Crystal StructuresCrystal Structures

FCC Unit cell HCP Unit cellBCC Unit cell

Ball-stick model: Ball – Atom, Stick -- Bonding

• Materials can be divided into Amorphous, Single Crystalline, Poly Crystalline according to their crystal structure.• Bulk Material composed a repeat pattern of unit cell and thus their properties are determined by their crystal structures

Crystalline direction and planeCrystalline direction and plane

What is What is Surface Surface and Surface and Surface EnergyEnergy

From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002)http://Materials.Usask.Ca

B u lk c ry s ta l

S u r fa c e

S u r fa c e a to m s

R e c o n s tru c te ds u r fa c e

OH

A b s o rb e dO x y g e n

H 2O

O

H 2D a n g lin g b o n d

Fig. 1.52: At the surface of a hypothetical two dimensional crystal, theatoms cannot fulfill their bonding requirements and therefore havebroken, or dangling, bonds. Some of the surface atoms bond with eachother; the surface becomes reconstructed. The surface can havephysisorbed and chemisorbed atoms.

Surface is the place where atoms terminating bulk, i.e. the termination of bulk crystalline structure (Repeating pattern)

Surface Energy is coming from unbonded electrons

What is the surface / interfaceWhat is the surface / interface

Atoms at surface / Interface has Atoms at surface / Interface has unbonded electrons which will cause extra unbonded electrons which will cause extra energy compared with atoms in bulkenergy compared with atoms in bulk This extra energy per area is defined as This extra energy per area is defined as surface / interface energy surface / interface energy The total energy associated with surface / The total energy associated with surface / interface E = interface E = SSThis energy becomes significant in nano This energy becomes significant in nano material systemmaterial system

Why surface properties are Why surface properties are important for Nano materialsimportant for Nano materials

Total Volume V=H*ATotal Surface Area S=A*H/d=V/dSurface-volume ratio S/V = 1/d

H

dd

Thin Films

Why surface properties are Why surface properties are important for Nano materialsimportant for Nano materials

Total Volume V=6*4/3R3=8R3

Total Surface Area S=6*4R2=24R2=3V/RSurface-volume ratio S/V = 3/R = 6/d

Nano Particles

What happen in real material system?What happen in real material system?

Surface reconfiguration will impact about 2-3 atomic Surface reconfiguration will impact about 2-3 atomic layers in depth. (or 6-8A)layers in depth. (or 6-8A)

In general, atoms in those regions are different than In general, atoms in those regions are different than their cousins in bulk and therefore included as their cousins in bulk and therefore included as surface. surface.

For Si, assume surface region are 2-3 atomic layers For Si, assume surface region are 2-3 atomic layers or roughly one unit cell in depth => Another reason or roughly one unit cell in depth => Another reason that surface properties are more important for nano that surface properties are more important for nano materials as most of atoms are at surfacematerials as most of atoms are at surface

What happen in real material system?What happen in real material system?% atoms in the surface = 100 * #% atoms in the surface = 100 * #surface atomssurface atoms / # / #bulk atomsbulk atoms

Thin layer: Thin layer:

##surface atomssurface atoms ~ # ~ #atoms in the unit cellatoms in the unit cell * s/a * s/a22 ~ ~ 8*V/d/25A8*V/d/25A22

##bulk atoms bulk atoms ~~ ##atoms in the unit cell*atoms in the unit cell*V/aV/a33~8*V/125A~8*V/125A33

% atoms in the surface = 500/d(A)% atoms in the surface = 500/d(A)

For particles:For particles:

##surface atomssurface atoms ~ # ~ #atoms in the unit cellatoms in the unit cell * s/a * s/a22 ~ ~ 8*8*4R2/25A/25A22

##bulk atoms bulk atoms ~~ ##atoms in the unit cell*atoms in the unit cell*V/aV/a33~8*(~8*(4/3)R3 /125A /125A33

% atoms in the surface = 1500/R(A)% atoms in the surface = 1500/R(A)Layer thicknessLayer thickness 11mm 10nm10nm 2nm2nm

% Atoms in the surface% Atoms in the surface 0.05% 5%5% 25%25%

Si Particle sizeSi Particle size 11mm 10nm10nm 2nm2nm

% Atoms in surface% Atoms in surface 0.3% 30%30% 88%88%

Surface Energy in BCC and FCC StructureSurface Energy in BCC and FCC Structure

Surface energySurface energy is determined by how many bonds we have to break in order to create a fresh surface (Cleavage) and thus depending upon crystalline planes?

Which crystalline plane shall have lowest Which crystalline plane shall have lowest surface energy? Why?surface energy? Why?

Closed packed plane has most atoms in plane and thus the out plane bonding is less and weak.

Surface Energy in Diamond Surface Energy in Diamond Structure such as Silicon Structure such as Silicon

Diamond Structures in Cubic (Si, C) Which one is close packed plane?

Surface ReconstructionSurface ReconstructionSurface atoms are re-arranging their position and bonding to minimize the surface energy

Adsorption and DesorptionAdsorption and Desorption• The adsorption of molecules on to a surface is a necessary prerequisite to any surface mediated chemical process. • In general, surface reaction can be broken down into the following steps :

Transferring of reactants to the active surface Adsorption of one or more reactants onto the surface Surface reaction Desorption of products from the surface Transferring of products away from the surface

• Absorption and Desorption are equally important.

The basis of distinction is the nature of the bonding between the molecule and the surface. With ...

•Physical Adsorption : the only bonding is by weak Van der Waals - type forces. There is no significant redistribution of electron density in either the molecule or at the substrate surface.

•Chemisorption : a chemical bond, involving substantial rearrangement of electron density, is formed between the adsorbate and substrate. The nature of this bond may lie anywhere between the extremes of virtually complete ionic or complete covalent character.

Physical and Chemical AbsorptionPhysical and Chemical Absorption

Chemisorption Physisorption

Temperature Range(over which adsorption occurs)

Virtually unlimited(but a given molecule may effectively adsorb only over a small range)

Near or below the condensation point of the gas(e.g. Xe < 100 K, CO2 < 200

K)

Adsorption EnthalpyWide range (related to the chemical bond strength) - typically 40 - 800 kJ mol-1

Related to factors like molecular mass and polarity but typically 5-40 kJ mol-1 (i.e. ~ heat of liquefaction)

Crystallographic Specificity

Marked variation between crystal planes

Virtually independent of surface atomic geometry

Nature of Adsorption Often irreversible Reversible

Saturation Uptake Limited to one monolayer Multilayer uptake possible

Kinetics of AdsorptionVery variable - often an

activated process Fast - since it is a non-activated process

Typical Characteristics of Adsorption Processes

Wetting and Contact Angle Wetting and Contact Angle

The engineering importance of surfacesThe engineering importance of surfaces Wetting -- Frying pans and car waxes, Detergents, LubricantsWetting -- Frying pans and car waxes, Detergents, Lubricants Bonding – Glues, SoldersBonding – Glues, Solders Catalysis -- AdsorptionCatalysis -- Adsorption Capillarity -- Tree sap and blood vessels Capillarity -- Tree sap and blood vessels

ß

ß

ßSL

LV

SVœ

V

S

L

cos() SV SL

LV

The “Young Equation” determines the “contact angle”The “Young Equation” determines the “contact angle” Balance of forces at the periphery of a drop on a rigid surfaceBalance of forces at the periphery of a drop on a rigid surface

The wetting angle, The wetting angle, ranges from 0 (wetting) to ranges from 0 (wetting) to (de- (de-wetting)wetting)

Film Formation (Spreading)Film Formation (Spreading)

Spreading: LV+SL interfaces have lower energy than SVSpreading: LV+SL interfaces have lower energy than SV Want for painting, coating, soldering, etc.Want for painting, coating, soldering, etc.

To promote spreadingTo promote spreading Raise Raise SVSV: e.g., clean the interface: e.g., clean the interface

Flux in soldering removes oxides from surfaceFlux in soldering removes oxides from surfaceClean the substrate interface prior to deposit thin filmClean the substrate interface prior to deposit thin film

Lower Lower SLSL: e.g., include reactive species in L : e.g., include reactive species in L Sn in solder forms intermetallic compounds with Cu, Ni or AuSn in solder forms intermetallic compounds with Cu, Ni or AuFlow catalyst or reactant to form an intermediate layer and prepare for depositionFlow catalyst or reactant to form an intermediate layer and prepare for deposition

Lower Lower LVLV: e.g., add surfactant (species that adsorbs at LV interface): e.g., add surfactant (species that adsorbs at LV interface)Flux in solder coats surface, lowers Flux in solder coats surface, lowers LVLV

S

L

V

SL LV SV

De-Wetting De-Wetting

De-wetting: film of vapor preferred between S and LDe-wetting: film of vapor preferred between S and L LV+SV interfaces have lower energy than SLLV+SV interfaces have lower energy than SL Want for “non-stick” coatings (frying pans, car wax).Want for “non-stick” coatings (frying pans, car wax). Passivation Layers to stop moisture. Passivation Layers to stop moisture.

To promote de-wettingTo promote de-wetting Lower Lower SVSV: e.g., add surfactants or low-: e.g., add surfactants or low- coatings to solid coatings to solid

Teflon on frying pansTeflon on frying pans Lower Lower LVLV: e.g., add surfactant (species that adsorbs at LV interface): e.g., add surfactant (species that adsorbs at LV interface) Raise Raise SLSL: e.g., remove any possible surfactants or reactive species : e.g., remove any possible surfactants or reactive species

SV LV SLS

LV

Hydrophilic vs. HydrophobicHydrophilic vs. HydrophobicHydrophilic (Water Loving): Materials exhibit an affinity for water The Hydrophilic (Water Loving): Materials exhibit an affinity for water The surface chemistry allows these materials to absorb water and be wetted surface chemistry allows these materials to absorb water and be wetted forming a water film. Hydrophilic materials also possess a high surface forming a water film. Hydrophilic materials also possess a high surface tension and have the ability to form "hydrogen-bonds" with water.tension and have the ability to form "hydrogen-bonds" with water.

Hydrophobic (Water hating): have little tendency to adsorb water and water Hydrophobic (Water hating): have little tendency to adsorb water and water tends to "bead" on their surfaces. Hydrophobic materials possess low tends to "bead" on their surfaces. Hydrophobic materials possess low surface tension values and lack active groups in their surface chemistry for surface tension values and lack active groups in their surface chemistry for formation of "hydrogen-bonds" with water. formation of "hydrogen-bonds" with water.