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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

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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.

Material Properties

• 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

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How to describe the material?

Property = Response to an external stimulus

Three Primary Material Types

Polymers (plastics, rubber) • Organic compounds (C, H)

and nonmetallic elements (O, N, Si,…)

• Large molecular structures with C backbone, e.g. PE, nylon, PVC, PC, silicone; low density

• Not as stiff nor strong (but per mass, similar to metals and ceramics), extremely ductile, pliable

• Easily soften or decompose to moderate temperature (but resistant to chemicals)

• Could be transparent, translucent, or opaque

• Low conductivity

• Non-magnetic

Metals • One or more metallic

elements (Au, Cu, Fe, Ni), [plus a tiny amount of nonmetallic elements (C, N, O)]

• Orderly structures; dense

• Mechanically stiff, strong, ductile, not easily fracture

• Good conductor of heat • Slivery or lustrous looking;

not transparent to light

• Usually good conductor of electron b/c nonlocalized e- not bonded to specific atoms

• Most are magnetic

Ceramics • Compounds of metallic and

nonmetallic elements, usually oxides, nitrides, carbides, e.g. alumina Al2O3, silica SiO2 [Also, clay minerals, cement, glass]

• Relatively stiff, strong, very hard but brittle, easily fracture

• Insulator to heat, resistant to high temperature (and to chemicals)

• Could be transparent, translucent, or opaque

• Poor electrical conductor

• Some are magnetic

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Two Complex Material Types

Composites • Two or more primary

materials to create properties not found in individual materials,

• Natural: wood and bone; • Synthetic: fiberglass - glass

fiber in epoxy or polyester – stiff & strong plus flexible & ductile; carbon fiber reinforced polymer (CFRP) – C fiber in a polymer – stiffer and stronger, & more expensive than fiberglass; used in aircraft, snowboards, iPod case

Advanced Materials • Materials used in high-tech applications; • Primary materials with properties enhanced and/or new

properties added • Semiconductors: Electrical properties in between

conductors (metal and alloys) and insulators (ceramics and polymers); properties sensitive to minute amount of doplants

• Biomaterials: Materials used in implants; with components from metals, ceramics, polymers, composites, & semiconductors

• “Future” materials: (a) Smart materials respond to changes in the external stimuli in a predictable way, e.g. shape-memory alloys, piezoelectric ceramics, magnetostrictive materials, electrorheological/magnetorheological fluids. (b) Nano-engineered materials or nanoscale materials, e.g. Carbon nanotubes

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Source: W. D. Callister, "Materials Science and Engineering: An Introduction", 7th ed., Wiley, New York (2006).

Nanoscale Phenomena

Physics at the Nanoscale • Electromagnetic force are

predominant

• Wave-particle duality of matter

• Quantum mechanical tunnelling – penetration of electron wavefunction into an energy barrier that is classically forbidden

• Quantum confinement > increased bandgap, leading to different electrical and optical properties

• Quantization of energy – discrete energy levels

• Internal magnetic field and coercive force – size dependent

• Random molecular motion – Brownian motion

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Chemistry at the Nanoscale • Intra-molecular bonding or chemical

interaction: ionic bonds, covalent bonds, metallic bonds – important to structure

• Inter-molecular bonding or physical interaction: ion-ion and ion-dipole, van-der Waals, hydrogen bond, plus hydrophobic interactions and repulsive forces (steric repulsions)

• Increase in surface-to-volume ratio > increased reactivity due to high surface energy of the surface atoms –important to catalysis and sensing

• Shape can also change surface area

• Increase in surface energy > decrease in melting point + increase in heat capacity

Spherical Fe Nanocrystals

How to calculate % of surface atoms in a nanocube? For a typical “bulk” object:

Volume density = 1023 atoms/cm3

Surface density = 1015 atoms/cm2

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%

02- 8 Source: Klabunde et al., J. Phys. Chem. 100 (1996) 12142.

Reduction in M.P.

02- 9 Source: B. D. Fahlman, "Materials Chemistry", 2nd ed., Springer, New York (2011), Ch. 6.

• 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

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Source: B. D. Fahlman, "Materials Chemistry", 2nd ed., Springer, New York (2011), Ch. 6.

Increase in Reactivity

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Side Length Number of Cubes in a 1 m3

Cube

Total Surface Area

1 m 1 6 m2

0.1 m 103 60 m2

0.01 m = 1 cm

10-3 m = 1 mm

10-9 m = 1 nm

Source: B. D. Fahlman, "Materials Chemistry", 2nd ed., Springer, New York (2011) ; Ch. 6

Increase in Heat Capacity

• Specific heat capacity: C = ∆Q/(m ∆T)

• For polycrystalline materials at high T: CV = 3 R /M ~ 26 J mol

-1 K-1 , 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

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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.

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Source: http://spie.org/x33323.xml

The Lycurgus Cup 400 AD

The optical properties are due to light scattering effect resulting from surface plasmons of appropriate nanoparticle size and distribution – “Roman” Nano-plasmonics?

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Chemical composition:

Silica (SiO2): 73%, Na2O: 14%, lime (CaO): 7% + 0.5% Mg

+ ~40 ppm Au + ~330 ppm Ag – These are present as colloidal nanoalloy (~70 nm) dispersed in the soda-lime-silica glass.

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.

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.

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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.

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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.

Exciton

Above: (A) Nanocrystal quantum dots are surrounded by a layer of organic molecules (surfactants) that allows precise size control, prevent conduction electrons from getting trapped at the surface, and make nanocrystals soluble. (B) The process of carrier multiplication—the freeing of two electrons (or creation of two excitons) with one high-energy photon—is depicted two ways: on the energy ladder and within the crystal lattice. In the latter, the valence electons orbit the atomic nuclei, and the conduction electrons move freely.

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Source: http://www.lanl.gov/science/1663/june2010/story2.shtml

Material m*e m*

h er Eex [meV]

rex [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

InSb 0,014 0,42 17,3 0,6 67,5

ZnS 0,34 1,76 8,9 49,0 1,7

ZnO 0,28 0,59 7,8 42,5 2,2

ZnSe 0,16 0,78 7,1 35,9 2,8

CdS 0,21 0,68 9,4 24,7 3,1

ZnTe 0,12 0,6 8,7 18,0 4,6

CdSe 0,11 0,45 10,2 11,6 6,1

CdTe 0,096 0,63 10,2 10,9 6,5

HgTe 0,031 0,32 21,0 0,87 39,3

Si 7,5

To be useful for solar cell application, exciton binding energy must be greater than kT = 25 meV

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Other models include Coulombic interaction of excitons and the correlation energy

Source: B. D. Fahlman, "Materials Chemistry", 2nd ed., Springer, New York (2011) ; Ch. 6

Intentionally produced Nanomaterials

• Carbon-based materials: Mostly composed of C. Common forms are hollow spheres and ellipsoids (fullerenes) and tubes (C nanotubes). Used for improved films and coatings, stronger and lighter materials [CNT >117x stronger than steel, 30x stronger than Kevlar (g per g)], and electronics.

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• Metal-based materials: Quantum dots, nano-particles (nanogold, nanosilver), metal oxides (TiO2).

• Dendrimers: Nanosized polymers built from branched units. The surface of a dendrimers has numerous chain ends that can be designed to perform specific chemical functions (e.g. catalysis). The interior cavities inside 3D dendrimers can be used to store drug molecules (drug delivery system).

• Composites: Nanoparticles combined with other nanoparticles or with larger bulk-like materials (host). Used for auto parts, packaging, heat barrier, flame-retardant materials.

Lab Tour: WATLab

Homework 1C: We will be stopping by several instrument clusters to be discussed in the four Modules, including: • wet chemistry (C2-061), • CVD (ovens) (C2-080), PVD (magnetron sputtering (C2-080), • PLD (C2-066), MBE (C2-066); plus • X-ray photoelectron spectroscopy (C2-064) • optical and electron microscopy lab (C2-060) • X-Ray diffraction (C2-060) • HIM and SIMS (C2-080) • Characterization of electrical and magnetic properties characterization (C2-080) (a) Provide the name and brief description of one technique in each of these Modules. (b) Using no more than TWO tweets (1 tweet = 140 characters), give a general impression of these instruments and techniques.

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