Abstract
Ultraviolet irradiation was used to tailor the surface plasmon band of the densely distributed aluminium nanoparticle arrays fabricated by gas-phase deposition. We showed that the broad surface plasmon resonance band of the as-prepared sample could be tuned to a sharp and strong resonance band in the deep ultraviolet optical range, with a large blue shift of the peak wavelength. The evolution of the surface plasmon resonance properties was attributed to the ultraviolet irradiation-improved surface oxidation of the nanoparticles, which eliminated the near-field couplings between the closely spaced nanoparticles by increasing their interspacing.
1. Introduction
Aluminium nanoparticles (Al NPs) are of interest to a variety of applications, such as photocatalysts [1], optical coatings [2], and transparent conductive films [3], as well as propellant and explosive materials [4]. They have been receiving considerable interest lately as plasmonic materials alternative to gold and silver NPs, with their attractive properties such as low cost, high natural abundance, and advantages in device performance, design flexibility, processing, and tenability [5–11]. Al NPs are especially attractive for ultraviolet (UV) plasmonics because of their surface plasmon resonance (SPR) properties in the full UV range. Particularly, the SPR of small Al NPs is located in the deep ultraviolet (DUV) region of the optical spectrum, which is of great interest in numerous applications, e.g., ultrasensitive organic molecule sensing [5, 6] and photocatalysis [7, 8]. Short wavelength UV light is capable of breaking organic bonds, a key factor in biological applications.
The surface plasmons (SPs) of Al NPs and their assemblies are extremely sensitive to the geometrical characteristics [9, 12–14]. The SPR bands can be tuned in a wide spectrum range from the DUV to the IR by varying the particle size, shape, and interparticle spacing. For an individual Al NP, the SPR wavelength increases with its diameter and reaches 300 nm at nm [9], which means it is difficult to generate SPR at DUV wavelengths with Al NPs fabricated with standard lithography techniques [15]. For NPs prepared by various bottom-up synthetic methods, distribution in particle size and impurities induce broadened or even featureless SPR spectra. The impact of oxidation is huge to the smaller NPs, which are concerned mainly in DUV applications. Furthermore, aggregation of the NPs in the dense array induces large red shift and broadening of the SPR bands due to the near-field coupling among the closely spaced NPs. Therefore, challenges still remain to synthesize well-controlled Al NPs suitable for DUV plasmonic applications.
As a poor metal, aluminium is easily oxidized when expose to the atmosphere. A thin native oxide layer can be formed on the surface, and then, the Al NP is wrapped with a shell. The formation of alumina layer is an important property. It acts as a passivation layer and prevents further oxidation of the Al NPs. As a result, highly stable and discrete Al NPs can be prepared. Furthermore, the SPR of the Al NPs also depends sensitively on the presence of the dielectric oxide shell [16, 17]. Consequently, oxidation can be a method to tune the SPR of the Al NPs by controllable growth of the dielectric shell. In this paper, we report the evolution of the UV SPR bands of the densely distributed Al NPs induced by surface oxidation assisted by UV light irradiation. We show that the UV irradiation can sufficiently tune the SP spectra of the gas-phase synthesized Al NP arrays into a sharp and strong resonance in the DUV optical range.
2. Materials and Methods
2.1. Preparation of Al NPs
We used a magnetron gas aggregation cluster source [18] to generate Al NPs in gas phase. Atoms were sputtered from the Al target with Al clusters formed through the aggregation process in the argon gas. A stable argon gas flow was introduced into the liquid nitrogen-cooled aggregation tube to maintain a constant carrier gas pressure for cluster growth. The cluster size was controlled by the carrier gas pressure. The clusters were then swept by the gas stream into a high vacuum chamber through a nozzle and deposited on the UV-grade fused silica substrate surface.
2.2. Characterization and Optical Measurements
The size and microstructure of the Al NPs were characterized with a transmission electron microscope (TEM). The morphology of Al NPs was characterized with a scanning electron microscope (SEM). The extinction spectra of the Al NP arrays are collected in a transmission configuration using a UV-vis spectrophotometer equipped with a deuterium lamp light source. The measurement was performed at normal incidence.
3. Results and Discussion
Figure 1(a) shows the TEM image of the Al NPs prepared under argon gas pressure of 50 Pa. As shown in the figure, the Al NPs distributed on the substrate surface randomly and aggregations occurred among most of the particles. In Figure 1(b), the average diameter of the NPs was measured to be 19 nm, with a size distribution of about 6 nm. Figure 1(c) shows the SEM image of the Al NPs deposited on a silicon substrate. The Al NPs were approximately spherical and dispersed randomly on the silicon substrate, consistent with the TEM image.
(a)
(b)
(c)
Figure 2 shows an extinction spectrum of the Al NP arrays collected in a transmission configuration using a UV-vis spectrophotometer equipped with a deuterium lamp light source. The measurement was performed at normal incidence. The spectrum is dominated by a very broad resonance peak, covering the wavelength range from about 210 nm to longer than 400 nm. The peak wavelength is about 270 nm. The spectrum displayed small change when measured following several days of atmospheric exposure, implying that the NPs were passivated effectively with the self-terminating native Al oxide. Also shown in Figure 2 is the extinction coefficient for an individual 14 nm Al sphere encapsulated with a 3 nm oxide shell calculated using the finite difference time domain (FDTD) method. Significant discrepancies appear when comparing the experimental and calculated spectra. The experimental spectrum displays a large red shift (>60 nm) and becomes very broadened. Its shape also departures from the characteristic Lorentzian resonance of a dipolar oscillator. For small Al NPs, the SPR bands red shift with increasing particle diameter, so that the size distribution exists in the NP arrays which may induce SPR band broadening. However, the size distribution-induced broadening should not dominate the experimental spectrum because only less than 25 nm red shift could be expected [19] when the size of the NP changes from 15 nm to 25 nm, the size region where most of the Al NPs located. We thus attribute the red shift and broadening of the experimental spectrum observed herein mainly to the near-field couplings occurring among the closely spaced NPs in the dense arrays. It has been reported that the near-field couplings between NPs in close proximity will not only broaden the resonant peak but also red shift the resonant wavelength [20]. The fractional plasmon red shift (, where denotes the SPR wavelength and is the red shift induced by near-field coupling) decays near-exponentially over a ratio of interparticle spacing , that is, , where is a constant [21]. This means shorter interparticle spacing has a stronger near-field coupling, which generates a larger red shift. Previous research [22] showed that in the dense array of 8 nm sized Ag NPs generated by cluster beam deposition, with the increase of the NP density, the SPR wavelength demonstrated a red shift from less than 400 nm to more than 570 nm, in accompanying with an increased broadening of the resonance peak.
From the TEM and SEM images, most of the Al NPs were not ideally spherical but with corners and edges, which would shift the plasmon to the red in comparison with spherical particles of similar dimensions. Thus, in addition to the near-field coupling between the nanoparticles, the nonspherical morphology of the Al NPs may also be a factor that cause the significant red shift of the plasmon resonance peaks.
Although near-field coupling and shape changing could significantly tune the SPR bands in a wide wavelength region, important for many applications [13, 15], for DUV plasmonics, the red shift and broadening of the SPR band still need to be further improved to satisfy the practical application conditions, especially when a high density of NPs is required to provide sufficient enhancement.
We found that the SPR band of the Al NP arrays could be tailored by UV light irradiation. To determine this, the Al NP samples were attached on the rotatable sample stage of the UV-vis spectrophotometer equipped with a 30 W deuterium lamp light source. Real-time extinction spectra were collected in a transmission configuration at room temperature every 5 minutes for holding times up to 140 min. Meanwhile, the NPs were exposed to the UV illumination of the deuterium lamp continuously. Shown in Figure 3 are the representative extinction spectra recorded during UV irradiation. With UV irradiation, a sharp extinction band between 220 nm and 250 nm rose at the shorter wavelength edge of the original broad band, in accompanying with a continuous decrease in the extinction peak intensity at longer wavelength. The new extinction band increased with the increase of the UV irradiation time. The new band peaked at 240 nm with 5 min irradiation and monotonously shifted to shorter wavelength when increasing the UV exposure time. The other evidence was the continuous narrowing of the extinction band. With 140 min UV irradiation, the original broad SPR band almost vanished, remaining a sharp intense SPR band peaks at 232 nm, with a full width at half maximum (FWHM) of about 20 nm.
As the spectrum displayed small change when measured after several days of atmospheric exposure, the evolution of the SPR band in Al NP arrays under UV irradiation might be attributed to the growth of the oxide layers on the NP surfaces inspired by UV light. As the Al2O3 shell is transparent in the deep UV to the IR region, it influences the peak position but does not reduce the extinction cross section. However, the change on the oxide shell thickness of the NPs varied both the size and interspacing of the NPs. In Figure 3, the broad band in longer wavelength was attributed to the near-field coupled SPs of closely spaced Al NPs, while the new arisen sharp band in shorter wavelength DUV region could be attributed to the intrinsic SPs of the isolated Al NPs. UV irradiation induced thicker Al oxide layers, which increased the interspacing and vanished the near-field couplings between them. With the increase of the oxide shell thickness, the proportion of the isolated NPs became larger, which resulted in the pronounced shorter wavelength SPR band that corresponded to the isolated Al NPs. Furthermore, the oxidation of the Al NPs would produce more rounded shapes, which can be confirmed by the HRTEM images shown in Figure 4. This will generate a further blue shift of the SPR band. Besides, it was reported that the blue shift induced by the oxidation is larger for the isolated spherical nanoparticle than for the nonspherical nanoparticles [17]. The SPR wavelength of an isolated Al NP depended sensitively on the presence of the oxide shell on its surface. The variation caused by NP oxidation depended on two factors, which generated opposite changes: the reduction of the metallic core size led to a blue shift and narrowing of the SPR band, whereas an increase of the effective refractive index surrounding the core resulted in a red shift and broadening of the SPR band. For smaller Al NPs, as the oxide shell increased, the blue shift and sharping compensated and dominated the red shift and broadening [10, 17]. Therefore, the time evolution of the SPR for the Al NP arrays under UV irradiation appeared consistent with the trends predicted by the above analysis, i.e., the rising of the DUV SPR band, and the decrease of FWHM and the blue shift of its peak.
(a)
(b)
(c)
(d)
The increase of the oxide shell thickness of the Al NPs under UV irradiation can also be verified with high-resolution TEM (HRTEM). Figures 4(a)–4(c) show the HRTEM images of three individual Al NPs with different diameters ( nm). Prior to the HRTEM observation, the NPs had been exposed to UV irradiation in atmospheric ambient for 140 min. From these images, spherically shaped core/shell nanoparticles could be clearly observed. Lattice images were distinguished in the cores, implying they were Al nanocrystals. We believed that the amorphous shells were most likely Al2O3. Although the oxide shell was not very uniform for each individual Al NP, its thickness kept around 6 nm on average and did not vary with the NP size. It should be noted that the oxide layer observed here was considerable thicker than those reported for small Al particles previously (typically 2.5 nm) [23, 24]. In Figure 4(d), a HRTEM image of an individual Al NP without UV irradiation is also shown. The sample had been exposed to atmospheric ambient for four days before the observation. Comparing with the NPs shown in Figures 4(a)–4(c), its oxide shell was much thinner and obscure.
Figure 5 shows the extinction coefficient of two closely touched 20 nm Al nanoparticles using FDTD method, where represents the thickness of the oxide shell on the surface of the nanoparticles. When , two Al nanoparticles are in contact, resulting in a wide band that extends to over 400 nm at the long wavelength due to near-field coupling. When two nanoparticles have the oxide layer, the wide extinction band at the long wavelength disappears, while a sharp extinction peak appears at the wavelength below 250 nm. With the thickening of the oxide layer, the extinction peak further blue shifts. This calculation is in line with the previous experimental results.
4. Conclusions
In summary, we have fabricated dense arrays of Al NPs with an average size of about 19 nm by the gas-phase cluster beam deposition method. The NP arrays exhibited broad SPR spectra in the UV region, owing to the red shift induced by the near-field couplings between the closely spaced nanoparticles. UV irradiation has been used to tailor the plasmonic properties in Al NP arrays. We have shown that the UV irradiation induced a large blue shift of the SPR band, resulted in a sharp and strong SPR band in the DUV region, peaked at 232 nm with a FWHM of 20 nm. The evolution of the SPR property was attributed to the growth of the oxide shells on the NP surfaces inspired by UV light, which vanished the near-field couplings between the closely spaced Al NPs by increasing their interspacing. The increase of the oxide shell thickness of the Al NPs under UV irradiation has been verified by HRTEM. We have demonstrated an easy way to realize intense DUV SPR in stable Al NP arrays, which might find broad applications, such as ultraviolet Raman spectroscopy, sensing, and photovoltaics.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.
Acknowledgments
We thank the financial support from the National Natural Science Foundation of China (Grant nos. 11627806, 11604161, and 61301015) and the National Basic Research Programme of China (973 Program, Grant no. 2014CB932302). This research was also supported by a project funded by the Priority Academic Programme Development of Jiangsu Higher Education Institutions.