How would you go about proving whether or not a monodispersed semiconductor nanoparticle is actually a semiconductor? Why does the performance of the bulk form of the anatase polymorph of TiO_2, when used as a photocathode in a dye sensitized solar cell, improve when synthesized as nanoparticles? Surface plasmon resonance is an important tool for studying biomolecular interactions in drug development. Explain briefly the physical principles behind this method. Why do you think it is affected by the size of metal nanoparticles? What are other factors that can influence the surface plasmon? Solution 1. A first solution for templating nanoparticles in a nanostructured film is to grow a deposit by physical means at the film surface. The atoms thereby deposited can diffuse into the mesoporous structure to fill the pores, but the penetration is low. Another strategy is to grow the nanoparticles chemically in two stages: first adsorption of metal cations at the pore surface by impregnation in solution, followed by precipitation of the particles by reaction with a gas. A homogeneous, 3D periodic lattice of CdS nanoparticles can be synthesised in this way, and the method has been extended to sulfides such as ZnS, PbS, and Ag2S, with potential applications where optical or photocatalytic properties are required like semiconductors. 2. The most widely used and best known photocatalyst is titanium dioxide TiO2 in the anatase form, which scatters light better. With a band gap of 3.0 eV, which corresponds to a wavelength of 400 nm, anatase still absorbs a significant fraction of the sun’s light. Anatase is remarkably stable under illumination and easy to obtain in nanostructured form, with a high specific surface area. An unexpected property of nanostructured anatase films is their superhydrophilicity, making them very effective anti-condensation coatings. 3. The surface plasmon resonance is a purely dielectric effect. It results from the enhancement of the amplitude of the internal field by confinement in a nanoparticle, with respect to the incident field, the intrinsic properties of the metal being barely modified by size reduction. The characteristics of surface plasmon resonance, such as spectral width and position, and sensitivity to light polarisation, depend not only on the intrinsic properties of the nano-objects (composition, structure, size, shape), but also on their environment. The surface plasmon resonance does not directly involve quantum confinement of the electronic states but is nevertheless influenced by it. One important conclusion of the quasi-static model is that the frequency of the surface plasmon resonance is independent of size for a given metal provided that its dielectric constant does not vary.Its spectral position is determined by the nature of the metal and its surrounding matrix through their dielectric constants..

1. How would you go about proving whether or not a monodispersed semiconductor nanoparticle
is actually a semiconductor? Why does the performance of the bulk form of the anatase
polymorph of TiO_2, when used as a photocathode in a dye sensitized solar cell, improve when
synthesized as nanoparticles? Surface plasmon resonance is an important tool for studying
biomolecular interactions in drug development. Explain briefly the physical principles behind
this method. Why do you think it is affected by the size of metal nanoparticles? What are other
factors that can influence the surface plasmon?
Solution
1. A first solution for templating nanoparticles in a nanostructured film is to grow a deposit by
physical means at the film surface. The atoms thereby deposited can diffuse into the mesoporous
structure to fill the pores, but the penetration is low. Another strategy is to grow the nanoparticles
chemically in two stages: first adsorption of metal cations at the pore surface by impregnation in
solution, followed by precipitation of the particles by reaction with a gas. A homogeneous, 3D
periodic lattice of CdS nanoparticles can be synthesised in this way, and the method has been
extended to sulfides such as ZnS, PbS, and Ag2S, with potential applications where optical or
photocatalytic properties are required like semiconductors.
2. The most widely used and best known photocatalyst is titanium dioxide TiO2 in the anatase
form, which scatters light better. With a band gap of 3.0 eV, which corresponds to a wavelength
of 400 nm, anatase still absorbs a significant fraction of the sun’s light. Anatase is remarkably
stable under illumination and easy to obtain in nanostructured form, with a high specific surface
area. An unexpected property of nanostructured anatase films is their superhydrophilicity,
making them very effective anti-condensation coatings.
3. The surface plasmon resonance is a purely dielectric effect. It results from the enhancement of
the amplitude of the internal field by confinement in a nanoparticle, with respect to the incident
field, the intrinsic properties of the metal being barely modified by size reduction. The
characteristics of surface plasmon resonance, such as spectral width and position, and sensitivity
to light polarisation, depend not only on the intrinsic properties of the nano-objects (composition,
structure, size, shape), but also on their environment.
The surface plasmon resonance does not directly involve quantum confinement of the electronic
states but is nevertheless influenced by it. One important conclusion of the quasi-static model is
that the frequency of the surface plasmon resonance is independent of size for a given metal
provided that its dielectric constant does not vary.Its spectral position is determined by the nature
of the metal and its surrounding matrix through their dielectric constants.