Photodetectors have wide applications in optical communications, imaging, sensing, and integrated photonics due to their ability to convert optical signals to electrical signals [1–6]. In the development of next-generation photodetectors, two-dimensional (2D)-material-based photodetectors have been playing a pivotal role by taking advantages of 2D materials in band tunability, integration flexibility, device fabrication, etc. [7–10]. The performance of 2D-material photodetectors however is largely restricted by their poor light absorption [7,11] and the limited spectral response range constrained by the intrinsic band structures [12]. In order to improve the performance of 2D-material photodetectors, various strategies have been explored, such as heterostructure construction [13–16], material doping [17], interface engineering [18], and photonic-nanostructure-induced enhancement [19–21]. Among these strategies, photonic-nanostructure-induced enhancement has attracted extensive interests because of its significant improvements in photodetection performance, flexible degree of freedom in design, and compatibility of cooperation with other enhancing strategies [22,23].

In a nanostructure-enhanced 2D photodetector, photonic nanostructures supporting optical resonant modes can concentrate light around the nearby 2D materials, enhancing its absorption and hence improving the performance of photodetectors [24]. To date, a variety of nanostructures, including plasmonic nanostructures [25–27], Mie resonators [28,29], and optical microcavities [30,31], have been reported and proven to be effective in achieving high-performance 2D photodetectors. Particularly, plasmonic nanostructures can bring about further improvement by an additional mechanism, hot electron injection [32–34], which not only enhances detectivities and responsivities of 2D photodetectors, but also shows potential in expanding their response spectral range [35]. However, most of the previous nanostructures used in 2D photodetectors only support narrowband resonances, restricting their validation in the development of highly desired broadband photodetectors [35].

On the other hand, from the viewpoint of applications, the fabrication and integration of nanostructures in the photodetectors need to be scalable and site-controllable. When it comes to this case, fabrication processes involving electron beam lithography or focused ion beams can produce photonic nanostructures with precise control of positions, but are expensive and limited in large-area device fabrication [36–38]. On the contrary, the used scalable fabrication techniques, such as thermal dewetting [39], AAO templating [40], vapor deposition [41], liquid phase synthesis [42], colloidal particles [43], and nanosphere lithography [44,45], can generate large-scale nanostructures with low cost, but are deficient in the capability of position controlling. Therefore, it is highly desired to develop scalable and site-controllable nanostructures in the developing of high-performance 2D photodetectors.

Here, we propose an enhanced broadband 2D photodetector enabled by a plasmonic array, which is fabricated by direct writing photolithography, a scalable technique with on-demand control of positions. The broadband enhancements are realized by the cooperation of three mechanisms, including hot electron injection, optical absorption enhancement, and strain effect. By taking advantages of these mechanisms, distinctly improved performance of a WS2-based 2D photodetector is demonstrated, with a broadened spectral response range from 405 to 1310 nm, an excellent detectivity up to 3.9×1014 Jones, and a high responsivity of 242 A/W. In addition, self-powered photodetection is also supported by the presented plasmonic 2D photodetector, which is highly desired in power-saving and portable optoelectronic devices. Our work demonstrates a scalable method for constructing high-performance broadband 2D photodetectors, which has great potential for boosting the applications of 2D photodetectors in imaging, broadband sensing, and optoelectronic systems.

From the optical and optoelectronic point of view, optical field enhancement and plasmonic hot electron injection hold the key to improving performance of the constructed 2D photodetector. As schemed in Fig. 1(a), on one hand, enhanced local light fields can improve optical absorption of the 2D materials, increasing the carriers directly generated in the materials [Fig. 1(a), I and II]. On the other hand, localized surface plasmons (LSPs) interact strongly with photons and utilize light energy to generate high-energy hot electrons that consequently transfer to the nearby 2D materials [Fig. 1(a), III], contributing to the enhancement of photocurrent. Therefore, in order to improve the performance of 2D photodetectors throughout a broad spectral range, it is important to develop plasmonic nanostructures with broadband resonances.

To this end, a plasmonic disk array with broadband enhancements is constructed, as shown by Figs. 1(b)–1(d). The plasmonic disk array exhibits no narrowband resonance from visible to near infrared bands except for the interband transition of gold around 500 nm [Fig. 1(b)]. Nevertheless, such a disk array possesses the capabilities to distinctly increase the optical absorption of nearby 2D materials in the short-wavelength range [Fig. 1(c)] and support electric field enhancements throughout visible to near infrared bands [Figs. 1(d) and 6 (Appendix E)]. The electric field distributions exhibit similar characteristics, including strong fields at vicinities of disk edges, which is typical for localized surface plasmons (LSPs), ring-shaped areas of enhanced field outside the disks, and standing wave patterns of breathing modes on the disk surface, which are more notable for the short-wavelength band. The latter two patterns are not haunted by the high optical loss of LSPs, making them more beneficial to the absorption enhancement of nearby 2D materials. With increased optical absorption, more carriers will be generated inside the 2D materials, benefiting the performance of 2D photodetectors. As for the wavelengths without obvious absorption enhancement, hot electron injection stemming from LSPs can also increase the carriers in the nearby 2D materials and thus improve device performance.

Experimentally, the designed plasmonic disk array needs to be integrated into a 2D photodetector, which can be accomplished in a facile and scalable way, as shown in Fig. 2(a). In particular, the as-proposed broadband 2D photodetector consists of an layer of WS2 on an h-BN-covered plasmonic disk array, with a pair of gold electrodes [Fig. 2(a)]. First, an array pattern is formed in a photoresist by standard direct writing photolithography, and the plasmonic disk array is then obtained by electron beam evaporation and a lift-off procedure [Fig. 2(a), I]. Next, a monolayer h-BN is wet transferred onto the gold disk array to avoid direct contact of plasmonic nanostructures and 2D materials [Fig. 2(a), II] [46]. Subsequently, a few-layer WS2 nanosheet is dry transferred onto the h-BN-coated disk array and made to form a conformal shape with underlying structures by a thermal treatment of 90°C for 5 min [Fig. 2(a), III and IV]. Afterward, a pair of gold electrodes is added on opposite sides of the WS2 sheet by photolithography and deposition, forming a 2D photodetector [Fig. 2(a), V]. Using these procedures, the plasmonic 2D photodetector with a simple electrical structure can be fabricated facilely, containing only two electrodes without involving a third gate electrode. The stability of the used 2D materials regarding the thermal treatment is verified by Raman spectra measurements (Fig. 7 in Appendix E).

As a demonstration, we fabricated an as-described broadband 2D photodetector, as shown by the optical photograph in Fig. 2(b). Within the photodetector, the plasmonic disk array was designed with geometrical parameters of D=1.5μm, h=40nm, and P=2.5μm, with well-defined morphology shown by the atomic force microscopy (AFM) image in Fig. 2(c). To fully take advantages of the plasmonic disk array, close contact is beneficial between the h-BN/WS2 sheet and the disk array, which can be achieved by the thermal treatment mentioned above. After the thermal treatment, the h-BN/WS2 sheet appeared to be flat on the plasmonic disks, and curved and closer to the substrate at other locations, proving fine contact between the 2D materials and the underlying plasmonic disks [Fig. 2(d)]. In Fig. 2(d), the disk areas with 2D materials were distinctly higher than those without covering, which is because the used WS2 sheet was about 30 nm in thickness. More information regarding the morphology of the fabricated device can be found in Fig. 8 (Appendix E).

In an as-fabricated device, the close vicinity of WS2 and the disk array can enable strong interactions between them both optically and electronically, making it effective to benefit from the light field enhancement and hot electron injection aroused by the plasmonic disks. As a consequence of their interactions, the photoluminescence (PL) of the WS2 on the plasmonic disk array was enhanced to 2.82 times of intensity under excitation of a 532 nm laser, compared to that of a pristine WS2 [Fig. 3(a)]. In addition, the spatial distribution of the PL intensities of the device was obviously modulated by the plasmonic disks, showing the strongest intensities around the disk edge [Fig. 3(b)], which is in accordance with the electric field distributions of the plasmonic disks shown in Fig. 1.

When it comes to the optoelectronic aspect, the interactions with the plasmonic disks bring about distinctly improved performance of the 2D photodetector. As shown in Fig. 3(c), the photocurrents of WS2 with plasmonic disks [red curves in Fig. 3(c)] were obviously larger than that without plasmonic disks [black curves in Fig. 3(c)] under illumination of a 635 nm laser with varied power densities. To be quantitative, there was a maximum amplification of 49 times in responsivity with the assistance of the plasmonic disks, as illustrated by Fig. 3(d) plotting the responsivity of the WS2 photodetectors with (red curve) and without (black curve) plasmonic disks illuminated using a 635 nm laser with varied power densities. The responsivity of the plasmonic 2D photodetector decreased obviously as incident light power densities increased [Figs. 3(d) and 9 (Appendix E)], which was caused by the saturation effect of carriers [47].

In the assessment of a photodetector, detectivity is another crucial parameter, which represents the capability of detecting weak light signals. Hence, the detectivity of the studied photodetectors under 635 nm illumination was also measured and shown in Fig. 3(e). In this case, the detectivity was able to be enhanced by 47 times in the presence of the plasmonic disks, as shown by the detectivity data of the 2D photodetectors with [red curve in Fig. 3(e)] and without [black curve in Fig. 3(e)] the plasmonic disks. As for the dependence of detectivity on power density, it was similar to that of responsivity, where decreases appeared as incident power densities increased [Figs. 3(e) and 10 (Appendix E)].

In fact, the constructed plasmonic 2D photodetector possessed high performance throughout a broad spectral range from visible to near infrared bands (Figs. 9–12 in Appendix E). Particularly, at its best working wavelength (405 nm), the plasmonic 2D photodetector supported a remarkably large detectivity up to 3.9×1014 Jones and a high responsivity of 242 A/W (Figs. 9 and 10 in Appendix E). Figure 3(f) depicts the detectivity and responsivity of the typical 2D photodetectors working with plasmonic nanostructures in previous reports [12,19–21,48–56], showing that our plasmonic 2D photodetector outruns the existing counterparts in detectivity while maintaining a relatively high responsivity. When compared with the existing WS2-based 2D photodetectors, the overall performance of our plasmonic 2D photodetector also holds significant advantages, the details of which can be found in Table 1 (Appendix E).Table 1.

Performance Comparison of the Plasmon-Coupled WS₂ Photodetector with Other Reported WS₂-Based Photodetectors^a

To comprehensively investigate the broadband detecting capability, the photocurrent behavior and temporal response of the plasmonic 2D photodetector were measured. As shown by the current-voltage (I−V) characteristics in Fig. 4(a), with the involvement of the plasmonic disks, notable amplifications of photocurrents can be observed throughout a broad wavelength range from 405 nm to 1060 nm. In the I−V measurements, higher light intensities were employed in the near infrared region, which can be attributed to the lower yield of photogenerated electrons in the NIR band, given the inherent limitation of WS2 in terms of its bandgap. More detailed I−V data are given by Figs. 3(c), 4(b), and 11 (Appendix E). Accordingly, the responsivity and detectivity of the plasmonic 2D photodetector were also magnified in a wide spectral band (Fig. 11).

In addition, the plasmonic 2D photodetector can support self-powered photodetection [12,57] without bias voltage, i.e., Vds=0, especially in the cases of longer wavelengths [Figs. 4(a) and 4(b)]. To further verify the self-powered effect, a current-time measurement without bias voltage was carried out under periodic laser pulse incidence. As shown in Fig. 4(c), the photocurrent rose when the laser pulse turned on, and fell as the pulse turned off, verifying the capability of self-powered photodetection. The demonstrated self-powered photodetection with enhanced photocurrent indicates low energy consumption and high responsivity, and is highly desired for the next-generation photodetectors.

In the experiments, self-powered photodetections were notable for light incidence with wavelengths of 808 nm and longer, but not obvious for that of 635 nm and shorter [Fig. 4(a)]. The reason for this may be the difference in the dominant channel of carrier generation. According to the theoretical results in Fig. 1(c), the optical absorption of WS2 is enhanced by the presence of the plasmonic disks in the short-wavelength band, contributing to the generation of more electron–hole pairs in the 2D materials. On the contrary, for wavelengths longer than 700 nm, there is no noticeable enhancement in the optical absorption of WS2, indicating that in the long-wavelength band the photocurrent enhancements mainly stem from the electrons transferred from the plasmonic disks. Because of the lack of a bonding effect of electron–hole pairs, plasmonic hot electrons possess ultrafast diffusion behavior [58]. The hot electrons may diffuse quickly to all directions after they are injected into the WS2. Since the contact lengths between the WS2 and the two electrodes are different [Fig. 2(b)], different quantities of electrons will be accumulated at the two electrodes. This will cause an electric potential difference that can enable notable photocurrent without external bias voltage. To further confirm the origin of the self-powered photodetection, a pump-probe technique with ability to measure temporal and spatial signals may be used to observe the transient evolution of the plasmonic hot electrons [58].

The biased time-dependent response of the plasmonic 2D photodetector under different incident wavelengths was measured and is shown in Fig. 4(d). The photocurrent of the device rose immediately and then stayed balanced after the laser turned on, and decayed fast when the laser turned off [Fig. 4(d)]. For instance, when working at 635 nm, the temporal response of the plasmonic 2D photodetector was quite fast, with a rise/fall time of 11/17 ms [Fig. 4(e)]. More discussions on the temporal performance and stability of the plasmonic 2D photodetector can be found in Figs. 13 and 14 (Appendix E).

It is worth noting that, for incident light at wavelengths longer than 1060 nm, photocurrents were only observed in the plasmonic 2D photodetector [the two upper colored curves in Fig. 4(d)], while no observable photoresponse appeared in the case of pristine WS2 [black curves in Fig. 4(d)]. This means that the photoresponse for wavelengths longer than 1060 nm is caused by the involvement of the plasmonic disks. Furthermore, the temporal measurements verify that the detection of 1310 nm light signal is also supported by the plasmonic 2D photodetector [the top sub-figure of Fig. 4(d)]. Therefore, the demonstrated photodetector can work throughout a distinctly broadened spectral response range from 405 to 1310 nm, which is rare but highly desired for 2D photodetectors.

The hot electron transfer from the plasmonic disks is the main cause bringing about such a distinct broadening effect of the response band. Apart from this cause, the strain effect plays an auxiliary role in the spectral broadening, as investigated in some previous works [12,59]. To be specific, the bandgap of WS2 is narrowed when a tensile strain is enforced on it, and the narrower bandgap arouses a response to smaller photon energy and longer wavelength. Figures 5(a)–5(c) and 15 (Appendix E) calculate the theoretical results of such a stain effect, showing that the bandgap of WS2 is narrowed by applying a strain [Fig. 5(b)] and becomes narrower as the applied strain increases [Figs. 5(c) and 15]. In experiment, the height differences of WS2 in Fig. 2(d) indicate the existence of tensile strain, and the redshift of PL peak wavelength in Fig. 3(a) (marked by the dashed lines) reflects the bandgap narrowing of the WS2 on plasmonic disks [60]. In the measurement of Raman scattering, the E2g1 peak of WS2 on plasmonic disks had a redshift of 1.6cm−1 when compared with that of a pristine WS2, which is corresponding to a 0.3% strain according to previous studies [12]. Such a strain can cause a theoretical bandgap narrowing of 40 meV in WS2 (Fig. 15), indicating photoresponse to longer wavelengths. It should be noted that in a real case the strain is not spatially uniform but correlated with the morphology pattern of the Au disk, which can be indicated by the spatial distribution of the relative Raman shift (Fig. 16 in Appendix E).

In summary, we proposed a scalable method to construct a high-performance broadband 2D photodetector based on a plasmonic disk array. Hot electron injection and optical absorption enhancement aroused by plasmonic disk arrays played important roles in improving the performance of the 2D photodetector. The tight contact between the 2D material and the plasmonic disk array caused a strain effect that, together with hot electron injection, broadened the spectral response range of the 2D materials. Under the cooperations of these effects, a WS2-based 2D photodetector exhibited distinctly improved performance with a broadened spectral response range from 405 to 1310 nm, an excellent detectivity up to 3.9×1014 Jones, and a high responsivity of 242 A/W. Besides, self-powered photodetection can be supported by the presented plasmonic 2D photodetector, which is attractive for power-saving and portable systems. In addition, the plasmonic disk array was fabricated by direct writing photolithography, a scalable technique with on-demand control of positions, which made it promising for real-world industrial manufacturing and applications. In future studies, the demonstrated scalable approach of building broadband 2D photodetectors may cooperate with other improving methods, such as heterostructure construction and material doping, to construct 2D photodetectors with unprecedentedly excellent performance, bringing about promising advantages in related optoelectronic devices and systems.

Acknowledgment. The authors thank the Center of Campus Network & Modern Educational Technology, Guangdong University of Technology, Guangdong, China, for providing computational resources and technical support for this work. The authors also thank the Analysis and Test Centre of Guangdong University of Technology for providing characterization instruments, including the confocal Raman spectrometer (Horiba, LabRAM HR Evolution) and scanning electron microscope (Tescan, LYRA 3 XMU).