Nanomaterials II

Synthesis strategies for controlled

nucleation and growth of colloidal

inorganic nanocrystals

Delia J. Milliron

The Molecular Foundry, Lawrence Berkeley National Lab

Preparative Strategies in Solid State and Materials Chemistry

UCSB-ICMR Summer School

August 12, 2010

MOLECULAR

FOUNDRY

Thursday, August 12, 2010

Outline

Lecture I: Fundamentals of nanocrystal synthesis

Basic apparatus & techniques

Minimizing polydispersity

Size control

Crystal phase control

Lecture 2: Complex structures

Shape control

Heterostructures & chemical conversion

Oriented attachment

MOLECULAR

FOUNDRY


Outline

Lecture I: Fundamentals of nanoparticle synthesis



Size control



Shape control


Oriented attachment

MOLECULAR

FOUNDRY


Surface energy of crystal facets

determines lowest energy shape

∆G surf =(γ a A a + γ b A B )

cube:

[100] facets

MOLECULAR

FOUNDRY





cube:

[100] facets

MOLECULAR

FOUNDRY





Wulff shape minimizes

energy given Ɣ of each

facet

cube:

[100] facets

cuboctohedron:

Ɣ [100] = 1.0

Ɣ [111] = 0.85

MOLECULAR

FOUNDRY


Ligands can modify relative surface

ies of ligands to different facets of CdSe quantum dots,

rovide evidence to support the recent hypothesis that

facets most strongly bound to ligands are the slowest

ow. This suggests that the geometry of nanoparticles

be controlled by altering the concentration of surfac-

. 10,12

veloping a theoretical model of II-VI colloidal quandot

structures represents a formidable challenge to statee-art,

first-principles electronic structure techniques. To

the majority of first principles quantum dot studies have

entrated on group IV (e.g., silicon, 15,16 germanium, 17,18

carbon 19 ) systems where the electronic structure of the

conductor core can be described in terms of the s and p

ce electrons and the dangling bonds at the surface can

fectively passivated with covalently bonded hydrogen

s. 20 In contrast, we have recently demonstrated that a

uantum mechanical description of CdSe nanoparticles

include the s, p, and d valence electrons to correctly

mine Manna, the crystal L., et structure al. of the semiconductor core and

ce J. atoms. Phys. 21 Chem. In addition, B (2005) the subtle interaction between

ore and its organic surfactants is central to the properties

energies of facets: Wurtzite CdSe

Figure 9. Surface energies for the facets passivated by MPA: (a) Cdpoor

case; (b) Cd-rich case.

As an example, Figure 4 reported the optimized geometries for

the 101h0 and the 0001 facets covered by MPA. On the 101h0

facet, for instance, both surface Cd and Se atoms tend to


Figure 1. Ideal wurtzite and relaxed

the (0001),(0001h), (011h0), and (112h0

attached. Cd (Se) atoms are shown i

energy Figure 10. configuration. Surface energies The for the initial facet

the poor Cdcase; 33 Se(b) 33 Cd-rich clustercase.

are shown in

of these clusters from the ideal

MOLECULAR

are passivated by a surfactant, while

lowest energy configuration FOUNDRY

is di

was shown that the ideal wurtzite

unpassivated, as it is already filled. T

configuration, when for instance th

Ligands can Anisotropic modify Growth relative of CdSe Nanocrystals surface

ies of ligands to

energies

different facets of CdSe

facets:

quantumWurtzite dots,

CdSe

w

rovide evidence to support the recent hypothesis that

co

or

facets most strongly bound to ligands are the slowest

ow. This suggests that the geometry of nanoparticles

be controlled by altering the concentration of surfac-

. 10,12

veloping a theoretical model of II-VI colloidal quandot

structures represents a formidable challenge to statee-art,

first-principles electronic structure techniques. To

the majority of first principles quantum dot studies have

entrated on group IV (e.g., silicon, 15,16 germanium, 17,18

carbon 19 ) systems where the electronic structure of the

conductor core can be described in terms of the s and p

Figure 1. Ideal wurtzite and relaxed

the (0001),(0001h), (011h0), and (112h0

ce electrons and the dangling bonds at the surface can

attached. Cd (Se) atoms are shown i

fectively passivated with covalently bonded hydrogen

s. 20 In contrast, we have recently demonstrated that a



the Cd 33 Se 33 cluster are shown in

energy configuration. The initial

uantum mechanical description of CdSe nanoparticles

include the s, p, and d valence electrons phosphonic to correctly acid ligands of these clusters from the ideal

mine Manna, Asthe crystal example, L., et structure al. Figure 4 reported of the semiconductor the optimized (0001) geometries core facet andof for CdSe lowest energy configuration is di

ce the 101h0 and the 0001 facets coveredFigure by MPA. 4. On the 101h0

J. atoms. Phys. 21 Chem. In addition, B (2005) the subtle interaction between

ore and

facet,

itsfor organic

instance,

surfactants

both surface

is central

Cd and

to

Se

theatoms properties

tend to was shown that the ideal wurtzite


co

po

by

fo

on

00

no

th

bo

su

00

∼

be

en

T

at

Figure 10. Surface energies for the facet

poor case; (b) Cd-rich case.

co

MOLECULAR at


FOUNDRY

de


si


ar

Optimized geometry showing the binding of the MPA

molecule to the 101h0 facet (A, B) and to the 0001Cd facet (C, D). For

ease of visualization, the side view for both facets reports only a 2×2

RACT

Growth of CdSe nanorods: Minimizing

varying from 3.0 to 6.5 nm and length from 7.5 to 40 nm. A qualitative

presented. the area of the high energy facet

50 nm

Alivisatos, et al. (2000-2004).

MOLECULAR

FOUNDRY


RACT

Growth of CdSe nanorods: Minimizing

varying from 3.0 to 6.5 nm and length from 7.5 to 40 nm. A qualitative

presented. the area of the high energy facet

Manna et al.

ligands to different facets of CdSe quantum dots,

evidence to support the recent hypothesis that

most strongly bound to ligands are the slowest

is suggests that the geometry of nanoparticles

trolled by altering the concentration of surfacng

a theoretical model of II-VI colloidal quanctures

represents a formidable challenge to stateirst-principles

electronic structure techniques. To

jority of first principles quantum dot studies have

d on group 50 IV (e.g., nm silicon, 15,16 germanium, 17,18

9

) systems where the electronic structure of the

tor core can be described in terms of the s and p

ctrons and the dangling bonds at the surface can

ly passivated with covalently bonded hydrogen

contrast, Alivisatos, we have et al. recently (2000-2004). demonstrated that a

Thursday, mechanical August 12, 2010 description of CdSe nanoparticles

high Ɣ end facet

low Ɣ

side facets

Figure 1. Ideal wurtzite and relaxed structur

the (0001),(0001h), (011h0), and (112h0) facets

attached. Cd (Se) atoms are shown in green

MOLECULAR

FOUNDRY

energy configuration. The initial and re

What about kinetics?









MOLECULAR


FOUNDRY




Even without ligands,

lowest energy shape of

CdSe should be similarly

elongated









MOLECULAR


FOUNDRY




ots,

that

est

cles

fac-

an-

ate-

. To

ave

17,18

the

d p

can

gen

at a

cles

Figure 1. Ideal wurtzite and relaxed structure of Cd 33 Se 33 showing

the (0001),(0001h), (011h0), and (112h0) facets to which ligands are

attached. Cd (Se) atoms are shown in green (gray).

G. Galli, energy et al. configuration. Nano Lett (2004). The initial and relaxed structures of

the Cd Se cluster are shown in Figure 1. The relaxation


MOLECULAR

FOUNDRY


ots,

that

est

cles

fac-




elongated

an-

ate-

. To

ave

17,18

the

d p

can

gen

at a

cles







MOLECULAR

FOUNDRY


ots,

that

est

cles

fac-

an-

ate-

. To

ave

17,18

high Ɣ end facet:

0.67 eV

Ligand binding

energies

side facets:

1.45 eV

1.26 eV

the

d p

Figure lower 1. Ideal Ɣ end wurtzite facet: and relaxed structure of Cd 33 Se 33 showing

the (0001),(0001h), 1.11 eV (011h0), and (112h0) facets to which ligands are

can


gen

at a


cles

Thursday, August 12,

the

2010Cd

Se cluster are shown in Figure 1. The relaxation




elongated

Binding energy of

phosphonic acid to side

facets is much larger

than end facets

MOLECULAR

FOUNDRY


ots,

that

est

cles

fac-

an-

ate-

. To

ave

17,18

the

d p

can

gen

at a

cles

high Ɣ end facet:

0.67 eV

side facets:

1.45 eV

1.26 eV




elongated

Binding energy of

phosphonic acid to side

facets is much larger

than end facets

Nanorod shape is

(largely) a kinetic

Figure lower 1. Ideal Ɣ end wurtzite facet: and relaxed structure of Cd 33 Se 33 showing

product

the (0001),(0001h), 1.11 eV (011h0), and (112h0) facets to which ligands are





Ligand binding

energies

MOLECULAR

FOUNDRY

CdTe tetrapods: Crystal phase + shape

control

Alivisatos, et al. Nature Mater. (2003)

MOLECULAR

FOUNDRY



control


100 nm

MOLECULAR

FOUNDRY



control

ODPA

–

WZ(0001)

ZB(111)


Figure 1 Proposed model of a CdTe tetrapod.The exploded view of one arm illustrates

the identical nature of the (111) zinc blende (ZB) and (0001 – ) wurtzite (WZ) facets of the


100 nm

MOLECULAR

FOUNDRY

Variation of arm length and diameter

100 nm!


MOLECULAR

FOUNDRY


Current model of CdTe tetrapod

ARTICLES

structure: 100% wurtzite

Carbone et al.

High resolution TEM

shows multiply

twinned wurtzite core

Figure 5. (a) Matchstick-shaped nanocrystal revealing several twins in the

head of the matchstick. The region indicated by the arrow is shown in (c)

at higher magnification. This area is close to the [11h00] zone axis. The

defect contrast can be identified as a (112h2) twin boundary, with a headto-tail

configuration of the crystal polarity. The arrangement of the atoms

close to the boundary corresponds to the one shown in the model of Figure

1d.

twins (Figure 4). In all cases the domains were elongated along

the c axis. The presence of sphalerite nanocrystals was very

seldom observed. This in addition indicates that the presence

of MPA decreased the probability of formation of sphalerite

multiply

twinned

wurtzite

core

phase considerably.

At higher MPA:OPDA ratios, the branching increased further,

as well as the anisotropic growth (Figure 2c-g). Ratios in the

range 0.08-0.09 yielded mainly tripods and tetrapods, whereas

nanocrystals with multiple branching points dominated at ratios

higher than 0.10.

4 of 8 facets are high

energy 000-1

4 nanorods grow on

high energy facets

Figure 6. (a) HRTEM image of a CdTe tetrapod with one arm pointing

upward. (Inset) Orientation of the nanocrystal with respect to the electron

beam (white arrow). The analysis of the symmetry and of the lattice spacing

of the arms of several tetrapods indicates a wurtzite structure (space group

(P6 3 mc) with lattice parameters corresponding to a ) 0.458 nm and c )

0.75 nm, respectively). (b) Tilted tetrapod. (Inset) Orientation again of the

nanocrystal with respect to the electron beam (white arrow). In particular,

olumn X-ray shows diffractionan spectra exploded of the branched view samples of three (for in this geometry two arms are superimposed. (c) Central region of the

different types of nuclei (middle column) that could

tetrapod shown in (b) is imaged here at higher magnification. The rectangular

instance, the samples shown in Figure 2d and 2f, see Supporting

ds (c). Two types of domains are considered incontour this the model region that (as is theshown object of thein image a, simulation. left side). (d) Supercell

Information), resembled their shape those of wurtzite nanorods.

17,43 They show, fact, a high degree of distortion of the central region of the tetrapod, In a multiple-twin viewed along its 247 zone nanocrystal axis and rotated the two

One domain

containing the atomistic model used to simulate the HRTEM image of the

the other Manna, domainL. exposes et al J. a (0001) Am. facet Chem. (typeSoc. II domain). (2006)

intensity ratios due to shape anisotropy, in particular with an around this axis (which is perpendicular to the plane of the image) so that

ndary. Thursday, August Of these 12, 2010 two types of domains, only one can grow to an elongated structure, although our

the model has the same orientation as the real nanocrystal in the HRTEM

MOLECULAR

FOUNDRY

Ripening of CdTe tetrapods reveals

kinetic structure of nanorod arms

MOLECULAR

FOUNDRY




After forming

tetrapod shape,

holding at high

temperature forms

balls on the ends of

arms

MOLECULAR

FOUNDRY




After forming

tetrapod shape,

holding at high

temperature forms

balls on the ends of

arms

Once

supersaturation

drops, ripening

tends toward lower

energy shape

MOLECULAR

FOUNDRY


Outline




Size control



Shape control


Oriented attachment

MOLECULAR

FOUNDRY


Free energy of homogeneous vs

heterogeneous nucleation

∆G =4πr 2 γ − 4 3

πr3

RT ln S

V m

ΔG

+ r 2

rc

Surface

Energy

Bulk

Energy

0

ΔGc

r

- r 3

r

MOLECULAR

FOUNDRY


Free energy of homogeneous vs

heterogeneous nucleation

∆G =4πr 2 γ − 4 3

πr3

RT ln S

V m

ΔG

+ r 2

rc

Surface

Energy

Bulk

Energy

0

ΔGc

r

- r 3

r

substrate

MOLECULAR

FOUNDRY


Contact angle depends on surface

energies

γ A = γ AB + γ B cos θ

ƔB

B

ƔAB

θ

ƔA

A

MOLECULAR

FOUNDRY


Free energy for heterogeneous vs

homogeneous nucleation

f (θ) = 1 2 − 3 4 cos θ + 1 4 cos3 θ

ΔG

0

+ r 2

rc

ΔGc

homo

- r 3

r

r

MOLECULAR

FOUNDRY




f (θ) = 1 2 − 3 4 cos θ + 1 4 cos3 θ

ΔG

+ r 2

rc

∆G hetero = f (θ)∆G homo

ΔGc

homo

0

- r 3

r

r

MOLECULAR

FOUNDRY




f (θ) = 1 2 − 3 4 cos θ + 1 4 cos3 θ

∆G hetero = f (θ)∆G homo

∆G hetero

c

= f (θ)∆G homo

c

ΔG

0

+ r 2

- r 3

rc

ΔGc

homo

ΔGc hetero

r

r

MOLECULAR

FOUNDRY


Variation of heterogeneous nucleation

activation energy with contact angle

∆G hetero

c

= f (θ)∆G homo

c

r

r

θ = 60°

r

θ = 90°

θ = 180°

f (θ)∆G homo

c

MOLECULAR

FOUNDRY




∆G hetero

c

= f (θ)∆G homo

c

r

r

θ = 60°

r

θ = 90°

θ = 180°

f (θ)∆G¼

homo

c

½

MOLECULAR

FOUNDRY




∆G hetero

c

= f (θ)∆G homo

c

r

r

θ = 60°

r

θ = 90°

θ = 180°

f (θ)∆G¼ homo

½ c

1

MOLECULAR

FOUNDRY




∆G hetero

c

= f (θ)∆G homo

c

r

r

r

θ = 60°

θ = 90°

θ = 180°

f (θ)∆G¼ homo

½ c

1

When θ < 180°C, S c for heterogeneous nucleation is less than

for homogeneous nucleation and selective heterogeneous

growth is achievable.

MOLECULAR

FOUNDRY


Epitaxial strain

thin film

substrate

MOLECULAR

FOUNDRY


Epitaxial strain

thin film

Strain due to lattice

mismatch adds to

free energy

substrate

MOLECULAR

FOUNDRY


Epitaxial strain

thin film

Strain due to lattice

mismatch adds to

free energy

8.360 ARTICLES

/NNANO.2008.360

NO.2008.360

Epitaxial

(core) thin shell

elaxedRelaxed

substrate

ore) shell (core) shell

Epitaxial Epitaxial

(core) thin (core) shell thin shell

ARTICLES ARTICLES

Nano-differences:

Epitaxial

deformation of core,

Epitaxial Epitaxial

isotropic vs biaxial

(core) thick (core) shell thick shell

strain, increase in

surface area

(core) thick shell

nanocrystal

core/shell

core

Nie, et al. Nature Nano. (2009)

MOLECULAR

FOUNDRY


toon, Joanna L. Casson, Donald J. Werder,

limov, and Jennifer A. Hollingsworth*

onal Laboratory, Los Alamos, New Mexico 87545

Controlling morphology of core/shell

growth

r 23, 2007; E-mail: jenn@lanl.gov

red

ble

nd

as

ed

cal

nd

CdSe

CdSe/CdS

to

ng

cal

ly,

D

by

nd

in

nd

Hollingsworth, et al. JACS (2008); Manna, et al. JACS (2009).


Figure 1. Transmission electron microscopy (TEM) images for (a) CdSe

NQD cores, (b) CdSe/19CdS g-NQDs, and (c) CdSe/11CdS-6Cd x Zn y S-2ZnS

g-NQDs. (d) Absorption (dark blue) and PL (light blue) spectra for CdSe

NQD cores. (e) Absorption (dark red) and PL (light red) spectra for CdSe/

MOLECULAR

FOUNDRY





growth


red

ble

nd

as

ed

cal

nd

CdSe

to

ng

cal

ly,

D

by

nd

in

nd

Keep supersaturation

low to avoid secondary

nucleation







CdSe/CdS

MOLECULAR

FOUNDRY





growth


red

ble

nd

as

ed

cal

nd

CdSe

to

ng

cal

ly,

D

by

nd

in

nd



nucleation







CdSe/CdS

MOLECULAR

FOUNDRY





growth


red

ble

nd

as

ed

cal

nd

CdSe

to

ng

cal

ly,

D

by

nd

in

nd



nucleation

Strain energy is lower,

surface energy higher in

nanorod core/shells

Use surfactants and

(some) supersaturation

effects to adjust kinetics

to grow spheres or

nanorods







CdSe/CdS

MOLECULAR

FOUNDRY

Combining phase control, shape

(103)

uated

(002)

nanossigncryst

240

shows

eflecn

the

olved

to the

phos-

A), a

OPS)

uality

orods

S1). 24

slow

action

CdS

control, and heterostructure growth

dSe nanocrystals with wurtzite and zinc blende structures. (a) Absorption spectrum and TEM image of 4.4 nm CdSe nanocrystals

e lattice. (b) Absorption spectrum and TEM image of 4.0 nm CdSe nanocrystals with zinc blende lattice. (c,d) Powder X-ray

ation) diffraction patterns of CdSe nanocrystals with wurtzite and zinc blende structures, correspondingly.

t or “mixed dimensionality” where holes are

CdSe while electrons can move freely between

dS phases, spreading over the entire nanostruceviously,

we synthesized structures consisting of

CdSe nanocrystal connected to a CdS nanorod. 17

ucture, hole is three-dimensionally confined,

lectron is confined only in two dimensions.

synthesized CdSe/CdS nanorods exhibited excelscent

properties with room-temperature photoce

quantum efficiencies (PL QE) above 50% and

Talapin, et al. Nano Lett (2007).


larized emission. The CdS nanorod can behave

capping ligands. The size of zb-CdSe nanocrystal seeds can

be tuned by varying the duration of nanocrystal growth at

240 °C. w-CdSe nanocrystals capped with HDA, TOPO,

TOP, and a small amount of n-octyl- or n-tetradecylphosphonic

acid have been synthesized as described in ref 13.

The detailed synthetic recipes are given in the Experimental

Details Section. Both approaches provide nearly spherical

MOLECULAR

FOUNDRY

CdSe nanocrystals with size distribution below 10% and

sharp excitonic features in the absorption spectra (Figure

1a,b). Both w-CdSe and zb-CdSe nanocrystals form stable

colloidal solutions in nonpolar solvents like hexane, toluene,


(103)

uated

(002)

nanossigncryst

240

shows

eflecn

the

olved

to the

phos-

A), a

OPS)

uality

orods

S1). 24

slow

action

CdS


dSe nanocrystals with with wurtzite wurtzite and and zinc zinc blende blende structures. (a) (a) Absorption spectrum and and TEM TEM image image of 4.4 of 4.4 nm nm CdSe CdSe nanocrystals

e lattice. (b) (b) Absorption spectrum and and TEM TEM image image of 4.0 of 4.0 nm nm CdSe CdSe nanocrystals with with zinc zinc blende blende lattice. lattice. (c,d) (c,d) Powder Powder X-ray X-ray

ation) diffraction patterns patterns of CdSe of CdSe nanocrystals with with wurtzite wurtzite and and zinc zinc blende blende structures, correspondingly.

or t or “mixed “mixed dimensionality” where where holes holes are are

CdSe while while electrons can can move move freely freely between

dS phases, phases, spreading over over the the entire entire nanostruceviously,

we we synthesized structures consisting of of

CdSe nanocrystal connected to atoCdS a CdS nanorod. 17 17

ucture, hole hole is is three-dimensionally confined,

lectron is is confined only only in in two two dimensions.

synthesized CdSe/CdS nanorods exhibited excel-

excelscent

with photoce

efficiencies (PL (PL QE) QE) above above 50% 50% and and

Talapin,

propertieset with

al.

room-temperature

Nano Lett (2007).

photoquantum



capping ligands. The The size size of zb-CdSe of nanocrystal seeds seeds can can

be be tuned tuned by by varying varying the the duration of of nanocrystal growth growth at at

240 240 °C. °C. w-CdSe nanocrystals capped capped with with HDA, HDA, TOPO, TOPO,

TOP, TOP, and and a small a small amount amount of of n-octyl- or or n-tetradecylphosphonic

acid acid have have been been synthesized as as described in ref in ref 13. 13.

phonic

The The detailed synthetic recipes recipes are are given given in the in the Experimental

Details Details Section. Both Both approaches provide provide nearly nearly spherical

MOLECULAR

CdSe CdSe nanocrystals with with size size distribution below below10% 10% FOUNDRY and and

sharp sharp excitonic features in in the the absorption spectra spectra (Figure (Figure

1a,b). 1a,b). Both Both w-CdSe and and zb-CdSe nanocrystals form form stable stable



(103)

uated

(002)

nanossignocrys-

240

shows

eflecinn

the

olved

att to the

lphos-

A), a

OPS)

uality

orods

S1). 24

slow

action

CdS


dSe nanocrystals with with wurtzite wurtzite and and zinc zinc blende blende structures. (a) (a) Absorption spectrum and and TEM TEM image image of 4.4 of 4.4 nm nm CdSe CdSe nanocrystals

e lattice. (b) (b) Absorption spectrum and and TEM TEM image image of 4.0 of 4.0 nm nm CdSe CdSe nanocrystals with with zinc zinc blende blende lattice. lattice. (c,d) (c,d) Powder Powder X-ray X-ray

ation) diffraction patterns patterns of CdSe of CdSe nanocrystals with with wurtzite wurtzite and and zinc zinc blende blende structures, correspondingly.

or t or “mixed “mixed dimensionality” where where holes holes are are

CdSe while while electrons can can move move freely freely between

dS phases, phases, spreading over over the the entire entire nanostruceviously,

we we synthesized structures consisting of of

CdSe nanocrystal connected to atoCdS a CdS nanorod. 17 17

ucture, hole hole is is three-dimensionally confined,

lectron is is confined only only in in two two dimensions.

synthesized CdSe/CdS nanorods exhibited excel-

excelscent

with photoce

efficiencies (PL (PL QE) QE) above above 50% 50% and and

Talapin,

propertieset with

al.

room-temperature

Nano Lett (2007).

photoquantum



wz

capping ligands. The The size size of zb-CdSe of nanocrystal seeds seeds can can

be be tuned tuned by by varying varying the the duration of of nanocrystal growth growth at at

240 240 °C. °C. w-CdSe nanocrystals capped capped with with HDA, HDA, TOPO, TOPO,

TOP, TOP, and and a small a small amount amount of of n-octyl- or or n-tetradecylphosphonic

acid acid have have been been synthesized as as described in ref in ref 13. 13.

phonic

The The detailed synthetic recipes recipes are are zbgiven in the in the Experimental

Details Details Section. Both Both approaches provide provide nearly nearly spherical

MOLECULAR

CdSe CdSe nanocrystals with with size size distribution below below10% 10% FOUNDRY and and

sharp sharp excitonic features in in the the absorption spectra spectra (Figure (Figure

1a,b). 1a,b). Both Both w-CdSe and and zb-CdSe nanocrystals form form stable stable


Longitudinal heterostructures

A

B

B

Alivisatos, et al. Nature

MOLECULAR

(2004); Banin, et al.

FOUNDRY

Nature Mater. (2005)



A

BEnergetics vs core/shell:

less strain, smaller

interface area, larger

overall surface area

B


MOLECULAR


FOUNDRY




A

BEnergetics vs core/shell:

less strain, smaller

interface area, larger

overall surface area

B

Kinetic control of

heterogeneous

nucleation on higher

energy facets

∆G hetero

c

= f (θ)∆G homo

c


MOLECULAR


FOUNDRY



Chemical conversion: “Seeded growth”

of Ag 2 X (X = S, Se, or Te)

room temp

Ag2S

S

!"#$%#

Ag

Te

Se

20 nm

Ag2Te

Ag2Se

TOP-Te

room temp

12 h

adapted from: Li,

et al. JACS

(2008).

!"#$%#

10 nm

MOLECULAR

FOUNDRY


Chemical conversion: “Seeded growth”

of Ag 2 X (X = S, Se, or Te)

room temp

Ag2S

S

!"#$%#

!"#$"

Ag

Te

Se

2 nm

20 nm

Ag2Te

Ag2Se

TOP-Te

room temp

12 h

adapted from: Li,

et al. JACS

(2008).

!"#$%#

!"#$"

10 nm

2 nm

MOLECULAR

FOUNDRY


001, 123, 11500-11501

Se

Chemical conversion: Topotactic

transformation of Se to Ag 2 Se

AgNO3

Ag2Se

!

'(!)*+!,-.!#'(!)/+!01,2%3!45!30-26%!7893:,660-%!-,-4;08%3!45!

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Chemical conversion: Topotactic

transformation of Se to Ag 2 Se

001, 123, 11500-11501

Se

Topotactic

transformation

between

structurally related

single crystals

Ag2Se

AgNO3

Minimal

rearrangement of

Se lattice required

to convert to Ag 2 Se

!

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sion-controlled reaction

reaction time is roughly

square of the size (26).

Topotactic cation exchange reaction in

nanocrystals

To obtain a more conclusive answer to

this question, we performed cation exchange

reactions on nanocrystals with highly aniso-

G and H) a

CdSe

AgNO3

40 nm

Ag2Se

Alivisatos, et al. Science (2004)

MOLECULAR

FOUNDRY


sion-controlled hollow spheres and reaction branched tetrapods To obtain (9, 12), a more conclusive answer to G and H) a

reaction

maintain

time

their

Topotactic is roughly

overall shapes

this question, cation

throughout

weexchange performed cationreaction exchange in

complete cation exchange

square of the size (26). reactions

cycles, provided

on nanocrystals with highly anisothey

have ananocrystals

dimension thicker than È5 nm

riable

(Fig. 3).

than

CdS

nanorod

hollow spheres maintain overall

e

morphology

shape change.

during the cation exchange,

although CdSe a smoothing of theCdSe

CdTe

plex and high-energy

rough surface

f nanocrystals,

and a small increase

such as

in volume are observed.

hed

In

tetrapods

the case of

(9,

CdTe

12),

tetrapods, slight expansion

(È5%)

shapes throughout

of the width of each branch is

ge

observed

cycles,

after

provided

the transformation to Ag 2

Te.

The observed40 changes nm

thicker than È5 nm

in size can be

accounted for by changes in the crystal AgNO3

eres maintain overall

unit

e

cell

cation

symmetry

exchange,

and lattice parameters during the Fig. 3. (A to C) TEMimagesof(A)initialCdS

transformation.

Ag2Se

In Fig. 4A, the Ag2Se

of the rough surface

structures of

Ag2Te

Se 2– sublattices in wurtzite CdSe and various TEM images of (D) initial CdTe tetrapods, (

volume are observed.

phases of Ag 2

Se are presented to show the CdTe, and (F) recovered CdTe from the reve

rapods, slight expanth

topotaxial

of each branch

relationship

is

between the reactant

ormation

and product

to Ag

phases and associated changes in

dimension. Small 2

Te.

40 nm

increases in the width

40 nm

ges in size can be

es

observed

in the crystal

in the transformation

unit

of thicker CdSe

parameters

rods to tetragonal

during the

Ag 2

Se rods (Fig. 2, F and J)

4A,

reflect the changes in dimension upon change

Alivisatos, the structures

of the crystal

et

structure

al. ofScience cation

as shown

(2004) exchange of CdS, and (C) recovered CdS from the reverse cation

ite CdSe and various TEM imagesin of Fig. (D) 4A. initial CdTe tetrapods, (E) Ag 2

Te tetrapods produc


These observations reveal a second fundamen-

Downloaded from www.sciencemag.org

cation exchange of CdS, and (C) recovered C

MOLECULAR

Fig. 3. (A to C) TEMimagesof(A)initialCdShollowspheres,(B)Ag FOUNDRY 2

S ho

ned units that are attached to a

Partial Cation Exchange

bedded in a solid medium (1–5).

1 Materials Science Division, Lawrence Berkeley National

lloidalnanocrystalresearchisto Laboratory, Berkeley, CA 94720, USA.

2 Department of

me structures while leveraging the

Chemistry, University of California, Berkeley, CA 94720,

Richard D. Robinson, 1 Bryce Sadtler, USA.

solution-phase fabrication, such as

3 Computational 2 * Denis O. Research Demchenko, Division, Lawrence 3 * Can Berkeley K. Erdonmez, 2

Lin-Wang Wang,

National Laboratory, Berkeley, CA 94720, USA.

esis and compatibility 3 A. Paul Alivisatos

in disparate

1,2 †

*These authors contributed equally to this work.

[e.g., for use in biological labeling

olution-processed Lattice-mismatch strains light-emitting are widely †To whom correspondence should be addressed. E-mail:

alivis@berkeley.edu

known to control nanoscale pattern formation in heteroepitaxy, but

such effects have not been exploited in colloidal nanocrystal growth. We demonstrate a colloidal route

to synthesizing CdS-Ag 2 Snanorodsuperlatticesthroughpartialcationexchange.Straininducesthe

spontaneous A formation of periodic B structures. Ab initio calculations of the interfacial energy and

modeling of strain energies show that these forces drive the self-organization of the superlattices. The

nanorod superlattices exhibit high stability against ripening and phase mixing. These materials are

tunable near-infrared emitters with potential applications as nanometer-scale optoelectronic devices.

C

Partial cation exchange: CdS + Ag +

The ability to pattern on the nanoscale has quantum-confined units that are attached to a

led to a wide range of advanced artificial substrate or embedded in a solid medium (1–5).

materials with controllable quantum energy

levels. Structures such as quantum-dot arrays create these same structures while leveraging the

Atargetofcolloidalnanocrystalresearchisto

and nanowire heterostructures can be fabricated advantages of solution-phase fabrication, such as

by vacuum- and vapor-deposition techniques low-cost synthesis and compatibility in disparate

such as molecular beam epitaxy (MBE) and environments [e.g., for use in biological labeling

vapor-liquid-solid

20

(VLS)

nm

processes, resulting in (6, 7) and solution-processed

20 nm

light-emitting

Downloaded from www.scie

tal and the quantum dot creates a region of s

surrounding the dot. Ingeniously, this local s

has been used to create an energy of intera

between closely spaced dots; this use of “s

engineering” has led, in turn, to quantum-do

rays that are spatially patterned in two (and

three) dimensions (2–4). We demonstrate

application of strain engineering in a coll

quantum-dot system by introducing a method

spontaneously creates a regularly spaced

rangement of quantum dots within a coll

quantum rod.

Alineararrayofquantumdotswithinan

rod effectively creates a one-dimensional

1 Materials Science Division, Lawrence Berkeley Na

Laboratory, Berkeley, CA 94720, USA.

2 Departme

Chemistry, University of California, Berkeley, CA 9

USA. 3 Computational Research Division, Lawrence Be



†To whom correspondence should be addressed. E

alivis@berkeley.edu

CdS

nanorod

10 CdS nm

Cd 2+

Ag +

Counts

CdS-Ag 2

S

60

20

0 16 32

Separation (nm)

Fig. 1. TEM images of superlattices formed through partial

cation SCIENCE exchange. VOL 317 (A) The 20 JULY original 2007 4.8–by–64-nm CdS

355

nanorods. (B and C) Transformed CdS-Ag 2 S superlattices.

(Inset) Alivisatos, Histogram ofet Ag 2 al. S segment Science spacing (2007) (center-tocenter).

The average spacing is 13.8 ± 3.8 nm. The sample

set for the histogram was greater than 250 spacings.


Ag +

A

B

MOLECULAR

FOUNDRY

ned units that are attached to a

Partial Cation Exchange

bedded in a solid medium (1–5).


lloidalnanocrystalresearchisto


2 Department of

me structures while leveraging the

Chemistry, University of California, Berkeley, CA 94720,

Richard D. Robinson, 1 Bryce Sadtler, USA. solution-phase fabrication, such as

3 Computational 2 * Denis O. Research Demchenko, Division, Lawrence 3 * Can Berkeley K. Erdonmez, 2

Lin-Wang Wang, esis and compatibility 3 A. Paul Alivisatos

National

in disparate

1,2 †



[e.g., for use in biological labeling

†To whom correspondence should be addressed. E-mail:

olution-processed Lattice-mismatch strains light-emitting are widely alivis@berkeley.edu

known to control nanoscale pattern formation in heteroepitaxy, but

such effects have not been exploited in colloidal nanocrystal growth. We demonstrate a colloidal route

to synthesizing CdS-Ag 2 Snanorodsuperlatticesthroughpartialcationexchange.Straininducesthe

spontaneous A

formation of periodic B

structures. Ab initio calculations of the interfacial energy and

modeling of strain energies show that these forces drive the self-organization of the superlattices. The

nanorod superlattices exhibit high stability against ripening and phase mixing. These materials are


Partial cation exchange: CdS + Ag +

The ability to pattern on the nanoscale has

quantum-confined units that are attached to a

led to a wide range of advanced artificial

substrate or embedded in a solid medium (1–5).

materials with controllable quantum ener-


gy levels. Structures such as quantum-dot arrays

create these same structures while leveraging the

and nanowire heterostructures can be fabricated

advantages of solution-phase fabrication, such as

by vacuum- and vapor-deposition techniques

low-cost synthesis and compatibility in disparate

such as molecular beam epitaxy (MBE) and

environments [e.g., for use in biological labeling

vapor-liquid-solid 20 (VLS) nm processes, resulting in

(6, 7) and solution-processed 20 nm

light-emitting

C





between closely spaced dots; this use of “s



three) dimensions (2–4). We demonstrate





quantum rod.





2 Departme


USA. 3 Computational Research Division, Lawrence Be



†To whom correspondence should be addressed. E

alivis@berkeley.edu

CdS

nanorod

10 CdS

nm

Cd 2+

Ag +

Counts

CdS-Ag 2 S

60

20

CdS-Ag2S

nanorod

0 16 32

Separation (nm)

Fig. 1. TEM images of superlattices formed through partial

cation SCIENCE exchange. VOL 317 (A) The 20 JULY original 2007 4.8–by–64-nm CdS

355


(Inset) Alivisatos, Histogram ofet Ag 2 al. S segment Science spacing (2007)

(center-tocenter).


set for the histogram was greater than 250 spacings.


Ag +

A

B

MOLECULAR

FOUNDRY

ned region units andthat all other are attached Ag

in which distances

2 Sregionsonarodwere

to a

Partial Cation between Exchange

bedded measured. in aThe solid organization medium (1–5). of the Ag 2 Sregions

each Ag 2 S



lloidalnanocrystalresearchisto

into superlattices is seen in the periodicity Laboratory, of the

Berkeley, CA 94720, USA.

n and all other Ag 2 Sregionsonarodwere

2 Department of


me histogram structures(Fig. while3F), leveraging extending the over

Chemistry,

several

University of California, Berkeley, CA 94720,


Richard D. Robinson, ured. The organization 1 Bryce Sadtler, USA.

solution-phase nearest-neighbor fabrication, distances. such In as the superlattices,

of 3 Computational 2 * Denis O. Research Demchenko, Division, Lawrence the Ag 2 Sregions

3 * Can Berkeley K. Erdonmez, 2

the

Lin-Wang

Ag Partial cation exchange: CdS + Ag +

superlattices

2 Sregionsarespacedevenlyalongthe

Wang,

esis and compatibility 3 A. Paul Alivisatos

National

in disparate

1,2 between closely spaced dots; this use of “s

†




[e.g., rod, for whereas use inno biological periodicity is seen labeling is seeninfor the lower periodicity of the

Ag gram + †To whom correspondence should be addressed. E-mail:


olution-processed Lattice-mismatch concentration strains light-emitting

(Fig. 3E). are widelyalivis@berkeley.edu

known to control nanoscale pattern formation in heteroepitaxy, but three) dimensions (2–4). We demonstrate

suchThe effects mechanism

(Fig. have not by3F), been which exploited the

extending

initial arrange-

colloidalover nanocrystal several

growth. We demonstrate a colloidal route


ment st-neighbor to synthesizing of randomly CdS-Ag distributed distances. 2 Snanorodsuperlatticesthroughpartialcationexchange.Straininducesthe

small islands In the of superlattices,


Ag spontaneous 2 ASevolvesintoaperiodic,1Dpatternisof

formation of B

structures. Ab initio calculations of the interfacial energy and


particular modeling g 2 Sregionsarespacedevenlyalongthe

interest. of strainBecause energiesthere showexists that a these posi-

forces drive self-organization of the superlattices. The


tive nanorod CdS-Ag superlattices whereas no

2 S interface exhibit formation high stability energy against ripening and phase mixing. These materials are

(~1.68 periodicity is seen for the lower

eV per Cd-Ag-S elementary interface

quantum rod.


unit, from our ab initio calculations), it is


concentration (Fig. 3E).

energetically favorable to merge small Ag he 2 S


islands The mechanism into ability larger to Ag pattern 2 Sbysegments. on which the nanoscale Thethe fast

has initial quantum-confined arrange-

units that are attached to a

diffusion ledof tocations a wideleads range to of a advanced situation where

artificial

substrate or embedded in a solid medium (1–5).

t Ostwald of randomly ripening between distributed the initially formed small islands of


materials with controllable quantum energy

levels. of Ag Structures evolvesintoaperiodic,1Dpatternisof



2 Departme

islands 2 Scanoccur,sothatlargerislands

such as quantum-dot arrays create these same structures while leveraging the


grow at the expense of nearby smaller ones.

USA. and nanowire heterostructures can be fabricated advantages of solution-phase fabrication, such as

3 Computational Research Division, Lawrence Be


cular

Diffusion by vacuum- interest.

of the and cations vapor-deposition Because

is allowed because

there techniques both

exists a posi-

Ag + and Cd 2+ Fig. low-cost 4. Theoretical synthesis modeling and compatibility and experimental in disparate optical characterization. (A) Cubic-cutout

are considered fast diffusers

*These authors contributed equally this work.

such as molecular beam epitaxy (MBE) andrepresentation environments of cells [e.g., used for for useab in biological initio energy labeling calculations.

(23–25). A distorted monoclinic Ag CdS-Ag Also, 2

Ag S chalcogenides interface exhibit formation superionic

conductivity in their high-temperature

†To whom correspondence should energy

2 S be(100)

addressed. E

vapor-liquid-solid

20

(VLS)

nm

processes, resulting inplane(6, connects 7) andwith solution-processed 20 the nm wurtzite CdS (001) light-emitting plane. (B) Elastic alivis@berkeley.edu

energy of the rod as a function of

segment separation (center-to-center). (C) Z-axis strain for the case of two mismatched segments at

8phases CeV(25). per A critical Cd-Ag-S juncture occurselementary when the interface

a center-to-center separation distance of 14.1 nm (top) and 12.1 nm (bottom). The elastic

regions of Ag

from our 2 Sgrowtothepointwherethey

ab initio calculations), interaction between it issegments is greatly reduced for separations >12.1 nm. Arrows show the

span the diameter of the rod. At this point,

further etically Ostwald favorable ripening is kinetically to merge placement of mismatchedAsegments. The CdS rods used Bfor VFF calculations (B and C) were 4.8 nm

Cd 2+

prohibited,

because an atom-by-atom exchange of the mismatched segments were from ab initio calculations for monoclinic Ag 2 S. (D) Visible and (E)

insmall diameter, Ag with 2

two S 4.8–by–4.0-nm lattice-mismatched segments. Effective elastic constants for

ds Ag + 60

among into segments largerwill Ag not reduce 2 S segments. the total NIR PLThe spectra atfast

l = 400- and 550-nm excitation, respectively. Coupling between the CdS and Ag 2 S

interfacial area. This leads to Ag sion of cations leads

2 Ssegmentsof is evident by the complete quenching of the visible PL (D) in the heterostructures. The shift in NIR PL

20

nearly equal size (fig. S3). The rodto is ain asituation (E) is due towhere

quantum confinement of the Ag 2 S.

10 CdS

nm

0 16 32

CdS Ag + CdS-Ag2S 2

S

ald ripening between Ag the initially formed

ds of nanorod

+

Ag 2 Scanoccur,sothatlargerislands nanorod

Counts

Separation (nm)

Fig. 1. TEM images of superlattices formed www.sciencemag.org through partial SCIENCE VOL 317 20 JULY 2007 357

cation MOLECULAR

SCIENCE exchange. FOUNDRY

at the

VOL expense

317 (A) The 20 of

JULY original

nearby

2007 4.8–by–64-nm CdS

smaller ones.

355


sion (Inset) Alivisatos, of Histogram

the cations ofet Ag 2 al. S segment

isScience allowed spacing

because (2007)

(center-tocenter).


set for the histogram was greater than 250 both

spacings.

and Cd 2+


are considered fast diffusers


Fig. 4. Theoretical modeling and experi

Generating heterostructures by partial

cation exchange: Segmented nanorods

Alivisatos, et al. JACS (2009)


CdS-Cu2S nanorods

by cation exchange

MOLECULAR

FOUNDRY





CdS-Cu2S nanorods

by cation exchange

MOLECULAR

FOUNDRY



Two contributions to energy

cost of forming an interface

Strain (θ)

Chemical (ΔƔ AB )

θ

ΔƔ

Cu small > 0



Ag large < 0

CdS-Cu2S nanorods

by cation exchange

MOLECULAR

FOUNDRY

t

n

of

of the

this shift

For

may

example,

in fact be

a structure

attributed

with

to

a

effects

PbSe nanocrystal embedded in a

ize orward change. For instance, back-exchange of the

PbS rod has not yet been described in the literature. Such a structure

turecouldresultinastrainedCdSe/CdSinterface,

a seed would enable band gap engineering and tuning of electron/hole

ed, casethe

of the larger seed, leading to a blue shift in

transport unrelated propertiesto in the crystal near-infrared structure

(NIR) region, potentially

o. 17,23 moreIn addition, the 3.9-nm seed is exposed to

dwever,

environment, making it sensitive to dielectric

gand effects shell and increase in surface traps resulting

xchange of the process, 1 possibly causing CdSe/CdS the shift and

terface, ission at longer wavelengths in the recovered PL

shift 2.3-nm in seed, in which case the sensitivity of its

osed to

Cu2Se/Cu2S

e external surface/ligand environment is expected

electric ue to complete embedment in the rod, shows

Lsulting

maximum position to within 0.5 PbSe/PbS nm, implying

e

ift

change.

and

This demonstrates that the anionic

red PL

eserved during the exchange without significant

y of its

nions across the interface, allowing the original

pected

orphology to be conserved.

shows

nular

plying

dark-field (HAADF) Cu and + high resolution Pb +2

ctron anionicmicroscopy CdSe CdS (HRTEM) of the CdCu2Se and Cu Cu2S

PbSe PbS

nificant ure 2, Supporting Information) show that they

eriginal

and shape during exchange. In HAADF images

selenide

Alivisatos,

seed exhibits

et al. JACS

higher

(2010)

Z-contrast compared

olution , allowing visualization of seed location. This is


Using cation exchange to make shapes

MOLECULAR

FOUNDRY

Strain-free core/single-crystal shells by

chemical conversion

Ouyang, et al Science (2010)

MOLECULAR

FOUNDRY



chemical conversion

amorphous shell

single xtal shell

+ M n+

TBP


MOLECULAR

FOUNDRY



chemical conversion

amorphous shell

single xtal shell

+ M n+

TBP

Au/CdSe

20 nm 5 nm


MOLECULAR

FOUNDRY



chemical conversion

II-2. Theory of hard- soft acids and bases

amorphous shell

single xtal shell

+ M n+

TBP

Au/CdSe

20 nm 5 nm


MOLECULAR

FOUNDRY


Outline




Size control



Shape control


Oriented attachment

MOLECULAR

FOUNDRY


(Figure 3 A) aggregated quasi-spherical partiized

beside already formed short rods. Most of

se experiby

the

by the

ssembled

gand-free

oparticles

however,

s could be

ration or

ZnO, Co,

n requires

zed when

llographic

sis Puntes

the case

e ligands,

e surface

ity single

ed attachicles.

The

raphically

to almost

nsmission

c acetate

ditions. [16]

eveloped

the ZnO

tration of

of below

as mainly

100 nm

Oriented attachment of nanocrystals

with anisotropic structures

40 nm

COMMUNICATIONS

Weller, ntrations. Figure 1. TEM images of ZnO. A) starting sol; B) after one day of reflux of

et al. Angew. Chem. (2002)

the concentrated sol. The insets show high-resolution TEM images of

l. for the

individual nanoparticles.


fused together, the lattice planes are aligned relative to e

other. Thus,

early

oriented

stageattachment of growth,

seems

where

to be

the

thewidth prerequ

o

for the condensation identical tostep theleading diameter to crystallite of the particles fusion un

our experimental starting conditions. sol. This is indeed observed as sho

This type in Figure of oriented 4. [21] attachment implies that the as

ratio of the rods should peak at integer numbers, at least i

early stage of growth, where the width of the rods is alm

identical to the diameter of the particles in the concentr

starting sol. This is indeed observed as shown in the histog

in Figure 4. [21]

Wurtzite ZnO

nanocrystals form

“attached” chains, then

single crystalline rods

upon heating

No bulky surfactants

means lower kinetic

barrier

Figure 4. Histogram (with n ˆ frequency) show

(length/width) of 2000 ZnO nanorods at an ear

reflux). The width of the rods at this stage is

Figure 4. Histogram (with n ˆ frequency) showing the aspect ra

(length/width) diameter of 2000 ofZnO the quasi-spherical nanorods at an early nanoparticles. stage MOLECULAR of growth

FOUNDRY

reflux). The width of the rods at this stage is almost identical to

diameter of the quasi-spherical nanoparticles.

In summary the presented experime

(Figure 3 A) aggregated quasi-spherical partiized

beside already formed short rods. Most of

se experiby

the

by the

ssembled

gand-free

oparticles

however,

e surface

ity single


with anisotropic structures

COMMUNICATIONS

s could be

ets of CdSe quantum dots,

ration or

ZnO, Co,

the recent hypothesis that

n requires

zed when

to ligands 100 are nmthe slowest

llographic

sis Puntes

eometry of nanoparticles

the case

e ligands,

concentration of surfac-

ed attachiclesofThe

II-VI colloidal quanidable

challenge to state-

raphically

to almost

nsmission

ic structure techniques. To

c acetate

ditions.

quantum dot studies 40 nmhave

[16]

eveloped

ilicon, 15,16 germanium, 17,18

the ZnO

tration of

electronic

of below

structure of the

as mainly

Weller, ntrations.

ed in terms Figure 1. TEM

et al. ofimages the of ZnO.

Angew. s A) and startingp

sol; B) after one day of reflux of

Chem. (2002)

the concentrated sol. The insets show high-resolution TEM images of

l. for the

g bondsindividual at the nanoparticles. surface can


fused together, the lattice planes are aligned relative to e

other. Thus,

early

oriented

stageattachment of growth,

seems

where

to be

the

thewidth prerequ

o

for the condensation identical tostep theleading diameter to crystallite of the particles fusion un

our experimental starting conditions. sol. This is indeed observed as sho

This type in Figure of oriented 4. [21] attachment implies that the as

ratio of the rods should peak at integer numbers, at least i

early stage of growth, where the width of the rods is alm

identical to the diameter of the particles in the concentr

starting sol. This is indeed observed as shown in the histog

in Figure 4. [21]

high Ɣ end facet

Wurtzite ZnO

nanocrystals form

“attached” chains, then

single crystalline rods

upon heating

No bulky surfactants

means lower kinetic

barrier

low Ɣ

Figure 4. Histogram (with n ˆ frequency) show

(length/width) side facets of 2000 ZnO nanorods at an ear

reflux). The width of the rods at this stage is

Figure 4. Histogram (with n ˆ frequency) showing the aspect ra

(length/width) diameter of 2000 ofZnO the quasi-spherical nanorods at an early nanoparticles. stage MOLECULAR of growth

FOUNDRY

reflux). The width of the rods at this stage is almost identical to

diameter of the quasi-spherical nanoparticles.



attached. Cd (Se) atoms are In shown summary in green the (gray). presented experime


with isotropic structures

Figure 2. Current-voltage characteristics (drain current, I d , versus

voltage, V ds , as a function of gate voltage, V g ) of field-effect tran

made of (a) p- and (b) n-doped PbSe nanowires aligned across 2 µm

between source and drain electrodes (inset to a).

PbSe

Figure 2. Current-voltage characteristics (drain current, I d , versus drain

formed in the presence of oleic acid.

voltage, V ds , as a function of gate voltage, V g ) of field-effect transistors

made of (a) p- and (b) n-doped PbSe nanowires aligned across 2 µm gaps


Information summarizes the effect of reagent

diameter distribution

concentration

form in the presence

and

of n-tetradecyl

phonic acid (TDPA) (Figure 1c-f). The average length o

reaction temperature on the yield and length nanowiresof is typically PbSe10-30 nanowires µm, while the average diam

can be tuned from ∼3.5 to ∼18 nm with a ∼10% stan


deviation by adjusting the temperature profile during gr

When oleic acid is used as the sole stabilizer,

and controllingPbSe the reaction

nanowires

time. Figure 1d shows the HR

Figure 3. PbSe nanorods formed in a colloidal solution of PbSe nanocrystals

washed from an excess of stabilizing exhibit agent significant (oleic acid). shape Inset shows and diameter resolved. undulations, The insets display as the shown corresponding selected

image of two nanowires with (100) and (110) lattice p

PbSe “oligomers” formed at early stages in ina Figure hot colloidal 1b. However, solution ofthe PbSe

electron diffraction (SAED) patterns. Surveys of num

nanowiresamples shape

usingand X-raymorphology

diffraction (Figure S2 of the Suppo

nanocrystals.

can be controlled by introducing cosurfactants Information), into HRTEM, theand reaction SAED indicate that the

nanowires are consistently !100" oriented single crystals

mixture. For example, long, straight the rocksalt wiresstructure.

with a narrow

lattice. 16 The suggested aggregationdiameter mechanism distribution basedform on the

Straight PbSe nanowires can be employed in p-type

in the presence of n-tetradecylphosphonic

reactivity High-resolution

n-type FETs, as shown in graphs a and b of Figu

difference in surface structure and Figure 1.

acid

(a) SEM of (TDPA)

and the (b) TEM(002) images

(Figure

of PbSe and nanowires

1c-f). The respectively, average and, therefore, length canof be used thein complementary

grown in solution in the presence of oleic acid. (c) Overview and (d-f)

circuits. The type of conductivity is determined by nano

high-resolution TEM images of PbSe nanowires formed in the presence of

(002h) faces of the wurtzite lattice. nanowires is typically 10-30 µm, while the average diameter

oleic acid and

Tang,

n-tetradecylphosphonic

Kotov,

acid.

and

Selected area

Giersig

electron diffraction doping.

from a film of PbSe nanowires to c) and single nanowires imaged

can be tuned from ∼3.5 to ∼18 nm with

One-Dimensional

a ∼10%

Wires

standard

from Zero-Dimensional N

demonstrated that wurtzite CdTealong nanowires the (100) and (110) can zone axes be(insets formed to d). The diameter fromof PbSe

particles. Several indirect observations suggest PbSe nano

nanowires can be tuned from (e) ∼4 nm to (f) ∼18 nm.

deviation by adjusting the temperature shown profile in Figure during 1 are formed growth by spontaneous alignmen

zinc blend CdTe nanocrystals after partial removal of the

fusion of PbSe nanoparticles. For example, the formatio

°C, theand same reactants controlling and stabilizing theagents reaction form monodisperse

stabilizing agents from the nanocrystals’

time. Figure 1d shows the HRTEM

nanocrystals.

image 26 PbSe nanorods is observed during the aging of coll

The surfaces. 15 optimal concentration The of oleic cubic, acid used for

nanowire synthesisof about two80% nanowires of that typically with employed(100) solutions

in and (110) of PbSelattice nanocrystals planes where the excess stabil

zinc blend CdTe nanocrystals formed

the synthesischainlike of PbSe nanocrystals.

aggregates

resolved. The insets 26 agents have been washed away (Figure 3).

High Pb:Se reactant

due

ratios

display the corresponding selected area

increase the yield of the nanowires and the average nanowire

Figure 4a shows the HRTEM images of dimers forme

to induced dipole-dipole interactions at the early stage of wire

length. electron For example, injecting diffraction precursors(SAED) with a Pb:Se molar

oriented attachment of PbSe nanocrystals along the !100"

patterns. Surveys numerous MOLECULAR

ratio of 1:3 at 250 °C yields ∼20% (by volume) of wires with

of the rocksalt lattice. This type of oriented attachment i

formation and then slowly recrystallized

FOUNDRY

an average samples

into

length ∼3 using

hexagonal,

µm and80%particlesafter2minof

X-ray

wurtzite

diffraction (Figure most typical S2 of forthe PbSe Supporting

nanocrystals; however, in the pres

growthInformation), 170-190 °C. WhenHRTEM, the Pb:Se ratio isand increased

SAED

to

of primary amines, attachment along !110" axis is also obs

CdTe nanowires.

indicate that the PbSe

3:1, the same growth conditions lead to 98% nanowires with

(Figure 4b).

an average nanowires length over are 30 µm. consistently Table S1 of the Supporting !100" oriented Figure The formation

single 4. High-resolution of nanowires

crystals

via self-assembly

with TEMof nanopar imag

CB Murray, et al JACS (2005)


The inherent anisotropy of crystal structure or crystal surface

Information summarizes the effect of reagent concentratio

reaction temperature on the yield and length of PbSe nano

When oleic acid is used as the sole stabilizer, PbSe nano

exhibit significant shape and diameter undulations, as sh

in Figure 1b. However, the nanowire shape and morpho

can be controlled by introducing cosurfactants into the rea

mixture. For example, long, straight wires with a na

has been studied for several materials. 14-18 Weller et al. rep







PbSe




made of (a) p- and (b) n-doped PbSe nanowires

PbSe has

aligned

rock

across

salt

2 µm gaps




concentration


and

of n-tetradecyl

phonic acid (TDPA) (Figure 1c-f). The average length o


Oriented can be tunedattachment

from ∼3.5 to ∼18 nm with a ∼10% stan





nanowires



washed from an excess of stabilizing exhibit agent significant (oleic acid). shape Inset shows and diameter axes resolved. as undulations, a Thefunction insets display as the shown of corresponding selected

observed along different







nanocrystals.

which surfactants are



mixture. For example, long, straight present

the rocksalt wiresstructure.

with a narrow







acid

(a) SEM of (TDPA)


(Figure







oleic acid and

Tang,


Kotov,

acid.

and

Selected area

Giersig




One-Dimensional

a ∼10%

Wires

standard


demonstrated that wurtzite CdTealong nanowires the (100) and (110) can zone axes be(insets formed to d). The diameter fromof PbSe




zinc blend CdTe nanocrystals after partial removal of the





nanocrystals.


The surfaces. 15 optimal concentration The of oleic cubic, acid used for

nanowire synthesisof about two80% nanowires of that typically with employed(100) solutions




aggregates


High Pb:Se reactant

due

ratios











FOUNDRY

an average samples

into

length ∼3 using

hexagonal,


X-ray

wurtzite



growthInformation), 170-190 °C. WhenHRTEM, the Pb:Se ratio isand increased

SAED

to


CdTe nanowires.



(Figure 4b).



crystals

via self-assembly












structure - cubic








PbSe






made of (a) p- and (b) n-doped PbSe nanowires

PbSe has

aligned

rock

across

salt

2 µm gaps









concentration


and

of n-tetradecyl

Nanowires and Nanorings

phonic acid (TDPA) (Figure 1c-f). ARTICLE

The average length o


Oriented can be tunedattachment

from ∼3.5 to ∼18 nm with a ∼10% stan





nanowires



washed from an excess of stabilizing exhibit agent significant (oleic acid). shape Inset shows and diameter axes resolved. as undulations, a Thefunction insets display as the shown of corresponding selected

observed along different







nanocrystals.

which surfactants are



mixture. For example, long, straight present

the rocksalt wiresstructure.

with a narrow







acid

(a) SEM of (TDPA)


(Figure







oleic acid and

Tang,


Kotov,

acid.

and

Selected area

Giersig




One-Dimensional

a ∼10%

Wires

standard


demonstrated that wurtzite CdTeTransient along nanowires the (100) and (110) can symmetry

zone axes be(insets formed to d). The diameter fromof PbSe




zinc blend CdTe nanocrystals afterbreaking partial removal of of the





nanocrystals.


cuboctohedra The surfaces. 15 optimal concentration The of oleic acid used for

nanowire synthesisof about two80% nanowires

cancubic,

of that typically with employed(100) solutions




aggregates


High Pb:Se reactant

due

produce a dipole

ratios











FOUNDRY

an average samples

into

length ∼3 using

hexagonal,


X-ray

wurtzite



CB Murray, et al JACS (2005) growthInformation), 170-190 °C. WhenHRTEM, the Pb:Se ratio isand increased

SAED

to


CdTe nanowires.



(Figure 4b).



crystals

via self-assembly




structure - cubic


Combining shape control and oriented

attachment

Cho et al.

Cho et al.

Change surfactant to get

Figure 7. (a) Octahedral PbSe nanocrystals octahedral grown PbSe in the (only

presence of HD

nanowires grown in the presence of HDA. The cartoons show packing of o

crystal helical PbSe nanowire grown in 111 oleic facets)

acid/HDA/trioctylamine mixtu

shown in Figure 1c, in the presence of trioctylamine.

Attach to form zig-zag

wires

planes, thus allowing more time for diffusion of atoms on

nanowire surfaces. To investigate the role of surface diffusion,

MOLECULAR

FOUNDRY

the rough wires (the wires similar to those shown in Figure 6a)


are heated on a heating stage in the TEM. As the temperature


chloroalkanes (fig. S1). The nanosheets

lateral dimensions of several hundred

meters and stack showing Moiré patterns

d by the interference between the crystalline

es of the individual attachment

sheets (see fig. S2 for

é patterns and corresponding simulations).

Generating 2D sheets by oriented

1. TEM image of stacked PbS nanosheets using

-trichloroethane.

H. Weller, et al Science (2010)

rg


MOLECULAR

FOUNDRY








particles (as shown in Fig. 2B), which should

Generating exhibit2D thesheets reactive by {110} oriented surfaces. Beca


-trichloroethane.


rg


MOLECULAR

FOUNDRY








particles (as shown in Fig. 2B), which should

chloride compound), whereas in the presence of

chloride compounds, which are known to act as

lead complexing agents (19), the kinetics of nucleation

and growth are altered, leading to ~3-nm

particles (as shown in Fig. 2B), which should still

exhibit the reactive {110} surfaces. Because

Generating exhibit2D thesheets reactive by {110} oriented surfaces. Beca

Downloaded fr


-trichloroethane.


rg


Fig. 3. Schematic illustration of large-particle (A)

MOLECULAR

FOUNDRY

Lecture 2 summary: Complex structures

Shape control

Surface energy of different facets determines lowest energy shape

Ligand-facet interactions change lowest energy shape AND growth kinetics

Heterostructures

Strain and interfacial energy impact achievable morphology

Chemical conversion

Post-synthetic conversion provides access to new compositions and

morphologies

Oriented attachment

Orientation driven by dipoles, attachment eliminates high energy facets

MOLECULAR

FOUNDRY


Lecture 2 summary: Complex structures

Shape control

Surface energy of different facets determines lowest energy shape

Ligand-facet interactions change lowest energy shape AND growth kinetics

Heterostructures

Strain and interfacial energy impact achievable morphology

Chemical conversion

Thermodynamic drivers and kinetic down-selection of growth pathways

Post-synthetic conversion provides access to new compositions and

morphologies

Oriented attachment

Nanocrystal morphologies derive from:

Complex interplay between surface, interfacial, and “bulk” free energy

Orientation driven by dipoles, attachment eliminates high energy facets

MOLECULAR

FOUNDRY


What have I left out?

Size-dependent properties & applications

Nanocrystal assembly and device/systems integration

Compositional complexity: Doping and ternary/quarternary

compositions

Templated shape control (e.g. inverse micelles)

Nanocrystal surface chemistry

Chemical mechanisms and pathways

MOLECULAR

FOUNDRY


Nanomaterials II

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