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Register a new userLayer breathing and shear modes in multilayer graphene: A DFT-vdW study
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In this work, we study structural and vibrational properties of multilayer graphene using density-functional theory (DFT) with van der Waals (vdW) functionals. Initially, we analyze how different vdW functionals compare by evaluating the lattice parameters, elastic constants and vibrational frequencies of low energy optical modes of graphite. Our results indicate that the vdW-DF1-optB88 functional has the best overall performance on the description of vibrational properties. Next, we use this functional to study the influence of the vdW interactions on the structural and vibrational properties of multilayer graphene. Specifically, we evaluate binding energies, interlayer distances and phonon frequencies of layer breathing and shear modes. We observe excellent agreement between our calculated results and available experimental data, which suggests that this functional has truly predictive power for layer-breathing and shear frequencies that have not been measured yet. This indicates that careful selected vdW functionals can describe interlayer bonding in graphene-related systems with good accuracy.
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Atomically thin NbSe2 is a metallic layered transition metal dichalcogenide (TMD) with considerably different crystallographic structure and electronic properties from other TMDs, such as MoS2, MoSe2, WS2 and WSe2. Properties of TMD atomic layers are sensitive to interlayer coupling. Here we investigate the interlayer phonons of few-layer NbSe2 by ultralow-frequency Raman spectroscopy. We observe both the interlayer breathing modes and shear modes at frequencies below 40 cm-1 for samples of 2 to 15 layers. Their frequency, Raman activity, and environmental instability depend systematically on the layer number. We account for these results utilizing a combination of the linear-chain model, group-theory analysis and first-principles calculations. Although NbSe2 possesses different stacking order from MoS2, MoSe2, WS2 and WSe2, it exhibits the same symmetry and Raman selection rules, as well as similar interlayer coupling strength and thickness dependence of interlayer phonon modes.
The results of micro-Raman scattering measurements performed on three different ``graphitic materials: micro-structured disks of highly oriented pyrolytic graphite, graphene multi-layers thermally decomposed from carbon terminated surface of 4H-SiC and an exfoliated graphene monolayer are presented. Despite its multi-layer character, most parts of the surface of the graphitized SiC substrates shows a single-component, Lorentzian shape, double resonance Raman feature in striking similarity to the case of a single graphene monolayer. Our observation suggests a very weak electronic coupling between graphitic layers on the SiC surface, which therefore can be considered to be graphene multi-layers with a simple (Dirac-like) band structure.
Viscous phenomena are the hallmark of the hydrodynamic flow exhibited by Dirac fermions in clean graphene at high enough temperatures. We report a quantitative calculation of the electronic shear and Hall viscosities in graphene based on the kinetic theory combined with the renormalization group providing a unified description at arbitrary doping levels and non-quantizing magnetic fields. At charge neutrality, the Hall viscosity vanishes, while the field-dependent shear viscosity decays from its zero-field value saturating to a nonzero value in classically strong fields. Away from charge neutrality, the field-dependent viscosity coefficients tend to agree with the semiclassical expectation.
Fluorine adatoms on graphene induce local changes in electronic and magnetic properties, and subtle correlation effects. We investigate the GGA and GGA+U approaches as possible solutions to describe the magnetic moment and electronic band structure of graphene sheets with fluorine adatoms, and compare to experiments. We show that, due to a lack of strong electronic correlations, GGA fails to reproduce the measured magnetic moment in this structure. In particular, the GGA incorrectly predicts a nonmagnetic ground state with a zero band gap. On the other hand, GGA+U is a computationally efficient tool which provides physically reasonable properties. Using Hubbard U and exchange J parameters of 5 eV and 0.1 eV provides a magnetic moment and optical gap in agreement with experiments. Our results imply that the magnetic moment observed in the experiment is injected by fluorine in carbon pz orbitals throughout the graphene sheet. The spin- orbit coupling (SOC) has almost no influence (ca. 2%) on the magnetism. No Rashba effect is detected and the magnetic moment induced by fluorine strongly dominates the electronic properties. Our findings explain the anisotropic magnetic behavior observed experimentally.
With the motivation of improving the performance and reliability of aggressively scaled nano-patterned graphene field-effect transistors, we present the first systematic experimental study on charge and current distribution in multilayer graphene field-effect transistors. We find a very particular thickness dependence for Ion, Ioff, and the Ion/Ioff ratio, and propose a resistor network model including screening and interlayer coupling to explain the experimental findings. In particular, our model does not invoke modification of the linear energy-band structure of graphene for the multilayer case. Noise reduction in nano-scale few-layer graphene transistors is experimentally demonstrated and can be understood within this model as well.
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