imec joins Graphene Flagship
To coincide with Graphene Week 2014, the Graphene Flagship announced that today one of the largest-ever European research initiatives is doubling in size. Sixty-six new partners are being invited to join the consortium following the results of a €9 million competitive call. While most partners are universities and research institutes, the share of companies, mainly SMEs, involved is increasing. This shows the growing interest of economic actors in graphene. The partnership now includes more than 140 organizations from 23 countries. It is fully set to take “wonder material” graphene and related layered materials from academic laboratories to everyday use.
“Imec aims to show that graphene can form the basis of practical optoelectronic devices, such as high speed modulators and detectors, for use in low power optical interconnects,” said Cedric Huyghebaert, team leader of imec’s graphene group. “During the past five years, we have built a strong knowledge in graphene device making, focused on the generic building blocks like contacting, doping and gate engineering, which are essential to progress in any graphene application. This knowledge, combined with our unique experience in integrating novel materials into CMOS-processes, and our optoelectronic silicon waveguide platform, makes imec a very suitable place to develop hybrid-silicon-graphene optoelectronic devices compatible with CMOS.”
Collecting light with artificial “moth eyes”
All over the world researchers are investigating solar cells which imitate plant photosynthesis, using sunlight and water to create synthetic fuels such as hydrogen. Empa researchers have developed such a photoelectrochemical cell, recreating a moth’s eye to drastically increase its light collecting efficiency. The cell is made of cheap raw materials – iron and tungsten oxide.
Rust – iron oxide – could revolutionize solar cell technology. This usually unwanted substance can be used to make photoelectrodes which split water and generate hydrogen. Sunlight is thereby directly converted into valuable fuel rather than first being used to generate electricity. Unfortunately, as a raw material iron oxide has its limitations. Although it is unbelievably cheap and absorbs light in exactly the wavelength region where the sun emits the most energy, it conducts electricity very poorly and must therefore be used in the form of an extremely thin film in order for the water splitting technique to work. The disadvantage of this is that these thin-films absorb too little of the sunlight shining on the cell.
Empa researchers Florent Boudoire and Artur Braun have now succeeded in solving this problem. A special microstructure on the photoelectrode surface literally gathers in sunlight and does not let it out again. The basis for this innovative structure are tiny particles of tungsten oxide which, because of their saturated yellow colour, can also be used for photoelectrodes. The yellow microspheres are applied to an electrode and then covered with an extremely thin nanoscale layer of iron oxide. When external light falls on the particle it is internally reflected back and forth, till finally all the light is absorbed. All the entire energy in the beam is now available to use for splitting the water molecules.
In principle the newly conceived microstructure functions like the eye of a moth, explains Florent Boudoire. The eyes of these night active creatures need to collect as much light as possible to see in the dark, and also must reflect as little as possible to avoid detection and being eaten by their enemies. The microstructure of their eyes especially adapted to the appropriate wavelength of light. Empa’s photocells take advantage of the same effect.
In order to recreate artificial moth eyes from metal oxide microspheres, Florent Boudoire sprays a sheet of glass with a suspension of plastic particles, each of which contains at its center a drop of tungsten salt solution. The particles lie on the glass like a layer of marbles packed close to each other. The sheet is placed in an oven and heated, the plastic material burns away and each drop of salt solution is transformed into the required tungsten oxide microsphere. The next step is to spray the new structure with an iron salt solution and once again heat it in an oven.
A silicon replacement? USC Viterbi School of Engineering overcomes major issue in carbon nanotube tech
When it comes to electronics, silicon may one day have to share the spotlight. In a paper recently published in Nature Communications, researchers from the USC Viterbi School of Engineering describe how they have overcome a major issue in carbon nanotube technology by developing a flexible, energy-efficient hybrid circuit combining carbon nanotube thin film transistors with other thin film transistors. This hybrid could take the place of silicon as the traditional transistor material used in electronic chips, since carbon nanotubes are more transparent, flexible, and can be processed at a lower cost.
Electrical engineering professor Dr. Chongwu Zhou and USC Viterbi graduate students Haitian Chen, Yu Cao, and Jialu Zhang developed this energy-efficient circuit by integrating carbon nanotube (CNT) thin film transistors (TFT) with thin film transistors comprised of indium, gallium and zinc oxide (IGZO).
“I came up with this concept in January 2013,” said Dr. Chongwu Zhou, professor in USC Viterbi’s Ming Hsieh Department of Electrical Engineering. “Before then, we were working hard to try to turn carbon nanotubes into n-type transistors and then one day, the idea came to me. Instead of working so hard to force nanotubes to do something that they are not good for, why don’t we just find another material which would be ideal for n-type transistors—in this case, IGZO—so we can achieve complementary circuits?”
Carbon nanotubes are so small that they can only be viewed through a scanning electron microscope. This hybridization of carbon nanotube thin films and IGZO thin films was achieved by combining their types, p-type and n-type, respectively, to create circuits that can operate complimentarily, reducing power loss and increasing efficiency. The inclusion of IGZO thin film transistors was necessary to provide power efficiency to increase battery life. If only carbon nanotubes had been used, then the circuits would not be power-efficient. By combining the two materials, their strengths have been joined and their weaknesses hidden.
Zhou likened the coupling of carbon nanotube TFTs and IGZO TFTs to the Chinese philosophy of yin and yang.
“It’s like a perfect marriage,” said Zhou. “We are very excited about this idea of hybrid integration and we believe there is a lot of potential for it.”