1
1
Reflection Anisotropy Spectroscopy (RAS)
and molecular assembly at ordered surfaces
Epioptics-9 Erice July 2006
David Martin
• What is RAS? Basics of RAbsorptionInfraRedS.
• RAS of Cu(110) and molecular assembly (carboxylates) on Cu(110).
• RAS of Au(110) and molecular assembly (amino acid) on Au(110).
Department of Physics and SSRC
A précis of the two lectures given at the Epioptics 9 Summer School
2
What is Reflection Anisotropy Spectroscopy?
Reflected light yields information on anisotropy of the surface
‘The study of the interaction of radiation with matter’
• A linear optical probe.
• Generally non-destructive – visible light used.
• Surface sensitive.
Cubic material
Surface anisotropy
Bulk linear optical response to normal incidence light is
isotropic
• Study surfaces in:
-Ultra-high vacuum (UHV)-High pressure-Ambient or Liquid environments.
Recent Review of RAS: P. Weightman, D.S. Martin, R.J. Cole, T. Farrell, Rep. Prog. Phys. 68, 1251 (2005)
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3
Polariser
Xenon lamp
WindowAnalyser
Monochromator
Detector
Azzam and Bashara, ‘Ellipsometry and Polarised Light’ Elsevier (1977)
2/)( yx
yx
rr
rr
r
r
+−
=∆
rxry
45°The RAS spectrometer
Photoelastic modulator (PEM)
Propagation of light through spectrometer described by Jones vector/matrices:
D.E. Aspnes et al, App. Phys. Lett. 52, 957 (1988)
V.L. Berkovits et al, Solid State Com. 56, 449 (1985)
P. Weightman, D.S. Martin, R.J. Cole, T. Farrell, Rep. Prog. Phys. 68, 1251 (2005)
4
Photo of RAS kit.
Relation of RAS to other Epioptic techniques:
J. McGilp, Prog. Surf. Sci. 49, 1 (1995) W. Richter, Phil. Trans. R. Soc. A 344, 453 (1993)
d) PEM
e) Analyserb) Polariser
a) Xenon lamp
f) Monochromator
g) Detector
c
a
bd
e fg
c) Window –access to sample
3
5
Surfaces giving ‘zero RAS’ signal – optically isotropic
• Ideal FCC (001) surfaces
Bulk crystal
(001) plane
(001) surface
FCC unit cell
-5-4
-3-2
-1012
34
5
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re(
r/r)
250300350400450500550600650700750800
Wavelength (nm)
2/)( yx
yx
rr
rr
r
r
+−
=∆
6
θε(θ)
x
y
θεθεθε 22 sincos)( yyxx +=
22
2
3
2
1
+
= yyxxxx εεε yyxx εε =
=
3
2)0(
πεε
Optical response in direction θ:
components of the dielectric tensor.
(111) surface has three-fold symmetry axis:
So from [1]
[1]
and hence
• Ideal FCC (111) surfaces
Surfaces giving ‘zero RAS’ signal
so ‘zero RAS’
True only for macroscopically ordered ‘perfect terminations’ of (001) and (111). Not true for vicinal surfaces, reconstructions, dimers on Si…
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7
FCC (110) surfaces have intrinsic structural anisotropy
(110) plane
(110) surface
[001]
[1 1 0]
FCC unit cell
-6
-4
-2
0
2
4
6
8
10
12
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re( ∆r
/r)
A
B
C
RA spectrum of the Cu(110)
surface
10-3
Re(
∆r/
r)
Photon energy (eV)
So far, most RAS investigations of metals have been with FCC and BCC (110) surfaces.
]001[]011[
]001[]011[)(2
rr
rr
r
r
+
−=
∆
8
Low temperature STM images of the occupied surface state on Cu(111) after confinement by circle of Fe atoms.
STM data by M.F. Crommie et al Science 262 (1993) 218
Surface state can be described as a 2D nearly-free electron gas.
Surface states on clean Cu
5
9
-6
-4
-2
0
2
4
6
8
10
12
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re
( ∆r/
r)
A
B
C
LW Γ
EF
Ene
rgy B C
2L′
L1u
RAS of the clean Cu(110) surface
rxry
A~1.8eV
~0.4eVEF
Energy
Symmetry
10-3
Re(
∆r/
r)
Polarisationselection rules
y x
z
s + pz
py +_
p-like
s-like
Come and ask me during the school!
+ other contributions
10
Suggested reading:
RAS of Cu(110)
Ph. Hofmann et al., Phys. Rev. Lett. 75, 2039 (1995)
J. Bremer et al., Surf. Sci. 436, L735 (1999)
K. Stahrenberg et al., Phys Rev B 61, 3043 (2000)
P. Monachesi et al., Phys Rev B 64, 115421 (2001)
D.S. Martin et al., Phys Rev B 62, 15417 (2000)
L.D. Sun et al., Surf. Sci. Lett. 527, L184 (2003)
63, 155403 (2001) 72, 35408 (2005)
Surface states
Review: N. Memmel, Surf. Sci. Rep. 32, 91 (1998)
M.F. Crommie et al., Science 262, 218 (1993)
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11
RAS can be used to monitor molecular assembly on macroscopic ordered surfaces where the surface induces order in the molecular layer
– substrate-influenced molecular assembly
Molecular assembly on Cu(110)
Di-carboxylic acids – opportunity to create a chemically functionalised surface with a second acid group at vacuum interface – available for further interaction.
Terephthalic acid(TPA)
12
Reflection absorption infrared spectroscopy (RAIRS)
-Measures absorptions due to bond vibrations.
-O-H
-C=O
-NH2
Group Wavenumber (cm-1)
3400
17503600
Intensity
Wavenumber (cm-1)
Number of wavelengths per cm
-Distinguish chemical bonds/groups by their vibrational signatures.
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13
RAIRS
++
+__
+
_
_
Metal
Cancelled Enhanced (x2)
Only vibrational modes exhibiting dynamic dipoles perpendicular to a metallic surface are observable.
Greenler et al Surf. Sci. 118 (1982) 415
-Metal surface selection rule:
Banwell and McCash, Fundamentals of molecular spectroscopy (1994)
Side view
-IR reflected from metal surface >> absorbed by molecular layer
Subtraction: (surface + absorbate) – clean surfacespectrum spectrum
p-polarisedGrazing incidence
14
Symmetric OCO(s)~1420 cm-1
Asymmetric OCO(as)~1650 cm-1
Observe both modes if tilted
Carboxylate – stretching signatures
8
15
60010001400180022002600300034003800
Wavenumber (cm-1)
Inten
sity (
arbitra
ry un
its)
a
b
c
d
e
RAIRS
TPA on Cu(110)
Increasing coverage
OCO(s)OCO(as)
O-H
D.S. Martin, R.J. Cole, S. Haq, Phys. Rev. B 66 (2002) 155427
[110]
C=O
[110]
Out-of-plane mode of acid group
Low coverage
First monolayer
16
-8
-6
-4
-2
0
2
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Photon energy (eV)
10-3
Re(
∆r/r
)
TPA on Cu(110) – RAS data
First monolayer
Low coverage
D.S. Martin, R.J. Cole, S. Haq, Phys. Rev. B 66 (2002) 155427
4.2 eV
[110]
[110]
4.2 eV
TPA - no optical transitions in range 1.5-5.0 eV.
Looking at influence on the Cu(110) surface RAS response.
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17
Functionalised surface:– control over chemistry
LEED shows p(2 x 1) structure.
Top view
D.S. Martin, R.J. Cole, S. Haq, Phys. Rev. B 66 (2002) 155427
RAIRS shows little evidence of H-bonding.For the first complete layer:
TPA on Cu(110) – first monolayer
– wide-ranging applications: immobilisation of biomoleculesmolecular recognition / biosensors
[001]
[11 0]
X
X
X
XX
XSide view
[11 0]
18
Zoomin
Prepare flat and smooth metal surfaces for molecular adsorption in aqueous environments.
Au surfaces are resistant to oxidation
Challenge:
1 µµm(1 x 2)
(1 x 1)
High resolution STM
The Au(110)-(1 x 2) surface
Au(110) standard polishing finish
1 µµm1 µµm
Imaged in ambient air, after ion bombardment
/annealing in UHV
G.E. Isted and D.S. Martin, Appl. Surf. Sci. 252, 1883 (2005)
AFM AFM
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RAS of the Au(110)-(1 x 2) surface
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re(
r/r)
B C
D
A
2 LW Γ
EF
Ene
rgy C D
′ L 2
L1u
B
Clean (UHV)ambient air
D.S. Martin et al, J. Phys.: Condens. Matter, 16, S4375 (2004)
L3
There is strong evidence that peaks B,C,D result from transitions between bulk bands near the L point that are modified by the anisotropic surface…
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What happens to polarised light reflected by a surface, and the RAS measurement...
Why it happens – link to a property of the material
∆
≈r
r
I
I
dc
Re2ω
- Jones analysis
- Interaction of light with a material is described by the dielectric tensor ε of the material.
- Can model RA spectra using a simple three-phase model, with each phase having its own ε.
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Clue 1: Model RA spectra - energy derivative model
JDE McIntyre and DE Aspnes, Surf. Sci. 24, 417 (1971)
εb
εv=1
Three-phase model
∆Eg = Energy and ∆Γ = linewidthdifference between x and y polarisationsfor the interband transitions.
At surface, εs is approx. proportional to the energy derivative of εb
d
Rossow et al, J. Vac. Sci. Technol. B 14, 3070 (1996)
(for d << λ)
εb from spectroscopic ellipsometry (SE) data.
xsεy
sε ys
xss εεε −=∆
K Stahrenberg et al, Phys Rev B, 65, 35407 (2001)
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1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re(
r/r)
B C
D
A
2
From SE data using an Au(110) crystal*
*NP Blanchard et al, Phys. Stat. Solidi. c 0, 2931 (2003)
012345678
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Energy (eV)
εb"
dE
d b''ε
''bε
RAS
-14
-12
-10
-8
-6
-4
-2
0
2
4
2.0 2.5 3.0 3.5 4.0 4.5 5.0
B?
C?
D?
x(-1)
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Clue 2: Au(110) as function of temperature
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re(
∆r/
r)
2
300K
580K
690K
790K
900K
1000K
A
B C
D
D.S. Martin et al, J. Phys.: Condens. Matter, 16, S4384 (2004)
Thermovariation optical spectroscopy results
-5.3x10-4 eV/K
P. Winsemius et al, J. Phys. F: Met. Phys. 6, 1583 (1976)
Main features of εb derive from interband transitions near L
3.0
3.1
3.2
3.3
3.4
3.5
3.6
250 450 650 850 1050
Temperature (K)
Pea
k po
sitio
n (e
V) -6.9x10-4 eV/K
-5.9x10-4 eV/K
Peak
pos
ition
(eV
)
Temperature (K)
RAS Peak C
RAS
uF LE 1→ Energy (eV)
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1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re(
∆r/
r)
2
a
b
c
ii
i
iiiSTM
RAS
D.S. Martin et al, Surf. Sci. 532-535 (2003) 1
(100nm x 100nm)
(100nm x 40nm)
(100nm x 100nm)
0.21
0.12
RMS roughness
i
iii
ii
Clue 3: Ar ion bombardment of Au(110)
13
25
-0.006
-0.004
-0.002
0.000
0.002
0.004
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
Re(
r/r)
B
C
D
E
F
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re(
r/r)
B C
D
A
2E
F
D.S. Martin et al, J. Phys.: Condens. Matter, 16, S4375 (2004)
bombarded
Well ordered
Experimental resultsEnergy derivative
model
∆Γ=0.0, ∆Eg=0.05 eV ( 1.7-3.0 eV) ∆Γ=0.0, ∆Eg=0.30 eV ( 3.0-4.5 eV)
∆Γ=-0.3 eV, ∆Eg=0.0 eV ( 3.0-4.5 eV)Ar ion bombarded
Well ordered
− ∆Γ increases due to ion bombardment induced disorder, but only above 3 eV?.
− The simple three-phase model does quite well – importance of εb.
All clues together…
LW Γ
EF
Ene
rgy C D
′ L 2
L1u
B
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L-Cysteine
thiol group
(HS-CH2-CH(NH2)-COOH)
RAS of amino acid (L-Cysteine) thin film on Au(110)
Proteins - structure on range of length scales:
Tertiary (quaternary) structure – 3D conformation
Human serum albumin
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XPS: Sulphur 2p line – thiolate bonding
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re(
∆r/r
)
1
B C
2saturated
UHV deposition
RAS of L-Cysteine on Au(110)-(1 x 2)
Electrochemical depositionL-Cysteine in 0.1M phosphate buffer (pH 7)
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re(
∆r/
r)
2
-0.6V0.0V
B C
Clean
RAS sensitivity to Au-S bondAu Au
?
AD
No clear LEED pattern.
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Change in B Consistent with assignment of
L-Cys/Au(110) following heating to 580 K and cooling to RT: S/Au(110)
B: molecule.
No change in C
XPS: only Au-S
C: Au-S bond.
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Photon energy (eV)
10-3
Re(
∆r/
r)
1
B C
S/Au(110)
(UHV)
L-Cys/Au(110)
CleanCC
Au
(100 x 60) nm
Au-S
G.E. Isted, et al., Phys. Stat. Sol. (c), 2, 4012 (2005)
LEED shows c(4 x 2) + (1 x 2)
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Summary
• Highlighted the basics of RAS (and RAIRS).
• RAS sensitive to substrate-influenced molecular assembly:
- ordering of molecular structures
- bonding (Cu-carboxylate, Au-S)
- orientation (flat lying/upright)
• RAS shows similar behavior for L-Cysteine/Au(110) in aqueous and UHV environments.
AcknowledgementsDr. Jamie Cole (Edinburgh)
Dr. Sam Haq
Dr. Caroline Smith
Dr. Rozenn Le Park (Montpellier)
Greg Isted
Prof. Peter Weightman
Dr. Nick Blanchard
UK EPSRC + Royal Society