Nanotubules and nanowires

Over the last one and a half decades, the world of conducting polymers has been enriched with a number of nanostructured objects in different forms like nanotubules, nanowires, nanorods, etc. The prime objective of synthesis in most of the cases, however, was to achieve the perfect control over the growth of polymers, which ultimately helps to reveal the unexplored fundamental properties of them. Inorganic and polymeric substrates having nanodimensional pores, channels, and gaps were used as a support for this purpose and conducting polymers were led to grow chemically or electrochemically within them. These ICP-loaded substrates are examples of a different kind of nanocomposite, in which the nanodimensional conducting polymer is the guest. After removal of the supporting matrix by selective dissolution, the nanomaterials are set free and can be collected in solid state and/or subjected to different studies. Depending upon the nature of the support and some other factors, these materials can be classified into two major classes viz. intercalative polymerization and template guided polymerization. There are two major differences between the intercalative nanocomposites, which depend on whether the supporting substrate itself is oxidative or not. In the former category, there are only a few layered materials possessing substantial oxidative property that is utilized for oxidative polymerization of the monomer within the substrate. The majority of the hosts are devoid of oxidative property and require oxidant (for the polymerization) to be applied from outside after incorporation of the monomer into the host. In each case, the reaction gives rise to solids with nanodimensional conducting polymers confined in the host materials. The synthesis techniques are obviously named as redox intercalative polymerization (RIP) and post-intercalative polymerization (PIP), respectively.

Exercise 6. Translate the text in written form.

Generation and agglomeration behaviour of size-selected sub-nm iron clusters as catalysts for the growth of carbon nanotubes

Controlling the individual diameters of carbon nanotubes (CNTs) is still one of the major challenges in current CNT research, and it is particularly important as it determines crucially their physical and electronic properties. High quality single-walled and double-walled CNTs are currently prepared on a large scale with the aid of nm-sized transition metal catalysts, by using high-temperature chemical vapour deposition (CVD) techniques above 750 °C, despite the fact that several modifications of the CVD technique exist that allow growth processes under moderate conditions even below 400 °C. Therefore obtaining more-selective catalysts that may allow reduction of the synthesis temperature in thermal CVD processes even further, while at the same time maintaining a high quality of the CNT material, would be a challenging goal. The introduction of defined metal-cluster catalysts with size-selected dimensions, deposited with a low surface coverage onto a suitable growth substrate to maintain their integrity, thus provides a new route into size-selective CNT growth. Moreover, this allows us to tackle the lower size limit of catalyst particles necessary for CNT growth. The smallest CNT diameter realized so far is 0.4 nm, which was found to be stable only in a template environment, either of a double walled CNT, or the interior of a porous zeolite AlPO₄-5 single crystal. Size-defined, sub-nm, ligand-free metal clusters would be an interesting synthetic alternative to obtain CNTs with controlled diameter. However, such small metal clusters can only be generated by gas-phase techniques, typically in a ligand-free environment as naked clusters. A controlled decrease in catalyst particle size below 1 nm and their deposition with a very low substrate surface coverage would allow studying of the lower size regime of such clusters and their catalytic activity in CNT growth. This could thus give information on the critical size of the catalyst particles and allow insight as to what extent catalyst agglomeration under thermal annealing of these particles occurs under typical CVD growth conditions. Larger catalyst particles in the size range of 3–10 nm can maintain their size throughout the growth process of CNTs, which typically operates at 750 °C or higher.

UNIT 2

Read and translate the text and learn terms from the Essential Vocabulary:

Electron Beam Writing in Nanoparticle Films

In recent years, significant progress has been made in creating a variety of functional, nanometer-scale architectures, such as molecular-scale wires and switches, molecular scale transistors based on nanotubes, and semiconductor nanowires. Even logic circuits, constructed from individual carbon nanotube molecules and semiconductor nanowires present dramatic advances toward the realization of nanoelectronics and especially nanocomputers. The main challenge to the advance of molecular-scale electronics from the single-device level to the circuit level lies in the assembling together of the component devices, each of which must be addressable. Fabrication techniques coping with the size region from the nanometer to the micrometer are essential to assemble the nanocomponents and link them to the macroscopic world. Here is introduced one of these techniques, namely, electron beam (e-beam) writing in passivated nanoparticle films. Passivated nanoparticles are the building materials for the films modified by the direct e-beam writing process. They are nanometer sized colloidal particles chemically terminated by organic monolayers, typically alkanethiol molecules. They can be produced from many materials and show a diversity of functional behavior beyond that of the corresponding bulk materials, with many

of their electronic, optical , and catalytic properties originating from their quantum-scale dimensions. The organic layer, typically bound to the nanoparticle core by a metal–sulphur (thiolate) bond, prevents the particles from aggregating together, in solution or on a surface, which is more likely to occur in the case of bare particles (clusters), so that they retain their unique, size-dependent functionality.

Self-Assembly of Passivated Nanoparticles.

Passivated metal (e.g., gold) nanoclusters can be used as “building blocks” to form ordered monolayers and thin films. The diameter of these nanoparticles (1–10 m) makes them ideal for constructing nanoscale architectures and devices. X-ray diffraction of the superlattice formed by a three-dimensional assembly of passivated gold nanocrystals shows a sequence of intense, sharp Bragg peaks in the

small angle region, in addition to the peaks arising from the planes of Au atoms within individual nanocrystal cores. The low angle peaks arise from the ordered layers of nanocrystals stacked to form a giant three-dimensional superlattice. The macroscopic properties of the nanoparticle superlattice are determined by both the properties of each individual particle and the coupling/interaction between the

nanocrystals (via the ligands). Thus the organic ligands serve not only as a protective layer for the particles, preventing direct contact between the cores, but also regulate interparticle bonding. For example, the tuneable interparticle

spacing, derived by changing the length of the molecular chain, allows control over structural, optical, and transport properties . Since a variety of organic molecules can be employed to passivate the nanoparticle cores, it is possible to choose ligands with a range of functionalities, allowing attachment to particular substrates or molecules. For example, one can envisage the selective attachment of individual nanoparticles to selected substrate regions, as part of the fabrication of a nanodevice. The electronic properties of single nanoparticles are already the subject of active research , which exploits the size-dependent properties of nanoparticles to fabricate individual single-electron devices and similar structures.

Patterning the Nanoparticle Assembly.

In order to manufacture nanostructures or, ultimately, devices using nanoparticles, one needs to be able to assemble these “artificial atoms” from solution onto technically interesting substrates and possibly also to process these assemblies further to create robust structures with practical functions. Exciting advances toward the self-assembly of nanoparticles into ordered structures on various substrates have been reported in recent years. As an example, linear structures of width ∼1–50 nm are of technological importance for potential applications and also hold great fundamental interest for quantum confinement behavior. A number of such “wires,” based on gold and silver nanoparticles, have been prepared by various methods. The spontaneous alignment of silver nanoparticles to form wires several micrometers long and 20–300 nm wide was observed at the water/air interface . Spontaneous formation of a wire consisting of a single line of clusters have been reported by some researchers. A metal particle chain, which formed a bridge between a pair of nanoelectrodes, was created by self-assembly under directional flow conditions. Another approach to the fabrication of nanowires is self-assembly of nanoparticles along linear templates. One example is the self- assembly of gold nanowires from ligand stabilized gold nanoparticles drop cast onto a corrugated amorphous carbon thin film with topographical features of nanometer dimensions, followed by heat treatment. The continuous wire segments resulting from the sintering of the nanoparticles were typically 3 to 5 nm wide and up to 100 nm long. Another example of the use of linear templates was reported by the researches who decorated bundles of single walled carbon nanotubes with gold particles and obtained continuous, polycrystalline gold nanowires after sintering the particles under heat treatment. Passivated gold nanoparticles have also been deposited from solution onto silicon dioxide surfaces prepatterned by photolithography and preferential cluster accumulation is observed along the edges of the resist structure after a lift-off of the photoresist. A guided flow along the boundary between a resist and a silicon surface can extend for over 600n m across the surface, producing cluster chains as narrow as ∼120 nm.

Essential Vocabulary

electron beam writing электронно-лучевая запись

passivated пассивированный

bulk material вещество в объеме, сыпучий материал

core ядро

bare particle голая частица

coupling связь, спаривание

superlattice сверхструктура, сверхрешетка

bragg peak максимум Брэгга, брэгговский пик

ligand лиганд

tuneable перестраиваемый

spacing расстояние, период (решетки)

robust прочный, устойчивый

template шаблон

corrugated гофрированный, рифленный

deposit осаждать

precipitate выпадать в осадок

Exercise 1. Answer the following questions:

1. What nanometer-scale architectures are developed at present?

2. What is the main challenge to the advance of molecular electronics from the single-device level to the circuit level?

3. What are passivated nanoparticles?

4. What prevents particles from aggregating?

5. What makes nanoparticles ideal for constructing nanoscale architectures and devices?

6. What are the macroscopic properties of the nanoparticle superlattice determined by?

7. What regulates interparticle bonding?

8. What is the function of the organic ligands?

9. What is necessary to manufacture nanostructures or devices using nanoparticles?

10. Give some examples of advances toward the self-assembly of nanoparticles into ordered structures on various substrates.

Exercise 2. Translate the following international words:

Individual, functional, architecture, logic, organic, electronic, individual, assemble, component, technique, region, optical, unique, ideal, diffraction, contact, regulate, transport, select, technically, practical, stabilize, accumulate, active.

Exercise 3. Match each word with its meaning:

1. Ligand a. to make or become solid or hard

2. Deposit b. to undergo or cause to undergo a process in

which a dissolved substance separates from

solution as a fine suspension of solid particles

3. To precipitate c. to render (a metal) less susceptible to corrosion

by coating the surface with a substance, such as an oxide

4. To solidify d. a coating produced on a surface, esp a layer of

metal formed by electrolysis

5. Passivate e. an atom, molecule, radical, or ion forming a

complex with a central atom