The advancement of heterostructures in two-dimensional (2D) materials plays a pivotal role in enhancing their optoelectronic capabilities. Nevertheless, existing heterostructures encounter limitations concerning light absorption efficiency and band alignment, impeding the generation and transportation of photo-generated charge carriers. To overcome these challenges, we have pioneered the development of a self-powered photodetector operating in the near-infrared (NIR) spectra. This innovative photodetector is built upon nitrogen-doped graphene quantum dots (N-GQDs), three-dimensional (3D) graphene, and germanium (Ge) Schottky junctions. By leveraging the remarkable light absorption capabilities of N-GQDs and 3D-graphene, combined with precisely aligned heterostructure band alignment, as-fabricated device manifests a robust built-in electric field. Consequently, it facilitates efficient generation and separation of photo-generated charge carriers, thereby significantly amplifying overall optoelectronic performance. The as-fabricated photodetectors exhibit exceptional responsivity (2.3 × 103 A/W) and specific detectivity (6.2 × 1014 Jones) when illuminated at 2200 nm. Extensive studies validate the outstanding reproducibility, long-term stability, and microsecond-level rise/fall times of the proposed photodetectors. Particularly noteworthy is their remarkable image sensing capability, enabling the capture of high-resolution images in the NIR range. Furthermore, we have successfully demonstrated the integration of as-fabricated photodetector into optically controlled digital logic circuits. This groundbreaking research paves the way for designing and fabricating high-performance 3D heterostructures, opening up exciting possibilities for diverse optoelectronic applications.
Article
One-Pot Synthesized Mixed-Dimensional Graphene for Photodetectors in Near-Infrared Image Sensors and Logic Circuits
https://doi.org/10.21203/rs.3.rs-4186645/v1
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Photodetectors have found extensive utility across various domains, including image sensing, communications, environmental monitoring, remote control, and autonomous driving.1-5 However, achieving optimal performance in near-infrared (NIR) optoelectronic devices necessitates meticulous optimization and a delicate balance between performance, reliability, and manufacturing cost. One promising avenue for enhancing photodetector performance lies in the construction of heterojunctions using two-dimensional (2D) materials.6-8 These heterojunctions can augment light absorption, electron transport, and separation, thereby broadening the spectral response range. Consequently, 2D materials have emerged as highly sought-after candidates for the next-generation optoelectronic devices.9-11 Despite extensive research on 2D material heterostructures, challenges persist in achieving optimal performance across various types of optoelectronic devices. Therefore, the selection of novel 2D materials and the construction of heterostructures play a pivotal role in enhancing device performance and enabling practical applications.
Graphene, as the earliest discovered 2D layered material, possesses exceptional electrical and optical properties, rendering it a favored material for broadband photodetection.12-13 However, its widespread application in NIR photodetectors is hindered by inherent defects such as weak light absorption capability (only 2.3%) and a short photocarrier lifetime.14-16 Recently, attention has shifted towards three-dimensional (3D) graphene, which is composed of stacked graphene layers, offers distinct advantages compared to traditional 2D-graphene.17-18 This unique structure imparts unparalleled physical and chemical properties, including high conductivity, carrier mobility, a large surface area, and a natural nano-resonant cavity, thereby significantly enhancing light absorption capacity and carrier generation in photodetectors.19-20 These attributes position 3D-graphene as a promising candidate for high-performance optoelectronic devices.
In contrast to traditional 2D layered materials, graphene quantum dots (GQDs) exhibit optical stability, low-cost manufacturing, tunable bandgaps, and high absorption coefficients.21-23 These qualities make GQDs ideal light-active materials in NIR photodetectors. Additionally, GQDs hold potential for large-scale solution processing, facilitating effective integration with other materials and substantially boosting photodetector performance.24 However, the existing preparation method for GQDs through solution processing may introduce impurities, and the application process involving drop coating may lead to agglomeration challenges.25-26
In our work, we have pioneered the development of a self-driven NIR photodetector by integrating nitrogen-doped graphene quantum dots (N-GQDs) with a 3D-graphene/germanium (Ge) heterostructure. The N-GQDs and 3D-graphene were directly synthesized on Ge wafer using plasma-assisted chemical vapor deposition (PACVD) in a one-pot process. These N-GQDs possess highly crystalline, stable, and low-defect characteristics, serving as the heterostructure without necessitating any post-growth transfer process. Remarkably, they exhibit excellent light absorption capabilities in the NIR spectra. Moreover, the integration of N-GQDs with 3D-graphene enables tunable modulation of the 3D-graphene Fermi energy levels, ensuring perfect alignment of the energy bands in the heterostructures. This confers the device with a potent built-in electric field, promoting abundant generation and highly efficient separation of photoinduced charge carriers. Consequently, the overall optoelectronic performance of the fabricated photodetector is significantly enhanced, displaying outstanding responsivity (2.3 × 103 A/W) and specific detectivity (6.2 × 1014 Jones) under 2200 nm illumination. Furthermore, the fabricated photodetector demonstrates remarkable imaging capabilities, capturing high-resolution images between 1850 and 2200 nm. Additionally, it exhibits high feasibility in optically controlled digital logic circuits. This research opens up exciting possibilities for designing and manufacturing high-performance 3D heterostructures, enabling diverse optoelectronic applications.
The schematic depicting the one-pot synthesis process for the preparation of the N-GQDs/3D-graphene/Ge heterostructure is depicted in Fig. 1a. Initially, 3D-graphene is grown on Ge, followed by the growth of N-GQDs on top of the 3D-graphene. Further details regarding the growth process can be found in the method section. After the initial step, the temperature is raised to 650 °C, and methane (CH4) and ammonia (NH3) are introduced for the growth of N-GQDs. This one-pot synthesis strategy offers improved control over temperature and growth processes for each layer compared to conventional methods, eliminating the need for sample removal. This reduces the potential introduction of defects during the post-growth transfer process. Additionally, N-GQDs synthesized through this method exhibit exceptional crystallinity, stability, and uniformity. Fig. 1b displays an atomic force microscope (AFM) image of the N-GQDs/3D-graphene Schottky junctions. The 3D-graphene exhibits a distinct 3D structure with a certain height in the vertical direction. Moreover, it possesses an exceptionally large surface area (Supplementary Fig. S1), effectively enhancing the light absorption capacity of the photodetector and promoting efficient carrier generation.27 Cross-sectional scanning electron microscopy (SEM) observations further confirm the structural characteristics of the 3D-graphene (Fig. 1c). The 3D-graphene exhibits a uniformly arranged vertical structure with a height of approximately 700 nm (Supplementary Fig. S2). The surface SEM image is depicted in Supplementary Fig. S3. Transmission electron microscope (TEM) images demonstrate the uniform distribution of N-GQDs (Fig. 1d). A high-resolution TEM (HR-TEM) image displays remarkably clear lattice edges, as shown in the inset of Fig. 1d, indicating that the diameter of the N-GQDs measures approximately 8 nm, consistent with the dimensions obtained from AFM imaging (Supplementary Fig. S4). Energy dispersive X-ray spectroscopy (EDS) tests confirm the uniform integration of N-GQDs with the 3D-graphene/Ge Schottky junctions, revealing the presence of carbon (C), N, and Ge elements (Supplementary Fig. S5). Raman scattering spectra of Ge, 3D-graphene/Ge, N-GQDs, and N-GQDs/3D-graphene/Ge were compared, demonstrating the uniform integration of N-GQDs with 3D-graphene/Ge Schottky junctions (Supplementary Figs. S6-S7). High-resolution X-ray photoelectron spectroscopy (XPS) spectra of C-1s and N-1s further characterize the chemical states of the N-GQDs/3D-graphene/Ge and 3D-graphene/Ge heterojunctions, revealing distinctive peaks corresponding to different C and N species (Fig. 1e, Supplementary Fig. S8).
The combination of 3D-graphene with N-GQDs significantly enhances the light utilization efficiency of the heterostructures. Finite-difference time-domain (FDTD) simulations were performed to analyze the localized electric field distribution in the N-GQDs/3D-graphene heterostructure (Fig. 1f, Supplementary Fig. S9). The results show significantly improved local electric field distribution in the N-GQDs/3D-graphene structures compared to the 3D-graphene structure. The formation of 3D “hot spot” gaps between the N-GQDs is revealed in Fig. 1g, indicating enhanced light absorption. Experimental tests confirm the significantly higher absorption of N-GQDs/3D-graphene compared to 3D-graphene (Supplementary Fig. S10).
Detailed theoretical calculations were conducted using the density functional theory (DFT) to investigate charge transfer within the heterojunction structures. The calculations reveal that the work function of the 3D-graphene/Ge heterojunction (Fig. 2a) is significantly lower compared to the N-GQDs/3D-graphene/Ge heterojunction (Fig. 2d). This indicates that the introduction of N-GQDs improves band alignment, reduces the influence of interface states, and enhances device performance.28 Supplementary Figs. S11-S12 provide models of the slab structures for 3D-graphene/Ge and N-GQDs/3D-graphene/Ge heterostructures, offering further insights. Supplementary Fig. S13 illustrates the difference in charge density between the two controlled heterojunctions, highlighting the charge rearrangement and transfer pathway within the structures. To further evaluate charge transfer between the two heterostructures, Bader charge analysis was performed, quantitatively analyzing the difference in electronic distribution before and after bonding with other atoms (Fig. 2b and Fig. 2e). The results indicate that the introduction of N-GQDs increases electron transfer from Ge to 3D-graphene. The band diagrams of the 3D-graphene/Ge heterojunction before and after introducing N-GQDs are plotted to explore the photoelectric charge transfer mechanism (Fig. 2c and Fig. 2f). After introducing N-GQDs, the Fermi level difference between 3D-graphene and Ge increases significantly. This is attributed to the different Fermi levels of 3D-graphene and N-GQDs. When they come into contact, electrons tend to transfer from 3D-graphene to N-GQDs to achieve the same Fermi level, resulting in a reduction of the Fermi level of 3D-graphene. As a result, the Fermi level difference between 3D-graphene and Ge expands, enhancing the built-in potential of the 3D-graphene/Ge heterojunction. Under light illumination, the built-in electric field efficiently and rapidly separates photogenerated carriers, thereby improving the optical response of heterojunction photodetectors.29 Furthermore, ultraviolet photoelectron spectroscopy (UPS) tests conducted on both heterostructures (Fig. 2g and Fig. 2h, Supplementary Fig. S14) validate the close alignment between experimentally obtained work functions and theoretical calculations.
To analyze the depletion layer size and charge transfer distribution in the two heterostructures, scanning Kelvin probe microscopy (SKPM) was employed to measure potential variations at their cross sections and interfaces (Supplementary Fig. S15). Fig. 3a illustrates the cross-sectional potentials of the 3D-graphene/Ge heterostructure (left) and the N-GQDs/3D-graphene/Ge heterostructure (right). A distinct potential drop is observed in the contact region between 3D-graphene and Ge (Fig. 3b), indicating the presence of a depletion layer caused by the recombination of electron-hole pairs, resulting in a reduction in charge density. Beyond this layer, the concentration of charge carriers remains relatively stable.30-31 Furthermore, the analysis of potential lines reveals that the depletion width of the N-GQDs/3D-graphene/Ge heterojunction exceeds that of the 3D-graphene/Ge heterojunction by 0.8 μm. This result highlights the significant extension of the depletion region due to the integration with N-GQDs. Fig. 3c presents the interfacial potential distributions of the 3D-graphene/Ge heterostructure (left) and N-GQDs/3D-graphene/Ge heterostructure (right) under light illumination. The distinct interface between 3D-graphene and Ge arises from their differing work functions. The Fermi levels difference (defined as ΔEf), extracted using the following equation:32-33
eP3D-graphene = Wtip - W3D-graphene (1)
ePGe = Wtip - WGe (2)
ΔEf = W3D-graphene - WGe = ePGe - eP3D-graphene (3)
where WGe, W3D-graphene, and Wtip are the work functions of Ge, 3D-graphene, and tip, respectively, e is the value of the electron charge, and PGe and P3D-graphene are the potential surface differences between the AFM tip and Ge and between the AFM tip and 3D-graphene, respectively. The ΔEf between the two heterostructures significantly increases from 102 meV (3D-graphene/Ge) to 139 meV (N-GQDs/3D-graphene/Ge) (Fig. 3d). This rise suggests that electrons in 3D-graphene tend to transfer into N-GQDs after their modification, thereby lowering the Fermi energy level of 3D-graphene. Consequently, the difference between the Fermi energy levels of 3D-graphene and Ge increases, indicating that the introduction of N-GQDs enhances the built-in potential in the 3D-graphene/Ge heterojunction. Experimental results confirm that the N-GQDs/3D-graphene/Ge heterostructure extends the depletion region, generating a robust built-in electric field. This facilitates more efficient dissociation and extraction of electrons and holes induced by photoexcitation under light, greatly enhancing the optoelectronic performance of the device.34 These findings align with previous theoretical calculations.
Conductive AFM (CAFM) was conducted to further validate the charge transfer in N-GQDs/3D-graphene/Ge heterojunctions (Fig. 3e and Fig. 3f). The experimental results demonstrate a significant increase in current under light conditions at zero bias voltage, indicating effective separation and transfer of numerous photogenerated charges. Furthermore, the uniform conductivity distribution across the sample surface in the CAFM images confirms the excellent conductivity of the N-GQDs/3D-graphene/Ge heterostructure. This uniformity is crucial for the consistency and reliability of the device. Importantly, the presence of current in the heterostructure even at zero bias voltage demonstrates its self-powered capabilities, enhancing device operability and integration.
Fig. 4 shows the performance of the as-fabricated photodetector. Fig. 4a presents the I-V curves measured under laser illuminated at various wavelengths (ranging from 440 nm to 2200 nm) with the same light intensity. The results demonstrate a decrease in the generated photocurrent as the wavelength increases, indicating the broad optical response capability of the as-fabricated photodetector based on N-GQDs/3D-graphene/Ge heterojunction across visible and NIR regions. To assess the stability and reversibility of the optical response under 2200 nm illumination at different power levels. Fig. 4b and Supplementary Fig. S16 present the corresponding results. Remarkably, the optical response remains detectable even under zero bias conditions, highlighting the self-powered characteristics of as-proposed photodetectors. The I-V curves of the as-fabricated photodetector in the dark and under 2200 nm illumination (power ranging from 0.25 to 1.25 µW) are shown in Supplementary Fig. S17. The as-fabricated photodetector exhibits excellent performance under 2200 nm illumination, with a specific detectivity (D*) of 1.5 × 1013 Jones and responsivity of 55.4 A/W at zero bias. Moreover, under -1 bias condition, the performance further improves, with a high D* and R of 6.21 × 1014 Jones and 2.3 × 103 A/W, respectively. These values surpass those of previously reported 2D heterojunction photodetectors (Supplementary Figs. S18-S19, Supplementary Table S1). R and D* are estimated by the equations:35-36
where Ip is the photocurrent, Id is the dark current, Pin is the intensity of the light, A is the effective area of the photodetector, B is the electrical bandwidth and in is the noise current (Fig. 4c). The response time of the as-fabricated photodetector is evaluated by testing the photo-response at different frequencies under 2200 nm illumination (Fig. 4d). The results demonstrate that the response is rapid and stable at both low and high frequencies, indicating excellent optoelectronic performance at different switching frequencies. The rise time (tr) and fall time (tf) are measured to be 82 µs and 93 µs, respectively, which are slightly slower than the devices with unmodified N-GQDs (tr/tf = 71/74 µs) (Fig. 4e). This discrepancy is attributed to the prolonged carrier lifetime resulting from the introduction of N-GQDs. The transient photocurrent (TPC) decay was examined before and after the introduction of N-GQDs into the 3D-graphene/Ge heterojunction, further confirming the extended carrier lifetime (Fig. 4f). The cut-off frequency of the as-fabricated photodetector, which determines its ability to respond to high-frequency signals, is also evaluated. The cut-off frequency at -3 dB was estimated to be 3.8 kHz (Fig. 4g). Apart from performance parameters, the long-term stability and reusability of the as-fabricated device are crucial factors to consider. The stability of the device under light conditions was evaluated over 100 cycles, demonstrating no significant degradation in performance (Fig. 4h). This indicates excellent long-term stability, enabling the device to maintain high-level performance even under continuous use. The uniformity of the device is also tested through light and dark current mapping plots (Supplementary Fig. S20). Additionally, the device was stored in a dry environment for up to 8 months and tested to assess its performance (Fig. 4i, Supplementary Fig. S21). The results show minimal changes, further confirming the excellent reusability and long-term stability of the photodetector.
To validate the functionality of the as-fabricated photodetector based on N-GQDs/3D-graphene/Ge heterostructure for image sensing, a single-pixel image sensor was designed and developed (Fig. 5a). The sensor setup comprises a mask plate with a pattern of either “G” or “r”, positioned between the illumination source and the photodetector. This mask plate is affixed to a motor-controlled x-y planar moving platform to ensure precise alignment of the photodetector unit with the center of the light source. Only the light passing through the hollow area of the mask is projected onto the pixels, while the rest of the pixels remain in a dark state without illumination. The current of each pixel was measured and plotted to generate a 2D comparative current map. Analyzing the photoelectric current of each pixel successfully yielded high-resolution images of the pattern “G” and “r” under illumination with wavelengths of 1850 nm and 2200 nm at zero bias (Fig. 5b and Fig. 5c). The high light-to-dark current ratio and rapid switching speed of the as-fabricated device enable accurate imaging of patterns. Extracting current levels along the lines in Fig. 5b and Fig. 5c facilitated the assessment of consistency between the scanned images and the target patterns (Fig. 5d). The rapid change in current values before and after passing through the image mask further demonstrates capability of the as-fabricated device for rapid light extraction and imaging over a wide dynamic range.
Furthermore, the application of the as-fabricated photodetectors based on N-GQDs/3D-graphene/Ge heterojunction in optically controlled digital logic circuits was explored. Optically controlled logic gates enable the interaction between optical and electrical signals, enhancing the integration density of logic circuits and expanding the multifunctionality of optoelectronic integrated circuits. A binary digital code is inputted into the signal generator, where a “1” represents the presence of light and a “0” denotes the absence of light. Corresponding optical signals are then directed into the photodetector through a modulated laser source (1850 nm and 2200 nm), converting the optical signal into an electrical current. Finally, an analog-to-digital converter efficiently transforms the analog signal into digital codes, effectively converting optical signals into digital codes (Fig. 5e). Thorough testing and analysis were conducted to assess the capability of device in converting optical signals to digital codes. The as-fabricated device accurately distinguishes the corresponding output signals, allowing for the precise extraction of the letter “G”, “r”, and “Gr” (Fig. 5f and Fig. 5g, Supplementary Fig. S22). These results demonstrate the device’s rapid and accurate performance in converting optical signals to digital codes.
In conclusion, this study successfully addresses the challenges of low light absorption efficiency in 2D materials and band alignment. It introduces the pioneering self-driven NIR photodetector based on N-GQDs/3D-graphene/Ge heterostructures. The combination of N-GQDs and 3D-graphene imparts high light absorption capabilities, while the well-aligned band structure of the heterostructure facilitates efficient electron-hole separation, thereby enhancing optoelectronic performance. The proposed photodetectors exhibit excellent responsivity (2.3 × 103 A/W) and specific detectivity (6.2 × 1014 Jones) at 2200 nm. Additionally, the photodetector demonstrates remarkable imaging capability, enabling high-resolution imaging within the NIR range. Furthermore, the feasibility of integrating this photodetector in optically controlled digital logic circuits is demonstrated. This work presents a highly promising approach for designing and fabricating high-performance 3D heterostructures for various optoelectronic applications.
Data availability.
The data that support the findings of this study are available from the corresponding author on reasonable request.
Acknowledgements
This study was supported by the National Natural Science Foundation of China (No. 62174093), the Ningbo Youth Science and Technology Innovation Leading Talent Project under Grant (No. 2023QL006), and the Open Research Fund of China National Key Laboratory of Materials for Integrated Circuits (No. NKLJC-K2023-01). L. Zheng acknowledges the support from Youth Innovation Promotion Association CAS and Shanghai Rising-Star Program (No. 21QA1410900). C. Ye acknowledges the support from Natural Science Foundation of Guangdong Province (No. 2022A1515110628), Guangdong Provincial Key Laboratory of Computational Science and Material Design (No. 2019B030301001). Computing resources were supported by Center for Computational Science and Engineering at Southern University of Science and Technology.
Author contributions
G. W., S. Z. conceived the project. S. Z., G. W., and L. Z. designed and performed the experiments. C. Y. performed DFT simulations. S. T. performed FDTD simulations. H. W., B. W., J. Z., S. L., G. Z., S. Y., G. D., and S. Z. performed the data analysis. S. Z., G. W., and L. Z. co-wrote the paper. All authors discussed the results and commented on the manuscript.
Conflicts of Interest
The authors declare no competing financial interests or non-financial interests.
Additional information
Supplementary Information is available for this paper.
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There is NO Competing Interest.