Tuning the Direction of Rectification by Adjusting ... - ACS Publications

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Tuning the Direction of Rectification by Adjusting the Location of the Bipyridyl Group in Alkanethiolate Molecular Diodes Guang-Ping Zhang,*,†,‡,∥ Shan Wang,†,∥ Ming-Zhi Wei,†,§ Gui-Chao Hu,†,‡ and Chuan-Kui Wang*,† †

Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, School of Physics and Electronics and Institute of Materials and Clean Energy, Shandong Normal University, Jinan 250014, China § School of Materials Science and Engineering, Qilu University of Technology, Jinan 250353, China ‡

S Supporting Information *

ABSTRACT: Controlling and optimizing the performance of molecular electronic devices is a great challenge in molecular electronics. By using a first-principles method, here we show that rectifying direction and rectification performance of molecular diodes, consisting of a single bipyridyl-embedded alkanethiolate molecule sandwiched between two parallel Ag(111) electrodes, can be precisely controlled by placing the bipyridyl group at different locations of the alkanethiolate backbone. The analysis reveals that the monotonic shift of energy levels of frontier molecular orbitals induced by the electrostatic effect of external bias voltage on the strongly localized wave functions is responsible for the features of rectification. The spatial distributions of frontier molecular orbitals are highly dependent on the location of the bipyridyl group. Hence, varying the bipyridyl location in the alkanethiolate backbone essentially changes the evolving behavior of the frontier molecular orbitals under external bias voltages. This work is helpful for the rational design of molecular diodes.

1. INTRODUCTION The molecular rectifier, which manifests like a traditional p−n junction in the molecular scale, has attracted a lot of attention since it was first proposed by Aviram and Ratner in 1974.1 Many kinds of molecular rectifiers have been designed and synthesized experimentally and theoretically, such as devices comprised of D-σ-A,2−5 D-π-A,6,7 and even more simple D− A8−13 molecules with intrinsic asymmetries. However, the low rectifying performance and stability of the molecular rectifier limit its further applications. Therefore, exploiting novel molecular rectifiers with more efficient mechanisms and better stability and developing new fabrication techniques with larger yields are highly required. Recently, Whitesides and Nijhuis14,15 have used eutectic indium−gallium alloy covered with a skin of gallium oxide Ga2O3 (noted as Ga2O3/EGaIn) as the top electrode and enabled a stable van der Waals contact between the top electrode and self-assembled monolayers (SAMs) that are covalently bonded to the bottom electrodes. Using this setup, they realized molecular rectification with a rectification ratio up to ∼1.0 × 10 2 in SAMs of ferrocene (Fc) capped undecanethiolates (SC11Fc). They further demonstrated that the distinct rectification of SC11Fc SAMs was not from the asymmetric contacts between the molecule and electrodes but from the molecule itself.14 Then, they carried out a lot of studies on SAMs with similar structures, where the saturated alkanethiolate backbone was covalently bonded to the bottom © XXXX American Chemical Society

Ag electrode, and the electron-rich headgroup was connected to the Ga2O3/EGaIn electrode through van der Waals contact, to check the robustness of rectification and the range of corresponding mechanisms that can produce rectification in similar systems.15−19 Among these investigations, Yoon et al. reproduced the high rectification ratio by SAMs with more simple structures,18 where the Fc headgroup was replaced by the bipyridyl group (BIPY). Also, they have found that the rectification ratio was highly headgroup dependent, which indicates that the rectification mechanisms in such systems need to be further explored. In this work, the rectification performance of bipyridyl-embedded undecanethiolates is modulated and optimized based on the understanding of underlying rectification mechanisms. The numerical results suggest that the rectifying direction and rectification performance of the molecular diodes can be elaborately controlled by tuning the location of the bipyridyl group in the alkanethiolate backbone.

2. THEORETICAL MODEL AND COMPUTATIONAL DETAILS To elucidate the rectification of SCnBIPYCm is an intrinsic property of the molecule itself but not from the asymmetric Received: December 14, 2016 Revised: February 27, 2017 Published: March 22, 2017 A

DOI: 10.1021/acs.jpcc.6b12595 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

the central region of the junction is optimized again. During this optimization, all atoms in the molecule and the adjacent two layers of Ag(111) electrodes at each side are fully optimized. Atoms in other Ag(111) layers in the central region are restricted to move rigidly along the transport direction to allow relaxation of the distance between two electrodes. In the optimization and following electron transport calculations, the improved Troullier−Martins type norm-conserving pseudopotentials22 are used to describe the core electrons, and the Perdew−Burke−Ernzerhof (PBE)23 generalized gradient approximation (GGA) is adopted for the exchange-correlation functional. A single-ζ plus single polarization (SZP) basis set is employed for Ag atoms, while a double-ζ plus single polarization (DZP) basis set is used for the other atoms. A 300 Ry mesh cutoff for the real space grid is chosen. The convergence criterion for the Hamiltonian matrix is chosen as 1.0 × 10−4 Hartree. A 6 × 6 k-point grid is used for the Brillouin zone (BZ) sampling after a convergence analysis. The electron transport properties are investigated using the nonequilibrium Green’s function (NEGF) method combined with density functional theory (DFT), which is implemented in the Atomistix ToolKit package.20,24 The current through the molecular junction is calculated according to the Landauer− Büttiker formula.25

contacts between the molecule and electrodes, here symmetric Ag(111) electrodes at both sides of the modeled molecular junction are used. As illustrated in Figure 1, the molecular

Figure 1. Schematic of the single molecular junction comprised of a 2,2′-bipyridyl alkanethiolate molecule sandwiched between two Ag(111) electrodes. Here m denotes the number of carbon atoms at the right side of the bipyridyl group in the alkanethiolate backbone, which is used to characterize the location of the bipyridyl group in the alkanethiolate backbone.

junction is comprised of a 2,2′-bipyridyl group embedded SCnBIPYCm molecule sandwiched between two parallel Ag(111) electrodes. The entire molecular junction is divided into three parts, the left electrode, the central region, and the right electrode. Each semi-infinite Ag(111) electrode is simulated by a 4 × 4 × 3 super cell with periodic boundary condition. The SCnBIPYCm molecule is chemically adsorbed at the hollow site of the left Ag(111) electrode through the terminal thiol sulfur atom, while it is connected to the right Ag(111) electrode by the terminal methyl group in the form of van der Waals interaction. The location of the bipyridyl group is characterized by the number of carbon atoms m at its right side in the alkanethiolate, and the corresponding molecular junction is denoted by Cm, where 1 ⩽ m ⩽ 11. The structure of the bare HSCnBIPYCm molecule is optimized first in the Atomistix ToolKit package20,21 with a maximum residual force of 0.02 eV/Å. Then, after removing the hydrogen atom of thiol group the molecule is coupled to the Ag(111) electrodes to form the single molecular junction, and

I=

2e h

∫ T(E , V )[f (E − μL ) − f (E − μR )]dE

(1)

Here e is the electron charge, h the Planck’s constant, T(E, V) the transmission coefficient of incident electrons at energy E under bias voltage V. f(E − μL/R) is the Fermi−Dirac distribution of electrons in the left/right electrode with chemical potential μL/R. The chemical potentials of the electrodes relate to an applied bias voltage V according to |e| V μL/R (V ) = μ0 ∓ 2 , where μ0 is the chemical potential under zero bias voltage. That is to say, for example, a positive bias voltage V will suppress the chemical potential of the left

Figure 2. Current−voltage curves for junctions with bipyridyl at different locations: (a) C1, (b) C4, and (c) C11. The insets are the rectification ratio R or inverted rectification ratio 1/R. (d) Variations of R and 1/R at 2.0 V with respect to the location of the bipyridyl group m. B


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The Journal of Physical Chemistry C electrode by |e| V , and then the current flows from the left 2 electrode to the right one.

was tuned by changing the ferrocenyl−electrode coupling in ferrocenyl−alkanethiol self-assembled monolayers (SAMs).26 To understand the mechanism of rectification, we explored the molecular eigenstates by diagonalizing the molecular projected self-consistent Hamiltonian (MPSH) at every bias voltage27 as well as the bias-dependent electronic transmission spectra. We start from the case of C1. As shown in Figure 3a,

3. RESULTS AND DISCUSSION After the optimization of the central region, we calculate the current−voltage (I−V) curve for each molecular junction Cm. The results are presented in Figure 2, from which it is easily seen that a pronounced rectifying effect is present for m = 1. In the positive bias region, there is hardly any current when the bias voltage is below 1.5 V. However, the current is rapidly enhanced as the bias voltage goes beyond 1.5 V, and the magnitude of the current reaches about 0.49 nA at 2.0 V. On the contrary, the junction keeps blocked in the investigated range of negative bias voltage. Therefore, the forward current for C1 is along the positive bias voltage; i.e., it prefers to flow from the left electrode to the right one. The rectifying performance of molecular junctions can be characterized by the rectification ratio R defined as R(V) = |I(V)/I(−V)| or the inverted rectification ratio 1/R as 1/R(V) = |I(−V)/I(V)|. The bias-dependent R of C1 is shown in the inset of Figure 2a. It is found that the curve of R follows the trend of the I−V curve, where the value of R is increased sharply when the applied bias voltage is beyond 1.5 V and reaches about 89 at 2.0 V. The maximum value of R is in good agreement with that of the experimental observation,18 which is reported to be 85 ± 2. The good agreement between the theoretical calculation and experimental measurement suggests that the rectifying effect is indeed not from the asymmetric electrodes used in the experiments but an intrinsic property of the molecule itself. When the bipyridyl group moves toward the left electrode with the increase of m, e.g., m = 4, it is noted that the junction begins to conduct at larger negative bias voltages. On the other hand, the values of current under positive bias voltages drop by one order in the magnitude compared with those of C1. Therefore, the magnitude of current under negative bias voltage is comparable with the counterpart under positive bias voltage. For example, the magnitude of current under −2.0 V is about one-fourth as large as that of 2.0 V. This is clearly reflected as a significant decrease in R shown in the inset of Figure 2b. The largest R is only about 3.9 at 2.0 V. Interestingly, it should be pointed out that the R is less than 1 when the bias voltage is smaller than 2.0 V, which means the direction of rectification is reversed in the range of [0, 1.8 V] for C4. The maximum inverted rectification ratio 1/R can reach about 2.4 at the bias voltage of 1.4 V. Even more remarkably, when m further increases to 11, complete reversal of rectification is observed, as shown in Figure 2c. The maximum 1/R of C11 is about 422 at 1.8 V, which is far higher than the maximum R of C1. Therefore, both the rectification ratio and rectifying direction can be elaborately controlled by tuning the location of the bipyridyl group in the alkanethiolate backbone. Figure 2d shows the dependence of rectification ratio R and inverted rectification ratio 1/R at 2.0 V on the location m of the bipyridyl group. It can be clearly seen that R decreases roughly as m increases, and the rectifying direction is reversed from positive bias voltage to negative bias voltage at m = 4. In addition, R is not always monotonic with respect to m. The R or 1/R fluctuates for smaller (m < 3) and larger (m > 8) m with a maximum R of 298.6 for m = 3 and a maximum 1/R of 388.6 for m = 10. The bipyridyl location-dependent direction of rectification is very similar to a very recent experimental observation by Nijhuis et al., where the direction of rectification

Figure 3. (a) Evolution of MPSH eigenvalues under bias voltages for C1 junction (the solid black lines are calculated self-consistently, and the solid red squares are evaluated using eq 2). (b) Spatial distributions of MPSH molecular orbitals at zero bias voltage (the isovalue is 0.01) and (c) electronic transmission spectra in logarithmic scale at different bias voltages for C1 junction. The light magenta shaded area indicates the bias window.

the frontier molecular orbitals are a little far away from the Fermi energy (EF) under zero bias voltage, where the HOMO locates at 1.37 eV below EF, while the LUMO resides at 2.07 eV above EF. This is why there is a large threshold bias under positive bias voltage in the I−V curve of C1. However, the evolutions of frontier molecular orbitals under bias voltage are strictly monotonic. The rigid shift of HOMO under bias voltage results in the entrance into the bias window (shown as the light magenta shaded area) at a higher positive bias voltage. Conversely, there are no molecular orbitals included in the bias window under negative bias voltages. This can preliminarily help to understand the phenomenon in C1 that the current under positive bias voltage is much larger than that under the reversal bias voltage. C


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Figure 4. (a) Eigenvalues of HOMO and LUMO versus the location of the bipyridyl group in alkanethiolate. (b) and (c) are spatial distributions of MPSH molecular orbitals at zero bias voltage for C4 and C11 junctions, respectively (the isovalue is 0.01).

out of the bias window. Thus, a pronounced rectification with the forward current along the positive bias is manifested. From the above discussion, the rectification of C1 is caused by the asymmetric energy shift of HOMO under positive and negative bias voltages. In the following content, we inspect the underlying mechanism of inversion of rectifying direction as the m increases. For this purpose, we first check the effect of bipyridyl location in the alkanethiolate backbone on the frontier molecular orbitals. From Figure 4a, it is found that the variations of eigenvalues for HOMO and LUMO against the changing of bipyridyl location are negligible under zero bias voltage. It is thus further inferred that the spatial distributions of frontier molecular orbitals in other Cm’s are also solely governed by either the conjugated bipyridyl block or the thiol group as is the case with C1. This speculation can be easily verified by examining the spatial distributions of frontier molecular orbitals for each m. As seen in Figure 4b, taking C4 and C11 as examples, the wave functions of HOMO and LUMO as well as other frontier molecular orbitals keep localized and are extremely similar to their counterparts in C1. In addition, molecular orbitals affiliated to the bipyridyl group would change their positions in the alkanethiolate backbone with varying m. Since the energy evolution of molecular orbitals is sensitive to the relative position of localized wave functions in molecular junctions, a dramatic change in the evolutions of frontier molecular orbitals under external bias voltage upon varying m is therefore expected. Next, the effect of bipyridyl location in alkanethiolate backbone on the evolutions of frontier molecular orbitals under bias voltage will be discussed. In Figure 5a and 5b, for C4 and C11, we note that the HOMO−3 and HOMO− 4 have nearly the same evolutions under bias voltage as those presented in Figure 3a for C1. This can be easily understood by comparing the wave functions shown in Figure 4b and 4c for C4 and C11 and Figure 3b for C1. In all cases, the wave functions of both HOMO−3 and HOMO−4 are highly localized on the left thiol group, which leads to the eigenvaules following EF of the left electrode under external bias voltage. However, as m goes from 1 to 4, the corresponding wave functions of HOMO, HOMO−1, and HOMO−2 also approach to the middle of the molecule. Therefore, the electrostatic effect of external bias voltage on the wave functions is weakened as shown by the evaluated energies in Figure 5a. So, the evolutions of HOMO, HOMO−1, and HOMO−2 under bias voltage in C4 have much smaller slopes than those in C1, which results in decreased rectification ratios. When m further increases, for example m = 11, the wave functions of HOMO, HOMO−1, and HOMO−2 are strongly coupled to the left electrode instead (see Figure S3 in the

The rigid shift of frontier molecular orbitals in C1 can be attributed to the electrostatic effect of external bias voltage on the highly localized wave functions. To elucidate this, spatial distributions of frontier molecular orbitals are plotted in Figure 3b. Clearly, LUMO, HOMO, HOMO−1, and HOMO−2 are mainly located on the π-conjugated bipyridyl block, which is in close proximity to the right electrode. Meanwhile, HOMO−3 and HOMO−4 are localized on the thiol group segment that is close to the left electrode. Hence, the energy shift of LUMO, HOMO, HOMO−1, and HOMO−2 will be strongly affected by the variation of EF of the right electrode, while HOMO−3 and HOMO−4 follow the EF of the left electrode. To see the electrostatic effect of external bias voltage clearly, an equation is proposed here to approximately fit the energy variation of the spatially localized wave functions at a given bias voltage with respect to the eigenvalue under zero bias voltage. ΔE(V ) =

∫ φi*(r)ΔU(V , r)φi(r)dr

(2)

where ΔE is the energy variation under bias voltage V with respect to the eigenvalue under zero bias voltage for the ith molecular orbital, of which the wave function under zero bias voltage is φi(r). ΔU(V,r) is the voltage drop in real space, which is defined as the difference between the self-consistent electrostatic potentials at bias voltage V and zero bias. The evaluated energies for frontier molecular orbitals by eq 2 are shown as solid red squares in Figure 3a. One can easily see that the evaluated energies are in good agreement with those obtained by self-consistent calculations. This suggests that although the external bias voltage can polarize the wave functions and hence modify the couplings between the molecule and electrodes (see Figures S1−S3 in Supporting Information) the electrostatic effect is the main source for the energy variation of localized molecular orbitals under external bias voltage. The rectifying effect of molecular junction C1 can be further checked by the bias-dependent electronic transmission spectra, of which the logarithms are present in Figure 3c. It is noted that the current of C1 is absolutely dominated by the transmission peak originated from HOMO. The HOMO-mediated transmission peak locates at −1.37 eV under zero bias voltage. Meanwhile, it has a very low intensity with the peak value of 9.1 × 10−3 and a very small broadening due to the high localization in spatial distribution of the HOMO. When a positive bias voltage is applied, this peak approaches to the bias window with a suppressed intensity, and it enters the bias window at around 1.8 V making an evident contribution to the current. On the contrary, although the intensity of this peak is enhanced under negative bias voltages it departs from the EF and keeps excluded D


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which is important for further rational design of molecular diodes.

4. CONCLUSIONS In summary, we have theoretically demonstrated that the rectifying direction and rectification performance of molecular diodes SCnBIPYCm, which consist of a single bipyridyl embedded alkanethiolate molecule sandwiched between two parallel Ag(111) electrodes, can be precisely controlled by tuning the location of the bipyridyl group in the alkanethiolate backbone. When the number of carbon atoms at the right side of bipyridyl is 1 ⩽ m ⩽ 3, the rectifying direction is along the positive bias, while it is reversed to be along the negative bias voltage when 5 ⩽ m ⩽ 11, where m = 4 is the critical location. It is found that the rectification originates from the strictly monotonic evolution of the strongly localized frontier molecular orbitals under bias voltage. Meanwhile, the spatial distributions of frontier molecular orbitals can be elaborately controlled by gradually tuning the location of the bipyridyl group in the alkanethiolate backbone, which in turn realizes a control of the rectifying direction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12595. Figures S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guang-Ping Zhang: 0000-0001-7928-4146 Author Contributions

Figure 5. (a) and (b) are evolutions of MPSH eigenvalues under bias voltages for C4 and C11 junctions, respectively (the solid black lines are calculated self-consistently, and the solid red squares are evaluated using eq 2). (c) Electronic transmission spectra in logarithmic scale at different bias voltages for C11 junction. The light magenta shaded area indicates the bias window.

∥ Guang-Ping Zhang and Shan Wang contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the National Natural Science Foundation of China (Grant Nos. 11547252 and 11374195), the Natural Science Foundation of Shandong Province (Grant No. ZR2014AM017), the Taishan Scholar Project of Shandong Province, the Excellent Young Scholars Research Fund of Shandong Normal University, and the “Dresden Junior Fellowship” of Technische Universität Dresden are gratefully acknowledged.

Supporting Information). That is, the eigenvaules of HOMO, HOMO−1, and HOMO−2 then follow EF of the left electrode, and the evolutions under bias voltages are reversed completely as presented in Figure 5b. Therefore, an inverted direction of rectification will be anticipated. The inverted direction of rectification for medium and large m also can be confirmed by the electronic transmission spectra. We take C11 as an example, of which the bias-dependent electronic transmission spectra are shown in Figure 5c. We can see that there is a HOMO contributed transmission peak at 1.34 eV below EF with an intensity of 1.2 × 10−3 under zero bias voltage. When a positive bias voltage is applied, this peak moves far away from the bias window although the peak intensity is enhanced. On the contrary, it is included in the bias window at around −1.8 V, contributing to the current. Therefore, the direction of rectification is reversed compared to that in C1. Finally, a control of the rectifying direction of single-molecular diodes by gradually tuning the location of the bipyridyl group in the alkanethiolate backbone is realized,

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REFERENCES

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