Band gap - Page 7

Researchers from Rice University discovered a way to create Bernal-stacked bi-layer graphene sheets. These kinds of structures (in which every other carbon atom in the six-carbon rings of the top graphene layer sits over the middle of the hexagonal space created by a six-carbon ring of the bottom layer) exhibit a small band gap.

To create these sheets with controlled thickness on copper substrates, the researchers used pressure-tuned CVD chambers, while keeping constant the pressure ratio of hydrogen to methane. The higher the pressure, the thicker the graphene film. They created all sorts of sheets, and the bi-layer ones indeed were Bernal-stacked. They created a transistor and checked the electronic properties to make sure there's a band gap.

Researchers from the Georgia Institute of Technology managed to create an substantial electronic bandgap in graphene nanoribbons - by coating bi-layer graphene on silicon carbide nanometer-scale steps. This could lead the way towards graphene based electronics.

The 1.4-nanometer ribbons created a bandgap of about 0.5 electron-volts. The researchers do not yet understand why the bent graphene creates the bandgap.

This is a guest post by Thanasis Georgiou, the Director of MoS2 Crystals, a Manchester based MoS2 Crystals and flakes provider.

Graphene has mesmerized the labs of hundreds of research groups worldwide. With its exceptional set of properties it is widely expected that it would find applications in a variety of areas. However, looking back on the work of the Nobel Laureates, their highly cited PNAS publication was entitled Two-dimensional atomic crystals(Novoselov et al., 2005). Indeed, graphene was just the first of many different crystalline materials that can be exfoliated and investigated. Graphene justifiably attracted such intense research due to its exotic Dirac-cone nature of its electronic spectrum.

Researchers from IBM and University of California Riverside managed to make the slimmest graphene nanoribbon (GNR) ever - just 10 nm in width. Making one is virtually impossible, and the team created a large number of GNRs in parallel. The researchers say that the arrays cover about 50% of the prototype device channel area, which means that integrated circuits based on GNRs with the required high current densities are now possible. The narrow GNRs have a bandgap of about 0.2 eV.

The process the researchers used consists of two main steps: a top-down e-beam lithography step and a bottom-up self-assembly step involving a block copolymer template comprising alternating lamellae of the polymers PS and PMMA.

Researchers from Europe say they have managed to synthesize silicene - a new Silicon allotrope that forms 2D single-atom sheets. Silicene could rival graphene and can be used to create transistors easily compared to graphene (which has no band gap). The researchers grew the silicene on silver substrates. Some researchers already claimed to have made silicene, but now it is the first time that there is microscopic proof.

In a silicene sheet some atoms are arranged above and below the main "panel" (this is called a buckled honeycomb structure). This creates the band gap and so silicene can be used as an on/off transistor.

Researchers from the Rensselaer Polytechnic Institute have discovered new graphene based materials that can be customized to produce specific band gap and magnetic properties (i.e. have tunable functionality in electronics). The materials may be used to enable new nanoelectronics, optics, and spintronics devices.

The researchers found out that graphitic nanoribbons can be segmented into several different surface structures called nanowiggles. Each of these structures produces highly different magnetic and conductive properties. This means that you can basically create a new graphene nanostructure that is customized for a specific task or device.

IBM has managed to produce RF integrated circuits on an 8" graphene wafer. IBM says that this demonstration is a "major step in transitioning this promising material from a scientific curiosity into a real technology". The graphene was grown on copper foil from high-temperature vapor and later coated with the polymer PMMA.

These are RF devices - as it's still difficult to create logic using graphene (it has no natural bandgap), although some researchers are working towards methods to fix this issue.

Foa Torres, a scientist from the National University of Córdoba in Argentina, says that Graphene's Achilles heel is the fact that it does not have a band gap. This means that Graphene cannot be 'turned-off', and so you can't use it for active electronic devices such as switches and transistors.

Foa Torres predicts that shining a mid-infrared laser on graphene can produce band gaps in its electronic structure, and this band gap could be tuned by controlling the laser polarization. The next crucial step is experimental verification.

Researchers from the National Institute of Standards and Technology (NIST) have shown that the electronic properties of two layers of graphene vary on the nanometer scale - not only in the strength of the electric charges between the two layers but they also actually reverse in sign to create randomly distributed puddles of alternating positive and negative charges.

This means that bi-layer graphene (two stacked graphene sheets) acts more like a semiconductor that a single sheet. A band gap may also form on its own due to variations in the sheets' electrical potential caused by interactions among the graphene electrons or with the substrate.

Researchers from the Beijing National Laboratory for Condensed Matter Physics and Peking University in China developed a simple and scalable method for manufacturing graphene nanoribbons. The idea is to cover a sheet of graphene with 1 µm polysterene spheres, which self-assembled on the surface into tightly packed arrays, and then etching with oxygen plasma.

The etching process patterned the graphene into complex shapes including dumbbells (see photo above), ribbons, chains and polygonal rings. Using different etching time and sphere packing order you can control which shape you'll get. When you wash away the spheres, you can recover the sheet in the shape you created. The narrow ribbons exhibit a bandgap, as opposed to large graphene sheets, and this can make them useful for electronic components.