Annealing Induced Morphology of Silver Nanoparticles on Pyramidal

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Annealing Induced Morphology of Silver Nanoparticles on Pyramidal Silicon Surface and Their Application to Surface Enhanced Raman Scattering ABHIJIT ROY, Arpan Maiti, Tapas Kumar Chini, and Biswarup Satpati ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08493 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 15, 2017

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ACS Applied Materials & Interfaces

Annealing Induced Morphology of Silver Nanoparticles on Pyramidal Silicon Surface and Their Application to Surface Enhanced Raman Scattering Abhijit Roy, Arpan Maiti, Tapas Kumar Chini and Biswarup Satpati*

Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, HBNI, 1/AF Bidhannagar, Kolkata-700064, India

*E-mail: [email protected]

ABSTRACT: This paper reports on a simple and cost-effective process of developing a stable surface enhanced Raman scattering (SERS) substrate based on silver (Ag) nanoparticles deposited on silicon (Si) surface. Durability is an important issue for preparing SERS active substrate as silver nanostructures are prone to rapid surface oxidation when exposed to ambient conditions, which may result in the loss of the enhancement capabilities in a short period of time. Here, we employ galvanic displacement method to produce Ag nanoparticles on Si(100) substrate pre-patterned with arrays of micro-pyramids by chemical etching and subsequently

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separate pieces of such substrates were annealed in oxygen and nitrogen environments at 550°C. Interestingly, while nitrogen annealed Si substrates were featured by spherical shaped Ag particles, the oxygen annealed Si substrate were dominated by the formation of triangular shape particles attached with the spherical one. Remarkably, the oxygen annealed substrate thus produced shows very high SERS enhancement compared to the either un-annealed or nitrogen annealed substrate. The hitherto unobserved co-existence of triangular morphology with the spherical one and the gap between the two (source of efficient hot-spots) are the origin of enhanced SERS activity for the oxygen annealed Ag particle covered Si substrate as probed by the combined finite-difference time domain (FDTD) simulation and cathodoluminesensce (CL) experiment. As the substrate has already been annealed in oxygen environment, further probability of oxidation is reduced in the present synthesis protocol that paves the way of making a novel long-lived thermally stable SERS substrate.

KEYWORDS: Galvanic displacement reaction, Rapid thermal annealing, Surface enhanced Raman scattering, Electron microscopy, Cathodoluminesensce.

INTRODUCTION

Surface enhanced Raman scattering (SERS) is an excellent tool to determine the molecular finger print up to single molecule level.1 In recent years, much attention has been paid towards the design and fabrication of metallic nanostructures with varied of morphological characteristics for their use as surface enhanced Raman scattering (SERS)-active substrates.2–11 The optical 2 ACS Paragon Plus Environment

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response of noble metal nanoparticles (NMNPs) is governed mainly by the collective oscillation of conduction electrons in NMNPs, known as localized surface plasmon (LSP). The interaction of electromagnetic (EM) waves with the metal particles produces localized surface plasmon resonance (LSPR) which is the collective oscillation of conduction electrons of metal under the excitation of external EM field (light or fast moving electron beam). The LSPR is localized mainly in the sharp tips, edges, apexes of metal nanostructures within subwavelength regions with substantial enhancement of local electric field, commonly called as “hot spots” that are responsible for large enhancements in SERS signals.12 LSPR is dependent on various parameters like size,13 shape14-16 and inter-particle distances between the metal nanoparticles.12 Gold and silver are mostly used noble metals for the preparation of SERS active substrate because they have surface plasmons which lie in the visible region of the EM spectrum which coincides with the commonly used Raman excitation wavelengths, in addition to their thermal stability and good resistance to corrosion. Ag generally shows better enhancement over Au due to the increased scattering efficiency in the UV-visible region.17 Unfortunately, Ag nanostructures are prone to rapid surface oxidation when exposed to atmosphere resulting in the loss of the enhancement capabilities in a short period of time.18 Further, heating the SERS substrate at elevated temperature will aggravate decay of the enhancement of the Raman signal because high temperature will speed up the surface oxidation process and lead to the destruction of the nanostructure through aggregation processes.19 Noble metal/semiconductor heterojunctions have some advantages over colloidal metal nanoparticles. As semiconductor acts as a support to hold metal nanoparticles, agglomeration cannot occur, which is very important to obtain a stable SERS substrate.20 In heterogeneous SERS approach, 3D SERS substrates are produced which show higher SERS enhancement compared to their 2D counterparts. This is due to the fact that



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3D substrate possesses larger surface area to provide more hot spots and can capture more absorbate molecules.21 Lee et al.22 have observed that the SERS signal enhancement measured at the 3D plasmonic nanostructures was 3.9 times the signal measured at the 2D plasmonic nanostructures. In recent years, three dimensional (3D) SERS substrates have been introduced to show impressive sensitivities.23-25 These 3D substrates are widely used for SERS application and they also show higher stability and easy tailoring.26-27 3D SERS substrate can be obtained via different processes like metal assisted chemical etching (MACE),28 anisotropic etching,29 reactive ion etching (RIE)30 and laser interference lithography (LIE).31 Among these processes, RIE and LIE are of high cost and time consuming. However, chemical etching and MACE are of low cost processes and can also give high throughput and higher uniformity. Galvanic displacement reaction (GDR) being in the electroless family, is a very popular method for the fabrication of metallic nanostructures on Si surfaces and is also a very clean process12 providing a direct interface between the deposited metal and the substrate.32 The standard potential for Ag is 0.8V and that for Si is -0.875V w.r.t standard hydrogen electrode (SHE), implying an easy Ag deposition on Si. Pinkhasova et al.33 have reported that for Ag particles deposited on Si substrate the enhancement factor (EF) of SERS decreases with annealing temperature up to 300°C due to the formation of silver oxide which reduces the number of binding sites for the analyte molecules at the particle surface. Further annealing at 400°C, the EF increases due to the decomposition of silver oxide but does not reache up to the level of un-annealed sample. In contrary to this finding, however, in the present work, we show enhancement in the SERS signal from a Ag nanoparticle deposited Si substrate that is annealed under oxygen environment. Here, we have adopted GDR method to deposit Ag nanoparticles on a Si surface pre-patterned with arrays of pyramid 4 ACS Paragon Plus Environment

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structures by chemical etching. The Ag nanoparticle covered pyramidal Si substrate prepared thus when subject to rapid thermal annealing (RTA) in oxygen atmosphere generates formation of some triangular shaped particles along with the spherical one and shows a very high SERS enhancement. We have employed cathodoluminescence (CL) spectroscopy and imaging in a field emission gun scanning electron microscope (FEG-SEM), where site specific electron-beam excitation of nanoparticles provides information on the surface plasmon assisted photon emission, both in the spectral and spatial domains for identifying the hotspots to explain the origin of high SERS activity in the present work. Additionally FDTD simulation is also employed for better understanding the SERS enhancement. The detailed analysis of the crystalline structure and the morphological features developed under different annealing environment was also studied by using plan view scanning electron microscopy (SEM), crosssectional SEM (XSEM) and cross-sectional transmission electron microscopy (XTEM) techniques.

EXPERIMENTAL SECTION Preparation of patterned Si surface and subsequent Ag particle growth. To produce Si pyramidal arrays, a piece of Si(100) substrates were first ultrasonically cleaned using ethanol, acetone and deionized water for 5 minutes. The cleaned substrates were then etched using saw damage removal (SDR) method reported previously.34 In this process substrates were immersed in 30wt% NaOH solution and kept at 75°C for 3 minutes which removed 5-6 μm thick layers from both the sides of Si substrate. The substrates were then immersed in a mixed solution of NaOH (2wt%) and isopropyl alcohol (IPA, 10%) in deionized water at 80°C for 45 minutes to



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produce pyramidal Si (P-Si) array finally. The sizes of the Si pyramids depend on the immersion time. The substrates were then rinsed with ethanol, deionized water and stored in vacuum desiccators until further use. Ag nanoparticles were deposited by GDR where pyramidal Si substrates were first immersed in a 2% HF solution for 1 min to remove the native oxide layer and it was then immediately immersed in a solution containing 5ml AgNO3(5mM) solution and 5ml HF (4.8M) solution. Deposition was carried out for different time intervals (10s, 30s, 60s and 90s) to observe the growth and optimum coverage of Ag nanoparticles on the pyramidal Si arrays. Separate pieces of such substrates having uniform coverage after 90s growth were then annealed in oxygen and nitrogen environments at 550°C for 1 minute in a rapid thermal annealing unit (Model: JETFIRST 100, jipelec). The temperature ramp was 10°C per second in both the cases. For convenience, henceforth we will use notation S1 for Ag deposited P-Si (PSi/Ag), S2 for P-Si/Ag annealed at 550°C for 1 minute in N2 atmosphere and S3 for P-Si/Ag annealed at 550°C for 1 minute in O2 atmospheric condition. These three samples (S1, S2 and S3) are used in the present work for further structural and optical characterizations. SEM and TEM Characterization. SEM analysis was carried out using 200 Quanta, FEG (FEI) in high vacuum mode. The secondary electron (SE) images were acquired in normal plan view condition as well in cross-sectional view condition (XSEM) for detail morphological characterization. In some cases, we also used back scattering electron (BSE) imaging for clear visualization of the image contrast (so called Z contrast image) arising out of the high difference in the atomic number of the species, i.e., between Ag and Si. The operating voltage of the SEM during SE imaging was varied between 5kV to 15kV according to resolution requirement. Crosssectional TEM foils were prepared in the [110] and [-110] projections as well as in the [100] and [-100] projections. For XTEM measurement two cut pieces (3mm × 6mm) were bonded face to 6 ACS Paragon Plus Environment

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face and inserted into a brush tube from which several discs of 0.5 mm thickness were cut. The cut discs were then subjected to mechanical thinning followed by double dimpling and a final milling by Ar+ ion using precision ion polishing system (PIPS, GATAN Inc.) at energy of 3.5keV and cleaning at 1.5keV. The detailed structural analysis was carried out using a TEM (FEI Tecnai G2 F30-ST) operated at 300keV. The same TEM is also equipped with high angle annular dark field (HAADF) detector from Fischione (Model 3000) and energy dispersive X-ray spectroscopy (EDX) attachment for compositional analysis.12 For TEM observation the samples those were prepared in the [110] and [-110] projections were aligned along [110] zone axis and others prepared in the [100] and [-100] projections were aligned along [100] zone axis. SERS measurements. For SERS measurement rhodamine 6G (R6G) was used as the analyte molecule. A 20 μL solution of 10 μΜ R6G in deionised (DI) water was spread on the substrate until the substrate gets immersed inside the solution and kept for 12 hours. The substrate was then taken out from the solution and dried in ambient condition before SERS measurement. The SERS and normal Raman spectra were acquired using a LABRAM HR 800 operated at laser wavelength 632.8nm (He-Ne laser) and Labspec 4 software was used for data acquisition. Raman peak of clean Si substrate at 525 cm-1 was used for calibration of the spectrometer. The data were recorded for 10s integration time and 3 accumulations by using a 100× aperture (Numerical aperture, NA = 0.9). 10-1 M R6G in DI water deposited on Si substrate was used as reference sample. NBulk, the number of molecules used in the bulk was computed from this sample by considering number of molecules within the focal volume assuming R6G molecules deposited on patterned Si uniformly. Assuming R6G molecules deposited on patterned Si uniformly, the number of molecules adsorbed and sampled on the SERS-active substrate, NSERS was considered to be the fraction which falls within the laser spot area. The SERS spectra were 7 ACS Paragon Plus Environment


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taken from different regions of the substrate to ensure the homogeneity of the sample. The SERS data were recorded at an interval of 7 days while keeping the samples in ambient condition to observe the effects of ageing on SERS enhancement. CL measurements. CL spectroscopy and imaging were performed on isolated single Ag particle in a ZEISS SUPRA40 SEM equipped with a Gatan MonoCL335 cathodoluminescence system. All the CL data presented in this paper were recorded with an electron acceleration voltage of 30 kV and a beam current of about 12 nA with a beam diameter of ~ 5 nm. The CL system can be operated in monochromatic (mono) and panchromatic (pan) modes. In the monochromatic mode, the focused e-beam is either scanned over the sample or positioned on a desired spot while the emitted light from the sample passes through a monochromator allowing us the emission spectra to be recorded serially in the wavelength range 300–600 nm by a high sensitivity photomultiplier tube (HSPMT) detector. The monochromatic photon map is then built up at a selected peak wavelength of the CL spectrum by scanning the e-beam over the sample. The bright pixels of the photon maps correspond to the areas where the strongly excited plasmon mode emits the photons. In the panchromatic mode of CL operation, on the other hand, the monochromator is bypassed allowing light of all the emitted wavelengths to be collected. FDTD Simulations: 3D finite-difference time domain (FDTD) simulations (from Lumerical FDTD Solutions, Canada) were performed in order to understand the near field intensity and SERS enhancement from the coupled Ag nanoparticles (NPs) (triangular Ag particles with the spherical Ag particle) grown over the 3D Si substrate. The coupled nanostructures were modelled as a single Ag sphere of radius 100 nm situated above a triangular Ag NPs with side edge length of 155 nm. The coupled nanostructure system was laid on a pyramidal shape Si


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substrate with dimensions of 2000 nm x 2000 nm x 2000 nm. The coupled Ag NPs were excited by a total field scattered field (TFSF) plane wave source with injection in the z-axis direction with a polarization in the x-axis direction. The total simulation region was set to 1500 × 1500 × 1500 nm3, and perfectly matched-layer (PML) boundary conditions were used. The simulation time was adjusted to 500 fs so that the energy field decays fully. The mesh size was set as 1.5 nm x 2 nm x 2 nm during all the numerical simulations. In order to understand the near field intensity map the field monitors were placed at y-z and x-z and x-y plane around the nanostructures. 632 nm excitation wavelength was used during the FDTD simulations.

RESULTS & DISCUSSIONS Analysis of morphology: SEM observations. Ultrasonically cleaned p-type Si(100) wafers chemically etched in NaOH was used to produce self-assembled pyramid structures using a method reported previously.34 Figure 1(a) show the SE image of such micro-pyramids structures on Si. The size distributions of the pyramids are plotted in Figure 1(b) with average base length 2.87 μm. The SE image of samples prepared for XSEM view is shown in Figure 1(c) and the height distribution is plotted in Figure 1(d) where the average height peaked at 2.78 μm.



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Figure 1. (a) SE images of micro-pyramids on Si, (b) histogram of base length distribution, (c) XSEM view, (d) histogram of height distribution.


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Figure 2. Plan view SE image of Ag deposited on Si pyramids for different reaction time: (a) 10s, (b) 30s, (c) 60s and (d) 90s, (e) magnified view of (d) showing dendritic Ag nanostructure formation on top of pyramid, (f) XSEM view of (d) showing the presence of dendritic structure at the top in most of the pyramids. To have uniform coverage of Ag nanoparticles on Si pyramids we have grown Ag at room temperature (RT) for different reaction time such as 10s, 30s, 60s and 90s and the corresponding morphological evolution is shown in the SE images of Figure 2(a)-(d). From these SE images one can see that the growth rate is higher at the top and sharp edges and is lower at the middle and bottom surface of the pyramids. At the initial period of growth at RT, the coverage of the Ag particles on the side walls of the pyramids were characterized by the presence of some oval shaped gaps devoid of any particle growth as is clear from Figure 2(a) and (b). It can be seen that the radius of the gap decreases and coverage increases with growth time. A uniform coverage of 11 ACS Paragon Plus Environment


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Ag nanoparticles on pyramidal Si (P-Si) surface was obtained that finally takes spaghetti like morphology forming a percolated network after the reaction time of 90 s [Figure 2(c)]. Similar growth pattern was reported earlier36 for Ag deposition on Si (111) surfaces at room temperature in UHV environment. We also observe some dendritic structures formed at the top of pyramid structures as revealed in the plan view and XSEM view SE images of Figure 2(e) and 2(f), respectively. The uniform Ag nanoparticles deposited P-Si sample prepared for 90s reaction time were cut into several pieces for further detail study to assess the role of annealing on morphology of Ag nanoparticles deposited at RT on P-Si surfaces. However, for the particle growth on Si surface without pyramids the time evolution of the growth pattern is opposite to that of pyramidal Si surfaces, i.e., initial growth (for reaction time of 60s) is characterized by uniform coverage whereas the final growth (for reaction time of 180s) pattern contains gap regions devoid of particles (Figure 3).

Figure 3. (a) Ag nanoparticles deposited by galvanic displacement method on Si (100) substrate for reaction time of (a) 60s, and (b) 180s.


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The SE image of S2 sample annealed at 550°C for 1 min in N2 atmospheric condition in XSEM view is shown in Figure 4(a). Corresponding BSE images of Figure 4(b, c) show the clear Z contrast between Si and Ag. In this case, the particle size is bigger than the as-grown sample at RT and interestingly, particles size increases gradually from bottom to the top of the pyramid. Also the edges contain bigger particles compared to the middle part of the pyramid wall [indicated by arrows in Figure 4(a)]. The increase in particle size and inter particle distance indicate that the particles get bigger through a process called “Ostwald ripening”37 which was reported earlier for Ag particles grown on different substrates were subjected to RTA.38 In case of Ostwald ripening, number of smaller particles reduced due to their higher surface to volume ratio and formation of bigger particles occurred due to coalescence. Here we observed some small particles [see, Figure 4(c)] along with the bigger one which indicates an incomplete Ostwald ripening process. However during annealing the average size is dictated by the diffusivity of the nanoparticles on the substrate.39 For the sample S3 that was annealed in O2 environment at 550 °C, it was observed that the small particles those were present in case of S2 were drastically reduced in number. From the magnified view of the XSEM images in Figure 4(f), it can be seen that a triangular shaped Ag island is formed at the bottom of each of the spherical particles. Recently, D. Wall and coworkers40 have observed Ag(111) triangular island formation on Si(111) when they annealed the sample above 500 °C and number of islands exhibiting a triangular shape increases with annealing temperature. At lower temperature this formation is kinetically limited. It is observed that triangular etch pits are formed on Si (111) in presence of Ag when the annealing temperature is more than 450°C.41 We have also observed etch pits on Si (100) in presence of Ag at annealing temperature 550°C [Figure S1(e), SI]. This is mainly attributed to the localized stress introduced 13 ACS Paragon Plus Environment


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by higher diffusion of Ag into Si, resulting Si lattice defects.41 It is also well known that kinetics of Ag particle is enhanced in presence of O2.42 This is due to the fact that an adspecies AgmO formed in presence of O2 atmosphere which can transfer Ag nanoparticle more easily compared to Ag adatoms due to their lower detachment barrier.

Figure 4. XSEM images: (a) SE image of S2 sample showing the particle size variation from bottom to top of the pyramid, (b)-(c) corresponding BSE images showing the atomic contrast between Si and Ag, (d) SE image of S3 sample showing the formation of spherical shaped particles along with triangular one, (e)-(f) corresponding BSE images confirm that the triangles are also made of Ag.

Due to reversible oxygen detachment-attachment process AgmO will decompose whenever they impinge on the surface of any other Ag nanoparticle. At high annealing temperature, however, 14 ACS Paragon Plus Environment

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AgmO clusters have an increased likelihood of reacting with Si on the surface of Si.42 Atoms arriving at surface and diffusing through it prefer to stick to the edges rather than to the facet of the island, which is possible if diffusion of particles through substrate is very much higher than through island.43 This possibly leads to the formation of triangular particle in O2 atmosphere along with Si etch pits. Since AgmO prefers Si surface over Ag sphere surface we observed small size of Ag sphere in S3. Triangle-shaped etch pits are the nucleation centres for triangle-shaped Ag nanoparticles which is a result of larger diffusion of Ag particles in O2 atmosphere and formation of bigger spherical particle through the process of “Ostwald ripening". As the kinetics of Ag particle is enhanced in presence of O2 during spherical particle formation which is also the root cause of triangle-shaped etch pit formation are thus gets the formation of triangle and nanosphere in pair. A thin layer of SiOx is seen to cover almost all the triangular particles and it also provides a separation between the triangular particle and the spherical one [Figure S3, SI]. This SiOx layer provides a higher stability to the triangular particle.44 Also as Ag diffuses slowly though SiO2 compared to Si, this may be the reason for the formation of spherical particle on top of the triangular particle by Ostwald ripening. Hence we must conclude that the mobility of the particles does get enhanced in O2 atmosphere compared to N2 atmosphere due to the formation of the adspecies AgmO. Analysis of crystalline structure: TEM observations. We carried out extensive TEM analysis to study the crystalline structure analysis and the related epitaxy between the deposited Ag particles and Si substrate in addition to particle size distribution. Figure 5(a) shows the low magnification TEM images of sample S1. The particle size distribution is shown in Figure 5(e) with an average particle size of 35±5 nm. It is also clear that particle size increases from base to the top of the pyramid [Figure 5(a, b)] which indicates an increase in surface energy along this 15 ACS Paragon Plus Environment


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direction. The EDX elemental mapping of S1 sample using high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) mode from a region in Figure 5(b) is shown in Figure 5(d), where a small amount of Si is observed beyond the substrate which may be due to water soluble SiF62- formed in the reaction and attached to Ag nanostructure. Figure 5(f) represents the HRTEM image of the interface showing clearly the high crystalline nature of Ag. The Fourier filtered image of the interface marked by red rectangular box in Figure 5(f) shows that Ag (111) plane is rotated by an angle 36.14° w.r.t Si (111) plane. This kind of tilted growth was also observed for Ag deposited on Ge (111) by galvanic displacement method and for Ag on Si (001) in UHV environment without forming dislocations.12, 43


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Figure 5. (a) Low magnification bright-field XTEM image of S1 sample, (b) STEM-HAADF image, (c) EDX spectrum from region 1 is shown in (b), (d) elemental map from region 2 using Si-K and Ag-L energy, (e) histogram of particle size distribution, (f) HRTEM image at the interface. Inset, Fourier filtered image of the red dotted region of the interface showing crystallographic relation between Si (111) and Ag (111) planes.



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Figure 6. (a) Low magnification bright-field XTEM image of S2 showing formation of spherical particle on Si due to N2 annealing. Particle size increases with the pyramid height. (b) HRTEM image showing the direct formation of 3D island of Ag on Si, (c) histogram of the particle size distribution, (d) selected area electron diffraction (SAED) pattern of the interface showing epitaxial growth of Ag on Si. Figure 6(a) shows the low magnification bright-field TEM image of S2 sample that was annealed in N2 environment. The increase in contact angle of Ag nanoparticle on Si pyramid from base to the summit is due to increase in surface energy along this direction. This results in increase in the particle size from base to the top of the pyramid. The particle size distribution [Figure 6(c)] indicates that the smaller size particles decreases in number compared to S1sample and the average size of the particles is 85 ±13 nm. HRTEM image of S2 sample shows that the particles 18 ACS Paragon Plus Environment

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obtained a quasi spherical shape [Figure 6(b)]. The 3D islands are in direct contact with Si substrate and no necking is observed between them indicating a growth process similar to Volmer-Weber growth mode. The [110] zone-axis selected area electron diffraction (SAED) pattern in Figure 6(d) clearly shows that the Ag (111) planes are in epitaxial relation with the Si (111) planes. Figure 7 show the TEM images of S3 sample that was annealed in O2 environment. Particle size distribution is shown in Figure 7(b). Here particles are even bigger (105±14 nm) compared to S2 sample indicating a higher rate of Ostwald ripening in O2 environment. The presence of triangular particle under the spherical one is clearly seen from the low magnification bright-field XTEM image in Figure 7(c). Magnified view in Figure 7(d) clearly shows the presence of triangular particles below the spherical one and presence of rough spherical surfaces. Slight deviation from triangular symmetry is also observed in some cases. The spherical particles are mostly residing on one side of the triangles. Thus it is clear that annealing at different atmospheric condition plays a very crucial role in controlling particle size and shape. The mobility of particles on a substrate during annealing is a very important factor that plays a critical role in particle size and shape formation. The property which is directly related to the mobility is adhesion energy and is given by, Ead=γm(1+cosθ), where γm and θ are the surface energy of the metal and equilibrium contact angle (CA) of the particles, respectively.38 From a set of measurement of CA from XTEM images the average contact angle of the nanoparticles for S2 sample and S3 sample is estimated as 113.6° and 133.2°, respectively. This clearly shows that adhesion is lower and hence mobility is higher for nanoparticles in O2 atmosphere.



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Figure 7. (a) Low-magnification bright-field XTEM image of S3, (b) Histogram of the particle size distribution, (c) bright-field XTEM image showing almost all the spherical particles are associated with triangular particle underneath, (d) high-magnification XTEM image of S3, (e) SAED pattern from a region in (c) showing epitaxial relation between Ag and Si. [110] zone axis diffraction pattern where diffraction spots of Ag are indexed by red and Si by yellow color. (f) HRTEM image of the portion marked by red square in (d) showing the facet is along {422} direction and Moiré fringe of spacing 9.67±0.04 Å. 20 ACS Paragon Plus Environment

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SAED pattern from a bright-field XTEM image of S3 is shown in Figure 7(e). There is a clear indication of an epitaxial relationship between Si and Ag. Ag (111) planes are in epitaxial relation with the Si (111) planes. Thus in the present study epitaxial growth of Ag nanoparticles take place on Si(100) surface irrespective of annealing environment or particle shape. This type of epitaxy is observed previously between Si and Ag and can be explained by domain matching epitaxy (DME). The initial mismatch between Si (111) and Ag (111) is about 25% but during DME a 3×3 domain of Si matches with a 4×4 domain of Ag with a mismatch of only 0.6% leading to an epitaxial growth [shown in Figure S2, SI]. HRTEM image of one triangle marked by dotted box in Figure 7(d) is shown in Figure 7(f). Detailed analysis of this triangular structure revealed that the side walls are bounded by high-index {422} facets [Figure 7(f)]. The lattice spacing of the Moiré pattern is measured to be 9.67 ± 0.04 Å, which is very close to the lattice spacing (9.4 Å) of the translational Moiré fringe formed by the overlap of Si (111) and Ag (111) plane.12 Hence, the flat top faces of the triangles are formed by Ag (111) facets. These triangular particles are not embedded inside the Si matrix as confirmed from the formation of Moiré pattern and STEM-HAADF-EDX analysis [Figure S3, SI]. SERS activity. To test the SERS abilities of the Ag nanoparticles deposited on Si pyramids, we first examined the enhancement of R6G using S1, S2 and S3 samples. For this purpose, we calculated the enhancement factor (EF) for all spectra using the equation, EF = [(ISERS) / (IBulk)] × [(NBulk)] / (NSERS)], where, ISERS is the integral intensity of a certain vibrational mode (Raman peak) of the analyte in the presence of nanoparticles, and IBulk is the intensity of the same in the bulk Raman spectrum from the analyte alone. NBulk is the number of molecules used in the bulk, and NSERS is the number of molecules adsorbed and sampled on the SERS-active substrate [see, SERS calculation, SI].45-46 The samples for SERS measurement were obtained by immersing the 21 ACS Paragon Plus Environment


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substrates into a solution of R6G of required concentration for 12 hours. We have taken the concentration of R6G as 1×10-1 M on P-Si as reference spectra. At first, enhancement for samples S1, S2 and S3 were tested with R6G of concentration 1×10-5 M and it is observed [Figure 8] that all the three samples giving Raman peaks at 613, 774, 1185, 1312, 1364, 1509 cm-1 which are the well established Raman peaks of R6G.47-48 The most intense enhancement is observed [Figure 8] for the peaks at 613 cm-1 (corresponds to C-C-C in plane vibration of R6G molecule) and 1364 cm-1 (corresponds to the aromatic C-C stretching vibration mode). It is interesting to note that the intensities of SERS spectra are much stronger for S3 than those of S2 or S1. It is observed that the calculated EF of sample S3 is about 3.5 and 2 times higher than that of sample S2 and S1, respectively for almost all the characteristics peaks of R6G [Table S1, SI]. The highest enhancement was obtained for S3 at 613 cm-1 [(9.49 ± 0.76)×107] which is comparable to the literature values.12,17,48 It is also observed that the SERS effect of S1 is higher than that of S2 [Figure 8(a)]. This is reasonable as S1 contains Ag particles which are irregular in shape and have more edges and corners (compared to S2 sample) suitable for electric field enhancement. In case of S3 sample, Ag particles are bigger in size and spherical in shape with rough surface along with the presence of triangular particles beneath the spherical one which increases the SERS signal immensely compared to the others.


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Figure 8. (a) SERS spectra of 1×10-5 M R6G taken for S1, S2 and S3 samples just after preparation (day 1). (b) SERS spectra of same three samples after 28 days. It is known that surface plasmon induced electric field enhancement at the edges, sharp corners and also at the gaps between two particles12 which have been verified by spatially resolved CL imaging and FDTD simulation will be discussed in the next section. To ensure the homogeneity and stability of the substrate as SERS active substrate, the SERS data were taken from several different regions of the three samples. To test the stability of the substrate, we took the SERS data at an interval of 7 days up to 28 days while keeping the substrates in ambient conditions [Table S2, SI]. Decrease in peak intensity is observed after 28 days for all the three samples [Figure 8(b)]. However, decrease in peak intensity is less in S3 (18%) compared to S1 (31%) and S2 (26%) for 613 cm-1 peak. This indicates that S3 sample is a superior SERS active substrate compared to S1 and S2 sample in terms of EF and stability. The coupling between triangular and spherical Ag particles (as shown in FDTD simulation) is the main reason behind the enhancement in S3 and formation of SiOx layer on top of triangular Ag particles [Figure S3, SI] provides the stability against oxidation. This indicates that S3 sample is a superior SERS active



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substrate compared to S1 and S2 sample. The SERS spectra were taken from three different regions of S1, S2 and S3 sample to ensure the homogeneity of SERS substrate [Figure S4, SI]. As sample S3 gave the highest enhancement and also higher stability, we test the lowest concentration of R6G (1×10-8 M) for SERS active substrate using S3 sample. Significant enhancement was obtained even for the lowest concentration (1×10-7 M) of R6G [Figure 9]. However this limit of detection of the SERS substrate is much higher than the work reported in the literature.49-50 Li et al. have observed low detection limit of 10 −11 M R6G.50

Figure 9. SERS spectra of different concentration of R6G for S3 and R6G (10-1M) on P-Si. Considering the spherical model by Lakowicz et al.,51 where the molecules are considered as a dipole and the metal particles are simplified by spherical particle, the induced electric field is written as, E=E0r3(εm+εd)/(d+r)3 +(εm+2εd), where r is the radius of the metal particles, d is the 24 ACS Paragon Plus Environment

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distance between molecule and metal particles, εm and εd are the dielectric constants for metal and substrate respectively. So there will be change in magnitude of induced dipole with size of metal particles. However such huge difference in Raman enhancement cannot be explained by only considering the size effect. CL Observations. In CL experiment radiative emission from metal nanostructures is generated by electrom beam excitation at nanoscale regime. As photon emission is highest in the region where resonant modes of LSP occurs, CL images thus map near E-field distribution at the surface of metal nanostructures.52-53 Use of electron irradiation to determine the spatiallyresolved plasmonic mode distribution from the detection of CL in Ag nanoparticles was first reported by Yamamoto et al.54 To find out exact location of the hot spots we have carried out CL imaging apart from spectroscopy. Among different kind of Ag nanostructures of the present samples, S3 sample in plan-view mode as shown in the SE image of Figure 10(a) and (f) was selected for CL study.



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Figure 10. (a) Secondary electron (SE) image (b) Pan CL image showing the formation of hot spots on both the spherical particle and triangular particle (c) CL emission spectra from the spherical particle (red curve) and from bottom triangle (blue curve). (d)-(e) Mono CL image at 372 nm and 525 nm respectively. (f) SE image of second particle (g) Mono CL image showing the hotspot is contained mainly by the sphere at 424 nm (h) Mono CL image where the hotspot is also contained by the bottom triangle at 464 nm. Since triangular particles are at the bottom of the spherical particles, plan view SEM could not clearly resolve the triangular particles. Under such constrained condition we have taken CL measurements from the combined spherical and triangular particle. In Figure 10(b), we show the 26 ACS Paragon Plus Environment

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panchromatic CL map of this structure. It is interesting to note that the “hot spot”s (region of local enhancement of electric field) is sustained by the spherical as well as by the triangular particles. To obtain spectrally resolved features, we performed CL spectroscopy and imaging in the monochromatic mode. The CL spectrum shown in Figure 10(c) was acquired for the electron beam impact at spherical particle (marked as blue dot) as well as triangular particle (marked as red dot). The CL spectrum of Figure 10(c) shows a strong peak at 372 nm (3.39 eV) which is close to the well-known surface plasmon resonance (SPR) peak of Ag.12 A few others less intense peaks are also observed at wavelengths 424 nm, 464 nm for the spherical particle and an intense peak at 525 nm for the triangular particle underneath. It is reported that the peaks higher than 372 nm are due to higher order modes of excitation which depends on the size and shape of the nanoparticles.54 The monochromatic CL images [Figure 10(d)-(e) and (g)-(h)] recorded for two different spherical and triangular particle combination at four wavelengths, i.e., 424 nm, 464 nm, 372 nm and 525 nm show a clear trend that strong enhancement of light emission occurs when the electron beam scans over the corners and edges of the Ag particles, the effect being most pronounced for the peak wavelength 372 nm. The spatial variations of the emission pattern as observed in the monochromatic photon maps in Figure 10 are a direct probe of resonant modes of plasmonic nanostructures and, consequently, provides a direct way to map the local electric fields (hot spots). The spatial variation of emission here is caused when the field produced by the electron beam couples strongly when the electron beam is located near the largest electromagnetic fields of the resonant surface plasmon modes of a nanostructure. So we can identify the peak wavelengths 372 nm, 424 nm, 464 nm and 525 nm of the CL spectrum in Figure 10(c) as the surface plasmon resonant modes of the combined spherical and triangular 27 ACS Paragon Plus Environment


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particle system. Thus, for spherical particles containing S3 sample, the hot spots are dominated by 372 nm, 424 nm and 464 nm resonant modes. However, the combined spherical plus triangular particles containing S3 sample synthesized in oxygen environment is dominated by extra hot spots due to the 464 nm and 525 nm resonant modes contributed by the triangular particles. Hence, the CL monochromatic images directly confirm the cause of experimentally observed SERS enhancement for oxygen annealed Ag nanoparticles on Si samples of the present study.

Figure 11. UV-visible spectra of three samples: S1, S2 and S3.

UV-Vis spectroscopy: The UV-Vis absorption spectra of the three samples are represented in Fig. 11. The spectra were taken using Perkin Elmer lambda 750 spectrophotometer in reflectance mode. It is clearly observed that the LSPR peak of Ag is red shifted (from 368 nm) due to annealing and also have broadened as a result of increased particle size and is also broadened due to radiation damping.22 For O2 annealed sample some peaks at higher wavelength appears which are very close to CL peak taken from a single combination of spherical and triangular particle. In 28 ACS Paragon Plus Environment

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N2 annealed sample the peak from 475 nm is also present but less intense compared to the O2 annealed sample.

Electromagnetic Field Distribution: FDTD simulation: To further investigate the origin of the experimentally observed large SERS enhancement from the O2 anneal sample, we have performed FDTD simulation from the coupled Ag NPs (triangular Ag particles with the spherical Ag particle) grown over the 3D Si substrate. Figure 12 (a) shows the x-z view of electric field distribution of the coupled nanostructure at 632 nm excitation wavelength. It is clear from Figure 12 (a) that a large field enhancement takes place at the Ag nanosphere and Ag nanotriangle junction. Thus the large field enhancement caused by coupling of localized surface plasmons of the Ag nanosphere and Ag nanotriangle is believed to induce the large SERS enhancement from O2 anneal sample. That the nanoscale gaps between metal nanostructures are the primary source for highest possible enhancement in Raman scattering was also reported earlier.55-56 Figure 12 (d) also shows the calculated EF for for 632 nm excitation. The EF is usually defined as (E/E0)^4 where E is the local maximum electric field, and E0 is the amplitude of input source electric field in a linear simulation.57 The large field enhancement (2.8 ×107) is clearly observed at the junction between the nanostructures. The discrepancy in experimental and calculated EF from FDTD simulations are due to the fact that (i) SERS enhancement is a combined effect of EM enhancement and chemical enhancement effect but in FDTD simulation only EM enhancement effect is considered, (ii) The Ag sphere-triangle pair comes in different size and distances between different pairs reduces whenever we go from top to the bottom of the pyramid, which is clear from the SEM images shown in Figure 4(e). This size variation and reduced distance



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between them will further affect the incident EM radiation. But in FDTD simulation EF from a single pair is only considered.

Figure 12. (a) FDTD calculated electric field distribution of triangular Ag particles with the spherical Ag particle combination on 3D Si substrate under 632 nm excitation wavelength, (b) Calculated enhancement factor (EF) in the x-z plane from the coupled Ag nanoparticles.

CONCLUSIONS

In conclusion, we have developed a simple and cost-effective approach to produce a stable SERS active substrate. Our synthesis process concerns the growing of Ag nanoparticles on Si pyramids using galvanic displacement method at room temperature and subsequent annealing at O2 atmosphere. We have shown that formation of combined triangular and spherical shaped Ag nanostructures and gap in between (source of hotspots) increases the SERS enhancement immensely relative to the either un-annealed or N2 annealed substrate. To the best of our knowledge, this is the first report that envisages controlling of enhancement in SERS by tuning 30 ACS Paragon Plus Environment

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the morphology of the noble metal nanoparticles under appropriate thermal environment to create SERS active substrate. Moreover, the co-existence of sharp edged triangular Ag particles with the spherical Ag particles on Si surface as observed in the present work entails the probability of increased forward scattering by spherical particles and enhancement of absorption by the triangular particle, an aspect, that have strong implication in plasmon based solar cell application.

ASSOCIATED CONTENT Supporting Information Plan view SEM image of S2 and S3 showing etch pit formation during annealing at O2 atmosphere (Figure S1); HRTEM image showing the interface of S3 sample (Figure S2); [100] XTEM, tomography and elemental mapping of S3 using STEM-HAADF-EDX technique (Figure S3); SERS spectra of 10-5 M R6G at three different random site for three different substrates; SERS enhancement factor calculation; EF from SERS measurement for different Raman peak of R6G (Table S1); EF for different interval of time for S3 (Table S2). The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.



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ACKNOWLEDGEMENTS The authors sincerely thank Prof. Satyaranjan Bhattacharayya, SINP, Kolkata for his help in extending the rapid thermal annealing facility, Prof. P. M. G. Nambissan and Soma Roy, SINP, Kolkata for their help in UV-Vis measurements and Dr. Ramkrishna Dev Das, SINP, Kolkata for SERS measurements.

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Annealing Induced Morphology of Silver Nanoparticles on Pyramidal

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