Unraveling the Evolution and Nature of the Plasmons in (Au Core)–(Ag Shell) Nanorods

Abstract

Evolution of the sizes and plasmonic properties of (Au core)−(Ag shell) nanorods is studied. Four plasmon bands are observed on the core−shell nanorods and their properties are investigated. The lowest-energy one belongs to the longitudinal dipolar plasmon mode, the second-lowest-energy one belongs to the transverse dipolar plasmon mode, and the two highest-energy ones are ascribed to octupolar plasmon modes. Localized surface plasmon resonances, which arise from the collective oscillations of the near-Fermi-level electrons in noble metal nanostructures, have received intense attention in recent years due to their rich, intriguing, and complex optical properties. The localized plasmon resonances of Au and Ag nanostructures are spectrally located in the visible to near-infrared range. In addition, Au and Ag nanostructures are relatively stable in ambient environments. They have, therefore, been intensively studied from the perspectives of both fundamental sciences1-3 and potential applications in optics,4-6 sensing,7-10 imaging,11, 12 information processing,13 photothermal therapeutics,11, 12 solar energy harvesting,14, 15 and photocatalysis.16, 17 Gold nanocrystals, especially Au nanorods, have been widely used as detection agents for analytical and biochemical applications as well as photothermal therapy,11, 12 owing to their chemical stability, synthetically tunable plasmon resonance bands within the visible and near-infrared spectral regions, and facile surface functionalization using thiol-containing molecules. Moreover, due to their chemical stability and various well- developed preparation methods, Au nanocrystals have also been frequently employed in fundamental plasmonic studies, such as plasmon resonance coupling,3, 18 plasmonic−molecular resonance coupling,19 and plasmon-controlled fluorescence.20, 21 On the other hand, Ag nanocrystals have been mostly utilized to enhance Raman scattering and fluorescence. Their performance in enhancing Raman and fluorescence signals has been found to be generally better than that of Au nanocrystals.22-24 This is because Ag nanocrystals exhibit much stronger electric field enhancements than Au nanocrystals. Figure 1 shows the finite-difference time-domain (FDTD) simulation results of the electric field intensity enhancements of the same sized Ag and Au nanorods of 30 nm in length and 20 nm in diameter. The longitudinal plasmon wavelengths of the Ag and Au nanorods are 463 and 564 nm, respectively. The field intensity enhancement contours were calculated under the excitation at their longitudinal plasmon wavelengths. The field intensity enhancement factors along the central length axis are plotted in Figure 1C. They decay rapidly away from the nanorod surface. The enhancement factors of the Ag nanorod are 3.5−6.8 times those of the Au nanorod, depending on the distance from the nanorod surface. Besides the larger electric field enhancements, Ag nanocrystals also exhibit higher refractive index sensitivities25, 26 and larger solar energy conversion efficiencies than Au nanocrystals.14 Moreover, compared to gold, silver has a higher interband transition energy and is able to support localized plasmon resonances in the near-ultraviolet spectral range. Ag nanocrystals are therefore very attractive in various plasmonic applications. However, although the growths of Ag nanocrystals with highly symmetric shapes, such as spheres, cubes, octahedrons, and triangular plates, have been widely demonstrated, the preparation of elongated anisotropic Ag nanocrystals of uniform shapes and narrow size distributions has proven challenging. As revealed by their extinction spectra, previously prepared anisotropic Ag nanorods are not as good as Au nanorods in terms of the shape and size uniformity.27, 28 In addition, Ag nanorods are rather instable and change automatically into nanospheres in aqueous solutions.28, 29 Moreover, the synthetic control of the aspect ratio of Ag nanorods and thus their longitudinal plasmon wavelengths has remained difficult. These obstacles severely hinder the use of the attractive plasmonic properties of Ag nanocrystals in many plasmon-based applications. Electric field intensity enhancement contours of a Ag and a Au nanorod. A) Ag nanorod. B) Au nanorod. The intensity enhancement contours are drawn at the logarithmic scale. C) Electric field intensity enhancement profiles along the length axis and passing through the center of the nanorod. Gold nanorods, owing to their attractive advantages, including facile growth methods, tunable longitudinal plasmon wavelengths, and chemical stability, can function as excellent supports for the formation of Ag shells. The resultant (Au core)−(Ag shell) nanorods are expected to exhibit plasmonic properties that are determined by silver. At the same time, the plasmon wavelengths of the core−shell nanorods can be tailored in the visible to near-infrared spectral regions by varying the Au core size and the Ag shell thickness. On the basis of this reasoning, (Au core)−(Ag shell) nanostructures in different shapes, including dumbbells,30, 31 rods,32-35 and octahedrons,36 have been prepared by different methods. Upon Ag coating, the lowest-energy plasmon band is generally blue-shifted and new plasmon bands are generated simultaneously at higher energies.32-35 The lowest-energy plasmon band is usually attributed to the longitudinal mode. The number of the newly generated plasmon bands is dependent on the shape of the core−shell nanostructures. However, the exact nature of the newly generated plasmon bands has not been investigated systematically and still remains controversial. Unraveling the nature of these plasmon bands is of vital importance because different plasmon modes exhibit distinct optical properties. For example, dipolar plasmon modes can radiate efficiently to the far field while quadrupolar plasmon modes only take effect in the near-field region mostly. Here we started with two differently sized Au nanorod samples and coated them with Ag shells systematically at varying thicknesses. The evolution of the plasmon bands, their peak wavelengths and extinction intensities as functions of the shell thickness was studied. The nature of each plasmon band was ascertained unambiguously with FDTD simulations. So far this has been the first systematical study on the plasmon resonances of (Au core)−(Ag shell) nanorods. Our results will be useful for applying (Au core)−(Ag shell) nanorods in the construction of optical devices and in various plasmon-enhanced spectroscopies. The uncoated Au nanorod samples were prepared using a seed-mediated growth method in aqueous solutions and are stabilized with cetyltrimethylammonium bromide (CTAB). The longitudinal and transverse plasmon resonance wavelengths of the large Au nanorod sample dispersed in aqueous solutions are 868 and 512 nm, respectively, as revealed by the extinction spectrum shown in Figure 2A. The particle concentration of the large nanorod sample is estimated according to its extinction coefficient37, 38 at the longitudinal plasmon peak to be 2.3 nM. Figure 2B shows the transmission electron microscopy (TEM) image of the uncoated large Au nanorods. The nanorods have narrow shape and size distributions. Their average length, diameter, and aspect ratio determined from the TEM images are 73 ± 8 nm, 18 ± 2 nm, and 4.1 ± 0.6, respectively. Extinction spectra and TEM images of the large Au nanorod sample and the obtained (Au core)−(Ag shell) nanorod samples. A) Extinction spectra. The samples 1−8 were produced from the growths with 0.04, 0.12, 0.24, 0.4, 0.56, 0.72, 0.88, and 1.2 mL of AgNO3 at 0.01 M, respectively. The volume of the added ascorbic acid solution at 0.1 M was equal to a half of that of AgNO3 for each sample. B) TEM image of the uncoated large Au nanorods. C−G) TEM images of the samples 3, 5, 6, 7, and 8, respectively. The (Au core)−(Ag shell) nanorods were prepared according to a previously reported method35 with slight modifications. Briefly, 2 mL of the uncoated Au nanorod solution was centrifuged and redispersed into an aqueous cetyltrimethylammonium chloride (CTAC) solution at the same volume. Varying volumes of aqueous AgNO3 and ascorbic acid solutions were then added to produce Ag coating at different thicknesses. Figure 2C−G show the TEM images of five representative (Au core)−(Ag shell) nanorod samples. The shapes and sizes of the nanorod samples are relatively uniform. As the amount of AgNO3 is increased, the (Au core)−(Ag shell) nanorods gradually change from a cylindrical to a cuboidal shape, with the edges and corners becoming sharper and sharper. The Ag shells at the side of the nanorods are seen to be thicker than those at the ends, resulting in a decrease in the aspect ratio. The extinction spectra of the (Au core)−(Ag shell) nanorod samples grown with increasing amounts of AgNO3 are shown in Figure 2A. For these extinction spectra, the dilution caused by the addition of the AgNO3 and ascorbic acid solutions have been corrected. Therefore, the particle concentrations in the solutions of all the samples are the same. Four plasmon resonance bands are They stronger as the of the Ag is the plasmon bands from to as respectively. The peak which has been ascribed to the longitudinal dipolar plasmon in The peak which has been ascribed to the transverse dipolar plasmon first and then at a The and the amount of AgNO3 a They only very as the amount of AgNO3 is the plasmon resonance of a Ag is generally than that of a Au of the same shape and the of the peak 1 can be as from both the in the aspect ratio of the nanorods and the increasing effect of the optical properties of silver those of The of the peak 2 can be attributed to the coating of the on the nanorod surface is from to silver the of the plasmon resonance peak wavelengths and intensities on the sizes of the (Au core)−(Ag shell) nanorods, we the length and as well as the Ag shell thickness of each sample from the TEM For each were their average were the thickness of the Ag shell at the side is different from that at the ends, the shell were at both the side and and The wavelengths and extinction intensities of the plasmon resonance were determined from the extinction spectra of the obtained are in in the Figure shows the of the of the Ag shell at the side and the volume of the AgNO3 solution for the core−shell nanorods prepared with the large Au nanorods as the The of the Ag shell at both the side and with increasing amounts of the in the thickness of the Ag shell at the side is than that at the This results in the of the aspect ratio of the (Au core)−(Ag shell) nanorods. of the plasmon peak wavelengths and intensities on the sizes of the core−shell nanorods with the large Au nanorods as the A) of the Ag shell at the side and the volume of the AgNO3 of the wavelengths and extinction intensities of the plasmon bands with the AgNO3 respectively. Evolution of the plasmon resonance wavelengths with the nanorod aspect ratio. Extinction intensities of the plasmon bands the particle volume. The and in the for the respectively. The on the are Figure the evolution of each plasmon band with the volume of the AgNO3 The peak located at 868 nm, first rapidly and then to The peak at 512 nm, first and then very The at a volume of the peak is The average of the (Au core)−(Ag shell) nanorods at the are and nm for the Ag shell at the side and ends, respectively. As the of the peak 2 is caused by the change of the from to silver. In addition, the peak 2 has been to arise from the transverse dipolar plasmon mode. Therefore, the of the that the effect of on the plasmon bands is the Ag shell thicker than The and on the extinction spectra the AgNO3 volume the average of the Ag shell at the side and are and nm, respectively. The of these two that the plasmonic properties of silver are the Ag shell thicker than As the Ag shell thickness is increased, the and only show very and nm, respectively. The extinction intensities of all the plasmon bands with increasing AgNO3 volumes The peak 2 is than the peak 1 at volumes of stronger than the peak 1 the AgNO3 volume is the average shell thickness is nm in the transverse and nm in the longitudinal the of the core−shell nanorods are still larger than the This that the (Au core)−(Ag shell) nanorods can support longitudinal and transverse dipolar plasmon modes with which is usually not in other elongated metal nanostructures. This the (Au core)−(Ag shell) nanorods useful for plasmonic applications. For example, in plasmon-controlled because of the large of the from the of the peak 2 can be used to enhance the excitation and the peak 1 can be utilized to enhance the 21 Figure the of the plasmon resonance wavelengths the aspect ratio of the nanorods. The peak 1 a of the peak on the aspect ratio. As the aspect ratio is the peak 2 first and then very The of the 1 and 2 the of the change on the plasmonic properties of the (Au core)−(Ag shell) nanorods, because the longitudinal dipolar plasmon usually a with the aspect ratio for nanorods of a The and exhibit very as the aspect ratio is also obtained the of the extinction intensities as functions of the particle volume The of the extinction intensities of the plasmon bands the particle volume are very to those of the extinction intensities as functions of the AgNO3 volume. This that the Ag is to silver on the core−shell nanorods at a conversion of the nanorod FDTD to the nature of the plasmon modes of the (Au core)−(Ag shell) nanorods, especially the and The average sizes of the sample 8 were The FDTD were in a of which has a refractive index of The (Au core)−(Ag shell) nanorod was as a with corners and and the Au nanorod core was as a with a at each were in the simulations. They are along the longitudinal the longitudinal the transverse the transverse and the respectively. The calculated and extinction spectra for each are shown in Figure in the The peak 1 can be with the and the other can be with the the and calculated extinction spectra that the spectrum can be by the spectrum the spectrum the one the one The relatively large in the plasmon of the peak 1 can be ascribed to that the spectrum is from the sample while the calculated ones are on the average The plasmon of the peak the longitudinal plasmon mode, is to the nanorod size than those of the other and also that the extinction spectrum different from the one in the spectral region the and and of this the spectrum is better from the and spectra than from the and In addition, is a on the side of the peak on the extinction spectrum from the and This is on the spectrum The slight the and spectra is caused by the the collective electron oscillations with the two which will be FDTD simulations. A) and calculated extinction spectra of the sample 8 shown in Figure 2A. The spectra have been the at the peak contours calculated for the respectively. extinction spectra for a nm nm Ag under different excitation in and in a with a refractive index of contours calculated at the for the nm nm Ag the of the the peak 1 is under the and the 2 are along the The and in the contours for and respectively. The nature of the different plasmon modes can be from their distributions. Figure shows the contours of the plasmon modes. For the peak the excitation is along the For the the excitation is along the amounts of are observed at the the Au core and Ag The of the has not been observed In to the plasmonic properties are by the as well as by the Au we FDTD on a Ag and a core)−(Ag shell) with the same The simulation results that the extinction spectra of the Ag and the core)−(Ag shell) are very to that of the (Au core)−(Ag shell) nanorod and Therefore, the and the Au core a on the plasmonic properties of the (Au core)−(Ag shell) nanorod. The plasmonic properties are determined by the Ag Figure in the shows the contours of the for the Ag They the same as those of the (Au core)−(Ag shell) nanorod the of the to the the 1 and 2 can be unambiguously attributed to the longitudinal and transverse dipolar respectively. The and can both be to an octupolar mode. In addition, the contours obtained under the two are also have shown that Ag also exhibit plasmon In the from to the plasmon modes of Ag have been attributed to a two and an octupolar mode, respectively. The in the nature of the plasmon modes the (Au core)−(Ag shell) nanorod and the Ag can be by the different by a and a The of a belongs to the a has the The nature of the plasmon modes by a metal has been to be dependent on the of the In addition, plasmon modes have also been observed on (Au core)−(Ag shell) They are ascribed to be and respectively, in the of to with the exact nature of the two modes As the calculated extinction spectra under the and exhibit a slight in the spectral region the and In to this we the core−shell nanorod to nm For a Ag was The has been proven to be in the of plasmon resonances, because the and caused by large Figure shows the calculated extinction spectra of the Ag under the and the only the peak and the other can be by the The and extinction spectra are the the spectrum is by one peak and two and In two distinct 2 and and one peak are observed under the the excitation field can be into the two with the same the the and extinction spectra for the Ag that the the collective electron oscillations along the two is for the of the peak The peak of the Ag is very under both the and The that the peak of the (Au core)−(Ag shell) nanorod is stronger can therefore be ascribed to the this we calculated the extinction spectrum of the Ag under the excitation in a a higher refractive index of this the peak is a metal in a with a higher refractive index can the effect due to the of the excitation The effect is therefore for the peak on the (Au core)−(Ag shell) nanorod. Moreover, the contours of the for the Ag are the same as those for the (Au core)−(Ag shell) nanorod This of the different plasmon modes to the on the extinction spectra of the (Au core)−(Ag shell) nanorods. prepared an of (Au core)−(Ag shell) nanorod samples using a Au nanorod sample as the core in to the of the different plasmon modes on the sizes of the Au The transverse and longitudinal plasmon wavelengths of the Au nanorod sample dispersed in aqueous solutions are and nm, The particle concentration of the Au nanorod sample is estimated according to the longitudinal plasmon peak extinction to be nM. average length and are determined from the TEM images to be ± and ± 1 nm, The average aspect ratio is from for the large Au nanorod sample to for the Figure shows the extinction spectra of the (Au core)−(Ag shell) nanorod samples that were grown from 2 mL of the Au nanorod sample with increasing amounts of The evolution of the extinction spectra is very to that observed for the core−shell samples prepared from the large Au nanorod sample. plasmon bands are The TEM images of representative core−shell samples are in Figure The core−shell nanorods gradually from a cylindrical to a cuboidal shape, with the edges and corners becoming relatively as the of AgNO3 is These results that the growth and the evolution of the plasmonic properties are relatively to the sizes of the Au nanorods within a range. On the other hand, we that core−shell nanorods are on the TEM They under TEM The is seen for the large core−shell nanorods. is due to the of the core−shell nanorods. In addition, the Au are seen to be from the center for many core−shell nanorods. This is not observed at all for the large core−shell nanorods. also prepared Au nanorod sample and the Ag The of the Au from the center was also will be to the for this Extinction spectra and TEM images of the Au nanorod sample and the obtained (Au core)−(Ag shell) nanorod samples. A) Extinction spectra. The samples were produced from the growths with 0.04, 0.12, 0.24, 0.4, 0.56, 0.72, and mL of AgNO3 at 0.01 M, respectively. The volume of the added ascorbic acid solution at 0.1 M was equal to a half of that of AgNO3 for each sample. The dilution caused by the addition of AgNO3 and ascorbic acid solutions has been corrected. B) TEM image of the uncoated Au nanorods. TEM images of the samples 3, 5, and 7, respectively. Figure the of the extinction peak wavelengths and intensities of the (Au core)−(Ag shell) nanorod samples. As the amounts of AgNO3 is increased, the Ag shell at both the side and with the thickness of the shell at the side increasing than that at the The of the peak at nm, first rapidly and then to nm The peak at nm, first and then The at a AgNO3 volume of the peak is nm and the average shell are and nm at the side and ends, respectively. This that the effect of on the plasmonic properties is the Ag shell thickness is larger than The and the AgNO3 volume the shell is at the side and at the The wavelengths of the and are and nm, respectively, on the amount of of the peak wavelengths the large and (Au core)−(Ag shell) nanorods that the of the peak 1 of the core−shell nanorods is and that the wavelengths of the are to each other for the two of core−shell nanorod samples. The extinction intensities of all the of the (Au core)−(Ag shell) nanorods with the amount of with those of the 1 and 2 increasing than those of the and Figure shows the of the wavelengths of the the aspect ratio. As the aspect ratio is the peak 1 a the peak 2 first and then owing to the change at the and the and in terms of the spectral The extinction intensities of the all with the particle volume the large and (Au core)−(Ag shell) nanorods that the for the shell and the peak wavelengths are in the two that are slight in the of the peak of the plasmon peak wavelengths and intensities on the sizes of the core−shell nanorods with the Au nanorods as the A) of the Ag shell at the side and the volume of the AgNO3 of the wavelengths and extinction intensities of the plasmon bands with the AgNO3 respectively. Evolution of the plasmon wavelengths with the nanorod aspect ratio. Extinction intensities of the plasmon bands the particle volume. The and in the for the respectively. The on the are In (Au core)−(Ag shell) nanorods of varying Ag shell were prepared using Au nanorods as the Four plasmon bands are observed on the (Au core)−(Ag shell) nanorods. The evolution of the peak wavelengths and extinction intensities of the plasmon bands were studied as functions of the nanorod sizes and the amount of the Ag that the lowest-energy peak belongs to the longitudinal dipolar plasmon mode, the second-lowest-energy peak is the transverse dipolar plasmon mode, and the two highest-energy can be ascribed to octupolar plasmon modes. The effect and the the along the two are for the of the highest-energy and octupolar plasmon respectively. are at the the Au core and Ag the Au core not the plasmonic properties of the core−shell nanorods the Ag shell thickness is than as the Ag shell the longitudinal plasmon the transverse plasmon first and then and the two octupolar plasmon modes in terms of their spectral The extinction intensities of all the plasmon bands with the particle The results obtained from study will in the localized plasmon resonances of (Au core)−(Ag shell) nanorods. They will also be useful for the different plasmon modes of (Au core)−(Ag shell) nanorods simultaneously in various plasmon-based applications. of the (Au The Au nanorod samples were obtained from They are stabilized by in aqueous The particle concentrations of the large and Au nanorod samples are estimated to be 2.3 and respectively. The (Au core)−(Ag shell) nanorods were prepared according to a reported with slight for the coating of the large Au nanorod of the Au nanorod solution were centrifuged and redispersed into aqueous solutions at the same volume. 0.04, 0.12, 0.24, 0.4, 0.56, 0.72, 0.88, and 1.2 mL of AgNO3 were added into the respectively, by the addition of ascorbic acid solutions The volume of the ascorbic acid solution was a half of the AgNO3 solution for each The resultant solutions were in an at for The same was to the Au nanorod that of the Au nanorod solution were were observed within 20 the addition of the ascorbic acid solution into the Au nanorod the of a Ag shell the Au nanorods. The growth of was used in all the growths in to the of AgNO3 as as Extinction spectra were on a with that an optical length of TEM was on an at FDTD The FDTD were using FDTD which was developed by the an in the from to nm was into a a size of nm was employed in the electric field enhancements of the Au and Ag nanorods and in the large core−shell nanorods, including the (Au core)−(Ag core)−(Ag and core)−(Ag shell) nanorods. size of nm was in the of the Ag The refractive index of the was to be which is the index of In one simulation for the Ag the index was to be to the The sizes of the nanostructures were according to the average sizes from the TEM images the Ag The uncoated Au and Ag nanorods in Figure 1 were as a with a at each The core in all the core−shell nanorods was as a with a at each The and length of the core were and 73 nm, respectively. The of the were and nm along the length and thickness of the core respectively. The core−shell nanorod was as a with corners and with the of both the and to be The length and of the core−shell nanorod were and nm, respectively. The functions of and silver were from previously is from the from the This was by and The FDTD in this were in the which is by and of importance to are as are not They are as by the The is not for the of information by the than be to the for the