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Highly sensitive surface enhanced Raman scattering substrates based on Ag decorated Si nanocone arrays and their application in trace dimethyl phthalate detection
Accepted Manuscript Title: Highly sensitive surface enhanced Raman scattering substrates based on Ag decorated Si nanocone arrays and their application in trace dmethyl phthalate detection Author: Zewen Zuo Kai Zhu Lixin Ning Guanglei Cui Jun Qu Ying Cheng Junzhuan Wang Yi Shi Dongsheng Xu Yu Xin PII: DOI: Reference:
S0169-4332(14)02579-3 http://dx.doi.org/doi:10.1016/j.apsusc.2014.10.181 APSUSC 29143
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-8-2014 26-9-2014 27-10-2014
Please cite this article as: Z. Zuo, K. Zhu, L. Ning, G. Cui, J. Qu, Y. Cheng, J. Wang, Y. Shi, D. Xu, Y. Xin, Highly sensitive surface enhanced Raman scattering substrates based on Ag decorated Si nanocone arrays and their application in trace dmethyl phthalate detection, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.10.181 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights SERS substrates with high enhancement and excellent uniformity were prepared. The methods of fabrication are completely compatible with Si device
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technologies.
The enhancement performance of substrates can be tuned by Ag sputtering
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duration.
Multiple-type high density hot spots are responsible for the high performance.
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Trace dimethyl phthalate is well detected by this SERS substrate.
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Highly sensitive surface enhanced Raman scattering substrates based on Ag decorated Si nanocone arrays and their application in trace
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dmethyl phthalate detection
Zewen Zuoa,b,, Kai Zhua, Lixin Ninga, Guanglei Cuia, Jun Qua, Ying Chengc,
Center for Nano Science and Technology, College of Physics and Electronics
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Junzhuan Wangc, Yi Shic, Dongsheng Xud, Yu Xind
Information, Anhui Normal University, Wuhu, 241000, China
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing,
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210093, China c
School of Electronic Science and Engineering, and Key Laboratory of Photonic and
School of Physical Science and Technology, Soochow University, Suzhou 215006,
China
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Electronic Materials, Nanjing University, Nanjing, 210093, China
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ABSTRACT: Wafer-scale three-dimensional (3D) surface enhancement Raman scattering (SERS) substrates were prepared using the plasma etching and ion
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sputtering methods that are completely compatible with well-established silicon device technologies. The substrates are highly sensitive with excellent uniformity and reproducibility, exhibiting an enhancement factor up to 1012 with a very low relative
standard deviation (RSD) around 5%. These are attributed mainly to the uniform-distributed, multiple-type high-density hot spots originating from the structural characteristics of Ag nanoparticles (NPs) decorated Si nanocone (NC) arrays. We demonstrate that the trace dimethyl phthalate (DMP) at a concentration of 10-7 M can be well detected using this SERS substrate, showing that the AgNPs-decorated SiNC arrays can serve as efficient SERS substrates for phthalate acid esters (PAEs) detection with high sensitivity.
Corresponding author at: College of Physics and Electronics Information, Anhui Normal University, Wuhu, 241000, China. Tel.: +86 5533883561. E-mail address: [email protected] (Z. W. Zuo)
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Keywords: Surface-enhanced Raman scattering (SERS); three-dimensional substrate;
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silicon nanocone array; hot spots; trace detection
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1. Introduction Surface enhanced Raman scattering (SERS) has been developed as a powerful spectroscopy tool for chemical and biological detection and identification due to its high sensitivity and rapid response [1−5]. Electromagnetic enhancement that is
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maximal in between adjacent metal nanostructures with the gap on the order of a few
nanometers (called “hot spots”), is established as the dominant mechanism of the
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SERS [6−8]. When located inside the hot spots, Raman-active molecules will be effectively activated, with a significant enhancement of their Raman signal.
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Though much effort has been devoted to preparing high quality SERS substrate [7−9], the fabrication of large-area uniform, reproducible and highly sensitive SERS
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substrates with cost-effective techniques remains as one of the main challenges for the wide applications of SERS. For example, by assembling chemical synthesized noble
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metal nanoparticles or nanorods on flat substrates, one can make highly sensitive SERS substrates, but with the difficulty of controlling the structure uniformity (and
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thus the signal uniformity) over a large area [10−12]; focus ion beam or electron beam lithography can produce well-controlled metal nanostructure arrays for efficient SERS
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fabrication [13−15].
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substrates, but these techniques are expensive and time-consuming for large-area
Considering the well established semiconductor manufacturing technologies, it is
highly desirable to produce SERS substrates using techniques compatible with silicon device technologies. This will greatly reduce the production cost and the produced sensors can be readily integrated into existing silicon devices. The maskless plasma etching is one such technique that allows for simple, uniform, large-area, and inexpensive preparation of silicon nanostructure arrays [16−19]. In this paper, we report on the production of high performance SERS substrates with high sensitivity and excellent reproducibility based on high density Si nanocone (SiNC) arrays formed by the maskless plasma etching of crystalline Si wafer. Silver nanoparticles (AgNPs) were then deposited onto the surfaces of the arrays by ion sputtering. The techniques used here are completely compatible with silicon device technologies. The effect of the morphology and size distribution of the AgNPs on the performance of the SERS
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substrates was investigated. The results show that the vertically-aligned nanocone array is readily for the AgNPs to be decorated onto the side surfaces and the top ends of the cones, further, to generate multiple-type efficient SERS hot spots. The optimized three dimensional (3D) SERS substrate was also used to detect trace
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dimethyl phthalate (DMP), a class of concerning pollutant [20,21]. The results in this
study demonstrate that the AgNPs-decorated SiNC arrays can serve as efficient SERS
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substrates for phthalate acid esters (PAEs) detection with high sensitivity. 2. Experimental details
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The fabrication process of the SERS substrates is schematically shown in Fig. 1. First, a (100) Si wafer of four inches diameter was ultrasonic cleaned with acetone
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and deionized water and blew drying with nitrogen. Then, the wafer was placed inside the chamber of a plasma etcher to perform maskless plasma etching. The plasma was
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generated using a single frequency of 40.68 MHz at a power of 200 W. The etcher was operated at a pressure of 2 Pa. The flow rates for SF6, and O2 were 100 sccm and
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3 sccm, respectively. After the etching, the wafer was treated with 10 % HF solution for 1 min, and then Ag deposition was performed using an ion sputtering system with
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the current of about 2 mA at room temperature. During the sputtering, no protective
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gas was used. The morphology of the arrays was characterized using scanning electron microscopy (SEM, Hitachi S-4800). The resulting wafer of Ag-decorated SiNC array was diced into small pieces for
SERS measurement. Rhodamine 6G (R6G) and DMP were diluted in ethanol with different concentrations for SERS test. The substrates were immersed in the test solutions for 3 hours at room temperature and were then rinsed with deionized water to remove physically adsorbed molecules, followed by blow drying with nitrogen. The SERS measurements were performed at room temperature on a Raman system (HORIBA Jobin Yvon, LabRAM HR) using 514 nm Ar+ laser line for excitation. The laser spot size was about 2 µm with an incident power of 50 µW through a 100× objective. The acquisition time was 5 s for all Raman measurements. 3. Results and discussions After the plasma etching for 15 min, a black surface appears on the wafer as
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shown in the inset of Fig. 2a, which is due to its antireflection property from the nanostructure surface [19,22]. The Si nanostructure array is clearly observed in the SEM images. Fig. 2a reveals that the array has a very high areal density of about 650 µm-2 under the present etching conditions, noted that the areal density of nanopillars
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obtained by electron beam lithography is less than 30 µm-2 [23]. The high areal
density of the array leads to a close proximity of the cones to each other with an
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average spacing of about 30 nm at the top. From the cross-sectional SEM image shown in Fig. 2b, one can see that the as-etched Si nanostructure is vertically aligned
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with cone-shaped morphology. The average height of the cones is about 140 nm, and their average diameters are 14 and 35 nmm at the top and base, respectively.
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The SiNC arrays were covered with Ag by ion sputtering to form SERS substrates, and then R6G molecules were used as probe molecules to reveal the
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sensitivity of these 3D substrates. Fig. 3a presents the SERS spectra of 10-4 M R6G absorbed on the 3D substrates with different Ag sputtering duration, along with a
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spectrum from an optimized 2D substrate, which was produced by sputtering 500 s Ag on a flat Si wafer (see the inset in Fig. 3a), for comparison. Evidently, the 3D
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substrates show great enhancement of the Raman signal compared to the 2D one.
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Many fingerprint Raman bands of R6G were clearly observed from the 3D SERS substrates. The bands at 612, 773 and 1126 cm-1 can be assigned to C-C-C ring in-plane, out-of-plane bending, and C-H in-plane bending vibrations, respectively, while the bands at 1183, 1362, 1508, and 1650 cm-1 are all associated with the
symmetric modes of C-C in-plane stretching vibrations [24,25]. The intensities of the Raman bands of 612, 1362, and 1650 cm-1 as functions of Ag sputtering duration were plotted in Fig. 3b. It shows that the SERS activity of the substrate improves with the increase of the Ag sputtering duration, reaching a peak enhancement with 1100 s Ag sputtering and then decreasing for the substrates with longer sputtering duration. This dependence of the SERS activity on Ag sputtering duration can be ascribed to the structure evolution of the SERS substrates. Fig. 4 gives the morphologies of 3D substrates with different Ag sputtering duration. The images show that AgNPs are formed on the top and side surface of the SiNCs due to a weak interaction energy
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between the absorbed Ag atom and the Si (Volmer-Weber mode) [26, 27], and their size and spacing are dependent on the sputtering duration. With a short sputtering duration of 100 s, only a few AgNPs with an average diameter of about 10 nm are formed on the top ends of the cones, as shown in Fig. 4a, while AgNPs are hardly
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observed on the side surface from the cross-sectional SEM image shown in Fig. 4b, resulting in a low density of hot spots and thus low SERS signal. With the increase of
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the sputtering duration, AgNPs on the top ends of SiNCs grow up, and also start to
emerge on the side surfaces of the cones, as shown in Fig. 4c. Then, the size and the
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density of the AgNPs both on the top ends and side surfaces of the SiNCs continually increase with increasing sputtering duration, forming more hot spots responsible for
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the further enhancement of the Raman signal. For longer sputtering durations, the AgNPs on top end agglomerate to form big sphere-shape particles due to Ostwald
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ripening. The lateral growing of the Ag spheres decrease the amount of Ag reaching the side surfaces of the cones due to shadowing effect, resulting in almost no further
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change in the density of hot spots on the side surfaces. As the size of the Ag spheres becomes larger, the gap between the Ag spheres on the top ends of two neighboring
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cones becomes smaller. For the sputtering duration of 1100 s, the average diameter of
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the Ag spheres is about 40 nm, and the average gap distance between two adjacent spheres is about 10 nm, as shown in Figs. 4g and 4h. These values are highly favorable for Raman enhancement;7 most effective enhancement was consequently
obtained. The inset in Fig. 4h presents the energy dispersive spectrometry (EDS) taken from the side surface of the array, which shows strong Ag signal, also certifying the formation of AgNPs on it. Further increase in sputtering duration leads to the agglomeration of the Ag spheres on the top ends of neighboring cones (see Fig. 5), which decreases the density of hot spots and thus accounts for the decreased SERS signal of the substrate with 1300 s Ag sputtering duration, as shown in Fig. 3. Based on above results, optimized SERS substrates with 1100 s Ag sputtering duration were prepared and soaked in R6G solutions at low concentration. Fig. 6 shows the measured Raman spectra of R6G with the concentrations of 10-7, 10-10, and
10-13 M. It can be seen that strong SERS signal can be obtained with a 10-10 M
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solution. The inset shows the enlarged SERS spectrum of R6G with the concentration of 10-13 M, in which fingerprint bands of R6G at 1181, 1362, 1509, and 1650 cm-1 can be clearly identified though a relatively low signal-to-noise ratio. To estimate the enhancement factor (EF) of Ag-decorated SiNC array SERS
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substrate, we employed a SiNC array without AgNP decorating as a reference substrate, with the following equation [25,28]: I SERS N SERS I Raman N Raman
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EF
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where ISERS and IRaman are the integrated Raman intensities of a specific SERS bands of R6G adsorbed on the substrates with and without AgNPs, respectively. NSERS and
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NRaman are the numbers of R6G molecules within detection volume on the substrates with and without AgNPs, respectively. The integrated intensity of the Raman band at
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1650 cm-1 was usded, and averaged over 10 random spots. The intensity ISERS for 10-13 M R6G on the substrates with AgNPs is about 9200, while the IRaman for 10-3 M R6G
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on the substrates without AgNPs is about 90, leading to ISERS/IRaman≈102. Considering that the conditions of the SiNC array preparation and the Raman measurement are the
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same for these two substrates, that is, identical detection volume and similar surface
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area for molecules absorbed, NSERS and NRaman are mainly determined by the concentration of R6G, giving the value of NRaman/NSERS equal to the ratio of
concentration, 1010. Accordingly, the EF of AgNPs-decorated SiNC array substrate is estimated to be as high as 1.02×1012. This high EF confirms that the AgNPs-decorated
SiNC array is an effective SERS substrate with very high sensitivity. It has been well demonstrated that the high SERS sensitivity can be ascribed
dominantly to the abundance of nanogaps between noble metal nanoparticles, which generates high density hot spots responsible for electromagnetic enhancement [7,8,12]. Because of the bushy SiNCs and dense AgNPs, multiple-type nanogaps are formed within the 3D array. The gaps are formed between the large Ag spheres on adjacent SiNCs, between the AgNPs on the side surface of a single cone, and between the AgNPs on the side surfaces of adjacent cones, as can be seen from Figs. 4g and 4h, all these gaps are enough small to induce effective electromagnetic coupling between
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AgNPs. Furthermore, the 3D array of the substrates might enhance the trapping of excitation light [25], and provide a much larger surface area for adsorbing probe molecules within the detection volume [24,25]. All these features contribute to the high SERS sensitivity.
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To investigate the uniformity and reproducibility of the 3D SERS substrates, Raman mapping (3 μm step size, 17×17 = 289 spectral lines in all) was performed
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using 10-4 M R6G as test molecules. All the 289 spectra from the mapping area (51 × 51 µm2) exhibit almost identical spectral lines, as shown in Fig. 7a. The left panels in
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Figs. 7b and 7c illustrate the measured rectangular region with the brightness of the grid being proportional to the signal intensity at 612 cm-1 and 1362 cm-1, respectively,
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while the right panels are the corresponding histograms of SERS spectral intensities along the two crossed lines within the left panels. The relative standard deviations
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(RSD) of the spectral intensity calculated from the histograms are 4.46 %, 4.14 % for 612 cm-1, and 5.91%, 6.73% for 1362 cm-1, respectively. These results unambiguously
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reveal excellent uniformity and reproducibility of this SERS substrate, which can be mainly attributed to the highly uniform structural characteristics of the 3D substrate,
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that is, uniformly aligned SiNC array and evenly distributed AgNPs on the cones.
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Furthermore, the 3D distributed high density hot spots can make more molecules be activated and probed within the detection volume and, as a consequence, improve the signal uniformity over the SERS substrate by neglecting local structure variation [27,29].
PAEs are most widely used in industrial production as plasticizers, additives, and
solvents [21,30]. During the production and utilization, PAEs diffuse everywhere and have been detected in water, air, food, and rainwater, etc., which are toxic to a variety of wildlife and also cause human’s endocrine system disorder, even at low-concentration levels [31,32]. As a result, PAEs are becoming a class of concerning pollutants. Short-chained esters such as DMP are among the most frequently identified PAEs in various water environments [20,21], therefore, the development of technologies that can effectively detect DMP at low concentration in solution is urgently required. In this work, we used AgNPs-decorated SiNC array
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SERS substrate to detect trace DMP diluted in ethanol. Fig. 8 shows the SERS spectrum from 10-7 M DMP solution, along with a normal Raman spectrum from 10-1 M DMP solution on a flat Si wafer. It is difficult to identify the Raman bands from the normal Raman spectrum, while the 3D substrate shows great enhancement of the
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Raman signal, making many fingerprint bands of the DMP to be clearly observed. Besides the Raman bands of above-mentioned various vibrations of C-C-C ring, C-H,
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and C-C, the bands associated with the vibrations of the orthophenyl group at 659, 1040, 1574, and 1600 cm-1 can be clearly identified [33]. These results demonstrate
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that AgNPs-decorated SiNC arrays have great potential as high sensitive SERS substrate for the detection of trace PAEs.
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4. Conclusions
In summary, maskless plasma etching technique was used to prepare large-area
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uniform SiNC array, and applied to SERS substrates by decorating AgNPs onto the array surface using ion sputtering. Bushy SiNCs and dense AgNPs in the 3D arrays generate multiple-type high density hot spots, making the SERS substrates highly
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sensitive. The substrates exhibit a very high enhancement factor up to 1012 with the
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detection limit as low as 10-13 M for R6G. The very low RSD around 5% of the
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Raman band intensities indicates excellent uniformity and reproducibility across the substrates. Furthermore, trace DMP at a concentration of 10-7 M was well detected
using this SERS substrate. The results in this work demonstrate that the AgNPs decorated SiNC arrays can serve as efficient SERS substrates for PAEs detection with high sensitivity. More importantly, the production of this SERS substrate is completely compatible with the well-established silicon device technologies, which is very promising to promote the wide applications of SERS substrates. Acknowledgements This study is partly supported by the National Natural Science Foundation of China under Grant Nos. 61106011 and 11374015, and the Anhui Province Natural Science Foundation under Grant No. 1308085QF109.
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Figure captions: Fig. 1 Schematic fabrication process of AgNPs-decorated SiNC array SERS substrate. Fig. 2 SEM images of (a) top view and (b) cross-sectional view of the SiNC array. Inset in (a) is the photograph of the wafer after SiNC array formed on the surface.
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Fig. 3 (a) SERS spectra of 10-4 M R6G from the substrates with different Ag sputtering durations (indicated on the upper-right of each spectrum), a spectrum from
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an optimized Ag-decorated Si wafer (2D substrate) is also shown for comparison. (b) The relationship between the sputtering duration and the intensities of three Raman
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bands. Inset in (a) is the SERS spectra of 10-4 M R6G from the 2D substrates with different Ag sputtering durations.
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Fig. 4 SEM images of the 3D SERS substrates with (a) and (b) 100 s, (c) 300 s, (d) 500 s, (e) 700 s, (f) 900 s, (g) and (h) 1100 s Ag sputtering duration. Inset in (h) is the
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energy dispersive spectrometry (EDS) taken from the side surface of the array. Fig. 5 SEM image of the 3D SERS substrates with 1300 s Ag sputtering duration.
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Fig. 6 SERS spectra of R6G with the concentrations of 10-7, 10-10, and 10-13 M obtained from the optimized substrates with 1100 s Ag sputtering duration. The inset
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is the enlarged SERS spectrum of 10-13 M R6G.
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Fig. 7 (a) All the 289 spectra from the mapping area. (b) and (c) Left panels: the measured rectangular regions (51×51 µm2) with the brightness of the grid being
proportional to the signal intensity at 612 cm-1 and 1362 cm-1, respectively; right
panels: the corresponding histograms of SERS spectral intensities along the two crossed lines within the left panels. Fig. 8 SERS spectrum of DMP with a concentration of 10-7 M obtained from the
optimized substrate with 1100 s Ag sputtering duration, a normal Raman spectrum from 10-1 M DMP solution on a flat Si wafer is also shown for comparison.
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S0169-4332(14)02579-3 http://dx.doi.org/doi:10.1016/j.apsusc.2014.10.181 APSUSC 29143
To appear in:
APSUSC
Received date: Revised date: Accepted date:
7-8-2014 26-9-2014 27-10-2014
Please cite this article as: Z. Zuo, K. Zhu, L. Ning, G. Cui, J. Qu, Y. Cheng, J. Wang, Y. Shi, D. Xu, Y. Xin, Highly sensitive surface enhanced Raman scattering substrates based on Ag decorated Si nanocone arrays and their application in trace dmethyl phthalate detection, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.10.181 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights SERS substrates with high enhancement and excellent uniformity were prepared. The methods of fabrication are completely compatible with Si device
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technologies.
The enhancement performance of substrates can be tuned by Ag sputtering
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duration.
Multiple-type high density hot spots are responsible for the high performance.
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Trace dimethyl phthalate is well detected by this SERS substrate.
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Highly sensitive surface enhanced Raman scattering substrates based on Ag decorated Si nanocone arrays and their application in trace
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dmethyl phthalate detection
Zewen Zuoa,b,, Kai Zhua, Lixin Ninga, Guanglei Cuia, Jun Qua, Ying Chengc,
Center for Nano Science and Technology, College of Physics and Electronics
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Junzhuan Wangc, Yi Shic, Dongsheng Xud, Yu Xind
Information, Anhui Normal University, Wuhu, 241000, China
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing,
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210093, China c
School of Electronic Science and Engineering, and Key Laboratory of Photonic and
School of Physical Science and Technology, Soochow University, Suzhou 215006,
China
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Electronic Materials, Nanjing University, Nanjing, 210093, China
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ABSTRACT: Wafer-scale three-dimensional (3D) surface enhancement Raman scattering (SERS) substrates were prepared using the plasma etching and ion
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sputtering methods that are completely compatible with well-established silicon device technologies. The substrates are highly sensitive with excellent uniformity and reproducibility, exhibiting an enhancement factor up to 1012 with a very low relative
standard deviation (RSD) around 5%. These are attributed mainly to the uniform-distributed, multiple-type high-density hot spots originating from the structural characteristics of Ag nanoparticles (NPs) decorated Si nanocone (NC) arrays. We demonstrate that the trace dimethyl phthalate (DMP) at a concentration of 10-7 M can be well detected using this SERS substrate, showing that the AgNPs-decorated SiNC arrays can serve as efficient SERS substrates for phthalate acid esters (PAEs) detection with high sensitivity.
Corresponding author at: College of Physics and Electronics Information, Anhui Normal University, Wuhu, 241000, China. Tel.: +86 5533883561. E-mail address: [email protected] (Z. W. Zuo)
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Keywords: Surface-enhanced Raman scattering (SERS); three-dimensional substrate;
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silicon nanocone array; hot spots; trace detection
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1. Introduction Surface enhanced Raman scattering (SERS) has been developed as a powerful spectroscopy tool for chemical and biological detection and identification due to its high sensitivity and rapid response [1−5]. Electromagnetic enhancement that is
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maximal in between adjacent metal nanostructures with the gap on the order of a few
nanometers (called “hot spots”), is established as the dominant mechanism of the
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SERS [6−8]. When located inside the hot spots, Raman-active molecules will be effectively activated, with a significant enhancement of their Raman signal.
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Though much effort has been devoted to preparing high quality SERS substrate [7−9], the fabrication of large-area uniform, reproducible and highly sensitive SERS
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substrates with cost-effective techniques remains as one of the main challenges for the wide applications of SERS. For example, by assembling chemical synthesized noble
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metal nanoparticles or nanorods on flat substrates, one can make highly sensitive SERS substrates, but with the difficulty of controlling the structure uniformity (and
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thus the signal uniformity) over a large area [10−12]; focus ion beam or electron beam lithography can produce well-controlled metal nanostructure arrays for efficient SERS
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fabrication [13−15].
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substrates, but these techniques are expensive and time-consuming for large-area
Considering the well established semiconductor manufacturing technologies, it is
highly desirable to produce SERS substrates using techniques compatible with silicon device technologies. This will greatly reduce the production cost and the produced sensors can be readily integrated into existing silicon devices. The maskless plasma etching is one such technique that allows for simple, uniform, large-area, and inexpensive preparation of silicon nanostructure arrays [16−19]. In this paper, we report on the production of high performance SERS substrates with high sensitivity and excellent reproducibility based on high density Si nanocone (SiNC) arrays formed by the maskless plasma etching of crystalline Si wafer. Silver nanoparticles (AgNPs) were then deposited onto the surfaces of the arrays by ion sputtering. The techniques used here are completely compatible with silicon device technologies. The effect of the morphology and size distribution of the AgNPs on the performance of the SERS
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substrates was investigated. The results show that the vertically-aligned nanocone array is readily for the AgNPs to be decorated onto the side surfaces and the top ends of the cones, further, to generate multiple-type efficient SERS hot spots. The optimized three dimensional (3D) SERS substrate was also used to detect trace
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dimethyl phthalate (DMP), a class of concerning pollutant [20,21]. The results in this
study demonstrate that the AgNPs-decorated SiNC arrays can serve as efficient SERS
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substrates for phthalate acid esters (PAEs) detection with high sensitivity. 2. Experimental details
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The fabrication process of the SERS substrates is schematically shown in Fig. 1. First, a (100) Si wafer of four inches diameter was ultrasonic cleaned with acetone
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and deionized water and blew drying with nitrogen. Then, the wafer was placed inside the chamber of a plasma etcher to perform maskless plasma etching. The plasma was
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generated using a single frequency of 40.68 MHz at a power of 200 W. The etcher was operated at a pressure of 2 Pa. The flow rates for SF6, and O2 were 100 sccm and
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3 sccm, respectively. After the etching, the wafer was treated with 10 % HF solution for 1 min, and then Ag deposition was performed using an ion sputtering system with
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the current of about 2 mA at room temperature. During the sputtering, no protective
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gas was used. The morphology of the arrays was characterized using scanning electron microscopy (SEM, Hitachi S-4800). The resulting wafer of Ag-decorated SiNC array was diced into small pieces for
SERS measurement. Rhodamine 6G (R6G) and DMP were diluted in ethanol with different concentrations for SERS test. The substrates were immersed in the test solutions for 3 hours at room temperature and were then rinsed with deionized water to remove physically adsorbed molecules, followed by blow drying with nitrogen. The SERS measurements were performed at room temperature on a Raman system (HORIBA Jobin Yvon, LabRAM HR) using 514 nm Ar+ laser line for excitation. The laser spot size was about 2 µm with an incident power of 50 µW through a 100× objective. The acquisition time was 5 s for all Raman measurements. 3. Results and discussions After the plasma etching for 15 min, a black surface appears on the wafer as
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shown in the inset of Fig. 2a, which is due to its antireflection property from the nanostructure surface [19,22]. The Si nanostructure array is clearly observed in the SEM images. Fig. 2a reveals that the array has a very high areal density of about 650 µm-2 under the present etching conditions, noted that the areal density of nanopillars
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obtained by electron beam lithography is less than 30 µm-2 [23]. The high areal
density of the array leads to a close proximity of the cones to each other with an
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average spacing of about 30 nm at the top. From the cross-sectional SEM image shown in Fig. 2b, one can see that the as-etched Si nanostructure is vertically aligned
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with cone-shaped morphology. The average height of the cones is about 140 nm, and their average diameters are 14 and 35 nmm at the top and base, respectively.
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The SiNC arrays were covered with Ag by ion sputtering to form SERS substrates, and then R6G molecules were used as probe molecules to reveal the
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sensitivity of these 3D substrates. Fig. 3a presents the SERS spectra of 10-4 M R6G absorbed on the 3D substrates with different Ag sputtering duration, along with a
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spectrum from an optimized 2D substrate, which was produced by sputtering 500 s Ag on a flat Si wafer (see the inset in Fig. 3a), for comparison. Evidently, the 3D
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substrates show great enhancement of the Raman signal compared to the 2D one.
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Many fingerprint Raman bands of R6G were clearly observed from the 3D SERS substrates. The bands at 612, 773 and 1126 cm-1 can be assigned to C-C-C ring in-plane, out-of-plane bending, and C-H in-plane bending vibrations, respectively, while the bands at 1183, 1362, 1508, and 1650 cm-1 are all associated with the
symmetric modes of C-C in-plane stretching vibrations [24,25]. The intensities of the Raman bands of 612, 1362, and 1650 cm-1 as functions of Ag sputtering duration were plotted in Fig. 3b. It shows that the SERS activity of the substrate improves with the increase of the Ag sputtering duration, reaching a peak enhancement with 1100 s Ag sputtering and then decreasing for the substrates with longer sputtering duration. This dependence of the SERS activity on Ag sputtering duration can be ascribed to the structure evolution of the SERS substrates. Fig. 4 gives the morphologies of 3D substrates with different Ag sputtering duration. The images show that AgNPs are formed on the top and side surface of the SiNCs due to a weak interaction energy
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between the absorbed Ag atom and the Si (Volmer-Weber mode) [26, 27], and their size and spacing are dependent on the sputtering duration. With a short sputtering duration of 100 s, only a few AgNPs with an average diameter of about 10 nm are formed on the top ends of the cones, as shown in Fig. 4a, while AgNPs are hardly
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observed on the side surface from the cross-sectional SEM image shown in Fig. 4b, resulting in a low density of hot spots and thus low SERS signal. With the increase of
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the sputtering duration, AgNPs on the top ends of SiNCs grow up, and also start to
emerge on the side surfaces of the cones, as shown in Fig. 4c. Then, the size and the
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density of the AgNPs both on the top ends and side surfaces of the SiNCs continually increase with increasing sputtering duration, forming more hot spots responsible for
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the further enhancement of the Raman signal. For longer sputtering durations, the AgNPs on top end agglomerate to form big sphere-shape particles due to Ostwald
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ripening. The lateral growing of the Ag spheres decrease the amount of Ag reaching the side surfaces of the cones due to shadowing effect, resulting in almost no further
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change in the density of hot spots on the side surfaces. As the size of the Ag spheres becomes larger, the gap between the Ag spheres on the top ends of two neighboring
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cones becomes smaller. For the sputtering duration of 1100 s, the average diameter of
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the Ag spheres is about 40 nm, and the average gap distance between two adjacent spheres is about 10 nm, as shown in Figs. 4g and 4h. These values are highly favorable for Raman enhancement;7 most effective enhancement was consequently
obtained. The inset in Fig. 4h presents the energy dispersive spectrometry (EDS) taken from the side surface of the array, which shows strong Ag signal, also certifying the formation of AgNPs on it. Further increase in sputtering duration leads to the agglomeration of the Ag spheres on the top ends of neighboring cones (see Fig. 5), which decreases the density of hot spots and thus accounts for the decreased SERS signal of the substrate with 1300 s Ag sputtering duration, as shown in Fig. 3. Based on above results, optimized SERS substrates with 1100 s Ag sputtering duration were prepared and soaked in R6G solutions at low concentration. Fig. 6 shows the measured Raman spectra of R6G with the concentrations of 10-7, 10-10, and
10-13 M. It can be seen that strong SERS signal can be obtained with a 10-10 M
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solution. The inset shows the enlarged SERS spectrum of R6G with the concentration of 10-13 M, in which fingerprint bands of R6G at 1181, 1362, 1509, and 1650 cm-1 can be clearly identified though a relatively low signal-to-noise ratio. To estimate the enhancement factor (EF) of Ag-decorated SiNC array SERS
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substrate, we employed a SiNC array without AgNP decorating as a reference substrate, with the following equation [25,28]: I SERS N SERS I Raman N Raman
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EF
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where ISERS and IRaman are the integrated Raman intensities of a specific SERS bands of R6G adsorbed on the substrates with and without AgNPs, respectively. NSERS and
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NRaman are the numbers of R6G molecules within detection volume on the substrates with and without AgNPs, respectively. The integrated intensity of the Raman band at
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1650 cm-1 was usded, and averaged over 10 random spots. The intensity ISERS for 10-13 M R6G on the substrates with AgNPs is about 9200, while the IRaman for 10-3 M R6G
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on the substrates without AgNPs is about 90, leading to ISERS/IRaman≈102. Considering that the conditions of the SiNC array preparation and the Raman measurement are the
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same for these two substrates, that is, identical detection volume and similar surface
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area for molecules absorbed, NSERS and NRaman are mainly determined by the concentration of R6G, giving the value of NRaman/NSERS equal to the ratio of
concentration, 1010. Accordingly, the EF of AgNPs-decorated SiNC array substrate is estimated to be as high as 1.02×1012. This high EF confirms that the AgNPs-decorated
SiNC array is an effective SERS substrate with very high sensitivity. It has been well demonstrated that the high SERS sensitivity can be ascribed
dominantly to the abundance of nanogaps between noble metal nanoparticles, which generates high density hot spots responsible for electromagnetic enhancement [7,8,12]. Because of the bushy SiNCs and dense AgNPs, multiple-type nanogaps are formed within the 3D array. The gaps are formed between the large Ag spheres on adjacent SiNCs, between the AgNPs on the side surface of a single cone, and between the AgNPs on the side surfaces of adjacent cones, as can be seen from Figs. 4g and 4h, all these gaps are enough small to induce effective electromagnetic coupling between
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AgNPs. Furthermore, the 3D array of the substrates might enhance the trapping of excitation light [25], and provide a much larger surface area for adsorbing probe molecules within the detection volume [24,25]. All these features contribute to the high SERS sensitivity.
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To investigate the uniformity and reproducibility of the 3D SERS substrates, Raman mapping (3 μm step size, 17×17 = 289 spectral lines in all) was performed
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using 10-4 M R6G as test molecules. All the 289 spectra from the mapping area (51 × 51 µm2) exhibit almost identical spectral lines, as shown in Fig. 7a. The left panels in
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Figs. 7b and 7c illustrate the measured rectangular region with the brightness of the grid being proportional to the signal intensity at 612 cm-1 and 1362 cm-1, respectively,
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while the right panels are the corresponding histograms of SERS spectral intensities along the two crossed lines within the left panels. The relative standard deviations
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(RSD) of the spectral intensity calculated from the histograms are 4.46 %, 4.14 % for 612 cm-1, and 5.91%, 6.73% for 1362 cm-1, respectively. These results unambiguously
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reveal excellent uniformity and reproducibility of this SERS substrate, which can be mainly attributed to the highly uniform structural characteristics of the 3D substrate,
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that is, uniformly aligned SiNC array and evenly distributed AgNPs on the cones.
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Furthermore, the 3D distributed high density hot spots can make more molecules be activated and probed within the detection volume and, as a consequence, improve the signal uniformity over the SERS substrate by neglecting local structure variation [27,29].
PAEs are most widely used in industrial production as plasticizers, additives, and
solvents [21,30]. During the production and utilization, PAEs diffuse everywhere and have been detected in water, air, food, and rainwater, etc., which are toxic to a variety of wildlife and also cause human’s endocrine system disorder, even at low-concentration levels [31,32]. As a result, PAEs are becoming a class of concerning pollutants. Short-chained esters such as DMP are among the most frequently identified PAEs in various water environments [20,21], therefore, the development of technologies that can effectively detect DMP at low concentration in solution is urgently required. In this work, we used AgNPs-decorated SiNC array
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SERS substrate to detect trace DMP diluted in ethanol. Fig. 8 shows the SERS spectrum from 10-7 M DMP solution, along with a normal Raman spectrum from 10-1 M DMP solution on a flat Si wafer. It is difficult to identify the Raman bands from the normal Raman spectrum, while the 3D substrate shows great enhancement of the
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Raman signal, making many fingerprint bands of the DMP to be clearly observed. Besides the Raman bands of above-mentioned various vibrations of C-C-C ring, C-H,
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and C-C, the bands associated with the vibrations of the orthophenyl group at 659, 1040, 1574, and 1600 cm-1 can be clearly identified [33]. These results demonstrate
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that AgNPs-decorated SiNC arrays have great potential as high sensitive SERS substrate for the detection of trace PAEs.
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4. Conclusions
In summary, maskless plasma etching technique was used to prepare large-area
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uniform SiNC array, and applied to SERS substrates by decorating AgNPs onto the array surface using ion sputtering. Bushy SiNCs and dense AgNPs in the 3D arrays generate multiple-type high density hot spots, making the SERS substrates highly
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sensitive. The substrates exhibit a very high enhancement factor up to 1012 with the
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detection limit as low as 10-13 M for R6G. The very low RSD around 5% of the
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Raman band intensities indicates excellent uniformity and reproducibility across the substrates. Furthermore, trace DMP at a concentration of 10-7 M was well detected
using this SERS substrate. The results in this work demonstrate that the AgNPs decorated SiNC arrays can serve as efficient SERS substrates for PAEs detection with high sensitivity. More importantly, the production of this SERS substrate is completely compatible with the well-established silicon device technologies, which is very promising to promote the wide applications of SERS substrates. Acknowledgements This study is partly supported by the National Natural Science Foundation of China under Grant Nos. 61106011 and 11374015, and the Anhui Province Natural Science Foundation under Grant No. 1308085QF109.
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Figure captions: Fig. 1 Schematic fabrication process of AgNPs-decorated SiNC array SERS substrate. Fig. 2 SEM images of (a) top view and (b) cross-sectional view of the SiNC array. Inset in (a) is the photograph of the wafer after SiNC array formed on the surface.
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Fig. 3 (a) SERS spectra of 10-4 M R6G from the substrates with different Ag sputtering durations (indicated on the upper-right of each spectrum), a spectrum from
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an optimized Ag-decorated Si wafer (2D substrate) is also shown for comparison. (b) The relationship between the sputtering duration and the intensities of three Raman
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bands. Inset in (a) is the SERS spectra of 10-4 M R6G from the 2D substrates with different Ag sputtering durations.
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Fig. 4 SEM images of the 3D SERS substrates with (a) and (b) 100 s, (c) 300 s, (d) 500 s, (e) 700 s, (f) 900 s, (g) and (h) 1100 s Ag sputtering duration. Inset in (h) is the
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energy dispersive spectrometry (EDS) taken from the side surface of the array. Fig. 5 SEM image of the 3D SERS substrates with 1300 s Ag sputtering duration.
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Fig. 6 SERS spectra of R6G with the concentrations of 10-7, 10-10, and 10-13 M obtained from the optimized substrates with 1100 s Ag sputtering duration. The inset
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is the enlarged SERS spectrum of 10-13 M R6G.
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Fig. 7 (a) All the 289 spectra from the mapping area. (b) and (c) Left panels: the measured rectangular regions (51×51 µm2) with the brightness of the grid being
proportional to the signal intensity at 612 cm-1 and 1362 cm-1, respectively; right
panels: the corresponding histograms of SERS spectral intensities along the two crossed lines within the left panels. Fig. 8 SERS spectrum of DMP with a concentration of 10-7 M obtained from the
optimized substrate with 1100 s Ag sputtering duration, a normal Raman spectrum from 10-1 M DMP solution on a flat Si wafer is also shown for comparison.
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Figure 3
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Figure 6
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Figure 7
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Figure 8
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