13_Properties of Nanomaterials.pptx

It describes how different properties of materials changes when reduced to nano. Property includes electrical, optical, mechanical, magnetic, thermal etc.

1. Properties of Nano Materials

2. Au - , Au Ref: Prof A K Gsnguli slides

3. The properties of materials can be different at the Nanoscale for two main reasons: First , nanomaterials have a relatively larger surface area when compared to the same mass of material produced in a larger form. Nano particles can make materials more chemically reactive and affect their strength, magnetic or electrical properties. Second, quantum effects can begin to dominate the behaviour of matter at the Nanoscale Reason of Properties Change in Nano

4. Size Nanoparticles exhibit unique properties due to their high surface area to volume ratio. A spherical particle has a diameter (D) of 100nm. Calculate the volume (V) and surface area (SA)  

5. Surface Area:Volume Ratio This gives an approximate surface area to volume ratio of >10 7 :1 which is significantly larger than a macro sized particle. As the surface area to volume ratio increases so does the percentage of atoms at the surface and surface forces become more dominant. Generally accepted material properties are derived from the bulk, where the percentage of atoms at the surface is miniscule. These properties change at the nanoscale.

6. Size Nanoparticle Nanoparticle Volume Surface Area SA:Vol Ratio Diameter (nm) Diameter (um) (nm 3 ) (nm 2 ) (nm 2 /nm 3 ) 1 0.001 0.524 3.14 6 10 0.01 524 314 0.6 100 0.1 523598 31416 0.06 1000 1 5.24E+08 3.14E+06 0.006 10000 10 5.24E+11 3.14E+08 0.0006 100000 100 5.24E+14 3.14E+10 0.00006 1000000 1000 5.24E+17 3.14E+12 0.000006 Some example calculations for volume and surface area of nanoparticles. These calculations use nm as unit of length.

7. Surface Area:Volume Ratio In this graph: SA = nm 2 Vol = nm 3 SA:Vol Ratio = nm 2 /nm 3 The ratio increases dramatically when the nanoparticle diameter drops below about 100 nm

8. Crystal Structure The spatial arrangement of atoms in a crystal lattice is described by its unit cell. The unit cell is the smallest possible volume that displays the full symmetry of the crystal. Many materials have a “preferred” unit cell.

9. Crystal Structure In 3 dimensions, unit cells are defined by 3 lattice constants and 3 angles. This leads to 14 Bravais lattices, each having characteristic restrictions on the lattice constants, angles, and centering of atoms in the unit cell. b a c

10. Crystal Structures Simple Cubic (SC) Body Centered Cubic (BCC) Face Centered Cubic (FCC) For cubic unit cells, there are three centering types:

11. Size & Crystal Structure Most metals in the solid form close packed lattices Ag, Al, Cu, Co, Pb, Pt, Rh are Face Centered Cubic (FCC) Mg, Nd, Os , Re, Ru, Y, Zn are Hexagonal Close Packed (HCP) Cr, Li, Sr can form Body Centered Cubic (BCC) as well as (FCC) and (HCP) depending upon formation energy

12. Size & Crystal Structure How does crystal structure impact nanoparticles? Nanoparticles have a “ structural magic number ” , that is, the optimum number of atoms that leads to a stable configuration while maintaining a specific structure. Structural magic number = minimum volume and maximum density configuration If the crystal structure is known, then the number of atoms per particle can be calculated.

13. Close-Packed Magic Number Clusters Number of atoms (y) in shell (n): y = 10n 2 + 2 (n = 1,2,3…) Maximum number of nearest neighbors (metal-metal hcp packing) Decreasing percentage of surface atoms as cluster grows

14. Size & Crystal Structure For n layers, the number of atoms N in an approximately spherical FCC nanoparticle is given by the following formula: N = 1/3[10 n 3 – 15 n 2 + 11 n - 3] The number of atoms on the surface N surf N Surf = 10 n 2 – 20 n +12

15. Size & Crystal Structure Poole, C., Owens, F. Introduction to Nanotechnology . Wiley, New Jersey. 2003 Example Calculations: How many atoms (N) are in idealized Au NP’s with the following diameters? 5 nm Au NP: With 9 shells, n = 9 and NP diameter = 17d = 4.896 nm N = 1/3[10 n 3 – 15 n 2 + 11 n - 3] N = 2057 Other Approximate Values 10 nm = 17,900 20 nm = 137,000 30 nm = 482,000 40 nm = 1.1 million 50 nm = 2.2 million

16. The Nano particles affects many properties such as Magnetic properties Electrical properties Mechanical properties Thermal properties Band gap Optical properties Dielectric properties Size Dependence of Properties

17. Mechanical Properties

18. Mechanical Properties dislocations grain boundaries atomic defects atomic defects, dislocations and strains grain boundaries and interfaces porosity connectivity and percolation short range order connectivity and percolation porosity

19. Yield strength refers to an indication of maximum stress that can be developed in a material without causing plastic  deformation . It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of the  elastic limit . Hardness  is a measure of the resistance to localize plastic deformation induced by either mechanical indentation or abrasion. Macroscopic hardness is generally characterized by strong intermolecular bonds. Elastic modulus is the ratio of stress, below the proportional limit, to the corresponding strain. It is the measure of rigidity or stiffness of a material.

20. Mechanical properties of nanomaterials may reach the theoretical strength, which are one or two orders of magnitude higher than that of single crystals in the bulk form . Mechanical Properties The mechanical properties of nanomaterials increase with decrease in size, because smaller the size, lesser is the probability of finding imperfections such as dislocations, vacancies, grain boundaries Strength of material improves significantly as the particle size decrease due to perfect defect free surface . Hardness and yield strength of material also increases as particle size is decreased. Elastic modulus and toughness of material also increases as particle size is decreased.

21. Thermal properties

22. Melting Point (Microscopic Definition): Temperature at which the atoms, ions, or molecules in a substance have enough energy to overcome the intermolecular forces (Binding Energy) that hold the them in a “fixed” position in a solid Thermal properties melt at the same temperature. At macroscopic length scales (Bulk) , the melting temperature of materials is size-independent. ice cube glacier

23. Thermal properties melting point decreases Nanocrystal size decreases surface energy increases Surface atoms require less energy to move because they are in contact with fewer atoms of the substance Example: 10 nm Au NP melts at 964 o C bulk Au at 1064 o C <1.4 nm Au NPs melts below room temperature 3 nm CdSe nanocrystal melts at 700 K bulk CdSe at 1678 K In contact with 3 atoms In contact with 6 atoms volume Binding energy: surface Electrostatic repulsion Lack of symmetry Parity

24. Thermal properties Melting point of the surface (1 st layer) T M  Melting point of bulk materials  Solid-liquid interface energy  Enthalpy r  Radius of the particle

25. Melting point as a function of size Source: Nanoscale Materials in Chemistry, Wiley, 2001

26. Melting point as a function of size Source: Nanoscale Materials in Chemistry, Wiley, 2001 Au

27. Thermal Conductivity bulk Heat is transported in materials by two different mechanisms: Lattice vibration waves (phonons) and Free electrons. In metals, the electron mechanism of heat transport is 8 significantly more efficient than phonon processes. In the case of nonmetals, phonons are the main mechanism of thermal transport. In both metals and nonmetals, as the system length scale is reduced to the nanoscale , there are quantum confinement and classical scattering effects.

28. Optical Properties

29. Optical Properties The size dependence on the optical properties of nanoparticles is the result of two distinct phenomena: Surface plasmon resonance (SPR) -- metal nano-structures b)Increased energy level spacing due to the confinement of delocalized energy states. -- prominent in semiconductors

30. Optical Properties -SPR Surface Plasmons Recall that metals can be modeled as an arrangement of positive ions surrounded by a sea of free electrons . The sea of electrons behaves like a fluid and will move under the influence of an electric field + - + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + - + + + + + + + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - E-field

31. Optical Properties -SPR Surface Plasmons If the electric field is oscillating (like a photon), then the sea of electrons will oscillate too. These oscillations are quantized and resonate at a specific frequency. Such oscillations are called plasmon. Source: MRS Bulletin 2005, 30(5), 338. Resonance at a metal surface Resonance in metal NP

32. Optical Properties - SPR Surface Plasmons Formal definition: Plasmons are the coherent excitation of free electrons in a metal. The plasmon resonance frequency (f) depends on particle size, shape, and material type. It is related to the plasmon energy (E) by Planck’s constant. E=h*f Surface plasmons are confined to the surface of the material. The optical properties of metal nanoparticles are dominated by the interaction of surface plasmons with incident photons. If the plasmons oscillation matches with the frequency of incident radiation, resonance occurs and called surface plasmon resonance. • It takes place when size of nanoparticle is smaller than wavelength of incident radiations.

33. Optical Properties -SPR Surface Plasmons Metal nanoparticles like gold and silver have plasmon frequencies in the visible range. When white light impinges on metal nanoparticles the wavelength corresponding to the plasmon frequency is absorbed. The spectral locations, strengths, and number of plasmon resonances for a given particle depend on the particle’s shape and size.

34. Optical Properties Optical absorption spectrum of 20- and 80-nm gold nanoparticles embedded in glass. [Adapted from F. Gonella et al., in Handbook of Nanostructured Material and Nanotechnology , H. S. Nalwa, ed., Academic Press, San Diego, 2000, Vol. 4, Chapter 2, p.85.] Absorption spectra of spherical Au nanoparticles

35. Martin, Olivier J.F. “Spectral response of plasmon resonant nanoparticles with non-regular shape”. Optics Express Col. 6. No. 11 May 2000 Surface Plasmons : Shape dependence of absorption spectra The amount of light that is scattered into the far field is described by the scattering cross section (SCS). The SCS is plotted against the wavelength of light used to illuminate a particle from a specific angle. The arrows indicate the illumination angle, and their colors correspond to the different plot lines. Optical Properties

36. Optical Properties- SPR Martin, Olivier J.F. "Plasmons". Plasmons . 22 Mar. 2006. Ecole Polytechnique Fédérale de Lausanne. 26 Jan. 2003. Surface Plasmons: Shape dependence of absorption spectra. Resonant frequency of different shapes NPs are different.

37. Optical Properties – Band Gap Energy levels: from atoms to bulk materials… The Pauli Exclusion Principle states that electrons can only exist in unique, discrete energy states. In an atom the energy states couple together through spin-orbit interactions to form the energy levels . When atoms are brought together in a bulk material, the energy states form nearly continuous bands of states, or in semiconductors and insulators, nearly continuous bands separated by an energy gap. N Energy Energy Atoms: Discrete Energy Levels Bulk Materials: Band Structures

38. Optical Properties- Band gap Energy levels In semiconductors and insulators, the valance band corresponds to the ground states of the valance electrons. In semiconductors and insulators, the conduction band corresponds to excited states where electrons are a free to move about in the material and participate in conduction. In order for conduction to take place in a semiconductor, electrons must be excited out of the valance band, across the band gap into the conduction band. This process is called carrier generation. Conduction takes place due to the empty states in the valence band (holes) and electrons in the conduction band. E c E v Electron excited into conduction band band gap

39. Optical Properties- Band gap Energy level spacing In semiconductors and insulators a photon with an energy equal to the band gap energy is emitted when an electron in the conduction band recombines with a hole in the valance band. The electronic band structure of a semiconductor dictates its optical properties. GaP , a material commonly used for green LEDs, has an intrinsic band gap of 2.26 eV. Carrier recombination across the gap results in the emission of 550 nm light. E g = 2.26 eV λ =550 nm

40. Optical Properties- Band gap Energy level spacing and quantum confinement The reduction in the number of atoms in a material results in the confinement of normally delocalized energy states. Electron-hole pairs become spatially confined when the dimensions of a nanoparticle approach the de Broglie wavelength of electrons in the conduction band. As a result the spacing between energy bands of semiconductor or insulator is increased (Similar to the particle in a box scenario, of introductory quantum mechanics.) Energy E g E g Bulk Materials Nano Materials Increased band gap

41. Band gap The band gap is increases with reducing the size of the particles Bandgap is the energy needed to promote an electron from the valence band to the conduction band In bulk materials, there are 10 23 atoms on surface, large no. of atoms means large energy states, so band gap is less. As we go in nanorange , no. of atoms decrease to 10-1000 atoms, so energy states decrease, band gap is more.

42. Optical Properties - Band gap Energy level spacing and quantum confinement As semiconductor particle size is reduced the band gap is increased. Absorbance and luminescence spectra are blue shifted with decreasing particle size. CdSe quantum dots Jyoti K. Jaiswal and Sanford M. Simon. Potentials and pitfalls of fluorescent quantum dots for biological imaging. TRENDS in Cell Biology Vol.14 No.9 September 2004

43. For semiconductors such as ZnO , CdS , and Si, the bandgap changes with size Optical Properties- Band gap

44. Electrical and Electronic Properties

45. Electrical and Electronic Properties Effect of structure on conduction If nanostructures have fewer defects, one would expect increased conductivity vs macroscale Other electrical effects on the nanoscale: Surface Scattering Change in Electronic Structure Ballistic Conduction Discrete Charging Tunneling Conduction Microstructural Effects

46. Surface Scattering Electrons have a mean-free-path (MFP) in solid state materials. MFP is the distance between scattering events as charge carriers move through the material. In metals, the MFP is on the order of 10’s of nanometers. If the dimensions of a nanostructure are smaller than the electron MFP, then surface scattering becomes a factor.

47. Source of resistance: scattering Total resistivity , ρ T , of a metal is a combination of the contribution of individual and independent scattering, known as Matthiessen’s rule: thermal resistivity defect resistivity Electron collisions with vibrating atoms (phonons) displaced from their equilibrium lattice positions are the source of the thermal or phonon contribution, which increases linearly with temperature. Impurity atoms, defects such as vacancies, and grain boundaries locally disrupt the periodic electric potential of the lattice and effectively cause electron scattering, which is temperature independent. Source

48. Considering individual electrical resistivity directly proportional to the respective mean free path (λ) between collisions, the Matthiessen’s rule can be written as: Increase in crystal perfection or reduction of defects, which would result in a reduction in defect scattering and, thus, a reduction in resistivity. However, the defect scattering makes a minor contribution to the total electrical resistivity of metals at room temperature, and thus the reduction of defects has a very small influence on the electrical resistivity. Create an additional contribution to the total resistivity due to surface scattering, which plays a very important role in determining the total electrical resistivity of nanosized materials. If the mean free electron path, λ S , due to the surface scattering is the smallest, then it will dominate the total electrical resistivity. In nano, the surface scattering of electrons results in reduction of electrical conductivity. + ρ s Nano

49. Reduction in material’s dimensions will increase crystal perfection or reduction of defects, which would result in a reduction in defect scattering and, thus a reduction in resistivity and conductivity increases. In nanowires and thin films, the surface scattering of electrons results in reduction of electrical conductivity. When the critical dimension is smaller than the mean free path, motion of electron will undergo elastic and inelastic scattering.  Elastic scattering : electron reflects same way as photon reflects from mirror. Both momentum and energy is conserved. Direction of motion of electron is parallel to surface. Electrical conductivity is same as bulk materials.  Inelastic scattering: In this electron mean free path is terminated by impinging on surface. The electron loses its kinetic energy and electrical conductivity decreases. Electrical and Electronic Properties Surface scattering:

50. Electrical and Electronic Properties Change of Electronic Structure: Reduction in characteristic dimension below a critical size, i.e. below De Broglie wavelength results in change of electronic structure , leading to widening of band gap . Such a change results in reduced electrical conductivity . Some metal nanowires undergo transition to become semiconductors and semiconductors might become insulators when their diameters are reduced below a critical diameter.

51. Bulk Nano Reduced dimension Conductor/semiconductor Insulator Increased Band-gap Electrical and Electronic Properties Change of Electronic Strucutre :

52. Electrical and Electronic Properties Quantum Transport: It occurs when length of conductor is smaller than electron mean free path. In this case, each transverse waveguide mode or conducting channel contributes G = 2e 2 h = 12.9kΩ- 1 In ballistic transport there is no energy dissipation and no elastic scattering takes place. Ballistic conduction, Coulomb blockade and Tunnelling Ballistic conduction:

53. Electrical and Electronic Properties It occurs when length of conductor is smaller than electron mean free path. Ballistic conduction: Transport of a conductor Ohm’s law  

54. Coulomb Blockade & Single Electron Transistor

55. Electrical and Electronic Properties Coulomb Blockade & Single Electron Transistor:  It occurs when length of contact resistance is larger than resistance of nanostructures and total capacitance of object is so small that adding a single electron requires significant energy. Metal or semiconductor nanocrystals exhibit quantum effects that give rise to discrete charging of metal particles. Coulomb Blockade can be observed by making the device very small, like quantum dot (i.e. 3D confinement). In this 3D confined state, electrons inside the QD will create a strong Coulomb repulsion preventing other to flow. This, the device will no longer follow Ohms law. It require too much Coulomb energy to add extra electron. This is called Coulomb blockade. G The Coulomb Energy Ec = e 2 /2C Electronic charge – e Total Capacitance - C

56. Single Nanowire Transistor:

57. Electrical and Electronic Properties Tunelling : It involves charge transport through an insulating medium seperating two conductors that are closely spaced. This is because electron wave function from two conductors overlap inside insulator, when thickness is thin. As thickness of layer increases, electrical conductivity decreases

58. Magnetic Properties

59. Magnetic Properties due to the huge surface energy bulk materials nanostructured materials Magnetic properties are distinctly different superparamagnetism Ferromagnetism Ref: Prof A K Gsnguli slides

60. Magnetic Properties I A Magnetic moment, (m or µ) = IA Origin of Magnetism Macroscopic Microscopic (charge current) (atomic scale) Magnetization (M) = Magnetic moment per unit volume Magnetic field strength (H)  measure of the strength of the externally applied magnetic field. Magnetic moment ( m or µ ) is the measure of the strength of magnet can the ability to produce magnetic field. Magnetic induction / Magnetic flux density (B)  Magnetic flux per unit area. Magnetic Susceptibility , Gives physical idea about the magnetic material Energy of the magnetic moment (E) : E= - m.B

61. Magnetic Properties Origin of Magnetism (atomic) Nuclear Spin Orbital motion of electrons Electron Spin (Small effect) Magnetic moment arising due to nucleus is very small compared to electron. or Am 2 Electron Nucleus

62. Magnetic Properties Spin Orbital motion Lattice weak weak strong In bulk, contribution of magnetic moment due to orbital motion of crystalline solid is small In Nano  orbital – lattice coupling decreases due to reduced surface energy magnetization increases Side reduced Fundamental component of magnetism Magnetism of atom Magnetism of Molecule Magnetism of solid Magnetism of hybrids

63. Magnetic Properties

64. Spin Orbital motion Lattice weak weak strong Contribution of magnetic moment due to orbital motion of crystalline solid is small in bulk In Nano  orbital – lattice coupling decreases due to reduced surface energy magnetization increases Side reduced Increase in Magnetization in nano

65. Decrease in Magnetization in further Nano

66. A Survey of Magnetic Nanoparticle Applications A method for early diagnosis of brain cancer under development uses magnetic nanoparticles and nuclear magnetic resonance (NMR) technology. The  magnetic nanoparticles attach to particles in the blood stream called microvesicles  which originate  in brain cancer cells. NMR is then used to detect these microvesicle /magnetic nanoparticle clusters, allowing an early diagnosis. Iron oxide nanoparticles can be used to improve Magnetic Resonance Imaging (MRI) of cancer tumors . The nanoparticle is coated with a peptide that binds to a cancer tumor . Once the nanoparticles are attached to the tumor , the magnetic property of the  iron oxide enhances the images from the MRI scan . Researchers at MIT have shown that iron oxide nanoparticles in water can be used to increase the amount of  heat transfer out of a system at localized hot spots . The researchers believe this technique could be applied to cooling a wide range of devices, from electronics devices to fusion reactors. Magnetic nanoparticles can attach to cancer cells in the blood stream. These nanoparticles may allow doctors to remove  cancer cells before they can establish new tumors . Using  nanoscavengers , in which a layer of reactive nanoparticles coat a synthetic core which is designed to be easily magnetized. The nanoparticles , for example silver nanoparticles if bacteria is a problem, attach to or kill the pollutants. Then when a magnetic field is applied the nanoscavengers are removed from the water. Nanoparticles containing iron oxide that allows the nanoparticles to be  directed, by a magnetic field, to stents . This could allow drugs to be delivered directly to stents placed in arteries. Iron oxide nanoparticles can used to improve MRI images of cancer tumors . The nanoparticle is coated with a peptide that binds to a cancer tumor , once the nanoparticles are attached to the tumor the magnetic property of the  iron oxide enhances the images from the Magnetic Resonance Imagining scan .

67. Thank You