New Developments in Surface Science

1.Complex 2D Systems (Graphene and beyond…)

2.Biosurfaces

3.Magnetic systems (new sort of…)

~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

2-D Systems Beyond Graphene:

1.BN, MoSe2, MoS2….

2.Stacks combining the above with graphene

3.Spintronics and Graphene, BN, etc.

Weck, et al. Phys. Chem. Chem. Phys., 2008, 10, 5184-5187

Boron nitride, isostructural and isoelectronic with graphene, but different

Watanabe, et al.

p. 404

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

Britnell, et al., Science 335 (2012) 947

Note, relatively small increase in I with Vg. (interf. Charge screening? MoS2 give higher on/off ratios

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.

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

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.

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

Lowest energy interfacial structure:

Giovanneti, et al., DFT calcns on graphene/BN interface

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.

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

Evidence of orbital mixing, Fermi level broadening

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

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

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

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

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!

C Co Si

Lattice Overlay:

Graphene (BN) on CoSi2(111)BN/graph 3x3~ CoSi2(2x2)

AMC 2012 21

BN bilayer on CoSi2(111)

AMC 2012 22

4 BCl3/NH3 cycles at 550 K, anneal to 850 K in UHV

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

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