Effect of amino on spin-dependent transport through a junction of fused oligothiophenes between graphene electrodes

Accepted Manuscript Effect of amino on spin-dependent transport through a junction of fused oligothiophenes between graphene electrodes Liemao Cao, Xiaobo Li, Guang Liu, Ziran Liu, Guanghui Zhou PII: DOI: Reference:

S0301-0104(16)30656-5 http://dx.doi.org/10.1016/j.chemphys.2017.03.004 CHEMPH 9758

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Chemical Physics

Please cite this article as: L. Cao, X. Li, G. Liu, Z. Liu, G. Zhou, Effect of amino on spin-dependent transport through a junction of fused oligothiophenes between graphene electrodes, Chemical Physics (2017), doi: http://dx.doi.org/ 10.1016/j.chemphys.2017.03.004

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Effect of amino on spin-dependent transport through a junction of fused oligothiophenes between graphene electrodes Liemao Cao, Xiaobo Li, Guang Liu, Ziran Liu, and Guanghui Zhou∗

1

Department of Physics, Key Laboratory for Low-Dimensional Structures and Quantum Manipulation (Ministry of Education), and Synergetic Innovation Center for Quantum Effects and Applications of Hunan, Hunan Normal University, Changsha 410081, China The influence of chemical side groups is significant in physical or chemical understanding the transport through the single molecular junction. Motivated by the recent successful fabrication and measurement of a single organic molecule sandwiched between graphene electrodes [Nano Lett. 11, 4607 (2011)], here we study the spin-dependent transport properties through a junction of a fused oligothiophenes molecule embedded between two zigzag-edged graphene nanoribbon (ZGNR) electrodes. The molecule with and without an attached amino NH2 side group is considered, respectively, and external magnetic fields or FM stripes are applied onto the ZGNRs to initially orient the magnetic alignment of the electrodes for the spin-dependent consideration. By the ab initio calculations based on the density functional theory combined with nonequilibrium Green’s function formalism, we have demonstrated the remarkable difference in the spin-charge transport property between the junctions of the molecule with and without NH2 side group. In particular, the junction with side group shows more obvious NDR. In addition, it exhibits an interesting dual spin-filtering effect when the magnetic alignment in electrodes is initially antiparallel-oriented. The mechanisms of the results are revealed and discussed in terms of the spin-resolved transmission spectrum associated with the frontier molecular orbitals evolution, the molecular projected self-consistent Hamiltonian eigenvalues, and the local density of states.

I.

INTRODUCTION

Recently, with the continuous miniaturization of conventional electronic devices and the rapid development of molecular and nanoelectronics, it is increasingly practicable to use a single molecule to construct electronic devices. A lot of relevant both experimental1−4 and theoretical5−22 works have been reported. Rocha et al. have demonstrated theoretically that organic spin valves, obtained by sandwiching an organic molecule between magnetic contacts, can show a large biasdependent magnetoresistance.9 Recently, Jia et al. successfully build a fully reversible, two-mode, single-molecule electrical switch with unprecedented levels of accuracy, stability, and reproducibility.10 It is known that various molecular devices with many particular and interesting effects have been came to light in addition to the above mentioned, such as spin crossover,5 rectifying,6−8 dual spin-filtering,11−15 negative differential resistance (NDR),18−21 and so forth. These effects show that unimolecular electronics may play a key role in the design of future circuits. However, to accurately understand the corresponding mechanisms of overall complexes is still a challenge. It depends on many factors, such as overall crystal (molecule) structure, spin configuration, positions of the terminal atoms on electrode surfaces, distance between the electrodes and so on.15,18,22 In addition, how to improve the functional performance is also very important after designing a molecular electronic device. Graphene, a single-layered two-dimensional crystal with a honeycomb lattice structure, has been fabricated recently.23 Studies have revealed that it has lots of excellent properties in electronics and optics, such as high carrier mobility,23,24 long spin relaxation times,25 and room-temperature quantum Hall effect.24 Graphene has also been considered as a

∗ Electronic

address: [email protected]

good candidate for electrodes in spintronic devices.26,27 Further, carbon atoms are fundamentally important for creating functional organic electronic devices, and they have been directly carved out experimentally from a graphene.28,29 It is known that an infinite graphene sheet can be cut into two typical graphene nanoribbons: zigzag-edged graphene nanoribbon (ZGNR) and armchair-edged graphene nanoribbon (AGNR).30,31 AGNR is a nonmagnetic semiconductor, but a ZGNR can be in either nonmagnetic, ferromagnetic or antiferromagnetic state when its two edges are passivated depending upon the external means. Nevertheless, recent experiments showed that graphene can exhibit an intrinsic ferromagnetic (FM) due to the existence of various defects or topological structures.32−34 And the application of a magnetic field35 or a transverse electrical field36 can also make the FM state more stable. Other experiments demonstrated that the spin-polarized antiferromagnetic (AFM) and FM states would become unstable with respect to the spin-unpolarized state at finite temperature37 or in the presence of a ballistic current through a GNR.38 Obviously, in order to simulate the experimentally detectable transport behavior under bias voltages, considering various magnetic configurations of ZGNRs is necessary. Even though there have been many studies on the spin-charge transport for organic molecular bridges linking ZGNRs,12,14,15,39 there still lacks the investigation on the transport property for the junction of a oligothiophenes molecule between two ZGNR electrodes. Here we address this issue by proposing a possible molecular electronic device model. On the other side, fused oligthiophenes are a main class of organic semiconductor materials for new technologies.40,41 In recent years, oligothiophenes molecules have attracted enormous attentions in molecular electronic devices.42−46 The obvious NDR effect has been predicted or measured in these works. However, all of them used gold as electrodes, and the planar graphene electrode has not been considered yet. Nevertheless, the recent successful fabrication and measurement

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Fig. 1: Schematic view of the two device models, where M1 is a junction consisting of a fused oligothiophenes molecule without side group (upper panel), and M2 a fused oligothiophenes molecule sided by NH2 (lower panel) linking carbon chain sandwiched between two ZGNR electrode leads. The shaded areas indicate the leads (with two repeated carbon unit cells along the transport z direction) which are modulated by an external magnetic field along the + or -y direction.

of single organic molecule sandwiched between graphene electrodes has been realized.47 In this paper, we investigate the spin-dependent electron transport for a one-dimensional molecular junction device, where a fused oligothiophenes molecule is sandwiched between two ferromagnetic ZGNR electrodes. It is well known that amino is an electrondonating group which releases electrons into a reaction center. This group has a huge modulation effect on the molecular orbitals, which changes the molecular transport property obviously.42,43 Hence, we also add a NH2 side group to the molecule and investigate its impact on the transport through the junction. Using the approach of the fully selfconsistent ab initio density-functional theory (DFT) combing with the nonequilibrium Green’s function (NEGF), we firstly predict that a fused oligothiophenes molecule sandwiched between graphene electrodes exhibits an obvious NDR effect, and when the electron spin is taken into account it shows a good dual spin-filtering effect. The influence of chemical side group on the transport property of a single molecular junction, in addition to the significance in basic chemical physics, it may give us an orientation for the design of multi-functional molecular devices which reduce greatly the integrating cost of circuits. This paper is organized as follows: In Sec. II, we give the model description of our molecular junction and the calculational method. In Sec. III, we present our results and the discussions with physical/chemical mechanisms, and we finally point out our concluding remarks in Sec. IV.

examined junction can be divided into three parts, namely, left lead, scattering region, and right lead. The left and right electrodes are semi-infinite ZGNR electrodes where all edge carbon atoms are saturated with hydrogen.12−17 Two external magnetic fields or ferromagnetic stripes are applied onto the ZGNRs to initially orient the magnetic alignment of the electrodes.12 When the magnetic field at two electrodes points in the same or opposite direction, the magnetization of the leads can be set to parallel (P) or antiparallel (AP) spin configuration. Moreover, the fused oligothiophenes molecule in our device is tied up to ZGNR electrodes with a five-membered ring. And the bond length of carbon chain linked to graphene is 1.477 Å. This structure has a perfect atomic interface leading to small contact resistance.16 For convenience, our proposed junction model for the oligothiophenes molecule without side group is called device M1 shown in the top panel of Fig. 1, and that with a NH2 side group is called M2 shown in the bottom panel. Based on self-consistent density-functional theory (DFT) combing with the nonequilibrium Green’s function (NEGF), the optimization of structures and the charge transport calculations for the model junctions have been carried out by a developed ab initio software package Atomistix ToolKit (ATK).48,49 Further, the geometries are optimized until all residual forces on each atom are smaller than 0.05 eVÅ−1 .In our calculation, the local spin density approximation proposed by Perdew and Zunger is adopted to describe the exchange and correlation energy. Single-zeta polarized (SZP) basis set is used for all elements including carbon, hydrogen, nitrogen and sulfur atoms. The Brillouin zone of the electrode is sampled as a Monkhorst-Pack grid using 1×1×100 k points. Moreover, the grid integration is defined by a cutoff energy of 150 Ry to achieve the balance between calculation efficiency and accuracy. The electrode temperature is set to be 300 K, and the convergence criterion of self-consistent calculations is 10−5 eV in total energy. The spin-dependent current is calculated by using the Landauer-B¨uttiker formula15,17,48,49,50 Iσ (V) = (e/h)

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where e is the electron charge, h the Planck’s constant, and fL/R (E) = 1/[1 + e(E−µL/R )/KB T ] the Fermi-Dirac distribution function in the left/right electrode with the electrochemical potential µL/R (V) = µL/R ± eV/2 when a bias V is applied. Moreover, σ =↑ / ↓ denotes electron spin-up/down and the spin-dependent transmission probability as a function of energy E and bias V can be calculated from the Green function approach T σ (E, V) = Tr[ΓLσ (E, V)Gσ (E, V)ΓRσ (E, V)G†σ (E, V)],

II.

MODEL AND CALCULATION METHOD

The proposed molecular junction is a two-probe system, as illustrated in Fig. 1, in which a fused oligothiophenes molecule is sandwiched between two ZGNR electrodes. The

where Gσ (E, V) is the Green function of central scattering region with complex conjecture G†σ (E, V), and ΓL/Rσ = i(ΣL/Rσ − Σ†L/Rσ ) the contact broadening function associated with ΣL/Rσ of the left/right electrode’s self-energy.

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RESULTS AND DISCUSSIONS

Figure 2 presents the calculated current as a function of the applied bias voltage from -2.0 to 2.0 V for the molecular junction with or without NH2 side group. From the current-voltage (I − V) curves, firstly, we find that the fused oligothiophenes molecular junction with or without NH2 exhibits resemble I−V characteristics, except that the magnitude of the current is different. This result means that the NH2 side group enhances the magnitude of the current through the junction, which is in agreement with other similar molecular junctions.19,51 Interestingly, the currents for M1 and M2 under the positive bias increase rapidly with the bias voltages from 0 to 0.6 V, while they smoothly decrease from 0.6 to 0.9 V. Further, the current rises monotonically from 0.71 to 1.30 µA for M1 and from 1.08 to 1.61 µA for M2 as the bias increase from 0.9 to 1.4 V, while it quickly drops from 1.02 to 0.23 µA for M1 and 1.61 to 0.47 for M2 µA when the bias voltage is increased further continuously from 1.4 to 1.8 V. In addition, the two molecular junctions show qualitatively the same I − V characteristic under the negative bias voltage. It’s worth noting that the current for M2 is always larger than that for M1 even in the very small bias of (0, 0.01)V as shown in the inset at right bottom of Fig. 2. Particularly, the difference is more obvious in the bias range from 0.01 to 1.4 V. The above I − V characteristics indicate that a fused oligothiophenes molecular junction device displays a good NDR behavior, but the one with NH2 side group is more effective. However, the current through junction with NH2 side group under negative bias is much lower

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Fig. 3: (Color online) Transmission probability as a function of energy for M1 [(green) solid line] and M2 [(red) dashed line] under different bias (a) 0, (b) 0.6 and (c) -1.5 V, where the left insets are the corresponding LDOS around zero energy (with isovalue 0.005 a.u.), respectively. The right upper inset in (a) is an enlarged view of transmission spectrum near the Fermi level.

than that without side group as the gold electrodes are used.42 To explore the above-mentioned I − V characteristics, in Fig. 3 we give the transmission spectra for the two molecular junctions at several fixed bias voltages (a) 0, (b) 0.6 and (c) -1.5 V, where the (green) solid line for M1 without side group and (red) dashed line for M2 with NH2 side group. We select these voltages as they are special voltage points: the origin, the biggest difference of the current for two junctions at 0.6 V, and the two values of current are exactly equal at -1.5 V. The left insets show the corresponding local density of states (LDOS) around zero energy under different biases, respectively. It can be clearly seen that the value of transmission probability for M2 is generally greater than that for M1 at zero bias. Meanwhile, as shown in the inset of Fig. 3(a), the LDOS for M2 is more delocalized than that for M1 (see the circle marks in the insets of Fig. 3). In consequence, it is more conductive to the electronic transport for M2 at the Fermi level, which results from the differences in LDOS. The first relative peak of currents at 0.6 V, the transmission spectrum and LDOS have the same performance. So the value of current for M2 is larger than M1 . However, the current value for M1 is equal to that for M2 at -1.5 V, so both transmission spectra are almost identical at the bias window. The symmetry structure of M1 leads to the symmetric current variation. The side group NH2 destroyed the symmetry of structure, so it increases the transmission probability and influences the magnitude of the current. Since junction M2 with NH2 side group is superior to M1 in NDR effect, next we focus our analysis on the explantation of I − V characteristic for M2 . In Fig. 4, we give the transmis-

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Fig. 4: (Color online) Transmission probability [(red) solid line] of M2 with NH2 side group under bias (a) V=1.4 and (b) 1.8 V, where the vertical (black) dashed lines indicate the range of bias window and the up-arrows show the positions of eigenenergy of orbitals HOMO, HOMO-1 and LUMO. (c)-(h) The associated the eigenstate distributions under the corresponding biases (the isovalues are all 0.05 a.u.) are displayed, respectively. The red and blue colors indicate the positive and negative signs of the wave functions, respectively.

sion spectrum and the corresponding molecular projected selfconsistent Hamiltonian (MPSH) of the four frontier molecular orbitals LUMO (lowest unoccupied molecular orbital), LUMO+1, HOMO (highest occupied molecular orbital), and HOMO-1 for M2 under bias voltages of (a) 1.4 and (b) 1.8 V. And these two biases correspond to the peak and valley of currents in the junction. As we known, the transmission spectrum of a device is the most intuitive representation of charge transport behavior because that the value of current depends on the integral area under the transmission probability within the bias window according to the Landauer-B¨uttiker formula. When the bias is set to be 1.4 V, the transmission peaks come into the bias window, and there are orbitals of HOMO, HOMO-1 and LUMO within the bias window (as indicated by up-arrows) which may make contribution to the transmission. But the LUMO+1 have not in bias window which can not contribute to the transmission. Generally, the more delocalized of the MPSH distribution on the central region, the molecule orbital can more easily transport the electrons. In Figs. 4(c-e), we can see that only the HOMO orbital is fully delocalized in the whole molecule. However, the HOMO-1 and LUMO orbitals are localized on the right and left sides, respectively. Therefore, it means that the HOMO-1 and LUMO orbitals almost not contribute to transport. In contrast, the current for M2 appears a valley at the bias 1.8 V. Although the bias window broaden, the total integral area in transmission gets smaller. The HOMO-LUMO gap is increased 0.1 eV compared to the case of lower bias 1.4 V. Meanwhile, we can also see that the HOMO orbital of MPSH eigenstate is no longer delocalized, but is localized on one side together with the HOMO-1 and LUMO orbitals [see Figs. 4(f-h)]. There are not many molecular orbitals contributing to the electron transport, the current

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Fig. 5: (Color online) Calculated spin-dependent I − V curves for M2 junction with (a) P and (b) AP configurations, respectively, where the (red) up-triangle solid lines for the spin-up current and the (blue) down-triangle dashed ones for the spin-down current.

is suppressed. Therefore, the M2 junction exhibits more obvious NDR effect. Further, spintronics may play a vital role in next-generation spin electronics systems. For the fused oligothiophenes molecular junction, the influence of the molecular length and the substituent group species on transport have been explored with gold electrodes, but the spintronic effect was ignored.42 Therefore, here in Fig. 5 we turn our attention to the spin-dependent transport for M2 with parallel (P) and antiparallel (AP) configuration of magnetization in two ZGNR electrodes,12,14 respectively, where several interesting features are exhibited. We have also calculated the spin-dependent transport properties for M1 , and it showed the same result as that for M2 . There is only quantitative difference compared with M2 , so it is not shown here. Our calculated result shows that the total energy of the P configuration is larger than that of AP case by 2.24 eV. It indicates that the AP configuration is more stable than P configuration. For the P spin configuration in Fig. 5(a), both the spin-up and -down currents show symmetric behavior. The spin-up current shows almost linear dependence under small bias less than 0.4 V, but it decreases quickly with the increase of the bias from 0.5 to 1.8 V after reaching a maximum value. However, the spin-down current is blocked in the whole bias range. This result indicates that the device shows a perfect spin-filtering effect in the P spin configuration. More interestingly, for the AP spin configuration in Fig. 5(b), both the spin-up and -down currents show highly asymmetric behavior because of the asymmetry magnetic configuration in two electrodes. The spin-down current is forbidden under the negative bias range and it increases up to maximum 3.3 µA with the increase of positive bias to 0.8 V. Then it drops slowly near to zero as the bias is further increased. However, the spin-up current can easily flow through the device under the negative bias, while it is almost forbidden under the positive one. This result implies that the molecular junction in AP configuration can act as a dual spin-filter.

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IV. SUMMARY AND CONCLUSION

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it is generally known that the molecule orbital has a great effect on transmission spectrum for a junction. From Figs. 6(a) and 6(b), we can see that the LUMO/HOMO comes into the bias window in P/AP configuration, which shows that the electronic transporting modes in two cases are not the same. However, the molecular orbital into the bias window does not necessarily contributing to transport. Look through Fig. 6(c) to 6(j) for the eigenstate distributions, we observe that only the LUMO of the spin-up current for P configuration at 0.4 V [see Fig. 6(c)] and the HOMO of spin-down one in AP case at 0.8 V [Fig. 6(j)] are delocalized. On the contrary, the other orbitals are localized. The spin-up current in P configuration can flow through device M2 , but spin-down one is forbidden. And the situation in AP configuration is just opposite. That is to say, the spin filtering can happen in cases of both P and AP, but their detailed physical/chemical origin is different.

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Moreover, a NDR effect can also be observed from Fig. 5. For the P spin configuration as shown in Fig. 5(a), when the bias goes to higher than 0.4 or -0.6 V, the spin-up current drops dramatically, which indicates the onset of NDR. However, for the AP configuration [see Fig. 5(b)], the spin-up and -down currents exhibit a NDR behavior at bias ±0.8 V. Finally, to understand the spin-filtering effect, in Fig. 6, we present the spin-dependent transmission spectrum near the Fermi energy for M2 , where the bias 0.4 V corresponding to the maximum value of spin-up current for P configuration and 0.8 V to the maximum value of spin-down one in AP case. It is clearly seen that the transmission spectra for spin-up and -down electrons, respectively, display remarkably different behavior around the Fermi level. In the P case, as can be seen in Fig. 6(a), the spin-up and -down transmission spectra are remarkably different. The spin-up transmission in the bias window has a broad peak near the Fermi level, whereas the spin-down one is small within the bias window. From the Landauer-B¨uttiker formula, we know that the transmission within the bias window makes contribution to the current. This leads to a phenomenon that the spin-up electrons are transporting at the bias of 0.4 V, but the spin-down electrons are blocked. However, the situation in the AP case is just opposite with that in the P case under bias 0.8 V. This may well explained the dual spin-filtering effect for M2 . Moreover,

In summary, we have studied the spin-dependent transport properties for a junction of a fused oligothiophenes molecule embedded between two semi-infinitive long ZGNR electrodes. The junctions of molecule with and without NH2 side group are considered, respectively, and external magnetic fields or FM stripes are applied onto the ZGNRs to initially orient the magnetic alignment of the electrodes for the spin-dependent consideration. By the ab initio calculations based on the DFT combined with NEGF formalism, we have demonstrated the remarkable difference in spin-charge transport properties between the junctions of molecule with and without NH2 side group. In particular, the junction of molecule with NH2 side group shows more obvious NDR than the one without side group. In addition, it exhibit an interesting dual spin-filtering effect when the magnetic alignment in two electrodes are initially oriented. This is because that the oligothiophenes molecule attaching side group influences on the both spin and charge transporting channels through the junction, thereby improving the spin-dependent electronic transport performance. The corresponding mechanisms of the above mentioned results are analyzed and discussed in terms of the evolution of the frontier molecular orbitals, the molecular projected self-consistent Hamiltonian eigenvalues, spinresolved transmission spectrum, and local density of state. The influence of chemical side group in a molecular junction, in addition to the significance in basic chemical physics, it may give us an idea of future study orientation for multifunctional molecular device which reduces greatly the integrating cost of circuits.

V. ACKNOWLEDGMENTS

This work was supported by the Research Foundation of Hunan Provincial Education Commission (Grant No. 16A124) and Hunan Provincial Innovation Foundation for Postgraduate (Grant Nos. CX2015B124 and CX2016B165).

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Highlights: 1. 2. 3. 4.

First report for molecular junction of oligothiophenes contact graphene electrodes. Obvious difference in transport between molecules with and without amino . Coexisted negative differential resistance and dual spin-filtering. Ideal dual spin-filtering effect for molecule with NH2 side group.