Submicron patterns obtained by thermal-induced reconstruction of self-assembled monolayer of Ag nanoparticles and their application in SERS
Abstract
The layer-by-layer (LbL) self-assembly technique was employed for the deposition of poly(diallyldimethylammonium chloride) (PDDA) and triangular Ag nanoplates on glass substrates. A thermal-induced reconstruction of these polyelectrolyte-linked nanoparticle (NP) films was presented. Before the reconstruction, triangular Ag nanoplates were distributed uniformly on the surface with an average interval of 50 ± 15 nm. After the reconstruction, the triangular Ag nanoplates accumulated into discrete stacks with an average interval of 90 ± 25 nm. The temperature-dependent experiments were done and the optimal temperature for the formation of the reconstructed patterns was 120 °C. The possible mechanism of the NP movement and stacking was analyzed. Under the experimental conditions, a hydrophobic environment was formed because of the vacuum and heating. As a result the polyelectrolyte-linked Ag NPs preferred to congregate due to the lowered surface energy. Finally the submicron patterns were formed. The ultraviolet-visible (UV-vis) absorption and surface-enhanced Raman scattering (SERS) properties of the films before and after the reconstruction was investigated. The reconstructed films with submicron patterns had better SERS enhancement ability, which was 1300 times to the original films. The reconstruction method of the monolayer films showed great potential in the surface design and related applications.
AFM images were obtained to clarify the three dimensional structures of the reconstructed films obtained at 120 °C. As shown in Fig. 2, the Ag NP stacks had an average diameter of 1.0 ± 0.2 μm and an average height of 170 ± 30 nm. The diameter and height of the stacks were shaped by the aggregates of tens of triangular Ag nanoplates. The AFM cross-sectional contour showed the clear intervals of the stacks, which was corresponding to the SEM characterization.XRD patterns of the polyelectrolyte-linked NP films before and after thermal post-treatment are showed in Fig. 3. The peak at 38.1° was corresponding to the diffraction of the {1 1 1} plane of Ag NPs. No other diffraction peaks was observable because the Ag NPs grew at the {1 1 1} plane preferred in our synthesis method. The intensity of the peaks changed slightly, which is attributed to the change of the orientation of NPs.The possible mechanism for the reconstruction was discussed. In our experiments, the as-prepared polyelectrolyte-linked NP films were located in a very hydrophobic environment in the post-treatment process. To minimize interfacial free energy, both the polar groups of PDDA and the Ag NPs preferred to accumulate to lower the surface area, viz. surface tension [22,23]. As a result, the uniform films converted to the discrete islands on glass substrates. The mechanism is illustrated in Fig. 4.It should note that the NPs did not undergo an obvious phase change in our experimental conditions. From the SEM in Fig. 1(b) it can be seen that the shape of triangular silver nanoplates did not change perceptibly. Moreover, the XRD patterns in Fig. 3 also did not show big changes in the peak intensities. The slight intensity change of the diffraction peaks at 38.1° is assigned to the change of the orientation of NPs, which happens usually in any reconstruction of NP films.The reconstruction of the NP films provides an opportunity to produce surface modifications which have applications in optical properties. The SPR and enhanced spectroscopy properties were investigated by UV-vis absorption and SERS, respectively. Fig. 5(a) shows the UV-vis absorption spectra of the Ag colloid, Ag NP films before and after the post-treatment at 120 °C. In the visible region the triangular Ag nanoplates have two adsorption bands centered at 675 and 490 nm. These absorption bands are assigned to be the in-plane dipole and the in-plane quadruple plasmon resonances, separately [24]. After the deposition of the triangular Ag NPs on glass slides, the UV-vis absorption bands appeared to be 627 and 454 nm, which are similar to the reported results [24]. When the Ag NP films underwent the thermal post-treatment at 120 °C, an absorption band at 410 nm was generated, which is generally assigned to be the strong coupling SPR of Ag NP aggregates. The relative stronger absorption near 514.5 nm comparing with the Ag NP films before the reconstruction could contribute more enhancement with the excitation line of 514.5 nm when the films were used as SERS substrates.Fig. 5(b) shows the SERS spectra of 4-ATP adsorbed on the Ag NP films before and after the reconstruction. The SERS measurements were prepared by drop coating of 10 μL of 4-ATP (10-5 mol/L) ethanol solution onto the reconstructed films and 10 μL of 4-ATP (10-3 mol/L) ethanol solution onto the original films, separately. Then they were dried naturally. 10 (±0.5) mm diameter circular blots were formed on the substrates. The peak at 1077 cm-1 was chosen for quantitative comparison of the enhancement ability of these films. Because the enhancement is proportional to the concentration of the 4-ATP molecule and the intensity of the SERS signal in our experiments, it is easy to obtain the quantitative comparison for the NP films. As results, the enhancement of the reconstructed films is 1300 times to the original films. The reconstruction of the NP films created more “hot spots” by the aggregation of Ag NPs, which has great contribution to the enhancement according to the electromagnetic mechanism of SERS. Other delicate works also showed the importance for the engineering fabrication of nanogaps with plasmonic materials [25-27]. Our method shows potential application to the fabrication of high-active SERS substrates via a convenient and inexpensive way.- Publication:
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Applied Surface Science
- Pub Date:
- August 2014
- DOI:
- 10.1016/j.apsusc.2014.05.034
- Bibcode:
- 2014ApSS..309..295R