DOE OSTI2017
Graphene has attracted great interest for future electronics due to its high mobility and high thermal conductivity. However, a two-dimensional graphene sheet behaves like a metal, lacking a bandgap needed for the key devices components such as field effect transistors (FETs) in digital electronics. It has been shown that, partly due to quantum confinement, graphene nanoribbons (GNRs) with ~2 nm width can open up sufficient bandgaps and evolve into semiconductors to exhibit high on/off ratios useful for FETs. However, a challenging problem has been that, such ultra-narrow GNRs (~2 nm) are difficult to fabricate, especially for GNRs with smooth edges throughout the ribbon length. Despite high on/off ratios, these GNRs show very low mobility and low on-state conductance due to dominant scattering effects by imperfections and disorders at the edges. Wider GNRs (>5 nm) show higher mobility, higher conductance but smaller bandgaps and low on/off ratios undesirable for FET applications. It is highly desirable to open up bandgaps in graphene or increase the bandgaps in wide GNRs to afford graphene based semiconductors for high performance (high on-state current and high on/off ratio) electronics. Large scale ordering and dense packing of such GNRs in parallel are also needed for device integration but have also been challenging thus far. It has been shown theoretically that uniaxial strains can be applied to a GNR to engineer its bandgap. The underlying physics is that under uniaxial strain, the Dirac point moves due to stretched C-C bonds, leading to an increase in the bandgap of armchair GNRs by up to 50% of its original bandgap (i.e. bandgap at zero strain). For zigzag GNRs, due to the existence of the edge states, changes of bandgap are smaller under uniaxial strain and can be increased by ~30%. This work proposes a novel approach to the fabrication of densely aligned graphene nanoribbons with highly smooth edges afforded by anisotropic etching and uniaxial strain for bandgap engineering of GNRs towards high on/off ratio and high on-state current GNR devices. First, we will develop a novel approach for the fabrication of high density GNR arrays (pitch <50 nm, tunable down to 30nm) with pre-defined edge orientation and smooth edges using a free standing nano-mask derived from diblock copolymer assembly for patterning of graphene sheets. Anisotropic graphene edges will be developed to afford smooth edges along crystallographic lattice directions. Then, we will fabricate GNR devices on flexible substrates and apply uniaxial strain to engineer the bandgap of the GNRs. The bandgap of GNRs could be increased by up to 50% under uniaxial strain according to theoretical calculations and will be investigated through electrical transport measurements. Micro-Raman spectroscopy of single GNRs and parallel arrays will be used to probe and quantify the uniaxial strain. Electrical measurements will be used to probe the on/off ratio of GNR FET devices and confirm the bandgap tuning effects. Finally, we plan to use dense parallel arrays of GNRs to demonstrate strained GNR field effect transistors with high on/off ratios and high on-state current, and compare strained GNR FETs with carbon nanotube and Si based field effect transistor (FET) devices.