Nanorobotics for NEMS Using Helical Nanostructures
Nanorobotics for NEMS Using Helical
Nanostructures
Didi Xu1, Li Zhang1, Lixin Dong2 and
Bradley J. Nelson1
1
Institute of Robotics and Intelligent Systems, ETH
Zurich, Zurich, Switzerland
2
Electrical and Computer Engineering, Michigan State
University, East Lansing, MI, USA
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nanostructures of semiconducting zinc oxide (ZnO)
were grown using a vapor-solid process. A rigid structural alteration, as a result of superlattice formation
[17], gives rise to the formation of a helical structure,
either left-handed or right-handed, as shown in Fig. 2.
The diameters, widths, and pitch distances of the
nanohelix range between 300–700, 100–500, and
500–2,500 nm, respectively. The length of the
as-fabricated ZnO nanohelix can be as long as
100 mm and the thickness of the helical ribbon is less
than 20 nm [17].
Synonyms
Helical nanobelt; Nanocoil; Nanohelix; Nanospring;
Rolled-up nanostructure; Scrolled nanostructure
Definition
Three-dimensional helical nanostructures, such as carbon nanotube coils (CNTs) [1], zinc oxide helical
nanobelts [2], and rolled-up helical nanostructures
[3–7] (Fig. 1 [5, 8–10]) have attracted intensive
research interest because of their potential applications
in nanoelectromechanical systems (NEMS) as springs,
electromagnets, inductors, resonators, sensors, and
actuators [11–14]. These NEMS will serve as the
tools for fabricating future nanorobots, and in the
meantime basic components constructing those
nanorobots. Three-dimensional helical nanostructures
could be manipulated and characterized by the
nanorobotic tools, which could be a good candidate
for fascinating applications, such as, mass sensors in
femto-gram ranges, force sensors at pico-Newton
scales, and resonators at GHz ranges.
Physical and Chemical Properties
Bottom-up Fabrication of Helical Nanostructures
Top-down and bottom-up nanofabrication strategies
are being independently investigated by various researchers to realize helical nanostructures. Bottom-up
strategies are assembly-based techniques, resulting in
controllable nanopatterns at large scales, such as
carbon nanotube coils [1], silica nanosprings [15],
and SiC nanohelices [16]. For example, helical
Top-down Fabrication of Helical Nanostructures
Top-down approaches are based on conventional
fabrication process, including nanolithography,
nanoimprinting, and chemical etching. Nanocoils are
created through a top-down fabrication process in
which a strained nanometer-thick heteroepitaxial
layer of semiconductor compounds curls up to form
3-D structures [4]. Using GaAs-InAs bilayer as an
example, when the basic bilayer film is released from
an InP substrate by selective etching, the elastic forces
of two layers are oppositely directed, generating
a nonzero moment of forces, therefore its top layer
contracts and its bottom layer expands, causing the
film to curl and roll up [4]. The preferential direction
for the film to roll up depending on the orientation of
a mesa stripe on the substrate, either tube-like or helical nanobelts, can be formed [5]. A helical geometry
based on the bilayer SiGe/Si can be achieved through
this process (Fig. 3) [9]. The gray arrows indicate the
< 100 > directions, that is, the smallest Young modulus direction on a < 001 > substrate, in which the
bilayer will start to roll up when it has been freed from
the substrate. Several techniques have been proposed
to improve the control over the fabrication process in
terms of the resulting length, shape, and orientation of
structures. It has been demonstrated that the
misaligned angle between the stripe and etching direction and the edge effect of the helical-shaped ribbon
are crucial to determine the chirality and pitch of
a helical nanobelt [5, 9]. Moreover, various materials,
such as metal, dielectrics, and polymers, can be integrated into the helical nanobelt by thin film deposition
and lithographic patterning [9]. It is worthy of note
that nanohelices with a minimum diameter of 7 nm
were obtained using 6-monolayer-thick InGaAs/GaAs
bilayers [4].
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Nanorobotics for NEMS Using Helical Nanostructures
Nanorobotics for NEMS
Using Helical
Nanostructures,
Fig. 1 Various helical
nanostructures. Bottom left is
reproduced with permission
from ref. [8], (Copyright 2003
APEX/JJAP. The others are in
courtesy of L. Zhang and D.
Gr€utzmacher)
Key Research Findings
Nanorobotic manipulation [18] enables property characterization to be performed after intermediate processes, and in situ characterization can be performed
using manipulation rather than conventional static
observations. Nanorobotic approach extends the lower
limit of robotic manipulation to the dimension of nanometers, and it paves the way for the design and fabrication of nanocomponents having applications in NEMS.
Mechanical Properties of Helical Structures
By in situ manipulation using a nanoprobe, mechanical
properties of various helical structures have been investigated, such as superlattice-structured ZnO nanohelices,
carbon nanocoils, and rolled-up nanosprings [11, 12, 14].
It is found that the superelasticity (shape memory) of
superlattice-structured ZnO nanohelices enables the
nanohelix to recover its shape after an extremely large
axial stretching and compressing, which could be of great
interest for fabricating nanoscale elastic energy storage in
microelectromechanical systems (MEMS) and NEMS.
The manipulation experiments of rolled up helical
nanostructures can be performed by a nanomanipulator
and an atomic force microscope (AFM) cantilever inside
a scanning electron microscope (SEM). The nanohelix
was picked up by welding its one end using platinum (Pt)
deposition onto the tip of a tungsten nanoprobe, and the
mechanical deformation was recorded via extending or
compressing the nanohelix using the nanoprobe. As a
result, its spring constant can be increased continuously
for up to 300–800%.
In Fig. 4a, b, it is evident that the welded nanohelix
can be pulled to an almost straightened shape, with its
twist cycles preserved. Next, the welded nanohelix is
released and started to restore its original shape, as
shown in Fig. 4c. With comparison to the initial
nanohelix dimensions in Fig. 4a, it indicates that the
nanohelix has an almost identical dimensionality
including pitch and radius, suggesting a complete elastic recovery from stretching. Similar mechanical
behavior, that is, the superelasticity (shape memory)
behavior, has also been observed when applying a
compression load on the welded nanohelix (Fig. 4d,
e). The spring constant of ZnO nanohelices is in
a range of 0.07–4.2 N/m, and the transverse fracture
force is in the micro-Newton range [12, 17].
Nanomanipulators can also be used to carry out the
mechanical characterization of rolled-up helical structure. A probe picked up an InGaAs/GaAs nanospring
and attached it to the AFM tip (Fig. 5a, b). Then,
a tensile force was applied to the nanospring, while
continuous SEM images were taken to detect the AFM
tip displacement and the nanospring deformation. When
the tensile force was increased dramatically, the attachment between the nanospring and the AFM cantilever
Nanorobotics for NEMS Using Helical Nanostructures
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3D view
a
10)
(01
right hand helix
right handed
b
ring
left hand helix
left handed
Top view
)
(0001
Cross-section
view
O2+
Zn2+
O2+
Zn2+
Nanorobotics for NEMS Using Helical Nanostructures,
Fig. 3 Illustration of the effect of the angle between the mesa
stripe and preferred <100> etch direction (gray arrows) on
the formation of rings or helices with right- and left-handed
chirality. The thickness of the SiGe/Si/Cr ribbons and the diameter of the helices are 40 nm and 3 mm, respectively (Reproduced
with permission from ref. [9], Copyright 2006 ACS. Reproduced
with permission from ref. [5], Copyright 2005 IOP)
O2+
Zn2+
Nanorobotics for NEMS Using Helical Nanostructures,
Fig. 2 Initiation and formation process of a nanohelix.
(a) Low magnification SEM image of a ZnO nanohelix showing
its starting point and finishing end. (b) A three-dimensional (3-D)
schematic model of a nanohelix, showing its initiating point and
finishing end. The periodicity of the superlattices may result in the
formation of periodic piezoelectric domains (Reproduced with
permission from ref. [17], Copyright 2005 AAAS)
broke, and the nanosprings returned to their initial shape
at the zero-displacement position. From the displacement data and the known stiffness of the AFM cantilever, the tensile force acting on the nanosprings versus
the nanospring displacement was plotted (Fig. 5c). For
the nanospring, an overall elastic strain of 48% was
measured when the attachment to the AFM tip broke.
Ultra-flexible rolled-up Si-based nanospring with
a spring constant of 0.003 N/m was also reported
which exhibits long-range linear elasticity (with ca.
189% relative elongation) and excellent mechanical
stability [19]. The rolled-up structure is promising for
use in electromechanical sensors, such as force or pressure sensors, because of intrinsic piezoresistive and
piezoelectric properties of these materials.
Linear-to-Rotary Motion Converter
The conversion between various forms of motions will
play an important role in future NEMS applications.
Among various nano building blocks, 3-D helical
nanostructures, for example, dual-chirality helical
nanobelts (DCHNBs), could realize conversion
between linear and rotary motion [20]. As schematically shown in Fig. 6a, the motion converter consists of
a DCHNB with a left-handed and a right-handed part.
By linearly stretching both ends of the DCHNB, the
central part was mobilized in rotary motion; allowing
for linear-to-rotary motion conversion, which has not
been previously demonstrated in other nanostructures.
With an extended arm, the output can be linear (small
displacement) or rotary (large displacement) motion,
as seen in Fig. 6b. When a tensile force F is applied to
the DCHNB, it elongates and rotates about the unwinding direction.
To implement and characterize motion converters,
experimental investigations have been performed in
the SEM using a nanomanipulator equipped with
a tungsten probe and an AFM cantilever. Manipulation
of an as-fabricated DCHNB was performed by cutting
the lower end of the DCHNB shown in Fig. 7a.
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Nanorobotics for NEMS Using Helical Nanostructures
Nanorobotics for NEMS
Using Helical
Nanostructures,
Fig. 4 Manipulation process
of a nanohelix during axial
stretching and compressing.
(a) One end of a nanohelix was
welded with Pt onto a tungsten
nanoprobe. (b) A SEM image
shows the extremely stretched
nanohelix. (c) A complete
restoring of the nanohelix
shape after releasing.
(d, e) Compressed
deformation process of
a nanohelix induced by the
nanoprobe. Both stretching
and compressing suggest the
superelasticity behavior
(Reproduced with permission
from ref. [12], Copyright 2006
ACS)
A “sticky” probe was then prepared by dipping
a tungsten probe (Picoprobe, T-4-10-1 mm) into a
silver tape and attached on the lower end of the
DCHNB. Rotation was then generated in the central
part by moving the probe downward (Fig. 7b–e).
Examples of Application
A fundamental application of the rolled up nanohelix is
the 3-D microscopy based on the linear-to-rotary
motion conversion [20], in which samples are rotated
so that different views are exposed to light, electron
beams, or focused-ion beams (FIBs). Due to their
extremely compact size, the dual-chirality helical
nanobelts are well suited to serve as rotary scanners
for creating 3-D scanning probe microscope (SPMs),
rotary stages for a microgoniometer for observing an
object from different crystal surfaces, and components
of nanomachines.
Figure 8 shows 3-D imaging of a pollen grain. The
pollen grain is picked up by a Picoprobe and attached
Nanorobotics for NEMS Using Helical Nanostructures
Nanorobotics for NEMS
Using Helical
Nanostructures,
Fig. 5 Mechanical
characterization. (a, b) Break
and pick up nanospring and
attach it to AFM tip.
(c) Simulation and
experimental results of axial
force versus nanospring
displacement (Reproduced
with permission from ref. [14],
Copyright 2006 ACS)
a
b
5
c
Nanospring 1
Nanospring 2
Nanospring 3
Simulation N1
Simulation N2
Simulation N3
0.6
4.5
0.5
Force [μN]
4
3.5
Force [μN]
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3
2.5
0.4
0.3
0.2
0.1
2
0
1.5
0
6
4
Displacement [μm]
2
1
8
10
0.5
0
0
5
10
15
20
Displacement [μm]
a
Nanorobotics for NEMS
Using Helical
Nanostructures,
Fig. 6 Motion converter
based on the dual-chirality
helical nanobelts (DCHNB).
(a) Linear-to-rotary motion
converter using a DCHNB.
(b) Transmission converting
linear motion to extended
linear (small displacement)/
rotary (large displacement)
motion (Reproduced with
permission from ref. [20],
Copyright 2009 IEEE)
F,d
t, f
N
F,d
b
F, d
to the DCHNB. The motion converter has been characterized before loading the sample. Figure 8a–d
shows that the sample rotates counterclockwise (top
view) for approximately 180 when releasing the
extended DCHNB. Figure 8e–h shows further
stretching of the DCHNB, and the sample rotates
clockwise (top view) for another 180 . It can be seen
that the different aspects of the sample can be exposed
t, f
F, d
T, F
P, D
to the electron beam simply by extending or releasing
the DCHNB with a small displacement. Further possibilities for this converter include goniometry for
a transmission electron microscope (TEM), tomography for a SEM or a TEM, and 3-D SPM. By integrating
in the fabrication processes of DCHNBs, rolled-up
spirals can serve as claws for holding relatively large
samples [6].
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Nanorobotics for NEMS Using Helical Nanostructures
Nanorobotics for NEMS
Using Helical
Nanostructures, Fig. 7 In
situ characterization of motion
conversion using nanorobotic
manipulation. (a) One end of
an as-fabricated DCHNB is
cut and attached to a “sticky”
probe. (b, e) Rotation of the
central part while moving the
probe downward (Reproduced
with permission from ref. [20],
Copyright 2009 IEEE)
Nanorobotics for NEMS Using Helical Nanostructures,
Fig. 8 Three-dimensional microscopy of a pollen grain. (a–d)
When releasing the extended DCHNB, the sample rotates
counterclockwise (top view). (e–h) When stretching further the
DCHNB, the sample rotates clockwise (top view) (Reproduced
with permission from ref. [20], Copyright 2009 IEEE)
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Nanoscale Printing
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Nanorods
▶ Nanostructures for Energy
▶ Physical Vapor Deposition3
Nanoscaffold
▶ Ligand-Directed Gold-Phage Nanosystems
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Nanoscale Fluid Mechanics
▶ Computational Micro/Nanofluidics: Unifier of Physical and Natural Sciences and Engineering
Nanoscale Heat Transport
▶ Thermal Conductivity and Phonon Transport
Nanoscale Particle
▶ Gas Phase Nanoparticle Formation
Nanoscale Printing
Timothy J. Merkel1 and Joseph M.
DeSimone1,2,3,4,5,6,7,8
1
Department of Chemistry, University of North
Carolina, Chapel Hill, NC, USA
2
Department of Pharmacology, Eshelman School of
Pharmacy, University of North Carolina, Chapel Hill,
NC, USA
3
Carolina Center of Cancer Nanotechnology
Excellence, University of North Carolina, Chapel Hill,
NC, USA
4
Institute for Advanced Materials, University of North
Carolina, Chapel Hill, NC, USA
5
Institute for Nanomedicine, University of North
Carolina, Chapel Hill, NC, USA
6
Lineberger Comprehensive Cancer Center,
University of North Carolina, Chapel Hill, NC, USA
7
Department of Chemical and Biomolecular
Engineering, North Carolina State University, Raleigh,
NC, USA
8
Sloan–Kettering Institute for Cancer Research,
Memorial Sloan–Kettering Cancer Center, New York,
NY, USA
Synonyms
Nanoscale Drug Vector
▶ Fullerenes for Drug Delivery
Dip-pen nanolithography;
Microcontact
printing;
Imprint lithography;
Molecular
printing;
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