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Quantum Dots - Artificial Atoms

Paul MulvaneyUniversity of Melbourne

http://www.nanoparticle.com

School of Physics Public Lecture Series

University of Melbourne11th July, 2008

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Building Blocks of 21st Century Materials

TL: Atom assembly using STMTR: Nanotube Gears

BL: Quantum DotBR: Fullerene “Current”

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The Hydrogen Atom

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The Bohr Modelr =

n2h2εo

πme2Un =−me4

8n2h2εo2 =

−13.6n2

• Hydrogen structure can be understood using the Bohr model, which is classical.

• Energies of an H atom are fixed by universal constants so H atoms are the same anywhere.

• H atoms could emit any colour if we could tweak these constants - BUT, atoms are Untweakable!

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Dielectric Constant

V

+Q -Qd

V =Ed =Qd

Aεrεo

+

+

+

+

+

+

+

+

+

-

--

----

---

+

-

-

-

-

- +

+

+

+

+

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An Exciton is an H atom in a crystal..

H+

e-

En =-me4

8n2h2εo2 =

-13.6n2 eV

En =-me4

8n2h2εr2εo

2 ~-13.6εr

2n2 eV

Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2- Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+ Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2- Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+ Cd2+Se2-Cd2+Se2-Cd+ Se2-Cd2+Se2-Cd2+Se2- Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+ Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2- Se2-Cd2+Se2-Cd2+Se2-Cd2+Se- Cd2+Se2-Cd2+ Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2- Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+ Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2- Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+Se2-Cd2+

r =

n2h2εrεo

πm* e2 : 3−30nm r =

n2h2εo

πme2 : 0.05nm

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Excitons are formed with Light Just less than the band Gap

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8 λ =

hmv

Particle in a box – Quantization of Energy

En =h2n2

8mL2

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Artificial Atoms by Playing with The Box Size

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1 10 100NanoCrystal Radius (nm)

CuCl

ZnSe

CdS

CdSe

GaAs

a > 8 nm (bulk)

a < 2 nm a < 3 nm

E=E

g+

π 2h2

2me*a2

+π2h2

2mh*a2

-1.786e2

4πεrεoa

Eg

exciton

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Nucleation - The Final Frontier

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Nucleation - The Final Frontier

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A photograph of the sound barrier being broken by a US Navy Jet as it crosses the Pacific Ocean at the speed of sound just 75 feet above the water. Condensation of water is caused

by the rapid expansion and consequent adiabatic cooling of air parcels induced by the shock (expansion/compression) waves caused by the plane outrunning the sound waves in front of it. Shot by John Gay from the top of an aircraft carrier. The photo won First Prize in

the science and technology division of the World Press Photo 2000 contest.

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Nothing Nucleates smaller than ~1-2nm.

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CdSe Nanocrystals

HRTEM CdSe Nanocrystal(Courtesy M Bawendi, MIT)

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Large brown-black, single crystals of CdSe grown for optics and electronics industry

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The Big Bang - Nucleation of Semiconductor Nanocrystals

Cd(oleate)2 + TOPSe + Oleic Acid (CdSe)NC(oleate)x Ksp

1. Bullen, C.R. et al. “Nucleation and Growth Kinetics of CdSe Nanocrystals in octadecene”, Nanoletters 4, 2303-7 (2004).

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Slowing Down the Big Bang by a Factor of Million

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Quantum Size Effects produces“Artificial Atoms”

1.6 nm

6 nm

CdSe passivated with dodecylamine in chloroform

PL Abs

5nm bar

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From UV to Visible to NIR…

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Michalet et al. Science 307, 538 (2005)

•Size tunable emission from the UV to NIR•Continuous excitation spectra

•Narrow emission spectra

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Single QD Spectroscopy

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There’s Something Wrong with our Atoms

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Time Dependent PL of a single Nanocrystal:

• Photoluminescence randomly cycles between periods where there is no emission (OFF) and periods with “fluctuating” intensity levels.

• Phenomenon not restricted to CdSe: CdS, InP, Si, PbSe and various molecular systems

• Characterized by the statistics of “On / Off times”

tON tOFF

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Power laws and distributed kinetics

• In molecules, off state is a single well defined level

• It leads to exponential on/off time histograms1

• In nanocrystals the power laws imply “distributions” of states

• Hypothesis: Distribution of surface states

1. R. J. Cook and H. J. Kimble, Phys Rev Lett, 54, 1023 (1985)

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Pauli’s Principle (1900-1958)

“ God made solids…

…but surfaces

were the work of the devil…”

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Surface “passivation” and blinking:CdSe

• Adsorption of Octylamine (OA) enhances the photoluminescence of CdSe nanocrystals•Reduces the probability of “long” off times•Increases the length of on-times

Saturation of surface trap states Saturation of surface trap states Is not possible unfortunately due Is not possible unfortunately due to steric limitations.to steric limitations.

CdSe

CdSe/OA

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hmv

Particle in a box – Quantization of Energy

En =h2n2

8mL2

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Carrier Escape in Spherical QW

ZnS CdSe ZnSCdS CdSe CdS

Hole is trapped but electron can escape Both charge carriers confined

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Wavefunction Engineering

Electron wavefunctionHole wavefunction

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Hofstadter’s Law (1945-)(author GEB, Pulitzer Prize 1980)

“It always takes longer than you expect …

… even when you take into account Hofstadter’s Law …”

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...3 years later…

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Monodisperse CdSe/CdS/ZnS Nanocrystals

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Type I heterostructures:CdSe vs CdSe@CdS

• Length of on – times is extended• Emission pattern tends towards “binary” blinking

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Almost Perfect..

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Ensemble: 60% QY, 488nm excitation, 5ms bin-time, ~ 1 photon /QD/100ns.

Review: Goméz, Califano, Mulvaney, PCCP 8, 4989-5011 (2006)

Blinking CdSe-CdS-ZnS on glass in air (590nm core)

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Properties of QDs• High molar extinction coefficient• Resistance to photobleaching• Long fluorescence lifetime• Excitation by simple UV source• Sharper spectral emission - multiplexing• Similar surface chemistry and conjugation for all colours

Wu, X. et al (2003) Nat Biotech 21, 41

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Non-Invasive Imaging

Gao, X. et al (2004) Nat Biotechnol 22, 969

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Solid-state Lighting• Lighting consumes 22% of electricity in the USA

• DOE is targeting 50% reduction in power consumption by 2020 – savings of 525 TW-hr/year ($35 Billion/year savings)

Incandescent bulb 17 lumens per watt

Fluorescent bulb 50 lumens per watt

Current generation white LED 40-50 lumens per watt

Next generation white LED (under development)

150-200 lumens per watt

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Economics of LED lighting

• Installation of LED lighting is approx 100x the cost of incandescent bulbs

Incandescent bulb $0.002 per lumen

Current generation white LED $0.2 per lumen

• Cost arises from the high cost of semiconductor wafer production – requires stringent vacuum processing conditions

• Typical LED chip has dimensions of less than 1 mm2

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Photoluminescence

UV visible

Electroluminescence

PL vs EL

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QD LEDs

Mueller, A.H., et al., MulticolorLight-Emitting Diodes Based on Semiconductor Nanocrystals Encapsulated in GaN

Charge Injection Layers. Nano Letters, 2005. 5(6): p. 1039-1044.

Mashford et al. Aust. Prov Pat 2008

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OLEDs QD-LEDs

• More than 1500 OLED patents filed since 1987

• Colour range arises from different molecules – varying degrees of stability

• Emission FWHM ~ 100-150 nm

• Approx 20-30 patents filed since 1996

• Colour range arises from single material – tunable by size

• Emission FWHM ~ 50 nm – highest colour purity for display applications

Typical OLED emission profile Our QD-LED emission profile

Head to Head

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Conclusions

• Quantum Dots are a superb example of quantum mechanics in action.

•Semiconductor nanocrystals have energy atoms smilar to those in the hydrogen atom and hence are called “artificial atoms”.

•Combining QM with improved materials design and synthesis also leads to better control of the interactions of light with matter.

•Thanks to: Prof Jamieson, Jude Prezens, Roger Rassool, Nick Nicola,

Ben Mashford, Daniel Gómez and Dr.Tich Lam Nguyen and to you the

audience..