ARTICLE pubs.acs.org/JPCC
Au-Coated Polystyrene Nanoparticles with High-Aspect-Ratio Nanocorrugations via Surface-Carboxylation-Shielded Anisotropic Etching for Significant SERS Signal Enhancement Hsin-Yi Hsieh,† Jian-Long Xiao,‡,§ Chau-Hwang Lee,‡,§ Tsu-Wei Huang,|| Chung-Shi Yang,||,^ Pen-Cheng Wang,*,|| and Fan-Gang Tseng*,†,‡,|| †
Institute of NanoEngineering and MicroSystems (NEMS), National Tsing Hua University, Hsinchu 30013, Taiwan R.O.C. Research Center for Applied Sciences, Academia Sinica, Taipei 11529, Taiwan R.O.C. § Institute of Biophotonics, National Yang-Ming University, Taipei 11221, Taiwan R.O.C. Department of Engineering and System Science, National Tsing Hua University, Hsinchu 30013, Taiwan R.O.C. ^ Center for Nanomedicine Research, National Health Research Institutes, Miaoli 35053, Taiwan R.O.C.
)
‡
bS Supporting Information ABSTRACT: Increasing the hot-spot area with high enhancement ability on SERS-active particles is generally acknowledged as one of the efficient ways to significantly improve the average SERS signal of nanoparticles. A method to create roughness on the surface of nanoparticles was proposed by oxygen plasma etching noncarboxylated polystyrene beads. However, the mechanism of nanocorrugation formation was not clear. Thus, in this paper, we employ argon-based reactive ion etching (RIE) incorporated with carboxylated polystyrene nanoparticles to investigate the roles of nanocorrugations’ morphologies for SERS signal enhancement. The formation mechanism of the nanocorrugations has been investigated thoroughly through a comparison with those formed by oxygen-based RIE processes from their high resolution X-ray photoelectron spectra and surface morphologies with or without hydrazine reduction treatment. Moreover, polystyrene beads with more intrinsic carboxyl groups and etched by argon plasma produce higher nanocorrugations. It is suggested that carbonyl groups with high bond energy become nanomasks on polystyrene bead surfaces and provides high selectivity between carboxyl and polystyrene surfaces under RIE. Raman intensity enhancement on a 20-nm gold coated nanocorrugated polystyrene bead array is summarized by three factors: (1) the effect of plasmonic coupling among neighboring particles, (2) the nanocorrugation-contributed roughness, and (3) the pitch size of nanocorrugations, through the analysis of SEM images, AFM height images, and LSPR signals. Among these factors, the pitch size of nanocorrugations (ranging from ∼6 nm to ∼12 nm on the surface of polystyrene beads) dominates the SERS enhancement. The 870 nm/120s oxygen plasma etched polystyrene beads (OPSBs) with a minimum pitch size of 6 nm provides the highest Raman intensity enhancement (measured by 632.8-nm He Ne laser), which is 12 times greater than the intensity of nontreated (870 nm/0s) polystyrene beads (while the Au/Ti coating is 20 nm/5 nm).
’ INTRODUCTION SERS is a surface sensitive detection technology that can enormously enlarge the Raman scattering signal intensity to distinguish the chemical structure of an unknown sample. In contrast to the fluorescent dye labeled ELISA assay,1 Raman spectroscopy is a nonlabeling method to analyze chemical bonds based on their inelastic Raman scattering shift, originating from the rotation or vibration interaction of a molecule with the excitation laser light. A single nanoparticle possessing a rough metal surface was reported with a sufficient enhancement factor (EF) to recognize single molecules adsorbed on the hot-spot of the nanoparticle.2 Owing to its enhanced signal-to-noise ratio of the Raman spectrum without interference from water,3,4 SERSactive nanoparticle tags with self-assembled monolayer coatings r 2011 American Chemical Society
become a multifunctional carrier for noninvasive glucose monitoring,5 cellular molecules probing,6,7 tumor targeting,8,9 nearinfrared imaging,9,10 and photothermal heating9,11 under in vivo conditions, even more so in live animals. The major SERS nanoparticles employed in the early 2000s were mostly gold or silver nanospheres. They played the role of resonance media to locally increase the intensity of the electric field surrounding the surface of the nanospheres.12 14 An individual gold or silver nanosphere has an EF of ∼3 orders magnitude of enhancement15,16 (although the EFs of the gold or Received: February 8, 2011 Revised: July 19, 2011 Published: July 19, 2011 16258
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The Journal of Physical Chemistry C silver nanosphere aggregates were approximately 10 12 orders of magnitude enhanced in liquid12,17). Metal-shelled or coreshelled smooth nanospheres were then reported to further promote the EF to maximum 2 orders of magnitude.16,18 Besides, to obtain nonaggregated nanoparticles with an ultrahigh EF of 5 11 orders of magnitude enhancement, the various radii sizes of the nanobowl structures19 21 and nano dimers/trimers18,22,23 with sub-10 nm edge or gap were also reported by simulations and experiments. Despite being a powerful tool, SERS-active nanoparticles, fabricated by the synthesis of gold or shelled metal coating on nanocolloids, still encounter many challenges, such as the difficulties of forming highly rough surface conformation,18,24 increasing the occurrence rate of SERS hot-spots with high EF,25 or controlling the nanostructure morphology of a nanoparticle.26 These difficulties result in the low ratio of hot-spot to overall surface and the 2/98% phenomenon; that is, samples on hotspot area (less than 2% of the particle surface) dominate over 98% of whole Raman intensity.27,28 Therefore, several methods were reported to synthesize gold nanoflower particles with many nanogaps (on a single particle)24,26 or grow smaller gold (or silver) nanoparticles on the surface of a larger core nanoparticle3,29 for further SERS enhancement, giving at least 1 order of magnitude more hot-spot area when compared to those of nanobowls or smooth nanogold nanospheres with a similar size. In contrast, a nanocoral structure was reported to provide many sub-10-nm gaps on a polystyrene bead using oxygen plasma etching,30 carrying out a better EF because of the gold shell and the surface roughness, matching the simulation results by Talley et al.18 Indeed, the increment of surface roughness by plasma etching methods30,31 contributed significantly on SERS from many prior excellent works.18,24,26,30,32 Besides, plasmon coupling effect generated by the gaps between adjacent metal nanostructures, such as nanohole33,34 or nanostructure array,35 37 has also been shown to support extremely intense local electromagnetic fields on the hot-spot to significantly increase the Raman intensity. To form these particle arrays, self-assembled monolayer assistant interparticle gap controlling,35 electron beam lithography (EBL),38,39 and film over etched nanospheres (FON) by RIE combined with nanosphere lithography NSL36,40,41 have been employed to produce reproducible nanoparticle arrays. Among those methods, FON is one of the most popular methods36,40,41 because of the robustness for the formation of dense-packed monolayer array, such as Langmiur Blodgett (LB) technique42 and spread-coating colloidal nanoparticle.43 The appropriate gap-to-diameter ratio of silver over oxygen plasma etched polystyrene arrays was suggested around 0.2 which contributes a maximum 4 times relative Raman enhancement,40 in which the surface roughness, generated by oxygen plasma etching, of polystyrene beads also promoted the Raman intensity.40,41 However, the reason for surface roughness formation was little-understood40,41 and the surface roughness contribution to Raman intensity of FON substrate has not been quantitatively investigated. Therefore, this study proposes a controllable way to engineer carboxyl polystyrene bead surfaces under argon/oxygen plasma etching44 using the carboxyl groups as nanomasks. It is a reliable method for fabricating nanostructures of controllable surface roughness on nanopolystyrene beads to increase hot-spot areas for SERS. This method, different from chemical synthesis,24,26 provides a simpler fabrication process on commercially available
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polystyrene beads with different sizes. In this paper, we not only demonstrate the fabrication process of corrugated polystyrene beads with controllable surface roughness on the surface for SERS enhancement but also investigate the possible formation mechanism by comparing the processes with different gas combinations during etching. Furthermore, we quantify the surface roughness of nanocorrugations on polystyrene beads to understand the degree and the mechanism of Raman enhancement. As a result, the formation of nanocorrugations on polystyrene bead surfaces are attributed to the existence of carboxyl groups.
’ EXPERIMENTAL SECTION Materials. Ten types of polystyrene beads were categorized into two groups, including bare polystyrene beads of five different diameters without carboxyl groups on the surface (named as “PSBs” in the following content) and carboxyl groups modified polystyrene beads of five different diameters (named “PSBsCOOH” in the following content). Four sizes of PSBs without fluorescent dye, including 870 nm (PS03N/5714), 640 nm (PS03N/4136), 420 nm (PS02N/ 2141), and 300 nm (PS02N/5378), were purchased from Bangs Laboratories, Inc. The fifth type of PSB is of 250 nm (G250) with green fluorescence (ex/em 468/508 nm) that was purchased from DUKE Scientific Corporation. For the series of PSBs-COOH, 920-nm (9.3 carboxyl groups/ nm2, PC03N/6499), 680-nm (2.6 carboxyl groups/nm2, PC03N/ 4121), 320-nm (4.5 carboxyl groups/nm2, PC02N/9172), and 220nm (2.0 carboxyl groups/nm2, PC02N/6481) carboxylic polystyrene beads with no fluorescence and 510-nm carboxylic polystyrene beads with the fluorescence of ex/em 480/520 nm and a density of 14.7 carboxyl groups/nm2 (FC03F/7049) were all purchased from Bangs Laboratories, Inc. Hydrazine monohydrate, 98+%, was purchased from Alfa Aesar, A14005. One side of the polished silicon wafer diced into 5-mm square chips and 50 mm (L) 10 mm (W) 5 mm (H) glass chips were employed as the substrates. The glass substrate was specially used for transmission light observation in LSPR experiments. Polydimethylsiloxane (PDMS, SYLGARD 184) was purchased from Sil-More Industrial Ltd. Rhodamine 6G was purchased from Sigma-Aldrich, R4127. AFM tips with less than 8-nm tip radius of curvature (NCH-50, PointProbe Plus) were purchased from NANOSENSORS. Polystyrene Bead Arrangement and Plasma Treatment. The diluted polystyrene beads were arranged as a densely packed thin film with a hexagonal crystal structure on the substrate to evaluate the amount of particles per unit area for SERS analysis, as shown in Figure 1 and Figure S-1. First, untreated substrates were cleaned by immersion in a freshly prepared piranha solution, 7:1 mixture of 96% sulfuric acid and 30% hydrogen peroxide, at 90 C for 10 min (Figure 1a and S-1(a)). Then the cleaned substrates were treated hydrophilically by oxygen plasma under 100-W RF power, 75-mTorr atmosphere, and 10sccm oxygen flow rate. As soon as the hydrophilic treatment was completed, the substrate was covered by a PDMS film with a hole, made with a general leather punch, of 2.75 mm or 4.65 mm in diameter and 225 μm in depth on the surface (Figure 1b, Figure S-1(b), and Figure S-1(c)). The 2.75-mm PDMS hole was only used for the SEM morphology comparison among three types of plasma treatment (Ar, O2, and Ar/O2 mixture), SEM morphology comparison after hydrazine reduction and plasma 16259
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Figure 1. Schematic illustration of reactive ionic etched polystyrene bead processes. (a) The cleaned silicon substrate is treated by oxygen plasma to form a hydrophilic surface. (b) A 225-μm height PDMS well is adhered onto the substrate. (c) 2 6 μL of solution with a concentration of ∼0.3% polystyrene beads is dropped in the well. (d) After several hours of drying the liquid, the polystyrene beads are arranged in a hexagonal dense package. (e) The PDMS is removed, and then, the beads are etched by pure argon, pure oxygen, or a mixture of argon and oxygen plasma, respectively.
treatment, and XPS analysis. The 4.65-mm PDMS hole was used for the other experiments, including the gap-to-diameter ratio calculation of the bead array, AFM images, UV vis spectrum, and Raman intensity measurement. The PDMS layer was made by spin coating 5 g of uncured PDMS at 1000 rpm for 30 s onto a 4-in. glass wafer, precoated with a 1% Teflon solution and then heated at 45 C overnight to obtain a final thickness of 225 μm. The purpose of the PDMS film is not only to confine the surface area that polystyrene beads cover but also to give a wall for supporting a concave solution meniscus during the polystyrene beads’ self-assembly process.45 Then ∼3 mg/mL of the polystyrene bead solution was added into the well (in Figure 1c). When water was evaporated, the meniscus force assisted the formation of a close-packed polystyrene bead monolayer starting from the center part to the PDMS wall by the cooperation of capillary forces and convective forces.46,47 Therefore, a hexagonal polystyrene bead arrangement was formed in Figure 1d. After peeling off the PDMS well (Figure S-1(d)), the samples were etched in a reactive ionic etching (RIE) system to obtain nanostructures on the bead surfaces (Figure 1e). All RIE processes were employed with the same RF power of 60 W and the chamber pressure of 60 mTorr but with various gas types (argon, oxygen, or the mixture), etching time (0 120 s), and gas flow rate. Vapor-Phase Hydrazine Reduction. To understand the relationship between the surface carboxyl density and the surface morphology after various plasma etching, hydrazine, a volatile reduction agent, was used to reduce the carboxyl groups on the polystyrene beads for the comparison with non-hydrazinereduced samples. Here, the densely packed polystyrene beads were reduced by vapor-phase hydrazine after the completion of the polystyrene bead arrangement. The densely packed polystyrene bead samples were put in a glass dish containing 1 mL of hydrazine solution and covered with a glass lid for 85 min. Then the lid was removed from the glass dish and the dish was put into a desiccator connected to a cold trap device for drying up the sample. As soon as the hydrazine was removed on the samples, polystyrene beads were etched under argon or oxygen plasma with the parameter of 60-W RF power, 60-mTorr chamber pressure, 60-s exposure time, and 10 Sccm gas flow rate. Surface Morphology Analysis after Plasma Treatment. Before and after the plasma etching treatment, the samples were first observed by a scanning electron microscope (SEM) (JSM6330F or JSM-6380F, Japan Electron Optics Laboratory Co., Ltd., Japan) to investigate the surface morphology and calculate the gap (s) to diameter (D) ratio, in which the gap size (s), equivalent to one-half of interparticle distance (g), was obtained
by subtracting the final diameter from the original diameter of the bead based on SEM images (Figure S-2). Then, the 420-nm PSBs before and after oxygen plasma etching were analyzed by highresolution X-ray photoelectron spectroscopy (HRXPS) (PHI Quantera SXM, ULVAC-PHI, Inc., Japan) for the observation of surface transformation. In addition to the survey scan, the O1s and C1s of the XPS multiplex scans were measured to analyze the chemical shift. To fit the XPS spectra, XPSPEAK41 software was used by setting 80% of the Lorentzian/Gaussian distribution. The nanostructure area on the polystyrene beads was analyzed based on the gray scale intensity line profile on a nanosphere in the top-view SEM images by image-pro plus 6.0 software to calculate the pitch size (p) and bead diameter (D) in average. Besides, the nanocorrugation-contributed roughness (abbreviated as “NCR”), including Ra (average roughness), Rq (root-meansquare roughness), and Rv (peak-to-valley roughness), was also taken using a JPK NanoWizard atomic force microscope (AFM) (NanoWorld, Neuchatel, Switzerland), operated in tapping mode with a 512 512 pixels resolution. For roughness calculation by the software of JPK Data Processing, the selected region of each polystyrene bead was the center n square area (in which n represents the 40% of bead diameter for Ra and Rq and the 20% of bead diameter for Rv) (Figure S-2). Gold-Shell Coating for Raman Spectroscopy and LSPR. To give a plasmon resonant characteristic on the surface, the polystyrene bead samples after RIE etching were continuously coated with two metal layers of 5 nm of Ti and then 20 or 60 nm of Au by electron beam evaporation (Figure S-1(e)). The orientation of all of the substrate surfaces was set in the same axis of the evaporation direction to ensure poor step coverage on oxygen plasma etched PSBs (OPSBs) and argon plasma etched PSBs-COOH (ArPSBs-COOH). To verify the thickness of the Au/Ti materials on the polystyrene beads, dummy substrates were coated with 5 nm of Ti and 10 30 nm of Au thin films for calibration. These calibration samples were measured by an atomic force microscope (PicoPlus 5500, Agilent, U.S.A.) in the contact mode on a 100 100-μm surface area. The SERS Raman intensity of Rhodamine 6G solution was measured based on 20-nm thickness of the gold shell with a titanium adhesion layer of 5 nm, and the LSPR spectrum was based on Au/Ti 60 nm/5 nm. LSPR Spectrum Measurement. To investigate the plasmonic coupling effect based on gold-coated OPSBs and ArPSBsCOOH samples, a UV vis NIR spectrometer (Jasco V-670, Japan) was used to measure transmission light intensity in the range of 350 850 nm. The baseline was taken by measuring a flat Au/Ti (60 nm/5 nm) film on a glass substrate. 16260
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Table 1. SEM Pictures of Three Polystyrene Beads, PSB, PSB-COOH (with 2.6 groups/nm2 Carboxyl Groups), and PSB-COOH (with 14.7 groups/nm2 Carboxyl Groups) after RIE Treatments with Different Gas Contents of Plasma (Scale Bar: 500 nm)
SERS Intensity Measurement. To evaluate the pillar sizes on the corrugated polystyrene beads under different etching conditions, nonfunctionalized polystyrene beads (PSBs) and carboxyl polystyrene beads (PSBs-COOH) were self-assembled on silicon substrates or glass chips for the etching process. Then, the nanoparticles were covered with a thin layer of gold. A PDMS plate consisting of a through-hole of 4.65 mm in diameter and 225 μm in depth was then pasted onto the substrate as a reservoir for containing a Rhodamine 6G (R6G) solution (Figure S-1(f)). After 5.7 μL of 100 μM R6G solution was added into the reservoir, a ∼150-μm coverslip was placed on the PDMS well to eliminate the lens effect from the curved meniscus for laser focusing. A MicroRaman spectrometer (LabRAM HR, HORIBA Jobin Yvon, France) with a 50 objective (MPLAN 50X/0.75, OLYMPUS) and a 632.8-nm He Ne laser source (3 mW out from the laser source/∼200 μW out from the objective) was used to measure the SERS intensity of different samples (Figure S-1(g)). The laser spot size was adjusted to 2.86 μm through a 200-μm pinhole and a grating of 600 grooves/mm, and the exposure time for the SERS measurement was set for 10 s with a 632-nm notch filter to filter out Rayleigh scattering signals. In the SERS comparison, we evaluated the relative Raman intensity by quantifying the maximum peak intensity at ∼1509 cm 1, representing the symmetric modes of in-plane C C stretching vibrations,2 of all the spectra. Then the signal intensities of the R6G solution enhanced by 20-nm gold-shelled polystyrene beads (PSBs or PSBs-COOH) and with or without plasma treatment were obtained by subtracting the baseline intensity from the maximum count intensity at 1509 ( 5 cm 1.
’ RESULTS AND DISCUSSION Formation Mechanism of the Corrugated Surface. In the RIE etching process on PSBs, the plasma can cleave and remove low-molecular-weight backbone segments,48,49 such as the
Figure 2. C1s spectra of high resolution X-ray photoelectron spectroscopy (HRXPS). (a) The spectrum of commercial nonfunctionalized polystyrene beads without plasma treatment, PSBs. (b) The spectrum of oxygen plasma treated polystyrene beads (OPSBs).
C C C bonds of polystyrene, which are formed by the ethylene groups of styrene monomers for linking neighboring benzene rings. The carbon atoms can also be converted to carboxyl functional groups by oxygen plasma treatment.48,49 Therefore, it is suspected that the carboxyl functional groups possessing higher bond energy can potentially become nanomasks for selective etching on polystyrene beads. For PSBsCOOH, the surface of the polystyrene beads has already been modified with carboxyl groups. Therefore, regardless of what kind of plasma is employed to etch the PSBs-COOH, surface corrugation is always observed after the etching process. To verify if the carboxyl groups play the key role in this reaction, 16261
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Table 2. SEM Pictures of Three Vapor-Phase Hydrazine Reduced Polystyrene Beads (PSB and PSB-COOH with 2.6 Groups/nm2 Carboxyl Groups and PSB-COOH with 14.7 Groups/nm2 Carboxyl Groups) before and after Oxygen and Argon Plasma Etching (Scale Bar: 500 nm)
three different types of nanobeads with various densities of carboxyl groups on the surfaces, PSBs, PSBs-COOH with 2.6 carboxyl groups/nm2, and PSBs-COOH with 14.7 carboxyl groups/nm2, were etched by oxygen plasma (10 sccm), argon plasma (10 sccm), and/or a mixture of argon (10 sccm) and oxygen (1 sccm). Additionally, the surface morphologies on the polystyrene beads were compared in two conditions: (i). Without Hydrazine Reduction. The surface structures of three types of PSBs under various RIE etching conditions are tabulated in Table 1. In this experiment, oxygen plasma is involved in a combination of anisotropic physical etching of PSB and isotropic chemical reaction to break C C C bonds for carboxyl group formation, while argon plasma is involved only in the anisotropic physical etching, in agreement with others’ findings.48 As a result, the argon plasma cannot generate carboxyl groups on the PSB surfaces, and if there are no intrinsic carboxyl groups appearing on the PSB surface, argon plasma etching only results in reducing the polystyrene bead size without generating any surface nanostructures, as shown in Table 1: image Ar-PSB. In contrast, the oxygen plasma-etched PSB surface does produce nanostructures, as shown in Table 1: image O2 PSB. Ar plasma does not generate surface nanomasks; however, if there are intrinsic surface nanomasks (here, it is the carboxyl groups), then Ar plasma can also produce nanostructures on the polystyrene bead surfaces, as shown in Table 1: images Ar-PSBCOOH with 2.6-grp/nm2 and 14.7-grp/nm2 carboxyl groups. The density of the surface nanostructures is dependent on the numbers of intrinsic carboxyl groups on the PSB surface, as shown in Table 1: Ar plasma column. O2 plasma also demonstrates a similar trend as Ar plasma but with more nanostructures, as shown in Table 1: O2 plasma column, which is attributed to the
generation of new nanomasks during O2 plasma etching. When O2 is mixed into argon, the mixing plasma also demonstrates the ability to generate nanomasks and high-density nanostructures, as shown in Table 1: Column Ar:O2 mixture. From the above observation, we can conclude that the carboxyl groups are highly related to the formation of nanomasks to resist plasma etching for the generation of surface nanostructures. The above findings can be further verified by the HRXPS chemical shift spectra of C1s in Figure 2 to reveal the transformation of chemical structures before and after oxygen plasma treatment. In Figure 2a, the XPS spectrum presents that nontreated PSBs merely contain C C groups (284.5 eV), C H groups (284.1 eV), and π π* shakeup satellites of the benzene ring (291.6 eV), in agreement with the chemical structure of polystyrene.48,50 Nevertheless, after oxygen plasma treatment, C O groups (286.2 eV), CdO groups (287.6 eV), and O CdO groups (289.0 eV), appear in the XPS spectrum in Figure 2b, representing alcohol/ether, carbonyl, and carboxyl/ester groups, respectively.48,50 This result provides further evidence that the nanostructures formed are highly related to the carboxyl groups on the surface. To oxidize polystyrene or other polymers to form carbonyl and carboxyl surface groups, the use of either UV, ozone, oxygen plasma, or acidic solution has also been reported in previous studies;48,49,51 however, the treatment of polystyrene with argon plasma can only etch the surface without functionalization of the polymer,48 in concurrence with our finding. The etching selectivity for polystyrene beads with different carboxyl group densities might be that the CdO bond energy (799 kJ/mol) in a carboxyl group is much higher than any other bond in nonfunctional PSBs, i.e., benzene rings (152 kJ/mol), 16262
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Figure 3. Relative Raman intensity of 100 μM rhodamine 6G solution on 20 nm gold coated (a) ArPSB-COOH and (b) OPSB substrates. The bead diameters are labeled near the maximum relative intensity of each series of initial bead diameter with different plasma etching time.
Table 3. Summary of the Relative Raman Intensity (Enhancement (%)), Raman Intensity, Gap-to-Diameter (s/D), Pitch Size (p), and NCR (Ra, Rq, and Rv) at the Maximum Relative Raman Intensity of Each Polystyrene Beads
ArPSB-COOH
220 nm/80s
320 nm/100s
510 nm/100s
680 nm/120s
920 nm/100s
(2.0 grp/nm2)
(4.5 grp/nm2)
(14.7 grp/nm2)
(2.6 grp/nm2)
(9.3 grp/nm2)
enhancement (%)
125
235
443
178
195
Raman intensitya
1589
2561
3421
742
3426
s/D p (nm)b
0.2840
0.2424
0.1307 8.07 ( 0.82
0.1316
0.0736 10.20 ( 1.70
Ra (nm)
4.57 ( 1.12
7.53 ( 1.26
19.90 ( 3.71
5.75 ( 2.21
13.01 ( 1.37
Rq (nm)
5.51 ( 1.17
9.39 ( 1.22
25.63 ( 4.49
6.40 + 1.89
16.54 ( 1.44
Rv (nm)
11.87 ( 1.28
26.79 ( 3.11
83.31 ( 2.65
23.35 ( 8.06
53.45 ( 3.09
OPSB
250 nm/100s
300 nm/100s
420 nm/120s
640 nm/120s
870 nm/120s 1224
enhancement (%)
21
328
631
163
Raman intensitya
3905
1791
2607
775
2803
s/D p (nm)
0.6331 9.71 ( 1.31
0.8523 8.99 ( 0.90
0.4449 7.28 ( 1.78
0.2409 11.55 ( 1.58
0.1570 6.14 ( 1.64
Ra (nm)
6.89 ( 0.61
4.78 ( 0.84
13.73 ( 1.88
10.72 ( 1.55
9.65 ( 1.44
Rq (nm)
8.66 ( 0.81
5.78 ( 0.87
17.8 ( 2.02
13.06 ( 1.57
12.19 ( 1.69
Rv (nm)
13.53 ( 0.83
13.16 ( 7.66
55.35 ( 4.62
41.10 ( 5.20
32.66 ( 5.20
a
Raman intensity is the average values from 3 to 5 different spots each sample, and the standard deviation is
