Mixed-Dimensional Heterostructure Photodetector Based on Bi₂O₂Se Nanosheets and PbS Quantum Dots

Abstract

Due to their exceptional electronic and optical properties, two-dimensional materials have emerged as one of the most promising candidates for future optoelectronic detection. However, optoelectronic detectors based on two-dimensional transition metal materials still face challenges due to factors such as limited absorption coefficients and carrier recombination. In this study, we combine two-dimensional Bi₂O₂Se with PbS quantum dots to prepare a hybrid heterojunction, effectively broadening the detection range and significantly enhancing the photoresponse rate. The hybrid photodetector exhibited a remarkable photoresponsivity of 14.89 A/W at 450 nm and demonstrated broadband detection capabilities from visible (405 nm) to near-infrared (1350 nm) light illumination. Moreover, the hybrid device showed reduced photocurrent response and recovery times, highlighting its improved performance over bare Bi₂O₂Se photodetectors. This work underscores the potential of hybrid heterojunctions for enhancing optoelectronic detection capabilities, paving the way for advanced applications in various fields.

1. Introduction

Due to its crucial applications in fields like video imaging, biomedical imaging, network security, optoelectronic communication, and motion detection, there is a pressing need for optoelectronic devices with high response speed, broad-wavelength detection, and versatile integration [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. Conventional detectors are unable to meet the escalating demands in various domains, including high-frequency communication, information security, and cutting-edge biomedical imaging. Two-dimensional (2D) materials, as layered low-dimensional substances, enable efficient optoelectronic detection through interlayer van der Waals interactions. Moreover, traditional optoelectronic detectors typically require low-temperature operation and complex fabrication processes, but using 2D materials simplifies this process. Two-dimensional semiconductor materials have unique advantages in the field of optoelectronic detection: (I) The preparation methods of two-dimensional semiconductor materials are relatively simple and can be prepared by methods such as chemical vapor deposition (CVD) [15] or mechanical exfoliation [2,16]. (II) The stacking between layers of two-dimensional layered semiconductor materials is not limited by lattice matching [17]. (III) The bandgap of most two-dimensional layered semiconductor materials can be adjusted by controlling the number of material layers [18]. Different types of two-dimensional semimetal materials have their unique advantages in the field of optoelectronic detection. Graphene, due to its extremely narrow bandgap, exhibits a wide optical response range, but this also results in a significant dark current [19,20,21]. Transition metal dichalcogenides (TMDs) come in various types, and their bandgaps can be adjusted based on the number of layers, yet most materials primarily respond to light in the ultraviolet and visible regions [22,23,24,25,26]. Black phosphorus (BP) demonstrates excellent optoelectronic properties, but it is prone to volatilization in air [27]. Hexagonal boron nitride (hBN) can be used for ultraviolet photodetection due to its wide bandgap and is often employed as a dielectric material. However, the preparation process for hBN is quite intricate [28]. To enhance the optoelectronic performance of two-dimensional transistors, researchers have explored methods such as etching and doping to modify the structure of two-dimensional materials. Among these, two-dimensional transition metal compounds, particularly Bi₂O₂Se, stand out due to their exceptional optoelectronic properties [29].

Bi₂O₂Se possesses excellent electron mobility and environmental stability. Nevertheless, Bi₂O₂Se photodetectors face challenges like a limited response range and higher dark current [30]. Constructing heterostructures, such as 2D/2D and 0D/2D junctions, offers a promising approach to enhance Bi₂O₂Se photodetectors, improving their on/off ratio and reducing dark current [31,32,33,34,35,36,37,38,39,40]. Heterojunctions between Bi₂O₂Se and other 2D or zero-dimensional (0D) materials hold potential to extend the detection range and boost photoresponsivity [30]. Due to the bandgap limitation, Bi₂O₂Se detection performance in the infrared spectrum exhibits a noticeable decline. Quantum dots (QDs), on the other hand, can achieve effective response in the infrared spectrum by controlling magnetization. There are many types of 0D narrow-bandgap colloidal quantum dots, whose bandgap size is related to the size of the QDs, and they possess a high extinction coefficient in the near-infrared band but relatively low electron mobility. Heterojunctions formed by QDs and 2D materials can simultaneously leverage the advantages of both materials: the high-mobility 2D materials serve as carrier transport channels, while the QDs provide the device with broadband light absorption capabilities. Additionally, the band bending formed between QDs and 2D materials due to the heterojunction structure can facilitate the efficient separation of photogenerated charges, reducing recombination probability and thus enhancing the photodetection performance. Therefore, a combination of the two can effectively broaden the detection range and improve the responsivity of photodetection.

In this work, we present the construction of a type-II Bi₂O₂Se/PbS QDs hybrid heterojunction by using chemical vapor deposition and spin-coating. Comparative analysis with the 2D Bi₂O₂Se photodetector reveals a substantial enhancement in photoresponsivity for the hybrid heterojunction, achieving a remarkable 14.89 A/W at 450 nm. Furthermore, the Bi₂O₂Se-PbS QD-based photodetector demonstrates broadband photoresponsivity spanning from visible (405 nm) to near-infrared (1350 nm) light illumination. Notably, compared to the bare Bi₂O₂Se photodetector, the hybrid photodetector exhibits significantly reduced photocurrent response and recovery times.

2. Materials and Methods

2.1. Synthesis of Bi₂O₂Se Nanosheets

Few-layer Bi₂O₂Se was prepared via the chemical vapor deposition (CVD) method. The furnace tube for dual-temperature zone CVD has a diameter of 10 cm and a length of 25 cm. Bi₂O₃ powder and Bi₂Se₃ powder were placed at the hot center of the first temperature zone and 5 cm upstream from it, respectively, with a molar mass ratio of 2:1 between Bi₂O₃ and Bi₂Se₃. Mica substrates were placed at the hot center of the second temperature zone. First, we flushed the quartz tube with Ar gas three times to expel the air from the tube. The temperature of the first heating zone was set to increase from room temperature to 580 °C over 35 min and then maintained at 580 °C for 30 min. The temperature of the second heating zone was set to increase from room temperature to 680–730 °C over 35 min and then maintained at 680–730 °C for 30 min. The entire CVD process for preparing the two-dimensional Bi₂O₂Se film was conducted under Ar conditions, with an argon flow rate of 120 sccm. The pressure inside the quartz tube was maintained at 250 torr by controlling the vacuum pump.

2.2. Characterization

To evaluate the prepared Bi₂O₂Se nanosheets, PbS QDs, and hybrid PbS/Bi₂O₂Se heterostructure, atomic force microscopy (AFM) (Innova, Bruker, Billerica, MA, USA), laser micro-Raman spectroscopy (DXR, Thermo Fisher, Waltham, MA, USA), and multi-functional X-ray photoelectron spectroscopy (XPS) (ESCALAB QXi, Thermo) were employed. To obtain the energy band alignment information between PbS QDs and Bi₂O₂Se, ultraviolet photoelectron spectroscopy (UPS) (ESCALAB 250XI, Thermo) measurements were conducted. To investigate the photodetection performance of Bi₂O₂Se photodetectors and hybrid photodetectors, we used a photodetection testing platform to test the devices. The photodetection testing platform includes a xenon lamp light source (GLORIA X500A, Zolix, Beijing, China), a probe station (DCH-E2, Metatest Corporation, Nanjing, China), an optical system (Omni-λ-i, Zolix), and a source meter (MODELS 2614B, Keythley, Cleveland, OH, USA).

2.3. Preparation of the Hybrid Photodetector

The Bi₂O₂Se and hybrid Bi₂O₂Se/PbS photodetectors were fabricated using electron beam evaporation, with Au (50 nm) as the contact electrodes. The electrode preparation requires the design of a mask for the sample material. The distance between electrodes is approximately 30 μm. Prior to electrode fabrication, alignment of the electrode pattern on the mask with the Bi₂O₂Se sample material is achieved using an optical microscope, followed by securing the mask to the substrate using high-temperature resistant tape. Finally, the assembly is placed into the electron beam evaporation equipment for depositing a 50 nm thick layer of gold.

To sensitize the device with PbS, the bare Bi₂O₂Se device was spin-coated with a layer of PbS QDs. A 25 μL solution of octane PbS–OA was spin-coated onto the entire substrate at 2500 rpm for 30 s. This process was repeated twice, and the sample was then annealed at 100 °C for 15 min. All electrical and optical measurements were performed using a probe station with a source meter. The devices were illuminated with a xenon lamp light source, and power calibration was conducted using a standard silicon detector.

3. Results and Discussion

Bi₂O₂Se forms a tetragonal structure at room temperature. With a spatial group symmetry of I4/mmm (a = b = 3.88 Å, c = 12.16 Å), eight Bi protones are located at the top point of the cube, as illustrated in Figure 1a. The [Bi₂O₂]_n²ⁿ⁺ cation layer and [Se]_n²ⁿ⁻ anionic layer are alternately stacked along the C-axis by weak electrostatic interaction, with a layer thickness of 0.608 nm [29,39]. Unlike the common van der Waals layered materials, Bi₂O₂Se is ion-layered, but it still exhibits the characteristics of typical 2D materials. The experiment utilized chemical vapor deposition (CVD) to synthesize two-dimensional Bi₂O₂Se, with Bi₂O₃ and Bi₂Se₃ powders as precursors. Since Bi₂O₂Se is the only stable phase on the eutectic line of Bi₂Se₃ and Bi₂O₃, the reaction will not result in the formation of other impurities or the occurrence of side reactions. Bi₂O₃ powder is adsorbed and diffused onto the substrate to react with Bi₂Se₃ to form Bi₂O₂Se; this mechanism can provide an improved local environment for high-quality growth. Typically, CVD preparation of 2D materials uses silicon dioxide as the substrate. The interlayer bonding force in Bi₂O₂Se is electrostatic. Using silicon dioxide as the substrate would promote the vertical growth of Bi₂O₂Se, preventing the formation of few-layer nanosheets. The choice of freshly cleaved fluorophlogopite mica [KMg₃(AlSi₃O₁₀)F₂] as the substrate for preparing 2D Bi₂O₂Se is due to its flat surface, surface inertness, and high thermal stability. Additionally, there is a strong electrostatic interaction between the K⁺ layers in fluorophlogopite mica and the Se²⁻ layers in Bi₂O₂Se, which promotes the lateral growth of Bi₂O₂Se [40,41]. A diagram of the CVD system processing the sample here with a double-temperature zone is shown in Figure 1b. Bi₂Se₃ powder and Bi₂O₃ powder are used as source materials and placed in a hot center for evaporation; the distance between the two quartz boats is 5 cm. Freshly cleaved fluorophlogopite mica [KMg₃(AlSi₃O₁₀)F₂] is placed in the hot center of the second warm zone, 10–13 cm away from the center of the first warm zone. As illustrated in Figure S1, when the substrate temperature is 680 °C, the precursor absorption rate on the substrate is relatively fast, allowing for the formation of larger Bi₂O₂Se nanosheets [29]. As the temperature increases, the precursor absorption rate decreases, resulting in smaller Bi₂O₂Se nanosheets. Further increases in temperature can lead to nanosheet decomposition and poorer crystallinity. In the experiment, electrodes were prepared using electron beam evaporation. Given that the mask electrode spacing is 30 μm, we chose to control the substrate temperature between 680 and 690 °C to obtain large-area Bi₂O₂Se nanosheets.

The two-dimensional Bi₂O₂Se nanosheets selected for heterojunction fabrication are shown in Figure 1c. The morphology of individual Bi₂O₂Se nanosheets is characterized by regular square shapes, with a side length of approximately 35 μm. The thickness of the nanosheets, as measured by atomic force microscopy (AFM), is approximately 6 nm, as shown in Figure 1d. The Raman spectra of two-dimensional Bi₂O₂Se is shown in Figure 1e. Due to the A_1g vibration modes of bi, the spectra show a single characteristic peak with a center of ≈159 cm⁻¹, indicating good uniformity of the 2D Bi₂O₂Se [35,36,37]. To further confirm that the synthesized material is Bi₂O₂Se, it was also characterized by energy-dispersive X-ray photoelectron spectroscopy (XPS). As described in Figure 1f–h, the peaks of Bi 4f_7/2 and Bi 4f_5/2, which are located at 158.8 eV and 164.2 eV, respectively. By performing Lorentzian fitting on the broad peak near 54 eV using XPS software (Avantage V5.9931), it is possible to resolve the peaks at 53.2 eV and 54.2 eV, which correspond to the Se 3d_5/2 and 3d_3/2 peaks, respectively. The O 1s spectrum contains two peaks that are located at 532.1 eV and 529.8 eV; these two peaks correspond to adsorbed oxygen and lattice oxygen [37]. All bonding energies are consistent with the elements in Bi₂O₂Se.

Subsequently, a Au electrode with a thickness of 50 nm was fabricated via electron beam evaporation deposition for establishing an electrical connection with Bi₂O₂Se (Figure S3a). As shown in Figure 2a, the photocurrent of the Bi₂O₂Se photodetector increases with the enhancement of light intensity at 550 nm. Due to the bandgap of about 0.8 eV in few-layer Bi₂O₂Se, the device’s dark current reaches 10⁻⁶ A. As shown in Figure 2b, Bi₂O₂Se exhibits excellent photoresponse curves in the 405–1000 nm range, while almost no photocurrent is detected beyond 1000 nm. Figure 2c shows the responsivity and detectivity of the Bi₂O₂Se photodetector from 405 nm to 1000 nm. The experiment found that the maximum responsivity and detectivity of the few-layer Bi₂O₂Se photodetector are both at 600 nm, with a responsivity reaching 0.604 A/W and a detectivity reaching 7.61 × 10⁹ Jones. Thus, the detection range of the few-layer Bi₂O₂Se photodetector prepared in the experiment is 405–1000 nm, mainly demonstrating its excellent detection capability in the visible light region. In the near-infrared region, however, the photodetection capability of Bi₂O₂Se is very weak, mainly due to the intrinsic indirect bandgap of Bi₂O₂Se [42,43], which limits its application in the field of photodetection. As shown in Figure 2d, the τ_rise/τ_decay values of the Bi₂O₂Se photodetector are 0.63/1.27 s (V_ds = 1 V).

However, by taking advantage of the high mobility of Bi₂O₂Se, an efficient broadband photodetector can be constructed by integrating it with QDs with high extinction coefficients [30]. As shown in Figure 3a, we further prepared photosensitive PbS QDs with a characteristic diameter of approximately 4.13 nm on the pre-fabricated Bi₂O₂Se transistor. These QDs possess a narrow bandgap of 1.1 electron volts, enabling spectral detection up to 1300 nm. In Figure 3b, the absorption of the quantum dots exhibits a first exciton absorption peak at approximately 1100 nm. The results are consistent with the predicted bandgap. The experiment involved spin-coating PbS QDs onto Bi₂O₂Se nanosheets, as illustrated in Figure 3c. Subsequently, the sample was annealed at 90 °C under Ar gas conditions for 15 min to induce solvent removal and atomic restructuring of the quantum dot surfaces, releasing surface partial stress defects, thereby enhancing the photodetection performance of the quantum dots. After two repeated spin-coating cycles, the optimal thickness of the PbS quantum dot layer was achieved, ensuring a dense distribution of the PbS quantum dots on the detector.

Next, we will investigate the performance of the PbS QDs/Bi₂O₂Se photodetector in detecting signals in the visible light and infrared spectrum. The schematic representation of this device is shown in Figure 4a. As shown in Figure 4b, introducing PbS QDs into Bi₂O₂Se was observed to increase the channel current by approximately 10 times under dark conditions, a phenomenon attributed to the electron transfer from PbS QDs to Bi₂O₂Se. Under excitation at 550 nm, the photocurrent (3648 nA) in the hybrid photodetector was nearly 13 times that of bare Bi₂O₂Se (280 nA). This significant enhancement in photocurrent is attributed to the charge transfer between PbS and Bi₂O₂Se, along with a higher electron mobility in Bi₂O₂Se compared to PbS QDs. This enhancement is due to the increased number of carriers within Bi₂O₂Se, leading to a reduction in the transit time within the transport channel and thereby enhancing the photodetection performance of the hybrid photodetector [44]. Figure 4c illustrates the variation of photocurrent at 550 nm under different light intensities.

Responsivity and detectivity are important metrics for assessing the performance of photodetectors, and they can be obtained based on the following equations [45,46,47]:

$$R=\frac{I_{ph}}{PA}$$

$$D^{*}=\frac{\sqrt{A}R}{\sqrt{2e{I}_d}}$$

where P is the light power intensity, R is the responsivity, e is the electron charge, I_d is dark current, and A is the effective device area. As the light intensity increases, the photocurrent of the hybrid photodetector also increases continuously, and the hybrid heterojunction photodetector also exhibits a fast response speed. However, we observed a gradual decrease in the responsivity and detectivity of the hybrid photodetector with increasing light intensity (Figure 4d). This phenomenon can be attributed to two reasons: (I) When the light intensity reaches a certain level, the responsivity and detectivity of the photodetector itself have already reached their maximum values, and further increases in light intensity do not correspondingly enhance the responsivity and detectivity. (II) At high light intensities, photogenerated carriers in the photodetector undergo recombination, where carriers excited by light recombine, leading to a decrease in the photodetection performance of the photodetector.

As the key figures of merit of photodetectors, the response speed (τ_rise: photocurrent increases from 10 to 90% of its final value)/decay time (τ_decay: photocurrent falls from 90 to 10% of its beginning value) are also characterized. As displayed in Figure 2d, τ_rise/τ_decay values of 0.105/0.651 s (V_ds = 1 V). Compared to the 0.63/1.27 s of bare Bi₂O₂Se (Figure 2d), the response speed and decay time of the hybrid heterojunction have been significantly improved. Figure 4f shows the photoresponse spectra of the compared devices. Compared to bare Bi₂O₂Se, the hybrid photodetector exhibited a 20-fold improvement in responsiveness below 1000 nm due to the detection range of PbS quantum dots covering 405–1350 nm (Figure S4). At λ > 1000 nm, the responsiveness of bare Bi₂O₂Se is nearly undetectable, whereas the PbS/Bi₂O₂Se hybrid photodetector exhibits significant photoresponse.

The charge transfers between Bi₂O₂Se and PbS QDs is determined by the band alignment at the interface. To obtain the band information of the prepared PbS QDs and Bi₂O₂Se, ultraviolet photoelectron spectroscopy (UPS) measurements were conducted. In Figure 4g, valence band spectra and the second cutoff energy of Bi₂O₂Se and PbS are presented. The work functions (W) of PbS and Bi₂O₂Se can be calculated from the second cutoff energy (E_cut) using W= hν − E_cut, where hν = 21.22 eV represents the photon energy. The work functions of PbS and Bi₂O₂Se were found to be 4.2 eV and 4.35 eV, respectively. Additionally, the valence band positions of PbS and Bi₂O₂Se were determined to be 0.49 eV and 0.51 eV below the Fermi level (E_F) of each material based on the valence band spectra, thus establishing the valence band positions of PbS quantum dots and Bi₂O₂Se. Therefore, based on the bandgaps of PbS (1.1 eV) and Bi₂O₂Se (0.8 eV), along with the estimated work functions and valence band positions, the energy band diagram shown in Figure 4h was constructed. PbS and Bi₂O₂Se form a type-II hybrid heterojunction, where electrons are transferred from PbS to Bi₂O₂Se due to the lower work function of PbS, forming an internal electric field that enhances the photodetection performance of Bi₂O₂Se.

Figure 5a illustrates the charge flow diagram of the PbS/Bi₂O₂Se hybrid photodetector under a 1 V bias. When the device is illuminated, both PbS quantum dots and Bi₂O₂Se generate photogenerated carriers. Due to the type-II heterojunction formed between PbS quantum dots and Bi₂O₂Se, photogenerated electrons in the PbS quantum dots transfer to the Bi₂O₂Se. Bi₂O₂Se acts as the current transport layer throughout the device. The transfer of electrons from the PbS quantum dots enhances the electron mobility in Bi₂O₂Se, shortening the carrier transport time (τ_transit), thereby increasing the photoconductive gain of the entire device [44]. Consequently, we can observe that the responsivity and specific detectivity of the PbS/Bi₂O₂Se hybrid photodetector have improved by an order of magnitude compared to the standalone Bi₂O₂Se photodetector.

The PbS QDs and two-dimensional Bi₂O₂Se form a type-II heterojunction, resulting in the creation of an internal electric field within the device. This field can promote the separation of carriers, improving the device’s photoresponse speed. However, experimental results reveal that the response time of the hybrid heterojunction device is slower compared to devices constructed with Bi₂O₂Se and other two-dimensional materials in type-II heterojunctions. This slower response time is attributed to Förster resonance in the 0D/2D heterojunction. Förster resonance energy transfer (FRET) is a highly distance-dependent non-radiative energy transfer mechanism [48,49]; the energy transfer diagram is shown in Figure 5b. In the PbS QDs/Bi₂O₂Se hybrid photodetector, when PbS quantum dots, acting as energy donors, are photoexcited to an excited state, they can transfer their energy non-radiatively to Bi₂O₂Se through dipole–dipole interactions. This causes electrons in the valence band of Bi₂O₂Se to transition to the conduction band. In the PbS QDs/Bi₂O₂Se hybrid photodetector, the primary factor enhancing photoresponse time is the capture of photogenerated carriers by defect states in Bi₂O₂Se rather than the separation of photogenerated carriers at the interface. Therefore, while the photoresponse speed of the hybrid heterojunction photodetector is improved compared to a standalone Bi₂O₂Se photodetector, it does not match the improvement observed in other type-II heterojunctions.

4. Conclusions

In summary, we successfully synthesized 2D layered Bi₂O₂Se samples using low-pressure chemical vapor deposition and demonstrated a broadband and high-performance infrared photodetector based on the hybrid structure of high-mobility 2D Bi₂O₂Se and PbS QDs. The fast photoresponse (rise/fall time) of the hybrid photodetector reached 0.15/0.651 s (compared to 0.63/1.27 s for bare Bi₂O₂Se), attributed to the rapid photogenerated carrier separation at the formed type-II interface. Compared to few-layer Bi₂O₂Se photodetectors, the hybrid photodetector expanded its photoresponse range to near 1350 nm. The responsivity (R) of the hybrid photodetector within 1000 nm was nearly 20 times higher than that of few-layer Bi₂O₂Se (reaching 14.89 A/W at 450 nm). The construction of the PbS quantum dots and Bi₂O₂Se heterojunction further validates the feasibility of the theoretical approach of combining photosensitive materials with channel materials. By utilizing the high light response of the photosensitive material, incident photons are efficiently converted into photogenerated electrons, and the high-speed charge transfer of the channel material allows photogenerated carriers to cycle multiple times within their lifetime, achieving high gain. This device structure holds broad prospects for future applications in the field of photodetection, offering wide spectral response range, high responsivity, miniaturization, and portability. To further enhance the optoelectronic performance of the hybrid heterojunction, future research should focus on the intrinsic properties of 2D materials and the surface defects of QDs. Improving the preparation and surface treatment processes of QDs and 2D materials will broaden the spectral response range and enhance the device’s performance.