High–energy resolution core level photoelectron
spectroscopy and diffraction:
powerful tools to probe physical
and chemical properties of solid surfaces.
Alessandro Baraldi
hν
Grado, September 21, 2013
e-
For info, do not hesitate to contact me!
Prof. Alessandro Baraldi
Physics Department, University of Trieste , ITALY
Deputy-chair Doctorate School in Nanotechnology, University fo Trieste
Head of the Surface Science Laboratory, Elettra-Sincrotrone Trieste
E-mail: [email protected]
website: alessandrobaraldi.weebly.com
Phone: +39 040 375 8719 (off)
8331 (lab)
Grado, September 21, 2013
Outline
Core-level spectroscopy is used to ...
… probe the properties of first-layer surface atoms.
… determine energetics and atomic mechanisms in diffusion processes.
… identify in a direct way the adsorption site of atoms and molecules.
… evaluate the chemical reactivity trends on model surfaces.
… investigate the role of surface defects.
… study molecular dissociation processes in nanostructured surfaces.
… monitor the surface chemical composition during chemical reactions.
… investigate the growth mechanism of graphene
… probe the unsual physical and chemical properties of graphene.
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One of the most widely employed
experimental techniques
Number of papers published in the last twenty years in peer-review international
scientific journals on X-ray Photoelectron Spectroscopy, according to the
Grado, September 21, 2013 Thomson Reuters ISI Web of KnowledgeR .
The main advantages
Down to the
100 ms time-scale
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In the range
40-100 meV
20
Between
K and 1300 K
0.5% of ML
Ir(111)
Details from the core level lineshape
Core-level photoemission lineshape is usually described by using the
Doniach-Šùnjić function
J. Phys. C: Sol. State Phys. 3, 285 (1970)
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A. Baraldi et al., Phys. Rev. B 67, 205404 (2003).
Many-body effects in Doniach-Šùnjić
Lorentzian distribution
arising from the finite corehole lifetime.
A convolution of a DoniachŠùnjić function and a
Gaussian, which account for
the vibration/phonon and
the contribution of the
instrumental resolution.
Asymmetry parameter,
describing the contribution
of electron-hole pairs
excitation.
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Core-level shift and atomic coordination number
Surface Core Level Shift of single crystal surfaces
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A. Baraldi et al., J. Phys.: Condens. Matter 20, 93001 (2008).
Surface reconstruction and atomic diffusion
Oxygen-induced reconstruction on Rh(110)
(1x2)
(2x2)pg
Q=0.5 ML
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(1x3)
c(2x6)
Q=0.66 ML
c(2x8)
(1x4)
Q=0.75 ML
A. Baraldi et al., Phys. Rev. B 72, 75417 (2005).
(1x4)+(1x1)
Atomic diffusion and surface reconstruction
(1x2)
-445
Rh3d5/2
308
surface
-675
307
(1x1)
-715
bulk
Photoemission Intensity [arb. units]
-700
306
Binding Energy [eV]
305
Atomic diffusion and surface reconstruction
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Atomic diffusion and surface reconstruction
Qt   Ie t
  e  E / K T
1.0
426 K
B
0.8
ln(K) [ x10-4]
Normalized (1x2) Rh Population
A
0.6
0.4
451 K
-2
-3
-4
-5
-6
2.1
0.2
2.2
2.3
2.4
-3
1/T [ x 10 ]
2.5
473 K
0.0
0
500
1000
Time [s]
1500
EA  0.95  0.13 eV
Mechanism of surface deconstruction
EXCHANGE
HOPPING
TS
TS = 2.56 eV
Final state= 1.09 eV
Fs
TS= 2.03 eV
Final state= 0.99 eV
Defect mediated diffusion mechanism
Step
1.14 eV
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1.16 eV
Molecular adsorbates on surfaces
Carbon Monoxide adsorption on Rh(111)
on-top bridge
bridge
on-top
A. Baraldi et al., Surf. Sci. Rep. 49, 169 (2003).
Localized vibrations in adsorbed molecules
C*O
Harmonic approximation
Probing the vibration of
C*O excited molecules
O
C
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Photoemission
Intensity
CO
(√3x√3)R30°
ΔE0,1=231.2 meV
ΔE0,2=461.7 meV
CO on Ir(111)
C1s core-level spectrum of adsorbed CO
c(2√3x4)rect
ΔE0,1=232.8 meV
ΔE0,2=466.2 meV
Atomic and molecular adsorption.
on transition metals
CO su Pt(111)
(4x4)
Q=0.06 ML
h
c(4x2)
Q=0.50 ML
O1s
C1s
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Carbon monoxide adsorption on Pt(111)
h
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Pt4f
Carbon monoxide adsorption on Pt(111)
Molecular adsorption site determination
dPt 0
 1
dQ
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dPt1
1
dQ
Oxygen adsorption on Rh(100)
MOVIE
Rh-1O
Rh-2O
Atomic adsorption site determination
dRh0
 4
dQ
four-fold hollow
adsorption site
A. Baraldi et al., Phys. Rev. Lett. 93, 046101 (2004).
SCLS vs chemical reactivity
1.2
-0.8
1.0
-1.0
0.8
-1.2
NO Adsorption energy [eV]
SCLS (eV)
C3,3
C2,3
0.6
0.4
C2,4
C1,4
0.2
Rh(111)
0.25 ML
-1.4
-1.6
-1.8
C1,3
0.5 ML
0.5 ML
0.0 ML
O/Rh(100)
0.25 ML
-2.0
0.0
0.0
0.0 ML
0.4
0.8
1.2
1.6
0.0
0.2
0.4
0.6
SCLS (eV)
0.8
Ed (eV)
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A. Baraldi et al., Phys. Rev. Lett. 93, 046101 (2004).
Surface defects vs chemical reactivity
Role of steps in N2 activation on Ru(0001)
S. Dhal et al., Phys. Rev. Lett. 83, 1814 (1999)
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Controlling the catalytic bondbreaking selectivity on Ni
surfaces by step blocking
R.T. Vang et al., Nature 4, 260
(2005)
Catalytic activity of gold nanoparticles,
B. Hvolbæk et al., Nanotoday 2, 14 (2007)
The role of adatoms on solid surfaces
2D adatom lattice-gas
Pd, Cu and Ag adatoms densities are 1-6 % on W at 700 K.
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Acetylene Cyclotrimerization
Adatom and addimer-induced Rh3d5/2 SCLS
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A. Baraldi et al., New J. Phys. 9, 143 (2007).
2.67 Å
Adatom and addimer
local geometrical
structure
d=2.50 Å
d12=1.82 Å
Rh(100)
-3.7 %
-6.4 %
d1
d1=2.59 Å
d2=2.51 Å
d3=2.55 Å
-3.0 %
-6.2 %
-4.8 %
d3
d2
2.59 Å -3.0 %
0.03 Å
dA=1.60 Å
dD=1.60 Å
d12=1.84 Å -2.7 %
dD=1.63 Å
d12=1.87 Å
Adatom and addimer-induced Rh3d5/2 SCLS
bulk
II-layer Rh(111)
II-layer Rh(100)
Undercoordinated atoms and
surface chemical reactivity
A careful analysis of CLSs provides a spectroscopic measure
of chemical reactivity changes at the atomic level.
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A. Baraldi et al., J. Phys. Chem. C 115, 3378 (2011).
Nanostructured surfaces and chemical reactivity
Tuning the Rh(110)
surface morphology
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[110]
[001]
Carbon Monoxide adsorption on Rh surfaces
CO↔C+O
flat
step
Ediss
(111)
(211) step
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Rh(111)
Rh-step
Rh-kink
1.17
0.30
0.21
kink
Eads
1.84
1.18
1.09
CN
9
7
6
The rhomboidal nanopyramids
Energy
b)
400eV
eV
400
c)
220
eV
250
eV
15
10
5
0
-5
-10
K<1-10> (% BZ)
a)
700eV
eV
700
-15
-15-10 -5 0 5 10 15
K<00 1> (% BZ)
Terrace width G=6.6 Å
Facet slope τ=11±2°
Spatial Periodicity L=14.7 nm
Thermal stability up to ~500 K
Thermal dissociation of Carbon Monoxide
9.4±0.5 %
F. Buatier de Mongeot et al., Phys. Rev. Lett. 97, 56103 (2006).
22±3 %
80±14 %
Surface segregation during chemical reactions
Adsorbates-induced
surface segegation.
Surface Concentration [%] Coverage [ML]
H2+O2 su Pt50Rh50(100)
QO
QH
H2O
0.4
0.2
0.0
T=330 K
100
T=520 K
T=400 K
Ptsurf
80
60
40
Rhsurf
20
0
0
2000
4000
A. Baraldi et al., J. Am. Chem. Soc. 127, 56713 (2005). Time [s]
6000
8000
A new dimension for carbon
● Two dimensional allotrope of carbon
● Basic-building block of graphite, carbon nanotubes and large fullerenes
● New electronic structural properties
● Single electron transistors
● Hydrogen storage devices
● Chemical sensors
● Ultracapacitors
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The methods for graphene production
● Mechanical exfoliation
● Liquid suspension graphene oxide followed by chemical reduction
● Epitaxial growth by thermal desorption of Si atoms from the SiC surface
● Unzipping carbon nanotubes
Epitaxial growth by chemical vapor deposition on
transition metals
● High quality carbon layers
● Tunable properteis
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Ethylene adsorption on
Transition Metals
Graphene-induced Surface Core Level Shift
C 1s
Ir 4f7/2
SCLS= -545 meV EXP
-550 meV THEO
HCP
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TOP
SCLS= -535 meV EXP
-549 meV THEO
-551 meV THEO
P. Lacovig et al., Phys. Rev. Lett. 103, 166101 (2009).
Kinetics of C2H4 adsorption
Time-lapsed
C 1s spectra
at
1273 K
400 ms/spectrum
Time-lapsed
C 1s spectra
at
823 K
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Temperature evolution of the adsorbate layer
C 1s components
CA= 284.12 eV
CB= 283.94 eV→284.10 eV
CC= 283.61 eV
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Morphology of carbon nano-islands from DFT
1.62 Å
Θ=0
2.53 Å
Θ=11
Dome-shaped carbon nano-islands formation
2.63 Å
Θ=16
3.13 Å
Θ=21
Nano-islands induced Ir4f7/2 core-level shifts
-225/-390 meV
Surface = -550 meV
Bulk
Ir1= +498 meV
Ir2= -325 meV
Ir3= -551 meV
Ir4= -132 meV
Ir5= -270 meV
Nano-islands induced C 1s core-level shifts
C4
C1= +268 meV
C2= -348 meV
C3= -439 meV
The energetics of nano-islands
Clusters formed with different number P of Honeycomb Rings (HRs)
P=19, n=54
P=7, n=24
P=3, n=13
graphene
P=1, n=6
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Thermal expansion of graphene
K.V. Zakharchenko et al.
Phys. Rev. Lett. 102, 046808 (2009)
C1s temperature dependence
Graphene is strongly anharmonic
due to soft bending modes.
Up to 900 K, graphene is
anomalous since its lattice
parameter decreases going over
to normal behavior at higher
temperatures.
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● Phonon-induced broadening
● Core level binding energy shift
M. Pozzo et al., Phys. Rev. Lett. 106, 135501 (2010).
Lattice constant versus interatomic distance
Ab initio molecular dynamics calculations
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Thermal expansion of graphene
Gas-phase data
CV
G1
CV
G
Thermal properties of graphene
SHIFTS
EXP =+70 meV
THEO =+20 meV
Karl-Franzens Universität, GRAZ, April 2010
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C1s core level shifts of epitaxial graphene
C 1s
A correlation between
graphene-substrate bond strength
and
graphene corrugation
weak interaction
Strong chemical bond
Why just two components?
0.2 Å
1.5 Å
Graphene growth on Re(0001)
Formation of a high–quality single-layer of graphene is strongly opposed
by two competing processes, namely surface carbide formation and
carbon bulk dissolution.
Time-lapsed
spectral sequence
of C1s spectra
taken during
ethylene exposure
and surface
annealing to high
temperature.
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E. Miniussi et al., submitted
Graphene corrugation on Re(0001)
2 μm
strong
interaction
moirè pattern
weak interaction
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E. Miniussi et al., Phys. Rev. Lett. 106, 216101 (2011).
C-C bond stretching on Re(0001)
humps
C-C bond length, obtained as
the average distance from the
three nearest neighbouring
atoms for each C atom
valleys
1.467 Å
1.460 Å
(0.5 %)
Graphene on Ru(0001): C1s spectrum
W
S
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S
W
D. Alfè et al., Sci. Rep. 3, 2430 (2013)
Graphene growth mechanism
Fast-data acquisition allows to monitor the C1s
spectral evolution while dosing ethylene at high
temperature.
LEEM (10 μm f.o.v.)
The carbon lattice-gas:
precursor to graphene formation
Eb(eV)
C 1s(eV)
MH2
7.53
283.02
MH1
MF2
7.78
283.55
DA1
MH4
7.31
282.97
DA2
MF3
7.45
283.37
DT1
MH5
7.38
282.89
DT2
MF4
7.15
282.92
DB1
Eb(eV)
C 1s(eV)
7.67
282.82
283.14
15.78
283.35
283.28
14.73
283.39
283.29
15.52
MF1
6.98
282.89
DB2
Three-fold hcp site on the terraces (MH1)
and the C monomer at the steps (MF2)
have very similar adsorption energies.
283.17
Monomers form a 2D lattice gas which
supplies C atoms for GR formation.
D. Alfè et al., Sci. Rep. 3, 2430 (2013)
Fine-tuning of graphene-metal adhesion
Bimetallic surface alloying provides a viable route for governing the interaction
between graphene and metal through the selective choice of the elemental
composition of the surface alloy.
C1s
The formation of PtRu surface alloys by
deposition of sub-monolayer
Pt films on Ru(0001) and subsequent
annealing to HT
H. E. Hoster et al., Phys. Chem. Chem. Phys. 10,
3812 (2008).
Fine-tuning of graphene-metal adhesion
Ru(0001)
C-substrate
distance
Charge
density
difference
Simulated
C1s
spectra
0.1 ML
0.2 ML
0.5 ML
For those who are interested in graphene
P. Lacovig, et al., Growth of dome-shaped carbon nanoislands on Ir(111): the intermediate between carbidic clusters and quasi free-standing graphene,
Phys. Rev. Lett. 103, 166101 (2009).
S. Lizzit, et al., Band dispersion in the deep 1s core levels of graphene,
Nature Physics 6, 345 (2010).
S. Lizzit, et al., High resolution fast x-ray photoelectron spectroscopy study of ethylene interaction with Ir(111): from chemisorption to dissociation and
graphene formation,
Catal. Today 154, 68 (2010).
M. Pozzo, et al., Thermal expansion of supported and free-standing graphene: lattice constant versus interatomic distance,
Phys. Rev. Lett. 106, 135501 (2011).
E. Miniussi, et al., Thermal stability of corrugated epitaxial graphene grown on Re(0001),
Phys. Rev. Lett. 106, 216101 (2011)
R. Larciprete, et al., Dual Path Mechanism in the thermal reduction of graphene oxide.
J. Am. Chem. Soc. 133, 17315 (2011).
A. Cavallin, et al., Local Electronic Structure and Density of Edge and Facet Atoms at Rh Nanoclusters Self-Assembled on a Graphene Template,
ACSNano 6, 3034 (2012).
S. Lizzit, et al., Transfer-free electrical insulation of epitaxial graphene from its metal substrate,
Nano Letters 12, 4503 (2012).
S. Ulstrup et al., High-temperature behavior of supported graphene: Electron-phonon coupling and substrate-induced doping,
Phys. Rev. B 86, 161402R (2012).
R. Larciprete, et al., Oxygen switching of the epitaxial graphene–metal interaction,
ACS Nano 6, 9551 (2012)
D. Alfè, et al., Fine-tuning of graphene-metal adhesion by surface alloy,
Sci. Rep. 3, 2430 (2013)
Grado, September 21, 2013