Ultra‑Sensitive UV Photodetector Using Graphene Quantum Dot‑Decorated ZnO Nanorods on GaN Isotype Heterojunctions

Background

Ultraviolet (UV) photodetectors are essential for missile launch detection, space exploration, environmental monitoring, UV calibration, and optical communication. Wide‑bandgap semiconductors such as GaN, CdS, ZnO, Ga₂O₃, ZnS, and SiC are commonly employed due to their strong UV absorption. ZnO, with a bandgap of ~3.37 eV and a high exciton binding energy (~60 meV) at room temperature, has become a focus of research for short‑wavelength optoelectronics.

ZnO‑based UV photodetectors have been explored using single crystals, thin films, and nanostructures. One‑dimensional ZnO architectures, including nanorods and nanowires, enhance photodetection by improving carrier transport and surface interaction. Moreover, heterostructures—such as n‑ZnO/n‑GaN—isotype junctions—benefit from lattice matching and comparable bandgaps (3.37 eV for ZnO and 3.39 eV for GaN), leading to efficient carrier generation under illumination.

Quantum dots (QDs) are frequently integrated with ZnO to increase charge separation and transport. Graphene quantum dots (GQDs), with size‑dependent bandgaps and strong optical absorption, have emerged as promising light‑absorbing materials for broadband photodetectors and photovoltaics. Prior work has shown that GQDs enhance photocurrent and response speed when decorated on ZnO nanorods or polymer Schottky junctions. However, the influence of GQDs on n‑ZnO/n‑GaN isotype heterojunctions has not been reported.

In this study, we present an n‑ZnO/n‑GaN isotype heterojunction UV photodetector decorated with GQDs, fabricated via a straightforward method. The GQD‑decorated device exhibits markedly higher photocurrent and improved reproducibility compared to the bare heterojunction, owing to the synergistic interaction between ZnO, GaN, and GQDs.

Methods/Experimental

Preparation of n‑ZnO/n‑GaN Heterojunction

Analytical‑grade reagents from Sigma‑Aldrich were used without further purification. n‑GaN thin films were deposited on Al₂O₃ substrates by metal‑organic chemical vapor deposition (MOCVD). n‑ZnO nanorod arrays were then grown directly on the GaN film via a hydrothermal process. An aqueous solution containing 0.025 M zinc acetate dihydrate and 0.025 M hexamethylene tetramine was transferred to a Teflon‑lined autoclave and heated at 95 °C for 12 h. After cooling, the samples were rinsed with deionized water and dried in air.

Synthesis of GQDs

GQDs were prepared by hydrothermal carbonization of citric acid (CA) in alkaline conditions. A mixture of 0.21 g (1 mmol) CA and 0.12 g (3 mmol) NaOH was dissolved in 5 mL water, stirred to clear, and placed in a 20‑mL Teflon‑lined autoclave. The solution was heated to 160 °C for 4 h, then cooled. GQDs were isolated by adding ethanol, centrifuging at 10 000 rpm for 5 min, and ultrasonically cleaning with ethanol three times. The resulting solid dispersed readily in water.

Fabrication of UV Photodetector

The Al₂O₃ substrate with the n‑ZnO/n‑GaN heterojunction was cleaned with deionized water and ethanol, then dried at 60 °C. GQDs were spin‑coated onto the heterojunction, followed by a PMMA layer and inductively coupled plasma (ICP) etching. The device was capped with indium tin oxide (ITO) and an Ag electrode was applied to GaN for Ohmic contact. The effective area of the heterojunction is ~5 × 5 mm². A schematic of the fabrication process is shown in Scheme 1.

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

Characterization

Field‑emission scanning electron microscopy (FE‑SEM) examined the ZnO nanorod morphology. High‑resolution transmission electron microscopy (HRTEM) evaluated GQD size distribution. UV‑vis spectra were recorded on a Lambda 25 spectrophotometer. Photoluminescence (PL) was measured with a Shimadzu RF‑5301 Fluorescence spectrometer. X‑ray photoelectron spectroscopy (XPS) used a ThermoFisher‑250XI with Al Kα radiation. X‑ray diffraction (XRD) employed a Bruker D8 Advance. Raman spectra were obtained with a PerkinElmer 400F. Photocurrent measurements used a Keithley 4200 system under a 300 mW cm⁻² Xenon lamp (365 nm).

Results and Discussions

Figure 1a shows the SEM image of uniformly grown ZnO nanorods across the Al₂O₃ substrate coated with GaN. Cross‑sectional SEM (Figure 1b) reveals layer thicknesses of 20 µm (substrate), 6 µm (GaN), and 4 µm (ZnO). XRD (Figure 1c) displays merged (002) peaks for GaN and ZnO, confirming wurtzite structures and growth along the [001] direction. Raman spectra (Figure 1d) exhibit D and G bands at ~1360 cm⁻¹ and ~1600 cm⁻¹, indicative of sp² graphitic structure and edge defects in GQDs.

a FE‑SEM image of ZnO nanorod arrays on GaN/Al₂O₃ (45° tilt). b Cross‑sectional FE‑SEM. c XRD pattern with high‑resolution rocking curve (inset). d Raman spectra of GQDs‑decorated ZnO/GaN.

Figure 2a,b present TEM and HRTEM images of GQDs, revealing a uniform size distribution with an average lateral dimension of 3.0 ± 0.6 nm and a lattice fringe of 0.21 nm. UV‑vis (Figure 2c) shows a pronounced π–π* peak at 240 nm and a shoulder near 310 nm (n–π* of C=O). PL peaks at 442 nm arise from π→π* transitions. XPS survey (Figure 2d) confirms C 1s at 284.5 eV and O 1s at 531.4 eV; high‑resolution spectra (Figures 2e,f) reveal C=C (284.8 eV) and O=C–O (288.7 eV) bonds, consistent with aromatic GQDs.

a TEM image (size distribution inset). b HRTEM. c UV‑vis and PL spectra (excitation 365 nm). d XPS survey. e C 1s. f O 1s.

Figure 3a demonstrates uniform GQD decoration on ZnO nanorods. UV‑diffuse reflectance spectra (Figure 3b) reveal ~20 % higher UV absorption for GQD‑decorated nanorods compared to bare ZnO, enhancing photoresponse. PMMA contributes negligible absorption in the 300–350 nm range, so its effect on device performance is minimal.

a TEM of a representative GQD/ZnO nanorod (inset: HRTEM). b UV‑DRS of GQD/ZnO, bare ZnO, and PMMA.

Figure 4a,b plot I–V characteristics in dark and under 365 nm illumination. The dark current remains low, with a slight increase after GQD coating. Under UV, the GQD‑decorated device exhibits a photocurrent of 0.4 mA at 1.5 V—over 40× higher than its dark current—while the bare device shows negligible change.

a I–V curves under dark and UV (with/without GQDs, inset). b I–V under varying UV power densities. c Photoresponse vs. incident power density. d Responsivity (red) and detectivity (blue) vs. power density.

Under 10 V bias, the time‑dependent photocurrent (Figure 5a) shows a rapid rise and fall, indicating fast response. Rise times are 100 ms (GQD) vs. 260 ms (bare); decay times are 120 ms (GQD) vs. 250 ms (bare). Responsivity values at 25–120 mW cm⁻² range from 34 to 12.9 mA W⁻¹. The maximum detectivity reaches ~10¹² Jones, surpassing many ZnO‑based detectors.

a Reproducible on/off switching under 365 nm light (20 s cycles, 10 V). b Enlarged transition curves (with/without GQDs).

Scheme 2 illustrates the photoresponse mechanism. Surface oxygen adsorption forms a depletion layer; UV illumination generates electron–hole pairs, and photogenerated holes neutralize surface‑adsorbed oxygen, reducing the depletion width and increasing conductivity. GQDs enhance electron injection into ZnO, further raising carrier density and accelerating response.

a ZnO/GaN UV photodetector with/without GQDs. b Band diagram of GQD/ZnO/GaN composite and carrier transport under UV.

Conclusions

GQD decoration on n‑ZnO/n‑GaN isotype heterojunctions markedly improves photocurrent and response speed under UV illumination. The hybrid device reaches a photocurrent of 0.4 mA at 1.5 V—over 40× the dark current—and exhibits millisecond‑scale switching. The performance gains arise from efficient GQD immobilization, which serves as a light absorber and electron donor, and from favorable band alignment that facilitates rapid carrier separation. This work demonstrates the potential of GQD‑sensitized heterostructures for high‑performance UV photodetectors and suggests a pathway for future multi‑composite optoelectronic devices.

Abbreviations

FE-SEM

Field‑emission scanning electron microscope

GQDs

Graphene quantum dots

HR-TEM

High‑resolution transmission electron microscopy

ICP

Inductively coupled plasma

ITO

Indium tin oxide

MOCVD

Metal‑organic chemical vapor deposition

PMMA

Polymethylmethacrylate

XPS

X‑ray photoelectron spectroscopy

XRD

X‑ray diffractometer

ZNRA

ZnO nanorod array