Elongation of gold nanoparticles by swift heavy ion irradiation: Surface plasmon resonance shift dependence on the electronic stopping power

Abstract

Gold nanoparticles embedded in a silica matrix were irradiated with 2 to 40 MeV Cu or Si ions at fluences ranging from 1 × 10¹³ to 4 × 10¹⁵ ions/cm², and their deformation from spheres to prolate ellipsoids with major axis parallel to the ion beam was studied using P and S polarized light. For fixed ion energy, the longitudinal surface plasmon resonance (SPR) at 520 nm is red-shifted with an increase of the ion fluence up to a certain value where it reaches a plateau indicating that a maximum aspect ratio is obtained. This saturation in the wavelength shift was found to depend on the ion energy and reaches a maximum of 40 nm. The SPR shift was also used to measure the electronic stopping power dependent deformation rate and to deduce the electronic stopping power threshold of (1.9 ± 1.3) keV/nm required for shape transformation of the embedded gold nanoparticles. Ion track diameters of 0.18 to 1.4 nm were inferred from the fluence dependence of the SPR shift. Analysis by transmission electron microscopy shows that large (d > 10 nm) particles are more elongated than smaller ones. Our data are consistent with a mechanism of gold nanoparticle elongation requiring both the silica matrix and the nanoparticles to melt following the passage of the swift heavy ion and with elongation being due to the relief of stress in the gold nanoparticle which had built up as a consequence of the deformation of the surrounding silica matrix.

Introduction

Ion irradiation produces many effects on condensed matter such as anisotropic deformation at constant volume of single colloidal particles [1], [2] and glass foils [3], [4], wafer curvature of thin films constrained on substrates [5], [6], [7], densification [6] and phase transformation [8]. Among these examples, we investigated previously the spherical gold–silica core–shell colloid system [9]. The silica shells were shown to transform into oblate ellipsoids with its minor axis parallel to the ion beam whereas the gold cores deformed into prolate ellipsoids with its major axis parallel to the ion beam. Simply put, the silica shell shrinks in the direction of the ion beam whereas the gold core elongates. In order to go beyond an incomplete description of this process, we have prepared and investigated Au nanoparticles embedded in silica films [10], [11], [12] as it was shown that Au particles deform more efficiently if the surrounding silica shell is thick [9] and not at all when surrounded by a non-deforming crystalline matrix such as AlAs [13]. The mechanism responsible for the deformation of the nanoparticles has not been uniquely identified, but it appears that both anisotropic deformation of the surrounding matrix [1], [9], [14], [15] and melting of the nanoparticle play an essential role [15], [16], [17], [18]. The electronic, magnetic, and optical properties of metallic nanoparticles embedded in a dielectric matrix are very interesting and depend strongly on the nanoparticle size, shape, orientation, interparticle distance, and environment [10], [12]. The fabrication, modification, and characterization of these heterogeneous nanocomposite materials is an active field of research motivated by the potential applications in optical and optoelectronic devices [19], [20], biosensing [21], and in medical applications [22].

When ions are slowed down in condensed matter they deposit a vast amount of energy within a small material volume, during a short time. This energy is lost via two distinct mechanisms: i) elastically by scattering of the projectile on the target nuclei, and ii) inelastically by ionization and electronic excitation of target atoms [3], [23]. Both mechanisms strongly depend on the kinetic energy of the incident ion. For high energy heavy ions, electronic interactions dominate and the ion loses energy at a rate of a few to a few tens of keV per nm. During a short period of time, the system within a small localized region around the ion trajectory is far from equilibrium due to a high density of highly excited electrons. These electrons are initially accelerated towards the center of the ion track and will either undergo scattering or overshoot the track center and end up moving outward while an energy transfer to the atomic network takes place. This energy transfer was explained by two models: the thermal spike model [24], [25], [26] and the ion spike model, also known as Coulomb explosion [27], [28]. In the first model, the energy is thermalized by electron–phonon coupling and leads to the increase of local temperature. In the second one, kinetic energy of electrons allows them to move far from the ion track leaving behind a cylinder of positively charged ions. The mutual electric repulsive forces between the positive ions lead to ionic Coulomb explosion. Hybrid models have also been proposed [29], [30] and validated by simulations [31].

The deposited energy during ion irradiation can affect the crystalline structure, the morphology and other properties of irradiated materials. In particular, amorphous targets can suffer macroscopic deformation, known as anisotropic growth or ion hammering, whereby a thin film becomes thinner and wider upon ion irradiation. This plastic deformation process has been described by a viscoelastic model [32], [33] and a hammering model [34], [35]. In the first model, the phenomenon is attributed to energy loss and it occurs efficiently at high electronic stopping powers. For intense electronic excitations a cylindrical region around the ion trajectory is heated and the shear stresses induced by the thermal dilatation are relaxed. The residual strains freeze-in upon cooling down. In the second one, a computer simulation of relaxation of mechanically polarized material is used to estimate the plastic flow. The viscoelastic model and ion hammering explain the deformation of amorphous colloids but not the resistance to deformation of crystalline ones. Other models were also proposed to explain material plastic deformation like: Ostwald ripening and creep deformation [36], and generation–relaxation of stress [6].

The viscoelastic model does not explain the deformation of core–shell colloids or nanoparticles embedded in an amorphous planar matrix. However, from the earliest observation of nanoparticle deformation it has been surmised that the visco-elastic response of the matrix plays an important role [1], [9], [14], [15] and thermal spike models involving melting of the nanoparticle have been invoked as well [15], [16], [17], [18]. In the current paper, we present experimental results that shed more light on possible physical deformation mechanisms of gold nanoparticles embedded in a silica matrix. We will show by modeling the SPR spectral position that the anisotropic deformation induced by ion irradiation can be controlled by the fluence, the energy and the nature of the ion beam. We will focus our attention on the role of the electronic stopping power in the modification of the optical and structural properties of gold/dielectric nanocomposite films.