Article updated on 16 January 2020.
Crystalline nanoparticle arrays and superlattices – structures as small as a few nanometers (billionths of a meter) thick – 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 are isostructural with CuAu intermetallic compound. Right: superlattice assembled from magnetic Fe2O3 and Au nanoparticles are isostructural with CaB6.
It is possible to produce superlattices with several distinct geometries using these approaches, and the number of achievable lattices can also be increased by formulating a suitable strategy.
Nanoparticles in a binary lattice can be substituted with spacer entries that imitate the behavior of the 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 recently created ten 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 which resulted in the nanoparticles self-assembling into ordered structures. The superlattice constituents included lead selenide (PbSe), palladium (Pd), gold (Au), iron oxide (Fe2O3), lead sulfide (PbS) and silver (Ag) and triangular nanoplates of lanthanum fluoride (LaF3). The superlattices comprised several crystalline 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
The researchers found that they could produce a large family of new materials by changing the combinations of the nanoparticle building blocks, packing them into many structures. This was achieved using the self-assembly (bottom-up) process. Talapin added that this is one of the major challenges of nanotechnology and nanoscience – to produce new materials, generating new characteristics by engineering the material composition at the nanoscale, 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 superlattice enables a modular approach to material design. These meta-materials combine useful properties of the building blocks and generate completely new properties as well.
Properties of nanoparticle superlattices
Binary nanoparticle superlattices are periodic nanostructures with lattice constants that are shorter than the wavelength of light and can be used for preparing multifunctional meta-materials. These superlattices are fabricated from synthetic nanoparticles and, even though some biohybrid structures have been developed, including biological binary blocks into binary nanoparticle superlattices, remains a challenge. Protein-based nanocages 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, resulting in the formation of superlattices 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 magnetic resonance imaging (MRI) used in medicine.
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 as well as quantum computer components, while 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 centers.
Sources and further reading
Synthetically programmable nanoparticle superlattices using a hollow three-dimensional spacer approach – Nature Nanotechnology
Electrostatic assembly of binary nanoparticle superlattices using protein cages – Nature Nanotechnology