me382: experiments in micro/nano science and engineering a review on carbon nanotube probes for...

ME382: Experiments in Micro/Nano Science and Engineering

A Review on Carbon Nanotube Probes for Microscopy Applications

Wei Chen, Yuan Yao and YunYoung Kim

Department of Mechanical EngineeringNorthwestern University, Illinois 60201, USA


Contents

1. Introduction (YunYoung Kim)

2. Gluing Method (Wei Chen)

3. Growng Method (Yuan Yao)

4. Biological Applications (YunYoung Kim)


Atomic Force Microscopy (AFM) - Indispensable device for micro/nano scale phenomena studies - Surface topology measurement - Micro/nano structure manipulation

Introduction

Carbon Nanotube (CNT) - A new material that has exceptional properties

- Promising possibilities of applications (composites, FEP display, chemical sensors, etc.)

Hafner et al., Prog. Biophys. Mol. Biol. 77, 73-110 (2001).



CNT-Probed AFM - A CNT (either SWNT or MWNT) is attached on the apex of the

conventional silicon AFM cantilever tip.

Tang et al., Nano Lett. 5(1), 11-14 (2005).

Si Tip

CNT Probe

Wong et al., J. Am. Chem. Soc. 120, 603-604 (1998).

Si Tip

CNT Probe

Introduction


The Synergy Effect of CNT-Probed AFM - Resolution enhancement

- Precise measurements of high aspect ratio structures

Chang et al., Jpn. J. Appl. Sci. 43(7B), 4517-4520 (2004). Guo et al., Appl. Surf. Sci. 228, 53-56 (2004).

Dai et al., Nature 384(14), 147-150 (1996).

Introduction

Si Tip

Si TipCNT Probe CNT Probe

CNT ProbeSi Tip


- High wear-resistance

Chang et al., Jpn. J. Appl. Phys. 43(7B), 4517-4520 (2004).

Si Tip

CNT Probe CNT Probe Si Tip

Introduction

- Biomolecule manipulation

Chang et al., Jpn. J. Appl. Phys. 43(7B), 4517-4520 (2004).

Biotin Ligand

Streptavidin Receptor


Overview: Two Ways of CNT-Tip Fabrication - Gluing Method

- Growing Method


Tang et al., Nano Lett. 5(1), 11-14 (2005).

Introduction


Gluing Method

Pick-up method

SWNT0.9~2.8nm in diameter

Lieber et al. J. Phys. Chem. B 2001

First trial-”glue”

MWNT5~20nm in diameter0.25~1μm in length

Dai, H. et al. Nature 1996


Gluing Method

Disadvantage

• Manual assembly method is time-consuming, needs proficient experimental technique, the fabrication yield is restricted.

• Spacial resolution is compromised by the large diameters of attached CNTs, since only thick CNTs can be viewed under optical microscope.


Gluing Method

Magnetic field-aligned• Sputter coat the Au film

on AFM probes;• Fix the AFM tips;• Prepare and stir CNTs

suspension (5mL);• Apply alternative

magnetic field (Amplitude B0=0.1T);

• CNTs are aligned and attached onto the tips.

Hall, A. et al. Appl. Phys. Lett. 2003

Average protruding angle: 35o

Length: 100-500nm


Gluing Method

Physical Principle oflux BAChange in flux ~(Induced potential)

L

BAAI ocurrentflux

Induced Current

fluxAIm Dipole moments

3

0

2)(

r

mmrU CNTtip

Potential Energy

Attraction Force

Aflux

Magnetic

Flux

Induced

Current


Gluing Method

Dip-Coating & Dielectrophoresis• Introduce the CNTs into

the TiO2 SG solution;• W tip was dipped into SG

solution containing CNTs;• Apply DC voltage between

the tip and the solution;• CNTs was deposited on

the tip, and the SG was coated on it as well,

• Withdraw the tip and anneal it under infra-red lamp.

A. Brioude et al. Appl. Surf. Sci. 221 (2004)

Diameter: ~9.2nm Length: ~950nm


Strong adhesion

A. Brioude et al. Appl. Surf. Sci. 221 (2004)

• Many CNTs entangle together into fiber, CNTs break near the apex of tips since the sudden strong current;

• The SG thin film works like a sheath, which integrates the CNTs and tip as a whole.

Potential for large-scale fabrication

Gluing Method


Growing Method

Pore-growth method (first direct growth, Hafner et al., Nature 1999)

1. Flatten the conventional silicon (Si) tip at its apex by contact AFM imaging;

2. Anodize the tip in hydrogen fluoride to create nano-pores of 50–100 nm diameter along the tip axis;

3. Electrodeposit iron catalyst into the nano-pores from FeSO4 solution;

4. Use Chemical Vapor Deposition (CVD) to grow carbon nanotubes with ethylene and hydrogen at 750 for 10 min. ℃


Growing Method

• The pore-growth method demonstrated the great potential of thermal CVD to grow directly controlled diameter nanotube tips.

1μm 20nm


Growing Method

Limitation:The preparation of a porous layer can be time consuming and may not place individual SWCNTs at the optimal location on the flattened apex.


Surface-growth method (Hafner et al., J. Am. Chem. Soc. 1999)

1. Deposit catalyst onto the pyramidal tip of a commercial cantilever; (Fe-Mo and colloidal Fe-oxide catalysts are electrophoretically deposited on the tip)

2. grow the SWCNT probe using thermal CVD at 800 ℃ for 3 min. (ethylene is used as carbon source -- C2H4 : H2 : Ar = 1 : 200 : 300)

Growing Method


Growing Method

Surface-growth method

SEM cannot provide an accurate measure of the tip diameters. TEM analysis was able to demonstrate that the tips consist of individual SWCNT and small SWCNT bundles.

20nm200nm


Individual SWNT tips can be prepared by lowering the catalyst density on the surface such that only 1 nanotube reaches the apex.

Hence, by controlling the catalyst density , it is possible to produce well-defined individual SWNT tips.

Surface-growth method

Growing Method

10nm


Growing Method

Mass production ?Erhan Yenilmez, et al., Appl. Phys. Lett. 2002

How to place catalyst on each of the 375 Si tips on a 4-in wafer ?

How to grow SWCNTs on a wafer-scale large substrate using CVD?


Growing Method

Catalyst patterning

1. Spin coat polymethylmethacrylate (PMMA) onto the wafer at a low spin speed of 1000 rpm and bake the wafer on a hot plate at 180 ℃ for 5 min. Repeat the PMMA spin coating and baking step for three times.

2. Spin coat the catalyst suspension onto the wafer at a low spin speed of 250rpm

3. Liftoff the PMMA layer in acetone. The catalyst coated around the pyramidal surfaces of the tips will be left.


Growing Method

The system is heated to 900 in an Ar flow after a thorough ℃purge of the system by Ar. Then ultrahigh purity methane (99.999% purity, flow rate of 1500 sccm) is piped through the system together with hydrogen (flow rate of 125 sccm) for a growth time of 7 min. At last, the system is cooled to room temperature in a H2 flow.


Growing Method

Their method yields SWCNTs protruding from more than 90% of the 375 Si tips on a wafer.


• CNT location, density, length, and orientation can not be welled controlled by the thermal CVD methods. Although we can get many tips through one thermal CVD process, seldom of them has individual free-standing and well-oriented CNTs. The yield of readily usable tips is very low.

• After CNT tip fabrication, a one-at-a-time manipulation approach is required to shorten the extruding CNTs for AFM use. This shortening process decreases the efficiency of mass production.

• In addition, nearly all of the previous approaches (including both attaching methods and direct growth methods) rely on commercially available silicon tips or prefabricated commercial silicon probe wafers.

Limitations:

Growing Method


Growing Method

Qi Ye et al., Nano Lett. 2004

They integrated nanopatterning and nanomaterials synthesis with traditional silicon cantilever microfabrication technology, and produced 244 CNT probe tips on a 4-in. wafer with controlled CNT location, diameter, length, and orientation.


Growing Method


PECVD – Plasma Enhanced CVD



Growing Method


Growing Method

1. No prefabricated silicon probe wafers needed;2. Better control of the CNT location, density, length,

and orientation by PECVD; 3. No need to conduct post-fabrication treatment to

remove and/or shorten the CNT tips.

Advantages of their approach:



Biological Applications

Biomolecule Structure Characterization

Wong et al., J. Am. Chem. Soc. 120, 603-604 (1998)

- Amyloid-β fibrils are measured with CNT tips.

- The small effective tip radii result in 12~30% increase of resolution.

- Observed widths are 3~8 nm smaller for MWNT tips than those for Si tips.

An image of Aβ40 fibril

A Y-branch A defect

A staggered end



- Significant resolution improvement for SWNT tips (full width at half maximum is 5±1 nm for SWNT tip and 15±1 nm for Si tip)

- More refined measurements are possible with SWNT tips.

An image of double-stranded DNA on mica Height cross-section of the DNA

SWNT Tip

FWHM=5.6 nm

Si Tip

FWHM=14.4 nm

Wong et al., Appl. Phys. Lett. 73, 3465-3467 (1998)



AFM image of DNA taken using a CNT tip AFM image of DNA taken using a Si tip

- Improvement in resolution by a factor of 2

- The diameter ranges from 12 to 15 nm with a Si tip, but 7 to 8 nm with a CNT tip.

- Blurred image is observed when the CNT tip with a ~10 nm diameter is longer than 500 nm

Nishijima et al., Appl. Phys. Lett 74, 4061-4063 (1999)


An Issue about the Length of CNTs


- To avoid a mechanical instability, the length should be less than 500 nm for MWNTs and 50nm for SWNTs1.

- Akita et al. also reports that L < 500 nm is required for a stable and high-resolution imaging2.

)(424.0

44

32

baY

kTLu

Vibration amplitude,

a: outer radii of the nanotube (5 nm)

b: inner radii of the nanutube (2.5 nm)

Y: Young’s modulus (1TPa)

- For example, L > 500 nm results in u > 0.5 nm and degradation of resolution by a factor of ten. 1) Chang et al., Jpn, J. Appl. Phys. 43, 4517-4520 (2004)

2) S. Akita et al., J. Phys. D.: Appl. Phys. 32, 1044-1048 (1999)



Manipulation of Wettability of CNTS - Hydrophobic CNTs are chemically incompatible in aqueous e

nvironments.

- Stevens et al. reported a failure of MWNT probe during the measurement in DI H2O.

A MWNT tip before immersing in DI water A MWNT tip after immersing in DI water

Stevens et al., IEEE Trans. Nanobio. 3, 56-60 (2004)



- The wettability can be adjusted by depositing ethylendiamine (DE) on the sidewalls of CNTs.

- Amine groups can be absorbed onto CNT sidewalls by exchanging of electrons, and render the CNT to be hydrophilic.

- The deposition do not change the chemical property of the tip end, and thus do not affect the probe resolution.

Si tip, in air CNT tip, untreated, in air CNT tip, treated, in air CNT tip, treated, in H2O

Stevens et al., IEEE Trans. Nanobio. 3, 56-60 (2004)


Summary

1. CNT-probed AFM is reviewed.

2. CNT-probes can be fabricated by

- the gluing method, in which a grown CNT is picked up by a Si tip using adhesives, electric or magnetic fields.

- the growing method, in which a CNT is directly grown on the pyramidal surfaces of a silicon tip.

3. Examples of CNT-probe applications for biological studies are presented.

me382: experiments in micro/nano science and engineering a review on carbon nanotube probes for...

Documents