key: cord-1004983-x6cnl9sm
authors: Singh, Alok; Shishodia, Manmohan Singh
title: Graphene vs. silica coated refractory nitrides based core-shell nanoparticles for nanoplasmonic sensing
date: 2020-06-12
journal: Physica E Low Dimens Syst Nanostruct
DOI: 10.1016/j.physe.2020.114288
sha: f3edcdf4621e525edb4d69e63d6f6e5b4d73421b
doc_id: 1004983
cord_uid: x6cnl9sm

Plasmonic nanoparticles based on conventional metals like gold (Au) and silver (Ag) has attracted significant attention of biosensor researchers. Core-shell nanoparticles (CSNP) have shown specific advantages by virtue of unique combination of strong field enhancement and wide ranging spectral tuneability of localized surface plasmon resonances (LSPR). In view of the remarkable plasmonic properties of refractory nitrides (e.g., ZrN and TiN) like higher degree of spectral tuneability, growth compatibility, high melting point, inherent CMOS and biocompatibility etc., and reported high surface area, excellent bio-molecular compatibility, improvement in the speed, higher sensitivity in graphene, the present work assess the feasibility of graphene coated refractory nitrides based CSNP as an efficient refractive index sensor. Mie theory is employed for the theoretical analysis and simulation of such plasmonic structures. The results reported in the present work have been corroborated using COMSOL. The comparison of plasmonic properties and sensing characteristics e.g., FWHM, quality factor, sensitivity and figure of merit is presented for graphene and silica based sensors. It is reported that the sensitivity = 171.68 (nm/RIU) and figure of merit = 3.57 × 10(4) (nm/RIU) can be attained. The present work suggests that graphene coated refractory nitrides based core-shell structures may emerge as ultrasensitive biosensor.

In the backdrop of the fast growing world population, the need of developing efficient healthcare systems and devices is at its peak [1] [2] . In turn, it is therefore mandatory to explore the possibility and scope for developing efficient, long lasting, economical and practically feasible solutions to the existing as well as future medical challenges posing threat to the existence of humanity like the pandemic spread of COVID-19. Plasmonic systems and devices exploiting unique features of hybrid quanta known as surface plasmons are emerging as the backbone of medical diagnostics e.g., large efforts of physicists, chemists, biologists, material scientists etc.

are focused on the development of ultrasensitive plasmonic detectors for biosensing applications [3, 4] . For the sake of completeness, this is to be emphasized that the plasmonic systems based on both, the propagating surface plasmons as well as localized surface plasmons are being investigated and considered for their applications in optoelectronic integration, light harvesting, energy transfer, cancer therapy, food safety, environmental monitoring, surface enhanced Raman spectroscopy (SERS) etc. [5] [6] [7] [8] [9] [10] [11] [12] [13] . As far as localized surface plasmon resonance (LSPR) based sensing is considered, gold (Au) has been the most widely used conventional plasmonic material for theoretical exploration and experimental investigations. Since long, the plasmonics has been suffering from the limited number of plasmonic materials, resulting in the underutilized potential of plasmonics. In order to expand the domain of plasmonics, it is obligatory to expand the list of plasmonic materials beyond conventional plasmonic materials e.g., gold, silver, copper etc.

Refractory nitrides such as TiN and ZrN has recently been explored for their applications in biosensing [5] . Moreover, graphene coated nanoparticles have also emerged as effective plasmonic systems based on alternative plasmonic materials [14] . Due to their exceptional optical, electrical, thermal and mechanical properties, graphene and its derivatives are considered to be the promising materials for biosensing [15] [16] . In order to improve the performance of nanoparticle based plasmonic sensors, it is required that the scattering efficiency increase and the line-width or full width half maximum (FWHM) of plasmonic resonances decrease. As reported in Ref. [5] , this was attained using plasmonic nanoparticle structures and optical gain incorporation in the dielectric layer. Although the gain incorporation is a viable alternative, this will lead to the enhanced complexity and the cost of the sensor. Therefore, it will be of great use if one can enhance the sensor performance without optical gain incorporation. There are different ways to secure reduce full width at half maximum. The use of Fanomodes closely packed nanodisk clusters are among such approaches [17] . Obviously, the sensitivity and other sensing characteristics also depend on the choice of material, size, shape and the nature of embedding medium. The graphene layer can be combined with the conventional plasmonic nanostructures to improve the interaction between the graphene and the incident electromagnetic radiation. These reports and literature survey suggests that the use of graphene coated nanoparticles are extremely advantageous for sensing applications [18, 19] . Graphene can also be placed in different kinds of optical microcavities [20] . Also, the patterned periodic graphene islands improve the plasmonic interactions and effects significantly [21] . Since the successful isolation and first characterization of graphene in 2004, there has been the quest to explore the applications of graphene in energy harvesting, display panels, solar cells, sensing etc. [22] [23] [24] [25] [26] [27] [28] [29] . Graphene shows fairly good plasmonic response MIR and FIR spectral region. A large volume of efforts are focused on increasing the light absorption using graphene. For example, it has been shown that the light absorption can be enhanced by placing a graphene layer in nano-cavities. By integrating the graphene with metal gratings, and photonic crystal cavity, enhancement ~ 70%, and 85%, respectively has been reported [30, 31] . Increasing the graphene absorption normally results in narrower absorption peaks and thus higher figure of merit of the sensor. In Ref.

[32], graphene is used as a spacer between the Au film and Au nanoparticles to obtain larger FOM relative to that one without graphene spacer. Furthermore, the size and geometry of the nanoparticle are another two main parameters that affect the sensitivity and the FOM of a LSPR sensor [7] [8] [9] [10] [11] [12] [13] [14] [38] . In view of the remarkable plasmonic properties of refractory nitrides (e.g., ZrN and TiN) like higher degree of spectral tuneability, growth compatibility, high melting point, inherent CMOS and biocompatibility etc. [5] , and reported high surface area, excellent bio-molecular compatibility, improvement in the speed, higher sensitivity in graphene, the present work assess the feasibility of graphene coated refractory nitrides based CSNP as an efficient refractive index sensor. Mie theory is employed for the theoretical analysis and simulation of such plasmonic structures. The results reported in the present work have been corroborated using COMSOL. The comparison of plasmonic properties and sensing characteristics e.g., FWHM, quality factor, sensitivity and figure of merit is presented for graphene and silica based sensors. Specially, the case of sensing blood with different plasma concentrations is taken as an example for demonstration.

The objective of the present article is to: (I). estimate the sensing characteristics of ZrN (core)-graphene (shell) and TiN (core)-graphene (shell) structure, (II).estimate the sensing characteristics of ZrN (core)-silica (shell) and TiN (core)-silica (shell) structure, (III).compare sensing characteristics of graphene assisted structure with silica assisted structure, (IV).

investigate the role of aspect ratio (r 1 /r 2 ) on sensing characteristics, (V).establish the dependence of sensing characteristics on aspect ratio through fitting equations, perceived to be of key importance for developing appropriate designs, (VI).propose different design strategies for obtaining improved sensor performance.

The manuscript is organized as follows. Section II describes the theoretical description, giving simple details of the theory. Section-III presents the results of our investigations. The concluding remarks are given in Sec. IV.

The schematic diagram of representative core-shell nanoparticle (CSNP) based plasmonic sensor is shown in Fig. 1 . The structure consists of transition metal nitride (e.g.,TiN, ZrN) core(permittivity ε 1 and radius r 1 ), and graphene or SiO 2 shell (permittivity ε 2 , radius r 2 and thickness d).The core-shell system is surrounded by thesensing medium (dielectric constant, ε 3 and refractive index, n) . The optical properties of this system are investigated using Miescattering theory, which is an effective analytical toolfor theoretical analysis of spherical, core-shell, cylindrical nanoparticlesetc. [3, 33, 39] .Besides these particle shapes, Miescattering theoryis extendible to multilayered core-shell nanoparticles (MCSNP) also [39] . For core-shell nanoparticle system considered in the present work, Mie theory expression for scattering efficiency ( sca Q )is expressed in the following form [5, 33] :

where, x is the size parameterand the coefficientsfor scattered fields n a and n b are defined as [5] , a   2  n  n  2  n  n  2  2  n  n  2  n  n   2  n  n  2  n  n  2  2  n  n  2  n  n  n  2  2  2  2  2  2   2  2  2  2 

where, 1 m and 2 m are the refractive indices of core and the shell relative to the surrounding medium. Also, 

In this section, results of our investigations concerning design considerations for graphene/SiO 2 coated ZrN/TiN based plasmonic structure as shown in Fig. 1 are presented.The thickness dependent optical response of graphene is considered using Kubo's formula [27, 30] .

The conductivity ( σ ) of an infinitesimal thin graphene sheet depends on the frequency of interacting light, and the surface conductivity is the sum of intra-conductivity ( intra σ ) and interconductivity ( inter σ ), described as follows [39, 40] :

Here, ω is the frequency of incident radiation, c µ is the chemical potential [31],τ is the relaxation time (τ -1 ≤ 1.0 meV), Tis the temperature, and h is the reduced Planck's constant. The thickness (d) dependent relative permittivity of grapheneis expressed as [42] , Fig. 7 (a-b) (solid lines). The particle is considered to be surrounded by the medium of refractive index, n=1.339. This is found that the resonance peaks in scattering efficiency spectra can be fitted into the Lorentzian expression, The effect of aspect ratio (r 1 /r 2 ) on resonance wavelength (λ R ), quality factor (QF), sensitivity (S) and figure of merit (FOM) for TiN/SiO 2 and ZrN/SiO 2 core-shell nanoparticles are shown in Fig. 9 (a-d) . All these variations are also fitted (into polynomial) and the fitted data is shown through circles. Similarly, the effect of aspect ratio (r 1 /r 2 ) on resonance wavelength (λ R ), quality factor (QF), sensitivity (S) and figure of merit (FOM) for TiN/Graphene and ZrN/Graphene core-shell nanoparticle is shown in Fig. 9 (e-h). All these variations are also fitted and the fitted data is shown as circles.Unless mentioned otherwise, µ c = 1.0 eV, and T=300-K for all calculations.This is observed that λ R vs.aspect ratio (r 1 /r 2 ) follow straight line for SiO 2 as well as graphene shell. The variation of QF with aspect ratio follows straight line (first order polynomial)for SiO 2 shell while for graphene, the variation is quadratic (second order polynomial). Also, in case of SiO 2 shell, the QF decrease with aspect ratio, while for graphene shell, QF increase with aspect ratio. Interestingly, it can be seen that the variation of S with aspect ratio is quadratic for both, SiO 2 as well as graphene shell. However, the dependence in twocases is opposite. In SiO 2 shell, S increase with aspect ratio and decrease with aspect ratio for graphene shell. Similar variation can be seen for FOM also. It is observed that the graphene assisted core-shell nanoparticle structure produces higher sensitivity (S) and figure of merit (FOM). Also it is evident that ZrN core produces higher sensitivity in case of SiO 2 shell and it is nearly equal in case of graphene shell. Moreover, higher FOM is obtained for ZrN core compared to TiN shell.The fitting parameters for TiN/SiO 2 , ZiN/SiO 2 , TiN/Graphene and ZrN/Graphene core-shell nanoparticles. These are fitted into the polynomial,

, where y represents λ R , QF, S and FOM. In summary, ZrN core shows clear advantage compared to TiN core for SiO 2 shell. Graphene assisted CSNP shows clear edge over dielectric shell such as SiO 2 . The analysis suggests that compared to dielectric shell, better sensing characteristics can be obtained for graphene coated structure.The following conclusions can be drawn: (I). resonance wavelength can be fine tuned by controlling aspect ratio, (II). aspect ratio can be optimized to obtain best sensitivity or FOM, (III). graphene coated CSNP produces higher sensitivity and FOM compared to that of dielectric coated CSNP, (IV).

ZrN exhibit sensing characteristics superior than that of TiN, (V). sensing characteristics are more sensitive to the graphene thickness compared to the case of dielectric.

In conclusion, TiN-SiO 2 , ZrN-SiO 2 ,TiN-Graphene and ZrN-Graphene-based core-shell nanoparticle systems are assessed for their suitability as plasmonic sensors. The analysis suggests that, (I). resonance wavelength can be fine tuned by controlling aspect ratio, (II). aspect ratio can be optimized to obtain best sensitivity or FOM, (III). graphene coated CSNP produces higher sensitivity and FOM compared to that of dielectric coated CSNP, (IV). ZrN exhibit sensing characteristics superior than that of TiN, (V). sensing characteristics are more sensitive to the graphene thickness compared to the case of dielectric. Moreover, refractory nitrides like TiN, and ZrN can serve as alternative plasmonic materials. The sensitivity of graphene assisted sensors can be significantly enhanced without gain incorporation and hence maintaining the simplicity of the system. 

All data generated or analysed during this study are included in this published article. reported in Ref. [42] .The excellent agreement between two data sets validates the use of present data for subsequent analysis. TiN-Graphene and ZrN-Graphene with graphene thickness d G =2.5 nm. The calculations are done at five different blood plasma concentrations, 10g/l, 20g/l, 30g/l, 40g/l and 50g/l. Other parameters are: r 1 =30 nm, r 2 =32.5 nm. The sensitivity is simply the slope of these lines. TiN/Graphene and ZrN/Graphene CSNP as a function of aspect ratio (r 1 /r 2 ). Also shown is the polynomial fitting (circles). The refractive index of embedding medium is 1.339 and T=300 K.

Biodemography of human ageing

Prognosis-a wearable health-monitoring system for people at risk: Methodology and modeling

Silver-Based Plasmonic Nanoparticles for and Their Use in Biosensing

Wearable and miniaturized sensor technologies for personalized and preventive medicine

Estimation of sensing characteristics for refractory nitrides based gain assisted core-shell plasmonic nanoparticles

Heterojunctionplasmonicmidinfrared detectors

Graphene-Based Sensors for Human Health Monitoring

Localized surface plasmon mediated energy transfer in the vicinity of core-shell nanoparticle

Surface plasmon enhanced IR absorption: design and experiment

Plasmonics for future biosensors

Compensation of coulomb blocking and energy transfer in the current voltage characteristic of molecular conduction junctions

Au nanomatryoshkas as efficient near-infrared photothermal transducers for cancer treatment: benchmarking against nanoshells

Linear optical properties of gold nanoshells

Numerical simulation on the LSPR-effective core-shell copper/graphenenanofluids

Shape and size dependence of radiative, non-radiative and photothermalproperties of gold nanocrystals

The interaction of light and graphene: basics, devices, and applications

Asymmetrically engineered metallic nanodisk clusters for plasmonic Fano resonance generation

Laser-generated bimetallic Ag-Au and Ag-Cu core-shell nanoparticles for refractive index sensing

Strong enhancement of light-matter interaction in graphene coupled to a photonic crystal nanocavity

Microcavity-integrated graphene waveguide: A reconfigurable electro-optical attenuator and switch

Quantum nonlocal effects in individual and interacting graphenenanoribbons

Carbon-based nanomaterials/allotropes: A glimpse of their synthesis, properties and some applications

Optical properties of metal clusters

Gold nanoshells with gainassisted silica core for ultra-sensitive bio-molecular sensors

Silver nanocube aggregation gradient materials in search for total internal reflection with high phase sensitivity

Design of one dimensional refractive index sensor using ternary photonic crystal waveguide for plasma blood samples applications

Gain-Assisted Transition Metal Ternary Nitrides (Ti 1−x Zr x N) Core-Shell Based Sensing of Waterborne Bacteria in Drinking Water

Theoretical investigation of intensity-dependent optical nonlinearity in graphene-aided D-microfiber

Plasmon-enhanced light-matter interactions and applications

Electrically tunable fano resonance from the coupling between interband transition in monolayer graphene and magnetic dipole in metamaterials

Damping-induced size effect in surface plasmon resonance in metallic nano-particles: Comparison of RPA microscopic model with numerical finite element simulation (COMSOL) and Mie approach

Local heating control of plasmonic nanoparticles for different incident lights and nanoparticles

Tapered optical fiber sensor based on localized surface plasmon resonance

Optical fiber sensor based on localized surface plasmon resonance using silver nanoparticles photodeposited on the optical fiber end

Preparation of localized surface plasmon resonance sensing film with gold colloid by electrostatic assembly

Nanoplasmonic Sensing

Graphene vs. Silica Coated Refractory Nitrides based Core-Shell Nanoparticles for Nanoplasmonic Sensing

ZrN and TiN) like higher degree of spectral tuneability, growth compatibility, high melting point, inherent CMOS and biocompatibility etc., and reported high surface area, excellent bio-molecular compatibility, improvement in the speed, higher sensitivity in graphene, the manuscript assess the feasibility of graphene coated refractory nitrides based core-shell nanoparticle as an efficient plasmonic sensor

To the best knowledge of authors, the manuscript present the first time investigation of graphene assisted refractory nitrides (TiN and ZrN) based core-shell nanoparticles

The manuscript reports that, (I). the plasmonic resonance wavelength can be fine tuned by controlling aspect ratio of core-shell nanoparticle, (II). aspect ratio can be optimized to obtain best sensitivity and figure of merit, (III). graphene coated core-shell nanoparticle produces higher sensitivity and figure of merit compared to that of dielectric

ZrN exhibit sensing characteristics superior than that of TiN, (V). sensing characteristics are more sensitive to the graphene layer compared to the case of dielectric

The fitting expressions useful for designing graphene/silica assisted core-shell nanoparticles based plasmonic sensor are presented

Graphene assisted core-shell nanoparticle based sensor exhibit higher sensitivity and figure of merit compared to that of silica

It is reported that the sensitivity=171.68 (nm/RIU) and figure of merit=3.57×10 4 (nm/RIU) can be attained without optical gain incorporation

This is to certify that for manuscript entitled, "Graphene vs. Silica Coated Refractory Nitrides based Core-Shell Nanoparticles for Nanoplasmonic Sensing", by Alok Singh, and myself (corresponding author) for publication in Physica-E; Low dimensional Systems and Nanostructures, there is no CONFLIC OF INTERST.If any additional information is needed, please contact me by Phone: +91-120-2344363 (office), Email: manmohan@gbu.ac.in Thanking You.

Dr. Manmohan Singh Shishodia (Assistant Professor)