Introduction

Photodetectors (PDs), one of the most important types of optoelectronic devices, can covert incident light into an electrical signal1. They have received considerable research interest, as this technology is essential for a variety of industrial and scientific applications and has a large impact on our daily lives. These applications include imaging, optical telecommunication, remote control technologies, day- and night-time surveillance and environmental monitoring2,3,4. In comparison to narrow-band PDs, broadband PDs can meet the demands of ultraviolet–visible–infrared light communication, wide spectral switches or memory storage by using only a single device. To date, significant efforts have been focused on the development of broadband PDs based on quantum dots5, inorganic nanomembranes6 and organic perovskite7, but these developed devices are still limited by a slow response speed and relatively short detection range. To date, a challenge still remains for simultaneously achieving a faster response speed and broader detection range by exploring new materials. For example, the traditional silicon has been a common and excellent material of choice for commercial PDs owing to its low cost and well-developed fabrication processes for large-area detectors8. Compared with their thin film counterparts, vertically standing silicon nanowire array (SiNWA) structures can strongly suppress light reflection, increase the interfacial area, facilitate fast charge transport and enhance charge collection by shortening the travel paths of minority carriers9,10. However, the spectral response of Si-based PDs is limited by the natural bandgap (~1.12 eV), corresponding to a range of 400–1100 nm6,11. To develop Si-based PDs with a broadband response, heterostructures with various designs have been suggested, including a heterojunction between Si and an organic semiconductor for extending photodetection to the ultraviolet (UV) region12, and heterogeneous integration of Si with Ge to provide sensitivity to the infrared (IR) region13. However, the instability and poor performance of these heterostructures limit their practical applications.

Recently, the increasingly popular two-dimensional (2D) materials have become of great interest for application in future micro- and nano-optoelectronics owing to their outstanding electronic, optical and mechanical properties14,15,16,17. In particular, the newly discovered PtSe2, a promising member of the Group-10 transition metal dichalcogenides (TMDs), has been theoretically predicted to have a high carrier mobility and tunable bandgap ranging from 1.2 eV (monolayer) to 0.21 eV (bilayer) at room temperature. However, the multilayer 1T-phase PtSe2 exhibits a broad light absorption spectrum because of its semi-metallic character18,19. The current approaches for fabricating PtSe2 layers are limited to mechanical exfoliation19 and chemical vapor deposition20, which usually yield PtSe2 nanosheets with uncontrollable sizes. Another approach to obtaining single-crystalline PtSe2 and a large film of layered PtSe2 is molecular beam epitaxy, but this method suffers from a high cost and slow growth rate18,21. Recently, scalable and horizontal 2D layered PtSe2 films prepared by either an aqueous-phase reaction22 or thermally assisted conversion23,24 are considered as prerequisites for electronic and optoelectronic device applications. For example, Yim et al.24 reported that vertically stacked heterostructures were successfully realized by directly transferring PtSe2 films onto a Si substrate, resulting in enhanced photoelectrical properties such as rectifying and photovoltaic behaviors. However, comprehensive investigations of PtSe2-based photodetectors and their key parameters, including the spectral selectivity, response speed and specific detectivity, have not yet been conducted.

Here, wafer-scale, high-quality and continuous 2D PtSe2 films with vertically standing layered structures were successfully synthesized via selenization of sputtered Pt films, which paves the way for obtaining high-growth-rate wafer-scale films with precise thickness control. The obtained vertical PtSe2 films were then transferred to an n-type SiNWA to form broadband photodetectors. The light absorption of the PtSe2 film was enhanced by an increase in the film thickness, whereas the vertically standing layered structure greatly facilitated the transport of photogenerated carriers along the in-plane direction to the top electrode25. Significantly, the PDs revealed a multiband detection range (200–1550 nm) and fast response time (10.1/19.5 μs). Our study demonstrated the feasibility of utilizing these proposed materials for future high-speed and broadband optoelectronic devices.

Materials and methods

Preparation and characterization of SiNWA and PtSe2 films

Vertical SiNWA were prepared on n-Si wafers (resistivity: 1–10 Ω cm) by using a Ag-assisted chemical etching approach26. The as-etched SiNWA were dipped in a dilute solution of HNO3 and HF to dissolve the Ag and Si oxide on the surface. The SiNWA were then cleaned in deionized water. The PtSe2 film with a vertically standing layered structure was prepared by magnetron sputtering followed by direct selenization (see Supplementary Fig. S1a, b). In brief, a Pt film was deposited on SiO2/Si using a magnetron sputtering system. The deposition process was carried out at a power of 20 W and a base pressure below 10–7 Pa. After magnetron sputtering, the as-prepared Pt film was placed in the center zone of a growth furnace, and elemental selenium (Se) powder was placed at the upstream side. The Se powder was evaporated at 220 °C, and Ar with a flow rate of 50 SCCM (standard cubic cm per min) was used to transport the vaporized Se to the Pt sample. The center temperature of the tube furnace was set to 450 °C and kept stable for 1 h, which was followed by natural cooling to room temperature to complete the annealing process.

Device fabrication and characterization

Poly (methyl methacrylate) (PMMA) as a supporting polymer was spin-coated onto the synthesized PtSe2 films. The SiO2 layer under the PtSe2 was removed by a wet-etching process using a 4 M NaOH solution (Supplementary Fig. S1c). To construct the PtSe2/SiNWA heterojunction photodetector, a Au (50 nm) top electrode and a Ag (50 nm) bottom electrode were deposited onto the front and back sides of a SiO2/Si substrate using magnetron sputtering. After being cleaned in deionized water, the PMMA-supported PtSe2 films were directly transferred onto the top of the SiNWA. After drying at 100 °C for 10 min, the residual PMMA on the PtSe2 film was removed by acetone (Supplementary Fig. S1d). The absorption spectra of the PtSe2 film on quartz glass, the SiNWA and the PtSe2/SiNWA heterojunction were measured by a UV–visible–near-infrared (NIR) spectrophotometer (UH4150, Hitachi, Japan). Raman spectra measurements were carried out on a HORIBA Raman spectrometer with a 488 nm argon ion laser. The X-ray diffraction (XRD) pattern was recorded using a RigakuSmartLab X-ray diffractometer. The Pt 4f and Se 3d core levels were recorded by X-ray photoemission spectroscopy (XPS), which was performed on a VG ESCALAB 220i-XL analysis system equipped with a monochromatic Al X-ray (1486.6 eV) source. The morphology of the as-prepared SiNWA sample was observed by scanning electron microscopy (SEM, JEOL Model JSM-6490). The morphology, crystal structure and chemical composition of PtSe2 were investigated using a field emission transmission electron microscope (TEM, JEOL Model JEM-2100F) equipped with an energy-dispersive spectrometer (EDS). The optoelectronic characterization of the devices was performed using a semiconductor parameter analyzer system (Keithley 4200-SCS) at room temperature, and a laser diode (780 nm) was used as the illumination source. To determine the spectral response and time response of the heterojunction devices, a home-built system composed of a light source (LE-SP-LS-XE), a monochromator (LE-SP-M300), an oscilloscope (Tektronix, TDS2012B) and an optical chopper (LE-oc120) was used.

Results and discussion

The crystal structure of PtSe2 is similar to that of MoS2 and is composed of one layer of Pt atoms sandwiched between two layers of Se atoms, as determined from the XRD pattern shown in Fig. 1a. All diffraction peaks at 16.78°, 32.87°, 44.32° and 56.25° can be indexed to the (001), (101), (102) and (200) planes, respectively. XPS characterization was also conducted on the PtSe2 films. The peaks of the 4f7/2 and 4f5/2 core levels of Pt are located at 73.35 and 76.65 eV, respectively, and the 3d5/2 and 3d3/2 core levels of Se are observed at 54.65 and 55.45 eV (Fig. 1b, c)18. The typical Raman spectrum of the PtSe2 film excited by 488 nm light shown in  Fig. 1d contains two main peaks at approximately 175.7 and 204.5 cm−1, which are assigned to the E2g in-plane and A1g out-of-plane vibration modes, respectively; these results indicate that PtSe2 has a similar crystal structure as other MX2 compounds based on comparison with the Raman bands of other MX2 species27. The coverage of the film grown by simple selenization is scalable and controllable even for 2D materials according to previous reports25. As shown in  Fig. 1e, the SiO2/Si substrate with a size of approximately 12 × 12 mm was covered uniformly by the PtSe2 film. To measure the uniformity of the film, seven spots randomly distributed on the film were analyzed by Raman spectroscopy, as shown in Fig. 1f. All spots showed the same two characteristic peaks of PtSe2 with similar profiles and intensities, revealing the homogeneous growth of PtSe2 on the wafer with a size of more than a centimeter.

Fig. 1: Characterization of the PtSe2 films.
figure 1

a XRD pattern of the as-grown PtSe2 film. XPS spectrum of b the Pt 4f core level and c the Se 3d core level. d Raman spectrum of the PtSe2 thin film. e Photograph of a 12 × 12 mm SiO2/Si substrate covered with the PtSe2 film. f Raman spectra of seven random spots on the film surface

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Fig. 2: Device structure characterization.
figure 2

a Schematic diagram of the device structure. b SEM images of as-prepared SiNWA array, where the inset shows the corresponding plane view of the nanoarray. c Height profile of the PtSe2 film; the inset shows the corresponding AFM image. d, e TEM images of the PtSe2 film under different magnifications and cross-sectional TEM image of the PtSe2 film (inset of (d)). f High-resolution TEM image of PtSe2, where the distance between two layers is approximately 0.52 nm

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Figure 2a illustrates the device structure of the PtSe2/SiNWA heterojunction photodetector. The SEM image of the SiNWA revealed that the well-aligned SiNWA was approximately 8–10 μm long with a high density and uniformity (Fig. 2b). The atomic force microscopy (AFM) profile across the edge of the PtSe2 nanofilm is shown in Fig. 2c. The thickness of the PtSe2 nanofilm was calculated to be ~37.2 nm, which is fairly consistent with the height acquired from the cross-sectional TEM image of the ordered PtSe2 film (inset of  Fig. 2d). The TEM images with different magnifications shown in Fig. 2d, e reveal that the films are composed of highly dense and vertically grown 2D PtSe2 layers with an intermolecular spacing of ~0.52 nm (right inset of  Fig. 2f), corresponding to the (001) plane of PtSe2. This unique structure of PtSe2 is probably due to the formation of thick precursor Pt films with a high density, leading to the formation of PtSe2 with a vertical alignment during the selenization process28. To further identify the chemical composition of the PtSe2 samples, EDS mapping was conducted, as shown in Supplementary Fig. S2a-c, and confirmed that platinum (Pt) and selenium (Se) were homogeneously distributed over the prepared sample.

Figure 3a depicts the current vs. voltage (I–V) characteristics of the PtSe2/SiNWA PD in the dark and under light illumination (780 nm, 39.3 mW/cm2). According to the I–V curve in the dark, typical rectifying characteristic was observed with a rectification ratio approaching 104 at ±5 V, suggesting that a heterojunction was formed between PtSe2 and the SiNWA because the Au and Ag films can provide good Ohmic contact with PtSe2 and the SiNWA, respectively (see Supplementary Fig. S3). By using the thermionic emission theory-based diode equation (Supporting information), the barrier height of the heterojunction was calculated to be 0.82 eV. Interestingly, the zero bias barrier (Φb) of PtSe2/SiNWA, similar to that of the monolayer graphene/Ge Schottky junction29, dramatically decreased under light illumination, giving rise to an enormous photocurrent under reverse bias, which is consistent with the previous report that light illumination can substantially lower the potential barriers of heterojunctions30. The photodetector with the thin PtSe2 film, which was selenized from a 1 nm Pt film, consisted of horizontally aligned nanosheets, as revealed by the TEM images shown in Supplementary Fig. S4a-d. The corresponding thickness of the PtSe2 film was determined to be ~4.3 nm by AFM measurements (Supplementary Fig. S4e). This photodetector also showed similar photoresponse characteristics in the reverse bias region, but its photocurrent was much smaller than that of the thicker PtSe2/SiNWA heterojunction with a vertically aligned layered structure, as shown in Supplementary Fig. S4f. Therefore, we can conclude that the device with a thicker PtSe2 film generated a higher photocurrent, indicating that a substantial amount of carriers was generated in the PtSe2 film24. By further investigating the response properties at different voltages, we found that the ON/OFF ratio could be adjusted by the reverse bias. As shown in Fig. 3b, the dark current showed a more significant increase than the photocurrent from 0 to −5 V, and the ON/OFF ratio decreased by two orders of magnitude, reaching the maximum value (4 × 104) at zero bias. The linear dynamic range (LDR) reached 92 dB under 39.3 mW/cm2 light illumination at zero bias according to the equation LDR = 20 log (Ip/Id); this value is superior to the LDRs of other similar PDs, such as WS2/n-Si (42 dB)31, InGaAs (66 dB)32, GaSe/GaSb (77.51 dB)4, GaS/PET (78.73 dB)33 and graphene/n-Si (90 dB)34. To fully understand the excellent performance of the PtSe2/SiNWA heterojunction, the energy-band diagrams of the heterojunction under zero and reverse bias are shown in  Fig. 3c, d. The energy levels of the PtSe2 film were analyzed by ultraviolet photoelectron spectroscopy characterization (see Supplementary Fig. S5), revealing that the Fermi level of the PtSe2 film is located at 5.1 eV35. The multilayer 1T-phase PtSe2 film (thickness ≈37.2 nm) could be considered a semimetal19,36. Therefore, to achieve an equilibrium state in the dark, electrons would diffuse from the n-SiNWA to PtSe2 due to the difference in Fermi levels between the two materials, which leads to band bending at the n-SiNWA side because of the semi-metallic nature of PtSe2. As a result, a strong built-in electric field is formed in the depletion region, which efficiently separates the photogenerated electron–hole pairs under light illumination, resulting in a sizeable photocurrent. Under reverse bias, an external electric field oriented towards the built-in field increases the separation efficiency of the photogenerated electron–hole pairs. In addition, the reverse bias can also extend the depletion region, allowing more photocarriers to participate in the generation of photocurrent37.

Fig. 3: Photoresponse characterisitics and working mechanism of photodetector.
figure 3

a I–V curves of the PtSe2/SiNWA heterojunction in the dark and under light illumination (780  nm, 39.3  mW/cm2). b Time-dependent photocurrent under different bias voltages. Energy-band diagrams of the PtSe2/n-SiNWA heterojunction under zero bias (c) and reverse bias (d) under illumination

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Quantification of the photodetector response to various light intensities is an important experiment for determining the photodetection performance of a heterojunction38, so we measured the current vs. bias voltage (logI–V) characteristics of the photodetector with respect to different light intensities ranging from the dark to 39.3 mW/cm2. Clearly, the photocurrent increased with increasing light intensity under reverse bias, resulting in reduced rectifying ratios (Fig. 4a). The reason for the high sensitivity under reverse bias is that photogenerated carriers greatly change the concentration of minority carriers dominating the photocurrent under reverse bias39. In addition, the time-dependent photocurrent of the photodetector at −5 V was investigated under light illumination with different power densities of 39.3, 31.3, 20.2, 11.1 and 1.38 mW/cm2, as shown in  Fig. 4b. The photocurrent increased from 1.29 mA/cm2 to 31.6 mA/cm2 with the light intensity was tuned from 1.38 to 39.3 mW/cm2 (illuminated area, S = 0.0025 cm2). To evaluate the performance of the PtSe2/SiNWA heterojunction, the responsivity (R) and specific detectivity (D*), which are two key parameters, were measured according to the following equations:

$$R = \frac{{I_p - I_d}}{{P_{opt}}},$$
(1)
$$D^ \ast = \frac{{A^{1/2}R}}{{\left( {2eI_d} \right)^{1/2}}},$$
(2)

where Ip, Id, Popt, A and e are the photocurrent, dark current, incident light power, effective device area (A = 0.25 cm2) and absolute value of electron charge (1.6 × 10−19 C). We obtained the largest R and D* values of 12.65 A/W and 2.5 × 1013 Jones (Jones = cm Hz1/2/W), respectively, under a weak light intensity of 0.008 mW/cm2 at −5 V. These values are comparable to the values reported for WS2, MoSe2 and MoS2-based PDs37,40,41,42. Moreover, in Fig. 4c, it is noted that these two metrics decreased with increasing light intensity owing to the increased recombination rate of the photoexcited carriers43. Next, the photovoltaic characteristics of the PtSe2/SiNWA heterostructure-based detector were further examined in the dark and under 780 nm light illumination in the light intensity range of 0.75–39.6 mW/cm2, as shown in  Fig. 4d. It is noted that the device exhibits pronounced photovoltaic behavior under light illumination at a zero external bias voltage, indicating that the PtSe2/SiNWA device can function as a self-driven photodetector. With the increase in the power density, the photocurrent increased substantially, as shown in Fig. 4e. The fitting curve of the power law of I = APθ yielded θ = 0.95, which is close to one at zero bias, indicating the presence of low trap states in the PtSe2/SiNWA PD44.

Fig. 4: Optoelectronic characteristics of photodetector.
figure 4

a I–V curves of the PtSe2/SiNWA PD under various light intensities. b Photoresponse of the PtSe2/SiNWA PD under various power intensities. c Plots of the responsivity and specific detectivity as a function of the power intensity. d Photovoltaic characteristics of the PtSe2/SiNWA device under different power intensities. e Zero bias photocurrent under different incident intensities and the fitting relationship between the photocurrent and the light intensity

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To assess the broad and sensitive photoresponse performance of the PtSe2/SiNWA photodetector, the spectral response was investigated, as shown in  Fig. 5a, which revealed a wide spectral response in the range of 200–1400 nm with the strongest response observed at approximately 800 nm for the PtSe2/SiNWA. As Si-based photodetectors are more sensitive in the range of 300–1100 nm34, the broad spectral response to light with wavelengths shorter than 300 nm and longer than 1100 nm was due to the strong light absorption of the PtSe2 thin film on the SiNWA according to  Fig. 5b. Moreover, the representative response characteristics of the PtSe2/SiNWA PD to deep UV (250 nm) and NIR (1310 and 1550 nm) light were also investigated. Obviously, the device showed a pronounced photoresponse at all these wavelengths. Even though the photoresponse at 250 nm (13.8 μW/cm2) was not as strong as that at 1310 (36 μW/cm2) and 1550 nm (2.6 mW/cm2) due to the lower power density, the photodetector still exhibited an excellent switching stability and reproducibility with an obvious photoresponse, as shown in  Fig. 5c. More importantly,  Fig. 5d, e shows the remarkable photoresponse of the photodetector at optical telecommunication wavelengths of 1310 nm (O band) and 1550 nm (C band), respectively, whose photocurrents are smaller than that obtained at 780 nm, which is probably caused by the low photon excitation energy and relatively weak light absorption at long wavelengths (Fig. 5b). However, these response properties have already surpassed those of the conventional Si-based PD, which is insensitive to 1310 and 1550 nm NIR light45. The expensive InGaAs-based photodetectors are used in commercially available products for 1550 nm practical detection applications46. The high performance of the PtSe2/SiNWA PD with a strong NIR response indicates its good potential for use in optical communication and fiber optic cable testing47.

Fig. 5: Spectral selectivity and temporal response.
figure 5

a Wavelength-dependent photoresponse of the PtSe2/SiNWA PD measured in the spectral range from 200 to 1400  nm. b UV–VIS–NIR absorption spectra of PtSe2, the SiNWA and the PtSe2 film on the SiNWA. Photoresponse of the photodetector under 250 nm (c), 1310 nm (d) and 1550 nm (e) light illumination

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Photodetectors are an essential component of some important applications, e.g., optical telecommunication and imaging, for which the response speed of the detector determines the information capacity and frame rate48,49,50. To study the response speed of the fabricated device, an optical signal with varied pulsed frequencies was used, as illustrated in Fig. 6a. As indicted by the schematic diagram of the measurement setup, the temporal photoresponse signal was recorded by a digital oscilloscope as 780 nm light was pulsed at several different frequencies from 1 Hz to 120  kHz by a function generator. Fig. 6b–d show some representative results of the photocurrent produced by the fabricated photodetector under light illumination with frequencies of 1, 10 and 40 kHz. On the other hand, the relative balance (Imax−Imin)/Imax of the photocurrent as function of the frequency is depicted in  Fig. 6e. It is shown that the relative balance (Imax−Imin)/Imax of the photocurrent did not decrease significantly at frequencies as high as 104 Hz, indicating that PtSe2/n-SiNWA-based PDs can actually operate at much higher frequencies. Significantly, a 3 dB bandwidth roll-off point occurs at 40 kHz, which was much larger than the small bandwidth of only 50 Hz for the SnS2 nanosheet/PbS CQD hybrid photodetector5. In the time domain, the response speed is defined as the total time required for the output to rise from 10 to 90 % of the pulse peak and fall from 90 to 10%. Therefore, by magnifying the shape of the response pulse at 10 kHz, the rise time τr and fall time τf were measured to be 10.1 and 19.5 μs, respectively (see  Fig. 6f). It is worth pointing out that this achieved response speed is much faster than that of other 2D TMD material-based photodetectors50,51,52. We believe that such a fast response speed can be attributed to the strong built-in electric field formed by the PtSe2-SiNWA Schottky junction and the distinct vertically standing layered structure of PtSe239. Furthermore, the long-term stability of a photodetector remains an important concern for the device reliability, so we investigated the air stability of the PtSe2/SiNWA device by repeating the performance test after storing the device under ambient conditions for a month without any protection. The photodetector showed high durability and almost no change when the device was measured again at 40 kHz (3 dB), and the ultrafast rise/fall time (τr/τf) of the device was up to 4.1/9.4 μs (see Supplementary Fig. S6), which indicates the high stability of the device under ambient conditions19.

Fig. 6: Response speed of photodetector.
figure 6

a Schematic illustration of the setup for measuring the photoresponse speed of the device. Photoresponse characteristics of the PtSe2/SiNWA heterojunction to pulsed light irradiation at frequencies of b 1 kHz, c 10 kHz and d 40 kHz. e Relative balance (Imax−Imin)/Imax vs. switching frequency. f Rising and falling edges for estimating the rise time (τr) and the fall time (τf) of the photodetector

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The main parameters of the PtSe2/SiNWA PD obtained under 780 nm light illumination are shown in Table 1. Notably, the general performance of the present device is much better than that of other 2D material-based photodetectors previously reported, achieving an optimal balance among all the important features. We believe that the excellent device performance can be ascribed to the following factors. (1) The PtSe2 film with a fully vertically standing layered structure greatly facilitated the transport of photoexcited carriers25,39. (2) The aligned SiNWA structure strongly suppresses light reflection and enhances light harvesting, which can promote the electron transport efficiency at the interface between the PtSe2 film and the SiNWA53. (3) The high-quality heterojunction along with the strong built-in electric field in the depletion layer are beneficial for prolonging the carrier lifetime and shortening the transit time of the photon-generated carriers in the device54.

Table 1 Performance comparison of our PtSe2/n-SiNWA heterostructure photodetector with that of other 2D material-based photodetectors
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Conclusion

In summary, a simple selenization approach was developed for the large-area preparation of a high-quality 2D PtSe2 film with a vertically standing layered structure. The as-grown PtSe2 film that was integrated with a SiNWA as a high-performance photodetector exhibited a high responsivity of 12.65 A/W at −5 V. Additionally, other key figure-of-merit parameters of the photodetector, such as the detectivity (2.5 × 1013), rise/fall time (10.1/19.5 μs) and linear dynamic range (92 dB), were all comparable to or better than those of other TMD-based photodetectors. The device showed high air stability over 1 month. Moreover, the PtSe2/SiNWA PD with a broadband photoresponse in the wavelength range from 200 to 1550 nm should have promise for a wide range of applications, including imaging, optical communications and fiber optic cable testing. The large-scale fabrication of PtSe2 thin films could open a myriad of opportunities for fabricating various high-performance integrated optoelectronic devices.