With the advantages of selectivity, spectral resolution and reduction of interference on account of light scattering, synchronous luminescence spectroscopy (SLS) is successfully applied to analyze complex mixtures with overlapped emission and/or excitation spectra. In fact, it is difficult to clearly distinguish the contributions of various luminescence centers to low-energy band of semiconductor nanocrystals (NCs). Herein, we report the application of SLS method to detect luminescence centers in colloidal Cdsub0.3/subZnsub0.7/subS NCs. Their conventional luminescence and synchronous luminescence spectra were comparatively investigated. Differently from conventional luminescence spectrum, the emission peaks at 460 and 515 nm were found using SLS method. They are attributed to the emission transitions related to sulfur and zinc/cadmium vacancies. The obtained results are useful to clarify the nature of luminescence centers as well as relaxation mechanism in Cdsubx/subZnsub1-x/subS NCs

Lieu, Nguyen Thi Thuy

Nghia, Nguyen Xuan

Quang, Vu Xuan

Thang, Phi Van

Thanh, Nguyen Trong

Tuyen, Ho Van

English

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 30, No. 2 (2020), pp. 181-187
DOI:10.15625/0868-3166/30/2/13819
DETECTION OF LUMINESCENCE CENTERS IN COLLOIDAL Cd0.3Zn0.7S
NANOCRYSTALS BY SYNCHRONOUS LUMINESCENCE SPECTROSCOPY
PHI VAN THANG1, HO VAN TUYEN2, VU XUAN QUANG2, NGUYEN THI THUY LIEU3,
NGUYEN TRONG THANH4,†AND NGUYEN XUAN NGHIA5,†
1Quy Nhon University, 170 An Duong Vuong, Quy Nhon, Binh Dinh
2Duy Tan University, 03 Quang Trung, Hai Chau, Da Nang
3Posts and Telecommunications Institute of Technology, Km 10 Nguyen Trai, Thanh Xuan, Hanoi
4Institute of Materials Science, 18 Hoang Quoc Viet, Cau Giay, Hanoi
5Institute of Physics, 10 Dao Tan, Ba Dinh, Hanoi
†E-mail: thanhnt@ims.vast.ac.vn; nxnghia@iop.vast.vn
Received 16 May 2019
Accepted for publication 14 May 2020
Published 19 May 2020
Abstract. With the advantages of selectivity, spectral resolution and reduction of interference on
account of light scattering, synchronous luminescence spectroscopy is successfully applied to an-
alyze complex mixtures with overlapped emission and/or excitation spectra. Herein, we report the
application of synchronous luminescence spectroscopy to detect luminescence centers in colloidal
Cd0.3Zn0.7S nanocrystals and the emission peaks at 460 and 515 nm which are attributed to the
emission transitions related to sulfur and zinc/cadmium vacancies. The obtained results are use-
ful to clarify the nature of luminescence centers as well as relaxation mechanism in CdxZn1−xS
nanocrystals.
Keywords: Colloidal Cd0.3Zn0.7S nanocrystals, Synchronous luminescence spectroscopy, Lumi-
nescence centers.
Classification numbers: 78.67.Bf, 78.55.Et, 81.07.Bc.
©2020 Vietnam Academy of Science and Technology
182 DETECTION OF LUMINESCENCE CENTERS IN COLLOIDAL Cd0.3Zn0.7S NANOCRYSTALS . . .
I. INTRODUCTION
Photoluminescence (PL) measurements yield information about the energetic positions of
the electronic states in the band gap of semiconductors. Such localized states can be due to various
types of structural imperfections like vacancies, interstitial atoms, and atoms at surfaces and grain
boundaries. The imperfections can create either donor or acceptor levels in the band gap, and it is
often difficult to determine their exact energy positions because the donor or acceptor levels may
be very close in energy and cannot be resolved in the luminescence bands [1-3]. It is possible that
a significant narrowing of emission peaks is achieved by using synchronous luminescence spec-
troscopy (SLS) [4]. In the conventional PL spectroscopy, the spectrum is obtained by scanning the
emission wavelength (λemiss) under the photoexcitation at a chosen wavelength (λexc). The exper-
iment by SLS is performed by varying synchronously both the photoexcitation λexc and emission
λemiss wavelengths at a constant difference ∆λ = λemiss−λexc.
Recently, SLS was successfully applied for size dependent simultaneous analysis of CdTe
nanocrystals (NCs) and their mixtures [5], to study the midgap electronic states in nanocrystalline
SrTiO3 [6], CaTiO3 [7], anatase and rutile TiO2 [8,9]. Herein, we report the application of SLS to
detect luminescence centers in homogeneous Cd0.3Zn0.7S NCs prepared by wet chemical method.
II. EXPERIMENT
Synthesis and purification of Cd0.3Zn0.7S NCs
Initial chemicals, including zinc stearate (Zn(St)2, 98%), cadmium oxide (CdO, 99.99%,
powder), sulfur (S, 99.98%, flour), 1-octadecene (ODE, 90%), and stearic acid (SA, 90%), were
purchased from Aldrich and used as received without further purification.
For the synthesis of Cd0.3Zn0.7S NCs, a mixture of S (1 mmol) and ODE (24 mmol) was
stirred at 100˚C for 60 min in a vessel. Meanwhile, 0.3 mmol of CdO, 1.4 mmol of SA, and
60 mmol of ODE were put in a three-neck flask, which was then heated up to 280˚C for 60 min
in order to form a transparent Cd solution. Zn solution was obtained by dissolving 0.7 mmol of
Zn(St)2 in mixture of SA (0.036 mmol) and ODE (60 mmol) at 180 oC for 60 min. After that,
S precursor solution was swiftly injected into a reaction flask containing a half of Zn and Cd
precursor solutions at 280 oC. Small amounts of remaining Zn and Cd precursor solutions then
were alternately injected into reaction flask. The NC sample was prepared with reaction time of
270 min. The precursor solutions and NCs are prepared under the condition of the nitrogen flow.
For simplicity of presentation, the calculated Cd and Zn contents (0.3 and 0.7, respectively) will
be used in the chemical formula of the sample.
Crude solution obtained after preparing NC sample was mixed with isopropanol (according
to the ratio of 1/3 in volume). NCs were collected by using a centrifuge, which worked with
speed of 15000 rpm for 3 min. After purification, a part of the product in powder was used to
investigate crystal structure. Other parts were dispersed in toluene for checking morphology, size
and spectroscopic measurements.
Measurements
Transmission electron microscopy (TEM) image of Cd0.3Zn0.7S sample was recorded using
JEM 1010 microscope (Jeol). The sample was mounted on a carbon-coated cooper-mesh grid. X-
ray diffraction (XRD) pattern was obtained from an X-ray diffractometer (Siemens, D5005), using
PHI VAN THANG et al. 183
a Cu Kα radiation source with λ = 1.5406 A˚. Optical absorption spectrum was recorded with a
Jasco 670 spectrometer.
The PL spectrum was collected using Fluorolog FL3-22 spectrometer (Horiba Jobin Yvon).
This instrument is equipped with dual monochromator gratings on the excitation and emission
light paths. To minimize optical artifacts due to primary and secondary absorption of light in
solid specimen, the PL spectrum was obtained in the Front Face (FF) geometry. To eliminate the
consequences of fluctuations of intensity of the photoexcitation source, the emission signal S from
the sample was divided by the reference signal R generated by the excitation beam before reaching
the sample, and the (S/R) ratio was always used as an Y axis of the spectrum. The PL emission
spectrum was recorded with excitation and emission slits at 5 nm.
SLS measurement was conducted using the same Fluorolog FL3-22 spectrometer in the FF
geometry as described above. The (S/R) ratio was used to construct the Y axis of all synchro-
nous luminescence spectra, and the sample was in the same physical form as in conventional PL
measurement. The ∆λ parameter was varied at the 10 nm increments. Synchronous luminescence
spectra were also recorded with excitation and emission slits set at 5 nm.
III. RESULTS AND DISCUSSION
Figure 1 shows the TEM image of Cd0.3Zn0.7S NCs. They have dot-like shape with the
average diameter of ∼ 4.5 nm and narrow size distribution.
 
Fig. 1. TEM image and size distribution of Cd0.3Zn0.7S NCs.
XRD pattern of Cd0.3Zn0.7S sample is shown in Fig. 2(a). The shoulders on both sides
of diffraction peak centered at about 27.5˚ reveal the superposition of diffraction peaks of zinc
blende (Zb) and wurtzite (Wz) phases in Cd0.3Zn0.7S NCs. To separate the diffraction peaks
corresponding to Zb and Wz phases, Rietveld refinement analysis was performed using FullProf
program modified with atomic scattering factors for electrons (Wz in space group P63mc and Zb
in space group F−43m) [10–12].
184 DETECTION OF LUMINESCENCE CENTERS IN COLLOIDAL Cd0.3Zn0.7S NANOCRYSTALS . . .
20 30 40 50 60
(1
10
)
(3
11
)
(1
12
)
(1
03
)
(2
20
)
(1
02
)
(1
11
)
(1
01
)
(0
02
)
In
te
ns
ity
 (a
.u
.)
 Iexp.
 IZb+IWz
 IZb
 IWz
 
 
2q (degree)
(1
00
)
(a)
 
0.0 0.2 0.4 0.6 0.8 1.0
0.40
0.45
0.50
0.55
0.60
0.65
La
tti
ce
 p
ar
am
et
er
 (a
ng
st
ro
m
)
Composition (x)
cWz
aZb
aWz
(b)
 
Fig. 2. (a) Rietveld refinement analysis of XRD pattern, and (b) the evidence on homo-
geneous alloying of Cd0.3Zn0.7S NCs. The Miller indices of Zb phase are shown by italic
numbers in parentheses.
2.0 2.5 3.0 3.5
A
bs
or
ba
nc
e 
(a
.u
.)
PL
 in
te
ns
ity
 (a
.u
.)
Energy (eV)  
Fig. 3. Absorption and PL
spectra of Cd0.3Zn0.7 S NCs.
The Zb phase of Cd0.3Zn0.7S NCs is char-
acterized by the diffraction peaks centered at 27.8,
46.2 and 54.8 ˚ corresponding to the Miller in-
dices (111), (220) and (311), respectively. Mean-
while, the remaining part of XRD pattern indi-
cates the Wz phase with the diffraction peaks cen-
tered at 26.1, 27.7, 29.6, 38.5, 46.1, 50.3 and
54.7 ˚ , which correspond to the Miller indices
(100), (002), (101), (102), (110), (103) and (112),
respectively. As displayed in Fig. 2(b), the ob-
tained lattice parameters of Zb and Wz phases are
in good agreement with the lattice parameters of
bulk Cd0.3Zn0.7S material, determined based on
Vegard’s law [13, 14]. This confirms the homo-
geneous alloying of Cd0.3Zn0.7S NCs. Moreover,
the Zb phase fraction calculated basing on the in-
tegrated intensities of diffraction peaks of Wz and
Zb phases is ∼ 73%. It should be noted that, be-
side the native defects of CdxZn1−xS material, the
interface regions between the different structural
phases usually contain the additional lattice de-
fects, these may dominate the optical spectrum.
PHI VAN THANG et al. 185
The absorption and PL spectra of Cd0.3Zn0.7S NCs are depicted in Fig. 3. The optical
band gap energy of Cd0.3Zn0.7S NCs is determined at the position of first absorption peak and
has value of 3.02 eV. Since the optical band gap energies of Zb and Wz phases are different [15,
16], the coexistence of both these structural phases in same NC leads to convolution of their
absorption spectra. Based on the equation for nonlinear dependence of the optical band gap energy
of CdxZn1−xS NCs on the composition and particle radius [17,18], it can be deduced that 3.02 eV
is the value of the optical band gap energy of the investigated sample. Simultaneously, the peak
at 3.02 eV is attributed to the absorption transition of Zb phase because the Zb phase fraction
is about 2.7 times larger than the Wz phase one. It also means that the absorption spectrum of
Wz phase is covered by that of Zb phase. Remarkably, the absorption spectrum of Cd0.3Zn0.7S
NCs has a tail in the low energy region. This tail is originated by absorption transitions from
valence band to the localized states, from the localized states to conduction band, and between the
localized states [19–21], which have smaller energy in comparison to the optical band gap energy.
In other words, the appearance of the absorption tail is related not only to the surface states, but
also to the native defects as well as the additional defects in the interface region between the Zb
and Wz phases of Cd0.3Zn0.7S NCs. The PL spectrum of Cd0.3Zn0.7S NCs consists of a strong
peak at 2.86 eV (433 nm) and broad emission band centered at 2.2 eV (563 nm). The full width
at half maximum of high energy peak is 0.2 eV (23 nm), exhibiting the narrow size distribution of
Cd0.3Zn0.7S sample.
400 450 500 550
Sy
nc
hr
on
ou
s s
ig
na
l (
a.
u.
)
Wavelength (nm)
 Dl=10 nm
 Dl=20 nm
 Dl=30 nm
 Dl=40 nm
 Dl=50 nm
 Dl=60 nm
 Dl=70 nm
 Dl=80 nm
 Dl=90 nm
 Dl=100 nm
(a)
 
420 480 540 600 660
43
3 
nm
51
5 
nm
Sy
nc
hr
on
ou
s s
ig
na
l (
a.
u.
)
Wavelength (nm)
 Dl=110 nm
 Dl=120 nm
 Dl=130 nm
 Dl=140 nm
 Dl=150 nm
 Dl=160 nm
(b)
46
0 
nm
 
Fig. 4. (Color online) Synchronous luminescence spectra at ∆λ parameter of: (a) 10-100
nm; and (b) 110-160 nm. To more clearly identify the peak at 515 nm, the range of
490-550 nm of the spectrum with ∆λ = 140 nm is shown in Fig. 4(b).
Figure 4 shows synchronous luminescence spectra at variable ∆λ parameter. As can be
seen in Fig. 4(b), large values of ∆λ > 100 nm are suitable for detection of luminescence centers.
186 DETECTION OF LUMINESCENCE CENTERS IN COLLOIDAL Cd0.3Zn0.7S NANOCRYSTALS . . .
Differently from Fig. 4(a), two spectral shoulders at 460 nm (2.69 eV) and 515 nm (2.41 eV) are
clearly observed at ∆λ = 140 nm. The shoulder at λemiss = 460 nm originates from photoexcitation
at λexc = λemiss – ∆λ = 460 nm - 140 nm = 320 nm (3.87 eV). Similarly, the shoulder at λemiss =
515 nm originates from photoexcitation at λexc = 375 nm (3.31 eV). Because the optical band gap
energy of Cd0.3Zn0.7S NCs is 3.02 eV, both two spectral shoulders at 460 and 515 nm originate
from the photoexcitation across the direct band gap in NCs.
The PL spectra of CdxZn1−xS (0 ≤ x ≤ 1) NCs may contain peaks due to self-trapped
exciton, sulfur vacancy (denoted VS), zinc/cadmium vacancy (VZn/VCd), interstitial sulfur (IS),
interstitial zinc/cadmium (IZn/ICd), and surface states. Based on the positions of absorption and
emission peaks in Fig. 3, the emission peak centered at 433 nm is attributed to exciton recom-
bination. The emission peak at 460 nm is due to the recombination of carriers between sulfur
vacancy VS related donor level and the valence band edge, and the emission peak at 515 nm can
be attributed to the radiative transition from zinc/cadmium interstitial IZn/ICd related donor level
to zinc/cadmium vacancy VZn/VCd related acceptor level [3, 22, 23] (see Fig. 5).
 
Fig. 5. Energy level diagram of 460 nm and 515 nm radiative transitions in Cd0.3Zn0.7S NCs.
IV. CONCLUSION
Synchronous luminescence spectra are superior to conventional PL spectra in resolving
characteristic emission peaks of homogeneous Cd0.3Zn0.7S NCs. Beside the emission peak at 433
nm due to the exciton recombination and luminescence related with the surface states, two spectral
shoulders at 460 and 515 nm have been detected by synchronous luminescence measurement. The
460 nm shoulder is attributed to the recombination of carriers between sulfur vacancy related donor
level and the valence band edge. Meanwhile, the 515 nm shoulder is originated due to the radiative
recombination from zinc/cadmium interstitial related donor level to zinc/cadmium vacancy related
acceptor level. The obtained results are useful to clarify the nature of luminescence centers as well
as relaxation mechanism in CdxZn1−xS NCs.
PHI VAN THANG et al. 187
ACKNOWLEDGMENTS
This research is funded by Vietnam National Foundation for Science and Technology De-
velopment (NAFOSTED) under grant number 103.02-2017.54.
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Detection of Luminescence Centers in Colloidal Cd\(_{0.3}\)Zn\(_{0.7}\)S Nanocrystals  by Synchronous Luminescence Spectroscopy

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Detection of Luminescence Centers in Colloidal Cd $_{0.3}$ Zn $_{0.7}$ S Nanocrystals by Synchronous Luminescence Spectroscopy

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Vietnam Academy of Science and Technology: Journals Online

Detection of Luminescence Centers in Colloidal Cd0.3_{0.3}0.3​Zn0.7_{0.7}0.7​S Nanocrystals by Synchronous Luminescence Spectroscopy

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Full text

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Vietnam Academy of Science and Technology: Journals Online

Detection of Luminescence Centers in Colloidal Cd $_{0.3}$ Zn $_{0.7}$ S Nanocrystals by Synchronous Luminescence Spectroscopy