**3.1 CWLR imaging for characterization of gold nanoarrays**

288 Advanced Photonic Sciences

where P is the total power for the incident white light beam, 0 *x* is the centre of the incident light beam and *w* is the desired 1/e2 half width. Thus the full width at half maximum (FWHM) of the incident beam (spot size) can be obtained from 2ln 2*w* , which was defined

Fig. 3. Typical intensity versus graphene edge position data with the best fitting to Eq. (1) superimposed to determine the white light spot size. The inset gives the spatial resolution fitting results along with the fitting errors by using different aperture and collection fiber

Through tuning the collection fiber core diameter (D1) and the aperture diameter (D2), different spatial resolutions were obtained from Eq. (1) as shown in the inset table of Figure 3. It reveals the best spatial resolution about 0.410 µm, obtained using the 25 µm core diameter collection fiber and setting the aperture diameter to 200 µm. This doubles the previously reported best spatial resolution (~0.800 µm) for white light scanning (Youk and Kim, 2006) and is even better than those of laser scanning techniques (Rembe & Dräbenstedt, 2006; Gütay and Bauer, 2007). It also indicates that the aperture size, and especially the diameter of collection pinhole (the diameter of the collection fiber in our case), plays a significant role in improving the system resolution. All the white light reflection images shown below were obtained by setting D1 and D2 to 200 µm and 25 µm, respectively, unless stated otherwise. Scanning electron microscope (SEM) images of

**3. Confocal white light reflection (CWLR) imaging for characterization of** 

Following, we will discuss our proposed applications in (1) resolving gold nanoarrys, (2) distinguish the resonance energy difference between the isolated single and dimer gold nanoparticles' LSP and revealing the strength of the near-field coupling between individual gold nanospheres and their supporting SiO2/Si substrate, and (3) correlating the polarization dependent CWLR images of single silver nanowires with the nanowire

samples were taken with field emission SEM (JEOL JSM-6700F).

polarization dependent excitation of surface plasmon (SP).

as the spatial resolution.

sizes.

**metal nanostructures** 

Gold nanoparticle arrays were fabricated by nanosphere lithography (Jensen et al., 1999) on cover glass substrates. Polystyrene (PS) microspheres with diameter 1 µm and 0.500 µm were used as masks, which will self-assemble into monolayer spheres on substrates. Gold thin film with thickness of about 0.050 µm was deposited by DC coater sputtering, and then the spheres were lifted off.

Fig. 4. The CWLR image at the wavelength of 0.480-0.520 µm for gold nanoarrays on cover glass which were fabricated by using 0.500 µm diameter PS as the lithographic mask.

Figure 4 gave a typical 5.0 x 5.0 µm2 CWLR image for the obtained gold nanoarrays on cover glass fabricated by using 0.500 µm diameter PS as a lithographic mask. The image was extracted from the white light reflection intensity between the wavelength of 0.480-0.520 µm from the samples. Owing to smaller reflection of the cover glass substrate than that of gold particles, hexagonal bright rings in the image of Figure 4 correspond to gold nanoparticles while black areas correspond to the cover glass. The periodicity for the gold particle arrays, which is 0.500 µm, can be clearly resolved, demonstrating the high resolution of our technique. However, the gold particle size and centre-to-centre distance between two nearest gold particles is measured to be about 0.150 µm and 0.100 µm, respectively, by SEM images (not shown), which are out of the range of the system's spatial resolution. This can explain why the image for six gold particles in one hexagonal cell merges to form a hexagonal ring. The dots in Figure 4 work just as a guide for the eye labelling where the particle is. Meanwhile, different defects in the sample as indicated by the rectangular circles in Figure 4 were imaged as well, which reveals that the present imaging method can also be used to test the sample quality, similar to reports elsewhere (Ormonde et al., 2004), but here with much higher spatial resolution.

For comparison, in Figure 5, we have also given the CWLR images for gold nanoarrays on cover glass fabricated by using 1 µm diameter PS as a lithographic mask. Similar to Figure 4, owing to the smaller reflection of the substrate than metal particles, hexagonal bright dots in images Figures 5a to 5f correspond to gold particles while black areas correspond to the cover glass. From the images, we found that the image at 0.480-0.520 µm gives us the best resolution. All the images in this chapter were selected by this way. The gold particle size and centre-to-centre distance between two nearest gold particles is measured to be about 0.300 µnm and 0.200 µm, respectively, by SEM images (not shown). From images in Figures 5a to 5(f), the six gold nanoparticles in one hexagon cell can be resolved clearly as labelled by pink dots in Figure 5a for guiding. The size of these six nanoparticles in CWLR images is spatial resolution determined, which is ~0.410 µm. From these images, it is very easy to obtain the white light reflection spectra for the substrate and gold particles, respectively, as shown in Figure 5g. The contrast spectra are defined as

Confocal White Light Reflection Imaging for Characterization of Nanostructures 291

Fig. 6. SEM image (a) and CWLR images at wavelength 510-550 nm of gold nanospheres on 200 nm SiO2 films with collection fiber core diameter 25 µm (b) and 100 µm (d). (c) The contrast spectra for single (black lines) and dimer spheres for incident light parallel (solid lines) and perpendicular (dotted lines) to the dimer axis. (Du et al., 2008).

Figure 6c plotted the contrast spectra for the single and dimer gold nanospheres after locating their positions. Two contrast dips (labelled 1 and 2, respectively) were observed from Figure 6. It is noticed that the position of dip 2 of isolated single spheres (at about 525 nm) coincides with that of the LSP of gold spheres with diameter 50 nm (Dijk et al., 2005). However, the same dip shows red shifts to 548 nm and 542 nm for the incident polarization parallel and perpendicular to the dimer axis, respectively. This results from the coupling effect between the two nanospheres of the dimer (Moores & Goettmann, 2006). The larger red shift for the parallel polarization than that of the perpendicular case is understandable considering the stronger coupling of the dimer for parallel polarization. Figure 6c also reveals a weak dip 1 for the single and dimer located at about 470 nm, which originates from the multi-polar SP excitation of the gold nanospheres. Firstly, SP excitation at about 470 nm has been observed for gold nanospheres with diameters close to 40 nm, although it was ascribed to false spectral lines that arose from using a 488 nm argon laser (Benrezzak et al., 2001). Secondly, similar multi-polar SP excitation has been reported for other isolated metal nanoparticles (Dijk et al., 2005). Moreover, the excitation of LSPs leads to the enhancement of the absorption of the nanospheres, which consequently has the effect of reducing the reflection intensity (Kawata, 2001), making the nanospheres dark in the corresponding images in Figure 6b. The different dip 2 position for the single and dimer spheres also reflects their different dipolar LSP resonant energies, further implying that the CWLR imaging method is capable of resolving the LSP energies for individual noble metal

Then, to determine the decay length of the electromagnetic (EM) coupling between individual gold nanopspheres and its supporting substrate, different thicknesses (*d*) SiO2 film on Si were obtained by annealing single-crystalline Si (*d* < 20 nm) in air or by RF

nanoparticles.

sputtering SiO2 (*d* > 20 nm) to serve as the substrate.

$$\text{Contrast} = \left( I\_{sample} - I\_{substate} \right) / \left( I\_{substate} \right) \tag{2}$$

where *sample I* and *substrate I* refer to the white light reflection intensity of the sample and the substrate, respectively. The result was shown as Figure 5h, which confirms that gold particles always have larger reflection than that of the substrate, as well as the role of SP in reflection, which is discussed in detail in the next section.

Fig. 5. The CWLR images (a-f) at different wavelength ranges for gold nanoarrays on cover glass fabricated by using 1 µm diameter PS as the lithographic mask while (g) and (h) are the reflected white light spectra and reflection contrast between the gold particle and substrate, respectively. Wavelength ranges for images (a) to (f) correspond to 0.440-0.480 µm, 0.480-0.520 µm, 0.520-0.560 µm, 0.560-0.600 µm, 0.600-0.640 µm and 0.640-0.680 µm, respectively.
