tween the glass samples ͑SNAB: xMn and SNAB:
Cd
1−x
Mn
x
S NCs͒ is observed in the EPR spectra ͓Figs. 3͑b͒
and 3͑c͔͒. Note that the hyperfine interaction due to Mn
2+
ions results from the presence of a crystalline field in CdS
NCs. The hyperfine interaction observed in the spectrum of
glass samples doped with Mn
2+
is not as evident as those
observed for Cd
1−x
Mn
x
S NCs. This difference could possibly
be attributed to the formation of small islands of crystalline
phase MnO.
17
It has already been reported that Mn
2+
ions are incorpo-
rated into two distinct sites of NCs; when found at the dot
core, it produces the EPR signal S
I
, when located on or near
the NCs surface it produces the EPR signal S
II
.
16,18
This
analysis is strongly supported by good agreement between
the experimental EPR spectrum and the simulated one. In
Fig. 4, the EPR spectrum of Cd
0.95
Mn
0.05
S NCs, measured
in the X-band and at room temperature, is shown as a solid
blue line and the calculated one as a solid green line. The
EPR simulation was obtained by a sum of two spectra with
A=7.6 and 8.2 mT, corresponding to Mn
2+
ions located in-
side the NC ͑labeled as S
I
͒ and near the NC surface ͑labeled
as S
II
͒ for a dot system with parameters: S=5/ 2, I =5/ 2,
D=40 mT, E = 5 mT, and g
e
=2.005. On the other hand, as
the annealing time increases, the probability of finding mag-
netic ions inside NCs occupying neighboring lattice sites as
well as the number of antiferromagnetic spin correlated clus-
ters increases. This combination of effects enhances the di-
polar interaction and increases the lattice distortions on the
Mn
2+
sites. Furthermore, the accumulation of Mn
2+
ions on
the NCs surfaces also strengthens Mn–Mn interactions.
16,18,19
As a consequence, the intensity of the broader background
peak is increased.
In conclusion, Cd
1−x
Mn
x
S NCs were synthesized in a
glass matrix by the fusion method and investigated by OA,
AFM, and EPR measurements. Due to the incorporation
of Mn
2+
ions into CdS NCs, we observed an OA resonance
blueshift, which increases with increasing Mn
2+
ion con-
centration. The theoretical estimated average NC size
͑R ϳ2.2 nm͒ was in agreement with the values determined
by AFM ͑R ϳ2.3 nm͒. EPR spectra provided evidence that
Mn
2+
ions are incorporated at two distinct sites: at the core
and/or at the surface of CdS NCs. Influences of CdS NCs
crystalline field and the presence of MnO cluster phases
could be confirmed by the presence of six hyperfine lines
assigned to the Mn
2+
ions in the samples. We believe that
these results may inspire further investigation of this system
for possible device applications.
The authors acknowledge the financial support from the
Brazilian agencies MCT/CNPq and FAPEMIG.
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FIG. 3. ͑Color online͒ Panel ͑a͒ EPR spectra of Cd
1−x
Mn
x
S NCs for samples
with concentration: x =0, 0.005, 0.01, 0.05, and 0.10. Panels ͑b͒ and ͑c͒ EPR
spectra of glass matrix doped with xMn and Cd
1−x
Mn
x
S NCs for x = 0.05 and
0.10, respectively.
310 320 330 340 350 360 370 38
S
I
SNAB: Cd
0.95
Mn
0.05
SNCs
EPR Intensity
a. u.
Ma
netic Field
mT
Exp
Sim
S
II
FIG. 4. ͑Color online͒ Experimental and simulated EPR spectra of
Cd
0.95
Mn
0.05
S NCs. The simulation uses superposition of two spectra with
A= 7.6 mT and 8.2 mT, corresponding to Mn
2+
ions located inside ͑labeled
as S
I
͒ and on the surfaces ͑labeled as S
II
͒ of NCs for a system with param-
eters: S=5/ 2, I =5/ 2, D =40 mT, E = 5 mT, and g
e
=2.005.
193115-3 Dantas et al. Appl. Phys. Lett. 93, 193115 ͑2008͒
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