Near-field imaging and spectroscopy of electronic states in single-walled carbon nanotubes

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1 Early View publication on (issue and page numbers not yet assigned; citable using Digital Object Identifier DOI) Original phys. stat. sol. (b), 1 5 (2006) / DOI /pssb Near-field imaging and spectroscopy of electronic states in single-walled carbon nanotubes Huihong Qian 1, Tobias Gokus 1, Neil Anderson 2, Lukas Novotny 2, Alfred J. Meixner 1, *, 1, 2 and Achim Hartschuh 1 Institute of Physical and Theoretical Chemistry, University of Tuebingen, Auf der Morgenstelle 8, Tuebingen, Germany 2 University of Rochester, The Institute of Optics, Rochester, New York 14627, USA Received 21 April 2006, revised 30 May 2006, accepted 1 August 2006 Published online 19 September 2006 PACS f, Na, Tr Near-field photoluminescence spectroscopy was used to study the electronic properties of semiconducting Single-Walled Carbon Nanotubes in different environments. A sharp laser-illuminated metal tip was raster scanned over the sample and served as a strongly confined excitation source. We observed localization of photoluminescence and variations of emission energies along nanotubes on a length scale of about 30 nm. 1 Introduction Single-Walled Carbon Nanotubes (SWNTs) are promising candidates for applications in photonics and opto/nano-electronics because of their unique electronic properties [1]. For applications as 1D wires in long range energy and charge transfer, possible variations of these properties along individual nanotubes play an important role. The spatial resolution achieved in conventional confocal microscopy, on the other hand, is limited by diffraction to about half the wavelength of the excitation light. The spectroscopic data obtained thus only represents an average over a nanotube section of about 300 nm. In this work, we present an investigation on both photoluminescence imaging and spectroscopy of SWNTs embedded in different environments using near-field microscopy with a resolution below 15 nm. This technique allows us to visualize the spatial extent of electronic states and to probe their emission energy. 2 Experimental The experimental setup is based on the combination of an inverted confocal microscope shown schematically in Fig. 1, with an x, y scan stage, and a tuning fork unit for shear force detection. A radially polarized laser beam [2] serves as excitation source providing a strong longitudinal field component in the focus that is required for field enhancement [3, 4]. Both, Raman scattered and photoluminescence light are detected either by a charge coupled device, or by two single-photon counting avalanche photodiodes simultaneously in our setup. A more detailed description of the experimental setup can be found in [5, 6]. Two different samples were studied: DNA-wrapped nanotubes were prepared according to [7] and dropped on a glass cover slip from aqueous solution; SDS encapsulated nanotubes [8] were spin cast on a thin layer of freshly cleaved MICA. * Corresponding author: achim.hartschuh@uni-tuebingen.de, Phone: , Fax:

2 2 Huihong Qian et al.: Near-field imaging and spectroscopy of electronic states in SWNTs Fig. 1 (online colour at: ) Schematic of the experimental setup. A sharp metal tip is scanned through a tightly focused laser beam. BS: Non-polarizing beam splitter. FM: Flip mirror. DBS: Dichroic beam splitter. 3 High resolution imaging of SWNT: Localization of photoluminescence Figure 2 shows simultaneously acquired near-field Raman (b) and photoluminescence (c) images of DNA-wrapped nanotubes on glass. The Raman image was detected by integrating the Raman G-band intensity around 700 nm after laser excitation at nm. The photoluminescence (PL) signal represents the intensity around 950 nm. The topography (a) of the same sample area was detected simultaneously. Figure 2(d) (f) show topographical and optical cross sections taken along the dashed lines in (a) (c) respectively. The high spatial resolution of about 14 nm as indicated in the PL image is far below the diffraction limit. In general, the resolution in near-field optical microscopy is limited by the tip size. Fig. 2 (online colour at: ) Topography (a) for DNA-wrapped SWNTs on glass and simultaneously detected Near-field Raman (b) and PL image (c). (d) (f) are topographic and optical cross sections taken along the dashed lines in (a) (c), respectively.

3 Original phys. stat. sol. (b) (2006) 3 In the topographic image, a nanotube is seen to extend from the upper left to the lower right. The measured height in the cross section along the dash line is about 2.5 nm. This is in agreement with the expected height of a single DNA-wrapped nanotube [9]. The Raman signal occurs along the nanotube, but disappears before the nanotube ends in the topography image. The PL signal, on the other hand, is localized within about 90 nm and occurs just where the Raman signal ends. This observation can be explained by a change in chirality (n, m) along the nanotube. In the upper section, the nanotube s electronic states, given by (n, m), are in resonance with the laser energy leading to resonance Raman scattering. Here, the nanotube is either metallic, i.e. non-luminescent, or the emission energy is outside of the detection window of 950 nm ± 20 nm. The nanotube chirality (n, m) in the lower section is associated with weaker resonance Raman enhancement while non-resonant excitation leads to PL about 950 nm. Structural transitions along individual nanotubes have been reported before based on Raman data [10, 11]. The dark spot in the lower part of the Raman image (b) is caused by a large particle that can be seen in the topography image with a height of 14 nm. At this position, the particle prevents laser excitation of the gold tip and therefore reduces the photoluminescence from gold which is detected as a background signal [12]. This is a well-known effect in near-field optical microscopy. The reduced optical signal that is observed on the tube in the upper left part is also a result of this effect. In addition, the signal enhancement at this position is effectively reduced because of an increased tip-nanotube distance [5]. 4 Near-field spectroscopy of SWNT: Variation of emission Figure 3 shows simultaneously acquired topography and near-field PL images of SWNTs in SDS on MICA. In the present case, the PL is extended over about 400 nm. The flakes in the background of the topography image with a height of about 1.4 nm are expected for a single layer of sodium dodecyl sulfate (SDS) surfactant. The height of the nanotube measured at the dashed line in the topography image is about 3.5 nm. Based on the topographic data, it is not possible to distinguish between a single SDSwrapped nanotube and a thin bundle. Near-field photoluminescence spectra detected by probing at 6 different positions along the nanotube separated by 30 nm are shown in Fig. 3(c). The spectra show a significant variation of the emission energy ranging from ~950 nm to ~975 nm. In standard confocal microscopy, only a spatial average could be observed leading to a broadened emission band. Most of the near-field images of SWNTs on substrates we recorded up to now exhibit a varying degree of localization of the PL. In general, localization could result from chirality variations along the nanotube leading to luminescent and non-luminescent sections as discussed in part 3 of this paper. Furthermore, defect related non-luminescent trap states can quench the emissive state [13]. Spatial variations Fig. 3 (online colour at: ) Near-field PL spectra (c) taken along positions 1 6 indicated in the near-field PL image of the SWNT in (b). The topography (a) of this tube was obtained simultaneously.

4 4 Huihong Qian et al.: Near-field imaging and spectroscopy of electronic states in SWNTs Fig. 4 (online colour at: ) Tip-enhanced Raman (a) and photoluminescence (b) spectra of SWNTs detected with and without tip. of the emission energy, on the other hand, could also result from inhomogeneous dielectric environments. Changes of the dielectric constant of the surrounding media have been reported to shift the emission energy by several tens of mev [14 16]. Transitions in DNA conformation, for example, lead to shifts of up to 15 mev for DNA-wrapped nanotubes. From the topographic measurement, it is clear that the wrapping by SDS and DNA is not uniform along the nanotubes for our samples and considerable fluctuations of the dielectric constant can be expected. Raman and PL spectra in the presence and the absence of the tip are taken to illustrate the signal enhancement achieved in our experiment (shown in Fig. 4). For Raman scattering, the signal is increased by an enhancement of both the incident field and the scattered field. The PL intensity, on the other hand, depends on the excitation rate and the radiative rate of SWNTs. Both rates can be increased by the metal tip acting as an antenna for radiation. The total detected signal in tip-enhanced microscopy consists of this tip-enhanced near-field and a confocal farfield contribution. For both Raman and PL, the near-field contribution clearly dominates in Fig. 2(b) and (c). While in the Raman image a weak and broad farfield signal can be seen following the nanotube, no farfield signal is observed in the PL image. From this it is evident that the signal enhancement for PL must be stronger than that for Raman scattering. A quantitative comparison of the signal enhancements achieved for Raman scattering and PL from the same nanotube indicated that the tip enhancement is in fact more efficient for PL [17]. 5 Summary In this contribution, we present a high-resolution near-field method for studying photoluminescence and Raman scattering of SWNTs. Upon localized excitation, we observed strongly confined emission signals and a variation of emission spectra along isolated nanotubes. Acknowledgements The authors thank G. Schulte for valuable experimental support. This work was supported by the DFG through Grant ME1600/5-1 and the U.S. Department of Energy through Grant DE-FG02-01ER References [1] P. G. Collins and P. Avouris, Sci. Am. 283, 62 (2000). [2] R. Dorn, S. Quabis, and G. Leuchs, Phys. Rev. Lett. 91, (2003). [3] L. Novotny, E. J. Sánchez, and X. S. Xie, Ultramicroscopy 71, 21 (1998). [4] A. Bouhelier, M. R. Beversluis, A. Hartschuh, and L. Novotny, Phys. Rev. Lett. 90, (2003). [5] A. Hartschuh, M. R. Beversluis, A. Bouhelier, L. Novotny, Philos. Trans., Math. Phys. Eng. Sci., 807 (2004). [6] A. Hartschuh, E. J. Sánchez, X. S. Xie, and L. Novotny, Phys. Rev. Lett. 90, (2003). [7] M. Zheng et al., Nature 2, 338 (2003).

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