phys. stat. sol. (b), 1–5 (2006) / DOI 10.1002/pssb.200669179

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Original

Paper

Near-field imaging and spectroscopy of electronic states

in single-walled carbon nanotubes

Huihong Qian1, Tobias Gokus1, Neil Anderson2, Lukas Novotny2, Alfred J. Meixner1,

and Achim Hartschuh*, 1, 2

1 Institute of Physical and Theoretical Chemistry, University of Tuebingen, Auf der Morgenstelle 8,

72076 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 73.22.–f, 78.30.Na, 78.66.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.

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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 pre-

sent 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 al-

lows 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 sche-

matically 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 photo-

diodes 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: e-mail: achim.hartschuh@uni-tuebingen.de, Phone: +49 7071 29 76171, Fax: +49 7071 29 5490

2 Huihong Qian et al.: Near-field imaging and spectroscopy of electronic states in SWNTs

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-b.com

Fig. 1 (online colour at: www.pss-b.com) 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 632.8 nm. The photoluminescence (PL) signal repre-

sents the intensity around 950 nm. The topography (a) of the same sample area was detected simultane-

ously. 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: www.pss-b.com) Topography (a) for DNA-wrapped SWNTs on glass and si-

multaneously 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.

phys. stat. sol. (b) (2006) 3

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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 elec-

tronic states, given by (n, m), are in resonance with the laser energy leading to resonance Raman scatter-

ing. 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 en-

hancement 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 SDS-

wrapped 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 en-

ergy 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 de-

gree 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. Fur-

thermore, defect related non-luminescent trap states can quench the emissive state [13]. Spatial variations

Fig. 3 (online colour at: www.pss-b.com) 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 simultane-

ously.

4 Huihong Qian et al.: Near-field imaging and spectroscopy of electronic states in SWNTs

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-b.com

Fig. 4 (online colour at: www.pss-b.com) 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 environ-

ments. Changes of the dielectric constant of the surrounding media have been reported to shift the emis-

sion 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 en-

hancement 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 quantita-

tive comparison of the signal enhancements achieved for Raman scattering and PL from the same nano-

tube 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-FG02-01ER15204.

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