Surface-Enhanced Raman Spectroscopic Studies of Metal

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Surface-Enhanced Raman Spectroscopic Studies of Metal− Semiconductor Interfaces in Organic Field-Effect Transistors Danish Adil and Suchi Guha* Department of Physics and Astronomy, University of Missouri, Columbia, Missouri 65211, United States S Supporting Information *

ABSTRACT: The performance of organic field-effect transistors (OFETs) largely depends on the nature of interfaces of dissimilar materials. Metal−semiconductor interfaces, in particular, play a critical role in the charge injection process. This work demonstrates the unique potential of Raman and surface-enhanced Raman scattering (SERS) for the investigation of physical phenomena at the nanoscale in pentacene− metal interfaces in OFETs. A large enhancement in the Raman intensity (SERS) is observed from pentacene films under thermally evaporated Au films. Comparing experiments with density functional theoretical calculations of the Raman spectrum of pentacene indicates the presence of disordered sp2 carbons. Changes in the Raman spectra are further tracked after biasing the devices. Raman maps across the pentacene−Au interface provide a powerful visualization tool for correlating the device performance with structural changes of the molecule. contribution to the SERS effect,7,8 which is a direct correlation of the nanostructured size and shape of the metal layer on top of the organic films. Since most p-type OFETs utilize Au contact electrodes, a question that may be raised is whether SERS, if observable, may be used as a diagnostic tool to probe the OS−metal interface in terms of its electronic and chemical makeup and provide a platform to understand any degradation related mechanisms in OFET performance. Optical spectroscopy has been instrumental in understanding charge injection and transport in OFETs: infrared spectroscopy in poly(3hexylthiophene)-based OFETs has been effectively used to study charge injection;9 charge modulation spectroscopy in polyfluorene-based OFETs has been used to image chargeinduced absorption.10 In this work, we explore the interface between Au and pentacene using Raman spectroscopy by varying the thickness of the Au layer in pentacene-based OFET structures. With a near-infrared (IR) excitation source, SERS from the pentacene layer is clearly seen beyond a certain thickness of Au. Formation of a nanostructured Au layer is confirmed by electron microscopy. Raman maps across the pentacene−Au interface provide a strong visualization tool to study the impact of the metal layer on the pentacene film. This technique further provides insights into device performance and defect induced mechanisms when gated. The pentacene results are also compared with other phenyl-based conjugated polymers and crystalline graphite, where Au evaporation results in a SERS effect enhancing the features from disordered carbons.

1. INTRODUCTION Organic field-effect transistors (OFETs) constitute the building block of organic electronics. Achieving low-operating-voltage, stable, and high-mobility OFETs has been a challenge. Charge injection across the metal−organic semiconductor (OS) interface plays a major role in the overall performance of OFETs.1 Since the work function of Au matches the highest occupied molecular orbital (HOMO) of many OSs, it is typically used as a source/drain contact for p-type OFETs. Thermal evaporation is a common mode of depositing Au contacts for both bottom and top contact OFET geometries. Investigations of the molecular nature of the interface between the OS and vapor-deposited Au therefore constitute an important direction toward understanding the nature of the interfacial phenomena that influence the overall device performance. There have been a few investigations of the interfaces of organic molecules with metals such as Ag, Al, and Mg using Raman spectroscopy. Most of these studies involved vacuum evaporated thin metal films. Depending on the choice of the metal, the Raman spectrum of tris-8-hydroxyquinoline aluminum (Alq3) shows an enhancement of intensities due to surface-enhanced Raman scattering (SERS) effect,2 changes in relative intensities due to complexes formed by weak interaction between the metal and the conjugated ring of the ligand,3 and also partial graphitization of the Alq3 film.4 Another organic molecule based on a perylene derivative with Ag shows a SERS effect only for the internal modes.5 Recently, a SERS substrate based on nanometer-scale electromigrated gaps has demonstrated very high sensitivity for trace chemical detection.6 The electromagnetic field enhancement due to the local field of surface plasmons excited by incident light is a major © 2012 American Chemical Society

Received: April 3, 2012 Revised: May 11, 2012 Published: May 22, 2012 12779

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Figure 1. Optical image of the channel/contact region of a pentacene-based OFET. The normalized Raman spectrum of pentacene in the 1120− 1600 cm−1 is shown through the Au pad (top) and from the channel region (bottom).

A comparison of the experimental results with first-principles vibrational frequency calculations of pentacene clearly show that Au deposition results in a change in the aromaticity of the phenyl rings and introduction of disordered carbons, where the Raman intensity from the disorder is further enhanced by SERS. The thermal evaporation of the pentacene film itself retains the aromaticity and structure of the molecule, evident from the Raman spectrum itself. Atomic force microscopy images (not shown in this work) of pentacene films on Si substrates exhibit typical dendritic structure with layer-by-layer formation and large grain sizes (∼1 μm), similar to what we observe on other substrates11 and comparable to other reports.12

were measured at room temperature with two source meters, Keithley 2400 and Keithley 236, configured together using a program written in LabView. The source−drain current and the source−gate leakage currents were measured with the Keithley 236, which has a resolution of 10 fA.

3. THEORETICAL METHOD The Raman spectrum of a pentacene molecule (including spectrum calculations of disordered pentacene) has been computed using density functional theory (DFT) within Gaussian 09.13 We employed the B3LYP (Becke’s three parameter hybrid)14 functional to perform geometry optimization and calculations of force constants, dipole moments, and polarizability derivatives combined with the polarized 6-311G or 3-21G* basis set. The two basis sets give comparable results for the Raman spectrum of a pentacene molecule. The Raman spectrum (frequencies and intensities) calculations were performed in two steps: first the force constant matrix was evaluated and then the eigenvalue equation was solved to obtain the eigenvalues and eigenvectors. The Raman intensities are theoretically obtained from the first derivatives of the polarizability with respect to the nuclear coordinates. The Gaussian output is in terms of the Raman activity, which is essentially the square of the polarizability tensor. In order to compare the theoretical spectrum with experiment, one needs to multiply the activities of each mode with the appropriate prefactors comprising the scattered and phonon energies, and the thermal average occupation number. We refer to ref 15 for details on how to obtain the first-order Stokes−Raman intensities.

2. EXPERIMENTAL METHODS Pentacene OFETs were fabricated using a top-contact bottomgate structure with gold source and drain electrodes. Heavily pdoped silicon (100) wafers, with resistivity of 0.001−0.005 Ω·cm, were used as the gate electrode with a 200 nm bare SiO2 layer as the gate dielectric. The substrates were cleaved, ultrasonicated in acetone, rinsed with isopropyl alcohol, and finally submerged in piranha solution (7:3 H2SO4:H2O2) at 115 °C for 15 min. OTS (octadecyltrichorosilane) from SigmaAldrich was used to form self-assembled monolayers on the SiO2 surface. Pentacene, purchased from Tokyo Chemicals Inc., was thermally evaporated on top of the SiO2 without any further purification. Top contacts of Au with various thicknesses were deposited for the source (S)−drain (D) contacts via thermal evaporation at a base pressure of 10−6 mbar through a shadow mask. Typical S−D channel length and width for OFETs ranged from 0.050 to 0.100 mm and from 0.50 to 4.0 mm, respectively. The Au thickness varied from 0.5 to 20 nm. Pentacene films were also deposited on Si wafers (stripped of any oxide layer) followed by Au evaporation using masks similar to the S/D contacts in OFETs. All thicknessdependent SERS studies were done with Si-based specimens while other studies, such as bias-stress measurements, were done with the SiO2-based OFETs. The Raman spectra were found to be independent of substrate used. The Raman spectra were collected by an Invia Renishaw spectrometer attached to a microscope with a 50× lens. The system is equipped with two lasers: a 785 nm diode laser and an argon ion laser with 514 nm as the excitation source. Twodimensional Raman maps were generated by measuring several individual Raman spectra in the channel region of the OFET and through the Au contact immediately adjacent to the channel region at 5 μm intervals. The OFET characteristics

4. RESULTS AND DISCUSSION 4.1. Au Thickness Dependent SERS Studies. Figure 1 shows the representative Raman spectra of a pentacene film in the 1100−1700 cm−1 region from the channel region and through the Au pad of an OFET using the 785 nm laser line as the excitation source. The Au thickness of this device was 5 nm. A detailed description of the Raman frequencies of pentacene is found in ref 16. The Raman peaks in the 1140−1190 cm−1 region originate from the C−H in-plane bending motion, and the C−C aromatic stretching vibration lies in the 1340−1400 cm−1 range. The two peaks at ∼1160 and 1178 cm−1 are related to the motion of the H atoms at the end and the side of the molecule. The 1371 cm−1 peak is seen as the strongest C−C stretching vibration. The changes in the peak intensity of the C−C stretch vibration at 1597 cm−1 have been related to the thickness of the pentacene film.16 As discussed in the 12780

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Supporting Information, we find there is also a correlation of the intensity of the 1597 cm−1 peak to the oxidation of pentacene films. A quick observation from Figure 1 is that the 1380 and 1560 cm−1 regions are broadened with the appearance of new peaks when probed through the Au pad. Additionally, the intensity of the 1380 cm−1 peak is greatly enhanced through Au, which is not seen here as the two spectra are normalized to the intensity of the 1380 cm−1 peak. The relative intensities of the 1160 and 1178 cm−1 Raman peaks also change with the thickness of the Au film. These changes in the Raman spectrum of pentacene with Au are observed for different underlying polymer dielectric layers as well in OFET structures. To understand the role of Au in the Raman scattering of pentacene, we have systematically varied the thickness of the Au layer and measured the Raman spectrum from the channel−Au pad region using both the 785 and 514 nm excitation sources. The largest SERS enhancement due to Au nanoparticles/ nanostructures occurs when excitation wavelengths are in the near-infrared, out of resonance with the surface plasmons.17 Comparing the Raman spectrum of pentacene through the Au pad using both 785 and 514 nm further distinguishes the SERS effect from normal Raman scattering. Figure 2 shows the

Au layer. Since the powers of the two lasers were slightly different, the integration times were adjusted accordingly. Being close to resonance, the overall Raman intensity of pentacene is higher with the 514 nm line. Qualitatively, the Raman spectrum measured with the 514 nm line through Au is similar to that without Au except for the shoulders at 1390 and ∼1570 cm−1, denoted by the dotted lines. The overall charge coupled detector (CCD) counts through the Au layer is lower compared to the pentacene-only signal due to attenuation of the green laser light. The Raman spectrum of pentacene through Au with the 785 nm line is greatly enhanced compared to the pentacene-only signal, as evident from Figure 3b. The 1380 and 1560 cm−1 peaks dominate and completely overwhelm the other pentacene Raman peaks in this region. The ratio of the 1160 cm−1 to the 1178 cm−1 Raman intensities changes as a function of the Au thickness. The intensity ratio increases until ∼3 nm of the Au layer with a slight drop as the Au thickness is further increased (Figure 3d); beyond 20 nm of the Au thickness, the ratio remains a constant. As discussed in the Supporting Information, the intensity ratio of the 1380 cm−1 peak to the 1371 cm−1 peak changes as well. By comparing the Raman spectrum of pentacene through Au with the two laser excitations, it is clear that there is a SERS effect with the 785 nm excitation source. Two immediate questions arise: Does Au form some kind of a nanostructured layer during evaporation, which could result in surface plasmons? What is the origin of the two strongly enhanced new peaks at 1380 and 1580 cm−1? We have some evidence from scanning electron microscope images that a very thin layer of Au on pentacene shows grains in the nanometer length scale, thus providing a viable mechanism for SERS (see the Supporting Information). Nanostructured surface of evaporated Au layers has been recently reported for pentacene OFETs fabricated on metallic fiber substrate.18 As an additional check, we have deposited Al on pentacene, and these metal evaporated films do not show any SERS or changes in the Raman spectrum of pentacene with the 785 nm excitation source. As shown in Figure 2, there are almost no changes in the Raman spectrum of pentacene when the Au layer thickness is less than 0.5 nm. However, Figure 3b,c suggests a permanent change due to a structural disorder of the pentacene molecule upon evaporation of thicker films of Au. To some extent these signatures are weakly observable even with the 514 nm line, but they are strongly enhanced with the 785 nm excitation source, consistent with a SERS effect due to the Au plasmons. To understand the changes in the Raman spectrum of pentacene with Au, we have calculated the Raman spectrum of pure and perturbed pentacene molecules by disrupting the aromaticity of the phenyl rings as well as partially oxidizing the molecule. As a first attempt to incorporate structural disorder in the pentacene molecule, an additional H atom was added to have one of the C atoms with sp3 hybridization. An addition of a H atom may not be the most realistic pathway, but it allows us to introduce distortion in the phenyl rings to emulate the experimental conditions. 4.3. Calculated Raman Spectra of Disordered Pentacene. The theoretical Raman frequencies of pentacene obtained from DFT calculations were not scaled in this work, so there are discrepancies between the experimental and theoretical energies. Moreover, since the calculations are for the gas phase, the C−H vibrational energies at 1170 cm−1 show a larger difference between experiment and theory, compared to

Figure 2. Raman spectra of pentacene films with three different thicknesses of Au on top.

Raman spectra of pentacene films (of the same thickness) in the 1150−1600 cm−1 region for three different Au layer thicknesses measured with the 785 nm excitation. No changes are observed in the Raman spectrum of pentacene for Au film thickness of 20 nm), these two peaks broaden and dominate the Raman spectrum. Moreover, the intensity of the 1380 cm−1 region keeps increasing with the Au layer thickness and saturates at about 10 nm. 4.2. Confirmation of the SERS Effect. To understand the origin of the new features observed in the Raman spectrum of pentacene with increasing thickness of the Au layer, several of these films were measured with the 514 nm laser line. Figure 3a shows a schematic of the setup, where either the 785 nm line or the 514 nm line was incident on the sample in a backscattering geometry. Parts b and c of Figure 3 show the Raman spectra of pentacene measured with and without the Au layer using the 785 and 514 nm lasers, respectively. Both sets of spectra were measured from the same region in the sample that had a 20 nm 12781

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Figure 3. (a) Schematic of the Raman setup in a backscattering geometry. (b) Raman spectra from two regions in the sample: through a 20 nm thick Au layer and without any Au layer with the 785 nm excitation source. (c) Raman spectra from the same regions as in (b) but measured with the 514 nm laser line. (d) Ratio of the Raman intensity of the 1160 cm−1 peak to the 1178 cm−1 peak (measured with the 785 nm laser line) as a function of the thickness of the Au layer on top of the pentacene film. The red dashed line is a guide to the eye.

the C−C stretch vibrations. Nonetheless, the overall calculated Raman spectrum, including the relative intensities, agrees quite well with experiment. The theoretical simulations were also carried out by introducing an additional sp3 hybridization in one of the rings. The Raman spectrum was calculated for three different scenarios: unperturbed pentacene, pentacene with one sp3 C in the outermost ring, and pentacene with one sp3 C in the center ring (Figure 4a). The three calculated spectra were weighted evenly and then averaged. Figure 4b shows the calculated spectra from a pure pentacene molecule along with a pentacene molecule with an sp3 defect site in one of the edge phenyl rings. In the distorted molecule, the 1218 cm−1 peak gets enhanced and the aromatic C−C stretch vibration at ∼1390 cm−1 is slightly hardened compared to the pristine molecule. The calculated spectrum in Figure 4c is an average of the three calculated spectra, as explained previously. This average spectrum agrees quite well with experiment: the energies are slightly different but the experimental 1170 cm−1 region is well reproduced by our calculations; similarly, the main features in the 1380 cm−1 region also corroborate experiment. By weighting the individual spectrum equally from the three pentacene molecules (in Figure 4), we are not suggesting that the distorted molecules to the pristine molecule are in a 2/3 ratio. Clearly, upon Au deposition the interface pentacene layer is most likely to be affected. The experimental data (with the 785 nm laser) probes the bulk of the pentacene film. The SERS signal from the distorted molecules seems to be much higher compared to the regular Raman intensity from pentacene. Using the 514 nm laser excitation source (with no SERS), we clearly see that the pristine pentacene C−C stretch at 1370 cm−1 is still stronger than the high energy shoulder in Figure 3c. The calculations here do not consider any electromagnetic enhancement; an equal contribution of the three spectra in

Figure 4. (a) Chemical structure of a pentacene molecule and pentacene with disordered sp2 rings. (b) Calculated Raman spectra of a pristine molecule and disordered molecule with a sp3 C atom at the edge. (c) Experimental Raman spectrum of pentacene through a Au layer. The calculated spectrum is the average of the three spectra corresponding to the different schemes in (a). The dashed line correlates the experimental Raman peaks in the 1170 cm−1 region with the calculated spectrum.

Figure 4c may thus be justified since the SERS signal of the C− C stretch vibration from the distorted molecule dominates the experimental result. One should not stress too much the actual ratios of the weighted spectrum used here. The important thing to realize is that an sp3 hybridization of the end ring in a pentacene molecule predicts the appearance of the 1381 cm−1 peak in our experiment, along with showing an increase in intensity of the 1160 cm−1 peak relative to the 1178 cm−1 peak 12782

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as seen in the experimental results (keeping in mind that the frequency positions are slightly shifted between the calculated and experimental peaks in this region). The increase in the 1560 cm−1 region (Figures 2 and 3b) most likely corresponds to the G-band of sp2 carbons.19 This band appears in many disordered carbon structures: carbon fibers, glassy carbon blacks, and pregraphitic carbon.20 Overall, Au evaporation on the pentacene layer results in modifying the molecule and introducing some disorder in the sp2 carbon network. This may affect a small fraction of the pentacene molecules near the interface, but since the SERS effect is strong for the disordered molecule, the experimental Raman spectrum is overwhelmed by the 1380 and 1560 cm−1 regions. In other words, the SERS effect shows a sensitivity to the disorder that is otherwise undetectable. As a check, we also evaporated a thin layer of Au (10 nm) on a piece of highly oriented pyrolytic graphite (HOPG) and measured the Raman spectrum with both the 785 and 514 nm excitation sources. The results are shown in the Supporting Information. With the 514 nm excitation, no changes are observed in the Raman spectrum of HOPG when probed through the Au film or without Au. The 785 nm excitation however shows features at 1306 and 1540 cm−1 (Figure S3 in the Supporting Information). The 1306 cm−1 feature is clearly the D-band of disordered graphite; only SERS picks up the signature from the disorder in HOPG. We note that the Raman peak position of the D-band depends on the excitation wavelength;21 the frequency upshifts with increasing energy of the excitation source. The D-band appears at 1350 cm−1 with the 488 nm excitation source.20 The relative intensity of the Dband to the G-band is also an indicator of the degree of disorder in graphene.22 4.4. SERS as a Probe for Bias Stress Effect in FieldEffect Transistors. The next question that arises is, can one use SERS as a diagnostic tool to study degradation/structural changes of organic films in OFETs when they are biased? It is known that OFET operation for many different polymers and molecules deteriorates upon bias stress, which is usually observed as lowering of the current, decreased mobility, and enhanced threshold voltages.23 As a next step, we fabricated regular pentacene-based OFETs. The Au source/drain pads of the devices shown here were ∼5 nm thick. Thin injecting electrodes were chosen so as to maximize the SERS sensitivity. The OFETs were not very well optimized, and the typical charge carrier mobility (μ) was in the mid-10−3 cm2/(V s) range. Raman line scans were measured across the pentacene− Au lateral interface before the I−V characteristics were measured (denoted as prebias). The devices then underwent a bias stress by running them in the saturation regime for 20 min in air, and the Raman spectra (maps) were immediately remeasured. Figure 5 shows the transfer characteristics of a pentacene FET both before and after being put under the bias stress. The transistor was biased by placing it in the saturation regime (VDS = −30 V and VG = −40 V) for 20 min. A 40% drop in the drain−source current was observed over the course of the bias stress. The transfer characteristics were measured under ambient conditions with VDS = −30 V. Device parameters such as charge carrier mobility, threshold voltage, and on/off ratio were estimated by using the standard current−voltage characteristics of FETs given by

Figure 5. Transfer plots of a pentacene OFET before and after biasing for 20 min at VDS = −30 V and VG = −40 V.

IDS =

μWC0 (VG − Vth)2 2L

(1)

where C0 is the effective capacitance, W is the channel width, L is the channel length of the transistor, VG and IDS are the gate voltage and source−drain current, respectively, and Vth is the threshold voltage. After biasing the device, the charge carrier mobility was found to decrease from 2.0 × 10−3 to 1.8 × 10−3 cm2/(V s). The threshold voltage increased from −4 to −12 V, and the on/off ratio decreased from ∼104 to ∼103. Each Raman line scan map consists of ∼13 spectra spanning 60 μm; the first measurement (0 μm) was taken in the channel itself, directly on pentacene, and the last measurement was on the Au pad. Figure 1 shows the optical image of the OFET. The Raman spectra from the different points were then collected to plot an intensity map, which provides an immediate snapshot as to how the spectrum changes across the interface. After bias stress, the same device and region were selected and Raman line scan maps were generated. Figure 6 shows the Raman maps from a pentacene OFET before and after bias stress using the 785 nm excitation source. The incident laser power and integration times were the same for both measurements. Data set 1 refers to the spectrum measured on the pentacene (see Figure 1), and data set 13 refers to the Raman spectrum measured well within the Au pad. All the spectra were measured in the 1100−1700 cm−1 region. The Raman intensities significantly increase when probed through the Au contact. The intensity scales are slightly different for the two maps, but one clearly observes that the 1380 and 1560 cm−1 regions are significantly enhanced upon bias stress. The background of the Raman spectra also increases slightly for the “after bias” measurements, but a careful analysis of the Raman peaks shows that there is a significant enhancement of the peak intensity (1380 and 1560 cm−1) by noting the area under the curve, as well as an increase in line broadening. Our theoretical Raman spectrum calculations of pentacene indicate that an enhancement of the 1380 cm−1 region is a signature of the distortion of the molecule. By biasing the OFETs at −40 V, we subject them to an electric field of ∼108 V/m; the force due to this field may be adequate in distorting the molecules with an increasing fraction of disordered sp2 carbons. Another scenario could relate to the actual transport through the OFET channel, which results in structural modifications to the molecule. At this stage, it is difficult to identify the exact reason for the structural changes, but our methodology shows that SERS is a very sensitive technique to 12783

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measuring the Raman spectra from the same device after a week, which underwent bias stress, shows features very similar to seen in the “after bias stress” device in Figure 6. Our observations suggest that, while interpreting the changes in the threshold voltages upon bias stress, we must also consider permanent structural changes, especially in pentacene. Structural phase transition in pentacene OFETs due to the sourceto-drain electric field have been observed in other works and has been attributed to a change in the herringbone packing of the pentacene molecule.28 Not all conjugated polymers and molecule show structural changes upon Au evaporation and biasing. Our recent work on diketopyrrolopyrrole (DPP)-based OFETs shows no signature of disordered carbons upon Au evaporation.29 Upon biasing the devices, almost no changes were observed in the Raman line scan maps. Moreover, the current−voltage characteristics of DPP-based OFETs do not degrade and the carrier mobilities/ threshold voltages remain almost constant over many weeks of operation.

5. CONCLUSION In summary, we demonstrate that Au evaporation on pentacene films induces disorder, which is enhanced by SERS. A comparison of our experimental results with theoretical DFT Raman spectrum calculations suggests the presence of sp2 disordered carbons in pentacene. The SERS signal dramatically increases when the Au thickness changes from 0.5 to 5 nm. The enhancement is evidence of a nanoscale roughness in the morphology of the Au interface, which is further confirmed from electron microscopy images. Other fluorene-based polymers (not shown here) show a similar SERS signal from disordered phenyl rings upon Au evaporation. These features in the Raman spectrum can be observed only with a near-IR laser (a condition that is sensitive to SERS). In most cases, a green excitation source does not pick up any signature from such a disordered carbon network, especially when the concentration of disorder/defects is very small. Furthermore, SERS may be exploited to study the structural/vibrational changes in an organic semiconductor in an OFET upon biasing. SERS/ Raman mapping across the pentacene−Au interface provides a strong visualization tool to study the impact of bias on the structural changes of the molecule. The methodology shown in this work may be used as a general diagnostic tool for any conjugated polymer/molecule in an OFET structure utilizing Au source/drain contacts to correlate the degradation in transport properties with structural changes.

Figure 6. Raman line scan maps of a pentacene OFET before and after bias stress. Data set 1 corresponds to the Raman spectrum measured in the channel region, and data set 13 corresponds to the Raman spectrum well within the Au pad of one of the contacts.

probe this disorder. The Raman studies can be further correlated to the actual device performance. Bias stress in organic thin film transistors manifests itself as shifts in the threshold voltages24 and has been attributed to the polar nature of adsorbed water at the organic semiconductor−dielectric interface for solution processable polymers.25,26 Furthermore, the shift of negative threshold voltages to more negative values upon bias stress has also been attributed to oxygen, which induces both acceptor-like and donor-like states.23 Increasing the thickness of the Au layer to ∼30 nm significantly improves the device performance. When subjected to a bias stress, the thicker Au contacted OFETs also show a change in the Raman maps, similar to Figure 6, with a concomitant increase in the threshold voltage. Theoretical calculations of a partially oxidized pentacene molecule show an enhanced intensity for the 1597 cm−1 peak, consistent with a peroxide treated pentacene film (Figure S2 in the Supporting Information). We point out that, upon bias stress, the relative intensity of the 1597 cm−1 Raman peak is almost the same as before biasing (throughout the interface region), indicating that oxidation effects are minimal.27 The main difference observed in the Raman spectrum, however, is enhanced 1380 and 1560 cm−1 regions (Figure 6). Rather than an ingression of oxygen/moisture in the pentacene film, our results indicate that bias stress results in some amount of distortion of the pentacene molecules. These changes are typically not sensitive to regular Raman scattering, but are easily detected by SERS. The structural changes are permanent;

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ASSOCIATED CONTENT

S Supporting Information *

SEM picture of top surface of pentacene specimen with 2 nm layer of gold; experimental and theoretical Raman spectra of partially oxidized pentacene films; SERS spectra of highly oriented pyrolytic graphite with 10 nm layer to gold; ratio of the Raman intensity of the 1380/1371 cm−1 peaks as a function of the Au thickness. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Tel.: (573) 884-3687. Fax: (573) 882-4195. 12784

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Notes

(25) Chua, L. L; Zaumseil, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194. (26) Kim, C. S.; Lee, S.; Gomez, E. D.; Anthony, J. E.; Loo, Y.-L. Appl. Phys. Lett. 2008, 93, 103302. (27) Vollmer, A.; Weiss, H.; Rentenberger, S.; Salzmann, I.; Rabe, J. P.; Koch, N. Surf. Sci. 2006, 600, 4004. (28) Cheng, H. L.; Chou, W. Y.; Kuo, C. W.; Wang, Y. W.; Mai, Y. S.; Tang, F. C.; Chu, S. W. Adv. Funct. Mater. 2008, 18, 285. (29) Adil, D.; Kanimozhi, C.; Ukah, N. B.; Paudel, K.; Patil, S.; Guha, S. ACS Appl. Mater. Interfaces 2011, 3, 1463.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of this work through the National Science Foundation under Grant ECCS-0823563. We thank the electron microscopy core at MU for the SEM images.

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REFERENCES

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dx.doi.org/10.1021/jp3031804 | J. Phys. Chem. C 2012, 116, 12779−12785