Controlling Rectification Performance by Tuning Molecule-Electrode

Subscriber access provided by University of Birmingham

C: Energy Conversion and Storage; Energy and Charge Transport

Controlling Rectification Performance by Tuning Molecule-Electrode Coupling Strength in Ferrocenyl-Undecanethiolate Molecular Diodes Ming-Zhi Wei, Xi Yu, Xiao-Xiao Fu, Zi-Qun Wang, Chuan-Kui Wang, and Guang-Ping Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08833 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Controlling Rectification Performance by Tuning Molecule-Electrode Coupling Strength in Ferrocenyl-Undecanethiolate Molecular Diodes Ming-Zhi Wei,†,¶ Xi Yu,∗,‡ Xiao-Xiao Fu,† Zi-Qun Wang,† Chuan-Kui Wang,∗,† and Guang-Ping Zhang∗,† †Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, School of Physics and Electronics, Shandong Normal University, Jinan 250358, China ‡Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Physics and Department of Chemistry, School of Sciences, Tianjin University, Tianjin 300072, China ¶School of Materials Science and Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China E-mail: [email protected]; [email protected]; [email protected]

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract

Controlling and optimizing rectifying performance of single-molecule diodes remains a formidable challenge in the field of molectronics. As is known, molecule-electrode interfaces play critical role in determining the charge transport properties and functionalities of molecular devices. Definitely, the molecule-electrode interfaces also should be paid special attention in designing molecular diodes with high performance. Recent experiments showed that rectification ratios of ferrocenyl-alkanethiolate diodes can vary by orders of magnitude through controlling the materials and surface topography of bottom electrode, orientation of ferrocenyl group with respect to the top electrode as well as orderliness of the self-assembled monolayers. Here we theoretically investigate effects of the orientation (i.e., tilt angle α) of ferrocenyl group with respect to the electrode surface on the rectifying performance of ferrocenyl-undecanethiolate singlemolecule diodes based on first-principles calculations. It is revealed that rectification ratios of the diodes are dramatically modulated by two orders of magnitude when α is varied. Further analysis shows that the conducting molecular orbitals move away from the Fermi energy due to enhanced coupling between the molecule and right electrode when α decreases. This is found to be responsible for the considerable reduction in rectifying performance of the diodes. Our results suggest that the coupling strength between the molecule and right electrode is of great importance on the rectification performance of ferrocenyl-alkanethiolate diodes and a weaker coupling strength is in favor of improvement of rectification ratios. Hence, our results provide a design principle for high performance molecular diodes based on SCn Fc or structurally similar systems.

1. Introduction The pursuit of continually miniaturizing silicon-based electronic devices opens up a new exciting field called molecular electronics, which aims to construct the circuits using single or a few collections of molecules as building blocks. 1 Over the past decades, various kinds of 2


Page 2 of 21

Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


functional molecular devices have been theoretically designed and experimentally fabricated, such as molecular wires, 2,3 molecular rectifiers, 4,5 molecular transistors, 6–8 and molecular switches. 9–11 Among these functional molecular devices, molecular diodes, which behave like traditional p-n junctions conducting under forward biases while blocking under reverse biases, have drawn especially extensive attention for its importance in circuits since it was first prosed by Aviram and Ratner. 12–20 In general, for a molecular junction to rectify, conducting orbitals around Fermi energy (EF ) having distinct coupling strengths with two electrodes are required, which respond differently to forward and reverse biases. Such asymmetric coupling strengths for a conducting orbital can be produced by various kinds of ways. For example, In 2009 Chen et al. have experimentally fabricated heterometallic nanogaps made of Pt and Au nanorods separated by a nanometer-sized gap as small as 2 nm by chemistry-based on-wire lithography nanofabrication technique. Molecular junctions comprised of such heterometallic nanogaps, in which symmetric thiol-terminated oligo(phenylene ethynylene) (OPE) molecules were assembled, exhibit rectifying behavior. The rectification was attributed to the stronger coupling strength for Au-S bonding than Pt-S. 4 In 2015, Dyck et al. have theoretically proposed a new strategy to design molecular rectifiers with significant rectification performance, in which two conjugated fragments are decoupled by a saturated bridge and connected to gold electrodes by asymmetric thiol and nitrile anchoring groups, respectively. In this way, strong Fermi pinning effect for the thiol favored HOMO and nitrile favored LUMO will lead to resonant and non-resonant electron tunneling under forward and reverse biases, respectively. In fact, molecules with intrinsic asymmetric structures have attracted more focuses in the past investigations, such as molecules with D-σ-A 21–23 and D-π-A structures. 24,25 Besides, Yu et al. have experimentally proved that a direct connection of the donor and acceptor segments (namely D-A molecules) can also produce efficient rectification. 13,26–28 More recently, Ding et al. have demonstrated that two amide-bridged phenyl groups connected to gold electrodes by thiolate anchors can also rectify. 14 The dominant conducting orbital of the molecule, which

3



has asymmetric spatial distribution induced by amide group, evolve monotonically under bias, shifting toward (away from) EF under forward (reverse) bias. Further experimental investigation showed that spatial distribution of the dominant transport channel as well as the rectification ratio strongly depended on the electronic coupling strength at the moleculeelectrode interface, which can be modulated by replacing the anchoring groups. 18 Moreover, they found that the rectification ratio could also be largely enhanced by moving the dominant conducting orbital closer to EF , which could be realized by asymmetrically functionaling the phenyl groups. 16 As is revealed, large difference in coupling strength at the two molecule-electrode interfaces is favorable for improvement of rectification performance. 29 However, in the above mentioned molecular rectifiers, all the molecules are chemically bonded to two electrodes to ensure the yields and stability of the fabricated devices. To some extend, this will prevent from obtaining molecular diodes with high performance. In order to acquire large difference in molecule-electrode coupling strength, the combination of chemical bonding at one interface and physical contact through van der Waals interaction at the other one is preferred. But the van der Waals interaction between the molecule and electrode in turn will cause low yield and high instability of the synthesized molecular devices. For this formidable challenge, a technique has been specially developed by Whitesides and Nijhuis et al., in which the molecular junctions are fabricated by growing a thiolated self-assembled monolayer (SAM) on a template-stripped ultraflat silver (AgTS ) electrode and then covering a eutectic alloy of gallium and indium with thin layers of gallium oxide (Ga2 O3 /EGaIn) as the top electrode. With this approach, molecular junctions having chemical bonding at the bottom molecule-electrode interface and van der Waals contact at the top can be easily obtained with high yield (usually 70∼90%) and stability. Taking the advantage of this approach, they have investigated a series of ferrocenyl-n-alkanethiolate (SCn Fc, Fc=ferrocenyl) diodes. 30–32 A significant rectification ratio of up to 1.0 × 102 has been observed for SC11 Fc. However, for a structurally similar system SC9 Fc, the rectifica-

4


Page 4 of 21

Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


tion ratio has been found only 10, 31,32 which is one order of magnitude smaller than SC11 Fc. By theoretical modeling, Cui et al. have attributed the much smaller rectification ratio in SC9 Fc to the non-negligible direct tunneling current between two electrodes due to a shorter interelectrode distance in the molecular junction compared to the case of SC11 Fc. 33 However, in more recent studies, Nijhuis et al. have reported that the rectification ratios for SC11 Fc and SC9 Fc reach 1.5 × 102 and 1.2 × 102 , respectively. 34,35 This suggests that the difference in alkyl chain lengths of SC11 Fc and SC9 Fc should not be the major origin for their large discrepancy in earlier reported rectification ratios. It is noted that electrochemical data in Nijhuis’ earlier experiments suggest a less ordered SAM for SC9 Fc than SC11 Fc. 31 This can also be confirmed by the large discrepancy in the log-standard deviations of the measured rectification ratios (6.8 for SC9 Fc compared to 2.1 for SC11 Fc). However, in latter experiments the reported log-standard deviations for rectification ratios are very close (1.6 for SC9 Fc and 2.1 for SC11 Fc) 34,35 suggesting the nearly same quality of SAMs for SC9 Fc and SC11 Fc. It has also been pointed out that the orderliness of the SAM plays an important role in determining the rectification performance and a less ordered SAM of SCn Fc gives low rectification ratio. 34,35 More specifically, they have demonstrated that the grain boundaries on the bottom electrode are the major source for the less ordered SAM, and the rectification ratio of SC11 Fc can be reduced to nearly unity by decreasing the surface topography quality of the bottom electrode. 36 The effects of a less ordered SAM can be approximately attributed to two main aspects, i.e., the changes in distance between Fc head group and top electrode and the orientation of Fc group with respect to the top electrode, both of which can fundamentally modify the coupling strength between the Fc head group and top electrode as well as the rectification performance. Here, taking the latter aspect as an example and choosing the SC11 Fc diode as the prototype, we theoretically illustrate effects of the orientation, namely the tilt angle, of Fc group with respect to electrode surface on the rectifying performance by using the state-of-the-art Keldysh nonequilibrium Green’s function (NEGF) formalism in combination

5



Figure 1: Model of the SC11 Fc molecular junction. α and γ respectively denote the tilt angle of the terminal Fc group and alkyl chain with respect to the normal of Ag(111) surface. with the density functional theory (DFT). The numerical results demonstrate a remarkable modulation (from nealy unity to ∼ 1.4 × 102 ) on rectification ratio of the diodes by the tilt angle of Fc group, which can be attributed to the energy level shift relative to EF for the conducting orbital induced by the tilt angle dependent coupling strength at the moleculeelectrode interface.

2. Modeling and computational details As schematically displayed in Figure 1, the investigated molecular junctions are modeled according to the two-probe methodology in Atomistix ToolKit (ATK) package, 37,38 which is comprised of three parts: a central region and two semi-infinite electrodes. In the central region, a single SC11 Fc molecule chemisorbs to the hollow site of the left Ag(111) surface through terminal thiolate group and physically contacts with the right electrode via Fc head group in forms of van der Waals interaction. Periodic boundary conditions are used along the x and y directions (refer to the compass in Figure 1). And a (4 × 4) Ag(111) supercell containing only one SC11 Fc molecule is constructed to avoid interactions between the molecule and its images. The geometric structure of the central region is optimized with

6


Page 6 of 21

Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1: The relaxed α, γ (in degree), and distance d (in Angstrom) from Fe atom to the right electrode for each molecular junction. junction

α

γ

d

MJ-50 MJ-40 MJ-30 MJ-20 MJ-10

47.9 33.9 22.9 9.9 5.1

24.8 34.4 44.3 54.0 65.1

4.917 4.654 4.617 4.579 4.469

a force criterion of 0.04 eV/Å, where the whole molecule and the innermost two layers of each Ag(111) electrode surface are fully relaxed and the remaining Ag(111) layers at each side are constrained to move rigidly to relax the distance between two electrodes. In all calculations, the PBE exchange-correlation functional and Troullier-Martins type norm-conserving pseudopotentials are used. A hybrid basis set of DZP for H, C, S, and Fe atoms and SZP for Ag atoms is employed. 300 Rydberg is adopted for mesh cutoff and the convergence criterion is 1.0 × 10−4 Hartree for Hamiltonian. The k -point sampling grids are 6 × 6 × 100 and 6 × 6 for the electrode and molecular junction calculations, respectively. The current-voltage (I-V ) curve of each junction is obtained according to the Landauer-Büttiker formula. 39 More details about the calculations can be found in our previous papers. 40–42

3. Results and discussion It is worth pointing out that since periodic boundary conditions are used in the x and y directions in our calculations, the model can be regarded as ideal ordered SC11 Fc SAMs with a coverage density of ∼ 1.43×10−10 mol/cm2 (or approximately single-molecule diode), which is about one-third of that (∼ 4.94 ± 0.7 × 10−10 mol/cm2 ) used in experiments. 31,34 However, our previous studies suggest that, for SCn Fc SAMs or similar systems, this coverage density of molecules in simulation can generally reproduce the main characteristics of the charge transport and rectification properties. 20,40 Here, results on the orientation effect of Fc group can, at least partially, provide some insights into the role of molecule-electrode coupling on 7



the rectification performance of SCn Fc SAMs, and can also suggest a useful design principle for high performance molecular diodes based on SCn Fc or structurally similar systems. To investigate the effects of tilt angle α of Fc group (defined as the angle between the line through two centers of cyclopentadienyl rings and z -axis) on charge transport properties of molecular junctions, tilt angle γ of the alkyl backbone (defined as the angle between the z -axis and the line through two ending carbon atoms of the alkyl chain, i.e., the first and eleventh carbon atoms counted from left in Figure 1) is varied to make the angle of Fc group α have different initial values, namely 50◦ , 40◦ , 30◦ , 20◦ , and 10◦ , for which the molecular junctions are correspondingly denoted as MJ-50, MJ-40, MJ-30, MJ-20, and MJ-10. Since the central region of each junction contains a few hundreds of atoms, the optimized structure of the central region is expected to be highly dependent on the initial one, which is clearly verified by the relaxed values of α and γ for each molecular junction in Table 1. One can find that the relaxed α are very close to the initial ones. As shown in Figure 1, the plane of the outer cyclopentadienyl in Fc group gets more parallel to the right electrode surface as α becomes smaller, which will lead to a stronger interaction between the π electrons of Fc group and those at the right electrode surface. On the other hand, as shown in Table 1, the decrease of α moves the Fc group slightly closer to the right Ag(111) surface (represented as a small decrease of the distance d from Fe atom in Fc group to the right electrode), which synergistically facilitates a stronger coupling between the molecule and right electrode. To investigate the influence of α on the coupling strength between the molecule and right electrode, the molecule junction is divided into two parts: the left part containing the left electrode and the SC11 Fc molecule (labeled as “L+M”) and the right part containing only the right electrode (labeled as “R”). The xy plane integrated electron difference density ∆ρ (defined as ∆ρ = ρjunction − ρL+M − ρR , where ρjunction , ρL+M , and ρR are the self-consistent electron densities integrated over xy plane for “junction”, “L+M”, and “R” systems, respectively) for each molecular junction are analyzed. ∆ρ represents the electron transfer between “L+M” and “R” systems after their contact and

8


Page 8 of 21

Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2: The xy plane integrated electron difference density ∆ρ for each single-molecule junction. The number above each line in (b) denotes the charge transfer between the SC11 Fc molecule and electrodes. hence its magnitude can reflect the interaction strength between the SC11 Fc molecule and the right electrode. From Figure 2, one can see a noticeable electron transfer process occurs at the interface between the SC11 Fc molecule and the right electrode during the formation of molecular junctions. An evident amount of electrons transfers from the molecule (almost from the Fc group) to the right electrode surface. An analysis of molecular orbitals reveals that the transferred electrons are mainly contributed by HOMO, HOMO-1, and HOMO-2 of the SC11 Fc molecule. The amount of transferred electrons increases as the α decreases. For instance, there is only 0.073 electron transferred for MJ-50 while it is 0.117 electron for MJ-10. This suggests that the coupling strength between the Fc group and the right electrode becomes stronger when α decreases. Then, the I-V curves for five investigated molecular junctions are calculated in [−1.0 9



Figure 3: Current-voltage curves for molecular junctions of (a) MJ-50, (b) MJ-30, and (c) MJ-10. The insets are the bias-dependent rectification ratio R. (d) The current at ±1.0 V and corresponding rectification ratio R as a function of α. V, 1.0 V], and those for three representative junctions, i.e., MJ-50, MJ-30, and MJ-10, are displayed in Figure 3. Each molecular junction manifests a clear rectification feature, where the junction prefers to conduct under positive bias while blocking under reverse bias. Specifically, for MJ-50 (see Figure 3a), the current is neglectable under negative bias, while it starts to increase sharply when the positive bias is over 0.4 V. For MJ-30, the I-V profile is very similar to that of MJ-50. But the magnitude of the positive current for MJ-30 is obviously suppressed, which is about 75% of MJ-50. In addition, the negative current, especially at −1.0 V, is easy to be noticed compared to the case of MJ-50. When it turns to MJ-10, the positive current is further reduced, which is now only about 5% of MJ-50. Meanwhile, the magnitude of negative current is more comparable to the positive counterpart. Therefore, the asymmetry of the I-V curve for MJ-10 is remarkably weakened. To quantitatively describe the asymmetry of the I-V curve, rectification ratio R = |I(V )/I(−V )| is accordingly defined. From the insets of Figure 3, one can see that in the investigated bias range the maximum R of MJ-50 is about 141.2 at 0.7 V while the maxima of R for both MJ-30 and

10


Page 10 of 21

Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: (a) Evolutions of frontier MPSH orbitals under bias and (b) their wavefunctions at zero bias for MJ-50. (c) The electronic transmission spectra for MJ-50 at 0.0, ±1.0 V. The blue dashed lines indicate the bias window and the isovalue is chosen as 0.002 for the wavefunction plots. MJ-10 occur at 1.0 V and they are only 31.0 and 1.3, respectively. Thus, the rectification performance of the SC11 Fc molecular diode can be highly modulated by controlling the tilt angle α of Fc group, and a significant reduction in the rectification ratio will be observed when α is decreased. To clarify the close dependence of rectification performance on the tilt angle α, the values of current at ±1.0 V for each molecular junction as a function of α have been plotted in Figure 3d. It shows that the current under −1.0 V keeps nearly unchanged when α varies. On the contrary, the current under 1.0 V closely depends on α and it decreases from about 1.25 nA to 0.06 nA as α goes from 47.7◦ to 9.7◦ . Therefore, rectification performance of the junction is lowered and R at 1.0 V decreases from 98.2 ( for α = 47.7◦ ) to 1.3 (for α = 9.7◦ ). So, one can conclude that remarkable variation in rectification performance of the molecular diodes is related to the notable modulation of the current under positive bias by tilt angle α of Fc group. Rectification mechanism of the SCn Fc diode has been well investigated in previous theoretical studies 33,40–43 by analyzing electronic transmission spectra and eigenstates of molecular projected self-consistent Hamiltonian (MPSH). Here, in order to further understand the large modulation of the rectification performance in SC11 Fc junctions by α, the rectification 11



mechanism is still first summarized. In Figure 4, it shows the evolutions of frontier MPSH orbitals for MJ-50 under external bias. One can find that there are three MPSH orbitals, namely HOMO, HOMO-1, and HOMO-2, below EF at zero bias, which respectively locate at −0.475 eV, −0.514 eV, and −0.626 eV. These three MPSH orbitals manifest a monotonic evolution under bias. Specifically, energies of these three MPSH eigenstates under bias vary in compliance with the chemical potential µR of the right electrode. And they enter the bias window when the applied positive bias reaches about 0.6 V, while under negative bias they always keep away from the bias window. Consequently, a unidirectional conducting feature with a remarkable rectification has been demonstrated for MJ-50 as seen in Figure 3a. Further analysis of the wavefunctions for these MPSH eigenstates reveals that their spatial distributions are highly localized on the Fc group (see Figure 4b), and thus these MPSH orbitals are expected to couple more strongly to the right electrode. This can help to understand why their energies are mainly dominated by the chemical potential µR of the adjacent right electrode. However, from Figure 4c, one can see that only two transmission peaks are located at −0.480 eV and −0.630 eV under zero bias. By comparing the locations of transmission peaks and MPSH eigenvalues under zero bias, it is easy to find that the two transmission peaks stem from HOMO and HOMO-2, while HOMO-1 hardly contributes to any transmission due to its much more localizing spatial distribution compared to HOMO and HOMO-2. The monotonic variations of MPSH eigenvalues under bias and the prominent rectification of MJ-50 can also be easily verified by the bias-dependent transmission spectra in Figure 4c. The HOMO and HOMO-2 mediated transmission peaks approach to EF under positive bias and both of them enter the bias window under 1.0 V contributing to a considerable forward current through MJ-50. On the contrary, they move away from EF and are always out of the bias window under negative bias resulting in an invisible leakage current. Now, we turn to explore the underlying mechanisms of the close dependence of rectification performance on α in SC11 Fc molecular diodes. It is generally known that the

12


Page 12 of 21

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5: (a) Eigenvalue variations of frontier MPSH orbitals versus tilt angle α. (b) and (c) are the electronic transmission spectra at 0.0, ±1.0 V for MJ-30 and MJ-10, respectively. conductance of a single-molecule junction is determined by two aspects of frontier molecular orbitals, which actually bridge the left and right electron reservoirs: (1) alignments of the molecular orbitals with respect to EF of metal electrodes; (2) spatial distributions of the corresponding wavefunctions. The latter is mainly governed by the nature of frontier molecular orbitals in the molecule. However, the former can be controlled by various factors, such as materials of metal electrodes and coupling strength between the molecule and electrodes. It has been clearly demonstrated in previous studies 44,45 that the conductance and even the functionality of a single-molecule junction can be elaborately controlled by tuning the alignments of conducting molecular orbitals with respect to EF through the way of mechanically changing the coupling strength between electrodes and sandwiched molecule. Here, variation of tilt angle α of Fc group with respect to the Ag(111) surface is proved to alter the coupling strength between Fc group and the right electrode and hence is expected to further modify the alignments of frontier molecular orbitals relative to EF . This can be explicitly confirmed from Figure 5a, where the energies of frontier MPSH orbitals decrease with α being smaller. For example, the HOMO is at −0.475 eV for α = 47.7◦ while it moves to −0.932 eV for α = 9.7◦ . It means that the conducting molecular orbitals go further away from EF as the tilt angle α decreases. This can also explain why there is an unambiguous increase in the bias threshold for the positive current with the decrease of α (the threshold biases are about 0.4 V, 0.6 V, and 0.8 V for MJ-50, MJ-30 and, MJ-10, respectively).

13



The modification in the alignments of conducting molecular orbitals with EF will substantially affect the bias-dependent transmission spectra and even the rectification performance of the diodes. More specifically, the HOMO and HOMO-2 mediated transmission peaks for MJ-30 under zero bias move to −0.780 eV and −0.935 eV, respectively (see Figure 5b). Only the HOMO transmission peak moves into the bias window and contributes to the forward current at 1.0 V. This is the reason why a 25% decrease in the magnitude of positive current and a suppressed rectification ratio have been observed for MJ-30 compared to MJ-50. However, when the tilt angle α further decreases, taking MJ-10 as an example, the HOMO and HOMO-2 mediated transmission peaks go on deviating from the EF . And they are respectively located at −0.935 eV (see Figure 5c) and −1.050 eV (not shown in Figure 5c) under zero bias, which results in neither of them entering the bias window under positive bias. Consequently, as seen in Figure 3c and 3d, the magnitude of the positive current and the rectification ratio of MJ-10 are sharply reduced in comparison to MJ-50.

4. Conclusions In summary, by applying the non-equilibrium Green’s function method in combination with the density functional theory, effects of tilt angle of the Fc group with respect to electrode surface on the rectification performance of SC11 Fc molecular diodes have been investigated. It has been found that the coupling strength between the Fc group and adjacent electrode can be efficiently tuned by the tilt angle, and the coupling strength is evidently enhanced as the tilt angle decreases. Since a stronger coupling leads the conducting molecular orbitals to move away from the Fermi energy, the rectification performance of SC11 Fc diodes is dramatically reduced as the tilt angle decreases. The rectification ratios of the diodes have been found to be largely modulated by nearly up to two orders of magnitude. Our results suggest that a weaker coupling strength between the Fc group and right electrode facilitates the improvement of rectification performance of SCn Fc diodes. As a result, a design rule for high performance molecular diodes based on SCn Fc or structurally similar systems is 14


Page 14 of 21

Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


proposed here.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 11704230, 11874242, and 21773169), China Postdoctoral Science Foundation (Grant No. 2017M612321), and the Taishan Scholar Project of Shandong Province.

References (1) Xiang, D.; Wang, X.; Jia, C.; Lee, T.; Guo, X. Molecular-Scale Electronics: From Concept to Function. Chem. Rev. 2016, 116, 4318–4440. (2) Kuang, G.; Chen, S.-Z.; Wang, W.; Lin, T.; Chen, K.; Shang, X.; Liu, P. N.; Lin, N. Resonant Charge Transport in Conjugated Molecular Wires Beyond 10 nm Range. J. Am. Chem. Soc. 2016, 138, 11140–11143. (3) Leary, E.; Limburg, B.; Alanazy, A.; Sangtarash, S.; Grace, I.; Swada, K.; Esdaile, L. J.; Noori, M.; González, M. T.; Rubio-Bollinger, G. et al. Bias-Driven Conductance Increase with Length in Porphyrin Tapes. J. Am. Chem. Soc. 2018, 140, 12877–12883. (4) Chen, X.; Yeganeh, S.; Qin, L.; Li, S.; Xue, C.; Braunschweig, A. B.; Schatz, G. C.; Ratner, M. A.; Mirkin, C. A. Chemical Fabrication of Heterometallic Nanogaps for Molecular Transport Junctions. Nano Lett. 2009, 9, 3974–3979. (5) Capozzi, B.; Xia, J.; Adak, O.; Dell, E. J.; Liu, Z.-F.; Taylor, J. C.; Neaton, J. B.; Campos, L. M.; Venkataraman, L. Single-Molecule Diodes with High Rectification Ratios Through Environmental Control. Nat. Nanotechnol. 2015, 10, 522–527. (6) Song, H.; Kim, Y.; Jang, Y. H.; Jeong, H.; Reed, M. A.; Lee, T. Observation of Molecular Orbital Gating. Nature 2009, 462, 1039–1043. 15



(7) Pan, Y.; Wang, Y.; Wang, L.; Zhong, H.; Quhe, R.; Ni, Z.; Ye, M.; Mei, W.-N.; Shi, J.; Guo, W. et al. Graphdiyne-Metal Contacts and Graphdiyne Transistors. Nanoscale 2015, 7, 2116–2127. (8) Pan, Y.; Dan, Y.; Wang, Y.; Ye, M.; Zhang, H.; Quhe, R.; Zhang, X.; Li, J.; Guo, W.; Yang, L. et al. Schottky Barriers in Bilayer Phosphorene Transistors. ACS Appl. Mater. Interfaces 2017, 9, 12694–12705. (9) Fu, Q.; Yang, J.; Luo, Y. Mechanism for Tautomerization Induced Conductance Switching of Naphthalocyanin Molecule. Appl. Phys. Lett. 2009, 95, 182103. (10) Roldan, D.; Kaliginedi, V.; Cobo, S.; Kolivoska, V.; Bucher, C.; Hong, W.; Royal, G.; Wandlowski, T. Charge Transport in Photoswitchable Dimethyldihydropyrene-Type Single-Molecule Junctions. J. Am. Chem. Soc. 2013, 135, 5974–5977. (11) Jia, C.; Migliore, A.; Xin, N.; Huang, S.; Wang, J.; Yang, Q.; Wang, S.; Chen, H.; Wang, D.; Feng, B. et al. Covalently Bonded Single-Molecule Junctions with Stable and Reversible Photoswitched Conductivity. Science 2016, 352, 1443–1445. (12) Aviram, A.; Ratner, M. A. Molecular Rectifiers. Chem. Phys. Lett. 1974, 29, 277–283. (13) Díez-Pérez, I.; Hihath, J.; Lee, Y.; Yu, L.; Adamska, L.; Kozhushner, M. A.; Oleynik, I. I.; Tao, N. Rectification and Stability of a Single Molecular Diode with Controlled Orientation. Nat. Chem. 2009, 1, 635–641. (14) Ding, W.; Negre, C. F.; Vogt, L.; Batista, V. S. Single Molecule Rectification Induced by the Asymmetry of a Single Frontier Orbital. J. Chem. Theory Comput. 2014, 10, 3393–3400. (15) Metzger, R. M. Unimolecular Electronics. Chem. Rev. 2015, 115, 5056–5115. (16) Ding, W.; Koepf, M.; Koenigsmann, C.; Batra, A.; Venkataraman, L.; Negre, C. F.; Brudvig, G. W.; Crabtree, R. H.; Schmuttenmaer, C. A.; Batista, V. S. Computational 16


Page 16 of 21

Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Design of Intrinsic Molecular Rectifiers Based on Asymmetric Functionalization of N Phenylbenzamide. J. Chem. Theory Comput. 2015, 11, 5888–5896. (17) Van Dyck, C.; Ratner, M. A. Molecular Rectifiers: A New Design Based on Asymmetric Anchoring Moieties. Nano Lett. 2015, 15, 1577–1584. (18) Koepf, M.; Koenigsmann, C.; Ding, W.; Batra, A.; Negre, C. F.; Venkataraman, L.; Brudvig, G. W.; Batista, V. S.; Schmuttenmaer, C. A.; Crabtree, R. H. Controlling the Rectification Properties of Molecular Junctions through Molecule-Electrode Coupling. Nanoscale 2016, 8, 16357–16362. (19) Zhang, G.-P.; Xie, Z.; Song, Y.; Hu, G.-C.; Wang, C.-K. Towards Rectifying Performance at the Molecular Scale. Top. Curr. Chem. 2017, 375, 85. (20) Zhang, G.-P.; Wang, S.; Wei, M.-Z.; Hu, G.-C.; Wang, C.-K. Tuning the Direction of Rectification by Adjusting the Location of the Bipyridyl Group in Alkanethiolate Molecular Diodes. J. Phys. Chem. C 2017, 121, 7643–7648. (21) Metzger, R. M. D-σ-A Unimolecular Rectifiers. Mat. Sci. and Eng. C-Mater. 1995, 3, 277–285. (22) Guo, C.; Zhang, Z. H.; Kwong, G.; Pan, J. B.; Deng, X. Q.; Zhang, J. J. Enormously Enhanced Rectifying Performances by Modification of Carbon Chains for D-σ-A Molecular Devices. J. Phys. Chem. C 2012, 116, 12900–12905. (23) Pan, J. B.; Zhang, Z. H.; Q., D. X.; M., Q.; C., G. The Transport Properties of D-σ-A Molecules: A Strikingly Opposite Directional Rectification. Appl. Phys. Lett. 2011, 98, 13503. (24) Martin, A. S.; Sambles, J. R.; Ashwell, G. J. Molecular Rectifier. Phys. Rev. Lett. 1993, 70, 218–221.

17



Page 18 of 21

(25) Ashwell, G. J.; Tyrrell, W. D.; Whittam, A. J. Molecular Rectification: Self-Assembled Monolayers in Which Donor-(π-Bridge)-Acceptor Moieties Are Centrally Located and Symmetrically Coupled to Both Gold Electrodes. J. Am. Chem. Soc. 2004, 126, 7102– 7110. (26) Ng, M.-K.; Lee, D.-C.; Yu, L. Molecular Diodes Based on Conjugated Diblock CoOligomers. J. Am. Chem. Soc. 2002, 124, 11862–11863. (27) Jiang, P.; Morales, G. M.; You, W.; Yu, L. Synthesis of Diode Molecules and Their Sequential Assembly to Control Electron Transport. Angew. Chem., Int. Ed. 2004, 43, 4471–4475. (28) Morales, G. M.; Jiang, P.; Yuan, S.; Lee, Y.; Sanchez, A.; You, W.; Yu, L. Inversion of the Rectifying Effect in Diblock Molecular Diodes by Protonation. J. Am. Chem. Soc. 2005, 127, 10456–10457. (29) Taylor, J.; Brandbyge, M.; Stokbro, K. Theory of Rectification in Tour Wires: The Role of Electrode Coupling. Phys. Rev. Lett. 2002, 89, 138301. (30) Nijhuis, C. A.; Reus, W. F.; Whitesides, G. M. Molecular Rectification in Metal-SAMMetal Oxide-Metal Junctions. J. Am. Chem. Soc. 2009, 131, 17814–17827. (31) Nijhuis, C. A.; Reus, W. F.; Whitesides, G. M. Mechanism of Rectification in Tunneling Junctions Based on Molecules with Asymmetric Potential Drops. J. Am. Chem. Soc. 2010, 132, 18386–18401. (32) Reus, W. F.;

Thuo, M. M.;

Shapiro, N. D.;

Nijhuis, C. A.;

White-

sides, G. M. The SAM, Not the Electrodes, Dominates Charge Transport in MetalMonolayer//Ga2 O3 /Gallium-Indium Eutectic Junctions. ACS Nano 2012, 6, 4806– 4822.

18


Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33) Cui, B.; Xu, Y.; Ji, G.; Wang, H.; Zhao, W.; Zhai, Y.; Li, D.; Liu, D. A Single-Molecule Diode with Significant Rectification and Negative Differential Resistance Behavior. Org. Electron. 2014, 15, 484–490. (34) Nerngchamnong, N.; Yuan, L.; Qi, D.-C.; Li, J.; Thompson, D.; Nijhuis, C. A. The Role of van der Waals Forces in the Performance of Molecular Diodes. Nat. Nanotechnol. 2013, 8, 113–118. (35) Yuan, L.; Thompson, D.; Cao, L.; Nerngchangnong, N.; Nijhuis, C. A. One Carbon Matters: The Origin and Reversal of Odd-Even Effects in Molecular Diodes with SelfAssembled Monolayers of Ferrocenyl-Alkanethiolates. J. Phys. Chem. C 2015, 119, 17910–17919. (36) Yuan, L.; Jiang, L.; Thompson, D.; Nijhuis, C. A. On the Remarkable Role of Surface Topography of the Bottom Electrodes in Blocking Leakage Currents in Molecular Diodes. J. Am. Chem. Soc. 2014, 136, 6554–6557. (37) Brandbyge, M.; Mozos, J.-L.; Ordejón, P.; Taylor, J.; Stokbro, K. Density-Functional Method for Nonequilibrium Electron Transport. Phys. Rev. B 2002, 65, 165401. (38) Atomistix ToolKit version 2015.1, QuantumWise A/S. https://quantumwise.com (accessed December 1, 2018). (39) Datta, S. Electronic Transport in Mesoscopic Systems; Cambridge University Press: U.K., 1995. (40) Wang, S.; Wei, M.-Z.; Hu, G.-C.; Wang, C.-K.; Zhang, G.-P. Mechanisms of the OddEven Effect and Its Reversal in Rectifying Performance of Ferrocenyl-n-Alkanethiolate Molecular Diodes. Org. Electron. 2017, 49, 76–84. (41) Zhang, G.-P.; Mu, Y.-Q.; Wei, M.-Z.; Wang, S.; Huang, H.; Hu, G.-C.; Li, Z.L.; Wang, C.-K. Designing Molecular Rectifiers and Spin Valves Using Metallocene19



Functionalized Undecanethiolates: One Transition Metal Atom Matters. J. Mater. Chem. C 2018, 6, 2105–2112. (42) Wei, M.-Z.; Wang, Z.-Q.; Fu, X.-X.; Hu, G.-C.; Li, Z.-L.; Wang, C.-K.; Zhang, G.-P. Theoretical Understanding of the Inversion of Rectification Direction in FerrocenylEmbedded Tridecanethiolate Single-Molecule Rectifiers. Physica E 2018, 103, 397–402. (43) An, Y.; Zhang, M.; Wang, T.; Wang, G.; Fu, Z. Rectifications in Organic SingleMolecule Diodes Alkanethiolate-Terminated Heterocyclics. Phys. Lett. A 2016, 380, 923–926. (44) Bruot, C.; Hihath, J.; Tao, N. Mechanically Controlled Molecular Orbital Alignment in Single Molecule Junctions. Nat. Nanotechnol. 2012, 7, 35–40. (45) Zhang, G.-P.; Hu, G.-C.; Song, Y.; Xie, Z.; Wang, C.-K. Stretch or Contraction Induced Inversion of Rectification in Diblock Molecular Junctions. J. Chem. Phys. 2013, 139, 094702.

20


Page 20 of 21

Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic.

21


Controlling Rectification Performance by Tuning Molecule-Electrode

Recommend Documents