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Liquid/Semiconductor
Interfaces through
Vibrational

Spectroscopy

R. Kramer
Campen

Physical Chemistry Department

Fritz Haber Institute of the Max Planck Society

1 / 37

25/3/13

w
/ Maria
Sovago
, Cho
-
Shuen

Hsieh,
Mischa

Bonn, Ana Vila Verde

Outline

I.
Justification:
why
vibrational

spectrocopy
?

II.
Background:
making
vibrational

spectroscopy
interface specific:
vibrational

sum frequency
spectroscopy

III.
Interfacial Solvent (Water):
structure and
dynamics

IV.
Semiconductor Interface:
optical detection of
surface phonons

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Outline

Why
vibrational

spectroscopy?

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Vibrations

report, label free, on inter/
intramolecular

forces (and structure)

I. Justification

Barth and
Zscherp

(2002)
Quarterly Reviews of Biophysics
,
35
, 4

from

Zscherp

and
Heberle

(1997)
Journal of Physical Chemistry B
,
101
, 10542

Spectrum of local oscillators
modified by environment,

1.
Through
-
bond coupling

2.
Hydrogen bonding

3.
Transition dipole coupling


Macromolecules (proteins)

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Why
vibrational

spectroscopy?

Vibrations

report, label free, on
interatomic
/molecular
forces (and structure)

Liquids (water)

I. Justification

OH stretch modulated by
anharmonic

coupling to low
frequency modes.

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II. Background

Interface specific
vibrational

spectroscopy

Vibrational

Sum Frequency Spectroscopy

ir

vis

sfg


ν
=0

forbidden in bulk water

c
(2)
=0

c
(2)
=0


ν
=1

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II. Background

What is detected?

The field radiated by the second order induced polarization

Induced polarization can be expanded in a Taylor series

Assuming two incident plane waves and just considering
the second order term,

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Origin of Interfacial Specificity

Even order nonlinear susceptibilities

Second order nonlinear susceptibilities are third rank tensors



Changing the sign of all indices is equivalent to inverting the
axes: the material response must change sign…

But, with inversion symmetry, all directions are equivalent so…

For materials with inversion symmetry (
χ
(2)
=0),
inversion
symmetry is always broken at interfaces.

II. Background

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Origin of Chemical Specificity

Raman scattering off an excited, coherent, vibration

II. Background

VSF active modes must be Raman and IR active.

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III. Solvent
to

Interface: Structure

III. Approaching the Interface from
the Solvent Side: Interfacial Water
Structure

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The OH stretch at the air/water interface

Two (or three) qualitative populations

Free OH

0.6

0.5

0.4

0.3

0.2

0.1

0.0

3800

3600

3400

3200

IR frequency (
cm
-
1
)

3000

IR Abs: Liquid Water

Hydrogen Bonded OH

I
SFG

(arbitrary units)

free

III. Solvent
to

Interface: Structure

VSF spectral features
are apparent that are
absent in bulk IR.

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III. Solvent
to

Interface: Structure

Is the double peaked feature general?

Yes!

Double peaked feature appears at all interfaces

Kataoka
,…, Cremer (
2004
)
Langmuir
, 20(5), 1662

Becraft

and Richmond (2005
)
Journal of Physical Chemistry B
, 109(11), 5108

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III. Solvent
to

Interface: Structure

Is the free OH feature general?

It appears at all hydrophobic interfaces

CCl
4
/ H
2
O



Hexane / H
2
O

Air/
H
2
O

Scatena
, Brown, Richmond (2001)
Science
, 292, 908

Silica/OTS/Water Interface

Ye,
Nihonyanagi
,
Uosaki

(2001)
PCCP
, 3(16), 3463

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III. Solvent
to

Interface: Structure

0.6

0.5

0.4

0.3

0.2

0.1

0.0

3800

3600

3400

3200

IR frequency (
cm
-
1
)

liquid water

3000

I
SFG

Du
,…,
Shen(1993)
Physical Review
Letters
, 70(15), 2313

ice

weak

‘water
-
like’

strong

‘ice
-
like’

What causes the double peaked feature ?

One idea: interfacial structural heterogeneity

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What causes the double peaked feature ?

A second idea: symmetric/asymmetric stretch

O
-
D

O
-
D

asym

symm

(same water molecule)

III. Solvent
to

Interface: Structure

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Two peaks should collapse to one

SS

AS

Sym/
Asym

hypothesis

D
2
O

HDO

SS

AS

Strong

Weak

D
2
O

HDO

‘Ice/Liquid
-
like’ hypothesis

The amplitude of

the whole
spectrum should change.

How can we distinguish these scenarios
experimentally

Isotopic dilution

III. Solvent
to

Interface: Structure

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Two peaks collapse to one

double peaked spectral feature ≠ water structure

2

1

0

SFG intensity (
arb
. units)

3200

3000

2800

2600

2400

2200

2000

IR frequency (
cm
-
1
)

O
-
D

C
-
H

water /

lipid
÷
10

water/air

H
2
O:D
2
O 2:1

pure D
2
O

H
2
O:D
2
O 2:1

pure D
2
O

III. Solvent
to

Interface: Structure

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3000

2800

2600

2400

2200

2000

IR frequency (cm
-
1
)

water/air

bend overtone (
d
OH
=2)

O
-
D

O
-
D

asym

symm

d
OH
=2

*

*

*

D
2
O


䡏䐠H睩w捨敳c潦映捯異u楮i

H

D

*

Two

peaks

have
same

symmetry
!
#

#
Gan
,

,
Wang (2006)

Journal of Chemical
Physics
, 124
, 114705
.


Are the two peaks sym/
asym
?

III. Solvent
to

Interface: Structure

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frequency

stretch mode DOS

coupling off

coupling on

Evans

window

bend overtone (
d
OH
=2)

3000

2800

2600

2400

2200

2000

IR frequency (cm
-
1
)

water/air

bend overtone (
d
OH
=2)

*

*

Hypothesis one: two peaks are from a
Fermi Resonance with the Bend Overtone

III. Solvent
to

Interface: Structure

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Hypothesis two: low frequency peak from
collective effects (intermolecular coupling)

III. Solvent
to

Interface: Structure

Buch

et al. (2007)
Journal of Chemical Physics
, 127(20), 204710

coupling switched off

Computation suggests the low frequency peak is a the result
of intermolecular coupling (
vibrational

delocalization).

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In either case using HDO allows more
straightforward access to structure

Fitting the HDO data

III. Solvent
to

Interface: Structure

25/3/13

III. Solvent
to

Interface: Dynamics

III. Solvent to Interface: Dynamics

0.6

0.5

0.4

0.3

0.2

0.1

0.0

3800

3600

3400

3200

IR frequency (
cm
-
1
)

3000

IR Abs: Liquid Water

I
SFG

(arbitrary units)

free

Free OH

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III. Solvent
to

Interface: Dynamics

Probing dynamics: IR pump


VSF Probe

IR (100
fs
)

vis

(100
fs
)

SFG

v = 0

v = 1

IR pump
(100
fs
)

t
wait


SFG signal decreases due to
depletion of ground state.


Recovery reflects
vibrational

relaxation and possibly
reorientation.

t
wait

SFG

pump

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III. Solvent
to

Interface: Dynamics

Probing reorientation of the free OH

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III. Solvent
to

Interface: Dynamics

different rates

the same rate

Signals relax at…

At long times…

signals do not converge

signals converge

Developing intuition for the signal

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III. Solvent
to

Interface: Dynamics

Free OH relaxation is intermediate

τ
p
, pump
= 640
±

20
fs

τ
s
. pump
= 870
±

50
fs


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III. Solvent
to

Interface: Dynamics

A brief simulation interlude


Simulation box
is 30*30*60

Å.


Periodic boundary conditions.


NVE
at
T = 300 K.


Simulation
run
for
2
ns (step =
1
fs
).


SPC/E potential.




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III. Solvent
to

Interface: Dynamics



= yes

θ

=
maybe (diffusion within a potential)


ϕ
2


free
OH

bulk H
2
O


ω
2


Free OH reorientation is diffusive (in MD)

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III. Solvent
to

Interface: Dynamics

Consistent with diffusion in a potential

Free OH has a preferred distribution in
θ

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III. Solvent
to

Interface: Dynamics

Model reorientation as diffusive in a
potential

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III. Solvent
to

Interface: Dynamics

D


= 0.32 rad
2
/ps,
D
q

= 0.36 rad
2
/ps

Fitting 2D diffusion model to simulation
output

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III. Solvent
to

Interface: Dynamics

MD results describe data without
adjustable parameters

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Bulk (
arb

orient)

D
ϕ

= 0.1 rad
2
/ps

D
θ

= 0.1 rad
2
/ps

Free OH

D
ϕ

= 0.32 rad
2
/ps

D
θ

= 0.36 rad
2
/ps

On average
, free OH reorient ≈ 3x
faster than bulk

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III. Solvent
to

Interface: Dynamics

Free OH reorientation relative to bulk

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IV. Solid
to

Interface

Phonon spectroscopy in bulk
α
-
qtz

Gonze
, Allan,
Teter

(1992)
Physical Review Letters
, 68(24), 3603

Probing these modes optically
(IR or Raman) depends on:

1.
Mode symmetry

2.
Tranverse

(couples to IR)
v
.
longitudinal (does not)

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VSF probes both IR and Raman active transverse optical phonons: a
simplified phonon spectrum that reflects bulk symmetry.

VSF phonon spectroscopy in bulk
α
-
qtz

Liu and
Shen

(2008)
Physical Review B
, 78(2), 024302

IV. Solid
to

Interface

795 cm
-
1

1064 cm
-
1

1160 cm
-
1

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VSF surface phonon spectroscopy on
α
-
qtz

and amorphous SiO
2

IV. Solid
to

Interface

Resonances appear in VSF response at,

1.
nonbulk

frequencies

2.
nonbulk

symmetries

3.
for amorphous material

4.
and are perturbed by surface chemistry

Liu and
Shen

(2008)
Physical Review Letters
, 101(1), 016101

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IV. Solid
to

Interface

VSF surface phonon spectroscopy on
α
-
qtz

for tracking surface reconstruction

wavenumber

(cm
-
1
)

after being baked
at 100
°
C

rehydroxylated

in
ambient air

after being baked
at 500
°
C

rehydroxylated

in
ambient air

rehydroxylated

in
boiling water

S
i

S
i

O

S
i

OH

Liu and
Shen

(2008)
Physical Review Letters
, 101(1), 016101

S
i

OH

2

S
i

S
i

O

H
2
O

+

surface
damage


= quartz

= amorphous silica

+

Conclusions

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Conclusions

1.
Using VSF spectroscopy one can probe
both

interfacial solvent (most work on water)
and

surface
phonons at
buried interfaces
.

2.
Adding additional pulses to VSF experiments makes
it possible to probe structural and energy relaxation
dynamics with interfacial specificity.

3.
Future work: optically probing the association of
interfacial molecules with particular surface sites.

Related Publications (from our previous work the
results of which were discussed above)

1.
Sovago
,
Campen
,
Wurpel
,
Müller
, Bakker, Bonn (2008)
Vibrational

Response of
Hydrogen
-
Bonded Interfacial Water is Dominated by Interfacial Coupling
,
Physical
Review Letters
, 100, 173901

2.
Sovago
,
Campen
, Bakker, Bonn (2009)
Hydrogen bonding strength of interfacial
water determined with surface sum
-
frequency generation
,
Chemical Physics
Letters
, 470, 7

3.
Hsieh,
Campen
, Vila Verde,
Bolhuis
,
Nienhuys
, Bonn (2011)
Ultrafast Reorientation
of Dangling OH Groups at the Air
-
Water Interface Using
Femtosecond

Vibrational

Spectroscopy
,
Physical Review Letters
, 107(11), 116102

4.
Vila Verde,
Bolhuis
,
Campen

(2012)
Statics and Dynamics of Free and Hydrogen
-
Bonded OH Groups at the Air/Water Interface
,
Journal of Physical Chemistry B
,
116(31), 9467

bonus page

25/3/13