Long-term evolution of luminescent properties in CdI₂ crystals
Fresh and aged melt-grown or gas-phase grown CdI₂ crystals are studied by means of low-temperature photoluminescence spectroscopy. Noticeable transformations of emission spectra are observed after long-term aging. The formation of nanostructures containing cadmium oxide and cadmium hydroxide as well...
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| description | Fresh and aged melt-grown or gas-phase grown CdI₂ crystals are studied by means of low-temperature photoluminescence spectroscopy. Noticeable transformations of emission spectra are observed after long-term aging. The formation of nanostructures containing cadmium oxide and cadmium hydroxide as well as the changes in local surrounding of iodine atoms and the possible growth of polytypic modifications of CdI₂ are taken into account when considering the diversity of optical spectra.
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Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 7, pp. 756–759
Long-term evolution of luminescent properties
in CdI2 crystals
I. Karbovnyk, I. Bolesta, I. Rovetskyi, V. Lesivtsiv, Ya. Shmygelsky, and S. Velgosh
Department of Electronics, Ivan Franko National University of Lviv
107 Tarnavskogo Str., Lviv 79017, Ukraine
E-mail: ivan_karbovnyck@yahoo.com
A.I. Popov
Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga LV-1063, Latvia
Received April 6, 2016, published online May 25, 2016
Fresh and aged melt-grown or gas-phase grown CdI2 crystals are studied by means of low-temperature photo-
luminescence spectroscopy. Noticeable transformations of emission spectra are observed after long-term aging.
The formation of nanostructures containing cadmium oxide and cadmium hydroxide as well as the changes in lo-
cal surrounding of iodine atoms and the possible growth of polytypic modifications of CdI2 are taken into account
when considering the diversity of optical spectra.
PACS: 78.55.–m Photoluminescence, properties and materials;
78.67.–n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures.
Keywords: СdI2, luminescence, nanostructures, spectroscopy.
Introduction
Cadmium iodide crystals are the representatives of MX2
halide family. A notable feature of MX2 crystals is their lay-
ered structure formed by a hexagonal close packing of iodine
atoms, while a half of octahedral interstices in the lattice are
filled with cadmium atoms. The principal structural element
of the lattice is a 2+ 4–
6[Cd I ]− octahedral complex formed
by 2+Cd in an octahedral interstice [1–4]. Previous investi-
gations of CdI2 crystals have been performed both theoretical-
ly [5–8] and experimentally [9–15].
The interaction between cadmium atoms and iodine at-
oms (that are easily polarisable) leads to the formation of
І–Сd–І triple layers with a strong ionic-covalent bonding
within the individual layer. The bonding energy, as estimat-
ed from the Cd–I dissociation energy is about 2.2 eV. Adja-
cent triple layers are weakly bonded by Van der Waals
(VdW) interaction, therefore there is a significant anisotro-
py of bonding Сd–І/I–І forces [16].
A weak bonding between the adjacent layers allows the
shifts of a part of the crystal along (0001) base planes. This
circumstance, consequently, creates a one-dimensional
(along c axis) structural disordering due to stacking faults.
Being repeated periodically, such disordering results in
polytypic modifications.
One-dimensional disordering can be revealed in x-ray
diffraction (XRD) patterns as diffusion bands around 10l
reflexes (where l = 0, 1, …). Another type of CdI2 structural
deformation is connected with the appearance of vertical
walls formed by dislocations that govern the block-like
structure of the crystal. It was revealed, that XRD patterns of
CdI2 undergo changes over time because of the movement
of edge and partial dislocations due to a low binding energy
(0.02 eV) between the adjacent І–Сd–І layers.
Aging processes are known to affect the functionality of
various devices such as functional elements based on ce-
ramics [17–19], polymer hybrid structures [20] or scintilla-
tors [21]. It was shown [22–24] that nanostructures are
formed on the surface of CdI2 crystals in the process of
their aging in air. The formation mechanisms of nanostruc-
tures involve several stages: inception, growth and interac-
tion between individual structures. In Ref. 22 it is estab-
lished that these nanostructures contain cadmium oxide and
cadmium hydroxide. A similar effect was observed in the
photoluminescence studies of porous silicon (PS) [25],
where quantum dots and quantum wires are formed from
PS nanocrystals due to surface oxidation.
Since optical properties are also determined by the struc-
ture, one should expect evolution of optical response of
CdI2 over a long period of time. It has to be noted that all
© I. Karbovnyk, I. Bolesta, I. Rovetskyi, V. Lesivtsiv, Ya. Shmygelsky, S. Velgosh, and A.I. Popov, 2016
mailto:ivan_karbovnyck@yahoo.com
Long-term evolution of luminescent properties in CdI2 crystals
CdX2, (X = Br, Cl, I) crystals strongly differ from alkali
and alkaline earth halides, where self-trapped holes (so-
called Vk), F centers and defect formation via exciton decay
are well established [26].
This work aims to study the emission spectra of CdI2
crystals in the process of long-term aging. The principal
tool for this study is a low-temperature luminescence
spectroscopy.
Experimental
Melt-grown cadmium iodide crystals were obtained by
means of the Bridgman–Stockbarger technique from a raw
material, which was previously purified by applying zone
meting [27]. Thin single crystalline plates of gas-phase
grown CdI2 were formed in the upper part of the growth
ampule.
The low-temperature luminescence measurements were
performed at SUPERLUMI beamline (HASYLAB at
DESY, Hamburg) using 4–20 eV synchrotron radiation
from the DORIS storage ring for excitation [28]. This ex-
perimental set-up is a unique tool for investigations of dif-
ferent types of wide band gap materials [29–34]. Synchro-
tron radiation intensity was 1012 photons per second.
BM50 monochromator operating in a spectral range from
1.3 to 6.2 eV was employed to disperse the light emitted
from the sample. The emission was eventually detected by
the SI-440-UV photomultiplier operating in а photon
counting mode.
Nuclear quadrupole resonance (NQR) spectra of 127I
isotope were recorded with the IS-3 radiospectrometer be-
tween 10 and 20 MHz at liquid nitrogen temperature.
Results and discussion
Time evolution of the CdI2 emission spectra at a long-
term (several years) aging in air is shown in Fig. 1. Fresh-
ly grown crystals (a bottom plot in Fig. 1) show an inten-
sive band peaked at 2.48 eV (501 nm, G-band) with a
weak component on the long-wavelength shoulder (2.25 eV,
551 nm, Y-band). This emission does not depend on the
excitation wavelength and is related to the radiative re-
combination of self-trapped excitons in 2+ 4–
6[Cd I ]− mo-
lecular complexes [35,36].
Long-term aging significantly changes the emission
spectra of CdI2. Middle and top plots in Fig. 1 show СdI2
emission spectra after 2-year and 4-year aging, respective-
ly. One can observe the shift of G- and Y-bands towards
long-wavelength region. In the middle plot, the G-band is
peaked at 2.42 eV (514 nm) and Y-band shifts to 2.04 eV
(609 nm). Furthermore, the intensities of these bands are
redistributed.
In the freshly grown crystal G-band is a dominant one.
However, in the process of aging in air the intensity of G-band
becomes comparable to that of the Y-band (see middle and
bottom plots). It has to be mentioned that similar changes
were observed in the luminescence spectra of СdI2 films
after their annealing at Т = 420 K: in addition to G-band,
an intensive luminescence around 2.2 eV (565 nm) was
detected [37].
Further aging of CdI2 (top plot) leads to the appearance
of a narrow band with the maximum at 1.87 eV (665 nm)
as well as of a long-wavelength emission around 1.68 eV
(740 nm). The transformation of CdI2 photoluminescence
spectra over time, as supported by atomic force microsco-
py results, may be connected with the creation of nano-
structures on the crystal surface [4,22–24] and with the
modification of the crystal structure: the annihilation of
moving defects and the appearance of PbI2 nanophases.
Let us discuss the possible nature of the 1.87 eV emis-
sion. A specific feature of this narrow band is its band-
width of FWHM = 0.05 eV, which is dramatically different
from the respective values for other bands (see Table 1). It
is plausible to assume that this band is related to СdO and
Сd(OH)2 nanoinclusions.
Nanophases are characterized by discrete energy spec-
tra. Some of energy levels may be localized within the
band gap of CdI2. Optical transitions between these levels
can be responsible for the 1.87 eV (665 nm) lumines-
cence. The fact that the band position does not depend on
temperature speaks in favor of such model, since the
spectrum of nanophases is to a large extent determined by
their small size, which is not strongly affected by tempe-
rature. The band at 1.68 (740 nm) is related to uncon-
trolled lead impurity.
Fig. 1. Photoluminescence spectra of freshly melt-grown (c),
2-year aged (b) and 4-year (a) aged СdI2 crystals. Spectra are
recorded under the excitation by 12.4 eV (100 nm) photons at 8 K.
Table 1. Peak position and respective FWHM parameter for
luminescence bands of aged melt-grown CdI2 crystals
Energy, eV FWHM, eV
1.68 0.08
1.87 0.05
2.13 0.18
2.44 0.09
Low Temperature Physics/Fizika Nizkikh Temperatur, 2016, v. 42, No. 7 757
I. Karbovnyk, I. Bolesta, I. Rovetskyi, V. Lesivtsiv, Ya. Shmygelsky, S. Velgosh, and A.I. Popov
We have also examined the changes in the emission
spectra of aged melt-grown and gas-phase-grown СdI2
crystals (see Fig. 2). In both spectra the band at 1.87 is
clearly identified, whereas there are some noticeable dif-
ferences in the short-wavelength region of the spectra.
One of the reasons for these differences is the crystal
thickness: melt-grown samples are 0.1 to 0.5 mm thick,
whereas the thickness of single-crystalline plates of gas-
phase-grown CdI2 does not exceed 10 µm. Thin crystals
possess increased ratio of the number of surface atoms (Ns)
to the number of bulk (Nv) atoms. Thus, surface atoms
should have the prevailing contribution to the optical prop-
erties of the crystal. Geometrical order of atom distribution
in the bulk (volume) and on the surface is also different.
This conclusion is confirmed by nuclear quadrupole reso-
nance (NQR) frequency studies of 127I in CdI2 crystals.
NQR measurements of 127І in melt-grown СdI2 crystals
show two maxima at 14.34 and 4.74 MHz, while four NQR
maxima at 14.07, 14.33, 14.74 and 15.02 MHz were ob-
served in gas-phase-grown crystals (see Fig. 3).
Thus, the emission spectra for melt- or gas-phase-
grown СdI2 crystals should be different as well, since they
are defined by both the electronic structure of centers and
the local surrounding. Hence, the structure of 2+ 4–
6[Cd I ]−
complexes localized in the volume (bulk) or on the surface
is different.
Two pronounced maxima at 1ν = 14.34 MHz and 2ν =
= 14.74 MHz (exact frequencies may slightly vary from
sample to sample) correspond to two unequal crystallo-
graphic positions of iodine in the unit cell of СdI2. The
other two maxima at 1′ν < ν and 2′′ν > ν may be related to
surface iodine atoms that have a different crystallographic
surrounding.
Another possible reason for the observed differences in
optical spectra can be related to the emission from the
nanophases formed on the СdI2 surface. The contribution
of these nanophases to the total luminescence yield should
be larger in case of gas-phase-grown CdI2, since due to a
small thickness the concentration of centers inside
nanophases is increased with respect to the concentration
of intrinsic emission centers. The latter conclusion is con-
firmed by different intensities of intrinsic emission of CdI2
in a spectral region about 3.38 eV (see Fig. 2).
At the same time, various polytypic modifications could be
formed in gas-phase-grown СdI2 crystals and their emission
spectra are sensitive to the structure of a specific polytype.
It is very likely that all mentioned factors occur simul-
taneously, governing the complex time evolution of a low
temperature photoluminescence in CdI2.
Conclusions
Long-term aging of CdI2 crystals leads to the modifica-
tion of their low-temperature luminescence spectra. In the
freshly grown crystals, green luminescence is prevailing,
while upon aging in air, the intensity of yellow emission
grows considerably. This fact can be associated with the for-
mation of nanostructures on the crystal surface and their trans-
formation over time. Different structure of 2+ 4–
6[Cd I ]−
complexes localized in the bulk and on the surface deter-
mines the distinctions in the emission spectra of melt-
grown and gas-phase-grown CdI2 crystals.
Acknowledgements
A.I. Popov would like to thank the support of State re-
search program IMIS-2. The authors are grateful to Prof.
A. Voloshinovskii and Dr. V. Vistovskyy for the assistance
with the low-temperature luminescence experiment.
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Fig. 3. NQR spectra of 127I isotopes in melt-grown and gas-
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Introduction
Experimental
Results and discussion
Conclusions
Acknowledgements
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| id | nasplib_isofts_kiev_ua-123456789-129196 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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| last_indexed | 2025-12-02T02:19:56Z |
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| record_format | dspace |
| spelling | Karbovnyk, I. Bolesta, I. Rovetskyi, I. Lesivtsiv, V. Shmygelsky, Ya. Velgosh, S. Popov, A.I. 2018-01-16T18:03:43Z 2018-01-16T18:03:43Z 2016 Long-term evolution of luminescent properties in CdI₂ crystals / I. Karbovnyk, I. Bolesta, I. Rovetskyi, V. Lesivtsiv, Ya. Shmygelsky, S. Velgosh, A.I. Popov // Физика низких температур. — 2003. — Т. 42, № 7. — С. 756-759. — Бібліогр.: 37 назв. — англ. 0132-6414 PACS: 78.55.–m, 78.67.–n https://nasplib.isofts.kiev.ua/handle/123456789/129196 Fresh and aged melt-grown or gas-phase grown CdI₂ crystals are studied by means of low-temperature photoluminescence spectroscopy. Noticeable transformations of emission spectra are observed after long-term aging. The formation of nanostructures containing cadmium oxide and cadmium hydroxide as well as the changes in local surrounding of iodine atoms and the possible growth of polytypic modifications of CdI₂ are taken into account when considering the diversity of optical spectra. A.I. Popov would like to thank the support of State research program IMIS-2. The authors are grateful to Prof. A. Voloshinovskii and Dr. V. Vistovskyy for the assistance with the low-temperature luminescence experiment. en Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України Физика низких температур Low-Temperature Radiation Effects in Wide Gap Materials Long-term evolution of luminescent properties in CdI₂ crystals Article published earlier |
| spellingShingle | Long-term evolution of luminescent properties in CdI₂ crystals Karbovnyk, I. Bolesta, I. Rovetskyi, I. Lesivtsiv, V. Shmygelsky, Ya. Velgosh, S. Popov, A.I. Low-Temperature Radiation Effects in Wide Gap Materials |
| title | Long-term evolution of luminescent properties in CdI₂ crystals |
| title_full | Long-term evolution of luminescent properties in CdI₂ crystals |
| title_fullStr | Long-term evolution of luminescent properties in CdI₂ crystals |
| title_full_unstemmed | Long-term evolution of luminescent properties in CdI₂ crystals |
| title_short | Long-term evolution of luminescent properties in CdI₂ crystals |
| title_sort | long-term evolution of luminescent properties in cdi₂ crystals |
| topic | Low-Temperature Radiation Effects in Wide Gap Materials |
| topic_facet | Low-Temperature Radiation Effects in Wide Gap Materials |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/129196 |
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