Article pubs.acs.org/JPCC
Conductance of a Single Magnesium Porphine Molecule on an Insulating Surface Kun Peng Dou,† Jung-Shen Tai,† and Chao-Cheng Kaun*,†,‡ †
Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan Department of Physics, National Tsing-Hua University, Hsinchu 30013, Taiwan
‡
ABSTRACT: Using first-principles calculations based on density functional theory and nonequilibrium Green’s function formalism, we study the electron transport through a magnesium porphine molecule adsorbed on an ultrathin NaCl bilayer. The conductance of the tip−vacuum−molecule−NaCl−metal junction depends on the orientation of the molecule on the insulating surface and the tip position above the molecule, which is mediated largely by the molecular pz orbital. The movement of molecule results in a perturbation to the spatial extension of these orbitals, leading to different conductions.
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INTRODUCTION The properties of molecules absorbed on a metal surface have been probed by using STM measurement and interesting results are reported.1−5 In the junction consisting of tip− vacuum−molecule−metal, the metal surface states hybridize with the molecule and often largely contribute to the junction conductance.5 However, the molecule can be separated relatively from the metal surface through a thin isolating layer to form a double-barrier tunneling junction so that the molecular nature can be better demonstrated. For example, a copper phthalocyanine (CuPc) molecule is adsorbed on a thin alumina film, where the relative intensities of individual conduction channels associated with different vibronic states of the molecule are revealed.6 By replacing the CuPc molecule with a magnesium porphine (MgP), a photosynthesis-involved molecule, the single-molecule junction can be coupled to photons and provides a pathway to explore molecular dynamics.7 A similar system is also used to investigate the voltage-controlled conductance hysteresis and switching in the junction because of the charge bistability of the molecule.8 Moreover, the MgP molecule is adsorbed on an ultrathin NaCl bilayer to study the resonant-tunneling-induced rotation dynamics of a single-molecule switch, where the angledependent high and low conductance occurs.9 Although the conductance bistability in a MgP single-molecule junction is simulated,10 the correlation between the orientation of the molecule on an isolating surface and its conductance has not, however, been addressed computationally to date. In this work, we carry out systematic computational investigations to obtain an insight into the interplay between atomic arrangement and conduction properties of MgP molecule on an ultrathin NaCl bilayer. The conductance of the double-barrier tunnel junction consisting of the STM tip, vacuum, the molecule, the salt bilayer, and the gold substrate are calculated. The effects of the molecular orientation on the © 2015 American Chemical Society
isolating surface and the tip location above the molecule on the junction conductance are shown. The conduction mediated molecular orbital is also identified.
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METHODS
A slab was adopted to model the substrate, shown in Figure 1a, that consists of a NaCl bilayer (25 atoms per layer) and a
Figure 1. (a) Schematic illustration of the model. A slab consisting of a NaCl bilayer and an Au (111) bilayer with a nanowire is used to simulate the substrate. (b) Top view of the MgP molecule on the substrate. There are gold (dark orange), chlorine (green), sodium (yellow), magnesium (silver), nitrogen (blue), carbon (bright orange), and hydrogen (white) atoms. (c) Orientation-dependent conductance, rotational potential energy, and coupling strength between the tip and molecule (blue triangles). The Mg atom is adsorbed on the Cl ion and under the tip. Received: September 9, 2015 Revised: October 14, 2015 Published: October 15, 2015 25129
DOI: 10.1021/acs.jpcc.5b08795 J. Phys. Chem. C 2015, 119, 25129−25133
Article
The Journal of Physical Chemistry C Au(111) bilayer. Two semi-infinite Au(111) nanowires were used as the top and bottom electrodes to reduce computational cost.11,12 The cone-shaped tip was obtained by sharpening the terminal of the top electrode. The orientation of the MgP molecule with respect to the NaCl lattice was defined by the angle θ between the Mg−N and Na−Cl bonds, as shown in Figure 1b. To focus on the role of electronic coupling between the NaCl upper layer and the MgP molecule, only these two parts were relaxed by using density functional theory (DFT) implemented in the SIESTA package,13 within the generalized gradient approximation14 and an energy cutoff of 400 Ry. The distance dTS (d2) between the tip apex atom (the NaCl lower layer) and the gold surface was fixed at 13.65 Å (2.73 Å). Valence electrons were expanded in a double-ζ plus polarization (a single-ζ) basis set for the NaCl bilayer and the MgP molecule (for Au electrode atoms). Structural relaxations were continued until the force acting on each atom was less than 0.05 eV/Å. Then, transport calculations were carried out with Nanodcal package,15 which combined DFT with the nonequilibrium Green’s function (NEGF) formalism. Three Au buffer layers of electrode were included in the scattering region. The Brillouin zone was sampled by 100 k-points. The conductance value G = T(Ef) G0 in the linear response regime, where T(E) is the transmission spectrum, Ef is the Fermi energy, and G0 = 2e2/ℏ is the conductance quantum.16
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RESULTS AND DISCUSSION The Mg atom (the center of the MgP molecule) favors adsorption on the Cl ion, 0.193 eV lower than on the Na ion, and is pulled downward from the molecular plane. After relaxations, as shown in Figure 1a, the distance d1 between the tip apex atom and the Mg atom is changed from an initial value of 5.00 Å to 5.47 Å (θ = 0°; rotational potential energy, 0.191 eV), 5.44 Å (14°; 0.047 eV), 5.32 Å (25°; 0.008 eV), 5.42 Å (30°; 0 eV), and 5.41 Å (45°; 0.045 eV). In Figure 1c, as the angle increases, the conductance changes from 6.37 × 10−7G0 (θ = 0°) to 4.93 × 10−7G0 (θ = 14°), 13.7 × 10−6G0 (θ = 25°), 8.28 × 10−6G0 (θ = 30°), and 9.16 × 10−6G0 (θ = 45°), demonstrating that the rotation of the molecule can lead to considerable change in the junction conductance. To gain insight into the orientation-dependent conductance, we show the transmission spectra of configurations with θ = 0, 25, 30, and 45° in Figure 2a. The transmission through the θ = 25, 30, and 45° configurations are higher than that through the θ = 0° configuration in the energy window between −0.01 and 0.01 eV. All spectra exhibit one peak around 0.003 eV. The conductance (the transmission coefficient at Ef) is related to this peak and hence shows the nearly one-order difference in the conductance among them. To understand the origin of discrepancy among the four transmission spectra, we plot the projected density of states (PDOS) of the MgP molecule and the NaCl upper layer in Figure 2b−d. The PDOS of the MgP molecule is further decomposed into in-plane orbitals (sum of s, px, and py) and out-of-plane orbitals (pz) in Figure 2b,c, with respect to their different spatial extension. As the MgP molecule is rotated from θ = 0°, its PDOS is lowered over the in-plane orbitals, as shown in Figure 2b, but heightened over out-of-plane orbitals, as indicated in Figure 2c. However, the PDOS over former orbitals are nearly one order of magnitude weaker than those over the latter orbitals. Thus, the pz orbitals provide the dominant contribution to the conduction. The PDOS of the NaCl upper layer present one peak around 0.003 eV in all
Figure 2. (a) Transmission spectra as a function of rotational angle. Projected density of states (PDOS) of the MgP molecule (b) on the s, px, and py orbitals and (c) on the pz orbitals. (d) PDOS of the NaCl upper layer. The black solid, red dashed, green solid, and blue dashed lines are for θ = 0, 25, 30, and 45° configurations, respectively. Local density of states (LDOS) at the Fermi energy for (e) θ = 0° and (g) θ = 30° configurations are shown in brown. (f and h) Corresponding contour plots of the LDOS, along the cross sections indicated by the dashed lines in e and g, respectively.
configurations. The similarity between Figure 2a,d indicates that the orbitals over the NaCl upper layer dominate the shape of transmission spectra. However, the height difference among the transmission spectra is correlated to the variation of the pz orbitals on the MgP molecule. Therefore, the features of transmission spectra are essentially the product of the pz orbitals of the molecule and the orbitals of the NaCl upper layer. To visualize the conduction channels, the evolution of the local density of states (LDOS) on the MgP molecule at the Fermi energy is investigated. The LDOS of the MgP molecule have π characters, consisting of the pz orbitals in both θ = 0 and 30° configurations as plotted in Figure 2e,g, respectively. The contours of the LDOS, plotted in Figure 2f,h, are along the cross sections indicated by the dashed lines in Figure 2e,g, respectively. The result implies that the orbital hybridization at the molecule−NaCl interface mainly tends to occur between the carbon atoms and the Cl ions in both configurations. For the LDOS of the bare NaCl under the tip, the density of states located at the Cl site are higher than those at the Na site, in 25130
DOI: 10.1021/acs.jpcc.5b08795 J. Phys. Chem. C 2015, 119, 25129−25133
Article
The Journal of Physical Chemistry C agreement with a previous study.17 Different spatial extension of the molecular pz orbitals toward the NaCl substrate gives an estimate for different transport barriers between two configurations. Upon rotating from θ = 0°, the pz orbital of the MgP molecule couples strongly with the orbitals of the substrate and gives a smaller barrier. This confirms that the molecular pz orbital governs the conduction. The conductance increases as the MgP molecule is rotated from θ = 0°, which is mainly due to the variation of the pz orbitals. Since the STM tip can move the molecule, the Mg atom (the center of the MgP molecule) adsorbed on the Na ion is also considered. After relaxations, the Mg atom is pushed upward from the molecular plane. The distance d1 between the tip apex atom and the Mg atom is changed from the initial value of 5.00 to 4.88 Å (θ = 14°; 0 eV) and 4.84 Å (θ = 30°; 0.073 eV). The transmission spectra shown in Figure 3a are also orientationdependent. The transmission coefficient of the θ = 30° configuration is higher than that of the θ = 14° one in the energy window between −0.015 and 0.015 eV. As a consequence, two distinct conductances, 2.39 × 10−6G0 (θ = 14°) and 1.87 × 10−5G0 (θ = 30°), are obtained. The PDOS of the NaCl upper layer of θ = 14−30° is weakened, as shown in Figure 3d, but the PDOS over the MgP orbitals of θ = 14−30° is enhanced, as plotted in Figure 3b,c. Thus, the order of the transmission coefficient is determined by the variation of the orbitals over the MgP molecule. The LDOS of the MgP molecule at the Fermi energy also shows π characters in both configurations, as shown in Figure 3e,g. Thus, the molecular pz orbitals provide the dominant contribution to conductance. The LDOS contours of Figure 3f,h indicate that the molecular pz orbitals extend further toward the NaCl substrate in the θ = 30° configuration than those in the θ = 14° one. This elucidates that the conductance of the MgP molecule increases, as the angle increases from 14 to 30°. Rather than being located above the Mg atom, the STM tip also can sit on other atoms. To explore the tip-locationdependent conductance, the MgP molecule and the NaCl bilayer shown in Figure 1b are translated together to the right bottom of the Au slab; then, the MgP molecule is rotated by θ = −8° (30°) so that the tip (marked by a big pink circle in Figure 4e−h) locates on the top (hollow) site of the bridge carbon atom (the pyrrole ring). As shown in Figure 4a, the transmission coefficient for the tip located on the top site of the bridge carbon atom is higher than that on the hollow site of the pyrrole ring. Different locations of the tip result in two conductance values: 1.20 × 10−5G0 (θ = −8°; 0.055 eV) and 6.17 × 10−6G0 (θ = 30°; 0 eV). The PDOS of the CT atom (marked by a small pink circle in Figure 4e−h), instead of the whole MgP molecule, are plotted in Figure 4b,c as the contours of the LDOS shown in Figure 4f,h; the hybridization between orbitals over the Cl ion and the pz orbitals of the CT atom provides the channels for electron to pass. Therefore, the transmission coefficients for different tip locations are determined by the overlap between orbitals of the CT atom and the Cl ion (in the second NaCl layer). Although DOS plays a role in shaping transmission spectra, the coupling strength between the molecule and the tip (substrate) Γm−tip (Γm−substrate), extracted from the calculated self-consistent Hamiltonian where only the molecular pz orbitals are involved, also contributes to the magnitude of conductance in this double-barrier tunneling junction. The wider vacuum barrier (Γm−tip), however, dominates the
Figure 3. (a) Transmission spectra as a function of rotational angle for the Mg atom on the Na ion. Projected density of states (PDOS) of the MgP molecule (b) on the s, px, and py orbitals and (c) on the pz orbitals. (d) PDOS of the NaCl upper layer. The black solid and red dashed lines are for θ = 14 and 30° configurations, respectively. Local density of states (LDOS) at the Fermi energy for (e) θ = 14° and (g) θ = 30° configurations are shown in brown. (f and h) Corresponding contour plots of the LDOS, along the cross sections indicated by the dashed lines in e and g, respectively.
conduction. As shown in Figure 1c, the conductance indeed correlates with Γm−tip (1.28 × 10−2eV, θ = 0°; 4.86 × 10−3eV, θ = 14°; 2.35 × 10−2eV, θ = 25°; 1.59 × 10−2eV, θ = 30°; and 1.64 × 10−2eV, θ = 45°). We note that the molecule with θ = 0° configuration has smaller PDOS on the pz orbital than others (Figure 2c), which can lead to the smaller conductance. For the cases of the Mg-on-Na and different tip location configurations, the conductance value also relates to Γm−tip (9.34 × 10−2eV, θ = 14°; 1.07 × 10−1eV, θ = 30°) and ΓCT−tip (5.14 × 10−4eV, θ = −8°; 5.34 × 10−5eV, θ = 30°), respectively.
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CONCLUSIONS We have investigated the electronic transport properties of the MgP molecule adsorbed on a NaCl bilayer above a gold 25131
DOI: 10.1021/acs.jpcc.5b08795 J. Phys. Chem. C 2015, 119, 25129−25133
The Journal of Physical Chemistry C
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the Ministry of Science and Technology, Taiwan, through grant no. 104-2112-M-001008-MY3, the National Center for Theoretical Sciences, Taiwan, and the NTU-AS Laker project. We thank Dr. Vladimir U. Nazarov for fruitful discussions.
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REFERENCES
(1) Pan, S.; Zhao, A. D.; Wang, B.; Yang, J. L.; Hou, J. G. Controlling Electronic States and Transport Properties at the Level of Single Molecules. Adv. Mater. 2010, 22, 1967−1971. (2) Brede, J.; Atodiresei, N.; Kuck, S.; Lazíc, P.; Caciuc, V.; Morikawa, Y.; Hoffmann, G.; Blügel, S.; Wiesendanger, R. Spin- and Energy-Dependent Tunneling through a Single Molecule with Intramolecular Spatial Resolution. Phys. Rev. Lett. 2010, 105, 047204. (3) Yang, H.-H.; Chu, Y.-H.; Lu, C.-I.; Yang, T.-H.; Yang, K.-J.; Kaun, C.-C.; Hoffmann, G.; Lin, M.-T. Digitized Charge Transfer Magnitude Determined by Metal-Organic Coordination Number. ACS Nano 2013, 7, 2814−2819. (4) Chu, Y.-H.; Hsu, C.-H.; Lu, C.-I.; Yang, H.-H.; Yang, T.-H.; Luo, C.-H.; Yang, K.-J.; Hsu, S.-H.; Hoffmann, G.; Kaun, C.-C.; Lin, M.-T. Spin-Dependent Molecule Symmetry at a Pentacene-Co Spinterface. ACS Nano 2015, 9, 7027−7032. (5) Hsu, C.-H.; Chu, Y.-H.; Lu, C.-I.; Hsu, P.-J.; Chen, S.-W.; Hsueh, W.-J.; Kaun, C.-C.; Lin, M.-T. Spin-Polarized Transport through Single Manganese Phthalocyanine Molecules on a Co Nanoisland. J. Phys. Chem. C 2015, 119, 3374−3378. (6) Wu, S. W.; Nazin, G. V.; Chen, X.; Qiu, X. H.; Ho, W. Control of Relative Tunneling Rates in Single Molecule Bipolar Electron Transport. Phys. Rev. Lett. 2004, 93, 236802. (7) Wu, S. W.; Ogawa, N.; Ho, W. Atomic-Scale Coupling of Photons to Single-Molecule Junctions. Science 2006, 312, 1362−1365. (8) Wu, S. W.; Ogawa, N.; Nazin, G. V.; Ho, W. Conductance Hysteresis and Switching in a Single-Molecule Junction. J. Phys. Chem. C 2008, 112, 5241−5244. (9) Yan, S. C.; Ding, Z. J.; Xie, N.; Gong, H. Q.; Sun, Q.; Guo, Y.; Shan, X. Y.; Meng, S.; Lu, X. H. Turning on and off the Rotational Oscillation of a Single Porphine Molecule by Molecular Charge State. ACS Nano 2012, 6, 4132−4136. (10) Maddox, J. B.; Harbola, U.; Mayoral, K.; Mukamel, S. Conductance Bistablilty in a Single Porphyrin Molecule in a STM Junction: A Many-Body Simulation Study. J. Phys. Chem. C 2007, 111, 9516−9521. (11) Dou, K. P.; Fan, W.; Niehaus, T. A.; Frauenheim, T.; Wang, C. L.; Zhang, X. H.; Zhang, R. Q. Electron Transport Suppression from Tip-π State Interaction on Si(100)-2 × 1 Surfaces. J. Chem. Theory Comput. 2011, 7, 707−712. (12) Sen, A.; Kaun, C.-C. Effect of Electrode Orientations on Charge Transport in Alkanedithiol Single-Molecule Junctions. ACS Nano 2010, 4, 6404−6408. (13) Soler, J. M.; Artacho, E.; Gale, J. D.; García, A.; Junquera, J.; Ordejón, P.; Sánchez-Portal, D. The SIESTA Method for Ab Initio Order-N Materials Simulation. J. Phys.: Condens. Matter 2002, 14, 2745−2779. (14) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (15) Taylor, J.; Guo, H.; Wang, J. Ab Initio Modeling of Quantum Transport Properties of Molecular Electronic Devices. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 245407. (16) Datta, S. Electrical Transport in Mesoscopic Systems; Cambridge University Press: Cambridge, U.K., 1997.
Figure 4. (a) Transmission spectra as a function of rotational angle for different tip locations. Projected density of states (PDOS) of the CT atom (b) on the s, px, and py orbitals and (c) on the pz orbitals. (d) PDOS of the NaCl upper layer. The black solid and red dashed lines are for θ = −8 and 30° configurations, respectively. Local density of states (LDOS) at the Fermi energy for (e) θ = −8° and (g) θ = 30° configurations are shown in brown. (f and h) Corresponding contour plots of the LDOS, along the cross sections indicated by the dashed lines in e and g, respectively. The tip locates on the top (hollow) site of the bridge carbon atom (the pyrrole ring) for the θ = −8° (θ = 30°) configuration.
substrate. The observed molecular-orientation- and tiplocation-dependent conductance can act as a electronic switch because of different conduction channels consisting of the molecular π orbitals with their spatial extension along the transport direction (particularly, the pz orbitals). The nature of a molecule on an insulating surface can be better demonstrated, whereas the correlation between its configurations and functions offers opportunities for designing molecular electronic devices. Moreover, understanding how electrons tunnel through an insulating film is the key to control the gate leaking current in a nanoscale transistor,18 and our results provide an insight into this issue of nanoelectronics. 25132
DOI: 10.1021/acs.jpcc.5b08795 J. Phys. Chem. C 2015, 119, 25129−25133
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The Journal of Physical Chemistry C (17) Hebenstreit, W.; Redinger, J.; Horozova, Z.; Schmid, M.; Podloucky, R.; Varga, P. Atomic Resolution by STM on Ultra-Thin Films of Alkali Halides: Experiment and Local Density Calculations. Surf. Sci. 1999, 424, L321−L328. (18) Kaun, C.-C.; Seideman, T. The Gating Efficiency of SingleMolecule Transistors. J. Comput. Theor. Nanosci. 2006, 3, 951−956.
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DOI: 10.1021/acs.jpcc.5b08795 J. Phys. Chem. C 2015, 119, 25129−25133
