Andersen, Nick Papior

Brandbyge, Mads

Gunst, Tue

Stradi, Daniele

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 21, 2017DFT-NEGF calculations of gated graphene nano-structuresPapior, Nick Rübner; Gunst, Tue; Stradi, Daniele; Brandbyge, MadsPublication date:2014Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Andersen, N. P., Gunst, T., Stradi, D., & Brandbyge, M. (2014). DFT-NEGF calculations of gated graphenenano-structures. Poster session presented at Graphene 2014, Toulouse, France.DFT-NEGF calculations of gated graphene nano-structuresNick Papior Andersen, Tue Gunst, Daniele Stradi, Mads BrandbygeDTU Dept. of Micro- and Nanotechnology & Center for Nanostructured Graphene, DK-2800 Kgs. Lyngby, DenmarkAbstractTwo dimensional (2D) materials such as graphene are expected to be prominent candidates for nanoscale electronics applications. In particular the possibility to electrostatically gate 2D atomic scalestructures is a key feature. Here we present non-equilibrium simulations of graphene nanostructures to illustrate a novel electrostatic gating implementation in SIESTA [1].We test this model on a graphene nanoconstriction and investigate the doping and the applied bias dependence on the transmission.Implementation detailsGatene−I Different gate geometries possibleI 2D materials onlyI Electric field obtained from PoissonI Self-consistent solution in SIESTA [1]Colour convention:+e− −e−Transmission properties of gating nanoconstriction [2]I Intrinsic transmission at 0 V1st peak 2nd peak−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.800.51E − EF [eV]TI n = −2.0e− transmission at different applied bias (V shifted vertically),symmetric about applied bias−1.2 −1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.60240.511.52.533.54.5−1.0 V−0.75 V−0.5 V−0.25 V0.0 V0.25 V0.5 V0.75 V1.0 VE − EF [eV]TI 0.5 V transmission at different doping levels (n shifted vertically)−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.80240.511.52.533.54.5−2.0n−1.5n−1.0n−0.5n0.0n0.5n1.0n1.5n2.0nE − EF [eV]TI Transmission peak doping compared to graphene doping−2 −1 0 1 2−0.4−0.200.20.4→ p-dopingn-doping←Gating[√n9.4× 1012 cm−2]Peakshift[eV]Bulk 0.0 V 0.25 V 0.5 V−2 −1 0 1 2→ p-dopingn-doping←LiFGating[√n9.4× 1012 cm−2]I Gating nanoconstriction does not follow linear√n dependenceI Transmission peak position dependent on applied bias and dopingExplicit doping and bias analysisAnalysis of 0.5 V and n = −2.0e−I Electrostatic potential tuned by dopingn = 0e−0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 400510152025AngstromAngstromV (r) = V (r,0.5 V,n)− V (r,0 V,n)−0.25−0.2−0.15−0.1−5 · 10−205 · 10−20.10.150.20.25eVn = −2.0e−0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 400510152025AngstromAngstromI Charge redistribution is consistent with the doping effect of the electrostatic potentialρ(r) = ρ(r,0.5 V,n)− ρ(r,0 V,n)n = 0e−n = −2.0e−Iso-value0.0001I Gating nanoconstriction shifts potential drop to one endI Possible enhancement of reactivity at localised potential dropDoping via adsorbatesI Doping with Fluor and Lithium in positionsCenter adsorption Adsorption outside of constriction−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.800.511.522.53F rightF centerIntrinsicLi centerLi leftE − EF [eV]TWe examine the effect of a donor Li or acceptor F adatom either inside the constriction, at the center position (CP),or outside (OP); see example positions in the potential plot.The Li and F atoms are positioned above the center of a hexagon or ontop a carbon atom, respectively.The transmission plot indicates that there is no significant scattering (except for the hole peaks in the case where Fis at CP).From the most significant peak shift we find that Li donates at least 0.2 electrons to graphene while F accepts atleast 0.3 electrons from graphene, from looking at the bulk doping profile.In addition, most of the doping effect is still present if the adatoms are moved outside the constriction.ConclusionI New electrostatic gating model implemented for SIESTA/TranSIESTAI Investigating the model on a graphene nanoconstriction. Gating is not the same as shifting Fermi-level. Features of transmission spectra does not shift linearly with√n. Electronic structure can be tuned via doping. Potential drop across constriction changes with doping, we can therefore tune the localpotential drop by gatingI Localised potential drop could influence the reactivityI Gating other graphene nanostructures could also shownon-linear effectsI Currently investigating oxygen terminated nanoconstriction[1] J. M. Soler et al. Journal of Physics: Condensed Matter 14.11 (Mar.2002).[2] T. Gunst et al. (May 2013). arXiv: 1305.5907.[3] M. Brandbyge et al. Physical Review B 65.16 (Mar. 2002).[4] W. Humphrey, A. Dalke, and K. Schulten. Journal of molecular graphics14.1 (Feb. 1996).[5] A. Kokalj. Journal of Molecular Graphics and Modelling 17.3-4 (June1999).nickpapior@gmail.com

DFT-NEGF calculations of gated graphene nano-structures

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DFT-NEGF calculations of gated graphene nano-structures

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