Atomic scale characterization techniquesAFM & 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.

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

p.376 Springer Handbook of Nanotechnology

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

p.379 Springer Handbook of Nanotechnology

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

p.380 Springer Handbook of Nanotechnology

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.

p.404 Springer Handbook of Nanotechnology

Molecular Recognition AFM

p.475 Springer Handbook of Nanotechnology

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