By Liam Critchley (chemistry and nanotechnology writer)
Graphite is a stack of millions of layers of graphene held together by invisible electrostatic attraction called “van der Waals forces”. Avadain’s electrochemical exfoliation process unstacks the graphene by gently overcoming the van der Waals forces to free large, thin and nearly defect free (LTDF) graphene flakes that can be used as an additive to improve thousands of products.
But what, exactly, are these mysterious van der Waals forces? This is an important – indeed, critical – question for graphene, as we will explore in this article.
Background
Dutch physicist Johannes Diderik van der Waals (above) won the Nobel Prize for Physics in 1910 for creating an equation that defined the concept of molecules attracting one another. These attractive forces hold many forms of atomic and molecular matter together that are not attached via chemical bonds (e.g., covalent bonds). They play a fundamental role in fields as diverse as nanotechnology, supramolecular chemistry, structural biology, surface science and condensed matter physics. (Take that, Sheldon Cooper!)
The most famous intermolecular attraction is probably the hydrogen bond because of its presence in water. It occurs when a hydrogen atom that is part of a molecule, experiences an electrostatic force of attraction with another molecule that has a lone pair of electrons. This is commonly oxygen, but it can also be other electronegative atoms such as nitrogen or fluorine—and, in some cases, sulfur, chlorine or carbon (but these tend to be comparatively weak).
Hydrogen bonds aren’t the only example of intermolecular force, though. Intermolecular forces exist in a range of forms depending on the atoms in a material and their structural arrangement. Different materials have different intermolecular forces.
Van der Waals forces play a vital role in graphene materials. They affect how different graphene layers not only interact with each other to bind them, but also how those layers interact with any other materials that they are deposited on to, admixed or integrated with.
What are van der Waals Forces?
Van der Waals forces are the most important intermolecular force as they play a key role in the stability of all materials and chemical systems.
Derjaguin-Landau-Verwey-Overbeek (DLVO) theory states that all particle interactions are based on two major intermolecular forces—attractive van der Waals forces and repulsive electrostatic double layer forces. As discussed above, van der Waals forces draw neutral molecules (including graphene) to one another. When these particles come within a threshold distance, electrons from one particle are increasingly pulled toward the nucleus of the other particle. When the atoms or molecules pass this threshold distance, a polarization occurs between the electron rich and the electron deficient domains. This generates an induced dipole with a δ+ on one atom and a δ− on the other. The δ+ side attracts the δ−, overlapping the electron cloud of the two atoms, creating a bond.
But there is also a countervailing repulsive force. At 1nm-3nm, repulsive electrostatic double layer forces push the particles apart.
Because van der Waals forces are relatively weak, mechanical motion can break the intermolecular attraction. The best-known example is graphite pencils, where the mechanical motion of the pencil going across the paper cleaves off graphite layers onto the paper (it doesn’t cleave graphene as many layers come off at once, otherwise we wouldn’t be able to see it).
Van der Waals consist of a number of dipole forces. The main ones are Keesom forces (dipole-dipole), Debye forces and London dispersion forces. Keesom forces occur when two polarized molecules have different charge distributions. Debye forces occur from the redistribution of charges when a molecule with a permanent dipole interacts with a molecule with no dipole. London dispersion forces are the fluctuation of electron clouds that alter the charge distribution between molecules when neither have a permanent dipole. All are classified as van der Waals forces.
Van der Waals forces can occur between uniformly sized and non-uniform particles, but when there’s a non-uniform particle system, it causes the bigger particles to be surrounded by smaller ones. While van der Waals forces are important for a lot of chemical systems, they are also crucial for nanomaterial systems to hold layers of nanomaterials in place and even for some nanomaterials to self-assemble.
Every molecule exhibits some level of van der Waals interaction with its neighboring media, material or environment. This can be as simple as van der Waals forces having a role in the way that water interacts with different surfaces to interactions being utilized for holding complex protein structures together.
Because van der Waals forces are the mechanism by which all molecules interact with each other in some way (when they come into close contact), van der Waals forces play a key role in the properties that many materials and chemical molecules exhibit, as well as in the way they interact with their surroundings.
Van der Waals Forces in Graphene
Graphene is an additive material. Van der Waals forces play a key role in how graphene interacts when added to other materials (such as composite ink or coating formulation) as well as overall stability.
The carbon atoms in graphene are sp2 hybridized, so there is one free π-electron per carbon atom in a graphene particle. This means that the graphene particle can attract other graphene particles, and other materials, toward it. The ability to form these van der Waals interactions with other materials is beneficial because they provide a stronger interaction that ultimately makes the end-product more stable and provides a better transfer of properties from graphene to the host.
Van der Waals forces also play a key role in how graphene interacts with its surroundings. For example, van der Waals forces determine the effectiveness of doping with different atoms. It can: govern the interaction of the particle with different substrates (such as different metals or different polymers); facilitate charge transfer mechanisms in electronic devices; and, when graphene is used as a sensor (especially in gas sensing), attract stimulus particles of interest to the particle’s surface.
In any material system, larger particles form stronger intermolecular forces with the surrounding particles, and this is no different with graphene. For large graphene particles, such as LTDF graphene flakes, the larger flakes can facilitate more intermolecular interactions with the surrounding media/environment because the large particle size of LTDF graphene flakes has a larger electron cloud to interact with other substrates and molecules. This provides a tighter intermolecular link with the surrounding environment, making them more stable in applications where graphene is used as an additive—such as in composites, inks and coating formulations.
Van der Waals forces in 2D Heterostructures
One of graphene’s cutting-edge opportunities is to create new materials with novel hybrid properties. When two or more graphene-like materials are stacked, their properties change, opening the possibility of designing novel nano-devices. The properties of these hybrid materials can be controlled by twisting the two stacked layers, providing a unique degree of freedom for the nanoscale control of next generation composite materials and nano-devices without any chemical bonds as they are bound together only by van der Waals forces.
One way to think about van der Waals heterostructures is to consider graphite. Graphite is many graphene layers held together by van der Waals forces. Van der Waals heterostructures use the same principle. The only difference is that not all the layers have the same molecular structure and the number of layers in the heterostructure tend to be much fewer than in graphite.
The van der Waals interactions hold the different 2D material layers in place and the properties of these heterostructures can be vastly different than the sum of its parts. Without van der Waals forces, it wouldn’t be possible to stack graphene and other 2D materials.
The versatility of van der Waals heterostructures is vast—as is the material properties that you can generate. This is due to the sheer number of 2D materials out there that can be stacked on top of one another in different configurations, with each layer laid on top of each other directly, or in some cases, twisted slightly to change the electronic properties of the heterostructure.
The ability for van der Waals heterostructures to be created using these intermolecular interactions has opened up new 2D materials to a broad swath of applications—including in a range of sensor, transistor, optoelectronic, photonic, LED, energy storage, electrocatalytic and quantum technology devices.
Conclusion
Van der Waals forces are a key aspect of graphene materials because they hold the different layers of graphene together without the need for chemical bonds. Most graphene materials in the marketplace are few- or multi-layered, so van der Waals forces are more important than many think.
This layered structure not only helps to define the properties of graphene itself, but it also is a critical intermolecular force that underpins how graphene materials interact with their surroundings when integrated into other materials. It is also a key force that enables the creation of novel materials with unique properties, such as 2D material heterostructures.
Larger graphene particle sizes can facilitate more intermolecular interactions with the surrounding environment, which is why LTDF graphene flakes are an ideal choice for additive-matrix-based applications (composites etc) where the van der Waals forces play a key role in product stability and performance.
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