**Confocal White Light Reflection Imaging for Characterization of Nanostructures**

C. L. Du1, Y. M. You2, 3, Z. H. Ni4,J. Kasim2 and Z. X. Shen2 *1College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing 2Division of Physics and Applied Physics, School of Physical and Mathematical Sciences Nanyang Technological University 3Department of Chemistry, Yale University, CT 4Department of Physics, Southeast University, Nanjing 1,4PR China 2Singapore 3USA* 

#### **1. Introduction**

284 Advanced Photonic Sciences

Dutier, G., Yarovitski, A., Saltiel, S., Papoyan, A., Sarkisyan, D., Bloch, D. & Ducloy, M.

Romer, R. & Dicke, R. (1955)New technique for high-resolution microwave spectroscopy,

Sargsyan, A., Hakhumyan, G., Papoyan, A., Sarkisyan, D., Atvars, A., & Auzinsh, M. (2008,

Sargsyan, A., Sarkisyan, D., Papoyan, A., Pashayan-Leroy, Y., Moroshkin, P., Weis, A.,

Sarkisyan, D., Bloch, D., Papoyan, A. & Ducloy, M. (2001) Sub-Doppler spectroscopy by sub micron thin Cs vapour layer, *Optics Communications,* Vol. 200, pp.201-208. Sarkisyan, D., Varzhapetyan, T., Sarkisyan, A., Malakyan, Yu., Papoyan, A., Lezama, A.,

Todorov, P., Vaseva, K., Cartaleva, S., Slavov, D., Maurin, I. & Saltiel, S. (2008) Absorption

Varzhapetyan, T., Sarkisyan, D., Petrov, L., Andreeva, C., Slavov, D., Saltiel, S., Markovski,

Vaseva, K., Slavov, D., Todorov, P., Taslakov, M., Saltiel, S. & Cartaleva, S. (2011) High

nanocell, *Journal of Physics: Conference Series*, Vol.19, pp.20-29.

*Physical Review,* Vol.99, pp.532-536.

Vol.93, 021119

*Physics*, Vol.18, 749-755.

*Review A,* Vol.69, 065802.

*Engineering,* Vol.7027, 7027R

*Society for Optical Engineering,* Vol.5830, pp.196-200.

development of frequency reference, *submitted for publication* 

(2003, b) Collapse and revival of a Dicke-type coherent narrowing in a sub-micron thick vapor cell transmission spectroscopy, *Europhysics Letters*, Vol.63, pp.35-41. Maurin, I., Todorov, P., Hamdi, I., Yarovitski, A., Dutier, G., Sarkisyan, D., Saltiel, S., Gorza,

M.-P., Fichet, M., Bloch, D. & Ducloy, M. (2005) Probing an atomic gas confined in a

a) A novel approach to quantitative spectroscopy of atoms in a magnetic field and applications based on an atomic vapor cell with L=λ, Applied Physics Letters,

Khanbekyan, A., Mariotti, E. & Moi, L. (2008, b) Saturated absorption spectroscopy: elimination of crossover resonances by use of a nanocell, *Laser* 

Bloch, D. & Ducloy, M. (2004) Spectroscopy in an extremely thin cell: Comparing the cell-length dependence in fluorescence and in absorption techniques, *Physical* 

and fluorescence in saturation regime of Cs-vapour layer with thickness close to the light wavelength, *Proceedings of SPIE - The International Society for Optical* 

A., Todorov, G. & Cartaleva, S. (2005) Sub-Doppler spectroscopy and coherence resonances in submicron Cs vapour layer, *Proceedings of SPIE - The International* 

resolution spectroscopy of Cesium-vapor layer with micrometric thickness for

The ability to image nanostructures with a high spatial resolution as well as spectral resolution is very important for a host of both fundamental and practical studies (Grigorenko et al., 2005; Dixon et al., 1991; Verveer et al., 2007; Yoshifumi et al., 2006; Singh et al., 2007; Patel & McGhee, 2007; Laurent et al., 2006). Recently, optical imaging and spectroscopic studies of metal nanoarrays, individual metal nanostructures, and graphene (one monolayer thick carbon atoms packed into a two-dimensional honeycomb lattice, which is the basic building block for other *sp2* carbon nanomaterials.) sheet have attracted much attention (Du et al., 2008, 2010; Laurent et al., 2005, 2006; Ni et al., 2007; Wang et al., 2010). As an example, scanning near-field optical microscope (SNOM) has provided high resolution and been widely used in nanostructure study. However, collecting an image by SNOM is very time-consuming and relies heavily on the equipment as well as the skill of the operator. SNOM is also ill-suited for spectroscopic measurements due to the weak signals. Comparatively, far-field techniques are simpler and generate much stronger signals, which have been successfully used to study localized surface plasmons (LSPs) of gold nanoparticle arrays (Laurent et al., 2005, 2006). Moreover, far-field white light scanning is a simple and low cost method, which also offers multiplewavelength advantage and is suitable to study spectral properties. White light confocal scanning microscopy has also been used to characterize material morphology, refractive index profile of fibers, etc (Ribes et al., 1995; Youk & Kim, 2006), where aperture or fiber were used as confocal pinhole. The best spatial resolution for normal confocal white light scanning optical microscope (not including that from a super continuum light source (Lindfors et al., 2004)) has been improved from 1.500 µm to about 0.800 µm (Youk & Kim, 2006). However, improvement of the spatial resolution is still much desired for the study of small-scale materials.

Confocal White Light Reflection Imaging for Characterization of Nanostructures 287

adopted in this work. The collection fiber also works as a pinhole, which is confocal with the illuminated spot on the sample. The reflected light was directed to a 150 grooves/mm grating and detected by a TE-cooled charge-coupled device (CCD). Typical integration time for imaging was 100 ms/pixel. The stage movement and data acquisition were controlled

Fig. 2. (a) optical image of 1-, 2-, and 3-layer graphene. (b) Raman spectra of different layer graphene sheets and HOPG. (c) Raman image plotted by the intensity of G-band. (d) Cross

To determine the spatial resolution of the confocal reflection imaging system, we referred to the scanning knife edge method (Veshapidze et al., 2006) by using a two-layer graphene sheet on SiO2/Si substrate as the edge (Ni et al., 2007). Thickness of a single layer graphene sheet is ~0.34 nm. The graphene sample was prepared by micromechanical cleavage on a silicon wafer with a 300 nm of SiO2 capping layer (Novoselov et al., 2004). The optical microscope (Figure 2a) was used to locate the graphene sheet, which thickness was further confirmed by Raman spectroscopy. Figure 2b gives the Raman spectra of the graphene sample while its Raman image plotted by the intensity of G band was shown as Figure 2c. Figure 2d plots the cross section of the Raman image from the dashed line in Figure 2c, showing distinct difference in the Raman G-band intensity from the different thicknesses of graphene sample. Hence, the 2 layer graphene sheet can be identified easily, which provides an ideal edge sample because it has strong enough Raman signal with sharp edge, and most importantly it is thin such that

there is no ambiguity in determining the spatial resolution caused by the edge effect.

well with the following equation:

The scanning confocal white light reflection spectrum by using 25 µm core diameter collection fiber and 200 µm diameter aperture was shown in Figure 3, which was fitted quite

> <sup>0</sup> 2( ) () 1 ( ) <sup>2</sup> *<sup>P</sup> x x I x erf*

*w*

(1)

section of Raman image, which corresponds to the dashed line in (c).

using ScanCtrl Spectroscopy Plus software from WITec GmbH, Germany.

In this chapter, a new confocal white light reflection imaging technique is proposed by combining a confocal white light scanning microscope with a spectrometer. By decreasing the diameters of the incident light beam and the collection fiber, a spatial resolution of about 0.410 µm was achieved, which doubly enhances the previously reported best spatial resolution (~0.800 µm) of white light scanning and is even higher than those of laser scanning techniques (Rembe & Dräbenstedt, 2006; Gütay and Bauer, 2007). This system can provide both sample images extracted from reflection within a selective range of wavelength and their white light reflection spectra at each point. The simplicity in carrying out experiments makes this technique attractive, easy and fast. Metal nanoarrays, individual metal nanostructures (including single, dimer gold nanospheres and silver nanowires) and graphene sheet were characterized by the proposed system, demonstrating the strong capabilities in resolving nanometre structures, distinguishing different LSP resonant energies between different individual metal nanostructures and determining the graphene number layers, even the refractive index information of graphene.

## **2. Instrumentation and experiment**

The schematic diagram of the experimental setup was shown in Figure 1. Light from a normal white light source (Xenon lamp) was polarized after passing through a polarizer, which serves as the incident light and was focused onto the sample through a holographic beam splitter and an OLYMPUS microscope objective lens (100X, NA=0.95). A tuneable aperture with a minimum diameter of 200 µm was introduced in the incident light path to tune the spatial resolution of the optical system. Different nanostructure samples were placed on a translation stage which provides coarse movement along the *x* and *y* axes, while the fine movement is offered by a piezostage with 100 μm travel distance along the *x* and *y* directions and 20 μm along the *z* direction. The piezostage also works as a mapping stage.

Fig. 1. Schematic diagram of the proposed confocal white light reflection imaging system. (Du et al., 2008).

The reflected light from the sample was collected by the same lens and directed to a spectrometer through a fiber. Fibers with various core diameters of 100, 50 and 25 µm were

In this chapter, a new confocal white light reflection imaging technique is proposed by combining a confocal white light scanning microscope with a spectrometer. By decreasing the diameters of the incident light beam and the collection fiber, a spatial resolution of about 0.410 µm was achieved, which doubly enhances the previously reported best spatial resolution (~0.800 µm) of white light scanning and is even higher than those of laser scanning techniques (Rembe & Dräbenstedt, 2006; Gütay and Bauer, 2007). This system can provide both sample images extracted from reflection within a selective range of wavelength and their white light reflection spectra at each point. The simplicity in carrying out experiments makes this technique attractive, easy and fast. Metal nanoarrays, individual metal nanostructures (including single, dimer gold nanospheres and silver nanowires) and graphene sheet were characterized by the proposed system, demonstrating the strong capabilities in resolving nanometre structures, distinguishing different LSP resonant energies between different individual metal nanostructures and determining the graphene

The schematic diagram of the experimental setup was shown in Figure 1. Light from a normal white light source (Xenon lamp) was polarized after passing through a polarizer, which serves as the incident light and was focused onto the sample through a holographic beam splitter and an OLYMPUS microscope objective lens (100X, NA=0.95). A tuneable aperture with a minimum diameter of 200 µm was introduced in the incident light path to tune the spatial resolution of the optical system. Different nanostructure samples were placed on a translation stage which provides coarse movement along the *x* and *y* axes, while the fine movement is offered by a piezostage with 100 μm travel distance along the *x* and *y* directions and 20 μm along the *z* direction. The piezostage also works as a mapping stage.

Fig. 1. Schematic diagram of the proposed confocal white light reflection imaging system.

The reflected light from the sample was collected by the same lens and directed to a spectrometer through a fiber. Fibers with various core diameters of 100, 50 and 25 µm were

number layers, even the refractive index information of graphene.

**2. Instrumentation and experiment** 

(Du et al., 2008).

adopted in this work. The collection fiber also works as a pinhole, which is confocal with the illuminated spot on the sample. The reflected light was directed to a 150 grooves/mm grating and detected by a TE-cooled charge-coupled device (CCD). Typical integration time for imaging was 100 ms/pixel. The stage movement and data acquisition were controlled using ScanCtrl Spectroscopy Plus software from WITec GmbH, Germany.

Fig. 2. (a) optical image of 1-, 2-, and 3-layer graphene. (b) Raman spectra of different layer graphene sheets and HOPG. (c) Raman image plotted by the intensity of G-band. (d) Cross section of Raman image, which corresponds to the dashed line in (c).

To determine the spatial resolution of the confocal reflection imaging system, we referred to the scanning knife edge method (Veshapidze et al., 2006) by using a two-layer graphene sheet on SiO2/Si substrate as the edge (Ni et al., 2007). Thickness of a single layer graphene sheet is ~0.34 nm. The graphene sample was prepared by micromechanical cleavage on a silicon wafer with a 300 nm of SiO2 capping layer (Novoselov et al., 2004). The optical microscope (Figure 2a) was used to locate the graphene sheet, which thickness was further confirmed by Raman spectroscopy. Figure 2b gives the Raman spectra of the graphene sample while its Raman image plotted by the intensity of G band was shown as Figure 2c. Figure 2d plots the cross section of the Raman image from the dashed line in Figure 2c, showing distinct difference in the Raman G-band intensity from the different thicknesses of graphene sample. Hence, the 2 layer graphene sheet can be identified easily, which provides an ideal edge sample because it has strong enough Raman signal with sharp edge, and most importantly it is thin such that there is no ambiguity in determining the spatial resolution caused by the edge effect.

The scanning confocal white light reflection spectrum by using 25 µm core diameter collection fiber and 200 µm diameter aperture was shown in Figure 3, which was fitted quite well with the following equation:

$$I(\mathbf{x}) = \frac{P}{2} \left\{ 1 + \text{erf}(\frac{\sqrt{2}(\mathbf{x} - \mathbf{x}\_0)}{w}) \right\} \tag{1}$$

Confocal White Light Reflection Imaging for Characterization of Nanostructures 289

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

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

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

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

the spheres were lifted off.

with much higher spatial resolution.

shown in Figure 5g. The contrast spectra are defined as

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 as the spatial resolution.

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 sizes.

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

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

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 polarization dependent excitation of surface plasmon (SP).
