**2. The preparation and theoretical mechanism of spun-coated WS2-based photodetectors**

### **2.1 The preparation process**

The WS2 film was grown on sapphire substrate by chemical vapor deposition (CVD) method and transferred to Si/SiO2 substrate by wet transfer method. The molecular configurations of WS2 are shown in **Figure 1a**. Raman spectra (**Figure 1b**), PL spectrum (**Figure 1c**) and atomic force microscopy (AFM) files (**Figure 1d**) reveal monolayer feature of WS2 film.

The 3D and cross-section view of the photodetector is shown in **Figure 2a, b**, respectively. The light incident normally from the top of the device. The fabrication was carried out by the following steps. First of all, a 200-nm-thick molybdenum (Mo) layer was deposited and patterned on WS2 layer. Afterwards, the WS2 was patterned (size: 30 × 100 μm) by photolithography and plasma etching to form active area. Finally, Au NPs solution was spun coating onto the channel and dried in air. Nearly spherical Au NPs can be seen in the low-resolution transmission electron microscopy (TEM) image (**Figure 2c**). The mean diameter of Au

**83**

**Figure 1.**

*Simple Preparations for Plasmon-Enhanced Photodetectors*

are well distributed on the top of the WS2 film.

0 to 2 V. The irradiance power is 20.5 mW/cm<sup>2</sup>

the on/off current (*I*DS/ *I*dark) reached nearly 10<sup>3</sup>

The responsivity can be calculated by

irradiance power is both 0.2 mW/cm2

The current gain is defined as

sured at the irradiance power of 20.5 mW/cm2

**2.2 The performance of the WS2-based photodetector**

NPs is ~20 nm (**Figure 2d**) as shown in the statistical analysis of the TEM images. Typical low-resolution (**Figure 2e**) and high-resolution (**Figure 2f**) scanning electron microscope (SEM) images of the Au NPs are also presented. The Au NPs

*The microscopic molecular structures and characterization of monolayer WS2 film [22]. (a) Schematic molecular structure of 1 L-WS2. The blue and yellow balls present sulfur and tungsten, respectively. (b) The Raman spectrum consisted of several characteristic peaks of the WS2 film on Si/SiO2 substrate acquired with laser excitation of λ = 532 nm. (c) PL spectra of 1 L WS2 layer. The band gap is about 1.96 eV as shown in the inset. (d) The AFM height profiles and corresponding AFM image of 1 L-WS2. The thickness is about 0.8 nm.*

The drain-source current (*I*DS) under illumination at room temperature without Au NPs are shown in **Figure 3a**. It is concluded that *I*DSincrease as *V*DS increase from

(590, 740 and 850 nm). *I*DS decrease with the increase of wavelength. The ratio of

*R* = *I*Ph/P (1)

where *I*Ph is the photocurrent, P is the irradiance power. The responsivity at *V*DS = 2 V is illustrated in **Figure 3b**. The responsivity decrease with the increase of the power which are typical for photodetectors [24, 25]. *R* reached 35 A/W at the wavelength of 590 nm, and reached 1.8 A/W at the wavelength of 850 nm when

decoated with Au NPs is shown in **Figure 3c**. The drain-source current are mea-

highest value at 590 nm and decreased with the increase of the wavelength (λ).

*G* = *I*pe/ *I*ph (2)

in all of these three wavelength

. The performance of the photodetector

under 590 nm light illumination.

. The enhanced *I*DS also reached the

*DOI: http://dx.doi.org/10.5772/intechopen.89251*

*Simple Preparations for Plasmon-Enhanced Photodetectors DOI: http://dx.doi.org/10.5772/intechopen.89251*

#### **Figure 1.**

*Nanoplasmonics*

deleterious to the device.

performed in the chapter.

**WS2-based photodetectors**

(**Figure 1d**) reveal monolayer feature of WS2 film.

**2.1 The preparation process**

Hence, WS2-based high stable photodetectors can be used in many attractive applications such as extreme environment detection. Furthermore, Two-dimensional stacked molybdenum disulfide (MoS2) has attracted many research interests for applications in optoelectronic devices, due to its outstanding merits of electronic and optical properties, especially photodetectors. But the few layers 2D materialbased photodetectors suffer from low photoresponsivity mainly due to the poor

A number of 2D material-based photodetectors have been enhanced using resonance microcavity, PbS quantum dots, tunneling effect, heterojunctions or perovskite [10–14]. The narrow band absorption or complex preparation process limits the application of these attractive methods. Recently, localized surface plasmon resonance (LSPR)-enhanced photodetector has been demonstrated, and we have also provided effective ways to enhance the efficiency of light-harvesting [15–17]. LSPR can be excited by Ag or Au nanoparticles (NPs) such as deep metallic grating, nanodisk array, bowtie array, hybrid antenna and fractal metasurface [18–20]. Furthermore, patterning 2D materials into periodic structure can also excite plasmon resonance [21]. The methods for preparing the nanostructure involve electron beam lithography, hydrothermal synthesis, and template-based electrochemical method, which are generally complicated, expensive and may

For the current existing problems, we have demonstrated two methods for easy-preparation and high-performance 2D material-based photodetectors [22, 23]. (i) Au NPs solution was directly spun coated onto WS2-based photodetectors. The performance has been enhanced by the LSPR of Au NPs, and reach quite high responsivity of 1050 A/W at the wavelength of 590 nm. The diameters and distance of Au NPs will affect the resonant wavelength and absorption of the device. (ii) We have demonstrated a MoS2 plasmonic photodetector by depositing Au NPs on MoS2 sheet using magnetron sputtering without need of template, which shows a significant improvement of photo-response in near-infrared region. The spectral response of pure MoS2 was in visible light and which was extended to near-infrared region (700–1600 nm). Furthermore, the responsivity reaches up to 64 mA/W when the incident light is 980 nm. Detailed preparation and discussion of the mechanism are

**2. The preparation and theoretical mechanism of spun-coated** 

The WS2 film was grown on sapphire substrate by chemical vapor deposition (CVD) method and transferred to Si/SiO2 substrate by wet transfer method. The molecular configurations of WS2 are shown in **Figure 1a**. Raman spectra (**Figure 1b**), PL spectrum (**Figure 1c**) and atomic force microscopy (AFM) files

The 3D and cross-section view of the photodetector is shown in **Figure 2a, b**, respectively. The light incident normally from the top of the device. The fabrication was carried out by the following steps. First of all, a 200-nm-thick molybdenum (Mo) layer was deposited and patterned on WS2 layer. Afterwards, the WS2 was patterned (size: 30 × 100 μm) by photolithography and plasma etching to form active area. Finally, Au NPs solution was spun coating onto the channel and dried in air. Nearly spherical Au NPs can be seen in the low-resolution transmission electron microscopy (TEM) image (**Figure 2c**). The mean diameter of Au

optical absorption in the atomic layered materials.

**82**

*The microscopic molecular structures and characterization of monolayer WS2 film [22]. (a) Schematic molecular structure of 1 L-WS2. The blue and yellow balls present sulfur and tungsten, respectively. (b) The Raman spectrum consisted of several characteristic peaks of the WS2 film on Si/SiO2 substrate acquired with laser excitation of λ = 532 nm. (c) PL spectra of 1 L WS2 layer. The band gap is about 1.96 eV as shown in the inset. (d) The AFM height profiles and corresponding AFM image of 1 L-WS2. The thickness is about 0.8 nm.*

NPs is ~20 nm (**Figure 2d**) as shown in the statistical analysis of the TEM images. Typical low-resolution (**Figure 2e**) and high-resolution (**Figure 2f**) scanning electron microscope (SEM) images of the Au NPs are also presented. The Au NPs are well distributed on the top of the WS2 film.

### **2.2 The performance of the WS2-based photodetector**

The drain-source current (*I*DS) under illumination at room temperature without Au NPs are shown in **Figure 3a**. It is concluded that *I*DSincrease as *V*DS increase from 0 to 2 V. The irradiance power is 20.5 mW/cm<sup>2</sup> in all of these three wavelength (590, 740 and 850 nm). *I*DS decrease with the increase of wavelength. The ratio of the on/off current (*I*DS/ *I*dark) reached nearly 10<sup>3</sup> under 590 nm light illumination. The responsivity can be calculated by

$$\mathbf{R} = I\_{\rm Pl}/\mathbf{P} \tag{1}$$

where *I*Ph is the photocurrent, P is the irradiance power. The responsivity at *V*DS = 2 V is illustrated in **Figure 3b**. The responsivity decrease with the increase of the power which are typical for photodetectors [24, 25]. *R* reached 35 A/W at the wavelength of 590 nm, and reached 1.8 A/W at the wavelength of 850 nm when irradiance power is both 0.2 mW/cm2 . The performance of the photodetector decoated with Au NPs is shown in **Figure 3c**. The drain-source current are measured at the irradiance power of 20.5 mW/cm2 . The enhanced *I*DS also reached the highest value at 590 nm and decreased with the increase of the wavelength (λ).

The current gain is defined as

$$\mathbf{G} = I\_{\rm pe} / I\_{\rm ph} \tag{2}$$

#### **Figure 2.**

*Characterization of the fabricated photodetector [22]. (a) The schematic 3D view of 1 L-WS2-based photodetector was presented. The drain/source electrodes are fabricated by Mo. Au NPs (red balls) were spun coated on the channel. (b) The cross-section view of the photodetector. (c) TEM image of Au NPs. The Au NPs are well distributed in the solution. (d) The statistics size distribution of Au NPs based on the TEM image. We can conclude that the mean size of Au NPs is* ∼*20 nm. (e) Low-resolution and (f) high-resolution SEM images of the photodetector. Clear electrodes and An NPs can be seen from these images.*

where *I*pe is the enhanced photocurrent of the photodetector. The current gain reveals the improvement of the device by Au NPs as shown in **Figure 3d** when P =0.2 mW/cm<sup>2</sup> and *V*DS = 2 *V*. The photoresponsivity was enhanced ~30 times and reached 1050 A/W when λ = 590 nm. The photoresponsivity was enhanced ~11 times and reached 55 A/W when λ = 740 nm. And the photoresponsivity was enhanced ~5 times at near infrared light (λ = 850 nm) and reached 8 A/W. In general, the switching behaviour, which reflect the response speed and high-frequency characteristic, is very important for photodetectors. The detectors also need to quickly refresh in some applications such as instant display. **Figure 3e** presents the switching behavior at near infrared light (λ = 850 nm) of the WS2 photodetector. The photodetector shows a good repeatability during on-off cycles. Moreover, the on-off characteristic in a period is shown in **Figure 3f**. The rise and decay time are about 100 and 200 ms, respectively.

### **2.3 The theoretical mechanism**

To explain the mechanism of the enhancement by Au NPs, we take finite-difference time-domain (FDTD) method to investigate the distribution of electric field of Au NPs. According to the Förster's expression for energy *W* transferred from donor to acceptor [24, 25].

**85**

fixed as 20 nm.

**Figure 3.**

*Simple Preparations for Plasmon-Enhanced Photodetectors*

\_ *W Wd*

*current of the photodetector with Au NPs at the same irradiance of 20.5 mW/cm<sup>2</sup>*

*photodetectors with and without Au NPs when the irradiance is 0.2 mW/cm<sup>2</sup>*

*by the drain-source bias from −2 to 2 V. The optical power is 20.5 mW/cm2*

<sup>β</sup> = *k*0 √

β = √

nanoparticles, which can be approximately presented by

wavelength depends only on the dimension of Au NPs.

\_

εd *k*<sup>0</sup> sinθ +

where θ, λ*g*,*n* are the horizontal angle of incident wave vector, the grating period, and an integer, respectively. From Eq. (5), we can see that the absorption

The results for the illumination at λ =590, 740, and 850 nm are shown in **Figures 4a**–**c**, respectively. It is clear that the intense electromagnetic fields were introduced by LSPR of Au NPs. The electric field near the Au NPs was significantly enhanced and stronger than the rest region, revealing that electromagnetic energy was compactly confined by the Au NPs. The electromagnetic field was enhanced more significant when the illumination was under λ = 590 nm. The

= \_ 9 8*π*∫ \_ *d k*4

vector of the source, σ*a*(ω) is the absorption of acceptor, *D* = *q*/ *r*<sup>3</sup>

*fd*(ω) σ*a*(ω) |*D*|

\_ \_ ε<sup>d</sup> + ε*<sup>m</sup>* εd ε*<sup>m</sup>*

> \_2*n* λ*g* (*x* ∧ + *y* ∧

where *k*0 is the vacuum wavevector, ε*d* is the relative permittivity of dielectric, ε*m* is the dielectric function of gold, which can be represented by Drude model. The mismatch of SPPs and incident wavevector can be compensated by the gold

where *Wd* is the donor's energy, *fd*(ω) and *k* are the spectral function and wave

*Visible to NIR light response of the fabricated photodetector [22]. (a) The drain-source current (IDS ) changed* 

*The photo-responsivity changed as a function of the illumination power when VDS = 2* V*. (c) The drain-source* 

*on/off behavior of the photodetector when λ = 850 nm. (f) The on-off characteristic of the Au NPs coated photodetector in a period time. The rise time is about 100 ms and the decay time is about 200 ms.*

coefficient, and *r* is the distance. The performance could be changed by key parameters of Au NPs such as diameter *d* and distance between two particles *s*. In order to give an intuitive description, simplified models were built. The distance between the edges of two adjacent nanospheres, s, was fixed on 10 nm. The diameter *d* was

The resonant wavelength can be acquired by SPPs dispersion equation.

<sup>2</sup> (3)

 *for these three wavelength. (b)* 

*. (d) The current gain of the* 

 *and the voltage is 2 V. (e) The* 

(4)

) (5)

is the coupling

*DOI: http://dx.doi.org/10.5772/intechopen.89251*

*Simple Preparations for Plasmon-Enhanced Photodetectors DOI: http://dx.doi.org/10.5772/intechopen.89251*

#### **Figure 3.**

*Nanoplasmonics*

**84**

acceptor [24, 25].

P =0.2 mW/cm<sup>2</sup>

**Figure 2.**

about 100 and 200 ms, respectively.

**2.3 The theoretical mechanism**

where *I*pe is the enhanced photocurrent of the photodetector. The current gain reveals the improvement of the device by Au NPs as shown in **Figure 3d** when

To explain the mechanism of the enhancement by Au NPs, we take finite-difference time-domain (FDTD) method to investigate the distribution of electric field of Au NPs. According to the Förster's expression for energy *W* transferred from donor to

and reached 1050 A/W when λ = 590 nm. The photoresponsivity was enhanced ~11 times and reached 55 A/W when λ = 740 nm. And the photoresponsivity was enhanced ~5 times at near infrared light (λ = 850 nm) and reached 8 A/W. In general, the switching behaviour, which reflect the response speed and high-frequency characteristic, is very important for photodetectors. The detectors also need to quickly refresh in some applications such as instant display. **Figure 3e** presents the switching behavior at near infrared light (λ = 850 nm) of the WS2 photodetector. The photodetector shows a good repeatability during on-off cycles. Moreover, the on-off characteristic in a period is shown in **Figure 3f**. The rise and decay time are

*Characterization of the fabricated photodetector [22]. (a) The schematic 3D view of 1 L-WS2-based photodetector was presented. The drain/source electrodes are fabricated by Mo. Au NPs (red balls) were spun coated on the channel. (b) The cross-section view of the photodetector. (c) TEM image of Au NPs. The Au NPs are well distributed in the solution. (d) The statistics size distribution of Au NPs based on the TEM image. We can conclude that the mean size of Au NPs is* ∼*20 nm. (e) Low-resolution and (f) high-resolution SEM images* 

*of the photodetector. Clear electrodes and An NPs can be seen from these images.*

and *V*DS = 2 *V*. The photoresponsivity was enhanced ~30 times

*Visible to NIR light response of the fabricated photodetector [22]. (a) The drain-source current (IDS ) changed by the drain-source bias from −2 to 2 V. The optical power is 20.5 mW/cm2 for these three wavelength. (b) The photo-responsivity changed as a function of the illumination power when VDS = 2* V*. (c) The drain-source current of the photodetector with Au NPs at the same irradiance of 20.5 mW/cm<sup>2</sup> . (d) The current gain of the photodetectors with and without Au NPs when the irradiance is 0.2 mW/cm<sup>2</sup> and the voltage is 2 V. (e) The on/off behavior of the photodetector when λ = 850 nm. (f) The on-off characteristic of the Au NPs coated photodetector in a period time. The rise time is about 100 ms and the decay time is about 200 ms.*

$$\frac{d\mathcal{W}}{d\mathcal{W}\_d} = \frac{\mathcal{\mathcal{G}}}{8\pi} \text{[}\frac{d\boldsymbol{\alpha}}{k}\text{f}\_d(\boldsymbol{\alpha})\,\boldsymbol{\sigma}\_d(\boldsymbol{\alpha})\,\left|\boldsymbol{D}\right|^2\tag{3}$$

where *Wd* is the donor's energy, *fd*(ω) and *k* are the spectral function and wave vector of the source, σ*a*(ω) is the absorption of acceptor, *D* = *q*/ *r*<sup>3</sup> is the coupling coefficient, and *r* is the distance. The performance could be changed by key parameters of Au NPs such as diameter *d* and distance between two particles *s*. In order to give an intuitive description, simplified models were built. The distance between the edges of two adjacent nanospheres, s, was fixed on 10 nm. The diameter *d* was fixed as 20 nm. \_

The resonant wavelength can be acquired by SPPs dispersion equation.

ed as 20 nm.

The resonant wavelength can be acquired by SPS dispersion equation.

$$
\beta = k\_0 \sqrt{\frac{\varepsilon\_d + \varepsilon\_m}{\varepsilon\_d \varepsilon\_m}}\tag{4}
$$

where *k*0 is the vacuum wavevector, ε*d* is the relative permittivity of dielectric, ε*m* is the dielectric function of gold, which can be represented by Drude model. The mismatch of SPPs and incident wavevector can be compensated by the gold nanoparticles, which can be approximately presented by

$$\mathbf{x}\_1 = \mathbf{x}\_2 \mathbf{x}\_1 + \dots + \mathbf{x}\_{\text{TF}} \mathbf{x}\_1 \mathbf{x}\_2 + \dots + \mathbf{x}\_{\text{TF}} \mathbf{x}\_1 \mathbf{x}\_2 \mathbf{x}\_2 + \dots + \mathbf{x}\_{\text{TF}} \mathbf{x}\_1 \mathbf{x}\_2 \mathbf{x}\_1 \mathbf{x}\_2 \tag{5}$$

$$\boldsymbol{\beta} = \sqrt{\boldsymbol{\varepsilon}\_{\text{cl}}} \, \mathbf{k}\_0 \sin \theta + \frac{2\pi n}{\lambda\_\text{\text{t}}} \left( \hat{\mathbf{x}} \star \hat{\boldsymbol{\eta}} \right) \tag{6}$$

where θ, λ*g*,*n* are the horizontal angle of incident wave vector, the grating period, and an integer, respectively. From Eq. (5), we can see that the absorption wavelength depends only on the dimension of Au NPs.

The results for the illumination at λ =590, 740, and 850 nm are shown in **Figures 4a**–**c**, respectively. It is clear that the intense electromagnetic fields were introduced by LSPR of Au NPs. The electric field near the Au NPs was significantly enhanced and stronger than the rest region, revealing that electromagnetic energy was compactly confined by the Au NPs. The electromagnetic field was enhanced more significant when the illumination was under λ = 590 nm. The

**Figure 4.**

*LSPR and carrier transfer of the presented enhanced photodetector [22]. Cross-section distribution of the square of electric field (|E| 2) near Au NPs under the illumination at the wavelength of (a) 590 nm, (b) 740 nm, and (c) 850 nm. (d) The charge transfer between Au NPs and WS2 film.*

enhanced electric fields could excite the generation of the carriers in the WS2 film, resulting a prominent photoresponse. The highest responsivity obtained at λ = 590 nm (**Figure 3b, d**) is consistent with the most intense LSPR at λ = 590 nm (**Figure 4a**). The generation and transportation of the electrons are shown in **Figure 4d**. The photons were absorbed by WS2 film and the excited electrons were driven by the drain-source voltage. There are more electrons around the Au NPs as **Figure 4d** shows.
