www.elsevier.com/locate/apsusc

Applied Surface Science 237 (2004) 444–449

Molecule orientation in self-assembled monolayers determined

by infrared-visible sum-frequency generation spectroscopy

A.A. Mania, Z.D. Schultzb, B. Champagnec, C. Humberta, L. Dreesena,A.A. Gewirthb, J.O. Whiteb, P.A. Thirya, A. Peremansa, Y. Caudanoa,�

aLaboratoire de Spectroscopie Moleculaire de Surface, Facultes Universitaires Notre-Dame de la Paix,

Rue de Bruxelles, 61 B-5000 Namur, BelgiumbFrederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign,

104 S. Goodwin Ave., Urbana, IL 61801, USAcLaboratoire de Chimie Theorique Appliquee, Facultes Universitaires Notre-Dame de la Paix,

Rue de Bruxelles, 61 B-5000 Namur, Belgium

Available online 14 August 2004

Abstract

The molecular orientation in self-assembled monolayers of thiophenol (TP) on Ag(1 1 1) and of para-nitroanilino-

dodecane-thiol (NAT) on Au was investigated by infrared-visible sum-frequency generation between 500 and 1500 cm�1.

Ab initio calculations were performed to determine the interface nonlinear response. The combined experimental and theoretical

results reveal that the S–C bond of TP is tilted 37� from the Ag(1 1 1) surface normal, with the aromatic ring plane perpendicular

to the surface. On Au, the tilt angle of the NAT molecule stabilizes below 60� at the end of film growth.

# 2004 Elsevier B.V. All rights reserved.

PACS: 68.43.Hn; 42.65.Ky; 68.43.Pq; 68.43.Fg

Keywords: Sum-frequency generation; Self-assembled monolayer; Adsorbate orientation; Vibration; Thiophenol; para-nitroanilino-dodecane-

thiol

1. Introduction

Organic thin films and, in particular, self-

assembled monolayers (SAM) of conjugated mole-

cules are promising materials for building electronic,

� Corresponding author. Tel.: +32 81 725487;

fax: +32 81 724718.

E-mail address: yves.caudano@fundp.ac.be (Y. Caudano).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2004.07.025

optoelectronic, and photonic nanometer-scale devices

[1,2]. Many of these applications rely on the repro-

ducible manufacture of organized molecular arrays on

metallic substrates. Hence, knowing the molecule

orientation is vital and advocates the development

of investigation tools adapted to the determination

of interface order and adsorbate structure.

Infrared-visible sum-frequency generation (SFG)

spectroscopy makes use of two laser beams, one

.

A.A. Mani et al. / Applied Surface Science 237 (2004) 444–449 445

visible and one tunable in the infrared, to record

interface-selective vibrational spectra. The intensity

of the directional SFG beam generated at the sum of

the incident frequencies increases resonantly when the

infrared beam excites an adsorbate vibration that is

both infrared and Raman active. The signal depends

on the relative orientations of the vibration infrared

and Raman-induced dipoles, with respect to the polar-

izations of the incident and SFG beams. These strict

selection rules lead to simple SFG signatures for SAM

on metals, that are highly sensitive to film order and to

adsorbate orientation and conformation.

By comparing ab initio calculations of the mono-

layer theoretical nonlinear optical response to experi-

mental SFG spectra acquired in the 500–1500 cm�1

range, we determined the orientation of the head

group in para-nitroanilino-dodecane-thiol SAM on

Au (NAT: p-NO2–C6H4–NH–(CH2)12–SH) and of

thiophenol on Ag(1 1 1) (TP: C6H5–SH). In order

to probe the out-of-plane deformations of the TP

carbon skeleton, the spectral range of our tunable

tabletop laser system was extended in the mid-IR

beyond the reach of conventional SFG set-ups (lim-

ited by the source to 900 cm�1 unless a free-electron

laser is used [3–5]).

2. Experimental SFG set-up

The visible and tunable, infrared beams used for

SFG are produced by converting the 9400 cm�1 radia-

tion of an all-solid-state Nd:YAG laser that is pumped

by flashlamps and mode locked by a frequency-dou-

bling nonlinear mirror combined with a two-photon

absorber (GaAs platelet) [6]. The green visible beam

(18800 cm�1) is obtained by frequency doubling one

third of the YAG laser output in a BBO crystal. The

remaining of the pump generates synchronous infrared

radiation in a picosecond optical parametric oscillator

(OPO).

In order to perform SFG spectroscopy as far as

500 cm�1, we used a KTP/CdSe OPO [7]. The first

stage of the duo OPO relies on a KTP crystal opti-

mized for the 9400 cm�1! 5030 cm�1 + 4370 cm�1

(o! o þ e) SFG process. In the second step, 30 % of

the OPO idler beam (4175–4450 cm�1) is mixed with

the signal beam (5225–4950 cm�1) in a CdSe crystal

to produce, by difference-frequency generation, infra-

red radiation tunable from 1050 to 500 cm�1. The

power at 670 cm�1 is2 mW (� 0.8 mgJ per8-ps pulse).

For SFG spectroscopy, the p-polarized, co-propa-

gating IR and visible beams are synchronized and

focused on the sample with 65� and 55� incidence

angles to the surface normal, respectively. The p-

polarized SFG beam is detected by a photomultiplier

after spatial and spectral filtering using a Raman filter

and a double monochromator.

3. Theoretical SFG spectra of molecular layers

Extracting the precise orientation of adsorbed

molecules from IR-visible SFG spectra requires a

detailed knowledge of the molecule vibrational first

hyperpolarizability b. In the harmonic approximation,

far from electronic resonances, the contribution of

each vibration Qv is proportional to its Raman scatter-

ing and infrared absorption cross-sections [8]:

bv;lmn ¼ � 4

�h

@alm

@Qv

@mn

@Qv

1

vIR � vv þ iG v(1)

where a and m are the molecular polarizability and

dipole moments. We can thus predict the SFG effi-

ciency from the computation of the vibration Raman

tensors and dynamic dipoles.

The adsorbed TP and NAT orientations are defined

by the angles u, the molecule tilt with respect to the

surface normal, and j, the rotation of the benzene ring

around the molecular axis (so that, if u ¼ j ¼ 90�, the

molecules lie flat on the surface). If we assume a

uniform adsorption structure on the surface and an

isotropic molecule distribution among randomly

oriented domains, the SFG signal is evaluated to:

IðvIRÞ/�����xð2Þnr þ 1

2NS

Xq

v¼1

Xijk

Fijkppp

�Xlmn

Tlmnijk bv;lmn

* +�����2

; (2)

where Tðu; jÞ converts the molecular hyperpolariz-

ability to the laboratory frame, F holds the interface

Fresnel factors for ppp polarizations [9], NS is the

molecule density on the surface, and xð2Þnr is the

effective, nonlinear, non-resonant susceptibility of

the interface, emulating the substrate signal. The angle

A.A. Mani et al. / Applied Surface Science 237 (2004) 444–449446

Fig. 1. In situ (bottom) and theoretical (top) SFG spectra of para-

nitro-anilino-dodecane-thiol SAM on Au, for ppp polarizations with

65� (IR) and 55� (visible) incidence angles. The substrate exposure

time to the NAT solution was 2 h. The continuous curve passing

through the experimental data results from a least square fit based on

a lorentzian model of the four SFG resonances [15]. The NO2–

C6H4–NH–CH3 molecule on which the calculations were performed

is shown.

brackets average the domain contributions, so that

only the xð2Þzzz , x

ð2Þxxz, x

ð2Þxzx, and x

ð2Þzxx components of the

monolayer second-order susceptibility yield ppp SFG

signal. In the simulations, xð2Þnr is adjusted to reproduce

the off-resonance SFG intensity in concordance with

the experimental lineshape of the modes, and the

vibration homogeneous broadening is set to 6 cm�1.

Because all the resonant and non-resonant contribu-

tions to the interface susceptibility interfere, SFG

resonances appear either as peaks or dips, or adopt

more complex shapes [10]. In practice, we neglect the

influence of the adsorbed monolayer on the interface

Fresnel factors. This approach, truly valid in the low

coverage limit, must be used as a first approximation,

since the monolayer dielectric constant is unknown.

The TP and NAT vibrational first hyperpolarizabil-

ities were calculated ab initio. The NAT alkane chain

was truncated to make the calculations tractable. The

use of the shorter NO2–C6H4–NH–CH3 molecule

bears little influence on the SFG signature because

the highly symmetric and apolar chain presents a weak

SFG cross-section. The molecule geometries were

fully optimized in Gaussian [11] using the 6–311

þþG(d,p) (TP) or 6–311G** (NAT) atomic basis sets

within the hybrid B3LYP [12] DFT scheme (TP) or at

the restricted Hartree-Fock level (NAT). When the

residual forces were less than 10�5 a.u., the harmonic

vibrational modes were determined analytically using

the coupled-perturbed Kohn-Sham approach [13].

Then, their dynamic dipoles @m=@Qv were found

analytically from the first-order nuclear coordinate

derivatives of the Kohn–Sham orbitals. The Raman

tensors @a=@Qv were evaluated by a numerical differ-

entiation of the static polarizability tensor involving

four distorted molecule geometries per normal modes

[14].

4. para-Nitroanilino-dodecane-thiol/Au

The SAM were produced by immersing polycrys-

talline Au in a 2mgmol/l solution of NAT in ethanol

[15]. The substrates were mounted into a Kel-F spec-

tro-chemical cell sealed with a prismatic BaF2 win-

dow. The NAT synthesis [16] and the procedure used

to control the film growth [15] are described else-

where. The NAT spectra were referenced to the spec-

trum of bare gold in pure ethanol.

The in situ SFG spectrum of NAT/Au evolves

gradually during film growth before stabilizing [15].

The final spectrum (Fig. 1) exhibits four resonances at

1327 cm�1 (a), 1307 cm�1 (b), 1281 cm�1 (g), and

1187 cm�1 (d). The first two are the strongest. The a

and d peaks have been assigned by ex situ infrared

absorption spectroscopy to the NO2 symmetric stretch

of NAT and to an in-plane deformation of the ring CH

bonds, respectively [16].

Our calculations predict 6 NAT vibrations between

1150 and 1340 cm�1, at 1329 cm�1 (A mode),

1326 cm�1 (B), 1312 cm�1 (C), 1279 cm�1 (D),

1191 cm�1 (E), and 1173 cm�1 (F). Due to the the-

oretical procedure limitations [17], the mode frequen-

cies were scaled by 0.82 to reproduce the experimental

spectra (Fig. 1). The calculations confirm that the a

resonance corresponds to the symmetric stretch of

NO2 concomitant with an in-plane ring deformation

(A and B modes). The b vibration matches a mode that

involves the NO2 symmetric stretch as well (C mode).

The D, E, and F modes involve in-plane deformations

of the ring CH bonds and carbon skeleton. The g

resonance coincide with the D mode, while the E and F

A.A. Mani et al. / Applied Surface Science 237 (2004) 444–449 447

Fig. 2. Normalized experimental (lowest curve) and theoretical

SFG spectra of tiophenol adsorbed on Ag(1 1 1), for ppp polariza-

tions with 65� (IR) and 55� (visible) incidence angles. The middle

curve is the better fit (u ¼ 37� and j ¼ 0�) to experiment.

modes contribute to the d feature, in agreement with its

previous attribution [16].

All these vibrations present stronger components

for their infrared dipoles and Raman tensors along the

molecule vertical axis, joining the two N atoms (i.e.

bzzz dominates). Therefore, the molecule tilt u is

constrained to small angles to yield detectable SFG

intensity. Indeed, if the molecule cants towards the

surface, the vibrations are screened by the metal sur-

face, and the SFG signal behaves like � cos6 u due to

the Fresnel factors. The calculated intensity of the

strong a resonance drops rapidly beyond u ¼ 40� (to

15% of its maximum at 40�, and 0.8% at 60�), so that

the molecule tilt should likely be below � 40� to

enable the observation of the weaker modes. This

constraint agrees well with the value of 36� deduced

by SHG for the same surface in ethanol [18].

Unfortunately, all the NAT vibrations investigated

here present the same pattern for their infrared dipoles

and Raman tensors. As a result, the relative intensity

of the modes is largely insensitive to the molecule

orientation. In particular, the SFG spectra are virtually

independent of j when u< 60�, as expected since

nearly all the molecular IR and Raman activity occurs

along the N–N axis. A typical spectrum is presented in

Fig. 1. The resolution of the precise molecule orienta-

tion requires thus to investigate spectral ranges where

the vibrational first hyperpolarizability has a richer

structure. This has been successfully achieved for

thiophenol adsorbed on Ag(1 1 1).

Table 1

Dominant components of the vibration dynamic dipoles and Raman

tensors in TP, below 1050 cm�1 (y: strong;§: weak)

Mode Frequency (cm�1) IR dipole Raman tensor

A 1014 ez ayxx, ayy, a

yzz

B 9840:8ex � 0:6e

xz a

yxx, ayy, a

yzz

C 903 ex along 0:5ex � 0:9eyz

D 718 eyy a

xyz

E 685 ez ayxx;ayy

The ez direction is the molecule vertical axis (the S–C bond), ey is

normal to the aromatic ring, and ex is in the ring plane, perpendicular

to the molecule axis. Theoretical frequencies were scaled by 0.97

[17].

5. Thiophenol/Ag(1 1 1)

The thiophenol SAM were produced by immersing

Ag substrates for 24 h in solutions of 1.0 mM of TP in

ethanol. The predominantly Ag(1 1 1) surfaces were

formed by evaporating 200 nm of Ag on mica.

The ppp SFG spectrum of TP SAM recorded

between 500 and 1050 cm�1 (Fig. 2) exhibits two

major resonances, around 690 and 1000 cm�1, in

agreement with previous experiments performed with

an OPO at 1000 cm�1[3] and a free electron laser [4].

According to our calculations, they must be attributed

to two in-plane deformations of the TP ring: the A

mode at 1014 cm�1 and the E mode at 685 cm�1. The

major features of the dynamic dipoles and Raman

tensors of the most intense SFG-active vibrations

found in the 500–1050 cm�1 spectral interval are

summarized in Table 1. They will allow us to establish

the TP orientation on the Ag(1 1 1) surface. Only the

ppp polarization signal was measured because the

strong screening of the parallel component of the

electric field on the Ag metal surface prevents the

use of the other polarization combinations to extract

the molecule orientation. (This is confirmed by our

theoretical SFG spectra of TP for the other polariza-

tions, which predict intensities below our detection

level.)

The two modes observed present a strong vibra-

tional dipole moment along the molecule S–C axis and

A.A. Mani et al. / Applied Surface Science 237 (2004) 444–449448

share a similar shape of the Raman tensor around it.

On the other hand, their Raman activities differ very

significantly along that axis. Their relative SFG inten-

sity is therefore governed by the tilt of the molecule on

the surface: the intensity of the A mode is maximum

when the S–C bond is vertical, while the E mode

intensity should peak around 55� given that its Raman

and IR activities occur in perpendicular directions.

The comparison between the experimental and theo-

retical SFG spectra (Fig. 2) points to an angle u of

� 37�.

The D mode described in Table 1 corresponds to an

out-of-plane deformation of the ring CH bonds. It has

a much weaker Raman activity (mainly in the plane

containing the S–C axis and the ring normal). Contra-

rily to the A and E modes, its dipole moment is

oriented perpendicular to the TP ring. Consequently,

its intensity depends on both u and j, and, in particular,

the flatter the molecule lies on the surface, the stronger

the D mode. Since this mode, and the more compli-

cated B and C modes, are not detected, the molecule

orientation must prevent their observation. This occurs

when the two angles are set to u ¼ 37� � 10� and

j ¼ 0� � 20�.

These findings agree well with previous angle-

resolved X-ray photoemission studies of TP/

Ag(1 1 1) that found u ¼ 24� � 5� [19]. They confirm

as well the smaller tilt of the molecule on Ag(1 1 1)

than on Au(1 1 1), where u� 49�. Finally, the per-

pendicular orientation of the TP ring with respect to

the substrate surface (i.e. j ¼ 0�) matches that of the

herringbone packing, often adopted by adsorbed aro-

matic chromophores [20,21].

6. Conclusions

The para-nitroanilino-dodecane-thiol and thiophe-

nol molecules form ordered films on Au and

Ag(1 1 1), respectively. The tilt angle of the NAT

molecule adsorbed on Au and studied in situ at the

end of the film growth is smaller than 60� and prob-

ably inferior or equivalent to � 40�. The comparison

of experimental and ab initio theoretical SFG spectra

of TP SAM indicates that the molecule S–C bond is

tilted � 37� from the surface normal, with its aromatic

ring oriented perpendicular to the surface. The accu-

racy on the TP orientation results from the vibrational

range inspected, between 500 and 1050 cm�1, where

the molecule vibrational first hyperpolarizability exhi-

bits pronounced spectral differences. Correspond-

ingly, the determination of the NAT molecule

orientation is impaired by the similar shape of the

hyperpolarizability tensor of the vibrations found

between 1150 and 1400 cm�1.

Acknowledgements

We thank Dr. G.-Q. Lu for assistance during the

SFG experiment and Dr. J.J. Wolff for preparing the

NAT solutions. A.P., B.C., C.H., and Y.C. are Research

Associate (RA), Senior RA, Scientific Research

Worker, and Postdoctoral Researcher, respectively,

of the Belgian National Fund for Scientific Research

(FNRS). This work was supported by the Belgian

Fund for Joint Basic Research, by the Belgian Inter-

university Program on ‘‘Quantum size effects in

nanostructured materials’’ PAI/IUAP 5/1 initiated

by the Belgian Office for Scientific, Technical, and

Cultural Affairs (OSTC), and by the U.S. Department

of Energy, Division of Materials Sciences under

Award No. DEFG02-91ER45439, through the Freder-

ick Seitz Materials Research Laboratory at the Uni-

versity of Illinois at Urbana-Champaign.

References

[1] A.J. Heeger, Rev. Mod. Phys. 73 (2001) 681.

[2] A.G. MacDiarmid, Rev. Mod. Phys. 73 (2001) 701.

[3] P. Guyot-Sionnest, N. Cheuk-Yu (Eds.), Laser Techniques for

State-Selected and State-to-State Chemistry, Proc. SPIE, vol.

1858 (1993).

[4] R. Braun, B.D. Casson, C.D. Bain, E.W.M. van der Ham,

Q.H.F. Vrehen, E.R. Eliel, A.M. Briggs, P.B. Davies, J. Chem.

Phys. 110 (1999) 4634.

[5] A. Peremans, A. Tadjeddine, R. Prazeres, F. Glotin, D. Jar-

oszynski, J.-M. Ortega, Nucl. Instrum. Meth. A 341 (1994)

146.

[6] A.A. Mani, L. Dreesen, P. Hollander, C. Humbert, Y. Caudano,

P.A. Thiry, A. Peremans, Appl. Phys. Lett. 79 (2001) 1945.

[7] A.A. Mani, Z.D. Schultz, Y. Caudano, C. Humbert, L. Dreesen,

A.A. Gewirth, J.O. White, P.A. Thiry, A. Peremans, Opt. Lett.

29 (2004) 274.

[8] S.H. Lin, A.A. Villaeys, Phys. Rev. A 50 (1994) 5134.

[9] T.F. Heinz, H.-E. Ponath, G.I. Stegeman, in: Nonlinear Surface

Electromagnetic Phenomena, Elsevier Science, Amsterdam,

1991.

A.A. Mani et al. / Applied Surface Science 237 (2004) 444–449 449

[10] L. Dreesen, C. Humbert, M. Celebi, J.-J. Lemaire, A.A. Mani,

P.A. Thiry, A. Peremans, Appl. Phys. B 74 (2002) 621.

[11] M.J. Frisch, et al. GAUSSIAN 98, Revision A.1, Gaussian

Inc., Pittsburg, PA, 1998.

[12] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.

[13] A. Komornicki, G.J. Fitzgerald, J. Chem. Phys. 98 (1993)

1398, and references therein.

[14] B. Champagne, Chem. Phys. Lett. 58 (1996) 261.

[15] C. Humbert, M. Buck, A. Calderone, J.-P. Vigneron, V. Meu-

nier, B. Champagne, W.-Q. Zheng, A. Tadjeddine, P.A. Thiry,

A. Peremans, Phys. Stat. Sol. (a) 175 (1999) 129.

[16] A. Wesch, O. Dannenberger, C. Woll, J.J. Wolff, M. Buck,

Langmuir 12 (1996) 5330.

[17] A.P. Scott, L. Radom, J. Chem. Phys. 100 (1996) 16502.

[18] F. Eisert, O. Dannenberger, M. Buck, Phys. Rev. B 58 (1998)

10860.

[19] S. Frey, V. Stadleer, K. Heister, W. Eck, M. Zharnikov, M.

Grunze, B. Zeysing, A. Terfort, Langmuir 17 (2001) 2408.

[20] A.-A. Dhirani, R.W. Zehner, R.P. Hsung, P. Guyot-Sionnest,

L.R. Sita, J. Am. Chem. Soc. 118 (1996) 3319.

[21] S.-C. Chang, I. Chao, Y.-T. Tao, J. Am. Chem. Soc. 116 (1994)

6792.