Crystalline nanoparticle arrays and superlattices can be synthesized with well-defined geometries using appropriate electrostatic, hydrogen bonding or biological recognition interactions.
Examples of binary nanoparticle superlattices. Left: superlattice assembled from PbSe and Au nanoparticles is isostructural with CuAu intermetallic compound. Right: superlattice assembled from magnetic Fe2O3 and Au nanoparticles is isostructural with CaB6.
Even though it is possible to produce superlattices with a number of distinct geometries using these approaches, the number of achievable lattices can be increased by formulating a suitable strategy.
By developing a strategy that will allow certain nanoparticles in a binary lattice to be substituted with spacer entries that imitate the behavior of nanoparticles being replaced, even if they do not have an inorganic core. Including spacer entities within a known binary superlattice will effectively delete one set of nanoparticles without impacting the positions of the other set.
Nanoparticle Superlattices with New Properties
Superlattices with two kinds of nanoparticles enable the creation of new materials with beneficial physical properties. However, It has been difficult to make the structures thermodynamically stable. A team from the University of Michigan, IBM and Columbia University created 10 novel binary nanoparticle superlattice materials.
In 2003 our teams at IBM and Columbia University reported the first binary superlattice assembled from semiconductor (PbSe) and magnetic (Fe2O3) nanoparticles. Since that time we learned how to grow about twenty binary superlattice structures using all possible combinations of about fifteen different materials.
Dmitri Talapin, Lawrence Berkeley National Laboratory
The superlattices were made by placing a substrate in a colloidal solution of two kinds of nanoparticles. The solvent was evaporated in a low-pressure chamber causing self assembly of the nanoparticles into ordered structures. The superlattice constituents included lead selenide (PbSe), palladium, gold, iron oxide, lead sulfide and silver and also triangular nanoplates of lanthanum fluoride. The superlattices had several crystal structures.
The self assembly process was directed by adjusting the charge state of the nanoparticles. This was achieved by adding tri-n-octylphosphine oxide (TOPO), carboxylic acids or dodecylamine to the nanoparticle solution. For instance, the addition of oleic acid to PbSe nanocrystals converted some negatively charged and neutral nanocrystals into positively charged ones. On the other hand, the addition of TOPO increased the population of negatively charged lead selenide nanocrystals. The addition of oleic acid to gold nanoparticles caused them to become negatively charged.
Talapin stated that by monitoring the self-assembly process it was possible to produce a large family of new materials changing the combinations of the building blocks and packing them into a number of structures. He also added that this is one of the major challenges of nanotechnology and nanoscience – to produce new materials and generate new characteristics by engineering the material composition at the nano scale and by using natural self-assembly phenomena.
The fine-tuning of the nanoparticle shape and the use of varied proportions of the two nanoparticle constituents enabled the researchers to control the self-assembly of lattices.
The combination of two or more materials in a super lattice enables a modular approach to material design. These meta-materials are then capable of combining useful properties of the building blocks and generating completely new properties due to intermixing of components.
Properties of Nanoparticle Superlattices
Binary nanoparticle superlattices are periodic nanostructures having lattice constants that are shorter than the wavelength of light and can be used for preparing multifunctional metamaterials. These superlattices are fabricated from synthetic nanoparticles and even though bio hybrid structures have been developed, including biological binary blocks into binary nanoparticle superlattices is still a challenge. Protein-based nano cages offer a complicated yet monodisperse and geometrically well-defined hollow cage that can be used for the encapsulation of different materials.
Video Courtesy of Mauri Kostiainen YouTube Channel
These protein cages have been utilized for programming the self-assembly of encapsulated materials for the formation of free-standing crystals. Recent research showed that electrostatically patchy protein cages such as ferritin cages and cowpea chlorotic mottle virus can be used for directing the self-assembly of three-dimensional binary superlattices. Super-paramagnetic iron oxide nanoparticles or RNA can be encapsulated in the negatively charged cages and superlattices are formed with positively charged gold nanoparticles. Viruses and gold nanoparticles form a non-isostructural crystal structure. Gold nanoparticles as well as nanoparticle-loaded or empty ferritin cages form a simple cubic AB structure. These magnetic assemblies ensure contrast enhancement in MRI.
Applications of Nanoparticle Superlattices
Talapin added that in their binary superlattices, they were able to combine metals, semiconductors, ferroelectric, magnetic, dielectric and other materials. For instance, binary superlattices of semiconducting and magnetic nanoparticles are suitable for spintronic and magneto-optic data storage devices and superlattices comprising two different semiconductors can be used for a new generation of thermoelectric devices and solar cells. Binary superlattices can also be used for designing new effective catalysts with an accurate arrangement of catalytic centres.
Sources and Further Reading