Tuesday, October 17, 2017 - PowerPoint Presentation

1. Tuesday, October 17, 2017Nanotechnology1

2. Nanoscince and NanotechnologyThe word ‘nano’ is a Greek prefix meaning dwarf or something very small and depicts one billionth (10-9) of a unit. Nanomaterials, therefore, refer to the class of materials with at least one of the dimensions in the nanometric range.Tuesday, October 17, 20172

3. Tuesday, October 17, 2017In the case of polycrystalline materials, the grain size is typically of the order of 1–100 microns (1 micron = 10-6 m). Nanocrystalline materials have a grain size of the order of 1−100 nm, and are therefore 100–1000 times smaller than conventional grain dimensions, so, they can no longer be treated as infinite systems and the resultant boundary effects lead to fascinating and useful properties, which can be explored and tailored for a variety of structural and functional applications.3

4. nanomaterials may be classified as those materials which have at least one of their dimensions in the nanometric range, below which there is significant variation in the property of interest compared to microcrystalline materials.Nanomaterials can be metals, ceramics, polymers or composites. Nanotechnology is an umbrella term for many areas of research dealing with objects that have one of their dimensions in the realm of a few hundreds of nanometres. The term ‘nanotechnology’ was first coined by Norio Taniguchi in 1974.Table (1) refer to the small scales.Tuesday, October 17, 20174

5. Table (1):- The world of small dimensions  Number Name Symbol0.1 deci d0.01 centi c0.001 milli m0.000 001 micro μ0.000 000 001 nano n0.000 000 000 001 pico p0.000 000 000 000 001 femto f0.000 000 000 000 000 001 atto a0.000 000 000 000 000 000 001 zepto z0.000 000 000 000 000 000 000 001 yocto y Tuesday, October 17, 20175

6. Important terms:- Nanomaterial: class of materials in which at least one of the dimensions is on the nanoscale (<100 nm) Nanotechnology: study of manipulating matter on an atomic and molecular scale; generally deals with structures sized between 1 and 100 nanometres in at least one dimension, and involves developingmaterials or devices possessing at least one dimension within that size.Tuesday, October 17, 20176

7. Nanosize and PropertiesWhen particle sizes of solid matter in the visible scale are compared to what can be seen in a regular optical microscope, there is little difference in the properties of the particles. But when particles are created with dimensions of about 1–100 nanometers (where the particles can be “seen” only with powerful specialized microscopes), the materials’ properties change significantly from those at larger scales. This is the size scale where so-called quantum effects rule the behavior and properties of particles. Properties of materials are size-dependent in this scale range. Thus, when particle size is made to be nanoscale, properties such as melting point, fluorescence, electrical conductivity, magnetic permeability, and chemical reactivity change as a function of the size of the particle.Tuesday, October 17, 20177

8. Nanoscale gold illustrates the unique properties that occur at the nanoscale. Nanoscale gold particles are not the yellow colour with which we are familiar; nanoscale gold can appear red or purple. At the nanoscale, the motion of the gold’s electrons is confined. Because this movement is restricted, gold nanoparticles react differently with light compared to larger-scale gold particles. In fact, gold can be used as a prime example: a colloid of gold nanoparticles is no longer ‘golden’ but ruby-red in colour as in figure (1).Tuesday, October 17, 20178

9. Figure (1):- The effect of nanosized particles on the colour of gold.Tuesday, October 17, 20179

10. Nanoscale materials have far larger surface areas than similar masses of larger-scale materials. As surface area per mass of a material increases, a greater amount of the material can come into contact with surrounding materials, thus affecting reactivity.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 (2).Tuesday, October 17, 201710

11. Figure 2: Schematic drawing showing how surface to volume increases as size is decreased.Tuesday, October 17, 201711

12. The consequence is that the surface-to-volume ratio of the material — compared to that of the parent bulk material — is increased.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 cubes? Table (2) summarizes the results. Table (2).Tuesday, October 17, 201712

13. Table (2).Tuesday, October 17, 201713

14. Scale at which Surfaces and Interfaces Play a Large Role in Materials Properties and Interactions Why should the material behaviour vary so significantly by a mere reduction in grain size? Nanostructured materials are composed of grains and grain boundaries. Nanometre-sized grains contain only a few thousands of atoms within each grain. A large number of atoms reside at the grain boundaries, as shown in Figure (3). As the grain size decreases, there is a significant increase in the volume fraction of grain boundaries or interfaces and triple junctions, as shown in Figure (4). With increase in defect density, or in other words, when the fraction of atoms residing at defect cores like dislocations, grain boundaries and triple junctions becomes comparable with that residing in the core, the properties of the material are bound to be governed to a large extent by defect configurations, dynamics and interactions. Hence the mechanical and chemical properties of nanomaterials are significantly altered due to defect dynamics.Tuesday, October 17, 201714

15. The elastic modulus of nanomaterials can be significantly different from that of bulk alloys, due to the presence of increased fraction of defects. Nanocrystalline ceramics are tougher and stronger than those with coarse grains. Nano-sized metals exhibit significant increase in yield strength and the toughness decreases. It has also been shown that electrical, optical and magnetic properties are influenced by the fine-grained structure of these materials. As the technical capability to tailor and modulate dimensions at the nanoscale has improved greatly it has become possible to realise the fascinating properties of nanostructuresTuesday, October 17, 201715

16.   Figure (3):- The hypothetical structure of a nanomaterial. The black circles indicate atoms in the grain, while the white circles indicate atoms at the grain boundaries.Tuesday, October 17, 201716

17.    Figure (4):- Increase in the intercrystalline region (grain boundaries) and triple junctions with decrease in grain size of nanomaterials.Tuesday, October 17, 201717

18. So, the changing in the properties of the materials when it become in nanoscale can be summarised by:-  (i) Large fraction of surface atoms; (ii) High surface energy; (iii) Spatial confinement; (iv) Reduced imperfections, which do not exist in the corresponding bulk materials.Tuesday, October 17, 201718

19. Therefore ,due to their small dimensions, nanomaterials have extremely large surface area tovolume ratio, which makes a large to be the surface or interfacial atoms, resulting in more “surface” dependent material properties. Especially when the sizes of nanomaterials are comparable to length, the entire material will be affected by the surface properties of nanomaterials. This in turn may enhance or modify the properties of the bulk materials.Tuesday, October 17, 201719

20. Large surface area also makes nanostructured membranes and materials ideal candidates for water treatment and desalination .It also helps support “functionalization” of nanoscale material surfaces (adding particles for specific purposes), for applications ranging from drug delivery to clothing insulation.Reduced imperfections are also an important factor in determination of the properties of the nanomaterials. Nanosturctures and Nanomaterials favors of a selfpurification process in that the impurities and intrinsic material defects will move to near the surface upon thermal annealing. This increased materials perfection affects the properties of nanomaterials. Due to their nanometer size, nanomaterials are already known to have many novel properties. Many novel applications of the nanomaterials rose from these novel properties have also been proposed. Tuesday, October 17, 201720

21. Important terms :- Catalyst: a substance that does not chemically take part in the reaction but results in increase in rate of reaction by decreasing the activation energy Colloid: a homogenous suspension of a dispersoid in a continuous medium; it may be a solid, liquid or gasTuesday, October 17, 201721

22. Tuesday, October 17, 2017Principle of Nanotechnology Classification of NanomaterialsNanomaterials have extremely small size which having at least one dimension (100 nm or less). Nanomaterials can be nanoscale in one dimension (e.g. surface films), two dimensions (e.g. strands or fibres), or three dimensions (e. g. particles). They can exist in single, fused, aggregated or agglomerated forms with spherical, tubular, and irregular shapes. Common types of nanomaterials include nanotubes, dendrimers, quantum dots and fullerenes. Nanomaterials have applications in the field of nano technology, and displays different physical chemical characteristics from normal materials (i.e., silver nano, carbon nano tube, fullerenet, carbon nano, silica).22

23. Tuesday, October 17, 2017According to Siegel, Nanostructured materials are classified as Zero dimensional, one dimensional, two dimensional, three dimensional nanostructures.Nanomaterials are materials which are characterized by an ultra fine grain size or by a dimensionality limited to 50nm. Nanomaterials can be created with various modulation dimensionalities as defined by Richard W. Siegel , zero, one, two and three dimensions as shown in the above Figure 1.23

24. Tuesday, October 17, 2017Figure 1: Classification of Nanomaterials (a) 0D spheres and clusters (b) 1D nanofibers, wires, and rods (c) 2D films, plates, and Networks (d) 3D nanomaterials.24

25. Tuesday, October 17, 2017Considering the three dimensions, we can classify each system by the number ofdimensions that are not in the nanometric range: 0-dimensional systems: in these materials pores have all three dimensions in nanometric range and, often, have spherical shapes and are directly connected with the external ambient. They are generally used to store single molecules, stabilized inside cavities by three-dimensional interactions with the walls. An example of this type of materials are zeolites. 1-dimensional systems: these materials are formed by parallel channels of micrometric length, often organized in regular patterns (such as hexagonal). Channels could be straight or curved, isolated or communicating, but generally they retain the same pattern along all the material. Materials with nanochannels could be used as nanoreactors, as diffusion systems in purifying membranes or for gas storage. An example of this type is carbon nanotubes CNTs.25

26. Tuesday, October 17, 20172-dimensional systems: with two dimensions over the nanometric range, these materials are constituted by large lamellae of nanometric thickness, often separated by pillars to avoid collapsing of the structure. Main applications of these materials are intercalation with other materials, Typical materials belonging to this group are nanostructured clays. 3-dimensional systems: the materials of this group are constituted by complex structures of channels and/or cages interconnected to create a three-dimensional structure. They are mainly used as storage systems or as scaffolds to obtain porous materials in metal or carbon. Examples of this group are numerous three-dimensional structures in silicon or metal oxide.26

27. Tuesday, October 17, 2017Classification of nanomaterials Depending on the dimension in which the size effect on the resultant property becomes apparent, the nanomaterials can be classified as zerodimensional (quantum dots) in which the movement of electrons is confined in all three dimensions, one-dimensional (quantum wires) in which the electrons can only move freely in the X-direction, two-dimensional (thin films) in which case the free electron can move in the X-Y plane, or three dimensional (nanostructured material built of nanoparticles as building blocks) in which the free electron can move in the X, Y and Z directions.27

28. Tuesday, October 17, 2017Figure (2) Density of states for 3D, 2D, 1D, 0D .28

29. Tuesday, October 17, 2017Different Types of Nanostructures (NSs)All bulk materials can be transformed at nanoscale including polymers; metals, glass, semiconductors and ceramic etc. There are different classifications of NSs innanotechnology, NSs usually classified by their geometrical properties. NSs usually consist of nanocages, nanocrystallites, nanobelts, nanoneedles, nanocomposites, nanofabrics, nanofibers, nanoflakes, nanoflowers, nanofoams, nanomeshes, nanoparticles, nanopillars, nanorings, nanorods, nanoshells, nanopowders, nanoclusters, nanowires, nanotubes, quantum dots.29

30. Tuesday, October 17, 2017Types of Nanomaterials (NMs)For understanding, nanomaterials are designed into four types as follows:(i) Carbon based materials(ii) Metal based materials(iii) Dendrimers(iv) Composites(i) Carbon based materials: These are composed of carbon, taking the form of hollow spheres, ellipsoids or tubes. The spherical and ellipsoidal forms are referred as fullerenes, while cylindrical forms are called nanotubes.30

31. Tuesday, October 17, 2017Figure (3)Carbon nanostructures31

32. Tuesday, October 17, 2017GrapheneGraphene is a crystalline allotrope of carbon with two- dimensional, atomic scale, hexagonal pattren. It is the basic structural element of other allotropes like graphite, fullerene, nanotubes, nanocones, etc. hence called mother of all carbon nanomaterials (Figure 4). Nowdays, it is commonly used in semiconductors, batteries, electronics, composite industries, and many more.32

33. Tuesday, October 17, 2017Figure (4) Graphene and other carbon nanomaterials33

34. Tuesday, October 17, 2017(i) Metal based materials: These include quantum dots, nanogold, nanosilver and metal oxides like TiO2. A quantum dot is a closely packed semiconductor crystal comprised of hundreds or thousands of atoms, whose size is on the order of a few nanometers to a few hundred nanometers. (ii) Dendrimers: Dendrimers are repetitively branched molecules. The name comes from the Greek word ‘dendron’ (tree). These nanomaterials are nanosized polymers built from branched units. The surface of dendrimer has numerous chain ends, which can perform specific chemical functions. Dendrimers are used in molecular recognition, nanosensing, light harvesting, and opto- electrochemical devices. They may be useful for drug delivery. (iii) Composites: Composites are combination of nanoparticles with other nanoparticles or with larger., bulk- type materials. Nanoparticles like nanosized clays are added to products (auto parts), packaging materials, etc.) to enhance mechanical, thermal, and flame–retardant properties.34

35. Tuesday, October 17, 2017Figure (5) Some types of nanomaterials35

36. Tuesday, October 17, 2017Optical properties in Nanomaterials:-Nanocrystalline systems have attracted much interest due to their novel optical properties, which differ remarkably from bulk crystals. Applications based on optical properties of nanomaterials include optical detector, sensor, imaging, display, solar cell, photocatalysis, photoelectrochemistry and biomedicine.36

37. Tuesday, October 17, 2017Interaction 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).37

38. Tuesday, October 17, 2017Some nanomaterials display very different optical properties, such as colour and transparency, com­pared to bulk materials. In fact the key contributory factor include quantum confinement of electrical carriers within nanoparticles, this behaviour can be cleared by Plasmons phenomena. What is the Plasmons of the surface? When a metal particle is exposed to light, the oscillating electromagnetic field of the light induces a collective coherent oscillation of the free electrons (conduction band electrons) of the metal. This electron oscillation around the particle surface causes a charge separation with respect to the ionic lattice, forming a dipole oscillation along the direction of the electric field of the light. The amplitude of the oscillation reaches maximum at a specific frequency, called surface plasmon resonance (SPR). The SPR induces a strong absorption of the incident light and thus can be measured using a UV–Vis absorption spectrometer.38

39. Tuesday, October 17, 2017The SPR band is much stronger for plasmonc nanoparticles (noble metal, especially Au and Ag) than other metals.The SPR band intensity and wavelength depends on the factors affecting the electron charge density on the particle surface such as :-the metal type, particle size, shape, structure, composition and the dielectric constant of the surrounding medium.From this reason, it can be easily understand the different in the colour of gold metal(for example) when the gold particles become in nanoscale.39

40. Tuesday, October 17, 2017Thermal Conductivity in Nanomaterials:- In general, increasing the number of grain boundaries will enhance phonon scattering at the disordered boundaries, resulting in lower thermal conductivity. Thus, nanocrystalline materials would be expected to have lower thermal conductivity compared to conventional materials. However, as the grain sizes assume nanodimensions, their size becomes comparable to the mean free paths of phonons that transport thermal energy. Thus, nanomaterials can show widely different properties compared to coarse-grained materials, due to the photon confinement and quantization effects of photon transport.40

41. Tuesday, October 17, 2017 It has been observed that in addition to the grain size, the shape also has an influence on the thermal properties of nanomaterials. For example, one-dimensional nanowires may offer ultralow thermal conductivities. In nanowires, quantum confinement of phonons in 1D can result in additional polarization modes compared to that observed in bulk solids. The strong phonon–phonon interactions and enhanced scattering at grain boundaries result in a significant reduction in thermal conductivity of nanostructures. Silicon nanowires are known to exhibit thermal conductivity at least about two orders of magnitude smaller than that of bulk silicon.41

42. Tuesday, October 17, 2017In contrast, the tubular structures result in an extremely high thermal conductivity along the axial direction. However, high anisotropy in their heat transport property is observed, making the thermal transport direction dependent.In multilayered coatings, many collective modes of phonon transport may appear besides thephonon modes in each single layer; when the phonon coherence length becomes comparable to the thickness of each layer, the transport properties are significantly influenced. When the mean free path of phonons spans multiple interfaces, the phonon dispersion relation is modified, resulting in enhanced scattering due to decrease in phonon group velocity. 42

43. Tuesday, October 17, 2017Further, if the multilayer is designed to have a superlattice structure, and alternate films have a large mismatch in the phonon dispersion relations, it is possible that phonons in a certain frequency range may not propagate to the neighbouring layers unless there are mode conversions at the interface. Also, the presence of interface dislocations and defects can contribute to enhanced boundary scattering. All these factors can contribute to the lower thermal conductivity of multilayered nanostructured films.43

44. Tuesday, October 17, 2017The use of a nanofluid to enhance thermal transport is another promising application of the thermal properties of nanomaterials. Nanofluids represent the class of liquids that have a stable colloidal dispersion of nanoparticles distributed uniformly in the medium. It has been observed that dispersion of a wide variety of nanoparticles of oxides, nitrides, metals, metal carbides and nanofibres, such as single- and multi-walled carbon nanotubes, can significantly enhance the thermal conductivity of the fluid. 44

45. Tuesday, October 17, 2017To obtain stable colloidal suspensions, the particle size should normally be in the range of 1–100 nm and an anti-coagulant may also be added to enhance the stability of the nanofluid. The idea of enhancing the thermal conductivity of liquids using solid dispersions is not completely new. This is because we know that solids in general have much higher thermal conductivity than liquids and gases.Thus, it is obvious that thermal conductivity of a fluid can be enhanced by having particles of better heat transport properties dispersed in it. However, in the early years of development of particle dispersed fluids, microcrystalline dispersoid particles were used.45

46. Tuesday, October 17, 2017These fluids suffered from:-inferior stability of suspension, leading to their coagulation and precipitation. Erosion of the walls of the pipes by the particles was also observed to be a major problem. So with the advent of techniques to synthesize nanoparticles with controlled grain size, nanofluids with improved stability have been developed.46

47. Tuesday, October 17, 2017Melting Point Property:-Melting-point depression is the phenomenon of reduction of the melting point of a material with reduction of its size. This phenomenon is very prominent in nanoscale materials, which melt at temperatures lower than bulk materials.The melting temperature of a bulk material is not dependent on its size. However as the dimensions of a material decrease towards the atomic scale, the melting temperature scales with the material dimensions. The decrease in melting temperature can be on the order of tens to hundreds of degrees for metals with nanometer dimensions. 47

48. Tuesday, October 17, 2017Melting-point depression is most evident in nanowires, nanotubes and nanoparticles, which all melt at lower temperatures than bulk amounts of the same material. Changes in melting point occur because nanoscale materials have a much larger surface-to-volume ratio than bulk materials, drastically altering their thermodynamic and thermal properties.The melting temperature of a nanoparticle decreases sharply as the particle reaches critical diameter, usually < 50 nm for common engineering metals. Figure (1) shows the shape of a typical melting curve for a metal nanoparticle as a function of its diameter.48

49. Tuesday, October 17, 2017Figure (1). A normalized melting curve for gold as a function of nanoparticle diameter. The bulk melting temperature and melting temperature of the particle are denoted TMB and TM respectively. Experimental melting curves for near spherical metal nanoparticles exhibit a similarly shaped curve.49

50. Tuesday, October 17, 2017Melting point depression is a very important issue for applications involving nanoparticles. Nanoparticles are currently used or proposed for prominent roles in catalyst, sensor, medicinal, optical, magnetic, thermal, electronic, and alternative energy applications. Nanoparticles must be in the solid state to function at elevated temperatures in several of these applications.Two techniques allow measurement of the melting point of nanoparticle. The first one is by electron beam of the transmission electron microscope (TEM) which can be used to melt nanoparticles. The melting temperature is estimated from the beam intensity, while changes in the diffraction conditions to indicate phase transition from solid to liquid. This method allows direct viewing of nanoparticles as they melt, making it possible to test and characterize samples with a wider distribution of particle sizes. 50

51. Tuesday, October 17, 2017The second one is by developed nanocalorimeters that directly measure the enthalpy and melting temperature of nanoparticles. Nanocalorimeters provide the same data as bulk calorimeters, however additional calculations must account for the presence of the substrate supporting the particles.Nanoparticles have a much greater surface to volume ratio than bulk materials. The increased surface to volume ratio means surface atoms have a much greater effect on chemical and physical properties of a nanoparticle. Surface atoms bind in the solid phase with less cohesive energy because they have fewer neighboring atoms in close proximity compared to atoms in the bulk of the solid. 51

52. Tuesday, October 17, 2017Each chemical bond an atom shares with a neighboring atom provides cohesive energy, so atoms with fewer bonds and neighboring atoms have lower cohesive energy. The average cohesive energy per atom of a nanoparticle has been theoretically calculated as a function of particle size according to Equation (1). .......................................................................... (1) Where: D=nanoparticle sized=atomic sizeEb=cohesive energy of bulkAs Equation (1) shows, the effective cohesive energy of a nanoparticle approaches that of the bulk material as the material extends beyond atomic size range (D>>d).52

53. Tuesday, October 17, 2017Atoms located at or near the surface of the nanoparticle have reduced cohesive energy due to a reduced number of cohesive bonds.  The cohesive energy of an atom is directly related to the thermal energy required to free the atom from the solid. According to Lindemann’s criterion, the melting temperature of a material is proportional to its cohesive energy, av (TM=Cav). Since atoms near the surface have fewer bonds and reduced cohesive energy, they require less energy to free from the solid phase. Melting point depression of high surface to volume ratio materials results from this effect. For the same reason, surfaces of bulk materials can melt at lower temperatures than the bulk material. 53

54. Tuesday, October 17, 2017The theoretical size-dependent melting point of a material can be calculated through classical thermodynamic analysis. The result is the Gibbs–Thomson equation shown in Equation (2).............................................................. (2) Where: TMB=Bulk Melting temperatureσsl=solid–liquid interface energyHf=Bulk heat of fusionρs=density of solidd=particle diameter54

55. Tuesday, October 17, 2017On the other hand, there is another important factor; it is the shape of nanoparticles. Nanoparticle shape impacts the melting point of a nanoparticle. Facets, edges and deviations from a perfect sphere all change the magnitude of melting point depression. These shape changes affect the surface to volume ratio, which affects the cohesive energy and thermal properties of a nanostructure. Equation (3) gives a general shape corrected formula for the theoretical melting point of a nanoparticle based on its size and shape. . . (3)Where: c=materials constantz=shape parameter of particleThe shape parameter is (1) for sphere and (3/2) for a very long wire, indicating that melting-point depression is suppressed in nanowires compared to nanoparticles.55

56. Tuesday, October 17, 2017Electrical Properties:-The properties like conductivity or resistivity are come under category of electrical properties. These properties are observed to change at nanoscale level like optical properties. The examples of the change in electrical properties in nanomaterials are:Conductivity of a bulk or large material does not depend upon dimensions like diameter or area of cross section and twist in the conducting wire etc. However it is found that in case of carbon nanotubes conductivity changes with change in area of cross section. It is also observed that conductivity also changes when some shear force (in simple terms twist) is given to nanotube. 3) Conductivity of a multiwalled carbon nanotube is different than that of single nanotube of same dimensions. 4) The carbon nanotubes can act as conductor or semiconductor in behaviour but we all know that large carbon (graphite) is good conductor of electricity.56

57. Tuesday, October 17, 2017In electrically conducting carbon nanotubes, only one electron wave mode is observed which transport the electrical current. As the lengths and orientations of the carbon nanotubes are different, they touch the surface of the mercury at different times, which provides two sets of information:(i) The influence of carbon nanotube length on the resistance; and (ii) The resistances of the different nanotubes. As the nanotubes have different lengths, then with increasing protrusion of the fiber bundle an increasing number of carbon nanotubes will touch the surface of the mercury droplet and contribute to the electrical current transport.57

58. Tuesday, October 17, 2017Figure (2) Electrical behavior of naotubes58

59. Tuesday, October 17, 2017There are three categories of materials based on their electrical properties: (a) conductors; (b) semiconductors; and (c) insulators. The energy separation between the valence band and the conduction band is called Eg (band gap). The ability to fill the conduction band with electrons and the energy of the band gap determine whether a material is a conductor, a semiconductor or an insulator.59

60. Tuesday, October 17, 2017In conducting 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 band gap is a few electron volts. If an applied voltage exceeds the band gap 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 3).60

61. Tuesday, October 17, 2017Figure 3: Schematic illustration of the valence and conduction bands in materials based on their electrical properties: insulator, semiconductor and conductor61

62. Tuesday, October 17, 2017Quantum confinement and its effect on material electrical properties:- Quantum confinement causes the energy of the band gap to increase as illustrated in Figure 4. Furthermore, at very small dimensions when the energy levels are quantified, the band overlap present in metals disappears and is actually transformed into a band gap. This explains why some metals become semiconductors as their size is decreased62

63. Tuesday, October 17, 2017Figure 4: The image compares the energy of the band gap (arrow) in a bulk semiconductor, a quantum dot and an atom. As more energy states are lost due to the shrinking size, the energy band gap increases.63