Preparation of carbon nanoparticles with strong optical ... - NUS Physics

JOURNAL OF APPLIED PHYSICS

VOLUME 95, NUMBER 3

1 FEBRUARY 2004

Preparation of carbon nanoparticles with strong optical limiting properties by laser ablation in water G. X. Chen, M. H. Hong,a) and T. C. Chong Data Storage Institute, DSI Building, 5 Engineering Drive 1, Singapore 117608, and Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576

H. I. Elim, G. H. Ma, and W. Ji Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542

共Received 7 May 2003; accepted 10 November 2003兲 Carbon nanoparticle colloids were prepared by Nd:YAG laser ablation of a carbon target immersed in water. The nanoparticles were collected on substrates and analyzed with micro-Raman spectroscopy and electron microscopy. Optical limiting properties of the carbon nanoparticle colloids towards 532 nm wavelength were characterized using a nanosecond Nd:YAG laser. A strong optical limiting response was detected for the carbon nanoparticle colloids with C60 as reference at input laser fluence above 0.6 J/cm2. The effects of carbon nanoparticle concentrations upon the optical limiting behavior were also investigated. A convenient method for preparing carbon nanoparticle based optical limiters is proposed in this investigation. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1637933兴

I. INTRODUCTION

as protection of human eyes and optical sensors for high power laser irradiation. For protection of personnel, a solid optical limiting device is often desirable. But studies show that the limiting performance of nanoparticle solutions is better.12 Therefore, preparing carbon nanoparticles in solutions, optimizing their nonlinear optical properties, and developing related devices become important topics of research. Various carbon nanoparticle related studies were reported. The formation and transformation of carbon nanoparticles were predicted theoretically.13 Diverse experimental methods were developed to produce carbon nanoparticles according to the broad range of applications. For example, the carbon arc technique,4 microwave-plasma chemical-vapor deposition,6 supersonic cluster beam deposition,7 and pulsed laser deposition8 were used successfully to prepare carbon nanoparticles in different ambients. As for carbon nanoparticles applied as optical limiters, they are usually prepared by diluting and filtering a commercially available ink.11 However, prepared colloids contains large particles and flocs. It was proposed that smaller particles may enhance the sensitivity.14 In the present work, we use a simple method to produce smaller carbon nanoparticles in water with remarkable optical limiting properties by laser ablation. Recently it was suggested to prepare nanoparticles in solution by laser ablation of targets immersed in liquid media.15 Gold and nickel nanoparticles were obtained by ablating targets in a solvent using a strong laser beam. In the present work we demonstrate that this method can be used directly to produce carbon nanoparticle based optical limiters. The carbon nanoparticles obtained are studied by various methods.

Carbon-based nanomaterials have attracted great interest in recent decades, especially after the discovery of fullerenes. A broad range of carbon nanostructures such as carbon nanoparticles, nanotubes, onions, and cubic nanocages were observed thereafter.1 Understanding their properties and exploring their promising application have led to an explosion of research worldwide. In particular, carbon nanoparticles have attracted much attention due to their significant role in different fields, from arc generated soot and vacuum deposited thin films to interstellar dust.2 One type of carbon nanoparticle, carbon black, has been widely used as a filler in rubber, dye in paint, and catalyst carrier in chemistry for centuries.3 Potential newly found applications of carbon nanoparticles occur in such diverse areas as protective coatings, electronic devices, field emission devices, supercapacitors, microsensor preconcentrators, and nonlinear optical devices, further prompting more intensive research.4 –10 The potential of using carbon nanoparticles as nonlinear optical materials is generating much research interest.9,10 Carbon nanoparticles suspended in solution are among the best nonlinear materials for manipulation of optical beams in passive mode.11 The transmittance of carbon nanoparticle colloids decreases significantly with an increase in light fluence due to photoinduced thermal processes in solutions. It provides strong optical limiting responses for nanosecond laser beams over visible and near infrared bands at high laser fluence without severe visual loss in comparison with colored optical limiters. This feature makes carbon nanoparticle solutions promising nonlinear materials for applications such a兲

Author to whom correspondence should be addressed; electronic mail: hongគ[email protected]

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FIG. 1. Experimental setup for fabricating carbon nanoparticles in water by laser ablation.

II. EXPERIMENTAL DETAILS A. Preparation of carbon nanoparticles

In this study, a pure glassy 共vitreous兲 carbon plate 共Johnson Matthey Co.兲 was used as the target for preparing carbon nanoparticles in a glass Petri dish container. Both the target and Petri dish were rinsed with ethanol and acetone followed by de-ionized water in an ultrasonic cleaner prior to preparation. The target was immersed in de-ionized water to a depth of around 5 mm below the water’s surface during laser ablation. The target was rotated constantly during laser ablation for uniform ablation, ensuring that the nanoparticles that formed dispersed uniformly into the water. A 7 ns Nd:YAG laser 共B. M. Industries, Series 5000兲 operating at a wavelength of 532 nm was used to ablate the target as depicted in Fig. 1. The repetition rate was 10 Hz. The laser fluence used in this experiment was 0.8 J/cm2 and it was monitored by a Scientech Vector H310 laser power meter. To avoid loss of energy of the laser pulse when the concentration of the nanoparticles was high, all preparations were held to 5 min. The as-prepared solution had nearly 100% transparency at 532 nm in an UV-visible 共UV-vis兲 measurement. Then the colloidal solution obtained was collected and condensed to increase the concentration of carbon nanoparticles. Before measurement, the solution was vibrated in an ultrasonic cleaner to ensure the uniformity of particle dispersion and to avoid possible aggregations formed during heating. B. Sample characterization

The morphology of the carbon nanoparticles was studied with a transmission electron microscope 共TEM兲共JEOL JEM 2010兲 by depositing the nanoparticles onto copper grids. Raman spectra were also taken by drying drops of carbon nanoparticle colloids on silicon substrates. A micro-Raman spectroscope 共Renishaw 2000兲 working at room temperature was used to collect the spectra. A 514.5 nm argon ion laser operating at 10 mW was used as the light source. The incident polarized laser light and scattering propagation were perpendicular to the substrate. During measurements, the spectral width selected was around 1.5 cm⫺1.

FIG. 2. TEM image of carbon nanoparticles obtained by laser ablation in water.

Linear transmission spectra of the carbon nanoparticle colloids were recorded using an UV-vis spectrophotometer 共Shimadzu 1601兲 operating in the wavelength range of 300– 1000 nm. UV-vis absorption spectra of carbon nanoparticle colloids before and after the optical limiting measurement were measured. Linearly polarized 7 ns pulses from a Q-switched, frequency-doubled Nd:YAG laser were used to-do the optical limiting measurements. After passing through a spatial filter, the spatial distribution of the laser beam was nearly Gaussian. Then the incident laser beam was divided by a splitter: The reflected part was taken as a reference that represented the incident light energy and the beam transmitted was focused into a 1 cm cuvette by a focusing mirror ( f ⫽250 mm) 16 to do the optical limiting measurements were conducted with the colloids filling in the cuvette. Both the reference beam and the beam transmitted through the carbon nanoparticle colloids were monitored simultaneously with a dual-channel energy meter. The transmittance was determined by the ratio of the transmitted pulse energy and incident pulse energy to analyze the optical limiting response.

III. RESULTS AND DISCUSSION A. Morphology

A typical TEM image of the carbon nanoparticles is presented in Fig. 2. The size of the nanoparticles ranges from 10 to 20 nm with few individual nanoparticles larger than 20 nm found in the image. It is suggested that the liquid environment favored growth of the carbon nanoparticles during laser ablation.17 The plasma species produced during laser ablation are quenched by heated vapor under very high pressures. The kinetic energy of carbon atoms is reduced by multiple collisions with the surrounding molecules and atoms through expansion. The liquid media induce quick nucleation followed by rapid growth of the nanoparticles. Selected area diffraction 共SAD兲 shows that the structure of the nanoparticles is amorphous.


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FIG. 4. UV-vis transmission spectra of carbon nanoparticles in water at different concentrations 共each line is a superimposition of spectra taken before and after optical limiting measurement兲. The concentrations of samples 1, 2, and 3 are 3, 10, and 36 mM, respectively.

FIG. 3. Raman spectra of as-received glassy carbon plate and prepared carbon nanoparticles. Both Raman spectra are fitted with Gausian lines.

B. Raman shift

The normalized visible Raman spectrum corresponding to the carbon nanoparticles is presented in Fig. 3. Usually two significant bands, labeled D for disorder and G for graphite, respectively, dominate the Raman spectrum of disordered carbon. The D band is near 1350 cm⫺1. It refers to the A 1g symmetry of disordered graphite and indicates the existence of nanocrystalline graphite. The G band is near 1580 cm⫺1. It is attributed to a zone-center mode of E 2g symmetry of single crystal graphite. Figure 3 shows a comparison between the glassy carbon target and the fabricated carbon nanoparticles. The Raman spectrum of the target and its Gaussian fit clearly show the D band near 1360 cm⫺1 and the G band at 1585 cm⫺1. However, the D band for the fabricated carbon nanoparticles is shifted upward to 1414 cm⫺1, while the G band remains near 1585 cm⫺1. There is also an increase in the full width at half maximum 共FWHM兲 of the D band in the carbon nanoparticle Raman spectrum with respect to that of the target material. The phenomenon of shifting upward and broadening in the D band implies an increased level of disorder and a decrease in the graphitic domain size.18 The recombination of bonds during carbon nanoparticle formation introduces smaller graphitic domain sizes, leading to size distributions with a variety of bonding structures. The distribution of clusters with different sizes introduces superposition of the different Raman modes, resulting in a broader linewidth and a shift of the Raman peaks.

The integrated intensity ratio between the D peak and G peak was proposed to be inversely proportional to the grain size of the graphite.19 The relationship between the grain size and the intensity ratio is given by L a ⫽44/关 I(D)/I(G) 兴 Å, where L a is the grain size, and I(D) and I(G) are the integrated intensity associated with the D peak and G peak, respectively. Using this formula, the average graphitic domain size of the glassy carbon plate is estimated to be 38.7 Å, while that of the nanoparticles is 15.6 Å. This result supports very well broadening of the D band and shrinking of the domain size. Assuming the carbon nanoparticles obtained are spherical with average size of 15 nm, the number of graphitic clusters per nanoparticle can be estimated to be on the order of 103 .

C. UV-vis spectra

Normalized UV-vis transmission spectra of carbon nanoparticle colloids at different concentrations are displayed in Fig. 4. The transmission spectra before and after optical limiting measurement for every sample are superimposed. The superimposition suggests that no severe damage of the samples occurred during optical limiting measurement, which shows good photostability of the samples. It demonstrates that the observed limiting effect represents the intrinsic property of carbon nanoparticle colloids. The linear transparency at 532 nm decreases from around 90% to 80% to 60% as the nanoparticle concentration increases. The corresponding concentrations are estimated as 3, 10, and 36 mM, respectively. The linear transparency changes monotonically with the particle concentration. Therefore, we can control the particle concentration by in situ monitoring of the linear transparency at 532 nm throughout particle generation.


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FIG. 5. 共a兲 Comparison of optical limiting performances of carbon nanoparticles in water with C60 in toluene at 532 nm measured towards a 7 ns laser beam. 共b兲 Optical limiting performance at different carbon nanoparticle concentrations. The linear transparency at 532 nm of sample 1 is 90%, while that of sample 2 is 80% and that of sample 3 is 60%. The corresponding concentrations are 3, 10, and 36 mM, respectively.

D. Optical limiting properties of carbon nanoparticle colloids

It is very interesting to find that the optical limiting properties of laser-synthesized carbon nanoparticles are strong using C60 as a reference. As shown in Fig. 5共a兲, at the same linear transparency of 80%, a quicker decrease in transparency towards the 532 nm laser beam is expected for carbon nanoparticles than for C60 when the incident laser fluence is above 0.6 J/cm2. Optical limiting properties of C60 have been investigated extensively.11 Due to its excellent optical limiting responses, C60 is generally taken as a benchmark in research of the optical limiting effect. However, C60 solution gives the appearance of color to the liquid. This effect introduces visual loss for eyes. For carbon nanoparticle colloids with the same transmittance 共80%兲 as that of C60 , the visual loss is noticeably reduced. On the other hand, carbon nanoparticle colloids have broadband transmittance as shown in Fig. 4. The performance of laser-synthesized carbon nanoparticles indicates that they are also good optical limiters. The improvement in optical limiting performance of carbon nanoparticles is mainly contributed by strong nonlinear scattering, while C60 follows a reverse saturable absorption process. The limiting effect of carbon nanoparticles in water has been explained as due to two mechanisms.14 One mecha-

nism involves the phase transition from solid to gas with the carbon nanoparticles generating explosive growth of microplasmas. The other one corresponds to heat transfer from carbon nanoparticles to surrounding liquid and expansion of the microplasma, leading to solvent bubble growth. During nanosecond pulse irradiation, carbon nanoparticles act as initiation sites for optical breakdown in the solution. The absorption of laser light induces a very high rise in the temperature of the nanoparticles, which leads to the formation of microsized plasmas. The growth and rapid expansion of this microplasma in turn give rise to an increase in the absorption and scattering, and hence, optical limiting occurs. At higher fluences 共normally higher than 1 J/cm2兲, the heat in the nanoparticles and microplasmas is quickly transferred to the surrounding liquid and bubbles form. For the repeated transmission spectra after the optical limiting measurements showed no damage in the samples, the possible mechanism is that scattering of these bubbles in the surrounding liquid dominates the limiting behavior of the carbon nanoparticles. The process is quite similar to that observed in carbon black suspensions and carbon nanotubes.11 The effects of particle concentration on the optical limiting properties of carbon nanoparticles were also studied. The optical limiting response of carbon nanoparticles is enhanced with an increase in nanoparticle concentration. As shown in Fig. 5共b兲, optical limiting begins when the fluence of the incident laser is increased above 0.3 J/cm2. The higher the concentration, the stronger the limiting effect. It is plausible to understand the enhancement of the limiting response at high concentrations as an increase in nonlinear scattering sites. At higher nanoparticle concentration, carbon nanoparticle colloids provide denser initial sites for the formation of bubbles. Thus, more bubbles in the focal volume are to be expected during laser irradiation of high concentration colloids, and this will promote a quicker increase in nonlinear scattering. IV. CONCLUSIONS

Amorphous carbon nanoparticles were fabricated by laser ablation of a glassy carbon target in water. The sizes of the nanoparticles ranged from 10 to 20 nm. UV-vis spectra demonstrated that the carbon nanoparticle colloids are photostable. The carbon nanoparticle colloids have strong optical limiting behavior towards the 532 nm wavelength of a nanosecond laser beam with C60 as the reference. It was demonstrated that higher concentrations of carbon nanoparticles introduce stronger optical limiting effects. Laser ablation of a glassy carbon target in water is shown to be a convenient approach by which to fabricate carbon based optical limiters. ACKNOWLEDGMENTS

This work was supported by A*STAR, Singapore. The authors would like to thank Ms Ji Rong for help in Raman measurement. 1

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