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Abstract
The weak plasmonic coupling intensity in an aluminum (Al) nanostructure has limited potential applications in excellent low-cost surface-enhanced Raman scattering (SERS) substrates and light harvesting. In this report, we aim to elevate the plasmonic coupling intensity by fabricating an Al nanoparticle (NP)−film system. In the system, the Al NP are fabricated directly on different Al film layers, and the nanoscale-thick alumina interlayer obtained between neighboring Al films acts as natural dielectric gaps. Interestingly, as the number of Al film layers increase, the plasmonic couplings generated between the Al NP and Al film increase as well. It is demonstrated that the confined gap plasmon modes stimulated in the nanoscale-thick alumina region between the adjacent Al films contribute significantly to elevating the plasmonic coupling intensity. The finite-difference time-domain (FDTD) method is used to carry out the simulations and verifies this result.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Collective oscillations of conduction electrons in plasmonic nanomaterials, regarded as localized surface plasmon resonance (LSPR) are capable of the enhancement of electromagnetic field near the metal materials surfaces, resulting in promising properties in the fields, such as photocatalysis, solar energy conversion and spectroscopic sensors [1–5]. Nanoscale gaps called “hot spots”, between plasmonic nanostructures can generate tremendous electric field enhancement, which significantly elevates the performance of SERS, making it possible for the detection of trace analyte [6–9]. Thus, it is reasonable to design plasmonic nanostructures with strong hot spots to promote the plasmon enhanced Raman spectroscopic sensing. These nanostructures, generally made of noble metals (Au, Ag, and Cu), are well represented and perform excellent SERS effect in the visible to infrared wavelength range [10–14]. However, the low abundance and high costs have seriously hindered their applications in practical.
Recently, aluminum (Al), the most abundant metal on earth, has attracted lots of interests as an emerging plasmonic material for the promise to be a substitute to the noble metals [15–17]. Besides its inexpensive and sustainable property, the plasmon resonance of Al support a broader spectrum range from UV to visible light compared with noble metals [18–20]. Moreover, the inherent nanoscale surface oxidation layer of Al is capable of providing binding sites for various functional groups (amides, phosphoric/carboxylic acids, and silanes), which supports affiliative molecule−substrate interactions that completely different from noble metal substrates [21]. Thus, Al-based nano-materials have also been used as SERS substrates in both the UV and visible region for ultra-sensitive sensing recently [21–27]. Especially in the UV region, the Al-based plasmonic materials possess the highest near-field electric field enhancement (EM) and absorption efficiency in this range.
Currently, the fabrication of Al-based SERS substrates can be achieved by several techniques including chemical synthesis, e-beam lithography and template stripping method [28–30]. Chemical synthesis of various-shaped Al nanocrystals have been well conducted, which faces the challenges from complex reactants and aggregation of Al nanocrystals. Meanwhile, e-beam lithography and template stripping method can ensure a high uniformity of the Al nanostructure but along with time-consuming and complicated processes. Moreover, the Al nanostructures, reported so far, are usually 2D arrays and still difficult to stimulate high enough EM for the weak plasmonic couplings between the isolated units. Hence, for these Al nanostructures, the weak hot spot intensity has limited their potential applications as excellent low-cost SERS substrates.
Here, we propose an Al NP−film system aiming at promoting the hot spot intensity for SERS. The systems were constructed with the Al NP on different number of Al film layers and the Al NP was fabricated by facile thermal annealing way. In this system, the nanoscale-thick alumina interlayer obtained between neighboring Al films acted as dielectric gaps. The adenine and crystal violet (CV) molecules were used as probe molecules for the SERS test. We observed that the Al NP−film system exhibit satisfactory SERS performance and optical absorption enhancement from deep UV to the visible region. Generally, the Ag or Au nanostructures exhibit stronger plasmonic resonances for electromagnetic enhancement than Al. And for this structure, the experimental non-resonance EF is calculated to be 3.62 × 105 for adenine and 4.1 × 105 for CV, which is even come up to the noble metals detection levels for non-resonant molecules. The FDTD method is used to carry out the simulations and elucidate the results. The SERS performance can be attributed to the coupling between the localized surface plasmons (LSP) from upper Al NP and the surface plasmon polaritons (SPP) from bottom Al film. And interestingly, the confined gap plasmon modes stimulated in the nanoscale-thick alumina region between the adjacent Al films also make a significant contribution. Thus, our results demonstrate the advantages of Al NP−film system for UV SERS and contribute to broadening the potential application of Al-based plasmonic material in SERS detection and light harvesting.
2. Experiment
2.1 Preparation of different number of Al film layers
The silica substrates were initially cleaned with ultrasonic processing in acetone, alcohol and deionized water for 1 hour respectively to remove the surface contamination. A magnetron sputtering equipment was used to deposit the Al film. In order to avoid the presence of oxide within the Al, the presputter process was first conducted to eliminate the surface oxidation layer of Al target. The Al film with the thicknesses around 6 nm were deposited on the SiO2 substrate with a deposition rate of 1 Å /s under low pressure 1 × 10−6 Pa. To obtain a high-quality, self-terminating, densified oxide layer on top of the Al film, the samples were transferred with the protection of nitrogen to a high vacuum chamber to carry out oxidation treatment by exposing the Al film to high-purity oxygen under low pressure (5 × 10−3 Pa) for 10 min. Repeat the steps above and the different number of Al film layers with the densified alumina layer as nanoscale gap can be obtained.
2.2 Fabrication of the Al NP on different number of Al film layers
The Al nanosheets with the thickness around 2 nm with a deposition rate of 8 Å /s under low pressure 1 × 10−6 Pa were deposited on the surface of Al films. The samples were then transferred with the protection of nitrogen to carry out annealing treatment in 150 °C for 5 min. After the annealing process, the Al nanosheets were transformed into Al NP, and the Al NP was also treated by the oxidation treatment. The Al NP−film systems with different Al film layers were obtained.
2.3 Sample characterization
The morphology of the Al NP on the Al films was obtained by using scanning electron microscope (SEM) (Zeiss Gemini Ultra-55) and Atomic force microscopy (AFM, Park XE100). The Raman spectrometer (Horiba HREvolution 800) was used to collect the SERS signals. The spectrophotometer (Hitachi, U-4000) was selected to collect the absorption bands of the Al NP−film systems. The Transmission Electron Microscope (TEM) and high Resolution TEM (HRTEM) images of Al NP coating with alumina layer were obtained using a transmission electron microscope (JEM-2100).
To evaluate the SERS performance of Al NP−film systems with different Al film layers, adenine and crystal violet (CV) molecules were used. The adenine and CV solution with the concentration from 10−1 to 10−7 M were obtained with the analytes powders dissolving in deionized water. 2 µL adenine or CV solution was dropped directly on the Al NP−film systems for the SERS test. After the droplets dried totally, the SERS signals were collected from each substrate under the conditions (325 nm excitation light, ×40 objective lens, 4 s acquisition times, gratings with 1800 grooves per 1 mm, and 1 µm laser spot).
3. Results and discussion
The fabrication process of the Al NP−film systems is illustrated schematically in Fig. 1, and the details have been exhibited in the Experimental Section. The Al film was first deposited on the SiO2 substrate with the thickness of the Al film around 6 nm. To obtain a densified, high-quality, self-terminating oxide layer on surface of the Al film, the samples are transferred with the protection of nitrogen to a high vacuum chamber to carry out oxidation treatment for 10 min by exposing the Al film to high-purity oxygen under low pressure. The obtained densified alumina layer consumes around 3 nm of Al [31]. Repeat the steps above and we can obtain the different number layer of Al film with the densified alumina layer as nanoscale gap. Then, the Al nanosheets with the thickness around 2 nm with a faster deposition rate are deposited on the alumina/Al films. The samples were carried out an annealing treatment in 150 °C immediately.
Fig. 1. Schematic illustration of the fabrication process of Al NP−film systems with strong hot spots. Apart from the plasmonic couplings between the Al NP and Al film, the confined gap plasmon modes generated in the nanoscale-thick alumina region between the adjacent Al films also elevate the hot spot intensity effectively.
After the annealing process, the Al NP appeared as shown in the SEM image in Fig. 2(b). However, the alumina/Al films (control sample) treated with same annealing process keep nearly no change as shown in Fig. 2(a). This can be ascribed to two factors: first, the annealing process is conducted at a relatively low temperature. Secondly, the coating of the alumina layer can enhance the thermal stability, improving the heat resistance of the Al film effectively, which limits the mobility of Al atoms during the annealing treatment [32]. The Al NP located on the Al film possess around 55 nm diameter as shown in Fig. 2(c). The corresponding AFM top-view of the Al NP−film system is shown in Fig. 2(d), and we found that the simple, low-temperature annealing method results in the hemispherical Al NP distributed uniformly on the whole substrate. In order to prevent scattering losses of SPP, and better support plasmonic resonances coupling, the high smooth surfaces of Al films are required [31,33]. In this work, the bottom Al film in the Al NP−film system presents a root-mean-square (rms) roughness of 0.26 nm, which represents atomically smooth Al films as shown in the Fig. 2(e), indicating the obtained Al film can act as an ideal platform for plasmonic applications. Transmission electron microscopy (TEM) analysis for the upper Al NP after oxidation treatment is also showed in Fig. 2(f). The samples prepared for TEM go through the ultrasonic and centrifugal treatment. The inset in Fig. 2(f) shows the high-resolution TEM image of nearby interface of the Al NP, exhibiting the metallic core and the surrounding shell of Al oxide with the thickness around 3 nm. This nanoscale surface oxidation layer can prevent the Al NP from further oxidation and support affiliative molecule−substrate interactions that completely different from noble metal substrates.
Fig. 2. (a) The Al film with the nanoscale surface oxide layer after the annealing treatment. (b) The obtained Al NP−film systems using the oxide layer as nanometer spacer. Inset is the high-magnification SEM image. (c) The corresponding diameter distribution histogram of the resulting Al NP. (d) AFM side-views of Al NP−film systems. (e) The corresponding height profile of the bottom Al film from the white line drawn in the inset. (f) TEM image of the Al NP. Inset is the high-magnification TEM image (arrows indicate the oxide shell).
Figure 3(a) demonstrates the optical properties of the Al NP−film systems with different Al film layers. The Al NP−film system exhibits a prominent light absorption enhancement across a broadband spectrum especially in the UV range, as compared to the Al NP on the SiO2, indicating the strong plasmonic coupling in the Al NP−film system. Therefore, in this work, the nanoscale-thick Al oxide layer between Al NP and Al film potentially plays a role as a proper dialectic gap to effectively concentrate and enhance the electric field. The light absorption coefficient keeps increasing, as the number of Al film layers increase. There exist two main factors for the efficient optical absorption enhancement: the strong plasmonic coupling in nanoscale gaps between the Al NP and Al film and the confined gap plasmon modes between Al film layers. The Al NP coupling with mutilayer Al film possess much stronger plasmonic coupling and the emergence of confined gap plasmon modes between Al film layers should be responsible for the higher efficient optical absorption. To confirm this point, the comparison of optical absorption spectra between Al NP−film systems and the simple Al films are conducted as shown in Figs. 3(b)–3(d). We found that the two-layer or three-layer Al NP−film systems exhibit an obviously enhanced light absorption compared with the simple Al films layers, which is attributed to the plasmonic coupling effect between Al NP and Al film. However, for the four-layer Al films coupling with Al NP, the absorption is nearly equal to the simple four-layer Al films. The excessively thick Al film is thought to result in this phenomenon, where the confined gap plasmon modes between Al film layers play a main role in optical absorption and weaken the plasmonic resonances factor. To further understand the origin of the light harvesting enhancement of Al NP−film systems and the elevation of the plasmonic coupling intensity as the increase of Al film layers, the simulations based on the FDTD method were carried out.
Fig. 3. (a) Absorption spectrum obtained from the Al NP−film systems with different Al film layers. (b) Absorption spectrum collected from the Al NP/ two-layer Al film and two-layer Al film, for comparison. (c) Absorption spectrum collected from the Al NP/ three-layer Al film and three-layer Al film, for comparison. (d) Absorption spectrum collected from the Al NP/ four-layer Al film and four-layer Al film, for comparison.
A simplified simulation setup of Al NP−film system is designed for calculation as shown in Fig. 4(a). The Al NP−film systems are modeled as two hemispheric Al NP with diameter 55 nm on the Al films based on the AFM and SEM images. Besides, the Al NP and Al films are coated by alumina shells with a thickness of 3 nm. The 3 nm thick Al film is piled up along the z-direction spaced by 3 nm alumina layer. The mesh with 0.1 nm spacing is used for discretization of the calculative domain, and the constructed model is simulated using the optical constants tabulated in Ref. [34]. The 325 nm incident wave polarizes along the y-direction and the 2D electric field monitor is placed in the y–z plane at the center of the hemispheric Al NP. The electric field distributions of the Al NP−film systems in the y–z views with different Al film layers: 1, 2, 3 and 4 are shown in Figs. 4(c)–4(f), respectively. From this model, it is clearly observed that the maximum electric field enhancement (EM) in the Al NP−film systems occurs at the alumina gap region between the Al NP and Al film. Figure 4(g) shows the maximum EM variation (E/E0) of Al NP−film systems with different number of Al layers. The enormous increase of EM is found after the first Al film layer deposited compared with the Al NP on SiO2, indicating the plasmonic coupling between the LSP from upper Al NP and the SPP of the underlying Al film play an important role in the SERS substrate. More interesting, for the Al NP−film systems with more than one-layer Al film, the confined gap plasmon modes are generated in the nanoscale-thick alumina region between the adjacent Al films as shown in Figs. 4(c)–4(f). The entire confined gap plasmon modes in the Al–alumina multilayer are defined as the confined bulk plasmon polaritons (BPP). And it is reasonably understood that the existence of BPP should be responsible for this more significant EM compared with the one-layer Al NP−film system. These results suggest that the EM of Al NP/ multilayer Al films can be attributed to the coupling between LSP from upper Al NP and the SPP as well as highly confined BPP of the underlying Al films. However, the EM is smaller than that for three or four layers Al film stacking. This factor can be explained by the finite penetration depth of the surface plasmon evanescent field into the Al NP−film systems in the UV range. It is observed that the gap plasmon modes between the adjacent Al films become weak gradually as the Al film layers stacking shown in Figs. 4(e) and 4(f). Hence, a finite depth that the surface plasmon evanescent field can penetrate and stimulate should exist. In general, the simulation results suggest the Al NP−film systems could elevate the electric field effectively, explaining the origin of the light harvesting enhancement, and should offer outstanding and sensitive SERS detection.
Fig. 4. (a) The simplified simulation setup of the Al NP−film systems. (b) The y–z plane of electric field distribution of Al NP on SiO2 substrate. The y–z views of electric field distribution of Al NP−film systems with (c) one-layer Al film, (d) two-layer Al films, (e) three-layer Al films and (f) four-layer Al films under the 325 nm incident laser beam. (g) Electric field enhancement (E/E0) of Al NP−film systems with different Al film layers. (h) SERS spectra of 1 mM adenine aqueous solution collected from Al NP on SiO2 and the Al NP−film systems with different Al film layers. Intensity scale bar: 500
The increase of Al layers in the Al NP−film systems is proved to support the plasmonic hot spot intensity in the simulation results. Therefore, to figure out the SERS effect related to the number of Al layers, the Al NP−film systems are applied as SERS substrates for trace molecules detection using a 325 nm excited laser. The adenine is chosen as probe molecule. To avoid any photo-degradation of adenine, the excited laser powers with 1 mW was used. 2 µL adenine aqueous solutions (1 mM) was dropped on top of Al NP−film systems and dried naturally. As shown in Fig. 4(h), the SERS spectra collected from the Al NP−film systems clearly exhibited four Raman features (724, 1327, 1485, and 1601 cm−1), which is in accordance with the Raman spectrum of adenine reported previously [23]. Moreover, it is also noted that the absorption peaks of adenine all locate below 325 nm [35]. Thus, there is no resonance Raman contributing to the SERS enhancement factor under the 325 nm excitation. As seen in Fig. 4(h), the SERS signals from Al NP−film systems are obviously higher than that collected from the Al NP on SiO2 whose peaks are hardly observed. This can be explained by the fact that the coupling between the LSPR from upper Al NP and the SPP of the underlying Al films enable the multiplicative intensity enhancement effect. For the SERS spectra obtained from the Al NP−film systems, it is noteworthy that the Al NP/three-layer Al film exhibits the best SERS effect, which is consistent with the simulation results. It is proved experimentally the fact that the SERS effect of the Al NP−film systems should be related closely to the number of Al film layers. The confined gap plasmon modes generated in the nanoscale-thick alumina region between the adjacent Al films could elevate the intensity of the hot spots. However, the SERS signal intensity of the adenine decreased for more than three-layer Al films. This should be related to the finite penetration depth of the surface plasmon evanescent field into the Al NP−film systems in the UV range as analyzed above [5,23].
The detection limit (LOD) of Al NP−film systems was investigated by performing the SERS test of adenine with the concentration ranging from 10−2 M to 10−6 M as shown in Fig. 5(a). We note that for 10−6 M adenine, the main characteristic peak at 1327 cm−1 can be still clearly observed, indicating that the LOD is approximate at the order of 10−6 M magnitude. The standard calibration plot of SERS signal at 1327 cm−1 is also shown in Fig. 5(b) and it can be expressed quantitatively by the empirical criterion equation (log I = 0.29logC + 3.79), where I represents the intensity of SERS signal and C represents the concentration of analytes. The coefficients of determination for the standard calibration plot are 0.989, suggesting a good linear response within the concentration.
Fig. 5. (a) UVSERS spectra collected from 10−2 M to 10−6 M adenine deposited onto the Al NP−film systems. Intensity scale bar: 500 (b) Calibration plot of SERS intensity at 1327 cm−1 as a function of adenine concentrations. The error bars indicate the standard deviation from ten different spectra. (c) SERS spectra collected from 10−3 M to 10−7 M CV deposited onto the Al NP−film systems. Intensity scale bar: 1000 (d) Calibration plot of SERS intensity at 1616 cm−1 as a function of CV concentrations. The error bars indicate the standard deviation from ten different spectra. (e) The SERS spectra of 10−6 M adenine (10−7 M CV) collected on Al NP−film systems and 10−1 M adenine (CV) obtained on SiO2 substrate for comparison. (f) The long time stability of the adenine SERS spectra measured from the Al NP−film systems stored for different time period.
CV was chosen as another probe molecule, whose absorption bands locate below 325 nm and between 450 and 650 nm. There is also no resonance SERS test under the 325 nm excitation [25]. As shown in Fig. 5(c), the SERS spectra of CV with the concentration ranging from 10−3 M to 10−7 M collected from the Al NP−film systems clearly exhibited three Raman features (917, 1179 and 1616 cm−1), which is in accordance with the Raman spectrum of CV aqueous solution (10−1M) on SiO2. The LOD for CV is approximate at the order of 10−7 M magnitude. The standard calibration plot of SERS signal at 1616 cm−1 is also obtained in Fig. 5(b) and expressed quantitatively by the empirical criterion equation (log I = 0.38logC + 4.06). The coefficients of determination for the standard calibration plot are 0.991, suggesting a good linear response within the concentration.
The enhancement factor (EF) is regarded as an important nature calculated to evaluate the intensity of hot spots and SERS effect. Since the EF is in proportion to |E|4 /|E0|4, the EF calculated theoretically outside the alumina layer for three-layer Al NP−film system is around 106 at 325 nm in the FDTD simulation, which is at least two orders of magnitude higher than that of previous reported Al-based systems shown in Table 1 [23,24,35]. The experimental EF of Al NP−film systems for adenine and CV was estimated using the equation: EF = (ISERS × NSiO2)/(ISiO2 × NSERS), where ISERS, ISiO2, represent the intensity of the Raman signal from the SERS substrate and from SiO2 respectively, and NSERS,and NSiO2 express the number of probe molecules within the laser spot on the SiO2 substrate and on the SERS substrate. The calculations in details are referred to previous works of our group [5,12]. The SERS spectra of 10−6 M adenine and 10−7 M CV on three-layer Al NP−film systems are selected for EF calculation and 10−1 M adenine or CV on SiO2 substrate was used for reference as shown in Fig. 5(e). The experimental EF is calculated to be 3.62 × 105 for adenine and 4.1 × 105 for CV, which is approximate to the theoretical calculation and even come up to the noble metals detection levels for nonresonant molecules as shown in Table 1 [36–38].
Table 1. Comparison between the Al NP−film systems and noble-metal or Al nanostructures in terms of the non-resonant SERS enhancement factor.
Last but not the least, the longtime stability of the Al NP−film systems is also assessed in terms of the SERS performance as shown in Fig. 5(f). The SERS spectra barely decreased over time, even after one month, which should be ascribe to the cladding of dense surface oxidation layer that can prevent Al NP−film systems from further oxidization.
4. Conclusion
In conclusion, we proposed a new method to improve the SERS effect and broadband optical absorption of the Al-based nanostructures, which is realized by the design of Al NP−film systems with different Al film layers. The confined gap plasmon modes generated in the nanoscale-thick alumina region between the adjacent Al films, and it remarkably increase the EM of the hot spots between the Al NP and Al film layers. Besides, the Al NP−film systems exhibit enormous optical absorption enhancement from ultraviolet to visible range, and realize the low-concentration detection for adenine and CV molecules. The experimental EF is at least two orders of magnitude higher than that of previous reported Al-based structures. Furthermore, the longtime stability of the Al NP−film systems is also verified. Considering all these factors, the Al NP−film system should contribute significantly to the potential applications of low-cost Al-based materials in SERS detection.
Funding
National Natural Science Foundation of China (11674199, 11747072, 11774208, 11904214); China Postdoctoral Science Foundation (2019M662423).
Disclosures
The authors declare no conflicts of interest.
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