Accepted: February 13, 2023 | Published Online: February 15, 2023

Citation: Jing-Xin Y, Zhi-Yu H, Wan-Jie D, Xiu-Ying L, Xiao-Dong L (2023) Effect of Asymmetric Terminal Group on the Transport Properties of Molecule Junctions: A First-Principles Study. Int J Nanoparticles Nanotech 8:041.

Copyright: © 2023 Jing-Xin Y, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Abstract

Single-molecule quantum transport calculations have been investigated using non-equilibrium Green's function method combined with the density functional theory (DEF-NEGF) for phenyl molecules symmetrical or asymmetrically anchored to Au(100) electrodes via different terminal groups. Among the considered 4 structures, the electronic coupling of C₆H₅S is slightly stronger than thiophenol. Because the -S as terminal group provides more free electrons than -SH, this results in significant current rectification, but the stability of responsivity is poorer than thiophenol molecule. Thiophenol molecules show good diode parameters in terms of asymmetry, nonlinearity and responsiveness above 0.5V the applied voltage. Although there is only one H atom differences, the thiophenol and C₆H₅S molecular show different property. The structure and diode characteristics of Thiophenol molecule are conducive to become interface modification materials and improve the photoelectric performance of devices.

PACS numbers: 71.15.Mb;61.46.-w; 73.40.c

Keywords

Molecular junction, Electron transport, Non-equilibrium Green's function

Introduction

In recent decades, to investigate the intrinsic quantum phenomena, the charge transport characteristics of single molecule devices have been widely studied. As we all know, different terminal groups of molecules will lead to different interactions between molecular junctions and electrodes for molecular junction systems [1-3]. Molecular coupling at the interface plays an important role in determining the performance of molecular electronics, such as the immobilization of molecular devices [4-9]. Molecules with phenyl groups have been widely employed in molecular electronics because of their high electron transfer properties [10-12]. Thiol(SH) group is considered as a potential anchoring group, which it can construct mechanically stable single molecule junctions in molecular junctions. The bridged methylene group is also commonly used to regulate the molecular coupling between the phenyl molecules and surface free electrons [13]. Due to the surface catalysis, the H atom in -SH is easy to be dissociated. Therefore, for the molecular system with -SH as the terminal group, the S atom forms a covalent bond with the gold electrode. In order to understand the difference of terminal groups on the electron transport properties, we have designed different molecular systems with gold as electrode, benzene as molecular skeleton, -S, -SH as terminal groups and -CH₂ bridging respectively.

In addition, the asymmetric anchoring between the molecular junction and electrode can change the transport properties, such as it can be used to create molecular rectifiers [14]. Many novel phenomena also have obtained, current rectification, negative differential resistance, etc [15-21]. Obviously, it is necessary to study the asymmetric anchoring and the coupling between molecular containing phenyl groups and free electrons on the metal surfaces.

We present the combined electrode-molecular system together with seven and six Au(100) atomic layers respectively to the left- and right-hand surface plane, each layer containing 3 × 3 atoms in the xy plane with periodic boundary conditions. Figure 1 shows the considered four molecular structures.

a) Thiophenol (TP), which shows good diode parameters in terms of asymmetry, nonlinearity and responsiveness.

b) C₆H₅S. Compared with TP molecule, it is observed the larger current and significant current rectification.

c) Benzyl mercaptan (BMP). The methylene group improves mechanical stability of the junctions [22].

d) C₆H₄S₂. We use symmetric structure as a comparison of the transport properties of the other three asymmetric molecular junction system. We have analyzed the effect of H bond in terminal groups on the transport properties, and analyzed the influence of different terminal groups on the diode parameters of Thiophenol molecular.

Model and Method

The electronic transport properties of molecular junctions have been calculated with the SMEAGOL software [23-27]. We first have performed primary geometric optimization for the molecular of the central region, the molecules along with nearest two layers Au are relaxed and four and five outer Au layers respectively to the left- and right-hand surface plane are frozen in their bulk position. Then the transport properties of molecular junctions, including the electron transmission coefficients of zero-bias, current- voltage (I-V) curves, the project density of states (PDOS), are calculated on the basis of optimization results [28]. We also calculated the resistance, nonlinearity, responsivity, and asymmetry of all the considered systems:

$\text{R = }{\left(\frac{dI}{dV}\right)}^{-1}$ (differential resistance)

$\text{N = }\frac{dI/dV}{I/V}$ (nonlinearity)

$\beta \text{ = }{}^{\frac{1}{2}\frac{d^{2}I}{d{V}^2}}/{}_{\frac{dI}{dV}}$ (responsivity)

$\text{A = }\left|\frac{I\left(+V\right)}{I\left(-V\right)}\right|$ (asymmetry) [4].

Our calculations are carried out using DFT-NEGF with the Perdew-Zunger form [29] of the local-density approximation (LDA) to the exchange-correlation functional. Troullier-Martins pseudo potentials [30] are used for Au (5d¹⁰6s¹). A single-zeta polarization (SZP) is used for Au atoms, whereas a double-zeta polarization (DZP) basis set is adopted for the orbitals of S, C and H atoms.

Results and Discussion

The ground state energy of the electrode-molecular system is calculated. By comparing the (a) TP and (b) C₆H₅S molecules, we find that their optimized structures are similar. Due to the presence of H in sulfhydryl group, the Au-S bond (about 2.59 Å) in (a) TP molecule is longer than the (b) C₆H₅S molecule (about 2.39 Å). In (a) TP molecule, the gold electrode at the other end is bonded with two C atoms in benzene ring. However, the gold electrode is bonded to a C atom in the benzene ring in (b) C₆H₅S molecule. The (a) TP molecule is more stable than (b) C₆H₅S molecule. In (c) BMP molecule, the Au-S bond about 2.58 Å and the gold electrode at the other end is bonded with two C atoms in benzene ring. This is similar to (a) TP molecule.

In Figure 2, we display the transmission spectra of all the considered systems at zero bias. The conductance at zero bias is 0.01 G₀(G₀ = 2e²/h), 0.03 G₀,0.001 G₀ and 0.22 G₀ for the (a) TP, (b) C₆H₅S, (c) BMP and (d) C₆H₄S₂ molecules, respectively. Because of the weak resonance, the highest occupied molecular orbital (HOMO) transmission peaks of all the molecular junctions is small near the Fermi level (E_F), but the effect of terminal groups on the conductivity of the molecular junctions is clearly observed. As expected, (d) shows a higher peak, so it corresponds to a large transmission probability, the essential reason of the large conductance due to strong sulfur-gold bonding with both electrodes. In the case of molecule structure (b), -S as terminal groups bonding with left electrode provides one more free electron than -SH, so the electronic coupling is slightly stronger than the structure (a). In the case of molecule structure (c), the molecular coupling is weakest because of the existence of bridged methylene group. In fact, the molecule structure (c) has the smallest transmission among all the considered systems. The above result is also agreement with the previous experimental results [14].

Figure 3 presents the calculated I-V curves, differential resistance and asymmetry of all models. As depicted in Figure 3(1), because of the strong S-Au bonds and better electronic level alignment [31], the molecule structure (d) have been observed the larger current in the whole bias voltage range (filled-magenta triangles in Figure 3(1)). Interestingly, the current in structure (d) has no obvious advantage as compared to the structure (b) despite only covalent S-Au bonding to left electrode. For the structure (c) (filled-blue triangles in Figure 3(1) and Figure 3(2)), the -CH₂ bridging may cause weak interfacial electronic coupling, it displays the smallest current but the largest differential resistance. In Figure 3(2), the differential resistance is small except for the structure (c), indicating that they belongs to the molecular resistance. The structure (a) and (b) shows obvious asymmetric characteristics because of the structural asymmetry, the current is larger for the positive bias and the current is smaller for the negative voltages (filled-black squares and filled-red circles in Figure 3(1)). Figure 3(3) presents the asymmetries of all models, the larger current rectification is obtained for structure (a) and (b), the result is also good agreement with the Figure 3(1). Although there is only the difference of one atom, the asymmetry of the structure (b) increases more than the structure (a) above 0.5 V the applied voltage. The structure (a) shows the better stability in a symmetry for positive voltage biasing. So the hydrogen bond coordination in molecular junction maybe plays an important role in electronic coupling.

Figure 4(1,2) show the nonlinearity and responsivity of all the models. For the structure (b), it is observed clear current rectification because of the -S as terminal group, and the maximum rectification ratio is 1.8. But its stability of responsivity is poorer than the structure (a). The structure (c) and (a) shows better nonlinearity and responsivity in the whole bias voltage range (filled-blue triangles). However, the structure (c) has no practical value due to the small current. In summary, the structure (a) shows better diode parameters in all the considered molecular junctions.

The electronic transport mechanism also can be attributed to the PDOS of the molecular region (Figure 5). Here, we present the results only for (a) TP and (b) C₆H₅S molecular because of their better conductance and diode characteristics. Embedded graphics is the DOS of the considered molecular (a) and (b), there is no obvious difference and the trend is consistent with the transport calculation results. For the molecular junction (a) (see Figure 5(1)), the effective electron transport channel is mainly originate from the Au dxz and dyz orbitals. For the molecular junction molecule (b), the conductance is larger than (a). The effective electron transport channel is mainly formed not only by the Au dxz and dyz orbitals but also by the S Px and Py orbitals, mainly because there are more free electrons -S as terminal groups. This also explains that the current is larger for the positive bias but the current is smaller for the negative voltages (i.e., larger rectification ratio) (see Figure 3(3)).With the application of bias voltage, the S atom Px and Py orbitals plays a decisive role in the difference asymmetry, nonlinearity and responsivity of the molecular (a) and (b).

Conclusions

In the present work, we have examined the effect of different terminal groups on the electronic transport properties using DEF-NEGF for phenyl molecules symmetrical or asymmetrically anchored to Au(100) electrodes. The larger current, asymmetry, nonlinearity and responsivity are obtained for thiophenol molecule above 0.5V the applied voltage. The -S as terminal group of phenyl molecule also is observed clear current rectification, but its stability of responsivity is poorer than thiophenol molecule. Detailed analysis of PDOS shows that the obtained current rectification formed by the effective electron transport channel of Au and S. Although there is only one H atom differences, the thiophenol and C₆H₅S molecular show different property. The structure and diode characteristics of thiophenol molecule are conducive to become interface modification materials and improve the photoelectric performance of devices. The results have practical importance for understand electron transport behavior and construct the molecular electronics devices.

Acknowledgments

We thank the support plan for key teachers of Henan University of Technology. We also thank Dr. Sanvito for his providing us the SMEAGOL code.

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