in the bulk but are discrete, due to the confinement of the electron wavefunction
to the physical dimensions of the particles [5]. This phenomenon is called quantum confinement and, therefore, nanocrystals are also known as quantum dots.
In other words, a small nanocrystal could be a very bad conductor although it is
a tiny silhouette of the conducting bulk. Likewise, a tiny nanocrystal of a ferromagnet can be paramagnetic in nature. In the case of semiconductors, besides
discretization of levels, the band gap increases due to quantum confinement. In
several respects, small nanocrystals behave like molecules. The nanocrystals can be
discretely charged with electrons with characteristic charging energies. This means
that a nanocrystal carrying an extra electron can exhibit properties different from a
neutral species.
The electronic absorption spectrum of metal nanocrystals in the visible region is
dominated by the plasmon band. This absorption is due to the collective excitation
of the itinerant electron gas on the particle surface and is characteristic of a nanocrystal of a given size. In metal colloids, surface plasmon excitations impart characteristic colors to the metal sols, the beautiful wine-red color of gold sols being
well-known [6–8]. The dependence of the plasmon peak on the dielectric constant
of the surrounding medium and the diameter of the nanocrystal was predicted
theoretically by Mie and others at the turn of the last century [9–12]. The dependence of the absorption band of thiol-capped Au nanocrystals on solvent refractive
index was recently verified by Templeton et al. [13]. Link et al. found that the absorption band splits into longitudinal and transverse bands in Au nanorods [6, 7].
In contrast to metals, exciton peaks dominate the absorption of semiconductor
nanocrystals. Thus, yellowish CdS, exhibits an excitonic absorption around 600 nm,
which gradually shifts into the UV region as the nanocrystal diameters are varied
below 10 nm (see Chapter 1). The absorption band can be systematically varied
across ranges of a few 100 nm by changing the size of the semiconductor nanocrystal [14–17]. Brus and others proposed an independent theory to describe size
quantization effects in semiconductor nanocrystals, based on the effective mass
approximation [18, 19] after recognizing the failure of Mie’s theory. Since then,
theories have grown in sophistication and rigour to include key effects like surface
structure and coupling of electronic states [20–22]. However, our understanding of
the optical properties of semiconductor nanocrystals is still incomplete and careful
experiments on monodisperse nanocrytals are currently being pursued to unravel
the mystery [23]. In addition to interesting absorption properties, the semiconductor nanocrystals also exhibit luminescent behaviour [24–27]. The emission from
mono-disperse semiconductor nanocrystals such as CdSe is intense, narrow and
can by brought about by excitation in a broad range of wavelengths [27]. The
emission can be tuned by altering the diameter of the nanocrystal (see Figure 4.1).
Further, control over the emission can be exercised by varying the surface structure
and controlling the diameter distribution. The above factors have led to the exploration of a wide range of applications for luminescent semiconductor nanocrystals.
The shrinking dimensions of the current microelectronic devices and the realization that current lithographic processes cannot extend to the nanoworld [28]
have lent tremendous thrust to research aimed at ordering nanocrystals into functional networks [29–34]. The nanocrystals akin to covalent systems, self-assemble
4 Mesoscopic Assembly and Other Properties of Metal and Semiconductor Nanocrystals
52
to the physical dimensions of the particles [5]. This phenomenon is called quantum confinement and, therefore, nanocrystals are also known as quantum dots.
In other words, a small nanocrystal could be a very bad conductor although it is
a tiny silhouette of the conducting bulk. Likewise, a tiny nanocrystal of a ferromagnet can be paramagnetic in nature. In the case of semiconductors, besides
discretization of levels, the band gap increases due to quantum confinement. In
several respects, small nanocrystals behave like molecules. The nanocrystals can be
discretely charged with electrons with characteristic charging energies. This means
that a nanocrystal carrying an extra electron can exhibit properties different from a
neutral species.
The electronic absorption spectrum of metal nanocrystals in the visible region is
dominated by the plasmon band. This absorption is due to the collective excitation
of the itinerant electron gas on the particle surface and is characteristic of a nanocrystal of a given size. In metal colloids, surface plasmon excitations impart characteristic colors to the metal sols, the beautiful wine-red color of gold sols being
well-known [6–8]. The dependence of the plasmon peak on the dielectric constant
of the surrounding medium and the diameter of the nanocrystal was predicted
theoretically by Mie and others at the turn of the last century [9–12]. The dependence of the absorption band of thiol-capped Au nanocrystals on solvent refractive
index was recently verified by Templeton et al. [13]. Link et al. found that the absorption band splits into longitudinal and transverse bands in Au nanorods [6, 7].
In contrast to metals, exciton peaks dominate the absorption of semiconductor
nanocrystals. Thus, yellowish CdS, exhibits an excitonic absorption around 600 nm,
which gradually shifts into the UV region as the nanocrystal diameters are varied
below 10 nm (see Chapter 1). The absorption band can be systematically varied
across ranges of a few 100 nm by changing the size of the semiconductor nanocrystal [14–17]. Brus and others proposed an independent theory to describe size
quantization effects in semiconductor nanocrystals, based on the effective mass
approximation [18, 19] after recognizing the failure of Mie’s theory. Since then,
theories have grown in sophistication and rigour to include key effects like surface
structure and coupling of electronic states [20–22]. However, our understanding of
the optical properties of semiconductor nanocrystals is still incomplete and careful
experiments on monodisperse nanocrytals are currently being pursued to unravel
the mystery [23]. In addition to interesting absorption properties, the semiconductor nanocrystals also exhibit luminescent behaviour [24–27]. The emission from
mono-disperse semiconductor nanocrystals such as CdSe is intense, narrow and
can by brought about by excitation in a broad range of wavelengths [27]. The
emission can be tuned by altering the diameter of the nanocrystal (see Figure 4.1).
Further, control over the emission can be exercised by varying the surface structure
and controlling the diameter distribution. The above factors have led to the exploration of a wide range of applications for luminescent semiconductor nanocrystals.
The shrinking dimensions of the current microelectronic devices and the realization that current lithographic processes cannot extend to the nanoworld [28]
have lent tremendous thrust to research aimed at ordering nanocrystals into functional networks [29–34]. The nanocrystals akin to covalent systems, self-assemble
4 Mesoscopic Assembly and Other Properties of Metal and Semiconductor Nanocrystals
52