Gianluca Giovannetti

Doping Graphene with Metal Contacts

Making devices with graphene necessarily involves making contacts with metals. We use density functional theory to study how graphene is doped by adsorption on metal substrates and find that weak bonding on Al, Ag, Cu, Au, and Pt, while preserving its unique electronic structure, can still shift the Fermi level with respect to the conical point by approximately 0.5 eV. At equilibrium separations, the crossover from p-type to n-type doping occurs for a metal work function of approximately 5.4 eV, a value much larger than the graphene work function of 4.5 eV. The numerical results for the Fermi level shift in graphene are described very well by a simple analytical model which characterizes the metal solely in terms of its work function, greatly extending their applicability.

Substrate-induced band gap in graphene on hexagonal boron nitride: Ab initio density functional calculations

We determine the electronic structure of a graphene sheet on top of a lattice-matched hexagonal boron nitride (h-BN) substrate using ab initio density functional calculations. The most stable configuration has one carbon atom on top of a boron atom, the other centered above a BN ring. The resulting inequivalence of the two carbon sites leads to the opening of a gap of 53 meV at the Dirac points of graphene and to finite masses for the Dirac fermions. Alternative orientations of the graphene sheet on the BN substrate generate similar band gaps and masses. The band gap induced by the BN surface can greatly improve room temperature pinch-off characteristics of graphene-based field effect transistors.

First-principles study of the interaction and charge transfer between graphene and metals

Measuring the transport of electrons through a graphene sheet necessarily involves contacting it with metal electrodes. We study the adsorption of graphene on metal substrates using first-principles calculations at the level of density-functional theory. The bonding of graphene to Al, Ag, Cu, Au, and Pt (111) surfaces is so weak that its unique “ultrarelativistic” electronic structure is preserved. The interaction does, however, lead to a charge transfer that shifts the Fermi level by up to 0.5 eV with respect to the conical points. The crossover from p-type to n-type doping occurs for a metal with a work function ~5.4 eV, a value much larger than the work function of free-standing graphene, 4.5 eV. We develop a simple analytical model that describes the Fermi-level shift in graphene in terms of the metal substrate work function. Graphene interacts with and binds more strongly to Co, Ni, Pd, and Ti. This chemisorption involves hybridization between graphene pz states and metal d states that opens a band gap in graphene, and reduces its work function considerably. The supported graphene is effectively n-type doped because in a current-in-plane device geometry the work-function lowering will lead to electrons being transferred to the unsupported part of the graphene sheet.

Above-room-temperature ferroelectricity in a single-component molecular crystal

Ferroelectrics are electro-active materials that can store and switch their polarity (ferroelectricity), sense temperature changes (pyroelectricity), interchange electric and mechanical functions (piezoelectricity), and manipulate light (through optical nonlinearities and the electro-optic effect): all of these functions have practical applications. Topological switching of pi-conjugation in organic molecules, such as the keto-enol transformation, has long been anticipated as a means of realizing these phenomena in molecular assemblies and crystals. Croconic acid, an ingredient of black dyes, was recently found to have a hydrogen-bonded polar structure in a crystalline state. Here we demonstrate that application of an electric field can coherently align the molecular polarities in crystalline croconic acid, as indicated by an increase of optical second harmonic generation, and produce a well-defined polarization hysteresis at room temperature. To make this simple pentagonal molecule ferroelectric, we switched the pi-bond topology using synchronized proton transfer instead of rigid-body rotation. Of the organic ferroelectrics, this molecular crystal exhibits the highest spontaneous polarization ( approximately 20 muC cm(-2)) in spite of its small molecular size, which is in accord with first-principles electronic-structure calculations. Such high polarization, which persists up to 400 K, may find application in active capacitor and nonlinear optics elements in future organic electronics.Ferroelectrics are electro-active materials that can store and switch their polarity (ferroelectricity), sense temperature changes (pyroelectricity), interchange electric and mechanical functions (piezoelectricity), and manipulate light (through optical nonlinearities and the electro-optic effect): all of these functions have practical applications. Topological switching of π-conjugation in organic molecules, such as the keto-enol transformation, has long been anticipated as a means of realizing these phenomena in molecular assemblies and crystals. Croconic acid, an ingredient of black dyes, was recently found to have a hydrogen-bonded polar structure in a crystalline state. Here we demonstrate that application of an electric field can coherently align the molecular polarities in crystalline croconic acid, as indicated by an increase of optical second harmonic generation, and produce a well-defined polarization hysteresis at room temperature. To make this simple pentagonal molecule ferroelectric, we switched the π-bond topology using synchronized proton transfer instead of rigid-body rotation. Of the organic ferroelectrics, this molecular crystal exhibits the highest spontaneous polarization (∼20 μC cm-2) in spite of its small molecular size, which is in accord with first-principles electronic-structure calculations. Such high polarization, which persists up to 400 K, may find application in active capacitor and nonlinear optics elements in future organic electronics.

Diisopropylammonium Bromide Is a High-Temperature Molecular Ferroelectric Crystal

Environmentally Friendly Ferroelectrics Ferroelectrics—which are widely used as piezo elements, sensors, and actuators—maintain charge polarization even in the absence of an external electric field. The best ferroelectric properties are found in perovskites such as barium titanate (BTO) and lead zirconate titanate; however, environmentally friendly, lead-free alternatives are highly desirable. Fu et al. (p. 425; see the Perspective by Bonnell) find that the organic molecular crystal diisopropylammonium bromide has ferroelectric properties comparable to those of BTO and may represent a viable alternative to perovskites. An organic molecular crystal is found to have ferroelectric properties comparable to those of barium titanate. [Also see Perspective by Bonnell] Molecular ferroelectrics are highly desirable for their easy and environmentally friendly processing, light weight, and mechanical flexibility. We found that diisopropylammonium bromide (DIPAB), a molecular crystal processed from aqueous solution, is a ferroelectric with a spontaneous polarization of 23 microcoulombs per square centimeter [close to that of barium titanate (BTO)], high Curie temperature of 426 kelvin (above that of BTO), large dielectric constant, and low dielectric loss. DIPAB exhibits good piezoelectric response and well-defined ferroelectric domains. These attributes make it a molecular alternative to perovskite ferroelectrics and ferroelectric polymers in sensing, actuation, data storage, electro-optics, and molecular or flexible electronics.

Theoretical prediction of perfect spin filtering at interfaces between close-packed surfaces of Ni or Co and graphite or graphene

The in-plane lattice constants of close-packed planes of fcc and hcp Ni and Co match that of graphite almost perfectly so that they share a common two-dimensional reciprocal space. Their electronic structures are such that they overlap in this reciprocal space for one spin direction only allowing us to predict perfect spin filtering for interfaces between graphite and (111) fcc or (0001) hcp Ni or Co. First-principles calculations of the scattering matrix show that the spin filtering is quite insensitive to amounts of interface roughness and disorder which drastically influence the spin-filtering properties of conventional magnetic tunnel junctions or interfaces between transition metals and semiconductors. When a single graphene sheet is adsorbed on these open d-shell transition-metal surfaces, its characteristic electronic structure, with topological singularities at the K points in the two-dimensional Brillouin zone, is destroyed by the chemical bonding. Because graphene bonds only weakly to Cu which has no states at the Fermi energy at the K point for either spin, the electronic structure of graphene can be restored by dusting Ni or Co with one or a few monolayers of Cu while still preserving the ideal spin-injection property.

Selective Mott Physics as a Key to Iron Superconductors

We show that electron- and hole-doped BaFe(2)As(2) are strongly influenced by a Mott insulator that would be realized for half-filled conduction bands. Experiments show that weakly and strongly correlated conduction electrons coexist in much of the phase diagram, a differentiation which increases with hole doping. This selective Mottness is caused by the Hunds coupling effect of decoupling the charge excitations in different orbitals. Each orbital then behaves as a single-band doped Mott insulator, where the correlation degree mainly depends on how doped is each orbital from half filling. Our scenario reconciles contrasting evidences on the electronic correlation strength, implies a strong asymmetry between hole and electron doping, and establishes a deep connection with the cuprates.

Multiferroicity in rare-earth nickelates RNiO3

We show that charge ordered rare-earth nickelates of the type RNiO3 (R = Ho, Lu, Pr and Nd) are multiferroic with very large magnetically-induced ferroelectric (FE) polarizations. This we determine from first principles electronic structure calculations. The emerging FE polarization is directly tied to the long-standing puzzle of which kind of magnetic ordering is present in this class of materials: its direction and size indicate the type of ground-state spin configuration that is realized. Vice versa, the small energy differences between the different magnetic orderings suggest that a chosen magnetic ordering can be stabilized by cooling the system in the presence of an electric field.

Electronic correlation effects in superconducting picene from ab initio calculations

We show, by means of ab-initio calculations, that electron-electron correlations play an important role in potassium-doped picene (

Dipole Formation at Interfaces of Alkanethiolate Self-assembled Monolayers and Ag(111)

K_x