Electron Transport in Butane Molecular Wires with Different Anchoring

J. Phys. Chem. C 2009, 113, 21911–21914

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Electron Transport in Butane Molecular Wires with Different Anchoring Groups Containing N, S, and P: A First Principles Study X. Y. Feng, Zhenyu Li,* and Jinlong Yang* Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui, 230026, China ReceiVed: August 29, 2009; ReVised Manuscript ReceiVed: NoVember 19, 2009

Using butane molecular wire as an example, based on density functional theory and the nonequilibrium Green’s function technique, we study the effect of anchoring groups on the transport properties of the corresponding molecular junctions. Consistent with available experimental data, we observe a conductance increase from amine to sulfide and phosphide anchoring groups. This behavior can be understood with the tunneling barrier model, where the p orbital energy of N, S, or P determines the energy of the highest occupied molecular orbital and thus the barrier height. Our results demonstrate the critical role of anchoring group chemistry in molecular electronics. 1. Introduction Because of rapid progress in microscale fabrication technology, molecular electronics has attracted more and more attentions recently. It is very desirable to construct electronic devices using single molecules,1,2 whereas there is a big challenge to make a proper linkage between a single molecule and a much larger electrode. Typically, an anchoring group is used to make such a connection.3-7 Frequently used anchoring groups include thiol,8-13 amine,12-18 carboxylic acid,19 sulfide,16,17 phosphide,16,17 dithiocarbamate,20,21 and dithiocarboxylate22,23 groups. The amide group has also been used as an anchoring group when carbon nanotubes are used as the electrode.24-27 The transport properties of a molecular junction may strongly depend on the anchoring groups used to connect the molecule with electrodes.9,16,17,19,21,23,28-31 Geometrically, an anchoring group provides a connection between the molecule and electrode. With similar binding strengths, the connection with stronger orientation preference leads to a more reliable and reproducible molecular junction. For example, a molecular junction with an amine anchoring group is more stable in conductivity than those with a thiol group,31 because the amine group prefers to adsorb on an undercoordinated apex atom (adatom) on the surface, while the thiolated group can be attached equally well to the undercoordinated atoms and clean surface of the electrodes.9 Physically, an anchoring group also provides a coupling between the connected molecule and electrode. The stronger the coupling, the larger the mixing between discrete molecular levels and the continuum of metal electronic states and the larger the broadening of resonances near the Fermi energy in the electronic transmission probability. For example, first predicted theoretically21 and then confirmed experimentally,20 the conjugated thiol anchoring group (dithiocarbamate) is able to introduce stronger molecule-electrode coupling than that of the widely used thiol group, thus leading to conductance enhancement. Chemically, the electronic structure of an anchoring group itself may be manifested into the transport property of the corresponding molecular junction. The conductance enhancement of dithiocarboxylate-terminated molecular junctions com* Corresponding authors. E-mail: [email protected]; [email protected].

Figure 1. Schematic structure of the studied molecular junctions. Yellow: gold atoms; blue: anchoring groups; gray: C atoms; white: H atoms.

pared to that of thiol linked junctions22 is exactly based on such an anchoring group chemistry. First principles calculations indicate that orbital interaction between the dithiocarboxylate group and the molecular wire leads to a resonant peak very close to the Fermi level of the Au electrode.23 Because of its importance for electron transport in molecular junctions, anchoring groups have attracted a broad research interest. Recently, Venkataraman and co-workers have measured conductance of alkane molecular wires terminated with amine (NH2), methyl sulfide (SCH3), and dimethylphosphine (P(CH3)2) groups;16,17 the junction with dimethylphosphine has the highest conductance. All three anchoring groups provide similar geometric linkage motifs with lone-pair electrons bound to undercoordinated Au atoms, and the observed conductance difference most probably comes from the different N, S, and P chemistries. To understand the experimental results, in this article, we systematically study the transport properties of butane molecular wire connected to two gold electrodes with these different anchoring groups (Figure 1). Three similar anchoring groups not considered in the experiment, dimethylamine (N(CH3)2), hydrogen sulfide (SH), and phosphine (PH2), have also been studied. Our calculations show that the relative energy of the p orbitals of N, S, and P in these anchoring groups strongly affects the conductance of the molecular junctions.

10.1021/jp908347s  2009 American Chemical Society Published on Web 12/07/2009

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Feng et al.

TABLE 1: The Au-X (X ) N, S, P) Bond Length (Å), Transmission, Energy (eV) of the Highest Occupied MPSH Orbital (HOMO), and Charge Transfer (CT) in Unit Charge anchoring group

Au-X

conductance

εHOMO

CT

N(CH3)2 SCH3 P(CH3)2 NH2 SH PH2 NH2(3 × 3)

2.216 2.376 2.296 2.216 2.400 2.296 2.210

0.00129 0.00311 0.00482 0.00227 0.00304 0.00738 0.00176

-1.20 -1.10 -0.54 -1.15 -1.02 -0.66 -1.34

0.075 0.161 0.209 0.170 0.163 0.378 0.266

2. Computational Details Density functional theory (DFT) calculations were performed to describe electronic structures, using the SIESTA package.32 The Kohn-Sham equations were solved with numerical atomic basis sets: the single-ζ with polarization (SZP) basis set for Au and the double-ζ with polarization (DZP) basis set for all other atoms. The exchange-correlation potential was described by Perdew-Zunger local density approximation (LDA).33 Normconserving pseudopotentials were used to describe the interaction between ions and valence electrons. Geometry optimization was performed with a force tolerance of 0.02 eV/Å. A mesh cutoff of 150 Ry and an electronic temperature of 300 K were adopted. The nonequilibrium Green’s function method implemented in the ATK software34 was used for transport property calculations. Geometry structure of a molecular junction was obtained by the following two steps. First, we optimized the R-C4H9 molecule adsorbed on the Au (111) surface, where R is one of the anchoring groups studied in this work. The surface was modeled by a five-layer thick 4 × 4 slab, of which the lowest two layers were fixed to its bulk coordinates during the geometry optimization. With a lone pair on R, the R-C4H9 molecule prefers to adsorb at an apex Au atom on the surface (Figure 1), as also suggested by our previous results on the amine group-anchored benzene molecule.9 Among the optimized structures of all R-C4H9 molecules, the lone pair electrons extend toward the apex Au atom. With this common feature, from different initial geometries, we could still obtain different orientations. For example, the alkane chain can be parallel or perpendicular to the Au-Au bond direction. However, the energy differences for different orientations are very small (typically smaller than 0.1 eV), and the transmission spectrum of the corresponding molecular junction is nearly not affected. With the optimized lowest-energy adsorption geometry, a molecular junction can be constructed by an inversion operation relative to the center of the butane. The obtained molecular junction was then optimized with the electrode-electrode distance fixed. Our test calculations indicate that a small change of electrode-electrode gap distance almost does not affect the transport properties. All atoms between the two vertical dashed lines in Figure 1 were fully relaxed. 3. Results and Discussion First, we consider the following three anchoring groups: dimethylamine, methyl sulfide, and dimethylphosphine. In Table 1, the lengths of Au-X (X ) N, S, and P) bonds in the three optimized molecular junctions are listed. The Au-S bond is the longest and then follows that of Au-P and Au-N. Such a result is in accord with the radius of the X (X ) N, S, P) atom. Transmission spectra of the three molecular junctions with different anchoring groups are plotted in Figure 2a. Conductance G can be obtained from the transmission coefficient T by G )

Figure 2. (a) The zero-bias transmission spectra of dimethylamine (black), methyl sulfide (red), and dimethylphosphine (blue). (b) Corresponding self-consistent current-voltage curves.

Figure 3. The MPSH orbitals which dominate the electron transport properties of the three junctions. The eigen-energies in eV are marked below the corresponding orbitals. From top to bottom: dimethylamines, methyl sulfides, and dimethylphosphines.

G0T(EF), where G0 ) 2e2/h is the quantum of conductance.35 The dimethylphosphine anchoring group gives the highest conductance and then follows methyl sulfide and dimethylamine, which is in accord with the experimental observation.16 Current-voltage (I-V) curves (Figure 2b) are obtained by integrated self-consistent transmission spectra at different bias voltages in the energy window from -V/2 to V/2. Transmission under different external bias voltages are very similar in the integration energy window, because there is no strong transmission peak there. As a result, the NEGF I-V curve is featureless. However, the conductance order for the three anchoring groups can also be obtained by comparing the slopes of I-V curves, which is consistent with that obtained from transmission curves at zero bias voltage. To extract more information from the transmission spectra, we project the self-consistent Hamiltonian to the molecular wire (including the two apex Au atoms). The obtained Hamiltonian is called molecular projected self-consistent Hamiltonian (MPSH), which is defined in the space spanned by basis functions of the center molecule but contains information of electrodes. The eigenstates of MPSH can be considered as molecular orbitals renormalized by the molecule-electrode interaction. The corresponding energies of such renormalized orbitals are marked in Figure 2a. A well delocalized MPSH orbital generally leads to a transmission peak. In Figure 3, we plot the highest occupied MPSH orbital (HOMO) and HOMO-1. These two orbitals are close to the Fermi energy, and they are mainly contributed by a hybridization of the p orbital of X (X ) N, S, P) and the 5d orbital of the two apex Au atoms. There is also a small contribution from the C4H8

Electron Transport in Butane Molecular Wires

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Figure 4. The zero-bias transmission spectra of amine (black)-, hydrogen sulfide (red)-, and dihydrogen phosphine (blue)-anchored molecular junctions.

Figure 5. The zero-bias transmission spectra of the amine-anchored group junctions. Black: 3 × 3 unit cell Au electrode; red: 4 × 4 unit cell.

molecular wire in HOMO and HOMO-1. Their energy differences are small, because the difference between these two orbitals mainly comes from the small contribution by the C4H8 molecular wire. The energy order of HOMO is consistent with the p orbital energy of N, S, and P atoms (-7.25, -7.13, and -5.62 eV, respectively). The higher HOMO leads to stronger interaction between apex Au and the anchoring group, and the higher conductance. A stronger interaction between Au and P than that between Au and N is also supported by the charge transfer (CT) between Au electrode and the molecular wire. CT listed in Table 1 is calculated as the change of the total charge on both Au electrodes (including the two apex Au atoms and Au atoms of the surface layers) compared to the sum of neutral atomic values. Positive values mean that there are electron transfers from the X (X ) N, S, P) atoms to Au electrodes. Our calculations indicate that CT between Au and P is the largest. In the experiment,16 amine was used instead of dimethylamine. It is thus interesting to study the effects by substituting CH3 in the previous three anchoring groups with H, which leads to another three junctions with amine, hydrogen sulfide (thiol), and phosphine anchoring groups. It is still under debate if dehydrogenation will happen when thiol adsorbs on a gold surface;36-39 for simplicity, this issue is not discussed here. In the three CH3 substituted junctions, the Au-X (X ) N, S, P) bond length almost does not change compared to the previous three junctions. The zero bias transmission spectra in the energy window [-4,4] eV of these three junctions are plotted in Figure 4. They are overall very similar to those of the previous CH3 attached junctions. Consistently, HOMO and HOMO-1 MPSH orbitals are very similar in the methyl and the hydrogen attached cases. The conductance (transmission at zero) of the phosphineanchored junction is still the largest and then follows that for hydrogen sulfide and amine. For both N and P, substituting CH3 with H leads to a conductance enhancement. However, the conductance for S-based anchoring groups is not very sensitive to such a substitution. This may be caused by the two lone-pairs of S compared to one lone-pair of N and P. The change of conductance is consistent with the change of CT (Table 1). In our junction model, interactions between neighboring molecular wires are expected to affect the transport properties. For the smallest amine anchoring group, we also consider a 3 × 3 Au electrode model. As shown in Figure 5, a notable difference between transmissions from different electrode models can be observed. The conductance of the 3 × 3 junction is smaller than that of the 4 × 4 junction. Because individual molecular wires are expected in the experiment,16 the 4 × 4 junction is a more reasonable model to simulate STM break

junction experiments. Nevertheless, we notice that in all seven junctions we studied here, the calculated conductances can be divided into three groups according to their magnitude, corresponding to N, S, and P contained anchoring groups, respectively. Modifications on side groups and the electrode model do not change such a conductance ordering. For staturated hydrocarbon molecular wire, the transport properties can be described by the tunneling barrier model40

[


I ∝ exp -2l

2

]

m* Φb p2

(1)

where l is the length of the molecular wire, m* is the effective mass of the carriers, and Φb is the barrier height. The lengths of the molecule are similar for all seven junctions. Since the HOMO is much closer to the Fermi level than the lowest unoccupied MPSH orbital for all junctions considered here, their transport is hole dominated, and the relative position of HOMO compared to the electrode Fermi level can be used to approximate the tunnel barrier height Φb. Actually, the conductance and HOMO energy values (listed in Table 1) from our calculations qualitatively agree well with the tunneling barrier model. 4. Conclusions In conclusion, we have calculated transport properties of butane molecular wire with different anchoring groups, which play a very important role in molecular electronics. P-based anchoring groups lead to higher conductance compared to Sand N-based anchoring groups. This is originated from the different p orbital energies of N, S, and P. The different anchoring group chemistries then lead to different coupling strengths between the molecule and the gold electrodes, as indicated by the different CT values. The HOMO level alignment can also give a rough indicator to the barrier height that an electron should overcome when tunneling through the molecular wire. Both the chemical group (methyl or hydrogen) connected to the N, S, or P atom and the electrode model can slightly affect the conductance, but the N-, S-, and P-based anchoring group chemistry dominates. Acknowledgment. This work is partially supported by the National Natural Science Foundation of China (20933006, 20803071, 50721091, 20873129), by Ministry of Education (FANEDD-2007B23, NCET-08-0521), by National Key Basic Research Program (2006CB922004), by the USTC-SCC project, and by the SCCAS and Shanghai Supercomputer Center. References and Notes (1) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384. (2) Joachim, C.; Ratner, M. A. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8801.

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