By Will Soutter
Topics Covered
Introduction
Miniaturization of Electronics
The "Bottom-Up" Approach
Molecular Switches
Types of Molecular Switch
Crown Ether Switches
Rotaxanes
Photochromic Switches
Nanoparticle Switches
Current State of Molecular
Switch Research
References
Introduction
Miniaturization of Electronics
One of the main drivers for nanotechnology research is to uncover
ways to produce nanoscale electronic circuits and components. The
computing industry demands ever smaller, faster, and more efficient
processors, and we are beginning to encounter limits to just how small
the
necessary components can be made.
The major manufacturers of microprocessors expect to reach the
limits of the resolution achievable on silicon chips quite soon - 14 nm
resolutions are expected to be commercial in 2014, and beyond that
point silicon wafers will become unusable due to quantum tunnelling
effects.
In
order to continue to continue the trend of decreasing feature size in
processors, and in electronics generally (known as Moore's Law),
technology companies are always eager to find new ways
to fabricate electronic components at the nanoscale.
The "Bottom-Up" Approach
One methodology which has been applied to nanoelectronics research
is the "bottom-up" approach. In essence, this involved trying to
assemble the fundamental components to be as small as possible from
individual atoms and molecules, rather than attempting to adapt
materials and manufacturing methods to ever smaller devices.
Molecular Switches
One of these molecular devices, on which significant progress has
been made in recent years, is the molecular switch. Although a very
simple device, a suitable molecular switch could be the basis for more
complicated molecular machines.
A molecular switch usually consists of a single molecule which can
shift controllably between two stable states. The trigger used to
switch between the states can be an electrical current, a change in
temperature or chemical environment, or even light.
Types of Molecular Switch
In the last few years, many research groups all over the world have
produced molecules which can be switched between stable states, with a
myriad of structures and mechanisms. Here are a few of the most common
structures which these molecular switches have been based around.
Crown Ether Switches
Crown ethers are cyclic organic compounds which can bind very
strongly to positive ions. Their flexible structure made them the
target for some of the earliest research into switchable molecules.
Most crown ether switches work by using a trigger, such as light or
a pH change, to alter the conformation of the cyclic ring. The drastic
change in affinity for certain ions which this can cause can be
measured as a step change in the ion current across the sample.
Rotaxanes
A rotaxane is a large molecule with a "thread and bead" structure. A
long, linear chain is inserted through a macrocyclic ring so that the
ring can slide freely along the length of the chain, and bulky
end-groups prevent the ring from sliding off. Under normal conditions,
an the ring structure on an unmodified rotaxane will shuttle backwards
and forwards along the chain at a thermally controlled speed.
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Rotaxanes consist of a molecular ring trapped on a long chain.
The
structure can be used as a nanoscale switch by moving the ring between
binding sites. Image credit: Berkely Lab
Molecular Foundry
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A rotaxane can be simply modified to give it switching
functionality. Adding two functional groups within the chain which have
an
affinity for the ring creates stable positions which the ring switches
between.
If one of these functional sites binds more strongly to the ring
than the other, the ring will sit preferentially in that position. The
site can also be designed so that a simple trigger, such as a pH
change, or an electrical current, will deactivate it, making the ring
shift to the other site. This system works as a simple two position
switch.
Whilst the prospect of a functional switch made up of only a few
dozen atoms may seem enticing, particularly with regard to
nanoelectronics, there are many limits to the usefulness of a
rotaxane-based system:
- Switching speed on the order of kHz, compared to the MHz
switching available from existing silicon devices
- Limited stability over long periods means limited applicability
to data storage
- Data storage density limited, despite small size or individual
switches, as rotaxanes must be isolated from each other to avoid
interactions which may lead to data corruption
Photochromic Switches
A photochromic switch is a molecule which can be optically switched
between two isomers (different structural forms made up of the same
atoms) with measurably different optical properties.
Tuned laser light can then be used to selectively transform the
molecule from one isomer to the other, and the changed property, such
as absorption wavelength, luminescence, or optical rotation.
Whilst there is a large range of photochromic compounds available,
their properties must be selected carefully to ensure good performance
as switches. The following criteria are important:
- Absence of thermal isomerization
- Significant differences in properties between isomers
- Good long-term stability, whether switched rapidly or left in one
state
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In 2011, researchers at the Technische Universität München (TUM) created a
single-molecule switch from a porphyrin ring. The molecule has four
discrete states, which can be switched with a single electron from an
scanning tunnelling microscope.
With a surface area of just one nanometer, this switch is one
of the smallest ever implemented. To make larger scale use of this sort
of switch practical, more research is required into easier switching,
and into stabilising the individual states at higher temperatures.
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Nanoparticle Switches
Nanoparticles of some specific materials have been found to exhibit
switchable optical properties. This is usually due to a structural
phase transition which results in a measureable change in properties,
and can be triggered optically.
For example, research published in 2005 demonstrated a film of
bistable gallium nanoparticles. Excitation of the nanparticles with a
continuous wave pump laser excites the particle into a different
structural phase. This higher-energy phase has a much higher
reflectivity than the initial phase, which allows straightforward
determination of the state using a second probe laser.
The interesting thing about this particular system is that the
higher-energy phase is very stable - the nanoparticles have to be
super-cooled to get them to return to the ground state. This
demonstrates great potential for memory applications - optical memory
based on bistable nanoparticles could lead to high-density, long-lived
data storage devices.
Current State of
Molecular Switch Research
Recent research has produced a variety of single-molecule switches.
The majority of these have taken advantage of the capabilities of
modern scanning probe microscopes (SPMs), using the probe tip to
manipulate the states of switchable molecules.
Carbon nanostructures like graphene and carbon nanotubes are also
prevalent in this field. A switch based on a rotatable molecule
embedded in a carbon nanotube, which allows current to pass in one
orientation and blocks it in another, was proved theoretically viable
by a team at Oak Ridge National Laboratory. Graphene has even been used
to assemble basic circuits made of transistors, and rapid progress is
being made in the fabrication of graphene-based electronics.
Whilst this allows us to increase our understanding of how best to
handle switchable molecules, at some stage the focus of development
will have to shift to practical implementations of switchable molecules
that could be used in nanoelectronics, nanoprocessors and high-density
data storage.
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here for more from AZoNano on molecular switches.
References