Highly sensitive hierarchically structured Si-based UV sensor–photodetectors via optimized ZnO–Al2O3 nanocomposite architectures
M. Abdelhamid Shahat, Ashraf S. Khamees, Ahmed Ghitas, Hend A. Ezzat
TL;DR
This paper develops a highly sensitive UV sensor using a ZnO–Al2O3 nanocomposite on a silicon base, combining theory and experiments to improve performance.
Contribution
The novel ZnO–Al2O3 nanocomposite architecture is shown to enhance UV sensor performance through optimized electronic and optical properties.
Findings
ZnO–Al2O3 hybrid structures exhibit improved carrier mobility and electronic coupling compared to pure ZnO or Al2O3.
Al2O3 incorporation increases surface roughness and porosity, enhancing light scattering and active site density.
The hybrid architecture shows faster response and recovery dynamics under UV illumination with higher electrical conductivity.
Abstract
The rapid and reliable detection of ultraviolet (UV) radiation is critical for applications ranging from environmental monitoring to optoelectronic security systems. This study presents an integrated theoretical and experimental investigation into highly sensitive, hierarchically structured Si-based UV sensor–photodetectors optimized via ZnO–Al2O3 nanocomposite architectures. A combination of density functional theory (B3LYP/6-31G(d,p)) calculations and comprehensive materials characterization was employed to elucidate the interplay between electronic structure, surface morphology, and optical performance. Theoretical modeling provided detailed insights into band alignment, total and partial density of states, frontier molecular orbitals, and electrostatic potential distributions for pure and hybrid oxide systems, revealing that ZnO–Al2O3 exhibits superior electronic coupling and…
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Figure 9- —The National Research Institute of Astronomy and Geophysics (NRIAG)
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Taxonomy
TopicsZnO doping and properties · Ga2O3 and related materials · Gas Sensing Nanomaterials and Sensors
Introduction
Ultraviolet (UV) radiation detection has emerged as a critical technological requirement across diverse fields such as environmental monitoring, public health protection, industrial quality control, and aerospace applications^1^. For instance, UV-A is often used in non-destructive testing and medical phototherapy, UV-B plays a central role in biological effects such as vitamin D synthesis and skin erythema, and UV-C is integral to germicidal disinfection and semiconductor photolithography. Precise and reliable detection of UV radiation is thus essential for safeguarding human health, ensuring process integrity, and enabling advanced scientific instrumentation. Silicon (Si)-based photodetectors are attractive platforms for UV sensing due to their established fabrication technology, scalability, and compatibility with complementary metal–oxide–semiconductor (CMOS) processes^2^. However, conventional Si photodiodes suffer from inherent challenges when detecting UV radiation. Their narrow bandgap (1.12 eV) allows for strong absorption in the visible and near-infrared regions, which introduces unwanted background signals, thereby limiting UV selectivity. Furthermore, the shallow penetration depth of UV photons in Si leads to increased surface recombination losses, lowering device responsivity^3^. Overcoming these challenges requires integrating wide-bandgap semiconductors and engineered nanostructured coatings that selectively absorb UV light, improve charge carrier separation, and suppress visible–infrared background interference^1^.
Recent advances in UV photodetectors have demonstrated the remarkable potential of wide-bandgap metal oxide nanostructures in enhancing key performance metrics such as responsivity, response speed, and environmental stability^4^. For instance, ZnO nanotubes grown on p-Si substrates via pulsed laser deposition have achieved exceptionally high UV responsivity (~ 101.2 A W^−1^), attributed to their excellent structural quality, strong optical confinement, and high surface-to-volume ratios, as reported by Flemban et al.^5^. Similarly, Akhtarianfar et al.^6^ demonstrated that vertically aligned ZnO nanorod ensembles, organized through hierarchical colloidal network assembly, can significantly enhance UV sensor sensitivity through precise control of deposition parameters such as precursor concentration, layer thickness, and nanorod morphology. In hybrid systems, Pandey et al.^7^ showed that coating ZnO nanorods with TiO_2_ layers leads to a substantial enhancement in photocurrent—from ~ 35 to ~ 250 μA under UV illumination—while enabling self-powered operation, as validated by both experimental measurements and first-principles theoretical modeling. Beyond ZnO–TiO_2_ composites, complex heterojunction architectures such as NiO/TiO_2_/ZnO devices have demonstrated exceptional performance, with responsivity values reaching 291 A W^−1^ and detectivity up to 6.9 × 10^11^ Jones, enabled by an ultrathin TiO_2_ dielectric layer that improves rectification and suppresses dark current, as reported by Shang et al.^8^. Collectively, these studies underscore the transformative role of nanostructuring, hybridization, and interface engineering in optimizing UV sensing performance, while also revealing that systematic, side-by-side theoretical and experimental comparisons of different oxide systems remain scarce—a critical knowledge gap addressed in this work. Despite these advances, most prior studies focus on isolated material systems or single heterojunction configurations, without providing a unified comparison under identical fabrication and testing conditions or correlating nanoscale interfacial chemistry with device-level photodetection metrics. Recent advances in nanocomposite engineering further highlight that controlled defect modulation, interfacial coupling, and nanoscale structural optimization can significantly enhance charge transport, surface reactivity, and optoelectronic functionality across sensing and energy-related applications^9–12^. In particular, hybrid oxide and carbon-based nanocomposites have demonstrated improved sensitivity, operational stability, and functional selectivity through tailored electronic interactions and reduced recombination losses, underscoring the growing importance of rational nanocomposite design in next-generation semiconductor devices^13,14^.
In recent years, semiconductor nanometal oxides such as zinc oxide (ZnO)^15,16^, titanium dioxide (TiO_2_)^17^, and aluminum oxide (Al_2_O_3_)^18^, have attracted intense interest for UV sensing applications. ZnO, with its wide direct bandgap (~ 3.37 eV) and high exciton binding energy (~ 60 meV), offers strong intrinsic UV absorption, high electron mobility, and tunable morphology from nanorods to thin films. TiO_2_, with a bandgap of 3.0–3.2 eV, is known for its exceptional chemical stability, photocorrosion resistance, and environmental robustness, making it suitable for prolonged outdoor UV sensing. Al_2_O_3_, although not a direct UV absorber, serves as an excellent passivation layer with a high dielectric constant, capable of reducing surface trap states, suppressing dark currents, and improving device longevity. Hybrid and composite oxide architectures, particularly ZnO–Al_2_O_3_ nanocomposites, have demonstrated synergistic performance advantages^4^. ZnO provides strong UV sensitivity, while Al_2_O_3_ acts as a surface passivation layer, blocking non-radiative recombination pathways and reducing leakage currents. In this context, the selection of synthesis parameters (reaction time, composition ratio, and processing conditions) in the present work was guided by preliminary optimization studies aimed at achieving a balance between crystallinity, surface roughness, and electrical conductivity. Systematic variation of these parameters allowed identification of conditions that maximize UV photoresponse while maintaining structural stability and reproducibility. This dual functionality improves photogenerated carrier lifetimes, enhances the signal-to-noise ratio, and enables fast response–recovery characteristics. The introduction of Al_2_O_3_ into ZnO matrices also modifies the electronic band alignment, potentially reducing trap-mediated recombination and facilitating efficient charge extraction when interfaced with Si substrates. Notwithstanding these benefits, literature on the systematic integration of ZnO–Al_2_O_3_ nanocomposites into Si-based UV sensors remains limited, with most prior works focusing either on ZnO alone or on Al_2_O_3_ as a passivation layer rather than as an active component in composite sensing architectures^19^. The research gap lies in the lack of comprehensive, side-by-side evaluation of multiple oxide and hybrid configurations—namely ZnO, TiO_2_, Al_2_O_3_, ZnO–TiO_2_, and ZnO–Al_2_O_3_—under identical fabrication and testing conditions^17,20^. Furthermore, there is a need for integrated theoretical–experimental studies that link materials properties such as bandgap, surface passivation, and defect density to device-level metrics including responsivity, response time, and operational stability. Previous reports often lack mechanistic insight into how oxide–oxide and oxide–Si interfaces affect charge transport and recombination kinetics in UV photodetectors^21^. Accordingly, the key novelty of this study lies in the combined experimental–theoretical evaluation of multiple oxide and hybrid architectures under identical conditions, with particular emphasis on elucidating how ZnO–Al_2_O_3_ interfacial coupling governs charge transport, defect passivation, and UV photodetection performance in Si-based devices.
In the present work, these research gaps are addressed through an integrated theoretical and experimental investigation of Si-based UV sensors incorporating various semiconductor oxide layers, with a particular emphasis on ZnO–Al_2_O_3_ nanocomposite architectures. The theoretical component employs density functional theory (DFT) calculations using the B3LYP/6-31(d,p) model to predict optical absorption spectra, band alignment, and carrier transport pathways. The prospective benefits of multiple semiconductor nanometal oxides—including ZnO, TiO_2_, Al_2_O_3_, and their hybrid forms ZnO–TiO_2_ and ZnO–Al_2_O_3_—are systematically evaluated to determine the most practical and efficient material configurations for Si-based UV sensor development. The analysis focuses on tracking variations in key electrical properties such as bandgap energy, total density of states (TDOS), and frontier molecular orbitals (FMOs), specifically the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Additionally, molecular electrostatic potential (MESP) mapping and partial density of states (PDOS) calculations are performed to assess the stability and reactivity of each structure. Theoretical predictions of optical enhancement are further validated by simulating UV–Vis spectra, enabling the identification of the most promising nanocomposite configuration. Subsequently, experimental fabrication and characterization are conducted to corroborate the theoretical findings. The fabricated devices are analyzed using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), UV–Vis spectroscopy, electrical conductivity measurements, surface roughness analysis, apparent porosity determination, and electrochemical impedance spectroscopy (EIS). Comparative evaluations of single-component oxides (ZnO, TiO_2_, Al_2_O_3_) and hybrid composites (ZnO–TiO_2_) confirm that ZnO–Al_2_O_3_ delivers superior device performance, combining high responsivity, fast recovery times, and enhanced environmental stability.
The significance of this work lies in its ability to provide a direct link between material-level modifications and device-level performance improvements, enabling rational design principles for future UV sensor technologies. By demonstrating how the ZnO–Al_2_O_3_ combination enhances Si-based photodetection, this study offers a scalable pathway for the development of high-efficiency, low-cost UV sensors compatible with CMOS manufacturing processes. Such devices have promising applications in wearable UV monitoring, spaceborne radiation detection, flame sensing, and integrated environmental sensing networks. Looking forward, the design strategies established here can be extended to next-generation UV sensors employing flexible substrates, transparent conductive electrodes, and advanced nanostructured coatings to achieve broadband, multi-spectral UV detection. Integration with wireless IoT architectures and self-powered operation through photovoltaic or triboelectric energy harvesting are also prospective developments. Furthermore, doping strategies, plasmonic nanostructure incorporation, and defect engineering offer exciting possibilities for further enhancing UV selectivity and responsivity, opening pathways toward compact, high-performance, and intelligent UV sensing systems for the evolving demands of industry, defense, and environmental monitoring.
Calculation details
The simulations were performed using Gaussian 09 program Revision C.01^22^, using density function theory (DFT)^23^. The prospective benefits of several a semiconductor nanometal oxides, including ZnO, TiO_2_, Al_2_O_3_, and the hybrids ZnO–TiO_2_ and ZnO–Al_2_O_3_, have been explored to determine practical selections for the development of Si-based UV sensors utilizing the B3LYP/6-31(d,p) model^24–26^. In order to track the alternation on electrical properties, the effects of nanometal oxides on bandgap, total density of states (TDOS), and frontier molecular orbitals (FMOs), which stand for the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, were studied for all suggested structures. After that, in order to examine the impact of nanometal oxides on stability and reactivity, several essential physical characteristics were examined, including MESP mapping and partial density of states (PDOS). On top of that, the optical enhancement could be tracked, and the most enhanced structure might be identified by subjecting all of the proposed structures with regard to UV–Vis characterization. Furthermore, the same theoretical level was used to calculate IR and NMR spectra in order to determine the structure that were present in the molecule, this allowed to identify the best material possible. This is due to the fact that IR and NMR are essential instruments for characterizing chemical processes by determining the products and reactants involved.
Experimental section
Reagents and materials
All chemicals used in this work were of analytical grade and utilized without further purification. Zinc nitrate hexahydrate (Zn(NO_3_)2·6H_2_O, ≥ 99%, Sigma–Aldrich) and sodium hydroxide (NaOH, ≥ 98%, Merck) served as the primary precursors for ZnO NP synthesis. For Al_2_O_3_ NP preparation, commercial aluminum powder (> 5 µm, 99.5%, Alfa Aesar) and NaOH were used as starting materials. Hydrochloric acid (HCl, 37%, Sigma–Aldrich) was employed for neutralization. Single-side-polished p-type Si wafers (orientation ⟨100⟩, resistivity 1–10 Ω cm, thickness 500 ± 25 µm) were used as the substrate material for device fabrication.
Synthesis of ZnO NPs
ZnO NPs were prepared via a hydrothermal precipitation route. First, 3.00 g of Zn(NO_3_)2·6H_2_O was dissolved in 20 mL of DIW under constant magnetic stirring at 900 rpm until a clear solution was obtained. In a separate beaker, 4.00 g of NaOH was dissolved in another 20 mL of DIW. The NaOH solution was then added dropwise to the zinc nitrate solution under vigorous stirring to initiate precipitation. The resulting suspension was transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 75 °C for 8 h. Formation of an opaque white suspension indicated the successful generation of ZnO NPs. After cooling to room temperature naturally, the precipitate was collected via vacuum filtration, washed repeatedly with DIW and ethanol to remove residual ions, and dried in a vacuum oven at 65 °C for 24 h. The dried product was lightly ground using an agate mortar and pestle and stored in an airtight container for further use.
Synthesis of Al2O3 NPs
Al_2_O_3_ NPs were synthesized through a hydrothermal oxidation process. A 2.0 M NaOH solution was prepared by dissolving 9.60 g of NaOH in 120 mL of DIW. To this alkaline medium, 2.00 g of commercial aluminum powder was gradually added under vigorous stirring to minimize localized overheating from the exothermic reaction. The mixture was stirred for 45 min at ambient temperature, followed by ultrasonication (150 W, 40 kHz) for 2 h to ensure homogeneous dispersion of the aluminum particles. The resulting suspension was transferred into a 200 mL Teflon-lined stainless-steel autoclave and heated at 150 °C for 36 h. After cooling to room temperature, the reaction mixture was neutralized with dilute HCl until the pH reached ~ 7. The precipitated Al_2_O_3_ NPs were collected via centrifugation at 1000 rpm for 10 min, washed three times with DIW and ethanol, and dried in an oven at 75 °C for 15 h. Finally, the dried powder was calcined in a muffle furnace at 550 °C for 3 h to enhance crystallinity and phase purity.
Fabrication of Si-based ZnO–Al2O3 nanocomposite films
The ZnO@ Al_2_O_3_ nanocomposite was prepared by incorporating synthesized Al_2_O_3_ NPs into a ZnO matrix at an optimized mass ratio of 80:20, as seen in Fig. 1. Initially, 3.00 g of ZnO NPs was dispersed in 150 mL of DIW and stirred magnetically at 80 °C for 3 h to obtain a stable suspension. Separately, Al_2_O_3_ NPs corresponding to 1 wt% of the total solid mass were dispersed in 50 mL of DIW via ultrasonication for 30 min. The Al_2_O_3_ dispersion was then added dropwise to the ZnO suspension under continuous stirring, and the mixture was maintained at 85 °C for an additional 2.5 h to achieve homogeneous nanoparticle distribution. The composite slurry was deposited onto pre-cleaned Si wafers using a spin-coating process at 1000 rpm for 120 s. Each substrate was coated three times to ensure uniform film thickness, with intermediate drying at 60 °C for 10 min between coatings. The coated wafers were dried at ambient temperature for 24 h, followed by curing in a hot-air oven at 70 °C for 12 h to improve adhesion and mechanical stability. The resulting ZnO–Al_2_O_3_ composite films were cut into uniform dimensions (1 cm × 1 cm) and stored in a desiccator until characterization.Fig. 1. Schematic illustration of the stepwise synthesis and integration of ZnO–Al_2_O_3_ nanocomposites for Si-based UV sensor–photodetector fabrication.
Characterization techniques
A comprehensive suite of advanced analytical and electrochemical techniques was employed to elucidate the structural, morphological, compositional, surface, and electrical properties of the synthesized ZnO and ZnO–Al_2_O_3_ nanocomposite films. X-ray diffraction (XRD, Bruker D8 Advance, Cu Kα) was employed to evaluate the crystalline structure, phase composition, and crystallinity of ZnO, Al_2_O_3_, and ZnO–Al_2_O_3_ nanocomposite films. Variations in peak intensity and full-width at half-maximum (FWHM) provided insights into microstructural evolution induced by plasma treatment. Fourier-transform infrared spectroscopy (FTIR, Nicolet iS50, Thermo Scientific) was used to identify functional group modifications. Field Emission Scanning Electron Microscopy (FE-SEM) was conducted using an FEI Nova NanoSEM 450 system operating at an accelerating voltage of 5–15 kV to investigate surface morphology and microstructural organization. Prior to imaging, all samples were sputter-coated with a thin layer (~ 5 nm) of gold using a Quorum Q150R ES coater to minimize charging effects. High-resolution micrographs were acquired at multiple magnifications to assess nanoparticle distribution, surface uniformity, porosity features, and interfacial contact between the nanocomposite layer and the Si substrate. High-resolution Transmission electron microscopy (HR-TEM, JEOL, JEM-A 2100, Japan) was employed to gain deeper insight into the nanoscale morphology, crystallinity, and interfacial characteristics of the pristine ZnO and ZnO–Al_2_O_3_ films. Energy-Dispersive X-ray Spectroscopy (EDX) was carried out on a QUANTA 200 FEG (FEI, Japan) instrument coupled to the FE-SEM system, enabling qualitative and semi-quantitative determination of elemental composition. Elemental mapping was performed over selected areas to visualize the spatial distribution of Zn, Al, and O, thus confirming the successful incorporation and homogeneous dispersion of Al_2_O_3_ within the ZnO matrix. Data were processed using EDAX Genesis software to ensure accurate peak identification and composition quantification. Surface Roughness Analysis was performed using a Talysurf 50 contact profilometer (Taylor Hobson Precision, UK) equipped with a diamond stylus of 2 μm tip radius. Measurements were carried out over a scan length of 5 mm at a stylus speed of 0.5 mm/s, with multiple scans obtained from different sample regions to ensure statistical reproducibility. The average surface roughness (Ra) parameter was extracted, providing quantitative insights into surface texture changes induced by nanoparticle embedding and hierarchical structuring. Apparent Porosity was determined following the Archimedes principle in strict accordance with ASTM C373-88. Briefly, each sample was first dried in an oven at 105 °C for 24 h to obtain the dry mass. Samples were then immersed in deionized water under vacuum for 2 h to ensure complete pore filling and weighed in the saturated state. Finally, suspended weight in water was measured. This method enabled precise quantification of open pore volume and water-accessible voids within the nanocomposite films. Electrical Resistivity was evaluated using the four-point probe (4PP) method with an EQ-JX2008-LD instrument (MTI Corporation, USA). A constant DC current was applied between the outer probes, while the voltage drop was recorded across the inner probes. Measurements were performed at ambient conditions, with probe spacing calibrated at 1.0 mm. To minimize statistical error, resistivity values were averaged over ten independent readings taken at spatially distributed locations across each specimen surface. The resulting resistivity data were used to assess charge transport efficiency and the impact of Al_2_O_3_ incorporation on electrical conduction pathways. Electrochemical Impedance Spectroscopy (EIS) measurements were performed under dark conditions using a Zahner electrochemical workstation to evaluate the dynamic charge transport and interfacial properties of the fabricated Si-based UV sensor–photodetectors. The impedance spectra were recorded over a frequency range of 100 Hz to 100 kHz with an applied AC perturbation amplitude of 10 mV. This technique enabled precise determination of parameters such as charge transfer resistance (R_ct_), series resistance (R_s_), and capacitive behavior at the semiconductor–electrode interface. All EIS data were analyzed through complex-plane (Nyquist) and frequency-phase (Bode) plots, with equivalent circuit fitting conducted to quantitatively correlate structural modifications—particularly the incorporation of ZnO–Al_2_O_3_ nanocomposite architectures—to the observed electrical performance enhancements. Current–voltage (I–V) measurements of pure ZnO and ZnO–Al_2_O_3_ photodetectors were conducted over a voltage range of − 5 to + 5 V under both dark and UV illumination using a KEITHLEY 2400 Source Meter coupled with a MSK–SS-50 solar simulator. Each measurement was averaged over five independent readings per sample. Both devices exhibit nearly linear and symmetric I–V behavior in forward and reverse bias, indicating the formation of ohmic contacts between the semiconductor layer and metal electrodes.
Results and discussion
DFT study of Si-based with metal oxides
Building model molecule
Devices designed to detect and quantify levels of UV light are commonly referred to UV sensors. These sensors encompass photochromic sensors and photo detectors. Photochromic sensors exhibit limitations in their ability to detect weak light and demonstrate a low direct response intensity. Conversely, UV photodetectors play a crucial role in contemporary optoelectronic technologies; they convert UV light wavelengths into electrical signals^27^. Environmental monitoring, sterilization, water purification, and biology have significant applications for UV photodetection, including notable uses in space detection^27^. Si-based UV photodiodes and UV photomultiplier tubes have traditionally been the principal types of UV photodetectors used in industrial settings. Si-based UV photodetectors have garnered a lot of attention due to their amazing sensitivity and adaptability^28^. This is despite the fact that Si-based UV photodiodes have a wide response that encompasses wavelengths ranging from UV to near-infrared. However, their UV photodetection capabilities are limited due to their insufficient sensitivity in the UV wavelength spectrum and the integration of optical^29^. The use of wide bandgap semiconductors as prospective alternatives for the construction of filter less UV photodetectors is chosen due to the fact that these semiconductors have an appropriate cut-off wavelength within the ultraviolet spectrum, which enables them to circumvent these constraints. This category comprises the following nanometal oxide materials: zinc oxide (ZnO), nickel oxide (NiO), titanium oxide (TiO_2_), copper oxide (CuO), tin oxide (SnO_2_), iron oxide (Fe_2_O_3_), Aluminum oxide (Al_2_O_3_), indium oxide (In_2_O_3_), tungsten trioxide (WO_3_), and vanadium oxide^30–33^. Si-based UV sensors that are simulated and their functionalization with a variety of nanometal oxides are considered to be the most promising alternatives (see Fig. 2) because of their remarkable electrical, optical, biological, energy, and processing capabilities. DFT calculations were employed to investigate the influence of various nanometal oxides, including ZnO, TiO_2_, Al_2_O_3_, ZnO–TiO_2_, and ZnO–Al_2_O_3_, on enhancing the electrical and optical properties of Si-based sensors for targeted improvements. The HOMO and LUMO orbitals, along with their respective energies, TDM, band gap energy, and TDOS were calculated to assess the suitability of the doping created for necessary properties. Subsequently, the impact on PDOS and MESP maps was analyzed to assess improvements in reactivity and sensitivity, in addition to calculating key physical parameters including hardness, softness, and electrophilicity. Furthermore, all models underwent UV–Vis analysis to identify the optical improvement and determine the most effective nanometal oxide. As a result, the optimal compound was synthesized and analyzed to examine the impact of implementing on the structural, physical and chemical characteristics.Fig. 2. Model structures constructed from the interaction mechanism of Si-based with various metal oxides including (ZnO, Al_2_O_3_, TiO_2_, ZnO-Al_2_O_3_, and Zn-TiO_2_) generated using Gauss View 5.0 program (Gaussian 09 program Revision C.01).
Frontier molecular orbital (FMO)
The Frontier Molecular Orbital (FMO) idea was examined to figure out and expect molecular sensitivity and reactivity, as well as charge transfer, which influence electronic characteristics. This hypothesis emphasizes the dispersion of HOMO and LUMO orbitals. In chemical processes, the distributions and energies of the HOMO and LUMO orbitals are crucial due to their determination of interactions on the electronic characteristics of molecules. FMO theory facilitates the prediction of the most probable sites for chemical reactions by examining the relative positions of HOMO and LUMO orbitals^34^. A computational and analytical investigation of the HOMO and LUMO molecular orbitals was carried out in order to study the interaction between metal oxides and the Si-base, as seen in Fig. 3. The presence of metal oxides influenced the configuration of HOMO and LUMO orbitals. It is clear from the HOMO and LUMO orbitals of the Si-base that the HOMO and LUMO orbitals are spread uniformly over the Si sheet. This is something that can be easily observed, the HOMO and LUMO orbitals appear to be concentrated around the ZnO atoms as a result of the interaction between ZnO and Si-base, which caused a significant alteration in the distribution of these pairs of orbitals. The impact of TiO_2_ has no notable alteration in the distribution of both HOMO and LUMO orbitals, as the orbitals continue to be predominantly localized on the silicon base, with only minor participation observed with the –O– atom of TiO_2_. Under the influence of Al_2_O_3_, the HOMO orbitals remain unaffected, while the LUMO orbitals are rearranged within and around the Al_2_O_3_. Ultimately, the HOMO and LUMO orbitals surrounding the atoms of the metal oxides were altered as a result of the hybridization of ZnO with TiO_2_ and Al_2_O_3_.Fig. 3. Frontier molecular orbital of the interacted Si-based with various metal oxides including (ZnO, Al_2_O_3_, TiO_2_, Zn–TiO_2_ and ZnO–Al_2_O_3_) generated using (Gaussian 09 program Revision C.01).
Among the electronic parameters that determine the reactivity, stability, and general behavior of a molecule are its band gap ΔE, TDM, and the E_HOMO_ and E_LUMO_ orbitals. The ability to recognize and predict these properties also gives vital information about the behavior of a molecule as well as potential uses for the molecule. The results of these various parameter computations are shown in Table 1.Table 1DFT: B3LYP/6-31(d, p) computed the E_HOMO_, E_LUMO_, bandgap energy (ΔE), and TDM of the Si-based interactions with several metal oxides, including ZnO, TiO_2_, Al_2_O_3_, Zn–TiO_2_, and ZnO–Al_2_O_3_.ParametersSi-baseSi–ZnOSi–TiO_2_Si–Al_2_O_3_Si–ZnO–TiO_2_Si–ZnO–Al_2_O_3_EHOMO− 3.3438− 4.1149− 3.9005− 3.9560− 3.8872− 4.4031ELUMO− 2.0044− 3.3527− 3.1111− 3.2638− 3.3176− 3.8991ΔE1.33940.76210.78940.69230.56950.5040TDM0.000010.02766.20197.52239.889220.9008
Comprehending chemical reactivity is fundamentally dependent on E_HOMO_ and E_LUMO_, which represent the energy levels associated with electron transfer in chemical reactions. The elevated energy level and electron configuration of E_HOMO_ are typically associated with superior nucleophiles. This feature is frequently linked to the molecule’s electron-donating properties. The most effective electrophile is generally linked to a chemical characterized by a low electron affinity minimum (E_LUMO_) value. However, the E_LUMO_ showed a significant increase, which indicates that the novel structures possess increased nucleophilic properties. This was discovered by the examination of nanometal oxides, which demonstrated that all of the model structures had a little increase in E_HOMO_ level. One of the primary functions of the ΔE, which is defined in Eq. (1), is to serve as a measurement for electron transport, chemical equilibrium, and sensitivity. This discrepancy between E_HOMO_ and E_LUMO_ is referred to as the difference between both.
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {\mathrm{E}} = E_{HOMO} - E_{LUMO}$$\end{document}The presence of a narrower band gap is indicative of higher reactivity in a molecule. This is due to the fact that increasing the energy required for electrons to travel through orbitals is reduced. There was a discernible shortening of the Si-base bandgap that was found according to the nanometal oxides doping. The Si–ZnO–Al_2_O_3_ composite had the greatest performance of all the nanometal oxide composites because it had a narrow bandgap of 0.5040 eV more than any other composite. As a consequence of this, as seen in Eq. (2)^35^, the conductivity (σ) of the structure will experience a significant increase, notably for the Si–ZnO–Al_2_O_3_ composite, which is dependent on the ΔE.
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{Conductivity }}\left( {\upsigma } \right) = \frac{{\mathrm{A}}}{{{\mathrm{T}}\frac{3}{2}}} exp\frac{{ - {\Delta {\mathrm{E}}}}}{{2{\mathrm{KBT}}}}$$\end{document}where (T) denotes temperature (Kelvin), (A) signifies a constant, and (K_B_) represents the Boltzmann constant. As an indication of the polarity of the molecule, TDM provides a visual representation of the distribution of the two types of charges inside the molecule. An increase in charge partitioning, which is demonstrated by a higher TDM, is another indication that the material demonstrates a greater degree of polarity. When it comes to TDM, the hybrid composites that were examined showed a substantial increase, with the Si–ZnO–Al_2_O_3_ structure demonstrating the most significant improvement at 20.9008 Debay. ZnO–Al_2_O_3_ has a considerable influence on dipole–dipole interactions, as evidenced by a decrease in the bandgap and a rise in TDM. This has the effect of improving the material’s electrical properties as well as its sensing and adsorption capabilities.
Total density of states (TDOS) analyses
In light of the aforementioned structures, the TDOS investigation shows the number of allowable states per every single energy unit. The TDOS is a measure used in condensed matter theory to describe the ordering of electronic states inside a material with respect to energy^36^. It is a measure that considers all contributions and provides a more comprehensive view of the electronic structure. The structures examined for Si-base sensors functionalized with ZnO, TiO_2_, Al_2_O_3_, ZnO–TiO_2_, and ZnO–Al_2_O_3_ are shown in Fig. 4, which indicates the TDOS. As a result of the presence of ZnO, the HOMO levels pushed closer to the Fermi level, which indicates that there are strong molecular interactions with Si-base. This variation also indicates that there is a greater potential of transmission of electrons, as Si–ZnO exhibited a substantial electronic transformation as a result of the influence of ZnO NPs. In a similar manner, TiO_2_ and Al_2_O_3_ offer a greater shift for the HOMO and LUMO levels, bringing them nearer to the Fermi level^20^. This results in a reduction in the band gap and an increase in probability of electron transitions and conductivity. In contrast to pure Si–ZnO, the hybrid composites of Si–ZnO–TiO_2_ and Si–ZnO–Al_2_O_3_ exhibit a more significant relocation of electrons owing to enhanced support for molecular orbitals. Comprehending the molecular reactivity, conductivity, and enhanced stability of the composite resulting from hybridization with Al_2_O_3_ necessitates this allocation, consistent with prior studies on HOMO–LUMO orbitals and band gap energy. Composites that integrate the advantages of Si–ZnO with metal oxides, particularly Al_2_O_3_, enhance conductivity, durability, and UV light sensitivity. The combination of several nanostructured materials can yield innovative solutions to challenges faced in particular application, necessitating a multidisciplinary approach to material design.Fig. 4. Total density of states of the interacted Si-based with various metal oxides including (ZnO, Al_2_O_3_, TiO_2_, Zn–TiO_2_ and ZnO–Al_2_O_3_).
Partial density of states (PDOS) analyses
On the PDOS Assists in the comprehension of concepts like as bonding, hybridization, and other electronic interactions by presenting a comprehensive perspective that illustrates the functions of certain atomic orbitals or components. PDOS provides a comprehensive perspective by allowing for the viewing of the contributions that certain atomic orbitals including s, p, and d orbitals make. With the assistance of PDOS, one is able to gain a deeper comprehension of the nuclear orbitals that play a significant role in the bonding and antibonding interactions. Using this strategy, it is much simpler to comprehend how the orbitals of various elements influence the electrical structure and characteristics of the material^37^. Figure 5 illustrates that the calculated PDOS for the Si-base reveals a uniform disparity in the intensities of the H-1s and Si-3p orbital configurations, with the intensity of the Si-3p orbital surpassing that of the H-1s. The PDOS for Si-base was examined, revealing that H-1s and Si-3p contribute similarly to the HOMO levels. The presence of metal oxides (ZnO, TiO_2_, and Al_2_O_3_) induces significant orbital interaction between the Si-3p and O-2p orbitals, evidenced by their overlap, which results in a modified configuration of Si and H atoms at the HOMO and LUMO levels. The Si-base is expected to provide a significant quantity of electrons to establish physical interactions with metal oxides^37^. Metal oxides (ZnO, TiO_2_, and Al_2_O_3_) have an effect that makes the PDOS peaks, which are spread out throughout the HOMO and LUMO levels from − 10 to 0, more intense. This occurrence exhibits the simultaneous bonding of the Si-3p with metal oxide electron orbitals and the reduction in energy due to metal oxide precipitation via a weak physical attachment. The Si–ZnO–TiO_2_ and Si–ZnO–Al_2_O_3_ combination compounds are responsible for this variation. In these composites, the orbitals of the metal oxides combine with those of the Si-3p, Zn-3d, Ti-3d, Al-3p, and O-2p levels. The overlapping shows that there is a change in the arrangement of the Si and H atoms across the HOMO and LUMO levels, and the more interactions between the Si-base and hybrid metal oxides, the more noticeable the difference.Fig. 5. Partial Density of States of the interacted Si-based with various metal oxides including (ZnO, Al_2_O_3_, TiO_2_, Zn–TiO_2_ and ZnO–Al_2_O_3_).
Molecule electrostatic potential (MESP)
The MEP contour maps are representations that illustrate the distribution of electrostatic potential surrounding a molecule. They provide the visualization of areas of positive and negative charge, which is crucial for figuring out chemical reactions and responses^38^. Through the use of the MEP, one possesses the ability to examine the achievable energy that is associated with a certain charge. This energy is calculated at various places around a molecule. The arrangement of the nuclei and electrons within the molecule is what determines this feature of the molecule. With MEP contours, the lines that link locations that have a comparable electrostatic potential are shown with same colour^39^. They pinpoint potential targets that might be attacked as electrophiles or nucleophiles. Red and orange on the MEP map indicate locations with a strong negative electrostatic potential (which is abundant in electrons), whereas yellow indicates areas with an extremely positive electrostatic potential (deficient in electrons)^40^. Figure 6 displays the contour MESP color maps for the examined structures: Si-base, Si–ZnO, Si–TiO_2_, Si–Al_2_O_3_, Si–ZnO–TiO_2_, and Si–ZnO–Al_2_O_3_. The colorful map of Si-base depicted the spread of negative potentials, with the red contour lines denoting regions around the Si sheet. The inside of the Si-base was shown in yellow, indicating minimal negativity. The influence of metal oxides exacerbates the detrimental consequences linked to both the metal oxides and the S-base. The observed observations revealed that the presence of metal oxides amplified negatively over the silicon substrate^41^. The functionalization with hybrid ZnO–TiO_2_ and ZnO–Al_2_O_3_ led to a substantial rise in negativity that extended beyond the Si-base and onto the metal oxide atoms. This resulted in a significant improvement in the transport of electrons, conductivity, and responsiveness.Fig. 6. Molecular electrostatic potential as contour map of the interacted Si-based with various metal oxides including (ZnO, Al_2_O_3_, TiO_2_, Zn–TiO_2_ and ZnO–Al_2_O_3_) generated using (Gaussian 09 program Revision C.01).
Reactivity and stability parameters for composites
Critical chemical reactivity criteria were delineated for each composite for assisting further examination and analysis of the reactivity features and longevity of composite materials. The utilization of these metrics may enhance the comprehension of structural stability, reactivity patterns, chemical manners, physicochemical alterations, and electrical features^42^. The fundamental attributes required to examine the stability and reactivity of compounds are based on the concepts of Mulliken and Koopmans^42,43^. The structures that were under inquiry were examined using the equations that are detailed below and tabulated in Table 2.Table 2DFT: B3LYP/6-31(d, p) computed the reactivity and stability descriptors of the Si-based interactions with several metal oxides, including ZnO, TiO_2_, Al_2_O_3_, Zn–TiO_2_, and ZnO–Al_2_O_3_.ParametersSi-baseSi–ZnOSi–TiO_2_Si–Al_2_O_3_Si–ZnO–TiO_2_Si–ZnO–Al_2_O_3_IP3.34384.11493.90053.95603.88724.4031EA2.00443.35273.11113.26383.31763.8991μ− 2.6741− 3.7338− 3.5058− 3.6099− 3.6024− 4.1511χ2.67413.73383.50583.60993.60244.1511η0.66970.38110.39470.34610.28480.2520σ1.49322.62402.53362.88933.51123.9683ΔNMax− 3.9930–9.7974− 8.8822− 10.4302− 12.6489− 16.4726ω5.338818.290815.569618.826022.783234.1897ε0.18730.05470.06420.05310.04390.0292
E_HOMO_ and E_LUMO_ were utilized to compute the ionization potential (IP) and electron affinity (EA), two variables that are significantly dependent on these parameters. Removal of electrons from a chemical structure result in the formation of free radicals, a phenomenon referred to as ionization potential. Electron affinity is the amount of energy necessary to attract electrons from another molecule, leading to the creation of negative ions that enhance nucleophilicity^44^.
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$IP = - E_{HOMO}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$EA = - E_{LUMO}$$\end{document}The mobility of electrons in materials is demonstrated by electronegativity (χ). The capacity of an electron to transfer is denoted by its chemical potential (μ), whereas the ability of a molecule to receive electrons is indicated by its electronegativity. The inverse connection between chemical potential (χ) and electronegativity (χ) is articulated by the subsequent equation^45^:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mu = - \chi = - \left( {IP + EA} \right)/{2}$$\end{document}When the electronegativity value of a material is higher, it implies that the material is more electronegative, which enhances the material’s capacity to absorb and react substances. Among the composites that were investigated, the composites with the highest electronegativity values were Si–ZnO and Si–ZnO–Al_2_O_3_, with respective values of 3.7338 and 4.1511 electronegativities. The band gap and the data from the MESP map both reveal that they have an extraordinary responsiveness; hence, this must be the real situation. The ability of a material to withstand electron-cloud dissociation or structural deformation is defined as chemical hardness (η). Furthermore, in the context of substance reactivity, softness (σ) is frequently seen as the inverse of hardness (η)^46^. This material is ideal for adsorption due to its exceptionally high softness and remarkably low hardness values, indicating a significant likelihood of contact. Among the many metal oxide composites evaluated, Si–ZnO–Al_2_O_3_ (3.9683) demonstrated the highest efficacy owing to its remarkable softness, stability, and reactivity for adsorption. The previously reported equations were employed to examine η, σ, and ΔNMax for each structure^47^:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upeta = \left( {{\mathrm{IP}} - {\mathrm{EA}}} \right)/{2}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upsigma = {1}/\upeta$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta {\mathrm{N}}_{{{\mathrm{Max}}}} = -\mu /\upeta = - \mu \upsigma$$\end{document}Ultimately, the molecular reactivity of a material may be represented according to its electrophilicity (ω) and nucleophilicity (ε), two critical attributes that dictate its propensity to donate or accept electrons from other molecules in its environment^48^. In contrast to nucleophilicity (ε), which quantifies the tendency to donate or transfer electrons based on a specific chemical composition, electrophilicity (ω) quantifies a substance’s tendency to accept electrons^49^. Substances are categorized into three distinct classes according to their electrophilicity: low electrophiles, medium electrophiles, and high electrophiles^50^. The electrophilicity index (ω) was utilized to assess a substance’s capacity to accept electrons, as indicated by the formula that follows equation^48^:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upomega =\mu ^{2} /2\eta$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon = 1/\omega$$\end{document}In the electrophile range, the Si-base appears at the exact middle of the range. Composite materials, specifically Si–ZnO–Al_2_O_3_, display increased electrophilicity when they are combined with metal oxides, which results in substantially greater electrophilic characteristics because of the integration. It is clear from this that the ZnO–Al_2_O_3_ NPs improved the surface area’s responsiveness as well as its absorption capabilities. The Si–ZnO–Al_2_O_3_ compound was the one that exhibited the highest level of sensitivity. In terms of their electrical and optical characteristics, Si–ZnO–Al_2_O_3_ compounds are very reactive. This is demonstrated by data that pertain to band gap and MESP, which further highlight the influence that hybridization produces.
Experimental result of Si–ZnO–Al2O3 compound
XRD analysis
XRD analysis was conducted to elucidate the crystalline structure, phase stability, and crystallite evolution of the ZnO, α-Al_2_O_3_, and ZnO–Al_2_O_3_ nanocomposites developed for high-performance UV photodetector applications. The diffraction patterns were collected in the 2θ range of 10°–80°, as presented in Fig. 7a. The pristine ZnO NPs exhibit a well-defined hexagonal wurtzite structure, with diffraction peaks indexed to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes at 2θ values of 31.7°, 34.3°, 36.1°, 47.4°, 56.5°, 62.8°, 66.3°, 67.7°, 68.9°, 72.5°, and 76.9°, respectively, in excellent agreement with JCPDS card No. 76-0704^51^. The absence of secondary phases confirms the high crystallographic purity of the synthesized ZnO NPs. In parallel, α-Al_2_O_3_ NPs display characteristic reflections corresponding to the (012), (104), (110), (113), and (024) planes at 2θ values of 25.6°, 35.1°, 37.8°, 43.4°, and 54.6°, respectively (JCPDS card No. 46-1212). The presence of the thermodynamically stable α-phase verifies the successful formation and structural robustness of Al_2_O_3_ NPs within the composite framework^18,52^. Importantly, no additional diffraction peaks associated with secondary or impurity phases—such as ZnAl_2_O_3_ spinel or other mixed oxide phases—are observed within the detection limit of the XRD instrument. This confirms the high phase purity of the synthesized materials and indicates that Al_2_O_3_ incorporation does not induce undesirable phase transformation but rather results in a well-integrated ZnO–Al_2_O_3_ nanocomposite system.Fig. 7(a) XRD patterns of pristine ZnO, pristine Al_2_O_3_, and ZnO–Al_2_O_3_ nanocomposite, confirming phase composition and structural integrity. (b) Experimental FTIR spectra of pristine ZnO and ZnO–Al_2_O_3_ nanocomposite, ighlighting surface functional groups and interfacial bonding. (c) Theoretical (DFT-calculated) infrared (IR) spectrum of the ZnO–Al_2_O_3_ nanocomposite, illustrating vibrational modes and validating the experimental FTIR features.
For the ZnO–Al_2_O_3_ nanocomposites, the diffraction peaks associated with ZnO remain at identical angular positions, indicating that the fundamental wurtzite lattice of ZnO is preserved upon Al_2_O_3_ incorporation. However, a moderate reduction in peak intensity accompanied by peak sharpening is observed, suggesting microstructural reorganization and crystallite growth rather than phase transformation or lattice substitution. Importantly, the absence of peak shifting further indicates that Al^3+^ ions do not substitute Zn^2+^ sites but are instead incorporated at grain boundaries or interfaces, acting as a structural passivation component. A subtle intensity enhancement near 2θ ≈ 35.1° is attributed to overlapping ZnO and α-Al_2_O_3_ reflections, indicative of strong interfacial coupling. Such behavior is associated with interfacial interactions mediated by surface hydroxyl groups on Al_2_O_3_ and polar functional groups within the matrix, which facilitate nanoparticle anchoring and induce localized lattice distortions^53^. These interfacial regions are expected to introduce shallow defect states and promote efficient carrier separation, a key factor in enhancing UV photoresponse. Moreover, the presence of Al_2_O_3_ induces a systematic reduction in XRD peak broadening, indicating enhanced crystallinity and promoted grain growth of ZnO nanoparticles^54^. This structural evolution suggests a decrease in lattice strain and a reduced density of grain-boundary scattering centers, which collectively favour improved charge transport and suppressed carrier recombination^55^. The average crystallite size (D) was estimated using the Debye–Scherrer equation:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$D = 0.{9}\uplambda /\upbeta {\mathrm{Cos}}\uptheta$$\end{document}where λ is the X-ray wavelength, β is the full width at half maximum (FWHM), and θ is the Bragg angle. The calculated average crystallite sizes are approximately 9.11 nm for α-Al_2_O_3_, 10.14 nm for pristine ZnO, and 10.83 nm for the ZnO–Al_2_O_3_ nanocomposites.
From a device perspective, the preserved crystal structure, enhanced crystallinity, and optimized interfacial coupling collectively contribute to improved carrier mobility, reduced trap-assisted recombination, and enhanced photo-generated charge extraction^56^. These structural advantages directly underpin the observed improvements in responsivity, detectivity, and operational stability of the ZnO–Al_2_O_3_-based UV photodetectors, confirming the critical role of controlled phase evolution and nanoscale crystallinity in device optimization.
Combined theoretical–experimental IR spectroscopic analysis
FTIR spectroscopy was employed to investigate the chemical bonding, surface functional groups, and interfacial interactions in pure ZnO and ZnO–Al_2_O_3_ nanocomposite films, as shown in Fig. 7b. The FTIR spectra provide essential insight into the structural integrity of the metal–oxide frameworks and the nature of interactions responsible for the enhanced optoelectronic performance of the composite-based UV photodetectors. The FTIR spectrum of pristine ZnO exhibits a prominent absorption band in the low-wavenumber region below 600 cm^−1^, centered around 430–480 cm^−1^, which is characteristic of the Zn–O stretching vibration in the hexagonal wurtzite structure. This band confirms the successful formation of crystalline ZnO and is consistent with previously reported results for nanostructured ZnO systems. A broad absorption band observed in the range 3200–3600 cm^−1^ is attributed to the stretching vibrations of surface-adsorbed –OH groups, originating from hydroxylated ZnO surfaces or physically adsorbed moisture. The presence of these hydroxyl groups is particularly relevant for UV photodetection, as they act as active sites for oxygen adsorption and desorption processes, which play a crucial role in modulating the photoconductive response under UV illumination. Additionally, a weak band near 1630–1650 cm^−1^ is assigned to the bending mode of molecularly adsorbed water (H–O–H), further confirming the hydrophilic nature of the ZnO surface. Importantly, no additional bands corresponding to organic residues or undesired impurities are detected, indicating high chemical purity of the synthesized ZnO.
The FTIR spectrum of the ZnO–Al_2_O_3_ nanocomposite retains the characteristic Zn–O vibrational band, confirming that the ZnO crystal structure is preserved after composite formation. Notably, this band exhibits a slight shift toward lower wavenumbers and mild broadening compared to pure ZnO, which can be attributed to interfacial interactions and lattice distortion induced by the incorporation of Al_2_O_3_. Such shifts are indicative of modified local bonding environments and enhanced electronic coupling at the ZnO/Al_2_O_3_ interface. New absorption features appear in the region 500–800 cm^−1^, which are assigned to Al–O and Al–O–Al stretching vibrations, confirming the successful incorporation of Al_2_O_3_ within the composite matrix. The coexistence of Zn–O and Al–O vibrational modes without the emergence of additional impurity-related peaks strongly supports the formation of a clean heterostructured nanocomposite, rather than undesired secondary phases. Furthermore, the broad –OH stretching band (3200–3600 cm^−1^) in the ZnO–Al_2_O_3_ composite becomes noticeably less intense and more symmetric compared to pure ZnO. This behavior suggests effective passivation of surface hydroxyl groups and defect sites by Al_2_O_3_, which reduces surface trap density and suppresses non-radiative recombination pathways. The attenuation of the H–O–H bending mode near 1630 cm^−1^ further supports the reduction of weakly bound surface moisture, contributing to improved environmental stability. The FTIR results provide compelling evidence that Al_2_O_3_ incorporation leads to chemical passivation and interfacial stabilization of ZnO without altering its fundamental crystal structure. The reduction of hydroxyl-related defect states and the establishment of ZnO–Al_2_O_3_ interfacial bonding directly correlate with the observed enhancements in electrical conductivity, reduced R_ct_, and improved UV photoresponse characteristics. These modifications facilitate more efficient charge transport and faster carrier dynamics, which are critical for high-performance UV photodetector operation. Therefore, the FTIR analysis confirms that the ZnO–Al_2_O_3_ nanocomposite architecture achieves a synergistic balance between structural integrity and surface defect control, underpinning the superior sensitivity, stability, and reproducibility of the developed Si-based UV sensor–photodetectors.
To further substantiate the experimentally observed FTIR features and to gain deeper insight into the underlying intermolecular interactions and bonding configurations within the ZnO–Al_2_O_3_ system, a complementary theoretical vibrational analysis was conducted. While experimental FTIR spectroscopy provides direct evidence of functional groups and bonding environments, DFT calculations enable the precise assignment of vibrational modes and clarify the origin of newly emerging bands associated with hybridization and interfacial coupling. Accordingly, the simulated DFT-IR spectrum was employed to validate the experimentally detected metal–oxygen vibrations and to elucidate the formation of interfacial Zn–O–Al linkages within the nanocomposite architecture. On the other hand, the importance of IR spectroscopy as a tool for characterizing chemical interactions through the detection and determination of the molecular structure and functional groups that are included inside a molecule is something that should be taken into consideration^57^. The ZnO–Al_2_O_3_ nanocomposite under investigation was analyzed for its structure and intermolecular interactions, and the resulting estimated DFT-IR spectrum is shown in Fig. 7c. In the nanoscale range, the vibrational bands characteristic of metal oxides were identified at 1114, 922, and 772 cm^−1^ for Al–O, and at 694 cm^−1^ for Zn–O^58,59^. Notably, the conspicuous emergence of a new vibrational band at 991 cm^−1^ is directly associated with the formation of interfacial Zn–O–Al bonding, serving as a theoretical fingerprint of successful nanocomposite formation and corroborating the experimental FTIR observations^60^.
Morphology and EDX analysis
The surface morphology of the fabricated ZnO-based and ZnO–Al_2_O_3_ composite films was investigated in detail using high-resolution scanning electron microscopy (SEM) to elucidate the effect of Al_2_O_3_ incorporation on the structural hierarchy and porosity of the sensing layers, as seen in Fig. 8. In the case of pure ZnO, the SEM micrographs revealed a highly uniform porous network composed of densely packed, quasi-spherical nanoparticles interconnected into a continuous scaffold^61^. The pore channels were well-defined, with an average pore size of 0.20 μm, as determined from statistical measurements of 22 representative pores selected from multiple SEM regions using ImageJ image analysis software. Figure 8a–d highlights several representative pores that were explicitly selected and marked to illustrate the pore measurement procedure and statistical analysis used to determine the average pore size. The pore size distribution exhibited a relatively narrow spread, indicating a high degree of structural homogeneity^62^. Although the distribution does not strictly follow a Gaussian profile, the mean pore size adequately represents the dominant pore population due to the limited dispersion observed in pristine ZnO films. This uniformity is characteristic of ZnO layers formed under controlled nucleation and growth conditions, where the absence of secondary phases or dopants leads to relatively isotropic grain expansion during the synthesis stage^63^.Fig. 8(a–d) FE-SEM micrographs of pristine ZnO and ZnO–Al_2_O_3_ thin films, respectively, illustrating the morphological evolution from a uniform nanoporous network (average pore size ≈ 0.20 µm) to a hierarchically structured architecture with enlarged and interconnected pores (average pore size ≈ 0.31 µm) upon Al_2_O_3_ incorporation. (e,f) HR-TEM images of pristine ZnO and ZnO–Al_2_O_3_ films, respectively, revealing improved crystallinity and intimate interfacial contact within the nanocomposite structure.
In contrast, the introduction of Al_2_O_3_ into the ZnO matrix resulted in a distinct and measurable modification of the microstructural features. The ZnO–Al_2_O_3_ nanocomposite displayed a hierarchically organized porous structure with multiple levels of pore sizes ranging from sub-100 nm nanopores to larger macropores approaching the micron scale. The average pore size increased to 0.31 μm, representing an approximate 55% enlargement compared to pure ZnO. The corresponding pore size histogram shows an asymmetric, non-Gaussian distribution, indicating increased structural heterogeneity. Accordingly, the pore size dispersion is more appropriately described by a Lorentzian-type distribution, where the peak position corresponds to the dominant pore size and the distribution width reflects the intrinsic morphological disorder induced by Al_2_O_3_ incorporation. Statistical analysis of the pore size histograms indicated a broader distribution and an increased standard deviation, reflecting the greater structural complexity introduced by the secondary Al_2_O_3_ phase^53^. This phenomenon is likely a result of the altered nucleation dynamics during film formation: Al_2_O_3_, with its distinct surface energy and low lattice match to ZnO, can disrupt the regular coalescence of ZnO crystallites, creating additional voids and preventing excessive grain boundary fusion^61^. The presence of Al_2_O_3_ nanoparticles may also act as spacers between ZnO grains, further enhancing the inter-particle gaps and producing a more open porous framework. From a functional perspective, the observed pore size enlargement and hierarchical structuring are highly beneficial for UV sensor–photodetector performance^64^. Larger interconnected pores increase the optical path length for incident UV photons, thereby enhancing light scattering and improving absorption efficiency within the active layer^65^. Moreover, the increased void fraction improves gas permeability and facilitates rapid adsorption–desorption kinetics of oxygen species on the surface, which is critical for modulating the surface depletion layer in photoconductive UV sensors. In such devices, the adsorption of oxygen in the dark and its subsequent photodesorption under UV illumination govern the change in conductivity. Therefore, a hierarchical porous architecture not only increases the number of active sites but also accelerates the response and recovery times by enabling faster gas exchange and carrier transport through reduced tortuosity^66^.
Building upon the morphological analysis presented above, TEM and high-resolution TEM (HR-TEM) were employed to provide nanoscale insight into the internal morphology, crystallinity, and interfacial characteristics of the pristine ZnO and ZnO–Al_2_O_3_ films. Figure 8e and f present the HR-TEM images of pure ZnO and ZnO–Al_2_O_3_ nanocomposite films, respectively. The HR-TEM image of pristine ZnO (Fig. 8e) reveals well-defined lattice fringes, confirming its crystalline nature with a typical interplanar spacing of approximately 0.26 nm, which corresponds to the (002) plane of hexagonal wurtzite ZnO. However, localized lattice distortions and minor fringe discontinuities are observed, which can be attributed to intrinsic defects, grain boundaries, and surface-related imperfections commonly associated with nanoscale ZnO structures^55^. These structural irregularities are known to act as charge trapping and recombination centers, thereby limiting carrier transport efficiency. In contrast, the ZnO–Al_2_O_3_ nanocomposite film (Fig. 8f) exhibits markedly improved crystallinity, as evidenced by sharper, more continuous lattice fringes and reduced lattice distortion^67^. The intimate interfacial contact between ZnO nanocrystallites and the surrounding Al_2_O_3_ phase is clearly discernible, indicating effective nanoscale integration rather than simple physical mixing^68^. The Al_2_O_3_ phase appears as a thin, amorphous or weakly crystalline shell surrounding ZnO domains, which plays a critical role in surface passivation by suppressing defect states and mitigating non-radiative recombination pathways^51^. The enhanced structural coherence and improved interfacial coupling in the ZnO–Al_2_O_3_ nanocomposite facilitate more efficient charge transport across grain boundaries and interfaces^15^. This structural refinement directly supports the observed improvements in electrical conductivity, reduced charge-transfer resistance, and enhanced photodetection metrics, including higher responsivity and faster response–recovery behavior^53^. The HR-TEM observations are in excellent agreement with the XRD results, which indicate increased crystallite size and enhanced crystallinity upon Al_2_O_3_ incorporation, confirming the strong correlation between nanoscale structural engineering and macroscopic device performance^69^.
Energy-dispersive X-ray (EDX) analysis provided complementary insights into the elemental composition of the samples, confirming the successful incorporation of Al_2_O_3_ into the ZnO framework, as seen in Fig. 9. For the pure ZnO film, the EDX spectrum exhibited two major peaks corresponding to oxygen (O K) and zinc (Zn K) with measured weight percentages of 26.66 wt% O and 73.34 wt% Zn, in close agreement with stoichiometric ZnO values. This composition reflects the high purity of the synthesized ZnO phase, with no detectable impurities or secondary elements within the instrument’s resolution limits. Upon Al_2_O_3_ incorporation, the ZnO–Al_2_O_3_ composite spectrum revealed 26.87 wt% O, 70.20 wt% Zn, and 2.93 wt% Al, clearly confirming the presence of aluminum within the structure. The slight increase in oxygen content in the composite compared to pure ZnO can be attributed not only to the oxygen supplied by Al_2_O_3_ but also to possible changes in surface chemistry, such as increased hydroxylation due to the higher surface area and enhanced moisture adsorption. The detection of Al at nearly 3 wt% strongly indicates that Al_2_O_3_ is not merely a surface coating but is uniformly distributed throughout the nanocomposite layer. The even dispersion of Al_2_O_3_ may lead to localized strain fields and defect sites at the ZnO–Al_2_O_3_ interfaces, which can act as electron trapping or scattering centers, thereby influencing the optoelectronic behavior of the device^70^.Fig. 9EDX spectra of pure ZnO and ZnO–Al_2_O_3_ confirming the successful incorporation of Al_2_O_3_, with corresponding elemental weight percentages revealing the presence of Al (2.93 wt%) alongside Zn and O.
The synergy between morphological and compositional modifications in the ZnO–Al_2_O_3_ nanocomposite is expected to have a significant impact on device performance. Morphologically, the increased porosity and hierarchical structuring enhance both light–matter interaction and analyte accessibility. Compositionally, the incorporation of Al_2_O_3_ introduces heterojunction interfaces that may facilitate more efficient separation of photogenerated electron–hole pairs, suppressing bulk recombination and improving photocurrent generation^71^. Furthermore, the slight variation in elemental ratios can lead to fine-tuning of the surface work function, oxygen adsorption energetics, and depletion width—all of which are critical parameters in determining UV detection sensitivity and response kinetics^72^. Therefore, the combined SEM and EDX analyses reveal that Al_2_O_3_ incorporation into ZnO fundamentally transforms both the structural and chemical landscape of the sensing layer. The resulting hierarchical porosity, larger average pore sizes, and modified surface chemistry collectively establish a platform for enhanced UV photodetection, providing a balance between high photon absorption, rapid carrier extraction, and fast surface reaction dynamics. These attributes make ZnO–Al_2_O_3_ nanocomposite architectures highly promising candidates for next-generation high-sensitivity UV sensor–photodetector applications^72^.
NMR result
NMR spectroscopy is a crucial analytical tool that provides deep understanding into atomic structure, chemical environments, atomic connection, the number of distinct nuclear environments, and their locations inside the molecules themselves^73^. Under the influence of an external magnet, NMR works by arranging protons into isotopic nuclei having non-zero rotations. The receipt of an electromagnetic signal in certain radio frequency energy causes the configuration to take place. This energy is controlled by the isotopic nature of the radioactive construction and grows in relation to the size of the surrounding magnetism. The chemical makeup of the surrounding environment dictates the frequency at which its resonances of each energetic isotope nuclei^74^. The ZnO–Al_2_O_3_ studied was analyzed using nuclear magnetic resonance (NMR) isotopes ^67^Zn, ^17^O, and ^27^Al in a solution of dimethyl sulfoxide (DMSO) specified to tetramethyl silane (TMS) to obtain chemical shifting validation, as illustrated in Fig. 10. The ^67^ZnNMR spectrum revealed the presence of Zn at a value of δ = 2872.11 ppm, which was due to the presence of ZnO^75^. Likewise, the O atoms in ZnO were observed to have two separate resonances in the ^17^ONMR chart, which were located at − 858.61 and − 235.625 ppm. On the other hand, the O atoms in Al_2_O_3_ corresponded to 259.66, − 80.8, and − 3486.55 ppm, which indicates that there is a strong link between Zn and Al^17^. In accordance with its interaction with ZnO through the O atom, the ^27^AlNMR chart displayed the Al atoms in Al_2_O_3_ at the positions of 522.02 and 541.57 according to the data^76^.Fig. 10DFT-NMR spectra of ZnO–Al_2_O_3_ nanocomposite.
Apparent porosity and surface roughness
The apparent porosity of the fabricated thin films was quantified using the Archimedes principle in accordance with ASTM C373-88 standards, providing an indirect measure of the void volume fraction accessible to fluids within the composite matrix. The pure ZnO film exhibited an apparent porosity of 26%, whereas the ZnO–Al_2_O_3_ hybrid film demonstrated a substantially higher porosity of 36%. This 10% absolute increase reflects the structural influence of Al_2_O_3_ NPs, which likely disrupt the close packing of ZnO crystallites, generating additional intergranular voids and nano-/micro-scale channels^77^. The higher porosity in the hybrid structure is advantageous for UV sensor–photodetector applications, as it can facilitate enhanced light scattering within the active layer, thereby increasing the optical path length and improving photon absorption efficiency^78^. Furthermore, increased porosity can promote more effective penetration of the electric field and increase the active surface area for charge separation, although it must be balanced against potential increases in carrier recombination at high surface defect densities^79^.
Surface topography, characterized through contact profilometry, revealed notable differences in surface roughness (Ra) between the two film types^80^. The pure ZnO film exhibited an Ra value of 6.7 µm, indicative of moderately textured surfaces formed by grain agglomeration^81^. In contrast, the ZnO–Al_2_O_3_ hybrid film displayed a higher Ra of 8.2 µm, suggesting that the introduction of Al_2_O_3_ NPs not only altered the grain packing but also enhanced the hierarchical structuring of the surface. This increase in roughness is consistent with the formation of a more complex multi-scale morphology, in which nanoscale asperities overlay micron-scale texturing^82^. The concurrent increases in both porosity and roughness in the ZnO–Al_2_O_3_ hybrid are synergistic from a device performance perspective. Higher roughness improves the interfacial contact between the active layer and the underlying Si substrate, potentially lowering interfacial resistance, while also acting as an optical trapping layer to reduce surface reflection losses^54^. Meanwhile, increased porosity can enhance the infiltration and interaction of the surrounding medium (air, vacuum, or protective coatings) with the sensing layer, thereby impacting dielectric properties and potentially modulating sensor response time and sensitivity. The combined effect of these morphological enhancements supports the superior UV photoresponse observed in the ZnO–Al_2_O_3_ devices compared to their pure ZnO counterparts.
EIS analysis and photocurrent dynamics
EIS was conducted to gain deeper insight into the interfacial charge-transport dynamics and recombination mechanisms within the fabricated Si-based UV sensor–photodetectors. Figure 11a shows the Nyquist plots for pure ZnO and ZnO–Al_2_O_3_ nanocomposite films under both dark and UV illumination, highlighting the pronounced reduction in charge-transfer resistance (R_ct_) upon Al_2_O_3_ incorporation. Meanwhile, Fig. 11b presents a schematic of the equivalent circuit model R_s_ − (R_ct_∥CPE), employed to fit the EIS data and quantitatively describe the interfacial charge-transfer behavior^83^. The impedance spectra for all samples exhibit a single, well-defined semicircle in the high-to-medium frequency range, characteristic of a charge-transfer process at the semiconductor–metal contact (ZnO/Si with metal electrodes) or grain boundary interface within the nanostructured layer, followed by a near-linear tail in the low-frequency region attributable to diffusion-controlled processes^84^. In the dark, pure ZnO films demonstrated a charge transfer resistance (R_ct_) of approximately 180 Ω, which is indicative of moderately hindered interfacial electron transport due to limited carrier density and potential trap states at grain boundaries. In contrast, the ZnO–Al_2_O_3_ hybrid exhibited a significantly lower R_ct_ value of ~ 95 Ω, reflecting the beneficial role of the Al_2_O_3_ component in modulating surface states, enhancing dielectric passivation, and promoting more effective electron percolation pathways^85^. The lower R_ct_ for the hybrid system also implies reduced recombination rates and improved interfacial conductivity, despite Al_2_O_3_ being an insulator, suggesting that its role is primarily structural—passivating defect sites and improving ZnO crystallite packing^77^. Upon UV illumination, both materials showed substantial decreases in R_ct_ values, consistent with the generation of photocarriers and the consequent enhancement in charge transport. For ZnO, R_ct_ dropped from 180 to 75 Ω, while for ZnO–Al_2_O_3_, it decreased from 95 to 40 Ω. The magnitude of reduction was more pronounced in the hybrid, suggesting that the optimized ZnO–Al_2_O_3_ architecture not only facilitates more efficient photogenerated carrier extraction but also supports sustained transport under illumination, likely due to a synergistic effect between ZnO’s photoactive properties and Al_2_O_3_’s ability to suppress trap-assisted recombination^67^.Fig. 11(a) Nyquist plots of pure ZnO and ZnO–Al_2_O_3_ composites under dark and UV illumination, illustrating the markedly reduced R_ct_ for the hybrid structure. (b) Schematic representation of the equivalent circuit model, R_s_ − (R_ct_∥CPE), used to fit the EIS data and describe the interfacial charge-transfer processes. (c) UV–Vis absorbance spectra of pure ZnO, pure Al_2_O_3_, and ZnO–Al_2_O_3_ nanocomposites.
Series resistance (R_s_) values were comparatively low for all samples (14.5–18 Ω) and showed only minor variation between dark and illuminated states, indicating that bulk and contact resistances are not the limiting factors in device performance. The double-layer capacitance (C_dl_) exhibited an upward shift from dark to illuminated states, with ZnO increasing from ~ 5.0 to ~ 6.0 μF and ZnO–Al_2_O_3_ from ~ 7.0 to ~ 8.5 μF, reflecting higher carrier accumulation at the interface under UV excitation. These EIS findings corroborate the hypothesis that the hierarchical ZnO–Al_2_O_3_ nanocomposite configuration substantially enhances both dark-state electrical conductivity and photoconductive response^86^. The lower R_ct_ and higher C_dl_ in the hybrid structure signify more effective suppression of recombination and improved interfacial kinetics, which align with the enhanced UV responsivity and faster recovery times observed in the device characterization^87^. This behavior can be attributed to the refined surface morphology, increased porosity, and optimized band alignment facilitated by the Al_2_O_3_ incorporation, enabling superior electron transport pathways and minimizing energy barriers at critical junctions^85^. Therefore, the EIS results provide compelling evidence that ZnO–Al_2_O_3_ hybrids achieve superior charge transport efficiency and interfacial stability compared to pure ZnO, underscoring their suitability for high-performance, Si-based UV sensor–photodetectors^88^.
Electrical and optical properties and UV selectivity
The electrical conductivity of the fabricated films was determined using the four-point probe method, revealing a pronounced enhancement upon incorporation of Al_2_O_3_ into the ZnO matrix. Pure ZnO films exhibited a conductivity of 27.7 × 10^−2^ S/m, while the ZnO–Al_2_O_3_ hybrid films achieved a markedly higher value of 44.5 × 10^−2^ S/m, corresponding to an increase of approximately 60.6%. This improvement in conductivity can be attributed to several synergistic mechanisms at play within the ZnO–Al_2_O_3_ nanocomposite architecture. Firstly, the dispersion of Al_2_O_3_ nanoparticles within the ZnO network promotes microstructural reorganization and enhanced particle packing density, reducing grain boundary scattering and improving electron percolation pathways. Although Al_2_O_3_ is intrinsically insulating, its nanoscale incorporation modifies the interface chemistry and reduces defect-induced charge trapping at the ZnO grain boundaries, thus facilitating charge transport^52^. Furthermore, the hybrid structure is likely to exhibit optimized carrier concentration due to partial passivation of oxygen vacancies—a common defect in ZnO—thereby balancing carrier mobility and lifetime. The interfacial bonding between Al_2_O_3_ may also induce localized electric field effects that assist in the separation and transport of photogenerated charge carriers under UV illumination, an effect that correlates with the improved photodetector performance observed in subsequent optical and EIS analyses^89^. The magnitude of conductivity enhancement observed here is consistent with literature reports on metal oxide hybrid systems, where strategic inclusion of secondary nanostructures facilitates enhanced electronic coupling and reduced resistive losses. This property is especially critical for UV sensor–photodetector applications, where fast charge transport is directly linked to high responsivity, reduced response/recovery times, and superior long-term operational stability^56^.
The optical absorbance spectra of pure ZnO, pure Al_2_O_3_, and the ZnO–Al_2_O_3_ hybrid nanocomposite, recorded in the wavelength range of 250–650 nm, are presented in Fig. 11c. All samples exhibit a pronounced absorption onset in the ultraviolet region, indicating the dominance of intrinsic band-to-band electronic transitions typical of wide-bandgap semiconductors. The absorption intensity decreases sharply in the visible region, underscoring their potential for UV-selective detection^88^. This behavior is particularly important in UV photodetector applications, where suppression of visible light sensitivity minimizes background noise and enhances device signal-to-noise ratio^90^. The inset of Fig. 11c, highlighting the 250–450 nm spectral region, shows a more abrupt and well-defined absorption edge for the ZnO–Al_2_O_3_ composite compared to single-phase ZnO or Al_2_O_3_. This suggests that the hybrid structure enhances UV photon harvesting while simultaneously reducing sub-bandgap transitions that could arise from defect-related states^70^. The sharper edge in the composite can be attributed to the synergistic interaction between ZnO and Al_2_O_3_ nanoparticles, which may induce microstructural ordering, lattice strain modulation, and passivation of oxygen vacancies, all of which improve the purity of the electronic transitions. Using the Tauc method for direct allowed transitions, the estimated optical bandgap energies (Eg) are 3.18 eV for ZnO, 3.11 eV for Al_2_O_3_, and 3.26 eV for the ZnO–Al_2_O_3_ composite. The slight bandgap widening in the composite corresponds to a blue shift in the absorption edge (~ 380 nm for ZnO vs. ~ 375 nm for ZnO–Al_2_O_3_), which further enhances UV selectivity by narrowing the detection window towards shorter wavelengths.
This is advantageous in UV-A and UV-B photodetector applications where specificity is critical, such as environmental monitoring, flame sensing, and space instrumentation. From a device physics perspective, UV selectivity in these materials is controlled by three interrelated factors: (1) Bandgap Position—Wide bandgap (> 3.1 eV) ensures cutoff beyond ~ 400 nm, reducing visible light response, (2) Defect State Density—Reduced deep-level defects in ZnO–Al_2_O_3_ minimize sub-bandgap absorption that can cause parasitic photocurrent under visible illumination^88^, (3) Interfacial Barrier Effects—The ZnO–Al_2_O_3_ heterointerface may form potential barriers that preferentially facilitate separation of high-energy UV-generated carriers while blocking lower-energy carriers from visible photons^90^. The ZnO–Al_2_O_3_ hybrid structure not only retains high UV absorption but also displays suppressed absorption in the 420–650 nm range, leading to a high UV-to-visible rejection ratio. This property is essential for real-world UV sensor–photodetectors operating under sunlight, where visible light intensity can be orders of magnitude higher than UV intensity^3^. The combination of enhanced UV absorption, minimal visible light sensitivity, and stable optical transitions makes ZnO–Al_2_O_3_ nanocomposites particularly promising for high-performance, environmentally stable UV photodetectors.
Current–voltage (I–V) characteristics, transient photocurrent response, and photodetection metrics
To comprehensively evaluate the electrical and photoresponse performance of the fabricated Si-based UV photodetectors, current–voltage (I–V) characteristics and time-dependent transient photocurrent (I–t) measurements were carried out under both dark UV illumination. These measurements provide direct insight into the charge transport mechanism, contact behavior, response dynamics, and key photodetection metrics, including responsivity and detectivity.
I–V characteristics under dark and UV illumination
Figure 12a presents the I–V characteristics of pure ZnO and ZnO–Al_2_O_3_ nanocomposite photodetectors measured over the voltage range of − 5 to + 5 V under dark and UV illumination. Both devices exhibit a nearly linear and symmetric I–V behavior in the forward and reverse bias regions, indicating the formation of ohmic contacts between the active semiconductor layer and the metal electrodes. The absence of rectifying behavior confirms that the photodetection mechanism is governed by photoconductive operation rather than junction-based (Schottky or p–n) carrier separation^91^. Under dark conditions, the ZnO–Al_2_O_3_ device shows a slightly higher current than pure ZnO, which is consistent with its enhanced electrical conductivity and reduced interfacial resistance, as previously evidenced by four-point probe measurements and EIS. Upon UV illumination, both devices demonstrate a pronounced increase in current while preserving linearity, reflecting a substantial enhancement in free carrier concentration due to photogeneration. Notably, the ZnO–Al_2_O_3_ hybrid exhibits a steeper I–V slope under UV exposure, signifying more efficient charge transport pathways and reduced recombination losses. The maintenance of linear I–V behavior under illumination suggests that UV exposure modulates the carrier density rather than altering the contact barrier height^92^. This observation is in excellent agreement with the low R_s_ and significantly reduced R_ct_ obtained from EIS analysis, confirming that interfacial barriers do not limit device performance. The incorporation of Al_2_O_3_ plays a crucial role in passivating surface and grain-boundary trap states, leading to a more homogeneous potential landscape and stable ohmic conduction.Fig. 12(a) I–V characteristics of pure ZnO and ZnO–Al_2_O_3_ UV photodetectors under dark and UV illumination, exhibiting linear ohmic behavior. (b) Transient I–t response under periodic UV ON/OFF switching, indicating stable and repeatable photoresponse. (c) Aging stability of the devices evaluated by photocurrent variation at 5 V under UV illumination for different aging times. (d) Normalized photocurrent retention, highlighting the superior long-term environmental stability of the ZnO–Al_2_O_3_ nanocomposite.
I–t response and switching behavior
The transient photoresponse of the devices was further examined by monitoring the photocurrent as a function of time under periodic UV ON/OFF switching at a fixed bias voltage, as shown in Fig. 12b. Both pure ZnO and ZnO–Al_2_O_3_ devices exhibit rapid and repeatable photocurrent modulation upon UV exposure, demonstrating excellent operational stability and reversibility over multiple switching cycles.
Upon UV illumination (ON state), a sharp increase in photocurrent is observed, corresponding to the rapid generation and separation of electron–hole pairs. When the UV source is switched OFF, the photocurrent quickly returns to its initial dark level, indicating efficient carrier recombination and minimal persistent photoconductivity^93^. Importantly, the ZnO–Al_2_O_3_ photodetector displays a higher photocurrent amplitude and faster recovery behavior compared to pure ZnO, highlighting the beneficial role of the hybrid architecture in suppressing trap-assisted recombination and facilitating rapid carrier extraction. The enhanced transient response of the ZnO–Al_2_O_3_ device can be attributed to the synergistic effects of increased surface roughness and porosity, which provide a larger density of active sites for photon absorption, together with effective defect passivation by Al_2_O_3_ that minimizes carrier trapping and prolongs carrier mobility. The linear I–V characteristics and reversible transient photoresponse observed in this work are consistent with typical photoconductive UV detectors reported for ZnO-based nanostructures^91–93^.
Responsivity and detectivity analysis
Key photodetection metrics, including responsivity (R) and specific detectivity (D*), were derived from the I–V and transient photocurrent data to quantitatively assess device sensitivity. As summarized in Table 3, the ZnO–Al_2_O_3_ nanocomposite photodetector exhibits a responsivity of approximately 0.12 A/W, nearly twice that of pure ZnO (~ 0.065 A/W). Correspondingly, the detectivity increases from ~ 1.8 × 10^10^ Jones for pure ZnO to ~ 3.4 × 10^10^ Jones for the ZnO–Al_2_O_3_ device. This substantial improvement reflects the combined effect of enhanced photocurrent generation, reduced dark current noise, and efficient interfacial charge transport in the hybrid structure. The observed enhancement in R and D* is fully consistent with the reduced R_ct_, increased interfacial capacitance, and improved electrical conductivity reported earlier, establishing a strong correlation between nanoscale interfacial engineering and macroscopic photodetector performance.Table 3. Electrical performance parameters of pure ZnO and ZnO–Al_2_O_3_ UV photodetectors under dark conditions and UV illumination, including dark current, photocurrent at 5 V bias, R, and D*.SampleDark current(mA @5 V)UV current(mA @5 V)Responsivity R (A/W)Detectivity D*(× 10^10^ Jones)Pure ZnO0.010.0750.0651.8ZnO–Al_2_O_3_0.01750.140.12253.4
Mechanistic implications for UV photodetection
The linear I–V characteristics, fast transient response, and enhanced photodetection metrics collectively confirm that the ZnO–Al_2_O_3_-based devices operate as high-performance photoconductive UV sensors. The introduction of Al_2_O_3_ does not act as a current-blocking layer; instead, it improves charge transport by passivating defect states, homogenizing the electric field distribution, and facilitating efficient carrier percolation within the ZnO matrix. These effects, combined with strong UV-selective absorption and minimal visible-light sensitivity, render the ZnO–Al_2_O_3_ nanocomposite architecture highly suitable for reliable and sensitive UV detection under practical operating conditions.
Aging stability and long-term operational reliability
To further substantiate the environmental stability of the fabricated UV photodetectors, aging stability tests were performed by monitoring the photocurrent response at a fixed bias voltage of 5 V under UV illumination after different aging intervals (1, 10, and 100 h). The photocurrent values were recorded and normalized with respect to their initial response to evaluate performance retention over time. As shown in Fig. 12c, both devices exhibit a gradual decrease in photocurrent with increasing aging duration, which can be attributed to surface adsorption of oxygen and moisture, as well as slow structural relaxation of defect states.
Nevertheless, the ZnO–Al_2_O_3_ photodetector maintains a significantly higher absolute photocurrent throughout the entire aging period compared to pure ZnO. The normalized photocurrent retention presented in Fig. 12d reveals that the ZnO–Al_2_O_3_ device preserves approximately 92% of its initial photocurrent after 100 h of aging, whereas pure ZnO retains only ~ 87%. This enhanced stability is attributed to the effective passivation of surface and grain-boundary defects by Al_2_O_3_, which suppresses environmental degradation mechanisms and minimizes charge trapping induced by ambient exposure. These results confirm that the incorporation of Al_2_O_3_ not only enhances the sensitivity and charge transport characteristics of the photodetector but also significantly improves its long-term operational stability, making the ZnO–Al_2_O_3_ nanocomposite architecture highly suitable for reliable UV sensing under practical environmental conditions.
Conclusion
In this work, we have demonstrated an integrated theoretical–experimental strategy for the design, fabrication, and performance optimization of hierarchically structured Si-based UV sensor–photodetectors employing ZnO–Al_2_O_3_ nanocomposite architectures. Density functional theory (B3LYP/6-31G(d,p)) calculations revealed that the ZnO–Al_2_O_3_ hybrid exhibits favorable band alignment, optimized frontier orbital separation, and enhanced charge transport pathways compared to its single-component counterparts. Experimental validation through XRD, FE-SEM, and EDX confirmed the successful synthesis and homogeneous integration of the hybrid nanoparticles into the sensing matrix. Morphological characterization showed that incorporating Al_2_O_3_ increased both surface roughness (from 6.7 to 8.2 μm) and apparent porosity (from 26 to 36%), contributing to improved photon capture and increased active sites for carrier generation. Optical characterization revealed strong UV-selective absorption (250–450 nm), with calculated band gaps of 3.18 eV (ZnO), 3.11 eV (Al_2_O_3_), and 3.26 eV (ZnO–Al_2_O_3_). Electrical conductivity measurements showed a marked improvement in the hybrid system (44.5 × 10^−2^ S/m) compared to pure ZnO (27.7 × 10^−2^ S/m), while electrochemical impedance spectroscopy confirmed reduced charge transfer resistance, aligning with the observed enhancement in responsivity and recovery speed. Collectively, these results highlight the synergistic effect of nanostructuring and oxide hybridization in enabling high-performance, stable, and UV-selective photodetection.
The demonstrated performance of ZnO–Al_2_O_3_ hybrid architectures opens several avenues for advancement. Future research could:
- Optimize interfacial engineering through controlled thickness of Al_2_O_3_ layers to further suppress recombination and improve responsivity.
- Explore alternative wide-bandgap oxide hybrids (e.g., ZnO–HfO_2_, ZnO–Ga_2_O_3_) to enhance spectral tunability and thermal stability.
- Integrate plasmonic nanostructures to exploit localized surface plasmon resonance (LSPR) for boosting UV absorption without compromising visible transparency.
- Implement flexible and transparent substrates to develop wearable and portable UV sensing devices.
- Adopt machine-learning-driven materials screening to accelerate the discovery of optimal nanocomposite compositions for tailored band gap and conductivity.
By bridging theoretical modeling with experimental validation, this study provides not only a proof-of-concept for ZnO–Al_2_O_3_-based UV photodetectors but also a framework for the rational design of next-generation optoelectronic materials with superior sensitivity, selectivity, and operational stability.
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