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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

 

Exercise 4. Fill in the blanks using terms given below:

The preparation of nanocomposites with an inorganic … such as metal nanoparticles and a protective organic … of small molar mass … or polymers is a route to obtain materials suitable for hierarchical self-organization into … . Incorporation of … into polymer matrices is particularly advantageous for materials engineering and the study of nanoparticle–matrix … . The adsorption of polymers onto a wide variety of nanoparticles is of broad industrial relevance in such areas as pharmaceuticals, photography, paints and coatings, foods, wastewater treatment, rubbers, and … and is ubiquitous in natural aqueous environments as well as in many biological organisms. Recently, … of conjugated polymers and inorganic particles have received much attention , due to the interesting high electrical … and redox properties of conjugated polymers and their extensive applications ranging from batteries to … .

 

Terms:core, shell, compound, nanoassemblies, nanoparticles, interactions, composite materials, nanocomposites, conductivity, light-emitting devices.

 

 








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