Title: First-Principles Study of the Electronic and Magnetic Properties of Defects in Carbon Nanostructures
Author(s): Santos E.J.G., Ayuela A., Sánchez-Portal D.
Carbon Materials: Chemistry and Physics , 7, pp. 41-76 (24 April 2013)
Understanding the magnetic properties of graphenic nanostructures is instrumental in future spintronics applications. These magnetic properties are known to depend crucially on the presence of defects. Here we review our recent theoretical studies using density functional calculations on two types of defects in carbon nanostructures: substitutional doping with transition metals, and sp3-type defects created by covalent functionalization with organic and inorganic molecules. We focus on such defects because they can be used to create and control magnetism in graphene-based materials. Our main results are summarized as follows:
Substitutional metal impurities are fully understood using a model based on the hybridization between the d states of the metal atom and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes, associated with the occupation of distinct hybridization levels, which determine the magnetic properties obtained with this type of doping.
A spin moment of 1.0 μ B is always induced by chemical functionalization when a molecule chemisorbs on a graphene layer via a single C–C (or other weakly polar) covalent bond. The magnetic coupling between adsorbates shows a key dependence on the sublattice adsorption site. This effect is similar to that of H adsorption, however, with universal character.
The spin moment of substitutional metal impurities can be controlled using strain. In particular, we show that although Ni substitutionals are nonmagnetic in flat and unstrained graphene, the magnetism of these defects can be activated by applying either uniaxial strain or curvature to the graphene layer.
All these results provide key information about formation and control of defect-induced magnetism in graphene and related materials.
Title: Electric-Field Dependence of the Effective Dielectric Constant in Graphene
Author(s): Santos E.J.G., Kaxiras E.
Nano Letters, 13, No. 3, pp. 898-902 (22 January 2013)
The dielectric constant of a material is one of the fundamental features used to characterize its electrostatic properties such as capacitance, charge screening, and energy storage capability. Graphene is a material with unique behavior due to its gapless electronic structure and linear dispersion near the Fermi level, which can lead to a tunable band gap in bilayer and trilayer graphene, a superconducting-insulating transition in hybrid systems driven by electric fields, and gate-controlled surface plasmons. All of these results suggest a strong interplay between graphene properties and external electric fields. Here we address the issue of the effective dielectric constant (ε) in N-layer graphene subjected to out-of-plane (Eext⊥) and in-plane (Eext∥) external electric fields. The value of ε has attracted interest due to contradictory reports from theoretical and experimental studies. Through extensive first-principles electronic structure calculations, including van der Waals interactions, we show that both the out-of-plane (ε⊥) and the in-plane (ε∥) dielectric constants depend on the value of applied field. For example, ε⊥ and ε∥ are nearly constant (∼3 and ∼1.8, respectively) at low fields (Eext < 0.01 V/Å) but increase at higher fields to values that are dependent on the system size. The increase of the external field perpendicular to the graphene layers beyond a critical value can drive the system to a unstable state where the graphene layers are decoupled and can be easily separated. The observed dependence of ε⊥ and ε∥ on the external field is due to charge polarization driven by the bias. Our results point to a promising way of understanding and controlling the screening properties of few-layer graphene through external electric fields.
Title: Electrical Spin Switch in Hydrogenated Multilayer Graphene
Author(s): Santos E.J.G.
The Journal of Physical Chemistry C, 117, No. 12, pp. 6420-6425 (15 March 2013)
Electric field control of magnetism has become increasingly important for the next generation of graphene-based spintronic devices. We predict that the magnetic moment induced by chemisorbed H atoms on the top layer of a few-layer graphene system is tunable by an external electric field. Through accurate first-principles electronic structure calculations, we show that this magnetoelectric effect is negligible in one-layer graphene, but becomes pronounced in bilayer and trilayer graphene, saturating in magnitude in quadrilayer graphene. The effect is due to shifting of the Dirac cone of the pure graphene layers relative to the bands of the hydrogenated layer, induced by the external field. The calculated magnetoelectric coefficient (α) has values comparable to those found for ferromagnetic films or perovskite interfaces. The value of α was also used to identify a half-metallic state at low gate bias, which suggests a new class of spin-polarized materials based on hydrogenated multilayer graphene. Our results point to an experimentally feasible way to create a magnetoelectric coupling in graphene using the interplay between covalent functionalization and electric fields.
Title: Tuning the Electronic and Chemical Properties of Monolayer MoS2 Adsorbed on Transition Metal Substrates
Author(s): Chen W., Santos E.J.G., Zhu W., Kaxiras E., Zhang Z.
Nano Letters, 13, No. 2, pp. 509-514 (15 January 2013)
Using first-principles calculations within density functional theory, we investigate the electronic and chemical properties of a single-layer MoS2 adsorbed on Ir(111), Pd(111), or Ru(0001), three representative transition metal substrates having varying work functions but each with minimal lattice mismatch with the MoS2 overlayer. We find that, for each of the metal substrates, the contact nature is of Schottky-barrier type, and the dependence of the barrier height on the work function exhibits a partial Fermi-level pinning picture. Using hydrogen adsorption as a testing example, we further demonstrate that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS2 layer. The enhanced binding of hydrogen, by as much as ∼0.4 eV, is attributed in part to a stronger H–S coupling enabled by the transferred charge from the substrate to the MoS2 overlayer, and in part to a stronger MoS2-metal interface by the hydrogen adsorption. These findings may prove to be instrumental in future design of MoS2-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.
Title: Magnetoelectric effect in functionalized few-layer graphene
Author(s): Santos E.J.G.
Physical Review B, 87, No. 15, pp. 155440- (15 April 2013)
We show that the spin moment induced by sp3-type defects created by different covalent functionalizations on a few-layer graphene structure can be controlled by an external electric field. Based on ab initio density functional calculations, including van der Waals interactions, we find that this effect has a dependence on the number of stacked layers and concentration of point defects, but the interplay of both with the electric field drives the system to a half-metallic state. The calculated magnetoelectric coefficient α has a value comparable to those found for ferromagnetic thin films (e.g., Fe, Co, Ni) and magnetoelectric surfaces (e.g., CrO2). The value of α also agrees with the universal value predicted for ferromagnetic half-metals and also points to a novel route to induce half-metallicity in graphene using surface decoration.
Title: Carrier-Mediated Magnetoelectric Coupling in Functionalized Graphene
Author(s): Santos E.J.G.
ASC Nano, 7, No. 11, pp. 9927-9932 (11 November 2013)
Materials in which magnetic order and electric fields can be coupled are of high fundamental and technological interests. Electrical control of magnetism is not only important for ultralow power consumption applications, but also enables control over intrinsic material properties that may have a major step in new developments in spintronic and magnetoelectric devices. Here we show that the magnetism induced by aryl-radicals covalently functionalized on top of multilayer graphene is sensitive to external electric fields which coupled to the interlayer charge-imbalance yields a strong magnetoelectric coupling. We used first-principles simulations, taking into account van der Waals dispersion forces, to show that this effect is thickness-dependent: it increases dramatically to thicker graphene structures reaching magnetoelectric coefficients comparable to perovskite interfaces. The interplay between electric fields and magnetism also leads functionalized graphene layers to a fully polarized spin state (half-metallicity). Efficiency nearly to 100% spin-polarization is observed at low electric bias, and the selection of the spin-conducting channel is determined by the field polarization.
Title: Electrically Driven Tuning of the Dielectric Constant in MoS2 Layers
Author(s): Santos E.J.G., Kaxiras E.
ACS Nano, 7, No. 12, pp. 10741-10746 (11 November 2013)
The properties of two-dimensional materials, such as molybdenum disulfide, will play an important role in the design of the next generation of electronic devices. Many of those properties are determined by the dielectric constant which is one of the fundamental quantities used to characterize conductivity, refractive index, charge screening, and capacitance. We predict that the effective dielectric constant (ε) of few-layer MoS2 is tunable by an external electric field (Eext). Through first-principles electronic structure calculations, including van der Waals interactions, we show that at low fields (Eext < 0.01 V/Å) ε assumes a nearly constant value ∼4 but increases at higher fields to values that depend on the layer thickness. The thicker the structure, the stronger the modulation of ε with the electric field. Increasing of the external field perpendicular to the dichalcogenide layers beyond a critical value can drive the system to an unstable state where the layers are weakly coupled and can be easily separated. The observed dependence of ε on the external field is due to charge polarization driven by the bias. Implications on the optical properties as well as on the electronic excitations are also considered. Our results point to a promising way of understanding and controlling the screening properties of MoS2 through external electric fields.