The hierarchical multiscale structures developed in this study combine macro-, micro-, and nanoscale design principles to modulate stress distribution and amplify light-matter interaction. On the macro scale, enabled by the hierarchical structures, millimeter-scale two-dimensional mechanoluminescent patterns can be embedded in an elastic thin-film without changing the mechanoluminescent material composition (Fig. 1a). The mechanism of ML pattern display is a combination of in-film stress concentration and enhanced light extraction. At the microscale, distinct arrays of convex and concave elements are patterned to redistribute mechanical stress within the thin-film (Fig. 1b). The nanoscale circular honeycomb arrays (2 μm in diameter, 320 nm in height, and 230 nm spacing) contribute to improved optical scattering and higher photon escape efficiency (Fig. 1c).
This hierarchical design achieves two key objectives: (1) about a threefold enhancement in luminescence intensity compared to planar films, owing to stress concentration at structural interfaces and improved photon extraction efficiency (Fig. 1d); and (2) the programmability of ML light-emitting patterns, enabling dynamic stress visualization through a responsive three-dimensional (3D) surface relief pattern under mechanical stimuli (Fig. 1e).
To achieve such precision, a three-step fabrication pipeline was developed, combining SU-8 photolithography, nanoimprint lithography (NIL), and soft lithography (Fig. 1f and Supplementary Figs. 5-8). This method ensures reproducibility across scales and offers tunability in key structural parameters, such as height, diameter, and spacing.
The ML film consists of PDMS and ZnS:Cu particles, with a particle diameter of ≈ 17 μm (Supplementary Fig. 12). When the ML film is subjected to compression or tension (Fig. 2a), the relative motion between the ML particles and PDMS induces contact-separation and lateral sliding (Fig. 2b). This motion generates an electric field that excites electrons in the ZnS:Cu particles. The excited electrons transition to a higher energy state and emit light as they return to their ground state (Fig. 2c). The PDMS matrix surrounding the ZnS:Cu particles plays a crucial role in facilitating mechanical deformation and enabling relative motion between particles, which enhances stress localization and further promotes luminescence.
The optimization of ML intensity is closely tied to the mechanical properties of the PDMS-ZnS:Cu composite, specifically its Young's modulus. To systematically evaluate this, the ZnS:Cu content and PDMS-to-curing agent ratio were varied to balance luminescence performance and mechanical durability. Experimental results showed that increasing the ZnS:Cu content enhances luminescence intensity; with an optimal mixing ratio of 6:4 (ZnS:Cu: PDMS), the particle dispersion and film uniformity are maximized (Fig. 2d). Beyond this threshold, excessive viscosity introduces defects, resulting in uneven film thickness and luminescence non-uniformity. Similarly, the PDMS-to-curing agent ratio optimization indicated that, with a ratio of 20:1, relatively high luminescent intensity and strain tolerance can be realized (Fig. 2e). Under these optimized conditions, the ML film achieved a maximum strain of 146.38% while maintaining a relatively high Young's modulus (Fig. 2f).
To further elucidate the interaction between structural design and ML intensity, finite-element analysis was performed to model the stress distribution in columnar and groove geometries (Supplementary Fig. 15). The results reveal that surface topography plays a pivotal role in modulating stress and luminescence. The elevated convex structures concentrate stress at their apex, resulting in localized luminescence enhancement, while recessed groove regions distribute stress along edges, producing lower luminescence intensity (Fig. 2g, h). As a practical demonstration, the letters "SJTU" (abbreviation of Shanghai Jiao Tong University) were patterned onto the ML film using arrays of pillars ("S" and "J") and grooves ("T" and "U") (Supplementary Fig. 16). Stress-induced birefringence mapping clearly indicated brighter regions corresponding to high residual stress areas, which correlated with enhanced luminescence (Fig. 2g, h). Quantitative analysis (Supplementary Fig. 17) confirmed that higher stress regions consistently exhibited greater luminescent intensity, providing direct experimental validation of the stress-luminescence relationship.
These findings demonstrate that macroscale surface structuring -- through convex (pillar) and concave (groove) geometries -- provides an essential framework for manipulating local stress distribution and establishing the basis for subsequent multiscale enhancements. At the macroscale, raised pillar and recessed groove architectures redistribute mechanical stress across the film, guiding and amplifying stress localization at finer scales. Raised pillars are particularly effective at concentrating stress, resulting in regions of enhanced luminescence, while recessed grooves lead to broader stress distribution and thus lower luminescence contrast. This initial macroscale modulation of mechanical stress establishes a foundation for precise control of mechanoluminescent output. By enabling both localized stress concentration and broader mechanical responsiveness, macrostructuring plays a critical role in optimizing overall mechanoluminescent performance.
Figure 3 explores the influence of microstructure parameters -- shape, feature size, and the perimeter-to-area ratio (P/A, µm⁻¹) -- on the ML intensity of mechanoluminescent films. These parameters are critical for controlling the luminescent performance of ML films. Microstructure arrays with feature sizes of 40, 60, 80, and 100 µm (Fig. 3a and Supplementary Figs. 18-20) were fabricated using high-aspect-ratio SU-8 photolithography and soft lithography, with a uniform structural height of 90 µm (Fig. 3b). Spectral analysis at the emission peak (521 nm) revealed significant variations in ML intensity, demonstrating that microstructure design enables controllable ML contrast.
Films with convex pillar arrays exhibited higher ML intensity than planar films. For convex pillars, the 40 µm array (P/A = 25 µm⁻¹) achieved the maximum ML intensity of 1462 (arb.u.), representing 130% of the planar-film value. The ML intensity of convex pillar arrays increased with decreasing feature size and increasing P/A. In contrast, concave hole arrays showed the opposite trend: ML intensity increased with increasing feature size and decreasing P/A, reaching a maximum of 1366 (arb.u.) for the 100 µm array (P/A = 4 µm⁻¹). Spectral measurements were performed for feature sizes of 40, 60, 80, and 100 µm (Fig. 3c and Supplementary Fig. 21), and the relationships between ML intensity, feature size, and P/A were established for both convex and concave structures (Fig. 3d, e).
Clear trends emerged from the results: for convex pillar arrays, smaller feature sizes and higher P/A ratios produced stronger luminescence intensities. In contrast, for concave hole arrays, larger features and lower P/A ratios resulted in higher intensities. These findings demonstrate that the luminescent properties of ML films can be systematically controlled by tuning the shape, size, and P/A ratio of microstructure arrays, enabling the fabrication of films with customizable brightness levels and spatial resolution. This provides a versatile toolbox for programming ML thin films as visual force sensors and force-responsive display units.
The incorporation of nanostructures onto the films forms hierarchical multiscale structures, further increasing photon escape efficiency. As demonstrated in Fig. 4, this figure elucidates the mechanism by which these multiscale structures enhance luminescent intensity by modifying the interaction between mechanical deformation and light emission. This design leads to an improvement in the overall performance of mechanoluminescent films.
During the fabrication process -- including coating, heating, curing, and peeling -- intrinsic stresses develop within the material as a result of molecular rearrangement and local density variations (Fig. 4a). The geometry of the microstructures plays a critical role in determining the distribution and magnitude of both residual and externally applied stresses. In films with pillar-like microstructures, external mechanical loading induces pronounced stress concentration at the bases and apexes of the pillars due to geometric confinement, resulting in highly localized elastic deformation (Fig. 4b). This enhances mechanical activation of ZnS:Cu particles and increases interfacial contact-separation and sliding, facilitating triboelectric charge generation and strong, spatially localized mechanoluminescent emission. In contrast, concave (hole-type) microstructures distribute mechanical stress more broadly toward their edges, forming a ring-shaped stress pattern. This geometry leads to a more diffuse and lower-intensity luminescent response compared to the sharply localized, high-intensity emission of pillar-based structures (Supplementary Fig. 23). Overall, the geometry of the microstructures fundamentally determines the local stress landscape and, consequently, the efficiency and spatial distribution of mechanoluminescent output.
Under cyclic stretching, internal mechanical stress dynamics further influence luminescence behavior. As shown in Fig. 4c, the luminescence intensity reaches its peak during the first stretch cycle, owing to the rapid release of pre-existing stresses introduced during fabrication. In subsequent cycles, the luminescence gradually decreases and stabilizes at a steady-state bimodal pattern, indicating stress redistribution and mechanical equilibrium. This cyclic luminescence behavior occurs independently of the testing system equipment displacement or stretching speed (Supplementary Fig. 3), confirming that internal stress dynamics dominate the observed behavior. To visualize and quantify these stress patterns, stress birefringence imaging was applied (Fig. 4d and Supplementary Fig. 24). Before stretching, stress naturally accumulates at the bases of convex pillars and the edges of concave holes, reflecting the inherent stress concentration induced by residual stress accumulation during microfabrication. Upon mechanical stretching, stress redistributes directionally along the loading axis, forming highly localized stress concentrations. These regions appear as bright directional stripes in birefringence images, aligning closely with luminescent gradients observed under mechanical excitation.
At the nanoscale, hierarchical structures primarily enhance photon escape by increasing multiscale scattering and reducing total internal reflection at the film-air interface. Nanopatterned features such as nanopores and nanopillars enlarge the effective surface area, promoting multiple scattering events that randomize photon trajectories and facilitate light out-coupling (Fig. 4f). Finite-difference time-domain (FDTD) simulations (Supplementary Figs. 25-27) demonstrate that these nanopatterns increase the proportion of photons escaping from the film, leading to a substantial improvement in overall luminescent output (Fig. 4g). The observed enhancement is thus mainly attributed to improved scattering and modulation of local refractive index boundaries.
The hierarchical multiscale-structured ML films achieve substantial luminescence enhancement by integrating microscale stress concentration with nanoscale photon management. Microscale convex and concave patterns efficiently localize mechanical stress, leading to stronger activation of ZnS:Cu luminescent centers and introducing pronounced angle-dependent emission (Supplementary Fig. 28), with structured films maintaining high brightness over a wider range of viewing angles than planar controls. Nanoscale features, such as nanopillars and nanopores, act as scattering centers that disrupt total internal reflection and facilitate photon escape, as confirmed by both FDTD simulations and experimental data. This synergistic structural design enables precise spatial control of mechanical activation and maximizes light extraction, resulting in up to about a threefold increase in emission intensity and providing a clear framework for the rational engineering of high-performance mechanoluminescent devices.
Multiscale-structured two-dimensional pattern arrays offer a dynamic and real-time approach for visualizing in-film stress distribution. By introducing hierarchical checkerboard architectures, the stress and strain distributions can be effectively visualized with high spatial resolution. These designs utilize the stress concentration effects of microstructures to amplify luminescence under localized stress while enhancing light-material interactions through increased surface complexity. This combination optimizes strain visualization, where the precise adjustment of size, shape, spacing, and hierarchy of microstructures ensures enhanced stress mapping performance (Supplementary Fig. 32).
The checkerboard structure integrates macroscopic square blocks with embedded microscale features, forming a hierarchical design that operates across multiple scales. At the macroscopic level, regularly arranged raised and recessed square blocks introduce foundational stress modulation. At the microscale, features such as microholes and micropillars enhance light-surface interactions by increasing the effective surface area and generating localized optical effects. When subjected to external forces, the hierarchical microstructures exhibit varying brightness levels corresponding to local strain, thereby enabling real-time stress visualization (Fig. 5a). This design significantly improves stress detection at both macro and micro levels, producing detailed stress maps through luminescence variations as the film deforms (Fig. 5d, e).
The hierarchical design achieves refined microscale stress detection by creating high-contrast bright and dark regions at stress concentration points, allowing for precise stress localization (Supplementary Figs. 33, 34). Moreover, variations in microstructure height and luminescent intensity introduce a quasi-3D effect, enabling the visualization of stress gradients with a depth-like appearance (Fig. 5f, g). This capability differentiates point, line, and surface stress characteristics, supporting accurate static and dynamic stress monitoring.
By fine-tuning the microstructure geometry and arrangement, the hierarchical design enables the creation of gradient stress patterns with enhanced resolution and contrast. For instance, simulated depth perception is achieved by adjusting luminous intensity and stress gradients across different regions (Fig. 5h, i, and Supplementary Fig. 35). Notably, dynamic patterns -- such as the giant panda pattern -- are generated, showcasing spatially graded luminescence and intricate 3D effects (Fig. 5i-k). The stress gradients vary responsively under external forces, thereby producing a detailed and adaptive 3D stress map.
The hierarchical multiscale structures facilitate the creation of complex visual patterns, including checkerboard and giant panda arrays, within ML films. By modulating light intensity, these patterns achieve quasi-3D visualization with high precision. Importantly, the technology demonstrates pixelated mechanoluminescence with a minimal pitch of 40 μm on a single light-emitting film, encoding 281,250 pixels with a resolution of 637 PPI (Fig. 5h-l).
This design improves stress engineering and light extraction in ML films, supporting applications in real-time stress visualization, passive force sensors, and mechanical imaging at μm-scale pixel pitch. These findings indicate that hierarchical architectures enable pixelated mechanoluminescence for intelligent sensing and dynamic force mapping systems.