Seongkyu Song

Color Manipulation of Mechanoluminescence from Stress‐Activated Composite Films

The prospective application of luminescence to imaging devices is shown using a combination of color-tunable and patternable mechanoluminescent materials. A white light source is demonstrated by using an alternative color tuning method, induced under high vibration conditions. As the implementation is fairly straightforward, it is expected that the present results will find a number of potential uses in current industrial applications.

Bright, wind-driven white mechanoluminescence from zinc sulphide microparticles embedded in a polydimethylsiloxane elastomer

A variety of mechanoluminescent (ML) materials have recently reinvigorated studies of luminescence activated by mechanical stress, but few practical applications have been demonstrated due to the destructive nature of the process. To overcome these shortcomings, elastico-mechanoluminescent (elastico-ML) materials, which generate luminescence under elastic deformation, have been suggested with a view to their use in practical devices. However, the weak brightness and limited white colour expression of these materials must be resolved before they can be employed in practical applications. Here, we report a wind-driven ML device that produces significant brightness and emits warm/neutral/cool white light over a range of colour temperatures from zinc sulphide (ZnS) microscopic particles embedded in a polydimethylsiloxane (PDMS) composite. Harnessing wind-activated mechanoluminescent devices in practical displays or lighting systems could pave the way to new environmentally friendly lights, which reduce energy waste and promote sustainability.

Mechanically driven light-generator with high durability

Mechanically activated luminescence from solids (mechanoluminescence) is a classical optical phenomenon induced in a substance when stressed or cleaved. However, no practical application has been realized due to its low luminescent intensity and lack of reproducibility. We demonstrate highly bright and durable mechanoluminescent flexible composite films with a brightness of ∼120 cd/m2 and durability over ∼100 000 repeated mechanical stresses by using a combination of copper-doped zinc sulfide (ZnS:Cu) particles and polydimethylsiloxane. Furthermore, the possibility of mechanoluminescent color-tuning by changing the repetitive stress rate on the composite films is also suggested. These findings can open a window for developing smart systems and opto-mechanical devices.

Luminance enhancement of electroluminescent devices using highly dielectric UV-curable polymer and oxide nanoparticle composite

A flexible hybrid structure electroluminescent (HSEL) device was fabricated from ZnS:Cu phosphor microparticles dispersed in a UV-curable polymer matrix. We observed a maximum luminance of 111 cd/m2 at 10 kHz and 170 V from a device wherein the mixing ratio between the phosphor particles and highly dielectric polymer binder was 70:30 wt%. Furthermore, by uniformly dispersing highly dielectric BaTiO3 nanoparticles within the polymer matrix, we were able to obtain a luminance of up to 211 cd/m2 in the HSEL device. Compared to the conventional thermal curing process, this UV process greatly simplifies the fabrication steps by combining phosphors and dielectric materials at room temperature. This process also demonstrates a promising pathway toward creating flexible and printed EL devices in the future.

Conductive, flexible transparent electrodes based on mechanically rubbed nonconductive polymer containing silver nanowires

This paper explores the use of rubbing for alleviating the problem of lost electrical conductivity, which is typically caused by the use of nonconductive polymers, to improve the substrate adhesion and surface roughness of metal nanowire networks. This process is used to create composite transparent electrodes based on a network of silver nanowires (AgNWs) fully-embedded in PVA, which, after mechanical rubbing, exhibit both a smoother surface and superior electromechanical stability.

Bidirectional two colored light emission from stress-activated ZnS-microparticles-embedded polydimethylsiloxane elastomer films

Bidirectional two-colored mechanoluminescent light emission has been demonstrated by unifying two polydimethylsiloxane elastomer layers functionalized with zinc sulfide doped with Cu (ZnS:Cu) or Cu and Mn (ZnS:Cu,Mn). The bilayered composite films are simply fabricated by dispensing uncured ZnS:Cu,Mn + PDMS onto previously spin-coated and hardened ZnS:Cu + PDMS film. The robust PDMS-PDMS bonding yields a film which can simultaneously emit light with color coordinates of (0.25, 0.56) and (0.50, 0.48), similar to the intrinsic colors of ZnS:Cu and ZnS:Cu,Mn, respectively. Composite films can emit light in upper and lower directions without fracture when it is stretched.

Electroluminescent devices with function of electro-optic shutter

The polymer-dispersed liquid crystal (PDLC) was used as a dielectric layer of electroluminescent (EL) device to provide multi-function of electroluminescence and electro-optic shutter. A 50 μm-thick PDLC layer was formed between a transparent electrode and a ZnS:Cu phosphor layer. The electro-optic properties of the EL device were not distorted by the introduction of the PDLC layer. The extraction efficiency of luminescence was improved by more than 14% by PDLC layer. The transmittance of the PDLC was also founded not to be degraded significantly by excitation frequency. Therefore, the electroluminescence of the device was ignited by excitation frequency at a given voltage for full transparency of the PDLC. This device has great potential for applications in transparent displays with the function of a privacy window.

Stretchable, alternating-current-driven white electroluminescent device based on bilayer-structured quantum-dot-embedded polydimethylsiloxane elastomer

In this work, we demonstrate neutral/pure/cool white light due to color conversion from quantum dot (QD)-embedded, polydimethylsiloxane (PDMS)-based alternating-current-driven electroluminescent (ACEL) devices. White light generation is achieved by the spontaneously formed bilayer-structure emitting layer (b-EML) comprising QDs embedded in PDMS containing Cu-doped zinc sulfide (ZnS:Cu) particles (EL emitting layer), without ZnS:Cu (color conversion layer). Red light is generated by CdSe/CdZnS QDs, which partially absorbs electrically excited blue-green EL. This study examines the use of spontaneously formed b-EML as a means of alleviating the problems resulting from QD aggregation in PDMS and its effects on electro-optical performance of ACEL devices under stretching conditions.

Electrically tunable birefringence of a polymer composite with long-range orientational ordering of liquid crystals

We report an optical film with electrically tunable birefringence in which the liquid crystals (LCs), mixed with the host polymer, form long-range ordering. The film was prepared through polymerization without phase separation between the LCs and polymers. Driving voltage below 30 V for full switching of birefringence is achieved in a 6 μm-thick film. Electro-optical investigations for the film suggest that the long-range ordering of the LCs mixed in the film caused by polymerization lead to rotations of the LCs as well as optical anisotropy in the film. These films with electrically tunable birefringence could have applications as flexible light modulators and phase retardation films for 2D-3D image switching.

Rewritable, Light-Driven Recordings in a Full-Colour Fluorescent Polydimethylsiloxane Elastomer

Current approaches to demonstrate light-driven writing enabling multiple “write–erase” cycles are mostly based on photochromic materials requiring complex synthesis processes. Here, we show a new approach to realize rewritable, light-driven recordings in conventional dye-incorporating poly(dimethylsiloxane) (PDMS) by using a laser-induced photobleaching technique. Upon selective photobleaching of red dyes, the PDMS film emits patterned letters of different colours (blue or green) from their backgrounds. We also found that the diffusion of dye molecules into the bleached region promoted a self-healing characteristic that could erase the patterned letters, which enables us to rewrite different letters using the same PDMS. This novel method of light-driven writing can be applied effectively in many research fields, including secure printing and optical storage devices, and will significantly advance future writing applications.