The electron and spin polarized transport in wide-voltage-ranges through colbaltporphyrin-based molecular junctions , , Jue-Fei Cheng, Qiang Yan, Liping Zhou , Qin Han, and Lei Gao
Citation: J. Chem. Phys. 144, 084707 (2016); doi: 10.1063/1.4942923 View online: http://dx.doi.org/10.1063/1.4942923 View Table of Contents: http://aip.scitation.org/toc/jcp/144/8 Published by the American Institute of Physics
THE JOURNAL OF CHEMICAL PHYSICS 144, 084707 (2016)
The electron and spin polarized transport in wide-voltage-ranges through colbaltporphyrin-based molecular junctions Jue-Fei Cheng,1,2 Qiang Yan,1 Liping Zhou,1,a) Qin Han,1 and Lei Gao1,a)
1
College of Physics, Optoelectronics and Energy and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China 2 School of Electronics and Information Engineering, Suzhou Vocational University, Suzhou 215104, China
(Received 9 November 2015; accepted 16 February 2016; published online 29 February 2016) The electron and spin polarized transport properties of Co benzene-porphyrin-benzene (BPB) molecule coupled to gold (Au) nanowires in a wide voltage range (0–3.0 V) are investigated. By successively removing the front-end Au atoms, we construct Au nanowires with different moleculeelectrode contact symmetries. Multiple negative differential resistance (NDR) peaks emerge at different bias voltage regions. It is found that the low-voltage NDR effect at 0.4 V can only be found in the junctions with S–Au top bindings. High-bias NDR effects intrinsic to central molecule at 2.8 V are observed in all the six structures. In particular, both the electron and spin polarized current-voltage (I–V) curves depend strongly on the contact configurations between Co-BPB molecule and the Au electrodes. And the top-binding may result in spin dependent transport properties and will be the priority selection in the design of molecular devices. C 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4942923] I. INTRODUCTION
Electron transport through molecular-scale devices has attracted considerable attentions among both experimentalists and theorists.1–7 Most studies of these systems have focused on molecules connected to metallic leads.8–10 Although some level of agreement between the experiment and theory has been reached, several controversies remain, especially regarding the magnitude of the current, the mechanism of the negative differential resistance (NDR), and the effect of junction geometry.11–14 The previous calculation results revealed that hollow-hollow contact configuration usually had the biggest conductance because of the strongest coupling strength.15 Kondo et al. studied the contact-structure dependence of transport properties of dithiolbenzene (DTB) molecule between Au electrodes and found that the hollow contact has the strongest molecule-electrode interaction.16 However, Bai et al. studied the effect of metal-molecule interface conformations on the electron transport of single molecule and found that the on-top contact exhibits the best molecule-metal coupling when an external bias lower than 1.4 V is applied.17 Understanding the relationship between the electronic transport properties of single molecular device and the contact sites is fundamental to the creation of functional nanoscale devices. Therefore, we give systematic investigations on the contact effect in a wide bias range (0–3.0 V) on porphyrinbased molecule wires. Porphyrin-based molecule wires are promising candidates for nanoelectronic and photovotatic devices due to highly conjugated structure, rigid planar geometry, the chemical stability, and unique optoelectronic properties. In particular, porphyrin can coordinate with various metal atoms which affect the electron transport through it.18,19 a)Authors to whom correspondence should be addressed. Electronic ad-
dresses: [email protected] and [email protected] 0021-9606/2016/144(8)/084707/7/$30.00
Transport properties of a system with transition metal in porphyrin molecule are quite interesting in relation to the spintronics.20,21 The field of spintronics is an important application topic since spintronic devices are now used in recording magnetoresistance, random access memory for high density hard drives.22–26 Pontes et al. show that the insertions of a single transition metal atom in the Au nanowire (NW) can result in strong spin dependent transport properties, which can be dramatically changed by the local symmetry of the nanowires.27 Therefore the potential spintronic behaviors of a Co atom in porphyrin based molecular junctions deserve to be investigated systematically. In the present paper, we start from pyramidal type Au electrodes based on the benzeneporphyrin-benzene (BPB) molecule with a Co atom in the center, and then successively remove the front-end Au atoms until we get the planar type electrode. Six Co-BPB junctions with different molecule-electrode contact binding modes are constructed. The current-voltage curves are quite different in these structures in the wide bias ranges. In particular, the low-bias NDR effect and the spin-dependent transport properties can only be observed in the junctions of S–Au top binding.
II. COMPUTATIONAL MODELS AND METHODS
The representative models of Co-BPB molecular junction bonded to Au nanowire by using a terminal S atom are shown in Fig. 1. Each computational model was divided into three regions: the left electrode (LE), the central molecular (CM), and the right electrode (RE). The Au nanowire is the electrode with the diameter of 4.5 Å. Six Co-BPB junctions with different molecule–electrode contact symmetries were constructed after the isolated CoBPB molecule was optimized. There were no conformation
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I (V) =
2e h
+∞
Tσ (E,V ) [ f l (E − µl ) − f r (E − µr )] dE,
(1)
−∞
where µl and µr are the electrochemical potentials of the two electrodes and f l and f r are the corresponding electron distributions of the two electrodes. Tσ (E,V ) is the spinresolved transmittance function with σ =↑,↓ as the spin index, which is obtained by Tσ (E,V ) = Tr ΓL Gσ ΓRG+σ . (2) Here Gσ is the spin-dependent retarded Green’s function of the molecule, ΓL/R is the coupling matrix between the molecule and the left/right electrode. FIG. 1. Up panel: Schematic illustrations of Co-BPB molecular junctions used in calculations. Down panel: Six representative structures of Co-BPB molecular junctions. The molecule is sandwiched between Au electrodes through the thiol atom with different contact modes, which are denoted as C1–C6. Three site types are considered and labeled as “top (T),” “bridge (B),” and “hollow (H),” respectively. Terminal S atom is anchoring to the tip Au atom in the top type. In the hollow type, the S atom connects directly on the hollow position of the Au atom plane. “Bridge site” means that the S atom is on the bridge location of two Au atoms. Dark yellow sphere: Au atom; light yellow sphere: S atom; dark red sphere: Co atom; blue sphere: N atom; gray sphere: C atom; white sphere: H atom.
variations for the molecular cores thereafter. The first configuration C1 refers to the junction geometry, where the S atoms were directly on top of the Au atoms to form a pyramidal configuration. The first layer Au atoms at the front-end on both sides were removed to obtain the C2 configuration. One of the second layer Au atoms on both sides were removed to get C3 configuration. Then, all the second layer Au atoms on both sides were deleted to acquire C4. The Au atoms at the front-end of C4 were removed to obtain the C5 configuration. Also, the second layer Au atoms on right side of C1 were removed to get the asymmetric configuration C6. The geometry of the free Co-BPB molecule was optimized using GAMESS at the B3LYP/6-31G level. After geometry optimization, the orientations of the two benzenes on the ends, relative to the central porphyrin plane were 60.6◦ and −63.4◦. The S atom was bonded to the front two surface-layers of the Au electrodes to build supercells for different contact cases. The S–Au binding modes were then determined by optimizing the supercells. The distances between the S and Au atoms were 1.71 Å for the hollow binding case, 2.01 Å for the bridge case, and 2.38 Å for the top binding case. Finally, the optimized Co-BPB molecule core and S–Au supercells were combined and translated between the Au electrodes to obtain the six Co-BPB junctions. The optimization and transport simulations for the junctions were carried out with the Atomistix Toolkit, which combines density-functional theory with the nonequilibrium Green’s function technique.28–30 The wave functions were expanded on a double-ξ plus polarization numerical basis set for all atoms. Convergent results were achieved using the Monkhorst–Pack grid with (1, 1, 200) k-points in the Brillouin zone within the LDA approximation. The current through the junction is calculated from the corresponding Green’s function and self-energies using Landauer-Büttiker formula
III. RESULTS AND DISCUSSION
The calculated I–V characteristics for different junction geometries (C1–C6) are presented in Fig. 2, where the bias varies in a wide range of 0–3.0 V. Several intriguing features are noticeable. First, in the low bias regime (0–1.0 V), the magnitude of the current under the same voltage of C1, C4, C6 (T–T) is larger than that of C2 (H–H), C3 (B–B), and C5 (H–H). For C1, C4, and C6, there is an increase in the current immediately after a bias voltage is applied. But for C2, C3, and C5, the currents are small over the whole low bias regime. Second, with the increase of the applied voltage, the currents of C2, C3, and C5 increase greatly and exceed that of C1, C4, C6 since 1.5 V. Third, when the applied voltage is larger than 0.4 V in the low bias regime, the currents for C1, C4, C6 decrease with the increasing voltage, showing the NDR effect. Such very low-bias NDR is scarcely reported. In previous papers, the NDR effect was not observed until the applied bias was above 1.0 V.31–33 In our simulations, low-bias regime NDR behavior around 0.4 V can only be seen in S–Au top binding configurations (C1, C4, C6). Besides, at moderate voltages between 1.0 and 2.0 V, two other NDR regions with different characteristics can be seen. NDR peak at 1.2 V only emerges in top configurations and the NDR effect at 1.6 V is shown in different contact modes. In particular, the high-bias regime NDR around 2.8 V can be seen in all the six structures. To penetrate the mechanism of the observed I-V behaviors, the transmission spectra as functions of the electron energy and applied bias for representative top-binding
FIG. 2. Calculated I–V characteristics for C1–C6.
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FIG. 3. Calculated transmission spectra as functions of the electron energy and applied bias for C1 (left panel) and C2 (right panel). The region between the solid lines is referred to as bias window.
configuration C1 (left panel) and hollow-binding configuration C2 (right panel) are given in Fig. 3. The region between the solid lines is referred to as bias window. The transmission spectra show different characteristics in low-bias and highbias regions for C1 and C2. Intense transmission peaks can be seen in C1 in negative low-bias region and become weaker with the increase of applied bias. For C2, the transmission peaks do not emerge until 1.5 V and become stronger only in positive high-bias region, leading to the increase of the current. We narrow the analysis to a specific voltage and summarize the representative transmission spectra of C1 (T–T) and C2 (H–H) under the applied bias of 0.4 V and 2.8 V in Fig. 4. The chemical potential window (CPW) is indicated with bold blue lines and the Fermi energy is set to zero on the energy scale. The molecule projected self-consistent Hamiltonian (MPSH) of the Co-BPB molecule (black triangles
for C1 and red for C2) is calculated for comparison. The conduction properties of C1 and C2 at 0.4 V are dominated by the transmission peaks which are near the Fermi level shown in Fig. 4(a). The S–Au couplings via top-binding (−3.6 eV) make High Occupied Molecular Orbital (HOMO) energy closer to Fermi level, which is the main cause of the transmission peak of C1 at 0.4 V. However, this transmissions peak near Fermi level is absent for C2 at 0.4 V, implying that this peak is top-contact dependent. The main transmission peak of C2, with hollow binding energy (−1.1 eV), is relevant to Low Unoccupied Molecular Orbital (LUMO), which is the out of CPW at 0.4 V. The transmission spectra of C1 at 1.2 V and 1.6 V are also given in Fig. 4(c). The calculations indicate that the transmission spectrum of C1 at 1.2 V is similar to that of 0.4 V. But when the applied bias increases to 1.6 V, the top-contact dependent transmission peak shifts
FIG. 4. Transmission spectra of C1 and C2 at (a) 0.4 V and (b) 2.8 V and transmission spectra of C1 (c) at 1.2 V and 1.6 V. The HOMO and LUMO of isolated Co-BPB are indicated with triangles for C1 (black) and C2 (red).
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TABLE I. MPSH states of HOMO-1, HOMO, LUMO, LUMO+1, LUMO+2 of C1 (T–T), and C2 (H–H) at 0.4 V and 2.8 V. HOMO-1
HOMO
LUMO
LUMO+1
LUMO+2
−0.27 eV
−0.26 eV
1.07 eV
1.13 eV
1.31 eV
−0.22 eV
−0.11 eV
0.78 eV
1.05 eV
1.12 eV
−1.45 eV
−1.22 eV
0.03 eV
0.16 eV
1.17 eV
−1.23 eV
−1.18 eV
0.07 eV
0.20 eV
0.37 eV
C1 (0.4 V)
C2 (0.4 V)
C1 (2.8 V)
C2 (2.8 V)
to the left and shrinks and more transmission peaks far away from Fermi level emerge. As the bias further increases to 2.8 V as shown in Fig. 4(b), the HOMO peak shifts away from the Fermi energy and only a small part of transmission peak is within the bias window. However, the transmission peaks of C2 at 2.8 V shift into CPW and become stronger than C1 because the MPSH levels intrinsic to the Co-BPB molecule govern the transport, leading to the increase of current. In this
process, the opening of the CPW and the shift of different transmission peaks resulted in the changes of the currents in wide-range-bias regions, leading to the multiple NDR effects. In the following, we focus on the discussion of the spaceresolved states of C1 and C2 at 0.4 V and 2.8 V, shown in Table I. The frontier molecular orbitals of Co-BPB molecule for C1 shift toward the lower energy with the increase of the applied voltage. For example, the HOMO level of C1 at
FIG. 5. I-V curves for the six structures displayed in Figure 1. All figures show Iup (black) and Idown (red).
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FIG. 6. Transmission spectra of the six structures at 0.2 V. All figures show spin up channel (black) and spin down channel (red).
0.4 V corresponds to HOMO-1 of C1 at 2.8 V. The HOMO is delocalized through the whole molecule at 0.4 V and coupling with the Au electrodes leading to the main transmission peak of C1. While the HOMO of C2 is only localized at the left side, thus no transmission peak near Fermi level can be seen in Fig. 4(a). Therefore, the top-binding configuration C1 displays much better low bias transport properties. When the applied bias increases from 0.4 V to 2.8 V, the LUMO (LUMO+1, LUMO+2) of C1 shifts to HOMO (LUMO, LUMO+1), far away from the Fermi level, and HOMO of C1 is localized at 2.8 V. However, with the increase of applied bias, the LUMO+4 (1.36 eV) of C2 originates from the central CoBPB molecule shift into CPW leading to the rapid increase of current at 2.8 V. Thus we can infer that the low bias NDR effect in the top contact modes is ascribed to the change in the coupling interactions between the molecular orbitals and the electrodes, which is due to the shift of the molecular orbitals caused only by top binding. With the increase of applied bias, the intrinsic frontier molecular orbitals far from the Fermi level govern the electron transport of Co-BPB molecular junctions. Considering the spin dependent transport properties related to the contact effect is scarcely reported so far; we give the spin polarized I–V curves for all the six structures in Fig. 5. The black lines indicate spin up transport channels and the red lines indicate spin down transport channel, respectively. The top binding (C1, C4, C6) structures show distinctive spin dependent transport properties. In the low bias regime (
Citation: J. Chem. Phys. 144, 084707 (2016); doi: 10.1063/1.4942923 View online: http://dx.doi.org/10.1063/1.4942923 View Table of Contents: http://aip.scitation.org/toc/jcp/144/8 Published by the American Institute of Physics
THE JOURNAL OF CHEMICAL PHYSICS 144, 084707 (2016)
The electron and spin polarized transport in wide-voltage-ranges through colbaltporphyrin-based molecular junctions Jue-Fei Cheng,1,2 Qiang Yan,1 Liping Zhou,1,a) Qin Han,1 and Lei Gao1,a)
1
College of Physics, Optoelectronics and Energy and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China 2 School of Electronics and Information Engineering, Suzhou Vocational University, Suzhou 215104, China
(Received 9 November 2015; accepted 16 February 2016; published online 29 February 2016) The electron and spin polarized transport properties of Co benzene-porphyrin-benzene (BPB) molecule coupled to gold (Au) nanowires in a wide voltage range (0–3.0 V) are investigated. By successively removing the front-end Au atoms, we construct Au nanowires with different moleculeelectrode contact symmetries. Multiple negative differential resistance (NDR) peaks emerge at different bias voltage regions. It is found that the low-voltage NDR effect at 0.4 V can only be found in the junctions with S–Au top bindings. High-bias NDR effects intrinsic to central molecule at 2.8 V are observed in all the six structures. In particular, both the electron and spin polarized current-voltage (I–V) curves depend strongly on the contact configurations between Co-BPB molecule and the Au electrodes. And the top-binding may result in spin dependent transport properties and will be the priority selection in the design of molecular devices. C 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4942923] I. INTRODUCTION
Electron transport through molecular-scale devices has attracted considerable attentions among both experimentalists and theorists.1–7 Most studies of these systems have focused on molecules connected to metallic leads.8–10 Although some level of agreement between the experiment and theory has been reached, several controversies remain, especially regarding the magnitude of the current, the mechanism of the negative differential resistance (NDR), and the effect of junction geometry.11–14 The previous calculation results revealed that hollow-hollow contact configuration usually had the biggest conductance because of the strongest coupling strength.15 Kondo et al. studied the contact-structure dependence of transport properties of dithiolbenzene (DTB) molecule between Au electrodes and found that the hollow contact has the strongest molecule-electrode interaction.16 However, Bai et al. studied the effect of metal-molecule interface conformations on the electron transport of single molecule and found that the on-top contact exhibits the best molecule-metal coupling when an external bias lower than 1.4 V is applied.17 Understanding the relationship between the electronic transport properties of single molecular device and the contact sites is fundamental to the creation of functional nanoscale devices. Therefore, we give systematic investigations on the contact effect in a wide bias range (0–3.0 V) on porphyrinbased molecule wires. Porphyrin-based molecule wires are promising candidates for nanoelectronic and photovotatic devices due to highly conjugated structure, rigid planar geometry, the chemical stability, and unique optoelectronic properties. In particular, porphyrin can coordinate with various metal atoms which affect the electron transport through it.18,19 a)Authors to whom correspondence should be addressed. Electronic ad-
dresses: [email protected] and [email protected] 0021-9606/2016/144(8)/084707/7/$30.00
Transport properties of a system with transition metal in porphyrin molecule are quite interesting in relation to the spintronics.20,21 The field of spintronics is an important application topic since spintronic devices are now used in recording magnetoresistance, random access memory for high density hard drives.22–26 Pontes et al. show that the insertions of a single transition metal atom in the Au nanowire (NW) can result in strong spin dependent transport properties, which can be dramatically changed by the local symmetry of the nanowires.27 Therefore the potential spintronic behaviors of a Co atom in porphyrin based molecular junctions deserve to be investigated systematically. In the present paper, we start from pyramidal type Au electrodes based on the benzeneporphyrin-benzene (BPB) molecule with a Co atom in the center, and then successively remove the front-end Au atoms until we get the planar type electrode. Six Co-BPB junctions with different molecule-electrode contact binding modes are constructed. The current-voltage curves are quite different in these structures in the wide bias ranges. In particular, the low-bias NDR effect and the spin-dependent transport properties can only be observed in the junctions of S–Au top binding.
II. COMPUTATIONAL MODELS AND METHODS
The representative models of Co-BPB molecular junction bonded to Au nanowire by using a terminal S atom are shown in Fig. 1. Each computational model was divided into three regions: the left electrode (LE), the central molecular (CM), and the right electrode (RE). The Au nanowire is the electrode with the diameter of 4.5 Å. Six Co-BPB junctions with different molecule–electrode contact symmetries were constructed after the isolated CoBPB molecule was optimized. There were no conformation
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J. Chem. Phys. 144, 084707 (2016)
I (V) =
2e h
+∞
Tσ (E,V ) [ f l (E − µl ) − f r (E − µr )] dE,
(1)
−∞
where µl and µr are the electrochemical potentials of the two electrodes and f l and f r are the corresponding electron distributions of the two electrodes. Tσ (E,V ) is the spinresolved transmittance function with σ =↑,↓ as the spin index, which is obtained by Tσ (E,V ) = Tr ΓL Gσ ΓRG+σ . (2) Here Gσ is the spin-dependent retarded Green’s function of the molecule, ΓL/R is the coupling matrix between the molecule and the left/right electrode. FIG. 1. Up panel: Schematic illustrations of Co-BPB molecular junctions used in calculations. Down panel: Six representative structures of Co-BPB molecular junctions. The molecule is sandwiched between Au electrodes through the thiol atom with different contact modes, which are denoted as C1–C6. Three site types are considered and labeled as “top (T),” “bridge (B),” and “hollow (H),” respectively. Terminal S atom is anchoring to the tip Au atom in the top type. In the hollow type, the S atom connects directly on the hollow position of the Au atom plane. “Bridge site” means that the S atom is on the bridge location of two Au atoms. Dark yellow sphere: Au atom; light yellow sphere: S atom; dark red sphere: Co atom; blue sphere: N atom; gray sphere: C atom; white sphere: H atom.
variations for the molecular cores thereafter. The first configuration C1 refers to the junction geometry, where the S atoms were directly on top of the Au atoms to form a pyramidal configuration. The first layer Au atoms at the front-end on both sides were removed to obtain the C2 configuration. One of the second layer Au atoms on both sides were removed to get C3 configuration. Then, all the second layer Au atoms on both sides were deleted to acquire C4. The Au atoms at the front-end of C4 were removed to obtain the C5 configuration. Also, the second layer Au atoms on right side of C1 were removed to get the asymmetric configuration C6. The geometry of the free Co-BPB molecule was optimized using GAMESS at the B3LYP/6-31G level. After geometry optimization, the orientations of the two benzenes on the ends, relative to the central porphyrin plane were 60.6◦ and −63.4◦. The S atom was bonded to the front two surface-layers of the Au electrodes to build supercells for different contact cases. The S–Au binding modes were then determined by optimizing the supercells. The distances between the S and Au atoms were 1.71 Å for the hollow binding case, 2.01 Å for the bridge case, and 2.38 Å for the top binding case. Finally, the optimized Co-BPB molecule core and S–Au supercells were combined and translated between the Au electrodes to obtain the six Co-BPB junctions. The optimization and transport simulations for the junctions were carried out with the Atomistix Toolkit, which combines density-functional theory with the nonequilibrium Green’s function technique.28–30 The wave functions were expanded on a double-ξ plus polarization numerical basis set for all atoms. Convergent results were achieved using the Monkhorst–Pack grid with (1, 1, 200) k-points in the Brillouin zone within the LDA approximation. The current through the junction is calculated from the corresponding Green’s function and self-energies using Landauer-Büttiker formula
III. RESULTS AND DISCUSSION
The calculated I–V characteristics for different junction geometries (C1–C6) are presented in Fig. 2, where the bias varies in a wide range of 0–3.0 V. Several intriguing features are noticeable. First, in the low bias regime (0–1.0 V), the magnitude of the current under the same voltage of C1, C4, C6 (T–T) is larger than that of C2 (H–H), C3 (B–B), and C5 (H–H). For C1, C4, and C6, there is an increase in the current immediately after a bias voltage is applied. But for C2, C3, and C5, the currents are small over the whole low bias regime. Second, with the increase of the applied voltage, the currents of C2, C3, and C5 increase greatly and exceed that of C1, C4, C6 since 1.5 V. Third, when the applied voltage is larger than 0.4 V in the low bias regime, the currents for C1, C4, C6 decrease with the increasing voltage, showing the NDR effect. Such very low-bias NDR is scarcely reported. In previous papers, the NDR effect was not observed until the applied bias was above 1.0 V.31–33 In our simulations, low-bias regime NDR behavior around 0.4 V can only be seen in S–Au top binding configurations (C1, C4, C6). Besides, at moderate voltages between 1.0 and 2.0 V, two other NDR regions with different characteristics can be seen. NDR peak at 1.2 V only emerges in top configurations and the NDR effect at 1.6 V is shown in different contact modes. In particular, the high-bias regime NDR around 2.8 V can be seen in all the six structures. To penetrate the mechanism of the observed I-V behaviors, the transmission spectra as functions of the electron energy and applied bias for representative top-binding
FIG. 2. Calculated I–V characteristics for C1–C6.
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FIG. 3. Calculated transmission spectra as functions of the electron energy and applied bias for C1 (left panel) and C2 (right panel). The region between the solid lines is referred to as bias window.
configuration C1 (left panel) and hollow-binding configuration C2 (right panel) are given in Fig. 3. The region between the solid lines is referred to as bias window. The transmission spectra show different characteristics in low-bias and highbias regions for C1 and C2. Intense transmission peaks can be seen in C1 in negative low-bias region and become weaker with the increase of applied bias. For C2, the transmission peaks do not emerge until 1.5 V and become stronger only in positive high-bias region, leading to the increase of the current. We narrow the analysis to a specific voltage and summarize the representative transmission spectra of C1 (T–T) and C2 (H–H) under the applied bias of 0.4 V and 2.8 V in Fig. 4. The chemical potential window (CPW) is indicated with bold blue lines and the Fermi energy is set to zero on the energy scale. The molecule projected self-consistent Hamiltonian (MPSH) of the Co-BPB molecule (black triangles
for C1 and red for C2) is calculated for comparison. The conduction properties of C1 and C2 at 0.4 V are dominated by the transmission peaks which are near the Fermi level shown in Fig. 4(a). The S–Au couplings via top-binding (−3.6 eV) make High Occupied Molecular Orbital (HOMO) energy closer to Fermi level, which is the main cause of the transmission peak of C1 at 0.4 V. However, this transmissions peak near Fermi level is absent for C2 at 0.4 V, implying that this peak is top-contact dependent. The main transmission peak of C2, with hollow binding energy (−1.1 eV), is relevant to Low Unoccupied Molecular Orbital (LUMO), which is the out of CPW at 0.4 V. The transmission spectra of C1 at 1.2 V and 1.6 V are also given in Fig. 4(c). The calculations indicate that the transmission spectrum of C1 at 1.2 V is similar to that of 0.4 V. But when the applied bias increases to 1.6 V, the top-contact dependent transmission peak shifts
FIG. 4. Transmission spectra of C1 and C2 at (a) 0.4 V and (b) 2.8 V and transmission spectra of C1 (c) at 1.2 V and 1.6 V. The HOMO and LUMO of isolated Co-BPB are indicated with triangles for C1 (black) and C2 (red).
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TABLE I. MPSH states of HOMO-1, HOMO, LUMO, LUMO+1, LUMO+2 of C1 (T–T), and C2 (H–H) at 0.4 V and 2.8 V. HOMO-1
HOMO
LUMO
LUMO+1
LUMO+2
−0.27 eV
−0.26 eV
1.07 eV
1.13 eV
1.31 eV
−0.22 eV
−0.11 eV
0.78 eV
1.05 eV
1.12 eV
−1.45 eV
−1.22 eV
0.03 eV
0.16 eV
1.17 eV
−1.23 eV
−1.18 eV
0.07 eV
0.20 eV
0.37 eV
C1 (0.4 V)
C2 (0.4 V)
C1 (2.8 V)
C2 (2.8 V)
to the left and shrinks and more transmission peaks far away from Fermi level emerge. As the bias further increases to 2.8 V as shown in Fig. 4(b), the HOMO peak shifts away from the Fermi energy and only a small part of transmission peak is within the bias window. However, the transmission peaks of C2 at 2.8 V shift into CPW and become stronger than C1 because the MPSH levels intrinsic to the Co-BPB molecule govern the transport, leading to the increase of current. In this
process, the opening of the CPW and the shift of different transmission peaks resulted in the changes of the currents in wide-range-bias regions, leading to the multiple NDR effects. In the following, we focus on the discussion of the spaceresolved states of C1 and C2 at 0.4 V and 2.8 V, shown in Table I. The frontier molecular orbitals of Co-BPB molecule for C1 shift toward the lower energy with the increase of the applied voltage. For example, the HOMO level of C1 at
FIG. 5. I-V curves for the six structures displayed in Figure 1. All figures show Iup (black) and Idown (red).
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J. Chem. Phys. 144, 084707 (2016)
FIG. 6. Transmission spectra of the six structures at 0.2 V. All figures show spin up channel (black) and spin down channel (red).
0.4 V corresponds to HOMO-1 of C1 at 2.8 V. The HOMO is delocalized through the whole molecule at 0.4 V and coupling with the Au electrodes leading to the main transmission peak of C1. While the HOMO of C2 is only localized at the left side, thus no transmission peak near Fermi level can be seen in Fig. 4(a). Therefore, the top-binding configuration C1 displays much better low bias transport properties. When the applied bias increases from 0.4 V to 2.8 V, the LUMO (LUMO+1, LUMO+2) of C1 shifts to HOMO (LUMO, LUMO+1), far away from the Fermi level, and HOMO of C1 is localized at 2.8 V. However, with the increase of applied bias, the LUMO+4 (1.36 eV) of C2 originates from the central CoBPB molecule shift into CPW leading to the rapid increase of current at 2.8 V. Thus we can infer that the low bias NDR effect in the top contact modes is ascribed to the change in the coupling interactions between the molecular orbitals and the electrodes, which is due to the shift of the molecular orbitals caused only by top binding. With the increase of applied bias, the intrinsic frontier molecular orbitals far from the Fermi level govern the electron transport of Co-BPB molecular junctions. Considering the spin dependent transport properties related to the contact effect is scarcely reported so far; we give the spin polarized I–V curves for all the six structures in Fig. 5. The black lines indicate spin up transport channels and the red lines indicate spin down transport channel, respectively. The top binding (C1, C4, C6) structures show distinctive spin dependent transport properties. In the low bias regime (
