carbon nanotube thin-films nanoparticle assembly · pdf file• electron-beam lithography...

CKC Symposium 2006.9.27

Seung-Beck Lee

Division of Electronics and Computer Engineering &Department of Nanotechnology, Hanyang University, Seoul, Korea

Seung-Beck Lee


Nanodevices using Nanomaterials :

Carbon Nanotube Thin-Films&

Nanoparticle Assembly

Nanodevices using Nanomaterials :

Carbon Nanotube Thin-Films&


D. G. Hasko and G. A. C. JonesD. G. Hasko and G. A. C. JonesCavendish Laboratory, University of Cambridge, Cambridge, U.K.Cavendish Laboratory, University of Cambridge, Cambridge, U.K.


Heong-Suk Jeon, Chi-Won Cho, Bonghyun Park, and Seung-Beck Lee


Heong-Suk Jeon, Chi-Won Cho, Bonghyun Park, and Seung-Beck Lee


Template Assisted Electrostatic Assembly of Colloidal Au Nanoparticles Template Assisted Electrostatic Assembly of Colloidal Au Nanoparticles

D. G. Hasko and G. A. C. JonesD. G. Hasko and G. A. C. JonesCavendish Laboratory, University of Cambridge, Cambridge, U.K.Cavendish Laboratory, University of Cambridge, Cambridge, U.K.


Motivation

• As more memories are packed closer together, the charge on neighboring memory start to affect charge coupling in the floating gate changing the threshold voltage shift.

Parasitic Capacitive CouplingFlash Memory ScalingOne limit of Flash memory scaling

http://go.reuters.com/newsArticle.jhtml?type=technologyNews&storyID=9620463&src=rss/technologyNews


• Using pre-synthesized nanoparticles, size distribution may be reduced

Si nanowire split gate nano-floating gate memorySi nanowire split gate nano-floating gate memory

Gate

Gate

Au colloidalnanoparticles

Nanowire

Source Drain

Motivation

• Si nanowire geometry may increase floating gate charge induced scattering by limiting the transport channel to 1D

• Using nanoparticles, parasitic coupling may be reduced• Using metallic nanoparticles, charge density may be increased


OutlineOutline

• Nanoparticle Floating-Gate Characteristics• Nanoparticle Floating-Gate Characteristics

• Summary• Summary

Nanoscale Floating-Gate Memory using Colloidal Au Nanoparticles Electrostatically Assembled on Si Nanowires

Nanoscale Floating-Gate Memory using Colloidal Au Nanoparticles Electrostatically Assembled on Si Nanowires

• Nanoparticle Assembly•• Nanoparticle AssemblyNanoparticle Assembly

• Template Assisted Electrostatic Assembly• Template Assisted Electrostatic Assembly



• Au nanoparticle assembly on amino functionalized oxide surfaces

Surface functionalization



• Increased sedimentation time increases Au nanoparticle density with localized sedimentation

Surface functionalization

More than 6 hours deposition time Too Slow

6 h6 h 6 h


• Spin-coating on ATPS results in relatively even distribution

5 nm Au nanocrystal spin-coating at 3000 rpm 30 sec

2 times : even distribution

AFM study

10 times : Thickness increase


• However, difficult to increase density above 1010 without agglomeration

Spin-Coating on APTS


Si

SiO2

100 nm

2 min electrophoresis

200 nm

10 min electrophoresis


Electrostatic assembly (electric field guided assembly)

• Electric-field applied between the substrate and the cathode guides the charged nanoparticles towards the substrate

• The process is much faster than convection drying or sedimentation methods

• Selective deposition is also possible• 2 min process produces ~ 1011 cm-2 density


NFG CharacteristicsNFG Characteristics

SiSiO2

Si

PMMA

• Silicon-on-Insulator wafer (SIMOX)(n~1x1014/cm2) n-type Si : 70 nm, SiO2 : 100 nm

• PMMA resist spin-coatingA4 (4%PMMA in Anisole)5000 rpm for 30 sec, Thickness ~ 200 nm

• Electron-Beam Lithography10 nm spot size at 80 kV, 10 pA800 μC/cm2 exposure dosage

Reactive Ion Etching

SiCl4 (20 sccm) + CF4 (20 sccm) 20 mTorr, 300 W, 40 secSi 2.3 nm/s, SiO20.8 nm/s etch rates

• Dry Oxidation1000 oC for 10 min ~15 nm thermal oxide

Nanofabrication done at Microelectronics Research Centre, Cavendish Laboratory


50 nmGate

Gate

Si nanowire

Si

SiO2

100 nm 200 nm

Selective Electrostatic AssemblySelective Electrostatic Assembly


Selective Electrostatic AssemblySelective Electrostatic Assembly



10-9

10-8

10-7

10-6

-4 -2 0 2 4VGS (V)

I DS/

W (A

/μm

)

VDS = 2 V

nanoparticledensity

~ 1011 /cm2

VDS = 1.4 V

10-9

10-8

10-7

10-6

-4 -2 0 2 4

I DS/

W (A

/μm

)

VGS (V)

Before Au nanoparticle assembly

Measurements performed in vacuum at room temperatures

• Original device characteristics shows hysteresis due to surface charge• After Au nanoparticle electrostatic assembly, the threshold voltage shift is increased from 0.14 to 1.9 resulting in ΔVT ~ 1.5 V

Nanoscale Floating-Gate Memory Characteristics


OutlineOutline

S D

G

Singe Nanoparticle Memory(?)

• Requires a method to place individual nanoparticles in predesignated locations


• Template assisted electrostatic assembly of 20 nm Au nc in 60 nm line pattern• nc diameter dnc / linewidth lW = 0.33 ; nc number per column ~ 2 • Selectivity to Al surface over resist surface is very high• High nc density resulted in multilayer nc assembly with fluctuation in layer number

Template Assisted Electrostatic AssemblyTemplate Assisted Electrostatic Assembly

Si

Si

AlOxPMMA

Template Assisted Electrostatic Assembly

Capillary force


Single Nanocrystal Array assemblySingle Nanocrystal Array assembly


• Template assisted nc electrostatic assembly of 20 nm Au nc in square array pattern• nc diameter dnc / linewidth lW = 0.5 ; average site density ~ 1.3 • High percentage(>70%) of holes filled with a single nanocrystal• Density fluctuation is reduced by limiting the template dimensions

40 nm40 nm

Single Nanocrystal Array assemblySingle Nanocrystal Array assembly

1 μm1 μm

Template Assisted Electrostatic AssemblyTemplate Assisted Electrostatic Assembly


Summary

40 nm40 nm

• We have demonstrated electrostatic selective assembly of colloidal Au nanoparticles on Si nanowires

• Nanoscale Floating Gate Memory characteristics were investigated with high threshold voltage shift, however retention time needs to be increased

• By using template assisted electrostatic assembly, it may be also be possible to use a single nanocrystal for NFGM

• Template assisted electrostatic assembly demonstrates that by using electrostatic assembly on patterned surfaces selective deposition of Au nanoparticles on device position may be possible

S D

G

Singe Nanoparticle Memory(?)


Highly Flexible and Transparent Single Wall Carbon Nanotube NetworkGas Sensors Fabricated on PDMS Substrates

Highly Flexible and Transparent Single Wall Carbon Nanotube NetworkGas Sensors Fabricated on PDMS Substrates

Chang-Seung Woo, Yoon-Sun Hwang, Chae-Hyun Lim, and Seung-Beck Lee

Division of Electronics and Computer Engineering &Department of Nanotechnology,Hanyang University, Seoul, Korea

D G Hasko and K B K TeoUniversity of Cambridge, Cambridge, U.K.


Motivation

Future Electronics yet fully developedFuture Electronics yet fully developed

• RFID• Mobile Sensor Network• Wearable Electronics• Smart Dust• Bendable (Flexible) Electronics• Transparent Electronics

• RFID• Mobile Sensor Network• Wearable Electronics• Smart Dust• Bendable (Flexible) Electronics• Transparent Electronics

requires

Low power consumptionHigh sensitivityMechanical stabilityChemical stabilityFlexible, transparent substratesFlexible, transparent interconnectsCompatible with Si processingMass producible (at low cost)

Mobile, Wearable, Flexible, Transparent Mobile, Wearable, Flexible, Transparent


Motivation

Carbon nanotube based Future ElectronicsCarbon nanotube based Future Electronics

Low power consumptionHigh sensitivityMechanical stabilityChemical stabilityFlexible, transparent substratesFlexible, transparent interconnectsCompatible with Si processingMass producible (at low cost)

Potential


• Energy band structure and density of states (DOS)

Metallic nanotubes

m-n = 0, Eg=0

DOS

Semiconducting nanotubes(wide gap)

m-n <>3p, Eg= 0.5 ~ 1 eV

(small-gap semiconductingnanotubes) Zigzag metals

Structure of SWCNT


OutlineOutline

• Film characteristics: Transparency & Conductivity• Film characteristics: Transparency & Conductivity

• Summary• Summary

Flexible and Transparent SWCNTNetwork Gas Sensors on PDMS substrates

Flexible and Transparent SWCNTNetwork Gas Sensors on PDMS substrates

• Fabrication: Flexible SWCNT thin film•• Fabrication: Fabrication: Flexible SWCNT thin filmFlexible SWCNT thin film

• Sensor Operation : NH3 Gas Sensing Operation• Sensor Operation : NH3 Gas Sensing Operation


Alumina filter

500 nm500 nm

Alumina filter surface

Flexible Thin FilmFlexible Thin Film

SWCNT thin film fabrication by Vacuum Filteration

Dispersion: SWCNT bundles in 0.1% Sodium Dodecylbenzene -sulfonate (SDS) solution

Arc-discharge SWCNTusing Fe catalyst(Φ:1.3 ~ 1.7 nm, EG : 0.59 ~ 0.45 eV)Chem.Phys.Lett.373, 266 (2003)

High power ultrasonic agitation at 30 ~70 W(10 s pulses for 1 hour)

Hu et al., Nano Lett. 4, 2513 (2004)

1 inch

before

after

Vacuum

Moore et al.,Nano Lett. 3, 1379 (2003)(Whatman; 20 nm pore size)

Density Control : SWCNT density in solution and filteration volume determines the thin film density


Transfer of SWCNT thin film to PDMS substrate surface

Flexible & Transparent SWCNT thin film

SWCNT Thin Film on PDMSFlexible Thin FilmFlexible Thin Film

1. PDMS mixed and placed in a vacuum chamber to remove air bubbles

2. Filter membrane placed on the surface of the uncured PDMS

3. PDMS curing in 100oC oven for 1 hour

PDMS poly(dimethyl siloxane)

4. Removal of the filter membrane

• Flexibility depends on PDMS thickness• Transparency depends on SWCNT density


SWCNT Thin Film on PDMSFlexible Thin FilmFlexible Thin Film

0.16 mg/ml5 μm5 μm

1 μm

• Due to some SWCNTs trapped in the nanoporesacting as ankersduring filter removal process, the SWCNT thin film transferred to the PDMS shows bundle ends lifted from the surface plain (grass like)

• Most of the SWCNT bundles have been transferred to the PDMS surface• For high density films, however, some percentage of SWCNTs remained untransferred


Film characteristicsFilm characteristics

Transparency, Conductivity and Flexibility testing

SWCNT Thin Film on PDMS : Characterization

Visible range optical transparency depending on SWCNT density

Optical Transmittance

• Higher SWCNT density results in low transparency

• Frequency dependence is believed to be due to PDMS thickness

I-V characteristics during bending (thin film complete folded)

Mechanical deformation

• Only a small decrease in conductance is observed• Returns back to original state when stress is removed

Film Resistance

Electrical resistance of the SWCNT thin film on PDMS depending on SWCNT density

• 5 x 5 mm2 between Au contacts

• Not all of the bundles are making contact resulting in resistance saturation

• Linear I-V characteristics

10

20

30

40

50

60

70

80

90

400 450 500 550 600 650 700

0.04 mg/ml0.12 mg/ml0.16 mg/ml

Tran

spar

ency

(%)

Wavelength (nm)

Tran

smitt

ance

(%)


Sensor OperationSensor Operation

NH3 gas sensitivity testing

Gas Sensing Measurement System

Flexible SWCNT Gas Sensor

Density dependent conductance change

• 5 min exposure to 1% NH3 results in ~ 10% change in film conductance

• The reduction in conductance shows variation on SWCNT density


Tunneling


possible sensing mechanism

Effect of NH3 gas adsorption on SWCNT energy band


EFEF

Metal p-type• Electron donation from NH3 raises the Fermi level

(direct adsorption) - reduced hole concentration• Adsorbed NH3 increases local scattering of holes

(adsorption on surfactant) - reduced carrier mobility

Tcnttot σσσ11

+=1

• Conduction in a network depends on nanotube interface properties and SWCNT density

High SWCNT density (percolation) 2D

• Many conduction paths – reduced scattering

Low SWCNT density ~ quasi 1D

• Few conduction paths – enhanced scattering

SWCNT network conduction reduced SWCNT density may increase sensitivity

(thermally assisted tunneling reduces the effect)




SWCNT density dependence

• There may be a correlation between SWCNT density and gas sensitivity due to the network conduction properties of SWCNT thin films

Low partial pressure detection

• It was possible to detect 10 ppm NH3 at 10 s exposure time ( 0.2 % conduction decrease)

• Bias voltage dependence within error margins

NH3 gas sensitivity testing


Summary

Thank you very much!

Flexible SWCNT thin film on PDMS was fabricatedwith Grass like surface

Conductivity vs Density Flexible SWCNT Gas Sensor NH3 sensitivity

Density dependence 10 ppm level detection

Transparency vs Density

10

20

30

40

50

60

70

80

90

400 450 500 550 600 650 700

0.04 mg/ml0.12 mg/ml0.16 mg/ml

Tran

spar

ency

(%)

Wavelength (nm)

carbon nanotube thin-films nanoparticle assembly · pdf file• electron-beam lithography...

Documents