Electrical Rectification in a Monolayer of Zwitterions Assembled by

Electrical Rectification in a Monolayer of Zwitterions Assembled by Either Physisorption or ..... By continuing to use the site, you are accepting our...
0 downloads 0 Views 697KB Size
Download PDF
Langmuir 2003, 19, 9043-9050

9043

Electrical Rectification in a Monolayer of Zwitterions Assembled by Either Physisorption or Chemisorption Archana Jaiswal, Ramiya R. Amaresh,‡ M. V. Lakshmikantham, Andrei Honciuc, Michael P. Cava, and Robert M. Metzger* Laboratory for Molecular Electronics, Department of Chemistry, Box 870336, The University of Alabama, Tuscaloosa, Alabama 35487-0336 Received January 15, 2003. In Final Form: June 10, 2003

Rectification by a monolayer of (acetylthioundecyl)quinolinium tricyanoquinodimethanide, 6, sandwiched or physisorbed or chemisorbed on gold electrodes, was studied by scanning tunneling microscopy and scanning tunneling spectroscopy, and also by macroscopic “gold | monolayer | gold” pads, using either physisorption (Langmuir-Blodgett deposition, followed by partial chemisorption) onto a Au substrate or chemisorption (“self-assembly” onto Au surface). Rectification ratios of 5 to 7 were seen by scanning tunneling spectroscopy of individual molecules, as expected, but the macroscopic films are too disordered to show dramatic rectification ratios.

1. Introduction In the last several years, the goal of measuring electrical rectification by a single organic molecule either by nanoscopic methods (by scanning tunneling microscopy (STM)) or by macroscopic methods (by measuring a “metal | organic monolayer | metal” sandwich) has been achieved for several molecules.1-3 In particular, three unimolecular rectifiers have been measured in our laboratories: 1,4-10 2,11 and 3,12 and elsewhere.13-30 In addition, other workers have measured asymmetrical conduction

by STM26 or by macroscopic means in several systems of interest.33

* To whom correspondence may be addressed: tel, 1-205-3485952; fax, 1-205-348-9104; e-mail, [email protected]. ‡ Present address: Department of Chemistry, University of Virginia, Charlottesville, VA 22904. (1) Metzger, R. M. J. Mater. Chem. 1999, 9, 2027-2036. (2) Metzger, R. M. J. Mater. Chem. 2000, 10, 55-62. (3) Metzger, R. M. Acc. Chem. Res. 1999, 32, 950-957. (4) Metzger, R. M.; Chen, B.; Ho¨pfner, 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. (5) Baldwin, J. W.; Chen, B.; Street, S. C.; Konovalov, V. V.; Sakurai, H.; Hughes, T. V.; Simpson, C. S.; Lakshmikantham, M. V.; Cava, M. P.; Kispert, L. D.; Metzger, R. M. J. Phys. Chem. 1999, B103, 42694277. (6) Chen, B.; Metzger, R. M. J. Phys. Chem. 1999, B103, 4447-4451. (7) Vuillaume, D.; Chen, B.; Metzger, R. M. Langmuir 1999, 15, 40114017. (8) Xu, T.; Peterson, I. R.; Lakshmikantham, M. V.; Metzger, R. M. Angew. Chem., Intl. Ed. 2001, 40, 1749-1752. (9) Metzger, R. M.; Xu, T.; Peterson, I. R. J. Phys. Chem. 2001, B105, 7280-7290. (10) Xu, T.; Morris, T. A.; Szulczewski, G. J.; Amaresh, R. R.; Gao, Y.; Street, S. C.; Kispert, L. D.; Metzger, R. M.; Terenziani, F. J. Phys. Chem. 2002, B106, 10374-10381. (11) Baldwin, J. W.; Amaresh, R. R.; Peterson, I. R.; Shumate, W. J.; Cava, M. P.; Amiri, M. A.; Hamilton, R.; Ashwell, G. J.; Metzger, R. M. J. Phys. Chem. B 2002, 106, 12158-12164. (12) Metzger, R. M.; Baldwin, J. W.; Shumate, W. J.; Peterson, I. R.; Mani, P.; Mankey, G. J.; Morris, T.; Szulczewski, G.; Bosi, S.; Prato, M.; Commito, A.; Rubin, Y. J. Phys. Chem. B 2002, 106, 12158-12164. (13) Ashwell, G. J.; Sambles, J. R.; Martin, A. S.; Parker, W. G.; Szablewski, M. J. Chem. Soc., Chem. Commun. 1990, 1374-1376. (14) Martin, A. S.; Sambles, J. R.; Ashwell, G. J. Phys. Rev. Lett 1993, 70, 218-221. (15) Fischer, C. M.; Burghard, M.; Roth, S.; v. Klitzing, K. Europhys. Lett. 1994, 28, 129-134. (16) Fischer, C. M.; Burghard, M.; Roth, S. Synth. Met. 1995, 71, 1975-1976. (17) Fischer, C. M.; Burghard, M.; Roth, S. Synth. Met. 1996, 76, 237-240.

At present, three mechanisms are thought34 to contribute to a potential asymmetry of I-V curves measured for “metal 1 | molecule | metal 2” sandwiches, either by nanoscopic or by macroscopic methods: (1) Schottky barriers (interface dipoles) at the “metal 1 | molecule” or at the “molecule | metal 2” interfaces, (2) asymmetric placement of the active “chromophore” of the molecule

10.1021/la034073i CCC: $25.00 © 2003 American Chemical Society Published on Web 09/06/2003

9044

Langmuir, Vol. 19, No. 21, 2003

between the two metal electrodes, and finally (3) the electrical asymmetry of the molecule itself.34 For instance, for molecule 1 mechanisms 2 and 3 are deemed to be operative.35 In our own experience, having used exclusively physisorbed Langmuir-Blodgett (LB) monolayers and multilayers, we have been plagued by the experimental result that, upon repeated measurements of the same “metal | LB monolayer | metal” pad, the electrical asymmetry between currents at positive and negative bias steadily decreased, and the rectification ratio, i.e., the ratio of the current at maximum positive bias divided by the absolute value of the current at the corresponding maximum negative bias, decreased steadily cycle after measuring cycle.4,10-12 It was thought that this decrease was due to the progressive reorientation of the polar molecules within the monolayer, under the relatively large forward fields (2.4 V across 2.4 nm corresponds to an electric field of 1 GV m-1, which is much greater than, for instance, the breakdown potential of air (3 MV m-1). Our particular interest was to improve the electrical durability of derivatives of 1, to obtain a molecular device with persistent electronic properties. One expedient was to attach potential rectifiers covalently onto a solid metallic or semiconducting substrate; the other was to link the rectifying molecules covalently one to the other. The first efforts shied away from using a thiolate-gold bond. It was thought that thiol derivatives (-SH) of 1 would be problematic, since the gold-thiol bond is about 50% ionic: this would add to the potential polarity of the molecule an additional Schottky barrier of a permanent Au+0.5S-0.5 dipole (of the order of 4.184 × 0.5 × 2 ) 4 D) at the Au-molecule interface. Besides this, a thiol derivative of 1 would be incompatible with the dicyanomethide end of 1. Therefore a silane bridge was planned for. We first synthesized a triethoxysilyl (-Si(OEt)3) derivate of 1, namely, molecule 4, which was then chemisorbed onto a silicon substrate and studied by scanning tunneling microscopy. Unfortunately, the coverage of the silicon surface resulted in clumps of molecule 4 attaching themselves onto a few sites on the surface, rather than (18) Pomerantz, M.; Aviram, A.; McCorkle, R. A.; Li, L.; Aschott, A. G. Science 1992, 255, 1115-1118. (19) Geddes, N. J.; Sambles, J. R.; Jarvis, D. J.; Parker, W. G.; Sandman, D. J. Appl. Phys. Lett. 1990, 56, 1916-1918. (20) Geddes, N. J.; Sambles, J. R.; Jarvis, D. J.; Parker, W. G.; Sandman, D. J. J. Appl. Phys. 1992, 71, 756-768. (21) Zhou, C.; Deshpande, M. R.; Reed, M. A.; Jones, L., II; Tour, J. M. Appl. Phys. Lett. 1997, 71, 611-613. (22) Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2000, 77, 1224-1226. (23) Brady, A. C.; Hodder, B.; Martin, A. S.; Sambles, J. R.; Ewels, C. P.; Jones, R.; Briddon, P. R.; Musa, A. M.; Panetta, C. A.; Mattern, D. L. J. Mater. Chem. 1999, 9, 2271-2275. (24) Ashwell, G. J.; Gandolfo, D. S. J. Mater. Chem. 2001, 11, 246248. (25) Ashwell, G. J.; Gandolfo, D. S.; Hamilton, R. J. Mater. Chem. 2002, 12, 416-420. (26) Ng, M.-K.; Yu, L. Angew. Chem., Intl. Ed. 2002, 41, 3598-3601. (27) Hu, W.; Liu, Y.; Xu, Y.; Liu, S.; Zhou, S.; Zhu, D. Synth. Met. 1999, 104, 19-26. (28) Zhou, S.; Liu, Y.; Qiu, W.; Xu, Y.; Huang, X.; Li, Y.; Jiang, L.; Zhu, D. Adv. Funct. Mater. 2002, 12, 65-69. (29) Zhang, Y. J.; Li, Y.; Liu, Q.; Jin, J.; Ding, B.; Song, Y.; Jiang, L.; Du, X.; Zhao, Y.; Li, T. J. Synth. Met. 2002, 128, 43-46. (30) Cingolani, R.; Rinaldi, R.; Maruccio, G.; Biasco, A. Physica 2002, E13, 1229-1235. (31) Baldwin, J. W.; Lakshmikantham, M. V. Unpublished results. (32) Baldwin, J. W.; Amaresh, R. R. Unpublished results. (33) Chabinyc, M. P.; Chen, X.; Holmlin, R. E.; Jacobs, H.; Skulason, H.; Frisbie, C. D.; Mujica, V.; Ratner, M. A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 11730-11736. (34) Metzger, R. M. Submitted to Chem. Rev. (35) Krzeminski, C.; Delerue, C.; Allan, G.; Vuillaume, D.; Metzger, R. M. Phys. Rev. 2001, B64, # 085405.

Jaiswal et al.

having individual molecules of 4 attaching themselves onto close sites on the silicon substrate, so the effort was abandoned.31 Next, we synthesized the N-dodecylbis(pyrrole) derivative 5, in the hope that the molecule could be polymerized either by electrochemical or by chemical means either at the air-water interface before transfer onto a substrate or chemically after a monolayer of 5 was already deposited on an aluminum substrate, at the positions indicated by the double arrows. However, all efforts to polymerize 5 failed. The bis(pyrrole) by itself should be readily electropolymerized, but the presence of even only a quinolinium ring covalently attached to the bis(pyrrole) impeded polymerization; this result explained why the electrochemical polymerization of a bispyrrole connected to a zwitterion that included the quinolinium moiety could not be achieved.32 Therefore, we decided to return to the gold-thiolate bond, despite its added dipole. A thioacetyl (-SC(O)CH3) analogue was targeted. This resulted in the synthesis of molecule 6. We report below on the chemical and physical properties of molecule 6. 2. Experimental Section 2.1. Instrumentation. NMR spectra of the powder samples were recorded on a Bruker AM360 NMR spectrometer using CDCl3 as the solvent. Mass spectra were obtained on a VG Micromass mass spectrometer. X-ray photoelectron spectra of LB monolayers and self-assembled monolayers (SAMs) were obtained with a Kratos Analytical Axis 165 scanning Auger/Xray photoelectron spectrometer. Monochromatized Al KR photons (E ) 1486.6 eV) were used as the exciting radiation; the intrinsic spectrometer resolution was less than (0.2 eV. Scanning tunneling micrographs (STMs) were taken on Digital Instruments Nanoscope II and Nanoscope IIIa scanned probe microscopes (Veeco, Santa Barbara, CA) using a Pt/Ir tip and the scanning tunneling spectroscopy (STS) software and the CITS software. The current (I)-voltage (V) measurements were performed both on macroscopic “Au | (LB or SA) monolayer | Au” sandwiches using a Keithley model 236 source measuring unit and also by nanoscopic STS (Digital Instruments). During these measurements the samples were placed in a Faraday cage. 2.2. Materials. N-[11-Mercaptoacetylundeca]quinolinium tricyanoquinodimethanide (CH3COSC11H22Q-3CNQ) was synthesized in four steps, as shown in Figure 1. Step I involves the synthesis of 11-tosyl-1-undecamercaptol from the reaction of 11mercapto-1-undecanol (Aldrich, 1.0 g, 4.8 mmol) and tosyl chloride (Aldrich, 1.11 g, 5.88 mmol) in methylene chloride (18 mL) at 0 °C in the presence of pyridine (0.59 mL, 7.3 mmol). The resulting product was washed with water and dilute HCl. The residue from the organic layer was purified by column chromatography to yield the tosylate (1.01 g, 58% yield). 1H NMR (CDCl3, 360 MHz): δ 7.778 (d, 2H, J ) 8.28 Hz), 7.34 (d, 2H, J ) 8.28 Hz), 4.0 (t, 2H, J ) 6.12 Hz), 2.50 (t, 2H, J ) 7.2 Hz), 2.44 (s, 3H), 1.64-1.57 (m, 4H), 1.34-1.21 (m, 14H). 11-Tosyl-1-undecamercaptol (Aldrich, 1.0 g, 2.71 mmol) thus obtained was stirred overnight with triethylamine (0.58 mL, 4.18 mmol) and acetic anhydride (0.21 mL, 3.07 mmol) at 0 °C in step II. The resulting product was washed with water and the solvent was then evaporated, to get solid 11-S-acetylundeca-1-tosylate (1.01 g, 99% yield). 1H NMR (CDCl3, 360 MHz): δ 7.78 (d, 2H, J ) 8.28 Hz), 7.34 (d, 2H, J ) 8.28 Hz), 4.01 (t, 2H, J ) 6.84 Hz), 2.85 (t, 2H, J ) 7.2 Hz), 2.44 (s, 3H), 2.31(s, 3H), 1.64-1.53 (m, 4H), 1.341.21 (m, 14H). MS (m/z, % relative intensity): 401 (M+ + 1, 83), 357 (89), 340 (33), 229 (67), 185 (100), 173 (56), 155 (72), 142 (80), 129 (37), 115 (46). In step III, 11-S-acetylundeca-1-tosylate (0.51 g, 1.27 mmol) was heated with lepidine (0.18 g, 1.27 mmol) for 12 h in an oil bath under nitrogen at 120 °C. The reaction mixture was cooled to room temperature. The gummy residue was triturated with anhydrous ether, and the ether layer was decanted; the product, N-[(11-mercaptoacetyl)undecyl]lepidinium tosylate, was obtained as a yellowish green liquid (0.59 g, 85% yield). 1H NMR (CDCl3, 360 MHz): δ 9.8 (d, 2H, J ) 5.4 Hz), 8.31

Electrical Rectification in a Monolayer of Zwitterions

Langmuir, Vol. 19, No. 21, 2003 9045

Figure 1. Reaction scheme for the synthesis of CH3COSC11H22Q+-3CNQ-, 6. (d, 2H, J ) 8.64 Hz), 8.14 (m, 1H), 7.94-7.92 (m, 2H), 7.79 (d, 2H, J ) 5.4 Hz), 7.09 (d, 2H, J ) 7.56 Hz), 5.10 (t, 2H, J ) 6.84 Hz), 2.93 (s, 3H), 1.96-1.595 (m, 2H), 1.55-1.52 (m, 2H), 1.341.19 (m, 14H). Finally, CH3COSC11H22Q-3CNQ was prepared from the reaction of N-[(11-mercaptoacetyl)undecane]lepidinium tosylate (0.61 g, 1.12 mmol) and LiTCNQ (0.475 g, 2.24 mmol) in DMSO (6 mL) (step IV); pyridine (0.35 mL) was slowly added to the above reaction mixture, and the mixture was heated at 95-100 °C for 4 h. The reaction mixture was then cooled, and a small amount of acetonitrile was added to the above mixture, to separate compound 6 as a blue-green solid (0.36 g, 58% yield). 1H NMR (CDCl , 360 MHz): δ 9.43 (d, 1H, J ) 6.12 Hz), 8.75 (d, 3 1H, J ) 8.28 Hz), 8.56 (d, 1H, J ) 8.64 Hz), 8.47 (t, 1H, J ) 6.48 Hz), 8.25 (t, 1H, J ) 7.92 Hz), 8.02 (t, 1H, J ) 7.92 Hz), 7.74 (d, H, J ) 8.28 Hz), 6.89 (d, 2H, J ) 8.64 Hz), 2.80 (t, 2H, J ) 7.2 Hz), 2.30 (s, 3H), 1.97 (m, 2H), 1.47-1.45 (m, 4H), 1.22 (m, 12H). 13C NMR (CDCl , 90 MHz): δ 150.25, 147.82, 147.07, 137.51, 3 135.18, 129.56, 127.97, 127.54, 127.15, 123.89, 123.16, 121.57, 121.47, 119.45, 119.26, 118.36, 116.80, 79.16, 57.03, 30.57, 29.09, 28.78, 28.43, 28.30, 28.08, 25.81. MS (m/z, % relative intensity): 548 (M+, 19), 293 (13), 255 (100), 227 (14), 179 (39), 141 (48), 127 (10), 116 (32). UV-vis (solution in CHCl3) λmax/nm (log10 ): 319 (3.87), 427 (3.71), 465 (3.74), 752 sh (4.32), 838 (4.71), 930 (4.87). 2.3. Preparation of LB Monolayers. LB monolayers of CH3COSC11H22Q+-3CNQ-, 6, were deposited onto gold substrates. These gold substrates were prepared by thermal deposition of an Au layer (200 nm thick) onto clean silicon wafers with a prior Cr adhesion layer (ca. 6 nm thick) at the rate of about 1 nm s-1 under a vacuum of 10-6 Torr, using an Edwards model EL306 coater. The gold substrates were stored in deionized water. Before use, the substrates were cleaned in UV/O3 atm for 2 min, to remove any organic contamination. A millimolar solution of CH3COSC11H22Q+-3CNQ- in chloroform (concentrated 1.4-10-4 M)

Figure 2. Surface pressure (Π)-molecular area (A) isotherm of CH3COSC11H22Q+-3CNQ-, 6. was spread onto an aqueous subphase in a NIMA model 622D2 Langmuir trough. Deionized water with a resistivity of 18 MΩ cm was used as the subphase. The compression was performed at a surface temperature of 14 ( 1 °C with a continuous speed of 20 cm2 min-1. The monolayers were transferred on the upstroke onto the gold substrates at a surface pressure of 12 mN m-1, at a speed of 5 mm min-1. All the LB experiments were performed under a green safe light. The samples were dried for 2 days in a vacuum desiccator containing P2O5 before the measurements.

9046

Langmuir, Vol. 19, No. 21, 2003

Jaiswal et al.

Figure 3. (a) XPS survey scan (0-500 eV binding energy) of CH3COSC11H22Q+-3CNQ-, 6, deposited as a SAM on Au. (b) Highresolution S(2p) XPS of a SAM of CH3COSC11H22Q+-3CNQ-, 6, on Au. (c) High-resolution S(2p) XPS of CH3COSC11H22Q+-3CNQ-, 6, on a Si substrate. (d) High-resolution Si(2s) XPS of CH3COSC11H22Q+-3CNQ-, 6, and of the Si substrate. (e) High-resolution S(2p) XPS of LB monolayer of CH3COSC11H22Q+-3CNQ-, 6, on Au. A weak feature at 171 eV (signal-to-noise ratio of 1.5 to 1) indicates a S atom that is not bound to Au; the feature at 161-164 eV (signal-to-noise ratio of about 3:1) indicates a S atom partially ionically bonded to Au (thiolate). 2.4. Preparation of SAM. The substrates used for the deposition of SAMs were obtained commercially from Molecular Imaging (Tempe, AZ). These substrates were prepared from

cleaved mica with a epitaxially grown gold (ca. 150 nm thick). The SAMs of 6 was prepared using a so-called base-promoted adsorption.36 The compound was dissolved in acetone to a

Electrical Rectification in a Monolayer of Zwitterions

Langmuir, Vol. 19, No. 21, 2003 9047

Figure 4. (a) STM image of SAM of CH3COSC11H22Q+-3CNQ-, 6, on Au (111). (b) STM image of LB monolayer of CH3COSC11H22Q+3CNQ-, 6, on Au(111). The molecules are fairly ordered in the bottom right-hand quadrant; they are visible as irregular parallelograms of sides 9 and 5.2 Å. The diagram shown on the right shows schematically how, at least for the somewhat ordered domain that is visible, a foreshortened molecule, with the thioacetyl group closest to the top and the dicyanomethide group closest to the Au substrate on bottom, forms a periodic array. It is possible that elsewhere in the sample attachment of a thiolate to Au occurs, but such an arrangement is not visible in this micrograph. concentration of 0.5 × 10-3 M. The solution was kept in an ultrasonic bath (Fisher Scientific FS5) for 10 min and filtered through a syringe filter. Concentrated NH4OH (20 µL) was then added, and the mixture was incubated for 20-30 min, to deprotect the thiol group. A gold substrate was then immersed into the above solution for a period of 24-36 h. All the solutions were freshly prepared and prepurged with nitrogen for 30 min, to (36) Fan, F. F.; Yang, J.; Cai, L.; Price, D. W., Jr.; Drik, S. M.; Kosynkin, D. V.; Yao, Y.; Rawlett, A. M.; Tour, J. M.; Bard; A. J. J. Am. Chem. Soc. 2002, 124, 5550-5560.

minimize dissolved oxygen. The solutions were kept in the dark during immersion. After the assembly, the samples were removed from solution, rinsed thoroughly with acetone, and blown dry with a stream of nitrogen gas.

3. Results and Discussion 3.1. Pockels-Langmuir Monolayers at the AirWater Interface. The surface pressure (Π) versus molecular area (A) isotherm of a monolayer of 6 at the airwater interface exhibits (Figure 2) a phase transition,

9048

Langmuir, Vol. 19, No. 21, 2003

Jaiswal et al.

which is represented by a “plateau” at Π ) 12-17 mN m-1. The area per molecule extrapolated to zero surface pressure (Π ) 0) was calculated as 60 ( 1 Å2. Similar results have also been observed by Ashwell and co-workers, who found that the Π-A isotherms of CnH2n+1Q-3CNQ homologues, with n between 8 and 14, occasionally show a plateau.37 A possible explanation is that some of the molecules organize themselves at the air-water interface in an antiparallel arrangement, which is favored by the dipole-dipole interactions between the molecules.38 These interactions between the molecules cause the surface pressure, as a function of decreasing molecular area, not to rise as high as would be expected, if the van der Waals interactions between molecules were dominant. However, considering the decrease in the area/molecule (45-30 Å2) in the above range, the plateau may also be attributed to

a slow collapse or to the incipient formation of bilayers or multilayers, which then gives a fairly compact but “unstable layer” floating at the air-water interface. These monolayers were stable only at low surface pressure, i.e., up to 12 mN m-1. The transfer pressure, therefore, was selected as 12 mN m-1, to minimize the deposition of bilayers, multilayers, or aggregates. The monolayer was fairly stable over about 1 h at 12 mN m-1, with no appreciable decrease in the area during that time interval. The transfer was performed on upstroke onto a gold substrate, with a transfer ratio of about 35%. 3.2. X-ray Photoelectron Spectroscopy Studies. SAMs and LB monolayers of 6 were studied by X-ray photoelectron spectroscopy (XPS) on a polycrystalline Au surface. Survey XPS spectra were recorded for a SAM of 6 absorbed onto Au, in the binding energy range of 0-500 eV; the presence of the appropriate chemical elements in each sample was confirmed. Figure 3a is a survey scan of a SAM of CH3COSC11H22Q3CNQ on Au. N(1s), C (1s), and S(2p) are found in the survey scan. To obtain the details of the binding energy, high-resolution ((0.1 eV) XPS spectra were also obtained. Figure 3b shows a high-resolution S(2p) XPS of a SAM of 6 on Au. The binding energy of S(2p) is found at 162.5 eV, which is due to S(2p3/2).39,40 However, a small shoulder peak is also found at about 163.5 eV, which is due to S(2p1/2).39,40 In addition, the S(2p) and Si(2s) XPS of pure CH3COSC11H22Q-3CNQ (with no base added) on Si were measured as a control experiment (Figure 3c). The S(2p) in SCOCH3 appears as a broad peak between 174 and 164 eV. The binding energy of Si(2s) appears at 151.3 eV, indicating that there is no charging of the substrate (Figure 3d). The above facts indicate the breakup of the CH3CO-S bond and the formation of the S-Au bond during the formation of a SAM. The monolayer thickness was measured by scanning the Au(4f) region before and after a gradual sputtering away of the LB monolayer. The complete removal of the organic layer was determined by the disappearance of the C(1s) photoelectron peak. The film thickness was calculated from the ratio of the Au(4f) peaks with and without the LB monolayer, by using the formula d ) -λ ln(Id/I0), where d is the thickness of the LB monolayer, λ is the mean free path of the Au(4f) photoelectron, Id is the Au(4f) intensity with the LB monolayer present, and I0 is the Au(4f) intensity after the monolayer was judged to be completely removed. The resulting thickness of the LB monolayer was 18 ( 2 Å. The calculated length of a fully extended molecule of CH3COSC11H22Q+-3CNQ-, 6, absent the COCH3 group, is approximately 27 Å (Hyperchem), which indicates that the thioalkyl chain and/or the aromatic headgroup of the molecule must be tilted by cos-1(18/27) ) 48° from the surface normal. A similar tilt is also seen for LB films of 1.10 The XPS of an LB monolayer of 6 on Au, recorded under the same conditions as for the SAM, exhibits very weak signals, indicating a lower surface coverage for the LB monolayer. Figure 3e shows the high-resolution S(2p) peak of the LB film. The two sets of broad peaks in the binding energy range 171-174 eV and 161-164 eV are associated with S(2p) in S-COCH3 and S(2p) bonded to Au, respectively. This result indicates that the organization within the LB monolayers is such that some of the molecules have formed the covalent-ionic S-Au bond, i.e., are

(37) Ashwell, G. J.; Jefferies, G.; Dawnay, E. J. C.; Kuczynski, A. P.; Lynch, D. E.; Gongda, Y. J. Mater. Chem. 1995, 5, 957-980. (38) Xu, T.; Morris, T. A.; Szulczewski, G. J.; Metzger, R. M.; Szablewski, M. J. Mater. Chem. 2002, 12, 3167-3171.

(39) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727. (40) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152-7167.

Figure 5. (a) STS I-V curve for a SAM of CH3COSC11H22Q+3CNQ-, 6, on Au(111), measured with a Pt/Ir tip. The higher current in the forward bias corresponds to the electron flow from the tip through the molecule to the Au substrate. (b) STS I-V curve for a LB monolayer of CH3COSC11H22Q+-3CNQ-, 6, on Au, measured with a Pt/Ir tip. The higher current in the forward bias corresponds to the electron flow from the tip through the molecule to the Au substrate in the case where the molecule lies with the thio-acetyl end on the Au substrate. (c) STS I-V curve for a LB monolayer of CH3COSC11H22Q+-3CNQ, 6, on Au, measured with a Pt/Ir tip. The higher current in the reverse bias corresponds to the electron flow from the Au substrate through the molecule to the tip in the case where the molecule lies with the 3CNQ end on the Au substrate.

Electrical Rectification in a Monolayer of Zwitterions

Langmuir, Vol. 19, No. 21, 2003 9049

Figure 6. Favorable conditions for electron (e-) flow through CH3COSC11H22Q+-3CNQ- , 6, during STS. At forward bias, the Pt/Ir tip is negatively charged and Au is positively charged; at reverse bias, the Pt/Ir tip is positively charged and Au is negatively charged.

chemisorbed onto Au, whereas other molecules remain in their original S-COCH3 form, i.e., are physisorbed on Au. 3.3. Scanning Tunneling Microscopy (STM) Images. The STM images of SAM and LB monolayer are shown in parts a and b of Figure 4, respectively. It is observed that the SAM is more uniform and homogeneous, as compared to the LB monolayer. The light diagonal line shown in Figure 4a indicates a one-atom step in the Au(111) atomic plane. The LB monolayers are distributed on the gold substrates as small “domains”, where each domain contains a highly packed and ordered arrangement of the molecules (Figure 4b). Arrays of a repeat unit of 5.2 × 9 Å, consisting of a 2 × 9 Å lighter region, closer to the tip (presumably the alkyl chain), and a 3.2 × 9 Å darker region (presumably the aromatic part), are clearly seen in the lower right quadrant of the STM image of the LB monolayer (see schematic diagram at the right of Figure 4b). The scatter of the ordered domains in the STM image can be explained by the relatively low LB transfer pressure that had to be used to avoid layer aggregation (i.e. Π ) 12 mN m-1), where the floating monolayer was not sufficiently compact. A transfer ratio as low as 35% clearly indicates that the substrate is not fully covered. It seems that the molecules rearrange themselves during the transfer from the air-water interface onto the solid gold substrate, to form compact and ordered regions.37 The chemical affinity of the thiol group for the Au surface might have aided the reorganization of the monolayer during the transfer. 3.4. Nanoscopic Electrical Conductivity by Scanning Tunneling Spectroscopy (STS). SAMs and LB monolayers were studied by a Pt/Ir nanotip at room temperature in air by STM and STS. The I-V plots show clear asymmetries in both the cases (Figure 5). Figure 5a is a representative I-V plot obtained for a SAM. A higher current is observed for positive voltage (i.e., the tip is negatively biased and the Au(111) substrate is positively biased), indicating facile electron flow from the tip through the molecule to the Au substrate. The rectification ratio, defined by the ratio of forward to reverse bias current at the highest bias used (1.8 V in Figure 5a), was found to be 4.5 ( 1 for the SAMs.

Figure 7. (a) Current-voltage (I-V) plot for a sandwich “Au | LB monolayer of CH3COSC11H22Q+-3CNQ- | Au”. The curves are symmetric, presumably because the molecules are ordered with equal likelihood in either direction. (b) Currentvoltage (I-V) plot for a SAM of CH3COSC11H22Q+-3CNQ-, 6. The rectification ratio is 1.5, presumably because a slight excess of molecules is found to be oriented with the 3CNQ end facing the bottom electrode. If the chemisorption reaction had occurred as hoped, all molecules would have had the sulfur atom facing the bottom electrode and the 3CNQ end facing the upper electrode.

It is interesting to note that whereas the SAMs rectify only in the forward direction, LB monolayers show rectification both in the forward and in the reverse directions. Parts b and c of Figure 5 show two STS I-V plots of LB films, which show a positive and a negative rectification, respectively. The rectification ratio ranges from 2.5 to 7 in the case of LB films. The XPS results (Figure 3e) suggested that within the LB monolayer, some molecules are oriented so that they form an S-Au bond, whereas other molecules do not form an S-Au bond, probably because they are oriented differently. Further, the properties of CnH2n+1Q+-3CNQfilms, an analogue of CH3COSC11H22Q+-3CNQ-, have been reported to be dependent upon the alkyl chain lengths; the short chain length (n e 14) probably adopts an antiparallel arrangement within the Langmuir layer.37 The XPS results and the STS I-V plots together suggest

9050

Langmuir, Vol. 19, No. 21, 2003

that the CH3COSC11H22Q+-3CNQ- LB films may have an antiparallel arrangement of many, if not most, of the molecules. That is, within the LB monolayer, some molecules lie with their 3CNQ end closest to the Au substrate, whereas others lie with the S-COCH3 end closest to Au. During STS measurements, when the tip is at the 3CNQ end of the molecule, a higher current is observed at positive bias (similar to the behavior of the SAMs). When the tip is at the S-COCH3 end (i.e., at the molecule which has its long chain extended outward), the rectification is observed in reverse direction. The electron flow is enhanced from the negatively biased Au substrate through the molecule to the positively biased tip. The situation is summarized in Figure 6. 3.5. Macroscopic Conductivity. The top gold electrodes were deposited atop the LB and self-assembled monolayer by the so-called “cold gold evaporation technique” as 17 nm thick cylindrical pads, with a diameter of 0.6 mm.9 A drop of Ga/In eutectic was used to make contact with the Au wire electrode (see Figure 4 of ref 9). For LB films, 25 pads out of 30 pads exhibited electrical short circuits; for SAMs, 20 out of 30 pads gave good results and only 10 pads were short-circuited. More short-circuited pads in the case of LB films were probably due to the very low coverage and inhomogeneity of the film. SAMs, on the other hand, resulted in a more compact and homogeneous coverage, leading to a larger number of satisfactory pads. The current (I)-voltage (V) plots for LB monolayers and SAMs are shown in parts a and b of Figure 7, respectively. Symmetric I-V curves were obtained for LB monolayers (Figure 7a); the SAMs, however, showed slight

Jaiswal et al.

rectification in the forward bias with a rectification ratio 1.5 ( 1 (Figure 7b). The reason for a symmetric I-V curve is that the organization of the molecules in LB films is antiparallel to each other. Therefore, current flows with the same ease in both directions, leading to a “symmetric” I-V plot. However, within the SAM, the molecules are oriented in one direction only, i.e., the long tail attached to the gold substrate, causing electrons to flow preferentially in one direction. 4. Conclusion It has been demonstrated that a thiolate termination can be attached to a unimolecular rectifier like 1. This causes a very unsymmetric current-voltage curve for single molecules of CH3COSC11H22Q+-3CNQ-, 6, as seen by STS. The results with macroscopic pads (0.283 mm2 in area) were rather disappointing, presumably because the alkylthioacetyl chains were not long or hydrophobic enough to allow for parallel ordering of all the molecules within the monolayer. Experiments are in progress with a longer alkyl chain (the C11H22 part of CH3COSC11H22Q+3CNQ- is replaced by a C13H26 part) and will be reported on in due time. Acknowledgment. We are grateful to Mr. Tao Xu for constructive suggestions. This work was supported in part by the United States National Science Foundation (Grant DMR-00-95215). LA034073I