Transparent dynamic infrared emissivity regulators

doi:10.21203/rs.3.rs-2517977/v1

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Transparent dynamic infrared emissivity regulators

https://doi.org/10.21203/rs.3.rs-2517977/v1

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Dynamic infrared emissivity (DIE) regulators, which can efficiently modulate infrared radiation beyond vision, have emerged as an attractive technology in energy and information fields. However, current DIE regulators are usually visibly opaque, which limits their applications involving broad-spectrum requirements or multispectral compatibility. Therefore, it is necessary to propose new DIE mechanism and develop the desirable fully transparent DIE regulators for dynamically regulating infrared emissivity and solar spectral properties independently, although highly challenging. Here, we demonstrate DIE regulators based on a novel DIE mechanism with high visible transparency (84.7%), large emissivity regulation (0.51 in 3–5 µm, 0.42 in 7.5–13 µm), fast response (< 600 ms), and long cycle life (> 10⁴ cycles). This excellent performance is achieved by the reversible injection/extraction of electrons into/from aluminum-doped zinc oxide (AZO) nanocrystals to modulate infrared plasmonic in a capacitive-type device, and the DIE regulation is attributed to variation of carrier concentration in the depletion layer near the surface of AZO nanocrystals. This novel DIE regulation method and fully transparent DIE regulators provide great opportunities for the on-demand smart thermal management of buildings and spacecrafts, multispectral display and adaptive camouflage, and may in other infrared radiation related technologies.

Physical sciences/Materials science/Nanoscale materials/Nanoparticles

Physical sciences/Optics and photonics/Applied optics/Displays

Physical sciences/Optics and photonics/Applied optics/Mid-infrared photonics

Although infrared radiation is invisible to the human eye, any object in our surroundings continually emits thermal infrared electromagnetic radiation. Recently, the development of technology for regulating dynamic infrared radiation has emerged as an attractive research area in the energy and information fields. The Stefan–Boltzmann law states that infrared radiation can be regulated by controlling either temperature or emissivity. Controlling temperature requires devices with high energy consumption¹ and complex systems², while regulating the emissivity electrically is a promising method because of its flexible regulation, fast response speed, lightweight structure, and low energy consumption^3–9. Several electrically dynamic infrared emissivity (DIE) regulators have been proposed based on ion intercalation/extraction into/from materials (metal oxides³, conducting polymers⁴, carbon nanomaterials^5,6, etc.), electron injection/extraction into/from structures (quantum wells⁷, plasmonic resonators⁸, etc.), and reversible metal electrodeposition⁹. These existing DIE regulators are usually opaque owing to the strong absorption or reflection of visible light from the DIE materials or multilayer devices that are generally black (carbon nanomaterials^5,6), white (lithium titanate¹⁰), or colored (e.g., devices based on polyaniline⁴ or tungsten oxide³, etc.) (Supplementary Table 1). The device opacity limits advanced applications with broad-spectrum requirements or multispectral compatibility. For smart thermal management of buildings (roofs, windows, etc.) and spacecrafts (satellites, space stations, etc.), in addition to DIE regulation, it is also necessary to simultaneously achieve independent regulation of solar spectral properties to better meet on-demand thermal control requirements. For multispectral adaptive camouflage, the infrared emissivity and visible color should be dynamically regulated independently to counter multispectral reconnaissance in different spatial and temporal scenarios and achieve both visible and infrared chameleon-like camouflages. However, current DIE regulators can only achieve either DIE regulation or coupled dynamic regulation of infrared emissivity and solar spectral properties. Therefore, highly transparent DIE regulators are highly anticipated as they can be placed on top of a solar-spectrum dynamic regulation device or visible color-changing device to achieve decoupling of the dynamic regulation of infrared emissivity and solar-spectrum performance.

We propose a novel DIE regulation mechanism and a fully transparent DIE (TDIE) regulator based on aluminum-doped zinc oxide nanocrystals (AZO NCs) by electro-regulation of localized surface plasmon resonance (LSPR) at infrared wavelengths. The LSPR absorption intensity of the AZO NCs is enhanced or diminished by modulating the carrier concentration in the surface depletion layer of the AZO NCs. The developed TDIE regulators exhibit a DIE regulation of 0.51 and 0.42 at mid-wave infrared (MWIR; 3–5 µm) and long-wave infrared (LWIR; 7.5–13 µm) atmospheric windows, respectively, and the visible transmittance is always maintained at 84.7%. Meanwhile, TDIE regulators have a fast response time (< 600 ms) and long cycle life (10⁴ cycles). TDIE regulators demonstrate significant advantages in smart thermal management and multispectral display applications compared to the state-of-the-art devices.

Device structure and operating principle of TDIE regulators. By precisely controlling the Al doping of AZO NCs (diameter: 11.0 ± 0.6 nm) to 0.95%, we successfully induced LSPR of NCs to shift to infrared wavelengths (Supplementary Fig. 1, Supplementary Fig. 2 and Supplementary Note 2)¹¹. The full-width half-maximum of the LSPR peak covers the entire infrared atmospheric window owing to free carrier motion scattering¹², allowing broadband regulation by the TDIE regulator (Supplementary Fig. 2). The structure and operating principle of the TDIE regulators are shown in Fig. 1a. BaF₂ was used as the top infrared-transparent substrate. An upper AZO NC film (~ 1.14 µm; Fig. 1a, Supplementary Fig. 3) was used to achieve variations in the LSPR intensity in the infrared wavelengths. The upper visible transparent indium tin oxide (ITO) film (~ 330 nm; Fig. 1a, Supplementary Fig. 3) serves as both a working electrode and infrared reflectivity layer (R_{3 − 13µm} = 86.9%). The lower AZO NC layer and ITO film act as the ion storage layer and transparent counter electrode, respectively. The visible transmittances of BaF₂/AZO NC/ITO and AZO NC/ITO-glass half-devices were 81.2% and 88%, respectively (Supplementary Fig. 4). Finally, the visible transparency of a TDIE regulator was increased to 84.7% (Fig. 1b, c) using a transparent liquid LiTFSI/tetraglyme electrolyte with a refractive index (n ≈ 1.43) matching that of the ITO film (n ≈ 1.394) and AZO NCs (n ≈ 1.7). Moreover, the TDIE regulators maintained a high visible transmittance in both high- and low-emissivity states (Supplementary Fig. 5, Supplementary Video S1).

When the TDIE regulator is charged, electrons are injected into the AZO NCs, increasing the LSPR absorption and resulting in high-emissivity state of the regulator (Fig. 1a, c). Conversely, when the TDIE regulator is discharged, electrons are extracted from the AZO NCs, LSPR absorption is reduced, and the transmittance increases. Thus, the TDIE regulator exhibits the high infrared reflectivity of the upper ITO film, that is, the low emissivity state (Fig. 1a, c; Supplementary Eq. S3). The infrared absorption peaks in Fig. 1c were caused by the infrared absorption of the LiTFSI/tetraglyme electrolyte in the fingerprint region.

Performances of TDIE regulators. The emissivity of the TDIE regulators is regulated by the applied voltage, that is, the state of charge (SoC; Fig. 2a(i) and b(i)). The TDIE regulators exhibited the maximum emissivity regulation (Δε_MWIR = 0.51, Δε_LWIR = 0.42) at ± 2.5 V with a charge capacity of 0.27 ± 0.5 mC/cm² (Fig. 2a(ii) and b(ii), Supplementary Fig. 6). Emissivity regulation is not binary, and the intermediate state of infrared emissivity can be precisely regulated by applying different SoC values (Fig. 2a(i) and b(i), Supplementary Fig. 7). The memory time at 2.5 V (τ_m+ = 1.29 s) was significantly shorter than that at − 2.5 V (τ_m− = 1351 s; Supplementary Fig. 8), because the carrier depletion layer in the AZO NCs due to the positive voltage was rapidly compensated by free carriers from the ITO film¹³. The TDIE regulators exhibited a refresh rate of ~ 2 Hz (Response time: 578 and 361 ms; Fig. 2c). The cycling stability is a key indicator for DIE regulators. The developed TDIE regulators exhibited a cycle stability of > 10,000 cycles (Fig. 2d). Moreover, the visible transmittance of the TDIE regulator was maintained at 83.4% even after 10,000 cycles (Supplementary Fig. 10). The TDIE regulators exhibited high visible transmittance; to the best of our knowledge, this has never been reported for previous electrically DIE regulators (Supplementary Table 1). Moreover, the degree of emissivity regulation and spectral modulation range of TDIE regulators are comparable to those of existing DIE regulators. In particular, the response time and cycling stability of the TDIE regulators far exceed those of existing DIE regulators (Supplementary Table 1).

Regulation mechanism of TDIE regulators. Understanding the underlying DIE regulatory mechanism of AZO NCs is a key issue. The electrochromic properties of transition-metal oxides are usually caused by surface redox reactions, ionic intercalation, and/or capacitive charging¹⁴. The electrochemical response of NCs can be used to distinguish between surface redox and capacitive charging¹⁵. Rectangular cyclic voltammograms were measured for the AZO NCs (Supplementary Fig. 11), similar to that of a capacitor, indicating that the electrochemistry is mainly dominated by capacitive charging¹⁵. With increasing voltage, the Auger parameter of the Zn element in AZO NCs slightly increased (Fig. 3a), but remained within the Zn(II)O range^16,17, indicating that no surface redox reaction occurred in the AZO NCs. This was also demonstrated by the lack of significant redox peaks in the cyclic voltammograms. Tetrabutylammonium (TBA⁺) based electrolytes were used to determine if the DIE regulation of AZO NCs is triggered by ionic intercalation. Ionic intercalation into AZO NCs is not permitted because TBA⁺ (0.494 nm) is significantly larger than the crystal spacing of the AZO NCs (0.261 nm)^18,19. The AZO films exhibited the same DIE regulation properties in both TBA⁺-based electrolytes and Li⁺-based electrolytes, indicating that the DIE regulation of AZO NCs is not due to ionic intercalation (Supplementary Fig. 13). Therefore, capacitive charging/discharging is the dominant mechanism for the DIE regulation of AZO NCs. Moreover, owing to their capacitive characteristics, the response time of TDIE regulator was of the order of milliseconds¹⁵.

The diffusion of Li⁺ ions in the TDIE regulator under positive or negative applied voltages was investigated. Li⁺ ions pass through the porous working electrode film to balance the charge in some electrically DIE regulators with infrared high-reflectivity layer as the working electrode^4,10. We used a dense gold film (~ 500 nm) as the working electrode to clarify the migration of Li⁺ ions in the TDIE regulator because it is difficult for Li⁺ ions to pass through it (Supplementary Fig. 14). DIE regulation can also be observed in devices with gold film as the working electrodes (Supplementary Fig. 14), indicating that Li⁺ ions only balance the charge near the working electrode and do not surround the AZO NCs (Fig. 3b).

The intensity modulation of LSPR is primarily responsible for the DIE regulation of AZO NCs with only a 308 cm^− 1 shift in the LSPR frequency (ω_LSPR), which differs from the behavior of conventional electrochromic NCs caused by shifts in the LSPR peak position^20–22. The modulation of the LSPR intensity and frequency is explained by the formation of a dynamic depletion layer near the AZO NC surface. The surface depletion layer of a semiconductor NC is defined as the region where the carrier concentration differs significantly from that in the interior. The depletion layer is formed because the Fermi level is pinned to the surface potential by the natural surface state or applied voltage^13,23. The carrier concentration distribution and energy band profiles of the AZO NCs were calculated at various applied surface potentials (E_surf) using Poisson’s equation (Supplementary Note 4). The AZO NC energy band bends only in the surface layer of the carrier-depleted region (Fig. 3c). The width of the surface depletion layer (W_sd) of an AZO NC is ~ 1.27 nm when the highest oxidation potential (E_surf = 0.8 eV) is applied, occupying 54.3% of the NC volume (Fig. 3d). Electrons were injected into the AZO NC as the surface potential increased, resulting in a decrease in W_sd. Under a 2 eV surface potential, electrons are injected to a depth of 0.715 nm from the surface of a typical AZO NC. The carrier-concentration gradient did not penetrate deeply into the core of the NC (Fig. 3d, Supplementary Fig. 16).

The absorption spectra of different surface depletion layers, simulated for an individual AZO NC with carrier concentration variations confined to the shell of the NC, shows that ω_LSPR is moderately regulated (Δω_LSPR = 213 cm^− 1; Supplementary Fig. 18). We further simulated the effective dielectric function of the AZO NC film using the Maxwell–Garnett effective medium model (Supplementary Note 4). The LSPR modulation of AZO NC films at different surface potentials relative to that at E_surf = 0.8 eV was fitted (Fig. 3e). The LSPR intensity is primarily regulated by changes in the number of carriers during the charging/discharging of the surface depletion layer of the AZO NCs. Notably, the intensity and frequency modulation of the LSPR predicted by the model (Fig. 3e) are in excellent agreement with the observed experimental values (Fig. 3f). Therefore, the surface depletion layer is critical for the electrochemical DIE regulation of AZO NCs.

Application demonstration of TDIE regulators. The high visible transparency of the developed TDIE regulators makes this technology suitable for several application scenarios. To reduce the energy use of buildings, as an example of smart thermal management, TDIE regulators can achieve an infrared radiated power modulation of 64.9 W/m² in the 7.5–13 µm infrared atmospheric window at 298 K. Smart energy-saving buildings (SES) buildings are designed by combining the use of SES roofs and SES windows, which are prepared by covering TDIE regulators on White/Black devices and smart glass, respectively (Fig. 4a). As illustrated in Supplementary Fig. 19, SES roofs allow for the independent modulation of visible reflectance (white: high reflectance or black: high absorption) and infrared emissivity (high or low emissivity). SES windows allow for the independent control of visible transmittance (transparent or blue) and infrared emissivity (high or low emissivity; Supplementary Fig. 20). Unlike passive radiative-cooling materials^24,25 and temperature-adaptive radiative materials^26,27, which can only achieve radiative cooling or passively radiation modulation that is dependent of temperature, SES roofs and SES windows can operate in various modes, enabling on-demand thermal control and meeting building energy- efficiency requirements (Supplementary Fig. 21, Supplementary Fig. 22, and Supplementary Video S2).

We simulated the energy consumption of SES buildings for heating, ventilation, and air conditioning (HVAC) using EnergyPlus software. For more details, see Supplementary Note 5. A Medium Office prototype building model from the U.S. Department of Energy with a window-to-wall ratio of 33% and a total floor area of 4982.19 m² was applied²⁸. The Köppen climate classification divides the world into 30 climate zones represented by typical cities in different climate zones²⁹. For example, in Beijing, China, SES buildings can save 204 MBtu of average annual HVAC energy with an energy savings of 12.3% compared with standard buildings (Supplementary Fig. 24), resulting in a 59,625 kg reduction in CO₂ emissions (Supplementary Eq. S17). As shown in Fig. 4b, SES buildings can yield the highest HVAC energy savings in colder climate zones with energy savings of up to 464 MBtu, which is equivalent to a 111,076 kg reduction in CO₂ emissions. It is worth noting that both SES roofs and windows require only low driving voltages (< 3 V) and have low power consumption. The driving energy of the TDIE regulator is only 5.87–7.87 J/m² (Supplementary Fig. 6). The energy consumed by SES roof and window is negligible compared to the amount of HVAC energy saved by the SES buildings. Furthermore, the potential of the TDIE regulator for smart thermal management in spacecraft is demonstrated in Supplementary Note 6.

TDIE regulators can also be applied in multispectral displays and adaptive camouflage in visible and infrared wavelengths. Conventional optoelectronic displays, such as organic and quantum-dot light-emitting diodes (LEDs), are designed to operate with high efficiency in a certain wavelength range, making it difficult to achieve multispectral displays⁶. We prepared three-by-three arrays in which TDIE regulators served as independently addressable “pixels” and infrared animations were realized using an electrical circuit and programming (Supplementary Note 7). In principle, the pixel density of the infrared multiplexed array can be further increased by reducing the size of a single TDIE regulator. A multispectral display that can display images in the visible and infrared wavelengths simultaneously was realized by covering the multiplexed array on the LED display (Fig. 4c, Supplementary Video S3). The multispectral display exhibits multi-channel independent information dissemination, fast refresh rate, and low power consumption. We also realized adaptive visible–infrared compatible camouflage by covering the TDIE regulator on an electrochromic device, which changes its visible and infrared characteristics depending on the background environment (Supplementary Note 8, Supplementary Fig. 29).

In summary, we developed a novel DIE regulator with high visible transparency. This unprecedented functionality is expected to inspire more applications with very high potential to reshape DIE regulation technology, either as a standalone unit or by incorporation with an established visible light-manipulation device. Furthermore, the infrared plasmonic regulation in the NCs could enable the development of new technologies for active plasmonic³⁰, transparent electronics³¹, and other technologies based on infrared radiation related. In the future, we expect this technology to be applied in a broad range of applications with the development of flexible and large-area TDIE regulators.

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Chemical Reagents. Zinc acetate dihydrate (99%), oleic acid (OA, 90%), and oleylamine (OLA, 70%) were supplied by Sigma-Aldrich. Aluminum acetylacetonate [Al(Acac)₃, 99%] and oleyl alcohol (80−85%) were supplied by Alfa Aesar. Toluene was purchased from Sinopharm Co., Ltd. Tin-doped indium oxide (ITO) pellets (In:Sn =90:10, 99.99%) were purchased from ZhongNuo Advanced Material Technology Co., Ltd. as an evaporation material. Commercial ITO glass (25 mm × 25 mm × 1.1 mm; ≤15 Ω) was purchased from Kaivo Optoelectronic Technology Co., Ltd.

Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 98%), tetrabutylammonium perchlorate (TBAP, 98%), tetraethylene glycol dimethyl ether (98%), acetonitrile (98%), and propylene carbonate (PC, anhydrous, 99.5%) were purchased from J&K Scientific.

Fabrication of TDIE regulator. The AZO NCs were synthesized using a slightly modified process as previously explained by Wainer et al.¹¹. The synthesis temperature was set to 280 °C, and the nominal Al doping concentration was 1%. Toluene was used to disperse the AZO NCs at concentrations of ∼50 mg/mL. AZO NCs were spin-coated onto 25 mm × 25 mm × 1 mm BaF₂sheets at a spinning speed of 1000 rpm for 60 s, followed by 4000 rpm for 20 s. The spin-coating process was repeated several times to increase the film thickness to ~1.1 μm. The samples were then heated in an argon environment at 250 °C for 30 min. An ITO film with a thickness of ~330 nm was then evaporated (MEB-600, Beijing Chuangshiweina Technology Co., Ltd., China) onto the samples as the working electrode at a deposition rate of 1 Å/s. The substrate temperature was set to 300 °C, and the oxygen flow rate was 20 sccm.

LiTFSI (1 M) in tetraglyme was used as the electrolyte. As previously stated, 700 nm nanocrystals were spin-coated as an ion storage layer on the counter electrode ITO glass (25 mm × 25 mm × 1.1 mm; ≤15 Ω). Half-devices made of BaF₂/AZO NC film/ITO film and AZO NC film/ITO glass were encapsulated using a transparent silicone adhesive sealant with 0.1-mm silver wire as the lead. The electrolyte was then injected into the device. The TDIE regulator was prepared according to the steps outlined above.

Fabrication of smart glass and SES windows. WO₃ films (~400 nm) were deposited on a ITO glass (25 mm × 25 mm × 1.1 mm; ≤15 Ω) as an electrochromic layer using a high-vacuum four-target magnetron sputtering system (MSP-300BTI, Beijing Chuangshiweina Technology Co., Ltd., China). The deposition was performed at a pressure of 1 Pa and power of 120 W for 700 s. Half-devices made of ITO glass/WO₃film and ITO glass were encapsulated using a transparent silicone adhesive sealant. The smart glass was prepared after injecting the electrolyte (1 M LiTFSI in tetraglyme). SES windows were assembled by stacking the TDIE regulators on top of the smart glass.

Fabrication of White/Black devices and SES roofs. The fabrication process for White/Black devices proposed by Imamura³² was followed with a slight modification. A transparent silicone adhesive sealant was used as the encapsulation material instead of the Teflon spacer. SES roofs were built by stacking TDIE regulators on top of a White/Black device.

Fabrication of visible-transparent infrared displays. Nine TDIE regulators in a 3×3 array were arranged on an LED display. The wires were then extended and attached to the circuit. The circuit and programming are presented in Supplementary Note 7.

Fabrication of adaptive visible-infrared compatible camouflage device. The preparation of the electrochromic device has been previously described by Wu et al.³³. A visible-infrared-compatible camouflage device was assembled by stacking a TDIE regulator on the electrochromic device.

Characterization. The total infrared reflectance spectra of the samples (2.5–25 μm) were measured using a Fourier transform infrared (FTIR) spectrometer (Bruker Vertex 70) equipped with a mid-IR integrating sphere (A562). The transmittance and reflectance spectra of the samples were measured using an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophotometer (Agilent Cary 5000) equipped with a diffuse integrating sphere.

The samples were photographed and filmed using a digital camera (Nikon D3100). A FLIR SC7300M with a working range of 3.7–4.8 μm was used to capture the MWIR images (the predefined emissivity was set to 1). LWIR images and videos were recorded using FLIR T1050sc with a working range of 7.5–14 μm (the predefined emissivity was set to 0.95). FLIR Tools V 5.6 and FLIR ResearchIR Max 4.0 were used to analyze the LWIR video data.

The Al content of AZO NCs was determined using inductively coupled plasma optical emission spectrometry (ICP-OES, Agilent 720ES). The relative error in the extracted Al content was reduced by averaging six replicate measurements for each batch of AZO NCs.

The surface morphologies of the AZO NCs films were examined using field-emission scanning electron microscopy (FE-SEM; TESCAN MIRA) with a beam energy of 20 keV. Low- and high-resolution transmission electron microscopy (TEM) measurement were carried out on an FEI Talos F200S instrument. To prepare the samples for TEM analysis, a drop of a toluene solution containing the NCs was dried on the surface of an ultrathin carbon film on a copper grid. The cross-section of the BaF₂/AZO NC film/ITO film half-device was prepared using a focused ion beam (FIB; FEI STRATA 400S). The cross-sectional morphologies were then studied using TEM (FEI Talos F200s) and energy-dispersive spectroscopy (EDS; super-X).

X-ray photoelectron spectroscopy (XPS) (Thermo ESCALAB 250Xi) was performed using a monochromatic Al Kα source (1486.6 eV). Various voltages (-2.5 to 2.5 V) were applied to the AZO nanocrystalline film for 10 min using 1 M LiTFSI as the electrolyte and saturated Ag/AgCl as the counter electrode. After the electrolyte was wiped clean, the samples were sent to the XPS sample chamber for quasi-in-situ XPS analysis. The thickness of the AZO NC film was measured using a Bruker Dektak XT Profiler.

A PARSTAT 4000 Advanced Electrochemical System (Princeton Applied Research, USA) was used to perform cyclic voltammetry (CV) on the AZO NC films and evaluate the cycling performance of a TDIE regulator. In the device performance demonstration, a DC-stabilized power supply (UTP1306-II) was used.

Acknowledgements

We thank E D Gaspera (RMIT University, Australia), Mingyang Li, Mei Zu, Zi Wang (National University of Defense Technology) for their valuable advice. We also thank Y Chen (National University of Defense Technology) for assistance with the Profiler analyses. We acknowledge financial support from National Natural Science Foundation of China (52073303), Natural Science Foundation of Hunan Province (2021JJ10049), and Postgraduate Scientific Research Innovation Project of Hunan Province (CX20210054).

Author contributions

D. L. conceived the idea. D. L. and H.C. supervised the work. Y. J. and D. L. fabricated the TDIE regulators, conducted the electrical and spectral characterizations, developed the optical modelling and analyzed the application of TDIE regulators. Y. J. and D. C. synthesized the AZO NCs under the Y. Jin’s supervision. T. L., Z. M., and B. C. fabricated White/Black devices and smart glass. J.T. designed the circuit and wrote the program of infrared displays. S. Z. and X. W. assisted in the analysis of mechanism of infrared emissivity regulation. C. C. participated in data analysis. Y. J. and D. L. analysed the data and wrote the manuscript. All the authors discussed the results and commented on the manuscript.

There is NO Competing Interest.

SInp.docx
Supporting information for: Transparent dynamic infrared emissivity regulators
VideoS1.mp4
Real-time visible and infrared thermal video of TDIE regulators, recorded by a Nikon D3100 digital camera and FLIR T1050sc infrared camera.
VideoS2.mp4
Real-time visible and infrared thermal video of SES roofs and SES windows, showing different mode in Supplementary Fig. 19 and Fig. 20.
VideoS3.mp4
Real time visible and infrared thermal video of multispectral display.

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