All-inorganic HIP Multilayer With Excellent Thermostability for High-Performance Broadband Radiative Cooling

All-inorganic HIP Multilayer With Excellent Thermostability for High-Performance Broadband Radiative Cooling | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article All-inorganic HIP Multilayer With Excellent Thermostability for High-Performance Broadband Radiative Cooling Atsha Ambar This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7730333/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Passive daytime radiative cooling (PDRC) enabling energy-free thermal regulation by solar reflection and thermal emission, remains unrealized for widespread adoption owing to rapid photodegradation of organic materials and inorganic material's narrowband spectral response. Existing solutions also underutilize the broad mid-IR (8–20 µm) spectrum, restricting space applications where no atmosphere aids cooling. To address these challenges, we present an all-inorganic broadband emitter comprising SiO 2 -overlaid periodic Si 3 N 4 /Al 2 O 3 high-index pair (HIP) backed by AlN-protected Ag reflector over quartz substrate via magnetron sputtering. HIP broadens mid-IR emission via constructive interference enabled by its high refractive-index—presumed unattainable in prior inorganic approaches. SiO 2 interface induces strong impedance mismatch through low-/high-refractive-index contrast to enhance atmospheric-window emissivity. This multilayer structure achieves 96.04% solar reflectivity, 91.06% atmospheric-window emissivity, and a remarkable 90.03% broadband emissivity, yielding 225 W·m - 2 cooling power with a 19.8 K sub-ambient drop—an apex in inorganic metrics. TG-DSC confirmed thermostability and chemical inertness up to 1450°C, confirming film's stability for extreme environments. Outdoor tests despite localized feedback show sub-ambient drops of 9.7°C (horizontal) and 3.2°C (vertical) under 1100 W·m - 2 and 426 W·m - 2 solar irradiance, respectively, validating real-world efficacy. This scalable design overcomes material scarcity, structural complexity, and spectral trade-offs, paving the way for terrestrial and extra-terrestrial thermal management. Physical sciences/Materials science/Nanoscale materials/Organic–inorganic nanostructures Physical sciences/Nanoscience and technology/Nanoscale materials/Organic–inorganic nanostructures Radiative cooling All-inorganic Broadband emissivity High-index pair (HIP) Vertical surface Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 INTRODUCTION Passive daytime radiative cooling (PDRC) offers a glimmer of hope against anthropogenic warming as the planet confronts record-breaking heatwaves, glacier retreat, and a shrinking cryosphere. 1 It reflects incoming sunlight and ejects surplus heat into space by leveraging an atmospheric transparency window (8–13 µm) without external energy. 2 – 6 This selective emission coincides with the transparent spectral range of the atmosphere, which is tied to the heat escape. 7 Initially confined to human-centric domains, including buildings, 8–10 electronics, 11 textiles, 12 and arid-region water harvesting, 13–15 the PDRC circumvents traditional refrigeration. 16 Amid the escalating climate crisis, it has evolved from a niche to a planetary-survival imperative to combat the cascading climate impacts. 17 For instance, Antarctica's Thwaites Glacier 18 is alarmingly receding at an unprecedented pace, shedding 50 billion tons of ice annually and driving 4% of the global sea-level rise. 19 Its potential collapse could surge the sea level to 65 cm, endangering coastal communities worldwide. Concurrently, oceans which absorb nearly 90% of global heat, 20 are triggering a silent deep-sea catastrophe as cryptic abyssal species 21 flee oxygen-depleted 'dead zones' toward uninhabitable shallows 22 in desperate bid of survival. 23 Projections forecast that a 1°C rise in abyssal depths over decades 24 may risk the dissolution of carbonate sediments that have sequestered CO 2 for millennia, unleashing irreversible changes. 25 , 26 These interlinked crises underscore the urgency to slash energy-guzzling cooling, 27 decouple it from carbon-intensive processes, and counteract ecological disruption while advancing industrial decarbonization. However, its practical implementation 28 , 29 remains constrained by unresolved material challenges, 30 requiring > 95% solar reflectivity and > 90% emissivity for optimal cooling. 31 – 33 A turning point emerged in 1D photonic crystals with an early breakthrough by Raman et al. (2014), showcasing seven-layered HfO 2 /SiO 2 stacks on Ag-coated Si, unveiling promising optical functionalities. 34 HfO 2 as a high-index material, improves solar reflectivity and infrared (IR) emission, while SiO 2 as a low-index material, adds transparency and absorption around 9 µm, with top layers emitting IR and bottom layers reflecting solar radiation. Although this design achieved 94% solar reflectivity, its low IR emissivity fell short of exploiting a full atmospheric window with cooling power plateauing at 40.1 W·m - 2 , which is insufficient for real-world applications. Furthermore, worse reliance on rare, narrowband response materials such as HfO 2 with intricate layering poses massive scalability barriers, 35 driving up costs, and rendering impracticality. 36 A decade later, Liu et al. streamlined this approach with four-layered TiO 2 /SiO 2 high-low-index inorganic multilayers on SiN x protected Ag over a glass substrate. 37 Their design improved the atmospheric emissivity to 90.9% with 91.6% solar reflectivity, delivering 114 W·m - 2 cooling power. While effective, it still fell short of the 95% reflectivity threshold 38 and 150 W·m - 2 cooling power, achieving only a marginal 2.5°C sub-ambient cooling. Such inorganic structures offer stability, 39–41 but face roadblocks, including scarce nanostructures, structural intricacy, suboptimal emissivity, and the struggle to balance high solar reflectivity, compelling the need for high-performance architectures. 42 – 48 Moreover, traditional 1D photonic crystals composed of low-index (absorbing < 9 µm) and high-index (absorbing 11–16 µm) materials inherently struggle to exhibit broadband emissions. Consequently, their narrowband emissions restrict their performance in space applications, where direct exposure to frigid voids demands broadband 8–20 µm emissions for effective thermal regulation. However, metamaterials hold great promise, yet involve elevated costs, complex and lengthy etching-based fabrication, mechanical fragility, weak adhesion, and low hydrophobicity, prompting a shift toward compact and scalable alternatives. Organic ones like polymeric µDE 49 and nanoPE sawtooth gratings, 50 not only provide broad thermal emission but also adapt to vertical surfaces, where nearby obstructions, vegetation, and the ground typically reduce the emissive performance. However, their longevity is undermined by rapid photodegradation from prolonged exposure to sunlight and heat. 51 To bridge this gap, future designs must use structurally simple, all-inorganic, broad-spectrum, and earth-abundant 52 – 55 materials for effective thermoregulation of planar and vertical surfaces. Our study directly addresses these challenges by introducing a periodic quad-layer high-index pair (HIP: H 1 /H 2 /H 1 /H 2 ) using low-cost Si 3 N 4 /Al 2 O 3 materials. Their high extinction peaks at 11 and 15 µm exploit the constructive interference from phonon absorption to achieve broadband mid-IR emissivity. A SiO 2 capping layer amplifies the atmospheric-window emissivity through an impedance mismatch at the HIP interface driven by low-/high-refractive-index contrasts, a mechanism prevalent in earlier 1D photonic designs 34 , 37 , 56 . The SiO 2 /HIP/AlN/Ag stack exhibited broadband emissivity while offering high solar reflectivity and UV durability comparable to those of HfO 2 /TiO 2 -based structures at a fraction of the cost. The AlN barrier prevents Ag oxidation while retaining its emissivity, which is a persistent flaw in the silver-based reflectors. This configuration resolves the reflectivity–emissivity trade-off by achieving 96.04% solar reflectivity (𝑅 0.3-2.5 µm ), 91.06% atmospheric emissivity (𝜀 8-13 µm ) and 225 W·m − 2 net cooling power, enabling 19.8 K sub-ambient temperature. Remarkably, a 90.03% broadband mid-IR emissivity (𝜀 8-20 µm ) is achieved—an achievement previously unattainable in the realm of inorganic and 1D photonic approaches. With remarkable aerospace-grade thermostability withstanding temperatures up to 1450°C, this design fulfills the pressing requirement for space-compatible emitters. Outdoor experiments demonstrate a maximum sub-ambient temperature reduction of 9.7°C under 1100 W·m − 2 solar irradiance (horizontally), and 3.2°C under 426 W·m − 2 solar irradiance (vertically), highlighting its applicability across various orientations, including vertically deployable setups. This HIP-based innovation stands out unique by combining broadband emission, excellent thermostability, and vertical performance—setting new benchmarks to propel sustainable efforts toward carbon neutrality pursuit. RESULTS AND DISCUSSION Multilayer Fabrication: In this study, an all-inorganic multilayer film (SiO 2 /Si 3 N 4 /Al 2 O 3 /Si 3 N 4 /Al 2 O 3 /AlN/Ag), optimized using MATLAB simulations [see Supplementary text], was deposited onto a pre-cleaned quartz substrate (Fig. 1 ) following ultrasonic cleaning and vacuum chamber evacuation protocols. The deposition was carried out under controlled parameters to achieve uniform and optimally thick layers (ignoring the interfacial roughness). A 100 nm Ag layer was deposited first to serve as a reflective base for minimizing solar absorption, followed by sequential deposition of a 20 nm AlN barrier to prevent oxidation during the reactive deposition of subsequent layers while retaining optical attributes, as shown in Fig. S3A. Although AlN can be omitted in non-reactive or oxygen-free environments, its inclusion ensures structural stability under degradable interfacial conditions. A periodic quad-layered Si 3 N 4 /Al 2 O 3 high-index pair (HIP) was strategically introduced at the core to expand the mid-IR emissivity via constructive interference, capitalizing on their respective 11 µm and 15–17 µm absorption peaks. Capping the stack, a SiO 2 layer with strong absorption near 9 µm amplified the atmospheric-window emissivity by inducing an impedance mismatch at low-/high-refractive-index contrast, as shown in Fig. S3B. Overall, this study embodies an effective impedance-engineered multilayer design that provides a fabrication pathway for efficient radiative cooling. Characterization The broadband-emitting all-inorganic multilayer film demonstrates a solar reflectivity (0.3–2.5 µm) of 96.04% (Fig. 2 a), and atmospheric emissivity (8–13 µm) of 91.06% (Fig. 2 b), making it highly suitable for terrestrial radiative cooling. This closely matches the MATLAB-simulated parameters of 96.86% reflectivity and 92.36% atmospheric emissivity, thus validating the transfer matrix method and fabrication fidelity. Notably, it also achieved a broadband emissivity (8–20 µm) of 90.03%, potentially suitable for extra-terrestrial applications. The strong agreement between computationally simulated and experimental absorption spectra (0.3–20 µm) is depicted in Fig. 2 c, with a shift at 2.5 µm demarcating the transition between UV–Vis.–NIR and FTIR instrumentation. Minor spectral deviations are attributed to interfacial roughness or measurement uncertainties. The NIR/SWIR reflectance dip (900–2500 nm) stems from Ag's free-electron (Drude) absorption compounded by destructive interference as the refractive index contrasts with the wavelength shift, creating localized minima. Despite this dip, high solar reflectivity (> 96%) was sustained by strong impedance mismatches originating from the low/high indices among SiO 2 (1.45–1.55), Si 3 N 4 (1.9–2.1), and Al 2 O 3 (1.6–1.77) at the SiO 2 /HIP and Si 3 N 4 /Al 2 O 3 dielectric interfaces. Overall, HIP-based inorganic design achieves high solar reflectivity and strong atmospheric and broadband emissivity for terrestrial and extraterrestrial applications. The film attained thermal equilibrium when the net cooling power approached zero, signifying the maximum attainable temperature gradient between the film and its surroundings. Under a nonradiative heat transfer coefficient (ℎ 𝑐 ) of 6.9 W·m - 2 K - 1 , this hierarchical all-inorganic photonic structure exhibits remarkable radiative cooling performance, achieving a substantial net cooling powers (𝑃 𝑐𝑜𝑜𝑙 ) of 261.1 W·m − 2 experimentally, when integrated over the 0.3–14 µm spectral range at 300 K ambient temperature. The structure achieves a maximum temperature drop (Δ𝑇) of 24.2 K under equilibrium, as shown in Table 1 alongside the corresponding cooling parameters. Noteworthy, this all-inorganic HIP structure employs broadband emissivity to outperform the selective-emitting 1D photonic structures and sidestepping unstable organic broadband emitters. Intriguingly, extending the experimental spectral range to 20 µm, the net cooling power is slightly differ at 225 W·m − 2 with maximum temperature drop (Δ𝑇) of 19.8 K. This deviation is likely due to additional far-infrared radiation captured in real-world measurements, amplifying real-world performance beyond computational estimates. Both simulations and experiments reveal a consistent decline in cooling power as the cooler's temperature drops, reflecting reduced thermal driving forces as depicted in Fig. 3 a. Figure 3 b quantifies power densities (𝑃 𝑐𝑜𝑜𝑙 , 𝑃 𝑟𝑎𝑑 , 𝑃 𝑠𝑘𝑦 , 𝑃 𝑠𝑢𝑛 , 𝑃 𝑐𝑜𝑛𝑑+𝑐𝑜𝑛𝑣 ) relative to temperature at 𝑇(K) at ℎ 𝑐 = 6.9 W·m - 2 K − 1 , where 𝑃 𝑐𝑜𝑜𝑙 drops from 225 W·m − 2 to 0.9 W·m − 2 as the film cools from 300 K to 280.2 K, reaching equilibrium (𝑃 𝑐𝑜𝑜𝑙 = 0). This inverse correlation reveals a strong dependence of net cooling power on film's temperature and environmental heat transfer dynamics. Figure 3 c illustrates that as ℎ 𝑐 increases, both the maximum achievable temperature gradient (Δ𝑇) and 𝑃 𝑐𝑜𝑜𝑙 diminish, indicating an inverse correlation. At constant Δ𝑇, rising ℎ 𝑐 reduces 𝑃 𝑐𝑜𝑜𝑙 due to enhanced parasitic heat gain; conversely, at constant ℎ 𝑐 , increasing Δ𝑇 leads to a progressive drop in 𝑃 𝑐𝑜𝑜𝑙 , emphasizing thermodynamic constraints and delicate balance between heat dissipation and ambient thermal influx. Remarkably, under ideal conditions with zero nonradiative heat gain (ℎ 𝑐 = 0), the film attains its peak temperature reduction of 54 K, reaching a minimum temperature drop of 246 K solely through radiative cooling. This combination of high cooling power, UV durability, and broadband emissivity highlights its promise for energy-efficient cooling technologies, as further substantiated by the comparative performance metrics listed in Table S2. Table 1 MATLAB-optimized cooling parameters from measured spectra 𝑅 0.3−2.5µm (%) 𝜀 8−13µm (%) 𝑃 𝑐𝑜𝑜𝑙 W·m −2 𝑃 𝑟𝑎𝑑 W·m −2 𝑃 𝑠𝑘𝑦 W·m −2 𝑃 𝑠𝑢𝑛 W·m −2 𝑃 𝑐𝑜𝑛𝑑+𝑐𝑜𝑛𝑣 W·m −2 Δ𝑇 ℎ𝑐=6.9 (K) Simulation 96.86 92.36 289.6 409.4 88.5 31.2 0 24.7 Experiment 96.04 91.06 261.1 389.2 75.9 52.1 0 24.2 The cross-sectional morphology of the proposed multilayer film, acquired via high-resolution scanning electron microscopy (SEM, ZEISS Gemini 500), confirmed that the thickness alignment was consistent with theoretical predictions, as shown in Fig. 4 a. The SEM image reveals a clear columnar crystal structure with distinct layer boundaries, including prominent SiO 2 , Si 3 N 4 , and Al 2 O 3 layers, and a protective AlN layer over Ag, confirming the deposition precision. The FTIR-ATR spectrum spanning 0–4500 cm - 1 provided insights into the chemical composition and bonding interactions within the multilayer. The overlapping vibrational modes, interfacial effects, and environmental contaminants influenced the key spectral peaks, underscoring the multilayer complexity, as shown in Fig. 4 b. The peak observed at 800 cm - 1 assigned to Si–O–Si symmetric stretching 57 , 58 overlapping with Al–O bending 59 in Al 2 O 3 and Al-N stretching 60 in AlN, indicating bonding interactions. The 950 cm - 1 band aligns with the Si–O 61,62 and Si–N 63 stretching in SiO 2 and Si 3 N 4 , suggesting structural integration or interdiffusion between the sequential layers. The peak at 1015 cm - 1 further indicates Si–O–Al bonding 59 between Si 3 N 4 and Al 2 O 3 . The peak at 1175 cm - 1 reflects asymmetric Si-O-Si stretching 59 , 64 in SiO 2 . A minor peak at 3360 cm - 1 corresponds to (2.9 µm) arising from O–H stretching, 65 indicative of hydroxyl incorporation from ambient moisture on the exposed surface of the multilayer film. The absence of Ag-related peaks is consistent with their nontransmitting nature. Importantly, the absence of Ag–O vibrational peaks (400–600 cm - 1 ) further confirmed its unoxidized state. Certain bond vibrations are concentrated within the 769–1250 cm - 1 range, contribute significantly to the film's increased absorptivity within the atmospheric window. These results collectively demonstrated the coexistence of layer-specific vibrational modes, cross-layer interactions, and environmental influences, probing the importance of HIP multilayers (Table 2 ). Table 2 Vibrational type of each absorption peak in the FTIR-ATR diagram absorption peak 800 cm − 1 950 cm − 1 1015 cm − 1 1175 cm − 1 3360 cm − 1 Vibration type Si-O-Si symmetric stretching Si-O stretching Si–O–Al bonding Si-O-Si asymmetric stretching O–H stretching Al-O bending Si-N stretching Al–N stretching Thermogravimetric (TG) and Differential Scanning Calorimetry (DSC) analyses of the multilayer film on a 5×5 mm silicon substrate were conducted using a NETZSCH STA449F3 instrument under a N 2 /O 2 atmosphere heated from room temperature to 1450°C at a constant rate of 10 K/min. The TG curve showed an insignificant 0.01% mass change, indicating exceptional thermostability to structural degradation and strong resistance to decomposition (Fig. 5 a). The DSC profile revealed no thermal events such as phase transitions, chemical reactivity, or volatilization until an endothermic peak at 1428.2°C (Fig. 5 b), which corresponds to the melting point of the silicon substrate, confirming the thermostability of the material at elevated temperatures. The slight deviation beyond the theoretical melting point of silicon (1414°C) is credited to experimental factors, such as refractory oxides and interfacial engineering, which suppress interdiffusion and reactions. These properties ensure that the film can endure structural integrity to withstand extreme thermal stress, implying its viability for intense thermal load scenarios, including solar energy systems and aerospace coatings. In horizontal skyward-facing configurations, enhanced vertical thermal emissions facilitate efficient heat dissipation, further reinforcing reliable and consistent cooling performance. Angular-resolved analysis across incident angles (0°–80°) assesses the film's strong angular tolerance, examining emission spectra, angular-dependent solar reflectivity, and angular-dependent emissivity while maintaining spectral selectivity in both solar and thermal regimes. Figure 6 a shows the angle-dependent absorption spectra (0.3–14 µm), while Fig. 6 b-c depicts angular variations in solar reflectivity and atmospheric-window emissivity, respectively, emphasizing the angular sensitivity of these optical performances. The film exhibited near-angle-agnostic solar reflectance (~ 97%) and stable emissivity up to 60° with a marked drop at steeper angles. This trend reflects that atmospheric transmittance peaks at minimal pathlengths near the zenith and declines with increasing angle toward the horizon, enabling the assessment of angular impact. This characteristic trend ensures effective radiative cooling on both vertical and horizontal surfaces, such as building façades and vehicles, even under direct sunlight, thereby expanding avenues. The normalized energy flow density quantifies the transmitted intensity (I) relative to the incident light (I 0 ), revealing the optical behavior: I/I 0 < 1 signifies absorption/scattering, whereas I/I 0 = 1 indicates unattenuated transmission or no light interaction. In the 8–14 µm range, the capping layer demonstrated the highest magnitude, positioning it as pivotal in facilitating IR dissipation within the multilayer film owing to the minimized attenuation, as depicted in Fig. 7 a. Figure 7 b depicts the CIE 1931 chromaticity diagram derived from the experimental reflectivity spectrum, showing the film's coordinates at (𝑥 = 0.323, 𝑦 = 0.328). The film exhibits high solar reflectance and near-optical neutrality, closely matching the CIE Standard Illuminant D65 white point (𝑥 = 0.333, 𝑦 = 0.333), with minimal chromatic deviation and a subtle greenish hue. PDRC Performance : An outdoor experiment in Beijing (39.9°N, 116.4°E) evaluated radiative heat exchange under real-world conditions, such as high solar irradiance and varying atmospheric conditions (humidity, wind, and cloud cover), by testing two chambers: one inclined at a 40° southward tilt (sky-facing) and one mounted vertically (side-facing), as shown in Fig. S4a. 66 The multilayer film was enclosed in a wooden box chamber with polystyrene base insulation, low-density polyethylene top cover, and aluminum foil outer shielding to suppress nonradiative losses, while K-type thermocouples placed inside recorded the film's sub-ambient temperature drop, as shown in Fig. S4a-b. A maximum temperature reduction of 9.7°C (horizontally) and 3.2°C (vertically) was recorded under solar irradiance of 1100 W·m - 2 and 426 W·m - 2 , respectively, measured using an SM206 Solar Power Meter and NIR meter (Fig. 8 a-b). All measurements were conducted under cloudy, humid, and windy conditions; the corresponding wind speed and relative humidity data are presented in Fig. S4c-d. Notably, higher ambient temperatures tended to yield a greater maximum cooling power. Discrepancies in the simulations arose from atmospheric transmittance disparities, cloud cover, wind, humidity, and local obstructions (e.g., buildings and vegetation). These findings reaffirm the strong solar reflectivity and validate the practical potential of the film in reducing solar thermal gain for effective real-world PDRC deployment. CONCLUSIONS We engineered a high-efficiency, all-inorganic multilayer radiative cooling structure with an innovative architecture comprising periodic quad-layered Si 3 N 4 /Al 2 O 3 high-index pairs (HIP), overlaid low-index SiO 2 and backed by an AlN-protected Ag reflector. This structure achieves a notable 96.04% solar reflectivity and enhance mid-IR emissivity, targeting both the atmospheric window (𝜀 8-13 µm = 91.06%) and broadband range (𝜀 8-20 µm = 90.03%)—a spectral selectivity elusive to conventional inorganic designs. Impedance mismatch at the SiO 2 /HIP interface enhances atmospheric-window emission, while HIP's phonon absorption at longer wavelengths broadens the mid-IR emissivity. The AlN interlayer preserves the Ag reflector from oxidation without compromising optical attributes. This unique spectral broadness enables a sub-ambient temperature reduction of 19.8 K with 225 W·m − 2 net cooling power. Structural fidelity is verified via high-resolution cross-sectional microscopy, affirming deposition precision. Simultaneously, TG-DSC analysis indicates outstanding thermostability and chemical inertness up to 1450°C with a mass loss of only 0.01% and negligible sensitivity, confirming its robustness for harsh and high-temperature applications. Angular-resolved spectra reveal angle-insensitive solar reflectance (> 96%) and stable emissive response up to 60° incidence. Outdoor test confirms a maximum temperature drop of 9.7°C (horizontally) and 3.2°C (vertically) under the solar irradiance of 1100 W·m −2 and 426 W·m −2 , respectively, demonstrating resilience against wind, cloud cover, humidity, and other dynamic environmental conditions. This distinctive HIP-based multilayer paradigm stands apart from prior studies by aiming thermal management from human comfort to global climate stabilization including coral reef and Arctic ice preservation. Declarations Funding: This work was supported by the National Natural Science Foundation of China (NSFC) [No. 52272264], the Chinese-German Mobility Programme [Project No. M-0273], and the Fundamental Research Funds for Central Universities. The authors also acknowledge the Center for Micro-Nano Innovation (Beihang Nano) and the Analysis and Testing Center at Beihang University for their facility access and technical support. Author contributions : Conceptualization: A. A.; Methodology: A. A.; Investigation: A. A.; Visualization: A. A.; Formal analysis: A. A.; Validation: A. A., H. L., Y. Y., Y. S., and C. W; Data curation: A. A., H. L., Y. Y., Y. S., and C. W; Funding acquisition: C. W.; Project administration: C. W. and Y. S.; Supervision: Y.S., C. W., and Y. S.; Writing – original draft: A. A.; Writing – review editing: H. L., Y. Y., Y. S., and C. W; Resources: C. W. Competing interests: The authors declare no conflicts of interest. 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Toxics 8 . https://doi.org/10.3390/TOXICS8030051 Hwang J (2024) Daytime Radiative Cooling under Extreme Weather Conditions. Advanced Energy and Sustainability Research 5 . https://doi.org/10.1002/aesr.202300239 Additional Declarations There is NO Competing Interest. Supplementary Files Supplementary.docx All-inorganic HIP Multilayer With Excellent Thermostability for High-Performance Broadband Radiative Cooling Materials and Methods Supplementary Text Figs. S1 to S4 Table S1 to S2 References (6 7 –81) Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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1","display":"","copyAsset":false,"role":"figure","size":152605,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of radiative cooling film illustrating energy flow in the radiative cooler: 𝑃\u003csub\u003e𝑠𝑢𝑛\u003c/sub\u003e is the absorbed solar power, 𝑃\u003csub\u003e𝑠𝑘𝑦\u003c/sub\u003e is the absorbed atmospheric power, 𝑃\u003csub\u003e𝑟𝑎𝑑\u003c/sub\u003e is the thermal radiation, and 𝑃\u003csub\u003e𝑐𝑜𝑛𝑑+𝑐𝑜𝑛𝑣\u003c/sub\u003e is non-radiative heat transfer via convection and conduction, with a multilayer structure of HIP capping SiO\u003csub\u003e2\u003c/sub\u003e onto AlN/Ag/Quartz substrate.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/a17dc85b10ab03a9cd1f0cc7.png"},{"id":96605113,"identity":"28f09f35-0607-4298-993e-b1954472204b","added_by":"auto","created_at":"2025-11-24 09:18:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":40360,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Solar reflectivity in the 0.3–2.5 µm range; (b) Atmospheric emissivity within 8–13 µm window and broadband emissivity from 8–20 µm; (c) Simulated and experimental FT-IR spectra of the multilayer film over 0.3–20 µm, along with AM1.5 solar irradiance and atmospheric emission spectrum.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/ded2581e5c456123acaf5b72.png"},{"id":96587945,"identity":"e15cc311-4b3c-4d11-a808-e23649df2bb5","added_by":"auto","created_at":"2025-11-24 05:37:22","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":16907,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Simulated vs. experimental net cooling power as a function of temperature 𝑇. (b) 𝑃\u003csub\u003e𝑐𝑜𝑜𝑙\u003c/sub\u003e, 𝑃\u003csub\u003e𝑟𝑎𝑑\u003c/sub\u003e, 𝑃\u003csub\u003e𝑠𝑘𝑦\u003c/sub\u003e, 𝑃\u003csub\u003e𝑠𝑢𝑛\u003c/sub\u003e, and 𝑃\u003csub\u003e𝑐𝑜𝑛𝑑+𝑐𝑜𝑛𝑣\u003c/sub\u003e as a function of temperature 𝑇 at ℎ\u003csub\u003e𝑐\u003c/sub\u003e = 6.9; (c) Net cooling power vs. gradient temperature Δ𝑇 at varying ℎ\u003csub\u003e𝑐\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/6d01f60f2e3202f07afebaa3.png"},{"id":96587950,"identity":"05977486-98a1-4bd4-976d-a02066510a5b","added_by":"auto","created_at":"2025-11-24 05:37:22","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":286424,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Cross-sectional SEM image of the SiO\u003csub\u003e2\u003c/sub\u003e/Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/AlN/Ag multilayer film on quartz substrate, highlighting uniform layer interfaces. (b) FTIR-ATR spectrum of the multilayer structure in the 0–4500 cm\u003csup\u003e-1\u003c/sup\u003e range revealing vibrational modes and observed characteristic peaks.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/95724c7d8b64dc3c1edbe0b9.png"},{"id":96587948,"identity":"836764ba-b4fa-4801-bdde-2050c12f1bc8","added_by":"auto","created_at":"2025-11-24 05:37:22","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":22640,"visible":true,"origin":"","legend":"\u003cp\u003eA 14.323 mg sample heated from ambient temperature to 1450°C at a constant rate of 10 K/min. (a) TG (blue) curve shows only 0.01% mass loss, and (b) DSC (red) curve indicates no phase transition up to 1428.2°C, corresponding to silicon substrate melting. Sensitivity curve (green) confirms negligible thermal events.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/6957d6054dc93e7d1c758ef3.png"},{"id":96587951,"identity":"78388109-74a0-4526-a7c5-d009739c5ae0","added_by":"auto","created_at":"2025-11-24 05:37:22","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":239764,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Emission spectra of the film at varying incident angles. (b) Angular-dependent reflectivity in the solar spectral region and (c) Angular-dependent emissivity within the atmospheric window across various incident angles.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/ab05ae468e7d9e2b6b68e77e.png"},{"id":96587960,"identity":"df09cff1-14cd-4f9d-9412-63cf85a2d547","added_by":"auto","created_at":"2025-11-24 05:37:22","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":175108,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Normalized energy flow density (I/I\u003csub\u003e0\u003c/sub\u003e) in the 8–14 µm range reveals capping layer's dominant role in IR dissipation due to minimal attenuation. (b) Chromaticity diagram from experimental reflectance with coordinates (x = 0.323, y = 0.328), closely match the D65 white point, indicating a neutral and high reflective appearance.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/1298149a0deef3aa04a1545e.png"},{"id":96605761,"identity":"f2319e58-ba1f-40db-8ea0-3fcff4abcbb2","added_by":"auto","created_at":"2025-11-24 09:24:00","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":119336,"visible":true,"origin":"","legend":"\u003cp\u003eMeasured temperature profiles showing maximum sub-ambient temperature drops of (a) 9.7°C (horizontal) and (b) 3.2°C (vertical) under natural solar and infrared irradiance.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/24a403d33f3d3c8d6b47c21a.png"},{"id":96708603,"identity":"3256fede-918d-40cc-b8f4-eb349818878d","added_by":"auto","created_at":"2025-11-25 10:04:44","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1465238,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/e5fbdfb3-7f13-46ad-8ec9-9ea5734faca8.pdf"},{"id":96605320,"identity":"a6dd57e4-0e7d-41ba-b50e-eb81e495ff13","added_by":"auto","created_at":"2025-11-24 09:22:18","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":967870,"visible":true,"origin":"","legend":"\u003cp\u003eAll-inorganic HIP Multilayer With Excellent Thermostability for High-Performance Broadband Radiative Cooling\u003c/p\u003e\n\u003cp\u003eMaterials and Methods\u003c/p\u003e\n\u003cp\u003eSupplementary Text\u003c/p\u003e\n\u003cp\u003eFigs. S1 to S4\u003c/p\u003e\n\u003cp\u003eTable S1 to S2\u003c/p\u003e\n\u003cp\u003eReferences (6\u003cem\u003e7\u003c/em\u003e–81)\u003c/p\u003e","description":"","filename":"Supplementary.docx","url":"https://assets-eu.researchsquare.com/files/rs-7730333/v1/c3c6ece3e0283ae67e770496.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"All-inorganic HIP Multilayer With Excellent Thermostability for High-Performance Broadband Radiative Cooling","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003ePassive daytime radiative cooling (PDRC) offers a glimmer of hope against anthropogenic warming as the planet confronts record-breaking heatwaves, glacier retreat, and a shrinking cryosphere.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e It reflects incoming sunlight and ejects surplus heat into space by leveraging an atmospheric transparency window (8–13 µm) without external energy.\u003csup\u003e\u003cspan additionalcitationids=\"CR3 CR4 CR5\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e–\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e This selective emission coincides with the transparent spectral range of the atmosphere, which is tied to the heat escape.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e Initially confined to human-centric domains, including buildings,\u003csup\u003e8–10\u003c/sup\u003e electronics,\u003csup\u003e11\u003c/sup\u003e textiles,\u003csup\u003e12\u003c/sup\u003e and arid-region water harvesting,\u003csup\u003e13–15\u003c/sup\u003e the PDRC circumvents traditional refrigeration.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e Amid the escalating climate crisis, it has evolved from a niche to a planetary-survival imperative to combat the cascading climate impacts.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e For instance, Antarctica's Thwaites Glacier\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e is alarmingly receding at an unprecedented pace, shedding 50\u0026nbsp;billion tons of ice annually and driving 4% of the global sea-level rise.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Its potential collapse could surge the sea level to 65 cm, endangering coastal communities worldwide. Concurrently, oceans which absorb nearly 90% of global heat,\u003csup\u003e20\u003c/sup\u003e are triggering a silent deep-sea catastrophe as cryptic abyssal species\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e flee oxygen-depleted 'dead zones' toward uninhabitable shallows\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e in desperate bid of survival.\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e Projections forecast that a 1°C rise in abyssal depths over decades\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e may risk the dissolution of carbonate sediments that have sequestered CO\u003csub\u003e2\u003c/sub\u003e for millennia, unleashing irreversible changes.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e These interlinked crises underscore the urgency to slash energy-guzzling cooling,\u003csup\u003e27\u003c/sup\u003e decouple it from carbon-intensive processes, and counteract ecological disruption while advancing industrial decarbonization. However, its practical implementation\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e remains constrained by unresolved material challenges,\u003csup\u003e30\u003c/sup\u003e requiring \u0026gt; 95% solar reflectivity and \u0026gt; 90% emissivity for optimal cooling.\u003csup\u003e\u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e–\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eA turning point emerged in 1D photonic crystals with an early breakthrough by Raman et al. (2014), showcasing seven-layered HfO\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e stacks on Ag-coated Si, unveiling promising optical functionalities.\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e HfO\u003csub\u003e2\u003c/sub\u003e as a high-index material, improves solar reflectivity and infrared (IR) emission, while SiO\u003csub\u003e2\u003c/sub\u003e as a low-index material, adds transparency and absorption around 9 µm, with top layers emitting IR and bottom layers reflecting solar radiation. Although this design achieved 94% solar reflectivity, its low IR emissivity fell short of exploiting a full atmospheric window with cooling power plateauing at 40.1 W·m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, which is insufficient for real-world applications. Furthermore, worse reliance on rare, narrowband response materials such as HfO\u003csub\u003e2\u003c/sub\u003e with intricate layering poses massive scalability barriers,\u003csup\u003e35\u003c/sup\u003e driving up costs, and rendering impracticality.\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e A decade later, Liu et al. streamlined this approach with four-layered TiO\u003csub\u003e2\u003c/sub\u003e/SiO\u003csub\u003e2\u003c/sub\u003e high-low-index inorganic multilayers on SiN\u003csub\u003ex\u003c/sub\u003e protected Ag over a glass substrate.\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e Their design improved the atmospheric emissivity to 90.9% with 91.6% solar reflectivity, delivering 114 W·m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e cooling power. While effective, it still fell short of the 95% reflectivity threshold\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e and 150 W·m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e cooling power, achieving only a marginal 2.5°C sub-ambient cooling. Such inorganic structures offer stability,\u003csup\u003e39–41\u003c/sup\u003e but face roadblocks, including scarce nanostructures, structural intricacy, suboptimal emissivity, and the struggle to balance high solar reflectivity, compelling the need for high-performance architectures.\u003csup\u003e\u003cspan additionalcitationids=\"CR43 CR44 CR45 CR46 CR47\" citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e–\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e Moreover, traditional 1D photonic crystals composed of low-index (absorbing \u0026lt; 9 µm) and high-index (absorbing 11–16 µm) materials inherently struggle to exhibit broadband emissions. Consequently, their narrowband emissions restrict their performance in space applications, where direct exposure to frigid voids demands broadband 8–20 µm emissions for effective thermal regulation. However, metamaterials hold great promise, yet involve elevated costs, complex and lengthy etching-based fabrication, mechanical fragility, weak adhesion, and low hydrophobicity, prompting a shift toward compact and scalable alternatives. Organic ones like polymeric µDE\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e and nanoPE sawtooth gratings,\u003csup\u003e50\u003c/sup\u003e not only provide broad thermal emission but also adapt to vertical surfaces, where nearby obstructions, vegetation, and the ground typically reduce the emissive performance. However, their longevity is undermined by rapid photodegradation from prolonged exposure to sunlight and heat.\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e To bridge this gap, future designs must use structurally simple, all-inorganic, broad-spectrum, and earth-abundant\u003csup\u003e\u003cspan additionalcitationids=\"CR53 CR54\" citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e–\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e materials for effective thermoregulation of planar and vertical surfaces.\u003c/p\u003e\u003cp\u003eOur study directly addresses these challenges by introducing a periodic quad-layer high-index pair (HIP: H\u003csub\u003e1\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e/H\u003csub\u003e1\u003c/sub\u003e/H\u003csub\u003e2\u003c/sub\u003e) using low-cost Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e materials. Their high extinction peaks at 11 and 15 µm exploit the constructive interference from phonon absorption to achieve broadband mid-IR emissivity. A SiO\u003csub\u003e2\u003c/sub\u003e capping layer amplifies the atmospheric-window emissivity through an impedance mismatch at the HIP interface driven by low-/high-refractive-index contrasts, a mechanism prevalent in earlier 1D photonic designs\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. The SiO\u003csub\u003e2\u003c/sub\u003e/HIP/AlN/Ag stack exhibited broadband emissivity while offering high solar reflectivity and UV durability comparable to those of HfO\u003csub\u003e2\u003c/sub\u003e/TiO\u003csub\u003e2\u003c/sub\u003e-based structures at a fraction of the cost. The AlN barrier prevents Ag oxidation while retaining its emissivity, which is a persistent flaw in the silver-based reflectors. This configuration resolves the reflectivity–emissivity trade-off by achieving 96.04% solar reflectivity (𝑅\u003csub\u003e0.3-2.5 µm\u003c/sub\u003e), 91.06% atmospheric emissivity (𝜀\u003csub\u003e8-13 µm\u003c/sub\u003e) and 225 W·m\u003csup\u003e−\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e net cooling power, enabling 19.8 K sub-ambient temperature. Remarkably, a 90.03% broadband mid-IR emissivity (𝜀\u003csub\u003e8-20 µm\u003c/sub\u003e) is achieved—an achievement previously unattainable in the realm of inorganic and 1D photonic approaches. With remarkable aerospace-grade thermostability withstanding temperatures up to 1450°C, this design fulfills the pressing requirement for space-compatible emitters. Outdoor experiments demonstrate a maximum sub-ambient temperature reduction of 9.7°C under 1100 W·m\u003csup\u003e−\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e solar irradiance (horizontally), and 3.2°C under 426 W·m\u003csup\u003e−\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e solar irradiance (vertically), highlighting its applicability across various orientations, including vertically deployable setups. This HIP-based innovation stands out unique by combining broadband emission, excellent thermostability, and vertical performance—setting new benchmarks to propel sustainable efforts toward carbon neutrality pursuit.\u003c/p\u003e"},{"header":"RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eMultilayer Fabrication:\u003c/h2\u003e\u003cp\u003eIn this study, an all-inorganic multilayer film (SiO\u003csub\u003e2\u003c/sub\u003e/Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e/AlN/Ag), optimized using MATLAB simulations [see Supplementary text], was deposited onto a pre-cleaned quartz substrate (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) following ultrasonic cleaning and vacuum chamber evacuation protocols. The deposition was carried out under controlled parameters to achieve uniform and optimally thick layers (ignoring the interfacial roughness). A 100 nm Ag layer was deposited first to serve as a reflective base for minimizing solar absorption, followed by sequential deposition of a 20 nm AlN barrier to prevent oxidation during the reactive deposition of subsequent layers while retaining optical attributes, as shown in Fig. S3A. Although AlN can be omitted in non-reactive or oxygen-free environments, its inclusion ensures structural stability under degradable interfacial conditions. A periodic quad-layered Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e high-index pair (HIP) was strategically introduced at the core to expand the mid-IR emissivity via constructive interference, capitalizing on their respective 11 \u0026micro;m and 15\u0026ndash;17 \u0026micro;m absorption peaks. Capping the stack, a SiO\u003csub\u003e2\u003c/sub\u003e layer with strong absorption near 9 \u0026micro;m amplified the atmospheric-window emissivity by inducing an impedance mismatch at low-/high-refractive-index contrast, as shown in Fig. S3B. Overall, this study embodies an effective impedance-engineered multilayer design that provides a fabrication pathway for efficient radiative cooling.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eCharacterization\u003c/strong\u003e\u003cp\u003eThe broadband-emitting all-inorganic multilayer film demonstrates a solar reflectivity (0.3\u0026ndash;2.5 \u0026micro;m) of 96.04% (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea), and atmospheric emissivity (8\u0026ndash;13 \u0026micro;m) of 91.06% (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), making it highly suitable for terrestrial radiative cooling. This closely matches the MATLAB-simulated parameters of 96.86% reflectivity and 92.36% atmospheric emissivity, thus validating the transfer matrix method and fabrication fidelity. Notably, it also achieved a broadband emissivity (8\u0026ndash;20 \u0026micro;m) of 90.03%, potentially suitable for extra-terrestrial applications. The strong agreement between computationally simulated and experimental absorption spectra (0.3\u0026ndash;20 \u0026micro;m) is depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, with a shift at 2.5 \u0026micro;m demarcating the transition between UV\u0026ndash;Vis.\u0026ndash;NIR and FTIR instrumentation. Minor spectral deviations are attributed to interfacial roughness or measurement uncertainties. The NIR/SWIR reflectance dip (900\u0026ndash;2500 nm) stems from Ag's free-electron (Drude) absorption compounded by destructive interference as the refractive index contrasts with the wavelength shift, creating localized minima. Despite this dip, high solar reflectivity (\u0026gt;\u0026thinsp;96%) was sustained by strong impedance mismatches originating from the low/high indices among SiO\u003csub\u003e2\u003c/sub\u003e (1.45\u0026ndash;1.55), Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e (1.9\u0026ndash;2.1), and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (1.6\u0026ndash;1.77) at the SiO\u003csub\u003e2\u003c/sub\u003e/HIP and Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e dielectric interfaces. Overall, HIP-based inorganic design achieves high solar reflectivity and strong atmospheric and broadband emissivity for terrestrial and extraterrestrial applications.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe film attained thermal equilibrium when the net cooling power approached zero, signifying the maximum attainable temperature gradient between the film and its surroundings. Under a nonradiative heat transfer coefficient (ℎ\u003csub\u003e\u0026#119888;\u003c/sub\u003e) of 6.9 W\u0026middot;m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003eK\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, this hierarchical all-inorganic photonic structure exhibits remarkable radiative cooling performance, achieving a substantial net cooling powers (\u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119900;\u0026#119897;\u003c/sub\u003e) of 261.1 W\u0026middot;m\u003csup\u003e\u0026minus;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e experimentally, when integrated over the 0.3\u0026ndash;14 \u0026micro;m spectral range at 300 K ambient temperature. The structure achieves a maximum temperature drop (Δ\u0026#119879;) of 24.2 K under equilibrium, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e alongside the corresponding cooling parameters. Noteworthy, this all-inorganic HIP structure employs broadband emissivity to outperform the selective-emitting 1D photonic structures and sidestepping unstable organic broadband emitters. Intriguingly, extending the experimental spectral range to 20 \u0026micro;m, the net cooling power is slightly differ at 225 W\u0026middot;m\u003csup\u003e\u0026minus;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e with maximum temperature drop (Δ\u0026#119879;) of 19.8 K. This deviation is likely due to additional far-infrared radiation captured in real-world measurements, amplifying real-world performance beyond computational estimates. Both simulations and experiments reveal a consistent decline in cooling power as the cooler's temperature drops, reflecting reduced thermal driving forces as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb quantifies power densities (\u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119900;\u0026#119897;\u003c/sub\u003e, \u0026#119875;\u003csub\u003e\u0026#119903;\u0026#119886;\u0026#119889;\u003c/sub\u003e, \u0026#119875;\u003csub\u003e\u0026#119904;\u0026#119896;\u0026#119910;\u003c/sub\u003e, \u0026#119875;\u003csub\u003e\u0026#119904;\u0026#119906;\u0026#119899;\u003c/sub\u003e, \u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119899;\u0026#119889;+\u0026#119888;\u0026#119900;\u0026#119899;\u0026#119907;\u003c/sub\u003e) relative to temperature at \u0026#119879;(K) at ℎ\u003csub\u003e\u0026#119888;\u003c/sub\u003e = 6.9 W\u0026middot;m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003eK\u003csup\u003e\u0026minus;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, where \u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119900;\u0026#119897;\u003c/sub\u003e drops from 225 W\u0026middot;m\u003csup\u003e\u0026minus;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e to 0.9 W\u0026middot;m\u003csup\u003e\u0026minus;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e as the film cools from 300 K to 280.2 K, reaching equilibrium (\u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119900;\u0026#119897;\u003c/sub\u003e = 0). This inverse correlation reveals a strong dependence of net cooling power on film's temperature and environmental heat transfer dynamics. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec illustrates that as ℎ\u003csub\u003e\u0026#119888;\u003c/sub\u003e increases, both the maximum achievable temperature gradient (Δ\u0026#119879;) and \u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119900;\u0026#119897;\u003c/sub\u003e diminish, indicating an inverse correlation. At constant Δ\u0026#119879;, rising ℎ\u003csub\u003e\u0026#119888;\u003c/sub\u003e reduces \u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119900;\u0026#119897;\u003c/sub\u003e due to enhanced parasitic heat gain; conversely, at constant ℎ\u003csub\u003e\u0026#119888;\u003c/sub\u003e, increasing Δ\u0026#119879; leads to a progressive drop in \u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119900;\u0026#119897;\u003c/sub\u003e, emphasizing thermodynamic constraints and delicate balance between heat dissipation and ambient thermal influx. Remarkably, under ideal conditions with zero nonradiative heat gain (ℎ\u003csub\u003e\u0026#119888;\u003c/sub\u003e = 0), the film attains its peak temperature reduction of 54 K, reaching a minimum temperature drop of 246 K solely through radiative cooling. This combination of high cooling power, UV durability, and broadband emissivity highlights its promise for energy-efficient cooling technologies, as further substantiated by the comparative performance metrics listed in Table S2.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eMATLAB-optimized cooling parameters from measured spectra\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"9\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e\u0026#119877;\u003csub\u003e0.3\u0026minus;2.5\u0026micro;m\u003c/sub\u003e (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e\u0026#120576;\u003csub\u003e8\u0026minus;13\u0026micro;m\u003c/sub\u003e (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e\u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119900;\u0026#119897;\u003c/sub\u003e W\u0026middot;m\u003csup\u003e\u0026minus;2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e\u0026#119875;\u003csub\u003e\u0026#119903;\u0026#119886;\u0026#119889;\u003c/sub\u003e W\u0026middot;m\u003csup\u003e\u0026minus;2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e\u0026#119875;\u003csub\u003e\u0026#119904;\u0026#119896;\u0026#119910;\u003c/sub\u003e W\u0026middot;m\u003csup\u003e\u0026minus;2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\"\u003e\u003cp\u003e\u0026#119875;\u003csub\u003e\u0026#119904;\u0026#119906;\u0026#119899;\u003c/sub\u003e W\u0026middot;m\u003csup\u003e\u0026minus;2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c8\"\u003e\u003cp\u003e\u0026#119875;\u003csub\u003e\u0026#119888;\u0026#119900;\u0026#119899;\u0026#119889;+\u0026#119888;\u0026#119900;\u0026#119899;\u0026#119907;\u003c/sub\u003e W\u0026middot;m\u003csup\u003e\u0026minus;2\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c9\"\u003e\u003cp\u003eΔ\u0026#119879;\u003csub\u003eℎ\u0026#119888;=6.9\u003c/sub\u003e (K)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSimulation\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e96.86\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e92.36\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e289.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e409.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e88.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e31.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e24.7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExperiment\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e96.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e91.06\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e261.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e389.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e\u003cp\u003e75.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e\u003cp\u003e52.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\u003cp\u003e0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\u003cp\u003e24.2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe cross-sectional morphology of the proposed multilayer film, acquired via high-resolution scanning electron microscopy (SEM, ZEISS Gemini 500), confirmed that the thickness alignment was consistent with theoretical predictions, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. The SEM image reveals a clear columnar crystal structure with distinct layer boundaries, including prominent SiO\u003csub\u003e2\u003c/sub\u003e, Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e, and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e layers, and a protective AlN layer over Ag, confirming the deposition precision. The FTIR-ATR spectrum spanning 0\u0026ndash;4500 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e provided insights into the chemical composition and bonding interactions within the multilayer. The overlapping vibrational modes, interfacial effects, and environmental contaminants influenced the key spectral peaks, underscoring the multilayer complexity, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. The peak observed at 800 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e assigned to Si\u0026ndash;O\u0026ndash;Si symmetric stretching\u003csup\u003e\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e,\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e overlapping with Al\u0026ndash;O bending\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u003c/sup\u003e in Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Al-N stretching\u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e in AlN, indicating bonding interactions. The 950 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e band aligns with the Si\u0026ndash;O\u003csup\u003e61,62\u003c/sup\u003e and Si\u0026ndash;N\u003csup\u003e63\u003c/sup\u003e stretching in SiO\u003csub\u003e2\u003c/sub\u003e and Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e, suggesting structural integration or interdiffusion between the sequential layers. The peak at 1015 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e further indicates Si\u0026ndash;O\u0026ndash;Al bonding\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u003c/sup\u003e between Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. The peak at 1175 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e reflects asymmetric Si-O-Si stretching\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e,\u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e\u003c/sup\u003e in SiO\u003csub\u003e2\u003c/sub\u003e. A minor peak at 3360 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e corresponds to (2.9 \u0026micro;m) arising from O\u0026ndash;H stretching,\u003csup\u003e65\u003c/sup\u003e indicative of hydroxyl incorporation from ambient moisture on the exposed surface of the multilayer film. The absence of Ag-related peaks is consistent with their nontransmitting nature. Importantly, the absence of Ag\u0026ndash;O vibrational peaks (400\u0026ndash;600 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e) further confirmed its unoxidized state. Certain bond vibrations are concentrated within the 769\u0026ndash;1250 cm\u003csup\u003e-\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e range, contribute significantly to the film's increased absorptivity within the atmospheric window. These results collectively demonstrated the coexistence of layer-specific vibrational modes, cross-layer interactions, and environmental influences, probing the importance of HIP multilayers (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eVibrational type of each absorption peak in the FTIR-ATR diagram\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eabsorption peak\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e950 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1015 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1175 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003e3360 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\" morerows=\"2\" rowspan=\"3\"\u003e\u003cp\u003eVibration type\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSi-O-Si symmetric stretching\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSi-O stretching\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eSi\u0026ndash;O\u0026ndash;Al bonding\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003eSi-O-Si asymmetric stretching\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003eO\u0026ndash;H stretching\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAl-O bending\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eSi-N stretching\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eAl\u0026ndash;N stretching\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThermogravimetric (TG) and Differential Scanning Calorimetry (DSC) analyses of the multilayer film on a 5\u0026times;5 mm silicon substrate were conducted using a NETZSCH STA449F3 instrument under a N\u003csub\u003e2\u003c/sub\u003e/O\u003csub\u003e2\u003c/sub\u003e atmosphere heated from room temperature to 1450\u0026deg;C at a constant rate of 10 K/min. The TG curve showed an insignificant 0.01% mass change, indicating exceptional thermostability to structural degradation and strong resistance to decomposition (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). The DSC profile revealed no thermal events such as phase transitions, chemical reactivity, or volatilization until an endothermic peak at 1428.2\u0026deg;C (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb), which corresponds to the melting point of the silicon substrate, confirming the thermostability of the material at elevated temperatures. The slight deviation beyond the theoretical melting point of silicon (1414\u0026deg;C) is credited to experimental factors, such as refractory oxides and interfacial engineering, which suppress interdiffusion and reactions. These properties ensure that the film can endure structural integrity to withstand extreme thermal stress, implying its viability for intense thermal load scenarios, including solar energy systems and aerospace coatings.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn horizontal skyward-facing configurations, enhanced vertical thermal emissions facilitate efficient heat dissipation, further reinforcing reliable and consistent cooling performance. Angular-resolved analysis across incident angles (0\u0026deg;\u0026ndash;80\u0026deg;) assesses the film's strong angular tolerance, examining emission spectra, angular-dependent solar reflectivity, and angular-dependent emissivity while maintaining spectral selectivity in both solar and thermal regimes. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea shows the angle-dependent absorption spectra (0.3\u0026ndash;14 \u0026micro;m), while Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb-c depicts angular variations in solar reflectivity and atmospheric-window emissivity, respectively, emphasizing the angular sensitivity of these optical performances. The film exhibited near-angle-agnostic solar reflectance (~\u0026thinsp;97%) and stable emissivity up to 60\u0026deg; with a marked drop at steeper angles. This trend reflects that atmospheric transmittance peaks at minimal pathlengths near the zenith and declines with increasing angle toward the horizon, enabling the assessment of angular impact. This characteristic trend ensures effective radiative cooling on both vertical and horizontal surfaces, such as building fa\u0026ccedil;ades and vehicles, even under direct sunlight, thereby expanding avenues.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe normalized energy flow density quantifies the transmitted intensity (I) relative to the incident light (I\u003csub\u003e0\u003c/sub\u003e), revealing the optical behavior: I/I\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;1 signifies absorption/scattering, whereas I/I\u003csub\u003e0\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1 indicates unattenuated transmission or no light interaction. In the 8\u0026ndash;14 \u0026micro;m range, the capping layer demonstrated the highest magnitude, positioning it as pivotal in facilitating IR dissipation within the multilayer film owing to the minimized attenuation, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb depicts the CIE 1931 chromaticity diagram derived from the experimental reflectivity spectrum, showing the film's coordinates at (\u0026#119909; = 0.323, \u0026#119910; = 0.328). The film exhibits high solar reflectance and near-optical neutrality, closely matching the CIE Standard Illuminant D65 white point (\u0026#119909; = 0.333, \u0026#119910; = 0.333), with minimal chromatic deviation and a subtle greenish hue.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003ePDRC Performance\u003c/b\u003e: An outdoor experiment in Beijing (39.9\u0026deg;N, 116.4\u0026deg;E) evaluated radiative heat exchange under real-world conditions, such as high solar irradiance and varying atmospheric conditions (humidity, wind, and cloud cover), by testing two chambers: one inclined at a 40\u0026deg; southward tilt (sky-facing) and one mounted vertically (side-facing), as shown in Fig. S4a.\u003csup\u003e\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e The multilayer film was enclosed in a wooden box chamber with polystyrene base insulation, low-density polyethylene top cover, and aluminum foil outer shielding to suppress nonradiative losses, while K-type thermocouples placed inside recorded the film's sub-ambient temperature drop, as shown in Fig. S4a-b. A maximum temperature reduction of 9.7\u0026deg;C (horizontally) and 3.2\u0026deg;C (vertically) was recorded under solar irradiance of 1100 W\u0026middot;m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and 426 W\u0026middot;m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e, respectively, measured using an SM206 Solar Power Meter and NIR meter (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea-b). All measurements were conducted under cloudy, humid, and windy conditions; the corresponding wind speed and relative humidity data are presented in Fig. S4c-d. Notably, higher ambient temperatures tended to yield a greater maximum cooling power. Discrepancies in the simulations arose from atmospheric transmittance disparities, cloud cover, wind, humidity, and local obstructions (e.g., buildings and vegetation). These findings reaffirm the strong solar reflectivity and validate the practical potential of the film in reducing solar thermal gain for effective real-world PDRC deployment.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"CONCLUSIONS","content":"\u003cp\u003eWe engineered a high-efficiency, all-inorganic multilayer radiative cooling structure with an innovative architecture comprising periodic quad-layered Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e high-index pairs (HIP), overlaid low-index SiO\u003csub\u003e2\u003c/sub\u003e and backed by an AlN-protected Ag reflector. This structure achieves a notable 96.04% solar reflectivity and enhance mid-IR emissivity, targeting both the atmospheric window (\u0026#120576;\u003csub\u003e8-13 \u0026micro;m\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;91.06%) and broadband range (\u0026#120576;\u003csub\u003e8-20 \u0026micro;m\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;90.03%)\u0026mdash;a spectral selectivity elusive to conventional inorganic designs. Impedance mismatch at the SiO\u003csub\u003e2\u003c/sub\u003e/HIP interface enhances atmospheric-window emission, while HIP's phonon absorption at longer wavelengths broadens the mid-IR emissivity. The AlN interlayer preserves the Ag reflector from oxidation without compromising optical attributes. This unique spectral broadness enables a sub-ambient temperature reduction of 19.8 K with 225 W\u0026middot;m\u003csup\u003e\u0026minus;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e net cooling power. Structural fidelity is verified via high-resolution cross-sectional microscopy, affirming deposition precision. Simultaneously, TG-DSC analysis indicates outstanding thermostability and chemical inertness up to 1450\u0026deg;C with a mass loss of only 0.01% and negligible sensitivity, confirming its robustness for harsh and high-temperature applications. Angular-resolved spectra reveal angle-insensitive solar reflectance (\u0026gt;\u0026thinsp;96%) and stable emissive response up to 60\u0026deg; incidence. Outdoor test confirms a maximum temperature drop of 9.7\u0026deg;C (horizontally) and 3.2\u0026deg;C (vertically) under the solar irradiance of 1100 W\u0026middot;m\u003csup\u003e\u0026minus;2\u003c/sup\u003e and 426 W\u0026middot;m\u003csup\u003e\u0026minus;2\u003c/sup\u003e, respectively, demonstrating resilience against wind, cloud cover, humidity, and other dynamic environmental conditions. This distinctive HIP-based multilayer paradigm stands apart from prior studies by aiming thermal management from human comfort to global climate stabilization including coral reef and Arctic ice preservation.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e This work was supported by the National Natural Science Foundation of China (NSFC) [No. 52272264], the Chinese-German Mobility Programme [Project No. M-0273], and the Fundamental Research Funds for Central Universities. The authors also acknowledge the Center for Micro-Nano Innovation (Beihang Nano) and the Analysis and Testing Center at Beihang University for their facility access and technical support.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003cstrong\u003e:\u0026nbsp;\u003c/strong\u003eConceptualization: A. A.; Methodology: A. A.; Investigation: A. A.; Visualization: A. A.;\u0026nbsp;Formal analysis: A. A.; Validation:\u0026nbsp;A. A., H. L., Y. Y., Y. S., and C. W;\u0026nbsp;Data curation:\u0026nbsp;A. A., H. L., Y. Y., Y. S., and C. W; Funding acquisition: C. W.; Project administration: C. W. and Y. S.; Supervision: Y.S., C. W., and Y. S.; Writing \u0026ndash; original draft: A. A.; Writing \u0026ndash; review \u0026nbsp;editing: H. L., Y. Y., Y. S., and C. W;\u0026nbsp;Resources: C. W.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u0026nbsp;\u003c/strong\u003eThe authors declare no conflicts of interest. The authors have no competing financial interests or personal relationships that could have influenced the work reported in this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData and materials availability:\u003c/strong\u003e All data needed to evaluate the conclusions in the paper are presented in the paper and Supplementary Materials.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eYue L, Chao N, Chen G, Chen L, Zhang B, Sun R, Zhang Y, Wang S, Wang Z, Li F et al (2023) Reconstructing Continuous Ice Sheet Elevation Changes in the Amundsen Sea Sector During 2003\u0026ndash;2021 by Merging Envisat, ICESat, CryoSat-2, and ICESat-2 Multi-Altimeter Observations. 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Toxics \u003cem\u003e8\u003c/em\u003e. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.3390/TOXICS8030051\u003c/span\u003e\u003cspan address=\"10.3390/TOXICS8030051\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eHwang J (2024) Daytime Radiative Cooling under Extreme Weather Conditions. Advanced Energy and Sustainability Research \u003cem\u003e5\u003c/em\u003e. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1002/aesr.202300239\u003c/span\u003e\u003cspan address=\"10.1002/aesr.202300239\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Radiative cooling, All-inorganic, Broadband emissivity, High-index pair (HIP), Vertical surface","lastPublishedDoi":"10.21203/rs.3.rs-7730333/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7730333/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePassive daytime radiative cooling (PDRC) enabling energy-free thermal regulation by solar reflection and thermal emission, remains unrealized for widespread adoption owing to rapid photodegradation of organic materials and inorganic material's narrowband spectral response. Existing solutions also underutilize the broad mid-IR (8\u0026ndash;20 \u0026micro;m) spectrum, restricting space applications where no atmosphere aids cooling. To address these challenges, we present an all-inorganic broadband emitter comprising SiO\u003csub\u003e2\u003c/sub\u003e-overlaid periodic Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e high-index pair (HIP) backed by AlN-protected Ag reflector over quartz substrate via magnetron sputtering. HIP broadens mid-IR emission via constructive interference enabled by its high refractive-index\u0026mdash;presumed unattainable in prior inorganic approaches. SiO\u003csub\u003e2\u003c/sub\u003e interface induces strong impedance mismatch through low-/high-refractive-index contrast to enhance atmospheric-window emissivity. This multilayer structure achieves 96.04% solar reflectivity, 91.06% atmospheric-window emissivity, and a remarkable 90.03% broadband emissivity, yielding 225 W\u0026middot;m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e cooling power with a 19.8 K sub-ambient drop\u0026mdash;an apex in inorganic metrics. TG-DSC confirmed thermostability and chemical inertness up to 1450\u0026deg;C, confirming film's stability for extreme environments. Outdoor tests despite localized feedback show sub-ambient drops of 9.7\u0026deg;C (horizontal) and 3.2\u0026deg;C (vertical) under 1100 W\u0026middot;m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e and 426 W\u0026middot;m\u003csup\u003e-\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e solar irradiance, respectively, validating real-world efficacy. This scalable design overcomes material scarcity, structural complexity, and spectral trade-offs, paving the way for terrestrial and extra-terrestrial thermal management.\u003c/p\u003e","manuscriptTitle":"All-inorganic HIP Multilayer With Excellent Thermostability for High-Performance Broadband Radiative Cooling","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-24 05:37:17","doi":"10.21203/rs.3.rs-7730333/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"4f9cc624-e366-467f-a1a7-0e0cbe75e5e8","owner":[],"postedDate":"November 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":58365533,"name":"Physical sciences/Materials science/Nanoscale materials/Organic\u0026#x2013;inorganic nanostructures"},{"id":58365534,"name":"Physical sciences/Nanoscience and technology/Nanoscale materials/Organic\u0026#x2013;inorganic nanostructures"}],"tags":[],"updatedAt":"2025-11-24T05:37:17+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-24 05:37:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7730333","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7730333","identity":"rs-7730333","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}