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P. Mushonga et al. / Physica B 439 (2014) 189–192
(25 mL) was added precipitating silver palmitate as a white
product. The product was dried at 60 1C overnight in air.
on a TECNAI F30ST TEM. The samples for TEM studies were
prepared by placing a hexane solution of the nanocrystals on
ultrathin carbon-film-coated copper grids. XRD analysis was
carried out at iThemba Labs, Cape Town using a Bruker AXS D8
Advanced Diffractometer equipped with a CuKα (λ¼1.5418 Å)
X-ray source.
2.1.1.3. Preparation of cobalt(II) palmitate. Cobalt(II) palmitate was
synthesized following a procedure by Acharya and Pradhan with some
modifications [13]. Palmitic acid (5.128 g, 0.02 mol), was added to
methanol (20 mL) in a two-necked round bottomed flask and heated
to 60 1C to obtain a clear solution. Tetramethylammonium hydroxide
pentahydrate (TMAH) (3.625 g, 0.02 mol) was dissolved separately in
methanol (10 mL) and transferred into a dropping funnel. The TMAH
solution was then added drop wise to the palmitic acid solution with
stirring over 20 min to complete the reaction. Separately, CoCl2 ꢀ 6H2O
(2.379 g, 0.01 mol) was dissolved in methanol (10 mL) and again
added drop wise to the above solution with vigorous stirring. A faint
pink product, cobalt(II) palmitate was obtained. The product was
filtered, washed with methanol and dried under vacuum.
3. Results and discussion
3.1. Optical characterization
Luminescent transition metal doped InP/ZnSe NCs were success-
fully synthesized using a growth-doping method. Fig. 1(a)–(c) shows
the normalized PL spectra of Ag, Co and Fe-doped InP/ZnSe NCs. The
introduction of Ag dopant caused a red-shift in the emission spectra
of the InP/ZnSe NCs. The emission peak positions were 592, 598 and
613 nm for Ag-dopant levels of 0%, 5% and 10%, respectively. A
similar red-shift was also reported by Banin and co-workers when
they prepared Ag-doped InAs NCs [15]. They reported that Agþ ions
act as substitutional impurities (as opposed to interstitial impuri-
ties) in III–V semiconductors. This is due to their large ionic radii
(129 pm) compared to In3þ ions (94 pm). The Agþ ions distort the
crystal structure of the III–V systems leading to band-tailing and
consequently to the red-shift in the emission spectrum. A pro-
nounced decrease in PL intensity with increasing Ag dopant levels
was realized. Georgekutty et al. reported a similar observation
where Ag-dopant induced the quenching of PL intensity in the
ZnO nanoparticles. This phenomenon was attributed to a decrease
in electron–hole recombination [16]. Further, doping with cobalt
gave no change in the emission spectrum. However, a decrease in PL
intensity with increasing Co-doping level from 15% to 20% was
observed. A similar observation was made by Vatankhah et al. [17].
They reported that the quenching point for Co:ZnS nanocrystals was
0.1% of the dopant. An increase in the level of cobalt impurity to 5%
resulted in continuous decrease in the fluorescence signal. In
another report of Co:CdS NCs, the emission intensity decrease for
a 6% dopant level was attributed to an even dopant incorporation in
the CdS nanocrystals [18]. Peng et al. equally reported the suppres-
sion of exciton emission with increasing Co dopant levels in a ZnO
system [19]. This was because of an increase in the distortion of host
lattice and defects as more dopant ions displaced host cations. The
PL spectra of Fe-doped InP/ZnSe NCs (Fig. 1(c)) exhibited a blue-shift
with increase in dopant level from 1% to 5%. The emission peaks
were 592 and 566 nm for the Fe dopant levels of 1% and 5%
respectively. A similar blue-shift was observed by Yang et al. when
they fabricated Fe:ZnSe NCs [20]. However, the blue-shift was
reported to be due to a decrease in the reaction rate with the
incorporation of the dopant ions. This was confirmed by nanopar-
ticle size reduction and bandgap increase. A decrease in emission
intensity was also observed with an increase in dopant level from
1% to 5% indicating that the iron dopant acted to quench the
fluorescence from the InP core. The quenching effect of iron in Fe-
doped ZnS was ascribed to the trapping of electrons by Fe centers
culminating in non-radiative recombination of excitons [21].
2.1.1.4. Preparation of iron(II) palmitate. Iron(II) palmitate was
synthesized following the same procedure as for silver palmitate.
Iron(II) chloride tetrahydrate (4.012 g, 0.02 mol) was dissolved in
methanol (100 mL). Palmitic acid (10.313 g, 0.04 mol) and
methanol (100 mL) were added to a two-necked flask, heated at
80 1C with stirring and slowly added to the above solution. NaOH
(0.815 g, 0.02 mol) in water (20 mL) was added precipitating
brown iron(II) palmitate. The product was filtered, washed with
methanol and dried under vacuum.
2.1.2. Synthesis of M:InP/ZnSe (M¼Ag, Co, and Fe)
All experiments involving the doping of InP/ZnSe nanocrystals
with transition metal elements (Ag, Fe and Co) were carried out
following a literature procedure, with some modifications [14].
The Ag-doped InP/ZnSe NCs were synthesized at a doping level of
5% Ag with respect to P. In the synthesis, indium acetate
(0.2 mmol) was mixed with palmitic acid (0.8 mmol) and ODE
(4 mL) in a three-necked flask in the glove box. The flask was
sealed and transferred to a Schlenk line and heated to 120 1C and
kept under vacuum for 1.5 h. The system was then purged with
argon and then heated to 300 1C. On reaching 300 1C, a freshly
prepared injection solution of P(TMS)3 in TOP (0.1 mmol in 0.5 mL
TOP prepared in the glove box) was injected rapidly under argon
flow. For the growth of the InP core, the reaction mixture was kept
at around 270 1C for 1 h. The temperature was lowered to 130 1C
and silver palmitate (0.5 mL, 0.01 M) was added. The temperature
was raised to 210 1C for 1 h to effect the doping. The temperature
was then lowered to 150 1C for the addition of zinc and selenium
precursors for the ZnSe shell. Zinc undecylenate (0.6 mL, 0.1 M)
and TOPSe (0.6 mL, 0.1 M) were added with a 10 min interval
between the two injections. The temperature was raised to 230 1C
for 1 h. The temperature was again lowered to 150 1C and zinc
undecylenate (0.8 mL, 0.1 M) and TOPSe (0.8 mL, 0.1 M) were
added with a 10 min interval between the injections. The tem-
perature was raised to 230 1C for 1 h. The Ag dopant levels were
varied 0%, 5% and 10% with respect to moles of phosphorus. Co:
InP/ZnSe and Fe:InP/ZnSe NCs were synthesized in a similar
manner with doping levels of 0%, 15% and 20% Co as well as 0%,
5% and 10% Fe using cobalt(II) palmitate and iron(II) palmitate as
dopant sources respectively.
3.2. Structural characterization
2.2. Characterization
3.2.1. TEM studies
Fig. 2(a)–(c) shows the TEM micrographs of the Ag, Co and Fe-
doped InP/ZnSe NCs. The micrographs exhibit lattice fringes
indicating the good crystallinity of the nanocrystals. The particle
sizes as determined by TEM were 3.73, 4.49 and 4.23 nm for Ag
(5%), Co (15%) and Fe (5%) doped InP/ZnSe NCs respectively. The
EDX analysis showed the presence of dopant ions.
Photoluminescence (PL) spectra were recorded on a HORIBA
Nanolog FL3-22-TRIAX. Samples for PL analysis were dispersed in
hexane. The values of PL intensity were normalized simply by
dividing the PL intensity by the highest PL intensity value
corresponding to the emission peak. TEM studies were performed
Mushonga, Paul
