Recent Publications

Elton J Santos

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  1. Title: Efficient Blue Electroluminescence Using Quantum-Confined Two-Dimensional Perovskites

    Author(s): Kumar S., Jagielski J., Yakunin S., Rice P., Chiu Y., Wang M., Nedelcu G., Kim Y., Lin S., Santos E.J.G., Kovalenko M.V., Shih C.,

    ACS Nano (29 September 2016)

    doi: 10.1021/acsnano.6b05775
    Abstract


    Full Text

    Solution-processed hybrid organic–inorganic lead halide perovskites are emerging as one of the most promising candidates for low-cost light-emitting diodes (LEDs). However, due to a small exciton binding energy, it is not yet possible to achieve an efficient electroluminescence within the blue wavelength region at room temperature, as is necessary for full-spectrum light sources. Here, we demonstrate efficient blue LEDs based on the colloidal, quantum-confined 2D perovskites, with precisely controlled stacking down to one-unit-cell thickness (n = 1). A variety of low-k organic host compounds are used to disperse the 2D perovskites, effectively creating a matrix of the dielectric quantum wells, which significantly boosts the exciton binding energy by the dielectric confinement effect. Through the Förster resonance energy transfer, the excitons down-convert and recombine radiatively in the 2D perovskites. We report room-temperature pure green (n = 7–10), sky blue (n = 5), pure blue (n = 3), and deep blue (n = 1) electroluminescence, with record-high external quantum efficiencies in the green-to-blue wavelength region.

  2. Title: Multiscale Analysis for Field-Effect Penetration through Two-Dimensional Materials

    Author(s): Tian T., Rice P. Santos E.J.G., Shih C.,

    ACS Nano Letters, 16, No. 8, pp. 5044-5052 (10 August 2016)

    doi: 10.1021/acs.nanolett.6b01876
    Abstract


    Full Text

    Gate-tunable two-dimensional (2D) materials-based quantum capacitors (QCs) and van der Waals heterostructures involve tuning transport or optoelectronic characteristics by the field effect. Recent studies have attributed the observed gate-tunable characteristics to the change of the Fermi level in the first 2D layer adjacent to the dielectrics, whereas the penetration of the field effect through the one-molecule-thick material is often ignored or oversimplified. Here, we present a multiscale theoretical approach that combines first-principles electronic structure calculations and the Poisson–Boltzmann equation methods to model penetration of the field effect through graphene in a metal–oxide–graphene–semiconductor (MOGS) QC, including quantifying the degree of “transparency” for graphene two-dimensional electron gas (2DEG) to an electric displacement field. We find that the space charge density in the semiconductor layer can be modulated by gating in a nonlinear manner, forming an accumulation or inversion layer at the semiconductor/graphene interface. The degree of transparency is determined by the combined effect of graphene quantum capacitance and the semiconductor capacitance, which allows us to predict the ranking for a variety of monolayer 2D materials according to their transparency to an electric displacement field as follows: graphene > silicene > germanene > WS2 > WTe2 > WSe2 > MoS2 > phosphorene > MoSe2 > MoTe2, when the majority carrier is electron. Our findings reveal a general picture of operation modes and design rules for the 2D-materials-based QCs.

  3. Title: Dielectric Screening in Atomically Thin Boron Nitride Nanosheets

    Author(s): Li L.H., Santos E.J.G., Xing T., Cappelluti E., Roldán R., Chen Y., Watanabe K., Taniguchi T.

    Nano Letters, 15, No. 1, pp. 218-223 (2 December 2015)

    doi: 10.1021/nl503411a
    Abstract



    Two-dimensional (2D) hexagonal boron nitride (BN) nanosheets are excellent dielectric substrate for graphene, molybdenum disulfide, and many other 2D nanomaterial-based electronic and photonic devices. To optimize the performance of these 2D devices, it is essential to understand the dielectric screening properties of BN nanosheets as a function of the thickness. Here, electric force microscopy along with theoretical calculations based on both state-of-the-art first-principles calculations with van der Waals interactions under consideration, and nonlinear Thomas–Fermi theory models are used to investigate the dielectric screening in high-quality BN nanosheets of different thicknesses. It is found that atomically thin BN nanosheets are less effective in electric field screening, but the screening capability of BN shows a relatively weak dependence on the layer thickness.

  4. Title: Electric Field Effects on Graphene Materials

    Author(s): Santos E.J.G.

    Carbon Materials: Chemistry and Physics , 8, pp. 383-391 (8 March 2015)

    doi: 10.1007/978-94-017-9567-8_14
    Abstract



    Understanding the effect of electric fields on the physical and chemical properties of two-dimensional (2D) nanostructures is instrumental in the design of novel electronic and optoelectronic devices. Several of those properties are characterized in terms of the dielectric constant which play an important role on capacitance, conductivity, screening, dielectric losses and refractive index. Here we review our recent theoretical studies using density functional calculations including van der Waals interactions on two types of layered materials of similar two-dimensional molecular geometry but remarkably different electronic structures, that is, graphene and molybdenum disulphide (MoS2). We focus on such two-dimensional crystals because of they complementary physical and chemical properties, and the appealing interest to incorporate them in the next generation of electronic and optoelectronic devices. We predict that the effective dielectric constant (ε) of few-layer graphene and MoS2 is tunable by external electric fields (E ext). We show that at low fields (E ext < 0.01 V/Å) ε assumes a nearly constant value ∼4 for both materials, but increases at higher fields to values that depend on the layer thickness. The thicker the structure the stronger is the modulation of ε with the electric field. Increasing of the external field perpendicular to the layer surface above a critical value can drive the systems 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, which show several similar characteristics despite of the layer considered. All these results provide key information about control and understanding of the screening properties in two-dimensional crystals beyond graphene and MoS2.

  5. Title: A boron nitride - graphene - C60 heterostructure

    Author(s): Ojeda-Aristizabal C., Santos E.J.G., Onishi S., Rasool H., Velasco J. Jr., Kahn S., Yan A., Zettl A.

    APS March Meeting 2015, abstract #S16.004 (2015)

    doi: 2015APS..MARS16004O
    Abstract



    We have fabricated a new van-der-Waals heterostructure composed by BN/graphene/C60. We performed transport measurements on the preliminary BN/graphene device finding a sharp Dirac point at the neutrality point. After the deposition of a C60 thin film by thermal evaporation, we have observed a significant n-doping of the heterostructure. This suggests an unusual electron transfer from C60 into the BN/graphene structure. This BN/graphene/C60 heterostructure can be of interest in photovoltaic applications. It can be used to build devices like p-n junctions, where C60 can be easily deposited in defined regions of a graphene junction by the use of a shadow mask. Our results are contrasted with theoretical calculations.

  6. Title: Abstract: M8.00007 : Graphene Template for Epitaxial Growth of Pentacene and C60 Thin Film

    Author(s): Kwanpyo K., Santos E.J.G., Lee T.H., Nishi Y., Bao Z.

    Bulletin of the American Physical Society, 60, No. 1 (2015)

    Abstract


    The study and reliable control of molecular packing structures at the graphene-molecule interface are of great importance for various applications. We utilize suspended graphene as an assembly template to investigate thin-film epitaxial growth of various organic molecules. Thin-film packing structures of pentacene and C60 on graphene are investigated using transmission electron microscopy. For pentacene thin-film, we observe an unusual polymorph growth on graphene, which shows significant strain along the c-axis of pentacene crystals. Moreover, the strained film exhibits a specific molecular orientation and a strong azimuthal correlation with underlying graphene lattice. For C60 crystals, we observe large grain sizes and somewhat strong azimuthal correlation with respect to underlying graphene lattice direction. Utilizing \textit{ab initio} electronic structure calculations with van der Waals interactions, we understand the observed molecular growth behavior mainly with graphene-molecule interaction.

  7. Title: Epitaxially Grown Strained Pentacene Thin Film on Graphene Membrane

    Author(s): Kim K., Santos E.J.G., Lee T.H., Nishi Y., Bao Z.

    Small, 11, No. 17, pp. 2037-2043 (7 January 2015)

    doi: 10.1002/smll.201403006
    Abstract



    Organic-graphene system has emerged as a new platform for various applications such as flexible organic photovoltaics and organic light emitting diodes. Due to its important implication in charge transport, the study and reliable control of molecular packing structures at the graphene–molecule interface are of great importance for successful incorporation of graphene in related organic devices. Here, an ideal membrane of suspended graphene as a molecular assembly template is utilized to investigate thin-film epitaxial behaviors. Using transmission electron microscopy, two distinct molecular packing structures of pentacene on graphene are found. One observed packing structure is similar to the well-known bulk-phase, which adapts a face-on molecular orientation on graphene substrate. On the other hand, a rare polymorph of pentacene crystal, which shows significant strain along the c-axis, is identified. In particular, the strained film exhibits a specific molecular orientation and a strong azimuthal correlation with underlying graphene. Through ab initio electronic structure calculations, including van der Waals interactions, the unusual polymorph is attributed to the strong graphene–pentacene interaction. The observed strained organic film growth on graphene demonstrates the possibility to tune molecular packing via graphene–molecule interactions.

  8. Title: Toward Controlled Growth of Helicity-Specific Carbon Nanotubes

    Author(s): Santos E.J.G., Nørskov J.K., Harutyunyan A.R., Abild-Pedersen F.

    The Journal of Physical Chemistry Letters, 6, pp. 2232-2237 (28 May 2015)

    doi: 10.1021/acs.jpclett.5b00880
    Abstract



    The underlying mechanisms for the nucleation of carbon nanotubes as well as their helicity, remain elusive. Here, using van der Waals dispersion force calculations implemented within density functional theory, we study the cap formation, believed to be responsible for the chirality of surface-catalyzed carbon nanotubes. We find the energetics associated with growth along different facets to be independent of the surface orientation and that the growth across an edge along the axis of the metal particle leads to a perfect honeycomb lattice in a curved geometry. The formation of defects in the graphene matrix, which bend the carbon plane, requires that two or more graphene embryos with significantly different growth axis merge. Such scenario is only possible at the front- or back-end of the metal particle where growth symmetry is broken. The graphene embryos reconstruct their hexagonal structure into pentagons, heptagons, and octagons counterpart to accommodate the tube curvature.

  9. Title: Structural and Electrical Investigation of C60–Graphene Vertical Heterostructures

    Author(s): Kim K., Lee T.H., Santos E.J.S., Jo P.S., Salleo A., Nishi Y., Bao Z.

    ACS Nano (1 June 2015)

    doi: 10.1021/acsnano.5b00581
    Abstract



    Graphene, with its unique electronic and structural qualities, has become an important playground for studying adsorption and assembly of various materials including organic molecules. Moreover, organic/graphene vertical structures assembled by van der Waals interaction have potential for multifunctional device applications. Here, we investigate structural and electrical properties of vertical heterostructures composed of C60 thin film on graphene. The assembled film structure of C60 on graphene is investigated using transmission electron microscopy, which reveals a uniform morphology of C60 film on graphene with a grain size as large as 500 nm. The strong epitaxial relations between C60 crystal and graphene lattice directions are found, and van der Waals ab initio calculations support the observed phenomena. Moreover, using C60–graphene heterostructures, we fabricate vertical graphene transistors incorporating n-type organic semiconducting materials with an on/off ratio above 3 × 103. Our work demonstrates that graphene can serve as an excellent substrate for assembly of molecules, and attained organic/graphene heterostructures have great potential for electronics applications.

  10. Title: Direct Observation of a Long-Lived Single-Atom Catalyst Chiseling Atomic Structures in Graphene

    Author(s): Wang W.L., Santos E.J.G., Jiang B., Dogus Cubuk E., Ophus C., Centeno A., Pesquera A., Zurutuza A., Ciston J., Westervelt R., Kaxiras E.

    Nano Letters, 14, No. 2, pp. 450-455 (21 January 2014)

    doi: 10.1021/nl403327u
    Abstract



    Fabricating stable functional devices at the atomic scale is an ultimate goal of nanotechnology. In biological processes, such high-precision operations are accomplished by enzymes. A counterpart molecular catalyst that binds to a solid-state substrate would be highly desirable. Here, we report the direct observation of single Si adatoms catalyzing the dissociation of carbon atoms from graphene in an aberration-corrected high-resolution transmission electron microscope (HRTEM). The single Si atom provides a catalytic wedge for energetic electrons to chisel off the graphene lattice, atom by atom, while the Si atom itself is not consumed. The products of the chiseling process are atomic-scale features including graphene pores and clean edges. Our experimental observations and first-principles calculations demonstrated the dynamics, stability, and selectivity of such a single-atom chisel, which opens up the possibility of fabricating certain stable molecular devices by precise modification of materials at the atomic scale.

  11. Title: Epitaxial Growth of Molecular Crystals on van der Waals Substrates for High-Performance Organic Electronics

    Author(s): Lee C.H., Schiros T., Santos E.J.G., Kim B., Yager K.G., Kang S.J., Lee S., Yu J., Watanabe K., Taniguchi T., Hone J., Kaxiras E., Nuckolls C., Kim P.

    Advanced Materials, 26, No. 18, pp. 2812-2817 (23 January 2014)

    doi: 10.1002/adma.201304973
    Abstract



    Epitaxial van der Waals (vdW) heterostructures of organic and layered materials are demonstrated to create high-performance organic electronic devices. High-quality rubrene films with large single-crystalline domains are grown on h-BN dielectric layers via vdW epitaxy. In addition, high carrier mobility comparable to free-standing single-crystal counterparts is achieved by forming interfacial electrical contacts with graphene electrodes.

  12. Title: Graphene/MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics

    Author(s): Yu L., Lee Y., Ling X., Santos E.J.G., Shin Y.C., Lin Yuxuan, Dubey M., Kaxiras E, Kong J., Wang H., Palacios T.

    Nano Letters, 14, No. 6, pp. 3055-3063 (8 May 2014)

    doi: 10.1021/nl404795z
    Abstract



    Two-dimensional (2D) materials have generated great interest in the past few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2), and insulating boron nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency, and favorable transport properties for realizing electronic, sensing, and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor deposition. We have fabricated high-performance devices and circuits based on this heterostructure, where MoS2 is used as the transistor channel and graphene as contact electrodes and circuit interconnects. We provide a systematic comparison of the graphene/MoS2 heterojunction contact to more traditional MoS2-metal junctions, as well as a theoretical investigation, using density functional theory, of the origin of the Schottky barrier height. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. These high-performance large-scale devices and circuits based on this 2D heterostructure pave the way for practical flexible transparent electronics.

  13. 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)

    doi: 10.1007/978-94-007-6413-2_2
    Abstract



    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: 1. 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. 2. 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. 3. 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.

  14. 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)

    doi: 10.1021/nl303909f
    Abstract



    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.

  15. 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)

    doi: 10.1021/nl303611v
    Abstract



    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.

  16. 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)

    doi: 10.1021/jp310463k
    Abstract



    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.

  17. 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)

    doi: 10.1103/PhysRevB.87.155440
    Abstract



    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.

  18. 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)

    doi: 10.1021/nn4037877
    Abstract



    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.

  19. 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)

    doi: 10.1021/nn403738b
    Abstract



    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.

  20. Title: Strain-Tunable Spin Moment in Ni-Doped Graphene

    Author(s): Santos E.J.G., Ayuela A., Sánchez-Portal D.

    Journal Physical Chemistry C, 116, No. 1, pp. 1174-1178 (29 November 2012)

    doi: 10.1021/jp2077374
    Abstract



    Graphene, due to its exceptional properties, is a promising material for nanotechnology applications. In this context, the ability to tune the properties of graphene-based materials and devices with the incorporation of defects and impurities can be of extraordinary importance. Here, we investigate the effect of uniaxial tensile strain on the electronic and magnetic properties of graphene doped with substitutional Ni impurities (Nisub). We have found that, although Nisub defects are nonmagnetic in the relaxed layer, uniaxial strain induces a spin moment in the system. The spin moment increases with the applied strain up to values of 0.3–0.4 μB per Nisub, until a critical strain of ∼6.5% is reached. At this point, a sharp transition to a high-spin state (∼1.9 μB) is observed. This magnetoelastic effect could be utilized to design strain-tunable spin devices based on Ni-doped graphene.

  21. Title: Magnetism of Single Vacancies in Rippled Graphene

    Author(s): Santos E.J.G., Riikonen S., Sánchez-Portal D., Ayuela A.

    The Journal of Physical Chemistry C, 116, No. 13, pp. 7602-7606 (2 March 2012)

    doi: 10.1021/jp300861m
    Abstract



    Using first-principles calculations, the dependence in the properties of the monovacancy of graphene under rippling controlled by an isotropic strain was determined, with a particular focus on spin moments. At zero strain, the vacancy shows a spin moment of 1.5 μB that increases to ∼2 μB when the graphene is in tension. The changes are more dramatic under compression in that the vacancy becomes nonmagnetic when graphene is compressed more than 2%. This transition is linked to the structural changes that occur around vacancies and is associated with formation of ripples. For compressions slightly greater than 3%, this rippling leads to formation of a heavily reconstructed vacancy structure consisting of two deformed hexagons and pentagons. Our results suggest that any magnetism induced by vacancies that occurs in graphene can be controlled by applying strain.

  22. Title: Universal magnetic properties of sp3-type defects in covalently functionalized graphene

    Author(s): Santos E.J.G., Ayuela A., Daniel Sánchez-Portal D.

    New Journal of Physics, 14, No. 4, pp. 043022- (19 April 2012)

    doi: 10.1088/1367-2630/14/4/043022
    Abstract



    Using density-functional calculations, we study the effect of sp3-type defects created by different covalent functionalizations on the electronic and magnetic properties of graphene. We find that the induced magnetic properties are universal, in the sense that they are largely independent of the particular adsorbates considered. When a weakly polar single covalent bond is established with the layer, a local spin moment of 1.0 μB always appears in graphene. This effect is similar to that of H adsorption, which saturates one pz orbital in the carbon layer. The magnetic couplings between the adsorbates show a strong dependence on the graphene sublattice of chemisorption. Molecules adsorbed at the same sublattice couple ferromagnetically, with an exchange interaction that decays very slowly with distance, while no magnetism is found for adsorbates at opposite sublattices. Similar magnetic properties are obtained if several pz orbitals are saturated simultaneously by the adsorption of a large molecule. These results might open new routes to engineering the magnetic properties of graphene derivatives by chemical means.

  23. Title: Magnetism of covalently functionalized carbon nanotubes

    Author(s): Santos E.J.G., Sánchez-Portal D., Ayuela A.

    Applied Physics Letters, 99, No. 6, pp. 062503- (2011)

    doi: 10.1063/1.3623755
    Abstract



    We investigate the electronic structure of carbon nanotubes functionalized by adsorbates anchored with single C-C covalent bonds. We find that despite the particular adsorbate, a spin moment with a universal value of 1.0 μ B per molecule is induced at low coverage. Therefore, we propose a mechanism of bonding-induced magnetism at the carbonsurface. The adsorption of a single molecule creates a dispersionless defect state at the Fermi energy, which is mainly localized in the carbon wall and presents a small contribution from the adsorbate. This universal spin moment is fairly independent of the coverage as long as all the molecules occupy the same graphenic sublattice. The magnetic coupling between adsorbates is also studied and reveals a key dependence on the graphenic sublattice adsorption site.

  24. Title: Magnetism of substitutional Co impurities in graphene: Realization of single π vacancies

    Author(s): Santos E.J.G., Sánchez-Portal D., Ayuela A.

    Physical Review B, 81, No. 12, pp. 125433- (26 March 2010)

    doi: 10.1103/PhysRevB.81.125433
    Abstract



    We report ab initio calculations of the structural, electronic, and magnetic properties of a graphene monolayer substitutionally doped with Co (Cosub) atoms. These calculations are done within density-functional theory using the generalized gradient approximation. We focus in Co because among traditional ferromagnetic elements (Fe, Co, and Ni), only Cosub atoms induce spin polarization in graphene. Our results show the complex magnetism of Co substitutional impurities in graphene, which is mapped into simple models such as the π-vacancy and Heisenberg model. The links established in our work can be used to bring into contact the engineering of nanostructures with the results of π models in defective graphene. In principle, the structures considered here can be fabricated using electron irradiation to create defects and depositing Co at the same time.

  25. Title: First-principles study of substitutional metal impurities in graphene: structural, electronic and magnetic properties

    Author(s): Santos E.J.G., Ayuela A., D Sánchez-Portal D.

    New Journal of Physics, 12, pp. 053012- (May 2010)

    doi: 10.1088/1367-2630/12/5/053012
    Abstract



    We present a theoretical study using density functional calculations of the structural, electronic and magnetic properties of 3d transition metal, noble metal and Zn atoms interacting with carbon monovacancies in graphene. We pay special attention to the electronic and magnetic properties of these substitutional impurities and find that they can be fully understood using a simple model based on the hybridization between the states of the metal atom, particularly the d shell, and the defect levels associated with an unreconstructed D3h carbon vacancy. We identify three different regimes associated with the occupation of different carbon–metal hybridized electronic levels: (i) bonding states are completely filled for Sc and Ti, and these impurities are non-magnetic; (ii) the non-bonding d shell is partially occupied for V, Cr and Mn and, correspondingly, these impurities present large and localized spin moments; (iii) antibonding states with increasing carbon character are progressively filled for Co, Ni, the noble metals and Zn. The spin moments of these impurities oscillate between 0 and 1μB and are increasingly delocalized. The substitutional Zn suffers a Jahn–Teller-like distortion from the C3v symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct position at the border between regimes (ii) and (iii) and shows a more complex behavior: while it is non-magnetic at the level of generalized gradient approximation (GGA) calculations, its spin moment can be switched on using GGA+U calculations with moderate values of the U parameter.

  26. Title: Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

    Author(s): Hummelshøj J.S., Landis D.D., Voss J., Jiang T., Tekin A., Bork N., Dułak M., Mortensen J.J., Adamska L., Andersin J., Baran J.D., Barmparis G.D., Bell F., Bezanilla A.L., Bjork J., Björketun M.E., Bleken F., Buchter F., Bürkle M., Burton P.D., Buus B.B., Calborean A., Calle-Vallejo F., Casolo S., Chandler B.D., Chi D.H., Czekaj I, Datta S., Datye A., DeLaRiva A., Despoja V, Dobrin S., Engelund M., Ferrighi L., Frondelius P., Fu Q., Fuentes A., Fürst J., García-Fuente A., Gavnholt J., Goeke R., Gudmundsdottir S., Hammond K.D., Hansen H.A., Hibbitts D., Hobi E., Howalt J.G., Hruby S.L., Huth A., Isaeva L., Jelic J., Jensen I.J.T., Kacprzak K.A., Kelkkanen A., Kelsey D., Kesanakurthi D.S., Kleis J., Klüpfel P.J., Konstantinov I, Korytar R., Koskinen P., Krishna C., Kunkes E., Larsen A.H., Lastra J.M.G., Lin H., Lopez-Acevedo O., Mantega M., Martínez J.I., Mesa I.N., Mowbray D.J., Mýrdal J.S.G., Natanzon Y., Nistor A., Olsen T., Park H., Pedroza L.S., Petzold V, Plaisance C., Rasmussen J.A., Ren H., Rizzi M., Ronco A.S., Rostgaard C., Saadi S., Salguero L.A., Santos E.J.G., Schoenhalz A.L., Shen J., Smedemand M., Stausholm-Møller O.J., Stibius M., Strange M., Su H.B., Temel B., Toftelund A., Tripkovic V, Vanin M., Viswanathan V, Vojvodic A., Wang S., Wellendorff J., Thygesen K.S., Rossmeisl J., Bligaard T., Jacobsen K.W., Nørskov J.K., Vegge T.

    The Journal of Chemical Physics, 131, pp. 014101- (2009)

    doi: 10.1063/1.3148892
    Abstract



    We present a computational screening study of ternary metal borohydrides for reversible hydrogen storage based on density functional theory. We investigate the stability and decomposition of alloys containing 1 alkali metal atom, Li, Na, or K (M1); and 1 alkali, alkaline earth or 3d/4dtransition metal atom (M2) plus two to five (BH4)− groups, i.e., M1M2(BH4)2–5, using a number of model structures with trigonal, tetrahedral, octahedral, and free coordination of the metal borohydride complexes. Of the over 700 investigated structures, about 20 were predicted to form potentially stable alloys with promising decomposition energies. The M1(Al/Mn/Fe)(BH4)4, (Li/Na)Zn(BH4)3, and (Na/K)(Ni/Co)(BH4)3 alloys are found to be the most promising, followed by selected M1(Nb/Rh)(BH4)4 alloys

  27. Title: Switching on magnetism in Ni-doped graphene: Density functional calculations

    Author(s): Santos E.J.G., Ayuela A., Fagan S.B., Mendes Filho J., Azevedo D.L., Souza Filho A.G., Sánchez-Portal D.

    Physical Review B, 78, No. 19, pp. 195420- (19 November 2008)

    doi: 10.1103/PhysRevB.78.195420
    Abstract



    Magnetic properties of graphenic carbon nanostructures, which are relevant for future spintronic applications, depend crucially on doping and on the presence of defects. In this paper we study the magnetism of the recently detected substitutional Ni (Nisub) impurities. Nisub defects are nonmagnetic in flat graphene and develop a nonzero-spin moment only in metallic nanotubes. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of Nisub. A similar magnetic-nonmagnetic transition of Nisub can be expected by applying anisotropic strain to a flat graphene layer.

  28. Title: Evolucao estrutural de nanotubos de oxidos de vanadio: efeito da temperatura

    Author(s): Ferreira O.P., Alves O.L., Souza Filho A.G., Santos E.J.G., Mendes Filho J.

    Chemical Physics Letters, 437, No. 1-3, pp. 79-82 (22 March 2007)

    doi: 10.1016/j.cplett.2007.01.071
    Abstract



    The interaction of 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (dioxin) with carbon nanotubes is analyzed by ab initio methods. The structural and electronic properties of the dioxin interacting with the pristine, defective and B-, N-, and Si-doped SWNTs are considered by studying different geometries. We have found that the dioxin interacts with a pure carbon nanotube although it depends on the geometry of the molecule approximation, increasing if the tube is defective.

  29. Title: Raman Spectra in Vanadate Nanotubes Revisited

    Author(s): Souza Filho A.G., Ferreira O.P., Santos E.J.G., Mendes Filho J., Alves O.L.

    Nano Letters, 4, No. 11, pp. 2099-2104 (23 October 2004)

    doi: 10.1021/nl0488477
    Abstract



    In this letter we report the Raman spectra of vanadate nanotubes (VONTs). The spectra present a clear signature that can be used for probing the tubular structure. The temperature effects on the structure of dodecylamine- and Cu-intercalated VONTs were studied by changing the laser power density during the Raman measurements. We have found that low laser power densities promote the decomposition of VONTs, leading to the collapse of the tubular structure and converting the nanotubes into V2O5 oxide. The decomposition occurs through an intermediate compound that is isostructural to V2O5 xerogel. The Raman experiments in VONT-based systems should be performed at extremely low laser power densities.