Coupled Surface-Enhanced Raman Spectroscopy and Electrical

Simultaneous Measurements of Electronic Conduction and Raman Response in .... Vibrational Signature of Double-End-Linked Molecules at Au Nanojunctions...
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Anal. Chem. 2006, 78, 120-124

Coupled Surface-Enhanced Raman Spectroscopy and Electrical Conductivity Measurements of 1,4-Phenylene Diisocyanide in Molecular Electronic Junctions Archana Jaiswal, Kusha G. Tavakoli,† and Shouzhong Zou*

Department of Chemistry & Biochemistry, Center for Nanotechnology, Miami University, Oxford, Ohio 45056

Probing the structure of molecules in a metal-moleculemetal junction under an applied voltage is critical for understanding molecular electron transport properties. We present an approach that allows recording surfaceenhanced Raman spectra simultaneously with electrical measurements of a monolayer of molecules in molecular electronic junctions. 1,4-Phenylene diisocyanide in two different types of junctions was used to illustrate the approach. The results show that the molecular integrity was intact in the molecular junctions and under the applied bias. The monolayer sensitivity of the approach provides a new powerful tool for characterizing molecular structure in a molecular electronic junction. Recently, molecular electronics has been a subject of active research driven mainly by the need of further miniaturizing electronic devices.1,2 Interesting molecular electron transport properties, such as rectification,3-5 conductance switching,6-8 and negative differential resistance (NDR)9-11 have been reported for several molecular systems. A good understanding of these properties, though crucial for developing future molecular electronic devices, is largely absent due partly to the lack of direct molecular structure information. For example, the NDR effect is * Corresponding author. Phone: 513-529 8084. Fax: 513-529 5715. E-mail: [email protected]. † NSF-REU student from the University of Pennsylvania. (1) McCreery, R. L. Chem. Mater. 2004, 16, 4477. (2) Kagan, C. R.; Ratner, M. A. MRS Bull. 2004, 29, 376. (3) Metzger, R. M.; Chen, B.; Hopfner, U.; Lakshmikantham, M. V.; Vuillaume, D.; Kawai, T.; Wu, X. L.; 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. (4) Ng, M. K.; Lee, D. C.; Yu, L. P. J. Am. Chem. Soc. 2002, 124, 11862. (5) Honciuc, A.; Jaiswal, A.; Gong, A.; Ashworth, H.; Spangler, C. W.; Peterson, I. R.; Dalton, L. R.; Metzger, R. M. J. Phys. Chem. B 2005, 109, 857. (6) Chen, F.; He, J.; Nuckolls, C.; Roberts, T.; Klare, J. E.; Lindsay, S. Nano Lett. 2005, 5, 503. (7) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303. (8) McCreery, R.; Dieringer, J.; Solak, A. O.; Snyder, B.; Nowak, A. M.; McGovern, W. R.; DuVall, S. J. Am. Chem. Soc. 2003, 125, 10748. (9) Reed, M. A.; Chen, J.; Rawlett, A. M.; Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2001, 78, 3735. (10) Dupraz, C. J. F.; Beierlein, U.; Kotthaus, J. P. Chemphyschem 2003, 4, 1247. (11) Lee, J. O.; Lientschnig, G.; Wiertz, F.; Struijk, M.; Janssen, R. A. J.; Egberink, R.; Reinhoudt, D. N.; Hadley, P.; Dekker, C. Nano Lett. 2003, 3, 113.

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often explained on the basis of molecular structure change or even redox reactions under an applied electric field.6,12,13 However, these explanations are based on either theoretical modeling or indirect experimental evidence.6,12-14 It is highly desirable to probe the structural or chemical changes that occur while molecules are under applied electric fields. This is challenging because currentvoltage measurements are often conducted with the molecule sandwiched between two metal electrodes or with a scanning probe microscope. The molecules are either buried under a metal electrode or situated where the electric field is applied in too small an area for conventional spectroscopic methods, which provide rich molecular information. Nonetheless, several groups have recently reported obtaining molecular structural information from molecules in a device by various vibrational spectroscopic techniques. Wang et al.15 and Kushmerick et al.16 independently reported inelastic electron tunneling spectroscopic (IETS) studies of alkanethiols on Au at low temperatures (∼4 K). Jun and Zhu proposed the use of attenuated total internal reflection infrared spectroscopy to probe molecular structure in a molecular electronic device.17 Sometime ago, Hipps and co-workers reported Raman spectra of phthalocyanine sandwiched between Al/AlOx and a thin film of Pb under two applied biases, but did not observe any applied potential-dependent spectral change.18,19 Of particular interest are a series of recent coupled electrical and resonance Raman spectral studies from McCreery’s group.8,20 In these studies, resonance Raman spectroscopy has shed light on the I-V characteristics of multilayer nitroazobenzene on carbon electrodes.8,20 In the present work, we report on an approach that can be used to simultaneously perform surface-enhanced Raman spectroscopic (SERS) and current (I)-voltage (V) measurements of (12) Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am. Chem. Soc. 2000, 122, 3015. (13) Derosa, P. A.; Guda, S.; Seminario, J. M. J. Am. Chem. Soc. 2003, 125, 14240. (14) Seminario, J. M.; Derosa, P. A.; Bastos, J. L. J. Am. Chem. Soc. 2002, 124, 10266. (15) Wang, W. Y.; Lee, T.; Kretzschmar, I.; Reed, M. A. Nano Lett. 2004, 4, 643. (16) Kushmerick, J. G.; Lazorcik, J.; Patterson, C. H.; Shashidhar, R.; Seferos, D. S.; Bazan, G. C. Nano Lett. 2004, 4, 639. (17) Jun, Y. S.; Zhu, X. Y. J. Am. Chem. Soc. 2004, 126, 13224. (18) Hoagland, J. J.; Dowdy, J.; Hipps, K. W. J. Phys. Chem. 1991, 95, 2246. (19) Hipps, K. W.; Dowdy, J.; Hoagland, J. J. Langmuir 1991, 7, 5. (20) Nowak, A. M.; McCreery, R. L. J. Am. Chem. Soc. 2004, 126, 16621. 10.1021/ac051318i CCC: $33.50

© 2006 American Chemical Society Published on Web 11/17/2005

a monolayer of molecules sandwiched between two metal film electrodes. The enhanced Raman signal comes from a carefully prepared gold film electrode. One advantage of this method is that the Raman spectrum is obtained through surface enhancement, rather than electronic resonance effect. Therefore, the approach is applicable to a wide range of molecules and is able to detect a single monolayer of molecules as is typically used in molecular electronics. Additionally, compared to the IETS, the approach is applicable at both low and ambient temperatures. To demonstrate the approach, we used 1,4-phenylene diisocyanide (PDI) because the molecule has been shown to form a compact monolayer on Au with one of the NC groups attached to the surface and the other pendant, which can therefore bind to a second surface.11,21 Moreover, the molecule shows interesting electron transport characteristics at low temperatures.10,11 EXPERIMENTAL SECTION Materials. 1,4-Phenylene diisocyanide (99%), (3-mercaptopropyl)trimethoxysilane (MCTMS; 95%), and aluminum foil (99.999%) used as the Al deposition source were from Aldrich. Gold shots (99.999%) were purchased from Alfa Aesar. ACS grade potassium hydroxide, chloroform, acetone, and smooth glass slides were from Fisher Scientific. Absolute ethanol (ACS grade) and methanol (HPLC grade) was from Pharmco. Device Fabrication. The device used in the I-V measurements consists of two metal film electrodes. The bottom electrode was prepared by thermal evaporation (CV301, Cooke Vacuum Products) of Au on glass slides with a deposition rate of 0.3 Å s-1. The base pressure of deposition was typically 5 × 10-7 Torr. Before Au deposition, the glass substrates were cleaned by 30min sonication in acetone, chloroform, and 2.5 M aqueous KOH solution successively and rinsed with methanol three times before being dried in an oven. The dried, cleaned glass slides were then coated with a self-assembled monolayer of MCTMS as an adhesion layer by soaking in a 1:10 (v/v) MCTMS/ethanol solution overnight. Au films deposited on MCTMS-covered glass slides have been shown to yield excellent SERS activity.22 To obtain optimal substrates for both SERS and I-V measurements, a variety of Au films were prepared by mounting the glass slides at different heights with respect to the gold source in the evaporation chamber. At each height, Au films with thicknesses ranging from 14 to 25 nm were fabricated in order to obtain an optimal thickness for SERS. The thickness was monitored using a quartz crystal microbalance (Sycon Instruments). Right after the Au film evaporation, the slides were immersed in 1 mM ethanolic solution of PDI overnight to form the PDI monolayer. The slides were then washed with ethanol and blown dry with nitrogen before being used. The top metal pads were vapor deposited onto the PDI-coated slides using a shadow mask containing arrays of circular holes of 0.5-mm radius (Figure 1A). A thickness of 15 nm is chosen for the top electrode to ensure good optical transparency and still allow for good electric conductivity. To prevent the damage of the PDI layer during the top pad deposition, the deposition chamber was back-filled with a few milli-Torr of nitrogen.23 The measurements on the devices were conducted within 2 days after they were fabricated. (21) Robertson, M. J.; Angelici, R. J. Langmuir 1994, 10, 1488. (22) Mosier-Boss, P. A.; Lieberman, S. H. Appl. Spectrosc. 1999, 53, 862. (23) Metzger, R. M.; Xu, T.; Peterson, I. R. J. Phys. Chem. B 2001, 105, 7280.

Figure 1. (A). Photograph of the top view of an array of 1-mm gold circular pads. (B). Schematic diagram of the experimental setup for coupled SERS and I-V measurements on a Au/PDI/Au molecular electronic junction.

Instrumentation and Measurements. Raman spectra were recorded using a portable Raman microprobe (Spectra Code) equipped with a SpectraPro 300i triple-grating spectrometer (Acton Research) coupled to a liquid nitrogen-cooled CCD detector (Spec10, Princeton Instruments). Typical spectral resolution is 4 cm-1. The Raman excitation at 785 nm was from a diode laser (Process Instrument). A long working distance 20× objective was used to focus the laser beam onto a top pad where the electrical measurements were being performed. The Raman spectra were collected with the same objective in a backscattering configuration (Figure 1B). The laser spot was ∼100 µm in diameter with a typical power of 5 mW on the sample. Current (I)-voltage (V) curves were recorded using an electrochemical analyzer (CHI 630, CH Instruments) with the samples placed in a home-built Faraday cage. The cage was suspended in air with bungee cords to minimize the building vibration. The electrical contacts for both the top and bottom electrodes were made through a drop of Ga/In eutectic at the end of gold wires connected to the external circuit.23 The gold wire was positioned on the top electrode by a micromanipulator (World Precision Instruments). All the measurements were done at room temperature (23 ( 1 °C). RESULTS AND DISCUSSION Bottom Electrode Preparation and Characterization. It is well known that the surface-enhanced Raman effect requires certain roughness on the substrate.24 However, if the surface is too rough, the possibility of obtaining electrically short-circuited device is very high due to the high density of defects in the molecular layer. To find the optimal conditions, various gold films were deposited with different deposition conditions and their SERS activity was examined using PDI as a probe molecule. Figure 2 shows a set of SER spectra obtained on Au-coated glass slides with the Au layer deposited at (a) 29, (b) 20, and (c) 12 cm above (24) Tian, Z. Q.; Ren, B. Annu. Rev. Phys. Chem. 2004, 55, 197.

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Figure 2. SER Spectra of PDI on 16-nm Au thin films deposited with the glass substrate (a) 29, (b) 20, and (c) 12 cm above the Au source. Spectrum acquisition time, 60 s.

Figure 3. AFM images (500 × 500 nm2) of 16-nm Au films deposited on the MCTMS-coated glass slides with different sourcetarget distances: (A) 12 cm; (B) 29 cm.

the source. In Figure 2a, three sharp and intense Raman peaks at 1168, 1206, and 1602 cm-1 were observed and are assigned to the benzene ring 9a, 7a, and 8a modes, respectively.25 A broad feature at 2180 cm-1 with a weak shoulder at 2130 cm-1 can be assigned to the NtC stretching modes. The main peak is from the NC group attached to the surface while the shoulder from the free NC at the other end. These bands are similar to those reported for PDI adsorbed on Au nanoparticles and electrodes.26,27 The features between 1300 and 1500 cm-1 are mainly from the glass slide fluorescence, as they are absent in the spectrum from a bulk gold electrode.27 Decreasing the distance between the glass substrate and the evaporation Au source results in decreasing Raman intensity, but the band positions are largely unaffected. Atomic force microscopic (AFM) images reveal that these substrates have different morphologies, as shown in Figure 3. The Au films prepared with a source-target distance of 29 cm show large (∼60 nm) grains with a relatively smooth surface, while those prepared at 12 cm above the source contain a large amount of smaller (20 nm) particles and yield a rather rough surface. As shown by Gupta et al. on Au and Ag films deposited on glass slides, a correlation between film morphology and surface plasmon resonance wavelength can be established.28 The 785-nm laser excitation used here lies near the peak of the surface plasmon extinction band of 60-nm particles and far away from that of 20nm particles.29 Van Duyne’s group has shown elegantly, using Ag nanoparticle arrays fabricated by nanosphere lithography, that the (25) Han, H. S.; Han, S. W.; Joo, S. W.; Kim, K. Langmuir 1999, 15, 6868. (26) Kim, H. S.; Lee, S. J.; Kim, N. H.; Yoon, J. K.; Park, H. K.; Kim, K. Langmuir 2003, 19, 6701. (27) Gruenbaum, S.; Henney, M. H.; Kumar, S.; Zou, S. In preparation. (28) Gupta, R.; Dyer, M. J.; Weimer, W. A. J. Appl. Phys. 2002, 92, 5264. (29) Wei, A.; Kim, B.; Sadtler, B.; Tripp, S. L. Chemphyschem 2001, 2, 743.

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Figure 4. SER spectra of PDI on Au in the ring vibration region (A) and the N-C stretching region (B). Spectrum (a) before deposition of the top electrode; (b) after deposition of 15-nm top Au layer; (c) after deposition of 10-nm Al + 5-nm Au top layer deposition. Spectrum acquisition time, 60 s.

surface enhancement is stronger with the excitation wavelength closer to the peak of the surface plasmon band.30,31 Wei et al. have shown the same by using Au nanoparticles.29 It is therefore expected that a stronger Raman signal will be obtained on the films deposited at 29 cm. Based on the above, all of the bottom Au electrodes in the devices were prepared at 29 cm. Another factor affecting the bottom electrode morphology and hence the Raman intensity is the film thickness. To examine this, Au film thicknesses varied from 14 to 25 nm were prepared. We found that the SERS activity increases as the Au film thickness decreases, leading to the most intense PDI Raman signal on 14nm Au. However, the I-V measurements performed on this set of devices revealed that all of the pads examined were electrically shorted. Fortunately, the number of shorted pads decreases as the thickness of the bottom gold electrode increases and the maximum number of nonshorted pads was found on samples with a 25-nm Au bottom electrode. This characteristic of the gold substrates can be understood on the basis of surface morphology. The thicker Au layer is smoother leading to a less defective PDI layer, but a lower Raman intensity. Consequently, a compromise between the Raman intensity and the number of nonshorted pads has to be made. For the present work, 16 nm was chosen, which provides a good Raman signal and a reasonably good number of nonshorted pads (vide infra). SER Spectra of PDI Sandwiched between Two Metal Electrodes. Next, we examine the integrity of the PDI monolayer after the deposition of the top electrode. SER spectra of PDI adsorbed on 16-nm Au substrates with and without the top metal pad are shown in Figure 4. Two types of top electrode were used, one with 15-nm Au (Figure 4, spectrum b) and the other with 10-nm Al plus 5-nm Au (Figure 4, spectrum c). Comparison between the spectra obtained before and after the deposition of the top electrode reveals that, after the deposition, the ring vibration modes are unchanged except for the decreasing intensity due to the decreasing transparency, indicating the molecule is intact. However, significant changes were observed in the NtC stretching region. The lower frequency shoulder at 2130 cm-1 present on the samples without the top electrode disappeared, and the main NtC stretching band blue-shifted 10 cm-1 in the presence of Au pad and 20 cm-1 in the presence of Al/Au pad. (30) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 11279. (31) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2003, 107, 7426.

Figure 5. Current-voltage curves of PDI sandwiched between two Au electrodes. Two overlapping consecutive segments are shown in the figure. Voltage scan rate, 50 mV s-1.

Both the absence of the 2130-cm-1 shoulder and the blue-shift of the main NtC stretching band indicate that the free NC group is chemically bound to the top electrode. The blue-shift of the NtC stretching frequency was also observed when Au nanoparticles were attached to a PDI layer coated Au surface, presumably coming from the strong vibrational coupling between the two isonitrile groups. I-V Characteristic of the Au/PDI/Au Junction. Once the molecular integrity in the device is confirmed, we examine individual pads to determine whether they are electrically shortcircuited. Here a short-circuited device is defined as a pad that yields a current greater than a few milliamperes at a small bias voltage (1000 times) than that observed on Au/PDI/Au junctions. The rectification is not stable. Upon repeated voltage scans, the rectification ratio becomes smaller, and after ∼10 repetitive potential cycles, the rectification essentially disappears. The smaller current can be attributed to the formation of a thin (2-3 nm) Al2O3 layer34,35 because the Al film was briefly exposed to the ambient environment36,37 after the deposition in order to replace the Au source in the deposition chamber. Although the deposition chamber contains three source holders, the formation of an alloy between the evaporated Al and Au requires the two sources placed in the chamber separately. Even if the samples were not exposed to the ambient environment, the formation of aluminum oxide seems to be unavoidable, as was demonstrated recently.35 The presence of the aluminum oxide layer makes the junction nominally Au/PDI/ Al/Al2O3/Au. The weak rectification is likely to arise from the asymmetric nature of both the Au/PDI/Al and the Al/Al2O3/Au junctions. A much larger rectification effect of the latter junction has been observed in the corresponding metal/insulator/metal diodes with a much thicker (12-54 nm) Al2O3 layer.36,37 Figure 7B displays a potential step current (I)-time (t) plot of the Au/PDI/Al/Au junction along with the SER spectra recorded at +2 (i) and -2 V (ii). The I-t curve was obtained by stepping the voltage between +2 and -2 V with a 200-s holding time at each voltage. A Raman spectrum was recorded at each potential. Similar to the I-V measurements, the I-t curve also shows a weak rectification effect; i.e., the current peak and plateau at +2 V are larger than those at -2.0 V. The I-t curve can be (34) McGovern, W. R.; Anariba, F.; McCreery, R. J. Electrochem. Soc. 2005, 152, E176. (35) Kalakodimi, R. P.; Nowak, A. M.; McCreery, R. L. Chem. Mater. 2005, 17, 4939. (36) Fisher, J. C.; Giaever, I. J. Appl. Phys. 1961, 32, 172. (37) Hickmott, T. W. J. Appl. Phys. 2000, 87, 7903.

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fitted with a double-exponential function, which suggests the device is equivalent to a circuit with two RC components in series, confirming the device has a double junction structure. The SER spectra taken at the two different applied voltages are similar to those obtained without the external electric field, indicating the molecular layer is intact under the applied voltages. The NtC stretching frequency and band shape are again independent of the applied voltage. CONCLUSIONS We have shown that, by carefully designing the bottom Au electrode, surface-enhanced Raman spectra of the molecules sandwiched in molecular electronic junctions can be obtained simultaneously with the electron transport characterizations. Two types of junction were tested. The Au/PDI/Au yields a symmetric nonlinear I-V curve which resembles that reported in the literature. The asymmetric Au/PDI/Al/Au junction shows a weak rectification effect that can be attributed to the double asymmetric structure of the junction. The simultaneously recorded Raman spectra demonstrate that the molecular structure is intact. Although the PDI Raman spectra do not change with the applied voltage, the ability to obtain molecular information from a molecular electronic junction under applied voltage paves a way to better understand its electron transport characteristics. The approach depends on the surface-enhanced Raman effect and should be applicable to most molecules studied thus far in molecular electronics. Compared to the IETS, the approach is applicable at both low and ambient temperatures. The monolayer sensitivity of SERS makes the method a powerful tool for probing molecular structure in the metal-monolayer molecule-metal type of molecular electronic devices. We are applying the approach to molecules that show interesting electron transport properties, such as negative differential resistance.9 ACKNOWLEDGMENT This work is partially supported by Miami University through startup funds and by National Science Foundation under ECS0403669. We thank Professor Jan Yarrison-Rice for critical reading of the manuscript and Mr. Barry Landrum of the Instrumentation Laboratory for making the Faradaic cage used in this work. Received for review July 25, 2005. Accepted October 24, 2005. AC051318I