The structure of the nanocrystalline materials plays an important role in determining electronic properties. The same material can crystallize in different
structures upon size reduction, depending on the reaction conditions. Such structural transformations are discussed in Section 11.2.
As already pointed out, the most direct consequence of a reduction in the nanocrystallite size on the electronic structure of semiconducting materials is a pronounced increase in the band gap due to the quantum confinement effect. While
there are several ways to quantitatively understand this phenomenon from a theoretical standpoint, the experimental determination of the band gap variation as a
function of size is most directly performed by ultraviolet–visible absorption spectroscopy, with the experimental absorption threshold corresponding to the direct
band gap in the material. As the band gap shifts to higher energy, the blue-shift in
the absorption edge signals the formation of progressively smaller sized nanocrystals. Therefore, UV–Vis absorption spectroscopy has played an immensely important role in the study of these systems and we discuss the essential aspects in
Section 11.3.
The UV–Vis absorption process generates an excited state of the system by generating an electron in the conduction band and a hole in the valence band; this
state most often de-excites via a radiative process, giving rise to fluorescence. The
emission characteristics depend on various aspects of the electronic structure of
the nanocrystallites. Thus, the tuning of the fluorescence wavelength in nanocrystals can be achieved by a variety of methods as discussed in Section 11.4.
A lot of theoretical work has been done in order to explain the size dependent
properties of semiconducting nanocrystals. These methods are primarily based on
the effective mass approximation, pseudopotential approaches or the tight binding
scheme. Each of these methods has certain advantages and disadvantages. We shall
explore these methods in some detail in Section 11.5.
Photoemission experiments that probe the electronic structure of the nanocrystals are indispensable if one wishes to gain insight into the electronic
structure–property relationship. Though very few studies have been carried out on
semiconducting nanocrystalline systems to date, techniques such as photoemission and X-ray absorption spectroscopies are of immense value in probing the
electronic structure and also in verifying various theories proposed for the nanocrystals. In Section 11.6 we discuss these spectroscopic studies.
11.2
Structural Transformations
Solids are known to exist in different structural phases depending on the temperature, pressure and other ambient conditions. Semiconductors, especially the
group IV, II–VI and III–V that are more covalent than other compounds, prefer
tetrahedral coordination around the ions, giving rise to phases such as the zinc
blende or the hexagonal phase. Ionic solids are stable in higher symmetry octahe11 Electronic Structure and Spectroscopy of Semiconductor Nanocrystals
372
structures upon size reduction, depending on the reaction conditions. Such structural transformations are discussed in Section 11.2.
As already pointed out, the most direct consequence of a reduction in the nanocrystallite size on the electronic structure of semiconducting materials is a pronounced increase in the band gap due to the quantum confinement effect. While
there are several ways to quantitatively understand this phenomenon from a theoretical standpoint, the experimental determination of the band gap variation as a
function of size is most directly performed by ultraviolet–visible absorption spectroscopy, with the experimental absorption threshold corresponding to the direct
band gap in the material. As the band gap shifts to higher energy, the blue-shift in
the absorption edge signals the formation of progressively smaller sized nanocrystals. Therefore, UV–Vis absorption spectroscopy has played an immensely important role in the study of these systems and we discuss the essential aspects in
Section 11.3.
The UV–Vis absorption process generates an excited state of the system by generating an electron in the conduction band and a hole in the valence band; this
state most often de-excites via a radiative process, giving rise to fluorescence. The
emission characteristics depend on various aspects of the electronic structure of
the nanocrystallites. Thus, the tuning of the fluorescence wavelength in nanocrystals can be achieved by a variety of methods as discussed in Section 11.4.
A lot of theoretical work has been done in order to explain the size dependent
properties of semiconducting nanocrystals. These methods are primarily based on
the effective mass approximation, pseudopotential approaches or the tight binding
scheme. Each of these methods has certain advantages and disadvantages. We shall
explore these methods in some detail in Section 11.5.
Photoemission experiments that probe the electronic structure of the nanocrystals are indispensable if one wishes to gain insight into the electronic
structure–property relationship. Though very few studies have been carried out on
semiconducting nanocrystalline systems to date, techniques such as photoemission and X-ray absorption spectroscopies are of immense value in probing the
electronic structure and also in verifying various theories proposed for the nanocrystals. In Section 11.6 we discuss these spectroscopic studies.
11.2
Structural Transformations
Solids are known to exist in different structural phases depending on the temperature, pressure and other ambient conditions. Semiconductors, especially the
group IV, II–VI and III–V that are more covalent than other compounds, prefer
tetrahedral coordination around the ions, giving rise to phases such as the zinc
blende or the hexagonal phase. Ionic solids are stable in higher symmetry octahe11 Electronic Structure and Spectroscopy of Semiconductor Nanocrystals
372