Anomalous photoelectric emission from Ag on zinc-phthalocyanine film
Senku Tanaka, Tomohiro Otani, Ken Fukuzawa, Koji Ogawa, Junpei Azuma, Isamu Yamamoto, Kazutoshi
Takahashi, Masao Kamada, and Ichiro Hiromitsu
Citation: Applied Physics Letters 104, 193304 (2014); doi: 10.1063/1.4876956
View online: http://dx.doi.org/10.1063/1.4876956
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/19?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Ag +12 ion induced modifications of structural and optical properties of ZnO-PMMA nanocomposite films
AIP Conf. Proc. 1512, 394 (2013); 10.1063/1.4791077
Structural modifications of zinc phthalocyanine thin films for organic photovoltaic applications
J. Appl. Phys. 111, 103117 (2012); 10.1063/1.4721409
Investigation on the orderly growth of thick zinc phthalocyanine films on Ag(100) surface
J. Chem. Phys. 133, 144704 (2010); 10.1063/1.3489658
Anomalously enhanced Raman scattering from longitudinal optical phonons on Ag-nanoparticle-covered GaN
and ZnO
Appl. Phys. Lett. 96, 033109 (2010); 10.1063/1.3291041
Effects of coatings on temporal cathodoluminescence quenching in ZnS:Ag,Cl phosphors
J. Appl. Phys. 95, 7873 (2004); 10.1063/1.1748861

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 163.51.64.90
On: Wed, 14 May 2014 10:34:05

APPLIED PHYSICS LETTERS 104, 193304 (2014)

Anomalous photoelectric emission from Ag on zinc-phthalocyanine film
Senku Tanaka,1,a) Tomohiro Otani,2 Ken Fukuzawa,2 Koji Ogawa,3 Junpei Azuma,3Isamu Yamamoto,3 Kazutoshi Takahashi,3 Masao Kamada,3 and Ichiro Hiromitsu2

1
Department of Electric and Electronic Engineering, Faculty of Science and Engineering, Kinki University,
Higashiosaka 577-8502, Japan2
Department of Physics and Materials Science, Graduate School of Science and Engineering,
Shimane University, Matsue 690-8504, Japan
3
Synchrotron Light Application Center, Saga University, Saga 840-8502, Japan(Received 6 March 2014; accepted 4 May 2014; published online 13 May 2014)
Photoelectric emission from organic and metal thin films is generally observed with irradiation ofphoton energy larger than 4 eV. In this paper, however, we report photoelectric emission from Ag
on a zinc-phthalocyanine (ZnPc) layer at a photon energy of 3.4 eV. The threshold energy for thisphotoelectric emission is much smaller than the work function of Ag estimated by conventional
photoelectron spectroscopy. The photoelectric emission by low-energy photons is significant forAg thicknesses of less than 1 nm. Photoelectron spectroscopy and morphological study of the
Ag/ZnPc suggest that the anomalous photoelectric emission from the Ag surface is caused by avacuum level shift at the Ag/ZnPc interface and by surface plasmons of the Ag nanoparticles.
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4876956]
V
An important issue in developing organic electronic
devices is the control of the properties of organic/organicand metal/organic interfaces. It is known that the propertiesof these interfaces affect the performance of the final organic
electronic device. For example, vacuum level shift is often
observed at organic/organic and metal/organic interfaces,1–5
and many studies have indicated that this shift affects theefficiency of carrier injection or extraction at suchinterfaces.6–8 While past studies have focused on the effectof the vacuum level shift on the electronic properties oforganic electronic devices, the effect on the barrier heightfor photoelectric emission has been paid little attention. In
this paper, we report photoelectric emission from a metal
surface on an organic layer under irradiation with near-UV
light having an energy of less than 4 eV. In general, the work
function of thin films of metals, except alkali metals, and theionization potential of organic thin films are no less than 4 eV.
It is therefore expected that light energy of less than 4 eV cannot generate photoelectric emission from the surfaces of metalor organic films. From the results of photoelectron spectroscopy, we conclude that the vacuum level shift at the metal/organic interface is correlated with a decrease in the effective
threshold energy for photoelectric emission from the sample
surface. The effect of metal surface plasmons on the anomalous photoelectric emission is also discussed.
Zinc-phthalocyanine (ZnPc) was used as the organic
layer in this work. ZnPc is a typical small-molecule semiconductor used in organic solar cells. The ZnPc was purchasedfrom Kanto Chemical and sublimed three times for purification. ZnPc thin films were fabricated by vacuum deposition
on an indium tin oxide (ITO) substrate purchased fromGEOMATEC. The pressure used during the deposition wasin the range of 10 5 Pa. Ag was used as the metal layer on
the ZnPc film. The thicknesses of the ZnPc and Ag layers
were measured using a quartz microbalance. The values ofa)Author to whom correspondence should be addressed. Electronic mail:senku@ele.kindai.ac.jp0003-6951/2014/104(19)/193304/4/$30.00

the thickness noted in this paper are, therefore, nominal
ones. The photoelectric emission by low-energy photo-irradiation and the photoelectron spectra under high-energy excitation of the film were measured using an experimental
photoelectron spectroscopy setup. The photoelectron spectroscopy was performed at two synchrotron facilities (Beamline 8B, UVSOR, Institute for Molecular Science, Okazaki,
Japan, and Beam line 13, SAGA-LS, Kyushu SynchrotronLight Research Center, Tosu, Japan). Both beam lines areequipped with a photoelectron measurement chamber connected to a preparation chamber. Thus, the samples could betransferred from the preparation chamber to the measurement chamber without breaking the vacuum. A white lightsource (a Xe lamp with an air mass 1.5 filter, HAL-320,
Asahi Spectra Co. Ltd.) and a tunable Ti:sapphire laser(Chameleon, COHERENT, Inc.) were used as UV-visible
region light sources. The UV-visible light was introduced tothe measurement chamber through a glass viewport that cut
off the light with wavelength shorter than approximately
320 nm. The power of the white light on the sample surface
was 100 6 10 mW/cm2. The repetition rate of the second
harmonic generation of the Ti:sapphire laser was 80 MHz,and the spot size and the intensity of the laser pulse were
7 mm in diameter and 0.2–0.5 nJ, respectively.
Figure 1(a) shows the photoelectron spectra of the Ag(0.2 nm)/ZnPc (10 nm)/ITO sample under laser irradiation ofdifferent wavelengths. The abscissa indicates the kinetic
energy of the electron. Note that a bias voltage of 10.0 V
was applied to the sample to detect the electrons near the vacuum level. The wavelength dependence of the intensity of thephotoelectric emission is shown in Fig. 1(b). The thresholdwavelength for the emission was estimated to be around370 nm. By taking the spectral width of the irradiated laser(approximately 10 nm) into account, the threshold energy ofthe photoelectric emission of the sample surface was estimated to be approximately 3.4 eV. This value is substantiallysmaller than both the ionization potential of the ZnPc thin film
and the work function of the Ag film, which were estimated to104, 193304-1

C 2014 AIP Publishing LLC
VThis article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 163.51.64.90
On: Wed, 14 May 2014 10:34:05

193304-2

Tanaka et al.

Appl. Phys. Lett. 104, 193304 (2014)

FIG. 2. Variation of the photoelectron spectra of the ZnPc film under incremental Ag deposition (0.5, 1.0, 3.0, 10.0, 20.0 nm). Left panel: Secondary
electron cutoff region. The sample was biased at 5.0 V during the measurements. Right panel: Around the Fermi level. No bias voltage was applied tothe sample.

FIG. 1. (a) Photoelectron spectra of the Ag(0.2 nm)/ZnPc sample under laser
irradiation of different individual wavelengths. The sample was biased at
10.0 V. (b) Intensities of the photoelectron peak under irradiation of theindividual laser wavelengths.

be 5.1 eV and 4.1 eV, respectively, from the conventional photoelectron measurements discussed later.
To investigate the mechanism of this anomalous photoelectric emission from Ag/ZnPc, the electronic structure of
the Ag/ZnPc interface was observed by photoelectron spectroscopy using synchrotron radiation. Fig. 2 shows the variation in the photoelectron spectrum of the ZnPc thin film
(15 nm) during the incremental deposition of Ag. All photoelectron measurements were carried out at room temperature
with synchrotron radiation of 39.6 eV. The overall energy resolution was approximately 0.15 eV. The bottom spectrum isthe photoelectron spectrum of the ZnPc film (15 nm) on the
ITO substrate. The Ag thickness increases from the bottom
to the top. The left panel shows the photoelectron spectra of
the secondary electron cutoff, for which a bias voltage of
5.0 V was applied to detect the photoelectrons near thevacuum level. The secondary electron cutoff was shifted
approximately 0.25 eV toward lower kinetic energy bydeposition of 0.5 nm Ag, and then shifted toward higher
kinetic energy with increasing Ag thickness. The right panel
shows the photoelectron spectra near the Fermi level. (Notethat no bias voltage was applied to the sample for the photoelectron measurement near the Fermi level.) The peak ataround 34 eV is attributed to the highest occupied molecularorbital (HOMO) of ZnPc. The step at 35.3 eV in the photoelectron spectrum measured after Ag deposition is due to the
Fermi level. The HOMO peak of ZnPc showed a similar variation with increasing Ag thickness to that of the secondary
electron cutoff. The smallest work function of 3.9 eV wasobserved for Ag (0.5 nm)/ZnPc (15 nm). The shift of thesecondary electron cutoff by the deposition of Ag wasobserved also on the sample in Fig. 1 by the conventional
photoelectron spectroscopy using synchrotron radiation. The
secondary electron cutoff was shifted approximately 0.35 eV
toward lower kinetic energy by deposition of 0.2 nm Ag on
the ZnPc substrate.9
From the photoelectron spectroscopy results, the shift inthe secondary electron cutoff was considered to be a key factor in the anomalous photoelectric emission. The variation of
the secondary electron cutoff was basically correlated withthe shift in the vacuum level of the sample surface. Thus, toinvestigate the correlation between the variation of the vacuum level and the anomalous photoelectric emission, the dependence of the photoelectric emission intensity on Ag
thickness was observed under white light illumination. The
results are shown in Fig. 3. The maximum emission intensity
was observed for an Ag thickness of 0.5 nm. The intensity ofthe photoelectric emission decreased with further increases
in Ag thickness. The Ag thickness dependence of the vacuum level shift estimated from Fig. 2 is plotted in Fig. 3. The
value of the vacuum level shift was evaluated against thevacuum level of the ZnPc film. The emission intensity was
negatively correlated with the vacuum level shift. The present results infer that a decrease in the vacuum level, whichindicates a decrease in the effective work function, of the Ag
on the ZnPc surface is the key factor accounting for the
anomalous photoelectric emission.
Vacuum level shift at metal/organic interfaces has beenreported in many systems, and is attributed to the formation

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 163.51.64.90
On: Wed, 14 May 2014 10:34:05

193304-3

Tanaka et al.

FIG. 3. Ag thickness dependence of the photoelectron signal intensity (black
circles) and the vacuum level shift (DVac) (red squares).

of an interfacial dipole layer.1–5 Several mechanisms havebeen proposed as to the origin of this interfacial dipole.2,10,11
In the present case, weak chemical interaction between ZnPc
and Ag is probably the main mechanism. The shape of the
HOMO peak of ZnPc showed no significant change after thedeposition of Ag. A strong chemical interaction may distortthe shape of the HOMO peak.12 From the direction of thevacuum level shift at the Ag/ZnPc interface, it is expectedthat an interfacial dipole layer was formed with the positively charged Ag and the negatively charged ZnPc. Notethat the interfacial dipole was localized at the Ag/ZnPc interface. The effect of this interfacial dipole on the vacuum level
shift at the surface decreased with increasing Ag thickness.
The effective barrier height for the photoelectric emission
was therefore smallest for the smallest Ag thickness andincreased with Ag thickness. In addition, the dispersion of
Ag in the ZnPc layer may have disturbed the effect of theinterfacial dipole. In Fig. 2, the HOMO peak of ZnPc
appears in the spectrum of the Ag (20 nm)/ZnPc sample.
Photoelectron spectroscopy is a surface-sensitive technique
because of the short mean-free path of the photoelectron in
the solid; an electron with a kinetic energy of 30–40 eV has amean-free path of only a few nanometers. Thus, the presenceof the HOMO peak of ZnPc in the Ag (20 nm)/ZnPc spectrum suggested that part of the ZnPc remained near the sample surface even after the deposition of thick Ag. We
suppose Ag particles during the vacuum deposition have alarge momentum and may diffuse into the ZnPc layer.
Indeed, the migration of metal particles into organic layers
during thermal deposition has been reported.13 A disordered
distribution of the Ag particles in the ZnPc layer can disturb
the formation of the aligned interfacial dipole.
Next, we discuss potential mechanisms of the anomalous photoelectric emission other than the interfacial dipole.
As described, the work function of the Ag (0.5 nm)/ZnPc(15 nm) film was estimated to be 3.9 eV by the photoelectron
spectroscopy. There is obvious deviation from the thresholdenergy for the anomalous photoelectric emission (3.4 eV).
Other probable mechanisms accounting for photoelectric
emission at smaller photon energy than the work function of
the individual materials include two-photon photoelectron
emission (2PPE),14–16 exciton fusion,17 and surface-plasmon-assisted (SPA) photoemission.18,19
In general, the 2PPE process is observed under high concentrations of light created by high-intensity laser light with

Appl. Phys. Lett. 104, 193304 (2014)

femtosecond-order pulse width. In the present measurement,
the photon concentration was too small to cause 2PPE. In
addition, in the case of 2PPE, the photoelectron intensity
should be proportional to the square of the light intensity. The
light power dependence of the photoelectric emission intensityof the Ag (0.5 nm)/ZnPc (15 nm) sample is shown in Fig. 4.
The emission intensity was linearly dependent on the light intensity. This result indicates that the observed photoelectric
emission was the result of a single-photon process. Moreover,the photoelectron spectrum caused by 2PPE would have a relatively long tail toward higher kinetic energy. The spectral
width of the present anomalous photoelectric emission wasless than 0.3 eV with no tail at higher kinetic energy, as seenin Fig. 1. These results exclude the possibility of 2PPE in thepresent phenomenon. For exciton fusion, the photoelectron intensity should also be increased by the square of the light
power.17 Exciton fusion is therefore also excluded as the origin of the present anomalous photoemission.
SPA photoemission is an enhancement of photoemission
yield in the range of surface-plasmon absorption of metal
surfaces. Callcott and Arakawa have demonstrated theenhancement of the photoemission yield of thin films of Al bysurface-plasmon resonance.18 Koschmieder and Thompson
have also reported that the incident energy dependence of thephotoemission yield from (4-methyl pyridine)-covered Ag
films showed peaks at the surface and the bulk plasmon resonance.19 Although anomalous photoelectric emission was notobserved in either case, these studies suggest that surface plasmons affect the photoelectric emission in the range of theplasmon resonance. In general, Ag surface plasmons are
excited by light of around 400 nm in wavelength. In the consideration of surface plasmons, the structure of Ag on the sample surface is important. To investigate the structure of the
Ag/ZnPc surface, we observed the morphology of the Ag on
the ZnPc surface using atomic force microscopy (AFM) andtransmission electron microscopy (TEM). For AFM measurements, the sample consisting of Ag (0.5 nm)/ZnPc (20 nm) on
ITO substrate was examined. Ag particles of less than 10 nm
in diameter were found to be dispersed on the ZnPc grains.9

FIG. 4. Light intensity dependence of the intensity of the photoelectron signal from the Ag (0.5 nm)/ZnPc (15 nm) sample under white light irradiation.
The intensity of the photoelectron signal has been normalized to that of the
100% value.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 163.51.64.90
On: Wed, 14 May 2014 10:34:05

193304-4

Tanaka et al.

As mentioned above, the Ag thickness measured by the quartz
microbalance was nominal. Thus, it was possible that the Ag
existed on the ZnPc layer in the form of small particles having
sizes in the range of the small thickness. The TEM image ofthe sample consisting of Ag (0.5 nm)/ZnPc (10 nm) on carbon
grid also showed a similar structure.9 The average crosssectional size of the Ag particles on the ZnPc surface wasapproximately 5 nm. It has been reported that free Ag clustersof 6–30 nm in size show surface-plasmon absorption at around
3.4–3.7 eV.20 The surface plasmons of Ag on ZnPc may also
be generated by absorption of light of similar energy range,
although the resonance energy for these surface plasmons
would differ from that of free Ag clusters. Tatsuma and coworkers have proposed the occurrence of plasmon-induced
charge separation between TiO2 and Ag nanostructures.21,22 A
similar mechanism may have assisted the escape of the electrons from the Ag to the vacuum. At this point, the effect of
the SPA photoemission on the anomalous photoelectric emission is not clear. However, the discrepancy between the work
function of the sample estimated by photoelectron spectroscopy and the threshold energy of the anomalous photoelectric
emission infers that the energy of the probe light may affectthe process of electron escape from solid surfaces. Further
research is now in progress.
In conclusion, we have demonstrated that anomalous
photoelectric emission from an Ag/ZnPc surface occurs
under 3.4 eV light irradiation. Although the reason for thediscrepancy observed between the threshold energy of thephotoelectric emission (3.4 eV) and the estimated work function of the surface by synchrotron photoelectron spectroscopy (3.9 eV) is not yet clear, a decrease in the effective
work function of Ag caused by the interfacial dipole at the
Ag/ZnPc interface and SPA photoemission are probably the
key mechanisms of the anomalous photoelectric emission.
From an application viewpoint, the present phenomenon
may be useful for the fabrication of photodiodes or photocathodes that work in the near-UV region. The simple structure and preparation method of this system are additional
advantages for such applications. For example, the present
work may be applicable to the fabrication of a planar electron source. It would also be interesting to combine thisAppl. Phys. Lett. 104, 193304 (2014)

system with a transparent flexible substrate, such as an ITOcoated polyethylene terephthalate film.
The authors thank Dr. Kazuto Arakawa at Shimane
University for TEM imaging and fruitful discussions. Part ofthis work was supported by JSPS KAKENHI Grant No.
24656037, Research Grant of Murata Science Foundation,
Kyushu Nanotechnology Network support, and the Use-ofUVSOR Facility Program (BL8B, 2011-2012) of theInstitute for Molecular Science, Okazaki, Japan.
1

H. Ishii and K. Seki, IEEE Trans. Electron Devices 44, 1295–1301 (1997).
H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater. 11, 605–625 (1999).
3
I. G. Hill, D. Milliron, J. Schwartz, and A. Kahn, Appl. Surf. Sci. 166,
354–362 (2000).
4
L. Yan and Y. Gao, Thin Solid Films 417, 101–106 (2002).
5
N. Koch, ChemPhysChem 8, 1438–1455 (2007).
6
T. Mori, H. Fujikawa, S. Tokito, and Y. Taga, Appl. Phys. Lett. 73,
2763–2765 (1998).
7
S. T. Lee, Y. M. Wang, X. Y. Hou, and C. W. Tang, Appl. Phys. Lett. 74,
670–672 (1999).
8
S. Tanaka, T. Hanada, K. Ono, K. Watanabe, K. Yoshino, and I.
Hiromitsu, Appl. Phys. Lett. 97, 253306 (2010).
9
See supplemental material at http://dx.doi.org/10.1063/1.4876956 for thephotoelectron spectra and the morphology of Ag on the ZnPc surface.
10
H. Vazquez, F. Flores, and A. Kahn, Org. Electron. 8, 241–248 (2007).
11
M. T. Greiner, M. G. Helander, W.-M. Tang, Z.-B. Wang, J. Qiu, and
Z.-H. Lu, Nature Mater. 11, 76–81 (2012).
12
T. Schwieger, H. Peisert, M. S. Golden, M. Knupfer, and J. Fink, Phys.
Rev. B 66, 155207 (2002).
13
K. Suemori, M. Yokoyama, and M. Hiramoto, J. Appl. Phys. 99, 036109(2006).
14
C. D. Lindstrom and X.-Y. Zhu, Chem. Rev. 106, 4281–4300 (2006).
15
M. Cinchetti, D. A. Valdaitsev, A. Gloskovskii, A. Oelsner, S. A. Nepijko,
and G. Sch€
onhense, J. Electron Spectrosc. Relat. Phenom. 137–140,
249–257 (2004).
16
A. Gloskovskii, D. A. Valdaitsev, M. Cinchetti, S. A. Nepijko, J. Lange,
M. Aeschlimann, M. Bauer, M. Klimenkov, L. V. Viduta, P. M. Tomchuk,and G. Schonhense, Phys. Rev. B 77, 195427 (2008).
17
M. Ono, R. Katoh, and M. Kotani, Proc. SPIE 2362, 219–224 (1995).
18
A. Callcott and E. T. Arakawa, Phys. Rev. B 11, 2750–2758 (1975).
19
T. H. Koschmieder and J. C. Thompson, Phys. Rev. B 50, 7845–7850(1994).
20
K.-P. Charle, L. K€
onig, S. Nepijko, I. Rabin, and W. Schulze, Cryst. Res.
Technol. 33, 1085–1096 (1998).
21
Y. Tiana and T. Tatsuma, Chem. Commun. 2004, 1810–1811.
22
E. Kazuma, N. Sakai, and T. Tatsuma, Chem. Commun. 47, 5777–5779(2011).
2This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 163.51.64.90
On: Wed, 14 May 2014 10:34:05