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# Effect of H2O Adsorption on Negative Differential Conductance Behavior of Single Junction

## Introduction

Utilizing single molecule as functional device in electronic circuit is an ultimate goal of molecular electronics, which has motivated scientists to devote themselves to the investigations of molecular devices for tens of years1,2,3,4,5,6,7,8,9,10. Due to the rapid development of single molecular technologies11,12,13,14, great progresses have been achieved for single-molecule-device fabrications in recent years15,16,17,18. At the meantime, different strategies are designed to control and improve the functional properties of single molecular device19,20,21,22. In order to gain insights into the controlling mechanism of single molecular functional characteristics23,24,25, the effects of external ambient26,27,28, electrode distance29, 30, molecule-electrode interface31,32,33,34,35, molecular anchor36,37,38,39,40,41,42,43, side group44,45,46,47, doping48, 49 and external field50,51,52,53,54,55 have been studied intensively. In experimental studies, molecular devices are often fabricated in solution and measured in vacuum or in gas circumstance, thus the effect of the surrounding molecule on the functional properties of molecular device should also be discussed56,57,58,59. Generally, the surrounding molecules play negative effects on the molecular junction, for example, aqueous solution or the water vapor can suppress the electronic transport of molecular junction dramatically28, 56,57,58. However, sometimes the surrounding molecules can also have positive effects, our recent research reveals that the small ambient molecules can make molecular junction more stable due to their suppressing the thermal vibrations of the molecular junction14. Therefore, studying the influence of surrounding molecules on molecular device is very helpful to improve the functional characteristics of the molecular device by properly applying surrounding molecules.

## Results

In order to understand the NDC behavior of the TADHA molecular junction and the H2O-adsorbate effects, we presented the transmission spectra for the bias voltages of 0.0 V, ±0.25 V, ±0.5 V and the voltage of peak-current in Fig. 3. Due to the contributions of the highest occupied molecular orbital (HOMO) and HOMO-1 to the electronic transmission and the degenerate of these two molecular orbitals, a high transmission peak is presented at about −0.2 eV for the transmission curve with zero bias as Fig. 3(a) shows. When the bias is applied, the degenerate of the two molecular orbitals is destroyed by the Stark effect of the bias, which further splits the high transmission peak into two lower transmission peaks. Because of the split of the transmission peak, the transmission probability at Fermi energy level slightly increases with the increase of the bias in the lower bias regime. However, due to the rapid decrease of the height of the split transmission peaks with the increase of the bias, the area under the transmission curve in the bias window increases at first and reaches a peak value at about 0.25 V, and then decreases with the increase of the bias, which depends on the rivalry of the width of the bias window and the mean height of transmission spectra in bias window. Thus a current peak appears at about 0.25 V as well as the NDC behavior for the TADHA molecular junction without H2O adsorbate.

In order to gain deep insight into the NDC behavior, we presented spatial distributions of the HOMOs and HOMOs-1 for TADHA molecular systems at 0.0 V, ±0.5 V and at the peak-current voltages in Fig. 4. The figure shows that, at 0.0 V, due to the approximate degeneration, the HOMO and HOMO-1 are both delocalized over the whole TADHA molecule not only for the molecular junction without H2O molecule, but also for the molecular junction with Type I-1, 2 and 3 configurations, which results in the high transmission peaks at about −0.2 eV for these molecular junctions at 0.0 V. However, there are still some differences for the effects of the H2O molecule on the orbital distributions at 0.0 V, such as for Type I-2 and 3 molecular systems. Attributing to the depressing of the H2O molecule, the gaps between the HOMO and HOMO-1 are slightly enlarged compared with the molecular system without the H2O molecule, and the spatial distributions of the orbitals are obviously asymmetric to the two branches of TADHA molecule. However, for Type I-1 molecular system, the orbitals are little influenced by the H2O molecule since the adsorption of H2O molecule is very weak. For the molecular junctions at peak-current voltage or at ±0.5 V, the HOMOs and the HOMOs-1 have been pulled apart by bias and each only locates on one branch of the TADHA molecule. Thus one can easily understand why the height of the transmission peaks decrease very quickly in the split process with the increase of the bias voltage. We should mention that the orbitals presented in the figure are the molecular projected self-consistent Hamiltonian (MPSH) eigenstates, which are often used to discuss the electronic transport properties of molecular junctions in literatures7, 23. The MPSH is the self-consistent Hamiltonian of the functional molecule including the H2O adsorbates with the influence of the electrode, which contains the electrode-molecule coupling effects but does not contain the Hamiltonian of the gold electrode, so the energies of the MPSH eigenstates are not perfectly consistent with the positions of the transmission peaks7.

### More H2O molecules or H2O aggregate adsorption

Our calculations show that, if we first set two H2O molecules on the two conjugated rings of the 9,10-dihydroanthracene core respectively and perform geometric optimization, the two H2O molecules will move and aggregate with each other, until at last formed Type II-3 configuration, which obviously due to the electrostatic attraction originated from the strong polarity of H2O molecule. From Fig. 5 one can see that, the current and the differential conductance curves of Type II-3 system are more asymmetric than those of Type I-3. The peak current value is further enhanced and the peak NDC value is more than doubled of that without H2O adsorbate for the junction in the positive bias regime, whereas for the negative bias, which is obviously depressed by the influence of the H2O adsorbates. Thus, for TADHA molecular junction with Type II-3 configuration, the adsorption of H2O molecules induces apparent rectifier behavior with the maximum rectification ratio of 2.74 at 0.35 V.

Figure 6(c) shows that, for Type II-3 molecular system, due to the depressing of the two H2O molecules to the molecular orbitals, the HOMO and HOMO-1 are not degenerated at zero bias, thus the transmission spectrum shows two peaks at about −0.15 eV and −0.24 eV corresponding to the HOMO and HOMO-1. With the increase of the positive bias, the HOMO is depressed and simultaneously the HOMO-1 is enhanced. When the bias is increased to about 0.15 V, the HOMO and HOMO-1 are re-degenerated, which results in the combination of the two transmission peaks into one higher transmission peak at 0.19 eV. Hence, the TADHA molecular junction with Type II-3 configuration is more conductive in the positive bias regime. Different from positive bias, the negative bias further splits and depresses the two transmission peaks which makes the molecular junction less conductive when the bias less than −0.20 V, so the TADHA molecular junction shows rectifier behavior. In fact, from the asymmetric evolution of the HOMO and HOMO-1, especially, the degeneration point of the HOMO and HOMO-1 obviously deviates from 0.0 V (Figure S2), one can also understand the rectifier behavior of Type II-3 molecular system, because the degeneration of the HOMO and HOMO-1 results in the delocalization of the HOMO and HOMO-1 (Figure S1), and consequently enhances the current of positive bias regime.

Similar to one-H2O-molecule adsorption on S atom site, for the Type II-4 molecular system that each terminal S atom adsorbs one H2O molecule, the NDC behavior of TADHA molecular junction has been destroyed absolutely in the lower bias regime. Due to the strong influence of the adsorbates, the conducting orbitals have been further depressed to lower than −0.5 eV (Fig. 6(d) and Figure S1), which results in much poorer conductance of Type II-4 molecular system compared with the molecular system without H2O adsorbate or with one H2O molecule adsorbed on the terminal S atom. The numerical results show that, for the bias lower than 0.25 V, the current of Type II-4 molecular system is about two orders lower than that of the system without the H2O adsorbate in the magnitudes, which is in good agreement with Long’s experiment28. Although with the increase of the positive bias, the transmission peak corresponding to the HOMO is shifted to higher energy regime, it is still out of the bias window when the bias is enhanced to 0.5 V (Figure S2). So the current is very weak for the Type II-4 molecular junction.

## Discussion

As to the accuracy of our work, we found that our calculations successfully reproduced the low-bias NDC behavior of TADHA molecular junction that was investigated experimental by Perrin et al.60. However, the absolute values of the current in the lower bias regime are about one order of magnitude larger than the experimental results. According to the studies of Quek et al.61,62,63, the HOMO-LUMO (LUMO: lowest unoccupied molecular orbital) gap is usually underestimated in standard density functional theory (DFT), which results in overestimate of conductance of single molecular junction. Thus an alternative DFT-based approach61,62,63,64,65 (e.g. DFT + Σ approach) is needed to give more accurate and physically meaningful understandings of electron-transport properties of TADHA molecular junction in the future. Since the HOMO is much closer to the Fermi level than the LUMO, the electron-transport properties are mainly governed by the HOMO. According to the DFT + Σ method in the literatures61,62,63 and the experimental results60, we can roughly estimate that, for the TADHA molecular junction without H2O adsorbate, the HOMO is overestimated by not more than 0.3 eV relative to the Fermi level, which is much smaller than those of weak coupling systems61,62,63.

## Methods

The geometric structures of TADHA molecular systems without or with H2O molecules were optimized using the SIESTA package with a maximum force 0.02 eV/Å66, 67. The Troullier–Martin type norm-conserving pseudopotentials are applied to represent the core electrons68, the generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE) formulation is applied as the exchange-correlation functional69. For Au atoms, a single-ζ plus polarization basis set is used, and for other atoms, a double-ζ plus polarization basis set is employed.

The current through the molecular device with different bias voltage is obtained according to the Landauer–Buttiker formula70

$$I=\frac{2e}{h}\int T(E,V)\,[f(E-{\mu }_{L})-f(E-{\mu }_{R})]dE,$$
(1)

which was calculated with the TranSIESTA module of the SIESTA package. In Eq. (1), T(E, V) is the transmission probability, which depends on the incident energy E of the transmission electrons and the applied bias voltage V. $${\mu }_{L}$$ and $${\mu }_{R}$$ in the Fermi–Dirac distribution functions f(E) are the electrochemical potentials of the two electrodes. The transmission probability T(E, V) is calculated by NEGF method. The differential conductance is defined as $$G=\partial I/\partial V$$. In the electron transport calculations, a 300 Ry mesh cutoff for the real space grid was chosen. The convergence criterion for density matrix was set to 1.0 × 10−4. A 4 × 4 k-point grid was used for the Brillouin-zone (BZ) sampling in the transverse directions. A 300 K smearing was applied for the electronic Fermi-Dirac distribution.

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## Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant Nos. 11374195 and 11304185) and the Natural Science Foundation of Shandong province, China (Grant No. ZR2013FM006). Thanks to the supporting of Taishan scholar project of Shandong Province.

## Author information

All authors provided essential contributions to the manuscript and the project. All authors have given approval to the final version of the manuscript. Z.L.L., X.H.Y. and R.L. contributed the ideas and discussed the manuscript in detail. X.H.Y., R.L., J.J.B. and H.Y.F. performed the calculations and the data analysis. G.P.Z., Y.Z.S. and C.K.W. discussed some details with the corresponding author.

Correspondence to Zong-Liang Li.

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