Multifunctional cadmium single source precursor for the selective deposition of CdO or CdS by a solution route

Article abstract of DOI:10.1039/b509623e

We report on the interesting properties of a novel single precursor, Cd(tta)₂·tmeda (Htta = 2-thenoyl-trifluoroacetone, tmeda = N,N,N′,N′-tetramethylethylenediamine), ideally suited for the selective and reproducible fabrication of pure quality films of CdS or CdO through a simple solution process. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b509623e

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COMMUNICATION
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Multifunctional cadmium single source precursor for the selective
deposition of CdO or CdS by a solution route{
a
Graziella Malandrino,* Sebastiana T. Finocchiaro, Patrizia Rossi, Paolo Dapporto and
a
b
b
a
Ignazio L. Fragala`
Received (in Cambridge, UK) 7th July 2005, Accepted 21st September 2005
First published as an Advance Article on the web 19th October 2005
DOI: 10.1039/b509623e
We report on the interesting properties of a novel single
precursor, Cd(tta) ?tmeda (Htta 5 2-thenoyl-trifluoroacetone,
and CdO films by an accurate tuning of processing parameters.
The synthesis has been successfully attempted through a one pot
reaction without any harmful handling of noxious and hazardous
reagents. In particular, the vigorous shaking of a water solution
2
tmeda 5 N,N,N9,N9-tetramethylethylenediamine), ideally suited
for the selective and reproducible fabrication of pure quality
films of CdS or CdO through a simple solution process.
3 2 2 2
containing Cd(CH COO) and of a CH Cl solution containing
the Htta and tmeda (2 : 1) ligands causes the formation of the
required precursor. The adduct is then extracted into the CH Cl
Cadmium-containing films have attracted continuously growing
attention due to commercial applications in various strategic fields.
2
2
solution and isolated as yellow crystalline powder by evaporation
1
In particular, cadmium sulfide (CdS) thin films are used as a wide
of the solvent.{
1
13
band-gap (2.42 eV) window layer in photovoltaic solar cells,
2
providing one half of the p–n junction. Cadmium oxide (CdO) is
It has been characterised by H, C NMR, mass spectrometry,
IR spectroscopy, elemental analysis and single crystal X-ray
diffraction analysis. The single crystal X-ray diffraction§ showed
a transparent conductive oxide (TCO) material, and due to its high
conductivity, high transmission and low band-gap, represents one
of the most promising and appealing candidates for optoelectronic
devices such as light emitting diodes, solar cells, flat panel displays,
2+
(Fig. 1) that in the Cd(tta) ?tmeda complex (1), the Cd cation is
2
hexacoordinated by the four oxygen atoms of the two tta anions
and by the two nitrogen atoms of one tmeda ancillary ligand. The
resulting coordination geometry is a distorted octahedron. Cd–O
and Cd–N distances are in agreement with those found
photovoltaics and anti-reflection coatings. Cadmium fluoride
3
(
CdF
2
)
based thin films, doped and properly annealed, are
1
3
important materials for optics and optoelectronics as light emitters
and lasers due to their well pronounced semiconducting properties.
Several techniques have been used to produce Cd-containing
(
Cambridge Structural Database (CSD) v. 5.25) for cadmium
acetylacetonate derivatives or complexes having a tmeda
molecule as ligand. It is worth noting that the distances Cd1–O1
and Cd1–O2 are significantly longer than the Cd1–O3 and
Cd1–O4 ones. The planes defined by the tta ligands, namely the
planes defined by the atoms (O1, C1, C2, C3, O2) and (O3, C6,
C7, C8, O4) form an angle of 59.7(3)u. The plane (N1, C19, C20,
4
,5
materials in thin film form, such as molecular beam epitaxy,
6
pulsed laser deposition, MOCVD,
7–10
and chemical bath deposi-
11
tion. Solution routes are very appealing in view of their ease, low
cost and quick industrial scale-up. Nevertheless, cadmium alkyls,
12
in some cases used as precursors, are well known for being highly
toxic and hazardous sources, not only because of their heavy metal
content, but also because of their high volatility and pyrophoricity
that requires cautious manipulation and precautions. In addition,
the synthesis of CdS is even more complex since it also requires a
sulfide source, usually thioacetamide or thiourea. All these issues
fuel the stringent demand for the development of novel versatile
and ‘‘safer’’ single source precursors.
In this communication, we report on the synthesis and
characterisation of a novel multipurpose cadmium single source
precursor for the deposition of Cd-containing films through
2
solution processes. Thus, the novel Cd(tta) ?tmeda (Htta 5 2-
thenoyl-trifluoroacetone, tmeda 5 N,N,N9,N9-tetramethylethylen-
diamine) adduct (1) has been synthesized and both reproducibly
and selectively applied to the preparation of nanostructured CdS
2
Fig. 1 ORTEP drawing of the mononuclear complex Cd(tta) ?tmeda (1).
˚
Selected distances (A) and angles (u): Cd1–O1 2.284(3), Cd1–O2 2.281(3),
Cd(1)–O(3) 2.255(3), Cd(1)–O(4) 2.258(3), Cd(1)–N(1) 2.393(4), Cd(1)–
N(2) 2.401(4), O(1)–Cd(1)–O(2) 79.6(1), O(1)–Cd(1)–O(3) 97.2(1), O(1)–
Cd(1)–O(4) 167.8(1), O(1)–Cd(1)–N(1) 94.3(1), O(1)–Cd(1)–N(2) 98.0(1),
O(2)–Cd(1)–O(3) 103.6(1), O(2)–Cd(1)–O(4) 89.0(1), O(2)–Cd(1)–N(1)
a
Dipartimento di Scienze Chimiche, Universit a` di Catania and INSTM,
UdR Catania, V.le Andrea Doria 6, I-95125, Catania, Italy.
E-mail: gmalandrino@dipchi.unict.it
Dipartimento di Energetica ‘‘S. Stecco’’, Universit a` di Firenze, Via
b
Santa Marta 3, I-50139, Firenze, Italy
165.5(2), O(2)–Cd(1)–N(2) 90.4(2), O(3)–Cd(1)–O(4) 81.1(1), O(3)–
{
Electronic supplementary information (ESI) available: Characterization
details of the complex and EDX spectra of Cd-containing films. See DOI:
0.1039/b509623e
Cd(1)–N(1) 90.2(2), O(3)–Cd(1)–N(2) 160.9(2), O(4)–Cd(1)–N(1) 97.9(1),
O(4)–Cd(1)–N(2) 86.4(1), N(1)–Cd(1)–N(2) 77.4(2).
1
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N2), containing the tmeda ligand, is almost orthogonal in respect
to the two previously mentioned mean planes [87.2(3) and 88.0(3)u
for (O1, C1, C2, C3, O2) and (O3, C6, C7, C8, O4), respectively].
The C–C distances found in the two tta anions, as well as the
values retrieved for the dihedral angles O2–C3–C5–C11 and
O4–C8–C10–C14 (2179.8(4) and 178.5(5)u, respectively), point to
an extended conjugation in the ligand anions involving the
thiophene moieties. As expected (both the nitrogen atoms
coordinate the metal cation), the tmeda molecule shows a gauche
conformation (N1–C19–C20–N2 5 260.7(8)u). The rather high
thermal factors of some atoms of the thiophene rings could be
indicative of a degree of disorder which cannot be modelled.
Finally, no intermolecular contacts are present in the crystal lattice.
2
The Cd(tta) ?tmeda precursor, due to its clean, single step
thermal decomposition and solubility in organic solvents such as
dichloromethane, has been successfully applied to the deposition of
Cd-containing films on glass or Si substrates using solution
processes through spin coating in air and post-thermal treatment.
No decomposition of the complex occurs at this stage. A 0.3 M
solution of Cd(tta) ?tmeda precursor in CH Cl has been used for
2 2 2
all the deposition experiments.
Thermal treatments have been carried out at atmospheric
pressure in the 300–700 uC temperature range under a controlled
atmosphere. The reaction atmosphere has been tuned using
nitrogen and oxygen to investigate the precursor decomposition
under different processing environments. It is quite interesting that
14
different films, namely CdO, CdS and CdF , have been
2
A similar complex, Co(tta)
2
?tmeda, was retrieved from the CSD.
?tmeda
reproducibly and selectively, in the case of CdO and CdS phases,
obtained, depending on the processing conditions used to pyrolyse
the adduct.
The main difference between the Cd(II) and Co(II) M(tta)
2
complexes is due to the disposition of the thiophene rings. In fact,
if we consider the plane defined by the cadmium cation and the
two CH atoms of the tta anions, the two thiophene rings lie on the
same side with respect to it, giving rise to a cis conformation. On
the other hand, in the cobalt complex they lie on opposite sides,
giving rise to a trans conformation.
The nature of the obtained films has been investigated by X-ray
diffraction (XRD) analysis in a grazing incidence mode. The CdO
phase has been observed right across the investigated temperature
range (300–700 uC) when the thermal treatments have been carried
out under O
oxygen (100 sccm) at 600 uC shows three peaks 33.02, 38.35 and
5.30u, corresponding respectively to 111, 200 and 220 reflections
of the CdO cubic lattice. The sample is polycrystalline and
randomly oriented, as indicated by the relative intensities of the
XRD reflections, which agree fairly well with those reported for a
2
. The XRD pattern (Fig. 3a) of a sample heated in
The thermal behaviour of the adduct, investigated through
atmospheric pressure thermogravimetric (TG) analyses carried out
5
under N , indicates a single step weight loss in the 200–360 uC
2
temperature range, as confirmed by the first TG derivative (DTG),
with a 22.40% residue remaining (Fig. 2). The chemical nature of
this residue cannot be easily attributed since three different phases
can form upon the precursor’s thermal decomposition: cadmium
fluoride, cadmium sulfide or cadmium oxide. In addition, even
2
though the theoretical residue value for CdF (22.41%) could point
to a CdF residue, the other phases, CdS and CdO, cannot be
2
unequivocally ruled out considering their similar theoretical residue
values of 21.53% and 19.14%, respectively. Nevertheless, the
energy dispersive X-ray analyses (EDX) of the residue unequi-
2
vocally points to it being CdF .
Fig. 3 XRD patterns and corresponding SEM images of films obtained
under different processing conditions: (a) CdO obtained at 600 uC under
O
2
, (b) CdF
2
obtained at 350 uC under N and (c) CdS obtained at 600 uC
2
Fig. 2 TG–DTG curve of Cd(tta) ?tmeda (1).
2
under N
2
.
5
682 | Chem. Commun., 2005, 5681–5683
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polycrystalline CdO powder (ICDD no. 05-0640; Powder
Diffraction File, International Centre of Diffraction Data,
Newton Square, PA).
presence of O (Ka at 0.525 keV) and S (Ka at 2.307 keV),
respectively. No peaks relative to other light elements have been
detected, thus confirming for CdO and CdS their monophasic
nature, assessed through XRD measurements, and the absence of
any C contamination.
Thermal treatments under nitrogen (100 sccm) result in two
different phases, depending on the reaction temperature. In
particular, the XRD pattern of films heated at 350–450 uC
In summary, the novel adduct Cd(tta) ?tmeda represents, to our
2
(Fig. 3b) show several peaks at 2h 5 28.75, 33.30, 47.75 and
knowledge, the first example of a multi-functional single source
5
6.70u, corresponding to the 111, 200, 220 and 311 reflections
precursor which reproducibly and selectively yields CdO or CdS
films, depending on the processing parameters. Its thermal
properties point to its potential application as a single precursor
for the synthesis of Cd-containing nanoparticles through a
respectively of the CdF2 cubic lattice, in accordance with the
nature of the residue found in the TG analysis. Films are also
polycrystalline and randomly oriented, in accordance with data
reported in the ICDD database (ICDD no. 23-0864).
15
surfactant-based template approach, and studies are currently
in progress. In addition, the easy synthetic procedure and simple
deposition process make the new cadmium adduct an attractive
candidate, not only for laboratory solution processes, but also for
large scale applications.
Thermal treatments under nitrogen at temperatures higher than
550 uC give rise to a different XRD pattern, with several peaks that
point to the formation of a new Cd-containing phase, namely CdS.
In fact, all the peaks (Fig. 3c) observed at 2h 5 24.85, 26.55, 28.20,
3
6.75, 43.85, 47.90, 51.90 and 52.80u can be associated with
The authors thank the MIUR for the financial support. Dr. G.
G. Condorelli is acknowledged for FT-IR measurements. CRIST
(Centro Interdipartimentale di Cristallografia Strutturale),
University of Florence is also gratefully acknowledged.
the reflections of the hexagonal CdS lattice (ICDD no. 41-1049).
The presence of all the expected reflections also points to the
polycrystalline nature of the CdS films. Actually, the low intensity
of these peaks, together with the broad peak centered around 20u,
indicate that the CdS films are of a poorly crystalline nature.
Notes and references
The change from CdF
in the 450–550 uC range under N
films.
2
to CdS is gradual, and at temperatures
{
Elemental analysis of 1. Calc. for C22
N, 4.18; S, 9.56. Found: C, 39.15; H, 3.76; N, 4.25; S, 9.35%.
Crystallographic data for 1: C22 24CdF , M 5 670.95, mono-
clinic, space group P2 /c, a 5 17.582(5), b 5 8.335(2), c 5 18.610(7) A˚ ,
6 2 4 2
H24CdF N O S : C, 39.38; H, 3.61;
2
, both phases are present in the
§
H
6 2 4 2
N O S
The SEM images (Fig. 3) show completely different morpho-
logies in accordance with the XRD data. Larger rounded grains
1
3
23
˚
b 5 92.61(3)u, V 5 2724.4(14) A , T 5 293 K, Z 5 4, D
c
5 1.636 g cm , m
Mo-Ka) 5 1.026 mm , 4804 reflections collected, 4640 unique
21
(
and square shaped grains are observed for the CdO and CdF
2
2
int 5 0.0327). F refinement, R1 5 0.0510 (I . 2s(I)), wR2 5 0.1467
(R
films, respectively, while a nanostructured surface with grain
dimensions less than 100 nm in diameter is observed for CdS films.
To better understand the decomposition mechanism under a N2
atmosphere, the precursor decomposition process has been
investigated through gas phase FT-IR (Fourier Transform-
Infrared) analysis using nitrogen as the reaction gas. IR spectra
show that precursor decomposition starts at 250 uC and is
(all data). CCDC 277427. For crystallographic data in CIF or other
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2
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2
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2
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2
2
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1
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1
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1
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2
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Chem. Commun., 2005, 5681–5683 | 5683