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Analyzing the surface of a nanomaterial is important for many reasons, but the biggest reason is because most nanomaterials have an active surface that defines how the nanomaterial reacts in the presence of other materials and in given scenarios. By understanding the composition, atomic positioning and properties of the surface, it enables researchers to better predict its behaviour. In this article, we look at some of the common techniques used to analyze the surface of nanomaterials
There are two photoelectron spectroscopy techniques which are widely used to analyze the surface of a nanomaterial. These are X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). These techniques are very similar to each other, and whilst they can be used as standalone techniques, many scientists will use both techniques to analyze the surface of a nanomaterial.
In both techniques, a radiation source is used to excite the electrons at the surface of a nanomaterial causing them to be ejected from the surface. These electrons then travel along an ultra-high vacuum chamber, where they are collected by an electron analyzer. The methods differ in two key ways. The first is the radiation source and the second is the information that is obtainable with each technique. In XPS, MgK or AlK radiation is used to create monochromatic X-rays that excite the sample, whereas UPS uses a gas discharge lamp to introduce helium radiation to the surface.
Moving on to the analysis. UPS is widely used to determine the energies of molecular orbitals in the valence band, the types of bonding orbitals present in the nanomaterial, the valence band hybridization, the position of the valence band maximum, electron affinity, ionization energy, activation energy, and work function of a nanomaterial. On the other hand, XPS is widely used to determine the elemental composition of a surface, the chemical states, the empirical formula of a nanomaterial, the surface layer depth, the binding energy of electronic states, and the density of electronic states in a nanomaterial. Both techniques are used in conjunction with each other to build up a picture of both the elemental composition and the properties of a nanomaterial.
There are also many microscopy techniques that can be used either image the surface or provide a topographic map of the relative position of atoms on the surface of a nanomaterial. The most widely used microscopy techniques for analyzing the surface of a nanomaterial are scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM).
SEM fires high-energy electrons at the surface of nanomaterial and the electrons interact with the surface causing them to reflect by elastic scattering. Secondary electrons and electromagnetic rays are also released. All these processes are observed and contribute to SEM being able to produce an image of the surface, and for determining the composition and topology of the surface. Cathodoluminescence principles can also be applied to SEM and are used to determine many properties of a nanomaterial, including the electrical conductivity/insulating properties of the nanomaterial, and if any defects, impurities, vacancies, or contamination are present at the surface of the nanomaterial.
TEM is similar to SEM in that it fires high energy electrons towards the surface of a nanomaterial. However, in this instance, the electrons are fired at thin nanomaterials and this enables the electrons to pass through the nanomaterial. The transmission of these electrons through the nanomaterial enables an image to be generated. It is very similar to optical microscopy but uses electrons instead of light. TEM provides information on the crystal structure, morphology and stress state of the surface of a nanomaterial (as well as the internal structure).
AFM is different to the other microscopy techniques, in that it uses a tip attached to cantilever which interacts with the atoms on a surface—where the tip will move closer to the surface when it is over an atom. This method enables a topographic map to be generated that shows the relative position of each atom at the surface of the nanomaterial. The tips can also be replaced in AFM with a micron-sized particle—otherwise known as a colloidal probe—to analyze the properties of a surface. Properties that can be measured include adhesion forces, other surfaces forces, chemical interactions, and how the surface interacts with particles. In these instances, a topographic map of the atoms is also generated, and the properties and atomic maps can be superimposed to show the localized properties of a nanomaterial’s surface.
Low Energy Electron Diffraction (LEED)
Low energy electron diffraction (LEED) is another widely used technique for analyzing the surface of a nanomaterial. In LEED, the surface of a nanomaterial is bombarded with low energy electrons which diffract off the surface. How they diffract depends on the crystallographic properties of the surface. Once diffracted, the diffraction patterns are observed on a fluorescent screen composed of multiple layers which filters the electrons by their energy. LEED has become a common technique for analyzing the surface of nanomaterials because it enables the surface structure, overlayer structure, lattice type, unit cell, point defects, bonding and the surface processes of a nanomaterial to be determined.
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