The POSTECH professor said tests have shown that the efficiency level of the magnetoresistance of graphene nanoribbons reaches into the million-percent range, compared to few hundred percent for devices created in the past.
Greater efficiency in magnetoresistance translates into smaller memory devices that can store more data.
This achievement may pave the way for the application of graphene to VLSI.
Technology update
Sep 8, 2008
STM cuts graphene to size
Researchers in Hungary and Belgium have developed the most precise nanolithography technique ever. Levente Tapasztó of the Research Institute for Technical Physics and Materials Science in Budapest and colleagues have already used the method, which employs the tip of a scanning tunnelling microscope, to pattern tiny nanostructures (ribbons) into a graphene sheet. The technique makes it possible to build entire working circuits and avoids the disadvantages of "bottom up" methods that rely on assembling individual building blocks, such as carbon nanotubes.
"We have realized truly nanometre precision lithography," Tapasztó told nanotechweb.org. "This allows us to fabricate nanostructures with the desired atomic structure and therefore good electronic properties."
The technique allows materials to be "cut" to the required shape and dimensions at the nanoscale. This is a huge step forward, says Tapasztó, because until now researchers had to rely on finding suitable blocks, like carbon nanotubes with the correct structure, to fabricate nanoscale electronic devices.
The team, which includes researchers from Facultés universitaire Notre Dame de la Paix in Namur, Belgium, made nanostructures by bombarding a graphene sheet with electrons emitted from an atomically sharp tip positioned just a few angstroms above the surface of the material. This "local access" ensures that the technique is precise. By moving the tip along a given geometry, different shapes can be patterned. A big advantage of the technique is that it allows in-situ atomic-scale resolution imaging of the sample immediately after it has been shaped.
By controlling both the width and crystallographic orientation of the nanoribbons, the method is the first to be able to fully engineer the electronic band gap of graphene. "Moreover, the accuracy of STM lithography also allows for downscaling, which is important since it enables us to open energy gaps large enough for room-temperature operation of graphene-based electronic devices," explained Tapasztó. Indeed, the team has managed to reduce a nanoribbon to just 2.5 nm wide (about 20 carbon atoms). This is far beyond the capabilities of current state-of-the-art electron beam lithography, which can only go down to around 20 nm.
Finally, "edge disorder" of ribbons is a big problem in these nanostructures because it affects their electronic properties, resulting in poor reproducibility. The new STM method produces smooth edges and so overcomes this challenge too.
"In principle, our method provides all of the solutions needed to fabricate functional nanoelectronic circuits from graphene," said Tapasztó. "It can be considered as the 'next step' in nanoscale electronic device fabrication since not only individual nanostructures but more complex nanoarchitectures can be made".
The proof-of-principle method now needs to be scaled up so that it can be used in industrial processes – a goal that is very challenging but not unrealistic, says the team.
The researchers now plan to make more complex structures from graphene and perform transport measurements to demonstrate that the circuits operate. And, although the method was originally developed for patterning graphene, it could be adapted and optimized for other materials, adds Tapasztó.
The work was reported in Nature Nanotechnology.
News
Feb 11, 2009
Argon atmosphere enhances graphene-on-SiC
Innovative carbon monolayer production method avoids high-vacuum annealing and opens the subject up to a wider range of researchers.
German and US researchers have driven forward methods for producing graphene by annealing SiC substrates, producing the largest homogeneous epitaxial domains of the carbon material yet described.
The team, led by scientists from the University of Erlangen-Nuremberg, found that heating SiC in argon produces significantly better quality graphene than methods using ultra-high vacuum conditions.
Analyses of the resulting samples at Erlangen and at the Fritz-Haber-Institute, Berlin, and Lawrence Berkeley National Laboratories, California, showed graphene terraces up to 3 µm wide and 50 µm long. This compares with 30 to 200 nm for the vacuum approach.
The group's progress towards effectively producing the material comprised of a single monolayer of graphite was published online in Nature Materials on February 8. “We can unambiguously conclude that the large atomically flat macro-terraces are homogeneously covered with a graphene monolayer,” wrote author Thomas Seyller in the paper.
“We were looking for practical ways to avoid the ultra-high vacuum environment for graphene growth, which is almost impossible to scale up to mass production,” Seyller told compoundsemiconductor.net. Using a type of vertical cold wall reactor that has previously performed post-growth annealing, it seems that the goal has been attained.
“It appears much simpler than the growth under ultra-high vacuum,” commented Pierre Mallet, a leading graphene researcher at Institut Néel in France. “I believe many labs and companies will try to synthesize their own epitaxial graphene-on-SiC using this method.”
Hot topic
Graphene is formed as silicon evaporates from the SiC substrate, and the 900 mbar argon atmosphere the researchers use reduces the rate at which this occurs. The argon deflects silicon atoms back towards the substrate, meaning that silicon desorption doesn't begin until 1500°C, compared to 1150°C for the vacuum method.
Erlangen's annealing method therefore occurs at 1680°C, enhancing diffusion of silicon and carbon atoms and improving surface morphology compared to the 1280°C vacuum method.
A homogeneous layer is important because a single monolayer of graphene is a gapless semiconductor, but additional carbon monolayers change the film's electronic structure.
Previously, large epitaxial graphene films have been formed on the transition metal ruthenium, but for this to be useful electronically the fragile graphene layer must be transferred to an insulating substrate. By using the insulating, on-axis, Si-terminated, Si(0001) wafers produced by substrate vendor SiCrystal, Seyller and his colleagues had a clear advantage.
Electron mobility measurements showed a highest value of 2000 cm2V-1s-1 for graphene grown in argon, compared to 710 cm2V-1s-1 for vacuum-grown material. This is well short of the 200,000 cm2V-1s-1 reported by the discoverers of graphene at the University of Manchester.
“Compared to exfoliated graphene, the mobility of our epitaxial graphene films is still much below expectations,” Seyller conceded. “We need to understand the reasons for this and develop strategies for improvement.”
Mallet suggests that the price of SiC substrates and the vertical cold-wall reactor used could also be offputting to some, but he is generally enthusiastic about the method.
“The impact will be very high,” Mallet said. “Not only the graphene-on-SiC community should be interested, but the entire condensed matter community.”
Regards, Hans(PhysOrg.com) said:-- Recent research into the properties of graphene nanoribbons provides two new reasons for using the material as interconnects in future computer chips. In widths as narrow as 16 nanometers, graphene has a current carrying capacity approximately a thousand times greater than copper—while providing improved thermal conductivity.
Regards, Hans(PhysOrg.com) said:One advantage of graphane is that it could actually become easier to make the tiny strips of graphene needed for electronic circuits. Such structures are currently made rather crudely by taking a sheet of the material and effectively burning away everything except the bit you need. But now such strips could be made by simply coating the whole of a graphene sheet - except for the strip itself - with hydrogen. The narrow bit left free of hydrogen is your conducting graphene strip, surrounded by a much bigger graphane area that electrons cannot go down.
physicsworld.com said:The new material is made from layers of graphene -- sheets of carbon atoms just one atom thick -- stacked on top of one another in such a way that each layer is electronically independent. The researchers claim that the material, dubbed multilayer epitaxial graphene (MEG), could be used in carbon electronics instead of costly single and double layer graphene sheets.