As a typical transition metal disulphide, MoS2 has a band gap ranging from an indirect bandgap (1.2 eV) to a single-layer direct bandgap (1.9 eV) [1,2], which brings about broadband light absorption capacity and medium carrier mobility [3–5]. Based on these characteristics, MoS2 has been studied and applied in many fields, such as phototransistor [6–8], solar cell [9–11], and photodetector [12–15]. Nevertheless, the light absorption capacity of few-layer or single-layer MoS2 is relatively weak [14,16], resulting in unsatisfactory photocurrent generation.

To improve the optical absorption in MoS2, many methods have been proposed, such as adopting heterostructures [17–24], photonic crystals [25,26], and transistors [27]. One promising strategy is to integrate MoS2 with noble metal nanoparticles and couple the surface plasmon resonance (SPR) with MoS2 through hot electron injection to enhance the light absorption [28,29]. However, most of the noble nanoparticles in such devices are drop-coated, which leads to uneven signal enhancement. Furthermore, the conventional surface plasmon resonance produced in such devices has a weak optical resonance and large radiation loss; thus, the enhancement effect is limited [30].

Here, we proposed a method to enhance the light absorption of MoS2 in the visible and near-infrared regime using Fano resonance. In particular, Fano resonances originated from the interference between light directly coupled to a bright mode (continuum), and light indirectly coupled to a dark mode (subradiant) excited via near-field interactions with the bright mode [31,32]. The bright mode is associated with optical scattering, while the dark mode preferentially traps the light energy and therefore suppresses scattering. As a result, a Fano resonance is typically characterized as a narrowband dip embedded within a broadband peak in the scattering spectrum [33]. The dark mode of Fano resonance corresponds to strong near-field trapping and enhanced absorption of light energy compared to conventional SPR [33–35]. To fabricate the Fano resonance structure, the capillary-assisted particle assembly (CAPA) method is used to assemble the oligomer of gold nanoparticles (AuNPs) on the silicon substrate with a uniform nanohole array (Appendix B). As the diameter of the nanohole is fixed to ∼520nm, with the spatial limitation, heptamers, hexamers, and pentamers can be formed by using AuNPs each with a diameter of 160 nm. A few layers of MoS2 were transferred above the oligomer structures and formeda heterojunction with the underlying silicon wafer (Fig. 6 in Appendix B). The oligomers can generate strong Fano resonance from ∼620nm to 830 nm, and the light absorption of MoS2 is enhanced through the hot electron ejection from the Fano resonance generated by the oligomer structures. The proposed photodetector exhibits a broadband photoresponse ranging from 450 to 1064 nm and a large responsivity up to 52 A/W under 3 V bias voltage at 785 nm light illumination, which is four times higher than that of a photodetector without Fano resonance enhancement.

The dispersion of gold nanoparticles is directly purchased from Biotyscience (ABNW-1-160). The few-layer MoS2 is prepared through mechanical exfoliation from bulk MoS2 (SixCarbon Technology Co., Ltd.). The porous silicon substrate is prepared using the electron beam lithography (EBL) process on silicon wafers.

To fabricate the photodetector, the preparation of the substrate with hole array was performed. A bare silicon wafer was placed in a beaker and washed with acetone, isopropyl alcohol (IPA), and deionized water for 5 min each in turn, and dried with nitrogen gas. The cleaned wafer was then vacuum-adsorbed on the spin coater, with positive EB resists ZEP520A dripped on the surface of the wafer, and spin-coated at 6000 r/min for 2 min. The wafer was then transferred to a hot plate and heated at 180°C for 3 min. The wafer was exposed using an electron beam lithography system with an accelerating voltage (EHT) at 20 kV and a dose of 90μC/cm2. After the exposure was completed, the wafer was developed with an amyl acetate solution for 80 s and IPA fixation for 20 s and then blow-dried with nitrogen gas. The hole array was etched using an inductively coupled plasma (ICP) etching system. The lithographically patterned sample was placed in the ICP etching system, and the etching parameters were set: pressure 8 mTorr, ICP power 600 W, RF power 100 W, gas type and flow rate CHF₃ 45 sccm/SF₆ 15 sccm, temperature 20°C, etching time 95 s. The total etching depth is ∼200nm. After etching, the wafer is washed with acetone, and the substrate with a hole array is prepared (Figs. 6 and 7 in Appendix B).

The photocurrent characteristics of the fabricated MoS2 photodetector are measured using laser diodes and a Keithley 2601 source meter.

To illustrate the Fano resonance of the oligomer structure, the designed structure was calculated using the finite element method (FEM). The designed heptamer structure is shown in Fig. 5 in Appendix A, where R is the radius of the AuNPs, and g is the distance between the center nanoparticle and the outer ones. Figures 1(a) and 1(b) are the extinction cross-section of the heptamer under different R and g values, respectively. The existence of the Fano dip confirms the generation of Fano resonance. In addition, the position of the Fano dip can be effectively adjusted by changing the radius of the nanoparticles and the spacing between the nanoparticles. By changing the radius from 70 nm to 90 nm, the Fano resonance wavelength can be adjusted from 700 nm to 830 nm. By changing the spacing from 5 nm to 15 nm, the Fano resonance wavelength can be adjusted from 830 nm to 720 nm. To improve the responsivity of the photodetector in the near-infrared wavelength regime, a heptamer with a radius of 80 nm and spacing of 7.5 nm is adopted to generate strong Fano resonance at 785 nm. By changing the number of nanoparticles, other oligomers can be formed, such as hexamer and pentamer, as shown in Fig. 5 (Appendix A).

Figure 1(c) shows the extinction cross-sections of heptamers, hexamers, and pentamers. As the number of particles decreases, the Fano resonance wavelength of the hexamers and pentamers gradually blue shifted to 620 nm and 670 nm, respectively. To study the Fano resonance mechanism of the heptamer, the surface charge distribution at wavelengths of 600 nm and 785 nm [circles in Fig. 1(b)] are calculated. Figure 1(d) shows the surface current density at the wavelength of 600 nm. The seven nanoparticles oscillate in phase, which originated from the interference between light directly coupled to a bright mode. Figure 1(e) shows the surface current density at 785 nm. The central nanoparticle and six surrounding nanoparticles oscillate in different phases and form two displacement current loops, which indicates the emergence of the dark mode. The destructive coupling between the bright mode and dark mode in the near field results in the enhancement of the maximum electromagnetic field at 785 nm, which can be confirmed by the strong electric “hot” spots between the central sphere and the surrounding spheres in Fig. 1(f).

The scheme of the photodetector is shown in Fig. 2(a). The AuNPs oligomers were assembled in a nanohole array on the silicon substrate and covered with few-layered MoS2, and the electrodes were prepared on the two ends of the MoS2. To fabricate the photodetector, a nanohole array with specific sizes was prepared on the silicon substrate using the electron beam lithography process. The fabricated 3×3 nanohole array can be seen in Fig. 7 in Appendix B, and the diameter of the nanohole was ∼520nm. The oligomer structures were assembled in the nanohole array using the CAPA process (Fig. 8 in Appendix B). With the designed nanohole spatial confinement and the AuNPs each with a diameter of 160 nm, many heptamers were assembled inside the nanoholes. Some other oligomers such as hexamer and pentamer were also assembled in the nanoholes due to the size variation of the nanohole and AuNPs during fabrication and synthesis processes, as shown in Fig. 9 in Appendix B; mechanically exfoliated few-layered MoS2 was transferred to the nanohole arrayed silicon substrate. Finally, electrodes were prepared at both ends of MoS2 using photolithography and magnetron sputtering. Figure 2(b) shows the scanning electron microscopy (SEM) image of the prepared MoS2 photodetector with oligomer structures, and the electrodes are mounted at both ends of the MoS2. The MoS2 flake has an area of 4.9×10−9m2. There were ∼87 oligomers covered by the MoS2 flake; ∼78% of them were heptamers, hexamers, and pentamers (Appendix B). For comparison, a MoS2 photodetector without an oligomer structure was fabricated as a control, and the SEM image is shown in Fig. 2(c). The Raman spectra of the MoS2 in both devices are shown in Fig. 2(d); two characteristic peaks of the MoS2 can be seen at ∼383nm and 408 nm. Note that the positions of the two peaks are shifted compared to that of the monolayer MoS2, which is consistent with the case of the few-layer MoS2 [36]. In addition, under the same test conditions, the Raman intensity of the MoS2 photodetector with oligomer structures was seven times higher than that of the control group, which indicates a strong electric field enhancement generated by Fano resonance. For this MoS2 photodetector, there is a Schottky barrier at the contact between the gold oligomers and the MoS2. When light illuminates on the oligomers, the Fano resonance generates hot electrons which have higher energy than the Schottky barrier. Thus, the hot electrons can be emitted from the gold oligomers to the MoS2, resulting in hot electron injection [Fig. 2(e)] [37]. Figure 2(f) shows the energy band diagram of the Si/MoS2 heterojunction [18]. Under light illumination, the heterojunction absorbs photons to generate carriers. The carriers drifted under bias voltage and created a photocurrent; together with the hot electron injection, this generates a strong photocurrent.

The photocurrent characteristics of the fabricated MoS2 photodetector are measured using laser diodes and a Keithley 2601 source meter. The diameter of the laser spot is ∼3mm, which can fully cover the photodetector. Figure 3 shows the current−voltage (I-V) relation of the device under different optical powers at different wavelengths. The black curve represents the dark current of the device measured in the dark state. With the bias varied between −3 and 3 V, the dark currents are changed in a range with a magnitude of ∼0.5μA. Because the increase in the optical power excites a larger number of photo-generated carriers, a stronger photocurrent can be generated under a bias voltage. It can be seen that a much greater photocurrent of the device is generated compared to the dark current, which is proportional to the absorbed optical power. The photocurrent increases with the bias voltage, which reaches a maximum at the bias of 3 V. As the larger bias voltage was applied, more photo-generated carriers drifted to the electrodes, resulting in an increased photocurrent. The device has a response at a light wavelength ranging from 450 nm to 1064 nm. It is found that at 785 nm, the device exhibits the strongest light response under 785 nm illumination, and its photocurrent magnitude is about two orders of magnitude higher than that of no illumination, as shown in Fig. 3(d). Most of the oligomers assembled are heptamers, which generate Fano resonance at incident light with a 785 nm wavelength. The strong electromagnetic field induced by Fano resonance created many hot electrons on the surface of the nanoparticles, which cross the Schottky barrier and are injected into the MoS2. Under the bias voltage, the injected free carriers drifted to electrodes and lead to a strong current.

We further evaluated the responsivity (R) and detectivity (D*) of the photodetectors, which are described by R=Ii−IdPopt×S,(1)D*=RS1/2(2qId)1/2,(2)where Ii is the current under illumination, Id is the dark current, Popt is the incident light intensity, S is the effective area, and q is the absolute value of the electron charge (1.6×10−19C).

Figure 4(a) shows the responsivity and detectivity calculated under 785 nm illumination and 3 V bias. Note that the responsivity of the device decreases with an increase in the optical power, which resulted from the saturation of the trap states either in the MoS2 or the MoS2/substrate interface [38]. Note that our devices have the highest responsivity and detection rate under an incident light power of 5.3 mW, which is up to 52 A/W and 4.4×109 Jones, respectively. To evaluate the dynamic response of the MoS2 photodetector, we measured its impulse response. Figure 4(b) is the I-T response curve of the device measured with a bias voltage of 3 V. The incident light intensity is 33.8 mW with a wavelength of 785 nm, from which it can be seen that the prepared device presented a typical photodiode switching behavior depending on the laser on/off state. Furthermore, the rise and fall times of the device were measured by an oscilloscope. The test results show that the response time of the device is less than 0.6 ms, demonstrating its fast response and high sensitivity. Figure 4(c) shows the responsivity of our device compared to the control group (tested with a 785 nm light source with 3 V bias); the control device has a responsivity of 13 A/W and a detection rate of 1.8×109 Jones under the same conditions. Note that the responsivity of the proposed device is about four times that of the control group.

In summary, we demonstrated a Fano resonance-enhanced Si/MoS2 heterojunction photodetector, which is enabled by the internal hot-electron injection process in the gold oligomers–MoS2 junction. By integrating a MoS2 flake on a silicon substrate with an oligomer array and aligning two Au electrodes on two ends of the MoS2 flake, the oligomers generate a strong Fano resonance upon incident light. Hot electrons are generated on the oligomers and transferred to the MoS2 channel above due to the low Au−MoS2 Schottky barrier, which yields a considerable photocurrent. A photoresponsivity of 52 A/W is obtained at a wavelength of 785 nm with a bias of 3 V, while the detection rate reaches 4.4×109 Jones. The photodetector has a fast response speed; the rise and fall time is less than 0.6 ms. The demonstrated Fano resonance-enhanced Si/MoS2 heterojunction photodetector outperforms other Si/MoS2 heterojunction photodetectors, especially in the near-infrared regime. The performance of similar photodetectors is summarized in Table 1. Our research breaks the limitations of conventional enhancement with gold nanoparticles and will provide an idea for future research on the use of gold nanoparticles to enhance the near-infrared light band.Table 1.

Performance Comparison of Photodetector-Based MoS2