Letter pubs.acs.org/JPCL
The Effect of Composition, Morphology, and Susceptibility on Nonlinear Light Scattering from Metallic and Dielectric Nanoparticles Grazia Gonella,* Wei Gan,† Bolei Xu, and Hai-Lung Dai Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *
ABSTRACT: To facilitate second-harmonic light scattering as an effective tool for sensing and imaging nanoparticles, a fundamental understanding of how particle properties affect the nonlinear light scattering process is necessary. The angle-resolved second harmonic scattering patterns, measured in various polarization combinations, from spheroidal Ag particles (80 nm in diameter) are presented for the first time and compared with those from similarly sized spherical polystyrene particles adsorbed with nonlinear-optically active malachite green molecules. Comparison of the data with theoretical models is used to determine how optical constants (related to the particle composition), nonlinear susceptibility tensor elements, and shape may affect secondharmonic scattering from nanoparticles. SECTION: Physical Processes in Nanomaterials and Nanostructures
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size. It has been found that for larger particles (micrometer size) there are multiple maxima in the AR-SHS intensity due to contributions in the form of higher multipoles to the SH intensity and that the angle of the maximum intensity is close to the fundamental light propagation direction.24 In contrast, for smaller particles (≲ 100 nm), there is only a single maximum in the SHS intensity at a 90° angle away from the fundamental propagation direction that is related to the emission from the nonlocally excited dipole.22−25 In this Letter, we will show that, in addition to the size, various material properties such as composition, susceptibility, and morphology also affect the SHS behavior of the particles. In particular, we will examine the effect of such properties by comparing AR-SHS results from Ag and MG/PS NPs. In general, the second-order nonlinear polarization for a centrosymmetric system can be described as the sum of a surface and a bulk term. However, as explained below, for both Ag and MG/PS NPs the great majority of the SH signal is generated at the particle surface and, for isotropic surfaces, the independent nonvanishing elements of the second-order surface susceptibility tensor χ⃡S(2) are reduced to χ⊥⊥⊥, χ⊥∥∥, and χ∥⊥∥ (⊥ and ∥ refer to perpendicular and parallel to the surface of the particle, respectively). In the case of MG/PS, the SH signal arises from MG adsorbed at the PS particle surface. No SH signal was measured from the bare PS particles in the acidic aqueous solution using our low laser power. The SH signal, however, can be clearly detected when MG dye molecules are adsorbed on the PS particle.22−24 This is expected because MG, in its ionic form in
econd-harmonic light scattering (SHS), a particular kind of nonlinear light scattering (NLS), is a coherent nonlinear optical phenomenon, that has recently emerged as a versatile tool for characterizing nano- and micro-objects such as solid particles and liquid droplets.1−4 SHS is a second-order process that is facilitated in a medium without inversion symmetry, a condition always satisfied at an interface where the inversion symmetry is naturally broken. This makes the second-harmonic scattering phenomenon uniquely suitable for probing surfaces/ interfaces of particles buried deep in the colloidal medium when their bulk presents a center of inversion symmetry. However, for this technique to be effective, it is important to understand how particle properties affect the observations made in NLS so that the characteristics of the surfaces/ interfaces can be properly deduced. Nano- and microparticles find applications in a wide range of fields such as optoelectronics, medicine, and pharmaceutics. The properties of these mesoscopic systems are unique and differ from those of their atomic/molecular constituents and the larger dimension bulk forms. Some of the unique properties of the particles arise from their high surface-to-volume ratio, and it is thus important to characterize their surface. Among the many different kinds of nanoparticles (NPs), metallic NPs have been the focus of intensive studies5,6 because of their potential applications in sensing and imaging,7−14 fields that have been recently extended to include second-order optical phenomena.15−21 We have extensively studied the effect of the particle size on NLS from particles in the nano- and micrometer range.22−24 The experimental measurements were conducted on acidic aqueous colloids of polystyrene (PS) particles adsorbed with the nonlinear optically active molecule malachite green (MG). The angle-resolved second-harmonic scattering (AR-SHS) pattern was shown to change as a function of the particle © 2012 American Chemical Society
Received: August 6, 2012 Accepted: September 18, 2012 Published: September 18, 2012 2877
dx.doi.org/10.1021/jz301129h | J. Phys. Chem. Lett. 2012, 3, 2877−2881
The Journal of Physical Chemistry Letters
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MG/PS NPs at 420 nm is shown in the inset. In both measurements the fundamental and SH lights are p-polarized (electric field polarized horizontally in the scattering plane). The difference between the scattering behaviors of the Ag and MG/PS NPs is apparent. The angular pattern of MG/PS NPs shows the presence of two lobes due to a dominant contribution from the nonlocally excited electric dipole emission.25 The pattern of Ag NPs shows four lobes, suggesting that the major contribution to the SHS signal from these Ag NPs is due to the emission from the surface quadrupole, in agreement with polarization-resolved measurements39 and theoretical predictions.21,32,40 In general, the shape of the scattering pattern is the result of constructive/destructive interference at the detector position of the SH fields emitted from different points of the particle. As in linear scattering, the conditions of interference are sensitive to the size-to-wavelength ratio, the refractive index, and the particle shape.4 The small difference of a few percent in the size-to-wavelength ratio between the MG/PS and Ag NPs experiments is, however, not enough to explain the difference in the scattering patterns in Figure 1. Such different behaviors can, instead, be justified by the substantial difference in optical constants between metallic and dielectric NPs, as shown below through the theoretical simulations. Because the refractive indices of metallic particles and the aqueous medium are very different and because the shape of the Ag NPs is in first approximation spheroidal, we apply the previously developed NLM theory,24 assuming an isotropic centrosymmetric spherical particle, for calculating the AR-SHS patterns and evaluating the contributions from individual susceptibility tensor components. The results from the NLM calculations for Ag NPs are presented in Figure 2. The calculations were performed using three different values of the Ag optical constants: Figure 2a is using values from Palik,41
the acidic solution, provides a resonant enhancement at the SH frequency of 420 nm due to the presence of the electronic S2← S0 transition. In the case of Ag NPs, there is also a resonance enhancement at the SH frequency of 400 nm due to the presence of the localized surface plasmon.5 Proof that the majority of the SH signal from the Ag NPs arises from the particle surface was obtained from the observation of the reduction in the SH intensity following the addition of thiol molecules into the colloidal solution. The SH signal is quenched by more than 70% as strong S−Ag bonds are formed at the particle surface that localize the metal electrons and reduce the surface polarizability. From this observation, we concluded that at least 70% of the SH signal is generated at the surface.26,27 The remaining 30% of the SH intensity is signal generated either by the particle bulk or by the surface that the thiols were unable to quench. Several theoretical frameworks have been developed to describe the SHS from particles.4 The two most widely used approaches are the nonlinear Rayleigh−Gans−Debye theory (NLRGD)28−31 and the nonlinear Mie theory (NLM).24,32−37 The NLRGD model can be applied to particles of any shape as long as the refractive indices of the particle and the ambient medium are similar, and the particle is small compared with the wavelength. The NLM theory does not suffer from these restrictions and can be used for particles of any size and material as long as the shape is spherical. Another useful approach recently developed for describing the SH from a particle with noncentrosymmetric shape is based on the finite element method (FEM).39 In the following, we will use established principles from all of these different theoretical approaches, as appropriate, to explain the NLS characteristics. In Figure 1, we present the first observation of the AR-SHS pattern from 80 nm diameter Ag NPs measured at a SH wavelength of 400 nm. As a comparison, AR-SHS from 88 nm
Figure 2. SHS intensity in the horizontal plane as a function of the scattering angle calculated using the NLM theory with different susceptibility components for (a−c) Ag NPs and (d) MG/PS NPs. Red, blue, and green lines represent the sole contribution from χ⊥⊥⊥, χ⊥∥∥, and χ∥⊥∥, respectively. For Ag NPs, the calculations were performed using the optical constants from (a) Palik,41 (b) Johnson and Christy,42 and (c) Hagemann.43 The angular resolution of the calculation has been set to a solid angle corresponding to N.A. = 0.1, and the intensity has been normalized for all calculations.
Figure 1. AR-SHS pattern in the horizontal plane at 400 nm for Ag NPs (80 nm diameter). The intensity unit is in photon counts/s. Inset: AR-SHS at 420 nm from MG/PS NPs (88 nm diameter). In both graphs, fundamental and SH field are p-polarized, and the 0 angle indicates the fundamental propagation direction (the forward direction). 2878
dx.doi.org/10.1021/jz301129h | J. Phys. Chem. Lett. 2012, 3, 2877−2881
The Journal of Physical Chemistry Letters
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Figure 2b values from Johnson and Christy,42 and Figure 2c values from Hagemann.43 The calculated normalized AR-SHS pattern for MG/PS NPs24 is presented for comparison in Figure 2d.44 It was previously proven that the dominant surface susceptibility component for MG/PS NPs is χ⊥∥∥.23,24 This component, however, appears to be far from effective in describing the scattering behavior of Ag NPs because it produces only two lobes. Between the two remaining components, χ⊥⊥⊥ and χ∥⊥∥, it seems that, independently from the optical constants chosen, χ⊥⊥⊥ is the only component able to produce four almost equally intense and well-resolved lobes as those experimentally measured and reported in Figure 1. These deductions on the relative importance of susceptibility elements of Ag NPs, namely, the fact that χ⊥∥∥ and χ∥⊥∥ give little contribution to the SH intensity while the χ⊥⊥⊥ component is dominant, are in agreement with the results obtained for Au NPs in solution in polarization-resolved experiments at a 90° detection angle for which the octupolar contribution to the SH emission was also observed.46 Those data were explained by assuming only the presence of the χ⊥⊥⊥ component,46 suggesting, together with the results from this work, a common trend in the relative importance of the different susceptibility tensor elements of metallic NPs. However, for small spherical particles, when the nonlocally excited dipole and locally excited quadrupole solely contribute to the SH emission, it is possible to single out the contribution of a specific susceptibility component only by analyzing ARSHS data. However, this would not be possible by analyzing data from polarization-resolved experiments in a 90° detection geometry as, in this latter case, all the components produce polarization patterns with the same shape.27 A closer comparison of the experimental data for Ag NPs in Figure 1 and the theoretical calculations in Figure 2a−c shows, however, that there are discrepancies, in particular, in the intensity at scattering angles (θ) of 0 and ±90°. The discrepancy at θ = ±90° could be explained by the fact that even if χ⊥⊥⊥ is the dominant tensor element, there might still be contributions from the other elements. This explanation, however, does not apply to θ = 0° (the forward direction) because in this direction there should be no contributions from any of the susceptibilities components. The SH intensity observed in the forward direction can only be explained by the presence of particles whose shape is noncentrosymmetric, allowing for the emission from the locally excited dipole. Experimentally, it has been observed for a Au nanoscopic tip, presenting a noncentrosymmetric shape, that a local dipole contribution to the SH is allowed and results in SH emission in the forward direction for the p-in/p-out polarization configuration.47 This leads us to conclude that the shape of at least some of the NPs must be noncentrosymmetric. This deduction is also in agreement with the results from FEM calculations.38 To investigate further the possibility of imperfect spherical shape, we show in Figure 3 the s-polarized (vertically polarized electric field) SHS intensity in the horizontal plane from Ag NPs induced by either linear (3a) p-polarized or (3b) spolarized fundamental beam. For MG/PS NPs, we find that the SH intensity measured for these two polarization combinations (Figure 3c,d) is comparable to the noise level (average intensity
