ete444-lec2-atomic_scale_characterization_techniques.pdf

Atomic scale characterization

techniques

AFM & STM

ETE444 / ETE544

Nanotechnology

Lecture 2

Book:

-Nanotechnology For Dummies Page 54 – 62

- Springer Handbook of Nanotechnology Page 331 - 369

22 June 2009 at NSU Bosundhora Campus

Introduction

• Seeing is believing.

• We want to see what is happening in mol

Microscope today

SPM histrory

• 1981: The Scanning Tunneling Microscope (STM) developed byDr.Gerd Binnig and his colleagues at the IBM Zurich Research Laboratory, Rueschlikon, Switzerland.

• 1985: Binnig et al. developed an Atomic Force Microscope (AFM) to measure ultra-small forces (less than 1µN) present between the AFM tip surface and the sample surface

• 1986: Binnig and Rohrer received a Nobel Prize in Physics

Rohrer in a Conference at Japan

Atomic force microscope (AFM)

• phonograph record

• crystal-tipped stylus (―needle‖)

• spinning vinyl platter

• when the motion vibrated the needle, the

machine translated that vibration into

sound.

• tiny tip made of a ceramic or semiconductor material as

it travels over the surface of a material. When that tip,

positioned at the end of a cantilever (a solid beam), is

attracted to or pushed away from the sample’s surface, it

deflects the cantilever beam — and a laser measures

the deflection.

Features of AFM

• It can get images of samples in air and

underneath liquids.

• The fineness of the tip used in an AFM is an

issue — the sharper the tip, the better the

resolution.

• While STMs require that the surface to be

measured be electrically conductive, AFMs are

capable of investigating surfaces of both

conductors and insulators on an atomic scale.

Contact mode

• Known as static mode or repulsive mode.

• A sharp tip at the end of a cantilever is

brought in contact with a sample surface.

• During initial contact, the atoms at the end

of the tip experience a very weak repulsive

force due to electronic orbital overlap with

the atoms in the sample surface.

Dynamic mode AFM

• noncontact imaging mode: the tip is brought in close

proximity (within a few nm) to, and not in contact with the

sample.

• The cantilever is deliberately vibrated either in

– amplitude modulation (AM) mode or

– frequency modulation (FM) mode.

• Very weak van der Waals attractive forces are present at

the tip–sample interface.

• Although in this technique, the normal pressure exerted

at the interface is zero (desirable to avoid any surface

deformation), it is slow, and is difficult to use, and is

rarely used outside research environments.

More

• In the contact (static) mode, the interaction force

between tip and sample is measured by

measuring the cantilever deflection.

• In the noncontact (or dynamic) mode, the force

gradient is obtained by vibrating the cantilever

and measuring the shift of resonant frequency of

the cantilever.

• In the contact mode, topographic images with a

vertical resolution of less than 0.1nm (as low as

0.01 nm) and a lateral resolution of about 0.2 nm

have been obtained

Measuring scale

• With a 0.01 nm displacement sensitivity,

10 nN to 1 pN forces are measurable.

These forces are comparable to the forces

associated with chemical bonding, e.g.,

0.1μN for an ionic bond and 10 pN for a

hydrogen bond.

AFM tips

Commercial AFM

• Digital Instruments Inc., a subsidiary of Veeco

Instruments, Inc., Santa Barbara, California

• Topometrix Corp., a subsidiary of Veeco Instruments,

Inc., Santa Clara, California;

• Molecular Imaging Corp., Phoenix, Arizona

• Quesant Instrument Corp., Agoura Hills, California

• Nanoscience Instruments Inc., Phoenix, Arizona

• Seiko Instruments, Japan

• Olympus, Japan.

• Omicron Vakuumphysik GMBH, Taunusstein, Germany.

Tools for observation in nanoscale

• Scanning Probe Microscopy

– scanning tunneling microscopy

– atomic force microscopy

– AFM instrumentation and analyses:

• Noncontact mode

• Contact mode

• Dynamic Force Microscopy

• Molecular Recognition Force Microscopy

AFM tips

AFM tips

p.373 Springer Handbook of Nanotechnology

A schematic overview of the fabrication of Si and Si3N4 tip fabrication

AFM tip :: electron beam deposition


A pyramidal tip before (left,2-µm-scale bar) and after (right,1-µm-scale bar) electron

beam deposition

Carbon nanotubes for AFM tips

• Because the nanotube is a cylinder, rather than a pyramid, it can move more smoothly over surfaces. Thus the AFM tip can traverse hill-and-valley shapes without getting snagged or stopped by a too-narrow valley (which can be a problem for pyramid-shaped tips).

• Because a nanotube AFM tip is a cylinder, it’s more likely to be able to reach the bottom of the valley.

• Because the nanotube is stronger and more flexible, it won’t break when too much force is exerted on it (as some other tips will)

• Carbon nanotube tips having small

diameter and high aspect ratio are used

for high resolution imaging of surfaces and

of deep trenches, in the tapping mode or

noncontact mode. Single-walled carbon

nanotubes (SWNT) are microscopic

graphitic cylinders that are 0.7 to 3 nm in

diameter and up to many microns in

length.

Carbon Nanotube Tips

diameters ranging from3 to 50 nm


TEMof a nanotube

protruding from the

pores

(scale bar is 20 nm)

Pore-growth CVD

nanotube tip

fabrication.

SEM image of such a tip with a

small nanotube protruding

fromthe pores

(scale bar is 1µm).

Surface-growth nanotube tip fabrication

(a)Schematic represents

the surface growth

process in which

nanotubes growing on

the pyramidal tip are

guided to the tip apex.

(b)SEM(200-nm-scale

bar)

(c) TEM (20-nm-scale

bar) images of a

surface growth tip


Application of AFM

• AFM imaging

• Molecular Recognition AFM

• Single-molecule recognition event

• Nanofabrication/Nanomachining

AFM image

DNA on mica by

MAC mode AFM

(scale 500 nm)

Source: MSc thesis of Mashiur

Rahman, Toyohashi University of

Technology

The constant frequency-shift

topography of aDNAhelix on a

mica surface.


Molecular Recognition AFM


Single-molecule recognition event

Raw data from a force-distance cycle with 100 nm z-amplitude at 1Hz sweep

frequency measured in PBS. Binding of the antibody on the tip to the antigen on

the surface during approach (trace points 1 to 5) physically connectstip to probe.

This causes a distinct force signal of distinct shape (points 6 to 7) during tip

retraction, reflecting extension of the distensible crosslinker-antibody-antigen

connection. The force increases until unbinding occurs at an unbinding force of

268 pN (points 7 to 2).

Nanofabrication/Nanomachining

References

• G. Binnig, H. Rohrer, C. gerber, E. Wiebel,

Phys. Rev. Lett. 49, 57 (1982)

• R. Wiesendanger, Scanning Probe

Microscopy and Spectroscopy, Methods

and applications, Cambridge University

Press, 1994

ete444-lec2-atomic_scale_characterization_techniques.pdf

Documents