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Characterization of Tetrahexahedral Gold Nanocrystals: A Combined Study by Surface-Enhanced Raman Spectroscopy and Computational Simulations Peng-Gang Yin,* Ting-Ting You, En-Zhong Tan, Jing Li, Xiu-Feng Lang, Li Jiang, and Lin Guo* School of Chemistry and Environment, Beihang University, Beijing 100191, China ABSTRACT: Tetrahexahedral (THH) and elongated tetrahexahedral (ETHH) gold nanocrystals (NCs) were fabricated and were studied for their unique surface plasmon (SP) excitation. UVvis absorption spectra and surface-enhanced Raman scattering (SERS) were introduced to experimentally investigate the far-field optical properties and near-field enhancement ability of nanoparticles. Calculation of electric field distribution on the basis of three-dimensional finite-difference time domain (3D-FDTD) method revealed that the E-field enhancement is largely confined to tips with strong dependences on geometry of tip and polarization of incident light. Enhancement factors were estimated, and several influence factors such as coupling effect were mentioned with discussion for the potential advantages of polyhedron-like structure in plasmon-related application.
1. INTRODUCTION Tetrahexahedral (THH) nanocrystals have been successfully synthesized15 and have drawn great interest for their high biocompatibilities and catalyzed properties as a result of enhanced chemical activities of high-index planes.68 Apart from that, these polyhedron-like nanostructures enclosed by 24 facets were also expected to provide unique near-field enhancement properties because of their pointy shape which is less mentioned in recent research. While plasmons have been studied for decades, the appearance of nanoscience has created a new revolution in this field. The interactions between light and metallic nanostructures became increasingly interesting as experimental fabrication techniques became mature. These subwavelength structures enable unique manipulation of electromagnetic fields at the nanoscale because of their ability to support so-called surface plasmons (SPs). Surface plasmons are oscillations of free electrons in metal that could couple with the electromagnetic field, which leads to unique optical properties of various nanostructures dominated by localized surface plasmon resonances (LSPR). Because of both strong field localization and unique optical properties induced by LSPR, nanostructures could function as ultrasensitive chemical and biological sensors, as surface-enhanced spectroscopy substrates, in nonlinear optics applications, and even as elements in optical nanocircuits. Since the plasmonic effects are highly geometry dependent, nanoparticles with shapes other than solid sphere, such as cubic, star, octopod, rice, and flowerlike structures,914 have drawn great interest in plasmon-related studies and applications, which remains a hot topic of current research. Many applications such as surface-enhanced raman scattering (SERS), which improves the sensitivity by amplifying the original signal for several or even tens of orders of magnitude, rely on the near-field characteristics of nanoparticles caused by LSPR especially in the hot spot.15 Hot spot represents a particularly large enhancement of the local field that is highly confined at a special r 2011 American Chemical Society
position on the surface because of geometry factor, symmetry breaking, and coupling effect. Since SERS was discovered in 1974, this technique has been greatly improved as substrates with great sensitivity and reproducibility have been well designed and fabricated.1619 Gold nanoparticles have been extensively reported as the substrate in SERS for advantages in local field enhancement, surface chemical activities, and biocompatibility. Not only is SERS widely used in the fields of molecule detection, sensor application, and surface process study as a branch of Raman spectroscopy, it also becomes an important characterization tool in nanoscience study since the enhancement process is extremely morphology sensitive.2024 As is generally considered, two sorts of mechanisms are used to describe the overall SERS effect: electromagnetic mechanism (EM) and chemical mechanism (CM).2528 Our work mainly focused on the former one for insights into the plasmonic properties of specific nanostructures with detailed characterization through scanning electron microscopy (SEM) and absorption spectra and further comparison and discussion via numerical calculations. Numerical methods were frequently applied to explore the optical properties of nanostructures and, on the other hand, to promote development of nanomaterial into the application of optics. Several numerical methods, including discrete dipole approximation (DDA), finite difference time domain (FDTD), finite element method (FEM), and boundary element method (BEM), have been widely applied in nano-optics study as efforts were made for obtaining the near-field and far-field information of newly synthesized nanomaterial to further understand its nearfield enhancement properties.2934 Among these simulation methods, 3D-FDTD simulation was generally conducted for Received: May 5, 2011 Revised: June 24, 2011 Published: July 04, 2011 18061
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Figure 1. 3D-FDTD models of THH NCs. (a) THH, (b) ETHH. Red dot: 6-facet-tip. Blue dot: 4-facet-tip. Green dot: 3-facet-tip. SEM image of THH NCs: (c) THH and (d, e) ETHH. The scale bars in the SEM images are 100 nm.
electromagnetic calculation to achieve E-field distribution and relevant optical spectrum of nanostructure especially for complex shapes and systems. In this paper, we investigated the near-field enhancement properties of THH and elongated tetrahexahedral (ETHH) gold nanocrystals (NCs) experimentally through SERS using p-mercapto benzoic acid (PMBA) as probe molecule and theoretically by 3D-FDTD solution. The successfully synthesized THH and ETHH gold nanoparticles were first characterized by SEM and UVvis absorption spectroscopy for model building process and for comparison with calculated spectra. Simulations were then conducted to obtain near-field distribution of nanostructure following which SERS spectra were measured for further characterization with assistance of quantum chemical simulations such as density functional theory (DFT).3537 Enhancement factors of SERS experiments and FDTD simulations were estimated to see both agreements and differences. Influence factors such as polarization and coupling effect were also discussed to explain phenomena observed in experiments and for better understanding on these extraordinary polyhedron-type nanostructures and to explore their potential for applications such as sensitive detection, sensor, nanoantenna, and photonics circuit element.
2. RESULTS AND DISCUSSION A. Synthesis and Characterization. THH and ETHH Au NCs were prepared by facile seed-mediated growth method as reported in previous work.38 During the two-step processes conducted in aqueous solution, the growth of NCs took place in a binary surfactant system of cetyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) with a molar ratio of different values, which was crucial for shape controlling of THH and ETHH NCs. The corresponding morphology of successfully synthesized THH and ETHH Au nanoparticles was investigated by scanning electron microscopy (SEM) as shown in Figure 1ce. Detailed structures were demonstrated as models built during simulations as illustrated in Figure 1a and b. The UVvis absorption spectra of THH
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Figure 2. UVvis absorption spectra of THH Au nanoparticles (blue line) and ETHH Au nanoparticles (red line).
(blue line) and ETHH (red line) Au nanoparticles are presented in Figure 2. There was a single SPR absorption around 556 nm in the spectrum of THH sample, and it split into two peaks separately located at 562 and 622 nm as transverse and longitudinal SPR mode, respectively, for ETHH sample because of anisotropy in geometry. B. 3D-FDTD Simulation. On the basis of 3D-FDTD solution, simulations were introduced to calculate electromagnetic field distribution around nanoparticles. The basic principle of FDTD is to numerically solve Maxwell’s differential equations.3942 In FDTD approach, both space and time are divided into discrete segments. Space is segmented into cells, which are known as the Yee cell. The electric fields are located on the edges of the Yee cell, while the magnetic fields are positioned on the faces. Electric fields and magnetic fields are thus calculated alternately step by step at these sequenced time and space segments. To approximate the experimental condition, models in this work were constructed according to SEM images. Symmetrical THH model was constructed by an 80 80 80 nm cubic body and 16 nm height pyramids on top of each facet as shown in Figure 1a. ETHH model shown in Figure 1b consisted of an 80 80 136 nm cubic body with two pyramids on top of the top and bottom facets and four rooflike blocks on top of the rest of the facets. The parameters were all measured from SEM images. Incident plane wave was propagated from the z axis and was polarized along the x direction. The wavelength was set to be 633 nm in all simulations. Data of dielectric constants were from Johnson and Christy.43 The Yee cell size in FDTD simulation was carefully considered to meet the accuracy needed by both wavelength and object parameters and to avoid too large memory resources and computation time required. Thus, 1 nm 1 nm 1 nm mesh was applied over the object region. We first calculated the optical spectra of THH and ETHH nanoparticles. The simulated extinction spectra are depicted in Figure 3. For ETHH nanoparticles, two different polarization directions along the transverse asymmetric axis and longitudinal asymmetric axis are included as shown in Figure 3b and c. A dipole SPR peak located at around ∼540 nm could be seen in extinction spectrum of THH nanoparticles, while the peaks located at ∼540 nm and ∼620 nm could be seen for transverse polarization and longitudinal polarization, respectively, for 18062
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Figure 3. Calculated optical spectra of (a) THH nanoparticles and (b) ETHH nanoparticles polarized along transverse asymmetric axis and (c) polarized along longitudinal asymmetric axis.
Figure 4. E-field amplitude (|E|, the amplitude of the local electric field normalized by the incident electric field) patterns of (a, b) THH and (c, d) ETHH nanoparticle. (a) E-field distribution of xz plane containing 4-facet-tip. (b) E-field distribution of xz plane containing 6-facet-tip. (c) E-field distribution of xz plane containing 4-facet-tip. (d) E-field distribution of xz plane containing 6-facet-tip. The scale bar is 30.
elongated THH one. This approach meets good agreement with experiment allowing us to further calculate the near-field distribution of nanoparticles. Simulated E-field distributions are shown in Figure 4. The distributions indicated that enhancement was highly restricted to a hot spot near the tips of nanocrystals and that it decayed away quite fast from the surface. In THH and ETHH nanoparticles, there were three types of tips with different geometries and various degrees of sharpness: 4-facet-tip (top of pyramid part, as
blue dot shown in Figure 1), 6-facet-tip (joining point of three pyramid parts, as red dot shown in Figure.1), and 3-facet-tip in the case of ETHH nanoparticle (as green dot shown in Figure 1). Among these tips, 6-facet-tip showed the strongest E-field enhancement with a maximum value more than 102. The dominate reason for this result was the degree of sharpness as when height was fixed, the area size under 6-facet-tip was smaller than that under any other tips and, thus, charges were easier to aggregate and penetrate into surrounding medium. Compared 18063
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Figure 5. Raman and SERS spectra: Raman spectra of solid PMBA (black line); surface-enhanced Raman spectra of PMBA absorbed on THH Au nanoparticles (blue line); surface-enhanced Raman spectra of PMBA absorbed on ETHH Au nanoparticles (red line).
with spherical nanoparticles, THH NCs could be considered as a single particle with several tips assembled on it. Thus, this polyhedron-like structure had more hot spots with larger E-field enhancement, which enabled possibilities for applications in sensing like tip-enhanced Raman scattering (TERS). As for ETHH nanoparticle, we saw similar distribution patterns like THH ones, but the maximum E-field enhancement Emax near 6-facet-tip is smaller than that of THH nanostructure with a value around 70. However, the enhancement region increases with the appearance of longer edge, which is able to enlarge the effective enhancement to some extent. C. Surface-Enhanced Raman Spectroscopy. As nearby E-field distributions of single THH and ETHH nanoparticles were already calculated for basic characterization, SERS measurement would provide direct evidence for further verification. SERS spectra of PMBA adsorbed on THH and ETHH Au nanoparticles are shown in Figure 5 (blue) and (red), respectively. Solid PMBA Raman spectrum (black) is presented as well for comparison. The predominant bands in the spectrum of solid PMBA are located at 1100, 1187, and 1596 cm1, which are assigned to a1 modes of νCS, δCH, and νCC. In SERS spectrum, the band νCS shifted from 1100 cm1 to 1078 and 1079 cm1 for THH and ETHH NCs, respectively, because of the formation of AuS bonding. The νCC band at 1596 cm1 shifted to 1587 and 1589 cm1, and the δCH band shifted from 1187 cm1 to 1183 and 1183 cm1 as a result of bonding and electronic structure changes. For band assignment and process monitoring to avoid structure changing during measurement, DFT calculations were performed with the Gaussian 03 package using B3LYP as exchange correlation function. The basis sets for carbon, hydrogen, and sulfur atoms were 6-311+G**, which included a polarization function and a diffuse function. For gold atoms, the valence electrons and the internal shells were described by LANL2DZ basis set.44 Au20 cluster was built on the basis of reference,45,42 which was used to represent gold nanoparticle in experiment. The structures of PMBA and PMBA-Au20 complex formed via SAu bonding were optimized and are shown in Figure 6 inset. Two different configurations were built as surface complex (S-complex) and vertex complex (V-complex) according to reference.46,47 Optimized structures were then used to obtain simulation spectra for comparison with experiment results. On the basis of DFT simulation, structures of PMBA and PMBA-Au20 were optimized, and the corresponding Raman
Figure 6. Simulated normal Raman spectra of PMBA (black line); simulated S-type complex (blue line) and V-type complex (red line) SERS spectra; inset: PMBA-Au20 S-type complex (left) and V-type complex (right).
spectra were calculated. Intensities of spectra are presented as differential Raman scattering cross section that is derived from obtained Raman scattering factors Si (in Å4/amu) through the equation48,49 dσ ð2πÞ4 h ðν~0 ν~i Þ4 Si ¼ ð1Þ dΩ i 45 3 8π2 cν~i 3 1 expðhcν~i =kB TÞ 3 Here, h, kB, c, and T are Planck's constant, Boltzmann's constant, ~i are light speed, and Kelvin temperature, respectively. ν~0 and ν the frequencies (in cm1) of incident light and the ith vibrational mode. In Figure 6, we plotted the results of calculated normal Raman spectrum of the solid PMBA (Figure 6 black line) and SERS spectra of PMBA-Au20 S-type (Figure 6 blue line) and V-type (Figure 6 red line) complex. Corresponding structures are shown in the inset. Detailed frequencies are listed in Table 1. It can be seen that three predominant bands in the simulated spectrum of PMBA are located at 1116, 1193, and 1644 cm1, while in the calculated spectrum of V-complex, the bands of νCS, δCH, and νCC shifted to 1074, 1187, and 1619 cm1, respectively. These results were in good agreement with experimental data for both normal Raman and SERS spectra except for the fact that simulations tended to overestimate in high-frequency regions. For different complexes in simulations, we saw no obvious differences in frequencies, whereas V-type complex had lower energy and higher Raman intensities indicating better stability and additional chemical enhancement ∼10 in SERS. Simulations also indicated that the PMBA molecules did not change during SERS measurement, and the assigned vibrational modes were confirmed for further comparison. D. Enhancement Factors. As is generally considered, the enhancement of a1 modes in SERS mainly originated from EM mechanism. Though it is not possible to completely exclude the enhancement from chemical mechanism such as static chemical enhancement and charge-transfer resonance, the obtained enhancement could still be considered as evidence of 18064
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Table 1. Simulated and Experimental Frequencies (cm1) and Assignments of Vibrations of Solid PMBA and PMBA on Gold Substrate solid PMBA experiments
PMBA adsorbed on Au simulation
experiments THH sample
a
assignmentsb
simulation
ETHH sample
V-complex
S-complex
988
a
1000
1006
1005
πCH
1116
1078
1079
1074
1072
νCS
1187
1193
1183
1183
1187
1187
δCH
1291
1329
1309
1286
1310
1310
δCH+ δCS
a
1371
1367
1349
1362
1365
γCOO
a
1525
1478
1483
1502
1495
δCH
1596
1644
1587
1589
1619
1617
νCC
a
1100
Not observed. b ν, stretch; δ, γ, in-plane bending or deformation; π, out-of-plane bending or deformation.
electromagnetic enhancement because of E-field amplifying of nanostructures. To evaluate the enhancement factor (EF) of SERS, three predominant a1 bands discussed earlier in the paper were chosen to compare the measured SERS intensity and the solid PMBA Raman intensity by using the following equation:50 EF ¼
ISERS =Nads Ibulk =Nbulk
ð2Þ
In eq 2, measured intensity was divided by the number of PMBA molecules under laser illumination for SERS and bulk sample to fairly compare the intensity of single molecule in both situations. ISERS and Ibulk, which represent measured SERS and bulk Raman intensities, could be obtained from experiments. The solid sample, Nbulk, which represents the number of PMBA molecules under laser illumination in bulk sample, was inferred from bulk density, mass of PMBA, and illuminated volume N bulk ¼ Ahnbulk ¼ Ah
Fbulk NA M bulk
ð3Þ
The illuminated volume was calculated as the product of the area of the laser spot A (∼1.9 μm2) and the penetration depth h of the focused laser which was estimated by the microscope feature (∼15.4 μm). On the basis of the density (1.49 g/cm3) and mass (154.19 g/mol) of bulk PMBA, Nbulk was calculated as 1.7 1011. As for SERS samples, NSERS could be obtained via N SERS ¼
AN sub Asub σ
ð4Þ
A is the area of the focal laser spot (∼1.9 μm2). Asub is the occupied area of individual nanoparticle (∼6.4 103 μm2). σ represents the surface area occupied by one adsorbed PMBA molecule, which was about 0.3 nm2 according to the literature.51 Assuming that a layer of nanoparticles deposited on the substrate homogeneously, number density of particles (Nsub) could be counted from the SEM figure. On the basis of the parameters above, NSERS was finally estimated as 1.5 106 and 3.0 106 for THH and ETHH samples, respectively. Considering different exposure times during Raman measurement, the obtained enhancement value of ETHH sample would be further multiplied by 2. By substituting measured and calculated values into eq 1, and by averaging over three predominant bands, enhancement factor (EF) of PMBA adsorbed on THH and ETHH gold
nanoparticles was estimated to be about 2.3 106 and 3.7 106. By excluding the static chemical enhancement (∼10) obtained via DFT, an enhancement factor of 105∼106 that was mainly contributed by electromagnetic mechanism was finally estimated. According to our simulations based on FDTD, |E|max was found to be near tips with a value of about 102. Since the enhancement factor of SERS was roughly proportional to |E|4, the maximum EF could reach a magnitude of 8. However, average E-field enhancement on the surface was much smaller because of fast decay from hot spot. According to research, a few molecules that are located in hot sites actually contribute the most in SERS spectrum.52 Thus, it is reasonable to consider tips with the maximum enhancement as the main contributors of substrates in SERS. On the basis of the size of the enhancement region, we assumed that the area of the hot site took up only 1% of the whole area, and thus the maximum EF needed to be divided by 100 to ignore the Raman signal contributed by parts that were not hot, which means that the EF was around 106 after considering the average effect.42 This conclusion shows general coherency to SERS results but still might be a bit overestimating because of slight distortion of tips and relatively limited enhancement region in simulation results. Another noteworthy point is that ETHH sample provided higher enhancement in SERS measurement than symmetrical THH sample, while the calculation results indicated that the maximum enhancement of ETHH nanoparticles was actually a bit lower than THH ones under the same condition leading to a weaker Raman enhancement in all. This phenomenon observed in experiments might be caused by three reasons: a larger enhancement region of ETHH because of the appearance of longer edge as already mentioned, polarization along elongated longitudinal direction because of lightening rod effect as ellipsoid, and most importantly, the coupling effect of aggregation. To further explain these differences between experiments and calculations, more simulations and discussion will be conducted in the next part. E. Influence Factors. It is generally accepted that near-field enhancement properties of nanoparticles is dependent on several influence factors such as particle geometry, incident light, and aggregation arrangement. According to simulations based on 3D-FDTD, polarization indeed played an important role in surface plasmon resonance (SPR) related phenomenon of THH and ETHH nanoparticles reflecting a polarization dependence of hot 18065
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Figure 7. E-field amplitude (|E|) patterns for a group of THH nanoparticles illustrated by incident light with different polarization directions varying from x-axis to y-axis. The direction changed 15° each time. (a) XZ plane contains 4-facet-tip points, (b) XZ plane contains 6-facet-tip points, (c) polar plot of polarization direction and normalized E-field enhancement of 4-facet-tip points (red solid line, red dash line) and 6-facet-tip points (blue solid line, blue dash line). (d) E-field amplitude (|E|) patterns for ETHH nanoparticle illustrated by light incident from y-axis and polarized along z-axis. XY plane that was 40 nm from center was plotted. (e) E-field amplitude (|E|) patterns for ETHH nanoparticle illustrated by light incident from y-axis and polarized along z-axis. XZ plane containing center was plotted. The scale bar is 30.
spots. As is clearly shown in Figure 7a and b, the near-field distribution of THH NCs changed dramatically with polarization direction varying from x-axis to y-axis. Here, we see that the position of hot site rotated with the rotation of polarization direction from one tip to another because of a promotional role polarization played in the charge accumulation process. When the polarized direction was parallel to the protruding direction of the tip, the E-field enhancement reached a maximum value, whereas when polarized direction was vertical to the tip, the field enhancement had a minimum value as shown in Figure 7c. Thus, all the tips showed different polarization dependences for different extruding directions; this characteristic implied that the hot spots in this pointy polyhedron-like structure might be controllable and manipulated by polarization of excitation source, which might draw great interest in nano-optics study and related applications. For ETHH structure, when light was polarized along the z-axis, each edge along the z-axis showed equal and even larger enhancement than the x-polarized situation as depicted in Figure 7d and e. The near-field enhancement and the optical spectral characteristics of ETHH nanocrystals were compared
with rodlike structures as nanorice.14 The 4-facet-tips on the top and bottom of ETHH showed stronger enhancement under longitudinal polarized excitation because of drastically penetrating electromagnetic energy into the air at the ends, which was quite similar with nanorice reported in a reference. Other than tips, the enhancements along the elongated edges of ETHH were also worth noticing, which indicated the unique near-field characteristics of this polyhedron-like structure. As ETHH could be approximately considered as a rod, similar spectral properties such as broad longitudinal SPR peak and lower transverse peak could also be observed in UVvis spectrum of ETHH NCs. However, high-order SPR modes as presented by nanorice have not been observed so far in our work. Thus, characteristics such as multipolar SPR modes and length-dependent SPR spectral shift might require more comparisons and are worth further studying. The plasmonic coupling between adjacent nanostructures is also considered as the key and necessary factor for local field enhancing. When two or more nanoparticles attached to each other in a very close way, hot spots occurred at the gap region of the dimer. As shown in Figure 8, the coupling produced 18066
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Figure 8. (a) 3D-FDTD model of THH (tip-to-tip dimer). (b) 3D-FDTD model of elongated THH (edge-to-edge dimer). (c, e, g, i) E-field amplitude (|E|) patterns for tip-to-tip THH dimer. The gap distance is 2, 10, 20, and 30 nm, respectively. XZ plane was plotted. (d, f, h, j) E-field amplitude (|E|) patterns for edge-to-edge ETHH dimer. The gap distance is 2, 10, 20, and 30 nm, respectively. XZ plane was plotted. The scale bar is 100.
extremely large E-field amplification around 102∼103 which gave rise to SERS signal enhancements reaching a maximum value as high as 1010 theoretically. From the SEM image, ETHH nanoparticles seemed to be arranged more tightly than THH ones to form an alveolate aggregation, which might be an important reason for the better enhancement the ETHH sample provided during SERS measurements. Apart from this, the ETHH dimer itself also gave a larger effective enhancement region because of the elongated edges that enabled more molecules to be enhanced under illumination. Thus, the coupling effect might be the most important factor to explain the good performance of ETHH in experiments. The enhancement region in the gap of the dimer, as presented in near-field distribution, is also worthy for further discussion as it was supposed to make this region controllable by different positions and arrangements of dimers because of varied interactions between tips, edges, and facets. Further practical explorations on precise arrangement for substrates were demanded to promote better development of THH and ETHH NCs in the fields of chemical analysis and plasmon-related applications.
3. CONCLUSIONS In this paper, THH and ETHH gold NCs were fabricated and studied for their unique surface plasmon (SPs) excitation. UVvis absorption spectra and surface-enhanced Raman scattering (SERS) were introduced to experimentally investigate the far-field optical properties and the near-field enhancement ability of nanoparticles. Calculation of electric field distribution on the basis of three-dimensional finite-difference time domain (3D-FDTD) method revealed that the E-field enhancement is largely confined to tips with strong dependences on geometry of tip and polarization of incident light. Enhancement factors were estimated, and several influence factors such as coupling effect were also mentioned with discussion for the potential advantages of polyhedron-like structure in plasmon-related application. In conclusion, we presented three major findings: (1) Simulations indicated that enhancements were highly restricted to tips or edges with a maximum E-field enhancement of more than 102 occurring near 6-facet-tip of THH nanoparticles, which was compared with the enhancement factor around 106∼ 107 obtained from SERS. (2) Experimental results indicated that
ETHH NCs samples exhibited larger enhancement than THH ones, whereas the calculated maximum E-field enhancements of ETHH nanoparticles were a bit smaller than THH one under the same condition. There were probably three reasons to explain this: the appearance of longer edges which provide more effective enhancement, elongation effect under longitudinal polarized illumination, and better coupling enhancement because of closer alveolate arrangement in ETHH sample and larger enhancement region in ETHH dimer. (3) Influence factors such as particle geometry, incident polarization, and coupling effect were also discussed implying a huge potential of these unique polyhedronlike nanostructures in plasmon-related application and research including multitip structure, polarization sensitivity, and controllable coupling effect. These experimental and theoretical studies are expected to promote practical explorations on the fabrication and characterization of gold nanostructures to thus benefit better understanding of nano-optics for the sake of extensive application and research such as ultrasensitive chemical and biological sensor, surface-enhanced spectroscopy substrate, nonlinear optics applications, and even elements in optical nanocircuits.
’ EXPERIMENTAL SECTION Synthesis. THH and ETHH Au NCs were prepared by facile seed-mediated growth method as reported in previous work.38 Two steps were involved during the process conducted in aqueous solution: seed-mediated growth of quasi-THH NCs in didodecyldimethylammonium bromide (DDAB) surfactant system and subsequent growth of the quasi-THH Au particles into well-shaped THH Au NCs in a binary surfactant system of cetyltrimethylammonium bromide (CTAB) and DDAB. During these processes, CTAB was dissolved in DDAB to make the binary surfactant solution with molar ratio of CTAB to DDAB of different values, which was crucial for shape controlling of THH and ETHH NCs. The morphology of nanoparticles was investigated by scanning electron microscopy (SEM, Hitachi S-4800). UVvis absorption spectra were recorded with a UVvis spectrophotometer (Cintra 10e). SERS Sample Preparation. The SERS substrate was prepared by dropping 10 μL of THH and ETHH NC sample synthesized 18067
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The Journal of Physical Chemistry C as described earlier onto a clean silicon plate and by leaving it to dry naturally in air. Subsequently, the silicon wafer was submerged into 1 103 mol/L p-mercaptobenzoic acid (PMBA) ethanol solution for 30 min. After thoroughly rinsing with absolute ethanol and drying in the air, the sample was subjected to Raman characterization by Raman spectrometer. For comparison, the normal Raman spectrum of bulk PMBA measured from powder sample was also presented. Instruments. Raman spectrum was collected using a Jobin Yvon (Laboratory RAM HR800) confocal micro-Raman spectrometer equipped with multichannel charge coupled detector. The HeNe laser emitting at a wavelength of 632.8 nm was used as the source of excitation. The number of grating in the Raman spectrometer was 600 grooves per mm, and the numerical aperture of the 50 objective was 0.5. Raman spectra of PMBA was obtained by using a 50 objective lens with 10 s exposure time and two accumulations, while surface-enhanced Raman spectra of PMBA adsorbed on THH Au and elongated THH Au were measured with 10 s and 5 s exposure time, respectively.
’ AUTHOR INFORMATION Corresponding Author
*Phone & fax: +86-10-82338987; e-mail:
[email protected] (P.-G.Y.);
[email protected] (L.G.).
’ ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (50725208, 20973019, and 51002007), National Basic Research Program of China (2010CB934700), and the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2006CB932301). ’ REFERENCES (1) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity. Science 2007, 316, 732–735. (2) Ming, T.; Feng, W.; Tang, Q.; Wang, F.; Sun, L. D.; Wang, J. F.; Yan, C. H. Growth of Tetrahexahedral Gold Nanocrystals with HighIndex Facets. J. Am. Chem. Soc. 2009, 131, 16350–16351. (3) Ding, Y.; Gao, Y. F.; Wang, Z. L.; Tian, N.; Zhou, Z. Y.; Sun, S. G.; Wang, L. Facets and surface relaxation of tetrahexahedral platinum nanocrystals. Appl. Phys. Lett. 2007, 91, 121901. (4) Kim, D. Y.; Im, S. H.; Park, O. O. Synthesis of Tetrahexahedral Gold Nanocrystals with High-Index Facets. Cryst. Growth Des. 2010, 10, 3321–3323. (5) Yu, Y.; Zhang, Q. B.; Liu, B.; Lee, J. Y. Synthesis of Nanocrystals with Variable High-Index Pd Facets through the Controlled Heteroepitaxial Growth of Trisoctahedral Au Templates. J. Am. Chem. Soc. 2010, 132, 18258–18265. (6) Tian, N.; Zhou, Z. Y.; Yu, N. F.; Wang, L. Y.; Sun, S. G. Direct Electrodeposition of Tetrahexahedral Pd Nanocrystals with High-Index Facets and High Catalytic Activity for Ethanol Electrooxidation. J. Am. Chem. Soc. 2010, 132, 7580–7581. (7) Lu, C. L.; Prasad, K. S.; Wu, H. L.; Ho, J. A.; Huang, M. H. Au Nanocube-Directed Fabrication of Au-Pd Core-Shell Nanocrystals with Tetrahexahedral, Concave Octahedral, and Octahedral Structures and Their Electrocatalytic Activity. J. Am. Chem. Soc. 2010, 132, 14546– 14553. (8) Ma, F.; Ma, S. L.; Xu, K. W.; Chu, P. K. Surface stability of platinum nanoparticles surrounded by high-index facets. J. Phys. Chem. C 2008, 112, 3247–3251.
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(9) McLellan, J. M.; Siekkinen, A.; Chen, J. Y.; Xia, Y. N. Comparison of the Surface-enhanced Raman scattering on sharp and truncated silver nanocubes. Chem. Phys. Lett. 2006, 427, 122–126. (10) Nehl, C. L.; Liao, H. W.; Hafner, J. H. Optical properties of starshaped gold nanoparticles. Nano Lett. 2006, 6, 683–688. (11) Naumov, I. I.; Li, Z. Y.; Bratkovsky, A. M. Plasmonic resonances and hot spots in Ag octopods. Appl. Phys. Lett. 2010, 96, 033105. (12) Liang, H. Y.; Li, Z. P.; Wang, W. Z.; Wu, Y. S.; Xu, H. X. Highly Surface-roughened “Flower-like” Silver Nanoparticles for Extremely Sensitive Substrates of Surface-enhanced Raman Scattering. Adv. Mater. 2009, 21, 1–5. (13) Liang, H. Y.; Yang, H. X.; Wang, W. Z.; Li, J. Q.; Xu, H. X. HighYield Uniform Synthesis and Microstructure-Determination of RiceShaped Silver Nanocrystals. J. Am. Chem. Soc. 2009, 131, 6068–6069. (14) Wei, H.; Reyes-Coronado, A.; Nordlander, P.; Aizpurua, J.; Xu, H. X. Multipolar Plasmon Resonances in Individual Ag Nanorice. ACS Nano 2010, 4, 2649–2654. (15) Xu, H. X.; Bjerneld, E. J.; K€all, M.; B€ orjesson, L. Spectroscopy of Single Hemoglobin Molecules by Surface Enhanced Raman Scattering. Phys. Rev. Lett. 1999, 83, 4357–4360. (16) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. SurfaceEnhanced Raman Spectroscopy of Self-Assembled Monolayers: Sandwich Architecture and Nanoparticle Shape Dependence. Anal. Chem. 2005, 77, 3261–3266. (17) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman spectra of pyridine adsorbed at a silver electrode. Chem. Phys. Lett. 1974, 26, 163–166. (18) Haynes, C. L.; Yonzon, C. R.; Zhang, X. Y.; Van Duyne, R. P. Surface-enhanced Raman sensors: early history and the development of sensors for quantitative biowarfare agent and glucose detection. J. Raman Spectrosc. 2005, 36, 471–484. (19) Li, K. B.; Clime, L.; Tay, L. L.; Cui, B.; Geissler, M.; Veres, T. Multiple Surface Plasmon Resonances and Near-Infrared Field Enhancement of Gold Nanowells. Anal. Chem. 2008, 80, 4945–4950. (20) McMahon, J. M.; Gray, S. K.; Schatz, G. C. Optical Properties of Nanowire Dimers with a Spatially Nonlocal Dielectric Function. Nano Lett. 2010, 10, 3473–3481. (21) Xu, H. X.; Aizpurua, J.; K€all, M.; Apell, P. Electromagnetic contributions to single-molecule sensitivity in surface-enhanced Raman scattering. Phys. Rev. E 2000, 62, 4318–4324. (22) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668–677. (23) Wei, H.; Hao, F.; Huang, Y. Z.; Wang, W. Z.; Nordiander, P.; Xu, H. X. Polarization Dependence of Surface-Enhanced Raman Scattering in Gold Nanoparticle-Nanowire Systems. Nano Lett. 2008, 8, 2497–2502. (24) Gellner, M.; K€ustner, B.; Schl€ucker, S. Optical properties and SERS efficiency of tunable gold/silver nanoshells. Vib. Spectrosc. 2009, 50, 43–47. (25) Franzen, S.; Folmer, J. C. W.; Glomm, W. R.; O’Neal, R. Optical Properties of Dye Molecules Adsorbed on Single Gold and Silver Nanoparticles. J. Phys. Chem. A 2002, 106, 6533–6540. (26) Schatz, G. C.; Young, M. A.; Van Duyne, R. P. Electromagnetic Mechanism of SERS. Top. Appl. Phys. 2006, 103, 19–46. (27) Kahl, M.; Voges, E. Analysis of plasmon resonance and surfaceenhanced Raman scattering on periodic silver structures. Phys. Rev. B 2000, 61, 14078–14088. (28) King, F. W.; Van Duyne, R. P.; Schatz, G. C. Theory of Raman scattering by molecules adsorbed on electrode surfaces. J. Chem. Phys. 1978, 69, 4472. (29) Lee, S. J.; Guan, Z. Q.; Xu, H. X.; Moskovits, M. SurfaceEnhanced Raman Spectroscopy and Nanogeometry: The Plasmonic Origin of SERS. J. Phys. Chem. C Lett. 2007, 111, 17985–17988. (30) Camden, J. P.; Dieringer, J. A.; Wang, Y. M.; Masiello, D. J.; Marks, L. D.; Schatz, G. C.; Van Duyne, R. P. Probing the Structure of Single-Molecule Surface-Enhanced Raman Scattering Hot Spots. J. Am. Chem. Soc. 2008, 130, 12616–12617. 18068
dx.doi.org/10.1021/jp2041586 |J. Phys. Chem. C 2011, 115, 18061–18069
The Journal of Physical Chemistry C (31) Li, Z. Y.; Xia, Y. N. Metal Nanoparticles with Gain toward Single-Molecule Detection by Surface-Enhanced Raman Scattering. Nano Lett. 2010, 10, 243–249. (32) Hao, E. C.; Li, S. Y.; Bailey, R. C.; Zou, S. L.; Schatz, G. C.; Hupp, J. T. Optical Properties of Metal Nanoshells. J. Phys. Chem. B 2004, 108, 1224–1229. (33) Wiley, B. J.; Chen, Y. C.; Mclellan, J. M.; Xiong, Y. J.; Li, Z. Y.; Ginger, D.; Xia, Y. N. Synthesis and Optical Properties of Silver Nanobars and Nanorice. Nano Lett. 2007, 7, 1032–1036. (34) McLellan, J. M.; Li, Z. Y.; Siekkinen, A. R.; Xia, Y. N. The SERS Activity of a Supported Ag Nanocube Strongly Depends on Its Orientation Relative to Laser Polarization. Nano Lett. 2007, 7, 1013– 1017. (35) Muniz-Miranda, M.; Pagliai, M.; Cardini, G.; Schettino, V. Role of Surface Metal Clusters in SERS Spectra of Ligands Adsorbed on Ag Colloidal Nanoparticles. J. Phys. Chem. C 2008, 112, 762–767. (36) Cardini, G.; Muniz-Miranda, M.; Schettino, V. SERS and DFT Study on 4-Methylpyridine Adsorbed on Silver Colloids and Electrodes. J. Phys. Chem. B 2004, 108, 17007–17011. (37) Sarkar, J.; Chowdhury, J.; Ghosh, M.; De, R.; Talapatra, G. B. Experimental and Theoretical Surface Enhanced Raman Scattering Study of 2-Amino-4-methylbenzothiazole Adsorbed on Colloidal Silver Particles. J. Phys. Chem. B 2005, 109, 22536–22544. (38) Li, J.; Wang, L. H.; Liu, L.; Guo, L.; Han, X. D.; Zhang, Z. Synthesis of Tetrahexahedral Au Nanocrystals with Exposed High-Index Surfaces. Chem. Commun. 2010, 46, 5109–5111. (39) Yang, Z. L.; Aizpurua, J.; Xu, H. X. Electromagnetic field enhancement in TERS configurations. J. Raman Spectrosc. 2009, 40, 1343–1348. (40) Chen, X. W.; Sandoghdar, V.; Agio, M. Highly Efficient Interfacing of Guided Plasmons and Photons in Nanowires. Nano Lett. 2009, 9, 3756–3761. (41) Ming, T.; Zhao, L.; Yang, Z.; Chen, H. J.; Sun, L. D.; Wang, J. F.; Yan, C. H. Strong Polarization Dependence of Plasmon-Enhanced Fluorescence on Single Gold Nanorods. Nano Lett. 2009, 9, 3896–3903. (42) Yang, Z. L.; Li, Y.; Li, Z. P.; Wu, D. Y.; Kang, J. Y.; Xu, H. X.; Sun, M. T. Surface enhanced Raman scattering of pyridine adsorbed on Au@Pd core/shell nanoparticles. J. Chem. Phys. 2009, 130, 234705. (43) Johnson, P. B.; Christy, R. W. Optical Constants of the Noble Metals. Phys. Rev. B 1972, 6, 4370–4379. (44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr., T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.; Nakatsuji, A. H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratemann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuk, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03 User’s Reference; Gaussian, Inc.: Carnegie, PA. 15106, USA, 2003. (45) Li, J.; Li, X.; Zhai, H. J.; Wang, L. S. Au20: A Tetrahedral Cluster. Science 2003, 299, 864–867. (46) Zhao, L. L.; Jensen, L.; Schatz, G. C. Pyridine-Ag20 Cluster: A Model System for Studying Surface-Enhanced Raman Scattering. J. Am. Chem. Soc. 2006, 128, 2911–2919. (47) Zhao, L. L.; Jensen, L.; Schatz, G. C. Surface-Enhanced Raman Scattering of Pyrazine at the Junction between Two Ag20 Nanoclusters. Nano Lett. 2006, 6, 1229–1234. (48) Neugebauer, J.; Reiher, M.; Kind, C.; Hess, B. A. Quantum chemical calculation of vibrational spectra of large molecules—Raman
ARTICLE
and IR spectra for Buckminsterfullerene. J. Comput. Chem. 2002, 23, 895–910. (49) Wu, D. Y.; Liu, X. M.; Duan, S.; Xu, X.; Ren, B.; Lin, S. H.; Tian, Z. Q. Chemical Enhancement Effects in SERS Spectra: A Quantum Chemical Study of Pyridine Interacting with Copper, Silver, Gold and Platinum Metals. J. Phys. Chem. C 2008, 112, 4195–4204. (50) Cai, W. B.; Ren, B.; Li, X. Q.; She, C. X.; Liu, F. M.; Cai, X. W.; Tian, Z. Q. Investigation of surface-enhanced Raman scattering from platinum electrodes using a confocal Raman microscope: dependence of surface roughening pretreatment. Surf. Sci. 1998, 406, 9–22. (51) Xia, L. X.; Wang, H. B.; Wang, J.; Gong, K.; Jia, Y.; Zhang, H. L.; Sun, M. T. Microwave-assisted synthesis of sensitive silver substrate for surface-enhanced Raman scattering spectroscopy. J. Chem. Phys. 2008, 129, 134703. (52) Fang, Y.; Seong, N. H.; Dlott, D. D. Measurement of the Distribution of Site Enhancements in Surface-Enhanced Raman Scattering. Science 2008, 321, 388–393.
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dx.doi.org/10.1021/jp2041586 |J. Phys. Chem. C 2011, 115, 18061–18069
