Abstract
Infrared polarization-sensitive photodetectors have attracted considerable interest for night vision, remote sensing and imaging applications. Traditional bulk infrared photodetectors suffer from integration challenges and high-power consumption induced by the cryogenic cooling requirement. Here, we demonstrate a tunneling-dominant triple-junction broadband polarization-sensitive photodetector based on a van der Waals heterostructure, operating from the near-infrared (NIR) to the long-wave infrared (LWIR) band. The device exhibits low noise current, low power consumption and high detectivity. Benefiting from the photogating-assisted tunneling, it reaches a responsivity of ~ 8 × 104 A/W and a response speed of 590 ns under NIR illumination. Apparent blackbody response and high photoresponse up to 10.6 μm is achieved with a room temperature responsivity and detectivity of 0.47 A/W and over 109 Jones. Remarkably, bias-tunable polarization detection capability and high polarization ratios are observed from NIR to LWIR, which further boost target detection and imaging capabilities. Our results offer a promising approach for multidimensional imaging applications and device miniaturization.
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Introduction
Infrared photodetectors with polarization imaging capability possess diversified application prospects in night vision, environmental monitoring, astronomy, and military surveillance, etc.1,2,3,4,5,6. The state-of-the-art infrared photodetectors are dominated by III−V compounds or bulky narrow-bandgap semiconductors, including In1−xGaxAs (1–3 µm)7,8, InSb (3–6 µm)9, and Hg1−xCdxTe (6–-15 µm)10 that operates in different wavelength bands. However, they suffer from poor compatibility with the current complementary metal-oxide-semiconductor (CMOS) technology, high power consumption caused by cryogenic cooling requirements, and complicated fabrication processes11. Besides, the polarization capability is typically implemented by adding additional polarization optics components such as polarizers and waveplates, which severely restricts their expansion of application fields12.
Two-dimensional (2D) materials and their van der Waals heterogeneous integration have become an attractive way to solve the above problem, owing to their exotic optoelectronic properties13,14,15, intrinsic in-plane anisotropic crystal structure16,17 and easy integrability with silicon-based techonology18. The infrared light-absorbing 2D materials, such as black phosphorus (BP)19,20 and b-AsP2, have been intensively investigated for polarized photodetection, which is manifested by their decent polarization ratio. Nowadays, the reported polarization ratio can reach existing application level by introducing specific plasmonic structures or building heterostructures, respectively21,22,23,24. However, their inherent poor environmental stability is still a tremendous challenge for practical applications25,26. As alternative candidate, topological semimetals with nontrivial electronic band structures27, such as Td-MoTe2, TaIrTe4, and PdSe2 exhibits high air stability, distinct broadband detection capabilities and anisotropic absorption, emerging as attractive polarized infrared photodetection platform28,29,30,31. Among them, PdSe2, an air-stable Group-10 noble transition metal dichalcogenide, has a thickness-dependent indirect bandgap with widely tunable bandgap (decreases from 1.3 eV for the monolayer to 0.03 eV for bulk) and possesses ultra-high theoretical room temperature carrier mobility (more than 1000 cm2/V s)32,33. This good feature broadens the detection range from the visible to the LWIR region34. Meanwhile, PdSe2 crystallizes in an orthorhombic structure (a = 5.74 Å, b = 5.86 Å, and c = 7.69 Å) with a unique puckered pentagonal structure within the layer and an interlayer distance of 0.4 nm (Fig. 1a)35. The low-symmetry pentagonal unit ring is formed with PdSe4 tetragonal units connected by Se atoms along the in-plane direction, which endows it with fascinating optical and electrical properties such as high in-plane anisotropy30,36. Nevertheless, the high carrier density in these semimetal materials induces large dark current, high power consumption, and poor responsivity of only several mA/W13,37,38. Besides, their low intrinsic anisotropy and lack of an effective modulation of polarization sensitivity result in low polarization ratios in devices with typical values of around 2 at infrared band. All the above bottlenecks severely impede their potential applications39,40. Thus far, it is still on demand to explore promising platforms with feasible optical tunability, strong anisotropy and good stability to achieve previously unrevealed low power consumption, high polarization ratio and high responsivity uncooled infrared photodetector.
a Schematic of the crystal structures of PdSe2 and MoSe2. b Schematic of a MoSe2/PdSe2 heterostructure device. D, S and G represent the drain, source and gate electrodes of the device, respectively. Vds and Vgs are drain–source voltage and gate voltage, respectively. c Ids−Vds curves under 785 nm illumination with different light power density. Vds and Ids are drain–source voltage and drain–source current, respectively. d R and D* as a function of light power density at Vds = 0 V under 785 nm. R and D* are responsivity and detectivity, respectively. Inset: Iph as a function of incident light power. The dots represent the experiment data, and the solid line is fitting of the data to a power function in the inset. α is an exponent of the power function. e Comparison of responsivity and response time of the MoSe2/PdSe2 device at zero bias with previously reported self-powered photodetectors based on 2D semiconductors (green ellipsoid). Details of quoted references should refer to Supplementary Table 1 in the Supplementary Information. f External quantum efficiency (EQE) as a function of incident light power at different wavelengths from 785 to 2200 nm under different Vds.
In this work, we proposed a tunneling-dominant triple-junction infrared photodetector based on MoSe2 and PdSe2 heterostructure with type-I band alignment, which can fulfill all the above requirements on low-power consumption and high polarization detection capability. The heterostructure features a low noise current on the order of pico-ampere (pA) and a notable rectification ratio of above 103 without the need of a gate voltage. Moreover, it displays favorable broadband self-powered performance and possesses pronounced bias-dependent photoresponse with a responsivity over 104 A/W and a fast response speed of hundreds of ns under positive bias at NIR band, which is better than the most existing 2D materials and their heterostructure photodetectors. Remarkably, an ultra-broadband photoresponse from NIR to LWIR was achieved at room temperature, with a decent specific detectivity of 1.53 × 109 Jones at the wavelength of 10.6 μm. Furthermore, owing to the unique puckered pentagonal atomic structure of PdSe2, we also observed bias-tunable polarization sensitivity of the device with a high polarization ratio at NIR and MIR band, which endows the device broadband high-resolution polarization imaging capabilities. Our findings may pave the way for the future development of miniaturized, multifunctional and high-performance uncooled infrared photodetectors, where application fields might involve imaging, miniaturized spectrometers and polarization information recognition.
Results
Characterization of the MoSe2/PdSe2 Heterostructure
Figure 1b shows the schematic diagram of the MoSe2/PdSe2 heterostructure photodetector. The layered MoSe2 and PdSe2 nanosheets were mechanically exfoliated from their bulk materials, and the metallic electrodes were composed of titanium/gold (10/80 nm). As a non-contact characterization method, angle-resolved polarized Raman spectroscopy was used to determine the crystallographic direction of exfoliated PdSe2 (Supplementary Note 1)41. Further combined with optical microscopy images, it is shown that the direction of the long edge of the preferential exfoliated PdSe2 corresponds to the a-axis of the material. Supplementary Fig. 2a illustrates the optical microscope image of a fabricated heterostructure device, in which the MoSe2 flake was transferred selectively on top of the PdSe2 flake on 300 nm SiO2/Si substrate. MoSe2 features sizeable bandgaps (1.3–1.6 eV), good ambient stability and high optical absorption42,43,44. Thereby, the construction of the heterojunction can greatly suppress the inherently large noise current of the narrow bandgap PdSe2. The thicknesses of multilayer PdSe2 and MoSe2 flakes were recorded by the atomic force microscopy (AFM) (Supplementary Fig. 2b and c). The high quality of materials and the efficient charge transfer at the MoSe2/PdSe2 interface were well demonstrated by optical (e.g., PL mapping and Raman spectroscopy in Supplementary Note 2) and high-resolution transmission electron microscope (HRTEM) measurements (Supplementary Fig. 3).
Figure 1c plots the Ids−Vds characteristics under 785 nm light irradiation with varying the power density. The electrical output characteristics of the heterostructure device in dark at room temperature exhibit typical rectification characteristics (>103) with an ideality factor of ~1 at a small bias voltage range (Vds = ±1 V, Vgs = 0 V), demonstrating its high-quality junction interface. The device shows obvious photovoltaic behaviors with the open-circuit voltage (Voc) and the short-circuit current (Isc) increase synergistically as the light power. As a vital performance indicator for evaluating photovoltaic performance, the fill factor (FF) value is 0.41 under the power density of 0.38 mW/cm2 (Supplementary Note 3), showing that our device maintains desirable photovoltaic performance. By fitting the power-dependent photocurrents with a power-law relationship, Iph ∝ Pα, the short-circuit current exhibits a sublinear (α = 0.86) dependence on the power of incident light (inset of Fig. 1d), indicating the presence of trap states and high optical gain45,46. High optical gain leads to high photoresponse and potential weak light signals detection capabilities. Accordingly, the responsivities (R) and detectivities (D*) as a function of light power are acquired at zero bias voltage, as illustrated in Fig. 1d. Both of them are decreased with promoting light power, due to the shortened photoinduced carrier lifetimes by the saturation of trap states under higher light intensities47,48. The maximum values of R of the device is 10.1 A/W obtained at a light intensity of 0.001 mW/cm2 at 785 nm (Vds = 0 V), and the significantly high R is repeatable for our self-powered MoSe2/PdSe2 photodetectors (Supplementary Note 4), which is better than commercial Si photodetectors (≤1 A/W)49. And the D* of the device is as high as 0.6 × 1011 Jones according to the noise spectral densities (Sn) of the device (Supplementary Note 5), demonstrating the weak light detection capability of our device at NIR band.
Additionally, notable power-dependent responsivity and photovoltaic characteristics (significant Voc and Isc) are achieved in a wide spectral range (Supplementary Note 3), manifesting its favorable broadband self-powered photoresponse with low power consumption. The response speed of the device is evaluated by extracting the high-resolution photocurrent signals measured through a digital oscilloscope. The self-powered photodetector exhibits a fast rising/decay time of 256/278 µs under 785 nm illumination (Supplementary Fig. 6). The significant detection properties including fast response and high responsivity, exhibits prominent advantages compared with other 2D materials-based self-powered photodetectors (Fig. 1e and Supplementary Table 1), and the device possesses desirable self-powered imaging capability (Supplementary Fig. 7), which highlights its application potential in energy-efficiency optoelectronic applications. To further evaluate the performance of the device over a wide spectrum, we measured the EQE as a function of optical power for different wavelengths at various Vds as depicted in Fig. 1f. The device exhibits bias-dependent photoelectric conversion efficiency in the infrared spectral range. The external quantum efficiency (EQE) decreases with promoting incident optical power and achieves a maximum value of 1.6 × 103% at 785 nm and Vds = 0 V under the light intensity of 0.001 mW/cm2, which far exceeds 100%50,51. We interpret this result as the effect of interfacial electron trapping at the heterojunction barrier. This charge confinement establishes a dynamic equilibrium governed by charge conservation, wherein holes undergo multiple drift cycles driven by the built-in field, ultimately leading to the optical gain (Supplementary Fig. 5). It should be noted that a superior EQE value of 3509% is obtained at Vds = 1 V (1064 nm), which is nearly three orders of magnitude improvement in performance compared to that at Vds = 0 V. The high photoresponse gain further confirms the existence of the photogating effect caused by the trap states and provides effective detection of light with lower photon energies (i.e., longer wavelengths)52. And the bias-dependent photoresponse of the device may lead to potential applications in different circumstances, for instance bias-tunable polarization photodetection.
Bias-tunable polarization photoresponse of MoSe2/PdSe2 device
The strong intrinsic in-plane anisotropy of the PdSe2 enables the device capable of polarization-sensitive photodetection. Figure 2a illustrates the schematic diagram of the polarized photoresponse measurement of the device. The polarization angle θ is defined as the angle between the polarization direction of the incident light and the direction of the a-axis of PdSe2 flake, which can be adjusted from 0° to 360° via a half-wave plate. The plot of the polarized photocurrent distribution of the MoSe2/PdSe2 heterojunction related to the polarization angles and bias voltages under 1550 nm polarized light illumination are shown in Fig. 2b. The photocurrent at different bias voltages varies periodically with the polarization angle with a period of 180°, demonstrating that the photocurrent depends strongly on the bias voltage and polarization angle. To visualize the polarization-sensitive photoresponse, the polarized photocurrent as a function of polarization angle θ is also illustrated in Fig. 2c. The experimental data dots were fitted by the curve in the figure via the sinusoidal function as Iph(θ) = Iphmax cos2(θ + φ) + Iphmin sin2(θ + φ), where the Iphmax and Iphmin represent the maximum and minimum photocurrent, θ is the polarization angle and φ is the fitting parameter53. It is apparent that the periodic variation of photocurrent occurs when θ is rotated counter-clockwise from 0° to 360°, and reaches the maximum while the direction of polarization is parallel to the elongated axis of PdSe2, which is corresponding to the direction of a axis of the flake. The decent polarization ratio (PR = Iphmax/Iphmin) of the device at 1550 nm was extracted to be 14.7, which favorably satisfies the demand of high resolution photodetection (PR > 10)54.
a Schematic diagram of the experimental setup for polarization-resolved photocurrent measurement. θ is the angle between the polarization direction of incident light and the y direction (a-axis of PdSe2 nanosheets). b Photocurrent distribution as a function of polarization angles under 1550 nm illumination at various bias voltages Vds (from −1 to +1 V). c The polarized photocurrents under 1550 nm light illumination. The red dots are experimental data and the black solid line is fitting curve. d Polarization ratios of the MoSe2/PdSe2 heterostructure device under positive and negative bias (Vds = ±0.3 V) conditions. The photoresponse of MoSe2/PdSe2 photodetector can be tuned by the Vds. e Comparison of polarization ratios among the reported polarized photodetectors based on anisotropic 1D, 2D, and 3D materials or their heterojunctions. Details of quoted references should refer to Supplementary Fig. 18. f NIR imaging results of target letters with polarization angles of 0°, 30°, 60° and 90°, respectively.
Taking into account the effect of the applied voltage on the EQE of the device as discussed in the previous section, the polarization-dependent photocurrents of the MoSe2/PdSe2 heterojunction were measured over a wide spectral range under different bias conditions (Supplementary Note 6). The polarization ratios were then extracted at positive and negative Vds (± 0.3 V) in NIR range, as depicted in Fig. 2d. It is worth noting that the polarization ratio of the device exhibits a pronounced dependence on bias voltages and wavelengths, which is manifested by the phenomenon that the polarization ratio of the device possesses a remarkable increase with the redshift of the incident wavelength and remarkable bias-tunability. The polarization ratio of the device increases from around 1.5 (visible 532 nm) to 2 (NIR 808 nm) and 15 (≥1550 nm) as the wavelength is redshifted, which is mainly attributed to the weakening of the isotropic absorption of MoSe2 at longer wavelengths and the absorption of PdSe2 progressively dominated. Specifically, as illustrated in region I of Fig. 2d, the device has almost no polarization detection capability at negative bias (PR ∼ 1), and in stark contrast, a slightly higher polarization ratio (PR > 1.5) is obtained at positive bias, exhibiting a pronounced bias-tunable polarization detection performance under the illumination range from 532 to 808 nm. This distinctive phenomenon is mainly attributed to the different mechanisms of operation at different biases, details of which will be discussed in the following section. As the wavelength is redshifted to region II, the device features significant polarization detection capabilities at both positive and negative bias, which is mainly attributed to the fact that the illumination wavelengths are close to the absorption band edge of MoSe2, resulting in the dominance of anisotropic absorption of PdSe2, so that even if the device operates at negative bias the photocurrent anisotropy remains remarkable. In addition, the device exhibits better polarization detection performance compared to region I, because the limited number of isotropic photogenerated carriers from MoSe2 results in a weak contribution to the photocurrent. As the wavelength is further increased, the polarization ratio is substantially improved and high polarization ratios of ~15 are obtained in region III at above 1310 nm. In region III, the photogenerated carriers almost originate from the anisotropic absorption of the PdSe2 because the incident light wavelength far exceeds the absorption range of the multilayer MoSe2, and hence the polarization ratio is drastically improved without the participation of the isotropic carriers from MoSe2. The dependence of the polarization ratio with wavelength at different bias voltages exhibits consistent phenomenon (Supplementary Fig. 12), further demonstrating that the device features a pronounced bias and wavelength-tunable polarization ratio. Besides, the device exhibits a slight variation of the polarization ratio with the gate voltage, and a high polarization ratio of 16 is obtained at Vds = 1 V, Vgs = −5 V (Supplementary Note 7). Notably, there is an order of magnitude improvement in the polarization ratio compared to the single material PdSe2 (PR = 1.56) we measured. The MoSe2/PdSe2 heterostructure can take advantage of the different enhancement effects of the photogating effect at different polarization directions, resulting in the improvement of the polarization ratio (Supplementary Note 8)55, which will be further elucidated in following section. Moreover, the calculated correlation between different wavelengths from 532 to 1550 nm is shown in Supplementary Fig. 17, demonstrating our MoSe2/PdSe2 heterostructure device possess considerable potential for future applications such as spectral discriminations56. On the basis of the unusual bias-tunable polarization characteristics, our device can simultaneously meet the application requirements in different scenarios of light intensity imaging and polarization imaging, holding further potential that not only fulfills the purpose of target enhancement, but also improves the capability of detecting and identifying target objects.
Figure 2e demonstrates the comparison of polarization ratios among the reported polarized photodetectors based on anisotropic 1D, 2D and 3D materials or their heterojunctions. It should be noted that majority of the reported polarization 2D photodetectors have poor polarization ratios <10, which is mainly attributed to the weaker intrinsic anisotropy absorption of the material. Notably, our photodetector presents much superior polarization ratios over a broadband spectral range, displaying the favorable polarization-sensitive photodetection performance. To exploit the potential applications57, the infrared polarization imaging capability is further investigated (Supplementary Note 9). The polarization angle-dependent infrared images with polarization angles from 0°, 30°, 60°, 90° at Vds = 1 V are shown in Supplementary Fig. 19b–e. The high-resolution images of the target letters “UCAS” were obtained, and the imaging results at different polarization angles are demonstrated in Fig. 2f. As the polarization angle changes from 0° to 90°, the image exhibits a significant difference in contrast and indicates the robust polarization-sensitive optoelectronic imaging capabilities of the device, which is promising for future high-fidelity imaging and multi-dimensional optoelectronic applications.
Mechanism of MoSe2/PdSe2 device at different biases
To elucidate the working mechanism of the MoSe2/PdSe2 device at different biases, we performed measurements on the modulation of output current of the device at different bias voltages. Figure 3a depicts the transfer curves (Ids-Vgs) of the device obtained in dark and under the illumination of NIR light with various incident power densities. The device exhibits n-type transfer property and significant on/off switch states under gate voltage modulation with a high on/off ratio larger than 106, and a high room temperature carrier mobility of 116 cm2/V s. The threshold voltage (Vth) shifts significantly to a more negative value as the light power increases (shaded area in Fig. 3a). This further confirms the remarkable photogating effect of our device under light illumination, which will be elucidated in detail in following58. The variability of the threshold voltage (∆Vth) increases with the increase of the incident optical density, from which the variation of the electron concentration (∆Nd) can be calculated as illustrated in Supplementary Fig. 20f59. Notably, the photoresponse has a prominent gate voltage and power density dependence at fixed Vds = 1 V in Supplementary Fig. 20i, and the maximum responsivity was evaluated as 4.7 × 105 A/W at Vgs = 5 V under the power density of 0.001 mW/cm2, which may due to the increased majority carrier (electron) concentration of the material at positive gate voltage.
a Transfer curves (Ids−Vgs) of the MoSe2/PdSe2 device under 785 nm illumination with different power densities (Vds = 1 V). ∆Vgs and the shaded area represent the specific value and range of the change of the charge neutrality point, respectively. b Output current distribution as a function of bias voltages Vds and back gate voltages Vgs. Inset: The change in the on/off ratio of the device at various source–drain voltages. c Extracted device responsivities, R (solid lines), and detectivities, D* (dashed lines), at Vds = −1, 0, and 1 V, respectively. d Relative response with different light modulation frequency measured. The −3 dB cutoff is marked by the horizontal dashed line. Inset: Single magnified response curves at Vds = −1, 0, and 1 V. The vertical dashed lines in the inset represent 10% and 90% of the photocurrent used to evaluate the rise/fall time of the device. e Fowler–Nordheim plots of the device at positive Vds in the dark and under illumination. The dashed lines are the fits to the experimental data. f Temperature-dependent transport characteristics of the MoSe2/PdSe2 heterostructure device in the dark.
Then, we performed measurements of the transfer characteristic curves (Ids−Vgs) at different source–drain voltages (Vds) from −1 to 1 V in the dark. The transfer characteristic curves of the device at different bias voltages (Vds = −1, 0, 1 V) are shown in Fig. 3b and Supplementary Fig. 20. The device possesses a significant difference in the switching characteristics at negative bias voltage compared to positive bias voltage. The inset of Fig. 3b depicts the extracted on/off ratio as a function of source–drain bias. The on/off ratio can be efficiently modulated over five orders of magnitude from 4 at Vds = −1 V to 8.3 × 105 at Vds = 1 V, revealing a remarkable bias-dependent electrical transfer characteristic. We further investigated the photoresponse performances of our device at different bias voltages (Supplementary Fig. 21). According to the power-dependent photocurrent curves, the power exponents are extracted as 0.8 (Vds = −1 V) and 0.21 (Vds = 1 V), respectively, which means that the device possesses more recombination centers or traps participating in the photoresponse at positive Vds, and the dominant photocurrent generation mechanism changes from photoconduction to photogating effect60. It is worth noting that the device has a distinct difference in responsivity under positive and negative bias, as manifested by the fact that a value of 105 A/W is achieved when Vds = +1 V, while the responsivities at −1 and 0 V are calculated to be 25 and 10.1 A/W, respectively (Fig. 3c), which is four orders of magnitude lower compared to the positive bias. The D*, calculated from the noise spectral densities (Sn), presents similar trends as R and obtains a decent maximum value of 4.1 × 1013 Jones at 785 nm (@0.001 mW/cm2) with the low-energy consumption of 8.3 nW (detailed process shown in Supplementary Note 5), which is competitive in most previous 2D materials and their heterostructure photodetectors61,62. The response speed through the measurement of frequency dependence photocurrent gives us the clue of the faster speed under positive bias. The 3 dB cutoff frequency was extracted as 0.27 MHz (corresponding response time τr ~ 1.3 µs) at Vds = 1 V according to the modulation frequency when the photocurrent signal reduced to 0.707 times the maximum value63, as shown in Fig. 3d. The temporal response curves of the device under different bias voltages are also shown in Supplementary Note 11 and typical single magnified curves are depicted in the inset of Fig. 3d, which demonstrates faster response under positive bias. Notably, the device response can reach a higher speed of 590 ns and a responsivity of around 8 × 104 A/W on h-BN substrate in the NIR band (Supplementary Fig. 24). Combined with the analysis of the responsivity and response speed of the device under different bias conditions, a simultaneous improvement in the responsivity (four orders of magnitude) and response speed (two orders of magnitude) of the device is achieved, breaking the traditional trade-off between responsivity and speed. Similar phenomena were also discovered under the illumination of other wavelengths (from 532 to 2200 nm in Supplementary Notes 12 and 13).
To further confirm the device mechanism, Fig. 3e demonstrates the ln (I/V2) versus 1/V curves at positive bias voltages are well modeled by a tunneling barrier with the Simmons approximation, where the dominant tunneling occurs with direct tunneling (DT) at low bias voltage and Fowler–Nordheim tunneling (FNT) at high voltage64. And similar phenomenon occurred under 1550 nm irradiation (Supplementary Note 14). The tunneling-mediated transport is also confirmed with temperature-dependent measurements in Fig. 3f, exhibiting almost temperature-independent electrical output characteristics. The spatially resolved photocurrent mappings were then measured (Vgs = 0 V) at various biases, Vds (Supplementary Note 15). When the device is forward biased (Vds = 1 V), there exists a more pronounced photocurrent in the Au/PdSe2 and Au/MoSe2 contact area, while the photocurrent is almost completely distributed within the overlapping active regions of the heterojunction at Vds = −1 V, which further illustrates the significantly bias-dependent working mechanism.
To explore the bias-dependent working mechanism, the charge transfer of the device under illumination at forward bias is shown in Supplementary Fig. 28. The photogenerated electrons generated at the Au/PdSe2 Schottky contact interface can be rapidly collected by the electrodes under the combination of the built-in electric field and the applied bias voltage, whereas, the photogenerated holes in PdSe2 drifting towards the heterojunction interface under the action of the electric field are blocked by the large hole barriers at the contact interface of MoSe2/PdSe2, which induces a considerable number of electrons to be generated at Au/MoSe2 side and increases the probability of their tunneling from the source to MoSe2. This triple-junction mode of operation empowers the device with high photoresponse and fast photodetection capabilities. In stark contrast, the device operates in the photoconductive mode under reverse biased (Vds = −1 V) conditions, and the photogenerated carriers are effectively separated by the action of the electric field (Supplementary Fig. 28a).
Based on the above, the responsivity and response speed of the MoSe2/PdSe2 heterostructure device operating under forward bias is simultaneously improved under the combined contributions of photogating-assisted carrier tunneling and triple-junction operation, of which the working mechanism of the photogating-assisted tunneling has been explored in our previous work59. The results here further confirm the effect of the mechanism on improving the performance of the device.
High-sensitivity LWIR photoresponse
Due to the enhanced performance of the device and wide intrinsic absorption range of multilayer PdSe2, the infrared photodetection performance of the MoSe2/PdSe2 heterojunction device was further evaluated by investigating photoresponses of the device under LWIR illumination. Figure 4a and the inset demonstrate the plots of the I–V characteristic curves measured in the dark and under various light powers of 7.4 μm incident light. The device exhibits a significant photoresponse at 7.4 μm, as manifested by the power-dependent photocurrent change, i.e., the photocurrent increases monotonically from 20.4 to 126 nA with the power density rising from 1.3 to 26 mW/mm2 at Vds = 1 V. The power exponent α was fitted to 0.61 at 7.4 µm in Supplementary Fig. 30a, and less than an ideal value of 1, indicating the complex carrier recombination processes in the device. Figure 4b shows the output characteristics of the device under light irradiation of 10.6 μm, which presents distinct photoresponse characteristics as well as forward bias. Similarly, the Ids−Vds curves shift upward monotonically increasing the laser power density with a power exponent α of 0.74 (Supplementary Fig. 30b). Besides, the device shows prominent repeatable temporal photoresponse with the incident light periodically switching on and off at Vds = 1 V (Supplementary Fig. 30c, d). The light intensity-dependent R and D* at 7.4 and 10.6 µm are depicted in Fig. 4c. Both the R and D* values decrease with increasing light intensity because of the carrier recombination, in which the maximum R values obtained for the device under 7.4 µm (@1.3 mW/mm2) and 10.6 µm (@0.86 mW/mm2) irradiation are 0.39 and 0.47 A/W, respectively, under the combined operating mechanisms of photogating effect and photothermoelectric (PTE) effect (Supplementary Note 16). Notably, the values of the D* calculated from the measured power spectral densities are 1.28 × 109 Jones (7.4 µm) and 1.53 × 109 Jones (10.6 µm) at room temperature. Figure 4d plots the wavelength-dependent specific detectivities D* of the MoSe2/PdSe2 heterostructure device and compares it with the photodetectors based on 2D materials in prior reports and the traditional commercial detectors as well. The device exhibits an ultra-broadband photoresponse spectrum range of up to 10.6 μm. Furthermore, the obtained room temperature D* is not only superior to most 2D material-based heterostructure photodetectors such as b-AsP (1.06 × 108 Jones at 8.05 µm),2 PdSe2/WSe2 (1.8 × 108 Jones at 10.6 µm) and the commercial thermistor bolometers (~108 Jones)65 but also close to some commercial infrared photodetectors that operate at low temperatures including InAsSb, InSb, InAs/GaSb66, which presents a distinct advantage in terms of the room temperature detection performance over currently investigated LWIR photodetectors (Fig. 4d and Supplementary Table 2).
a Output curves (Ids−Vds) in dark and under 7.4 µm illumination with different power densities. Inset: Semi-logarithmic plot of Ids−Vds characteristic curves in dark and under 7.4 µm illumination with different power densities. b Output curves (Ids−Vds) in dark and under 10.6 µm illumination with different power densities. Inset: Semi-logarithmic plot of Ids−Vds characteristic curves in dark and under 10.6 µm illumination with different power densities. c Responsivities and detectivities at Vds = 1 V under 7.4 and 10.6 µm light illuminations, respectively. d Room-temperature specific detectivity D* as a function of wavelength. The D* is compared with previously reported infrared 2D materials and the state-of-the-art LWIR photodetectors34,38,70,71,72. The black dotted line represents the theoretical limits of D* calculated for the photodetectors operating under background-limited infrared performance (BLIP). The PC represents the photoconductive mode and SL represents the superlattice. FOV and Tb represent the field of view and background temperature, respectively. e Time-dependent photoresponse of the device under different polarized light illumination of 4.5 µm. The arrow represents the tendency for the photocurrent change as the angle of polarized light changes from 0° to 90°. f The polar plots of polarized photocurrents under 4.5 µm illumination.
For the purpose of evaluating the potential of our MoSe2/PdSe2 heterojunction photodetector for real applications, we characterized the performance of the device using a surface-source blackbody as a radiation source. The radiation distribution of blackbody radiation source in accordance with Planck’s law at different color temperatures and the schematic diagram of the blackbody response measurement system are shown in Supplementary Fig. 33a and b, respectively67,68. The corresponding Ids−Vds curves were measured both in the dark and under 300 K blackbody source illumination at room temperature (Supplementary Fig. 33c). The device exhibits significant current variations under lower blackbody radiation intensity at Vds = 1 V. The time-resolved radiative response in Supplementary Fig. 33d further confirms that the response is from the intrinsic absorption of the materials rather than the effect of the noise current of the device, demonstrating that the MoSe2/PdSe2 heterostructure photodetector has promising potential in LWIR detection applications. To further explore the broadband polarization sensitivity of the device, we performed measurements of the polarized photoresponse under LWIR irradiation in Fig. 4e. Similar to the performance of the device in the NIR, the device exhibits significant angle-dependent photocurrent variations at 4.5 μm illumination, and a superior polarization ratio of 15.5 is obtained as illustrated in Fig. 4f. Combining with the decent photoresponse of our devices at room temperature in the LWIR band, our results provide a prospective platform for high-performance polarization-resolved broadband imaging and target identification applications in complex environments.
Discussion
In summary, we propose a high-performance and polarization-sensitive uncooled NIR-LWIR photodetector based on MoSe2/PdSe2 heterostructures with the aid of photogating-assisted carrier tunneling-dominated triple-junction operation. Benefiting from the existence of a powerful built-in electric field at the interface, the device exhibits a broadband photovoltaic effect with a remarkable responsivity of up to 10.1 A/W at the NIR band. Furthermore, the device exhibits apparent blackbody (300 K) response and high sensitivity for LWIR up to 10.6 µm with a room temperature responsivity and detectivity of 0.47 A/W and 1.53 × 109 Jones, respectively. Notably, our heterostructure device features bias-dependent operating mechanisms and optoelectronic performance, with a simultaneously improved responsivity and speed realized at positive Vds. In addition, the device was demonstrated to be highly sensitive to broadband polarized light, giving rise to a high polarization ratio of 15.5 at 4.5 µm, and high-resolution polarization imaging capability was further confirmed. Interestingly, the polarization sensitivity of the device is bias-tunable, where the anisotropy ratio is modulated from around 2–15 from NIR to LWIR. The bias-tunable photoresponse and polarization ratio give us insight into the spectral and polarization information of incident lights. Our results combine broadband room-temperature photoresponse with high-resolution polarization detection and imaging, offering the potential to explore miniaturized and multifunction on-chip integrated optoelectronic applications, such as in polarization imaging, miniaturized polarization spectrometer, logic circuits, etc.
Methods
Device fabrication
PdSe2 and MoSe2 nanosheets were fabricated via the mechanical exfoliation method from single crystals grown by the chemical vapor transport (CVT) method. Firstly, multilayer flakes of PdSe2 were exfoliated from its bulk crystals using scotch tape and then transferred onto a silicon substrate (with 300 nm SiO2). Then, thin MoSe2 was exfoliated onto the polydimethylsiloxane (PDMS) film and transferred selectively on top of the PdSe2 flake under the optical microscope assisted by a 3D positioning system. To fabricate the device for measurements, the source/drain electrodes were patterned by ultraviolet photolithography, and Ti/Au (10/80 nm) metals were deposited by thermal evaporation.
Characterization of MoSe2/PdSe2 heterostructure
The morphologies of the MoSe2/PdSe2 heterostructure were investigated by an optical microscope (BX51, OLMPUS). The Raman mapping and PL mapping were carried out at room temperature by a confocal Raman/PL system (Alpha 300R, WITec) equipped with 532 and 633 nm laser sources. The thicknesses of the MoSe2 and PdSe2 nanoflakes were measured using atomic force microscopy (Cypher S, Asylum Research).
Optoelectrical measurements
The electrical measurements were performed under ambient conditions at room temperature. All static behaviors of the photodetector were characterized by a semiconductor parameter analyzer (Keithley 4200) on a probe station (EVERBEING, C-4) in dark and under illumination by different lasers: IR (2200, 1550, 1310, 1064, 980 and 808 nm), red (635 nm), green (532 nm), MIR (4.5, 7.4, 10.6 µm). The device has been measured multiple times to ensure the consistency of the dark current, and the spot area of incident light was confirmed based on an optical microscope. The temporal responses of the device were recorded by a current meter after the light illumination switching on–off. The device's 3 dB bandwidth is measured by modulating the light switching frequency through a signal generator (RIGOL, DG822), and the modulated optical signal was focused on the photodetector through an optical microscope.
Performance evaluation of the device
In order to quantitatively measure and compare the overall performance of photodetectors, a series of figures of merits are used to characterize the performance of our device, including R, D*, and response time.
The R is defined as the ratio of the photocurrent to the incident optical power on the active area (A) of the device, which can be expressed as
where Iph = Ilight−Idark is the photocurrent, Ilight and Idark indicate the output current with and without light, respectively, P is the light intensity. The photo-absorption active area A was estimated through the photocurrent mapping method69.
The D* is a parameter to describe the sensitivity of the photodetector, which was calculated by the following formula:
where A is the effective device area, B is the electrical bandwidth, and NEP denotes the ability to distinguish the optical signals from the background noise, and defined as: NEP = in/R (in is the dark current noise).
The response speed (τr) is another crucial figure of merit for a practical photodetector. In general, rise time and fall time are the time intervals for the photocurrent to rise from 10% to 90% and decay from 90% to 10%, respectively. An alternative characterization is the light modulation frequency (f3dB) when the photocurrent drops from a steady level to −3 dB69. The response time τr of the device is estimated using the equation:
Photocurrent mapping
The spatial-resolved photocurrent mapping was conducted using scanning photocurrent microscopy built on a confocal Raman/PL system (WITec, Alpha 300R) with a high spatial resolution of about 350 nm. The device was laterally moved with steps of 0.5 μm, where a focused laser beam (532/633 nm) was raster-scanned over the whole device area. The source–drain current Ids were recorded by a current meter under various bias voltages Vds.
Polarization-sensitive characterization
A linear polarizer (Thorlabs, LPVIS050) and half-wave plate were used to generate polarized light impinging on the sample in order to measure the polarization-dependent characteristics. The polarization angle was changed by rotating the polarizer.
Polarization imaging
A steel plate with hollow letters of “UCAS” controlled by a computerized 2D motorized platform is placed between the collimating lens and the polarizer. Following, the position-related photocurrents were measured in real-time with a source meter and further transformed into mapping images.
Data availability
Relevant data supporting the key findings of this study are available within the article and the Supplementary Information file. All raw data generated during the current study are available from the corresponding authors upon request.
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Acknowledgements
S.L. acknowledges the financial support from the National Natural Science Foundation of China (Nos. 62334010, 62121005, 62022081, 62204240, and 62304221), the National Key Research and Development Program (2021YFA0717600), and the International Fund Program of Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences.
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S.L. conceived of the original concept and supervised the project. M.L. and S.L. performed most of the experiments. M.L. and S.L. contributed to device fabrication, data collection, and photoresponse measurements. M.L., Y.Z., L.Q., N.Z., H.X., F.Z., Z.L., M.Q., X.S., Y.Z., C.L., D.L., and S.L. analyzed the data and co-wrote the paper. All authors discussed the results and commented on the manuscript. All authors have approved the final version of the manuscript.
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Liu, M., Qi, L., Zou, Y. et al. Uncooled near- to long-wave-infrared polarization-sensitive photodetectors based on MoSe2/PdSe2 van der Waals heterostructures. Nat Commun 16, 2774 (2025). https://doi.org/10.1038/s41467-025-58155-0
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DOI: https://doi.org/10.1038/s41467-025-58155-0