COMMUNICATION
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Excitation-wavelength-dependent photoluminescence of a pyromellitic
diimide nanowire networkw
Hairong Zhang,a Xiaohe Xub and Hai-Feng Ji*b
Received (in Cambridge, UK) 7th September 2009, Accepted 14th December 2009
First published as an Advance Article on the web 13th January 2010
DOI: 10.1039/b918297g
Nanowires from deposition of pyromellitic diimide (PMDI) from
the gas phase and their unique excitation-wavelength-dependent
photoluminescence were demonstrated. The luminescence peaks
of the PMDI nanowires red-shifted as the excitation wavelength
increased. The relationship between the luminescence peak and
the excitation wavelength is nearly linear in a broad range of
excitation.
The observation of excitation-wavelength-dependent photoluminescence (EWDP) from nanostructures of metal oxides
and semiconductors has attracted attention because of the
nanostructures’ unique luminescent behavior and potential
applications in photoluminescence, solar energy conversion,
optoelectronic and chemical sensor devices. Examples of these
nanostructures are nanocolloids,1 nanowires of ZnO,2 porous
silicon,3 anodized alumina membranes,4 et al. In general, the
emission peaks of these nanostructures shift toward longer
wavelength when the excitation wavelength increases. Several
mechanisms have been proposed to explain the EWDP phenomenon, such as incomplete solvation of the excited state of
the species,1 size distribution for the nanocrystallites,3 level
splitting by doped ions,4 etc. All of these involve the existence
of a size or change distribution of molecules or the nanostructures. It has also been suggested1 that the presence of the
heterogeneous, energetically different molecules or nanostructures
alone is not enough for a system to exhibit EWDP behavior.
Another common requirement is that the species should be
excited selectively and there is no energy transfer between the
energetically different species. These may be the reasons why
EWDP behavior has been observed from a only few nanostructures so far. In this work, we report the EWDP phenomenon of a pyromellitic diimide (PMDI) nanowire network,
which is the EWDP behavior observed in an organic material
nanostructure.
During the past decade, considerable attention has been
focused on self-assembled or supramolecular electronics that
are based on polycyclic aromatic hydrocarbons.5 The need for
a
Laboratory of Biochemical Analysis, Xinzhou Teachers’ College,
Shanxi, 034000, P.R. China
b
Department of Chemistry, Drexel University, Philadelphia,
PA 19104, USA. E-mail: hj56@drexel.edu; Fax: +1 215-895-1405;
Tel: +1 215-895-2562
w Electronic supplementary information (ESI) available: Additional
figures. See DOI: 10.1039/b918297g
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higher quantum efficiency and ease of fabrication in nanoelectronics drives the search for new materials. Polycyclic
aromatic hydrocarbons have been a subject of great interest
as potential candidates for self-assembly into nanostructures
having semiconducting and photoluminescence properties.
Most of the self-assembling processes of small organic molecules
to form nanostructures have been carried out in solution.
Many polycyclic aromatic hydrocarbons, however, have a
very low solubility in any solvent due to the high lattice energy.
The traditional approach is to modify the polycyclic aromatic
hydrocarbons with side chains to increase the solubility.5
Recently, alternative approaches, such as reprecipitation,6
thermal evaporation,7 microemulsion,8 laser ablation9 et al.
have been applied to assemble polycyclic aromatic hydrocarbons into nanostructures without modifying the molecules.
The assembly of polycyclic aromatic hydrocarbons through
the thermal evaporation and deposition method is in its infancy
and only a few studies have been reported. In our previous
work, we reported10 the formation of perylenediimide
(PTCDI) and naphthalenediimide (NPDI) nanostructures in
the gas phase with a thermal evaporation method. Here, we
applied the same method to grow a PMDI nanowire network.
Briefly, 5 mg of PMDI power was placed in a 50 mL quartz
tube. The tube was heated in a vacuum to 500 1C for 1 h.
The tube was allowed to cool to room temperature. PMDI
nanowires that were self-assembled on glass substrates were
collected for analysis.
Fig. 1, left, shows a scanning electron microscopy (SEM)
image of the nanowire network of PMDI. The PMDI nanowires have a dimension of B60–500 nm, and a length greater
than 10 mm. Transmission electron microscopy (TEM) experiments indicated that the PMDI nanowires (Fig. 1 right) have
an amorphous structure. The surface of the nanowires is
rough. Small grains, with diameter of 10–20 nm, were widely
distributed on the nanowire surfaces. No electron diffraction
pattern (figure not shown) of the PMDI nanowires was
observed. More TEM images with different sizes and morphologies of PMDI nanowires are shown in the ESI.w
The PMDI nanowires were further studied by attenuated
total reflectance infrared (ATR-IR), photoluminescence, and
UV-visible spectroscopy. The ATR-IR experiment showed
that the nanowires are comprised of the diimides, but not
decomposed chemicals. Fig. 2 shows the PMDI nanowire
network has two photoluminescent peaks (Fig. 2a and c).
When the network was excited at 360 nm, two photoluminescent
peaks at 477 and 564 nm were observed.
Since the PMDI powder does not show fluorescence in the
visible range, the photoluminescence from the PMDI nanowire network may be attributed to the band gap of the
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Fig. 1 SEM (left) and TEM (right) images of PMDI nanowire
networks from thermal vacuum evaporation of PMDI powders at
500 1C.
Fig. 2 Photoluminescence of PMDI nanowires. (a–b) Spectra of the
first peaks at various excitation wavelengths from 340 nm to 680 nm at
ambient temperature. (c) Spectra of the second peaks at various
excitation wavelengths from 340 nm to 500 nm (those from 520–600 nm
are not shown for clarity). (d) Luminescence peaks as a function of the
excitation wavelength.
nanostructures. Various mechanisms, such as the quantum
size effect,3 have been proposed to explain the photoluminescence properties of nanostructures.11,12 According to these
mechanisms, nanostructures with different morphologies,
shapes, and sizes have a photoluminescent wavelength, which
varies over a wide spectral range from the infrared to the UV.
However, since the band gap between the levels of free
electrons and holes of organic nanostructures is generally
greater than the intramolecular band gap (around 4 eV in
PMDI), it is not clear at this stage why the emission can occur
in the visible range.
The unique property of this nanowire network is its EWDP
behavior. When the nanowire network was excited at various
wavelengths from short to long with an interval of 20 nm, the
photoluminescence peaks showed a synchronous red-shift with
an interval of 20 nm. The shift of the first photoluminescence
peaks exhibited in a wide range from 438 to 794 nm with the
excitation wavelength from 320 to 680 nm. The PMDI in
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Fig. 3 (Left) Excitation spectra of the PMDI nanowire network at
various emission wavelengths from 320 nm to 600 nm at ambient
temperature; (right) adsorption spectra of the nanowires and PMDI
powder.
solutions has a weak fluorescence, but EWDP behavior was
not observed (see ESIw).
Although the photoluminescence phenomenon of nanostructures is not fully understood, it is generally recognized1–4
that the EWDP phenomenon is due to the size distribution of
nanostructures, which can also be used to explain the EWDP
behavior of the PMDI nanowire network. The TEM images
have shown the existence of a size distribution of the PMDI
nanowires. It is expected that nanowires of different sizes can
be excited only at different wavelengths. This has been confirmed from the excitation spectra of the PMDI nanowire
network (Fig. 3), which showed that the photoluminescence at
various wavelengths originated from the excitation of different
sized nanowires. Beside the size distribution, the observation
of EWDP from the PMDI nanowire network is also attributed
to the separation of the nanowires so the energy transfer
between a structure with a higher vibrational level and a
structure with a lower vibrational level of the excited states
is inefficient.
PMDI powder does not show adsorption between 300 and
1000 nm. On the other hand, the PMDI nanowire network has
a broad absorption spectrum from 300 to 1000 nm, suggesting
broad energy levels or band gaps. This result further confirmed
the existence of a size distribution of the nanowires. From the
molecular level, the band gap differences may originate from
the intermolecular potentials and the hydrogen bonding
between the PMDI molecules.
This photoluminescence of PMDI nanowires, to the best of
our knowledge, is the first EWDP phenomenon observed for a
nanostructure of an organic material. It should be noted that
the photoluminescence of PMDI nanowires is extremely weak,
especially at longer excitation wavelengths (Fig. 2). The fact
that the EWDP phenomenon has not been observed from
other organic nanostructures, such as the PTCDI nanowires,
may be due to the fluorescence of organic molecules themselves, which overlapped the weak photoluminescence of the
nanostructures. The PMDI nanowires might be, so far, the
only nanostructure to demonstrate a nearly straight line
between the luminescence peak and the excitation wavelength
in a broad range (greater than 300 nm) of excitation. This
linear relationship is different from other EWDP behaviors
reported1,13 and will be further investigated in the future. For
instance, it is possible that the EWDP effect is caused from
excitons as the PMDI molecules pack, or a charge transfer
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state. Another possibility is a combination of a broad range of
excitonic and solvation effects between clusters of PMDI
molecules leading to a broad range of localized singlet
energies, which depends directly on the short range packing
of the PMDI clusters.
In summary, we reported the EWDP behavior of a PMDI
nanowire network. The luminescence peak shifts towards red
linearly as the excitation wavelength increases. This observation
has been attributed to the presence of energetically different
forms of the nanowires. Potential applications of these nanowires include in optoelectronics and light emitting devices. Our
results continue to demonstrate the uniqueness of the nanostructures obtained by the simple yet effective self-assembly
technique from a gas phase deposition method.
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