Investigating pressure induced conductive states on the nanoscale : A novel route to nano-circuitry

The ability to transfer nanometer scale metallic patterns at low cost, high throughput and high resolution has huge implications for slashing the manufacturing costs of semiconductors and data storage devices. Techniques like nanoimprint lithography allow fabrication of such nanometer scale patterns by mechanical deformation of imprint resist and subsequent processes. Even though the technique is considered one of the simplest lithography approaches, it still comprises of several complicated steps during pattern transfer. If the transfer step could be eliminated and only the imprint step could directly result in the printing of a few nanometer sharp circuit pattern on the chosen material, that would represent a dramatic leap in terms of throughput and reproducibility of patterns for nano-circuitry. In order to achieve this visionary goal of directly imprinting circuitry, it is necessary as a first step to understand the physical phenomena in materials that would allow localised pressure to be used as a tool to sketch and control sharp conductive channels in an otherwise insulating material. There are atleast two different mechanisms that could give rise to local pressure induced conductive states in an insulating material. Ferroelectrics with conducting domain walls and materials undergoing metal-insulator phase transitions are the two primary material systems where nanoscale pressure can be used to realise confined conducting states in the material and potentially achieve deterministic control of such interfaces. Both systems allow co-existence of conducting walls or phases in the bulk but the mechanisms through which localised stress results in the formation of conductive interfaces or channels in these materials remains to be well understood before the effect itself can be fully exploited. For potential applications, it is also necessary to evaluate the ease of channel formation under pressure, their stability and reconfigurability. To address these issues, the primary goal of this proposal is to establish the pressure mediated control of localised nanoscale conductive states and develop a fundamental understanding of the physics associated with this behaviour so that reliable control of conductive interfaces can be achieved as a first step towards nano-circuitry. Pressure applied via an atomic force microscope (AFM) tip will be used to inject conductive states in three different materials : an improper ferroelectric, a mixed phase ferroelectric and a material undergoing metal-insulator phase transition, each representing a unique type of conductive interface created through pressure induced writing. In each of the three cases, the achievable degree of control of tip pressure induced conductive walls/phases will be evaluated and experiments will be performed to identify the physical origin and the mechanisms underlying the conductivity of the created interfaces. With a grasp on the mechanism of pressure induced conductivity in these materials, we aim to be able to precisely control the formation and annihilation of these confined walls or phases. The complementarity of pressure mediated control of conductive behaviour with other stimuli will be evaluated for optimal reconfigurability of conductive channels and read/erase capability. Proof-of-concept demonstration of pressure induced conductive channels between lateral electrodes will be performed. The AFM based approach developed here would thus help establish the underpinning physics of pressure induced conductive states in the discussed material systems and provide key insight for developing other nanoimprint methods for direct writing of nano-circuitry.

The proposal, with its theme focussed around the investigation of pressure induced conducting states in ferroelectrics and materials undergoing phase transitions, has great potential towards delivering significant impact in terms of novel physics as well as development of new technologies based upon the phenomena. The investigated phenomena represents an exciting but relatively unexplored direction and the physics underpinning localised pressure induced conductivity mediated by interfaces like domain walls or metallic phases in materials is novel. The physical understanding of the evolution of ferroelectric domain structures and their control with local mechanical pressure could provide an alternative and complementary route to controlling wall motion, ideas that are likely to create a widespread impact in the domain wall functionality research community as well as researchers working in the area of domain wall nanoelectronics. With these topics emerging as 'hot' areas of research across the world, this proposal will give UK research the potential for making ground-breaking progress. The proposal will also help foster strong international collaborations with teams engaged in research on relevant topics. The proposal will also enable training the next generation of scientists in the evolving and competitive field of nanoscale functional materials. It is foreseen that results and understanding gleaned from the project would generate future projects which are likely to permeate towards undergraduate teaching, especially in the form of MSci projects.

Impact of this proposal to the society will be through successive implementation of our proof-of-concept results towards advanced applications. The broader vision of the proposal, of which the proposed experiments are only a first step, is to develop nanoimprint methods which allow direct writing of nanometers sharp circuitry. This vision, when realised to its full potential, could enable transfer of nanometer scale metal-like patterns at low cost, high throughput and high resolution and has huge implications for slashing the manufacturing costs of semiconductors and data storage devices. The methodology and understanding of the phenomena developed in the proposal could thus enable cheaper and better processing techniques which in turn could bring down the cost of electronic devices and make them more accessible to society increasing the quality of life in UK. As discussed earlier, the ability to utilise localised pressure to control domain walls also opens up possibilities towards the field of domain wall nanoelectronics, a research area which is likely to attract interest from industry. The domain walls can be injected or annihilated by external fields which gives way to their potential use in "active" two dimensional electronic devices and localised stress confined to nanoscale volumes can be used to control their functionality. The potential for control of metal-insulator transition in oxides via localised pressure has significant implications towards novel sensor applications.