Optical Properties of Lead Sulfide Nanoclusters: Effects of Size, Stoichiometry and Surface Alloying
- PDF / 316,416 Bytes
- 5 Pages / 414.72 x 648 pts Page_size
- 51 Downloads / 196 Views
lx10-4
2xlO-4
2.5 ±0.5
2.9 ±0.6
I
5x10 5.0±1.0
I
lxl0-3 4.5±1.0
265 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
3
14
E12 10
-0-- #82 (.0005 M)
=6
c
PbS #81 .0002 M
-
--o-#84 (.0010 M) -- o--#83 (.00075M)
C) H
6 #81 (.0002 M)
8
--
-
I
.
I
molecular absorpti4on
2
w
C)
o9 #80 (.0001 M)
0 0 .0 IC U) .0 'I
S4 2
1
-.
.
,
.
. , .
.
.
.
." '
0
0 200 250 300 350 400 450 500 550 600 wavelength (nm)
200
250 300 350 400 450 wavelength (nm)
500
Figure 2. Comparison of absorption between nanoclusters and matrix isolated diatomic PbS molecules [3].
Figure 1. Optical absorbance of PbS clusters for a range of Pb concentrations.
The diameters of the particles were measured from TEM micrographs. To determine the size and the dispersion of sizes accurately, good contrast and a large number of clusters are required. Our preliminary results consist of a limited number of samples, so the results summarized in Table I should be considered rough estimates of the average size. The errors represent the inaccuracy in determining the average diameter and n= the polydispersity of sizes. The general trend is that the particle size increases with reagent concentration in the range of 2-5 nm. Figure 1 shows the optical absorbance of the PbS clusters synthesized at 1:1 stoichiometry. Samples #80 and #81 have an absorption peak at 4.37 eV (284 nm). These are the smallest clusters, synthesized at the lowest concentration. As the concentration is increased a background absorption develops and the relative intensity of the 284 nm peak decreases. Note that this peak is present in all the samples and that the position does not shift. The fact that the peak position does not depend on cluster size suggests that it is a molecular level and not an excitonic feature. Figure 2 compares the absorption spectrum of 2-3 nm clusters to that of matrix isolated diatomic PbS monomers [3]. The strongest UV absorption feature of the diatomic PbS matches this peak in energy. This state is labeled E and has been tentatively identified as the transition to the excited singlet molecular orbital x 3 o 2 x, 11+ [4]. The E transition has no vibrational structure
Emission - 520 nm
J
200
250
Excitation
300 nm
II
300
350
400
450
500
wavelenath (nm) Figure 3. PL emission and excitation spectra of PbS # 81. 266
550
600
and is thought to be a pre-dissociative excitation since the dissociative energy, 4.7 eV, for PbS is greater than the photon energy, hv = 4.4 eV. The photoluminescence excitation (PLE) and emission (PL) spectra are shown in figure 3. A strong emission band occurs at 520 nm. The PLE spectrum for the 520 nm emission exhibits two peaks at 270 and 300 nm. Since the emission occurs at a much lower energy than either the excitation or absorption features it is interpreted to result from a surface state or defect. Figure 4 shows a comparison of a typical PLE and absorption spectrum. The fact that the absorption peak lines up with the valley between the two excitation peaks suggests
Data Loading...
