[MUSIC] Hello everyone. Today, we will learn about the smart sensors using the nano materials. So, in this slide, we will see the synthesis of the graphene and graphite. So, simultaneously, the graphene and graphite can be synthesized for the application in the sensors. If we use CVD method, Chemical Vapor Deposition method, Typically, a copper foil or copper thin film is used for the synthesis of the graphene. So, at high temperature above 1000 Celsius, the methane gases can be decomposed so the carbon atoms can be diffused inside of the copper foil. So, in that case, if we cool down the temperature rapidly, then the dissolved carbon atoms become precipitated, So, they become located on the top surface of the copper foil. That precipitated carbon atoms make a graphene filling. So, graphene is typically the single atomic layer of the carbon. So, in that case, that is possible because the solubility of a carbon in copper is not very high. Instead of the copper, if we use other metals, for example cobalt or nickel, then the carbon's solubility in the nickel or cobalt is much higher compared to the copper case. So, more amount of the carbon becomes precipitated on the metal catalyst film. So, in that case, our graphite structure can be formed instead of graphene. So, we can use copper film for the synthesis of graphene. And also, we can use a cobalt or nickel for the synthesis of the graphite. And that's dependent on the carbon solubility. So, if we make metal catalyst of patterns on any kind of substrate for example, some alphabet is patterned using the cobalt and some alphabet is patterned using the nickel and Also, the background area is copper filling. And then if we synthesize using the CVD method, then graphite and graphene patterns can be synthesized altogether, simultaneously. So thus, this synthesize result. So, in the optical micrograph image, the green color shows the graphite film and then you'll see the purple colored area that's the graphene area. And also, from the Nickel surfaces' a little bit thicker, the graphite can be synthesized. So, we can control the number of the graphene layers in this way by using heterogeneous metal catalyst. In this image, the copper and nickel were used instead of the cobalt. So, the background area is graphene synthesized from the copper area. And then electrode patterns shows the nickel patterns so they make some graphite with the six or eight layers of the graphene. So, we can control the number of graphene layers by using the heterogeneous metal catalyst. So that kind of number of graphene layers can be detected using the Raman or TM images. So, in the Raman image, JPG is dependent. You will see the Raman shift here and also in the right-side image, it shows the TM. Cross sectional TM image so graphite layer and a few layers of graphene they are linked continuously. So, the number of graphene layers changes gradually as shown in this TM image. And electrical properties can be dependent on the number of graphene layers. So, as the number of graphene layers increase, its sheet resistance becomes decreases because graphene is conductive. So, more graphene layers make a lot higher conductivity. So, its resistance becomes decreases. So, the graphite is more conductive than graphene. But the transconductance is different. So, in the case of transconductance, graphene becomes higher so you see the larger modulation of the current from the graphene area. But in the case of graphite, the large number of graphene layers screen the electric field so its transconductance becomes negligible. I mean the change in the current becomes negligible. So, graphite can be used as electrode and graphene can be used as the channel of the field effect transistors. So, we can synthesize the kind of Graphene-Graphite field effect transistors using the CVD method. So, for the electrode part, we can use the cobalt. Then for the channel part, we can use copper and then if we synthesize that our patterns. Using the CBD method, large number of graphene I mean the graphite can be synthesized from the cobalt area for the electrode patterns. And very thin graphene layer can be synthesized from the copper area for the. Channel or the field effect transistors. After synthesis, we can remove the metal catalyst the films. And then we can transfer the kind of monolithic graphene and graphite structures on to other substrate for example, silicon wafer with SiO2 electric layers. In the case, by simply transferring the synthesized patterns can form the transistor's structure itself. So, bottom silicon layer can act as bottom-gate. And SiO2 layer can act as the electric layer. And graphite act as the electrode. Graphene part becomes channel over the transistor. And by forming the kind of monolithic Graphene-graphite transistor, we can check the current behavior. So It's shown the bottom of the graph. It's a whole mobility, it's pretty high, it's about 1,800 and Also, electro mobility is also similar. And these mobility values comparable to the Conventional graphene transistors. So here, the conventional graphene transistor means, graphene synthesized graphing channel, with the normal gold electrode or other metal electrode. So, for the monolithic transistors, we use graphite as the electronic part instead of metal, but the graphene´s mobilities are similar. And we can compare the contact properties I mean contact Resistances of the graphene with the other metal electron, for example, Chrome or gold electrode or Graphite electrode, in the case of monolithic graphene and graphite, its content resistance is slightly better than the other electrode apart. So, but the difference is not very significant. So, we can use the kind of graphite electrode for the graphene transistors. But the difference between the graphite and the lower metal is its large flexibility. Relatively graphite is more flexible compared to the other metals. So, the Monolithic graphene-graphite transistor becomes flexible. So, it shows very high flexibility. So, as shown in this graph by applying the tensor string up to 2% or 3% is the register's change is not significant. So, the synthesized graphene-graphite field effect transistor array can be bent, as it is shown in this photo images. So here, all electrodes were form using the graphite. For example, source, Drain and interconnected gate Is graphite. But the channel material is graphene. So, in this way, we can make a top gate monolithic graphene-graphite field effect transistors (FET). And this kind of transistor is very flexible. So, although the sample bents with the bending radius of the 7 millimeters, the current change is not significant. And another application of this graphene and graphite field effect transistor Is the use for the sensor for example, it can be used as the pH sensor. So, graphene´s Resistance is dependent on the pH. So, by measuring the current change in the graphene part, we can find out the pH value. So, in this way, the right-side graph shows the kind of pH monitoring using this graphene and graphite field effect transistors. So, by changing the pH of the solution, you see the Dirac point of the graphene transistor becomes shifted. So, by monitoring the current change, we can find out the pH. and another interesting thing is the graphene and graphite monolithic field effect transistor is Hydrophobic because it is composed of all carbon layers. So, carbons are hydrophobic. So, this is the size graphene and graphite the transistor is hydrophobic, so it can be floating on the water for example. So, this image shows that kind of floating layer over this graphene-graphite transistor. On water this kind of synthesize grapheme-graphite patterns can be floating and Also, we can transfer it using this kind of floating theory. So, the floating sample can be located on to another desired Substrate, so in this way you can transfer the synthesized devices. For example, it can be transferred on to the gloved fingers or coins, non-planar surfaces. And also, you can be transported on to the cylindrical tube as well. And also, you can be transported to the soft contact lenses as well In this image shows that that is monolithic grapheme and graphite devices were attached on to the outer surface over that insect as well. So, by attaching the devices on to the insect body, We can use that insect as kind of live substrate.
