Colloidal Nanocrystals: Synthesis, Properties, Applications ...

Colloidal Nanocrystals: 

Synthesis, Properties, Applications, Perspectives 

Liberato Manna 

NNL-INFM, Lecce (Italy) 

University of California, Berkeley (USA) 

Center for Nanoscience at LMU, Munich (Germany) 

1

Outline 

•Colloidal nanocrystals: synthesis, scaling laws and optical properties 

• Growth kinetics: spherical nanocrystals 

•Shape control : CdSe nanorods (synthesis, optical properties, self-assembly) 

•Shape + phase control: CdTe tetrapods (synthesis, optical properties) 

•Shape + phase + composition control: nanojunctions, inorganic dendrimers, 

coupled dots 

• Applications and Perspectives: nanoelectronics, optoelectronics, 

photovoltaics, biology 

2

P =O 

P= O 

3 

“Physical” and “chemical” approaches to nanostructures 

Electrically defined nanostructures lithographically defined nanostructures 

a1) gates 

a2) 

metal 

GaAs AlGaAs 

2DEG 

one individual 

quantum dot 

b) c) 

one individual 

quantum dot 

P=O 

O=P 

O=P 

P=O 

O=P 

O=P 

P=O 

P=O 

P=O 

P=O 

P = O 

P=O 

P= O 

P=O 

P=O 

P=O 

P=O 

P=O 

P=O 

GaAs 

P =O 

P =O 

O= P 

O=P 

O=P 

O=P 

O= P 

P=OO= P 

P=O 

O=P 

O=P 

P=O 

P=O 

P=O 

P=O 

O=P 

O=P 

P =O 

P=O 

InAs 

P=O 

P=O 

P=O 

P=O 

O=P 

O=P 

P=OO=P 

P=O 

P=O 

P=O 

P=O 

O=P 

O=P 

O=P 

GaAs 

Self-assembled islands Solution grown nanocrystals 

Molecular Beam Epitaxy 

of Quantum Structures

Synthesis of colloidal nanocrystals 

Ar 

Ar 

thermocouple 

Inject 

organometallic 

precursors 

P=OCd 

P=O Cd 

P Se 

P=O 

P=O 

P=O 

P=O 

“high T,” 250-350 C 

P=OCd 

P=O 

P=O 

P=O 

P=O 

Heating mantle 

350 set 

348 cur 

temperature 

controller 

P Se 

P=O 

P=O 

P=O 

Mixture of 

surfactants 

Highly crystalline 

Narrow size 

distribution 

4

Nanocrystal size control: 

Colloidal nanocrystals of different materials 

ZnSe 

PbSe 

Fe 2 O 3 CoPt 3 

5

A wide field view of 7nm diameter surfactant capped CdSe nanocrystals 

6

Co nanocrystals-9nm-monolayer 

7

Co nanocrystals-9nm-bilayer 

8

A comparison of defects 

in extended solids and nanocrystals 

Nonequilibrium 

grain 

boundary 

Equilibrium 

vacancy 

•One defect can affect an entire 

bulk solid 

•On average, nanocrystals contain 

no equilibrium defects 

•Easier to anneal out nonequilibrium 

defects in nanocrystals 

9

Some SCALING LAWS for nanocrystal properties 

Energy level spacing (band gap) ~1/r^2 

Density of states, oscillator strength (~1/V) 

Charging energy (~1/r) 

Melting temperature (~1/r) 

Magnetic relaxation time (~N) 

Control of size and shape will be an 

important variable 

in the design of new materials 

% Surface atoms 

80 

60 

40 

20 

0 

CdSe 

0 5 10 15 20 

Size (nm) 

10

Quantum Confinement 

• Particle-in-a-box: E ∝ 1 / R 2 

Absorbance (a.u.) 

25Å 

35Å 

45Å 

CdSe 

PL Intensity (a.u.) 

energy 

p 

sp 3 σ * 

conduction 

band 

energy gap 

300 500 700 

Wavelength (nm) 

s 

atom 

σ 

molecule 

quantum 

dot 

bulk solidstate 

body 

valence 

band 

number of connected atoms 

11

Size Dependent Photoluminescence of Semiconductor Nanocrystals 


1780 1030 730 560 

InAs InP CdSe 

460 

60Å 46 36 28Å 

46Å 40 35 30 46 36 31 24 21Å 

0.7 1.2 1.7 2.2 2.7 

Photon Energy (eV) 

Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P., Science 1998, 281, 2013-2016. 

12

Thermodynamics and Kinetics of colloidal nanocrystal growth 

∆G 

growth 

= 

( µ − µ ) − k T ln c + γdA 

cryst 

K 

D ⎛ 1 1 

Growth rate = dr dt = ⎜ − 

∗ 

r ⎝ r r 

sol 

r* = critical size, depends on c 

B 

⎞ 

⎟ 

⎠ 

c is large, r* is small, so small crystals are stable 

and they will grow 

c is small, r* is large, so small crystals become 

unstable and they will dissolve 

13

Time dependent nanocrystal growth kinetics 

A) Just After Injection 

Broadening 

zone 

Narrowing 

zone 

You are here 

dr 

dt 

K 

⎛ 1 

⎜ 

⎝ r 

D 

1 

= 

∗ 

r 

⎞ 

− ⎟ 

r ⎠ 

Growth Rate 

+ 

0 

r * 

3r * 5r * 7r * 9r * 

- 

_ 

Nanocrystal Size 

High Monomer Concentration = Small Critical Size r* 

r* is small 

B) Well After Injection 

Broadening 

zone 

Narrowing 

zone 

dr 

dt 

K 

⎛ 1 

⎜ 

⎝ r 

D 

1 

= − 

∗ 

r 

r 

⎞ 

⎟ 

⎠ 

Growth Rate 

+ 

0 

- 

_ 

You are here 

r * 3r * 

5r * 

Nanocrystal Size 

Low Monomer Concentration = Large Critical Size r* 

r* is large 

14

Std.Dev (%) Ave. Size (nm) 

6 Focusing Defocusing 

4 

14 

6 

Manipulation of the growth kinetics: Size Distribution Focusing 

Refocusing 

injection 

Time 

injection 

15

Epitaxial growth of Stable, Highly Luminescent Core/shell Nanocrystals 

CdS 

CdSe 

e - 

h + 

0.27eV 

0.51eV 

Absorbance(a.u.) 

2.5 

2.0 

1.5 

1.0 

0.5 

7x 

40x 

0.0 

1.8 

2.2 2.6 

3 3.4 

Photoluminescence(a.u.) 

84% 

35% 

14% 

2% 

1% 

%QY 

Photon Energy (eV) 

16

Nanocrystal shape control: CdSe nanorods 

TOPO 

+ 

Hexylphosphonic acid 

(HPA) 

+ 

Tetradecylphosphonic acid 

(TDPA) 

Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E., Kadavanich, A. P. Alivisatos, 

Nature 2000, 404, 59-61 

Manna, L.; Scher, E. C.; Alivisatos, A. P., J. Am. Chem. Soc. 2000, 122, 12700-12706. 

17

Monolayer of 4x40 nm CdSe nanorods 

18

Nanorod Growth Mechanism 

Anisotropic 

Growth 

(001) 

Se 

Cd 

(001) 

Manna, L.; Scher, E. C.; Alivisatos, A. P., J. Am. Chem. Soc. 2000, 122, 12700-12706. 

19

XRD of dots and rods 

c axis 

002 110 

sphere 

rod 

20

Evolution of optical properties in rods 

100 nm 

1.4 

1.4 

1.4 

1.2 

1.2 

1.2 

1.0 

1.0 

1.0 

AB/PL (AU) 

0.8 

0.6 

AB/PL (AU) 

0.8 

0.6 

AB/PL (AU) 

0.8 

0.6 

0.4 

0.4 

0.4 

0.2 

0.2 

0.2 

0.0 

500 

550 

600 

650 

700 

0.0 

500 

550 

600 

650 

700 

0.0 

500 

550 

600 

650 

700 

Waveleng(nm) 

Waveleng(nm) 

Waveleng(nm) 

Hu, J. T.; Li, L. S.; Yang, W. D.; Manna, L.; Wang, L. W.; Alivisatos, A. P., Science 2001, 292, 2060-2063. 

21

Polarized Emission from Single Nanorods 

I // 

I ⊥ 

0° 180° 

0.8 

Beam Splitting crystal 

0.6 

0.4 

Polarization 

0.2 

0.0 

-0.2 

-0.4 

-0.6 

-0.8 

20 

40 

60 

80 

100 

120 

140 

160 

180 

Detection Angle 

p = (I ⊥ -I // ) / (I ⊥ + I // ) 

Hu, J.; L.-S. Li,: Yang, W.; Manna, L.; L.-W. Wang; A.P. Alivisatos, Science, 2001, 2060-3 

22

3.7x18.5 nm CdSe Rod Monolayer -“Smectic Phase” 

23

Self-assembly of CdSe nanorods: Liquid Crystals 

Crossed polarizers 

Nanorod solution 

sealed in a capillary tube 

• Nucleation and Growth of Liquid Crystals 

20µm 

Polarized Microscopy 

• Schlieren pattern from CdSe nanorod 

Liquid Crystal 

20 µm 

20µm 

L.-S. Li,: J. Walda, L. Manna, A.P. Alivisatos, Nanoletters, 2002 (2), 557-560 

24

3.7x18.5nm 

TEM of “liquid crystalline phases” 

4x40nm 

N 

e 

m 

atic 

S 

50 nm 

m 

ectic 

N 

ematic 

S 

mectic 

50 nm 

3.7x18.5nm 

50 nm 

50 nm 

4x40nm 

25

Spontaneous Assembly of Ribbons of Magnetic Nanorods 

Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P., Science 2001, 291, 2115-2117. 

26

Core/shell CdSe/CdS/ZnS nanorods 

Cores 

3.3 x 22.8 nm 

Core/shell 

4.4 x 24.2 nm 

Higher ratio of Cd:Zn in first 

few monolayers of shell 

CdSe 

cores 

CdS-rich 

shell 

ZnS-rich 

shell 

Core/shell 

6.0 x 27.0 nm 

Core/shell 

7.3 x 29.8 nm 

Manna, L.; Scher, E. C.; Li, L.S.; Alivisatos, A. P., J. Am. Chem. Soc. 2002, 124, 7136-7145 

27

c axis 

X-ray diffraction studies of the 

stress and strain in core-shell nanorods 

compression 

CdSe bulk 

CdSe core 

3-4 nm 

compression 

Thin shell 

20-40 nm 

Med. shell 

Thick shell 

ZnS bulk 

Manna, L.; Scher, E. C.; Li, L.S.; Alivisatos, A. P., J. Am. Chem. Soc. 2002, 124, 7136-7145 

28

Nanocrystal phase + shape control: tetrapods 

Zinc-blende (ZB) 

Wurtzite (WZ) 

Manna, L.; Milliron, D., Meisel; A., Scher, E; Alivisatos, A.P. Nature Materials, 2(6), 382-386 

29

Controlled growth of CdTe tetrapods 


30

The band gap depends on the arm’s diameter 


31

Nanocrystal phase + shape + composition control 

Growing more complex nanostructures 

Cd-VI rods and tetrapods 

growing facet 

growing facet 

Nano bar codes 

Coupled quantum dots, artificial dipoles, inorganic dendrimers 

32

Another example: CdS/PbSe heterostructures 

cb 

cb 

cb 

vb 

vb 

50 nm 

PbSe 

vb 

CdS 

PbSe 

PbSe 

CdS 

PbSe 

Kudera, S.; Parak, W; Casula, M.; Manna, L in preparation 

33

Nanocomposites 

•Reinforced polymers 

•Wear resistant materials 

•Flame retardant materials 

•Self-cleaning surfaces 

Environmental science 

•Degradation of pollutants 

•Sensors 

Chemical industry 

•Catalysis 

•Additives 

Colloidal 

Nanocrystals 

Biomedical research 

•Biological labeling 

•Biosensing 

•Cancer Research 

•Drug Delivery 

Optoelectronics 

•LEDs 

•Photovoltaics 

•Lasers 

Nano-Electronics 

•Single electron devices 

•High density storage 

•Quantum computing 

34

Exploitation of individual nanostructures 

100 

I (pA) 

0 

-100 

-200 

V g = 6.4 V 

V g = 6.9 V 

V g = 7.4 V 

V g = 7.7 V 

-60 -40 -20 0 20 40 60 

V sd 

(mV) 

Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L., Nature 1997, 389, 699-701. 

Park, H.; Park, J.; Lim, A. K. L.; Anderson, E. H.; Alivisatos, A. P.; McEuen, P. L., Nature 2000, 407, 57-60. 

35

Perspectives in self-assembly and applications of complex nanocrystals 

Self-assembly of nanocrystals 

in solution and on substrates 

R s 

Single tetrapod electronic devices 

Au 

+ - 

Au 

Au 

Electrostatic trapping of a single 

CdTe tetrapod between two 

electrodes 

R. Krahne, L.Manna, R. Cingolani 

36

Semiconductor Nanocrystals and Polymers 

Band Offsets and Electrical Devices 

PVs 

h+ e- 

Polymer 

3.0 

5.35 

E.A. 

e- 

4.4 

6.2 

I.P. 

vac 

light 

in 

I 

T 

O 

Al 

nanocrystal/polymer 

blend 

LEDs 

h+ e- 

h+ 

Nanocrystal 

light 

out 

I 

T 

O 

P 

P 

V 

Cd 

Se 

Mg 

Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P., Nature 1994, 370, 354-357. 

Schlamp, M. C.; Peng, X.; Alivisatos, A. P., J. Appl. Phys. 1997, 82, 5837-5842. 

nanocrystal 

multilayer 

37

Nanocrystal/Polymer Cells 

Al 

S 

S 

S 

CdSe/P3HT Blend 

S 

P3HT 

n/4 

Advantages of polymer cells: 

•Easy to produce, inexpensive 

Disadvantages: 

• Low power conversion efficiency 

ITO 

Glass 

100nm 

Huynh, W.; Dittmer, J.; Alivisatos, A. P., Science 2002, 242-4 

38

Self-assembled Nanorod-Polymer Photovoltaics 

Exciton 

Diffusion 

Length 

ITO 

Absorption 

100 nm 

Absorption 

Depth 

20 nm 

e - h + 

Exciton 

Diffusion 

Charge 

Transfer 

P3HT 

Polymer 

Al 

CdSe Nanorods 

Charge 

Transport 

S 

n 


39

Best Plastic/Nanorod Solar Cells 

(Obtained with 7 nm x 60 nm CdSe rods) 

1.2 

EQE 

0.8 

0.4 

0 

400 500 600 700 


Under AM 1.5 Global solar conditions: 

Power Conversion: 1.7% 


40

Self-Assembled Polymer-Nanorod Photovoltaic 

CdSe 

R 

O 

P X 

R 

e - h + 

S 

S 

S 

S 

S 

S 

S 

S 

S 

S 

S 

S 

S 

S 

S 

S 

Control on Many Length Scales: 

•Photogenerated excitons charge separate across the rod/polymer interface 

•Quantum Size Effect: Rod diameter determines bandgap (2-10 nm) - tandem cells 

•Electrically Active Surfactant enables rod dispersion in polymer and charge separation 

•distance between rods - polymer exciton diffusion length (50 nm) 

•inter-rod distance determined by polymer/surfactant interaction 

•thickness - full solar absorption (100-200) nm 

41

Biological applications of colloidal nanocrystals 

HS 

SH 

SiO 2 

SH 

SH 

NH 2 

42

Fluorescence Labeling Widely Used in Biology 

1 

0.8 

0.6 

0.4 

0.2 

0 

excitation 

Fluorescein 

(AU) 

350 400 450 500 550 600 650 

wavelength (nm) 

emission 

2 

1.8 

1.6 

1.4 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

•Discrete Excitation and Emission 

•Rapid Photobleaching 

Size Dependent Absorbance 

CdSe/CdS/ZnS 

and Emission 

2.2 nm 

CdSe 

5.0 nm 

CdSe 

350 450 550 650 


•Continuous excitation 

past threshold 

Qdots Enable Multiple Channels of Emission in Parallel 

43

Water solubilization of nanocrystals 

SH 

- 

- 

SiO 2 

SH 

CH 3 

- 

SH 

As grown nanocrystals: 

hydrophobic 

COOH 

COOH 

COOH 

COOH 

COOH 

COOH 

COOH 

COOH 

Silanization 

COOH 

COOH 

COOH 

COOH 

Surfactant exchange 

biological 

molecules -> 

aqueous solution 

Polymer coating 

44

Bio-conjugation 

of core-shell nanocrystals 

Streptavidin 

- 

HS 

Silanize 

SH 

SiO 2 

SH 

SH 

biotin 

O 

SA 

NH 2 

HN 

N 

Antibody 

Conjugation 

+ 

- different functional groups 

- positive / neutral / negative charge 


D. Gerion, F. Pinaud, S.C. Williams, W.J. Parak, D. Zanchet, S.Weiss, A.P. Alivisatos 

J Phys Chem B 105(37), 8861-8871 2001 

45

DNA conjugation of Au nanocrystals 

DNA can be directly 

attached to Au surface 

via thiol-linkage 

+ HS 

- 

Gel electrophoresis of Aunanocrystal 

/ DNA conjugates 

10 nm Au 

100 bases DNA 

2% agarose gel 

1h @ 7.1 V/cm 

Au and DNA are 

negatively charged 

DNA/Au ratio 

-> Au nanocrystals with controlled number of DNA molecules 

+ 

Zanchet, D.; Micheel, C. M.; Parak, W. J.; Gerion, D.; Alivisatos, A. P.;Nano Letters 2001; 1(1); 32-35 

46

T A 

C G G C 

A C 

G C T G 

DNA-directed assemblies (Au) 

- 

100 nm 

100 nm 

100 nm 


+ 

47

DNA-directed assemblies (Au) 

T A 

C G G C 

A C 

G C T G 

Formation of trimers and other building blocks 

- 

100 nm 

+ 


48

DNA-directed sorting (CdSe/ZnS) 

Sample top view 

60 µm 

gold 

8 µm 

silicon 

Au/SiO 2 

Gerion, D.; Parak, W. J.; Williams, S. C.; Zanchet, D.; Micheel, C. M.; Alivisatos, A. P.; 

J. Am. Chem. Soc.;2002; 124(24); 7070-7074 

49

Two Color Labeling of Mouse 3T3 Fibroblast 

idea: 

conjugate fluorescent nanocrystals 

with biological ligands, that specifically 

recognize receptor sites: 

molecular recognition 

i.e. protein: antibodies, avidin 

-> multicolor labeling of cells: 

- reduced photobleaching 

- only one excitation wavelength 

- parallel use of colors 


50

Time-Gated Enhancement 

1 

ensemble measurement 

arb. units 

0.1 

0.01 

0 20 40 60 80 100 120 140 160 

time (ns) 

NC: emission at 575 nm, double shell (CdS/ZnS) 

1 

fluorescence (a.u.) 

0.1 

single dot 

0.01 

0 20 40 60 80 100 120 140 160 

time (ns) 

51

Phagokinetic tracks with CdSe/ZnS nanocrystals 

Pellegrino, T. , Parak, W, et at., Differentiation, December 2003 

52

Conclusions and perspectives 

• Colloidal nanocrystals can be easily and cheaply fabricated 

•High quality (monodispersity, crystallinity, size dependent high 

fluorescence QY), growth kinetics well understood 

•Shape control via selective attachment 

•Phase control through surface energy manipulation 

•Compositional control (graded core-shell structures, 

heterostructures, barcodes…, coupled dots) 

•Novel applications: composite materials, biology, nano-electronics, 

photovoltaics, display technology. 

53

Colloidal Nanocrystals: Synthesis, Properties, Applications ...

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