Lectures in Nanoscience & Technology

1. Nanomaterials & StructuresK. Sakkaravarthi

Department of PhysicsNational Institute of Technology

Tiruchirappalli – 620 015Tamil Nadu

India

sakkaravarthi@nitt.eduksakkaravarthi.weebly.com

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ObjectivesEducation and Research for a better scientificunderstanding on the Preparation & Characterizationof Materials in Nanoscales towards enhanced Utilization& Applications.

Normal/Bulk materials Nanomaterials

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My sincere acknowledgments toIntroduction to Nanoscience and Nanotechnology,Chris Binns, Wiley Publications, New Jersey (2009).Nanotechnology: Basic Sciences and EnergyTechnologies, M. Wilson, K. Kannangara, G. Smith, M.Simmons and B. Raguse, CRC Press, New York (2005).Introduction to Nanotechnology,C.P. Poole and F.J. Ownes, Wiley India, New Delhi (2007).Introduction to Nanomaterials and Nanotechnology(Lecture Notes: University of Tartu), Vladimir Pokropivny,Rynno Lohmus, Irina Hussainova, Alex Pokropivny, SergeyVlassov (2007).Many other free & copyright internet resources.

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Nanoscience & TechnologyDiscussion?

NANO: Definition for nano?NanoscienceTechnology in NanoscienceNeed for NANO

Utilize the possibility to vary fundamental properties ofmaterials without changing their chemical compositions.

The size can change the behavior of the material!

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NanomaterialsPeculiar Reasons?

Nanomaterials have a relatively larger surface areacompared to the same mass of material produced in alarger/bulk form.

This makes materials more chemically reactive(sometimes inert materials in larger bulk form can becomereactive when produced in their tiny nanoscale), and affecttheir (mechanical/electrical/optical/magnetic) properties.

Quantum effects begin to dominate the behavior.

The mechanical, thermal, optical, electrical and magneticbehaviour of materials exhibit huge difference in thenanoscale size.

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Nanomaterials: PropertiesMechanical properties: Surface to volume ratio increaseswith decreasing size. Grain size, Cavities, dislocation,Strength of materials...Thermal properties: Surface atoms are less constrainedthan interior atoms and thus can vibrate more freely abouttheir equilibrium.Better specific heat and thermal conductivity innanomaterials. Also, have lower melting temperature/point.Electrical conductivity: Free electrons yield an energycontinuum with no forbidden energy levels. The energylevels become discrete/quantized and then form bands withforbidden zones when move from bulk to nano scale. Hence,bulk metals are good electrical conductors.In the reverse, in nanoscale materials the energy levels willhave more forbidden zones & reduces the electricalconductivity of nanomaterials.

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Nanomaterials: Properties...Electrical conductivity: If size decreases bandgap increases!

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Types of NanomaterialsClassification Based on Confined dimensions?

1D nanomaterial: One-dimension in nanoscale range &two other dimensions free (any scale).Ex.: Surface coatings and thin films.2D nanomaterial: Any two dimensions in nanoscale range& one dimension free.Ex.: Biopolymers, nanotubes, and nanowires.3D nanomaterial: All three dimensions in nanoscale.Ex. Nanoparticles, Colloids, Quantum dots, fullerenes, etc.0D nanomaterial: Zero dimensions in nanoscale :-(i.e. Bulk materials in macroscale ) NOT NANO.Violates basic definition ) NOT acceptable

But, actual/acceptable classification??Should be based on the

“Number of Free Dimensions!"K. Sakkaravarthi Lectures in Nanoscience & Technology 8/50

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Types of NanomaterialsClassification: Based on the Number of free dimensions!

0D nanomaterial: None can be outside nanoscale.All the three dimensions are in nanoscale range.Ex. Nanoparticles, Colloids, Quantum dots, fullerenes, etc.

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Types of Nanomaterials...Classification: Based on the Number of free dimensions!

1D nanomaterial: One dimension outside the nanoscale& two other dimensions in the nanoscale range.Ex.: Nanowires, Nanorods, Nanotubes & Biopolymers.

2D nanomaterial: Any two dimensions can be outside thenanoscale & one dimension in nanoscale range.Exhibit plate-like shapes!Ex.: Nanolayers, Surface coatings and thin films.

3D nanomaterial: Bulk nanomaterials & all threedimensions can be outside nanoscale. But, made up of acollection of nano-particles/materials.Ex.: Dispersions of nanoparticles, bundles of nanowires/nanotubes & multiple nanolayers.

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Types of Nanomaterials...Classification: Based on the Number of free dimensions!

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0D: Nanoparticles (single/composite/coated)

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1D: Nanowires/Nanorods/Nanotubes

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All-in-One ExampleCarbon: Basic element of life & its special ability to bond withmany elements in different ways.

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Allotropes of carbon: 0D to 3D

Eight allotropes of carbon:a) diamond, b) graphite, c) lonsdaleite,

d) C60 buckminster fullerene, e) C540, Fulleritef) C70, g) amorphous carbon, and h) single-walled carbon

nanotube.

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1. NanowiresOne to few-tens of nanometer (109 m).The ratio of the length-to-width greater than 1000(aspect ratio: Length

Thickness ' 1000).Structures with a thickness/diameter constrained to tens ofnanometers or less and an unconstrained length.At these scales, quantum mechanical effects are important) “quantum wires".Many different types of nanowires exist.Superconducting, metallic, Semiconducting, & insulating.

Ex. Semiconducting nanowires: Silicon, Gallium Nitrideand Indium Phosphide.Remarkable optical, electronic and magnetic characteristics!(Silica nanowires can bend light around very tight corners).

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1. Nanowires: ApplicationsHigh-density data storage, either as magnetic read heads oras patterned storage media.Electronic and opto-electronic nanodevices.Metallic interconnects of quantum devices and nanodevices.

1. Nanowires: SynthesisSelf-assembly processes, where atoms arrange themselvesnaturally on stepped surfaces.Chemical vapor deposition (CVD) onto patternedsubstratesElectroplating or molecular beam epitaxy (MBE)

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2. NanorodsA solid wire-like structure in nanometer-scale.Diameter: One to 100 nanometer (1� 100 nm).The ratio of the length-to-width is very small.(aspect ratio: Length

Thickness ' 2� 20).Structures with a thickness/diameter constrained to tens ofnanometers or less and an unconstrained length.

2D view 3D view

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3. NanotubesA tube-like (hollow) structure in nanometer-scale.Diameter in the order of nanometer (109 m).The ratio of the length-to-width huge.Aspect ratio: Length

Thickness up to 132,000,000(significantly larger than for any other material).Exceptional strength and stiffness.Nanotubes maybe single-walled or multi-walled! Mainlymade of carbon!!

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Synthesis Approaches1 Top-down:

* Breaking down bulk matter into basic building blocks.* Slicing or successive-cutting (crushing, milling orgrinding) of a bulk material to get nano sized particles.* Not suitable for preparing uniformly shaped materials.Mainly, chemical or thermal or mechanical methods.1. Ball milling 2. Plasma arching3. Laser sputtering 4. Vapour deposition

2 Bottoms-up:* Building complex systems by combining simpleatomic-level components.* Devices ‘create themselves’ by self-assembly.* Much cheaper than top-down methods.* Able to generate a uniform size, shape and distribution.* Normally made by chemical synthesis.1. Sol-gel 2. Colloidal3. Electrodeposition 4. Solution phase reductions

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Synthesis Approaches...

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Synthesis Approaches...

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Synthesis Approaches...

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0D Nanoparticles: Ball Milling1 Grinder used to grind & blend materials.2 Principle of impact and attrition: Size reduction by impact

(balls drop from near the top of the shell).3 A hollow cylindrical shell rotating about its axis.

It is partially filled with balls (made of steel, stainless steel,ceramic, or rubber).

4 The length of the cylindrical mill ⇡ its diameter.5 Grinding can be carried out either wet or dry (speed).6 Effective for production of amorphous materials.

7 Properties of balls: size, density, hardness, & composition.

8 Advantages: Cost effective; Suitable for both open &closed grinding and batch & continuous operation;Materials of all degrees of hardness

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0D Nanoparticles: Ball Milling - Schematic

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0D Nanoparticles: Ball Milling - Factors to consider

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Introduction Classification Synthesis0D Nanoparticles: Gas CondensationInert Gas evaporation-Condensation (IGC) technique:Formation of nano-powders/particles on a metallic source/substrate without chemical reactivity (inert gas environment).

1 Evaporating a metallic source using resistive heating(radio frequency heating or electron gun or laser beam)inside an evacuated chamber (to about 10�7 torr) filledwith inert gas at a low pressure.

2 The metal vapour migrates from the hot source into thecooler inert gas by a combination of convective flow anddiffusion. Here the evaporated atoms collide with the gasatoms within the chamber, thus losing kinetic energy.

3 The particles are collected usually by deposition on acold surface (cooling the substrate with liquid nitrogen toenhance the deposition efficiency).

4 Highly concentrated deposition on the substrate.Have complex aggregate morphology: need to be classifiedbased on the size (smaller or larger structures).

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0D Nanoparticles: Gas Condensation

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Gas Condensation: 1. Thermal evaporationMost popular technique: easy handling & good yield.Suitable to synthesis several pure nanocrystalline metals(Cu, Au, Pd, Ni, Fe,) with a narrow particle size 5-20 nm.Able to change particle size by varying gas pressure & temp.Cons.: Alloys with different vapor pressures of constituentelements are difficult to make :-(

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Gas Condensation: 2. Magnetron-sputteringSuitable for elements with different vapour pressures.Good for metals with very high melting point (Mo andoxides like ZrO2) - very difficult in thermal evaporation.For thin films: at very low pressures (around 10�3 mbar).For powders/particles: Pressure around 10�1 mbar in IGC.Cannot change the particle size by changing gas pressure :-(

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Introduction Classification SynthesisGas Condensation: 3. Pulsed-laser-ablationHigh power laser pulses to melt, evaporate and ionizematerial from the surface of a target.Pico-second pulsed laser irradiation in a vacuum chamber(base pressure 10�7 mbar) with helium or argon.Ablated atoms condense into clusters/nanoparticles &collected on a liquid nitrogen cooled collection device.Production of superconducting & insulating circuitcomponents to improved wear and biocompatibility.Wider range of nanopowders of metals, metallic alloys,semiconductors, and metal oxides can be synthesized :-)

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Gas Condensation: 3. Pulsed-laser-ablationLaser parameters: Laser energy/fluence [Joule/cm2] andionization degree.Surface temperature: Has a large effect on the nucleationdensity. Nucleation density decreases as the temperature isincreased. Heating of the surface can involve a heatingplate or the use of a CO2 laser.Substrate surface: Surface preparation, the miscut of thesubstrate, and roughness of the substrate.Background pressure: Common in oxide deposition, anoxygen background is needed to ensure stoichiometrictransfer from the target to the film. This will affect thenucleation density and film quality.

Three growth modes are possible:1. Step-flow growth 2. Layer-by-layer growth, 3. 3D growth.

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Electron beam (physical vapor) deposition

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Synthesis: 2D films/layers

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Synthesis: Sol-GelWet-chemical process: Formation of an inorganic colloidalsuspension (sol) and gelation of the sol in a continuous liquidphase (gel)!

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Synthesis: Sol-Gel

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Synthesis: Sol-Gel

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Spray PyrolysisVertical spray set-up

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Spray Pyrolysis45o spray set-up

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Dip Coating1 Coating process to manufacture bulk products.2 Thin-film coatings: Single layer or Multilayer.3 Suitable for4 Mainly involves five stages:

1. Immersion: The substrate is immersed in the solution ata constant speed.2. Start-up: Pulled up back (constant speed) after a while.The speed determines the thickness of the coating.(faster withdrawal gives thicker coating material)3. Deposition: Thin layer deposits on the substrate.4. Drainage: Excess liquid is drained from the surface.5. Evaporation: The solvent evaporates from the liquid.For volatile solvents, such as alcohols, evaporation startsalready during the deposition and drainage steps.

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Dip Coating: Schematic 1

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Dip Coating: Schematic 2

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Dip Coating: Factors to considerThe dip coating technique can give uniform, high quality filmseven on bulky, complex shapes.

1 Initial substrate surface2 Submersion time3 Withdrawal speed4 Number of dipping cycles5 Solution composition6 Concentration and temperature7 Environmental factors

Dip Coating: Few AdvantagesSingle/Multilayer coatings for sensor applicationsHydrogels/ Sol-Gel nanoparticle coatingsLayer-by-layer nanoparticle assemblies

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Multilayer Materials

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Multilayer Coating: Semiconductor Applications

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Molecular-beam epitaxyThin-film deposition of single crystals!

1 Epitaxy: Method of depositing a mono crystalline film ordeposition and growth of mono crystalline layers.

2 Growing crystalline layers on a crystalline substrate.Similar to Ink-Jet-Color printing!!

3 Widely used in the manufacture of semiconductor devices.4 1. Homoepitaxy: Same type substrate & material (Si-Si).

2. Heteroepitaxy: Different substrate & material (Ga-As).3. Pseudo-homoepitaxy: Same material with a doping layer.

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Molecular-beam epitaxy...1 MBE is an ultra high vacuum (UHV) based based

production of high quality epitaxial structures with monolayer (ML) control.

2 “Beam” molecules do not collide to either walls of thevacuum chamber (pressure: 10-11 Torr) or existing gasatoms.

3 Very slow process: Growth rate: 1 m/hr.4 To produce layers of metals, insulators & superconductors.5 1. Ultra pure elements are heated in separate quai-effusion

cells until they begin to sublimate slowly.6 2. Epitaxial growth takes place due to the interaction of

molecular or atomic beams on the surface of heatedcrystalline substrate.

7 3. Atoms on a clean surface are free to move until findingcorrect position in the crystal lattice to bond.

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Introduction Classification SynthesisMolecular-beam epitaxy...1 Basic elements of MBE system:

* Heated substrate. * Effusion cells and shutter.* Reflection High Energy Electron Diffraction (RHEED)system: RHEED gun & screen.* Ultra High Vacuum * Liquid Nitrogen cryopanelling.

2 MBE Working Conditions:* Mean free path of the particles > size of the chamber.* Ultra-high vacuum (UHV= 10-11 Torr) for clear epilayer.* Chemically stable as crucibles even at high temperature.* Molybdenum and tantalum are widely used for shutters.* Ultrapure materials are used as source.

3 Pros: Clean surfaces & free from oxide layer. Good controlof layer thickness & composition. Low growth rate(1µm/h) gives high purity. Precisely controllable thermalevaporation of each component.

4 Cons: Expensive (106 $ per MBE chamber).Very complex system. Ultra-high vacuum conditions.

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Molecular-beam epitaxy...

Molecular-beam epitaxy: Applications* Heterojunction bipolar transistors (HBT’s) used in satellitecommunications.* Electronic and optoelectronic devices (LED’s for laserprinters, CD/DVD players).* Construction of quantum wells, dots and wires.* To build a thin film of a photo voltaic solar cells.* Low temperature Superconductor.

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