Hello everyone. The class today is about stretchable electronics. The most conventional electronic devices are rigid and flat. So, they are not stretchable, But the effort to make a stretchable electronic device has been carried out very significantly. So that kind of stretchable electronics has been developed very well. in the research area, at the fabrication methods to form the stretchable geometries are very popular and well-known. today, let's learn about that conventional method to make these stretchable geometry of electronic devices. The best way to make these stretchable electronic devices is to use stretchable materials. But in order to make optoelectronic devices such as this plate, we need various electronic materials including the organic materials or inorganic materials such as the metals, or glasses, and so on. But all of the materials cannot be stretchable. I mean, there are some materials that can be stretchable, but it is very difficult to make entire materials as stretch forms. So, we need to find another alternative methods to simplify the fabrication or materials issues. So, the most conventional methods to make stretchable devices is to transfer the prefabricated devices as it is explained in this slide. So as shown in this slide, we can use the conventional fabrication method for the displays for example. we can use the conventional glass substrate. Also, we can use the conventional rigid types of the silicon wafers or inorganic wafers. Also, we can use the exactly the same materials for the displays for example, optoelectronic devices. So, we can use the conventional materials with the conventional fabrication process based on photolithography and vacuum processes to make the displays. So here, the only different thing is, we can use the sacrificial layer. For example, we can deposit the sacrificial layer on the rigid glass or the rigid silicon wafers. Then we can fabricate the displays, or transistors, or other electronic devices. After that, we can remove the sacrificial layers selectively. Then the fabricated device part can be separated from the substrate area. So here, we can call the substrate as the mothers of straight. So, using the PDMS stamp. PDMS is elastomer and its surface is very sticky. So, we can prepare data to the devices selectively, So, the device's part can be lifted off from the substrate area because the sacrificial layers were removed. After that, we can transfer the devices using the PDMS onto another substrate, for example, another elastomer substrate. So here, the elastomeric soft substrate is Pre-strained and then we transfer the devices using the PDMS stamp. After transferring, then we can release the stamp, So, the devices part can be remained on the pre-strained, the PDMS substrate area. After that, if we release the pre-strained strings, then the device part shrinks. in that case, the interconnection part to link the individual pixels becomes popped off and then we can form the wavy geometry of the interconnects. So, because of that wavy structure of the interconnection electrode, Then the entire geometry of the electronic devices becomes stretchable. So, this method is very very powerful and very simple because it can use exactly the same materials with exactly the same fabrication process. The only thing that we need is to transport process. For example, in this slide, you will see the LED arrays with the wavy geometry of the interconnections. So, the entire structure of this LED array becomes stretchable and it is attached onto the PDMS elastomer substrate. in this way, we can stretch the entire geometry of the LED array because of that freestanding wavy structure of the interconnections. The exact same method can be applied for the transparent electrode as well. For example, two-dimensional graphene. So, we can synthesize two-dimensional graphene using the chemical vapor deposition method, which is called as the CVD. So, for this CVD synthesis, the copper film or a copper foil can be used and then that copper foil becomes inserted inside the hot furnace, and typically, its temperature is around 1,000 Celsius or above the 1,000 Celsius and then if we flow the methane gases for example, then the methane gases can be decomposed, then the carbon Sigma bond can be diffused inside the copper foil. So here, the solubility of the carbon is very low in the case of the copper, so very small amounts of the carbon can be melted inside the copper foil, particularly near the surface region. Then if we cool the temperature, then the melt heat, I mean, the diffuse, the carbon Sigma bond can be precipitated onto the surface area of the copper foil. in this way, we can form a very thin graphene layer on the copper foil. So then, the synthesized graphene film can be transferred on to the another substrate by removing the copper foil selectively. So here we can use the stretchable, elastomer such as PDMS as the transporting the substrate. So, in the case, the graphene is transferred onto the pre-strained PDMS substrate, and then if we release that pre-strained string, then the graphene film then shrinks with the freestanding, the wavy structures. So, in this way, we can form the wrinkled, the graphene film as the transparent electrode. So, if we stretch this graphene film, wrinkled the graphene film, then the wrinkle becomes flat. So, in this way, we can form stretchable transparent electrode using the graphene, CVD synthesize graphene. But the problem of this approach is the wrinkled geometry of this graphene film itself. So, the light becomes scattered on to the surface of this wrinkled geometry. So, the transmitters decrease slightly. So, for example, you will see the transmittance curve dependent on stretching the conditions. So, in the case of the flat CVD, the synthesize graphene is transmittance is above 97 percent. So, it's very transparent, but by forming that kind of wavy geometry on PDMS surface, because of that light-scattering, the transmittance decreases very significantly to around 70 percent [inaudible] dependent on the wavy structures. So, in this way, we can form the stretchable function for the graphene, but its transmittance decreases. So that's the disadvantage of this approach. the best way, I think for the transparent electrode, the best way might be to form the stretchable geometry with the influenced structures. Then how can you make that kind of inflamed geometry with the stretch functions. we can use the mesh structure as shown in this movie. The left side of movie shows just paper. the paper is not very stretchable, but if we cut the paper with the random mesh structures, then we can stretch this on stretchable material. So here the diameter of this mesh is very thick, we can see the network geometries, but if the diameter of this network is very small in to the nanometer scales, then the entire geometry becomes invisible. So, it becomes transparent and also, it becomes stretchable as well. So, in this way, we can make stretchable geometries, using on stretchable materials. So, we have two different approaches, one is to make wavy structures and the other is to make mesh random network structures. So, these two methods are very powerful for the stretchable electrodes. The most conventional transparent electrode is ITO, which is indium tin oxide. That's the oxide, so it's very fragile and also, it's very rigid structure. Thus, it's not stretchable. Instead of the indium tin oxide, we can use diverse nanomaterials with that kind of random mesh structures. So, for that purpose, we can use random networks of the carbon nanotubes or metal nanowires for example, silver nanowires. Also, we can form the metal grid patterns and that metal grid pattern can be stretchable as well. Also, two-dimensional, the graphene can be stretchable, although its stretch ability is limited to around four percent. Also, we can form a one-dimensional, two-dimensional hybrid structures to form these stretchable and transparent electrode. So, you can form the graphene and metal nanowire hybrid structures and also, we can form the graphene and metal-grid patterns for the transparent stretchable electrode.
