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New Developments in Surface Science 1.Complex 2D Systems (Graphene and beyond…) 2.Biosurfaces 3.Magnetic systems (new sort of…)

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~1900 Catalysis Haber Langmuir Electronic Materials 1960- 1980’s LEED (1927) TPD AES XPS Micro/nano electronics Complex catalysts Bell, Somorjai, Ertl Bardeen, Sigbahn, Bell Labs, IBM Research Seitz, etc…. STM, AFM Spin-polarized PES MOKE, SFG Spin-polarized LEED, STM 2D systems, new materials Spintronics Binnig, Rohr (STM) Fert, Grunberg (GMR) Bader (MOKE) Nanocatalysts and particles Goodman Biomaterials Development of Surface Science Techniques Materials ~1985

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2-D Systems Beyond Graphene: 1.BN, MoSe 2, MoS 2 …. 2.Stacks combining the above with graphene 3.Spintronics and Graphene, BN, etc.

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Weck, et al. Phys. Chem. Chem. Phys., 2008, 10, 5184-5187 Boron nitride, isostructural and isoelectronic with graphene, but different

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Watanabe, et al. p. 404

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Multilayer BN tunneling barrier Application of gate voltage induces increase in carrier densities in cond. Bands of both graphene layers (weak screening). Note, graphene low DOS yields much greater increase in E F for given V g Application of V B induces tunneling between graphene layers Britnell, et al., Science 335 (2012) 947

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Note, relatively small increase in I with Vg. (interf. Charge screening? MoS 2 give higher on/off ratios

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Tunneling transit time ~ femtoseconds, better than electron transit time in modern planar FETs Conclusion: Graphene/BN And Graphene/MoS 2 (MoSe 2 ) stacks have exciting photonic/nanoelectronic applications.

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Alternative proposed design for a graphene tunneling transistor (BN could be used as the base…) Graphene has band gap in vertical direction: monolayer thickness favors ballistic transport with applied bias High on/off ratios (> 10 5 ) and THZ switching predicted in simulations

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Issues: 1.Orbital overlap/hybridization—band gap formation 2.Growth Multilayer BN, precise thickness control?? Graphene on BN (or MoS2) and vice versa 3. Interfacial Effects, Charge transfer, mass transfer, etc.

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A B A B HOMO and LUMO Orbitals in Graphene at Dirac point (adopted from Cox: The Electronic Structure and Chemistry of Solids (1991) = +1 = -1/2 WHY A BAND GAP? LEED is C3V: A site/B sites different electron densities Degeneracy of HOMO,LUMO at Dirac Point due to chemical equivalency of A and B lattice sites kk A, B equivalent (C 6v ) no band gap kk A ≠ B (C 3V ) band gap 11

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Lowest energy interfacial structure: Giovanneti, et al., DFT calcns on graphene/BN interface

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Band gap of 0.05 eV predicted. How does this compare to RT? Prediction, O.1 eV band gap for graphene on Cu, but huge charge transfer.

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EFEF EFEF EgEg Isolated Graphene Sheet E k Graphene/BN—band gap, with Fermi level in middle of gap EFEF EgEg Graphene on Cu: charge transfer masks the gap, moves Fermi level well above the gap Giovanetti, et al; DFT results

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Evidence of orbital mixing, Fermi level broadening

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Cu 3d/BN π mixing: weaker than in Ni (Cu d’s more localized)

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Why don’t we see a band gap for BN/Ru???

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Can we grow BN multilayers? Yes! Atomic layer deposition (see Ferguson, et al. Thin Sol. Films 413 (2002) 16 BCl 3 + (surface) BCl 2(ads) BCl 2(ads) + NH 3 B-N-H (ads) + 2HCl (desorbed) BNH (ads) + BCl 3 B-N-B-Cl 2 + HCl (desorbed) BNBCl 2 + NH 3 BNBN BCl 2 BNH

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AMC 2012 19 BN/Si(111): ALD Growth Characteristics BN films are stoichiometric (1:1) for thin films (<5 ML) and become slightly B-rich (?) as film thickness increases

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AMC 2012 20 h-BN(0001): ALD/BCl 3 +NH 3 vs CVD/Borazine ALD: Epitaxial Multilayers CVD/Borazine: Flat or puckered monolayers Ru(0001)Ni(111) We need multilayers for applications, and not just on Ru!

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CCoSi Lattice Overlay: Graphene (BN) on CoSi 2 (111) BN/graph 3x3~ CoSi 2 (2x2) AMC 2012 21

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BN bilayer on CoSi 2 (111) AMC 2012 22 4 BCl 3 /NH 3 cycles at 550 K, anneal to 850 K in UHV

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Anneal of BN/CoSi 2 at 1000K: LEED analysis: E=78ev 187 243 BN implant lattice constant =2.5(±0.1)Å CoSi2 implant lattice constant=3.8(±0.1)Å Expected values

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Interesting results, but: 1.Anneal to 1000 K to induce order, but CoSi 2 is slightly unstable at this temp. (slow Co diffusion) Can we go to lower temperatures, other silicides? 2.Carbon buildup is worrisome. Clean up our act? 3.Heteroepitaxy (BN 3x3 vs. silicide 2x2) demonstrated. 4.What about BN/transition metals vs. silicides? Spintronics? {Spin filtering predicted in MTJs} AMC 2012 24

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