emissions over a range of photon energies, in other words, giving rise to a broad
and consequently a low intensity emission. Moreover, it is in general difficult to
tailor-make or tune the energies arising from such defect states, rendering it difficult to influence the emission, as required by any specific application, by rational
synthesis. Thus, synthesis strategies are often adopted to passivate these dangling
bonds, so that the dominant part of the de-excitation process takes place via the
band edge recombination, thereby allowing for tuning of the emission characteristics from a nanocrystal sample by tuning the band gap via size-control. Such surface passivation can be achieved by different methods. In one approach, the organic
capping agent, that is used to limit the growth of the nanocrystals, is so chosen
that it has a strong interaction with the surface states, leading to an effective removal of the surface states from the band gap region. Using cysteine ester, it has
been shown [19] that CdS nanocrystals that normally emit in the orange due to
such defect states can be made to emit in the blue. Figure 11.5 shows the case of ZnSe
with the surface states suitably passivated to give rise to band edge emission. An
alternative method to remove the surface states is to overcoat the nanocrystallite
with a thin layer of another material of a higher band gap. This has been carried
out in the case of CdSe nanocrystals overcoated with a few layers of ZnS [10]. This
results in the proper passivation of the trap states giving rise to an enhanced fluorescence compared to CdSe nanocrystals coated with an organic passivating layer.
Another method to induce sharp fluorescence at a chosen energy is to dope impurity ions into the host semiconductor lattice. This results in energy transfer from
the band edge to the impurity ion, thus giving rise to fluorescence from the impurity levels. As the energy levels of the impurity ion must be below the bottom of
the conduction band for the electron to de-excite into these levels, the emission
from impurity levels is, in general, red-shifted with respect to the band edge. The
most extensively studied doped systems are those of Mn-doped ZnS [14, 20–34]
and Mn-doped CdS [26–28] nanocrystals. Figure 11.7 shows typical fluorescence
emission spectra of various percentages of Mn-doped ZnS nanocrystals of 18 A ˚ diameter [20]. The excitation wavelength is 290 nm (4.3 eV). The ZnS surface states,
which appear at 440 nm, are almost totally quenched for the 9% Mn-doped ZnS
sample and only the Mn d–d orange emission is observed at 590 nm (2.1 eV).
The orange emission is slightly weakened at higher temperatures due to exciton–
phonon coupling [29]. At temperatures below 50 K the luminescence intensity remains constant indicating a phonon frequency of 101 cm
À1 [29]. At room temperature the luminescence efficiency drops to about half the value at 50 K, as studied
in a glass matrix, due to phonon coupling [29]. Bhargava et al. concluded that the
lifetime of the Mn d–d transition in the case of nanocrystals reduces to the order of
a few nanoseconds compared to the bulk material where the transition takes place
on a timescale of a few milliseconds [31]. This was attributed to an enhanced
overlap of the sp–d exchange interactions between the host bands and the Mn d
levels due to the larger overlap of the host sp and Mn d wavefunctions in the confined nanocrystals compared to the bulk material. Bol and Meijerink [34] studied
the time resolved fluorescence of Mn-doped ZnS nanocrystals. For a zero time delay they observe only the ZnS defect emission at @400 nm, indicating that this
11 Electronic Structure and Spectroscopy of Semiconductor Nanocrystals
380
and consequently a low intensity emission. Moreover, it is in general difficult to
tailor-make or tune the energies arising from such defect states, rendering it difficult to influence the emission, as required by any specific application, by rational
synthesis. Thus, synthesis strategies are often adopted to passivate these dangling
bonds, so that the dominant part of the de-excitation process takes place via the
band edge recombination, thereby allowing for tuning of the emission characteristics from a nanocrystal sample by tuning the band gap via size-control. Such surface passivation can be achieved by different methods. In one approach, the organic
capping agent, that is used to limit the growth of the nanocrystals, is so chosen
that it has a strong interaction with the surface states, leading to an effective removal of the surface states from the band gap region. Using cysteine ester, it has
been shown [19] that CdS nanocrystals that normally emit in the orange due to
such defect states can be made to emit in the blue. Figure 11.5 shows the case of ZnSe
with the surface states suitably passivated to give rise to band edge emission. An
alternative method to remove the surface states is to overcoat the nanocrystallite
with a thin layer of another material of a higher band gap. This has been carried
out in the case of CdSe nanocrystals overcoated with a few layers of ZnS [10]. This
results in the proper passivation of the trap states giving rise to an enhanced fluorescence compared to CdSe nanocrystals coated with an organic passivating layer.
Another method to induce sharp fluorescence at a chosen energy is to dope impurity ions into the host semiconductor lattice. This results in energy transfer from
the band edge to the impurity ion, thus giving rise to fluorescence from the impurity levels. As the energy levels of the impurity ion must be below the bottom of
the conduction band for the electron to de-excite into these levels, the emission
from impurity levels is, in general, red-shifted with respect to the band edge. The
most extensively studied doped systems are those of Mn-doped ZnS [14, 20–34]
and Mn-doped CdS [26–28] nanocrystals. Figure 11.7 shows typical fluorescence
emission spectra of various percentages of Mn-doped ZnS nanocrystals of 18 A ˚ diameter [20]. The excitation wavelength is 290 nm (4.3 eV). The ZnS surface states,
which appear at 440 nm, are almost totally quenched for the 9% Mn-doped ZnS
sample and only the Mn d–d orange emission is observed at 590 nm (2.1 eV).
The orange emission is slightly weakened at higher temperatures due to exciton–
phonon coupling [29]. At temperatures below 50 K the luminescence intensity remains constant indicating a phonon frequency of 101 cm
À1 [29]. At room temperature the luminescence efficiency drops to about half the value at 50 K, as studied
in a glass matrix, due to phonon coupling [29]. Bhargava et al. concluded that the
lifetime of the Mn d–d transition in the case of nanocrystals reduces to the order of
a few nanoseconds compared to the bulk material where the transition takes place
on a timescale of a few milliseconds [31]. This was attributed to an enhanced
overlap of the sp–d exchange interactions between the host bands and the Mn d
levels due to the larger overlap of the host sp and Mn d wavefunctions in the confined nanocrystals compared to the bulk material. Bol and Meijerink [34] studied
the time resolved fluorescence of Mn-doped ZnS nanocrystals. For a zero time delay they observe only the ZnS defect emission at @400 nm, indicating that this
11 Electronic Structure and Spectroscopy of Semiconductor Nanocrystals
380