Interactions between Single-Walled Carbon Nanotubes and

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2007, 111, 3539-3543 Published on Web 02/14/2007

Interactions between Single-Walled Carbon Nanotubes and Tetraphenyl Metalloporphyrins: Correlation between Spectroscopic and FET Measurements Douglas R. Kauffman, Oleksandr Kuzmych, and Alexander Star* Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: December 29, 2006; In Final Form: January 29, 2007

The noncovalent functionalization, or complexation, of single-walled carbon nanotubes (SWNTs) with metallotetraphenyl porphyrins (M-TPPs) was monitored with UV-vis-NIR absorption spectroscopy and electroanalytical measurements using nanotube field-effect transistors (NTFETs). From combining both techniques, we have found that M-TTPs donate electron density exclusively into semiconducting SWNTs. Moreover, thin-film UV-vis-NIR absorption spectroscopy on SWNT/M-TPP complexes provides a well-resolved picture of the interaction between the SWNT S1 band and the electron-donating M-TPP. In total, the electrondonating properties of four different M-TPPs containing Mn(III), Co(II), Cu(II), and Zn(II) were compared, and the electronic affects of each are discussed.

Introduction Porphyrins are an extensively studied group of organic molecules, and much interest has been generated in exploiting their natural biochemical behavior.1,2 Research into this rich field has produced developments such as gas3-6 and electrochemical sensors7,8 and the design of integral components for solar energy conversion.9 Compared to porphyrin research, the field of carbon nanotube chemistry is still in its infancy,10 and much can be learned from coupling the well-studied porphyrin to the relatively new nanotube system. Interest has been gaining in carbon nanotube research due, in part, to the carbon nanotube’s potential for such applications as molecular electronics,11,12 sensing devices,13,14 and catalytic platforms,15 to name a few. Because of carbon nanotube’s environmentally sensitive electronic properties, which can be probed by various spectroscopic16,17 and electronic11,12,18-20 methods, they represent an ideal candidate for nanoscale devices for extremely sensitive applications. Pairing the carbon nanotube with a system having photoactive properties, molecular selectivity, and electrondonating characteristics invites the possibility for the development of ultrasensitive and compact devices for a host of energy conversion, analyte sensing, and catalytic capabilities on a truly molecular level. Several studies have been conducted on the functionalization of single-walled carbon nanotubes (SWNTs) with porphyrin systems.21,22 Recently, the interaction between SWNT and porphyrins has led to the creation of supramolecular assemblies23 and the study of light-induced charge transfer24 for applications in solar energy conversion. In light of the budding interest in SWNT-porphyrin hybrids, it is important to understand the result of the complexation between the two species in terms of the impact on the electronic structure of the newly formed complex. Combining two techniques such as optical spectroscopy and electroanalytical measurements using nanotube field* Corresponding author. E-mail: [email protected]; phone: 412-624-6493.

10.1021/jp0690289 CCC: $37.00

effect transistors (NTFETs) creates a powerful tool for probing SWNT electronic structure. The extreme sensitivity of these techniques has been illustrated in recent studies monitoring the hybridization25,26 and conformational changes27 of DNA molecules immobilized on SWNT surfaces. In this study we combine UV-vis-NIR thin-film absorption spectroscopy and NTFET measurements to quantitatively address the electronic consequences of porphyrin complex formation on the SWNT electronic structure, specifically how the lower energy first and second semiconducting electronic transitions are modified due to complexation with the porphyrin. Experimental Section As-purchased SWNTs (Carbon Solutions) were dispersed in DMF and sonicated for approximately 30 min until the solution became uniform in color, indicating good nanotube dispersion. 5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride (MnTPP), zinc(II) (ZnTPP), copper(II) (CuTPP), and cobalt(II) (CoTPP) tetraphenyl porphyrins (Sigma-Aldrich) were dissolved in DMF and introduced in excess to the SWNT solution. An additional 5 min sonication was conducted to ensure even distribution of SWNT and M-TPP and facilitate complex formation. After stirring overnight, the solution was filtered and rinsed with copious amounts of DMF, water, and acetone to remove any free M-TPP and dried in vacuum. The dried SWNT/M-TPP filtrate was redispersed in DMF by brief sonication and used for solution UV-vis absorption spectroscopy or sprayed onto a heated quartz substrate with a commercial air brush (Iwata) to create SWNT/M-TPP thin films, which afterward were dried in vacuum and used for spectroscopic measurements on a Lamda 900 UV-vis-NIR spectrophotometer (Perkin-Elmer). NTFET devices were constructed as described elsewhere,25 but briefly, SWNTs were grown via CVD process onto Si wafers and interdigitated Au/Ti electrodes were photolithographically patterned onto the SWNT network creating multiple devices on the Si chips. For experiments, chips © 2007 American Chemical Society

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Figure 1. (a) Solution UV-vis absorption spectra of free MnTPP (dashed), pristine SWNT (blue), and the resuspeneded SWNT/MnTPP complex (red) in DMF. (b) Conceptualized depiction of the SWNT/M-TPP complex.

were wire-bonded and packaged in a 40-pin ceramic dual-inline package (CERDIP) and tested using a custom NTFET electronic test fixture.25 NTFET devices were complexed with M-TPP by dropcasting a small volume of metalloporphyrin solution (1 µL; 10-4 M in THF or DMF) onto the NT network, and NTFET source-drain conductance versus gate voltage (G-VG) transfer characteristics were recorded. Results In DMF, free MnTPP has major and minor peaks in the Soret region at 466 and 436 nm, respectively (Figure 1a). Upon complexation with a ligand, the major and minor MnTPP peaks reversed, showing a simultaneous increase at 436 nm and decrease at 466 nm. This spectroscopic behavior has previously been used to study the complexation of metalloporphyrins and ligands.4,5,28,29 Pristine SWNTs do not show prominent spectroscopic features in the MnTPP Soret absorption region, but after complexation with MnTPP, filtration, rinsing, and resuspension, two small peaks developed at 436 and 466 nm. Interestingly, these features correspond to the ligated form of MnTPP, showing a larger peak at 436 nm and a small peak at 466 nm in accordance with literature results.30,31 This peak formation suggests two things: (1) MnTPP complexes to the SWNT, and (2) little to no free MnTPP is in solution after filtration and resuspension, indicating good coverage on the nanotube surface. In addition, SWNT/CoTPP, SWNT/ZnTPP, and SWNT/CuTPP complexes were investigated, but it was found that only CoTPP demonstrated similar behavior to MnTPP in the Soret region in the presence of SWNTs. To determine if the solvent played a role in ZnTPP and CuTPP ligation, DMF was added to a solution of ZnTPP and CuTPP dissolved in THF; it was found the Soret peaks of both porphyrins red-shifted, indicating an interaction between DMF and the ZnTPP and CuTPP species. Additional absorption spectra of M-TTP/ SWNT and ZnTPP and CuTPP response to DMF can be found in the Supporting Information, Figures S1 and S2. Furthermore, SWNT/M-TPP thin films were used to investigate the complexes spectroscopically. Using thin films to make spectroscopic measurements is advantageous for two reasons: (1) accurate and fairly well-resolved UV-vis-NIR spectra can be taken for lower energy electronic transitions of the SWNT/ M-TPP complex,32 and (2) thin films more accurately represent the nanotube environment present in random network NTFET devices, as illustrated in Figure 2. Even though thin-film absorption spectra may lack some fine resolution, it is a technique that warrants further development because SWNT films immobilized on transparent substrates can serve as useful

Figure 2. SEM image of a SWNT thin film on quartz (left) showing a random SWNT network similar to that present in the network NTFET devices (right).25

tools for modeling the photoactivity of solid-state optical devices such as third-generation photovoltaics.33 Figure 3a shows the UV-vis-NIR spectra of thin films on a thin (∼1 mm) quartz substrate of pristine SWNT and the SWNT/MnTPP complex, clearly showing the first three semiconducting and metallic electronic transitions, referred to as the S1, S2, S3, and M1 bands, respectively. The origin and significance of SWNT transition bands is well-described in the literature.34-36 All UV-vis-NIR spectra are normalized at 1425 nm, a spectral region lacking SWNT absorption bands. The thinfilm absorption spectra reveal two interesting characteristics about the interaction between SWNT and MnTPP. First, a substantial red-shift in the S1 and S2 absorption bands can be seen. The pristine SWNT film has S1 and S2 absorption bands centered around 1845 and 1020 nm, agreeing with previously reported results.37 Upon complexation with MnTPP, the S1 band is shifted to 1895 nm, a 50 nm (16 meV) red-shift; and the S2 is shifted to 1030, a 10 nm (12 meV) red-shift. Second, the M1 and S3 were not shifted by the addition of MnTPP. In addition, an increase in absorption intensity is seen for the semiconducting absorption bands, but no trend was found between different M-TPP complexes. Figure 3b shows the results of NTFET G-VG measurements of an unfunctionalized (blue) and MnTPP-functionalized (red) NTFET. Two important observations can be made from this data, the first being an obvious negative shift in the gate voltage of approximately 2.2 V. Second, no significant change in minimum conductance was seen beyond a positive gate voltage of 5.0 V. Gate-voltage shifts were found to be consistent for all device geometries functionalized with the same M-TPP and data for devices with different network geometries, that is, separations between interdigitated electrodes (pitch sizes) and network areas. Consistent gate-voltage shifts for MnTPPfunctionalized NTFET devices of different geometries and exhibiting different ON-OFF ratios are summarized in the

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Figure 4. Density of states (DOS) diagram showing electron donation from M-TTP into the partially vacant SWNT S1.

Figure 3. (a) Thin-film UV-vis-NIR absorption spectra of pristine SWNT (blue) and SWNT/MnTPP (red) showing red-shifts in the S1 and S2 nanotube absorption bands (normalized at 1425 nm). (b) Conductance vs gate voltage (G-VG) transfer characteristics of bare (blue) and MnTPP-functionalized (red) NTFET devices showing a negative shift in gate voltage (arrow) and no significant change in the minimum conductance region positive of +5.0 V.

TABLE 1: Summary of UV-vis-NIR Spectroscopic and G-VG Measurements of SWNT/M-TPP Complexes as Compared to Bare SWNT SWNT porphyrin complex

S1 band red-shift (meV)

S2 band red-shift (meV)

average G-VG shift (V)

ZnTPP CuTPP MnTPP CoTPP

16 15 16 15

14 13 12 11

-4.3 ( 0.12 -2.0 ( 0.45 38 -2.2 ( 0.14 -4.3 ( 0.092

Supporting Information, Figure S3. Additionally, equivalent G-VG results were obtained with NTFET chips functionalized with M-TTP solutions made in DMF and THF. In addition to MnTPP, the thin-film UV-vis-NIR spectra and NTFET G-VG responses of CoTPP, CuTPP, and ZnTPP complexes were taken. Interestingly, each metalloporphyrin caused an equivalent red-shift in the nanotube S1 absorption band and a smaller shift in nanotube S2 absorption band. The spectroscopic and G-VG results are summarized in Table 1, and individual plots can be found in the Supporting Information, Figures S4 and S5. Discussion Upon complexation with M-TPP, the SWNTs demonstrated a spectral red-shift in the S1 and S2, whereas the M1 and S3 bands were unshifted. Previous studies have concluded that certain molecules preferentially interact with either semiconducting or metallic SWNTs39 and metalloporphyrins have been shown to selectively interact with semiconducting SWNT.40 Our spectroscopic results support this finding in that complexation only modified the S1 and S2 absorption bands while the M1 band remained unaffected. Interestingly, the spectroscopic red-shifts were consistent over the entire S1 and S2 bands, indicating that

M-TPP did not show a strong preference in nanotube chirality or diameter. An apparent increase in semiconducting absorption band intensity was seen for SWNT/MnTPP, but no consistent trend was found between the four SWNT/M-TPP complexes. It is well established that metalloporphyrins are electrondonating systems,24,30,31,41 and because SWNTs are hole-doped in ambient conditions18 the S1 band will be partially vacant.42 As a result, electronic transfer is expected to occur at the top of the SWNT valence band,42,43 meaning that the S1 transition should be strongly effected. As M-TPP donates electronic density into the SWNT S1, the band gap will decrease (Figure 4), resulting in the observed S1 red-shift. The red-shift in the S2 has been reported previously in charge-transfer experiments and has been attributed to noncovalent interactions between the charge-transfer molecule and the SWNT sidewall.44 The small differences in the S2 red-shifts may be a result of the metal center in the different MTPPs. Unlike previous experiments, the SWNT S1 did not experience bleaching; the noncovalent π-π interactions allow electronic donation into the SWNT while maintaining a coherent absorption spectra. From G-VG measurements, a negative shift in gate voltage and no significant change in minimum conductance were seen. Under ambient conditions, NTFET devices are p-type,18,45 meaning that electron donation from M-TPP would result in a decrease of the carrier concentration, and the negative gate voltage shifts in M-TPP-functionalized NTFETs G-VG transfer characteristics are in line with previous findings.24a The observed negative shift in gate voltage corroborates spectral red-shifts resulting from electronic donation into the SWNT S1 valence band. It was found that gate-voltage shifts were consistent for all device geometries functionalized with the same M-TPP, indicating that device geometry did not play a role in charge transfer. Last, there was no change in the region of minimum conductance, which has been shown to represent the contribution of metallic SWNT in the NTFET random network.12 Because the region of minimum conductance, residing at positive potentials greater than 5.0 V, represents the metallic portion of the SWNT network, a lack of change in G-VG characteristics there coupled with no M1 spectral-band red-shifting indicates no interaction between metallic SWNT and M-TPP. In the evaluation of this data, we assumed the surface adsorption of M-TTP on the SWNT, but a definitive determination of the structural configuration of M-TTP/SWNT samples could not be made in the absence of rigorous surface studies. However, we do feel confident in assuming surface adsorption owing to previous porphyrin/SWNT studies. Studies utilizing similar complexation techniques have found porphyrins capable of individual SWNT dispersion in DMF.22a The ability of

3542 J. Phys. Chem. C, Vol. 111, No. 9, 2007 porphyrins to maintain intimate contact with the SWNT sidewall is due to π-π stacking between the SWNT and aromatic porphyrin, and previous STM studies have found excellent porphyrin adsorption on HOPG,46 which has been accepted as a suitable model for nanotube sidewalls. Controversy does exist, however, on the orientation of adsorbed porphyrins because previous studies have concluded that porphyrins in aqueous solutions can assemble in monolayers or form extended rodlike aggregates on HOPG.47 To address this controversy, we feel that porphyrin aggregation was avoided in thin-film preparation due to thorough rinsing of the SWNT/M-TTP complex filtrate, allowing fairly well-resolved spectroscopic evidence of semiconducting SWNT selective charge transfer. A previous method for functionalizing NTFETs with M-TPP was followed,24a and we feel that this method probably resulted in the formation of aggregate porphyrin structures on the nanotube surface due to solvent evaporation. Regardless of M-TTP aggregation in NTFET devices, all measurements indicated electronic donation exclusively into the semiconducting SWNT. Conclusions We have demonstrated upon complexation with metalloporphyrin, SWNTs experience an injection of electronic density into the S1 band, while confirming spectroscopically and electronically that M-TPP species selectively interact with the semiconducting SWNT. Thin-film UV-vis-NIR absorption spectroscopy was employed to directly monitor the electronic changes in the lower energy SWNT transitions resulting from the complexation with the electron donating M-TPP. A better understanding of the fundamental charge-transfer events present in the interaction between SWNT and M-TPP will hopefully lead to further progress in the study and application of such complexes for a host of novel sensing and optoelectronic platforms. Acknowledgment. We thank Nanomix Inc. for supplying NTFET devices for this study. The authors thank the Department of Materials Science and Engineering for the provision of access to the electron microscopy instrumentation and Dr. P. D. Kichambare and A. Stewart for assistance with the execution of this part of our research. Supporting Information Available: Additional solution UV-vis absorption spectra, thin-film UV-vis-NIR absorption spectra, and G-VG curves of SWNT/M-TPP complexes. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000. (2) The Porphyrins: Volume VII: Biochemistry, Part B; Dophin, D., Ed.; Academic Press: New York, 1979. (3) Rakow, N. A.; Suslick, K. S. Nature 2000, 406, 710. (4) Morales-Bahnik, A.; Czolk, R.; Ache, H. J. Sens. Actuators, B 1994, 18-19, 493. (5) Czolk, R. Sens. Actuators, B 1996, 30, 61. (6) Rodgers, K. R. Curr. Opin. Chem. Biol. 1999, 3, 158. (7) Quintino, M. S. M.; Winnischofer, H.; Nakamura, M.; Araki, K.; Toma, H. E.; Angnes, L. Anal. Chim. Acta 2005, 539, 215. (8) Ozoemena, K. I.; Zhao, Z.; Nyokong, T. Electrochem. Commun. 2005, 7, 679. (9) Schwab, A. D.; Smith, D. E.; Bond-Watts, B.; Johnston, D. E.; Hone, J.; Johnson, A. T.; de Paula, J. C.; Smith, W. F. Nano Lett. 2004, 4, 1261. (10) Iijima, S. Nature 1991, 354, 56.

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