First Author:Ray permanently.
Corresponding Author:Chang Shuai, Li Yunchuan, Xiao Bohuai.
Correspondence unit:Faculty of Materials Science, Wuhan University of Science and Technology.
Key Highlights:
The effects of three different substituents modified on the molecular-electrode binding configuration of the thiophene-anchored group were studied in detail by scanning tunneling microscopy cleavage technique. It was found that there were two conductive states of high conductance (GH) and low conductance (GL) of the three molecules, which were formed by the interaction between thiophene orbitals and thiophene S atoms and gold electrodes, respectively. Among them, the low conductance corresponding to the AUS configuration is susceptible to the frontline orbital of the substituent;However, the molecular bonding rate in the Au configuration is susceptible to steric hindrance of the substituents.
Background:
As an important building block in organic semiconductor materials, thiophene can also be attached between nanoelectrodes as an anchor group of molecules to form metal molecules and metal junctions. For a long time, most researchers have focused on the overall charge transport capacity of thiophene anchor molecules, but few studies have been conducted on the effect of thiophene anchor groups on the molecule-electrode binding configuration. The team designed and synthesized three organic conjugated molecules (BT-H, BT-HEX, and BT-Cl) containing different thiophene anchor groups through chemical manipulation, and then explored the dominant configurations and corresponding electrical properties of the three molecules binding to gold electrodes by scanning tunneling microscopy (STM-BJ).
The results show that BT-H, BT-Hex and BT-Cl can form two binding configurations (AU configuration and AUS configuration) with gold electrodes, which produce high and low conductivity states, respectively. Among them, the molecular conductance in the au configuration is independent of the molecular structure;The molecular conductance in the AUS configuration is related to the molecular structure. More importantly, the team found that the monomolecular junction (structure) constructed by these three molecules is closely related to the substitution effect on the thiophene anchor group. When the steric hindrance of the substituents is large, the AUS configuration is the dominant configuration of the monomolecular junction;Whereas, when the steric hindrance of the substituent group is small, the au configuration is the dominant configuration of the single molecular junction. This study is of great significance for regulating the charge transport performance of organic semiconductor materials and promoting the development of single-molecule devices.
Core content
1.Single-molecule conductivity test results and analysis
The conductance plateau shown by the scanning tunneling microscopy cleavage curve (Fig. 1a c)** demonstrates that molecules can be attached between the electrodes and form stable molecular junctions, and two conductance steps in the typical curves of the three molecules of bt-h, bt-hex, and bt-cl indicate that the molecules form two molecular junctions between the electrodes. In the corresponding one-dimensional conductance histogram (Fig. 1d f), two conductance peaks appear for each of the three molecules, indicating that each of the three molecules can form two different stable junction configurations between the gold electrodes.
Among them, the low conductance of three molecules showed the following trend: BT-HEX > BT-H > BT-Cl. This may be because the substituents have different electronic properties, which affect the charge transport efficiency of the molecule. As an electron-withdrawing group, the S atom can reduce the electron density of the S atom on thiophene through the electron-withdrawing effect, thus reducing the charge transport efficiency between the S atom and the gold. However, the n-hexyl group has a weak electron-donor property, which can increase the electron density of thiophene sulfur and increase the conductance. Their high conductance values are much higher than those of low conductance and are very close to each other, and a comprehensive literature survey has determined that the high conductance is generated by the Au connection configuration.
Figure 1 (a c) shows the typical conductances of BT-H, BT-HEX, and BT-Cl, respectivelyDistance curve;(d f) Histograms of conductance for BT-H (red), BT-HEX (blue), BT-CL (green), respectively.
2.Analysis of electrical transport characteristics in two configurations
To further explain the conductance trend under the AUS junction configuration, the team analyzed the anterior orbitals of the three molecules and found that the homo energy levels of these three molecules are closer to the gold Fermi level, suggesting that they mainly transport charge between gold electrodes through the homo orbitals. In addition, the smaller the energy range difference of the Homo level relative to the Au Fermi level, the smaller the charge transport resistance, so the numerical trend of low conductance is: BH-HEX > BT-H > BT-CL. This indicates that the chlorine atom increases the difference between the molecular homo and the Au Fermi level, while the n-hexyl group narrows the difference.
For the high conductance dominated by AU, the conductance measurement results show that the energy level distribution of the substituent has no obvious effect on its conductivity. The nics values of different thiophenes were obtained by theoretical calculations. This value can be used to assess the degree of delocalization of the molecule's electrons. The corresponding nics values of these three molecules are relatively close (thiophene: 28.)08；C6-thiophene: 2481;Cl-thiophene: 2352), indicating that the degree of electron delocalization of electrons is not significantly different due to the different substituents.
Further, the team optimized the geometry of the three molecules and calculated the AU interaction energies of the three molecules with the gold electrode (Fig. 3). The results showed that the interaction energy of thiophene modified by different substituents with gold clusters was similar. This also supports the phenomenon that the high conductance does not change significantly to a certain extent.
Fig.2 Energy level diagram of a molecule relative to the Au Fermi level.
Fig.3 Optimized structures and AU of the three moleculesInteraction energy.
3.Conductance of a single molecular junctionDistance two-dimensional distribution intensity statistics
The team further investigated the effect of substituents on the AU S and au junction configurations by plotting the conductance distance 2D intensity plot (Fig. 4). Two darker regions appear in the intensity plots for all three molecules, visually showing the magnitude of the conductance and the relative distance at which the molecular junction is formed. The step lengths corresponding to the AUS linkage configuration are approximately equal, indicating that the molecular junction configuration is similar when the three molecules are combined with the gold electrode by AUS. The step length corresponding to the Au connection configuration showed a trend of bt-h > bt-hex > bt-cl. Au ligation is a non-covalent interaction, and the strength of the binding of the molecule to the electrode is weaker relative to the covalent bonding of Au s, so the tensile length of the Au junction may be shorter for the same backbone length. In addition, the steric hindrance of substituents also inhibits the generation of Au binding configurations to a certain extent.
Fig.4 Molecular conductance of BT-H, BT-Hex and BT-ClDistance two-dimensional distribution intensity plot.
4.Count the knotting rate of different configurations of molecules
The statistical results of the junction formation rate of the three molecules (Table 1) show that the junction formation rate corresponding to the AU configuration decreases with the increase of the atomic radius of the substituent, which is consistent with the trend of the junction length of the AU molecule. However, the effect of substitutive groups on the adhesion rate of AUS was not obvious. This indicates that the steric hindrance effect of substituents is an important factor affecting the tensile length and knotting probability of AU molecular junctions.
Table 1 The cohesion rates of the three molecules of BT-H, BT-Hex and BT-Cl in different configurations.
Conclusions and prospects
The team used STM-BJ technology to study the conductance of 1,4-bis(thiophene-2-yl)benzene model molecules, and analyzed the effect of thiophene 4 substitution effect on molecular conductance and molecule-electrode binding configuration. The following conclusions are obtained: (1) There are two stable binding configurations for the interaction between these three molecules and gold electrodes, which are Au S and Au binding configurations, respectively. (2) The electronic properties of the substituents affect the molecular junction conductance size of the AUS configurationThe increase of steric hindrance effect of substituents will lead to a decrease in the probability of molecular knotting in the Au configuration. These findings provide structural design ideas for the design of molecular devices with different performance, and also reveal the influence of substitutives with different properties on the charge transport efficiency of molecular junctions, which is of great significance for promoting the basic research of molecular electronics and the development of molecular devices.
Link to original article
Lei Yongheng, Wang Xu, Wang Zhiye, Zhou Jianghao, Chen Haijian, Liang Lei, Li Yunchuan, Xiao Bohuai, Chang Shuai. Study on the effect of thiophene anchor group on the binding configuration of molecule and electrode. Acta Physico-Chimica Sinica,39 (11), 2212023. doi: 10.3866/pku.whxb202212023
lei, y.; wang, x.; wang, z.; zhou, j.; chen, h.; liang, l.; li, y.; xiao, b.; chang, s. effect of modified thiophene anchor on molecule-electrode bonding. acta phys. -chim. sin.39 (11), 2212023.doi:10.3866/pku.whxb202212023