книги / Nanotechnology Read and Discuss

Fig. 3. Steps for making a quantum dot using lithography (Poole and Owens, 2003, courtesy of Wiley-Interscience)

CNTs are also highly flexible – when bent, they are able to recover their original shape without permanent changes. They are ultra strong and stiff, with breakage stress exceeding tens of GPa and Young’s modulus of the order of TPa. CNTs are therefore being explored as strengthening fibers in composites. The French Babolat tennis rackets, made of CNT-reinforced composites, are said to be several times stiffer than current carbon–composite rackets.

Speaking 2

10. Answer the questions:

Why have fullerenes attracted a lot of attention? What do the electronic properties of CNTs depend on? How do CNTs exhibit fieldemission effects?

Why can CNT find a potential application in high-speed computers?

What are the I–V characteristics of a CNT field-effect transistor strongly affected by?

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quantum well structure. Irradiation (optical light, UV, electrons or even neutrons) is then shone onto the region of the resist where the nanostructure (e.g. a quantum dot) is to be made, either through a mask or shield, as shown in fig. 3, a, or, in the case of electron-beam lithography, by directing the electron beam accordingly.

Fig. 4. Schematic showing the dependence of band gap on size of a semiconductor

After washing away the exposed region of the resist with a developer, an etching mask is deposited back on the feature region. The remaining photo-resist not covered by the etching mask is then removed to form the desired pattern.

Unlike free electrons in a metal, which are unconfined, the electrons in quantum wells, wires and dots are spatially confined. As an example, the discrete energy states of an infinite potential well of width L are given by E = n2h2 / (8 mL2) and, likewise, the states in other confined structures such as finite wells, wires and dots are confined and are different from those in the bulk condition. Physical properties that depend on the density of states of electrons, such as the specific heat capacity, magnetic susceptibility, etc. are also expected to be different

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from bulk behavior. In semiconductors a measure of the confinement effect is the radius ae of an exciton. An exciton is a bound state of an electron–hole pair, and its radius ae measures the effective range of Coulomb-type interaction between the electron and the hole. For typical semiconductors such as GaAs, ae is of the order of 10 nm, and so a particle with size comparable to ae will exhibit strong quantum confinement effects. In strongly confined situations, the band gap of the semiconductor will increase relative to the bulk condition (fig. 4). The band gap of a bulk semiconductor must be smaller than the difference between the occupied and unoccupied energy levels in the free atomic state, and since a nanocrystal can be thought of as an intermediate case between the bulk form and the single atom state, its band gap is higher than that of the bulk state. Semiconductors are important light-emitting materials and one important development focus is to fine-tune their band gaps to cover the entire visible wavelength spectrum, i.e. the so-called ‘band-gap engineering’. Visible light has a wavelength range of 375– 740 nm, corresponding to a photon energy range of 1.68–3.32 eV. Pure GaN, for example, has a direct band gap of about 3.4 eV, which falls in the ultraviolet range, but alloying with InN andAlN allows the band gap to be tunable between 1.9 and 6.2 eV, which now overlaps with the optical range. An alternative method of band-gap engineering is to make use of the quantum confinement effect shown in fig. 4, in which the increase of the band gap as the material size decreases causes ‘blue shift’ of the emitting light. In practice, a mixture of the alloying and quantum confinement techniques is used in band-gap engineering.

An example being intensively investigated is the ZnxCd1−x Se alloy system. Nanoparticles with a core–shell structure, in which a core of higher band-gap ZnSe (2.7 eV) is surrounded by a shell of lower bandgap CdSe (1.75 eV), is able to emit in the range between red and blue for x between 0.1 and 0.5. The discrete nature of the density of states and the widened band gap in a size-confined semiconductor have other applications, including infrared detectors and lasers. As an infrared detector, the incoming radiation excites electrons in the lower bound states to higher energy conduction states.

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Fig. 5. Schematic of a quantum-dot laser

(from Center for Quantum Devices, Northwestern University)

Once in the conduction states, the electrons can conduct electricity and so the power of the radiation is measurable in terms of the resultant current. A conventional laser works by virtue of the existence of discrete energy levels, the difference of which corresponds to the laser emission, and a ‘population in-

version’ mechanism by which more electrons can reside at the upper energy level than the lower one so that, upon triggering, they fall down simul-

taneously to give a coherent emission of radiation. In a quantum-dot laser source, the discrete energy levels in the quantum dots provide these two requirements. Fig. 5 shows the layout of a quantum-dot laser, in which the active volume consists of multiple layers of InGaAs/GaAs quantum dots.

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13. Match the words from the two boxes to make up phrases that you encountered while reading the text.

Speaking 3

14. Answer the questions:

How are quantum wells, wires and dots typically produced? What’s the difference between free electrons in a metal and the

electrons in quantum wells, wires and dots?

Since semiconductors are important light-emitting materials, what is their important development focus?

What techniques are used in band-gap engineering?

What’s the difference between a conventional laser and a quantumdot laser source?

15. Enumerate the existing applications of quantum wells, wires and dots. Think of any potential ways to exploit them.

Reading 6

16. You are going to read the forth part of a chapter about nanomaterials taken from a course book ‘Physical Metallurgy and Advanced Materials’ by R. E. Smallman and A. H.W. Ngan (2007). Before you read it look at the following three sentences taken from

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the text. They are split. Restore the word-order, then read and check:

•materials Nanocrystalline with polycrystalline are solids grains nanometer-sized.

•can They be by metallurgy powder made techniques.

•Nanocrystalline exhibit solids mechanical unusual properties.

Bulk nanostructured solids

Another important class of nanomaterials includes solids that have nano-scale microstructures. Examples include nanocrystalline solids, and tailor-made structures such as photonic crystals. Nanocrystalline solids are simply polycrystalline materials with nanometer-sized grains. They can be made by powder metallurgy techniques and thin films can also be made using physical deposition techniques such as magnetron sputtering, but low-porosity nanocrystalline metals with grain sizes approaching the nm limit are more commonly made using electrodeposition. Nanocrystalline solids have attracted considerable interest in the past because they exhibit unusual mechanical properties when compared to ordinary polycrystalline materials. In the emerging field of photonics, light, as opposed to electrons in electronics, is used to transmit signals through waveguides. Just as electrons are scattered by crystals and exhibit phenomena such as Bragg reflection governed by dispersion surfaces, similar properties can be exploited for monochromatic light, but then the ‘crystal’ concerned must have a periodicity comparable to the wavelength of light. A photonic crystal is therefore a tailor-made 2-D or 3-D periodic structure of dielectric material, with a lattice periodicity comparable to that of the optical wavelength. A simple application is a waveguide bend, shown in fig. 6, а, which is a 2-D lattice of particles of a dielectric material (silica, alumina, etc.), but with missing particles along a bent path. The spacing of the particles is such that strong Bragg reflection is experienced by the incident light, so that the latter is prevented from entering the lattice. The light is thus forced to travel along the curved path of missing particles. More complicated 3-D structures have also been proposed (fig. 6, b) which exhibit interesting and useful dispersion properties.

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Fig. 6. Schematic illustrating a defected photonic crystal being used as a waveguide bend (a); a proposed 3-D photonic crystal (b) (Povinelli and co-workers, 2001, by permission of American Physical Society)

17. Answer the following questions on the text:

•What are nanocrystalline solids?

•By what techniques can they be made?

•Why do they attract considerable interest?

•What unusual properties doe they have?

Writing 2

18. Complete the gaps.

Another class of nanomaterials includes solids that have ...

Nanocrystalline solids are polycrystalline materials with ... They can be made by … techniques, physical deposition techniques such as …, or by electrodeposition. Nanocrystalline solids have attracted considerable interest in the past because they … Their dispersion properties can be used in signals transmission.

19. Imagine you were asked to write a description of bulk nanostructured solids to be published in a Young Researcher Journal. Use the text you’ve read, additional resourses and follow the given plan and phrases to organize your writing:

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Plan

–bulk nanostructured solids

–techniques to produce

–properties

–existing and potential ways to exploit

Useful phrases

Opening:

This article will address / is concerned with… The aim / purpose of this article is …

Organising the main points:

Firstly, next, finally…

We now turn to / At this point it is important to look at… We shall see below that… The following example shows… Consider Figure 22, which shows…

Conclusion

To bring the paper to a close, I summarise the main points here… We may summarise the description in a few words…

The final point to stress is that …

Reading 7

20. Complete the gaps with the translation of the term:

Strength – прочность

Hardness – ____________

Toughness – вязкость

Elasticity – ____________

Plasticity – пластичность

Brittleness – ____________

Ductility – пластичность

Malleability – __________

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21. Before you read the next part, addressing mechanical properties of small material volumes, complete the definitions of mechanical properties given below:

Strength, hardness, toughness, elasticity, plasticity, brittleness, and ductility and malleability are mechanical properties used as measurements of how metals behave under a load.

___________ is the property that enables a metal to resist deformation under load. Fatigue strength is the ability of material to resist various kinds of rapidly changing stresses. Impact strength is the ability of a metal to resist suddenly applied loads.

____________ is the property of a material to resist permanent indentation.

_____________ is the property that enables a material to withstand shock and to be deformed without rupturing. Toughness may be considered as a combination of strength and plasticity.

______________ is the ability of a material to return to its original shape after the load is removed.

______________ is the ability of a material to deform permanently without breaking or rupturing. This property is the opposite of strength.

______________ is the opposite of the property of plasticity. A brittle metal is one that breaks or shatters before it deforms.

______________ is the property that enables a material to stretch, bend, or twist without cracking or breaking. This property makes it possible for a material to be drawn out into a thin wire.

______________ is the property that enables a material to deform by compressive forces without developing defects. A malleable material is one that can be stamped, hammered, forged, pressed, or rolled into thin sheets.

22. Now read and say what mechanical properties are mentioned in the text. While reading translate some terms.

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