Self-Assembled Oligo(phenylene-ethynylene) Molecular Electronic

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Langmuir 2003, 19, 8245-8255

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Self-Assembled Oligo(phenylene-ethynylene) Molecular Electronic Switch Monolayers on Gold: Structures and Chemical Stability Joshua J. Stapleton,† Philipp Harder,†,§ Thomas A. Daniel,† Michael D. Reinard,† Yuxing Yao,‡ David W. Price,‡ James M. Tour,*,‡ and David L. Allara*,† Department of Chemistry and The Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, and Department of Chemistry and Center for Nanoscale Science and Technology, Rice University, Houston, Texas 77005 Received July 1, 2003 Self-assembled monolayers (SAMs) of the nitro-substituted oligo(phenylene-ethynylene) (OPE) 4,4′(diethynylphenyl)-2′-nitro-1-benzenethiolate on Au{111} were prepared, and the structures were characterized by multiple techniques, including infrared spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy. Assembly of the nitro-OPE SAM, either via acidic hydrolysis of the thioacetate derivative or from the thiol in pure solvent, produces a well-ordered SAM with a (x3 × x3) superlattice structure and an average molecular tilt of 32-39° from the surface normal. In comparison, SAMs prepared from the unsubstituted OPE show the same lattice structure and a similar tilt of ∼33°. In contrast, when the nitro-OPE SAM is assembled by hydrolysis of the thioacetate derivative under basic conditions, extensive redox reactions arise in which oxidation of the S atoms occurs with accompanying reduction of -NO2 to -NH2, apparently via intermediates including -NH(OH), to form mixed composition SAMs typically containing ∼30% of the amino-substituted molecule. Further, the nitro-OPE SAM, regardless of the preparation method, shows significant chemical instability under storage in air and/or light exposure. Since the nitro-OPE molecule and molecules with related structures are of considerable interest for molecular electronics applications, these results indicate that extreme diligence must be used in designing conditions for the fabrication of devices utilizing these SAMs.

1. Introduction The electrical conduction properties of organic molecules have become of intense interest recently,1-38 driven mainly by the possibilities for molecule-based electronics.39 Avi-

ram and Ratner originally proposed in 1974 that functionalized aromatic molecules could be used in the construction of electronic devices.40 Over two decades later, reports appeared on the demonstration of such devices,

* Corresponding author. E-mail: [email protected]. † Pennsylvania State University. ‡ Rice University. § Present address: SuNyx Surface Nanotechnologies GmbH, Stolberger Strasse 370, D-50933 Ko¨ln/Cologne, Germany. E-mail: [email protected].

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Stapleton et al. Table 1. Summary of Chemical Structures and Notations of SAM Precursor Molecules

that currently appear to offer the greatest potential for SAM-based logic and memory devices. In particular, the family of aromatic organic molecules that has demonstrated particularly interesting electrical properties is based on derivatives of the molecule 4,4′-di(phenyleneethynylene)benzenethiol.70 The fully conjugated and rigid nature of these oligo(phenylene-ethynylene) (OPE) compounds has been shown to enhance electrical conduction when compared to alkanethiol films of similar thickness.18,71 Further, changing ring substituents can significantly alter the current-voltage (i-V) response.7,8,19 Single molecule scanning tunneling microscopy (STM) measurements of Dhirani and co-workers have shown that the unfunctionalized OPE 1 (see Table 1), chemisorbed on polycrystalline Au(111), behaves like a molecular rectifier.33 By substitution on the central phenyl ring with -NH2 and -NO2 groups, negative differential resistance (NDR) has been observed in a nanopore structure at 60 K7 while a SAM made of OPE molecules with a single -NO2 group on the central phenyl ring, compound 2 (Table 1), shows NDR behavior at 260 K.8 These results indicate that a wide variety of device properties may be achievable by changing the nature and positions of substituents.70 While these types of results demonstrate significant potential for new types of exploratory devices, a detailed understanding of the molecular and interfacial structures and the chemical behavior is needed to provide a firm basis for rational design and efficient engineering. The only characterization of these OPE SAMs appears to be studies of 1a on Au{111} in which STM images revealed ordering28,48 while aqueous electrochemical measurements showed poor current blocking characteristics62 when compared to SAMs of alkanethiols. In this paper, we present a detailed study of the preparation and characterization of SAMs of the nitrosubstituted OPE (2) on Au{111}. A study of the unfunctionalized OPE (1) also was done in order to provide a comparison sample and to establish further important (70) Tour, J. M. Acc. Chem. Res. 2000, 33, 791-804. (71) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J. Y.; Gozin, M.; Kayyem, J. F. J. Am. Chem. Soc. 1999, 121, 1059-1064.

Molecular Electronic Switch Monolayers

details on this SAM that are not available in the previous reports.28,48,62 Our approach incorporates multiple characterization probes including quantitative infrared reflection spectroscopy (IRS), X-ray photoelectron spectroscopy (XPS), molecular resolution atomic force microscopy (AFM), single wavelength ellipsometry (SWE), cyclic voltammetry, and contact angles. Our results show that when assembled using either acid cleavage of the precursor thioacetate in ethanol solution or an ethanol solution of the pure thiol, both SAMs form essentially identical hexagonally ordered monolayers with the approximate Au{111} x3 × x3 lattice spacing and average molecular tilt angles of ∼32-39°. In contrast, assembly of the 2-based SAM via deprotection of the thioacetate 2b under basic conditions can result in significant redox side reactions of the Ar-S and Ar-NO2 groups (where Ar ) the aromatic ring structure) with replacement of the -NO2 group by -NH2, thereby changing the intrinsic chemical nature of the SAM. Finally, the SAMs show significant sensitivity to environmental degradation in light and air. These results show that the chemical stabilities of these OPE types of SAMs must be carefully considered in designing methods for fabrication of reliable molecular devices. 2. Experimental Section 2.1. Sample Preparation. Methyl alcohol (Aldrich, 99.8% A.C.S. reagent), sulfuric acid (J. T. Baker, A.C.S. reagent), and ammonium hydroxide (Aldrich, 30% NH4OH in water) were used as received. Tetrahydrofuran (THF; Aldrich, 99.9%, anhydrous, inhibitor free) and dichloromethane (Aldrich, HPLC grade) were purified before use. Dichloromethane was distilled over P2O5 under a nitrogen atmosphere. Tetrahydrofuran was purified by distillation over metallic sodium and benzophenone under a nitrogen atmosphere. Ethyl alcohol (Pharmco, A.C.S./U.S.P. grade) was degassed through multiple FPT (freeze-pump-thaw) cycles. Water was purified to be organic and ion free (Milli-Q grade water; Millipore Products, Bedford, MA). Two types of gold substrates were used: Au/mica for AFM and Au/Cr/SiO2/Si for IRS. All Au films were deposited from resistive boats at background pressures below 1 × 10-8 Torr during deposition. The Au/Cr/SiO2/Si films were deposited on room temperature substrates and typically showed a root-mean-square (rms) roughness of ∼1.0 nm by tapping mode AFM. The Au/mica films were deposited with the mica heated to 340 °C. The freshly evaporated Au/Cr/SiO2/Si films were immediately characterized by SWE and then used within 30 min of ambient exposure to form the SAM. The Au/mica samples were flame annealed prior to being placed into the SAM-forming solution. Procedures for the synthesis of 1b, 2b, and 3b have been reported elsewhere.72 Previous reports have indicated the best method for forming monolayers from acetyl-terminated OPEs is via an in situ cleavage of the acetyl group in dilute NH4OH in THF.8,50,59 While we find this method works well for some compounds, it will be shown that significant problems can arise when these conditions are used in the preparation of NO2substituted-molecule SAMs. For this, base-free procedures were explored,73 and the one described herein is designated as our standard procedure and was used for preparation for monolayers of 1 and 2, unless otherwise noted. All procedures were carried out in a nitrogen-purged glovebox with the O2 concentration kept below 2 ppm to avoid oxidative degradation of the reagents and the SAMs. All solutions were mixed and stored in screwtop fluoroware containers. SAMs of 1 and 2 were formed in a standard procedure as follows. First, 0.1-0.5 mM solutions were prepared by dissolving the thioacetate (1b or 2b) in ethanol, typically involving a 5 min sonication period. After filtration (0.20 µm syringe filter), 10 mL aliquots were mixed with 120 µL of concentrated H2SO4 and (72) Tour, J. M.; Rawlett, A. M.; Kozaki, M.; Yao, Y. X.; Jagessar, R. C.; Dirk, S. M.; Price, D. W.; Reed, M. A.; Zhou, C. W.; Chen, J.; Wang, W. Y.; Campbell, I. Chem.sEur. J. 2001, 7, 5118-5134. (73) Cai, L.; Yao, Y.; Yang, J.; Price, D. W., Jr.; Tour, J. M. Chem. Mater. 2002, 14, 2905-2909.

Langmuir, Vol. 19, No. 20, 2003 8247 incubated for 1.5 h to allow cleavage of the thioacetyl group. The gold substrates then were added, and the solution was stored overnight. During the SAM formation, the container top was secured tightly and the container was covered with aluminum foil to prevent photoinduced degradation of the nitro group.74 After monolayer formation, the wafers were rinsed with copious amounts of distilled THF and dried under a stream of N2. Monolayers of 3 were prepared using the same procedure as for 1 and 2 except that the thioacetyl group was cleaved using a 120 µL aliquot of NH4OH. Monolayers of 3 formed using the acidic cleavage process were generally of lower coverages than those made using the basic cleavage, as shown by IRS and SWE measurements. The reasons for this were not investigated but could be related to the possible protonation of the amine group in the acid during monolayer formation. 2.2. Characterization Methods. The SWE measurements were recorded at 632.8 nm using a null ellipsometer (Rudolph Research, Auto EL-II; Flanders, NJ) set at a 70° angle of incidence. The experimental polarization angles were used to determine film thickness using well-established modeling methods.75,76 For simplicity, the thicknesses were based on isotropic film models in which the complex refractive index is described as a scalar, n ) n + ik. More rigorous anisotropic models77 were not considered since accurate values for the elements of the diagonalized refractive index tensor are not available and the variations from the isotropic model were assumed to be close to the error limits of our measurements (ca. (0.1 nm). In addition, the inclusion of an optically distinct Au-S interface layer was not considered.78 For all compounds, k (at 632.8 nm) was set to zero while n was estimated from atom-fragment quantitative structure-activity relationships (QSAR)79 to give values of 1.759, 1.749, and 1.768 for 1a, 2a, and 3a, respectively. The required mass densities were obtained using the atom-fragment density estimation method.80 This method has predicted mass densities within 5% of the known values for similar compounds.50 The IR spectra were collected using a custom, in-housemodified Fourier transform infrared (FTIR) spectrometer (Mattson RS/1, Madison, WI) as described in detail elsewhere.81 The spectrometer and external optics were purged with CO2-free, dry air or nitrogen gas. All spectra were taken at an 87° angle of incidence with p-polarized light and the instrument resolution set to 2 cm-1. Reflection spectral intensities are reported as -log(R/R0), where R is the power reflectivity of the IR beam and R0 is the reflectivity of a reference sample. Several different reference samples were used depending on the spectral region of interest and included C12D25S-/Au SAMS, C16H33S-/Au SAMS, and wafers of evaporated gold films cleaned by UV-ozone exposure or by immersion in H2O2 (30%)/NH4OH mixtures (at room temperature). Transmission spectra were recorded for pressed disk samples of dispersions of the pure thiols in KBr. The corresponding spectral intensities are reported as -log(T/T0), where T and T0 are the transmitted peak power through KBr disks with and without the molecule added, respectively. The XPS analyses were performed on a monochromatic Al KR source instrument (Kratos, Axis Ultra; United Kingdom). All spectra were referenced to the Au 4f7/2 binding energy at 84.00 eV. Samples were subjected to X-ray beam exposure for varying times to determine the amount of exposure that resulted in film (74) Morrison, H. A. The photochemistry of nitro and nitroso groups. In Chemistry of the Nitro and Nitroso Groups; Feuer, H., Ed.; Interscience: New York, 1969; pp 165-214. (75) Collins, R. W.; Kim, Y. T. Anal. Chem. 1990, 62, 887-890. (76) Shi, J.; Hong, B.; Parikh, A. N.; Collins, R. W.; Allara, D. L. Chem. Phys. Lett. 1995, 246, 90-94. (77) Parikh, A. N.; Allara, D. L. Effects of Optical Anisotropy on Spectro-Ellipsometric Data for Thin Films and Surfaces. In Optical Studies on Real Surfaces and Inhomogeneous Thin Films; Francombe, M., Ed.; Physics of Thin Films Series; Academic Press: New York, 1994; Vol. 19, pp 279-323. (78) Previous spectroscopic ellipsometry studies have shown the presence of a weakly metallic Au-S interface layer of ∼0.2 nm thickness (ref 76). Experiments are currently in progress in our laboratory to characterize the properties of the interface layers for the current SAMs. (79) Ghose, A. K.; Crippen, G. M. J. Chem. Inf. Comput. Sci. 1987, 27, 21-35. (80) Immirzi, A.; Perini, B. Acta Crystallogr. 1977, A33, 216-218. (81) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 96, 927-945.

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degradation. In all cases, collection times were shown to be shorter than the onset of measurable degradation. The AFM images were recorded using a home-built scan head, based on the design of Salmeron and co-workers,82 which was operated using AFM 100 and SPM 100 control electronics (RHK Technology, Troy, MI). The cantilevers (TM Microscopes, Sunnyvale, CA) were V shaped with spring constants of 2.1 and 1.6 N/m. The lateral dimensions of the AFM were calibrated using a hexadecanethiolate/Au{111} SAM (a ) 5.0 Å; γ ) 60°). The images were taken under ambient conditions, and both normal and lateral forces were recorded. Lattice resolution images were taken in the repulsive contact mode under a constant normal force, and the periodicity was revealed in the lateral force. In typical images, the normal force varied from 20 to 60 nN and the scan rate varied from 200 to 400 nm/s. Cyclic voltammetry was performed using a VoltaLab PGZ 100 potentiostat (Radiometer Analytical, Copenhagen, Denmark). All experiments were performed in the three-electrode mode using a Ag/AgCl reference electrode (MF-2052, Bioanalytical Systems, West Lafayette, IN) and a platinum mesh counter electrode. A custom Teflon sample cell was used for all measurements. In this configuration, a polyimide mask is used to define an electrode area of 0.38 cm2. An O-ring is then pressed between the mask and the solution cell to form a seal. A 1.0 mM K3Fe(CN)6 (Aldrich) solution made with Milli-Q water (Millipore Products) and also containing 100 mM KNO3 (Aldrich) as a supporting electrolyte was prepared freshly before each use. A scan rate of 20 mV/s was used for all measurements. Dynamic advancing and receding contact angles were measured using a home-built video-interfaced apparatus. A 20 µL drop of Milli-Q water (Millipore Products) was dispensed onto the surface using a flat-tipped micrometer syringe (GS-1200, Gilmont Instruments, Barrington, IL). The advancing contact angle was measured as the drop expanded. The drop was then partially retracted into the syringe, and the receding angle was measured. Images of the drop were captured digitally using a CCD camera, and contact angles were analyzed using Scion Image for Windows software Beta 4.0.2 (Scion Corp., Frederick, MD). The 1H and proton decoupled 13C NMR spectra were recorded with a Bruker DRX 400 spectrometer. Samples were prepared inside a N2-purged glovebox with an O2 level of less than 10 ppm. Each sample tube was dissolved in 1 mL of 99.5% THF-d8 (CIL, DL-36). The δ scale of the 1H and 13C spectra was calibrated with literature values for the THF proton and carbon signals. 2.3. Quantum Calculations of Vibrational Spectra. The vibrational structure of each SAM molecule was investigated by performing density functional theory (DFT) calculations on the isolated thiol. The calculations were performed on a custombuilt 10-PC linux cluster (Los Alamos Computers, Los Alamos, NM) using the Gaussian 98 program package.83 Vibrational modes were analyzed using the Molden molecular visualization program.84 Optimized geometries and frequencies were calculated using the B3LYP85,86 functional with the 6-31G(d,p) basis set.87 Several authors have shown frequencies calculated by such gradient corrected functionals to be in good agreement with experimental spectra for various aromatic compounds.88-91 (82) Kolbe, W. F.; Ogletree, D. F.; Salmeron, M. B. Ultramicroscopy 1992, 42-44, 1113. (83) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9; Gaussian, Inc.: Pittsburgh, PA, 1998. (84) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000, 14, 123-134. (85) Becke, A. D. J. Chem. Phys. 1993, 98, 1372-1377. (86) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785-789. (87) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; DeFrees, D. J.; Pople, J. A.; Binkley, J. S. J. Am. Chem. Soc. 1982, 104, 5039-5048.

Stapleton et al.

Figure 1. The XPS S 2p region spectra of the 1-, 2-, and 3-SAMs, shown as curves a, b, and c, respectively. The dotted lines show the curve-fitted components of the two peaks. Spectra were collected with a takeoff angle of 90° and a pass energy of 40 eV.

3. Results and Discussion 3.1. Molecular Structure of SAMs Prepared by the Standard Procedure. 3.1.1. Thickness and Coverage. Based on multiple measurements at different spots on several different samples for each type of SAM, the average SWE thicknesses are 1.94 ((0.1), 1.91 ((0.1), and 1.88 ((0.1) nm for SAMs of 1, 2, and 3, respectively. The total length of the molecules from the terminal hydrogen to the sulfur atom is 1.94 nm based on the optimized geometries calculated using the B3LYP85,86 functional with the 6-31G(d,p) basis set.87 Given the approximations in the modeling of the SWE data (particularly the lack of modeling a discrete Au-S interface layer and using an isotropic approximation; see section 2.2), we regard these values only as a close approximation to the actual physical thicknesses within 0.1 nm or so. An independent estimate of film thickness was determined from our XPS measurements (spectra shown in Figures 1-4). In previous studies, the thickness of SAMs on gold has been determined using C 1s/Au 4f peak area ratios in conjunction with alkanethiolate/Au SAM references.52,92 We extended this analysis method by assuming that the previously determined attenuation lengths of 3.5 and 4.5 nm for the C 1s and Au 4f photoelectrons,93 respectively, in the n-C22H45S-/Au SAM apply to the 1-, 2-, and 3-SAMs. This analysis yields thicknesses of 1.93 ((0.1), 1.81 ((0.1), and 1.85 ((0.1) nm for the 1-, 2-, and 3-SAMs, respectively.94 These values are consistent with the SWE values. The XPS data also verify the chemical integrity of the SAMs prepared by the standard procedure. The S 2p region of monolayers of the 1-, 2-, and 3-SAMs (Figure 1a-c, respectively) shows the expected S 2p3/2, S 2p1/2 doublet at 162, 163.2 eV with a 2:1 intensity ratio, typical for alkanethiols chemisorbed on gold.34 The C 1s region of monolayers of 1, 2, and 3, given in Figure 2a-c, respectively, shows the respective C 1s peaks at 284.3, 284.5, (88) El-Azhary, A. A.; Suter, H. U. J. Phys. Chem. 1996, 100, 1505615063. (89) Kwiatkowski, J. S.; Leszczynski, J. J. Phys. Chem. 1996, 100, 941-953. (90) Gellini, C.; Moroni, L.; Muniz-Miranda, M. J. Phys. Chem. A 2002, 106, 10999-11007. (91) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502-16513. (92) Wesch, A.; Dannenberger, O.; Woll, C.; Wolff, J. J.; Buck, M. Langmuir 1996, 12, 5330-5337. (93) Hansen, H. S.; Tougaard, S.; Biebuyck, H. J. Electron Spectrosc. Relat. Phenom. 1992, 58, 141-158.

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Figure 2. The XPS C 1s region spectra of monolayers of the 1-, 2-, and 3-SAMs, shown as curves a, b, and c, respectively. Spectra were collected with a photoelectron takeoff angle of 45° and a pass energy of 40 eV.

Figure 3. The XPS O 1s region spectrum of the 2-SAM. Spectra were collected with a photoelectron takeoff angle of 45° and a pass energy of 40 eV.

and 284.2 eV. The asymmetry of these peaks indicates a spread in the intrinsic binding energies of the various ring C atoms. The O 1s region of a monolayer of 2 is shown in Figure 3 with a peak centered at 532.3 eV. The N 1s spectra for monolayers of 2 and 3 are seen in Figure 4. The 2-SAM has a single peak at 405.5 eV, assigned to the -NO2 group (Figure 4a), while the 3-SAM has a single peak at 399.1 eV, assigned to an -NH2 group (Figure 4c). Both of these peak positions are in agreement with results previously published for aromatic nitro and amine groups.55 In the case of the 2-SAM, the spectra indicate an area ratio of O 1s/N 1s of 2.1/1 which is within experimental error of the expected 2/1 ratio. Advancing (θadv) and receding (θrec) water contact angles of 78 ( 2° and 69 ( 2°, respectively, were obtained for all (94)

[

] [

]

IC -dsample -dreference (sample) exp 1 - exp IAu λC λAu ) IC -dsample -dreference (reference) exp 1 - exp IAu λAu λC

[

]

[

]

In this equation, IC and IAu are the integrated areas of the C 1s and Au 4f peaks. λC and λAu are the attenuation lengths for the C 1s and Au 4f photoelectrons. dreference is the thickness of the reference sample as determined by single wavelength ellipsometry. dsample is the thickness of the sample and was the value of interest.

Figure 4. The N 1s region of nitro and amino SAMs: (a) 2-SAM prepared via acid cleavage of 2b, (b) 2-SAM prepared via base cleavage of 2b (note the intensity has been multiplied by 2.0 relative to the other plots), and (c) 3-SAM prepared via base cleavage of 3b. The spectra were collected with a photoelectron takeoff angle of 45° and a pass energy of 40 eV.

three SAMs. The advancing contact angle measured for the 1-SAM is within experimental error of previously reported data for that made from 1a.62 While these data cannot directly be interpreted in terms of absolute coverage, the 9° hysteresis is reasonable for well-packed SAMs with molecularly smooth surfaces.95 The general packing density of the SAMs was probed using cyclic voltammetry, a method that gives a qualitative measure of the ion transport blocking property of a film.96 In all cases, FeIII(CN)63- was used as the redox active species. The cyclic voltammograms (CVs) for bare gold and a CH3(CH2)15S-/Au SAM are shown for reference in Figure 5a. The CVs for the 1-, 2-, and 3-SAMs are shown in Figure 5b. When compared to the blocking properties of a CH3(CH2)15S-/Au SAM, the blocking properties of 1-, 2-, and 3-SAMs are relatively poor. The 1- and 2-SAMs allow a current of ∼50 µA/cm2 at -0.15 V compared to