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Abstract
Plasmonic nanomaterials with a high density of accessible hot spots over a large area are highly desirable for ultrasensitive near-infrared surface-enhanced Raman scattering (NIR SERS) sensing, especially those based on low-cost and earth-abundant noncoinage metals. Here, we report a facile thermal evaporation approach to fabricate 3D hierarchical aluminum cloud-like nanostructures decorated with nanoparticles on a wafer-sized substrate, which can provide high enhancement of Raman signals using 785 nm excitation. This enhancement is attributed to its absorption peak close to the excitation wavelength and high density of hot spots constructed by adjacent large cloud-like nanostructures and/or small nanoparticles. Systematic measurements and statistical analysis show that the obtained NIR SERS substrates exhibit high enhancement factor (1.48×105), long-term stability, and excellent reproducibility with less than 7% standard deviation over a 4-inch wafer surface. The combination of significant enhancement, long-term stability, good reproducibility, and scalable production process suggest that this type of 3D hierarchical aluminum nanostructures hold promise as a robust low-cost plasmonic material for applications in the near infrared SERS operation.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface-enhanced Raman scattering (SERS) [1], as a powerful sensing technique, has been widely applied in biosensing [2–7], material science [8–10], and food safety [11–15], as it can be used to identify analytes with their vibrational fingerprints down to single molecule sensitivity [16,17]. Studies on SERS mechanism indicated that Raman intensity can be greatly enhanced on the noble metal nanoparticles (NPs) surface with tremendous electric field enhancement due to the localized surface plasmon resonance (LSPR) [18], as the Raman scattering is proportional to the forth power of the electric field [19]. Considering that fluorescence background can be effectively reduced in the infrared region and the SERS enhancement increases when both the LSPR and the excitation laser shift to longer wavelengths. Therefore, in recent years, the use of infrared wavelengths for SERS has attracted much attention [20–25]. For fabrication of NIR SERS substrate, noble metal nanostructures (i.e., Au and Ag) with different shapes such as nanoparticles and nanostars have been fabricated [21,22]. In addition, gold/silver bimetallic nanoparticles were also reported for NIR SERS application [20,23–25]. However, noble metals suffer from high cost and low abundance, severely hindering their practical applications.
Very lately, aluminum (Al), as an emerging non-noble-metal plasmonic material, has been under intense investigation due to its low-cost, high abundance, and especially its plasmon resonance across a broad spectrum range from UV to visible light [26–28]. The studies on Al nanomaterials have demonstrated their potential in the ultraviolet (UV) [29–33] and visible range [34,35]. But, by tunning the size and aspect ratio of Al nanoparticles [27], and optimizing the preparation of Al nanostructures arrays [36], plasmon resonances can shift to lower energies and appear in the NIR range. In addition, the formation of aluminum oxide can also induce shift of the plasmon resonance to longer wavelength [37]. This makes aluminum desirable for application in NIR region [38,39]. It is well known that the hot spots, i.e., the nanoscale gap between neighboring NPs, can generate tremendous electric field enhancement, normally two orders of magnitude larger than that of isolated NPs, due to strong plasmonic resonance couplings [40]. Therefore, it is of great importance to design and construct plasmonic nanostructures with high-density and large area hot spots in the plasmon-enhanced spectroscopic sensing. Nevertheless, the Al-based nanostructures, reported so far, were mainly 2D arrays [41,42] unable to provide near-field coupling between neighboring structural units. Ideally, an Al-based NIR SERS substrate should have high enhancement effect comparable to the noble metal nanostructures while maintaining excellent uniformity and high reproducibility for Raman applications. There is always a trade-off between the signal enhancement and product reproducibility — when being concerned with higher enhancement, it is difficult to achieve uniformity and reproducibility over wafer-scale areas because of technical limitations in large-area nanofabrication. It is therefore essential that Al nanostructures with high-density and highly accessible hot spots for NIR SERS analysis can be fabricated uniformly over wafer-scale areas in a cost-effective way.
Here, we propose a novel 3D cloud-like Al hierarchical nanostructure that contains dense plasmonic hot spots with high reproducibility over wafer-scale areas by using one-step thermal evaporation method. The hierarchical nanostructure was composed of larger cloud-like nanostructures and decorated smaller NPs, which can provide two types of hot spots, i.e., abundant air nanogaps and ideal dielectric gaps. In particular, we want to explore SERS capabilities of this film with NIR excitation at 785 nm. The results show that the Al hierarchical nanostructures have superior performance in NIR SERS — very low limit of detection (10−6 M) for nonresonant molecule Rhodamine 6G (R6G), high enhancement factor (EF, more than 105) comparable to that of noble metals, excellent reproducibility of less than 7%, and long-term stability of less than 2%. Thus, the prepared high performance 3D Al hierarchical nanostructure over wafer-sized areas could be beneficial for commercial production.
2. Experimental details
2.1 Fabrication of a 3D cloud-like Al hierarchical nanostructure as a NIR SERS substrate
The 3D cloud-like Al hierarchical nanostructures were fabricated by a one-step process utilizing thermal evaporator technique. First, the 4-inch Si wafers were ultrasonically cleaned in acetone, alcohol, and deionized water, each for 10 min. Afterward, Al was deposited on the pre-washed wafer by the thermal evaporator system (DZS500, SKY Company, China) using high-purity (99.99%) aluminum wire as the raw material. The diameter of the Al wires was determined to be φ 0.6 mm. Twelve 10 mm Al wires, each folded three times, are hooked to a tungsten filament. During the evaporation process, the coatings were carried out at a current of 50 A and under an operating pressure of 1.0×10−4 Pa. Finally, the prepared nanostructures were kept in the evaporation chamber for 1 h in order to stabilize the crystalline Al film.
2.2 SERS measurements
Rhodamine 6G (R6G) purchased from Sigma-Aldrich was used to evaluate the detection performance of the NIR SERS substrate. SERS measurements were recorded with a Renishaw Laser Raman Spectrometer using a 785 nm laser and a CCD detector of 1 cm−1 resolution. The analysis was done using a 20×objective lens, and the spot size obtained was about 2 µm in diameter. Integration time for the analysis was 10 s, and the substrates were scanned 1 time under 100% (30 µW) laser power to obtain SERS spectrums. It should be noted that in the measurement all 5 µL-analytes were detected after being evaporated.
2.3 Instruments
The plasmon resonance spectra of the Al film were confirmed by optical reflectance spectra in the range of 350-800 nm by a HITACHI U-3310 spectrophotometer. The morphology and element justification were analyzed by using FESEM-EDS (JEOL, model JSM-6380LV). The surface morphologies were also obtained by Atomic force microscope (SEIK SPA 300HV). The Raman measurements were performed at room temperature by the Renishaw (Gloucestershire, UK) RM-2000 Laser Raman Spectrometer.
3. Results and discussion
Figure 1 shows an NIR SERS-active substrate with an Al film consisting of compact cloud-like nanostructure arrays on a 4-inch silicon wafer and fabricated using a rapid thermal evaporation process. Contrary to conventional nanolithography [30,33] or self-assembly technologies [29], which are time-consuming and suffer from small sample size for making SERS substrates, the thermal evaporation technique, widely used in industrial laboratories for making thin films, is fast and scalable. Al, as a non-noble metal, has advantages such as high natural abundance and low cost, and more importantly, Al films produced by thermal evaporation have naturally-formed nanostructures in a large area [43,44], thus are ideal for mass production. Considering that a reduction in vacuum pressure is accompanied by a decrease in surface topography [43], and on the other hand, Al oxide is more easily formed at higher pressure, we set the background pressure in the evaporation chamber at a trade-off value of 1×10−4 Pa. Previous studies have shown that the rate at which aluminum is deposited onto a surface can greatly affect the properties of the resulting film — as the rate of evaporation is increased and the arrival of aluminum at the tip surface outpaces the arrival of background gasses, decrease in both surface roughness and distribution is observed [44]. Therefore, in our study, the silicon wafer was coated at the evaporation rate 10 Å/s.
Fig. 1. Photograph of an aluminum film consisting of 3D cloud-like hierarchical nanostructure arrays on a 4-inch silicon wafer.
To illustrate the morphologies of the prepared Al hierarchical nanostructures, we systematically investigate the reproducibility of structure over the 4-inch diameter substrate. For this purpose, four concentric rings with radii of 0.6, 1.6, 2.6, and 3.6 cm divide the 4-inch wafer into five regions designated as R1, R2, R3, R4, and R5, respectively. From Figs. 2a-e, i.e., the top-view SEM images of the Al surface morphology, we see that long-range closed-packing of cloud-like nanostructures are evident, and similar structures and distribution cover the whole wafer surface. In addition, plenty of sub-10 nm gaps are present among these cloud-like nanostructures, which is useful for the formation of dense hot spots. More interestingly, each of the larger cloud-like nanostructures is decorated with several smaller NPs. Atomic force microscopy (AFM) was also used to analyze the Al film surface morphology and roughness. Typical images are shown in Figs. 2f and 2 g, and it can be seen that there are a lot of protruding nanostructures which were packed tightly to produce a high density structure on the surface and the average roughness (Ra) of the substrate is 1.774 nm. A statistical analysis on the hierarchical nanostructures in the SEM images clearly reveals a bimodal size distribution — the dimensions of cloud-like nanostructures and nanoparticles are 84.3 ± 9.2 nm and 24.1 ± 3.3 nm, respectively, as shown in Fig. 2 h. The film thickness was examined by cross-section SEM analysis, as shown in Fig. 2i. It can be seen that the film thickness is approximately 240 nm, and the surface appeared to have an undulating, rather than flat, morphology. The elemental analysis of the hierarchical nanostructures by energy dispersive X-ray spectroscopy (EDS) is provided in Fig. 2j, which suggests that the region primarily contains Al and trace-amounts of O, confirming the existence of alumina shell on the Al particle surface. Previous studies have shown that this thin oxide surface layer can be advantageous for SERS because it can not only provide binding sites for a variety of functional groups but also tune shifts of the plasmon resonance to NIR wavelengths [39]. It is well known that electromagnetic energy can be effectively confined in the subwavelength-sized air or dielectric gaps between plasmonic nanostructures due to the excitation of strong plasmonic couplings [45,46]. Therefore, in the current work, the nanoscale-thick alumina interlayer in each hierarchical nanostructure could potentially serve as an appropriate dialectic gap to greatly concentrate and enhance the electric field as demonstrated in literature [29]. Thus, such 3D hierarchical nanostructures featuring by multi-scale roughness, including that generated by the undulating surface as well as that generated by the protruding nanostructures, can provide high-density and highly accessible hot spots for NIR SERS Analysis.
Fig. 2. Characteristics of the 3D cloud-like Al hierarchical nanostructures on the 4-inch wafer substrate. (a-e) SEM images of hierarchical nanostructures arrays at different locations of the substrate separated by concentric rings with radii of 0.6, 1.6, 2.6, and 3.6 cm. (f-g) AFM images of Al hierarchical nanostructures. Distribution of structure size, determined from the SEM images, for (h) large Al cloud-like nanostructure, and small Al NPs. (i) The cross-section SEM image of Al film. (j) Elemental analysis of the Al hierarchical nanostructures by EDS.
The 3D cloud-like Al hierarchical nanostructures, which have advantages of dense hot spots over large areas, are very promising plasmonic nanomaterials for applications in high performance NIR SERS. Therefore, we adopt the prepared Al hierarchical nanostructure as the NIR SERS substrate for molecule detection with an excitation of 785-nm wavelength. Rhodamine 6G is chosen as the test molecule, which is not resonant at 785 nm. The limit of detection (LOD) of the 3D Al hierarchical nanostructure substrate is determined by carrying out SERS measurements for different concentrations of R6G ranging from 10−3 to 10−6 M. As shown in Fig. 3a, the SERS spectrum of 10−6 M R6G yields clear Raman features of R6G, in agreement with the Raman spectrum of R6G in literature [47]. The peaks at 611, 780, 1190, 1312, 1365, and 1510 cm−1 are respectively assigned to C-C-C ring in-plane bending, C-C-C out-of-plane bending, C-H in-plane stretching, N–H in-plane bending, aromatic C-C stretching and aromatic C-C stretching, suggesting that the LOD is at the order of 10−6 M. The standard calibration plotted in Fig. 3b shows a good correlation coefficient of 0.93, indicating linear response over a wide range of analyte concentration. The SERS detection performance of the prepared 3D Al hierarchical nanostructure is comparable to that of Ag/Au-based nanomaterials under nonresonant Raman conditions [48], which should be attributed to the high density of hot spots over the entire large area as shown in Fig. 2.
Fig. 3. (a) NIR SERS spectra of R6G aqueous solution with different concentrations ranging from 10−3 to 10−6 M, excited at 785 nm. (b) Calibration plot of Raman intensity at 1510 cm−1 as a function of concentrations.
To calculate the enhancement factor (EF) of the NIR SERS substrate, the silicon wafer is chosen as a reference substrate, and it is estimated on the accepted relationship formula: EF=(ISERS/IRaman)×(NRaman/NSERS) [49], where ISERS and IRaman correspond to peak intensities of the same band in the SERS and reference substrate; NRaman refers to the amount of R6G molecules probed in a bulk sample, and NSERS is the number of monolayer R6G molecules adsorbed on the substrate surface under the laser spot area. Here, the band centered at 1510 cm−1 obtained from the SERS spectra of 10−3 M R6G on the corresponding substrates is chosen to calculate the I values, as shown in Fig. 4a. In the SERS measurements, for the original substrate, the peak intensity IRaman is measured to be ∼312 after the 5 µL-R6G droplet evaporates naturally; for the SERS substrate, the value of ISERS is measured to be ∼1.28×104. For our Raman setup, the illumination focus has a diameter of ∼2 µm and the penetration depth of laser beam is ∼3 mm, which gives an illuminated volume of ∼9.42 × 103 µm3 and NRaman of ∼5.67×109. To determine NSERS, we assume that monolayer R6G molecules are absorbed on the surface of NIR SERS substrate. The surface area of one R6G molecule (length 1.37 nm, and width 1.43 nm) is ∼2.0 nm2 [50]. Dividing the illuminated area by the surface area of one R6G molecule, we find NSERS to be 1.57×106. Accordingly, the prepared Al-based NIR SERS substrate exhibits a large EF of ∼1.48×105 at the excitation of 785 nm, which is comparable to the noble metals for NIR SERS analysis as shown in Fig. 4b. The enhancement factor of this NIR SERS substrate is on the order of 105, lower than other Al nanostructures SERS substrate shown in Fig. 4b. This difference is due primarily to the lower excitation power and shorter integration time but may also be affected by the electronic structure of Al at the laser excitation wavelength.
Fig. 4. (a) SERS spectra of R6G (10−3 M) molecules absorbed on the silicon wafer, cloud-like Al nanostructure arrays. (b) A comparison between the 3D Al hierarchical nanostructures and recently reported noble-metal or Al nanostructures in terms of the SERS enhancement factor.
We have also systematically investigated the reproducibility of NIR SERS performance over the 4-inch diameter substrate. Figure 5 compares the SERS spectra of R6G molecules adsorbed on the 3D Al hierarchical nanostructure in different regions (R1-R5). In each region, we have randomly obtained at least 15 SERS spectra, and found that the peak position for any specific vibrational mode is almost identical. The corresponding Raman intensity and standard deviation for different SERS peaks are listed in Table 1. From Table 1, it is evident that the SERS enhancement is reproducible from place to place within each region. We have also calculated the standard deviation of the averaged Raman counts for each vibrational mode across the 4-inch wafer. The results are 19.8% (612 cm−1), 36% (780 cm−1), 32% (1190 cm−1), 17% (1312 cm−1), 7.6% (1365 cm−1), and 6.5% (1510 cm−1). The raw Raman signal is often obscured by a broad background curve (or baseline) due to the intrinsic fluorescence of the organic molecules [51], which leads to unpredictable negative effects in quantitative analysis of Raman spectra ranging from 612 to 1365 cm−1. Therefore, we conclude that the 3D hierarchical Al nanostructure substrates exhibit high SERS reproducibility with less than a 7% (6.5% at 1510 cm−1) standard deviation over a 4-inch wafer surface.
Fig. 5. SERS spectra recored for 10−5 M R6G adsorbed on five areas (R1-R5) of 4-inch Al film. In each region, 15 SERS spectra were obtained at randomly positions.
Table 1. Assignment of SERS peaks and corresponding Average Raman Counts
In order to further investigate the long-term stability of our NIR SERS substrates, the concentration of 10−5 M of R6G is selected to investigate the stability. The SERS spectra were obtained from the same piece of the 3D hierarchical Al nanostructure during one month. As shown in Fig. 6, the average SERS peak intensity at 1510 cm−1 barely degrades with time and is dropped only 2% even after one month. This demonstrates that the prepared NIR SERS substrate possesses good stability, which could be attributed to surface coating of the dense alumina layer that can isolate Al from the surrounding environment, and protects the nanostructure from further oxidization.
Fig. 6. The Raman intensity changes with time obtained from the NIR SERS substrate. The arrow points to the band intensity at 1510 cm−1 measured from 10−5 M R6G on the same piece of NIR SERS substrate.
In order to understand the NIR SERS behaviors of Al hierarchical nanostructure substrate, we measure the optical reflection at normal incidence with wavelength ranging from 350 nm to 800 nm. The reflection spectra in Fig. 7 have been obtained at three random locations on the 4-inch Al film. The position of the laser excitation wavelength, 785 nm, is indicated by the dashed line. It is evident that the laser excitation wavelength almost coincides with the absorbance valley located at ∼785 nm. Therefore, the interesting observation here is that 3D cloud-like Al hierarchical nanostructure can provide hot spots in the near infrared range. This might support that high-performance SERS behaviors of our NIR substrate are dominantly attributed to the strong plasmonic resonance (close to the excitation wavelength 785 nm) around the dense hot spots in the 3D cloud-like Al hierarchical nanostructure.
4. Conclusions
In summary, 3D cloud-like Al hierarchical nanostructures supporting dense hot spots over a wafer-scale have been successfully prepared by conventional thermal evaporation method. The obtained hierarchical nanostructures, with 785 nm laser excitation, exhibit excellent SERS enhancement factor of ∼1.48×105, R6G detection limit of ∼10−6 M, superior reproducibility of less than 7%, and long-term stability. The ultrahigh enhancement is attributed to abundant air nanogaps and ideal dielectric gaps illuminated at the wavelengths close to the plasmon resonance. Considering the low-cost and abundance of the Al-based plasmonic nanomaterials and the high SERS performance of the 3D cloud-like Al hierarchical nanostructures over wafer-scale areas, our study could lead to important technological applications in NIR biosensors.
Funding
Natural Science Foundation of Shandong Province (ZR2016FQ05, ZR2018MA044); Scientific Research Starting Foundation of Liaocheng University (318051542); National Natural Science Foundation of China (NSFC) (11504386, 61574071, 61775089); Alliance Fund of Shandong Provincial Key Laboratory (SDKL2016038).
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