Nanoscience is often referred to as ‘the science of small’. The question we address in this chapter is: Why does size matter? What is so special about nano-sized materials? How are their properties different from those of ‘conventional’ bulk materials? Here, a complete review of the topic, which would involve an in-depth review of quantum physics, cannot be provided, but some fundamental aspects of nanoscience that are essential to understanding the exceptional properties of nanomaterials are highlighted. Rather than focusing on the theory behind these effects, the focus is on the implications that these effects have on the properties of materials.
What happens at the nanoscale?
The macroscopic physical properties of a substance (melting point, boiling point, conductivity, etc.) are determined by studying a pure sample in quantities big enough to be measured under normal laboratory conditions. One mole of any material contains 6.022 x 1023 molecules; one mole of water, for instance, weights 18 g. Therefore, when the boiling point of one mole of water is determined, in reality, the value which is obtained represents an average value based on the behavior of billions and billions of molecules of water; we assume that the result should be true for any size of group of water molecules. This is not correct for many materials: as the size of the material is reduced, and the nanoscale level is reached, the same material may display totally different properties (different melting point, conductivity, etc.). This is because the matter at the nanoscale no longer follows Newtonian physics but rather quantum mechanics.
In other words, the properties of materials can be size–dependent! This might be a rather new concept to bring into the classroom as, conventionally, the properties of a substance (solid, liquid or gas) are related to the atoms and molecules that make up the substance and the way they are connected to one another (chemical bonds). Size is not normally mentioned as a key factor. Students will probably expect a piece of gold to be golden in color however big or small it is. This is correct at the macro and micro-scale levels: but at the nanoscale, things start to change dramatically due to quantum effects. In fact, gold can be used as a prime example: a colloid of gold nanoparticles is no longer ‘golden’ but ruby-red in color (Figure 1).
TIP FOR TEACHERS: The concept that a material can have properties that are size-dependent can be illustrated through ‘macro’ analogies. For instance, a glass half-filled with water makes a different sound to a glass totally filled with water. In this analogy, the quantity (volume) of the water determines the sound that is emitted. Similarly, a guitar string under more or less tension will also make a different sound.
What You Will Learn
- 0.1 A different kind of small
- 0.2 Physics at the nanoscale
- 0.3 Chemistry at the nanoscale
- 0.4 Surface energy
- 0.5 Reactions where surface properties are very important
- 0.6 Nanomaterials with exceptional electrical properties
- 0.7 Interaction of light with matter
- 0.8 Colour generation from nanoparticles and nanostructures
- 0.9 From white to transparent materials
- 0.10 Improving existing materials
- 1 Microscopy
- 2 Spectroscopy methods
- 3 Non-radiative and non-electron
A different kind of small
Materials that belong to the ‘nanoscale’ are made of at least clusters of atoms and molecules, not just single atoms: for example, 3.5 atoms of gold or eight hydrogen atoms lined up in a row are one nanometre long. A glucose molecule is about 1 nm in size.
Nanomaterials represent a ‘different kind of small’ compared to other ‘small’ objects that a student might be familiar with. Nanomaterials are not as small as electrons or single atoms, and are bigger than other ‘very small objects’, such as a cell or bacterium, that a student can probably think of. Nanostructures are at the confluence of the smallest human-made objects (e.g. latest-generation transistors) and the largest molecules of living things (e.g. DNA, proteins). Nanomaterials are intermediate in size between isolated atoms and molecules, and bulk materials. At this scale, matter shows exceptional properties.
Thanks to their unique properties, nanomaterials offer two exciting possibilities:
- nanomaterials can be used to improve current materials or create new ones that have exceptional properties;
- as nanomaterials have dimensions in the range of the largest molecules found in the natural world, it is possible to integrate them and interact actively with them in a
The fabrication and utility of a range of nanomaterials are further discussed in Module 1, Chapter 5: Overview of nanomaterials.
Physics at the nanoscale
Nanomaterials are closer in size to single atoms and molecules than to bulk materials, and to explain their behavior, it is necessary to use quantum mechanics. Quantum mechanics is a scientific model that was developed for describing the motion and energy of atoms and electrons. As quantum mechanics concepts are normally included in secondary science (physics or chemistry) school curriculum, they are not described here in detail. Here, just a brief summary of the most salient quantum effects, together with other physical properties that are relevant at the nanoscale is provided.
- Due to the smallness of nanomaterials, their mass is extremely small and gravitational forces become Instead, electromagnetic forces are dominant in determining the behavior of atoms and molecules.
- The wave-corpuscle duality of the matter: for objects of very small mass, such as the electron, wavelike nature has a more pronounced Thus, electrons exhibit wave behavior and their position is represented by a wave (probability) function.
- One of the consequences is a phenomenon called tunneling. Classic physics states that a body can pass a barrier (potential barrier) only if it has enough energy to ‘jump’ over, Therefore, if the object has lower energy than that needed to jump over the energy barrier (the ‘obstacle’), in classic physics, the probability of finding the object on the other side of the barrier is zero. In quantum physics, a particle with energy less than that required to jump the barrier has a finite probability of being found on the other side of the barrier. Figuratively, one can imagine that the particle passes into a ‘virtual tunnel’ through the barrier (Figure 2). It should be noted that in order to have a tunnel effect, the ‘thickness’ of the barrier (i.e. energy potential) must be comparable to the wavelength of the particle and, therefore, this effect is observed only at the nanometre level. So, in simple terms, electron (or quantum) tunneling is attained when a particle (an electron) with lower kinetic energy is able to exist on the other side of an energy barrier with higher potential energy, thus defying a fundamental law of classic mechanics. Tunneling is the penetration of an electron into an energy region that is classically forbidden.
Tunneling is a fundamental quantum effect and it is the basis of a very important instrument for imaging nanostructured surfaces called the Scanning Tunnelling Microscope (STM). The same instrument can be used as a nanofabrication tool (movement of single atoms). The operation principle and capabilities of the STM are discussed in Module 1, Chapter 6: Characterisation methods.
- Quantum confinement: in a nanomaterial, such as a metal, electrons are confined in space rather than free to move in the bulk of the
- Quantisation of energy: electrons can only exist at discrete energy levels. Quantum dots are nanomaterials that display the effect of quantization of
- Random molecular motion: molecules move due to their kinetic energy (assuming the sample is above absolute zero). This is called random molecular motion and is always At the macroscale, this motion is very small compared to the sizes of the objects and thus is not influential on how the object moves. At the nanoscale, however, these motions can be of the same scale as the size of the particles and thus have an important influence on how they behave. One example of random kinetic motion is Brownian motion.
- Increased surface-to-volume ratio: one of the distinguishing properties of nanomaterials is that they have an increased surface area. This characteristic is described in more depth in the next section.
Chemistry at the nanoscale
It has already been stated that a nanomaterial is formed of at least a cluster of atoms, often a cluster of molecules. It follows that all types of bonding that are important in chemistry are also important in nanoscience. They are generally classified as:
- intramolecular bonding (chemical interactions): these are bondings that involve changes in the chemical structure of the molecules and include ionic, covalent and metallic bonds;
- intermolecular bonding (physical interaction): these are bondings that do not involve changes in the chemical structure of the molecules and include ion-ion and ion-dipole interactions; van der Waals interactions; hydrogen bonds; hydrophobic interactions; repulsive forces (such as steric repulsions).
It is assumed that the description of chemical and physical bonds is part of the secondary science school curriculum, so here some chemical and physical interactions that often used to describe the properties of nanomaterials are highlighted.
Nanomaterials often arise from a number of molecules held together or large molecules that assume specific three-dimensional structures through intermolecular bonding (macromolecules). Therefore, nanoscience also deals with supramolecular chemistry (i.e. the chemistry that deals with interactions among molecules), which is just a sub-area of the general field called ‘chemistry’. In these macromolecules, intermolecular bonding often plays a crucial role.
- Intermolecular bondings, such as hydrogen bonding and van der Waals bonding are weak interactions but in a large number, they can have total energy that can be quite significant. Consider, for instance, the structure of DNA (which has a cross-section of 2 nm): the two helixes are held together by numerous hydrogen This point becomes particularly relevant in nanoscience, where materials can have very large surface areas and, consequently, small forces can be applied to very large areas.
- Intermolecular bondings often hold macromolecules (such as proteins) together in specific three-dimensional structures with which precise biological functions are Disruption of these interactions in a protein irreversibly affects its 3D structure (quaternary structure) and leads to a total loss of function (protein denaturation).
- One type of intermolecular bonding particularly significant in nanoscience is the hydrophobic effect. This is a process basically driven by entropy and which has a major role in biological materials. In simple terms, it is the property by which non-polar molecules (e.g. oil) tend to form aggregates of like molecules inTIP FOR TEACHERS: Students will be familiar with the hydrophobic effect. A simple example is oil drops in water.
Molecules as devices
In nanoscience, macromolecules are often considered as ‘devices’ that, for instance, can trap or release a specific ion under certain environmental conditions (pH, etc.). A biological example of such a macromolecule is ferritin. Therefore, in nanoscience and nanotechnologies, where molecules can themselves be devices, bonds may also be device components. One area of study is, for example, the use of molecules as molecular switches, actuators and electronic wires. This is further discussed in Module 2, Chapter 4: Information and Communication Technologies.
Unique material properties
at the nanoscale
Regardless of whether we consider a bulk material or a nanoscale material, its physical and chemical properties depend on many of its surface properties. Surfaces perform numerous functions: they keep things in or out; they allow the flow of a material or energy across an interface; they can initiate or terminate a chemical reaction, as in the case of catalysts. The branch of science that deals with the chemical, physical and biological properties of surfaces is called surface science. In this context, the term interface, rather than surface, is often used, to emphasize the fact that it is a boundary between two phases: the material and the surrounding environment (liquid, solid or gas).
If a bulk material is subdivided into an ensemble of individual nanomaterials, the total volume remains the same, but the collective surface area is greatly increased. This is shown schematically in Figure 3.
The consequence is that the surface-to-volume ratio of the material — compared to that of the parent bulk material — is increased.
TIP FOR TEACHERS: A very simple example to mention in class is granular sugar and caster sugar. Caster sugar is finer, stickier (more surface absorption) and dissolves faster in water.
How would the total surface area increase if a cube of 1 m3 were progressively cut into smaller and smaller cubes until it is composed of 1 nm3 cube? Table 1 summarises the results.
The importance of surface atoms
In surface science, the chemical groups that are at the material interface determine its properties. Properties like catalytic reactivity, electrical resistivity, adhesion, gas storage and chemical reactivity depend on the nature of the interface. Nanomaterials have a significant proportion of atoms existing at the surface. This has a profound effect on reactions that occur at the surface such as catalysis reactions, detection reactions, and reactions that, to be initiated, require the physical adsorption of certain species at the material’s surface.
The fact that in a nanomaterial a larger fraction of the atoms is at the surface influences some physical properties such as the melting point. Given the same material, its melting point will be lower if it is nano-sized. Surface atoms are more easily removed than bulk atoms, so the total energy needed to overcome the intermolecular forces that hold the atom ‘fixed’ is less, thus the melting point is lower.
Shape also matters
Given the same volume, the extent of the surface area depends on the shape of the material. A simple example is a sphere and a cube having the same volume. The cube has a larger surface area than the sphere. For this reason, in nanoscience, not only the size of a nanomaterial is important, but also its shape. Figure 4 illustrates this concept. In the section on catalysis, an example is given of a nanomaterial whose properties are determined not only by size but also by shape.
Atoms and molecules that exist at the surface or at an interface are different from the same atoms or molecules that exist in the interior of a material. This is true for any material. Atoms and molecules at the interface have enhanced reactivity and a greater tendency to agglomerate: surface atoms and molecules are unstable, they have high surface energy.
TIp FOR TEACHERS: A simple example to use in class to demonstrate the concept of ‘high surface energy’ is to coat some ping pong balls with Velcro (putting the two sides on different balls). If you place the balls in a plastic bag and shake, the balls will stick together. If normal balls are used, no attachment occurs.
As mentioned in the previous section, nanomaterials have a very large fraction of their atoms and molecules on their surface. On the other hand, a fundamental chemical principle is that ‘systems of high energy will strive to attain a state of lower energy, by whatever means possible’. So how is it possible to have nanomaterials? Nanomaterials are abundant in nature (proteins, DNA, etc.). Nanomaterials are inherently unstable, therefore there are various methods that nanomaterials adopt to minimize their inherent high surface energy.
One of the ways of reducing the surface energy in nanoparticles is agglomeration. Surface energy is an additive quantity. The surface of 10 identical nanoparticles is equal to the sum of the surface energy of each individual nanoparticle. If these were to agglomerate, and become one large particle, the overall surface energy would be reduced. The concept is illustrated in Figure 5. If a generic surface energy value γ is associated with each lateral surface of cube A, then its total surface energy is 6γ. The same applies to cube B. Therefore, the total surface energy of both cubes, A and B, separated, is 2 x 6γ = 12γ. The total surface energy of the parallelepiped C, on the other hand, is 10γ.
Nanoparticles have a strong intrinsic tendency to agglomerate. To avoid this, surfactants can be used. This also explains why when nanoparticles are used in research and industry they are often immobilized on a solid support or mixed within a matrix. Even in commercial products that claim to contain nanoparticles (such as sunscreens) microscope images show that they are actually present in the form of agglomerates of > 100 nm dimensions.
Reactions where surface properties are very important
In this last section, two reactions are briefly reviewed: catalysis and detection, where the surface properties of the material are particularly important, and what nanoscience can do to improve their outcome is highlighted.
A catalyst is a substance that increases a chemical reaction rate without being consumed or chemically altered. Nature’s catalysts are called enzymes and are able to assemble specific endproducts, always finding pathways by which reactions take place with minimum energy consumption. Man-made catalysts are not so energy efficient: they are often made of metal particles fixed on an oxide surface, working on a hot reactant stream (to reduce the phenomenon, ‘catalyst poisoning’, which occurs when species dispersed in the atmosphere, such as CO, occupy the active sites of the catalysts is used). One of the most important properties of a catalyst is its active surface where the reaction takes place. The ‘active surface’ increases when the size of the catalysts is decreased: the smaller the catalyst particles, the greater the surface-to-volume ratio (Figure 5). The higher the catalysts’ active surface, the greater the surface reactivity. Research has shown that the spatial organization of the active sites in a catalyst is also important. Both properties (nanoparticle size and molecular structure/ distribution) can be controlled using nanotechnology. Hence, this technology has great potential to expand catalyst design with benefits for the chemical, petroleum, automotive, pharmaceutical and food industries. The use of nanoparticles that have catalytic properties allows a drastic reduction in the amount of material used, with resulting economic and environmental benefits.
A good example of how nanoscience can impact the development of catalytic materials is that of gold. Bulk gold is a noble metal: it is stable, non-toxic, and resistant to oxidation and chemical attack. For these reasons, it is widely used in jewelry. On the other hand, nanoscale gold particles can catalyze chemical reactions. It has been found that finely dispersed gold nanoparticles on oxide supports are catalytically very active. In many cases, the catalytic activity and selectivity of dispersed gold nanoparticles exceed those of the commonly used transition metal catalysts such as platinum, rhodium and palladium. This is an exciting result because metals like platinum and palladium (commonly used in catalysis such as car catalytic converters) are toxic and are also very rare metals, hence very expensive.
The detection of a specific chemical or biological compound within a mixture represents the basis for the operation of numerous devices, such as chemical sensors, biosensors and microarrays. These devices will be described in more detail in Module 2, Chapter 1: Medicine and healthcare. As with catalysis, a detection reaction occurs at the material interface. The rate, specificity and accuracy of this reaction can be improved using nanomaterials rather than bulk materials in the detection area. The higher surface-to-volume ratio of nanomaterials increases the surface area available for detection with a positive effect on the rate and on the limit of detection of the reaction. In addition, nanomaterials can be designed to have specific surface properties (chemical or biochemical), tailored at a molecular level. This way, the active sites on the material surface can act as ‘locks’ to detect specific molecules (the ‘keys’). Figure 7 illustrates this concept. Scaling down using nanomaterials allows more detection sites to be packed into the same device, thus allowing the detection of multiple analytes.
This scaling-down capability, together with the high specificity of the detection sites obtainable using nanomaterials, will allow the fabrication of super-small ‘multiplex detection devices’ (i.e. devices that can test and detect more than one analyte at the time).
There are three categories of materials based on their electrical properties: (a) conductors; semiconductors; and (c) The energy separation between the valence band and the conduction band is called Eg (bandgap). The ability to fill the conduction band with electrons and the energy of the bandgap determine whether a material is a conductor, a semiconductor or an insulator. Inconducting materials like metals, the valence band and the conducting band overlap, so the value of Eg is small: thermal energy is enough to stimulate electrons to move to the conduction band. In semiconductors, the bandgap is a few electron volts. If an applied voltage exceeds the bandgap energy, electrons jump from the valence band to the conduction band, thereby forming electron-hole pairs called excitons. Insulators have large bandgaps that require an enormous amount of voltage to overcome the threshold. This is why these materials do not conduct electricity (Figure 8).
Quantum confinement and its effect on material electrical properties
Quantum confinement causes the energy of the bandgap to increase as illustrated in Figure 9. Furthermore, at very small dimensions when the energy levels are quantified, the band overlap present in metals disappears and is actually transformed into a bandgap. This explains why some metals become semiconductors as their size is decreased.
The increase in bandgap energy due to quantum confinement means that more energy will be needed in order to be absorbed by the bandgap of the material. Higher energy means shorter wavelengths (blue shift). The same applies to the wavelength of the fluorescent light emitted from the nano-sized material, which will be higher, so the same blue shift will occur. Thus, a method of tuning the optical absorption and emission properties of a nano-sized semiconductor over a range of wavelengths by controlling its crystallite size is provided. The optical properties of nano-sized metals and semiconductors (quantum dots) are described in the section of this chapter on optical properties.
Nanomaterials with exceptional electrical properties
Some nanomaterials exhibit electrical properties that are absolutely exceptional. Their electrical properties are related to their unique structure. Two of these are fullerenes and carbon nanotubes. For instance, carbon nanotubes can be conductors or semiconductors depending on their nanostructure. These materials are discussed in Module 1, Chapter 5: Overview of nanomaterials. Another example is that of supercapacitors materials in which there is effectively no resistance and which do not obey Ohm’s law.
Some nanomaterials display very different optical properties, such as color and transparency, compared to bulk materials. In this section, the reason for this behavior is discussed and some examples provided. Before going into detail, some fundamentals are reviewed.
Interaction of light with matter
The ‘colour’ of a material is a function of the interaction between the light and the object. If a material absorbs light of certain wavelengths, an observer will not see these colours in the reflected light. Only reflected wavelengths reach our eyes and this makes an object appear a certain colour. For example, leaves appear green because chlorophyll, which is a pigment, absorbs the blue and red colours of the spectrum and reflects the green.
In general light (I) incident on a material can be transmitted (T), absorbed (A) or reflected (R):
I = T+A+R
As the size of the materials is reduced, scattering (S) of light can also contribute to its colour (or transparency). A short summary of each process follows.
- Reflection (R) occurs when lightstrikes a smooth surface and the incident wave is directed back into the original medium. The reflected wave has same geometrical structure as the incident
- Absorption (A) is a process that involves energy The energy levels of a substance determines the wavelengths of light that can be absorbed. It is a molecular phenomenon, dependent on the chemical identity and structure of the substance (not on the size of the molecules or clusters), and involves electronic transitions, vibrations and rotations. Chromophores and fluorophores are examples of organic materials that have specific electronic transitions.
- Transmission (T) is the ability of light to pass through a material: it is complementary to absorption. Transmission of light is what is left after reflection, scattering and absorption have
Scattering (S) is the phenomenon that occurs when radiation hits a structure with dimensions comparable to the incident wavelength. Therefore, it is a physical process that depends on cluster size, the refractive index of the cluster and the refractive index of the suspension medium. It is a physical interaction only no energy transformations occur during scattering (as opposed to absorption), energy is simply redirected in many directions. The wavelength of the incoming light and that of the outgoing light are the same. After the light hits the clusters in the colloid and is redirected once, it can encounter another cluster and be redirected again. This phenomenon is called multiple scattering. At the ‘macro’ level, the overall effect can be that light is sent back the way it came (backscattering) or moves forward in the same direction it was moving initially (front scattering). Maximum scattering occurs for wavelengths twice as large as the cluster size. Therefore, if the cluster is about 200 nm, the maximum scattering is observed at 400 nm, which lies within the range of the visible spectrum.
The formula indicated above still holds if scattering occurs. Scattering simply contributes to the ‘reflection’ (backscattering) and ‘transmission’ (front scattering) parts of the equation. The light that has been absorbed cannot be scattered.
Colour generation from nanoparticles and nanostructures
Nanomaterials, in general, can have peculiar optical properties as a result of the way light interacts with their fine nanostructure. An overview follows.
Colour in metal colloids (surface plasmons)
One of the distinguishing properties of metal nanoparticles, in general, is their optical properties, which are different from those of their bulk counterpart. This is due to an effect called localized surface plasmon resonance. In simple terms, when light hits a metal surface (of any size) some of the light waves propagates along the metal surface giving rise to a surface plasmon — a group of surface conduction electrons that propagate in a direction parallel to the metal/dielectric (or metal/vacuum) interface. When a plasmon is generated in a conventional bulk metal, electrons can move freely in the material and no effect is registered. In the case of nanoparticles, the surface plasmon is localized in space, so it oscillates back and forth in a synchronized way in a small space, and the effect is called Localised Surface Plasmon Resonance (LSPR). When the frequency of this oscillation is the same as the frequency of the light that it generated it (i.e. the incident light), the plasmon is said to be in resonance with the incident light.
LSPR energy is sensitive to the dielectric function of the material and the surroundings, and to the shape and size of the nanoparticle. This means that if a ligand, such a protein, attaches to the surface of the metal nanoparticle, its LSPR energy changes. Similarly, the LSPR effect is sensitive to other variations such as the distance between the nanoparticles, which can be changed by the presence of surfactants or ions. The LSPR effect has been observed not only on metal nanoparticles but also in nanorings, voids in metal films and other nanostructures (Figure 10).
QDs are currently used as an alternative to conventional dyes in fluorescence microscopy and in other methods where dyes are used (e.g. dye-sensitized solar cells). QDs are also being studied as alternative light-emitting sources.
From white to transparent materials
The scattering of visible light is responsible for the white appearance of high-protection sunscreens. Students will be familiar with these thick, white paste-like sunscreens that are often used on children and on adults with sensitive skin. These sunscreens contain ZnO and TiO2 clusters of about 200 nm. Visible light interacts with these clusters and all of its wavelengths are scattered. The combination of the visible spectrum is white: therefore, the sunscreen appears white (as illustrated in the curve shown in Figure 12).
If the dimensions of the cluster are reduced, for instance from 200 to 100 nm, maximum scattering occurs around 200 nm and the curve is shifted towards shorter wavelengths, which are no longer in the visible spectrum: the effect is that the same material (e.g. ZnO), now in a smaller size (100 nm), no longer appears white but transparent (Figure 12).
The magnetic properties of a magnet are described by its magnetization curve. In general terms, the magnetization curve of a ferromagnetic material is a plot of the total magnetization of the sample versus the applied DC field with strength H, as illustrated in Figure 13.
Initially, as H increases, M increases until a saturation point Ms is reached. When H is decreased from the saturation point, M does not decrease to the same value it had before: rather, it is higher on the curve of the decreasing field. This is called hysteresis. When the applied field H is returned to zero, the magnet still has a magnetization, referred to as remnant magnetization Mr. In order to remove the remnant magnetization, a field Hc has to be applied in the direction opposite to the field applied the first time. This field is called the coercive field.
Nanostructuring of bulk magnetic materials can be used to design the magnetization curve of the material, leading to soft or hard magnets with improved properties.
In general, the magnetic behavior of a material depends on the structure of the material and on its temperature. In order to ‘feel’ a magnetic field, a material must have a non-zero net spin (transition metals). The typical size of expected magnetic domains is around 1 µm. When the size of a magnet is reduced, the number of surface atoms becomes an important fraction of the total number of atoms, surface effects become important, and quantum effects start to prevail. When the size of these domains reach the nanoscale, these materials show new properties due to quantum confinement, for example the giant magnetoresistance effect (GMR). This is a fundamental nano-effect which is now being used in modern data storage devices. The effect of nanostructuring on magnetic materials in discussed in detail in this module, Chapter 5: Overview of nanomaterials.
Some nanomaterials have inherent exceptional mechanical properties that are connected to their structure. One such material is carbon nanotubes: these are extremely small tubes having the same honeycomb structure of graphite, but with different properties compared to graphite. They can be single-walled or multi-walled, as illustrated in Figure 14. Carbon nanotubes are 100 times stronger than steel but six times lighter!! The different structures, properties and potential applications of carbon nanotubes are reviewed in the next chapter of this module, Chapter 5: Overview of nanomaterials.
Improving existing materials
Nanomaterials can also be used to improve the mechanical properties of existing materials. In this case, nanocomposites are formed.
One example is nanocrystalline materials, which are polycrystalline (i.e. made of many crystals which are identical but connected without orientation) and defined as materials with grain sizes from a few nanometres up to 100 nm. In contrast, the grain size in industrial metallic materials is about 10 000 nm or greater. These materials generally show improved mechanical properties (toughness, hardness, etc.).
Why is this so? A polycrystalline material (illustrated in Figure 15) has large pockets of regularity (crystals) in a ‘sea’ of atoms that are not ordered (amorphous region).
Within the crystalline structures, there can be defects (cracks or dislocations). If subjected to mechanical stress, the polycrystalline materials can fracture because these defects allow the crack to propagate. To impede the movement of cracks and dislocations tiny particles (nanoparticles) of another material can be added in the lattice. Nanocrystalline materials can have vastly improved mechanical, magnetic, electrical, and catalytic properties and greater corrosion resistance compared to conventional materials with large grains.
‘Seeing is believing’ … therefore, imaging of nanomaterials is an essential part of nanoscience and nanotechnologies. Imaging in nanoscience does not just mean ‘to create an image’, but to understand its meaning. Scientists nowadays have access to a variety of truly amazing instruments that allow them to see objects at the nanoscale. This was a dream for scientists until just a couple of decades ago, a dream that came true in the mid 1980s when a revolutionary instrument was invented, the scanning tunnelling microscope, and shortly after, the atomic force microscope. As a matter of fact, it was the invention of these instruments that truly opened the doors to the nano-world. Once scientists were able to see nanoscale objects, they started to be able to analyse them, understand their behaviour, and imagine ways of manipulating them.
This chapter summarises some of the methods used for imaging and characterisation of nanomater- ials, meaning materials with at least one dimension atthe nanoscale level (1–100 nm). These include nanostructured surfaces, nanoparticles, nanoporous materials, etc. The aim of this chapter is to answer the question: How are nanomaterials imaged and characterised?
There are many methods available to image nanostructured materials (e.g. a nanostructured surface) and to characterise their physical and chemical properties. Here, only a short review and description of these methods is provided, but the interested teacher can find more resources at the end of the chapter.
In general, two fundamental types of characterisation methods exist: imaging by microscopy and analysis by spectroscopy. The methods employed have been developed specifically to meet the characterisation needs of nanomaterials.
An optical microscope uses visible light (i.e. electromagnetic radiation) and a system of lenses to mag- nify images of small samples. For this reason, it is also called a light microscope. Optical microscopes are the oldest and simplest of the microscopes. The resolution limit of an optical microscope is gov- erned by the wavelength of visible light (5). Visible light is the part of the electromagnetic spectrum with wavelengths between 400 and 700 nm and the resolving power of an optical microscope is around
0.2 µm or 200 nm: thus, for two objects to be distinguishable, they need to be separated by at least 200 nm. Single objects smaller than this limit are not distinguishable: they are seen as ‘fuzzy objects’. This is known as the ‘diffraction limit’ of visible light.
In order to overcome the limitations set by the diffraction limit of visible light, other microscopes have been designed which use other beams: rather than light, they use electron beams to illuminatethe sample. Electron microscopes have much greater resolving power than light microscopes that use electromagnetic radiation and can obtain much higher magnifications of up to two million times, while the best light microscopes are limited to magnifications of 2 000 times. Both electron and light microscopes have resolution limitations, imposed by the wavelength of the radiation used. The greater resolution and magnification of the electron microscope is because the wavelength of an electron (its de Broglie wavelength) is much smaller than that of a photon of visible light.
There are various types of electron microscopes, such as the scanning electron microscope (SEM) and or the transmission electron microscope (TEM). Conceptually, these microscopes are similar to an optical microscope in the sense that they use radiation to visualize a sample: photons in the case of an optical microscope, and electrons (i.e. particles) in the case of electron microscopes.
In 1981, a totally new concept of imaging was introduced by Binning and his co-workers from IBM. They used a small metal tip placed at a minute distance from a conducting surface: when the two are placed very close together, but not actually touching, a bias between the two can allow electrons to tunnel through the vacuum between them. This creates a tunneling current (6), which can be measured and which is a function of the electron density on the surface. Electron density is the probability of finding an electron in a particular place: there is high electron density around the atoms and bonds in molecules.
This type of microscope is called the Scanning Tunnelling Microscope (STM). Variations in current as the probe passes over the surface are translated into an image. The STM can create detailed 3D images of a sample with atomic resolution. This means that the resolution is actually so high that it is possible to see and distinguish the individual atoms (0.2 nm = 2 * 10-10 m) on the surface. The invention of the STM earned Binning and his co-worker Heinrich Rohrer (at IBM Zürich) the Nobel Prize in Physics in 1986.
Scanning tunneling microscope
The STM is a fundamental tool in nanoscience and nanotechnologies. It is used in both industrial and fundamental research to obtain atomic-scale images of metal and semiconducting surfaces (Figure 1). It provides a three-dimensional profile of the surface roughness, allowing the observation of surface defects and the determination of the size and conformation of molecules and aggregates.
Another astonishing property of the STM is that it can be used to manipulate (move!) individual atoms, trigger chemical reactions, as well as performing electronic spectroscopy.
The operational principle of the STM
STM is a Scanning Probe Microscopy (SPM) technique. SPM provides images of surfaces by scanning the surface line by line with a probe. Scanning works very similarly to the way the blind read Braille, line by line, by moving a finger over buds on the paper. In an STM, the probe is a very thin needle called the ‘tip’ that is so small that its point is just a few atoms across. The tip is made of conducting material (e.g. metal, typically tungsten). The precise movement of the tip is controlled by a piezo motor.
The tip of an STM is about 3 mm (3 * 10-3 m) long and should be located very close to the surface to be scanned. In practice, the distance between the end of the tip and the surface must be less than 0.1 nm (10-10 m), without the tip actually hitting the surface. To visualize how small and precise this actually is, it corresponds to placing the 300 m tall Eiffel Tower (3 * 102 m) top-down with a distance of 0.01 mm (1 * 10-5 m) over a neighborhood and scanning across it without actually touching it! (Figure 2). One of the fundamental elements of the STM is the tip of the probe that scans the surface, which must be sharpened to a very fine tip (Figure 3). The fabrication of sharper probes allows for better resolution of surface features. Ultimately, a probe tip sharpened to one atom would provide the best resolution.
When a conducting tip is brought very near to a metallic or semiconducting surface, at a distance of about 0.1 nm, it can induce the formation of a tunnel current between the tip and the surface: a bias between the two atoms (tip and surface) can allow electrons to tunnel through the vacuum between them and induce the formation of a current. Variations in current as the probe passes over the surface are translated into an image (Figure 4).
As the conducting tip of an STM scans over a conducting or semiconducting surface, a ‘tunnel current’ is formed, which arises from electrons jumping from the surface to the tip of the STM probe. The probability of this happening depends largely on the distance d between the surface and tip, thus the size of the current depends on this distance. Small changes in the distance between the probe tip and the substrate surface translate into large changes in tunnel current: atomic-scale resolution by STM is possible in the x, y, and z directions due to this phenomenon.
How are images created?
One way of using the STM to image the surface of the substrate is to keep the tunnel current constant, typically nanoamps (nA) (10-9 A) — by applying a constant tunnel current, the tip of the probe is kept at a specific distance above the surface. When the tip scans a surface, it will rise when it scans over an atom and drop when scanning between two atoms in the surface, as shown in Figure 5, where the STM tip moves from left to right.
The movement of the tip can be transformed into a colored height map of the surface. This map corresponds to an atlas map, where each color indicates a specific height, as in Figure 6.
Using the STM, surfaces can be scanned by moving the tip in steps of 0.1 nm (10-10 m), thus providing a very accurate representation of the surface. For the technique to work, it is necessary that, as the tip scans the surface, a tunnel flow is induced. Thus, the surface must be conductive to some extent (the substrate must be a conductor or semiconductor).
If it is necessary to scan a surface which in itself is not electrically conductive, it can be coated with a very thin layer of a conductive material such as gold. This does, however, imply that the STM is less suitable for some studies (e.g. to study biological molecules such as DNA (which is not conductive)). For these types of samples, other SPM techniques are more suitable, such as the Atomic Force Microscope (AFM). An AFM does not measure the tunnel current, but the forces between the tip and
the surface and, therefore, does not require the surface to be conductive. The AFM was developed in 1985, also by Binning and co-workers at IBM Zürich. It was developed specifically to image materials that are insulating.
Atomic force microscope
The Atomic Force Microscope (AFM) was developed specifically to overcome the intrinsic limitations of the STM, which is not suitable for imaging surfaces coated with biological entities such as DNA or proteins. The AFM operates in air and not under a vacuum. Some versions of the instrument also allow operation in liquid, which is very advantageous when imaging biological samples that often need buffers to remain biologically active.
The AFM measures the interaction force (attractive or repulsive) between the probe and the surface. The solid probe is located at the end of a very flexible cantilever; an optical system detects the deflection of a laser beam that bounces off the reflective back of the cantilever, thus reporting cantilever fluctuations, which are proportional to the applied force. The probe is continuously moved along the surface and the cantilever deflection is constantly monitored. A feedback loop continuously changes the height of the probe on the surface in order to keep the applied force constant. The vertical movement of the probe is recorded to create a topographic map of the surface under study.
The AFM probe tip is very sharp, with a radius of curvature in the range of tens of nanometres. If the surface under analysis is soft, the probe can penetrate it, with the risk of damaging it and degrading the spatial resolution of the resulting micrograph. To overcome this limitation, instruments working in dynamic modes have been developed. In these systems, the probe is not simply dragged on the surface but oscillated vertically with respect to the surface while it is scanned. These techniques (tapping mode and non-contact mode) significantly reduce the damage that can be caused by the probe and allow the imaging of soft, compressible samples, such as biomolecules and cells.
On the other hand, the tip of an AFM can be used to deliberately ‘scratch’ and remove some molecules from a surface or to write with an ‘ink’. Both are ‘writing’ methods in the sense that they allow the creation of nanostructures on a surface with any geometry. This technique is called Dip Pen Nanolithography® (DPN®) and is discussed in Chapter 7: Fabrication methods since it is a fabrication method.
Spectroscopy is defined as the branch of science that is concerned with the investigation and measurement of spectra produced when matter interacts with or emits electromagnetic (EM) radiation. Depending on the wavelength of the electromagnetic used and the type of interaction with matter that occurs (absorption, scattering, etc.), different spectra are measured from which much information can be inferred.
Next, the spectroscopy methods that are most relevant in the characterization of nanomaterials (particles and surfaces) are briefly reviewed.
X-ray methods involve exciting a sample either with X-rays (creating more X-rays) or with electrons (creating X-rays). X-rays can be also generated by bombarding a sample with alpha particles. The energy of emitted X-rays is equal to the difference between the binding energies of the electrons involved in the transition. There are various methods that use X-rays: X-ray fluorescence (XRF), X-ray diffraction (XRD), etc. In the context of nanomaterials, the most important method is small-angle X-ray scatter- ing (SAXS) analysis. Like XRD, this method is based on the principle of scattering of X-rays. The diffraction of X-rays is a result of scattering from atoms configured in regular arrays. In conventional XRD, only crystalline materials can be visualized, as it is necessary to have a periodicity in the structure in the long-range, which nanomaterials lack (owing to their size). XRD is used for bulk crystals. With SAXS, particle sizes of the order of 1–100 nm can be analyzed. The method can be used to image powders in the dry state or suspended in a medium. The method can also be used to measure nanoparticle size.
UV-visible plasmon absorption and emission
Metal nanoparticles, in particular gold and silver, are characterized by a plasmon resonance absorption that gives rise to intensely colored solutions.
The absorption band is due to electrons confined at the particle surface that collectively oscillate at a specific frequency, commonly referred to as the surface plasmon resonance frequency. As examples, the plasmon band of a 20 nm silver (Ag) particle is centered at 395 nm, resulting in a yellow solution, while a 20 nm gold (Au) particle absorbs at 520 nm resulting in a red solution. The plasmon absorption effect occurs for particles up to approximately 50 nm in diameter and scales with particle volume. Absorption can in be in the visible and UV area of the spectrum. Particles can be visualized by absorbance in solution at nanomolar and picomolar concentrations.
Plasmon resonance light scattering
In larger metal nanoparticles (> 30 nm) another effect, light scattering, is observed. When illuminated with white light, metal nanoparticles in the 50–120 nm diameter size range scatter light of a specific color at the surface plasmon resonance frequency. This effect is called plasmon resonance light scattering. As in the case of plasmon absorbance, light scattering scales with particle volume, but the scattered light can be detected at much lower concentrations than the absorbed light. For example, light scattered by a solution of 80 nm diameter gold particles is detectable down to 5 fM concentration (fM = 10-15 M). For this reason, metal nanoparticles are interesting materials for use in techniques that rely on labeling (such as microarray technology).
Surface-enhanced Raman scattering
Metal surfaces with nanometre-scale roughness have the property of amplifying the Raman scattering signals of absorbed molecules. In simple terms, Raman scattering is the inelastic scatter- ing of photons. Normally, when light is scattered from an atom or molecule, it has the same energy (frequency) and wavelength as the incident light (Rayleigh scattering). This is an elastic scattering. However, a small fraction of the scattered light (approximately 1 in 10 million photons) is scattered by excitation, with the scattered photons having energy (frequency) different to the frequency of the incident photons. Metal surfaces with nanoscale roughness increase the Raman scattering of molecules absorbed on them. This effect is due to chemical and electromagnetic factors, as well as increased surface area. The details of this effect will not be considered here: what is important is that the surface-enhanced Raman scattering (SERS) effect can induce a signal enhancement of up to 108 times. In one specific case, it has been possible to achieve a Raman enhancement effect of 1015 times! This means that the SERS effect makes it possible to push the detection limit of surface detection techniques. The SERS signal depends on the characteristics of the nano-substrate: the size, shape, orientation and composition of the surface nano-roughness. Advancements in SERS technology will allow detection at the attomole (10-18 mol) level and single-molecule detection.
Non-radiative and non-electron
There are numerous methods used to characterize nanomaterials that do not rely on the use of EM radiation. They include methods to determine particle size, surface area, and porosity; thermodynamic methods (such as thermogravimetric analysis, TGA) to evaluate the temperature dependence of the nanomaterial (melting, etc.); and mass spectroscopy, to determine the chemical composition of the nanomaterial. An important surface method is the quartz crystal microbalance (QCM), which can measure mass changes as small as a few nano grams per square centimeter. This is sensitive enough to detect monolayers of deposited materials. It can be used to measure the amount of metal deposited on a surface after sputtering or evaporation or to measure the amount of protein absorbed on a surface. Due to this great sensitivity, the QCM is used in the design of biosensors.