A nanoparticle (or nanopowder or nanocluster or nanocrystal) is a microscopic particle with at least one dimension less than 100 nm. Nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic or molecular structures. Nanoparticles exhibit a number of special properties relative to bulk material.Nanoparticles of many other materials, including metals, metal oxides; carbides, borides, nitrides, silicon, and other elemental semiconductors are available.
Mechanism
Their unique physical properties are due to atoms residing on the surface. The excitation of an electron from the valance band to the conduction band creates an electron hole pair. Recombination can happen two ways as radiative and non-radiative leading to radiative recombination to photon and non-radiative recombination to phonon (lattice vibrations).
Also the band gap gradually becomes larger because of quantum confinement effects giving rise to discrete energy levels, rather than a continuous band as in the corresponding bulk material. Further, problem of particle agglomeration is overcome by passivating (capping) the “bare” surface atoms with protecting groups for providing electronic stabilization to the surface. The capping agent usually takes the form of a Lewis base compound covalently bound to surface metal atoms.
Synthesis of Nanoparticles
There are various methods for the synthesis of nanoparticles and synthesis technique is a function of the material, desired size, quantity and quality of dispersion.
Synthèses techniques are  Vapor phase (molecular beams, flame synthesis etc) and solution phase synthesis (Aqueous Solution and Nonaqueous Solution). Semiconductor Nanoparticles Synthesis typically occurs by the rapid reduction of organmetallic precursors in hot organics with surfactants.
Few semiconductor nanoparticles are:
II-VI: CdS, CdSe, PbS, ZnS
III-V: InP, InAs
MO: TiO2, ZnO, Fe2O3, PbO, Y2O3
Applications
Nanoparticles often possess unexpected optical properties as they are small enough to confine their electrons and produce quantum effects. For example gold nanoparticles appear deep-red to black in solution. Nanoparticles of yellow gold and grey silicon are red in color. Gold nanoparticles melt at much lower temperatures (~300 °C for 2.5 nm size) than the gold slabs (1064 °C). Absorption of solar radiation is much higher in materials composed of nanoparticles than it is in thin films of continuous sheets of material. In both solar PV and solar thermal applications, controlling the size, shape, and material of the particles, it is possible to control solar absorption. Clay nanoparticles when incorporated into polymer matrices increase reinforcement, leading to stronger plastics, verifiable by a higher glass transition temperature and other mechanical property tests. These nanoparticles are hard, and impart their properties to the polymer (plastic). Nanoparticles have also been attached to textile fibers in order to create smart and functional clothing.
Researchers at University College of London have reported in Science that a suspension of coated titanium dioxide nanoparticles that can be spray-painted or dip coated onto a range of hard and soft surfaces, including paper, cloth, and glass, yield super hydrophobic coatings that resist oil and are self-cleaning in air. The coatings resisted rubbing, scratching, and surface contamination, factors often exacerbated in most self-cleaning technologies.
They further report that nanoparticle additives indicate a major opportunity to improve the energy efficiency of large industrial, commercial, and institutional cooling systems known as chillers.
Silver nanoparticles have unique optical, electrical, and thermal properties and are being incorporated into products that range from photovoltaics to biological and chemical sensors. Examples include conductive inks, pastes and fillers which utilize silver nanoparticles for their high electrical conductivity, stability, and low sintering temperatures. Additional applications include molecular diagnostics and photonic devices, which take advantage of the novel optical properties of these nanomaterials. An increasingly common application is the use of silver nanoparticles for antimicrobial coatings, and many textiles, keyboards, wound dressings, and biomedical devices now contain silver nanoparticles that continuously release a low level of silver ions to provide protection against bacteria.( See more at: http://www.sigmaaldrich.com/materials-science/nanomaterials/silver-nanoparticles.html#sthash.WGzJEuKE.dpuf)
Colloidal gold nanoparticles have been utilized for centuries by artists due to the vibrant colors produced by their interaction with visible light. More recently, these unique optical-electronics properties have been researched and utilized in high technology applications such as organic photovoltaics, sensory probes, therapeutic agents, drug delivery in biological and medical applications, electronic conductors and catalysis.( See more at: http://www.sigmaaldrich.com/materials-science/nanomaterials/gold-nanoparticles.html#sthash.8pgtk6eI.dpuf)
Q-dots
Semiconductor nanoparticles also known as Q-dots are generally particles of material with diameters in the range of 1 to 20 nm.
Properties of Q - dots
Quantum Dots have high quantum yield of often 20 times brighter, possess a narrower and more symmetric emission spectra, 100-1000 times more stable to photo bleaching, possess high resistance to photo-/chemical degradation and have tunable wave length range of 400-4000 nm.
Capping Quantum Dots
Due to the extremely high surface area of a nanoparticle there is a high quantity of “dangling bonds” and by adding a capping agent consisting of a higher band gap energy semiconductor (or smaller) can eliminate dangling bonds and drastically increase quantum yield. With the addition of CdS/ZnS the quantum yield can be increased from ~5% to 55%
Applications
Due to their unique physical properties there are many potential applications in the areas such as nonlinear optics, luminescence, electronics, catalysis, solar energy conversion, and optoelectronics.