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Featured paper: Electronic Structure

Tunable Excitons in Biased Bilayer Graphene

 

Cheol-Hwan Park and Steven G. Louie

 

Nano Lett., 2010, 10 (2), pp 426–431

Recent measurements have shown that a continuously tunable bandgap of up to 250 meV can be generated in biased bilayer graphene [Y. Zhang et al., Nature 459, 820 (2009)], opening up pathway for possible graphene-based nanoelectronic and nanophotonic devices operating at room temperature. Here, we show that the optical response of this system is dominated by bound excitons. The main feature of the optical absorbance spectrum is determined by a single symmetric peak arising from excitons, a profile that is markedly different from that of an interband transition picture. Under laboratory conditions, the binding energy of the excitons may be tuned with the external bias going from zero to several tens of meV’s. These novel strong excitonic behaviors result from a peculiar, effective “one-dimensional” joint density of states and a continuously-tunable bandgap in biased bilayer graphene. Moreover, we show that the electronic structure (level degeneracy, optical selection rules, etc.) of the bound excitons in a biased bilayer graphene is markedly different from that of a two-dimensional hydrogen atom because of the pseudospin physics.

Featured paper: CVD growth

Wafer-Scale Synthesis and Transfer of Graphene Films

Nano Lett., 2010, 10 (2), pp 490–493

 

Youngbin Lee, Sukang Bae, Houk Jang, Sukjae Jang, Shou-En Zhu, Sung Hyun Sim, Young Il Song, Byung Hee Hong and Jong-Hyun Ahn

 

We developed means to produce wafer scale, high-quality graphene films as large as 3 in. wafer size on Ni and Cu films under ambient pressure and transfer them onto arbitrary substrates through instantaneous etching of metal layers. We also demonstrated the applications of the large-area graphene films for the batch fabrication of field-effect transistor (FET) arrays and stretchable strain gauges showing extraordinary performances. Transistors showed the hole and electron mobilities of the device of 1100 ± 70 and 550 ± 50 cm2/(V s) at drain bias of 0.75 V, respectively. The piezo-resistance gauge factor of strain sensor was 6.1. These methods represent a significant step toward the realization of graphene devices in wafer scale as well as application in optoelectronics, flexible and stretchable electronics.

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Featured paper: Mechanical Properties

Tunable Excitons in Biased Bilayer Graphene

 

Constanze Metzger, Sebastian Rmi, Mengkun Liu, Silvia V. Kusminskiy, Antonio H. Castro Neto, Anna K. Swan and Bennett B. Goldberg

 

Nano Lett., 2010, 10 (1), pp 6–10

Measurements on graphene exfoliated over a substrate prepatterned with shallow depressions demonstrate that graphene does not remain free-standing but instead adheres to the substrate despite the induced biaxial strain. The strain is homogeneous over the depression bottom as determined by Raman measurements. We find higher Raman shifts and Grneisen parameters of the phonons underlying the G and 2D bands under biaxial strain than previously reported. Interference modeling is used to determine the vertical position of the graphene and to calculate the optimum dielectric substrate stack for maximum Raman signal.

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