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

Development of Surface Science Techniques

 

Haber

Materials Catalysis

Langmuir

~1900 Electronic Materials

LEED (1927) TPD

~1985

Binnig, Rohr (STM) Fert, Grunberg (GMR) Bader (MOKE) Bell, Somorjai, Ertl

Complex catalysts

Goodman

Nanocatalysts and particles Biomaterials Micro/nano electronics 2D systems, new materials

Bardeen, Sigbahn, Bell Labs, IBM Research Seitz, etc….

AES XPS

1960 1980’s Spintronics

STM, AFM Spin-polarized PES MOKE, SFG Spin-polarized LEED, STM

2-D Systems Beyond Graphene:

1. BN, MoSe 2 , MoS 2 ….

2. Stacks combining the above with graphene 3. Spintronics and Graphene, BN, etc.

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

Watanabe, et al.

p. 404

Britnell, et al., Science 335 (2012) 947 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 V g for given

Application of V tunneling between graphene layers

B

induces

Britnell, et al., Science 335 (2012) 947 Note, relatively small increase in I with Vg. (interf. Charge screening? MoS 2 give higher on/off ratios

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.

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

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.

= +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 A , B equivalent (C 6v ) no band gap

k 

A B B A ≠ B (C 3V ) band gap A

k  HOMO and LUMO Orbitals in Graphene at Dirac point (adopted from Cox:

The Electronic Structure and Chemistry of Solids (1991)

11

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

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.

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

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???

BCl 3 Can we grow BN multilayers?

Yes! Atomic layer deposition (see Ferguson, et al. Thin Sol. Films 413 (2002) 16 + (surface) BCl 2(ads) + NH 3   BCl 2(ads) 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

BN/Si(111): ALD Growth Characteristics

14 12 10 8 6 4 2 0 0

BN Film Thickness

2 4 AB Cycle # 10

BN Avg Theoretical

1,8 1,6 1,4 1,2 1 0,8 0,6 0,4 0,2 0 0

B/N Ratio

2 4 6 #AB Cycles 8

BN films are stoichiometric (1:1) for thin films (<5 ML) and become slightly B-rich (?) as film thickness increases AMC 2012

10

19

h-BN(0001): ALD/BCl

3

+NH

3

vs CVD/Borazine

ALD: Epitaxial Multilayers CVD/Borazine: Flat or puckered monolayers

Ru(0001) We need multilayers for applications, and not just on Ru!

Ni(111) AMC 2012 20

Lattice Overlay: Graphene (BN) on CoSi 2 (111) BN/graph 3x3~ CoSi 2 (2x2) C

AMC 2012

Co Si

21

BN bilayer on CoSi

2

(111)

4 BCl 3 /NH 3 cycles at 550 K, anneal to 850 K in UHV

AMC 2012 22

Anneal of BN/CoSi

2

at 1000K: LEED analysis:

BN implant lattice constant =2.5(±0.1)Å CoSi2 implant lattice constant=3.8(±0.1)Å Expected values

E=78ev

B 35000 30000 25000 20000 15000 10000 243 187 0 50 100 150 X Axis Title 200 250 300

Interesting results, but: 1. Anneal to 1000 K to induce order, but CoSi 2 unstable at this temp. (slow Co diffusion) is slightly

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