Self-assembly of nanoparticles
[3] For nanoparticles, this spontaneous assembly is a consequence of interactions between the particles aimed at achieving a thermodynamic equilibrium and reducing the system’s free energy.
[6] To do so correctly, an extremely high level of direction and control is required and developing a simple, efficient method to organize molecules and molecular clusters into precise, predetermined structures is crucial.
[7] In 1959, physicist Richard Feynman gave a talk titled “There’s Plenty of Room at the Bottom" to the American Physical Society.
He imagined a world in which “we could arrange atoms one by one, just as we want them.” This idea set the stage for the bottom-up synthesis approach in which constituent components interact to form higher-ordered structures in a controllable manner.
"[8] Another definition by Serge Palacin & Renaud Demadrill is "Self-assembly is a spontaneous and reversible process that brings together in a defined geometry randomly moving distinct bodies through selective bonding forces.
The fundamental thermodynamic and kinetic mechanisms of self-assembly are poorly understood - the basic principles of atomistic and macroscale processes can be significantly different than those for nanostructures.
Concepts related to thermal motion and capillary action influence equilibrium timescales and kinetic rates that are not well defined in self-assembling systems.
[11] By controlling local intermolecular forces to find the lowest-energy configuration, self-assembly can be guided by templates to generate similar structures to those currently fabricated by top-down approaches.
This so-called bridging will enable fabrication of materials with the fine resolution of bottom-up methods and the larger range and arbitrary structure of top-down processes.
Quantum dots - most commonly CdSe nanoparticles having diameters of tens of nm, and with protective surface coatings - are notable for their ability to fluoresce over a broad range of the visible spectrum, with the controlling parameter being size.
[17] In addition, the flexibility and the lower free energy conformation is usually a result of a weaker intermolecular force between self-assembled moieties and is essentially enthalpic in nature.
Small assemblies are formed because of their increased lifetime as the attractive interactions between the components lower the Gibbs free energy.
The necessity of the self-assembly to be an equilibrium process is defined by the organization of the structure which requires non-ideal arrangements to be formed before the lowest energy configuration is found.
Dislocations caused during the assembling of nanomaterials can majorly affect the final structure and in general defects are never completely avoidable.
[23] In most cases, the thermodynamic driving force for self-assembly is provided by weak intermolecular interactions and is usually of the same order of magnitude as the entropy term.
In self-assembly, regular structural arrangements are frequently observed, therefore there must be a balance of attractive and repulsive between molecules otherwise an equilibrium distance will not exist between the particles.
[19] For hard particles, Pauling's rules are useful in understanding the structure of ionic compounds in the early days, and the later entropy maximization principle shows favor of dense packing in the system.
[20] Different particle shapes / polyhedra create diverse complex packing structures in order to minimize the entropy of the system.
As these ligands tend to be complex and sophisticated, self-assembly can provide a simpler pathway for nanoparticle organization by synthesizing efficient functional groups.
For instance, DNA oligomers have been a key ligand for nanoparticle building blocks to be self-assembling via sequence-based specific organization.
[25] However, to deliver precise and scalable (programmable) assembly for a desired structure, a careful positioning of ligand molecules onto the nanoparticle counterpart should be required at the building block (precursor) level,[26][27][28][29] such as direction, geometry, morphology, affinity, etc.
[23] Nanoparticles can be programmed to self-assemble by changing the functionality of their side groups, taking advantage of weak and specific intermolecular forces to spontaneously order the particles.
[23] While these aggregations are based on intermolecular forces, external factors such as temperature and pH also play a role in spontaneous self-assembly.
By incorporating active sites to the surfaces of nanotubes and polymers, the functionalization of these templates can be transformed to favor self-assembly of specified nanoparticles.
Understanding the behavior of nanoparticles at liquid interfaces is essential for integrating them into electronics, optics, sensing, and catalysis devices.
They observed that the micron-sized colloids generated a resistant film at the interface between the two immiscible phases, inhibiting the coalescence of the emulsion drops.
[citation needed] Nanoscale objects have always been difficult to manipulate because they cannot be characterized by molecular techniques and they are too small to observe optically.
Nanostructure characterization tools include advanced optical spectro-microscopy (linear, non-linear, tipenhanced and pump-probe) and Auger and x-ray photoemission for surface analysis.
The microphase separation of block copolymers shows a great deal of promise as a means of generating regular nanopatterns at surfaces.
[40] These copolymers offer the ability to self-assemble into uniform, nanosized micelles[41][42] and accumulate in tumors via the enhanced permeability and retention effect.
