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Masashi Chiba, associate professor at Tokai University, explains the new transparent conductive film by using a model of Mg(OH)2 crystal.
The film's characteristic values, including its electric resistivity, are still low. But the researchers aim to replace ITO (indium tin oxide), transparent conductive film used for LCD panels and others, with the new transparent conductive film, taking advantage of the fact that the cost to procure its materials is low and its manufacturing process is simple.
The transparent conductive film was developed by Toshiro Kuji and Masashi Chiba, professor and associate professor, respectively, of the School of High Technology for Human Welfare at Tokai University.
The chemical elements constituting the transparent conductive film are magnesium (Mg), oxygen (O), hydrogen (H) and carbon (C). As for the film's structure, the results of an X-ray diffraction method showed that carbon exists in the crystal of magnesium hydrate, Kuji said.
The crystal of magnesium hydrate is originally transparent. Therefore, "We think that the crystal of magnesium hydrate provides transparency and the network of carbon in the crystal provides conductivity," Chiba said.
Mg reacts with H2O and becomes transparent
In the manufacturing process, the RF magnetron sputtering method is used. Magnesium metal and graphite are sputtered in a low vacuum to form a film of the composite of magnesium and carbon. The temperature is not controlled in the process.
Then, the opaque film turns transparent after being left in a water vapor atmosphere for 10-15 minutes, with H2O gradually reacting with Mg.
"It works in the same way as magnesium metal starts an exothermic reaction in water and generates Mg(OH)2 and H2 (except for the existence of carbon)," Kuji said.
The film is 2.4μm thick. And the particle diameter of the crystal ranges from several tens to several hundreds of nanometers.
When carbon is not used in the sputtering process, insulating crystal with very high specific resistance is formed. Because carbon was used this time, a specific resistance value of about 10-1Ωcm was observed. This value is much larger than ITO's 10-4Ωcm.
Nevertheless, "The specific resistance values of ITO and ZnO were as low as 10-1Ωcm when they were first developed," Chiba said. "I believe that the value of the transparent conductive film can be reduced too as we continue our research."
"By controlling the ratio of carbon to optimize the carbon's network, we possibly can increase the conductivity of the film beyond that of ITO while keeping the transparency of the film," Kuji said.
On the other hand, the light transmission of the film is relatively high from the beginning. When carbon is added, the light transmission of the filmed materials is 89.8% in average in the wavelength range of 380nm-1μm. In the infrared domain, the light transmission is even higher.
"From now on, we will diminish the size of particle diameter to increase the uniformity and reduce the degree of reflection," Chiba said.
There are many challenges in making the film fit for practical use.
"We will test the film's adhesive properties to substrates, the stability of resistance value, the temperature dependency and whether it becomes a semiconductor material," Kuji said.
Though Mg(OH)2 decomposes into MgO and H2O at 330°C, it is fairly stable at 100°C and below, he said.
Working toward practical use in collaboration with manufacturers
"We do not intend to confine our research to the university," Kuji said. "We want to actively work toward practical use of it in collaboration with manufacturers."
Aisekku Nano Tyubu, developer of carbon nanotubes, will be one of the participants in the joint research.
A black film turns transparent after being left in a water vapor atmosphere for 10-15 minutes.
The average light transmission is 90% in visible light range. The red line shows the transmission of the new material.
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