8934
J. Phys. Chem. C 2007, 111, 8934-8941
Dependence of the Threshold Energy of Femtosecond Laser Ejection of Gold Nanoprisms from Quartz Substrates on the Nanoparticle Environment† Christopher Tabor, Wei Qian, and Mostafa A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: January 12, 2007; In Final Form: February 24, 2007
Recently, it was reported that gold nanoprisms in a monolayer array on a quartz substrate were ejected in air when irradiated with femtosecond laser pulses near their surface plasmon absorption maximum. It was deduced from the measured reduction in particle thickness upon irradiation that the ejection mechanism involved ablation of surface atoms from the gold particle, which generates an intense pressure at the particle-substrate interface. The present study reports on this phenomenon when the substrate-bound nanoparticle is immersed in a liquid environment. In this system, it is found that the nanoparticle ejection requires less than one tenth the energy required if the system was irradiated in air. The ejected nanoparticle is also found to increase in thickness instead of the decrease observed in air. These results suggest another photoinitiated ejection mechanism, different from surface ablation, when the particles are surrounded by a liquid environment. From this and other spectroscopic and microscopic results on the ejected nanoprisms, we suggest a mechanism that involves energy transfer from the photoexcited nanoprism to the solvent within cavities and defects at the particlesubstrate interface. The hot-solvent molecules result in an intense pressure at the particle-substrate interface, resulting in particle ejection. Ejection is proposed to consist of two processes, namely nanoparticle-substrate dissociation and nanoparticle solvation and diffusion away from the substrate. These two processes have independently been studied as a function of solvent property.
I. Introduction Directed colloidal nanoparticle synthesis was reported in the early 1950s1 in which metallic ions are reduced to form small clusters of metal particles. Particle growth is controlled by use of a capping agent that binds to the particle surface. Investigation of this process has continued over the past 50 years to produce a wealth of particles by varying the identity and concentration of the metal salt, reducing agent, and/or surfactant. These studies have produced an increasingly large amount of nanomaterials with a range of shapes and sizes, such as cubes,2-4 prisms,5-7 rods,5,8,9 and shells.10,11 One of the most useful and intriguing properties of plasmonic nanoparticles (gold and silver) is their ability to strongly absorb and scatter light at a frequency that is resonant with their surface plasmon oscillation. This ability stems from a coupling of the particle’s conduction band electrons with the electric field of the incident light that oscillate in phase with one another, termed the particle’s localized surface plasmon resonance (LSPR). This resonance frequency is determined by the shape, size, and density of the particle’s electron distribution and the surrounding dielectric environment. The LSPR frequency can selectively be tuned by changing one or both of these parameters.5,8,11-13 These plasmonic colloidal particles can be used in a wide range of applications such as imaging and biomedical therapy,14-16 optical enhancing,17-19 nanorulers,20,21 and catalysis.22-24 One unfortunate disadvantage of the colloidal synthetic method is that it produces nanoparticles with large particle size distribution. Lithographic techniques are known to produce †
Part of the special issue “Kenneth B. Eisenthal Festschrift”. * To whom correspondence should be addressed. E-mail:
[email protected].
nanoparticles with very narrow size distributions (highly monodisperse) on substrates. Some of the nanolithographic techniques used today are electron beam lithography,25,26 extreme UV photolithography,27-29 nanosphere lithography (NSL),30,31 and nanoimprint lithography.32-34 The lithographic scheme is composed of designing a mask overlaying a substrate that is coated with a vaporized metal. The metal adheres to the underlying substrate through interstitial holes in the mask and once the mask is removed leaves a “metallic negative” of the mask image bound to the substrate surface. This technique affords the designer opportunity to create highly uniform nanosystems of nearly any shape, size, and/or pattern while eliminating the need for a stabilizing surfactant agent. Similar plasmonic properties exist with lithographically fabricated particles as they do in colloidal solutions. They offer superior ability over colloids to study fundamental attributes of particles35,36 as well as offering a wealth of applications for plasmonic devices,37,38 optical waveguides,39 and enhanced spectroscopy platforms.40,41 While nanoparticles supported on substrates are useful in these areas of study, nanoparticles in solution are useful in a broader range of applications in the fields of chemistry, nanobiology, and nanomedicine. For this reason, it is advantageous to explore the possibility of fabricating nanoparticles by lithographic techniques and then releasing the particles from the substrates into solution. The undertaking to combine the advantages of both techniques to simultaneously afford the range of monodispersed shapes that lithography offers and the solution specific applications that synthetic techniques display has to date not been fully investigated. Van Duyne et al.42 have successfully transferred silver nanoprisms fabricated by NSL into solution by utilizing the weak interaction between Ag nanoparticles and the fabrication
10.1021/jp070282q CCC: $37.00 © 2007 American Chemical Society Published on Web 03/31/2007
Femtosecond Laser Ejection of Gold Nanoprisms substrate. By first capping the surface of the silver prisms with an alkanethiol surfactant and sonicating them in a solution of ethanol, they were able to achieve the first transmission electron microscopy (TEM) micrograph images of NSL particles in solution as well as absorption spectra of the particles in situ. While this technique is useful, it is restricted to particles that poorly adhere to their substrate, and it is advantageous to possess multiple transfer techniques to choose the appropriate method for the desired application. Previously, nanoparticle ejection of NSL-fabricated nanoprism from a substrate into air was reported to occur with a nanosecond pulse at 532 nm.43 This “jump” was accompanied by particle melting to the most thermodynamically stable spherical shape, proposed to be mechanistically required for the particle to leave the surface. The fast change in shape generates a rapid change in the particle’s center of mass as the tips retract inward, thrusting the particle away from the substrate and rendering a solution of fully melted, spherical particles. In more recent studies, our group has discovered that nanoparticle ejection can be initiated in air by exposing the sample to an ultrafast femtosecond pulse at or near the nanoparticle’s LSPR that results in a decrease in the particle bisector and thickness but with no loss in nanoparticle prismatic shape.45 The mechanism for ultrafast ejection was proposed to result from fast evaporation/ sublimation of gold atoms from the prism’s surface as a pathway to cool the particle in a time domain much faster than external thermalization (e-ph coupling) of the nanoparticle affords. The ablation process cools the particle prior to melting and generates an intense pressure gradient at the particle-substrate interface. This intense pressure is believed to result in a particle-substrate separation and nanoparticle ejection. However, the alternative mechanism of heating the quartz area under the nanoparticle, causing water evaporation, was not eliminated. The present study examines the process of NSL-fabricated Au nanoprism ejection into various liquid media. This femtosecond-pumped nanoparticle ejection is compared with the results in air at lower laser fluencies. Various solvents are used with different intrinsic properties to understand the nature of the nanoparticle coupling with the solvent and its influence on the ejection process. The solvents used span a wide range and have various intrinsic properties, namely, water, a group of three alkane nitriles, and a group of four alkane alcohols. II. Experimental Section Quartz microscope slides from Technical Glass Products were cut into 1 × 1.5 cm pieces. Polystyrene spheres (D ) 450 nm) were purchased from Duke Scientific with a size distribution less than 3%. All other chemicals were used as received. NSL was used to fabricate gold nanoprisms on a quartz substrate. A nanosphere (NS)-mask was created by drop coating a 1:1 solution of aqueous polystyrene spheres (D ) 450 nm) and absolute ethanol onto a glass cover slide, which was suspended on the surface of DDI water. A diluted drop of sodium dodecyl sulfate was used to alter the surface tension of the water and arrange the spheres into a well-ordered, closely packed array on the water’s surface. This array was then transferred to a quartz substrate and allowed to dry. Deposition of gold through the interstitial spaces in the PS mask was done with a PVD75 filament evaporator at a chamber pressure no greater than 5.0 e-6 Torr. Mask removal was accomplished by sonicating the sample ( kT and the particle is sufficiently stabilized in solution. IV. Conclusion In conclusion, photothermal ejection of gold nanoprisms was observed for the first time in a liquid medium, occurring at a
Femtosecond Laser Ejection of Gold Nanoprisms fraction of the energy required for the ejection of nanoparticles in air. Ejected particles in solution were observed to increase in thickness upon irradiation instead of decreasing as ejected particles in air were previously observed to do. This suggested a different ejection mechanism in solution involving the hotsolvent molecules present in the cavities located at the particlesubstrate interface. These hot-solvent molecules result from the transfer of photothermal nanoparticle energy to the solvent molecules upon excitation at the particle’s LSPR. The mechanism was supported by an observed decrease in nanoparticle ejection upon annealing the substrate-bound particles prior to irradiation. Nanoprism ejection was proposed to occur in two separate processes, nanoparticle-substrate dissociation and nanoparticle solvation and diffusion into solution. By studying these two phenomena individually, we were able to qualitatively explain how the solvent properties and the immediate environment of the particle might affect the ejection process. Nanoparticlesubstrate dissociation was not seen to be sensitive to the solvents used, and the energy threshold for dissociation was relatively constant, suggesting little dependency of the dissociation on the solvent. Particle solvation was observed to be extremely sensitive to the solvent properties, namely, the nature of the coupling between the metal and the solvent and the solvent polarizability. Alcohols were suggested to be loosely coupled to the particle through an induced dipole. As their polarizability increased (by increasing the alkane chain length), the solvent molecules preferentially favor one another over the metallic nanoparticle surface, resulting in a decrease in solvation of the nanoparticles and a subsequent increase in the nanoparticle aggregation. Nitriles were suggested to couple to the nanoparticle surface in a fundamentally different way by donating electrons to the particle surface. This anchors the solvent molecule through the nitrile group to the particle surface, functioning as a chemically bound ligand that caps and solvates the particle. Increased length in the alkane chain of the nitriles has little to no effect on the solvation of the nanoparticle. Acknowledgment. This work is supported by the Materials Research Division of the National Science Foundation (No. 0138391). The authors would like to thank Wenyu Huang for valuable discussion on the research presented here. References and Notes (1) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (2) Hu, M.; Petrova, H.; Sekkinen, A. R.; Chen, J.; McLellan, J. M.; Li, Z.-Y.; Marquez, M.; Li, X.; Xia, Y.; Hartland, G. V. J. Phys. Chem. B 2006, 110, 19923. (3) Murphy, C. J. Science (Washington, D.C.) 2002, 298, 2139. (4) Sun, Y.; Xia, Y. Science (Washington, D.C.) 2002, 298, 2176. (5) Ha, T. H.; Koo, H.-J.; Chung, B. H. J. Phys. Chem. C 2007, 111, 1123. (6) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science (Washington, D.C.) 2001, 294, 1901. (7) Shankar, S. S.; Ahmad, A.; Pasricha, R.; Sastry, M. J. Mater. Chem. 2003, 13, 1822. (8) Chang, S.-S.; Shih, C.-W.; Chen, C.-D.; Lai, W.-C.; Wang, C. R. C. Langmuir 1999, 15, 701. (9) Perez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Mulvaney, P. Coord. Chem. ReV. 2005, 249, 1870.
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