DIELECTROPHORETIC ARCHITECTURES - Bio-Inspired and Nanoscale Integrated Computing - page 158

(K )

(K II ) E e ±

E e

L

R

Figure 5.2. Clausius-Mossotti definitions for orientations of a cylindrical particle.

This factor, in both sign and magnitude, is essential to characterizing the
dielectrophoretic behavior of a particle in a medium. When the factor is positive,
the particle moves toward stronger electric fields; when the factor is negative, the
particle moves toward weaker electric fields. For rotationally asymmetric parti-
cles, the magnitude can depend strongly on particle orientation relative to the field
direction and, in particular, roughly corresponds to the elongation of the particle
along the field axis. The nanodevices we focus on in this chapter are cylindrical, so
we now present approximate Clausius-Mossotti expressions for cylindrical parti-
cles parallel and perpendicular to a field, as shown in Figure 5.2:

e p e m
e m þðe p e m Þð 1 ð 1 þ R 2

K jj ð f Þ

Þ 1 = 2 = L 2 Þ

and

e p e m
e m 1

8 3 e p e m

þ e p

K ? ð f Þ¼

e p þ 2 e m :

p
8

p

5.2. DIELECTROPHORETIC ASSEMBLY AND TRANSPORT
OF NANODEVICES

Having completed our discussion of the theory of dielectrophoresis, we are now
ready to explore its practical applications to nanocomputation. Let us first consid-
er the problem of assembling individual nanodevices. Various approaches for
manipulating electronic nanostructures have been developed, including mechanical
[7], optical [8, 9], electrostatic [10, 11], and dielectrophoretic [6] methods. Dielec-
trophoresis is particularly attractive for inexpensive and massively parallel manip-
ulation [12] of neutral microscale and nanoscale objects using only standard
semiconductor fabrication technologies. It has been used to trap a variety of
structures from suspensions, including NiSi nanowires [13], CdS nanowires [14],
GaN nanowires [15], carbon nanotubes [16], silicon microblocks [17], ZnO