**3.2 The structure of the MoS2-based photodetector**

In order to investigate chemical composition of the prepared materials, the X-ray photoelectron spectroscopy (XPS) was employed. As shown in **Figure 7**, The peaks at 229.2 and 232.3 eV correspond to the doublet of Mo 3d5/2 and Mo 3d3/2, respectively. And the peaks of 226.3, 162.1 and 163.2 eV of the binding energy attach to the S 2s, S 2p3/2 and S 2p1/2, respectively [26, 27]. For Au 4f, the peak positions of 83.6 and 87.2 eV bind to the Au 4f 7/2 and Au 4f5/f, indicating the Au NPs are directly introduced into the exfoliated MoS2 sheet [28]. More importantly, the banding energies of Mo and S in Au decorated MoS2 maintain the same values as that of bare MoS2 sheet, indicating the introduction of Au NPs has non-influence on the crystal structures of exfoliated MoS2 sheet.

Further, we used Raman spectroscopy of 532 nm laser to confirm the structural properties of the fabricated devices. For MoS2, the difference of E1 2g and A1g, Δ, corresponding to in-plane and out of plane energy vibrations, is used to index the layer number of obtained MoS2. **Figure 8a** shows the Δ of bulk MoS2 and our exfoliated MoS2 are 27.8 and 25.3 cm<sup>−</sup><sup>1</sup> , respectively, indicating that the thickness of bare MoS2 is about 10 layers [29], which is highly consistent with the AFM results. After decorating Au NPs with MoS2, it exhibits the Δ maintains nearly same as bare MoS2 but the intensities obviously increase, shown in **Figure 8b**, **c**.

### **3.3 The performance of the MoS2-based photodetector**

The photoelectric performance of the fabricated photodetector was studied at room temperature, which applied a 980 nm laser source with controllable incident power. In order to produce the laser beam pulses, we combined an oscilloscope to

#### **Figure 6.**

*AFM images of as-prepared bare MoS2 and MoS2 modified with Au NPs [23]. (a) Height of exfoliated bare MoS2 sheet. (b) Height vs. distance plot of bare MoS2, correspond to the Section A1 in (a). (c) Morphology of bare MoS2, correspond to the Section A2 in (a). Height of Au NPs decorated MoS2 sheet for (d) LPP1, (e) HPP1 and (f) LPP2.*

**Figure 7.** *XPS plots of bare MoS2 and Au NPs decorated MoS2 [23].*

the incident laser source. **Figure 9a** shows the photocurrent plots of photodetector, where the illumination power and bias voltage are 1.60 mW and 2 V, respectively. Obviously, the Au NPs/MoS2 heterostructure-based photodetector exhibits an ultrahigh photocurrent (8.6 nA) compared with that of bare MoS2-based photodetector (0.59 nA).

**Figure 9b** shows the plots of photocurrent vs. applied bias voltage ranging from 0.1 to 15 V. We obtained a photocurrent up to ~480 nA, when the applied incident laser power and bias voltage were 7.50 mW and 15 V, yielding an improved responsibility of 64 mA/W. Moreover, the *I*-*V* plots indicate the photocurrent owns a good linear relationship with applied bias voltage, when the illumination intensities tuned from 0.85 to 7.5 mW. For another, **Figure 9c** shows the dependence plots of photocurrent (*I*ph, nA) on laser power irradiation (*P*in, μW). It is found that the

**89**

*Simple Preparations for Plasmon-Enhanced Photodetectors*

photocurrent follows a nonlinear dependence to the incident power intensity, *aP*in

*Photoelectrical performances of Au NPs/MoS2-based photodetector [23]. (a) I-V plots of photodetectors based on bare MoS2 and Au NPs decorated MoS2. (b) I-V scatters of Au NPs/MoS2-based photodetector with different laser power irradiation ranging from 0.85 to 7.50 mW. (c) Photocurrent as a function of laser power under bias of 15 V. (d) Stability of the fabricated Au NPs/MoS2-based photodetector. (e) Time response of the* 

*Raman spectroscopy of the prepared materials. (a) Typical spectra of bulk MoS2 and exfoliated MoS2. Raman shift of exfoliated bare MoS2 sheet and Au NPs decorated MoS2 with different (b) sputtering electric current* 

where a and b are constant for different bias voltage. For example, when the bias

With respect to the stability of the photodetector, we performed the extended duration photocurrent measurements by periodically switching the incident laser under illumination of 3.20 mW at bias of 5 V, and the periods of both on and off state are 5 s. **Figure 9d** shows the photocurrents over 500 circles of continuous operation, exhibiting a well stability. Moreover, in order to characterize the response time for detecting infrared wavelengths of our designed device, we

applied an oscilloscope in the process of laser excitation to produce laser pulses with a duration of 10 ms. **Figure 9e** shows the time-response of Au decorated MoS2 based device under illumination of 7.50 mW at bias of 15 V. It indicates that the rise time (*t*rise) and fall time (*t*fall) are 2.4 and 2.6 ms, respectively, which are signifi-

and 1.34, respectively.

voltage are 15 V, the fitting *a* and *b* are 3.37 × 1o<sup>−</sup><sup>2</sup>

*fabricated Au NPs/MoS2-based photodetector.*

cantly superb to that of other reported MoS2 photodetectors.

*b* ,

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

*(LPP1, HPP1) (c) deposition period (LPP1, LPP2) [23].*

**Figure 8.**

**Figure 9.**

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

#### **Figure 8.**

*Nanoplasmonics*

**88**

(0.59 nA).

**Figure 7.**

*XPS plots of bare MoS2 and Au NPs decorated MoS2 [23].*

**Figure 6.**

*HPP1 and (f) LPP2.*

the incident laser source. **Figure 9a** shows the photocurrent plots of photodetector, where the illumination power and bias voltage are 1.60 mW and 2 V, respectively. Obviously, the Au NPs/MoS2 heterostructure-based photodetector exhibits an ultrahigh photocurrent (8.6 nA) compared with that of bare MoS2-based photodetector

*AFM images of as-prepared bare MoS2 and MoS2 modified with Au NPs [23]. (a) Height of exfoliated bare MoS2 sheet. (b) Height vs. distance plot of bare MoS2, correspond to the Section A1 in (a). (c) Morphology of bare MoS2, correspond to the Section A2 in (a). Height of Au NPs decorated MoS2 sheet for (d) LPP1, (e)* 

**Figure 9b** shows the plots of photocurrent vs. applied bias voltage ranging from 0.1 to 15 V. We obtained a photocurrent up to ~480 nA, when the applied incident laser power and bias voltage were 7.50 mW and 15 V, yielding an improved responsibility of 64 mA/W. Moreover, the *I*-*V* plots indicate the photocurrent owns a good linear relationship with applied bias voltage, when the illumination intensities tuned from 0.85 to 7.5 mW. For another, **Figure 9c** shows the dependence plots of photocurrent (*I*ph, nA) on laser power irradiation (*P*in, μW). It is found that the

*Raman spectroscopy of the prepared materials. (a) Typical spectra of bulk MoS2 and exfoliated MoS2. Raman shift of exfoliated bare MoS2 sheet and Au NPs decorated MoS2 with different (b) sputtering electric current (LPP1, HPP1) (c) deposition period (LPP1, LPP2) [23].*

#### **Figure 9.**

*Photoelectrical performances of Au NPs/MoS2-based photodetector [23]. (a) I-V plots of photodetectors based on bare MoS2 and Au NPs decorated MoS2. (b) I-V scatters of Au NPs/MoS2-based photodetector with different laser power irradiation ranging from 0.85 to 7.50 mW. (c) Photocurrent as a function of laser power under bias of 15 V. (d) Stability of the fabricated Au NPs/MoS2-based photodetector. (e) Time response of the fabricated Au NPs/MoS2-based photodetector.*

photocurrent follows a nonlinear dependence to the incident power intensity, *aP*in *b* , where a and b are constant for different bias voltage. For example, when the bias voltage are 15 V, the fitting *a* and *b* are 3.37 × 1o<sup>−</sup><sup>2</sup> and 1.34, respectively.

With respect to the stability of the photodetector, we performed the extended duration photocurrent measurements by periodically switching the incident laser under illumination of 3.20 mW at bias of 5 V, and the periods of both on and off state are 5 s. **Figure 9d** shows the photocurrents over 500 circles of continuous operation, exhibiting a well stability. Moreover, in order to characterize the response time for detecting infrared wavelengths of our designed device, we applied an oscilloscope in the process of laser excitation to produce laser pulses with a duration of 10 ms. **Figure 9e** shows the time-response of Au decorated MoS2 based device under illumination of 7.50 mW at bias of 15 V. It indicates that the rise time (*t*rise) and fall time (*t*fall) are 2.4 and 2.6 ms, respectively, which are significantly superb to that of other reported MoS2 photodetectors.
