ARTICLE pubs.acs.org/JPCA
First-Principles Study of Rectification in Bis-2(5-ethynylthienyl)ethyne Molecular Junctions Shundong Yuan,†,‡ Shiyan Wang,‡ Qunbo Mei,† Qidan Ling,*,†,§ Lianhui Wang,†,|| and Wei Huang† †
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Jiangsu Key Laboratory of Organic Electronics & Information Displays and Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210046, People's Republic of China ‡ College of Physics Science and Technology, China University of Petroleum, Dongying 257061, People's Republic of China § Fujian Key Laboratory of Polymer Materials and College of Chemistry and Materials Science, Fujian Normal University, Fuzhou 350108, People's Republic of China Laboratory of Advanced Materials, Fudan University, 2205 Songhu Road, Shanghai 200438, People's Republic of China
bS Supporting Information ABSTRACT: Using density functional theory (DFT) combined with the first-principles nonequilibrium Green’s function (NEGF), we investigated the electron-transport properties and rectifying behaviors of several molecular junctions based on the bis-2-(5-ethynylthienyl)ethyne (BETE) molecule. To examine the roles of different rectification factors, asymmetric electrodemolecule contacts and donoracceptor substituent groups were introduced into the BETE-based molecular junction. The asymmetric currentvoltage characteristics were obtained for the molecular junctions containing asymmetric contacts and donoracceptor groups. In our models, the computed rectification ratios show that the mode of electrodemolecule contacts plays a crucial role in rectification and that the rectifying effect is not enhanced significantly by introducing the additional donoracceptor components for the molecular rectifier with asymmetric electrodemolecule contacts. The currentvoltage characteristics and rectifying behaviors are discussed in terms of transmission spectra, molecular projected self-consistent Hamiltonian (MPSH) states, and energy levels of MPSH states.
1. INTRODUCTION Rectification is one of the most attractive features in the field of molecular electronics for its potential application in logic circuits, organic memory, and so on. The first suggestion for a molecular diode is the AviramRatner proposal1 that a unimolecular rectifier could be obtained by a DσA molecule, where D is an electron donor, A is an electron acceptor, and σ is a saturated covalent sigma bridge. The σ bond acts as an insulating barrier for the electron transfer between the wires. When the DσA molecule is placed between two metallic electrodes, the electron transfer is more favorable from A to D, rather than from D to A; thus, rectification of current occurs. Inspired by the AviramRatner proposal, plenty of experimental26 and theoretical712 investigations were performed over the past decades to explore molecular rectification. Now, some foundational topics, such as the rectification mechanism and rectification process, have been clarified in some investigations,3,13 and some other reasonable views14,15 on molecular rectification have also been suggested. The rectifying behavior might have different origins, such as built-in asymmetry of the molecular structure (e.g., DσA and DπA molecules),2,3,7,9,12,14,1618 asymmetric anchoring groups or electrodemolecule contacts,10,15,1924 suitable electrode materials,2527 and changes in molecular conformation induced by an external electric field.2830 Generally, the basic r 2011 American Chemical Society
way to yield molecular rectification is to introduce asymmetry into the molecular device. The rectifying behaviors caused by introducing different rectification factors into one certain pristine molecule system can be significantly different. For one molecular rectifier, it is expected that the rectification effect can be enhanced by modifying the components of the rectifier. One possible approach is to introduce different rectification factors into the device simultaneously. However, the role of only one rectification factor is the focus in most experimental and theoretical investigations on molecular rectifiers, especially for rectifiers based on short-chain molecules. There are still few reports in which two or more factors are focused coequally in one molecular rectification device to examine whether the rectifying behavior can be influenced significantly. Therefore, it is necessary to discuss the roles of different rectification factors in the rectifying behavior of one particular molecule system. A molecular junction based on a single molecule is a promising subject in the investigation of molecular electronics because it is most likely to be applied to a practical functional device in the future. In experiments, some advanced implements and techniques, Received: May 5, 2011 Revised: June 17, 2011 Published: June 30, 2011 9033
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including scanning tunneling microscopy (STM), conducting probe atomic force microscopy (CP-AFM),31 and mechanically controllable break junctions (MCBJs),32,33 have been employed to prepare molecular junctions and measure their electrical properties. Great efforts have also been made to study singlemolecule junctions.3442 However, there are still great challenges to obtaining perfect electrodemolecule contact because of the ultrasmall size. The corresponding measurements might have some uncertainties. It is assumed that theoretical simulations can offer useful help in investigating the electrical properties of single-molecule junctions under controllable and ideal conditions. Therefore, in this work, we carried out a theoretical investigation of molecular rectification in which asymmetric electrode molecule contacts and donoracceptor groups were introduced into one particular single-molecule rectifying model separately or simultaneously. The computational method was based on the first-principles nonequilibrium Green’s function (NEGF) combined with density functional theory (DFT), which has been employed extensively in the theoretical studies of the electrontransport properties in molecular junctions.12,15,4348 The coupled DFT-NEGF method has been described in detail in some articles.43,44 It is considered to be highly reliable for studying electron-transport properties. Moreover, its reliability has been demonstrated in some works, in which the calculated results have reasonably reproduced experimental data.4951 The prototype molecule in this work was bis-2-(5-ethynylthienyl)ethyne (BETE), a short segment of oligo(2,5-thiophene ethynylene) (OTE) molecular wire. Theoretical calculations employing larger molecule usually encounter computational difficulties, such as complicated models and longer calculation times. Conjugated electron-transport materials based on thiophene moieties are good candidates for applications in the field of organic electronics.52 They are used frequently by researchers all over the world because of the stability of the conjugated chain, excellent charge-transport properties, and processability, among other features. The ethynylene moiety makes the OTE molecule a rigid-rod conjugated molecular wire that can serve in current promising studies of molecular devices. Pearson and Tour53 reported the synthesis of a series of OTE molecules, and they also prepared monothiol-terminated systems and dithiol-terminated systems so that these oligomers can adhere to the surface of a metal (Au, Al, Li, etc.) for the study of molecular electrical conduction. Here, thiols act as so-called “alligator clips” for adhesion to the surface of the metal.
Figure 1. Schematic illustrations of molecular junctions ae: thiolateended molecules self-assembled on the Au(111)(3 3) surface. Yellow, gray, white, blue, and red balls in the central region represent S, C, H, N, and O atoms, respectively. The dashed frame part in system a is defined as the scattering region.
2. COMPUTATIONAL DETAILS Figure 1 illustrates the models of molecular junctions ae with metal/molecule/metal structures. In each so-called two-probe system, a thiolate-ended molecule based on BETE is sandwiched between two gold electrodes. The thiolate end group is employed widely in the field of molecular devices.5,10,20,24,54,55 Molecules containing thiol end groups can be self-assembled on the Au substrate because the hydrogen atom in the thiol group will be dissociated and strong AuS covalent bonds will form when the thiol group interacts with Au surface. In the present work, five different two-probe systems were investigated. The molecule in the central region of system a is the dithiolate-terminated BETE. The two thiolate end groups can offer symmetric electrode molecule contacts. The molecules in systems b, d, and e are monothiolate-terminated to introduce asymmetric electrode molecule contacts. Nitro and amino groups were introduced into the molecules as acceptor and donor functional components in
systems ce, thus forming three DπA molecules. Moreover, the nitro group was located on the right thiophene moiety, and the amino group was located on the left thiophene moiety. We considered that the roles of asymmetric contacts and donor acceptor groups in the rectifying behaviors of BETE-based molecular wires could be understood by comparing the transport properties of these different molecules. The whole computation was composed of two main procedures. The first procedure was geometry optimization. First, the geometry optimizations of the isolated molecules in the central region in Figure 1 were performed using the Gaussian 03 program56 at the hybrid DFT/B3LYP57,58 level of theory with the 6-31G(p,d) basis set. Second, two gold clusters were added separately to the two sides of the optimized isolated molecule. Each gold cluster consisted of three Au atoms that were arranged as an equilateral triangle with an AuAu bond length of 2.88 Å to model the Au(111) surface. According to the preceding discussion, the terminal sulfur 9034
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The Journal of Physical Chemistry A atoms were bonded directly to the gold clusters. The terminal hydrogen atom was not dissociated in the case of nonthiol contact in our models. The isolated molecule and the two gold clusters represent an extended molecule. The terminal sulfur/hydrogen atoms were located on top of the center of the gold triangle, that is, at a hollow site, which agrees with the result suggested by Gr€onbeck et al.59 The particular distance between the gold triangle and the terminal sulfur (or hydrogen) atom was obtained by examining the total energies of the extended molecule as a function of different distances. The optimal distance corresponds to the lowest total energy. In this work, the optimal distance was 2.35 Å. Third, the relative positions of the gold atoms in each gold cluster and the AuS (or AuH) distance were fixed. Then, the geometries of the extended molecules were relaxed completely using the Gaussian 03 program56 at the hybrid DFT/B3LYP57,58 level of theory with the 6-31G(p,d) basis set for all atoms, except for Au atoms, which used the LANL2DZ basis set. The next procedure was the transport computation after the above geometry optimizations. The geometries of the isolated molecules were extracted from the optimized extended molecules and then translated into the central region between the two gold electrodes, as illustrated in Figure 1. The two Au(111)(3 3) surfaces (i.e., each layer consisting of nine gold atoms) with periodic boundary conditions were used to model the left and right electrodes. The Au/molecule/Au configuration was divided into three parts: left electrode, right electrode, and central scattering region. In our models, there were three gold layers in each the left and right electrode unit cells. The scattering region was composed of the isolated molecule together with the respective two gold layers on the left and right sides. The distance between the Au(111) surface and the terminal S (or H) atom, which was located at the hollow site of the Au surface, was 2.35 Å. The electron-transport properties of the Au/molecule/Au systems were investigated using the ATK 2008.10 program,60 which is based on density functional theory (DFT) combined with the first-principles nonequilibrium Green’s function (NEGF).43,61 In this work, a double-ζ with polarization (DZP) basis set was used for all atoms of the molecule, and a single-ζ with polarization (SZP) basis set was used for Au atoms. The PerdewZunger local density approximation (LDA.PZ)62,63 was chosen to describe the exchangecorrelation potential. The convergence criterion was set to 1 105 for grid integration to obtain accurate results. For the Brillouin zone integration parameters in the directions along the Au(111) surface (x and y directions), our previous work suggested that 2 2 k-point sampling for self-consistent calculation and 4 4 k-point sampling for transmission calculation are the most favorable combinations for more accurate results and less computational time.64 A k-point sampling of 500 was used in the electron-transport direction (z direction). The currentvoltage (IV) characteristics were obtained from the LandauerB€uttiker formula6567 Z 2e2 μR IðVb Þ ¼ ½ f ðE μL Þ f ðE μR ÞTðE, Vb Þ dE ð1Þ h μL where 2e2/h is the conductance quantum G0, h is the Planck constant, e is the elementary charge, and f is the Fermi function. μL and μR are the electrochemical potentials of the left and right electrodes, respectively: μL(Vb) = EF eVb/2, and μR(Vb) = EF + eVb/2. EF is the Fermi energy of the electrode. The energy range [μL(Vb), μR(Vb)] contributing to the current integral was defined as the bias window. T(E,Vb) is the total transmission probability
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Figure 2. IV curves of systems ae in the bias range from 3.0 to +3.0 V. The positive current means that the current flows from the left electrode to the right electrode and vice versa.
for an electron incident at energy E through the molecular junction under potential bias Vb.
3. RESULTS AND DISCUSSION 3.1. IV Characteristics and Rectification. The computed currentvoltage (IV) curves of the five two-probe systems ae are plotted in Figure 2. It is evident that the currents of the dithiolate-terminated systems a and c were over an order of magnitude larger than those of the monothiolate-terminated systems b, d, and e. The large difference should be attributed to different moleculeelectrode coupling strengths. It is known that the coupling strength affects the overlapping of electronic states between electrode and molecule. In the monothiolateterminated systems b, d, and e, the AuH coupling was very weak, and the electronic state on the terminal H atom overlapped slightly with that on the Au electrode. Therefore, transmission from electrode to molecule (or from molecule to electrode) was difficult. In the dithiolate-terminated systems a and c, however, the two strong AuS couplings in each system could well afford overlapping of the electronic states on both sides; thus, the electron transfer between molecule and electrode was more efficient. As a result, the currents in monothiolate-terminated two-probe systems were lower than those in dithiolate-terminated systems. Our computational results are also in agreement, to some extent, with the experimental measurements by Cui et al.68 and the theoretical results of Taylor et al.,10 which state that a molecular junction with two chemical bond contacts is several orders of magnitude less resistive than a junction with only one chemical bond contact. The difference can also be demonstrated by the transmission spectra of the two-probe systems at zero bias voltage. The transmission coefficient at a given energy E corresponds to the transmission probability of an electron incident at this energy E through the two-probe system. Figure 3 illustrates the transmission spectra of the five systems at zero bias voltage. The results show that the transmission coefficients of systems a and c at the Fermi level were over 0.5. In contrast, for systems b, d, and e, the transmission coefficients were less than 0.02, which is due to the presence of a tunnel barrier between the Au electrode and the terminal H atom. Apparently, the difference in transmission coefficients led to the difference in currents. 9035
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Figure 3. Transmission spectra of systems ae at zero bias voltage. The black, red, green, blue, and magenta lines indicate systems ae, respectively. The energy origin is set to be the Fermi level of the gold electrode.
Figure 4. Rectification ratio (R) as a function of applied bias voltage for systems ae. The inset shows the rectification curves: 1/R for system d and R for system e.
In Figure 2, an asymmetry in the IV curves is evident for all systems except system a. For system a, in Figure 1, the central molecule is geometrically symmetric, and the two electrode molecule contacts on the left- and right-hand sides are equivalent. Therefore, the IV curve of system a is symmetric about zero bias voltage. In systems be, the asymmetric IV curves can be attributed to the asymmetric configurations of these two-probe systems, such as the intrinsically asymmetric DπA molecule and the asymmetric electrodemolecule contacts. The rectification ratios of all of the systems were computed, so that the detailed characteristics of the asymmetric currents could be compared. The rectification ratio is defined as Ið þ V Þ ð2Þ RðV Þ ¼ Ið V Þ
weak-coupling side. Because the two semi-infinite electrodes can be regarded as two electron reservoirs, the electron prefers transferring from the right electrode to the terminal S atom rather than from the left electrode to the terminal H atom. Zhao et al.22 investigated the rectification of contact-asymmetrical molecular junctions based on the porphyrin molecule. In their models, the favored direction of electron transport was from the one-thiol electrode to the two-thiol electrode. That is, the electron again favored transfer from the weak-coupling side to the strong-coupling side. A similar case was also found in the work of Li et al.15 However, Deng et al.26 reported the opposite result that, in contact-asymmetrical molecular junctions using Au electrodes based on the terphenyl molecule, the electron was found to favor transfer from the two-thiol electrode to the one-thiol electrode. The inconsistency might originate from the intrinsic properties of the molecular junctions, such as the built-in molecular electronic structure and the moleculeelectrode separation. Hao et al.69 investigated the electron-transport properties of the AuS(CH2)7CH3Au molecular junction theoretically. Their results suggested that different separations between the terminal CH3 group and the electrode can give rise to contrasting rectifying effects, that is, opposite preferential transfer directions for the electron. Perhaps, their suggestion can be used to explain the above inconsistency. The central molecule of system c was dithiolate-terminated, and nitro and amino groups were introduced. The rectification ratio of system c was slightly below 0.6 V (i.e., 1 means that the current in the positive direction is larger than that in the negative direction, and vice versa. Figure 4 shows the dependence of the rectification ratio on the applied bias voltage. For the five systems, the rectifying behaviors showed clear differences. The rectification ratio of system a remained constant at R = 1, as a result of the symmetric configuration. In system b, the variation of the central molecule compared to system a was only from a dithiolate end to monothiolate end. This variation reduced the magnitude of the current, but at the same time, evident rectification was obtained. The rectification ratio, R(V), of system b increased as the bias voltage was increased from 0 to 1.4 V and reached a maximum of 5.2 at 1.4 V. Then, R(V) decreased with increasing bias voltage. The magnitude of R(V) was less than 1 in the bias range of 2.02.4 V, and the minimum was about 0.7 at 2.2 V. Subsequently, the rectification ratio became greater than 1 again above 2.5 V. Generally, the asymmetric electrodemolecule contacts can bring about an evident rectification for the BETE molecule. It should be noted that, in system b, the right electrodemolecule contact was thiolate-bonded, and the left electrodemolecule contact was nonthiolate-bonded. Considering the characteristic of R(V) > 1 on the whole in system b, it seems that the current occurred in the direction from the nonthiolated molecular end to the thiolated molecular end rather than in the opposite direction. This means that, in our models, the preferential transfer direction for the electron is from the strong-coupling side to the
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The Journal of Physical Chemistry A showed a change tendency opposite to that of system b: R(V) < 1 in the bias range of 01.4 V and R(V) > 1 in the bias range of 1.62.6 V. The maximum rectification ratio in system d was 3.3 at 1.8 V, and the minimum was about 0.24 at 0.8 V. In fact, the reciprocal of the rectification ratio, that is, 1/R = |I(V)/I(+V)|, can also describe rectifying behavior. Therefore, the actual maximum of R(V) in system d was 4.2 at 0.8 V. The configurations of systems e and d differed only in the electrodemolecule contacts. The contacts in system e were same as those in system b. In Figure 4, the rectification curve of system e is also very similar to that of system b, and the rectifying effect of the former is slightly greater that of the latter. This result demonstrates that the mode of electrodemolecule contacts plays a crucial role in the rectification in our models and that the rectifying effect is not enhanced significantly by introducing additional donoracceptor components for a molecular rectifier with asymmetric electrodemolecule contacts (AuS versus AuH). A comparison of the rectifying effects in systems d and e with that in system c also demonstrates that the use of asymmetric electrodemolecule contacts is an efficient method to obtain molecular rectification, in agreement with the above suggestion. This conclusion might be helpful for the design of molecular rectification devices. According to the comparison of the transport properties of systems d and e, one can understand simply the influence on the rectifying behavior of the different configurations in which donor acceptor substituent groups and asymmetric electrodemolecule contacts coexist in a two-probe system. The inset in Figure 4 shows 1/R for system d and R for system e against the applied bias voltage. Apparently, the rectifying effect of system e was more prominent than that of system d throughout most of the bias range. This result suggests that the relative positions of the donor and acceptor groups in a molecular backbone that contains two asymmetric termini can influence the molecular rectification. In system e, the electron-withdrawing group (NO2) was located close to the terminal S atom, and the electron-donating group (NH2) was located close to the terminal H atom. The electronwithdrawing function of the nitro group made the electron transfers from the electrode to the S atom through AuS bond easier. Here, the nitro group acted as a “pump”, and the electron was extracted from the “reservoir” (electrode) by the nitro group and then injected into the molecule. However, the nitro group was located close to the terminal H atom in system d, and its effect was not as prominent as that in system e. As a result, the rectification effect in system e was better than that in system d. The comparison of the rectification behaviors between systems e and b can also be interpreted in the same way. 3.2. Transmission Spectrum. According to the Landauer B€uttiker formula shown in section 2, the current is directly dependent on the transmission amplitude. Therefore, the current flowing through a two-probe system is determined by the transmission spectra within the bias window [μL(Vb), μR(Vb)]. The region of the bias window will be [Vb/2, +Vb/2] when the Fermi level is set to zero. Generally, the transmission is determined by the molecular electronic structure, the moleculeelectrode coupling, and so on. The IV characteristics and the rectifying behaviors of the five systems can be understood through an analysis of their transmission spectra. Figure 5 depicts the transmission spectra of all of the systems in the energy range from 2.0 to +2.0 eV under a bias of 3.0 to +3.0 V. Obviously, the transmission peaks within the bias window in systems a and c are higher and broader than those
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in the other systems, and this resulted in the large difference in electric currents, just as shown in Figure 2. For system a, there are four main transmission peaks in the energy range from 2.0 to +2.0 eV, including three peaks below the Fermi level and one peak above the Fermi level. The transmission spectra at positive and negative biases are symmetric and make equivalent contributions to the respective current integrals. Therefore, as discussed in the preceding section, the IV curve of system a is symmetric about zero bias, and the rectification ratio remains constant at R = 1. In system b, the transmission spectra within the bias window were weak. The tiny peak below the Fermi level remained inside the bias window throughout the positive bias range, whereas it shifted toward the lower-energy region with rising bias in the negative bias range. It was located at the edge of the bias window and never entered the bias window completely. Moreover, no other peak entered the bias window in the bias range from 1.8 to +1.8 V. Consequently, the currents at biases of 0(+1.8) V were larger than those at biases of 0(1.8) V. As a result, the rectification ratio was clearly greater than 1 at biases of 01.8 V, as shown in Figure 4. The first peak above the Fermi level entered the bias window above 2.0 V, and it resulted in a decrease in the rectification ratio to R < 1 above 2.0 V, until the two peaks at about (1.4 eV entered the bias window above +2.6 V, resulting in R > 1 again. The transmission spectra of system c do not vary much in comparison with those of system a. The distinct difference between the two systems is in the first peak below the Fermi level, which is the main peak contributing to the current integral. It is evident that the peak in system c was not symmetric about zero bias. In most cases, the transmission peaks within the bias window at Vb > 0 V were not only higher but also broader than those at Vb < 0 V. Therefore, the corresponding rectification ratios of system c were greater than 1 throughout most of the bias range. For systems d and e, the transmission peaks within the bias window were not prominent, similarly to that in system b. In system d, the transmission within the bias window at 0(1.4) V was stronger than that at 0(+1.4) V, and the latter was very weak. This resulted in R < 1 in the bias range from 0 to 1.4 V. The very prominent peak above the Fermi level entered the positive bias window when the bias was above 1.6 V and made the rectification ratio greater than 1, until that peak and another peak located at about 1.4 eV entered the negative bias window near 3.0 V. The transmission within the bias window in system e was similar to that in system b. Therefore, as illustrated in Figure 4, the rectification curve of system e shows change tendencies similar to those of system b. 3.3. Molecular Projected Self-Consistent Hamiltonian. The orbitals of the molecule in the scattering region can be modified by electrodemolecule interactions. The resonance peaks in transmission spectra are related to these molecular orbitals. It is therefore necessary to identify which orbitals on the central molecule are involved in the transmission, so that the features of transmission spectra can be understood and the rectifying behavior can be further interpreted. In the present work, a method that was demonstrated to be effective in previous theoretical studies12,51,64,70,71 was employed. In this method, the self-consistent Hamiltonian of the molecular junction is projected onto the molecule. Then, the molecular projected self-consistent Hamiltonian (MPSH) matrix is diagonalized. The MPSH state is the eigenstate of the molecule within the two-probe system, and the self-energies of two electrodes are not considered. MPSH states delocalized over the 9037
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Figure 5. Transmission spectra of systems ae in the energy range from 2.0 to +2.0 eV under a bias of 3.0 to +3.0 V. The region confined by the blue dashed lines and energy axis is defined as the bias window.
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Figure 6. Spatial distribution of MPSH states of systems a, b, and d at (1.0 V bias.
whole scattering region will contribute to the transmission spectra, whereas MPSH states localized near only one of the electrodes will not make any contribution to the transmission spectra.12,51,64,70,71 In Figure 4, the rectification curves of systems a, b, and d exhibit different change tendencies. The rectification ratios of the three systems were found to be Ra(1.0 V) = 1, Rb(1.0 V) > 1, and Rd(1.0 V) < 1, respectively. Figure 6 presents the spatial distributions of the MPSH states of the three systems at (1.0 V. Here, we consider the MPSH states that are projected onto the whole scattering region so that the moleculeelectrode coupling can also be observed directly. For the transmission spectra of system a at (1.0 V bias shown in Figure 5, the two broad peaks below the Fermi level are mainly due to MPSH states 242, 247, 248, and 250. The peak at 1.7 eV is a superposition of states 234 and 236, whereas the peak at 1.5 eV is due to state 253. The two strong AuS bonds offer large probabilities for the electron transferring and forming delocalized MPSH states, thus giving broad and high peaks in the transmission spectra. The MPSH states of system a at positive and negative biases are almost symmetric except for states 247 and 248. The states that contribute to the currents at (1.0 V are states 247, 248, and 250, because their energies are closest to the bias window. Obviously, the contributions to the positive and negative currents are equivalent, resulting in R = 1 at 1.0 V. For system b, the small transmission peak around the Fermi level at +1.0 V bias (shown in Figure 5) is mainly due to state 248. It is somewhat difficult for the electronic state to enter the left electrode in MPSH state 248 because of the weak interaction between the left electrode and the terminal H atom, even though the electronic state is well-delocalized on the central molecule
and the right electrode. Therefore, transmission through state 248 is difficult and gives only a small peak in the transmission spectrum at 1.0 V bias. MPSH states 239, 241, and 242 are the origins of the transmission peak at 1.0 eV. The sharp transmission peak at 1.8 eV is the result of delocalization of state 250. In the case of 1.0 V bias, the transmission peak at 0.6 eV is due to states 241, 242, and 247, whereas the peak at 1.4 eV is due to states 234, 238, and 239. The peak at about 1.4 eV results from state 250. The states contributing to the current at +1.0 V bias are states 241, 242, and 248, whereas those at 1.0 V bias are mainly states 242 and 247. Consequently, the contribution to the current at +1.0 V bias is larger than that at 1.0 V bias, which leads to the result Rb(1.0 V) > 1. For system d, it is expected that the MPSH states at +1.0 V make lesser contributions to the transmission spectrum than at 1.0 V because the rectification ratio at 1.0 V is less than 1. In the MPSH states of system d, the main states contributing to the current at +1.0 V bias are states 254 and 258, and the states contributing to the current at 1.0 V bias are 252 and 259. It is evident that the delocalization of states 252 and 259 at 1.0 V is better than that of states 254 and 258 at +1.0 V. Therefore, Rd(1.0 V) < 1. The MPSH states that are responsible for the transmission peaks are also easily identified, just as discussed above. Some corresponding MPSH states at +1.0 and 1.0 V that make no contribution to transmission peaks are not shown. Pure electrodemolecule coupling without any influence from the external electric field can be observed in the MPSH states at zero bias, and accordingly, the preferential electrontransfer directions mentioned above can be understood intuitively. Therefore, we calculated several MPSH states around the 9039
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Figure 7. Energy levels of the MPSH states for systems ae at different biases.
Fermi level for each system at zero bias. The results and a related brief discussion are given in the Supporting Information. 3.4. Energy Levels of the Molecular Orbitals. The transmission peaks shifted with varying applied bias, as shown in Figure 5.
Essentially, the shifts originated from changes in the energy levels of the molecular orbitals with respect to the Fermi level of the electrode. To understand the distribution of transmission peaks, energy levels of the MPSH states at different biases are plotted in 9040
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The Journal of Physical Chemistry A Figure 7. For simplicity and clarity, the self-consistent Hamiltonian of the molecular junction was projected onto only the central isolated molecule, which is somewhat different from the projection method employed in section 3.3. The gray region shows the bias window, and the Fermi level was set to zero. Obviously, the distribution of energy levels is asymmetric about zero bias for all of the systems except for system a. The asymmetric features of the energy levels reflect the built-in asymmetry of the four systems, namely, the asymmetric molecular structure in system c and the asymmetric electrodemolecule contacts in systems b, d, and e. As shown in Figure 7, the energy levels in systems b, d, and e were influenced more prominently than those in system c. This finding corresponds with the suggestion in section 3.1 that asymmetric electrodemolecule contacts are more effective than donoracceptor components for the rectifying behavior of the same unsubstituted BETE molecule. Comparing Figure 5 with Figure 7, one can see that the distribution of transmission peaks in each system is in good agreement with the change in energy levels under various applied biases. This agreement demonstrates that each molecular orbital around the Fermi level corresponds to one potential electrontransport channel and that there will be a prominent peak in the transmission spectrum if the transmission probability at this molecular orbital is large. In addition, the correlation between the energy levels of the molecular orbitals and the transmission peaks can be confirmed easily by examining Figures 5 and 7. Here, only system a is considered. There are four prominent, resolvable transmission peaks in the energy range from 2 to +2 eV. For convenience, they are denoted as peaks a1, a2, a3, and a4, respectively, in order from low energy to high energy. Peak a1 originates mainly from orbital 38 at lower bias voltage and mainly from orbital 39 at higher bias voltage. Peak a2 originates from orbitals 39, 40, and 41 at lower bias voltage and from orbitals 40 and 41 at higher bias voltage. Peak a3 originates from orbital 42 [i.e., the highest occupied molecular orbital (HOMO)]. Peak a4 originates from orbital 43 [i.e., the lowest unoccupied molecular orbital (LUMO)].
4. CONCLUSIONS In summary, the electron-transport properties of several molecular junctions based on the bis-2-(5-ethynylthienyl)ethyne (BETE) molecule were investigated through density functional theory (DFT) combined with the first-principles nonequilibrium Green’s function (NEGF). The molecular junctions were different in their configurations, including the electrodemolecule contacts and the donoracceptor substituent groups. In this work, the currents of the dithiolate-terminated systems were found to be over an order of magnitude larger than those of the monothiolate-terminated systems. Asymmetric IV characteristics were obtained for the systems containing asymmetric contacts and donoracceptor groups. In addition, the rectification ratios of all of the systems were computed. The results showed that the mode of electrodemolecule contacts plays a crucial role in rectification in our models and that the rectifying effect is not enhanced significantly by introducing additional donoracceptor components for a molecular rectifier with asymmetric electrodemolecule contacts. Our results suggest that the preferential direction for electron flow in monothiolateterminated system is from the thiolate-anchoring side to the nonthiolate-anchoring side. Finally, the IV characteristics and rectifying behaviors of BETE-based molecular junctions were
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also analyzed through their transmission spectra, their MPSH states, and the energy levels of their MPSH states.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional text and one figure showing spatial distribution of MPSH states of the five systems at 0 V bias. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: +86-25-8586 6333. Fax: +86-25-8586 6396. E-mail: iamqdling@ njupt.edu.cn.
’ ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (973 Program, 2009CB930601), National Natural Science Foundation of China (NSFC 60976019, 90813010), Program for New Century Excellent Talents in University (NCET-07-0446), Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP 20093223110002), and Scientific and Technological Innovation Teams of Colleges and Universities in Jiangsu Province (TJ207035, TJ208027). ’ REFERENCES (1) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277–283. (2) Martin, A. S.; Sambles, J. R.; Ashwell, G. J. Phys. Rev. Lett. 1993, 70, 218. (3) Metzger, R. M.; Chen, B.; Hopfner, U.; Lakshmikantham, M. V.; Vuillaume, D.; Kawai, T.; Wu, X.; Tachibana, H.; Hughes, T. V.; Sakurai, H.; Baldwin, J. W.; Hosch, C.; Cava, M. P.; Brehmer, L.; Ashwell, G. J. J. Am. Chem. Soc. 1997, 119, 10455–10466. (4) Ashwell, G. J.; Tyrrell, W. D.; Whittam, A. J. J. Am. Chem. Soc. 2004, 126, 7102–7110. (5) Ashwell, G. J.; Mohib, A. J. Am. Chem. Soc. 2005, 127, 16238– 16244. (6) Zhao, J.; Zeng, C.; Cheng, X.; Wang, K.; Wang, G.; Yang, J.; Hou, J. G.; Zhu, Q. Phys. Rev. Lett. 2005, 95, 045502. (7) Andriotis, A. N.; Menon, M.; Srivastava, D.; Chernozatonskii, L. Phys. Rev. Lett. 2001, 87, 066802. (8) Krzeminski, C.; Delerue, C.; Allan, G.; Vuillaume, D.; Metzger, R. M. Phys. Rev. B 2001, 64, 085405. (9) Kornilovitch, P. E.; Bratkovsky, A. M.; Williams, R. S. Phys. Rev. B 2002, 66, 165436. (10) Taylor, J.; Brandbyge, M.; Stokbro, K. Phys. Rev. Lett. 2002, 89, 138301. (11) Stokbro, K.; Taylor, J.; Brandbyge, M. J. Am. Chem. Soc. 2003, 125, 3674–3675. (12) Staykov, A.; Nozaki, D.; Yoshizawa, K. J. Phys. Chem. C 2007, 111, 11699–11705. (13) Metzger, R. M. Chem. Rev. 2003, 103, 3803–3834. (14) Oleynik, I. I.; Kozhushner, M. A.; Posvyanskii, V. S.; Yu, L. Phys. Rev. Lett. 2006, 96, 096803. (15) Li, Z.; Kosov, D. S. J. Phys. Chem. B 2006, 110, 19116–19120. (16) Larade, B.; Bratkovsky, A. M. Phys. Rev. B 2003, 68, 235305. (17) Morales, G. M.; Jiang, P.; Yuan, S.; Lee, Y.; Sanchez, A.; You, W.; Yu, L. J. Am. Chem. Soc. 2005, 127, 10456–10457. (18) Okuno, Y.; Yokoyama, S. Thin Solid Films 2008, 516, 2630– 2634. (19) Kushmerick, J. G.; Whitaker, C. M.; Pollack, S. K.; Schull, T. L.; Shashidhar, R. Nanotechnology 2004, 15, S489–S493. (20) Lee, Y.; Carsten, B.; Yu, L. Langmuir 2009, 25, 1495–1499. 9041
dx.doi.org/10.1021/jp204161z |J. Phys. Chem. A 2011, 115, 9033–9042
The Journal of Physical Chemistry A (21) Martin, S.; Manrique, D. Z.; Garcia-Suarez, V. M.; Haiss, W.; Higgins, S. J.; Lambert, C. J.; Nichols, R. J. Nanotechnology 2009, 20, 125203. (22) Zhao, J.; Yu, C.; Wang, N.; Liu, H. J. Phys. Chem. C 2010, 114, 4135–4141. (23) Pan, J. B.; Zhang, Z. H.; Ding, K. H.; Deng, X. Q.; Guo, C. Appl. Phys. Lett. 2011, 98, 092102. (24) Gao, D.; Scholz, F.; Nothofer, H.; Ford, W. E.; Scherf, U.; Wessels, J. M.; Yasuda, A.; von Wrochem, F. J. Am. Chem. Soc. 2011, 133, 5921–5930. (25) Dalgleish, H.; Kirczenow, G. Phys. Rev. B 2006, 73, 245431. (26) Deng, X. Q.; Zhou, J. C.; Zhang, Z. H.; Tang, G. P.; Qiu, M. Appl. Phys. Lett. 2009, 95, 103113. (27) Chen, X.; Yeganeh, S.; Qin, L.; Li, S.; Xue, C.; Braunschweig, A. B.; Schatz, G. C.; Ratner, M. A.; Mirkin, C. A. Nano Lett. 2009, 9, 3974–3979. (28) Troisi, A.; Ratner, M. A. J. Am. Chem. Soc. 2002, 124, 14528–14529. (29) Kornilovitch, P. E.; Bratkovsky, A. M.; Williams, R. S. Phys. Rev. B 2002, 66, 245413. (30) Troisi, A.; Ratner, M. A. Nano Lett. 2004, 4, 591–595. (31) McCarty, G. S.; Weiss, P. S. Chem. Rev. 1999, 99, 1983–1990. (32) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (33) Halbritter, A.; Csonka, S.; Mihaly, G.; Jurdik, E.; Kolesnychenko, O. Y.; Shklyarevskii, O. I.; Speller, S.; van Kempen, H. Phys. Rev. B 2003, 68, 035417. (34) Li, X.; He, J.; Hihath, J.; Xu, B.; Lindsay, S. M.; Tao, N. J. Am. Chem. Soc. 2006, 128, 2135–2141. (35) Quinn, J. R.; Frank, W.; Foss, J.; Venkataraman, L.; Hybertsen, M. S.; Breslow, R. J. Am. Chem. Soc. 2007, 129, 6714–6715. (36) Venkataraman, L.; Park, Y. S.; Whalley, A. C.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nano Lett. 2007, 7, 502–506. (37) Zhou, J.; Chen, F.; Xu, B. J. Am. Chem. Soc. 2009, 131, 10439–10446. (38) Søndergaard, R.; Strobel, S.; Bundgaard, E.; Norrman, K.; Hansen, A. G.; Albert, E.; Csaba, G.; Lugli, P.; Tornow, M.; Krebs, F. C. J. Mater. Chem. 2009, 19, 3899–3908. (39) Wang, Y. F.; Kr€oger, J.; Berndt, R.; Vazquez, H.; Brandbyge, M.; Paulsson, M. Phys. Rev. Lett. 2010, 104, 176802. (40) Mishchenko, A.; Vonlanthen, D.; Meded, V.; B€urkle, M.; Li, C.; Pobelov, I. V.; Bagrets, A.; Viljas, J. K.; Pauly, F.; Evers, F.; Mayor, M.; Wandlowski, T. Nano Lett. 2010, 10, 156–163. (41) Xing, Y.; Park, T.; Venkatramani, R.; Keinan, S.; Beratan, D. N.; Therien, M. J.; Borguet, E. J. Am. Chem. Soc. 2010, 132, 7946–7956. (42) Ko, C.; Huang, M.; Fu, M.; Chen, C. J. Am. Chem. Soc. 2010, 132, 756–764. (43) Brandbyge, M.; Mozos, J.; Ordejon, P.; Taylor, J.; Stokbro, K. Phys. Rev. B 2002, 65, 165401. (44) Ke, S.; Baranger, H. U.; Yang, W. Phys. Rev. B 2004, 70, 085410. (45) Wang, Z.; Kadohira, T.; Tada, T.; Watanabe, S. Nano Lett. 2007, 7, 2688–2692. (46) Li, Y.; Yin, G.; Yao, J.; Zhao, J. Comput. Mater. Sci. 2008, 42, 638. (47) Cho, Y.; Kim, W. Y.; Kim, K. S. J. Phys. Chem. A 2009, 113, 4100–4104. (48) Liu, H.; Ni, W.; Zhao, J.; Wang, N.; Guo, Y.; Taketsugu, T.; Kiguchi, M.; Murakoshi, K. J. Chem. Phys. 2009, 130, 244501. (49) Nielsen, S. K.; Brandbyge, M.; Hansen, K.; Stokbro, K.; van Ruitenbeek, J. M.; Besenbacher, F. Phys. Rev. Lett. 2002, 89, 66804. (50) Paulsson, M.; Frederiksen, T.; Brandbyge, M. Nano Lett. 2006, 6, 258–262. (51) Bao, Q.; Lu, Z.; Li, J.; Loh, K. P.; Li, C. M. J. Phys. Chem. C 2009, 113, 12530–12537. (52) Mishra, A.; Ma, C.; Bauerle, P. Chem. Rev. 2009, 109, 1141–1276. (53) Pearson, D. L.; Tour, J. M. J. Org. Chem. 1997, 62, 1376–1387. (54) Yoshizawa, K.; Tada, T.; Staykov, A. J. Am. Chem. Soc. 2008, 130, 9406–9413. (55) Long, M.; Chen, K.; Wang, L.; Qing, W.; Zou, B. S.; Shuai, Z. Appl. Phys. Lett. 2008, 92, 243303.
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(56) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.01; Gaussian, Inc.: Wallingford, CT, 2004. (57) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (58) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (59) Gr€onbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122, 3839–3842. (60) Atomistix ToolKit, version 2008.10; QuantumWise A/S: Copenhagen, Denmark, 2008 (www.quantumwise.com). (61) Taylor, J.; Guo, H.; Wang, J. Phys. Rev. B 2001, 63, 245407. (62) Ceperley, D. M.; Alder, B. J. Phys. Rev. Lett. 1980, 45, 566. (63) Perdew, J.; Zunger, A. Phys. Rev. B 1981, 23, 5048. (64) Yuan, S.; Dai, C.; Weng, J.; Mei, Q.; Ling, Q.; Wang, L.; Huang, W. J. Phys. Chem. A 2011, 115, 4535–4546. (65) B€uttiker, M; Imry, Y.; Landauer, R.; Pinhas, S. Phys. Rev. B 1985, 31, 6207. (66) B€uttiker, M. Phys. Rev. Lett. 1986, 57, 1761. (67) B€uttiker, M. Phys. Rev. B 1988, 38, 9375. (68) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571–574. (69) Hao, H.; Shi, X. Q.; Zeng, Z. Microelectron. J. 2009, 40, 773. (70) Stokbro, K.; Taylor, J.; Brandbyge, M.; Mozos, J. -L.; Ordejon, P. Comput. Mater. Sci. 2003, 27, 151–160. (71) Sen, S.; Chakrabarti, S. J. Phys. Chem. C 2008, 112, 1685–1693.
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