Chapter 7 Characterization of (Cd, Mn)S quantum dots
7.4 Correlation of Mn local structures and their luminescence .1 Origin of signal S I
3000 3200 3400 3600 3800
x = 0.001
SI
EPR intensity (a. u.)
Magnetic field (Gauss)
Figure 7.6 EPR spectrum of extremely lightly doped CdS:Mn with Mn concentration 0.001.
To avoid the effect of the background signal, which come from the Mn clustering or dipole interaction due to high Mn concentration, we prepared an extremely lightly doped sample for EPR measurement. The doping concentration is so low that it can not be determined by EDX, however from the preparation it is estimated to be about x = 0.001. A well structured signal SI without any background signal is obtained (see figure 7.6). Therefore it is clear that SI is from isolated Mn substitute Cd cites in cubic CdS quantum dots.
7.4.2 Origin of signal SII
To investigate whether SII is related to Mn on the surface, we doped the nanocrystals in two different ways: one is the commonly used method as described in chapter 2, i.e., first prepare Cd2+ and Mn2+ microemulsion, and then mix with the other part contains S2-, this process is indicated by Cd+Mn+S; the other is Cd+S+Mn process, i.e., first prepare CdS microemulsion by mixing Cd2+ and S2-, and then dope the dots with Mn2+. We expect that in the latter process Mn will mainly be located on the surface. To decrease the diffusion of Mn from the surface to the core, PL spectra were soon measured after the mixing. Figure 7.7 (a) shows the EPR spectra of dots prepared the two different processes. For the Cd+Mn+S process, SI dominates though the signal SII can be also seen. However for the Cd+S+Mn process, SI is strongly
Chapter 7 Characterization of (Cd, Mn)S quantum dots
surpressed, and SII dominates. Therefore we are convinced that SII is from Mn located at the surface.
7.4.3 Contributions of SI and SII to Mn luminescence
(a)
SI
3000 3200 3400 3600 3800
Cd+S+Mn Cd+Mn+S
SII
EPR intensity (a. u.)
Magnetic field (Gauss)
1.5 2.0 2.5 3.0 3.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2
(b)
Cd+S+Mn Cd+Mn+S
PL intensity (a.u.)
Energy (eV)
Figure 7.7 EPR (a) and PL (b) spectra of Cd0.96Mn0.04S nanocrystal prepared via Cd+S+Mn and Cd+Mn+S processes.
2.0 2.5 3.0 3.5
0.0 0.2 0.4 0.6 0.8
1.0 as-grown
100oC
200oC
300oC
500oC
700oC
PL Intensity (a. u.)
Energy (eV)
700 600 500 400
W avelength (nm )
Figure 7.8 PL spectra of Cd0.96Mn0.04S nanocrystal annealed at different temperatures.
Figure 7.7 (b) shows the corresponding PL spectra of the two processes. The dramatic decrease of Mn luminescence by SII (process Cd+S+Mn) immediately demonstrates that when Mn is present at the surface (SII), the Mn luminescence quenches.
The conclusion that SII quenches the Mn emission is further confirmed by an annealing study, see figure 7.8. When annealed, the Mn luminescence band sharply decreases at temperatures higher than 200oC. Corresponding EPR spectra show a great decrease of SI and increase of SII and background signal. It indicates that Mn ions located in the core have lower energy state, while higher energy state when located on the surface. This may account for the appearance of SII in ZnS nanocrystals upon aging [15].
7.4.4 Evolution of SI and SII
Obviously there is an evolution from SI to SII with increasing dot size and Mn concentration (look back fig. 7.5). For a quantitative analysis, we need to know how many Mn ions are in each dot with different average size. This can be obtained with a simple calculation. For simplicity, we assume each nanocrystal is spherical in shape, and the cell volume of nanocrystals is identical with the value of the bulk case. The primitive cell volume of sphalerite CdS, Vc, is 198.46×10-3 nm3. Therefore we have
R3 =n⋅Vc 3
4π , i.e. D3 =n⋅Vc 6
π (7.1) where R and D are the radius and diameter of the nanocrystals, and n the cell number that each dots possess. The number of Mn, nMn, is therefore
D x D x n V
x n
c
Mn = ⋅ = ⋅ 3⋅ =2.636 3⋅ 6
π (7.2)
with x is the concentration of Mn. The number of Mn is proportional to concentration (x) and the cube of diameter (D). The bigger D, and the higher x, the larger . Supposing the Mn ions are homogeneously dispersed in the dots, the longest distance between two neighbor Mn ions within one dot
nMn
d is d = Dsin(π /nMn).
Table 7.2 shows the number of Mn in each dot (n ) and the distance between two neighboring Mn ions (
Mn
d ) with different dot size and Mn concentration. For lightly doped sample, the smallest dots have average 2 Mn ions per dot with 2.7 nm distance, a distance four times larger than the lattice parameter a0 = 0.583 nm. Therefore the Mn dipole interaction
Chapter 7 Characterization of (Cd, Mn)S quantum dots
is rather weak, and this may account for the well resolved EPR signal (fig. 7.5 (a)). With increasing dot size, more Mn is present within one dot while being closer, resulting in stronger Mn-Mn interactions (background), and diffusion to the surface (SII). When heavily doped, there are so many Mn within one dot that the distance is even smaller than the lattice distance a0, indicating formation of Mn clusters. As a consequence mainly the broad background can be seen.
Table 7.2 Number of Mn in each dot (nMn) and the distance between two neighbor Mn ions (d ) with different dot size and concentration.
w Diameter (nm)
nMn
(x =0.04)
nMn
(x = 0.23)
d (nm) (x = 0.04)
d (nm) (x = 0.23) 2.5 2.7 2.07 11.93 2.7 0.69
5 2.9 2.56 14.78 2.51 0.71 7 3.5 4.52 25.99 2.45 0.43 10 4.2 7.81 44.85 1.61 0.29
SI SII SIII
CdS nanoparticles Mn
. . .
.
. .
. .
. .
. .
. . . . . . . .
. . .
.
. . ..
.. .
. . . . .
..
. . .
. . .
. . . .
.. .
.
. . . .
.
.
Figure 7.9 Schemes of different locations of Mn ions in CdS nanocrystals. SI is related to Mn2+
located substitutionally on Cd sites; SII to Mn2+ located near the surface; and SIII to Mn clusters or Mn-Mn pairs.
Schemes of different Mn locations in CdS quantum dots are shown in figure 7.9. SI is related to Mn2+ located substitutionally on Cd sites; SII to Mn2+ located near (or at) the surface; and SIII to Mn clusters or Mn-Mn pairs.
1.5 2.0 2.5 3.0 3.5 4.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
x = 0.04
x = 0.23
II III I
PL intensity (a.u.)
Energy (eV)
Mn(4T1)
Mn(6A1) surface trap
Figure 7.10 PL spectra of lightly (x=0.04) and heavily (x=0.23) Mn doped CdS quantum dots. The inset shows radiative recombination processes. Process I is bandgap emission; process II emission from surface traps; and process III Mn 4T1 to 6A1 emission.
To summarize the results of the this section, the PL spectra of lightly (x=0.04) and heavily (x=0.23) Mn doped CdS quantum dots are reproduced in figure 7.10. The luminescence concludes three recombination processes, as schematically depicted in the inset. While the bandgap (process I) and surface (process II) emissions are observed for both the lightly and heavily doped samples, the 4T1 to 6A1 emission of Mn ions (process III) is strongly correlated to the their concentration, i.e. Mn local structures. Only when Mn is located in the dots, substitutionally on Cd sites, the emission can be observed.