{"id":120,"date":"2018-12-06T07:02:05","date_gmt":"2018-12-06T07:02:05","guid":{"rendered":"http:\/\/msp08.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=120"},"modified":"2018-12-06T09:57:57","modified_gmt":"2018-12-06T09:57:57","slug":"quantum-confinement-size-effects-and-properties-of-nanostructures","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp08\/chapter\/quantum-confinement-size-effects-and-properties-of-nanostructures\/","title":{"rendered":"Quantum Confinement-Size effects and properties of nanostructures"},"content":{"raw":"
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Nanoscience is generally described as \u201cthe science of small\u201d. Size dependent variations in the properties of materials are central to the progress of this discipline. After learning the important quantum confinement effects, we will discuss the most exotic properties of the nanoscaled materials in this module.<\/span><\/div>\r\n
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\r\n\r\n \r\n\r\nNanoscale<\/em><\/strong>\r\n\r\n \r\n

The macroscopic properties of any material (e.g., melting and boiling point, etc.) can be measured by examining the sample in conventionally convenient quantities under standard lab conditions. These quantities can vary for different materials as well as the purpose of investigations. One of the most often defined quantity is \u2018mole\u2019 such that one mole of any substance consists 6.022 x 1023 molecules. Thus, if any property of one mole of the substance is measure, this value usually represents the average value of 6.022 x 1023 molecules of that substance. It is often deduced that this value remains same regardless of the size of the group of molecules under investigation. However, this does not hold for all materials. That is, as the size of the material is decreased to reach nanoscale, the same material may demonstrate radically different properties. In other words, materials start demonstrating size dependent properties after reaching the nano-dimensions. This happens because matter at nanoscale does not follow Newtonian principles and quantum mechanics needs to be applied to describe the behavior of materials. This is explained for gold in Module-1 <\/strong>of this unit.<\/p>\r\n \r\n\r\nWhat kind of small is this?<\/em><\/strong>\r\n\r\n \r\n

Nanoscaled materials are considered to be consisting of clusters of atoms\/molecules, not the single atoms\/ molecules. For instance, 8 hydrogen atoms or 3.5 gold atoms placed adjacent forming a row become one nanometre long. In this regard, the size of nanomaterials is intermediate to a single atom or molecule, and their bulk forms. At these dimensions, matter exhibits novel properties. Therefore, nanomaterials offer the following opportunities:<\/p>\r\n\r\n

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  1. They can be used to improve existing materials as well as to produce new materials with exceptional properties.<\/li>\r\n \t
  2. As nanomaterials have the size resembling that of the largest molecules present in nature (such as proteins, DNA, etc.), they can be integrated into a device to interact with these molecules.<\/li>\r\n<\/ol>\r\n<\/div>\r\n
    \r\n\r\n\u00a0 \u00a0 Physics at Nanoscale<\/em><\/strong>\r\n\r\n \r\n

    Owing to their nanoscaled dimensions, nano materials are dimensionally more close to individual atoms and molecules than to the bulk materials, and therefore, their behavior is described using quantum mechanics. Quantum mechanics is the scientific model employed to describe the motion and energy of individual atoms and electrons. The significant quantum effects and the properties relevant at nanoscale are described below:<\/p>\r\n \r\n\r\n1. At nanoscale, electromagnetic forces dominate whereas the gravitational forces are negligible\r\n\r\n \r\n\r\n\"\"\r\n\r\n \r\n

    Since the mass of nanoscaled objects becomes very small, gravitational force, which scales linearly with the mass of the particles, becomes negligible.<\/p>\r\n \r\n\r\n\"\"\r\n

    Since electromagnetic force is independent of the mass of the particles, and is determined by the charge and distance of the particles, it is strong even for the nanosized particles. As an example, between two protons, electromagnetic force is 1036 times stronger than the gravitational force between them.<\/p>\r\n \r\n\r\n2.\u00a0\u00a0 Wave-particle duality\r\n\r\n \r\n

    For extremely small objects having very low mass (e.g. electron), wave nature becomes pronounced. Thus an electron exhibits wave-like properties and its position can be described by the wave probability function.<\/span><\/p>\r\n\r\n<\/div>\r\n

    \r\n\r\n \r\n\r\n3.\u00a0\u00a0 Tunneling\r\n\r\n \r\n

    Tunneling is an extremely important consequence in nanoscaled materials. As per classical physics, an object can pass a potential barrier if its energy is greater than that of the barrier. Thus, if the object has energy lower than the barrier potential, the object cannot cross the barrier. In this case, there is zero probability of locating the object on other side of the potential barrier (see Figure 1<\/strong>). As per quantum physics, the object can tunnel through the barrier. Also, the thickness of the barrier is crucial in determining the tunneling probability of the particle. That is, the thickness of the barrier should be of the order of particle\u2019s wavelength. Thus, even if the object energy is lower than the potential of the barrier, there is a finite probability of locating the particle at the other side of the barrier.<\/p>\r\n \r\n\r\n\"\"\r\n\r\n \r\n

    Figure 1 Schematic illustration of tunneling across a potential barrier.<\/strong><\/p>\r\n \r\n

    Tunneling is an important quantum effect and forms the basic principle of scanning tunneling microscope (STM) used to image the nanostructured materials.<\/p>\r\n \r\n\r\n4.\u00a0\u00a0 Quantum Confinement\r\n\r\n \r\n

    In nanostructured materials, including metals, electrons are not free to move within the material, but are confined in space.<\/p>\r\n \r\n\r\n5.\u00a0\u00a0 Quantization of energy\r\n\r\n \r\n

    The electrons in the nanomaterials do not have continuous energy bands; rather they exist only at some discrete energy states. This is effect called the quantization of energy, and is most pronounced in quantum dots.<\/p>\r\n \r\n\r\n6.\u00a0\u00a0 Random molecular motion assumes importance\r\n\r\n \r\n\r\nAbove absolute zero, the molecules move owing to their kinetic energy. This motion is described as the random molecular motion and occurs in materials at a temperature above 0K. In bulk materials, this movement can be neglected in comparison to the size of the object; therefore do not influence object\u2019s movement. However, in nanoscaled objects, these motions are comparable to the particles\u2019 dimensions and thus affect their behavior.<\/span>\r\n\r\n<\/div>\r\n

    \r\n\r\n \r\n\r\n7.\u00a0\u00a0 Larger surface area to volume ratio\r\n\r\n \r\n

    Nanomaterials possess very large surface areas. The smaller the object is, the larger the surface area to volume ratio. Large surface-to-volume ratio is an extremely important characteristic of a nanoparticle. This property is described in detail in the subsequent sections.<\/p>\r\n \r\n\r\n8.\u00a0\u00a0 Energy of confined electrons\r\n\r\n \r\n

    In nano-crystals, the energy levels of electrons do not remain continuous as is the case with bulk materials. Rather, owing to the confined electron wave functions, they become discrete resulting in finite DoS. These effects appear when the dimensions of the potential well approach de Broglie wavelength of electrons resulting in change in energy levels. The effect is described as Quantum confinement and consequently, the nano-crystals are often called quantum dots or QDs. These effects influence the electrical, optical as well as mechanical behaviour of material. Depending on the QD size, confined electrons have higher energy than the electrons in bulk materials. This shift in energy (Figure 2<\/strong>), is given by:<\/p>\r\n \r\n\r\n\"\"\r\n\r\n \r\n

    here, represents the principal quantum number; \u210e is Planck\u2019s constant; is effective mass and is the radius of the QD.<\/p>\r\n \r\n\r\n\"\"\r\n\r\n \r\n

    Figure 2 Variation of energy with size of the quantum dot (QD).<\/strong><\/p>\r\n \r\n\r\nChemistry at Nanoscale<\/em><\/strong>\r\n\r\n \r\n

    Since a nanoparticle is essentially a cluster of atoms\/molecules, all forms of bindings relevant in chemistry, are relevant at <\/span>nanoscale as well. These are described below:<\/span><\/p>\r\n\r\n<\/div>\r\n

    \r\n\r\n \r\n\r\n1.\u00a0\u00a0 Intramolecular bonding\r\n\r\n \r\n

    These are also called chemical interactions and cause a change in the chemical structure of the molecule. Examples are ionic, metallic, and covalent bonding.<\/p>\r\n \r\n\r\n2.\u00a0\u00a0 Inter-molecular bonding\r\n\r\n \r\n

    These are the physical interactions causing no change in chemical structure of molecule. Examples are ion-ion bonds, ion-dipole bonds, van der Waals bonds, hydrogen bonds, hydrophobic bonds, repulsive interactions, etc.<\/p>\r\n \r\n

    Nanomaterials are formed of a variety of molecules joined together or large molecules which take 3D structures via inter-molecular bonding (such as macromolecules). Thus nanoscience includes supramolecular chemistry as well. Additionally, intermolecular bonding plays a significant role in these macromolecules.<\/p>\r\n \r\n