Novel Two‐Dimensional NbWO6 Nanosheets for High Performance UV Photodetectors

A new ultraviolet photodetector (UV PD) is successfully fabricated based on 2D NbWO6 (NWO) nanosheets for the first time, which is synthesized by a facile solid‐state reaction, ion exchange and liquid exfoliation process. The NWO nanosheet‐based PDs exhibit excellent UV detecting performance at 1 V at 290 nm, high responsivity (378 A W−1), high external quantum efficiency (EQE, 1.6 × 104%), high spectral selectivity (R290/R400 = 8.84 × 103) and fast speed (1.05/88 ms). Furthermore, the NbWO6 nanosheets film based PD shows great potential for the application in UV image sensing. This work provides a feasible strategy for the development of new UV PDs based on other potential layered niobates.


Introduction
13][14] Over the past decades, substantial progress have been made to develop high-performance UV PDs based on 2D perovskite DOI: 10.1002/aelm.202300462[17] This progress has opened up many new possibilities for technological advances, including the use of wide-bandgap semiconductors for UV PDs. [16,17]27][28] As an all-inorganic wide-bandgap perovskites with a trirutiletype structure, HNbWO 6 (HNWO) has aroused increasing research interest owing to their unusual structure and properties. [29]Therefore, the HNWO derived materials have been proven as excellent materials for wide applications.For example, through partially replaced of tungsten in tetragonal tungsten bronzes by niobium (1:1), possible electron conduction caused by the mixed valency of W ions was suppressed; the HNWO product behaves as a very good protonic solid electrolyte. [30]The HNWO aggregated nanosheets have been also demonstrated as an effective homogeneous Lewis acid catalyst with high selectivity for the transformation of carbohydrates to furfurals in water. [31]Besides, by utilizing its properties of semiconductor, the HNWO nanosheets have been proven as excellent materials for photocatalysis, [32,33] electrocatalysis, [34,35] and water splitting. [29]Its strong chemical stability means that it is resistant to degradation when exposed to environmental factors, such as humidity and temperature, making it suitable for use in a variety of environments. [34]In addition to its chemical stability, NWO also has suitable bandgap and nontoxicity, which make it available for use in a range of optoelectronic and sensing applications. [34]Nevertheless, there is rarely report on the photodetecting application of the NWO nanosheets up to now.In this study, 2D all-inorganic perovskite NWO nanosheets were synthesized by high-temperature solid-state reaction, proton-exchange and liquid exfoliation process.The individual NWO nanosheet based UV PD shows high performance at 1 V at 290 nm light illumination, including high responsivity, high detectivity, fast speed, good spectral selectivity.Furthermore, the NWO nanosheets film PD exhibits great potential for the application in UV image sensing.Our work provides a feasible strategy for the development of new UV PDs based on other potential layered niobates.

Results and Discussion
Following the synthesis procedure described in the experiential section, the ultrathin NWO nanosheets can be obtained by a typical top-down approach based on high-temperature solid-state reaction and followed by a proton-exchange, intercalation process.The morphology of the products in each step was characterized by SEM and AFM (Figure 1).It is observed from the SEM image that the LiNbWO 6 (LNWO) parent compound after calcination displayed a lateral grain size in dozens of micrometers scale and possess a layered nature (Figure 1a).After proton-exchange process, the Li + of LNWO samples are replaced by H + in nitric acid, HNbWO 6 (HNWO) with a well-defined layered structure are obtained (Figure 1b).Finally, micrometer-sized NWO nanosheets are successfully prepared by the reaction of the acidic HNbWO 6 (HNWO) sample with alkaline Tetrabutylammonium hydroxide (TBAOH).Statistics indicates of the SEM results that the main distribution lateral of the NbWO 6 (NWO) nanosheets is up to several micrometers (Figure 1c).According to the atomic force microscopy (AFM) result (Figure 1d), the thickness of monolayer NWO nanosheets are 1.5, and 2.2 nm, consisting with our previous report. [36]he micro-structure and elemental composition of the NWO nanosheets are further characterized by TEM.As shown in Figure 2a, the ultrathin morphology of NWO nanosheets with micrometer lateral size are observed by low-resolution TEM, indicating the successful exfoliation of NWO nanosheets.The highresolution TEM (HRTEM) image in Figure 2b illustrates the interplanar spacing of 0.33 nm along the two perpendicular directions, corresponding to (100) and (010) planes of NWO, which demonstrates that the NWO nanosheets retain a high in-plane crystallinity.The obvious diffraction spots in selected area electron diffraction (SAED) pattern in Figure 2c prove the high crystallization quality of NWO nanosheets, and the marked spots are indexed as (110), (1-10), and (200) planes along the [001] zone axis, respectively, consist with previous report. [29]Furthermore, the corresponding energy-dispersive X-ray spectroscopy (EDS) mapping images in Figure 2d-g show the uniform distribution of Nb, W and O elements, respectively.
In order to investigate the crystal phase evolution during the preparation process, the X-ray powder diffraction (XRD) method is carried out for the bulk samples after calcination, proton-exchange and exfoliated NWO nanosheets.As shown in Figure 3a, XRD pattern of the synthesized LNWO sample matches well with the standard data (PDF#41-0378), with the three highest peaks at 26.9°, 34.7°, and 52.8°corresponding with (110), (103), and (213) planes.After the proton-exchange of process, the three highest peaks at 6.8°, 26.7°, and 30.8°corresponding with (002), (110), and (114) planes of the HNbWO 6 •1.5H 2 O (PDF#41-0110), suggesting a successful replacement of Li + with H + .The XRD pattern of the NWO nanosheets reveals (002) and (110) crystal planes, consisting with the structural results of the HRTEM and SAED.Furthermore, the UV-vis absorption spectrum of NWO nanosheets was conducted to investigate theirs optical property.As displayed in Figure 3b, the sharp absorption edge of NWO nanosheets is at ≈360 nm.The optical bandgap is estimated to be 3.45 eV by the Tauc plot [36][37][38][39][40][41] using the UVvis absorbance spectrum in the inset of Figure 3b.Due to their approximate absorption edge and considerable bandgap width, the NWO nanosheets based device may have strong coupling and unique photoelectric properties.
To study the photodetecting performances, the individual NWO nanosheet based photodetector was fabricated via a typi-cal photolithography, magnetron sputtering of Cr/Au electrodes, and lift-off process, and the optical image of the device is shown in Figure 4a. Figure 4b shows representative I-V curves in a semi-logarithmic plot measured with respect to the individual NWO nanosheet-based photodetector device by a two-probe approach under ambient conditions.It is observed that the device shows a low dark current (1.92 pA) and yields a drastically enhanced photocurrent (3.54 nA) at 1 V at 290 nm, more than three order-of-magnitude larger, suggests that the NWO PD responds effectively to UV light.[12][13][14] Then, the photogenerated electron-hole pairs flows to the opposite electrodes that driven by the applied bias.44] The current-time (I-t) curves of the device in Figure 4c represent the reversible time response of the NWO PD between low and high-conductance states, which show great periodicity and stability.As the 290 nm UV irradiation (0.16 mW cm −2 ) switching from off to on, the currents increase from 1, 10, 12 pA to 144, 764, 1860 pA at 0.1, 0.5, 1.0 V bias, respectively.Figure 4d displays a normalized pulse response of NWO PD at 1 V bias of the individual NWO nanosheets based device, where the rise and decay time can be estimated to be ≈1.05 and ≈88 ms, respectively.In order to evaluate the sensitivity of the NWO individual nanosheet PD, responsivity (R  ) and external quantum efficiency (EQE) are estimated from the experimental data, which can be calculated by the following equations: [45,46] where I p , I d , P  , and  are the photo current, dark current, incident power density and wavelength of the incident light, respectively, which can be collected from the measurement.While h, c, and e are the Plank constant, velocity of light and the charge of the electron.The effective illumination area S is estimated to be 0.0136 mm 2 for the device.As seen in Figure 4e,f, the maximum responsivity is up to 378 A W −1 under 290 nm illumination, and the calculated EQE and spectral selectivity (R 290 /R 400 ) are 1.6 × 10 4 % and 8.84 × 10 3 at 1 V, respectively, indicating the good sen-sitivity and selectivity of the NWO individual nanosheet based PD.As summarized in Table 1, the NWO individual nanosheet based PD shows good performance when compared with other recently reported photodetectors based on 2D perovskite.
To further investigate the performance of NWO nanosheets film based PD, the NWO nanosheets were drop coated on a quartz substrate and deposition of Cr/Au electrodes by mask (the channel length is kept at 20 μm). Figure 5a shows the I-V features of NWO nanosheets film PD under dark condition and UV light illumination with variable wavelengths.It is noteworthy that NWO nanosheets film based PD exhibits a dark current of 0.1 nA at 1 V bias, and yields the photocurrent of 30 nA under 290 nm UV illumination (0.16 mW cm −2 ). Figure 5b presents the I-t characteristics of NWO nanosheets film PD at 290 nm at 0.1 and 1.0 V, and the device shows fast photoresponse and good reproducibility.When illuminated with UV light switched (0.16 mW cm −2 ), the photocurrent of the PD quickly increases to 2.8 nA and then quickly decreases to 0.034 nA.The on/off ratio (I ph /I d ) of this device is calculated to be 82 at 1 V.
Furthermore, the ability of the NWO nanosheets film based PD for recording the image information under 290 nm light irradiation was investigated.An imaging system was constructed to investigate the UV light imaging capability of our PDs, as illustrated in Figure 5c.A hollow photomask with letter "E" graphics (2.5 × 3 cm) was mounted between the light and the PDs, which was utilized to collect the light signal.The photomask can be manually moved continuously in horizontal and vertical direction (X and Y direction), as shown in Figure 5d.The photocurrent from the PDs was recorded by a Keithley 4200 and then was analyzed by a computer.A 5 × 7 pixels' 290 mm image was obtained, via transforming the output signal current.The imaging results of NWO nanosheets film based PD are shown in Figure 5e.The image "E" was displayed in 290 nm illumination can be clearly seen, demonstrating its high potential of UV photodetecting performance.Figure 5f shows the corresponding I-t curves of NWO nanosheets film based PD when the photomask located at different pixels, illustrating that the NWO nanosheets based PD can be potentially applied in visible-blind UV imaging areas.

Experimental Section
Synthesis of NbWO 6 Nanosheets: NWO nanosheets was synthesized by a typical top-down method based on high-temperature solid-state reaction, then followed by proton-exchange, intercalation, and exfoliation of layered products.Specifically, the parent compound, LiNbWO 6 (denoted as LNWO) was prepared from the mixture of high purity Li 2 CO 3 (Aladdin, 99.99%), Nb 2 O 5 (Aladdin, 99.9%), and WO 3 (Aladdin, 99.99%), these three raw materials were mixed with the molar ratio of Li:Nb:W = 1.05:1:1 and thoroughly ground before heated at 800 °C for 24 h in air, and this calcination process was repeated for another two times.In order to compensate for the loss of Li 2 CO 3 at high temperature, 5 mol% Li 2 CO 3 was added into the white calcinated sample during the second and the third calcination processes.The exchange of Li + by H + reaction was carried out by treating of 0.5 g the parent compound LNWO with ≈50 mL of dilute nitric acid (4 mol dm −3 ) at room temperature for 3 days under stirring, and the acid aqueous solution was replaced with a fresh one every day.The proton-exchanged product HNbWO 6 (denoted as HNWO) was then washed five times with distilled water and dried at 60 °C in air.The HNWO suspensions were quickly obtained by adding 200 mg HNWO sample into an equimolar amount of TBAOH (Tetrabutylammonium hydroxide 30-hydrate) 50 mL aqueous solution.The NWO nanosheets were prepared by shaking the TBAOH-containing solution in a oscillator under room temperature for 2 weeks, the final suspension was then centrifuged and rinsed three times with distilled water.In order to rule out the influence of TBA + , (TBA)NbWO 6 nanosheets were drop-coated on various substrates and exposed to UV illumination before any characterizations and measurements.
Characterizations of the NbWO 6 Sample: The characterization of size and morphology of the exfoliated NWO nanosheets were conducted by a field-emission scanning electron microscopy (SEM, Zeiss Sigma).The atomic force microscopy (AFM) measurement was performed on a Bruker Dimension Icon atomic force microscope.Micro-structure of the NWO nanosheets were measured by transmission electron microscopy (TEM, Tecnai G2 F20/F30).The data of crystal structures of the samples were recorded by X-ray diffraction (XRD, D8 ADVANCE) using Cu K radiation ( = 1.5405Å).UV-vis spectra were recorded by measuring dispersed NWO nanosheets on quartz substrate using a UV-vis spectrophotometer (Hitachi U-3900H).
Photoelectric Measurements: The individual NWO nanosheet based PDs on Si/SiO 2 substrate were fabricated by photolithography, electron beam deposition, and lift-off process.The photolithography was performed on a Heidelberg μPG 501 direct writer system.NWO nanosheets based film PDs with centimeter scale can be easily constructed via drop coating the final suspension on a quartz substrate, while all the Cr/Au electrodes were patterned by using a shadow mask.All the photoelectric measurements of NWO-based PDs were conducted using a programcontrolled semiconductor characterization system with a four-probe station.A 450 W Xenon lamp equipped with a monochromator was used as the light source, and its power density was recorded by a NOVA II power meter (OPHIR Photonics).For pulse response characterization, a digital oscilloscope (Tektronix DPO 5140B) and a 355 nm Nd:YAG pulsed laser system with a pulse duration of 3-5 ns were used.

Figure 1 .
Figure 1.Morphology of the products in each step.a-c) SEM images of LNWO sample after calcination.b) HNWO sample after proton-exchanged SEM image.c,d) SEM and AFM images of NWO nanosheets after exfoliation and insert in (d) presents the height profile.

Figure 2 .
Figure 2. The micro-structure and elemental composition of the NWO nanosheets.a) TEM image of NWO nanosheets after exfoliation.b,c) HRTEM image and SAED pattern of the NWO nanosheets along the [001] zone axis.d-g) Corresponding elemental composition of the NWO nanosheets.

Figure 3 .
Figure 3. Structure and optical properties of the products in each step.a) XRD patterns of LiNbWO 6 , HNbWO 6 and NbWO 6 nanosheets, respectively.b) UV-vis absorption spectrum of NbWO 6 nanosheets film, inset is the Tauc plot from its absorbance spectrum.

Figure 4 .
Figure 4. Photodetecting performance of the NWO individual nanosheet PD. a) Optical microscope photograph of the NWO nanosheets based device.b) I-V curves of the individual NWO nanosheets based device.c) I-t curves of the NWO device at different bias voltages, respectively.d) The normalized pulse response of NWO PD at 1 V bias.The enlarged part pulse response of the rising edge is shown in the inset.e,f) The spectral responsivity and EQE of the device at 1 V bias.
In summary, 2D NWO nanosheets based UV PDs were fabricated for the first time.The individual NWO nanosheet-based PD ex-hibit excellent UV detecting performance at 1 V at 290 nm, high responsivity (378 A W −1 ), high EQE (1.6 × 10 4 %), high spectral selectivity (R 290 /R 400 = 8.84 × 10 3 ) and fast speed (1.05/88 ms).Furthermore, the NWO nanosheets film based PD shows great potential for the application in UV image sensing.This work provides a feasible strategy for searching new UV PDs based on numerous layered niobates and for many special application.Our future work will focus on the material and device structure optimization, such as fabricate the devices with different layers to study the thickness-dependent properties of 2D NWO materials, and measure the nanosheet-based back-gate field-effect transistors to get insightful understanding and further controlling the carrier transport properties, and fabricate the Au-NWO-Ti based devices with Schottky junction to study its rectification characteristics and self-powered performance.

Figure 5 .
Figure 5. Photodetecting performance and UV imaging application of the NWO nanosheets film PD.a) I-V at dark and different light wavelengths and b) I-t curves of the NWO nanosheets film based device NWO device at 0.1 and 1.0 V bias voltages, respectively.c) Schematic illustration of the imaging system using the NWO nanosheets film based photodetector as the sensing pixel.d) The moving X and Y direction of the object with letters "E" on photomask.e) The image of NWO nanosheets film based PD under 290 nm light irradiation.f) Corresponding I-t curves of NWO nanosheets film based PD when the photomask moving located at different positions.

Table 1 .
Comparison of the characteristic parameters of recently reported photodetectors based on 2D perovskite.