The Effects of Molecular Combination and Side Groups for Thiophene

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Effects of Molecular Combination and Side Groups for ThiopheneBenzene-Based Nanodevices Yujie Xia, Xingfan Zhang, Chao Yuan, Lishu Zhang, Xinyue Dai, Tao Li, Chengrui Fu, and Hui Li* Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, People’s Republic of China

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S Supporting Information *

ABSTRACT: The electron transport properties of thiophene-benzene-based molecules attached to zigzag graphene nanoribbon (ZGNR) electrodes are investigated using density functional theory and the nonequilibrium Green function. The results show that 3,6-(dithiophen-2-yl)benzene has better performance compared to 2,5-diphenylthiophene. Interestingly, both the devices exhibit two negative differential resistance (NDR) peaks while a thiophene or a benzene molecule tethered to ZGNR electrodes shows one NDR peak. Although the side groups −OH, −NO2, and −NH2 controllably repress the transmission and current of 3,6-(dithiophen-2-yl)benzene, the current rectification performance is found because of its asymmetry of molecule structure. Unexpectedly, 2,5-diphenylthiophene system added by a thiophene shows semiconductor behavior under a certain bias region. These results contribute to a deep understanding of the electron transport properties of molecular combination and are valuable for designing molecular devices with excellent and distinctive performance at nanoscale. bert27 demonstrated that the conductance of polyyne-based molecular wires between pyridine groups was lower than that of polyyne-based molecular wires between benzene-thiol groups, both showing NDR behavior. Troisi and Ratner28 found that the principle of the bridge-modulated conformational molecular rectifiers sandwiched with Au electrode were different compared to that of silicon structures. On the basis of the nonequilibrium Green’s function method and the density functional theory, Deng et al.29 presented that terphenyl molecule tethered on Li, Al, or Au electrodes showed distinct electron transport properties and the excellent rectifying performance was found when Au electrode was used. Although much work has been done to study the electron transport properties of multiple molecules that also contain three identical molecules, few studies are devoted to focusing on a molecule sandwiched between two identical molecules in nanoscale. For instance, Pati et al.21 studied the spin-polarized electron transport of the molecular wire comprised of benzene1,4-dithiolate (BDT) attached to two Ni or Mn clusters followed by nonmagnetic Au electrode (named as Au−Ni− BDT−Ni−Au or Au−Mn−BDT−Mn−Au) using density functional theory (DFT) combined with the Landauer− Büttiker formalism. They found that compared to Mn clusters, the system with Ni clusters exhibited smaller current in the antiparallel case. Dong et al.30 found that the benzene-

1. INTRODUCTION Over the past decade, extensive research studies have focused on studying the electron transport properties of molecular devices because of their excellent properties such as negative differential resistance (NDR) behavior,1−3 current rectification,4,5 field-effect characteristics,6 and electronic switching.7,8 Those molecular devices have been considered as potential materials for designing various functional devices like molecular switches,9,10 molecular rectifiers,11 field-effect transistors,12,13 memory devices,14−16 and so on.17−19 It is of central significance for comprehending the transport properties of molecular junctions20 constituted by one molecule attached to two identical molecules. Recently, a great deal of theoretical and experimental advances have been devoted to investigating the electron transport of a few molecules21 (molecular strands) because they have promising application in electronic devices.22 A set of phenyl-ethylene oligomers have been investigated by Reed and co-workers23,24 who reported that the molecular devices with different side groups could display NDR effect and a large on−off peak-to-valley ratio. Subsequently, Taylor et al.25 studied the electron transport of the three phenyl rings sandwiched between two Au(111) electrodes by using firstprinciples, showing that the currents of the molecular devices were not affected by side groups −NO2 and −NH2, while the interactions were sensitive to those side groups. Fan and Chen26 showed that the contact geometry of the phenalenyl molecule attached to the Au electrodes had a great impact on ́ its electronic transport properties. Garcia-Suá rez and Lam© XXXX American Chemical Society

Received: November 13, 2018 Revised: January 12, 2019 Published: January 17, 2019 A

DOI: 10.1021/acs.jpcc.8b11031 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C graphene field-effect transistors at the thiolate-gold coupling pattern displayed excellent switching behavior, NDR effect, and stable electron transport feature. These studies are important for us to understand the electron transport properties of one molecule attached to two identical molecules. Thiophenes discovered in benzene have been widely used in organic diodes, solar cells, sensors, spintronics, and other electronic devices because of their outstanding electrical, magnetic, and optical properties.31,32 In numerous studies, the benzene was usually used as a prototypical example to construct molecular devices theoretically and experimentally, showing excellent electron transport properties.33−36 Graphene nanoribbons (GNRs) have drawn attractive attention for constructing nanoscale electronic, optical, and sensor devices because of their outstanding electron properties.37−40 Motivated by thiophene, benzene, and GNRs showing outstanding properties that would present extraordinary performance, we propose 4-zigzag GNRs as electrodes to connect a thiophene-benzene-based molecule and study the effect of side groups on the transport properties of the molecular device.

the exchange-correlation potential. The mesh cutoff for electrostatic potentials is 75 Ha. 1 × 1 × 100 Monkhorst sampling is used in the Brillouin zone. The temperature is 300 K in the Fermi function. To avoid interactions between periodic images, all structures are modeled in a supercell with vacuum layers of at least 10 Å between neighboring cells. Before calculating the electron-transport properties of these devices, the geometries are optimized until the forces of the atoms are less than 0.05 Å−1. The device current Id is calculated by the Landauere− Büttiker formula:42 2e ∞ dE(T (E , V )(f1 (E) − f2 (E))) I= ℏ −∞ where f1,2(E) represents the Fermi functions of the source and drain electrodes, e is the electron charge, ℏ is Planck’s constant, and T(E,V) is the quantum mechanical transmission probability of electrons as a function of E and V, which is calculated by42

∫

T (E , V ) = tr[ΓL(E , V )GR (E , V )ΓR (E , V )G A (E , V )]

where ΓL and ΓR are the coupling functions to the left and right electrodes, respectively. GR and GA are the retarded and advanced Green functions of the conductor part, respectively.

2. MODEL AND CALCULATION METHOD The structures of the molecular devices are shown in Figure 1. The transformation of C−H provides a way to enable synthetic

3. RESULTS AND DISCUSSION For simplicity, a thiophene or a benzene molecule attached to 4-atoms width of two narrow ZGNR electrodes are denoted as the devices Mt and Mb, respectively, which are shown in Figure S1 in the Supporting Information The Id−Vd curves of the four devices in the bias region [−2 V, 2 V] show that the current of the device M1 gets much smaller than that of the devices Mt and Mb while the current of the device M2 gets relatively smaller than that of the devices Mt and Mb in Figure 2. It is worth mentioning that the devices Mt and Mb show

Figure 1. Different molecules sandwiched between two identical ZGNR electrodes, which are named as the devices M1, M2, M3, M4, and M5, respectively.

disconnections which were previously unachievable,41 indicating that expected molecules could be obtained by opening the C−H bond and connecting the C−C bond in the experiment. The five optimized 2,5-diphenylthiophene, 3,6-(dithiophen-2yl)benzene, 3,6-(dithiophen-2-yl)-5-hydrobenzene, 3,6-(dithiophen-2-yl)-5-nitrobenzene, and 3,6-(dithiophen-2-yl)-5aminobenzene are attached to the narrow zigzag graphene nanoribbon (ZGNR) electrodes to construct the molecular devices which are named as the devices M1, M2, M3, M4, and M5, respectively. The molecular devices are constructed by three parts: left electrode, central scattering region, and right electrode. A carbon atom, which is positioned symmetrically above the surfaces of the narrow electrode, is bonded to another carbon atom to be the alligator clip. Both the geometric optimization and the electron transport calculation are performed by the DFT and the nonequilibrium Green function supplied in the software package AtomistixToolKit. Double-zeta single polarized basis sets and norm-conserving pseudo potentials are used. The generalized gradient approximations (GGA) with Perdew−Burke−Ernzerhof (PBE) parametrization of correlation energy is adopted for

Figure 2. Id−Vd curves of the devices M1, M2, Mt, and Mb in the bias region [−2 V, 2 V].

one NDR peak while the devices M1 and M2 exhibit two NDR peaks, indicating that the molecule combinations produce different electron-transport properties. Next, we analyze the electron transport properties of the devices M1 and M2 systematically. Figure 2 clearly shows that the devices M1 and M2 are symmetric. We just analyze the current under the positive bias voltages range [0 V, 2 V] because of their symmetry at the positive and negative biases. It is interesting to notice that the current−voltage curves of the devices M1 and M2 exhibit B


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The Journal of Physical Chemistry C

Figure 3. Transmission spectra of the devices M1 and M2 at 0.1, 0.2, 0.3, and 0.4 V, respectively. The dotted lines indicate the bias window [−V/2, +V/2].

Figure 4. Transmission spectra of the device M2 under different typical biases. The transmission peaks are marked by P1, P2, and P3 in the bias window [−V/2, +V/2].

linear feature under the lower bias, indicating ohmic behavior which means a nonresonant tunneling phenomenon.29,43 Under the higher bias, the currents exhibit nonlinearly and correspond to the resonant conduction. The important feature in the Id−Vd curves is its NDR effect with voltage increasing. Materials possessing NDR features are widely used as fundamental electronic components containing the resonant tunneling and Esaki diode.44,45 The device M2 has better performance in molecular electronics because the NDR of the device M2 is more notable than that of the device M1. Meanwhile, by comparison of the Id−Vd curves between the device M1 and the device M2, we clearly find that the current of the device M2 is larger than that of the device M1 under the lower bias. However, with the voltage increasing, the current of the device M2 is relatively smaller than that of the device M1. To shed light on the origin of NDR of the device M1, the transmission spectra at several typical biases of 0.1, 0.2, 0.3, and 0.4 V are presented in Figure 3. The Fermi level is set as 0. We analyze the areas of the transmission spectra which enter

the bias window [−V/2, +V/2] because they determine the current. Under 0.1 V, some transmissions move into the bias window, leading to an initial increase in current. When the bias increases up to 0.2 V, the area of transmission gets larger and the current increases. With the bias increasing further, the area of transmission continuously gets larger and reaches maximum value of 0.3 V, resulting in the largest current. The area of transmission decreases gradually with the bias increasing from 0.3 to 0.4 V, so the current drops gradually and leads to the first NDR peak. It is clear that the transmission area of the device M2 is larger than that of the device M1 under the same bias, so the first NDR peak of the device M2 is more notable than that of the device M1. To further illustrate the NDR effect, we take the device M2 as an example to analyze in detail. The transmission spectra at several typical biases of 0.4, 0.5, 1.0, 1.2, 1.5, 1.7, 1.9, and 2.0 V are shown in Figure 4. The peak around EF has an important role in the electron transport properties of the devices, so we study three transmission peaks around EF which are named as C


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Figure 5. (a) Transmission eigenstates of the P1, P2, P3, and MPSH states of six frontier orbitals for the device M2 at zero bias. (b−d) Transmission eigenstate of the P1 and MPSH states of the HOMO and LUMO for the device M2 at 0.2, 0.5, and 0.6 V, respectively. (e−g) Transmission eigenstate of the P2 and MPSH states of the HOMO and LUMO for the device M2 at 1.5, 1.7, and 1.8 V, respectively. (h, i) Transmission eigenstate of the P3 and MPSH states of the LUMO and LUMO+1 for the device M2 at 1.9 and 2.0 V, respectively.

P1, P2, and P3 within the energy range [−2.5 V, 2.5 V]. From Figures 3 and 4, we can see that the P1 enters the bias window and gradually becomes smaller and eventually disappears within the voltage region [0 V, 0.5 V], leading to the first NDR peak. After that the P2 comes into the bias window and starts to work, implying that the first NDR disappears. Beyond 0.6 V, the area of transmission gets larger along with the P2 gradually entering the bias window, causing larger current. Continuing to increase the bias voltage, the P3 replaces the P2 eventually and the transmission area of the P3 is smaller than that of the P2 in the bias window, resulting in the second NDR. The area of the P2 at 1.8 V is larger than that of the P1 at 0.3 V (in Figure 3), so the current becomes larger when the second NDR peak occurs. The isolated and prominent resonance transmission peak is dependent on the delocalization of the orbital. Delocalization of an orbital means that an electron has a high moving probability, and thus a corresponding transmission peak appears.46,47 To explore the origin of the transmission peaks P1, P2, and P3, we calculate the transmission eigenstates in the bias window and the molecular projected self-consistent Hamiltonian (MPSH) eigenstates26,48 onto the M2 at different typical bias voltages mentioned above in Figure 5. Comparing the transmission eigenstates with the MPSH states at zero bias, we find that the P1, P2, and P3 correspond to the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and the LUMO+1, respectively. Therefore, the P1, P2, and P3 are responsible for the HOMO, the LUMO, and the LUMO+1, respectively. Interestingly, the eigenstates of the HOMO, the LUMO, and the LUMO+1 have no change at different biases, respectively. Meanwhile, in the bias voltage range [0 V, 0.5 V], the eigenstates of the transmission in the bias window correspond to that of the HOMO, indicating that the HOMO contributing to the P1 determines the current. In fact, the eigenstate of the

transmission is somewhat like the LUMO at 0.5 V, indicating that the bias is a turning point. In the bias voltage range [0.6 V, 1.8 V], the eigenstates of the transmission in the bias window correspond to that of the LUMO, confirming that the P1 disappears and the P2 dominated by the LUMO enters, which causes the current. Beyond 1.8 V, the eigenstates of the transmission in the bias window correspond to that of the LUMO+1, showing that the P3 dominated by the LUMO+1 governs the current. We can see that the LUMO and the LUMO+1 are more delocalized than the HOMO, leading to larger current under the higher bias than that under the lower bias. To further understand the difference of the electronic transport properties of the devices M1 and M2, Figure 6a−c shows the equilibrium total density of states (DOS) of the molecule, the partial density of states (PDOS) on hydrogen, sulfur, and carbon of the two molecules, and the transmission eigenstates of the matrix at the equilibrium condition at zero bias in Figure 6. We can see that the DOS-C of the two molecules almost coincide with the DOS compared to the DOS-H and the DOS-S, showing that the DOS-C contributes greatly to the DOS. Generally speaking, high DOS at EF leads to strong coupling, resulting in larger transmission at EF.20,49−51 It is worth mentioning that there is a large energy gap below the EF in the DOS, which is due to a few electronic states in those energy regions, further resulting in a large energy gap below EF in the transmission spectrum. We can see that the DOS of the M2 is larger than that of the M1 at EF, and the eigenstates surrounding the carbon atom attached to the M2 are larger than that of the M1, which means stronger coupling between the molecule and ZGNR electrodes, resulting in larger transmission of the device M2 at EF. The large transmission at EF indicates strong transport performance under the zero bias, indicating that the electronic transport properties of the device M2 is better than that of the device D


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Table 2. Binding Energy (Eb) of the Devices M3, M4, and M5 device Eb

device M3 −11.26 eV



M5 are all negative values. Therefore, the devices M3, M4, and M5 are stable. Different side groups would cause special performance of the devices. Figure 7 shows that the structures of the devices M3,

Figure 6. (a, b) Total DOS of the molecules M1 and M2 and PDOS on hydrogen, sulfur, and carbon of the two molecules in the energy range [−3 eV, 3 eV]. (c) Total DOS of the M1 and M2 in the energy range [−3 eV, 3 eV]. (d) Transmission spectra of the devices M1 and M2 in the energy range [−3 eV, 3 eV]. (e, f) Transmission eigenstates of the devices M1 and M2 at EF. The isovalue is 0.16.

Figure 7. Id−Vd curves of the devices M2, M3, M4, and M5 in the bias region [−2 V, 2 V].

M1. It is well-known that the more delocalized the eigenstates, the larger the transmission. Although the transmission eigenstates of the devices M1 and M2 are both delocalized in the whole central region, the delocalization degree of the device M2 is larger than that of the device M1, leading to larger transmission of the device M2 at EF. We also can see that the positions of the HOMO and the LUMO of the device M2 have little change compared to that of the device M1. In fact, our calculations show that the HOMO−LUMO gaps (HLGs) of the M1 and M2 are similar, which are 2.48 and 2.52 eV, indicating that the positions of the HOMO and the LUMO of the devices M1 and M2 are similar. Next, we study the electronic properties of three other devices functionalized with side groups −OH, −NO2, and −NH2, named as devices M3, M4, and M5, respectively. Table 1 gives the total energy of the M1, M2, M3, M4, and M5, and it can be seen that the total energy is decreasing, indicating that the structures of the M3, M4, and M5 are more stable compared to those of M1 and M2. The three molecules are arranged according to their structural stabilities as M4 > M3 > M5, showing that the side group −NO2 favors improving the structural stability. Table 2 shows the stability of the device, and the binding energy Eb is calculated by using the following formula:52

M4, and M5 are asymmetrical and their currents get smaller than that of the device M2, showing that all side groups repress the current. The Id−Vd curve of the device M4 is distinctly different from the others, indicating that side group −NO2 has a great influence on the current. It can be seen that there are minor differences in the first NDR peak for the devices M3, M4, and M5, while there are larger differences in the second NDR peak under the higher bias. For example, the current of the device M4 decreases dramatically with the bias increasing from 1.6 to 1.7 V in comparison to that of the devices M3 and M5. To understand the discrepancy in the Id−Vd curves, we calculate the transmission eigenstates at typical bias of ±0.3 and ±1.9 V shown in Figure 8. We can see that the electronic states are also distributed on side groups, resulting in lower transmission, further leading to the lower current. At ±0.3 V, the currents of the devices M3, M4, and M5 can be arranged as M3 > M4 > M5 because of the electronic states size distributed on the side groups, which can be expressed as M5 > M4 > M3. When the biases are ±1.9 V, the current of the device M4 is the smallest compared to those of the devices M3 and M5 because of the largest electronic states distribution on the group −NO2, which suppresses the transmission in some way. The distribution of electronic states on the groups −OH and −NH2 are almost the same, so the currents of the devices M3 and M5 have little difference. It is well-known that the more delocalized the eigenstates, the larger the current. The delocalization degrees of the devices M3 and M5 at higher bias are larger than those at lower bias, leading to a larger current at higher bias. We can see that the transmission

E b = Ecenter − (Esurface + Emolecue)

where Ecenter is the total energy of the isolated heterojunction unit, Esurface is the total energy of the isolated heterojunction unit excluding the molecule, and Emolecule is the total energy of the molecule. The negative Eb of the device represents its stability.53 We can see that the Eb of the devices M3, M4, and

Table 1. Total Energy of the Molecules M1, M2, M3, M4, and M5 molecule total energy

M1 −3027.55 eV

M2 −3038.71 eV

M3 −3473.91 eV E

M4 −4160.90 eV

M5 −3326.71 eV DOI: 10.1021/acs.jpcc.8b11031 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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the rectification ratio is 5, indicating that the switching behavior can be improved to some extent under certain bias. To further investigate the rectification effect, the transmission spectra at several typical bias voltages of ±0.3, ±1.2, and ±1.9 V are calculated in Figure 10. We can see that the area of transmission entering the bias window under the positive bias is different from that under the negative bias. If the area of the transmission entering the bias window under the positive bias is larger than that under the negative bias, the rectification ratio is more than 1, which is called as the forward rectification. In contrast, the rectification ratio is less than 1, which is denoted as the reversed rectification. The area of transmission of the device M4 at −1.9 V, which enters the bias window, is considerably larger than that at 1.9 V, leading to the largest rectification ratio of the device M4 compared to that of the devices M3 and M5. To further investigate the discrepancy in rectifying behavior, we calculate the electron density and electrostatic difference potential of the devices M3, M4, and M5 at zero bias shown in Figure 11. We can see that the electron density and electrostatic potential of the devices M3, M4, and M5 are asymmetric. All the N and O atoms of the three side groups show higher electron density and lower electrostatic potential compared to the left atom, so the two regions produce two barriers, which are similar to the p−n junction. For the three asymmetrical devices, the barriers of the N and O atoms of the three side groups are higher than that of the left atom, especially apparent for that of the device M4, so the N and O atoms of the three side groups play an essential role in rectifying behavior. When the positive bias is applied on the left electrode, electron transfers become easy because the N and O atoms of the three side groups are far from the applied positive bias compared to the left atom, resulting in the reducing barrier of the N and O atoms of the three side groups. In contrast, when the negative bias is applied on the left electrode, electron transfers are burdensome through the barrier because if the increasing barrier of the N and O atoms of the three side groups.54,55 Therefore, the current is asymmetrical under the positive and negative biases, resulting in rectifying behavior. However, when the considerably large bias is applied on the left electrode of the device M4, the two barriers are reversed because the distance is not the dominant reason. By the way, their difference magnitude of the electron density and electrostatic potential can be described as M4 > M3 > M5, resulting in the most pronounced rectifying behavior of the device M4. When we add a thiophene to M1, the corresponding molecule is named as M6 attached to ZGNR electrodes, which is shown in Figure S2 in the Supporting Information and its Id−Vd curve is displayed in Figure 12. We can see that the current decreases abruptly in the bias regions [−1.9 V, 1.9 V]. The current−voltage of the device M6 exhibits linear behavior in the bias regions [−0.3 V, 0.3 V], but it is very small. It is clear that the current of the device M6 decreases to zero in the bias regions [−1.5 V, −0.5 V] and [0.5 V, 1.5 V]. Interestingly, the current increases rapidly beyond 1.9 V or below −1.9 V. This phenomenon illustrates that the device displays semiconductor behavior under certain bias region. Therefore, the length of thiophene-benzene-based molecule attached to the narrow ZGNR electrodes has a great impact on the current− voltage characteristics.

Figure 8. (a) Transmission eigenstates in the bias window for the device M3 at ±0.3 V and ±1.9 V, respectively. (b) Transmission eigenstates in the bias window for the device M4 at ±0.3 V and ±1.9 V, respectively. (c) Transmission eigenstates in the bias window for the device M5 at ±0.3 V and at ±1.9 V, respectively. The isovalue is 0.15.

eigenstate of the device M4 at 1.9 V is uneven which mainly localizes on the left part of the device compared with that at −1.9 V, resulting in the lower current at 1.9 V. It is worth noting that the electronic states are asymmetrical, leading to the current under the positive bias being different from that under the negative bias. The structures of the devices M3, M4, and M5 are asymmetrical, which lead to asymmetrical currents under the positive and negative biases. This phenomenon is called rectification effect. The symmetrical devices can be tuned by different side groups to have different rectifying behaviors. The rectification ratios can offer an explanation to the rectification behaviors and be defined as follows: R (V ) =

I (V ) I(−V )

where I(V) and I(−V) are the current under the positive and negative biases. The absolute value of the bias voltage is identical. To visualize the comparison, we take the absolute value. From Figure 9, we can see that the rectification ratios of

Figure 9. Rectification ratios of the devices M3, M4, and M5 as a function of the applied bias voltage.

the device M4 are insignificant under the lower bias compared to that under the higher bias. The rectification ratios of the device M3 are more pronounced than that of the device M5; meanwhile, the maximum rectification ratios of the devices M3 and M5 appear at high bias. Compared to those of the devices M3 and M5, the rectification ratios of the device M4 are very distinct. In the bias region [0.9 V, 2.0 V], the rectification ratios of the device M4 are pronounced, especially at ±1.9 V; F


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Figure 10. (a) Transmission spectra of the device M3 at ±0.3, ±1.2, and ±1.9 V, respectively. (b) Transmission spectra of the device M4 at ±0.3, ±1.2, and ±1.9 V, respectively. (c) Transmission spectra of the device M5 at ±0.3, ±1.2, and ±1.9 V, respectively. The dotted lines indicate the bias window [−V/2, +V/2].

molecular electronics. Interestingly, two devices exhibit two NDR peaks while a thiophene or a benzene molecule tethered to ZGNR electrodes show one NDR peak. The two NDR peaks are caused by three peaks which originate from the considerably different electronic states of dominating orbitals entering the bias window. The electron transport properties of 3,6-(dithiophen-2-yl)benzene system can be controllably modulated by the side groups. Although all the side groups −OH, −NO2, and −NH2 repress the transmission and current, rectification performance appears in those devices because of its structure asymmetry which causes asymmetry of transmission eigenstate, asymmetry of electron density, and asymmetry of electrostatic difference potential. Unexpectedly, when a thiophene is added to 2,5-diphenylthiophene system attached to ZGNR electrodes, the system shows semiconductor behavior under a certain bias region. These results provide deep insight into the electron transport properties of the molecular combination.

Figure 11. Electron density distributions and electrostatic difference potential distributions of the devices M3, M4, and M5 at zero bias.

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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.8b11031. Figure 12. Id−Vd curve of the device M6 in the bias region [−2 V, 2 V].

4. CONCLUSION By using the DFT and the nonequilibrium Green function, we investigate the electron transport properties of the M1, M2, M3, M4, M5, and M6 attached to the narrow ZGNR electrodes. The 3,6-(dithiophen-2-yl)benzene has better performance compared to 2,5-diphenylthiophene applied in

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A thiophene or a benzene molecule attached to 4-atoms width of two narrow ZGNR electrodes; the corresponding molecular device when a thiophene has been added to M1, which is attached to ZGNR electrodes (PDF)

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86 531 88395011. E-mail: lihuilmy@hotmail. com (H.L.). G


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The Journal of Physical Chemistry C ORCID

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Xingfan Zhang: 0000-0003-0852-4194 Chao Yuan: 0000-0001-7323-085X Hui Li: 0000-0002-1457-8650 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support from the National Natural Science Foundation of China (Grant Nos. 51671114 and U1806219). This work is also supported by the Special Funding in the Project of the Taishan Scholar Construction Engineering.

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The Effects of Molecular Combination and Side Groups for Thiophene

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