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Article

Broadband Near-Infrared Reflective Film from Stacked Opposite-Handed Chiral Liquid Crystals with Pitch Gradients

by
Hyeon Seong Hwang
1,†,
Jongsu Lee
2,†,
Byungsoo Kang
3,
Minhye Kim
1,
Doyo Kim
1 and
Se-Um Kim
1,*
1
Department of Electrical and Information Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea
2
Sensor System Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
3
Advanced Photovoltaics Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Crystals 2025, 15(7), 597; https://doi.org/10.3390/cryst15070597
Submission received: 30 May 2025 / Revised: 18 June 2025 / Accepted: 22 June 2025 / Published: 25 June 2025
(This article belongs to the Collection Liquid Crystals and Their Applications)
Browse Figures
Versions Notes

Abstract

Broadband near-infrared (NIR) reflective films are widely used in architecture and the automotive and aerospace industries for energy saving and thermal regulation. For large-area and flexible applications, it is essential to develop cost-effective, solution-processable, and long-term-stable NIR-reflective films. Here, we present a polymer-stabilized chiral liquid crystal (CLC) film that achieves broadband NIR reflection by stacking opposite-handed CLC layers with pitch gradients. We experimentally established optimal formulations of materials for both right-handed and left-handed CLCs. The resulting film exhibits high-degree broadband reflection (~95%) in the 1000–1800 nm wavelength range, while maintaining visible transmittance (~80%) in the 450–850 nm range. The concept proposed here will be widely applicable for scalable and practical NIR-filtering applications in smart glasses, sensors, and optoelectronic devices.

1. Introduction

A significant portion of solar energy, over 50%, lies in the near-infrared (NIR) spectrum, which contributes to excessive thermal load. This increases the cooling demand in buildings and, in the case of optoelectronic devices, accelerates performance degradation in outdoor environments. Therefore, the development of optical coatings or films that selectively reflect NIR wavelengths while maintaining high transmittance in the visible range is critical for improving thermal management performance in various applications [1,2,3].
Low-emissivity (low-e) coatings [4,5] and distributed Bragg reflectors (DBRs) [6], which are designed to strongly reflect in the NIR wavelength while remaining transparent in the visible range, have been widely employed to mitigate photothermal effects caused by NIR radiation. Low-e coatings incorporate thin, low-emissivity metal layers such as silver [4,5], aluminum [5,7], or copper [8,9] to reflect near-infrared (NIR) light, with metal oxide layers enhancing visible transmittance and protecting the metal layers. Their reflection typically spans from the NIR to IR region with high reflectance (over 90%), but the presence of metal layers often leads to reduced visible transmittance (typically 60–80%) [10]. DBRs, on the other hand, form a photonic bandgap by alternating high- and low- refractive-index dielectric layers to create destructive interference that blocks the propagation of NIR wavelengths [11,12,13]. DBRs generally offer a high visible transmittance of 80–90%. However, their NIR reflection bandwidth is relatively narrow (typically 100–200 nm) so that broadband photothermal effects remain constrained [14,15].
Despite their widespread use, the optical performance of both low-e coatings and DBRs strongly depends on the precise thickness of individual layers. For low-e coatings, thicker metal layers significantly reduce visible transmittance, while those thinner than a critical limit may result in incomplete conformal coverage on substrates [4,16]. Similarly, in DBRs, even slight deviations in the thickness of sub-layers shift the central reflection wavelength and reduce its intensity [17,18,19]. Moreover, achieving broadband reflection requires a substantial increase in the number of stacked layers [20,21]. These thin layers are typically fabricated via vacuum-assisted evaporation processes, which inherently limit scalability for large-area applications.
Cholesteric liquid crystals (CLCs) serve as polarization-dependent photonic crystals that enable thickness-independent NIR reflection owing to the helical structure of liquid crystal (LC) molecules. CLCs selectively reflect the circularly polarized light with the same handedness as the helical twist [22,23]. In CLCs, the central reflection wavelength λ and reflection bandwidth Δλ are given by λ = (ne + no) × P/2 and Δλ = (ne − no) × P, where no and ne are the ordinary and extraordinary refractive indices of the LC molecules, respectively, and P is the helical pitch. CLCs generally comprise host nematic liquid crystals doped with a small concentration of chiral compounds, and incorporating reactive monomers enables the formation of polymer-stabilized structures that can be fabricated into solid-state films [24,25,26]. These CLC films can be self-assembled through simple solution-based processes such as spin coating [27,28,29,30], spray coating [31,32], or bar-coating [33,34].
Since the reflection bandwidth of single-pitch CLCs is intrinsically narrow, various strategies have been proposed to broaden the reflection band by introducing a pitch gradient in CLCs through electrical modulation [25,35], thermal diffusion [36,37], or doping with ultraviolet (UV)-absorbing dyes [38,39,40]. CLCs are compatible with solution processing, large-area fabrication, and flexible substrates. However, their intrinsic polarization selectivity limits the maximum theoretical reflectance to 50%, as they only reflect circularly polarized light of a single handedness. To circumvent this, it is necessary to assemble CLC layers with opposite handedness into a monolithic structure capable of reflecting both left- and right-circularly polarized NIR light. Precise alignment of both the bandwidth and central wavelength of the reflection bands from the two CLC layers is essential to achieve almost-complete broadband NIR reflection. It is also important to maintain a high visible-light transmission in the assembled structure. Furthermore, the long-term reliability of such films under continuous exposure to intense NIR radiation is crucial for their practical implementation.
In this study, we demonstrate a broadband NIR-reflective film fabricated by stacking two opposite-handed CLC layers with pitch gradients. The pitch gradient was introduced by doping the CLCs with a UV-absorption dye, and the reflection characteristics were controlled by adjusting the concentrations of the dye and a reactive mesogen used for polymer stabilization. Each CLC layer was fabricated separately on different substrates and subsequently assembled into a single monolithic film. The resulting film exhibits high visible-light transmittance while achieving broad NIR reflection from 1000 to 1800 nm, with a maximum reflectance exceeding 95%. This combination of broadband NIR reflectance and high visible-light transmittance is comparable with those of established low-e and DBR-based films. The thermal durability of the film was evaluated, along with an assessment of its NIR reflection performance through infrared thermography to examine feasibility under practical conditions.

2. Materials and Methods

2.1. Materials

Reactive Mesogen RM257, host LC E7, and chiral dopant LC756 were purchased from Henan Wentao Chemical Product Co. (Zhengzhou, China). Chiral dopant S811 and UV-absorption dye UV-327 were purchased from Chemical Industry Co., Ltd. (Tokyo, Japan). Photoinitiator 2,2-Dimethoxy-2-phenylacetophenone (DMPA) was purchased from Sigma-Aldrich (St. Louis, MO, USA). The chemical structures of these materials are illustrated in Figure 1. Polyvinyl alcohol (PVA) and ethanol were purchased from Duksan General Science (Seoul, South Korea). Polyimide (PI) SE-7492 was provided by Nissan Chemical Co. (Tokyo, Japan) UV-curable adhesive NOA81 was purchased from Edmund Optics (Barrington, NJ, USA). All materials were used as received without further purification.

2.2. Sample Preparation

The CLC mixtures with various component ratios were prepared as described in Table 1. All sample groups contained 0.5 wt% of DMPA as the photoinitiator. The mixtures were stirred uniformly for 1 h at 100 °C and then slowly cooled to room temperature. Top and bottom glass substrates were cleaned with acetone and subsequently treated with UV-ozone for 5 min. After surface treatment, polyimide was spin-coated on the top substrate at a rate of 3000 rpm for 30 s and annealed at 230 °C for 2 h. A 1 wt% solution of PVA in distilled water was spin-coated on the bottom substrate at a rate of 3000 rpm for 30 s and annealed at 100 °C for 1 h. To achieve a homogeneous orientation of the CLC film, the PI-coated top substrate and the PVA-coated bottom substrate were rubbed in anti-parallel directions and then assembled with a 25 µm cell gap, using 25 µm spacer particles dispersed in NOA81. The prepared CLC mixtures were inserted into the empty cell by capillary action at 35 °C, followed by 365 nm UV irradiation using a UIT250 UV source (SP-9, Ushio, Tokyo, Japan) with an output power of 1200 mW for 15 min to form a polymer network with a gradient pitch. The measurement of UV intensity (UV-340A, Lutron, Coopersburg, PA, USA) at the exposed surface was 2 mW/cm2. The LC cells were then immersed in ethanol for 1 h to disassemble the substrates. The CLC layers were selectively retained on the top PI-coated substrate, which faced the UV light, due to stronger adhesion. Two types of CLC layers, left-handed (LH) CLC and right-handed (RH) CLC, were stacked with their CLC sides facing each other. In Table 1, the sample groups A and C are LH CLCs and the sample groups B and D are RH CLCs.

3. Results and Discussion

3.1. Structure and Chemical Composition of NIR-Reflective Films

The proposed NIR-reflective film is composed of two CLC layers with opposite handedness, an LH CLC layer and a RH CLC layer, as illustrated in Figure 1a. The helical pitches of these layers are designed to reflect NIR light while transmitting visible light. Figure 1b schematically shows the molecular arrangement of the LH CLC (left) and RH CLC (right). To achieve a chiral nematic phase at room temperature prior to polymerization, the nematic LC mixture, E7, was used as the host material. E7 consists of four mesogenic compounds: 4-cyano-4′-n-pentyl-biphenyl, 4-cyano-4′-heptyl-biphenyl, 4-cyano-4′-n-oxyoctyl-biphenyl, and 4-cyano-4″-n-pentyl-p-terphenyl (Figure 1c).
Figure 1d shows the chemical structures of the additional components: RM257, S811, LC756, and UV-327, which are dispersed in the host LC. The UV-assisted polymerization of the reactive mesogen RM257 stabilizes the overall LC structure. RM257 is a bifunctional linear reactive mesogen containing diacrylate groups, which forms the structural backbone of the CLC network and co-polymerizes with LC756. S811 and LC756 were used as LH and RH chiral dopants, respectively. It is important to note that S811 is unable to participate in the polymerization process, yet it induces a stabilized chiral nematic phase in the E7/RM257 composite during polymerization. As a result, the LH CLC layer retains its helical structure even after the removal of S811.
UV-327 was incorporated as a UV absorber [41,42] that modulates the UV light intensity throughout the film thickness, inducing a vertical gradient in the helical pitch [20,43]. This gradient arises from the diffusion behavior of monomers during polymerization, which depends on the local UV intensity. The presence of UV-absorption dye leads to reduced UV penetration, resulting in a higher monomer concentration near the UV-irradiated surface. This in turn lowers the relative concentration of chiral dopant, producing a longer helical pitch near the top substrate. Conversely, the helical pitch decreases near the bottom side of the film. This gradual variation in helical pitch across the film thickness produces a broadened reflection band in the NIR region [7,44,45].

3.2. Fabrication of the Optimal LH CLC and RH CLC Layers

An empty cell comprising two glass substrates, coated with thin layers of PI and PVA respectively, was prepared as shown in Figure 2a. A 25 µm cell gap was maintained using polystyrene (PS) spacer particles embedded at the four corners of the substrates. NOA81, serving as the dispersion medium for the spacer particles, was pre-cured under UV light to fix the gap. The prepared CLC mixtures were subsequently injected into the cell. After a second UV exposure to polymerize the injected mixtures, the sample was immersed in ethanol. After disassembling the cell, both LH (Figure 2b) and RH (Figure 2c) CLC layers were successfully fabricated on the PI-coated glass substrates. The PVA-coated glass substrate was easily detached without surface residues due to the weak adhesion between the CLC layer and the PVA surface [46,47,48]. The transmittance spectra of these layers are expected to exhibit full transmission in the visible region and polarization-dependent reflection of up to 50% in the NIR region.
The CLC layers can achieve near-theoretical maximum reflectance when the number of the helical pitches typically exceed 10 [49,50]. Based on the equation for the central reflection wavelength, λ = (ne + no) × P/2, and the approximate value (ne + no)/2 ~ 1.62 given by the primary LC materials E7 [51] and RM257 [52], a 10 µm-thick layer can cover the maximum reflectance. However, bandwidth broadening was limited at this thickness (Figure 2d). A 25 µm-thick layer showed saturated bandwidth broadening without significant degradation in visible transmittance. Layers thicker than 25 µm may suffer reduced transmittance due to instability in LC alignment.
To target NIR reflection in the 1000–2000 nm range, the weight ratio of LC756 was fixed at 2 wt%. LC756 has a high helical twisting power (HTP) of 64 µm−1, which is greater than that of S811 (11 µm−1) [53,54]. Accordingly, the required S811 concentration to match the target reflection range is theoretically expected to be around 12 wt%. However, the actual matched concentration may vary depending on the host materials used [55,56]. To determine the optimal value, a series of samples was prepared with S811 concentrations ranging from 10 to 12 wt% in 1 wt% intervals. As shown in Figure 2e, the LH CLC layer containing 10 wt% S811 exhibited a reflection bandwidth well matched to that of the RH CLC layer, achieving broadband reflection in the 1000–1800 nm range. When the LH and RH CLC layers were stacked, the resulting NIR-reflective film demonstrated highly selective visible light transmission along with polarization-independent, significantly enhanced NIR reflectance (Figure 2e). Given the low optical absorption of CLC materials in the visible to NIR wavelength range [57], the reflectance spectrum (R) can be reliably inferred from the measured transmittance spectrum (T) as R = 1 − T. The stacked CLC layers exhibited approximately 80% visible transmittance at normal incidence. The remaining transmittance loss is primarily attributed to Fresnel reflections at the glass–air and internal layer interfaces, as well as intrinsic absorption from the glass substrate. No visible haze was observed even at oblique viewing angles (30° and 60°), confirming that the stacked CLC configuration achieved broadband NIR reflectance without introducing noticeable light scattering or haze in the visible range.
Although the reflectance spectrum shifts toward shorter wavelengths at oblique incidence, the broad NIR reflection bandwidth ensures that this spectral displacement remains within the effective operating range. Also, the reflection band was designed with sufficient angular tolerance by positioning its lower bound above 1000 nm so that the film remains fully transparent even at such oblique viewing directions.
The total reflectance of the stacked CLC configuration arises from the complementary polarization selectivity of the individual LH and RH CLC layers [58,59]. By combining opposite-handedness CLC layers, near-complete reflection of both circular polarization components is achieved, resulting in polarization-independent broadband NIR reflectance.

3.3. Optimization of the Compositional Ratio and Fabrication Conditions of the CLC Mixture

To ensure the formation of a homogeneous polymer network capable of inducing a pitch gradient over a broad range in the NIR spectrum, it is essential to prevent phase separation between RM257-rich and E7-rich domains. Such phase separation leads to a decrease in visible transmittance due to haze, as well as distortion of the helical structure [60,61]. To determine the optimal weight ratio of RM257, its concentration was varied from 15 wt% to 30 wt% in 5 wt% increments for both LH CLC and RH CLC layers. As shown in the transmittance spectra for LH CLC (Figure 3a), increasing the RM257 concentration led to improved transmittance in the visible range and enhanced selective reflection in the NIR region. Polarized optical microscopy (POM) images reveal significant phase separation at lower RM257 concentrations of 15 wt% (Figure 3b) and 20 wt% (Figure 3c). At a concentration of 15 wt%, the domain size was smaller than the resolution limit of the microscope, appearing as subtle color fluctuations without clearly defined boundaries. In contrast, at 20 wt%, phase separation domains became physically distinguishable and were widely distributed across the sample.
More uniform texture emerged at 25% RM257 (Figure 3d), with optimal uniformity and optical characteristics observed at 30% (Figure 3e). It should be noted that the oil streak textures observed in POM images were formed due to flow-induced alignment during LC injection [62,63].
For the RH CLC layer, a similar trend was observed. Increasing the RM257 concentration led to higher transmission in the visible range and broader NIR reflection (Figure 3f). At 15% RM257, a heterogeneous texture produced from phase separation was clearly visible (Figure 3g). In contrast to the LH CLC, a homogeneous phase appeared as early as 20% RM257 in the RH CLC, likely due to the copolymerization between RM257 and LC756 (Figure 3h). Both the uniformity and optical characteristics further improved at 25% (Figure 3i) and reached optimal levels at 30% RM257 (Figure 3j).
With a fixed RM257 concentration of 30 wt%, the UV-327 concentration was varied at 0%, 1%, 2%, and 3%, and the corresponding transmittance spectra were measured (Figure 4). For the LH CLC layer, visible-range transmittance decreased significantly when the UV-327 concentration reached 2% or higher (Figure 4a). Comparison of POM images at 0% (Figure 4b) and 1% (Figure 4c) UV-327 showed uniform phases in both cases. However, 1% UV-327 notably broadened the NIR reflection band by inducing a pitch gradient within the CLC structure. Compared with the undoped sample, the reflection bandwidth increased approximately four to five times. These results suggest that within the 0–1 wt% range, the pitch gradient magnitude increased approximately linearly with the UV-327 concentration [64]. However, higher UV-327 concentrations (2% and 3%) disrupted effective monomer polymerization and caused phase separation, as shown in Figure 4d and 4e, respectively.
For the RH CLC layer, visible transmittance gradually decreased, and the NIR reflection band narrowed as the UV-327 concentration increased (Figure 4f). As with the LH CLC layer, uniform LC phases were observed at both 0% (Figure 4g) and 1% UV-327 (Figure 4h), whereas phase separation occurred at 2% (Figure 4i) and 3% (Figure 4j). Therefore, a UV-327 concentration of 1% was identified as the optimal condition for both LH and RH CLC layers when combined with 30% RM257. It should be noted that in both LH and RH CLCs, an increase in the degree of phase separation with higher UV-327 concentrations appears as a growth in domain size observed in the POM images, particularly between 2% and 3% (Figure 4d,e,i,j).
With 30% RM257 and 1% UV-327 established as the optimal compositions for both LH and RH CLC layers, the effect of UV exposure time was further investigated. For the LH layer, the transmittance spectra exhibited a gradual broadening of the NIR reflection band and a slight decrease in visible transmission as the UV exposure time increased from 0 to 20 min (Figure 5a). However, considering sample-to-sample variations, the spectra appeared to saturate after approximately 10 min of exposure. This is supported by POM images, which show a clear difference between the unexposed sample and the one exposed for 10 min (Figure 5b,c). From 10 to 15 min of exposure, the LH CLC layer maintained its optical properties (Figure 5d), with a noticeable overall decrease in transmittance observed at 20 min (Figure 5e).
A similar trend was observed for the RH CLC layer, where the transmittance spectra also saturated after 10 min of UV exposure (Figure 5f). POM images reveal a significant color change between 0 and 10 min of exposure (Figure 5g,h), stability between 10 and 15 min (Figure 5i), and a distinct color change after 20 min (Figure 5j). We selected 15 min as the optimized processing time to ensure complete polymerization and minimized potential instabilities caused by residual unreacted materials.

3.4. NIR Reflection Performance of the Stacked CLC Layers

To fabricate the final NIR-reflective film, the optimized LH and RH CLC layers were stacked together, as illustrated in Figure 2d. The stacked configuration exhibited high transmittance in the visible range (450–850 nm) and broadband reflection exceeding 50% in the NIR range (1000–1800 nm). The wavelength range between 850 and 1000 nm represents a transition from transmission to reflection. Upon lamination, the primary reflection peak of the individual CLC layers, initially centered near 1100 nm, shifted toward longer wavelengths. This redshift may be attributed to the diffusion of unreacted chiral dopants across the layer interface. The LH chiral dopant S811 remained unreacted, whereas the RH chiral dopant LC756 was fully consumed during polymerization. The resulting concentration gradient of S811 drove its migration into the RH CLC layer, partially relaxing the RH helical twisting and thereby shifting the reflection band with a new peak near 1200–1300 nm.
Durability to thermal stress was evaluated by heating the film up to 40 °C and 100 °C for 10 min. The film retained the visible light transmittance and NIR reflection properties after thermal treatment (Figure 6a). This excellent stability is primarily attributed to the acrylate-based thermoset polymer network formed by RM257. Although the host LC becomes optically isotropic at high temperatures, it preserves the LC phase due to stabilization within the polymer network. Removing E7 would degrade visible transmittance by increasing haze. The consistent transmittance spectra indicate that no evaporation or phase change of the host LC occurred upon heating.
To evaluate the durability to sunlight exposure, the NIR-reflective film was exposed to a broadband NIR light source (OSL2IR furnished with a cut-on filter at 550 nm, Thorlabs) ranging from 550 to 2000 nm (~30 mW/cm2 at 980 nm) for durations between 4 and 48 h. As shown in Figure 6b, the transmittance spectra remained stable over the entire 48-h period, indicating no degradation in optical performance. Furthermore, the film exhibited no noticeable spectral degradation after being exposed to ambient conditions for over 500 h (Figure 6c). Considering the intrinsic hydrophobic nature of the constituent materials in the CLC layers, the film is expected to preserve its optical performance and potentially serve as an encapsulation layer [65,66].
To demonstrate the NIR-reflection performance of the stacked CLC layer, infrared thermography images of bare glass (Figure 6d) and the stacked CLC layer (Figure 6e) were captured under NIR illumination from above. The maximum temperatures recorded were 27.2 °C for bare glass and 43.4 °C for the stacked CLC layer. Since the recorded temperature correlates with the intensity of reflected NIR light, the stacked CLC layer exhibited higher NIR reflectance. In transmission mode, with NIR light incident from behind, a stronger NIR signal was detected passing through the bare glass (Figure 6f), whereas the stacked CLC layer effectively blocked the incoming NIR light (Figure 6g). The maximum temperatures recorded in this mode were 64.3 °C for bare glass and 34.7 °C for the stacked CLC layer.

4. Conclusions

We have developed a broadband NIR-reflective film composed of stacked LH and RH CLC layers. Through optimization of polymerizable monomer RM257 concentration, UV absorber UV-327 concentration, and UV exposure time, we realized a gradient-pitch helical structure with uniform phase in both LH and RH CLC layers. The optimized LH CLC layer was prepared with E7, RM257, S811, and UV-327 in weight ratios of 58.5:30:10:1, respectively (sample A4). The optimized RH CLC film was prepared with those materials in weight ratios of 66.5:30:2:1, respectively (sample B4). We selected the optimized UV intensity and exposure time to achieve a stable pitch gradient while preventing transmittance degradation caused by either incomplete polymerization or overexposure. The stacked CLC configuration achieved visible light transmission of up to 80% and broadband NIR reflection spanning 1000 to 1800 nm, with a maximum reflectance of 95%. The film demonstrated a high degree of durability under continuous sunlight exposure and thermal stress. This strategy of stacking two opposite-handed CLC layers offers an effective and versatile NIR-reflective solution suitable for energy-efficient and protective coatings in optoelectronic applications.

Author Contributions

Conceptualization, H.S.H., J.L., B.K. and S.-U.K.; methodology, J.L.; software, H.S.H. and J.L.; validation, B.K.; investigation, H.S.H., M.K. and D.K.; data curation, H.S.H., M.K. and D.K.; writing—original draft, H.S.H. and J.L.; writing—review and editing, J.L. and S.-U.K.; visualization, J.L. and B.K.; supervision, S.-U.K.; project administration, S.-U.K.; funding acquisition, S.-U.K., H.S.H. and J.L. contributed equally to this study. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by Seoul National University of Science and Technology.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 15 00597 g001
Figure 1. (a) Schematic illustration of the proposed NIR-reflective film structure. (b) Structural schematics of LH CLC (left) and RH CLC (right) layers. (c) Composition and chemical structures of the nematic LC mixture E7. (d) Chemical structures of the reactive mesogen RM257, the LH chiral dopant S811, the RH chiral dopant LC756, and the UV-absorption dye UV-327.
Figure 1. (a) Schematic illustration of the proposed NIR-reflective film structure. (b) Structural schematics of LH CLC (left) and RH CLC (right) layers. (c) Composition and chemical structures of the nematic LC mixture E7. (d) Chemical structures of the reactive mesogen RM257, the LH chiral dopant S811, the RH chiral dopant LC756, and the UV-absorption dye UV-327.
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Crystals 15 00597 g002
Figure 2. (a) Schematic illustration of the fabrication process for CLC layers. (b,c) Photographs of the fabricated (b) LH and (c) RH CLC layers with their corresponding transmittance spectra. (d) Transmittance spectra of LH CLC layers with 10 µm and 25 µm cell gap. (e) Transmittance spectra of LH CLC layers with varying concentrations of S811 (10, 11, and 12 wt%). (f) Schematic of the stacking process for LH and RH CLC layers, along with sample photographs taken at sample tilt angles of 0°, 30°, and 60°, and the representative transmittance spectrum. Scale bar, 1 cm.
Figure 2. (a) Schematic illustration of the fabrication process for CLC layers. (b,c) Photographs of the fabricated (b) LH and (c) RH CLC layers with their corresponding transmittance spectra. (d) Transmittance spectra of LH CLC layers with 10 µm and 25 µm cell gap. (e) Transmittance spectra of LH CLC layers with varying concentrations of S811 (10, 11, and 12 wt%). (f) Schematic of the stacking process for LH and RH CLC layers, along with sample photographs taken at sample tilt angles of 0°, 30°, and 60°, and the representative transmittance spectrum. Scale bar, 1 cm.
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Crystals 15 00597 g003
Figure 3. (a) Transmittance spectra of LH CLC layers with RM257 concentrations ranging from 15% to 30% (A1–A4). (b–e) POM images of LH CLC layers with RM257 concentrations of (b) 15%, (c) 20%, (d) 25%, and (e) 30%. (f) Transmittance spectra of RH CLC films with RM257 concentrations ranging from 15% to 30%. (g–j) POM images of RH CLC layers with RM257 concentrations of (g) 15%, (h) 20%, (i) 25%, and (j) 30%. In (g), the white arrows indicate representative regions of phase separation. Scale bar, 500 µm.
Figure 3. (a) Transmittance spectra of LH CLC layers with RM257 concentrations ranging from 15% to 30% (A1–A4). (b–e) POM images of LH CLC layers with RM257 concentrations of (b) 15%, (c) 20%, (d) 25%, and (e) 30%. (f) Transmittance spectra of RH CLC films with RM257 concentrations ranging from 15% to 30%. (g–j) POM images of RH CLC layers with RM257 concentrations of (g) 15%, (h) 20%, (i) 25%, and (j) 30%. In (g), the white arrows indicate representative regions of phase separation. Scale bar, 500 µm.
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Figure 4. (a) Transmittance spectra of LH CLC layers with UV-327 concentrations ranging from 0% to 3%. (b–e) POM images of LH CLC layers with UV-327 concentrations of (b) 0%, (c) 1%, (d) 2%, and (e) 3%. (f) Transmittance spectra of RH CLC layers with UV-327 concentrations ranging from 0% to 3%. (g–j) POM images of RH CLC layers with UV-327 concentrations of (g) 0%, (h) 1%, (i) 2%, and (j) 3%. In (e,j), the white arrows indicate representative regions of phase separation. Scale bar, 500 µm.
Figure 4. (a) Transmittance spectra of LH CLC layers with UV-327 concentrations ranging from 0% to 3%. (b–e) POM images of LH CLC layers with UV-327 concentrations of (b) 0%, (c) 1%, (d) 2%, and (e) 3%. (f) Transmittance spectra of RH CLC layers with UV-327 concentrations ranging from 0% to 3%. (g–j) POM images of RH CLC layers with UV-327 concentrations of (g) 0%, (h) 1%, (i) 2%, and (j) 3%. In (e,j), the white arrows indicate representative regions of phase separation. Scale bar, 500 µm.
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Figure 5. (a) Transmittance spectra of LH CLC layers under UV exposure times ranging from 0 to 20 min. (b–e) POM images of LH CLC layers with UV exposure times of (b) 0 min, (c) 10 min, (d) 15 min, and (e) 20 min. (f) Transmittance spectra of RH CLC layers under UV exposure times ranging from 0 to 20 min. (g–j) POM images of RH CLC layers with UV exposure times of (g) 0 min, (h) 10 min, (i) 15 min, and (j) 20 min. Scale bar, 500 µm.
Figure 5. (a) Transmittance spectra of LH CLC layers under UV exposure times ranging from 0 to 20 min. (b–e) POM images of LH CLC layers with UV exposure times of (b) 0 min, (c) 10 min, (d) 15 min, and (e) 20 min. (f) Transmittance spectra of RH CLC layers under UV exposure times ranging from 0 to 20 min. (g–j) POM images of RH CLC layers with UV exposure times of (g) 0 min, (h) 10 min, (i) 15 min, and (j) 20 min. Scale bar, 500 µm.
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Figure 6. (a) Transmittance spectra of the stacked CLC layer before and after applying thermal stress at 40 °C and 100 °C for 10 min. (b) Transmittance spectra of the stacked CLC layer after NIR exposure for 0 to 48 h. (c) Transmittance spectra of the samples in (b) after being stored under ambient conditions for 500 h, following NIR exposure durations of 4, 24, and 48 h. (d,e) Infrared thermography images showing NIR reflection of (c) bare glass and (d) the stacked CLC layer (d). (f,g) Infrared thermography images showing NIR transmission through (f) bare glass and (g) the stacked CLC layer. In (d–g), a broadband NIR light source (550–2000 nm) with an intensity of 30 mW/cm2 at 980 nm was used. Camera exposure time was automatically set between 8 and 12 ms. White dashed boxes indicate the sample positions. Scale bar, 2 cm.
Figure 6. (a) Transmittance spectra of the stacked CLC layer before and after applying thermal stress at 40 °C and 100 °C for 10 min. (b) Transmittance spectra of the stacked CLC layer after NIR exposure for 0 to 48 h. (c) Transmittance spectra of the samples in (b) after being stored under ambient conditions for 500 h, following NIR exposure durations of 4, 24, and 48 h. (d,e) Infrared thermography images showing NIR reflection of (c) bare glass and (d) the stacked CLC layer (d). (f,g) Infrared thermography images showing NIR transmission through (f) bare glass and (g) the stacked CLC layer. In (d–g), a broadband NIR light source (550–2000 nm) with an intensity of 30 mW/cm2 at 980 nm was used. Camera exposure time was automatically set between 8 and 12 ms. White dashed boxes indicate the sample positions. Scale bar, 2 cm.
Crystals 15 00597 g006
Table 1. Composition ratios of materials for CLC mixture preparation.
Table 1. Composition ratios of materials for CLC mixture preparation.
SampleE7RM257S811LC756UV-327DMPA
A173.51510 10.5
A268.52010 10.5
A363.52510 10.5
A458.53010 10.5
B181.515 210.5
B276.520 210.5
B371.525 210.5
B466.530 210.5
C159.53010 00.5
C258.53010 10.5
C357.53010 20.5
C456.53010 30.5
D167.530 200.5
D266.530 210.5
D365.530 220.5
D464.530 230.5
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Hwang, H.S.; Lee, J.; Kang, B.; Kim, M.; Kim, D.; Kim, S.-U. Broadband Near-Infrared Reflective Film from Stacked Opposite-Handed Chiral Liquid Crystals with Pitch Gradients. Crystals 2025, 15, 597. https://doi.org/10.3390/cryst15070597

AMA Style

Hwang HS, Lee J, Kang B, Kim M, Kim D, Kim S-U. Broadband Near-Infrared Reflective Film from Stacked Opposite-Handed Chiral Liquid Crystals with Pitch Gradients. Crystals. 2025; 15(7):597. https://doi.org/10.3390/cryst15070597

Chicago/Turabian Style

Hwang, Hyeon Seong, Jongsu Lee, Byungsoo Kang, Minhye Kim, Doyo Kim, and Se-Um Kim. 2025. "Broadband Near-Infrared Reflective Film from Stacked Opposite-Handed Chiral Liquid Crystals with Pitch Gradients" Crystals 15, no. 7: 597. https://doi.org/10.3390/cryst15070597

APA Style

Hwang, H. S., Lee, J., Kang, B., Kim, M., Kim, D., & Kim, S.-U. (2025). Broadband Near-Infrared Reflective Film from Stacked Opposite-Handed Chiral Liquid Crystals with Pitch Gradients. Crystals, 15(7), 597. https://doi.org/10.3390/cryst15070597

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