Synthesis, characterization and application of Ni(tta)2· tmeda to MOCVD of nickel oxide thin films

Article abstract of DOI:10.1039/b511317b

A novel nickel β-diketonate adduct, Ni(tta)₂·tmeda, has been synthesized using 2-thenoyltrifluoroacetone as the β-diketonate and N,N,N,′N′-tetramethylethylendiamine as the Lewis base. It has been characterized by elemental analyses, IR, ¹H NMR, ¹³C NMR spectroscopy and single-crystal X-ray diffraction studies. Physical and thermal properties of Ni(tta)₂·tmeda precursor have been also extensively investigated. Its efficacy as a metal-organic chemical vapour deposition (MOCVD) precursor for the growth of nickel oxide films has been fully tested by applying it to the deposition of NiO films on quartz substrate. NiO thin films have been characterized by X-ray diffraction (XRD), scanning electron microscopy and UV spectroscopy. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b511317b

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Synthesis, characterization and application of Ni(tta) ·tmeda to MOCVD of
2
nickel oxide thin films
a
a
a
a
Graziella Malandrino,* Laura M. S. Perdicaro, Giuseppe Condorelli, Ignazio L. Fragal a` ,
b
b
Patrizia Rossi and Paolo Dapporto
Received 8th August 2005, Accepted 7th November 2005
First published as an Advance Article on the web 28th November 2005
DOI: 10.1039/b511317b
A novel nickel b-diketonate adduct, Ni(tta)
2
·tmeda, has been synthesized using
ꢀ ꢀ
2
-thenoyltrifluoroacetone as the b-diketonate and N,N,N, N -tetramethylethylendiamine as the Lewis
1
13
base. It has been characterized by elemental analyses, IR, H NMR, C NMR spectroscopy and
single-crystal X-ray diffraction studies. Physical and thermal properties of Ni(tta)
·tmeda precursor
have been also extensively investigated. Its efficacy as a metal–organic chemical vapour deposition
MOCVD) precursor for the growth of nickel oxide films has been fully tested by applying it to the
2
(
deposition of NiO films on quartz substrate. NiO thin films have been characterized by X-ray
diffraction (XRD), scanning electron microscopy and UV spectroscopy.
Introduction
fast production of films with a high uniformity degree in both
thickness and composition over large areas due to the reliability
and reproducibility of the process.
Nickel oxide (NiO) is among the most widely investigated tran-
sition metal oxides. NiO thin films have a wide range of applica-
tions due to their excellent chemical stability as well as optical,
17
Nevertheless, reproducible MOCVD processes require suitable
source precursors, thermally stable and volatile enough to provide
molecular integrity during the mass transport. Low-melting
architectures are also desired to allow constant evaporation from
the liquid phase for long time intervals. To date, b-diketonate com-
plexes with 2,2,6,6-tetramethyl-3,5-heptanedione (Hthmd) and
acetylacetone (Hacac) ligands have been widely used as MOCVD
1
electrical and magnetic properties. They have, also, various
interesting properties including crystallinity, transparency asso-
ciated with low resistivity and controllable transmittance for
incident visible light. These films have been used as functional
2
layers for chemical sensors, materials for electrochromic display
3
4
devices and antiferromagnetic materials.
14a
sources for NiO films. These precursors suffer, however, from
NiO films are also of great interest because they represent a
good example of p-type transparent semiconducting oxides. In
fact, transparent conducting oxide films such as indium oxide, tin
oxide and zinc oxide, of practical use as transparent electrodes and
window coatings, are all n-type semiconductors.
Recently, nickel oxide thin films have attracted attention as
promising electrochromic materials after tungsten oxide. Elec-
trochromic materials enable dynamic control of the throughput of
radiant energy and play a significant role in energy efficient “smart
windows” to reduce the cooling and lighting cost of buildings.
Other important applications include preparations of alkaline
◦
some drawbacks associated with their high melting points [228 C
5
18
◦
19
for Ni(tmhd)
of H O molecules in the coordination sphere.
In the present paper, we report on the synthesis and char-
2
and 240 C for Ni(acac)
2
] and with the presence
2
6
acterization of a novel nickel precursor Ni(tta)
2
ꢀ
·tmeda (Htta =
ꢀ
2
-thenoyltrifluoroacetone, tmeda
=
N,N,N ,N -tetramethyl-
ethylendiamine), where the tmeda acts as an ancillary Lewis base
ligand. The single-crystal X-ray diffraction analysis points to a
monomeric, water-free complex. The thermogravimetric analyses
have shown that this adduct possesses mass transport properties
well suited for MOCVD applications. As a consequence it has
been reproducibly applied to the MOCVD of NiO films on quartz
substrates.
7
8
batteries (as cathode materials) and buffer layers for depositions
of other oriented films, such as superconducting films with well
matched lattice parameters, since NiO has a cubic structure with
9
˚
a = 4.195 A.
Nickel oxide thin films have been prepared by various tech-
Results and discussion
10
11
niques: pulsed laser deposition, vacuum evaporation, mag-
netron sputtering, sol–gel,
spray pyrolysis. Quite recently atomic layer epitaxy has been
successfully applied to NiO depositions. Metal Organic Chemical
Vapor Deposition (MOCVD) is an attractive challenge for the
12
3a,13
14
chemical vapor deposition and
The reaction of Ni(CH COO) ·2H O with the b-diketone Htta
3
2
2
15
in the presence of the Lewis base tmeda yields the water-free
16
Ni(tta) ·tmeda adduct as indicated in eqn. (1):
2
Ni(CH
3
COO)
2
·2H
2
O + 2 Htta + tmeda
COOH + 2 H
→
Ni(tta)
2
·tmeda + 2 CH
3
2
O
(1)
a
Dipartimento di Scienze Chimiche, Universit a` diCatania, and INSTM, UdR
Catania, Viale A. Doria 6, I-95125, Catania, Italy. E-mail: gmalandrino@
dipchi.unict.it
Dipartimento di Energetica “S. Stecco” Universit a` di Firenze, Via Santa
Marta 3, I-50139, Firenze, Italy
The reaction can be easily conducted in a single step under the most
usual and simple laboratory conditions. The complex is soluble
b
in EtOH, acetone and CH
2
Cl
2
, it is insoluble in H
2
O, while it
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is partially soluble in solvents such as pentane and hexane. The
adduct is non-hygroscopic and can be handled in air.
Table 2 Cell parameters and space groups of complexes having general
formula M(tta)
2
·tmeda
2
+
8
The Ni ion having a d electronic configuration gives rise,
in an octahedral environment, to a high-spin complex and
Ni(II)
Zn (II) (ref. 20) Cd (II) (ref. 20) Co (II) (ref. 23)
1
therefore a paramagnetic complex. As a consequence, the H
˚
a/A
b/A˚
11.867(7) 11.947(2)
15.421(3) 15.368(3)
15.174(5) 15.132(3)
92.48(2) 92.94(2)
17.582(5)
8.335(2)
18.610(7)
92.61(3)
12.033(6)
15.565(6)
15.339(6)
92.58(1)
NMR spectrum shows paramagnetic shifts with respect to the
2
0
21
˚
homologous Zn(tta)
2
·tmeda or Cd(tta)
2
·tmeda complexes, with
c/A
◦
b/
line broadening due to unpaired electrons.
Space group P2
1
/c
P2
1
/c
P2
1
/c
1
P2 /c
X-Ray crystallographic studies of Ni(tta)
2
·tmeda
20
In previous works we have studied Zn(tta)
2
·tmeda and
·tmeda, homologues complexes of Ni(tta) ·tmeda (gen-
eral formula M(tta) ·tmeda complex
·tmeda). Another M(tta)
M = Co(II)] was retrieved from the CSD. Of these four complexes
three (Zn, Ni and Co) are almost isostructural and isomorphous
see Table 2). The rms obtained superimposing the metal cation
and the six donor atoms of the three complexes ranges from 0.031
21
Cd(tta)
2
2
The ORTEP-3 view of Ni(tta)
2
·tmeda is reported in Fig. 1. In the
2
2
Ni(tta)
2
·tmeda complex the Ni(II) cation is hexacoordinated by
24
[
the oxygen atoms of two tta anions and by the nitrogen atoms
of one tmeda molecule. The coordination geometry is a distorted
octahedron. The tmeda molecule, acting as a chelating ligand,
shows a gauche conformation.
(
˚
and 0.041 A. The main difference between the Ni(II), Zn(II) and
Co(II) M(tta)
2
·tmeda complex and the Cd(II) complex is due to
the disposition of the thiophene rings. In fact, if we consider the
plane defined by the metal cation and the two CH carbon atoms of
the two tta anions, in the nickel-like complexes, the two thiophene
rings lie in opposite sides with respect to it, giving rise to a trans
conformation. On the other hand in the cadmium complex they
lie on the same side (Fig. 2).
Fig. 2 Pluto view of complexes Ni(tta)
2
·tmeda and Cd(tta)
2
·tmeda.
Fig. 1 ORTEP view of the compound Ni(tta)
2
·tmeda.
Finally, in the crystal lattice no intermolecular contacts are
The Ni–O and Ni–N distances are in agreement with those
present.
22
found in the Cambridge Structural Database (CSD, v5.26) for
nickel complexes having as ligands acetylacetonate derivatives
and amine molecules, respectively (see Table 1). In particular,
Physico-chemical and mass transport properties of Ni(tta)
2
·tmeda
18
23
The FT-IR spectrum of Ni(tta)
in the 3500–3600 cm interval as expected for a water-free
2
·tmeda does not show any band
tmeda adducts of Ni(tmhd)
2
and Ni(acac)
2
have been recently
−
1
reported. The tmeda molecule shows a gauche conformation found
−
1
species. Peaks at 1619 cm (C=O stretching vibrations) and
also in the Ni(tmhd)
2
·tmeda and Ni(acac)
2
·tmeda adducts.
−
1
at 1596 cm (C=C stretching vibrations) are typical of the
b-diketonate framework. The other bands, observed at 1300–
◦
Table 1 Selected bond lengths ( A˚ ) and angles ( ) for Ni(tta)
·tmeda
2
−
1
1100 cm , represent C–F and C–S stretchings of the tta ligand.
Ni(1)–O(1)
Ni(1)–O(3)
Ni(1)–O(4)
2.000(7)
2.010(7)
2.033(7)
Ni(1)–O(2)
Ni(1)–N(1)
Ni(1)–N(2)
2.045(7)
2.145(9)
2.166(9)
−1
Peaks at 1230 and 1030 cm (C–N stretching vibrations) are
typical of tmeda. Other features (at 2923, 1460 and 1377 cm )
are typical of the Nujol used to prepare the mull.
−1
O(1)–Ni(1)–O(3)
O(1)–Ni(1)–O(4)
O(3)–Ni(1)–O(4)
O(1)–Ni(1)–O(2)
O(3)–Ni(1)–O(2)
O(4)–Ni(1)–O(2)
O(1)–Ni(1)–N(1)
O(3)–Ni(1)–N(1)
173.2(3)
85.3(3)
88.7(3)
89.5(3)
87.7(3)
93.4(3)
88.8(3)
94.6(3)
O(4)–Ni(1)–N(1)
O(2)–Ni(1)–N(1)
O(1)–Ni(1)–N(2)
O(3)–Ni(1)–N(2)
O(4)–Ni(1)–N(2)
O(2)–Ni(1)–N(2)
N(1)–Ni(1)–N(2)
91.7(4)
174.4(4)
95.1(3)
91.2(3)
176.0(3)
90.5(3)
84.3(4)
The thermal behavior of the present Ni precursor have
been fully investigated by thermogravimetric analyses (TG). The
Ni(tta)
2
·tmeda (Fig. 3)adductis thermallystable andthe derivative
curve of TG (DTG) consists of a single peak thus indicating that it
evaporates quantitatively in the 120–250 C range, with a residue
left at 350 C of lower than 2%. Fig. 3 shows the TG curves of
◦
◦
Ni(acac)
2
and Ni(tmhd)
2
for comparison.
1
102 | Dalton Trans., 2006, 1101–1106
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Fig. 3 TG profiles of Ni(tta)
flow in the 25–350 C temperature range at 20 Torr.
2
·tmeda, Ni(acac)
2 2 2
and Ni(tmhd) under N
◦
The TG of [Ni(acac)
2
·(H
2
O) ] shows more steps and a residue
2
◦
at 250 C of 20.9%. The first step observed in the range 50–
Fig. 4 (a) Mass percent vs. time for vaporization of Ni(tta)
2
·tmeda under
◦
◦
N
2
flow in the 140–170 C temperature range at 20 Torr. (b) Arrhenius
1
20 C is associated with H
2
◦
O loss while the major weight loss
plots for the vaporization of Ni(tta) ·tmeda under N flow at 20 Torr.
2
2
found in the range 120–250 C is associated with decomposition
of the complex. This behaviour may find a counterpart in
the transformation from the octahedral monomeric hydrated
advantages are precluded with first-generation Ni(tmhd) that
2
must be sublimed from the solid phase in MOCVD experiments.
In these cases, the crystallite sizes, in fact, may be responsible for
changes of evaporation rates in different experiments and, even
more important, of variations during the same experiment.
25
[
Ni(acac)
2
·(H
2
O)
2
] species to the trimeric form [Ni(acac) ] . The
2
3
final residue may be likely associated with metallic Ni, since the
corresponding theoretical residue is about 20 wt%. The TG of the
Ni(tmhd)
2
indicates that vaporization occurs in a single step with a
All the investigations clearly indicate that Ni(tta)
2
·tmeda is
residue (4%) greater than that left from the Ni(tta)
2
·tmeda adduct.
thermally stable and volatile, therefore, this new synthesized nickel
adduct is expected to be a good precursor for the deposition of
nickel containing films.
Isothermal thermogravimetric curves are shown in Fig. 4(a).
The mass loss of the cylindrical sample boat has been recorded
as a function of time, at constant temperature under reduced
pressure (20 Torr) in N
2
atmosphere for 60 min. Temperatures have
MOCVD depositions of NiO from the Ni(tta)
2
·tmeda precursor
◦
been varied from 140 to 170 C. Note that for every investigated
temperature, a linear behaviour is observed, thus pointing to a
constant vaporization rate during the experimental time.
Fig. 4(b) shows the Arrhenius relationships obtained by plotting
the vaporization rate as a function of reciprocal temperature.
When no decomposition takes place, the activation energy of
the evaporation process can be calculated from the slope of the
Arrhenius plots and it represents the apparent enthalpy of vapor-
ization (DH). On the basis of experimental curves the vaporization
Applications of NiO films as TCO electrodes requires, of course,
transparent substrates for optical characterization of films. There-
fore, MOCVD depositions have been attempted on quartz sub-
strates. Experiments to grow Ni oxide films were first attempted at
◦
◦
450 C substrate temperature and 160 C precursor vaporization
temperature.
The above reported experiments yielded films with growth rate
−
1
of about 2 nm min . EDX analyses is reported in Fig. 5. The
nickel Kaand Kb peaks are observed at 7.460 and 8.250 keV while
the L lines are spread in the 0.86–1.020 keV range. In addition, the
windowless EDX detector allowed to detect the O Kapeak at 0.520
keV, and to exclude any F contamination, since no peak is observed
at 0.670 keV. The films do not contain F within the detectability
limit of the EDX technique (∼1%). In addition films have been
analyzed through wavelength dispersive X-ray analysis (WDX).
All the experiments on the present films indicate negligible F
(0.1%) contamination. The WDX detection limit for F is about
400 ppm.
−
1
enthalpy has been estimated to be 113.51 ± 2.43 kJ mol .
In this context, it is of relevance to note that the rather
◦
low melting point (147–149 C) of the presently synthesized
Ni(tta)
2
·tmeda adduct allows its use in the liquid phase at the
processing temperature, in contrast to Ni(tmhd)
2
which is always
solid during deposition experiments. This represents an important
issue for MOCVD application, since a liquid precursor is the most
desirable source due to the greater stability of the vapour pressure
and to the absence of any effects of crystallite sizes on the precursor
evaporation rate and, hence, on the film growth rate. These
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Fig. 5 EDX spectrum of a NiO film deposited on a quartz substrate using
the Ni(tta)
·tmeda precursor.
2
Fig. 7 FESEM image of a NiO film deposited on a quartz substrate using
The XRD pattern of the NiO films deposited in these conditions
is shown in Fig. 6. All the reflections characteristic of the NiO
phase are observed thus indicating that the film has no preferential
the Ni(tta)
2
·tmeda precursor.
◦
orientation. The pattern is recorded up to 64 , because this is the
maximum value achievable for the grazing incidence mode. Peaks
◦
found at 37.48, 43.55 and 63.21 are associated with the 111, 200
and 220 reflections, respectively. Note that only the reflection with
h,k,l all odd or all even are observed due to the F-face-centered
26
cubic lattice.
Fig. 8 Optical transmittance of a NiO film deposited on a quartz substrate
using the Ni(tta)
·tmeda precursor.
2
NiO has a six-fold octahedral coordination in the so-called rock
salt structure, the absorption at 450 nm can be related to the
3
3
transition from the A
2
fundamental state to the T excited state.
1
Conclusions
Fig. 6 XRD pattern of a NiO film deposited on a quartz substrate using
the Ni(tta)
·tmeda precursor.
2
A novel adduct of nickel, Ni(tta)
·tmeda, has been synthesized
2
through a single-step reaction and characterized. X-Ray single-
crystal diffraction indicates that the complex is water free and
presents a six-coordinate quasi-octahedral geometry. Thermal
analysis by dynamic TG and isothermal measurements showed
higher thermal stability for the novel precursor than conventional
The NiO films have mirror-like surfaces and are homogeneous
in terms of morphology over the whole area. The morphologies
have been observed through field emission scanning electron
microscopy (FESEM). The FESEM image (Fig. 7) indicates a
homogeneous surface with aggregates of grains having dimensions
of about 10 nm. The presence of very small grains and the
consequent low roughness account for a low radiation scattering
[Ni(acac)
Ni(tta)
·tmeda adduct, in contrast to both [Ni(acac)
Ni(tmhd) could be vaporized in the liquid state due to its lower
2
·(H
2
O)
2
]. In addition, it should be noted that the
2
2
·(H O) ] and
2
2
2
1
and therefore, the high transmittance.
melting point, thus representing a liquid source at processing
temperature.
The measured spectral transmittance of an as-deposited NiO
◦
film grown on quartz at 450 C is shown in Fig. 8. This spectrum
The definitive functional validation of the complex as a precur-
sor for MOCVD has been assessed by applying it to the efficient
growth of nickel oxide films on quartz substrates. The films have a
good morphology with very small grains, which find a counterpart
in the interesting optical properties in terms of high transmittance
in the visible region.
shows that the NiO film is highly transparent in the visible region
with optical transmission nearly 90% between 550 and 800 nm.
Note that the absorption around 450 nm, usually observed for
thin NiO films and related to the d–d transition, still leaves a
transmittance higher than 70% in the 380–550 nm range. Since
1
104 | Dalton Trans., 2006, 1101–1106
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These data indicate the great potentiality of this precursor to
grow good quality NiO films through a versatile, low cost and,
therefore, industrially appealing MOCVD process. Further studies
are in progress to establish the relationship between NiO films and
various substrates of interest for specific purposes.
The hydrogen atoms were introduced in calculated position and
their coordinates refined in agreement with those of the linked
atoms. All the non-hydrogen atoms were refined anisotropically.
Atomic scattering factors and anomalous dispersion corrections
for all the atoms were taken from ref. 30.
31
Geometrical calculations were performed by PARST97. The
32
molecular plot was produced by the ORTEP-3 program.
Experimental
Crystal data for Ni(tta) NiO
2
·tmeda.
C
22
H
24
F
6
N
2
4
S
2
, mono-
˚
Reagents
clinic, space group P2
1
/c, M = 617.26, T = 293 K, k = 0.71069 A,
◦
˚
a = 11.867(7), b = 15.421(3), c = 15.174(5) A, b = 92.48(2) , V =
The chemical reagents Htta, tmeda and Ni(CH
purchased from Aldrich and used without any further purification.
3
COO)
2
·2H
2
O were
3
−3
˚
2
774(2) A , Z = 4, D
c
= 1.478 g cm , R [I > 2r(I)] = 0.0837,
wR2 = 0.1426.
CCDC reference number 280382.
General procedures
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b511317b
Infrared transmittance spectra were recorded using a Jasco
FT/IR-430 spectrometer as Nujol mulls between NaCl plates.
Elemental microanalyses were performed in the analytical labora-
tories of the University of Catania using a Carlo Erba elemental
analyzer EA 1108. The melting points were measured in air with a
Koffler microscope. H NMR spectra were recorded on a Varian
Inova 500 spectrometer.
Thermogravimetric analyses were performed by using a
MOCVD experiments
MOCVD depositions of NiO thin films were carried out in a
low-pressure, hot-wall MOCVD reactor on quartz substrates. The
1
Ni(tta)
2
·tmeda precursor was evaporated from the molten state at
◦
1
60 C. It was transported by 150 sccm argon flow, while a flow of
e
O (150 sccm) was used as reaction gas. The substrate temperature
Mettler Toledo TGA/SDTA 851 . Thermal investigations of
Ni(tta) (H O) ] and Ni(tmhd) were carried out
·tmeda, [Ni(acac)
in purified nitrogen flow (30 sccm). The temperature was measured
2
◦
was maintained at 450 C for 90–120 min, while the pressure of
2
2
2
2
3
the system was about 2–3 Torr.
◦
◦
−1
The atomic composition of the films was analyzed by energy
dispersive X-ray analysis (EDX), using an IXRF windowless
detector, and by wavelength dispersive X-ray analysis (WDX)
using an Oxford Instruments WDX-3PC analyzer. The WDX
analyses were carried out at 15 kV accelerating voltage, 10
nA current, and 100 s acquisition time. A TAP (thallium acid
phthalate) crystal was used as X-ray selector for F.
with an accuracy of ± 0.1 C. The heating rate was 5 C min .
Thermal investigations were carried out at 20 Torr. The weights of
the sample for TG measurements were about 5–15 mg, while the
cylindrical sample boat (12.56 mm cross sectional area) was filled
with ∼10 mg of Ni(tta)
2
2
·tmeda for the isothermal investigations.
Synthesis of Ni(tta) tmeda
2
XRD patterns were recorded on a Bruker-AXS D5005 h–h
diffractometer, using Cu-Ka radiation operating at 40 kV and 30
The complex has been synthesised through a one-pot re-
action between an aqueous solution (30 ml) containing
◦
mA in grazing incidence (0.5 ) mode. Film surface morphology
−
2
Ni(CH
solution (35 ml) containing Htta (8.31 g, 3.14 × 10 mmol) and
tmeda (2.82 ml, 1.87 × 10 mmol). The resulting biphasic system
was shaken for 30 min and the product recovered from the CH Cl
3
COO)
2
·2H
2
O (4.65 g, 1.87 × 10 mmol) and a CH
2
Cl
2
was examined using a field emission scanning electron microscope
ZEISS SUPRA 55 VP.
Spectrophotometric measurements from 200 to 800 nm were
taken at near-normal incidence on a Perkin-Elmer Lambda 20
spectrophotometer.
−
2
−
2
2
2
solution, in which it is soluble. The product was purified using
ethanol to yield pale green crystals. Yield: 85%. The melting point
of the crude product was 147–149 C.
◦
Acknowledgements
NiC22
H
24
F
6
N
2
S
2
O
4
(M.W. = 617.26): calc.: C, 42.81; H, 3.92; N,
4
.54; S, 10.39; found: C, 43.21; H, 3.74; N, 4.80; S, 10.63%.
The authors thank the MIUR for the financial support. CRIST
(
Centro Interdipartimentale di Cristallografia Strutturale, Uni-
X-Ray crystallographic studies of Ni(tta)
2
·tmeda
versity of Florence), which provided the X-ray equipment, is
gratefully acknowledged
Pale green crystals (0.2 × 0.4 × 0.6 mm) were obtained
from ethanol solutions. Cell parameters and intensity data for
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2
·tmeda were obtained on a Nonius CAD4
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(
k = 0.71069 A). Cell parameters were determined by least-squares
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