1
Molecular scale organic electroluminescence from tunnel junctions
Z.-C. DONG*, X.-L. GUO*, A. S. TRIFONOV*, P. S. DOROZHKIN*,
S. YOKOYAMA†, S. MASHIKO†, T. OKAMATO‡
*
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan
†Communication Research Laboratory, Kobe, Hyogo 651-2401, Japan
‡RIKEN (Institute of Physical and Chemical Research), Wako, Saitama 351-0198, Japan
Abstract: Ultrasmall light sources are valuable for high-resolution optical microscopy [1,2] and quantum
information processing [ 3 , 4 ]. While optical pumping of dilute dye molecules in solids can generate
single-molecule light sources [1-5], electrically driven analogues are desirable for electronic applications.
However, the demonstration of single-molecule electroluminescence is very rare, amounting mainly to
fluorescence from specific molecules on an oxide-buffered surface [6] and electroluminescence from different
sizes of silver nanoclusters [7,8]. The reason behind this is the quenching of molecular fluorescence when a
molecule is close to a surface [9,10]. The realization of molecular scale organic electroluminescence on
surfaces would thus depend on how effectively the luminescent core can be decoupled from the substrate and
how well a localized excitation source can be applied to individual molecules. Here we report a thin multilayer
decoupling approach with nano-probe excitation to generating well-defined molecular fluorescence that is
vibrationally resolved and on a molecular scale. Our approach features simplicity via self-assembly, room
temperature ‘bipolar’ operation, low operation voltages in the tunnelling regime, and good reproducibility. The
research offers new understanding on how molecules behave optoelectronically in a nanoscopic environment.
Keywords: organic electroluminescence, scanning tunnelling microscopy, molecular scale light sources.
A scanning tunnelling microscope (STM) is
capable of more than just imaging with atomic
resolution; the highly localized tunnelling current can
also be used for excitation of light emission [11]. In
contrast to fine understanding of STM-induced
luminescence (STML) on metal [ 12 , 13 ] and
semiconductor surfaces [ 14 , 15 , 16 ], the role of
molecules, acting as spacers or emitting fluorescence
directly, remains controversial for molecule-covered
surfaces [6,17,18]. Despite good optical contrast in
photon maps [17,19], no direct evidence of molecular
fluorescence is available from neutral molecules
excited by STM. Our fundamental motivation is to
understand the optoelectronic behavior of molecules
in a nanoscopic environment, which is a critical issue
not only for interface control in organic electronics
but also for the development of molecular scale
electronics.
Here we report a thin multilayer self-assembly
approach to generating molecular based fluorescence
with an STM tip operated in the tunnelling regime.
The multi-monolayer stacking of a selected molecular
architecture reduces the nonradiative energy transfer
from excited molecules to the substrate, while the
enhanced electric field strength at the tip apex excites
efficiently the molecule directly underneath.
Well-defined vibrationally resolved fluorescence is
observed that can be well justified by the standard
photoluminescence (PL) spectra of the molecule.
Fluorescence on the molecular scale is suggested by
the linewidth narrowing of spectral peaks with
increased fluorophoresubstrate spacing and exciton
confinement underneath the tip apex. Through
investigation on the interfacial energy level
alignment, a model is proposed to explain the
occurrence of STM-induced molecular fluorescence.
The experiments were performed with an ultrahigh
vacuum (UHV) JEOL-STM. Atomically clean
Au(100) surfaces were routinely obtained after cycles
of
argon
ion
sputtering
and
annealing.
Meso-tetrakis(3,5-di-tertiarybutylphenyl)porphyrin
(H2TBPP) molecules were deposited onto the surface
in UHV by sublimation from a Knudsen cell. The
coverage was monitored by a quartz oscillator with
the deposition rate calibrated by STM images below
two monolayers (ML). STM images were taken in the
constant current topographic mode with the sample
biased. Tunnelling spectroscopy was performed with
the tipsample distance regulated at +3.0 V and 50
pA. Tips used for imaging and photon emission were
2
prepared by electrochemical etching of a tungsten
(W) wire followed by in-situ cleaning in vacuum by
argon ion sputtering.
STM-induced luminescence from molecules was
generated in the tunnelling regime at low bias
voltages (typically <3 V) and small tunnel currents
(0.5 nA or less). The tip was positioned statically
with a stable tunnel resistance of several G. Photons
emitted from the tunnel junction were collected by a
lens located near the tipsample region [18] and
detected
by
either
a
photomultiplier
(Hamamatsu-R943-02,
300800 nm)
or a
spectrophotometer (Hamamatsu PMA-100). The
latter operates in the pulse counting mode at a
spectral resolution of 9 nm over 350850 nm. Spectra
presented here were acquired at room temperature
over typically 10 min. The PL spectra of H2TBPP
were measured from a different sample of H2TBPP
thin film (20 nm) on Cu(100) by using a He-Cd
laser at 442 nm. The full-width-at-half-maximum
(FWHM) of spectral peaks was obtained via fitting to
a Lorentzian line shape. A sharp highpass cutoff filter
at 620 nm was used to remove most of the surface
plasmon contribution other than tails during intensity
measurements.
Figure 1 shows how H2TBPP molecules are packed
layer-by-layer on Au(100) and typical tunnelling
spectra for the molecular multilayer. The steric
repulsion between t-butyl substituents and
pyrrole-ring H atoms (Fig. 1a) drives the phenyl ring
rotate 60 out of the porphyrin plane (Fig. 1b). The
STM images exhibit characteristic four-lobe feature
for each molecule (2 nm) [20], indicating that the
porphyrin core is oriented almost parallel to the
surface (Fig. 1c). The molecules are found to stack in
either “eclipsed” (left in Fig. 1c) or “staggered” (right
in Fig. 1c) manner. The tilt of the phenyl rings shore
up the porphyrin core 0.7 nm above the substrate,
but the moleculesubstrate interaction is still strong
for the first monolayer. Relatively well-defined
molecular states were observed at higher coverage. A
typical tunnelling spectrum is shown in Fig. 1d for
coverage above 3 ML and reveals localized states
possibly associated with the highest occupied
molecular orbital (HOMO) at -1.4 V and the lowest
unoccupied molecular orbital (LUMO) at +1.7 V.
Such orbital-mediated tunnelling spectra (OMTS) are
comparable
to
those
reported
for
alike
tetraphenylporphyrins via OMTS and ultraviolet
photoelectron spectroscopy (UPS) [21].
Figure 1 Stacking and tunnelling spectra of H2TBPP
on Au(100). a, Molecular structure. b, Side-view of
space-filling geometry optimized by energy
minimization via molecular mechanics methods. c,
STM topography of molecular stacking at 2.5 ML
(2020 nm2, +2.5 V, 50 pA). The inset shows the
adsorption conformation of the molecules on the
striped Au(100) (98 nm2, 2.8 V, 50 pA). The
four-lobe quadrilateral shape is a typical registry of
such molecule with the lobes attributed to the
uppermost t-butyl groups. d, Typical normalized
differential tunnelling spectra (above 3 ML).
Optical spectroscopy is most authoritative in
judging the origin and nature of STM-induced
luminescence. The spectrum from 1 ML H2TBPP
molecules on Au(100) shows a single-peak feature
(Fig. 2a, blue) that is characteristic of
plasmon-mediated emission from the Au substrate
(Fig. 2a, brown) except the enhancement and
blue-shift effects [12,17,18]. Molecular fluorescence
is quenched, an indication of the molecule acting
merely as a spacer to modify the STM junction
geometry for the one monolayer case. Further
electronic decoupling is required to weaken the
moleculesubstrate interaction for molecular
fluorescence to emerge. Since energy transfer near a
surface
depends
dramatically
on
the
fluorophoresubstrate distance [9], we deposit more
molecules on the surface and use bottom molecules
as a spacer layer. The green curve in Fig. 2a shows a
typical spectrum at 3 ML. In addition to the surface
plasmon band around 600 nm, a broad band at 658
nm starts to emerge (with a small bump at 723 nm),
suggesting direct luminescence contribution from
molecules. These two additional peaks become
sharper with increased coverage (and thus increased
3
Figure 2 STM-induced luminescence (STML) spectra
of H2TBPP. a, Spectra at +2.8 V and 0.5 nA with the
tip above Au(100) (brown) and those at different
coverage of H2TBPP (blue: 1 ML; green: 3 ML; red: 6
ML). The black curve shows a theoretical simulation
for the pristine Au(100). The spectra for Au(100) and 1
ML were acquired using the “same” tip and alignment.
Their
intensities
are
scaled
according
to
counts-per-second per-nA to show the enhancement
effect of molecules on local surface plasmons (LSP). b,
STML spectrum (red, 6 ML, +2.8 V, 0.2 nA) compared
with the PL spectrum (blue) and photoluminescence
excitation spectrum (PLE, black). c, Distance
dependence of spectral linewidths for STM-induced
molecular fluorescence. The fluorophoresubstrate
spacing (d) is based on the first layer spacing of 0.7
nm and molecular height of 0.8 nm. The four data
points were obtained for 3, 4, 5, and 6 ML. The inset
shows the schematic junction geometry.
thickness up to 5 nm for 6 ML) while the
plasmon-mediated emission band is suppressed (Fig.
2a, red, 6 ML).
The molecular origin of the STM-induced emission
bands at 658 nm (1.88 eV) and 723 nm (1.72 eV)
(Fig. 2b, red) is clearly established upon comparison
with the standard PL spectrum of H2TBPP (Fig. 2b,
blue). The matching of two sharp peaks is nearly
perfect, which also suggests the same decay channel
of excited states despite different excitation
mechanisms. The luminescence of porphyrins
originates from * transitions with nearly
degenerate a1u() and a2u() MOs as HOMO and
degenerate eg(*) MO as LUMO [22]. The peak at
658 nm can be assigned to the Qx(0,0) zero-phonon
fluorescence while the 723-nm peak to the vibronic
overtone Qx(0,1) [22]. Such assignment is further
supported by (1) the coincidence between the Qx(0,0)
absorption and fluorescence transitions and (2) the
same peak spacing of 0.16 eV for the Q-band
vibronic structure in both fluorescence and absorption
spectra (Fig. 2b, black). Such vibrational mode is also
confirmed by the infrared spectrum of the molecule
with absorption at 1290 cm-1, and can usually be
assigned to the Cpyrrolephenyl vibration [23,24]. Note
that STM-induced luminescent spectra from polymers
were previously observed on indium-tin oxide, but
from a relatively thick film (200 nm) and under field
emission conditions (100 V) with the tip
micrometers above the surface [24].
It is striking that such well-defined vibrationally
resolved fluorescence occurs for a very thin layer (<5
nm) of organic molecules on a metal substrate.
Furthermore, the fluorescence appears to arise from
the few or even single molecules directly underneath
the tip apex. The linewidths of spectral peaks at 658
nm narrow down as the molecules stack up
layer-by-layer (Fig. 2c), a strong indication of the
fluorescence from the top-layer molecules. With
increased thickness (d), the fluorophoresubstrate
coupling is weakened, which results in decreased
lifetime-broadening and narrowed linewidths (w).
The power-law fit to the wd plot yields an
approximate w1/d3 relation, indicating a
Forster-type dipoledipole energy transfer in nature
from molecular excited states to the bulk substrate
[10]. The classical dipole theory [9] still holds for
distances less than 5 nm. The radiative decay of
excited H2TBPP molecules becomes highly
competitive against the nonradiative decay to surface
plasmons as thickness increases.
The overwhelming dominance of molecular
fluorescence from the top layer is not a surprise in the
tunnelling regime. A sharp STM nano-probe is
generally thought to feature a single-atom-like tip
apex. There exists strongly enhanced electric field
strength at the tip apex since the electric field drops
off rapidly away from it. Only those molecules in
proximity to the tip apex can be effectively excited.
(No molecular fluorescence was detected by using
blunt tips intentionally crashed.) Moreover, the
exciton energy appears to be confined within the
molecule directly underneath the tip apex. The lateral
electron/exciton travel distance, l, has been estimated
to be less than 1 nm at its high limit (l  Eτ, where E
is the electric field, 1 V/nm;  is the electron
mobility, <10-5 cm2/V-s;  is the lifetime of molecular
excited states, 10500 ps for the thin multilayer
4
[25]). Therefore, we believe that the STM-induced
fluorescence phenomenon observed here occurs on
the molecule scale. Further research on the
single-molecule behavior is underway.
As expected for molecular fluorescence, the peak
positions at 658 and 723 nm remain constant for
different excitation voltages, e.g., from 1.9 to 3.5 V,
because the radiative decay is always associated with
the HOMOLUMO gap of the molecule. The
fluorescence intensity increases at higher bias,
presumably owing to increased number of vibronic
states available for electrons to tunnel into. It is also
noteworthy that the H2TBPP molecule fluoresces for
both polarities, presenting a unique demonstration of
‘bipolar’ organic electroluminescence. A quantum
efficiency of 10-5 photons per tunneling electron has
been obtained for the molecular fluorescence at +2.5
V. It is worthy to mention that the fluorescence
intensity appears to drop above +3.5 and -3.0 V,
probably because of the damage of molecules at high
bias.
While we are in a process to theoretically model
the STM-induced molecular fluorescence, the
occurrence of light emission from molecules
definitely requires that both HOMO and LUMO
states be partially occupied upon application of an
appropriate bias. In other words, the relative energy
level alignment of molecular electronic states at the
interface and junction has to be in such a manner that,
above certain bias voltage, electrons can be injected
into the LUMO and holes into the HOMO (or
electrons tunnelling out). From this point of view, the
STM-excited molecular fluorescence may simulate
the operation of organic light emitting diodes. On the
basis of the occurrence of molecular fluorescence at
low bias and suppression of plasmon-mediated
emission at increased thickness, the mechanism of
STM-induced luminescence from a thin molecular
multilayer is proposed schematically in Fig. 3a. The
emission of the surface plasmon bands at 600 nm
and the molecular fluorescence at 658 and 723 nm
arises from two simultaneous but substantially
different decay mechanisms [6,26]. Process I refers to
the inelastic tunnelling events that lead to
plasmon-mediated emissions. Such oscillating surface
charges are generated via either inelastic tunneling
excitation between the tip and substrate or dipole
coupling between molecular excited states and
substrate, or both. Both excitation effects are
suppressed at increased thickness because of
Figure 3 Schematic diagrams showing the light
emission mechanism from a thin molecular layer on a
metal substrate. a, Two kinds of concurrent STML
mechanisms. Process I leads to plasmon-mediated
emission via inelastic tunnelling; Process II refers to
molecular fluorescence via hot electron injection. b,
Molecular fluorescence via Frank-Condon transitions
from S1 (excited state) to S0 (ground state).
weakened electric field and moleculesubstrate
interaction.
Molecular fluorescence is generated by the hot
electron injection excitation (Process II). Once bias
voltages are above the electron injection barrier,
electrons can tunnel elastically through the vacuum
barrier into unoccupied * molecular orbitals, leaving
the molecule in an electronically excited vibronic
state. Nevertheless, as shown in Fig. 3b, no matter
where the electrons are injected into the vibronic
states, the excited states will first go through fast
radiationless decay to the ground vibrational level of
the lowest * state. Then, a radiative decay follows
through Frank-Condon transitions to the electronic
ground state, giving rise to the vibronic Qx(0,0) and
Qx(0,1) fluorescence.
The mechanism proposed in Fig. 3a is essentially a
double-barrier model with a vacuum barrier on the W
tip side and a Schottky barrier at the moleculegold
interface. When the substrate is negatively biased,
electrons from the gold substrate have to tunnel
through the Schottky barrier to create electronically
excited states of molecules. The excited molecule
may experience a transient anionic state before the
injected electrons recombine with the holes in the
molecule or driven close by the local electric field.
In conclusion, we have developed a thin multilayer
decoupling approach with nano-probe excitation to
generating molecular fluorescence associated with
HOMO-LUMO transitions. The fluorescence is
vibrationally resolved and believed to be on the
molecular scale. These results reveal new
optoelectronic behavior of molecules in a
nano-environment. The combination of STM with
optical techniques may open new routes to
single-molecule spectroscopy and high-resolution
5
chemical mapping as well as to the determination of
important interfacial parameters in organic
electronics.
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Acknowledgements
This work was supported by the Ministry of
Education, Science and Technology of Japan. We
thank K. Amemiya for providing the simulation
program for metal surfaces and T. Kamikado for
providing the molecules.
Correspondence and requests for materials should
be addressed to Z.C. Dong.
(email: dong.zhen-chao@nims.go.jp).