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Quantum Confinement • Types of Nanomaterials • Lab Tour

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Quantum Confinement • Types of Nanomaterials • Lab Tour 02- 1 Lecture 02 MNS 102: Techniques for Materials and Nano Sciences • Material Properties and Types of Materials • Review of nanoscale phenomena • Examples of nano-specific properties: m.p., reactivity, quantum confinement • Types of nanomaterials • Lab tour Homework 2A: Read B. D. Fahlman, "Materials Chemistry", 2nd ed., Springer, New York (2011), Ch. 6. In less than 1 page for each part, (a) discuss the difference between nanoparticles and colloidal particles; (b) explain the concept of band diagram by using concepts from MO Theory and MO diagram. 02- 2 Material Properties How to describe the material? Property = Response to an external stimulus • Mechanical : Deformation to an applied force – elastic modulus, strength • Thermal : Heat or change in temperature – heat capacity, thermal conductivity • Optical : Light or electromagnetic radiation – refraction index, reflectivity (colour?) • Electrical : Response to an external electric field - electrical conductivity, dielectric constant • Magnetic : Response to an external magnetic field – magnetic susceptibility, coercivity [Coercive force (or magnetic memory) = force needed to reverse an internal magnetic field] • Deteriorative : External environment - chemical reactivity and stability 02- 3 Three Primary Material Types Metals Ceramics Polymers (plastics, rubber) • One or more metallic • Compounds of metallic and • Organic compounds (C, H) elements (Au, Cu, Fe, Ni), nonmetallic elements, and nonmetallic elements [plus a tiny amount of usually oxides, nitrides, (O, N, Si,…) nonmetallic elements (C, N, carbides, e.g. alumina • Large molecular structures O)] Al2O3, silica SiO2 [Also, clay with C backbone, e.g. PE, • Orderly structures; dense minerals, cement, glass] nylon, PVC, PC, silicone; low density • Mechanically stiff, strong, • Relatively stiff, strong, very • Not as stiff nor strong (but ductile, not easily fracture hard but brittle, easily per mass, similar to metals • Good conductor of heat fracture and ceramics), extremely • Slivery or lustrous looking; • Insulator to heat, resistant ductile, pliable not transparent to light to high temperature (and to • Easily soften or decompose • Usually good conductor of chemicals) to moderate temperature electron b/c nonlocalized e- • Could be transparent, (but resistant to chemicals) not bonded to specific translucent, or opaque • Could be transparent, atoms • Poor electrical conductor translucent, or opaque • Most are magnetic • Some are magnetic • Low conductivity • Non-magnetic 02- 4 Two Complex Material Types Composites Advanced Materials • Two or more primary • Materials used in high-tech applications; materials to create • Primary materials with properties enhanced and/or new properties not found in properties added individual materials, • Semiconductors: Electrical properties in between • Natural: wood and bone; conductors (metal and alloys) and insulators (ceramics and • Synthetic: fiberglass - glass polymers); properties sensitive to minute amount of fiber in epoxy or polyester – doplants stiff & strong plus flexible & • Biomaterials: Materials used in implants; with components ductile; from metals, ceramics, polymers, composites, & carbon fiber reinforced semiconductors polymer (CFRP) – C fiber in • “Future” materials: a polymer – stiffer and (a) Smart materials respond to changes in the external stronger, & more expensive stimuli in a predictable way, e.g. shape-memory alloys, than fiberglass; used in piezoelectric ceramics, magnetostrictive materials, aircraft, snowboards, iPod electrorheological/magnetorheological fluids. case (b) Nano-engineered materials or nanoscale materials, e.g. Carbon nanotubes 02- 5 Source: W. D. Callister, "Materials Science and Engineering: An Introduction", 7th ed., Wiley, New York (2006). 02- 6 Nanoscale Phenomena Physics at the Nanoscale Chemistry at the Nanoscale • Electromagnetic force are • Intra-molecular bonding or chemical predominant interaction: ionic bonds, covalent • Wave-particle duality of matter bonds, metallic bonds – important to • Quantum mechanical tunnelling – structure penetration of electron wavefunction • Inter-molecular bonding or physical into an energy barrier that is interaction: ion-ion and ion-dipole, classically forbidden van-der Waals, hydrogen bond, plus • Quantum confinement > increased hydrophobic interactions and bandgap, leading to different repulsive forces (steric repulsions) electrical and optical properties • Increase in surface-to-volume ratio > • Quantization of energy – discrete increased reactivity due to high energy levels surface energy of the surface atoms – important to catalysis and sensing • Internal magnetic field and coercive force – size dependent • Shape can also change surface area • Random molecular motion – • Increase in surface energy > decrease Brownian motion in melting point + increase in heat capacity 02- 7 Spherical Fe Nanocrystals How to calculate % of surface atoms in a nanocube? For a typical “bulk” object: Volume density = 1023 atoms/cm3 15 2 Surface density = 10 atoms/cm NB: 1 nm = 10-7 cm For a nanocube of 1 nm side length: Total # of atoms in a nancube = 1023/cm3 × (10-7 cm)3 ~ 100 Total # of atoms on the surface of nanocube = 1015/cm2 × 6 × (10-7 cm)2 ~ 60 % of surface atoms = 60/100 = 60% Source: Klabunde et al., J. Phys. Chem. 100 (1996) 12142. 02- 8 Reduction in M.P. • Atoms on the surface have higher energy (i.e. more like atoms in liquid). • Surface energy increases as size decreases • Lowering in MP ∝ 1/r • Decrease ~ a few hundreds K for <10 nm Source: B. D. Fahlman, "Materials Chemistry", 2nd ed., Springer, New York (2011), Ch. 6. 02- 9 Source: B. D. Fahlman, "Materials Chemistry", 2nd ed., Springer, New York (2011), Ch. 6. 02- 10 Increase in Reactivity Side Length Number of Total Surface Area Cubes in a 1 m3 Cube 1 m 1 6 m2 0.1 m 103 60 m2 0.01 m = 1 cm 106 600 m2 -3 9 2 10 m = 1 mm 10 6000 m ~ 1 football field (5390 m2) 10-9 m = 1 nm 1027 6x109 m2 = 6000 km2 1,500 x UW campus Source: B. D. Fahlman, "Materials Chemistry", 2nd ed., Springer, New York (2011) ; Ch. 6 02- 11 Increase in Heat Capacity • Specific heat capacity: C = ∆Q/(m ∆T) • For polycrystalline materials at high T: -1 -1 CV = 3 R /M ~ 26 J mol K , where M is mol wt, R is gas constant [ 1 J = 0.239 cal, 1 cal = heat needed to raise 1 g of H2O by 1 deg] • For nanocrystalline material at high T: CV is higher than the bulk due to quantum confinement effects and higher surface energy. • Examples: Pd 6 nm : +48% 25-37 J mol-1 K-1 at 250 K Cu 8 nm : +8% 24-26 J mol-1 K-1 at 250 K Ru 6 nm : +22% 23-28 J mol-1 K-1 at 250 K 02- 12 As the size of the object gets into the nanoscale… In addition to changes in the mechanical property (related to physical structure), thermal property, and in reactivity (deteriorative property), changes in optical, electrical, and magnetic properties also occur. Re mechanical property… Surface and interfacial forces (e.g. adhesion forces, capillary forces, strain forces) dominate at the nanoscale. These forces could overcome forces at macro scale (e.g. gravity). E.g. superhydropho- bicity of lotus leaf; surface coating; NEMS. Source: http://spie.org/x33323.xml 02- 13 The Lycurgus Cup 400 AD Reflected light = Appeared as green like jade Transmitted light = Appeared as red like ruby “The mythological scenes on the cup depict the death of Lycurgus, King of the Edoni in Thrace at the hands of Dionysus and his followers. A man of violent temper, Lycurgus attacked Dionysus and one of his maenads, Ambrosia. Ambrosia called out to Mother Earth, who transformed her into a vine. She then coiled herself about the king, and held him captive. The cup shows this moment when Lycurgus is enmeshed in vines by the metamorphosing nymph Ambrosia, while Dionysus with his thyrsos and panther (Fig 2), a Pan and a satyr torment him for his evil behaviour. It has been thought that the theme of this myth - the triumph of Dionysus over Lycurgus - might have been chosen to refer to a contemporary political event, the defeat of the emperor Licinius (reigned AD 308-24) by Constantine in AD 324.” Source: “The Lycurgus Cup – A Roman Nanotechnology” I. Freestone, N. Meeks, M. Sax, C. Higgitt, Gold Bulletin, 40 (2007) 270. Chemical composition: The optical properties are due to Silica (SiO2): 73%, Na2O: 14%, lime (CaO): 7% light scattering effect resulting + 0.5% Mg from surface plasmons of + ~40 ppm Au + ~330 ppm Ag – These are appropriate nanoparticle size and present as colloidal nanoalloy (~70 nm) distribution – dispersed in the soda-lime-silica glass. “Roman” Nano-plasmonics? 02- 14 Size-dependent Properties of Gold Surface plasmons (SPs) are natural collective oscillations of the electron gas in metals. The localized SP resonance frequency depends on the size and shape of the nanoparticle (and its dielectric function). Absorption peak broadens and shifts to longer wavelengths as size increases above 50 nm. Reflection (caused by scattering) is weaker at smaller sizes. 02- 15 Quantum Confinement When an electron is promoted from valence to conduction bands, an electron-hole pair known as an exciton is created in the bulk lattice. The physical separation between e- and h+ is known as the exciton Bohr radius (rB). When the size (or diameter D) of an nano-object (quantum dot), i.e. D 2rB, this leads to quantum confinement of the exciton. Unlike bulk semiconductor crystal, the dimensions of a quantum dot (or an nano-object) could be quite small. Adding or removing an atom could change the dimension of the nanocrystal a lot, causing significant change in the bandgap of the bulk, Eg. 02- 16 Exciton * * Material m e m h er Eex rex [meV] [nm] BN 0,752 0,38 5,1 131 1,1 GaN 0,20 0,80 9,3 25,2 3,1 InN 0,12 0,50 9,3 15,2 5,1 GaAs 0,063 0,50 13,2 4,4 12,5 InP 0,079 0,60 12,6 6,0 9,5 GaSb 0,041 0,28 15,7 2,0 23,2 GaP InAs 0,024 0,41 15,2 1,3 35,5 Above: (A) Nanocrystal quantum dots are surrounded by a layer of organic InSb 0,014 0,42 17,3 0,6 67,5 molecules (surfactants) that allows precise size control, prevent conduction ZnS 0,34 1,76 8,9 49,0 1,7 electrons from getting trapped at the surface, and make nanocrystals soluble.
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