High-Performance Ultraviolet Photodetector Based on Graphene Quantum Dots Decorated ZnO Nanorods/GaN Film Isotype Heterojunctions

Abstract

A novel isotype heterojunction ultraviolet photodetector was fabricated by growing n-ZnO nanorod arrays on n-GaN thin films and then spin-coated with graphene quantum dots (GQDs). Exposed to UV illumination with a wavelength of 365 nm, the time-dependent photoresponse of the hybrid detectors manifests high sensitivity and consistent transients with a rise time of 100 ms and a decay time of 120 ms. Meanwhile, an ultra-high specific detectivity (up to ~ 10¹² Jones) and high photoresponsivity (up to 34 mA W^-1) are obtained at 10 V bias. Compared to the bare heterojunction detectors, the excellent performance of the GQDs decorated n-ZnO/n-GaN heterostructure is attributed to the efficient immobilization of GQDs on the ZnO nanorod arrays. GQDs were exploited as a light absorber and act like an electron donor to effectively improve the effective carrier concentration in interfacial junction. Moreover, appropriate energy band alignment in GQDs decorated ZnO/GaN hybrids can also be a potential factor in facilitating the UV-induced photocurrent and response speed.

Keywords: Graphene quantum dots; Heterojunction; UV photodetector; ZnO nanorod arrays.

Scheme 1

Schematic diagram of the fabrication process of the isotype heterojunction UV photodetector

Fig. 1

a The FE-SEM image of ZnO nanorod arrays grown over GaN film on Al₂O₃ substrate (45° tilted). b The cross-sectional FE-SEM image of the device. c The X-ray diffraction pattern of ZnO/GaN sample (inset: high resolution rocking curve of the (002) reflection resolving ZnO and GaN peaks). d Raman spectra of n-ZnO/n-GaN heterojunctions decorated with GQDs

Fig. 2

a TEM image (inset: size distribution of GQDs). b HRTEM image of GQDs. c UV-vis spectra and PL spectra of the GQDs (the excitation wavelength is 365 nm). d XPS survey spectra. e C 1s high-resolution XPS spectra. f O 1s high-resolution XPS spectra

Fig. 3

a TEM image of a representative GQDs/ZnO nanorod (inset: HRTEM image of the green circle in (a)). b UV-DRS absorption spectra of the GQDs/ZnO nanorods, bare ZnO nanorods, and PMMA

Fig. 4

a The I–V characteristic curves of the UV photodetectors under dark and UV light irradiation decorated with/without GQDs (inset: the magnified I–V characteristic curves of the UV photodetectors). b The I–V characteristic curves illuminated with UV light of different incident power densities (mW/cm²). c The photoresponse at different incident light power densities (mW/cm²). d The responsivity (red) and detectivity (blue) as a function of the incident light power density, respectively

Fig. 5

a The reproducible on/off switching of the device decorated with/without GQDs upon 365 nm light illumination with a 20-s cycle under 10 V bias, respectively. b The enlarged portions of the light-off to light-on and light-on to light-off transitions with/without GQDs decoration, respectively

Scheme 2

a The schematic diagrams of the ZnO NRs/GaN UV photodetector decorated without and with GQDs. b Energy band diagram of the GQD-ZnO NRs composite and its carrier transport mechanism in the interfacial region irradiated by UV light