9/21/04 - S McGarry 1 of some large but finite number

Semiconductor Nanocrystal Quantum Dots

S. McGarrySept. 23, 2004

9/21/04 - S McGarry 2 of some large but finite number

Outline

1. Quantum Devices- Particle in a Box

2. Quantum Dots- Particle in a Sphere

3. Quantum Size Effect

4. Nanocrystal Growth

5. Biological Tagging

9/21/04 - S McGarry 3 of some large but finite number

Quantum Devices

9/21/04 - S McGarry 4 of some large but finite number

Quantum Well Review

Thin semiconductor layer(s) with reduced bandgapresult in quantum confinement

2

*

2

2

=

zn L

nm

Eπh

( ) ( ) ( ) ( )zEzzVdz

zd

mnψψψ =+− 2

2

*

2

2h

( ) ( )zEdz

zd

mnψ

ψ =− 2

2

*

2

2h

Time-independent Schrödinger eqn –

Infinite well –

Boundary conditions –

Eigenfunctions –

Energy levels (Eigenvalues) –

( )

=

znn L

znAz

πψ sin

( ) ( ) 00 == zLψψ

=22

2

*

11hdk

Ed

m

9/21/04 - S McGarry 5 of some large but finite number

Size Quantization Effect

Nann, T., Polymers and Adhesives in Microelectronics and Photonics, 2001. First International IEEE Conference on , 2001, 49-53

Molecules Nanoparticles Bulk Semiconductors

Bad Gap

Ener

gy

LUMO

HOMO

ConductionBand

ValanceBand

Density of states versus dimensionality

Available energy levels versus material type

9/21/04 - S McGarry 6 of some large but finite number

Single QW Semiconductor Structure

n=3

n=2

n=2

n=1

n=1

well bandgap energy, Eg

conductionband offset

valenceband offset

barrierbandgapenergy

Lz

exhn

enga BEEEE −++=

2

*

2

2

=

zn L

nm

Eπh

HRTEM of Single Quantum Well

+

-

Bohr radiusof exciton, aB

Exciton Orbit

9/21/04 - S McGarry 7 of some large but finite number

Bulk Free Space EA Structure

Optical modulator based on a PIN diode structure

p+-cap

i-active

n--contact

N- or SIsubstrate

P+-contact

Light in

Light out

Anode

Cathode

Generic Transmission EA Device Structure

Transmission Devices– grown doped layers are

transparent at operatingwavelength

– may be necessary to etch awaysubstrate in some materialsystems (i.e. GaAs/AlGaAs)

– waveguide devices more common

9/21/04 - S McGarry 8 of some large but finite number

Bulk Semiconductor Absorption

Absorption and band structure

9/21/04 - S McGarry 9 of some large but finite number

Bulk Semiconductor Absorption

Optical absorption is important for many types of semiconductor device– e.g. electroabsorption (EA) modulators– rely on change in absorption with applied reverse bias to a PIN diode structure– early EAs used the Franz-Keldysh Effect (FKE) in bulk material– Applied field causes band sloping →→ change in absorption edge

Vd=0

Vd=-5

Vd=-10

λop

abs.

ER

FKE EA Modulator Characteristic

hν

A

Photon Energy

Franz-Keldysh EffectTypical band structure of a direct-gap semiconductor

(Actually is a little more complicated - there are excitonic effects but Eb~4.2meV)

9/21/04 - S McGarry 10 of some large but finite number

MQW- EA (or SEED) Structure

Also based on a PIN diode structure

p+-cap

i-MQW

n--contact

N- or SIsubstrate

P+-contact

Light in

Light out

Anode

Cathode

Generic Transmission MQW-EA/SEED Device Structure

Transmission Devices– quantum wells grown using

MBE, MOCVD or CBE– reflection and F-P devices also

possible through mirror stackgrowth

9/21/04 - S McGarry 11 of some large but finite number

Quantum Well Confinment

Modern EAs (and SEEDs) utilize the Quantum-Confined StarkEffect (QCSE)

– stronger effect than FKE -> smaller interaction length required» still need many wells for a vertical device

– trade-off with wavelength sensitivity

Vd=0

Vd=-5

Vd=-10

λop

abs. ER

QCSE EA Modulator Characteristic

hν

A

Photon Energy

Ý Behaviour is a little more complex in the case due toquantum effects

Ý Note there are two regimes in which this can be used

9/21/04 - S McGarry 12 of some large but finite number

QCSE - Putting it all together

EgEω Eω’

Vapp=0 Vapp>0

Vd=0

Vd=-5

Vd=-10

λop

abs. ER

QCSE EA Modulator Characteristic

hν

A

Photon Energy

QCSE Band Tilt Shift

QCSE Exciton Energy Shift (also causes field ionization)

9/21/04 - S McGarry 13 of some large but finite number

Quantum Dots

9/21/04 - S McGarry 14 of some large but finite number

What are Quantum Dots

X Quantum dots are nanometer-sized semiconductorcrystals with size-dependent optical, physical, electronicand chemical properties

– highest degree of quantum confinement available

9/21/04 - S McGarry 15 of some large but finite number

Properties of Semiconductor QDs

ä Size-tuneable properties

ä Discrete optical exciton transitions

ä Large oscillator strengths & nonlinear response

ä Highly luminescent

ä Photochemically robust

ä Compatible with a variety of hostsä(e.g. SiO2, polymer, etc.)

ä Act as a “molecular semiconductor”

9/21/04 - S McGarry 16 of some large but finite number

Particle in a Sphere

9/21/04 - S McGarry 17 of some large but finite number

Particle in a Sphere

Effects seen in a spherical nanoparticle can be modeledas quantum confinement in a sphere

2

*

2

2

=

amE nl

r

nl

κh

( )( ) ( ) ( ) 011 222

2

=

−++− rRk

r

ll

dr

rrRd

rnl

nl

T-I Schrödinger eqn separable –

Radial T-I Schrödinger eqn –

Boundary condition –

Eigenfunctions –

Energy levels (Eigenvalues) –

( ) ( )( )nll

nllnl j

arj

arR

κκ

13

2+

=

( ) 0=aRnl

+=

***

111

her mmm

( ) ( ) ( )φθψ ,lmnlnlm YrRz =

9/21/04 - S McGarry 18 of some large but finite number

Particle in a Sphere

Spherical Harmonics

9/21/04 - S McGarry 19 of some large but finite number

P. K. SEN, J. T. ANDREWS, Superlattices and Microstructures, Vol. 29, No. 4, 2001

Rnl

r

goe Ea

eam

248.0786.12

22

*

2

−−

+=

εκωω h

hh 2

2

2 Bt

Ram

Eh=

boeob E∆−= ωω hh 2

Excitons in a Sphere

NC exciton energy -

NC biexciton energy -

where

+

-

Bohr radiusof exciton, aB

Exciton Orbit

+-

Biexciton “Molecule”

+-

+-

9/21/04 - S McGarry 20 of some large but finite number

Exitons in a Sphere - other issues

ä Optical phononsäpolar coupling depends on charge distribution

ä Acoustic phononsädeformation modes of the sphere - E~1/a

ä Phonon bottleneckädoes it really exists - none observed in II-VIs (Moire effect?)

ä Augeräa moderate optical powers biexciton-Auger can occur

äone high energy carrier ejected to the matrix

älong return time for ejected carrier (up to 10 min. observed)

äsignificant problem for NLO applications (including amps)

9/21/04 - S McGarry 21 of some large but finite number

Effect of confinement on mass*

ä For little or no confinement with a > aB

the Coulomb force dominates and theexciton acts as a single particle so:

a > aB a < aB

a → ∞

2

*

2

2

=

amE nl

r

nl

κh

***her mmm +=

***111

her mmm+=

ä For strong confinement with a < aB thewave functions of the electron and holeare decoupled by the dominantquantizations effect so:

9/21/04 - S McGarry 22 of some large but finite number

Quantum Size Effect

9/21/04 - S McGarry 23 of some large but finite number

Engineering Desired Properties

äControl of NC size determines

äLinear optical properties

äNonlinear optical response

äLuminescence wavelength

äElectrical properties

äEtc.

9/21/04 - S McGarry 24 of some large but finite number

Bohr diameter and mass*

Exciton Bohr diameters and band gap energies forvarious semiconductors.

Electron and hole masses for varioussemiconductors.

Remember - ballistic) (also 21 ,25 where ==∝ −∗impurityphononm ααµ α

T. J. Bukowski et al, Critical Reviews in Solid State and Materials Sciences, 27(3/4):119–142(2002)

9/21/04 - S McGarry 25 of some large but finite number

Quantum Well Review

Relative degree of bandgap energy shifts due

to quantumconfinement for several

direct and indirectsemiconductors.

Energy band diagrams for germanium, Ge,and cadmium telluride, CdTe.

Absorbance vs.wavelength for a Ge film

and for various Gequantum dots 150 Å, 46 Å,12 Å, and 4 Å in diameter.

T. J. Bukowski et al, Critical Reviews in Solid State and Materials Sciences, 27(3/4):119–142(2002)

9/21/04 - S McGarry 26 of some large but finite number

Quantum Confinement

9/21/04 - S McGarry 27 of some large but finite number

Finite Size Distribution - Polydispersity

9/21/04 - S McGarry 28 of some large but finite number

Nanocrystal Growth

9/21/04 - S McGarry 29 of some large but finite number

Island Growth of QDs

Structural characteristics of In Ga As–GaAs self-organized QD’s:(a) AFM image,(b) cross-sectional TEM image of single dot and its schematic,illustrating a near-pyramidal shape, and(c) XTEM image of 4 layers of vertically coupled dots with 15 Å ofGaAs barrier layers in between.

Typical ridge-guide structure containingarrays of InGaAs QDs clad in GaAs

grown by MOCVD.

BHATTACHARYA et al., IEEE JOUR OF SEL TOP IN QUANT ELECTS, VOL. 6, NO. 3, MAY/JUNE 2000

BORRI et al, IEEE JOUR OF SEL TOP IN QUANT ELECTS, VOL. 8, NO. 5, Sept/Oct 2002

9/21/04 - S McGarry 30 of some large but finite number

Chemical Synthesis of Nanocrystals

C. B. MURRAY ET AL. IBM J. RES. & DEV. VOL. 45 NO. 1 JANUARY 2001

(a) synthesize NC samples by high-temperature solution-phase routes - usually using hot surfactant, n-trctylphosphine oxide (TOPO)

(b) narrow the NC sample size distribution by size-selective precipitation

(c) deposit NC dispersions that self-assemble

(d) form ordered NC assemblies (superlattices) or disperse

9/21/04 - S McGarry 31 of some large but finite number

Colloidal Materials

äColloidal NCs are mostly II-VI materials

äCdSe, CdTe, CdS

äHgSe, HgTe, HgS

äPbSe, PbS

äZnS

äEtc.

äMost common - CdSe & CdTe

9/21/04 - S McGarry 32 of some large but finite number

Chemical Synthesis of Nanocrystals

Alivisatos, J. Phys. Chem., Vol. 100, No. 31, 1996

Transmission electron microscopy study of the growth of a CdS/HgS/CdS quantum dot quantum well. The micrograph of a CdS core cluster (a2) exhibits tetrahedral morphology which is in agreement with TEMsimulation (a3). The corresponding molecular model (a1) shows that all surfaces are Cd terminated (111). Picture (b) shows a model of the CdS particle after surface modification with Hg. A typical micrograph of atetrahedral CdS/HgS/CdS nanocrystals is shown in (c2) along with a corresponding model (c1). Model (d1) and micrograph (d2) represent a CdS/HgS/CdS nanocrystal after winned epitaxial growth. The arrowmarks the interfacial layer exhibiting increases contrast due to the presence of HgS, in agreement with the simulation (d3). No contrast is seen in a simulation of a model with all Hg replaced by Cd (d4).

9/21/04 - S McGarry 33 of some large but finite number

CdSe Nanorods and Fractal Nanocrystals

Evolution from CdSe fractals to nanorods based on reaction temperature

XRD patterns of CdSe nanocrystals

HRTEM images of CdSe fractals:

(a) a typical fractal tip

(b) crossed branches

(c) partial enlarged detail of two crossedbranches in the black frame of b.

Qing Peng et al, Inorganic Chemistry, Vol. 41, No. 20, 2002

9/21/04 - S McGarry 34 of some large but finite number

Dispersal of NCs in Opal Matrix

S.V. Gaponenko et al., Journal of Luminescence 87-89 (2000) 152-156

Opal structure - spheres representvoids connected by channels

Modification of the spontaneous emission of CdTe nanocrystals embeddedin opal. Nanocrystal mean diameter is 2.4 nm.(a) Optical reflection spectrum of opal sample(b) modified spontaneous emission spectrum of CdTe nanocrystals in opal(c) reference emission spectrum of CdTe nanocrystals in free space.

9/21/04 - S McGarry 35 of some large but finite number

Growth of CdTe Nanocrystals

Absorption spectra (normalized to the firstabsorption maximum, except for 5 and 6.5 min)

Emission spectra (normalized to the emissionmaximum, λex = 400 nm)

Average size of CdTe QDs (diluted in toluene)taken for a synthesis at 165 C at different timeintervals.

All spectra recorded at room temperature.

S. F. Wuister et al, Phys. Chem. Chem. Phys., 2003, 5, 1253–1258

- synthesized in a mixture of TOP and DDA- fast initial growth is observed in the first 30 min followed by a slower growth to the final particle size- luminescence lifetimes of up to approximately 10 ns

9/21/04 - S McGarry 36 of some large but finite number

Biological Tagging

9/21/04 - S McGarry 37 of some large but finite number

An Array of QD Luminescent Tags

Ten distinguishable emission colors of ZnS capped CdSeQDs excited with a near-UV lamp. From left to right (blue to

red), the emission maxima are located at:

443, 473, 481,500, 518, 543, 565, 587, 610, and 655 nm.

Luminescent quantum dots for multiplexed biological detection and imaging Chan et al.

9/21/04 - S McGarry 38 of some large but finite number

QDs vs Dye Molecule Tags

ä Protein coated QDs very stable (>2yrs)

ä Narrow spectral width (FWHM~25nm)

ä Broad excitation spectrum

ä High quantum yields (40-50%)

ä High cross-section

ä Low photobleaching

ä Compared to rhodamine 6G

ä 20x brighter

ä 100-200x more stable

Excitation

Emission

ZnS-capped CdSe QDs grown in TOPO

9/21/04 - S McGarry 39 of some large but finite number

Attaching the Tag

Tagging can be accomplished in anumber of ways:

(a) Use of a bifunctional ligand linking QDs tobiomolecules

(b) TOPO-capped QDs bound to a modifiedacrylic acid polymer by hydrophobicforces.

(c) QD solubilization and bioconjugation usinga mercaptosilane compound

(d) Positively charged biomolecules are linkedto negatively charged QDs by electrostaticattraction

(e) Incorporation of QDs in microbeads andnanobeads

Micro/nanobeads can individually carry a“code” to expand tag range

9/21/04 - S McGarry 40 of some large but finite number

QD Flourescent Tags in Action

Fluorescence micrograph of amixture of CdSe/ZnS QD-taggedbeads emitting single colorsignals at:

484, 508, 547, 575, and 611 nm

Fluorescence imaging of folate-conjugated QDsinside human cancer cells.

(a) Brightfield image of control KB cell (withoutQDs).

(b) KB cell incubated with folate-conjugated QDs.

(c) KB cell incubated with bovine serumalbumin-conjugated QDs.

Receptor-mediated endocytosis occurs onlywhen the QDs are conjugated to folic acid, whichis recognized by folate receptors overexpressedon the surface of cancer cells.

(a) (b) (c)

9/21/04 - S McGarry 41 of some large but finite number

Sorting with DNA

Gerion et al., J. AM. CHEM. SOC. 9 VOL. 124, NO. 24, 2002

The fluorescence of the solution(in black) is the superposition ofthe fluorescence of four differentDNA-nanocrystal samples. Allspectra are normalized.

The fluorescence spectrum ofthe squares (color) showssignificant narrowing comparedto that of the solution (black),and each set of squares has acharacteristically differentspectrum.

The same solution is exposed tofour substrates, each beingactivated with a differentoligonucleotide. The goldpatterns exhibit a strongfluorescence with a minimalbackground signal. The capturetime is 5 s.

9/21/04 - S McGarry 42 of some large but finite number

Conclusions

• A very cross-disciplinary field requiring a varietyof scientific and engineering expertise

• Many exciting applications emerging in a numberof disparate fields

• enhanced optoelectronic devices• NLO “all-optical” elements• Biological cross-over applications• etc.

• Semiconductor nanocrystals/QDs now “ready forprime-time” in some applications