2-(N,N-Dimethylamino)ethylselenolates of cadmium(II): Syntheses, structure of [Cd3(OAc)2(SeCH2CH2NMe2 )4] and their use as single source precursors for the preparation of CdSe nanoparticles

Article abstract of DOI:10.1016/j.poly.2006.02.011

The reaction of Cd(OAc)₂ · 2H₂O with NaSeCH₂CH₂NMe₂ gave a homoleptic cadmium selenolate, [Cd(SeCH₂CH₂NMe₂)₂]. The latter complex, on treatment with Cd(OAc)₂ · 2H₂O, afforded [Cd₃(OAc)₂(SeCH₂CH₂NMe₂ )₄], which was structurally characterized by single-crystal X-ray diffraction analysis. Pyrolysis of [Cd(SeCH₂CH₂NMe₂)₂] either in a mixture of hot hexadecylamine (HDA) and tri-n-octylphosphine oxide (TOPO) or in a furnace (180 and 200 °C) gave CdSe nanoparticles with average sizes varying between 3 and 21 nm. Both cubic and hexagonal phases of CdSe nanoparticles have been isolated under different experimental conditions. The CdSe nanoparticles were characterized by UV-Vis, photoluminescence, X-ray diffraction and electron microscopy. Time resolved luminescence measurements showed three different decay times for both band edge and trap state emissions.

Full text of DOI:10.1016/j.poly.2006.02.011

Polyhedron 25 (2006) 2383–2391
www.elsevier.com/locate/poly
2-(N,N-Dimethylamino)ethylselenolates of cadmium(II):
Syntheses, structure of [Cd3(OAc)2(SeCH2CH2NMe2)4] and their use
as single source precursors for the preparation of CdSe nanoparticles
a
a
a,
b
c
G. Kedarnath , Sandip Dey , Vimal K. Jain *, Gautam K. Dey , Babu Varghese
a
Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharasthra 400085, India
b
Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India
c
RSIC, Indian Institute of Technology, Chennai 600036, India
Received 8 November 2005; accepted 2 February 2006
Available online 9 March 2006
Abstract
The reaction of Cd(OAc)2 Æ 2H2O with NaSeCH2CH2NMe2 gave a homoleptic cadmium selenolate, [Cd(SeCH2CH2NMe2)2]. The
latter complex, on treatment with Cd(OAc)2 Æ 2H2O, afforded [Cd3(OAc)2(SeCH2CH2NMe2)4], which was structurally characterized
by single-crystal X-ray diffraction analysis. Pyrolysis of [Cd(SeCH2CH2NMe2)2] either in a mixture of hot hexadecylamine (HDA)
and tri-n-octylphosphine oxide (TOPO) or in a furnace (180 and 200 ꢀC) gave CdSe nanoparticles with average sizes varying between
3 and 21 nm. Both cubic and hexagonal phases of CdSe nanoparticles have been isolated under different experimental conditions.
The CdSe nanoparticles were characterized by UV–Vis, photoluminescence, X-ray diffraction and electron microscopy. Time resolved
luminescence measurements showed three different decay times for both band edge and trap state emissions.
ꢁ 2006 Elsevier Ltd. All rights reserved.
Keywords: Cadmium selenolate; CdSe; Nanoparticles; Crystal structure; TEM
1. Introduction
and distribution of nanoparticles has been a challenging
task. These materials have been synthesized by various
techniques [16–23], which include pyrolysis of organome-
tallic compounds [8,17–20], single source precursors [21]
and solvothermal reactions [22]. Simple organochalco-
genolates of zinc and cadmium are in general polymeric
in nature and hence have limited utility as molecular
precursors for II–VI materials [24]. We have recently
reported the chemistry of internally functionalized chalco-
genolate ligands, viz. N,N-dimethylaminoethylchalcogeno-
lates which assist in suppressing polymerization [25,26].
For instance, the zinc complex [Zn(SeCH2CH2NMe2)2],
isolated as a discrete monomer, on pyrolysis afforded ZnSe
[27]. This has prompted us to prepare the analogous
cadmium complex and assess its suitability as a molecular
precursor for CdSe nanoparticles. Results of this work are
reported herein.
Semiconductor nanoparticles [1,2], in particular II–VI,
have been a subject area of considerable current research
due to their promising applications in areas like catalysis
[3], photovoltaics [4], materials for tunable LEDs [2,5]
and biological imaging agents [6]. Among the II–VI nano-
particles, CdS and CdSe have been extensively studied
owing to their tunable bandgap, making them potential
candidates for bio-marking [7]. Shape selective synthesis
of CdSe nano-rods [8–11], nano-wires [12], nano-tubes
[13], nano-saw [14] and nano-arrows [15], have been
reported, although simultaneous control of the shape, size
*
Corresponding author. Tel.: +91 22 2559 5095; fax: +91 22 2550 5150.
E-mail addresses: jainvk@apsara.barc.ernet.in (V.K. Jain), varghese@
iitm.ac.in (B. Varghese).
0277-5387/$ - see front matter ꢁ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.02.011

2384
G. Kedarnath et al. / Polyhedron 25 (2006) 2383–2391
2. Experimental
and the oily residue was washed with hexane and dried
again in vacuo. The residue was extracted with CH2Cl2
(3 · 10 cm3), filtered to remove NaOAc and the filtrate
was evaporated under reduced pressure to give a white
solid, which was recrystallized from dichloromethane-ethyl-
acetate mixture. Yield: 702 mg (68%). mp 93–95 ꢀC. Anal.
Calc. for C8H20N2CdSe2: C, 23.2; H, 4.8; N, 6.8. Found:
2.1. Materials and methods
Cadmium acetate, mercury acetate, tmeda (N,N,N0,N0-
tetramethylethylenediamine),
tri-n-octylphosphineoxide
(TOPO) and hexadecylamine (HDA) were obtained from
commercial sources and were used without further purifica-
tion. HgCl2 Æ tmeda was prepared by refluxing HgCl2 and
excess tmeda in dichloromethane under an inert atmo-
sphere for 2 h. Bis(2-dimethylaminoethyl)diselenide was
prepared according to the literature method [25]. Solvents
were dried and distilled before use under a nitrogen atmo-
sphere. HPLC grade toluene was used in all experiments
involved with a capping agent and pyridine was used in
pyrolysis experiments for optical studies. Melting points
were determined in capillary tubes and are uncorrected.
Elemental analyses were carried out by the Analytical
1
C, 23.0; H, 4.9; N, 6.6%. H NMR (in CDCl3) d: 2.28 (s,
NMe2); 2.56 (br, SeCH2); 2.74 (br, NCH2). 13C{1H}
NMR (in CDCl3) d: 15.5 (s, SeCH2); 45.8 (s, NMe2); 62.9
(s, NCH2). The NMR spectra (1H) in some samples showed
additional peaks of low intensity (<5%) which may be
attributed to [Cd3(OAc)2(SeCH2CH2NMe2)4]. The micro-
analysis of the latter (Calc. for C20H46Cd3N4O4Se4: C,
22.6; H, 4.3; N, 5.3%) is not significantly different from
the bis derivative.
2.2.2. [Cd3(OAc)2(SeCH2CH2NMe2)4]
Chemistry Division of B.A.R.C. H and 13C{1H} NMR
To a dichloromethane solution (5 cm3) of [Cd(SeCH2-
CH2NMe2)2] (187 mg, 0.45 mmol), a methanolic solution
(5 cm3) of Cd(OAc)2 Æ 2H2O (59 mg, 0.22 mmol) was added
and the whole was heated at 40 ꢀC with stirring for 2 h. The
solvents were evaporated under vacuum and the residue
was extracted with dichloromethane (5 cm3 · 4) and fil-
tered through a fine filter. The filtrate was concentrated
under vacuum to give a colorless powder (230 mg, 96%),
1
spectra were recorded on a Bruker DPX-300 NMR spec-
trometer operating at 300 and 75.47 MHz, respectively.
Chemical shifts are relative to the internal chloroform peak
at d 7.26 for 1H and d 77.0 for13C{1H}. UV–Vis absorption
spectra were recorded on a Chemito Spectrascan UV 2600
double beam UV–Vis spectrophotometer. Fluorescence
spectra were recorded using a Hitachi F-4010 spectrofluo-
rometer. Time-resolved fluorescence measurements were
carried out using a diode laser based time-correlated-
single-photon-counting (TCSPC) spectrometer. In the pres-
ent investigation, a 408 nm diode laser (1 MHz) was used
as the excitation source and a TBX4 detection module cou-
pled with a special Hamamatsu PMT was used for fluores-
cence detection. The instrument response function was
ꢀ230 ps at the FWHM. IR spectra were recorded on a
Bomem MB-102 FT-IR spectrometer. Thermogravimetric
analyses (TGA) were carried out on a Mettler TA–3000
instrument, which was calibrated with CaC2O4 Æ H2O. TG
curve was recorded at a heating rate of 10 ꢀC minꢁ1 under
a flow of nitrogen. X-ray powder diffraction data were col-
lected on a Philips X-ray diffractometer (Model PW 1729)
using Cu Ka radiation. A JEOL-2000FX transmission elec-
tron microscope operating at accelerating voltages up to
200 kV was used for TEM studies. The samples for TEM
and SAED were prepared by placing a drop of a dilute
solution of nanoparticles in acetone or pyridine on a
carbon coated copper grid. EDAX was performed using
a JEOL JSM-T330A instrument.
1
mp 170 ꢀC (dec. turns red). mmax/cmꢁ1 1560 (C@O). H
NMR (in CDCl3) d: 2.05 (s, O2CMe); 2.33 (s, NMe2);
2.63 (br, SeCH2); 2.82 (br, NCH2). 13C{1H} NMR (in
CDCl3) d: 16.4 (s, SeCH2, 1J(Se–C) = 57 Hz); 21.6 (s,
O2CMe); 45.5 (s, NMe2); 62.0 (s, NCH2); 179.4 (s, C@O).
2.2.3. Attempted preparation of [Hg(SeCH2CH2NMe2)2]
A dichloromethane solution (20 cm3) of HgCl2 Æ tmeda
(201 mg, 0.51 mmol) was added to a freshly prepared meth-
anolic solution of NaSeCH2CH2NMe2 [prepared from
(Me2NCH2CH2Se)2 (159 mg, 0.53 mmol) and NaBH4
(44 mg, 1.16 mmol)] with continuous stirring at room tem-
perature. After 3 h, a black precipitate settled down. The
supernatant was decanted and the black precipitate was
washed with hexane, acetone in sequence and then with
water, acetone and dried in vacuo. Found: C, 1.8; H, 0.2;
N, <0.2%. XRD pattern of this black powder was well in
agreement with the pattern for cubic HgSe (JCPDS File
No.73-1668). Similar results were obtained when Hg(OAc)2
was used in place of [HgCl2 Æ tmeda].
2.3. Synthesis of CdSe nanoparticles
2.2. Synthesis of complexes
2.3.1. Pyrolysis of [Cd(SeCH2CH2NMe2)2] in
HDA–TOPO mixture
2.2.1. [Cd(SeCH2CH2NMe2)2]
A dichloromethane suspension (20 cm3) of Cd(OAc)2 Æ
2H2O (664 mg, 2.49 mmol) was added to a freshly prepared
methanolic solution of NaSeCH2CH2NMe2 [prepared from
(Me2NCH2CH2Se)2 (757 mg, 2.51 mmol) and NaBH4
(193 mg, 5.10 mmol)] with continuous stirring at room tem-
perature. After 3 h the colorless solution was dried in vacuo
In a three necked flask fitted with a thermometer, a con-
tinuously stirred mixture of HDA (2.39 g) and TOPO
(185 mg) was degassed at 120 ꢀC under argon atmosphere
for 1 h. The temperature was slowly raised to 187 ꢀC and
stabilised at this temperature. To this, a solution of
[Cd(SeCH2CH2NMe2)2] (140 mg, 0.34 mmol) in a mixture

G. Kedarnath et al. / Polyhedron 25 (2006) 2383–2391
2385
of dichloromethane (2 cm3), toluene (2 cm3) and TOPO
(200 mg) was injected rapidly. The temperature dropped
to 160 ꢀC and was slowly raised to and maintained at
185 ꢀC. Fractions were collected at 4 min (fraction 1) and
6 min (fraction 2) in vials containing methanol (5 cm3).
The hot solution at the end of 30 min (fraction 3) was
cooled down to 70 ꢀC and methanol was added to precipi-
tate red nanoparticles. The red floculate was given a thor-
ough washing with methanol followed by centrifuging and
drying under vacuum.
5.6; H, 1.0; N, 1.6%), leading to the formation of a dark
red CdSe coat. For optical measurements, small quantities
of fractions 5–6 were immediately dissolved in pyridine.
2.4. Crystallography
X-ray data of a colorless crystal of [Cd3(OAc)2(SeCH2-
CH2NMe2)4] were collected at room temperature on an
Enraf-Nonius CAD-4 diffractometer using graphite mono-
˚
chromated Mo Ka radiation (k = 0.71069 A) and employ-
Under similar conditions,
a
larger quantity of
ing the x–2h scan technique. The unit cell parameters
(Table 1) were determined from 25 reflections measured
by a random search routine. The intensity data were
corrected for Lorentz, polarization and absorption effects
(w scans). The structure was solved by direct methods using
the SIR-92 program. The structure was completed by succes-
sive Fourier synthesis and refined using the SHELXL-97 pro-
gram. The non-hydrogen atoms were refined with
anisotropic thermal parameters. Hydrogen atoms could
not be located reliably through difference Fourier maps
due to disorder of the carbon atoms and hence were ignored.
[Cd(SeCH2CH2NMe2)2] (414 mg, 1.00 mmol) was pyroly-
sed in a mixture of HDA (7.09 g) and TOPO (1.00 g) and
the reaction was quenched after 20 min (fraction 4) by add-
ing methanol.
2.3.2. Pyrolysis of [Cd(SeCH2CH2NMe2)2] in a furnace
Pyrolysis of [Cd(SeCH2CH2NMe2)2] was carried out in
a furnace under different conditions. In each experiment
a weighed quantity (ꢀ80 mg) of the complex was dispersed
in a small quartz boat by using dichloromethane and was
subjected to pyrolysis. In one experiment the boat contain-
ing the complex was rapidly introduced into a preheated
(180 ꢀC) furnace and was kept at this temperature for 1 h
under flowing nitrogen whereupon a yellowish-orange coat
of CdSe (fraction 5: Found: C, 6.3; H, 1.8; N, 2.8%) was
formed. In other experiments the furnace temperature
was raised from room temperature to 200 ꢀC in 5 min
and the samples were heated at this temperature for 1 h
either under flowing N2 (fraction 6: Found: C, 6.9; H,
1.2; N, 1.6%) or under vacuum (fraction 7: Found: C,
3. Results and discussion
3.1. Synthesis and spectroscopy
Reaction of Cd(OAc)2 Æ 2H2O with NaSeCH2CH2NMe2,
prepared by the reduction of (Me2NCH2CH2Se)2 with
NaBH4 in methanol, gave a colorless homoleptic selenolate
complex [Cd(SeCH2CH2NMe2)2] (Eq. (1)). The freshly pre-
pared complex is soluble in CH2Cl2, CHCl3, benzene, tolu-
ene and methanol, but solubility decreases with aging,
finally leading to a poorly soluble product. The NCH2
1
Table 1
and SeCH2 proton resonances in the H NMR spectrum
Crystallographic and refinement data for [Cd3(OAc)2(SeCH2CH2NMe2)4]
appeared as broadened triplets which were shielded from
the corresponding resonances for (Me2NCH2CH2Se)2. Sev-
eral attempts to recrystallize the product from a mixture of
solvents (CH2Cl2–hexane, CH2Cl2–MeCOOEt) gave either
a colorless powder or flakes from which in some cases a
few cubic crystals were separated which were identified by
X-ray crystallography as a trinuclear cadmium complex,
[Cd3(OAc)2(SeCH2CH2NMe2)4]. The latter complex, how-
ever, can readily be prepared by a redistribution reaction
between Cd(OAc)2 Æ 2H2O and the bis complex,
[Cd(SeCH2CH2NMe2)2]. Attempts to prepare the mercury
complex, [Hg(SeCH2CH2NMe2)2] using reaction route 1
led to excessive decomposition with the formation of cubic
HgSe at room temperature.
Chemical formula
Formula weight
Crystal size (mm3)
C20H46Cd3N4O4Se4
1059.65
0.2 · 0.2 · 0.1
0.71069
monoclinic
C2/c
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions
˚
a (A)
24.700 (5)
13.309 (3)
11.575 (3)
115.38 (4)
3437.8 (14)
2.047
˚
b (A)
˚
c (A)
b (ꢀ)
3
˚
V (A )
Dcalc (g cmꢁ1
)
Z
4
l (mmꢁ1
F(000)
)
6.012
2024
h Range for data collection (ꢀ)
2.33–25.00
0 6 h 6 29;
0 6 k 6 15;
ꢁ13 6 l 6 12
3245/3034 (0.0559)
w scan
3034/128/193
0.0579, 0.1586
0.1277, 0.1895
1.002 and ꢁ0.774
Limiting indices
(Me2NCH2CH2Se)2 + 2 NaBH4
2 NaSeCH2CH2NMe2
[Cd(OAc)2].2H2O
−2 BH3
Reflections collected/unique (Rint
Absorption correction
Data/restraints/parameters
Final R1 [I > 2r(I)], xR2
R1, xR2 (all data)
)
[Cd(SeCH2CH2NMe2)2] + 2 NaOAc + 2 H2O
½ [Cd(OAc)2].2H2O
½ [Cd3(OAc)2(SeCH2CH2NMe2)4]
ꢁ3
˚
Largest difference in peak and hole (e A
)
ð1Þ

2386
G. Kedarnath et al. / Polyhedron 25 (2006) 2383–2391
Table 2
3.2. Crystal structure of [Cd3(OAc)2(SeCH2CH2NMe2)4]
˚
Selected bond lengths (A) and angles (ꢀ) for [Cd3(OAc)2(SeCH2CH2-
NMe2)4]
The molecular structure of [Cd3(OAc)2(SeCH2CH2-
NMe2)4], established unambiguously by X-ray diffraction
analysis, is shown in Fig. 1 and selected interatomic param-
eters are listed in Table 2. The molecule has a twofold rota-
tion axis passing through the central cadmium atom and
hence half the molecule forms the crystallographic asym-
metric unit. The Me2NCH2CH2Se fragments in the mole-
cule are disordered and act in a chelating bridging
fashion. The two terminal cadmium atoms adopt a skew
trapezoidal planar geometry in which the basal plane is
defined by two O and two Se atoms. The two remaining
positions are occupied by the N atoms which are almost
linear (\N(1)–Cd(1)–N(2) 178.2 (3)ꢀ). The acetate ligand
is asymmetrically chelated with two slightly different
Cd–O distances, whereas the two C–O distances are essen-
tially similar. The two Cd–Se distances differ marginally.
The coordination around the central cadmium is defined
by four selenium bridges and adopts a pyrimidalized con-
figuration with various angles varying between the ideal
values of tetrahedral (109ꢀ) and square plane (90ꢀ). The
two Cd–Se bonds are shorter than the other two. All the
Cd–Se [28,29], Cd–N [28,30] and Cd–O [31] distances are
well in agreement with values reported in other complexes.
Cd(1)–O(1)
Cd(1)–O(2)
Cd(1)–Se(1)
Cd(1)–Se(10)
Cd(1)–Se(2)
Cd(1)–Se(20)
Cd(1)–N(1)
Cd(1)–N(2)
2.311 (9)
2.395 (9)
2.724 (3)
2.732 (3)
2.800 (3)
2.652 (2)
2.413 (9)
2.437 (12)
Cd(2)–Se(1)
Cd(2)–Se(10)
Cd(2)–Se(2)
Cd(2)–Se(20)
Se(1)–C(6)
Se(10)–C(100)
Se(2)–C(10)
Se(20)–C(60)
2.538 (3)
2.729 (3)
2.726 (3)
2.618 (3)
1.96 (2)
2.03 (3)
1.91 (3)
2.06 (3)
O(1)–Cd(1)–O(2)
O(1)–Cd(1)–N(1)
O(2)–Cd(1)–N(1)
O(1)–Cd(1)–N(2)
O(2)–Cd(1)–N(2)
N(1)–Cd(1)–N(2)
O(1)–Cd(1)–Se(1)
O(2)–Cd(1)–Se(1)
N(1)–Cd(1)–Se(1)
N(2)–Cd(1)–Se(1)
54.4 (3)
93.5 (4)
90.9 (3)
88.0 (4)
90.7 (4)
178.2 (3)
109.3 (3)
162.1 (2)
82.1 (2)
96.4 (3)
O(1)–Cd(1)–Se(2)
O(2)–Cd(1)–Se(2)
N(1)–Cd(1)–Se(2)
N(2)–Cd(1)–Se(2)
Se(1)–Cd(1)–Se(2)
Se(1)–Cd(2)–Se(2)
Se(10)–Cd(2)–Se(20)
Cd(1)–Se(1)–Cd(2)
Cd(1)–Se(10)–Cd(2)
Cd(1)–Se(2)–Cd(2)
151.5 (3)
101.5 (2)
102.7 (2)
76.3 (3)
96.14 (9)
102.61 (9)
99.51 (8)
82.74 (9)
79.20 (7)
78.07 (7)
100
90
80
70
60
50
3.3. Thermal studies
The TG curve (Fig. 2) of [Cd(SeCH2CH2NMe2)2]
showed that the complex decomposed in a single step at
155 ꢀC to CdSe as inferred from the weight loss (Found:
weight loss 52.6%; calcd. weight loss 53.8%).
Pyrolysis of [Cd(SeCH2CH2NMe2)2] was carried out
under different conditions, either in HDA/TOPO mixture
or in a furnace. Pyrolysis of [Cd(SeCH2CH2NMe2)2] in a
furnace gave either cubic (fractions 5 and 6) or hexagonal
(fraction 7) CdSe nanoparticles (from XRD, see later)
which were formed under a flowing inert gas or vacuum,
respectively. The microanalyses data of fractions 5–7 sug-
gest that the CdSe may be capped by the amine residues
evolved on pyrolysis of the complex.
40
0
50
100
150
200
250
300
350
Temperature (˚C)
Fig. 2. TG curve of [Cd(SeCH2CH2NMe2)2].
spectra of fractions 1–4 (Fig. 3) displayed a shift of m(N–H)
to lower wavenumbers compared to that of free HDA
(for free HDA m(N–H) = 3333 cmꢁ1), indicating coordina-
tion of HDA. The 31P{1H} NMR chemical shifts for
Pyrolysis of [Cd(SeCH2CH2NMe2)2] in a HDA/TOPO
mixture gave HDA capped CdSe nanoparticles. The IR
Fig. 1. ORTEP of [Cd3(OAc)2(SeCH2CH2NMe2)4] with the atomic numbering scheme. Inset shows skeletal atoms.

G. Kedarnath et al. / Polyhedron 25 (2006) 2383–2391
2387
100
80
fraction 4 (d 44.0 ppm) and free TOPO (d 43.5 ppm) are
comparable suggesting the absence of TOPO capping of
CdSe nanoparticles.
3.4. Powder X-ray diffraction
XRD patterns (Fig. 4) of CdSe prepared by pyrolysis
of [Cd(SeCH2CH2NMe2)2] under different conditions
showed broad peaks which are typical for particles in
the nano regime. The particle sizes estimated from
Scherrer’s formula come out to be 12, 6, 4, 9 and
8.3 nm for fractions 3, 4, 5, 6 and 7, respectively. The
pattern can be assigned to the cubic phase of CdSe in
the case of fractions 5 and 6 and the hexagonal phase
of CdSe in the case of fraction 7 (Table 3). In the case
of fraction 7, we have used X-ray peak fitting software
to confirm whether the first peak is a single peak or the
overlap of three peaks and it is fitted into three peaks,
which coincides with the hexagonal phase of CdSe.
Assignment of the hexagonal phase of CdSe to fraction
7 is further confirmed by its SAED pattern.
D
E
0
B
A
60
C
40
3500
3450
3400
3350
3300
3250
3200
Wavenumber (cm-1)
Fig. 3. IR spectra of A: free HAD, B: fraction 1, C: fraction 2, D: fraction
3 and E: fraction 4.
160
140
120
100
80
F
60
40
20
0
140
20
30
40
50
60
120
100
80
E
60
40
20
80
70
60
50
40
30
20
10
20
20
30
30
40
50
50
60
60
D
140
120
100
80
40
C
60
40
20
0
160
140
120
100
80
20
30
40
50
60
B
60
40
20
250
20
20
30
30
40
50
50
60
60
200
150
100
50
A
0
40
2θ (degree)
Fig. 4. XRD pattern of A: bulk CdSe obtained at 400 ꢀC, B: fraction 3, C: fraction 4, D: fraction 5, E: fraction 6 and F: fraction 7.

2388
G. Kedarnath et al. / Polyhedron 25 (2006) 2383–2391
Table 3
XRD data of CdSe nanoparticles prepared by different routes and the particle sizes of different fractions
Fraction
XRD data
Size calculated from
Brus expression (nm)
Size calculated from
Scherrer’s formula (nm)
Size obtained from
TEM (nm)
˚
˚
d (A)
Lattice parameter (A)
1
5
2
6.4
12.2
7.9
4.8
6.7
6.5
3
3.49
3.55
3.47
3.54
3.66
3.51
3.72
2.15
1.83
1.83
1.83
1.83
3.27
1.83
3.29
6.046
6.149
6.011
6.131
4.274, 6.958
6.077
4.299, 7.010
12
6
4
9
8.3
4
2.16
2.14
2.15
2.14
2.15
2.15
7.5
21c
5
6
7
CdSe (cubic)a
CdSe (hexagonal)b
a
JCPDS File No. 80-0021 (International Centre for Diffraction data – 1998).
JCPDS File No. 08-0459 (International Centre for Diffraction data – 1998).
TEM of the sample was recorded after one month and therefore ripening might have taken place and hence a larger size compared to the one estimated
b
c
from the XRD pattern.
3.5. Electron microscopy
fractions 3, 4, 6 and 7 exhibited peaks for Cd and Se, which
were in 1:1 ratios.
The selected area electron diffraction (SAED) patterns
of fractions 4 and 7 (Fig. 5) revealed the formation of crys-
talline CdSe. Clear diffraction rings corresponding to the
lattice planes (111), (220) and (311) in the SAED pattern
of the fraction 4 were attributed to cubic CdSe. The SAED
pattern of fraction 7 has been interpreted in terms of the
hexagonal phase of CdSe with diffraction rings corre-
sponding to the lattice planes (100), (101), (102), (110),
(103), (112), (202) and (203). The phases determined
from the SAED and XRD patterns were in agreement with
each other. The TEM images of fraction 4 (Fig. 6) and
fraction 7 (Fig. 7) shows the particles are spherical in
shape. The average diameters of particles of fraction 4
and fraction 7, estimated from TEM images, are
7.5 4% and 21 8% nm, respectively. In the case of frac-
tion 4, the particle size obtained from the TEM image is
closer to that of the particle size calculated from the
XRD pattern using Scherrer’s formula. EDX spectra of
3.6. Optical properties
CdSe nanoparticles, prepared by pyrolysis of
[Cd(SeCH2CH2NMe2)2] in a furnace, displayed absorption
maxima at 423 (fraction 5), 538, 580 (fraction 6) and
455 nm (fraction 7) (Fig. 8). The excitation spectrum of
fraction 7 at room temperature showed a broad exciton
peak at 400 nm, whereas a broad maximum at 499 nm
was observed in the emission spectrum (Fig. 8).
Absorption, excitation and emission spectra of HDA
capped CdSe nanoparticles, prepared by pyrolysis of
[Cd(SeCH2CH2NMe2)2] in HDA/TOPO mixture are given
in Figs. 9–11. Absorption spectra (Fig. 9) of HDA capped
CdSe nanoparticles displayed a typical band edge absorp-
tion with an exciton band and the spectra are well in agree-
ment with those reported earlier [17,19,32]. Fractions 1–4
showed exciton maxima at 490, 549, 625 and 582 nm,
Fig. 5. SAED pattern CdSe nanoparticles of A: fraction 4 and B: fraction 7.

G. Kedarnath et al. / Polyhedron 25 (2006) 2383–2391
2389
1.0
40
30
20
10
0
B
C
0.8
0.6
0.4
0.2
0.0
A
4
5
6
7
8
9 10 11 12 13 14 15
Diameterof the particles(nm)
350
400
450
500
550
600
650
700
750
800
Wavelength (nm)
Fig. 8. A: Absorption spectrum, B: excitation spectrum and C: emission
spectrum of CdSe nanoparticles of fraction 7.
10 nm
Fig. 6. TEM picture of capped CdSe nanoparticles of fraction 4. Inset
shows particle size distribution.
0.6
A
40
30
20
10
0
10
15
20
25
30
B
0.3
Diameterof the particles(nm)
D
C
0.0
400
500
600
700
800
30 nm
Wavelength (nm)
Fig. 9. Absorption spectra of HDA capped CdSe nanoparticles A:
fraction 1, B: fraction 2, C: fraction 3 and D: fraction 4.
The spectra showed narrow band edge emission peaks in
these fractions, implying that the particle size distribution
is narrow as the width of the emission peaks depend on
the size distribution of the particles [19]. The fractions 3
and 4, however, showed only a broad and a narrow peak
corresponding to band edge emissions at 637 and 605 nm,
respectively, implying a slightly broad and narrow distribu-
tion of the particles in these fractions. The particle sizes
estimated using Brus’s expression [37] for the fractions
1–4 are 5, 6.4, 12.2 and 7.9 nm, respectively, and compare
well with the size estimated employing Scherrer’s formula
(Table 3).
To understand the charge carrier dynamics, we have
carried out time-resolved luminescence measurements
(Fig. 12) of fractions 1 and 3. For all the samples, excitation
was carried out at 408 nm. For fraction 1, lifetimes for both
the band edge and trap state components were taken while
Fig. 7. TEM picture of CdSe nanoparticles of fraction 7. Inset shows
particle size distribution.
respectively, which are blue shifted with respect to bulk
CdSe (konset = 713 nm), confirming the quantum confine-
ment effect of the particles. The absorption maximum of
the fractions collected at different time intervals showed a
red shift with increasing time, which is in accordance with
earlier reports [33]. The excitation spectra (Fig. 10) for each
fraction are found to have maxima at 494, 549 and 625 nm,
respectively, at room temperature. These excitation max-
ima coincide well with the absorption band edges of the
three fractions 1–3 at 490, 547 and 625 nm, respectively.
The emission spectra (Fig. 11) displayed two peaks each
in fractions 1 (518, 640 nm) and 2 (573 and 719 nm) corre-
sponding to band edge and trap state emissions [34–36].

2390
G. Kedarnath et al. / Polyhedron 25 (2006) 2383–2391
104
1.0
0.8
0.7
0.5
0.3
0.2
0.0
B
C
A
103
C
B
A
102
101
50
100
150
200
0
Time (ns)
350
400
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 12. Lifetime measurements of A: band edge component of fraction 3,
B: band edge component of fraction 1 and C: trap state component of
fraction 1.
Fig. 10. Excitation spectra of HDA capped CdSe nanoparticles A:
fraction 1, B: fraction 2 and C: fraction 3.
1.0
4. Conclusion
A
B
D
C
0.8
0.7
0.5
0.3
0.2
0.0
The single source precursor [Cd(SeCH2CH2NMe2)2],
prepared conveniently, has been employed for the synthesis
of CdSe nanoparticles. The size and phase (cubic and hex-
agonal) of these particles is conveniently controlled by
varying the pyrolysis conditions. Both absorption and
emission bands of these nanoparticles showed size depen-
dence and were blue shifted with decreasing particle size.
The band edge and trap state showed three different decay
times.
5. Supplementary materials
500
550
600
650
700
750
800
CCDC-277552 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or
from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK [fax: (internat.)
+44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk].
Wavelength (nm)
Fig. 11. Emission spectra of HDA capped CdSe nanoparticles A: fraction
1, B: fraction 2, C: fraction 3 and D: fraction 4.
for fraction 3, the band edge component was studied. A
multi-exponential function has been used to fit the data
for a reasonable fit in all the cases. Three different decay
times for each band edge [0.5 (68%), 8.3 (24%), 61.5 (8%)
ns] and trap state components [1.8 (29%), 12.3 (36%),
73.7 (35%) ns] for fraction 1 were observed. Similarly three
decay times [2.9 (45%), 12.5 (46%) and 49.1 (9%) ns] for the
band edge emission for fraction 3 were identified. The
multi-exponential character of the decay, even when photo-
luminescence shows well resolved lines, can be attributed to
the overlap of various contributions arising from different
particles [38]. The decay times of band edge and trap state
emission of fraction 1 shows that the former component is
faster than the latter. The decay times of the band edge
component of fraction 1 is also faster than that of fraction
3, showing size dependence of life times and is in agreement
with the reported data [32].
Acknowledgements
We thank Drs. T. Mukherjee and S.K. Kulshreshtha
for encouragement of this work. We are also grateful
to Dr. M. Sudarsanan, Head, Analytical Chemistry Divi-
sion for providing microanalyses of the complexes and
Dr. G. Ramakrishna for helpful discussions.
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