Fwd: University PhD Dissertation Defense/ Randal J. Grow

Randal Grow randalg at gmail.com
Fri May 20 10:47:30 PDT 2005

---------- Forwarded message ----------
From: Claire Nicholas <claireni at stanford.edu>
Date: May 18, 2005 11:39 AM
Subject: Re: University PhD Dissertation Defense/ Randal J. Grow
Cc: apgradstudents at lists.stanford.edu, apfaculty at lists.stanford.edu


Speaker:                Randal J. Grow
           Research Advisor: Professor Hongjie Dai
Title:          Novel Atomic Force Microscope Cantilevers;
Piezoresistance of Carbon

                Nanotubes and Germanium Nanowires. 

Date:           May 25, 2005
Time:           10:30 A. M. 

 Place:          CIS-X Auditorium

There has been great interest in recent years in nanometer-scale
materials and tools for fabricating and characterizing them. One such
tool is the atomic force microscope (AFM), which has developed rapidly
since its invention in 1986. Imaging soft or fragile samples requires
low-spring-constant cantilevers to minimize the force on the sample.
Silicon nitride is well-suited to making soft cantilevers, but it is
not ideal for making sharp tips. We combined a silicon tip with a
silicon nitride cantilever to achieve a hybrid with both a low spring
constant and a sharp tip.
 Carbon nanotubes have remarkable mechanical and electrical
properties. Their electromechanical properties are also interesting,
as a few groups have determined by deforming suspended nanotubes.
Their change in resistance under strain is stronger than that of
silicon, which is commonly used in mechanical sensors such as pressure
sensors, accelerometers, and strain gauges. However, the fragility of
suspended tubes makes them impractical for mass-produced sensors.
Tubes on surfaces are more robust. We fabricated micro-machined
pressure sensors using a thin silicon nitride membrane with
metal-contacted carbon nanotubes on the surface. Deforming the
membrane introduced longitudinal strain in the nanotube, and we
characterized the resistance changes. We found responses stronger than
those of suspended nanotubes, probably because of local deformations
in the nanotube caused by interaction with the surface.
 Nanowires of inorganic semiconductors such as silicon, germanium, and
III-V semiconductors have evoked much interest for electronics and
optics applications. The piezoresistive effect in nanowires should be
larger than that of the bulk material, which could be useful for
increasing the sensitivity of mechanical sensors. We have fabricated
pressure sensors with germanium nanowires as the sensing elements, and
we have found the piezoresistance to be much stronger than in the
bulk. We understand this to be partly the result of the
one-dimensionality, but there may be other contributing factors. We
have also measured the mechanical properties of germanium nanowires
suspended across trenches by pressing on them with AFM cantilevers. We
found that their behavior matched what we expected for a simple
mechanical model.





Randy Grow
Ph.D. Candidate
Dept. of Applied Physics
Stanford University
Stanford, CA 94305-4090

rgrow at stanford.edu
(650) 387-2588 (cell)
(650) 725-9156 (lab)

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