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12 Surface-Controlled Actuation and Manipulation of the Properties of Nanostructures

12 Surface-Controlled Actuation and Manipulation of the Properties of Nanostructures

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6.12



Surface-Controlled Actuation and Manipulation of the Properties of Nanostructures



307



voltages could be of relevance for convenient design of magnetic data storage

devices. Here, some recent progress in charge-induced strain in nanocrystalline metals and carbon nanotube composites, modification of magnetic properties by electric

fields, and chemistry-driven actuation will be discussed.



6.12.1 Charge-Induced Reversible Strain in Nanocrystalline

Metals

Length changes in the order of 0.1% or more in response to an applied voltage have

been reported for many materials, including ceramics, polymers, and carbon nanostructures, which in these cases arise from atomic rearrangements or charge transfer

throughout the entire solid. In metals, voltage-induced length changes have been

observed making use of nanometer-sized porous metal nanostructures [6.121]. In

this case, the length change is due to a controlling of the surface charge density σ in,

e.g., a nanoporous Pt sample (grain size 6 nm; see Fig. 6.33a, b) through an applied



Fig. 6.33 (a) Scanning electron micrograph of the fracture surface of a nanoporous Pt sample.

(b) Schematic representation of an array of charged nanoparticles immersed in an electrolyte. (c)

Relative length change I/I, as measured by dilatometry, versus the surface charge density σ .

(d) Lattice parameter a determined by x-ray diffraction (right ordinate) and lattice strain a/a0

(left ordinate) versus E. The horizontal line indicates the lattice parameter of the dry powder.

The error bar refers to the reproducibility of a/a0 ; the uncertainty in the absolute value of a is

estimated to be ± 0.3 pm. (Reprinted with permission from [6.121]. © 2003 AAAS)



308



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Nanocrystalline Materials



potential E relative to an electrolyte impregnating the pores. The nanocrystalline Pt

sample shows a reversible macroscopic length change I/I = 0.0015 in a potential E which induces a variation σ ≈ 500μC/cm2 of the surface charge density

(Fig. 6.33c). This coincides with the voltage-induced variation of the lattice parameter a/a0 ≈ 0.0014 as measured by x-ray diffraction (Fig. 6.33d). A microscopic

discussion of the charge-induced strain in nanocrystalline Pt starts from the effects

of the electronic band structure on the interatomic spacing in late transition metals,

where the antibonding interaction due to the upper d-band states is balanced by the

bonding effect of the sp-hybridized states. Injecting electrons into the band structure

changes the population of both bands, and the lattice contraction found experimentally upon injecting electrons at the surface would be compatible with the notion of

a dominant effect on the bonding sp orbitals [6.121]. Even larger strain amplitudes

than at present can be envisioned in porous metals with higher surface-to-volume

ratios.



6.12.2 Artificial Muscles Made of Carbon Nanotubes

An aerogel – a lightweight (1.5 mg/cm3 ), sponge-like material consisting of bundles

of multiwalled carbon nanotubes (12 nm in diameter) – is light as air, yet stronger

than steel and bendier than rubber. These characteristics are combined in a material

that twitches like a bionic man’s biceps when a voltage is applied [6.120]. Applying

a voltage across the width of a ribbon of the material electrically charges the nanotubes that thread through the material (see [6.122]). This makes them repel one

another and the ribbon can expand sideways by up to three times its original width

with an actuation rate of 3.7×104 %s−1 , much faster than the 50% s–1 maximum rate

of natural muscle (see [6.120]).The maximum achieved work per actuation cycle is

∼30 J/kg, compared with the maximum capability of ∼40 J/kg for natural muscle.

Applying a voltage along the length of the ribbon has a very different effect. It triggers the nanotube structure to contract, making the material more dense and very

stiff. The material can withstand extreme temperatures between 80 and 1900 K and,

therefore, could be easily used in harsh environment.



6.12.3 Electric Field-Controlled Magnetism

in Nanostructured Metals

The magnetic anisotropy of a bcc Fe (001)/MgO 001 junction (see Fig. 6.34a) can

be modified by an electric field. Magnetic hysteresis loops, in a 0.48-nm-thick Fe

layer under the application of a bias voltage, obtained from Kerr ellipticity, ηK ,

measurements are shown in Fig. 6.34b. From these data, a change of the magnetic

anisotropy by 39% is derived when the electric field is switched from 200 to –200 V.

The effect is largest for an Fe film thickness of 0.48 nm and is tentatively attributed

to a suppression of the number of electrons in the d3z2 −r2 (mz = 0) electron states of

Fe atoms adjacent to the MgO barrier (Fig. 6.34c) under a negative voltage due to



6.12



Surface-Controlled Actuation and Manipulation of the Properties of Nanostructures



309



Fig. 6.34 (a) Schematic of the sample used for a voltage-induced magnetic anisotropy change. The

magnetic field was applied perpendicular to film plane for magneto-optical Kerr effect (MOKE)

ellipticity measurements, ηK . (b) Measurements of ηK for different applied voltages on a Fe/MgO

junction (Fe thickness 0.48 nm) as a function of the applied magnetic field. The change in the

hysteresis curve upon the change of the bias voltage indicates a large change in the perpendicular

magnetic anisotropy of the Fe film. (c) Schematic of the effect of the electric field on electron

filling of the 3d orbitals in the ultrathin Fe layer. (Reprinted with permission from [6.123]. © 2009

Nature Publishing Group)



an increase in energy of these electron states. Therefore, the electron occupancy in

the dxy and dx2 −y2 states could be changed relative to one another, leading to a modulation of the magnetic anisotropy [6.123]. This approach may provide a technique

for high-speed voltage-controlled magnetization switching.

The orientation of the magnetization in ferromagnetic Co, which is exchangebias coupled to multiferroic (see Sect. 8.9) antiferromagnetic BiFeO3 , can be

controlled and switched through the application of an electric field to the BiFeO3

structure [6.124], [6.125].



310



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Nanocrystalline Materials



The magnetization at the surface of a ferromagnetic electrode in an electrochemical cell has been demonstrated to be electric field tunable [6.126].



6.12.4 Surface Chemistry-Driven Actuation in Nanoporous Gold

The powering of actuation by chemical energy which is used in biological systems

has been demonstrated for man-made actuator technologies in high-surface area

nanoporous gold [6.127]. Reversible strain amplitudes of the order of a few tenths

of a percent were achieved by alternating exposure of nanoporous Au to ozone and

carbon monoxide. The effect can be explained by adsorbate-induced changes of the

surface stress and can be used to convert chemical energy directly into mechanical

response [6.127].



6.13 Summary

Nanocrystalline bulk materials are polycrystals with nanometer-sized crystallite

dimensions. Due to the high volume fraction of interfaces with a disordered atomic

structure they are expected to show novel mechanical, thermal, electrical, and diffusive properties. Molecular dynamics studies have contributed to the understanding

of the interface structure and the plastic behavior of bulk nanomaterials showing

a yield strength increasing with decreasing crystallite size (Hall–Petch behavior)

and turning to an inverse Hall–Petch behavior at small (∼15 nm) crystallite sizes.

Both ductility and strength can be enhanced in metals with nanotwin structures and

superplasticity is observed. Nanocomposites of inorganic and organic materials and

nanoceramics play an interesting industrial role. Due to the rapid atomic diffusion in

interfaces, the diffusion processes in nanocrystalline solids are strongly enhanced.

The properties of nanocrystalline materials can be surface controlled by charges and

electrical fields.



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Chapter 7



Nanomechanics – Nanophotonics – Nanofluidics



A number of novel phenomena emerge for mechanical, optical, or fluidic properties on nanometer scales. Nanoelectromechanical systems (NEMS) are developed

for high-sensitivity detection and for investigating the boundary between classical

mechanics and quantum mechanics. By the adhesion properties of nanostructured

gecko toe pads, the fabrication of highly sticky adhesive materials is stimulated.

In the rapidly expanding field of nanophotonics, single-photon sources and detectors may be of importance for cryptography and quantum computing. Quantum dot

lasers and amplifiers are expected to play a role in communication technology. The

field of plasmonics, where photons are coupled to the oscillations of conduction

electrons in metal nanostructures, is of relevance from the coloring of stained glass

in antiquity to future biomedical technology.

Fluidics on the nanoscale are governing many biological processes and have to

be taken into account for the scaling down of biomedical assays. Here, recent studies

of wetting and spreading, of enhanced transport rates in nanotubes, on nanodroplets,

and on nanobubbles will be outlined.

In addition, first steps of unifying the above fields, such as the combination

of nanomechanics and nanophotonics [7.1] or the fusion of nanophotonics and

nanofluidics [7.2–7.4] appear to be particularly promising (see Sect. 7.6).



7.1 Nanoelectromechanical Systems (NEMS)

Microelectromechanical systems (MEMS) have become mainstream devices such

as optical switches, inkjets, and accelerometers. As an advancement, nanoelectromechanical systems (NEMS) [7.5, 7.6] with nanoscale dimensions not only

show great progress in sub-single-charge electrometry [7.7], single-electron spin

paramagnetic resonance [7.8], nuclear spin relaxation (see [7.9]), zeptogram-scale

mass sensing [7.10] in vacuum and attogram-scale sensing in liquids [7.11],

zeptonewton-scale force sensing [7.12], sub-femtometer displacement sensing

[7.13], and high-sensitivity energy sensing [7.14] (see also Sects. 1.7 and 7.2)

but also provide a way to observe the imprint of quantum phenomena directly

[7.15]. Displacements of microscale cantilevers are typically measured optically

H.-E. Schaefer, Nanoscience, DOI 10.1007/978-3-642-10559-3_7,

C Springer-Verlag Berlin Heidelberg 2010



315



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7 Nanomechanics – Nanophotonics – Nanofluidics



by bouncing a laser off the sensor. Diffraction effects make it far more difficult to

apply this method at the nanoscale, so electric currents are used to drive and detect

the motion of tiny mechanical NEMS structures such as beams and cantilevers by

employing, e.g., effects of piezoelectricity discovered by the Curie brothers [7.16]

in 1880 [7.6]. Recent developments in the fields of nanoelectromechanical highfrequency resonators, rotational activators, and switches will be discussed in the

following.



7.1.1 High-Frequency Resonators

Self-sensing nanocantilevers with fundamental mechanical resonances up to some

hundred MHz [7.17, 7.18] can make use of integrated electronic displacement transducers based on piezoresistive thin metal films, (e.g., a 30 nm thick gold layer

on a nanosized SiC cantilever; see Fig. 7.1a), which undergo a resistance change

in response to the motion-induced strain. By circumventing optics, piezoresistive

transduction yields access to dimensions far below the diffraction limit of ∼ 200 nm.

Sufficiently small cantilevers with a resonance frequency of 127 MHz (Fig. 7.1a)

can be operated at atmospheric pressure and room temperature (mean free path of

air molecules ∼ 65 nm) because enhanced damping due to viscous flow can be suppressed [7.17]. Under these conditions, mass resolutions of ∼ 100 zg (10−19 g) are

achieved.

Substantially higher resonance frequencies up to 1.85 GHz have been observed

for 3.5 nm thick carbon nanotubes (CNTs) suspended over a trench (Fig. 7.1b)

where the vibrating motion is detected by changes in the source-drain current

through the carbon nanotube [7.19]. The resonance frequency of a CNT can be

tuned by employing a gate voltage (Fig. 7.1c) in a similar fashion like tightening a

guitar string [7.20].

Rotational actuator. Repeated rotary oscillations and free rotations of a 440 nm

diameter thin metal plate fixed on a multiwalled carbon nanotube (MWNT;

diameters 10–40 nm) have been demonstrated (Fig. 7.2), promising torsional

oscillator mechanical resonance frequencies of the order of tens to hundreds

of megahertz. For large-displacement rotary operation, the outer MWNT shells

between the rotor plate and the anchors (see Fig. 7.2a) were removed by a

strong torque past the elastic limit of the outer shells initiated by a ∼ 80 V dc

stator voltage (see Fig. 7.2a). Subsequently, due to the low intershell friction of

the atomically smooth MWNT surface, complete rotor-plate revolutions can be

performed controlled by quasi-static dc stator voltages (Fig. 7.2b). In principle very

high-frequency operation should be possible [7.21].



7.1.2 Nanoelectromechanical Switches

Several switching devices based on microelectromechanical (MEMS) and nanoelectromechanical (NEMS) systems have been proposed recently in order to suppress



7.1



Nanoelectromechanical Systems (NEMS)



317



Fig. 7.1 High-frequency nanoelectromechanical system (NEMS) resonators. (a) Piezoresistively

detected resonant response from a SiC nanocantilever (600 × 400 × 70 nm3 ) covered with a

30 nm thick gold film as piezoresistive transducer [7.17]; (b) Schematic cross-section and scanning electron micrograph (SEM) of a suspended CVD-grown carbon nanotube (CNT) resonator

crossing a trench [7.19]. (c) SEM image of a suspended CNT with a p-Si gate on the bottom of the

trench [7.20]. (Reprinted with permission from [7.17] (a), [7.19] (b), and [7.20] (c). © 2007 Nature

Publishing Group (a), © 2006 American Physical Society (b), and © 2006 American Chemical

Society (c))



power dissipation, parasitic leakage currents, and short-channel effects, emerging

in the scaling of complementary metal-oxide semiconductor (CMOS) devices (see

[7.22]). Cantilever-type switches with a 35 nm thick TiN beam could be fabricated

by and integrated into conventional CMOS technology [7.22]. The beam material TiN (see Fig. 7.3a, b) was selected owing to its low electrical resistivity of

20 μ cm, a high Young’s modulus of 600 GPa, and its chemical inertness. The

switch has a pull-in voltage of ∼ 13.45 V for switch closing, an essentially zero off

current, an excellent on/off current ratio of 105 (Fig. 7.3c), and can be stably operated over several hundreds of switching cycles under dc and ac biases at ambient

conditions.



318



7 Nanomechanics – Nanophotonics – Nanofluidics



Fig. 7.2 Integrated rotational NEMS actuator. (a) Conceptual drawing of the nanoactuator.

A metal plate rotor (R) of gold (90 nm thick) covered with chromium (10 nm) and a diameter

of ∼ 440 nm is attached to a multiwalled carbon nanotube (MWNT) which acts as a support shaft

and yields rotational freedom. Electrical contact to the rotor plate is made via the MWNT and its

anchor pads (A1, A2). Three stator electrodes, two on the SiO2 surface (S1, S2) and one buried

beneath the surface (S3), provide additional voltage control elements. The SiO2 surface has been

etched down to provide full rotational freedom for the rotor plate. The entire actuator assembly is

integrated on a Si chip. (b–i) Series of scanning electron micrographs (SEM) showing the actuator

rotor plate at different angular displacements. The MWNT, barely visible, runs vertically through

the middle of each frame. The schematic diagrams located beneath each SEM image illustrate

a cross-sectional view of the position of the nanotube/rotor-plate assembly. Scale bar, 300 nm.

(Reprinted with permission from [7.21]. © 2003 Nature Publishing Group)



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