Dual-Functional Halide-Based Double Perovskite for Memristive and Temperature-Responsive Optical Encryption

Dual-Functional Halide-Based Double Perovskite for Memristive and Temperature-Responsive Optical Encryption | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 11 February 2026 V1 Latest version Share on Dual-Functional Halide-Based Double Perovskite for Memristive and Temperature-Responsive Optical Encryption Authors : Myungkwan Song 0000-0001-7935-5303 [email protected] , Jae Ho Kim , Soo-Won Choi , Youngjin Kim , Myoi Kim , Jihye An , Jung-Dae Kwon , Yonghun Kim , Jin-Woo Oh , and Hyung Woo Lee Authors Info & Affiliations https://doi.org/10.22541/au.177077537.75873607/v1 150 views 60 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Lead-free double perovskite (Cs2CuSbX6, X = Br, I) was systematically investigated as a multifunctional stimulus-responsive material for electronic memory and physical encryption applications. Structural and spectroscopic analyses confirmed the presence of phase-pure Cs2CuSbX6 with halide-dependent electronic structures. Memristor devices using Cs2CuSbBr6 and Cs2CuSbI6 exhibited reliable bipolar resistive switching, governed by electrochemical metallization, with distinct switching characteristics depending on halide composition. While Cs2CuSbI6 enabled low-voltage and analog conductance modulation, Cs2CuSbBr6 demonstrated superior switching uniformity and energy-efficient synaptic behavior. Beyond electrically driven operations, Cs2CuSbBr6 precursor inks enabled reversible, temperature-gated information encryption. Written patterns were concealed on paper substrates at low temperatures and revealed upon heating. X-ray diffraction analyses confirmed that visibility changes originated from reversible physical modulation rather than irreversible chemical transformation. These results highlight Cs2CuSbBr6 as a versatile, stimulus-responsive material combining memristive electronics and physical security platforms. Dual-Functional Halide-Based Double Perovskite for Memristive and Temperature-Responsive Optical Encryption Jae Ho Kim, Soo-Won Choi, Youngjin Kim, Myoi Kim, Jihye An, Jung-Dae Kwon, Yonghun Kim, Jin-Woo Oh*, Hyung Woo Lee*, Myungkwan Song* ((Optional Dedication)) Dr. J. H. Kim Global Institute for Advanced Nanoscience & Technology (GIANT), Changwon National University, 20, Changwondaehak-ro, Uichang-gu, Changwon-si, Gyeongsangnam-do, 51140, Republic of Korea Dr. S-W. Choi, Y. Kim, M. Kim, J. An, J.-D. Kwon, Y. Kim, Dr. M. Song Department of Materials Science and Engineering & Department of Energy Engineering Convergence, Kumoh National Institute of Technology, 61, Daehak-ro, Gumi-si, Gyeongsangbuk-do, 39177, Republic of Korea E-mail: [email protected] J-W. Oh, H. W. Lee Department of Nano Fusion Technology and Research Center of Energy Convergence Technology, Pusan National University, Busan 46241, Republic of Korea Humanoid Olfactory Display Innovation Research Center, Pusan National University, Busan 46241, Republic of Korea H. W. Lee Research Institute for Convergence of Biomedical Science and Technology, Pusan National University Yangsan Hospital, Yangsan 50612, Republic of Korea Dr. M. Song Humanoid Olfactory Display Innovation Research Center, Pusan National University, Busan 46241, Republic of Korea Keywords: halide double perovskites, information security, thermochromic encryption and decryption, reversible responses, memristors Abstract Lead-free double perovskite (Cs 2 CuSbX 6 , X = Br, I) was systematically investigated as a multifunctional stimulus-responsive material for electronic memory and physical encryption applications. Structural and spectroscopic analyses confirmed the presence of phase-pure Cs 2 CuSbX 6 with halide-dependent electronic structures. Memristor devices using Cs 2 CuSbBr 6 and Cs 2 CuSbI 6 exhibited reliable bipolar resistive switching, governed by electrochemical metallization, with distinct switching characteristics depending on halide composition. While Cs 2 CuSbI 6 enabled low-voltage and analog conductance modulation, Cs 2 CuSbBr 6 demonstrated superior switching uniformity and energy-efficient synaptic behavior. Beyond electrically driven operations, Cs 2 CuSbBr 6 precursor inks enabled reversible, temperature-gated information encryption. Written patterns were concealed on paper substrates at low temperatures and revealed upon heating. X-ray diffraction analyses confirmed that visibility changes originated from reversible physical modulation rather than irreversible chemical transformation. These results highlight Cs 2 CuSbBr 6 as a versatile, stimulus-responsive material combining memristive electronics and physical security platforms. In an era defined by the explosive expansion of information technologies, ensuring the integrity and confidentiality of sensitive data has become an urgent global imperative. Conventional encryption strategies based on radio wave transmission or static fluorescent patterns suffer from intrinsic vulnerabilities, including interception risks, irreversible information exposure, and susceptibility to environmental perturbations. These limitations have spurred the rapid emergence of the use of stimuli-responsive functional materials in next-generation encryption platforms capable of encoding, concealing, and revealing information through external stimuli such as temperature, light, or chemicals. [1-3] Metal halide perovskites have attracted significant attention owing to their properties such as outstanding photoluminescence, tunable optical bandgaps, fast response dynamics, and ease of chemical modification. These characteristics make perovskite-based systems ideal candidates for multilevel, dynamic, and high-security information encryption. [4,5] Recent studies have demonstrated that perovskite materials can support orthogonal stimulus control, allowing information to be encrypted in a concealed, non-luminescent state and decrypted only after the activation of precise external triggers. [6,7] Strategies leveraging thermochromic microcapsules or anion-exchange reactions have already opened pathways toward dynamic multilevel encryption, significantly increasing the difficulty of unauthorized decryption. [8] For example, invisible ink systems based on halide exchange allow information to be concealed under ambient conditions and revealed only through a tightly controlled combination of light, chemical developers, and time gating, thereby creating multidimensional encryption frameworks. [7,9] However, most reported systems still rely heavily on lead-based perovskites and are often constrained to single-direction stimulus response or narrow thermal windows, which fundamentally limit their long-term stability, environmental safety, and practical deployment. [10–12] To address these limitations, lead-free double perovskites, particularly Cu-based halides, have emerged as a promising alternative. [13,14] These materials combine non-toxic composition, outstanding optical tunability, and robust thermal reversibility, providing an environmentally benign platform for high-level security applications. [15] In this study, a thermally reversible, stimulus-responsive encryption system based on multifunctional Cs 2 CuSbBr 6 double perovskite is proposed and demonstrated. Results show that encrypted text remains invisible at subzero temperatures (−10 °C) but becomes visible when subjected to elevated temperatures (120 °C). In addition, the material exhibits memristive switching behavior, enabling the co-integration of information encryption and electronic memory functions within a single platform. This dual-mode functionality provides a time-independent and thermally switchable mechanism for secure information control, while the memristive response allows for additional encryption keys at the electronic level to further strengthen the security architecture. By integrating thermochromic responsiveness, memristor-like characteristics, and the inherent stability and lead-free nature of Cs 2 CuSbBr 6 , the system provides a robust, reversible, and environmentally friendly encryption–decryption cycle. This work establishes a new paradigm for multifunctional, thermally triggered perovskite-based information security, paving the way for practical applications in anti-counterfeiting, confidential data storage, and smart wearable encryption technologies. The atomic structures of the double perovskites Cs₂CuSbBr 6 and Cs₂CuSbI 6 are shown in Figure 1 (a). Density functional theory (DFT) calculations predict that Cs₂CuSbBr 6 has a smaller unit-cell volume (334 Å 3 per formula unit) than Cs₂CuSbI 6 (410 Å 3 per formula unit), which is attributed to the larger ionic radius of I − compared to Br − . As shown in Figures 1 (b) and (c), simulations indicate that both compounds are semiconductors, with valence bands primarily originating from the hybridization of Cu 3d, Sb 5s, and halide p orbitals, while the conduction bands are primarily composed of Cu 4s, Sb 5p, and halide p states. Both materials exhibit indirect band gaps of 1.27 eV for Cs₂CuSbBr 6 and 0.70 eV for Cs₂CuSbI 6 . Direct band gaps, corresponding to the onset of strong optical absorption, are larger than the indirect gaps by 1.44 eV and 1.27 eV for Cs₂CuSbBr 6 and Cs₂CuSbI 6 , respectively. The UV–visible (UV-Vis) absorbance spectra of Cs 2 CuSbBr 6 and Cs 2 CuSbI 6 reveal a clear halide-dependent evolution of the optical response, as shown in Figure 1 (d). Cs 2 CuSbI 6 exhibits a distinct red-shift of the absorption edge compared to Cs 2 CuSbBr 6 , reflecting a reduced optical bandgap induced by Br − to I − substitution. The measured absorption onsets of the compounds, approximately 600 and 500 nm for Cs₂CuSbI 6 and Cs₂CuSbBr 6 , respectively, are in reasonable agreement with the calculated direct band gaps. This trend is typical of lead-free halide double perovskites, where the valence-band maximum is primarily governed by halogen p-orbitals. Replacement with the more polarizable and less electronegative iodide anion elevates the valence-band energy, narrowing the bandgap and extending light absorption towards longer wavelengths. [16] In contrast, Cs 2 CuSbBr 6 shows stronger absorption in the near-UV and blue spectral regions, which can be attributed to differences in the electronic density of states and transition probability at higher photon energies. Notably, both compositions display a gradual absorption onset rather than a sharp band edge, suggesting the presence of band-tail states associated with lattice disorder, electron–phonon coupling, and defect-related sub-gap states. [17–19] Such spectral characteristics are widely reported in lead-free double perovskite systems and have been recognized as key factors governing charge-carrier generation, recombination dynamics, and the overall optoelectronic performance of environmentally benign perovskite materials. [19] Figure 1 (e) and (f) present the X-ray diffraction (XRD) patterns of Cs 2 CuSbBr 6 and Cs 2 CuSbI 6 , respectively. These confirm the successful formation of phase-pure Cu-based lead-free double perovskite structures. All diffraction peaks can be indexed to the cubic double perovskite lattice (space group Fm3̅m), with the prominent reflections corresponding to the (220), (222), (400), (422), (440), and (622) planes, in agreement with previously reported Cs 2 CuSbX 6 phases. [20,21] Notably, Cs 2 CuSbI 6 exhibits a systematic shift of diffraction peaks toward lower 2θ angles compared to Cs 2 CuSbBr 6 . This may be attributed to lattice expansion induced by the substitution of larger iodide anions (I⁻) for bromide (Br⁻), as previously predicted by the simulations. [17,22] This halide-dependent lattice dilation reflects the increased ionic radius and polarizability of iodide, leading to an enlarged unit cell without altering the overall crystallographic symmetry. The well-defined diffraction peaks observed for both compositions indicate high crystallinity and low structural disorder, while the absence of secondary impurity phases suggest excellent phase stability upon halide substitution. [19,23] The preservation of the double perovskite framework across both halide compositions underscores the structural robustness of Cs 2 CuSbX 6 , which is essential for achieving reliable thermal reversibility and stable optoelectronic functionality in stimulus-responsive encryption and memory applications. Figure 1 (g) shows the wide-scan X-ray photoelectron spectroscopy (XPS) spectra of Cs 2 CuSbBr 6 and Cs 2 CuSbI 6 , where the presence of characteristic core levels (Cu 2p, Cs 3d, Sb 3d, and halogen Br 3d or I 3d) verifies the expected elemental composition of the Cu–Sb lead-free double-perovskite phases. [24] The clear metal and halogen core-level features indicate chemically well-defined lattices and enable assessment of chemical states and surface stoichiometry, which are essential for correlating thermally driven optical/electronic switching with interfacial/surface chemistry. [25] More broadly, XPS is a well-established tool to monitor halide perovskite bonding environments and stress-induced chemical evolution (e.g., via halogen core levels and valence-band spectra), supporting its suitability here to validate chemical integrity of Cs 2 CuSbX 6 prior to band-structure analysis and device integration. Following XPS analysis, ultraviolet photoelectron spectroscopy (UPS) was used to quantitatively determine the electronic energy levels of two independently prepared compositions, Cs 2 CuSbBr 6 and Cs 2 CuSbI 6 . Figures S2 and S3 show the UPS spectra of Cs 2 CuSbBr 6 and Cs 2 CuSbI 6 , respectively, from which the secondary electron cutoff and the valence-band onset were extracted. The secondary cutoff positions at 19.24 eV (Cs 2 CuSbBr 6 ) and 19.45 eV (Cs 2 CuSbI 6 ) correspond to work functions of 1.98 and 1.77 eV, respectively, indicating composition-dependent variations in surface energetics that are commonly associated with differences in halogen-derived electronic states and surface dipoles in metal-halide perovskites, as shown in Table S1. [26] The valence-band maxima (VBM), obtained by linear extrapolation of the leading edge of the UPS valence spectra, are located at 5.75 and 3.98 eV below the vacuum level for Cs 2 CuSbBr 6 and Cs 2 CuSbI 6, respectively. The higher VBM in the iodide-based film is consistent with the stronger contribution of I 5p orbitals to the valence band relative to Br 4p orbitals in halide perovskite, resulting in systematic variations in valence-band positions across different halide compositions. [22] The UPS-derived VBM positions and work functions with optical bandgaps obtained from UV–Vis absorption were combined to construct vacuum-referenced band-edge diagrams for each composition, as shown in Figure 1(h). The results show that Cs 2 CuSbI 6 exhibits a narrower bandgap and shallower band-edge positions than Cs 2 CuSbBr 6 , while both retain a consistent double-perovskite electronic framework. This composition-dependent band alignment provides a direct basis for interpreting the distinct optical response and device-relevant behavior of Cs 2 CuSbBr 6 and Cs 2 CuSbI 6 in stimulus-responsive encryption and memory applications. [27] A Cs 2 CuSbBr 6 - and Cs 2 CuSbI 6 -based memristor device was fabricated with a vertical sandwich architecture of ITO/Cs 2 CuSbX 6 /PMMA/Ag, as shown in Figure 2 (a). Indium tin oxide (ITO) was employed as the bottom electrode, while Ag served as the active top electrode. The Cu–Sb-based double perovskite layer acted as the resistive switching medium, with a thin PMMA interlayer was introduced to regulate Ag ion migration and stabilize conductive filament formation. This device architecture enabled reliable bipolar resistive switching through the reversible formation and rupture of Ag filaments under an external electric field. As shown in Figure 2(b), the resistive switching mechanism of the Cs 2 CuSbX 6 -based memristor is governed by the electrochemical migration of Ag ions and the formation of metallic conductive filaments. Under a positive bias applied to the Ag top electrode, Ag atoms are oxidized into Ag⁺ ions, which drift through the PMMA and Cu–Sb-based double perovskite layers towards the bottom electrode. The subsequent reduction and accumulation of Ag⁺ ions leads to the growth of a conductive filament bridging the two electrodes, resulting in a transition to a low-resistance state (LRS). When the bias is reversed, Joule heating and electrochemical dissolution annihilate the filament near the weakest region, returning the device to a high-resistance state (HRS). Figure 2(c) shows the bipolar resistive switching ( I–V ) characteristics of the ITO/Cs 2 CuSbBr 6 /PMMA/Ag structure, which is typical of memristor devices. During the positive sweep (region ①), Ag undergoes electrochemical oxidation at the active electrode, and the generated Ag⁺ ions drift through the dielectric/semiconducting stack, resulting in the progressive growth of a metallic filament and the SET (HRS to LRS) transition. After switching (region ②), the LRS is retained as the conductive pathway remains electrically connected. Upon polarity reversal (region ③), the filament ruptures close to the weakest constriction via electrochemical dissolution, assisted by localized Joule heating, returning to the RESET (LRS to HRS) transition. The subsequent sweep back to 0 V (region ④) preserves the OFF-state current governed by the insulating matrix, confirming reversible filament dynamics as the dominant switching origin. [28] Figure 2(d) summarizes the device-to-device distributions of the HRS and LRS for ten independent Cs₂CuSbBr₆-based memristor devices, recorded at a read voltage of 0.21 V. The two resistance states remained well separated with relatively narrow dispersion, indicating robust switching uniformity across cells. Such uniformity is consistent with an electrochemical metallization (ECM) process, wherein the PMMA interlayer regulates Ag oxidation/injection and subsequent ion transport, thereby reducing stochastic variations in filament nucleation and constriction geometry that typically broaden HRS/LRS distributions. [29] Figure 2(e) shows the analog conductance modulation of the memristor under repeated potentiation (0.6 V) and depression (−0.6 V) pulse trains. The conductance gradually increased and decreased during potentiation and depression, respectively, and the modulation trend remained reproducible even after extended cycling periods (up to 10³ cycles), indicating stable weight-update behavior suitable for synaptic operation. This continuous tuning is consistent with ECM dynamics, where partial growth/merging of Ag nanoclusters and filaments under positive pulses increases the effective conductive cross-section, while reverse pulses induce partial dissolution and neck thinning, enabling incremental conductance updates rather than abrupt binary switching. [30] Figure 2(f) displays the bipolar resistive switching ( I–V ) characteristics of the ITO/Cs 2 CuSbI 6 /PMMA/Ag memristor device. The Cs 2 CuSbI 6 -based memristor operates at lower voltages and currents, reflecting facilitated Ag ion migration and softer filament dynamics, which can be beneficial for low-power or analog switching applications. [31] However, when directly compared with the Cs 2 CuSbBr 6 -based device in Figure 2(c), the Cs 2 CuSbBr 6 memristor exhibits a wider switching window, higher operational current, and more abrupt SET and RESET transitions, indicating the formation of a thicker and more stable conductive filament. These characteristics are advantageous for reliable nonvolatile memory operation, where clear binary switching and robust resistance contrast are required. This distinction suggests that, although both devices operate via an ECM mechanism, the Br-based system promotes stronger filament confinement and improved LRS retention, making it more suitable for nonvolatile memory operation than flow analog modulation. Figure 2(g) summarizes the device-to-device distributions of the HRS and LRS for ten independent Cs 2 CuSbI 6 -based devices measured at a read voltage of 0.21 V. The Cs 2 CuSbI 6 -based devices maintained a clear separation between HRS and LRS, confirming stable filamentary switching governed by an ECM mechanism. In addition, the higher HRS levels are indicative of effective filament annihilation and sufficiently restored insulating gaps after RESET, which support the suppression of leakage currents. When compared with the Cs 2 CuSbBr 6 -based devices presented in Figure 2(d), the Cs 2 CuSbI 6 -based devices exhibit broader resistance distributions, particularly in the HRS. This increased variability suggests the occurence of more stochastic filament formation and annihilation processes, likely associated with enhanced Ag ion mobility in the iodide-based matrix. While such behavior may be advantageous for low-power or analog switching, it is less desirable for reliable binary nonvolatile memory operation, where tight resistance distributions and high uniformity are essential. [32] Figure 2(h) shows the conductance modulation behavior of the Cs 2 CuSbI 6 -based memristor under repeated potentiation and depression pulse cycles. In contrast to the Cs 2 CuSbBr 6 -based device, the Cs 2 CuSbI 6 -based device required higher pulse amplitudes of +0.8 V and −0.8 V to induce comparable conductance changes. The conductance gradually increased during potentiation and decreased during depression, with the modulation remaining reproducible over extended cycling, confirming stable analog switching behavior in the Cs 2 CuSbI 6 device. Additional single-cycle conductance evolution under identical pulse conditions is presented in Figure S4. Both devices exhibited monotonic conductance increases and decreases during potentiation and depression, respectively, within a single cycle, confirming well-defined synaptic responses without abrupt switching. The smoother conductance evolution observed in the Cs 2 CuSbI 6 device is consistent with softer filament modulation, whereas the Cs 2 CuSbBr 6 device shows a wider conductance window suitable for memory-oriented operation. The requirement for higher pulse voltages in the Cs 2 CuSbI 6 -based memristor suggests a different balance between filament stabilization and dissolution kinetics compared to the Cs 2 CuSbBr 6 counterpart, consistent with ECM-type filamentary switching. [33, 34] While the iodide-based device enables gradual and reversible filament modulation, weaker confinement of Ag conductive paths necessitates larger electric fields to achieve effective potentiation and depression. This voltage-dependent behavior is further supported by the pulse-amplitude-dependent conductance modulation shown in Figure S5(a–d), where increasing pulse voltages progressively enhance the conductance dynamic range but also introduce larger cycle-to-cycle variations, reflecting stronger yet less confined filament evolution. Although such characteristics can be beneficial for analog weight updates, the higher operating voltages and reduced conductance contrast render the Cs 2 CuSbI 6 device less favorable than Cs 2 CuSbBr 6 for low-voltage and energy-efficient synaptic operation. Notably, while the Cs 2 CuSbI 6 device exhibits stable analog conductance modulation under ±0.8 V pulses, the Cs 2 CuSbBr 6 device achieves comparable synaptic behavior at lower voltages (±0.6 V), highlighting its advantage in energy-efficient operation. Figure 3 (a) schematically illustrates a temperature-responsive encryption process realized by directly applying a Cs 2 CuSbBr 6 precursor ink onto paper substrates using a brush. After deposition, the ink remained nearly invisible at low temperatures (−18 °C), whereas thermal activation at elevated temperature (120 °C) significantly improved the visibility of the pattern. Importantly, this change was reversible following repeated cooling and heating cycles, indicating that the process is governed by stimulus-dependent physical modulation rather than irreversible chemical transformation. The choice of Cs 2 CuSbBr 6 rather than Cs 2 CuSbI 6 is motivated by its balanced stimulus sensitivity and structural robustness. As demonstrated in the memristive devices previously discussed, Cs 2 CuSbBr 6 exhibits more confined ionic and electronic responses under external stimuli, leading to stable and reproducible state changes. In contrast, iodide-based counterparts typically exhibit softer lattice characteristics and higher ionic mobility, which, although advantageous for low-voltage or analog modulation, may lead to less controlled and less stable state retention. For temperature-responsive encryption on porous paper substrates, Cs 2 CuSbBr 6 provides a favorable combination of thermal activation sensitivity and reversibility without excessive diffusion or permanent contrast degradation. Representative photographs in Figure 3(b) demonstrate the practical encryption capability of this approach, where written “KIMS” characters remained concealed at lower temperatures and became legible upon heating. In this context, the information was physically present but remained latent unless the appropriate thermal stimulus was applied, thereby functioning as a temperature-gated decryption key. Such behavior enables a simple yet robust physical encryption scheme based on reversible information hiding and revealing. The robustness and tunability of this temperature-responsive behavior are further examined in Figure 3(c), where patterned arrays prepared with different precursor concentrations and repeated writing cycles are compared. The results show that higher precursor concentrations lead to more pronounced optical contrast upon heating, while the concealment is maintained across multiple cycles. This concentration- and cycle-dependent response confirms that the encryption performance can be systematically controlled without sacrificing reversibility, highlighting the suitability of Cs 2 CuSbBr 6 for stimulus-responsive information encoding on flexible and porous substrates. Figure 3(d) presents a real-time demonstration of the reversible temperature-responsive encryption behavior of Cs 2 CuSbBr 6 patterns written on paper. When heated to 120 °C, the written characters became clearly visible owing to enhanced optical contrast. Subsequent cooling to −18 °C rapidly suppressed the contrast, making the information nearly invisible within 7 s. When the sample was reheated to 120 °C, the concealed information reappeared with comparable clarity, confirming the reversibility and robustness of the thermal response. The dynamic encryption and decryption process under repeated heating and cooling cycles is further illustrated in Supporting Video S1. This cyclic visibility transition indicates that the encryption and decryption processes are governed by reversible physical modulation rather than irreversible chemical change. The rapid response time and repeatable recovery further emphasize the practicality of Cs 2 CuSbBr 6 -based inks for temperature-gated information hiding and retrieval in simple, paper-based platforms. XRD was conducted to determine whether the temperature-dependent visibility change of Cs 2 CuSbBr 6 -written paper originates from a reversible physical process or an irreversible chemical or structural transformation, as shown in Figure 3(e). By comparing diffraction patterns collected at −18 °C and 120 °C, the measurements aimed to identify phase evolution (e.g., oxidation products) and assess whether the underlying crystalline signatures remained preserved during thermal cycling. Although Sb 2 O 3 is often discussed in the context of wide-bandgap optical materials, its optical appearance strongly depends on morphology, thickness, and scattering conditions rather than on bandgap alone. [35] In porous paper-based patterns, perceived contrast is frequently dominated by light scattering from micro- or meso-scale heterogeneities, where pores and coating structure act as primary scattering centers. [36] Therefore, the presence of an Sb 2 O 3 -related diffraction feature at 120 °C does not necessarily imply that the written trace becomes optically “invisible,” as scattering-driven contrast can remain significant even in materials with weak visible absorption. [37] XRD peak intensities can vary significantly with preferred orientation (texture), crystallite size, microstrain, and sample packing or densification, even at similar phase fractions. Accordingly, an Sb 2 O 3 peak appearing stronger at 120 °C may reflect improved crystallinity and/or texture changes upon heating, rather than a proportional increase in Sb 2 O 3 content or a direct optical contrast mechanism. [38] Because the encryption–decryption behavior is reversible upon repeated cooling/heating cycles, it is unlikely that an irreversible chemical conversion (e.g., bulk oxidation) is the origin of the visibility transition. A weak Sb 2 O 3 -related feature observed at 120 °C is more consistently interpreted as minor surface oxidation or a secondary process during heating, while the principal visibility modulation is governed by reversible physical changes on the porous substrate. [36] The temperature-gated visibility is attributed primarily to thermally driven drying and microstructural rearrangement of the Cs 2 CuSbBr 6 -derived deposits on the porous paper substrate. [39] At elevated temperatures (approximately 120 °C), rapid solvent and moisture removal, together with enhanced crystallization/densification, increases the refractive-index contrast and scattering efficiency within the written trace, resulting in pronounced optical contrast (decryption). Upon cooling (−18 °C), moisture uptake/condensation, together with associated microstructural changes (e.g., partial pore filling or refractive-index matching) can suppress scattering contrast, thereby rendering the trace faint or nearly invisible (encryption), while remaining reversible over repeated thermal cycling. In summary, Cs 2 CuSbX 6 -based materials were demonstrated to be versatile platforms capable of encoding information through both electrical and thermal stimuli. Comparative memristor investigations revealed that Cs 2 CuSbBr 6 provides more stable and energy-efficient resistive switching than Cs 2 CuSbI 6 , making it better suited for reliable memory and synaptic applications. Cs 2 CuSbBr 6 precursor inks were also shown to enable reversible, temperature-responsive physical encryption on paper substrates, displaying potential application beyond use in conventional device architecture. The thermally gated visibility was shown to arise from reversible physical processes associated with drying, microstructural rearrangement, and light scattering, rather than irreversible chemical reactions. By unifying electrically driven memristive behavior with thermally activated optical encryption within a single material system, this study demonstrates a new direction for multifunctional, stimulus-responsive materials that integrate electronic memory and physical information security. Figure 1. Optoelectronic and chemical properties of Cs 2 CuSbX 6 (X = Br, I). (a) Crystal structure of cubic double-perovskite Cs 2 CuSbX 6 , including the atomic arrangement of Cs, Cu, Sb, and halide (Br or I) ions in the Fm3̅m space group. (b,c) DFT-calculated electronic band structures of (b) Cs 2 CuSbBr 6 and (c) Cs 2 CuSbI 6 , revealing indirect bandgap characteristics. (d) UV–Vis absorbance spectra. XRD patterns of Cs 2 CuSbX 6 with (e) Br and (f) I, confirming phase-pure cubic double-perovskite formation indexed to the Fm3̅m space group. (g) XPS spectra verifying elemental composition and chemical states. (h) Energy band alignment diagrams derived from XPS valence-band spectra and optical bandgap measurements. Figure 2. Cs 2 CuSbX 6 -based memristor devices and resistive switching characteristics. (a) Schematic illustration of the ITO/Cs 2 CuSbX 6 (X = Br or I)/PMMA/Ag memristor device structure. (b) Schematic of Ag filament formation and annihilation under an external electric field. (c–e) Electrical characteristics of Cs 2 CuSbBr 6 -based devices: (c) typical bipolar resistive switching (I–V) curves, (d) statistical distributions of HRS and LRS measured at 0.21 V for 10 devices, and (e) conductance modulation under repeated potentiation (red, +0.6 V) and depression (blue, −0.6 V) pulses. (f–h) Corresponding electrical characteristics of Cs 2 CuSbI 6 -based devices: (f) bipolar resistive switching (I–V) curves, (g) HRS and LRS distributions measured at 0.21 V for 10 devices, and (h) conductance modulation under repeated potentiation (red, +0.8 V) and depression (blue, −0.8 V) pulses. Figure 3. Temperature-responsive physical encryption behavior of Cs 2 CuSbBr 6 on paper. (a) Schematic illustration of reversible information encryption and decryption achieved by applying a Cs 2 CuSbBr 6 precursor ink to paper. (b) Photographs showing a representative written pattern (“KIMS”). (c) Dependence of optical contrast on precursor concentration and the number of repetition cycles. (d) Real-time photographs demonstrating the reversible encryption–decryption process (see Supporting Video S2). (e) XRD patterns of the Cs 2 CuSbBr 6 -applied paper. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements J. H. Kim, S-W. Choi, and Y. Kim contributed equally to this study. This research was supported by the Fundamental Research Program (PNKA460) of the Korea Institute of Materials Science (KIMS); a National Research Council of Science & Technology (NST) grant from the Korean government (MSIT) (No. GTL24041-000); and a National Research Foundation of Korea grant funded by the Korean government (MSIT) (RS-2024-00406152). Received: (will be filled in by the editorial staff) Revised: (will be filled in by the editorial staff) Published online: (will be filled in by the editorial staff) References [1] L. 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DOI: 10.1016/j.scriptamat.2004.05.007. [39] R. Antonelli, T. E. Kodger, Coatings 2023, 13, 1873. DOI: 10.3390/coatings13111873. The lead-free double perovskite Cs 2 CuSbBr 6 serves as a versatile platform for dual device functionalities. Band-structure-controlled resistive switching enables reliable memory behavior, while thermally reversible writing allows visible information encryption. The coexistence of electrical and thermal responses within a single material highlights Cs 2 CuSbBr 6 as a multifunctional candidate for secure and neuromorphic hardware systems. Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018. Supporting Information Dual-Functional Halide-Based Double Perovskite for Memristive and Temperature-Responsive Optical Encryption Jae Ho Kim, Soo-Won Choi, Youngjin Kim, Myoi Kim, Jihye An, Jung-Dae Kwon, Yonghun Kim, Jin-Woo Oh*, Hyung Woo Lee*, Myungkwan Song* Dr. J. H. Kim Global Institute for Advanced Nanoscience & Technology (GIANT), Changwon National University, 20, Changwondaehak-ro, Uichang-gu, Changwon-si, Gyeongsangnam-do, 51140, Republic of Korea Dr. S-W. Choi, Y. Kim, M. Kim, J. An, J.-D. Kwon, Y. Kim, Dr. M. Song Department of Materials Science and Engineering & Department of Energy Engineering Convergence, Kumoh National Institute of Technology, 61, Daehak-ro, Gumi-si, Gyeongsangbuk-do, 39177, Republic of Korea E-mail: [email protected] J-W. Oh, H. W. Lee Department of Nano Fusion Technology and Research Center of Energy Convergence Technology, Pusan National University, Busan 46241, Republic of Korea Humanoid Olfactory Display Innovation Research Center, Pusan National University, Busan 46241, Republic of Korea H. W. Lee Research Institute for Convergence of Biomedical Science and Technology, Pusan National University Yangsan Hospital, Yangsan 50612, Republic of Korea Dr. M. Song Humanoid Olfactory Display Innovation Research Center, Pusan National University, Busan 46241, Republic of Korea Keywords: halide double perovskites, information security, thermochromic encryption and decryption, reversible responses, memristors 1-1. Materials Cesium iodide (CsI, 99.9 %) was purchased from Alfa Aesar. Cesium bromide (CsBr, 99.999 %) was purchased from Sigma-Aldrich. Copper iodide (CuI, 98%) was purchased from Sigma-Aldrich. Copper bromide (CsBr, 99.999 %) was purchased from Sigma-Aldrich. Antimony iodide (SbI, 98 %) was purchased from Sigma-Aldrich. Antimony(III) bromide (SbBr 3 , 99%) was purchased from Alfa-Aesar (Haverhill, MA, USA). Poly(methyl methacrylate) (PMMA), and dimethyl sulfoxide (DMSO, 99.9 %) were purchased from Sigma-Aldrich. Methanol was purchased from Chemitop. All the chemicals were used without further purification. 1-2. Preparation of Cs 2 CuSbBr 6 precursor solution The Cs 2 CuSbBr 6 precursor solution was prepared by dissolving CsBr, CuBr, and SbBr 3 in DMSO with a molar ratio of CsBr:CuBr:SbBr 3 = 2:1:1. The total precursor concentration was adjusted to 0.5 M, corresponding to the stoichiometry of Cs 2 CuSbBr 6 . The mixed precursor solution was stirred at 60 °C for overnight until all solid components were completely dissolved. yielding a homogeneous dark brown colored solution. Prior to film deposition, the precursor solution was filtered through a 0.45 µm PTFE syringe filter to remove any undissolved residues or particulates. The filtered solution was then directly used for the deposition of Cs 2 CuSbBr 6 thin films. The Cs 2 CuSbI 6 precursor solution was prepared following the same procedure, using CsI, CuI, and SbI 3 as iodine-based precursors. 1-3. Fabrication of Cs 2 CuSbBr 6 -based memristors Memristive devices were fabricated using a sandwich-type structure of ITO/ Cs 2 CuSbBr 6 /PMMA/Ag, as schematically illustrated in Figure 2(a). ITO glass substrates were sequentially cleaned by ultrasonication in acetone, IPA, and DI water, followed by UV–ozone treatment to improve surface wettability. The Cs 2 CuSbBr 6 active layer was deposited onto the cleaned ITO substrates by spin-coating the Cs 2 CuSbBr 6 precursor solution, prepared as described above. Subsequently, PMMA interfacial layer was spin-coated on top of the Cs 2 CuSbBr 6 film to function as a dielectric buffer and ion-modulating layer. Finally, silver (Ag) top electrodes were deposited through a shadow mask by thermal evaporation, defining the active device area and completing the device architecture. Devices based on Cs 2 CuSbI 6 were fabricated using the same procedure. 1-4. Characterization methods X-ray diffraction (XRD) patterns were obtained using an X-ray diffractometer (Empyrean, PANalytical, Malvern, UK; Cu K α radiation source at λ = 1.54 Å) under a voltage of 40 kV and current of 40 mA in a 2° angle range of 10–60°. The UV-visible absorption spectra were recorded using a UV-vis-NIR spectrophotometer (Cary 5000, AGILENT Technologies, Santa Clara, CA, USA). The infrared spectra of the samples were recorded using Fourier transform infrared (FT-IR; Nicolet iS10, Thermo Scientific, Waltham, MA, USA) spectroscopy. X-ray photoelectron spectroscopy (XPS) was conducted with a modified XPS system (ESCALab MK II, VG Scientific, Conroe, TX, USA) and an Al K α X-ray source (1486.6 eV) at a high voltage of 14 kV and a beam current of 30 mA. 1-5. Density functional theory calculations Density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP) with the projector-augmented wave (PAW) method to describe the core–valence interactions. [S1,S2] A plane-wave basis set with an energy cutoff of 300 eV was employed. Brillouin-zone integrations are carried out using a Γ-centered 5×5×5 k -point mesh. For the exchange–correlation functional, the HSE06 hybrid functional was adopted with the standard fraction of 0.25 for Fock exchange. [S3] The convergence criteria were set to 10 −6 eV for electronic self-consistency and 0.02 eV Å −1 for atomic relaxations. [S1] G. Kresse, J. Furthmüller, Phys. Rev. B 1996 , 54 , 11169. DOI: 10.1103/PhysRevB.54.11169. [S2] G. Kresse, D. Joubert, Phys. Rev. B 1999 , 59 , 1758. DOI: 10.1103/PhysRevB.59.1758. [S3] J. Heyd, G. E. Scuseria, M. Ernzerhof, J. Chem. Phys. 2006 , 124 , 219906. DOI: 10.1063/1.2204597. Figure S1. Schematic illustration of the synthesis of Cs 2 CuSbX 6 (X = Br, I). Figure S2. UPS spectra of Cs 2 CuSbBr 6 thin film. Figure S3. UPS spectra of Cs 2 CuSbI 6 thin film. Figure S4. Single conductance modulation cycles of (a) the Cs 2 CuSbBr 6 -based device under repeated ±0.6 V pulses and (b) the Cs 2 CuSbI 6 -based device under repeated ±0.8 V pulses. Figure S5. Conductance modulation under repeated potentiation and depression cycles, measured at different pulse amplitudes. Table S1. Parameters from UPS measurements of Cs 2 CuSbBr 6 and Cs 2 CuSbI 6 thin films. Cs 2 CuSbBr 6 19.24 1.98 5.75 Cs 2 CuSbI 6 19.45 1.77 3.98 Highlights • Cs 2 CuSbX 6 (X=Br, I) is a dual-functional material for memristive memory and thermal optical encryption. • Cs 2 CuSbBr 6 -based memristors exhibit superior switching stability and energy-efficient operation compared to Cs 2 CuSbI 6 . • Memrisitve behavior originates from confined Ag filament formation governed by an electrochemical metallization mechanism. • Cs 2 CuSbBr 6 inks enable reversible, temperature-gated information hiding and revealing on paper substrates. • The thermally induced visibility change is driven by reversible physical processes rather than irreversible chemical transformation. Information & Authors Information Version history V1 Version 1 11 February 2026 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords energy materials environmental materials materials science thin films Authors Affiliations Myungkwan Song 0000-0001-7935-5303 [email protected] Korea Institute of Materials Science View all articles by this author Jae Ho Kim Korea Institute of Materials Science View all articles by this author Soo-Won Choi Korea Institute of Materials Science View all articles by this author Youngjin Kim Korea Institute of Materials Science View all articles by this author Myoi Kim Korea Institute of Materials Science View all articles by this author Jihye An Korea Institute of Materials Science View all articles by this author Jung-Dae Kwon Korea Institute of Materials Science View all articles by this author Yonghun Kim Korea Institute of Materials Science View all articles by this author Jin-Woo Oh Pusan National University View all articles by this author Hyung Woo Lee Pusan National University View all articles by this author Metrics & Citations Metrics Article Usage 150 views 60 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Myungkwan Song, Jae Ho Kim, Soo-Won Choi, et al. 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