Transparency - Page 5

Researchers from Cornell University have managed to pattern single atom films of graphene and boron nitride, an insulator, without the use of a silicon substrate. They are using a technique they call patterned regrowth, and they say this could lead towards substrate-free, atomically thin circuits. These will be so thin that they could be transparent and flexible, and yet have great electrical performance.

Patterned regrowth uses the same basic photolithography technology used in silicon wafer processing, and it allows graphene and boron nitride to grow in perfectly flat, structurally smooth films. The researchers first grew graphene on copper and used photolithography to expose graphene on selected areas, depending on the desired pattern. They filled that exposed copper surface with boron nitride, the insulator, which grows on copper and fills the gaps. Then you simply peel off the entire structure.

Sony has developed a new graphene producing process that use a roll-to-roll method. They have built a machine that can produce graphene sheets up to 100 meters in length (23 cm width), and the resulting sheet is the largest area graphene sheet in the world by far (the previous record was Samsung's 40" sheet from back in 2011).

Sony's new process integrates CVD and roll-to-roll, at a temperature of 1,000°C. The idea is to directly apply a current to a copper (Cu) foil that is the catalyst of graphene and a substrate for the CVD method so that only the Cu foil is heated. As a result, the thermal load on the entire machine was drastically reduced.

Researchers from Rice University and Rensselaer discovered that graphene is essentially invisible to water: when a single layer of graphene is used to cover silicon or most metals - there is almost no change in the water behavior when compared to a silicon without a graphene coating.

The researchers explain that "A drop of water sitting on a surface 'sees through' the graphene layers and conforms to the wetting forces dictated by the surface beneath. It’s quite an interesting phenomenon unseen in any other coatings and once again proves that graphene is really unique in many different ways".

A team of researchers from the University of Cambridge in the UK demonstrated ink-jet printing as a viable method for large area production of graphene devices. The team produced a graphene-based ink by liquid phase exfoliation of graphite in N-Methylpyrrolidone, and used it to print thin-film transistors. The team also printed transparent conductive patterns.

This research paves the way towards all-printed flexible and transparent graphene devices, on any substrate.

Researchers from Korea and the USA developed a Ultraviolet nitride (UV) LED that uses a few layers of graphene as a transparent conducting layer. They say that Graphene may prove better than the currently-used ITO as it performs better in terms of cost, transparency, and, heat and current spreading. Graphene also sports improved transmission in the ultraviolet.

Graphene however suffers from reliability and degradation issues that must be further investigated.

Korean researchers fabricated a stretchable, transparent graphene-based transistor. They say that the new to transistor overcomes some of the problems faced by transistors made of conventional semiconductor materials - which simply cannot be made stretchable and transparent on substrates such as rubber slabs or balloons.

To make the transistor, the researchers synthesized single layers of graphene and then stacked them layer by layer on copper foil. Using photolithography and etching techniques, the researchers patterned some of the transistor’s essential elements, including the electrodes and semiconducting channel, onto the graphene. After transferring these components onto a stretchable rubber substrate, the researchers printed the remaining components gate insulators and gate electrodes onto the device using stretchable ion gel.

The following article was sent to us by Corey McCarren and Dr. Elena Polyakova from Graphene Laboratories (a Graphene-Info sponsor), discussing Graphene Oxide and its applications:

Graphene, a multi or single layer sheet of graphite, is considered a key material in producing the next generation of low-cost carbon-based transparent and flexible electronics. Graphene is the strongest material available, as well as being highly transparent, flexible, and the best conductor of heat and electricity. Great effort is devoted to developing an effective yet inexpensive way to produce graphene materials in industrial quantities.

Dr. Wlodzimierz Strupinski from the Institute of Electronics Materials Technology says that he developed an efficient and cheap method of harvesting Graphene, using commercially available equipment. The new method relies on the crystallization of carbon from an outer source, which means the carbon is settled down in the form of a one or two atom thick layer on the surface. That's an adaptation of an existing process called epitaxy, which may lead to commercial scale production - as he uses standard commercial machinery.

Researchers from Rice University created thin hybrid metal-graphene electrodes - that outperform ITO electrodes, are also more transparent and less resistance to electric current. These electrodes can be used to create non-glass touch displays, transparent and flexible OLEDs, solar cells and lighting products.

The new electrode is a thin film of single-layer graphene and a fine grid of metal nanowire. It's basically a hybrid-graphene electrode. The metal is used to enhance the conductivity at the required transparency. The metal grid strengthens the graphene, and the graphene fills all the empty spaces between the grid. The researchers found a grid of five-micron nanowires made of inexpensive, lightweight aluminum did not detract from the material's transparency.

Researchers from the University of California, Berkeley built an optical modulator (switches light on and off) using Graphene. This is the basis of network modulators (which use light to transmit data). The graphene based modulator is the world's smallest and fastest - which could help create faster communication devices. In fact Graphene can be used to create modulators that are up to ten times faster than any current technology based modulators.

The researchers found out that tuning graphene electrically (applying voltage) causes it to absorb light in wavelength that are used for data communications (it alters the Graphene's Fermi level). At certain voltages Graphene becomes transparent, and lets light through. If you change the voltage around that level you can change whether the material is transparent or not - and basically it becomes a light switch.