Award Abstract #0103476
NER: Chemically Modified Nanotube Tips for Selective Imaging with Scanning Tunneling Microscopy

NSF Org:	CMMI Division of Civil, Mechanical, and Manufacturing Innovation
Initial Amendment Date:	July 26, 2001
Latest Amendment Date:	July 26, 2001
Award Number:	0103476
Award Instrument:	Standard Grant
Program Manager:	Jorn Larsen-Basse CMMI Division of Civil, Mechanical, and Manufacturing Innovation ENG Directorate for Engineering
Start Date:	August 1, 2001
Expires:	November 30, 2002 (Estimated)
Awarded Amount to Date:	$97715
Investigator(s):	Philippe Buhlmann buhlmann@umn.edu (Principal Investigator)
Sponsor:	University of Minnesota-Twin Cities 200 OAK ST SE MINNEAPOLIS, MN 55455 612/624-5599
NSF Program(s):	DMR SHORT TERM SUPPORT, MATERIALS AND SURFACE ENG, ELECT, PHOTONICS, & DEVICE TEC
Field Application(s):	0308000 Industrial Technology
Program Reference Code(s):	MANU, 9146, 1676
Program Element Code(s):	1712, 1633, 1517

ABSTRACT

0103476

Buhlmann

This proposal was submitted in response to the solicitation "Nanoscale Science and Engineering " (NSF 00-119)

One of the great challenges of nanotechnology is the development of techniques for imaging of nanoobjects. This research develops the methodology to observe surfaces at the molecular and atomic level with chemical selectivity. It is based on the use of scanning tunneling microscope (STM) tips that chemically interact with the sample surfaces of interest. STM has revolutionized surface analysis because it allowsimaging with atomic resolution even in air and liquids, where many other analysis methods fail. However, the limited ability for chemical recognition, i. e., for discrimination between different types of atoms or functional groups, is a weakness ofconventional scanning tunneling microscopy. This problem can be solved by allowing an STM tip to interact chemically with a sample.

Recently, it has been shown that the modification of gold tips with self-assembled monolayers or polypyrrole can be used to selectively recognize functional groups that form hydrogen bonds. Preliminary results have shown that this method is also able to distinguish between functional groups that have different spatial orientations and to differentiate different metal atoms. The working principle of STMwith chemically modified tips resembles that of chemically modified electrodes in electroanalytical chemistry. While in the former case electrons are transferred between the STM tip and sample, the electron transfer in the latter caseoccurs between the sensor electrode and a molecule in the sample solution. In both cases, an overlap of the electronic wave functions of the electron-donating and accepting side is required for the electron transfer.In the latter case, the chemical modification of electrodes is used to control selected redox reactions. In the STM case, an analogous enhancement of electron transfer by chemical tip modification results in selective recognition of selected functional groups or atoms in a surface image.

To observe individual functional groups or atoms on a sample, a chemically modified tip must interact chemically only with one functional group of the sample at a time. Unfortunately, electrochemically etchedand chemically modified metal tips that are sharp at the molecular level cannot be produced with high reproducibility. Consequently, chemical interactions between the sample and several interaction sites on chemically modified tip used so far often occur simultaneously, impairing the resolution. To obtain very high resolution, this project explores the use of chemically modified carbon nanotubes as STM tips. Carbon nanotubesare ideally suited for chemically modified STM tips. Carbon nanotubes have a cylindrical shape with diameters that are typically between 0.8 and 15 nm. These extraordinarily small diameters provide for very slender and atomically sharp tips. Also, the rigid arrangement of the covalently linked carbon atoms that form a carbon nanotube results in great stiffness under conditions that are typical for STM imaging.

In this project, carbon nanotubes will be chemically modified in various ways and used to image well-understood test samples. The ability of chemically modified carbon nanotube tips to distinguish between different functional groups and atomsof the test samples will be investigated, and the experimental parameters determining the resolution will be studied.

The unique ability of STM to characterize samples at atmospheric pressure in liquids and gases, combined with the capability for chemical selec-tivity, should make this technique a very generaltool for nanosciences. Potential real-life applications are, for example, the characterization of nanodevices, self-assembled structures, catalytic surfaces, or electroanalytical sensor surfaces, as well asthe in-situ observation of chemical reactions and biological processes.

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