Molecular orientation in thin monolayer films by infrared spectroscopy

Journal of Electron Spectroscopy

and Related Phenomena,

30 (1983) 29-34

29

Blsevier Scientific Publishing Company, Amsterdam -Printed

in The Netherlands

MOLECULAR ORIENTATION IN THIN MONOLAYER BY INFRARED SPECTROSCOPY

FILMS

J. F. Rabolt, F. C. Bums, N. E. Schlotter and J. D. Swalen IBM Research

Laboratory,

San Jose, California

95193

ABSTRACT

Fourier transform infrared spectroscopic measurements have been made on monolayer samples of cadmium arachidate in order to determine orientation and molecular packing on the surface. This was accomplished by using both grazing angle reflection methods, where the polarization of the infrared radiation is very close to being perpendicular to the surface, and transmission methods, where the incident optical electric field is polarized parallel to the surface. Hence these two methods are sensitive to molecular vibrations whose change in Our results showed that independent of the dipole moment lies along different directions. substrate, silver for the reflection experiments and silver bromide for the transmission experiments, the chains of the fatty acid salt (no evidence for any free acid was found) are oriented within a few degrees of the normal to the surface of the substrate. From a detailed analysis of the observed vibrational bands in the two orientations, combined with the known literature values and assignments, we were able to make a “complete assignment” of the observed bands. Our experimental results and conclusions will be presented.

JNTRODUCTION Infrared

spectroscopy

of fatty acid monolayers (Ref.

1) employed

has been used to investigate

the orientation

and their salts relative to the substrate

a multiple reflection

technique

of the aliphatic chains

surface.

Francis and Ellison

in which the monolayers

were deposited

directly onto two silvered mirrors to improve the signal to noise ratio from the few surface molecules.

Takenaka

et al. (Ref. 2) used

attenuated

polarized IR spectra of 33 layers of stearic acid deposited estimated found

a tilt angle of 25 to 35“ between

no evidence

monolayers

and

measurements 8’*5“. incidence

of in-plane its

calcium

indicated

This conclusion

by

of cadmium arachidate

both

Chollet’s reflection

that the acid was inclined for salts was supported

IR at an angle of 86” (Greenler

reflection

(ATR)

studies and

(Ref. 3) of behenic variable

angle

acid

transmission

at 2S0rb40 and the salt was inclined

at

by AIlara and Swalen (Ref. 4) where grazing

reflection)

was used to investigate

on silver.

0368-2048/83/0006-6000/$03.00

to obtain

the stearic acid tail and the surface normal, but

anisotropy. salt

total

on a germanium ATR plate and they

0 1983 Elsevier Scientific Publishing Company

l-10 monolayers

30 Our work reported infrared

here (see Refs. 5 and 6 for other details) combines grazing incidence

(GIIR) spectroscopy

characterize

with IR transmission

molecular orientation

measurements

in a cadmium arachidate

to accurately

monolayer

determine

and

film.

EXPERIMENTAL All infrared

measurements

with a room temperature resolution

with the addition

incidence

IR technique,

field perpendicular

of corresponding

described

data points

by Greenler

microscope

monolayers

slides

for

transmission

measurements.

a pH=6.3),

a small amount The arachidic

ANALAB,

1024 scans.

magnitude

at 2 cm-l

The grazing

(Ref. 7), selects the component

of the electric

at the surface to obtain the

of fatty acid salts were prepared

in detail (Ref. 8). These monolayers

surface.

from

equipped

were recorded

spectra of thin films.

Langmuir-Blodgett

coated

Spectra of the monolayers

to the surface and use its enhanced

polarized absorption

described

were made with an IBM IR98 FTIR interferometer

DTGS detector.

reflection

were deposited

studies

and

The trough contained, of cadmium

on

by methods

previously

from a water trough onto silver silver

bromide

substrates

for

in addition to water and buffer (to provide

chloride

to form the cadmium

acid (CH3(CH2)lgCOOH)

arachidate

on the

used in this work was obtained

Inc. from which the cadmium arachidate

was synthesized

in this laboratory.

from Bulk

samples were prepared in KBr under high pressure. RESULTS AND DISCUSSION Arachidic structure

acid contains

sequence

assignments. (2800-3000

studied

the IR spectra

In addition cm-l)

of n-alkanes

to the localized

modes

a number of band progressions

and rocking vibrations

figure.

observed

Snyder (Ref.

found in the spectrum of an oligomer, can be understood activity

allowed

polyethylene

through

the

breakdown

and made

involving

appear in the 1150-1450

clearly seen in the accompanying

which exists in a trans planar

Snyder and Schachtschneider

in the solid state similar to the n-alkanes.

have systematically

twisting

a long (-CH2-)n

in the

complete

(Ref. 9) vibrational

in the CH stretching a mixture of -CH2-

cm-l

region;

region wagging,

these modes can be

10) has shown that these progressions, by consideration

optical

selection

of the spectroscopic rules

for

an infinite

chain.

Since, as mentioned, that after deposition

recent IR studies (Refs.

on a metal surface, the hydrocarbon

to the surface, a combination axis, and transmission

3,4) on fatty acid salt monolayers tail is oriented approximately

indicate normal

of GIIR, to obtain the IR spectrum with E parallel to the chain

measurements,

to obtain the spectrum with E perpendicular

to the chain

axis, was used to assign the observed bands. In the CH stretching to methyl

(-CH3)

and

region (2800-3000 methylene

(-CH2-)

cm-l)

there are a number of bands attributable

stretching

vibrations.

As shown

in the

accompanying (E,)

figure, five bands of similar intensity

to the substrate

intensity,

(or parallel to the hydrocarbon

with perhaps

the substrate

are observed

a weak shoulder,

(or perpendicular

tail) while only three bands of sizeable

are present

in the spectrum

to the hydrocarbon

or two parallel spectra.

with E parallel (El)

The spectrum

tail).

from a random sample is shown in the bottom spectrum. perpendicular

when E is perpendicular

to

with E unpolarized

This should be a combination

of one

Visually this can be seen to be approximately

correct,

that is, combining the two top spectra in the right ratio should give the bottom spectrum In monolayers expected

with the aliphatic

that the -CH2-

E,, spectrum asymmetric stretching

bands,

and symmetric

found

since the asymmetric

either perpendicular

associated

(vs(CH2))

respectively.

stretching

methyl stretching

plane.

moment

When E is perpendicular

to the plane of the

at 2931 cm-l has recently fundamental

Levin.**

and is consistent

of the rs(CH3)

with the assignment

When E is parallel to the surface

rendered

(the middle spectrum

modes when E is perpendicular from the E perpendicular

previously

the ra(CH2) center

at 2919 cm-l

and in the middle

and a combination of the zone.

11)

split by

by Spiker and

b) in addition to the 2962 This could either

at the average of the two symmetric methyl

but shifted to lower energy as the asymmetric

to parallel orientation.

(Ref.

at 2874 cm-l

cm-* mode, there is a shoulder at 2895 cm-’ on the strong 2919 cm-l band. be the vs mode of the methyl group, approximately

mode,

since its change in dipole

been assigned Fermi resonance

symmetry

appears in the E,

surface, a medium band found in the vicinity of v,(CH2) component

and two

Thus, the 2962 cm-l

in the E, spectrum

to a second

CH

modes can have a change in dipole

change in the plane of the backbone,

to the skeletal plane.

to the

The methyl to rs(CH3)

spectrum while the 2954 cm-l band is observed is perpendicular

respectively

vibrations.

are assigned

only in the

The latter two bands exhibit different

or parallel to the skeletal

with the dipole moment

it would be

would occur with sizable intensity

at 2874, 2962 and 2954 cm-l,

of the va(CH3),

and polarization moment

vibrations

to the substrate

and hence the bands at 2919 and 2850 cm-l are assigned (r,(CH2))

components

stretching

tail truly perpendicular

mode is in going

Or it could be a Fermi resonance of two 6(CH2)

The vs(CH3)

between

modes at the Brillouin zone

would then probably

be under the

2919 cm-l band. In the region below 1600 cm-l are found several strong bands whose intensities strong polarization CO2 stretching

dependence

vibrations

of the carboxylate

sample,

the asymmetric

intense

than the symmetric

mode the change va(C02)

and which are attributable

stretch,

va(C02),

stretch,

in dipole moment

mode it is approximately

group.

to the asymmetric

and symmetric

As seen in the figure in an isotropic

is found at 1541 cm-’ and is considerably

vs(C02),

located

is perpendicular

show a

at 1433 cm-l.

Since in the p.s(C02)

to the substrate

parallel to the surface, a significant

more

surface

polarization

while in the effect of an

32

0.021

’ 3200

I 2800

I

I

0.06

I

I

I

I

0.01

I 3200

I

I 2800

I

2.25

,

I

I

I

I

I 2400 I

I 2400 I

I 2000

I

I

I

I 2000

I

I

I

I

I 1600 I

I

I 1200

I

I

I 1200

I

I 1600

I

I

I

I

I

(c) E (unpolarized)

I

1

0.25

I 3200

I

I 2800

I

I

I

2400 Wavenumbers

I

2000 cm-’

INFRARED SPECTRA CADMIUM ABACHIDATK a) six monolayers silver with normal to substrate; bromide E parallel to and isotropic bulk unpolarized where is the bending vibrational mode, mode, is the wagging vibrational mode 01 refers carboxylate group

1 1600

eighteen sample is the the u

1200

monolayers on silver KBr pellet E twisting vibrational atom adjacent to

oriented sample is expected. earlier

work

(Refs.

perpendicular

3.4)

As seen in the figure, this is, in fact, observed.

that

monolayers

of an IR polarizer,

investigated. observed, trough

Within experimental

indicating

bending

perpendicular packing. oriented polarized

experiments

on an eighteen

orientation

differences

approximately

After

subsequent

measurements

and rocking region, orthorhombic

to the substrate

Two hydrocarbon

(orthorhombic

the monolayer

changes or frequency direction

of the crystal

at various plane

were

shifts were

from the monolayer field splitting

in the

subcell packing with the molecules

nearly

modification)

chains in an orthorhombic

IR beam, it would be expected anisotropy

layered multilayer fii

deposition

was identified

as the crystal

unit cell have their molecular

at an angle of 90’ with respect to one another.

no apparent

This supports

are oriented

within

error no relative intensity

that in this case no preferential

had occurred.

-CH2-

acid salts

to the substrate.

In a series of transmission positions

of fatty

planes

In sampling a large area with the

that on the average, both orientations

contribute

and

should be detected.

CONCLUSIONS Spectroscopic grazing incidence fatty

investigation and transmission

of Langmuir-Blodgett measurements

acid salt and not as the free

polarizations

acid.

monolayers

of cadmium arachidate

indicate that the deposited The measurements

by

layers exist as the

at these

two orthogonal

led to an almost complete assignment from which it can be concluded that the

hydrocarbon tails are, within a few degrees, oriented normal to the substrate surface whether they are deposited, in monolayer form, on either clean silver (for GIIR measurements) or on silver bromide crystals (for transmission studies). In-plane polarized transmission studies to determine the extent of orientation in the plane of the monolayers did not detect any anisotropy.

However, this was not surprising since

the perpendicular orientation of the molecular planes of the two molecules in the orthorhombic unit cell would lead to an effective averaging of the molecular contributions to the vibrational spectrum which in this case prevents differentiation between uniaxial or biaxial orientation. ACKNOWLEDGMENT We would lie

to thank M. Jurich for his preparation

of the monolayer

samples on silver

bromide. REFERENCES 1. 2. 3. 4.

S. A. Francis and A. H. Ellison, J. Opt. Sot. Amer., 49 (1950) 131. T. Takenaka, K. Nogami, H. Gotoh and R. Gotoh, J. Coll. and Interf. 395 and 40 (1971) 409. P. A. Chollet, Thin Solid Fii. 52 (1978) 343. D. L. Allara and J. D. Swalen, J. Phys. Chem., 86 (1972) 2700.

Sci., 35 (1971)

34 5.

6. 7. 8. 9. 10. 11. 12.

F. A. Burns, N. E. Schlotter, J. F. Rabolt and .I. D. Swalen, IBM Instruments, Inc., Application Note No. 1, (198 1). J. F. Rabolt, F. C. Burns, N. E. Schlotter and J. D. Swalen, J. Chem. Phys. (submitted) R. G. Greenler, J. Chem. Phys., 44 (1966) 310. “Insoluble Monolayers at Liquid-Gas Interfaces,” (Interscience, G. L. Gaines, New York, 1966). R. G. Snyder and J. H. Schachtschneider, Spectrochlm. Acta, 19 (1960) 85. R. G. Snyder, J. Mol. Spectrosc., 4 (1960) 411. R. G. Snyder, S. L. Hsu and S. Krimm, Spectrochim. Acta, 34A (1978) 395. R. C. Spiker and I. W. Levin, Biochim. Biophys. Acta, 388 (1975) 361.