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Langmuir–Blodgett Ag nanoparticle monolayer patterned by pulsed laser-induced selective desorption
Superlattices and Microstructures 44 (2008) 657–663 www.elsevier.com/locate/superlattices
Langmuir–Blodgett Ag nanoparticle monolayer patterned by pulsed laser-induced selective desorption Hyunkwon Shin a , Hyunjun Kim a , Jeongmin Ha a , Ki-Soo Lim b , Myeongkyu Lee a,∗ a Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-ku,
Seoul 120-749, Republic of Korea b Department of Physics, Chungbuk University, 12 Gaesin-dong, Cheongju 361-763, Republic of Korea
Available online 5 March 2008
Abstract The control of nanoparticle distribution on a solid substrate is an important and challenging issue in the development of novel nanostructured devices. Here we present a simple optical method for obtaining regular geometric nanoparticle arrangements. Monolayered Ag-nanoparticle films were deposited on glass substrates by the Langmuir–Blodgett (LB) technique. The nanoparticles could be quickly desorbed from the substrate without causing a surface damage, when exposed to pulsed Nd:YAG laser with an energy density over a threshold value. This made it possible to obtain well-aligned patterned arrays simply by a spatial modulation of the pulse energy density. The effects of pulse duration, exposure time, and particle size on this laser-induced selective desorption process are discussed. c 2008 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticle; Patterning; Langmuir–Blodgett film; Pulsed laser
1. Introduction Metal nanoparticles can serve as attractive building blocks for novel devices in such areas as biological sensing [1,2], catalysis [3,4], magnetic data storage [5], and photonics [6,7]. As the intrinsic properties of the nanoparticle are strongly dependent on its dimension, a great deal of the nanoparticle research has been focused on control of particle size and shape. Due to the ∗ Corresponding author. Tel.: +82 2 2123 2832; fax: +82 2 312 5375.
E-mail address: [email protected] (M. Lee). c 2008 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2008.01.022
658
H. Shin et al. / Superlattices and Microstructures 44 (2008) 657–663
Table 1 Experimental conditions used to synthesize Ag nanoparticles AgNO3 concentration (M)
PVP concentration (M)
Molar ratio (AgNO3 :PVP)
Reaction temperature (◦ C)
Reaction time (min)
Average particle size (nm)
0.5 0.5 0.8
1.125 1.125 1.441
1:6 1:6 1:45
160 150 150
30 30 30
200 120 50
substantial progress made in recent years, many metals can now be processed into monodisperse particles with controllable size, shape and structure [8–11]. A fundamental challenge is to control the spatial distribution of nanoparticles and fabricate regular geometric patterns on solid substrates. Various approaches have been proposed, such as patterning by photo or e-beam lithography [12–14], template-directed assembly [15,16], microcontact printing [17,18], thermal patterning of self-assembled particles [19], and pure self-assembly [20,21]. Patterning methods developed up to now require sacrifice of either the simplicity of process or the controllability of pattern geometry and period. Here, we describe an optical approach to nanoparticle patterning which is based on the pulsed laser-induced selective desorption. It is demonstrated that Ag nanoparticle Langmuir–Blodgett (LB) films can be patterned into well-aligned 1D and 2D arrays by a spatial modulation of the pulsed Nd:YAG laser beam without pre-defined templates. The effects of pulse duration, exposure time, and particle size on this laser-induced selective desorption process are discussed. 2. Nanoparticle synthesis and LB film deposition Ag nanoparticles of different sizes (average sizes of 50, 120, and 200 nm) were synthesized by a modified polyol process [8]. Silver nitrate (AgNO3 ) and polyvinylpyrolidone (PVP, Mw ∼ 55,000) were used as the precursor and stabilizer, respectively. Ethylene glycol (EG) was used as the solvent and reducing agent. PVP was dissolved in EG at room temperature, and AgNO3 was also dissolved in EG. The PVP–EG solution was heated inside a 3-necked flask at a constant rate of 3 ◦ C min−1 , and AgNO3 –EG solution was subsequently injected into the flask. The entire solution was maintained at 150–160 ◦ C for 30 min, while being stirred. After cooling to room temperature, the solution was centrifuged several times with a large volume of acetone to separate particles from the solvent and salts. Table 1 shows the experimental conditions used to synthesize Ag nanoparticles. The particle size was measured by scanning electron microscopy (SEM). Monolayered Ag films were deposited on glass substrates by the Langmuir–Blodgett (LB) method, as depicted in Fig. 1(a). The synthesized nanoparticles (about 1 g) were dispersed in 95 ml chloroform and 5 ml ethanol. A small volume of the nanoparticle solution (typically, 2 to 3 ml) was spread onto a DI water surface in a LB mini-trough (380 mm × 90 mm). Before spreading the solution, a slide glass substrate (Menzel-Glaser, Germany) was dipped into the water subphase (the substrate has hydrophilic surfaces with a contact angle of nearly zero and there is no need for any surface treatment). When a uniform film was formed on the subphase (approximately 30 min after particle spreading), the substrate was pulled up while maintaining a constant surface pressure. As shown in Fig. 1(b), the monolayer began to collapse when the surface pressure was above 20 mN/m. Thus, the pressure was kept constant in the range of 10–15 mN/m during the LB transfer. Monolayer films of uniform density were transferred to
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Fig. 1. (a) Mechanism of LB film deposition. (b) Surface pressure vs. area. (c) XRD pattern from an LB film.
Fig. 2. Video capture images showing nanoparticle desorption by a nanosecond Nd:YAG laser beam during the LB transfer onto a slide glass. During the LB transfer, Ag films are deposited on both sides of the substrate and thus Ag particles are removed from both surfaces by the normally incident laser beam. Since the images shown here were taken obliquely at an angle of about 45◦ with the direction of the laser beam, the particle-free lines appear to have a shadow.
both sides of the substrate. X-ray diffraction pattern from an LB film showed that the synthesized nanoparticles are crystalline silver (Fig. 1(c)). 3. Experimental results and discussion We have observed that Ag nanoparticles can be selectively desorbed from the substrate with illumination by a pulsed laser. Fig. 2 is the video capture images showing nanoparticle desorption by a nanosecond Nd:YAG laser beam (λ = 1064 nm, pulse width = 10 ns, repetition rate = 10 Hz, maximum average power = 3.5 W) during the LB transfer onto a slide glass. A laser beam of Gaussian profile slightly defocused on the film (particle size = 120 nm) during the LB transfer generated a transparent line in the film. There were only a few particles observed within the line, while the particle density outside it remained little changed. The width of this particle-free line increased with increasing laser power. This indicates that the desorption of Ag nanoparticles is a step-wise function of the pulse energy density. The desorption of Ag nanoparticles can be explained by the laser-induced thermal desorption (LITD), which emerged as an effective tool for the surface cleaning of microelectronic devices [22–24]. Particles adhered to the surface can be desorbed when the thermo-elastic force, caused by rapid thermal expansion of the particle and/or the substrate resulting from a pulsed laser irradiation, exceeds the adhesive force (predominantly van der Waals force) between the particle and the substrate [22–24]. When the same type of film (monolayer consisting of 120 nm-sized particles) was exposed to a millisecond Nd:YAG laser
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Fig. 3. (a) Setup for Ag nanoparticle patterning by pulsed laser interference. (b) Patterning mechanism. (c) Optical micrograph of a stripe pattern (period = 8.7 µm) generated by two-beam interference. Pulse energy density is 0.45 J/cm2 . Scale bar is 20 µm. (d) Pattern with the same period as “(c)” but with wider stripes. The stripe width could be controlled by tuning up the pulse energy. Pulse energy density is 0.3 J/cm2 . Scale bar is 20 µm. (e) 2D pattern of honeycomb structure obtained by three-beam interference. A trigonal pyramid shaped-prism was used. Scale bar is 20 µm. The inset shows SEM image of a honeycomb cell (scale bar is 5 µm). LB films used in the patterning consist of 120 nm-sized particles.
pulse (pulse width = 1 ms, repetition rate = 10 Hz, maximum average power = 10 W), melting was dominant over desorption. The effect of pulse width is discussed later. The desorption behavior was little influenced by the total exposure time, i.e., the number of pulse shots. Ag nanoparticles could be removed by a single pulse of 10 ns when the pulse energy density was above a threshold value. The threshold energy density of a pulse was measured to be approximately 0.3 J/cm2 , which is well below the damage threshold of the glass. LB films have been patterned by multi-beam interference, as illustrated in Fig. 3(a). This method does not require any pattern projection system. A 10 ns-Nd:YAG laser beam of Gaussian profile was expanded and then passed through an aperture (5 mm diameter) in order to take the nearly uniform central area. An interference pattern was generated by a refracting prism made of quartz and recorded in the film for 0.1 s. The film was prepared on the backside of a slide glass and the beam was made incident through the front surface. This is to avoid a potential contamination of the prism by desorbed particles. Front-side patterning was also possible in which a bare slide glass was inserted between the prism and the sample. Single-sided samples used in the patterning process were prepared by removing one of the films and then cleaning the surface. When transferred quickly, the LB films remained wet for a minute. The wet films could be easily wiped away. Fig. 3(b) illustrates the patterning mechanism. For LB films consisting of 120 nm and 200 nm particles, well-aligned stripe patterns were fabricated by two-beam interference using a prism of isosceles-triangle shape (Fig. 3(c) and (d)). For a fixed period, the width of individual stripes could be controlled by tuning up the pulse energy. 2D patterns can be generated if threebeams are made to interfere with one another on a plane. A hexagonal pattern of honeycomb structure was fabricated by three-beam interference using a prism of trigonal-pyramid shape, as shown in Fig. 3(e). Particles of 50 nm were agglomerated into irregular-shaped particles
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661
Fig. 4. Patterning of 50 nm-sized Ag nanoparticles. Scale bars of (a) and (b) are 20 µm. Scale bar in (c) is 5 µm.
after patterning (Fig. 4), while there was no significant change to larger ones. It is likely that agglomeration occurs as a result of the surface melting of remaining particles. We make a theoretical investigation of how the particle size and pulse width influence the thermal desorption and agglomeration (or melting) of particles. When a particle is irradiated with a pulse of energy density J (energy/area), the temperature rise in the particle is then expressed as 1Ta =
3J 1 ∝ , 4ρdC P d
(1)
where d, ρ, and C P are the diameter, density, and specific heat of the particle, respectively. The subscript “a” is to indicate the actual temperature rise in the particle. Considering a very short penetration depth of the near-infrared wave into the metal, the laser energy will be originally absorbed in the near-surface area of the particle, i.e., the surface facing the substrate in case of the back-side patterning. The thermal diffusion lengths of metals for 10 ns are on the order of micrometers [25], much larger than the particle sizes in question. Thus, the top surface of the particle is also heated in the duration of a single pulse, although the actual temperature rise depends on the vertical position within the particle. We neglect a heat loss into the air and consider that the absorbed laser energy is completely used up in heating the particle. Then, the mean thermal expansion (1l) of the particle due to the mean temperature rise (1T ) is given by 1l = αd1T where α is the thermal expansion coefficient. Consequently, the thermo-elastic force exerted on the particle is given by F = ma = ρ
π d 3 1l , 6 t P2
(2)
where t P is the pulse width and the acceleration (a) resulting from the thermal expansion is approximated as a = 1l . The predominant adhesive force between a spherical particle and a flat t2 p
dry substrate is the Van der Waals force given by [26] hd hda2 + , (3) 16π z 2 32π z 3 where h, da and z are the Lifshitz–Van-der Waals constant, the diameter of the contact surface area, and the atomic separation between the bottom surface of the particle and the substrate surface, respectively. Assuming that there is no surface deformation at the point contact surface, the second term on the right-hand side of Eq. (3) can be neglected. Particle desorption can occur when the thermo-elastic force exceeds the adhesive force (Van der Waal force) between the particle and the substrate surface. The temperature increase required for particle desorption is FV dW =
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H. Shin et al. / Superlattices and Microstructures 44 (2008) 657–663
Fig. 5. Simplified schematic of the expected temperature variation of particles located near the pattern edge. Tm is the melting temperature. Tr , and Ta represent the actual temperature variation and temperature variation required for desorption, respectively. t p is the pulse width.
then given by the following relation. F =ρ
π d 4 α1Tr hd = . 2 6 16π z 2 tP
(4)
For a fixed pulse width, 1 . (5) d3 The subscript “r ” here denotes the required temperature increase in comparison to the actual increase. We have observed that 120 nm-sized Ag particles could be desorbed by a nanosecond pulse but were melted by a millisecond pulse. As given in Eq. (2), the thermo-elastic force is proportional to how fast the temperature rises. This means that the rate of temperature increase should be above a specific rate in order to desorb the particles. When the total energy of a pulse is fixed, the temperature rise in a particle gets faster as the pulse width becomes shorter. The temperature rise with a millisecond pulse might be too slow to give a thermo-elastic force required for particle desorption. However, particles could reach the melting temperature with a millisecond pulse because melting is determined by how high the temperature rises, i.e., the total energy of a pulse. In addition, nanoparticles have a much lower melting temperature than the bulk material. We now discuss the effect of particle size. When the energy and width of a pulse are fixed, the actual temperature rise (1Ta ) in the particle is proportional to d1 . However, the temperature rise (1Tr ) required for thermal desorption is proportional to d13 . Although both 1Ta and 1Tr increase with decreasing particle size, 1Tr is more rapidly changing with particle size. Thus, higher pulse energy will be required to desorb smaller particles for a given pulse width. Fig. 5 illustrates a simplified schematic of how the temperatures of particles change when they are patterned by two-beam interference. Regardless of the particle size, particles near the pattern edge (marked with an arrow) have 1Ta lower than 1Tr because they remains undesorbed after patterning. Large particles might have 1Ta insufficient to reach the melting temperature at the end of pulse incidence, as depicted in Fig. 5(a). As the particle size decreases, 1Ta increases and the melting temperature is lowered. Thus, particles below a specific size (located near the pattern edge) may reach the melting temperature at the end of pulse incidence, giving rise to an 1Tr ∝
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agglomeration (Fig. 5(b)). It seems that the use of a shorter pulse (less than 10 ns) will enable patterning of 50 nm-sized Ag particles without melting or agglomeration because it allows pulse energy reduction without sacrifice of the thermo-elastic force. 4. Conclusion We have proposed an optical approach to the nanoparticle patterning which is based on pulsed laser-induced thermal desorption. Monolayered Ag-nanoparticle films were deposited on glass substrates by the Langmuir–Blodgett (LB) technique. The nanoparticles could be quickly desorbed from the substrate without causing a surface damage, when exposed to a pulsed Nd:YAG laser (1064 nm, 10 ns, 10 Hz) with an energy density over a threshold value. This made it possible to obtain well-aligned patterned arrays simply by a spatial modulation of the pulse energy density. Theoretical investigation predicted that as the particle size decreases, a shorter laser pulse is required to obtain patterned nanoparticle strictures without melting or agglomeration. Acknowledgement This research was supported by a grant from the Korea Research Foundation (Grant number: KRF 2005-8-1391). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
R. Sheila, et al., Science 294 (2001) 137. D. Maxwell, J. Taylor, S. Nie, J. Amer. Chem. Soc. 124 (2002) 9696. L. Lewis, Chem. Rev. 93 (1993) 2693. R. Scott, O. Wilson, R. Crooks, J. Phys. Chem. B 109 (2005) 692. S. Sun, C. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. S. Maier, et al., Adv. Mater. 13 (2001) 1501. P. Kamat, J. Phys. Chem. B 106 (2002) 7729. Y. Sun, Y. Xia, Science 298 (2002) 2176. T. Ahmadi, et al., Science 272 (1996) 1924. X. Peng, et al., Nature 404 (2000) 59. R. Jin, et al., Science 294 (2001) 1901. F. Hua, J. Shi, Y. Lvov, T. Cui, Nano Lett. 2 (2002) 1219. D. Yin, et al., Langmuir 21 (2005) 9352. P. Mendes, et al., Langmuir 20 (2004) 3766. A. van Blaaderen, R. Ruel, P. Wiltzius, Nature 385 (1997) 321. J. Liddle, Y. Cui, P. Alivisatos, J. Vac. Sci. Technol. B 22 (2004) 3409. H. Jacobs, G. Whitesides, Science 291 (2001) 1763. V. Santhanam, et al., Langmuir 19 (2003) 7881. H. Hamann, S. Woods, S. Sun, Nano Lett. 3 (2003) 1643. E. Rabani, et al., Nature 426 (2003) 271. J. Huang, et al., Nature Mater. 4 (2005) 896. M. Arronte, P. Neves, R. Vilar, J. Appl. Phys. 92 (2002) 6973. J. Lee, C. Curran, K. Watkins, Appl. Phys. A 73 (2001) 219. G. Vereecke, E. Rohr, M. Heyns, J. Appl. Phys. 85 (1999) 3837. E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, Oxford, 1992. R.A. Bowling, in: K.L. Mittal (Ed.), Particle on Surfaces. Vol. 1, Plenum Press, New York, 1988.
Langmuir–Blodgett Ag nanoparticle monolayer patterned by pulsed laser-induced selective desorption Hyunkwon Shin a , Hyunjun Kim a , Jeongmin Ha a , Ki-Soo Lim b , Myeongkyu Lee a,∗ a Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-ku,
Seoul 120-749, Republic of Korea b Department of Physics, Chungbuk University, 12 Gaesin-dong, Cheongju 361-763, Republic of Korea
Available online 5 March 2008
Abstract The control of nanoparticle distribution on a solid substrate is an important and challenging issue in the development of novel nanostructured devices. Here we present a simple optical method for obtaining regular geometric nanoparticle arrangements. Monolayered Ag-nanoparticle films were deposited on glass substrates by the Langmuir–Blodgett (LB) technique. The nanoparticles could be quickly desorbed from the substrate without causing a surface damage, when exposed to pulsed Nd:YAG laser with an energy density over a threshold value. This made it possible to obtain well-aligned patterned arrays simply by a spatial modulation of the pulse energy density. The effects of pulse duration, exposure time, and particle size on this laser-induced selective desorption process are discussed. c 2008 Elsevier Ltd. All rights reserved.
Keywords: Nanoparticle; Patterning; Langmuir–Blodgett film; Pulsed laser
1. Introduction Metal nanoparticles can serve as attractive building blocks for novel devices in such areas as biological sensing [1,2], catalysis [3,4], magnetic data storage [5], and photonics [6,7]. As the intrinsic properties of the nanoparticle are strongly dependent on its dimension, a great deal of the nanoparticle research has been focused on control of particle size and shape. Due to the ∗ Corresponding author. Tel.: +82 2 2123 2832; fax: +82 2 312 5375.
E-mail address: [email protected] (M. Lee). c 2008 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2008.01.022
658
H. Shin et al. / Superlattices and Microstructures 44 (2008) 657–663
Table 1 Experimental conditions used to synthesize Ag nanoparticles AgNO3 concentration (M)
PVP concentration (M)
Molar ratio (AgNO3 :PVP)
Reaction temperature (◦ C)
Reaction time (min)
Average particle size (nm)
0.5 0.5 0.8
1.125 1.125 1.441
1:6 1:6 1:45
160 150 150
30 30 30
200 120 50
substantial progress made in recent years, many metals can now be processed into monodisperse particles with controllable size, shape and structure [8–11]. A fundamental challenge is to control the spatial distribution of nanoparticles and fabricate regular geometric patterns on solid substrates. Various approaches have been proposed, such as patterning by photo or e-beam lithography [12–14], template-directed assembly [15,16], microcontact printing [17,18], thermal patterning of self-assembled particles [19], and pure self-assembly [20,21]. Patterning methods developed up to now require sacrifice of either the simplicity of process or the controllability of pattern geometry and period. Here, we describe an optical approach to nanoparticle patterning which is based on the pulsed laser-induced selective desorption. It is demonstrated that Ag nanoparticle Langmuir–Blodgett (LB) films can be patterned into well-aligned 1D and 2D arrays by a spatial modulation of the pulsed Nd:YAG laser beam without pre-defined templates. The effects of pulse duration, exposure time, and particle size on this laser-induced selective desorption process are discussed. 2. Nanoparticle synthesis and LB film deposition Ag nanoparticles of different sizes (average sizes of 50, 120, and 200 nm) were synthesized by a modified polyol process [8]. Silver nitrate (AgNO3 ) and polyvinylpyrolidone (PVP, Mw ∼ 55,000) were used as the precursor and stabilizer, respectively. Ethylene glycol (EG) was used as the solvent and reducing agent. PVP was dissolved in EG at room temperature, and AgNO3 was also dissolved in EG. The PVP–EG solution was heated inside a 3-necked flask at a constant rate of 3 ◦ C min−1 , and AgNO3 –EG solution was subsequently injected into the flask. The entire solution was maintained at 150–160 ◦ C for 30 min, while being stirred. After cooling to room temperature, the solution was centrifuged several times with a large volume of acetone to separate particles from the solvent and salts. Table 1 shows the experimental conditions used to synthesize Ag nanoparticles. The particle size was measured by scanning electron microscopy (SEM). Monolayered Ag films were deposited on glass substrates by the Langmuir–Blodgett (LB) method, as depicted in Fig. 1(a). The synthesized nanoparticles (about 1 g) were dispersed in 95 ml chloroform and 5 ml ethanol. A small volume of the nanoparticle solution (typically, 2 to 3 ml) was spread onto a DI water surface in a LB mini-trough (380 mm × 90 mm). Before spreading the solution, a slide glass substrate (Menzel-Glaser, Germany) was dipped into the water subphase (the substrate has hydrophilic surfaces with a contact angle of nearly zero and there is no need for any surface treatment). When a uniform film was formed on the subphase (approximately 30 min after particle spreading), the substrate was pulled up while maintaining a constant surface pressure. As shown in Fig. 1(b), the monolayer began to collapse when the surface pressure was above 20 mN/m. Thus, the pressure was kept constant in the range of 10–15 mN/m during the LB transfer. Monolayer films of uniform density were transferred to
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659
Fig. 1. (a) Mechanism of LB film deposition. (b) Surface pressure vs. area. (c) XRD pattern from an LB film.
Fig. 2. Video capture images showing nanoparticle desorption by a nanosecond Nd:YAG laser beam during the LB transfer onto a slide glass. During the LB transfer, Ag films are deposited on both sides of the substrate and thus Ag particles are removed from both surfaces by the normally incident laser beam. Since the images shown here were taken obliquely at an angle of about 45◦ with the direction of the laser beam, the particle-free lines appear to have a shadow.
both sides of the substrate. X-ray diffraction pattern from an LB film showed that the synthesized nanoparticles are crystalline silver (Fig. 1(c)). 3. Experimental results and discussion We have observed that Ag nanoparticles can be selectively desorbed from the substrate with illumination by a pulsed laser. Fig. 2 is the video capture images showing nanoparticle desorption by a nanosecond Nd:YAG laser beam (λ = 1064 nm, pulse width = 10 ns, repetition rate = 10 Hz, maximum average power = 3.5 W) during the LB transfer onto a slide glass. A laser beam of Gaussian profile slightly defocused on the film (particle size = 120 nm) during the LB transfer generated a transparent line in the film. There were only a few particles observed within the line, while the particle density outside it remained little changed. The width of this particle-free line increased with increasing laser power. This indicates that the desorption of Ag nanoparticles is a step-wise function of the pulse energy density. The desorption of Ag nanoparticles can be explained by the laser-induced thermal desorption (LITD), which emerged as an effective tool for the surface cleaning of microelectronic devices [22–24]. Particles adhered to the surface can be desorbed when the thermo-elastic force, caused by rapid thermal expansion of the particle and/or the substrate resulting from a pulsed laser irradiation, exceeds the adhesive force (predominantly van der Waals force) between the particle and the substrate [22–24]. When the same type of film (monolayer consisting of 120 nm-sized particles) was exposed to a millisecond Nd:YAG laser
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H. Shin et al. / Superlattices and Microstructures 44 (2008) 657–663
Fig. 3. (a) Setup for Ag nanoparticle patterning by pulsed laser interference. (b) Patterning mechanism. (c) Optical micrograph of a stripe pattern (period = 8.7 µm) generated by two-beam interference. Pulse energy density is 0.45 J/cm2 . Scale bar is 20 µm. (d) Pattern with the same period as “(c)” but with wider stripes. The stripe width could be controlled by tuning up the pulse energy. Pulse energy density is 0.3 J/cm2 . Scale bar is 20 µm. (e) 2D pattern of honeycomb structure obtained by three-beam interference. A trigonal pyramid shaped-prism was used. Scale bar is 20 µm. The inset shows SEM image of a honeycomb cell (scale bar is 5 µm). LB films used in the patterning consist of 120 nm-sized particles.
pulse (pulse width = 1 ms, repetition rate = 10 Hz, maximum average power = 10 W), melting was dominant over desorption. The effect of pulse width is discussed later. The desorption behavior was little influenced by the total exposure time, i.e., the number of pulse shots. Ag nanoparticles could be removed by a single pulse of 10 ns when the pulse energy density was above a threshold value. The threshold energy density of a pulse was measured to be approximately 0.3 J/cm2 , which is well below the damage threshold of the glass. LB films have been patterned by multi-beam interference, as illustrated in Fig. 3(a). This method does not require any pattern projection system. A 10 ns-Nd:YAG laser beam of Gaussian profile was expanded and then passed through an aperture (5 mm diameter) in order to take the nearly uniform central area. An interference pattern was generated by a refracting prism made of quartz and recorded in the film for 0.1 s. The film was prepared on the backside of a slide glass and the beam was made incident through the front surface. This is to avoid a potential contamination of the prism by desorbed particles. Front-side patterning was also possible in which a bare slide glass was inserted between the prism and the sample. Single-sided samples used in the patterning process were prepared by removing one of the films and then cleaning the surface. When transferred quickly, the LB films remained wet for a minute. The wet films could be easily wiped away. Fig. 3(b) illustrates the patterning mechanism. For LB films consisting of 120 nm and 200 nm particles, well-aligned stripe patterns were fabricated by two-beam interference using a prism of isosceles-triangle shape (Fig. 3(c) and (d)). For a fixed period, the width of individual stripes could be controlled by tuning up the pulse energy. 2D patterns can be generated if threebeams are made to interfere with one another on a plane. A hexagonal pattern of honeycomb structure was fabricated by three-beam interference using a prism of trigonal-pyramid shape, as shown in Fig. 3(e). Particles of 50 nm were agglomerated into irregular-shaped particles
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661
Fig. 4. Patterning of 50 nm-sized Ag nanoparticles. Scale bars of (a) and (b) are 20 µm. Scale bar in (c) is 5 µm.
after patterning (Fig. 4), while there was no significant change to larger ones. It is likely that agglomeration occurs as a result of the surface melting of remaining particles. We make a theoretical investigation of how the particle size and pulse width influence the thermal desorption and agglomeration (or melting) of particles. When a particle is irradiated with a pulse of energy density J (energy/area), the temperature rise in the particle is then expressed as 1Ta =
3J 1 ∝ , 4ρdC P d
(1)
where d, ρ, and C P are the diameter, density, and specific heat of the particle, respectively. The subscript “a” is to indicate the actual temperature rise in the particle. Considering a very short penetration depth of the near-infrared wave into the metal, the laser energy will be originally absorbed in the near-surface area of the particle, i.e., the surface facing the substrate in case of the back-side patterning. The thermal diffusion lengths of metals for 10 ns are on the order of micrometers [25], much larger than the particle sizes in question. Thus, the top surface of the particle is also heated in the duration of a single pulse, although the actual temperature rise depends on the vertical position within the particle. We neglect a heat loss into the air and consider that the absorbed laser energy is completely used up in heating the particle. Then, the mean thermal expansion (1l) of the particle due to the mean temperature rise (1T ) is given by 1l = αd1T where α is the thermal expansion coefficient. Consequently, the thermo-elastic force exerted on the particle is given by F = ma = ρ
π d 3 1l , 6 t P2
(2)
where t P is the pulse width and the acceleration (a) resulting from the thermal expansion is approximated as a = 1l . The predominant adhesive force between a spherical particle and a flat t2 p
dry substrate is the Van der Waals force given by [26] hd hda2 + , (3) 16π z 2 32π z 3 where h, da and z are the Lifshitz–Van-der Waals constant, the diameter of the contact surface area, and the atomic separation between the bottom surface of the particle and the substrate surface, respectively. Assuming that there is no surface deformation at the point contact surface, the second term on the right-hand side of Eq. (3) can be neglected. Particle desorption can occur when the thermo-elastic force exceeds the adhesive force (Van der Waal force) between the particle and the substrate surface. The temperature increase required for particle desorption is FV dW =
662
H. Shin et al. / Superlattices and Microstructures 44 (2008) 657–663
Fig. 5. Simplified schematic of the expected temperature variation of particles located near the pattern edge. Tm is the melting temperature. Tr , and Ta represent the actual temperature variation and temperature variation required for desorption, respectively. t p is the pulse width.
then given by the following relation. F =ρ
π d 4 α1Tr hd = . 2 6 16π z 2 tP
(4)
For a fixed pulse width, 1 . (5) d3 The subscript “r ” here denotes the required temperature increase in comparison to the actual increase. We have observed that 120 nm-sized Ag particles could be desorbed by a nanosecond pulse but were melted by a millisecond pulse. As given in Eq. (2), the thermo-elastic force is proportional to how fast the temperature rises. This means that the rate of temperature increase should be above a specific rate in order to desorb the particles. When the total energy of a pulse is fixed, the temperature rise in a particle gets faster as the pulse width becomes shorter. The temperature rise with a millisecond pulse might be too slow to give a thermo-elastic force required for particle desorption. However, particles could reach the melting temperature with a millisecond pulse because melting is determined by how high the temperature rises, i.e., the total energy of a pulse. In addition, nanoparticles have a much lower melting temperature than the bulk material. We now discuss the effect of particle size. When the energy and width of a pulse are fixed, the actual temperature rise (1Ta ) in the particle is proportional to d1 . However, the temperature rise (1Tr ) required for thermal desorption is proportional to d13 . Although both 1Ta and 1Tr increase with decreasing particle size, 1Tr is more rapidly changing with particle size. Thus, higher pulse energy will be required to desorb smaller particles for a given pulse width. Fig. 5 illustrates a simplified schematic of how the temperatures of particles change when they are patterned by two-beam interference. Regardless of the particle size, particles near the pattern edge (marked with an arrow) have 1Ta lower than 1Tr because they remains undesorbed after patterning. Large particles might have 1Ta insufficient to reach the melting temperature at the end of pulse incidence, as depicted in Fig. 5(a). As the particle size decreases, 1Ta increases and the melting temperature is lowered. Thus, particles below a specific size (located near the pattern edge) may reach the melting temperature at the end of pulse incidence, giving rise to an 1Tr ∝
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agglomeration (Fig. 5(b)). It seems that the use of a shorter pulse (less than 10 ns) will enable patterning of 50 nm-sized Ag particles without melting or agglomeration because it allows pulse energy reduction without sacrifice of the thermo-elastic force. 4. Conclusion We have proposed an optical approach to the nanoparticle patterning which is based on pulsed laser-induced thermal desorption. Monolayered Ag-nanoparticle films were deposited on glass substrates by the Langmuir–Blodgett (LB) technique. The nanoparticles could be quickly desorbed from the substrate without causing a surface damage, when exposed to a pulsed Nd:YAG laser (1064 nm, 10 ns, 10 Hz) with an energy density over a threshold value. This made it possible to obtain well-aligned patterned arrays simply by a spatial modulation of the pulse energy density. Theoretical investigation predicted that as the particle size decreases, a shorter laser pulse is required to obtain patterned nanoparticle strictures without melting or agglomeration. Acknowledgement This research was supported by a grant from the Korea Research Foundation (Grant number: KRF 2005-8-1391). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
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