13250
J. Phys. Chem. C 2007, 111, 13250-13255
Correlations between SFG Spectra and Electrical Properties of Organic Field Effect Transistors Hongke Ye,† Jia Huang,‡ Jung-Rae Park,† Howard E. Katz,‡,§ and David H. Gracias*,†,§ Department of Chemical and Biomolecular Engineering, Department of Materials Science and Engineering, and Department of Chemistry, Johns Hopkins UniVersity, 3400 North Charles Street, Baltimore, Maryland 21218 ReceiVed: April 9, 2007; In Final Form: June 20, 2007
Simultaneous sum frequency generation (SFG) vibrational spectra and electrical measurements were obtained on organic field effect transistors (OFETs) fabricated with the semiconductors: 5,5′-bis(4-hexylphenyl)-2,2′bithiophene (6pttp6), 5,5′-bis(4-ethylphenyl)-2,2′-bithiophene (2pttp2), and pentacene. In-situ measurements during gating of the OFETs showed strong correlations between vibrational spectra and electronic properties. One correlation involved structural changes in the hexyl and ethyl groups, of 6pttp6 and 2pttp2, respectively, and saturation source-drain current; the correlation was observed only at negative gate voltages (when carrier injection was possible) and was more pronounced for 6pttp6, with the introduction of gauche defects in the longer hexyl chains. A second correlation between the dependence of SF nonresonant background on gate voltage and electronic mobility was observed on OFETs of all three semiconductors, at both positive and negative gate voltages. This correlation suggests that a common molecular structural packing element may determine the magnitude of both the electronic mobility and higher order nonlinear optical susceptibilities in oligomeric thin films. These results also demonstrate the utility of SFG in probing molecular structural and electrical field effects at the buried semiconductor-dielectric interface of OFETs.
Introduction Transistors fabricated with organic semiconductors allow for the possibility of solution-based processing of electronic devices such as organic field effect transistors (OFETs).1,2 Typically the channel height is on the order of the Debye length which ranges between 0.1 and 1 nm;3 hence, nearly all the charge of the conducting channel resides in the first molecular layer next to the gate dielectric-semiconductor interface. The ability to monitor the channel region (semiconductor-dielectric interface) within these devices is critical to understanding structural and electronic changes occurring in the layer during gated conduction. Results from such studies could shed light on strategies to develop organic semiconductor devices that overcome some present day limitations such as large operational voltage, irreproducibility, and limited reliability.3,4 Monitoring the channel region during gated conduction in an operational organic semiconductor device, e.g., an OFET, however, is very challenging because of the fact that the semiconductor-dielectric interface is buried. Most surface probes rely on vacuum operation or place considerable restrictions on the types of substrates that can be analyzed.5 Hence, while there have been many elegant spectroscopic studies6-13 of organic semiconductors, many were conducted ex situ or using restrictive substrates. Additionally since the critical channel in the OFET, the semiconductor-dielectric interface, is buried, it is extremely challenging to probe, and previous surface spectroscopic studies have focused on characterizing the semiconductor-air interface or the bulk, which may have limited relevance to the operational * Author to whom correspondence should be addressed. † Department of Chemical and Biomolecular Engineering. ‡ Department of Materials Science and Engineering. § Department of Chemistry.
characteristics of the OFETs. Second order nonlinear optical spectroscopies, however, are inherently surface sensitive and allow nondestructive probing of buried interfaces.14 Iwamoto’s group15,16 has pioneered the use of second harmonic generation (SHG) to study electronic effects in operational OFETs and these studies demonstrate strong correlations between carrier density and electric field-induced SHG spectra. Recently, we demonstrated that it is also possible to utilize another nonlinear optical spectroscopy, i.e., IR+visible sum frequency generation (SFG) vibrational spectroscopy to probe the buried interface of an OFET during gating.17 Even though SFG is a vibrational spectroscopy and is sensitive to molecular structural and orientational changes (vibrational spectroscopy as opposed to electronic spectroscopy), we did observe two striking correlations between the structure of molecular end chains and the nonresonant SF background with electrical properties of the OFETs. In this paper, we probe the effect of length of the alkyl end chains on two thiophene molecules of the npttpn form, where pttp represents the core of the molecule composed of phenyl (p) and thiophene (t) groups, while n represents the number of carbon atoms on the alkyl end chains. The two molecules studied were 5,5′-bis(4-hexylphenyl)-2,2′bithiophene (6pttp6) and 1,1′-bis(4-ethylphenyl)-2,2′-bithiophene (2pttp2) with n ) 6 and n ) 2 carbon atoms, respectively, on each of the alkyl end groups. To provide a broader interpretation of the experimental results, we also studied the popular organic semiconductor pentacene. All the semiconductors studied are p-channel semiconductors and operate only at negative gate voltage when carrier injection is possible. Experimental Section SFG Set Up. In the experiment we used an SFG vibrational spectroscopy system purchased from EKSPLA. In our optical
10.1021/jp072767k CCC: $37.00 © 2007 American Chemical Society Published on Web 08/14/2007
Organic Field Effect Transistors
Figure 1. (A) A schematic layout of the combined SFG-electrical probe measurements on the OFETs. The three interfaces probed are I0, the semiconductor-air interface; I1, the semiconductor-SiO2 dielectric interface and I2, the SiO2-Si (dielectric-gate) interface. The SFG experiment was done in reflection configuration, with simultaneous electrical measurements on source, drain, and gate electrodes. (B) An optical top view image of the OFETs showing patterned gold source and drain electrodes (bright) on the semiconductor (dark) layer.
system, both the IR and visible beam were generated from a ps pulsed Nd:YAG laser. The pulse duration was about 30 ps, and repetition frequency was set at 10 Hz. A type I potassium dideuterium phosphate crystal was used to convert the base frequency (λ ) 1064 nm) to the second harmonic (λ ) 532 nm) which was used as the visible beam. A lithium triborate (LBO) crystal-based optical parametric generation/amplification (OPA/OPG) system was used together with a silver gallium sulfide (AgGaS2) crystal-based differential frequency generation (DFG) system to produce a tunable IR beam (λ ) 3.22 µm3.51 µm). The visible beam and IR beam were overlapped both spatially and temporally on the sample with incident angles of 60° and 55°, respectively. The incident power of the visible and IR beams were set at about 60 µJ and 200 µJ, respectively, to minimize the damage to the sample surface. The SFG signal was collected in a reflection configuration, and after spatial and frequency filtering, it was measured using a photomultiplier tube (PMT). All experiments were conducted using ssp polarization configuration, i.e., the visible beam was s-polarized, the IR beam was p-polarized, and we detected the s-polarized component of the output SF signal. Each spectrum obtained is an average result from five spectra. In each scan, we have 50 acquisitions for one data point. Electrical Conductivity Measurement. A four-probe electrical measurement station was integrated with the SFG system (Figure 1). The probe heads and the translation stages were purchased from Cascade Microtech and electrically controlled by two synchronized Keithley source meters (2400 series). One source meter was used to scan the source-drain voltage from 0 to -80 V and measure the source-drain current, while the other one was used to apply gate voltages in the range of 0 to -80 V. The probers were positioned with microscale accuracy on electrical contact pads using an optical imaging system. The imaging system CCD integrated an optical microscope (Navitar) and a video capture card (Dazzle Digital Video Creator 90, from Pinnacle Systems). The tip of each probe head was slightly blunted to prevent damage to the contact pads. The gate voltage was set to appropriate values during the experiment. Before and after each optical experiment, we scanned the source-drain voltage at full range (0 to -80 V) at different gate voltages (0 to -80 V in steps of -10 V) to ensure that there was no damage to the OFET operation by the SFG spectroscopy.
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13251 Sample Preparation. The semiconductor 5,5′-bis(4-n-hexylphenyl)-2,2′-bithiophene (6pttp6) and 5,5′-bis(4-ethyl-phenyl)2,2′-bithiophene (2pttp2) were synthesized according to published synthetic routes.18a The crystal structure of 2pttp2 has been submitted for publication elsewhere.18b Pentacene was purchased from Aldrich. All of the organic semiconductors were further purified by sublimation. All the OFETs were fabricated using heavily doped silicon (Si) wafers with 300 nm of thermally grown silicon dioxide (SiO2). We used the Si substrate as the gate electrode while the SiO2 functioned as the gate dielectric. The oxide-coated Si wafers were cleaned prior to deposition of the organic semiconductors by sonication for 20 min in acetone followed by a rinse with 2-propanol. Organic semiconductor thin films with a thickness of 50 nm (∼0.5 Å/s) were deposited using thermal evaporation at 4 × 10-6 mbar. The substrate temperature could be controlled from room temperature to 100 °C in order to form films with different grain sizes and hence electronic mobility. After semiconductor deposition, topcontact source and drain electrodes were fabricated by thermal evaporation of 50 nm gold through a shadow mask with a spacing of L ) 270 µm and W ) 6.5 mm. Plasma Surface Modification. In order to selectively modify the semiconductor-air interface of 6pttp6 OFETs, we utilized a plasma cleaner (PDC-32G, from Harrick Scientific Products). We experimented with different exposure times that were determined based on our prior studies on plasma-induced surface modification of polymers.19 OFETs exposed to the plasma for 2 min lost their semiconductor behavior, indicating significant damage to the bulk of the film. In our experiments we have utilized plasma exposure times of 1 min to selectively modify only the top semiconductor-air interface with minimal damage to the bulk of the film and especially the semiconductordielectric interface. There was no significant change observed in the electrical characteristics of OFETs exposed to the plasma for 1 min. Results and Discussion The schematic diagram of the combined SFG-electrical probe station is shown in Figure 1. The frequency scanning range of SFG was set to cover C-H vibrational modes. Since the organic semiconductor and SiO2 layers in the OFETs are very thin, i.e., 50 and 300 nm, respectively, it is possible for the optical beams to penetrate into these layers without significant attenuation. The resulting SFG signal detected thus involves interference of the SFG beams from all three interfaces 0, 1, and 2. Since the interface 2 is the SiO2-Si interface, with no C-H groups, no resonant peaks are expected from this interface, and it contributes only to the nonresonant SFG background. The C-H resonant peaks measured are a result of interference between the SF output beams generated from the semiconductor-air interface (I0), semiconductor-dielectric interface (I1), and the nonresonant signal from the SiO2-Si interface (I3). Since it is known that in these organic OFETs, only the molecular layers nearest the dielectric interface are most affected by the electrical field, the interface I1 is critical to operation of the OFET. We separated the resonant signal from the two interfaces 0 and 1, by fitting SFG spectra to an equation of the form,
|
ISFG ) F χNR + Be
iφ1
∑i
A0i
+
(ω - ωi) + iΓ0i
Ce
iφ2
∑i
A1i (ω - ωi) + iΓ1i
|
2
IvisIIR (1)
13252 J. Phys. Chem. C, Vol. 111, No. 35, 2007
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Figure 2. Molecular structures of the three semiconductors studied. 6pttp6 and 2pttp2 have the same conjugated core but with hexyl and ethyl end groups, respectively.
Figure 4. SFG spectra of 6pttp6 (left) and 2pttp2 (right) at 0V (bottom) and -80 V (top). Also shown are intensity contributions to the spectra from interfaces I0 and I1, these contributions by fitting the SFG spectra to eq 1.
Figure 3. Typical electrical characteristics of the OFETs showing gated conduction at negative voltages (typical p-type device operation).
where ISFG is the intensity of the SFG beam, Ivis and IIR are intensities of the incident visible and IR beam, respectively, F is the effective Fresnel’s coefficient for the SF beam, χNR is the nonresonant contribution to the SF signal with ssp polarization, ω is the frequency of the tuned IR beam, while ωi, Ai, Γi are the frequency, amplitude, and damping constant of the ith vibrational mode, the superscripts 0 and 1 refer to the contributions from the semiconductor-air and semiconductor-dielectric interfaces.20 This equation takes into account the interference between nonresonant contributions from all the interfaces and resonant contributions from the semiconductor-air and semiconductor-dielectric interface. By fitting this equation to the SFG spectra, it was possible to extract the resonant terms from these two interfaces. Figure 2 shows the molecular structure of 5,5′-bis(4-n-hexylphenyl)-2,2′-bithiophene (6pttp6), 5,5′-bis(4-ethyl-phenyl)-2,2′bithiophene (2pttp2), and pentacene. The two npttpn oligomers differ in the alkyl chain lengths only, with the same conjugated core. Shown in Figure 3 are characteristic IV curves measured on the three OFETs, at different negative gate voltages (VG) showing typical p-channel transistor functionality. The pentacene device had the highest field effect mobility. The SFG spectra for 6pttp6 and 2pttp2 at gate voltages of 0 and -80 V are shown in Figure 4. The important peaks have been assigned to the CH3, ss (symmetric stretch, 2880 cm-1) and the CH2, ss (2850 cm-1) groups present on the alkyl chains on the molecules.17 It should be noted that while we also observed the aromatic C-H peaks from the conjugated cores of the molecules, these had lower
peak intensities and did not change during gating. Hence, we only focused our SFG experiments in the spectral window of the C-H modes of the pendant alkyl groups of 6pttp6 and 2pttp2. The signals from I0 and I1 were separated by the fitting eq 1 and are plotted in Figure 4 (dashed lines). At VG ) 0 V, for both 6pttp6 and 2pttp2, the semiconductor-air interface I0 had a large intensity in the CH3 symmetric stretch peak and very small intensity in the CH2 symmetric stretch peak, while the semiconductor-dielectric interface I1 had larger CH2 peak intensities. We also observed that the ratio of CH2/CH3 peak intensities on 6pttp6 was lower as compared to that on 2pttp2 (on both interfaces). At first glance, since 6pttp6 has many more (five at each end) CH2 groups as compared to 2pttp2 (one at each end), one might have expected a larger CH2/CH3 peak intensity ratio on 6pttp6. However, we rationalize our observation of a smaller CH2/CH3 peak intensity ratio on 6pttp6 by cancellation of CH2 modes due to molecular orientation. We know from published X-ray diffraction results that the npttn molecules pack almost perpendicular to the SiO2 surface with their alkyl groups vertical. In order for the 6pttp6 molecules to pack in an ordered layer, the CH2 groups in the alkyl chain at each end of the molecule must align in a trans configuration. In SFG studies on well ordered vertical monolayers, it is known that this all-trans configuration in alkyl chains results in cancellation of the SFG contributions from the CH2 groups because of symmetry considerations21,22a and hence little or no CH2 peak signal is observed. In 6pttp6 at each end of the aromatic core there are five (odd number of) CH2 groups; hence, complete cancellation may not occur and we do observe some peak intensity. It should be noted that Nishi et al.22b have observed negligible SFG CH2 peak intensity (attributed to alltrans packing) in n-alkanethiols (adsorbed on gold), for molecules with both even and odd numbers of CH2 groups. Hence, we believe that the small CH2/CH3 peak intensity ratio on 6pttp6 is a consequence of the CH2 groups being packed in a trans configuration. In comparison, the 2pttp2 molecule has only one CH2 group on the alkyl chain at each end. Since we also observed consistently lower field effect mobility on the 2pttp2
Organic Field Effect Transistors
Figure 5. SFG spectra of 6pttp6 (left) and 2pttp2 (right) at different gate voltages (during negative gated conduction). For 6pttp6 the CH2/ CH3 peak intensity ratio at approximately 2850/2880 cm-1 increases with increasing negative gate voltage. No significant change is observed in the relative peak intensities at positive gate voltages on either 6pttp6 or 2pttp2.
OFET, it is also possible that the molecules in the native 2pttp2 film (i.e., at 0 V) may not be as well ordered as those in 6pttp6; this lack of ordering would explain the higher CH2 peak intensity (as a result of increased gauche defects) observed on 2pttp2 at 0 V. When the transistor was negatively gated at VG ) -80 V, so that carrier injection was possible, we observed significant changes of the SFG spectra from 6pttp6 sample. There was a large increase in the CH2/CH3 peak intensity ratio at C, while the peak ratio at I0 remained relatively unchanged. This observation is consistent with the fact that the OFET channel resides within the molecular layers closest to the semiconductor-dielectric interface I1 within the Debye length of the device. On 6pttp6, at the buried interface I1, the CH2 symmetric stretch peak grew dramatically VG ) -80 V, while the peak of CH3 symmetric stretch from I1 decreased a little. We explain the large increase in the CH2/CH3 peak ratio by noting that all the thin films studied are molecular crystals, and it is known that the molecules stand almost perpendicular with respect to the substrate. Since the molecules are well packed, have no strong intramolecular dipoles, and are in the solid state, they are not expected to undergo any significant field-induced reorientations during conduction. However, while this is true for the conjugated cores of the molecules, it may not be true for the floppy alkyl chains. One possible mechanism for the increase in the CH2/ CH3 peak ratio is the introduction of gauche defects into the alkyl group that breaks the symmetry of CH2 groups from trans to gauche and increases the CH2 SFG signal.21,22 It should be noted that the introduction of gauche defects should also decrease the absolute CH3 peak intensities because of disordering of the CH3 groups at the interface. We observe only a slight decrease in our experiments, which can be explained by the fact that the expected decrease is counteracted by a field-induced SFG signal (discussed later) that increases the absolute intensity values of all peaks as well as the nonresonant background. Hence, instead of focusing on absolute intensities, we have used the CH2/CH3 peak ratio as a measure of the changes occurring in the OFET during gating. Since the CH2 peak intensities were extremely small, within the noise level, for the 6pttp6 molecule in the sps polarization, we were unable to do an orientational analysis of the CH2 groups. Hence, we cannot rule out another
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13253
Figure 6. Graphs showing plots of peak amplitude ratios of the symmetric stretches of CH2/CH3 vs gate voltage from I0 and I1 interface. Also superimposed is a plot of ISD,sat vs gate voltage. No significant change was observed at interface I0 during gated conduction in both 6pttp6 and 2pttp2 OFETs. A remarkable correlation was observed between the CH2/CH3 peak intensity ratio from I1 and ISD,sat for the 6pttp6 OFET. The peak ratio does not increase as dramatically for 2pttp2 which has a single CH2 group at each alkyl end chains as compared to five in 6pttp6.
possibility for the observed increase in CH2/CH3, that of an orientational change of the CH2 groups toward the surface normal on application of gate voltage in 6pttp6. If this were to occur we would expect it to occur most likely in the CH2 group nearest to the charged conjugated core. In order to probe the changes observed more systematically, we measured both SFG and IV spectra at a series of voltages between 0 and -80 V in steps of 20 V (Figure 5). We observe a clear trend of an increase of the ratio of CH2/CH3 symmetric stretches in 6pttp6 sample, with minimal change in 2pttp2 sample. In Figure 6 we plot the strength ratio of CH2/CH3 symmetric stretches at different gate voltages for 6pttp6 and for 2pttp2 at the two interfaces I0 and I1. As expected there is minimal change observed at the semiconductor-air interface. To correlate the change in CH2/CH3 ratios to the electrical characteristics of the OFET which is the goal of these studies, we have also plotted the source-drain saturation current ISD,sat vs VG as a solid line in the same graph. In the saturation regime, the saturation source-drain current is governed by the equation,3
ISD,sat )
L Cµ(VG - VT)2 2W
(2)
where L and W are the length and width of the semiconductor channel, and µ is the carrier mobility, C is the capacitance across the dielectric layer, and VT is the threshold voltage. A quadratic dependence between the source-drain current and the gate voltage is expected from the equation and this is what was observed. Additionally, a remarkable correlation was observed between the CH2/CH3 peak ratio and ID,sat at different VG for 6pttp6. This correlation points to a strong connection between the structural changes occurring in the molecule and the electrical characteristics of the OFET. For 2pttp2 and pentacene we did not observe any dramatic structural changes in the SFG spectra to begin with; hence, no such correlations were expected. To further prove that the changes observed in the SFG spectra of 6pttp6 were coming from the buried interface I1 and not from the semiconductor-air interface I0, we selectively modified I0. This was done by placing the 6pttp6 OFET in a plasma chamber that was operated at approximately 200 mTorr. This plasma is
13254 J. Phys. Chem. C, Vol. 111, No. 35, 2007
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Figure 7. Results obtained from the plasma exposed 6pttp6 sample. (A) Schematic diagram showing the expected selective modification of the semiconductor-air (I0) interface during plasma exposure. (B) SFG spectra obtained at different gate voltages showing an increase in the relative peak intensity ratio of CH2/CH3 symmetric stretches similar to that seen on untreated samples (Figure 5). (C) Ratio of the peak amplitudes of CH2/CH3 vs gate voltage. Also superimposed is a plot of ISD,sat vs gate voltage. The remarkable correlation seen earlier at interface I1 between the SFG peak ratio and saturation current (Figure 6) is observed even on the plasma damaged sample. Since, for this graph, peak ratios were calculated assuming a single SFG interface, the result provides further evidence that the changes observed in the SFG spectra are largely a result of changes occurring in the buried I1 interface that we believe is unaffected during plasma exposure.
a low power system (18 W applied to the radio frequency coil) but contains reactive oxidative radicals and ions that can oxidize oligomers and polymers. From previous studies,19 it is known that this oxidation takes place from the surface down into the bulk and the extent of oxidation (thickness) depends on the duration of exposure to the plasma. We found that it was possible, by keeping the exposure of the plasma to 1 min, to selectively damage I0 without damaging I1 (Figure 7A). This conclusion was drawn based on the observation that the electrical characteristics of the OFET were retained. After plasma exposure, SFG spectra were recorded at different VG (Figure 7B). The CH2/CH3 peak ratio was extracted from the spectra (without separating the contributions from the two interfaces I0 and I1) and plotted against VG (Figure 7C). Plasma treated OFETs showed a very similar trend in the correlation of CH2/ CH3 ratios and ISD,sat to that observed from the untreated 6pttp6 interface I1 (Figure 6). Since we are sure that the plasma will significantly damage the semiconductor-air interface, but not the semiconductor-dielectric interface, our observations further confirm that the changes we are observing during gating of the OFET are indeed coming from the semiconductor-dielectric (I1) interface. Apart from the conformational change observed in the floppy molecular chains of 6pttp6 during gating, we observed a second correlation, that of the dependence of χNR, the nonresonant
Figure 8. Correlations observed between SFG non resonant background and mobility. (A) A plot of the SFG non resonant background, χNR, measured at different gate voltages on OFETs with 6pttp6 deposited at different substrate temperatures (to produce films with different intermolecular packing and grain characteristics, and hence different mobility, µ). The mobilities of the samples are 0.045, 0.055 and 0.12 cm2 V/s. We observe that the slope of the plot ofχNR, (i.e., the increase of χNR with gate voltage) is larger for 6pttp6 OFETs with larger carrier mobility. (B) A plot of the slope of χNR with gate voltage obtained on 6pttp6, 2pttp2 and pentacene OFETs with different mobilities. Within a given molecular OFET, larger mobility devices also show larger increases of gate voltage dependence of the SFG nonresonant background.
contribution to the SFG intensity, and the mobility (extracted from the IV curves of the OFETs). The output polarization of s-polarized SFG beam is given by,23,24
PSFG ) χ(2)EVISEIR + χ(3)EVISEIRE0
(3)
where PSFG is the nonlinear polarization of SFG beam, χ(2) is the second-order nonlinear susceptibility, χ(3) is the third-order nonlinear susceptibility, E0 is the background electric field, and EVIS and EIR are the electric fields of s-polarized visible and p-polarized infrared input beam, respectively. The equation implies that in the presence of an external electrical field a change in the SFG signal is expected, commonly referred to as
Organic Field Effect Transistors “field enhanced SFG”17 Iwamoto et al. have utilized a similar field dependent signal in SHG to probe carrier dynamics. We observe that the nonresonant background increases linearly with increasing VG, as expected based on the equation above. VG generates an intense electric field in the semiconductor layer, enabling a field-induced SFG change; as noted earlier, this change generally increases intensity levels of all peaks as well χNR. This voltage dependence of the nonresonant background is shown in Figure 8A. As opposed to the structural changes in the OFETs that were observed only at negative VG, the change in χNR was observed at both positive and negative VG. In order to investigate correlations between this increase in χNR and electrical properties of the OFETs, we fabricated 6pttp6, 2pttp2, and pentacene devices with different carrier mobilities (µ). The mobility was manipulated by changing the substrate temperature during evaporation of the semiconductor, since the substrate temperature affects molecular packing and grain growth. For each semiconductor, we observed that a sample with higher µ also resulted in a higher increase of χNR with increasing VG. We captured this trend by calculating the slope (b) of the linear dependence of χNR vs VG. Figure 8B shows a plot of b vs µ for the OFETs studied. A strong correlation is evident, but this correlation holds only within OFETs fabricated with the same semiconductor molecular film. For OFETs composed of the same molecular film, whenever µ was high, b was also high. Since b is related to the third-order optical susceptibility, this correlation points to the fact that a common structural factor probably influences both the electronic mobility and the third order nonlinear optical susceptibility. This result suggests that this SFG slope of χNR vs gate voltage (b) can be used as a local probe of electric field and mobility within OFET channels fabricated from a given molecule. Conclusion In conclusion, we have demonstrated that SFG can be used as a valuable tool to investigate a buried interface, which is of critical importance in spectroscopy of OFETs. While electronic conduction in organic molecules was always thought to occur in the absence of conformational changes, we do believe that conformational changes in these charged organic molecules can indeed occur in their long floppy alkyl chains. Moreover these conformational changes are strongly related to gating of the semiconductor. This conformation change probably occurs as a result of charging of the conjugated cores of the molecular layer at the semiconductor-dielectric interface, resulting in a mechanically stressed molecular interfacial layer. This stress results in the introduction of structural changes in the floppy alkyl end groups, e.g., the introduction of gauche defects in these chains. We have confirmed that the semiconductor-dielectric interface is indeed the critical interface in OFETs, and it is possible to even damage the semiconductor-air interface without any observable change to the performance of the device or SFG spectral trends. Finally, we observe strong field enhancement of the SFG signal, and the extent of this enhance-
J. Phys. Chem. C, Vol. 111, No. 35, 2007 13255 ment is related to the mobility of the device. We believe that SFG could prove a very valuable tool for investigating OFETs. In the present experiments, in situ measurements were all done at VSD ) 0. This experimental approach was followed since we were attempting to capture the effects of gate voltage which is responsible for charging the semiconductor layer and carrier injection. Attractive future directions would be to study the effects of varying source-drain voltages as well as to incorporate SFG microscopy to provide spatially resolved information about local electric fields, carrier mobility, and molecular conformational changes within the different spatial regions of the channels of operational OFETs. Acknowledgment. This material is based upon work supported by the National Science Foundation under Grant No. 0528472. The SFG system was acquired using funds from NSF MRI grant CHE-0421010. References and Notes (1) Andry, P., Kagan, C. R., Eds. Thin-Film Transistors; Marcel Dekker: New York, 2003. (2) Katz, H. E. Chem. Mater. 2004, 16, 4748. (3) Horowitz, G. J. Mater. Res. 2004, 19, 1946. (4) Dimitrakopoulos, C. D.; Mascaro, D. J. IBM J. Res., DeV. 2001, 45, 11. (5) Bubert, H., Jenett, H. Eds Surface and Thin Film Analysis: A Compendium of Principles, Instrumentation and Applications; WileyVCH: Weinheim, 2002. (6) Chabinyc, M. L.; Toney, M. F.; Kline, R. J.; McCulloch, I.; Heeney, M. J. Am. Chem. Soc. 2007, 129, 3226. (7) DeLongchamp, D. M.; Kline, R. J.; Lin, E. K.; Fischer, D. A.; Richter, L. J.; Lucas, L. A.; Heeney, M.; McCulloch, I.; Northrup, J. E. AdV. Mater. 2007, 19, 833. (8) Krapchetov, D. A.; Ma, H.; Jen, A. K. Y.; Fischer, D. A.; Loo, Y.-L. Langmuir 2006, 22, 9491. (9) Leung, S.-A.; Tojo. T.; Murata, H. Jap. J. Appl. Phys. 2005, 44, 3733. (10) Blochwitz, J.; Fritz, T.; Pfeiffer, M.; Leo, K.; Alloway, D. M.; Lee, P. A.; Armstrong, N. R. Org. Electron. 2001, 2, 97. (11) Kocharova, N.; Lukkari, J.; Viinikanoja, A.; Aaritalo, T.; Kankare, J. J. Phys. Chem. B 2002, 106, 10973. (12) Cahen, D.; Kahn, A. AdV. Mater. 2003, 15, 271. (13) Crouch, D. J.; Skabara, P. J.; Lohr, J. E.; McDouall, J. J. W.; Heeney, M. et al. Chem. Mater. 2005, 17, 6567. (14) Shen, Y. R. Nature 1989, 337, 519. (15) Manaka, T.; Lim, E.; Tamura, R.; Iwamoto, M. Appl. Phys. Lett. 2005, 87, 222107. (16) Lim, E.; Manaka, T.; Iwamoto, M. J. Appl. Phys. 2007, 101, 024515. (17) Ye, H.; Abu-Akeel, A.; Huang, J.; Katz, H. E.; Gracias, D. H. J. Am. Chem. Soc. 2006, 128, 6528. (18) (a) Mushrush, M.; Facchetti, A.; Lefenfeld, M.; Katz, H. E.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 9414. (b) Stokes, M. A.; Kortan, R.; Amy, S. R.; Katz, H. E.; Chabal, Y. J.; et al. J. Mater. Chem., in press. (19) Ye, H.; Gu, Z.; Gracias, D. H. Langmuir 2006, 22, 1863. (20) Wilson, P. T.; Briggman, K. A.; Wallace, W. E.; Stephenson J. C.; Richter, L. J. Appl. Phys. Lett. 2002, 80, 3084. (21) Du, Q.; Xiao, X.-d.; Charych, D.; Wolf, F.; Frantz, P.; Shen, Y. R.; Salmeron, M. Phys. ReV. B: Condens. Matter 1995, 51, 7456. (22) (a) Himmelhaus, M.; Eisert, F.; Buck, M.; Grunze, M. J. Phys. Chem. 2000, 104, 576. (b) Nishi, N.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. J. Chem. Phys. 2003, 118, 1904. (23) Bethune, D. S.; Smit, R. W.; Shen, Y. R. Phys. ReV. Lett. 1977, 38, 647. (24) Gragson, D. E.; Richmond, D. L. J. Phys. Chem. B 1998, 102, 3847.
