Lecture 14--new developments in surface science

Report
New Developments in Surface Science
1. Complex 2D Systems (Graphene and beyond…)
2. Biosurfaces
3. Magnetic systems (new sort of…)
Development of Surface Science
Binnig, Rohr (STM)
~1985 Fert, Grunberg (GMR)
Bader (MOKE)
Techniques   Materials
Haber
Catalysis
Langmuir
Electronic Materials
Goodman
Bell, Somorjai, Ertl
Nanocatalysts and particles
Complex catalysts
Biomaterials
Micro/nano electronics
Bardeen, Sigbahn, Bell
Labs, IBM Research
Seitz, etc….
LEED (1927)
TPD
~1900
2D systems, new
materials
Spintronics
AES
XPS
19601980’s
STM, AFM
Spin-polarized PES
MOKE, SFG
Spin-polarized LEED, STM
2-D Systems Beyond Graphene:
1. BN, MoSe2, MoS2….
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 EF for given
Vg
Application of VB induces
tunneling between
graphene layers
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
And
Graphene/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.
= +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
(C6v) no band gap
k
B
B
A
A≠B
(C3V) 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
Graphene/BN—band gap,
with Fermi level in middle of
gap
E
k
EF
Eg
EF
Giovanetti, et al;
DFT results
Eg
EF
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???
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
BN/Si(111): ALD Growth
Characteristics
14
12
10
8
6
4
2
0
1.8
B/N Ratio
1.6
1.4
B/N Ratio
Film Thickness (A)
BN Film Thickness
BN Avg
Theoretical
1.2
1
0.8
0.6
0.4
0.2
0
2
4
6
8
10
0
0
AB Cycle #
2
4
6
8
#AB Cycles
10
BN films are stoichiometric (1:1) for thin films (<5 ML) and
become slightly B-rich (?) as film thickness increases
AMC 2012
19
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!
AMC 2012
20
Lattice Overlay:
Graphene (BN) on CoSi2(111)
BN/graph 3x3~ CoSi2(2x2)
C
AMC 2012
Co
Si
21
BN bilayer on CoSi2(111)
4 BCl3/NH3 cycles at 550 K, anneal to 850 K in UHV
AMC 2012
22
Anneal of BN/CoSi2 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
Y Axis Title
30000
243
25000
187
20000
15000
10000
0
50
100
150
X Axis Title
200
250
300
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

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