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Yang et al. Microstructures 2023;3:2023005 https://dx.doi.org/10.20517/microstructures.2022.24 Page 7 of 10

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+)/SiO (300 nm) substrates via the solution method. Then, two Ti/Au electrodes were
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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).