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Tunable plasmonic properties of elongated bimetallic alloys nanoparticles towards deep UV-NIR absorbance and sensing
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Tunable Plasmonic Properties of Elongated Bimetallic Alloys Nanoparticles towards deep UV-NIR Absorbance and Sensing Pradeep Bhatia , S.S. Verma , M.M. Sinha PII: DOI: Reference:
S0022-4073(19)30553-9 https://doi.org/10.1016/j.jqsrt.2019.106751 JQSRT 106751
To appear in:
Journal of Quantitative Spectroscopy & Radiative Transfer
Received date: Revised date: Accepted date:
2 August 2019 8 November 2019 8 November 2019
Please cite this article as: Pradeep Bhatia , S.S. Verma , M.M. Sinha , Tunable Plasmonic Properties of Elongated Bimetallic Alloys Nanoparticles towards deep UV-NIR Absorbance and Sensing, Journal of Quantitative Spectroscopy & Radiative Transfer (2019), doi: https://doi.org/10.1016/j.jqsrt.2019.106751
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Highlights The present research is to highlight the studies of bimetallic alloys nanoparticles of magneto-plasmonic nanostructures. The influence of size, shapes, compositions and surrounding medium for Co-Ag and Co-Au alloy nanoparticles results in significant change in optical properties. The spectra are found between 211-964 nm wavelengths ranges and can be tuned in the deep UV-NIR region of the electromagnetic (EM) spectrum. Tunability in sizes and shapes opens new potential use with desired applications in biomedical fields.
Tunable Plasmonic Properties of Elongated Bimetallic Alloys Nanoparticles towards deep UV-NIR Absorbance and Sensing Pradeep Bhatia*, S. S. Verma and M. M. Sinha
Department of Physics, Sant Longowal Institute of Engineering and Technology, Sangrur148106, Punjab, India (November 08, 2019) *
Email: Pradeep Bhatia ([email protected])
Abstract Combining magnetic nanoparticles with noble metallic nanoparticles to construct multifunctional alloy nanostructures has become a powerful tool for imaging sensing, medicine, biology, and cancer treatment and photothermal therapy. This work reports a study regarding the plasmonic properties of Co-Ag and Co-Au bimetallic alloys nanostructures on the electromagnetic spectrum. In this paper, the optical properties of magnetic and plasmonic nanoparticles (NPs) with different shapes, sizes, compositions, and surrounding medium are investigated by using the discrete dipole approximation (DDA) technique. The absorption and scattering localized surface plasmon resonance (LSPR) peak positions are found between 211-964 nm wavelengths ranges and can be tuned in the deep UV-NIR region of the EM spectrum in accordance with the desired application. Further, we calculate the refractive index sensitivity (S) and figure of merit (FOM) of LSPR based nanosensors. Although, LSPR based sensors undergo low FOM as compared to conventional surface plasmon resonance (SPR) sensors due to high losses from the radiative damping of localized surface plasmons (LSPs) waves. However, LSPR based sensors have potential applications in gas and biosensors technology viz. medical and environmental monitoring applications. In this work, the main feature of LSPR peak is dependent on nanoparticle shapes, sizes, compositions, and ambient medium and have ordered as prolate < sphere < cube < rectangular shape NPs. These
results suggest that the nanostructures of Co-Ag and Co-Au alloys associated with LSPR tunability and sensitivity can be used in biomedical and sensing environment technology such as detection of chemical and catalytic events. Keywords: Nanoparticles, Sensor, Refractive Index Sensitivity, Figure of Merit, Localized Surface Plasmon Resonance
Introduction Metallic nanoparticles have shown remarkably properties in comparison to bulk counterpart due to their surface to volume ratio and its localized surface plasmon resonances (LSPRs) peak in electromagnetic (EM) spectrum [1]. The collective oscillation of conduction (free) electrons in metal nanostructures, known as localized surface plasmon (LSP), made much attraction since few decades due to its variety of physical phenomena and chemical processes. The collective oscillations of electrons in LSPRs phenomenon result in a strong enhancement in optical (scattering and absorption) intensities of nanoparticles and allowing their use in modern photonics, photothermal therapy, drug delivery and many more in biomedical and biotechnology [2, 3]. The displacement of plasmon peak on the spectral scale is very sensitive to metallic NPs (an effective dielectric function of materials), size, shape, and surrounding environment [4-6]. For fundamental and practical research, modeling of nanoparticle (NPs) with different parameters and calculating their optical properties is an important aspect. Nanorods shaped nanostructures with varying aspect ratio set their examples of such tunability, in which the position of LSPR peaks varies from visible to infrared region on the EM spectrum [7-9]. In addition, other alloy nanostructures like a triangular prism, nanocubes, prolate, rectangular geometry and varying core-shell thickness have been shown their LSPR peaks tunability [1013]. Furthermore, LSPR based sensors performance has been described, including a variation
of geometry, and structure configuration (alloy or core-shell). Although, SPR based sensor is common and commercialized used in gas sensors and bio-sensor technology to monitor the environmental and medical issues. Tittl A et al. introduced a probing technique based on plasmonic NPs for monitoring local chemical reactions [14]. Bingham J. M. and co-workers studied the LSPR spectroscopy to detect the small changes (< 3 x 10-4 RIU) in bulk refractive index by NPs. H2, N2, and Ar were used to control the NPs surrounding environment [15]. In contrast, the LSPR sensor offers low-cost production, simple setup, and detection of local change in the refractive index, which is not the case with SPR sensors [16, 17]. Our main aim of the research is to enhance the efficiency of alloy nanostructures and attain LSPR peaks tunability between UV and NIR regions of EM spectrum. The present work is carried out with a theoretical understanding of cobalt alloys NPs. Transition metal cobalt (Co) is a peculiar choice because it has good thermal stability and ferromagnetic properties which may introduce versatile and tunable properties [18]. However, high toxicity and less optical properties of pure cobalt limit their use. Although, cobalt NPs showing their potential in magnetic separation and magnetic tape [19, 20]. The configuration of alloy nanostructures, which mixed the transitions metal (Co) with noble metal and become a new approach with remarkable applications. Introducing transition metal (Co) into noble (Ag and Au) metallic system not only reduces the cost but also increases their stability and multifunctional properties. Wang L. et al. studied the enhancement in plasmonic and magneto-optical (MO) Faraday rotation of core-shell Co-Ag with varying concentration ratios from 30:70, 50:50 and 80:20 in 1.5 refractive indexes of the ambient medium [21]. Recently, some published work showed the direct effect of magneto-plasmonic bimetallic alloys on LSPR peaks and its based sensor by tuning the shapes, sizes and surrounding medium [22, 23]. Also, many studies have been
reported on the simple (spherical) configuration of Co-Au and Co-Ag as core-shell and alloy [24, 25], studies on the mechanism of LSPR-based nanosensing are still needed. In this paper, we investigated the optical properties of elongated Co-Ag and Co-Au alloy nanoparticles by increasing the size of the sphere and cube nanostructure in one direction to see the influence of sizes, composition, and the surrounding medium. Further, the effect of sizes and shapes on the sensing performance of bimetallic NPs is considered. Therefore, this work focus on the theoretical understanding of unique features of elongated nanostructures made of Co-Ag and Co-Au alloys nanoparticles. Finally, we explored the optical properties of bimetallic alloys NPs by using the discrete dipole approximation (DDA) method. Absorption and scattering spectra of the nanostructures in the UV-NIR region of EM spectrum may open new potential in imaging, cancer therapy, and medicine. Further, the optical characteristics of presently considered nanostructures can find significant applications in solar cells and sensors.
Theoretical Method It is well known that Mie scattering theory, when metal NPs interact with incident light with the help of Maxwell’s equations, is widely used and an exact analytical solution of the scattering of electromagnetic waves, to calculates optical properties for only simple geometry i.e. sphere. For complex geometry studies and theoretical research of metallic nanoparticles to practical application, it is important to understand the interaction of light with NPs at the nanoscale, necessary to use the approximation methods for their calculations. To calculate the optical properties of NPs of various shapes, composition, and sizes, the DDA method is employed. Since DDA is a flexible method and a powerful tool for calculating the optical properties of NPs with various geometry. The various quantitative characteristics calculated by using the DDA method which available as DDSCAT software in open access [26, 27]. The following quantities are calculated by DDSCAT: (I) absorption efficiency factor; Qabs ≡
Cabs/πr2eff, where Cabs is the absorption cross-section, (II) scattering efficiency factor; Qsca ≡ Csca/πr2eff, where Csca is the scattering cross-section, and (III) extinction efficiency factor; Qext ≡ Qsca + Qabs, where reff is defined below as an effective radius. DDA method solves Maxwell’s equation for the electromagnetic wave interaction with NPs in which NPs are discretized into arrays of dipoles or polarized points. In our calculation, the number of dipoles is used ~7 X 104 in order to accurately represent the target geometry and for better convergence and volume of the target is defined by the effective radius (reff = (3V/4π)1/3), the radius of an equal volume sphere. For spherical shape, the volume ‘V’ is 4/3πr3, thus, aeff = (3*(4/3πr3)/4π)1/3 hence, aeff = ~ r; in this way, we have considered radius ‘r’ =20, 30, 40, 50, 60 nm. For non-spherical (rectangular) shape, we considered a*b*c, with a : b : c :: 20 : 60 : 60 with aspect ratio 1 : 3 : 3, then V = abc = 9a 3. Thus, aeff ≡ (3V/4π)1/3 = (27/4π)1/3a = 1.2905*a. Now, our target should have thickness ‘a’ = 0.016 nm in the x-direction to make the same effective radius as considered in case of sphere i.e. 20 nm. The complex dielectric function of nanomaterials considered as input parameter and is taken from Johnson and Christy [28, 29] and further assumed to be the same as that of the bulk materials. This dielectric function of individual metal NPs is used to form the alloy with different compositions and hence, the effective complex dielectric function is responsible for the change in the optical properties of alloy NPs. In the previous work, we made a comparison of experimentally available imaginary (ε2) part of particular alloy composition with our calculated average weight percentage method and concluded that experimental ε2 is remarkably higher than our calculated method [22]. However, the numbers of studies are reported in the literature for noble metal alloy composition based on average weight percentage method and this method is satisfactory for simulation and evaluating the trend in
multimetallic nanostructures. Hence, average weight percentage method is used in the present studies to analyze the optical properties of bimetallic nanoparticles. Furthermore, the effective refractive index value for both Cox-Ag1-x and Cox-Au1-x bimetallic alloys with closed composition x= 0.30, 0.50 and 0.80 are calculated by using the weight percentage method. Different refractive indices (n)= 1.00, 1.33, and 1.53 of the ambient medium are considered. Variation of the refractive index of surrounding in increasing order results increases in the screening of the surface electrons. Consequently, restoring force increases and hence, low energy required to excite the free (charges) electrons. Therefore, NPs can be used as LSPR sensors where both the polarization and restoring force affect the sensitivity of sensors. The sensing performance of nanosensors can be evaluated by refractive index sensitivity (S) and FOM measurements. The refractive index sensitivity (S) is defined as the ratio of LSPR resonance wavelength shift (∆λres) to the change in the refractive index of the surrounding medium (∆nm) i.e. S = (∆λres/∆nm). The FOM is defined as FOM = S/FWHM, where FWHM stands for full width at half maximum. In addition, we have explored the composition effects on LSPR peaks in a considered alloy of different shapes and sizes. All the simulations are made by the DDA method in the wavelength range from 200 to 1300 nm. Thus, the present calculation is carried out for bimetallic alloys NPs.
Results and Discussion The calculation for optical characteristics in terms of Qabs and Qsca of magnetic and plasmonic (Co-Ag and Co-Au) elongated bimetallic alloy nanostructures is performed. The elongated structures e.g. prolate and rectangular is modeled from basic structures i.e. sphere and cube by increasing their size in one direction. Furthermore, the optical properties of nanostructures are theoretically calculated by considering the linear polarization only in one (i.e. y-axis) direction and the directional polarization effect of incident light has not been
considered. The present calculations are carried out by using the DDA method. From the practical point of view, great interest presents the research of absorption, scattering, and extinction in optical properties of metallic NPs depending on their shapes. Therefore, NPs geometry plays an important and shows a direct impact on tuning the LSPR peak position. Furthermore, Ammari H. et al. [30-32] have also mathematically defined the notation of plasmonic resonance to analyze the shift and broadening of the plasmon resonance with changes in size and shape of the nanoparticles to investigate the scattering and absorption enhancements by plasmon resonant nanoparticles and obtained optimal bounds on the absorption and scattering enhancement. In case of symmetric shape NPs, only on the dipolar peak is induced, whereas asymmetric shape NPs, a higher order of electric dipole modes is induced. LSPR peaks not dependent on only shapes, sizes, and compositions but depend mainly on the dielectric function of materials. To achieve good plasmonic properties, the real part of dielectric function (ε1) of NPs requires negative value and imaginary part (losses) of dielectric function (ε2) of the NPs assumed to be small. Our calculated dielectric function (ε1
Fig. 1. Description of real and imaginary parts (a) – (d) of effective dielectric function for pure and different alloy compositions as a function of wavelength.
Sphere
Prolate
Cube
Rectangular
Fig. 2. Illustration of the elongated shape of alloy nanoparticles.
& ε2) for Cox-Ag1-x and Cox-Au1-x (x=0.30, 0.50 and 0.80) are shown in Fig. 1. All configured nanoparticles are visualized by ParaView; Visualization ToolKit (VTK) is a DDA extended subroutine program written by Jalel Chergui [33] and represented in Fig. 2. In general, plasmon oscillations in NPs experience both intrinsic and radiation damping due to electron-electron collisions and decay of collective oscillations into photons respectively. Noble metal NPs can be utilizing for resonant and helps to reduce intrinsic damping whereas radiation damping constitutes the dominating effect for larger particle size (few tens of nanometers). Therefore, we considered the Co-Ag and Co-Au bimetallic alloys of 20 nm to 60 nm particle sizes to investigate the optical properties of four different shapes nanostructures. The optical properties of the NPs depend on the sizes and shapes and can also be tuned by varying the compositions of metallic NPs. Thus, composition-dependent effective dielectric functions result in the different plasmonic properties on the electromagnetic (EM) spectrum of bimetallic alloy. Fig. 3 shows the simulated absorption (solid lines) and scattering (dotted line) efficiencies of Cox-Ag1-x, and Cox-Au1-x for 60 nm particle size with three different compositions (x) = 0.30, 0.50, and 0.80, in water ambient medium (n=1.33). As the concentration (composition) of Ag and Au in Co alloy increases, the peak position shift towards the longer wavelength region and further intensity of peak increases with the increasing concentration of Ag and Au. The efficiency of Co-Ag NPs is
remarkably high as compared to Co-Au NPs, clearly shown in Fig. 3 (a, b). Tuning in LSPR and their intensity peak shows dependency on the alloy composition. Therefore, all the further calculations are done with 50:50 composition of Co-Ag and Co-Au alloy to maintain low cost and obtain a decent optical response as well as magnetic effects at a reasonable cost.
Fig. 3. Effect of compositions on LSPR peak spectra (a) Co-Ag and (b) Co-Au alloys of 60 nm nanoparticles particle size.
The influence of size, shape and dielectric environment on optical properties of metal NPs is reported by Kelly K. L. et al. [34]. They studied the optical properties such as extinction, scattering cross-section, and local field of spherical and non-spherical shapes of silver nanoparticles. Thus, we have calculated the absorption and scattering LSPR peak of spherically shaped Co0.50-Ag0.50 alloy NPs as a function particle size as shown in Fig. 4(a, b). Furthermore, absorption and scattering LSPR peak of Co0.50-Au0.50 alloy NPs are calculated also and shown in Fig. S1(a, b) (supplementary information). The result shows that absorption LSPR at λmax is found in the range between 361 nm to 420 nm and 262 nm to 525 nm wavelength and scattering LSPR λmax 365 nm to 529 and 213 nm to 582 nm wavelength of both Co-Ag and Co-Au alloys, respectively, on EM spectrum. Hence, redshift is found in Co-Ag alloy nanostructures. The LSPR peaks of NPs exhibit a blue-shift & red-shift due to lower and higher dielectric constant (ε’’) of metallic NPs at nanoscale. The dielectric constants i.e. imaginary part (ε’’) of Au is 1.624, Ag is 0.403, and Co is17.05 as given in ref. [29]. Under similar conditions (size, shape, and surrounding medium) the LSPR peaks of Au
or Ag NPs is red or blue-shifted whereas it will be red-shifted for Co NPs. Thus, the LSPR peaks for Co-Au and Co-Ag alloy NPs are red-shifted as compared to pure Au or Ag NPs. Furthermore, as the particle size increase, absorption peak efficiency increases for 20 and 30 nm sizes and then decreasing for 40 nm to 60 nm particle sizes with shifting of LSPR peak at λmax towards higher wavelength region. On the other hand, scattering efficiency increases along with LSPR at λmax peak with an increment of particle size. Furthermore, scattering efficiency systematically increases and absorption efficiency decreases with large Co-Ag alloy particle size. With large particle size, retardation effects: retardation damping and dynamic polarization are associated and cannot be ignored. Since these effects reduce the intensity of the plasmon along with broadening of the peak. Thus, surface scattering and radiation damping effects cause a decrease in absorption efficiencies. Moreover, scattering efficiency of Co-Ag and Co-Au bimetallic alloy NPs linear increases with the nanoparticle sizes. However, the absorption and scattering efficiency of Co-Ag NPs is remarkably high as compared to Co-Au NPs. Hence, LSPR peaks at λmax shifted towards longer wavelength region and their corresponding efficiency increases of scattering spectra as particle size increases.
Fig. 4. Calculated LSPR spectra for (a) absorption and (b) scattering peak of spherical Co.50-Ag.50 alloys as a function particle sizes. The refractive index of an ambient medium is 1.33.
Fig. 5. Calculated spectra for (a) absorption and (b) scattering of Co-Ag alloy nanoparticle with different shapes.
In order to analyze the influence of shapes on optical properties, we considered the four different geometry of bimetallic alloy. The absorption and scattering spectra with 60 nm particle size of Co.50-Ag.50 alloy composition for different considered shapes as shown in Fig. 5(a, b) (refractive index of the surrounding medium is 1.33). The LSPR peaks shifting towards the higher wavelength regime associated with the change in NPs geometry. Besides, optical efficiency also shifted with a change in NPs shapes. As the shapes of bimetallic alloy nanoparticles vary from sphere to prolate and cube to rectangular, the rectangular nanostructure yield the maxima of LSPR peak at λmax and their efficiency in comparison to other considered nanostructures in the order of prolate < sphere < cube < rectangular. The absorption and scattering LSPR peak at λmax are tuned in the wavelength range of 341-814 nm and 438-787 nm, respectively, on the EM spectrum of four different considered alloy nanostructures. This calculated LSPR peak position spectrum lies in the different regions of the EM spectrum due to the variation of free electron oscillations in the different geometry volume. Hence, LSPR peaks at λmax and their corresponding efficiencies show dependence on NPs size and geometries of the NPs. Thus, we have investigated the absorption and scattering spectra of Co-Ag and Co-Au bimetallic alloy nanoparticles with shapes and sizes in three different ambient medium.
Fig. 6. Calculated absorption LSPR peak position of Co-Ag and Co-Au alloys nanoparticles as a function of particle size (a, b) represent LSPR wavelength in refractive index of an ambient medium is 1.00, (c, d) represent LSPR wavelength in refractive index of an ambient medium is 1.33 and (e, f) represent LSPR wavelength in refractive index of an ambient medium is 1.53.
Fig. 6 shows the absorption LSPR peaks of Co-Ag and Co-Au alloys as a function of sizes with four different shapes. The absorption LSPR peak of the sphere, prolate, cube and rectangular shapes are found in the wavelengths range of 327-602 nm, 338-821 nm, 356-948 nm of Co-Ag alloys and 211-623 nm, 219-831 nm, 266-964 nm of Co-Au alloys within different ambient (n=1.00 1.33 and 1.53) medium, respectively as shown in Fig. 6(a-f). As the NPs size increases and shape changes, LSPR peaks at λmax shifted towards the longer wavelength regime on the EM spectrum. For both sphere and prolate geometry of Co-Au
alloy NPs, no LSPR peaks have been appearing for 20 and 30 nm particle size because of small oscillation of free electrons and hence, no variation in plasmon wavelength. Similarly, Fig. 7(a-f) shows the efficiency (intensity) corresponding to LSPR peak of Co-Ag and Co-Au alloys for the sphere, prolate, cube and rectangular-shaped NPs in three different surrounding medium. The absorption intensity value increases of 20 to 30 nm and decreasing for above 30 nm particle size with a change in NPs geometry and the surrounding medium. Thus, it is concluded that rectangular nanostructure has maximum LSPR peak at λmax and intensity (efficiency) as compared to other considered nanostructures and has to order rectangular > cube > sphere > prolate. It is well known that the absorption or scattering efficiency is a function of NPs volume and geometry. The theoretical description of rectangular shape has larger efficiency as compared to other considered shapes because of its large volume. Moreover, we have considered the elongated nanostructures viz. prolate shape (from sphere) and rectangular shape (from cube). Thus, rectangular shape gives high efficiency as compared to other considered shapes. For details study of LSPR peaks, the values of absorption LSPR peaks at λmax of Co-Ag & Co-Au alloy with different shapes, sizes, and the different surrounding medium is given in Table 1 & 2.
Fig. 7. Calculated absorption efficiency of Co-Ag an Co-Au bimetallic alloys nanoparticles as a function of size (a, b) represent LSPR wavelength in refractive index of an ambient medium is 1.00, (c, d) represent LSPR wavelength in refractive index of an ambient medium is 1.33 and (e, f) represent LSPR wavelength in refractive index of an ambient medium is 1.53. Table 1 Summarized values of LSPR peaks at λmax of Co-Ag alloy NPs of various shapes and sizes in the different surrounding medium (n). reff Absorption LSPR peak at the maximum wavelength (nm) of Co-Ag nanoparticles (nm)
20 30 40 50 60
Ambient medium (n) = 1.00 Ambient medium (n)= 1.33 Sphere Prolate Cube Rect Sphere Prolate Cube Rect angular angular 334.08 332.64 349.88 407.72 361.35 339.23 411.07 510.97 335.37 334.08 365.41 436.15 384.14 350.69 444.74 575.85 345.49 333.40 384.95 478.78 420.96 356.37 493.72 652.17 351.84 333.40 403.11 537.57 361.35 338.30 533.38 722.51 332.90 327.68 382.32 602.40 371.45 342.11 476.81 821.16
Ambient medium (n) = 1.53 Prolate Cube Rect angular 390.24 360.00 407.97 594.28 427.50 377.76 443.92 673.45 493.56 370.65 486.71 755.22 390.24 356.6 531.28 845.30 415.57 368.96 476.46 948.86
Sphere
Table 2 Summarized values of LSPR peaks at λmax of Co-Au alloy NPs of various shapes and sizes in the different surrounding medium (n). reff Absorption LSPR peak at the maximum wavelength (nm) of Co-Au nanoparticles (nm)
20 30 40 50 60
Ambient medium (n) = 1.00 Sphere Prolate Cube Rect angular 211.86 517.32 248.31 269.54 525.50 304.59 250.03 346.15 535.82 365.03 311.55 409.76 576.70 461.18 351.46 513.38 623.62
Ambient medium (n) = 1.33 Prolate Cube Rect angular 262.68 219.73 275.53 588.32 344.84 279.62 524.52 623.88 507.31 360.02 546.72 684.10 520.27 490.85 585.07 752.63 525.30 500.97 601.43 831.79
Sphere
Ambient medium (n) = 1.53 Prolate Cube Rect angular 333.54 266.58 520.09 656.84 515.37 359.25 526.89 710.83 533.67 506.61 549.86 786.37 572.62 512.65 583.54 867.35 505.06 314.86 588.89 964.43
Sphere
Furthermore, simulation for scattering LSPR at λmax peaks for the sphere, prolate, cube and rectangular geometry of Co-Ag and Co-Au alloys with varying (20 - 60 nm) particle size
is presented in Fig. S2 (a-f) of supplementary information. The scattering spectra for considered elongated bimetallic nanostructures is found between 333-588 nm, 347-622 nm, 363-920 nm wavelengths of Co-Ag alloys and 214-626 nm, 213-816 nm, 211-939 nm wavelengths of Co-Au alloys with the varied NPs size in three different ambient (n=1.00 1.33 and 1.53) medium, respectively. The LSPR peaks shift towards the longer wavelength region with the particle size of range between 20 nm to 60 nm and the geometry of the NPs also shows a direct impact on the LSPR peaks. Thus, a rectangular-shaped nanostructure shows maximum LSPR peak at λmax in comparison to other considered nanostructures. Likewise, the scattering efficiency of Co-Ag and Co-Au alloy is shown in Fig. S3 of supplementary information. The scattering efficiency of LSPR peaks increases, when the NPs size increases. In addition, rectangular shape nanostructure shows maximum efficiency as a function of sizes in the different surrounding medium and is depicted in Fig. S3 (a-f) of supplementary information. The greatest LSPR peaks positions and efficiencies are obtained for rectangular nanostructure of Co-Au alloy in comparison to Co-Ag alloy NPs and their spectra of four different shapes are as follows: prolate < sphere < cube < rectangular. Hence, the main feature of a magneto-plasmonic bimetallic alloy of optical properties is depending on geometry and size of NPs. Thus, the absorption and scattering peaks are found in the range of 211-964 nm and 211–939 nm wavelengths and can be tuned in UV, visible, and near-infrared regions with changing the considered shapes and sizes. For details study, the value of scattering LSPR peaks of Co-Ag & Co-Au alloy with different shapes, sizes, and the surrounding medium is given in Table 3 & 4. Table 3 Summarized values of LSPR peaks at λmax of Co-Ag alloy NPs of various shapes and sizes in the different surrounding medium (n). reff Scattering LSPR peak at the maximum wavelength (nm) of Co-Ag nanoparticles (nm)
20 30 40
Ambient medium 1.00 Sphere Prolate Cube Rect angular 338.42 333.91 364.13 417.06 349.76 336.30 337.46 435.17 363.43 341.08 395.48 475.47
Sphere 365.67 385.33 421.72
Ambient medium 1.33 Prolate Cube Rect angular 347.98 410.38 500.49 364.71 443.69 556.49 381.44 492.72 632.70
Ambient medium 1.53 Prolate Cube Rect angular 387.19 363.98 408.89 571.48 421.50 386.66 444.61 653.89 474.80 413.02 493.79 734.48
Sphere
50 60
381.76 412.55
357.79 372.89
424.99 463.05
527.83 588.38
467.15 529.30
410.93 445.63
550.37 622.06
676.19 789.22
535.76 613.87
447.32 506.23
548.41 615.74
826.56 920.79
Table 4 Summarized values of LSPR peaks at λmax of Co-Au alloy NPs of various shapes and sizes in the different surrounding medium reff Scattering LSPR peak at the maximum wavelength (nm) of Co-Au nanoparticles (nm)
20 30 40 50 60
Ambient medium 1.00 Sphere Prolate Cube Rect angular 529.43 214.21 215.76 535.82 268.13 215.76 280.2 556.26 312.32 274.17 380.85 588.96 384.36 314.47 419.20 626.66
Sphere 213.43 271.25 377.15 544.78 582.74
Ambient medium 1.33 Prolate Cube Rect angular 598.08 258.01 277.17 630.28 280.40 564.43 678.39 380.46 605.32 737.83 535.82 654.19 816.22
Sphere 511.23 527.28 549.86 594.88 647.99
Ambient medium 1.53 Prolate Cube Rect angular 211.68 527.09 656.33 269.50 542.83 703.48 376.76 568.09 769.10 542.05 607.42 853.19 579.24 656.28 939.48
Fig. 8 shows the variation of full width at half maxima (FWHM) with the size of Co-Ag and Co-Au alloys NPs for the sphere, prolate, cube and rectangular-shaped nanostructures in three different surrounding medium. Since FWHM is an important parameter for potential use in solar cells and sensing. A linear change in FWHM is observed with the particle size of Co-Ag alloy and further FWHM decreases, above the 50 nm particle size of Co-Au alloy for all considered nanostructures in 1.00 ambient medium and is clearly shown in Fig. 8 (a, b). The prolate shape nanostructure shows the minimum value of FWHM in comparison to other considered nanostructure and further, FWHM value decreases and increases for above 50 nm size of Co-Ag and Co-Au alloys respectively, in three different surrounding medium. Large size effects such as radiation damping and dynamic polarization show the important role and direct impact towards large width value. Therefore, these large size effects are responsible for a larger increase in FWHM of 50 nm and 60 nm sizes. Furthermore, depending upon the sizes, shapes and different surrounding medium, the FWHM has been controlled from 41-415 nm, 33-215 nm, 74-361 nm, 129-452 of Co-Ag and 129-512 nm, 86-405 nm, 83-406 nm, 293-454 nm for Co-Au alloy NPs in the broad region of the spectrum with sphere, prolate, cube and rectangular-shaped nanostructures.
Fig. 8. FWHM of Co-Ag and Co-Au bimetallic alloys nanoparticles as a function of size (a, b) represent LSPR wavelength in refractive index of an ambient medium is 1.00, (c, d) represent LSPR wavelength in refractive index of an ambient medium is 1.33 and (e, f) represent LSPR wavelength in refractive index of an ambient medium is 1.53.
Further studies are carried out for LSPR based sensors technology because of low-cost fabrication and simple sensors setup [16, 35, 36]. The LSPR of any material is sensitive to the shape, size of the NPs as well as their surrounding medium. There are different mathematical and numerical frameworks to study the sensing properties of NPs [37-39]. However, in the present study, sensitivity analysis is carried out on the basis of ratio of peak shift to change in refractive index of the embedded medium. In general, sensing performance of NPs and nanoarray is analyzed based on scattering and absorption or reflection spectroscopy
respectively. Thus, the performance of sensors can be evaluated through sensitivity (S) and figure of merit (FOM) measurements. Therefore, S is calculated by using slope of the line (S=bn+a). Fig.9 shows the effect of shapes on sensing performance as a function of sizes (20 nm to 60 nm). Fig. 9 (a) demonstrate the sensitivity of all considered nanostructures of CoAg alloy and order as follows: S (rectangular) > S(cube) > S(sphere) > S (prolate). Sensitivity corresponding to considered nanostructures of Co-Ag alloy as a function of size increases, when the size of NPs increases. Also, prolate nanostructure Co-Ag alloy presents the lowest sensitivity with an increment of particle size. In the case of Co-Au alloy, as the NPs size increases, corresponding sensitivity decreases for the sphere, prolate and cube nanostructures as shown in Fig. 9 (b). However, ∆λres (peak shift) is also strongly dependent on the size of the NPs. The sensitivity curve of different considered shapes exhibits large swing and crossover due to the combined effect of both: size of NPs and refractive index of surrounding medium. Moreover, the sensitivity of rectangular nanostructure increases when NPs size increases. Furthermore, we compared the sensing performance of all considered nanostructures as a function size of Co-Ag and Co-Au bimetallic alloys. The highest sensitivity is found of Co-Ag alloy for rectangular in comparison to Co-Au alloy rectangular nanostructure. Thus, it is concluded that the sensitivity of Co-Ag alloy of all considered geometries shows their dependence on the particle size and for Co-Au alloy sensitivity shows the opposite trend with NPs size. Moreover, the sensitivity of rectangular nanostructure shows the direct impact of particle size of Co-Ag and Co-Au alloys and hence, Co-Ag alloy has the highest sensitivity.
Fig. 9. Calculated sensitivity of (a) Co-Ag and (b) Co-Au alloys nanoparticle with different shapes as a function of sizes.
Fig. 10. Calculated FOM of (a) Co-Ag and (b) Co-Au alloys nanoparticle with different shapes as a function of sizes.
Fig. 10 (a, b) demonstrates the figure of merit (FOM) for all considered Co-Ag and Co-Au alloys nanostructures as a function of sizes in the liquid surrounding medium i.e. 1.33 refractive index. The result shows that the sensitivity (S) increases, while the FOM decreases as the size of NPs increases. In the case of Co-Ag alloy, the FOM shows the direct impact on sensitivity with particle size. If the sensitivity increases of a particle size then FOM also increases for the same size of all nanostructures. On the other hand, FOM presents the same trend as shown by the sensitivity (S) parameter with a particle size of Co-Au alloy. For example, sensitivity is decreasing for sphere, prolate and cube as a function of size then FOM also decreasing for the same structures with particle size. Hence, the order of FOM as follows: FOM (cube) > FOM (rectangular) > FOM (sphere) > FOM (prolate) for Co-Ag and FOM (prolate) > FOM (sphere) > FOM (cube) > FOM (rectangular) for Co-Au alloy nanostructures.
Conclusions The nanostructures presented here shows that it is now possible to describe many optical properties in terms of absorption scattering of Co-Ag and Co-Au alloy nanoparticles with complex geometry. The calculation of other properties, such as LSPR (λ max,) FWHM, RIS, and FOM, provides new challenges for elongated particles that are an important component of our current research. From studies, it is concluded that the optical properties of both CoAg and Co-Au bimetallic alloy nanoparticles are enhanced and tuned between UV-NIR region of EM spectrum with varying the sizes, shapes, compositions and the refractive index of the surrounding medium. Furthermore, the sensitivity of all considered nanostructures of Co-Ag alloy increases when the size of NPs increases and prolate shape depicts the lowest sensitivity. The highest sensitivity is found for the Co-Ag alloy of rectangular shape in contrast Co-Au alloy. From results, the nanostructures of Co-Ag and Co-Au alloys NPs open their potential in desired applications such as plasmonic devices, optical imaging, solar cells, and further progressive work for nanosensors, based on sensitivity and figure of merit, can be used in sensing environment technology.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment The author (P. Bhatia) would like to acknowledge the S.L.I.E.T. Longowal for providing financial support. The author thanks B.T. Draine and P.J. Flatau for using their DDA code DDSCAT 7.0.
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Tunable Plasmonic Properties of Elongated Bimetallic Alloys Nanoparticles towards deep UV-NIR Absorbance and Sensing Pradeep Bhatia , S.S. Verma , M.M. Sinha PII: DOI: Reference:
S0022-4073(19)30553-9 https://doi.org/10.1016/j.jqsrt.2019.106751 JQSRT 106751
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Journal of Quantitative Spectroscopy & Radiative Transfer
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Please cite this article as: Pradeep Bhatia , S.S. Verma , M.M. Sinha , Tunable Plasmonic Properties of Elongated Bimetallic Alloys Nanoparticles towards deep UV-NIR Absorbance and Sensing, Journal of Quantitative Spectroscopy & Radiative Transfer (2019), doi: https://doi.org/10.1016/j.jqsrt.2019.106751
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Highlights The present research is to highlight the studies of bimetallic alloys nanoparticles of magneto-plasmonic nanostructures. The influence of size, shapes, compositions and surrounding medium for Co-Ag and Co-Au alloy nanoparticles results in significant change in optical properties. The spectra are found between 211-964 nm wavelengths ranges and can be tuned in the deep UV-NIR region of the electromagnetic (EM) spectrum. Tunability in sizes and shapes opens new potential use with desired applications in biomedical fields.
Tunable Plasmonic Properties of Elongated Bimetallic Alloys Nanoparticles towards deep UV-NIR Absorbance and Sensing Pradeep Bhatia*, S. S. Verma and M. M. Sinha
Department of Physics, Sant Longowal Institute of Engineering and Technology, Sangrur148106, Punjab, India (November 08, 2019) *
Email: Pradeep Bhatia ([email protected])
Abstract Combining magnetic nanoparticles with noble metallic nanoparticles to construct multifunctional alloy nanostructures has become a powerful tool for imaging sensing, medicine, biology, and cancer treatment and photothermal therapy. This work reports a study regarding the plasmonic properties of Co-Ag and Co-Au bimetallic alloys nanostructures on the electromagnetic spectrum. In this paper, the optical properties of magnetic and plasmonic nanoparticles (NPs) with different shapes, sizes, compositions, and surrounding medium are investigated by using the discrete dipole approximation (DDA) technique. The absorption and scattering localized surface plasmon resonance (LSPR) peak positions are found between 211-964 nm wavelengths ranges and can be tuned in the deep UV-NIR region of the EM spectrum in accordance with the desired application. Further, we calculate the refractive index sensitivity (S) and figure of merit (FOM) of LSPR based nanosensors. Although, LSPR based sensors undergo low FOM as compared to conventional surface plasmon resonance (SPR) sensors due to high losses from the radiative damping of localized surface plasmons (LSPs) waves. However, LSPR based sensors have potential applications in gas and biosensors technology viz. medical and environmental monitoring applications. In this work, the main feature of LSPR peak is dependent on nanoparticle shapes, sizes, compositions, and ambient medium and have ordered as prolate < sphere < cube < rectangular shape NPs. These
results suggest that the nanostructures of Co-Ag and Co-Au alloys associated with LSPR tunability and sensitivity can be used in biomedical and sensing environment technology such as detection of chemical and catalytic events. Keywords: Nanoparticles, Sensor, Refractive Index Sensitivity, Figure of Merit, Localized Surface Plasmon Resonance
Introduction Metallic nanoparticles have shown remarkably properties in comparison to bulk counterpart due to their surface to volume ratio and its localized surface plasmon resonances (LSPRs) peak in electromagnetic (EM) spectrum [1]. The collective oscillation of conduction (free) electrons in metal nanostructures, known as localized surface plasmon (LSP), made much attraction since few decades due to its variety of physical phenomena and chemical processes. The collective oscillations of electrons in LSPRs phenomenon result in a strong enhancement in optical (scattering and absorption) intensities of nanoparticles and allowing their use in modern photonics, photothermal therapy, drug delivery and many more in biomedical and biotechnology [2, 3]. The displacement of plasmon peak on the spectral scale is very sensitive to metallic NPs (an effective dielectric function of materials), size, shape, and surrounding environment [4-6]. For fundamental and practical research, modeling of nanoparticle (NPs) with different parameters and calculating their optical properties is an important aspect. Nanorods shaped nanostructures with varying aspect ratio set their examples of such tunability, in which the position of LSPR peaks varies from visible to infrared region on the EM spectrum [7-9]. In addition, other alloy nanostructures like a triangular prism, nanocubes, prolate, rectangular geometry and varying core-shell thickness have been shown their LSPR peaks tunability [1013]. Furthermore, LSPR based sensors performance has been described, including a variation
of geometry, and structure configuration (alloy or core-shell). Although, SPR based sensor is common and commercialized used in gas sensors and bio-sensor technology to monitor the environmental and medical issues. Tittl A et al. introduced a probing technique based on plasmonic NPs for monitoring local chemical reactions [14]. Bingham J. M. and co-workers studied the LSPR spectroscopy to detect the small changes (< 3 x 10-4 RIU) in bulk refractive index by NPs. H2, N2, and Ar were used to control the NPs surrounding environment [15]. In contrast, the LSPR sensor offers low-cost production, simple setup, and detection of local change in the refractive index, which is not the case with SPR sensors [16, 17]. Our main aim of the research is to enhance the efficiency of alloy nanostructures and attain LSPR peaks tunability between UV and NIR regions of EM spectrum. The present work is carried out with a theoretical understanding of cobalt alloys NPs. Transition metal cobalt (Co) is a peculiar choice because it has good thermal stability and ferromagnetic properties which may introduce versatile and tunable properties [18]. However, high toxicity and less optical properties of pure cobalt limit their use. Although, cobalt NPs showing their potential in magnetic separation and magnetic tape [19, 20]. The configuration of alloy nanostructures, which mixed the transitions metal (Co) with noble metal and become a new approach with remarkable applications. Introducing transition metal (Co) into noble (Ag and Au) metallic system not only reduces the cost but also increases their stability and multifunctional properties. Wang L. et al. studied the enhancement in plasmonic and magneto-optical (MO) Faraday rotation of core-shell Co-Ag with varying concentration ratios from 30:70, 50:50 and 80:20 in 1.5 refractive indexes of the ambient medium [21]. Recently, some published work showed the direct effect of magneto-plasmonic bimetallic alloys on LSPR peaks and its based sensor by tuning the shapes, sizes and surrounding medium [22, 23]. Also, many studies have been
reported on the simple (spherical) configuration of Co-Au and Co-Ag as core-shell and alloy [24, 25], studies on the mechanism of LSPR-based nanosensing are still needed. In this paper, we investigated the optical properties of elongated Co-Ag and Co-Au alloy nanoparticles by increasing the size of the sphere and cube nanostructure in one direction to see the influence of sizes, composition, and the surrounding medium. Further, the effect of sizes and shapes on the sensing performance of bimetallic NPs is considered. Therefore, this work focus on the theoretical understanding of unique features of elongated nanostructures made of Co-Ag and Co-Au alloys nanoparticles. Finally, we explored the optical properties of bimetallic alloys NPs by using the discrete dipole approximation (DDA) method. Absorption and scattering spectra of the nanostructures in the UV-NIR region of EM spectrum may open new potential in imaging, cancer therapy, and medicine. Further, the optical characteristics of presently considered nanostructures can find significant applications in solar cells and sensors.
Theoretical Method It is well known that Mie scattering theory, when metal NPs interact with incident light with the help of Maxwell’s equations, is widely used and an exact analytical solution of the scattering of electromagnetic waves, to calculates optical properties for only simple geometry i.e. sphere. For complex geometry studies and theoretical research of metallic nanoparticles to practical application, it is important to understand the interaction of light with NPs at the nanoscale, necessary to use the approximation methods for their calculations. To calculate the optical properties of NPs of various shapes, composition, and sizes, the DDA method is employed. Since DDA is a flexible method and a powerful tool for calculating the optical properties of NPs with various geometry. The various quantitative characteristics calculated by using the DDA method which available as DDSCAT software in open access [26, 27]. The following quantities are calculated by DDSCAT: (I) absorption efficiency factor; Qabs ≡
Cabs/πr2eff, where Cabs is the absorption cross-section, (II) scattering efficiency factor; Qsca ≡ Csca/πr2eff, where Csca is the scattering cross-section, and (III) extinction efficiency factor; Qext ≡ Qsca + Qabs, where reff is defined below as an effective radius. DDA method solves Maxwell’s equation for the electromagnetic wave interaction with NPs in which NPs are discretized into arrays of dipoles or polarized points. In our calculation, the number of dipoles is used ~7 X 104 in order to accurately represent the target geometry and for better convergence and volume of the target is defined by the effective radius (reff = (3V/4π)1/3), the radius of an equal volume sphere. For spherical shape, the volume ‘V’ is 4/3πr3, thus, aeff = (3*(4/3πr3)/4π)1/3 hence, aeff = ~ r; in this way, we have considered radius ‘r’ =20, 30, 40, 50, 60 nm. For non-spherical (rectangular) shape, we considered a*b*c, with a : b : c :: 20 : 60 : 60 with aspect ratio 1 : 3 : 3, then V = abc = 9a 3. Thus, aeff ≡ (3V/4π)1/3 = (27/4π)1/3a = 1.2905*a. Now, our target should have thickness ‘a’ = 0.016 nm in the x-direction to make the same effective radius as considered in case of sphere i.e. 20 nm. The complex dielectric function of nanomaterials considered as input parameter and is taken from Johnson and Christy [28, 29] and further assumed to be the same as that of the bulk materials. This dielectric function of individual metal NPs is used to form the alloy with different compositions and hence, the effective complex dielectric function is responsible for the change in the optical properties of alloy NPs. In the previous work, we made a comparison of experimentally available imaginary (ε2) part of particular alloy composition with our calculated average weight percentage method and concluded that experimental ε2 is remarkably higher than our calculated method [22]. However, the numbers of studies are reported in the literature for noble metal alloy composition based on average weight percentage method and this method is satisfactory for simulation and evaluating the trend in
multimetallic nanostructures. Hence, average weight percentage method is used in the present studies to analyze the optical properties of bimetallic nanoparticles. Furthermore, the effective refractive index value for both Cox-Ag1-x and Cox-Au1-x bimetallic alloys with closed composition x= 0.30, 0.50 and 0.80 are calculated by using the weight percentage method. Different refractive indices (n)= 1.00, 1.33, and 1.53 of the ambient medium are considered. Variation of the refractive index of surrounding in increasing order results increases in the screening of the surface electrons. Consequently, restoring force increases and hence, low energy required to excite the free (charges) electrons. Therefore, NPs can be used as LSPR sensors where both the polarization and restoring force affect the sensitivity of sensors. The sensing performance of nanosensors can be evaluated by refractive index sensitivity (S) and FOM measurements. The refractive index sensitivity (S) is defined as the ratio of LSPR resonance wavelength shift (∆λres) to the change in the refractive index of the surrounding medium (∆nm) i.e. S = (∆λres/∆nm). The FOM is defined as FOM = S/FWHM, where FWHM stands for full width at half maximum. In addition, we have explored the composition effects on LSPR peaks in a considered alloy of different shapes and sizes. All the simulations are made by the DDA method in the wavelength range from 200 to 1300 nm. Thus, the present calculation is carried out for bimetallic alloys NPs.
Results and Discussion The calculation for optical characteristics in terms of Qabs and Qsca of magnetic and plasmonic (Co-Ag and Co-Au) elongated bimetallic alloy nanostructures is performed. The elongated structures e.g. prolate and rectangular is modeled from basic structures i.e. sphere and cube by increasing their size in one direction. Furthermore, the optical properties of nanostructures are theoretically calculated by considering the linear polarization only in one (i.e. y-axis) direction and the directional polarization effect of incident light has not been
considered. The present calculations are carried out by using the DDA method. From the practical point of view, great interest presents the research of absorption, scattering, and extinction in optical properties of metallic NPs depending on their shapes. Therefore, NPs geometry plays an important and shows a direct impact on tuning the LSPR peak position. Furthermore, Ammari H. et al. [30-32] have also mathematically defined the notation of plasmonic resonance to analyze the shift and broadening of the plasmon resonance with changes in size and shape of the nanoparticles to investigate the scattering and absorption enhancements by plasmon resonant nanoparticles and obtained optimal bounds on the absorption and scattering enhancement. In case of symmetric shape NPs, only on the dipolar peak is induced, whereas asymmetric shape NPs, a higher order of electric dipole modes is induced. LSPR peaks not dependent on only shapes, sizes, and compositions but depend mainly on the dielectric function of materials. To achieve good plasmonic properties, the real part of dielectric function (ε1) of NPs requires negative value and imaginary part (losses) of dielectric function (ε2) of the NPs assumed to be small. Our calculated dielectric function (ε1
Fig. 1. Description of real and imaginary parts (a) – (d) of effective dielectric function for pure and different alloy compositions as a function of wavelength.
Sphere
Prolate
Cube
Rectangular
Fig. 2. Illustration of the elongated shape of alloy nanoparticles.
& ε2) for Cox-Ag1-x and Cox-Au1-x (x=0.30, 0.50 and 0.80) are shown in Fig. 1. All configured nanoparticles are visualized by ParaView; Visualization ToolKit (VTK) is a DDA extended subroutine program written by Jalel Chergui [33] and represented in Fig. 2. In general, plasmon oscillations in NPs experience both intrinsic and radiation damping due to electron-electron collisions and decay of collective oscillations into photons respectively. Noble metal NPs can be utilizing for resonant and helps to reduce intrinsic damping whereas radiation damping constitutes the dominating effect for larger particle size (few tens of nanometers). Therefore, we considered the Co-Ag and Co-Au bimetallic alloys of 20 nm to 60 nm particle sizes to investigate the optical properties of four different shapes nanostructures. The optical properties of the NPs depend on the sizes and shapes and can also be tuned by varying the compositions of metallic NPs. Thus, composition-dependent effective dielectric functions result in the different plasmonic properties on the electromagnetic (EM) spectrum of bimetallic alloy. Fig. 3 shows the simulated absorption (solid lines) and scattering (dotted line) efficiencies of Cox-Ag1-x, and Cox-Au1-x for 60 nm particle size with three different compositions (x) = 0.30, 0.50, and 0.80, in water ambient medium (n=1.33). As the concentration (composition) of Ag and Au in Co alloy increases, the peak position shift towards the longer wavelength region and further intensity of peak increases with the increasing concentration of Ag and Au. The efficiency of Co-Ag NPs is
remarkably high as compared to Co-Au NPs, clearly shown in Fig. 3 (a, b). Tuning in LSPR and their intensity peak shows dependency on the alloy composition. Therefore, all the further calculations are done with 50:50 composition of Co-Ag and Co-Au alloy to maintain low cost and obtain a decent optical response as well as magnetic effects at a reasonable cost.
Fig. 3. Effect of compositions on LSPR peak spectra (a) Co-Ag and (b) Co-Au alloys of 60 nm nanoparticles particle size.
The influence of size, shape and dielectric environment on optical properties of metal NPs is reported by Kelly K. L. et al. [34]. They studied the optical properties such as extinction, scattering cross-section, and local field of spherical and non-spherical shapes of silver nanoparticles. Thus, we have calculated the absorption and scattering LSPR peak of spherically shaped Co0.50-Ag0.50 alloy NPs as a function particle size as shown in Fig. 4(a, b). Furthermore, absorption and scattering LSPR peak of Co0.50-Au0.50 alloy NPs are calculated also and shown in Fig. S1(a, b) (supplementary information). The result shows that absorption LSPR at λmax is found in the range between 361 nm to 420 nm and 262 nm to 525 nm wavelength and scattering LSPR λmax 365 nm to 529 and 213 nm to 582 nm wavelength of both Co-Ag and Co-Au alloys, respectively, on EM spectrum. Hence, redshift is found in Co-Ag alloy nanostructures. The LSPR peaks of NPs exhibit a blue-shift & red-shift due to lower and higher dielectric constant (ε’’) of metallic NPs at nanoscale. The dielectric constants i.e. imaginary part (ε’’) of Au is 1.624, Ag is 0.403, and Co is17.05 as given in ref. [29]. Under similar conditions (size, shape, and surrounding medium) the LSPR peaks of Au
or Ag NPs is red or blue-shifted whereas it will be red-shifted for Co NPs. Thus, the LSPR peaks for Co-Au and Co-Ag alloy NPs are red-shifted as compared to pure Au or Ag NPs. Furthermore, as the particle size increase, absorption peak efficiency increases for 20 and 30 nm sizes and then decreasing for 40 nm to 60 nm particle sizes with shifting of LSPR peak at λmax towards higher wavelength region. On the other hand, scattering efficiency increases along with LSPR at λmax peak with an increment of particle size. Furthermore, scattering efficiency systematically increases and absorption efficiency decreases with large Co-Ag alloy particle size. With large particle size, retardation effects: retardation damping and dynamic polarization are associated and cannot be ignored. Since these effects reduce the intensity of the plasmon along with broadening of the peak. Thus, surface scattering and radiation damping effects cause a decrease in absorption efficiencies. Moreover, scattering efficiency of Co-Ag and Co-Au bimetallic alloy NPs linear increases with the nanoparticle sizes. However, the absorption and scattering efficiency of Co-Ag NPs is remarkably high as compared to Co-Au NPs. Hence, LSPR peaks at λmax shifted towards longer wavelength region and their corresponding efficiency increases of scattering spectra as particle size increases.
Fig. 4. Calculated LSPR spectra for (a) absorption and (b) scattering peak of spherical Co.50-Ag.50 alloys as a function particle sizes. The refractive index of an ambient medium is 1.33.
Fig. 5. Calculated spectra for (a) absorption and (b) scattering of Co-Ag alloy nanoparticle with different shapes.
In order to analyze the influence of shapes on optical properties, we considered the four different geometry of bimetallic alloy. The absorption and scattering spectra with 60 nm particle size of Co.50-Ag.50 alloy composition for different considered shapes as shown in Fig. 5(a, b) (refractive index of the surrounding medium is 1.33). The LSPR peaks shifting towards the higher wavelength regime associated with the change in NPs geometry. Besides, optical efficiency also shifted with a change in NPs shapes. As the shapes of bimetallic alloy nanoparticles vary from sphere to prolate and cube to rectangular, the rectangular nanostructure yield the maxima of LSPR peak at λmax and their efficiency in comparison to other considered nanostructures in the order of prolate < sphere < cube < rectangular. The absorption and scattering LSPR peak at λmax are tuned in the wavelength range of 341-814 nm and 438-787 nm, respectively, on the EM spectrum of four different considered alloy nanostructures. This calculated LSPR peak position spectrum lies in the different regions of the EM spectrum due to the variation of free electron oscillations in the different geometry volume. Hence, LSPR peaks at λmax and their corresponding efficiencies show dependence on NPs size and geometries of the NPs. Thus, we have investigated the absorption and scattering spectra of Co-Ag and Co-Au bimetallic alloy nanoparticles with shapes and sizes in three different ambient medium.
Fig. 6. Calculated absorption LSPR peak position of Co-Ag and Co-Au alloys nanoparticles as a function of particle size (a, b) represent LSPR wavelength in refractive index of an ambient medium is 1.00, (c, d) represent LSPR wavelength in refractive index of an ambient medium is 1.33 and (e, f) represent LSPR wavelength in refractive index of an ambient medium is 1.53.
Fig. 6 shows the absorption LSPR peaks of Co-Ag and Co-Au alloys as a function of sizes with four different shapes. The absorption LSPR peak of the sphere, prolate, cube and rectangular shapes are found in the wavelengths range of 327-602 nm, 338-821 nm, 356-948 nm of Co-Ag alloys and 211-623 nm, 219-831 nm, 266-964 nm of Co-Au alloys within different ambient (n=1.00 1.33 and 1.53) medium, respectively as shown in Fig. 6(a-f). As the NPs size increases and shape changes, LSPR peaks at λmax shifted towards the longer wavelength regime on the EM spectrum. For both sphere and prolate geometry of Co-Au
alloy NPs, no LSPR peaks have been appearing for 20 and 30 nm particle size because of small oscillation of free electrons and hence, no variation in plasmon wavelength. Similarly, Fig. 7(a-f) shows the efficiency (intensity) corresponding to LSPR peak of Co-Ag and Co-Au alloys for the sphere, prolate, cube and rectangular-shaped NPs in three different surrounding medium. The absorption intensity value increases of 20 to 30 nm and decreasing for above 30 nm particle size with a change in NPs geometry and the surrounding medium. Thus, it is concluded that rectangular nanostructure has maximum LSPR peak at λmax and intensity (efficiency) as compared to other considered nanostructures and has to order rectangular > cube > sphere > prolate. It is well known that the absorption or scattering efficiency is a function of NPs volume and geometry. The theoretical description of rectangular shape has larger efficiency as compared to other considered shapes because of its large volume. Moreover, we have considered the elongated nanostructures viz. prolate shape (from sphere) and rectangular shape (from cube). Thus, rectangular shape gives high efficiency as compared to other considered shapes. For details study of LSPR peaks, the values of absorption LSPR peaks at λmax of Co-Ag & Co-Au alloy with different shapes, sizes, and the different surrounding medium is given in Table 1 & 2.
Fig. 7. Calculated absorption efficiency of Co-Ag an Co-Au bimetallic alloys nanoparticles as a function of size (a, b) represent LSPR wavelength in refractive index of an ambient medium is 1.00, (c, d) represent LSPR wavelength in refractive index of an ambient medium is 1.33 and (e, f) represent LSPR wavelength in refractive index of an ambient medium is 1.53. Table 1 Summarized values of LSPR peaks at λmax of Co-Ag alloy NPs of various shapes and sizes in the different surrounding medium (n). reff Absorption LSPR peak at the maximum wavelength (nm) of Co-Ag nanoparticles (nm)
20 30 40 50 60
Ambient medium (n) = 1.00 Ambient medium (n)= 1.33 Sphere Prolate Cube Rect Sphere Prolate Cube Rect angular angular 334.08 332.64 349.88 407.72 361.35 339.23 411.07 510.97 335.37 334.08 365.41 436.15 384.14 350.69 444.74 575.85 345.49 333.40 384.95 478.78 420.96 356.37 493.72 652.17 351.84 333.40 403.11 537.57 361.35 338.30 533.38 722.51 332.90 327.68 382.32 602.40 371.45 342.11 476.81 821.16
Ambient medium (n) = 1.53 Prolate Cube Rect angular 390.24 360.00 407.97 594.28 427.50 377.76 443.92 673.45 493.56 370.65 486.71 755.22 390.24 356.6 531.28 845.30 415.57 368.96 476.46 948.86
Sphere
Table 2 Summarized values of LSPR peaks at λmax of Co-Au alloy NPs of various shapes and sizes in the different surrounding medium (n). reff Absorption LSPR peak at the maximum wavelength (nm) of Co-Au nanoparticles (nm)
20 30 40 50 60
Ambient medium (n) = 1.00 Sphere Prolate Cube Rect angular 211.86 517.32 248.31 269.54 525.50 304.59 250.03 346.15 535.82 365.03 311.55 409.76 576.70 461.18 351.46 513.38 623.62
Ambient medium (n) = 1.33 Prolate Cube Rect angular 262.68 219.73 275.53 588.32 344.84 279.62 524.52 623.88 507.31 360.02 546.72 684.10 520.27 490.85 585.07 752.63 525.30 500.97 601.43 831.79
Sphere
Ambient medium (n) = 1.53 Prolate Cube Rect angular 333.54 266.58 520.09 656.84 515.37 359.25 526.89 710.83 533.67 506.61 549.86 786.37 572.62 512.65 583.54 867.35 505.06 314.86 588.89 964.43
Sphere
Furthermore, simulation for scattering LSPR at λmax peaks for the sphere, prolate, cube and rectangular geometry of Co-Ag and Co-Au alloys with varying (20 - 60 nm) particle size
is presented in Fig. S2 (a-f) of supplementary information. The scattering spectra for considered elongated bimetallic nanostructures is found between 333-588 nm, 347-622 nm, 363-920 nm wavelengths of Co-Ag alloys and 214-626 nm, 213-816 nm, 211-939 nm wavelengths of Co-Au alloys with the varied NPs size in three different ambient (n=1.00 1.33 and 1.53) medium, respectively. The LSPR peaks shift towards the longer wavelength region with the particle size of range between 20 nm to 60 nm and the geometry of the NPs also shows a direct impact on the LSPR peaks. Thus, a rectangular-shaped nanostructure shows maximum LSPR peak at λmax in comparison to other considered nanostructures. Likewise, the scattering efficiency of Co-Ag and Co-Au alloy is shown in Fig. S3 of supplementary information. The scattering efficiency of LSPR peaks increases, when the NPs size increases. In addition, rectangular shape nanostructure shows maximum efficiency as a function of sizes in the different surrounding medium and is depicted in Fig. S3 (a-f) of supplementary information. The greatest LSPR peaks positions and efficiencies are obtained for rectangular nanostructure of Co-Au alloy in comparison to Co-Ag alloy NPs and their spectra of four different shapes are as follows: prolate < sphere < cube < rectangular. Hence, the main feature of a magneto-plasmonic bimetallic alloy of optical properties is depending on geometry and size of NPs. Thus, the absorption and scattering peaks are found in the range of 211-964 nm and 211–939 nm wavelengths and can be tuned in UV, visible, and near-infrared regions with changing the considered shapes and sizes. For details study, the value of scattering LSPR peaks of Co-Ag & Co-Au alloy with different shapes, sizes, and the surrounding medium is given in Table 3 & 4. Table 3 Summarized values of LSPR peaks at λmax of Co-Ag alloy NPs of various shapes and sizes in the different surrounding medium (n). reff Scattering LSPR peak at the maximum wavelength (nm) of Co-Ag nanoparticles (nm)
20 30 40
Ambient medium 1.00 Sphere Prolate Cube Rect angular 338.42 333.91 364.13 417.06 349.76 336.30 337.46 435.17 363.43 341.08 395.48 475.47
Sphere 365.67 385.33 421.72
Ambient medium 1.33 Prolate Cube Rect angular 347.98 410.38 500.49 364.71 443.69 556.49 381.44 492.72 632.70
Ambient medium 1.53 Prolate Cube Rect angular 387.19 363.98 408.89 571.48 421.50 386.66 444.61 653.89 474.80 413.02 493.79 734.48
Sphere
50 60
381.76 412.55
357.79 372.89
424.99 463.05
527.83 588.38
467.15 529.30
410.93 445.63
550.37 622.06
676.19 789.22
535.76 613.87
447.32 506.23
548.41 615.74
826.56 920.79
Table 4 Summarized values of LSPR peaks at λmax of Co-Au alloy NPs of various shapes and sizes in the different surrounding medium reff Scattering LSPR peak at the maximum wavelength (nm) of Co-Au nanoparticles (nm)
20 30 40 50 60
Ambient medium 1.00 Sphere Prolate Cube Rect angular 529.43 214.21 215.76 535.82 268.13 215.76 280.2 556.26 312.32 274.17 380.85 588.96 384.36 314.47 419.20 626.66
Sphere 213.43 271.25 377.15 544.78 582.74
Ambient medium 1.33 Prolate Cube Rect angular 598.08 258.01 277.17 630.28 280.40 564.43 678.39 380.46 605.32 737.83 535.82 654.19 816.22
Sphere 511.23 527.28 549.86 594.88 647.99
Ambient medium 1.53 Prolate Cube Rect angular 211.68 527.09 656.33 269.50 542.83 703.48 376.76 568.09 769.10 542.05 607.42 853.19 579.24 656.28 939.48
Fig. 8 shows the variation of full width at half maxima (FWHM) with the size of Co-Ag and Co-Au alloys NPs for the sphere, prolate, cube and rectangular-shaped nanostructures in three different surrounding medium. Since FWHM is an important parameter for potential use in solar cells and sensing. A linear change in FWHM is observed with the particle size of Co-Ag alloy and further FWHM decreases, above the 50 nm particle size of Co-Au alloy for all considered nanostructures in 1.00 ambient medium and is clearly shown in Fig. 8 (a, b). The prolate shape nanostructure shows the minimum value of FWHM in comparison to other considered nanostructure and further, FWHM value decreases and increases for above 50 nm size of Co-Ag and Co-Au alloys respectively, in three different surrounding medium. Large size effects such as radiation damping and dynamic polarization show the important role and direct impact towards large width value. Therefore, these large size effects are responsible for a larger increase in FWHM of 50 nm and 60 nm sizes. Furthermore, depending upon the sizes, shapes and different surrounding medium, the FWHM has been controlled from 41-415 nm, 33-215 nm, 74-361 nm, 129-452 of Co-Ag and 129-512 nm, 86-405 nm, 83-406 nm, 293-454 nm for Co-Au alloy NPs in the broad region of the spectrum with sphere, prolate, cube and rectangular-shaped nanostructures.
Fig. 8. FWHM of Co-Ag and Co-Au bimetallic alloys nanoparticles as a function of size (a, b) represent LSPR wavelength in refractive index of an ambient medium is 1.00, (c, d) represent LSPR wavelength in refractive index of an ambient medium is 1.33 and (e, f) represent LSPR wavelength in refractive index of an ambient medium is 1.53.
Further studies are carried out for LSPR based sensors technology because of low-cost fabrication and simple sensors setup [16, 35, 36]. The LSPR of any material is sensitive to the shape, size of the NPs as well as their surrounding medium. There are different mathematical and numerical frameworks to study the sensing properties of NPs [37-39]. However, in the present study, sensitivity analysis is carried out on the basis of ratio of peak shift to change in refractive index of the embedded medium. In general, sensing performance of NPs and nanoarray is analyzed based on scattering and absorption or reflection spectroscopy
respectively. Thus, the performance of sensors can be evaluated through sensitivity (S) and figure of merit (FOM) measurements. Therefore, S is calculated by using slope of the line (S=bn+a). Fig.9 shows the effect of shapes on sensing performance as a function of sizes (20 nm to 60 nm). Fig. 9 (a) demonstrate the sensitivity of all considered nanostructures of CoAg alloy and order as follows: S (rectangular) > S(cube) > S(sphere) > S (prolate). Sensitivity corresponding to considered nanostructures of Co-Ag alloy as a function of size increases, when the size of NPs increases. Also, prolate nanostructure Co-Ag alloy presents the lowest sensitivity with an increment of particle size. In the case of Co-Au alloy, as the NPs size increases, corresponding sensitivity decreases for the sphere, prolate and cube nanostructures as shown in Fig. 9 (b). However, ∆λres (peak shift) is also strongly dependent on the size of the NPs. The sensitivity curve of different considered shapes exhibits large swing and crossover due to the combined effect of both: size of NPs and refractive index of surrounding medium. Moreover, the sensitivity of rectangular nanostructure increases when NPs size increases. Furthermore, we compared the sensing performance of all considered nanostructures as a function size of Co-Ag and Co-Au bimetallic alloys. The highest sensitivity is found of Co-Ag alloy for rectangular in comparison to Co-Au alloy rectangular nanostructure. Thus, it is concluded that the sensitivity of Co-Ag alloy of all considered geometries shows their dependence on the particle size and for Co-Au alloy sensitivity shows the opposite trend with NPs size. Moreover, the sensitivity of rectangular nanostructure shows the direct impact of particle size of Co-Ag and Co-Au alloys and hence, Co-Ag alloy has the highest sensitivity.
Fig. 9. Calculated sensitivity of (a) Co-Ag and (b) Co-Au alloys nanoparticle with different shapes as a function of sizes.
Fig. 10. Calculated FOM of (a) Co-Ag and (b) Co-Au alloys nanoparticle with different shapes as a function of sizes.
Fig. 10 (a, b) demonstrates the figure of merit (FOM) for all considered Co-Ag and Co-Au alloys nanostructures as a function of sizes in the liquid surrounding medium i.e. 1.33 refractive index. The result shows that the sensitivity (S) increases, while the FOM decreases as the size of NPs increases. In the case of Co-Ag alloy, the FOM shows the direct impact on sensitivity with particle size. If the sensitivity increases of a particle size then FOM also increases for the same size of all nanostructures. On the other hand, FOM presents the same trend as shown by the sensitivity (S) parameter with a particle size of Co-Au alloy. For example, sensitivity is decreasing for sphere, prolate and cube as a function of size then FOM also decreasing for the same structures with particle size. Hence, the order of FOM as follows: FOM (cube) > FOM (rectangular) > FOM (sphere) > FOM (prolate) for Co-Ag and FOM (prolate) > FOM (sphere) > FOM (cube) > FOM (rectangular) for Co-Au alloy nanostructures.
Conclusions The nanostructures presented here shows that it is now possible to describe many optical properties in terms of absorption scattering of Co-Ag and Co-Au alloy nanoparticles with complex geometry. The calculation of other properties, such as LSPR (λ max,) FWHM, RIS, and FOM, provides new challenges for elongated particles that are an important component of our current research. From studies, it is concluded that the optical properties of both CoAg and Co-Au bimetallic alloy nanoparticles are enhanced and tuned between UV-NIR region of EM spectrum with varying the sizes, shapes, compositions and the refractive index of the surrounding medium. Furthermore, the sensitivity of all considered nanostructures of Co-Ag alloy increases when the size of NPs increases and prolate shape depicts the lowest sensitivity. The highest sensitivity is found for the Co-Ag alloy of rectangular shape in contrast Co-Au alloy. From results, the nanostructures of Co-Ag and Co-Au alloys NPs open their potential in desired applications such as plasmonic devices, optical imaging, solar cells, and further progressive work for nanosensors, based on sensitivity and figure of merit, can be used in sensing environment technology.
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment The author (P. Bhatia) would like to acknowledge the S.L.I.E.T. Longowal for providing financial support. The author thanks B.T. Draine and P.J. Flatau for using their DDA code DDSCAT 7.0.
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