Abstract
At the nanoscale physics follows familiar principles that lead to unfamiliar and even unlikely responses. The change in the balance of a range of physical features results in behaviour that can differ wildly from the same materials at the macroscale. In this issue Di Ventra and Pershin examine some of the memory effects that have attracted increasing interest in investigations of nanoscale electronic systems [1]. The work builds on the familiar premise that external perturbations cannot have an instantaneous effect on any condensed matter system. As they point out, 'This is even more so in systems of nanoscale dimensions where the dynamics of a few atoms may affect the whole structure dramatically'. In this way they explain that the response of these systems will always have some degree of memory present and that memristive, memcapacitive and meminductive systems are simply examples where this feature is particularly prominent.
In the late 1990s investigations into the use of carbon nanotubes and SiC nanorods revealed that the moduli of these structures changes with diameter, highlighting the eccentricities of mechanical properties at the nanoscale. These results prompted Miller at the University of Saskatchewan and Shenoy at the Indian Institute of Technology to study the properties of nanotubes and nanorods in detail [2]. 'In the eyes of an engineer these structures are essentially little beams', they explained, 'Albeit they are "little" to a degree that challenges our traditional notions of continuum mechanics'. In their work they developed one of the first simple models for explaining the behaviour of the Young's modulus of nanostructures, verified by direct atomistic simulation of axial loading of these structures. Since then, consideration of different nanoscale structures and the dissipation of energy under stress and strain have also demystified the extraordinary mechanical properties of natural materials such as collagen [3] and spider's silk [4].
The increasing availability of nanomaterials provided a number of advantages for research in field emission for displays, microwave and x-ray generation, electron-beam lithography and photonic devices [5, 6]. However the results reported in these studies have often misapplied the Fowler–Nordheim theory for describing field-emission behaviour, as changes in scale alter the validity of the widely cited simplified equation. As Forbes at the University of Surrey points out, this equation over-predicts the large-area field-emission average current density by a large highly variable factor thought to usually lie between 103 and 109. In his paper he explains how to modify the Fowler–Nordheim type equations so that they can be applied to large-area field emitters with recommendations for improved practice in this area of research [7].
The unusual nuances of behaviour in electronic circuits at the nanoscale has piqued avid interest in 'the memristor', the fourth circuit element reported 'missing' by Leon Chua in 1971 [8] and 'found' by researchers at HP Labs in 2008 [9]. The past five years have seen intense research into the mechanisms governing memristor behaviour [10], as well as the potential to apply this behaviour in novel electronic devices capable of mimicking the biological synapses that implement human learning [11]. (Keep an eye out for Nanotechnology's special issue on synaptic electronics later this year.) However, as Di Ventra and Pershin point out, 'Although this whole field of research has been growing at a fast pace, there is still much confusion about the fundamental physical properties that realistic systems with memory (as opposed to ideal ones) satisfy'. In this issue they derive expressions for memristances, memcapacitances and meminductances from the Kubo response and microscopic theories and show that they are indeed simply response functions that satisfy well defined physical properties.
In the midst of concrete facts, cutting edge research often exploits cracks that allow new ideas and a deepening understanding to take root. Over the last few decades nanoscale research has matured into a science with generally well understood principles, allowing technology companies to develop next-generation devices with functions that dwarf some of the most creative fancies of science fiction. Yet some of the greatest leaps of inspiration have come where observations tax the limits of what current theory can explain, forcing researchers to question and examine historic reasoning. As Di Ventra and Pershin demonstrate in this issue, current memristor research is proving very fertile ground for such questioning and examining, ultimately leading to a deepening understanding of the physical properties of these nanoscale systems.
References
[1] Di Ventra M and Pershin Y V 2013 On the physical properties of memristive, memcapacitive, and meminductive systems Nanotechnology 24 255201
[2] Miller R E and Shenoy V B 2000 Size-dependent elastic properties of nanosized structural elements Nanotechnology 11 139–47
[3] Buehler M J 2006 Nature designs tough collagen: explaining the nanostructure of collagen fibrils Proc. Natl Acad. Sci. USA 103 12285–90
[4] Keten S, Xu Z, Ihle B and Buehler M J 2010 Nanoconfinement controls stiffness, strength and mechanical toughness of B-sheet crystals in silk Nature Mater. 9 359–67
[5] Misewich J A, Martel R, Avouris Ph, Tsang J C, Heinze S and Tersoff J 2003 Electrically induced optical emission from a carbon nanotube FET Science 300 783–6
[6]Jiang H, Hu J, Gu F and Li C 2009 Stable field emission performance from urchin-like ZnO nanostructures Nanotechnology 20 055706
[7] Forbes R G 2012 Extraction of emission parameters for large-area field emitters, using a technically complete Fowler–Nordheim-type equation Nanotechnology 23 095706
[8] Chua L O 1971 Memristor—the missing circuit element IEEE Trans. Circuit Theory 18 507–19
[9]Strukov D B, Snider G S, Stewart D R and Williams R S 2008 The missing memristor found Nature 453 80–3
[10] Yang J J, Miao F, Pickett M D, Ohlberg D A A, Stewart D R, Lau C N and Williams R S 2009 The mechanism of electroforming of metal oxide memristive switches Nanotechnology 20 215201
[11] Seo K, Kim I, Jung S, Jo M, Park S, Park J, Shin J and Hwang H 2011 Analog memory and spike-timing-dependent plasticity characteristics of a nanoscale titanium oxide bilayer resistive switching device Nanotechnology 22 254023