J. Phys. Chem. C 2009, 113, 10947–10955
10947
Coherent Acoustic Oscillations of Nanorods Composed of Various Metals J. C. Owrutsky,*,† M. B. Pomfret,† and D. J. Brown‡ Chemistry DiVision, US NaVal Research Laboratory, Washington, D.C. 20375, and Chemistry Department, U.S. NaVal Academy, Annapolis, Maryland 21402 ReceiVed: March 17, 2009; ReVised Manuscript ReceiVed: May 6, 2009
Ultrafast transient absorption spectroscopy has been used to characterize coherent acoustic oscillations for a series of nanorods composed of a variety of metals. The nanorods are produced by electrochemical synthesis in polycarbonate templates with several pore diameters of 25). The measurements extend our previous studies on Au, Ni, and Pd (Sando, G. M.; et al. J. Chem. Phys. 2007, 127, 074705) as well as Al nanorods (Pomfret, M. B.; et al. Chem. Mater. 2008, 20, 5945) to include those composed of Ag, Cu, Pt, Fe, Rh, and Co. The acoustic breathing mode periods measured are consistent with those predicted based on classical elastic theory using the bulk speeds of sound. The phase and damping of the oscillations are discussed in terms of the nanorod structures and excitation mechanisms. Introduction There is intense interest in characterizing and utilizing unique properties of metal nanoparticles as a function of their shape, size, and composition for a variety of applications including sensing, e.g., surface enhanced infrared absorption (SEIRA)1 and surface enhanced Raman scattering (SERS),2-4 photonics, catalysis, and photodynamic therapy.5-8 Studies were initially concentrated on spherical nanoparticles of coinage metals (Au, Ag, Cu),9-11 which have well-resolved surface plasmon bands in the visible or near UV. Work has now expanded to nanostructures with complex shapes and compositions, such as core-shell12,13 and hollow14 structures, single particles,15-18 arrays,19-28 segmented structures,6,29 cubes,17,30 prisms,15,20,21,31 and crescents.32 Nanorods, which can be considered as ellipsoids or cylinders, represent one of the simplest anisotropic structures and offer the opportunity to investigate the effects of reduced symmetry on the properties of nanomaterials.5,6,10,11,13,33 A popular method for producing nanorods and arrays is template synthesis, including electrodeposition in anodized aluminum oxide or polycarbonate (PC) membranes.6,19,28,34-44 This approach is particularly effective for fabricating high aspect ratio and aligned nanorods and it has been used for various metals, alloys, and segmented nanorods. Nanoparticles have unique modes and energy levels for both electronic and vibrational degrees of freedom. For small particles (R , λ) coherent motions of electrons are responsible for localized surface plasmon resonance (SPR) bands. While isotropic nanoparticles of some materials, such as coinage metals, have a single SPR band, nanorods have two bands due to excitation along the short (SPRT, transverse) and long (SPRL, longitudinal) axes. When metal structures are longer than the wavelength, plasmons are not localized but are traveling in nature, so they conform to a dispersion curve rather than a resonance condition. Nanoparticles also possess elastic or acoustic vibration modes with periods on the order of a few to tens of picoseconds in which the period scales with the radius (e.g., for Au and Ag, the period in picoseconds is close to the particle radius in nanometers).45,46 There is a single breathing * Corresponding author. E-mail:
[email protected]. † US Naval Research Laboratory. ‡ U.S. Naval Academy.
mode for spheres, whereas rods have a breathing as well as an extensional mode. The breathing mode is analogous to the Raman breathing mode of carbon nanotubes47 except that the frequencies tend to be lower since the diameters and masses of metal nanoparticles tend to be larger. For nanoparticles, which have low vibrational frequencies, the vibrations are difficult to detect and study by low frequency Raman scattering. They are more commonly observed using ultrafast pump-probe spectroscopy to detect coherent acoustic oscillations (CAOs) as in earlier ultrafast acoustic studies of films.48 Static optical studies are informative for measuring SPR band parameters, such as their positions and damping, and how they depend on nanoparticle properties, which are critical for evaluating their potential in applications such as enhanced spectroscopic sensing.40,49-54 While the band for spherical nanoparticles and the transverse band of nanorods are somewhat sensitive to radius and aspect ratio, some modes, such as for core-shell structures and the longitudinal band of nanorods, provide greater tunability by varying the dimensions of aspect ratio and shell thickness. Time-resolved spectroscopy provides additional information, such as on the acoustic properties. Investigating CAOs provides a method to characterize electron relaxation, electron-phonon coupling, and energy dissipation to the surroundings, including how these phenomena depend on properties of nanoparticles.20,21,30,42,45,46,55-65 Understanding these effects is important for numerous applications, including how energy is transferred to adsorbates in phonon-mediated and hot-electron-mediated photodesorption processes that are more broadly pertinent to surface photochemistry.48 Using template synthesis with commercially available PC membranes, we have produced Au, Ni, Pd, and Al nanorods and then characterized them using ultrafast spectroscopy.42-44 The nanorods produced have high aspect ratios (300) and extending the SPRL band to wavelengths longer than 20 µm, which are not observable with our spectrometer. Mid-IR SPRL bands are seen with shorter deposition times. Also, the it4ip membrane pores have a narrower angular distribution, so they are more parallel and oriented with the long axis along the propagation direction so that the SPRL transition dipole direction is perpendicular to the field of the incident radiation. Furthermore, we did not observe a significant absorption by tilting the membranes, which has been reported for other nanorods in templates. The CAOs are observed using pump-probe spectroscopy. The pump pulse energy is maintained below 1 µJ to avoid modifying the nanorods due to heating. It is possible to photothermally induce changes in nanoparticles. This effect has been studied and used to fabricate or modify nanoparticles.13,33,69 It has also been shown that morphology changes of nanoparticles occur at higher temperatures for ultrafast laser heating than for slow heating (700 vs 250 °C for Au).70 It is possible to estimate the temperature increase for the lattice based on the lattice heating capacity and energy absorbed per nanorod. It is reasonable to assume that all the energy is absorbed since absorbances at 400 nm are g1 for all but the P10. As a result, the estimated temperature increase is 150 °C for a 1 µJ pulse energy with a 225 nm diameter beam at the sample for L ) 3 µm, d ) 70 nm (P30) Ag nanorods. A table of the results of this calculation for all the metals and nanorods is provided in the Supporting Information (Table S1). The temperature increases for the various metals range from 99 to 150 °C. The P10 nanorods have the highest temperature increases (up to ∼225 °C) because they have the least amount of material in the irradiated area. The temperature rises are predicted to be smaller for the I30 and I50 nanorods because they are longer and have a higher pore density. As pointed out by Jerebtsov et al.,71 if the nanorods absorbed predominantly on the end, rather than at enough of an angle for the entire length to be excited, then the energy would be restricted to a region of about the penetration depth, which is
