Surface-enhanced Raman scattering properties of multi-walled carbon nanotubes arrays-Ag nanoparticles
Introduction
The phenomenon of surface-enhanced Raman scattering (SERS) was firstly discovered on rough Ag electrode surface by Fleischmann M. in 1974 [1]. Owing to its specificity of molecular fingerprint identification and high sensitivity, SERS is an established analytical tool for chemical and biological sensing [2], capable of single-molecule detection [3], [4]. There are two main mechanisms which are commonly believed to generate: electromagnetic enhancement (EM, of the order 106–108) and chemical enhancement (CM, of the order 102) [5]. The latter is based on the interaction between the organic molecules and the proximal end of nano-metallic structures, and it is of smaller magnitude [3], [5]. The EM effect is related to local electromagnetic field enhancement in the hot spots, which are found in gaps between metal nanoparticles (MNPs) in dimers, aggregates and large fractal structures [6], [7], [8], also found in the metal nanowires and nanoclusters [9], [10], [11]. Fabrication of MNP-based structures with cost-effectiveness, multiple access to such characterizations still remains a challenge. It can be addressed by incorporating another source of surface plasmons (SPs) with those of noble metals [12].
Graphene and carbon nanotubes (CNTs) have been suggested as potential SP enhancers [13], [14], [15], [16]. Graphene can be grown only on a few specific metallic substrates and then transferred onto a target substrate, mostly using the polymer supportive layer-based transfer [14]. However, this transfer process has some drawbacks such as the presence of polymer residues, cracks and non-uniformity [17], [18]. Carbon nanotube forests, on the contrary, can be fabricated on a wider variety of substrates with good quality [19], [20], [21]. Therefore, the CNT forests structure is a good choice as plasmonic component to meet the requirements for intensive light scattering properties. Recently, the plasmonic properties of CNTs have been studied on individual tubes coated by gold nanoparticles (AuNPs) [22], [23], [24], horizontal networks of CNTs on gold surface [25], [26], nanotube forests covered by gold nanoparticles [27], [28], [29], hybrid of CNT forests and AuNPs [12] and CNT arrays coated by Ag nanoparticles (AgNPs) [30], two-dimensional single-walled carbon nanotube (SWNT) network decorated with Ag nanoparticles [31], [32], SWNT coated by gold nanoparticles [33], and carbonaceous nanotubes modified with titania nanocrystals [34]. However, the limitation of one dimensional (1D) and two dimensional (2D) CNTs SERS substrates is insufficient quantity of hot spots in planar geometry, and the incident laser needs to be accurately focused on the correct plane to obtain optical Raman signal. The three dimensional (3D) CNT hybrids with noble metal nanoparticles actively increase the number and utility of SERS hot spots [35], [36], [37], in which more target molecules can be adsorbed and detected in 3D laser focus volume along the wave vector direction [38], [39], [40]. Therefore, the 3D hybrids have higher tolerance for inaccurate focus along the wave vector direction [41]. In our previous studies, the vertically aligned multi-walled carbon nanotubes (MWNTs) arrays were used as a framework for AuNPs/AgNPs to act as SERS substrates [30], [42], and the sensitivity, reproducibility and stability were tested.
In this paper, we further investigate the properties of the multi-walled carbon nanotubes arrays decorated with silver nanoparticles (MWNTs-AgNPs) from the following aspects: (1) we characterize the structures by transmission electron microscopy (TEM) and Raman measurements, and analyze in detail the amplification of Raman signal intensity caused by Ag nanoparticles SPs. (2) We test the SERS properties using 4-mercaptobenzoic acid (4-MBA) as standard analyte. SERS mapping is adopted to study the uniformity of the hybrid structures and investigate the influence induced by molecule solution during evaporation process. The detection concentration level of our study is down to 10−10 M. (3) Using finite-difference time-domain (FDTD), we study the electric field distribution and enhancement on a hybrid system of MWNTs and five layers AgNPs supported by a silica substrate. There is a strong electromagnetic interaction among nanoparticles, and between MWNTs and nanoparticles, which is favorable for Raman enhancement.
Section snippets
Preparation of MWNTs-AgNPs
Our 3D SERS substrate is shown in Fig. 1, vertically aligned MWNTs arrays coated by Ag nanoparticles. The vertically aligned MWNTs arrays were synthesized on a Si substrate by chemical vapor deposition (CVD). The Ag nanoparticles were formed by magnetron sputtering and high temperature annealing process; detailed process was described in Supplementary Materials.
Characterization and Raman measurements
The surface morphologies of the prepared samples were characterized using a field emission scanning electron microscopy (FESEM, JEOL
Characterization of MWNTs-AgNPs
The FESEM image of the side of vertically aligned MWNTs arrays coated with Ag nanoparticles is shown in Fig. 2(a). The average length and diameter of the MWNTs are approximately 15 μm and 15 nm, respectively. The average diameter of the Ag nanoparticles is 15 nm, and nanoparticles consist of different shapes such as rod, sphere and spheroid. Highly dense AgNPs deposited along the perpendicular axis of MWNTs are also illustrated in Figs. S1 and S2 (see Supplementary Materials). It is clear that
Conclusions
In conclusion, we have developed a hybrid structure of MWNTs-AgNPs as potentially effective 3D SERS substrate, which yielded extremely high SERS activity, with the detection limit of 4-MBA down to 10−10 M. Meanwhile, the calculated EF is up to 4.1 × 107. With the results of FDTD simulations, two kinds of hot spots contribute to the EFEM of about 2.7 × 107. This work implies that the MWNTs-AgNPs hybrid structures have potential applications in SERS. For more stable enhancement with higher
Acknowledgments
We would like to thank Prof. L.W. Lin at University of California @Berkeley for MWNTs arrays sample help, Mr. X.Q. Qi from the College of Chemistry and Chemical Engineering in Chongqing University for SEM and Raman spectrometer help. This research is funded by National Natural Science Foundation of China (No. 61376121), National Natural Science Foundation of Chongqing (CSTC2015JCYJBX 0034), and National High-tech R&D Program (863 Program, No. 2015AA034801).
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