Localized surface plasmon resonance (LSPR), known as the collective oscillation of electrons and incident light in metallic nanostructures, has been applied in high performance photodetectors over the past few years. But the preparation process is complex and expensive due to the introduction of electron beam lithography (EBL) for preparing nanostructures. In the past few months, we have demonstrated two simple methods to prepare plasmon-enhanced photodetectors: (i) Au nanoparticles (Au NPs) solution were directly spun coated onto the WS2-based photodetectors. The performance has been enhanced by the LSPR of Au NPs, and reached an excellent high responsivity of 1050 A/W at the wavelength of 590 nm. (ii) Au NPs were deposited on MoS2 by magnetron sputtering. The spectral response of pure MoS2 was located in visible light and which was extended to near-infrared region (700–1600 nm) by Au NPs. Further, the responsivity reaches up to 64 mA/W when the incident light is 980 nm. In this book chapter, more details for developing those two simple methods and the discussion of the enhanced mechanism are performed, which can be very useful for the next generation photodetection.
- surface plasmon resonance
- Au nanoparticles
- enhanced photodetectors
- simple preparations
- high responsivity
Photodetectors are one of the most important devices in photonic chips, which show a great potential in optical communication, flame sensing, environmental monitoring, and astronomical studies . A lot of semiconductors have been exploited for photodetection such as silicon, GaN, PbS, InGaAs and HgCdTe, operating from ultraviolet to far-infrared region . It is significant to decrease the dimension of photodetectors down to the sub-nano scale for the next generation highly integrated photonic chips.
Two-dimensional (2D) materials such as graphene and layered transition metal dichalcogenides (TMDs) have attractive electronic and optical properties such as flexibility, transparency and metal-oxide-semiconductor (CMOS) compatibility [3, 4, 5, 6, 7]. Tungsten disulfide (WS2), a typical member of the TMDs group, has a higher carrier mobility than other TMD materials due to smaller electron effective mass . Moreover, WS2 enjoys excellent thermal stability for extensive applications . 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 material-based photodetectors suffer from low photoresponsivity mainly due to the poor optical absorption in the atomic layered materials.
A number of 2D material-based photodetectors have been enhanced using resonance microcavity, PbS quantum dots, tunneling effect, heterojunctions or perovskite [10, 11, 12, 13, 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, 16, 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, 19, 20]. Furthermore, patterning 2D materials into periodic structure can also excite plasmon resonance . The methods for preparing the nanostructure involve electron beam lithography, hydrothermal synthesis, and template-based electrochemical method, which are generally complicated, expensive and may deleterious to the device.
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 performed in the chapter.
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 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 () under illumination at room temperature without Au NPs are shown in Figure 3a. It is concluded that increase as increase from 0 to 2 V. The irradiance power is 20.5 mW/cm2 in all of these three wavelength (590, 740 and 850 nm). decrease with the increase of wavelength. The ratio of the on/off current () reached nearly under 590 nm light illumination. The responsivity can be calculated by
is the photocurrent, P is the irradiance power. The responsivity at 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 enhancedalso reached the highest value at 590 nm and decreased with the increase of the wavelength (λ).
The current gain is defined as
where 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 =0.2 mW/cm2 and . 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].
where is the donor’s energy, and are the spectral function and wave vector of the source, is the absorption of acceptor, is the coupling coefficient, and 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.
where is the vacuum wavevector, is the relative permittivity of dielectric, 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
where 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 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.
3. The preparation and theoretical mechanism of magnetron-sputtering-based MoS2 photodetectors
3.1 The preparation process
Figure 5a,b shows the schematic and optical image of our designed MoS2 plasmonic photodetector by introducing Au NPs, respectively. The few-layered MoS2 sheet was obtained using mechanical exfoliation method. Then, we transferred the exfoliated MoS2 to the SiO2/Si substrate which contacted with Au/Ni electrodes. Next, we fabricated the Au NPs on exfoliated MoS2 sheet using magnetron sputtering technique. This facile method offers the convenience of without need of template compared with other reported MoS2 plasmonic photodetectors.
Figure 6 shows the morphology of as-prepared materials was characterized using an AFM. As shown in Section A1 in Figure 6a, b, it indicates the thickness of exfoliated bare MoS2 is just about 6 nm (about 10 folds). In comparison, the surface morphology of the MoS2 after depositing with Au NPs by magnetron sputtering was shown in Figure 6c–f. It clearly exhibits the physical size and particle distribution of Au NPs can be easily tuned by sputtering technique. When the sputtering current increased from 30 mA (LPP1, Figure 6d) to 35 mA (HPP1, Figure 6e) with deposition period fixed to 1 s, we obtained the controllable Au NPs with lateral size increasing from ~3 to ~5 nm and vertical height increasing from ~5 to ~8 nm, respectively. For another, if we fixed the applied current to 30 mA and prolonged the deposit period to 2 s (LPP2, Figure 6f), the Au NPs maintain almost same physical size as LPP1 but the gap of adjacent deposited Au NPs sharply drops compared with LPP1.
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 4f7/2 and Au 4f5/f, indicating the Au NPs are directly introduced into the exfoliated MoS2 sheet . 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 E12g 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−1, respectively, indicating that the thickness of bare MoS2 is about 10 layers , 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 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 ultra-high 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 (Iph, nA) on laser power irradiation (Pin, μW). It is found that the photocurrent follows a nonlinear dependence to the incident power intensity, aPinb, 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−2 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 (trise) and fall time (tfall) are 2.4 and 2.6 ms, respectively, which are significantly superb to that of other reported MoS2 photodetectors.
3.4 The operational mechanism
In order to study the potential mechanism of our fabricated photodetector, we simulated the electric field distribution of Au decorated MoS2 using finite element method in the infrared region. We assumed that the Au NPs with a gap of 6 nm and a diameter of 5 nm under illumination of a linearly polarized plane wave with an electric field amplitude of 1 Vm−1 for the simulated model. And the thicknesses of MoS2 sheet and incident laser wavelength were 6 and 980 nm, respectively. Figure 10a, b shows the cross-section of the simulated electric field distribution of Au NPs decorated MoS2. Benefiting from the LSPR effect excited by the Au NPs, when the diameter matched to the incident wavelengths, the intensities of electric field at interfaces of air/Au/MoS2, up to ~3.96 × 105 V/m, are obviously higher than other districts. In comparison, interfaces of air/MoS2 show a poor intensity of electric field, only about 6.72 × 104 V/m. This tendency can be also observed in the recent researches of Au NPs guided MoS2 sheets for photo-detection . We further experimentally proved the absorption by using UV-visible-NIR spectroscopy analysis. Figure 10c shows the absorption spectrum of bare MoS2 sheet Au NPs decorated MoS2. The normalized absorptance plots indicate that the Au NPs/MoS2 is obviously enhanced than that of bare MoS2 ranging from 700 to 1600 nm.
The above results and discussion clearly unveil the introduction of Au NPs plays a key role in enhancing the light matter interactions of MoS2 with infrared wavelengths. The significantly improved sensitivity of the fabricated photodetector could be attributed to the LSPR effect, shown in Figure 10d, induced by the periodically aligned Au NPs, resulting in obvious improvement of local electric field. When the incident infrared wavelengths highly confined by the deposited Au NPs, the local electric field at the interface of Au/MoS2 is greatly improved by the surface plasmon waves. Firstly, the Au surface plasmons effectively excite a coupling effect at the interface of Au/MoS2 for absorbing photons, yielding a much more photo-induced carriers to improve the photo sensing [30, 31]. Moreover, the additional local electric field generated by the LSPR effect of Au NPs accelerates the photogenerated carriers to separate for producing photocurrent [32, 33]. This facile method, tuning Au NPs by sputtering method to excite LSPR effect for fabricating the unique device structure, is expected to be practical applications in other 2D materials such as WS2 and MoSe2 [34, 35, 36], thus offers a new route on a variety of high-performance optoelectronic devices.
High performance photodetectors are very important in a lot of applications. We have successfully developed two simple methods to prepare plasmon-enhanced photodetectors. (i) Au nanoparticles (Au NPs) solution were directly spun coated onto the WS2-based photodetectors. The performance has been enhanced by the LSPR of Au NPs, and reached an excellent high responsivity of 1050 A/W at the wavelength of 590 nm. (ii) Au NPs were deposited on MoS2 by magnetron sputtering. The spectral response of pure MoS2 was located in visible light and which was extended to near-infrared region (700–1600 nm) by Au NPs. Further, the responsivity reaching up to 64 mA/W when the incident light is 980 nm. These photodetectors achieved excellent responsivity and response speed. The results not only promote the development of high-performance photodetectors, but also provide a simplified method for the fabrication of other hybrid structure devices.
The project was supported by grants from the National Basic Research Program of China (No. 2015CB351905), the National Key Research and Development Program of China (No. 2016YFA 0302300, No. 2016YFA0200400), the National Natural Science Foundation of China (No. 61306105).
Conflict of interest
The authors declare no conflict of interest.