Spectroscopic ellipsometry was used to characterize the complex refractive index of chemical vapor deposition (CVD) graphene grown on copper foils and transferred to glass substrates. Two ellipsometers, with respective wavelength ranges extending into the ultraviolet and infrared (IR), have been used to characterize the CVD graphene optical functions. The optical absorption follows the same relation to the fine structure constant previously observed in the IR region, and displays the exciton-dominated absorption peak at ∼4.5 eV. The optical functions of CVD graphene show some differences when compared to published values for exfoliated graphene.
The field of plasmonics relies on light coupling strongly to plasmons as collective excitations. The energy loss function of graphene is dominated by two peaks at ∼5 and ∼15 eV, known as π and π + σ plasmons, respectively. We use electron energy-loss spectroscopy in an aberration-corrected scanning transmission electron microscope and density functional theory to show that between 1 to 50 eV, these prominent π and π + σ peaks are not plasmons, but single-particle interband excitations.
We find that optical second-harmonic generation (SHG) in reflection from a chemical-vapor-deposition graphene monolayer transferred onto a SiO2/Si(001) substrate is enhanced about 3 times by the flow of direct current electric current in graphene. Measurements of rotational-anisotropy SHG revealed that the current-induced SHG from the current-biased graphene/SiO2/Si(001) structure undergoes a phase inversion as the measurement location on graphene is shifted laterally along the current flow direction. The enhancement is due to current-associated charge trapping at the graphene/SiO2 interface, which introduces a vertical electric field across the SiO2/Si interface that produces electric field-induced SHG. The phase inversion is due to the positive-to-negative polarity switch in the current direction of the trapped charges at the current-biased graphene/SiO2 interface.
Graphene is a single layer of carbon atoms arranged in a hexagonal lattice. The high carrier mobility and mechanical robustness of single layer graphene make it an attractive material for "beyond CMOS" devices. The current work investigates through high-resolution transmission electron microscopy (HRTEM) image simulation the sensitivity of aberration-corrected HRTEM to the different graphene stacking configurations AAA/ABA/ABC as well as bilayers with rotational misorientations between the individual layers. High-angle annular dark field-scanning transmission electron microscopy simulation is also explored. Images calculated using the multislice approximation show discernable differences between the stacking sequences when simulated with realistic operating parameters in the presence of low random noise.
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