to tunnels through the junction via the HOMO orbital of the Mn-phthalocyanine molecule. Detailed analysis of spin-dependent electron tunneling mechanism as well as the electronic/magnetic properties of the junction is also presented. Key words: SS-DFT, quantum transport, manganese phthalocyanine. The key idea of molecular electronics is that single molecules can be employed as functional units of a circuit.[1] To facilitate molecular device design, it is crucial to theoretically (or computationally) understand the transport properties of a molecular junction that are closely related to electronic structures of the junction under study.[2, 3, 4, 5] It was recently shown that, when a molecular junction is under a finite bias, its electronic and transport properties can be described by a so-called steady-state density functional theory (SS-DFT)[6] that takes into account the bias induced non-equilibrium effects. In SS- DFT, the steady-state properties of a molecular junction under a bias are uniquely determined by two electron densities, the usual total electron density ρ and the current-carrying electron density ρ n defined as ρ n (r) = i µl 2π µ R dϵg < (r, ϵ),where G < is the lesser Green s function of the interacting system and µ L/R is the chemical potential of left/right electrode. The SS-DFT can be naturally generalized to spin-polarized cases that the non-equilibrium steady state is determined by (ρ σ,ρ σ n), where σ denotes the spin channel (either up or down). The spin-dependent current-carrying electron density should be calculated from spin-dependent lesser Green s function as Eq. 1. In this paper, we apply the generalized spin SS-DFT to investigate electron coherent transport through a magnetic metal-phthalocyanine (MPc) molecule sandwiched between two graphene nanoribbon (GNR) leads, and show that the molecular junction can be used as a intriguing molecular device. Shuanglong Liu 1 and Na Guo. Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore Yongjie Xi and Kah Meng Yam. Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore Chun Zhang. 2 Departments of Physics and Chemistry, National University of Singapore, 2 Science Drive 3, Singapore Centre for Advanced 2D Materials, National University of Singapore, 2 Science Drive 3, Singapore Present Address: Department of Physics, University of Florida, 21 Museum Rd, Gainesville, FL 3263, USA 2 Corresponding author ( phyzc@nus.edu.sg). [1] ρ σ n(r) = i µl dϵg <σ (r, ϵ), 2π µ R Molecular scale spintronics devices recently have attracted enormous attention. To find appropriate electrodes and a molecular scale center is the key for the design of a high performance molecular spintronic device.[7, 8, 9] GNR has been regarded as one of most promising building blocks for nano electronics or spintronics.[1] It was reported that zigzag GNRs (ZGNRs) can be conductive thus can be used as leads in a circuit.[11, 12] MPc molecules recently attracted a lot of interests. By changing the embedded metal atom, one can tune the properties of MPc molecules for various desired applications. It has been demonstrated that MPc molecules can be used to make lightemitting diodes, photovoltaic cells and molecular electronic devices.[13] Here, using spin SS-DFT, we show that the combination of ZGNR and a magnetic MPc molecule can serve as an ideal candidate for molecular scale spin filter. 2. Computational Details We relaxed the atomic structure of the junction using SIESTA package[14] and simulated the transport properties of the junction based on SS-DFT as implemented in SIESTA. In our calculations, we employed norm-conserving psudopotential as generated with the Troullier-Martins scheme[15] to describe the inner atomic core. The generalized gradient approximation (GGA) in PBE format[16] with non-equilibrium corrections[6, 17, 18] is adopted to approximate the XC energy functional. Double ζ polarized basis set with.2 Ry energy shift was applied. Since the junction is non periodic, only Γ point in reciprocal space was used. The tolerance for density matrix and energy is set to and ev respectively. The force was converged below.2 ev/å. 3. Results and Discussion As shown in Fig. 1, the junction under study consists of a MnPc molecule sandwiched between two zigzag graphene unknown 99: 1 4 (216) DOI: /Zxx-xxx c 216 NRC Canada

3 Page 2 of 4 2 unknown Vol. 99, 216 nanoribbon leads. The vertical lines in the figure mark the boundary between the leads and the scattering region. 3.5 and 3 GNR unit cells are used as screen layers on the left and right sides respectively. In both left and right leads, there are two GNR unit cells. One obvious advantage of this device is that two contact regions are all made of carbon that are just natural transitions from GNR to the MnPc molecule. To build the atomic structure of the junction, we first relaxed the periodic ZGNR by DFT and found its lattice constant. Then we inserted the MnPc molecule between the left and right leads and optimized the distance between the leads with the center region fully relaxed at each separation. Note that the final atomic structure of the whole junction is planar. Fig. 1. (Colour online) Relaxed atomic structure of the GNR- MnPc-GNR junction. The vertical lines mark the scattering region. Color scheme: C, dark grey; H, light grey; Mn, Purple. Fig. 2. (Colour online) Red solid lines show the spin resolved DOS for the isolated MnPc molecule. The blue dash lines show the spin resolved projected DOS for the MnPc molecule which is coupled to leads. Positive (negative) DOS is for spin-up (spin-down) electrons. DOS Isolated MnPc Coupled MnPc Eenergy (ev) Scattering Region To compare the electronic structures of the MnPc molecule coupled to the leads and the isolated MnPc molecule, we plot in Fig. 2, the density of states (DOS) for both of them. As shown in the figure, the blue dash lines and the red solid lines represent the DOS for the coupled MnPc molecule and the isolated MnPc molecule respectively. The positive curves (above x axis) are for spin-up (minority spin) electrons while the negative ones (below x axis) are for spin-down (majority spin) electrons. Here the DOS for the coupled MnPc molecule is calculated from the molecular projected self-consistent Hamiltonian (MPSH) for the junction under.25 V. This figure shows that the local electronic structure around the MnPc molecule in the junction is rather similar to that of the isolated molecule under small biases. It indicates that the local magnetic momentum around the isolated MnPc molecule is the same with that of the isolated molecule, which is 3. µ B. The current voltage characteristics for the GNR-MnPc-GNR junction under study are shown in Fig. 3. For the calculation of the electric current, we applied bias voltages between zero and.25 V. In this bias range, the spin-up electric current is almost zero while the spin-down electric current is significantly larger. Thus the junction can filter spin-up electrons and allow spindown electrons to pass. The spin filtering efficiency [2] ξ = I I I + I varies between 99.4% and 99.6% for the whole bias range. When the junction is under zero bias, both spin-up and spindown currents are zero. To calculate the spin filtering efficiency at zero bias, we substitute the electric currents by the transmissions at the Fermi energy in Eqn. 2. Fig. 3. (Colour online) Spin resolved electric currents for the GNR-MnPc-GNR based junction (left y axis) and the spin filtering efficiency (right y axis) % Current ( 1 2 na) % 99.5% 99.4% % Bias Voltage (V) In order to understand the spin filtering effect, we plot in Fig. 4a the spin resolved transmission function for the GNR-MnPc- GNR juncion under.25 V. For the energy in the x axis of Fig. 4a, the middle of the left and right chemical potentials is set to zero. As shown in the figure, the transmission function for spin-up electrons is vanishing while the spin-down one is fairly large within the bias window. In addition, there is a transmission peak right above the higher chemical potential whose left tail crosses the bias window. It is clear that the difference in the transmission function leads to the large spin filtering efficiency. In the next step, we calculated the the transmission eigenchannel corresponding to the transmission peak mentioned earlier and plot it in Fig. 4b. For comparison, the HOMO orbital for the isolated MnPc molecule is also plotted in the inset of Fig. c 216 NRC Canada

4 Page 3 of 4 Liu et al. 3 4a. As can be seen, the transmission eigenchannel and HOMO orbital appears quite similar around the MnPc molecule. This similarity indicates that the spin-down electrons tunnel through the junction via the HOMO orbital of the MnPc molecule. By the way, the presence of the tail of the transmission peak implies that the HOMO orbital is broadened when the molecule is coupled to the leads. Fig. 4. (Colour online) (a) Transmission function for the GNR-MnPc-GNR junction under.25 V. The inset shows the HOMO orbital of the isolated MnPc molecule. (b) Transmission eigenchannel at the energy.16 ev. Transmission Spin Up Spin Down HOMO (a) Fig. 5. (Colour online) non-equilibrium electron density for (a) spin-up electrons and (b) spin-down electrons with the same isovalue Bohr 3. (a) (b) Energy (ev) (b) DFT. We found that the junction can work as a spin filter with a spin filtering efficiency around 99.5%, suggesting that the combination of GNR and magnetic MPc molecule is a good candidate for high performance spintronics devices. For the particular junction under study, we showed that the HOMO orbital of the MnPc molecule played an important role in the spin filtering effect. In the end, the distribution of spin-dependent current carrying electron densities are also presented. Acknowledgements In SS-DFT, the current-carrying electron density (or the nonequilibrium electron density) originates from the occupied states within the bias window and provides information about the distribution of current-carrying electrons. In Fig. 5a (5b), we plot the current-carrying electron density for spin-up (spindown) electrons. As shown in Fig. 5a, the spin-up currentcarrying electrons are mostly accumulated on the left side and can hardly get to the right side. Note that current-carrying electrons come from left in the case since the left chemical potential is higher than the right one. The plot in Fig. 5a is consistent with the transmission function presented before. Both of them show that the spin-up electrons are mostly reflected back to the left side. In contrast, the spin-down current-carrying electron density spreads over both left and right sides, which can viewed as a consequence of the high transmission for spindown electrons. As a minor point, the spin-down current-carrying electron density mainly resides on the ZGNR edges. It indicates that the spin-down electric current flows along the ZGNR edges. 4. Conclusions In conclusion, we have studied the electronic and transport properties of a GNR-MnPc-GNR juncion based on spin SS- We acknowledge the support from Singapore National Research Foundation (NRF-CRP ), Singapore Ministry of Education (R ) and NUS academic research grants (R ). Computations were performed at the Graphene Research Centre at NUS. References 1. Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, doi:1.116/9-2614(74) Taylor, J.; Guo, H.; Wang, J. Phys. Rev. B 21, 63, doi:1.113/physrevb Zhang, C.; Du, M. H.; Cheng, H. P.; Zhang, X. G.; Roitberg, A. E.; Krause, J. L. Phys. Rev. Lett. 24, 92, doi:1.113/physrevlett Zhang, C.; Barnett, R. N.; Landman, U. Phys. Rev. Lett. 28, 1, doi:1.113/physrevlett Cai, Y. Q.; Zhang, A. H.; Feng, Y. P.; Zhang, C. J. Chem. Phys. 211, 135, doi:1.163/ Liu, S.; Nurbawono, A.; Zhang C. Sci. Rep. 215, 5, doi:1.138/srep Barraza-Lopez, S.; Park, K.; García-Suárez, V.; Ferrer, J. Phys. Rev. Lett. 29, 12, doi:1.113/physrevlett c 216 NRC Canada

5 Page 4 of 4 4 unknown Vol. 99, Dai, Z. X.; Nurbawono, A.; Zhang, A. H.; Zhou, M.; Feng, Y. P.; Ho, G. W.; Zhang, C. J. Chem. Phys. 211, 134, doi:1.163/ Zhou, M.; Cai, Y. Q.; Zeng, M. G.; Zhang, C.; Feng, Y. P. Appl. Phys. Lett. 211, 98, doi:1.163/ Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. Rev. Mod. Phys. 29, 81, 19. doi:1.113/revmodphys Ezawa, M. Phys. Rev. B 26, 73, doi:1.113/physrevb Fujita, M.; Wakabayashi, K.; Nakada, K.; Kusakabe, K. Phys. Soc. Jpn. 1996, 65, 192. doi:1.1143/jpsj Kadish, K. M.; Smith, K. M.; Guilard, R. Academy Press, San Diego 23, Application of phthalocyanines vol 19 of The Porphyrin Handbook. ISBN: Soler, J. M.; Artacho, E.; J. D. Gale, García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. J. Phys.: Condens. Matter 22, 14, doi:1.188/ /14/11/ Troullier, N.; Martins J. L. Phys. Rev. B 1991, 43, doi:1.113/physrevb Perdew, J. P.; Burke K.; Ernzerhof M. Phys. Rev. Lett. 1996, 77, doi:1.113/physrevlett Liu, S.; Feng, Y.-P.; Zhang, C. J. Chem. Phys. 213, 139, doi:1.163/ Zhang, C. J. At. Mol. Sci. 214, 5, 95. doi:1.428/jams a. c 216 NRC Canada