Based on section 5.1 and section 5.2, I demonstrate now that simultaneously obtained near-field Raman scattering and PL signals from nanotubes can be used to study the correlation between phonon properties and electronic properties of nanotubes. Figure 34 shows simultaneously acquired near-field Raman (b) and photoluminescence (c) images of DNA-wrapped nanotubes on glass. The same laser excitation was used. The Raman image was detected by integrating the G-band intensity using a bandpass filter centered at 700 nm. The photoluminescence (PL) signal represents the intensity around 950 nm detected by using a bandpass filter centered at 950 nm. The topography (a) of the same sample area was detected simultaneously. Figure 34(d), (e) and (f) are zoom in images in the area marked with white rectangles in (a), (b) and (c). Figure 34(g), (h) and (i) show topographic and optical cross sections taken along the dashed lines in (a), (b) and (c), respectively. The optical resolution is about 14 nm as indicated in the PL image, which is far below the diffraction limit. In the topographic image (a), a nanotube can be seen to extend from the upper left to the lower right. The measured height in the cross section along the dashed line is about 2.5 nm. This is in agreement with the expected height of a single DNA-wrapped nanotube [95, 160, 161]. In both topography (a) and (d) images we can see the roughness of the glass cover slide, being about 5˚A. The patterns formed by surfactant as seen in Figure 28(a) fortunately do not exist in this sample area. The surfactant-free area gives the possibility to observe DNA-segments, as shown in Figure 14(b).
The Raman signal occurs along the nanotube, but disappears before the nanotube ends as determined from 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 beyond the detection window of 950±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 at about 950 nm. Structural transitions along individual nanotubes have been reported before based on Raman data [30, 176]. PL has been observed at the defects rich position along the nanotubes and attributed to local doping identified by the change of G’ band shape [158]. The D-band intensity along the present nanotube however reflect the low D-band intensity at the end of the nanotube, where the near-field PL occurs, as shown in Figure 35. The absence of PL in the upper part can also be explained by the quenching effects due to the presence of
5.3 Simultaneous Raman scattering and PL of single SWNTs 65
Figure 34: Simultaneously acquired topography (a), near-field Raman (b) and photolumines- cence (c) images of DNA-wrapped nanotubes spin-coated on glass. (d), (e) and (f) are zoom in images corresponding to the areas marked with white rectangles in (a), (b) and (c), respectively. (g), (h) and (i) show topographical and optical cross sections taken along the dashed lines in (a), (b) and (c). The optical resolution is 14nm indicated by the cross section through the PL signal which is only determined by the diameter of the gold tip. The PL signal starts where the Raman signal ends indicating a possible intramolecular junction along the nanotube.
66 5 NEAR-FIELD RAMAN AND PL IMAGING AND SPECTROSCOPY OF SWNTS
Figure 35: Simultaneous topography (a), near-field Raman G-band (b) and D-band (c) images of the same nanotube as in Figure 34. The G-band and D-band images are obtaind by acquiring a spectrum at each pixel and integrating the intensity around 700 nm and 690 nm, respectively.
defects, similar discussion is presented in section 8.
It is notable that besides the signals from the nanotube, there are several big features appearing in the topography giving negative signals in the Raman image, e.g. the dark spot in the lower part of the Raman image (b) located next to the white rectangle is induced by the large particle that can be seen in the topography with a height of 14 nm. In near-field optical microscopy, the background signal results mainly from gold tip luminescence. At the position of the dark spot, the particle reduces the laser excitation of the gold tip and therefore reduces the luminescence from the gold, leading to a hole in the background signal [154]. Moreover, when the gold tip meets such big particle with 14 nm height, the feedback control will retract the tip 14 nm off the surface, which will also reduce the detectable luminescence from the gold tip. This negative effect can also compete with the near-field signal. As an example, the big particle sticked on the nanotube on the upper left part, significantly reduces the signal enhancement by increasing the tip-sample distance. This is a well-known tip artifact in near-field Raman microscopy that can be avoided or at least reduced by the procedure described in the following paragraph [154].
Instead of imaging Raman and PL signals using bandpass filters centered at different spectral windows, spectroscopic imaging provides characterization of different bands simultaneously with spectral information at each pixel. The method has been used for both Figure 31 and Figure 35. An example for spectroscopic imaging is shown in Figure 36(a). Raman images of different Raman features and the PL image of emission around 940 nm are displayed. This method avoids the influence of overlapping bands by selecting specific spectral windows with narrow
5.3 Simultaneous Raman scattering and PL of single SWNTs 67
Figure 36: Raman images and PL image (a) obtained by integrating intensities at different spectral windows as shown in (b). Raman images showing the D-band (≈690 nm), G-band (≈703 nm), D*-band (≈706 nm) and G’-band (≈760 nm) intensities, the spectral positions marked are in (b). PL image is from emission band ≈940 nm, seen also in (b). Spectra are taken from each pixel during scanning. The spectral exposure time is 40 ms. The excitation power is 100 µW. Spectrum (b) is taken from the pixel marked with the black circle in the PL image in (a).
band width. The G-band image gives three nanotubes in this area, two on the left and one on the upper right. The D-band image shows weak intensity, indicating the presence of defects in all three nanotubes. While the second-order G’-band does not scale with the D-band intensity image as it is not defect related (see section 2.4). However, the D*-band is missing from the nanotube on the upper right, as well as in the PL image. The intensity of the D*-band (1620
cm−1 to 1630cm−1) is also defect related [177]. Whether there is a correlation between D*-band and PL of nanotubes is still unclear. The missing PL from the upper right nanotube can also indicate a metallic nanotube.
Simultaneous Raman scattering and PL images reveal that both phonon modes and excitonic states are non-uniform along the nanotubes. The variation of Raman scattering signals are mani- fested by the varying intensity along the nanotube, can be attributed to the change of resonance or chiralities. While the non-uniform PL signals are appeared as either variable PL energies
68 5 NEAR-FIELD RAMAN AND PL IMAGING AND SPECTROSCOPY OF SWNTS
or localized PL signals, evidenced by near-field PL spectroscopy. The variations of PL energy results from inhomogeneous dielectric environments (see section 2.5). Changes of the dielectric constant of the surrounding media have been reported to shift the emission energy by several tens of meV [27, 97, 100, 101]. Transitions in DNA conformation, for example, lead to shifts of up to 25 meV for DNA-wrapped nanotubes. From the topographic measurement, it is clear that the wrapping by DNA (or SDS) is uniform along the nanotubes for our samples. Considerable fluctuations of the dielectric constant can be expected. How nanotubes respond locally to the DNA-wrapping has high potential in nanoscale sensing applications, will be discussed in details in section 6. PL localization could result from chirality variations along the nanotube leading to luminescent and non-luminescent sections as discussed in Figure 34. Furthermore, defect related non-luminescent trap states can quench the emissiv state. As nanotubes have surface atoms only, PL appears more sensitive to both intrinsic and extrinsic factors.