Real time observation on the evolution silver nanoparticles film

We used dark field microscopy and optical spectrum-meter to record the dynamics of the silver film under a annealing process. Comparing the information from dark field micrograph and real time absorbance spectra, the consistent can be found due to the optical information. This real time optical measurement provides a destructive method to observe the evolution of metal nanoparticle differed from SEM and AFM analysis.

4.4.1 Dark field Mirocscopy

Figure 4.7 shows the real time micrographs of silver nano-particles film observed in dark field optical microscopy. The dark-field micrograph shows interesting optical phenomena of particular scattering. The scanning electron micrograph and optical micrograph corresponded to various temperature was analyzed due to the different size. The dynamic spectra reflected the real situation of the growth of metal nanoparticles in real time.

Figure 4.3: (a) Dark-field micrograph of silver nanoparticles in various sizes. (b) Collecting the scattering light with dark-field objective.

Before annealing process, the probe light can not be scattered by silver film and showed dark color in the dark field microscopy. It exhibited the silver was in a film structure, no light scattered into the object. When the temperature was increased to 150 ℃, the nano-particles film revealed a weak yellow-orange light in the microscopy.

The weak yellow-orange light was corresponded to the absorbance spectrum of at 150℃. The maximum absorbance occurred at around the 560 nm. As the temperature arrived to 200 and 250 degrees, the color was changed into strong blue color, which

means the localized surface plasmons were excited dramatically in this wavelength band region. At the temperature of 300 degree, the scattering light intensity was turned weaker. The density of silver nano-particles decreased according to the aggregation of nano-particles.

Figure 4.4: Dark field optical micrographs of silver nano-particles film at every annealing step.

4.5.2 Real time optical absorbance spectra

Figure 4.9 was the experimental setup for the measurement of optical absorbance of silver nanoparitcles film. The white light emerged from a W lamp was colleted into the silver nanoparticles film. The spectrum was measured by a spectrometer in the right side.

Figure 4.5: Experimental setup for absorbance spectra measurement

Figure 4.10 shows the first stage of growth evolution of silver nanoparticles film. The absorbance spectrum before annealing was a flat curve in the spectrum range from 450 to 750 nm. As the temperature was increased gradually, the broad absorbance spectrum appears, the seed of nano-particle has been formed in this stage. Since the growth of particle led to the decrease of contact area with the ITO substrate surface, the effective index of environment was decreased and caused the peak position shifted to the shorted wavelength band.

Figure 4.6: Real time absorbance spectra of silver nano-particles film at various heating temperature of 28, 50, 100, 150, 200, and 250 ℃. A broad peak appeared at around 552 nm (100 ℃) and then shifted to shorter wavelength band at around 500 nm (250 ℃).

In Figure 4.11, the peak wavelength shifted to longer wavelength. The peak shifted to longer wavelength with the increase of particle size. The intensity was also decreased with the decrease of particle density. The higher absorbance intensity replied the stronger localized surface plasmon resonance. At 300 ℃ for an hour, the absorbance was drop up to zero. In this stage, the particles grew up stably to about 250 nm, but the lower density leaded to the lower absorbance.

Figure 4.7: Absorbance spectra at the heating temperature of 250 and 300 ℃, then the temperature was kept at 300 ℃ for an hour. The absorbance peak (500 nm at 250 ℃) shifted to longer wavelength band around 572 nm (300 ℃ after an hour).

Fig. 4.12 shows that the peak position and FWHM of absorbance spectra of silver nano-particles film varied in whole annealing process. Due to the excitation of localized surface plasmon was measured at every annealing step. We divided the evolution of growth into two stages according to the peak position shift of the real time absorbance spectra. The peak position shifted to short wavelength when the temperature lower than 250 ℃. The absorbance spectra provide a convenient and simple method to observe the optical formation of silver nano-particles. The effective dielectric property was decreased due to the contact area with the ITO substrate. The resonance wavelength was shifted to the shorted wavelength.

Figure 4.8: Variations of the peak position and the full width half maximum of the absorbance spectra at every annealing step.

In the first stage, the silver film was gradually split to the nuclear formation.

After the nuclear, the particle was stably aggregated together as a larger article. The real time absorbance spectra revealed a optical dynamics of silver nano-particles during the annealing process.

Optical property of metal nano-particles was theoretically explained by electromagnetic interaction on the basis of Mie’s theory. The extinction loss included the light scattered and absorbed by the metal nano-particles. According to the Mie’s theory, the extinction can be easily simulated due to the electromagnetic theory. On the other hand, the exposition of electron motion in the metal nano-particles determined due to the external optical field was also applied to discuss the features of the optical absorbance spectrum. The increased of effective mass of electron leads to the resonance wavelength shifted to longer wavelength.

Particular features revealed in the absorbance spectrum exhibited several clues of the localized surface plasmon excited on the metal nano-particles. The surface

plasmon resonance is strongly dependent on the shape, size and dielectric property of the surrounding environment. The peak position, bandwidth, and intensity were determined by the particles size, size distribution, and the density, respectively. Due to these properties, the optical observation based on the absorbance spectrum of metal nano-particles provided a conventient approach to observation of the nano-structured metal material.

In summary, the real time absorbance spectra due to optical dynamics of silver nano-particles film were studied. In the annealing process, the peak position and FWHM were significantly dependent on the annealing temperature and time. The variation of particle size and density were confirmed by SEM and dark-field microscopy analyses. The real time measurement of absorbance spectra leaded to a direct and non-destructed method to observe the optical behavior of metal nano-particles system.

References

1. T. L. Ferrell, ‘‘Surface-enhanced Raman scattering in Ag-pyride sols,’’ Phys.

Rev. B 25, 2930 (1982).

2. T. A. Callcott and E. T. Arakawa, ‘‘Volume and surface photoemission processes from plasmon resonance fields,’’ Phys. Rev. B 11, 2750 (1975).

3. S. W. Kennerly, J. W. Little, R. J. Warmack, and T. L. Ferrell, ‘‘Optical properties of heated Ag films,’’ Phys. Rev. B 29, 2926 (1984).

4. J. W. Little, T. L. Ferrell, T. A. Callcott, and E. T. Arakawa, ‘‘Radiative decay of surface plasmons on oblate spheroids,’’ Phys. Rev. B 26, 5953 (1982).

5. M. C. Buncick, R. J. Warmack, and T. L. Ferrell, ‘‘Optical absorbance of silver ellipsoidal particles,’’ J. Opt. Soc. Am. B 4, 927 (1987).

6. M. J. Bloemer, T. L. Ferrell, M. C. Buncick, and R. J. Warmack, ‘‘Optical properties of submicrometer-size silver needles,’’Phys. Rev. B 37, 8015 (1988).

7. R. J. Warmack and S. L. Humphrey, ‘‘Observation of two surface plasmon modes on gold particles,’’ Phys. Rev. B 34, 2246 (1986)

Chapter 5

Laser pulse induced gold nanoparticles gratings

Various technologies involving the use of nano-particles are becoming increasingly important in many areas of applications. This leads to a significant increase in the studies of the physical properties of optical systems involving nano-particles. A unique property of metallic nano-particles is the presence of surface plasmons and the resulting enhanced absorption and/or scattering. Various methods are developed for the fabrication of nano-particles. These include chemical systhesis with precise control of the shape and size of the particles [1, 2]. The enhanced absorption and scattering due to the surface plasmons can be employed for optical applications [3, 4]. The realization of the nano-scale device systems can lead to many interesting and potentially useful phenomena, such as sub-diffraction-limit image, optical signal propagation via metal nanoparticles array, nano-material exploration based on surface enhanced Raman scattering, and so on [5-10]. However, the current fabrication process is complicated. A simple and economic fabrication of nano-scale structures and nano-particles is desirable for practical applications.

5.1 Deposition of nano thick gold film

In this paper, we report the results of our experimental investigation on the optical properties of gold nano-particle gratings produced by a single shot of a pair Nd-YAG laser pulse beams. In our experimental investigations, the gratings were obtained via a spatially periodic laser heating of a thin gold film. The heating by the laser pulse leads to the formation of a periodic distribution of gold nano-particles. The surface morphology of gold nano-particle gratings was analyzed by using scanning electron microscope (SEM) and dark-filed optical microscope. It was noted that the grating configuration and the geometry of nano-particles were strongly dependent on the fluence of the laser pulse. We also investigated the diffraction property of the nano-particle gratings in the visible spectral regime.

In our experimental investigations, we first deposited a polymethy1 methacrylate (PMMA) layer with a thickness of 2.5 μm on a glass substrate via spin coating. We then thermally deposited a 6 nm thick gold film on the PMMA layer.

Figure 5.1 shows the experimental setup for the creation of a gold nano-particle grating using a Q-switch Nd-YAG pulse laser at 532 nm with a pulse width of 6 ns.

The fluences of the laser pulse were in 70 and 110 mJ/cm².

Figure 5.1: Fabrication process of nano thick gold film on PMMA substrate

Figure 5.2 shows the experimental and theoretical absorbance spectrum of gold film with nano-thickness of deposited. The experimental spectrum is consistent with the theoretical prediction. On the base of this consistent, the deposited thickness is confirmed.

Figure 5.2: (a) Experimental and (b) theoretical absorbance spectrum of gold film.

Experimental setup was shown in Figure 5.3. The incident beam was polarized perpendicular to the plane of incidence (s-polarization) by using a polarizer (P) and a half-wave plate (λ/2). The laser beam was split into two beams by using a beam splitter (BS). These two beams were directed toward the thin film sample by using a mirror (M), creating an interference pattern on the film surface. The angle (θ) between two beams was about 3 degrees. The gold film/PMMA sample was oriented so that the beams enter the sample from the glass substrate. After the exposure with the laser pulse, we examined the nano-particle grating by using a linearly polarized beam of He-Ne laser.

Figure 5.3: Experimental setup for the formation of nano-particle grating via a spatially periodic heating process on a thin gold film. The periodic intensity pattern is obtained via the interference of two Nd-YAG pulse beams.

5.2 Transformation between optical and thermal energy

Upon exposure of the laser pulse, the optical energy deposited in the gold film was converted to thermal energy. This led to an increase of the temperature in the gold film. The temperature gradient in the film plane along the direction of the fringe pattern can be estimated by assuming local heating only. In other words, we can ignore the heat conduction in a non-equilibrium state. This assumption is legitimate as the nano-particles are formed during the extreme short duration of pulse. The spatial intensity (W/m²) distribution of the optical beam at the golf film can be written [11]

⎟⎠ where the I0 is the intensity of each of the incident beams, Λ is the period of the fringe pattern and the x is the coordinate along the direction of the interference fringe pattern.

For a symmetric incidence, the period is given by Λ =λ 2sin(θ/2). For λ=532 nm, a period of Λ ~ 10 μm requires an angle of θ=3 degrees.

Assuming local heating only, the temperature (T) on the surface of gold film during the exposure can be written [12]

2 specific heat (J/kg·℃), respectively. These physical parameters of gold are listed in Table 1. It is important to note that the time t in Eq. (2) is limited to the 6ns-duration

of the laser pulse.

Figure 5.3 shows the spatial distribution of the beam intensity and the temperature on the target surface. We note that an estimated maximum surface temperature of 1450 and 2278 ℃ at the target surface were obtained from the laser pulse of 70 and 110 mJ/cm², respectively. These temperatures are in the range between the melting point of 1064 and boiling point of 2660 of gold. This ℃ ℃ temperature range is ideal for the formation of nano-particles. Due to the extremely short duration of the laser pulse, the energy deposited in the film leads to a non-equilibrium distribution of the thermal energy, resulting a rapid increase of temperature at the high optical intensity locations (bright fringes).

Figure 5.4: Spatial distributed optical density and temperature along interference pattern.

As a result, the gold film in the bright regions is converted into nano-particles, while the gold film in the dark regions remains intact. Thus, the exposure of the gold film with a periodic intensity pattern leads to the formation of a nano-particle grating.

This is illustrated in Figure 5.5

Table 5.1: Physical thermal properties of gold material at 25 ℃

Thermal conductivity(K) Density (ρ) Specific heat (C) Thermal diffusivity( k) 3.17 W/(cm· )℃ 19.32 (g/cm³) 0.128 J/(g· )℃ k=K/C (cm²/s)

Figure 5.5: Spatial distribution of the surface temperature due to the periodic variation of the incident beam intensity on the gold film.

In our experimental investigations, the PMMA layer played an important role in the formation of gold nano-particles. The absorption of optical energy at 532 nm in the PMMA layer is virtually zero. Furthermore, the PMMA layer also plays the role of an insulating layer preventing the flow of heat into the glass substrate. This ensures that the optical energy is deposited at the interface between the gold film and the PMMA layer.

5.3 Gold nanoparticles gratings

In our experimental investigation, we employed laser pulses at two different fluences (70 and 110 mJ/cm²). Figure 5.6 shows the scanning electron micrograph and dark-field optical micrograph of gold nano-particle grating induced by a laser pulse with a fluence of 70 mJ/cm². As shown in Figure 5.6 (a), the grating consists of a periodic bands of gold nano-parrticles, separated by a periodic array of stripes of gold film. In the region of stripes of gold films, the lower thermal energy due to the destructive interference of optical wave was insufficient to raise the temperature of the gold film to form gold nano-particles. The size of gold nanoparticles is in the range of ~ 100 nm in diameter, with a variation of about 20%. Several gold nano-particles were found on the stripes of gold film. They may be a result of sputtering from the nano-particles formed in the adjacent high-temperature regions.

The few gold nanoparticles appeared on this non-heated zone and the presence of gold films are consistent with the assumption of local heating with a rapid raise of the local temperature.

Figure 5.6: (a) Scanning electron and (b) dark-field optical micrograph of gold nano-particle gratings induced with a Nd-Yag laser pulse with a fluence of 70 mJ/cm².

Figure 5.6(b) shows the dark-field optical micrograph of the grating. The bright spots represent strong scattering of light from the gold nano-particles. The enhanced scattering of light at selected wavelength can be due to the excitation of localized surface plasmons in the gold nano-particles. The figure also shows that the nano-particles are indeed formed in the spatial regions of bright fringes. The few nano-particles found in the regions of dark fringes are possibly due to sputtering from neighboring bands of nano-particles.

Figure 5.7: (a) Scanning electron and (b) dark-field optical micrograph of gold nano-particle gratings induced with a Nd-Yag laser pulse with a fluence of 110 mJ/cm².

5.3.1 Diffraction Property of gold nanoparticles gratings

Figure 5.7 illustrates a similar set of micrographs showing the grating configuration using a laser pulse with a fluence of 110 mJ/cm². At such a higher fluence level, the residual energy in the regions of dark fringes can be sufficient to raise the local temperature so that nano-particles are formed. As a result, nano-particles are formed in both the bright regions and dark regions. This leads to a

nano-particle grating with alternating high and low nano-particle populations. In this case, the excitation of surface plasmons occurs in both regions of the grating.

Figure 5.8: First order diffraction efficiency of gold nano-particle gratings as a function of probe wavelength.

We also investigated the diffraction property of the nano-particle grating formed with two different fluence levels. A linearly polarized beam of monochromatic light was directed toward the gold nanoparticles grating. The polarization state of the incident beam was perpendicular to the x-direction. The first-order diffraction spot was measured by using an optical detector. The monochromatic beam was obtained by using a monochromator in conjunction with a tungsten lamp. Figure 5.8 shows the first order diffraction efficiencies of gold nano-particle gratings formed with laser pulses of fluence levels 70 and 110 mJ/cm². The diffraction efficiencies were measured in the spectral range from 450 nm to 750 nm. The two diffraction spectra are significantly different. Maximum diffraction efficiencies occur at around 680 and 620 nm, respectively. The significant difference can be due to the difference of the population distribution of the nano-particles in the two gratings. Since the presence of nano-particles of gold can lead to the excitation of surface plasmons and the resulting absorption and scattering of light, the nano-particle gratings can be modeled as an amplitude gratings.

Figure 5.9: Absorbance of a thin layer of gold nanoparticles with a size (diameter) of ~ 100 nm. The insert shows the SEM of an array of gold nano-particles distributed on the PMMA layer.

Figure 5.9 shows the measured absorbance (defined as A = - log (T/T0), where T and T0 are, respectively, the transmittances of the sample with and without the presence of a thin layer of 2D array of gold nano-particles on the PMMA layer) as a function of wavelength. The insert in Fig. 5.9 is the SEM of an array of the gold nano-particles. The density and size of the nano-particles are typical of that presented in the bright zone of Figure 5.6 (b). We note that the absorbance spectrum consists of a broad absorbance band with a peak at around 630 nm. The broad absorption band at 630 nm is possibly due to the excitation of surface plasmons on the gold nano-particles. The nano-particle gratings are due to the periodic variation of the population of nano-particles. In addition to the variation of population of nano-particles, the averaged size of the nano-particles can be spatially dependent. A periodic modulation of the scattering and/or absorption leads to an optical grating with a first order diffraction efficiency which peaks at around 630 nm. Further analysis is needed to include the size distribution, the presence of a phase grating due to the difference in the phase shift in the two regions of nano-particles.

In summary, we have carried out experimental investigation on the formation of gold nano-particles via the laser ablation of a thin film. Using a periodic variation of the laser intensity, we obtained nano-particle gratings via a single shot of Nd-YAG

laser pulse. The spatial variation of the local heating leads to a periodic variation of the population of nano-particles. We also investigated the diffraction property of the nano-particle gratings. The diffraction spectra show a strong wavelength dependence, possibly due to the presence of the gold nano-particles and the excitation of surface plasmons. The method of the formation of the nano-particles via the illumination of laser pulses may provide a fast and simple fabrication processes of metal nano-particle devices.

References

References