Optical Resonances of Colloidal Gold Nanorods: From Seeds to

Article pubs.acs.org/JPCC

Optical Resonances of Colloidal Gold Nanorods: From Seeds to Chemically Thiolated Long Nanorods F. J. Recio,*,†,‡,§,¶ N. Zabala,*,∥,⊥ A. Rivacoba,*,∥ P. Crespo,*,†,‡ A. Ayuela,*,∥ P. M. Echenique,*,∥ and A. Hernando*,†,‡ †

Instituto de Magnetismo Aplicado, UCM-CSIC-ADIF, Las Rozas, P.O. Box 155, Madrid 28230, Spain Departmento Fı ́sica de Materiales, Universidad Complutense, Madrid, Spain § Departamento de Quı ́mica Inorgánica, Facultad de Quı ́mica, Pontificia Universidad Católica de Chile, Avda. Vicuña Mackenna 4860, Macul, Santiago, Chile ∥ Centro de Fı ́sica de Materiales CFM-MPC, CSIC-UPV/EHU, Donostia International Physics Center (DIPC), Departamento de Fı ́sica de Materiales, Facultad de Quı ́micas, Universidad del Pais Vasco UPV-EHU, 20018 San Sebastián, Spain ⊥ Departamento de Electricidad y Electrónica, Facultad de Ciencias, Universidad del Pais Vasco UPV-EHU, Bilbao, Spain ‡

ABSTRACT: Following an adapted three-step seed-mediated method, we synthesize colloidal Au thin and long (L > 100 nm) nanorods (NRs) and characterize the metallic nanostructures evolving from the initial Au nanoparticle (NPs) seeds to thiolatefunctionalized NRs, using HRTEM and ultraviolet, visible, and nearinfrared absorption spectroscopy (UV−vis−NIR). For the long NRs we analyze the role of several solvents and rod concentration on the spectral features of the assembled products, which are further studied with simulated spectra. Superimposed to a broad resonance in the range 700−900 nm, which corresponds to short (L < 100 nm) interacting nanorods, the growth of long nanorods is clearly identified with the emergence of a robust resonance at 960 nm, linked to the three half-wavelength antenna plasmon mode. This mode is enhanced when the nanorod concentration decreases and splits into two peaks when the thiolate coverage chemically modifies the rod surface by a thin layer of nanometer size. This behavior is explained with a dielectric model based on ab initio calculations of thiolate−gold cluster surfaces. the high electromagnetic field enhancement in the interparticle cavity. Chemical detection methods are based on the shift of the plasmonic resonances with the refractive index of the embedding medium. For example, the shift toward longer wavelengths for nanoparticles covered by oxides was studied years ago.20−22 The absence of the surface plasmon resonance band has been reported23 in gold NPs by substituting the surrounding medium of weak interacting molecules by covalent bonded thiolates, due to the electron transfer between the capping or stabilizing agents and the NPs. Furthermore, Au NPs covered by thiolates have been observed to show a peculiar magnetic behavior,24−28 which is explained by sp electrons transferred from thiolates to gold underneath the interface.28 In the present work, we explore the role played by these molecules attached to elongated gold NPs. Whereas spherical Au NPs exhibit surface plasmon resonances in the visible region of the spectrum, a shift of the absorption band to the near-infrared (NIR) can be achieved by synthesizing more sophisticated NPs, such as nanoshells (dielectric core covered by a thin Au shell),

1. INTRODUCTION Metal nanoparticles are attractive because of their size- and shape-dependent properties, distinct to those of bulk counterparts, that make them useful for applications in many different fields such as biochemistry, molecular detection in biosensing, nanolectronics, catalysis and biomedicinal diagnosis, and photothermal therapy in hyperthermia treatments.1−4 In particular, noble metal nanoparticles exhibit outstanding optical properties associated with the tunability of their localized surface plasmon resonances (LSPRs) in the visible part of the spectrum. NPs of size smaller than the wavelength of exciting light are known to absorb at the energy of the dipolar surface plasmon peak. Higher multipolar modes are clearly observed using a localized source of electrons instead of light, e.g., by electron energy loss spectroscopy (EELS).5−8 It is well-known that the size, shape, interparticle distance, and the surrounding medium determine the LSPR wavelengths.9−12 The high tunability of the NP resonances has boosted the development of different methods to synthesize nanostructures for specific applications.13,14 For instance, the dependence on the distance between nanoparticles allows the design of novel biosensors for biodetection.15 New modes emerge when two particles approach7,16−19 at distances close to 1 nm, associated with © XXXX American Chemical Society

Received: November 24, 2014 Revised: March 9, 2015

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DOI: 10.1021/jp5117273 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C nanocages29−31 (hollow porous gold nanoparticles), and nanorods with different aspect ratios (ARs).32,33 In particular, there has been a growing interest in colloidal gold nanorods for NIR photothermal therapy. However, the different synthesis methods, capping agents, dimensions, and geometry of these nanostructures in previous studies make the comparison between them a difficult task. Nanorods have typically two distinct LSPR absorption bands associated with transverse and longitudinal electron oscillations. The longitudinal LSPR produces the longest-wavelength band, which is sensitive to not only the nanorod aspect ratio34 but also the diameter.35 For infinite cylindrical nanowires these modes can be understood as fundamentally stemming from surface plasmon modes with azimuthal numbers m = 0 and m ≠ 0, well studied in the bibliography.5,36−38 Nanorods with large aspect ratios, having the major dimension of the order of light wavelengths, behave like nanoantennas, with the λ/2 modes in the near-IR range.19,35,39−43 However, we are herein focusing on the observed absorption at the intermediate range between the longitudinal and transverse plasmons for the thin and long nanorods with dimensions below the typical visible-light wavelengths. In this letter, we report results on the synthesis of thin NRs according to a seed-mediated growth method followed by a filtering process. In this way, anisotropic gold NRs have different aspect ratios with sizes covering the range between tenths to a few hundred of nanometers. Because of our interest in medical applications, gold NRs are dispersed in colloidal solutions, so that some molecules are bound to their surfaces and keep them partially separated. We use the well-known solvents: first a CTAB−water solution, where gold NRs of different sizes are grown from Au NP seeds, and then a thiolate solution. We characterize the short and long Au NRs using HRTEM and UV−vis−NIR spectroscopy. We then focus on long NR samples (L > 100 nm), obtained after filtering. In particular, understanding the origin of a peak that appears at about 960 nm for several solvents and its evolution with the Au NR colloidal concentrations is one of the main challenges concerning the characterization of their optical response. We perform this study with the help of a dielectric model based on ab initio calculations of thiolate−gold cluster surfaces.

Figure 1. Absortion spectra of the as-prepared gold nanorod solution (full red line) and the final filtered solution dominated by long (L > 100 nm) NRs (blue dashed line). The spectra of the initial NP seed solution (dotted line) and the intermediate solution with short Au NRs (gray dashed line) are shown for comparison.

100 nm [gray line in Figure 1] and long ones with L > 100 nm [blue line in Figure 1]. NPs used for the NR growth remained in the colloidal solution, and their diameter also characterizes the transversal width of all the Au NRs. A drop of the samples (centrifuged and washed) was placed on a HRTEM grid for characterization. HRTEM was performed on a transmission electronic microscope JEOL JEM-3000F working at 300 kV, so that NRs were characterized by high-resolution transmission electron micrographs (HRTEMs). Particle diameters from a sample size of 100 nanoparticles were determined through measurements with imaging software. The results are shown in Figure 2. The images point out that the short nanorods, in Figure 2(b), have an average length of about 70 nm following a good Gaussian distribution. More precisely the histograms of the short NRs shown in Figure 2(d) fit to a Gaussian distribution with average aspect ratio AR = 2.4 and diameter D = 36.2 nm. So their average length, including hemispherical ends, is L = 86.9 nm. Although long Au NRs, in Figure 2(c), have a wider dispersion in the size distribution [Figure 2(e)], they have also been fitted to a Gaussian distribution centered at AR = 12.2 and D = 26 nm, so that L = 317.2 nm. It is noteworthy that nanorods of shorter lengths have polyhedral ends, while at larger sizes they become round, a finding that agrees well with recent works focusing particularly on the end shape of rods during their growth.34

2. SYNTHESIS OF NRS AND MORPHOLOGY BY HRTEM A set of NR samples of different sizes were systematically grown and then characterized by HRTEM and standard UV− vis−NIR spectroscopy. The synthesis of Au NRs was performed following a three-step process.44,45 Gold seeds were first synthesized by chemical reduction of gold salt (HAuCl4, chloroauric acid) with a strong reducing agent (NaBH4, sodium borohydride) in the presence of a capping agent (citrate). These seeds were then added to a water solution containing more metal salt, a weak reducing agent (e.g., ascorbic acid), and a surfactant-directing agent (cetyltrimethylammonium bromide, CTAB). The initial seed diameter was about 30 nm. In a second step NRs of different lengths were grown. At this stage of the process three types of nanostructures coexist in colloidal solution (NPs, short and long NRs), as shown in the absorption spectra (red line) and inset HRTEM images of Figure 1. Finally, in a third step, the solution was centrifuged and vacuum filtered several times, using a membrane of pore size about 100 nm. Two different NR samples were obtained: short NRs with lengths L below

3. OPTICAL RESONANCES BY ABSORPTION UV−VIS−NIR SPECTROSCOPY For the different gold NP and NR products obtained during the growth process, absorption spectra were recorded in the vis− NIR range using unpolarized incident light. The spectra were obtained at room temperature using a Lambda 1050 wideband UV−vis−NIR spectrometer equipped with a diffuse reflectance attachment (Biconical DRA-CA-50M). The solid-state reflectance spectra were obtained and transformed to absorption data with the aid of the Kubelka−Munk function.46 The size distribution of both NPs and NRs, described in the previous section, causes some inherent broadening of the peaks in the B


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Figure 2. Morphological analysis of (a) thin gold NRs with short (b) and long (c) aspect ratios after repeating filtration with pore sizes around 100 nm. The rod solution is deposited and dried in a graphite substrate. (b,c) HRTEM detailed micrographs of the NRs. Histograms of measured aspect ratios for both samples of (d) short and (e) long NRs. The insets show the diameter of the NRs over the same measurements. The aspect ratio change from 2.4 in short rods to 12.2 in very long and thin gold nanorods.

intensity at λ = 960 nm is clearly observed in the red line spectrum of Figure 1 and is attributed to the growth of long NRs (of L > 100 nm). It is well-known that LSPRs in nanorods are dependent on their aspect ratio and diameter19,35,39 and that for longer NRs or plasmonic nanoantennas the dipolar LSPR shifts to the red because the induced dipole in the longitudinal direction increases. Furthermore, longer NRs are able to sustain new longitudinal plasmon resonances, so that the spectrum is richer and extends in a wider wavelength range. This is qualitatively well understood with a simple Fabry−Pérot cavity picture in which the resonances are given by the condition sin(keffL) =0, where keff = 2π/λeff is the plasmon mode wave vector along the direction of polarization. The approximate relation which describes the position of the different longitudinal LSPR modes is then L=nλeff/2, where n is an integer. The most intense LSPR is then the dipolar n = 1 mode or half-wave resonance, similar to the first stationary vibrational mode of a string, with a node in the center, studied in basic physics. Higher-order resonances for n = 2, 3, ... are excited in longer NRs, and their position in the spectrum is blue-shifted when the mode order n increases. Notice that λeff is the effective wavelength of the plasmonic mode and not the wavelength λ of the incoming light. Novotny39 proposed a simple link between both wavelengths,

optical spectra measured in the colloidal state, besides the broadening due to the plasmon damping.47 Another source of peak broadening, especially for long NRs, is their different orientations with respect to the incident light or the fact that the light is unpolarized, to be explained in the simulation section. The dipolar LSPR of the initial NP seeds, shown in Figure 1 with the dotted line, is centered at λ = 530 nm, a value that corresponds well with the size of NPs used as seeds, which have diameters around D = 30 nm. A new peak appears in the range λ = 650−700 nm and is the signature of the growth of elongated NPs or short NRs (spectra in full red and gray dashed lines in Figure 1). NRs of length L < 100 nm present two main LSPRs, corresponding to oscillations along the two main symmetry axes, called longitudinal and transverse plasmons.19,35,39 The transverse one, due to the charge oscillations along the short symmetry axis of the NRs, matches the initial NP dipolar LSPR, and the new peak due to charge oscillations between the two ends of the NRs is the longitudinal dipolar plasmon mode. The HRTEM images confirm the presence of NRs shorter than 100 nm. The concentration dependence for short nanorods was monitored as well, and as expected when rods become closer, the dipolar longitudinal resonance shifts to the red.19,48,49 A third peak of lower C


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The Journal of Physical Chemistry C which is controlled by the shape, the dielectric responses of the NR material, and its surroundings. A more quantitative description of these modes is discussed in the simulation section. Next we focus on the long NR samples refining much better during filtering. In this case only two peaks at around λ = 540 and 960 nm are observed in the dashed blue line in Figure 1, separated by a nearly continuous absorbance background. We systematically study the long NR samples to determine the dependence of absorbance in the NIR region on gold nanorod concentration and the surrounding media. Several samples of long Au NRs covered by CTAB in water were prepared with decreasing load amount of NRs by repeated dilutions (Co, Co/ 2, Co/4, Co/16, and Co/32, Co being the initial concentration). A similar study on the concentration of NRs was performed in another set of samples but with a solvent of thiolates (Co, Co/2, Co/4, Co/16). The samples were depured by filtering several times, and the same size study was performed by HRTEM. Figure 3 shows the UV−vis−NIR spectra of a number of filtered colloidal suspensions of gold NRs at several concentrations and different media. In CTAB solution in water, the spectra in Figure 3(a) show a steady increase of the uniform plateau between the two main plasmon peaks with

increasing Au load, showing a strong dependence of the absorption on Au nanorod concentration. These experiments use the same setup of Figure 1. The shown rod absorption subtracts the absorption signal from the solvent itself averaged over several runs. The remaining absorption is due to rods and nanoparticles and should be proportional to the metal filling fraction. Therefore, the spectra are scaled multiplying them by the number of times that the input sample has been diluted. Au NRs remain covered by a few CTAB molecules, as sketched in the right of the figure. It is observed that the peak at 960 nm increases and gets more prominent as the Au NR solution is more and more diluted, up to a limit. Then the increase in the absorption peak becomes difficult to detect due to high dilution because we are already hitting the accuracy of the experimental setup. Even when the average volume fraction of NRs may be small for all concentrations, the average separation seems large after the repeated filtering. The rapid increase of absorbance about 960 nm with decreasing Au NR concentration in CTAB solution is clearly related to the larger distance between nanorods: when nanorods are uniformly distributed their average distance decreases with concentration. Close side-toside nanorods are experimentally known to change the absorbance in a rather large wavelength region due to the interaction between surface−surface plasmons, as seen for pairs of rods on substrates.48 Coupling of tip-to-tip rods separated below 2 nm shows a further red shift in the LSPR peaks and enhances the near electromagnetic field in the spatial gap.19,50 A plasmon hybridization model has been proposed to account for the plasmon resonances in complex nanostructures, including tip-to-tip interactions.17,51,52 Note that for the diluted cases the spectra are dominated by the longitudinal modes because of their higher polarizabilities. In the present study, although the diameters of nanorods are an order of magnitude smaller than in previous studies, rod−rod interactions are also involved and are important for reducing the uniform signal in absorbance with concentration. The high dependence of absorbance on concentration thus indicates that the plasmons of gold nanorods in CTAB solution are indeed side-to-side interacting. Next we repeated the procedure for thiolated NRs within a thiolate solvent, by removing CTAB to form the thiolate cover. The absorption spectra recorded for the thiolate-functionalized NRs are shown in Figure 3(b) versus NR concentration. NRs covered by thiolates are further diluted with toluene with a very close but slightly higher absorbance in the whole range. The spectra present a prominent peak around λ = 960 nm, as for the case presented in Figure 3(a). Most significantly the absorbance remains almost unaffected with nanorod concentration, which is decreasing divided by two between curves (in the sequence of black, red, green, and blue). Note that the absorption of the spectra increases as a whole when samples are more diluted, even if they are normalized by the amount of gold, due to the larger amount of toluene. Well-known ligands like thiols dissociate over the gold surface and guarantee the self-assembling of thiolate molecules as a fur. Thiolates react and bind to gold, so the dependence of absorbance with concentration becomes much smaller than in the previous case of water. The coated gold−gold NR surfaces are far enough, at minimum distances of 5.54 nm, as imposed by the length of the thiol chains, which is about 2.27 nm. Furthermore, it is observed that the peak in the range λ = 900− 1000 nm appears now at shorter wavelengths than in the samples with water solvent of Figure 3(a), and a new lower peak emerges over 1000 nm. In fact, it seems that the plasmon

Figure 3. Absorption spectra of the long NR sample, as characterized in Figure 2(b), in different solutions. (a) Au NRs in a solution of CTAB molecules in water. The concentration of NRs has been continuously decreased (divided by two) following the sequence of black, red, green, blue, and yellow curves. (b) Au NRs when fully covered by thiols in a thiolate solution. The concentration of NRs has also been decreased as in (a) following the same color sequence. D


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Figure 4. Simulated extinction cross sections of NRs of two distinct sizes and different incidence angles and polarization of the incoming light, as sketched in the insets with colors. (a) Spectra of short gold NRs (L = 87 nm, D = 36 nm, AR = 2.4) for three polarizations of the incident light. The blue line has the electric field polarization along the transversal short axis, the red line along the long axis and the green one at 45° (see incidence schemes in the inset). (b) Extinction cross-section of long Au NRs (L = 390 nm, D = 26 nm, AR = 15) for four different angles of the electric field of incident light with respect to the long axis: 90° (blue), 45° (green), 30° (black) and 0° (red). Letters a−f label the main peaks. The dashed line denotes the sum over the four angles. (c) Calculated near-field patterns around a long Au NR considered in (b) at the wavelengths corresponding to the main observed peaks.

way due to the presence of the corners.34,56 Furthermore, the present study is more focused on the long NRs described above and having more structure in the experimental absorption spectra. The solvent consisting of water with organic molecules is characterized as a homogeneous medium with a real dielectric constant of value ϵ = 2.1 considered to be nearly the same for CTAB or thiolate solutions, commonly used in the literature. The effect of increasing the refractive index of the surrounding medium is to red-shift the plasmonics resonances.32 A detailed study of the effect of the solvent in the absorption spectra of gold nanorods can be found in the literature.32 However, for the thiol-functionalized NRs described at the end of the previous section, we propose a more sophisticated modeling with details found below. The experimental absorption spectra use unpolarized light and nonaligned NRs. Nevertheless, the simulations were performed for incident plane waves with the electric field polarized in a given direction with respect to a single NR axis, as sketched in the insets of Figure 4. The plasmon resonances of an elongated nano object as the NRs considered in the present study, contrary to spherical NPs, have a strong dependence on the exciting light polarization and incident angle.57 In Figure 4 we show BEM simulations performed for two values of the NRs corresponding to the average sizes of the two samples characterized by HRTEM and the histograms of Figure 2. In Figure 4(a) we consider short NRs of length L = 87 nm and diameter D = 36 nm, i.e., AR = 2.4, and in Figure 4(b) we display the calculated extinction spectra for long NRs of length L = 390 nm and D = 26 nm, i.e., with the aspect ratio of 15. When the incident light is polarized with the electric field E along the short symmetrical axis of the NR (blue lines in Figure 4), the position of the plasmonic

peak in Au rods covered by thiolates splits, so further explanations are required to elucidate the role of the thiolate−gold chemical bond, which are given in next section.

4. SIMULATIONS AND DISCUSSION OF RESULTS. ASSIGNMENT OF NIR PEAKS TO PLASMON RESONANCES In order to investigate the different absorption spectra of Au nanorods described above, full electrodynamical calculations were performed using the boundary-element method (BEM)53,54 to solve the Maxwell equations in the frequency domain. Within this numerical method, the electromagnetic fields are obtained in terms of the charges and currents induced at the boundaries of the nanostructure. Then the total extinction cross section, as a sum of absorption and scattering, is calculated using the optical theorem. It is illustrative to map the near-field distributions around the NRs at the excitation wavelengths of interest to understand their plasmonic origin. A large number of discretization points were used to ensure full convergence of the obtained results. Notice the importance of performing full electrodynamics calculations, which incorporate retardation effects, especially for long NRs. Nanorods were described by a local frequency-dependent dielectric function obtained by interpolating tabulated experimental optical data,55 and they were supposed to have axial symmetry. So they were modeled as cylinders capped by hemispheres, as sketched in Figure 2(a). This seems a suitable model for the long rods of the HRTEM images shown in Figure 2(d−e). It is noteworthy that the short rods present rather conical or polyhedrical end shapes, but the results on absorption peaks are known to just move slightly in an effective E


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(labeled f), which is the main longitudinal mode in the calculated spectrum, is shown at the right side. It has an n = 3 pattern with three nodes: one at the center and other two symmetrically placed between the center and both ends of the NR. This is the main peak of the spectrum after the dipolar n = 1 mode which has the highest dipolar moment. Below the n = 1 dipolar mode, lower intensity symmetric and asymmetric modes appear, which would be summing together to a background signal. The next peaks for the longitudinal excitation, at shorter wavelengths, labeled d and b, correspond to the n = 5 and n = 7 LSP modes, with 5 and 7 nodes in the near-field pattern, respectively. Notice that the near fields of peaks c and e present an asymmetric shape with a maximum induced charge at the NR center, instead of a minimum, as corresponds to the even modes n = 6 and 4, respectively. Going back to experiments with colloidal NRs of different orientations and with a size distribution, discussed in a previous section, we conclude that these small peaks merge into a uniform spectrum background. The experimental spectra measured for long NRs in CTAB solution [Figure 3(a)] present a peak at 960 nm, with increasing intensity as rods are more and more diluted, emphasizing the absorption of isolated gold nanorods. Therefore, this peak is better distinguished in this case. When the NR density is increased, we note that the peak decreases, merging in the background absorption. Apart from the λ = 960 nm and the transversal mode, another three bumps superimposed upon the background are observed in Figure 3(a) for the two most diluted samples (blue and yellow lines), which might correspond to the secondary higher-order modes discussed above. Nevertheless, the main peak at λ = 960 nm is clearly distinguished for diluted samples, in perfect agreement with the calculated main n = 3 mode. We have included the total extinction calculated as a sum for the different orientations considered in the plot for long rods [dashed line in Figure 4(b)]. The total sum over different directions of the incident field would produce a broadening of the peaks, as in experiments. This would change the weight of the peaks appearing in the plots of Figure 4(b). Anyhow, from that figure we can conclude that the peak labeled as f is the main one, and the other ones have a lower weight in the total spectrum. The spectrum is dominated by the longitudinal LSPR peak as it is the strongest of all. Interpreting the absorption spectra of thiolate-covered long NRs displayed in Figure 3(b) still requires a further analysis. Now the rod−rod interaction is minimized because the minimum distance between thiolated NRs is about 5 nm. The main peak at 960 nm in Figure 3(a) is blue-shifted to 930 nm. Since the thiolate solution has a slightly higher refractive index than the CTAB−water solution, one would expect this plasmon peak to shift to even longer wavelengths.32 On the contrary, the experimental results suggest that the coverage by a thin layer of thiolate molecules reshapes the main plasmon peak that splits into two: a main peak at lower wavelength, λ = 930 nm, but also a less intense peak at λ = 1040 nm. The bluish peak cannot be explained neither by the effects of a different surrounding medium20−22 nor by NR touching or proximity19,48 since both effects would contribute regularly to a red shift. Thus, we have developed a more sophisticated modeling of the thiolated NRs. On the basis of first-principles calculations performed on thiolated gold clusters, we found that sp electrons from the thiolate−Au2 complex pass below gold cluster surfaces, forming a conductor shell28 [see sketch shown

resonance is about the same wavelength as the dipolar LSPR of one NP with a diameter of similar size to the transversal width of the NR. For the short NRs considered in the present study the transverse mode is centered at λ = 520 nm and for the long ones at λ = 515 nm. The longitudinal resonance moves to higher wavelengths for larger sizes because the dipolar moment associated with the allowed plasma oscillations increases. The corresponding near fields are shown in the inset of Figure 4(a) for the short NR and in Figure 4(c) for the long one. The charge oscillations associated with these modes are sketched with plus and minus marks. When incident light is polarized with E along the long axis of the NR, the dipolar plasmonic peak wavelengths are shifted toward the red, as shown in Figure 4(a) specifically for short gold rods (red line). In this case the peak is at λ = 720 nm. The near-field for this wavelength clearly shows the dipolar pattern. In the figure, additional curves are comparing the extinction spectra obtained for different incidences (k vector direction) and polarization angles with respect to the long axis of the NR. Note that the spectrum at 45° (green) has two peaks at the same positions as the transverse and longitudinal LSPRs. Even without aligning the electric field of light with the rod axis, as is the case in colloidal assemblies, the longitudinal dipolar modes seem to be present at the same wavelength independently of the polarization angle, when light has a longitudinal component along the NR. The spectra for the long and thin Au nanorods of large AR extend up to longer wavelengths, with the dipolar peak being the most intense one, around 2400 nm, beyond the range considered in Figure 4(b). Furthermore, when rods are particularly long, as in the present case, more plasmon modes enter in resonance at wavelengths between the previously discussed longitudinal dipolar and transverse modes, giving rise to new peaks such as the ones shown in Figure 4(b). For the long NRs of AR = 15, three peaks are observed in the calculated spectrum when the incident light is polarized along the NR, at λ = 960, 705, and 614 nm, labeled as f, d, and b, respectively. At oblique incidence (E forming 45° and 30° with the long axis), symmetry-breaking arises, and due to retardation effects, now the spectrum is not just the superposition of longitudinal and transverse modes. New peaks appear, labeled c and e, not present for the longitudinal nor for the traversal excitations. Excitation of the so-called dark modes (with n an even integer) becomes possible at oblique incidence because of the phase difference of the incoming wave at the two nanorod ends. These modes are excited in symmetry-breaking conditions such as in the presence of defects42 or single dipolar emitters positioned near NRs.43,58 They are clearly observed in the EELS spectra and plasmon mapping, when an electron beam passes close to a nanowire, a NR, or other nanostructures.59,60,60 Indeed, investigations with other angles reveal that these peaks shift with asymmetric light incidence and have smaller intensity values, so in the experiments with colloidal samples they are broadened and added with lower intensity to a background signal. In order to understand the origin of the peaks obtained in the spectra simulated with BEM, we have also calculated the nearfield distributions around the NR for all of them. The results are displayed in Figure 4(c), where the signs of the induced charge densities of the corresponding modes are superposed to the bright-field spots. The peak at λ = 515 nm, labeled a, remains as the main transversal LSP mode, as observed at the left of Figure 4(c). The near field of the λ = 960 nm peak F


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cross-section of thiolated NRs, modeled as described above, are displayed in Figure 5(b). The size is the same one considered in Figure 4(b). The calculated spectrum of the uncoated NR of Figure 4(b) is also displayed for comparison. The shell electrons couple to the n = 3 peak of the Au NR splitting into two peaks centered at 824 and 1040 nm. The changes induced by close values of Ωp are also included in Figure 5(b), where the simulated spectra are still robust enough to show the two peaks splitting. A more careful modeling should be performed in order to get a more quantitative agreement with the experiments, for example, by considering the coupling to shorter NRs from the size distribution of the sample represented in Figure 2(b), which would present the n = 3 peak at shorter wavelength and would couple more efficiently to the shell electrons. Nevertheless, the experimentally measured blue-shifted peak in Figure 3(b) may be explained qualitatively by the present model of Au rods covered with a shell of different electron density coupled to gold by an energy of about 1.5 eV, as obtained from the DFT calculations. Plasmons of this conducting shell exhibit a stronger damping than for noble metals such as gold, given by the DFT calculated energy difference between the backdonated electron with respect to the former gold levels. These findings suggest that thiolates chemically adjust and alter the plasmon resonances of gold nanorods in a controlled way. At first, one would think of a further characterization by EELS experiments to settle the exact topography of the plasmon modes, but the typical energies of electron beams are enough to remove thiolate molecules from the gold nanorods’ surfaces during measurement time. The response of the thiolated nanorods chemically modified could be accessible by nondestructive analysis using scanning near-field optical microscopy. However, these experiments need depositing these long gold nanorods covered by thiolates in substrates. They constitute, in fact, future work beyond the scope of the present study focusing on colloidal gold nanorods and the uncommon light response of the long nanorods in different media (typified here by water and thiolates).

in Figure 5(a)]. Thiolates chemically attached to gold NRs are thus supposed to form a conducting shell of different electron

Figure 5. (a) Sketch of the bonding mechanism of thiolates to gold surfaces (upper panel). Lower panel represents the density of states of a gold−thiolate cluster projected on the sp and d states in the gold atoms not bound to sulfur, for both spin channels. Just below the gold surface, the extra sp electrons are coupled to gold d and other lowlying sp electrons with an energy difference of about 1.5 eV. (b) Calculated extinction cross-section spectra for a long NR (L = 390 nm, D = 26 nm, AR = 15 nm) covered by a shell of 1 nm mimicking the thiolates chemically attached to the nanorod surface, described by a Drude−Lorentz model with the characteristic electron binding energy of 1.5 eV and Ωp = 6 (violet line) and 5 eV (green line). The spectrum for the uncovered NR with the same size (red line) has been also plotted for reference.

density around gold nanorods. We have assumed the thickness of this shell to be d = 1 nm, and the dielectric response function associated with these electrons has been described by a simple Drude−Lorentz model ϵ(ω) = 1 −

Ω2p ω 2 − ω02 + iωγ

(1)

5. CONCLUSIONS Following a seed-mediated three-step method, gold nanorods of different aspect ratios and lengths of several hundreds of nanometers have been grown and isolated from NRs shorter than 100 nm by a filtration process. The samples of the different stages have been characterized by HRTEM and UV− vis−NIR absorption spectroscopy followed by full electrodynamics BEM calculations to interpret the results. For the intermediate small rods during growth, with length below 100 nm, we showed that the main plasmon peaks corresponding to colloidal nanorods can still be seen individually as the longitudinal and transverse modes. By increasing the aspect ratio, for nanorods of big aspect ratio and length L > 100 nm, a plasmon mode at 960 nm is measured superimposed upon a spectrum background between the longitudinal and transverse modes, when the solvent is water with CTAB molecules. We have identified this mode as the n = 3 longitudinal plasmon mode. Other higher plasmon modes below this one and above the transverse surface plasmon mode have lower intensity and would contribute to a background as a number of factors such as rod−rod interactions, size distribution, and different orientations would contribute to a considerable broadening of these peaks forming an absorption band. Additionally, when looking at the solutions of several solvents, we see that in

where Ωp = 6 eV is the plasma resonance associated with the electron density in the shell; ωo = 1.5 eV is the binding energy of these electrons; and γ = 0.1 eV is a damping factor. The considered value of Ωp corresponds to the electron density in the shell, which is one-half of the electron density in bulk gold, and therefore Ωp = ωp/21/2, where ωp = 8.5 eV has been considered for the Au bulk plasmon. The resonance energy ω0 of this model is obtained from the energy difference between sp electrons back-donated from thiolates to gold and gold electrons themselves. We have performed DFT calculations for the whole system of thiolates already deposited on gold clusters up to 13 atoms and extracted the parameter ω0 from the local density-of-states projected on inner Au atoms. Our calculations use the value of ω0 obtained with a PBE functional. Anyhow, other functionals28 have close values for ω0 that would drive to qualitatively similar results. Note that Au nanoclusters in the range of a few nanometers and protected with a monolayer of thiolates can be calculated today by ab initio methods.61 All these parameters could be adjusted in order to obtain a more quantitative agreement with the experiments, but this is beyond the scope of the present work. Figure 5(a) shows the schematics of the density of states projected in energy, instead of spatially, from DFT calculations, to get such characteristic energy. The results for the extinction G


Article

The Journal of Physical Chemistry C CTAB−-water, this fine structure emerges when decreasing the NR concentration, as rods are almost isolated. On the contrary, when the concentration is increased this peak dims in the background. Furthermore, we found that in a solution with thiol molecules thiolates attached to the surface of the NRs can change the surface chemistry and the plasmonic resonances noted in the absorption spectrum, so that the main absorption plasmon peak at 960 nm splits now into two peaks, one main peak slightly shifted toward the blue and another less intense one toward the red. We have explained qualitatively this behavior by considering a conductor shell model based on DFT calculations, where thiolates donated additional electrons below the gold surface. This shell is mimicked by a Drude−Lorentz dielectric function, with parameters obtained from DFT in clusters. These pioneering results constitute a promising approach to adjust the optical response of gold nanorods by chemical means in the near-infrared region, as followed for hybrid thiolate−noble metal nanostructures.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail: *E-mail: *E-mail: *E-mail:

[email protected]. [email protected]. [email protected]. patricia.crespo@fis.ucm.es. [email protected]. [email protected]. [email protected].

Present Address ¶

(F.J.P.) Centro de Nanotecnologı ́a y Materiales Avanzados, CIEN-UC, Pontificia Universidad Católica de Chile.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the Basque Departamento de Educación and the UPV/EHU (Grant No. IT-366-07) and Spanish Ministry of Economy and Competitiveness MINECO (FIS2013-48286-C2-1-P and FIS201341184-P) and the ETORTEK research program funded by the Basque Departamento de Industria and the Diputación Foral de Guipúzcoa (nanoGUNE 2014). F.J. Recio thanks a Fondecyt 3130538 Postdoctoral Grant. A.A. acknowledges Prof G. Bryant for the careful reading of the manuscript.

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Optical Resonances of Colloidal Gold Nanorods: From Seeds to

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