Synthesis, X-ray Single-Crystal Structural Characterization, and Thermal Analysis of Bis(O-alkylxanthato)Cd(II) and Bis(O-alkylxanthato)Zn(II) Complexes Used as Precursors for Cadmium and Zinc Sulfide Thin Films

Article abstract of DOI:10.1021/acs.inorgchem.1c01110

This work investigates tuning of the molecular structure of a series of O-alkylxanthato zinc and cadmium precursor complexes to enhance production of ZnS and CdS materials. The structures of several bis(O-alkylxanthato) cadmium(II) complexes (8-13) and bis(O-alkyl xanthato)zinc(II) complexes (18 and 19) are reported based on single crystal X-ray diffraction data. CdS and ZnS films were produced by the spin-coating of these metal complexes followed by their thermal decomposition to the corresponding metal sulfides. Thin films of CdS were deposited by spin-coating the bis(O-alkylxanthato) cadmium(II) precursors (7-13) on glass substrates, followed by annealing at 300 °C for 60 min. Thin films of ZnS were deposited by spin-coating bis(O-alkylxanthato) zinc(II) (14-20), followed by annealing at 200 °C for 60 min. The molecular complexes and solid state materials are characterized using a range of techniques including single-crystal X-ray diffraction, pXRD, EDS and XPS, DSC and TGA, UV-vis and PL spectroscopies, and electron microscopy. These techniques provided information on the influence of alkyl chain length on the thermal conditions required to fabricate metal sulfide films as well as film properties such as film quality, and morphology. For example, the obtained crystallite size of metal sulfide films formed is correlated to the hydrocarbon chain length of xanthate ligands in the precursor. The behavior of the complexes under thermal stress was therefore studied in detail. DTA and TGA profiles explain the relationship between hydrocarbon chain length, decomposition temperatures, and the energies required for decomposition. A higher decomposition temperature for complexes with longer hydrocarbon chains is observed compared to complexes with shorter hydrocarbon chains. Band-gap energies calculated from the optical absorption spectra alongside steady state and time-resolved photoluminescence studies are reported for CdS films.

Full text of DOI:10.1021/acs.inorgchem.1c01110

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Synthesis, X‑ray Single-Crystal Structural Characterization, and
Thermal Analysis of Bis(O-alkylxanthato)Cd(II) and Bis(O-
alkylxanthato)Zn(II) Complexes Used as Precursors for Cadmium
and Zinc Sulfide Thin Films
Ali A.K. Bakly, David Collison, Ruben Ahumada-Lazo, David J. Binks, Matthew Smith, James Raftery,
George F. S. Whitehead, Paul O’Brien, and David J. Lewis*
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ABSTRACT: This work investigates tuning of the molecular structure of a
series of O-alkylxanthato zinc and cadmium precursor complexes to enhance
production of ZnS and CdS materials. The structures of several bis(O-
alkylxanthato) cadmium(II) complexes (8−13) and bis(O-alkyl xanthato)zinc-
(II) complexes (18 and 19) are reported based on single crystal X-ray diffraction
data. CdS and ZnS films were produced by the spin-coating of these metal
complexes followed by their thermal decomposition to the corresponding metal
sulfides. Thin films of CdS were deposited by spin-coating the bis(O-
alkylxanthato) cadmium(II) precursors (7−13) on glass substrates, followed
by annealing at 300 °C for 60 min. Thin films of ZnS were deposited by spin-
coating bis(O-alkylxanthato) zinc(II) (14−20), followed by annealing at 200 °C
for 60 min. The molecular complexes and solid state materials are characterized
using a range of techniques including single-crystal X-ray diffraction, pXRD,
EDS and XPS, DSC and TGA, UV−vis and PL spectroscopies, and electron microscopy. These techniques provided information on
the influence of alkyl chain length on the thermal conditions required to fabricate metal sulfide films as well as film properties such as
film quality, and morphology. For example, the obtained crystallite size of metal sulfide films formed is correlated to the hydrocarbon
chain length of xanthate ligands in the precursor. The behavior of the complexes under thermal stress was therefore studied in detail.
DTA and TGA profiles explain the relationship between hydrocarbon chain length, decomposition temperatures, and the energies
required for decomposition. A higher decomposition temperature for complexes with longer hydrocarbon chains is observed
compared to complexes with shorter hydrocarbon chains. Band-gap energies calculated from the optical absorption spectra alongside
steady state and time-resolved photoluminescence studies are reported for CdS films.
along the c-axis.10 Moore et al., showed that phase trans-
INTRODUCTION
■
formation between zinc blende and wurtzite occurs through
Alkylxanthato cadmium and zinc complexes have been used as
changes in stacking sequence.11
single source molecular precursors toward thin films of
Precursors of Cd2+ and Zn2+ with xanthate ligands are
cadmium sulfide (CdS) and zinc sulfide (ZnS), respectively.1−7
typified by the formation of weak polymeric structures in
CdS is a II−VI compound semiconductor with a wide direct
solution. A monoclinic crystal structure of bis(methoxyethyl-
band gap (2.4 eV), making it useful in a variety of electronic
xanthato)cadmium, Cd(S2COCH2CH2OCH3)2, which is a
and photovoltaic applications.8 CdS crystallizes in both
metal xanthate precursor toward CdS materials formation, for
hexagonal (wurtzite) and cubic (zinc blende) polymorphs.
example, has been determined by Abrahams et al.12 Such
Ramasamy et al.,2 have reported that CdS film growth depends
complexes have attractive features by which to act as
on the precursors, deposition methods, and synthetic
precursors for thin film deposition of metal sulfides including
conditions employed. The successful synthesis of face-centered
cubic CdS nanoparticles was reported by Wang and coauthors9
Received: April 10, 2021
Published: May 5, 2021
through a synthetic process of liquid−solid-solution phase
transfer and separation. ZnS is another II−VI semiconductor
with an even wider direct band gap of 3.7 eV.1 ZnS exists as
hexagonal wurtzite or cubic zinc blende polymprphs. Ma et al.,
described wurtzite ZnS as being composed of alternating
planes of tetrahedrally coordinated S2− and Zn2+ ions, stacked
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a preformed network of metal−sulfur bonds that enables low
temperature decomposition to the corresponding metal
sulfides, which a is relatively clean process due to the
generation of gaseous side products. Bakly et al.,1 demon-
strated a new way to produce CdS, ZnS, and CdZnS thin films
by an inexpensive synthetic technique based on spin-coating
followed by annealing. Thomas et al.,13 reported high quality
nanocrystalline thin films of CdS, ZnS, CuS, and PbS
nanoparticles from thiobiuret complexes [M(SON-
(CNiPr2)2)2] as a single source precursor. Lewis et al.,14
reviewed the synthesis of metal chalcogenide materials using
‘reactive melts’ of precursors and concluded that self-capping
reactions are exceedingly simple and potentially scalable for
nanoparticle synthesis. Afzaal et al.,15 reviewed synthetic
methods for the production of binary metal chalcogenides
from single molecule precursors. Ramasamy et al.,16 reviewed
developments in synthetic routes to metal sulfide materials that
have applications in solar cells: emerging materials include
bismuth sulfide, ternary materials such as copper indium
sulfide, and quaternary materials such as copper zinc tin sulfide
all provide promising sustainable alternatives for solar energy
generation via the photovoltaic effect.
Metal chalcogenide compounds with thiolate ligands may
possess polymeric structures in solution and the solid state but
are often involatile. The lack of volatility in general means
these chalcogenide compounds are not useful as precursors for
thin film deposition techniques such as LP-MOCVD.15,17
However, these single source molecular precursors of precisely
defined composition can potentially provide a high degree of
control during synthesis. Indeed, many studies have demon-
strated the growth of binary and ternary nanostructures of
transition metal chalcogenides as thin films, nanocrystals,
nanosheets, nanoplates, nanowires, nanoribbons, and others
for energy applications.14,18−20
displays a schematic process of formation of inorganic films of
ZnS and CdS by thermal decomposition of the corresponding
zinc and cadmium alkylxanthato precursors. The films were
prepared by the spin-coating of the metal xanthate complexes
followed by annealing at relatively low temperatures in a
nitrogen atmosphere. The study we present demonstrates that
the structure of the ligand is the predominant factor in
controlling the thermal decomposition of metal xanthate
complexes.
EXPERIMENTAL SECTION
■
n
Solvents and Reagents. Potassium hydroxide (≥85%), propa-
nol (≥99%), carbon disulfide (≥98.99%), petroleum ether
(≥98.99%), hexane (≥97%), acetone (≥99%), nbutanol (≥99%),
npentanol (≥99%), nhexanol (≥98%), nheptanol (≥98%), noctanol
(≥99%), potassium ethylxanthate (≥98%), cadmium chloride
(≥99%), zinc chloride (≥97%), acetonitrile (≥99.99%), methanol
(≥99.8%), ethyl acetate (≥99.5%), tetrahydrofuran (≥99.99%), and
N-methyl-2-pyrrolidone (≥99. %) were purchased from Sigma-
Aldrich or Fisher and used without further purification. Deuterated
solvents for NMR were purchased from Sigma-Aldrich.
Materials Characterization. Elemental analysis was conducted in
the microanalytical laboratory at the University of Manchester with a
Carlo Erba EA 1108/Flash 2000 Thermo Scientific elemental
analyzer. Thermogravimetric Analysis (TGA) and Differential
Scanning Calorimetry (DSC) profiles were recorded from 30 to
600 °C with a 10 °C min−1 heating rate under N2, using a Mettler
Toledo TGA-DSC1. Fourier transform infrared (FTIR) spectra were
obtained using a Nicolet iS5 IR Spectrometer-Thermo Fisher
Scientific; SI, Figures S21 and S22. Nuclear magnetic resonance
(NMR) spectra were recorded using a 400 MHz Bruker instrument;
see Figures S1−S20. Melting point measurements were carried out in
a capillary tube using a Stuart SMP10 melting point apparatus.
Powder X-ray diffraction measurements were carried out at room
temperature by using a Bruker D8 Advance diffractometer, using Cu−
Kα radiation, (1.5418 Å), 40 kV, 40 mA. The scanning range was
between 20° and 80° with a step size of 0.050° and a dwell time of 8 s.
The diffraction patterns were processed using X’Pert High Score Plus
software. High resolution transmission electron microscopy
(HRTEM) images and energy dispersive X-ray spectra were obtained
using an FEI Talos F200A microscope equipped with X-FEG electron
source and Super-X SDD EDS detectors. The experiment was
performed using an acceleration voltage of 200 kV and a beam current
of approximately 1 nA. TEM images were recorded with an FEI
CETA 4K CMOS camera. STEM images were acquired with a high
angle annular dark field (HAADF) detector. Photoluminescence (PL)
decays were recorded using a Time-Correlated Single-Photon
Counting (TCSPC) system, equipped with a mode-locked Ti:
sapphire laser (Spectra Physics Mai-Tai HP). The photoluminescence
from the samples was collected and focused into a monochromator
(Spex 1870c). Emission was detected using a multichannel detector
(Hamamatsu R3809U-50). The time correlation of detected photons
was performed with the use of a PC electronic card from Edinburgh
Instruments (TCC900). Absorption spectra were recorded with a
Shimadzu double beam spectrophotometer, model UV-1800, wave-
length range between 800 and 300 at 1 nm resolution. Single crystal
X-ray diffraction data for the compounds were collected on a dual
source Rigaku FR-X rotating anode diffractometer, using a Mo Kα
wavelength at 150 K, and reduced using CrysAlisPro version
171.39.21a. The structures were solved and refined using Shelx-
2016 implemented through Olex2 v1.2.8. CIF files were deposited
with the CCDC, 1812163 (8), 1812158 (9), 1812159 (10), 1812157
(11), 1812160 (12), 1812156 (13), 1812161 (18), 1812162 (19).
Synthesis of Molecular Precursors. The preparation of fourteen
different metal xanthato complexes (7−20) are described in this
section. All syntheses of the ligands (1−6) and precursors (7−20)
were performed under dry N2, using standard Schlenk techniques.
The ligands (1−6) and precursors (7−20) were prepared as
In this study, a series of metal-xanthato precursors (7−20)
were synthesized and their thermal decomposition to metal
chalcogenide materials is studied. The work is grouped into
three main sections; an X-ray crystallographic study of
precursor structure, thermal analyses of precursors, and film
deposition using spin-coating methods and subsequent
structural and optical characterization of the resulting solid
state materials. New single crystal structures of metal
alkylxanthato precursors are presented. We then focus on the
decomposition conditions and the thermal behavior of the
metal xanthate complexes. We find that the ligand structure
plays a leading role in dictating the energy required for thermal
decomposition of metal alkylxanthato precursors. Figure 1
Figure 1. Schematic showing the process of the formation of metal
sulfide films from bis(O-alkylxanthato)cadmium(II) or zinc(II)
compounds by spin-coating followed by annealing at low temper-
atures (T < 300 °C).
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Figure 2. Single crystal X-ray structures of alkylxanthato cadmium(II) compounds 8−13 (a) [Cd(S2COnPr)2], (b) [Cd(S2COnBu)2], (c)
[Cd(S2COnPen)2], (d) [Cd(S2COnHex)2], (e) [Cd(S2COnHep)2], and (f) [Cd(S2COnOct)2]. The longer Cd−S bonds in the ligand (parts a−e)
are represented as black and red dashed lines while the longer Cd−S bonds in part f are equivalent and appear as black dashed lines. Structures
were deposited as CCDC numbers, 1812163 (8), 1812158 (9), 1812159 (10), 1812157 (11), 1812160 (12), 1812156 (13).
described in the literature.1,21 The tuning of surface morphology of
synthesized by metathesis of zinc(II) salts in water. All
thin film nanostructures is an essential step in optimization of solar
precursors (7−20) were soluble in the common organic
cell layers and has a strong influence on the optical properties of the
solvents, e.g., THF or acetonitrile.
films.1 Toward this end, the impact precursor structure has on the film
Single Crystal X-ray Structures of Cd and Zn
Xanthates. Single crystals of precursors 8−13, 18, and 19
of suitable quality for X-ray diffraction were grown by
evaporation from acetonitrile. Compounds 8−11 and 13
crystallized in the monoclinic crystal system, and compound
12 crystallized in the orthorhombic system. The molecular
structures of [Cd(S2COnPr)2] (8), [Cd(S2COnBu)2] (9),
[Cd(S2COnPen)2] (10), [Cd(S2COnHex)2] (11), [Cd-
(S2COnHep)2] (12), and [Cd(S2COnOct)2] (13) are shown
in Figure 2a−f. In all the cases, the cadmium ion is coordinated
with four sulfur atoms with four bridging xanthate ligands to
form a network, with each sulfur atom spanning across two
adjacent cadmium atoms to form a distorted tetrahedral
environment at the metal center. The coordination environ-
microstructure/nanostructures has been widely reported.22−24 Full
synthetic methods, elemental analysis, NMR data, and other
characterization of ligands and precursors are described in SI.
RESULTS AND DISCUSSION
■
Seven bis(O-alkylxanthato)cadmium(II) [Cd(S2COR)2] pre-
cursors: [Cd(S2COEt)2] (7), [Cd(S2COnPr)2] (8), [Cd-
(S2COnBu)2] (9), [Cd(S2COnPen)2] (10), [Cd(S2COnHex)2]
(11), [Cd(S2COnHep)2] (12), and [Cd(S2COnOct)2] (13)
were synthesized by metathesis of cadmium(II) salts in water.
Bis(O-alkylxanthato)zinc(II) precursors: [Zn(S2COEt)2]
(14), [Zn(S2COnPr)2] (15), [Zn(S2COnBu)2] (16), [Zn-
(S2COnPen)2] (17), [Zn(S2COnHex)2] (18), [Zn-
(S2COnHep)2] (19), and [Zn(S2COnOct)2] (20) were
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Figure 3. Views of the intermolecular packing in a single crystal of [Cd(S2COnPen)2], (10), showing (a) the polymeric structure, (b) coordination
at the Cd center. Black and red dashed lines correspond to the longer bonds of 3.374 and 3.354 Å, respectively. CCDC number 1812159.
ment at Cd centers with other bonds omitted for clarity is
shown in SI Figure S25. The refinement data of crystals for
compounds 8−13 are presented in Table S1.
forming a two-dimensional network.27−30 To reflect this, we
compiled the differences in Cd−S bonds lengths correlated
with number of carbons in the xanthate ligands for (8−13) and
(15, 18, and 19) (Figure S23). Similarly, the correlation
between unit cell volume with the carbon chain length of the
xanthate ligand for (8−13) is shown in Figure S24.
The structure [Cd(S2COnPen)2] (10) shows Cd in a
distorted tetrahedral geometry, with additional electrostatic
interactions with noncoordinating oxygen and sulfur from
nearest neighbor xanthates. The four different coordinating S
have Cd−S bond distances in the range of 2.5373(9) to
2.5764(8) Å, with S−Cd−S angles ranging between 101.49(3)
and 122.95(3)°. There are two additional sulfur atoms and one
additional oxygen close to the cadmium centers at 3.3531(8),
3.3801(8) and 3.016(2), respectively. These are at the limits of
the sum of van der Waals radii for Cd and S (3.38 Å) and Cd
and O (3.1 Å), respectively;25 see Figure 3b and Figure 4.
The structure of [Zn(S2COnPr)2] (15) previously reported
by Lai et al.31 can be compared to those of [Zn(S2COnHex)2]
(18) and [Zn(S2COnHep)2] (19), and all are shown in Figure
5a−c. Compounds 18 and 19 crystallize in the monoclinic
crystal system with space-group Cc. In both cases, the zinc ion
is coordinated by four monodentate ligands, with a single
ligand bridging across two zinc ions in a distorted tetrahedral
environment forming a network similar to that which we
observe with the Cd xanthates we report. The asymmetric unit
of 15 has the 16-membered ring structure as described by Ito
et al.32 (Figure S28 and Table S4). The molecular packing in
crystals of 19 is shown in Figure 6a,b. The coordination
environments of the distorted tetrahedron at Zn centers with
other bonds omitted for clarity are shown in Figure S27. The
data from the X-ray analysis of Zn xanthates are compiled in
Table S3. Bond lengths and angles for complexes 15, 18, and
19 are reported in Table S4.
Thermal Analysis of Cd and Zn Xanthates. It has been
reported that xanthate ligands with longer hydrocarbon chains
have a weaker affinity for the metal ion in metal xanthate
complexes;33 hence, it may be expected that there will be
differences in the thermal profiles for each complex, and we
investigated this phenomenon in detail for our Zn and Cd
complexes. Thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) of the complexes was performed
up to 600 °C under a nitrogen atmosphere in all cases, and
from this, we generate data on decomposition pathway and the
energy required for each step. By using the series of complexes
with increasing alkyl chain length, we were able to infer the
effect of ligand structure on the nature of the decomposition
reactions for these complexes.
The TGA profile complexes 7−13 are shown in Figure 7.
The mass loss for these complexes occurs in general through
two steps with a rapid mass loss within the temperature range
of 145 to 170 °C and gradual mass loss between 175 to 300
°C. The mass loss for complexes 10−13 occurs through three
steps found at approximately 135 °C, 162 °C, and 262−300
°C, respectively. The measured mass loss of CdS corresponds
Figure 4. Intermolecular packing in single crystals of [Cd-
(S2COnPen)2], (10). The view is projected along the c axis of the
unit cell.
Hence, we suggest that these additional interactions are likely a
consequence of the imperfect tetrahedral geometry around the
Cd metal,12 which leads to asymmetric arrangements of
xanthate ligands in the structures.26 The bond lengths and
angles found in the X-ray structures of compounds (8−13) are
compiled in the SI Table S2. There are a number of reports of
similar coordination geometries of sulfur atoms in bis-
(ethylxanthato)cadmium(II) complexes where sulfur bridges
two adjacent cadmium atoms in two different directions
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Figure 5. Crystal structures of alkylxanthato zinc complexes. (a) [Zn(S2COnPr)2] as reported by Lai et al.31 (15), (b) [Zn(S2COnHex)2] (18), and
(c) [Zn(S2COnHep)2] (19). The longer bonds of the ligand are represented as a black and a red dashed line. Hydrogen atoms are omitted for
clarity. Structures of 18 and 19 were deposited as CCDC 1812161 and 1812162. The data for structure 15 is published in ref 31 and is reused with
permission from The Royal Society of Chemistry.
Figure 6. Intermolecular packing in single crystals of [Zn(S2COnHep)2] (19) showing (a) packing that forms a polymeric structure and (b)
coordination at Zn projected along the a axis of the unit cell. The longer Zn−S bonds are represented as a black line and a red dashed line of 3.459
and 3.342 Å, respectively.
(38.9:35.2), (36.0:32.9), (34.9:30.9), (33.0:29.2),
(33.5:27.62). Table S5 presents the compiled data for onset
temperature, the experimental and theoretical residual mass,
the melting points, and some melting points in literature for
bis(O-alkylxanthato)cadmium(II) precursors (7−13). MacLa-
chlan et al.,21 have described similar profiles for Cd xanthate
complexes with pyridine adducts, but these also show
additional mass loss steps corresponding to the loss of the
pyridine adducts. We note in some cases that above 200 °C,
there is some additional mass loss which could potentially
correspond to the loss of sulfur by sublimation.
Figure 8 shows TGA profiles for bis(O-alkylxanthato)zinc-
(II) compounds (14−20): [Zn(S2COnEt)2] (14), [Zn-
(S2COnPr)2] (15), [Zn(S2COnBu)2] (16), [Zn(S2COnPen)2]
(17), [Zn(S2COnHex)2] (18), [Zn(S2COnHep)2] (19), or
Figure 7. Thermogravimetric analysis (TGA) profiles of bis(O-
[Zn(S2COnOct)2] (20), in the range 30−600 °C. The initial
alkylxanthato) cadmium(II), complexes 7−13; heated in the range
100−300 °C with a temperature gradient of 10.0 °C min−1.
decomposition temperature of the compounds 14−20 were
144.7, 137.2, 137.7, 137.8, 138.7, 137.0, and 137.0 °C,
respectively. The mass loss of the compounds with longer
to the calculated value of precursors 7−13 as follows:
(measured: calculated) %; (41.3:40.7), (40.0:37.8),
alkyl chains, i.e., [Zn(S2COnPen)2] (17), [Zn(S2COnHex)2]
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metric enthalpies (ΔHca) of the phase transition were
expressed by multiplying the integrated peaks by calorimetric
constant. [36] The combined DSC and TGA data from these
complexes is presented in Table 1. Figure 9 shows the
Figure 8. Thermogravimetric analysis (TGA) profiles of bis(O-
alkylxanthato)zinc(II) complexes14−20; heated in the range 100−
300 °C with a temperature gradient of 10.0 °C min−1.
Figure 9. Approximately linear correlation between number of
carbons in the alkyl hydrocarbon chain of xanthate ligands and the
energy required for decomposition from differential scanning
calorimetry (DSC) for bis(O-alkylxanthato)cadmium(II) complexes
(7−13) and bis(O-alkylxanthato)zinc(II) complexes (14−20); the
samples were heated from ambient temperature to 600 °C, at rate of
10.0 °C min−1.
(18), [Zn(S2COnHep)2] (19), or [Zn(S2COnOct)2] (20)
tends to consist of three steps, because as the chains grow
longer, the organic side products from decomposition may
have higher boiling points and are not lost as easily. In all cases,
the experimental residual mass was approximately equal to the
theoretical residual mass of ZnS material, which demonstrates
that the decomposition in all cases is relatively clean and
potentially free of significant organic contamination.
It has been suggested that metal xanthate complexes
decompose to metal sulfides via a Chugaev elimination
pathway.1,33 Initially, this proceeds by skeletal rearrangement
of the ligand and thus the structure of ligand is therefore the
predominant factor to determine the onset temperature of
decomposition.40 The metal entity does not appear to affect
the reaction pathway to a large extent (iron being a major
exception).33 Having synthesized a family of both Cd and Zn
xanthates with ligands with monotonically increasing alkyl
chain lengths, we decided to investigate this phenomenon
systematically. The endothermic decomposition as a function
of temperatures changes in heat capacity were determined
using differential scanning calorimetry (DSC). The calori-
approximately linear correlation between alkyl hydrocarbon
chain length and the energy required for decomposition for
bis(O-alkylxanthato)cadmium(II) (7−13) and bis(O-
alkylxanthato)zinc(II) (14−20). The calorimetric enthalpies
(ΔHca) of the phase transition were expressed by multiplying
the integrated peaks by the calorimetric constant.36 It may be
concluded, from these data, that as alkyl chain length increases,
in general, the energy required to initiate decomposition is
increased in Cd and Zn xanthate complexes. Previously it has
been reported,34−38 that as the alkyl chain length of the ligand
increases, the enthalpy of fusion decreases and the enthalpy of
exothermic decomposition increases. For Cd complexes,
specifically, this is associated with the larger enthalpy of
Table 1. Combined DSC and TGA Data for bis(O-alkylxanthato) inc(II) Compounds 14−20
Ligand alkylchain Decomposition Experimental Residual
Theoretical Residual mass
Measured melting
Calculated melting point
Complex
length
steps
mass (%)
(final step) (%)
point (°C)
(onset) (°C)
(14)
C2
1st
2nd
1st
2nd
1st
2nd
1st
2nd
3rd
1st
2nd
3rd
1st
2nd
3rd
1st
2nd
3rd
99.16
35.24
99.5
33.77
99.48
32.42
99.64
77.26
31.15
99.8
73.34
29.5
99.48
82.77
28.5
140−142
139−141
128−130
126−128
143.1
142.3
128
31.67
29.02
26.78
(15)
(16)
(17)
C3
C4
C5
128.5
24.86
23.20
21.74
(18)
(19)
(20)
C6
C7
C8
128−130
124−126
123−125
130.4
125
99.76
83.57
27.4
122.1
20.46
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formation of the metal xanthato moiety in the case of ligands
with longer hydrocarbon chains.20,39
ratio of Cd:S; the results were shown in SI, Table S10. The
chemical composition and elemental ratios determined by X-
ray photoelectron spectroscopy (XPS) and the binding
energies observed are shown in SI, Table S11 and Figure
S32. The atomic ratio of Cd:S determined from EDS was
Synthesis and Structural Characterization of CdS and
ZnS Thin Films. Deposition of thin films on glass was
achieved by spin-coating metal sulfide precursors (7−20)
dissolved in tetrahydrofuran (THF) on glass substrates. Films
of the precursors were then annealed at different growth
temperatures (300 or 200 °C), in a furnace under a N2 flow for
60 min to furnish the metal chalcogenide thin films. The
resulting thin films of cadmium or zinc sulfides were uniform
and yellow or black in color.
To confirm that CdS films were successfully formed by the
thermal decomposition of bis(alkylxanthato)cadmium(II)
complexes, thin films from spin-coating were subjected to X-
ray powder diffraction (pXRD). The diffraction patterns of
films prepared from a tetrahydrofuran solution of [Cd-
(S2COnEt)2] (7), [Cd(S2COnPr)2] (8), [Cd(S2COnBu)2]
(9), [Cd(S2COnPen)2] (10), [Cd(S2COnHex)2] (11), [Cd-
(S2COnHep)2] (12), or [Cd(S2COnOct)2] (13) (labeled as
C2Cd−C8Cd for complexes 7−13, respectively, in Figure 10)
approximately 1.00
determined by XPS was 1.00
0.06, and the atomic ratio of Cd:S
0.08, hence there is good
agreement between these two techniques that probe the bulk
and surface (ca. 6−10 nm probe depth) respectively.
Powder-XRD patterns of zinc sulfide films (C2Zn−C8Zn)
were prepared from a tetrahydrofuran solution of [Zn-
(S2COnEt)2] (14), [Zn(S2COnPr)2] (15), [Zn(S2COnBu)2]
(16), [Zn(S2COnPen)2] (17), [Zn(S2COnHex)2] (18), [Zn-
(S2COnHep)2] (19), or [Zn(S2COnOct)2] (20) by spin-
coating onto glass substrates followed by annealing at 200 °C
for 60 min under N2 (Figure S33). The diffraction patterns of
thin films corresponded to zinc blende (cubic ZnS, ICDD no,
00−0005−0566). The chemical compositions and elemental
ratios of ZnS films by EDS measurements were recorded for
C4Zn, C6Zn, and C8Zn (see Table S10). The atomic ratio of
Zn:S in most cases was approximately 1.00 which suggests the
formation of stoichiometric zinc sulfide from this process.
The particle size and distribution of CdS films derived from
the spin-coating and annealing at 300 °C of precursors 9−11
and 13 were studied using HRTEM. The TEM images of films
from thermal decomposition of [Cd(S2COnBu)2] (9), [Cd-
(S2COnPen)2] (10), [Cd(S2COnHex)2] (11), and [Cd-
(S2COnOct)2] (13) are shown in Figure 12a−d, respectively.
Figure 12d shows the selected area electron diffraction
(SAED) pattern, high-resolution TEM image with the fast
Fourier transformation (FFT) pattern, and the particles’ size
́
distribution; the moire fringes of two overlapping crystal
lattices is imaged in Figure 11c,d.41 The SAED pattern is
hexagonal and shows the (100) and (101) planes. This is
consistent with p-XRD results, which indicate that the product
is hexagonal CdS.1
In Figure 12e−g, TEM images of zinc sulfide crystallites
from thin films that were produced by decomposition of
[Zn(S2COnBu)2] (16), [Zn(S2COnHex)2] (18), and [Zn-
(S2COnOct)2] (20) at 200 °C are shown. HRTEM images, fast
Fourier transformation FFT, and selected area electron
diffraction SAED patterns demonstrate the crystallinity of the
films. Figure 13g shows crystallites from the zinc sulfide film
derived from [Zn(S2COnOct)2] (20) have wave-like fringes,
inside and around the green border, as reported by Lackner.44
Pradhan et al., previously described these as important features
in the transition from nanorod to nanowire during crystal
growth.45
Photophysical Characterization of CdS Thin Films.
Absorption spectroscopy was carried out after the films were
deposited onto glass substrates with each of the resulting
spectra (see Figure S34) showing an absorption onset between
580 and 600 nm. The optical band gap, Eg, for each film was
determined via a Tauc plot (see Figure S34). This analysis
yielded band gap energies (Eg) in the range 2.25−2.43 eV (see
Table S12), consistent with our previous report that used
metal xanthate complexes to produce CdS.1
Figure 10. Grazing incidence X-ray diffraction (GIXRD) patterns of
cadmium sulfide thin films (C2Cd−C8Cd) from the series of bis(O-
alkylxanthato)cadmium(II) precursors (7−13) deposited by spin-
coating followed by annealing and conversion to CdS at 300 °C under
N2 for 60 min.
and annealed at 300 °C for 60 min were recorded. Figure 10
shows the pXRD patterns of the nanocrystalline CdS thin films
obtained (C2Cd−C8Cd). The diffraction peaks were indexed to
the (100), (002), (101), (102), (110), (103), (112), (203),
(211), and (105) planes of hexagonal CdS (a = 4.14 Å and b =
6.72 Å; ICDD no: 00−041−1049). Generally, peaks grow
sharper as the annealing temperature increases, which we
attribute to a decrease in Scherrer broadening indicating that
the size of the crystalline domains in thin films increases as
synthesis temperature increases.4 The p-XRD patterns of all
films deposited at 300 °C from a THF solution of precursors
7−13 show a similar trend. The p-XRD pattern of films
[Cd(S2COPr)2] (8) and [Cd(S2COnOct)2] (13) at 300 °C
can be indexed to hexagonal CdS (Figure 11). The lattice
parameters of hexagonal structure for films C2Cd−C8Cd are
shown in SI, Table S9. Energy-dispersive X-ray spectroscopy
(EDS) of cadmium sulfide films showed an approximately 1:1
Photoluminescence (PL) spectra are shown in Figure S35
for C2Cd−C8Cd films produced by spin-coating the complexes
(7−13) and annealing at 300 °C. Thin films C2Cd−C8Cd were
dispersed prior to the measurement in N-methyl-2-pyrrolidone
(NMP) solvent. In each case, a broad PL band spanning the
visible spectrum is observed, on top of which are superimposed
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Figure 11. Characterization of CdS crystallites taken from thin films produced at 300 °C for 60 min from [Cd(S2COR)2] precursors (9−11) and
13 by TEM. (a,1−4) TEM images of C4Cd film; (b,1−4) TEM images of C5Cd film; (c,1−4) TEM images of C6Cd film; (d,1−6) TEM images of
cadmium sulfide C8Cd film show (d1) particle size distribution; (d2) HRTEM image; (d3) FFT obtained from the area in panel (d2); (d4) CTEM
image; (d5) structure morphology; and (d6) SAED pattern with indexing, obtained from the area in panel d2 that is highlighted by a yellow circle
in panel (d5).
a number of peaks. In particular, peaks in the emission spectra
are observed at 2.35 eV (527 nm) and 2.2 eV (564.5 nm),
which are attributed to a transition across the band gap,
broadly consistent with the Eg values determined above, and a
transition from a band edge to a defect state, respectively. We
report next the luminescence properties of CdS thin films
produced at different temperatures, and find that the spectral
features of the films depend on the annealing temperature
postdeposition. For annealing at 300 °C, the observed PL had
a peak at 527 nm, consistent with C2Cd−C8Cd films (see
Figure S35). Mlowe et al., also reported the features of PL
spectra for CdS films; for an excitation wavelength of 350 nm,
they observed broad emission in the 450−600 nm range with a
peak at a similar wavelength of 515 nm.42 Photoluminescence
decay transients for the C2Cd−C8Cd films are shown in Figure
13. All decays can be fitted by a triexponential decay function
(see eq S1). The parameter values resulting from this fitting are
given in the SI (Figure S36 and accompanying text). The decay
is almost identical in all cases and can be described by a
triexponential function comprising three components with
fitted decays ∼0.15, ∼2, and ∼9 ns. The first of these
corresponds to the instrument response function of the system.
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Figure 13. PL decay transients of the C2Cd−C8Cd films for λex = 420
nm and λem = 530 nm.
CONCLUSIONS
■
In summary, this study describes the synthesis and properties
of a series of Cd(II) and Zn(II) complexes, and explores the
relationship between precursor structure and resulting thin film
metal sulfide. The precursors were crystallized; single crystals
were fully characterized, and compounds were used to
prepared CdS and ZnS semiconductor films by spin-coating
and thermal decomposition. An analytical study of the films
was carried out after the thermal decomposition.
The cadmium or zinc bis(alkylxanthato) compounds
[M(S2COnEt)2] (7 or 14), [M(S2COnPr)2] (8 or 15),
[M(S2COnBu)2] (9 or 16), [M(S2COnPen)2] (10 or 17),
[M(S2COnHex)2] (11 or 18), [M(S2COnHep)2] (12 or 19),
or [M(S2COnOct)2] (13 or 20) were successfully character-
ized.The X-ray structures of crystals [Cd(S2COnPr)2] (8),
[Cd(S2COnBu)2] (9), [Cd(S2COnPen)2] (10), [Cd-
(S2COnHex)2] (11), [Cd(S2COnHep)2] (12), [Cd-
(S2COnOct)2] (13), [Zn(S2COnHex)2] (18), and [Zn-
(S2COnHep)2] (19) were reported. We show that there is an
approximately linear relationship between the alkyl chain
length in these complexes and the energy required for
decomposition to the corresponding crystalline metal sulfide,
i.e., CdS or ZnS. Spin-coating of these precursors followed by a
thermal annealing step was therefore used to produce thin
films of cadmium or zinc sulfide at temperatures as low as 200
°C. Overall, the spin-coating/annealing method is a promising
method of producing transition metal sulfide films at relatively
low temperatures. With the advent of this systematic study, it
will be possible in future to judiciously tune decomposition
tempertaures of precursors to tailor any application being
targeted.
Figure 12. TEM characterization of crystallites from ZnS films
produced at 200 °C for 60 min from [Zn(S2COR)2] precursors 16,
18, and 20. (e,1−5) TEM images of C4Zn film; (f,1−4) TEM images
of C6Zn film; both films include SAED pattern with indexing, FFT,
HRTEM image, structural morphology, and the histogram of particle
length. (g,1−5) TEM images of zinc sulfide C8Zn film show (g1)
conventional transmission electron microscope CTEM image; (g2)
HRTEM image; the inset image is the structural morphology of the
selected area at 10 nm magnification scale; (g3) FFT of HRTEM
image of panel g2 with indexing; (g4) SAED pattern with indexing,
obtained from the highlighted area by a green circle in panel g2; and
(g5) the histogram showing particle size distribution.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01110.
(PDF)
Accession Codes
CCDC 1812156−1812163 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
We attribute the short component of 2 ns to nonradiative
recombination due to defects in the films, while the longer
component of 9 ns is ascribed to radiative recombination.43
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Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
David J. Lewis − Department of Materials, University of
Manchester, Manchester M13 9PL, U.K.; orcid.org/0000-
0001-5950-1350; Email: david.lewis-4@manchester.ac.uk
Authors
Ali A.K. Bakly − Department of Materials, University of
Manchester, Manchester M13 9PL, U.K.
David Collison − Department of Chemistry, University of
Manchester, Manchester M13 9PL, U.K.
Ruben Ahumada-Lazo − Department of Physics and
Astronomy and Photon Science Institute, University of
Manchester, Manchester M13 9PL, U.K.; orcid.org/0000-
0002-1524-9576
David J. Binks − Department of Physics and Astronomy and
Photon Science Institute, University of Manchester,
Manchester M13 9PL, U.K.; orcid.org/0000-0002-9102-
0941
(12) Abrahams, B. F.; Hoskins, B. F.; Tiekink, E. R. T.; Winter, G.
Investigation of a New Xanthate Ligand. The Crystal and Molecular
Structures of Nickel and Cadmium (Methoxyethyl) Xanthates. Aust. J.
Chem. 1988, 41 (7), 1117−1122.
Matthew Smith − Department of Materials, University of
Manchester, Manchester M13 9PL, U.K.
(13) Thomas, P. J.; Stansfield, G. L.; Komba, N.; Cant, D. J. H.;
Ramasamy, K.; Albrasi, E.; Al-Chaghouri, H.; Syres, K. L.; O’Brien, P.;
Flavell, W. R.; Mubofu, E.; Bondino, F.; Magnano, E. Growth of
Nanocrystalline Thin Films of Metal Sulfides [CdS, ZnS, CuS and
PbS] at the Water-Oil Interface. RSC Adv. 2015, 5, 62291−62299.
(14) Lewis, E.; Haigh, S.; O’Brien, P. The Synthesis of Metallic and
Semiconducting Nanoparticles from Reactive Melts of Precursors. J.
Mater. Chem. A 2014, 2 (3), 570−580.
(15) Afzaal, M.; Malik, M. A.; O’Brien, P. Chemical Routes to
Chalcogenide Materials as Thin Films or Particles with Critical
Dimensions with the Order of Nanometres. J. Mater. Chem. 2010, 20,
4031−4040.
James Raftery − Department of Chemistry, University of
Manchester, Manchester M13 9PL, U.K.
George F. S. Whitehead − Department of Chemistry,
University of Manchester, Manchester M13 9PL, U.K.;
orcid.org/0000-0003-1949-4250
Paul O’Brien − Department of Materials and Department of
Chemistry, University of Manchester, Manchester M13 9PL,
U.K.; orcid.org/0000-0001-8744-9174
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01110
(16) Ramasamy, K.; Malik, M. A.; Revaprasadu, N.; O’Brien, P.
Routes to Nanostructured Inorganic Materials with Potential for Solar
Energy Applications. Chem. Mater. 2013, 25, 3551−3569.
(17) Dance, I. G. The Structural Chemistry of Metal Thiolate
Complexes. Polyhedron 1986, 5 (5), 1037−1104.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(18) Matthews, P. D.; McNaughter, P. D.; Lewis, D. J.; O’Brien, P.
Shining a Light on Transition Metal Chalcogenides for Sustainable
Photovoltaics. Chem. Sci. 2017, 8, 4177−4187.
This work was supported by the Ministry of Higher Education
and Scientific Research of Iraq “MOHESR-IQ” and the
University of Babylon, Iraq. AAKB thanks Maria A.
Leontiadou, Department of Physics and Astronomy and
Photon Science Institute for useful discussions regarding
steady-state PL and TCSPC experiments.
(19) Ramasamy, K.; Malik, M. A.; Helliwell, M.; Raftery, J.; O’Brien,
P. Thio-and Dithio-Biuret Precursors for Zinc Sulfide, Cadmium
Sulfide, and Zinc Cadmium Sulfide Thin Films. Chem. Mater. 2011,
23, 1471−1481.
(20) McNaughter, P. D.; Saah, S. A.; Akhtar, M.; Abdulwahab, K.;
Malik, M. A.; Raftery, J.; Awudza, J. A. M.; O’Brien, P. The Effect of
Alkyl Chain Length on the Structure of Lead(II) Xanthates and Their
Decomposition to PbS in Melt Reactions. Dalton Trans. 2016, 45,
16345−16353.
(21) MacLachlan, A. J.; Rath, T.; Cappel, U. B.; Dowland, S. A.;
Amenitsch, H.; Knall, A. C.; Buchmaier, C.; Trimmel, G.; Nelson, J.;
Haque, S. A. Polymer/Nanocrystal Hybrid Solar Cells: Influence of
Molecular Precursor Design on Film Nanomorphology, Charge
Generation and Device Performance. Adv. Funct. Mater. 2015, 25
(3), 409−420.
(22) Lewis, E. A.; McNaughter, P. D.; Yin, Z.; Chen, Y.; Brent, J. R.;
Saah, S. A.; Raftery, J.; Awudza, J. A. M.; Malik, M. A.; O’Brien, P. In
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(23) Abdelhady, A. L.; Malik, M. A.; O’Brien, P. Colloidal Synthesis
of ZnS, CdS and ZnxCd12xS Nanoparticles from Zinc and Cadmium
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