Two main factors have caused nanostructured materials to behave differently from materials in normal dimensions: surface effects and quantum effects. These two factors of chemical reactivity of materials, mechanical, optical, electrical, magnetic, etc., and generally all their physical and chemical properties are under their influence, which we will examine further.

• Surface effects

Consider Figure 1. First, the first cube is divided into 8 equal parts. If we put these 8 cubes on top of each other, the same initial cube with the same volume will be obtained, but the difference with the first case is its area. By dividing the cube into 8 parts, a series of new levels have been created that did not exist in the beginning. In the second step, each of the obtained 8 cubes is divided into 8 other parts. Again, if we put the obtained 64 cubes on top of each other, the same volume as the initial cube is obtained, but the area has increased a lot.

Figure 1- The area increases as it shrinks.

In physics and chemistry, there is a difference between atoms that are on the surface of an object and atoms that are inside it. The atoms that are inside the substance have a full capacity and do not want to react due to the higher number of neighbors (the number of atoms around them is more). But the atoms on the surface may have some incomplete or incomplete bonds because they are connected with fewer atoms, so their reactivity is higher than the atoms inside the material, as shown in Figure 2.

Figure 2- The difference between atoms on the surface and inside the material

As the dimensions of the material become smaller and reach the nano dimensions, the surface of the material and consequently the atoms on the surface of the material increases a lot and as a result the material becomes extremely unstable. As you know, in nature, all organisms tend to be stable and have a lower energy level. A substance that has reached nano dimensions, due to its high instability, tends to go towards stability with different methods, which leads to a change in properties. One of these methods is changing the arrangement of atoms. As explained earlier, with a slight change in the arrangement of atoms (change in bond length or bond angle), the properties of the material also change. In the following, for better understanding, examples are given that can be used to calculate the increase in the number of atoms on the surface with the help of relations.

The reason for some changes in nanoscale properties can be explained by increasing the surface area relative to the volume. One of these phenomena is the reduction of the melting temperature with the reduction of dimensions. As shown in Figure 3, the melting point temperature of 3 nm gold nanoparticles is more than 300 degrees lower than the melting point temperature of gold in normal dimensions. As you know, at the melting temperature, the amount of heat energy necessary for the substance is provided to break the entire bond between the atoms in the solid state and the substance turns into a liquid. When the dimensions of the material become small and reach nano dimensions, the number of broken bonds increases due to the increase in surface area and atoms on the surface. Therefore, less energy is needed to break all the bonds and change the substance from solid to liquid, which leads to a decrease in the melting temperature.

Figure 3- Diagram of dependence of surface area and melting point on particle diameter in gold molecule

• Quantum effects

Quantum literally means discrete. In physics, quantities are divided into continuous and discrete (quantum). Continuous quantities can have any numerical value, such as the height and weight of people, but discrete quantities can only have certain values, such as the number of people in a class. From continuous physical quantities, we can refer to speed, kinetic energy, force, friction, etc., and from discrete physical quantities, we can refer to electric charge, which is an integer multiple of the electric charge of an electron (q=±ne).

Every material around us has a unique energy structure and the energy structure of different materials is different from each other. The energy structure of atoms consists of energy levels, but the energy structure of macroscopic and ordinary materials is in the form of energy bands, as shown in Figure 4. In different atoms, the distance between the levels is different from each other, and in ordinary materials, the width of the energy bands and the width of the forbidden region (energy gap) are different from each other.

Figure 4- The energy structure of atoms and ordinary substances

Many properties of materials depend on its energy structure, and with the change of energy structure, the properties also change. For example, to make diodes, impurity atoms are usually introduced in ordinary semiconductor materials. The entry of impurity atoms into the structure changes the energy structure and decreases the energy gap, which brings changes in the electrical properties.

In physics that exists in ordinary dimensions and is known as classical physics (the same physics we study in high school), energy and most quantities have continuous values ​​and can have any value, for example, the kinetic energy of a moving human can be 1 , 1/5, 2/7 or any other amount of joules. Now suppose, we want to cut a normal material with certain dimensions and bring it to nano dimensions. When a substance is reduced, its atoms are actually reduced. When the atom separates from the substance, the corresponding energy level also separates from the band structure. Below a certain dimension (usually below 100 nanometers), the number of atoms and energy levels decreases so much that energy bands become energy levels again. So, by shrinking and reaching nano dimensions, in addition to the huge increase in surface area compared to volume, the second thing that happens is the separation of energy bands and becoming an energy balance. Now, a quantity such as the energy of an electron cannot have any value and its energy must be equal to the energy levels. Therefore, the physics that is true in these dimensions (nano dimensions) and the dimensions below it, i.e. molecular and atomic dimensions, is called quantum physics or discrete physics. Figure 5 shows how to convert the bar into alignment.

Figure 5- A- The energy structure of a normal substance in the form of an energy band, B- The energy structure of large nanoparticles (between 80 and 100 nm) and C- The energy structure of large nanoparticles (between 80 and 100 nm)

Some changes in properties in nano dimensions, such as an increase in the absorption of electromagnetic waves or a change in color, are justified by the separation of energy levels.

Figure 6-a) TEM image of cadmiselenide nanoparticles and b) cadmiselenide nanoparticles in solution under ultraviolet light

In some models, due to the discrete energy levels of nanoparticles like atoms, nanoparticles are also called artificial atoms or superatoms. According to these models and contrary to what it seems, the reactivity of nanoparticles is not related to their size. Because as the size of materials decreases, their surface area increases and as the surface area increases, the broken bonds increase, so it seems that the reactivity is proportional to the size. While reactivity depends on the number of electrons. Nanoparticles also behave like atoms, that is, if their final energy balance is full, the nanoparticle has little reactivity, and if the final energy balance is empty, reactivity increases.