Many-electron effects on optical absorption spectra of strained graphene
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We use the first-principles GW + Bethe–Salpeter equation approach to study the electronic structure and optical absorption spectra of uniaxial strained graphene. Applied strain induces an anisotropic Fermi velocity and tilts the axis of the Dirac cone. As a result, the optical response of strained graphene is dramatically changed; the optical absorption is anisotropic; the characteristic single optical absorption peak of pristine graphene is split into two peaks with enhanced excitonic effects. Within the infrared regime, the optical absorbance of uniaxial strained graphene is no longer a constant because of the broken symmetry and anisotropic excitonic effects. Within the visiblelight regime, we observe a prominent optical absorption peak due to an enhanced red shift by electron–hole interactions, enabling us to change the visible color and transparency of stretched graphene. Finally, we also reveal enhanced excitonic effects within the ultraviolet regime, where a few nearly bound excitons are identified.
I. INTRODUCTION 1–4
Graphene, a single layer of graphite, has ignited tremendous research attention because of interesting physics associated with its unusual electronic structure and promising device applications.5,6 In particular, the optical response of graphene displays many intriguing features, such as a nearly constant optical conductivity in the infrared regime and gate-dependent optical absorbance.7–11 Meanwhile, graphene exhibits outstanding mechanical properties,12,13 e.g., it can sustain a huge uniaxial stretch (up to 20%),12 placing graphene among the hardest materials known. This impressive structural modulation through applying uniaxial strain gives hope to a useful way to tailor the electrical and optical properties of graphene for broad applications. To date, in addition to experimental advances, both model and density functional theory (DFT) calculations have revealed numerous strain effects on the electronic structure and optical response of graphene.14–18 However, to thoroughly understand its optical excitations, manyelectron effects, such as electron–electron (e–e) and electron–hole (e–h) interactions, have to be included in a more accurate way because they are known to be important factors in deciding excited properties of both pristine and doped graphene.9,19–21 In particular, since a)
Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2011.412 J. Mater. Res., Vol. 27, No. 2, Jan 28, 2012
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applied strain will inevitably change the hexagonal geometry, it is of fundamental interest to study e–h pairs (excitons) and their optical activities under such an anisotropic environment, which have not been well understood yet. In this work, we apply the first-principles GW + Bethe–Salpeter equation (BSE) approach to study quasiparticle energy and optical excitations of uniaxial strained graphene. Two typical stretching directions are considered, i.e.
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