Different Methods of Increasing the Mechanical Strength of Gold Nanocages Mahmoud A. Mahmoud, Paul Szymanski, and Mostafa A. El-Sayed* Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States * S Supporting Information ABSTRACT: Using the ultrafast coherent modulation of the surface plasmon band intensity with the totally symmetric lattice vibration of gold nanocages, we were able to determine and use their frequencies as a measure of the cage’s mechanical stability. The presence of an inner “stiff” transition-metal nanoshell with a higher value of the elastic modulus is found to increase the frequency of the lattice vibration of the outer soft gold nanoshell. This could also explain the observed increase in both the gold lattice vibrational frequency as well as the lattice vibration relaxation time in the Au-Pt and Au-Pd double-shell nanocages. It is also found that when these nanoparticles are assembled into monolayers on quartz substrates by the Langmuir-Blodgett technique, the oscillation frequency of the gold shell with the transition metal having the largest elastic constant suffers the least change in its oscillation frequency as a result of its resistance to distortion as a result of binding to the substrate. SECTION: Physical Processes in Nanomaterials and Nanostructures P lasmonic noble-metal nanostructures attract much interest because of their unique optical, 1-3 photothermal, photo- electromagnetic, 4 and photoacoustic 5 properties, which result from their large scattering and absorption cross sections due to their localized surface plasmon resonances (LSPRs). 1-3 The above properties are tunable by changing the shape, size, and the dielectric function of the medium and the composition of the nanoparticle. Different shapes have been prepared such as spheres, 6 cubes, 7-9 nanocages, 7,8 rods, 10,11 stars, 12 triangles, 13 shells, 14 rings, 15 and frames 16,17 having LSPR spectra that cover the visible and near-IR regions. Gold nanocages have many useful optical, 1-3 sensing, 18 photothermal, 19 and catalytic properties. 20,21 As hollow nano- particles, their mechanical stability is an important factor in determining their practical usefulness for future applications. The changes in the vibrational motions of nanoparticles can be used to follow the changes in their mechanical stability. The vibrational motions of metal nanoparticles cause periodic instantaneous changes in size and/or shape, which modulate the wavelength of the LSPR band. 22,23 Thus, the intensity of this band at a specific wavelength is modulated at a frequency corresponding to the lattice vibrational frequency of the nanoparticle. Totally symmetric vibrations of the lattice usually give the largest modulation of the electronic volume of the plasmonic nanoparticle and thus the largest modulation of the wavelength of the surface plasmon spectrum. This makes it possible to determine the symmetric lattice vibrations of plasmonic nanostructures in the optical region. 24,25 These measurements have been carried out on a wide variety of nanostructures in both ensemble 9,15,24-33 and single-par- ticle 26,34-39 studies. The dependence of the nanoparticle vibrational frequencies and the damping and dephasing of vibrational excitation on the nanoparticle composition and shape serve as important fundamental probes of the mechanical properties on the nanoscale. 26,28,31,32,40-42 Recently, a study has been published on Au nanorods coated with Pd. 32 A change in the mean elastic properties and particle size compared to those of uncoated nanorods affected the rod’s vibrational frequency. 32 An experimental and theoretical study has been published 31 comparing Au nanocages with Au-Ag nanoboxes, with both at multiple sizes. It was concluded that the elastic constants of metals do not change on the nanoscale. 31 Recently, we have synthesized hollow double-shell nano- particles made of two different metals in contact. 21,43,44 On the basis of the Au nanocage structure, an inner shell of Pt or Pd is formed, while the outer shell remains Au. In the present work, we present the results of studying the modulation of the gold plasmonic band intensity at a specific wavelength to determine the perturbation of the lattice vibrational frequency of the outer gold nanoshell by the stiffer inner Pt or Pd nanoshell. Our previously observed 21,43,44 high-resolution TEM and STEM coupled to X-ray energy dispersive microanalysis images showed that alloying processes have occurred between the two shells at their joint interfaces. We found that the outer gold lattice oscillation increases in frequency in proportion to the elastic constant of the transition metal inside of the nanoshell. Inner shells result in increasing the symmetric breathing lattice vibrational frequency of the outer soft gold nanoshell. In Received: September 24, 2012 Accepted: November 15, 2012 Published: November 15, 2012 Letter pubs.acs.org/JPCL © 2012 American Chemical Society 3527 dx.doi.org/10.1021/jz301503z | J. Phys. Chem. Lett. 2012, 3, 3527-3531