Shell Nanoplasmonic Structures with

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Fabrication of Si@Au Core-shell Nanoplasmonic Structures with Ultrasensitive Surface-enhanced Raman Scattering for Monolayer Molecules Detection Guo Wei Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5111482 • Publication Date (Web): 26 Dec 2014 Downloaded from http://pubs.acs.org on December 27, 2014

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Fabrication of Si@Au Core-shell Nanoplasmonic Structures with Ultrasensitive Surface-enhanced Raman Scattering for Monolayer Molecules Detection

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

The Journal of Physical Chemistry jp-2014-111482.R1 Article 24-Dec-2014 Liu, Pu; Sun Yat-sen University, Chen, Huanjun; Sun Yat-sen University, School of Physics and Engineering Wang, Hao; Sun Yat-sen University, School of Physics and Engineering Yan, Jiahao; Sun Yat-sen University, Lin, Zhaoyong; Sun Yat-sen University, Yang, Guo Wei; Sun Yat Sen University, State Key Laboratory of Optoelectronic Materials and Technologies

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Fabrication of Si/Au Core/shell Nanoplasmonic Structures with Ultrasensitive Surface-enhanced Raman Scattering for Monolayer Molecule Detection

Pu Liu, Huanjun Chen, Hao Wang, Jiahao Yan, Zhaoyong Lin, Guowei Yang * State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China *Corresponding author: [email protected]

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Abstract Nanoparticles of noble metals are typically plasmonic materials, but the pure metals have high non-irradiative ohmic losses at optical frequencies, leading to large absorption and unwanted heating effects. Additionally, spherical silicon nanoparticles (NPs) have a unique optical system with a high-refractive-index dielectric nanostructure. Combining these two components in one functional core/shell NP nanostructure of Si/M, where M represents a noble metal, to produce resonant optical electronic and magnetic responses has been rare. Herein, an approach for the assembly of Si/M core/shell NPs is developed based upon double-beam laser ablation in liquid, thereby fabricating a series of Si/M (M=Au, Ag, Pd and Pt) core/shell NPs and characterizing the intense visible-light scattering modes from the as-fabricated Si/Au NPs as a nanoplasmonic structure. Dark-field optical measurements show that these Si/Au NPs have a strong electromagnetic response in the visible-light region, and the resonant frequencies can be modulated by NP size and morphology. The dispersed NPs are used as a highly-sensitive surface-enhanced Raman scattering (SERS) active probe for detecting monolayer molecules, as proven herein using 4-MBT molecules. A SERS plasmonic enhancement factor of ~108 is found for these Si/Au NPs by correlating the SERS measurement with scanning electron microscopy analysis. These findings present the possibility of confining light in ubiquitous silicon-based semiconductor technologies and manipulating the optical properties of nontraditional plasmonic nanostructures, while also opening up new perspectives. This technique can be extended to other noble metals to form similar structures and 2


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fabricate low-loss metamaterials and nanophotonic devices.

Keywords: silicon nanospheres, nanoplasmonic structures, surface-enhanced Raman scattering.

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1. Introduction In recent years, the domain of plasmonic applications has expanded significantly with the introduction of the related fields of metamaterials and transformation optics, which induces many novel physical phenomena and device applications in advanced optics and optoelectronics.1 Among them, controlling and manipulating light on the micro- and nanoscale is one of the most challenging issues.2 At sub-wavelength dimensions, in fact, noble metal nanostructures such as nanoantennas and nanoparticles (NPs) made from Au and Ag have been investigated intensely in recent decades.3–5 These are mostly characterized by a strong resonant response to the electric field of the light, known as the localized surface plasmon resonance. In addition, specially designed metallic nanostructures such as the split-ring have attracted significant interest owing to their manifestation of a magnetic dipole response, which is not found in naturally occurring materials.6–9 However, at optical frequencies, pure metals have high non-irradiative ohmic losses, which lead to large absorption and unwanted heating effects.10 Because efficient control of the visible light at the nanoscale is vital for future light-on-chip integration, these intrinsic losses are the principal drawback of using pure metallic plasmonic nanostructures in the visible frequency range, strongly affecting their overall performance and limiting their scalability to practical application dimensions. To overcome such limitations and simultaneously exploit similar resonant properties, one possible alternative is to use high-refractive-index dielectric nanomaterials.11,12 Thus, semiconductors emerge as an alternative and potentially more flexible class of plasmonic materials.1,13 For example, 4


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the visible-range nanoantennas based on spherical silicon NPs have been theoretically analyzed recently, and have demonstrated a higher efficiency than their metallic analogues.14–15 Importantly, considering the ubiquity and indispensability of silicon in semiconductor device fabrication, Si should undoubtedly be chosen as an ideal system to manipulate surface plasmons. Although Si could provide an ideal platform for the marriage of these distinct technologies, the integration of Si and plasmonic components has been hampered by the lack of an intrinsic source of surface plasmon polaritons compatible with silicon-based fabrication techniques.13,16 Therefore, optical elements such as lenses and optical splitters working at the sub-wavelength scale require nanoscale optical prototypes that are breakthrough and conceptually new. Materials technology, and especially advanced materials techniques induced by new scientific concepts, has played an important role in the positive feedback loop established between science and technology, enabling novel applications and also assisting in the exploration of new science. That is, advancing a novel concept and prototypes into real, practical techniques are needed from materials technology. Nowadays, the fabrication of dielectric-based noble metal modified nanostructures still remains a challenge.17–19 Therefore, a suitable material with a high refractive index and small absorption coefficient together with a fabrication technology is urgent. This technology should allow generation of spherical NPs with electric and magnetic Mie resonances in the visible spectral range as well as precise control over their sizes, morphologies and composition.20–23 In light of these issues, we report herein the development of a novel and facile 5



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technique to fabricate Si/M (M=Au, Ag, Pd and Pt) core/shell NPs based on laser ablation in liquid (LAL). Using dark-field microscopy, we study the local field enhancement characteristics of the Si/Au core/shell NPs within their loosely-coupled system. We discover that the composite active NPs can create an extremely high surface-enhanced Raman scattering (SERS) enhancement factor (EF) of the local field intensity that exceeds a value on the order of 108, exhibiting an excellent SERS sensitivity performance that is sufficient for monolayer molecule detection. Importantly, this character is attributed to the amplification of the magnetic response of Si surface plasmon resonances by the gain medium of the Au shell under pumping of the incident electromagnetic field. Therefore, these Si/Au core/shell NPs possessing electric and magnetic multipole resonances in the visible spectral range are expected to be promising for the future realization of new types of efficient nanoantennas and metamaterials. 2. Experimental The experimental setup for the double-beam LAL is shown in Fig. 1. Using this technique, we easily fabricated a series of Si/M (M=Au, Ag, Pd and Pt) NPs at room temperature and ambient pressure by a one-step strategy by reacting a solid Si target in solvent environments. We take the fabrication of the Si/Au core/shell NPs as an example of how the developed approach worked for the assembly of Si/M core/shell NPs. The starting materials were a silicon target and HAuCl4 solution, where the original HAuCl4 (>99.9%) was purchased from Aladdin Industrial Corp. A single-crystal Si target with 99.99% purity was first fixed to the bottom of a 6


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rectangular quartz chamber possessing the dimensions of 10.0×3.8×7.0 cm3. Then, the 2.5 mM of HAuCl4 solution was poured slowly into the chamber until the target was covered by 3–5 mm of the solution. Finally, two pulsed lasers were guided to and focused on the same area of the surface of the solid target. One was a third harmonic laser beam produced by a Q-switched Nd:YAG laser with a wavelength of 355 nm, pulse width of 3 ns, repeating frequency of 10 Hz and energy of 40 mJ, which was focused finely for the major ablation of the target. The second was a second harmonic laser beam produced by a Q-switched Nd:YAG laser with a wavelength of 532 nm, pulse width of 10 ns, repeating frequency of 10 Hz and energy of 50 mJ, which was guided in a separate optical path as an assistant laser and adjusted to be in an under-focused state around the first laser focus. These lasers were synchronously controlled by a DG535 Digital Delay Generator, through which we achieved a double-beam laser co-ablation-in-one point status, as shown in Fig. 1. The whole system was maintained at room temperature during the entire experimental process, and two magnetic stirring rotors set on both sides of the target easily dispersed the fabricated products evenly in the liquid environment. After LAL for 30 min, the products dispersed in the liquid were collected, washed and dialyzed carefully with deionized water for 1 h to remove the remaining HAuCl4 residues, whereupon the collection was dried at 50°C in a vacuum oven for 12 h before further measurements. A scanning electron microscope (SEM; JSM-7600F field emission scanning electron microscope operated at 15 kV and equipped with an energy-dispersive X-ray spectrometer (EDS)), X-ray diffraction (XRD; D8 Advance X-ray diffractometer with 7



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Cu Kα radiation of λ=1.54056 Å, 40 kV, 30 mA), a transmission electron microscope (TEM) equipped with an EDS and a scanning (STEM) mode (FEI Tecnai G2 F30 instrument at an accelerating voltage of 300 kV), a high-sensitivity multifunctional microscopic and spectroscopic platform (monochromator of Acton SpectraPro, 2300i, integrated with Princeton Instruments, Pixis 512B) with a dark-field objective system (Olympus BX51 integrated with a 100 W quartz-tungsten-halogen lamp), and a Raman scattering spectroscope (Renishaw, inVia Reflex+Plus laser micro-Raman spectrometer with a laser source of 632.8 nm, 1.7 mW HeNe laser excitation) were employed to characterize the morphology, structure, optical scattering and spectroscopic properties, and the SERS sensitivity of the as-synthesized samples. 3. Results and Discussion 3.1 Morphology and structure of the Si/Au core/shell nanospheres A typical low-magnification SEM image of the as-synthesized samples dispersed individually on a Si substrate is shown in Fig. 2a, demonstrating that the products are numerous, well-defined spheres with diameters in the range of 100–500 nm. Two high-magnification SEM images (Fig. 2a insets) clearly show that most of the spheres have wrinkled surfaces and that the majority of the diameters are between 200 nm and 400 nm. Meanwhile, the corresponding EDS spectrum of one NP indicates that the synthesized NPs are composed of silicon and gold elements within the measurement error of 4%. To obtain a detailed description of the morphology of a single sphere, we carefully analyze more than 60 NPs in this fashion and find that there are four kinds of morphologies in the products, which are represented in Fig. 2b (i–iv). Specifically, 8


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Fig. 2b (i) indicates that a number of the spheres have wrinkled surfaces coated with tiny particles of Au, while Fig. 2b (ii) shows that some spheres have sparse wrinkles and thin slices covering the surface. Figure 2b (iii) illustrates a concentrated-wrinkle coverage form of the NPs, while Fig. 2b (iv) illustrates that a few of the samples have a thin shell covering the sphere. Based upon the SEM statistics, the size distribution of the synthesized NPs are measured and plotted as histograms in Fig. 2c, indicating that the maximal probability of the sphere diameter is about 250 nm. In addition to these SEM characterizations, the corresponding XRD pattern for the condensed NPs is given in Fig. 2d. The diffraction pattern of the samples demonstrate that these spheres have good crystallinity, and that all of the diffraction peaks can be indexed definitely to Si with a diamond structure (JCPDS Card File No. 65-1060) and to Au with a face-centered-cubic (fcc) phase. Note that the diffraction intensity of the peak appearing at 69.14°, originating from the (400) crystal planes of Si, is much stronger than any of the other peaks attributed to the Au component and the Si crystal planes. This strong peak from the (400) planes indicates that the Si spherical core of the as-synthesized NPs has high index crystal facets, which directly leads to the good uniformity of the sphere crystallinity. Therefore, combining the SEM analysis and the XRD results, we can preliminarily demonstrate that well-defined Si/Au NPs with a core/shell structure are successfully fabricated, and that no impurity component can be detected in the products. The best strategy to obtain a detailed analysis of the NP structure is to use TEM, and Figs. 3–6 show TEM images of the as-synthesized spheres representing the four 9



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typical structures that emerge in the products. Figure 3a shows a bright-field image of an individual sphere coated with wrinkled slices and tiny particles, whose corresponding selected area electronic diffraction (SAED) pattern (Fig. 3a inset) clearly reveals that the synthesized sphere is a single crystal with the diamond structure of Si (JCPDS Card File No. 65-1060), which is consistent with the XRD analysis results. Additionally, the corresponding SAED pattern identifies the existence of an Au crystal by the weak diffraction points (marked with white dots circles) indexed in the inset to Fig. 3a. Further, a high-magnification TEM image of one side of the sphere and the high-resolution TEM (HRTEM) image of the selected area are shown in Fig. 3b, where it can clearly be seen that tiny NPs are embedded in the surface of the sphere. Meanwhile, the HRTEM image (Fig. 3b inset) indicates that the interplanar spacing of one NP (circled in Fig. 3b) is d=0.20 nm, which is in good agreement with the d value of the (200) crystallographic plane of fcc Au. These results imply that the tiny particles around the surface are Au nanocrystals. We use STEM analysis to confirm the component distribution of the spheres (Fig. 3c–d), where Fig. 3c shows the high-angle annular dark-field (HAADF) STEM image of the individual sphere shown in Fig. 3a. Because HAADF imaging is a Z-contrast technique in which atoms with larger atomic number (Z) have a higher contrast, the less bright spherical center and the bright surface slices with tiny NPs clearly observed in Fig. 3c indicate that the light Si atoms are in the core and the heavy Au atoms are on the surface. Panels d1–d4 of Fig. 3d show the two-dimensional EDS elemental mapping images of the sphere, in which the Si element is colored green 10


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(Panel d2) and the Au detection is colored orange (Panel d3) and yellow (Panel d4). It is clear that the Si element is only found in the central area, which is distinguishable as a spherical core. When compared with the basic image in Panel d1, it is obvious that the detected Au element is surrounding the core area and can be identified as a shell in both Panels d3 and d4. Using a similar analysis as that used for Fig. 3, Figs. 4–6 exhibit the detailed TEM results of the other three types of spheres. In each series, the comparison of the bright-field image of Figs. 4a, 5a and 6a and the STEM analysis of Figs. 4c–d, 5c–d and 6c–d clearly indicate that all of the spheres possess the Si/Au core/shell structure, whose centers are an almost spherical Si core and whose Au shells are constructed as a wrinkled surface or a cover of tiny particles surrounding the Si core. Figures 4b, 5b and 6b show HRTEM analysis of various regions of the corresponding spheres, from which we can identify the interplanar spacing of the Au composition (d=0.2 nm for Au (200), shown in Fig. 4b; d=0.24 nm for Au (111), shown in Fig. 5b), and Si crystals (d=0.31 nm for Si (111), shown in Figs. 4b, 5b and 6b). The fast Fourier transform analysis (Figs. 4a and 5b insets) further confirms the HRTEM detection of an Au shell and Si core. Note that some Au nanocrystals not only exist on the surface of the Si core, but are also implanted in the spheres (Fig. 4b), and the direct contact of the two components likely in this arrangement implies that the interaction between these two components would be very strong. Considering that the interplanar spacing value of the (200) crystallographic plane of Au (0.2 nm) is very close to the Si (220) spacing (0.19 nm), the minimal crystal lattice mismatch further leads to the generation 11



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of a composite structure such as Si/Au NPs via double-beam LAL and the probability that they can exist stably. Therefore, using SEM, XRD and TEM/STEM analysis, we find that the synthesized spheres consist of Si and Au, where the Au elements mostly surround the Si as well as form a gold-net around the Si core. Moreover, because of the minimal crystal lattice mismatch between the (200) crystallographic plane of Au and the (220) plane of Si at the interface, the combination of materials in these special Si/Au nanospheres have a high stability in the structure. Therefore, unique spheres composed of Si/Au core/shell structures are truly fabricated by this developed technique. 3.2 Scattering properties of single Si/Au core/shell nanospheres The strong magnetic dipole response of spherical Si NPs in the visible spectral range has been demonstrated in recent years both in experiment and theory,20–21,23 but there have been few works involving silicon-metal systems.17 Therefore, we report on the scattering properties of the Si/Au core/shell NPs with four different average radius values of about 250, 450, 600 and 700 nm, measured together with the dark-field optical microscope and SEM images. Figure 7 shows SEM images (i and ii) of two individual Si/Au core/shell NPs with 250 nm diameters dispersed on an indium tin oxide (ITO) substrate, along with reflected dark-field optical microscope images recorded in the backward direction. The corresponding low-magnification SEM images are inset at the top right corners of (i) and (ii), which demonstrably confirm that the individual Si/Au core/shell NP maps one-for-one from the reflected dark-field optical microscope image to the 12


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corresponding dark-field scattering optical microscope images. Further, in both of the dark-field optical microscope images, the fabricated NPs can be seen to shine with intense colors, which corresponds to their strong magnetic and electric dipole scattering. In the case of the metallic dimer, electric dipoles excited in different NPs interfere with each other, providing Fano-type resonant anisotropic scattering.24 In the presence of a dielectric, when considering the Si core effects,21 electric and magnetic dipoles would both be excited simultaneously in the Si/Au core/shell NPs, giving rise to anisotropic scattering by a single NP. On the other hand, in the central spectral range between the electric and magnetic dipole resonances, the backward scattering may become dominant. To account for this effect, we perform a single NP spectroscopy measurement for each of two NPs in the backward scattering directions (Fig. 7(iii)). As it can be seen using the SEM images, each shiny object in Fig. 7 corresponds to an NP whose shape is close to spherical and whose diameter is about 230 (i) and 250 nm (ii). Interestingly, though there is a large difference between the surface structures of these two NPs, their backward scattering spectra are basically similar excepting a small red-shift of the spectrum of the NP in (ii) compared with that of the NP in (i). Using finite-difference time-domain (FDTD) simulations, whose spectrum of an Si/Au nanosphere 230 nm in diameter is shown in the inset of Fig. 7(iii), we confirm that the resonance maxima positions (450–600 nm) in the backward scattering of these two NPs are higher multipole modes of the magnetic and electric quadrupoles. We also confirm that their shape would be sensitive to the fine structure of the morphology of the NP. Therefore, using the 13



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spectrum measurements together with the corresponding SEM analysis, we can see that the spectrum is mainly defined by the sphere size, while the morphology of surface influences only the fine structure of the spectrum. Further, the center wavelength is red-shifted when the spheres become large. Using the analysis discussed for Fig. 7, Figs. 8–10 exhibit the detailed dark-field optical microscope images with the analyzed spectrum and the corresponding SEM images for the NPs possessing an average diameter of 450 nm (Fig. 8), 600 nm (Fig. 9) and 700 nm (Fig. 10). Meanwhile, the blue and red curves in each spectrum show the experimental data for the backward reflected spectrum of the NPs shown in the images labeled (i) and (ii). One can see that as the NP size increases, higher multipole modes of the magnetic and electric octupole resonance modes appear at the blue part of the spectra, while the resonance maximum of the magnetic quadrupoles and the dipole are seen to shift to the red and infrared frequencies. The spectrum defined by the strongest resonance peak exhibits an obvious change from the short wavelength range to the long wavelength range as the NP diameter increases from 250 to 450 nm (Figs. 7(iii) to 8(iii)), while the main resonance shifts from 600 to 800 nm. Therefore, we can conclude that the center wavelength of the intense brightness observed in the dark-field optical microscope image corresponds to the size-induced magnetic and electric multipole scattering of the Si/Au core/shell NPs. Comparing the present literature reports of single Si NPs20,21,23 and the theoretical spectrum calculated using FDTD simulation in this work, we find that the characteristics of the Au shell of these NPs may split the observed spectra into the 14


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separate contributions of the different multipole modes. We also find that they may impact the field distribution inside the sphere at each resonance maximum (Fig. 7(iii) inset). These analyses are done for each particle and the corresponding multipole contributions are identified. According to this analysis, the differences in the spectra shown in Panel (iii) of Figs. 7–10 can be easily attributed to the varying morphology of the Si/Au core/shell NPs. Based upon the dark-field scattering spectral analysis, we can conclude that, first, the entire state of the dark-field scattering spectra of a single Si/Au core/shell NPs is defined by the size of the spheres (Si core), and the magnetic quadrupole and octupole resonance is generally observed by increasing the size of the NPs. Second, the fine structure of the spectrum is highly dependent upon the Au morphology around the core-shell NPs, whereby the Au shell can be used to adjust the scattering spectra of the Si/Au core/shell NPs. Third, the spectrum of an NP can be continuously tuned throughout the entire visible-light range by varying the particle size, where the center wavelength red-shifts as the sphere increases in size. 3.3 SERS of an as-synthesized single Si/Au core/shell nanosphere Generally, monolayer molecule detection via SERS is a method that harnesses the strong surface plasmon resonance of light in complex metallic (mostly Ag) nanostructures, such as particle aggregates, two-particle gaps, sharp tips, and nanostructures with dendritic shapes.25–29 In this way, the performance of the SERS characteristic is an effective index to examine the plasmonic property of a particular nanostructure. Here we use SERS testing as a route to expand the nanoplasmonic 15



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applications of the synthesized Si/Au core/shell NPs with its nontoxic aspect and its compatibility

in

the

semiconductor

industry.

The

analyte

for

SERS

is

4-methylbenzenethiol (4-MBT), where the aromatic thiol molecules are known to form self-assembled monolayers (SAMs) only on metals (gold, silver, and copper),30 and covalently attach only to these metallic surfaces. The Si/Au core/shell NPs deposited on an ITO substrate are immersed in a 4 mM methanolic 4-MBT solution for 6 h, which is long enough to reach full surface coverage. This concentration is lower than that used for many SAM structures,27,31–32 and based upon this technique it is expected to yield a strictly monolayer molecule coverage.33 After the 4-MBT soak, the ITO substrate is first immersed in pure ethanol for over 5 h, and then thoroughly rinsed with copious amounts of ethanol to remove the multilayer, molecular crystals that may have formed on the sample and any physically adsorbed molecules on the surface.31–33 The sample is dried under a stream of dry nitrogen and preserved in a vacuum oven before it is used for the SERS measurement. The SERS characterizations are conducted on a commercial micro-Raman system, and the laser beam is focused by a 50× objective to a spot size of 1 µm. Note that Raman measurements are also taken on NPs of pure Au and Si for comparison using the same treatment conditions as used for the Si/Au core/shell samples. All spectra are obtained by integrating two times with a 10 s exposure for each, as shown in Fig. 11. Figure 11a gives a bright-field optical microscope image of the measured area of the Si/Au samples (indicated with a white circle). Figure 11b shows the corresponding low-magnification image with the area marked one-for-one to that used 16


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to detect the bright-field image. Figure 11c shows a high-magnification image of the circled area from Fig. 11b (from which 4 particles of the Si/Au core/shell NPs can be definitely identified). Figure 11d provides four examples of Raman spectra obtained from the Si/Au core/shell sample, condensed pure Au NPs reported elsewhere,34,35 condensed Si NPs prepared by laser ablation in pure water,36,37 and the standard analytically pure bulk 4-MBT sample. The standard Raman spectrum of 4-MBT (red line of D) exhibits two typical characteristic peaks at 1089.0 and 1579.9 cm−1.38,39. The peak at 1089.0 cm−1 is attributed to a combination of the phenyl ring-breathing mode, C–H in-plane bending, and C–S stretching,29 while the peak at 1579.9 cm−1, which can be as attributed to the phenyl stretching motion,40 is chosen for the subsequent SERS intensity comparison. It is clear that only the spectrum of the Si/Au core/shell NPs (black line of A) gives the SERS of 4-MBT bands at 1080.9 and 1567.2 cm−1, while one faint band can only just be distinguished in the spectrum of pure Au NPs (yellow line of B) and is undetectable in the spectrum of pure Si NPs (blue line of C). All spectra results definitely confirm that the SERS enhancement originates from the Si/Au core/shell nanostructure, and that the fabricated Si/Au core/shell NPs exhibit plasmonic features with a strong magnetic-electric resonance. A monolayer coverage assumption is used for the conservative estimates of the Raman EF in the following calculations. We employ the peak at 1567.2 cm−1 to estimate the experimental Raman EF for our sample, calculated using the standard equation

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I SERS EF =

N SERS I BULK

,

(1)

N BULK where ISERS and IBULK are the Raman intensities of the same band for the SERS and bulk spectra, respectively; and NSERS and NBULK are the number of molecules probed in SERS and probed for the bulk sample of 4-MBT, respectively. The number of molecules excited in our SERS case is determined by assuming the excitation volume to be a sphere of diameter equal to 230–500 nm, which can be ascertained exactly from the SEM images of the spherical sample shown in Fig. 11c. Considering that the laser is focused to a spot of 1 µm, the scale bar of Fig. 11c further implies that there may be only one or two spheres irradiated by the detecting laser during the acquisition of spectrum A. Taking the monolayer thickness of 4-MBT as 0.5 nm,31 we can obtain a quick estimate of the NBULK/NSERS ratio in the following way. Because the specimens of bulk 4-MBT used for resonance Raman scattering (RRS) and for SERS detection of the sample are prepared in the same way, the ratio of NSERS/NBULK can be estimated as N SERRS C SERRS , ∝ N BULK C BULK

(2)

where CSERS and CBULK are the concentrations of 4-MBT contributing to the SERS and the bulk signals, respectively. In this way, the EF values of the Si/Au nanosphere with a 4-MBT concentration of 4 mM are calculated based on the CSERS/CBULK ratio. The estimated value of the EF is found to be 1.2×108 when the spherical object is estimated to have a diameter of 230 nm. For a conservative estimate, the average EF of the Si/Au core/shell NPs is estimated to be 6.7×107, which is as good as or even 18


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better

than

the

observed

EF

values

reported

for

complex

metallic

nanostructures.32,41–43 It should be noted that we use the flat-area approximation while determining NSERS in this EF calculation. Based upon the dark-field scattering and SERS analysis given above, we can conclude that the active Si/Au core/shell NPs provide intriguing power to enhance local magnetic-electric field intensity to a high level, which induces an extremely high SERS EF on the order of 107–108 that enables single-molecule detection. These Si/Au core/shell NPs can justifiably be considered as active plasmonic nanodevices, paving the way for their application in SERS detection 4. Formation mechanism of Si/Au core/shell NPs upon laser ablation in liquid Laser ablation-induced NP generation in liquids has fulfilled the prospect of (i) simplicity of the procedure, (ii) versatility with respect to solid targets and solvents, and (iii) the absence of chemical reagents or ions in the final preparation. This fascination arising from the interaction of the solid-liquid interface with intense light has motivated numerous investigations all over the world focused on the laser irradiation of colloids and laser ablation of solids in liquids.36,44–47 However, there is only one kind of laser irradiation or ablation process mentioned in each of the studies,44–46,48–50 which shows that there is still a great capacity for growth and development in experimental and theoretical research of LAL.47,51 Laser-generated NPs in liquid is known to often be a process that is very fast and far from equilibrium, so that stable and even metastable phases of materials forming in LAL could be retained in the final products.52 The initial process of laser ablation at the liquid–solid 19



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interface is mainly an interaction between the laser and solid target where, in general, along with the laser pulse ablates at the interface, the species originate from the solid and liquids with a large initial kinetic energy that first form a dense region in a plasma plume. Subsequently, a shock wave is created at supersonic velocity in the front that induces extra pressure in the plasma plume, called laser-induced pressure, owing to the liquid confinement. In this way, the laser-induced pressure induces a temperature increase in the plasma plume.53 Finally, the plasma plume from LAL is in the high-temperature, high-density, and high-pressure state. Another result of the liquid confinement is that the quenching time of the plasma plume becomes so short that some metastable phase or structure can be frozen in the final products.54 Additionally, because the growth time (plasma quenching time) of the nuclei is very short, the size of the grown crystals is usually in the nanometer scale.53 In our case, the physics and chemistry involved in the fabrication of the Si/Au core/shell NPs upon LAL are basically similar to that of the general process given above, but there are further unique and specific reactions that would occur in this manufacture. The Si/Au core/shell nanostructure formation upon LAL can be divided into three processes as follows. (i) First, the LAL, using a 355 nm laser ablating the solid target, generates a plasma plume at the interface between the liquid and Si that contains Si ions and species under high temperature, high density and high pressure conditions, which ultimately leads to the formation of the crystalline Si core. The high-purity HAuCl4 solution dissipated in the deionized water around the plasma plume is ionized into the active species H+, OH−, and AuCl4−, which can combine 20


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with the Si species in the plasma plume in the HTHPHD state to form a complex Si mixed Au species, as shown in Fig. 12a. (ii) Second, the laser irradiation-induced reaction, accomplished by the additional pulse laser of 532 nm irradiation, causes the composite active AuCl4− species to be strongly transformed into crystalline particles or into nanostructures with more complex mixed Au/Si crystal phases around the Si species owing to the good absorption of the 532 nm laser by the Au ions, as shown in Fig. 12b. This technology thus provides a unique possibility for the laser tuning of optical properties of a common Si/Au core/shell nanostructure. (iii) For the Si/Au core/shell structure fabrication, the incident laser radiation of 532 nm (whose pulse width is longer than that of the 355 nm laser) will promote a continuous dynamic crystallization of the Si/Au core/shell NPs. Considering the rapid cooling and condensation of the plasma plume that occurs in the confining liquid, the nucleation and growth of the Si/Au core/shell NPs will occur. Meanwhile, the rapid quenching and growth of the Au component leads to the synthesis of metastable and stable morphology frozen in the final products, which are seen as the wrinkle slices and NPs around the Si core shown in Fig. 12c. Therefore, within the (ii) and (iii) stages, the size and crystallographic phase of the synthesized NPs can be controllably influenced, which enables the fabrication of optical elements with laterally varying optical properties. As stated above, the generality of the proposed strategy for the fabrication of Si/M (where M is a noble metal) core/shell NPs ensures that this novel technique will allow researchers to choose and design interesting solid targets and solution 21



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environments within a double-beam-induced approach to fabricate nanoplasmonic structures of semiconductor-noble metal compounds for the purpose of fundamental research and potential applications. The fabrication of the Si/Au core/shell NPs, therefore, demonstrates this strategy. 5. Conclusions In summary, we propose a facile and general approach of double-beam LAL to fabricate well-defined Si/M (where M is a noble metal) core/shell NPs as nanoplasmonic structures. We demonstrate that these fabricated nanoplasmonic structures have strong localized magnetic-electronic fields in the observable light region, whose magnetic-electronic resonance structures are based on the Si core and enhanced by the complex nanostructures of the noble metal surface. Our results show that the assembled Si/Au core/shell NPs can be used as ultrasensitive individual sphere substrates for monolayer molecule SERS detection, and the average EF of their SERS sensitivity is found to be on the order of 108, which can be attributed to the plasmon effect. Obviously, the developed technique and the generated Si/Au core/shell NPs will find broad applications in Si-based photonics. Therefore, these findings open the door to a range of new opportunities for the ubiquitous semiconductor Si, making it an ideal candidate to control magnetic-electronic resonance in the visible spectral range and to be further used for the realization of efficient nanoantennas, nanolasers and artificial metamaterials.

Acknowledgements.

The

National

Basic

Research

22


Program

of

China

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(2014CB931700), and the State Key Laboratory of Optoelectronic Materials and Technologies of Sun Yat-sen University supported this work. P. Liu thanks C. Li and G. W. Yang thanks A. Zayats and G. Wurtz for valuable discussions.

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Figure Captions

Figure 1. Schematic illustration of double-beam laser ablation in liquid.

Figure 2. (a) Low-magnification SEM image of the synthesized Si@Au core-shell NPs. (b) High-magnification SEM image of four Si@Au NPs. (c) Distribution histograms of the size ration of the Si@Au core-shell NPs. (d) XRD pattern of the synthesized Si@Au core-shell NPs.

Figure 3. (a) TEM bright-field image of a Si@Au core-shell NP with the wrinkled slices and tiny particles around the surface, with the corresponding SAED pattern is shown in the inset. (b) The corresponding HRTEM images. (c-d) The corresponding STEM analysis.

Figure 4. (a) TEM bright-field image of a Si@Au core-shell NP with the wrinkled surface, with an SAED pattern show the dark piece circled by white square can be indexed to Au crystal. (b) HRTEM images of the sample. (c-d) STEM analysis of the sample.

Figure 5. (a) TEM bright-field image of a Si@Au core-shell NP with the net-shape wrinkled surface around the particle. (b) HRTEM images of the sample, and the clear-cut crystal lattice and a corresponding FFT analysis in the inset of b, which 31



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clearly indicates the Au shell and Si core. (c-d) STEM analysis of the sample.

Figure 6. (a) TEM bright-field image of a Si@Au core-shell NP with a complete core-shell structure. (b) HRTEM images of the sample and a clear-cut crystal lattice analysis in the inset of b. (c-d) STEM analysis of the sample.

Figure 7. SEM images (i and ii) and their corresponding dark-field scattering spectra (iii) of the Si@Au core-shell NP with the wrinkled slices and tiny particles around the surface. The close-view dark-field microscope of each Si@Au core-shell NPs are shown in every panel (iii).

Figure 8. SEM images (i and ii) and their corresponding dark-field scattering spectra (iii) of the Si@Au core-shell NP with the wrinkled surface. The close-view dark-field microscope of each Si@Au core-shell NPs are shown in every panel (iii).

Figure 9. SEM images (i and ii) and their corresponding dark-field scattering spectra (iii) of the Si@Au core-shell NP with the net-shape wrinkled surface around the particle. The close-view dark-field microscope of each Si@Au core-shell NPs are shown in every panel (iii).

Figure 10. SEM images (i and ii) and their corresponding dark-field scattering spectra (iii) of the Si@Au core-shell NP with a complete core-shell structure. The close-view 32


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dark-field microscope of each Si@Au core-shell NPs are shown in every panel (iii).

Figure 11. (a) Bright-field microscopy image of the measured area of the Si@Au samples. (b) A corresponding low-magnification SEM image of the detection area. (c) The high-magnification SEM image of the circled area that shown in b. (d) SERS spectra of a SAM of 4-MBT taken from the Si@Au core-shell NPs (A) that shown in (c), condensed pure Au NPs (B), and condensed Si NPs (C) that be prepared by laser ablation in pure water. A standard Raman spectrum of 4-MBT molecular (D). Raman system has an excitation wavelength of 632.8 nm (measurement power = 1.7 mW) and accumulation time = 10 s.

Figure 12. Schematic illustration of fabrication mechanism of Si@M core-shell NPs upon double-beam laser ablation in liquid.

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Figure 1

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Figure 2

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(b)

(a)

(ii)

100 nm

100 nm

100 nm

(iii)

(iv)

100 nm

100 nm

Si

Au Au

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Au Au

100 nm

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Intensity（）（a. u.）

Abundance（）（%）

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(d)

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Figure 3 (a)

(b)

_ (311) Si _ (220) Si

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Figure 4

(b)

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(020)Au

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Figure 5

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Figure 6

(b)

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Figure 7

FDTD Simulation electric quadrupoles magnetic quadrupoles

400

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Figure 8

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Figure 10

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Figure 11

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Figure 12

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Shell Nanoplasmonic Structures with

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