characterization of thin films

8/12/2019 Characterization of Thin Films

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ChE 5535

Characterization of thin Films

Alexander Couzis


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Motivation

Surface or interfacial properties determinethe extent of interaction of a material withits surroundings.

It is very difficult to find materials that have

the right combination of bulk and surfaceproperties.


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Objective

Determine New Ways for ControllingSurface Properties of Materials.

Deliver Surface Functionality to a materialthat would allow its use in new applications.


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Surface Properties

Surface properties control processes such as :

Wettability

Adhesion

Adsorption

Biocompatability

Lubrication

Permeation

Colloidal Interactions and Stability

Catalytic Action

Technological Impact:

Printing Processes

Adhesion and sealing between coated

web material (Paper, Non-Wovens)

Flavor and aroma scavenging in foodpackaging

Absorbency

Xerography

Pesticide and Herbiside Delivery

Microelectronics Packaging


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Techniques for Modification of Surfaces

Coating Techniques: Paints, Lacquers, Primers on Metal Surfaces

Sizing Agents on Paper or Non-wovens

Vapor Deposition Techniques

Surface Reactions: Flame Treatment Corona treatment

Plasma treatment

Chromic acid treatment

Grafting

Bulk Techniques Blending of surface active compounds

Alloys


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Water

Langmuir-Blodgett Transfer

LateralPressure

LateralPressure

Air

Experimental Method

Infrared spectrum

measurement

Calculation of A Calculation of ads

Water

Langmuir Film

Lateral

Pressure

Lateral

PressureAir


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Preparation of Hydrophobic Surface byOTS Deposition

Solvent Mixture :- Hexadecane:CHCl3:CCl4=80:8:12 (Volume) OTS Concentration :- 2.0x10-3mol/lit Solvents kept in 80-90% relative humidity for 48 hours to absorb water

TIME FOR HYDROLYSIS

INCUBATION TIME

SOLVENT

OTS

SUBSTRATE

OTS SOLUTION

TIME FOR DEPOSITION

DEPOSITION TIME

RINSE IN CHLOROFORMDRYING WITH NITROGEN

q

CONTACT ANGLE

MEASUREMENT

AFM


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Layered Polyelectrolytes


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Layered Polyeletrolytes

qz( -1)


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Characterization Barriers

Typically thin film are deposited on opaquesubstrates.

Often the films themselves are opaque. Very wide range of refractive indices.

Thickness can range from a few ngstrmsto a fem micrometers.


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Interaction of Materials and Light Transmission

Reflection

Diffraction

Scattering


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Spectral Range

Violet

Ultaviolet

Red

Infrared

500550600650700750

nm

GammaRays

X-RaysUltravioletInfraredMicrowaveRadiowave

0.013 x 1020Hz

0.11

10 1240ev

10m0.124 ev or 2850 cal/mol

1 cm

10cm2.85 cal/mol0.1cm-1

Spin Alignments Molecular TransitionsElectronic

Transitions

Nuclear

Transitions


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Tool Classification

Optical TechniquesDirect optical observation

Measurement of refractive index

Spectroscopic TechniquesTransmission

Reflection

Absorption

Scattering TechniquesRaman


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Tool Classification

Diffraction TechniquesX-Ray

Neutron

Higher energy techniquesX-ray photoelectron spectroscopy

Neutron reflectivity

Electrochemical TechniquesSurface Potential

Gravimetric

Quartz crystal microbalance


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Molecular Orbitals


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Intermolecular Forces

ChE5570


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Interaction Potentials

Typical Interaction Potentials of twomolecules have the form:

w(r)Cm1m2r

n

The corresponding force between these

two molecules is then given by:F(r)

dw(r)

drn

Cm1m2

rn1


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Mies Interaction Pair Potential

w(r) A

rn

B

rm

Repulsive TermAttractive Term

Classical Example of this form of an interaction potentialis the Lennard-JonesPotential:

w(r)A

r6

B

r12


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Lennard-Jones Potential

In this description of a intermolecular potential theattractive (negative) term is the Van der Waalsinteraction potential.

Two species at equilibrium, re, will be at a distancefrom one another that results in a minimum of theinteraction potential:

dw(r)

dr rre0

Because F(r) is given by dw(r)/dr, the maximum forceoccurs at a distance that satisfies d2w/dr2 = 0.


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Lennard-Jones Potentialw(r)= min when dw(r)/dr= 0 r = re= (2B/A)

1/6

w(re)min= w(r = (2B/A)1/6) = -A2/4B = -A/(2re

6)

The ratio of the minimum total potential to the VDW potential isthen given by:

w(re)min/w(re)VDW= (-A/(2re6))/ (-A/re

6) = 1/2

The interaction potential is zero for rthat satisfies w(ro)= 0 :r = ro= (B/A)

1/6 andthis means that re/ro= 21/6 = 1.12


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-4 10-21

-2 10

-21

0

2 10-21

4 10-21

6 10-21

2 3 4 5 6 7 8

B

In

termolecularPotential(J)

Intermolecular Distance ()

re= (2B/A)1/6

ro= (B/A)1/6

Repulsive

Attractive


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Lenard-Jones Potential

The intermolecular force is maximum at r: dw2(r)/dr2= 0.

Substituting gives us r = rs= (26B/7A)1/6

For realistic Lennard - Jones parameter values, A = 10-77 J m 6 & B = 10-134 J m 12 we can calculate a maximum attractiveforce of

Fmax=1.89x10-11N

For the same values of A and B the minimum potential canalso be calculated: wmin = -A

2/4B = -2.5x10-21J = 0.61kT at T= 298 K


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-1 10-10

0

1 10-10

2 10-10

3 10-10

4 10-10

5 10

-10

2 3 4 5 6 7 8

IntermolecularFor

ce(N)

Intermolecular Distance ()

(26B/7A)1/6

Repulsive

Attractive


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Intermolecular Potential BetweenTwo Oxygen Molecules


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Strong Intermolecular Forces

Covalent Bonds

Coulombic Interactions


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Strengths of Covalent Bonds

Bond Type Strength

(kJ mole-1)

Bond Type Strength

(kJ mole-1)

CN 870 Si----O 370

C==O 690 C----C 360

C==C 600 C----O 340

O---H 460 N----O 200

C---H 430 F-----F 150


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Coulomb Forces or Charge-Charge Interactions

The electrical field at a distance raway from a charge Q1is thendefined by:

E1 Q1

40r2

Vm1

This field when acting on a second charge, Q2, at r, gives riseto a force:

FQ2E1

Q1Q2

40r2


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Coulomb Forces or Charge-

Charge Interactions

w(r) (1.6021019

)2

4(8.8541012)(0.276109)8.41019 Joules

For a typical system, eg Na+Cl-, the interaction potential is:

The equilibrium separation is 2.76

For a temperature of 300 K this turns out to be equivalent of200 kT per ion pair in vacuum, and this comparable to theenergies of a covalent bond.

Compare to VDW max potential of 0.61 kT


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Interactions Involving PolarMolecules

-5

0

5

10

2 4 6 8 10 12 14 16

InteractionPotential(kcal/mol)

Interatomic Distance ()

3.8186, -2.5253kcal/mol (1.755E-20 J)

Na+Cl-


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Ionic CrystalsFor accurate determination of the lattice energy the Coulomb

energy of an ion with all the other ions in the lattice has tobe summed, and not only with the nearest neighbors.

In a NaCl crystal each Na+has 6 Cl-

nearest neighbor at r=0.276nm, 12 Na+

next nearest neighbors at (

2)r, and 8 Cl-

more at (3)r.

The total interaction potential is then:

i

e2

40r6

12

2

8

3

6

2

...

1.748 e

2

40r 1.461018 J

The lattice cohesive energy: U = -N0i= 880 kJ/mol


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Born Energy of an IonWhen a single ion is in vacuum or in a medium, it still has an

electrostatic free energy, even though it is not interacting withother ions. If in vacuum this energy is referred to as the self-energyof the ion and if a medium it is the Born or solvationenergyof the ion.

The work done to increase the charge of a sphere of radius a bygradually bringing charges from infinity to r = a is given by:Q

1 q andQ

2 dq and r

dw qdq40

And the total free energy is then:

i dw qdq

400

Q

Q

2

80

ze 2

80

Ch f M di


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Change of Medium

i ze 2

80

1

1

1

2

[J]

28z2

1

1

1

2

kT per ion at 300K

and

G No i

69z2

1

1

1

2

kJ / mol

With agiven in nanometers. Because moving from low

dielectric to high dielectric results in G


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Polar Molecules

NH2

CH2

CO

OH

NH3

+

CH2

CO

O

Glycine Glycine in water

Most molecules carry no netcharge, but many possess anelectric dipole.

When a molecules shows a spatialdistribution of electron density theypossess a permanent dipole.


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Dipole Moment

Unit of Dipole moment: 1 Debye 3.336x10 -30 C m

The dipole moment of two electrons (e = 1.602x10-19 C)separated by 1 is 1.6 x 10-29 C m = 4.8 D

Permanent Dipoles only occur is asymmetric molecules:They arise from the asymmetric displacement of electronalong covalent bonds.

The dipole moment of a molecule can be found by vectorialsummation of its component dipole moments: e.g. H2O

uH2O= 2 cos(1/2 q) uOH= 2 cos(52.25o) x 1.51 =1.85 D

u

q


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Dipole Self-Energy

It is the sum of the Born energies to bring the two charges qtogether minus the Coulomb interaction of bringing the twocharges together to form the dipole:

i 1

40 q2

2 q2

2 q 2

r

, with r l 2

i q 2

80


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Ion-Dipole Interactions

ChargeQ=-ze

B

C

-q

+q

1/2 l

1/2 l

Dipole Momentu=ql

w(r) Qq

40

1

AB

1

AC

with

AB r1

2lcosq

2

1

2lsin q

2

1/2

AC r1

2

lcos q

2

1

2

lsin q

2

1/2

rA

q


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Ion-Dipole InteractionsAt separation r much larger that the dipoles length AB and AC

simplify to:AB r - 1/2 l cosq, andAC r + 1/2 l cosq

w(r) w(r,q) Qq

40 1

r1

2

lcos q

1

r 1

2

lcos q

Qq

40 lcos q

r2 1

4l2co 2sq

Qu cos q

40r2

ze u cos q40r

2

w(r,q) uE(r)cos q


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Ion-Dipole InteractionsIf qis 0 o then interaction energy is negative and thus there is

attraction, is the dipole points towards the charge, ie. q is 180 o then the interaction is repulsive.

Ion dipole interaction dictate the interactions of ions in solvents.


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Ions in Polar SolventsIn a solvent like water the electrostatic interactions are

reduced by a factor of 80. Still they are strong enough forthese interactions to be significant.

WHAT DOES THIS INTERACTION MEAN ??????

+ion + ?


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Ions in Polar Solvents

This process cannot involve any energy gain by bringing onesolvent molecule close to a ion, because that would requirethat one solvent molecule depart from the vicinity of the ion.

SO where is the gain ?

+

ORDER ING IS WHAT GIVESONE THE ENERGY GAIN

Near cations q=0 is favored

Near anions q=180 is favoredNumber of water molecules thatare bound is known as thehydration number


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Dipole-Dipole InteractionsWhen two polar molecules are close to one anotherthere is dipole-dipole interactions, analogous to theinteractions of two magnets.

q2q1

w r, q1 ,q2 , u1u2 2 cosq1 cos q2 sin q1 sin q2 cos

40r3

Maximum interaction occurs when the two dipoles arelying flat and are in line:

w r,0,0, 2u1u2

40r3


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Rotating Dipoles and Angle

Averaged Potentialse w r / kT

e w r, / kTdd

ew r, / kT , d sin qdqd

andso d d sin qdq 40

0

2

And so in general one can write

e w r / kT ew r, /kT

1

4 d e

w r,q, /kTsin qdq

0

0

2


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Bottom LineFor charge-dipole interactions:

w r Q2

u2

6 40 2

kTr4 for kT

Qu

40r2

w r u12

u22

3 40

2

kTr

6 for kT

u1u2

40r3

For Dipole-Dipole interactions:


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Entropic EffectsA UTSUT

A

T

but

T

A

T

A

A U A 1

2U

So half the total energy is absorbed internally during theinteraction.

Since A < 0 the entropic contribution is negative and so theinteraction is associated with a loss in entropy.


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Interactions In ol ing


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Interactions InvolvingPolarization of Molecules

Polarizability is defined according to the strength of theinduced dipole moment that a molecule acquires whenin a field of strength of E:

uind=E

+e

-e

l

E

E+e

-euind 0Ele

Fext eE

Fint e2

40R2

sin q e2l

40R3

e

40R3

uind

uind 40R3E 0E

0 40R3


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Polarizability of Polar MoleculesConsider a freely rotating dipolar molecule

+

E

uind ucosqeuEcosq

kT u

2E

kTcos

2 q u

2

3kTE, uE kT

Since the induced dipole is proportional to the electrical field, thefactor u2/(3kT)is an additional contribution to the molecularpolarizability, and is known as orientational polarizability

orient u2

3kTE

and

0

u2

3kTE (Debye Langevin Equation)

In low enough T, or high Ethe molecules dipole will

align totally with the field

Interaction Between Ion and


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Interaction Between Ion andUncharged Molecule

+-

CationAnion

q q

Development of

Attractive force

Er 2uind

40r3

2E

40r3

2 ze

40 2

r5

attractive force is: F ze Er 2 ze 2

40 2

r4

1

2E2

The energy is half what is expectedof an ion and a s similarly alignedpermanent dipole:

w r uE E2


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Net Ion-Induced Dipole

Intercation

w r ze 2

2 40 2 r4 ze 2

2 40 2 r4 0 u

2

3kT


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Dipole-Induced Dipole

InteractionsE

u 1 3cos2 q 1/240r

3

w r,q 1

2 0E2

u20 1 3cos2 q

2 40 2

r6

Averaging over all angles:

w r u20

2 40 2

r

6

Debye Interaction or Induction Interaction

w r u1

202 u2201

2 40 2

r6

Van Der Waals Forces


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Van Der Waals Forces(I) Origin

Force acting between all atoms and molecules,similar to gravitational forces-----> London Forcesor Dispersion forces

Play a significant role is phenomena such asadhesion, surface tension, wetting, etc.

Can be effective in ranges greated than 10nm or down tointeratomic spacings.

Can be attractive or repulsive, no simple power law.

Dispersion Forces tend to align molecules

Non-additivity


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Quantum Mechanical Nature of

VanDer Waals Forces

+

-F

e2

400 2h, h 2.2 1018J

Bohr Radius 0

Simply solving gives us the valueof this first radius: 0.053nm

A Bohr atom has no permanent dipole moment, but at anygiven moment an instanteneous dipole exists that is equal to:

u a0e


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Magnitude of Dispersion Forces

w(r=a)=10-21J=1kT

characterization of thin films

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