A hybrid-ligand exchange strategy for high-performance PbSe quantum dot short-wave infrared photodetectors

Abstract

Lead selenide (PbSe) colloidal quantum dots (CQDs) are promising candidates for short-wave infrared (SWIR) photodetectors due to their low-cost fabrication and solution processability. However, conventional ligand exchange strategies, such as treatment with 1,2-ethanedithiol (EDT), usually lead to incomplete defect passivation and undesirable doping characteristics. Here, we developed a hybrid-ligand strategy by combining EDT and zinc iodide (ZnI₂) to simultaneously passivate surface defects and modulate the doping type of PbSe CQD films. As a result, the photodetector responsivity improves from 0.04 A W⁻¹ to 0.40 A W⁻¹, and the specific detectivity increases from 3.4 × 10¹⁰ Jones to 2.8 × 10¹¹ Jones at 500 Hz under zero bias. The optimized device exhibits a wide linear dynamic range exceeding 114 dB and a fast response time of 7.3 μs. Finally, the infrared imaging applications of PbSe CQD photodetectors were successfully demonstrated. This work highlights the importance of synergistic surface passivation and doping modulation in enhancing the performance of CQD photodetectors.

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Huicheng Hu

Huicheng Hu is an associate researcher in the Yangtze Delta Region Academy of Beijing Institute of Technology, China. He received his PhD from the Institute of Functional Nano & Soft Materials at Soochow University in 2020. He worked as a visiting scholar in the Department of Chemistry at the University of Chicago (2018–2020). After being a postdoctoral fellow in Huazhong University of Science and Technology (2021–2023), he joined the Beijing Institute of Technology in 2023. His main research field is colloidal quantum dot infrared detection and imaging technology.

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Introduction

The past two decades have witnessed growing interest in colloidal quantum dot (CQD)-based infrared photodetectors owing to their low fabrication cost and high compatibility with silicon-based readout circuits.^1–4 Among various candidate materials, lead chalcogenide CQDs have emerged as a leading choice for short-wave infrared (SWIR) detection, owing to their suitable bandgap and good stability. In particular, lead sulfide (PbS) CQD infrared photodetectors have been extensively studied and show strong potential for commercialization.^5–8 Compared to PbS CQDs, lead selenide (PbSe) CQDs offer distinct advantages, including a narrower bulk bandgap (0.28 eV) and a larger exciton Bohr radius (∼47 nm),^9–12 which have attracted increasing attention. By precisely controlling their size, the absorption spectra of PbSe CQDs can cover the full SWIR range and even extend into the mid-wave infrared (MWIR).^13–15 Although there are an increasing number of studies on PbSe CQD photodetectors, their performance is still obviously worse than that of PbS CQD photodetectors.

To fabricate conductive films suitable for optoelectronic applications, the long and insulating ligands used in CQD synthesis need be replaced with shorter organic linkers or halide anions. Various small ligands, including 1,2-ethanedithiol (EDT), benzenedithiol, mercaptopropionic acid, and tetrabutylammonium iodine, have been demonstrated to remove the insulating ligands and passivate the surface defects of CQDs.^16–18 Nonetheless, conventional ligand exchange methods often lead to incomplete surface coverage or unintended defects. Recently, Zhang et al. developed a hybrid ligand co-passivation strategy that precisely positions halide anions at Pb sites and metal cations at Se sites, achieving targeted passivation of PbSe CQDs and further enhancing IR power conversion efficiency in the CQD solar cells.¹⁹ However, such multifunctional passivation strategies have been less extensively studied in PbSe CQD photodetectors, despite their potential to significantly reduce trap-assisted recombination and improve device performance.

Meanwhile, achieving high-performance photodetectors relies on optimizing the doping characteristics of the PbSe CQD active layer, as well as controlling the built-in field between the absorption and transport layers, which critically influences carrier separation and collection.^20–22 Sargent et al. introduced a cascade surface modification strategy, providing control over doping and enabling n-type and p-type CQDs.²³ Hence, ligands can offer a promising approach for controlling doping characteristics in CQDs, providing further opportunities to enhance device performance.^23–26

In this work, we developed a hybrid-ligand exchange strategy combining EDT and zinc iodide (ZnI₂) to simultaneously suppress surface defects and modulate the doping characteristics of PbSe CQD films. The incorporation of ZnI₂ plays a critical role by providing robust chemical passivation through the incorporation of Zn²⁺ and I⁻ ions onto the CQD surfaces, effectively reducing trap-state density. Ultraviolet photoelectron spectroscopy (UPS) measurement reveals that ZnI₂ treatment significantly modulates the Fermi levels of PbSe CQD films, promoting a shift from n-type to near-intrinsic behavior and enhancing carrier collection. As a result, the responsivity of the PbSe CQD photodetector increases from 0.04 A W⁻¹ to 0.40 A W⁻¹, and the specific detectivity (D*) improves from 3.4 × 10¹⁰ to 2.8 × 10¹¹ Jones at 500 Hz under zero bias. Additionally, the device demonstrates a wide linear dynamic range (LDR) exceeding 114 dB and a fast response time of 7.3 μs. Finally, the imaging capability of the PbSe CQD photodetectors was successfully demonstrated.

Results and discussion

The PbSe CQDs were synthesized in a PbCl₂-OLA complex with diphenylphosphine selenium (DPP-Se) and trioctylphosphine selenium (TOP-Se) as anion precursors.¹³ The transmission electron microscopy (TEM) images of the PbSe CQDs (Fig. 1A) reveal a high degree of monodispersity and size homogeneity, with a diameter of 5.00 ± 0.32 nm (Fig. S1, ESI†). The X-ray diffraction (XRD) peaks suggest that (200) facets are dominant in PbSe films (Fig. S2, ESI†). As shown in Fig. 1B, (200) facets are composed of an alternating arrangement of Pb and Se atoms. In the traditional passivation strategy, most Pb sites located on the (200) facets bond with EDT ligands providing partial surface passivation in the PbSe CQDs. However, the Se sites on the (200) facets, which possess lower surface energy, remain poorly passivated, leading to a high density of trap states that significantly degrade the electronic properties of PbSe CQDs. To address this issue, we developed a hybrid-ligand system in which EDT and ZnI₂ were co-dissolved in isopropanol. With the treatment of hybrid ligands, EDT and I⁻ anions jointly coordinate with Pb sites, and the Zn²⁺ cations selectively passivate the Se sites, resulting in a more comprehensive surface passivation.

Fig. 1 (A) TEM images of as-synthesized PbSe CQDs. (B) Schematic illustration of the ligand exchange for PbSe CQDs. (C) SEM images of PbSe-EDT (top) and PbSe-EDT/ZnI₂ (bottom) films. (D) FTIR spectra of PbSe-OA, PbSe-EDT and PbSe-EDT/ZnI₂ films. (E) Absorption spectra of PbSe-EDT and PbSe-EDT/ZnI₂ films.

PbSe CQD films with EDT and EDT/ZnI₂ treatment (denoted as PbSe-EDT and PbSe-EDT/ZnI₂) were fabricated via a layer-by-layer method (see the Experimental section for details). The scanning electron microscopy (SEM) images of PbSe-EDT films exhibit noticeable cracking, while that of PbSe-EDT/ZnI₂ films present a uniform, crack-free morphology (Fig. 1C). This stark contrast is primarily attributed to the distinct effects of the two ligand treatments on film stress relaxation and microstructural evolution, with the hybrid passivation approach providing improved interparticle connectivity.²⁷

Fourier transform infrared spectroscopy (FTIR) was performed to analyze the surface chemistry before and after ligand exchange. As shown in Fig. 1D, the intensities of C–H_x vibrations (2852–3011 cm⁻¹) and COO⁻ vibrations (1410–1512 cm⁻¹) are significantly reduced after ligand exchange,^28,29 indicating that most of the original oleic acid (OA) ligands were successfully replaced by the EDT or EDT/ZnI₂.

The absorption spectra before and after ligand exchange were recorded (Fig. 1E). Compared to the original OA-capped CQD solution (Fig. S3, ESI†), the excitonic absorption peaks of the PbSe-EDT and PbSe-EDT/ZnI₂ films show a redshift of 28 nm and 2 nm, respectively. This spectral shift primarily results from two concurrent phenomena occurring during ligand exchange: (1) enhanced inter-dot electronic coupling due to reduced inter-particle distances, and (2) partial inter-dot fusion.^30–33 The significantly smaller redshift observed in PbSe-EDT/ZnI₂ films indicates that the hybrid ligand treatment enables a more stable ligand exchange process, effectively inhibiting inter-dot fusion. Furthermore, as shown in Fig. S4 (ESI†), thermal treatment at 90 °C in nitrogen for 1 hour causes significant broadening and redshift of the excitonic peak in the PbSe-EDT film, while the PbSe-EDT/ZnI₂ film remains nearly unchanged. This stability improvement may be attributed to the synergistic effect of ZnI₂: the ZnI₂ facilitates deprotonation of the EDT molecules,³⁴ thereby strengthening their coordination to the CQD surface, while ZnI₂ itself also contributes additional surface coordination.

The detailed surface properties of the PbSe CQD films were investigated by X-ray photoelectron spectroscopy (XPS) measurement. No signals of Zn and I elements can be found in the PbSe-EDT film (Fig. 2A and B). The two distinct peaks appear at 1022.31 and 1045.41 eV in the EDT/ZnI₂ film, corresponding to the Zn²⁺ 2p_3/2 and 2p_1/2 orbitals, suggesting the possible incorporation of Zn²⁺ ions on the CQD surface.^35,36 Similarly, two clear peaks at 619.41 and 630.91 eV in the PbSe-EDT/ZnI₂ film are assigned to the I⁻ 3d_5/2 and 3d_3/2 orbitals, indicating the potential incorporation of I⁻ ions on the CQD surface.^35,37 The Pb 4f peaks of the PbSe-EDT/ZnI₂ film shift to higher binding energies compared to those of the PbS-EDT film (Fig. 2C), suggesting that I⁻ ligands effectively coordinate with Pb sites due to the higher binding energy of Pb–I compared to Pb–Se and Pb–S.^37–39 Additionally, as shown in Fig. 2D, the Se 3d peaks show an obvious shift toward higher binding energy, further confirming the passivation of exposed Se atom sites on the PbSe (200) facets by Zn²⁺ cations.^35,36,40

Fig. 2 (A)–(D) XPS spectra of Zn (A), I (B), Pb (C) and Se (D) of PbSe-EDT and PbSe-EDT/ZnI₂ films. (E) and (F) I–V characteristics of PbSe-EDT (E) and PbSe-EDT/ZnI₂ (F) devices according to the SCLC model. (G) and (H) UPS spectra for determination of E_F (G) and energy differences between E_VBM and E_F (H) of PbSe-EDT and PbSe-EDT/ZnI₂ films. (I) Diagram of the energy band of PbSe-EDT and PbSe-EDT/ZnI₂ films.

To verify the passivation effect of ZnI₂, the density of trap states of PbSe-EDT and PbSe-EDT/ZnI₂ films was studied using the space-charge-limited current (SCLC) measurements. For this purpose, a hole-only device structure with indium-doped tin oxide (ITO)/PbS-EDT CQDs/PbSe CQDs/PbS-EDT CQDs/ZnTe/ITO was fabricated. The trap density (n_trap) was estimated from the SCLC curves (Fig. 2E and F) based on the expression.⁴¹

n_trap = 2ε₀ε_CQDV_TFL/qL² (1)

where ε₀ is the permittivity of vacuum, ε_CQD is the permittivity of the CQD materials (∼11), V_TFL is the trap-filled limit voltage, q is the elementary charge, and L is the thickness of the CQD film.¹⁹ The calculated n_trap for the device based on PbSe-EDT/ZnI₂ CQDs is 8.95 × 10¹⁵ cm⁻³, which is significantly lower than the 1.11 × 10¹⁶ cm⁻³ measured for the PbSe-EDT device, confirming the superior trap state passivation provided by the EDT/ZnI₂ hybrid ligands.

To evaluate the band structure of PbSe CQD films, the Fermi level (E_F) and the valence band maximum (E_VBM) of PbS-EDT and PbSe-EDT/ZnI₂ films were determined by using UPS characterization. As shown in Fig. 2G and H, the E_F and E_V of the PbSe-EDT film are −4.68 and −5.20 eV, suggesting a slight n-doping for PbSe CQDs with a bandgap of ∼0.77 eV. In contrast, the E_F and E_V of the PbSe-EDT film are −4.80 and −5.19 eV, indicating quasi-intrinsic doping for PbSe CQDs with a bandgap of ∼0.79 eV. This result suggests that the ZnI₂ ligands significantly alter the doping characteristics, effectively adjusting the CQDs from n-type to a more intrinsic behavior (Fig. 2I). Such band alignment modifications are crucial for optimizing the performance of PbSe CQD photodetectors. Fig. S5 and S6, ESI† show the energy level information for the ZnO electron transport layer (ETL) obtained by transmission spectrum and UPS measurements. The results indicate that compared to the PbSe-EDT layer, the PbSe-EDT/ZnI₂ layer exhibits an enhanced built-in field at the heterojunction interface with the ETL and an improved carrier transport efficiency (Fig. S7, ESI†).

To demonstrate the advantages of the hybrid ligand exchange strategy, SWIR photodetectors were fabricated using a top-illuminated architecture, featuring an ITO/ZnO/PbSe CQDs/PbS-EDT CQDs/ZnTe/ITO.^34,42 Notably, the ZnTe film shows a high transmittance of 93.4% at 1560 nm, suggesting exceptional optical properties of the hole transport layer in SWIR photodetectors (Fig. S8, ESI†). As shown in Fig. 3A, the cross-sectional SEM image shows that the thickness of the ZnO, PbSe CQDs, PbS-EDT CQDs and ZnTe functional layers is approximately 60 nm, 400 nm, 45 nm, and 40 nm, respectively, with well-defined interfaces. The typical current density–voltage (J–V) curves of photodetectors based on PbSe-EDT and PbSe-EDT/ZnI₂ absorption layers are shown in Fig. 3B. For the photoresponse measurement, a 1550 nm LED with an illumination intensity of about 0.4 mW cm⁻² was employed. The PbSe-EDT/ZnI₂ device exhibits significantly higher photocurrent compared to the PbSe-EDT device, highlighting the superior charge extraction enabled by hybrid ligands. The improvement is further supported by the broadband external quantum efficiency (EQE) and responsivity from 400 to 2000 nm (Fig. 3C and Fig. S9, ESI†). Notably, the EQE of the PbSe-EDT/ZnI₂ device reaches 31.4%, which is far higher than 2.8% achieved by the PbSe-EDT device. The enhanced EQE is attributed to the incorporation of ZnI₂, which reduces the trap state density and improves the carrier transport. The performances of devices with different ZnI₂ concentrations were recorded (Fig. S10A, ESI†). The device with 10 mM ZnI₂ exhibited the lowest dark current and highest EQE (Fig. S10B and Table S1, ESI†), which may be attributed to insufficient passivation of CQDs at low ZnI₂ concentrations, while excessive ZnI₂ could form an insulating matrix that hinders charge carrier transport.

Fig. 3 (A) Typical cross-sectional SEM images of PbSe-EDT/ZnI₂ photodetectors. (B) J–V curves of PbSe-EDT and PbSe-EDT/ZnI₂ photodetectors. (C) and (D) EQE (C) and D* (D) spectra of PbSe-EDT and PbSe-EDT/ZnI₂ photodetectors at zero bias. (E) and (F) LDR spectrum (E) and response time curve (F) of PbSe-EDT/ZnI₂ photodetectors at zero bias.

We found that the integration of EDT ligands is crucial for achieving high-performance photodetectors. The PbSe CQD photodetector based on pure ZnI₂ ligands (denoted as PbSe–ZnI₂) was fabricated, and its performance was systematically evaluated. As shown in Fig. S11 (ESI†), compared to the PbSe–ZnI₂ device, the dark current density in the PbSe-EDT/ZnI₂ device is reduced by more than an order of magnitude, primarily due to the effective passivation provided by EDT ligands. This significant reduction in dark current also translates into a suppressed noise current, as evidenced by the frequency-dependent noise current spectra in Fig. S12 (ESI†). Moreover, the PbSe-EDT/ZnI₂ device exhibits considerably enhanced photocurrent extraction under zero bias, along with an improved EQE (Fig. S13, ESI†). These results clearly demonstrate that EDT ligands play a crucial role in passivating the surface trap states of PbSe CQDs, working synergistically with ZnI₂ to protect CQD surfaces from defects.

To expectantly realize efficient light utilization, we fabricated PbSe-EDT/ZnI₂ photodetectors with varying absorption layer thicknesses, ranging from 200 to 600 nm, via controlling the spin-coating processes. Interestingly, the photocurrent density and EQE of the devices did not increase monotonically with the thickness of the absorption layer, as illustrated in Fig. S14 and S15 (ESI†). When the absorption layer thickness increases from 200 to 400 nm, the EQE significantly improves from 21.0% to 31.4% at zero bias, indicating enhanced light harvesting and more efficient charge collection. However, further increasing the thickness to 600 nm results in a reduced EQE of 19%. At the same time, we conducted EQE spectral measurements of these detectors under varying bias voltages. We found that as the thickness increases, the EQE becomes more dependent on the applied bias (Fig. S16, ESI†).

Additionally, the dark current density of these devices follows a non-monotonic trend with increasing absorption layer thickness. For instance, when the PbSe-EDT/ZnI₂ CQD layer thickness is only 200 nm, the device exhibits a high dark current density of 2.26 μA cm⁻² at −0.5 V, which drops to 0.92 μA cm⁻² for a 400 nm absorption layer, before increasing again at 600 nm (Fig. S13, ESI†). This suggests that an optimal absorption layer thickness exists, balancing efficient light absorption, carrier transport, and reduced noise, resulting in the highest signal-to-noise ratio at 400 nm.

The performances of PbSe CQD photodetectors were further evaluated by D*, calculated using the following expression:^43–45

D* = R(AΔf)^1/2/i_n (2)

where R is the responsivity, A is the active area (0.04 cm²), Δf is the bandwidth (1 Hz), and i_n is the noise current collected at 500 Hz (Fig. S17, ESI†). As shown in Fig. 3D, the PbSe-EDT/ZnI₂ device exhibits a peak D* of 2.8 × 10¹¹ Jones at approximately 1560 nm, which is nearly an order of magnitude higher than that of the device based on PbSe-EDT CQDs (3.4 × 10¹⁰ Jones).

Meanwhile, the J–V curves of the optimized photodetector were measured under different incident light power densities using a 1550 nm LED as the light source (Fig. S18, ESI†). As the light power density received by the device ranges from 16 nW cm⁻² to 7.65 mW cm⁻², the photocurrent exhibits a linear dependence on the light intensity, resulting in a wide LDR exceeding 114 dB at zero bias (Fig. 3E). In addition, the response speed of the device with an active area of 0.04 cm² was tested at zero bias by a 1550 nm LED as a light source. The rise and fall times are determined to be 7.3 and 22.5 μs (Fig. 3F), suggesting a relative fast response of the photodetectors.

To investigate the infrared imaging capability of the PbSe CQD photodetectors, a single-point imaging system was constructed, as illustrated in Fig. 4A. This imaging system consists of several key components: a tungsten filament lamp as the infrared light source, a scanning lens for precise object imaging, and a PbSe CQD photodetector operated at zero bias. The scanning lens is controlled by dedicated software, allowing coordinated movement of both the lens and the target image to ensure accurate focus. When the scanned image reaches the active area of the photodetector, the optical signal is converted into an electrical signal. This signal, along with the corresponding displacement data from the scanning system, is captured by an acquisition card. Finally, the imaging software processes the collected data, generating the imaging results.

Fig. 4 (A) Schematic illustration of the single-point infrared imaging system. (B)–(D) The visible (top) and infrared (bottom) images of a mask engraved with the letter “BIT” (B), glass vials with H₂O and TCE (C) and the same vials blocked by the Si wafer (D).

The imaging capability of the PbSe CQD photodetector was first evaluated using a mask engraved with the letters “BIT” as the imaging object. As shown in Fig. 4B, the PbSe CQD photodetector clearly captures the pattern, demonstrating its imaging performance in comparison to visual observation. Furthermore, a demonstrative experiment was conducted to assess the detector's potential for chemical solvent discrimination. Water (H₂O) and tetrachloroethylene (TCE) were selected as representative samples. While both solvents appear visually transparent when imaged using a conventional silicon imager under natural light (Fig. 4C), they exhibit distinct infrared absorption characteristics. H₂O has a significantly higher absorption coefficient for infrared light compared to TCE. As a result, the PbSe CQD photodetector shows a darker image when capturing H₂O, while TCE appears brighter. This contrast highlights the potential of the PbSe CQD photodetector for substance screening applications. To further evaluate the material penetration imaging capability of the PbSe CQD photodetector, we placed a Si wafer as the screen on the glass bottles for H₂O and TCE. Due to the strong absorption of natural light by the Si wafer, a conventional silicon imager is unable to capture the image of the underlying glass bottles (Fig. 4D). In contrast, the PbSe CQD photodetector, using an infrared light source, successfully penetrates the Si wafer and clearly reveals the contours of the concealed glass bottles. This result demonstrates the PbSe CQD photodetector's ability for effective material penetration imaging, highlighting its potential for advanced imaging applications.

Conclusions

In this study, we developed a hybrid-ligand strategy to tune the surface properties and doping characteristics of PbSe CQDs. Combining EDT with ZnI₂ effectively reduces trap-state densities and enhances carrier collection, resulting in a significant improvement in device performance. As a result, the optimized PbSe CQD photodetector exhibits a peak detectivity of 2.8 × 10¹¹ Jones at 1560 nm, which is nearly an order of magnitude higher than that of conventional PbSe-EDT devices. The device also exhibits a wide LDR (>114 dB), and a fast response time (7.3 μs). Furthermore, successful imaging demonstrations highlight the potential of this approach for practical optoelectronic applications. This work provides key insights into the role of ligand engineering and doping modulation in enhancing the performance of CQD photodetectors, offering a pathway toward high-performance SWIR imaging and sensing technologies.

Experimental

Chemicals

Oleic acid (90%), oleylamine (90%), lead oxide (99.99%), 1-octadecene (≥90%), n-octane (≥99%), acetonitrile (≥99%), zinc iodide (≥99.99%), tri-n-octylphosphine (90%), diphenylphosphine (98%) and 1,2-ethanedithiol (98%) were purchased from Admas. Ethanol (≥99.9%) and isopropanol (≥99.9%) were purchased from Greagent. Lead chloride (99.99%) and selenium (99.99%) were purchased from Aladdin.

Synthesis of PbSe CQDs

PbSe CQDs were synthesized by a hot-injection method as reported by Lian et al.¹³

Synthesis of PbS CQDs

PbS CQDs were synthesized under Schlenkline-line conditions as reported by Tavakoli et al.⁴⁶

Fabrication of PbSe CQD films

PbSe CQD films were prepared via a layer-by-layer method. For each layer, 15 μL of PbSe CQD solution in n-octane (40 mg mL⁻¹) was spin-coated onto the substrate at 2500 rpm for 20 s. Then, 100 μL of EDT solution (0.1 vol% in IPA) or EDT/ZnI₂ mixed solution (10 mM ZnI₂ with 0.1 vol% EDT in IPA) were dropped on PbSe CQDs and maintained for 20 s, followed by two rinse–spin steps using IPA. This process was repeated 10 to 30 times to obtain a film with target thickness.

Fabrication of PbS CQD films

PbS CQD films were also prepared via a layer-by-layer method. For each layer, 15 μL of PbS CQD solution in n-octane (40 mg mL⁻¹) was spin-coated onto the substrate at 2500 rpm for 20 s. Then, 100 μL of EDT solution (0.02 vol% in acetonitrile) were dropped on PbS CQDs and maintained for 20 s, followed by two rinse–spin steps using acetonitrile.

Device fabrication

ITO-on-glass substrates were washed using a sonication bath in deionized water and ethanol. The ZnO film was first deposited on ITO by magnetron sputtering. The sputtering target was zinc oxide of 99.9% purity from Zhongnuo New Material Company. The sputtering power was 100 W, and the sputtering pressure was 2 Pa. Then the PbSe-EDT or PbSe-EDT/ZnI₂ films were fabricated on the ZnO layer. Subsequently, the PbS-EDT films were deposited on top of the PbSe CQD layer. Then, the ZnTe film was deposited on the PbS-EDT film by thermal evaporation at a deposition rate of 0.4 Å s⁻¹. Finally, the ITO top electrode was deposited by magnetron sputtering with a deposition pressure of 0.5 Pa and a power setting of 100 W. The active area of the CQD photodetectors was defined using a shadow mask, yielding a device area of 0.04 cm².

Materials and device characterization

The SEM images were obtained using a Gemini SEM 360 microscope. The UPS measurements were performed in an ultra-high vacuum (10⁻⁸–10⁻⁹ Torr) with a Thermo Fisher Scientific ESCALAB XI+ system and He I radiation (21.22 eV). The optical absorption spectra of the CQDs were obtained using a Shanghai Precision UVN-5000A spectrophotometer. The electrical characteristics and LDR were measured using a semiconductor analyzer (Primarius FS-PRO) under both dark and 1550 nm LED illumination conditions. The response time was measured on an oscilloscope instrument (Agilent DSOS054A) with a 1550 nm LED source. The noise current spectra were measured using Low-Frequency Noise Test System LFN-2000 (Wuxi XINJIAN Semiconductor Technology Co., Ltd). The EQE spectra were measured using an EQE test system (LST-QE) equipped with a halogen lamp and standard Si and InGaAs as standard detectors.

Author contributions

Y. W. and M. C. contributed equally to this work. H. H. supervised and directed the study. X. T. performed supervision. Y. W., M. C. and M. H. synthesized the materials and performed the material characterization. Y. W., A. J. and W. L. fabricated the device. F. W. and X. Z. performed the single-point infrared imaging. H. H., Y. W. and M. C. co-wrote the manuscript. All authors contributed to discussions regarding the manuscript.

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

All data generated or analyzed during this study are included in this published article and its ESI.†

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 62405024) and the China Postdoctoral Science Foundation (No. 2024M764156). We would like to thank Jing Liu and Xi Ran for their efforts on response time measurement and noise analysis.

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Footnotes

† Electronic supplementary information (ESI) available: Size distributions, XRD patterns, absorption spectrum of PbSe CQDs; J–V curves, broadband responsivity, and noise spectral density of PbSe CQD photodetectors. See DOI: https://doi.org/10.1039/d5tc02115d

‡ Ya Wang and Min Chen contributed equally to this work.

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