Download Study of excited states of fluorinated copper phthalocyanine by inner

Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 137–140
Study of excited states of fluorinated copper phthalocyanine
by inner shell excitation
K.K. Okudaira a,b,∗ , H. Setoyama a , H. Yagi a , K. Mase c , S. Kera a,b ,
A. Kahn d , N. Ueno b
a
b
Graduate school of Science and Technology, Chiba University, Chiba 263-8522, Japan
Faculty of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
c Institute of Materials Structure Science, Tsukuba 305-0801, Japan
d Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA
Available online 20 March 2004
Abstract
Near edge X-ray absorption fine structure (NEXAFS) spectra of hexadecafluoro copper phthalocyanine (FCuPc) films (thickness of 50 Å)
on MoS2 substrates were observed near the carbon (C) and fluorine (F) K-edges. From the analysis of the dependence of C and F K-edge
NEXAFS spectra on the photon incidence angle (α), the average molecular tilt angle was determined to be 30◦ . The lowest and second lowest
peaks in the F K-edge NEXAFS were assigned to the transition to ␴∗ . In the ion time-of-flight mass spectra of FCuPc excited by photons near
the F K-edge, F+ , CF+ , and CF3 + ions were mainly observed. These results indicate that C–C bonds as well as C–F bonds are broken by the
photon irradiation. From the analysis of the partial ion yield spectra of F+ and CF+ near the F K-edge, the lowest and second lowest peaks in
the F K-edge NEXAFS spectra could be assigned to transitions to ␴(C–F)∗ and ␴(C–C)∗ , respectively.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Fluorinated copper phthalocyanine; Inner shell excitation; Unoccupied state; Site-specific chemical bond scission
1. Introduction
In general, photoabsorption spectroscopy provides information on occupied states as well as unoccupied states. As
the inner shell electron is excited in near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, the character of unoccupied states can be easily studied. For the
assignment of the spectral structure of NEXAFS for large
and complex molecules, the building block approach is very
useful and has been widely used [1]. For ordered films of
planar ␲-conjugated organic molecules, the polarization dependence of NEXAFS spectra provides the symmetry of the
␲∗ and ␴∗ unoccupied states.
The analysis of the dependence of photon-stimulated
ion desorption (PSID) on photon energy (hν) is expected
to help in the assignment of NEXAFS spectra, since the
chemical bond scission by inner shell excitation depends
on the electronic configuration of the excited state. In
∗ Corresponding author. Tel.: +81-43-290-3446;
fax: +81-43-290-3449.
E-mail address: [email protected] (K.K. Okudaira).
0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.elspec.2004.02.078
fact, for poly(methyl methacrylate) and fluorocarbons such
as poly(tetrafluoroethylene) (PTFE) and perfluorinated
oligo(p-phenylene) (PF8P), it was reported that partial ion
yields (PIYs) depend on the photon energy near absorption edges [2–5]. These studies demonstrated that PSID in
molecular systems is highly dependent on the character of
the excited state.
Fluorinated copper phthalocyanine is a very interesting
material, since it is a promising electron-transport material
for organic devices [6]. To develop a highly efficient organic
light-emitting diodes (OLED), it is important to clarify the
characteristics of unoccupied states of the electron-transport
layer in the OLED consisting of organic molecules.
In this paper, we show the polarization dependence
of NEXAFS of a hexadecafluoro copper phthalocyanine
(FCuPc) film on a MoS2 substrate near the carbon and
fluorine K absorption edges. It provides both molecular orientation and characteristics of the unoccupied states. PIY
spectra of FCuPc near the fluorine K-edge are observed.
The comparison of FCuPc PIY spectra with those of fluorocarbons such as PTFE and PF8P provides the needed peak
assignment of NEXAFS spectra.
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K.K. Okudaira et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 137–140
2. Experimental
(a)
α
Total Electron Yield (arb. units)
(*)
α=70˚
α=70˚
α=55˚
α=55˚
α=35˚
α=0˚
α=35˚
α=0˚
280 290 300 310 320 330
280
290
Photon energy (eV)
(c)
hν
2
Iπ*(α,β)
Experiments were performed at the beamline 13 C at the
Photon Factory, Institute of Materials Structure Science.
This beam line was designed based on a cylindrical element
monochromator (CEM) concept using undulator radiation
from a 27-pole multiple wiggler/undulator. NEXAFS spectra were measured by the total electron yield (TEY) method.
TEY spectra were obtained by measuring the sample current. The incidence angle of the photons (α) was defined by
the angle between the direction of incidence of the photons
and the surface normal. PIY spectra were measured using a
time-of-flight (TOF) mass spectrometer system at α = 55◦
[2]. Soft X-ray pulses with a period of 624 ns, which is
obtained during single-bunch operation of the Photon Factory storage ring, were incident on the sample through a
photon-flux monitor [7]. Ion yields were normalized to the
incident photon flux. The incident photon flux spectrum was
recorded as the photocurrent at the photon-flux monitor consisting of a gold-evaporated mesh. All measurements were
performed at room temperature.
Commercially obtained FCuPc was purified by one-time
sublimation in an Ar gas stream of about 0.2 Torr. The thin
films were prepared by vacuum evaporation onto a MoS2
single-crystal surface. The thickness of the film was 50 Å.
(b)
E
hν
1
E
β=90˚
α
β
Molecular
Plane
300
β=0˚
β=20˚
β=30˚
β=45˚
β=60˚
00
20
40
60
Incidence Angle (α) (˚)
β=75˚
80
Fig. 1. (a, b) Carbon K-edge NEXAFS spectra of FCuPc film (50 Å
thick) on MoS2 at incidence angle (α) = 0◦ (normal incidence), 35, 55,
and 70◦ (grazing incidence). (c) Comparison between observed (solid
symbols) and calculated (solid curve) incidence angle dependence (α) of
the transition intensity from 1s to ␲∗ (the second lowest peak marked by
an asterisk (∗) in (b)).
3. Results and discussion
Figure 1a and b show carbon (C) K-edge NEXAFS spectra of FCuPc at α = 0◦ (normal incidence), 35, 55, and 70◦
(grazing incidence). Four sharp peaks at hν = 284.6, 285.5,
287.8, and 289.5 eV and one broad peak at about 295 eV
appear at grazing incidence. The first three peaks show
strong polarization dependence. The intensity of these peaks
becomes larger as the incidence angle decreases. On the
contrary, the broad peak at about 295 eV becomes larger at
normal incidence. From the assignment of the lowest peak
in the carbon K-edge NEXAFS spectrum of CuPc [8], which
is expected to be similar to that of FCuPc, the lowest peak of
the FCuPc NEXAFS spectrum is attributed to the transition
from C 1s to ␲∗ . Since the second and third peaks at 285.5
and 287.8 eV show a polarization dependence similar to that
of the first peak, these two peaks can also be assigned to
transitions from C 1s to ␲∗ . This polarization dependence of
the ␲∗ transition indicates that the angle between the FCuPc
molecular plane and the MoS2 substrate surface is small.
The opposite polarization dependence of the broad peak at
about 295 eV indicates that its origin is in the transition to
the ␴∗ state. Indeed, the ␴∗ orbitals are oriented along the
chemical bonds within the molecular plane, and the direction of the dipole moment of the ␴∗ state is perpendicular
to that of the ␲∗ state for a planar ␲-conjugated molecule.
In NEXAF spectroscopy, molecular orientation can be determined by analyzing the polarization dependence of either
the ␲∗ or ␴∗ transition intensities [1]. The intensity of the
1s → ␲∗ transition for a planar ␲-conjugated molecule under perfect polarization conditions has been given as
I(α, β) ∝ 2 cos2 β cos2 (90 − α) + sin2 β sin2 (90 − α)
(1)
where β is the tilt angle of the molecular plane with respect to
the substrate surface [9]. We assumed here that the azimuthal
orientation is disordered. The calculated results of Eq. (1)
for several tilt angles are presented as solid curves in Fig. 1c.
We compare the intensity of the second lowest peak at hv =
285.5 eV marked by an asterisk in Fig. 1b with calculated
results. The observed dependence of the intensity of the
second peak on the angle α is in good agreement with the
calculated result for β = 30◦ . It indicates that the average
molecular tilt angle of FCuPc is 30◦ for the 50 Å-thick film
on MoS2 substrate.
Figure 2a shows fluorine (F) K-edge NEXAFS spectra of
FCuPc at α = 0, 35, 55, and 70◦ . Two intense peaks appear
at hν = 689.8 and 695 eV. The intensity of these peaks is
larger at normal incidence than at grazing incidence. The
two peaks in the fluorine K-edge NEXAFS spectrum show
a polarization dependence opposite to that of the first three
peaks (the transitions to ␲∗ states) in the carbon K-edge
spectrum, indicating that these two peaks are attributed to the
transition from F 1s to the ␴∗ states. Since the direction of the
␴∗ state is perpendicular to that of the ␲∗ state for a planar
␲-conjugated molecule, the intensity of the transition from
1s to ␴∗ could be obtained by substituting β in Eq. (1) with
α
α=70˚
α=55˚
α=35˚
α=0˚
Iσ*(α,β)
690
2
1
0
0
(b)
700 710 720 730
Photon Energy (eV)
E
hν
α
Molecular
Plane
β
β=0˚
740
β=90˚
β=75˚
β=60˚
β=45˚
20
40
60
Incidence Angle (α) (˚)
β=30˚
β=20˚
80
Fig. 2. (a) Fluorine K-edge NEXAFS spectra of FCuPc film (50 Å thick)
on MoS2 at incidence angle α = 0◦ (normal incidence), 35, 55, and 70◦
(grazing incidence). (b) Comparison between observed (solid symbols) and
calculated (solid curve) incidence angle (α) dependence of the transition
intensity from 1s to ␴∗ (the lowest peak is marked by an asterisk (∗) in
(a)).
90−β. The calculated results of the 1s to ␴∗ transition for
several tilt angles are presented as solid curves in Fig. 2b. We
compare the intensity of the lowest peak at hν = 689.8 eV,
marked by an asterisk in Fig. 2a, with calculated results.
The observed dependence of the intensity of the first peak
on angle α is in good agreement with the calculated result
for β = 30◦ . This result is consistent with that obtained for
the carbon K-edge. From the analysis of the dependences of
the carbon and fluorine K-edge NEXAFS spectra on α, it is
found that the average molecular tilt angle of FCuPc is 30◦
for the 50 Å-thick film on the MoS2 . This tilt angle of FCuPc
is larger than that of the CuPc film on MoS2 (β = 10◦ ) [10].
A typical ion time-of-flight mass spectrum of FCuPc near
the fluorine K-edge is shown in Fig. 3. The main desorption ions are F+ , CF+ , CF3 + , and H+ . The H+ peak can be
ascribed to (i) surface contamination (the film was exposed
to air after the deposition) and (ii) high sensitivity of mass
spectrometer for H+ ion. The appearance of F+ and CF+
ions indicates that C–C bonds as well as C–F bonds are broken by the irradiation with photons near the fluorine K-edge.
The mass spectra of typical fluorocarbons such as hexafluoroethylene and hexafluorobenzene show relatively high intensity for the CF3 + ion [11]. Since the simple chemical
bond breaking can not account for the process of CF3 + ion
generation, we focus here on the ion yields of F+ and CF+ .
Figure 4a and b show partial ion yield (PIY) spectra of
F+ and CF+ ions, respectively, for FCuPc near the flu-
139
hν=689.8eV
F+
2
1.5
H+
+
CF
hν
1
0
CF3+
hν
200
400
600
Time of Flight (nsec)
Fig. 3. Typical ion time-of-flight mass spectrum of FCuPc at hν =
689.8 eV.
orine K-edge. The NEXAFS spectrum of FCuPc is also
shown for comparison. The PIY of F+ ions becomes larger
near the energy position of the lowest peak in the NEXAFS spectrum, and reaches a maximum at hν = 691.2 eV.
This indicates that C–F bond scission is effective due to
absorption of hν = 689.8 eV photons (corresponding to
the energy position of the lowest peak in fluorine K-edge
NEXAFS) and hν = 691.2 eV photons. In general, chemical bond scission by the excitation to ␴∗ is very effective.
We reported that for fluorocarbons such as PTFE and PF8P
thin films, F+ ion desorption shows an intense peak at
the photon energy corresponding to the lowest peak in the
fluorine K-edge NEXAFS spectrum and that these lowest
peaks in the fluorine K-edge NEXAFS of PTFE and PF8P
are assigned to transitions to ␴(C–F)∗ [4,5]. The fluorine
K-edge NEXAFS spectrum of FCuPc shown in Fig. 4a is
20
(a)
+
PIY(F )
TEY
PIY
10
TEY
0
(b)
PIY
+
PIY(CF )
TEY
0.8
TEY Intensity (arb. units)
hν
PIY Intensity (CPS/pA)
E
(*)
Total Electron Yield (arb. units)
(a)
Ion yield (normlized to photon flux)
(CPS/pA/channel)
K.K. Okudaira et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 137–140
0.6
TEY
0.4
680
690
700
710
720
730
Photon energy (eV)
Fig. 4. PIY spectra of (a) F+ and (b) CF+ for FCuPc near the fluorine
K absorption edge. NEXAFS spectra (broken curve) are also shown for
comparison. NEXAFS spectra are renormalized at hν = 682 and 732 eV
to fit the PIY intensities.
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K.K. Okudaira et al. / Journal of Electron Spectroscopy and Related Phenomena 137–140 (2004) 137–140
similar to those of PTFE and PF8P. From these results, the
lowest peak in the fluorine K-edge NEXAFS of FCuPc can
be assigned to the transition to ␴(C–F)∗ . This assignment
is consistent with the result of polarization dependence of
fluorine K-edge NEXAFS as discussed above.
We also observe a maximum in the F+ ion yield at hν =
691.2 eV, which is higher by 1.4 eV than the photon energy
of the lowest NEXAFS peak position. At hν = 691.2 eV, the
fluorine K-edge NEXAFS spectra do not show a peak at any
incidence angle (see Fig. 2a), indicating that (i) a transition
to an unoccupied state which is distributed at the C–F bond
in FCuPc molecule exists at hν = 691.2 eV and (ii) the
transition dipole moment from F 1s to this unoccupied state
is small. It is expected that a possible assignment of this
unoccupied state is ␴(C–F(s))∗ , since the transition from 1s
to unoccupied states with s-symmetry is forbidden.
Next we focus on the PIY spectrum of CF+ ion near the
fluorine K-edge shown in Fig. 4b. The hν-dependence of
CF+ yield is different from that of F+ . This difference suggests that the site-specific chemical bond scission by the
inner shell excitation occurs in FCuPc thin film near the
fluorine K-edge. The PIY spectrum of CF+ ion gives two
maximum at hν = 692.5 and 695 eV. It indicates that C–C
bond scission occurs effectively by the irradiation at these
two photon energies. The energy position of the second peak
in the PIY spectrum of CF+ is in agreement with that of
the second lowest peak of the fluorine K-edge NEXAFS
spectrum. The second lowest peak in the fluorine K-edge
NEXAFS at hν = 695 eV could be assigned to the transition to ␴(C–C)∗ . On the other hand, in the fluorine K-edge
NEXAFS spectrum no peaks appear at hν = 692.5 eV, corresponding to the energy position of the first peak in PIY
spectrum of CF+ ion. It indicates that (i) a transition to the
unoccupied state which is distributed at the C–C bond exists
at hν = 692.5 eV and (ii) the transition dipole moment from
F 1s to this unoccupied state is small. For a more detail assignment of the transition, we need extensive studies such as
theoretical analyses, since there are many candidates for the
unoccupied states which are distributed on the C–C bonds.
4. Conclusions
The polarization dependence of carbon and fluorine
K-edge NEXAFS spectra of FCuPc provides the molecular orientation. The average molecular tilt angle of FCuPc
molecules in a 50 Å thick film on MoS2 is 30◦ . The lowest peak in fluorine K-edge NEXAFS is assigned to the
transition to the ␴∗ unoccupied state, not to ␲∗ .
PIY spectra of FCuPc for F+ and CF+ ions near the fluorine K-edge are observed. The PIY spectra are different
from the total electron yield (NEXAFS) spectrum near the
fluorine K-edge. In particular, the PIY of F+ ions becomes
larger at hν = 689.8 eV, corresponding to the lowest peak
in the NEXAFS spectrum and reaches a maximum at hν =
691.2 eV. From the comparison of the PIY spectrum of F+
ions for FCuPc with those of PTFE and PF8P, the lowest
peak in the fluorine K-edge NEXAFS spectrum of FCuPc
can be assigned to the transition from F 1s to ␴(C–F)∗ .
Furthermore, it is expected that the transition from F 1s to
␴(C–F(s))∗ exists at hν = 691.2 eV. From PIY of the CF+
ion, the second lowest peak in the fluorine K-edge NEXAFS
spectrum can be assigned to the transition to ␴(C–C)∗ . The
analysis of the PIY spectra is useful for the assignment of
NEXAFS spectra.
Acknowledgements
This research was partially supported by the Ministry of
Education, Science, Sports and Culture; Grant-in-Aid for
Creative Scientific Research, 14GS021; and a grant from
the New Energy and Industrial Technology Development
Organization (NEDO).
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