Dr Shashi Paul reviews the existing academic research on nanocomposite electronic memory devices and looks to the future.
The flexible and printable electronic industry is continually growing and is now important to various consumer electronic devices. The interest in this market is mainly down to two reasons. Firstly, the ability to use flexible substrates such as metal foils and plastic sheets will introduce new futuristic products, including rollable displays, sensors or even wearable electronics and artificial skin. Secondly, the cost in fabrication and production line of electronic devices is drastically lower than existing silicon and other wafer-based technologies. Among other advantages, those devices will be lighter, conformably shaped and durable.
Electronic memory devices are associated with modern electronic products, such as personal computers, tablets and smartphones. There is a demand for a memory that can cope with all the new requirements of these products and researchers are looking into the possibility of building a ‘universal memory’. Some of the characteristics will need to be low cost, high speed, high capacity, scalability, nonvolatility and low power consumption. We are currently in an exploratory stage to discover the potential for electronic memory materials and devices.
The primary aim in an electronic memory device is to produce structures that exhibit two distinct states when a certain type of stimulus is applied – for example, electrical field in flash memory and magnetic field in magnetic hard-drives. These two states can be seen as the realisation of memory devices.
The current field of play
When it comes to memory devices comprising inorganic materials, a number of new structures have recently been reported. Examples of such configurations include phase change memory devices, floating dynamic random access memory (DRAM), nonfloating DRAM, single/few electron memories, Mott Memory and the memristor. These memory devices are mostly based on inorganic materials and their merits and limitations are documented in the International Technology Roadmap for Semiconductors (ITRS) report, Overview of Emerging Research Memory Devices 2015 ITRS-ERD Meeting.
All single-crystal semiconductor technologies are ultimately constrained by their restriction to two dimensions. Single crystals cannot currently be grown on top of plastic/glass substrates, so multiple active memory layers cannot be built on a single wafer or by exploiting the current technology route. The only way to enhance memory density in these structures is to reduce the feature size in the two dimensional plane – miniaturisation, which solicits increased production expenditure.
There are several issues related to the emerging field of memory device technology – although there is a clear demand for the next generation of non-volatile solid state memories, the upcoming memory devices have a lot to live up to – they must exceed the existing speed and cost constraints of today's entrenched technologies. They must also meet other critical performance criteria, such as long-term data retention – at least 10 years and up to 100 years for archiving – low power consumption and a large number of rewrite cycles. Flash memories are rated for 106 cycles while DRAM are rated for >1015 cycles.
It should be noted that the memory materials and devices discussed and presented here are based on the admixture of small molecules, nanoparticles and polymers, and such devices are called organic electronic memory devices. The first fully-organic memory devices, based on a poly-vinyl-phenol (PVP) and buckyball (C60) nanocomposite, were reported in 2004. Around the same time, memory devices using gold nanoparticles and 8-hydroxyquinoline, dispersed in a polystyrene matrix, were also demonstrated. Since then, the interest to use an admixture of nanoparticles, small molecules and polymers in the manufacture of electronic memory devices has been on the rise.
I published a working model of such devices in IEEE Transactions on Nanotechnology (2007), Volume 6, based on the creation of an internal electric field that had been verified using different systems. For example, Salaoru et al (2010) demonstrated bi-stable nonvolatile behaviour from a blend of two small organic molecules. One was an electronic donor tetrathiafulvalene (TTF) and the second one was an electron acceptor tetracyanoethylene (TCNE) in poly(vinyl acetate) (PVAc) matrix. Recently, Liu et al (2016) discussed the latest developments in polymer composite materials, including nanocomposite and their electrical switching behaviour, considering the role of these materials in memory devices.
However, there is hardly any mention of admixture-based organic electronic memory devices. This area certainly needs attention from theoretical physicists, materials scientists, chemists and device engineers, but there are a number of issues that need to be addressed before we can embark on extending these devices to the real world. Such issues involve a complete understanding of the electrical bistability mechanism in nanocomposites, maintaining the difference between low and high conduction states for a longer period of time by ensuring the stability of the high and low states, selecting environmentally friendly materials required for fabrication of nanocomposite materials and developing a cost-effective methodology for the fabrication of devices.
Is the future nano?
It will not be possible to replace silicon-based memory devices in the foreseeable future. However, there are a number of other applications where cheap electronic memory devices can play a vital role. For example, nanocomposite-based memory devices can be directly printed on medicine packaging and the information about the patient and schedule of taking medicine can be stored on the printed device.
The field of polymer memory devices is still in its infancy, although the potential of these devices is enormous. Once the physics underlying the behaviour of these devices is grasped and agreed by the memory community, we can fine-tune design, rendering them capable of revolutionising the technological market in memory devices. Such understanding is of profound academic importance and also enables the controlled design and ultimately, the industrial production of these structures.
Some academic papers on polymer memory devices report on the use of nanoparticles, such as gold and barium titanate, and question whether the lack of standardisation of the size of such nanoparticles could lead to variability from device to device. We might not see such variances when the device size is very large – in micrometres scale – but its significant effect will be felt at the nanoscale. An alternative approach should be looked into to alleviate such problems.
Following research conducted at De Montfort University, UK, we have done away with nanoparticles altogether. In their place, organic molecules were used. We exploited the molecular storage capability of C60 – a feature that will help to significantly reduce the invariability from one device to the next, as C60 molecules have an identical size.
It is quite clear from the ITRS 2015 report that there is a distinct urgency to develop new types of memory devices. The possibility and viability of the use of molecules in polymer memory devices need attention from the memory community – a simple chip of polymer memory devices has not yet been demonstrated. To date, the majority of published device characteristics has been measured on individual memory cells, but not on a memory chip.
Materials scientists should think about developing materials with a strong internal electric field either by designing ferro-electric polymers with a higher value of polarisation or a polymer blend with charge storage capability for an extended period of time. Such curiosity might also result in designer or self-destructive memory devices – for instance, if the sensitive information does not get to its destination in the expected time, the stored information would wipe out by itself.
If the suggestions noted above and the ideas discussed are further validated, this will open a new way for non-volatile information storage. More importantly, this technology would connect the future flexible, wearable memory devices and open up various opportunities, both in the academic and industrial world. The development will have an impact on society, economy, an individual and should bring us nearer to the dream of the designer and immortal memory devices.