11
Electronic Structure and Spectroscopy
of Semiconductor Nanocrystals
S. Sapra and D. D. Sarma
11.1
Introduction
There has been a tremendous increase in research in the field of nanocrystals
during the past decade [1–7]. Although research on nanocrystals started in the
early eighties, the spectacular level of activity was witnessed only in the nineties
with the emergence of improved techniques for the synthesis of high quality
nanocrystals and better instruments to characterize them. Also, the growing demand from the electronics industry to reduce the size of the integrated circuit
chips gave the field of nanotechnology a tremendous boost.
Solid materials can be categorized as metals or semiconductors, depending on
their ability to conduct electrical charge. On a microscopic level, this implies that
the metal has overlapping valence and conduction bands, allowing the electrons to
move freely, and thus giving rise to charge conduction. Semiconductors have fully
filled valence bands and the entirely empty conduction band is separated from the
valence band by a forbidden range of energies. Thus, conduction in such materials
requires a finite energy to promote the electrons from the valence to the conduction band. While metals do not show much change in their electronic properties
down to very low sizes, but semiconductors exhibit interesting variations in their
properties at sizes of about 10 nm (100 A ˚ ) and below. One thing that is most noticeable in the case of semiconductors is the variation of the band gap with a decrease in the size of the nanocrystals below the Bohr exciton radius, a B , of the
semiconductor. For example, the band gap of CdS, a II–VI semiconductor, can be
made to vary more or less continuously from the bulk value of 2.4 eV up to about
4.0 eV observed for a 15 A ˚ nanocrystal by merely tuning the size of the CdS nanocrystals [8]. Other properties such as the melting point also depend strongly on
size. For example, while the melting point of bulk CdS is @1600
C, 2.5 nm CdS
crystallites melt at a temperature as low as @400
C [9]. Optical properties such as
fluorescence emission are also critically dependent on the size of the nanocrystals.
For example, it has been shown that various sized CdSe nanocrystals can fluoresce
throughout the visible range of the electromagnetic spectrum [10] with suitable
control of the particle size.
371
Electronic Structure and Spectroscopy
of Semiconductor Nanocrystals
S. Sapra and D. D. Sarma
11.1
Introduction
There has been a tremendous increase in research in the field of nanocrystals
during the past decade [1–7]. Although research on nanocrystals started in the
early eighties, the spectacular level of activity was witnessed only in the nineties
with the emergence of improved techniques for the synthesis of high quality
nanocrystals and better instruments to characterize them. Also, the growing demand from the electronics industry to reduce the size of the integrated circuit
chips gave the field of nanotechnology a tremendous boost.
Solid materials can be categorized as metals or semiconductors, depending on
their ability to conduct electrical charge. On a microscopic level, this implies that
the metal has overlapping valence and conduction bands, allowing the electrons to
move freely, and thus giving rise to charge conduction. Semiconductors have fully
filled valence bands and the entirely empty conduction band is separated from the
valence band by a forbidden range of energies. Thus, conduction in such materials
requires a finite energy to promote the electrons from the valence to the conduction band. While metals do not show much change in their electronic properties
down to very low sizes, but semiconductors exhibit interesting variations in their
properties at sizes of about 10 nm (100 A ˚ ) and below. One thing that is most noticeable in the case of semiconductors is the variation of the band gap with a decrease in the size of the nanocrystals below the Bohr exciton radius, a B , of the
semiconductor. For example, the band gap of CdS, a II–VI semiconductor, can be
made to vary more or less continuously from the bulk value of 2.4 eV up to about
4.0 eV observed for a 15 A ˚ nanocrystal by merely tuning the size of the CdS nanocrystals [8]. Other properties such as the melting point also depend strongly on
size. For example, while the melting point of bulk CdS is @1600
C, 2.5 nm CdS
crystallites melt at a temperature as low as @400
C [9]. Optical properties such as
fluorescence emission are also critically dependent on the size of the nanocrystals.
For example, it has been shown that various sized CdSe nanocrystals can fluoresce
throughout the visible range of the electromagnetic spectrum [10] with suitable
control of the particle size.
371