Appl Phys A (2011) 102: 153–160 DOI 10.1007/s00339-010-6058-5
Control of plasmon resonance of gold nanoparticles via excimer laser irradiation Daria Riabinina · Jianming Zhang · Mohamed Chaker · Joëlle Margot · Dongling Ma · Peter Tijssen
Published online: 6 November 2010 © Springer-Verlag 2010
Abstract A significant shift of the surface plasmon resonance absorption spectrum of gold nanoparticles was obtained by the oxidation of the nanoparticle surface via pulsed excimer laser irradiation. The high UV-light absorption of gold nanoparticles chemically produced by citrate reduction led to the important surface oxidation up to 26%. As a result of laser irradiation, the gold/gold oxide core-shell nanoparticles with little variation of the nanoparticle size were produced. After only 5 min of laser irradiation, a 12-nm blue shift in surface plasmon resonance was obtained. The possible mechanisms governing the modification in surface plasmon resonance by laser irradiation of gold nanoparticles were discussed.
1 Introduction Surface Plasmon resonance (SPR) is a charge density oscillation excited by electromagnetic radiation at a metaldielectric interface. Plasmonics is a highly active area of research on such light-metal interactions [1], in particular on metallic nanoparticles (NPs) having a high surfaceD. Riabinina () · J. Margot Département de physique, Université de Montréal, C.P.6128, Succ. Centre-Ville, Montréal, Québec H3C 3J7, Canada e-mail:
[email protected] Fax: +1-450-9298102 D. Riabinina · P. Tijssen Institut Armand-Frappier, INRS, 531, boul. des Prairies, Laval, Québec H7V 1B7, Canada J. Zhang · M. Chaker · D. Ma INRS-Énergie, Matériaux et Télécommunications, Université du Québec, 1650 Lionel-Boulet, C.P. 1020, Varennes, Qc J3X 1S2, Canada
to-volume ratio [2, 3]. Many industrial applications such as optical devices, sensors as well as medical diagnostics and therapeutics have been proposed and actively developed [1, 4–7]. The tuning of optical properties of metallic nanoparticles is well controlled by precise variation of their size, shape, and spacings [8–14]. In addition, according to the theoretical description of a localized surface plasmon resonance, the optical properties of metallic nanoparticles can be controlled through various factors described hereafter [15]. First, the plasma frequency can be changed by charge localization at metal nanoparticles. It has been reported in literature that this phenomenon can yield a spectrum shift as large as about 10 nm [16]. Also, the red shift and broadening of a localized surface plasmon resonance (LSPR) spectrum can be a consequence of the modification of nanoparticle shape or aggregation, due to the changes in depolarization coefficient. Another factor is the dependence of resonance wavelength on the refractive index of the surrounding matter which led to the development of a large research field of synthesis of core-shell nanoparticles [17]. It has been clearly demonstrated that bimetallic core-shell nanostructures undergo important displacements of the LSPR peak depending on the thickness of the shell. For example, an important 100-nm blue shift was observed for solution-grown Au–Ag core-shell nanoparticles [17]. The electrical tuning of the LSPR spectrum of gold nanoparticles in liquid crystal due to the changes in refractive index of surrounding media has been reported [18]. It has also been demonstrated that electrical tuning of stabilized gold nanoparticles in various solutions yields to the significant peak shift of the absorption spectra of gold nanoparticles due to their electrochemical oxidation [15]. Due to the metastable nature of the gold oxide [19], the synthesis of gold/gold oxide core-shell nanoparticles constitutes a great challenge for the understanding of the influence of gold ox-
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ide shell on optical properties of gold nanoparticles. Also, it is extremely difficult to differentiate the influence of various parameters such as localized charge, refractive index of surrounding medium, and surface chemistry on optical properties of gold nanoparticles. We suggest here a novel method for tuning of the optical properties of gold nanoparticles, which is pulsed laser irradiation. It has been recently demonstrated that pulsed laser ablation in liquid media (PLAL) allows a synthesis of gold nanoparticles with a fair control of size, which ranges from 2 to 100 nm in diameter [20–23]. The laser-material interaction yields the partial oxidation of colloidal gold suspension during the ablation process [24]. The PLAL process involves two superimposed mechanisms: (1) the ablation of the material, and (2) the interaction between laser and ablated nanoparticles. Because two processes take place simultaneously, it is impossible to characterize their specific influence on the structural and chemical composition of the resulting colloids. In the present work, we have addressed this challenge by examining the influence of laser irradiation on gold colloids prepared independently by chemical synthesis. The correlation between structural, chemical and optical properties of irradiated gold nanoparticles was examined. In particular, the role of laser irradiation on the chemical composition of gold nanoparticles was investigated for the first time. In addition, we studied the influence of the structure and composition of irradiated nanoparticles on their plasmon-related optical properties.
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volume was 3 ml and the liquid layer thickness was 1 cm. Under laser irradiation, the solution progressively changed its color from clear red to light purple. Au nanoparticles were characterized by transmission electron microscopy (TEM), absorption spectroscopy, XRay diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). For TEM measurements, Au nanoparticles were centrifuged on Cu/C grids, and analyzed using a Hitachi H-7100 TEM operated at 75 KeV. High-resolution TEM (HRTEM) measurements were performed using a JEOL 2100F microscope operating at 200 KeV. XRD analysis was used to determine the size of Au nanoparticles using the Scherrer equation for the broadened XRD peaks [26, 27]. For XRD measurements, a drop of colloidal gold solution was dried out on a glass substrate in order to avoid the interference of data with peaks coming from crystalline substrate. XRD patterns were acquired at a grazing angle of 1◦ , in the region of the Au (111) diffraction peak at θ = 38.2◦ . XPS measurements were used to examine the oxidation process of irradiated gold as a function of the irradiation time. Sample preparation was identical to the one for XRD analysis. XPS spectra of Au colloids were recorded using a Twin Mg X-ray source and an electron energy analyzer operating in the constant pass energy mode (15 eV). The electron take-off angle was held at 90◦ . Finally, the absorption spectra of colloidal gold solutions were measured using UV-Vis-NIR spectrometer Varian 5000.
3 Results 2 Experimental methods Gold nanoparticles having a diameter of approximately 11 nm were prepared by sodium citrate reduction of HAuCl4 [25]. For this purpose, 75 mg of HAuCl4 diluted in 250 ml of distilled water (0.88 mM) was brought to a vigorous boil with stirring in a round-bottom flask fitted with a reflux condenser. A volume of 26.25 ml of 1% (w/v) sodium citrate solution was then rapidly added to flask. The solution was boiled for another 15 min, during which time the solution changed from pale yellow to deep red. The solution was allowed to cool to room temperature with continuous stirring. The suspension was then filtered using a 0.22-µm filter (Corning) to remove large aggregates and further stored at 4◦ C. Under these conditions, the solution remains stable for several months. As-prepared Au nanoparticles were irradiated with a pulsed KrF laser (wavelength 248 nm, pulse length 17 ns, repetition rate 20 Hz) during various time durations, from 0 to 5 min. Laser power was set at 50 mJ/pulse, and the laser was weakly focused on solution, with a spot diameter of about 1 cm. During irradiation, the colloidal solution was continuously mixed using a magnetic stirrer. The solution
Figure 1 shows TEM images of the original (0 s) and laserirradiated colloidal Au nanoparticles (5, 10, 30 s, 1 min, and 5 min after irradiation). The as-synthesized gold nanoparticles are isolated with a diameter of about 11 nm. After several seconds of laser irradiation, the TEM images show the presence of aggregates. These agglomerations can be possibly due to the sample preparation. The original absorption spectra of freshly irradiated nanoparticles do not reveal any important red shift (of the order of 100 nm) which could be associated to the precipitation of nanoparticles. One also notices that the individual nanoparticle size becomes slightly smaller as irradiation time increases. This results from a fragmentation effect under laser irradiation, as already reported in literature [28] for femtosecond pulse durations. Indeed, the authors observed the fragmentation and the narrowing of the nanoparticle size distribution of laser-ablated and irradiated nanoparticles. In our case, the initial distribution of chemically-synthesized nanoparticles was very narrow (11 ± 1) nm so that laser irradiation did not improve the size distribution width. The agglomeration of nanoparticles during the sample preparation of irradiated nanoparticles prevents the determination of the final size distribution by TEM analysis.
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Fig. 1 TEM images of colloidal gold nanoparticles before (0 s) and after (5, 10, 30 s, 1 min, 5 min) irradiation. In the inset, a high-resolution TEM image of Au nanoparticles irradiated during 5 min
Due to the difficulties encountered for determining the NP size from TEM measurements, we would rather rely on the XRD technique. It has been demonstrated before that XRD and Scanning Tunneling Microscopy (STM) analysis are in a good agreement concerning the size of Pt nanoparticles produced by laser ablation in an inert gas atmosphere [29]. This comparison of the XRD and STM techniques confirmed that XRD is a viable technique for determination of size for metallic nanoparticles synthesized by laser ablation and can thus be employed in the present work for analysis of gold nanoparticles rather than TEM characterization. In the inset of Fig. 2, a typical XRD pattern of Au nanoparticles lying on a glass substrate is displayed. Assuming the absence of strain in gold NPs, we calculated the average nanocrystal size using the Scherrer formula. This size is found to be equal to (10.5 ± 0.6) nm for original nonirradiated Au NPs, for the peak full width at half maximum (FWHM) equal to about 1◦ . Figure 2 shows the evolution of the Au nanocrystal average size extracted from XRD spectra as a function of irradiation time. The nanoparticle size decreases from 10.5 nm down to 8.5 nm for the irradiation times varied from 0 to 300 s. This behavior can be possibly related both to the nanoparticle fragmentation or partial oxidation of Au nanoparticles. XPS measurements were carried out to investigate the chemical changes induced by laser irradiation of the Au nanoparticles. Figure 3a shows the XPS spectra in the 4f region of irradiated nanoparticles for three different irradiation times (5, 60, and 300 s). The raw data were corrected for charging effect by using the binding energy of the C1s peak on hydrocarbon (284.8 eV). Two sets of doublet (4f7/2 and 4f5/2 ) are seen in the 4f core level spectra. The first set is located at 83.2 and 86.9 eV, while the second one is located at 85.1 and 88.8 eV. There is a 3.7-eV energy difference
Fig. 2 Au nanoparticle average size determined from XRD spectra as a function of irradiation time, for the original nanoparticle size of 11 nm. In the inset, a typical XRD spectrum of gold nanoparticles is shown
between the 4f7/2 and 4f5/2 components of each doublet, which corresponds to the separation expected for these two core shells [30]. In Fig. 3a, the doublet situated at 83.2 eV and 86.9 eV is assigned to gold in the metallic state. Based on the literature [30], value for the binding energy difference between the 4f7/2 and 4f5/2 components in XPS spectra of gold/gold oxide (1.9 eV) allows to identify the corresponding phase as gold oxide. The XPS spectra in the Au 4f regions were analyzed assuming that they are composed of four components corresponding to Au 4f7/2 and Au 4f5/2 doublets. The results of the curve-fitting of XPS spectra are displayed in Table 1. The relative atomic ratio between the metallic gold and the gold oxide components was evaluated by calculating the ratio between the area of the 4f7/2 and 4f5/2 components of each compound. Figure 3b displays the relative concentration of gold oxide (Au oxide vs. gold nor-
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Fig. 3 (a) XPS spectra of gold nanoparticles irradiation during 5, 60, and 300 s; (b) Relative concentration of oxidized gold as a function of laser irradiation time
Table 1 Curve-fitting parameters extracted from the analysis of the Au 4f core level region of irradiated Au nanoparticles
Irradiation time (s) 5
Au 4f7/2 P
Area
83.2 1250
malized XPS signal ratio) as a function of irradiation time. It can be seen that the laser-nanoparticles interaction in water results in oxidation of gold up to 26% of its initial concentration. This clearly demonstrates that laser irradiation of gold nanoparticles in water results in a metastable material such as gold oxide. The oxidation is progressive as it depends on irradiation time or on the number of laser pulses. This result clearly shows that laser irradiation not only modifies the nanoparticle size and the size distribution, as reported in the literature [31], but also significantly influences the chemical composition of Au nanoparticles. The nanoparticle oxidation revealed by XPS measurements has been confirmed by the HRTEM analysis of irradiated nanoparticles. The inset in Fig. 1 shows the HRTEM image of gold nanoparticles irradiated during 300 s. One can notice that the crystalline structure of gold can be observed mostly at the center of the nanoparticle, surrounded by a dark-gray amorphous area which can be interpreted by the presence of gold oxide on nanoparticle surface. This result supports the hypothesis of the formation of core-shell gold/gold oxide nanoparticles after laser irradiation. In resume, combining XRD and XPS analysis of Au nanoparticles submitted to laser irradiation in water, it is demonstrated that laser irradiation influences both the nanoparticle size and the oxidation degree of gold. It is well known that metal nanoparticles, in particular gold, strongly absorb wavelengths corresponding to their plasmon resonance which itself strongly depends on the nanoparticle size. In the present work, in order to understand the influence of laser irradiation on the plasmon-related optical properties of the nanoparticles, we have examined the absorption spectra of colloidal gold solutions. Figure 4a shows the absorption spectra of irradiated gold nanoparticles. A plasmon resonance peak of original gold nanoparticles is located at about 520 nm, which is in a good agreement with the data reported in literature for gold nanoparticles of the same size [13]. The FWHM of the absorption peak remains rather similar for all irradiated samples, which indicates that laser irradiation does not significantly modify the size distribution of gold nanoparticles. Figure 4b displays the position of the absorption peak of gold NPs as a function of the irradiation time. A significant blue-shift of the
Au 4f5/2 FWHM P 1.50
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Area 975
AuO 4f7/2 FWHM P 1.50
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AuO 4f5/2
Area FWHM P 41
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Fig. 4 (a) Absorption spectra of colloidal gold solutions for various irradiation times (initial nanoparticle size of 11 nm. (b) Absorption peak position as a function of irradiation time
peak position is observed which reaches 11 nm after irradiating the nanoparticles for 300 seconds. Combining the data on peak position and gold oxidation, we have determined the dependence of the absorption peak position on the relative concentration of gold oxide as shown in Fig. 5a. The peak position undergoes a significant blue shift when the gold oxidation degree increases from 0 to 26%. Combining the information obtained from absorption spectroscopy and XRD analysis yields the dependence of the absorption peak position on nanocrystal size shown in Fig. 5b. The peak position undergoes a blue shift with decreasing nanoparticle size. However, the observed size variation is rather small and should not cause such an important peak shift [13]. To verify the reproducibility of these results, we performed the same measurements on gold nanoparticles of different size synthesized by citrate reduction followed by laser irradiation. Figure 6 shows the nanoparticle size extracted from XRD measurements as a function of irradiation time. The original size of gold nanoparticles was 22 nm. Laser irradiation during 120 s leads to the nanocrystalline size decrease from 22 to about 16 nm. Figure 7a shows the absorp-
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Fig. 5 (a) Peak position as a function of relative concentration of gold oxide for Au nanoparticles (initial size of 11 nm). Open triangle correspond to the data for original non-irradiated Au NPs. (b) Absorption peak position as a function of nanoparticle size (initial size of 11 nm)
Fig. 6 Au nanoparticle average size determined from XRD spectra as a function of irradiation time, for the original nanoparticle size of 22 nm
tion spectra of gold nanoparticles as a function of irradiation time. Peak position undergoes an important blue shift which is similar to the peak shift observed for 11-nm gold nanoparticles (Fig. 4).
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Fig. 8 Absorption peak position as a function of nanoparticle size. Solid triangles show the peak position for laser irradiated Au nanoparticles as a function of size, for two different initial sizes of chemically synthesized nanoparticles, i.e., 11 nm and 22 nm. Dashed line is a fit of experimental data found in literature for Au nanoparticles produced via chemical synthesis (solid stars [25], cross [14], and solid squares [13]), dotted line is a fit of calculations (open circles [13])
4 Discussion
Fig. 7 (a) Absorption spectra of colloidal gold solutions for various irradiation times (initial nanoparticle size of 22 nm). (b) Absorption peak position as a function of irradiation time
The ensemble of results on 11-nm and 22-nm gold nanoparticles are compared to those well established that were observed for gold nanoparticles synthesized by the chemical method of citrate reduction [13, 14, 25, 32]. Figure 8 shows the superposition of experimental (solid stars [25], cross [14] and solid squares [13]) and calculated (open circles [13]) data found in literature for nanoparticle size range between 5 and 30 nm. The dashed line is a fit of experimental data while dotted line is a fit of calculation results. Although there is certain divergence between experiment and calculation found in literature, both curves show that there is no significant variation in absorption peak position within the investigated nanoparticle size range. Figure 8 also shows experimental data obtained in this work (open and solid triangles) superposed with those found in literature. The data corresponding to laser–irradiated NPs at two different original sizes, namely 11 nm and 22 nm (triangles) clearly emphasize major modifications of the optical properties of laser–irradiated gold nanoparticles. Therefore, it is clear that the peak shift observed for irradiated nanoparticles in Fig. 8 cannot be explained simply by the decrease in nanoparticle size.
Our studies demonstrated that the SPR-related absorption spectra of gold nanoparticles chemically synthesized by citrate reduction undergo a significant blue shift upon laser irradiation. The investigation of their structural properties and chemical composition revealed that many factors could have influenced this behavior of the plasmon resonance absorption spectrum. The theoretical expression of resonance wavelength λ of the localized surface plasmon can be expressed as follows [15]: 2πc (4π − N )n2 + N 1/2 λ= , (1) ωp N where ωp is the plasma frequency, N is the depolarization coefficient, n is the refractive index of the surrounding medium, and c is the velocity of light in vacuum. From formula (1), the resonance wavelength is a function of n, N , and ωp . The plasma frequency ωp is proportional to the square root of the density of the electron and to the elementary charge. The increase of the negative charge on gold nanoparticles, i.e., the injection of electrons into nanoparticles may result in the blue shift of the SPR spectrum [16, 33]. During laser irradiation of gold nanoparticles, a high-density UV energy is intensely absorbed by gold colloids, which can certainly induce ionization effects on nanoparticle surface. Indeed, it has already been observed in laser-ablation process that laser-matter interaction induces the ionization of the oxidized gold nanoparticles which contributes to their negative charge [24]. These additional laser-irradiation induced free electrons on gold nanoparticle surface can be thus one of possible explanations of the observed blue shift.
Laser irradiation of gold nanoparticles
The second possible mechanism influencing the plasmon resonance wavelength is the depolarization coefficient which depends on nanoparticle shape [34]. However, in our case, this mechanism seems to be improbable because the original gold nanoparticles are spherical and the only possible shape modification could be the elongation of nanoparticles which would result in a red shift of the absorption peak, contrarily to the observed blue shift. The third possible mechanism governing the peak shift of the SPR spectrum is the decrease in nanoparticle size [13, 14]. In our work, the irradiation-induced fragmentation of nanoparticles yields the decrease in nanoparticle size from 10.5 to 8.5 nm which should normally correspond to a 1–2 nm blue shift of SPR absorption peak. This little size variation of gold nanocrystals obtained using XRD analysis can partially explain the SPR blue shift but is not significant enough to justify such an important variation of the peak position as 11 nm (see Fig. 8). Another important mechanism that could influence the SPR spectrum is the variation of refractive index of matter close to the nanoparticle surface. These changes can be induced by various methods: the modification in refractive index of the surrounding medium, absorption/desorption of chemical products on nanoparticle surface, and the creation of core-shell structures with dielectric shells of different refractive index [15]. The blue-shift of the SPR spectrum may occur with the decrease in refractive index of surrounding medium. In our case, the surface oxidation of the gold nanoparticles cannot be considered as a plausible explanation of the blue-shift of the absorption peak. It has been recently demonstrated that the electrochemical oxidation of the gold nanoparticles results in a large red shift in localized SPR spectrum. The blue shift observed in our work can thus only be explained by two principle mechanisms related to the complex surface chemistry occurring during laser–matter interaction, i.e., ionization, decomposition, and absorption/desorption of surfactant products at the nanoparticle surface, and/or laser-induced chemical reactions, such as decomposition of citrate [35], leading to the changes in refractive index of the surrounding solution. Although laser irradiation involves a superposition of complex physicchemical processes induced by laser–matter interaction, the laser-irradiation of gold nanoparticles remains a remarkably efficient method for fine tuning of the optical properties and chemical composition of gold nanoparticles.
5 Conclusion In conclusion, using a combination of measurements (TEM, XRD, XPS, and absorption spectroscopy), we have demonstrated that UV laser irradiation induces important changes in the SPR absorption spectrum of colloidal gold solutions.
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We have determined that laser irradiation for only 5 min induced important changes in chemical composition of gold nanoparticles, i.e., their partial oxidation up to 26% and an important spectral blue-shift of about 11 nm. Possible mechanisms governing the modifications in the surface plasmon resonance of gold nanoparticles were discussed. We concluded that the blue shift of the absorption peak is most probably due to the superposition of several mechanisms, such as slight size modification, particle surface ionization, absorption and desorption of surfactant products at the nanoparticle surface, and possibly decomposition of chemical products leading to the changes in the refractive index of the surrounding media. Acknowledgements The authors acknowledge the financial support from the National Science and Engineering Research Council, the Fonds Québécois de Recherche en Nature et Technologies, and the Canada Research Chair program.
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