Three-dimensional multi-walled carbon nanotube arrays coated by gold-sol as a surface-enhanced Raman scattering substrate Jie Zhang,1,* Tuo Fan,1 Xiaolei Zhang,1 Chunhong Lai,1,2 and Yong Zhu1 1
The Key Laboratory of Optoelectronic Technology & System, Education Ministry of China, Chongqing University, 400044, China
2
College of Physics and Electronic Information, China West Normal University, 637000, China *Corresponding author: [email protected] Received 9 August 2013; revised 27 December 2013; accepted 24 January 2014; posted 28 January 2014 (Doc. ID 195495); published 19 February 2014
We demonstrated a three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrate consisting of large area carbon nanotube (CNT) arrays coated by gold-sol nanoparticles. A low-cost, simple process is used to prepare Au-decorated 3D CNT arrays. The SERS enhancement from the 3D CNT arrays, and two-dimensional (2D) CNT films substrates coated by different size gold-sol nanoparticles, was experimentally verified with Rhodamine 6G as the probe analyte. The experiments showed that the 3D CNT arrays substrate has a higher Raman enhancement compared with 2D CNT arrays substrate and planar glass substrate, due to the large specific surface area of CNT arrays and more gold nanoparticles on the CNT arrays sidewalls, which contribute the electromagnetic field and Raman intensity. Meanwhile, the 3D structure could enhance the excitation light trapping in CNT arrays, consequently increasing the light interaction with Au nanoparticles. © 2014 Optical Society of America OCIS codes: (280.4788) Optical sensing and sensors; (290.5860) Scattering, Raman. http://dx.doi.org/10.1364/AO.53.001159
1. Introduction
Since its discovery in the 1970s, surface-enhanced Raman scattering (SERS) has shown tremendous potential for many applications, especially bio/chemical molecular analysis at the trace, and even single molecule level [1–3]. The enhancement factor is believed due to the excitation of localized surface plasmon resonance on the rough metal surface, in which a SERS substrate plays a vital role. To achieve the sensitive detection of molecules by SERS, the substrates should have abundant “hot spots” to enhance the local electromagnetic fields, and huge surface area
1559-128X/14/061159-07$15.00/0 © 2014 Optical Society of America
to adsorb more molecules and, thus, contribute Raman intensity. Conventionally, SERS substrates are twodimensional (2D) structures, prepared by fabricating nanostructures (such as nanoparticles [4], nanograting [5], nanoporous silicon [6], and nanodomes [7]) on planar silicon wafers or glass. Some 2D substrates are prepared nanoparticles on disordered carbon nanotubes (CNTs) [8], or aligned carbon nanotubes [9]. However, there are some limits for 2D configurations: (1) the available density of SERS-active sites within the detection volume is limited; (2) the analytes in bulk solution diffuse slowly to the SERSactive sites. We know that optical scattering and light collection occur in a three-dimensional (3D) focus volume. Thus, SERS substrates should contain “hot spots” in 20 February 2014 / Vol. 53, No. 6 / APPLIED OPTICS
1159
a 3D focus volume to maximize the quantity of scattered light generated and detected [10]. 3D SERS detection has a much larger detection area and higher sensitivity than the 2D planar SERS substrates. Recently, 3D nanostructures, such as nanowires, nanorods, ordered Au particle arrays [11], nanoporous silicon [6], nanotube arrays [12], and cavity nano antenna arrays with dense plasmonic nanodots [13] have been proposed as SERS substrates. Silicon nanowires are usually grown through a high temperature vapor–liquid–solid route, which is expensive [14]. Ag and Au nanoparticles and structures exhibit plasmon resonances that result in enhanced optical fields near the metal surface and enhanced Raman intensity. However, Ag nanoparticles are easily oxidized, which leads to a limited lifetime. Suspended platinum-coated carbon nanotubes [15] and aligned carbon nanotube scaffolds/ Au nano particles [10] are used as SERS substrates. Therefore, it is still desirable to create densely packed metal nanoparticles in a 3D substrate for better SERS performance. In this paper, we describe a tunable 3D SERS substrate based on vertically aligned carbon nanotube arrays coated by Au nanoparticles. The effect of the different Au nanoparticle sizes to SERS intensity is also experimentally studied with 3D CNT arrays and 2D CNT films substrates.
N SERS and N 0 are the numbers of probe molecules contributing to the SERS signal and the non-SERS signal, respectively. I SERS can be obtained by [16] I SERS ωs NAΩ
where I SERS ωs is SERS intensity at frequency ωs, N the molecular surface density, A the excitation area, Ω the solid angle of photon collection, σωs the Raman scattering cross section, PL ωL the radiant flux at excitation frequency, εωL the energy of incident photon, Qωs the quantum efficiency of detector, T m the transmission efficiency of the spectrometer, and T 0 the transmission efficiency of the collection optics. In our CNT arrays/Au-sol nanoparticles substrate, due to the 3D structure, the total surface area for absorbing Au nanoparticles of the CNT arrays substrate is shown in Fig. 2, compared with CNT films substrate and planar substrate. If the center distance of two CNTs d is double D, then π π SCNTF L1 L2 SPlanar ; 2 2
2. Structure
Our 3D SERS substrate, as shown in Fig. 1, comprises vertically aligned CNT arrays coated by Au nanoparticles. The dimension of carbon nanotube (height, diameter, inter-distance, etc.) can be tuned using different conditions during chemical vapor deposition (CVD). The dimension of Au nanoparticles could be regulated with the process of Au-sol solution preparation. The sensitivity of a SERS substrate is characterized by the enhancement factor, defined by EF
I SERS N SERS I0 N0
;
dσωs PL ωL εωL −1 Qωs T m T 0 EF; dΩ (2)
SCNTA
πH πH L L S : 4D 1 2 4 D Planar
(3)
(4)
In our sample, H is at micrometer scale and D is at nanometer scale, so SCNTF ≈ 1.5SPlanar , SCNTA ≈ 103 SPlanar . That is to say, the CNT arrays substrate has a much larger surface area to adsorb more nanoparticles compared with planar substrates, which
(1)
where I SERS and I 0 are the intensity of the SERS signal and the non-SERS signal, respectively, and
Fig. 1. 3D CNT arrays coated by Au-sol nanoparticles in 3D laser incident focus volume. 1160
APPLIED OPTICS / Vol. 53, No. 6 / 20 February 2014
Fig. 2. Surface areas of three substrates on the same planar area (SPlanar L1 L2 ), where SPlanar , SCNTF , SCNTA are the total surface areas of the planar, CNT films, and CNT array substrates, respectively; L1 , L2 are the side lengths of the planar substrate, D is the diameter of the CNT, H is the length/height of the CNT, and d is the center distance of two CNTs when forming films and arrays.
could enhance the Raman intensity, brought by the local surface plasmon resonances of nanoparticles. As for CNT arrays structure, A is huge [Eq. (2)] and N would be bigger (more Au nanoparticles, more molecules adsorbed on the walls of Au), which would enhance the SERS intensity and EF. 3. Sample Preparations
Carbon nanotube arrays growth: The vertically aligned CNT arrays were synthesized on a Si wafer by CVD. A Si wafer, on which a 50 nm Mo, 10 nm Al, and 5 nm Fe layer were orderly deposited, using an e-beam evaporator in advance, was put into a quartz tube and heated to 720°C, in a buffer gas of H2 (50 sccm); then C2 H4 (150 sccm) was introduced into the quartz tube as the carbon source, with a proportion of 3∶1 to H2 . The flow of C2 H4 was stopped after 10 min of the CNT array growth, and then the buffer gas was cooled to room temperature. Au-sol solution preparation: A simple hydrothermal method was used [17–20]. First, 1% HAuCl4 · 4H2 O solution (0.5 mL) was added to pure water (50 mL), under heating in an oil bath bank. Continuously heating and stirring, at a temperature of 92°C 4°C, 1% HOCCOONaCH2 COONa2 (the volume can be changed according to the needed size of Au nanoparticles) was added, with temperature maintained at 92°C 4°C under stirring for 25 min. Afterward, the solution was cooled naturally to room temperature. Finally, the Au sol solution was sealed, light blocked and saved in refrigerator at 4°C. The volume of 1% HOCCOONaCH2 COONa2 affects the size of Au nanoparticles. In our experiments, the volumes used ranged from 0.75 to 0.35 mL, which resulted in Au nanoparticles with diameters from 30 to 80 nm. Au nanoparticle formation on CNT arrays: A simple drop-coat method is used [21]. A certain volume (10 μL) of gold-sol solution prepared was dipped to the CNT arrays to cause the gold to diffuse along
Fig. 3. SEM photo of Au nanoparticles decorated on the CNT arrays.
Fig. 4. Au element distribution on the sample by energy dispersive spectrometer (red dot means the distribution of Au element).
the nanotubes wall, to form 3D SERS substrate with dense Au nanoparticles. Au nanoparticles formation on the CNT walls is related to the sol solution concentration, wetting, and different sizes. Each CNT/ CNT intersection provides a site for Au nanoparticles, which leads to discrete Au nanoparticle on the surface of the CNT arrays walls, with small gaps between particles. Figure 3 shows the Au nanoparticles decorated on the walls of CNT arrays by scanning electron microscopy (SEM). Figure 4 shows the Au element distribution on the sample, where the most abundant element is C (weight at 84.21%, atomic at 92.72%) and the Au element is weight at 2.26%, atomic at 0.15%. 4. Experiments
SERS measurements: Raman spectra were recorded with a Raman microscope using a 20× objective of numerical aperture 0.4, at work distance of
Fig. 5. Raman intensity of R6G at different concentrations (10−3 M, 10−5 M, and 10−6 M) with CNT arrays coated by Au nanoparticles (65 nm diameter) as substrate. 20 February 2014 / Vol. 53, No. 6 / APPLIED OPTICS
1161
Table 1.
Characterized Raman Peaks
−1
Raman Shift (cm )
Reason
609, 771, 1180
Assigned to C–C–C ring in-plane bending, out-of-plane bending motion of the hydrogen atoms of the xanthene skeleton, and C–C stretching vibrations [24] C–H in-plane bend [25] The combination of the four stretching modes [25] The aromatic C–C stretching vibrations [24,25]
1125 1309 1361, 1505, 1566, 1645
5.46 mm, with input laser of wavelength 632.8 nm [22,23]; the laser power incident on the sample surface was typically in the range 0.5–4.0 mW. The integration time was 6 s. SERS measurements were carried out with Rhodamine 6G (R6G) as a probe molecule. To reduce the effect of background light, we used 4th polynomial fitting to the initial Raman spectrum data. A.
Different R6G Concentration Effect
To demonstrate the performance of our substrates and investigate whether the SERS signals are related to molecule concentration, we tested the SERS sensitivity using different concentrations of R6G solution of 10−3 M, 10−5 M, and 10−6 M. The result with 3D CNT arrays coated by Au nanoparticles (65 nm diameter) is shown in Fig. 5. Distinctive Raman shifts at 609, 771, 1125, 1180, 1309, 1361, 1505, 1566, and 1645 cm−1 were observed, which are associated with the characteristic vibration modes, shown in Table 1. For typical Raman peaks, we can see that lower concentrations produce smaller Raman peaks intensity (given in Table 2), which is due to fewer probe analytes within the laser focus volume.
large enhancement may be ascribed some reasons: (1) the 3D CNT arrays structure with high-density tubes provide much larger surface areas for loading Au nanoparticles. The increased adsorption of R6G molecule in the carbon nanotubes causes stronger Raman signal in the given excitation area. Thus, this geometric effect can contribute to the enhancement. (2) The inter-distance between Au nanoparticles and CNT arrays allows the formation of “hot spots”; as a result, the local electromagnetic effect is obtained. (3) The periodic order 3D nanostructures might
B. 3D CNT Substrates Compared with 2D CNT and Planar Substrates
To compare 3D CNT arrays/Au enhancement with 2D substrates, we fabricated 2D CNT films (detailed fabrication steps in Ref. [8]) and planar glass coated by Au nanoparticles. Figure 6(a) shows the SERS spectra of R6G at a concentration of 10−5 M adsorbed on the three substrates, with a Au nanoparticles diameter of 80 nm, and Raman peaks intensity data in Table 3. For 10−6 M R6G, the result is shown in Fig. 6(b) with 3D and 2D CNT substrates. We can see that the performance of 3D CNT arrays substrates is better than that of 2D CNT films and planar glass substrates. The maximum intensity ratio occurs at 1505 cm−1 , which is 5.67 (3D CNT/2D CNT) and 3.78 (3D CNT/Glass), respectively. The
Table 2.
Typical Raman Peaks Intensity at Different Concentrations
Raman Shift (cm−1 ) R6G 10−3 10−5 10−6
1162
M M M
∼1180
∼1309
∼1361
∼1505
∼1566
∼1645
1422 685.7 339.9
2697 1533 731
5683 2754 1290
6681 3584 1614
1826 1371 465.2
3337 1544 668.5
APPLIED OPTICS / Vol. 53, No. 6 / 20 February 2014
Fig. 6. Raman intensity of R6G at a concentration of (a) 10−5 M adsorbed on three different substrates (CNT arrays/Au, CNT films/ Au, glass/Au substrates), (b) 10−6 M adsorbed on two CNT substrates, with the Au nanoparticle diameter at 80 nm.
Table 3.
Typical Raman Peaks Intensity with Different Substrates
Table 4.
−1
Raman Shift (cm ) Substrates
∼1180
∼1309
∼1361
∼1505
∼1566
∼1645
3D CNT 2D CNT Glass
1713 767.5 481.1
4136 1233 1146
7009 1308 1940
9047 1596 2401
2152 851.6 577.7
3767 1270 1048
enhance the excitation light trapping in the CNT arrays, consequently increasing the light interaction with Au nanoparticles [26]. C.
Au Nanoparticle Size Effect
Au nanoparticle size relates to the substrate roughness and its own optical properties, which contribute to the SERS intensity [21]. In this paper, we prepared different diameter Au nanoparticles: 30, 65, and 80 nm. As for 3D CNT arrays, the Raman intensity of R6G (10−6 M) is given in Fig. 7, where we can see the intensity with 80 nm Au nanoparticles has the best performance (detailed data in Table 4). The same effects of Au size for 2D CNT films and planar glass are shown in Figs. 8(a) and 8(b). According to Eq. (4), the CNT arrays have much larger (∼103 X) surface areas than planar substrates, but the SERS experimental results indicate little enhancement, which might be due to the majority of Au nanoparticles being on the top surface of the CNT arrays, with relatively few Au nanoparticles penetrating down along the CNT walls. Generally, the SERS effects are strongly dependent upon materials, shape, excitation source, and environment. For very small particles, electronic scattering at the surface results in the depletion of electrical conductivity, thus degrading the plasmon resonance quality. Meanwhile, the SERS sensitivity exhibits a correlation with the size and the SERS enhancement diminishes for larger platelets [27]. The
−6
Fig. 7. Raman intensity of R6G (10 M) while 3D CNT arrays coated by Au nanoparticles (30, 65, and 80 nm diameter).
Typical Raman Peaks Intensity with Different Size Au Nanoparticles
Raman Shift (cm−1 ) Au Diameter (nm) ∼1180 ∼1309 ∼1361 ∼1505 ∼1566 ∼1645 80 65 30
647.1 1714 339.9 731 106.6 253.3
2928 3515 1290 1614 378.5 416
1054 465.2 213
1469 668.5 173.3
SERS intensity of pyridine continues to increase (range from 20 to 135 nm), with a maximum at about 135 nm, when the excitation wavelength is 632.8 nm. A calculated enhancement keeps on increasing with the increase in particle size until about 110 nm [21]. In our experiments, the Raman intensity with 80 nm diameter nanoparticle substrates has the better performance, which agrees with previously reported research. Here, due to aggregation phenomena, it is difficult to prepare larger Au nanoparticle sizes (∼100 nm). The present study describes only three
Fig. 8. Raman intensity of R6G (10−5 M) with (a) planar glass and (b) 2D CNT films coated by Au nanoparticles (30, 65, and 80 nm diameter) as substrates. 20 February 2014 / Vol. 53, No. 6 / APPLIED OPTICS
1163
different nanoparticle diameters; therefore, further experimentation with different diameters is required to get an optimization size for SERS enhancement. D.
CNT Arrays Effect
There are three obvious Raman peaks for CNTs: 1338 cm−1 (D band), 1571 cm−1 (G band), and 2673 cm−1 (G0 band) [28]. The samples here consist of three materials: CNT arrays, Au nanoparticles on the surface of CNT arrays walls, and R6G on the surface of Au nanoparticles. When the laser excites the sample, the molecules (R6G) at the top surface have the greatest contribution to the Raman signal inside the focus volume, which leads to no obvious CNTs Raman peaks in the R6G experiments. The optimization of CNT arrays height/length and optical system is not yet discussed, which should be carried on later.
7. 8.
9.
10.
11.
12.
5. Conclusion
We demonstrated a sensitive SERS substrate that can be easily fabricated in large areas and at low cost by CVD for CNT arrays, and a hydrothermal method for Au-sol solution preparations. Using different Au sizes, different R6G concentration effects on detection were experimentally studied, which demonstrated that 3D substrates with the same Au size are better than 2D substrates for this purpose. These results contribute to the body of knowledge with respect to SERS phenomena. We thank Prof. Liwei Lin and Dr. Yingqi Jiang at the University of California at Berkeley for sample fabrication; Prof. Gang Chen at Chongqing University for Raman experimental help; and Prof. Mingmei Wu at Sun Yat-sen University for SEM help. This study is funded by National Science Fund (61376121), CSTC 2011BB2076, the Fundamental research Funds for the central Universities (106112013CDJZR 125502, 20003) and Visiting Scholar Fund of Key Laboratory of Optoelectronic Technology & System, Education Ministry of China.
13.
14.
15.
16. 17. 18. 19.
References 1. M. Fleischmann, P. J. Hendra, and A. J. McQuillan, “Raman spectra of pyridine adsorbed at a silver electrode,” Chem. Phys. Lett. 26, 163–166 (1974). 2. D. L. Jeanmaire and R. P. Van Duyne, “Surface Raman spectroelectrochemistry: part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode,” J. Electroanal. Chem. 84, 1–20 (1977). 3. Y. B. Guo, M. K. Khaing Oo, K. Reddy, and X. D. Fan, “Ultrasensitive optofluidic surface-enhanced Raman scattering detection with flow-through multihole capillaries,” ACS Nano 6, 381–388 (2012). 4. L. Zhang, X. Lang, A. Hirata, and M. Chen, “Wrinkled nanoporous gold films with ultrahigh surface-enhanced Raman scattering enhancement,” ACS Nano 5, 4407–4413 (2011). 5. Y. Jiao, J. D. Ryckman, P. N. Ciesielski, C. A. Escobar, G. K. Jennings, and S. M. Weiss, “Patterned nanoporous gold as an effective SERS template,” Nanotechnology 22, 295302 (2011). 6. S. Chan, S. Kwon, T. W. Koo, L. P. Lee, and A. A. Berlin, “Surface-enhanced Raman scattering of small molecules from 1164
APPLIED OPTICS / Vol. 53, No. 6 / 20 February 2014
20. 21.
22.
23. 24.
25.
silver-coated silicon nanopores,” Adv. Mater. 15, 1595–1598 (2003). C. J. Choi, Z. Xu, H. Y. Wu, G. L. Liu, and B. T. Cunningham, “Surface-enhanced Raman nanodomes,” Nanotechnology 21, 415301 (2010). J. Zhang, Y. L. Chen, T. Fan, and Y. Zhu, “Large area Au decorated multi-walled CNTs film for surface enhanced Raman scattering,” Key Eng. Mat. 562, 826–831 (2013). Y. H. Sun, K. Liu, J. Miao, Z. Y. Wang, B. Z. Tian, L. N. Zhang, Q. Q. Li, S. S. Fan, and K. L. Jiang, “Highly sensitive surfaceenhanced Raman scattering substrate made from superaligned carbon nanotubes,” Nano Lett. 10, 1747–1753 (2010). S. Lee, M. G. Hahm, R. Vajtai, D. P. Hashim, T. Thurakitseree, A. C. Chipara, P. M. Ajayan, and J. H. Hafner, “Utilizing 3D SERS active volumes in aligned carbon nanotube scaffold substrates,” Adv. Mater. 24, 5261–5266 (2012). G. T. Duan, W. P. Cai, Y. Y. Luo, and Y. Lei, “Hierarchical surface rough ordered Au particle arrays and their surface enhanced Raman scattering,” Appl. Phys. Lett. 89, 181918 (2006). X. Li, G. Chen, L. Yang, Z. Jin, and J. Liu, “Multifunctional Au-coated TiO2 nanotube arrays as recyclable SERS substrates for multifold organic pollutants detection,” Adv. Funct. Mater. 20, 2815–2824 (2010). W. D. Li, F. Ding, J. Hu, and S. Y. Chou, “Three-dimensional cavity nanoantenna coupled plasmonic nanodots for ultrahigh and uniform surface-enhanced Raman scattering over large area,” Opt. Express 19, 3925–3936 (2011). M. Becker, V. Sivakov, U. G. Gösele, T. Stelzner, A. Gudrun, H. J. Reich, S. Hoffmann, J. Michler, and S. H. Christiansen, “Nanowires enabling signal-enhanced nanoscale Raman spectroscopy,” Small 4, 398–404 (2008). M. K. Lee, J. Seo, S. J. Cho, Y. Jo, S. Kim, Y. J. Kang, and H. Lee, “Novel 3D arrays of gold nanostructures on suspended platinum-coated carbon nanotubes as surface-enhanced Raman scattering substrates,” Mater. Lett. 81, 9–12 (2012). P. L. Stiles, J. A. Dieringer, N. C. Shah, and R. P. Van Duyne, “Surface-enhanced Raman spectroscopy,” Annu. Rev. Anal. Chem. 1, 601–626 (2008). G. Frens, “Controlled nucleation for the regulation of the particle size in monodisperse gold suspensions,” Nat. Phys. Sci. 241, 20–22 (1973). D. Y. Wu, J. F. Li, B. Ren, and Z. Q. Tian, “Electrochemical surface-enhanced Raman spectroscopy of nanostructures,” Chem. Soc. Rev. 37, 1025–1041 (2008). Z. Krpetic, L. Guerrini, I. A. Larmour, J. Reglinski, K. Faulds, and D. Graham, “Importance of nanoparticle size in colorimetric and SERS-based multimodal trace detection of Ni (II) ions with functional gold nanoparticles,” Small 8, 707– 714 (2012). L. Liz-Marzán, “Nanometals: formation and color,” Mater. Today 7(2), 26–31 (2004). P. P. Fang, J. F. Li, Z. L. Yang, L. M. Li, B. Ren, and Z. Q. Tian, “Optimization of SERS activities of gold nanoparticles and gold-core–palladium-shell nanoparticles by controlling size and shell thickness,” J. Raman Spectrosc. 39, 1679–1687 (2008). J. T. Krug, G. D. Wang, S. R. Emory, and S. M. Nie, “Efficient Raman enhancement and intermittent light emission observed in single gold nanocrystals,” J. Am. Chem. Soc. 121, 9208–9214 (1999). X. M. Lin, Y. Cui, Y. H. Xu, B. Ren, and Z. Q. Tian, “Surfaceenhanced Raman spectroscopy: substrate-related issues,” Anal. Bioanal. Chem. 394, 1729–1745 (2009). X. M. Zhao, B. H. Zhang, K. L. Ai, G. Zhang, L. Y. Cao, X. J. Liu, H. M. Sun, H. S. Wang, and L. H. Lu, “Monitoring catalytic degradation of dye molecules on silver-coated ZnO nanowire arrays by surface-enhanced Raman spectroscopy,” J. Mater. Chem. 19, 5547–5553 (2009). P. Hildebrandt and M. Stockburger, “Surface-enhanced resonance Raman spectroscopy of Rhodamine 6G adsorbed
on colloidal silver,” J. Phys. Chem. 88, 5935–5944 (1984). 26. C. W. Cheng, B. Yan, S. M. Wong, X. L. Li, W. W. Zhou, T. Yu, Z. X. Shen, H. Y. Yu, and H. J. Fan, “Fabrication and SERS performance of silver-nanoparticle-decorated Si/ZnO nanotrees in ordered arrays,” ACS Appl. Mater. 2, 1824–1828 (2010).
27. H. Ko, S. Singamaneni, and V. V. Tsukruk, “Nanostructured surfaces and assemblies as SERS media,” Small 4, 1576– 1599 (2008). 28. R. A. DiLeo, B. J. Landi, and R. P. Raffaelle, “Purity assessment of multiwalled carbon nanotubes by Raman spectroscopy,” J. Appl. Phys. 101, 064307 (2007).
20 February 2014 / Vol. 53, No. 6 / APPLIED OPTICS
1165
The Key Laboratory of Optoelectronic Technology & System, Education Ministry of China, Chongqing University, 400044, China
2
College of Physics and Electronic Information, China West Normal University, 637000, China *Corresponding author: [email protected] Received 9 August 2013; revised 27 December 2013; accepted 24 January 2014; posted 28 January 2014 (Doc. ID 195495); published 19 February 2014
We demonstrated a three-dimensional (3D) surface-enhanced Raman scattering (SERS) substrate consisting of large area carbon nanotube (CNT) arrays coated by gold-sol nanoparticles. A low-cost, simple process is used to prepare Au-decorated 3D CNT arrays. The SERS enhancement from the 3D CNT arrays, and two-dimensional (2D) CNT films substrates coated by different size gold-sol nanoparticles, was experimentally verified with Rhodamine 6G as the probe analyte. The experiments showed that the 3D CNT arrays substrate has a higher Raman enhancement compared with 2D CNT arrays substrate and planar glass substrate, due to the large specific surface area of CNT arrays and more gold nanoparticles on the CNT arrays sidewalls, which contribute the electromagnetic field and Raman intensity. Meanwhile, the 3D structure could enhance the excitation light trapping in CNT arrays, consequently increasing the light interaction with Au nanoparticles. © 2014 Optical Society of America OCIS codes: (280.4788) Optical sensing and sensors; (290.5860) Scattering, Raman. http://dx.doi.org/10.1364/AO.53.001159
1. Introduction
Since its discovery in the 1970s, surface-enhanced Raman scattering (SERS) has shown tremendous potential for many applications, especially bio/chemical molecular analysis at the trace, and even single molecule level [1–3]. The enhancement factor is believed due to the excitation of localized surface plasmon resonance on the rough metal surface, in which a SERS substrate plays a vital role. To achieve the sensitive detection of molecules by SERS, the substrates should have abundant “hot spots” to enhance the local electromagnetic fields, and huge surface area
1559-128X/14/061159-07$15.00/0 © 2014 Optical Society of America
to adsorb more molecules and, thus, contribute Raman intensity. Conventionally, SERS substrates are twodimensional (2D) structures, prepared by fabricating nanostructures (such as nanoparticles [4], nanograting [5], nanoporous silicon [6], and nanodomes [7]) on planar silicon wafers or glass. Some 2D substrates are prepared nanoparticles on disordered carbon nanotubes (CNTs) [8], or aligned carbon nanotubes [9]. However, there are some limits for 2D configurations: (1) the available density of SERS-active sites within the detection volume is limited; (2) the analytes in bulk solution diffuse slowly to the SERSactive sites. We know that optical scattering and light collection occur in a three-dimensional (3D) focus volume. Thus, SERS substrates should contain “hot spots” in 20 February 2014 / Vol. 53, No. 6 / APPLIED OPTICS
1159
a 3D focus volume to maximize the quantity of scattered light generated and detected [10]. 3D SERS detection has a much larger detection area and higher sensitivity than the 2D planar SERS substrates. Recently, 3D nanostructures, such as nanowires, nanorods, ordered Au particle arrays [11], nanoporous silicon [6], nanotube arrays [12], and cavity nano antenna arrays with dense plasmonic nanodots [13] have been proposed as SERS substrates. Silicon nanowires are usually grown through a high temperature vapor–liquid–solid route, which is expensive [14]. Ag and Au nanoparticles and structures exhibit plasmon resonances that result in enhanced optical fields near the metal surface and enhanced Raman intensity. However, Ag nanoparticles are easily oxidized, which leads to a limited lifetime. Suspended platinum-coated carbon nanotubes [15] and aligned carbon nanotube scaffolds/ Au nano particles [10] are used as SERS substrates. Therefore, it is still desirable to create densely packed metal nanoparticles in a 3D substrate for better SERS performance. In this paper, we describe a tunable 3D SERS substrate based on vertically aligned carbon nanotube arrays coated by Au nanoparticles. The effect of the different Au nanoparticle sizes to SERS intensity is also experimentally studied with 3D CNT arrays and 2D CNT films substrates.
N SERS and N 0 are the numbers of probe molecules contributing to the SERS signal and the non-SERS signal, respectively. I SERS can be obtained by [16] I SERS ωs NAΩ
where I SERS ωs is SERS intensity at frequency ωs, N the molecular surface density, A the excitation area, Ω the solid angle of photon collection, σωs the Raman scattering cross section, PL ωL the radiant flux at excitation frequency, εωL the energy of incident photon, Qωs the quantum efficiency of detector, T m the transmission efficiency of the spectrometer, and T 0 the transmission efficiency of the collection optics. In our CNT arrays/Au-sol nanoparticles substrate, due to the 3D structure, the total surface area for absorbing Au nanoparticles of the CNT arrays substrate is shown in Fig. 2, compared with CNT films substrate and planar substrate. If the center distance of two CNTs d is double D, then π π SCNTF L1 L2 SPlanar ; 2 2
2. Structure
Our 3D SERS substrate, as shown in Fig. 1, comprises vertically aligned CNT arrays coated by Au nanoparticles. The dimension of carbon nanotube (height, diameter, inter-distance, etc.) can be tuned using different conditions during chemical vapor deposition (CVD). The dimension of Au nanoparticles could be regulated with the process of Au-sol solution preparation. The sensitivity of a SERS substrate is characterized by the enhancement factor, defined by EF
I SERS N SERS I0 N0
;
dσωs PL ωL εωL −1 Qωs T m T 0 EF; dΩ (2)
SCNTA
πH πH L L S : 4D 1 2 4 D Planar
(3)
(4)
In our sample, H is at micrometer scale and D is at nanometer scale, so SCNTF ≈ 1.5SPlanar , SCNTA ≈ 103 SPlanar . That is to say, the CNT arrays substrate has a much larger surface area to adsorb more nanoparticles compared with planar substrates, which
(1)
where I SERS and I 0 are the intensity of the SERS signal and the non-SERS signal, respectively, and
Fig. 1. 3D CNT arrays coated by Au-sol nanoparticles in 3D laser incident focus volume. 1160
APPLIED OPTICS / Vol. 53, No. 6 / 20 February 2014
Fig. 2. Surface areas of three substrates on the same planar area (SPlanar L1 L2 ), where SPlanar , SCNTF , SCNTA are the total surface areas of the planar, CNT films, and CNT array substrates, respectively; L1 , L2 are the side lengths of the planar substrate, D is the diameter of the CNT, H is the length/height of the CNT, and d is the center distance of two CNTs when forming films and arrays.
could enhance the Raman intensity, brought by the local surface plasmon resonances of nanoparticles. As for CNT arrays structure, A is huge [Eq. (2)] and N would be bigger (more Au nanoparticles, more molecules adsorbed on the walls of Au), which would enhance the SERS intensity and EF. 3. Sample Preparations
Carbon nanotube arrays growth: The vertically aligned CNT arrays were synthesized on a Si wafer by CVD. A Si wafer, on which a 50 nm Mo, 10 nm Al, and 5 nm Fe layer were orderly deposited, using an e-beam evaporator in advance, was put into a quartz tube and heated to 720°C, in a buffer gas of H2 (50 sccm); then C2 H4 (150 sccm) was introduced into the quartz tube as the carbon source, with a proportion of 3∶1 to H2 . The flow of C2 H4 was stopped after 10 min of the CNT array growth, and then the buffer gas was cooled to room temperature. Au-sol solution preparation: A simple hydrothermal method was used [17–20]. First, 1% HAuCl4 · 4H2 O solution (0.5 mL) was added to pure water (50 mL), under heating in an oil bath bank. Continuously heating and stirring, at a temperature of 92°C 4°C, 1% HOCCOONaCH2 COONa2 (the volume can be changed according to the needed size of Au nanoparticles) was added, with temperature maintained at 92°C 4°C under stirring for 25 min. Afterward, the solution was cooled naturally to room temperature. Finally, the Au sol solution was sealed, light blocked and saved in refrigerator at 4°C. The volume of 1% HOCCOONaCH2 COONa2 affects the size of Au nanoparticles. In our experiments, the volumes used ranged from 0.75 to 0.35 mL, which resulted in Au nanoparticles with diameters from 30 to 80 nm. Au nanoparticle formation on CNT arrays: A simple drop-coat method is used [21]. A certain volume (10 μL) of gold-sol solution prepared was dipped to the CNT arrays to cause the gold to diffuse along
Fig. 3. SEM photo of Au nanoparticles decorated on the CNT arrays.
Fig. 4. Au element distribution on the sample by energy dispersive spectrometer (red dot means the distribution of Au element).
the nanotubes wall, to form 3D SERS substrate with dense Au nanoparticles. Au nanoparticles formation on the CNT walls is related to the sol solution concentration, wetting, and different sizes. Each CNT/ CNT intersection provides a site for Au nanoparticles, which leads to discrete Au nanoparticle on the surface of the CNT arrays walls, with small gaps between particles. Figure 3 shows the Au nanoparticles decorated on the walls of CNT arrays by scanning electron microscopy (SEM). Figure 4 shows the Au element distribution on the sample, where the most abundant element is C (weight at 84.21%, atomic at 92.72%) and the Au element is weight at 2.26%, atomic at 0.15%. 4. Experiments
SERS measurements: Raman spectra were recorded with a Raman microscope using a 20× objective of numerical aperture 0.4, at work distance of
Fig. 5. Raman intensity of R6G at different concentrations (10−3 M, 10−5 M, and 10−6 M) with CNT arrays coated by Au nanoparticles (65 nm diameter) as substrate. 20 February 2014 / Vol. 53, No. 6 / APPLIED OPTICS
1161
Table 1.
Characterized Raman Peaks
−1
Raman Shift (cm )
Reason
609, 771, 1180
Assigned to C–C–C ring in-plane bending, out-of-plane bending motion of the hydrogen atoms of the xanthene skeleton, and C–C stretching vibrations [24] C–H in-plane bend [25] The combination of the four stretching modes [25] The aromatic C–C stretching vibrations [24,25]
1125 1309 1361, 1505, 1566, 1645
5.46 mm, with input laser of wavelength 632.8 nm [22,23]; the laser power incident on the sample surface was typically in the range 0.5–4.0 mW. The integration time was 6 s. SERS measurements were carried out with Rhodamine 6G (R6G) as a probe molecule. To reduce the effect of background light, we used 4th polynomial fitting to the initial Raman spectrum data. A.
Different R6G Concentration Effect
To demonstrate the performance of our substrates and investigate whether the SERS signals are related to molecule concentration, we tested the SERS sensitivity using different concentrations of R6G solution of 10−3 M, 10−5 M, and 10−6 M. The result with 3D CNT arrays coated by Au nanoparticles (65 nm diameter) is shown in Fig. 5. Distinctive Raman shifts at 609, 771, 1125, 1180, 1309, 1361, 1505, 1566, and 1645 cm−1 were observed, which are associated with the characteristic vibration modes, shown in Table 1. For typical Raman peaks, we can see that lower concentrations produce smaller Raman peaks intensity (given in Table 2), which is due to fewer probe analytes within the laser focus volume.
large enhancement may be ascribed some reasons: (1) the 3D CNT arrays structure with high-density tubes provide much larger surface areas for loading Au nanoparticles. The increased adsorption of R6G molecule in the carbon nanotubes causes stronger Raman signal in the given excitation area. Thus, this geometric effect can contribute to the enhancement. (2) The inter-distance between Au nanoparticles and CNT arrays allows the formation of “hot spots”; as a result, the local electromagnetic effect is obtained. (3) The periodic order 3D nanostructures might
B. 3D CNT Substrates Compared with 2D CNT and Planar Substrates
To compare 3D CNT arrays/Au enhancement with 2D substrates, we fabricated 2D CNT films (detailed fabrication steps in Ref. [8]) and planar glass coated by Au nanoparticles. Figure 6(a) shows the SERS spectra of R6G at a concentration of 10−5 M adsorbed on the three substrates, with a Au nanoparticles diameter of 80 nm, and Raman peaks intensity data in Table 3. For 10−6 M R6G, the result is shown in Fig. 6(b) with 3D and 2D CNT substrates. We can see that the performance of 3D CNT arrays substrates is better than that of 2D CNT films and planar glass substrates. The maximum intensity ratio occurs at 1505 cm−1 , which is 5.67 (3D CNT/2D CNT) and 3.78 (3D CNT/Glass), respectively. The
Table 2.
Typical Raman Peaks Intensity at Different Concentrations
Raman Shift (cm−1 ) R6G 10−3 10−5 10−6
1162
M M M
∼1180
∼1309
∼1361
∼1505
∼1566
∼1645
1422 685.7 339.9
2697 1533 731
5683 2754 1290
6681 3584 1614
1826 1371 465.2
3337 1544 668.5
APPLIED OPTICS / Vol. 53, No. 6 / 20 February 2014
Fig. 6. Raman intensity of R6G at a concentration of (a) 10−5 M adsorbed on three different substrates (CNT arrays/Au, CNT films/ Au, glass/Au substrates), (b) 10−6 M adsorbed on two CNT substrates, with the Au nanoparticle diameter at 80 nm.
Table 3.
Typical Raman Peaks Intensity with Different Substrates
Table 4.
−1
Raman Shift (cm ) Substrates
∼1180
∼1309
∼1361
∼1505
∼1566
∼1645
3D CNT 2D CNT Glass
1713 767.5 481.1
4136 1233 1146
7009 1308 1940
9047 1596 2401
2152 851.6 577.7
3767 1270 1048
enhance the excitation light trapping in the CNT arrays, consequently increasing the light interaction with Au nanoparticles [26]. C.
Au Nanoparticle Size Effect
Au nanoparticle size relates to the substrate roughness and its own optical properties, which contribute to the SERS intensity [21]. In this paper, we prepared different diameter Au nanoparticles: 30, 65, and 80 nm. As for 3D CNT arrays, the Raman intensity of R6G (10−6 M) is given in Fig. 7, where we can see the intensity with 80 nm Au nanoparticles has the best performance (detailed data in Table 4). The same effects of Au size for 2D CNT films and planar glass are shown in Figs. 8(a) and 8(b). According to Eq. (4), the CNT arrays have much larger (∼103 X) surface areas than planar substrates, but the SERS experimental results indicate little enhancement, which might be due to the majority of Au nanoparticles being on the top surface of the CNT arrays, with relatively few Au nanoparticles penetrating down along the CNT walls. Generally, the SERS effects are strongly dependent upon materials, shape, excitation source, and environment. For very small particles, electronic scattering at the surface results in the depletion of electrical conductivity, thus degrading the plasmon resonance quality. Meanwhile, the SERS sensitivity exhibits a correlation with the size and the SERS enhancement diminishes for larger platelets [27]. The
−6
Fig. 7. Raman intensity of R6G (10 M) while 3D CNT arrays coated by Au nanoparticles (30, 65, and 80 nm diameter).
Typical Raman Peaks Intensity with Different Size Au Nanoparticles
Raman Shift (cm−1 ) Au Diameter (nm) ∼1180 ∼1309 ∼1361 ∼1505 ∼1566 ∼1645 80 65 30
647.1 1714 339.9 731 106.6 253.3
2928 3515 1290 1614 378.5 416
1054 465.2 213
1469 668.5 173.3
SERS intensity of pyridine continues to increase (range from 20 to 135 nm), with a maximum at about 135 nm, when the excitation wavelength is 632.8 nm. A calculated enhancement keeps on increasing with the increase in particle size until about 110 nm [21]. In our experiments, the Raman intensity with 80 nm diameter nanoparticle substrates has the better performance, which agrees with previously reported research. Here, due to aggregation phenomena, it is difficult to prepare larger Au nanoparticle sizes (∼100 nm). The present study describes only three
Fig. 8. Raman intensity of R6G (10−5 M) with (a) planar glass and (b) 2D CNT films coated by Au nanoparticles (30, 65, and 80 nm diameter) as substrates. 20 February 2014 / Vol. 53, No. 6 / APPLIED OPTICS
1163
different nanoparticle diameters; therefore, further experimentation with different diameters is required to get an optimization size for SERS enhancement. D.
CNT Arrays Effect
There are three obvious Raman peaks for CNTs: 1338 cm−1 (D band), 1571 cm−1 (G band), and 2673 cm−1 (G0 band) [28]. The samples here consist of three materials: CNT arrays, Au nanoparticles on the surface of CNT arrays walls, and R6G on the surface of Au nanoparticles. When the laser excites the sample, the molecules (R6G) at the top surface have the greatest contribution to the Raman signal inside the focus volume, which leads to no obvious CNTs Raman peaks in the R6G experiments. The optimization of CNT arrays height/length and optical system is not yet discussed, which should be carried on later.
7. 8.
9.
10.
11.
12.
5. Conclusion
We demonstrated a sensitive SERS substrate that can be easily fabricated in large areas and at low cost by CVD for CNT arrays, and a hydrothermal method for Au-sol solution preparations. Using different Au sizes, different R6G concentration effects on detection were experimentally studied, which demonstrated that 3D substrates with the same Au size are better than 2D substrates for this purpose. These results contribute to the body of knowledge with respect to SERS phenomena. We thank Prof. Liwei Lin and Dr. Yingqi Jiang at the University of California at Berkeley for sample fabrication; Prof. Gang Chen at Chongqing University for Raman experimental help; and Prof. Mingmei Wu at Sun Yat-sen University for SEM help. This study is funded by National Science Fund (61376121), CSTC 2011BB2076, the Fundamental research Funds for the central Universities (106112013CDJZR 125502, 20003) and Visiting Scholar Fund of Key Laboratory of Optoelectronic Technology & System, Education Ministry of China.
13.
14.
15.
16. 17. 18. 19.
References 1. M. Fleischmann, P. J. Hendra, and A. J. McQuillan, “Raman spectra of pyridine adsorbed at a silver electrode,” Chem. Phys. Lett. 26, 163–166 (1974). 2. D. L. Jeanmaire and R. P. Van Duyne, “Surface Raman spectroelectrochemistry: part I. Heterocyclic, aromatic, and aliphatic amines adsorbed on the anodized silver electrode,” J. Electroanal. Chem. 84, 1–20 (1977). 3. Y. B. Guo, M. K. Khaing Oo, K. Reddy, and X. D. Fan, “Ultrasensitive optofluidic surface-enhanced Raman scattering detection with flow-through multihole capillaries,” ACS Nano 6, 381–388 (2012). 4. L. Zhang, X. Lang, A. Hirata, and M. Chen, “Wrinkled nanoporous gold films with ultrahigh surface-enhanced Raman scattering enhancement,” ACS Nano 5, 4407–4413 (2011). 5. Y. Jiao, J. D. Ryckman, P. N. Ciesielski, C. A. Escobar, G. K. Jennings, and S. M. Weiss, “Patterned nanoporous gold as an effective SERS template,” Nanotechnology 22, 295302 (2011). 6. S. Chan, S. Kwon, T. W. Koo, L. P. Lee, and A. A. Berlin, “Surface-enhanced Raman scattering of small molecules from 1164
APPLIED OPTICS / Vol. 53, No. 6 / 20 February 2014
20. 21.
22.
23. 24.
25.
silver-coated silicon nanopores,” Adv. Mater. 15, 1595–1598 (2003). C. J. Choi, Z. Xu, H. Y. Wu, G. L. Liu, and B. T. Cunningham, “Surface-enhanced Raman nanodomes,” Nanotechnology 21, 415301 (2010). J. Zhang, Y. L. Chen, T. Fan, and Y. Zhu, “Large area Au decorated multi-walled CNTs film for surface enhanced Raman scattering,” Key Eng. Mat. 562, 826–831 (2013). Y. H. Sun, K. Liu, J. Miao, Z. Y. Wang, B. Z. Tian, L. N. Zhang, Q. Q. Li, S. S. Fan, and K. L. Jiang, “Highly sensitive surfaceenhanced Raman scattering substrate made from superaligned carbon nanotubes,” Nano Lett. 10, 1747–1753 (2010). S. Lee, M. G. Hahm, R. Vajtai, D. P. Hashim, T. Thurakitseree, A. C. Chipara, P. M. Ajayan, and J. H. Hafner, “Utilizing 3D SERS active volumes in aligned carbon nanotube scaffold substrates,” Adv. Mater. 24, 5261–5266 (2012). G. T. Duan, W. P. Cai, Y. Y. Luo, and Y. Lei, “Hierarchical surface rough ordered Au particle arrays and their surface enhanced Raman scattering,” Appl. Phys. Lett. 89, 181918 (2006). X. Li, G. Chen, L. Yang, Z. Jin, and J. Liu, “Multifunctional Au-coated TiO2 nanotube arrays as recyclable SERS substrates for multifold organic pollutants detection,” Adv. Funct. Mater. 20, 2815–2824 (2010). W. D. Li, F. Ding, J. Hu, and S. Y. Chou, “Three-dimensional cavity nanoantenna coupled plasmonic nanodots for ultrahigh and uniform surface-enhanced Raman scattering over large area,” Opt. Express 19, 3925–3936 (2011). M. Becker, V. Sivakov, U. G. Gösele, T. Stelzner, A. Gudrun, H. J. Reich, S. Hoffmann, J. Michler, and S. H. Christiansen, “Nanowires enabling signal-enhanced nanoscale Raman spectroscopy,” Small 4, 398–404 (2008). M. K. Lee, J. Seo, S. J. Cho, Y. Jo, S. Kim, Y. J. Kang, and H. Lee, “Novel 3D arrays of gold nanostructures on suspended platinum-coated carbon nanotubes as surface-enhanced Raman scattering substrates,” Mater. Lett. 81, 9–12 (2012). P. L. Stiles, J. A. Dieringer, N. C. Shah, and R. P. Van Duyne, “Surface-enhanced Raman spectroscopy,” Annu. Rev. Anal. Chem. 1, 601–626 (2008). G. Frens, “Controlled nucleation for the regulation of the particle size in monodisperse gold suspensions,” Nat. Phys. Sci. 241, 20–22 (1973). D. Y. Wu, J. F. Li, B. Ren, and Z. Q. Tian, “Electrochemical surface-enhanced Raman spectroscopy of nanostructures,” Chem. Soc. Rev. 37, 1025–1041 (2008). Z. Krpetic, L. Guerrini, I. A. Larmour, J. Reglinski, K. Faulds, and D. Graham, “Importance of nanoparticle size in colorimetric and SERS-based multimodal trace detection of Ni (II) ions with functional gold nanoparticles,” Small 8, 707– 714 (2012). L. Liz-Marzán, “Nanometals: formation and color,” Mater. Today 7(2), 26–31 (2004). P. P. Fang, J. F. Li, Z. L. Yang, L. M. Li, B. Ren, and Z. Q. Tian, “Optimization of SERS activities of gold nanoparticles and gold-core–palladium-shell nanoparticles by controlling size and shell thickness,” J. Raman Spectrosc. 39, 1679–1687 (2008). J. T. Krug, G. D. Wang, S. R. Emory, and S. M. Nie, “Efficient Raman enhancement and intermittent light emission observed in single gold nanocrystals,” J. Am. Chem. Soc. 121, 9208–9214 (1999). X. M. Lin, Y. Cui, Y. H. Xu, B. Ren, and Z. Q. Tian, “Surfaceenhanced Raman spectroscopy: substrate-related issues,” Anal. Bioanal. Chem. 394, 1729–1745 (2009). X. M. Zhao, B. H. Zhang, K. L. Ai, G. Zhang, L. Y. Cao, X. J. Liu, H. M. Sun, H. S. Wang, and L. H. Lu, “Monitoring catalytic degradation of dye molecules on silver-coated ZnO nanowire arrays by surface-enhanced Raman spectroscopy,” J. Mater. Chem. 19, 5547–5553 (2009). P. Hildebrandt and M. Stockburger, “Surface-enhanced resonance Raman spectroscopy of Rhodamine 6G adsorbed
on colloidal silver,” J. Phys. Chem. 88, 5935–5944 (1984). 26. C. W. Cheng, B. Yan, S. M. Wong, X. L. Li, W. W. Zhou, T. Yu, Z. X. Shen, H. Y. Yu, and H. J. Fan, “Fabrication and SERS performance of silver-nanoparticle-decorated Si/ZnO nanotrees in ordered arrays,” ACS Appl. Mater. 2, 1824–1828 (2010).
27. H. Ko, S. Singamaneni, and V. V. Tsukruk, “Nanostructured surfaces and assemblies as SERS media,” Small 4, 1576– 1599 (2008). 28. R. A. DiLeo, B. J. Landi, and R. P. Raffaelle, “Purity assessment of multiwalled carbon nanotubes by Raman spectroscopy,” J. Appl. Phys. 101, 064307 (2007).
20 February 2014 / Vol. 53, No. 6 / APPLIED OPTICS
1165
