Graphene and its few layer cousins are unique two-dimensional (2D) systems with extraordinary electrical, thermal, mechanical and optical properties, and they have become both fantastic platforms for exploring fundamental processes and some of the most promising material for next generation electronics. Here we present our transport studies of dual gated suspended bilayer and trilayer graphene devices. At the charge neutrality point, application of an electric field induces a gap in bilayer graphene’s band structure. For high mobility bilayer devices, we observe an intrinsic insulating state with a gap of 2-3 meV and a transition temperature ~5K, which arises from electronic interactions. In ABC-stacked trilayer devices, an insulating state with gap ~25 meV is observed. Our results underscore the rich interaction-induced collective states in few layer graphene and suggest a promising direction for THz technology and high speed low dissipation electronic devices.
Graphene is a promising material for optoelectronics and photonics. Recent experiments demonstrated graphene
photodectectors based on interband transitions working at Mid and Near-IR/Visible regions. Extension of spectral
range to longer wavelengths requires alternative photoresponse mechanisms. One of the mechanisms which has
been proven to be efficient for THz detection in "classical" semiconductor materials is the optically-induced
breakdown of quantum Hall effect. In our work we successfully demonstrated a graphene-based QHE
photodetector. Our result demonstrates the potential of graphene as a material for Far-IR photodetectors. Further
improvement in device design and use of more efficient radiation coupling solutions should enable graphene
photodetectors with broader spectral range, higher sensitivity, and elevated operating temperatures for a variety of
applications.
Spin transport in graphene devices has been investigated in both local and non-local spin valve geometries. In the nonlocal
measurement, spin transport and spin precession in single layer and bilayer graphene have both been achieved with
transparent Co contacts. Gate controllable non-local spin signal was also demonstrated in this system. For the local
graphite spin valve device, we report MR up to 12% for devices with tunneling contacts. We observe a correlation
between the nonlinearity of the I-V curve and the presence of local MR and conclude that tunnel barriers can be
employed to surmount the conductance mismatch problem in this system. These studies indicate that the improvement of
tunnel barriers on graphene, especially to inhibit the formation of pinholes, is an important step to achieve more efficient
spin injection into graphene.
The unique properties of graphene have recently attracted major attention leading to proposals in electronic,
optoelectronic, and detector applications. Micro-Raman spectroscopy has been utilized as a convenient tool for
identifying graphene layers. Most Raman studies were limited to layers on silicon substrates with an oxide layer
thickness of 300 nm, rendering graphene visible under an optical microscope. The development of graphene technology
requires its integration with different materials and strict control of the number of layers and defects. Thus it is
important to extend the nanometrology capabilities of Raman spectroscopy to arbitrary substrates and temperatures.
Here we report that the deconvolution of the 2D band allows one to count graphene layers even when placed on
"inconvenient" substrates such as glass or sapphire. We also show that even small excitation laser power typically used
for Raman spectroscopy may lead to strong heating in graphene. The determined temperature coefficients for graphene
allowed us to evaluate the temperature rise and decouple the temperature effects from those due to variations in the
graphene edges or substrates.
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