quantum dots, nanowires and nanotubes

8/8/2019 Quantum Dots, Nanowires and Nanotubes

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NCLT July 15th, 2005

Nanomaterials -Quantum Dots,Nanowires and Nanotubes

Tim Sands

Materials Engineering and Electrical &Computer Engineering

Birck Nanotechnology CenterPurdue University

100 nm

Si/SiGe nanowires grown by the vapor-liquid-solid mechanism - Y. Wu et. al., Nano Lett 2:83 2002

Note: The diameter of these relatively large nanowires is 1/1,000th the diameter of a human hair!


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Questions

What is a quantum dot? What is a nanowire?What is a nanotube?

Why are they (scientifically) interesting? What are their potential applications?

How are they made?

???


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Q: What is a quantum dot?

A: Any solid material in the form of a particlewith a diameter comparable to thewavelength of an electron (more later)

CdSe nanocrystals; Manna, Scher andAlivisatos, J. Cluster Sci. 13 (2002)521


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Q: What is a nanowire?

A: Any solid material in

the form of wire withdiameter smaller thanabout 100 nm

Transmission

electronmicrograph ofan InP/InAsnanowire

(M.T. Bjork et.al., Nanoletters,2:2 2002)


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Q: What is a nanotube?

A: A hollow nanowire, typically with a wall thickness onthe order of molecular dimensions

The smallest (and most interesting) nanotube is thesingle-walled carbon nanotube (SWNT) consisting of a

single graphene sheet rolled up into a tube

ScanningTunneling

Micrograph of asingle-walledcarbon nanotubeandcorrespondingmodel (Dekker)


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Q: How big is a nanometer?

1 billionth of a meter

1/50,000 the diameter of a human hair

40% of the diameter of a DNA molecule

5 times the interatomic spacing in a siliconcrystal

1 nm

PolySi image: C. Song, NCEM


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Dust mite~500 m

Ant~5 mm

Human hair10-50 m dia.

Fly ash~10-20 m dia.

Red blood cells

with white cell

2-5 m dia.

~10 nm dia.

ATP synthesis

DNA

2.5 nm dia. Atoms in silicon

0.2 nm spacing

Quantum corral of 48 iron atoms on

copper surface positioned one at a time

with an STM tip - Corral diameter 14 nm

Carbon nanotube

~2 nm diameter

Nanotube devices (C. Dekker)

Red blood cellsPollen grain

Microelectromechanical devices10-100 m wide

Head of a pin

1-2 mm

Assemble nanoscale

building blocks to make

functional devices,

e.g., a photosynthetic

reaction center with

integral semiconductor

storage

1 nanometer (nm)

1000

nanometers=1 micrometer(m)

Visiblespe

ctrum

0.1 nm

0.01 m10 nm

0.1 m100 nm

0.01 mm10 m

0.1 mm100 m

1,000,000

nanometers=

1 millimeter(mm)

1 cm

10 mm

TheNanoworld

TheMicroworld

10-2m

10-3

m

10-4m

10-5m

10-6m

10-7m

10-8m10-8m

10-9m

10-10m

Adapted from: NRC Report: Small Wonders , Endles s Frontiers: Review of the NationalNanotechnology Initiative (National Res earch Council , July 2002)


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Q: What makes nanostructured

materials (scientifically) interesting? Electronic & optical properties

Nanowires and nanotubes are the most confining electricalconductors - puts the squeeze on electrons

Can be defect free - electrons move ballistically

Quantum confinement - tunable optical properties Mechanical properties

Small enough to be defect-free, thus exhibiting ideal strength

Thermal properties Can be designed to conduct heat substantially better (or

much worse) than nearly every bulk material Chemical properties

Dominated by large surface-to-volume ratio

Nanowires and Nanotubes are New Materials!


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Electronic and Optical Properties

Quantum Confinement


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Electrons in solids

Core shell electrons are tightly bound to atoms, anddo not interact strongly with electrons in other atoms

Valence shell electrons are the outer electrons thatcontribute to bonds between atoms

core

valence

conduction

Electron

energ

y

If all of the bonds are satisfiedby valence electrons, and if

these bonds are strong, thenthe material does not conductelectricity - an insulator


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Electrons in solids

If all the bonds are satisfied, but the bonds are relativelyweak, then the material is an intrinsic semiconductor, andthermal energy can break a small number of bonds, releasingthe electrons to conduct electricity

core

valence

conduction

Ele

ctron

energy


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Electrons in solids

If impurities with one more or one fewer electrons than a hostatom are substituted for host atoms in a semiconductor, thenthe material becomes conductive - an extrinsic semiconductor

core

valence

conduction

Ele

ctron

energy


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Electrons in solids

If only a fraction of the bonds are satisfied (or alternatively, ifthere are many more electrons than are needed for bonding)then there is a high density of electrons that contribute toconduction, and the solid is a metal

core

valence

conduction

Ele

ctron

energy


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Electrons in nanostructured materials

Electrons can be considered to be both wavesand particles

Particle: momentum = mass x velocity (p =mv)

Wave: momentum = Plancksconstant/wavelength (p = h/

Particle or wave: kinetic energy = p2/2m


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Electrons in nanostructured materials

An electron (particle) can be described as a wave packet, i.e.the sum of sinusoidal waves, each with a slightly differentwavelength

Electron Wave Packet

-10

-8

-6

-4

-2

0

2

4

6

8

10

12

-30 -20 -10 0 10 20 30

Position

Amplitude

ofWavefunction

Sum of 10 cosinewaves with slightlydifferent wavelengths


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Electrons in nanostructured materials Each wave (wavefunction) is analogous to the shape of a

vibrating string (or jump rope).

If we hold two ends of the string, we constrain the string tovibrate within an envelope that contains an integer number ofhalf wavelengthsa standing wave.

Each half-wavelength exists between two nodes - points thatdo not move

The shorter the wavelength (shorter string or more nodes), themore energy it takes to vibrate the string

Envelope Function

-1.5

-1

-0.5

0

0.5

1

1.5

0 0.5 1 1.5 2 2.5 3 3.5

Position

Amplitude

Envelope Function

-1.5

-1

-0.5

0

0.5

1

1.5

0 1 2 3 4 5 6 7

Position

Amplitude


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Optical properties

Decreasing the size of a nanostructuredmaterial increases the energy difference, E,

between allowed electron energy levels

When an electron drops from a higherenergy state to a lower energy state, aquantum of light (photon) with wavelength, = hc/E may be emitted

LargerE implies shorter wavelength (blueshifted)

h is Plancks constant; c is the speed of light


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Example: Quantum Dots

CdSe nanocrystals; Manna, Scher andAlivisatos, J. Cluster Sci. 13 (2002)521

Left- absorption spectra ofsemiconductor nanparticles ofdifferent diameter (Murray, MIT).Right- nanoparticles suspended insolution (Frankel, MIT) - National

Science Foundation Report, SocietalImplications of NanotechnologyMarch 2001


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but what is the wavefunction?

Unlike the case of the vibratingstring (or jump rope), the electronswavefunction is a mathematicalabstraction that is not tangible

The square of the wavefunction,however, represents the probability

distribution describing the likelihoodof finding the electron at aparticular location

Multiplying the square of thewavefunction by the charge on anelectron, we have the chargedistribution

Charge density

Amplitude


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How does quantum confinement

work in a nanowire? If a nanowires diameter is comparable to the

typical wavelength of a conduction electronin the bulk material, then the nanowire

becomes a quantum wire, and the electronsare squeezed in two dimensions, but canmove freely in the third


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The carbon nanotube.neither diamond nor graphite

M. Terrones, Annual Rev, Mater. Res. 33 (2003)419

Nanotubes of different flavors (Charlier)

Scanning TunnelingMicrograph of a single-walled carbonnanotube andcorresponding model(Dekker)

1.4 nm diameter single-walled carbonnanotube across two electrodes (Dekker)


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Applications of nanostructured materials

Quantum dots as fluorescent markers for biomedicine

Nanotubes as structural reinforcement in compositematerials

Nanowires and nanotubes as conducting channelsfor future electronics

Nanowires for tunable lasers and light-emittingdiodes

Quantum dots, nanotubes and nanowires for high-surface area sensors

.


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Dont forget Nanopants

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or nano-tennis racquets

Free Babolat Tee Shirt AND a $20 Instantrebate on any Nanotechnology racket for alimited time only!

New high Modulus Carbon Graphite Carbon Nanotube

Headsize:118 sq. in. * Length:28 in. * Weight (strung):9.3 oz.* Stiffness (Babolat RDC):73 * Balance:14.63 in. Head Heavy *

Cross Section: 28mm Head/26mm Throat * Swingweight: 326

kg*sq. cm * "This year, Babolat introduced two Nano Carbon

Technology, or NCT, racquets, the Tour and the Power. The Tour,

above is geared toward high-level intermediates while the Power

(below) is for those with short strokes. Both models are reinforcedin the throat and the bottom half of the head with carbon nanotubes,

strings of carbon molecules constructed at the nano level, which

are five times stiffer than graphite. Net result: Babolat is able to

increase the stiffness and power of the racquets without adding

weight."Racquet Sports Industry.


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Application to nanoelectronics


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Si is already in the nano realm

From Extending Moores Law in the Nanotechnology Era by Sunlin Chou (www.intel.com)


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How far can we go with silicon?



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Future Nanotechnology



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How are nanostructured materials made?

Key challenge: high surface-to-volume ratio

Surface contributes excess surface freeenergy per unit area

Lowest free energy state for a material with acubic crystal structure is a single facetedspheroid

Must exploit kinetics (rates of reactions atsurfaces) to make small structures and/orstructures with very large aspect ratios


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One approach: catalysis

Au Au-Si

SiH4 H2SiH4 H2

SiH4 H2

Saturation/

nucleationEutectic

formation

Catalyst

deposition

Si nanowire

GrowthVapor-Liquid-Solid Epitaxy

Wagner & Ellis, 1964

Yang, P. et al., Chem.

Eur. J.8:6 (2002)

ZnO nanowires by VLS

A similar catalytic approach is used to make carbon nanotubes


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Questions and Answers

Societal impact of Nanotechnology (question is notaudible because the videotape was changed)

Carbon dispersed when making bucky balls

What is the protocol in the labs at Purdue when

carbon is extracted? How is the carbon nanotube/wire made?


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Safety Issues

If DNA is used to relay information, what keeps itfrom degrading along the way?

Mutations of DNA because of stray engery

Bases and backbones

Strength of carbon nanotubes


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Covalent bonds

Bond differences

Deformation mechanisms

Electricity and crystal deformation


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Materials science and engineering

Teaching materials science

Whats the big hold-up on the white LED?

What goes bad in a crystal to make it die out?

The continuous spectrum of the Sun


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Black-body spectrum & radiation

What is a fundamental way to explain the origin ofblack-body radiation?


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Nanostructuring in the 3rd dimension

By changing the vapor precursors during VLSgrowth, single crystal nanowire heterostructures canbe grown

InAs/InP nanowire (false colorscorrespond to different crystal latticespacings)

M.T. Bjork et. al., Nanoletters, 2:2 2002


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Another approach: nanoporous templates

top view

500 nm500 nm

side view

Pore bottoms

Porous anodic alumina (PAA) with 50 nm diameter pores on 100 nm centersproduced by anodizing Al foil at 40V at 4C in oxalic acid


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Pores can be filled by electrodeposition

Crystalline single-phase Bi2Te2.6Se0.4nanowires electrodeposited into 50nm pores in anodic alumina[Martn-Gonzlez et al. Nanoletters 3(2003) 973]

Au/Ag/Au nanowires produced byelectrodeposition into porous anodic alumina[Manuel DaSilva, unpublished]

Pt

Au

Au

Ag

PAA

2 m

100 nm


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quantum dots, nanowires and nanotubes

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