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

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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]

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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)

24

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)

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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

26

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