Nano Physics

7/28/2019 Nano Physics

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A Day in the Life of a Nanoparticle

Or how I learned to not sunburn and still look good.

http://www.wsu.edu/~jtd/physunder/physun2.jpg http://www.rdecom.army.mil/rdemagazine/200402/images/itl_arl_particles.jpg

http://media.photobucket.com/imag

e/sunscreen%20and%20nanoparticles/vivawoman/badger-spf30-sunscreen.jpg


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Nanoparticle Uses

Sunscreens

Make-up

Automotive Paint

Sporting Goods

anti-bacterial

Hong Dong

FE-SEM: Zeiss(1550)-Clark

This image shows electrospun nylon 6 nanofibers decorated with surface bound Ag nanoparticles.Immersing nylon 6 nanofibers into Ag colloidal solution with pH 5, Ag nanoparticles were assembledonto nylon 6 nanofibers via interaction between nylon 6 and protection groups of Ag nanoparticles.Future applications include antibacterial filtration.

Fiber Science and Apparel Design

Advisor Juan Hinestroza


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When physicists first began investigating the structure ofatoms in the early 1900s, they uncovered a strange newworld. The subatomic particles they found -- electrons,protons, and neutrons -- seemed to behave according toa completely different set of laws than those governingour everyday world.

Then, in the late 1920s, a team of young physicists ledby Niels Bohr introduced a theory that explained thebehavior of atoms and their particles. Not surprisingly,

the theory, called quantum mechanics, was as bizarreas the world it attempted to explain.


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Rather than identifying precisely where an electron shouldbe, for example, quantum mechanics predicts only theprobability of finding that electron in one place or another.

This description of unpredictability at the atomic level --

indeed, at any level -- was completely unacceptable toEinstein; it flew in the face of everything he believed, anddirectly contradicted his orderly theories of the universe.

Despite Einstein's disapproval, quantum mechanics has onlygrown in acceptance as a theory.


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The Quantum CafMichael Greene

http://www.pbs.org/wgbh/nova/programs/ht/qt/3012_qd_05.html


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Opinions on quantum mechanicsI think it is safe to say that no

one understands quantummechanics. Do not keep sayingto yourself, if you can possiblyavoid it, But how can it be likethat? because you will getdown the drain into a blindalley from which nobody has yetescaped. Nobody knows how itcan be like that.

- Richard Feynman

Richard Feynman (1918-1988)

Those who are not shockedwhen they first come acrossquantum mechanics cannot

possibly have understood it.

- Niels Bohr


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# How did our understanding of the atomchange in the 1920s?

# How did quantum mechanics contradictEinstein's view of physics? What did Einsteinmean when he said, "God does not throwdice"?

# What are some of the "bizarre" things thatquantum mechanics predicts?

Important Questions


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The Birth of Modern Physics

Classical Physics of the 1890s

The Kinetic Theory of Gases

Waves and Particles

Conservation Laws and Fundamental Forces

The Atomic Theory of Matter

Outstanding Problems of 1895 and NewHorizons

The more impo rtant fundamental laws and facts of physical sc ience have all

been discov ered, and these are now so f i rm ly establ ished that the possib i l i ty

of their ever being supp lanted in con sequence of new disco ver ies isexceedingly remote Our future discoveries must be looked for in the sixthplace of decimals. - Albert A. Michelson, 1894

There is nothin g new to b e discovered in phys ics now. Al l that remains is more

and mo re precise measurement. - Lord Kelvin, 1900

James Clerk Maxwell


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Classical Physics of the 1890s

Mechanics

Thermodynamics

Electromagnetism


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0 0B

t

BE

t

0B

0/E q

Electromagnetism culminatedwith Maxwells Equations

Gausss law :(electric field)

Gausss law: (magnetic field)

Faradays law:

Ampres law:

James Clerk Maxwell(1831-1879)

in the presence ofonly stationarycharges.


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Faraday saw the World in a new way!


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The Nature of Light

Newton promoted the corpuscular(particle) theory

Particles of light travel in straight linesor rays

Explained sharp shadows

Explained reflection and refraction

"I procured me a triangular glass prism to trytherewith the celebrated phenomena ofcolours." (Newton, 1665)

Newton in action


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The Nature of Light

Huygens promoted the wave theory.

He explained polarization,reflection, refraction, and double

refraction.

Double refraction

Christiaan Huygens(1629-1695)

He realized that light propagates as awave from the point of origin.

He realized that light slowed down on

entering dense media.


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Diffraction confirmed light to be a wave.

Diffraction patterns

One slit

Two slits

While scientists of Newtons time thought shadows were sharp, Youngs two -slit

experiment could only be explained bylight behaving as a wave. Fresnel

developed an accurate theory of diffraction

in the early 19th

century.

Augustin Fresnel


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Waves can interfere.


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Maxwell strove to prove his Mentor correct


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Light waves were found to be solutions toMaxwells Equations.

All electromagnetic wavestravel in a vacuum witha speed c given by:

infrared X-rayUVvisible

wavelength (nm)

microwave

radio

105106

gamma-ray

The electromagnetic spectrum is vast.

where 0

and 0

are the permeability and permittivity of free space


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Light is an electromagnetic wave.

The electric (E) and magnetic (B) fields are in phase.

The electric field, the magnetic field, and the propagation direction areall perpendicular.


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Triumph of Classical Physics:

The Conservation Laws

Conservation ofenergy: The sum of energy(in all its forms) is conserved (does not

change) in all interactions.

Conservation oflinear momentum: In the

absence of external forces, linearmomentum is conserved in all interactions.

Conservation ofangular momentum: In the

absence of external torque, angular

momentum is conserved in all interactions.

Conservation ofcharge: Electric charge is

conserved in all interactions.

These laws remain

the key to interpreting

even particle physics

experiments today.


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Problems in 19th-century physics

In a speech to the Royal Institution in 1900, Lord Kelvin himselfdescribed two dark clouds on the horizon of physics:

The question of the existenceof an electro-magnetic

mediumreferred to asether or aether.

The failure of classical

physics to explain blackbodyradiation.


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The Ultraviolet Catastrophe

Lord Rayleigh used the classical theories of electromagnetism

and thermodynamics to show that the blackbody spectrumshould be:

This worked at longer wavelengths but deviates badly at short ones.This problem became known as the ultraviolet catastrophe and was one of

the many effects classical physics couldnt explain.

Rayleigh-Jeans Formula


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More problems: discrete spectral lines

Wavelength

Emission

spectrafrom

gases ofhot

atoms.

For reasons then unknown, atomic gases emitted only certain narrow

frequencies, unique to each atomic species.


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Additional discoveries in 1895-7 contributed to thecomplications.

X-rays (Roentgen)

Radioactivity (Becquerel)

Electron (Thomson)

Zeeman effect

Roentgens x -ray

image of his wifes hand (with her wedding ring)


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The Beginnings of Modern Physics

These new discoveries and themany resulting complicationsrequired a massive revision offundamental physicalassumptions.

The introduction (~1900) of themodern theories ofspecialrelativity and quantummechanics became thestarting point of this most

fascinating revision. Generalrelativity (~1915) continued it.

Log(size)

Speed

0

c

19th-

centuryphysics

Generalrelativity

Q

uantumm

echanics

Special

relativity


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Triumph of Classical Physics:

The Conservation Laws

Conservation ofenergy: The sum of energy

(in all its forms) is conserved (does not

change) in all interactions.

Conservation oflinear momentum: In theabsence of external forces, linear

momentum is conserved in all interactions.

Conservation ofangular momentum: In the

absence of external torque, angularmomentum is conserved in all interactions.

Conservation ofcharge: Electric charge is

conserved in all interactions.

These laws remain

the key to interpreting

even particle physics

experiments today.


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For our sunscreen to work we will need to lookat an experiment designed to determine howtightly bound electrons are to a surface.

This requires coming up with Planck's Constant.

This also requires the determination of the work Function.


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Work function experiment.

http://www.walter-fendt.de/ph11e/photoeffect.htm

Workfunction for ZnO is ~4.5

Wh i Q Ph i ?


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What is Quantum Physics? Quantum physics is a branch of Science

that deals with discrete, indivisible units of

energy called quanta as described byQuantum Theory.

There are five main ideas represented in

Quantum Theory which are:

1. Energy is not continuous, but comes in

small, but discrete units.2. The elementary particles behave both

like particles and like waves.

3. The movement of these particles is

inherently random.

4. It is physically impossible to know both

the position and momentum of a particle at

any instant in time so that the more accurate

the measurement of one is, the more

inaccurate the measure of the other is.

5. The atomic world is NOTHING like the

world we live in.

Structure of the Atom


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The Atomic Models of Thomson andRutherford

Rutherford Scattering

The Classic Atomic Model

The Bohr Model of the Hydrogen Atom

Successes & Failures of the Bohr Model

Characteristic X-Ray Spectra and Atomic

Number

Atomic Excitation by Electrons


The opposite of a correct statement is a false statement. But the opposite of a

profound truth may well be another profound truth.

An expert is a person who has made all the mistakes that can be made in a very

narrow field.

Never express yourself more clearly than you are able to think.

Prediction is very difficult, especially about the future.

- Niels Bohr

Niels Bohr (1885-1962)


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Evidence in 1900 indicated thatthe atom was not a fundamental unit:

1) There seemed to be too many kinds

of atoms, each belonging to a distinct chemical

element (way more than earth, air, water, and fire!).

2) Atoms and electromagnetic phenomena were intimately

related (magnetic materials; insulators vs. conductors;

different emission spectra).

3) Elements combine with some elements but not with others,a characteristic that hinted at an internal atomic structure

(valence).

4) The discoveries of radioactivity, x rays, and the electron (all

seemed to involve atoms breaking apart in some way).


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Knowledge of atoms in 1900

Electrons (discovered in

1897) carried the negative

charge.

Electrons were very light,

even compared to the atom.

Protons had not yet been

discovered, but clearly

positive charge had to bepresent to achieve charge

neutrality.


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In Thomsons view, when the atom was heated, the electrons could

vibrate about their equilibrium positions, thus producing

electromagnetic radiation.

Unfortunately, Thomson couldnt explain spectra with this model.

Thomsons

Atomic Model

Thomsons plum-pudding

model of the atom had the

positive charges spread

uniformly throughout a

sphere the size of the atom,

with electrons embedded in

the uniform background.

Experiments of Rutherford Geiger and


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Experiments of Rutherford, Geiger and

Marsden

Rutherford, Geiger, and Marsden

conceived a new technique for

investigating the structure of

matter by scattering a particles

from atoms.

Experiments of Rutherford Geiger and


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Experiments of Rutherford, Geiger and

Marsden 2

Geiger showed that many a particles were scattered from thin

gold-leaf targets at backward angles greater than 90.

Rutherfords Atomic Model


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Experimental results

were not consistent with

Thomsons atomic model.

Rutherford proposed thatan atom has a positively

charged core (nucleus)

surrounded by the

negative electrons.

Geiger and Marsden

confirmed the idea in

1913.

Rutherfords Atomic Model

Ernest Rutherford

(1871-1937)

The Classical Atomic Model


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2 2

2

0

1 v

4e

e mF

r r

The Classical Atomic Model

Consider an atom as a planetary system.

The Newtons 2nd Law force of attraction on

the electron by the nucleus is:

where v is the tangential velocity of the

electron:

The total energy is then:

0

v4

e

mr

221 1

2 2

0

v4

eK m

r

This is negative, so

the system is bound,

which is good.

The Planetary Model is Doomed


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The Planetary Model is Doomed

From classical E&M theory, an accelerated electric charge radiates

energy (electromagnetic radiation), which means the total energy

must decrease. So theradius r must decrease!!

Physics had reached a turning point in 1900 with Plancks

hypothesis of the quantum behavior of radiation, so a radical

solution would be considered possible.

Electroncrashes

into the

nucleus!?



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Bohrs general assumptions:

1. Stationary states, in which orbiting

electrons do not radiate energy, exist in

atoms and have well-defined energies,

En. Transitions can occur between them,

yielding light of energy:

E=En En = h

2. Classical laws of physics do not apply

to transitions between stationary states,

but they do apply elsewhere.

3. The angular momentum of the nth state is:

where n is called the Principal Quantum Number.

n

n = 1

n = 3

n = 2

Angular

momentum is

quantized!

Consequences of the Bohr Model


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Consequences of the Bohr Model

The angular momentum is:

nrmL v

0

v4

e

mr

mrn /v

04

2 2 2

2 2

n e

m r mr But: So:

Solving forrn:2

0nr n a

So the velocity is:

00

4 2

2ame

where:

a0is called the Bohr radius. Its the diameter of the Hydrogen

atom (in its lowest-energy, or ground, state).

a0

Bohr Radius


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Bohr Radius

The Bohr radius,

is the radius of the unexcited hydrogen atom and is equal to:

The ground state Hydrogen atom diameter is:

00

4 2

2a

me

/

The Hydrogen


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y g

Atom Energies

So the energies of the stationary

states are:

whereE0 = 13.6 eV.

r

eE

0

2

8

04 2 2

n 2

nr

me

Use the classicalresult for the

energy:

and:

En = E0/n2or:

The Hydrogen Atom


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The Hydrogen Atom

Emission of light occurs when the atom is in an excited state

and decays to a lower energy state (nu n).

uh E E

1 h

c hc

R is theRydberg constant.

where

is the frequency of a photon.

3

0(4 )

4

2

meR

c

Transitions


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Transitions

in the

HydrogenAtom

The atom will remain

in the excited state

for a short time

before emitting a

photon and returning

to a lower stationary

state. In equilibrium,all hydrogen atoms

exist in n = 1.

Characteristic X-Ray


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Shells have letter names:

K shell forn = 1

L shell forn = 2

The atom is most stable in its

ground state.

When it occurs in a heavy atom, the radiation emitted is an x-ray.

It has the energyE(x-ray) =Eu E.

Characteristic X Ray

Spectra and Atomic

Number

An electron from highershells will fill the inner-shell vacancy at lower energy.

Th C d


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The Correspondence

Principle

In the limits where classical and

quantum theories should agree,

the quantum theory must reducethe classical result.

Bohrs correspondence principle

is rather obvious:

Successes and Failures of the Bohr


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Model

The electron and

hydrogen nucleus

actually revolve

about their mutual

center of mass.

The electron mass is replaced

by its reduced mass:

The Rydberg constant for infinite nuclear mass,R, is replaced byR.

Success:

Limitations of the


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Limitations of the

Bohr Model

Works only for single-electron (hydrogenic) atoms.

Could not account for the intensities or the fine structure of

the spectral lines (for example, in magnetic fields).

Could not explain the binding of atoms into molecules.

Failures:

The Bohr model was a great

step in the new quantum

theory, but it had its limitations.

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