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Article

Collinear Pulse Train PLD: Fabrication of High-Refractive-Index-Difference TiO2/ZnO Multilayers with Multifunctional Applications

by
Xiang Zhao
1,
Guoyan Dong
1,*,
Zheng Zhu
1,
Yutao Qin
1,
Jiaxiang He
1 and
Jin Yu
2,*
1
Center of Materials Science and Optoelectronics Engineering, School of Opto-Electronics, University of Chinese Academy of Sciences, Beijing 101408, China
2
College of Physics Science and Technology, Hebei University, Baoding 071002, China
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2026, 16(3), 1354; https://doi.org/10.3390/app16031354
Submission received: 4 December 2025 / Revised: 21 January 2026 / Accepted: 26 January 2026 / Published: 29 January 2026
(This article belongs to the Special Issue Recent Advances and Applications of Electromagnetic Metamaterials)
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Abstract

Pulsed laser deposition (PLD) is widely used for functional film fabrication, but traditional nanosecond-laser-induced thermal effects and interface roughness severely limit the quality of multilayer structures. To address this critical challenge, a picosecond pulsed laser with collinear pulse train output was adopted for TiO2/ZnO multilayer preparation, achieving dual advantages of thermal diffusion suppression and roughness reduction. A systematic investigation was conducted on the properties of TiO2 and ZnO films, establishing a “constant-deposition-rate multi-pulse regulation” strategy that yielded low roughness (4.43 nm for TiO2, 3.27 nm for ZnO) and optimized refractive index matching. Through 500 °C oxygen annealing, TiO2’s refractive index was enhanced to 2.6, forming a large refractive index difference (Δn = 0.77) with ZnO (~1.83) for efficient photonic band gap (PBG) regulation. Integral annealing was identified as the optimal post-treatment, enabling the four-layer TiO2/ZnO multilayer to reach a maximum reflectance of 75% with excellent structural uniformity. The multifunctional applications of the multilayers exhibit excellent ability in photocatalytic degradation of tetracycline hydrochloride (TCH) and fluorescence enhancement of CdSe quantum dots (QDs). This work pioneers a high-quality PLD-based multilayer fabrication route and opens new avenues for its application in environmental remediation and optoelectronic devices.

1. Introduction

Multilayer films based on semiconductor materials have attracted extensive attention due to their tunable PBG and multifunctional properties, which find applications in optical filters, photocatalysis, and optoelectronic devices [1,2,3]. PLD is widely used for fabricating functional films owing to its high deposition precision, stoichiometric transfer of target materials, and compatibility with various substrates [4,5]. However, traditional PLD systems using nanosecond lasers suffer from severe thermal effects during deposition. For targets, ns-lasers’ long pulse duration causes localized melting, vaporization, and phase decomposition, altering plasma plume composition and generating defective droplets. For substrates, prolonged thermal interaction induces lattice distortion, interfacial atomic diffusion, and residual stress, exacerbating interface roughness and defects in multilayers. These issues are particularly critical for multilayer structures, as cumulative roughness and interface diffusion can degrade the photonic band gap effect and functional performance. Picosecond pulsed lasers, with their ultra-short pulse width (<10 ps), can effectively suppress thermal diffusion by reducing the interaction time between laser and target materials, thereby minimizing thermal damage to the films [6]. Moreover, the collinear pulse train output mode of picosecond lasers allows for secondary heating of the plasma generated by the initial pulse, promoting the decomposition of nanoparticles and reducing film roughness [7]. TiO2 and ZnO are ideal candidates for constructing multilayer films due to their suitable band gap energies, high chemical stability, and excellent optical/electronic properties [8]. TiO2 (rutile phase) has a high refractive index (~2.45), while ZnO exhibits a moderate refractive index (~1.85) and good thermal stability, forming a significant refractive index difference that is beneficial for PBG regulation [9].
TiO2/ZnO multilayers have garnered extensive attention for their multifunctional potential [10,11]. In photocatalysis, their heterojunction structure is expected to suppress electron–hole recombination [12,13], but existing studies show inconsistent results—combinatorial atomic layer deposition (C-ALD)-prepared ZnO/TiO2 nanolaminates even failed to exhibit enhanced photoactivity compared to single-layer ZnO [14,15]. For antibiotic degradation, most chemical-synthesized TiO2/ZnO composites only achieve a 1–1.5-fold improvement in tetracycline degradation rate relative to single-layer TiO2, limited by interface defects and insufficient light–material interaction [16]. TiO2/ZnO films show advantages of surface state and surface atomic mobility enhancement for improving the photocatalytic activity [17,18]. In fluorescence enhancement, embedding QDs into TiO2 or ZnO matrices [19,20] can realize moderate luminescence improvement via a dielectric confinement effect, with CdSe/ZnS QDs embedded in TiO2 crystals showing a 2.3-fold spontaneous emission rate enhancement [21]. Current research on TiO2/ZnO multilayers primarily focuses on nanosecond-laser-based PLD [22,23] or chemical synthesis [11,24], with fewer studies exploring picosecond-laser-driven deposition to address thermal effect issues. Moreover, systematic optimization of PLD parameters (e.g., sub-pulse number) for balancing film quality and deposition efficiency, as well as in-depth integration of PBG regulation with functional performance, is still lacking. Notably, existing multilayer systems often rely on 6–10 layers to pursue acceptable performance, leading to increased preparation complexity and defect accumulation.
In this work, a picosecond-pulsed-laser-based PLD system was established to prepare TiO2/ZnO multilayers. The objectives of this work are: (1) to optimize the PLD process parameters for TiO2, Al2O3, and ZnO single layers by investigating the effects of laser energy, pulse frequency, and sub-pulse number on film properties; (2) to enhance the refractive index difference between TiO2 and ZnO through temperature control and annealing treatment; (3) to study the influence of annealing strategies and layer number on the reflectance performance of TiO2/ZnO multilayers; and (4) to evaluate the multifunctional applications of the multilayers in photocatalytic degradation of TCH and fluorescence enhancement of CdSe QDs. This research provides a theoretical and experimental basis for the fabrication of high-performance semiconductor multilayer films.

2. Experimental Section

2.1. PLD Experimental System

A solid-state picosecond pulsed laser (λ = 532 nm, RAY-IR(GR)2001, Rayto Laser, Shenzhen, China) was used as the ablation source, consisting of a seed oscillator, laser amplifier, and acousto-optic modulator for sub-pulse generation. As shown in Figure 1a, the laser emitted collinear picosecond pulse trains with a repetition frequency ranging from 50 to 1000 kHz, and the maximum single-pulse energy was 400 μJ. To facilitate precise alignment of the optical path, a He-Ne laser with a wavelength of 632.8 nm was employed as the guiding light, maintaining coaxial propagation with the picosecond laser beam throughout the optical path. The laser beam underwent a two-step optical processing: first, it was expanded using a Galilean beam expander (GCO-140113, Daheng Optics, Beijing, China) to adjust the beam divergence angle and optimize energy density, and then it passed through a focusing lens with a focal length of 350 mm. A high-speed scanning galvanometer (500 mm–2000 mm/s, YZD-6002HD, BRIT Optoelectronic Tech. Co., Ltd., Wuhan, China) was integrated into the optical path to achieve uniform scanning of the laser beam over the target surface, ensuring consistent ablation across the entire target area.
As shown in Figure 1b,c, the vacuum deposition chamber was evacuated to a base pressure below 10−4 Pa through a two-stage pumping system: a mechanical pump for preliminary evacuation and a turbomolecular pump (operating at a maximum rotation speed of 27,000 rpm and pumping speed of 1200 L/s, FF-200/1200, KYKY Co., Ltd., Beijing, China) for high-vacuum refinement. The targets used in the experiment included TiO2 (99.99%, 25.4 × 45 mm, JingMaiYan, Beijing, China) and ZnO (99.99%, 25.4 × 45 mm, JingMaiYan) and were fixed on a rotatable holder with a rotation speed of 10 rpm to ensure uniform ablation. The substrates were either single-crystal Si (100) or quartz, mounted on a rotating bracket with an adjustable rotation speed of 20 rpm and a temperature-controllable range from room temperature (approximately 25 °C) to 500 °C. The laser incident angle was set to 60° ± 1° relative to the target surface to optimize the ablation efficiency and plasma plume propagation.

2.2. Deposition Process

The PLD deposition process was systematically divided into five key steps to ensure the quality and reproducibility of the films:
(1)
Substrate and target preparation: The substrates were subjected to a rigorous cleaning process to remove surface contaminants. They were sequentially ultrasonicated in acetone, ethanol, and deionized water for 15 min each and then dried with high-purity nitrogen gas to avoid water stains. The targets were polished using fine abrasive paper to achieve a smooth surface with low roughness, ensuring uniform laser ablation and consistent film growth.
(2)
Laser adjustment: Prior to operation, the laser water chiller was activated and set to a constant temperature of 18.5 °C to maintain the stability of the laser output. The laser power supply was turned on, and the control software was launched on a connected computer to adjust parameters such as pulse energy, frequency, and pulse number. A laser power meter was used to calibrate the output power to ensure it matched the experimental requirements. The high-speed scanning galvanometer and its control card software were initialized, and a scanning mode was programmed (1200 mm/s for TiO2, 500 mm/s for ZnO) to guarantee that the laser spot irradiated the target center precisely and scanned uniformly across the target surface.
(3)
Vacuum evacuation: All windows and valves of the deposition chamber were sealed tightly. The mechanical pump and bypass valve were opened first, and the chamber was evacuated until the pressure dropped below 10 Pa. Subsequently, the bypass valve was closed, and the foreline valve and turbomolecular pump were activated. After the turbomolecular pump reached its maximum rotation speed of 27,000 rpm, the gate valve was opened and set to full open. The ionization vacuum gauge (ZJ-12/CF35, Ruibao Elec. Tech. Co., Ltd., Shanghai, China) of the coating chamber was turned on, and the evacuation process continued until the chamber pressure fell below 10−4 Pa.
(4)
Film deposition: A 5 min pre-deposition step was performed to stabilize the plasma plume and remove any residual contaminants on the target surface. During formal deposition, the laser parameters (energy, frequency, pulse number) were precisely controlled to achieve the desired film thickness. The substrate rotation speed was maintained at 20 rpm to ensure uniform film growth, and the deposition time was adjusted according to the target thickness requirements. For multi-pulse deposition experiments, the number of sub-pulses was adjusted while synchronously optimizing the pulse energy and frequency to maintain a stable deposition rate.
(5)
Post-deposition treatment: After the deposition was completed, the ionization vacuum gauge and turbomolecular pump were turned off, and the system was allowed to cool down until the turbomolecular pump stopped rotating. The gate valve, foreline valve, and bypass valve were closed in sequence, and the air release valve was opened to return the chamber pressure to atmospheric pressure. Finally, the samples were carefully collected. For annealing treatment, the films were placed in a tube furnace with an oxygen atmosphere, heated at a rate of 5 °C/min to 500 °C, and annealed for 60–150 min to improve film crystallinity, repair oxygen vacancies, and enhance optical properties.

2.3. Characterization and Performance Testing

A comprehensive suite of characterization techniques was employed to evaluate the structural, optical, and functional properties of the prepared films in this work. Surface morphology and roughness (Sa, defined as the arithmetic mean of height deviations from the reference surface according to ISO 25178-2:2021 [25]) were characterized using a confocal laser scanning microscope (CLSM, SENSOFAR S NEOX, Sensofar Metrology, Terrassa, Spain) and a scanning electron microscope (SEM, SU8010, HITACHI, Tokyo, Japan or ZEISS Sigma 500, Carl Zeiss AG, Oberkochen, Germany). The SEM images provided high-resolution observations of surface features such as particle size, distribution, and agglomeration, while the confocal microscope was used to measure three-dimensional roughness and generate surface topography maps. The refractive index and thickness of the films were measured using a spectroscopic ellipsometer (SE800, SENTECH Instruments GmbH, Berlin, Germany) at a wavelength of 500 nm. The ellipsometer fitting process yielded the mean squared error (MSE) value, which quantifies the deviation between theoretical and experimental ellipsometric parameters, reflecting the accuracy of the refractive index measurement and the quality of the film. A homogeneous layer model was used for the fitting to prioritize the film’s intrinsic uniformity, excluding artificial complexity like roughness gradients or birefringence. For non-optimized samples with higher MSE, their refractive indices were only used for process optimization comparisons. The crystal structure of the films was analyzed by X-ray diffraction (XRD, D8 ADVANCE Plus, Bruker Corporation, Billerica, MA, USA). The XRD patterns were used to identify the crystal phases (e.g., rutile phase of TiO2, polycrystalline phase of ZnO) and evaluate the crystallinity and preferred orientation of the films.
For photocatalytic performance testing, a 300 W xenon lamp was used as the simulated sunlight source. The films were immersed in 100 mL of tetracycline hydrochloride (TCH) solution with an initial concentration of 20 mg/L. Prior to illumination, the solution was stirred in the dark for 60 min to achieve adsorption–desorption equilibrium between the film surface and the TCH molecules. During illumination, 5 mL of the solution was sampled every 30 min, and the absorbance was measured using a UV–vis spectrophotometer (U-3900, HITACHI High-Technologies Corporation, Tokyo, Japan). The TCH concentration and degradation rate were calculated based on the absorbance data, and the reaction kinetics were analyzed using quasi-first-order reaction models. For fluorescence enhancement testing, CdSe quantum dots (QDs) with an excitation wavelength of 450 nm and an emission wavelength of 527 nm were deposited on the film surfaces. A fluorescence spectrometer was used to record the fluorescence spectra of the QDs on blank quartz substrates, single-layer TiO2 films, and TiO2/ZnO multilayer films. The fluorescence intensity was compared to evaluate the enhancement effect of the film structures on the QD fluorescence.

3. Results and Discussion

3.1. Optimization of Single-Layer Deposition Process

The selection of appropriate single-layer materials and optimization of their deposition processes are foundational to the fabrication of high-performance TiO2/ZnO multilayers. We systematically investigated the effects of key PLD parameters—laser energy, pulse frequency, and sub-pulse number—on the structural and optical properties of TiO2 (high-refractive-index layer) and ZnO (low-refractive-index layer) single films. The goal is to identify optimal process windows that balance deposition efficiency, film uniformity, and optical performance, laying the groundwork for subsequent multilayer fabrication.

3.1.1. TiO2 Single Layers

TiO2 was chosen as the high-refractive-index material due to its theoretical refractive index of 2.45 in the visible spectrum and excellent chemical stability. Given the 532 nm wavelength of the picosecond laser used in this study—closely matching the band gap of TiO2—the material exhibits efficient laser absorption, making it suitable for PLD deposition. To optimize its performance, the effects of three critical parameters were systematically studied: laser energy (30–55% of the maximum single-pulse energy, corresponding to an average power range of 5.31–8.45 W), pulse frequency (60–140 kHz), and sub-pulse number (1–7).
Laser energy directly governs the ablation intensity of the TiO2 target, thereby influencing film growth dynamics and quality. During the deposition process, the laser frequency was 100 kHz, and the deposition time was 40 min at room temperature for a single pulse. As shown in Figure 2a, when the laser energy was set to 30% of the maximum single-pulse energy (average power ≈ 5.31 W), the target surface only exhibited a faint plasma plume, indicating insufficient ablation. The resulting TiO2 film had a thickness of 15.4 nm, a roughness (Sa) of 6.1 nm, and a refractive index of 2.15. As the laser energy increased, the plasma plume became more intense, promoting increased material ejection from the target. This led to a significant increase in film thickness and a moderate rise in Sa, while the refractive index slightly decreased due to the initial formation of small particle aggregates on the film surface. The introduced abundant nanoscale voids between the clusters decreased the effective refractive index of the film. A critical transition was observed when the laser energy reached 40% (average power ≈ 7.08 W). The film growth rate surged from 0.15 Å/s (at 35% energy) to 0.26 Å/s, with the thickness increasing to 63.4 nm. However, this enhancement in deposition efficiency was accompanied by a sharp increase in Sa (31.0 nm)—more than double the value at 35% energy. The refractive index further decreased to 1.948, and the MSE of ellipsometric fitting rose to 3.449, reflecting a decline in film quality due to increased surface irregularities and reduced density. When the laser energy exceeded 55% (average power > 8.45 W), the film quality deteriorated dramatically. As shown in Figure 2b, the SEM image of the film surface reveals severe particle agglomeration, with large clusters of molten material forming on the surface. Consequently, Sa increased to 57.1 nm, and the refractive index plummeted to 1.657. The film also turned gray and lost its optical transparency, making it unsuitable for optical applications. These results indicate that a laser energy range of 35–45% (average power 6.19–7.96 W) can be identified as optimal for balancing deposition rate and film quality.
To further optimize film quality, the effect of pulse frequency (60–140 kHz) was investigated while maintaining the laser energy at 40% for single pulses at room temperature. Since the total laser power was kept constant within this range, increasing the frequency led to a proportional decrease in single-pulse energy. As shown in Table 1, this parameter adjustment had a significant impact on film properties. At a frequency of 60 kHz, the single-pulse energy was relatively high, resulting in a thick film (107.4 nm) but poor surface quality (Sa = 43.2 nm). The refractive index was ~1.83, and the MSE value was 7.913—indicative of a porous, irregular structure with numerous defects. As the frequency increased to 80 kHz, the single-pulse energy decreased, leading to a reduction in film thickness (83.18 nm) and Sa (35.0 nm). The refractive index improved to 1.881, and the MSE value dropped to 5.351, suggesting a more uniform film structure. At 100 kHz, the film thickness was 63.4 nm, Sa was 31.0 nm, and the refractive index reached ~1.95—representing a balance between deposition efficiency and quality. A further increase in frequency to 120 kHz and 140 kHz resulted in a continued decrease in single-pulse energy, leading to thinner films (50.1 nm and 30.3 nm, respectively) but significantly improved surface smoothness. At 140 kHz, Sa was reduced to 11.2 nm—less than one-quarter of the value at 60 kHz—and the refractive index increased to ~2.0, approaching the theoretical value. The MSE value also decreased to 2.19, indicating a better match between the experimental film and the theoretical ellipsometric model. This improvement is attributed to the reduced single-pulse energy, which minimized thermal ablation and the formation of large droplets, resulting in a more uniform, dense film. However, the trade-off between frequency and deposition rate must be considered. At 140 kHz, the deposition rate was only 0.126 Å/s—less than half of the rate at 60 kHz (0.447 Å/s). To achieve a film thickness comparable to that at 60 kHz (107.4 nm) at 140 kHz, the deposition time needs to be extended from 40 min to over 140 min, which is impractical for efficient fabrication. Thus, a pulse frequency of 100 kHz is selected as optimal, as it balances a reasonable deposition rate (0.264 Å/s) with good film quality (Sa = 31.0 nm, refractive index = 1.95).
To address the relatively high roughness of TiO2 films deposited at 100 kHz (Sa = 31.0 nm) without sacrificing deposition rate, the effect of sub-pulse number (1–7) was investigated while maintaining a constant deposition rate of ~0.264 Å/s (~laser energy at 40%). This was achieved by adjusting the laser energy percentage and frequency synchronously: as the number of sub-pulses increased, the laser energy percentage was increased to compensate for the reduced energy per sub-pulse, while the frequency was decreased to maintain the total average power. As shown in Table 2, increasing the sub-pulse number had a profound impact on film quality. For a single sub-pulse, the film exhibited a Sa of 31.0 nm, a thickness of 63.4 nm, a refractive index of 1.95, and an MSE of 3.449. When the sub-pulse number was increased to 3, Sa decreased to 22.1 nm, the refractive index improved to 2.04, and the MSE dropped to 3.291. This improvement is attributed to the secondary heating effect of additional sub-pulses: the second and third sub-pulses reheated the plasma plume generated by the first sub-pulse, increasing the ionization degree of the plasma and promoting the decomposition of large nanoparticles into smaller, more uniform species. This resulted in a smoother, denser film. Figure 2c–e present the surface particle morphologies and corresponding statistical histograms (analyzed via Nano Measurer 1.02.0005, Shanghai, China) of TiO2 films under different sub-pulse numbers at a comparable deposition rate. From the histogram statistics, under single-sub-pulse conditions, large particle clusters formed on the film surface, with particle sizes mainly concentrated in the range of 100–250 nm. When the sub-pulse number increased to three, the number of particles larger than 200 nm decreased significantly. For the five-sub-pulse case, the particle sizes were predominantly distributed below 100 nm. This trend confirms the secondary heating decomposition effect of additional sub-pulses, consistent with the reduced surface roughness (Sa) and improved refractive index observed in Table 2.
When the sub-pulse number reached seven, the film quality was optimized: Sa was reduced by 85% compared to the single-pulse case, dropping to 4.4 nm. The refractive index increased to 2.16, and the MSE value decreased to 1.945. Additionally, the deposition rate slightly increased to 0.296 Å/s, and the film thickness reached 71.24 nm after 40 min—slightly higher than the single-pulse case. This improvement is due to the cumulative secondary heating effect of seven sub-pulses, which maximized the plasma ionization and nanoparticle decomposition, resulting in a highly uniform, dense film. Notably, when the sub-pulse number was increased beyond seven, the laser energy per sub-pulse became too low to reach the ablation threshold of TiO2, leading to ineffective deposition. Thus, seven sub-pulses was determined as the optimal number for TiO2 single-layer deposition. The resulting film exhibited a linear thickness growth rate of ~1.7 nm/min, low roughness (4.4 nm), and a refractive index of 2.16—meeting the requirements for high-quality multilayer fabrication.

3.1.2. ZnO Single Layers

ZnO was selected as the low-refractive-index material for its theoretical refractive index of 1.85, excellent thermal stability, and band gap that closely matches the 532 nm laser—ensuring efficient absorption and ablation during PLD. Similar to TiO2, the deposition parameters for ZnO were systematically optimized, focusing on laser energy, pulse frequency, and sub-pulse number, to achieve a film with low roughness, stable refractive index, and compatible deposition rate for multilayer fabrication.
The effect of laser energy (40–65% of the maximum single-pulse energy, corresponding to an average power range of 6.0–9.75 W) on the ZnO film properties was investigated at a fixed pulse frequency of 100 kHz and at room temperature for single-pulse mode. As shown in Table 3, at 40% laser energy (average power ≈ 6.0 W), the resulting ZnO film was thin (16.1 nm) with a low Sa (~6.5 nm) and a refractive index of 1.81—close to the theoretical value. However, the deposition rate was only 0.06 Å/s, which is too low for efficient multilayer fabrication. As the laser energy increased to 50% (average power ≈ 7.5 W), the film thickness increased to 33.7 nm, with Sa rising to 11.1 nm and the refractive index decreasing to 1.77. This trade-off between thickness and roughness is typical of PLD, as increased ablation leads to more material deposition but also more particle ejection. The optimal laser energy was identified as 55% of the maximum single-pulse energy (average power ≈ 8.25 W). At this energy level, the film thickness reached 43.1 nm, with a balance Sa of 15.9 nm and a refractive index of 1.70. The deposition rate was 0.18 Å/s—sufficient for efficient multilayer fabrication—and the MSE value was 3.45, indicating a reasonable match with the theoretical model. This energy level provided sufficient ablation to drive continuous film growth while avoiding excessive thermal damage. When the laser energy exceeded 60% (average power > 9.0 W), the film quality deteriorated. The film turned gray and opaque, losing its optical transparency. Thus, 55% laser energy was determined as optimal for ZnO single-layer deposition.
At a fixed laser energy of 55%, increasing the pulse frequency led to a decrease in single-pulse energy, which influenced the film thickness, roughness, and refractive index. As shown in Figure 3a, at 60 kHz, the single-pulse energy was high, resulting in a thick film (69.2 nm) but high roughness (Sa = 35.1 nm). The refractive index was 1.60, and the MSE value was 6.34, indicating a porous, rough structure. As the frequency increased to 80 kHz, the film thickness decreased to 52 nm, Sa reduced to 21.7 nm, the refractive index improved to 1.68. At 100 kHz, the film thickness was ~43.1 nm, Sa was 15.9 nm, and the refractive index reached 1.70, representing a balance between deposition rate and quality. A further increase in frequency to 120 kHz and 140 kHz resulted in thinner films (32.1 nm and 24.6 nm, respectively) but significantly improved surface smoothness. However, similar to TiO2, the deposition rate at 140 kHz was too slow for practical preparation. Thus, a pulse frequency of 100 kHz was selected as optimal for ZnO, balancing a reasonable deposition rate (0.18 Å/s) with good film quality (Sa = 15.9 nm, refractive index = 1.7).
To reduce the roughness of ZnO films deposited at 100 kHz (Sa = 15.9 nm) while maintaining a constant deposition rate ~0.179 Å/s, the effect of sub-pulse number (1–7) was explored at room temperature. As shown in Figure 3b, increasing the sub-pulse number had a significant positive impact on the ZnO film quality. When the sub-pulse number reached seven, the optimized ZnO film exhibited a linear thickness growth rate of ~1.45 nm/min, low roughness (Sa = 3.27 nm), and a stable refractive index (1.84) at an average laser power of 8.5 W. This film quality is well-suited for multilayer fabrication, as it provided a smooth, uniform base for subsequent layer deposition and maintained a refractive index that complements the optimized TiO2 film (refractive index = 2.16), creating a significant refractive index difference (Δn = 0.32) that is critical for photon band gap (PBG) regulation in multilayers.

3.2. Effect of Temperature and Annealing on Film Properties

The refractive index difference between the high- and low-refractive-index layers is a key factor in determining the PBG width and reflectance of TiO2/ZnO multilayers. To maximize this difference, the effects of substrate temperature and post-deposition annealing were investigated on the refractive indices of TiO2 and ZnO single films. As shown in Figure 3c, the effect of substrate temperature at room temperature ~25 °C (blue dashed line), 400 °C (black dash-dot line), and 500 °C (red dot line) on the refractive index of TiO2 films was investigated under the optimized single-layer deposition parameters (TiO2: 40% laser energy, 100 kHz frequency, seven sub-pulses; ZnO: 55% laser energy, 100 kHz frequency, seven sub-pulses). The refractive index of TiO2 increased significantly with increasing substrate temperature. At room temperature, the refractive index was 2.16. When the temperature was increased to 400 °C, the refractive index rose to 2.23, and at 500 °C, it reached 2.3, representing a 6.2% increase compared to room temperature. This improvement is attributed to increased film crystallinity and density at higher temperatures: elevated substrate temperatures promote grain growth and reduce defect density, leading to a more ordered structure with a higher refractive index. However, when the substrate temperature exceeded 500 °C, the film quality deteriorated. The Sa of the TiO2 films increased to over 10 nm, and surface particle aggregation was observed—likely due to excessive grain growth and uneven deposition. This increase in roughness leads to increased optical scattering, reducing the accuracy of refractive index measurements and making the film unsuitable for multilayer fabrication. Thus, 500 °C was identified as the optimal substrate temperature for TiO2 deposition. In contrast, the ZnO films showed excellent thermal stability at substrate temperatures up to 500 °C. The refractive index of ZnO remained relatively constant (~1.84) across the temperature range of 25–500 °C. ZnO films can be deposited at 500 °C without compromising their quality, making them compatible with the optimal deposition temperature for TiO2.
To further enhance the refractive index of TiO2 films and improve film quality, post-deposition annealing was performed in an oxygen atmosphere. TiO2 and ZnO films deposited at 500 °C were annealed at 500 °C for 60 min with a heating rate of 5 °C/min. As shown in Figure 3d, annealing had a profound effect on the refractive index of TiO2: the refractive index increased from 2.3 (as-deposited at 500 °C) to 2.6, representing a 13% increase. This significant improvement is attributed to the repair of oxygen vacancies, phase transformation, and film densification. The significant refractive index difference between annealed TiO2 (2.6) and ZnO (1.83) (Δn = 0.77) is highly favorable for PBG regulation in multilayers. This large difference enhances the Bragg interference effect, leading to a wider PBG and higher reflectance—critical for the optical performance of the multilayers.

3.3. Preparation and Optimization of TiO2/ZnO Multilayers

3.3.1. Annealing Strategy Optimization

Two annealing strategies were compared for four-layer TiO2/ZnO multilayers (central wavelength = 500 nm): stepwise annealing and integral annealing. Stepwise annealing involves annealing each pair of TiO2 and ZnO layers (i.e., after depositing one TiO2 layer and one ZnO layer) at 500 °C for 60 min, while integral annealing involves annealing the entire four-layer structure at 500 °C for 120 min after all layers are deposited. The reflectance of the multilayers for s-polarized and p-polarized light incidence was measured by using a spectroscopic ellipsometer with an incidence angle of 70° ± 1°. As shown in Figure 4a, stepwise annealing resulted in a maximum reflectance of ~65% for s-polarized light and less than 20% for p-polarized light. In contrast, integral annealing significantly improved the reflectance, with maximum values of ~75% for s-polarized light and ~30% for p-polarized light. Stepwise annealing caused repeated heating and cooling of the film, leading to interlayer diffusion between TiO2 and ZnO. Grain-boundary- and defect-assisted diffusion coefficients are higher, and repeated heating/cooling promotes additional stress and curvature driven interdiffusion and roughening at the interfaces [26]. This diffusion forms a mixed interface layer with an intermediate refractive index, reducing the refractive index difference between the layers and weakening the PBG effect. In contrast, during integral annealing, the full stack is heated once, after the layers have already partially densified during growth at elevated substrate temperature. The lower defect density and more relaxed stress state reduce the effective diffusivity along grain boundaries and interfaces [27], so that the intermixed Ti–Zn–O region remains thinner and the TiO2/ZnO index contrast is better preserved, thus maximizing the refractive index difference and enhancing the PBG effect.
The surface roughness of the multilayers was characterized using a confocal laser scanning microscope. During PLD, the high-speed galvanometer scans the laser beam more uniformly over the central area of the target than the edge regions, leading to more consistent material ejection, resulting in a smoother central region of the deposited film. As shown in Figure 4b, stepwise annealing resulted in an optimal Sa of 10.1 nm, while integral annealing reduced the Sa to 7.2 nm. The XRD patterns of the four-layer multilayers after integral annealing are shown in Figure 4d. The intense peak at 2θ≈27.5° corresponds to the (110) crystal plane of rutile-phase TiO2 (JCPDS No. 21-1276), while the peak near 2θ ≈ 55° is indexed to the (220) plane of rutile TiO2. The high-crystallinity and (110) preferred rutile-phase TiO2 are the structural basis for its relatively high refractive index (ellipsometer test value ~ 2.6). In the multilayer film, the (211) and (103) peaks are the secondary diffraction peaks of rutile TiO2. For ZnO, the (100), (002), and (101) crystal plane diffraction peaks are of similar intensity, indicating that ZnO grows in a polycrystalline form without obvious preferred orientation. The peaks (102), (112), and (201) are secondary diffraction peaks of ZnO, further confirming that ZnO has a polycrystalline structure. This crystal structure stability confirms that integral annealing does not alter the desired phase composition of the multilayers.
To further optimize the integral annealing process, the effect of annealing time (90, 120, and 150 min) on four-layer TiO2/ZnO multilayers was investigated. As shown in Figure 4c, increasing the annealing time from 90 to 150 min did not significantly improve the reflectance: the maximum reflectance for four-layer multilayers remained at ~75%. This is because the key processes contributing to reflectance enhancement—oxygen vacancy repair, phase transformation, and densification—were completed within the first 90 min of annealing. Extending the annealing time beyond 90 min only led to excessive grain growth, which increased the surface roughness and caused slight optical scattering, offsetting any potential reflectance improvement. Thus, an integral annealing time of 90 min is determined as optimal, balancing reflectance enhancement and surface smoothness.

3.3.2. Effect of Layer Number on Reflectance

The number of layers is a critical parameter for multilayer reflectance, as it directly influences the Bragg interference effect. To investigate this, TiO2/ZnO multilayers with four, sox, and eight layers (central wavelength = 450 nm) were fabricated using the optimized single-layer parameters and integral annealing (500 °C, 90 min). As shown in Figure 5, the theoretical reflectance increased with the number of layers, because increasing the number of layers enhanced the constructive interference of light, leading to a more pronounced PBG effect. However, the experimental reflectance deviated significantly from the theoretical values. Notably, the deviation between experimental and theoretical reflectance increased with the number of layers, indicating that structural defects accumulated as the number of layers increased. Several factors of cumulative interlayer roughness, interlayer diffusion, defect accumulation, and thickness deviation contribute to the deviation between experimental and theoretical reflectance
Considering the trade-off between reflectance and structural integrity, the four-layer structure is identified as optimal. It exhibits the highest experimental reflectance (75%) and the smallest deviation from the theoretical value (10%). Six-layer and eight-layer multilayers show lower reflectance due to cumulative roughness, diffusion, and defects. Additionally, four-layer multilayers are more efficient to fabricate, requiring less deposition time and reducing the risk of process errors. Thus, four-layer (TiO2/ZnO)2 multilayers were selected for subsequent application studies.

3.4. Multifunctional Applications of TiO2/ZnO Multilayers

3.4.1. Photocatalytic Degradation of TCH

TCH is a widely used antibiotic that poses a significant environmental risk due to its persistence in water. TiO2 and ZnO are both excellent photocatalysts, and their combination in a multilayer structure with a PBG effect is expected to enhance photocatalytic efficiency. As shown in Figure 6a, a photocatalytic experiment was conducted using a 300 W xenon lamp as the light source (as natural light) and 100 mL of TCH solution (initial concentration = 20 mg/L) as the target pollutant. The tested samples included single-layer TiO2, single-layer ZnO, two-layer (TiO2/ZnO), four-layer (TiO2/ZnO)2, and six-layer (TiO2/ZnO)3 films with their central wavelengths at 380 nm or 450 nm. Prior to irradiation, the samples were immersed in TCH solution in the dark for 60 min to achieve adsorption–desorption equilibrium. The photocatalytic mechanism of TiO2/ZnO multilayers involves two synergistic effects of heterojunction charge separation and a PBG slow-light effect. As shown in Figure 6b, TiO2 and ZnO form a type-II heterojunction [28,29]. Under light irradiation, electrons transfer from the conduction band of ZnO (lower energy) to the conduction band of TiO2 (higher energy), while holes transfer from the valence band of TiO2 (higher energy) to the valence band of ZnO (lower energy). This charge separation suppresses recombination, increasing the number of reactive oxygen species (·O2− and ·OH) that degrade TCH. Meanwhile, the PBG blue edge (350–400 nm) of the multilayer overlaps with the absorption band of TiO2/ZnO (~387 nm). This overlap traps light within the film, increasing the light–material interaction time and promoting the generation of electron–hole pairs.
In Figure 6c,d, the points denote the measured ratio C/C0 and ln(C/C0) of TCH concentration C to the initial C0 at different degradation times, respectively. The colored solid lines denote the TCH concentration change trends for different films. The TCH degradation rate constant k (min−1) in Figure 6d was calculated from the slope of ln(C0/C) versus time t (min), where the four-layer (TiO2/ZnO)2-450 multilayer (central wavelength = 450 nm) exhibited the highest photocatalytic efficiency (k = 0.0125 min−1). After 240 min of irradiation, the TCH degradation rate was ~85%, with a rate constant (k) of 0.018 min−1—2.2 times that of single-layer TiO2 (k = 0.0082 min−1) and 2.9 times that of single-layer ZnO (k = 0.0062 min−1). The two-layer TiO2/ZnO multilayer showed a degradation rate of ~65% (k = 0.0115 min−1)—1.4 times that of single-layer TiO2. The lower efficiency compared to the four-layer structure is due to the weaker PBG effect and fewer heterojunction interfaces. The four-layer (TiO2/ZnO)2-380 multilayer with a central wavelength of 380 nm exhibited a degradation rate of ~55% (k = 0.0094 min−1)—only 1.14 times that of single-layer TiO2. This is because the PBG center wavelength (380 nm) is too close to the material absorption band, leading to PBG-induced reflection that reduces light absorption. The six-layer and eight-layer TiO2/ZnO multilayers showed reduced degradation efficiency (~50% and ~45%, respectively) due to cumulative roughness and PBG red-shift. The increased roughness led to increased light scattering, while the PBG red-shift moved the blue edge away from the material absorption band, weakening the slow-light effect. Although the optimal rate constant (k = 0.0125 min−1) is lower than that of some powder photocatalyst systems assisted by oxidants (k = 0.0197 min−1) [30] and UV-rich light sources (k = 0.018 min−1) [31], it is reasonable for an immobilized TiO2/ZnO film operated under simulated solar light without external oxidants. In addition, the film configuration enables facile recovery and reuse and avoidance of secondary pollution from photocatalyst powders, which is advantageous for practical applications.

3.4.2. Fluorescence Enhancement of CdSe QDs

Fluorescence enhancement is critical for applications such as biosensing and optoelectronic devices. TiO2/ZnO multilayers with a PBG effect can concentrate local electromagnetic fields [32], enhancing the fluorescence of QDs deposited on their surface. The quantum dots (QDs) for fluorescence analysis were CdSe QDs with an excitation wavelength of 450 nm and emission wavelength of 527 nm. The QD solution was prepared at a concentration of 15 mg/mL. Samples were prepared by dropping 2 μL of QD solution onto blank quartz glass, TiO2 monolayer film, and six-layer (TiO2/ZnO)3-450 multilayer (central wavelength = 450 nm). The optimal overlap between the PBG red edge (yellow area in Figure 6e) of the (TiO2/ZnO)3-450 multilayer and the emission wavelength 527 nm of the QDs enhanced the localized field through the slow-light effect and the efficient charge separation by the heterojunction. The single-layer TiO2 film enhanced the QD fluorescence by 2.3 times (intensity = ~600 a.u.). This enhancement is attributed to the localized energy levels formed by oxygen vacancies in TiO2, which promote energy transfer from TiO2 to the QDs. The six-layer (TiO2/ZnO)3-450 multilayer achieved a 4.5-fold fluorescence enhancement (intensity = ~1166 a.u.)—nearly double the enhancement of the single-layer TiO2 film. This superior performance is due to two synergistic effects of PBG field localization and heterojunction charge separation. The PBG band edge of the six-layer multilayer (yellow shaded area in Figure 6e) overlaps with the emission wavelength of the QDs (527 nm). This overlap concentrates the local electromagnetic field, increasing the excitation rate of the QDs and enhancing their fluorescence. The TiO2/ZnO heterojunction promotes the separation of photoexcited electrons and holes, reducing non-radiative recombination and increasing the radiative decay rate of the QDs. These results confirm that TiO2/ZnO multilayers have significant potential for fluorescence enhancement applications, outperforming single-layer films due to the combined effects of PBG field localization and heterojunction charge separation.

4. Conclusions

In this work, a picosecond-pulsed-laser-based PLD system was successfully applied to fabricate high-quality TiO2/ZnO multilayers. The optimal process parameters for single-layer films were determined as follows: TiO2 (laser energy = 40%, frequency = 100 kHz, seven sub-pulses, deposition rate = 0.264 Å/s) and ZnO (laser energy = 55%, frequency = 100 kHz, seven sub-pulses, deposition rate = 0.179 Å/s), resulting in low roughness and high refractive index (2.156 for TiO2, 1.835 for ZnO). Annealing at 500 °C in an oxygen atmosphere significantly improves the refractive index of TiO2 to 2.6 while maintaining the refractive index of ZnO at ~1.83, forming a large refractive index difference (Δn = 0.77) for PBG regulation. Integral annealing (500 °C, 90 min) is superior to stepwise annealing for multilayer films, resulting in smooth surfaces and a maximum reflectance of 75% for the four-layer structure. The four-layer (TiO2/ZnO)2 multilayer with the central wavelength at 450 nm exhibits the highest photocatalytic degradation rate for TCH (k = 0.018 min−1)—2.2 times that of single-layer TiO2. The six-layer multilayer achieves a 4.5-fold fluorescence enhancement for CdSe QDs, outperforming single-layer TiO2. These results demonstrate that TiO2/ZnO multilayers fabricated by PLD with optimized parameters have excellent optical and functional properties, making them suitable for a wide range of applications in photocatalysis, optoelectronics, and beyond. This technology can expand to water purification, biosensing, and optoelectronic devices, optimizing structure for target pollutants/wavelengths to enhance practicality and scalability.

Author Contributions

X.Z.: experimentation, simulation, and characterization. G.D.: supervision and editing. Z.Z.: characterization. Y.Q.: validation. J.H.: analysis. J.Y.: guidance and methodology. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (12374398), Key Project of the National Natural Science Foundation of China (52532007), National Key Research and Development Program of China (2022YFB3806003), Fundamental Research Funds for the Central Universities, Beijing Natural Science Foundation (4222081), and Hebei Yanzhao Golden Platform Talent Gathering Program Core Talent Project (Education Platform) (HJZD202502).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
PLDPulsed laser deposition
PBGPhotonic band gap
QDsQuantum dots
TCHTetracycline hydrochloride
SaSurface arithmetical deviation
MSEMean squared error
SDStandard deviation

References

  1. Wang, W.; Qi, L. Light management with patterned micro-and nanostructure arrays for photocatalysis, photovoltaics, and optoelectronic and optical devices. Adv. Funct. Mater. 2019, 29, 1807275. [Google Scholar] [CrossRef]
  2. Syed, W.A.; Rafiq, N.; Ali, A.; Din, R.-U.; Shah, W.H. Multilayer AR coatings of TiO2/MgF2 for application in optoelectronic devices. Optik 2017, 136, 564–572. [Google Scholar] [CrossRef]
  3. Wu, Y.; Shen, H.; Ye, S.; Zhao, X.; Zhang, K.; Zhang, J.; Yang, B. Fluorescence manipulation of carbon dots by 1D photonic crystals. Adv. Opt. Mater. 2018, 6, 1701262. [Google Scholar] [CrossRef]
  4. Schou, J. Physical aspects of the pulsed laser deposition technique: The stoichiometric transfer of material from target to film. Appl. Surf. Sci. 2009, 255, 5191–5198. [Google Scholar] [CrossRef]
  5. Melaibari, A.; Eltaher, M.A. High repetition rate deposition of boron nitride films using femtosecond pulsed laser. Mater. Res. Express 2020, 7, 096401. [Google Scholar] [CrossRef]
  6. Wang, Y.; Cheng, X.; Shao, J.; Zheng, C.; Chen, A.; Zhang, L. The damage threshold of multilayer film induced by femtosecond and picosecond laser pulses. Coatings 2022, 12, 251. [Google Scholar] [CrossRef]
  7. Hamad, A.H. Effects of Different Laser Pulse Regimes (Nanosecond, Picosecond and Femtosecond) on the Ablation of Materials for Production of Nanoparticles in Liquid Solution. In High Energy Short Pulse Lasers; IntechOpen: London, UK, 2016; p. 303. [Google Scholar] [CrossRef]
  8. Arooj, F.; Mukarram, A.; Abdul, S.; Maria, U.; Muhammad Hisham Al, N.; Asad, R.; Mati, U.; Maneeb Ur, R.; Mansoor Inayat, K.; Muhammad Asad, A. Photovoltaic Performance of Solar Cells Enhanced by TIO2 and ZnO Nanoparticle Layers. Spectr. Eng. Sci. 2025, 3, 1023–1031. [Google Scholar]
  9. AL-Rubaiawi, H.; Faisal Alwan, A.; Ayad Husain, A.; Muhamed Ibrahim, N.; Abdul Kareem Ali, D.; Tariq Mahmood, A. Structural and Optical Properties of Doped Polystyrene Thin Films by (NiO, TiO2, ZnO, MgO) Nanoparticles. Color. Coat. 2025, 18, 323–341. [Google Scholar]
  10. Haider, A.J.; Alawsi, T.; Haider, M.J.; Taha, B.A.; Marhoon, H.A. A comprehensive review on pulsed laser deposition technique to effective nanostructure production: Trends and challenges. Opt. Quantum Electron. 2022, 54, 488. [Google Scholar] [CrossRef]
  11. Kim, H.S.; Jung, S.-W.; Ahn, K.-S.; Kang, S.H. Enhanced light harvesting of CdSe quantum dot sensitized bilayered ZnO nanostar/TiO2 nanotubes. Curr. Appl. Phys. 2013, 13, S162–S167. [Google Scholar] [CrossRef]
  12. Kammoun, J.; Dhiflaoui, H.; Chayoukhi, S.; Gassoumi, B.; Benhayoune, H.; Hajjaji, A.; Ben Moussa, N. Elaboration of TiO2–ZnO Coatings Through Spray-Pyrolysis Technology and the Study of Their Microstructural, Nanoindentation and Optic Properties. Trans. Indian Inst. Met. 2025, 78, 164. [Google Scholar] [CrossRef]
  13. Duraisamy, M.; Sreekantan, S.; Govindasamy, G.A.; Murthe, S.S.; Appaturi, J.N.; Md Noor, S.S. An in-vitro study of antibacterial, cytocompatibility and physicochemical properties of TiO2-ZnO nanocomposite. J. Sol-Gel Sci. Technol. 2025, 115, 1156–1168. [Google Scholar] [CrossRef]
  14. Doyle, S.; Ryan, L.; McCarthy, M.M.; Modreanu, M.; Schmidt, M.; Laffir, F.; Povey, I.M.; Pemble, M.E. Combinatorial ALD for the growth of ZnO/TiO2 nanolaminates and mixed ZnO/TiO2 nanostructured films. Mater. Adv. 2022, 3, 2896–2907. [Google Scholar] [CrossRef]
  15. Dizaj, M.H. Comparison and Review of ALD Target Materials for Quantum Dot Solar Cells: Al2O3, TiO2, ZnO, HFO2, WN, and NiO. Third International Conference for Students and Engineers of Electrical and Clean Energy (mee.cdsts.ir), Tehran, Iran. 2024. Available online: https://zenodo.org/records/14607545 (accessed on 10 November 2025).
  16. Sawunyama, L.; Oyewo, O.; Onwudiwe, D.C.; Makgato, S.S. Photocatalytic degradation of tetracycline using surface defective black TiO2-ZnO heterojunction photocatalyst under visible light. Heliyon 2023, 9, e21423. [Google Scholar] [CrossRef]
  17. Hussin, R.; Zulkiflee, N.S.; Zakiah, K.; Ainun Rahmahwati, A.; Harun, Z.; Mohamed Hatta, M.N. Photocatalytic Activity of Bilayer TiO2/ZnO and ZnO TiO2 Thin Films. Mater. Sci. Forum 2020, 1010, 411–417. [Google Scholar] [CrossRef]
  18. Hu, L.; Xing, M.; He, X.; Yang, K.; Zhu, J.; Wang, J.; He, J.; Shi, J. Photocatalytic degradation of tetracycline hydrochloride by ZnO/TiO2 composite photocatalyst. J. Mater. Sci. Mater. Electron. 2023, 34, 2273. [Google Scholar] [CrossRef]
  19. Monika, S.; Mahalakshmi, M.; Pandian, M.S. TiO2/CdS/CdSe quantum dots co-sensitized solar cell with the staggered-gap (type-II) heterojunctions for the enhanced photovoltaic performance. Ceram. Int. 2023, 49, 8820–8826. [Google Scholar] [CrossRef]
  20. Devloo-Casier, K.; Geiregat, P.; Ludwig, K.F.; van Stiphout, K.; Vantomme, A.; Hens, Z.; Detavernier, C.; Dendooven, J. A Case Study of ALD Encapsulation of Quantum Dots: Embedding Supported CdSe/CdS/ZnS Quantum Dots in a ZnO Matrix. J. Phys. Chem. C 2016, 120, 18039–18045. [Google Scholar] [CrossRef]
  21. Iqbal, A.; Saidu, U.; Adam, F.; Sreekantan, S.; Jasni, N.; Ahmad, M.N. The effects of zinc oxide (ZnO) quantum dots (QDs) embedment on the physicochemical properties and photocatalytic activity of titanium dioxide (TiO2) nanoparticles. J. Phys. Sci. 2021, 32, 71–85. [Google Scholar]
  22. Theerthagiri, J.; Karuppasamy, K.; Lee, S.J.; Shwetharani, R.; Kim, H.-S.; Pasha, S.K.K.; Ashokkumar, M.; Choi, M.Y. Fundamentals and comprehensive insights on pulsed laser synthesis of advanced materials for diverse photo- and electrocatalytic applications. Light Sci. Appl. 2022, 11, 250. [Google Scholar] [CrossRef]
  23. Lau, M.; Reichenberger, S.; Haxhiaj, I.; Barcikowski, S.; Müller, A.M. Mechanism of Laser-Induced Bulk and Surface Defect Generation in ZnO and TiO2 Nanoparticles: Effect on Photoelectrochemical Performance. ACS Appl. Energy Mater. 2018, 1, 5366–5385. [Google Scholar] [CrossRef]
  24. Theerthagiri, J.; Karuppasamy, K.; Min, A.; Govindarajan, D.; Kumari, M.L.A.; Muthusamy, G.; Kheawhom, S.; Kim, H.-S.; Choi, M.Y. Unraveling the fundamentals of pulsed laser-assisted synthesis of nanomaterials in liquids: Applications in energy and the environment. Appl. Phys. Rev. 2022, 9, 041314. [Google Scholar] [CrossRef]
  25. ISO 25178-2:2021; Geometrical Product Specifications (GPS)—Surface Texture: Areal—Part 2: Terms, Definitions and Surface Texture Parameters. International Organization for Standardization: Geneva, Switzerland, 2021.
  26. Ghazaryan, L.; Handa, S.; Schmitt, P.; Beladiya, V.; Roddatis, V.; Tünnermann, A.; Szeghalmi, A. Structural, optical, and mechanical properties of TiO2 nanolaminates. Nanotechnology 2020, 32, 095709. [Google Scholar] [CrossRef] [PubMed]
  27. Jena, S.; Tokas, R.B.; Thakur, S.; Sahoo, N.K. Influence of annealing on optical, microstructural and laser induced damage properties of TiO2/SiO2 multilayer high reflection mirror. AIP Conf. Proc. 2017, 1832, 060005. [Google Scholar]
  28. Kumar, A.; Nayak, D.; Sahoo, P.; Kumar Nandi, B.; Thangavel, R. Synthesis of type-II TiO2 nanoparticle/ZnO nanorods heterostructure for enhanced photocatalytic activity. Mater. Lett. 2024, 367, 136672. [Google Scholar] [CrossRef]
  29. Behara, D.K.; Tammineni, J.; Maheswari, M.S. TiO2/ZnO: Type-II Heterostructures for electrochemical crystal violet dye degradation studies. Maced. J. Chem. Chem. Eng. 2020, 39, 217–226. [Google Scholar] [CrossRef]
  30. Shi, Y.; Li, J.; Wan, D.; Huang, J.; Liu, Y. Peroxymonosulfate-enhanced photocatalysis by carbonyl-modified g-C3N4 for effective degradation of the tetracycline hydrochloride. Sci. Total Environ. 2020, 749, 142313. [Google Scholar] [CrossRef]
  31. Malakootian, M.; Asadzadeh, S.N.; Mehdipoor, M.; Kalantar-Neyestanaki, D. A new approach in photocatalytic degradation of tetracycline using biogenic zinc oxide nanoparticles and peroxymonosulfate under UVC irradiation. Desalination Water Treat. 2021, 222, 302–312. [Google Scholar] [CrossRef]
  32. Akimoto, T.; Yasuda, M. Fluorescence enhancement and reflection of the excitation light observed with a multilayered substrate. Appl. Opt. 2010, 49, 80–85. [Google Scholar] [CrossRef]
Applsci 16 01354 g001
Figure 1. Schematic of PLD deposition (a) experimental optical path and (b) vacuum deposition equipment; (c) photograph of experiment setup.
Figure 1. Schematic of PLD deposition (a) experimental optical path and (b) vacuum deposition equipment; (c) photograph of experiment setup.
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Figure 2. Effects of PLD parameters on single-layer film properties: (a) TiO2 properties of refractive index (red), thickness (black), Sa (blue), and MSE (orange) vs. laser energy, while laser frequency was 100 kHz and deposition time was 40 min at room temperature for single pulse; (b) SEM images of TiO2 surface morphology with different laser energy; surface particulate distributions (SEMs and particle size) of TiO2 films prepared under different pulse number conditions of (c) single pulse; (d) 3 pulses; and (e) 5 pulses, with laser energy ~40% and laser frequency at 100 kHz at room temperature.
Figure 2. Effects of PLD parameters on single-layer film properties: (a) TiO2 properties of refractive index (red), thickness (black), Sa (blue), and MSE (orange) vs. laser energy, while laser frequency was 100 kHz and deposition time was 40 min at room temperature for single pulse; (b) SEM images of TiO2 surface morphology with different laser energy; surface particulate distributions (SEMs and particle size) of TiO2 films prepared under different pulse number conditions of (c) single pulse; (d) 3 pulses; and (e) 5 pulses, with laser energy ~40% and laser frequency at 100 kHz at room temperature.
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Figure 3. (a) ZnO properties vs. laser frequency; (b) ZnO properties vs. sub-pulse number (constant deposition rate ~ 0.179 Å/s). (c) Refractive index of TiO2 films vs. substrate temperature; (d) refractive index changes of TiO2 and ZnO films deposited at 500 °C substrate temperature before and after annealing in oxygen (500 °C).
Figure 3. (a) ZnO properties vs. laser frequency; (b) ZnO properties vs. sub-pulse number (constant deposition rate ~ 0.179 Å/s). (c) Refractive index of TiO2 films vs. substrate temperature; (d) refractive index changes of TiO2 and ZnO films deposited at 500 °C substrate temperature before and after annealing in oxygen (500 °C).
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Figure 4. (a) Reflectance of four-layer (TiO2/ZnO)2 multilayers vs. annealing strategies for S and P polarization; (b) 2D planar views (grey-scale) and 3D topography (colored) of confocal laser scanning microscopy images of (TiO2/ZnO)2 multilayers after step annealing and integral annealing; (c) reflectance and 3D topography confocal laser microscopy images of four-layer multilayers vs. integral annealing time; (d) XRD patterns after integral annealing under oxygen atmosphere of monolayer TiO2 film (top) and (TiO2/ZnO)2 multilayer film (bottom).
Figure 4. (a) Reflectance of four-layer (TiO2/ZnO)2 multilayers vs. annealing strategies for S and P polarization; (b) 2D planar views (grey-scale) and 3D topography (colored) of confocal laser scanning microscopy images of (TiO2/ZnO)2 multilayers after step annealing and integral annealing; (c) reflectance and 3D topography confocal laser microscopy images of four-layer multilayers vs. integral annealing time; (d) XRD patterns after integral annealing under oxygen atmosphere of monolayer TiO2 film (top) and (TiO2/ZnO)2 multilayer film (bottom).
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Figure 5. Comparison of simulated (dashed lines) and measured (solid lines) s- and p-polarization reflectances for TiO2/ZnO multilayers with different layer numbers (central wavelength = 450 nm, incident angle = 70°): (a) 4 layers; (b) 6 layers; (c) 8 layers.
Figure 5. Comparison of simulated (dashed lines) and measured (solid lines) s- and p-polarization reflectances for TiO2/ZnO multilayers with different layer numbers (central wavelength = 450 nm, incident angle = 70°): (a) 4 layers; (b) 6 layers; (c) 8 layers.
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Figure 6. (a) Schematic of the experimental setup; (b) principle of TiO2/ZnO heterojunction photocatalytic degradation of TCH; (c) ratio of TCH concentration C to the initial C0 vs. irradiation time for TiO2 and ZnO monolayer, (TiO2/ZnO), (TiO2/ZnO)2, and (TiO2/ZnO)3 films with their central wavelengths at 380 nm or 450 nm; (d) TCH degradation rate constants k (min−1) of different films, with the inset k values calculated from the slope of ln(C0/C) versus time t. (e) Reflectance spectrum of (TiO2/ZnO)3-450 six-layer film (450 nm central wavelength); (f) fluorescence spectra of CdSe QDs on quartz glass, TiO2, and (TiO2/ZnO)3-450 substrates.
Figure 6. (a) Schematic of the experimental setup; (b) principle of TiO2/ZnO heterojunction photocatalytic degradation of TCH; (c) ratio of TCH concentration C to the initial C0 vs. irradiation time for TiO2 and ZnO monolayer, (TiO2/ZnO), (TiO2/ZnO)2, and (TiO2/ZnO)3 films with their central wavelengths at 380 nm or 450 nm; (d) TCH degradation rate constants k (min−1) of different films, with the inset k values calculated from the slope of ln(C0/C) versus time t. (e) Reflectance spectrum of (TiO2/ZnO)3-450 six-layer film (450 nm central wavelength); (f) fluorescence spectra of CdSe QDs on quartz glass, TiO2, and (TiO2/ZnO)3-450 substrates.
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Table 1. TiO2 film quality vs. pulse frequency.
Table 1. TiO2 film quality vs. pulse frequency.
Laser Frequency (kHz)Sa (nm) ± SDThickness (nm) ± SDRefractive Index ± SDMSE
6043.2 ± 0.8107.4 ± 2.51.83 ± 0.047.913
8035.0 ± 0.583.2 ± 1.81.88 ± 0.035.351
10031.0 ± 0.563.4 ± 1.21.95 ± 0.033.449
12021.2 ± 0.450.1 ± 1.02.00 ± 0.022.867
14011.2 ± 0.330.3 ± 0.72.00 ± 0.022.190
Table 2. TiO2 film quality vs. sub-pulse number.
Table 2. TiO2 film quality vs. sub-pulse number.
Sub-PulsesSa (nm) ± SDThickness (nm) ± SDRefractive Index ± SDMSE
131.0 ± 0.563.4 ± 1.21.95 ± 0.023.449
322.1 ± 0.465.9 ± 1.32.04 ± 0.023.291
510.2 ± 0.368.1 ± 1.42.10 ± 0.022.876
74.4 ± 0.271.2 ± 1.52.16 ± 0.021.945
Table 3. ZnO film quality vs. laser energy.
Table 3. ZnO film quality vs. laser energy.
Laser Energy (%)Sa (nm) ± SDThickness (nm) ± SDRefractive Index ± SDMSE
406.5 ± 0.216.1 ± 0.51.81 ± 0.021.45
458.1 ± 0.322.9 ± 0.81.79 ± 0.021.88
5011.1 ± 0.433.7 ± 1.01.77 ± 0.022.17
5515.9 ± 0.543.1 ± 1.01.70 ± 0.023.45
6028.0 ± 0.658.0 ± 1.51.69 ± 0.035.68
6544.7 ± 0.878.5 ± 2.01.65 ± 0.057.35
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Zhao, X.; Dong, G.; Zhu, Z.; Qin, Y.; He, J.; Yu, J. Collinear Pulse Train PLD: Fabrication of High-Refractive-Index-Difference TiO2/ZnO Multilayers with Multifunctional Applications. Appl. Sci. 2026, 16, 1354. https://doi.org/10.3390/app16031354

AMA Style

Zhao X, Dong G, Zhu Z, Qin Y, He J, Yu J. Collinear Pulse Train PLD: Fabrication of High-Refractive-Index-Difference TiO2/ZnO Multilayers with Multifunctional Applications. Applied Sciences. 2026; 16(3):1354. https://doi.org/10.3390/app16031354

Chicago/Turabian Style

Zhao, Xiang, Guoyan Dong, Zheng Zhu, Yutao Qin, Jiaxiang He, and Jin Yu. 2026. "Collinear Pulse Train PLD: Fabrication of High-Refractive-Index-Difference TiO2/ZnO Multilayers with Multifunctional Applications" Applied Sciences 16, no. 3: 1354. https://doi.org/10.3390/app16031354

APA Style

Zhao, X., Dong, G., Zhu, Z., Qin, Y., He, J., & Yu, J. (2026). Collinear Pulse Train PLD: Fabrication of High-Refractive-Index-Difference TiO2/ZnO Multilayers with Multifunctional Applications. Applied Sciences, 16(3), 1354. https://doi.org/10.3390/app16031354

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