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.
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.
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.
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
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
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 stat
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.
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.
Sources and Further Reading