R. Mohan Sankaran

George B. Mayer Assistant Professor of EngineeringDepartment of Chemical Engineering

Case Western Reserve UniversityCleveland, OH 44106-7217Email: mohan@case.edu

http://microplasma.case.edu

Bottom-up solutions to plasma synthesis of nanomaterials

•

High purity•

High fidelity pattern transfer•

Low temperature processing•

Large volume (large area)

Applied Materials, Inc.

Plasma etching of Si

Plasma-enhanced CVD of amorphous Si

Xunlight, Inc.

1 µm

Plasma processing of materials

Powered electrode

Substrate

PLASMAA+ A+

A+

A·

e-

e-

e-

e- A·

50 nm 30 nm 10 nm 5 nmSi nanowire CNT

Chau et al., Intel (2007)

From top-down approaches to…

2007

2010

?

Manufacturing

Research

Development

…to bottom-up approaches to materials synthesis

Stoffels et al., J. Vac. Sci. Technol. A , Vol. 17, 3385 (1999)

Methane particles1 Å

1 nm

10 nm

0.1 μm

1 μm

Atoms, molecules,

radicals

Polycarbons

Clusters

Agglomerates

Powder

A new direction for plasma synthesis of materials

Wafer

Vacuum pump

Electrodes

Conventional plasma

•

Large volume, batch•

Low pressure (10-5-1 Torr)•

Non-thermal [> 10,000 K]•

Plasma-surface interactions

100 mm 5 mm

Microplasma

•

Microscale, continuous-flow•

High pressure (10-1000

Torr)•

Non-thermal [> 10,000 K]•

Gas-phase interactions

Gas flow

Microplasmas are a versatile source for nanomaterials synthesis

Gas-phase nucleation of nanoparticles

Non-lithographic micro/nanofabricationCatalytic growth of well-defined CNTS

Plasma-liquid electrochemistry

10 nm

2 nm

Chiang et al.,

JPCC 112, 17920 (2008)Chiang et al., Diam. Rel. Mater. 18, 946 (2009)Chiang et al., Nature Mater. 8, 882 (2009)Chiang et al., ACS Nano, ASAP online

Chiang et al., APL 91, 121503 (2007)Chiang et al., Adv. Mater 20, 4857

(2008)

Chiang et al., APL 91, 021501 (2007)Lee et al., in preparation

Richmonds et al., APL 93, 131501 (2008)Chiang et al., PSST, in press

Typical operating conditions (for single discharge): ►100 sccm Ar►Vplasma

=450 V DC, Iplasma

=5 mA►Dhole

=180 m, L=2 mm

4 mm

Microplasmas operate stably and continuously at atmospheric pressure

U.S. Patent No. 6,700,329 (Issued 3/2/04)

Continuous atmospheric-operation in Ar gas flow for at least 100 hrs!

0 200 400 600 800 1000 1200190

200

210

220

230

240

250

260

Pressure 50 Torr 100 Torr 200 Torr 500 Torr 760 Torr

Vol

tage

(Vol

ts)

Current (A)

RB

Gas

L

- +

Metal capillary tube(CATHODE)

Dhole

Counter electrode(ANODE)

Gas-phase nucleation of nanoparticles

Batch vs. continuous-flow synthesis of nanoparticles: The example of CdSe

Yen et al., Adv. Mater., Vol. 15, 1858 (2003)

Batch Continuous-flow

Talapin et al., J. Phys. Chem. B, Vol. 105, 12278 (2001)

Kinetic-control Reaction-control

Continuous-flow microreactors based on microplasmas

Characteristics of process

•

Non-thermal dissociation of reactive precursor molecules (EID)

•

Short residence times (10-3-10-6

seconds)

•

In situ monitoring (kinetic studies)

•

Generic –

precursor can be chosen to grow different materials

Microplasma

Inert gas (Ar) +

Vapor precursor

Nanoparticles1-3 nm

15-20% std. dev.- +

DC

Continuous-flow synthesis and in situ size classification of nanoparticles

INERT GAS+

VAPOR PRECURSOR

MICROPLASMA

CATHODE ANODE

PARTICLE COUNTER

SIZE CLASSIFIER N2►Continuous-flow, steady-state

►Atmospheric-pressure

►In situ

monitoring

Cylindrical differential mobility analyzer (TSI, Inc.)Detects particles 2 nm < Dp

< 100 nm

NANOPARTICLES

QUARTZ TUBE

0.1 1 10 100104

105

106

107

Dp (nm)

dN

/d(lo

gDp) (

cm-3)

0.1 1 10 100104

105

106

107

dN/d

(logD

p) (cm

-3)

Dp (nm)

Aerosol measurements of Fe and Ni nanoparticles

0.1 1 10 100104

105

106

107

dN/d

(logD

p) (cm

-3)

Dp (nm)

2.6 ppmDpg

=3.11

nm σg

=1.16

5.2 ppmDpg

=4.74

nm σg

=1.22

Fe NPs

0.1 1 10 100104

105

106

107

dN/d

(logD

p) (cm

-3)

Dp (nm)

1.5 ppmDpg

=2.89

nm σg

=1.14

3.0 ppmDpg

=4.52

nm σg

=1.20

4.5 ppmDpg

=6.82

nm σg

=1.28

Ni NPs

Mechanism for nucleation and growth of NPs from vapor-phase precursors

Molecular fragments from electron impact dissociation

(EID)

Fe

Fe

Microplasma volume

►For constant plasma parameters, Dp

is a function of precursor concentration

and enthalpy of dissociation►ΔHD

(ferrocene) ~ 662 kcal/mol; ΔHD

(nickelocene) ~ 686 kcal/mol

+ Vapor

5 nm 5 nm 5 nm2 nm

(220),0.127 nm

(111),0.203 nm

2.0 ppm 2.6 ppm 5.2 ppm

HRTEM analysis of as-grown Ni nanoparticles deposited by electrostatic precipitation

0 5 100

5

10

15

20

N=48

Cou

nts

Particle diameter (nm)

Mean diameter = 2.15 nmStandard deviation = ±0.60 nm

0 5 10

0

5

10

15

20

Particle diameter (nm)

Cou

nts

Mean diameter = 5.11 nmStandard deviation = ±2.01 nm

N=76

0 5 100

5

10

15

20

N=46

Cou

nts

Particle diameter (nm)

Mean diameter = 3.70 nmStandard deviation = ±0.75 nm

Dp

=5.1 nm ±2.0 nmDp

=3.7 nm ±0.75 nmDp

=2.2 nm ±0.60 nm

0.1 1 10 100104

105

106

107

Dp (nm)

Ni Ni0.88Fe0.12 Ni0.67Fe0.33

Ni0.27Fe0.73

dN

/d(lo

gDp) (

cm-3)

Compositionally-controlled growth of bimetallic (alloyed) nanoparticles

0.1 1 10 100104

105

106

107

dN/d

(logD

p) (cm

-3)

Dp (nm)

2.2 ppmDpg

=3.5 nm σg

=1.18

2.6 ppmDpg

=3.9

nm σg

=1.19

2.9 ppmDpg

=4.5 nm σg

=1.22

73%

27%

Dimensional tuning: Ni0.27

Fe0.73 Compositional tuning

EDS of as-grown Nix

Fe1-x

bimetallic nanoparticles

0 5 100

1x103

2x103

3x103

4x103

Cou

nts

(A.U

.)

Energy (keV)

C

O

NiNi

SiCu

NiCu

Ni Ni

NiFe

Fe

Ni NiNi

Fe

Fe

Ni

Ni0.67

Fe0.33

Ni0.27

Fe0.73

► Final atomic composition corresponds to relative metallocene concentration

3.0 nm

0 20 40 60 80 100

0

20

40

60

80

100 Experimental data Cm=Ca

Ni a

tom

ic c

once

ntra

tion

(Ca) (

%)

Nickelocene concentration (Cm) (%)

XRD of as-grown Nix

Fe1-x

bimetallic nanoparticles

40 45 50 55 60 65 70 75 800

50

100

150

200

Inte

nsity

(A. U

.)

2(degree)

(111)

(200)(220)

(111)

(200)

(220)

(111)

(200)

(220)

*

*

Ni

Ni0.67

Fe0.33

Ni0.27

Fe0.73

3.0 nm►Bulk Fe is bcc (α); bulk Ni is fcc (γ)

►NiFe alloys exhibit new distorted crystal structure

F. A. Shunk, Constitution of Binary Alloys (1991)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50

1

2

3

4

5

6

Fe Ni Ni0.67Fe0.33

Ni0.27Fe0.73

Dg(n

m)

Cm (ppm)

(111), 0.20 nm

(111), 0.20 nm

(111), 0.21 nm

(111), 0.21 nm

2 nm 2 nm

5 nm 5 nm

2 nm 2 nm

5 nm 5 nm

Ni,

3.2

nm

Ni,

2

nm Ni0.27

Fe0.73

, 2 nm

Ni0.27

Fe0.73

, 3.2 nm

A priori design and synthesis of dimensionally-

and compositionally-tuned bimetallic nanoparticles

Catalytic growth of carbon nanotubes

Conventional approaches to catalyst preparation for carbon nanotube growth

1. Floating catalyst method

2. Substrate growth

Catalyst precursor+

Hydrocarbon gas

Plasma

Yoshida et al., Nano Lett., Vol. 8, 2082 (2008)

A new two step, sequential process for catalytic growth of CNTs

INERT GAS+

METALLOCENE

MICROPLASMA

CATHODE ANODE

PARTICLE COUNTER

SIZE CLASSIFIER

TUBE FURNACE

SEED NANOPARTICLES

CNTS

N2

H2

+ C2

H2

Cylindrical differential mobility analyzer (TSI, Inc.)Detects particles 2 nm < Dp

< 100 nm

QUARTZ TUBE

►Continuous-flow, steady-state

►Atmospheric-pressure

►Independent control of growth parameters

►In situ

monitoring

0.1 1 10 100103

104

105

106

107

dN/d

log(

Dm) (

parti

cles

/cm

3 )

Dm (nm)

•700 oC + no catalyst C NPs

•Ni NPs room temp.

•500 oC CNTs?

•700 oC CNTs, C NPs?

<700 oC only catalytic CNT growth!

Aerosol monitoring of reactor effluent

CNTs?C NPs?

200 300 1000 1200 1400 1600 1800

Inte

nsity

(A. U

.)

Raman shift (cm-1)

Diameter-controlled growth of CNTs –

Towards high-purity synthesis of SWCNTs

3 nm

Ni 2.5 nm / DWCNTs

Ni 3.1 nm / MWCNTs

10 nm

5 nm

Ni 2.2 nm / SWCNTs

Ni 2.6 nm / TWCNTs

3 nm

CNT diameter1.6 nm.….....0.8 nm

D GRBM

2.2 nm

2.5 nm

2.6 nm

3.1 nm

75% purity!

metallic (m): n-m=3isemiconducting (s): n-m≠3i

s m m s m

(n,m) chirality

/

structure of SWCNTs (electronic type)

1 cm

Glassy

fiber

filter SWCNT dispersion

►

0.17

mg/hr (no

purification) (for

single microplasma

reactor)

► Sodium dodecyl sulfate (SDS)+ D2

O

Sonication (24 hrs)+

Ultracentrifuging (38000g, 2 hrs)

Collecting and dispersing SWCNTs for optical characterization

400 600 800 1000 1200 1400

Nor

mal

ized

Abs

orba

nce

(A.U

.)Wavelength (nm)

Ni, 2.0 nm Ni, 2.6 nm

S11S22M11

►

Relative peak intensities in UV-

Vis-NIR spectra are identical

400 600 800 1000 1200 14000.0

0.1

0.2

Abs

orba

nce

Wavelength (nm)

Ni, 2.0 nm Ni, 2.2 nm Ni, 2.4 nm Ni, 2.6 nm

Decreasing Dp

►Overall yield (absorbance) of SWCNTs decreases with increasing particle size

UV-Vis-NIR absorbance: Effect of catalyst size

S11S22M11

Increasing Fe

Dp

=2.0 nm

UV-Vis-NIR: Effect of catalyst composition

Lower metallic content in nanotubes catalyzed by nanoparticles with higher Fe at%Significant changes in semiconducting population distribution as

the nanocatalyst composition is varied

400 600 800 1000 1200 1400

Nor

mal

ized

Abs

orba

nce

(A.U

.)

Wavelength (nm)

Ni Ni0.80Fe0.20

Ni0.67Fe0.33

Ni0.50Fe0.50

Ni0.27Fe0.73

Ni0.20Fe0.80

Photoluminescence spectroscopy of SWCNTs

1.0

0.0

0.5

Pho

tolu

min

esce

nce

inte

nsity

(A.U

.)

900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400500

550

600

650

700

750

800

850

(10,2)

Exci

tatio

n W

avel

engt

h (n

m)

Emission Wavelength (nm)

Ni0.27Fe0.73

(8,3)(7,5) (7,6)

(8,4)(6,5)

900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400500

550

600

650

700

750

800

850

(9,4)

Emission Wavelength (nm)

(10,2)

(6,5)

Exci

tatio

n W

avel

engt

h (n

m) Ni

(8,3)(7,5) (7,6)

(8,4)

(10,3)

(8,6)

(9,5)

(8,7)

900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400500

550

600

650

700

750

800

850

(9,4)

Emission Wavelength (nm)

(6,5)

Exci

tatio

n W

avel

engt

h (n

m) Ni0.67Fe0.33

(8,3)(7,5) (7,6)

(8,4)

(10,3)

(8,6)

(9,5)

(8,7)(10,2)

900 950 1000 1050 1100 1150 1200 1250 1300 1350 1400500

550

600

650

700

750

800

850

(10,2)

(6,5)

Emission Wavelength (nm)

Exci

tatio

n W

avel

engt

h (n

m) Ni0.5Fe0.5

(8,3)(7,5) (7,6)

(8,4)

Fe

Relative abundance of different semiconducting chiralities changes with nanocatalyst composition

(6,5) (8,3) (7,5) (8,4) (10,2) (7,6) (9,4) (10,3) (8,6) (9,5) (8,7)0

5

10

15

20

25

30

35

40

45

50

A

bund

ance

(%)

CNT Chirality

Ni0.27Fe0.73

Ni0.5Fe0.5

Ni0.67Fe0.37

Ni

Decreasing SWCNT diameter

0.90 nm

0.83 nm

Model for SWCNT growth based on lattice matching (epitaxial growth)

S. Reich et al., Chem. Phys. Lett., Vol. 421, 469 (2006)

Ni(111) bond length: 0.249 nmGraphene lattice constant: 0.248 nm

0 20 40 60 80 1000.28

0.29

0.30

fcc

latti

ce c

onst

ant (

nm)

bcc

latti

ce c

onst

ant (

nm)

Ni (%)

0.34

0.35

0.36

Crystal structure of Nix

Fe1-x

nanocatalysts

Catalyst

fcc bcc Diffraction peak

(degree) afcc (nm)

Bond length (nm)

Diffraction peak (degree) abcc

(nm)

Bond length (nm) (111) (200) (220) (110) (200)

Ni 44.67 52.86 75.98 0.3507 0.2480 - - - - Ni0.67Fe0.33 44.20 51.16 75.38 0.3563 0.2519 - - - - Ni0.27Fe0.73 43.86 50.88 75.32 0.3578 0.2530 45.40 65.60 0.2836 0.2456

Fe - - - - - 44.76 - 0.2888 0.2501

Multi line excitation micro Raman spectroscopy of SWCNTs

488 nm

514 nm

M11 S22

150 200 250 300 350

HiPCO

633 nm

Nor

mal

ized

inte

nsity

(A.U

.)

Raman shift (cm-1)

Ni

Ni0.27Fe0.73

Diameter (nm) 1.4 1.2 1.1 1.0 0.9 0.8 0.7

633 nm

100 150 200 250 300 350

1.8 1.6 1.4 1.2 1.1 1.0 0.9 0.8 0.7

Ni

488 nm

Nor

mal

ized

inte

nsity

(A.U

.)

Raman shift (cm-1)

Ni0.27Fe0.73

Diameter (nm)

HiPCO

M11S33

100 150 200 250 300 350

Nor

mal

ized

inte

nsity

(A.U

.)

Raman shift (cm-1)

514 nm

Ni

Ni0.27Fe0.73

Diameter (nm) 1.8 1.6 1.4 1.2 1.1 1.0 0.9 0.8 0.7

HiPCO

M11S33

Relative amount of semiconducting and metallic nanotubes changes with catalyst composition

Excitation

wavelength (nm)

Temperature

(oC) Catalyst % semiconducting % metallic

FWHM of G-band

intensity (cm-1)

633 HiPCO 62.9 37.1 --

600 Ni 66.8 33.2 --

600 Ni0.27

Fe0.73 90.5 9.5 --

700 Ni0.27

Fe0.73 55.5 44.5 --

488 HiPCO 62.9 37.1 --

600 Ni 63.4 36.6 --

600 Ni0.27

Fe0.73 85.5 14.5 --

514 HiPCO 62.9 37.1 40

600 Ni 60.2 39.8 39

600 Ni0.27

Fe0.73 84.2 15.8 25

Fabrication of thin film SWCNT-based FETs

100 m 0.5 m

Si

SiO2

SWCNT

film

Al/Ti Al/Ti

2 μm

200 μm60 μm

Schematic of single device:

SEM of multiple devices fabricated in parallel:

Metal pads

Bottom -gate FET Source-Drain

Electrical characterization of SWCNT-based FETs

<1 1-2 2-3 >30

10

20

30

40

50

60

70

80

90

100

Perc

enta

ge (%

)

log(Ion/Ioff)

Ni0.27Fe0.73, N=25 Ni, N=22 HiPCO, N=30

Drain current vs. Gate voltage Overall statistics of on/off ratio

-10 -8 -6 -4 -2 0 2 4 6 8 1010-9

10-8

10-7

10-6

10-5

10-4

I ds (A

)

Vg (V)

HiPCO Ni Ni0.27Fe0.73

Vds=1V

p-type semiconductor

Plasma-liquid electrochemistry

A hybrid microplasma-based electrochemical cell

1.

Anodic dissolution of metal foil

2.

Reduction of metal ions at microplasma

3.

Nucleation, growth, and capping of particles

Microplasma-assisted electrochemical synthesis of colliodal Ag nanoparticles from bulk precursor

t=5 min t=7 min t=9 min

►Atmospheric-pressure, near room temperature

►Gas conduit for current flow (contact-less)

►Rapid reduction of aqueous metal cations

►Versatile (metal precursor, stabilizer molecule)

µplasma

Ag foil

Efficiency of cell operated with microplasma

2 3 4 50

10

20

30

Wei

ght l

ost (

mg)

Plasma current (mA)

Experimental data Theoretical (based on 100%)

Ag+

+ e-

Ag

2 3 4 50

5

10

Wei

ght l

ost (

mg)

Plasma current (mA)

Experimental data Theoretical (based on 100%)

Cu2+

+ 2e-

Cu

300 400 500 600 700 8000.00

0.05

0.10

0.15

0.20

0.25

Abso

rban

ce

Wavelength (nm)

Increasing timeTime (min)

Ag Au

300 400 500 600 700 8000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abso

rban

ce

Wavelength (nm)

Increasing time

5 10 15X 0.5

Time (min)

10 15 30

SPR

SPR

UV-visible absorbance of Ag and Au nanoparticle colloids

TEM of Ag and Au nanoparticles

0.20 nm

100 nm 5 nm

5 nm100 nm

Ag

Au

Microplasma reduction of metal salts –

Pt anode/HAuCl4

300 400 500 600 700 8000.0

0.1

0.2

0.3

0.4

0.5

1 min 2 min 3 min 4 min 5 min 10 min

Abs

orba

nce

Wavelength (nm)

Time (min)

0.0001 M HAuCl4

in water

Synthesis of Ag nanoparticle colloids for point-of-use SERS applications

AgNO3CV

Pt Anode

Crystal violet:

PlasmaCathode

Ar

400 600 800 1000 1200 14000

500

1000

1500

2000

2500

3000

3500

Inte

nsity

(A.U

.)

Raman shift cm-1

441 799 916 1176 1376 cm-1

10-7

M

5 min

10 min

15 min

20 min

Non-lithographic fabrication of patterned nanoparticle films

►Atmospheric-pressure, near room temperature

►~100 micron lateral resolution

►Polymer and metal salt are interchangeable

►Particle size depends on metal salt concentration

500 µm

~200 µm

AgNO3

+PVA

Reduction of wide range of metal salts is possible

30 40 50 60 70 80 90

2 (Degree)

Rel

ativ

e In

tens

ity (A

.U.)

Pt

Au

Ag

Ag NPsLow salt conc.

Pt NPs

10 nm 10 nm

50 nm

XRD spectra of thin films: TEM of NPs:

10 nm

Au NPs

Ag NPsHigh salt conc.

Flexible polymeric films with patterned metal nanoparticles

Acknowledgments

Funding:

Metal nanoparticle synthesis and characterization

•Amir Avishai (Materials Science)

•Prof. Frank Ernst (Materials Science)

Carbon nanotube characterization

•Regan Silvestri (Macromolecular Science)

•Prof. Jack Koenig (Macromolecular Science)

•Prof. Liming Dai (Chemical Engineering)

•Prof. Ken Singer (Physics)

FET fabrication and testing

•Mohammed Sakr (Physics)

•Prof. Xuan Gao (Physics)

Mentors

•C.C. Liu

•John Angus

Past group members

•Dr. Wei-Hung Chiang (Post Doc, AIST, Japan)

•Matthew Cochey (Product engineer, Venture Lighting)

•Christopher Carach (Ph.D. candidate, UCSB)

Current group members

•Pin Ann Lin (Ph.D. candidate)

•Seung Whan Lee (Ph.D. candidate)

•Carolyn Richmonds

•Tessie Panthani

•Raymond Rodgers

•Trent Hovis

•Megan Witzke

•Albert Xue

410 420 430 440 450 460 470 480 490

* ******

Inte

nsity

(A.U

.)

Wavelength (nm)

Microplasmas contain a high concentration of energetic electrons (> 10 eV)

d=178 m d=500 m

760 Torr Ar10 mA

200 sccm in d=180 μm hole

200 sccm in d=500 μm hole

450 460 470 480 490

* ******

Differential mobility analysis of aerosol nanoparticles

Bipolar charger (85Kr)Polydisperse particles

Sheath flow

High voltage rod-

-

--

-

-

-

-

-

-

-

-

-

-

--ExhaustParticle counter

+

+

+

+

+ --

-

+

Emerging applications of nanomaterials

Roy, Mater. Lett. (2006)Avouris, ACS Nano (2008)

Sailor, Nature Materials (2009)

Atwater, APL (2007)

Nanoelectronics: Catalysis: Nanomedicine:

Solar energy conversion:

0.1 1 10 100103

104

105

106

dN/d

log(

Dm) (

parti

cles

/cm

3 )

Dm (nm)

Aerosol size classification of CNTs catalyzed by Fe and Ni NPs

Fe catalyzed Ni catalyzed

Fe NPs

500 oC

600 oC

0.1 1 10 100103

104

105

106

dN/d

log(

Dm) (

parti

cles

/cm

3 )

Dm (nm)

Ni NPs

400 oC

500 oC600 oC

HRTEM analysis of carbon nanostructures

5 nm

50 nm

20 nm

20 nm

Fe catalyzed Ni catalyzed

500 oC

600 oC

Extracting kinetic parameters for CNT growth from aerosol measurements

CNT length calculation:

DA

Dt

Lt

t

mDDπ

tL 4

2

=

►Electrical mobility diameter (Dm

) corresponds to projected area diameter (DA

)

S.H. Kim et al., J. Aerosol Sci., Vol. 38, 823 (2007)