Nanomaterials Science: Creating, Characterizing, and Measuring Nanoscale Particles

Nanomaterials Science at PNNL

Nanomaterials science focuses on methods for creating, characterizing and measuring nanoscale particles. These techniques help scientists understand how their unique properties relate to chemical reactivity, transport and exposure.

Examples include fluorescent crystalline semiconductors, dendrimers (repetitively branched molecules) and carbon fullerenes (Bucky balls). Scientists are also developing methods to distinguish natural minerals and metal from engineered nanomaterials.

Synthetic Methods

In order to produce nanomaterials, several different synthesis techniques are used. These include top-down methods where the bulk materials are mechanically machined into nano dimensions, and bottom-up synthesis where fine particles are assembled to build the nanomaterials. The latter is usually accomplished through self-assembly or co-precipitation methods.

A recent example of a bottom-up nanomaterials is aerogels, which are polymers that can be dissolved in water to create a gel. These gels have low thermal conductivity because of their porous structure. However, it is possible to improve their thermal properties by adding an admixture of surfactants.

In addition to reducing their viscosity, these admixtures can also help stabilize the nanomaterials against shear stresses and vibrations. Another way to improve the physicochemical properties of these gels is by adding organic molecules or lipids to them. These additions may allow the use of these nanomaterials for theranostics, which is an emerging field that combines diagnostic and therapeutic approaches.

Nanoparticles

Nanomaterials are any materials with one or more dimensions less than 100 nanometers. They can be made from organic or inorganic substances and appear naturally, giving butterfly wings their iridescence or gecko’s footpads their sticking power. PNNL researchers explore the relationships between material building blocks, synthesis conditions and their resulting structures, properties and functions.

These tiny objects can exhibit dramatic effects compared with larger-scale counterparts because of their high surface-to-volume ratio and unusual atomic arrangements. These effects can be used to create new materials that perform valuable tasks, such as delivering drugs directly to cancerous growths or enabling carbon nanotubes to bend aircraft wings in response to an electric current.

PNNL researchers also explore ways to assemble nanomaterials into larger structures. These assemblies can be formed via oriented attachment, such as in a coating that attaches to a surface in a precise and predictable manner; patterned formation on a substrate; or chemical links between two different materials.

Monolayers

Using sophisticated thin film deposition equipment it is possible to create thin organic monolayers that are electrically, optically or biologically active. These are called Langmuir films and have sparked high expectations for their use as useful components in many practical applications such as sensors, detectors and displays.

Nanomaterials science methods are being used to understand and control their properties. For example, x-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) are commonly used to analyze the composition and identify individual molecules of a sample.

Another method of characterization is called Atomic Force Microscopy (AFM), which allows for the direct imaging of individual atoms and molecules on surfaces. AFM can also be used to measure the mechanical properties of monolayers. The mechanical properties of a lipid monolayer depend on the head groups of the lipids and their hydration shells. For a phospholipid monolayer the head groups conform an excluded volume per se and a second volume due to water molecules hydrating them.

Two-Dimensional Materials

The size of particles in a material can dramatically affect its properties, as observed for example in the temperature-dependent electronic conductivity of semiconductors and magnetic materials. The phenomenon is even more pronounced at the nanoscale, where the quantum effects become prominent.

Atomically thin two-dimensional (2D) materials like graphene, transition metal dichalcogenides (TMDs), monoatomic buckled crystals like boron nitride and molybdenum disulfide, and diatomic hexagonal boron nitride possess novel structural and electronic properties. They can be exfoliated or fabricated via epitaxial growth to achieve large-area 2D sheets with controlled thicknesses and minimal defects.

Unlike bulk materials, where the physical properties depend on the chemical composition, 2D materials can be engineered to have desired properties and functionalities for particular applications. They are also more tolerant of mechanical, electrical and optical manipulations, making them a promising candidate for future devices. However, due to the novelty of this class of materials, very little is known about their biocompatibility and how they interact with living systems.

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