new developments in surface science complex 2d systems (graphene and beyond…) biosurfaces
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New Developments in Surface Science Complex 2D Systems (Graphene and beyond…) Biosurfaces Magnetic systems (new sort of…). Development of Surface Science Techniques Materials. Binnig, Rohr (STM) Fert, Grunberg (GMR) Bader (MOKE). ~1985. Goodman. Haber. Bell, Somorjai, Ertl. - PowerPoint PPT PresentationTRANSCRIPT
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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
CatalysisHaber
LangmuirElectronic Materials
1960-1980’s
LEED (1927)TPD
AESXPS
Micro/nano electronics
Complex catalysts
Bell, Somorjai, Ertl
Bardeen, Sigbahn, Bell Labs, IBM ResearchSeitz, etc….
STM, AFMSpin-polarized PESMOKE, SFGSpin-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, MoSe2, MoS2….
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 EF for given Vg
Application of VB induces tunneling between graphene layers
Britnell, et al., Science 335 (2012) 947
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Britnell, et al., Science 335 (2012) 947
Note, relatively small increase in I with Vg. (interf. Charge screening? MoS2 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 AndGraphene/MoS2 (MoSe2) 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 (> 105) 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
k
A , B equivalent (C6v) no band gap
k
A ≠ B(C3V) 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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EF EFEg
Isolated Graphene Sheet
E
k
Graphene/BN—band gap, with Fermi level in middle of gap
EFEg
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
BCl3 + (surface) BCl2(ads)
BCl2(ads) + NH3 B-N-H(ads) + 2HCl(desorbed)
BNH(ads) + BCl3 B-N-B-Cl2 + HCl(desorbed)
BNBCl2 + NH3 BNBN
BCl2
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 increasesBN 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/BCl3+NH3 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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C Co Si
Lattice Overlay:
Graphene (BN) on CoSi2(111)BN/graph 3x3~ CoSi2(2x2)
AMC 2012 21
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BN bilayer on CoSi2(111)
AMC 2012 22
4 BCl3/NH3 cycles at 550 K, anneal to 850 K in UHV
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Anneal of BN/CoSi2 at 1000K: LEED analysis: Anneal of BN/CoSi2 at 1000K: LEED analysis:
E=78ev
0 50 100 150 200 250 300
10000
15000
20000
25000
30000
35000
Y A
xis
Titl
e
X Axis Title
B
187
243
BN implant lattice constant =2.5(±0.1)ÅCoSi2 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 CoSi2 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