Jan 31, 2014
Patterning technique prepares graphene nanoribbons for photodetector applications
Graphene nanoribbons (GNRs), quasi-one-dimensional structures of graphene, show promise over their two-dimensional (2D) counterparts for applications in electronic and optoelectronic devices. Their dimensionality allows their band-gap to be opened and their electronic properties to be tuned; greatly increasing their applicability. Now researchers at the University of Utah have developed a way to pattern large arrays of these GNRs and exploit the structure's beneficial properties in photodetectors.
Graphene, a 2D honeycomb network of carbon atoms, exhibits a high carrier mobility and a broad range of optical absorption. Despite this its zero band-gap limits its application in devices. Patterning GNRs from graphene opens the band-gap by reducing the diameter of structures to the level that induces the quantum confinement effect. This allows GNRs to be more widely applied.
Variations in dimension, edge quality and areal density of GNRs can impact their performance in devices, therefore a technology to fabricate large-area high quality narrow GNRs is essential. Reporting in Nanotechnology, the researchers have provided just that, by developing a technology to direct-write large-array 15nm graphene nanoribbons (GNR) on multilayer epitaxial graphene (EG) sheet using Focused Ion Beam (FIB).
By accelerating Ga+ ions to 30keV under vacuum in an FEI Helios NanoLab 650 dual-beam FIB machine, the researchers were able to precisely remove undesired carbon atoms with a 1.3 sputtering yield (carbon/Ga+ ratio). As shown in the diagram above, the accelerated Ga+ ion collides with and transfers part of its high kinetic energy to the carbon atom, allowing it to escape the graphene surface and eventually leave a GNR. This technology can be easily transferred to pattern other graphene nanostructures such as nanospheres, nanorings and nanoblocks.
GNRs have promising applications in photodetectors as a long-term goal is to achieve wavelength selectivity. Graphene film lacks this as it has a flat absorption spectrum, but patterning it into GNRs creates a beneficial band-gap opening and plasmonic effect that aids this type of device. Testing this, the researchers fabricated a photodetector using an array of three hundred 20nm GNRs and studied its photoresponse under different laser powers. Under zero-bias, the photoresponsivity of the device was estimated to be 7.32 mA/W.
Future research on how to further narrow down the feature size to sub-10nm and reduce pattern pitch size could lead to a direct observable band-gap and stronger plasmonic effect. This will significantly impact the development of graphene-based devices, such as further photodetectors, solar cells, light-emitting diodes (LEDs), and transistors.
More information can be found in the journal Nanotechnology 25 135301.
Making narrow nanoribbons the easy way (Aug 2013)
End-contacted graphene nanoribbons outperform side-contacted ones (April 2012)
Unzipped graphene reveals its secrets (May 2011)
New technique opens a gap in graphene (July 2010)
About the author
Principal Investigator Dr Feng Liu, Professor and Chair of the Deptartment of Materials Science and Engineering at the University of Utah and co-founder of Solan LLC, leads a multidisciplinary team in both computational and experimental researches on nanomaterials. Dr Ye Zhang, Dr Chao Hui and Dr Rujie Sun, members of the graphene FIB lithography technology development team, currently focus their research on graphene-based electronics.