Hello everyone. In this lecture, we are going to talk about two-dimensional nanomaterials. So two-dimensional nanomaterials are thin films, and you can find two different film nucleation and growth mechanism as shown here. The first one is layer by layer growth, which can be resulted in high-quality epitaxial films, and second one is 3D island in growth, which it can be visuality in the polycrystalline Colombian ratio. So as shown here, if you control the surface energy of substrate, surface energy of a film, an interface energy of substrate and film, you can control the film nucleation, and growth mechanism. The as layer by layer, sometime 3D allocating. The symptoms is that the position of a crystalline overlayer owner crystalline substrate. So you should know that some important terminology including epitaxy, homoepitaxy, and heteroepitaxy. The epitaxy is a registry between the overlayer and the substrate. The overlayer is an epitaxial film or epitaxial layer, and homoepitaxy is a epitaxy performed with only one material, a crystalline film is grown on a substrate or film of the same material. So homoepitaxy can be used to grow or more fewer film were a film with different toping levers. Heteroepitaxy is a epitaxy performed with different materials. A crystalline film grows on a crystalline substrate of a different material. This figure shows the one example for homoepitaxy. As shown here, the left side of the figure, the Zinc oxide film can be deposited directly onto the zinc oxide sulfate. But in this case, we cannot obtain the high-quality zinc oxide film due to the low quality of surface quality of zinc oxide suflphate. So as shown here by introduction of some upper layer such as high temperature zinc oxide or low temperature zinc oxide, we can obtain the high-quality thin film of zinc oxide. By using atomic force microscopy analysis, we can confirm that the quality of a film, as shown here, then you can find step and tell a structure in this AFM analysis. Then heteroepitaxy, actually there are three different relationship between substrate and overlayer. The first one is coherent, the coherent is the the exact atomic matching structure between substrate and overlayer, and second one is a semicoherent. You can find that there's some T part structure, at the interface between substrate and overlayer, there is dislocation. Third one is incoherent that there are no relationship between atomic registry between substrate and overlayer. So in heteroepitaxy, you can find the disk like three different the morphology at interface. The first one is well matched structure without any strain field, and the second one is the well-matched lattice structure but with strain field. These two type of interface is the coherent interface. The fourth one should have some relaxed interfaces structure by generation of dislocation this interface is semiconductor interface. Let's think about the one example of the combination of a homoepitaxy and heteroepitaxy. Then Firstly, we are preparing the gallium arsenide is substrate, but this gallium arsenide substrate should have intrinsic oxide layer. So the first step to public case the aluminum doped gallium arsenide. The symposium is the outgassing to deoxidize. So after deoxidization, to which you check the surfaces states of gallium arsenide substrate by using wind monotony. Then in order to increase the surface quality of gallium arsenide substrate, we should use homoepitaxy. It is the top monolayer T position of gallium arsenide film on to the gallium arsenide substrate. Then high quality epitaxial film, it is heteroepitaxy aluminum doped gallium arsenide the coolant can be deposited onto the gallium arsenide layer. This is one example of the combination of homoepitaxy and heteroepitaxy. This is Interface Engineering provided the or dependencies in the heteroepitaxy of complex oxide, by fabricating interface in oxide with atomic precision, it can generate electron system that nature does not produce in the bulk. Then sometimes, the electrons interact and other at the interface in unique ways. So this means that new generation of oxide electronic devices, so by connection of different material we can generate ultrathin or even 2-dimensional interfaces, and this approach provide possibility for generating noble electronic pieces based on the 2-dimensional nanomaterials. One important example is 2DEG (2-dimensional electron gas) which can be found in semiconductors. The tip current carrier density of these 2DEG is ranged from 10_10 per centimeter square to 10_12 per centimeter square. So by generation of interface between silicon doped aluminum gallium arsenide, and undoped gallium arsenide. So in order to transport the carriers, the Fermi energy level of these two materials should be matched. So by matching of the Fermi vapor we can induce disk-like band bending at the interface. So due to the, the band bending at the interface, we can realize dislike to the structure, and the mobility of these 2DEG is very high, is about 10_4 centimeters square per voltages second. So top values now exceed 10_7 centimeter square per voltage second at lower temperatures. So this high mobilities, and long beam pre paths, which can be realized in 2DEG materials provide us the possibility for the publication of high electron mobility transistors and this is like a 2DEG that can be found also in oxide interfaces. By reconstruction of the complex electronic structure of the oxide. In 2004, the professor Ohtomo and Hwang have reported this unique feature, which can be found in oxide interfaces between lanthanum animate and strontium titanate. As you know, lanthanum aluminates is a band insulator with larger bandgap, about 5.6 electron volt, and also strontium titanate is band insulator with large bandgap of 3.2 electron volt. But by combination of these two insulating material, they realized a very thin metallic interface. The generation of a 2DEG in oxide interfaces can be understood by the charging the relationship. So the titanic oxide layer and strontium oxide layer has already charged neutrality. However, that aluminum oxide layer and lanthanum oxide layer should have the instability of charge state. So the remained the charges from the lanthanum oxide layer can be accumulated at the interface between lanthanum oxide and titanium dioxide. So this can make the 2DEG structure in oxide interfaces. Another example of 2DEG which can be found in oxide interface pieces is the contempt where structure. So this material is the chapeau ratchet structured material. The conducting Diome doped strontium titanate are [inaudible] between the insulating strontium titanate real. So in this quantum confinement structure, we can obtain the dislike quantum well structure. So quantum well structure means the enlarging the density of state, and the one important pit car parameter seebeck coefficient is proportional to the density of states near the Fermi level. So a shown here, you can find significantly enhance the Seebeck coefficient in this 2DEG oxide hetero structured materials. So in this lecture, we briefly reviewed the 2-dimensional nanomaterials. Then in the next, we are talking about the 3-dimensional nanomaterials. Thank you.
