Synthesis of Ni(II) porphyrazine peripherally octa-substituted with the 4-tert-butylbenzylthio moiety and electronic properties of the Al/Ni(II)Pz/p-Si Schottky barrier diode

Article abstract of DOI:10.1016/j.poly.2012.02.033

Magnesium porphyrazinate substituted with eight 4-tert-butylphenylthio- groups on the peripheral positions has been synthesized by cyclotetramerization of 1,2-bis(4-tert-butylphenylthio)maleonitrile in the presence of magnesium butanolate. The metal-free

Full text of DOI:10.1016/j.poly.2012.02.033

Polyhedron 38 (2012) 121–125
Contents lists available at SciVerse ScienceDirect
Polyhedron
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Synthesis of Ni(II) porphyrazine peripherally octa-substituted with the
4-tert-butylbenzylthio moiety and electronic properties of the Al/Ni(II)Pz/p-Si
Schottky barrier diode
a
c,
Bahadir Keskin a, Cenk Denktasß b, Ahmet Altındal b, , Ulvi Avcıata , Ahmet Gül
⇑
⇑
a Department of Chemistry, Yildiz Technical University, TR34210 Istanbul, Turkey
b Department of Physics, Yildiz Technical University, TR34210 Istanbul, Turkey
c Department of Chemistry, Technical University of Istanbul, TR34469 Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnesium porphyrazinate substituted with eight 4-tert-butylphenylthio-groups on the peripheral posi-
tions has been synthesized by cyclotetramerization of 1,2-bis(4-tert-butylphenylthio)maleonitrile in the
presence of magnesium butanolate. The metal-free derivative was obtained by its treatment with triflu-
oroacetic acid, and further reaction of this product with nickel(II) acetate led to the metal porphyrazinate
(M = Ni). These new compounds have been characterized by elemental analysis, together with FT-IR, 1H
NMR and UV–Vis spectral data. The electronic properties of a spin coated film of NiPz have been studied
by fabricating metal–insulator–semiconductor (MIS) capacitors. Current–voltage (I–V) and capacitance–
voltage (C–V) measurements were carried out. It was observed that the Al/NiPz/p-Si structure exhibits
rectifying behavior with a barrier height value of 0.89 eV and with an ideality factor value of 1.81. It
was seen that this value of the obtained barrier height is remarkably higher than those given for
metal/Si semiconductor contacts in the literature. The Lien, So and Nicolet method, combined with con-
Received 1 February 2012
Accepted 19 February 2012
Available online 17 March 2012
Keywords:
Porphyrazine
Schottky barrier diode
Nickel
Thin films
Barrier height
Ideality factor
ventional forward I–V, was used to extract the series resistance value and it was found to be 26 kX. High
frequency C–V measurements were used to determine the mobile oxide charge in the NiPz layer and this
was found to be 1.6 ꢀ 1011 cmꢁ2
.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
additional functionalities for interaction with alkali or transition
metal ions, mesophase formation, etc. [10–12]. Nickel porphyr-
azine (NiPz), compared with other metal-centered tetrapyrroles,
e.g., metal phthalocyanines (Pc), offers new ways to induce, modify
and to control molecular properties and has a high metal/ligand
stability [4,13,14].
Porphyrazine macrocycles afford a skillful platform to build up
detailed molecular superstructures and this property, coupled with
a full and well-improved synthetic chemistry, has surpassed the
synthesis of different catalytically essential metallo porphyrazines
and original model compounds of various functions [1–3]. In con-
trast to the structural similarity between phthalocyanines and por-
phyrazines, the latter have been relatively less studied compared
to the phthalocyanines [4–6]. The diffuse nature of porphyrazines
Schottky barrier diodes (SBDs) play a crucial role in modern
semiconductor technology as the basis of a large number of
electronic devices such as field-effect transistors (FETs), solar cells
and photodetectors [15,16]. Recently there has been extensive
investigation of organic materials for their use in SB diodes to im-
prove the fundamental SB diode parameters such as the Schottky
barrier height UB and the ideality factor n. A SB diode with the de-
sired electronic properties can be obtained by means of the choice
of a suitable interlayer. With that regard, phthalocyanines, porphy-
rins and porphyrazines have been considered to be one of the most
stable organic semiconductors for various electronic and opto-
electronic applications. Considerable attention has been given in
recent years to the fabrication and characterization of SB diodes
using some phthalocyanine compounds [17]. Although porphyra-
zines are of interest because of their potential applications, such
as molecular electronics [18], sensors [19–21], organic solar cells
p
electronic structure would help us understand there are long-
range interactions between different parts of the molecule. The
presence of soft S donor atoms play an important role in affecting
the solid-state interactions, and an extensive series of derivatives
with physical and chemical properties comparable to those of
phthalocyanines have been reported [7–9]. Derivatization of por-
phyrazines has generally been accomplished by the addition of
various substituents to the peripheral positions. These substituents
enhance the solubility of the products (e.g., tert-butyl) and provide
⇑
Corresponding authors.
E-mail addresses: altindal@yildiz.edu.tr (A. Altındal), ahmetg@itu.edu.tr (A. Gül).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.02.033

122
B. Keskin et al. / Polyhedron 38 (2012) 121–125
[22], photodynamic therapy [23] and organic field effect transistors
[24,25], unlike some phthalocyanine compounds, the electronic
and interface properties of porphyrazines have not been widely
studied. Furthermore, although there are a lot of works on
heterojunctions with phthalocyanines and some other organic
compounds, as yet there are no reports on porphyrazine plus a
p-Si type junction.
The purpose of the present work is to synthesize nickel(II)
inserted peripherally tert-butylbenzylthio substituted porphyr-
azine (NiPz) and to investigate the electronic properties of the
Al/NiPz/p-Si SB diode by the insertion of the NiPz organic layer be-
tween the Si semiconductor and Al metal.
at 400 °C for 20 min in ambient nitrogen. The film of the Pz com-
pound was prepared by the spin coating method. The elipsometric
technique was used to measure the thickness of the NiPz film and
it was found to be 110 nm. After the film deposition process, the
substrate was immediately placed in a vacuum system for the pro-
cesses. Al metal contacts were formed on a NiPz layer by vacuum
thermal evaporation of Al at pressure of approximately 3.0 ꢀ
10ꢁ6 mbar using an Edwards Auto 500 thermal evaporator system.
The current–voltage (I–V) measurements were performed using a
KEITHLEY 6517A electrometer and data of current–voltage mea-
surements were recorded on a PC using a GPIB data transfer card.
The capacitance–voltage (C–V) measurements on the prepared
Schottky diodes were also carried out using an Agilent 4284A
LCR meter. All the measurements were performed under
10ꢁ3 mbar.
2. Experimental
2.1. Reagents, instruments and measurements
3. Results and discussions
[Octakis(4-tert-butylbenzylthio)porphyrazinato]Mg(II) (1) (MgP
z) and [2,3,7,8,12,13,17,18-octakis(4-tert-butylbenzylthio)21H,23
H
3.1. Synthesis and characterization
porphyrazine] (2) (H2Pz) were prepared according to the previ-
ously reported procedures and characterized by comparing their
spectral data to those reported earlier [26]. The Mg(II) complex
and the metal free porphyrazine (H2Pz) were stable at room tem-
perature, non-hygroscopic, insoluble in water, but soluble many
common organic solvents. Reagents: Chemicals employed were of
the highest grade available. Unless specified otherwise, reagent
grade reactants and solvents were used as received from the chem-
ical suppliers.
The starting point of this new nickel(II) porphyrazine structure
with eight (4-tert-butylbenzylthio) groups bound to the periphery
is 1,2-bis-(4-tert-butylbenzylthio) maleonitrile (TBBTMnt), which
was synthesized as a solid product in relatively high yield accord-
ing a previous report (Scheme 1) [26]. The presence of electron
donating S-groups is expected to shift the absorption range of
the porphyrazine Q-band to higher wavelength and the tert-butyl
groups are expected to enhance the solubility [4,13,14,27].
The cyclotetramerization process of the dinitrile derivative
(TBBTMnt) by the template effect of magnesium butanolate in
butanol resulted in the blue-green octakis(4-tert-butyl-benzyl-
thio)porphyrazinato magnesium 1 in very good yield (Fig. 1). The
metal-free derivative 2 was obtained in a reasonable yield of 30–
40% by treatment of 1 with trifluoroacetic acid at room tempera-
ture for 8 h. The change of color from dark blue-green to purplish
blue and the lowering of the solubility are apparent differences be-
tween the magnesium and metal-free products. Insertion of metal
ions into 2 with nickel(II) acetate was performed in THF and etha-
nol at reflux temperature for 18 h and afforded derivative 3 in
approximately quantitative yield (84%). The elemental analysis re-
sults closely follow the values calculated for 3.
The FT-IR spectra were recorded in the 4000–400 cmꢁ1 range on
a Perkin Elmer Spectrum One spectrometer using KBr pellets. The
electronic spectra and absorbance measurements were recorded
on an Agile 8453 UV–Vis spectroscopy system. Proton NMR spectra
were recorded on Brucker 250 MHz and 500 MHz Varian Inova
spectrometers. Elemental analyses were recorded on Thermo Flas-
hea 1112 series equipment.
2.2. Synthesis of {2,3,7,8,12,13,17,18-octakis[4-tert-butylbenzylthio]
porphyrazinato}Ni(II) (3)
To a solution of anhydrous Ni(CH3COO)2 (173 mg, 0.094 mmol)
in 10 ml of absolute ethanol was added a solution of 2 (80 mg,
0.046 mmol) in 10 ml of THF, and the resulting mixture was re-
fluxed under Ar for about 18 h. The precipitate that formed, com-
posed of the crude product and the excess metal salt, was
filtered then washed with hot THF. After evaporation of the sol-
vent, the remaining product was treated with hot water to remove
the unreacted metal salt, washed with methanol and dried. The
pure dark blue porphyrazine was obtained. The product was very
soluble in chloroform, THF and dichloromethane.
Spectroscopic investigations on the newly synthesized interme-
diates and porphyrazine are in accordance with the proposed
structures. In the FT-IR spectrum of TBBTMnt, the stretching vibra-
tion of C„N is observed at 2213 cmꢁ1, the tert-butyl peak around
2978 cmꢁ1, the S–CH2 peak around 680 cmꢁ1 and the aromatic C–
H peaks around 3028 cmꢁ1. These values comply with those
reported in the literature for similar compounds and with the pre-
vious report for TBBTMnt [26,28]. The sharp C„N vibration around
2213 cmꢁ1 disappeared after the formation of porphyrazine 1. The
N–H stretching vibration of the inner core of the metal-free por-
phyrazine 2 was observed around 3289 cmꢁ1 after demetallation
of 1 [25,29,30]. The FT-IR spectrum of the NiPz (3) derivative
showed a stretching vibration of the tert-butyl peak around
Yield: 71 mg (84%). FT-IR (KBr) m
max (cmꢁ1): 3028 (H–Ar), 2956
C–(CH3)3, 1636, 1513, 1461, 1263, 1191, 1105, 1018, 834. 1H NMR
(CDCl3) (d, ppm): 7.34–7.27 (d, m, 4H, Ar–H), 4.59 (m, s, 2H, CH2–
S), 1.33 (s, 9H, (CH3)–C). Anal. Calc. for C104H120NiN8S8: C, 69.50; H,
6.73; N, 6.23; Ni, 3.27; S, 14.27. Found: C, 68.35; H, 6.57; N, 5.84; S,
12.73%.
2863–2956 cmꢁ1 and aromatic C–H peaks around 3028 cmꢁ1
,
which are very similar with the literature (M = Cu, Co, Zn) as ex-
pected [4,26].
2.3. MIS diode fabrication
In the 1H NMR spectra of 1, 2 and 3, chemical shifts correspond-
ing to the tert-butyl protons came out at the expected values: a sin-
glet at 1.3 ppm in the ligand TBBTMnt, 1.14 ppm in 1, 1.17 ppm in
2 and 1.33 ppm in 3 [5,26,29,31,32].
For the MIS diode fabrication, p-type single crystal silicon (100)
with resistivity in the 8–10
X cm range was used. Prior to the
deposition, the silicon substrates were cleaned by ultrasonic treat-
ment in acetone, propanol and water for 10 min each, and subse-
quently etched in diluted HF solution, to remove the native SiO2
layer. An Ohmic back contact was established by the thermal evap-
oration of 200 nm high purity (99.999%) Al followed by annealing
To identify the structure of the porphyrazines (1–3), electronic
spectra are especially useful. The electronic absorption spectra of
the metallo-porphyrazines (1 and 3) exhibit a strong absorption
between 648 and 674 nm which is due to a
p ?
p⁄ transition and

B. Keskin et al. / Polyhedron 38 (2012) 121–125
123
Scheme 1. (i) Mg turnings, I2, n-BuOH; (ii) CF3CO2H; (iii) EtOH-THF and Ni(OAc)2.
Table 1
UV–Vis data for the porphyrazines in dichloromethane.
Compound
Band k, nm (log
e
, dm3 molꢁ1 cmꢁ1
Q
)
B
1
2
3
380 (4.42)
352 (3.97)
327 (3.84)
678 (4.38)
653 (3.75)
656 (4.13)
714 (3.88)
obtained, it was observed that the compound did not show any
appreciable aggregation in the concentration range 10ꢁ3–10ꢁ6 M.
Fig. 1. Schematic representation of the porphyrazines.
3.2. Electronic properties of Al/NiPz/p-Si
is commonly referred to as the Q-band, by which the first intense
bands of the porphyrazine core are dominated. A second intense
and broad
Current–voltage (I–V) and capacitance–voltage (C–V) measure-
ments are two fundamental characterization technique for SB
diodes. The forward bias I–V characteristic of the Al/Pz/p-Si struc-
ture at room temperature is given in Fig. 3. Although the reverse
bias current exhibits a weak voltage dependence, the forward cur-
rent of the diode increases exponentially with the applied voltage,
which is the characteristic behavior of rectifying contacts. This plot
also indicates that there is a deviation from linearity in the forward
bias ln I versus V graph for sufficiently large applied voltages. The
observed I–V characteristics indicate that the Al/Pz/p-Si structure
exhibits a rectifying behavior with a rectification ratio, which is
the ratio of the forward current to the reverse current at a certain
applied voltage, of 3.1 ꢀ 104 at 1 V. In this case, it is reasonable to
represent the dependence of the current I on the applied voltage V,
p ?
p⁄ transition in the near UV region in the range
327–380 nm, called the soret or B-band, is also a characteristic of
these tetrapyrole derivatives [4,5,26]. The UV–Vis spectra of the
porphyrazines (1, 2 and 3 as 4 ꢀ 10ꢁ5 M solutions in dichlorometh-
ane) prepared in the present work exhibited an intense single Q-
band absorption for the
p ?
p⁄ transition around 648–674 nm
and B-bands in the UV region around 326–346 nm (Fig. 2). The
change of symmetry of the porphyrazine core from D4h in the case
of the metallo-species to D2h in the metal-free derivative 2 is
apparent, showing a split Q-band at 647 and 714 nm, as expected,
for 2 (Table 1) [26,28–30]. The UV–Vis spectra were measured at
different levels of concentration to assess whether or not com-
pound 3 reveals any aggregation properties. In light of the results
1.2
NiPz
-10
-15
-20
-25
-30
H Pz
2
MgPz
0.9
0.6
0.3
0.0
300
400
500
600
700
800
-1.0
-0.5
0.0
V (V)
0.5
1.0
Wavelenght (nm)
Fig. 2. Electronic spectra of octakis(4-tert-butylbenzylthio) substituted porphyra-
zines 1, 2, and 3 as 4 ꢀ 10ꢁ5 M solutions in CH2Cl2.
Fig. 3. The current–voltage characteristic of the Al/Pz/p-Si Schottky barrier diode.

124
B. Keskin et al. / Polyhedron 38 (2012) 121–125
ꢀ
ꢁ
V
a
kT
q
I
according to thermionic emission of the SB diode model with a ser-
ies resistance, by the well-known expression [33]:
FðV; IÞ ¼
ꢁ
ln
ð5Þ
AAꢂT2
ꢀ
ꢀ
ꢁ
ꢁ
where
a is an arbitrary parameter greater than the ideality factor n.
According to this method, plots of F(V, I) versus current show a min-
qðV ꢁ IRsÞ
I ¼ I0 exp
ꢁ 1
ð1Þ
nkT
kT
imum for Ia
¼
ð
a
ꢁ nÞ. The plot of I versus
a is a straight line
a
qRs
whose slope leads to the value of the series resistance Rs. The plot
where
of F(V, I) versus I for various values of
shown in Fig. 4.
a ranging from 2 to 3.2 is
ꢀ
ꢁ
q
UB
I0 ¼ AAꢂT2 exp
ꢁ
kT
ð2Þ
Once the minimum point and corresponding value of the cur-
rent (I ) on each F(V, I) versus I plot was determined, we then plot-
a
and I0 is the saturation current, Rs is the series resistance of the
diode, V is the applied voltage, q is the electronic charge, k is the
Boltzmann constant, T is the absolute temperature in K, A is the area
of diode, A⁄ is the effective Richardson constant for p-type Si
(A⁄ = 32 cmꢁ2 Kꢁ2) [34], n is the ideality factor and UB is the effec-
tive Schottky barrier height at zero bias. Solving Eq. (2) for UB
yields,
ted the I versus
a graph using the obtained I value. Fig. 5 shows
a
a
the I –a plot of the Al/Pz/p-Si SB diode. From the slope of the I –a
a
a
plot, the value of the series resistance has been determined as
26 k . The high series resistance behavior may be ascribed to
X
the decrease in the increasing rate in current due to space charge
injection into the Pz organic thin film. The observed high value of
the series resistance indicates that the series resistance is a cur-
rent-limiting factor for the studied SB diode.
!
kT
q
AAꢂT2
I0
Many types of oxide charges, such as fixed oxide charge, oxide
trapped charge, mobile oxide charge and interface trapped charge,
can influence the capacitance–voltage (C–V) characteristics of the
SB diode. Therefore, the determination of the oxide charges, espe-
cially mobile oxide charges (Qm), is important for information
about the quality of the MIS type SB diodes. Here, high frequency
C–V measurements were used to determine the mobile oxide
charges. Fig. 6 shows the forward and reverse bias C–V character-
istic of the Al/NiPz/p-Si structure measured at signal frequency of
2 MHz at room temperature. As can be seen from the C–V curve
in Fig. 6, the capacitance towards high negative voltage increases
and reaches the capacitance of the NiPz layer alone. The number
of mobile oxide charges was determined by using the following
relation,
UB
¼
ln
ð3Þ
In most cases, it is not easy to decide which type of carrier transport
mechanisms is responsible for the observed conduction mechanism.
However, the value of the ideality factor n can help in determining
the possible conduction mechanism. For an ideal Schottky barrier
diode n = 1, but in a real SB diode, the ideality factor has a value
greater than unity for several reasons, such as image force lowering
of the Schottky barrier at the interface, the presence of an interfacial
thin native oxide layer and series resistance [33,35]. The ideality
factor n is given by
q
dV
n ¼
ð4Þ
kT dðln IÞ
C
OXDVFB
qA
With the aid of Eqs. (3) and (4), the barrier height and ideality factor
n of the diode were calculated from the intercept and slope of the
linear region of the forward bias ln I–V characteristic, and they were
found to be 1.81 and 0.89 eV, respectively.
Qm
¼
ð6Þ
where COX is the oxide capacitance,
D
VFB is the flat band voltage
shifts, q is the electronic charge and A is the area of the capacitor.
In an ideal metal–insulator–semiconductor (MIS) capacitor (with
no oxide charge), flat band capacitance occurs at zero applied volt-
age. However, most real oxides contain charges which give rise to
non-ideal C–V curves; and thus the flat band voltage is shifted from
zero, due to compensation for the charge in the oxide. The most
commonly used method to determine the value of the flat band
voltage, and which is used in this work, is the method based on
To obtain the significance of this value of UB, we compared our
findings with conventional MS contacts such as Al/p-Si and Au/n-Si
diodes. The current–voltage characteristics of p-Si/C junctions fab-
ricated by pulsed laser deposition at different temperatures were
investigated by Gupta et al. [36]. The values of various junction
parameters, such as ideality factor, barrier height and series resis-
tance, were determined from the forward bias I–V characteristics.
The values of the barrier height were found to be 0.37 and
0.41 eV for junctions fabricated at room temperature and 200 °C,
respectively. More recently, the current–voltage characteristics of
Au/n-Si Schottky barrier diodes over the wide temperature range
70–310 K were investigated by Sharma [37]. Sharma observed a
decreasing trend in barrier height (from 0.79 eV at 310 K to
0.27 V at 70 K) with the decrease in operating temperature. The
UB value of 0.89 eV that we have obtained for the Al/NiPz/p-Si de-
vice is remarkably higher than that achieved with conventional MS
contacts such as Al/p-Si and Au/n-Si diodes. The obtained results
reveal that the Pz organic film controls the carrier transport of
the diode at the contact interface and the conventional Al/p-Si
diode can be designed to exhibit the desired properties by means
of the choice of the organic molecule.
α = 2.0
1.05
α = 2.1
α = 2.3
1.00
0.95
0.90
0.85
0.80
0.75
α = 2.4
α = 2.5
α = 2.7
α = 2.9
α = 3.0
α = 3.2
The obtained high value of the ideality factor and the deviation
from the linearity in the forward bias current–voltage plot at suffi-
ciently large voltages suggest the presence of an organic interlayer
and series resistance. Several methods have been proposed to ob-
tain series resistance in a Schottky barrier diode. The Lien, So and
Nicolet method [38] was employed for an accurate evaluation of
the series resistance (Rs) from the standard ln I–V plot. This method
is based on an auxiliary function defined by
-11
-10
-9
-8
-7
-6
-5
10
10
10
10
I (A)
10
10
10
Fig. 4. Semi-logarithmic plot of F(V, I) vs. I for different values of a.

B. Keskin et al. / Polyhedron 38 (2012) 121–125
125
1.4x10-6
1.2x10-6
1.0x10-6
8.0x10-7
6.0x10-7
4.0x10-7
The Al/NiPz/p-Si structure has been fabricated and it is shown
that this structure exhibits rectifying behavior with a rectification
ratio of 3.1 ꢀ 104 at 1 V. The main SB diode parameters, such as
the ideality factor, series resistance and barrier height, were ob-
tained from the measured I–V data. It was observed that these val-
ues are remarkably higher than those given for metal/Si
semiconductor contacts in the literature, that is, it has been shown
that the NiPz interlayer produces an increase in the barrier height.
The oxide charge calculations on this MIS capacitors showed that
the combination of Al/NiPz/p-Si is a promising structure with
low oxide charges suitable for MIS based devices.
Acknowledgment
2.0x10-7
0.0
This research was carried out with the support of the Yildiz
Technical University. Project No.: 23-01-02-01.
2.0
2.2
2.4
2.6
2.8
3.0
3.2
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In conclusion, we have described the synthesis and spectral
characterization of a new Ni(II) porphyrazine surrounded by eight
tert-butylphenyl groups on the periphery. A distinctive property of
these compounds is that they have good solubility in many com-
mon organic solvents. The presence of the bulky tert-butyl groups
hinders aggregation at relatively higher concentrations (e.g.,
10ꢁ3 M).