Sensitivity Enhancement in Surface Plasmon Resonance Biochemical Sensor Based on Transition Metal Dichalcogenides/Graphene Heterostructure

In this work, a surface plasmon resonance (SPR) biosensor based on two-dimensional transition metal dichalcogenides (TMDCs) is proposed to improve the biosensor’s sensitivity. In this sensor, different kinds of two-dimensional TMDCs are coated on both surfaces of metal film. By optimizing the structural parameters, the angular sensitivity can reach as high as 315.5 Deg/RIU with 7-layers WS2 and 36 nm Al thin film, which is 3.3 times of the conventional structure based on single Al thin film. We also obtain maximum phase sensitivity (3.85 × 106 Deg/RIU) with bilayer WS2 and 35 nm Al thin film. The phase sensitivity can be further improved by employing Ag and removing air layer. The proposed configuration is of great potential for biochemical sensing.


Introduction
Surface plasmon resonance (SPR) is an optical phenomenon which occurs at the metal-dielectric surface. When light reflects at a SPR angle, free electrons on the metal surface can resonate and absorb light energy, consequently leading to a drastic attenuation of reflected light [1,2]. The SPR condition is sensitive to the environment variations and can be utilized as sensors. The biological molecules interactions in the sensing medium are detected by observing the refractive index changes of the sensor region. Due to advantages such as convenient detection, high sensitivity, real-time measurement, SPR sensors have been used to detect and analyze various biological molecules, such as proteins, nucleic acids and viruses, and have a broad prospect in practical applications [3][4][5][6]. Sensitivity is one of the most important aspects for biological sensing in particular, and how to enhance the sensitivity becomes a research hotspot for SPR biosensors.
Recently, 2D materials such as graphene and transition metal dichalcogenides (TMDCs) have are well-known for their use in constructing SPR sensors due to their unique electrical and optical properties [7]. This is because firstly, the high real part of the dielectric constant allows them to help metal absorb light energy [8]. Secondly, some features such as high surface to volume ratio and tunable biocompatibility can help the biosensor obtain sensitivity enhancements [9]. Finally, when coating these materials on the metal film, they can also protect the metal from oxidation as protective layers [10,11]. Based on these advantages, various 2D-material-assisted SPR sensors are proposed and investigated.
Graphene has been proposed for the enhancement of the sensitivity [12]. Zeng et al. presented a highly sensitive SPR biosensor based on graphene-MoS 2 hybrid nanostructures to enhance its sensitivity [13]. Air layer and graphene sheet for sensitivity enhancement was analyzed in [14]. Other TMDCs like WS 2 , MoSe 2 and WSe 2 are combined with silicon to enhance the sensitivity [15]. Wu et al. proposed a SPR biochemical sensor with heterostructures of few-layer BP and 2D materials (graphene/MoS 2 /WS 2 /MoSe 2 /WSe 2 ) [16]. According to the previous work, it is found that the sensor performances are highly related to the structures and functional materials. To further enhance the sensitivity, both of these two aspects should be properly optimized.
In this paper, SPR sensor constructed by TMDCs/metal/TMDCs/graphene heterostructure is used for both angular and phase sensitivity enhancement. By coating different 2D TMDCs (MoS 2 /MoSe 2 /WS 2 /WSe 2 ) at both sides of the metal, the sensitivity of the proposed sensor can be improved by the enhancement of light-material interaction. Angular sensitivity as high as 315.5 Deg/RIU which is nearly 3 times that of conventional configurations can be obtained. Furthermore, the proposed SPR configuration is suitable for phase detection as well and a pronounced phase sensitivity up to 3.85 × 10 6 Deg/RIU is predicted.

Sensor Configuration and Theoretical Model
The schematic diagram of the proposed SPR biosensor is shown in Figure 1a, the configuration contains seven layers and the operation wavelength is 633 nm which is popular for SPR applications [13,14,17]. BK7 glass with refractive index of n p = 1.5151 acts as the coupling prism [18]. The refractive index of the air layer is fixed at 1 with thickness of 35 nm. The metal employed in this configuration is Al with dielectric constant of −34.2574 + 0.9108i [19]. Various TMDCs, represented by MX 2 , are coated at both sides of Al thin film, the thickness and refractive index of TMDCs at 633 nm are shown in Table 1 [20,21]. The graphene layer is coated on the MX 2 /Al/MX 2 hybrid structure as the biomolecular recognition element and the refractive index of graphene is given as [22]: where λ is the wavelength and C 1 = 5.446 µm −1 . The thickness of the monolayer graphene is 0.34 nm. The refractive index of the sensing medium is given as n s = 1.33 + ∆n, where ∆n is the index change of the sensing medium. investigated. Graphene has been proposed for the enhancement of the sensitivity [12]. Zeng et al. presented a highly sensitive SPR biosensor based on graphene-MoS2 hybrid nanostructures to enhance its sensitivity [13]. Air layer and graphene sheet for sensitivity enhancement was analyzed in [14]. Other TMDCs like WS2, MoSe2 and WSe2 are combined with silicon to enhance the sensitivity [15]. Wu et al. proposed a SPR biochemical sensor with heterostructures of few-layer BP and 2D materials (graphene/MoS2/WS2/MoSe2/WSe2) [16]. According to the previous work, it is found that the sensor performances are highly related to the structures and functional materials. To further enhance the sensitivity, both of these two aspects should be properly optimized.
In this paper, SPR sensor constructed by TMDCs/metal/TMDCs/graphene heterostructure is used for both angular and phase sensitivity enhancement. By coating different 2D TMDCs (MoS2/MoSe2/WS2/WSe2) at both sides of the metal, the sensitivity of the proposed sensor can be improved by the enhancement of light-material interaction. Angular sensitivity as high as 315.5 Deg/RIU which is nearly 3 times that of conventional configurations can be obtained. Furthermore, the proposed SPR configuration is suitable for phase detection as well and a pronounced phase sensitivity up to 3.85 × 10 6 Deg/RIU is predicted.

Sensor Configuration and Theoretical Model
The schematic diagram of the proposed SPR biosensor is shown in Figure 1a, the configuration contains seven layers and the operation wavelength is 633 nm which is popular for SPR applications [13,14,17]. BK7 glass with refractive index of np = 1.5151 acts as the coupling prism [18]. The refractive index of the air layer is fixed at 1 with thickness of 35 nm. The metal employed in this configuration is Al with dielectric constant of −34.2574 + 0.9108i [19]. Various TMDCs, represented by MX2, are coated at both sides of Al thin film, the thickness and refractive index of TMDCs at 633 nm are shown in Table 1 [20,21]. The graphene layer is coated on the MX2/Al/MX2 hybrid structure as the biomolecular recognition element and the refractive index of graphene is given as [22]: where λ is the wavelength and C1 = 5.446 μm −1 . The thickness of the monolayer graphene is 0.34 nm. The refractive index of the sensing medium is given as ns = 1.33 + ∆n, where ∆n is the index change of the sensing medium.   In this paper, for the SPR curve calculation and sensing performance analysis, the transfer matrix method (TMM) is employed [23]. In the proposed structure, the thickness, the refractive index, and the dielectric constant of each layer are defined as d k , n k and ε k , respectively. The incident angle corresponding to the minimum reflectance is called resonance angle and the angular sensitivity is calculated by probing the spectral shifts of the resonance angle [3] and defined as S A = ∆θ res /∆n [24], where ∆θ res represents the change of resonance angle. Furthermore, we also discuss phase sensitivity which is defined as S p = ∆ϕ/∆n [13], where ∆ϕ is the differential phase changes corresponding to ∆n.

Results and Discussions
In order to obtain the optimal angular sensitivity for the proposed configuration, we firstly calculate the angular sensitivity with various number of MX 2 layers and Al thickness. It should be noted that in this calculation, the MX 2 layers at both sides are changed simultaneously. As shown in Figure 2a-d, when the refractive index of sensing medium changes from 1.330 to 1.335 (∆n = 0.005), the SPR curves shows three important features enumerated below.
(1) When the thickness of Al thin film is fixed, the sensitivity increases with more MX 2 layers mainly due to the enhanced light energy absorption. However, it will decrease rapidly when the number of MX 2 layers exceeds the optimal number which is defined as the number of MX 2 layers with the highest sensitivity. (2) The optimal numbers of MX 2 layers will increase when the thickness of Al increases.
(3) With the same thickness of Al thin film, the enhancement effect offered by different kinds of MX 2 are not the same.
It is known that the improvement effect caused by TMDCs is related to their dielectric constants. As illustrated in Table 1, MoS 2 has a larger real part of the dielectric constant than others, which means its absorption ability is stronger [15]. Nevertheless, the ability to absorb light is not the only crucial factor to affect sensitivity; electron energy loss related to the imaginary part of the dielectric constant can lead to a counteraction [13]. Comparing to other TMDCs, the WS 2 layers have much lower energy loss because of their small imaginary part of the dielectric constant. According to the conditions above, the optimized parameters and the corresponding angular sensitivity for different TMDCs are summarized in Table 2    It should be noted that the intensity of reflection light is very weak at resonance. In order to effectively measure the resonance shift, it is necessary to increase the incident power or adopt detectors with high sensitivity. For example, assuming 10 mW incident light and 0.1 deg angular resolution [25], the reflected power at the SPR dip is estimated to be 6.5 µW and the reflected power variation between the resonance angle and its nearest measurable neighbor is 0.32 µW, which can be easily detected by commercially-available photodiodes at a visible wavelength [26]. Also note that, in our calculation, the sensing medium is set to be homogeneous in order to make fair comparison with previously works [12][13][14][15][16]. In fact, when the SPR biosensors are used for detecting cells with size of several microns [27], a homogeneous sensing medium layer is reasonable. To further investigate the surface sensitivity, the index change caused by the sensing target is applied with finite thickness. Under the optimal condition for WS 2 , the sensitivities with different sensing layer thickness are shown in Figure 3. It is shown that the surface sensitivity increases with thicker sensing layer. Even with 10 nm thickness, the surface sensitivity can be still as high as 37 Deg/RIU.  It should be noted that the intensity of reflection light is very weak at resonance. In order to effectively measure the resonance shift, it is necessary to increase the incident power or adopt detectors with high sensitivity. For example, assuming 10 mW incident light and 0.1 deg angular resolution [25], the reflected power at the SPR dip is estimated to be 6.5 μW and the reflected power variation between the resonance angle and its nearest measurable neighbor is 0.32 μW, which can be easily detected by commercially-available photodiodes at a visible wavelength [26]. Also note that, in our calculation, the sensing medium is set to be homogeneous in order to make fair comparison with previously works [12][13][14][15][16]. In fact, when the SPR biosensors are used for detecting cells with size of several microns [27], a homogeneous sensing medium layer is reasonable. To further investigate the surface sensitivity, the index change caused by the sensing target is applied with finite thickness. Under the optimal condition for WS2, the sensitivities with different sensing layer thickness are shown in Figure 3. It is shown that the surface sensitivity increases with thicker sensing layer. Even with 10 nm thickness, the surface sensitivity can be still as high as 37 Deg/RIU. To further illustrate the contribution of MX2 on sensitivity, we have drawn the variation of the reflectance with incident angle varying from 60 Deg to 90 Deg for the different number of layers when the thickness of Al thin film is fixed at 30 nm and ns = 1.3300 in Figure 4a-d. It is indicated that the minimum reflectivity, representing the capability of light energy absorption [15], approaches zero firstly when the number of MX2 layers increase, which means the contribution of light energy absorption exceeds the electron energy loss. Meanwhile, the full width at half maximum (FWHM) becomes broader caused by electron energy loss of MX2 layers [13]. With the further increase of the MX2 layers, the FWHM keeps getting broader and the minimum reflectivity begins to diverge from zero. This is because the SPR process must satisfy the energy conservation T + R + A = 1, where T, R and A denote the transmission, reflection and absorption, respectively. Under SPR condition, since the total internal reflection is fulfilled, the transmission T is close to 0. When the number of TMDCs layers is insufficient, the absorbed light energy is not able to promote a strong SPR excitation. Therefore, increasing the TMDCs layers can enhance the light absorption, resulting in higher sensitivity. In this condition, the absorption A is enhanced while the reflection R is reduced. However, To further illustrate the contribution of MX 2 on sensitivity, we have drawn the variation of the reflectance with incident angle varying from 60 Deg to 90 Deg for the different number of layers when the thickness of Al thin film is fixed at 30 nm and n s = 1.3300 in Figure 4a-d. It is indicated that the minimum reflectivity, representing the capability of light energy absorption [15], approaches zero firstly when the number of MX 2 layers increase, which means the contribution of light energy absorption exceeds the electron energy loss. Meanwhile, the full width at half maximum (FWHM) becomes broader caused by electron energy loss of MX 2 layers [13]. With the further increase of the MX 2 layers, the FWHM keeps getting broader and the minimum reflectivity begins to diverge from zero. This is because the SPR process must satisfy the energy conservation T + R + A = 1, where T, R and A denote the transmission, reflection and absorption, respectively. Under SPR condition, since the total internal reflection is fulfilled, the transmission T is close to 0. When the number of TMDCs layers is insufficient, the absorbed light energy is not able to promote a strong SPR excitation. Therefore, increasing the TMDCs layers can enhance the light absorption, resulting in higher sensitivity. In this condition, the absorption A is enhanced while the reflection R is reduced. However, the absorption enhancement will be saturated due to the electron energy loss when further adding TMDCs layers. In this condition, the absorption A is degraded while the reflection R is increased.
Sensors 2018, 18, x FOR PEER REVIEW 6 of 10 the absorption enhancement will be saturated due to the electron energy loss when further adding TMDCs layers. In this condition, the absorption A is degraded while the reflection R is increased. The sensitivity of conventional structure based on single Al thin film is not high enough since the metallic layer cannot absorb enough light energy to excite a strong SPR. However, TMDCs has a larger real part of dielectric constant, which benefits SPR excitation [15]. Figure 5 shows the configuration and reflectance curves for the conventional SPR sensors and the proposed one with optimal parameters. Prominent sensitivity improvement up to 3.3 times can be observed in the WS2assisted configuration. Figure 6 plots the electric field distributions in these two structures. There is a stronger field enhancement in the WS2-assisted structure compared to the conventional one, which further verifies the positive contribution provided by the TMDCs. In addition to sensitivity, figure of merit (FOM) is also one of the important aspects that affects the sensing performance. According to our calculation, the structure without TMDCs demonstrates the highest FOM. This is because the energy loss induced by the TMDCs will broaden FWHM. As we know, this phenomenon is also reported in other works of TMDCs-based SPR sensors [12][13][14][15][16]. Therefore, introducing TMDCs into the SPR sensor will contribute to sensitivity enhancement but not FOM enhancement. The sensitivity of conventional structure based on single Al thin film is not high enough since the metallic layer cannot absorb enough light energy to excite a strong SPR. However, TMDCs has a larger real part of dielectric constant, which benefits SPR excitation [15]. Figure 5 shows the configuration and reflectance curves for the conventional SPR sensors and the proposed one with optimal parameters. Prominent sensitivity improvement up to 3.3 times can be observed in the WS 2 -assisted configuration. Figure 6 plots the electric field distributions in these two structures. There is a stronger field enhancement in the WS 2 -assisted structure compared to the conventional one, which further verifies the positive contribution provided by the TMDCs. In addition to sensitivity, figure of merit (FOM) is also one of the important aspects that affects the sensing performance. According to our calculation, the structure without TMDCs demonstrates the highest FOM. This is because the energy loss induced by the TMDCs will broaden FWHM. As we know, this phenomenon is also reported in other works of TMDCs-based SPR sensors [12][13][14][15][16]. Therefore, introducing TMDCs into the SPR sensor will contribute to sensitivity enhancement but not FOM enhancement.  Besides angular sensitivity, the differential phase change between p-polarized and s-polarized reflective wave is another approach to detect the analyte [28][29][30]. The variation of the phase sensitivity with respect to the different number of MX2 and thickness are showed in Figure 7a-d. In the structure showing in Figure 1a, we can obtain the highest sensitivity of 1.12 × 10 5 Deg/RIU for bilayer MoS2 with 30 nm Al film, 2.02 × 10 5 Deg/RIU for monolayer MoSe2 with 35 nm Al film, 1.37 × 10 5 Deg/RIU for 3-layer WS2 with 35 nm Al film and 4.56 × 10 5 Deg/RIU for bilayer WSe2 with 35 nm Al film, respectively. Comparing with angular sensitivity, the optimal number of MX2 layers for phase sensitivity is less.  Besides angular sensitivity, the differential phase change between p-polarized and s-polarized reflective wave is another approach to detect the analyte [28][29][30]. The variation of the phase sensitivity with respect to the different number of MX2 and thickness are showed in Figure 7a-d. In the structure showing in Figure 1a, we can obtain the highest sensitivity of 1.12 × 10 5 Deg/RIU for bilayer MoS2 with 30 nm Al film, 2.02 × 10 5 Deg/RIU for monolayer MoSe2 with 35 nm Al film, 1.37 × 10 5 Deg/RIU for 3-layer WS2 with 35 nm Al film and 4.56 × 10 5 Deg/RIU for bilayer WSe2 with 35 nm Al film, respectively. Comparing with angular sensitivity, the optimal number of MX2 layers for phase sensitivity is less. Besides angular sensitivity, the differential phase change between p-polarized and s-polarized reflective wave is another approach to detect the analyte [28][29][30]. The variation of the phase sensitivity with respect to the different number of MX 2 and thickness are showed in Figure 7a-d. In the structure showing in Figure 1a, we can obtain the highest sensitivity of 1.12 × 10 5 Deg/RIU for bilayer MoS 2 with 30 nm Al film, 2.02 × 10 5 Deg/RIU for monolayer MoSe 2 with 35 nm Al film, 1.37 × 10 5 Deg/RIU for 3-layer WS 2 with 35 nm Al film and 4.56 × 10 5 Deg/RIU for bilayer WSe 2 with 35 nm Al film, respectively. Comparing with angular sensitivity, the optimal number of MX 2 layers for phase sensitivity is less. To further improve the phase sensitivity, we propose another SPR biosensor based on Kretschmann configuration, as shown in Figure 1b. In this sensor, Ag is used to replace Al and the air layer is removed. The optimal conditions of Ag thickness and MX2 layers are summarized in Table 3. The best phase sensitivity as high as 3.85 × 10 6 Deg/RIU can be achieved with monolayer WS2 and 46 nm Ag film. For comparison, the performances of previously reported 2D-material-assisted SPR sensors are summarized in Table 4. Significant enhancements on both angular sensitivity and phase sensitivity can be obtained in the proposed sensors.  To further improve the phase sensitivity, we propose another SPR biosensor based on Kretschmann configuration, as shown in Figure 1b. In this sensor, Ag is used to replace Al and the air layer is removed. The optimal conditions of Ag thickness and MX 2 layers are summarized in Table 3. The best phase sensitivity as high as 3.85 × 10 6 Deg/RIU can be achieved with monolayer WS 2 and 46 nm Ag film. For comparison, the performances of previously reported 2D-material-assisted SPR sensors are summarized in Table 4. Significant enhancements on both angular sensitivity and phase sensitivity can be obtained in the proposed sensors.

Conclusions
In this paper, SPR biosensors by using 2D TMDCs are proposed to enhance the sensitivity. In such sensors, the functional materials are coated on both sides of the metal layer and the impacts of material type, layer number, and metal thickness on the sensing performance are investigated and analyzed in detail. The results show that the angular sensitivity and phase sensitivity can reach as high as 315.5 Deg/RIU with 7-layers WS 2 and 3.85 × 10 6 Deg/RIU with 1-layers WS 2 , respectively. The proposed configuration can be promising a candidate for high performance biosensing.