Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field
The conductance relaxation of Langmuir-Blodgett films of manganese phthalocyanine in inhomogeneous electrical field was studied. Inhomogeneous electrical field was achieved by using the lateral surface of reverse-biased Si p-n junction. The conductance of new film increases up to saturation with the...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2005
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| Cite this: | Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field / Z. Kazantseva, V. Kislyuk, I. Kozyarevych, V. Lozovski, O. Tretyak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 4. — С. 80-84. — Бібліогр.: 14 назв. — англ. |
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irk-123456789-1215692017-06-15T03:02:58Z Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field Kazantseva, Z. Kislyuk, V. Kozyarevych, I. Lozovski, V. Tretyak, O. The conductance relaxation of Langmuir-Blodgett films of manganese phthalocyanine in inhomogeneous electrical field was studied. Inhomogeneous electrical field was achieved by using the lateral surface of reverse-biased Si p-n junction. The conductance of new film increases up to saturation with the characteristic time about 10 hours. After that the film has been kept in air for a long time (about 50 days), on application of the back bias the conductance slowly decreased with the characteristic time more than 10 hours. These properties are associated with appearance or disappearance of the bonds between molecular stacks in the film. 2005 Article Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field / Z. Kazantseva, V. Kislyuk, I. Kozyarevych, V. Lozovski, O. Tretyak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 4. — С. 80-84. — Бібліогр.: 14 назв. — англ. 1560-8034 PACS 68.47.Fg, 68.47.Pe, 72.80.Le http://dspace.nbuv.gov.ua/handle/123456789/121569 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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The conductance relaxation of Langmuir-Blodgett films of manganese phthalocyanine in inhomogeneous electrical field was studied. Inhomogeneous electrical field was achieved by using the lateral surface of reverse-biased Si p-n junction. The conductance of new film increases up to saturation with the characteristic time about 10 hours. After that the film has been kept in air for a long time (about 50 days), on application of the back bias the conductance slowly decreased with the characteristic time more than 10 hours. These properties are associated with appearance or disappearance of the bonds between molecular stacks in the film. |
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Kazantseva, Z. Kislyuk, V. Kozyarevych, I. Lozovski, V. Tretyak, O. |
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Kazantseva, Z. Kislyuk, V. Kozyarevych, I. Lozovski, V. Tretyak, O. Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field Semiconductor Physics Quantum Electronics & Optoelectronics |
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Kazantseva, Z. Kislyuk, V. Kozyarevych, I. Lozovski, V. Tretyak, O. |
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Kazantseva, Z. |
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Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field |
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Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field |
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Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field |
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Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field |
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Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field |
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conductance relaxation in langmuir-blodgett manganese phthalocyanine (pcmn) films in inhomogeneous electrical field |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2005 |
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Conductance relaxation in Langmuir-Blodgett manganese phthalocyanine (PcMn) films in inhomogeneous electrical field / Z. Kazantseva, V. Kislyuk, I. Kozyarevych, V. Lozovski, O. Tretyak // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2005. — Т. 8, № 4. — С. 80-84. — Бібліогр.: 14 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 80-84.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
80
PACS 68.47.Fg, 68.47.Pe, 72.80.Le
Conductance relaxation in Langmuir-Blodgett manganese
phthalocyanine (PcMn) films in inhomogeneous electrical field
Z. Kazantseva1, V. Kislyuk2, I. Kozyarevych2, V. Lozovski1,2, O. Tretyak2
1Institute of Semiconductor Physics, NAS of Ukraine,
45, prospect Nauky, 03028 Kyiv, Ukraine; phone/fax: (+380) 44 5255530
2Taras Shevchenko Kyiv National University, Radiophysics Department,
2 build 5, prospect Academician Glushkov, 03022 Kyiv, Ukraine
Corresponding author: Igor Kozyarevych, e-mail: kio@univ.kiev.ua
Abstract. The conductance relaxation of Langmuir-Blodgett films of manganese
phthalocyanine in inhomogeneous electrical field was studied. Inhomogeneous electrical
field was achieved by using the lateral surface of reverse-biased Si p-n junction. The
conductance of new film increases up to saturation with the characteristic time about 10
hours. After that the film has been kept in air for a long time (about 50 days), on
application of the back bias the conductance slowly decreased with the characteristic
time more than 10 hours. These properties are associated with appearance or
disappearance of the bonds between molecular stacks in the film.
Keywords: manganese phthalocyanine, Langmuir-Blodgett film, conductance, molecular
stacks, silicon p-n junction.
Manuscript received 23.09.05; accepted for publication 25.10.05.
1. Introduction
A great interest is being shown in thin organic films,
because they are promising to be applied as materials for
molecular electronics [1], in particular, as material for
electroluminescent structures [2]. In this connection,
nowadays the interest for studies of phthalocyanine films
is arisen [3]. Phthalocyanine molecules have a specific
geometrical structure (Fig. 1) that is a reason for their
ability to form the metal-organic coordination
compounds, the so-called metal-phthalocyanine (PcM).
As a result, created are the molecular complex, in which
a metal atom is located in the centre of the molecule.
The metal atom and molecular frame form the so-called
coordination bonds that allow the metal to have ability
for binding the additional chemical bonds. This ability is
the central point of the idea of this study.
Nanowires formed of TCNQ-TTF salts have been
investigated for a long time as a prominent example of
using the Langmuir-Blodgett (LB) technique to form
nanoobjects [4, 5]. The phthalocyanine molecules have
the same properties. Namely, cofacially stacked
macrocycles can also form nanowires. Fig. 1 shows an
example of the macrocycle – a phthalocyanine molecule
in which metal occupies the central position (the “M”
designation in the structural formula in the left side of
Fig. 1). The molecular stacks of phthalocyanines can be
formed during the molecular crystal growth [6]. The
same stacks are formed during preparation of
monomolecular films with the LB technique that is well
known [7-11]. Phthalocyanine films were studied as
field-sensitive materials (i.e., interactions with applied
electric, magnetic or electromagnetic fields) and as
chemo-sensitive materials (i.e., interactions with other
chemical species) [12-14]. The anomalous behavior of
conductance of copper-phthalocyanine LB films was
observed earlier [9, 11]. In particulary, the super-high
lateral conductance increase was observed. Namely, the
lateral conductance was more than that normal to the
film surface by a factor of 106. This conductance
increase was related with the stack formation in the film
[7, 9, 11]. The idea that the high lateral conductance is
related with quasi-metal conductance along the
monoatomic one-dimension metal wires inside the stacks
can be fruitful for explanation of this phenomenon.
The behavior of lateral conductance in the LB tetra-
tertbutyl-manganese phthalocyanine (PcMn) films was
studied in this work. High sensitivity of the LB
phthalocyanine films to electric field effect is expected.
This work is devoted to the study of the inhomogeneous
electrical field influence on the lateral conductance of
PcMn LB films with the short hydrocarbon radicals
(R = C(CH3)3). Molecules of these films are stacked. The
central atoms of the molecules form monoatomic metal
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 80-84.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
81
Fig. 1. Metal-substituted phthalocyanine molecule (left) and
phthalocyanine molecules stacked cofacially in chain (right),
R = C(CH3)3.
Fig. 2. Method of preparation of the LB films. One monolayer
is deposited in series during unitary immersion.
+
V
LB-film
R
Fig. 3. Setup of the experiment.
wires. As a result, the film itself is in general non-
conductive except local regions. The particular attention
is given to the nanowire alignment. The nanowire
alignment arises due to electrical field effect. This
influence of homogeneous lateral electric field on the
lateral conductance of PcCu LB films was observed in
works [7, 9, 11]. A reverse-biased p-n junction is the
most appropriate substrate to study changes in
conductance associated with alignment of the nanowires
because there is inhomogeneous electrical field on the
lateral surface.
2. Experiment
Multilayer PcMn films were deposited on lateral
surfaces of silicon p-n junctions by the LB technique.
Silicon p-n junction is a handy substrate to investigate
the high-resistance PcMn films because a current at
reverse bias depends mainly on the conditions on the
lateral surface of p-n junction. A reverse-biased p-n
junction also provides an inhomogeneous electrical field.
A silicon p-n junction is just chosen because the
electrophysical properties of phthalocyanines such as
vanadium oxide phthalocyanine and copper
phthalocyanine on silicon surface are known [7].
Before deposition of LB films, substrates were
prepared. The lateral surfaces of the p-n junctions were
polished with diamond paste for imparting the necessary
shape and smooth surface to the substrates. The direction
of the polishing was both parallel and perpendicular to
the p-n junction since influence of the polishing
direction on the character of the conductance relaxation
was not known. Then all substrates were degreased in
boiling alcohol for about 20 min. The removal of the
thermal oxide in the boiling solution of the hydrogen
peroxide and ammonia took about 10 min. The etching
of the silicon p-n junction substrates in 40 % solution of
(NH4)2Ti2F6 was made to prepare microscopically
smooth surfaces.
PcMn LB films were prepared using a facility with
the automatic system of the maintenance of the constant
surface pressure inherent to the molecular monolayer on
the water surface. The scheme and construction of this
device are described in [7]. The structural formula of the
tetrasubstituted PcMn with substituting hydrocarbon
radicals is shown in Fig. 1. The first stage of producing
the LB film is formation of a monomolecular pellicle of
PcMn on the water surface. Then the transfer of the
monolayers was carried out onto the hydrophobic
substrates at the room temperature and constant surface
pressure by consecutive immersion and taking out these
substrates that were preliminary cleaned with ultrasound
in chloroform (Fig. 2). The molecules of PcMn are
transferred onto the hydrophobic substrate because of
the specific structure of molecules, which have
hydrophilic and hydrophobic groups. Two-, four-, and
ten-monolayer films were obtained on the lateral
surfaces of the p-n junctions.
Conductance relaxation of deposited PcMn
monolayers was studied by applying a permanent back
bias to the p-n junction (Fig. 3) and the registration of
changes in the current that flows through the series p-n
junction and a load resistor R (0.03 % precision). This
current was determined from a voltage drop on the load
resistor. Each sample was placed into the dark
thermostat before measurements. There was a room
temperature inside the thermostat.
3. Results and discussion
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 80-84.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
82
E
r
a
b
1 2
Fig. 4. For explanation of the conductance dependence on
the structure of the film consisting of molecular stacks
(marked as bold lines).
Fig. 5. The long-time current relaxation, two-layer sample,
parallel direction of the polishing, the back bias is 2.74 V.
Fig. 6. The long-time current relaxation, ten-layer sample,
parallel direction of the polishing, the back bias is 2.74 V;
dash line – the repetition of the measurement.
Results of our study of conductance relaxation are
shown in Figs 5 and 6. The unexpected results
characterising the long-time current (conductance)
relaxation can be explained as follows: as it was
mentioned above, the structure of MnPc LB films is
conceivably close-packed molecular stacks [9, 11]. The
conductance of these films is of a dielectric type with the
exception of local regions – nanowires (or molecular
assemblies) that are formed by atoms of manganese due
to molecular stacking. Nanowire conductance is quasi-
metal. But internanowire conductance has hopping
nature. It means that the number of the regions
separating the beginnings and the ends of the nanowires
defines the conducting properties of the film. Indeed, let
the regions connecting the end of one molecular stack
and beginning of the other one are characterized by the
hopping conductivity Hσ and the regions between
arbitrary inter-stack contacts (except the end-beginning
connections) – Sσ , and SH σσ > . Therefore, the
conductance between sections “a” and “b” along the
right complex is much higher than that of the left one
showed in Fig. 4. Here the molecular stack is designated
as a bold line. The electrical field on the lateral surface
of the p-n junction is inhomogeneous. Therefore, the
molecular assemblies are located in inhomogeneous
electrical field. Let us begin with consideration of two
molecular assemblies pinned by a weak binding force at
the surface in homogeneous electrical field. A
monoatomic needle located inside the phthalocyanine
assembly is polarized by the action of the electrical field
(i.e., the electron density on this metallic needle is
shifted along the direction opposite to the field, and
assembly acquires some considerable dipole moment). It
is well known that the moment of rotation (shown by
arrows in Fig. 7) θsinpEN = is applied to the dipoles
that are turned through the angle θ to the electrical
field, where p is the dipole moment, E is the electrical
field. As shown in Fig. 7a, the molecular stack 2
prevents turning the molecular stack 1, because the
molecular stack 2 is acted upon by about the same
magnitude of the moment of force as the molecular stack
1 but in the opposite direction. Therefore, molecular
assemblies do not turn round, and conductance of the
film keeps its value. Another process proceeds in the
case of inhomogeneous filed. Indeed, in the presence of
the nonzero gradients of the field, the
force EpF
rrr
grad⋅= acts on the molecular stack as a
whole. This force can pick dipole from the pinning
centre and shift it. If, for example, the force of the
pinning of the first dipole is less than that of the second
one, then the first dipole is shifted towards the field
gradient (let do it towards the field direction) (Fig. 7b))
until fixation on another pinning centre will be reached.
Then, both molecular complexes get the ability to turn
(Fig. 7c)), and the conductance of this region can
increase considerably. Molecular stacks will take up
I, μA
time, h
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 80-84.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
83
E
r
1
2
a) b) c)
Fig. 7. Moments and forces, acting upon the dipoles in
homogeneous (a) and inhomogeneous (b) electric fields.
b)
a)
E
r
p-n junction
surface film layers
Fig. 8. a) weakly-ordered film (till 8 hours) that has a bigger
conductance than (b) – layerwise aligned nanowires (after 8
hours) because of better conductivity between layers in case (a).
Fig. 9. The long-time current relaxation, the same sample as
in Fig. 5, the back bias is 2.74 V.
Fig. 10. The long-time current relaxation, non-measured
sample, the back bias is 2.74 V, the parallel direction of the
polishing, four layers were deposited.
positions that are optimal for present electrical field,
(i.e., the dipoles take a position corresponding to the
minimum of the energy), and after 9-14 hours keeping
the conductance of the film goes into saturation (see
Figs 5 and 6). The conductance is reduced for the first 7-
10 minutes because the distances between nanowires are
largest.
The current slump was observed in the ten-layer film
after keeping it for about eight hours (Fig. 6). This
phenomenon can be explained as follows. After
alignment of the stacks with preferred orientation along
the field direction in the plane of each molecular layer,
the processes of aligning in the planes perpendicular to
the molecular layer plane begin to proceed (see Fig. 8b).
Then, the electrical bonds between the stacks from
different (but adjacent) layers are destroyed. As a result,
the conductance of the film decreases when nanowires is
aligned layer-by-layer because of the conductance
between layers decreases.
The swing of some molecular stacks occurs during
the voltage drop across the p-n junction for a long time.
Repeated measurement shows conductance growth that
begins from about that point which corresponds to the
saturation on the previous curve.
The same results were obtained in samples that
differed from the described above ones in the direction
of polishing (i.e., with perpendicular to p-n junction
direction of polishing). The character of the current
relaxation remained also without influences at back
biases of p-n junctions equal to 5 and 10 V.
As time goes by, there arose some bonds between
stacks, and applying the back bias to the diode destroys
these bonds by causing stack rotation, so conductance is
decreased. The decreasing conductance, which can
achieve up to 17.6 and 31.5 %, is shown in Figs 9 and
10, respectively. It can be explained by the preliminary
ordering of the sample shown in Fig. 9, across which the
back bias was applied before. The results shown in
Fig. 10 are concerned the sample that wasn’t preliminary
ordered by the electrical field, then destruction of bonds
between molecular stacks result in decreasing the
conductance. It should be noted that applying the direct
bias across the field ordered film with heating
simultaneously, leads to the behavior of the conductance
like that in green films (see Fig. 11). Difference between
dashed and solid lines in this figure arises because of
insufficient duration of applying direct bias and heating
for total misorientation of the stacks as well as breaking
the bonds between it. Conductance relaxation was also
studied in the films formed by molecules with long
radicals. There was not observed this process in these
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2005. V. 8, N 4. P. 80-84.
© 2005, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
84
Fig. 11. The long-time current relaxation, solid line – the same
sample as in Fig. 5, the back bias is 2.74 V, dash line – repetition
after heating at about 55 °C and forward bias over 3 hours.
films. The behaviour of the back current through the p-n
junction does not differ from that in the p-n junction that
uncovered by the film. Therefore, it was supposed that
the molecules with long radicals do not form the stacks.
4. Conclusions
The relation between the structure and electrophysical
properties of the tetrasubstituted PcMn LB films with
substituting hydrocarbon radicals was ascertained. The
long-time conductance relaxation was observed in PcMn
LB films. This occurred because of the nonzero gradient
of the electrical field. The electrical field shifts
molecular stacks from the pinning centers, and then they
can align along the field. Aligning molecular stacks
increases the conductance. After 9-14 hours keeping, the
film conductance goes into saturation. The theory of
aligning the stacks is confirmed by similar experiments
with molecules that have long tails and can’t form the
molecular stacks. Applying the direct bias across the
field ordered film with heating simultaneously leads to
the behavior of the conductance like that in new films.
Acknowledgements
Authors are very grateful to K. Pokyd’ko and
O. Pokyd’ko for their help in preparation of the samples
and performing the measurements.
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