with each other and hence we would not observe sharp excitonic features in the
absorption spectra, but only a broad and featureless absorption edge.
Besides the monotonic shift in the band gap with size, an interesting observation
of a non-monotonic band gap variation was made for Mn-doped CdS nanocrystals
[28]. For a fixed Mn composition, the band gap increases with decreasing particle
size, as expected on the basis of quantum confinement. However, varying the Mn
concentration in a fixed size nanocrystal also results in a small but definite variation of the band gap. The band gap is indeed expected to vary as one goes from
CdS to MnS by doping with Mn
2þ ions because the band gaps of the two materials
are different; bulk CdS has a 2.4 eV band gap and MnS has a band gap of 3.4 eV
[13]. However, the band gap is not found to vary linearly between these two values
as a function of x in Cd 1Àx Mn x S; instead one notices a minimum in the band gap
at about 2–3% Mn doping [28]. This effect is found to be more pronounced for
smaller particles and has been ascribed to a stronger hybridization of the host sp
bands with the Mn d bands in the case of nanocrystals due to the confinement of
the wavefunctions [28].
Interestingly, a reversed type non-monotonic variation in the band gap has been
observed with Mn concentration in Mn-doped ZnS nanocrystals [14]. This is
shown in Figure 11.4; the main frame shows the UV–Vis absorption spectra and
the inset shows an initial increase in the band gap, also seen in the UV–Vis
absorption edges, with values of x in Zn 1Àx Mn x S up to x ¼ 6% followed by a
decrease on further increase in x. It is to be noted here that the band gap of ZnS,
in contrast to that of CdS, is larger than that in MnS, accounting for the reversed
Fig. 11.3. Low magnification TEM pictures of ZnS-3.5 showing
individual nanocrystallites. Also shown is the histogram for the
particle size distribution. Adapted from [11].
11 Electronic Structure and Spectroscopy of Semiconductor Nanocrystals
376
absorption spectra, but only a broad and featureless absorption edge.
Besides the monotonic shift in the band gap with size, an interesting observation
of a non-monotonic band gap variation was made for Mn-doped CdS nanocrystals
[28]. For a fixed Mn composition, the band gap increases with decreasing particle
size, as expected on the basis of quantum confinement. However, varying the Mn
concentration in a fixed size nanocrystal also results in a small but definite variation of the band gap. The band gap is indeed expected to vary as one goes from
CdS to MnS by doping with Mn
2þ ions because the band gaps of the two materials
are different; bulk CdS has a 2.4 eV band gap and MnS has a band gap of 3.4 eV
[13]. However, the band gap is not found to vary linearly between these two values
as a function of x in Cd 1Àx Mn x S; instead one notices a minimum in the band gap
at about 2–3% Mn doping [28]. This effect is found to be more pronounced for
smaller particles and has been ascribed to a stronger hybridization of the host sp
bands with the Mn d bands in the case of nanocrystals due to the confinement of
the wavefunctions [28].
Interestingly, a reversed type non-monotonic variation in the band gap has been
observed with Mn concentration in Mn-doped ZnS nanocrystals [14]. This is
shown in Figure 11.4; the main frame shows the UV–Vis absorption spectra and
the inset shows an initial increase in the band gap, also seen in the UV–Vis
absorption edges, with values of x in Zn 1Àx Mn x S up to x ¼ 6% followed by a
decrease on further increase in x. It is to be noted here that the band gap of ZnS,
in contrast to that of CdS, is larger than that in MnS, accounting for the reversed
Fig. 11.3. Low magnification TEM pictures of ZnS-3.5 showing
individual nanocrystallites. Also shown is the histogram for the
particle size distribution. Adapted from [11].
11 Electronic Structure and Spectroscopy of Semiconductor Nanocrystals
376