Surface-segregated monolayers: A new type of ordered monolayer for surface modification of organic semiconductors

Article abstract of DOI:10.1021/ja9057053

We report a new type of ordered monolayer for the surface modification of organic semiconductors. Fullerene derivatives with fluorocarbon chains ([6,6]-phenyl-C₆₁-buryric acid 1H,1H-perfluoro-1-alkyl ester or FC_n) spontaneously segregated as a monolayer on the surface of a [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) film during a spin-coating process from the mixture solutions, as confirmed by X-ray photoelectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS) showed the shift of ionization potentials (IPs) depending on the fluorocarbon chain length, indicating the formation of surface dipole moments. Surface-sensitive vibrational spectroscopy, sum frequency generation (SFG) revealed the ordered molecular orientations of the C₆₀ moiety in the surface FC_n layers. The intensity of the SFG signals from FC _n on the surface showed a clear odd-even effect when the length of the fluorocarbon chain was changed. This new concept of the surface-segregated monolayer provides a facile and versatile approach to modifying the surface of organic semiconductors and is applicable to various organic optoelectronic devices.

Full text of DOI:10.1021/ja9057053

Published on Web 11/13/2009
Surface-Segregated Monolayers: A New Type of Ordered
Monolayer for Surface Modification of Organic Semiconductors
Qingshuo Wei,† Keisuke Tajima,*,† Yujin Tong,‡ Shen Ye,*,‡,§ and
Kazuhito Hashimoto*,†,|
Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Catalysis Research Center, Hokkaido UniVersity,
Sapporo 001-0021, Japan, PRESTO, Japan Science and Technology Agency (JST), and
HASHIMOTO Light Energy ConVersion Project, Exploratory Research for AdVanced Technology
(ERATO), Japan Science and Technology Agency (JST), Japan
Received July 10, 2009; E-mail: k-tajima@light.t.u-tokyo.ac.jp; ye@cat.hokudai.ac.jp;
hashimoto@light.t.u-tokyo.ac.jp
Abstract: We report a new type of ordered monolayer for the surface modification of organic semiconductors.
Fullerene derivatives with fluorocarbon chains ([6,6]-phenyl-C61-buryric acid 1H,1H-perfluoro-1-alkyl ester
or FCn) spontaneously segregated as a monolayer on the surface of a [6,6]-phenyl-C61-butyric acid methyl
ester (PCBM) film during a spin-coating process from the mixture solutions, as confirmed by X-ray
photoelectron spectroscopy (XPS). Ultraviolet photoelectron spectroscopy (UPS) showed the shift of
ionization potentials (IPs) depending on the fluorocarbon chain length, indicating the formation of surface
dipole moments. Surface-sensitive vibrational spectroscopy, sum frequency generation (SFG) revealed
the ordered molecular orientations of the C60 moiety in the surface FCn layers. The intensity of the SFG
signals from FCn on the surface showed a clear odd-even effect when the length of the fluorocarbon
chain was changed. This new concept of the surface-segregated monolayer provides a facile and versatile
approach to modifying the surface of organic semiconductors and is applicable to various organic
optoelectronic devices.
Introduction
Recently, we have reported the formation process of a self-
organized layer on organic semiconducting materials, which
utilized the phenomenon of surface segregation.5,6 A novel
fullerene derivative with a fluorocarbon chain (phenyl-C61-
buryric acid 1H,1H-perfluoro-1-octyl ester or FC7 in Figure 1a)
was designed and synthesized. When a small amount of FC7
was mixed in a solution of another fullerene derivative, [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM) (Figure 1b), and
the solution was spin-coated on a substrate, FC7 spontaneously
migrated to the surface as a result of the low surface energy of
the fluorocarbon and formed a very thin layer on the PCBM
surface in a single step (Figure 1c). By using the FC7 layers as
buffer layers between the organic and metal layers, an improve-
ment in the performance of organic solar cells was achieved.
Interestingly, the FC7 layer on the PCBM film induced a large
shift in ionization potential (IP), suggesting the alignment of
the molecular dipole moment at the surface. This observation
raised the notion that the layer of FC7 could have a well-oriented,
ordered structure on the surface, despite the great ease of the
procedure. Although the surface segregation has been well
studied in the research field of polymer film structure and coating
process,7-10 there has been no report to control the surface
Well-ordered ultrathin films of organic molecules have been
extensively studied due to their potential applications in various
fields such as molecular electronics, nonlinear optical materials,
and biological sensors.1 To control the chemical and physical
properties of materials such as metals, metal oxides, and
inorganic semiconductors, Langmuir-Blodgett (LB) films and
self-assembled monolayers (SAMs) are frequently used for their
surface modifications.1–3 However, the surface of organic
semiconductors is difficult to modify using these methods due
to the instability of the organic substrates in solution and the
lack of specific interaction between the head groups of the
modifying molecules and the organic semiconductor surfaces.4
Since control of the surface and interfacial properties is of
critical importance for the realization of high-performance all-
organic electronic devices, the development of a novel and
simple approach to the surface modification of organic semi-
conductors is strongly desired.
† The University of Tokyo.
‡ Hokkaido University.
§ PRESTO, Japan Science and Technology Agency.
| ERATO, Japan Science and Technology Agency.
(1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic:
New York, 1991.
(5) Wei, Q. S.; Nishizawa, T.; Tajima, K.; Hashimoto, K. AdV. Mater.
2008, 20, 2211–2216.
(2) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch,
I.; Lahav, M.; Leiserowitz, L. Chem. ReV. 2001, 101, 1659–1696.
(3) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides,
G. M. Chem. ReV. 2005, 105, 1103–1169.
(6) Yamakawa, S.; Tajima, K.; Hashimoto, K. Org. Electron. 2009, 10,
511–514.
(7) Benrashid, R.; Nelson, G. L.; Linn, J. H.; Hanley, K. H.; Wade, W. R.
J. Appl. Polym. Sci. 1993, 49, 523–537.
(4) Calhoun, M. F.; Sanchez, J.; Olaya, D.; Gershenson, M. E.; Podzorov,
V. Nat. Mater. 2008, 7, 84–89.
(8) Chen, X.; Gardella, J. A. Macromolecules 1994, 27, 3363–3369.
9
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Wei et al.
Figure 2. (a) UV-vis absorption spectra of PCBM and FCn in CHCl3;
concentration is 1.6 × 10-5 mol L-1; (b) cyclic voltammograms of PCBM
and FCn. Anhydrous dichloromethane was used as an electrolyte, containing
0.1 M tetrabutylammonium perchlorate and approximately 1.0 × 10-3 mol
L-1 FCn or PCBM. The scan rate is 50 mV/s.
al.15 Molecular electronic properties of the fullerene deriva-
tives FCn (n ) 5-10) in solution were investigated by
UV-vis absorption spectroscopy and cyclic voltammetry. As
shown in Figure 2a, FCn (n ) 5-10) and PCBM had identical
UV-vis absorption profiles in chloroform (CHCl3) with the
absorption maxima of 260 and 330 nm attributed to the
fullerene absorption. Cyclic voltammograms of FCn (Figure
2b) and PCBM in dichloromethane (CH2Cl2) were also
identical with three reversible reduction peaks with half
potentials at -1.1, -1.5, and -2.0 V (vs Fc/Fc+) corre-
sponding to the one-, two-, three-electron reduction of the
fullerene moieties, respectively.15 These results indicate that
the fluorocarbon moieties have little effect on the electronic
properties of the methanofullerenes. This could be rational-
ized by the fact that the electron withdrawing fluorocarbon
chains were attached via an ester group and therefore
electronically isolated from the fullerene. Since the molecular
electronic properties of the fullerene groups are the same,
differences in the surface properties and spectra of FCn and
PCBM can be compared in the following sections.
Figure 1. (a) Chemical structure of FCn, where n represents the number
of fluorocarbons; (b) chemical structure of PCBM; (c) schematic representa-
tion of surface-segregated monolayer formation; (d) schematic representation
of a model of PCBM film with a densely packed surface-segregated FCn
monolayer.
properties of the organic semiconductors such as ionization
potential by utilizing this phenomenon.
In this study, we synthesized a series of fullerene derivatives
with different lengths of fluorocarbon chains (FCn in Figure 1a)
and studied in detail the structure of the surface-segregated FCn
thin layer by X-ray photoelectron spectroscopy (XPS), ultraviolet
photoelectron spectroscopy (UPS), and sum frequency genera-
tion (SFG) vibrational spectroscopy. As a second-order nonlinear
vibrational spectroscopy, SFG vibrational spectroscopy is now
widely used to investigate the structure, symmetry, and ordering
of molecules on various interfaces, given that such structural
information is difficult to obtain by conventional infrared (IR)
and Raman scattering methods.11-14 This technique was suc-
cessfully employed in this work to characterize the molecular
structures on the surface of the FCn thin film in combination
with other measurements.
Surface Segregation in FCn/PCBM Films. In the spin-coated
films from the mixture of FCn and PCBM, FCn molecules are
expected to segregate to the film surface during spin-coating
due to their lower surface energy.10 To elucidate the surface
segregation behavior of FCn, the mixing ratio was changed in
the FC8/PCBM blend solution and the atomic concentration of
F at the film surface was monitored by XPS. Films were
Results and Discussion
(13) (a) Liu, J.; Conboy, J. C. J. Am. Chem. Soc. 2004, 126, 8376–8377.
(b) Chen, X.; Tang, H.; Even, M. A.; Wang, J.; Tew, G. N.; Chen, Z.
J. Am. Chem. Soc. 2006, 128, 2711–2714. (c) Roke, S.; Roeterdink,
W.; Wijnhoven, J.; Petukhov, A.; Kleyn, A.; Bonn, M. Phys. ReV.
Lett. 2003, 91, 258302. (d) Tyrode, E.; Rutland, M. W.; Bain, C. D.
J. Am. Chem. Soc. 2008, 130, 17434–17445. (e) Ma, G.; Allen, H. C.
Photochem. Photobiol. 2006, 82, 1517–1529. (f) Stokes, G. Y.;
Buchbinder, A. M.; Gibbs-Davis, J. M.; Scheidt, K. A.; Geiger, F. M.
J. Phys. Chem. A 2008, 112, 11688–11698.
Electronic Properties of FCn Molecules. FCn were synthesized
by utilizing the synthetic protocol developed by Hummelen et
(9) Affrossman, S.; Bertrand, P.; Hartshorne, M.; Kiff, T.; Leonard, D.;
Pethrick, R. A.; Richards, R. W. Macromolecules 1996, 29, 5432–
5437.
(10) Iyengar, D. R.; Perutz, S. M.; Dai, C. A.; Ober, C. K.; Kramer, E. J.
Macromolecules 1996, 29, 1229–1234.
(14) (a) Nishida, T.; Johnson, M.; Holman, J.; Osawa, M.; Davies, P. B.;
Ye, S. Phys. ReV. Lett. 2006, 96, 77402. (b) Ye, S.; Noda, H.; Morita,
S.; Uosaki, K.; Osawa, M. Langmuir 2003, 19, 2238–2242. (c) Ye,
S.; Noda, H.; Nishida, T.; Morita, S.; Osawa, M. Langmuir 2004, 20,
357–365. (d) Li, G.; Morita, S.; Ye, S.; Tanaka, M.; Osawa, M. Anal.
Chem. 2004, 76, 788–795. (e) Ye, S.; Morita, S.; Li, G.; Noda, H.;
Tanaka, M.; Uosaki, K.; Osawa, M. Macromolecules 2003, 36, 5694–
5703.
(11) Shen, Y. R. The Principles of Nonlinear Optics; John Wiley & Sons,
Inc.: New York, 1984.
(12) (a) Bain, C. D. J. Chem. Soc. Faraday Trans. 1995, 91, 1281–1296.
(b) Richmond, G. L. Chem. ReV. 2002, 102, 2693–2724. (c) Chen,
Z.; Shen, Y. R.; Somorjai, G. A. Annu. ReV. Phys. Chem. 2002, 53,
437–465. (d) Holman, J.; Davies, P. B.; Nishida, T.; Ye, S.; Neivandt,
D. J. J. Phys. Chem. B 2005, 109, 18723–18732. (e) Shen, Y. R.;
Ostroverkhov, V. Chem. ReV. 2006, 106, 1140–1154. (f) Ye, S.;
Osawa, M. Chem. Lett. 2009, 38, 386–391.
(15) Hummelen, J. C.; Knight, B. W.; Lepeq, F.; Wudl, F.; Yao, J.; Wilkins,
C. L. J. Org. Chem. 1995, 60, 532–538.
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A R T I C L E S
the surfaces of all FCn/PCBM films at 690 eV. After 5 s of Ar+
etching on the surfaces (corresponding to an etching depth of
approximately 2 nm), the F 1s peaks completely disappeared.
Etching was carried out until the entire organic layer was
removed, but no F 1s peaks were detected except at the surface.
On the basis of the XPS results given above, it was concluded
that the fluorocarbon chains of FCn were present to a depth of
less than 2 nm from the surface.
Surface Density of FCn Molecules. To quantitatively inves-
tigate the surface coverage of FCn on PCBM films, we
constructed a model with a densely packed FCn layer on a
PCBM film, as shown in Figure 1d. In this model, the molecular
conformation was optimized by density functional theory (DFT)
calculations with extended fluorocarbon chains (Figure 5a). The
center-to-center distance of the fullerene groups of FCn in
the monolayer was assumed to be 0.985-1.013 nm by using
the shortest distance between neighboring fullerenes in PCBM
single crystals previously reported.21 The packing density of
FCn was estimated to be 1.6-1.7 × 10-10 mol cm-2 in the
model. These values are comparable to those of the fullerene-
Figure 3. (a) F atom concentrations and (b) water contact angles on the
surfaces of FC8/PCBM thin films plotted as a function of FC8 concentrations
in the spin-coating solutions.
prepared on ITO substrates by spin-coating chlorobenzene
solutions of FC8/PCBM blends with various concentrations of
FC8 from 0 to 3 g L-1 (2.3 × 10-3 mol L-1), while the
concentration of PCBM was fixed to 40 g L-1 (0.044 mol L-1).
F atom concentrations on the surface are plotted in Figure 3a
as a function of the FC8 concentrations in the solution. As the
FC8 concentration in the solution increased, the F atom
concentration on the surface first linearly increased but clearly
saturated at around 1 g L-1 (8 × 10-4 mol L-1). This result
suggests that, when the amount of FC8 in the solution is smaller
than the saturation point, all the FC8 molecules in the solution
segregate to the surface to partly cover the film surface. With
higher concentrations of FC8 than the saturation point, the
surface of the films is completely covered by the FC8 molecules.
In fact, XPS depth profiles of the films (see below) showed the
absence of F atoms in the bulk below the saturated concentra-
tion, while F 1s peaks with a constant intensity were observed
even in the bulk above the saturated concentration. This simple
picture of the segregation behavior was also confirmed by the
water contact angles of the film surface (Figure 3b). Increase
of the contact angles at the low FC8 concentration and clear
saturation behavior at the FC8 concentration of around 1 g L-1
were observed, which coincides well with the surface concentra-
tion of F atoms in Figure 3a. The contact angle at the saturated
region (102°) is close to the typical values of fluorocarbon SAMs
on flat substrates (105-125°)16 and larger than that of fullerene-
based SAMs (70-90°).17-19 This suggests dense coverage of
the surface with the FC8 molecules at the saturation point.20
based SAMs on Au surfaces previously reported (1.0-1.8 ×
19
10-10 mol cm-2
)
but lower than fluorocarbon-based SAMs
on a Au surface (5.0 × 10-10 mol cm-2).16 A comparison of
the F/C atom ratios observed by XPS measurement and
calculated from this model could provide an estimation of the
packing density of FCn. The fluorine to carbon atomic ratio from
the surface can be calculated as
L × XF
(λ - L) × XC
F/C ratio )
(1)
where λ is the attenuation length of photoelectrons that was
assumed to be 2.5 nm based on the previous report by Lee et
al.,22,23 L is the length of the fluorocarbon layer, and XF/XC is
the density ratio of fluorine to carbon. Since most photoelectrons
originate from the first one or two layers of molecules at the
surface, XF/XC was calculated from molar fractions of fluorine
and nonfluorinated carbon in FC7 molecules. The density ratio
of fluorine to carbon was
XF
XC
15/0.89
72/1.26
)
) 0.295
(2)
As summarized in Table 1, the observed F/C atom ratios
agree well with the calculated values for all the FCn
molecules. Although some approximations are involved in
the calculation, this result indicates that FCn molecules packed
densely on the surface to cover the whole surface of the
PCBM layer.
Angle-Dependent XPS. Angle-dependent XPS (ADXPS) was
performed on the films to quantitatively determine the thickness
of the segregated fluorinated carbon layer, following the
technique previously reported for SAMs, LB films, and spin-
coated polymer thin films.24-28 The F 1s peak of fluorocarbons
XPS Depth Profiles. To further confirm the surface segrega-
tion of FCn molecules, depth profiles of the atomic components
was evaluated by XPS and Ar+ plasma etching. The FCn/PCBM
films are prepared by spin-coating the blended solutions at the
saturated concentration (8 × 10-4 mol L-1) on glass substrates.
From the atomic force microscopy (AFM) height images (Figure
S3 of the Supporting Information), all the surfaces were found
to be very flat with a mean roughness less than 2 nm. As shown
in Figure 4, F 1s peaks were observed in the XPS spectra from
(16) Colorado, R.; Lee, T. R. Langmuir 2003, 19, 3288–3296.
(17) Lee, S. W.; Shon, Y. S.; Lee, T. R.; Perry, S. S. Thin Solid Films
2000, 358, 152–158.
(21) Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.; Sariciftci,
N. S.; Hummelen, J. C. Chem. Commun. 2003, 2116–2118.
(22) Colorado, R.; Lee, T. R. J. Phys. Chem. B 2003, 107, 10216–10220.
(23) Mg KR radiation was used in this measurement, and the KE of C 1s
was 968.6 eV.
(18) Sahoo, R. R.; Patnaik, A. J. Colloid Interface Sci. 2003, 268, 43–49.
(19) Bonifazi, D.; Enger, O.; Diederich, F. Chem. Soc. ReV. 2007, 36, 390–
414.
(20) The molar ratio between FCn and PCBM in the total film can be
calculated based on the model in Figure 1d. By using the packing
density of FCn on the surface (1.6-1.7 × 10-10 mol cm-2, see below)
and the film thickness (100 nm), the molar ratio between FCn and
PCBM was estimated as approximately 0.02 in the film. This value is
close to the molar ratio in the blended solution (0.018).
(24) Fadley, C. S. Prog. Surf. Sci. 1984, 16, 275–388.
(25) Ton-That, C.; Shard, A. G.; Bradley, R. H. Langmuir 2000, 16, 2281–
2284.
(26) Gerenser, L. J.; Pochan, J. M.; Mason, M. G.; Elman, J. F. Langmuir
1985, 1, 305–312.
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A R T I C L E S
Wei et al.
Figure 4. XPS spectra of FCn/PCBM films in F 1s region without etching, after etching for 2, 20, 40, 60, 80, and 100 nm. (a) FC5/PCBM; (b)
FC6/PCBM; (c) FC7/PCBM; (d) FC8/PCBM; (e) FC9/PCBM; (f) FC10/PCBM. The total film thickness was approximately 100 nm, as determined by
profilometry.
(690 eV) and the C 1s peak of nonfluorinated carbons (285 eV)
were used for the measurement so that there was no contribution
of the elements from one layer to the other. According to
previous reports,24–28
IF1s 1
d
λ
ln
+ 1 ) sec θ
(3)
(
)
IC1s
K
where IF1s and IC1s are the XPS signal intensities of F 1s and C
1s, respectively, K is a constant determined by the relative
sensitivity factor (RSF) for XPS and the atom density ratio of
F to C, λ is the attenuation length of photoelectrons, θ is the
takeoff angle of the measurement (Figure 5b), and d is the film
thickness. K was calculated from the RSF ratio and the density
ratio of fluorine to carbon. The RSF ratio, which was obtained
by XPS measurements, was 3.82. Thus,
F
RSF XF
K )
) 3.82 × 0.295 ) 1.13
(4)
C
RSF XC
A plot of ln((IF1s)/(IC1s)(1)/(K) + 1) as a function of sec θ is
expected to produce a straight line through the origin if the
assumed model is valid. The slope of this line is equal to the
thickness of the film in units of attenuation length. As shown
Figure 5. (a) Calculated optimized structure of FC7 molecule; (b) general
geometry for angle-dependence XPS measurement; (c) plot of ln(IF1s/IC1s
/
1.13 + 1) vs 1/cos θ for a FCn/PCBM film.
(27) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.;
Ulman, A.; Snyder, R. G. Langmuir 1991, 7, 2700–2709.
in Figure 5c, good correlation was achieved by fitting a straight
line for all the FCn/PCBM films, which indicates the validity
of the model. The slopes of these lines increase from FC5 to
(28) Solletti, J. M.; Botreau, M.; Sommer, F.; Brunat, W. L.; Kasas, S.;
Duc, T. M.; Celio, M. R. Langmuir 1996, 12, 5379–5386.
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A R T I C L E S
Table 1. Experimental and Calculated Data for FCn/PCBM Films
estimated
measured
F/C atom
ratio
(by XPS)a
length of
fluorinated
carbon chain
(nm)b
film
thickness
(by ADXPS)
(nm)
calculated
F/C atom
ratioa
ionization
potential
(eV)
composition
PCBM
-
-
-
-
5.69
5.95
5.96
5.90
5.99
6.03
6.04
FC5/PCBM
FC6/PCBM
FC7/PCBM
FC8/PCBM
FC9/PCBM
FC10/PCBM
0.10
0.13
0.16
0.20
0.24
0.30
0.08
0.11
0.15
0.20
0.25
0.27
0.63
0.75
0.88
1.00
1.13
1.25
0.38
0.48
0.60
0.82
0.92
1.11
a Fluorinated carbons were not considered here because fluorinated
carbon has a binding energy different from nonfluorinated carbon.
b Molecular length was estimated with the Cambridge Scientific Chem
3D software.
FC10, which suggests an increase in the thickness of the
fluorocarbon layer with increasing fluorocarbon chain length.
The thickness of the fluorocarbon layer was calculated by using
equation d ) λτ, where τ was the slope. As shown in Table 1
and Figure S4 of the Supporting Information, the thickness of
the fluorocarbon layer determined by ADXPS increases with
increasing fluorocarbon chain length. This result indicates that
FCn molecules formed densely packed monolayers on the
surface of PCBM with the fluorocarbon chains pointing in the
direction of the air. The thickness is less than that calculated
from the molecular models based on the assumption of the
extended conformation and orientation of fluorocarbons being
completely perpendicular to the surface (Figure 1d). This
suggests the tilting of fluorocarbons or the formation of a helical
structure of fluorocarbon in the monolayer.29,30 We could also
see a slight deviation from the linear relationship in Figure S4
of the Supporting Information when the fluorocarbon chains get
longer. This might be attributed to the change in the tilting angle
or the conformations of the fluorocarbon chains due to the
change of the interchain interactions on the surface.
Figure 6. UPS data for PCBM and FCn/PCBM films at (a) cutoff and (b)
Fermi edge regions using He (I) irradiation. (c) Ionization potentials plotted
as the function of the number of fluorocarbons in FCn.
UPS Measurements. XPS depth profiles and angle-dependent
XPS clearly show that FCn molecules spontaneously migrate
to the surface of PCBM during spin-coating and form a
monolayer in a single step. In these structures, the perfluoroalkyl
chains could form interfacial dipole moments that shift the
vacuum level of the solid state. To confirm the formation of
this dipole layer, UPS measurements were carried out on PCBM
and FCn/PCBM films. Figure 6a and b show the UPS data for
PCBM and FCn/PCBM films at the cutoff and Fermi edge
regions, respectively, using the He (I) irradiation. As the
fluorocarbon chain length increases, the cutoff of the photo-
emission spectra continuously shifts to higher kinetic energy.
By using the spectrum width determined from the distance
between the cutoff to the Fermi edge, the ionization potential
(IP) of the films can be extracted.31 As summarized in Figure
6c and Table 1, the IP of PCBM film was determined to be
5.69 eV. When a monolayer of FCn was introduced on the
surface of the PCBM films, the IP shifted to 5.90-6.04 eV.
This IP shift was also observed previously for the FC7/PCBM
film by using photoelectron yield spectroscopy.5 Since the same
reduction potential of FCn and PCBM was observed by CV,
Figure 7. (a) Calculated Raman and IR spectra of PCBM; red arrows
indicate the peaks at 1470 cm-1 and 1430 cm-1, respectively. (b) Optimized
structure of PCBM by DFT calculation. Arrows represent the dipole
derivative vectors of the vibration modes at 1470 cm-1 (Ag(2) mode) and
1430 cm-1 (F1u(4) mode), respectively.
the increase in IP can be attributed to a dipole layer induced by
the molecular alignment of FCn at the film surface. From the
direction of the IP shift, we conclude that the perfluoroalkyl
chain points toward the direction of air to increase the ionization
potential of the films.
SFG Measurements. As a second-order nonlinear vibration
spectroscopy, SFG is sensitive to the molecular structure on
the surface of materials.11 Since the intensity of the SFG signal
from a vibration mode is proportional to the product of its
Raman polarizability tensor and IR dipole moment,11 we first
calculated IR and Raman spectra for PCBM and FCn obtained
by using the density functional theory (DFT). The calculations
were carried out at the B3LYP/6-31G* level following the
previous report by Schettino et al.32 The same scaling factor
(0.98) for frequency as that in the paper was also used in our
calculation.
Figure 7a shows calculated IR and Raman spectra of PCBM.
Reasonable agreements have been reached between the calcu-
(29) Genzer, J.; Kramer, E. J.; Fischer, D. A. J. Appl. Phys. 2002, 92, 7070–
7079.
(30) Ji, N.; Ostroverkhov, V.; Lagugne-Labarthet, F.; Shen, Y. R. J. Am.
Chem. Soc. 2003, 125, 14218–14219.
(31) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605–
625.
(32) Schettino, V.; Pagliai, M.; Ciabini, L.; Cardini, G. J. Phys. Chem. A
2001, 105, 11192–11196.
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vibrational modes with IR dipole moments that contain a
component perpendicular to the surface. Two peaks were clearly
observed at 1470 and 1430 cm-1, which can be attributed to
the Ag(2) and F1u(4) vibration modes of C60 moiety in PCBM,
respectively, as expected from the DFT calculations. The peak
intensity at 1430 cm-1 is higher than that at 1470 cm-1
.
Figure 8a-ii shows the ssp-polarized SFG spectrum of FC7/
PCBM film. Two peaks at 1470 cm-1 and 1430 cm-1 were also
observed as in the case of the PCBM films. Interestingly, the
intensity of the peak at 1470 cm-1 from the FC7/PCBM film is
about ten times higher than that from the PCBM film. In
contrast, the peak intensity at 1430 cm-1 is comparable with
that of the PCBM film. Since the vibrational properties of the
C60 moiety in FC7 and PCBM obtained by the IR and Raman
measurements are very similar from the aforementioned DFT
calculations, the difference in SFG spectra in Figure 8a-i and
8a-ii can be attributed to the different surface structures between
the PCBM and the FC7/PCBM films. The SFG spectrum after
the removal of the surface FC7 layer by Ar+ plasma etching
gave further support for this attribution. As shown in Figure
8a-iii, after ∼2 nm of the surface on the FC7/PCBM film was
etched away, the intensity of the peak at 1470 cm-1 decreased
substantially, and the SFG spectrum became similar to that of
the pure PCBM film. These results again confirm that the SFG
signals from the FC7/PCBM film are mainly contributed by FC7
molecules segregated on the FC7/PCBM surface. The FC7
structure on the surface of the FC7/PCBM film that produced
the higher intensity of the peak at 1470 cm-1 was removed by
Ar+ plasma etching, and the newly produced surface consisting
of pure PCBM had a structure similar to that of the spin-coated
film of PCBM. These SFG spectral changes correspond well to
the surface segregation process of FC7 molecules to the surface
of the FC7/PCBM film proposed by the aforementioned XPS
measurements (Figure 4).
Figure 8. (a) SFG spectra (ssp) of PCBM films, as-cast FC7/PCBM films,
FC7/PCBM films after plasma etching, and FC7 LB films. (b) SFG spectra
(sps) of PCBM films, as-cast FC7/PCBM films, and FC7/PCBM films after
plasma etching.
lated and experimentally observed IR and Raman spectra for
PCBM (Figure S5 of the Supporting Information) and FC7
(Figure S6 of the Supporting Information). Two vibration modes
for the C60 moiety in PCBM at 1470 and 1430 cm-1 with
different intensities can be found in both calculated Raman and
IR spectra (indicated by red arrows in Figure 7a). The peak at
1470 cm-1 can be assigned to the Ag(2) mode of the C60 moiety,
which is a symmetry-forbidden vibration in C60 but becomes
active due to symmetry breaking in PCBM. This mode has also
been observed in several C60 derivatives with substitutions.33,34
The peak at 1430 cm-1 is attributed to an IR-active F1u(4) mode
33–36
of C60,
which has often been reported in the IR spectra of
C60 compounds. A similar peak has also been observed in the
SFG spectra of a C60 monolayer adsorbed on a Ag(111)
surface.37,38 As indicated by the arrows in Figure 7b, the dipole
moment of the Ag(2) mode is pointing from the center of C60
to the functionalized position of methanofullerene. On the other
hand, the dipole direction of the F1u(4) mode is almost
perpendicular to that of the Ag(2) mode (see also the animations
in the Supporting Information).
The DFT calculations showed that IR and Raman peaks for
the C60 moiety in FCn have quite similar peak positions and
intensity to those of PCBM (see Table S1 of the Supporting
Information). This insensitivity of the vibration properties to
the substitution can be explained by the weak electronic coupling
between C60 and ester moieties in the molecules, which coincides
with the identical electronic properties of the C60 moiety
observed by UV-vis absorption and the electrochemical
measurements described in the previous section.
On the basis of the spectral information of IR and Raman
spectra, we are able to understand and discuss our SFG
observations on PCBM and FC7/PCBM thin films. Figure 8a-i
shows a SFG spectrum for a spin-coated PCBM film (20 nm
thick for all the following samples unless otherwise stated)
obtained in the IR frequency region between 1300 cm-1 and
1600 cm-1. The spectrum was collected with an ssp polarization
combination (i.e., s-SFG, s-visible, p-IR), which is sensitive to
We also measured the SFG spectrum of a crystalline FC7
thin layer prepared by the LB method.39 As shown in Figure
8a-iv, a peak is clearly observed at 1470 cm-1 while the peak
at 1430 cm-1 is very faint. These results support the conclusion
that the strong peak at 1470 cm-1 is attributed to the well-
ordered structure of FC7 molecules on the surface.
Molecular Orientation at the Surface of FC7/PCBM Film.
The orthogonality of the dipole moments of the Ag(2) and F1u(4)
modes confirmed by the DFT calculation (Figure 7b) is
important in understanding the SFG spectra and the molecular
orientation of the FC7/PCBM and PCBM films, considering the
interaction between the electric field of the incident IR pulse
and the vibration dipole moments of the C60 moiety. The higher
SFG signal of the Ag(2) mode at 1470 cm-1 from FC7/PCBM
shows a stronger interaction between the electric field of the
incident p-polarized IR pulse and the dipole moment of the
vibration mode. This indicates that the dipole moment of the Ag(2)
mode in the FC7 monolayer contains a component perpendicular
to the surface. In contrast, the intensity of the SFG peaks at
1430 cm-1 are comparable between the FC7/PCBM and PCBM
films. This is reasonable if the F1u(4) mode is in a direction
that has a weaker interaction with the p-polarized IR pulse. To
(33) Konarev, D. V.; Lyubovskaya, R. N.; Drichko, N. V.; Yudanova, E. I.;
Shul’ga, Y. M.; Litvinov, A. L.; Semkin, V. N.; Tarasov, B. P. J.
Mater. Chem. 2000, 10, 803–818.
(34) Pichler, T.; Winkler, R.; Kuzmany, H. Phys. ReV. B 1994, 49, 15879–
15889.
(35) Winkler, R.; Pichler, T.; Kuzmany, H. Z. Phys. B 1994, 96, 39–45.
(36) Semkin, V. N.; Drichko, N. V.; Kumzerov, Y. A.; Konarev, D. V.;
Lyubovskaya, R. N.; Graja, A. Chem. Phys. Lett. 1998, 295, 266–
272.
(39) A Langmuir trough was employed for the preparation of the FC7 LB
films. An FC7 chloroform solution having a concentration of 1 mg/
mL was carefully spread on pure water at room temperature. The
insoluble Langmuir film of FC7 was formed by slowly compressing
the barrier after waiting for more than 30 min for the solvent to
evaporate. After a thin crystalline film formed on the surface of water,
the films were transferred onto CaF2 substrates.
(37) Peremans, A.; Caudano, Y.; Thiry, P. A.; Dumas, P.; Zhang, W. Q.;
LeRille, A.; Tadjeddine, A. Phys. ReV. Lett. 1997, 78, 2999–3002.
(38) Silien, C.; Caudano, Y.; Longueville, J. L.; Bouzidi, S.; Wiame, F.;
Peremans, A.; Thiry, P. A. Surf. Sci. 1999, 428, 79–84.
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Surface Modification of Organic Semiconductors
A R T I C L E S
on the FC7/PCBM film was 1.6-1.7 × 10-10 mol cm-2, which
is lower than that of the fluorinated carbon SAMs on Au surface
(5 × 10-10 mol cm-2).16 This should be related to the large
footprints of C60 molecules compared to the cross-section area
of the fluorocarbon chain. Since SFG signal is proportional to
the square of the molecular numbers on the surface, the lower
surface coverage will largely reduce the relative SFG intensity.
(2) Because of the same reason for (1), the van der Waals
interaction between the fluorocarbon chains of FC7 molecules
is expected to be much weaker than the fluorinated carbon SAMs
on Au surfaces. Although all the fluorocarbon chains averagely
point to air, they may have some disordered conformation that
can also significantly reduce the relative SFG intensities from
the CdO group and fluorocarbon chains. (3) A technical
problem of the tunable IR half-wave plate (PO-TWP-L2-25-
IR, Alphalas GmbH) used in the present SFG system, which
starts to absorb IR light as the frequency is lower than
approximately 1400 cm-1, can affect our SFG spectral behaviors
in lower wavenumber region. Detailed studies are still in
progress.
Figure 9. SFG spectra (ssp) of an as-cast FC7/PCBM film and gold surface
(before normalization).
support this model, SFG measurements were also carried out
with an sps polarization combination (i.e., s-SFG, p-visible, s-IR)
that is sensitive to vibrational modes with IR dipole moments
that contain a component parallel to the surface. As shown in
Figure 8b, the SFG spectra of PCBM, FC7/PCBM, and etched
FC7/PCBM thin films were similar to each other in this case,
and only one peak at 1430 cm-1 was observed with a small
shoulder at 1470 cm-1. These results indicate that the C60 moiety
of the FC7 monolayer aligned on the surface in an orientation
with Ag(2) mode more perpendicular to the surface. Considering
the relative directions of the dipole moment of the Ag(2) mode
and fluorocarbon chains, one can expect that the fluorocarbon
chains point in the direction of the air as that schematically
proposed in Figure 1d. All the experimental observations (XPS,
UPS, and SFG) independently support the structural model
(Figure 1d).
It should be emphasized here that the SFG peak intensity at
1470 cm-1 in the ssp-polarized SFG spectrum for the FC7/
PCBM films is so strong that it is comparable to that of
nonresonant signals from a gold substrate, which is normally
used for the optical alignment and normalization procedures of
SFG measurements (Figure 9). In addition to the molecular
density, packing structures, and ordering of the C60 moiety of
FC7 at the surface, very high second-order nonlinear susceptibil-
ity of the molecule should also significantly contribute to the
strong SFG signal. These features can be applied for develop-
ment of new nonlinear optical materials.
SFG Signal Intensity Dependence on Fluorocarbon Chain
Length. SFG vibrational spectroscopy has been shown above
to be a powerful tool for investigating molecular orientation in
the surface-segregated monolayer of FC7. To investigate the
orientation dependence on the fluorocarbon chain length, the
SFG spectra of the FCn/PCBM films were collected with an
ssp polarization combination, as shown in Figure 10a. The
intensity of the peak at 1470 cm-1 was plotted as a function of
the number of fluorinated carbons.41 As shown in Figure 10b,
a longer fluorocarbon chain gives a higher intensity of the SFG
signal from the vibration mode corresponding to the Ag(2) mode
On the other hand, neither SFG peaks from the carbonyl
(CdO) stretching mode in the ester moiety nor the C-F
stretching mode in the fluorocarbon chain from the FC7/PCBM
surface were observed in the present experiments although the
DFT calculation and the IR and Raman spectra in bulk predict
the appearance of such bands (Figures S5 and S6 of the
Supporting Information). Recently, Tyrode et al. reported an
SFG study on adsorption of ammonium perfluorononanoate at
the air-liquid interface and found reasonably strong C-F bands
in their SFG observations.40 This may be attributed to several
possible reasons: (1) The packing density of fluorocarbon chains
Figure 10. (a) SFG spectra (ssp) of PCBM, FC5/PCBM, FC6/PCBM, FC7/PCBM, FC8/PCBM, FC9/PCBM, and FC10/PCBM films; (b) plot of SFG signal
intensity for the peak at 1470 cm-1 as the function of the number of the fluorocarbons in FCn.
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A R T I C L E S
Wei et al.
in C60. The DFT calculation shows that the length of fluoro-
carbon chain did not significantly affect the IR and Raman
intensities for these vibration modes (Table S1 of the Supporting
Information). Therefore, the increase in the peak intensity with
the chain length should be attributed to the better packing of
molecules with long chains owing to the stronger van der Waals
interactions between the fluorocarbon chains, which can further
improve the packing and orientation of the C60 moiety in FCn
molecules.
such as UPS or XPS. Detailed investigations of the origin of
this odd-even effect are underway.
Conclusion
In conclusion, we have presented a new type of ordered
monolayer driven by surface segregation. It is worth emphasiz-
ing that the monolayers are spontaneously formed during a spin-
coating process from a mixture solution; this process has not
yet been considered to induce the formation of such oriented,
ordered monolayers. The surface-segregated FCn monolayer can
be easily applied to the control of the chemical or physical
properties of the PCBM film surface such as ionization potential
(IP). This concept is potentially applicable to other types of
organic semiconductors and therefore could be useful for various
organic devices. The surface-segregated FCn monolayers could
also provide a new perspective for understanding the relation-
ships between structural ordering, the local electric field, and
the SFG signal; therefore, it may also lead to a new strategy
for designing nonlinear optical materials.43,44
Interestingly, the molecules with even numbers of the
fluorinated carbons give a higher intensity of the signal than
those with odd numbers. The similar “odd-even” trend of the
SFG signal intensity was also observed in FCn/PCBM thin films
with a different film thickness of 100 nm (Figure S8 of the
Supporting Information), suggesting that this effect is due to
the surface structure. This so-called “odd-even effect” has been
observed in other monolayer systems42 where the layers have a
densely packed, well-oriented structure. The observation of the
odd-even effect for the surface-segregated FCn monolayer
supports our hypothesis that FCn molecules are well oriented
and densely packed on the surface. On the other hand, as
discussed above, the density of fluorocarbon chains in FCn/
PCBM is not as high as the fluorinated carbon SAMs on a Au
surface16 due to the large footprint of the C60 moieties. At this
point, the mechanism of the odd-even effect in the SFG signals
from FCn/PCBM is not yet clear; the extremely high sensitivity
of SFG vibration spectroscopy to the molecular alignment at
the interface might enable the detection of structural difference
between FCn with odd and even numbers of fluorinated carbons,
which might be too subtle to be detected by other measurements
Acknowledgment. We thank Dr. Takeshi Nishizawa, Dr.
Ryuhei Nakamura (The University of Tokyo), Dr. Chunhe Yang,
and Dr. Erjun Zhou (ERATO project, JST) for helpful discussion,
Dr. Koji Nakano (The University of Tokyo) for 19F NMR
measurements, and Dr. Kiyoshi Yagi (The University of Tokyo)
for his support in the DFT calculations. This work was supported
in part by the Global COE Program (Chemistry Innovation through
Cooperation of Science and Engineering), MEXT, Japan. This work
was partly supported by JST (PRESTO) and a Grant-in-Aid for
Scientific Research (B) 19350099 and Exploratory Research
21655074 from MEXT, Japan.
Supporting Information Available: General methods, char-
acterization, spectra, images, and animations of vibration modes
(.ppt file). This material is available free of charge via the
Internet at http://pubs.acs.org.
(40) Tyrode, E.; Johnson, C. M.; Rutland, M. W.; Day, J. P. R.; Bain,
C. D. J. Phys. Chem. C 2007, 111, 316–329.
(41) To eliminate the effects of the differences in CaF2 substrates and laser
intensity during different measurements, the peak at 1430 cm-1 was
normalized for comparison. When the FC7/PCBM films have been
prepared and measured independently three times, the deviation in
peak intensity ratio between 1430 and 1470 cm-1 was smaller than
20% (Figure S7 of the Supporting Information).
JA9057053
(43) Kaino, T.; Tomaru, S. AdV. Mater. 1993, 5, 172–178.
(44) Marder, S. R.; Perry, J. W. AdV. Mater. 1993, 5, 804–815.
(42) Tao, F.; Bernasek, S. L. Chem. ReV. 2007, 107, 1408–1453.
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