Synthesis and characterization of lead (IV) precursors and their conversion to PZT materials through a CVD process

Article abstract of DOI:10.1016/j.poly.2019.114270

New Pb(IV) precursors for lead zirconate titanate (PZT) thin films were synthesized, and their solid-state structures were elucidated by single-crystal X-ray crystallography. First, tetraphenyl lead (Ph₄Pb) was synthesized from the reaction of PbCl₂ with the Grignard reagent, PhMgBr, by a published method. Two of the phenyl ligands in Ph₄Pb were replaced by bromines, and then were substituted with bis(trimethylsilyl)amides (btsa) to yield Ph₂Pb(btsa)₂ (1), or with 2,2,6,6-tetramethyl-3,5-heptadiketonate (thd) ligands to yield Ph₂Pb(thd)₂ (2). Single crystals of these two new Pb(IV) precursors were obtained by recrystallization in hexane, and their chemical compositions were characterized by FT-IR and ¹H NMR. In TG analyses, both compounds exhibited sharp decomposition curves, with major mass losses in the region of 200–250 °C. PZT films were fabricated from one of the newly prepared Pb(IV) precursors by an MOCVD process involving Ti(OⁱPr)₄ and Zr(OPr)₄, and the development of the perovskite phase with temperature was monitored by an XRD method.

Full text of DOI:10.1016/j.poly.2019.114270

Polyhedron 177 (2020) 114270
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and characterization of lead (IV) precursors and their
conversion to PZT materials through a CVD process
a
b
a
c
a,b,
⇑
Euk Hyun Kim , Cheong Won Moon , Jung Gyu Lee , Myoung Soo Lah , Sang Man Koo
a
Department of Chemical Engineering, Hanyang University, Seoul 04763, Republic of Korea
Department of Fuel Cells and Hydrogen Technology, Hanyang University, Seoul 04763, Republic of Korea
Department of Chemistry, UNIST, Ulsan 44919, Republic of Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
New Pb(IV) precursors for lead zirconate titanate (PZT) thin films were synthesized, and their solid-state
structures were elucidated by single-crystal X-ray crystallography. First, tetraphenyl lead (Ph Pb) was
synthesized from the reaction of PbCl with the Grignard reagent, PhMgBr, by a published method.
Two of the phenyl ligands in Ph Pb were replaced by bromines, and then were substituted with bis
trimethylsilyl)amides (btsa) to yield Ph Pb(btsa) (1), or with 2,2,6,6-tetramethyl-3,5-heptadiketonate
thd) ligands to yield Ph Pb(thd) (2). Single crystals of these two new Pb(IV) precursors were obtained
Received 5 September 2019
Accepted 28 November 2019
Available online 16 December 2019
4
2
4
(
(
2
2
Keywords:
2
2
Lead precursors
b-Ketonate ligand
Silylamides
MOCVD process
PZT materials
1
by recrystallization in hexane, and their chemical compositions were characterized by FT-IR and H NMR.
In TG analyses, both compounds exhibited sharp decomposition curves, with major mass losses in the
region of 200–250 °C. PZT films were fabricated from one of the newly prepared Pb(IV) precursors by
i
an MOCVD process involving Ti(O Pr)
4
and Zr(OPr)
4
, and the development of the perovskite phase with
temperature was monitored by an XRD method.
Ó 2019 Elsevier Ltd. All rights reserved.
1
. Introduction
cursors are dissolved in a solvent, and a controlled amount of the
solution is injected into a flash evaporator. The evaporated gas
mixture is introduced into a deposition chamber, and a film is
deposited on a substrate, as in an ordinary CVD process.
One of the challenges in LD-MOCVD is the need for suitable pre-
cursors that have convenient vapor pressures at moderate temper-
atures, decompose at modest temperatures and are stable during
storage. Organometallic compounds have been found to satisfy
most of these demands, as the physical and chemical properties
of such precursors can be changed through the tailoring of the
organic ligands. Various organometallic precursors, including
metal alkoxides, beta-diketonates and aminoalcohols, have been
x 3
Lead zirconate titanates (PZT, Pb(Zr Ti1Àx)O ) are well-known
ferroelectric materials based on the perovskite structure, and are
of great interest for microelectronic applications such as pyroelec-
tric and piezoelectric devices, actuators, thermal image sensors,
non-volatile memory systems and optical waveguides [1,2]. Vari-
ous techniques for the preparation of PZT thin films have been
employed, such as reactive magnetron sputtering, multi-ion-beam
sputtering, spin coating, sol-gel processing, and pulsed laser abla-
tion [3–7]. For thin-film applications, the formation of crystalline
single-phase materials at low temperatures is desired. Chemical
routes such as sol–gel processing and MOCVD offer advantages
over physical deposition techniques, and have attracted wide
interest.
Among various chemical routes, liquid delivery metalorganic
chemical vapor deposition (LD-MOCVD) is regarded as one of the
most suitable techniques for the formation of PZT thin films due
to its good step coverage, high deposition rate, large area unifor-
mity and high throughput [8–10]. In the LD-MOCVD process, pre-
2 2
used for the MOCVD of TiO and ZrO [8–12]. However, there are
few precursors for lead sources that exhibit both high reactivity
at low temperatures and long-term stability in solution-based pro-
cesses. Aside from lead precursors that are frequently used for the
2 5 4
growth of PZT thin films, such as tetraethyllead (Pb(C H ) ) and Pb
(thd) (thd = 2,2,6,6-tetramethyl-3,5-heptadiketonate), most of the
2
Pb precursors developed so far have inadequate vapor pressures
for the CVD process compared to Ti and Zr sources, due to their
polymeric structure of lead (II) compound [13].
In this study, we describe the preparation of a new series of lead
(
IV) precursors containing phenyl ligands and the fabrication of
⇑
Corresponding author.
E-mail address: sangman@hanyang.ac.kr (S.M. Koo).
ferroelectric Pb(Zr Ti1Àx)O thin films by LD-MOCVD. For the syn-
x
3
https://doi.org/10.1016/j.poly.2019.114270
277-5387/Ó 2019 Elsevier Ltd. All rights reserved.
0

2
E.H. Kim et al. / Polyhedron 177 (2020) 114270
thesis, Ph
bromination of Ph
ligands onto the lead center (Ph
compounds in the solution and renders them less toxic than their
aliphatic counterparts [14]. Thus, the volatility or thermal behavior
of lead precursors could be markedly enhanced by the substitution
2
PbBr
2
was obtained as an intermediate through the
Pb. It is known that the introduction of phenyl
2.2.2. Preparation of diphenyl dibromo lead, Ph
Diphenyl dibromo lead (Ph PbBr ) was prepared by a published
procedure with some modifications [16]. First, 3.00 g of Ph Pb
(5.8 mmol) was dissolved in 75 mL of anhydrous chloroform. The
solution was cooled down to À64 °C in a chloroform/liquid-N
mixture bath, and 0.6 mL of bromine (Br , 11.6 mmol) was injected
2 2
PbBr
4
2
2
2+
2
Pb ) stabilizes the resulting
4
2
2
of the two bromines of Ph
2
PbBr
2
with bis(trimethylsilyl)amide
into the solution with a syringe. Upon thawing of the reaction bath,
the reaction mixture was stirred for 4 h at room temperature, and
the color of the solution changed from red to white. The resulting
solution was filtered to remove by-products. The filtered product
was washed with anhydrous benzene and anhydrous diethyl ether
several times, and was air-dried (yield: 80%).
(
btsa) or 2,2,6,6-tetramethyl-3,5-heptadiketonate (thd) ligands.
The newly prepared lead precursors were characterized by FT-IR,
1
H NMR, TGA and single-crystal X-ray crystallography. PZT films
were then fabricated by LD-MOCVD at 550 or 700 °C on a Si wafer,
and the development of PZT phases was investigated by XRD, SEM
and AFM methods.
À1
Anal. FT-IR (KBr, cm ): 3068 vw, 3049 vw, 1558 m, 1471 s,
1
439 s, 1321 w, 984 s, 717 vs, 673 s.
1
H NMR analysis for Ph
2 2
PbBr could not be carried out because
2
. Experimental section
Ph
2
PbBr is insoluble in most NMR solvents.
2
2.1. General methods
2.2.3. Preparation of diphenyl di{bis(trimethylsilyl)amide} lead, Ph
2
Pb
{
N(SiCH (1)
3 2 2
) }
All reactions were carried out in a dry nitrogen atmosphere by
A 1.28-g sample of lithium bis(trimethylsilyl)amide (btsa,
.6 mmol) was dissolved in anhydrous hexane, and 2.00 g of diphe-
standard Schlenk techniques, unless otherwise noted. Hexane,
toluene, diethyl ether and tetrahydrofuran (THF) were distilled
from sodium and benzophenone, and were stored under dry nitro-
gen. Chloroform was distilled with calcium hydride prior to use.
Anhydrous alcohols were also stored under dry nitrogen with a
7
2 2
nyl dibromo lead (Ph PbBr , 3.8 mmol) was added. After being stir-
red for 12 h, the reaction mixture was filtered, and the filtered
product was washed with methanol to remove the residual lithium
btsa salt. After the filtrate was reduced to 10 mL, it was stored at
4
Å molecule sieve (baked at 250 °C, 2 h). Lead chloride (PbCl
2
, Dae-
À20 °C overnight, and the crystalline product of Ph
2 3 2 2
Pb{N(SiCH ) }
jung Chemicals), phenyl magnesium bromide (PhMgBr, 3 M in
was obtained in cold hexane (yield: 80%).
i
diethyl ether, Aldrich), titanium isopropoxide (Ti(O Pr)
4
, 97%,
À1
Anal. FT-IR (KBr, cm ): 3064 w, 2951 m, 2895 m, 1572 w, 1475
Aldrich) and zirconium propoxide (Zr(OPr) , 70 wt% in n-propanol,
4
w, 1431 m, 1248 s, 933 vs, 858 vs, 837 vs, 721 s, 669 s, 617 m.
Aldrich) were purchased and used without further purification.
Deuterated benzene and chloroform for NMR measurements were
used as received. The synthesized precursors were characterized
1
H NMR (CDCl
3
, 25 °C, ppm): 7.68 (d, 4H, Ph), 7.47 (t, 4H, Ph),
). CCDC-1582108.
7
.36 (t, 2H, Ph), 0.17 (s, 36H, Si(CH
3
)
3
1
by FT-IR (ABB, FTLA 2000), H NMR (Varian, Mercury 300,
2
.2.4. Preparation of diphenyl di(2,2,6,6-tetramethyl-3,5-
3
00 MHz) and TGA (TA Instrument, SDT Q600). The structures of
heptadiketonate) lead, Ph
2
Pbthd (2)
2
the synthesized compounds were investigated by single-crystal
X-ray crystallography (Bruker, Smart CCD 1000). The fabricated
PZT films were analyzed by XRD (Rigaku, MiniFlex 600) at 40 kV,
5 mA, along with SEM (Jeol JEM-6340F) and AFM (Park Systems,
XE-100) methods.
First, 0.78 mL of 2,2,6,6-tetramethyl-3,5-heptadiketonate (thdH,
.6 mmol) and 0.18 g of sodium hydride (NaH, 7.60 mmol) was
7
added to 100 mL of anhydrous toluene. After the solution was stir-
red for several hours, 2.00 g of diphenyl dibromo lead (Ph PbBr
.8 mmol) was added, and the mixture was stirred for 12 h at room
temperature. The reaction mixture was filtered, the filtrate was
1
2
2
,
3
2
2
.2. Syntheses
stored at À20 °C overnight, and the crystalline product of Ph
2
-
Pbthd
2
was obtained after the filtration (yield: 80%).
À1
.2.1. Preparation of tetraphenyl lead, Ph
4
Pb
Anal. FT-IR (KBr, cm ): 3070 w, 3057 w, 2962 m, 2930 m,
Tetraphenyl lead (Ph
described method with some modifications [15]. 10.0 g of lead
chloride (PbCl , 35.9 mmol) was dispersed in 30 mL of anhydrous
diethyl ether. Then 1.0 equivalent of anhydrous iodo-benzene
I, 4.0 mL, 35.9 mmol.) was injected into the dispersed solu-
4
Pb) was prepared by
a
previously
1564 s, 1496 s, 1435 s, 1387 s, 1356 s, 1219 s, 1182 s, 1124 s,
1018 m, 993 m, 945 m, 868 s, 793 m, 735 s, 687 s, 602 m.
1
2
3
H NMR (CDCl , 25 °C, ppm): 7.61 (d, 4H, Ph), 7.43 (t, 4H, Ph),
7.31 (t, 2H, Ph), 5.52 (s, 2H, COCHCO), 1.08 (s, 18H, CC(CH
3 3
) ).
6 5
(C H
CCDC-1582107.
tion. With vigorous stirring, 12.0 mL of phenyl magnesium bro-
mide solution (3.0 M in diethyl ether, 107.7 mmol.) was slowly
added using a cannula, maintaining inert atmosphere. After addi-
tion of phenyl magnesium bromide, the reaction mixture was
refluxed for 8 h. Excess de-ionized (DI) water was slowly and cau-
tiously added into the stirring solution to remove unreacted phenyl
magnesium bromide. By heating the reaction medium, most of
diethyl ether was removed by evaporation. The gray precipitates
were washed with an excess amount of DI-water and dried in vac-
uum. The obtained powder was re-dissolved in a minimal amount
of anhydrous chloroform, and the resulting solution was filtered.
Clear solution was then stored in À30 °C refrigerator for 3 days
and the crystalline product of tetraphenyl lead was finally obtained
after filtration (yield: 70%).
2.3. Fabrication of a PZT film through the MOCVD process
2
Before deposition of PZT film, TiO layer was prepared on Si
wafer for bottom layer of PZT film. Si wafer was washed with ace-
tone and ethanol to remove organic impurities on the wafer and
i
dried prior to use. 1.0 mL of Ti(O Pr)
4
was used as Ti precursor,
the deposition time was 10 min. The temperatures of canister
and oven were 50 °C and 500 °C, respectively.
For the preparation of the PZT source for the MOCVD process,
1.0 g of Pb precursor 2 was dissolved with 0.5 M equivalent of Ti
i
(O Pr)
4
(0.20 mL) and Zr(OPr)
4
(70 wt% in n-PrOH, 0.30 mL) in
5.0 mL of butyl ether and 0.4 mL of tetraglyme. The canister and
line temperatures were set to 175 and 220 °C, respectively, and
the oven temperature was 550 or 700 °C. To control the flow rate,
10 sccm of argon gas was used, and the operating pressures ranged
from 2 to 15 torr. The reaction time was 1 h, and there was no addi-
tional sintering process. After the MOCVD process, the crystallinity
À1
Anal. FT-IR (KBr, cm ): 3060 m, 3036 m, 1473 s, 1425 s,
1
057 m, 1016 m, 991 s, 725 vs, 694 vs.
1
3
H NMR (CDCl , 25 °C, ppm): 7.53 (m, 4H, Ph), 7.26 (m, 4H, Ph),
7
.24 (m, 2H, Ph).

E.H. Kim et al. / Polyhedron 177 (2020) 114270
3
of the PZT-coated on Si wafer was characterized by an XRD
method, and its surface morphology was characterized by SEM
and AFM methods.
2.4. Crystallographic data collection and refinement of the structure
The crystal data, experimental details and refinement results for
Pb(IV) precursors 1 and 2 are listed in Table 1. Crystals of precur-
sors 1 and 2 were coated with paratone oil, and the diffraction data
Scheme 1. Synthetic routes for the two Pb (IV) precursors 1 and 2.
were measured at 298 K and 173 K, respectively, with Mo Ka radi-
ation on an X-ray diffraction camera system with an imaging plate
equipped with a graphite crystal incident beam monochromator.
RapidAuto software [S1] was used for data collection and data pro-
cessing. The structures were solved by a direct method and refined
by a full-matrix least-squares calculation with the SHELXTL soft-
ware package [S2].
S1. Rapid Auto software, R-Axis series, Cat. No. 9220B101,
Rigaku Corporation.
S2. G. M. Sheldrick, SHELXTL-PLUS, Crystal Structure Analysis
Package; Bruker Analytical X-Ray; Madison, WI, USA, 1997.
bromination of published method [16]. Two new Pb(IV) precursors
were synthesized by the ligand displacement reaction of Ph PbBr
with two equivalents of the monodentate ligand, bis(trimethylsi-
lyl)amide (btsa, N{SiMe ), for precursor 1, and two equivalents
of the bidentate ligand, 2,2,6,6-tetramethyl-3,5-heptadiketonate
thd), for precursor 2, as shown in Scheme 1. The ligand displace-
ment reactions of Ph PbBr proceeded readily at room temperature
2
2
3 2
}
(
2
2
through the use of alkali salts of the corresponding ligands, and the
reactions generated insoluble by-products, alkali metal bromides
(
LiBr or NaBr), in organic solvents. The newly synthesized Pb(IV)
precursors (1 and 2) easily dissolved in most organic solvents. In
contrast, Ph Pb is partially soluble only in CHCl , and Ph PbBr is
3
. Results and discussion
4
3
2
2
insoluble in most organic solvents. Therefore, the degree of ligand
3.1. Synthesis and characterization of Pb(IV) precursors
displacement reaction for Pb(IV) precursors 1 and 2 could be mon-
itored by the disappearance of the insoluble Ph
medium, such as toluene and hexane.
The two Pb(IV) precursors were characterized by FT-IR and H
NMR spectrometry as well as TGA. The FT-IR spectra for both Pb
(IV) precursors exhibited the characteristic absorption peaks at
2
PbBr
2
in an organic
Ph
nard reagent (PhMgBr) and iodobenzene by a previously described
process [15]. Ph PbBr was prepared of Ph Pb via liquid bromine at
mixture bath by a modified
4 2
Pb was obtained from the reaction of PbCl with the Grig-
1
2
2
4
À64 °C in a chloroform/liquid-N
2
Table 1
Crystal data and structure refinement of Pb (IV) precursors: precursors 1 and 2.
Precursor 1
Precursor 2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Α(°)
C
24
H
46
N
2
Si
4
Pb
34 48 4
C H O Pb
682.18
173(2)
0.71073
Monoclinic
P2 /c
13.332(3)
11.743(2)
20.392(4)
90
727.91
298(2)
0.71073
Monoclinic
P2 /c
1
1
17.387(4)
9.6407(19)
21.209(4)
90
b(°)
90.56(3)
90
102.31(3)
90
c
(°)
3
Volume (Å )
Z
Density (calculated)
Abs. coeff. (mm
F(0 0 0)
Crystal size
Theta range for data collection
Index ranges
3192.6(11)
4
3473.4(12)
4
1.419 Mg/m3
5.448
1.392 Mg/m3
4.889
À1
)
1368
1464
3
3
0.30 Â 0.25 Â 0.24 mm
3.05–27.45°
À17 ꢀ h ꢀ 17
À15 ꢀ k ꢀ 15
À26 ꢀ l ꢀ 26
30,018
0.75 Â 0.55 Â 0.24 mm
3.19–27.48°
À22 ꢀ h ꢀ 22
À12 ꢀ k ꢀ 11
À27 ꢀ l ꢀ 27
32,372
Reflections collected
Independent reflections
Completeness to theta = 27.45°
Absorption correction
Max. and min. transmission
Refinement method
7278 [R(int) = 0.0428]
99.80%
Semi-empirical from equivalents
0.3547 and 0.2918
Full-matrix least-squares on F2
7278/0/292
7933 [R(int) = 0.1471]
99.50%
Semi-empirical from equivalents
0.3866 and 0.1206
Full-matrix least-squares on F2
7933/48/396
Data/restraints/parameters
Goodness-of-fit on F2
1.039
1.013
Final R indices [I > 2sigma(I)]
R
1
= 0.0218
wR = 0.0475
= 0.0266
R
1
= 0.0838
wR = 0.1934
= 0.1013
wR = 0.2267
2
2
R indices (all data)
R
1
R
1
wR
2
= 0.0494
2
Extinction coeff.
Largest diff. peak/hole
–
0.0061(7)
3.560/À2.880 e Å
À3
À3
1.166/À0.833 e Å

4
E.H. Kim et al. / Polyhedron 177 (2020) 114270
À1
3
058, 1569, 1474, 1439, 984, 717 and 673 cm from the stretching
vibrations of the two phenyl groups. It was noticeable that the
v(C@C) stretching vibrational peak from the phenyl groups in pre-
À1
cursors 1 and 2 had red-shifted from 1600 cm (in free phenyl
À1
groups) to 1569 cm , due to the weakening of the C@C bond
energy by the electron withdrawal from the Pb(IV) center. For pre-
cursor 1, the absorption peaks of v(C–H) and v(Si–C) stretching
vibrations from the monodentate btsa ligands were observed at
À1
À1
2
951, 2895 cm
and 1248 cm , respectively, and those of
À1
v(Si–N) and v(Si–C) bending vibrations appeared at 933 cm
and 837 cm , respectively. For the bidentate b-ketonate ligands
À1
in precursor 2, the peak from C@O stretching vibrations had
À1
À1
red-shifted from 1604 cm (in free thdH) to 1564 cm , due to
the formation of a six-membered ring between a lead atom and
the b-ketonate ligands.
1
The H NMR spectra of Pb(IV) precursors 1 and 2 exhibited typ-
ical split resonance peaks from phenyl protons, ranging between
7
.3 and 7.7 ppm. For precursor 1, an additional singlet peak was
observed at 0.17 ppm from the two Si(CH units in the btsa
ligand, exhibiting an up-field shift compared to 0.38 ppm for the
Sr(btsa) complex [17]. For precursor 2, two singlet absorption
peaks appeared at 5.52 and 1.08 ppm, corresponding respectively
to protons of C(@O)CHC(@O) and the two C(CH units in the
b-ketonate ligands, also displaying a slight up-field shift from
.72 and 1.17 ppm for thdH. These up-field shifts in the positions
3 3
)
2
3 3
)
5
of the proton peaks for both the btsa and b-ketonate ligands in
the two Pb(IV) precursors might have been induced by shielding
effects, probably due to the electron-donating abilities of phenyl
ligands.
3.2. Single-crystal X-ray structures for Pb(IV) precursors
Single-crystal X-ray diffraction analyses revealed that precur-
sors 1 and 2 both crystallized in the monoclinic space group
P2 /c. The ORTEP diagrams of the two precursors are shown in
1
Fig. 1. As shown in Fig. 1a, precursor 1 existed as a slightly dis-
torted tetrahedral structure, in which the two btsa and two phenyl
ligands were coordinated to the central Pb(IV) atom. As shown in
Table 2, the C–Pb–C bond angle between the two phenyl ligands
around the metal center was 101.80(10)°, while the N–Pb–N bond
angle between the two btsa ligands around the metal center was
Fig. 1. ORTEP diagrams of the two Pb(IV) precursors: (a) precursor 1 and (b)
precursor 2.
1
09.93(8)°. The four C–Pb–N bond angles between the phenyl
and btsa ligands around the metal center were 105.01(9)°,
05.18(9)°, 116.68(9)° and 118.56(9)°. The average distances for
3.3. Conversion to PZT thin films (by the LD-MOCVD process)
1
the two Pb–N and two Pb–C bonds were 2.167(2) and 2.214(3) Å,
Fig. 2 exhibited TGA curves for the two lead precursors 1, Ph
2
Pb
respectively.
(btsa) , and 2, Ph Pb(thd) . The curve pattern for the precursor 1
2
2
2
In precursor 2, the lead atom was coordinated to two bidentate
b-ketonate ligands and two phenyl ligands, forming a slightly dis-
torted octahedral geometry. The C–Pb–C angle between the two
phenyl groups around the Pb atom was 163.9(3)°, deviating from
that of the ideal octahedral structure (180°). In addition, the two
inter O–Pb–O angles between the two b-ketonate ligands on the
equatorial plane of the octahedral geometry also exhibited
severely distorted values of 80.7(2) and 124.3(2)°, while the intra
O–Pb–O angles of the b-ketonate ligands were 77.2(2) and 77.7
shows that the weight decrease starts at 230 °C and finally ends
at below 300 °C. The pattern for precursor 2 looks quite similar
to that of Pb(thd) [8], although the window for the sublimation
2
for precursor 2 appears to be wider than precursor 1—The weight
change occurred from 220 °C to a temperature slightly above
300 °C. The sublimation points of precursors 1 and 2 were found
to be higher than the transition temperatures for other diphenyl
lead (IV) derivatives (Ph
229.5 °C) [20], Ph PbBr (decomposition at 248 °C) [21], and Ph
Pb(OAc) (m.p. 208.5 °C) [22]. The final residues of precursors 1
2 2 4
PbX ); for example, Ph Pb (m.p.
2
2
2
-
(
2)°. The distances of the two Pb–C bonds from the phenyl groups
were 2.134(7) and 2.173(9) Å, similar to previously reported values
18], while those of the four Pb–O bonds from the b-ketonate
2
and 2 were 4.6% and 5.5%, respectively, after the decomposition
of the samples under aerobic conditions during the TGA
measurements.
Precursor 2 was chosen as a precursor for the fabrication of PZT
thin films by the LD-MOCVD process, due to its higher volatility
and better stability than those of precursor 1. Butyl ether was used
as a main solvent for mixing PZT precursors and facilitating liquid
delivery, and a small amount (0.5 mL) of tetraglyme was used as a
thermal stabilizer for its high boiling point (~275 °C). Unlike the
[
ligands were 2.243(3), 2.253(5), 2.422(8) and 2.429(6) Å. The two
different set values of the Pb–O bonds may indicate that the dis-
torted structure was due to the anisotropic bidentate chelation of
the b-ketonate ligands to the metal center. These results differed
from those for Pbthd
were similar: 2.303 Å of Pb(1)–O(1) versus 2.287 Å of Pb(1)–O(2)
19].
2
, where the bond lengths of the four Pb-Os
[

E.H. Kim et al. / Polyhedron 177 (2020) 114270
5
Table 2
Bond lengths (Å) and angles (°) of Pb (IV) precursors 1 and 2.
Precursor 1
Precursor 2
Average bond length (Å)
Average bond angle (°)
Average bond length (Å)
Average bond angle (°)
Pb(1)–N(1) 2.168(2)
Pb(1)–N(2) 2.166(2)
Pb(1)–C(13) 2.219(2)
Pb(1)–C(19) 2.209(3)
C(13)–Pb(1)–C(19) 101.80(10)
C(13)–Pb(1)–N(1) 105.18(9)
C(13)–Pb(1)–N(2) 118.56(9)
C(19)–Pb(1)–N(1) 116.68(9)
C(19)–Pb(1)–N(2) 105.01(9)
N(1)–Pb(1)–N(2) 109.93(8)
Pb(1)–C(1) 2.134(7)
Pb(1)–C(7) 2.173(9)
Pb(1)–O(1) 2.243(6)
Pb(1)–O(2) 2.429(6)
Pb(1)–O(3) 2.422(8)
Pb(1)–O(4) 2.253(5)
C(1)–Pb(1)–C(7) 163.9(3)
C(1)–Pb(1)–O(1) 94.9(3)
C(1)–Pb(1)–O(2) 89.7(2)
C(1)–Pb(1)–O(3) 87.4(3)
C(1)–Pb(1)–O(4) 98.1(2)
C(7)–Pb(1)–O(1) 97.8(3)
C(7)–Pb(1)–O(2) 83.4(3)
C(7)–Pb(1)–O(3) 84.5(3)
C(7)–Pb(1)–O(4) 93.7(3)
O(1)–Pb(1)–O(2) 77.7(2)
O(1)–Pb(1)–O(3) 157.89(19)
O(1)–Pb(1)–O(4) 80.7(2)
O(2)–Pb(1)–O(3) 124.3(2)
O(2)–Pb(1)–O4) 157.7(2)
O(3)–Pb(1)–O(4) 77.2(2)
Fig. 2. TGA curves for the two Pb(IV) precursors: (a) precursor 1 and (b) precursor 2.
2 5 4
case of PZT films fabricated with Pb(C H ) [23,24], an additional
oxygen source such as O or ozone gas was not needed, due to
the availability of the thd ligands as an oxygen source.
2
Fig. 3. XRD spectra of PZT thin films; (a) standard PbZr0.5Ti0.5
(b) prepared at 550 °C and (c) 700 °C.
3
O (JCPDS 14-0031),
We note that the initial decomposition temperature of Pb(thd)
2
on a heated substrate is known to be 350 °C at vacuum) [9]. Con-
peaks, indicating the amorphous nature of a fabricated film at this
temperature.
i
sidering low boiling points of Ti(O Pr)
OPr) (208 °C at 0.1 torr) as well as TGA curve of precursor 2,
the sublimation of three molecular sources-Pb(thd)
OPr) , occurred initially at low temperature range between
00 and 300 °C without any structural decomposition. Then the
4
(232 °C at 1 atm.) and Zr
(
4
Okamura et al. investigated the relationship between the posi-
tion of diffraction peaks in the resulting PZT phases and the ratios
of Ti to Zr [25]. They found that two XRD peaks at 22.0° (1 0 0) and
31.1° (1 0 1) were shifted to lower angles (21.6° and 30.7°) when
the ratio of Ti/Zr decreased from 1 to 0.2, while the direction of
peak shift was reversed to higher angles (22.4° and 31.4°) when
the ratio of Ti/Zr increased from 1 to 5. When the ratio was 1,
two diffraction peaks appeared at 22.0° and 31.1°. Based on this
result, it could be concluded that PZT film obtained in this study
should have the Ti/Zr ratio of 0.8, because two XRD peaks from
(1 0 0) and (1 0 1) were observed at 21.9° and 31.0°. Okamura
et al. also examined the effect of the PT/(PZ + PT) ratio on the type
of PZT phases formed (such as orthorhombic, rhombohedral, and
tetragonal phases). They discovered that the orthorhombic phase
of PZT was first formed, when the PT/(PZ + PT) ratio was between
0 and 0.1. Next, the conversion of the PZT phase to rhombohedral
was observed when the PT/(PZ + PT) ratio was increased from
0.1 to 0.4. Finally, the transformation to tetragonal PZT was
occurred as the PT/(PZ + PT) ratio exceeded to 0.4. Considering
the Ti/Zr ratio of 0.8 (which corresponds to the PT/(PZ + PT) ratio
of 0.4, the phase of PZT film fabricated in this study should be
i
2
4
, Ti(O Pr) , Zr
(
4
2
decomposition of alkoxide ligands in titanium isopropoxides and
zirconium propoxides should be followed, while maintaining the
structural integrity of Ph Pb(thd) . Finally, the assembly to multi-
2 2
metallic cluster species containing lead, titanium, and zirconium
would be taken placed, resulting the formation of PZT phase upon
sintering. Further study will be necessary to elucidate the exact
decomposition process for the deposition of PZT films.
0.5 3
Fig. 3 exhibits the XRD patterns for standard PbZr0.5T O phase
and the PZT films grown at 500 and 700 °C in this study, respec-
tively. As shown in Fig. 3, the X-ray diffraction peaks from a fabri-
cated PZT film prepared at 700 °C were observed at 21.9° (1 0 0),
3
7
1.0° (1 0 1), 39.0° (1 1 1), 45.2° (2 0 0), and 51.2° (2 1 0). At
00 °C, no peak from pyrochlore phase, or unconverted metal
oxide, such as lead, zirconium and titanium oxide, was observed,
indicating the formation of fully developed PZT phase with a
(
1 0 1) preferred orientation. In a meanwhile, the XRD pattern of
a PZT film prepared at 550 °C shows no discernible diffraction

6
E.H. Kim et al. / Polyhedron 177 (2020) 114270
tetragonal with the expected lattice parameters of a = b = 4.061 Å,
c = 4.097 Å, respectively. The calculated tetragonality value (c/a)
was 1.009.
size of PZT thin film prepared at 700 °C was found to be 13.1 nm by
calculating with Debye-Scherrer equation using a 31.0° (1 0 1)
peak in XRD spectrum. In a meanwhile, the RMS roughness value
of the same deposited PZT film was obtained from the result of
AFM image analysis, as shown in Fig. 4. The estimated average
grain size of the deposited PZT film at 700 °C was 22.18 nm. It
appears that there might be no apparent relationship between
the size of primary particles from XRD analysis and grain sizes
obtained from RMS roughness value using AFM analysis. Nishida
et. al investigated the PZT film growth using the RF plasma-
assisted CVD [30]. They found that the RMS roughness values of
PZT film grown at 550 °C increased from 10 nm to 50 nm as the
thickness of deposited film increased from 46 nm to 400 nm. From
the result of SEM image analysis, the average size of secondary par-
ticles, obtained from counting sizes of 20 particles, in deposited
PZT film at 700 °C was 195(±46) nm. Veith et. al reported that they
observed SEM images of PZT thin films with a smooth surface with
grain sizes between 100 nm and 500 nm, after heat treatment at
700 °C [27]. It seems that the agglomeration of several primary
particles to the secondary particles occurred in a deposited film.
It has been known that the orientation of deposited PZT films
could be controlled by growth and crystallization conditions such
as chemical composition of substrates and crystal lattice matching
between PZT phase and substrate. Dormans et al. reported the rela-
tionship between the chemical composition of substrate and the
preferred orientation of resulting PZT film deposited using MOCVD
method [26]. They investigated the deposition of PZT thin films
using triethyllead tert-butoxide and titanium and zirconium
tetra-butoxides as precursors and observed that ferroelectric PZT
films with good crystallinity of preferred (0 0 1) orientation were
obtained at 700 °C, when a (1 1 1)-Pt coated oxidized (1 1 0) Si
was used as a substrate. Veith’s group also reported that (1 0 0)-
oriented PZT thin films were produced using a classical thermal
CVD process [27]. They employed quasi single-source precursor
solutions, prepared by mixing two mixed bimetallic species of
Pb-Ti and Pb-Zr complexes. After chemical vapor deposition on
2
Pt-Ti-SiO -Si substrates and subsequent thermal treatment, trans-
parent PZT thin films exhibiting (1 0 0)-oriented perovskite phase
were obtained. In the other hand, Zhou et. al obtained PZT films
4
. Conclusion
with different preferred orientation on a TiO
sol-gel process [28]. Highly preferred (1 0 1) orientation PZT thin
film was produced on the well crystallized TiO insulator layer
which was deposited by MOCVD method. Bai et al. fabricated
thin films on (1 0 1) and (1 1 0) RuO /SiO /Si
0 0 1) substrate respectively by MOCVD, and noticed that pre-
2
-Si substrate using
We successfully synthesized a new series of lead (IV) precursors
2
for the fabrication of ferroelectric Pb(Zr,Ti)O
MOCVD. Ph Pb is a thermally stable and volatile precursor, but
due to the absence of an oxygen source, Ph Pb is not fully con-
verted to PbO, inhibiting the fabrication of a PZT film of conformal
composition. Therefore, two of its phenyl ligands were substituted
with monodentate bis(trimethylsilyl)amide or bidentate 2,2,6,6-
tetramethyl-3,5-heptadiketonate for the synthesis of Pb(IV) pre-
cursors 1 and 2. These precursors were purified by recrystallization
3
thin films by LD-
4
PbZr0.5Ti0.5
O
3
2
2
4
(
ferred orientation of fabricated PZT films were decided from unit
cell matching between PZT layer and substrate, even though the
chemical compositions of substrates are identical [29]. When a
PZT thin film layer was grown on a (1 0 1)-textured RuO bottom
2
electrode layer, PZT phase with predominant (0 0 1) orientation
was observed, while a PZT layer deposited on (1 1 0)-textured
1
in an anhydrous solvent, and were characterized by FT-IR, H NMR
and TGA. In the TGA, precursors 1 and 2 evaporated at 272 °C and
2
RuO presented a randomly oriented PZT phase with a mixed
2
62 °C, respectively, under atmospheric pressure without any ther-
(
0 0 1)-(1 1 1)-(1 1 0) polycrystalline structure. Based on the above
mal decomposition, demonstrating their applicability as precur-
sors for MOCVD. The monomeric structures of precursors 1 and 2
were defined by single-crystal X-ray crystallography. Ph Pbthd
2 2
was chosen as a precursor for the fabrication of PZT thin films,
due to its higher volatility and better stability. The perovskite
PZT thin films were successfully deposited on a silicon wafer at
results, it should be reasonable that highly (1 0 1)-oriented direc-
tional growth was observed on fabricated PZT thin films on a
TiO–Si substrate used in this study.
The surface morphologies of PZT thin films prepared at 550 or
7
00 °C were characterized by SEM and AFM methods. As can be
seen in Fig. 4, the PZT thin film prepared at 550 °C consisted of
large numbers of small grains, coinciding with the amorphous fea-
ture shown in the XRD pattern. On the other hand, the crystalline
7
00 °C by the LD-MOCVD process. XRD, SEM and AFM methods
were used to characterize the crystallinity and surface morphology
of the PZT thin films.
5
. Contributions of author
First author and corresponding author are mainly contributed
and all other authors are contributed partially for preparing this
manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This research was supported by the Converging Research Center
Program through the Ministry of Science, ICT and Future Planning,
Korea (2013K000156), and by the Technology Innovation Program
(
2 4
10052730, Rutile TiO Powder Manufacturing Technology by TiCl
Oxidation) funded by the Ministry of Trade, Industry & Energy (MI,
Korea).
Fig. 4. SEM (bar = 1
and (b) 700 °C.
lm) and AFM images of PZT thin films prepared at (a) 550 °C

E.H. Kim et al. / Polyhedron 177 (2020) 114270
7
[
12] M.A. Cameron, S.M. George, Thin Solid Films 348 (1999) 90, https://doi.org/
0.1016/S0040-6090(99)00022-X.
Appendix A. Supplementary data
1
[
13] R.J. Puddephatt, G.H. Thistlethwaite, J. C. S. Dalton Trans. (1972) 570, https://
doi.org/10.1039/DT9720000570.
CCDC 1582107 and 1582108 contain the supplementary crys-
tallographic data for precursors 1 and 2. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/re-
trieving.html, or from the Cambridge Crystallographic Data Centre,
[14] U. Tuncel, W.J. Clerici, R.O. Jones, Hear. Res. 166 (2002) 113, https://doi.org/
0.1016/S0378-5955(02)00303-9.
[
1
15] H. Gilman, L. Summers, R.W. Leeper, J. Org. Chem. 17 (1952) 630, https://doi.
org/10.1021/jo01138a018.
1
2 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033;
[16] G. Grüttner, Ber. Dtsch. Chem. Ges. 51 (1918) 1298, https://doi.org/10.1002/
cber.19180510216.
17] S.M. George, B.K. Park, C.G. Kim, T.M. Chung, Eur. J. Inorg. Chem. 2014 (2014)
or e-mail: deposit@ccdc.cam.ac.uk.
[
[
[
[
[
[
[
[
[
[
[
[
2
002, https://doi.org/10.1002/ejic.201301296.
18] C. Schneider, M. Drager, J. Organomet. Chem. 415 (1991) 349, https://doi.org/
10.1016/0022-328X(91)80135-7.
19] M.A. Malik, P. O’Brien, M. Motevalli, A.C. Jones, T. Leedham, Polyhedron 18
References
(
1999) 1641, https://doi.org/10.1016/S0277-5387(99)00043-1.
[
[
[
[
1] D.W. Richerson, Modern Ceramic Engineering Properties, Processing, and Use
in Design, second ed., CRC Press, 1992.
2] Z. Ou, C. Lu, A. Yang, H. Zhou, Z. Cao, R. Zhu, Sens. Actuators, A 290 (2019) 8,
https://doi.org/10.1016/j.sna.2019.03.008.
3] Y. Wang, H. Cheng, J. Yan, N. Chen, P. Yang, F. Yang, J. Ouyang, Materialia 5
20] E.C. Juenge, D. Seyferth, J. Org. Chem. 26 (1961) 563, https://doi.org/10.1021/
jo01061a067.
21] S.N. Bhattacharya, A.K. Saxena, J. Indian, Chem. Sect. A: Inorg. Phys. Theor.
Anal. 17A (1979) 307.
22] A.W. Krebs, M.C. Henry, J. Org. Chem. 28 (1963) 1911, https://doi.org/10.1021/
jo01042a513.
23] Y. Sakashita, H. Segawa, K. Tominaga, M. Okada, J. Appl. Phys. 73 (1993) 7857,
https://doi.org/10.1063/1.353936.
(
2019), https://doi.org/10.1016/j.mtla.2019.100228 100228.
4] D.M. Kim, C.B. Eom, V. Nagarajan, J. Ouyang, R. Ramesh, V. Vaithyanathan, D.G.
Schlom, Appl. Phys. Lett. 88 (2006), https://doi.org/10.1063/1.2185614
1
42904.
5] L. Jian, A.S. Kumar, C.S.C. Lekha, S. Vivek, I. Salvado, A.L. Kholkin, S.S. Nair,
Nano-Struct. Nano-Objects 18 (2019), https://doi.org/10.1016/
j.nanoso.2019.100272 100272.
24] C.Y. Pan, Y.L. Chen, D.S. Tsai, Mater. Res. Soc. 17 (2002) 1536, https://doi.org/
[
1
0.1557/JMR.2002.0228.
25] S. Okamura, R. Takeuchi, T. Shiosaki, Jpn. J. Appl. Phys. 41 (2002) 6714, https://
doi.org/10.1143/JJAP.41.6714.
26] G.J.M. Dormans, M. de Keijser, P.J. van Veldhoven, Chem. Mater. 5 (1993) 448,
https://doi.org/10.1021/cm00028a009.
[
[
[
[
6] C.C. Leu, C.P. Hsu, C.G. Hong, C.T. Hu, J. Mater. Chem. 21 (2011) 12991, https://
doi.org/10.1039/c1jm11464f.
7] Y.D. Maio, J.P. Colombier, P. Cazottes, E. Audouard, Opt. Lasers Eng. 50 (2012)
27] M. Veith, M. Bender, T. Lehnert, M. Zimmer, A. Jakob, Dalton Trans. 40 (2011)
1
582, doi: j.optlaseng.2012.05.022.
1
175, https://doi.org/10.1039/C0DT00830C.
8] J.S. Zhao, D.Y. Park, M.J. Seo, C.S. Hwang, Y.K. Han, C.H. Yang, K.Y. Oh, J.
Electrochem. Soc. 151 (2004) C283, https://doi.org/10.1149/1.1676711.
9] D.H. Kim, W.Y. Yang, S.W. Rhee, J. Electrochem. Soc. 150 (2003) C516, https://
doi.org/10.1149/1.1584441.
28] C. Zhou, P. Peng, Y. Yang, T. Ren, in: 2011 6th IEEE International Conference on
Nano/Micro Engineered and Molecular Systems, 2011, p. 134. doi: 10.1109/
NEMS.2011.6017313.
[
[
29] G.R. Bai, A. Wang, I.F. Tsu, C.M. Foster, O. Auciello, Integr. Ferroelectr. 21 (1998)
[
10] D. Xie, R. Li, X. Han, Y. Ruan, T.L. Ren, L.T. Liu, Integr. Ferroelectr. 100 (2008)
14, https://doi.org/10.1080/10584580802540538.
11] M.V. Baryshnikova, L.A. Filatov, A.S. Petrov, S.E. Alexandrov, Chem. Vap. Dep.
1 (2015) 327, https://doi.org/10.1002/cvde.201507187.
2
91, https://doi.org/10.1080/10584589808202071.
1
30] K. Nishida, K. Shirakata, M. Osada, T. Katoda, J. Cryst. Growth 272 (2004) 789,
https://doi.org/10.1016/j.jcrysgro.2004.08.084.
[
2