What is the reason for changing properties and characteristics in nano dimensions?

Two main factors have caused nanomaterials to behave differently from macromaterials and micromaterials: surface effects and quantum effects (discontinuity behavior due to quantum confinement effects in materials with delocalized electrons).

Surface effects

Consider a sphere. The ratio of the area to the volume of the sphere is:

A/V = 4?r ² / (4/3) ?r ³ = 3/r

As it is clear from the above relationship, as the dimensions of a sphere decrease (its radius decreases), the ratio of area to volume increases. The greater the fineness, the more this ratio increases. As the surface increases, the number of atoms placed on the surface increases. In physics and chemistry, there is a difference between atoms that are on the surface and atoms that are inside the substance. 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 that are on the surface, because they are connected with fewer atoms, may have a number of incomplete or incomplete bonds, so their reactivity is higher than the atoms inside the material, as shown in the figure below.

Therefore, as the dimensions of the material become smaller and reach the nano dimensions, the surface of the material increases a lot and the atoms on its surface also increase a lot. As a result, the material becomes extremely unstable, so it tries to go towards stability with different methods due to its high instability. One of these methods is changing the arrangement of atoms, which causes the properties of matter to change.

Quantum effects

Every material around us has an energy structure that is characteristic of that material. The energy structure of different materials is different from each other. As shown in the figure below, 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. In atoms, the distance between levels is different from each other, and in normal and large-scale materials, the width of the energy bands and the width of the forbidden region (energy gap) are different from each other.

Many properties of materials depend on its energy structure, and with the change of energy structure, the properties also change. For example, in order 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.

As you know, materials are made up of atoms, when atoms are placed next to each other to form a substance (or particle), the energy levels of different atoms are also placed next to each other. When the number of atoms increases, the number of energy levels also increases, as shown in the figure, energy levels form regions called energy bands. In fact, the energy band consists of a large number of energy levels that are continuous. There are also areas in which there is no energy balance, and they are called forbidden areas or energy gaps.

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 actually decrease. When the atom separates from the substance, the corresponding energy level also separates from the band structure. Below a certain dimension (usually 100 nm), the number of atoms and energy levels decreases so much that energy bands become energy levels again. So, by getting smaller and reaching nano dimensions, in addition to the huge increase in surface area compared to the volume, the second thing that happens is the separation of the energy bands and the transformation into an energy balance. Now, a quantity like 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. The figure below shows how to convert the bar to alignment.

Some changes in nanoscale properties are justified by the dissociation of energy levels. For example, increasing the power of absorbing electromagnetic waves or changing color are among these things.