In Situ Raman Mapping of Charge Carrier Distribution in Electrolyte

Article pubs.acs.org/JPCC

In Situ Raman Mapping of Charge Carrier Distribution in ElectrolyteGated Carbon Nanotube Network Field-Effect Transistors Jana Zaumseil,* Florian Jakubka, Ming Wang, and Florentina Gannott Institute of Polymer Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany S Supporting Information *

ABSTRACT: Solution-processed networks of purely semiconducting single-walled carbon nanotubes (s-SWNTs) can be used to create high-mobility field-effect transistors (FETs) for flexible electronics. In order to optimize network alignment and density, an understanding of carrier distribution within the FET channel is necessary. Here, we used confocal Raman microscopy to investigate charge accumulation and doping in electrolyte-gated FETs with asymmetric layers of only (7,5) nanotubes, that were selected by dispersion in poly(9,9-dioctylfluorene). The nanotube FETs exhibited hole mobilities of up to 7.5 cm2 V−1 s−1 and on/off ratios of 105. All Raman modes decreased in intensity with hole accumulation. The G′-peak and D-peak shifted linearly with negative gate voltages to higher wavenumbers. Using the G′-peak shift, the charge carrier distribution in an operating FET was mapped at different gate and source-drain voltages with high spatial resolution (∼300 nm) and over large areas. With this simple technique, we were able to visualize directly how the assignment of source and drain electrodes determined channel pinch-off and the onset of the saturation regime in FETs with nonuniform carbon nanotube distributions along the channel. In situ Raman mapping could also be applied to other semiconductors that show significant changes in their Raman spectra with doping.

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INTRODUCTION Random and semialigned networks of semiconducting singlewalled carbon nanotubes (s-SWNTs) deposited from dispersion can be applied in high-mobility field-effect transistors (FETs) for future flexible electronics.1−8 Compared to other printable semiconductors, such as small organic molecules and conjugated polymers, inorganic nanoparticles and semiconducting oxides, carbon nanotubes exhibit very high charge carrier mobilities and transconductance while being environmentally stable and essentially transparent as thin films. Since the growth process of carbon nanotubes always produces mixtures of metallic and semiconducting nanotubes, it is necessary to separate them before further processing. By using density gradient ultracentrifugation,9−11 ion-exchange chromatography,12 gel-chromatography,13,14 or selective dispersion in conjugated polymers,15−18 it is nowadays possible to obtain dispersions with >98% semiconducting nanotubes or even monochiral nanotube inks. High-density, random network FETs produced from purely semiconducting SWNT dispersions exhibit high mobilities as well as high on/off ratios.2,16,18−22 An efficient way of tuning the conductivity of semiconducting carbon nanotubes in sparse or dense networks and arrays is electrolyte gating.18,23,24 Here the gate dielectric is replaced by an electrolyte, for example, LiClO4 in polyethylene glycol or ionic liquids and iongels.25,26 When a gate voltage is applied, the anions and cations move toward the gate electrode and the semiconductor surface, respectively, depending on the polarity of the voltage. At those interfaces, they form electric double layers (EDLs). As the bulk of the electrolyte remains © 2013 American Chemical Society

neutral, the entire applied voltage drops across the nanometerthick EDLs, resulting in very large effective capacitances in the μF/cm2 range. Such high capacitances have several important consequences for carbon nanotube FETs. The accumulated charge carrier density and the transconductance of these FETs are very high, thus enabling, for example, switching of currentdriven organic light-emitting diodes for display applications.27 Injection of both holes and electrons is strongly facilitated, and often ambipolar transport, even in air, is observed.18,24,28,29 Ambipolar SWNT-FETs can be used for complementary-like circuits5 and near-infrared light emission.29 The efficient gating of SWNTs with electrolytes, which essentially creates a wraparound gating effect for each nanotube,30,31 also improves the on/off ratios of dense network SWNT-FETs and thus allows for the combination of high mobilities (>10 cm2 V−1 s−1) and high on/off ratios (>105).5,18 However, due to the high capacitance of the EDL, the quantum capacitance of the onedimensional SWNT cannot be neglected anymore. If the quantum capacitance CQ of the nanotube network is lower than that of the EDL, the former will dominate. CQ can be estimated from the density of the SWNT network (∂N/∂w, N = number of nanotubes, w = width) and the quantum capacitance per unit length of a nanotube (CQl = 4 × 10−10 F/m, when only one subband is occupied) as CQ ≈ (∂N/∂w)CQl.23,32 This has to be considered when calculating field-effect mobilities. Another important advantage of electrolyte-gating is the fact that the Received: October 3, 2013 Revised: November 18, 2013 Published: November 27, 2013 26361

dx.doi.org/10.1021/jp409849w | J. Phys. Chem. C 2013, 117, 26361−26370

The Journal of Physical Chemistry C

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semiconducting) and its diameter.55,59−61 The origin of the observed frequency shifts of certain Raman modes is less well understood. In this study, we use electrolyte-gating with an iongel to accumulate charges (holes) in thin films of only semiconducting nanotubes. We investigate the influence of doping with holes on all relevant Raman modes and find a reproducible and linear shift of the G′-mode that can be used to map the charge carrier density within the FET channel over large areas (20 × 20 μm2 and larger) and with optical resolution (300 nm).

gate does not have to be on top of the channel but can be relatively far away if high switching speeds are not required. This way, a convenient side-gate geometry and unobstructed optical access to the channel area become feasible.33,34 For random and semialigned networks of carbon nanotubes, which can be dense or sparse, uniform or graded, the distribution of current density within the network is nontrivial even when no metallic nanotubes are present. Preferential current paths and trapping sites depending on density, type, and distribution of nanotubes are expected. In order to understand the impact of such features on channel formation and performance of an FET, a method to image charge transport through SWNT networks with high resolution but under realistic operating conditions is required. One such method is scanning Kelvin probe microscopy, which maps the surface potential of a sample and has been used widely to investigate organic field-effect transistors35−38 and also carbon nanotube FETs.39−41 The spatial resolution for surface potential profiles along a transistor channel is ∼50 nm. However, a major drawback of scanning Kelvin probe microscopy is the need for highly controlled measurement conditions (e.g., ultrahigh vacuum) and accessibility of the surface of the semiconductor. Hence, only back-gated devices can be studied. The same drawbacks apply to the closely related method of electrostatic force microscopy.41−43 Voltage-contrast scanning electron microscopy offers even higher spatial resolution for nanotube networks but again requires direct access to the channel surface and high vacuum.44 Optical techniques such as scanning charge modulation spectroscopy,45 second-harmonic generation,46,47 and the more indirect scanning photoresponse method48 also have been used to image and investigate charge transport and traps in organic semiconducting thin films. Their resolution is limited to several hundred nanometers, but measurement conditions are more relaxed and top-gate FETs are also feasible. For carbon nanotube networks, the indirect method of scanning photocurrent microscopy can give insight into built-in electric fields and potential drops within the network.49,50 However, it does not represent the active channel that is formed in a nanotube network FET during operation. Here we introduce in situ confocal resonant Raman microscopy as a simple and direct technique to image the charge distribution within an operating electrolyte-gated nanotube thin film transistor. This way we are able to show how the nanotube density and the assignment of source and drain electrode affect the distribution of charges along the channel during operation. Single-walled carbon nanotubes exhibit several characteristic Raman modes, that can be used to identify and investigate them: radial breathing mode (RBM, 100−350 cm−1), D-mode (1300−1350 cm−1), G− and G+-mode (1500−1600 cm−1), and G′-mode (also called 2D-mode, 2600−2700 cm−1).51,52 The intensity and, in some cases, the Raman shift (i.e., frequency) of these modes depend on the level of doping of the nanotubes. Doping of carbon nanotubes can be achieved chemically,53 electrochemically,54,55 or electrostatically.56,57 A large number of studies report on the effects of doping on the Raman features of SWNTs; see, for example, Kavan and Dunsch58 and references therein. However, the various mixtures of metallic and semiconducting nanotubes with different diameter distributions make it difficult to form a unified picture. The effect that is most consistently observed is the symmetric reduction of intensity of the RBM peaks with electron and hole doping, which depends on the type of nanotube (metallic/

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EXPERIMENTAL METHODS Materials. CoMoCat single-walled carbon nanotubes (Aldrich, diameter 0.7−0.9 nm), poly(9,9-dioctylfluorene) (PFO, Aldrich, Mw = 75 kg·mol−1, PD = 3.4), ionic liquid 1ethyl-3-methyl-imidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM][FAP], high purity grade, Merck), and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP), Aldrich, Mw ∼ 400 kg·mol−1, Mn ∼ 130 kg· mol−1) were all used as received without further purification. Selective Dispersion of (7,5) SWNT. CoMoCat SWNTs (1.4 mg·mL−1) were dispersed in 2 mg·mL−1 PFO in toluene solution and bath sonicated for 60 min followed by tip sonication for 5 min. The resulting dispersion was centrifuged at 75 000g for 50 min (Beckman Coulter Avanti J26XP). The supernatant was further ultracentrifuged at 268 000g for 30 min to remove any remaining bundles and then at 268 000g for 8 h to sediment the majority of the nanotubes (Beckman Coulter OptimaMax XP table top centrifuge with a swinging bucket rotor MLS-50, thick-walled polyallomer tubes). The pellet at the bottom of the centrifuge tube was washed several times with toluene to remove residual PFO and then redispered in toluene by mild bath sonication. Absorption spectra were recorded with a Varian Cary 6000i UV−vis-nIR spectrometer, and photoluminescence excitation−emission maps with a Horiba Jobin-Yvon Fluorolog-3 spectrometer with a Symphony-II InGaAs diode array detector. Device Fabrication. Interdigitated source/drain electrodes were patterned on glass substrates (Schott AF32 Eco) by photolithography (double layer photoresist LOR5B/S1813), electron-beam evaporation of 2 nm chromium and 30 nm gold, and lift-off (channel width W = 20 mm, channel length L = 20 μm). For SWNT thin films, the enriched SWNT dispersion without free polymer was drop-cast (5 μL) onto hot substrates (100 °C) while a bias of 20 V (or 100 V) was applied between the source/drain electrodes. This caused preferential alignment of the SWNTs during the solvent evaporation process. After SWNT deposition, the substrates were washed with copious amounts of tetrahydrofuran and isopropanol to remove any residual polymer. Tapping-mode atomic force microscopy images were obtained with a Veeco diDimension 5000 AFM. The ionogel was spin-coated on top at 2000 rpm from a solution of [EMIM][FAP]/P(VDF-HFP)/acetone (5:1:15) and annealed in vacuum at 110 °C for 12 h before measurements to remove any residual solvents. Current− voltage characteristics of the electrolyte-gated FETs were measured, and constant voltages during Raman mapping were applied with an Agilent 4155C semiconductor parameter analyzer in air. Raman Microscopy. Raman spectra and maps were recorded under ambient conditions with a Renishaw InVia Reflex confocal Raman microscope in back scattering configuration with a high speed encoded sample stage 26362



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(MS20), using a ×100 objective (NA 0.85), excitation with a 633 nm helium−neon laser (in resonance with (7,5) nanotubes), a 20 μm pinhole, and a grating with 1800 lines mm−1. The spatial resolution was experimentally determined to be ∼300 nm (Figure S1, Supporting Information). Low resolution maps were obtained in StreamLine Plus mode with a line focus lens and ×50 objective (NA 0.7). The laser was polarized parallel to the charge transport and nanotube alignment direction to maximize the signal. Note that similar maps and shifts were obtained for excitation with circularly polarized laser light. Laser power and integration time were adjusted to minimize local heating while maintaining sufficient signal-tonoise ratio even at negative gate voltages and then kept constant for all spectra and maps. After application of constant drain and gate voltages, the devices were allowed to reach a quasi-static state before Raman spectra were recorded, usually after 1 min. This was necessary due to the relatively slow redistribution of ions within the iongel and nanotube network and thus EDL formation.

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RESULTS AND DISCUSSION In order to simplify interpretation of the Raman spectra, we chose a nearly monochiral network of semiconducting SWNTs instead of a mixture of nanotube species. This was achieved by selectively dispersing CoMoCat nanotubes, which contain predominantly (6,5), (7,5), and (7,6) nanotubes in a solution of PFO (poly[9,9-dioctylfluorene]) in toluene as introduced by Nish et al.15 After ultrasonication and centrifugation, only (7,5) nanotubes remained in dispersion. The excess polymer was removed by sedimentation of the nanotubes at high centrifugation speeds (>250 000g) for several hours, washing of the obtained SWNT pellet, and redispersion in toluene by mild sonication. The absorption spectrum (Figure 1a) of the final dispersion shows the E11, E22, and E33 absorption peaks of the (7,5) nanotubes, and only minimal amounts of residual polymer (broad absorption around 380 nm) and minute traces of (7,6) nanotubes. The corresponding excitation−emission map (Figure 1b) confirms the monochiral character of the dispersion. The (7,5) nanotubes were deposited by dropcasting onto interdigitated source-drain electrodes (see Figure 2a and b) (channel length 20 μm) while a DC-voltage was applied to ensure preferential alignment in the direction of charge transport. Any remaining polymer was removed by washing with tetrahydrofuran. Tapping mode atomic force microscopy images of the channel area show a dense network with nonuniform coverage and single as well as bundled SWNTs (see Figure 2c). Electrolyte-gated FETs were completed by spin-coating a few micrometers thick layer of iongel containing the ionic liquid [EMIM][FAP] (1-ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate) as the electrolyte and P(VDF-HFP) as the network polymer. P(VDF-HFP) has a better thermal stability34 than the more commonly used triblock copolymers for iongels62 and is therefore better suited for laser exposure, which is likely to result in moderate local heating. The iongel layer was thin enough (∼2 μm) to allow for high-resolution confocal Raman mapping of the SWNT network. A large gold pad (1 mm2) adjacent to the sourcedrain electrodes served as a side-gate electrode. The current−voltage characteristics of an electrolyte-gated SWNT network-FET are shown in Figure 2d. All FETs exhibited clear hole accumulation and positive turn-on voltages (about +1 V), which indicates strong p-doping. This is expected under ambient conditions. The same devices measured in dry

Figure 1. (a) Absorption spectrum (cuvette length 1 cm) and (b) photoluminescence excitation−emission map of enriched (7,5) SWNT dispersion with minimal residual polymer (PFO). The dispersion shows only small traces of (7,6) nanotubes.

nitrogen showed negative turn-on voltages for hole transport and also electron transport at higher positive gate voltages (Figure S2, Supporting Information). Moderate current hysteresis is apparent for all measurement conditions, which is partially due to the slow redistribution of ions in the iongel with changing gate voltage and can be reduced by lowering the voltage sweep rate. The on/off ratios for (7,5) nanotube FETs reached 105. This high on/off ratio for a dense nanotube network results from the efficient electrolyte-gating as well as the selective dispersion process, which removed metallic nanotubes. Note that the source-drain current saturated at higher gate voltages due to the one-dimensional nature of the nanotubes indicating filling of the first subband (the E11 band gap is about 1.2 eV). The transconductance and thus mobility reached a single maximum between 0 V and −0.5 V (forward and reverse). We did not observe another increase of transconductance or mobility within our gate voltage range that would indicate access to the second subband. Due to the large E22 gap (∼1.9 eV), this was also not to be expected. The estimated maximum hole mobility reached 7.5 cm2 V−1 s−1 using an areal quantum capacitance for the nanotubes of 0.8 μF cm−2, calculated for a density of 20 SWNTs μm−1. In order to study the effect of accumulated charges on the Raman modes of the (7,5) nanotubes, the source and drain electrodes were both grounded and a constant gate voltage was applied for 1 min before Raman spectra were collected from a point in the center of the channel. Note that all data are given 26363



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Figure 2. (a) Schematic illustration (top view) of FET layout in side-gate geometry and (b) measurement setup (side view) for in situ Raman microscopy of electrolyte-gated networks of (7,5) nanotubes. (c) Tapping-mode AFM image of SWNT network after drop-casting. (d) Transfer characteristics of electrolyte-gated FET with dense network of (7,5) nanotubes (channel length L = 20 μm, channel width W = 20 mm), measured under ambient conditions (dashed lines: gate leakage current).

Figure 3. Progression of Raman spectra (excitation wavelength: 633 nm) obtained from channel area of electrolyte-gated FET with dense network of (7,5) nanotubes (source/drain electrodes grounded) at different applied gate voltages from +0.1 to −2.0 V in steps of 0.1 V (spectra from +2 to +0.2 V are not shown for clarity). (a) Radial breathing mode for (7,5) nanotubes, (b) D-peak, (c) G-mode, and (d) G′-mode. All spectra are offset vertically for clarity, but not normalized or scaled.

(Supporting Information). Kalbac et al. demonstrated that both phonon energy renormalization and modifications of the C−C bond strength play a role for the G+-mode of semiconducting nanotubes, and the diameter of the nanotubes influences the direction of the peak shift.64 We observe an additional sharp peak at 1540 cm−1, whose intensity decreases faster than that of the G+ and G− mode (see normalized spectra in Figure S4, Supporting Information) and vanishes completely at gate voltages lower than −1 V. The origin of this peak is unclear. In the literature, a peak at 1540 cm−1 was associated with LO phonons of metallic tubes.65 However, these should not be present in this sample according to the absorption spectra. Note that we observe no changes for any of the Raman modes at positive gate voltages, that is, for electron accumulation. Due to the measurement conditions (ambient air, presence of water and oxygen), electrons are strongly trapped and neither contribute to charge transport (see transfer characteristics) nor affect the Raman signal within the channel area.

with respect to this applied gate voltage, which is correlated with but does not represent the exact electrochemical potential at the interface.63 Figure 3 shows all relevant Raman modes at various positive and negative gate voltages. The radial breathing mode at 285 cm−1 for (7,5) nanotubes is the only observable mode in the RBM frequency range. Even when other laser wavelengths (532 and 785 nm) are used for excitation, no additional RBM peaks appear. The intensity of the (7,5) RBMpeak decreases with negative gate voltages (see Figure 4a), but frequency and full width at half-maximum (∼4 cm−1) remain unaltered, which is consistent with previous reports.55,59 While an abrupt change of intensity between 0 V and −1 V is always observed, slight changes of intensity are due to prolonged laser exposure of the same spot. This is shown in more detail for all Raman peaks for forward and reverse gate voltage sweeps in Figure S3 (Supporting Information). The intensity of the G+-mode at 1591 cm−1 decreases with negative Vg (see Figures 3c and 4b) but also shifts slightly by less than 3 cm−1 and not monotonically as shown in Figure S4 26364



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Figure 4. (a−d) Variation of normalized peak area with applied gate voltage from +2 to −2 V in steps of 0.1 V for all relevant Raman peaks. (e,f) Shift of fitted peak position with gate voltage for D-peak and G′-peak.

displacement current measurements to determine the capacitance and thus total number of injected charges. For a rough estimate, we assume again that the quantum capacitance of the nanotubes is the limiting capacitance. Taking into account the applied gate voltage (e.g., −1 V) and observed threshold voltages, we estimate a hole density of 0.5 holes nm−1 SWNT, which is rather high. Unlike the Raman mode intensities, the photoluminescence of the (7,5) nanotubes dropped sharply and instantly with the onset of hole accumulation (see Figure S6, Supporting Information). This decrease of photoluminescence efficiency is usually attributed to an Auger-type E11 exciton quenching mechanism,56 which is sensitive to small charge concentrations. Clearly, the impact of charges on the Raman spectra does not correlate with low charge carrier accumulation but seems to set in when the first subband is filled. The detailed interpretation of the Raman peak shifts is complex and beyond the scope of this work. Briefly, resonant Raman scattering of carbon nanotubes depends on the fulfillment of the resonance condition, the electron−phonon coupling matrix elements, and the optical electron−photon interaction coefficients as discussed by Dragin et al.55 and others.56,57,66 The D-peak arises from double-resonant Raman scattering and is activated by defects. The interaction of charges with defects is likely to be strong and thus an influence of doping on this mode due to trapped charges might be expected. However, the G′-mode, which results from the same twophonon, second-order scattering process, but is independent of defects, is equally affected by doping. The nature of the G′mode as an overtone of the D-mode is also reflected in the slopes of the peak shift versus gate voltage, with the G′-peak shift being twice as steep as that of the D-peak. For electrolytegated graphene, a shift of the 2D-peak (i.e., the G′- peak in

The dependencies of the D-peak and G′-peak on gate voltage are more revealing and have not been reported in detail for semiconducting nanotubes before. While also decreasing in intensity, both shift monotonically toward higher wavenumbers for increasing negative gate voltages (see Figures 3b,d and 4e,f). The G′-peak shifts from 2603 cm−1 at positive Vg to 2656 cm−1 at −2 V. The D-peak shifts from 1302 to 1325 cm−1, respectively. The onset of the shift is in both cases around −0.2 V, that is, at more negative gate voltages than the onset of charge transport and close to the point of maximum transconductance. The peak shifts increase linearly with a slope of −31 ± 2 cm−1 V−1 for the G′-mode and −14 ± 1 cm−1 V−1for the D-mode. The widths of both peaks also increase with −Vg (see Figure S5, Supporting Information). Note that the shift of peak position with increasing charge accumulation is very reproducible for forward and reverse voltage sweeps and shows no or very little hysteresis or dependence on measurement history unlike the peak intensities. Also, the intensity of the D-peak does not increase with measurement time, which rules out possible functionalization of the nanotubes by the ionic liquid or by oxygen/water during laser exposure and electrochemical charging. An approximation of the charge carrier density per unit length of nanotube or per carbon atom is difficult. In general, the increase of charge carrier density should be linear with Vg. For electrolyte-gated FETs, especially with one-dimensional semiconductors, the capacitance can vary with Vg as well. Thus the capacitance should be measured independently to obtain correct charge carrier densities. However, the employed sidegate geometry with a thin iongel and thus slow response of the system to voltage changes requires at least 30 s to reach an equilibrium state. This prevents standard impedance or 26365



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remained at high wavenumbers around 2623 cm−1. Close to the drain electrode the charge carrier density was lower and the peak position shifted back to lower wavenumbers depending on the applied drain voltage. To deconvolve position-dependent variations of the peak shift and the influence of the applied bias, Figure 5b shows the difference between the G′-peak position for Vds = 0 V and all other drain voltages. The obtained gradients along the channel are more distinct and increase with −Vds as expected for FET channels in the linear regime with accumulated charges. For higher −Vds (−1, −1.25 V) a more pronounced drop of the peak shift close to the drain electrodes is observed, which indicates pinch-off of the channel and onset of the saturation regime. That is, a depletion region forms next to the drain because the difference between the local potential and the gate voltage is now below the threshold voltage for transport. Holes are swept from the pinch-off point to the drain by the comparatively high electric field in the depletion region. Further increasing the source-drain voltage does not increase the current anymore because the voltage at the pinch-off point remains the same. The depletion region should expand with Vds toward the source electrode. The shape of these G′-peak position versus channel position curves are qualitatively similar to surface potential maps of thin film FETs obtained by scanning Kelvin probe microscopy36,41 and local polaronic absorption mapping of polymer FETs.45 We conclude that the shift of the G′-peak position appears to accurately reproduce the charge carrier density (trapped and mobile charges) along the transistor channel for these voltage conditions similar to other scanning probe techniques. In order to gain further insight into the distribution of charges depending on position within the channel region and density of the carbon nanotube network, two-dimensional Raman maps were collected for a 10 × 24 μm2 area (Figure 6) with a step distance of 200 nm in both directions. As shown in the intensity map of the G′-peak (Figure 6a), the distribution of nanotubes is inhomogeneous and there is a much higher density close to the right electrode than the left electrode. This SWNT density gradient is a result of the deposition of SWNTs from dispersion with applied constant bias. It not only aligns the charged SWNTs but also attracts them toward one electrode. The obtained asymmetric SWNT distribution should affect the distribution of charges within the channel as well as the current−voltage characteristics. In Figure 6b, the position of the G′-peak is plotted for negative gate voltage (−1 V) and zero source-drain bias. Slight variations of the peak position are evident, which coincide with low and high nanotube densities represented by the G′-peak area (Figure 6a). This is conceivable as the G′-peak shift should be sensitive to the concentration of charges per nanotube. When a negative drain bias was applied (Figure 6c and d), a clear gradient of the peak position from the source to the drain developed. Plotting the difference to the peak position for Vds = 0 V at each point (Figure 6e and f) removes some of the features caused by the SWNT density variation. One might expect that areas with a high density of nanotubes lead to higher conductivity, which should be reflected in a steeper gradient of carrier density along the channel. Although there are some variations, the gradient appears to be relatively uniform within the channel area. Strikingly, the overall distribution of charge density changed significantly when the assignment of the electrodes was reversed. In Figure 6c, the source is on the side of low

SWNT) was observed for hole and electron accumulation and was discussed in terms of changes in C−C bond strength, electron−phonon coupling, and electron−electron interaction.67−69 Regardless of its exact origin, the strong and linear G′-peak shift with applied gate voltage is an indicator for charge accumulation. Although the onset of the shift is somewhat more negative than that of charge transport, it should, within limits, be applicable to map the carrier density within an SWNT-FET channel depending on gate and source-drain voltage similar to other scanning microscopy techniques. To show this, Raman spectra were collected along a straight line from the source to the drain electrode with a step distance of 100 nm and the G′-peaks were fitted with Lorentz profiles to extract the center wavenumber. The resolution limit of the confocal microscope with 633 nm laser excitation is about 300 nm; hence, the data points were oversampled to allow averaging. Figure 5a shows plots of the fitted G′-peak position

Figure 5. Raman line scans across FET channel (step distance 100 nm) at different applied gate and source-drain voltages. (a) Fitted G′peak position, (b) difference between G′-peak position at Vg = −1 V, Vds = 0 V, and at Vg = −1 V and different Vds. Solid lines represent strongly smoothed data to guide the eye.

versus lateral position. For zero source-drain voltage, the peak center varied only slightly with lateral position and shifted uniformly to higher wavenumbers as a negative gate voltage was applied. Note that the uncertainty of the fit and thus noise increased with decreasing signal, that is, with more negative gate voltage. When a negative bias was applied to the drain electrode a gradient of the G′-peak position developed that became steeper with the absolute value of the drain voltage. At the source electrode, where charges were injected and the carrier density was at its maximum, the G′-peak center 26366



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Figure 6. 10 × 24 μm2 Raman maps (step size 200 nm) of channel area during FET operation. (a) Peak area of G′-peak at Vg = −1 V, Vds = 0 V indicating density of SWNT. (b) Fitted peak position (cm−1) of G′-peak at Vg = −1 V, Vds = 0 V, (c) at Vg = −1 V, Vds = −1 V with source electrode on the left, (d) with source electrode on the right. (e) Difference between peak position of G′-peak for Vg = −1 V, Vds = −1 V and Vg = −1 V, Vds = 0 V with source electrode on the left, (f) with source electrode on the right.

SWNT density, and in Figure 6d on the side of high SWNT density. The relatively uniform gradient in the former case suggests a continuously decreasing density of holes from the drain to the source as expected for a transistor channel with a dense nanotube coverage in the linear regime. In the latter case, however, the charge density stays relatively high for the main part of the channel and only drops relatively close to the drain electrode, which would suggest a pinched-off channel in saturation although the applied voltages are the same in both cases. The same dependence of G′-peak position maps on electrode assignment was found for all samples with a graded distribution of SWNT and was independent of the sequence of measurements. A closer look at the output characteristics (see Figure 7) of this FET depending on the source/drain electrode assignment revealed that, indeed, using the electrode on the high SWNTdensity side as the source resulted in early saturation. Exchanging source and drain led to output characteristics with no obvious saturation within the applied voltage range. Note, that this dependence on electrode assignment only became significant at higher source-drain bias and was negligible at |Vds| < 0.2 V. The transfer characteristics for these Vds did not differ noticeably. The differences in saturation behavior are clearly a result of the asymmetric nanotube distribution. A lower density of nanotubes corresponds to a

Figure 7. Output characteristics of FET in Figure 6 with asymmetric SWNT density depending on source/drain electrode assignment.

higher channel resistance and therefore a larger voltage drop in this area. This pushes the channel toward pinch-off when the low SWNT-density area is next to the drain electrode (see Figure 6f), although according to the gate (−1 V), source-drain (−1 V), and threshold voltage (+1 V), saturation should not yet be reached. In the reverse case, the channel is still in the linear regime as shown in Figure 6e. The two possible regimes 26367



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(linear and saturation) for the same voltage conditions can be clearly identified and visualized by mapping the G′-peak shift and are corroborated by the output characteristics. More examples of high resolution Raman maps can be found in the Supporting Information (Figures S7a and b). In addition to the G′-peak position, it was also possible to use the shift of the D-peak for imaging (as shown in S8, Supporting Information). Similar gradients and dependencies on electrode assignment were observed. However, due to the more shallow slope of the shift compared to that of the G′-peak and the given uncertainties of peak fitting for weak signals the obtained data were noisier, which reduced spatial resolution. Also the intensity of the D-peak depends on the defect density, while the G′-peak is always present. The demonstrated shifts of the G′-peak for (7,5) nanotubes were also observed for SWNT networks with more than just one type of nanotube. For example networks of five different semiconducting nanotube species, obtained from selective dispersion of HipCO nanotubes in PFO/toluene solution, showed the same overall effect. The distribution of charge density could be mapped equally well with high resolution and reproducibility as shown in Figures S9 and S10. While confocal Raman microscopy allows for high resolution mapping it can also be applied for fast mapping over very large areas when used in combination with a line focus lens to illuminate a line on the sample and continuous signal readout over the entire detector array. The spatial resolution in this mapping mode is reduced to 1.3 μm in vertical direction. The overall measurement time is drastically reduced and areas of hundreds of square micrometers can be mapped within minutes thus reducing bias stress for the sample. Large overview Raman maps were obtained in this imaging mode spanning several interdigitated electrode fingers and corresponding channel areas. The fitted G′-peak area and position shift maps for Vg = −1 V and Vds = −1 V versus Vg = −1 V and Vds = 0 V for an area of 170 × 130 μm are presented in Figure 8. They show the same dependence on source/drain electrode assignment as seen in Figure 6 for a larger part of the same device. Such large area scans are usually not possible with other scanning probe techniques and enable assessment of device uniformity during operation within reasonable time limits.

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Figure 8. 170 × 130 μm2 low-resolution Raman maps (step size 1.3 μm (vertical), 0.7 μm (horizontal)) of electrolyte-gated (7,5) SWNTFET. (a) Peak area of G′-peak (Vg = −1 V, Vds = 0 V) indicating density of SWNT. (b,c) Difference between peak position of G′-peak for Vg = −1 V, Vds = −1 V and Vg = −1 V, Vds = 0 V for both source and drain electrode assignments.

CONCLUSIONS In summary, we investigated the influence of hole accumulation by electrolyte-gating on resonant Raman spectra of thin films of purely (7,5) single-walled carbon nanotubes. In addition to the expected decrease of Raman scattering with charge accumulation, we found reproducible and linear shifts of the D-peak and G′-peak toward higher wavenumbers with increasing negative gate voltage and thus hole density. We demonstrated how the G′-mode shift can be used to map the hole distribution in the channel area of an electrolyte-gated nanotube field-effect transistor during operation with high spatial resolution and over large areas. Using this technique, we were able to directly observe the different conditions for pinch-off in FETs with nonuniform nanotube density depending on source/drain electrode assignment. This in situ mapping of charge carrier density in FETs, carried out with a standard confocal Raman microscope, could also be applied to other semiconductors that show significant changes in their Raman spectra when doped or charged, for example, graphene, reduced graphene oxide thin films, two-dimensional transition metal dichalcogenides, and organic semiconductors.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental determination of spatial resolution, FET transfer characteristics in dry nitrogen, variation of normalized peak areas and peak position for forward and reverse sweeps of gate voltage, detailed G-mode dependence on Vg, dependence of Dpeak and G′-peak width on Vg, photoluminescence spectra and dependence on Vg, additional Raman maps of SWNT-FETs during operation and high resolution Raman maps of SWNTFET with several semiconducting nanotube species. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49 9131 8525435. E-mail: [email protected]. 26368



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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Alfried Krupp von Bohlen und Halbach-Stiftung via the “Alfried Krupp Förderpreis für junge Hochschullehrer” and by the Deutsche Forschungsgemeinschaft (DFG) via the Collaborative Research Center “Synthetic Carbon Allotropes” (SFB 953). J.Z. also acknowledges general support by the Cluster of Excellence “Engineering of Advanced Materials” (EXC 315). The authors thank Udo Mundloch and the Institute of Advanced Materials and Processes (ZMP) for help with ultracentrifugation and photoluminescence measurements, and Prof. Eric Pop (Stanford University) for helpful discussions.

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In Situ Raman Mapping of Charge Carrier Distribution in Electrolyte

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