(Cu2O-Au) – Graphene - Au layered structures as efficient near Infra - Red SERS substrates

Near Infra-Red Surface Enhanced Raman Spectroscopy (NIR SERS) has gained huge attention in recent years as the conventional visible SERS suffers from overwhelming fluorescence background from the fluorophore resulting in the masking of Raman signals. In this paper, we propose a novel multi-layered SERS substrate- (Cu2O - Au) - Graphene – Au - for efficient NIR SERS applications. The proposed structure has a monolayer of Cu2O - Au core-shell particles on a Au substrate with 1 nm thick graphene spacer layer. Mie simulations are used to optimize the aspect ratios of core-shell particles to shift their plasmon resonances to NIR region using MieLab software. Further, Finite Difference Time Domain (FDTD) simulations using Lumerical software are used for the design of the multiparticle layered SERS substrate as MieLab software works only for single particle systems. Designed structure is shown to provide high field enhancement factor of the order of 108 at an excitation of 1064 nm thus ensuring the possibility of using the proposed structure as efficient NIR SERS substrate which could probably be used for various NIR sensing applications.

www.nature.com/scientificreports www.nature.com/scientificreports/ proposed earlier 25,26 . Plasmon resonances below 800 nm are made use of in the above works which is far below the excitation wavelength 1064 nm. Here we propose a layered NIR SERS substrate with monolayer of Cu 2 O -Au core-shell particles on Au substrate separated by 1 nm thick graphene layer. Layered structure is designed in such a way that the plasmon resonance exactly matches with 1064 nm which can provide enhanced NIR SERS properties. Mie computations and FDTD simulations are used to explore the potential of the proposed structure in NIR SERS applications. Mie calculations using MieLab software is used for the plasmon resonance study of single core-shell particle systems. MieLab software can only be used for the study of single particle systems. Due to this limitation, further modelling of multi core-shell particle embedded SERS substrate is carried out using Lumerical FDTD simulations. Mie theory used for the study of absorption efficiencies of core-shell particles is discussed in the next section.

Absorption Efficiency of Cu 2 O -Au Core-Shell Particles
Mie theory in the modified form for core-shell particles is used for the calculation of the absorption efficiency of Cu 2 O -Au core-shell particles in the near infra-red region 27 . According to Mie theory, where n s is the refractive index of the spherical particle and n m is the refractive index of the medium and a l and b l are the Mie coefficients.
For core-shell particles (core radius a and outer radius b) with core refractive index m 1 and shell refractive index m 2 , Mie coefficients a l and b l are calculated using the modified relations 21 ,  For each angular momentum l, the shift in the plasmon resonances of core-shell particles is in accordance with the relation 15 , ω ω + − represent the frequencies of antisymmetric and symmetric modes, p ω is the bulk plasma frequency and a and b are the inner and outer radii of the shell 28 .
Q abs values of Cu 2 O -Au core-shell particles in the near infra-red region are calculated using MieLab software. MieLab is a dedicated software to calculate the absorption and scattering properties single core-shell particles. FDTD can also be used for similar studies without any discrepancy of the results. Figure 1(a) shows the comparison of absorption efficiency calculations using MieLab and lumerical FDTD softwares. Absorption efficiency is calculated for Cu 2 O -Au core-shell particle with core radius 60 nm and shell thickness 10 nm using both MieLab and lumerical FDTD softwares. It is observed that there is no discernible disparity in the results obtained using both the softwares. The peak values are observed to be the same in both the simulations. Q abs value obtained using FDTD simulation (0.14) is found to be comparable with the value obtained using MieLab simulation (0.15). For the ease of simulation we have used MieLab for single particle studies. It is not possible to use MieLab for multiparticle systems. Hence we have used FDTD for further modelling of layered substrates and visualization of electric field. Refractive index data required for simulation for Au and Cu 2 O are obtained from Johnson and Christy data 29 and Querry et al. 30 respectively. We have performed Mie calculations on various aspect ratios of www.nature.com/scientificreports www.nature.com/scientificreports/ Cu 2 O-Au core-shell particles. Table 1 shows plasmon resonance positions obtained for Cu 2 O-Au core-shell particles with core radius 40 nm, 50 nm and 60 nm and with Au shell thickness in each case varying from 2 nm to 30 nm in steps of 4 nm. Plasmon resonance tunability with increase in Au shell thickness for Cu 2 O (40 nm)-Au, Cu 2 O (50 nm)-Au and Cu 2 O (60 nm) -Au is observed to be from 1325 nm to 505 nm, 1500 nm to 675 nm and 1630 nm to 712 nm, respectively.
Among the different aspect ratios, Cu 2 O -Au core-shell particle with core radius 50 nm and Au shell thickness (2 nm to 30 nm) are found to have tunable plasmonic resonance within the desired biological window of 650 nm to 1350 nm ( Fig. 2(a)) and the corresponding Q abs and Q sca values are shown in Fig. 2 Fig. 2(a) it can be inferred that the plasmon resonances of particles get blue shifted with increase in shell thickness (Eq. 8). For Cu 2 O (50 nm) -Au core-shell particles, plasmon resonances are found to be tunable from 675 nm to 1500 nm which falls within the desired NIR biological window. Absorption efficiency increases with increase in shell thickness till 14 nm beyond which the Q abs reduces ( Fig. 2(b)) since the coupling between spherical and cavity plasmons gradually reduces at larger thicknesses. Meanwhile, Q sca increases with increase in shell thickness and overrides Q abs for larger shell thicknesses (above 25 nm). From Fig. 2(b) it can be seen that for Cu 2 O (50 nm) -Au core-shell particle with shell thickness 6 nm, plasmon resonance is at 1040 nm which is very close to the desired excitation of 1064 nm.

FDTD Simulations On Cu 2 O (50 nm)-Au Core-Shell Particles
FDTD simulations are performed using Lumerical software to study the electric field distribution over Cu 2 O (50 nm) -Au core-shell particle at 1064 nm ( Fig. 3(a-f)). Simulations are performed by considering air as the surrounding medium. A gradual enhancement in the electric field around the particle with the increase in Au shell thickness is observed till 6 nm where the field intensity is maximum as the plasmon resonance at 6 nm shell thickness (1040 nm) is close to the excitation wavelength of 1064 nm. As the plasmon resonances move farther from the excitation wavelength, a gradual decrease in the near field is observed. We could also observe an abnormal field distribution in Fig. 3b.
In Fig. 3b, the thickness of Au shell is 2 nm (very close to the quantum regime) where the transverse motion of conduction electrons is quantized. Such quantum size effect results in the formation of two groups of electrons.  Table 1. Plasmon resonance tunability data of Cu 2 O -Au core-shell particles with different core radii and Au shell thicknesses.
One group provides non local contribution to the electromagnetic field response which can be explained classically and also can be studied using FDTD simulations. The other group executes quantum transition between states which are in resonance with the external wave. This can only be explained using the quantum approach and is not possible using FDTD which uses classical approach to solve Maxwell's equations. Abnormal field distribution observed in Fig. 3b is hence due to the quantum size effect 15 .

FDTD Simulations On NIR Absorption Enhancement of Cu 2 O (50 nm) -Au Particles
We have tried further to improve the NIR absorption efficiency by distributing Cu 2 O -Au core-shell particles on Au substrate using a graphene spacer layer. Graphene is chosen as spacer layer as it is a good absorber of NIR in combination with plasmonic nanostructures. It helps to effectively reduce reflections at substrate-particle interface and also renders efficient coupling of localized surface plasmons of core-shell particles with the Au substrate 31 . Graphene layer of thickness 1 nm (approximately three layers of graphene) is designed on Au substrate of thickness 50 nm.
Optimization of graphene spacer layer is done using FDTD simulations and the results are shown in Fig. 4. Simulations are performed with one to five layers of graphene as spacer layer. Electric field enhancement between two Cu 2 O (50 nm) -Au (6 nm) core-shell particles is studied in each case using two field monitors along XZ and YZ directions. Field intensity between the particles is observed to be the highest with three layers of graphene spacer which corresponds to an approximate thickness of 1 nm (Fig. 4(e,f)). Hence we have used the optimized thickness of 1 nm for graphene spacer in all simulations Figure 5 shows the schematic of the proposed layered structure. Design of the structure and the study of its NIR absorption properties are performed using Lumerical FDTD solutions. Cu 2 O (50 nm) -Au core-shell particles are selected for designing the NIR absorber due to their large Q abs values in the NIR region. Optimized Au shell is 6 nm for maximum absorption at 1064 nm for single Cu 2 O (50 nm) -Au core-shell particle. But the absorption scenario becomes entirely different when Cu 2 O -Au core-shell particles are distributed over the Au substrate. Plasmonic coupling between Au substrate and core-shell particles via graphene spacer layer significantly modifies the effective absorption by the designed structure. Hence the NIR absorption properties are further studied for different Au shell thickness values for layered structure. Single layer of Cu 2 O (50 nm) -Au particles are uniformly distributed over the graphene layer and the structure is then irradiated with NIR source (power = 1 mW). Two different power monitors are used to collect the transmitted and reflected signals from the structure when irradiated with plane wave source (λ = 0.7 µm to 2 µm). Perfectly matched layer (PML) boundary condition is employed in all simulations. The mesh size is taken as 0.2 nm which is well below the thickness of the spacer layer. The incident polarization is parallel to the substrate.

Effect of Au Substrate On Plasmon Resonance of Cu 2 O (50 nm)-Au Particles
In order to confirm the influence of Au substrate on NIR absorption, FDTD simulations are performed for two different configurations -Cu 2 O (50 nm) particles without Au substrate and Cu 2 O (50 nm) particles on Au substrate. Figure 6 shows absorbance of Cu 2 O (50 nm) in the presence and absence of Au substrate. Plasmonic field due to Au substrate is found to have a huge influence on the NIR absorption of Cu 2 O and is reflected as a clear enhancement in the NIR absorption over the entire NIR region. To prove the enhanced plasmonic coupling of core-shell particles with the substrate in the presence of spacer graphene layer, we have modelled two systems in which two Cu 2 O (50 nm) -Au (6 nm) particles are distributed on Au substrate with and without 1 nm thick spacer graphene layer. The excitation wavelength is chosen as 1064 nm in FDTD simulations. Field distribution in the XY and YZ directions without and with the spacer layer are shown in Fig. 7(a-d), respectively. Electric field between the particles is found to enhance by around 40%, and between particles and substrate by around 14% in the presence of graphene spacer layer thereby validating the stronger plasmonic coupling in this case compared to the bare Au substrate.

NIR Absorption in (Cu 2 O (50 nm)-Au)-Graphene-Au Layered Structure
Further, FDTD simulations are performed on the layered structure shown in Fig. 5 by varying the thickness of the Au shell. Figure 8 shows the absorbance of the layered structure with Cu 2 O (50 nm) -Au core-shell particles with different Au shell thicknesses. Huge enhancement in absorbance is observed in the NIR region due to the plasmonic coupling between Cu 2 O -Au with Au substrate through graphene spacer layer. We could observe a gradual enhancement in NIR absorption over the entire range 0.7 µm to 1.6 µm when the Au shell is added which is the highest at 30 nm beyond which it is a constant. NIR absorption in the window 0.9 µm to 1.1 µm for (Cu 2 O (50 nm) -Au) -Graphene -Au layered structure increases by 18% compared to (Cu 2 O (50 nm)) -Graphene -Au layered structures. Increase in absorption in the specified window is attributed to the effective plasmonic coupling between Au substrate and core -shell particles. Local field around the core -shell particle has contribution from both the localized plasmonic field and the incident electric field. Enhancement in NIR absorption due to Au shell results in the formation of hot spots with stronger local electric field confinement between the core-shell particles. It is inferred from simulations that the hotspots provide significant enhancement factor values which are proportional to the fourth power of locally generated electric field. Generation of such strong hotspots enable SERS detection of biomolecules even with very low Raman cross sections. Our specific interest is in the region 0.9 µm to 1.1 µm where we could observe a new plasmonic peak originating at higher Au shell thicknesses due to the effective plasmonic coupling between the particles and the substrate. At lower Au shell thickness values, the dipolar and quadrapolar resonances of core-shell particles are far apart where dipolar resonances fall in the IR and quadrapolar resonances fall in the UV region. As the thickness of Au shell increases, quadrapolar resonances get red shifted towards visible region while blue shift occurs to dipolar resonances to visible region making both the resonances much closer. This results in the coupling between these modes causing enhanced effective plasmonic absorption over a wide range and formation of new plasmonic peaks. Such a strong plasmonic absorption within the NIR biological window is highly useful for sensing applications particularly sensing based on NIR surface enhanced Raman spectroscopy (SERS). The possibility of using (Cu 2 O -Au) -graphene -Au layered structures as NIR SERS substrate using FDTD simulations is studied further.

FDTD Simulations on SERS activity of Cu 2 O-Au-Graphene-Au Multilayer Substrate
FDTD simulations are further used to study the scattering enhancement in Cu 2 O (50 nm) -Au particles when they are kept on Au substrate with graphene spacer layer. Scattering enhancement factor here is defined as (E/E 0 ) 4 where E is the maximum local electric field and E 0 is the incident electric field. Gap between Cu 2 O (50 nm) -Au particles and also that between Au substrate and Cu 2 O (50 nm) -Au particles are maintained as 1 nm in all simulations. This enabled the plasmons from the particles as well as the substrate interfere constructively to produce highly enhanced local electric field regions within the gaps called hotspots. The excitation source of wavelength  Figure 4(a,c,e,g,i) show field distribution in XZ direction and 4(b,d,f,h,j) in YZ direction with one, two, three, four and five layers of graphene spacer layers respectively. (2020) 10:4152 | https://doi.org/10.1038/s41598-020-60874-x www.nature.com/scientificreports www.nature.com/scientificreports/ 1064 nm to study the NIR SERS property of the layered structure is Total -field scattered -field. It helps to separate the scattered field from the incident field and helps to avoid the reflections from the simulation boundaries.
Simulations are carried out for various Au shell thicknesses ranging from 2 nm to 30 nm and the scattering field enhancement factor, which is a measure of SERS activity of the structure, is calculated in each case. Fixed separation of 1 nm between the particles and that between particles and substrate is maintained in all simulations. SERS enhancement factor (EF) which is a measure of SERS efficiency of substrate is defined as (E/E 0 ) 4 where E is the local maximum electric field and E 0 is the amplitude of input source electric field. With the addition of Au shell, EF is found to increase from 1.24 × 10 3 to 1.86 × 10 8 at larger shell thicknesses and the increase is observed to be very fast till the thickness value reaches 22 nm (Fig. 9). Beyond 22 nm, a slight increase in EF is observed giving rise to a stable order of magnitude 8 at 1064 nm excitation which points to the possibility of using the proposed structure for NIR SERS applications. Hence the proposed structure will serve as efficient NIR SERS substrate which in particular has applications in bio-sensing where most biomarkers such as fluorescein and AFP have their fluorescence in the visible region.

Conclusion
We have proposed a novel metal -dielectric -metal layered NIR SERS substrate which can find immense potential in NIR based bio sensing applications. NIR SERS substrate with Cu 2 O -Au core shell particles distributed over the Au substrate with graphene spacer layer with optimized aspect ratio (Cu 2 O (50 nm) -Au (22 nm)) is found to show large electric field enhancement factor of the order of 10 8 at 1064 nm excitation. Graphene spacer layer enables efficient plasmonic coupling between the upper layers of core-shell particles with Au substrate. Field enhancement factor values which are better than the reported NIR SERS substrates suggest the possibility of using the structure as efficient NIR SERS substrate for various applications. General drawbacks of visible SERS such as fluorescence background, photo bleaching and plasmonic heating which adversely affect the intensity of SERS signals can be overcome by the proposed structure as it is designed for a longer wavelength of 1064 nm.

Methods
MieLab software is used for the study of plasmon resonance tunability in Cu 2 O -Au core -shell particles. MieLab is a computational package which simulates the scattering of electromagnetic field of multilayered spheres based on Yang's algorithm 27 . Absorption efficiency (Q abs ) values are calculated for various aspect ratios of Cu 2 O -Au core -shell particles towards their optimization for maximum absorption in the near Infra-red (NIR) region.