Preparation and Properties.pdf - Preparation and properties...

This preview shows page 1 - 6 out of 58 pages.

Preparation and propertiesof nanomaterialsA. CrediNanotecnologia MolecolareLS-FCMUniversità di Bologna
Ananostructured material(ornanomaterial) is defined as a solid materialcharacterized by at least one dimension in the nanometer rangeA variety of materials1 – 1000 nm(thickness)Surfaces and thin filmsMetals, semiconductors, magnetic materialsSeveral nm in thethree dimensions3-D structures (superlattices)Metals, semiconductors, magnetic mater.Several nm2– μm22-D arrays (of nanoparticles)Zeolites, phosphates, etc.0.5 – 10 nm (porediam.)Nanoporous solidsCarbon, layered metal chalcogenides1 – 100 nm (diam.)NanotubesMetals, semiconductors, oxides, sulfides,nitrides1 – 100 nm (diam.)NanowiresCeramic oxides1 – 100 nm (diam.)Other nanoparticlesMetals, semiconductors, magnetic materials1 – 10 nm(diam.)Nanocrystals and clusters(quantum dots)MaterialsSize (approx.)MOLECULESBULK MATERIALSA. CrediNanotecnologia MolecolareLS-FCMUniversità di Bologna
Size effectsSize effects constitute a peculiar and fascinating aspect of nanomaterials.The effects determined by size pertain to the evolution of structural,thermodynamic, electronic, spectroscopic, electromagnetic and chemicalfeatures of these finite systems with changing size.The properties of a material depend on the typeof motion its electrons can execute, whichdepends on the space available for them. Thus,the properties of a material are characterized bya specific “length scale”, usually on the nmdimension. If the physical size of the material isreduced below this length scale, its propertieschange and become sensitive to size and shape.A. CrediNanotecnologia MolecolareLS-FCMUniversità di Bologna
Size effectsFigure 48.Observation of discrete electronic transitions inoptical absorption for a series of sizes of monodisperse CdSe.Figure 4. Melting temperature versus size for CdS nanocrystals.Figure 6. Size dependence of the wurtzite to rock salt pressure-induced structural transformation in CdSe nanocrystals.A. CrediNanotecnologia MolecolareLS-FCMUniversità di Bologna
Quantum confinementIn small nanocrystals, the electronic energy levels are not continuous asin the bulk but are discrete (finite density of states), because of theconfinement of the electronic wavefunction to the physical dimensions ofthe particles. This phenomenon is calledquantum confinementandtherefore nanocrystals are also referred to asquantum dots (QDs).In any material, substantial variation of fundamental electrical and opticalproperties with reduced size will be observed when the energy spacingbetween the electronic levels exceeds the thermal energy (kT).Moreover, nanocrystals possess a high surface are and a large fractionof the atoms in a nanocrystal are on its surface. Since this fractiondepends largely on the size of the particle (30% for a 1-nm crystal, 15%for a 10-nm crystal), it can give rise to size effects in chemical andphysical properties of the nanocrystal.

Upload your study docs or become a

Course Hero member to access this document

Upload your study docs or become a

Course Hero member to access this document

End of preview. Want to read all 58 pages?

Upload your study docs or become a

Course Hero member to access this document

Term
Fall
Professor
N/A
Tags
Band gap, Electronic band structure, a Credi

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture