[MUSIC] So from now let's learn about the ultra thin film based on the cellulose nano fibers. So for the foldable or flexible electronic devices, we need a high tourality of that kind of device performances. So these devices should be folded or bended for multiple times. But during that kind of bending motions or the folding motions, if the devices becomes broken then we have problems, right? So, we need very strong electronic devices against that kind of mechanical deformations. So, electronic devices are composed of various materials including semi conductors, and metal for electrode, and also the dialect layer for encapsulation as well. But also, we need substrate as well. But most electronic devices in the current state, they use the rigid type substrate, such as glasses or silicon wafers. So if we make that kind of electronic devices, for example this place on glasses. Then these devices can be operated for long time, for more than ten years in the case of the displays, right? But if we fabricated the identical devices on the plastic substrate, instead of the glasses. Then the electronic devices cannot be operated for that kind of long period. Why? So let's run about the region for the kind of weakness of the electronic devices on plastic substrate. So in the case of the substrate, we should consider the fabrication, processing, also performances, reliability. And also we should consider the cost of the final product. So the best way is to use the same identical glasses with flexible or a bendable geometries. So in the case of glass, it's transparent and also is somewhat stable. And also, if we make ultra thin glasses, then it becomes flexible as well. But the problem is the kind of ultra thin glasses are very expensive, and also they are fragile. So, that's not perfect for the flexible or foldable electronic devices. So people use plastic films such as the PET or transparent polyethylene thin films. Also we can use polycarbonate as well. And these plastic, engineering plastics can be transparent. And also these plastic films are flexible, but the problem is, they are thermally expandable. So their thermal expansion coefficients are relatively large. But in the case of the inorganic devices, displays or the semiconductors, they mostly use the inorganic materials. And their thermal expansion coefficient are much lower than the these plastic films. So, by thermal loading and unloading, the kind of electronic devices fabricated on the plastic films. Then these plastics becomes expanded thermally. But compare it to the device areas. So, that's the reason why the electronic devices are, they can be broken against their kind of thermal loading and unloading processes. If they are fabricated on the plastic substrate. And also these plastic films becomes expensive as well, particularly for the transparent. And also they are relatively very smooth, so they have the low hardness values as well. So, we need to find alternative, the flexible films. So instead of the thin glasses or plastics. So again, let's learn the current issues in flexible electronic devices. The most important issue here is the device liability. For example, let's say we have foldable electronic devices. So it is working initially well, but if we fold multiple times, for example, if we fold 10,000 times or 100. If we fold these electronic devices multiple times, then these devices can be broken eventually. So during the device operations, the temperature becomes increases so the flexible plastic films can be expanded because of their high thermal expansion coefficient. For example, in the case of the smartphones, the temperature can be increased to about 60 Celsius. And for the automobiles, the temperature can be increased to around 100 Celsius in summer. And also that temperature can be cooled down to minus 40 Celsius in winter, for example. And also during the device fabrication process, also we need the high temperatures to around 240 Celsius for the display fabrications. So during that kind of summer stress, because of the kinds of summer stress, the plastic films can be expanded because of their high thermal expansion coefficient. But most electronic devices are fabricated using the inorganic materials, like silicone, or the metals and so on. And these inorganic materials has relatively lower thermal expansion coefficient. So because of that kind of mismatch between these two thermal expansion coefficient, so the device part can be stressed thermally. So eventually, these devices can be broken due to the fatigue of failure. So we need to find another alternative plastic films or the transparent films to replace the plastics or the fragile glasses. So, in this slide you will see the thermal expansion coefficient values for the metals and typical ceramics, and glasses, and so on. These inorganic materials, they have the thermal expansion coefficients of around ten. But in the case of the plastic films, such as the pulley meter or PT and so on, these are thermal expansion coefficient is higher than 30, typically. So, because of this difference between their thermal expansion coefficient, so that kinds of mechanic failure of devices can be occurred. So, we need to reduce the, we need to engineer to make that kind of another alternative or transparent films with lower thermal expansion coefficient. Let's see cellulose. So in the case of the cellulose, it's abundant because most plants are based on the cellulose. And these are plants, or wood for example, they can be cellulose fibers can be obtained by wood. And if we engineer further then cellulose fibers can be narrowed down to the nanofibers structures or nanocrystal structures. So in the case of cellulose fibers, their typically larger, wider than 15 micrometers. But if we narrow down these fiber geometries then we can reduce its diameter, narrower than three nanometer scales, for example. So in this way, we can make very, very thin cellulose nanofibers. So by using that kind of cellulose nanofibers, we can make flexible films. And that kind of a flexible film is very flexible and also they have very, very low thermal expansion coefficient. For example, lower than one. So that's suitable for the inorganic devices. But the problem is, these cellulose nanofiber films are not transparent. But because they have many pores inside if they make the film structures. But these are porous structures can be occupied by the polymers. And if the reflected index of those polymers are similar to the cellulose nnofibers then, we can reduce the reflective index difference. So in this way, we can make a transparent and flexible cellulose films based on the cellulose nanofibers. And these cellulose nanofiber-based films are also mechanically robust as well. So that's a very good candidate for the flexible and transparent substrate. So by using that kind of cellulose nanofibers, we can make the cellulose base, the transparent flexible films. And we can use these cellulose-based films for the fabrication of the electronic devices, for example, touchscreen panels. And also, we can use that kind of cellulose films for the cover layers for the encapsulation layers instead of the substrate as well. Also that's another interesting approach. And also we can make a transparent electrode suitable for the kind of cellulose films. So, let's see the three different approaches in detail. So the first approach is to make the flexible substrate using the cellulose nanofibers. And the second approach is to use, to make the cover layers using that kind of cellulose or films for the touch screen panels. And the third approach is to make the transparent electrode using the kinds of cellulose films for the high temperature heaters, for example.
