Nanostructures

A “nanostructure” is an engineered object that extends in at least one of its dimensions for significantly less than one millionth of a meter (10-6 m, or one micron). “Nanotechnology” is the branch of applied science concerned with manufacturing, analysing and identifying useful applications for these objects

The most common form of nanotechnology is the modern very-large-scale integration microchip, where the size of elemental units (transistors) has shrunk below 100 nanometers (10-7 m). A standard microchip is a solid block of intricately composed materials, but techniques identical or very similar to those used in its fabrication can also produce freestanding structures, miniature mechanical (or optical) components, or manifest strong quantum mechanical effects in matter due to components of extremely small scale.

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Nanowires [Frank Glas, NanoEmbrace Meeting, Rome].

 Nanowires are very elongated micro-crystals of semiconductors, with two dimensions out of three below the “nano” threshold: they are also called “one-dimensional nano structures”, referring to the one direction (length) along which electrons are relatively free to move. (Electron confinement is important since it can give rise to quantum effects.)

An interesting feature of nanowires is their ability to naturally grow out of a seed particle (catalyst) of gold, or in some cases from a naturally occurring droplet of their own constituents. Fine control of the materials available to the growing crystals allows the creation of complex structures, such as multiple coaxial shells or many-layered stacks.

Diagram of nanowires, showing the core and shell

 Diagram of nano wires, showing the core and shell.

[K. Durose et al.  Appl. Phys. Lett.  V.104 Issue 5,  2014]

One technique which is widely used to grow nanowires is Molecular Beam epitaxy (MBE). This is an ultra-high vacuum (UHV) technology that can deposit thin films with atomic precision. The principle underlying MBE growth is relatively simple: beams of atoms or clusters of atoms are produced by heating up a crucible in UHV; a set of baffles directs them to impinge on a hot substrate surface, where they can diffuse and eventually incorporate into the growing film. Various diagnostic instruments can be inserted in the vacuum chamber to monitor this process.

Performing MBE deposition over a target prepared with catalyst particles will produce nanowires, as the constituent atoms dissolve in the particle and then condensate at the bottom. Since the particle is liquid at operating temperature, this is called Vapor-Liquid-Solid process.

When at least one of the nanowire constituents is itself liquid in the conditions of interest, the catalyst may be omitted. This self-catalyzed process is very attractive, due to its extreme simplicity, for high volume production as it may be required by solar cell applications.

Other techniques employed in nanowire technology are chemical vapour deposition (CVD), a deposition technique where a low-pressure plasma transfers material to the surface instead of ballistic beams in high vacuum; plasma based etching techniques, to selectively remove parts of the substrate; and lithography technologies, to transfer patterns (e.g. arrays of catalyst particles) to a substrate. These are usually based on optical projection techniques, not unlike film photography, but alternatives based on electron beams or extremely fine stamps exist.

The latter solution, nanoimprint lithography, is currently an active research topic for NanoEmbrace and could bring very high resolution nanolithography within the reach of teams with modest funding.

The possible applications of nanowire technology cover many topics in electronics: some examples are performance enhancement for solar cells, sensitive novel detectors for both ultraviolet  and far-infrared (terahertz) light, nanoscale opto-electronic devices such as lasers, and new types of chemically sensitive microprobes for Atomic Force microscopy (an imaging technique where an extremely sharp tip mounted on a flexible arm is dragged along the sample to be analysed, with minute deflections of the arm revealing surface details with nanometer precision).

 /images/NW detector-emitter copy.pngIntegrated photonics platform. Fabricaton using nano wires from two different samples : "detector" and "emitter". Connecton with an SiN waveguide defined by e‐beam lithography. [Tchernycheva et al, Nano Lett., 14, 3515 (2014)] 

 Electronic devices require materials in the form of crystals very close to perfection to achieve high efficiencies; the properties of crystals sometimes rule out combinations of materials that would have useful properties, because of the very large number of crystal defects that would be created by lattice mismatch – when two compounds with different natural spacing between atoms are joined, very strong forces will appear at the junction interface that will inevitably lead to breakage of atomic bonds and the formation of “dislocations” or other defects.

A nanowire, due to its extremely small radius, can tolerate a much high lattice mismatch than a thin planar film without the interface failing due to stresses, and is also more resistant to other defect-generating phenomena: these characteristics make the production of heterostructures (combinations of different semiconductors) easier.

This is especially interesting for solar cell work, since the ability to grow strongly dissimilar semiconductors on top of each other can be used to produce “multiple-junction” photovoltaic cells, that utilise the different colours of light in a more efficient manner. Having the option to grow different semiconductors on top of silicon, such as gallium arsenide or other III-V compounds, can be useful for many other optoelectronic devices, since the optical properties of silicon are often poor but the development investment in it for electronic applications has been tremendous.

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Nanowires for solar sells [K.Durose, iNOW Workshop]

The main challenges facing NanoEmbrace members, and nanowire users in general, starts with the control of the growth process; achieving better uniformity in the properties of VLS grown nanowires and more freedom in the design of complex structures is an ongoing task of many groups. Practical experiments together with computer simulations of the growth process are necessary to advance our understanding.

Manipulating individual nanowires is also very challenging and time consuming, though possible at the laboratory scale; NanoEmbrace intends to begin the scaling of nanowire powered electronic devices from one-off demonstration prototypes to series production, and is interested both in the utilisation of VLS nanowires as-grown and in techniques to align a large quantity of them along a desired direction on a substrate, for incorporation in predefined circuitry.