Langmuir 2009, 25, 1495-1499
1495
Understanding the Anchoring Group Effect of Molecular Diodes on Rectification Youngu Lee, Brian Carsten, and Luping Yu* Department of Chemistry and The James Franck Institute, The UniVersity of Chicago, Center for IntegratiVe Science, 929 E. 57th Street, Chicago, Illinois 60637 ReceiVed September 5, 2008. ReVised Manuscript ReceiVed NoVember 14, 2008 This paper describes the anchoring group effect of molecular diodes on rectifying behavior. Two molecular diodes with different anchoring groups, which are based on diblock co-oligomeric structures, have been synthesized and characterized. Scanning tunneling spectroscopy (STS) measurements revealed the correlation of rectifying effects in these molecular diodes with anchoring groups such as thiol and isocyanide. The combination of theoretical calculation and experimental results on these molecular diodes demonstrated that the rectifying effect could be affected by the nature of anchoring groups due to the bond dipoles at the interface and internal polarization inside the molecules.
Introduction The behaviors of charge transport through single molecule or molecular arrays are controlled by many factors associated with electrodes, molecules, and their interfacial properties.1-5 Understanding these factors is intellectually interesting and will enable further technological development in molecular electronics. Although numerous molecules with unique properties are synthesized and assembled to build the electrical components such as wires,6-9 diodes,10-15 switches,16-20 or transistors,21-27 * To whom correspondence should be addressed. E-mail: lupingyu@ uchicago.edu. (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. (3) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (4) Metzger, R. M. Chem. ReV. 2003, 103, 3803. (5) Tao, N. J. Nat. Nanotechnol. 2006, 1, 173. (6) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (7) He, J.; Chen, F.; Li, J.; Sankey, O. F.; Terazono, Y.; Herrero, C.; Gust, D.; Moore, T. A.; Moore, A. L.; Lindsay, S. M. J. Am. Chem. Soc. 2005, 127, 1384. (8) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nature 2006, 442, 904. (9) Cui, X. D.; Primak, A.; Zarate, X.; Tomfojr, J.; Sankey, O. F.; Moore, A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001, 294, 571. (10) Ng, M.-K.; Lee, D.-C.; Yu, L. J. Am. Chem. Soc. 2002, 124, 11862. (11) Ng, M.-K.; Yu, L. Angew. Chem., Int. Ed. 2002, 41, 3598. (12) Jiang, P.; Morales, G. M.; You, W.; Yu, L. Angew. Chem., Int. Ed. 2004, 43, 4471. (13) Morales, G. M.; Jiang, P.; Yuan, S.; Lee, Y.; Sanchez, A.; You, W.; Yu, L. J. Am. Chem. Soc. 2005, 127, 10456. (14) Gasyna, Z. L.; Morales, G. M.; Sanchez, A.; Yu, L. Chem. Phys. Lett. 2006, 417, 401. (15) Oleynik, I. I.; Kozhushner, M. A.; Posvyanskii, V. S.; Yu, L. Phys. ReV. Lett. 2006, 96, 096803. (16) Green, J. E.; Choi, J. W.; Boukai, A.; Bunimovich, Y.; Johnston-Halperin, E.; DeIonno, E.; Luo, Y.; Sheriff, B. A.; Xu, K.; Shin, Y. S.; Tseng, H.-R.; Stoddart, J. F.; Heath, J. R. Nature 2007, 445, 414. (17) Flood, A. H.; Stoddart, J. F.; Steuerman, D. W.; Heath, J. R. Science 2004, 306, 2055. (18) Collier, C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000, 289, 1172. (19) Collier, C. P.; Wong, E. W.; Belohradsky, M.; Raymo, F. M.; Stoddart, J. F.; Kuekes, P. J.; Williams, R. S.; Heath, J. R. Science 1999, 285, 391. (20) Blum, A. S.; Kushmerick, J. G.; Long, D. P.; Patterson, C. H.; Yang, J. C.; Henderson, J. C.; Yao, Y.; Tour, J. M.; Sashidhar, R.; Ratna, B. R. Nat. Mater. 2005, 4, 167. (21) Cai, L.; Cabassi, M. A.; Yoon, H.; Cabarcos, O. M.; Mcguiness, C. L.; Flatt, A. K.; Allara, D. L.; Tour, J. M.; Mayer, T. S. Nano Lett. 2005, 5, 2365. (22) Albrecht, T.; Guckian, A.; Ulstrup, J.; Vos, J. G. Nano Lett. 2005, 5, 1451. (23) Albrecht, T.; Moth-Poulsen, K.; Christensen, J. B.; Hjelm, J.; Bjornholm, T.; Ulstrup, J. J. Am. Chem. Soc. 2006, 128, 6574. (24) Chen, F.; Hihath, J.; Huang, Z.; Li, X.; Tao, N. J. Annu. ReV. Phys. Chem. 2007, 58, 535.
our control in charge transport through molecular assembly is very limited. We developed a new class of molecular diodes based on a conjugated diblock co-oligomer with a directly coupled donor-acceptor structure, which showed a significant rectifying effect.10-13 The molecular structures of the diblock co-oligomers give rise to a built-in electronic asymmetry resembling a p-n junction semiconductor. Scanning tunneling spectroscopy (STS) studies revealed that the observed asymmetry of electron transport was an intrinsic property of the conjugated diblock co-oligomers. In addition, the orientation of dipole moments of the molecular diodes played a crucial role in determining the direction of electron transport in the system.14,15 These molecular diodes can be tailored in their physical properties by adjusting parameters such as conjugation length, dipole moments, and anchoring groups in the molecules. Many studies have shown that the anchoring groups that bind molecules to metal electrodes play a key role in determining the electron transport properties of molecular wires.28-32 For example, Kim et al. reported that an aromatic isocyanide linker imposes higher tunneling barriers at the contact than the corresponding thiol compounds.28 It will be interesting to investigate the effect of anchoring groups on the rectification of our molecular diodes. Therefore, we synthesized two molecular diodes with different anchoring groups. Scanning tunneling spectroscopy (STS) measurements and theoretical calculations on these molecular diodes demonstrated that the anchoring groups affect the rectification through both the bond dipoles at the interface and internal polarization inside the molecules. (25) Albrecht, T.; Guckian, A.; Kuznetsov, A. M.; Vos, J. G.; Ulstrup, J. J. Am. Chem. Soc. 2006, 128, 17132. (26) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna, J. P.; Abruna, H. D.; McEuen, P. L.; Ralph, D. C. Nature 2002, 417, 722. (27) Liang, W.; Shores, M. P.; Bockrath, M.; Long, J. R.; Park, H. Nature 2002, 417, 725. (28) Kim, B.; Beebe, J. M.; Jun, Y.; Zhu, X.-Y.; Frisbie, C. D. J. Am. Chem. Soc. 2006, 128, 4970. (29) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J.-L. Nano Lett. 2007, 7, 932. (30) Heimel, G.; Romaner, L.; Bredas, J. L.; Zojer, E. Phys. ReV. Lett. 2006, 96, 196806. (31) Lang, N. D.; Kagan, C. R. Nano Lett. 2006, 6, 2955. (32) Zangmeister, C. D.; Robey, S. W.; van Zee, R. D.; Kushmerick, J. G.; Naciri, J.; Yao, Y.; Tour, J. M.; Varughese, B.; Xu, B.; Reutt-Robey, J. E. J. Phys. Chem. B 2006, 110, 17138.
10.1021/la802923a CCC: $40.75 2009 American Chemical Society Published on Web 12/16/2008
1496 Langmuir, Vol. 25, No. 3, 2009
Lee et al.
Experimental Section General. UV-vis spectra of molecular diodes in methylene chloride were recorded on a Shimadzu UV-2401PC UV-visible spectrophotometer. Ellipsometry measurements were performed with a Gaertner L116C single-wavelength optical ellipsometer equipped with a He-Ne laser operating at the wavelength of 632.8 nm with incidence angle of 70°. An index of refraction of 1.55 was used for the ellipsometric measurements. Substrate Preparation for STM. The gold substrates used in the STM studies were prepared by gold (99.999%) deposition from a tungsten boat onto freshly cleaved mica (highest quality V1, Ted Pella, Inc.). Before deposition, the mica was heated at a temperature of ∼450 °C overnight under high vacuum. Gold film was then annealed at 400 °C for 10 h under high vacuum. Before using the gold film, its surface was subjected to hydrogen flame annealing until the gold film radiated a dim orange color (in a dark room) in order to remove possible carbonaceous materials. The annealed gold film was quenched in pure ethanol saturated with argon. Monolayer Preparation. The self-assembly process was carried out at room temperature under N2 atmosphere. Immediately after the annealing process, the gold substrates were transferred from the ethanol to a ∼1 mM ethanol solution of dodecanethiol (DDT). After 24 h, the gold substrate was removed from the solution and rinsed five times with pure ethanol and twice with freshly distilled tetrahydrofuran (THF). The dodecanethiolate self-assembled monolayers (DDT SAMs)/Au electrode were immersed in a THF solution of the molecular diode (∼10-4 M) with an excess of freshly prepared NaOEt solution for 30 min. The electrode was then washed consecutively five times with ethanol and THF. This procedure removes the cyanoethyl (CNE) protection group in the molecules, leaving the trimethylsilylethyl (TMSE) protection group intact. The exposed S- or SH groups can be covalently bonded to the gold surface. The TMSE protection group was cleaved by dipping the electrode with the modified SAM into 0.01 M THF solution of tetrabutylammonium fluoride (TBAF) for 1 h. The resulting sample was rinsed several times with pure THF, ethanol, and toluene. Finally, the electrode was immersed in a freshly prepared solution of 5 nm gold nanoparticles (Au NPs) in toluene (absorbance 0.2 at 520 nm) for 1-2 h and subsequently rinsed with tetraoctylammonium bromide in toluene, pure toluene, THF, and ethanol. The samples were dried with a continuous and gentle flow of argon and immediately used for STM and STS measurements. Scanning Tunneling Microscopy (STM). STM Measurements were carried out by utilizing a NanoScope III (Digital Instruments) equipped with a low current STM head and Picoamp Boost Box (Digital Instruments), which allows STM measurements with tunneling currents in the pA range. The tips used in the measurements were fabricated by electrochemical etching of a Pt/Ir wire (Molecular Imaging, Phoenix, AZ) in aqueous 8 M NaOH solution. After etching, the tips were rinsed with deionized water, dried, and immediately mounted in the STM converter head. The experiments presented here were performed in atmospheric air at room temperature, and the images were recorded in constant current operating mode. Scanning Tunneling Spectroscopy (STS). STS data were acquired using the I-V mode of Nanoscope software version 5.12r4 (Digital Instruments). This operation mode allows us to monitor the variation of the tunneling current (I) corresponding to applied bias voltage (V). After the tip is positioned at a specific Au NP on the Au(111) surface, the feedback is shut off and a spectroscopic plot is acquired (the tunneling current is measured as the sample voltage is ramped at 10 V/s). In all the measurements, the bias potential was applied to the sample with respect to the grounded tip. In this configuration, a positive bias corresponds to an electron flow from the tip to the sample while a negative bias corresponds to an electron flow from the sample to the tip. The STS data shown here are averaged I-V curves, measured with at least 40 Au NPs attached to the molecules in 3 samples, which were independently prepared. The I-V data for each Au NP is the average of about 5 individual I-V curves, each of which is composed of 256 points. In order to avoid artifacts, the tips used for the STS measurements were previously
Figure 1. Molecular structures of molecular diodes with two thiols (I) and both an isocyanide and a thiol (II) anchoring group.
Figure 2. UV-vis spectra of molecular diode I (dotted line) and II (solid line) in methylene chloride.
examined as follows: In the DDT SAMs, the tip must be able to produce atomically resolved images with the characteristic (3 × 3)R 30° layer structure of an alkanethiolate SAM on a Au(111) surface. Finally, before and after the STS measurement on a Au NP, the I-V curve of the DDT SAMs should be almost symmetric. Theoretical Calculations. Ab initio calculations were performed using the Gaussian 03 program on molecular diodes. The hybrid HF-DFT functional B3LYP with the basis set LANL2DZ was used to calculate their molecular properties.
Results and Discussion To probe the effect of anchoring groups of molecular diodes on rectification, we synthesized two molecular diodes I and II, each of which contains an electron-rich biphenyl segment and an electron-deficient bipyrimidyl segment. As shown in Figure 1, molecular diode I possesses two thiol anchoring groups with cyanoethyl (CNE) and trimethylsilylethyl (TMSE) groups, while molecular diode II bears an isocyanide anchoring group and a thiol protected with TMSE group. Figure 2 illustrates UV-vis spectra for molecular diodes I and II. The absorption maximum of molecular diodes I and II appears at 333 nm, attributable to π-π* transition. However, molecular diode II exhibits a weak transition at longer wavelength, probably due to the extended conjugation or the n-π* transition involving the isocyanide functional group. The designed structures of molecular diodes I and II allowed us to use sequential assembly approaches to immobilize them on a Au(111) surface as described in Figure 3.12 The CNE protection group on molecular diode I was cleaved in situ with freshly prepared sodium ethoxide (NaOEt) solution to generate a free thiolate at one end, which was then inserted into defect sites of dodecanethiolate self-assembled monolayers (DDT SAMs). The TMSE protection group on the top of the surface was cleaved with a THF solution of tributylammonium fluoride (TBAF) to free the thiol group that was further connected to a gold nanoparticle (Au NP) solution (Figure 3a). The resulting
Anchoring Group Effect of Molecular Diodes
Langmuir, Vol. 25, No. 3, 2009 1497
Figure 3. Schematic illustration of sequential assembly of molecular diodes (a) I and (b) II.
assembly with the Au/molecular diode I/Au NP structure was used for scanning tunneling spectroscopy (STS) measurements. Since the isocyanide group can chemisorb onto a Au(111) surface, molecular diode II was directly inserted into defect sites of the DDT SAM. The remaining TMSE protection group was removed by TBAF treatment to free the top thiol, which can be connected to a Au NP to form a Au/molecular diode II/Au NP assembly (Figure 3b). The aromatic isocyanide functional group is known to chemisorb onto a gold substrate in a vertical fashion.33-36 Ellipsometric measurements indicated a thickness of 2.74 nm for the monolayer film of molecular diode II with the TMSE protection group onto the gold substrate, which is consistent with the theoretical thickness (2.79 nm) for vertical chemisorption onto the gold substrate. The sequential assembly process was also monitored by using scanning tunneling microscopy (STM). Figure 4 shows the STM images of the Au/molecular diode I/Au NP assembly in a DDT SAM on a Au surface. Control experiments using monolayers of molecular diodes I and II with the TMSE protection group showed no attachment of Au NPs, confirming that the images are an assembly of molecular diodes sandwiched between the Au substrate and Au NPs. The I-V characteristics of molecular diodes I and II were investigated by using scanning tunneling spectroscopy (STS). As shown in Figure 5a, the assembly of molecular diode I exhibited a significant rectifying behavior with an average rectification ratio (RR) of 7.4, where RR is defined as RR ) I(+1.5 V)/I(-1.5 V). When the orientation of molecular diode I was reversed, the rectification direction was also reversed as (33) Murphy, K. L.; Tysoe, W. T.; Bennett, D. W. Langmuir 2004, 20, 1732. (34) Henderson, J. I.; Feng, S.; Bein, T.; Kubiak, C. P. Langmuir 2000, 16, 6138. (35) Shih, K.-C.; Angelici, R. J. Langmuir 1995, 11, 2539. (36) Robertson, M. J.; Angelici, R. J. Langmuir 1994, 10, 1488.
Figure 4. Constant current STM topography of (a) dodecanethiol/ molecular diode I SAM on Au(111) after attachment of Au NPs to the termini of molecular diode I. Inset: (b) Image of a single Au(NP). STM imaging condition: Vbias ) 1.0 V, It ) 1 pA.
we expected (Figure 5b). The inset histogram shows the statistical distribution of RR for the different molecular diodes measured, indicating the reproducibility of charge transport behaviors of different molecular diodes. These results indicate that the rectification effect is an intrinsic property of the molecule and the orientation of dipole moments plays a key role in determining the rectifying direction. We also changed the current set point which reflects the distance between the STM tip and the Au NP in order to investigate the contact resistance between the STM tip and the Au NP. I-V curves at various current set points were consistently asymmetric and showed similar RR values. This indicates that the observed asymmetry originates from the molecular structure. However, the charge transport behavior of the assembly of molecular diode II was opposite to that of molecular diode I, although the only difference in the two assemblies is in one of the anchoring groups. The assembly of molecular diode II was more conductive at the negative bias region (Figure 6). The
1498 Langmuir, Vol. 25, No. 3, 2009
Lee et al.
Figure 7. (a) Schematic illustration of an Au/tetrabenzene/Au NP assembly. (b) Averaged I-V characteristics of tetrabenzene with a thiol and an isocyanide anchoring group at different tunneling current set points (set point Vbias ) 1.5 V, It ) 0.05, 0.1, and 0.2nA).
Figure 5. Averaged I-V characteristics of (a) molecular diode I and (b) reversed orientation of molecular diode I (set points Vbias ) 1.5 V, It ) 0.05, 0.1, and 0.2 nA).
Figure 6. Averaged I-V characteristics of molecular diode II at different tunneling current set points (set points Vbias ) 1.5 V, It ) 0.05, 0.1, and 0.2 nA).
average RR value of assembly of molecular diode II was rather small (only 1.9), where RR is defined as RR ) I(-1.5 V)/I(+1.5 V). The behavior for molecular diode II was observed consistently from various tunneling current set point conditions (Figure 6). This is a puzzling contrast in rectifying behavior at the first glance between molecular diodes I and II because the orientations of internal dipole moments in the two assemblies are the same. A control experiment was carried out using a tetrabenzene molecule with a thiol and an isocyanide anchoring group at each end. STS measurement showed that the tetrabenzene molecule with asymmetric anchoring groups exhibited a clear rectifying
effect, with a slightly larger average RR of 2.4, which is slightly larger than that of molecular diode II (Figure 7). To understand the difference of rectifying behaviors between molecular diodes I and II, the structural and electronic differences between two molecules were investigated. The molecular properties of diodes I and II were calculated by using the Gaussian 03 program in which the hybrid HF-DFT functional B3LYP with the base set LANL2DZ was used. It was found that molecular diode I possesses a strong internal dipole moment (6.3 D) from the bipyrimidyl segment to biphenyl segment. Molecular diode II possesses a significantly decreased internal dipole moment (2.5 D) due to an electron-withdrawing effect of the isocyanide functional group. The calculation also indicated that molecular diode I exhibits a significant polarization. Its highest occupied molecular orbital (HOMO) state is predominantly localized over the biphenyl segment, while the lowest unoccupied molecular orbital (LUMO) state is localized over the bipyrimidyl segment. However, the HOMO state of molecular diode II is almost evenly distributed over both biphenyl and bipyrimidyl segments. The LUMO state of molecular diode II is also evenly distributed over the entire molecule. The isocyanide anchoring group possesses a large orbital distribution in both HOMO and LUMO states of molecular diode II. Therefore, molecular diode II behaves more like a molecular wire (Figure 8). The charge transport results of these diode assemblies can qualitatively be explained by considering the effect of bond dipole (BD) on alignment of the Fermi energy level of electrodes. The bond dipole describes the decreased surface dipole of the metal electrode and local charge rearrangement induced by the anchoring group binding a metal electrode. Heimel and co-workers reported that the BD could play a key role in aligning the Fermi level of a metal electrode.29,30 When the isocyanide anchoring group forms a covalent bond with a gold electrode, a positive BD is developed, leading to an increase in the electrostatic potential at the interface between the isocyanide and gold electrode. In contrast, a negative BD is induced for the anchoring group, leading to a decrease of the electrostatic potential at the interface. Thus, the Fermi level of the gold electrode will rise
Anchoring Group Effect of Molecular Diodes
Langmuir, Vol. 25, No. 3, 2009 1499
energy gap of the molecules under a zero bias and no internal dipolar field. For molecular diode I, the negative BD values from two thiol anchoring groups cancel each other. Therefore, the molecular dipolar effect becomes the main factor that pushes down the Fermi energy level of the gold electrode when the assembly is connected with an outside circuit. This shift leads to electron tunneling through the HOMO of molecular diode I because the Fermi energy level of the gold electrode becomes closer to the HOMO of molecular diode I rather than the LUMO (E- > E+) (Figure 9a).37-41 On the contrary, the positive BD from the isocyanide of molecular diode II increases the Fermi energy level of the gold electrode at the interface, while the negative BD from the thiol anchoring groups decrease the Fermi energy level of the Au NP electrode. This alignment of the Fermi level makes the electron tunneling through the LUMO more favorable because the LUMO of molecular diode II is much closer to the Fermi energy level of the gold electrode (E+ > E-) (Figure 9b). Therefore, due to contrary BD effects between the isocyanide and thiol anchoring groups, the opposite rectifying behavior between molecular diodes I and II was observed. Figure 8. Frontier molecular orbitals of molecular diodes I and II.
Conclusion Significantly different charge transport behaviors were observed on two molecular diodes with different anchoring groups. These molecular diodes are conjugated diblock co-oligomers; one possesses two thiol anchoring groups at both ends, while the other possesses an isocyanide anchoring group at one end and a thiol anchoring group at the other end. Scanning tunneling spectroscopy (STS) measurements on the molecular assemblies made from these molecules revealed that the rectifying behaviors of two molecular diodes are completely opposite even though the direction of their dipole moments are the same. The combination of theoretical calculations and control experiments indicated that different anchoring groups induce different bond dipoles at the interface and internal polarization inside the molecules, which is the origin of the differences in rectifying effects in these molecules. Figure 9. Schematic energy level diagrams for molecular diodes (a) I and (b) II attached to a Au electrode and Au NP. Ed is the relative shift in the local vacuum level (VL) at the interface due to the combination of intrinsic molecular dipole and bond dipole. E+ and E- represent the energy barrier needed to align the Fermi energy level (EF) of the metal electrodes with the HOMO and LUMO levels, respectively.
up when a new covalent bond between the isocyanide anchoring group and gold electrode forms, while the Fermi level will be reduced by the reaction between thiol and gold electrode. Furthermore, the direction and degree of BD on the anchoring groups can affect the internal dipole moment of molecular diodes including conjugated molecular components and anchoring groups. Figure 9 schematically illustrates energy diagrams for molecular assemblies of diodes I and II attached to a gold substrate and a gold nanoparticle, respectively. It is assumed there that the Fermi level of gold electrodes would align at the middle of the
Acknowledgment. We gratefully acknowledge the financial support of the National Science Foundation and the NSF MRSEC program at the University of Chicago. Supporting Information Available: Syntheses of molecular diodes I and II. This material is available free of charge via the Internet at http://pubs.acs.org. LA802923A (37) Tao, N. J. Phys. ReV. Lett. 1996, 76, 4066. (38) Lenfant, S.; Krzeminski, C.; Delerue, C.; Allan, G.; Vuillaume, D. Nano Lett. 2003, 3, 741. (39) Guisinger, N. P.; Greene, M. E.; Basu, R.; Baluch, A. S.; Hersam, M. C. Nano Lett. 2004, 4, 55. (40) Rakshit, T.; Liang, G.-C.; Ghosh, A. W.; Datta, S. Nano Lett. 2004, 4, 1803. (41) Quek, S. Y.; Neaton, J. B.; Hybertsen, M. S.; Kaxiras, E.; Louie, S. G. Phys. ReV. Lett. 2007, 98, 066807.
