Acta Materialia 267 (2024) 119744

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Stabilization of high-pressure phase of CdO by nanoparticle formation in
CdxZn1-xO thin films
Arkaprava Das a, *, Camille Latouche b, c, Stephane Jobic b, Eric Gautron b, Amina Merabet b,
Marcin Zajac d, Akinori Shibui e, Peter Krüger e, Wei-Hsiang Huang f, Chi-Liang Chen f,
Asokan Kandasami g, Carla Bittencourt aaChimie des Interaction Plasma surface, University of Mons, Mons, Belgium
Nantes Université, CNRS, Institut des Matériaux de Nantes Jean Rouxel, IMN, Nantes F-44000, FranceInstitut Universitaire de France, Paris 75005, Franced
SOLARIS National Synchrotron Radiation Centre, Jagiellonian University, Krakow, PolandeGraduate School of Science and Engineering, Chiba University, Chiba, JapanfNational Synchrotron Radiation Research Center, Hsinchu, TaiwangDepartment of Physics & Centre for Interdisciplinary Research, University of Petroleum and Energy Studies (UPES) Dehradun, Indiabc

A R T I C L E I N F O

A B S T R A C T

Keywords:
Phase transformation
X-ray absorption spectroscopy
X-ray photoelectron spectroscopy

We report the observation of the high-pressure B2 phase in CdO nanoparticles obtained by temperature induced
phase transition from the CdO B1 phase in CdxZn1-xO films grown on a Si substrate. The structural transformation
occurs upon annealing the film from 700 to 900 ◦ C and is monitored by X-ray diffraction and Raman spec­
troscopy. Concomitantly, willemite Zn2SiO4 nanoparticles form at the CdxZn1-xO/Si interface and are evidenced
using scanning transmission electron microscopy, X-ray absorption and photoelectron spectroscopies. The
presence of Zn2SiO4 at the film-substrate interface is assumed to exert locally a high pressure on the CdO
crystallites. The B1 to B2 phase transition in CdO was previously only reported under hydrostatic pressure
conditions. By varying the Cd content and adjusting the growth conditions, we have succeeded in stabilizing themetastable B2 phase under ambient conditions, which holds significant potential for applications in energy
storage and stress sensing.

1. Introduction
Semiconductors have played a crucial role in the development ofnovel technologies over the last century. Among them, transparent
conducting wurtzite oxide Zinc oxide (WZ) (ZnO) has garnered signifi­
cant attention due to its wide direct bandgap (3.37 eV) and a highexciton binding energy (60 meV) compared to the thermal energy at RT(25 meV). Numerous studies have explored the different possible ap­
plications of ZnO, including its use as a pigment for anti-pathogenic
coatings [1], piezoelectric devices [2], acoustic wave transducers [3],and active layers in chemical sensors [4]. Similarly, CdO (rock-salt (B1)structure type, RS) is a wide band gap (2.2 eV) semiconductor oxide thatis recognized for its high electrical conductivity (>1014 S/cm) and highelectron mobility (>100 cm2/V/s) [5–7]. Consequently, Cd has been
identified as a key doping material to enhance the optoelectronic
properties of ZnO. The incorporation of cadmium (Cd) into ZnO

effectively narrows the bandgap, rendering the optoelectronic device
suitable for operation in the visible region [8–10].
The mixing of RS CdO and WZ ZnO would result in the formation of
mixed binary oxide [11]. However, obtaining a single crystallographic
phase in CdxZn1-xO thin films under normal growth conditions in the
mid-composition region is challenging as CdO and ZnO crystallize in the
RS and WZ phases, respectively. Additionally, the uniform mixing be­
tween CdO and ZnO is further impeded by the low atomic solubility (< 2%) of Cd in WZ ZnO) [12,13]. To overcome the miscibility limitation,
researchers have endeavored to synthesize CdxZn1-xO binary oxide withhigh Cd concentration using different non-equilibrium growth methods.
The synthesis of WZ CdxZn1-xO thin films, where the band gap varies
from 3.2 to 1.8 eV, are reported for x up to 0.7 using the remote plasma
enhanced metal organic chemical vapour deposition [14,15]. Using thepulsed filtered cathodic arc deposition (PFCAD) method, CdxZn1-xO
mixed binary oxide for x < 0.67 were obtained with high free electron* Corresponding author.
E-mail address: arkaprava.das@umons.ac.be (A. Das).
https://doi.org/10.1016/j.actamat.2024.119744
Received 3 December 2023; Received in revised form 20 January 2024; Accepted 6 February 2024
Available online 7 February 2024
1359-6454/© 2024 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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concentration (from mid-1019 to >1020/cc) due to native defects andappreciable mobility value (∼20 cm2V− 1S− 1) [16]. For DC magnetron
sputtering CdxZn1-xO thin film deposition, the dominance of the WZ
phase is observed for x up to 0.66 [17]. Therefore, we have focused on
the study of CdxZn1-xO thin films in mid-composition (i.e., x = 0.4)
synthesized by the sol-gel method. To obtain high nano-crystalline thinfilms, high-temperature annealing was used. Despite significant chal­lenges, the crystallographic phase identification demonstrated thepresence of RS CdO (B1) and a remarkable phase transformation (PT)from the CdO B1 (NaCl) to CdO B2 (CsCl) structural phase within thethin film as the post-synthesis annealing temperature increased. Thisobservation has prompted us to investigate the B1 to B2 type phasetransformation mechanism. Notably, this phenomenon has only beenreported using the diamond anvil cell (DAC) technique [18].
Theoretical modelling has demonstrated that 4d CdO and 3d ZnOtransition metal oxides can suffer a B1 to B2 type PT at 89 GPa and 256
GPa, respectively [19,20]. These pressure values are challenging toobtain experimentally. Therefore, only one experimental report isavailable for CdO with B1 to B2 type PT [18], and for ZnO, no experi­mental evidence is available to our knowledge. Such theoretically pre­dicted critical pressure (PTC) values for PT are questionable as
relativistic corrections, nonzero temperature, and zero-point motion canalter such predictions. Therefore, precise diffraction measurements are
essential to understand the B1 to B2 PT kinetics. Apart from Liu et al. in
2004, such investigation is yet to be done due to experimental limita­
tions in kinetically achieving the B2 phase in CdO [18]. Wurtzite torocksalt PT with ∼9 GPa pressure in ZnO bulk has been reported else­
where [21], and with compositional variation for x>0.67 in CdxZn1-xO
thin films [8]. So, the wurtzite to rocksalt PT has been reported for
undoped ZnO and for CdxZn1-xO (x>0.67) binary oxides as for thesematerials it requires <15 GPa pressure. However, using in situ
high-pressure angle dispersive synchrotron X-ray diffraction technique,
Liu et al. showed the B1 to B2 type PT for bulk CdO which requires
beyond 90 GPa pressure. [18]. Investigating the electronic structure andmicroscopic properties of a kinetically stable B2 phase under ambientconditions is a complex task that typically requires high-pressure tech­niques such as the diamond anvil cell (DAC). Currently, only a small
amount of research has been conducted in this field.
In our current study of the CdxZn1-xO (x = 0.4) thin film, we havesuccessfully achieved a kinetically stable B2 phase without using the
diamond anvil cell (DAC) technique. Through local pressure generationin the thin film, we have attained a thermodynamically stable B2 phaseat room temperature and pressure. This breakthrough has enabled us toinvestigate the modifications in electronic structure and changes in
microscopic properties during the B1 to B2 type phase transition in the
CdxZn1-xO (x = 0.4) thin film. The primary goal of this research is to
uncover the underlying source of the local pressure generation within
the lattice that triggers this PT. Ultimately, our findings on the corre­
lation between the phase transition, microscopic and electronic prop­
erties will be valuable in understanding the fundamental physiochemical interactions that play a crucial role in energy storage andstress sensing [8].

Spectroscopic measurements have been accomplished at RT with a
SENTERRA spectrometer (Bruker) using an Ar ion laser (532 nm) with
0.2 mW laser operating power. Scanning Electron Microscopy (SEM)
measurements have been performed with a SU8020 HITACHI modelwith a 3.0 keV electron beam energy. Topographical measurements
were performed using the Atomic Force Microscopy (AFM) techniqueusing NANOSCOPE II tapping mode. The microstructure was charac­terized using a Cs probe corrected STEM Themis Z G3 (Thermo Fisher
Scientific) equipped with a High Angle Annular Dark Field (HAADF)
detector (Fischione) operating at 300 kV accelerating voltage, with a 31
mrad convergence angle and 63–200 mrad collection angle. It isequipped with the 4-SDD detectors Super-X system, allowing EDS
analysis. Cross sections of the samples were prepared using a focused ion
beam (FIB) (ZEISS Crossbeam 550 L). Pt/C layers were deposited on topof the samples before FIB milling to protect them. Surface chemical
states were investigated with an ESCA-5000 (Physical Electronics) Versa
Probe system using an Al Kα (1486.7 eV) beam attached to a 124 mm
hemispherical electron analyzer. Calibration during XPS measurements
was done using C 1 s edge (284.6 eV). X-ray absorption near edge
spectroscopic (XANES) measurements have been performed for Zn L2,3
and O K edges in total electron yield (TEY) detection mode at PIRX
beamline [23] in the SOLARIS synchrotron, Poland [24]. Extended X-ray
absorption fine structure (EXAFS) data for the Zn K edge has beenrecorded in fluorescence detection mode at the TPS 44A beamline, Na­tional Synchrotron Radiation Research Center (NSRRC), Taiwan.
2.1. Computational methods
The density of states (DOS) calculation for the CdXZn1-xO (0≤x≤1)
system is performed with projected augmented wave (PAW) method
using Vienna Ab Initio simulation package (VASP) software [25,26].
12×12×6 Monkhorst-pack k-mesh sampling has been used to optimize
the geometries [27]. The kinetic energy cut-off of the plane wave basis
was set at 540 eV. The total energy convergence criterion for
self-consistent field cycles was 10− 5 eV. The PyDEF software was used torealize the graphical representation of the DOS [28,29]. The Zn L-edge
absorption spectra were computed in supercells of at least 64 atoms
using the so-called final state approximation, i.e., with a full core hole on
one Zn site. The energy splitting between the 2p3/2 and 2p1/2 core levels
was taken from photoemission data (23.3 eV). The L3 and L2 edge
spectra were added in the statistical branching ratio 2:1. Since absolutebinding energies are difficult to reproduce in DFT, we have shifted thespectra rigidly to align the L3-edge with the experiment.
3. Results and discussion
3.1. Crystallographic phase identification
Fig. 1(a) shows the X-ray diffractograms (XRD) for the annealed thinfilms. The Bragg reflections for the wurtzite ZnO (space group P6_3mc)
and the CdO rocksalt phases are indexed in Fig. 1(a). For the CZ700 thinfilm, B1 phase peaks, i.e., for CdO rocksalt phase (space group Fm-3 m),
are situated at 33.0∘ (111) and 38.3∘ (200). The reflections for the
wurtzite phase (space group P6_3mc) are situated at 31.6∘ (100), 34.3∘(002), 36.1∘ (101), 47.4∘ (102). Therefore, the CZ700 thin film consistsof a mixed phase containing ZnO wurtzite and CdO rocksalt crystallites[30]. A gradual evolution of the peak at 32∘ is observed with increasing
annealing temperature, indicating the NaCl B1 (space group Fm-3 m)
to CsCl B2 (space group Pm-3 m) phase transition (PT). For the CdO
powder, the B1 to B2 PT is reported experimentally [18] and theoreti­
cally [20] under hydrostatic pressure. For our CdxZn1-xO thin film, thecomplete transition takes place around 900 ◦ C. For the CZ900 thin film,
two low-intensity peaks are observed at 38.8∘ , and 42.1∘ corresponding
to the trigonal (223) and (202) reflections of the willemite Zn2SiO4phase (space group R-3). In the following, it is shown that the formationof the trigonal Zn2SiO4 phase is crucial for the B1 to B2 PT. In Figure S1

2. Experimental methods
The thin films of ZnO with 40 % Cd concentration were preparedwith the sol-gel chemical synthesis method using a spin coater. The
detailed process is described elsewhere [22]. The post synthesis
annealing was done in a tubular furnace for 700 ◦ C, 800 ◦ C, 850 ◦ C, and900 ◦ C in a flowing oxygen environment. The "CdxZn1-xO" thin films
annealed at these different temperatures are named CZ700, CZ800,
CZ850, and CZ900, respectively. ZnO thin films used as references wereannealed at 700 ◦ C, 800 ◦ C, and 900 ◦ C. They are named Z700, Z800,
and Z900, respectively.
X-ray diffraction (XRD) measurements were performed with a Bruker
High-resolution X-ray diffractometer system using Cu Kα beam. Raman
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Fig. 1. (a): X-ray diffraction patterns and (b) Raman spectra for ZnO and CdxZn1-xO (x = 0.4) thin films at different annealing temperatures showing B1 to B2 phasetransformation in CdO sublattice.

of SI (Supplementary information file) we have shown the XRD pattern
for CZ900 thin films in logarithmic scale where (223) and (202)reflection of willemite trigonal Zn2SiO4 phase is well visible. We have
performed the structural Reitveld refinement for Z700, CZ700, andCZ900 thin films. In Figure S2 of SI, the structural refinements for thethin films are stack plotted. The details of the Rietveld refinement, andthe calculated structural parameters, are mentioned in Table ST1 of theSI.
The lattice parameter ‘a’ for hexagonal wurtzite ZnO can be calcu­
lated with the following equation [31]:a=λ√3sinθ

expected with shifting of 0.2∘ of (100) reflection of ZnO towards a lower
value.
Fig. 1(b) shows the Raman spectra for the Z700, Z900, CZ700,
CZ800, and CZ900 thin films. The Raman modes observed at 303 cm− 1,
520 cm− 1, 620 cm− 1, and 671 cm− 1 correspond to the Si substrate [32].
The mode at 435.5 cm− 1, i.e., the E2 (high), indicates the hexagonal
wurtzite phase of ZnO. The mode at 867.2 cm− 1, is assumed to corre­
spond to the transformed B2 phase, which has yet to be reported in theliterature to our knowledge.

(1)

3.2. Grain growth mechanism
Fig. 2 displays the SEM images of the Z700, Z800, Z900, and CZ700,
CZ800, CZ900 films. The Z700, Z800, and Z900 films show grain growthwith increasing annealing temperature due to sintering process. In sin­tering heat treatment process where loose material is subjected to highHere θ corresponds to the position of (100) reflection of the wurtzite
ZnO phase in the XRD pattern.
From Eq. (1), an increment of ∼0.9 % in the ‘a’ lattice parameter isFig. 2. Scanning electron microscopic images for ZnO (a, b, c) and CdxZn1-xO (x = 0.4) thin films (aʹ, bʹ, cʹ) at 700 ◦ C, 800 ◦ C and 900 ◦ C respectively.
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temperature and pressure to compact it into a solid piece. From theobserved surface morphology, it is established that there is a homoge­
nous distribution of inter-connected grains with voids. The mean void
size increases with increasing annealing temperature as sintering pro­
cess induced synthesis of compact solid piece of mixed oxide. This
happens in Z900 thin film resulting a larger void space among the grains.
For the CZ900 thin film, annealed at 900 ◦ C, which is near the melting
temperature of CdO (∼1000 ◦ C), there is a probability that a portion of
the CdO melts and subsequently condensates at energetically favourablesites to minimize the overall surface energy. This would support theformation of larger voids or ribbon-like morphology.
Figure S3 (Supplementary information file) depicts the AFM micro­
graphs for the CZ700, CZ800, and CZ900 thin films. The topographicalcontrast shows that the local evaporation and agglomeration generated
voids. Previously, we have observed such a sheet-like structure for CdO
thin films [33]. Brenner et al. reported that nanosheet or ribbon-likenanostructure formation is driven by the vapour − solid (VS) process[34]. In the VS process, the vaporization of oxide materials occurs from ahigh-temperature zone and is further deposited on the surface at a
relatively lower temperature region. Similar when the sample isannealed at 900 ◦ C, local CdO evaporation and condensation, along withsubsequent nucleation at the energetically favorable site, might haveoccurred, causing such ribbon-like features.

following equation [36,37].
AP = BE(i) + KE(jkl)(2)

In the above equation, KE(jkl) is the kinetic energy of the Augertransition for jkl level and BE(i) is the BE of ith electron level. To
calculate the Auger Parameter (AP), the position of Zn L3M45M45 (LMM)
auger transition peak in the kinetic energy (KE) scale is used (Figure S4(a)). The position of the Zn 2p3/2 peak is considered as the BE of the ithelectron level. The calculated APs are plotted in Figure S4(b). The
calculated AP value for CZ900 is 2009.6 ± 0.1 eV, which agrees with thereported value for willemite Zn2SiO4 by Dake et al. [38]. As XPS probes
the ultimate surface, it suggests that the Zn2SiO4 has out-diffused fromthe film-substrate interface and reached the surface [39,40]. It is ex­pected that Zn2SiO4 might have possibly formed in the interface but
could not reach up to the surface for Z900 thin film due to the absence of
Cd.
Fig. 4(a) shows the Si 2p peak for the CZ900, CZ800, CZ700, Z900
and Z700 films. The Si 2p peak has closely-shifted spin-orbit components(Δ=0.63 eV), that can be ignored for Si 2p peaks from silicon com­pounds. Therefore, the deconvolution of the Si 2p is performed withsinglets for the different chemical environment. For the XPS spectrum of
the CZ900 film two components were used to reproduce the Si 2p peakthese are centred at 101.2 eV and at 102.5. The component at 101.2 eV
was reported to be associated to Si− O for SiOx [41]. The BE value for theSi 2p3/2 peak for willemite Zn2SiO4 is reported between 102.4 − 102.6
eV, which is compatible with our Si 2p peak deconvolution for the
CZ900 thin film [38]. The deconvolution of Si 2p peak evidences thepresence of willemite Zn2SiO4 at 102.5 eV for CZ900 thin film, whereas
there is no signature of the same in the Z900 thin film. A shift of Si 2p
peak in the CZ900 compared to the CZ700 thin film indicates the for­mation of Zn2SiO4. Such a shift is not observed in Z900 compared toZ700 thin film. The Si 2p peak intensity in the absolute scale is almost
seven times higher in the CZ900 thin film than in the CZ700 thin film. In
Z700, Z800, CZ700 and CZ800, the intensity is relatively small as the
thick ZnO film layer has caused the attenuation of the Si signal at thesurface.
Fig. 4(b) shows the O 1 s XPS spectra for all thin films. The compo­
nent centred at 529.2 eV corresponds to the contribution of Zn− Obonding, which does not shift with increasing annealing temperature.
The other components centered at 530.5 eV indicate oxygen vacancies(VO) [33,35,42]. For the CZ700 thin film, three components were used

3.3. Evidence for Zn2SiO4 phase formation from soft X-ray photoemission
and absorption spectroscopy
3.3.1. XPS analysis
In the XPS survey spectra in addition to the expected elements, i.e.,
Zn, Cd and O, we observed the silicon for the CZ900 thin film. Fig. 3(a)shows the Cd 3d5/2 XPS peak corresponding to Cd − O bonding for the
CZ700, CZ800 and CZ900 thin films. This peak shifts to higher binding
energy for increasing annealing temperature reflecting a change in thechemical environment [33,35]. In Fig. 3(b), a similar shift towards
higher BE is observed in the Zn 2p3/2 peak in the CZ900 sample
compared to the CZ700 thin film. While for Z700, Z900, and CZ700
samples the Zn 2p3/2 peak is centred at 1019.7 eV, indicating no change
in the chemical environment of Zn. However, for CZ800 and CZ900 thinfilms, Zn 2p3/2 has shifted to 1020.4 eV and 1021.2 eV, respectively. To
investigate the chemical structure change for the CdxZn1-xO film
deposited on Si, the Auger Parameter (AP) was calculated using the

Fig. 3. X-ray photoelectron spectra for Cd 3d (a) edge for CZ700, CZ800 and CZ900 thin film and Zn 2p (b) edges for Z700, Z900, CZ700, CZ800 and CZ900
thin films.
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Fig. 4. X-ray photoelectron spectra for Si 2p edges (a) and O 1 s edges (b) for Z700, Z900, CZ700, CZ800, and CZ900 thin films.

for deconvolution. Components at 528.5 eV and 529.2 eV correspond to
Cd − O and Zn − O, respectively, for solgel-derived undoped CdO andZnO thin films Cd − O and Zn− O peaks are positioned at 528.0 eV and
529.5 eV, respectively [35,43]. For the CZ800 thin film, using two
components, i.e., Zn − O, and Cd − O provides a good fitting without
considering VO. Longer annealing time in the oxygen atmosphere might
have reduced the VO density. In the CZ900 thin film, a notable shift of
1.6 eV towards a higher BE value is observed in the O1s peak compared
to CZ700. Three components were considered for the fitting: the O(iii)component centered at 531.8 eV is the signature of the Zn2SiO4 phase,
which has been reported by Mier et al. [41]. The O(i) and O(ii) com­ponents indicate the presence of Cd − O and Zn− O bonding, respec­tively. Therefore, the presence of the Zn2SiO4 phase in CZ900 thin films
is evidenced by the Si 2p, O 1 s, and Zn 2p core level spectra.

with the transformed B2 phase, the change in electronic structureassociated with the PT is probed with Zn L3,2 edge. In Fig. 5(a) for CZ900
thin film, a sharp spike-like characteristic feature (a4) rises at 1023.5
eV. A broad hump-like feature appears in the CZ900 thin film at 1035.3
eV (a5) and 1050.4 eV (a6) instead of the valley-like feature of the
CZ800 and CZ700 samples. Such spike-like characteristic feature has
been reported to the willemite Zn2SiO4 phase by Becker et al. [46]. The
Zn L3,2 measurements were performed with surface sensitive TEY mode,indicating that the Zn2SiO4 nanoparticles have out-diffused from thefilm-substrate interface towards the surface of the thin film. From pro­
jected DOS calculation for the trigonal Zn2SiO4 phase, it has been
observed that the valence band maxima (VBM) is formed mainly withhybridized O 2p and Zn 3d bands, and conduction band minima (CBM) is
formed with Zn 4 s and Si 3p states [47]. Consequently, the formation of
the Zn2SiO4 phase is not expected to cause any significant change in the
VBM [47]. Thus, the a4 feature at 1023.7 eV may be generated by thetransition from Zn 2p to coupled Zn 4 s and Si 3p unoccupied states. In
Fig. 5(b), we compare the experimental spectra Z700 and CZ900 withsimulated Zn L3,2 XANES spectra for a few possible structures.
Comparing the experimental ZnO spectrum (Z700) with the calculated
for wurtzite ZnO, we find very good agreement in peak positions andrelative intensities, confirming the structural analysis by XRD andvalidating the theoretical approach. The experimental ZnO spectra aresmoother than the theoretical ones, especially at high energy. This may
be attributed to the fact that we have used a Lorentzian broadening of

3.3.2. XANES at Zn L3,2 edge
Fig. 5(a) shows the pre-, post-edge corrected, and normalized Zn L2,3
edge XANES spectra for Z700, Z800, CZ700, CZ800, and CZ900 thinfilms. The feature a1 around 1023 eV is a signature of the transition fromZn 2p to vacant Zn 4 s [44]. Features a2 and a3 are observed at 1027.8
and 1033 eV for 700 ◦ C and 800 ◦ C annealed thin films. Feature a2
corresponds to the Zn 2p1/2 → 3d antibonding transitions. The a3 featurearises mainly due to the transition to an unoccupied s orbital in the
conduction band (CB) [45].
The XRD and XPS analysis, indicate the presence of Zn2SiO4 along

Fig. 5. (a): Zn L3,2 XANES spectra for Z700, Z900, CZ700, CZ800 and CZ900 thin films; (b) Theoretical Zn L3,2 XANES spectra for wurtzite ZnO, willemite Zn2SiO4
and ZnxCd1-xO in B1 and B2 phases.
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constant width, while energy-dependent self-energy effects, such asplasmon loss, are neglected. As possible structural candidates for the
CZ900 film, we consider willemite Zn2SiO4 and ZnxCd1-xO in the phases
B1 and B2 with x = 1/32 and x = 1/64, respectively. We chose these low
Zn concentrations because the B1 and B2 lattice structures observed in
XRD (Fig. 1(a)) correspond to CdO rather than to cubic ZnO, which isunstable, implying that only a small amount of Zn can be incorporated
into the CdO lattice. The calculated spectra of all three candidate
structures reproduce the pronounced peak at threshold (1024 eV) in theexperimental CZ900 spectrum. At higher energy, however, the theo­
retical spectra differ strongly. The Zn2SiO4 spectrum agrees remarkably
well with CZ900 over the whole energy range. The ZnxCd1-xO spectra(both B1 and B2), in contrast, differ strongly from the data, which ismost clearly seen at energy 1045–1050 eV, where the CZ900 spectrum
has a peak, while the ZnxCd1-xO spectra have a dip. From this XANES
analysis, we conclude that the local structure of the Zn ions in the CZ900
film is the same as in bulk Zn2SiO4, which proves the existence of the
Zn2SiO4 phase in these films.

3.3.4. K edge EXAFS
We have performed EXAFS modeling of Fourier transformed Zn K
edge for Z700, Z900, CZ700, CZ800, and CZ900 thin films with Athenasoftware package to determine the bond length around the photoabsorbing Zn atom and Debye-Waller factor (σ2 ). We did not find sig­
nificant changes in the local structure around the Zn atom between the
different samples. This indicates that during the PT the Zn ions keep atetrahedral ZnO4 coordination, which is common for ZnO wurtzite and
Zn2SiO4 willemite. This result rules out the possibility of substantial Cd
substitution by Zn ions in the CdO sublattice during the PT, since thiswould generate octahedral ZnO6 sites (for B1) or ZnO8 sites (for B2)
which would lead to observable changes in the EXAFS signal. Normal­
ized Zn K edge data and the best fit for magnitude and real part ofFourier transformed χ (k).k2 spectra for all the thin films are shown in
Figure S5(a, b) and S6(a, b) of SI. The details of the EXAFS fitting are
mentioned in the SI file. The values of Zn− O and Zn − Zn bond lengths
and σ2 are shown in Table ST2.
3.4. Diffusion of Zn2SiO4 from film-substrate interface to surface
observed with HAADF-STEM and TEM

3.3.3. XANES at O K edge
Fig. 6 portrays the O K edge XANES data for Z700, Z900, CZ700,
CZ800, and CZ900 thin films. The spectral features b1 and b2 between
531 and 537 eV are attributed to the electronic transitions from O 1 s to
hybridized O 2p and non-localized Zn 4 s states. The sharp b2 feature at
537 eV is due to the electronic transition to hybridized localized O 2pz
and 2px+y [48]. The absorption between 539 and 550 eV is caused by thetransition to the hybridized O 2p and Zn 4d states. Beyond 550 eV, i.e.,
for the b3 feature, electron transitions to hybridized O 2p and Zn 4dstates are responsible for absorption [48]. We do not observe any sig­
nificant change in b1, b2, and b3 features for Z700 and Z900 thin films.
For CZ700 and CZ800 thin films, the overall spectral features are similar
to features of Z700 and Z900 thin films. However, there is a slight in­tensity enhancement for CZ700 and CZ800 thin films compared to Z700
at b1 and b2 features. The transition to hybridized Cd 5 s and O 2p states
also takes part in the absorption process, which might have increased
the intensity. For the CZ900 thin film, the overall spectral feature hascompletely changed, and the intensity has also been reduced at the b1
and b2 regions. In the CZ900 thin film, evaporation of the CdO wasobserved from microscopy, which might have caused such intensity
reduction. There is a 1.1 eV right-hand side shift in the inflection point of
the absorption edge in CZ900 thin film compared to CZ700 thin film.
Such a shift indicates a shift in the CBM towards higher energy [33].
CBM pinning can be considered direct evidence of changing band
structure and possible PT [33].

STEM-EDX mapping for CZ900 thin film is shown in Fig. 7. Other
STEM-EDX mapping for CZ900, CZ700 and Z900 thin films are shown in
Figures S7, S8, and S9 of SI respectively. In Fig. 7, Zn comprised nano­particles are non-uniformly embedded in a matrix containing Cd at the
CZ900 thin film surface region. Element specific histograms at regions 1
and 3 in the bottom panel of Fig. 7 show the coexistence of Si, Zn, and O,
i.e., indicating the possible formation of the Zn2SiO4 phase above the Si
substrate. Si signal with 20 % atomic fraction is present beyond the 200
nm distance above the film-substrate interface in region 3 of Fig. 7.
Similar amount of Si signal is absent above the film-substrate interface
region in CZ700 and Z900 thin films. Even though the Zn2SiO4 phase ispresent in CZ700 and Z900 films, outward diffusion to the surface isfound to be only possible in the CZ900 film. These findings are in linewith previous reports on the formation of a zinc silicate phase in ZnO/Si
heterostructures annealed at 900 ◦ C, where orthorhombic ZnSiO3
nanoparticles embedded in the amorphous interfacial layer wereobserved [49]. The thickness of Si oxide layer increases with increasing
annealing temperature, giving rise to interdiffusion of Zn, Cd, O, and Si
atoms. In Figure S8 for the CZ700 thin film, a SiOx layer above the Si
substrate is more prominent than in S9 for the Z900 thin film. This in­dicates that the presence of Cd supports the inter-diffusion at thefilm-substrate interface.
The cross-sectional TEM image in Fig. 8(a) shows a 5–30 nm thick
SiOx layer just above the Si substrate. The region inside the square box in
Fig. 8(a) is shown at higher magnification in Fig. 8(b). Above the SiOx
layer, nanoparticles with Cd, Si, and O are present. From atomic fraction
profile in Fig. 7, we can also observe the presence of Cd just above the Si
substrate. Beyond this layer, Zn, Si, and O are observed, which confirms
the possibility of Zn2SiO4 formation. The inter-planner spacing value
determined from the lattice fringe in this layer is 0.41 nm, corresponding
to the (3–11) planes of the willemite Zn2SiO4 phase [50].
3.5. Discussion of Pt mechanism and its thermodynamic aspects
In the CdxZn1-xO (x = 0.4) mixed binary oxide system, hightemperature annealing beyond 800 ◦ C has caused a significant atomic
inter-diffusion between the substrate and the film [49]. The notablediffusion of Si, starting at the film-substrate interface and further in­jection into the thin film layer, forms Si− O bonds. The thermal treat­ment at 900 ◦ C annealing temperature enhances the thickness of theamorphous Si oxide layer caused by the interdiffusion of Si, O, Cd, andZn atoms near the SiOx layer [49]. However, the possible thermalinterdiffusion was initiated with 800 ◦ C annealing temperature. As we
can observe from Fig. 1(a), B1 to B2 PT started from 800 ◦ C. Therefore,

Fig. 6. O K XANES spectra for Z700, Z900, CZ700, CZ800 and CZ900
thin films.
6

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Acta Materialia 267 (2024) 119744

Fig. 7. Stacked HAADF-STEM image of CZ900 lamella at the top with elemental mapping at subsequent bottom layers. Atomic fraction profiles across the thin film
are extracted for 3 different regions in the bottom panel.

Fig. 8. (a) Cross-sectional TEM micrograph for CZ900 thin film; (b) The square boxed region in (a) has been shown in a magnified scale where atomic planes of thenanoparticles are visible.

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A. Das et al.

Acta Materialia 267 (2024) 119744

the formation of Zn2SiO4 nanoparticles is anticipated to take place at
800 ◦ C annealing temperature. Initially, an amorphous layer of
Zn2xSi1-xO2 is formed at the lower region of CdxZn1-xO thin film due to Si
diffusion, and the width of this layer would possibly increase due tooxygen diffusion. This oxygen diffusion occurs near the Si substrate
which further causes a deficiency in the Zn2xSi1-xO2 layer. Adjustment of
the stoichiometry embeds the trigonal Zn2SiO4 nanoparticles in the
Zn2xSi1-xO2 interface layer between thin film and Si substrate, which canprovide oxygen for the Zn2xSi1-xO2 interface layer [49]. Further, onehour of annealing at 900 ◦ C temperature provides sufficient energy to
the system, resulting in an outward diffusion of the trigonal Zn2SiO4nanoparticles toward the thin film surface. The formation of Zn2SiO4 in
the ZnO/Si heterostructure and subsequent diffusion towards the film
surface is well reported by Xu et al. [51]. At 900 ◦ C, possible evaporationof CdO nanoparticles from the thin film supports the outward diffusionof Zn2SiO4 nanoparticles, further minimizing the surface energy [33]. As
rhombohedral Zn2SiO4 has a larger unit cell volume, it will, expectedly,
produce compressional stress upon both CdO and ZnO sublattice. The
unit cell volume for ZnO wurtzite, CdO rocksalt, and Zn2SiO4 rhombo­
3

3

pressure/annealing temperature. In a nutshell, the tetrahedral coordi­
nation allows the presence of O 2p − Zn 3d repulsive interaction
throughout the BZ. In contrast, octahedral coordination in CdO withinversion symmetry doesn’t allow Cd 4d− O 2p repulsive interaction at Γ
point, causing shifting of VBM away from the Γ point [20,54]. Such p-d
repulsive interaction is potentially the origin of anomalous VB structure
for IIB-VIA compounds with Rocksalt structure. IIB-VIA compounds withstrong p-d repulsion are prone to structural PT as d orbital in the upper
VB triggers the anomalous dispersion making the system less stable.
Increasing Cd concentration (∼ 50 %), the system becomes prone to PT
in the presence of local pressure [55].
4. Conclusion
In conclusion, high temperature-driven B1 to B2 structural PT in
CdxZn1-xO (x = 0.4) has been reported. X-ray diffractograms and com­plementary Raman spectroscopic results revealed the evolution of theB2 phase with thermal annealing from ~700 ◦ C to ~900 ◦ C. In addition,
microscopic observations reflect grain growth, followed by agglomera­tion of grains and void formation with increasing annealing tempera­
ture. The formation of willemite Zn2SiO4 at the film and substrateinterface and its further out-diffusion with increasing annealing tem­
perature has caused a local pressure generation inside the lattice trig­
gering the PT. Core level spectroscopies XPS, XANES, HAADF-STEM,
and cross-sectional TEM techniques have evidenced the presence anddistribution of willemite Zn2SiO4 nanoparticles in a thin film matrix.
This study reports the first instance of a B1 to B2 phase transition
induced by local pressure in a solgel-derived CdxZn1-xO (x = 0.4) thinfilm. We will further investigate other Cd concentrations to evaluate thepossibility of a transformed B2 phase.

3

hedral lattice systems are 47.40 Å , 105.29 Å , 1569.22 Å , respectively.
At 900 ◦ C temperature, outward diffusion of Zn2SiO4 nanoparticles with
sufficient thermal energy exerts pressure upon the CdO sublattice andachieves a thermodynamically stable B2 phase. The local pressure
exerted by the Zn2SiO4 lattice is enough to trigger PT in the CdO sub­lattice due to lower PTC (89 GPa). However, the same cannot change thephase in ZnO due to higher PTC (256 Gpa). Zn2SiO4 phase formation atfilm-substrate interface is expected for Z900 thin film, but we do not
observe the trace of Zn2SiO4 at the surface with XPS and XAS. This
brings us to the conclusion that Cd incorporation in ZnO matrix supports
the outward diffusion Zn2SiO4 which is not found to be possible only for
ZnO. Such trigonal Zn2SiO4 has caused a mixed phase formation fromhexagonal ZnO to a mixture of hexagonal plus trigonal mixed phase at
800 ◦ C annealing temperature for ZnO/Si heterostructure [52]. The
enthalpy difference (ΔH) in the PT process can be subdivided into twocontributions, i.e., pressure term (ΔPV) and internal energy change(ΔU). B1 to B2 PT can occur if ΔPV becomes more negative than positive
ΔU, which is the barrier energy for stabilizing the B1 phase. The
enhanced local pressure from the Zn2SiO4 lattice causes ΔPV to be morenegative with a compressed CdO sublattice than positive ΔU, further
triggering the PT. Apart from CsCl, as per ab initio calculations, there arepossibly many high-pressure phases for CdO, such as NiAs, Zinc blende,
wurtzite, etc. The intermediate phase is observed in the pathway be­
tween B1 to B2 PT for some TMOs. In the present scenario, we did notobserve any other intermediate phase during B1 to B2 PT for CdO. Being
the end member, Cd is similar to the IIA group element with [Kr]4d105s2
ground state electronic configuration. The filled 4d level might be thereason for its different behavior from other TMOs and similar to the
IIA-VIA family of oxides such as CaO, SrO, and BaO among others [53].
To illustrate the bonding character and description of orbital hy­
bridization with changing Cd concentration, the partial density of states(PDOS) is calculated from ZnO to CdO supercell using GGA. The PDOS isdisplayed in Figure S10(a, b, c, d, and e). PDOS calculation has shownthat the VB in wurtzite ZnO is dominated by Zn 3d and O 2p states(Figure S10(a)). At the top of the VB, Zn 3d levels are hybridized with O
2p levels. In VB from − 4 to − 6 eV, PDOS is dominated by Zn 3d corestates where O 2p states are absent. From the top of the VB, i.e., from 0 to
− 4 eV, mainly Zn 3d and O 2p hybridized states are present. In
Figure S10(e), for Rocksalt CdO, the top of the VB is constructed with theO 2p states. The hybridized O 2p and Cd 4d states are confined within − 3
to − 4 eV below the VBM. In our earlier work, temperature-mediatedRocksalt to wurtzite phase transformation (PT) in CdO thin film has
been reported where the VBM has shifted from zone boundary to zone
center with PT [33]. So, with such Cd concentration in the ZnO matrix,
we remark that the system is on the verge of triggering a structural PT
with the possible external influence of change in external

Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgment
A. Das acknowledges Wallonie Bruxelles International’s postdoctoral funding for providing scholarship during this research work.
A. Das is thankful to PHC TOURNESOL’s short-term scholarship for
traveling to Nantes, France, to perform cluster computation. A. Das isthankful to the Director of IMN for providing access to high-performancecluster for performing computation. Authors are thankful to Glicid and
CCIPL for clustus computation at IMN. CB is a research associate of the
FNRS. This publication was partly developed under the provision of thePolish Ministry and Higher Education project Support for research anddevelopment with the use of research infrastructure of the “National
Synchrotron Radiation Centre SOLARIS” under contract nr 1/SOL/
2021/2. CLC acknowledges support by Taiwan Ministry of Science andTechnology (grants MOST 111–2112-M-213–030 - and NSTC 112–2112M-213–013 -). Authors are thankful to IMN’s equipment platform,
PLASSMAT, Nantes, France for STEM-EDX measurements.
Supplementary materials
Supplementary material associated with this article can be found, inthe online version, at doi:10.1016/j.actamat.2024.119744.
References[1] J.de O. Primo, J.de S. Correa, D.F.L. Horsth, A. Das, M. Zając, P. Umek, R. Wattiez,
F.J. Anaissi, R.C.A. Onderwater, C. Bittencourt, Antiviral properties against SARSCoV-2 of nanostructured ZnO obtained by green combustion synthesis and coatedin waterborne acrylic coatings, Nanomaterials 12 (2022) 4345.

8

A. Das et al.

Acta Materialia 267 (2024) 119744[2] R. Serhane, S. Abdelli-Messaci, S. Lafane, H. Khales, W. Aouimeur, A. Hassein-Bey,
T. Boutkedjirt, Pulsed laser deposition of piezoelectric ZnO thin films for bulk
acoustic wave devices, Appl. Surf. Sci. 288 (2014) 572–578.
[3] B. Bhat, S. Chakraborty, Acoustic-wave-induced analyte separation in narrow
fluidic confinements in the presence of interfacial interactions, Langmuir. 26(2010) 15035–15043.
[4] S. Agarwal, P. Rai, E.N. Gatell, E. Llobet, F. Güell, M. Kumar, K. Awasthi, Gassensing properties of ZnO nanostructures (flowers/rods) synthesized byhydrothermal method, Sens. Actuators. B Chem. 292 (2019) 24–31.
[5] K.M. Yu, M.A. Mayer, D.T. Speaks, H. He, R. Zhao, L. Hsu, S.S. Mao, E.E. Haller,
W. Walukiewicz, Ideal transparent conductors for full spectrum photovoltaics,
J. Appl. Phys. 111 (2012).
[6] Y. Yang, S. Jin, J.E. Medvedeva, J.R. Ireland, A.W. Metz, J. Ni, M.C. Hersam, A.
J. Freeman, T.J. Marks, CdO as the archetypical transparent conducting oxide.
Systematics of dopant ionic radius and electronic structure effects on chargetransport and band structure, J. Am. Chem. Soc. 127 (2005) 8796–8804.
[7] M. Yan, M. Lane, C.R. Kannewurf, R.P.H. Chang, Highly conductive epitaxial CdO
thin films prepared by pulsed laser deposition, Appl. Phys. Lett. 78 (2001)
2342–2344.
[8] Y. Chen, S. Zhang, W. Gao, F. Ke, J. Yan, B. Saha, C. Ko, J. Suh, B. Chen, J.
W. Ager III, Pressure-induced structural transition of CdxZn1− xO alloys, Appl.
Phys. Lett. 108 (2016) 152105.
[9] A. Janotti, C.G. Van de Walle, Fundamentals of zinc oxide as a semiconductor, Rep.
Progr. Phys. 72 (2009) 126501.
[10] A. Schleife, C. Rödl, J. Furthmüller, F. Bechstedt, Electronic and optical propertiesof MgxZn1− xO and CdxZn1− xO from ab initio calculations, New. J. Phys. 13(2011) 085012.
[11] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda,
H. Koinuma, Band gap engineering based on Mg x Zn 1− x O and Cd y Zn 1− y O
ternary alloy films, Appl. Phys. Lett. 78 (2001) 1237–1239.
[12] V. Venkatachalapathy, A. Galeckas, M. Trunk, T. Zhang, A. Azarov, A.Y. Kuznetsov,
Understanding phase separation in ZnCdO by a combination of structural andoptical analysis, Phys. Rev. B 83 (2011) 125315.
[13] A. Schleife, M. Eisenacher, C. Rödl, F. Fuchs, J. Furthmüller, F. Bechstedt, Ab initio
description of heterostructural alloys: thermodynamic and structural properties ofMg x Zn 1− x O and Cd x Zn 1− x O, Phys. Rev. B 81 (2010) 245210.
[14] J. Ishihara, A. Nakamura, S. Shigemori, T. Aoki, J. Temmyo, Zn1− xCdxO systems
with visible band gaps, Appl. Phys. Lett. 89 (2006).
[15] T. Ohashi, K. Yamamoto, A. Nakamura, T. Aoki, J. Temmyo, Optical properties ofwurtzite Zn1-xCdxO films grown by remote-plasma-enhanced metalorganic
chemical vapor deposition, Jpn. J. Appl. Phys. 46 (2007) 2516.
[16] D.M. Detert, S.H.M. Lim, K. Tom, A.V. Luce, A. Anders, O.D. Dubon, K.M. Yu,
W. Walukiewicz, Crystal structure and properties of CdxZn1− xO alloys across thefull composition range, Appl. Phys. Lett. 102 (2013).
[17] X. Ma, P. Chen, R. Zhang, D. Yang, Optical properties of sputtered hexagonal
CdZnO films with band gap energies from 1.8 to 3.3eV, J. Alloys. Compd. 509(2011) 6599–6602.
[18] H. Liu, H. Mao, M. Somayazulu, Y. Ding, Y. Meng, D. Häusermann, B 1-to-B 2 phasetransition of transition-metal monoxide CdO under strong compression, Phys. Rev.
B 70 (2004) 094114.
[19] J.E. Jaffe, J.A. Snyder, Z. Lin, A.C. Hess, LDA and GGA calculations for highpressure phase transitions in ZnO and MgO, Phys. Rev. B 62 (2000) 1660.
[20] R.J. Guerrero-Moreno, N. Takeuchi, First principles calculations of the groundstate properties and structural phase transformation in CdO, Phys. Rev. B 66
(2002) 205205.
[21] A.N. Baranov, P.S. Sokolov, V.A. Tafeenko, C. Lathe, Y. v Zubavichus, A.
A. Veligzhanin, M. v Chukichev, V.L. Solozhenko, Nanocrystallinity as a route tometastable phases: rock salt ZnO, Chem. Mater. 25 (2013) 1775–1782.
[22] N. Gautam, F. Singh, S.K. Gautam, R.G. Singh, S. Ojha, A. Kapoor, Growth of highly
transparent CdxZn1− xO (CZO) thin films: structural and optical studies, J. Alloys.
Compd. 650 (2015) 311–317, https://doi.org/10.1016/j.jallcom.2015.07.283.
[23] M. Zając, T. Giela, K. Freindl, K. Kollbek, J. Korecki, E. Madej, K. Pitala, A. KoziołRachwał, M. Sikora, N. Spiridis, The first experimental results from the 04BM
(PEEM/XAS) beamline at Solaris, Nucl. Instrum. Methods Phys. Res. B 492 (2021)
43–48.
[24] J. Szlachetko, J. Szade, E. Beyer, W. Błachucki, P. Ciochoń, P. Dumas, K. Freindl,
G. Gazdowicz, S. Glatt, K. Guła, SOLARIS national synchrotron radiation centre in
Krakow, Poland, Euro. Phys. J. Plus 138 (2023) 1–10.
[25] J.P. Perdew, M. Ernzerhof, K. Burke, Rationale for mixing exact exchange withdensity functional approximations, J. Chem. Phys. 105 (1996) 9982–9985.
[26] K. Mathew, R. Sundararaman, K. Letchworth-Weaver, T.A. Arias, R.G. Hennig,
Implicit solvation model for density-functional study of nanocrystal surfaces andreaction pathways, J. Chem. Phys. 140 (2014) 084106.
[27] H.J. Monkhorst, J.D. Pack, Special points for Brillouin-zone integrations, Phys.
Rev. B 13 (1976) 5188.
[28] E. Péan, J. Vidal, S. Jobic, C. Latouche, Presentation of the PyDEF post-treatment
Python software to compute publishable charts for defect energy formation, Chem.
Phys. Lett. 671 (2017) 124–130.
[29] A. Stoliaroff, S. Jobic, C. Latouche, PyDEF 2.0: an easy to use post-treatment
software for publishable charts featuring a graphical user interface, J. Comput.
Chem. 39 (2018) 2251–2261.

[30] Y. Caglar, M. Caglar, S. Ilican, A. Ates, Morphological, optical and electricalproperties of CdZnO films prepared by sol–gel method, J. Phys. D. Appl. Phys. 42(2009) 65421.
[31] A. Singh, D. Kumar, P.K. Khanna, A. Kumar, M. Kumar, Dielectric anomaly in Mg
doped ZnO thin film deposited by Sol–Gel method, J. Electrochem. Soc. 158 (2010)
G9.
[32] W.J. Lee, Y.H. Chang, Growth without postannealing of monoclinic VO2 thin filmby atomic layer deposition using VCl4 as precursor, Coatings 8 (2018) 431.
[33] A. Das, C.P. Saini, D. Singh, R. Ahuja, A. Kaur, S. Aliukov, D. Shukla, F. Singh, Hightemperature-mediated rocksalt to wurtzite phase transformation in cadmium oxide
nanosheets and its theoretical evidence, Nanoscale 11 (2019) 14802–14819.
[34] S.S. Brenner, G.W. Sears, Mechanism of whisker growth—III nature of growth sites,
Acta Metallurgica 4 (1956) 268–270.
[35] A. Das, D. Singh, C.P. Saini, R. Ahuja, A. Kaur, S. Aliukov, Orbital hybridizationinduced band offset phenomena in Ni x Cd 1− x O thin films, Nanoscale 12 (2020)
669–686.
[36] G. Moretti, Auger parameter and Wagner plot in the characterization of chemical
states by X-ray photoelectron spectroscopy: a review, J. Electron. Spectros. Relat.
Phenomena 95 (1998) 95–144.
[37] R. Mu, A. Steigert, N. Lin, R. Wilks, M. Bär, Y. Zhang, The chemical structure of the
ZnO/SiC heterointerface as revealed by electron spectroscopies, J. Phys. D. Appl.
Phys. 48 (2015) 305304.
[38] L.S. Dake, D.R. Baer, J.M. Zachara, Auger parameter measurements of zinccompounds relevant to zinc transport in the environment, Surf. Interface Anal. 14(1989) 71–75.
[39] J.G. Ma, Y.C. Liu, C.S. Xu, Y.X. Liu, C.L. Shao, H.Y. Xu, J.Y. Zhang, Y.M. Lu, D.
Z. Shen, X.W. Fan, Preparation and characterization of ZnO particles embedded in
Si O 2 matrix by reactive magnetron sputtering, J. Appl. Phys. 97 (2005) 103509.
[40] D. Gerlach, M. Wimmer, R.G. Wilks, R. Félix, F. Kronast, F. Ruske, M. Bär, The
complex interface chemistry of thin-film silicon/zinc oxide solar cell structures,Phys. Chem. Chem. Phys. 16 (2014) 26266–26272.
[41] U. Meier, C. Pettenkofer, Morphology of the Si–ZnO interface, Appl. Surf. Sci. 252(2005) 1139–1146.
[42] A. Kaur, D. Singh, A. Das, K. Asokan, C.L. Chen, I.B. Mishra, R. Ahuja, Spin andvalence variation in cobalt doped barium strontium titanate ceramics, Phys. Chem.
Chem. Phys. 24 (2022) 19865–19881.
[43] G. Dasi, T. Lavanya, S. Suneetha, S. Vijayakumar, J.J. Shim, K. Thangaraju, Raman
and X-ray photoelectron spectroscopic investigation of solution processed Alq3/
ZnO hybrid thin films, Spectrochim. Acta a Mol. Biomol. Spectrosc. 265 (2022)
120377.
[44] J.W. Chiou, K.P.K. Kumar, J.C. Jan, H.M. Tsai, C.W. Bao, W.F. Pong, F.Z. Chien, M.
H. Tsai, I.H. Hong, R. Klauser, Diameter dependence of the electronic structure ofZnO nanorods determined by x-ray absorption spectroscopy and scanningphotoelectron microscopy, Appl. Phys. Lett. 85 (2004) 3220–3222.
[45] M.W. Murphy, W.R. Diebel, Y.M. Yiu, T.K. Sham, Cr doped ZnO nanostructures:
synthesis, electronic structures, and magnetic properties, Can. J. Chem. 95 (2017)
1225–1232.
[46] C. Becker, M. Pagels, C. Zachäus, B. Pollakowski, B. Beckhoff, B. Kanngießer,
B. Rech, Chemical speciation at buried interfaces in high-temperature processed
polycrystalline silicon thin-film solar cells on ZnO: al, J. Appl. Phys. 113 (2013)
044519.
[47] S.Z. Karazhanov, P. Ravindran, H. Fjellvåg, B.G. Svensson, Electronic structure andoptical properties of ZnSiO 3 and Zn 2 SiO 4, J. Appl. Phys. 106 (2009) 123701.
[48] S. Krishnamurthy, C. McGuinness, L.S. Dorneles, M. Venkatesan, J.M.D. Coey, J.
G. Lunney, C.H. Patterson, K.E. Smith, T. Learmonth, P.A. Glans, Soft-x-ray
spectroscopic investigation of ferromagnetic Co-doped ZnO, J. Appl. Phys. 99(2006) 08M111.
[49] J.M. Yuk, J.Y. Lee, J.H. Jung, D.U. Lee, T.W. Kim, D.I. Son, W.K. Choi, Formation
mechanism of Zn Si O 3 nanoparticles embedded in an amorphous interfacial layer
between a ZnO thin film and an n-Si (001) substrate due to thermal treatment,
J. Appl. Phys. 103 (2008) 083520.
[50] A. Das, C. Balasubramanian, P. Orpe, G.M. Pugliese, A. Puri, A. Marcelli, N.L. Saini,Morphological, electronic, and magnetic properties of multicomponent cobalt
oxide nanoparticles synthesized by high temperature arc plasma, Nanotechnology.
33 (2021) 095603.
[51] X. Xu, P. Wang, Z. Qi, H. Ming, J. Xu, H. Liu, C. Shi, G. Lu, W. Ge, Formation
mechanism of Zn2SiO4 crystal and amorphous SiO2 in ZnO/Si system, J. Phys.
Condens. Matter 15 (2003) L607.
[52] X. Xu, C. Guo, Z. Qi, H. Liu, J. Xu, C. Shi, C. Chong, W. Huang, Y. Zhou, C. Xu,
Annealing effect for surface morphology and luminescence of ZnO film on silicon,
Chem. Phys. Lett. 364 (2002) 57–63.
[53] P. Toledano, K. Knorr, L. Ehm, W. Depmeier, Phenomenological theory of thereconstructive phase transition between the NaCl and CsCl structure types, Phys.
Rev. B 67 (2003) 144106.
[54] J.E. Jaffe, R. Pandey, A.B. Kunz, Electronic structure of the rocksalt-structure
semiconductors ZnO and CdO, Phys. Rev. B 43 (1991) 14030.
[55] Y. Chen, S. Zhang, W. Gao, F. Ke, J. Yan, B. Saha, C. Ko, J. Suh, B. Chen, J.W. Ager,
Pressure-induced structural transition of CdxZn1− xO alloys, Appl. Phys. Lett. 108(2016).

9