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Abstract
Using a self-encapsulation technology, we prepared a metal film–coupled nanoisland system (FCN), that is, Ag and Al2O3 film–coupled Ag nanoislands (AgNIs/Al2O3/Ag) composite SERS (surface-enhanced Raman scattering) substrate, through the anti-wetting of Ag film on the surface of Al film. The thickness of the Al2O3 film can be controlled within the range of 4 nm to 22 nm by tuning the annealing temperature. Three important properties were investigated. Firstly, the structure shows an excellent near-field and far-field enhancement using COMSOL Multiphysics simulation. Secondly, the experimental SERS analytical enhancement factor (AEF) of the AgNIs/Al2O3/Ag substrate can reach 3.9 × 108, two orders of magnitude larger than that of bare AgNIs. Thirdly, after exposed in air for 90 days, it can keep 55% enhancement capability, while the bare AgNIs can keep 16%.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) is considered as a powerful analytical tool in the detection of trace molecules with high sensitivity [1]. In SERS applications, highly enhanced electric field can be produced at the nanogap between metal nano structures due to the localized surface plasmon resonance (LSPR) [2,3], called “hot spots”. In general, noble metal (Ag/Au) is widely used as SERS-active substrate [4–7], its performance of LSPR is determined by its size, shape and gap between two nanoparticles [8–12].
Recently, metal nanoislands have exhibited high and reproducible SERS performance making them excellent candidates as SERS-active substrates [13–15]. However, the field enhancement (EF) of Ag nanoislands was generally considered to be rather modest (104–105) [16,17]. Meanwhile, the oxidation of metals also has been a difficult issue for researchers [18,19]. Hence, the fabrication of the SERS substrates with high-density, uniform, large area “hot spots”, and anti-corrosion is still a challenge [20–24]. To further improve the properties, Šubr M et al. [25] reported an Ag nanosiland/polytetrafluoroethylene/metal structure as a highly uniform and sensitive SERS-active substrate, the top layer (Ag nanoislands) is responsible for the SERS effect, the bottom layer (Ag film) is not only a reflective layer, but also a plasmonic material and plasmonic waveguide modes can be excited between the AgNIs and Ag film [26], the interlayer is a dielectric separation layer working as a Fabry-Perot (F-P) cavity, the experimental EF was improved by one order of magnitude. The F-P cavity structure can provide additional Raman enhancement, because it creates the interconnection between LSPR from Ag nanoislands and SPP due to the dielectric layer [27]. The dielectric layer can also be an isolation layer for oxygen. Obviously, organic materials can neither resist high temperatures nor isolate oxygen completely. Hence, a self-encapsulation technology [28–30] can be utilized to conveniently form a smooth metal oxide layer on the metal film by thermal diffusion effect, which ensures the stable resonant mode in the cavity, the isolation of oxygen, and the high temperature resistance. It could be a potential way to prepare SERS substrates.
In this paper, we proposed an AgNIs/Al2O3/Ag composite SERS substrate using self-encapsulation technology, in which a nanometer-thick Al2O3 film was employed as the F-P cavity. A finite element method (COMSOL Multiphysics) was used to simulate the optical characteristics of LSPR and SPP, and the distribution of the near-fields and far-fields of this hybrid structure. Moreover, The SERS activities were investigated with Rhodamine 6G (R6G) as probe molecules.
2. Preparation and characterization
The preparation steps are shown in Fig. 1(a). Firstly, using a magnetron sputtering method, an Al film (thickness of 15 nm) and Ag film (thickness of 150 nm) was subsequently deposited on a SiO2 (thickness of 300 nm) substrate. Secondly, using an annealing method in ultra-low concentration of O2 (cavum of 10 m Torr), the sample was annealed at 600°C for 1 h with 2 l/min Ar condition. Then the temperature was naturally cooled down to room temperature. Thirdly, another Ag film with thickness of 13 nm was deposited. Fourthly, second annealing was carried on at 400°C for 30 min with Ar condition. Finally, the Ag film coupled AgNIs was obtained, and its SEM image is shown in Figs. 1(b) and 1(c).
Fig. 1. (a) Preparation steps of FCN (Ag film coupled AgNIs) sample, SEM images of (b) FCN and (c) Al/Ag/Al2O3 composite SERS substrate, and (d) an ellipsometer characterization of Al2O3 thickness on the surface of Ag film along with the annealing temperature.
There are two points to be emphasized. (1) During the first annealing, ultra-low concentration of O2 is very important. (2) In order to keep the smoothness of Al2O3 film, the second annealing temperature must be lower than that during the first annealing. The temperature during the first annealing process affects the thickness of Al2O3. With an ellipsometer, the thickness of Al2O3 at different annealing temperatures is shown in Fig. 1(d).
In addition to the driving force of Al diffusion caused by the concentration gradient, the thermal expansion mismatch between the films and the substrate the high stress field caused by heating the sample must be considered as another important factor. Based on discussions above, upon annealing (600°C),the Al atoms would diffuse to the surface of Ag film, and react with oxygen atoms, finally, a Al2O3 film was formed on the surface of Ag film, called a self-encapsulation process. The self-encapsulation process of Al/Ag double metal films can be analyzed with planar layer diffusion theory within a limited volume. (1) During the diffusion process, Ag atom would catch Al atom resulting in low residual Al concentration in silver layer. When Al diffused to the surface of Ag surface, Al atom would be consumed. The amount of Al atom transferred through Ag film would determine the thickness of Al2O3. The thickness would increase quickly at the early stage, then tend to be stable. A longer annealing time leads to a thicker Al2O3 thickness. (2) In addition, at the same annealing temperature and during time, the thickness of Al2O3 is inversely proportional to the thickness of Ag film. (3) At the same annealing during time, higher annealing temperature leads to thicker Al2O3 film. (4) The diffusion energy of Al atom can effectively eliminate the stress inside Ag film, and prevent Ag atoms from agglomerating at high temperature to form Ag nanoislands. So that the surface of Ag film maintains its smoothness at high temperature.
3. Results and discussion
3.1 Near-field optical properties of FCN
Using COMSOL Multiphysics, we studied the electric field distributions of the FCN system. The incident laser is 633 nm, polarized at axis x, and transmitted at axis -z. The refractive index of Ag and Al2O3 is -14.461-1.1936·i and 1.75. The thickness of Ag film and Al2O3 is 150 and 10 nm.
3.1.1 Two plasmon resonance modes
It is well known that scattered light of different orders and directions will produce different wavevectors. Here, there are two localized surface plasmon resonance modes [31]. One is called LSPR mode in free space (mode_FS), and the other one is called plasmonic waveguide mode in nano cavity between AgNIs and Ag film (mode_NC).
The mode_FS includes a dipole mode and dipole-dipole interaction. The dipole mode refers to the electric field distributed at the upper hemisphere of the nanoislands, which comes from incident light excitation and reverse excitation by plasmonic waveguide mode [32]. Dipole-dipole interaction is the propagating modes (plasmonic waveguide modes) couple energy back into the AgNIs in a reciprocal process [32]. It can be estimated as fm(λ)σ(λ), where σ(λ) is the measured scattering spectrum of the isolated island layer, and fm(λ) is the fraction of energy emitted [33], each nanoisland can receive energy from its neighbors to achieve this coupling effect. As for the mode_NC, nanoisland contributes to an irrational source of SPP model of metal surface, resulting in a transmission SPP along the surface of Ag film. The nano cavity in the AgNIs/Al2O3/Ag structure is similar to a F-P cavity, which makes SPP form a huge electric field in the nano cavity.
When the diameter of nanoisland is smaller (40, 80 nm), the plasmonic waveguide mode will process multi-reflection and irradiation at the corners of the nanoisland [34]. The maximum value of E-field in these regions exceeds 100 (diameter of 80 nm, shown in Fig. 2(b)). With the increase of the AgNI diameter, the E-field at the corners of the nanoisland reduces sharply, and the localized E-field tends to be located at the nano cavity. In SERS applications, only the electric field region contacting the probe molecules can achieve the enhancement of Raman signal. Therefore, the hotspots which can achieve Raman enhancement are the hotspots at the corners of the nanoisland, dipole hotspots and dipole-dipole interaction.
Fig. 2. (a) FCN system simulation model; (b) simulation results of electromagnetic field with different diameters of AgNI from 40 nm to 480 nm, the electric field diagrams of 40 and 80 nm enlarged in size to be seen clearly.
3.1.2 SPP wavelength induced by Ag nanoislands
In the FCN system, the metal film can affect the dipoles interaction of AgNIs. There are three interaction coupling modes, a mode in near-field, a mode in far-field, and a mode based on SPP. The interaction coupling modes in near-field and far-field are similar to traditional AgNIs (without Al2O3 layer). However, the mode based on SPP is the energy transmission between AgNIs and Ag film, which can complete the interaction coupling between AgNIs. In order to see clearly the plasmon wave distribution, the electrical field distribution of Ez in the nano F-P cavity is shown in Fig. 3, with different diameters from 40 nm to 480 nm.
We know a classical plasma wavelength λspp is calculated by [35]
Based Eq. (1), the calculated λspp is 321 nm, shown in Fig. 3, there is a wave peak and wave valley at ∼-27 nm and ∼+27 nm with larger AgNI diameter, hence the calculated wavelength of SPP is ∼110 nm, which decreased sharply compared with the traditional λspp (321 nm), the reason can be attributed to the compression function of F-P cavity [36].
We know that for MIM (metal-insulation-metal) waveguide structures, we can tune the plasmon wavelength by tuning the thickness of the insulation layer. A greater thickness of the dielectric layer leads to a larger plasma wavelength [36]. The FCN has similar properties as well. In addition, the contact angle between the Ag nanoisland and the surface of the dielectric layer will also affect the plasma wavelength. Taking the thickness of Al2O3 and contact angle into consideration, the plasmon wavelength in the nano F-P cavity can be calculated by
Where f(T) represents the thickness influence factor, g(θ) is the angle influence factor.The E-field distributions of Ez with different thickness of Al2O3 (θ was set to 34.3°) and different contact angles (the thickness was set to 8 nm) are shown in Figs. 4(a) and 4(c), and the linear fits of plasmon wavelength as functions of the thickness and contact angle are shown in Figs. 4(b) and 4(d), λFPspp=34.8 + 34.5 T, λFPspp=-39.6 + 121.5sinθ, with R2 of 0.96533 and 0.91035, respectively. Note that the resonant wavelength λspp is proportional to sinθ [35].
Fig. 4. E-field distributions Ez with (a) different thickness and (c) different contact angles, the corresponding linear fits of the plasmon wavelength as a function of (b) thickness and (d) contact angle. The diameter of AgNI was set to 160 nm.
Based on the Eqs. (1) and (2) and the fitting results mentioned above, the plasmon wavelength of the FCN sample can be calculated by
3.2 Far-field optical properties of FCN sample
An improvement of Raman spectrum collective efficiency is a complementary method to enhance Raman scattering, some specially designed nanostructures can effectively improve the collection of Raman signals [37] by one order of magnitude [38]. Here, a Raman collective efficiency can be calculated using the beam efficiency (BE):
In order to study the collective efficiency of FCN sample, we investigated the far-field optical properties of FCN sample using COMSOL Multiphysics simulation. A probe molecule was set as a point dipole, and it was set at the center of two nanoislands in the FCN system and a bare AgNI system, respectively. The orientation of the electric dipole is positive along the x direction, and the dipole moment is the same in both systems. The free space radiation wavelength of the exciton is 658.5 nm, corresponding to the Raman peak of R6G molecule at 613 cm−1 (incident laser of 633 nm). The far-field radiation mode in the xz-plane and yz-plane of two systems is shown in Fig. 5.
Fig. 5. (a) Normalized radiation patterns simulation of the FCN system, far-field E-field of a bare AgNI system (black line) and FCN system (red line) at (b) xz-plane, and (c) yz-plane.
Compared with the bare AgNI system, the FCN system has a stronger radiation ability and a greater collection efficiency around 90° and 270°. Meanwhile, Raman collection efficiency in FCN system can reach close to 50% which was tested using several objectives with different numerical apertures. The result further illustrated the Raman enhancement capability of the SERS substrate.
3.3 Raman measurements
As for the Raman experimental sample, the thickness of Ag film and Al2O3 layer is 150 and 10 nm, respectively, the calculated size of AgNI is between 44.0-187.3 nm with averaged value of 119.2 nm, the RSD value is 32.4%, the distance between two AgNIs is about 47 nm, and the contact angle is 123°. Two substrates (bare AgNIs for reference sample) were immersed in 10−7 mol/l R6G aqueous solution for three hours, then they were washed with deionized water to remove unabsorbed molecules. Finally, the substrates were dried with a nitrogen gun.
The Raman spectra are shown in Fig. 6(a), where the signal from bare AgNIs sample is enlarged 200 times to be seen clearly. In order to show our sample’s Raman enhancement property, the analytical enhancement factor (AEF) is counted. The AEF represents a simple figure for the SERS EF, whose measurement is easily reproducible, expressed by [39]
Where IRS is a Raman signal under non-SERS condition with R6G concentration cRS. Under the same experimental conditions and preparation conditions, ISERS is a Raman signal under SERS condition with R6G concentration cSERS.Fig. 6. (a) SERS spectrum of 10−7 mol/l (M) R6G on samples of bare AgNIs and FCN. (b) Antioxidation stability of FCN and bare AgNIs as SERS substrates.
The calculated AEF at 1509 cm−1 is 3.9 × 108 and 1 × 106 for FCN and bare AgNIs substrate, respectively. Therefore, due to the existence of the self-encapsulation Al2O3 film in the FCN system, the electric field enhancement capability of the substrate has been greatly improved by two orders of magnitude compared with bare AgNIs system.
An ultra-thin self-encapsulation Al2O3 layer was formed on the surface of Ag film, which greatly improved the oxidation and corrosion resistance of Ag film, so the life of FCN substrate could be greatly extended. using R6G as a probe molecule, the FCN and bare AgNIs substrates were exposed in the air for different days. For bare AgNIs substrate, its Raman enhancement capability decayed rapidly with the increase of the exposure time, and its Raman signal capability decreased by 84% (90 days) due to the rapid oxidation of the Ag atoms. In comparison, the Raman enhancement ability of the FCN substrate decreased by only 45% (90 days) due to the protection of Al2O3 film for the underlying Ag film. The normalized Raman signals at 613 cm−1 are shown in Fig. 6(b).
4. Conclusion
The AgNIs/Al2O3/Ag structure was successfully prepared by a self-encapsulation technology. Its Raman enhancement properties were studied theoretically and experimentally. This system has two advantages: 1) high near-field and far-field enhancement, with the AEF of 3.9 × 108 for R6G; 2) high stability due to the Al2O3 film as a protection layer. Based on the E-field distributions, the diameter of AgNIs, the thickness of Al2O3 and the contact angle have large effects on the distribution of hotspots, researchers would pay more attention to the preparation procedure.
Funding
National Natural Science Foundation of China (61875024).
Acknowledgement
We would like to thank Dr. Gong Xiangnan at Analytical and Testing Centre of Chongqing University for his help in Raman measurement.
Disclosures
The authors declare no conflicts of interest.
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