In some cases, a layer of a noble metal is used as a buffer between the core
nanocrystal and the ligand shell [87]. Thus, a layer of gold lends special stability to
Fe nanocrystals and helps prevent oxidation and in preserving the magnetic properties of Fe (see Figure 4.7). A coating of a wider band gap material over a semiconductor nanocrystal aids in lifting the energy of the surface states from in between the highest occupied and the lowest unoccupied levels, thereby enhancing
the luminescence efficiency [14, 24]. The lower band gap material acts as a seed for
nucleation of the higher band bap material. Thus, core–shell nanocrystals such as
CdSeaZnS, SiaSiO 2 , HgSaCdS, PbSaCdS [45, 46], ZnSaCdSe [88], ZnSeaCdSe
[89], CdTeaHgTe nanocrystals have been obtained [90, 91].
Fig. 4.6. (a) TEM micrograph showing
tetrahedral Si nanocrystals. The inset shows a
selected area electron diffraction pattern typical
of a diamond structure. (b) A high resolution
TEM image of a triangular CdS nanocrystal.
The corresponding power spectrum is in (c).
(reproduced with permission from [79, 80]).
4.2 Synthetic Strategies 59
nanocrystal and the ligand shell [87]. Thus, a layer of gold lends special stability to
Fe nanocrystals and helps prevent oxidation and in preserving the magnetic properties of Fe (see Figure 4.7). A coating of a wider band gap material over a semiconductor nanocrystal aids in lifting the energy of the surface states from in between the highest occupied and the lowest unoccupied levels, thereby enhancing
the luminescence efficiency [14, 24]. The lower band gap material acts as a seed for
nucleation of the higher band bap material. Thus, core–shell nanocrystals such as
CdSeaZnS, SiaSiO 2 , HgSaCdS, PbSaCdS [45, 46], ZnSaCdSe [88], ZnSeaCdSe
[89], CdTeaHgTe nanocrystals have been obtained [90, 91].
Fig. 4.6. (a) TEM micrograph showing
tetrahedral Si nanocrystals. The inset shows a
selected area electron diffraction pattern typical
of a diamond structure. (b) A high resolution
TEM image of a triangular CdS nanocrystal.
The corresponding power spectrum is in (c).
(reproduced with permission from [79, 80]).
4.2 Synthetic Strategies 59