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Why are nanoparticles so special?

Our CEO Aaike van Vugt talking at Singularity University about what makes nanoparticles special 

High-tech materials are rapidly gaining importance in our society. Traditionally, most functionality is derived from the properties of bulk materials. To understand and enhance these properties, it is crucial to realize that these properties are derived from atoms of the elements in the periodic table. So by changing the composition of a material, it is possible to change the properties.

Since the engineering of nanomaterials, it is clear that we now have a new way of changing the properties of high-tech materials: by changing the size of the nanoparticles. While both bulk and atomic properties are well known, the properties of nanomaterials can be distinctly different and show unexpected and promising properties. More and more we are discovering that nanosizing materials can lead to enhanced functionality


Why properties change

The figure below shows some of the fundamental changes that can occur when the size of a particle is increased from a single atom on the left, to a 'bulk' crystal of 100 nm or larger. In the middle, there is a region that is characterized by atomic clusters. These clusters do not show crystallographic features like facets and have a high fraction of atoms at the surface. The outer atoms are surrounded by fewer neighboring atoms and have electronic states not accessible in the bulk. This plays a large role in catalysis and decreases the melting temperature.

For small clusters, even the electronic states in the interior are changed, as shown on the top. Individual atoms have discrete energy levels. In the case of metallic elements, the levels will eventually cross the Fermi level, giving rise to their metallic properties like electrical. This crossover takes place for clusters of several hundred atoms and depends on temperature. At room temperature, this is translated to a critical particle size of a few nanometers.

On the bottom, the magnetic properties are also shown to change. Unpaired electrons of single atoms have a magnetic moment and give rise to a (tiny) magnetic field. For ferromagnetic materials, these magnetic moments spontaneously align themselves. Due to thermal fluctuations the direction of the net magnetic moment is not stable. Only when enough spins are present can they form a stable single magnetic domain. When the domains become larger, they eventually arrange themselves in a multi-domain configuration where the net magnetization approaches zero.

Nanoparticle applications

By analysing scientific literature, many applications are revealed. The results is a pie chart containing the most common keywords used in the scientific literature concerning nanoparticles, quantum dots, nanoclusters etc. weighed by the number of publications. Small descriptions have been added to get a quick overview of the most common applications and scientific terms.

Catalysis facilitates chemical reactions by lowering the activation energy, allowing the reaction temperature and pressure to be lowered while increasing the rate of the reaction. Catalysis has an enormous technological significance, being important in energy production, the chemical industry and environmental technologies. The long history of a continuously growing demand for chemicals and fuel brought about a parallel demand for better catalysts. For heterogeneous catalysis, this means optimizing the interaction between the solid catalyst and the liquid or gas at the surface. A maximized relative surface area can be achieved by preparing thin films, using porous materials or by the use of nanoparticles.

For nanoparticles, downsizing materials does not only affect the fraction of atoms at the surface but also influences the electronic properties. This has opened new prospects. By tuning the particle size, the shape and electronic properties can both be changed to increase the turnover frequency of the chemical reaction. In addition, the chemical composition of the catalyst can also be tuned. The influence of the composition can be explained using the Sabatier principle. This principle states that for effective catalysis to occur, the interactions between catalyst and substrate (liquid or gas) should neither be too strong nor too weak. For most reactions the noble metals show the most optimal behavior.

Luminescence is emission of light by a material not resulting from heat. A well-known case is fluorescence, where a material only emits light of a particular wavelength, corresponding to a particular color. The energy of the emitted light, and therefore the wavelength, is dependent on the energy states within the material. For a semiconductor, the energy can be tuned so that the wavelength falls in the visible spectrum by tuning the size of the material. This is because the electronic structure changes drastically when the size of the particles is in the range of a few nanometers. These nanoparticles are also called quantum dots.

These quantum dots can be used for a variety of applications. The monochromatic light that is emitted makes them interesting for use in light emitting diodes (LEDs). When the emitted light falls in the so-called biologic window, the high brightness of the quantum dots makes them interesting for cellular imaging.

When the size of the nanoparticles is in the range of tens of nanometers, there is another process that gives rise to luminescence, called surface plasmon resonance. It occurs in metal nanoparticles when the wavelength of the incoming radiation is much larger compared to the diameter of the nanoparticle. This resonance scales with the dielectric constant of the material, the shape and volume of the particle. At the resonance frequency, absorption of (visible) light occurs and the extinction of light through a sample with suspended nanoparticles shows a peak. This is due to both absorption and scattering of the light by the plasmon. Scattering is most interesting for biolabeling, particle sensing and nano-lensing, while absorption is used for heating, like used in photothermal therapy.

Mixing of elements occurs spontaneously when the total free energy of the system is lowered. Different electronic properties of elements can prevent any alloying from taking place in the bulk. The table on the right illustrates this alloying behaviour in bulk materials:

  • Red indicates the combination of materials do not mix
  • Green indicates the materials form an alloy at a particular composition
  • M indicates there is a miscibility gap: the elements form an alloy at various compositions, but not all
  • S indicates the mixed elements form a solid solution, i.e. alloying at any composition

However, at the nanoscale, bulk interactions become less important and surface effects start to dominate. This means that elements that would not alloy in the bulk, can potentially alloy in nanoparticles. Calculations have shown that the miscibility gap for Au-Pt alloys disappears for particle sizes below 5 nm. This opens up new opportunities in material research and their applications.

miscibility_table

Nanoparticles can be used for health applications, but at the same time they also pose new risks. Beneficial properties are the anti-bacterial function of nano-silver and the various applications in biological systems mentioned in the other applications. The anti-bacterial function is based on the interaction of nanoparticles with the cell membrane and the DNA. The nanoparticles attach to these parts of the cell and disturb their proper function.

There are also risks that are posed by nanoparticles because they are found in our environment. For instance, additives like titanium oxide are used in many food, personal care and other consumer products and subsequently, enter the environment with our waste streams. Airborne fine dust (aerosol) pervades in both professional and everyday life, in emissions from cars, cooking, welding, office printers and many other artificial and natural sources; in many cases these dusts also include nanoparticles. There are still many unknowns regarding the potential hazard of the smallest size fractions of airborne particles. Synthetic nanoparticles are used to study the health impact of fine dust in both in vivo and in vitro studies. Research into the effects of the nanoparticles in our environment can help to protect us in the future.

The application of a magnetic field can change the properties of magnetic materials. For para- and ferromagnetic materials this changes the magnetization (direction). For magnetic hard drives this principle has been used to store information. Nanosizing the bits that make up the information leads to an increase in the information density, but is limited by the superparamagnetic limit. The superparamagnetic behavior causes the magnetization direction to fluctuate in time. For magnetic storage this is detrimental, but for other applications this is ideal. The nanoparticles behave as paramagnets and will only show a magnetic response after applications of a magnetic field.

Superparamagnetic nanoparticles can be used for magnetic hyperthermia, where an oscillating magnetic field rotates the particles, generating heat and neutralizing any hazardous tissues, like tumors. When suspended in a liquid, the rheological properties of change under influence of a magnetic field. This has been exploited in high-performance bearings and seals.

The small magnetic fields surrounding magnetic nanoparticles can also be used for MRI contrast enhancement. The contrast is based on the relaxation of spins of the nuclei of the atoms, and is affected by the magnetic fields produced by the nanoparticles. Nanomaterials based on Fe and Gd are commonly used for this purpose.

The availability of a pure and scalable source of nanoparticles makes it possible to deposit thin films of nanoparticles at an industrial scale. The resulting thin films are characterized by a porous structure resulting in a high surface-area. This makes the thin films receptive to a multitude of external stimuli, for example, temperature, light and chemical. The sensitivity to external stimuli can be used to create novel devices or fabrication processes. A few examples are discussed below:

  • Temperature
    Following an additive technique analogues to conductive ink printing, the thin films can be locally deposited and subsequently heated to form metallic interconnects. As the size of the nanoparticles is of the order of 10 nm, low temperatures are sufficient to form metallic interconnects. The use of pure nanoparticles as compared to conductive inks, result in enhanced material properties.
  • Light
    Subtractive fabrication methods are also possible. Here the photonic sensitivity of the nanoparticle film is used, eliminating the need of a resist process step. With the use of for example a laser, a pattern can be written in the thin film. The unpatterned nanoparticles can be subsequently removed, resulting in a patterned nanoparticle thin film which can be directly used as interconnect layer or as an metallic mask for later process steps.
  • Chemical
    The chemical sensitivity of the porous nanoparticle thin-films makes it possible to let them function as chemical sensors. The relatively simple method of creating the nanoparticles and deposition of the thin-films, can result in a CMOS compatible method for creating a wide range of chemical sensors.

There are many other applications of nanoparticles in various fields of research. In the overview given here and the data presented for each of the materials usable for spark ablation, we cannot discuss all applications exhaustively and focused on major application fields instead. If you feel a specific application is missing, please contact us so that we can include it as well.