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10 Magnetically Tunable Photonic Crystals of Superparamagnetic Colloids

10 Magnetically Tunable Photonic Crystals of Superparamagnetic Colloids

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8.11



Nanomagnets in Bacteria



417



Fig. 8.40 (a) Transmission electron micrograph and (b) schematic illustration of polyacrylatecapped Fe3 O4 colloidal nanocrystal clusters (CNCs) with a nanocrystal size of ∼ 10 nm; scale bar:

100 nm. (c) Magnetization curve of CNCs measured at room temperature exhibiting superparamagnetic behavior. (d) Photographs of solutions of colloidal photonic crystals formed in response

to an external magnetic field; the magnet–sample distance decreases gradually from right to left.

(e) Variation of the reflectance spectra at normal incidence on the colloidal photonic crystals with

the magnet–sample distance. Diffraction peaks blue shift (from right to left) as the magnet–sample

distance decreases from 3.7 to 2.0 cm in steps of 0.1 cm. (Reprinted with permission from [8.150].

© 2007 Wiley-VCH)



8.11 Nanomagnets in Bacteria

Magnetotactic bacteria, such as the bacterium Magnetospirillum magnetotacticum,

biomineralize iron into magnetite (Fe3 O4 ) nanoparticles (“magnetosomes”), which

enable the bacteria to respond to magnetic fields in their environments. These magnetosomes have considerable potential for use in nanotechnological, biotechnical,

and medical application because of their narrow size distribution and inherent biocompatibility (see [8.152]). Furthermore, magnetotactic bacteria are the simplest

single-cell organisms in which biomineralization occurs and, as such, offer a model

system to study biomineralization mechanisms. Models of the biomineralization of



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8 Nanomagnetism



Fig. 8.41 (a) Electron micrograph of a chain of magnetosomes in a magnetic bacterium where

a small magnetite crystal at the left end of the chain is still growing [8.153]. The inset shows a

model of magnetite formation: As the inner membrane (red) forms a membrane vesicle, iron can

be transported inside to eventually precipitate as magnetite [8.154, 8.155]. (b) Electron micrograph

of Co-doped magnetosomes of the bacterium M. gryphiswaldense. Insets: undoped magnetosome

chain (left) and undoped magnetosomes in a whole cell (right); scale bars are 500 nm. (c) M.

magnetotacticum MS1; scale bar is 500 nm. (d) Magnetic hysteresis loops of MS1 measured at

300 K. Closed squares, open triangles, and open circles indicate the magnetosomes corresponding

to [Fe], [FeCo], and [Co] growth conditions [8.152]. (Reprinted with permission from Refs. [8.155]

(a) and [8.152] (b–d). © 2008 Nature Publishing Group)



magnetosomes with a typical size of about 50 nm (Fig. 8.41a) propose that in a first

step iron, e.g., incorporated in the cage protein ferritin, is accumulated in the space

between the outer and inner cell membrane (see inset in Fig. 8.41a). In a second

step, the inner membrane expands inward to form an empty magnetosome vesicle

(see [8.155]), into which the stored iron can be released to eventually precipitate as

magnetite.



8.11.1 In Vivo Doping of Magnetosomes

Pure magnetite has the highest saturation magnetization of the iron oxides but

is magnetically soft and therefore less suitable for applications that require the

nanoparticles to remain magnetized in zero-field. It is, however, well known that

magnetite particles can be magnetically stabilized when small amounts of iron are



8.11



Nanomagnets in Bacteria



419



substituted by cobalt, a technique which has been employed for iron oxide nanoparticles used in videotapes and floppy disks (see [8.155]). Recently it could be shown

that for magnetosomes in bacteria fed in vivo with cobalt-containing quinate (CoQ),

the coercive field, which is a measure of the magnetic stability of the particles,

could be increased by up to 50% (see Fig. 8.41d). This is due to the biological

introduction of a Co content up to 1.4%, as determined by induced coupled plasma

optical emission spectroscopy (ICP-OES) and x-ray magnetic circular dichroism

[8.152] (see Sect. 8.1). This demonstrates the possibility of tuning the properties

of biosynthesized nanoparticles in vivo, but retaining the advantageous biological

growth control.



8.11.2 Magnetosomes for Highly Sensitive Biomarker Detection

A biomarker is a biochemical compound (see Sect. 12.2), such as a protein, which

can be used to specifically and sensitively monitor a health status. Furthermore, in

various fields such as agriculture, drug development, or doping control, biomarkers

are of interest (see [8.156]).

Magnetosomes have been developed for sensitive detection of the hepatitis B

surface antigen (HBs Ag) in human serum by employing a magneto immuno

polymerase chain reaction (M-IPCR) [8.156, 8.157]. Magnetite ferromagnetic

magnetosomes derived from the bacterium Magnetospirillum gryphiswaldense are

modified with oligonucleotides and antibodies (see Fig. 8.42) for the detection



Fig. 8.42 Schematic drawing of the magneto immuno polymerase chain reaction (M-IPCR)

assay. (a) Hepatitis B surface antigen (HBsAg)-specific magnetosome–antibody conjugate and

DNA–antibody conjugate are incubated with the serum sample containing HBsAg resulting in

a signal-generating detection complex. (b) The detection complex is concentrated using an external magnetic field. (c) The detection complex solution is transferred to a microplate containing

the polymerase chain reaction (PCR) amplification mastermix for highly sensitive detection of the

HBsAg antigen. (Reprinted with permission from [8.156]. © 2007 Elsevier)



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8 Nanomagnetism



of the HBsAg antigen biomarker where additionally a DNA–antibody conjugate

is attached to the HBsAg for PCR amplification (Fig. 8.42). A 125-fold increase

in sensitivity is observed for this M-IPCR assay as compared to enzyme-linked

immunosorbent analysis (M-ELISA [8.156]) and a 25-fold increased sensitivity

compared to the limit of HBsAg detection using commercial magnetic beads. Most

likely, the enhanced performance of the magnetosomes, compared to conventional

magnetic microbeads, results from their smaller size, monodispersity, and higher

magnetization [8.156].



8.12 Summary

The progress in the understanding of magnetism on the nanoscale has been stimulated by advances in film growth, by new imaging methods, and by the increase

in computer power for modeling. Imaging of magnetic nanostructures can be

performed by various scanning probe microscopy techniques, even down to the

magnetic behavior of single atoms on surfaces, by electron microscopy and holography, and by x-ray magnetic circular dichroism (XMCD) microscopy. Substantial

size and dimensionality effects are found in nanomagnetism going from single

atoms and clusters to nanowires and nanolayers. Soft- and hard-magnetic materials can be improved by nanostructuring. Antiferromagnetic structures are modified

and complex magnetic structures appear at the nanoscale and the motion of magnetic domain walls can be induced by electrical currents. Single-molecule magnets

may be exploited for information storage. Multiferroic behavior may be induced

in nanostructures. Magnetic nanostructures can be biomineralized and doped in

bacteria.



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423



Chapter 9



Nanotechnology for Computers, Memories,

and Hard Disks



The field of computer and data storage development is of particular importance in

nanoscience. The design and fabrication of computer components, such as transistors, or of data bits in storage media are governed by the principles of physics,

chemistry, and materials science on the nanoscale. On the other hand, the nanotechnical semiconductor industry with its current revenues of ∼200 billion US $

annually [9.1] is presumably the largest economic factor where nanotechnology

plays a central role.

The continuous development of computers is driven by scientific projects such

as the 1,000 Genomes Project or the Large Hadron Collider (LHC) at the CERN

European particle-physics lab, by search engines such as Google (see [9.2]), by

military supercomputers with petaflop (1015 ) operations per second [9.3], but also

by consumers’ demands for increasing computation power, for digital video, digital cameras in cell phones, interactive multimedia, game products, etc., with ever

increasing data storage densities and data transfer rates in addition to random access

and removability (the ability to separate the media from the drive) [9.4]. In a

computer, the memories that directly provide data bits to the microprocessor are

semiconductor devices known as the static random access memory (SRAM) and the

dynamic random access memory (DRAM). They are fast but need power to maintain the stored bits. When a personal computer (PC) is turned off, the information

stored in these memory devices vanishes. The only archival memory in a computer

today is a hard disc drive (HDD). Its access time, however, is six orders of magnitude slower than that of SRAM, as seen in the all too familiar wait when a computer

is turned on [9.5].

Established by Intel co-founder Gordon Moore in 1965 [9.6], the empirical rule

of Moore’s law states that the density of transistors on a silicon-based integrated

circuit (IC), and so the attainable computing power, doubles about every 18 months,

with similar rules for data storage. This had the consequence that the IC components, such as transistors or capacitors, or data bits on HDD shrank to nanometer

dimensions so that novel designs and materials concept had to and have to be developed in the future. The rapid development of computers has also initiated novel

mathematical techniques. Whereas the computation of a particular equation took

more than 2 days in 1980, this only took 10 ms in 2007 (20 million times faster),



H.-E. Schaefer, Nanoscience, DOI 10.1007/978-3-642-10559-3_9,

C Springer-Verlag Berlin Heidelberg 2010



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9 Nanotechnology for Computers, Memories, and Hard Disks



although the computer velocity increased in this period only by a factor of 4,000.

This demonstrates that a similar acceleration in computation has been contributed

by novel calculation methods [9.7].

Moore’s law has held for more than 40 years but there is a sobering consensus

in the industry that the miniaturization process or scaling can continue for only

another decade or so [9.8]. Therefore, in this section, the present state and future

prospects of integrated circuits including strategies beyond complementary metaloxide-semiconductor (CMOS) technology, of modern lithography technologies, of

solid state memory, and of hard disk drives will be discussed. In addition materials

(high k, low k) for ultrahigh-density circuit integration will be outlined.



9.1 Transistors and Integrated Circuits

After the invention of the transistor in 1947, the monolithic integrated circuit (IC)

was devised in 1958–1959 and in 1971 Intel unveiled the 4004 microprocessor

(2,300 transistors) [9.9]. Back in 1993, the Pentium processors with ∼ 10 million transistors were released and the current transistor count (see Fig. 9.1) is ∼ 4

billion. In 2009 the chip industries had an annual turnover of US$ 212 billion [9.10].

A modern factory costs about US$ 4.5 billion and for a successful operation of this

factory an annual output of US$ 7 billion is required [9.10].

The 45 nm technology is available in Penryn processors since November 2007

(P. Otellini, Intel). Prototypes of the 32 nm technology have been manufactured in

September 2007 and fabrication started in 2009.

The basic transistor structure in the gate conductors is going to be re-engineered

in the current decade. Figure 9.2 shows a transistor structure with a silicon base, a

top gate, and a few-monolayer standard SiO2 (1.2 nm) gate insulator. As the leakage

current increases substantially when the insulator is made thinner and thinner, the

SiO2 insulator has to be replaced by a higher-k material (Fig. 9.2, right). Since it

is much thicker than the SiO2 insulator, it has one-hundredth of the leakage current

(see Sect. 9.7).

For further scaling, a redesign of the transistor structure, with a very thin conduction channel (∼ 2.0 nm) is suggested (Fig. 9.3, left). The performance of a transistor,

which is usually compromised in disordered Si by a high-leakage current due to

defect states in the band gap, Eg , is enhanced in nanometer-thin films due to quantum confinement. This gives rise to band edge shifts in both the conduction and the

valence bands, and thereby an effective increase of Eg (see [9.11]), which results in

an enhanced ratio ION /IOFF > 1011 of the ON and OFF currents, the holy grail of IC

designers. Another example is the source-gated transistor (SGT) concept (Fig. 9.3,

right), which leads to much less susceptibility to short-channel effects and a higher

output independence due to the source barrier being screened from the drain field

by the gate (see [9.11]).

The exponential advances in the technologies of complementary metal oxide

semiconductor (CMOS) transistors and integrated circuitry predicted by Moore’s



9.1



Transistors and Integrated Circuits



427



Fig. 9.1 The rate of innovation in transistor density. (a) The generations 1999, 2001, 2003, and

2005 with cross sections of the transistors (upper panels) and cross sections of the metal interconnects at different magnifications (lower panels). LG is the gate length, “6 Al” means six layers

of aluminum, “8 Cu” means eight layers of Cu, etc., CoSi2 or NiSi is the materials of source,

drain, and gate electrodes. (b) Transistor generations of 2007, 2009, 2011 extending Moore’s law.

(Reprinted with permission from [9.1]. © 2006 Materials Research Society)



Fig. 9.2 High-resolution cross-sectional images of a transistor structure for SRAMs with a silicon

base, a gate on the top, and the dielectric in between. The dielectric layer in (a) is a standard SiO2

gate, 1.2 nm thick. The gate in (b) shows a high-k dielectric. Although it is much thicker than the

gate in (a), it has 60% more capacitance and, more importantly, one-hundredth the leakage current

because of the thicker gate dielectric. (Reprinted with permission from [9.1]. © 2006 Materials

Research Society)



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