Effect of Chromium Interlayer Thickness on Optical Properties of Au-Ag Nanoparticle Array

The effect of chromium interlayer thickness on optical properties of array of hybrid Au-Ag triangular nanoparticles is systematically investigated. The optical spectrum simulated by discrete dipole approximation (DDA) numerical method shows that with increase of the chromium interlayer thickness both refractive index sensitivity (RIS) and figure of merit (FOM) of localized surface plasmon resonance from the hybrid nanostructures experience remarkable change and the intensity of the extinction efficiency decreases. The nanosphere lithography (NSL) is used to fabricate the hybrid nanostructure arrays with different chromium interlayer thicknesses. The experiment demonstrates that the spectrum as measured from the as-fabricated hybrid nanostructure arrays is essentially in agreement with the simulated results.


Introduction
Noble metal nanoparticles are well known for their ability to exhibit localized surface plasmon resonance (LSPR) which can be applied in surface-enhanced spectroscopy [1], optical filters [2], plasmonic devices [3,4], sensors [5], and so forth.The LSPR refers to the excitation of surface plasmons by light from nanometer-sized metallic particles.The position and intensity of LSPR spectrum peaks are sensitively dependent on composition, size, shape, and interparticle spacing of the nanoparticles as well as the dielectric properties of their local environments [6].The discrete dipole approximation (DDA) is one of the most efficient computational numerical algorithms to simulate LSPR for the nanostructures with arbitrary shapes and dimensions.This method has significant advantages of occupying less computation resource, calculating the mutual action between the light and the metal nanostructures [7][8][9][10].On the other hand, the nanosphere lithography (NSL) is the most low-cost, high-throughput method for producing periodic, geometrically tunable nanostructure arrays [11].The NSL makes use of a template formed by the self-assembly of monodisperse nanospheres on flat surface acting as a deposition/etching mask.Nevertheless, even with the NSL, complicated physical and chemical processes are normally involved and thus render it difficult to achieve a designed structure.One of the major problems is the adhesive ability of the deposited metallic particles to the substrate.The cohesive force and wettability of a noble metal, for example, silver or gold, are normally very limited on a glass or silicon substrate.The noble metals are inclined to be lifted off due to the abrupt change of physical and chemical properties across the interface between the metal and the substrate.Both theory and experiments suggest that an interlayer can be deposited to promote the adhesion of metallic nanoparticles to the substrates [6,[12][13][14][15][16][17][18].For example, Haes et al. [6] applied chromium buffer thin film to increased adhesion of Ag nanoparticles to the glass substrate.Xilian and Jianda [13] introduced Cr intermediate layer, which reduced the surface roughness and promoted the reflectivity of Ag film.In addition, Zhu et al. [14] also focused on the study of the influence of Cr adhesion layer on detection of amyloidderived diffusible ligands based on localized surface plasmon resonance.To the concern of our knowledge, so far, there is no work investigating the effect of Cr interlayer thickness deposition on the periodic hybrid Au-Ag triangular nanoparticle array.The hybrid nanoparticles can prevent oxidation of pure Ag nanoparticles due to Au protective layer caped on the Ag nanoparticles [19][20][21].The refractive index sensitivity (RIS) and figure of merit (FOM) of the LSPR sensor [22][23][24][25][26][27] are sensitive to the thickness of the Cr interlayer, so the influence of the Cr interlayer thickness on the optical properties and adhesive ability of the hybrid nanostructure is important as the main topic of this paper.
With above considerations, in the present paper, the effect of Cr interlayer thickness deposition on the periodic hybrid Au-Ag triangular nanoparticle array is systematically investigated.DDA method is used to calculate RIS and FOM of the hybrid nanostructure arrays with different Cr interlayer thicknesses.The hybrid nanoparticles are fabricated by NSL with polystyrene (PS) nanospheres as a deposition mask.Thermal evaporation is applied to deposit Au-Ag and Cr films.The structures of the achieved hybrid nanoparticle arrays with different Cr interlayer thicknesses are characterized by scanning electron microscope (SEM).The results suggest that the adhesive ability of the hexagonally arranged triangular Au-Ag nanoparticles to silicon substrate can be remarkably promoted by introducing a certain thickness of Cr interlayer.According to the theoretical calculations and experimental results, we obtain a suitable Cr interlayer thickness.

DDA Method
The DDA provides a convenient method for describing light scattering from nanoparticles of arbitrary shape.In DDA formalism, the object of interest, usually called "target, " is described as a cubic array lattice of electric dipoles (point dipoles) in which the polarizability and position vector of each dipole are specified as   and r  , respectively.The induced dipole polarization P  in each element is determined from [7][8][9][10] where the local field E loc, (r  ) is the sum of the field radiated from all of the other  − 1 dipoles.For a given wavelength , including the contribution of all of the other dipoles, the local field can be written as where k and E 0 are the wave vector and the amplitude of the incident radiation, respectively.The contribution to the electric field at position  due to the dipole at position  is contained in the second term on the right side of (2) and is normally expressed in terms of the dipole-dipole interaction matrix A as where r  and   correspond to the dipole-dipole position difference vector and magnitude that are defined as r  − r  and |r  − r  |, respectively.Substituting ( 2) and ( 3) into (1), we can generate the system of equations where the off-diagonal elements of the matrix    are the same as   , and the diagonal elements of the matrix    are  −1 .
For a system with total dipoles, , E and P are 3dimensional vectors, and A  is a 3 × 3 matrix.Solving this set of 3 complex linear equations, the polarizations P  are determined, and from this we can calculate cross section of the extinction as

Results and Discussions
Using the DDA algorithm, we design and calculate the extinction spectra, RIS, and FOM of the hybrid Au-Ag triangular nanostructure arrays with different Cr interlayer thicknesses.Corresponding schematic illustration of the hybrid nanoprism is shown in Figure 1.The triangle in XY plane contains three equilateral edges, each of which is of 180 nm.The thickness of Au and Ag is fixed on ℎ Au = 5 nm and ℎ Ag = 25 nm and the Cr interlayer thickness, ℎ Cr , is varying from 4 nm to 20 nm.Here ℎ Au , ℎ Ag , and ℎ Cr are defined in Figure 1(b).
Extinction is a term used in physics to describe the absorption and scattering of electromagnetic radiation.Any changes in the parameters of metal nanoparticles could lead to the optical drift in extinction spectrum and thus influence the optical applications in practice [28][29][30][31].According to the calculated extinction spectra presented in Figure 2, when the Cr interlayer thickness is increased, the positions of peak wavelengths appear slightly blue shifted while the intensity of the extinction has obviously been decreased.The full width at half maximum (FWHM) increases rapidly by increasing thickness of Cr interlayer from 4 to 16 nm and then increases slowly with increasing Cr interlayer thickness to 20 nm.It is noted that, as can be seen from the extinction spectrum in Figure 2(a), each curve has several peaks independent of the thickness of Cr.This phenomenon not only relates to the hybrid Au and Ag material, but also relates to the optical    properties of triangular nanoparticle.For Ag or Au triangular nanoprism, the extinction spectrum always shows several peaks due to the in-plane and out-of-plane polarization [32].
In order to investigate the effect of the Cr interlayer thickness on the sensitivity of the hybrid nanostructure array, we calculated the extinction spectra of the effective refractive index of the medium surrounding the nanostructure array.The RIS is defined as  = Δ/Δ [33], where Δ and Δ denote the peak of the wavelength change and the refractive index change, respectively.For the hybrid nanostructure array with 4 nm Cr interlayer thickness, the peak wavelength has a red shift when the refractive index  increases as shown in Figure 3(a).For example, when the refractive index increases from 1.0 to 1.05, the peak wavelength shifts from 659 nm to 689 nm, exhibiting a refractive index sensitivity of 592 nm/RIU (refractive index unit), as indicated in Figure 3(b).Figure 3(c) shows the RIS of the hybrid nanostructure arrays with different Cr interlayer thicknesses.
The result shows that the RIS increases with the thickness of Cr interlayer from 4 to 12 nm and then decreases with further increase of Cr interlayer thickness to 20 nm.When the Cr interlayer thickness is increased from 10 to 14 nm, the structure is very sensitive for the refractive index from 1.0 to 1.2, and the RIS is higher than the other thickness.
The FOM for a metal nanostructure is defined as  = / [34], where  and  denote the RIS and FWHM, respectively.
According to Figures 2(c) and 3(c), we calculated the FOM and the calculation result is shown in Figure 4.It can be seen from Figure 4 that the thickness of the Cr interlayer causes the change of the FOM of the nanostructure array.The FOM generally decreases with the increasing thickness of Cr thickness; moreover, the FOM at 4-8 nm and 12-16 nm shows more abrupt change than those at 8-12 nm and 16-20 nm.From the calculated results, we can see that the FOM of the hybrid nanostructure array keeps at a more stable and relatively high value (the value less descended), when the thickness of Cr interlayer increases from 8 to 12 nm.

Fabrication of Nanostructures
The PS nanospheres with a mean diameter of 360 nm and a concentration of 10 wt% in solution are purchased from  Suzhou Nano-Micro Bio-Tech Co. Ltd.First of all, closepacked nanosphere is a prerequisite.The regular monolayer as a deposition mask is principal to achieve large-area hexagonal structure.To begin with, the PSNS solution is diluted to be 3 wt% with deionized water.The silicon substrates (-type, (100) orientation) are firstly ultrasonic cleaned, in toluene, acetone, and ethanol for 10 min, respectively, and then in piranha solution (H 2 SO 4 : H 2 O 2 = 3 : 1) for 2 hours to remove organic residues.To achieve a hydrophilic surface, the silicon substrates are ultrasonically bathed in NH 4 OH, H 2 O 2 , and H 2 O solution with the ratio of 1 : 1 : 5 for 2 hours.Every sonication followed rinsing with large amount of deionized water.The cleaned substrates are stored in deionized water until used.
Following drop-coating of the PS nanospheres on the substrate, Cr, Ag, and Au layers are deposited on the PS mask sequentially, as shown in Figure 5.When the Cr interlayer is deposited on the PS mask, the tips of the acquired triangle nanoparticles are much sharper, and the density array is much larger than the other cases.It is believed that the structure of the 2D hybrid Au-Ag nanoparticle array with Cr interlayer is better than that without interlayer.The hybrid Au-Ag particles can protect oxidation and sulfuration of the pure Ag particles from ambient environment.
The deposition of three metallic layers (3N Au, 3N Ag, and 3N Cr) is performed in a home-built thermal evaporator at a pressure of 5.0 × 10 −4 Pa.The substrates are rotated at a speed of 16.5 rpm all through the deposition.To achieve homogeneous deposition, the power for heating-up of the source materials is carefully increased.The deposition rate is ∼2.5 nm/s for Au and Ag layers and ∼4.0 nm/s for Cr film.The thickness had been monitored using a Dektak 3 Series surface profiler to achieve an identical depth for a low reflectance.It is controlled to be 5 nm for Au film, 25 nm for Ag film, 3, 4, 8, 12, 20, and 64 nm for Cr film, respectively.After deposition of Au film, the PS spheres are lifted off by immersing in absolute ethanol for about 5 s.The PS spheres are also removed by sonication (B3500S-MT, Branson, 140 W, 42 kHz) in absolute ethanol to examine the adhesive ability of the hybrid Au-Ag nanoparticles on silicon substrates.Nanostructures of the achieved PS mask and the hybrid Au-Ag nanoparticle arrays are characterized by LEO-1530 SEM.Ultraviolet visible (UVvis) spectra are obtained on a Varian Cary 5000 UV-Vis-NIR spectrophotometer.

Results and Discussion
Figure 6 shows the SEM images of the Cr triangular nanoparticle arrays in different thicknesses.Figure 7 shows the SEM images of the hybrid Au-Ag triangular nanoparticles arrays with the different thicknesses of the Cr films.
As showed in Figure 7(a), the hybrid nanoparticles exhibit a hexagonally arranged disc structure rather than triangular structure and many tiny cracks appear in the structure.When the thickness of deposited Cr film is 3 nm, the cohesive force between the PS nanospheres and the silicon substrate seems not to be strong enough, as shown in Figure 6(a).However, as the deposited Cr film thickness increases, the cohesive force between the PS nanospheres and the silicon substrate seems enhanced; thus the angular structure mounts and tends to be regular or well-defined, as shown in Figure 6(b).After the Ag and Au layers are deposited, the nanoparticles still exhibit similar morphology of the hybrid Au-Ag triangular nanoparticles arrays, as shown in Figure 7(b).As showed in Figure 6(c), when the Cr interlayer thickness is 8 nm, the angular structure turns into regular shape, and its tip becomes sharp.The Cr interlayer of this thickness can be used to improve the structure of the hybrid Au-Ag triangular nanoparticles arrays.After the Ag and Au layers are deposited, the nanoparticles still exhibit similar morphology of the hybrid Au-Ag triangular nanoparticles arrays, as shown in Figure 7(c).As can be seen from Figures 6(d  film, the Ag and Au deposition in the gap interspaces among the spheres seem to become increasingly blocked with the predeposited Cr nanopaticles, which makes the Au and Ag particles difficult to arrive at the silicon substrate.In doing so, after the PS spheres are removed by sonication in absolute ethanol, the nanoparticles still exhibit a triangular but less uniform structure than the previous cases, as in Figures from 7(d) to 7(f).When thickness of Cr is 3 nm and 8 nm, the hybrid nanoparticles both form a large-scale monolayer array with the hexagonally close-packed structure.Considering the theoretical calculations and experimental results, 8 nm Cr interlayer thickness is selected in the further experiment.
In the experiments, we detected the extinction efficiency of the representative hybrid Au-Ag triangular nanoparticle array with 8 nm thickness of the Cr interlayer.The experimental and the calculated results are shown in Figure 8.It can be seen apparently that when the wavelength is 621 nm, the extinction efficiency reaches a maximum value.When DDA calculation is taken to calculate the extinction efficiency, the maximum wavelength of extinction efficiency is 659.2 nm, as shown in Figure 8.The calculated result of the plasmon wavelength with the designed model agrees with recent experimental result.The only major difference with the experiment is that the experimental peak is shifted by 38 nm to the blue compared to the calculated result.This is likely caused by substrate effect (see [35]).The other reason is that fabrication error causes uniformity issue for both size and shape of the particles.In addition, in DDA calculated model, the edge of the triangular nanoparticles is straight while in experiment it is curving.Most of the results presented here are analysis and characterization of the Cr interlayer thickness variation.The results for the 2D hexagonally arranged hybrid Au-Ag triangular nanoparticles arrays show that the perfect Cr interlayer thickness is around 8 nm for our experimental fabrication.The experimental results are generally in agreement with the calculated results.

Summary
The effect of the Cr interlayer thickness deposition on the periodic hybrid Au-Ag triangular nanostructure is systematically investigated.DDA numerical method is used to study the RIS and FOM of the hybrid nanostructures.The calculation results show that the Cr interlayer thickness ranging from 8 to 12 nm is the better geometrical parameters of the hybrid nanostructures.The NSL is used to fabricate the hybrid nanostructure arrays with different Cr interlayer thicknesses.The experimental results demonstrate that when the thickness is fixed on 3 nm and 8 nm, the nanoparticles exhibit a triangular structure but that is more uniform than the other cases.From the above results, about 8 nm of Cr interlayer thickness is the suitable structure parameter which can be used to fabricate hybrid Au-Ag triangular nanoparticles array with sharp tip and regular angular structure.

Figure 1 :
Figure 1: Schematic view of a single hybrid Au-Ag triangular nanoparticle. 0

Figure 2 :
Figure 2: Calculation results ofthe hybrid Au-Ag triangular nanoparticle arrays with different Cr interlayer thicknesses.

2 (Figure 3 :
Figure 3: Sensing performance of the hybrid Au-Ag triangular nanoparticle arrays with different Cr interlayer thicknesses.

Figure 5 :
Figure 5: Schematic illustration of hybrid Au-Ag nanoparticle array fabrication with the following five steps: (1) drop-coating of monolayered PS nanospheres on substrate; (2) deposition of a thin buffer layer of Cr film over the as-coated monolayered PS nanospheres; (3) a further deposition of a layer of Ag film following the Cr deposition; (4) final deposition of a caped layer of Au thin film on top of the Ag film; (5) a 2D hybrid Au-Ag nanoparticle array formation on substrate after left-off of the monolayered PS nanospheres.
) to 6(f), with the further increase in the thickness of the predeposited Cr

Figure 8 :
Figure 8: Extinction spectra of experiment and calculation.
Figure 4: Figure of merit of the hybrid Au-Ag triangular nanoparticle arrays with different Cr interlayer thicknesses.