Two-step ligand exchange to promote charge transfer in PbSe quantum dot photodetectors for pulse monitoring

Abstract

Quantum dots (QDs) have emerged as promising materials for next-generation infrared semiconductors due to their facile solution processing, low-cost, tunable bandgap and superior optoelectronic properties. However, organic long-chain ligands that modify the surface of QDs hinder charge transfer, thus impairing the performance of QD infrared photodetectors. Here, we report a two-step ligand exchange strategy that decouples the native long-chain ligands from the QDs using specific molecules and then attaches the short-chain ligands, resulting in high response for lead-rich lead selenide (PbSe) QD photodetectors. During the layer-by-layer film deposition process, 1-octanethiol is first used for primary ligand exchange to remove stable ligands, followed by 3-mercaptopropionic acid for secondary exchange to ensure thorough passivation of surface defects. The two-step processing PbSe QD photodetector has a responsivity of up to 1.28 A W⁻¹, a detectivity of 9.65 × 10¹² Jones and a record high external quantum efficiency of 144.4% at a bias voltage of 0.5 V at 1100 nm. Benefitting from the high performance, the PbSe QD photodetector can be integrated into a pulse monitoring platform, achieving a physiological sign monitoring by capturing real-time pulse signals of human superficial arteries.

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New concepts

Quantum dots (QDs) have received widespread attention in the field of infrared photodetectors. However, long-chain ligands hinder charge transfer, reducing the performance of QD infrared photodetectors, and short-chain ligands may lead to aggregation and non-uniform film shrinkage. This work proposes a two-step ligand exchange strategy to enhance performance of PbSe QD infrared photodetectors. First, 1-octanethiol is used as a colloidal stabilizer to help surface reconstruction of PbSe QDs by regulating ligand exchange kinetics. Subsequently, the 3-mercaptopropionic acid ligand is introduced to enhance charge transfer channel continuity within the thin film, achieving an improvement in the optoelectronic performance. The proposed PbSe QD photodetector can accurately capture the pulsation signals of superficial arteries in the human body under illumination of 620 nm and 940 nm. These findings elucidate an effective method for PbSe QDs with organic ligands to promote the development of QD infrared photodetectors.

1. Introduction

With the rapid development of consumer electronics, the demands for infrared detection technology in industrial inspection, machine vision, communication remote sensing, medical imaging and other fields have been steadily increasing.^1–5 However, the traditional bulk materials such as HgCdTe severely hinder the development of consumer-grade electronics.^6–9 They not only require expensive epitaxial equipment such as molecular beam epitaxy to grow on substrates with strictly matched lattices, but also suffer from compatibility issues with Si-based readout integrated circuits in flip-chip bonding.^10–12 Therefore, there is an urgent need to search for new narrow bandgap semiconductor optoelectronic materials for infrared photodetectors.

Quantum dots (QDs) are distinguished by their facile solution processing, low-cost manufacturing and tunable infrared bandgap.^13–15 The unique optoelectronic properties make them one of the ideal narrow bandgap infrared semiconductor materials for the next generation. Among various QDs, lead selenide (PbSe) QDs attract extensive attention in the field of infrared photodetectors due to their unique properties: large Bohr radius (46 nm) enabling strong electronic coupling, infrared-tunable bandgap, and efficient multiple exciton generation.^16–20 In the synthesis process of PbSe QDs, strong coordinating solvents such as trioctylphosphine (TOP) and oleic acid (OA) which are usually introduced as ligands to maintain the stability of the PbSe QDs.^21–23 However, the length of long-chain ligand OA molecule can reach 2.5 nm, thereby weakening the coupling between QDs, significantly suppressing the carrier tunneling effect, and ultimately causing a decline in the response performance of the photodetector.^24–26 The widely adopted approach is currently used to replace long-chain ligands with short-chain ligands, among which 1,2-ethanedithiol (EDT) has become a research hotspot due to its small size.^27–32 When not treated with ligands, the trap states on the surface of QDs serve as the main transport pathways and recombination centers. Carriers are injected into these trap states under forward and reverse biases. After passivating the dangling bonds on the surface of the PbSe QDs, the carrier transport occurs through electronic states near the conduction and valence bands of the PbSe QDs, reducing the capture behavior of surface traps on carriers.^29–32 It has a maximum responsivity of 2 A W⁻¹ under 800 nm at a bias of 4 V.³⁰ The difference between 3-mercaptopropionic acid (MPA) and EDT is that one of the sulfhydryl groups is replaced by a carboxylic acid. Compared with thiols, the PbSe QDs passivated with carboxylic acid ligands have better charge transfer ability.³³ However, it is worth noting that although this modification method can improve the conductivity of the PbSe QDs film, it is not suitable for material systems with poor chemical stability.³⁴ Additionally, these short-chain ligands may induce QD aggregation, leading to non-uniform film shrinkage and low ligand exchange efficiency during solid-phase ligand exchange. This not only reduces the film quality but also exacerbates energy disorder.³⁵ Therefore, optimizing the preparation process of the PbSe QD film modified with strongly coordinated organic ligands and constructing high-performance photodetectors remain critical challenges that urgently need to be addressed in this field.

This work aims to address the above challenges by enhancing the performance of the PbSe QD photodetector via a two-step ligand exchange strategy. This method first employs 1-octanethiol (OTT) as a colloidal stabilizer to help surface reconstruction of QDs by regulating ligand exchange kinetics. Subsequently, the functional MPA ligands are introduced to enhance charge transfer channel continuity within the PbSe film, achieving a synergistic improvement in the optoelectronic performance.

2. Experimental section

2.1. Materials

The materials used include lead oxide (PbO, Aladdin, Shanghai, 99.999 wt%), selenium (Se, Aladdin, Shanghai, 99.99 wt%), OA (Aladdin, Shanghai, 90 wt%), 1-octadecene (ODE, Aladdin, Shanghai, 90 wt%), TOP (Aladdin, Shanghai, 90 wt%), OTT (Aladdin, Shanghai, ≥98.5 wt%), MPA (Aladdin, Shanghai, >99 wt%), isopropanol (IPA, Aladdin, Shanghai, ≥99.7 wt%), octane (Aladdin, Shanghai, ≥96.0 wt%), ethanol (Chron Chemicals, Shanghai, ≥99.7 wt%), and toluene (Chron Chemicals, Chengdu, 99.5 wt%).

2.2. Synthesis of PbSe QDs

First, PbO (0.8928 g, 4 mmol), OA (2 mL), and ODE (4 mL) were added in a 50-mL flask. The mixed solution was degassed by stirring under vacuum at 120 °C for 1 h. Subsequently, the mixed solution was kept at 130 °C under a nitrogen atmosphere. 0.0789 g (1 mmol) of Se powder and 1 mL of TOP were placed in a 10 mL glass vial and were stirred overnight in a glove box at 50 °C, finally forming a colorless, clear and transparent TOP-Se solution. Then, the TOP-Se solution was rapidly injected to the mixed solution and retained for 150 s. The reaction was quenched by injecting a solution of 4 mL of toluene. After quenching, the vial was quickly cooled by an ice bath. The solution was precipitated with ethanol and centrifuged at 8000 rpm for 3 min. Finally, the precipitate was resuspended in octane at a concentration of 100 mg mL⁻¹ and stored under ambient conditions.

2.3. Fabrication of PbSe QD photodetectors

The ITO glass substrate was thoroughly cleaned by deionized water, acetone and isopropanol, each for 30 min. For the ZnO layer, the presynthesized ZnO nanoparticles at a concentration of 80 mg mL⁻¹ were spin-coated onto the substrate at 4000 rpm for 40 s and subsequently annealed at 200 °C for 30 min to act as the electron transporting layer. The active layer was deposited on top of the ZnO layer using 40 μL of PbSe ink in the glove box, utilizing a spin-coating process with 2000 rpm for 20 s. Each layer was crosslinked with the OTT/IPA (1 : 50 by volume) solution for 30 s, rinsed with IPA and dried. And then it was crosslinked with the MPA/IPA (1 : 50 by volume) solution for 30 s, rinsed with IPA and dried. After the ligand exchange, the active layer was thermally annealed at 50 °C for 10 min in the glove box. Above steps were repeated 6–8 times to complete the construction of the active layer. Finally, the device was completed by thermal deposition of a 10 nm MoO₃ layer and a 130 nm silver electrode layer onto the active layer.

2.4. Characterizations of the PbSe QDs and devices

X-ray diffraction (XRD) patterns of the PbSe QDs were recorded by Rigaku Ultima IV using Cu-Kα1 irradiation in the range of 2θ from 10 to 90°, with a step size of 0.1°. The morphology and crystalline structure of the PbSe QDs were examined by using a JEOL 2100 Plus Transmission electron microscope (TEM) at 300 kV. Optical absorption spectrums of the PbSe QD solutions and film were collected using a Shimadzu UV-3600 plus spectrophotometer in the range of 400–1500 nm. The chemical composition and surface chemical state of the PbSe QDs were measured by using an X-ray photoelectron spectrometer (XPS) in an ultrahigh vacuum chamber (Kratos AXIS UltraDLD). Cross-sectional morphology of the PbSe QD photodetector was obtained using a scanning electron microscope (SEM, ZEISS Gemini 300), and the film of the same thickness as the device was spun coated on the substrate to show the microscopic morphology of the functional layers. Fourier transform infrared spectroscopy (FTIR) spectra were obtained using a Bruker Vertex 80v. Ultraviolet photoelectron spectrometry (UPS) measurements were performed on a Thermo Scientific-ESCALAB Xi+ electron spectrometer using the monochromatic HeI radiation (21.22 eV) under ultra-high vacuum conditions at 10 V bias.

2.5. Device performance testing

The current–voltage curves, photocurrent broadband response spectra, responsivity (R), specific detectivity (D*) and external quantum efficiency (EQE) were measured using a semiconductor characterization system (Keithley 4200) under a Xenon light source coupled with a monochromator, calibrated with a standard Si detector (S1337-1010BQ, Hamamatsu Photonics) and an InGaAs detector (Hamamatsu Photonics). The response bandwidth, linear dynamic range (LDR) and response/recovery time were measured by using a semiconductor parameter analyzer (PD-RS, Enli Technology). The light emitting diodes with 620 and 940 nm were used for the implementation of pulse detection. All the measurements were conducted under ambient air at room temperature (25 °C).

3. Results and discussion

3.1. Characterization results

Based on the modified hot-injection method,^23,36,37 we synthesized monodisperse PbSe QDs via elevated lead/selenium precursor ratios (Pb : Se = 4 : 1). The PbSe QDs display a sharp first-exciton absorption peak at 1306 nm due to the excess incorporation of Pb²⁺ (Fig. 1a). The rich metal ions regulate the nucleation-growth kinetics by suppressing nucleation rates and preferential growth along specific crystal planes, thereby achieving a uniform size distribution.³⁸ The TEM image in Fig. 1b reveals that the PbSe QDs have an average diameter of approximately 5 nm with excellent crystallinity and uniform dispersion. The interplanar spacing of (200) in the inset of Fig. 1b is 0.31 nm, in good agreement with the PbSe lattice parameter.³⁹ The XRD patterns of the PbSe QDs demonstrate that the diffraction peaks match well with the standard spectrum of the cubic (rock salt) structure of PbSe (Fig. 1c).²³ The above characterization results confirm the successful synthesis of the high-quality PbSe QDs.

Fig. 1 (a) Normalized absorption spectrum of the PbSe QDs. (b) TEM image of the PbSe QDs, and the inset shows a high-resolution TEM image. (c) XRD patterns of the PbSe QDs.

The excess metal ions form a stable coordination with native OA ligands, thereby ensuring the structural stability of the PbSe QDs in non-polar solvents. Although the long-chain OA ligands effectively stabilize the PbSe QDs, they increase the spacing between PbSe QDs and hinder the transport of charge carriers. To address this issue, we propose a two-step ligand exchange strategy as shown in Fig. 2a. In detail, OTT can enhance MPA ligand exchange efficiency by coordinating with Pb through its thiol groups, replacing the existing long-chain OA ligands on the surface of PbSe QDs. In addition, OTT can reduce the steric hindrance on the surface of PbSe QDs compared with OA, thereby promoting the coordination of MPA with Pb. Meanwhile, the steric hindrance effect of OTT effectively suppresses the aggregation of PbSe QDs, creating conditions for the subsequent sufficient passivation of MPA ligands. As a short chain ligand for solid-phase exchange, MPA reduces the inter-dot spacing in the PbSe QDs assembly, thereby improving the carrier transport efficiency.^40–42 To compare the differences of our innovative two-step ligand exchange strategy versus the conventional one-step method (using exclusively MPA ligands) on the surface chemistry modification of the PbSe QDs, XPS characterization was used to analyze the PbSe QDs treated with different ligand exchange methods. For untreated PbSe QDs, the XPS spectrum of Pb 4f orbitals exhibits the characteristic peaks at 142.3 eV (4f_5/2) and 137.4 eV (4f_7/2), while the binding energies of Se 3d orbitals are 55.4 eV (3d_3/2) and 54.7 eV (3d_5/2), respectively (Fig. 2b and c). Both two-step and one-step ligand exchange methods induce that the Pb 4f peaks of PbSe QDs shift towards lower binding energy, with the two-step method exhibiting more pronounced displacement magnitudes (Fig. S1). The binding energy shift indicates that the lead atoms on the surface have coordinated with stronger functional groups, and the carboxylate groups on the surface of PbSe QDs have been replaced by thiol groups. The enhanced displacement magnitude demonstrates a more complete ligand exchange efficiency achieved via the two-step method compared to the one-step treatment. By calculation of XPS results (Fig. S2), the S/Pb atomic ratio of PbSe QDs is 0.27, which is greater than that of one-step processing (S/Pb atomic ratio: 0.2). This also indicates that the OTT processing step can more thoroughly decouple OA and achieve more effective passivation. Although XPS analysis has confirmed the decoupling function of OTT toward OA ligands, the potential dual role as surface capping agents remains to be substantiated. To address the mechanistic ambiguity, the samples with two- and one-step treatment were characterized by FTIR to compare the coordination states of OTT on the PbSe QD surface, and the results are shown in Fig. 2d. The two peaks appearing at 1425 and 1560 cm⁻¹ are attributed to the symmetric and asymmetric stretching vibrations of the COO⁻ vibrational band caused by deprotonation of hydrophilic thiols. The absence of S–H stretching frequency of free hydrophilic thiols confirms that the ligands bind to the surface of the PbSe QDs through thiol salt groups.⁴³ Compared to the reported results, the C–H resonance intensity significantly decreased from 80% of the OA-capped PbSe QDs to 87% of the PbSe QDs after two-step ligand exchange treatment, indicating a decrease in the length of the PbSe QD hydrocarbon chain and confirming the attachment of OTT on the surface of the PbSe QDs.⁴⁴ This trend is consistent with the law of shortened ligand chain length, which directly confirms the success of ligand exchange. Therefore, we infer that in the two-step ligand exchange process, the larger groups of OTT provide sufficient spacing between PbSe QDs to prevent tight surface packing. However, the short chain functional groups of MPA have low steric hindrance and can penetrate into the gaps between OTT ligands, thus achieving more complete surface passivation of the PbSe QDs. In addition, OTT also contributes to the formation of high-quality PbSe QDs films (Fig. S3).

Fig. 2 (a) Schematic illustration of the ligand exchange. (b) Pb 4f XPS spectrum and (c) Se 3d XPS spectrum of the two-step treated PbSe QDs. (d) FTIR spectra of the one-step and two-step treated PbSe QDs. The pink area represents the asymmetric stretching vibration of the COO⁻ groups and the green area represents the stretching vibration of the C–H resonances.

3.2. Performance of the PbSe QD photodetectors

Fig. 3a shows the device structure of the PbSe QD photodetector with a layered composition of ITO/ZnO/PbSe QDs/MoO₃/Ag from bottom to top, where the PbSe QDs layer functions as the core light absorbing layer. Fig. 3b shows that each layer is vertically stacked with clear layering, in which the thickness of the PbSe QD layer is about 710 nm. The interface band arrangement of the PbSe QDs fundamentally determines the charge transfer under illumination. The absorption spectrum and bandgap of the PbSe QDs film are calculated by a Tauc method, and the results are shown in Fig. S4, with a bandgap value of 0.86 eV.⁴⁵ In addition, the maximum valence band (E_V) and Fermi level (E_F) of the PbSe QDs film were characterized by UPS. According to the W = hν − E_cut (hν: 21.21 eV, HeI light source), the work function (W) of the PbSe QDs is estimated to be 4.29 eV. Hence, the calculated E_V is 4.97 eV and E_F is 4.29 eV (Fig. S5). Based on the above data, the energy band diagram of the two-step treated PbSe QD photodetector is shown in Fig. 3c.

Fig. 3 (a) Structure diagram of the PbSe QD photodetector. (b) Cross-sectional SEM image of the PbSe QD photodetector. (c) Energy band diagram of the two-step treated PbSe QD photodetector. (d) Photocurrent response curves of the PbSe QD photodetector under different wavelengths at 0 V bias with one-step treatment, and (e) with two-step treatment. (f) Current density–voltage curves of the PbSe QD photodetector under different wavelengths ranging from −0.5 V to 0.5 V. (g) Transient response and (h) 3 dB bandwidth of the PbSe QD photodetector under 520 nm at 0 V. (i) 3 dB bandwidth of the PbSe QD photodetector under 520 nm at 0 V.

Fig. 3d and e show the current response curves of the PbSe QD photodetectors fabricated with two-step and one-step treatments under dark conditions and at different wavelengths. The two photodetectors exhibit distinct photocurrent response under different wavelengths from 400 to 1500 nm. At 0 V, the maximum photocurrent of the device treated with a one-step method is −2.1 × 10⁻⁹ A. The photocurrent of the device processed by a two-step method increases to −5.7 × 10⁻⁸ A while maintaining a low dark current level without significant enhancement (from −6 × 10⁻¹² to −9 × 10⁻¹² A). The results show that the photocurrent of the device processed in two steps is about 27 times that of the former. This is attributed to dual ligands which provide a dense protective ligand layer on the surface to effectively isolate environmental influences and significantly eliminate trap states. Hence, it is beneficial for inhibiting Auger recombination, consequently enhancing photocurrent generation. Furthermore, the current curve of the two-step-treated PbSe QD photodetector under different wavelengths at 0 V indicates that it can achieve a self-powered operation mode. Fig. 3f shows the curve of the current density–voltage relationship of the two-step treated PbSe QD photodetector under different wavelengths, demonstrating a better rectification phenomenon than the one-step-treated PbSe QD photodetector (Fig. S6).

To evaluate the performance of the two-step-treated PbSe QD photodetector, the core parameters including R, EQE and D* were calculated as depicted in Fig. S7–S9. Among them, R is defined as the ratio of the output current of the device to the incident light power. EQE characterizes the efficiency of the device in converting incident photons into collectable photo-generated carriers. D* reflects the weak signal detection capability of the photodetector and the D* in this work is calculated based on the shot-noise limit. Accordingly, R, EQE and D* are calculated using the following eqn (1)–(3), respectively.^46–49

where I_light is the photocurrent, I_dark is the dark current, P_in is the radiation intensity, A (0.024 cm²) is the effective area of the device, hν is the incident photon energy, and e is the electron charge (1.60 × 10⁻¹⁹ C). At a bias voltage of 0 V, the maximum R, D* and EQE of the one-step-treated PbSe QD photodetector are 0.28 mA W⁻¹, 1.64 × 10⁹ Jones and 0.087%, respectively. Due to the addition of the OTT processing step, the two-step ligand exchange device enhances the photocurrent response, thereby improving the R (9.24 mA W⁻¹), D* (6.96 × 10¹⁰ Jones) and EQE (2.86%). It is worth noting that regardless of whether it has been treated with OTT or not, the R, EQE and D* values of the PbSe QD photodetectors are very low at 0 V bias, mainly due to the insufficient intrinsic carrier mobility of the PbSe QD films.²¹

It is interesting that the PbSe QD photodetector with one-step treatment cannot operate under bias voltage, while the PbSe QD photodetector with two-step ligand exchange treatment can operate under a bias voltage of 0.5 V. This is because the quality of the PbSe QD film treated with one step is poor, which can easily cause excessive leakage current and electric field breakdown, making it difficult to distinguish between current under dark and light conditions. As shown in Fig. S8 and S9, the photocurrent significantly increases to the order of 10⁻⁵ A, resulting in maximum values of R (1.28 A W⁻¹), EQE (144.4%) and D* (9.65 × 10¹² Jones) at 1100 nm, respectively. At different biases, the R, EQE, and D* exhibit different curves, mainly caused by changes in the distribution of captured electrons near ITO or Ag electrodes.^50,51 More than 100% EQE is caused by trap-assisted carrier multiplication.^52–54 Under the action of an electric field, the carriers captured by the traps inside the PbSe QD photodetector overcome the limitations of trap energy levels. The collision between the deconfined carriers and those still confined trigger the generation of secondary carriers, ultimately leading to the collection of multiplied carriers at the electrode. This internal carrier gain mechanism enables the EQE of the device to exceed 100% under bias. Under 0 V bias, the trapped carriers lack sufficient energy to overcome their confinement and act as non-radiative recombination centers. The non-radiative recombination process suppresses carrier transport, resulting in the reduction of EQE.^52–54 The two-step treated PbSe QD photodetector demonstrates excellent infrared detection performances at a low bias voltage of 0.5 V.

The rise time directly reflects the real-time tracking capability for optical signals of the photodetector. Rise time (τ_rise) is defined as the time for the photocurrent to rise from 10% to 90% of the maximum photocurrent and the fall time (τ_fall) is defined as the time for the photocurrent to decay from 90% to 10% of the maximum photocurrent. The rise time of the two-step-treated photodetector is 9.8 μs, and the fall time is 6.7 μs at 520 nm, demonstrating rapid response capability to optical signal (Fig. 3g). In addition, by testing the response characteristics dependent on light intensity, the LDR of the device was calculated to be 94 dB according to eqn (4) as shown in Fig. 3h, where P_max is the maximum power in the linear current light power region and P_min is minimum power in the linear current light power region.^55,56

The 3 dB bandwidth refers to the frequency range corresponding to a decrease in signal power from its maximum value to half (i.e., a power attenuation of 3 dB) and is a key indicator for measuring the frequency response characteristics of the photodetector. The 3 dB bandwidth test was conducted on the device with two-step ligand exchange treatment, and the result is shown in Fig. 3i, with a bandwidth of 330 kHz. The response time and 3 dB bandwidth do not match the commonly used formula 0.35/τ, possibly because the PbSe QD photodetector is not an ideal circuit composed of a simple first-order pole low-pass filter RC device.⁵⁷ Trap distribution, surface states or recombination dynamics can influence the results. In addition, the observed anomalous “bump” phenomenon is mainly due to the superposition of multiple time constants of charge carriers, resulting in local signal enhancement at specific frequencies. This abnormal phenomenon has also been reported before.⁵⁸ Compared with many PbSe based photodetectors (Table 1),^30,59–67 our PbSe QD photodetector with two-step ligand exchange treatment has achieved significant breakthroughs in optoelectronic performances.

Table 1 Comparisons of the different PbSe QD photodetectors

Main materials	Main strategy	EQE (%)	Responsivity (A W^−1)	Detectivity (Jones)	Rise/fall time (ms)	LDR (dB)	3 dB bandwidth (kHz)	Ref.
PbSe-EDT	Solid-phase ligand exchange	—	2@800 nm	1 × 10^12@800 nm	87/63	—	—	30
PbSe/P3HT	Heterojunction	—	7.64@980 nm	5.23 × 10^14@980 nm	22.9/23.2	113.2	—	59
PbSe/PbS/metallic metasurface	Heterojunction	—	375@2710 nm	2 × 10^8@2710 nm	4270/5250	—	—	60
PbSe-TBAI/PbSe-EDT ^−	Homojunction	23.96@1460 nm	0.282@1460 nm	4.08 × 10^10@1460 nm	0.012/0.0118	—	25.67	61
CsPbI3:PbSe	Composite with perovskite	—	2.89@980 nm	5.22 × 10^13@980 nm	86.1/91.8	79.99	—	62
PbSe:CsPbBr1.5I1.5/P3HT	Heterojunction and composite with perovskite	18.22@532 nm	6.16@532 nm	5.96 × 10^13@532 nm	350/375	91.01	—	63
PbSe-TBAI/CuSCN	CuSCN as a hole transport layer	—	2 × 10^−4@1550 nm	4.54 × 10^9@1550 nm	235/157	—	—	64
PbSe/CdSe	Core–shell structure	—	—	2.49  ×  10^9 @4μm	—	—	—	65
CH3NH3PbI3/PbSe	Heterojunction	—	4.94 × 10^−3@1300 nm	4.32 × 10^11@1300 nm	0.001/0.008	53	—	66
PbSe/MoS2	Heterojunction	—	19.7@808 nm	2.65 × 10^10@808 nm	380/860	—	—	67
PbSe-OTT-MPA	Two-step ligand exchange	144.4@1100 nm	1.28@1100 nm	9.65 × 10^12@1100 nm	0.0098/0.0067	94	330	This work

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3.3. Application in pulse signal monitoring

The dynamic changes in pulse not only reflect the pumping function of the heart but are also closely related to vascular elasticity, systemic circulation status and metabolic levels.^68–71 Monitoring pulse characteristics (frequency, rhythm and intensity), combined with indicators such as blood pressure and symptoms, it can be used as a non-invasive method for early screening of health problems such as cardiovascular disease and endocrine disorders. As shown in Fig. 4a, the monitoring platform designed in this study uses dual-wavelength light sources of red light (620 nm) and infrared light (940 nm) to irradiate the fingers and then collects transmitted light signals through the PbSe QD photodetector. The measured signal waveform exhibits typical periodic fluctuations, driven by changes in blood volume, and includes three key sites: systolic peak A, diastolic systole notch B and diastolic secondary peak C.⁷¹ Fig. 4b and c both show that the PbSe QD photodetector can record pulse under normal and post exercise states, with the pulse characteristic peaks being more pronounced at 620 nm. By virtue of its high sensitivity, the PbSe QD photodetector is integrated into a pulse sensor visualization system, enabling precise heart rate monitoring with an error margin of less than 2% compared to a commercial pulse detector (Fig. S10). The monitoring platform based on our two-step treated PbSe QD photodetector has successfully achieved real-time extraction and dynamic tracking of pulse signals. Its non-invasive and continuous monitoring characteristics provide effective technical support for early identification of physiological signal abnormalities and disease warning.

Fig. 4 (a) Schematic diagram of the finger-clip pulse monitoring experimental platform. (b) Pulse output signal under 620 nm and (c) 940 nm in different states.

4. Conclusions

In summary, we proposed a PbSe QD photodetector based on an innovative two-step process with OTT and MPA. The obtained PbSe QD photodetector has a responsivity of up to 1.28 A W⁻¹, a detectivity of 9.65 × 10¹² Jones and a record-high EQE of 144.4% at a bias voltage of 0.5 V under 1100 nm. It also shows fast response and recovery speed (9.8 μs of rise time and 6.7 μs of fall time) and wide 3 dB bandwidth (330 kHz). Moreover, the proposed PbSe QD photodetector can be used for non-invasive and continuous healthcare monitoring by capturing real-time pulse signals of human superficial arteries. This work establishes a new pathway for developing high-performance PbSe QD photodetectors. However, in order to achieve practical applications of the proposed PbSe QD photodetector, further research is needed on its operational stability, such as the effects of water and oxygen.

Author contributions

Jingwen Yang: conceptualization, validation, methodology, investigation, writing – original draft. Zaihua Duan: conceptualization, supervision, writing – review & editing. Wenxin Zeng, Yichen Bu, Xing Tang, Guosheng Wang: methodology, investigation. Xin Zhou: validation, funding acquisition. Qian Dai, Zhen Yuan, Yadong Jiang: validation, writing – review & editing. Huiling Tai: supervision, writing – review & editing, funding acquisition.

Data availability

The data supporting this article have been included as part of the SI. See DOI: https://doi.org/10.1039/d5nh00495k

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Science Fund for Distinguished Young Scholars (Grant No. 62225106), Natural Science Foundation of China (Grant No. U24A20229), Sichuan Innovation Research Group Project (Grant No. 2025NSFTD0008), and Sichuan Provincial Youth Science and Technology Foundation (Grant No. 2023NSFSC0958).

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