Nanoparticles are of significant interest to academic and commercial fields, particularly those related to mechanical, electronic, photonic, and biological applications. The unique properties associated with nanoparticles originate from the substantial increase in surface area to volume ratio as particle dimension decreases, and the existence of quantum size effects not present in bulk materials.
For starters, let’s take a theoretical particle and shrink it down. As discussed earlier, the volume decreases much more rapidly than the surface. Once the particle is smaller than 100 nm or so, we can start thinking about it in terms of its atoms. In general, we can divide the particle into two sections - the outermost shell of atoms on the surface that interact with its environment, and the interior atoms (also called the bulk). The interior atoms are free to associate with each other, but generally do not play a role in chemical reactions, mechanical bonding, or other useful properties. In a sense, they are wasted (aside from the change in properties that would result if they were actually removed). The surface is where the action is, and as the particle size decreases, the fraction of atoms at the surfaces starts to increase rapidly.
Generally, a nanomaterial is defined as a particle with at least one dimension below 100 nm. These can generally be sheet-like (2-dimensional), rod or tube-like (1-dimensional), or spheres (so-called zero-dimensional, or quantum dots). The prototypical 2-D nanoparticle is graphene, which is about 1 nm thick and may be several thousand times longer and wider. Carbon nanotubes are 1-D nanoparticles, with diameter in the range of 2-30 nm (i.e., width and depth equal, like a cylinder), and length several hundred or thousand times larger.
Quantum dots are generally defined as particles with all dimensions below about 10 nm. There isn’t anything specific about this cut-off other than the fraction of atoms at the surface is huge compared to the bulk. One important property that starts to change in this time is the electronic band gap. For the purpose of this post, let’s not try and define the band gap, as it quickly becomes a quantum mechanical effect, but just say that it’s related to the energy needed to move electrons around in an atom (if that isn’t sufficient, here is a relatively friendly presentation of the band gap and its role in solid-state physics).
The electronic band gap determines a number of important physical properties of a material, including its interaction with light and conductivity. With conventional methods, band gap engineering requires controlling the composition of an alloy, constructing layered materials with alternating compositions (molecular-beam epitaxy), or changing temperature or pressure.
Quantum dots are unique because the band gap is size-dependent through a phenomenon called the quantum confinement effect. This is an extremely useful feature, because it means that the fundamental electronic and optical character of a material can now be tuned like a dial for a particular application — materials engineering at its finest.
In addition to tuning the size of the particles, the distance between neighboring quantum dots can also influence the optoelectronic properties due to short-range quantum coupling. Significant electronic energy transfer occurs at interparticle distances of up to 10 nm. As the distance decreases, the quantum size effects diminish, and their properties approach those of the bulk material. Nanoparticles also possess large surface area to volume ratios that are crucial for applications as catalysts, gas storage devices, efficient energy conversion, and barrier property enhancement. One example highlighting the challenges in mixing nanoparticles in a polymer medium is shown in the figure below. Preservation of particle dispersion during handling at all length scales is critical, particularly for sensor and multi-functional composite applications. The resulting higher-order microstructure must also be well controlled to realize the desired change in properties in the bulk.
Dazhi Sun and co-workers developed a new way to prevent clusters of zinc oxide quantum dots (ZnO QDs) by adding a plate-like nanoparticle, zirconium phosphate, to the mix. The plates help break clumps apart and prevent their re-formation. Photographs showing the hazy features of the clustered ZnO and the excellent transparency of the nanoclay-dispersed QDs shown at bottom [Sun et al., Macromolecules 2009 42(5): 1665-71].
An important research question - with significant practical applications - is how the characteristics of the nanoparticles (size, size distribution, aspect ratio, and chemical functionality), as well as the dispersion state (individually distributed particles or aggregated), and interactions with the suspending medium influence the properties of a system. This structure-property relationship is necessary to understand and potentially tailor the physical response of multi-functional materials.
One example of this research field is the work by Damasceno and colleagues (Science 2012, 337(6093):453-7). Using a molecular modeling approach, they showed that simple estimates of particle shape and local order in a fluid are sufficient to predict various categories of structural order for many convex polyhedral.
However, these discontinuous nano-scale modeling approaches implicitly assume local constitutive relationships. Or, to try and take some of the jargon out, the methods rely on basic equations developed from larger-scale systems that are not necessarily relevant on the nanoscale. This can be the source of considerable error if interfacial interactions or non-local coupling between mechanical fields are present. Extensive experimental investigations have been attempted, but little detail on the fundamental physics of the nanoparticles is possible because of challenges in achieving reproducible properties of an ensemble of individual nanoparticles.
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