Recent Publications

Elton J Santos

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  1. 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

    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.

  2. 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

    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.

  3. 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

    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.

  4. 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

    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.

  5. 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

    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.

  6. 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

    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.

  7. 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)


    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.