Photodetectors, converting light into electrical signals, have attracted enormous attention for applications in remote sensing, imaging, night vision, telecommunication, and environmental monitoring [1–3]. The emerging of two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDCs) offers a unique opportunity for high-performance photodetectors, with the advantages of superior carrier mobility [4–7]. Various graphene-based ultrafast response photodetectors have been reported; however, the gapless band structure of graphene also generated unsatisfactory dark currents [8]. In addition, the limited absorption of graphene makes graphene-based photodetectors difficult to realize the high photocurrent-to-dark-current-ratio (IP/ID) [9]. Although many techniques have been tried to regulate the bandgap of graphene, the by-product of loss of mobility is undesired [10–12]. By contrast, TMDCs with large bandgap ranges of 1–2 eV offer a competitive platform for efficient electron-hole pair generation under photoexcitation. Besides, TMDCs can form Schottky contact with the electrode, so that the dark current of TMDC-based photodetectors can be significantly suppressed. Compared to the widely used TMDC material, molybdenum disulfide (MoS2), tungsten diselenide (WSe2) stands out due to its superior carrier mobility. Because of the lower electron and hole effective masses, WSe2 has been widely acknowledged as one of the most promising candidates for the development of highly sensitive photodetectors [13–15]. Recently, Zhou et al. [16] demonstrated a WSe2 photodetector employing an asymmetric Schottky contact design, which yielded a high responsivity of 2.31 A/W at 650 nm wavelength and an exceptionally low dark current of 1 fA. Bu et al. [17] also fine-tuned the Schottky barrier height of the WSe2 photodetector, achieving notable results with a high IP/ID of 105 and an impressive responsivity of 5.16 A/W under 532 nm illumination.

Nevertheless, the limited absorption coefficients of WSe2 in the near infrared (NIR) wavelength range hinder its potential applications in infrared imaging, night vision, telecommunication [15], etc. In order to boost the photodetection performances in the NIR regime, the heterostructures composed of WSe2 and other narrow bandgap semiconductor materials have been proposed to serve as the photosensitive layer [18,19]. For example, Sun et al. [20] demonstrated the photocurrent of the WSe2 photodetector could be remarkably enhanced at 940 nm with the assistance of graphene quantum dots. Xue et al. [21] also reported a broadband photodetector based on the heterostructure of WSe2/SnSe2, which displayed an obvious photo-response under the telecommunication wavelength of 1550 nm. Despite this, heterostructured photodetectors that combine WSe2 with other semiconductor materials inevitably face interfacial challenges, leading to a degradation in performance, especially in dark current [18,19]. Alternatively, a metal nanostructure can excite plasmonic to enhance the absorption and photoelectric properties of optoelectronics [22–24]. Plasmonic nanoparticles (NPs) can also be used to boost the photo-response of WSe2 photodetectors in the NIR regime, via exciting plasmonic hot carriers [25–28]. For example, Guo et al. [29] integrated the noble metal Au NPs into the photodetector made of WSe2/MoS2, which could remarkably boost the light absorption over a broadband spectral range from 400 nm to 1100 nm. It suggests that the excited plasmonic hot carriers by Au NPs have the potential to overcome the bandgap limitation in semiconductors, ultimately enabling an enhanced photo-response in the NIR regime. In addition to noble metals, transition metal nitrides can serve as functionalized plasmonic materials capable of generating hot carriers. Furthermore, in contrast to Au NPs, transition metal nitride NPs boast a wider plasmonic resonant spectrum and a lower material cost, making them increasingly competitive as plasmonic components introduced into optoelectronic devices [30–32]. Until now, there have been no reports on the use of plasmonic transition metal nitride nanoparticles to enhance the performance of 2D-material-based photodetectors.

Herein, we demonstrate a plasmonic enhanced broadband WSe2 photodetector with the titanium nitride nanoparticles (TiN NPs) to generate plasmonic hot carriers. The results indicate a significant improvement in the absorption and the photoelectric performances of the WSe2 photodetector across a broad spectral range from UV to NIR with the incorporation of TiN NPs. Though the increased photocurrent is accompanied by the degradation of dark current, by introducing an atomically thick Al2O3 layer between WSe2 and TiN NPs, we can maintain the amplitude of the dark current close to its initial level. With the synergistic effects of TiN NPs and Al2O3 layer, the device exhibits a high IP/ID of ∼105 at 660 nm wavelength. In addition, a high external quantum efficiency (EQE) enhancement factor of 379.66% and a broad linear dynamic range (LDR) of 99 dB are demonstrated under 850 nm illumination. In literatures, only those heterostructure-based photodetectors comprising WSe2 and other narrow bandgap semiconductors exhibited a noticeable photoelectric response at 1550 nm wavelength. Surprisingly, the WSe2 photodetector developed in this work demonstrates responsiveness to the telecommunication illumination at 1550 nm, attributed to the well-regulated energy band diagrams. With the incorporation of TiN NPs and Al2O3 layer, a remarkable enhancement factor in EQE of 178.47% is achieved at 1550 nm. This work provides an alternative approach for developing cost-effective NIR photodetectors with potential applications in areas such as night vision, telecommunication, and more in the near future.

Figure 1 presents the fabrication processes of the plasmonic device, encompassing the functional layers of TiN NPs, Al2O3, WSe2, and TiN electrodes based on a PS nanosphere template (for details see Appendix A). The absorption spectra of TiN NPs with varying deposition thicknesses are depicted in Fig. 6(a) of Appendix D. It shows that the thickness will influence the surface plasmon resonance frequency and change the propagation characteristics of surface plasmon waves, causing shifts in absorption peaks and variations in the absorption intensity of TiN NPs. Thus, TiN NPs exhibit broadband absorption, ranging from 380 nm to 1100 nm, and achieve an average absorption of 45% when the nominal thickness of the TiN NPs is 40 nm. The optimal 40 nm thick TiN NPs exhibit a broad plasmon resonance absorption from 380 nm to 675 nm centered at 520 nm. Besides, the absorption spectra of 40 nm thick TiN NPs under varying incident angles are depicted in Fig. 6(b) of Appendix D. It shows that different incident angles lead to variation in the efficiency of surface plasmon wave excitation, reflection, and interference effects of incident light within the TiN NPs. Larger incident angles may weaken the distribution of the electromagnetic field on the TiN NP surface, decreasing the absorption properties of TiN NPs. In Fig. 2(a), the atomic force microscopy (AFM) image shows the well-ordered and closely-packed monolayer PS nanosphere template with the diameter around 100 nm. Figure 7 in Appendix D displays the AFM image of oxygen-etched monolayer PS nanospheres with a reduced diameter of approximately 70 nm. Figure 2(b) reveals the AFM image of a hexagonal arrangement with 40 nm thick TiN NPs after sputtering TiN and removal of the bottom PS nanosphere template using acetone.

Benefiting from the broadband plasmonic resonance excited by TiN NPs, the absorption of transition metal sulfides (WSe2) is expected to be enhanced when combined with TiN NPs. Figure 2(c) shows the experiment absorption spectra of single WSe2 and TiN NPs/WSe2 films. Besides, the absorption spectra of single WSe2 and TiN NPs/WSe2 films are also investigated theoretically [refer to Appendix E, Fig. 8(a)]. The broad plasmon resonance absorption of the TiN NPs is overlapped with the absorption peak of WSe2 and the TiN NPs are covered by WSe2, so the resonance absorption peaks of TiN NPs in the absorption spectrum of TiN NPs/Al2O3/WSe2 are not obvious. By integrating simulated and experimental results, we can infer a substantial enhancement in the absorption intensity of WSe2, spanning from the wavelength of 390 nm to 900 nm, and the absorption peak in the visible region is more obvious and a bit shifts upon incorporation with TiN NPs. Besides, the absorption curves of WSe2 become more gentle near 520 nm, with the strong plasmon resonance absorption of TiN NPs. Limited by the imprecision of the experiment, the size and the thickness of the TiN NPs and WSe2 cannot be precisely controlled compared with simulation. Thus, more typical WSe2 absorption peaks have been observed in the simulated absorption spectra. In order to gain a more detailed understanding of the absorption enhancement mechanism, we investigate the electric field distributions of the single WSe2 together with TiN NPs/WSe2 films at different positions for comparison, as shown in Fig. 2(d) and Fig. 9 of Appendix E (for simulation details see Appendix B). It is observed that a dipole-like plasmonic resonance is excited by the TiN NP with a strong localized electric field surrounding the TiN NP, which is responsible for the enhanced absorption in the neighborhood of TiN NPs/WSe2 film compared with single WSe2 film in a broad wavelength range. Additionally, the intensity of plasmonic resonance gets increased in the wavelength range from 385 nm to 850 nm. Consequently, the absorption enhancement factor [ΔAbs (%)] between TiN NPs/WSe2 and single WSe2 films increases from the UV to NIR regime, as shown in Fig. 8(b) of Appendix E, with the maximum ΔAbs (%) reaching 1685% at 850 nm. And the atomically-thick Al2O3 layer has less effect on the absorption of the TiN NPs/WSe2 film. The detailed calculation of ΔAbs (%) can be found in Appendix C.

The I−V characteristics of WSe2 photodetectors (PDs) with and without TiN-NPs are illustrated in Figs. 3(a) and 3(b), both in dark condition and under illumination of 660 nm with the intensity of 10.2mW/cm2. It is noteworthy that the photocurrent of the TiN-NPs/WSe2 PD significantly increases from 12 nA to 40 nA under an applied bias of 2 V. This improvement is attributed to the enhanced absorption of light facilitated by the excited plasmonic hot electrons. However, this enhancement introduces a new issue wherein the dark current of the TiN-NPs/WSe2 PD experiences a significant degradation to 1.5 pA, compared to the single WSe2 PD’s dark current of 522 fA. Previous studies have shown that the insertion of an atomic-level thick Al2O3 layer can effectively suppress the dark current without compromising the photocurrent of PDs [33,34]. To maintain the amplitude of the dark current at a level close to its initial state, a 1 nm thick Al2O3 layer is introduced between WSe2 and TiN NPs. Figure 3(a) demonstrates that the dark current of the TiN-NPs/WSe2 PD can be effectively reduced and brought close to its initial level, with the value of 830 fA under the bias of 2 V. Additionally, Fig. 3(b) indicates that the insertion of an atomically thick Al2O3 layer has minimal effect on photocurrent of the TiN-NPs/WSe2 PD. For a deeper understanding of the influence of the Al2O3 layer thicknesses on the performances of TiN-NPs based PDs, Fig. 10 in Appendix E presents the compared I−V curves of TiN-NPs/Al2O3/WSe2 PD with different Al2O3 layer thicknesses both in dark and under illumination of a 980 nm laser with the intensity of 10.2mW/cm2. It can be concluded that all PDs with the Al2O3 layer exhibit significantly higher photocurrent-to-dark-current ratios (IP/ID), and the largest ratio of IP/ID is achieved as 37.19 with a 1 nm thick Al2O3 layer. Consequently, we selected TiN-NPs/Al2O3/WSe2 PD with a 1 nm thick Al2O3 layer as the optimal device for comparison of other performances against the single WSe2 PD. Figure 11 in Appendix E displays the I−V curves of PDs in the dark and under different wavelength illuminations with the intensity of 10.2mW/cm2. The PD with the Al2O3 layer demonstrates an enhanced IP/ID from UV to telecommunication range of 1550 nm compared with single WSe2 PD. The transient responses for TiN-NPs/Al2O3/WSe2 and WSe2 PDs under different wavelength illuminations and applied with the bias of 2 V are also depicted in Figs. 3(c) and 3(d). It is evident that the signal contrast of the PD with TiN-NPs/Al2O3 is increased over a wide spectral region, ranging from 375 nm to 1550 nm, compared with single WSe2 PD, consistent with the results of the photocurrent shown in Fig. 11 of Appendix E. Besides, we calculated the response time of the TiN-NPs/Al2O3/WSe2 and WSe2 PDs under visible and NIR wavelengths, shown in Fig. 12 of Appendix E. It can be concluded that both WSe2 and TiN-NPs/Al2O3/WSe2 PDs have shorter response times to visible light (nearly 1 ms) than to NIR light (nearly 100 ms), and the PD with TiN NPs has faster response speed compared with a single WSe2 device. However, the fastest relaxation time of the TiN-NPs/Al2O3/WSe2 device is nearly 100 μs. Controlling the relaxation time of WSe2 device in ps, we need to make a prodigious effort. There are some strategies that can be considered to improve the relaxation time of WSe2 devices, such as band structure engineering, surface engineering, external optical excitation, and device structure engineering with transistor structure.

The external quantum efficiency (EQE) and responsivity (R) under bias of 2 V for TiN-NPs/Al2O3/WSe2 and WSe2 PDs are shown in Fig. 4(a). Detailed calculations can be found in Appendix C. The results indicate that both EQE and R exhibit an increase over a broad wavelength range from 375 nm to 1550 nm for the TiN-NPs/Al2O3/WSe2 PD. The EQE increases from 35% to 90%, and the highest R reaches 0.36 A/W at 505 nm for TiN-NPs/Al2O3/WSe2 PD, which is 2.5 times higher than that of the single WSe2 PD. Besides, high EQE enhancement factors (ΔEQE) of 379.66% and 178.47% at 850 nm and 1550 nm are obtained, respectively, as shown in Fig. 4(b). Linear dynamic range (LDR) is derived from the ratio of the highest and the lowest detectable illumination power densities (Phigh/Plow) within the linear response range. The LDRs of TiN-NPs/Al2O3/WSe2 PD under 505 nm and 850 nm with varied power density illuminations are presented in Figs. 4(c) and 4(d). The device demonstrates a broad LDR of 120 dB at 505 nm, and 99 dB at 850 nm. The limit of detection (LOD) is also a crucial parameter for evaluating the performance of the photodetectors. We also obtained the LOD of the TiN-NPs/Al2O3/WSe2 PD from the LDR curves; the results show that the weak-light LODs of the PD are 51nW/cm2 at 505 nm, and 1.02μW/cm2 at 850 nm.

To further understand the effect of the Al2O3 layer on the performances of WSe2 films and devices, Raman spectroscopy measurements of WSe2, Al2O3/WSe2, and TiN-NPs/Al2O3/WSe2 films are conducted. As can be seen in Fig. 5(a), the Raman characteristic peak of WSe2 film is located at 253cm−1, consistent with that in the reported results [29], and the full-width at half-maximum (FWHM) of Raman spectrum for WSe2 film is 32.70cm−1. When transferring the WSe2 on the surface of the Al2O3 layer, the Raman characteristic peak of Al2O3/WSe2 shifts to 252cm−1, indicating a 1cm−1 shift that suggests a weak reaction happened between WSe2 and Al2O3 [33,34,37]. This reaction reduces the interface defects between WSe2 and the substrate, improving the crystalline quality of WSe2, with the FWHM of the Al2O3/WSe2 film decreasing to 32.49cm−1 [38–41]. Furthermore, with the addition of TiN-NPs under the Al2O3/WSe2 film, the Raman intensity of WSe2 is enhanced due to the plasmonic resonance induced by TiN-NPs, in accordance with the photocurrent changes in Fig. 3(b). Besides, a 2cm−1 shift in the Raman characteristic peak occurs because of the reaction between the bottom surface of Al2O3 and TiN-NPs, and the alloying process between TiN-NPs and WSe2 is suppressed by inserting the Al2O3 layer [42]. These factors are also beneficial for enhancing the crystalline quality of WSe2 with the FWHM of 31.99cm−1 (details seen in Table 2 in Appendix E). As a result, the performances of the TiN-NPs/Al2O3/WSe2 device improved compared with single WSe2 device under both dark and illumination conditions.Table 2.

Raman Characteristics of WSe₂, Al₂O₃/WSe₂, and TiN-NPs/Al₂O₃/WSe₂ Films

The schematics in Figs. 5(b) and 5(c) show the flow of photogenerated carriers in WSe2 at the UV and visible wavelengths for the single WSe2 and TiN-NPs/Al2O3/WSe2 devices under an applied forward bias. In the case of the single WSe2 device, photogenerated holes and electrons yielded by WSe2 traverse the semiconductor driven by the electric potential, and are collected by the TiN cathode and anode, respectively, as depicted in Fig. 5(b). For the TiN-NPs/Al2O3/WSe2 device, the excitation of localized plasmonic resonance by TiN-NPs leads to the generation of more electrons and holes in the WSe2; facilitated by the enhanced electric field in the WSe2 around TiN-NPs, more photogenerated holes and electrons are transported to the corresponding electrode and collected, as illustrated in Fig. 5(c). Consequently, the absorption and photocurrent of the TiN-NPs/Al2O3/WSe2 films and the corresponding device exhibit enhancement in the UV and visible wavelength ranges. This enhancement results in higher EQE values for the TiN-NPs/Al2O3/WSe2 device in the wavelength range from 375 nm to 660 nm.

In the NIR or even the telecommunication range, the limited absorption nature of WSe2 results in an extremely weak photocurrent. Consequently, the major component in the photocurrent signals for the TiN-NPs/Al2O3/WSe2 device becomes the current induced by hot carriers generated by TiN-NPs. To better understand the type of hot carriers, Fig. 5(d) shows the energy band diagram of functional layers at the TiN-NPs/Al2O3/WSe2 interfaces without bias, depicting the transfer process of hot electrons generated from TiN-NPs to WSe2. The work function of TiN-NPs and WSe2 is characterized to be 5.08 eV and 4.31 eV, respectively, by the Kelvin probe force microscopy (KPFM) method. The KPFM images of TiN-NPs and WSe2 are shown in Fig. 14 of Appendix E, and the calculation details can be found in Appendix C. The schematics in Figs. 5(e) and 5(f) illustrate the flow of hot electrons within WSe2 for the single WSe2 and TiN-NPs/Al2O3/WSe2 devices under an applied forward bias in the NIR and telecommunication ranges. In the case of the single WSe2 device shown in Fig. 5(e), there is limited hot electron emission from the TiN cathode and only scarce hot electrons can be transported and collected by the anode, resulting in very low EQEs observed from 850 nm to 1550 nm in Fig. 4(a). Conversely, for the TiN-NPs/Al2O3/WSe2 device, abundant hot electrons are generated by TiN-NPs. Combined with the hot electrons produced from the TiN electrode, a substantial number of hot electrons are transported into WSe2 and collected by the anode as depicted in Fig. 5(f). This ultimately contributes to the significantly enhanced EQEs, as shown in Fig. 4(a).

In summary, we have demonstrated a novel method to prepare TiN NPs using PS nanospheres as a template and applied them in WSe2 photodetectors. The results demonstrated a significant enhancement in the absorption and the photoelectric performances of the WSe2 photodetector across a wide wavelength range from UV to NIR. The incorporation of an atomically thick Al2O3 layer helps maintain the dark current of the TiN-NPs/WSe2 PD at the same level as the single WSe2 PD, resulting in a high IP/ID of ∼105 at 660 nm wavelength. The improved performance in the UV and visible wavelength ranges originates from the increased absorption of light due to the localized plasmonic resonance of TiN NPs. Additionally, a substantial number of hot electrons generated by TiN NPs boost photocurrent responses, leading to the improved EQE at NIR wavelengths, with high enhancement factors of 379.66% and 178.47% at 850 nm and 1550 nm, respectively. Moreover, the device exhibits a broad LDR of 120 dB and 99 dB under 505 nm and 850 nm illuminations. Importantly, the TiN-NPs/Al2O3/WSe2 device shows much more significant responsivity in the telecommunication range compared with the singe WSe2 PD. This work introduces an alternative strategy for achieving cost-effective, broadband, and high-performance NIR photodetectors with potential applications in the fields of night vision, telecommunication, and infrared thermal imaging in the near future.