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Microstructures 2023;3:2023005. 10.20517/microstructures.2022.24 © The Author(s) 2023.
Open Access Research Article

An ultraviolet-visible distinguishable broadband photodetector based on the positive and negative photoconductance effects of a graphene/ZnO quantum dot heterostructure

Henan Key Laboratory of Diamond Optoelectronic Materials and Devices, Key Laboratory of Material Physics, Ministry of Education, School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450052, Henan, China.

#Authors contributed equally.

Correspondence to: Dr. Shaobo Cheng, Henan Key Laboratory of Diamond Optoelectronic Materials and Devices, Key Laboratory of Material Physics, Ministry of Education, School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450052, Henan, China. E-mail: chengshaobo@zzu.edu.cn ; Dr. Chong-Xin Shan, Henan Key Laboratory of Diamond Optoelectronic Materials and Devices, Key Laboratory of Material Physics, Ministry of Education, School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450052, Henan, China. E-mail: cxshan@zzu.edu.cn

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    Academic Editor: Zibin Chen | Copy Editor: Fangling Lan | Production Editor: Fangling Lan

    © The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, sharing, adaptation, distribution and reproduction in any medium or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

    Abstract

    Broadband photodetectors covering the ultraviolet (UV) to visible range are significant for applications in communication and imaging. Broadband photodetectors with the capacity to distinguish wavelength bands are highly desirable because they can provide additional spectral information. Herein, we report a UV-visible distinguishable broadband photodetector based on a graphene/ZnO quantum dot heterostructure. The photodetector exhibits negative photoconductance under visible illumination because the adsorbents on graphene act as scattering centers to reduce the carrier mobility. In contrast, under UV illumination, the photodetector shows positive photoconductance as the photogenerated electrons in the ZnO quantum dots transfer to the graphene, thereby increasing the conductivity. Thus, the detection and distinction of UV and visible illumination can be realized by utilizing the opposing photoconductivity changes. These results offer inspiration for the design of multifunctional broadband photodetectors.

    INTRODUCTION

    Photodetectors, which convert optical to electrical signals, play a key role in optoelectronic systems[1-6]. In contrast to photodetectors that respond only to a specific wavelength range, broadband photodetectors can detect light over a broad spectral range and it is therefore essential to various techniques, including image sensing, optical communications, environmental monitoring, and so on[7-9]. Broadband photodetectors responding to light over a wide spectral range from ultraviolet (UV) to infrared have been developed from a variety of materials, such as two-dimensional (2D) materials, perovskites, organic semiconductors, and so on. However, the specific band of the incident light usually cannot be distinguished[10-13]. Therefore, broadband photodetectors with the capacity to distinguish different wavelength bands are highly desirable and can provide additional spectral information. For instance, UV-visible distinguishable broadband photodetectors are required for astronomical detection, information storage and other applications[1,14-16]. However, it is challenging to detect and distinguish UV and visible light using a single photodetector.

    Graphene is a promising material for broadband photodetectors due to its wide-range absorption spectrum and high carrier mobility[17-22]. More interestingly, both positive and negative photoconductance responses have been reported for graphene photodetectors[23-25]. Ordinarily, light with photon energy larger than the band gap generates carriers in the valence and conduction bands of a semiconductor material, thereby increasing the conductivity. Such positive photoconductance (PPC) has been observed in most photodetectors[26,27]. In contrast to PPC, the conductivity in some low-dimensional materials may decrease under illumination, i.e., negative photoconductance (NPC)[24,28-30]. This abnormal phenomenon has been observed in photodetectors based on graphene, InAs nanowires, ZnS nanoparticles, carbon nanotubes, monolayer MoS2, and so on[23,30-32]. NPC in graphene is related to surface adsorbents, which act as scattering centers and decrease the carrier mobility under light illumination, leading to a decrease in conductivity[33,34]. In contrast, graphene photodetectors with PPC and high responsivity can also be obtained from van der Waals heterostructures composed of graphene and other materials[7,35]. The opposing photoconductivity changes of photodetectors with PPC and NPC can be easily distinguished[36]. Therefore, UV-visible distinguishable broadband photodetectors may be realized by integrating the two different response mechanisms in the same photodetector.

    Here, we present a UV-visible distinguishable photodetector composed of graphene and zinc oxide (ZnO) quantum dots (QDs). Bare graphene shows NPC under illumination from the UV to visible region. To make the response in the UV and visible region distinguishable, ZnO QDs are coated onto graphene to convert the NPC response under UV illumination to a PPC response. ZnO is chosen to absorb UV illumination due to its wide bandgap (3.3 eV, ~376 nm), low cost and abundant nanostructures[37-42]. Moreover, given that the surface states of ZnO are sensitive to the environment, the applications of ZnO photodetectors may further be extended to the chemical, medical and biological fields[43-48]. In the graphene/ZnO QD van der Waals heterostructure, electrons generated in the ZnO QDs by UV light can transfer to the graphene and enhance its conductivity, resulting in PPC in the UV region. Furthermore, the graphene/ZnO QD photodetector retains the NPC response in the visible region because the ZnO QDs do not absorb visible light. In this context, the graphene/ZnO QD photodetector shows PPC under UV light and NPC under visible light, thereby realizing the detection and distinction of UV and visible illumination simultaneously.

    RESULTS AND DISCUSSION

    Before investigating the spectral response characteristics of the graphene/ZnO QD photodetector, we first study the photoresponsive behavior of the bare graphene device. A schematic diagram of the bare graphene photodetector is displayed in Figure 1A. In this structure, graphene functions as the charge transport channel as well as the light-absorbing medium. The Raman spectrum of graphene is shown in Figure 1B. The two predominant peaks located at 1583 and 2680 cm-1 can be attributed to the graphene G- and 2D-bands, respectively. The small peak at 1344 cm-1 is the D-band Raman peak of graphene, related to defects or disorders in the hybrid structure[49,50]. The intensity ratio between the 2D and G peaks is ~0.6, indicating that the graphene is bilayer[51]. Figure 1C demonstrates the photoresponsive behavior of the bare graphene device under UV and visible illumination at a source-drain voltage (VDS) of 0.2 V and a gate voltage (VG) of 0 V. Under illumination at all wavelengths, the source-drain current (IDS) gradually decreases within 1000 s, indicating persistent NPC. The persistent NPC in graphene derives from the adsorbents (O2 or OH- groups) at its surface, which induce p-type conductance and act as scattering centers under illumination, thereby reducing its conductance[24,52]. The decrease in free carrier mobility of graphene under illumination can also be confirmed by Hall measurements. In dark conditions, the hole mobility of graphene is 1050 cm2 V-1 s-1, which decreases to 910 cm2 V-1 s-1 under 365 nm UV illumination. After the light is switched off, the IDS recovers slowly. For instance, after 4000 s, the current under 365 nm illumination only recovers to 24%[53]. It is notable that both the saturated current and transition time vary with the incident light wavelength, which is related to the wavelength-dependent absorption characteristics of graphene[24,54]. Figure 1D shows the responsivity of the graphene photodetector vs. the excitation wavelength. The responsivity is defined as R = (Iph-Idark)/(Pph*S), where Iph is IDS under illumination, Idark is IDS in the dark environment, Pph is the incident light intensity density and S is the active area of the device. The negative responsivity values represent the NPC behavior of the photodetector. The absolute value of the responsivity increases as the excitation wavelength decreases.

    Figure 1. Bare graphene photodetector. (A) Schematic diagram of device structure (device area is 100 µm in length and 1200 µm in width defined as the area between the two electrodes). (B) Raman spectrum of graphene. (C) Photoelectric on/off response under illumination in the UV to visible range (VDS = 0.2 V and VG = 0 V). (D) Responsivity at different wavelengths.

    To make the response of the photodetector to UV illumination distinguishable from that to visible illumination, the graphene is coated with ZnO QDs to form a graphene/ZnO QD photodetector, as shown in Figure 2A. Figure 2B presents the transmission electron microscopy (TEM) image of the ZnO QDs, which show a typical diameter of ~5 nm. The HRTEM image clearly identifies lattice fringes with a spacing of 0.26 nm, corresponding to the (002) plane of hexagonal wurtzite ZnO[55]. Figure 2C shows the XRD pattern of the ZnO QDs. There are six characteristic peaks, corresponding to the (100), (101), (102), (110), (103) and (200) planes of the hexagonal wurtzite structure of the ZnO QDs (P63mc, a = b = 0.325 nm, c = 0.521 nm, JCPDS Card No. 80-007). Figure 2D-F show the XPS survey, O 1s and Zn 2p spectra of the ZnO QDs. The O 1s spectrum is composed of three peaks at 530.8, 532.0 and 534.0 eV, corresponding to Zn-O, O-C=O and C-O-C/OH bonds, respectively[56]. The XPS spectrum for Zn 2p shows two peaks at 1023.4 and 1046.5 eV, which can be attributed to Zn 2p3/2 and Zn 2p1/2, respectively, thereby confirming the divalent state of Zn. Figure 2G shows the Raman spectrum of the graphene/ZnO QDs. In addition to the peaks from graphene, there are three obvious peaks at 99, 418 and 640 cm-1, which derive from the E2(low), E1(TO) and multi-phonon modes of ZnO, respectively[57,58]. Figure 2H presents the absorption of the graphene/ZnO QDs, which show an absorption edge at ~376 nm, corresponding to the bandgap of ZnO. The absorption in the visible region is due to scattering by the ZnO QDs.

    Figure 2. Graphene/ZnO QD heterostructure. (A) Schematic diagram of graphene/ZnO QD heterostructure. (B) TEM image of ZnO QDs. Inset shows HRTEM image of a single ZnO QD. (C) XRD pattern of ZnO QDs. (D) XPS survey spectrum and high-resolution spectra of (E) O 1s and (F) Zn 2p for ZnO QDs. (G) Raman and (H) absorption spectra of graphene/ZnO QDs.

    Figure 3A shows the response characteristics of the graphene/ZnO QD photodetector under UV (upper panel) and visible (lower panel) illumination. NPC behavior can be observed under visible illumination, similar to that of the bare graphene photodetector. However, under UV illumination, the IDS increases gradually, indicating PPC response to UV illumination. After the light is switched off, the IDS recovers to its original level slowly. The current under 650 nm illumination recovers to 21% after 1400 s, while the current under 313 nm illumination recovers to 4% after 700 s. The opposing photoconductance changes under UV and visible illumination confirm the spectrum-selective detection of the graphene/ZnO QD device. Figure 3B and C show the time-dependent response curves of the graphene/ZnO QD photodetector under 365 and 460 nm illumination, respectively. The light sources are turned on and off periodically to investigate the reproducibility of the photoresponse. Both the PPC and NPC responses are repeatable. However, the baselines change slightly after multicycle testing, which may derive from catalytic modification of the graphene.

    Figure 3. Graphene/ZnO QD photodetector. (A) Photocurrent response under UV and visible illumination (VDS = 0.2 V and VG = 0 V). Time-dependent photoresponse of photodetector under (B) 460 and (C) 365 nm illumination (VDS = 0.2 V and VG = 0 V).

    Figure 4A and B plot the responsivity of the graphene/ZnO QD photodetector as a function of light intensity. The responsivity decreases with the light intensity and the data can be fitted using REβ-1, where E is the light intensity and β is a constant[25]. The constant β values under visible (460 nm) and UV (365 nm) illumination are fitted to be 0.46 and 0.12, respectively. The different β values suggest different trap states for NPC under visible illumination and PPC under UV illumination[59].

    Figure 4. Responsivity of graphene/ZnO QD photodetector as a function of light intensity for (A) 460 and (B) 365 nm illumination.

    To obtain a better understanding of the response mechanism of the graphene/ZnO QD photodetector, we measured the transfer curves under UV (365 nm) and visible (460 nm) illumination, as shown in Figure 5A and B, respectively. In dark conditions, the transfer curve shows typical ambipolar transfer characteristics with the Dirac point at 1.7 V, due to the p-type doping effects from surface adsorbents. Under 460 nm illumination, the transfer curve shifts down, confirming the NPC response of the photodetector. Moreover, the Dirac point shifts gradually to 2.0 V with the incident light intensity increasing to 128 μW/cm2, indicating that the photogenerated electrons are trapped, which is also regarded as the photogating effect[59]. In contrast, the Dirac point moves to about -0.9 V under 365 nm illumination. The negative Dirac point indicates that the p-type conductivity of graphene turns into n-type conductivity because the photogenerated electrons in the ZnO QDs transfer to the graphene[42,60]. The electron transfer mechanism can be understood using the schematic diagram in Figure 5C. Under illumination with photon energy larger than the bandgap of ZnO, electrons and holes are generated in the ZnO QDs. The photogenerated holes are trapped by surface hole traps formed by adsorbed oxygen ions, which are sufficient due to the high surface-to-volume ratio of the ZnO QDs[15,42,61,62]. Furthermore, the photogenerated electrons transfer to the graphene, which are numerous enough to compensate for the holes in the graphene and convert the p- to n-type conductivity. Hall measurements also show that the hole sheet density and mobility of graphene coated with ZnO QDs in the dark are 6.39 × 1012 cm-2 and 360 cm2 V-1 s-1, respectively. The carrier mobility is smaller than that of the bare graphene (1050 cm2 V-1 s-1) due to scattering by the ZnO QDs. Under the illumination of 365 nm light with an intensity of 20 μW/cm2, the graphene exhibits n-type conductivity with an electron sheet density of 1.6 × 1013 cm-2 and a mobility of 150 cm2 V-1 s-1. The significant increase in carrier density under UV illumination results in an increase in conductivity, namely, PPC.

    Figure 5. Photoresponsive behavior of graphene/ZnO QD photodetector. Transfer curves under (A) 460 and (B) 365 nm illumination (VDS = 0.2 V). (C) Schematic diagram of charge transfer process between ZnO QDs and graphene. (D) Responsivity spectrum of graphene/ZnO QD photodetector and absorption spectrum of ZnO QDs.

    Figure 5D shows the responsivity spectrum of the graphene/ZnO QD photodetector accompanied by the absorption spectrum of the ZnO QDs. The absorption edge of the ZnO QDs (376 nm) is denoted by the vertical dashed line. It is apparent that the graphene/ZnO QD photodetector shows PPC under illumination with photon energy larger than the bandgap of the ZnO QDs, because a large amount of photon-generated electrons transfer from the ZnO QDs to the graphene, thereby increasing the conductivity. Furthermore, under illumination with photon energy smaller than the bandgap of the ZnO QDs, photoinduced impurity scattering plays the leading role, resulting in NPC. Thus, in this context, the NPC response of the graphene for UV illumination can be converted to a PPC response, thereby realizing the detection and distinction of UV and visible light using a single graphene/ZnO QD photodetector.

    CONCLUSIONS

    In conclusion, we have demonstrated a UV-visible distinguishable broadband photodetector utilizing the NPC and PPC response in a graphene/ZnO QD heterostructure. The photoresponsive mechanism of the photodetector under visible illumination is attributed to adsorbents on the graphene, which act as scattering centers in illumination conditions to decrease the conductivity. In contrast, under UV illumination, the photogenerated electrons in the ZnO QDs could transfer to the graphene, leading to an increase in conductivity. Thus, the current of the graphene/ZnO QDs photodetector decreases under visible illumination and increases under UV illumination, which can be used to detect and distinguish UV and visible illumination. Our results may expand the application area of broadband photodetectors.

    MATERIALS AND METHODS

    Device Fabrication: Graphene was grown on copper foils via a chemical vapor deposition method and transferred to Si(n+)/SiO2 (300 nm) substrates via the solution method. Then, two Ti/Au electrodes were prepared onto the graphene by thermal evaporation using a mask. Another Ti/Au electrode was deposited on the Si substrate as a gate electrode [Figure 1A]. The device active area is 100 µm in length and 1200 µm in width defined as the area between the two electrodes. ZnO QDs were synthesized by a traditional sol-gel method using zinc acetate and KOH as reactants. To fabricate the graphene/ZnO QD photodetector, the ZnO QDs dispersed in an ethanol solution (2 mg/mL) were spin-coated over the graphene at 2000 rpm for 20 s and baked at 70 °C for 10 min. This process was repeated three times to ensure that the graphene was covered by sufficient ZnO QDs. The final thickness of the ZnO QD film is ~120 nm.

    Device Characterization: The electrical transport characteristics of the device were studied using a semiconductor characterization system (Keithley 4200-SCS). Handhold lasers and LEDs with different wavelengths were employed as the light sources. The X-ray diffraction (XRD) pattern of the ZnO QDs was obtained using a diffractometer (X’Pert Pro, PANalytical). The Raman spectra were recorded using an SOL instrument spectrometer (Confotec MR520) using a 532-nm laser as the excitation source. X-ray photoelectron spectroscopy (XPS) spectra were carried out using an XPS spectrometer (Thermo ESCALAB 250) and calibrated based on the C 1s peak at 284.8 eV. The absorption spectra of the ZnO QDs were measured using a spectrophotometer (Hitachi UH4150). The morphology of the ZnO QDs was investigated using transmission electron microscopy (TEM, JEOL 2100). All the photoresponsive measurements were carried out at room temperature and in air.

    DECLARATIONS

    Authors’ contributions

    Design, writing review and editing: Yang X, Cheng S, Shan CX

    Data analysis: Yang X, Yang XG, Cheng S

    Data acquisition: Yang X, Wang CJ

    Sample fabrication: Wang CJ, Zang JH

    Availability of data and materials

    Not applicable.

    Financial support and sponsorship

    This work was supported by National Natural Science Foundation of China (Nos. 62271450, 12174348) and Henan Center for Outstanding Overseas Scientists (No. GZS201903).

    Conflicts of interest

    All authors declared that there are no conflicts of interest.

    Ethical approval and consent to participate

    Not applicable.

    Consent for publication

    Not applicable.

    Copyright

    © The Author(s) 2023.

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    Cite This Article

    OAE Style

    Yang X, Wang CJ, Cheng S, Yang XG, Zang JH, Shan CX. An ultraviolet-visible distinguishable broadband photodetector based on the positive and negative photoconductance effects of a graphene/ZnO quantum dot heterostructure. Microstructures 2023;3:2023005. http://dx.doi.org/10.20517/microstructures.2022.24

    AMA Style

    Yang X, Wang CJ, Cheng S, Yang XG, Zang JH, Shan CX. An ultraviolet-visible distinguishable broadband photodetector based on the positive and negative photoconductance effects of a graphene/ZnO quantum dot heterostructure. Microstructures. 2023; 3(1):2023005. http://dx.doi.org/10.20517/microstructures.2022.24

    Chicago/Turabian Style

    Yang, Xun, Chao-Jun Wang, Shaobo Cheng, Xi-Gui Yang, Jin-Hao Zang, Chong-Xin Shan. 2023. "An ultraviolet-visible distinguishable broadband photodetector based on the positive and negative photoconductance effects of a graphene/ZnO quantum dot heterostructure" Microstructures. 3, no.1: 2023005. http://dx.doi.org/10.20517/microstructures.2022.24

    ACS Style

    Yang, X.; Wang C.J.; Cheng S.; Yang X.G.; Zang J.H.; Shan C.X. An ultraviolet-visible distinguishable broadband photodetector based on the positive and negative photoconductance effects of a graphene/ZnO quantum dot heterostructure. Microstructures. 20233, 2023005. http://dx.doi.org/10.20517/microstructures.2022.24

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