Scientists have managed to develop a method to produce two materials that have three layers of graphene. They developed a quasi-free-standing trilayer graphene (TL) with ABA or ABC stacking and different unique electrical possibilities. This innovational discovery can lead to the creation of novel electronic devices, for example, a photo sensor, which has the ability to transform light into electricity.
Graphene is one of the allotropic forms of carbon, a monoatomic layer of carbon atoms with a hexagonal structure. It is stronger than steel at 200 times, flexible, furthermore, it is an excellent conductor of electricity. The bilayer graphene consists of two graphene layers with AB stacking. The sheets can be put in one of two positions: AA-stacking is based on the formation of the centres of hexagons of each sheet above one another; AB-stacking is based on the placement forward the centre of the hexagon when one sheet is above a carbon atom below it. In TL graphene, the third graphene layer has 2 possible stacking consistencies - A or C, when it is put on bilayer graphene. The TL graphene has a great potential for developing various the electronic devices. Despite this fact, it has some difficulties during the manufacturing process.
Scientists successfully fabricated the quasi-free-standing TL graphene with ABA or ABC stacking grown epitaxially on hydrogen-terminated silicon carbide. The team has used 2 methods, which are based on the silicon carbide (SiC) heating, to achieve this goal. The first method is the heating of SiC up to 1,510°C under pressurized argon (under a 0.1 MPa Ar atmosphere). The second way - SiC was heated up to 1,300°C in the high vacuum (1.0 × 10−7 Torr). The most important moment is that the temperature and pressure control is the key to producing ABA and ABC phases.
Scientists sprayed ‘cracked’ hydrogen gas onto the as-grown BL graphene film in a vacuum of 1.0 × 10−3 Torr by keeping the substrate at 500 °C. The duration of hydrogen gas flow was 120 min. The cracking of hydrogen molecules into hydrogen atoms was performed by a tungsten filament heated to 1600 °C22, 23 in front of the BL graphene sample. This hydrogenation process terminates chemical bonding from the SiC substrate and simultaneously converts the buffer layer into a single graphene layer, leading to the formation of quasi-free-standing G graphene.
Scientists applied the same hydrogenation procedure to both Ar-BL and Vac-BL graphene but obtained two different types of TL graphene with ABA and ABC stacking. This finding indicates that hydrogenation is not related to selective fabrication, and more importantly, the buffer layer is already arranged in the ABA or ABC stacking sequence before hydrogenation. Other parameters observed to control the sample-growth conditions in the present study are temperature and pressure (atmosphere).
The group have tested the physical features and discovered that electrons operate in different ways. The ABA graphene was a perfect electrical conductor, alike the monolayer graphene. The ABC grapheme has semiconductor features. Consequently, this innovative development can lead to the creation of modern electronic devices.