In recent years, the demand of energy-efficient solar-blind photodetectors arrays, which are suitable for large-scale manufacturing, integration, and low energy consumption, has grown significantly due to advancements in applications such as Internet of Things (IoT) sensors, autonomous driving, biosensors, smart medical systems, and other fields [1–7]. Gallium oxide (Ga2O3), known for its direct and wide bandgap (4.4–5.3 eV), high critical breakdown field strength (8 MV/cm), and excellent stability, has emerged as one of most competitive and prospective materials in this field [8–10]. Recent advancements have indeed shown promising developments in flexible array-type solar-blind photodetectors utilizing Ga2O3 film, achieved through techniques such as deposition on flexible substrates or supplementary methods like spray coating and transfer [11–13]. Nevertheless, despite these strides, Ga2O3 film often exhibits poor crystalline quality and a high density of internal defects during growth, which limits their overall performance and efficiency in practical applications [5,14]. Moreover, challenges such as lattice mismatch, film discontinuities, and inadequate mechanical strength pose additional hurdles. These issues could lead to microcracks and material degradation when subjected to prolonged bending or stress [15,16], thereby compromising the stability and reliability required for applications like flexible electronics, sensors, and optoelectronic devices.

To address these challenges and propel technology forward, current research is focusing on exploiting low-dimensional Ga2O3 micro/nanostructures [17–21]. These materials, including microwires, microbelts, nanowires, nanosheets, and nanocrystals, offer unique advantages such as quantum effects, surface/interface properties, multifunctionality, tunability, flexibility, and integration capabilities, making them alternative choices for developing high-performance, large-area, array, and flexible optoelectronic devices [22–25]. For instance, monocrystalline Ga2O3 microstructures are extensively employed in producing the single microdevice [26–28]. However, challenges remain in ensuring consistent performance across individual units and in the complex device fabrication processes, which hinder the arraying and large-scale integration of Ga2O3 microstructures [1,29]. Ga2O3 nanostructures, such as an individual nanowire and nanowire arrays, are also crucial components of high-performance optoelectronic detectors [30–32]. Yet, previously reported Ga2O3 nanowire arrays typically exhibit random distribution and disorderly arrangement. This randomness not only increases the complexity of integration and control, but also may lead to device performance variability and instability, making it challenging to accurately predict and manipulate the performance of devices or sensors. Developing highly ordered arrays of Ga2O3 nanowires is particularly crucial for making improvements, yet there are currently no relevant reports on this topic [33]. Furthermore, Ga2O3 nanocrystals typically exhibit poorer stability compared to large crystals, particularly under prolonged usage or extreme environmental conditions, thereby potentially leading to structural change, phase transition, or oxidation that affects long-term stability and reliability [34,35]. Therefore, achieving the synthesis of low-dimensional Ga2O3 micro/nanostructures with appropriate size, scalable production capability, and high crystalline quality, which meets the requirements of future integrated, large-array, and wearable electronic devices, poses a challenging and long-term endeavor.

In this study, we successfully synthesized high-quality monocrystalline Ga2O3 nanorods using a hydrothermal approach. Subsequently, a high-performance solar-blind ultraviolet heterojunction detector array was experimentally exhibited, which contained Pt nanoparticles-modified Ga2O3 nanorod film (PtNPs@Ga2O3) assembling on p-GaN film. The modulation effect of PtNPs on the photovoltaic properties of Ga2O3 nanorod films and detectors was scientifically investigated. The optimized PtNPs@Ga2O3/GaN heterojunction detector units exhibit impressive performance upon solar-blind ultraviolet radiation in self-operating mode. Besides, the PtNPs@Ga2O3/GaN detector arrays had a very high uniformity, which led to satisfactory results of the detector arrays for image sensing applications. This study offers new insights into the design and fabrication of large-scale, array, and integrated optoelectronic devices based on low-dimensional Ga2O3 nanorods.

The preparation of a growth solution by dissolving 0.3 mol of hydrated gallium (III) nitrate [Ga(NO3)3·nH2O] in 100 mL of deionized water was achieved. The solution was transferred to an oil bath and then heated up to 150°C. While maintaining the growth solution at 150°C, its pH was modulated to 3 using ammonium hydroxide (NH4OH). Continuously heating the solution for 4 h, the precipitation of white gallium hydroxide (GaOOH) nanorods can be realized. The solution cooled naturally to room temperature, and then we precipitated the filter and washed it with deionized water. The samples were cleaned and dried in an oven at 70°C for 5 h under ambient atmosphere conditions. Finally, the dried GaOOH nanorods were calcined at 400°C and 800°C for 3 h each to obtain Ga2O3 nanorods powder. The Ga2O3 nanorod powder can be mixed using ethanol in a 1:5 volume ratio and dispersed by ultrasonication to obtain a uniform colloidal ethanol solution of Ga2O3 nanorods [36,37].

(1) Initially, the p-GaN film prepared on a sapphire substrate was sliced into 1cm×1cm pieces, and 50 nm/55 nm Ni/Au nanofilms were deposited using electron beam evaporation on one side. The sample was annealed to establish Ohmic contact between the Ni/Au electrode and p-GaN film. (2) A tailored PDMS mold featuring an 8×8 array of window apertures with 500μm×500μm was used as mask plate. After affixing the mold onto the p-GaN film, a Ga2O3 nanorod film colloidal ethanol solution was deposited onto the GaN substrate using spin-coating at 500 r/min for 20 s. The specimens were then baked for about 30 min on a 150°C hot plate. (3) Radio-frequency (RF) magnetron sputtering technology was utilized to sputter a dense Pt nanofilm onto the pre-prepared Ga2O3 nanorod film, with a sputtering duration of 80 s. The samples were subsequently moved to a tube furnace and annealed for 30 min at 600°C in an argon atmosphere, resulting in a hybrid structure of Pt nanoparticles-modified Ga2O3 (PtNPs@Ga2O3) nanorods [38]. (4) T3C2Tx (MXene) solution (0.5 mg/mL) was spin-coated onto the PtNPs@Ga2O3 nanorods film and dried in an inert atmosphere [28,39]. (5) Ti/Au electrode patterns were deposited onto quartz substrates via electron beam evaporation and then transferred onto the samples. This procedure produced a photodetector array equipped with an 8×8 grid of photodetector units.

The surface morphology and elemental composition of the Ga2O3 samples were examined using a scanning electron microscope (SEM, TESCAN LAYRA3 GM), which is embedded with energy-dispersive X-ray spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) measurements were conducted to check the samples using a monochromatic AlKα source at 1486.6 eV, which was carried out with a Thermo Fisher Kα instrument running at 300 W. X-ray diffraction (XRD) patterns of Ga2O3 nanorod powder were obtained using a Malvern Panalytical Empyrean instrument. Raman spectra of Ga2O3 nanorods were obtained using a Raman spectrometer with a 532 nm laser as the excitation source. Optical properties of the samples were determined using a UV-visible absorption spectrometer. Lastly, the photoelectric properties of the manufactured detectors were evaluated using a photometric test system consisting of a xenon lamp, LEDs, a monochromator, a chopper, an oscilloscope, a signal generator, and a source meter.

Figure 1 illustrates the device fabrication process of the PtNPs@Ga2O3/GaN heterojunction detector array and the schematic structure of the detector units. With the assistance of a customized PDMS mold, the ethanol colloidal solution of Ga2O3 nanorods was spin-coated onto a p-GaN substrate, successfully producing grid-patterned nanorod film that serves as independent photosensitive units for a large-area photodetector array. The experimental section provides a detailed description of the specific fabrication procedure of the device.

The surface morphology of the Ga2O3 nanorod film on the p-GaN substrate was characterized using SEM. As shown in Fig. 2(a), the nanorods exhibit a relatively uniform arrangement, with an average length of 2 μm and a diameter of 300 nm, providing a solid foundation for subsequent device fabrication. SEM was utilized to examine the external morphology and geometric configuration of a Ga2O3 nanorod, while its elemental distribution was assessed using EDS. As illustrated in Fig. 2(b), the Ga2O3 nanorod displays a typical prismatic structure with distinct boundaries and smooth side surfaces, while its EDS analysis indicates the homogeneous distribution of Ga and O elements throughout the entire nanorod. In addition, micro-Raman spectroscopy was performed on an individual Ga2O3 nanorod to characterize the vibrational properties of the samples. The Raman spectrum presented in Fig. 2(c) reveals eleven characteristic peaks within the range of 100 to 800cm−1, corresponding to the Ga2O3 material. The strongest peak is located at approximately 200cm−1.

XPS was employed to analyze the chemical composition of the prepared Ga2O3 nanorods [40]. Shown in Fig. 2(d), a full spectrum scanning from 0 to 1200 eV was carried out, and the resulting sharp peaks correspond to C 1s, Ga 2p, Ga 3s, Ga 3p, Ga 3d, and O 1s signals, respectively, along with Ga LMM and O KLL Auger peaks. The C 1s signal originates from environmental carbon contamination adhering to the sample surface. No significant peaks of other elements could be observed, indicating that pure Ga2O3 nanorods were synthesized using the hydrothermal method, free from external contaminants. Fine scans of the Ga 2p core level spectrum are shown in Fig. 2(e). Characteristic peaks at binding energies of 1117.50 and 1144.50 eV correspond, respectively, to the Ga2p3/2 and Ga2p1/2 electron levels in Ga2O3, which are consistent with published literature [41,42]. XRD analysis was characterized for the Ga2O3 sample. The XRD pattern in Fig. 2(f) confirms the monoclinic phase β-Ga2O3, which matches well with standard values of Ga2O3 (JCPDS card No. 43-1012). In conclusion, Ga2O3 nanorods synthesized via the hydrothermal method exhibit significantly higher crystallization quality [26].

In previous research, metallic nanostructures have demonstrated a significant capability to enhance the local electric field via the localized surface plasmon resonance (LSPR) effect. The heightened electric field allows enhancement of the efficiency of light absorption and detection, particularly in the solar-blind ultraviolet spectral region. Noble metals, like Al and Pt, have been commonly utilized as the promising candidates. By comprehensive comparison, Al tends to undergo surface oxidation when in contact with Ga2O3, resulting in the formation of an Al2O3 layer, which can compromise the stability of photodetectors, while Pt is preferred due to its superior optical properties and chemical stability. In our experimental approach, PtNPs were obtained by depositing Pt nanofilms on pre-prepared Ga2O3 nanorods through magnetron sputtering, followed by high-temperature annealing. The SEM photograph in Fig. 3(a) depicts PtNPs with well-dispersed distribution, which have been prepared on the nanorods. The PtNPs show predominantly spherical or semi-ellipsoidal shapes with well-defined boundaries, averaging approximately 50 nm in diameter.

The optical absorption properties of unmodified and PtNPs-modified Ga2O3 nanorods were analyzed using a UV-Vis spectrometer. As depicted in Fig. 3(b), distinct absorption edges around 260 nm for both samples were exhibited. The optical bandgap (Eg) of the nanorod sample, determined using the Tauc equation, is estimated to 4.67 eV, as seen in the inset of Fig. 3(b). Notably, the surface-modified PtNPs lead to a significant enhancement in the light absorption capability of Ga2O3 nanorods within the wavelength range of 200–260 nm [43].

The impact of PtNPs on the electronic properties of Ga2O3 nanorod film was thoroughly investigated through the fabrication of the metal-semiconductor-metal (MSM) device employing MXene as symmetrical electrodes. Figure 3(c) presents the current-voltage (I−V) characteristics of Ga2O3 nanorods with and without PtNP decoration under both dark and illuminated conditions. In the dark, the I−V curves displayed in Fig. 3(c) exhibit symmetric and linear behavior in the semi-logarithmic scale, indicating an Ohmic contact between MXene electrodes and Ga2O3 nanorods. It suggests an efficient electron transport across the MXene/Ga2O3 interface. And an enhancement of electron transport properties was observed by following the Ga2O3 nanorod film coated with PtNPs. Moreover, when illuminated by a 254 nm LED, the PtNPs@Ga2O3 nanorod film MSM device demonstrated a significant increase in the photocurrent-to-dark current ratio, which is enhanced by approximately one order of magnitude. This enhancement underscores the improved light absorption capabilities of the device in the solar-blind UV region, thereby boosting the optical response [44]. These results highlight the potential of PtNPs@Ga2O3 nanorods for promising applications in high-sensitivity photodetection.

PtNPs with consistent size and excellent uniformity were synthesized on a quartz substrate using a standardized procedure, as illustrated in the inset of Fig. 3(d). The diagram clearly demonstrates that the PtNPs of specific sizes uniformly cover the entire surface of the quartz substrate. To characterize the optical properties of these PtNPs, their absorption spectra on quartz were examined using a UV-visible spectrophotometer. The absorption spectrum presented in Fig. 3(d) reveals a prominent peak at approximately 250 nm, indicating that the PtNPs exhibit strong absorption in the solar-blind ultraviolet wavelength region, particularly around 250 nm. This phenomenon confirms the successful fabrication of PtNPs displaying plasmonic resonance in the solar-blind ultraviolet range [45,46].

Finite-difference time-domain (FDTD) simulations were employed to investigate the effect of PtNPs on the optoelectronic enhancements of the Ga2O3 nanorod samples. These simulations were supported by precise dimensions of both the Ga2O3 samples and PtNPs obtained from SEM images, along with a resonance wavelength confirmation at 254 nm. Figure 3(e) provides insights into the spatial electric field pattern in the x–y plane around an individual PtNP on Ga2O3. The color map depicts the magnitude of the local electric field |E|2/|E0|2 along the x-axis, where the deep red indicates the highest intensity and deep blue the lowest. It illustrates the symmetric dipolar plasmonic resonance of a PtNP upon absorbing incident light. It reveals enhanced electric field zones at the PtNP/Ga2O3 interface that diminish gradually with distance, highlighting the localized enhancement effect.

Figure 3(f) further analyzes the electric field distribution in the x–z plane around a PtNP placed on Ga2O3, depicting |E|2/|E0|2 along the z-axis. A significantly enhanced electric-field distribution is observed along the boundaries of the PtNP, with partial penetration into the Ga2O3 nanorods along the sides of the x–z plane. This spatial distribution underscores how PtNPs effectively enhance the absorption of solar-blind ultraviolet light at the PtNPs/Ga2O3 interface through LSPR. The larger geometric profile of PtNPs facilitates efficient light absorption and energy transfer to Ga2O3 nanorods, thereby boosting their optoelectronic properties [38,47,48]. The findings align with experimental data, confirming the role of surface-modified PtNPs in enhancing the performance of Ga2O3 nanorods and their related photodetectors.

Research was conducted on the optoelectronic characteristics of the Ga2O3/GaN heterojunction unit. Figure 4(a) illustrates the I−V curve of the device, showcasing typical nonlinear behavior. Previous studies have confirmed that the Ni/Au metal electrodes form standard Ohmic contacts with the p-GaN film. Therefore, the nonlinear I−V curve can be attributed to the standard diode-like rectifying characteristics of the PtNPs@Ga2O3/GaN structure [49]. Figure 4(b) depicts the wavelength-dependent photocurrent of the PtNPs@Ga2O3/GaN heterojunction unit at 0 V bias upon light illumination with the irradiance of 1mW/cm2. The device exhibits excellent photoresponse in the solar-blind spectral region, with the dominant photocurrent peaking at 254 nm. Notably, despite using GaN as the hole transporting layer, the device shows a relatively weak response to ultraviolet light (280–380 nm), underscoring the effectiveness of Ga2O3 nanorods as solar-blind photosensitive materials in device fabrication [50].

Figure 4(c) depicts the I−V characteristics of the PtNPs@Ga2O3/GaN detector under 254 nm illumination at various intensities at 0 V bias. It is evident that higher-intensity light induces greater photocarrier generation, thus resulting in increased photocurrent. As the optical power density rises from 0.1mW/cm2 to 10.0mW/cm2, the device’s short-circuit current escalates progressively from 4.39 nA to 3.76 µA. Particularly at 10mW/cm2, the device exhibits an open-circuit voltage as large as 1.95 V, demonstrating an outstanding photovoltaic performance. Additionally, the device’s on/off switching response was measured, as shown in Fig. 4(d). It can be observed that under high optical power density illumination, the dark current of the detector exhibits a slight increase. This may be attributed to defect states generated during the synthesis of Ga2O3 nanorods and the device fabrication process, which results in a higher density of photogenerated carriers being captured by these defect states as their number increases. However, despite the rise in dark current, the ratio of photocurrent to dark current continues to grow steadily, increasing from 101 (1.0μW/cm2) to 105 (10.0mW/cm2). The detector’s light response exhibits a stable linear growth with an increase of the incident light intensity. The plotted I−T curves demonstrate that the detector unit possesses good stability and repeatability, with no significant signal variation after several exposures at the same light intensity. Of particular note, we subjected the device to prolonged operational testing three months later, as illustrated in Fig. 4(e). Experimental findings indicate that following extended cycles of operation or standby, the device’s performance did not exhibit notable degradation. It consistently manifested nearly equivalent photocurrent and retained full functionality [43].

The research on the energy band structure of PtNPs@Ga2O3/GaN heterojunction detectors is summarized in Fig. 4(f). The electronic band gaps and electron affinities of the materials involved are detailed as follows: Eg,Ga2O3=4.55eV, Eg,GaN=3.4eV, χg,Ga2O3=4.0eV, and χg,GaN=3.25eV [38,47]. Due to the drift and diffusion of charge carrier, a robust built-in electric field is established at the Ga2O3-to-GaN interface. Under 0 V bias and illuminated by a 254 nm LED, the photocarriers are generated at the Ga2O3/GaN interface. Additionally, the incorporation of PtNPs enables the enhancement of light absorption of the Ga2O3 nanorods film, which promotes the generation of additional photocarriers. This phenomenon significantly boosts the device’s photocurrent and enhances its solar-blind light detection capabilities and electrical conductivity [38,47].

In our investigation of the impact of PtNPs on photodetection performance, we conducted a detailed comparative analysis of the Ga2O3/GaN photodetection devices uncoated and coated with PtNPs modification. Figure 5(a) presents the semilogarithmic I−V characteristics of both the devices under dark condition and 254 nm light illumination (∼10mW/cm2). The graph indicates that the PtNPs-modified detector exhibits a higher dark current compared to the original heterogeneous junction device. And the enhancement in photocurrent is more pronounced. At 0 V bias, the ratio of photocurrent to dark current increases by approximately an order of magnitude. This underscores the effectiveness of the PtNPs’ LSPR effect in boosting the photodetector’s performances. Additionally, Fig. 5(b) displays the I−T curves of both devices under identical conditions. The PtNPs@Ga2O3/GaN detector unit demonstrates a photocurrent of approximately 3.76 µA at 0 V bias, which is 7.6 times higher than that of the pristine heterogeneous structure detector.

The photodetection figures-of-merit at 0 V bias were computed for both devices. In Fig. 5(c), the photocurrent (Iph) of both devices is shown to increase linearly with incident light intensity (P). The relationship can be expressed as Iph=APθ, where A is a constant and θ denotes the response characteristic of photocurrent to incident light. The values of θ obtained from fitting are 0.982 and 0.978 for the two devices, respectively, which closely approach the ideal value of 1. It indicates that Ga2O3 nanorods exhibit high crystalline quality with minimal interface states and defects, enabling efficient conversion of absorbed photons into photo-generated electron-hole pairs [51]. Importantly, the surface-modification of PtNPs on Ga2O3 nanorods enables the device to detect even much weaker signals down to 1μW/cm2. This enhancement in sensitivity underscores the capability of integrating PtNPs with detectors to significantly improve light absorption and consequently increase photocurrents.

The key parameters of the photodetector, such as R, D∗, and EQE, are determined by the following formulas: R=Iph−IdPS,(1)D∗=RS2eId,(2)EQE=Rhceλ.(3)

In these contexts, Id, S, h, c, e, and λ denote the dark current, light-receiving area, Planck’s constant, speed of light, elementary charge, and wavelength, respectively [52]. Figures 5(d)–5(f) compare the R, D∗, and EQE of both the two devices uncoated and coated with PtNPs decoration. It is evident that as incident optical power increases, the metrics R, D∗, and EQE exhibit a gradual decline, which are attributed possibly to phenomena such as saturation absorption and thermal effects at high radiation intensities [53,54]. With surface-modified PtNPs, the detector is notably enhanced across all three parameters compared to its original configuration. Under illumination from a 254 nm LED at an intensity of 10μW/cm2, which the original device can detect, the PtNPs@Ga2O3/GaN detector achieves R, D∗, and EQE values that are 5.6 times, 3.1 times, and 5.6 times higher than those of the pristine device, respectively. Additionally, under 1μW/cm2 illumination, the optimized device achieves maximum values of R, D∗, and EQE calculated as 189.0 mA/W, 4.0×1012 Jones, and 92.0%, respectively.

Integrated array image sensors play a pivotal role across various domains [4,62]. Considering the PtNPs@Ga2O3/GaN heterojunction units exhibit superior photosensitivity, exceptional stability, and ultra-fast response in solar-blind spectral regions, we investigated the imaging capabilities of an 8×8 array based on PtNPs@Ga2O3/GaN heterojunction detectors. Figure 6(d) depicts a schematic of the imaging system. A 254 nm LED was employed as the light source, vertically incident on the array surface, while metal masks with different hollow letters were positioned between the uniform illumination and the detector array. Partial PtNPs@Ga2O3/GaN heterojunction units passing through the hollow regions were affected by solar-blind irradiation, thus generating a notable photocurrent, while others remained dark. Figure 6(e) shows I−T curves under 254 nm light illumination and dark conditions when operating at 0 V bias. The array device demonstrates an exceptional uniformity, with photocurrent and dark current magnitudes approximately at 10−7A and 10−11A, respectively. Each detector unit exhibits remarkable stability in both photocurrent and dark current. We conducted measurements on the output currents of each detector unit, visualized as two-dimensional current contrast plots featuring the letters “F,” “P,” “G,” and “A,” which are, respectively, depicted in Fig. 6(f). These findings clearly indicate the excellent immunity of the PtNPs@Ga2O3/GaN detector array to interference in optical detection, highlighting its potential for initial visual imaging. These characteristics are particularly crucial for applications in machine vision, image sensing, and related fields.

In summary, we have designed and fabricated a Ga2O3 nanorod/GaN heterojunction solar-blind ultraviolet photodetector array, achieving significant enhancement in detector performance through surface modification with PtNPs. The optimized PtNPs@Ga2O3/GaN heterojunction detector units have demonstrated exceptional light detection efficiency in self-powered mode, with a high responsivity of 189.0 mA/W, specific detectivity of 4.0×1012 Jones, superior external quantum efficiency of 92.4%, and swift response time of 674/692 µs at 254 nm, establishing them as formidable competitors in the field of arrayed solar-blind photodetectors. Furthermore, the detector units have exhibited outstanding uniformity and stability, which have yielded satisfactory results in concept demonstrations for arrayed detector imaging. This research aims to advance the development of monocrystalline Ga2O3 nanostructure arrayed integrated optoelectronic devices and facilitate their widespread adoption in sensing, industrial production, and wearable devices.

Acknowledgment. We acknowledge the facilities in the Center for Microscopy and Analysis at Nanjing University of Aeronautics and Astronautics.