Broadband, Plasmon‐Modified SnSe2 Photodetector Based on LNOI Thin‐Film Platform

Lithium niobate on insulator (LNOI) is widely recognized as an essential optoelectronic integration platform due to its unique ferroelectric properties and photorefractive effect. However, the wide bandgap and weak absorption of lithium niobate limit its further application in integrated photodetection field. To address this issue, encapsulating silver nanoparticles within the LNOI structure are proposed to manipulate the light field distribution of modified lithium niobate through the localized surface plasmon resonance (LSPR) effect and utilize the modified lithium niobate thin film as a functional substrate to tailor the optoelectronic properties of surface SnSe2 nanosheets, significantly enhancing their photodetection capabilities. The photocurrent of the SnSe2 photodetector based on LNOI with embedded Ag nanoparticles is enhanced by up to 1912 times compared to that on the original LNOI under the same conditions, which represents the highest reported plasmonic‐induced photodetection enhancement. This work deepens the basic research on plasmonic‐modified 2D materials and ferroelectric materials, which promotes the development of on‐chip photodetectors and the realization of fully functional photonic circuits that integrate all essential components on a single chip.


Introduction
Lithium niobate (LiNbO 3 , LN) has been widely recognized as a material that holds great potential for revolutionizing photonics.Its exceptional electro-optic, acousto-optic, nonlinear DOI: 10.1002/admi.2023010943][4][5] In recent years, the emergence of lithium niobate-on-insulator (LNOI) technology has opened up new possibilities in integrated photonics.[8][9] By incorporating this thin-film platform, optical limiting capabilities can be further enhanced. [10,11][14] This results in the generation and asymmetric distribution of surface charges, leading to the formation of "built-in electric field" when exposed to light. [15,16]In recent years, researchers have made significant advancements in understanding the unique ferroelectric properties of LiNbO 3 .These properties have been found to have a profound impact on the transport behavior of photocarriers in 2D materials.By harnessing these properties, researchers have been able to manipulate and tailor the transport behavior of photocarriers, leading to improved photodetection capabilities.[19] Hong et al. demonstrate a graphene photodetector on LiNbO 3 bulk with a responsivity of 2.12 × 10 6 A W −1 at 2 V bias. [20]Liang et al. have achieved a self-powered LN/WS 2 photodetector with the high on/off ratio of 6.24 × 10 4 at 0 V bias. [21]Guan et al. further focus on LiNbO 3 thin films and combine them with their pyroelectric effect to achieve high-performance graphene photodetectors on the LNOI platform. [22]The LNOI thin film effectively extends the optical path length in the surface 2D materials by scattering and trapping light at multiple high angles, thereby increasing light absorption. [23]This makes the LNOI thin film a valuable addition to 2D material photodetection, as it demonstrates its ability to function as multi-functional substrate for on-chip photodetectors.However, LiNbO 3 has a wideband optical transparency window, leaving ample room for improvement in the performance of LNOI-based photodetectors. [24,25]The coherent oscillation of electrons in metallic particles, known as localized surface plasmon resonance (LSPR), has been shown to confine light at the nanoscale, offering a practical means to enhance the interaction between light and matter. [26,27][32] Chemical synthesis and electron beam exposure are two commonly used methods for synthesizing nanoparticles (NPs).However, both methods result in NPs that are deposited on the substrate surface, meaning that the NPs are directly exposed to the air and are prone to oxidation, leading to a decline in device performance.[35] Moreover, they are protected by the dielectric material, making them less prone to oxidation and ensuring long-term environmental stability. [36,37]The injection of embedded plasmonic NPs allows for controlled modulation of the spontaneous polarization effect in LNOI thin films, thereby further regulating the carrier balance process of 2D materials and improving the performance of 2D materials-based photodetectors.
Here, we realize a broadband and self-powered SnSe 2 photodetector based on an LNOI thin film with embedded Ag NPs (Ag:LNOI) synthesized by ion implantation.The laser spot irradiation induces local ferroelectric polarization of the LNOI, leading to asymmetric doping of free charges into the surface of the SnSe 2 material.Additionally, the Ag NPs embedded in the substrate improve the distribution and absorption of light through the strong LSPR effect, facilitating the separation of photogenerated electron-hole pairs.As a result, the photoresponse of SnSe 2 photodetector has been greatly enhanced from visible to nearinfrared region.At 473 nm, the maximum enhancement factor is 1912.Maximum responsivity of 300.67 mA W −1 and detectivity of 1.58 × 10 11 Jones are achieved at 0 V.This work provides a novel strategy for enhancing the optoelectronic properties of 2D materials and promotes the development of integrated optoelectronic devices based on LNOI thin-film platform.

Results and Discussion
Figure 1a schematically shows the construction of the SnSe 2 photodetector based on LNOI with embedded Ag nanoparticles (SnSe 2 /Ag:LNOI).The substrate is a layered structure consisting of x-cut LiNbO 3 thin film (with a thickness of 0.9 μm), silicon dioxide (2 μm) and LiNbO 3 bulk (500 μm) from top to bottom.A large number of Ag ions are implanted into the LNOI structure and embedded NPs are formed in the film layer.Then a fewlayer SnSe 2 nanosheet is deposited on the LNOI surface through chemical vapor deposition (CVD).The details about the device fabrication are shown in the Experimental Section.The thickness (≈10 nm) and surface topography of SnSe 2 nanosheets can be characterized by atomic force microscope (AFM), as shown in Figure 1b and Figure S1 (Supporting Information).In order to obtain the physical information of Ag NPs in the substrate, on one hand, we simulate the quantity distribution of Ag ions with SRIM software, which is basically within 100 nm from the surface and presents a normal distribution trend, shown in Figure 1c.On the other hand, we also use transmission electron microscope (TEM) to measure the cross-section of the sample for more detailed and accurate information.As shown in Figure 1d, part Ag ions have formed spherical particles of different sizes in the LiNbO 3 film, and they are only distributed within ≈100 nm, which is consistent with our simulation results.There is a clear boundary between ion-implanted and non-implanted LiNbO 3 region, and the intrinsic region can maintain perfect lattice structure.The diameter of Ag NPs is ≈2-15 nm, which can be obviously observed in the HRTEM image shown in Figure 1e.
Figure 2a shows the linear optical absorption spectra measured in the experiment.It can be observed that the SnSe 2 /Ag:LNOI sample combines the absorption peak of SnSe 2 at 332 nm and the typical LSPR peak of Ag NPs at 520 nm.Under the action of LSPR effect, the LNOI embedded with Ag NPs significantly enhances the light absorption of SnSe 2 and broadens its absorption band to the near-infrared region.To simulate the interaction of NPs, an array model is established by COMSOL, and the theoretical LSPR peak of Ag:LNOI is located at 525 nm, as shown in Figure 2b.The experimental values show a slight blueshift, which can be attributed to the non-crystalline transformation of the LiNbO 3 film.This transformation results in a decreased refractive index of the dielectric environment surrounding the Ag NPs.The Raman spectra of all the samples with a 473 nm excitation laser are presented in Figure 2c,d.For samples with the SnSe 2 nanosheets transferred, two strong Raman vibrational peaks at 114 and 183 cm −1 can be attributed to the in-plane mode (E g ) and out-of-plane mode (A 1g ), respectively. [38,39]Additionally, the intensity of SnSe 2 characteristic peaks on Ag:LNOI substrate is notably higher compared to that on LNOI substrate, which can be attributed to the electromagnetic mechanism of the metal NPs inducing Raman scattering enhancement.While for the LiNbO 3 , multiple peaks can be observed, among which the peak at 632 cm −1 corresponds to the A 1 (TO) mode, while the remaining marked peaks correspond to the E(TO) mode. [40,41]It is worth noting that the Raman intensity of Ag:LNOI dropped significantly by an order of magnitude due to the destruction of the lattice structure of LiNbO 3 .Similar results are also observed when the excitation laser is 633 nm, as shown in Figure S2 (Supporting Information).
Performance tests of SnSe 2 photodetectors based on different LNOI substrates are the most critical part of evaluating the devices.In the experiment, seven continuous lasers with wavelengths of 473, 520, 637, 980, 1342, 1550 and 1900 nm are used to carry photocurrent tests on SnSe 2 /LNOI and SnSe 2 /Ag:LNOI photodetectors, respectively.
Figures 3a and S3 (Supporting Information) show that SnSe 2 /Ag:LNOI and SnSe 2 /LNOI have inhomogeneous spatial photocurrent distribution at 0 V.This confirms the presence of a built-in electric field in the design structures, which is closely related to the unique bulk photovoltaic effect of ferroelectric materials, as described in Figure 3d.Unlike the usual photovoltaic effect, the bulk photovoltaic effect originates from the spontaneous polarization generated by the uniform orientation of the dipoles inside the ferroelectric. [42,43]When light is incident, the ferroelectric material undergoes an instantaneous temperature change due to laser heating.The reduced polarization strength weakens the capture effect of external charges on the LNOI surface, allowing electrons and holes to be injected into the SnSe 2 from both sides separately (see step I). [44,45]Considering that SnSe 2 deposited on the LNOI surface is an n-type semiconductor material, the majority of carriers are electrons. [46]Therefore, the electrons doped in the left region will lead to a higher majority carriers concentration, while the holes doped in the right region will be mostly neutralized by the electrons.Charges injection at the interface causes the electron concentration to be unevenly distributed in SnSe 2 , electrons spontaneously diffuse from along the concentration gradient to the right region, while holes do the opposite, forming an n+/n-homojunction (see step II). [21]When the transfer of electrons and holes in the LiNbO 3 thin film and the SnSe 2 layer reaches an equilibrium state, fixed positive and negative charge regions are formed on both sides.In other words, the spontaneous polarization effect of LiNbO 3 under illumination regulates the carrier concentration distribution of the surface SnSe 2 , ultimately forming two overlapping built-in electric fields in the same direction (see step III). [47]The presence of this electric field is the fundamental reason why the photodetectors can achieve self-powering. [48]n order to get more information about the built-in electric field through experiments, we select the marked point in Figure 3a as the test point.The higher current observed is associated with a more pronounced pyroelectric effect near the electrode edges.A gradually increasing negative voltage is applied to the electrodes in sequence, as shown in Figure 3b,c.When the bias voltage is <−48 mV, the photocurrent gradually decreases with increasing voltage but remains positive.The drift of photogenerated carriers in this range is caused by the synergistic effect of the built-in and the external bias electric field.When the bias voltage is >−48 mV, the direction of the photocurrent directly becomes negative, and thereafter the photoelectric effect of the device is mainly controlled by the external electric field.When the bias voltage is −48 mV, there is the obvious tip current caused by the pyroelectric effect at the moment of laser on/off, [49] and the dark current and the illuminated current are almost the same.At this moment, the external electric field and the built-in electric field cancel each other out, indicating an open-circuit voltage of 48 mV.
Figures 4a,c, and S4 (Supporting Information) depict the I-V curves of SnSe 2 /LNOI and SnSe 2 /Ag:LNOI in the dark and under illumination, respectively.The results show that SnSe 2 /LNOI has a good ohmic contact, while a Schottky barrier is formed in SnSe 2 /Ag:LNOI. [50,51]Therefore, the barrier is formed between the n-type semiconductor SnSe 2 and the Ag NPs, not the metal electrode.The inset of Figure 4a shows the enlarged I-V curves, where the photogenerated current is significantly higher than the dark current at 0 V, reflecting the typical self-powered behavior.Response time is an important parameter that reflects the response speed of the photodetector.The rise/fall time of the SnSe 2 /Ag:LNOI and SnSe 2 /LNOI photodetectors is 111/109 and 84/77 ms respectively shown in Figure 4b,d.The change in response speed may be due to the large number of defects produced by the implantation of Ag ions, which can serve as carrier-capture points and slow down the recombination rate of electron-hole pairs. [52,53]he time-dependent photocurrents of SnSe 2 /Ag:LNOI and SnSe 2 /LNOI devices at three typical wavelengths (473, 1342, and 1900 nm) without external bias are shown in Figure 4g-i.More photocurrent data of other wavelengths (520, 637, 980, and 1550 nm) are depicted in Figure S5 (Supporting Information).The I-T curves show that compared with the LNOI substrate, the photocurrent signal of the SnSe 2 photodetector based on the Ag:LNOI increases significantly from the visible to the near-infrared wavelengths.At 473 nm, the photocurrents of SnSe 2 /LNOI and SnSe 2 /Ag:LNOI systems are 0.032 and 60.01 nA (under 7.96 mW cm −2 and at 0 V), respectively.After Ag ion implantation into LNOI, the photoelectric current of SnSe 2 is enhanced by three orders of magnitude, which is one order of magnitude higher than that achieved with Ag ion implantation into SiO 2 substrates as reported previously. [54]At 1342 nm, the photocurrent rises from 0.017 to 0.526 nA.It is worth noting that due to the low optical absorption of intrinsic SnSe 2 , the photocurrent of SnSe 2 /LNOI at 1900 nm is too low to be detected.The localized surface plasmon resonance (LSPR) effect of Ag nanoparticles enhances the light-matter interaction of SnSe 2 , significantly improving its optical absorption in the near-infrared range.From Figure 4i, the photocurrent of SnSe 2 /Ag:LNOI system is 0.75 nA.What's more, we compare the responsivity (R) and detectivity (D * ) of SnSe 2 /LNOI and SnSe 2 /Ag:LNOI at various wavelengths respectively.They can be calculated as follows, [55,56]  where J, A and e are the power density of the incident illumination, effective device area, and electron charge.As shown in Figure 4e, the maximum photoresponse occurs at 473 nm after modification with Ag NPs.The R increases from 0.16 to 300.67 mA W −1 , representing a 1912-fold increase.The corresponding D * increases from 7.14 × 10 8 to 1.58 × 10 11 Jones.Figure 4f summarizes the relationship between the responsivity ratio of the two devices and the increasing wavelength, demonstrating an overall downward trend.Specifically, at wavelengths of 473, 520, 637, 980, 1342, and 1550 nm, the factors are 1912, 1656, 383, 142, 31, and 107, respectively.More detailed photodetection parameters are recorded in Table S1 (Supporting Information).
As shown in Figure 5a, the photodetector maintains a stable, sensitive, and repeatable self-powered response over multiple cycle tests.[59][60][61][62] It can be observed that the R enhancement factor value of this work is significantly better than that of other metal NPs-modified photodetectors.Moreover, a broadband response spanning from visible to near-infrared range is achieved at 0 bias voltage.Compared to other modification methods, our Ag NPs do not directly contact the 2D materials, reducing the possibility of introducing structural damage and additional defects.Therefore, our strategy not only reduces the impact on the intrinsic 2D materials but also achieves outstanding detection per-formance, paving a feasible path for applications in other 2D materials.
In order to better understand the LSPR effect of Ag NPs embedded in the LiNbO 3 film and the mechanism of enhanced photocurrent, we design a monolayer 2 × 2 array model using COM-SOL software.More details about model building are provided in the Experimental Section.The electric field enhancement distributions under the incident light wavelengths of 473, 1342, and 1900 nm are shown in Figure 6a-c, which indicates the presence of electric field enhancement across visible light to near-infrared wavelengths.The field enhancement factor (|E|/|E 0 |) can reach ≈9 at 473 nm.The collective oscillation of numerous free electrons results in a strong electric field around the nanoparticles, promoting the generation and recombination of photogenerated electron-hole pairs.Moreover, for Ag NPs that are partially located in the shallow layer of the substrate, the near field of the plasma under illumination will couple to SnSe 2 , increasing its effective absorption cross section. [63]The above results all confirm that the LSPR effect of Ag NPs can strengthen light-substance interactions and significantly improve light absorption.Metal plasmons promote the conversion of absorbed light energy into a large number of hot carriers, which can elevate the surface temperature of the LiNbO 3 photovoltaic layer.This accelerates the release and transfer of pyroelectric charges, thereby enhancing the modulation effect of the LiNbO 3 thin film substrate on the carrier concentration distribution in 2D materials.Therefore, the LNOI encapsulated with Ag NPs possesses a larger built-in electric field, driving the directional motion of photogenerated carriers and amplifying the photocurrent signal.
In addition to the plasmonic effect of Ag nanoparticles on the modulation of the optical field, there are also other possibilities that can enhance the photoelectric properties of SnSe 2 .The refractive index contrast between the LiNbO 3 film layer and the air and SiO 2 layer results in strong optical confinement, so the incident light can undergo multiple scattering and multiple-angle scattering, which helps LiNbO 3 to effectively transmit and manipulate optical signals. [6,64]According to our previous work, Ag ion-doped LNOI will introduce defect energy levels in the forbidden band, [65] which serve as electron acceptor centers to receive excited electrons from the valence band, leaving a significant number of free-moving holes in the valence band and increasing electrical conductivity.Therefore, the embedded Ag NPs in the substrate improve the performance of the SnSe 2 /Ag:LNOI photodetector through the combined effects of capturing and absorbing photons, exciting the LSPR effect to enhance the local electric field, and introducing defect states.

Conclusion
In conclusion, we have successfully constructed the SnSe 2 /Ag:LNOI photodetector, which effectively combines and utilizes the advantages of both ferroelectric LiNbO 3 and metallic Ag NPs.The ferroelectric properties of LiNbO 3 enable it to control the carrier dynamics process of surface SnSe 2 under illumination, and the introduction of Ag NPs further improves this process through the LSPR effect, thus enhancing the photodetection capability of SnSe 2 .Under laser irradiation from 473 to 1900 nm, the SnSe 2 /Ag:LNOI photodetector all shows a remarkable increase in photocurrent compared to the SnSe 2 /LNOI photodetector, up to 1912 times.A maximum responsivity of 300.67 mA W −1 and a detectivity of 1.58 × 10 11 Jones have been achieved at 473 nm.The combined application of ferroelectrics and 2D materials is a promising research area for next-generation high-performance optoelectronic devices.This work provides key support for the further development of LiNbO 3 photodetectors based on low-dimensional semiconductor materials.

Experimental Section
Device Fabrication: All samples used in the experiments were realized on commercially available 900 nm LiNbO 3 single crystal films (NanoLN, Jinan Jingzheng Electronics Co.).The x-cut LNOI thin film possessed the maximum electro-optic coefficient  33 .The polished surface of LNOI (size of 10 mm × 10 mm) was irradiated using Ag + with an energy of 160 keV and the fluence of 1 × 10 17 ions cm −2 by an ion-implanter (LC22-1C0-01) from Wuhan University.When the ion density exceeds the solid solubility limit, randomly distributed metallic Ag ions aggregate in the LNOI to form embedded spherical Ag NPs.Then, using high-purity SnO and Se powders as precursors, a large area of few-layer SnSe 2 was grown on the sapphire substrate through chemical vapor deposition (CVD).Finally, the SnSe 2 nanosheets were attached to the surface of the target substrate LNOI and Ag:LNOI by wet transfer, and contacted with 50 nm Ni electrodes pairs.Physical and Optical Characterization: AFM measurements of the SnSe 2 nanosheets were completed under the "tapping mode" of Bioscope Resolve (Bruker).Ag:LNOI was sampled and reduced to a thickness of <100 nm by Focused Ion Beam (FIB).And then FEI Strata 400 S was used to obtain TEM images of the sample under an accelerating voltage of 200 kV.Linear optical absorption abilities of all samples were obtained by testing through UV-vis-NIR microspectrophotometer (20/30 PV) by CRAIC Technologies.Raman spectra were measured using LabRAM HR800 (HORLBA JY) under optical excitation at 473 and 633 nm, respectively.
Photocurrent Response Tests: In the experiment, the transient photocurrent (I-T) tests, the variable bias output (I-V) tests, and the photocurrent mapping tests were all finished on the E2 Fiber-Coupled Optoelectronic Test Probe Station (Nanjing Metatest Co.).The picoammeter (Keithley 6482) was double-channel system digital sourcemeter used to provide a voltage source and to measure DC currents in device testing.
Electric Field Enhancement Simulation: The Ag NPs embedded in the LiNbO 3 thin film were simulated and analyzed using COMSOL software.First, a monolayer 2 × 2 Ag NPs array model was established, with a particle diameter of 2.5 nm and a distance between the centers of spheres of 8 nm.The SnSe 2 layer on the surface of Ag:LNOI was set as 10 nm, and the above parameters were set based on real experimental data.The material properties and parameters used in the model were all from the software's own database.Then, to simulate the large number of NPs, Floquet Periodic Boundary conditions were imposed on the boundaries of the array.Finally, the total electric field "E" and the background electric field "E 0 " were calculated by modeling with or without Ag NPs.Based on the above information, the field enhancement factor (|E|/|E 0 |) could be obtained for any point near the nanoparticle.

Figure 1 .
Figure 1.a) A schematic diagram of the SnSe 2 /Ag:LNOI photodetector device.b) The AFM characterization of few-layer SnSe 2 of the SnSe 2 /Ag:LNOI sample.c) The ions distribution of Ag + calculated by SRIM.d) Cross-sectional TEM image of the Ag NPs in the LNOI substrate.e) HRTEM image of the Ag NPs.

Figure 3 .
Figure 3. a) Photocurrent mapping of SnSe 2 /Ag:LNOI device in self-powered mode at 520 nm.Under 30 mW incident laser, photocurrent behavior changes at different bias voltages with an interval of b) 10 mV and c) 2 mV.d) The schematic diagram of the formation of the built-in electric field under illumination.

Figure 4 .
Figure 4. I-V response curves of the a) SnSe 2 /Ag:LNOI photodetector and c) SnSe 2 /LNOI photodetector at 520 nm.Photocurrent characteristics of two designed devices in self-powered mode: Rise time and decay time of b) SnSe 2 /Ag:LNOI and d) SnSe 2 /LNOI photodetector.e) Trend of responsivity with wavelength under 10 mW incident laser.f) Responsivity enhancement factor of the Ag NPs-modified device from visible to near-infrared region.The enhancement factor is defined as the ratio of the R of SnSe 2 /Ag:LNOI to SnSe 2 /LNOI.Comparison of the time-dependent photocurrent with the excited wavelength of g) 473 nm, h) 1342 nm, and i) 1900 nm.

Figure 5 .
Figure 5. a) 80-cycles switching stability test of the SnSe 2 /Ag:LNOI photodetector.b) Performance comparison of our work with other plasmon-induced 2D materials-based photodetectors.

Figure 6 .
Figure 6.Near-field distribution in the x-y plane around Ag NPs simulated by COMSOL under incident light of a) 473 nm, b) 1342 nm, and c) 1900 nm.