Modulation of Rectification in Diblock Co-oligomer Diodes by

Article pubs.acs.org/JPCC

Modulation of Rectification in Diblock Co-oligomer Diodes by Adjusting Anchoring Groups for Both Symmetric and Asymmetric Electrodes Guang-Ping Zhang, Gui-Chao Hu,* Yang Song, Zong-Liang Li, and Chuan-Kui Wang* College of Physics and Electronics, Shandong Normal University, Jinan 250014, China

ABSTRACT: The rectifying properties in dipyrimidinyl-diphenyl co-oligomer diodes with asymmetric anchoring groups were investigated using density functional theory combined with the nonequilibrium Green’s function method. Effects of asymmetric interfaces caused by both the anchoring groups and/or contact geometries of electrodes have been investigated. Our results showed that the rectifying behavior of the co-oligomer diode could be reversed or largely enhanced by adjusting asymmetric anchoring groups. Whether the asymmetric contact geometries play a positive or negative role in improving the rectifying behavior is closely related to each molecular diode. The mechanism of modulation was analyzed in terms of molecular projected self-consistent Hamiltonian states and transmission spectra. The theoretical simulations are helpful for understanding recent experimental results [Lee et al. Langmuir 2009, 25, 1495 and Hihath et al. ACS Nano 2011, 5, 8331]. Moreover, the mechanism of the rectification only due to the electrode asymmetry was explained, and a single-molecule diode with significant rectifying behaviors has been theoretically designed. such as protonation of the molecule in acid solution17,18 and the effect of the molecular length19,20 have been further investigated. Anchoring groups that bind molecules to electrodes play an important role in determining the charge transport of molecular devices, thanks to different electronic couplings between the molecules and the electrodes and different positions of the molecular orbitals with respect to the electrode’s Fermi energy caused by different anchoring groups.21−23 Accordingly, substituting appropriate anchoring groups presents a predominant approach to tune charge-transport properties of molecular junctions. For example, Lee et al. have studied the anchoring group effect of molecular diodes on rectifying behavior and found that the rectifying direction may be reversed by replacing the thiol group with an isocyanide group on the terminal of the diphenyl moiety.9 The introduction of asymmetric anchoring groups into co-oligomers looks attractive since it combines the two kinds of asymmetries from interfaces and molecules. In the

1. INTRODUCTION Molecular diodes have attracted considerable attention in the past decades since the first prototype was proposed by Aviram and Ratner in 1974.1 The Aviram-Ratner (A-R) molecular diodes consist of two donor and acceptor molecular fragments separated by a σ bridge (D-σ-A). However, the realization of AR molecular diode is challenged by further chemical synthesis and theoretical verification.2,3 An alternative way is to design a molecular device with asymmetric electrodes or asymmetric molecule/electrode interfaces.4−6 But such extrinsic molecular diodes seem to be inefficient due to the uncontrollable interfaces and the difficulty in further integration. Recently, diblock co-oligomer diodes with donor−acceptor (D−A) structure have captured a lot of attention as promising intrinsic molecular diodes.7−9 It has been reported that a clear and reproducible rectification can be achieved and the rectifying direction can be controlled by adjusting the molecular orientation.9,10 Furthermore, experiment has found that the rectifying is independent of the temperature in the range between 300 and 50 K.11 Understanding the mechanism of the rectifiers has been the subject of a great deal of theoretical works.12−16 The factors affecting the rectification properties, © 2012 American Chemical Society

Received: May 20, 2012 Revised: August 26, 2012 Published: September 21, 2012 22009

dx.doi.org/10.1021/jp304890p | J. Phys. Chem. C 2012, 116, 22009−22014

The Journal of Physical Chemistry C

Article

Figure 1. Schematic structures of the calculated three molecular junctions: M1 corresponds to the molecule connecting the electrodes via two thiol groups; M2 is the junction where the left thiol group is replaced by an isocyanide group while the isocyanide group appears in the right terminal for M3. The region between the two dashed lines is the scattering region. (a) Symmetric contact geometries and (b) asymmetric contact geometries.

(DZP) basis set for the other atoms. Current through the junctions is calculated by the Landauer-Büttiker formula, 2e T (E , V )[f (E − μL ) − f (E − μR )] dE I= h

present work, based on ab initio calculations and nonequilibrium Green’s function method, we theoretically studied the tuning of the rectification in dipyrimidinyl-diphenyl diodes with asymmetric thiol and isocyanide anchoring groups. To elucidate the experimental measurements,9,24 two kinds of electrodes’ arrangement, that is, the symmetric and asymmetric contact geometries for the two electrodes, were considered. Thus, the molecular intrinsic properties and their variation caused by other factors were investigated. The results indicate that the molecular rectifying behavior is largely modulated by adjusting the anchoring groups and altering contact structures of the electrodes.

∫

where e is the electron charge, h Planck’s constant, and T(E, V) the transmission function of the junctions at energy E under bias voltage V. f(E − μL/R) is the Fermi-Dirac distribution function with the electrochemical potential μL/R of the left (right) electrode. The electronic transport properties are calculated by the ab initio code TranSIESTA.28

3. RESULTS AND DISCUSSION After constructing the molecular junction, the extended molecule was optimized. The dihedral angles and the bond lengths of the terminal atoms and Au are shown in Table 1.

2. THEORETICAL MODEL AND COMPUTATIONAL DETAILS The molecular junctions with the symmetric and asymmetric arrangements of the two electrodes are illustrated in Figure 1. The case of symmetric electrodes is shown in Figure 1a, where a single dipyrimidinyl-diphenyl molecule is bonded to two Au(111) surfaces through different anchoring groups. While in Figure 1b, the right contact geometry is chosen to be a triangle structure with three gold atoms and the left contact geometry keeps a gold surface. Three cases are considered here which are denoted by M1, M2, and M3. In M1, two thiol groups are adopted where the sulfur atoms are chosen to be located at the hollow site. For M2 the left thiol group in M1 connected with the diphenyl part is replaced by an isocyanide group, while the right one is replaced for M3. Two layers of Au atoms in each electrode and the molecule constitute the scattering region, which is optimized in SIESTA package25 with a maximum force of 0.02 eV/Å. During the optimization, the anchoring atoms are only allowed to relax along the transport direction and the relative positions of the electrode surface layers are frozen except the distance between the two electrodes. The improved Troullier-Martins type norm-conserving pseudopotentials26 are used to describe the core electrons and the Perdew-BurkeErnzerhof (PBE) generalized gradient approximation (GGA)27 is adopted for the exchange-correlation functional. In all the calculations a single-ζ plus single polarization (SZP) basis set is employed for Au atoms and a double-ζ plus single polarization

Table 1. Selective Geometrical Parameters of the Molecular Junctionsa θ1/deg

θ2/deg

θ3/deg

d1/Å

d2/Å

symmetric contact geometry

M1 M2 M3

37.0 36.0 34.5

−37.5 −34.9 −35.4

0.4 0.2 0.8

2.635 2.284 2.657

2.633 2.657 2.285

asymmetric contact geometry

M1 M2 M3

36.2 36.0 34.6

−36.4 −34.9 −35.4

0.9 0.2 0.8

2.640 2.282 2.657

2.528 2.531 2.204

a

The dihedral angles between the neighboring rings from the left to right are noted as θ, and the bond length of the left (right) terminal atom and gold atom is noted as d1 (d2).

From Table 1, one can see that the molecule for each case is not in a planar state. However, the first phenyl ring and the two pyrimidinyl rings are almost coplanar. Both the anchoring group and the contact configuration have little influence on the angles. The bond length of C−Au is shorter than that of S−Au. 3.1. Symmetric Arrangement of the Two Electrodes. The two electrodes are assumed to be symmetric in order to investigate the intrinsic property of the molecules and also elucidate the experimental measurements conducted by 22010



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molecular eigenstates are investigated by diagonalizing the molecular projected self-consistent Hamiltonian (MPSH) at every bias voltage.29 Figure 3 shows the evolution of the MPSH

Hihath’s scheme where formation of symmetric electrodes would be deduced from the symmetric current−voltage (I−V) curves about the zero bias for tetraphenyl molecule.24 We start from I−V curves of the three junctions shown in Figure 2. It is

Figure 2. Current−voltage curves for molecular junctions M1, M2, and M3 as shown in Figure 1a. The insets are the rectification ratios.

found that, for M1, the I−V characteristic manifests a slight asymmetry. The current prefers to flow from the dipyrimidinyl block to the diphenyl one at a positive bias. The bias-dependent rectification ratio, defined as R(V) = |I(V)/I(−V)|, shows a maximum value of about 1.6 at the bias voltage of 1.0 V as depicted in the inserted plot of Figure 2a, which is consistent with the experimental result of 2.97 ± 1.2 at 1.5 V given by Hihath et al.24 The theorectial result is also consistent with the calculated value of 1.58 at 1.0 V given by Nakamura et al.16 However, this value is rather small compared to the experimental results given by Lee et al.9 This disagreement may result from their experimental techniques for preparation of molecular junctions and measurement, in which asymmetric electrodes responsible for the larger rectification ratio are introduced. When the left thiol group connected with the diphenyl terminal is replaced by an isocyanide group (M2), in which asymmetric interfaces between the molecule and electrodes caused by the different anchoring groups appear, the rectifying direction is obviously reversed accompanied with an enhanced magnitude of current as shown in Figure 2b. The maximum inverted rectification ratio (1/R) of M2 exceeds the maximum rectification ratio R of M1, and becomes as large as 4.62 at the bias voltage of 1.2 V. One can see that the rectifying direction is consistent with the experimental finding.9 However, the computational maximum R value is rather large compared to the experimental average R value of 1.9, showing a discrepancy between the simulation and measurement. On the basis of M2, we theoretically form a rectifying diode M3 by just exchanging the positions of the two anchoring groups in M2. As shown in Figure 2c, the rectifying direction of current keeps the same as that of M1, but the magnitude of current in the positive bias region is surprisingly increased by almost 1 order. More importantly, we found that the rectification ratio is much enhanced with the largest R of 15.4 at the bias voltage of 1.4 V. This once again indicates that the performance of dipyrimidinyl-diphenyl diodes can be largely modulated by selecting different anchoring groups connecting with the electrodes. The modulation of rectification from the anchoring groups can be understood by a molecular orbital analysis. The

Figure 3. Evolution of MPSH eigenvalues under bias voltages for the three molecular junctions as shown in Figure 1a. The dashed lines indicate the bias window.

eigenvalues of the three molecular junctions under bias voltages. For the case of M1, electron tunneling is mainly through the highest occupied molecular orbital (HOMO) because the Fermi energy level is close to the HOMO. When the bias is increased, the HOMO gradually enters the bias window and serves as a conducting channel. However, for both cases of M2 and M3, electron tunneling through the lowest unoccupied molecular orbital (LUMO) is more favorable. For M2 the LUMO and LUMO+1 soon enter the bias window at a negative bias while a similar situation occurs at a positive bias for M3. Furthermore, the LUMO of M3 is closer to the Fermi energy level than those of other two cases at a low bias voltage. This indicates that the MPSH eigenvalues are largely modified by the isocyanide group as well as its position. The modification of the wave functions of the frontier orbitals at zero bias voltage is also shown in Figure 4. For M1,

Figure 4. Spatial distributions of the perturbed frontier molecular orbitals for each molecular junction shown in Figure 1a at zero bias voltage. (a) M1, (b) M2, and (c) M3.

the HOMO and LUMO show a slight asymmetry while the HOMO-1 and LUMO+1 mainly distribute in the right dipyrimidinyl moiety. In the case of M2, the HOMO turns to be localized in the right dipyrimidinyl moiety and the LUMO and LUMO+1 in the left diphenyl moiety. An opposite spatial distribution of the above three orbitals with respect to M2 22011



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3.2. Asymmetric Arrangement of the Two Electrodes. Influences of the asymmetric contact geometries on the rectifying behavior of molecular junctions were considered here mainly for elucidating the experimental measurements given by Lee et al.9 In this experiment, an asymmetric setup was used. Here we used a triangle structure with three gold atoms at the right end of the molecules to simulate the nanoparticles. Thus, the asymmetry of the two electrodes is simulated. The I− V curves of the three junctions are shown in Figure 6. When the

appears when the isocyanide is connected on the right terminal in M3. The asymmetric spatial distribution of an orbital usually determines its response to the bias as well as the rectification property. As shown in Figure 3, the slight asymmetry of HOMO and LUMO in M1 induces a weakly asymmetric evolution of the two orbitals and then a weak rectification. For M2, an apparent asymmetric evolution of the HOMO, LUMO, and LUMO+1 is observed, where they are shifted to the EF under a negative bias voltage and away from the EF under a positive bias voltage. This is because the orbital localized in one moiety tends to follow the chemical potential of its adjacent electrode under a bias voltage. Such shift of energy levels also leads to an inversion of the rectification compared with M1. In the case of M3, the LUMO and LUMO+1 track evolution similar to that of M1, but their changes with bias voltage become more distinct because of the more asymmetric spatial distribution. Three orbitals completely enter the bias window under the bias voltage of 1.6 V. As a result, the current as well as the rectification ratio is significantly enhanced. The effect of the MPSH states change on the rectification can be further verified by the electronic transmission spectrum. The results are shown in Figure 5 where the MPSH eigenvalues are

Figure 6. Current−voltage curves for molecular junctions M1, M2, and M3 as shown in Figure 1b. The insets are the rectification ratios.

asymmetry setup is included, a maximum R value of about 4.82 at the bias voltage of 1.0 V shown in Figure 6a for M1 is observed, a large enhancement compared to the case with symmetric electrodes in Figure 2a. It is clear that this computational result is more comparable with the measurement.9 For the case of M2 shown in Figure 6b, it is surprisingly observed that the rectifying behavior is weakened by introducing the asymmetry setup. Furthermore, the maximum 1/R value is lowered to be 2.01 at the bias voltage of 1.4 V, which is highly consistent with the average value of 1.9 in the experiment.9 In this case, the asymmetric interfaces are attributed to both the asymmetric anchoring groups and the asymmetric electrodes. The simulation seems to demonstrate a negative role of the asymmetric electrodes for the asymmetric interfaces. Turning to the M3 case shown in Figure 6c, we can see that the rectifying behavior is a little weakened in comparison with the case in Figure 2c. The maximum R value is lowered to be 13.12 at 1.4 V. Nevertheless, the molecular junction still presents interesting and significant rectifying behaviors. The evolution of the MPSH eigenvalues of the three molecular junctions as bias voltages is given in Figure 7. For the cases of M1 and M2, when negative biases are applied, the HOMO does not enter the bias window even at a bias voltage of −1.6 V. This situation is quite different from that in Figure 3. Moreover, the evolution of the eigenvalues at positive biases has variation similar to that in Figure 3. As a result, the rectification ratio R for M1 is enhanced, while the inverted rectification ratio 1/R for M2 is lowered. For M3, the HOMO at negative biases is closer to the bias window that results in lower values of rectification ratio. The discussions shown above demonstrate that the electrode asymmetry has an obvious influence on the rectification behaviors of the molecular junctions. For the purpose of explicitly analyzing the mechanism of rectification due to the

Figure 5. Electronic transmission spectra for three molecular junctions shown in Figure 1a at different bias voltages in semilogarithmic coordinate. (a) M1, (b) M2, and (c) M3. For each molecular junction, curves corresponding to three bias values of −1.6, 0, and 1.6 V are listed from top to bottom, respectively. The dashed lines indicate the bias window and the triangles are the MPSH eigenvalues.

also labeled. For M1, the larger current under 1.6 V comes from the entrance of more proportion of the LUMO transmission spectrum into the bias window. For M2, the current at −1.6 V becomes larger than that at 1.6 V due to the presence of transmission band in the bias window contributed by HOMO, LUMO, and LUMO+1, which leads to the inversion of rectification. For M3, the HOMO, LUMO, and LUMO+1 contribute a broader and higher transmission band in the bias window than that in M1, which improves the magnitude of current at 1.6 V and rectification ratio. 22012



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direction of the external electric field. To illustrate this feature, we calculate the projected density of states (PDOS) for sulfur atoms at different bias voltages shown in Figure 9. For the case

Figure 9. Projected density of states to each terminal sulfur atom for the tetraphenyl-dithiol molecular junction with asymmetric electrodes under different bias voltages.

Figure 7. Evolution of MPSH eigenvaluses under bias voltages for the three molecular junctions as shown in Figure 1b. The dashed lines indicate the bias window.

of zero bias voltage, one can see that the PDOS for the left and the right sulfur atom are quite different. This difference comes from the electrode asymmetry. The interaction energy between the molecule and electrodes mainly arises from the coupling energy between the terminal sulfur atoms and the electrodes. Thus, the difference implies that the coupling energies for the left and right parts in the molecular junction are not equal. When bias voltage values of 0.8 and −0.8 V are applied, one can observe that the evolution of the PDOS for both S(L) and S(R) depends on the direction of the external field. It is thus concluded that the coupling energies are related to the direction of the external field. Accordingly, one can expect the asymmetric shift and asymmetric spatial distribution of the molecular orbitals under the external field, resulting in asymmetric characteristic of the current. On the basis of the discussions above, we conclude that the rectification due to the electrode asymmetry is mainly caused by the dependence of the coupling energies on the direction of the external field. However, for the donor−acceptor typed molecule shown in Figure 2a, the rectification mainly results from the external field direction dependence of the interaction between the polarized molecule and the field.

electrode asymmetry, we model a molecular junction with a symmetric tetraphenyl molecule and an asymmetric arrangement of the two electrodes shown above. The charge transport properties of this molecular junction are shown in Figure 8.

Figure 8. (a) Current−voltage curves and (b) transmission spectra at different bias voltages for the tetraphenyl-dithiol molecular junction with asymmetric electrodes. The inset is the rectification ratio.

4. CONCLUSIONS The modulation of asymmetric anchoring groups and asymmetric contact geometries on the rectification in diblock co-oligomer diodes is investigated by applying first-principles quantum transport calculations. By introduction of the isocyanide group on one of the two terminals of dipyrimidinyl-diphenyl diodes, the frontier molecular orbitals, especially their spatial distributions, are modulated significantly, which further leads to different evolutions of the orbitals under bias voltages. The rectifying direction or the rectification ratio of single dipyrimidinyl-diphenyl diodes was modulated by substituting an isocyanide group for a thiol group. The asymmetry of electrodes could give an enhanced rectifying effect for the dipyrimidinyl-diphenyl diode with thiol terminals, but a reduced one for the dipyrimidinyl-diphenyl diode as one thiol terminal was replaced by an isocyanide terminal. The mechanism of the rectification only due to the electrode asymmetry is attributed to the dependence of the coupling

From Figure 8a, it is observed that the molecular junction has an obvious rectifying characteristic, and the maximum R value of about 3.1 at the bias of 0.8 V is larger than that for the dipyrimidinyl-diphenyl molecular junction M1 with symmetric electrodes shown in Figure 1a. Turning to the transmission spectra at different bias voltage in Figure 8b, one can see that, when a positive bias is applied, the transmission peak over the Fermi level is shifted to the Fermi level. Although peak A is away from the Fermi level, one new peak B gradually appears as the positive bias value is increased. It is thus understandable that the current values at positive biases are larger than their counterparts at negative biases. The electrode asymmetry results in asymmetric interfaces in the junctions. It may be expected that the interaction between the molecule and electrodes depends on both the strength and 22013



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energies on the direction of the external field. The findings propose a feasible way to considerably improve the rectifying performance of intrinsic molecular diodes.

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(23) Kamenetska, M.; Koentopp, M.; Whalley, A. C.; Park, Y. S.; Steigerwald, M. L.; Nuckolls, C.; Hybertsen, M. S.; Venkataraman, L. Phys. Rev. Lett. 2009, 102, 126803. (24) Hihath, J.; Bruot, C.; Nakamura, H.; Asai, Y.; Díez-Pérez, I.; Lee, Y.; Yu, L.; Tao, N. ACS Nano 2011, 5, 8331−8339. (25) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745−2779. (26) Troullier, N.; Martins, J. L. Solid State Commun. 1990, 74, 613− 616. (27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (28) Brandbyge, M.; Mozos, J.-L.; Ordejón, P.; Taylor, J.; Stokbro, K. Phys. Rev. B 2002, 65, 165401. (29) Larade, B.; Taylor, J.; Zheng, Q. R.; Mehrez, H.; Pomorski, P.; Guo, H. Phys. Rev. B 2001, 64, 195402.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.-C.H.); [email protected] (C.-K.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 10804064, 10904084, and 10974121), the Shandong Province Middle-Aged and Young Scientists Research Awards Foundation (Grant No. 2009BS01009), and the Natural Science Foundation of Shandong Province (Grant No. ZR2010AZ002).

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Modulation of Rectification in Diblock Co-oligomer Diodes by

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