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Page 2 of 10 Yang et al. Microstructures 2023;3:2023005 https://dx.doi.org/10.20517/microstructures.2022.24

Keywords: Graphene, ZnO, positive photoconductance, negative photoconductance, broadband photodetector

INTRODUCTION
Photodetectors, which convert optical to electrical signals, play a key role in optoelectronic systems . In
[1-6]
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
[7-9]
sensing, optical communications, environmental monitoring, and so on . 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 MoS and so on [23,30-32] . NPC in graphene is related to surface adsorbents, which act as scattering
2,
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 . Therefore, UV-visible
[36]
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.