Letter pubs.acs.org/NanoLett
Spectroscopic Investigation of Electrochemically Charged Individual (6,5) Single-Walled Carbon Nanotubes Sebastian Schaf̈ er,† Nicole M. B. Cogan,† and Todd D. Krauss*,†,‡ †
Department of Chemistry and ‡Institute of Optics, University of Rochester, Rochester, New York 14627, United States S Supporting Information *
ABSTRACT: Individual single-walled carbon nanotubes (SWNTs) of (6,5) chirality were investigated by means of optical spectroscopy while their charge state was controlled electrochemically. The photoluminescence of the SWNTs was found to be quenched at positive and negative potentials, where the onset and offset varied for each individual SWNT. We propose that differences in the local environment of the individual SWNT lead to a shift of the Fermi energy, resulting in a distribution of the oxidation and reduction potentials. The exciton emission energy was found to correlate with the oxidation and reduction potential. Further proof of a correlation was found by deliberately doping individual SWNTs and monitoring their photoluminescence spectral shift. KEYWORDS: Carbon nanotubes, electrochemistry, photoluminescence, redox potentials particles, 19,20 molecular fluorophores,21,22 and quantum dots.23−25 Electrochemistry on the single molecule level has an exquisite sensitivity to discrete charging and discharging events and, therefore, might also offer additional insight into the electrical and optical properties of SWNTs. In this study, we utilize spectroelectrochemistry to investigate the redox properties of individual (6,5) SWNTs. We show that although the oxidation and reduction potentials differ in each individual SWNT, the mean electrochemical band gap matches the value found for an ensemble of (6,5) SWNTs.12 Additionally, we find that the PL wavelength shifts to the red (blue) for a low (high) Fermi energy. We attribute this effect to local charges that change the stability of the exciton and thereby shift its emission energy. The spectroelectrochemical experiments were performed on CoMoCAT SWNTs (Southwest Nanotechnologies, Inc.). For dispersion in aqueous solution, the SWNTs were wrapped in single-stranded DNA (GT)30 following procedures described in the literature.26,27 Fifty microliters of a highly diluted SWNT sample were drop coated on an indium tin oxide (ITO) covered quartz coverslip that was mounted in an electrochemical cell. ITO is conductive and transparent and hence allows us to simultaneously perform electrochemical and optical experiments. After the solvent evaporated, the electrochemical cell was transferred to a glovebox and filled with the electrolyte solution consisting of dry acetonitrile and 0.1 M tetrabutylammonium perchlorate (Sigma-Aldrich). During the drying process the SWNTs adsorb to the ITO so that they do not detach after the addition of the electrolyte solution. Finally, a Pt
T
here has been enormous progress synthesizing and understanding the optical and electrical properties of single-walled carbon nanotubes (SWNTs) since their discovery in 1993.1,2 Current research focuses on both embedding SWNTs in optoelectronic and electronic devices3,4 and understanding fundamental SWNT properties.5 In recent years, spectroelectrochemistry has been shown to be a powerful tool to investigate fundamental electrical properties of SWNTs.6,7 In spectroelectrochemistry, electrochemistry is combined with optical spectroscopy methods such as Raman,8,9 absorption,10 or photoluminescence (PL) spectroscopy.11,12 Those experiments have been successfully utilized to determine the oxidation and reduction potentials of different chiralities in a SWNT ensemble.10−12 Furthermore, it was recently demonstrated that the excitonic PL quenches under an applied potential, while a new emission peak from a trion simultaneously arises.13 Experiments by Yanagi et al. showed that the color of a metallic SWNT film can be changed by applying a potential, thereby opening the possibility of fabricating electrochromic devices without rare metals.14 All spectroelectrochemical experiments reported to date have been performed on ensembles or macroscopic samples of SWNTs. However, experiments carried out on individual molecules often differ substantially from ensemble measurements. For example, at low temperature the PL spectra of individual SWNTs exhibit a line width of down to 0.25 meV and spectral wandering.15 SWNTs of the same chirality have been shown to differ in spectral position and amplitude16 and there are reports on fluctuations in PL intensity of individual SWNTs.17,18 Although the origins of these phenomena are unknown, charges near the SWNT are commonly believed to play an important role. Spectroelectrochemistry has recently been extended to single molecules, including polymer nano© 2014 American Chemical Society
Received: January 29, 2014 Revised: April 24, 2014 Published: May 5, 2014 3138
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Figure 1. PL images of the same area containing individual SWNTs under different potentials. The potential dependence of the SWNTs marked in (e) is detailed in Figure 2.
and a Ag/AgCl electrode were placed in the electrolyte solution and the electrochemical cell was sealed, so that the electrochemical experiments could be performed under the exclusion of oxygen and water. For simultaneous electrochemical and optical experiments, the electrochemical cell was placed on top of an inverted microscope (Nikon Eclipse Ti). A potentiostat (CH Instruments Model 600D) was used in the three-electrode configuration, where ITO served as the working electrode and the Pt and Ag/AgCl wires were used as counter and reference electrodes, respectively. The potentials are given versus the Ag/AgCl electrode, which was calibrated using the Fc/Fc+ redox couple (found at 0.26 ± 0.01 V versus Ag/AgCl). Optical measurements were performed in widefield mode using a Nikon Plan Fluor objective (NA = 1.3, 100×). A Krypton-ion laser operating at 568.2 nm was used as an excitation source. Typical laser power densities were 10 kW/cm2. A Si CCD (Princeton Instruments Spec-10 400BR) coupled to a spectrometer was employed for the detection of the SWNT PL. We note that a CoMoCAT sample usually contains about 28% (6,5) SWNTs.28 To ensure that only (6,5) SWNTs (emission maximum at 975 nm)29 were investigated, we excited our sample in resonance with the E22 of (6,5) SWNTs (567 nm)29 and used a 950 nm long-pass filter in our optical setup to block emission from (6,4) SWNTs (emission maximum at 873 nm).29 Additionally, the Si CCD detection efficiency cuts off above 1050 nm, so that our setup is optimized for (6,5) SWNT PL detection. A PLE map of our sample (Supporting Information Figure S1) supports the conclusion that we only excite and detect (6,5) SWNTs under our experimental conditions, and indeed the detected individual NT species were all (6,5) SWNTs. PL spectra from 16 spots exhibited Lorentzian lineshapes and PL maxima between 975 and 1000 nm, as expected for single (6,5) SWNTs (cf. Figure 5). In addition, the sample was diluted until the detected PL spots were circularly shaped (note that SWNT lengths were below the diffraction limit) and the separation between emitters was sufficient to be confident that our SWNTs were individual (cf. Figure 1e). Sample areas and SWNTs that did not meet these criteria were disregarded. Control experiments (described in the Supporting Information) indicate that contamination from small bundles (e.g., a (6,4)-(6,5) bundle) is less than 14% and
thus has negligible effect on the results presented here. Hence, we are confident that we only probed individual (6,5) SWNTs. Figure 1 shows PL images of individual SWNTs under different applied potentials. The voltage scan rate was 5 mV/s and the integration time for an image was 10 s. Figure 1a shows that the PL of the SWNTs is quenched at a positive potential of about +1 V. When the applied voltage is decreased the SWNT PL recovers, where the PL of different SWNTs recovers at different potentials (Figure 1b−d). Figure 1e−h details the quenching of the SWNT PL at negative potentials. Similar to a positive potential, the PL of different SWNTs quenches at slightly different potentials. A video of the full potential range from +1 to −1.5 V can be found in the Supporting Information. The quenching of the E11 exciton PL peak at positive and negative potentials is associated with the extraction of electrons out of the valence band (VB) (oxidation) and filling of electrons in the conduction band (CB) (reduction), respectively. Previous reports on SWNT ensembles suggest that a trion, a quasi particle with three bound charge carriers, forms upon oxidation or reduction.11−13 For a positively (negatively) charged SWNT, the trion consists of two (one) holes and one (two) electrons. The trion recombination of (6,5) SWNTs exhibits a weak PL signal at 1160 nm (1.07 eV),13 which is beyond the spectral range of our detector. We note that our SWNT surfactant DNA is oxidized (reduced) only under more positive (negative) potentials than the ones we used for our experiments.30,31 Hence, the observed PL quenching can only be assigned to the SWNT itself and not to a redox reaction of its surfactant. The quenching curves in Figure 2 detail the dependence of the PL intensity on the applied potential for the two individual SWNTs marked in Figure 1e. To extract the SWNTs oxidation (Eox) and reduction (Ered) potentials, the data was fit using a Nernst equation similar to the one used by Tanaka et al.11 For measurements on SWNT ensembles the Nernst analysis assumes a fractional content of oxidized and reduced SWNTs, however, this situation does not apply to our SWNTs because our measurements were conducted on individual SWNTs. The simplest model in which the injection of one charge forms a trion and quenches the excitonic PL would predict a sharp step-function in the quenching curves. However, for our SWNTs the exciton diffusion length is smaller than their physical length of about 300 nm. Recent studies 3139
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character of SWNTs and softens the quenching curves of SWNTs compared to other single molecule experiments. We write Pox =
Pred =
1 nF ⎡ 1 + exp⎣ RT (E − Eox )⎤⎦
(1)
1 nF ⎡ 1 + exp⎣ − RT (E − Ered)⎤⎦
(2)
where Pox and Pred denote the probability of forming excitons in the SWNT, n is the number of transferred charge carriers, F is the Faraday constant, R is the ideal gas constant, and T is the temperature. For the measured PL intensity I we get I = Imax(Pox + Pred − 1)
(3)
where Imax is the maximum PL intensity. For the SWNTs in Figure 2, we find similar electrochemical band gaps as calculated from ΔEechm = Eox − Ered (1.06 and 1.03 eV for SWNT #1 and #2, respectively), however, their Eox and Ered differ significantly. For SWNT #1 we find Eox = 0.63 V and Ered = −0.42 V versus Ag/AgCl, while Eox = 0.89 V and Ered = −0.14 V versus Ag/AgCl for SWNT #2. Electrochemical data for a total of 104 SWNTs were recorded as shown in Figure 1 and analyzed as detailed in Figure 2. A statistical analysis of these results is shown in Figure 3. The distributions of Eox and Ered (Figure 3a,b) were fit with a Gaussian where the peak centers were found at 0.65 ± 0.02 and −0.52 ± 0.01 V versus Ag/AgCl with a full width at halfmaximum (fwhm) of about 372 ± 42 and 313 ± 30 meV, respectively. Figure 3c shows the distribution of ΔEechm. The peak center of the Gaussian fitted to ΔEechm is found at 1.17 ± 0.01 eV, while the fwhm is 244 ± 24 meV. The peak center corresponds exactly to the value found by Hirana et al. for (6,5) SWNTs in a SWNT ensemble in acetonitrile.12
Figure 2. Dependence of the PL intensity on the applied potential for the SWNTs labeled (a) #1 and (b) #2 in Figure 1e, respectively. The left and the right vertical lines are the reduction and oxidation potential, respectively; ΔEechm is the electrochemical band gap.
suggest exciton diffusion lengths of 50 to 200 nm,32−34 where the diffusion length depends on the SWNT surfactant.35,36 For DNA-wrapped SWNTs, Miyauchi et al. report a diffusion length of about 50 nm.33 Therefore, each injected charge carrier may not interact with the exciton, and there is a probabilistic interpretation of trion formation for each charge injection event. This behavior results from the unique one-dimensional
Figure 3. Distribution of (a) oxidation potentials, (b) reduction potentials, and (c) electrochemical band gap. (d) Correlation between measured oxidation and reduction potentials. The red line depicts the ensemble electrochemical band gap of (6,5) SWNTs in acetonitrile (1.17 eV).12 3140
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The data presented in Figure 1−3 shows that Eox and Ered of a single SWNT chirality exhibit a wide range of potentials that differ by several 100 meV and that ΔEechm is less than the optical band gap (ΔEopt). Determining the origins of these effects are important issues that are uniquely addressed by single molecule spectroscopy. A high resistance between the ITO layer and the SWNTs could, in principle, lead to a distribution in the measured ΔEechm values. However, a high resistance could only explain measured values that are higher than the actual ΔEechm value. Because we measure both higher and lower values and because our Gaussian fit reveals an average value that corresponds to the literature value measured on the ensemble,12 we expect that any differences in contact resistance are negligible. Alternatively, the distributions could arise from different Fermi energies among SWNTs (Figure 4).
Figure 4. Local charges shift the Fermi energy (EF) of the SWNTs and influence their oxidation (Eox) and reduction (Ered) potentials, see text for details.
Although all SWNTs are in the same macroscopic environment, the local environment could still be substantially different among SWNTs due to localized charges and SWNT defects. Local charges would be expected to influence the Fermi energy of the SWNT, pushing it closer to the VB or the CB and varying the potential required to inject holes into the VB or fill electrons into the CB. Because the Fermi energy affects the offset and not the magnitude of the energy gap, one would predict the Eox and Ered values would be highly correlated if the redox potentials varied due to differences in the Fermi energy. Experimentally we observe that the Eox and Ered values are, indeed, highly correlated, where a high Eox corresponds to a high (less negative) Ered (Figure 3d). This correlation also manifests in a smaller fwhm of the ΔEechm distribution compared to the Eox and Ered distributions. Most Eox and Ered combinations are found in close vicinity to the red line in Figure 3d which represents the ensemble band gap ΔEechm = 1.17 eV. To further quantify the impact of local charges, PL spectra were taken prior to the electrochemical analysis for a total of 16 SWNTs (Figure 5a). All of the analyzed SWNTs were of (6,5) chirality, however, the peak and amplitude of the PL spectra differed slightly.16,37 Figure 5b,c displays the dependence of ΔEopt on Eox and Ered, respectively. We find correlations between ΔEopt and both Eox and Ered (R2 values are 0.29 and 0.35 for Eox and Ered, respectively), where Eox and Ered are more positive for smaller ΔEopt. Surprisingly, we find only a weak correlation between ΔEopt and ΔEechm (R2 = 0.05) (Figure 5d). In order to understand our results, it is important to understand the difference between the electrical band gap ΔE, the optical band gap ΔEopt and the electrochemical band gap ΔEechm. Figure 5e shows a sketch of the different band gaps. ΔE is the difference between CB and VB, which has previously
Figure 5. (a) PL spectrum (left) and dependency of the PL intensity on the applied voltage (right) of the same individual SWNT. The PL spectrum was fit with a Lorentz fit. Optical band gap in dependence of (b) oxidation potential, (c) reduction potential, and (d) electrochemical band gap. The error bars represent the standard error in the fitting procedure. (e) Three different band gaps in a SWNT, ΔE is the difference between conduction band (CB) and valence band (VB). ΔEopt depends slightly on its local environment while ΔEechm may shift several 100 meV depending on the local environment.
been estimated using scanning tunneling spectroscopy.38,39 In (6,5) SWNTs, this value is found to be about 1.69 eV.40 Because of the strong electron−hole interaction in SWNTs the exciton binding energy is on the order of 400 meV,41 hence the excitonic level is significantly lower than ΔE; in (6,5) SWNTs ΔEopt is found to be about 1.27 eV.29 ΔEechm is not as welldefined and may differ from ΔEopt7,11 because any energy state within the band gap such as a defect state may be filled through the applied electrochemical voltage. Moreover, defect states may exhibit energies depending on the defect type and location and hence could lead to a variation in ΔEechm. The lack of correlation between ΔEechm and ΔEopt is surprising. Hirana et al. found that ΔEechm depends substantially on the chosen solvent (differences of about 200 meV), however, ΔEopt was essentially independent of the solvent.12 Another study suggests that ΔEopt may shift up to 40 meV depending on the solvent.42 Additionally, on the single molecule level it has been found that the PL peaks of individual SWNTs of the same chirality vary slightly (several 10 meV).16,37 This has been attributed to defects, electrostatic surface potential changes and localized charges.16 All of the above studies support the assumption that ΔEechm is strongly 3141
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Figure 6. (a) PL spectra of an individual SWNT under positive bias and (b) evolution of PL intensity and spectral position in dependence of applied voltage. (c) PL spectra of an individual SWNT under negative bias and (d) evolution of PL intensity and spectral position in dependence of applied voltage. The error bars represent the standard error in the fitting procedure. Because of the noise in the spectra only the Lorentzian fits of the spectra are shown, the spectra of the SWNTs at 0.05 and −0.05 V are displayed in the insets of (a) and (c), respectively.
influenced by its local environment, whereas ΔEopt is less dependent on its local environment. Therefore, we expect that the correlation between ΔEopt and ΔEechm is masked under the much stronger influence of the local environment and hence can not be seen in our measurements. Nonetheless, there are clear correlations between the redox potentials and ΔEopt. From Figure 5b,c we observe that negative charges that increase the Fermi energy are associated with a redshift (high Eox and low Ered), while positive charges are associated with a blueshift (low Eox and high Ered). To test whether the same trend holds in the same SWNT under an electrochemical bias, we deliberately doped individual SWNTs and monitored the shift of their PL spectra. Figure 6 details the spectral evolution of two different individual SWNTs under an applied potential. Figure 6a,b shows a slight blueshift of the spectra at a positive potential while the spectra taken at a negative potential shift to the red (Figure 6c,d). For both positive and negative bias, the PL wavelength is stable and only shifts once the PL intensity starts to quench. We note that our information on the spectral shift is limited by the quenching and we cannot detect spectra for highly quenched species. Nevertheless, the results presented here are consistent with the data presented before; a shift of the Fermi energy toward the CB leads to a redshift while a shift toward the VB results in a blueshift of the spectrum. Charge carrier and electric field-induced shifts of SWNT PL spectra have been previously described in literature. The shifts are reported to be exclusively to the red or to the blue depending on the strengths of the effects.43−47 For example, calculations suggest a blueshift due to band gap renormalization for electron- and hole-doped SWNTs.45 The same shift was found experimentally by applying a positive and a negative gate voltage.46 On the other hand, a gate-induced redshift has been observed and has been assigned to screening by induced charge
carriers.44 In addition, a Stark effect has been reported to lead to a redshift.43 Because we observe both a redshift and a blueshift of the PL none of the above effects can explain our observations sufficiently. We have previously argued that the distribution in redox potentials originates from differences in local charges. Therefore, we suggest that the shifts in emission energy are due to Coulomb interactions between the exciton and local charges. Our SWNT surfactant DNA is negatively charged. The exciton experiences a fraction of this negative charge due to dielectric screening, which stabilizes the exciton and increases its binding energy. Therefore, the SWNT PL undergoes a redshift in DNAwrapped SWNTs as compared to unwrapped SWNTs.48,49 In our experiments, we observe additional charges in the local environment of the SWNT or deliberately charge the SWNT. Because of a variation in defect state energies, some of the injected charge carriers do not form a trion with the exciton, however, they may still contribute to screening effects. We expect that additional negative charges near or on the SWNT would further stabilize the exciton and increase the redshift, whereas an addition of positive charges leads to a reduction of the stabilizing effect of the DNA charges, consequently blueshifting the PL. Principally, a Stark effect similar to the one found in CdSe quantum dots can explain our results,50 however, the fields required to result in the shifts we observe are significantly higher than the ones we generate during our experiments.43,46 The absolute energy of the Fermi level in SWNTs is currently an open question. Several reports, using differing experiment methodologies, conclude that the Fermi energy scales linearly with the SWNT diameter51,52 and can vary by up to 1.2 eV for SWNT diameters ranging from 0.75 to 1.2 nm. On the other hand, several other reports find that the Fermi energy is relatively flat over the same range of SWNT 3142
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diameters.11,40,53 Clearly, both of these viewpoints can not be correct. We found that the reduction potential and oxidation potential can vary by large fractions of an electronvolt, while the electrochemical potential is relatively constant, thus implying large changes in the Fermi level from SWNT to SWNT. However, these Fermi level variations are for the same SWNT species (i.e., not different SWNT (n,m) values), and thus we conclude that the Fermi level for the (6,5) SWNTs is varying due to environmental differences in the local electrostatic potential. Therefore, one possible way to rationalize the diverse set of experimental findings is that the reports of a varying Fermi level could be due to systematic changes in the local doping level for SWNTs due to environmental factors in these studies. On the other hand, it is also possible that the relatively flat Fermi levels reported are due to ensemble averaging over the inhomogeneous environment of an ensemble of SWNTs. Future spectroelectrochemical studies of single SWNTs could help elucidate a clearer picture of this phenomenon. In summary, we report spectroelectrochemistry studies on individual SWNTs. We have shown that the oxidation and reduction potentials in individual SWNTs of the same chirality vary significantly. This variation is mainly attributed to intrinsic defect states and localized charges that shift the Fermi energy. Additionally, we have shown that the position of the Fermi energy correlates with the exciton emission energy. This observation is consistent with a change of the exciton stabilization by surrounding charges. Our results show that charges in the vicinity of SWNTs largely affect their properties which makes it hard to study individual SWNTs accurately. In addition, the large variety of redox potential complicates an application of SWNTs, for example, as a catalyst in H2production.
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ASSOCIATED CONTENT
S Supporting Information *
A PLE map of our sample and a video of the full potential range of the data in Figure 1. 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]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Jeffrey Peterson for fruitful discussions. The authors gratefully acknowledge the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-FG0206ER15821 for financial support.
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REFERENCES
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dx.doi.org/10.1021/nl5003729 | Nano Lett. 2014, 14, 3138−3144
