Low Temperature Crystallization of Amorphous AlQ3 Nanoparticles and the
Transformation to Nanowires
Tsong P. Perng (彭宗平) tpperng@mx.nthu.edu.tw
Department of Materials Science and Engineering
National Tsing Hua University
Contract No :
93.04.01 - 94.03.31
Abstract
Amorphous AlQ3 nanoparticles could
directly grow into α-phase crystalline nanowires
by a one-step heat treatment. With the most
appropriate Ar pressure, heating time, and heating
temperatures between 150°C and 190°C, fine and
long nanowires could be obtained. The growth of
nanowires is dictated by the anisotropic bonding in
α-AlQ3 crystal. The growth mechanism can be
illustrated by the concept of nucleation and
molecular migration.
Two exotherms are
revealed from differential scanning calorimetry
analysis in the transformation process of
amorphous AlQ3 nanoparticles.
The first
exotherm is the transition from amorphous
nanoparticles to γphase, and the second exotherm
is the transition from γto αphase. By means of
Kissinger plots, the activation energies for the
crystallization of γphase and the transition from
γto αphase are first time caculated to be 9.7 and
12.1 kJ/mol, repectively. A blue shift and higher
intensity of photoluminescence after heat
treatment are also demonstrated.
Keywords: organometallic semiconductor, AlQ3,
nanoparticles, nanowires, phase transition
1. Introduction
The small size of nanomaterials may result
in different electronic, optical, magnetic, chemical,
and mechanical properties, and may be more
suitable for novel applications.[1-3] With a size
between those of molecules and bulk solid-state
structures, nanomaterials exhibit hybrid properties
that are still not completely understood today.
Nanoparticles can be taken as fundamental
building blocks in bottom-up approaches to
generate one-dimensional (1D) or
two-dimensional (2D) nanostructures via self- or
externally manipulated assembly.[4-7]
In this
work, a new method of achieving 1D
nanostructures from nanoparticles is disclosed.
By an appropriate heat treatment, nanowires of
tris-(8-hydroxyquinoline) aluminum (AlQ3), an
organometallic semiconductor, can grow directly
from amorphous nanoparticles at relatively low
temperature in short time without using catalyst or
template, adding any chemical reagent or solvent,
and purification. The heat treatment shows more
simplicity and accessibility compared with
previous methods,[8] and may provide a new
method for synthesizing 1D nanostructures. The
nanowires are α-phase predominant, and have a
nano-sized diameter and a long length up to
several m, rather than short and thick rod- or
needle-like crystallites.[9,10] The most appropriate
heating condition and growth mechanism in terms
of nucleation and molecular migration are
presented.
The activation energies for
crystallization of α and γ phases were first time
caculated.
Recently, more attention has been paid to
organometallic nanomaterials due to their unique
properties
such
as
flexibility,
high
photoconductivity and nonlinear optical effect that
may offer novel applications. Since the efficient
organic light emitting diodes (OLEDs) were
reported by Tang and VanSlyke,[11] AlQ3 has
become one of the most successful electron
transport and emitting materials. Until recently,
it was demonstrated that AlQ3 nanowires and
nanoscaled crystalline film exhibited field
emission characteristics, and AlQ3 nanoparticles
showed enhanced photoluminescence (PL)
intensity.[12-14] Owing to these advantages, it is
perceived that AlQ3 is a promising material with a
plurality of morphologies for application to
nano-optoelectronic devices.
Therefore, the
crystallographic characteristics of AlQ3 deserve
better
understanding,
and
the
possible
optoelectronic applications of AlQ3 nanostructures
need further exploration.
2. Experimental
Amorphous AlQ3 nanoparticles were
fabricated in He of 1.33 × 103 Pa in a vapor
condensation system, whose setup has been
presented elsewhere.[14] A graphite boat was
employed as a resistive heater, and the temperature
was regulated by a power supply and controlled by
a K-type thermocouple.
Commercial AlQ3
powder (TCI Ltd., T1527) was put in the graphite
boat, and the silicon substrate under a cold trap of
liquid nitrogen was placed 10 cm above the boat.
AlQ3 powder was sublimed as the temperature of
the graphite boat reached 410°C. Systematic heat
treatment was executed on amorphous AlQ3
nanoparticles at temperatures between 75°C and
190°C in Ar of 1.33 × 104 Pa for various durations,
to inspect the evolution of nanowires. A field
emission scanning electron microscope (FESEM,
JEOL JSM-6330F EM) and a low angle X-ray
diffraction (LAXRD) spectrometer (Cu-Kα1) were
used to examine the morphology and crystallinity
of AlQ3 nanostructures, respectively.
A
differential scanning calorimeter (SETARAM,
DSC131) was utilized to inspect the phase
transition of the amorphous nanoparticles. Four
heating rates, 5, 10, 20, and 40 K/min, were
selected to study the variation of the transition
temperatures.
3. Results and Discussion
The AlQ3 nanoparticles have a spherical
shape and a smooth surface. Their diameter
ranges from 50 to 200 nm, as shown in Fig. 1a.
With a systematic heat treatment, it has been
revealed that temperatures below 100oC make no
obvious structural transformation after heating for
more than 1 hour. On the contrary, temperatures
higher than 200oC vaporize off AlQ3 easily.
Consequently, temperatures between 150oC and
190oC are the most appropriate to transform
nanoparticles into fine and long nanowires.
Several representative examples are given herein
to show the evolution of the nanowires. Only
few nanowires grow from the nanoparticles as
heated at 100°C for 1 hour (Fig. 1b). When
heated at 120°C for 1 hour, most nanoparticles
transform into nanowires with a diameter of 40-80
nm and a length below 1m (Fig. 1c). The
original spherical shape of nanoparticles has
disappeared, and some residues are still present.
The initial growth of nanowires from nanoparticles
can be observed as heating at 150°C for 1 min (Fig.
1d). Heating at 150°C for 10 min transforms the
nanoparticles completely into nanowires, which
have a diameter of 40-100 nm and a length more
than 1 m (Fig. 1e). Some wires even grow
together as bundles. As heated at 190°C for only
2 mins, nanowires with a diameter of 50-100 nm
are formed (Fig. 1f).
Both higher heating
temperature and longer heating time promote the
growth of AlQ3 crystalline nanowires and result in
more complete transformation.
Similar heat
treatment on amorphous thin film also led to the
growth of nanowires, as revealed previously.[15]
The as-prepared nanoparticles are amorphous, and
they remain basically amorphous after heating at
100oC for 2 min, as revealed in Fig. 2, curves a
and b. The XRD patterns of the specimens
heated at higher temperatures are given in Fig. 2,
curves c to e.
Higher crystallinity can be
attributed to higher heating temperature. The
crystalline peaks, although broad and complex,
can be ascribed to α-phase AlQ3.[9,16,17]
In AlQ3 α-crystals, the molecular packing
goes along a specific direction that is controlled by
the strong π-π interaction between pairs of
quinoxaline ligands from neighboring AlQ3
molecules.[9] Thus it is deduced that AlQ3
molecules can migrate and adopt a favorable
orientation or conformation to pile up
preferentially along this direction, as they gain
enough thermal energy from heat treatment. The
building block for nanowires is composed of
several AlQ3 molecules with the same
configuration.
As the concentration of the
building blocks is sufficiently high, they aggregate
into small nuclei. These nuclei can serve as
seeds for further growth to form larger structures
with a continuous supply of AlQ3 molecules from
nearby regions. The growth is dictated by the
anisotropic bonding in α-crystal structure.
Similar growth process from amorphous AlQ3 thin
film has also been discussed previously.[15]
Four crystalline phases of AlQ3 (α, β, γ, and
δ) have been identified.[9,16,17] Both α and β
phases have low crystallization temperatures, and
γ and δ phases are high-temperature phases. In
addition to the large melting transition (Tm) at
419°C, several phase transitions have been
demonstrated.[9,17,18] Sapochak et al. reported a
glass transition (Tg) at 177°C, a recrystallization
exotherm at 251°C, and an exotherm at around
395°C, which was ascribed to the phase transition
of α to γ phase.[9,18] Cölle et al. reported that
annealing α-phase AlQ3 at temperatures between
380°C and 400°C with a subsequent slow cooling
resulted in a blue-luminescent δ phase.[17] No
transition regarding β phase has been reported so
far. Since amorphous nanoparticles transformed
to crystalline nanowires at such low temperatures,
DSC was utilized to explore the subtle change.
Fig. 4a shows a DSC curve of amorphous
nanoparticles heating from 50°C to 470°C with a
heating rate of 10K/min. The melting point of
amorphous nanoparticles is 416°C, and an
exotherm at around 362°C is similar to the
transition of α to δ phase reported by Cölle et
al.[9,17,18] A small broad peak located at about
120-140°C is also present, as marked by a circle.
In order to have more precise identification of this
small peak, more study was performed
subsequently. The DSC measurements with four
heating rates are shown in Fig. 4b.
Two
exothermic peaks are present in each curve, and
they shift to higher temperatures as the heating
rate increases. Two peaks imply two phase
transitions. The peak temperatures (Tp1 and Tp2)
increase from 117°C and 138°C to 143°C and
165°C, respectively, as the heating rate increases
from 5 K/min to 40 K/min.
The XRD patterns demonstrate that α-phase
is formed after heating at temperatures higher than
150°C, so the second exotherm of DSC curves can
be ascribed to a transition to αphase. The large
overlap of two exotherms implies the similarity
between two phases and the difficulty in isolating
one from another. In 1981, Khan et al. indicated
that the crystallization of glassy materials
generally proceeded from nuclei quenched in
during glass fabrication.[19,20]
The disorders
(high-temperature form) would crystallize out first,
followed by the orders (low-temperature form) at a
higher temperature in the heating process of glass.
Therefore, the highest-temperature phase (γ-phase)
is assumed to form first in the heating process of
glassy AlQ3 (amorphous nanoparticles). Two
isothermal heatings with a heating rate of 10
K/min were executed, followed by XRD analysis,
to verify whether the first exotherm is the
transition from amorphism to γ phase. With a
scanning rate of 0.25 degree per min, the XRD
patterns at 2 = 5° - 10° were collected and
compared with those of AlQ3 crystal powder in the
literature.[9,17]
The XRD patterns in this
low-angle region have higher intensity and more
differentiable features, and are regarded as the
most appropriate for identification.
AlQ3 amorphous nanoparticles were heated
at 120°C and 150°C for 1 min, 3 min, 5 min, and
10 min, respectively. As heated at 120°C for 1 or
3 min, the γ phase predominates, with less amount
of α phase. The characteristic XRD patterns are
mingled with some noisy peaks, perhaps from the
background or substrate, as shown in Fig. 5.
Even so, they can be discriminated by fitting with
the patterns of α and γ phases.[9,17] As heated for
5 min, both crystallinity and intensity of the
characteristic peaks of α phase increase. As the
heating at 120°C is prolonged to 10 min, the
crystallinity is even better, and the characteristic
XRD pattern is easier to identify.
The
predominance of α phase becomes clearer as the
heating temperature is raised to 150°C, as shown
in Fig. 6. The percentage of γ phase gradually
diminishes, and higher crystallinity is also
obtained, as the heating at 150°C is extended.
The above results reveal that γ phase is a
transitional phase that occurs prior to the
formation of α phase in the heating process of
glassy AlQ3. To obtain pure γ-phase AlQ3 is
difficult because it is easy to transform into more
stable α phase.
Accordingly, it can be
demonstrated that the first exotherm is the
transition from amorphous nanoparticles to γ
phase, and the second one is the transition from γ
The activation energies for crystallization of
γ and α phases are determined by means of the
Kissinger plot,[21]
E
 b 
ln  2  = - a  constant
T
R
T
 
[1]
where b is the heating rate (K/min), T is the
specific temperature, R is the gas constant, and Ea
is the activation energy (J/mol·K). By using the
values of peak temperatures (Tp) for different
heating rates, the plot of ln(b/Tp2) vs 1/Tp yields a
straight line. Figs. 3c and 3d show the Kissinger
plots of the transitions from amorphous
nanoparticles to γ phase and from γ to α phase,
respectively. From the slopes of the plots, the
activation energy for crystallization of amorphous
nanoparticles to γ phase is caculated to be 9.7
kJ/mol, and that for phase transition from γ phase
to α phase is 12.1 kJ/mol.
Although no
information concerning the activation energies for
crystallization of AlQ3 has been reported in the
literature, the caculated values are reasonable,
compared to those of small-molecule organic
polymers.[22]
Unlike the strong covalent
bondings in organic polymers, the attraction
among AlQ3 molecules are merely caused by van
der Waals force, so the activation energies for
crystallization and phase transition of AlQ3 are
relatively lower.
The heat treatment also changes the PL
property of AlQ3. After heating at 150°C for 6
and 10 min, a blue shift of 20 nm and a higher
intensity (~6 times) of PL were observed, as
shown in Fig. 7 (curves c and d). The lower
intensity (~3 times) of curve b may be attributed to
the uncomplete transformation in 1 min. The PL
maximums are located at 505 nm and very close to
that reported by Cölle et al.[10,17] Thus the blue
shift can be attributed to the preferentially formed
α-phase AlQ3 after heat treatment. Compared to
the polymorph of amorphous AlQ3, the molecular
packing in α phase has a looser interligand spacing
(3.9 Å) that reduces the orbital overlap and results
in a blue shift.[9,18,23] Moreover, the ordered
molecular arrangement caused by heat treatment
results in decreased scattering, leading to a higher
PL intensity. Similar blue shift and enhanced
intensity of PL have been also observed for the
nanowires grown from amorphous thin film.
4.
Conclusion
Amorphous AlQ3 nanoparticles were
synthesized by vapor condensation. Long and
fine crystalline AlQ3 nanowires could be
fabricated by a simple and accessible heat
treatment in Ar of 1.33 × 104 Pa at temperatures
between 150°C and 190°C for various durations.
Both higher temperature and longer heating time
promote the transformation from amorphous
nanoparticles to α-phase nanowires. The growth
of nanowires is dictated by the anisotropic nature
of AlQ3, and the growth mechanism can be
interpreted by a concept of nucleation and
molecular migration.
Two phase tranisitions
occur during the formation of nanowires. The
low-temperature transition is from amorphous
nanoparticles to γ phase, and the high-temperature
transition is from γ to α phase. The activation
energies for the two transitions are first time
caculated to be 9.7 and 12.1 kJ/mol, respectively.
A blue shift of 20 nm and a higher intensity of PL
are also observed after heat treatment. The blue
shift can be attributed to the predominant α-phase
AlQ3, and the higher intensity results from the
ordered molecular arrangement caused by heat
treatment.
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d
c
b
a
5
10
15
20
25
30
35
2
Fig. 2
LAXRD patterns of AlQ3.
(a)
amorphous nanoparticles; and nanoparticles heated
in 1.33 × 104 Pa of Ar at (b) 100oC for 1 hr, (c)
120oC for 1 hr, (d) 150oC for 10 min, (e) 190oC for
2 min.
Heat Flow (arb. unit)
Heat Flow (arb. unit)
a
5
0
-5
-10
-15
-20
-25
50 100 150 200 250 300 350 400 450
b
40K/min
20K/min
10K/min
5K/min
80
100 120 140 160 180 200
o
o
T ( C)
T ( C)
-6.0
c
d
-6.5
ln (b/T )
2
2
ln (b/T )
-6.5
-7.0
-7.5
-8.0
7.5
8.0
-3
-7.5
-8.0
Ea = 9.7 kJ/mol
7.0
-7.0
-1
1/T (10 K )
8.5
Ea = 12.1 kJ/mol
6.0
6.5
7.0
-3
-1
1/T (10 K )
Fig. 4
DSC curves of amorphous AlQ3
nanoparticles (a) heated from 50oC to 470oC with
a heating rate of 10K/min, (b) heated with various
heating rates from 5 K/min to 40 K/min; Kissinger
plots of (c) the crystallization from amorphous
nanoparticles to γ phase, and (d) the phase
transformation from γto α phase.
Fig. 6 LAXRD patterns of amorphous AlQ3
nanoparticles heated at 150oC in 1.33 × 104 Pa of
Ar for various durations. They are fitted with
those of γ and α phases.[9,17]
Intensity (arb. unit)
d
c
b
a
400
500
600
700
wavelength (nm)
Fig. 7
Fig. 5
LAXRD patterns of amorphous AlQ3
nanoparticles heated at 120oC in 1.33 × 104 Pa of
Ar for various durations. They are fitted with
those of γ and α phases.[9,17]
PL spectra of AlQ3.
(a) amorphous
nanoparticles; and nanoparticles heated in 1.33 ×
104 Pa of Ar at 150oC for (b) 1 min, (c) 6 min, and
(d) 10 min.