Applied Physics Express 3 (2010) 015201

Anomalous Light Emission from Metal Phthalocyanine Films on Au(111)Activated by Tunneling-Current-Induced Surface Plasmon
Arifumi Okada, Ken Kanazawa, Kiwamu Hayashi, Naohiro Okawa,Takehiro Kurita, Osamu Takeuchi, and Hidemi Shigekaway
Institute of Applied Physics, CREST-JST, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Received November 30, 2009; accepted December 16, 2009; published online January 15, 2010
We performed a scanning-tunneling-microscopy-induced light emission (STM-LE) study on Cu- and Zn-phthalocyanine films formed on a Au(111)
surface. Optical emission with transition between the highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO) was observed
at bias voltages lower than the energy corresponding to the molecular HOMO–LUMO gap. The voltage and current dependences of STM-LE
intensities show an inconsistency for both samples, i.e., although the former shows the existence of onsets below the HOMO–LUMO gap
energies, suggesting excitations by a two-electron process via an intermediate state, the latter shows a linear relationship expected from oneelectron excitation processes. # 2010 The Japan Society of Applied Physics
DOI: 10.1143/APEX.3.015201

or example, because of their durability and characteristic electronic properties, metal phthalocyanines are widely used as a hole injection layer forelectroluminescence devices.1) Therefore, it is of great
importance to clarify the detailed electronic transition
processes in the ﬁlms of these dye materials not only
for the understanding their scientiﬁc basis but also for
determining their applications. A scanning-tunneling-microscopy-induced light emission (STM-LE) technique is oneof the methods for studying such processes and for obtaining
a localized signal.
In recent studies of STM-LE from molecular layers
on metal substrates, such as copper phthalocyanine andporphyrine derivatives on a Au(111) surface, highest
occupied molecular orbital–lowest unoccupied molecularorbital (HOMO–LUMO) emission was observed despite thefact that the bias voltage for excitation was lower than theHOMO–LUMO gap energies.2,3) Various mechanisms, suchas excitation via an intermediate state2) and the energy levelshift,3) have been discussed and the eﬀect of surface plasmon
is suggested;2–9) however, their details have not yet beenclariﬁed.
To determine the basic mechanism, we investigated thebias voltage and tunneling current dependences of STM-LE
intensity using two types of sample, namely, Cu- and Znphthalocyanine [CuPc and ZnPc, the molecular structureis shown in Fig. 1(a)] ﬁlms formed on a Au(111) surface.
Despite the fact that the tunneling current dependence
provides the basis for determining a certain process, forexample, a one- or two-electron process, such an experimenthas never been performed, probably owing to the instabilityof the molecular ﬁlms at a high tunneling current. In thispaper, we present the results of the experiment; a stablemeasurement over the necessary range of tunnel current
intensities was realized with the ﬂow of nitrogen gas.
A custom-made STM-LE system was prepared for theexperiment, in which an STM head was set in a metallic
shading box. A mechanically cut PtIr tip was used for
injecting tunneling current, and the emitted photons werefocused onto the entrance edge of an optical ﬁber (singlecore, diameter: 600 m) using two fused-silica lenses, asF

Present address: Graduate School of Chemistry and Materials Technology,
Kyoto Institute of Technology, Kyoto 606-8585, Japan.
yE-mail address: hidemi@ims.tsukuba.ac.jp

(a)

NNNN
M

NNN

N

(b)

STM
To
monochromatorhνFig. 1. (a) Molecular structure of metal phthalocyanine (in this study, M
stands for Zn and Cu); (b) Schematic drawing of STM-LE light collection
system.

shown schematically in Fig. 1(b). The focal lengths andspacing of these lenses were carefully selected to minimize
chromatic aberration. The emitted photons were introduced
into a monochromator with a 150 grooves/mm grating anddetected with a Peltier-cooled charge-coupled device (CCD)with a high sensitivity (1024  127 pixels). The CCD
exposure time was ﬁxed at 128 s. STM topographic andspectroscopic measurements were started and ﬁnishedsimultaneously. A median ﬁlter was applied to remove
spike noises of the CCD. The scan area was 1  1 m2 .
When the measurements were performed in air, the obtained
signal was not stable and rapidly decreased in intensity
during the measurements. The dry N2 gas ﬂow aroundthe STM tip enabled stable measurements, probably by
preventing the sample from being damaged by oxidation.
We repeated the measurements several times over the same
area to verify the stability of the signal under the measurement conditions. Samples were prepared by the vacuum
deposition of 200-nm-thick CuPc and ZnPc ﬁlms on
Au(111)/mica substrates.

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Appl. Phys. Express 3 (2010) 015201

A. Okada et al.

(a)ZnPc

STM-LE+2.5 V, 0.5 nA
PL (in Ethanol)

(b)

S1
T1

1.85 eV
1.13 eV
S0Fig. 2. (a) Typical STM-LE and photoluminescence spectra fromZnPc/Au(111) system. The STM spectrum consists of tip-induced
plasmon and molecular electronic transition components. (b) Energy
diagram around ground and first excited states of ZnPc.10)

Figure 2(a) shows typical spectra obtained by observing
the STM-LE (+2.5 V and 0.5 nA) from a ZnPc/Au ﬁlm(upper spectrum) and by the photoluminescence (PL)measurement of ZnPc solution with excitation using a405 nm CW laser (lower spectrum). Figure 2(b) showselectronic levels around the ﬁrst excited state.10) The
emission components at 1.81 eV in the STM-LE spectrum
and at 1.84 eV in the PL spectrum are in good agreement andare attributed to the emission induced by the HOMO–LUMO
transition of ZnPc [S1 ! S0 in Fig. 2(b)]. The low-energy
component appearing in the STM-LE spectrum (dominantbelow 1.5 eV here) originates from the localized tip-induced
surface plasmon.
Figure 3(a) shows STM-LE spectra from CuPc/Au(111)
obtained at a bias voltage of +1.6 V for various tunnelingcurrents (0.5 nA step, shown with a certain oﬀset). Despite
the fact that the bias voltage is lower than the HOMO–
LUMO gap energy of the molecule, the spectra havea component corresponding to the HOMO–LUMO gap
energy, similar to the previous case.2) Spectral intensity
increases with the intensity of tunneling current usedfor excitation. For further analysis, signal intensity wasintegrated over the blue and red areas in Fig. 3(a) to give theintensity due to the HOMO–LUMO transition, AM , and that
due to the tip-induced Plasmon process, AP , respectively.
The overlapping component of the plasmon-related emission
on the HOMO–LUMO signal was removed by assuming alinear background.
Figure 3(b) shows the bias voltage dependences of AM
and AP . As shown in Fig. 3(b), AM rapidly increases withVs from approximately the voltage corresponding to theHOMO–LUMO gap energy of the molecule (1.77 eV,indicated by a purple dashed line): however, we can clearlyobserve that the signal appears at a voltage lower than theHOMO–LUMO gap energy. The onset is approximately+1.2 V, as shown in the inset, which is close to that obtainedin the previous study (+1.1 V).2) The threshold energies are(a)

(b)

(c)

(d)

(e)

(f)

Fig. 3. (a) Some STM-induced light emission spectra from CuPc/Au(111) obtained at different tunneling currents. (b,c) Changes in integrated
spectral intensities of molecular transition and decay of plasmon of CuPc/Au(111) with bias voltage and tunneling current. The inset of (b) shows adetail from 1 to 2 eV. (d) Some emission spectra from ZnPc/Au(111) at different tunneling currents. (e,f) Changes in integrated spectral intensities ofmolecular transition and decay of plasmon of ZnPc/Au(111) with bias voltage and tunneling current. Blue, green, and red squares indicate theintegrated areas of the hatched regions with the same colors as in (a) and (d). All the data of dependences on the tunneling current are obtained at lower
bias voltages than the molecular HOMO–LUMO energy gaps. Black dashed lines in (b) and (e) serve as visual guides, having intersection points close
to the HOMO–LUMO gap of each molecule.

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Appl. Phys. Express 3 (2010) 015201

A. Okada et al.

close to the triplet state of CuPc, suggesting an excitation
process via a triplet state as an intermediate state for a twoelectron excitation process, similar to that in the triplet–
triplet annihilation process.2) However, in consideration ofthe tunnel current intensity used for the excitation, the STMLE signal intensity is extremely high for the excitation
process including a triplet state.3)
To investigate the excitation process in detail, we, for theﬁrst time, examined tunnel current dependence of the STMLE signal intensity using AM and AP . For the measurementof the current dependence of the STM-LE signal intensity,
the bias voltage should be selected below the threshold of
the direct HOMO–LUMO transition. On the other hand, it
was diﬃcult to obtain a stable signal at a bias voltage lower
than 1.6 V when the current was increased, probably owing
to sample damage. Therefore, we set the voltage at 1.6 V,
which is lower than the threshold as shown in Fig. 3(b).
If the excitation is via an intermediate state, i.e., themechanism is a two-electron process, the emission intensity
is expected to have a quadratic dependence on the tunnel
current. As shown in Fig. 3(c), however, AM has a linear
current dependence, which is generally observed for a oneelectron process, in contrast to the Vs dependence of theemission intensity, i.e., the appearance of the signal with thebias voltage lower than the HOMO–LUMO gap energy that
results in a two-electron excitation process.
To conﬁrm the anomalous results obtained for the CuPc/
Au sample in this experiment, we performed similar
experiments on the ZnPc/Au sample. The obtained results
are shown in Figs. 2(d)–2(f). Signal intensity was integrated
over the green and red areas in Fig. 3(d) in the same manneras in the case of CuPc. The STM tip came into contact withthe sample at a bias voltage lower than +1.5 V in the caseof ZnPc. The saturation of AM observed for bias voltages
higher than +2.4 V [Fig. 3(e)] was reproducible, suggestingthat such saturation is not due to the damage of the sample
surface. The properties of a plasmon, such as electromagnetic decoupling for high-frequency components, may
induce the saturation to some extent.11,12) As shown inFigs. 2(e) and 2(f), the characteristic properties observed for
the ZnPc/Au sample are similar to those observed for theCuPc/Au sample. The voltage and current dependences ofSTM-LE intensities seem to show an inconsistency for both
samples, i.e., the former shows the existence of onsets below
the HOMO–LUMO gap energies, suggesting excitations bya two-electron process via an intermediate state, while thelatter shows a linear relationship expected from one-electronexcitation processes.

Note that when a highly oriented pyrolytic graphite(HOPG) was used as a substrate, no emissions wereobserved, which is similar to the results of other studies,2,6)
indicating that the molecular excitation is not due to a simple
mechanism, such as the tunneling of electrons from theFermi level of the tip to unoccupied molecular states3,13) or
the direct excitation of electrons from HOMO to LUMO
states by tunneling electrons. Namely, the process includes
energy transfer from a plasmon. Excitation through aplasmon may play an important role in modifying theexcitation processes, for example, via the triplet state as
an intermediate state. Enhancement by a localized surface
plasmon and the immediate saturation of the intermediate
triplet state may result in the observed one-electron-liketunneling current dependence. In the development of organic
EL devices, improvement in emission eﬃciency by energy
transfer using triplet states has been attracting interests,14)
and it is of great importance to study the basic processes inmore detail.
As has been clariﬁed, unique characteristics exist in theprocess of STM-induced light emission from metal phthalocyanine ﬁlms formed on a metal surface. To obtain acomprehensive understanding of the mechanism, further
theoretical analysis on the phenomena, such as the couplingof the surface plasmon with the excitation process including
a triplet state, is urgently required.

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015201-3# 2010 The Japan Society of Applied Physics