**3.3 PL manipulation of monolayer and bilayer WS2 gated by silicon nanostripes**

Realizing the electrical tuning is crucial for further application. To examine the electrical tuning performance, first, PL intensities of monolayer WS2 under different voltages were measured as shown in **Figure 6a**. When applying negative gate voltages from 0 to 10 V, the maximum PL intensity increases by 50%. On the contrary, the intensity of the PL peak decreases to half under positive gate voltages from 0 to 10 V. Compared with the normal WS2 monolayer gated by the flat gate [34–36], the PL enhancement effect is weaker, while the reduction effect is more obvious. This phenomenon indicates that the tuning effect is not pure electrostatic doping and there should be a new mechanism. PL changes of the bilayer WS2 under different voltages were also measured as shown in **Figure 6b** and **c**. Unexpectedly, the variation trends under positive and negative gate voltages are almost the same. The maximum PL intensity doubled when increasing the gate voltage from 0 to 10 V. Several groups have studied the electrically controlled PL of bilayer WS2, while no obvious effect has been observed [34–36]. Therefore, the obvious PL enhancement we observed may not arise from pure electrostatic doping. The gate voltage dependent intensity of excitonic peak is plotted in **Figure 6d**. The PL intensity of monolayer WS2 increases linearly with gate voltage, while the PL intensity of bilayer WS2 and the gate voltage follow a parabolic relationship.

When placing WS2 flakes on the Si nanostripe, there are three regions that experience different forces. The first part is WS2 on the Si nanostripe, the second part is WS2 near the edges of Si nanostripes, and the third part is fully suspended WS2. If the applied voltage is high enough, a great number of holes and electrons will be produced at WS2 layers and bottom Si nanostructures, respectively. Besides the electrostatic doping, the static electric field can also produce attractive forces. As shown in **Figure 7a**, there are three types of attractive forces F1, F2, and F3 which depend on different distances and capacitances. F1 is the attractive force between adherent WS2 and Si nanostripe through the 30 nm insulator layer. F2 is the attractive force between suspended WS2 around edges and the Si nanostripe. F2 at edges equals to F1 and decreases gradually away from the Si nanostripe, so this force will let WS2 at edges be curved and exert a large uniaxial tensile strain on WS2. The electrostatic attraction between the suspended WS2 and bottom Si (F3) is much weaker which can be ignored because of a larger distance and smaller capacitance. The deflection of few layer WS2 Δ*l* under electrostatic force can be calculated by

*band structure and dominated transitions before and after applying strain.*

*Electro-Optical Manipulation Based on Dielectric Nanoparticles*

*DOI: http://dx.doi.org/10.5772/intechopen.88616*

*Mechanism of electrical tuning. (a) Schematic setup showing how tensile strain can be generated by electrostatic gating. F1, F2, and F3 represent three types of electrostatic forces. (b) Schematic diagram showing the changes of*

*PL*<sup>2</sup> <sup>¼</sup> <sup>8</sup>*T*0*t*Δ*<sup>l</sup>* <sup>þ</sup>

deflection, the strain can be estimated by [40]:

*χ* ¼ 2Δ*l* 2 64 3

where *t* is the thickness that equals to 0.8 or 1.6 nm, *E* is the Young's modulus of WS2, and *L* is the effective length of strained WS2. The effective length is very small because only WS2 at edges experiences significant attractive forces. *<sup>P</sup>* <sup>¼</sup> *<sup>C</sup>*2*V*<sup>2</sup> ð Þ*<sup>g</sup> <sup>=</sup>*2*ε*<sup>0</sup> is the electrostatic pressure, where *C* is the capacitance per unit area and *ε*<sup>0</sup> is the permittivity of vacuum. After considering the relationship between strain and the

Based on the calculation above, the strain *χ* within effective area is larger than 2.8%. Such high strain is able to change the band structure of WS2 and influence the excitonic emission [41–43]. Therefore, in **Figure 7b**, we combine electrostatic doping and strain effect together to analyze the change of band structure and PL intensity. For 1L-WS2, if only electrostatic doping comes into effect, negative gating would enhance the PL intensity contributed by direct transition. However, in our case, larger strain generated from attractive forces changes the band gap of monolayer WS2. Without applied bias, the locations of valence-band maximum and conduction-band minimum are overlapped at the point Κ. Once strain is applied,

*Et <sup>L</sup>*<sup>2</sup> <sup>1</sup> � *<sup>υ</sup>*<sup>2</sup> ð Þ

Δ*l*

*<sup>=</sup>L*<sup>2</sup> <sup>¼</sup> 2 3 ½ � ð Þ *<sup>=</sup>*<sup>64</sup> ð Þ *PL <sup>=</sup>*ð Þ *Et* <sup>2</sup>*=*<sup>3</sup> (6)

<sup>3</sup> (5)

equation [40]:

**85**

**Figure 7.**

How to explain the abnormal PL manipulation in the proposed hybrid nanoantennas? The schematic shown in **Figure 7a** may give a better understanding.

#### **Figure 6.**

*Electrically controlled PL. (a) PL spectra of the monolayer WS2 on the Si nanostripe at different gate voltages. (b, c) PL spectra of the bilayer WS2 on the Si nanostripe at different gate voltages. (d) The gate voltage dependence of PL intensities.*

*Electro-Optical Manipulation Based on Dielectric Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.88616*

**Figure 7.**

effect and the optical resonant modes may be two reasons that lead to the change of line shapes. The Fabry-Perot mode in the unpatterned substrate and Fabry-Perot mode assisted by Mie resonances in fabricated nanostripes can both influence the

**3.3 PL manipulation of monolayer and bilayer WS2 gated by silicon nanostripes**

*Electrically controlled PL. (a) PL spectra of the monolayer WS2 on the Si nanostripe at different gate voltages. (b, c) PL spectra of the bilayer WS2 on the Si nanostripe at different gate voltages. (d) The gate voltage*

Realizing the electrical tuning is crucial for further application. To examine the electrical tuning performance, first, PL intensities of monolayer WS2 under different voltages were measured as shown in **Figure 6a**. When applying negative gate voltages from 0 to 10 V, the maximum PL intensity increases by 50%. On the contrary, the intensity of the PL peak decreases to half under positive gate voltages from 0 to 10 V. Compared with the normal WS2 monolayer gated by the flat gate [34–36], the PL enhancement effect is weaker, while the reduction effect is more obvious. This phenomenon indicates that the tuning effect is not pure electrostatic doping and there should be a new mechanism. PL changes of the bilayer WS2 under different voltages were also measured as shown in **Figure 6b** and **c**. Unexpectedly, the variation trends under positive and negative gate voltages are almost the same. The maximum PL intensity doubled when increasing the gate voltage from 0 to 10 V. Several groups have studied the electrically controlled PL of bilayer WS2, while no obvious effect has been observed [34–36]. Therefore, the obvious PL enhancement we observed may not arise from pure electrostatic doping. The gate voltage dependent intensity of excitonic peak is plotted in **Figure 6d**. The PL intensity of monolayer WS2 increases linearly with gate voltage, while the PL intensity of bilayer WS2 and the gate voltage follow a parabolic relationship. How to explain the abnormal PL manipulation in the proposed hybrid nanoantennas? The schematic shown in **Figure 7a** may give a better understanding.

PL line shapes.

*Applications of Nanobiotechnology*

**Figure 6.**

**84**

*dependence of PL intensities.*

*Mechanism of electrical tuning. (a) Schematic setup showing how tensile strain can be generated by electrostatic gating. F1, F2, and F3 represent three types of electrostatic forces. (b) Schematic diagram showing the changes of band structure and dominated transitions before and after applying strain.*

When placing WS2 flakes on the Si nanostripe, there are three regions that experience different forces. The first part is WS2 on the Si nanostripe, the second part is WS2 near the edges of Si nanostripes, and the third part is fully suspended WS2. If the applied voltage is high enough, a great number of holes and electrons will be produced at WS2 layers and bottom Si nanostructures, respectively. Besides the electrostatic doping, the static electric field can also produce attractive forces. As shown in **Figure 7a**, there are three types of attractive forces F1, F2, and F3 which depend on different distances and capacitances. F1 is the attractive force between adherent WS2 and Si nanostripe through the 30 nm insulator layer. F2 is the attractive force between suspended WS2 around edges and the Si nanostripe. F2 at edges equals to F1 and decreases gradually away from the Si nanostripe, so this force will let WS2 at edges be curved and exert a large uniaxial tensile strain on WS2. The electrostatic attraction between the suspended WS2 and bottom Si (F3) is much weaker which can be ignored because of a larger distance and smaller capacitance. The deflection of few layer WS2 Δ*l* under electrostatic force can be calculated by equation [40]:

$$PL^2 = 8T\_0 t \Delta l + \frac{64}{3} \frac{Et}{L^2 (1 - v^2)} \Delta l^3 \tag{5}$$

where *t* is the thickness that equals to 0.8 or 1.6 nm, *E* is the Young's modulus of WS2, and *L* is the effective length of strained WS2. The effective length is very small because only WS2 at edges experiences significant attractive forces. *<sup>P</sup>* <sup>¼</sup> *<sup>C</sup>*2*V*<sup>2</sup> ð Þ*<sup>g</sup> <sup>=</sup>*2*ε*<sup>0</sup> is the electrostatic pressure, where *C* is the capacitance per unit area and *ε*<sup>0</sup> is the permittivity of vacuum. After considering the relationship between strain and the deflection, the strain can be estimated by [40]:

$$\chi = 2\Delta l^2 / L^2 = 2[(\mathbf{3}/\mathbf{64})(\mathbf{PL})/(\mathbf{Et})]^{2/3} \tag{6}$$

Based on the calculation above, the strain *χ* within effective area is larger than 2.8%. Such high strain is able to change the band structure of WS2 and influence the excitonic emission [41–43]. Therefore, in **Figure 7b**, we combine electrostatic doping and strain effect together to analyze the change of band structure and PL intensity. For 1L-WS2, if only electrostatic doping comes into effect, negative gating would enhance the PL intensity contributed by direct transition. However, in our case, larger strain generated from attractive forces changes the band gap of monolayer WS2. Without applied bias, the locations of valence-band maximum and conduction-band minimum are overlapped at the point Κ. Once strain is applied,

the valence-band maximum will shift from Κ to Γ point, and indirect transition will happen. Therefore, the strain effect will weaken the PL intensity, which has opposite effect compared with electrostatic doping under negative gating. As a consequence, the PL enhancement is weaker under negative gating and more obvious under positive gating compared with previous works. For 2L-WS2, electrostatic effect can be ignored, and the strain effect is dominated. Without applied bias, the PL emission of bilayer WS2 contains both direct transitions and indirect transitions. If the strain becomes larger, valence-band maximum at the Κ point reduces, which promotes the direct transition along the Κ Κ direction [43, 44].

opportunity to apply voltages at nanoscale and collect the optical signals at single points. Interfaces are important in those hybrid nanodevices. The interfaces between plasmonic structures and Mie resonators bring new mechanism on carrier injection and changes of refractive index, while the contact between WS2 and Mie resonator generates unique PL active tuning arising from the synergistic effect between electrical doping and tensile strain under gate voltages. In the emerging applications based on dielectric Mie resonators, our findings provide an important and feasible method to build optoelectronic functional devices that can transfer electrical signal to optical signal. Furthermore, the excellent biosensing performance will expand the applications of Mie resonator-based optoelectronic devices.

*Electro-Optical Manipulation Based on Dielectric Nanoparticles*

*DOI: http://dx.doi.org/10.5772/intechopen.88616*

This work was supported by the National Natural Science Foundation of China

**Acknowledgements**

**Conflict of interest**

**Author details**

**87**

Jiahao Yan and Yuchao Li\*

(No. 11774135, 11874183, and 61827822).

The authors declare no competing financial interests.

Institute of Nanophotonics, Jinan University, Guangzhou, China

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: liyuchao@jnu.edu.cn

provided the original work is properly cited.

## **4. Dielectric nanoparticles for bionanosensing**

From the above analysis, we know that Mie resonators such as Si nanoparticles can combine with plasmonic nano-electrodes to obtain electrically controlled optical responses, and Mie resonators such as Si nanostripes can also interact with WS2 layers to realize abnormal electro-optical modulation based on electrostatic doping and strain effect. Further, it is necessary to utilize the unique properties of Mie resonators and analyze their application prospect in biosensing.

As we know, plasmonic nanostructures have been widely used in biosensing. Plasmon resonances experience redshift when increasing the surrounding refractive index, which is the most basic mechanism of biosensing. Dielectric Mie resonators have low-loss feature and strong directional scattering which also have a potential as biosensing nanoantennas. However, based on current reports and our experiments, we found the optical responses of single silicon nanostructures such as Si nanoparticles cannot exhibit obvious change when changing the surrounding refractive index. Therefore, the biosensor based on a single Si nanoparticle is insensitive.

Fortunately, we found the scattering spectra become very sensitive to surrounding refractive index if single Si nanoparticles combine to dimers or other oligomers. Based on our theoretical analysis, touching Si nanoparticles can produce strong electric field enhancement in the gap. This gap electric mode is a key factor for sensitive spectral change, because the gap electric mode would enhance and experience redshift with the increase of surrounding refractive index. As talked above, 1–2 nm silica layer is naturally grown on Si nanoparticles. Based on the mature biomarker technique, we can easily modify the silica surface with specific functional groups and realize the detection of many kinds of biomolecules. Furtherly, Si nanoparticles can be injected into living cells to realize the sensing in vivo. Finally, we can combine the biosensing and optoelectronic property of Mie resonators to build new type biosensors. On the one hand, biomolecules can change the electrical properties of dielectric nanostructures and then influence the optical signals. On the other hand, biomolecules can change the optical properties of nanoantennas and furtherly influence the electrical readout.

### **5. Conclusions**

In this chapter, we have introduced the electrically controlled scattering of individual Mie resonators and PL from the WS2-Mie resonator hybrid system. The strong magnetic responses and low-loss feature make silicon-based Mie resonators become important building blocks in nanophotonics. Combining top-down and bottom-up fabrication methods, plasmon-Mie hybrid nanostructures and WS2-Mie hybrid nanostructures are fabricated, respectively. These structures give us an

*Electro-Optical Manipulation Based on Dielectric Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.88616*

opportunity to apply voltages at nanoscale and collect the optical signals at single points. Interfaces are important in those hybrid nanodevices. The interfaces between plasmonic structures and Mie resonators bring new mechanism on carrier injection and changes of refractive index, while the contact between WS2 and Mie resonator generates unique PL active tuning arising from the synergistic effect between electrical doping and tensile strain under gate voltages. In the emerging applications based on dielectric Mie resonators, our findings provide an important and feasible method to build optoelectronic functional devices that can transfer electrical signal to optical signal. Furthermore, the excellent biosensing performance will expand the applications of Mie resonator-based optoelectronic devices.
